The Theses and Dissertations Abstracts from 2001 to Present are from masters and dissertations completed at the University of Texas in the field of Geological Sciences. See the Theses and Dissertations Index : 2001-2008 for a more concise listing of authors and titles. Dissertation abstracts published prior to 2001 may be found through Dissertations and Theses: Full Text.
INTEGRATED CORE, WELL LOG, AND SEISMIC INTERPRETATION OF ALBIAN PATCH REEFS IN MAVERICK BASIN, SW TEXAS
Enzo Sener Aconcha, M.S. GeoSci
The University of Texas at Austin, 2008
Supervisor: Charles Kerans
106 pages, 64 references
The Albian Glen Rose Formation has been well known as a shallow shelf carbonate with common patch reef development along the interface between the Comanche Shelf and the Maverick Basin. Stratigraphic analysis of 75 well logs and 7 seismic stratal slices formed the basic data for interpreting the Lower Glen Rose sequence framework and paleogeographic setting. The Lower Glen Rose units record the HST of sequence 6 and all of sequence 7. Patch reefs were found in the TST, near the MFS, and the HST of sequence 7, with three distinct reef levels found throughout units 2-4. Buildups in unit 2 change from transgressive isolated buildups into a flat biostrome during progressive onlap. Unit 3 buildups record a landward shift relative to the biostrome of unit 2, as shown on well logs and are coincident with the sequence 7 MFS. The third and uppermost buildup level is found in unit 4, in the late highstand of sequence 7. This unit shows a distinct seaward shift of the patch reefs, relative to unit 3, consistent with the highstand interpretation.
Detailed examination of facies in the two cores available for study indicates that the patch reefs formed during shoaling of the shelf. These patch reefs are characterized by a low API GR value (<20 GAPInorm) and strong negative seismic amplitudes. Off-reef deposits correlate with a higher, variable gamma-ray response (>20 GAPInorm) and positive seismic amplitudes. The patch reefs are proven hydrocarbon reservoirs with gas potential concentrated in units 2 and 4.
Geomorphometric analysis based on seismic stratal slices of more than 30 buildups from unit 4 determined that these are domal features with a distinct NNW-SSE elongation and steeper SW flanks. Asymmetry of the patch reefs may be the result of NW-directed far-field paleo-currents and NNE-directed, wind-driven paleo-waves. Independently of the size scale, the elongation axis of buildups is 1.5-2.5 times larger than its perpendicular direction. Results imply best reservoir facies in the SW side of the Lower Glen Rose patch reefs.
IMAGING THE LOWER SLOPE, OFFSHORE NICARAGUA AND COSTA RICA USING A NEW RESIDUAL MIGRATION VELOCITY ANALYSIS TECHNIQUE IN THE SPACE-OFFSET DOMAIN
Imtiaz Ahmed, Ph.D.
The University of Texas at Austin, 2003
Supervisors: Paul L. Stoffa and Kirk D. McIntosh
162 pages, 73 references
There is a dramatic variation in geochemical sediment tracer signal along the Central American volcanic arc. Two contradicting theories, one supporting sediment accretion and the other subduction erosion, have been suggested as possible explanations for these variations. My goal in this dissertation is to use seismic images of the lower slope off Nicaragua and Costa Rica to study the influence of subducting plate structure on sediment dynamics. I am particularly interested in documenting the efficiency of sediment subduction, its along strike variation, and to see if it corresponds with the geochemical anomalies.
Although high quality seismic data was acquired offshore Nicaragua, it remained difficult to image the lower slope in detail with conventional processing techniques. Several characteristics of this geologic environment pose seismic imaging problems: the area is heavily faulted, adjacent reflection boundaries have contrasting dips, and the velocity structure is complex. In this environment the common midpoint gathers do not represent a collection of true common subsurface reflection points. To get a clearer image, I needed to apply pre-stack depth migration (PSDM) techniques. An accurate velocity model is required to get a good PSDM image. Therefore one of the most important aspects of PSDM is velocity analysis.
Over the last few decades, residual migration velocity analysis (RMVA) has been an area of active research. Previous work on RMVA in the depth-offset domain required top down layer stripping migration in order to derive the interval velocities directly, hence making it very computationally intensive. Here I propose a new technique in which for each common image gather (CIG) we first create a table of offset-ray parameters-depth ( x-p-z ) using a local 1D assumption. Then I calculate the residual migration depth corrections in the p-z domain and finally map these depth corrections back to the x-z domain using the x-p-z table. Since I calculate the residual migration depth corrections in the p-z domain, the interval velocities are derived directly by top down residual migration. Hence I do not have to explicitly do the layer stripping migration followed by residual normal moveout to get the interval velocities.
The velocities generated by using the above RMVA technique produce interpretable depth images of the lower slope off Nicaragua and Costa Rica. From these images I interpret partial sediment accretion off Costa Rica and likely total sediment subduction off Nicaragua, and therefore provide a geophysical evidence for the observed geochemical disparity along this margin.
Trevor John Aitken, M.S. GeoSci
The University of Texas at Austin, 2005
Supervisor: Paul Mann
This thesis presents an integrated stratigraphic and tectonic evolution of the Grenada and Tobago basins using multi-channel seismic data collected in May, 2004, (BOLIVAR cruise), along with GULFREX seismic data collected by Gulf Oil Company in 1975. These reflection data, combined with UTIG OBS refraction data also collected with the BOLIVAR study in 2004 and with a compilation of previously published, onland geologic data in the southeastern Caribbean, constrain a multi-stage, Cenozoic tectonic history for the southern Lesser Antilles arc and flanking Grenada and Tobago basins. A new tectonic model for the Grenada and Tobago basins is based on three seismic megasequences. The striking similarity in the two basins’ half-graben structure, smooth basement character, deep-marine seismic facies, and similar Paleogene sediment thickness suggest that the two basins formed as a single, Paleogene forearc basin related to the now dormant Aves Ridge. This single forearc basin continued to open through flexural subsidence during the early to middle Eocene probably because of slow rollback of the subducting Atlantic slab. The Grenada and Tobago basins began to be divided during the early to middle Miocene, when the thinned crust of the forearc was inverted as a result of: 1) oblique convergence between the Caribbean plate and the passive margin of South America; and 2) intrusion of the Neogene Lesser Antilles arc. Observed transpressional shortening of the basins decreases from southwest to northeast. Total shortening in the southern Grenada basin varies from 5 km in the southern part of the study area to 1 km in the northern part of the study area. Shortening structures include inverted Paleogene normal faults, folds, and shale diapirism. The late Miocene to Recent period is characterized by divided depositional histories of the Grenada and Tobago basins. The Tobago basin is characterized by a 4-km-thick wedge of Plio-Pleistocene clastic sediments inferred to represent the distal progradation of the proto-Orinoco River. The Grenada basin becomes increasingly isolated from further continental sediment input by uplift of coastal Venezuelan ranges and the Neogene Lesser Antilles volcanic ridge.
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3D SEISMIC SURFACE MULTIPLE ATTENUATION: ALOGRITHMS AND ANALYSIS
Abdulaziz Saleh Alaslani, PhD
Supervisors: Paul L. Stoffa and Mrinal K. Sen
The aim of seismic exploration is to provide a comprehensive description of subsurface geologic structure in terms of its reflectivity function at the boundaries between geological units. Seismic multiples are coherent noise that obscure primary events and considerably degrade the quality of seismic images in the target zones. In spite of the fact that many methods have been designed to suppress multiples, only a limited success has been achieved. I have developed two different approaches to address the problem of seismic multiples. The first approach attempts to suppress multiples in terms of decomposition of the measured seismic wavefields into its upgoing and downgoing waves. The separation process is accomplished by using some statistical characteristics of the data in the plane-wave p domain. The ratio of these two components yields the true reflectivity function free of multiples. Although encouraging results are obtained in the separation process, instability occurs during the wavefield division step. As a result, the effectiveness of this approach is limited. I have also investigated seismic multiples for 3D geology and proposed a new methodology in which 3D multiples are predicted and attenuated successfully. The departure of the predicted multiple arrival times from the observed multiple arrival times explains why demultiple algorithms that assume two-dimensional multiple reflections often fail. In this approach, I employed 3D ray tracing to predict the arrival times of the primary and its multiples in individual shot gathers generated from a three-dimensional reflector. A non-linear optimization method, called Very Fast Simulated Annealing (VFSA) is used to determine geometry of the subsurface reflector in 3D. This is achieved by applying a ray traced normal moveout (NMO) correction to seismic reflections with respect to the zero offset time. Based on the optimized NMO-corrected shot gathers, the autoconvolution of the seismic trace is employed to predict the multiple reflections, which are then scaled and subtracted from the original data. The application of this technique to real data demonstrates that the new method successfully suppresses many surface multiples, and is able to recover several deep primary events. This algorithm is robust and computationally very efficient.
Return to listIMAGING OF R3 PROFILE OF CHICXULUB OFFSHORE SEISMIC DATA USING PRESTACK SPLIT-STIP FOURIER MIGRATION IN THE PLANE WAVE DOMAIN
Ali Mansour Aljadher, MSGeoSci
The University of Texas at Austin, 2008
Supervisor: Paul Stoffa
98 pages, 45 references, 1 table
Sixty-five million years ago, a bolide approximately 10 km in diameter traveling over 20 km/sec collided with earth in the Yucatan Peninsula leaving behind the wide multi-ring Chicxulub crater. Two-dimensional (2D) marine seismic reflection data were acquired in 1996 and 2005 to image the crustal deformation. Radial line R3, a 100 km seismic reflection profile, was processed using a conventional seismic data processing flow (McDonald, MS Thesis, 2006). In this study, line R3 is processed using a different scheme using prestack split-step Fourier migration in the plane wave domain. This new seismic imaging of the R3 data collapses the scattered waves, moves the temporal reflection events to their true structural position in depth and increases the signal to noise ratio. The field shot gathers are contaminated with low-frequency guided waves due to the shallow water column and the hard water bottom offshore Yucatan as well as the Scholte waves propagating along the seafloor interface. A 2D normal derivative operator was applied to remove this coherent noise for NMO corrected data. This multichannel filtering approach attempts to reveal the horizontal or nearly-horizontal reflections while non-horizontal evenets such as groundroll and Scholte waves are attenuated. Before migration of the reflections, the seismic shot gathers were mapped from the offset-time (X-T) domain to the vertical delay time, t, and the horizontal ray parameter, p, or simply t-p domain. In the t-p domain, predictive deconvolution often works better since multiples are periodic and hence easier to remove and this usually gives better results than applying the deconvolution in the original offset-time (X-T) domain. Moreover, groundroll and Scholte waves are mapped to points in the t-p domain and there can then be readily excluded for the imaging, improving the signal to noise ratio of the final depth section. For depth migration, a good velocity model is required to image the data to the correct position and depth. Thus, an optimized velocity model was used for prestack plane wave migration. Prestack depth migration was applied directly on the transformed t-p gathers that are sorted into constant ray parameter sections. Each plane wave component, i.e. constant p value, was imaged separately and prestack-migrated common-image-gathers (CIGs) are collected. They are in the depth and ray parameter z-p domain, at each shot position. The migrated and stacked results are obtained by stacking a selected range or all the traces in each CIG to generate the final image. Residual depth versus p “moveout” is then used to refine the interval velocity of the depth section. The result of this new processing is an improved image in depth of the crater which is important to understanding the actual structural geometry of this large impact event. The improved image can give a greater confidence in both the geologic structure and the velocity model than time migration since the events are now in their true spatial position.
MULTI-DIMENSIONAL LAND SEISMIC DATA-ACQUISITION TECHNIQUES AND RANDOM SURVEY DESIGN
Engin Alkan, MS. Geo. Sci.
The University of Texas at Austin , 2007
Supervisor : Bob Hardage
This study analyzes different techniques and innovations of three-dimensional seismic data acquisition and survey design. Multi-dimensional (both 2-D and 3-D) survey design requires objective consideration of survey goals, the range of expected Earth responses, crew and equipment accessibility, acquisition costs, instrument capabilities, experimental field conditions, and logistic considerations.
Planning a 3-D survey combines operational and technical issues which, in turn, depend on acquisition and design parameters. Because seismic source effort, crew and equipment availability, and size and shape of the survey affect survey cost, it is necessary to understand how all of these factors individually affect the overall data-acquisition program. The main goals of this thesis are to analyze the effect of receiving station coordinate randomness on different 3-D seismic data-acquisition and survey design characteristics, both operationally and technically, and to ultimately optimize the cost and data quality of seismic surveys.
Many advances have been made in imaging subsurface structures at both shallow and deep target locations through improved seismic data-acquisition and processing techniques in the past two decades. Service companies and oil companies continue to develop new techniques to create better 3-D images with higher resolution and improved signal-to-noise ratio.
A major problem that confronts onshore seismic exploration companies is the effects of the acquisition geometry on recorded data. Receiver and source line spacings, range of offsets, and azimuths, fold variations, and source-generated noise are all important issues to consider, as are culture, topography, and surface conditions.
This thesis consists of two parts: (1) station randomness effects on acquisition and survey parameters, and (2) experimental evaluation of Vibroseis sweep parameters. To perform the latter analysis, field data acquired across Tohonadla field at Bluff, Utah were processed, and data generated with various Vibroseis sweep parameters were compared. These analyses allowed data-acquisition and survey design parameters to be related to the cost of the survey and to data quality.
PRE-STACK INVERSION FOR POROSITY ESTIMATION FROM SEISMIC DATA IN AN OIL FIELD, EASTERN SAUDI ARABIA
Abdulaziz Mohammad AlMuhaidib, M.S. Geo. Sci.
The University of Texas at Austin , 2008
Supervisor: Mrinal K. Sen
107 pages, 41 references, 2 tables
The main objective of seismic inversion is to obtain earth model parameters from seismic reflection data. In other words, it is the process of determining what physical characteristics of rocks and fluids (i.e., P-impedance, shear impedance, and density) could have produced the seismic record. The aim of this study is to obtain reservoir properties, such as porosity both at the well locations and in the inter-well regions from seismic data and incorporated well logs. The target is a Jurassic carbonate reservoir from an oil field located to the East of Saudi Arabia. The purpose was to investigate the reliability of inferring the elastic properties (Zp, Zs, ρ) from seismic data in this field, and to build a geologic framework for flow simulation for better reservoir production forecasting and management.
The seismic data were processed with special attention to preserving the true reflection amplitudes, and were time migrated before stack. Residual moveout from multiples after NMO, however, is almost horizontal at near offset, and constructively add to the stacked amplitude. Therefore, we applied a pre-stack inversion technique on the seismic data, after careful processing, including removal of residual internal multiples. Such an inversion incorporates all of the offsets to obtain an optimum acoustic impedance model. We also investigated the stability of inverting shear impedance and density in the field of study.
The seismic inversion results were overall very good and stable for P-impedance. The match between borehole log and seismic impedance profiles was excellent for the high-contrast events and variable for the low contrast in acoustic impedance, depending on the location within the field. Inverted shear impedance results were less stable compared to P-Impedance, while density was totally unstable and has not been resolved. In general, areas of poor inversion coincided with the zones of poor quality seismic data.
The borehole log data showed a good impedance-porosity relationship. The Raymer-Hunt-Gardner impedance-porosity empirical relation fits the borehole data very well. Thus, I used the Raymer-Hunt-Gardner relation, with coefficients for this field derived from the log data, to convert inverted acoustic impedance into a porosity model for the field. Based on the new quantitative seismic reservoir characterization, I was able to identify additional areas of potentially good reservoir quality.
MILANKOVITCH ORBITAL FORCING CONTROLS ON SHALLOW-WATER CARBONATE CYCLICITY AND EARLY DOLOMITIZATION: INSIGHTS FROM THE LOWER CRETACEOUS CUPIDO PLATFORM, NE MEXICO
Younis Khamis Altobi, Ph.D
The University of Texas at Austin , 2007
Supervisor: Randall Marrett
High-resolution sequence stratigraphy coupled with understanding of diagenetic processes within carbonate strata improves our understanding of platform evolution controls, facies models, and reservoir quality. The Cupido platform (Barremian to Aptian) exhibits well developed cyclicity and provides unparalleled outcrops to investigate platform morphology, depositional settings, sequence stratigraphic development, origin of high-frequency cycles, and possible climatic controls on early dolomitization across the shelf.
High-frequency cycles stack into twelve depositional sequences (A-L) that correlate across the platform, which, in turn, build into the highstand part of composite sequence I, two complete composite sequences (II and III), and the transgressive based portion of composite sequence IV. Cupido platform interior cycles show evidence for an allocyclic origin, including (a) symmetric and asymmetric peritidal and subtidal cycles, (b) lateral continuity across facies tracts, and (c) incomplete subtidal cycles.
Spectral analyses of cycle thickness reveal a strong Milankovitch signal when sedimentation rate is assumed constant within cycles and cycles are tuned to orbital precession. High spectral density peaks are present for long and short eccentricity (E) period calculated for the Early Cretaceous times: E1 (264-392 kyr), E2 (89-98 kyr), and E3 (107-126 kyr) . Obliquity component with peaks at period (38-59 kyr ) are also present. Strong correlation of Milankovitch signals across the platform suggests that the lateral continuity of high-frequency cycles was caused by shelf-wide sea-level fluctuations.
Early dolomitization of the highstand lower Cupido deposits (LCu) possibly occurred in modified seawater within subtidal and supratidal zones, both at high-frequency cycle-scale and depositional sequence-scale. Dolomitization patterns appear to be linked to Milankovitch-induced 4 th -order sea-level events. The transgressive portions of depositional sequences are more pervasively or preferentially dolomitized compared to the highstand portions. Thickness and periodicity clusters of completely dolomitized cycles are equivalent to thickness and duration of depositional sequences. The results strengthen arguments for the role of Milankovitch-induced sea-level fluctuations in the development of greenhouse carbonate platforms and its control on early dolomite distribution in specific settings.
TECTONIC GEOMORPHOLOGY OF THE EASTERN TRINIDAD SHELF; IMPLICATIONS FOR INFLUENCE OF STRUCTURE ON RESERVOIR DISTRIBUTION AND NATURE IN OLDER BASIN FILL
Tricia Grier Alvarez, M.S. GeoSci
The University of Texas at Austin, 2008
Supervisor: Lesli J. Wood
Few deltaic systems in tectonically active areas of the world exhibit more data for detailed study than the Orinoco Delta extending along the hydrocarbon-rich, narrow, eastern shelf of the island of Trinidad. The entire region has undergone tectonic extension, compression and transpression during the late Tertiary and into the Quaternary. Paralic and shelf reservoir sand distribution and geometry have been significantly influenced by both structuring and strong offshore current activity, as well as large forced and unforced sealevel regressions. These sedimentary deposits hold significant shallow gas resources across the region, but the complexity of their distribution and architecture is poorly understood. A large merged 3D seismic survey (~9,000 sq km) was integrated with well penetrations across the modern shelf to examine the influence of structuring on near-modern basin fill depositional morphology and architecture.
Key reflecting horizons were mapped across the area and these document the structural opening of the basin by extension superimposed over ongoing uplift of compressional anticlines. Seismic attributes image reservoir elements such as channels and channel belts, tidally inundated interfluves and interdistributary areas which facilitate analysis of the scale and form of these features as well as enable evaluation of the influence of structure on deposition. Both larger channels (1-2 km wide) and smaller channels (less than 100 meters) show patterns of avulsion and lateral migration and appear to be ubiquitous in some areas of the shelf during lowstand times. The largest major sediment fairway (valley) is long-lived (~ 1.0 million years) and structurally confined by east-west trending anticlinal uplifts, funneling sediments down the axis of the basin. Alternations between surfaces with well defined depositional elements interpreted to be indicative of subaerial conditions, and surfaces devoid of such features which have been interpreted to be submarine in nature suggest large scale fluctuations in the depositional environment over time under the influence of changes in sea level.
Extensional faults, which show a complex pattern of displacement, both spatially and temporally; appear to remain active up to near present day time. Coupled with the apparent long-lived nature of the tectonic uplifts in this region, observations suggest that the structure is controlling sedimentation, as opposed to sedimentation driving deformation. Structure is playing a significant role in accommodation creation and therefore the location of the axis of sediment transport and accumulation throughout the Pleistocene history of the basin. This conclusion would be in keeping with the regional structural history of the basin showing transpression between the east-southeastward moving Caribbean Plate and stationary South American plate, initiated in the early Tertiary and continuing to the present day.
Quantitative data on systems tract architectural elements, including spatial orientation and distribution should significantly improve 3D modeling of these reservoirs and improve understanding of sand distribution and the processes of sediment transfer from proximal sources to shelf staging areas.
EVALUATION OF RISK ANALYSIS METHODS AND THE APPLICATIONS IN EXPLORATION AND PRODUCTION OF AN OFFSHORE PETROLEUM AND GAS FIELD
Ramiro Alberto Amaya, M.A.
The University of Texas at Austin, 2001
Supervisor: Willem C.J. van Rensburg
The aim of this thesis is to present a comprehensive evaluation of risk analysis tools and their application to the oil and gas industry. Risk analysis and decision tools have been largely applied to financial evaluation and oil and gas exploration projects, but their applications to technical risks have been limited. This work deals with technical risks and technical decisions in the operation and production phases. As the main concern of any business, including the oil industry, is the pay-off from financial investments, cash flow methods are also discussed.
Initially, the decision analysis cycle, as well as risk analysis methods and techniques, are described in detail and are applied in critical decision cases of technical and economic risk projects. The Bureau of Economic Geology provided the decision cases data. The information was taken from a current offshore project located in the Gulf of Mexico.
Probability theories and their applications in operations and exploration cases are discussed. An analysis of probability distributions is done to show how to assess exploration and operation uncertainties.
Conclusions presented are based on decision analysis results, supporting economics, and technical facts. Sensitivity analysis, probabilistic risk analysis, decision trees, and expected value are the tools used to make the best possible decision.
HIGH-PRESSURE METAMORPHISM IN THE WESTERN LLANO UPLIFT RECORDED BY GARNET-CLINOPYROXENITES IN MASON COUNTY, TEXAS
Susan Denise Anderson, M.S.Geo.Sci.
The University of Texas at Austin, 2001
SUPERVISOR: William D. Carlson
Garnet-clinopyroxenites, such as those exposed in Mason County, Texas, are rare in the Llano Uplift of central Texas and are thus important to interpretation of the region's metamorphic history. Because our understanding of the metamorphic history of the Llano Uplift has improved greatly since the garnet-clinopyroxenites in Mason County were first recognized as high-pressure remnants, a re-evaluation of their significance has become necessary. This study provides evidence of in situ metamorphism of the garnet-clinopyroxenites and re-evaluates previous estimates of pressure-temperature (P-T) conditions during early metamorphism of the western Llano Uplift. These interpretations place constraints on tectonic models currently under development for the southern margin of ancient Laurentia during the Grenville Orogeny.
Mapping of the Purdy Hill locality covered ~2 km2 and revealed for the first time substantial outcrop of retrogressed garnet-clinopyroxenites within the Lost Creek Gneiss. The mafic bodies crop out in elongate, pod-like bodies up to 15 m in length and 10 m in width. The bodies are aligned along two major trends that correlate strongly to the surrounding structures of the Lost Creek Gneiss. The orientations and occurrences of the garnet-clinopyroxenites, and the structural setting, of similar rocks in the eastern Uplift (at smaller scale but better exposed), lead to the conclusion that these rocks have been boudinaged, indicating high-grade deformation. These observations provide evidence that the garnet-clinopyroxenites were metamorphosed in situ, that they and their surrounding rocks experienced the same metamorphic history.
The retrogressed garnet-clinopyroxenites at the Sterling Jordan locality, ~2 km northwest of the Purdy Hill locality, contain the best-preserved primary metamorphic assemblage. Fe-Mg exchange thermometry based on interior compositions of garnet-orthopyroxene pairs and garnet-clinopyroxene pairs yields ~750°C, which represents peak or near-peak metamorphic temperatures. These temperatures are supported by the homogenization of garnet growth zoning profiles, the boudinage seen in the mafic rocks, and anatexis in the surrounding quartzofeldspathic Lost Creek Gneiss. Pressure estimates are based on the Al content in orthopyroxene and are complicated by the inhomogeneous distribution of Al within the orthopyroxene grains, particularly by increases in Al at the orthopyroxene rims toward grain boundaries. Compositional X-ray mapping of the spatial variation of Al in orthopyroxene has proven to be an effective method for analyzing such textural information.
Previous studies estimated peak metamorphic conditions of 8.0 - 11 kbar at an average temperature of 750°C based on inferred equilibrium between orthopyroxene rims and garnet interiors (Wilkerson et al., 1988). Re-evaluation of that study and application of a corrected garnet-orthopyroxene barometer have led in this study to estimates of 15-21 kbar at 750°C, within the eclogite facies, based on equilibrium between garnet and orthopyroxene cores, for the early metamorphism. These new P-T estimates for peak metamorphic conditions of the western Llano Uplift are significantly different from estimates for other localities in the north-central and southeastern uplift, perhaps suggesting important differences in their early metamorphic histories.
Compositional modification of adjacent orthopyroxene and garnet rims, producing an increase in Al at the margins of orthopyroxene, occurred as a result of the continued growth of orthopyroxene at the expense of garnet as the rocks began to re-equilibrate to lower P-T conditions subsequent to peak metamorphism. The resulting rim compositions of garnet, orthopyroxene, and clinopyroxene, which likely do not represent complete re-equilibration, yield 0.3 - 5.4 kbar at an average of 650º C; these conditions represent the amphibolite facies. Although it is also possible that this partial re-equilibration occurred along a late, retrograde portion of the same P- T path that produced peak metamorphism, evidence for hydration during this recrystallization episode suggests that the partial re-equilibration was likely associated with the later, lower-pressure amphibolite-facies metamorphism that occurred across the uplift.
MODELING OF RUNOFF-PRODUCING RAINFALL HYETOGRAPHS IN TEXAS USING L-MOMENT STATISTICS
By
William Harold Asquith, Ph.D.
The University of Texas at Austin, 2003
Supervisor: John M. Sharp
386 pages, 103 references, 0 plates
Temporal distributions of storm rainfall are known as hyetographs. Design hyetographs are important for cost-effective risk-mitigated ramfall-runoff modeling. The hyetographs considered are known to produce or generate runoff on small watersheds (typically about 50 square kilometers) in Texas. L-moment statistics and the nonparametric median are used to summarize the dimensionless representations of over 1,600 observed hyetograph distributions. A focus is made on storm depths in excess of about 25 mm and durations of 0-12, 12-24, and 24 hours and greater. Statistical distributions are fit to the L-moments of the dimensionless hyetographs including the newly described L-gamma. L-gamma hyetograph models are anticipated to be reliable predictors of expected hyetographs. Finally, a separate permeability-related L-moment application to the popular Carman-Kozeny equation is described.RESERVOIR CHARACTERIZATION OF THE MIOCENE STARFAK AND TIGER SHOAL FIELDS, OFFSHORE LOUISIANA THROUGH INTEGRATION OF SEQUENCE STRATIGRAPHY, 3-D SEISMIC, AND WELL-LOG DATA
Adrian Constantin Badescu, Ph.D.
The University of Texas at Austin, 2002
Supervisor: William L. Fisher
Many "mature" Gulf of Mexico (GOM) fields, due to their structurally and stratigraphically complex nature, possess significant remaining resources. Such is the case in the Starfak and Tiger Shoal fields, offshore Louisiana. In these fields, forty hydrocarbon reservoirs occur in a regressive Miocene-age succession that comprises 10 third-order and at least 58 fourth-order sequences. Reservoir-scale heterogeneity is controlled by the nature and distribution of sedimentary facies and is usually below the resolution of current subsurface seismic sampling. Sequence-stratigraphic analysis helps to improve predictions of spatial and temporal reservoir heterogeneity. This study addresses the application of the correlation between petrophysical properties (PP) (e.g., effective porosity and shale volume) and seismic attributes (SA) within a high-frequency sequence stratigraphic framework to identify untapped reservoir compartments in the two offshore Louisiana fields.
Synthetic modeling of the seismic data showed that the vertical resolution of the seismic is approximately 12 ms. The relief on the fourth-order sequence boundary (SB) is below seismic resolution. A new method of mapping fourthorder SBs was developed. This method is based on the ability to image planiform morphology along the SB that can be depicted from a sequence of 4-ms-thick stratal slices. This method was successfully applied in the study area, and showed increased resolution when compared to two other mapping methods (manual tracking and proportional slicing).
Three methods were tested to correlate SA with PP: (1) direct correlation between SAs and PPs through regression analysis, (2) seismic inversion, and (3) probabilistic neural network (PNN). Among the three methods, the PNN proved to be the best technique. Four uncontacted compartments targeting incised-valley sands in genetic sequence 30 were identified upon analyzing the inverted Vshale volume created using PNN method.
Starfak and Tiger Shoal are typical of mature shelf-bound fields in the GOM, with long and sustained production histories. Structural and stratigraphic heterogeneities present opportunities for reserve-growth potential if an improved process for mapping of geologic complexity can be unraveled.
METHODS COMPARISONS AND SPATIAL ANALYSIS OF FIELD PARAMETER QUANTIFICATION OF UNSATURATED FLOW AT AN EXISTING WASTEWATER
INFILTRATION SYSTEM; MINES PARK, GOLDEN, COLORADO
Danielle Margueritte Bailey, M.S. Geo. Sci.
The University of Texas at Austin , 2006
CO-SUPERVISORS: John E. McCray and Leon E. Long
Subsurface water reclamation systems (SWRS) in mountain watersheds commonly experience issues of water sustainability, and are generally designed through judgment and experience rather than through rigorous quantitative methods. I have made a detailed vadose-zone characterization of several saturated and unsaturated flow parameters in an excavated trench representative of the size of a typical domestic drainage field at a wastewater reclamation test site (WRTS) at Mines Park, Golden, CO with the objective to improve the pre-design techniques associated with SWRS. Most wastewater infiltration is an unsaturated water flow process; therefore, measurement of soil-hydraulic parameters, especially saturated and unsaturated hydraulic conductivity (K sat and K unsat ), is required. Toward this end, I compared the consistency of estimating or measuring K sat and K unsat through field, laboratory, and empirical methods.
Field data collection included Guelph Permeameter and Tension-disc Infiltrometer measurements in the excavated trench, and Slug Test measurements at nearby monitoring wells. Representative soil samples were collected for in-lab experimentation at the UT Bureau of Economic Geology, including the van Genuchten-Mualem transformations of capillary-pressure curves determined from hanging-column measurements. Additional estimates of K sat and K unsat were determined through nine empirical functions and the Rosetta neural network found within Hydrus-1D software. Spatial variations in K sat and K unsat determined from the various methods are illustrated by curves that plot K sat or K unsat according to location. Statistical analyses demonstrate such a large degree of spatial variability that no single measurement adequately represents the values of these parameters, whether along the excavated trench or between the upper and lower tiers of the trench.
The following main conclusions are offered: (1) spatial variations in K sat and K unsat occur along a trench and with depth, in which K unsat exhibits much larger variability with depth than does K sat ; (2) of the empirical functions, results of the Hazen (1893), Harleman et al. (1963), and Beyer (1964) methods compare most favorably to both field methods (Guelph Permeameter, Tension-disc Infiltrometer), providing data that agree with the field-measured values within a factor of two; (3) the Guelph Permeameter field method is the least time consuming and less complicated, but the most arduous to conduct of all field methods; (4) Rosetta estimates with either sand/silt/clay input or soil classification input data provide K unsat values most similar to the Hanging Column values using the van Genuchten-Mualem relationship ; and (5) a single measurement did not appear to represent the hydraulic properties of an entire area occupied by a typical wastewater infiltration trench. I found variation both horizontally along the length of each tier and vertically between Tier 1 (2 feet below ground surface) and Tier 2 (4 feet bgs). This research advances the use of rigorous quantitative measurements and methods in understanding the design of SWRS.
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GRAIN SIZES OF RECENT SILICICLASTIC DEPOSITS IN WAX LAKE DELTA, LOUISIANA
Elke Baitis, B.S.
The University of Texas at Austin, 2008
Supervisor: David Mohrig
Reconstruction of dominant sand transport conditions within the Wax Lake delta using grain-size distributions in recent delta deposits shows that most sand travels as suspended load through the system. When comparing three different bed elevations within the delta, sand deposits reveal a fining upward trend, which compares favorably with estimates of transport style. Very fine sands associated with full suspension predominate at the highest elevations associated with an island levee. Lower fine sands associated with the onset of significant suspension are found distributed in roughly equal proportions across all bed elevations. Upper fine sands linked to incipient suspension are found at all bed elevations but favor channel bottoms. Medium sands, the coarsest component recovered from sediment samples, are found only in the deep feeder channel of the delta and are interpreted to move as pure bedload. These findings along with observations made during average flow conditions in late May, 2007, which showed little to no sand movement through the channels, suggest that delta surfaces are inactive with regards to sand deposition except during infrequent periods of flooding when most of the sand is transported in suspension.
Return to listOsteological Description of an Embryonic Elephant Bird (Ratitae: Aepyornis) Using High-Resolution X-Ray Computed Tomography, With a Discussion of Growth in Aepyornis
Amy Michelle Balanoff, M.S. Geo. Sci.
The University of Texas at Austin, 2003
SUPERVISOR: Timothy Rowe
175 pages, 110 references
Very little is known regarding the embryonic skeleton of the extinct elephant bird, Aepyornis, largely due to our hesitation in breaking open rarely recovered intact eggs of this taxon.High-resolution x-ray computed tomography (HRCT), however, provides a non-destructive method for viewing the internal structures of such eggs.The combination of HRCT and three-dimensional rapid prototyping technology made possible the description and reconstruction of a disarticulated embryonic skeleton of Aepyornispreserved within a complete egg.Skeletal elements digitally retrieved from the egg include: the braincase, palate, rostrum, vertebral column, and both fore- and hindlimbs.The cranial morphology of the embryo is well preserved, however due to the early ontogenetic stage of the specimen, development of the postcranial skeleton is less advanced.Comparisons of morphological characters with other embryonic birds allowed an estimation of the ontogenetic maturity as approximately 80-90% through incubation.This specimen, in conjunction with adult material allocated to Aepyornis, was used to construct a growth trajectory that allows a preliminary comparison of growth between Aepyornis and extant ratites.These comparisons offer new insight into the question of how the world’s largest bird got so big.The data suggest that the growth of Aepyornis proceeded in a manner similar to other ratites with a period of rapid growth early in ontogeny that leveled off after hatching.PLIOCENE-QUATERNARY DEFORMATION AND MAGMATISM
AT THE SOUTHERN MARGIN OF THE PUNA PLATEAU, ARGENTINE ANDES
by
Austin Kyle Baldwin, M.S.Geo.Sci.
The University of Texas at Austin, 2005
Supervisor: Randall Marrett
New fault data and mapping of volcanic rocks from northwestern Argentina's Cordillera de San Buenaventura, a Pliocene-Quaternary, E-W trending volcanic range at the southern margin of the Puna plateau adjacent to the modern volcanic arc at 27°S, reveal numerous tectonomagmatic characteristics unique in the region. Faults in the Cordillera de San Buenaventura dominantly strike ENE-WSW, with lengths reaching at least 30-40 km. Fault motion, where measurable, has been normal, commonly with a right-lateral component. Some faults displace alluvium and dated Plio-Quaternary non-explosive lava domes and flows, intermediate in composition, with throws of 1-100 m, whereas other faults do not affect overlying volcanics. Previous studies in the surrounding southern Puna have reported Pliocene-Quaternary NNW-SSE extension along N-S to NNE-SSW striking right-slip faults commonly associated with volumetrically-minor mafic monogenetic cinder cones and flows. Explosive eruptions with intermediate compositions have also characterized the southern Puna during the Pliocene-Quaternary. Volcanics erupted during this time period in the Cordillera de San Buenaventura lack both mafic and explosive tendencies.
The new data, combined with existing data, suggest that the Cordillera de San Buenaventura overlies a shallow magma chamber, which thermally weakened the upper crust and promoted the growth of the observed ENE-WSW striking normal and oblique-slip faults during the Pliocene-Quaternary. The new faults created right-stepping extensional transfers by linking preexisting NNE-SSW striking right-slip faults. Decreased horizontal stress at these transfers facilitated magma ascent from the underlying chamber. Periodic breaches of the magma chamber cupola by slip on faults prevented magmatic fluid accumulation and explosive eruptions. Mafic magma ascending beneath the Cordillera de San Buenaventura pooled in the shallow magma chamber, where it assimilated silica-rich crustal material and differentiated, eventually reaching the surface with an intermediate composition. In surrounding areas, where no shallow magma chamber existed, rising magmas reached the surface with mafic compositions. The high topography of the Cordillera de San Buenaventura may be the combined effect of mechanical doming by the shallow magma chamber, thermal uplift, isostatic elevation of an extensional transfer block, and local accumulation of Plio-Quaternary volcanics.
HYDROTHERMAL FLUIDS AND CU-AU MINERALIZATION OF THE DEEP GRASBERG PORPHYRY DEPOSIT, PAPUA, INDONESIA
Lorraine Marie Baline, M.S. Geo. Sci.
The University of Texas at Austin , 2007
Supervisor: J. Richard Kyle
The Grasberg porphyry Cu-Au deposit is located in the Ertsberg District in the Central Range of Papua, Indonesia. The Deep Grasberg is the deepest explored part of the Grasberg Igneous Complex (GIC) at elevations between 2450 and 3050 m, more than 1100 m below the pre-mining surface. The deposit is hosted by three quartz-monzonite to diorite units, emplaced approximately 3 Ma: the Dalam, the Main Grasberg Intrusion (MGI), and the Kali.
In the Deep Grasberg, the intrusions contain abundant plagioclase phenocrysts with biotite and hornblende as the dominant mafic minerals. Plagioclase has undergone varying degrees of hydrothermal alteration to sericite. Complete alteration of plagioclase to sericite is common in the Dalam, whereas the Early and Late Kali are significantly less altered. Matrix minerals include potassium feldspar and quartz. Quartz occurs as widespread stockwork veins and also is the dominant mineral in the silicified zone, near the GIC-wallrock contact in the Deep Grasberg. Chalcopyrite, with lesser bornite and covellite are the most abundant economic minerals in the Deep Grasberg and are both vein hosted and disseminated. Bornite is most abundant near the center of the Deep Grasberg, whereas covellite abundance increases toward the periphery.
Hydrothermal fluids, supplied from a cupola at depth, created the Grasberg orebody. Due to its relative proximity to the cupola, the Deep Grasberg records a unique fluid evolution and constrains fluid processes during ore formation.
Some changes in the temperature and composition of the hydrothermal fluids in the Deep Grasberg are recorded in quartz crystals. Textures observed using scanned luminescence of 32 samples of quartz veins from the Dalam, MGI, and Kali reveal a complex vein history in the GIC. Alternating light and dark bands forming concentric growth zone textures record changing conditions during quartz precipitation. Point analyses were unable to detect compositional differences in the quartz bands, but X-ray mapping revealed bright bands had higher Al and Ti contents. Subtle compositional changes in quartz suggest that chemical changes in fluid composition may be responsible for the observed growth zonations. Irregular textures in some quartz crystals suggest a period of quartz dissolution occurred as hydrothermal fluids cooled, followed by the resumption of quartz precipitation.
Fluid inclusion populations from 27 samples from the Dalam, MGI, and Kali reveal that a range of fluid compositions were trapped during quartz precipitation. Five types of fluid inclusions are identified: liquid + vapor, vapor-rich, liquid + vapor + halite, liquid + vapor + multi-daughter crystal, and liquid + vapor + opaque. Liquid + vapor + opaque inclusions are believed to represent a supercritical fluid exsolved from a magma at depth, whereas the coexistence of vapor-rich inclusions and high salinity liquid + vapor + halite and liquid + vapor + multi-daughter crystal inclusions represent two separated phases of this fluid after changes in pressure and temperature caused the hydrothermal fluid system to move from the one-phase to the two-phase field. Homogenization temperatures recorded for liquid + vapor + halite, liquid + vapor + multi-daughter crystal, and vapor-rich inclusions are similar. The lowest homogenization temperatures recorded are about 225 °C, but some inclusions did not homogenize at temperatures as high as 700 °C, the maximum temperature able to be reached on the fluid inclusion stage. Two groups emerge from the data, a set of moderate temperature inclusions and a set of higher temperature inclusions. The moderate temperature inclusions have an average homogenization temperature of 368 °C. The higher temperature inclusions have an average homogenization temperature of >598 °C. Calculated salinities for liquid + vapor + halite and liquid + vapor + multi-daughter crystal inclusions indicate that the separated fluid phase from which these inclusions sourced was very saline, containing about 50 wt.% NaCl equivalent. Fluid inclusions in anhydrite from late-stage veins record a significantly cooler fluid, at approximately 250°C at a salinity near 10 wt.% NaCl equivalent.
Similarity in fluid inclusion compositions, temperatures, textures, and chemical zonations in quartz crystals in the veins in all three intrusive units suggest a common fluid source. It also suggests that similar fluid processes were prevalent during quartz vein development in these intrusive units. In the Deep Grasberg, vein quartz precipitated both directly from a supercritical fluid released from a cupola at depth and from a hydrothermal fluid post-boiling. The narrowing of the GIC at depth may have caused “throttling” of fluids in the Deep Grasberg, resulting in relatively increased pressure in the fractures. Decompression of the fluid was inhibited, and fluid inclusions of liquid + vapor + opaque record the earliest fluids . A decrease in pressure resulted in boiling, creating two fluids: a brine and a vapor. These changes in the hydrothermal fluid account for the variety of fluid inclusion types observed in the Deep Grasberg. Additionally, the repetitive growth zonation patterns in quartz suggests that the hydrothermal fluids were released from a cupola at depth into fractures in pulses during quartz precipitation. Fractures remained open, allowing for quartz growth with successive fluid pulses.
SEISMIC CHARACTERIZATION OF NATURALLY FRACTURED RESERVOIRS
Reeshidev Bansal, Ph.D.
The University of Texas at Austin , 2007
Supervisor: Mrinal K. Sen
Mrinal K. Sen
Many hydrocarbon reservoirs have sufficient porosity but low perme¬ability (for example, tight gas sands and coal beds). However, such reservoirs are often naturally fractured. The fracture patterns in these reservoirs can control flow and transport properties, and therefore, play an important role in drilling production wells.
On the scale of seismic wavelengths, closely spaced parallel fractures behave like an anisotropic media, which precludes the response of individual fractures in the seismic data. There are a number of fracture parameters which are needed to fully characterize a fractured reservoir. However, seismic data may reveal only certain fracture parameters and those are fracture orientation, crack density and fracture infill.
Most of the widely used fracture characterization methods such as S-wave splitting analysis or amplitude vs. offset and azimuth (AVOA) analysis fail to render desired results in laterally varying media. I have conducted a systematic study of the response of fractured reservoirs with laterally varying elastic and fracture properties, and I have developed a scheme to invert for the fracture parameters.
I have implemented a 3D finite-difference method to generate multicom-ponent synthetic seismic data in general anisotropic media. I applied the finite-difference algorithm in both Standard and Rotated Staggered grids. Stan¬dard Staggered grid is used for media having symmetry up to orthorhombic (isotropic, transversely isotropic, and orthorhombic), whereas Rotated Stag¬gered grid is implemented for monoclinic and triclinic media. I have also developed an efficient and accurate ray-bending algorithm to compute seismic traveltimes in 3D anisotropic media.
AVOA analysis is equivalent to the first-order Born approximation. However, AVOA analysis can be applied only in a laterally uniform medium, whereas the Born-approximation does not pose any restriction on the subsur¬face structure. I have developed an inversion scheme based on a ray-Born approximation to invert for the fracture parameters. Best results are achieved when both vertical and horizontal components of the seismic data are inverted simultaneously. I have also developed an efficient positivity constraint which forbids the inverted fracture parameters to be negative in value. I have im¬plemented the inversion scheme in the frequency domain and I show, using various numerical examples, that all frequency samples up to the Nyquist are not required to achieve desired inversion results.
THE EFFECTS OF NUTRIENT-BEARING MINERALS ON HYDROCARBON BIODEGRADATION
By
Joey Jacqualin Barker, M.S. GeoSci
The University of Texas at Austin, 2004
Supervisor: Philip C. Bennett
119 pages, 143 references
In recent decades researchers have found that native microbial communities are capable of degrading numerous environmental contaminants into more benign compounds. The ability of a microbial consortia to effectively slow the spread of contamination in an aquifer can be limited by the availability of the nutrients needed for metabolism and reproduction. Previous investigations into microbial colonization patterns indicate that microbes may be gaining the necessary nutrients from mineral grains in an aquifer.
The purpose of this study was to determine the effects of mineral chemistry on anaerobic hydrocarbon biodegradation in a nutrient-limited system. These laboratory experiments were conducted using microcosms constructed of native sediment and groundwater from the Bemidji, Minnesota U.S. Geological Survey Toxics Research Site, and minerals containing trace amounts of nutrients. Some microcosms had nutrients added in solution, instead of in mineral form. In the first trial experiment, the contamination of the sterile control with chloroform was found to have contaminated the microbes in the live microcosms during the sampling procedure. New microcosms were constructed for a second trial, and it was found that temperature fluctuations occurring during the sampling procedure affected the ability of the microbes to degrade the hydrocarbons. Once a steady temperature was maintained the experiments were able to proceed.
The results of the third trial suggest that mineral chemistry does affect hydrocarbon biodegradation rate. The addition of ferric iron, in both mineral and solution form, had the greatest individual effect on increasing biodegradation. Phosphorus and nitrogen also increased biodegradation. However, the combination of phosphorus and iron in the same mineral had the greatest effect on increasing biodegradation of toluene.
Trace amounts of nutrients in minerals {\em do} affect microbial toluene degradation. Additionally, the nutrient content of the minerals can provide information about the nutrients which are limiting the system. Degradation rates are controlled in part by small-scale variations in aquifer mineralogy and permeability. These variations affect which microbial populations are dominant, the concentration of contaminants and the availability of nutrients. These factors affect the biodegradation rate of contaminants. Providing minerals that contain the most limiting nutrients to a microbial community can improve intrinsic bioremediation and limit the spread of hydrocarbons in the groundwater.
THE SENSITIVITY OF LAND SURFACE MODEL SIMULATIONS TO BIAS CORRECTION OF THE EUROPEAN CENTRE FOR MEDIUM-RANGE WEATHER FORECASTS REANALYSIS
Aaron Andrew Berg, M.S. Geol. Sci.
The University of Texas at Austin, 2001
Supervisors: James S. Famiglietti, and Clark R. Wilson
After sea surface temperatures, the most important boundary condition for the prediction of climate at the seasonal-to-interannual timeframe is the soil wetness state. Unfortunately, large-scale soil moisture observations are not available, and must therefore be provided through a modeling/data-assimilation approach. Modeling the initial moisture status of a soil requires a comprehensive list of surface water and energy fluxes at consistent temporal and spatial scales. Such data are not presently available except in weather reanalysis products. However, reanalysis products are often biased due to errors in the weather forecast model. In the present study, an attempt to minimize errors to model predictions of the initial soil moisture status is realized through a bias reduction scheme to the reanalysis forcing. The bias reduction scheme uses difference and ratio-based corrections to the reanalysis with global observational datasets. The corrected and original forcing was used to simulate water and energy fluxes within a land surface modeling framework. Output from the land surface model was compared with observations to assess the impact of the bias correction on simulation accuracy. Results of this study demonstrate the implementation of a bias reduction scheme reduces errors in the simulation of runoff, soil moisture, and snow depth. Accordingly, the initializations produced should also be closer to actual surface conditions, and therefore more useful to the climate modeling community. Modelers using reanalysis products for driving land surface models especially for the establishment of initial conditions are cautioned on this approach, and advised to consider a bias reduction strategy.
STRAIN RATES AND CONSTRAINTS ON CHEMICAL HOMOGENEITY AND LENGTH SCALES OF EQUILIBRATION DURING ALPINE METAMORPHISM AT PASSO DEL SOLE, CENTRAL SWISS ALPS
Christopher Andrew Berg, Ph.D.
The University of Texas at Austin , 2007
Supervisor: William Carlson
Garnet-bearing gneisses from the vicinity of Passo del Sole, Central Swiss Alps, have undergone a complex history of metamorphism and deformation that has imbued them with unique compositional and textural variations. Complex, concentric zoning patterns in garnet may be correlated between porphyroblasts on the scale of a hand sample; however, the character of the Ca, Mn, and Y zoning patterns in garnet vary within single layers on the meter-scale. Within a hand sample, individual compositional zones can be correlated from crystal to crystal on the basis of chemical similarities (e.g., intricate yet identical variations in calcium concentration; equivalent manganese concentrations at zone boundaries) and textural similarities (e.g., initiation of inclusion-trail curvature). These relationships allow the identification of individual compositional zones as time markers during garnet growth.
Detailed examination of garnet growth zoning patterns, in combination with measurements of inclusion-trail curvature within garnet porphyroblasts and detailed thermodynamic models of the garnet growth history in selected samples, together with an assumed heating rate associated with Alpine metamorphism, allows quantification of strain-rates during prograde amphibolite-facies metamorphism along the northern margin of the Lucomagno nappe. Constraints on the nucleation and timing of garnet growth and garnet growth rate mechanisms permit further insight into the relationship between strain-rate and metamorphism at Passo del Sole than had previously been possible with this method: variations in strain-rate magnitude of over a factor of ten (10 -14 – 10 -13 s -1 ) are observed, which correlate with core-to-rim changes in compositional zoning.
The source of the incredible diversity of compositional zoning patterns in garnet is hypothesized to be open-system infiltration of ephemeral, channelized Ca- or Mn-rich fluids derived from magmatic sources or equilibrated with metamorphic rocks deeper in the section. Stages of garnet growth associated with increased strain rates are also correlated with the high-Ca or high-Mn zones within the garnet porphyroblasts, presumably as the result of strain softening associated with the passage of these fluids. In-situ oxygen isotope analysis using SIMS demonstrates that the changes in major-element zoning patterns correlate with small shifts in the isotopic composition of garnet.
SEQUENCE STRATIGRAPHY AND DEPOSITIONAL HISTORY OF THE UPPER CAÑON DEL TULE, LAS IMAGENES, AND LOWER CERRO GRANDE FORMATIONS, CENTRAL PARRAS BASIN, NORTHEASTERN MEXICO
Juan Clemente Bermúdez Santana, Ph. D.
The University of Texas at Austin, 2003
Supervisor: Richard T. Buffler
210 pages, 100 references, 20 plates
This dissertation describes the results of a field-oriented sequence stratigraphic study of a 900-m-thick Maastrichtian interval in the middle portion of the Upper Cretaceous Difunta Group. Basic analysis of rock features and their stratigraphic arrangement were used to subdivide the interval into transgressive (TST), highstand (HST), and falling stage (FSST) systems tracts and to recognize their bounding surfaces.
The progradational offshore to lower shoreface highstand (HST) deposits of the upper Cañón del Tule Formation and the underlying transgressive (TST) deposits, contain gastropods and bivalves common in shallow marine Lower Maastrichtian strata. However, the occurrence of the ammonite Coahuilites sheltoni restricts these rocks to the lower Upper Maastrichtian.
In the Late Maastrichtian, the final stage of shallow marine sedimentation of the upper Cañón del Tule Formation was characterized by an abrupt change. Accommodation space was reduced in proximal settings, and the shoreline and coarse-grained facies belts migrated basinward in response to a relative sea level fall (forced regression). Wave scouring of the seafloor, and rip and longshore currents produced a regionally extended forced regression surface and sequence boundary characterized by meter-scale gutter casts.
The FSST upper shoreface to foreshore deposits of the uppermost Cañón del Tule Formation include a ridge-forming sandstone of variable thickness. Its internal stratigraphic architecture suggests that high-frequency pulses of sea level risings punctuated the forced regression. During this process, thick, aggradational red deposits of the lower Las Imágenes Formation (FSST) accumulated over the adjacent coastal plain until an intermittent marine transgression began. T he initial transgressive pulses occurred across a rapidly subsiding coastal plain, which favored the development of charophytes in fresh and brackish water environments, associated with benthonic foraminifers, ostracods, oysters, Ophiomorpha and Thalassinoides . Mixed-load probably meandering rivers drained the coastal plain bounded by lagoons and extensive shoreface and offshore environments for this part of the Late Maastrichtian Gulf Coast of México. After this stage of intermittent marine transgression, shallow seas and transgressive (TST) deposits of the lower Cerro Grande Formation covered the region.
STUDIES ON POST-NATAL VARIATION AND VARIABILITY IN THE VERTEBRATE SKELETON AND ITS PALEONTOLOGICAL IMPLICATIONS
Gaberiel Stephen Bever, Ph.D.
The University of Texas at Austin, 2006
Supervisor: Christopher J. Bell
Developmental biology combined with phylogenetic systematics is broadening our perspectives on the role of intraspecific variation in morphology. These perspectives include conceptualizing variability as a reflection of the evolutionary history of the underlying developmental pathways. I conducted a series of studies explicitly addressing the implications that intraspecific variation and variability have for the description, analysis, and interpretation of fossil vertebrates.
The first chapter examines variation in the anuran ilium as a test case for the potential problems that intraspecific variation has on our perceptions of the fossil record. Results indicate that no distinctive morphologies in the ilium of extant North American Bufo are known that support species-level identifications. These observations indicate that our current understanding of the evolutionary history of many extinct and extant taxa based on the fossil record is highly influenced by our assumptions regarding the distribution of taxa through space and time. Restricting identifications to higher taxonomic levels that can be supported with apomorphies helps to avoid circularity if fossil identifications are used subsequently in analyses offaunal dynamics.
Chapters 3 and 4 examine the nature of intrapopulational variation in the crania of two extant turtles, Sternolherus odoratus and Pseudemys texana. Results indicate that the number of variable characters- and the levels at which they vary- are surprisingly high considering the conservative nature of both the taxonomic sample and the pool of examined characters. Variability can be unevenly distributed within and between age groups, sexes, and modular partitions of the skull. Complex patterns of post-natal ossification are a major source of intraspecific variation.
The final chapter examines the potential implications that polymorphic characters have for the phylogenetic assessment of fossils and the potential for objectively estimating variation in extinct lineages. Results indicate that polymorphic characters can significantly affect phylogenetic hypotheses that include fossils. However, a modem view of variability, combined with increasing data on variation in both extant and extinct lineages, not only provides an objective means to estimate variation in fossil specimens but represents a largely untapped source of phylogenetic information that is a potentially important research trajectory in evolutionary morphology.
THE ORIGIN AND EVOLUTION OF THE GRAN DESIERTO SAND SEA
Carrie Ann Beveridge
University of Texas at Austin, 2004.
Supervisor: Kocurek, Gary
The Gran Desierto Sand Sea of Sonora, Mexico is located on the northeastern shore of the Gulf of California, east of the Colorado River Delta. The modern sand sea covers an area of 5700 km² and is the largest active dunefield in North America. The Gran Desierto is well-suited for investigation into the timing and nature of aeolian activity and regional climate change as it contains multiple dune geomorphic elements that compose the complex pattern observed today. Individual generations of dune geomorphic features are identified and ‘backstripped’ through the utilization of several techniques. Methods include remote sensing and statistical analysis of physical parameters of the dune geomorphic forms, as well as absolute age determination of individual generations through optically stimulated luminescence (OSL) age dating techniques. Individual generations of dune geomorphic features are digitized from a Landsat 7 multispectral composite. Crestline lengths, spacings and orientations are statistically analyzed in order to identify distinct trends among individual groups of dunes. These groups are then compared with dune geomorphic features visible on a satellite image overlain on a vertically exaggerated digital elevation model. OSL ages are utilized to determine individual generations within each dune group. These generations are then correlated with global climatic events, and associated variability in predominant wind regimes. Aeolian units sit on top of alluvium that is sourced from distal alluvial fans of Tertiary granitic outcrops, and from fluvial sediments deposited as the ancient Colorado River avulsed westward across the region throughout the Pleistocene. Alluvium is dated from ~87 ka to ~26 ka. Six generations of dune geomorphic features ranging in age from 26,000 years to modern are identified. Based on gross bedform normal transport, wind regimes are reconstructed since LGM. From ~26 ka to ~12 ka, winds shifted from N and NNW to NW and S. From ~12 ka to ~ 10 ka, winds shifted to SE dominant with a subordinate NW component. From ~10 ka to ~6 ka, NW winds became dominant over the south winds and more northerly. Modern conditions were reached by (at latest) ~6 ka, with the exception of enhanced ENSO cycles ~2.4-2.8 ka.
Return to listANATOMY AND PHYLOGENY OF XENOSAURUS AND ITS EXTINCT RELATIVES
Bhart-Anjan Singh Bhullar, M.S.Geo.Sci.
The University of Texas at Austin, 2008
Supervisor: Christopher J. Bell
Xenosaurus is an enigmatic clade of Mexican and Central American lizards distinguished by knob-like scalation and flattening of the head and body associated with living in cracks within cliff faces. The position of Xenosaurus within the larger clade Anguimorpha is difficult to determine due to a combination of primitive features and unique, highly modified anatomy that obscures useful characters. Evidently, the phylogenetic stem of Xenosaurus represents a long independent history of evolution. Fortunately, several fossil taxa exist that can elucidate this history. These taxa include the extinct Exostinus lancensis (Cretaceous), Exostinus serratus (Oligocene), and Restes rugosus (Paleocene), the latter two known from substantial but inadequately described cranial material. Using osteological and alcohol-preserved specimens, fossils, and high-resolution x-ray CT scans thereof, I attempted to reconstruct the relationships of the three fossil taxa and the six extant species of Xenosaurus that are available in U.S. collections.
An exhaustive CT-data-based description of the skull elements of Exostinus serratus yielded several character states suggesting that this taxon is the immediate sister to Xenosaurus, as well as a host of unusual autapomorphic features. Special care was taken during the description to account for soft anatomical correlates of osteology.
Despite the considerable phylogenetic importance of Xenosaurus and its stem, this is the first phylogenetic analysis of the group. An exhaustive search of the skeleton, including osteoderms embedded in the skin visualized using CT scanning, yielded 274 new characters, substantially more than have been used previously in gross anatomy-based analyses of such a restricted group of reptiles. The great number of characters is largely the result of the availability of disarticulated skeletal material; CT scans showing internal bone structure and bones embedded in the skin; and attention to subtle anatomical differences whose validity could be assessed in terms of intraspecies variation because of the availability of large sample sizes for certain taxa.
My results suggested that Restes rugosus is sister to the other xenosaurs, resolving a polytomy with other Anguimorpha recovered by previous work. Exostinus lancensis is problematic in that it may represent several distinct taxa, but it was recovered as sister to E. serratus + Xenosaurus, making Exostinus paraphyletic. Exostinus serratus emerged as sister to Xenosaurus. Xenosaurus comprises a northern clade consisting of X. newmanorum and X. platyceps; the remaining taxa are united as a southern clade. Within the southern clade, X. agrenon and X. rectocollaris are sister to X. grandis and X. rackhami. North-south splits within Xenosauridae mirror those of several other lizard clades and may be the legacy of the equatorial contraction of early Tertiary tropical forests.
Return to listTIDAL SIGNATURES IN SAND PRONE, TECTONICALLY GENERATED JURASSIC STRAITS, SCOTLAND
Stuart Blackwood, MSGeoSci
The University of Texas at Austin , 2006
Supervisor: Ron J. Steel
The Mid-Jurassic Bearreraig Formation is superbly well exposed on the Isles of Skye and Raasay in NW Scotland. In southern Skye the Bearreraig displays a spectacular, cyclical hierarchy of sand rich, tide-generated bedforms:
1. Upward thickening to thinning compound and compound-compound trough cross-sets (3D dunes) up to 2.5m thick
2. Compound trough cross-sets (3D dunes) up to 10m thick
3. Upward thinning, simple and compound planer cross-sets (2D dunes) up to 50cm thick
Tectonically-generated sequences (up to 140m thick) consisting of these three dune types can be recognized within the formation.
Sedimentation occurred in NNE-SSW-aligned, North Skye/Raasay and South Skye half-grabens, defined by Aalenian-Bajocian faulting associated with the initial opening of the North Atlantic . SSW oriented (S. Skye) and NNE oriented (N. Skye/Raasay) fault-parallel paleocurrent directions (indicative of basin separation) suggest that the faults were active and had enough topographic expression to enhance the tidal regime in these very narrow, block-tilted zones.
Previous depositional interpretation of the Bearreraig is one of alternating tide-dominated deltaic and macro-tidal estuarine cycles with high-frequency regressions and transgressions. Facies cropping out at Trotternish in N. Skye, and Raasay display trends consistent with this regional interpretation. However, in southern Skye the overall thickness of the cross-stratified succession, anomalously large size of the bedforms (implying relatively deep water), intensely sand-rich character of the succession, together with the high bioclastic content of broken shell debris is indicative of a more complex, higher-energy depositional environment. The character and scale of the bedforms, and the absence of pro-delta, lagoonal or delta-plain deposits suggests a narrow seaway or tidal strait generated by active tilting of narrow fault blocks.
FIELD EXPERIMENTS FOR FRACTURE CHARACTERIZATION: STUDIES OF SEISMIC ANISOTROPY AND TRACER IMAGING WITH GPR
Nedra Danielle Bonal, Ph.D.
The University of Texas at Austin , 2007
Supervisor: Clark R. Wilson
Knowledge of fracture orientation and density is significant for reservoir and aquifer characterization. In this study, field experiments are designed to estimate fracture parameters in situ from seismic and GPR (radar) data. The seismic experiment estimates parameters of orientation, density, and filling material. The GPR experiment estimates channel flow geometry and aperture.
In the seismic study, lines of 2D data are acquired in a vertically fractured limestone at three different azimuths to look for differences in seismic velocities. A sledgehammer, vertical source and a multicomponent, Vibroseis source are used with multicomponent receivers. Acquisition parameters of frequency, receiver spacing and source-to-receiver offset are varied. The entire suite of seismic body waves and Rayleigh waves is analyzed to characterize the subsurface. Alford rotations are used to determine fracture orientation and demonstrate good results when geophone orientation is taken into account. Results indicate that seismic anisotropy is caused by regional faulting. Aaverage fracture density of less than 5% and water table depth estimates are consistent with field observations. Groundwater flow direction has been observed by others to cross the fault trend and is subparallel to a secondary fracture set. In this study, seismic anisotropy appears unrelated to this secondary fracture set. Vp/Vs and Poisson's ratio values indicate a dolomite lithology. Sledgehammer and Vibroseis data provide consistent results.
In the GPR experiment, reflection profiles are acquired through common-offset profiling perpendicular to the dominant flow direction. High frequency waves are used to delineate fluid flow paths through a subhorizontal fracture and observe tracer channeling. Channeling of flow is expected to control solute transport. Changes in radar signal are quantitatively associated with changes in fracture filling material from an innovative method using correlation coefficients. Mapping these changes throughout the survey area reveals the geometry of the flow path of each injected liquid. The tracer is found to be concentrated in the center of the survey area where fracture apertures are large. This demonstrates that spatial variations in concentration are controlled by fluid channel geometry.
DEVELOPMENT OF A PARALLEL RIVER TRANSPORT ALGORITHM AND APPLICATIONS TO CLIMATE STUDIES
Marcia Lynne Branstetter, PhD
The University of Texas at Austin, 2001
Supervisor: James S. Famiglietti
The global hydrologic cycle plays a central role in the interactive functioning of the Earth’s climate system. The theme of this study is land-oceanatmosphere interaction. Continental runoff has a notable effect on the global hydrologic cycle, both directly as freshwater forcing on the oceans and indirectly through effects on global patterns of precipitation and coincident feedbacks to continental runoff. To demonstrate this, a series of three projects involving both observations and modeling were completed.
The first phase involved the development of a parallel river transport model to deliver runoff from the land surface to the oceans at the appropriate location and time. Within each watershed, the river routing algorithm used cellto- cell routing by considering the mass balance of surface inflows and outflows. This river transport model was then incorporated into a climate system model.
The second and third phases involved the use of a climate system model to investigate the effect of continental runoff. To test the sensitivity of the oceans to freshwater input from runoff, in the second phase, a number of 70-year simulations were conducted, using the ocean and ice components of a highresolution climate model with observed runoff and modeled atmospheric forcing. A half-degree observed runoff data set, consisting of both annual and monthly averages, was used for this forcing. Differences in sea surface temperature and salinity between simulations with and without the addition of runoff were found in the Arctic, Tropical Atlantic, and North Atlantic Oceans. The differences were especially pronounced in the North Atlantic. By affecting sea surface temperatures and salinity, the addition of freshwater from river runoff led to a reduction in North Atlantic Deep Water formation and a corresponding slowdown of heat transport.
The third phase used a fully coupled land-ocean-atmosphere-ice model plus the river transport model from the first phase. Two 200-year simulations were conducted, with and without the river component. The simulation with rivers had reduced oceanic meridional heat transport. Reduced convective rainfall and runoff during January in the simulation with rivers indicated a feedback from the continental runoff flux into the oceans back to the land surface.
DELINEATING CONTRIBUTING AREAS FOR KARST SPRINGS USING NEXRAD DATA AND CROSS-CORRELATION ANALYSIS
Trevor Jones Budge, PhD
The University of Texas at Austin, 2008
Supervisor: John M. Sharp, Jr
189 pages, 88 references, 2 tables
The use of cross-correlation analysis on spring discharge and precipitation data in karst aquifer basins has been used for many years to develop a conceptual understanding of an aquifer and estimate aquifer properties. However, to this point, the application of these processes has relied on gaged precipitation at discrete locations. The use of spatially varying precipitation data and cross-correlation analysis provides a means of spatially characterizing recharge locations on a karst aquifer. NEXRAD provides a spatial estimate of precipitation based by combining reflectivity measurements from radar stations and traditional precipitation gages. This study combines NEXRAD precipitation data with spring discharge data to develop maps of contributing areas for two karst springs in Central Texas. By calculating the cross-correlation of each NEXRAD measurement to spring flow data for the same period of time a map showing the locations hydraulically connected to the spring can be developed. Both numerical experiments and field applications were conducted as part of the study. The numerical experiments conducted by Padilla and Pulido-Bosch are revisited using the numerical groundwater model MODFLOW. This allowed the introduction of spatially varying parameters into the model. The results show that spatially varying parameters can be inferred based on the results cross-correlation of spatially varying precipitation with respect to a single spring discharge location. Also, contributing area maps are prepared for both Barton Springs and Jacob’s Well. Barton Springs has a precise estimate of the recharge area. The current map of the recharge area and the NEXRAD derived map show good agreement with the cross-correlation results. Conversely, Jacob’s Well has not been sufficiently studied to delineate a contributing area map. This study provides an preliminary estimate of the area contributing to flow at Jacob’s Well. Finally, the development of these maps can also be applied to the construction of regional groundwater models. An application of this methodology with the groundwater availability model for the Barton Springs portion of the Edward’s aquifer is introduced. The application of spatial cross-correlation analysis to constrain recharge in the model showed a reduction in the objective function with respect to discharge at Barton Springs of 15%.
ESTIMATING BIOZONE HYDRAULIC CONDUCTIVITY IN WASTEWATER SOIL ABSORPTION SYSTEMS USING INVERSE NUMERICAL MODELING
Johnathan R. Bumgarner, M.S. Geo. Sci.
The University of Texas at Austin, 2005
Supervisors: John E. McCray and Bridget R. Scanlon
Wastewater soil absorption systems (WSAS) are commonly used in the United States to treat domestic wastewater prior. Soil clogging occurs at the infiltrative surface of WSAS due to the accumulation of suspended solids, organic matter, and chemical precipitates during continued wastewater infiltration. This low permeability zone is referred to as a biozone. A certain degree of clogging enhances wastewater treatment due to increased residence times and nutrient use by organisms. Conversely, too much clogging can cause WSAS to backup and ultimately fail. As such, determining a generalized set of biozone hydraulic parameters and whether those parameters vary in different types of WSAS or for different loading rates are integral to proper WSAS design and installation.
Because the system is unsaturated and the biozone-soil interface behaves like a capillary barrier, simply estimating infiltration using Darcy’s equation, which assumes saturation, may not be appropriate. Therefore, inverse modeling was conducted using the HYDRUS-1D multi-phase modeling software. Input data for the model simulations, which include infiltration rates and soil hydraulic parameters, were determined at a test site at Colorado School of Mines (CSM). The test site includes ten each of three different types of WSAS – bundle, chamber, and gravel – for a total of 30 test cells. Half of the test cells received a loading rate of 4 cm/day, while the other half received 8 cm/day for one year. Both of these loading rates are higher than the Jefferson County, CO, recommended loading rate of 2 cm/day. Use of different loading rates is an attempt to simulate systems of different ages because the biozone becomes less permeable as the WSAS ages. Constant head infiltration tests were conducted by researchers at CSM in each of the test cells at the time of site installation to calculate the saturated hydraulic conductivity (K s) of the soil, and falling head infiltration tests were conducted after 12 months to calculate the biozone K s.
HYDRUS-1D was used to establish the retention function parameters and K s for biozones at Mines Park via inverse modeling. Of the parameters estimated by HYDRUS-1D inverse simulations, K s has the greatest impact on the falling head test model results when the system is near saturation (volumetric water content > 90%). According to the simulation results, the biozone is near saturation and, therefore, K s weighted most heavily in the data analysis. Results of the modeling process indicate that there are no significant differences (α=0.10) between the mean K s of cells of different types or loading rates (total mean K s = 2.12 cm/day ± 0.78). On the other hand, there is a significant difference (α=0.10) between the mean K s calculated by HYDRUS-1D (2.12 cm/day ± 0.78) and that calculated by Darcy’s equation (3.02 cm/day ± 0.89), but that difference is physically inconsequential. Ponding did not occur in any of the WSAS simulations with 2 cm/day loading rates. Finally, an acceptable set of biozone hydraulic parameters for each of the test cells is established by the modeling process for use at the CSM outdoor laboratory. These parameters will be used to constrain transport in future investigations at the site including contaminant attenuation and loading rate studies.
MID-PLEISTOCENE TO PRESENT STRATIGRAPHIC RESPONSES IN A TECTONICALLY- DRIVEN DEPOSITIONAL SETTING: EEL RIVER BASIN, NORTHERN CALIFORNIA
Robert Lawrence Burger, Ph.D.
The University of Texas at Austin, 2002
Supervisors: William E. Galloway, Craig S. Fulthorpe, and James A. Austin, Jr.
The Eel River Basin of northern California is an actively deforming forearc basin, affected today by Gorda-North America Plate convergence, northward migration of the Mendocino Triple Junction, glacioeustatic sea-level fluctuations that periodically exposed the continental shelf, and high rates of sediment input. A high-resolution multichannel seismic reflection survey of the shelf and upper slope has been conducted as part of the Office of Naval Research STRATAFORM initiative, with the goal of understanding processes affecting sediment dispersal and preservation in this complex continental margin setting.
Interpretations of the seismic images indicate that, although tectonism strongly overprints preserved sequence morphologies, glacioeustacy is of primary importance in the distribution and preservation of sediments along this margin. On the shelf, stacked sequences dominated by interpreted highstand marine sediment are separated by prominent ravinement surfaces formed during sea-level transgressions. Folding and faulting locally modify sequence morphologies. At the south end of the seismic grid, localized uplift attributed to Mendocino Triple Junction encroachment overrides the glacioeustatic stacking pattern dominant over the rest of the shelf, inducing shelf incision and preferential preservation of vertically stacked channel-fills. An abrupt shift in shelf sequence morphology ~500 ka suggests that the triple junction began to affect the southern basin at that time, by decreasing accommodation space marginwide.
On the adjacent slope, effects of tectonism and glacioeustacy can also be discerned. In the south, the Humboldt Slide is identified as a long-lived masswasting feature, likely triggered by triple junction-related uplift and associated seismicity ~450-500 ka. However, a cyclical succession of contrasting lithologies is also apparent in the deformed slide sequences, which I attribute to glacioeustatic cyclicity. To the north, effects of regional tectonism are less apparent: the slope is dominated by channels and gullies that incise and infill due to presumed glacioeustatically-induced variations in slope sediment supply. However, shifts in lateral channel distributions through time may be an indication of marginal uplift.
Although ground truth is needed to test the broad stratigraphic development models proposed by this study, seismic images of the Eel River Basin confirm the complex interplay of processes influencing sediment deposition and preservation in this active margin setting.
A GENERAL TRANSFORM FOR REVERSIBLE SEISMIC DATA PROCESSING BY NONSTATIONARY FILTERING
William Andrew Burnett, M.S. Geo. Sci.
The University of Texas at Austin , 2007
Supervisor: Robert J. Ferguson
I use the nonstationary equivalent of the Fourier shift theorem to derive a general one-dimensional integral transform for the application and removal of certain seismic data processing steps. This derivation comes from the observation that many seismic data processing steps can be viewed as nonstationary shifts themselves. The nonstationary frequency and time distortions inherent to many seismic data processes are predicted and quantified by this transform. Once the transform is developed, I demonstrate matrix-vector multiplication as a valuable tool for implementation. I use the normal moveout (NMO) correction and Stolt migration as examples. Displaying the integration kernel matrices yields insight to the mechanics of their respective transforms, and also reveals symmetries that can be exploited to improve computational efficiency. Matrix-vector multiplication is an intensely studied field in applied mathematics and computer science, so there are many algorithms available to further increase efficiency. Most of these algorithms offer a trade-off between accuracy and efficiency, and some even offer to quantify the error associated with their approximation. Unlike conventional processing algorithms that require interpolation, data processing steps performed by matrix vector multiplication are reversible within computational accuracy. I show that interpolation schemes are a frequency band-limited approximation to the transform method I develop. Just as the Fourier inversion theorem justifies the Fourier domain as a valid processing domain, the reversible data processing transform quantitatively justifies using processed data as if it is in a new domain.
TEMPORAL AND SPATIAL SULFATE VARIABILITY IN GROUNDWATER AT A LIGNITE MINE, NORTHEAST TEXAS
Margaret Frances Cagle, M.S. Geo. Sci.
The University of Texas at Austin, 2007
Supervisor: John M. Sharp
Sulfate (SO 4 2- ) concentrations in the groundwater at the Luminant Monticello lignite mine, northeast Texas, vary with both time and space. SO 4 2- is monitored closely because it is a product of chemical reactions that can lead to acid mine drainage. Although acid mine drainage is not a problem at Monticello, SO 4 2- concentrations in some areas are high and correspond to high total dissolved solids (TDS), low pH groundwater. At Monticello, chloride, total dissolved iron, and total dissolved manganese concentrations also are variable. To examine this variability, 46 monitoring wells are divided into those: 1) screened in rock underneath the lignite seam (underburden); 2) screened in the rock over the lignite seam (overburden); and 3) screened in the reclaimed spoil. There is a wide range of SO 4 2- concentration across the mine with statistically significant differences between the SO 4 2- distributions for each well category. Three hypotheses may explain the spatial SO 4 2- variability: (1) The heterogeneity of the mined material (2) Flushing of SO 4 2- from the reclaimed section into the overburden (3) Exposing iron sulfides to oxidizing conditions when the water table is dropped by mine dewatering operations. Possible sources of SO 4 2- include the oxidation of pyrite and/or the dissolution of gypsum.
Temporal SO 4 2- variability is evident in the SO 4 2- time series for each well. In the underburden, SO 4 2- seems to be approaching a steady state; however, some wells in the unmined overburden and reclaimed area have SO 4 2- trends that are increasing or are highly variable with time. Water table fluctuations or flushing caused by seasonal meteorological changes may control these trends.
Correlation analyses show that for most underburden, overburden, and reclaim wells, SO 4 2- does not correlate with pH, indicating that acidity does not accompany the incorporation of SO 4 2- and that SO 4 2- may be advected from another part of the mine. Analysis of water level and SO 4 2- time series, as well as Cl - and SO 4 2- time series, show that advection from reclaimed areas cannot account for every high SO 4 2- value or increasing SO 4 2- concentration in the overburden and reclaim. Groundwater in most overburden and reclaimed areas are at equilibrium with gypsum, indicating gypsum precipitation if SO 4 2- is flushed into the area and gypsum dissolution if SO 4 2- is flushed out.
Time series analyses shows some of the wells with increasing SO 4 2- concentration to be approaching equilibrium. Also, most SO 4 2- time series have a seasonal component. Flushing from the reclaimed areas is considered to be the dominant process controlling SO 4 2- variability in the overburden. A combination of flushing and aqueous geochemical processes related to heterogeneity controls the SO 4 2- concentration in the reclaimed land.
DEVELOPMENT OF THE LOS CHIVOS PLATFORM, A PALEOCENE CARBONATE BUILDUP IN A SILICICLASTIC DOMINATED SALT BASIN, LA POPA BASIN, NORTHEAST MEXICO
by
Mathew Allan Campbell, MS Geo Sci
The University of Texas at Austin, 2003
Supervisor:William L. Fisher for R. K. Goldhammer (Deceased)
118 pages, 31 references, 4 plates
The stratigraphic development of carbonate platforms in the Gulf of Mexico region are subject to many controlling factors. While eustasy, regional subsidence, and sediment production are important elements, the effect of local syn-depositional tectonics, related to diapiric movement of mobile evaporite, is also locally vital. Uplift and subsidence caused by salt movement probably were critical factors in the development of the Los Chivos Lentil in the La Popa foreland basin of northeast Mexico.
The Los Chivos Lentil is a 16 sq. km isolated Paleocene carbonate platform that probably formed on a salt-supported anticline. Several other carbonate platforms in the basin formed on salt related topographic highs found within the Upper Mudstone Member (i.e.during maximum flooding conditions) of the Potrerillos Formation in the Difunta Group. The Difunta Group represents roughly 5,000 meters of Maastrichtian to Eocene fluvial-deltaic clastic sediment shed from an area west of the Sierra Madre Oriental. The low clastic influx at the time of deposition for the Upper Mudstone Member allowed for vertical migration of underlying, Jurassic-aged, evaporite. These evaporite diapirs provided the bathymetric relief necessary to initiate carbonate sedimentation.
The Los Chivos Lentil consists of two major carbonate outcrops, the Los Chivos mesa and the Chivos Ridge section. The Chivos Ridge outcrop represents in situ carbonate sedimentation consisting of isolated red algal and coral buildups found on the western margin of the outcrop. The section exposed on the Los Chivos mesa is approximately 80-100 meters thick and primarily consists of thin-bedded, mixed skeletal packstones and grainstones transported from the in situ buildup and deposited on the eastern limb of the Chivos Anticline. A well exposed face of the mesa reveals large, seismic scale, off-lapping clinoform geometries in the allodapic sediment beds. Because of its good preservation and exposure, this platform provides an excellent location to study how carbonate deposition is affected by active salt tectonics. Development of this, and other La Popa Basin buildups, closely resembles modern sedimentation found on salt highs in the southern Persian Gulf and the Gulf of Mexico. This outcrop can also be used as an analog for buildups found in the subsurface.
Integrating Seismic Geomorphology into improved reservoir models of Shelf Margin Deltas, Corallita/Lantana Field, Trinidad
by Terence Campbell, MS. GeoSci
The University of Texas at Austin , 2006
Supervisor : Lesli Wood
This project analyzes the reservoir architecture and compartmentalization of the as yet undeveloped Corallita and Lantana gas fields. Corallita-Lantana, discovered by Amoco Production Company in 1995, is an undeveloped dry gas discovery in the Greater Mahogany region offshore in the eastern shelf of the Columbus Basin , offshore S.E. Trinidad (Wood, 2004). It is currently estimated to contain 1.5 TCF of gas resource in Pleistocene age reservoirs.
Several factors complicate development of the Corallita-Lantana field, including fault seal, trap definition and reservoir compartmentalization. 1120 km 2 of 3D seismic combined with well log analysis have been used to explain the reservoir shape, size and distribution. Seismic coherency, amplitude analysis and spectral decomposition have been integrated to improve understanding of reservoir heterogeneity and compartmentalization, and spatial changes in reservoir petrophysics. Such knowledge will enable engineers to establish the best production design, and implement the most economical artificial drive mechanism for production.
I made statistically robust, geomophometric analysis of the reservoir by assigning binary codes to various facies and incorporating well log character using cumulate analysis software on interactive computer workstations. The reservoir of interest (TQ60) shows varying seismic facies architecture from shelf to slope. The seven seismic facies identified are conformable topset facies, interchannel facies, canyon facies (mass transport deposits), channel levee systems, ponded turbidities, conformable horizontal slope facies and faults which are identified on seismic attributes extractions. Assigning digitized codes to different facies is a fast and effective way to model a reservoir and compare different data sets.
The result of combining geomorphology from seismic attributes to reexamine the stratigraphic framework is a predictive tool that strengthens any model from sediment content, basin plays characterization, risking prospects and well placement models. Development of the field is now being planned and approximately 56% of the reserves are in the Pleistocene age TQ60 sand across both the Lantana and Corallita fault segments. Results from this study do not only apply to the development of the field, but explain how reservoir shape and distribution can be addressed in all shallow marine and fluvial-deltaic environments.
Return to listSTRUCTURAL AND PETROGRAPHIC ANALYSIS OF CAMBRIAN ROCKS AT BEAVERTAIL STATE PARK, NARRAGANSETT BASIN, RHODE ISLAND
Matthew J. Carter, M.S.
The University of Texas at Austin, 2008
Supervisor: Sharon Mosher
85 pages, 26 references, 3 glossy plates
Structural Mapping at 1:1000 scale of the Cambrian rocks within Beavertail State Park, Narragansett Basin, Rhode Island has revealed an early open to isoclinal fold generation (F1) and axial planar foliation (S1) followed by a later, more prominent, E-vergent, N-trending non-coaxial fold generation (F2) and associated foliation (S2). A third map-scale folding phase is inferred from the NNE-trending, broad warping of S2. Quartz veins are folded by and cross cut F2 folds. N-S to NE-SW extension resulted in 1-10 m scale boudins that deform S2. All of these structures are cross cut by NNE to ENE-striking faults and joints. Kinematic indicators show that the dominant motion on these faults was normal with minor sinistral and/or dextral motions. The Beaverhead Shear Zone that juxtaposes the Cambrian rocks with Pennsylvanian rocks of the Narragansett Basin deflects S2 in a dextral sense, consistent with recorded motion elsewhere. Kink bands associated with faulting trend NNE to ENE and display WNW to NNW side up.
Petrographic work reveals S1 is a penetrative foliation defined by alignment of white micas. S2 is a crenulation cleavage of S1 that is enhanced by pressure solution. S2 is the dominant foliation expressed in the field; however there are areas where S1 is locally better developed. Siderite and quartz nodules overgrow S1 and are wrapped by S2. Mica, quartz and iron carbonate pressure shadows on these nodules are parallel to S2.
Fabric development and orientation, style, and sequence of structures found in the lower greenschist Cambrian rocks within and up to 4 km north of Beavertail State Park, RI are dissimilar to those associated with the Taconic and Acadian events, but identical to Alleghanian structures in Pennsylvanian aged rocks of the Narragansett Basin. These similarities indicate that the Cambrian rocks have exclusively recorded the Pennsylvanian-Permian Alleghanian orogeny, and could not have been in place in North America during the Ordovician Taconic and Devonian Acadian orogenies. Timing constraints indicate the collision of the Avalon microcontinent occurred during the late Pennsylvanian, whereas progressive deformation of the Pennsylvanian rocks of the Narragansett Basin and the Cambrian rocks took place during the Permian.
Return to listEVOLVING SUBGLACIAL WATER SYSTEMS IN EAST ANTARCTICA FROM AIRBORNE RADAR SOUNDING
Sasha Peter Carter, PhD
The University of Texas at Austin, 2008
Supervisor: Donald D. Blankenship
241 pages, 162 references, 16 tables
The cold, lightless, and high pressure aquatic environment at the base of the East Antarctic Ice Sheet is of interest to a wide range of disciplines. Stable subglacial lakes and their connecting channels remain perennially liquid three kilometers below some of the coldest places on Earth. The presence of subglacial water impacts flow of the overlying ice and provides clues to the geologic properties of the bedrock below, and may harbor unique life forms which have evolved out of contact with the atmosphere for millions of years. Periodic release of water from this system may impact ocean circulation at the margins of the ice sheet.
This research uses airborne radar sounding, with its unique ability to infer properties within and at the base of the ice sheet over large spatial scales, to locate and characterize this unique environment. Subglacial lakes, the primary storage mechanism for subglacial water, have been located and classified into four categories on the basis of the radar reflection properties from the sub-ice interface: Definite lakes are brighter than their surroundings by at least two decibels (relatively bright), and are both consistently reflective (specular) and have a reflection coefficient greater than –10 decibels (absolutely bright). Dim lakes are relatively bright and specular but not absolutely bright, possibly indicating non-steady dynamics in the overlying ice. Fuzzy lakes are both relatively and absolutely bright, but not specular, and may indicate saturated sediments or high frequency spatially heterogeneous distributions of sediment and liquid water (i.e. a braided steam). Indistinct lakes are absolutely bright and specular but no brighter than their surroundings. Lakes themselves and the different classes of lakes are not arranged randomly throughout Antarctica but are clustered around ice divides, ice stream onsets and prominent bedrock troughs, with each cluster demonstrating a different characteristic lake classification distribution.
In the bedrock trough of Adventure Subglacial Trench, analysis of satellite altimetry is combined with radar sounding data to calculate a mass budget and infer a flow mechanism for a two cubic kilometer discharge reported to have traveled between two lakes in the region from 1996 – 1998. The volume released from the source lake exceeded the volume received by the destination lakes by one and a tenth cubic kilometers, indicating that some water must have escaped downstream from the lowest destination lake over the course of the event. Release of water from the source lake preceded arrival of the water at the destination lakes, 260 kilometers away, by about three months. Water continued draining from the destination lakes for several years after surface subsidence at the source lake had ceased. By 2003, a total of one and a half cubic km or nearly 75% of the water released by the source lake had traveled downstream from the destination lakes.
Hydraulic modeling work indicates that the initial release of water from the source lake could have been accommodated by a self-enlarging semicircular channel. Subsequent evolution of the discharge and the three-month delay between release of water from the source lake and arrival of that water at the destination lakes indicates that a shallower and broader distributed water system is responsible for the transport of subglacial water in this region. Such a system would be more stable for the given ice-bedrock geometry and may explain the observations of intermittent flat bright bedrock reflections in radar data acquired upstream from the destination lake in 2000.
For the purpose of better understanding the long-term water budget of the Dome C region, an area upstream of Adventure Trench, eleven dated isochronal internal layers within the ice penetrating radar data were tracked. An age-depth relationship, derived from the European ice core through Dome C is used to calculate strain, estimate melt, model ice temperature, and determine absolute basal reflectivity for the entire region which covers over 28,000 square kilometers. The two largest subglacial lakes within the survey, Concordia and Vincennes, are both associated with enhanced basal melting on their upstream shores at rates locally greater than two millimeters per year. Widely distributed melt rates in the major topographic valleys upstream of these lakes are generally less than one millimeter per year throughout the region with slightly higher melts in the basin draining into Vincennes Subglacial Lake. Although published estimates for geothermal flux are capable of explaining the behavior of ice and water in most of the area, an additional source of basal heat is required to explain melt anomalies and subglacial lakes along the Concordia Ridge. Lake Concordia is expected to discharge water on a similar scale and duration as that observed in Adventure Trench, with a repeat cycle of a few hundred years.
Return to listSEDIMENT VOLUME PARTITIONING, TOPSET PROCESSES AND CLINOFORM ARCHITECTURE –UNDERSTANDING THE ROLE OF SEDIMENT SUPPLY, SEA LEVEL AND DELTA TYPES IN SHELF MARGIN BUILDING AND DEEPWATER SAND BYPASS: THE LANCE-FOX HILLS-LEWIS SYSTEM IN S. WYOMING
Cristian Rene Carvajal, Ph.D.
The University of Texas at Austin, 2007
Supervisor: Ronald J. Steel
171 pages, 133 references, 3 tables
This research focuses on how sediment supply, sea level and delta processes control the partitioning of the sediment budget across and into the topset, slope and basin-floor compartments of deepwater basins. Addressing this problem provides significant insight to characterize source-to-sink systems, improve tectono-stratigraphic models and predict sand bypass to deepwater areas. The research was carried out in the Lance-Fox Hills-Lewis shelf margin formed during the Maastrichtian in the Washakie-Great Divide basin of southern Wyoming. I use a database with approximately 520 wells integrated with outcrops to develop a high resolution, dynamic stratigraphy approach for shelf-margin characterization.
The results emphasize the driving role of sediment supply in rapid shelf-margin building and deepwater sand emplacement. On the study margin, high sediment supply was able to outpace shelf accommodation even at times of relatively high and rising sea level. At these times, shelf margin clinoforms developed a more aggradational architecture with relatively thick and more marine influenced topsets formed in response to basin deepening due to rapid subsidence. The high supply and subsidence are interpreted to have resulted from crustal loading and significant erosion during prominent Laramide thrust-driven source uplift. The high supply caused the formation of highstand shelf-edge deltas with strong wave and river influences. These deltas resulted in extensive coastal sand belts at the shelf margin, and bypass of significant volumes of sand to deepwater areas. In contrast, during times of stable to very low rates of sea level rise, the basin developed more progradational clinoforms with more terrestrial and generally thinner topsets. More of the sediment was funneled to the basin floor and shelf-edge deltas were under strong river and tidal influence. Stable or even falling sea level resulted from decreased subsidence or slight basin uplift, interpreted to have resulted from decreasing uplift, tectonic quiescence or possibly slight tectonic rebound in the basin. The Lewis-Fox Hills margin is considered supply-dominated, a term to denote moderately deep shelf margins (< 1000 m) that prograde at high rates (several tens of km/my) and deliver sand to deepwater areas recurrently and in large volumes even at sea level highstand.
Return to listSTRUCTURAL ANALYSIS OF CENOZOIC FAULT SYSTEMS USING 3D SEISMIC DATA IN THE SOUTHERN MARACAIBO BASIN, VENEZUELA
Maria Veronica Castillo, PhD
The University of Texas at Austin, 2001
Supervisor: William L. Fisher
The Icotea and VLE 1 faults are two of the major faults in the Maracaibo Basin, Venezuela, and are known from seismic and well data to have been most active during Paleogene time. Both faults have linear traces over distances up to 100 km, are deeply buried under largely unfaulted Neogene sedimentary rocks, and are associated with localized continental growth strata of Paleogene age along their traces. The origin and displacement history of both faults has remained controversial partly because of along-strike complexities in fault structure, use of highly exaggerated 2D seismic lines, and the lack of synoptic views of both fault systems. Previous interpretations range from east dipping basement-involved thrust faults to sub-vertical left-lateral strike-slip faults controlling pull-apart basins. I use regional 2D seismic data crossing both faults, and 3D seismic data covering a 1600 km 2 area of the southern Maracaibo Basin to describe structures along the traces of both faults and fault termination structures at their southern ends. These seismic reflection data show that both faults are inverted normal faults that first formed during the late Jurassic-early Cretaceous. The abrupt termination of both faults in the southern part of the basin probably corresponds to the southern ends of two parallel rift structures. This study also uses 3D seismic reflection data to describe a karst horizon in the Aptian-Albian carbonate rocks that may have formed during a worldwide eustatic drop in Albian sea level that also produced the well-known mid-Cretaceous unconformity recognized in the Gulf of Mexico, France, and Middle East. The karst interpretation may allow a better understanding of reservoir characteristics at this level in the carbonate platform, which are generally attributed to fracturing rather than subaerial weathering. The presence of a regionally extensive karst surface at depth beneath other basins along the northern margin of South America may prove to be a useful oil exploration play concept.
1 VLE fault: named by the Petróleos de Venezuela filial MARAVEN S.A., to describe a NS striking fault in Block V located in the central area of the Lake Maracaibo.
THREE-DIMENSIONAL STRATAL DEVELOPMENT OF A CARBONATE-SILICICLASTIC SEDIMENTARY REGIME, NORTHERN CARNARVON BASIN, NORTHWEST AUSTRALIA
Donna Louise Cathro, Ph.D.
The University of Texas at Austin, 2002
Supervisors: William L. Fisher and James A. Austin, Jr.
Detailed stratigraphic interpretation of continental margin clinoforms is a necessary first step in understanding the link between this complex stratal architecture and large-scale processes resulting in progradation. Maps derived from a 3D seismic volume (40-55 Hz) nested within a regional 2D multichannel seismic grid (25-35 Hz), and tied to nine hydrocarbon exploration wells, show the detailed morphological evolution of five prograding clinoformal sequences in the Northern Carnarvon Basin (NCB), northwest Australia.
Depocenters concentrated along northeast-southwest oriented, linear clinoform fronts are governed by latitudinal variations in sediment productivity. Fronts change from smooth to highly dissected, with intense gullying apparent after the mid Miocene optimum. Bottomsets remain relatively sediment-starved without the development of aprons on the lower slope and basin. Small-scale variability suggests heterogeneous sediment dispersal through these slope conduits. Along-strike sediment transport superimposed on northwest-oriented progradation changes from south-directed in the late Oligocene to north-directed in the late mid-Miocene suggesting a reorganization of circulation in the southeastern Indian Ocean.
Prominent seismic discontinuity surfaces represent intervals of shallow paleo-water depth and flooding of the shelf. Exposure surfaces are subordinate. Rather than build to sea-level, progradation occurs with shelf paleo-water depths of 20-200 m. Therefore, onlap onto the clinoform front is not coastal and the sensitivity of the clinoforms to sea-level changes is muted. However, in the mid-Miocene, partial exposure of the shelf and development of karst topography indicate paleo-water depth falls of 60-180 m across two sub-sequence boundaries.
The sequence stratigraphic framework is combined with a two-dimensional, forward kinematic and flexural model for deformation of the lithosphere to determine the distribution, magnitude and history of Cretaceous-Tertiary compression-induced inversion across the Dampier Sub-basin in the NCB. Inversion simultaneously creates and destroys accommodation space at different wavelengths superimposed on long wavelength subsidence and eustatic variations that impact the entire margin. Inversion anticlines focused along earlier rift fault systems are small, temporally and spatially variable relative to the total accommodation space created in the sub-basins during rifting and thermal subsidence. Santonian inversion represents ~4 km shortening, whereas the four modeled events in the Miocene each represent ~200-400 m of shortening across the modeled section.
TIME-LAPSE (4-D), VP/VS INTERPRETATION OF RESERVOIR DEPLETION IN TEAL SOUTH FIELD, GULF OF MEXICO
Fernando Antonio Cerda, M. S. Geo Sci.
The University of Texas at Austin, 2001
Supervisor: William L. Fisher
Multicomponent seismic data over the producing reservoirs in Teal South
field, located 160 miles southwest of New Orleans, were analyzed to detect
changes and describe reservoir changes over time. The primary seismic objectives
of the analysis were to monitor: hydrocarbon movement; distinguish sand
fill from shale within the 4500 ft sand using the ratio of seismic wave
velocities (Vp/Vs); map actual reservoir boundaries; and compare Vp/Vs ratios
from different time-lapse observations. Conventional P-wave seismic velocity
is not a good lithologic indicator because of the overlap in P-wave velocities
ranges for various rock types. The additional information provided by the
converted-wave data reduces the ambiguity in lithological and hydrocarbon
interpretation.
In July 1997, an Ocean Bottom Cable (OBC) survey was performed at Teal
South using a sparse, fixed-position receiver grid (Phase I). In order to
achieve adequate fold, a dense grid of shots was acquired over an area extending
1000 m in all directions around the areal receiver grid. The dense source
grid covered an area of about 9 km2. The receiver spread was stationary for the entire source grid.
In April 1999 (Phase II), a repeat survey was recorded for time-lapse comparison
to the base survey.
After processing, the data quality observed in the prestack and migrated
data was good, although significant migration artifacts were visible at
the edges of the survey, especially on the mode converted-wave data. An
acquisition footprint effect was also observed shallow in the section, related
to the sparse receiver spacing and possibly affected by the shooting direction.
The interpretation of the P-wave data focused on identifying high reflection
contrast (4500 ft sand), and analyzing reflection amplitude differences
related to fluid properties of the reservoir between initial and repeat
surveys. This latest aspect reveals characteristics of the dynamic response
of the bulk rock/fluid properties to the changing fluid conditions. The
high production rate in the reservoir resulted in the expansion of the gas
cap, due to gas exsolving from the oil. As a result, a pressure loss may
have prevented production of a significant amount of remaining oil.
Interpretation of the mode-converted (P-SV) wave volume was guided by
the interpretation of the P-wave volume and proceeded by picking and mapping
a number of reflection events in both the orthogonal-component volumes (components
C1 and C2) for phases I and II, respectively. The strongest and most continuous
reflections were visually correlated from the P-wave data set to the mode-converted
wave data set to reduce ambiguity of misinterpreting weaker or minor events.
Experiments with different horizons were done to calculate the Vp/Vs ratio.
Large intervals between horizons were avoided so that the measurements
were not influenced by properties outside the interval of interest.
The interpretation of the velocity ratio Vp/Vs for phase I, shows lower
values of Vp/Vs that define the shape of the reservoirs and show a separation
between different producing reservoirs in Teal South. In phase II (21 months
later), it is possible to observe a expansion of low values of Vp/Vs all
around neighboring sands (Sand A). This spatial expansion of low Vp/Vs ratio
coincides with the gas cap expansion observed in the P-wave volume in phase
II. In general, correlation of P-wave and S-waves was complicated by the
lack of a multicomponent Vertical Seismic Profile (VSP) and well log information
that can define which P-wave reflections coincide with selected S-wave reflections.
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DISSOLVED NOBLE GASES IN GROUNDWATER
Bradley Donald Cey, Ph.D
The University of Texas at Austin, 2008
Supervisor: Bridget R. Scanlon
179 pages, 114 references, 9 tables
Atmospheric noble gases (He, Ne, Ar, Kr, and Xe) dissolved in groundwater are a valuable tool in hydrology. Numerous studies have relied on groundwater recharge temperatures calculated from dissolved noble gas data (noble gas temperatures, NGT) to infer paleoclimate conditions. This research investigated gas dissolution during groundwater recharge and critically examined the use of dissolved noble gas data in groundwater research. A detailed investigation of an agriculturally impacted shallow aquifer allowed comparison of measured water table temperatures (WTT) with calculated NGT. Results suggest that NGT calculated from widely used noble gas interpretive models do reflect measured WTT, supporting the use of dissolved noble gases to deduce recharge temperatures. Samples having dissolved gas concentrations below the equilibrium concentration with respect to atmospheric pressure were attributed to denitrification induced gas stripping in the saturated zone. Modeling indicated that minor degassing (<10% Δ Ne) may cause underestimation of groundwater recharge temperature by up to 2 °C. In another study a large dissolved noble gas data set (905 samples) from California was analyzed. Noble gas modeling using the same interpretive models indicates that multiple models may fit measured data within measurement uncertainty, suggesting that goodness-of-fit is not a robust indicator of model appropriateness. A unique aspect of this study was the high Ne and excess air concentrations associated with surficial artificial recharge facilities. A final study examined whether climatic/hydrologic changes occurring over glacial-interglacial time periods could impact the accuracy of NGT used in paleoclimate studies. Numerical modeling experiments estimated WTT sensitivity to changes in: 1) precipitation amount, 2) water table depth, and 3) air temperature. Precipitation and water table depth had a minor impact on WTT (~0.2 °C). In contrast, the impact of air temperature changes on WTT was more pronounced. Results suggest that air temperatures inferred from NGT data may underestimate actual air temperature change since the last glacial maximum by ~1 °C at sites having seasonal snowcover. These results suggest despite uncertainty in the exact physical processes controlling gas dissolution during groundwater recharge, NGT do reflect WTT. However, inferring paleo-air temperatures from NGT are subject to error, especially locations with seasonal snowcover.
Return to listEVOLUTION OF THE TOBAGO FOREARC BASIN: IMPLICATIONS FOR SEDIMENTATION AND HYDROCARBON PRODUCTIVITY
Nysha Alana Niela Chaderton, M.S.
The University of Texas at Austin, 2005
Supervisor: Lesli Wood
The relationship of the Lesser Antilles Island Arc, the Tobago Forearc Basin and the Barbados Accretionary Prism shows classic convergent margin/subduction zone geometry. The island of Barbados is the only emergent part of the accretionary prism, with 80% of the land area being covered by Pleistocene limestone which has been eroded in the NE to expose the older rocks of the accretionary prism. A 450 km 2 2D seismic data volume allows for the extension of these stratigraphic units offshore and allows for the definition of a structural framework for the region. The relationship between the unit identified onshore as the Early Eocene to Middle Miocene Oceanic Formation and the basal unit, the intensively folded and faulted Eocene Scotland Formation, has long been cause for debate. It has been previously proposed that the Oceanic Formation, which consists of pelagic clays with some ash beds, is allocthonous and has been thrusted into its present position, overlying the accreted sediments of the Scotland Formation. However the seismic data shows no evidence of the nappes which are the basis for this theory. The opposing theory is that the Oceanic Formation is related to the 12 km of sediment which fills the Tobago Forearc Basin and has been deposited in situ. This is supported by the relationships between the seismic facies which shows the onlap of the Oceanic Formation onto the older accreted sediments.
Return to listTHE RELATIONSHIP OF MAMMALIAN MORPHOMETRIC DIVERSITY TO ENVIRONMENTAL VARIATIONS AND ITS USE IN PALEOCLIMATIC RECONSTRUCTIONS
John Lee Chaille, PhD
The University of Texas at Austin, 2001
Supervisors: Ernest L. Lundelius, Jr. and Timothy B. Rowe
The morphometric variation in the skulls of six species of mammals from North America (Canis lupus, Castor canadensis, Lynx rufus, Mephitis mephitis, Odocoileus virginianus, Procyon lotor) with broad geographic ranges is statistically analyzed. The morphometric characters are used along with climatic temperature and rainfall variables to determine the degree of relationship between these two sets of variables. The number of morphometric variables varies from twelve to seventeen different measurements, depending on the species; whereas the climatic data set consists of four temperature-related and four rainfall-related variables. The morphometric variables for all of the species, except one, have a higher percentage of significant correlations with the temperature-related variables than with those for rainfall. The exception, Procyon lotor, has a slightly higher percentage of rainfall related correlations that are significant. The t test, Hotelling T2 test, and discriminant analysis indicate varying degrees of sexual dimorphism in the morphometric characters, from relatively weak in Castor canadensis to very strong in Canis lupus and Odocoileus virginianus. This prompts the analyses of the male and female specimens both separately and together in many of the statistical tests. The robustness of making climatic variable predictions using multiple regression varies between the species. Mephitis mephitis and Odocoileus virginianus have the best overall prediction results, although each species has some accurate along with some not-so-accurate prediction results. A higher percentage of accurate predictions occurs in the analyses related to temperature. The understanding of the relationship of morphometric variation in the skulls of Recent widespread mammalian species, and the relationship of that variation to the climatic variation over the species ranges can be useful in paleoclimatic reconstructions.
Return to listAN INTEGRATED GEOLOGIC MODEL OF VALHALL OIL FIELD FOR NUMERICAL SIMULATION OF FLUID FLOW AND SEISMIC RESPONSE
Samarjit Chakraborty, MS
The University of Texas at Austin, 2007
Supervisor: Dr. Robert Ferguson
Time-lapse seismic monitoring promises to be a valuable tool for reservoir engineering as it provides dynamic data over the entire ?eld rather than the spatially limited production data. In this thesis, I develop a link between computerized reservoir simulation, rock physics, and seismic analysis. I present an example study of time-lapse seismic e ? ects in a sequence of reservoir simulation, rock physics, and seismic forward modeling. The thesis includes a case-study of the Valhall ?eld which I propose be used for an integrated geologic model for ?uid ?ow and seismic simulation.
I combine ?uid ?ow simulation studies with a parallel ?ow simulation code IPARS to obtain computed pore pressure and oil saturation at di ? erent spatial location as a function of time. The reservoir model for ?uid ?ow simulation input is linear and isotropic. The reservoir model has an injection well below the oil-water contact and a producer well at a shallower level. The variations of pore pressure due to injection and production cause 3-D multi-phase ?uid ?ow in the reservoir with time.
I develop a rock physics mapping code to estimate the P-wave and S-wave seismic velocities and densities for seismic forward modeling from pore pressure and water and oil saturation obtained by ?uid ?ow simulation. The rock physics code uses Gassmann's relations for ?uid substitution to compute the seismic re?ection parameters.
Migrated depth sections show brightening of amplitude values near the producer well as a function of time. Re?ections from the production zone appear stronger indicating high oil saturation values with increasing production.
I develop a case-study of the Valhall Field to make an integrated geologic model for ?uid ?ow and seismic simulation. Based on an initial description of reservoir geology, I combine rock-physics measurements, ?uid properties, geomechanics, seismic, well, and checkshot data, to build an integrated model for simulations of subsurface ?uid-?ow and surface seismic data.
STRUCTURAL GEOLOGY OF THE PACHECO PASS AREA, DIABLO RANGE, CALIFORNIA: EXHUMATION OF A COHERENT BLUESCHIST TERRANE IN THE FRANCISCAN SUBDUCTION COMPLEX
Kirk Chatawanich, M.S. Geo. Sci.
The University of Texas at Austin, 2001
Supervisor: Mark Cloos
The world's largest terrane of coherently bedded blueschist-facies metamorphic rocks containing jadeitic pyroxene plus quartz is exposed in the Franciscan Complex core of the Diablo Range of Central California. Petrologic studies by W .G. Ernst and others have shown that peak metamorphic temperatures were in the range 150-250ºC and pressures were 7-8 kbar. Plate convergence explains how these sediments were subducted to depths of 25-30 km, but the exhumation of these rocks and juxtaposition against upturned strata of the Great Valley Group is problematical.
In the Diablo Range, Franciscan rocks deposited during the Late Jurassic Cretaceous are found in contact with Great Valley Group sedimentary rocks of roughly the same age across the high-angle Ortigalita Fault. North of San Francisco, the Great Valley strata are separated from the Franciscan by the sheared ultramafic and mafic igneous rocks of the Jurassic Coast Range Ophiolite. Isotopic age data indicate peak temperatures were attained around 100 Ma, and fission track thermochronology indicates a major period of cooling occurred at 100-70 Ma. Franciscan detritis began to appear in the geologic record in the Eocene at about 50 Ma. Volcanic rocks dated it 9-11 Ma are deposited on Franciscan rocks in the Diablo Range, indicating uplift and exhumation to surface levels had occurred by that time.
Near Pacheco Pass, California, Franciscan strata are exposed along a 4 km transect cut by California Highway 152 and a 3.1 km transect cut by the old road through the pass. Structural analysis along 4.3 km of total roadcut exposures was performed in order to determine average orientations of structures along the traverse. The Franciscan strata consist mainly of interlayered greywacke and shale with subsidiary beds of chert and rare conglomerate lenses. Paleotop indicators were found at only nine localities, and all but one indicate the nearby section is upright. In many roadcuts, individual beds can be traced for tens of meters. There is no evidence of a greenschist facies overprint, and the presence of aragonite veins indicates that exhumation to depths less than 10 km occurred under cool, syn-subduction geothermal conditions.
Franciscan strata exposed along the transacts contain numerous angular folds and quartz veins. Planar faults with thin cataclastic zones are common. The attitudes of 1280 bedding surfaces, 2297 metamorphic foliations, 1073 fold hinge lines, 4036 veins, and 109 faults were measured along the roadcuts. Metamorphic foliations are bedding-parallel and folded. Upright chevron folds with moderately to steeply dipping limbs are common in the central part of the field area along Highway 152 and near the Ortigalita Fault. Some veins are bedding/foliation parallel, but most are perpendicular to foliations and indicate NE-SW extension direction, roughly perpendicular to the Ortigalita Fault. Most fold axes have an average NE-SW trend. The bedding-parallel cleavage indicates bedding-perpendicular shortening, whereas many veins and normal faults indicate roughly NE-SW extension.
The structural geology of the jadeitic rocks near Pacheco Pass can be explained with a history in which strata deposited on the Farallon plate during the Mesozoic were subducted to depths of 20-30 km. Stratal coherency at the scale of tens of meters was largely maintained during subduction, deep accretion by underplating, uplift and exhumation. The folding of bedding-parallel cleavage, most veining, and faulting post-date peak temperatures. The metamorphic foliation., veins, normal faults, and folds are most simply explained as forming during a near-vertical syn-subduction exhumation driven by deeper-seated underplating. Over tens of m.y., this process drove deeply accreted sediments through the crystalline base of the overriding North American plate to form the Diablo Range anticlinorium in the forearc basin setting.
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SEISMICITY AND TECTONICS OF THE ARC SEGMENTATION IN THE SOLOMON ISLAND ARC, SW PACIFIC OCEAN AND THEIR RELATION TO SUBDUCTING BATHYMETRIC FEATURES
Ming-Chu Chen, M.S.Geo.Sci.
The University of Texas at Austin , 2007
Co-supervisors: Cliff Frohlich and Fred W. Taylor
For the Solomon Island arc we evaluate arc segmentation as defined by seismological and geological evidence. Seismological evidence that defines arc segment boundaries includes seismicity patterns, aftershock areas for large earthquakes, focal mechanisms, and variations in seismic moment release; the geologic evidence is mostly information about Holocene vertical movements, drowned coastlines and especially uplifted coral reef terraces, sometimes augmented by dated coral samples. We identify three major tectonic regimes or supersegments that correspond respectively to the areas surrounding Bougainville, New Georgia, and Guadalcanal-San Cristobal Islands . We speculate that subduction of the young Woodlark spreading center is what distinguishes the central New Georgia supersegment from the two neighboring supersegments. The New Georgia supersegment has relatively higher uplift rates, smaller arc segments, and more small islands; prior to the 1 April 2007 M B w B 8.1 earthquake it had generally lower rates of seismic activity than the neighboring supersegments. Altogether we identify twenty two segment boundaries, including eleven major segment boundaries, four minor boundaries, and seven possible boundaries in the Solomon Islands . Generally the mean along-arc lateral extent of Solomons arc segments are smaller than segments reported in other island arcs; this may be real but it may occur simply because the high seismicity rates and coral-friendly tropical environment provide an unusually favorable situation for observing variations in vertical tectonic activity and thus segment boundaries.
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MICROFACIES AND DEPOSITIONAL ENVIRONMENTS OF SELECTED PENNSYLVANIAN CALCAREOUS ALGAL DEPOSITS FROM SOUTHERN U.S.A., AND APPLICATION OF INFORMATION TECHNOLOGY FOR SEDIMENTARY PETROLOGY TEACHING AND RESEARCH
by
Suk-Joo Stephen Choh, Ph.D.
The University of Texas at Austin, 2004
Supervisor: William L. Fisher
168 pages, 209 references
Pennsylvanian calcareous algal bioherms dominated by Donezella, Beresellid, and Cuneiphycus could potentially form prolific hydrocarbon reservoirs. Understanding the biological makeup and depositional settings of these carbonate buildups is imperative. To provide an understanding of the geological and biological processes controlling the formation of these buildups, three localities in the southern United States were studied by integrating field sampling, microfacies analysis, and microstratigraphic analysis.
A Lower Pennsylvanian Donezella- and siliceous sponge-dominated carbonate buildup from the Wapanucka Formation, Oklahoma is composed of irregular pockets of boundstone dominated by Donezella, worm tubes, microbial peloids and micrite, siliceous sponge spicule, and peloidal wackestone with other bioclasts. The most significant finding of this study is the recognition of delicate a Donezella framework or colony of thalli enclosing and forming constructional pores.
Upper Pennsylvanian Beresellid algal colonies from the Holder Formation, New Mexico, are the product of an in situ growth fabric and constituents of Beresellid algal colonies. The Beresellid boundstone fabric was created by bundles of tangled algal thalli growing at an angle or parallel with the bedding surface, in association with phylloid algae, foraminifera, Tubiphytes, and rare brachiopods and crinoid fragments.
The Lower Pennsylvanian Cuneiphycus-dominated buildups from the Marble Falls Formation, Texas are composed of irregular pockets of boundstone dominated by Cuneiphycus, Donezella boundstone, and peloidal wackestone with other bioclasts. The first account of the red alga Cuneiphycus building a delicate framework of thalli and building a small-scale framework is documented.
In order to effectively handle large numbers of digital images of thin sections used in this study, a new method for creating and authoring virtual carbonate thin sections in PDF format was devised that it is a highly flexible, and platform independent image digitization and content creation method. The significance of this approach is that an image of the thin section is preserved/created and as well as preserves a variety of valuable information about a specimen that can be directly linked to the image of the specimen itself.
Another method of building a digital petrography image archive was devised to supplement the labor-intensive nature of teaching optical petrography. The architecture, process, and lessons learned from developing an interactive multimedia CD-ROM tutorial in sandstone petrology for undergraduates is documented. This approach provides possible models for similar development in other fields of petrography or in any other field that utilizes large quantities of visual material, such as seismic or remote-sensing image interpretation.
Return to listSILICATE SURFACE CHEMISTRY AND DISSOLUTION
KINETICS IN DILUTE AQUEOUS SYSTEMS
Wan-Joo Choi, Ph.D.
The University of Texas at Austin, 2004
Supervisor: Philip C. Bennett
268 pages, 116 references
Organic acids from the biosphere are important reactants in a number of weathering environments. Organic acids accelerate silicate dissolution, increase silicate solubility, mobilize aluminum and silica, and alter the equilibrium between the solution and precipitated secondary phases. The chemical dynamics of the weathering environment was examined by investigating the interaction between mineral surfaces and organic/inorganic electrolyte solutions. Organic acids, analogs of microbially generated siderophores, were examined for their effects on aluminosilicate dissolution kinetics at multiple temperatures in various electrolyte solutions.
Mineral surface titrations were performed for six mineral samples: quartz, gibbsite, feldspars, microcline, andalusite, and kyanite. Mineral powder/distilled water mixture samples were titrated by 0.1 N HCl in the basic pH region, and by 0.1 N NaOH in the acidic pH region. UV-difference spectroscopic analysis was performed on dissolved silica-organic acid mixtures to characterize solution complexes. Mineral dissolution experiments were performed using temperature controlled, continuous-flow mixed reactors. For inorganic dissolution experiment, the solution ionic strength was controlled using LiCl while solution pH was adjusted using either dilute HCl or LiOH solutions. Reagent grade citric acid, tropolone, and 3,4-dihydroxybenzoic acid were used for organic ligand dissolution experiments. A constant flow rate was maintained by using a peristaltic proportioning pump. The mineral surface titration results revealed important surface properties that can be critical to interpreting dissolution kinetics in natural environments. One of the most important results would be that the amount of active surface sites can vary in different solution pH conditions which have been normally assumed to be fixed numbers based on surface area measurements. The UV-difference spectroscopy result shows that some siderophores form stable solution complexes with silica as well as Al and Fe. The results imply that dissolution of aluminosilicate minerals can be significantly enhanced in natural environments by bacterial siderophores, as suggested by previous researchers.
Dissolution results in inorganic electrolyte solutions showed that the net effect of solution ionic strength on the aluminosilicate dissolution reactions is a decrease in the overall dissolution rates, opposite to the effect of ionic strength on quartz dissolution. When the solution ionic species interact with feldspar surfaces, the mechanism of lowering the dissolution rates may be by inhibiting the ion exchange reaction. However, when the solution ionic species do not interact with the silicate surfaces or no ion-exchangeable species are available on the mineral surface, the mechanism of lowering the dissolution rate may be attributed to the effect of activity changes in the neutral species in the solution.
Microcline dissolution increased in organic ligand solutions relative to inorganic electrolyte solutions while andalusite and kyanite dissolution rates decreased in organic ligand solutions. The increased dissolution rate of microcline suggests that feldspar dissolution may be a S N2 mechanism. The decreased or unaffected dissolution rates from kyanite and andalusite suggest that the mechanism for these minerals may be a S N1 mechanism. The effect of organic ligands on dissolution rate was greater in pH 5 solutions than in pH 3. This result suggests that the dominant reaction mechanism in the pH 3 region is proton-promoted, while it is ligand-promoted in the pH 5 region.
Lower activation energies in organic ligand solutions suggest that: (1) the metal-organic complex is more stable at lower temperatures; or (2) the dominant reaction mechanism at a high temperature region may be proton-promoted and is ligand-promoted at a lower temperature region.
CHARACTERIZATION OF BLUE GEM TOPAZ: A COMPARATIVE ANALYSIS OF TEXAS AND BRAZILIAN TOPAZ
Ana Collins, B.S.
The University of Texas at Austin, 2008
Supervisor: Mark Helper
The physical, optical and chemical characteristics of gem-quality Texas blue and colorless topaz were investigated with the goal of establishing sound criteria for differentiating Texas topaz from gem topaz from other pegmatite localities. Although higher prices are often associated with gem-quality Texas blue topaz, there is presently no qualitative or quantitative basis for this price difference. In addition, there is no way to independently verify a source of origin. It was determined that there are chemical and physical differences among gem topaz from the different localities, but these differences are not unique to each sample suite. The heterogeneity observed within the Texas and Brazilian samples themselves thus complicated the effort to find distinctive characteristics between the topaz from these two localities. Ultraviolet through infrared absorption spectra, X-ray diffraction-derived unit cell parameters, cathodoluminescence characteristics, and major- and trace-element concentrations were applied to arrive at a suite of identifying characteristics. Trace-element concentrations were expected to be the most important discriminator. Although trace-element concentrations are typically characteristic of the pegmatitic origins of the samples, no significant variation between sample suites was observed. Differences within and between sample suites likely represent differences in depth of emplacement, the amount of water present during crystallization, and/or trace-element concentrations within the pegmatitic melt.
Return to listDEPOSITIONAL AND STRUCTURAL EVOLUTION OF THE MIDDLE MIOCENE DEPOSITIONAL EPISODE, EAST-CENTRAL GULF OF MEXICO
by
Ricardo Ignacio Combellas Bigott, Ph.D.
The University of Texas at Austin, 2003
Supervisor: William E. Galloway
291 pages, 137 references, 38 plates
Two widespread, transgressive deposits associated with the faunal top Amphistegina B (15.5 Ma) and Textularia W (12.1 Ma) define the Middle Miocene depositional episode. An extensive stratigraphic correlation framework established in this study allowed tracing of the middle Miocene sediment dispersal system from the shelf through the slope to the basin floor in the complex paleogeography of the east-central Gulf of Mexico. The Middle Miocene depositional episode is recorded in four genetic cycles (~1 to 2 Ma), bounded by regional maximum flooding surfaces in the shelf and shelf margin setting, and three equivalent seismic sequences punctuated by condensed sections in the slope and basin floor.
Two principal, long-lived extrabasinal fluvial/delta axes, the ancestral Mississippi and the Tennessee systems, provided the bulk of sediments that infilled the middle Miocene depocenters. Salt-related structural provinces controlled the configuration of the depocenters. Structural linked systems, dominated by gravity spreading, and a minibasin province, driven by differential subsidence, were established during the Middle Miocene depositional episode. Sediment supply coupled with wave energy flux, high-frequency sea-level changes, and salt tectonism determined the time and space distribution of progradation, aggradation, and retrogradation of system tracts.
Middle Miocene shelf margins have prograded 20 to 40 miles from the relict lower Miocene shelf margin. Two depositional systems tracts characterize the constructional shelf margin: (1) a mixed-load fluvial-dominated platform delta/ shelf-margin delta/delta-fed apron systems tract; and (2) a strandplain/shelf/muddy slope systems tract. However, the constructional, offlapping shelf margin systems were locally punctuated by a large-scale phase of retreat and erosion, named the Harang collapse system, in which a large volume of sediments bypassed the shelf margin to be deposited on the slope and basin floor. The Harang collapse system is a type of large-scale slope failure produced by massive salt-withdrawal, retreat of major delta systems, and high-frequency sea-level fluctuations.
A large volume of sediment bypassed the confined minibasin province and the unconfined Florida slope at the flank of active deltaic depocenters, forming the long-lived MCAVLU submarine fan system (named for its location beneath the Mississippi Canyon, Atwater Valley, and Lund continental shelf (OCS areas) in the linked, primary minibasin corridor of the lower slope and basin floor. The MCAVLU submarine fan system evolved from a structurally-controlled, elongate sand-rich to mixed sand/mud fan to a large radial, mixed sand/mud fan. Significant untapped middle Miocene hydrocarbon resources remain in the confined channel fills and lobes of the Harang collapse system and sand-rich ponded facies assemblages of the MCAVLU submarine fan system.
Return to listMODAL ANALYSIS OF SAND FROM THE NANKAI ACCRETIONARY PRISM
Emily Comer, B.S.
The University of Texas at Austin, 2008
During the Cenozoic, complex plate interactions between the Philippine, Eurasian, and Pacific plates produced the Nankai Accretionary Prism off the southwest coast of Japan. As the Philippine plate subducts beneath the Eurasian plate, sediment from the volcanically active Izu collision zone northeast of the Nankai Trough travels down the trench and accretes tectonically into the prism. More quartz-rich forearc sediment from the Kii peninsula is also sedimented upon this prism by transportation down numerous submarine canyons.
Though the prism is deformed by numerous thrust faults, provenance identification is possible as documented in previous studies by Marsaglia (1992) and Fergusson (2005). In this study, 35 unconsolidated Cenozoic sand samples were thin-sectioned, point-counted, and analyzed. These samples were obtained in four sites (C0004, C0008, C0006, and C0007) by the Chikyu drilling vessel during NanTroSEIZE Expedition 316. QFL averages for Pleistocene samples is as follows; C0006 Q23F34L38; C0007 Q25F29L44; C0008 Q22F32L43; C0004 Q15F33L37. Only five Pliocene samples were collected in this study and all are more lithic-rich having an average composition of Q20F27L53. All 35 samples are classified as lithic arkoses or feldspathic litharenites within the Folk classification (Folk, 1982). QmKP and LmLvLs ternary diagrams show that plagioclase strongly dominates the feldspar assemblage and that volcanic and sedimentary lithics dominate over metamorphic lithics. Grain size between the four sites ranges from 3.75Ø (lower very fine) to .75Ø (lower coarse) with the mean being 2.75Ø (lower fine). Ternary plots dividing samples into 3 categories, 0.5-<2Ø, 2-<3Ø, and 3-4Ø show trends of increasing feldspar, quartz, sedimentary lithics abundance as grain size decreases.
Return to listSPATIAL DELINEATION, FLUID-LITHOLOGY CHARACTERIZATION, AND PETROPHYSICAL MODELING OF DEEPWATER GULF OF MEXICO RESERVOIRS THROUGH JOINT AVA DETERMINISTIC AND STOCHASTIC INVERSION OF 3D PARTIALLY-STACKED SEISMIC AMPLITUDE DATA AND WELL LOGS
Arturo Javier Contreras, Ph.D.
The University of Texas at Austin , 2006
Supervisor: William L. Fisher & Carlos Torres-Verdin
This dissertation describes a novel Amplitude-versus-Angle (AVA) inversion methodology to quantitatively integrate pre-stack seismic data, well logs, geologic data, and geostatistical information. Deterministic and stochastic inversion algorithms are used to characterize flow units of deepwater reservoirs located in the central Gulf of Mexico. A detailed fluid/lithology sensitivity analysis was conducted to assess the nature of AVA effects in the study area. Standard AVA analysis indicates that the shale/sand interface represented by the top of the hydrocarbon-bearing turbidite deposits generate typical Class III AVA responses. Layer-dependent Biot-Gassmann analysis shows significant sensitivity of the P-wave velocity and density to fluid substitution, indicating that presence of light saturating fluids clearly affects the elastic response of sands. Accordingly, AVA deterministic and stochastic inversions, which combine the advantages of AVA analysis with those of inversion, have provided quantitative information about the lateral continuity of the turbidite reservoirs based on the interpretation of inverted acoustic properties and fluid-sensitive modulus attributes (P-Impedance, S-Impedance, density, and LambdaRho, in the case of deterministic inversion; and P-velocity, S-velocity, density, and lithotype (sand-shale) distributions, in the case of stochastic inversion).
The quantitative use of rock/fluid information through AVA seismic data, coupled with the implementation of co-simulation via lithotype-dependent multidimensional joint probability distributions of acoustic/petrophysical properties, provides accurate 3D models of petrophysical properties such as porosity, permeability, and water saturation. Pre-stack stochastic inversion provides more realistic and higher-resolution results than those obtained from analogous deterministic techniques. Furthermore, 3D petrophysical models can be more accurately co-simulated from AVA stochastic inversion results. By combining AVA sensitivity analysis techniques with pre-stack stochastic inversion, geologic data, and awareness of inversion pitfalls, it is possible to substantially reduce the risk in exploration and development of conventional and non-conventional reservoirs.
From the final integration of deterministic and stochastic inversion results with depositional models and analogous examples, the M-series reservoirs have been interpreted as stacked terminal turbidite lobes within an overall fan complex (the Miocene MCAVLU Submarine Fan System); this interpretation is consistent with previous core data interpretations and regional stratigraphic/depositional studies.
DEFINING THE NORTHEASTERN BOUNDARY OF THE SUPERGIANT MARACAIBO FORELAND BASIN, VENEZUELA
David Rafael Contreras, MS
The University of Texas at Austin, 2008
Supervisor: Paul Mann
184 pages, 94 references, 3 tables
Differences in styles of structure and sedimentation and the distribution of hydrocarbons across the 140-km-long and 1-km-wide Burro Negro fault zone (BNFZ) constrain its origin as a deeply-rooted, right-lateral strike-slip fault that formed a major Paleogene tectonic and paleogeographic boundary along the northeastern edge of the supergiant Maracaibo basin. Complex subsurface structures and the lack of high quality seismic data and deep wells have made it difficult for previous workers to determine whether the BNFZ truncates the northeastern extension of the giant Eocene oil reservoirs in the Lake Maracaibo area or whether these reservoirs extend northeast of the BNFZ.
In Chapter 2, I integrate 2848 km of two dimensional (2D) seismic reflection data, 29 wells, gravity and magnetic data, and maps of outcrop geology to provide an improved structural interpretation of the BNFZ that can be used to assist future exploration programs in the region. Across the 1-km-wide BNFZ, the structural style seen on seismic lines changes abruptly from Eocene transtensional deformation in the Outer Maracaibo foreland basin to convergent deformation of the deepwater Inner Maracaibo foreland basin northeast of the BNFZ. I determined a right-lateral strike-slip fault character for the BNFZ that formed as a result of at least 7.2 km of Eocene right-lateral offset. Stratigraphic correlations using wells and seismic data spanning the BNFZ support the idea that giant Eocene oil reservoirs of the Lake Maracaibo area may continue towards the northeast into a complexly deformed Inner Maracaibo foreland basin consisting of Eocene deep-water sedimentary rocks. Late Eocene growth strata composed of deep-marine clastic rocks of the Agua Negra Group were deposited in piggyback basins in the Inner basin and accompanied Eocene thrusting and folding of the Lara nappes to the southeast
In Chapter 3, I present a subsurface geological interpretation of the subsurface of the northern edge of the Maracaibo foreland basin using 988 km of seismic data, 17 wells and 14,700 km2 of gravity and aeromagnetic data in the North Ambrosio area. Mapping of key surfaces in the basin that include the Eocene unconformity show that the primary structures in the subsurface are five, north to northeast-striking, right-lateral strike-slip faults of Eocene age (Icotea, East Urdaneta, West Urdaneta, San Ramón and La Concepción). On seismic data, these faults exhibit profiles typical of “positive flower zones”, where the strike-slip fault converges from a 2 to 4-km-wide fault zone at the surface to a narrow zone at the top of basement. The higher levels of the faults are composed of en echelon anticlines that form excellent structural traps for oil and gas. The 180 km2 Santa Rita pull-apart basin of Eocene age was identified at a left-step between the Icotea and East Urdaneta faults. Well logs show that Eocene sand bars were deposited in a tide-dominated deltaic system similar to that described by previous workers in areas to the south. I used temperature measurements from one well to calculate the depth of the oil window in the North Ambrosio area from 2.0 to 3.5 km and the gas window from 3.5 to 4.5 km. I also estimated the depth range of the “Golden Zone”, or depth interval where oil and gas is most likely to be preserved. The predicted Golden Zone corresponds well with reported depths of production and shows in the North Ambrosio area.
Gas potential for the North Ambrosio area could be determined form seismic and well data within a small area ~1800 km2 representing about 3.6% of the area of the Maracaibo foreland basin. The interaction of northeast-striking faults, such as the Icotea, East Urdaneta, West Urdaneta, and La Concepción fault zones controlled sedimentation during the Eocene foreland phase of the Maracaibo basin. Elongate Eocene sand bodies deposited in a tide-dominated deltaic system represent the main target for further exploration in the North Ambrosio area. The petroleum system is characterized by a strong foreland subsidence during Paleocene-Eocene times that buried Late Cretaceous source rocks of the La Luna Formation to depths of 4000 m below the surface. The source rocks reached gas generation window during two separate periods: middle-late Eocene and Miocene-Pliocene.
SOIL FORMATION AND EROSION IN CENTRAL TEXAS: INSIGHTS FROM RELICT SOILS AND CAVE DEPOSITS
Mary Jennifer Cooke, Ph.D.
The University of Texas at Austin, 2005
Supervisors: Libby Stern and Jay Banner
Cave deposits containing sediments of eroded soils and relict soils preserved on modern uplands provide some of the few insights into understanding the Quaternary evolution of central Texas landscapes and environments. This study investigates an ancient soil-erosion event on the Edwards Plateau of central Texas by using a new approach that relies on the strontium isotope composition of the ancient soil as an indicator of ancient soil thickness. I found the strontium isotope composition of soil on the Edwards Plateau today varies with soil thickness. I also found that variations in strontium isotope compositions of fossil plants and animals provide a record of temporal changes in soil thickness. Furthermore, I present stable carbon, oxygen, and nitrogen isotope data measured on sediments and fossils from a central Texas cave-fill deposit that support the hypothesis that increases in aridity and increases in precipitation intensity may have facilitated massive, late Quaternary soil erosion on the Edwards Plateau.
The origin of the former thick soil cover that was removed by the erosion is poorly understood. This study uses the geographic distribution and the textural and mineralogical properties of relict thick soils along with their neodymium isotope compositions, and rare earth element concentrations to determine the silicate source of the former thick soils. I propose that the Del Rio Clay, a locally-eroded, stratigraphically-higher, clay-rich strata provided the silicates to form thick soils overlying the silicate-pure Edwards Limestone.
Four important conclusions of this study are that: 1) in areas where the soil and bedrock are distinct in their strontium isotope composition, the strontium isotope composition of the soil varies with soil thickness, 2) the strontium isotope composition of terrestrial fossils can be a useful tool to reconstruct temporal changes in soil thickness, 3) climate variability can facilitate massive soil erosion with regional environmental consequences, and 4) soil formation from now-eroded bedrock of a higher stratigraphic unit may be more common than previously recognized. Thus, in light of future changes in climate and land use, soil conservation strategies should be considered in regions where soils rest on relatively pure limestones, because they are likely a “non-renewable” resource.
STRUCTURAL ANALYSIS OF THE SAN SIMEON FAULT ZONE, CALIFORNIA: IMPLICATIONS FOR TRANSFORM TECTONICS
Ryan Thomas Coppersmith, M.S. GeoSci
The University of Texas at Austin, 2008
Supervisor: Mark Cloos
The San Gregorio-Hosgri fault zone (SGH), located in the Southern Coast Ranges of California is a 420 kilometer long right-lateral strand of the San Andreas fault system. The San Simeon fault zone is a segment of the SGH that cross-cuts the Nacimiento block which is primarily composed of Franciscan Complex accretionary prism. The Nacimiento block is juxtaposed against the Salinian block, a portion of the Sierra Nevada batholith, by the Nacimiento Fault. The Nacimiento and Salinian blocks have been displaced from the south in a right lateral sense as part of movements within the San Andreas fault system. The San Simeon segment juxtaposes mid-Jurassic Coast Range Ophiolite with Cretaceous Franciscan accretionary prism material. These units are locally overlain by the Oligocene Lospe Formation and Miocene Monterey Formation.
To better understand the movement history near the San Simeon fault zone, 33 kilometers of outcrop were examined along the sea-cliff between Ragged Point in the north and Pico Creek to the south. Of this transect, 4 kilometers were buried under marine terrace and sand dunes. No data was collected along 1 kilometer of transect due to the presence of elephant seals. The 28 kilometers of bedrock examined include: 7 kilometers of ophiolitic material, 16 kilometers of Franciscan Complex, 2 kilometers of Lospe Formation, and 3 kilometers of Monterey Formation. In all, 466 minor faults and 254 major (≥0.5 meters exposure length) faults were mapped, and 22 of these major faults juxtapose different formations (n=8) or different units within the ophiolite (n=14). Slickenlines were measured on 517 faults, of which 237 record sense of slip. Of the faults measured, 199 are strike-slip (0-30°rake), 179 are dip-slip (60-90°rake), and 139 are oblique-slip (31-59°rake). Sense of slip indicators record a wide range of movements: 49 right-lateral, 47 left-lateral, 40 normal, 38 reverse, 18 reverse left-lateral, 17 normal left-lateral, 15 normal right-lateral and 13 reverse right-lateral faults.
The study transect was divided into structural domains based on fault kinematic patterns. Movement recorded in these data resulted from transform-related faulting. Fault kinematics that differ from the regional N35W strike of the San Simeon fault zone are explained by local variations in movement patterns near the San Simeon fault zone. This variations include local bends and splays off of the fault zone. The Lospe and Monterey Formations that make up 18% of the mapped transect contain 12% of the faults. These formations only experienced transform-related deformation. Faults in the Monterey Formation are parallel to the regional San Simeon fault zone. Faults in the Lospe Formation to the north primarily strike E-W. Ophiolite material contains 25% of the mapped transect and 37% of the faults. These faults primarily indicate right-lateral movement; however, reverse and normal faulting near perpendicular to the regional NW fault trend is common. The Franciscan Complex along 57% of the mapped transect contains 51% of the faults. Faults in the Franciscan Complex and the ophiolite potentially record subduction-related faulting, but evidence from fault kinematics from this study indicates transform-related faulting. Reverse and right-lateral faulting along the splays is indicated. East of San Simeon Point, a 1 kilometer wide San Simeon fault zone is indicated by a cluster of faults between the San Simeon Pier and Broken Bridge Creek, the eastern boundary of the fault zone. The complexity of fault patterns and kinematics in and near the San Simeon fault zone record a long and complex history of transform faulting.
THE ORIGIN OF PARTIAL-DISEQUILIBRIUM MAJOR- AND TRACE-ELEMENT ZONING IN GARNETS FROM THE PICURIS MOUNTAINS, NM
Wesley Dean Crawford, Jr., M.S.
The University of Texas at Austin , 2008
Supervisor: William D. Carlson
189 pages, 31 references, 2 tables
New evidence from X-ray mapping, quantitative major- and trace-element analyses, and garnet-inclusion suites suggests that the disequilibrium behavior in garnets from the Picuris Mountains, NM, is the result of changes in matrix assemblages at a very local scale (i.e., within the immediate vicinity of each growing porphyroblast). In addition, detailed petrologic study of metapelites with a range of bulk compositions reveals a wide variety of partial-disequilibrium garnet zoning patterns not previously known to exist. The complicated zoning patterns of garnets from samples containing the assemblage garnet + biotite + muscovite + quartz + plagioclase can be explained by a local change from a garnet-forming to a garnet- and biotite-forming reaction, coupled with the breakdown of allanite, the nucleation of apatite and monazite, and the temporary presence of xenotime. This interpretation is supported by mass-balance calculations that show that the breakdown of less than one modal percent allanite could supply the spikes in trace-element concentration observed in garnet. This study also provides plausible explanations for the unexpected zoning patterns found in garnet-staurolite schists and one calc-schist. The implications for petrologists of cryptic disequilibrium like that recorded by garnets of the Picuris Range are many fold. In particular, the incautious application of geothermometers and geobarometers, especially those which utilize accessory minerals, could lead to seriously flawed results.
Return to listSTRUCTURAL AND SEQUENCE STRATIGRAPHIC ARCHITECTURE OF THE TERTIARY SECTION IN THE RED FISH BAY AREA OF TEXAS
Luis Arturo Crespo, MA
The University of Texas at Austin , 2006
Supervisor: William L. Fisher & Robert G. Loucks
The main objective of this report is to describe the structural and sequence stratigraphic architecture of the Tertiary section in the Red Fish Bay area of Texas . The report was based upon previous investigations of the Tertiary section carried out in the Red Fish Bay subbasin area.
A high influx of sediments coupled with relative sea-level changes produces a thick sedimentary section in the Red Fish Bay area composed of deep- to shallow-water deposits. At a time of a third-order lowstand, river systems incising into the older shelf strata transported sediment beyond the shelf edge to accumulate on the slope and subbasin floor. The loading stress of these third-order deposits caused instability in the underlying muds of the older Vicksburg and Frio Formations and a growth fault was initiated. Deposition of the lower Frio unit continued to take place in the subbasin, and as the underlying mud migrated, the hanging wall of the growth fault underwent rotation and a shale ridge formed at the most basinward edge of the subbasin. Additionally, two minor growth faults activated which subdivided the Lower Frio unit of the subbasin into three “minibasins” which all have similar fourth- and higher-order systems tracts and are also chronostratigraphically correlative from one to the other. The long-term second-order lowstand systems tract ended, marked by the top of the Lower Frio unit and a long-term second-order transgressive systems tract began as indicated by the deposition of the middle to upper Frio and Anahuac units. The second-order transgressive systems tract contains significantly thick higher order transgressive and highstand systems tracts. A succeeding long-term second-order highstand systems tract continued through the remainder of the Anahuac , the Oakville Formation and passed the age range of this report. During this second-order highstand systems tract, well-log data and seismic data indicate the development of lowstand incised valleys in the Oakville Formation.
By interpreting and constructing the structural and sequence stratigraphic architecture of this area, the prediction of sand (and other reservoir rock) distribution can be improved. Applying correct interpretation models and techniques, as well as correlation tools such as the Site-Specific Sequence-Stratigraphic Section (S 5 ) benchmark composite log, erroneous correlations which could produce the wrong stratigraphic and structural architecture can be avoided.
SUBGLACIAL WEST ANTARCTIC VOLCANOES DEFINED BY AEROGEOPHYSICAL DATA AND THE POTENTIAL FOR ASSOCIATED HYDROTHERMAL SYSTEMS
Hunter-West Anderson Danque, M.S. Geo. Sci.
The University of Texas at Austin, 2008
Supervisor: Donald D. Blankenship
112 pages, 124 references, 3 tables
Subglacial hydrothermal systems beneath the West Antarctic Ice Sheet (WAIS) are important objects of research because meltwater associated with them can act as a lubricant beneath this potentially unstable ice sheet. In addition, subglacial hydrothermal systems on Earth may provide analogues for subglacial microbial habitats under the polar caps of Mars or other icy bodies. The West Antarctic subglacial volcanoes compared herein display a range of plausible hydrothermal systems. One end-member is Mt. CASERTZ which is a recently erupted subglacial volcano discovered in the early 1990's just upstream of a well lubricated ice stream. It sustains a depression in the ice surface, and it is associated with nearby subglacial lakes. Therefore, it has a high potential for sustaining a short-lived hydrothermal system. At the other end of the spectrum of plausible subglacial hydrothermal systems is a subglacial volcano, designated M, which appears to be inactive and to possess no substantial hydrothermal system. In the middle of the spectrum is a subglacial volcano, designated L, which lies near the ice divide between the Ross Sea and Amundsen Sea Embayments in West Antarctica. It has a well-defined intrusive body and a terraced morphology consistent with multiple eruptions. Subglacial Volcano L is distinctly larger than the other two volcanoes, and it is probably a greater heat source than M while being older than Mt. CASERTZ. Using newly developed radar techniques, I observed that Volcano L has several small (200-500 meter) subglacial lakes located over and on the boundaries of the modeled underlying intrusive body implying the intrusive body is associated with subglacial melting. Adjacent to volcano L, I also observed a large drawdown (designated H) in the internal layers of the ice sheet consistent with substantial basal melting. To better evaluate ice mass loss associated with anomaly H and Volcano L, I developed an algorithm that maps anticorrelations between internal layers within an ice sheet and subglacial topography. Using this algorithm, I discovered that the layer drawdown adjacent to L extended along a highly linear zone about 28 kilometers in length. The layer drawdown near Volcano L implies a mass loss at the base of the ice sheet of 25 cubic kilometers. This mass flux can be explained by localized subglacial melting associated with igneous processes. If the mass flux is due to local subglacial melting, then subglacial lakes identified on either side of the drawdown are hydraulically plausible destinations for the meltwater. I hypothesize that the linear zone associated with anomaly H is a fault supplying an avenue for the movement of magma to the base of the ice sheet that supports an associated hydrothermal system. Together these factors make the subglacial lakes that I have observed on Volcano L and adjacent to the hypothesized fault highly attractive targets for geochemical and biological sampling of a mature subglacial hydrothermal system.
Return to listHYDROGEOLOGICAL CHARACTERIZATION OF BASEFLOW TO JACOB’S WELL SPRING, HAYS COUNTY, TEXAS
Sarah Cain Davidson, M.S.Geo.Sci.
The University of Texas at Austin, 2008
Supervisor: Philip C. Bennett
Jacob’s Well is a karst spring that discharges from the Trinity aquifer in central Texas. Ongoing development and increased groundwater pumping in the area around the spring are thought to be impacting the volume of discharge at the spring—however, the source of spring discharge is poorly understood. The goals of this study are to characterize the chemistry of springflow and nearby surface waters and groundwaters and to identify the source or sources of water that contribute flow to the spring under low flow conditions.
The spring is located in the Middle Trinity aquifer within the Lower Cretaceous Trinity Group in central Texas. The Trinity Group is composed of alternating terrestrial and marine deposits, with most groundwater flow occurring through secondary porosity in the carbonate units. The cave leading up to the spring consists of at least two water-filled conduits that have been mapped down into the Cow Creek Limestone and several hundred meters to the north and northwest of the spring outlet.
Water samples and measurements of water levels and chemical field parameters were collected at 48 study sites representing stream, spring, and well waters over a 250-km2 area around the spring from May–September 2008. The area was in drought throughout this period and the results thus represent low flow conditions. Samples were analyzed for major and trace ions, organic and inorganic carbon, and isotopes of O, H, and Sr. Modeling using PHREEQC was used to calculate whether realistic phase changes in known aquifer minerals could explain the chemical differences between sampled waters along hypothesized flowpaths.
Water levels indicate that groundwater flows generally to the southeast within the study area. Chemical analyses of water samples show three chemical types: (1) Ca-Mg-HCO3 waters, which make up all spring water, all surface water, and most ground water samples; (2) high TDS Ca-Mg-SO4 ground waters; and (3) high TDS Na-Mg-Ca-SO4 ground waters. Measured element concentrations, strontium isotope data, and geochemical modeling results indicate that the high TDS waters most likely come from the upper member of the Glen Rose Limestone downgradient from the spring (type 2) and from the Hosston Formation in the Lower Trinity aquifer (type 3).
Physical and chemical properties of the sampled waters were used to define which well and stream samples might represent water within the area contributing to baseflow at the spring. Water samples taken from seven sites north and west of the spring were found to represent possible sources of springflow during the study period. These findings suggest the possibility of two distinct flowpaths connecting the aquifer to Jacob’s Well: (1) flow to the southeast within the Cypress Creek watershed along the regional groundwater flow path estimated from water level measurements, and (2) flow to the east along fracture or conduit paths that cross the surface divide between the Upper Blanco River and Cypress Creek watersheds.
Return to listCOMPLEX STRUCTURAL AND FLUID FLOW EVOLUTION ALONG THE GRENVILLE FRONT, TRANS-PECOS TEXAS
Benjamin R Davis, M. S. Geo. Sci.
The University of Texas at Austin, 2007
Supervisor: Sharon Mosher
A narrow (~7 km wide) fold and thrust belt in west Texas that represents the northernmost extent of the Grenville collision belt along the southern margin ofLaurentia (Grenville Front) records a complex history of deformation and fluid flow. The main Streeruwitz thrust that emplaced -1.35 Ga high-grade metamorphic rocks over -1.25 Ga foreland sedimentary and volcanic rocks postdates polyphase deformation in the footwall and is itself complexly folded into local domes and basins. Deformation can be characterized by four stress regimes. A pre-Streeruwitz ductile deformation phase, dextral oblique strike-slip motion, deformation associated with Streeruwitz thrusting, and formation of complex domes and basins resulting from continual transpression. Fluids with an evolving chemistry were channelized along the thrusts, metasomatically altering the adjacent rocks.
Deformation consisting of six phases of folding, metasomatic reactions, and veining within foreland rocks record a change in kinematics and evolving fluid chemistry with time. An early phase of siliceous fluids during D2 caused replacement of mafic dikes and dolostones that preserve fi/s] and formation of extensive talc lenses aligned axial planar to F2, forming the dominant 82. Subsequent upright vertical sheath folds, F4, indicate dextral oblique motion on high-angle west-northwest-trending faults. The Streeruwitz thrust and subsidiary imbricate thrusts truncate Fi-F4 folds at a high angle requiring a change in the overall kinematics of the deformation. Late chevron to box folds and kinks of 82 are generally associated with shears locally related to thrusting. Initial thrusting at depth produced mylonites in both footwall (syn-S2) and hanging wall rocks that were later brecciated in the final stage of thrusting along the Streeruwitz. Further evolution of fluids along the thrusts is recorded in the breccias and veins, starting with siliceous- and alkali-rich fluids. Lastly, carbonate-rich fluids replaced footwall rocks and cemented breccias in both the hanging and footwalls. Final deformation of the area folded the Streeruwitz and imbricate thrust sheets into complex domes and basins trending SW and SE.
The degree of structural complexity and fluid flow interaction requires a new kinematic model for the southern margin of Laurentia, one where a north-verging indenter collided with southern Laurentia and then under went clockwise rotation. The disparity in timing of deformation, and of kinematics, between west Texas and the Llano uplift of central Texas requires active subduction in west Texas after collision in central Texas. Furthermore, the complex structural histories and differences in kinematics with time between the hanging wall and the footwall of the Streeruwitz thrust can be explained by clockwise rotation of an indenting continent after docking on southern Laurentia.
SUBGLACIAL MORPHOLOGY AND STRUCTURAL GEOLOGY IN THE SOUTHERN TRANSANTARCTIC MOUNTAINS FROM AIRBORNE GEOPHYSICS
by
Marcy Brooke Davis, MSGeoSci
The University of Texas at Austin, 2001
Co-Supervisors: Donald D. Blankenship and Ian W. D. Dalziel
The Transantarctic Mountains (TAM) form the high western rift flank of the West Antarctic Rift System (WARS), a region of extended crust that separates East and West Antarctica. Little is known about the structural geology in the southern TAM due to limited accessibility and more ice cover than in the central and northern TAM. The objectives of this study are to characterize the subglacial bedrock morphology interior of the southern TAM and to define the structural geology along the southern TAM Front through analysis of airborne ice-penetrating radar data.
The airborne survey extended from the Ross Ice Shelf, southward over the TAM along the 150ºW meridian and between the Scott and Reedy Glaciers then over the high plateau and through the South Pole. Approximately 15,000 line km were flown and processed using finite difference migration techniques in a seismic processing software package. Subsequently, the ice and bed surfaces were picked along each line and known geology was interpreted on the radar records where it exists. Ice-penetrating radar sounding coupled with a compilation of field geologic studies has proven to be a powerful technique for examining the architecture and structural geological relationships of the southern TAM.
Four distinct morphological provinces are identified along the length of the survey, these include: 1) the South Pole Basin and Plateau Province, with low relief features and up to 4 km of ice; 2) the Alpine Glaciated Province, with well-preserved U-shaped valleys that show a glaciation network that flowed opposite of contemporary glaciers; 3) the TAM Massif, which includes three subglacial blocks and the outcropping portion of the TAM; and 4) the TAM Front, a normal fault zone north of the TAM to Ice Stream A.
The southern TAM have a southward tilted block structure with the area of maximum uplift in a region 30-50 km wide from the Watson Escarpment, the highest part of the TAM in this area, southward and is bounded by NW-SE-trending normal faults on both the north and south sides. Down-to-the-north normal faults north of the Watson Escarpment topographically downdrop the TAM from >3000 m to sea level over ~50 km and facilitate the development of the Leverett Glacier and Ice Stream A. The primary structural trend between the Scott and Reedy Glaciers is NW-SE, parallel to the TAM. Faults oriented obliquely to the TAM break the area of maximum uplift into three NNW-SSE trending blocks that appear offset ~10 km in a left-lateral sense relative to each other with range-parallel horst and graben features superimposed. One of these faults may control the eastern side of Scott Glacier. Structural relationships across the Watson Escarpment suggest that the southern TAM are a result of upward block faulting along the TAM Front rather than a result of regional upwarping.
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THE TECTONICS OF TRANQUITAS: A FIELD STUDY OF RIFT THROUGH PASSIVE MARGIN DEVELOPMENT AND LARAMIDE DEFORMATION IN TRIASSIC AND JURASSIC STRATA OF THE SIERRA MADRE ORIENTAL, NE MEXICO
Matthew H. Davis
The University of Texas at Austin, 2005
Supervisor: Randall Marrett
Exposed near Galeana, Nuevo Leon are sedimentary deposits and contemporaneous structures that record the rifting and opening of the Gulf of Mexico, passive-margin development, and Laramide compression in the region. A mapping, sedimentologic, and structural study utilizing thin sections, measured sections, aerial photos and kinematics has produced a detailed stratigraphic section that records the transition from terrestrial to open-marine deposition and the orientation and timing of deformational events. Triassic to Early Jurassic red beds in the Huizachal Group are composed of fluvial and marine sands. The La Boca and La Joya Formations, respectively, are separated by a polymictic cobble conglomerate. The Callovian Minas Viejas represents the initial stages of marine transgression and the overlying interbedded carbonates and evaporites of the Zuloaga and Olvido Formations respond to cyclic eustatic sea-level fluxuations. The increase of biodiversity through the upper Jurassic strata reflects a change from restricted to open-marine conditions. Rift-related tectonics and gravity driven brittle extension features within the carbonates have been overprinted by Laramide orogenesis. The red beds have been folded and faulted into a broad Laramide anticlinorium with smaller intrusion cored folds having amplitudes on the order of 10's of meters. Within the evaporite lies the decollement for Laramide thin-skinned deformation, thus there is a high degree of structural complexity represented in the overlying carbonates as tight folding patterns. By integrating concepts of depositional systems with structural deformation the hypothesis, “There is an exhumed preraft block in the Zuloaga Limestone around San Pablo de Tranquitas.” will be tested.
Return to listSTRATIGRAPHIC EVOLUTION AND FACIES ANALYSIS IN A PALEOCENE REEF MOUND, LA POPA
BASIN, NORTHEAST MEXICO
Jose Francisco Delgado Gutierrez, MS Geo. Sci.
The University of Texas at Austin, 2003
Supervisor: William L. Fisher for R. K. Goldhammer (Deceased)
111 pages, 65 references, color illustrations, CD-Rom
The Paleocene La Popa reef mound is an isolated carbonate platform exposed in the foot-wall of the La Popa salt-weld structure within the Late Cretaceous- Paleocene La Popa Salt Basin (northeast Mexico). A facies and sequence stratigraphic analysis was undertaken to define the three-dimensional geometry, facies architecture and the stratigraphic development of this world class, 3-D exposure of a Paleocene reef mound.
Microfacies analysis was performed with more than 240 hand samples and 91 thin sections from 12 stratigraphic sections using the methodology described by Flügel (1982) & Carozzi (1988). The results indicate that the La Popa reef mound is composed mainly of calcareous algae (Dasycladaceae and coralline red algae) and they reveal the significance of these types of organisms as the primary reef builders of Tertiary reefs, rather than corals. Microfacies analysis also indicates that windward-leeward effects played a major role in defining the facies architecture.
The facies architecture of the Reef Mound reveals a classic facies zonation similar to the modern-day coral-algal reefs, with the Paleocene facies tracts defined by depth-dependent green and red algae. In the mixed clastic-carbonate section (The La Popa Prow), 1-D stacking-pattern analysis and the facies proportions were performed between the clastics and carbonates to develop a hierarchical subdivision of the genetic units defined by: the fundamental cycle, the cycle sets, the high-frequency sequences and the second-order super sequences. In this unit, each sequence element regardless of scale (for example the fundamental cycle, or the high-frequency sequence, or the second-order supersequence) the TST tends to consist of clastic sediments and the HST consists of carbonate sediments.
1-D and 2-D stacking pattern analysis were conducted in the Tertiary part of the succession (the platform-top canyon sections), which is Paleocene in age and consists of one second-order supersequence defined by higher-frequency elements. In this unit the genetic packages consist of an amalgamation of the fundamental cycles and cycle sets within the Reef Mound.
The termination or drowning sequence consists of a second-order supersequence boundary, and corresponds to the position of "stratigraphic reversal" where the shallow-water reef mound top starts to drown in response to local (salt-induced subsidence?) or regional (eustatic?) accommodation increase.
The major phase of "reef mound" development is linked to the HST of a second-order supersequence, and the buildup termination occurs through drowning during the TST of a subsequent second-order supersequence.
Return to listTRAVERTINE DEPOSITS AS RECORDS OF GROUNDWATER EVOLUTION IN URBANIZING ENVIRONMENTS
Laura Michelle DeMott, M.S.Geo.Sci
The University of Texas at Austin, 2007
Supervisor: Jay L. Banner
Urban development has multiple impacts on groundwater, including changes in patterns of recharge, alteration of groundwater flow paths, and increased water contamination. For the most part, these impacts have only recently begun to be appreciated, and in most aquifers there is a lack of data available on recharge and water quality prior to urbanization for comparison. This lack of data across the period of urbanization makes the prediction of the impacts of future development difficult. I propose that recently deposited spring water calcite (travertine) may provide a proxy record of changing groundwater composition over time. The city of Austin, Texas has undergone a large increase in urbanization over the past 40 years. Because the chemistry of municipal water in Austin is distinctive from the local groundwater, I have examined the possibility that the amount of urban water input to local groundwater recharge can be assessed using the isotopic composition of strontium ( 87 Sr/ 86 Sr) in combination with elemental concentrations and ratios of Mg/Ca and Sr/Ca. This study examines the current depositional environment of three travertine-depositing springs in the Austin area. It makes use of the modern system, including experimental growth of calcite on artificial substrates in the springs, to interpret a 30-year temporal record provided by one of the three studied springs.
Three travertine-depositing springs were studied over the course of one year to examine water chemistry and strontium isotope composition changes on a seasonal basis. Elemental concentrations and strontium isotope ratios remained fairly constant throughout the study year, and showed the influence of urban water in the isotopic compositions of the spring water. Glass plate substrates were employed over a four-month time period, to examine monthly changes in travertine growth and isotopic and elemental compositions. Using these substrate experiments, I determine travertine growth rates and effective trace element partition coefficients. These partition coefficients are higher than values determined from laboratory experiments from the literature, indicating that partitioning behavior in natural systems may be different than that of controlled laboratory experiments. Applying these partition coefficients to the travertine temporal record provides a more accurate interpretation of past water compositions. Comparison of stable isotopes of carbon and oxygen between spring waters and plate calcites revealed that the plate carbonates (travertines) are not forming in isotopic equilibrium with their dripwaters. Based on this, interpretation of stable isotope records from these travertines should be treated with caution.
Isotopic and elemental analyses were conducted on a spring-fed travertine that preserved 30 years of deposition on a roadcut through Lower Cretaceous marine limestones. The spring is actively precipitating calcite, and appears to have grown continually since the excavation of the roadcut. The contributing area to this spring has undergone an increase in urban land use of approximately 40% since 1983. 87 Sr/ 86 Sr data for the travertine and spring waters show a small but resolvable increase over time, from 0.70790 to 0.70797. These values for 87 Sr/ 86 Sr are higher than those for Lower Cretaceous marine limestones, and lower than values for municipal water as well as soil exchangeable Sr. The increase in 87 Sr/ 86 Sr may be accounted for in three ways: 1) Through an increased relative proportion of municipal water input to the groundwater in the spring's contributing area; 2) Through increased overall recharge in the area, resulting in more water-soil interaction; or 3) A combination of the two. Modeling of karst flow processes (water-limestone interaction and fluid mixing) shows that the trend in elemental ratios and 87 Sr/ 86 Sr may be accounted for by processes of soil water-limestone interaction and mixing with urban waters. Models also indicate that this fluid mixing likely occurs in the epikarst zone, and that urban water may account for as much as 80% of the discharge from the spring. Values for 87 Sr/ 86 Sr in spring waters are likely dampened by limestone interaction during times of low recharge. This model of water-limestone interaction and urban water mixing implies that urbanization in this particular area resulted in increased recharge and that this recharge may be quantifiable over time. Travertine proxies represent a previously unused resource for examining urban recharge in karst terrains.
ORIGIN OF A COMPLEX DUNE PATTERN, ALGODONES, SOUTHEASTERN CALIFORNIA
Dana C. Derickson, M.S. Geo. Sci.
The University of Texas at Austin, 2005
Supervisor: Gary Kocurek
The Algodones dune field in southeastern California contains two unusual features which are cause for investigation. First, several different morphologies of sand dunes are situated within a relatively small area. Previous studies attribute this complexity to the dunes themselves, which modify the wind within the dune field, allowing different types of dunes to coexist in equilibrium. Yet this hypothesis has never really been proven. A different approach would be to consider the dunes as a pattern, and a complex pattern would be explained as simple patterns superimposed on each other. Each simple pattern would then correspond to a different generation of dunes.
Pattern analysis of the Algodones revealed two different generations of dunes within the dune field. The longer, more widely spaced dunes correspond to the compound crescentic dunes in the central portion of the dune field. The second generation includes the remaining morphologies, which are all interrelated. The arms of crescentic dunes can elongate and become linear dunes, and examples of this can be seen in the northern end of the dune field. Superimposed crescentic dunes have shorter crest lengths and spacings than the simple crescentic dunes because of sorter development time. Their location on the stoss on the stoss slopes of the compound crescentic dunes means that they are subject to avalanching over the lee slopes of these dunes. Their average orientations are different because the superimposed crescentic dunes are subject to secondary flow from the lee faces of the compound crescentic dunes. Zibars in the western edge of the dune field correspond to coarse-grained sediment.
The second unusual feature of the Algodones is the 400 m wide, 40 m thick topographic ramp located along the western margin of the dunefield. One or two more ramps exist east of the western margin, and each ramp is capped by a linear dune. Sweet et al. (1988) postulated that the westernmost ramp was the trailing margin of an eastward migrating dunefield. Ground penetrating radar (GPR) profiles along the western margin of the dunefield have contradicted this model. The subsurface cross-strata and surface topography are more consistent to that of migrating compound dunes. Whether these dunes are of the same age, or part of a transgressive dunefield system migrating from paleolake shorelines, the source of the dune sand, is yet to be proven. However, the intermittent nature of Lake Cahuilla and linear trends within the dune field support the hypothesis that the ramp is part of a transgressive dune field.
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STRUCTURAL ANALYSIS IN THE COAST RANGE OPHIOLITE NEAR PASKENTA, CALIFORNIA: IMPLICATIONS FOR TECTONIC UPLIFT PROCESSES
Andrew David Dewhurst, M.S. Geo. Sci.
The University of Texas at Austin, 2008
Supervisor: Mark Cloos
220 pages, 85 references, 4 tables
California has been a site of subduction since about 155 Ma. The Franciscan complex accumulated in front of, and beneath, the crystalline leading edge of the North American Plate: a belt of oceanic crust and mantle known as the Coast Range Ophiolite. Near Paskenta, California, the blueschist facies Franciscan rocks (South Fork Mountain Schist) are juxtaposed against the ophiolitic rocks along the Coast Range Fault. These Franciscan rocks have risen ~15 km and the ophiolitic rocks at least 3 km and perhaps as much as 10 km. The intervening Coast Range Fault has a near vertical dip. It has previously been interpreted as a rotated east-dipping thrust fault that formed by subduction underthrusting, an east-dipping décollement to a series of out-of-sequence imbricated thrusts, and a high-angle west-dipping reverse fault that formed by collisional overthrusting. To explain the unroofing of the Franciscan rocks, it has been interpreted as a high-angle normal fault. Notably, fault-related kinematic data in support of tectonic models for Coast Range Fault movement and ophiolite emplacement are sparse.
To constrain the geometry and kinematics of the Coast Range Fault and ophiolite emplacement, three geologic transects were made along sinuous roads near Paskenta. Approximately 30 km of roadcut is present along the ~60 km length of road. Exposures are typically less than two meters high, but locally as tall as 20 m, along roadcuts that extend for tens to a few hundred meters. Lithologies were mapped in the roadcut exposures and the attitudes of fault planes and direction and sense of slip was measured. Data were obtained on 275 faults traceable for ≥ meter. Within serpentinite shear zones, the attitudes of 1159 minor faults were measured.
Mapping revealed that although all parts of the ophiolite suite are present near Paskenta, the pieces are well shuffled. For example, slabs of layered radiolarian chert are found in the western part of the ophiolite near the Franciscan rocks and masses of serpentinite are present in the eastern part near the steeply eastwards dipping strata of the depositionally overlying Great Valley Group (Knoxville Formation). In addition, faults and serpentinite shear zones are heterogeneously distributed. In the Paskenta area, the ophiolite is not simply an upturned slab of oceanic crust. It is well described as an ophiolitic mélange. Stereographic analysis reveals the pattern of faulting along each transect is similar with most faults striking roughly north-south and steeply dipping - near-parallel to the Coast Range Fault. For major faults with full kinematic observations, normal offsets (n = 226) dominate over those with reverse offset (n = 97). Movement patterns recorded in the minor faults found in the serpentinite shear zones are less defined, but overall patterns appear similar.
Uplift of the Franciscan complex and Coast Range Ophiolite near Paskenta was accomplished not just by normal, near-vertical movement along the Coast Range Fault, but also by slip along the numerous faults and shear zones within the ophiolite belt. However, faulting associated with the ascent of Franciscan rock does not appear to be sufficient to account for the thorough disruption of the ophiolite terrane in this area. The ophiolitic mélange was probably generated prior to ophiolite emplacement in an oceanic transform setting.
Return to listFRACTURE TECTONICS, FRACTURE POROSITY EVOLUTION AND STRUCTURAL DIAGENESIS, CAMBRIAN ERIBOLL SANDSTONES, NORTHWESTERN SCOTLAND
Kira Diaz-Tushman , M.S. Geo. Sci.
The University of Texas at Austin , 2007
Supervisors: Stephen E. Laubach & Sharon Mosher
Cambran Eriboll Sandstones in Northwestern Scotland contain 5 sets of regional fractures; Sets A through E. Set A strikes north to NNE, Set B strikes NW-WNW, Set C strikes NE, Set D strikes east, and Set E strikes north. Crosscutting relationships and orientations with respect to bedding places the relative ages as Set A being the oldest and Set E being the youngest, with sets B through D in between. Based on orientation and intensity patterns I interpret Set A to have formed in a platformal setting before emplacement of the Moine Thrust Zone (MTZ). Sets B and C were likely formed during and shortly after emplacement of the MTZ. Set D may have formed after thrust emplacement, possibly during Late Devonian or later regional extension. Set E was likely caused by late faulting in the Cenozoic.
Sets A and D are comparable, in the fact that both are likely formed in a basinal setting, with similar cement patterns and structures. Set A and Set D differ in amount of preserved fracture porosity. Both sets contain characteristic localized cement deposits (synkinematic bridges) having crack seal and lateral growth textures. Such bridges are common in sedimentary basins. Lander et al. (2002) explained bridges as a consequence of greatly differing quartz growth rates on broken (fast) and idiomorphic (slow) quartz surfaces. Bridges form where incremental opening allows growth on some grains to span the fracture between opening steps. Local cross-fracture bonding and subsequent rebreaking perpetuates localized fast growth. Set A is the first documented case where former voids between bridges are filled with massive quartz having no crack-seal texture. This pattern indicates that Set A is a basinal system fossilized by protracted burial. Growth rate experimental data (Lander et al., 2002), fluid-inclusion data I collected from synkinematic quartz, burial history, and the 1.6 cm aperture size of the largest fractures allows me to calculate the time needed to fill fractures with slow growing quartz in Sets A and D. Results agree with my fracture timing estimates based on orientation and kinematic compatibility arguments. My observations and sealing estimates also show that fractures can remain open for millions of years in deep basinal settings, where they are potential conduits for fluid flow. Because sets A and D have identical attributes to basinal fracture systems in economically important fractured hydrocarbon systems, such as tight gas sandstones, my fracture observations from the ~150 km long Eriboll Sandstone outcrop belt provide a rare view of regional fracture characteristics typical in the subsurface.
Return to listGRAVITY ANALYSES FOR THE CRUSTAL STRUCTURE AND SUBGLACIAL GEOLOGY OF WEST ANTARCTICA, PARTICULARLY BENEATH THWAITES GLACIER
Theresa Marie Diehl, Ph.D.
The University of Texas, 2008
Supervisor: Donald D. Blankenship
209 pages, 162 references, 18 tables
The West Antarctic Ice Sheet (WAIS) is mostly grounded in broad, deep basins (down to 2.5 km below sea level) that are stretched between five crustal blocks. The geometry of the bedrock, being mostly below sea level, induces a fundamental instability in the WAIS through the possibility of runaway grounding line retreat. The crustal environment of the WAIS further influences the ice sheet’s fast flow through conditions at the ice-bedrock boundary. This study focuses on understanding the WAIS by examining the subglacial geology (such as volcanoes and sedimentary basins) at the ice-bedrock boundary and the continent’s deeper crustal structure- primarily using airborne gravity anomalies. The keystone of this study is a 2004-2005 aerogeophysical survey over one of the most negative mass balance glaciers on the continent: Thwaites Glacier (TG). The gravity anomalies derived from this dataset- as well as gravity-based modeling and spectral crustal boundary depth estimates- reveal a heterogeneous crustal environment beneath the glacier. The widespread Mesozoic rifting observed in the Ross Sea Embayment (RSE) of West Antarctica extends beneath TG, where the crust is ~27 km thick and cool. Adjacent to TG, spectrally-derived shallow Moho depths for the Marie Byrd Land (MBL) crustal block can be explained by thermal support from warm mantle. I assemble here new compilations of free-air and Bouguer gravity anomalies across West Antarctica (from both airborne and satellite datasets) and re-interpret the extents of West Antarctic crustal block and their boundaries with the rift system. Airy isostatic gravity anomalies reveal that TG is relatively sediment starved, in contrast to the sediment-rich RSE. TG’s fast flow velocities could be sustained in this sediment poor environment if higher heat flux in MBL was providing an ample source of subglacial melt water to the glacier. The isostatic anomalies also indicate that TG’s outlet rests on a bedrock sill that will impede future grounding line retreat (up to ~100 km) and temporarily stabilize the glacier.
Return to listCHARACTERIZATION OF HISTORIC LIME MORTARS
Kara Rose Dotter, MSGeoSci
University of Texas , 2006
Supervisor: Kitty Milliken
Historic lime mortars are an amalgam of natural and man-made materials. Thus, the study of lime mortars inherently requires interdisciplinary approaches. In an effort to evaluate the applicability of select analytical techniques typically utilized by geoscientists, fourteen samples of historic lime mortars are examined with transmitted polarized light microscopy, scanning electron microscopy (SEM), backscattered-electron imaging (BSE), and energy-dispersive X-ray analysis (EDS). The imaging and analysis tools prove very effective in determining mineralogical characteristics of the aggregate and binder materials, the binder-to-aggregate ratio, the pore structure, possible secondary cements, and the presence of hydraulic and/or pozzolanic constituents. This research highlights the many benefits gained from application of traditional geological analysis techniques to the field of conservation science. In addition, the research points towards areas of future research for integrating these techniques into a standardized analysis methodology for the characterization of historic lime mortars.
THE CRANIAL ANATOMY OF THE THEROPOD DINOSAUR SHUVUUIA DESERTI (COELUROSAURIA: ALVAREZSAURIDAE), AND ITS BEARING UPON COELUROSAURIAN PHYLOGENY
by
David Laurence Dufeau, M.S. Geo. Sci.
The University of Texas at Austin, 2003
Supervisor: Timothy Rowe
275 pages, 122 references, CD-Rom in pocket
Alvarezsaurids are a very unusual group of theropod dinosaurs whose bizarre anatomical novelties defy easy interpretation in the context of their relationship to other groups of theropod dinosaurs. The problem of resolving their affinites to any particular group or clade of theropods is compounded by the lack of any descriptions of cranial material for alvarezsaurids. Heretofore only two hypothesis for the placement of alvarezsaurids within a theropod phylogeny were published. Both analyses were conducted without the benefit of cranial material. In this thesis I describe the skulls of two specimens of the alvarezsaurid Shuvuuia deserti, and apply the data derived from that description to tests of the two competing hypotheses. The result of one of these tests is an entirely new and unexpected placement for alvarezsaurids on the theropod family tree.
Return to listLATEST QUATERNARY STRATIGRAPHY AND SEAFLOOR MORPHOLOGY OF THE NEW JERSEY CONTINENTAL SHELF
Catherine Lorraine Schuur Duncan, PhD
The University of Texas at Austin, 2001
Supervisors: William E. Galloway and John A. Goff
A transect of multi-frequency, high resolution acoustic images calibrated by surficial samples across the New Jersey shelf (20 to 150 m water depths) resolves the shallow stratigraphic record of the Wisconsinan cycle of glaciation and global eustatic sea level change. Simrad (95 kHz) swath bathymetry and sidescan images, overlapping chirp sonar (1-15 kHz), 2D/3D Huntec (1-3.5 kHz) single channel seismic and 3.5 kHz echosounder surveys, and sediment samples comprise the transect data. The following observed stratigraphic features have been interpreted in light of the well-constrained pattern of shoreline migration, ca. 120 ka to Present: (1) During early and mid-Wisconsinan regression, a gravel-rich outwash plain covered the exposed shelf, and a marine unconformity, "R", formed on the sediment-starved middle and outer shelf. (2) About 35 ka, a muddy, marine outer shelf wedge was deposited above "R". (3) During the last glacial maximum (LGM), ca. 22 ka, the fluvial style on the exposed shelf changed from a high-energy outwash plain to a lower-energy system of N-S oriented, dendritic channels. (4) Two observed suites of iceberg keel marks suggest that icebergs grounded on the outer shelf during, or soon after, the LGM. (5) A transgressive ravinement surface above "Channels", which I have named "T", marks the migration of the shore-zone across the shelf during the latest Pleistocene and Holocene transgression, ca. 22 ka to Present. (6) The mid-shelf wedge, a prograded sediment lobe above "T", was deposited in shallow water ca. 11.5 to 8 ka, perhaps as a result of the collapse of glacial Lake Hackensack in northern New Jersey. The mid-shelf scarp, previously interpreted as a Holocene paleo-shore, is the depositional edge of the mid-shelf wedge. (7) Shelf currents have reworked the modern New Jersey seascape into a complex array of superimposed relict and active bedforms. Features older than the LGM have no seafloor expression on this shelf; there is poor correlation between shallowly buried stratigraphy and modern bathymetry.
Return to listXENOLITH MINERALOGY AND GEOLOGY OF THE PRAIRIE CREEK LAMPROITE PROVINCE, ARKANSAS
Dennis Patrick Dunn, Ph.D.
The University of Texas at Austin, 2002
Supervisor: Douglas Smith
Three new Arkansas lamproites were delineated and evaluated during the 1980's. The intrusions were emplaced during the Middle Cretaceous Period based on stratigraphic relationships and published isotopic ages. Geological relationships record a dominant crater facies lamproite with minimal vent erosion. A 260 tonne sample taken from the two larger lamproite vents recovered 5 macro diamonds (>0.5mm) yielding a sub-economic diamond grade of ~0.04 carats per 100 tonnes. A diamond evaluation program undertaken at the Prairie Creek vent by the Arkansas State Parks Commission in the 1990's resulted in mapping of four major rock types: epiclastic rocks, olivine lamproite, phlogopite-rich tuff and olivine-rich tuff. Significant diamond contents were found only within the phlogopite-rich tuff (~0.11 carat/100 t) and olivine-rich tuff (~1.1 carat/100 t). Stratigraphic relationships indicate that the diamondiferous tuffs have undergone <50 meters of erosion. Extrapolation of the surface rock units and their diamond contents to the pre-erosion surface suggests that ~93,000 carats of diamonds were liberated and then concentrated as a natural surface enrichment.
Mantle xenoliths recovered from the Black Lick and Twin Knobs lamproite vents were analyzed for major element compositions. Their data were used to calculate a pressure-temperature array that record maximum pressures of ~5 GPa and maximum temperatures of ~1000°C for the xenolith source region. Comparisons between calculated pressures and olivine compositions indicate relatively shallow fertile mantle overlying more depleted mantle lithosphere. The two layers of mantle lithosphere may represent different ages based on their olivine composition. Crustal xenoliths include near-surface sedimentary rocks and abundant amphibolite with granitoids and rare meta-sedimentary and meta-volcanic rocks. K-Ar dates of 1.48-1.31 Ga were obtained from four amphibolite xenoliths. The cooling ages confirm that continental crust of ~1.42 Ga in age extends beneath southwestern Arkansas and that thermal effects of the younger Grenville and Ouachita orogenies were insufficient to reset the amphibole K-Ar systems. Xenolith data and published results are used to test two models for development of the Ouachita system. The first model proposes that the Ouachita trough is part of a mid-continent failed rift; the second model suggests the Ouachita System was formed at the rifted oceanic margin of the continental craton. The xenolith data support the intra-craton rift model over the oceanic-margin model.
AN INTEGRATED EVALUATION OF A MATURE LOWER MIOCENE SILICICLASTIC GAS FIELD INCORPORATING 3-D SEISMIC, NORTHWESTERN GULF OF MEXICO SHELF
Christopher Lane Edwards, B.S.
The University of Texas at Austin, 2003
Supervisor: William Fisher
A mature gas field located in the northwestern Gulf of Mexico Basin's lower Miocene outer shelf, offshore of southern Texas, has produced from three main pay zones since 1984. The reservoirs consist of Siphonina davisi age, upward-coarsening and -thickening, moderately well-sorted, fine- to medium-grained distal deltaic sandstone packages. Hydrocarbon trapping occurs between 10800 and 12600 feet, within a growth fault's resultant rollover anticline. Porosity is relatively high, but a range of permeabilities exist due to pore-filling cementation, quartz overgrowths, pyrite and chlorite grain linings, pore-filling clay, and interbedded shales.
Based on an interpretation incorporating 3-D seismic data, the field has produced only 71.5 billion cubic feet of its 114 billion cubic feet of total recoverable gas during its 23-year history. Volumetric calculations based on the new seismic mapping suggest that two of the three main pay zones retain at least 2/3 of their original gas in place, and that another unproduced zone contains 21 BCF of untapped recoverable gas. In spite of such large calculated remaining reserves, production from 16 of the field's 18 perforations had fallen below economic rates (many without increased water production or a pressure decrease to the point of abandonment), and were consequently shut in.
The explanation for such poor recovery is threefold: (a) the character of the reservoir rock (consisting of low resistivity pay) was not well understood, resulting in (b) the abandonment of perforations as production declined, (c) without the application of modern technologies. With a better understanding of field dimensions and the application of newer technologies (such as gravel packing, fracturing, and pressure stimulation), cumulative production from this mature field could effectively triple. The evaluation of mature fields, such as is done here, is especially applicable in a time when the US petroleum industry demands more reserve growth from a considerably smaller workforce.
Return to listGEOTECHNICAL AND ECONOMIC CONSTRAINTS OF THE US STRATEGIC PETROLEUM
Benjamin A Eisterhold M.A.
The University of Texas at Austin, 2008
Supervisor: Michael E Webber
84 pages, 29 references
The Strategic Petroleum Reserve (SPR) is a stockpile of oil that is set aside by the United States government to protect the US economy from oil supply disruptions. The oil is held in salt caverns. The 4 different sites that are currently in use are Bryan Mound, Big Hill, West Hackberry, and Choctaw Bayou. A new site is currently under construction in Richton, MS. The purpose of this manuscript is to provide an introduction on how and why the SPR was created, discuss the history of the SPR and how it works, and to make sound policy recommendations with respect to certain geotechnical and economic limitations that should influence the decisions on when to empty and fill the SPR.
Return to listQUATERNARY SEISMIC STRATIGRAPHIC INVESTIGATIONS, YAKUTAT BAY REGION, GULF OF ALASKA: SUBGLACIAL DRAINAGE MECHANICS AND GLACIAL EXPANSE
Christopher Ryan Elmore, M.S. Geol. Sci.
The University of Texas at Austin, 2008
Supervisor: Sean P.S. Gulick
Quaternary tectonic and climate interactions have and continue to contribute to a substantial stratigraphic record that preserves the key regional events that result from the interplay of these dynamic processes. Near Yakutat Bay, Alaska, temperate glacial cover continuously manipulates and shapes the southern Alaskan margin and delivers substantial amounts of sediment annually. During cooler climatic periods, these glaciers, like most glaciers in Alaska, experience increases in volume and advance across the shelf. Using an integrated seismic data set, we investigate stratigraphic signatures buried along the shelf, between the present Yakutat and Alsek Sea Valleys, in order to understand the mechanics governing the regional glacial system during advance phases.
Our investigations reveal four glacial unconformities, of which, two show preserved retreat sediment packages. The events are indicative of ice advance phases during the Little Ice Age (LIA), the Last Glacial Maxima (LGM), and two pre-LGM events. The events show dominance by ice expanse from the Malaspina and Alsek River districts. The LGM event transgresses the shelf and concentrates erosion in highly overdeepened troughs, somewhat analogous to ice stream behavior. In addition, associated with these regional events, we find a suite of large-scale channel forms, exhibiting widths between 0.6-4.2 km and depths between 49-353 m. The channels are highly eroded vertically relative to lateral extent; widths to thickness ratios of the buried channel deposits are 15.7 on average. The channels show good correlation with the interpreted erosional events and other stratigraphic signatures indicative of ice occupation, i.e. moraines, grounding lines, etc. The channels exhibit distinct facies and facies assemblages of variable thicknesses and occurrence and show common morphologic characteristics with one another, i.e. cross-cuts. The facies and assemblages appear dominated by glacio-proximal, glacio-fluvial and glacio-lacustrine/marine physical processes. The characteristic morphology of the channels implies they are subglacially derived, and facies suggest subsequent fill by subaerial/submarine processes after ice retreat. Based on morphologic interpretations of the channels and their associations with glacial stratigraphy and erosional events, we interpret them as tunnel valleys. Expanding on their predictable relationship with specific glacial stratigraphy, we conclude that the tunnel valleys are indicators of retreat following glacial advance periods, when significant amounts of ice and melt are available to initiate and generate large tunnel valley deposits. As such, we conclude that the tunnel valleys are useful paleoclimate indicators.
Return to listINTEGRATED SEQUENCE STRATIGRAPHY, DEPOSITIONAL ENVIRONMENTS, DIAGENESIS, AND RESERVOIR CHARACTERIZATION OF THE COTTON VALLEY SANDSTONES (JURASSIC), EAST TEXAS BASIN, USA
Tarek Abu Serie Elshayeb, Ph.D.
The University of Texas at Austin, 2004
Supervisor: Earle F. McBride
The cumulative production of the Cotton Valley Sandstones in East Texas Basin as of 1993 was 2,666 BCFG with estimated reserves of 24.2TCFG. The Upper Jurassic-Lower Cretaceous Cotton Valley Sandstone is a thick siliciclastic unit in East Texas Basin. It is characterized by low porosity (<6%) and low permeability (<0.5 md) and requires hydraulic fracturing to yield its gas. This study concerns the sequence stratigraphy, depositional environments and diagenesis of the Cotton Valley Sandstones in order to gain an insight into the processes that influenced their reservoir quality. The research involved the use of 4000 feet of conventional core, 350 well logs, and petrographic and geochemical data on sandstones, shales and organic matter. Various analytical techniques were used, including optical petrography, SEM/EDS, CL, EMP, XRD, stable and radiogenic isotope geochemistry and fluid inclusions. Six major third-order depositional sequences were identified by correlating SP and resistivity logs on 60 wells. Each sequence has identifiable highstand, transgressive, and/or lowstand system tracts. Sandstones and shales of the transgressive system tract were deposited in strike-oriented barrier island complexes and associated shelf and slope environments. Deposits of the highstand system tracts were most probably deposited in wave-dominated deltaic settings. No cores are available in the lowstand system tracts. Twenty five facies (F1-F25) have been grouped into four main facies associations that represent fully marine (FA1), marginal marine (FA2 and FA3) and back-barrier coastal plain (FA4) environments of deposition.
Cotton Valley sandstones are very fine-to fine-grained, moderately-to well-sorted quartzarenite and subarkose. The Ouachita Mountains and Arbuckle Mountains located north and northwest of the study area are most probably the main sources of sediments for the Cotton Valley clastics. Sandstones experienced extensive diagenesis that altered the original detrital composition of the sandstone and modified initial porosity and permeability.
The paragenetic sequence of the Cotton Valley sandstones include: 1) clay coatings around the detrital grains (chlorite and illite); 2) early pore-filling pyrite; 3) calcite 1 precipitation; 4) quartz cement; 5) dissolution of feldspars; 6) potash-feldspar overgrowths; 7) kaolinite and chlorite cement; 8) oil emplacement; 9) illite; 10) calcite 2; 11) dolomite and anhydrite; and 12) late diagenetic pyrite.
There are two principal internal sources of silica in the Cotton Valley Sandstones, intergranular pressure solution and dissolution along stylolites. The average quartz cement in the Cotton Valley is 11% and the calculated amount of silica derived internally from both pressure solution and stylolites constitute up to 48% of the total quartz cement. The remaining 52% come from sources that could not be quantified. Other significant local sources of silica include dissolution of quartz grains at clay laminae and at contacts between sandstone and interbedded shales, and dissolution of micron-scale detrital quartz within interbedded shales and clay laminae. High content of clay laminae are observed in cores and shale interbeds constitute up to 35% of the total CV rock volume. These sources could not be quantified. A possible external source, a silica-rich fluid expelled from the Bossier shales.
Authigenic carbonates occur as pore-filling cement and replacive phase. Calcite cement is the dominant carbonate type in the Cotton Valley Sandstones. The source of Ca for authigenic calcite is the local dissolution and reprecipitation of the carbonate shell fragments in the sandstones and the oyster-rich lagoonal limestone beds interbedded with the sandstone. The limestone beds, which range in thickness from 35 ft (10.5 m) to 50 ft (15.5 m), served as a good sealing rock as well as a potential source for calcite cement. The Cotton Valley has an average of 20%-35% shales interbedded with the sandstones; these shales could have been a significant source for iron as well as carbonates and quartz cements to the sandstones.
Original porosity loss by compaction was much more than that lost by cementation in the bioturbated Ophiomorpha-dominated sandstones and crypto-bioturbated sandstones, whereas porosity loss by cementation was more significant in the
cleaner sandstones of the laminated, tidally influenced shoreface and sand flat.
The reservoir quality of the studied sandstones is controlled mainly by depositional environment and subsequent diagenesis. Depositional environment controlled the distribution of clay matrix and oyster fragments. Clay coatings in the Cotton Valley Sandstones had a direct influence on the precipitation of quartz cement. Clay coatings retarded quartz cementation and resulted in a better reservoir quality. It was also observed that shell fragments were the primary source of carbonate cements. So, depositional environment indirectly controlled the distribution of quartz cement and carbonates in various facies.
Three main parameters influenced reservoir quality: (1) the amount of clay matrix introduced by bioturbation, (2) the amount of quartz cement, and (3) the percentage of clay coating. Cotton Valley Sandstones are divided into three main reservoir types: (1) poor reservoir quality: quartz-cemented sandstones, (2) poor reservoir quality: bioturbated sandstones with more than 15% clay matrix; and (3) good reservoir quality: clean bioturbated sandstones with less than 10% clay matrix.
Return to listGEOMICROBIOLOGY OF SULFURIC ACID SPELEOGENESIS: MICROBIAL DIVERSITY, NUTRIENT CYCLING, AND CONTROLS ON CAVE FORMATION
Annette Summers Engel, Ph.D.
The University of Texas at Austin, 2004
Supervisor: Philip C. Bennett
375 pages, 324 references
Much of the terrestrial subsurface is inaccessible for study, but caves represent distinctive shallow subsurface habitats where biogeochemical processes can be easily examined. Previously defined speleogenesis models are almost entirely based on abiotic chemical and hydrologic controls, as biological controls on cave formation have not been considered significant. Hydrogen sulfide-rich groundwater discharges from springs into Lower Kane Cave, Wyoming, and the sulfuric acid speleogenesis model was introduced in the early 1970s as a cave enlargement process resulting primarily from hydrogen sulfide autoxidation to sulfuric acid and replacement of carbonate by gypsum on subaerially exposed surfaces. The reduced sulfur compounds serve as rich energy sources for microorganisms that colonize the cave in both subaqueous and subaerial environments. Several evolutionary lineages of the class “Epsilonproteobacteria” dominate the microbial diversity of subaqueous mats, and these microbes support the cave ecosystem through sulfur cycling and chemolithoautotrophic carbon fixation. The “Epsilonproteobacteria” occupy microbial mats in additional sulfidic cave and spring habitats, expanding the evolutionary and ecological diversity of these previously unknown organisms. The interior of the Lower Kane Cave microbial mats is devoid of oxygen and this provides habitat for anaerobic metabolic guilds, dominated by sulfate-reducing and fermenting bacteria. These anaerobic groups are responsible for autochthonous hydrogen sulfide and volatile organosulfur gas production. Cycling of carbon and sulfur compounds by the subaqueous microbial communities affects sulfuric acid speleogenesis. Compared to the total flux of sulfide into the cave, little hydrogen sulfide volatilizes into the cave atmosphere or oxidizes abiotically. Instead, the primary loss mechanism is from subaqueous microbial sulfur oxidation. Consequently, despite the cave waters being slightly supersaturated with respect to calcite, the “Epsilonproteobacteria” generate sulfuric acid as a byproduct of their metabolism, locally depress pH, and focus carbonate dissolution. The hydrogen sulfide that volatilizes into the cave air is oxidized at the cave walls where interactions between cave-wall biological and physicochemical factors influence subaerial speleogenesis and low temperature authigenic quartz precipitation. The recognition of the geomicrobiological contributions to subaqueous and subaerial carbonate dissolution fundamentally changes the model for sulfuric acid speleogenesis and the mechanisms for subsurface porosity development.
Return to listREGIONAL TECTONICS, SEQUENCE STRATIGRAPHY AND RESERVOIR PROPERTIES OF EOCENE CLASTIC SEDIMENTATION, MARACAIBO BASIN, VENEZUELA
Alejandro Escalona, Ph.D.
The University of Texas at Austin, 2003
Supervisors: William L. Fisher and Paul Mann
222 pages, 137 references, 1 CD-Rom, 41 plates
The Maracaibo basin of Venezuela is one of the most prolific hydrocarbon basins in the world. During the Paleogene, oblique collision between the Caribbean and South American plates produced a 4-km-thick wedge of clastic sediments, where over 40 billion barrels of hydrocarbons have been produced.
Previous studies in the Eocene interval are focused either at a large regional scale or a field-size reservoir scale. Integration between both scales of observation has not been previously done, and, as a consequence, the effect of regional tectonics is not considered in the small-scale stratigraphic record. The aim of this dissertation is to study the interplay of tectonic and stratigraphic variables that controlled the Eocene sedimentation in the Maracaibo basin, and to establish a geologic model that incorporates data from a regional to reservoir scale.
Interpretation of 2-D and 3-D seismic data in the central and eastern Maracaibo basin reveals two major tectonic features formed during Paleogene collision between the Caribbean and the South American plates: 1) a late Paleocene-early Eocene foreland basin; and 2) a middle-late Eocene lateral ramp fault. The lateral ramp fault forms a paleogeographic facies boundary separating a less faulted and folded shelf area to the west from a fold-thrust belt to the east. In the Eocene Maracaibo shelf area, intraplate deformation occurs by NNE- striking left-lateral faulting with pull-apart basins localized at fault stepovers. Three-dimensional seismic time slice interpretation of more than 2000 km² of 3-D seismic data allows mapping of the Icotea pull-apart basin. Extension of the Icotea pull-apart basin is localized on pre-existing NW-SE-striking normal faults, formed by Paleocene-Eocene plate flexure during the foreland basin period.
Detailed sequence stratigraphic interpretation of the central Maracaibo basin was carried out using 330 wells and 3-D visualization methods that combined well and 3-D seismic data techniques providing greater vertical and lateral resolution (pseudo-seismic). These data reveal that Eocene clastic sedimentation is controlled by tectonic subsidence and to a lesser degree by changes in sediment supply and eustasy. Hydrocarbon reservoirs of the central Maracaibo basin are concentrated in distributary channels and tidal sand bar facies on structural highs produced by strike-slip motion of N-NE-striking faults. Depositional environments and fluid content of Eocene reservoirs are inferred from cross sections based on closely spaced well logs.
Return to listUSING CHEMICAL DATA TO DEFINE FLOW SYSTEMS IN CUATRO CIENEGAS, COAHUILA, MEXICO
Shanna B. Evans, M.S. Geo. Sci.
The University of Texas at Austin, 2005
Supervisor: John M. Sharp
The Cuatro Cienegas basin in Coahuila, Mexico was declared a National Protected Area in 1994 by the Mexican government. Its principle uniqueness is in its aquatic fauna, which are dependent on the abundant springs in this desert valley. However, the basin's hydrogeology is not very well understood. The springs in the basin have extreme variability in temperature, salinity, water chemistry, and discharge over small spatial scales. By gathering physical and chemical data in the field and using standard computer methods, the extremely complex flow systems in this critically sensitive desert wetland have been delineated. Five distinct flow systems are defined in the basin by the chemical data. These include the Churince system in the southwest, the Garabatal-Becerra-Rio Mesquites system flowing from the southwest to northeast portion of the basin, the Tio Candido-Hundidos system in the center of the eastern side of the basin, the Santa Tecla system in the southeast, and the Anteojo system in the north. The physical characteristics of the basin, including spring and canal locations, have been examined and show variable mixing within each system and complex physical flow systems. The Churince, Rio Mesquites, and Tio Candidio systems have separate source springs and terminal playa lakes. The Tio Candido and Rio Mesquites systems are related through canal and possible karst flow.
Chemical processes involved in the evolution of Cuatro Cienegas waters have been evaluated f rom the analyses of 35 samples collected in April 2004, June 2004, and January 2005. Calcium and sulfate dominate the water chemistry. However, high alkalinities are encountered in pools located closest to the mountains, especially in the Anteojo system. The Anteojo and Santa Tecla systems have lower conductivity/TDS than other systems. The water chemistry for the basin has been integrated into a Geographic Information System (GIS) to examine the spatial variations in chemistry and temperature throughout the basin. Highly saline waters (31.4 mg/L Na to 810.7 mg/L Na) are encountered throughout the basin, but the highest salinities occur toward the end of the flow systems. Furthermore, the temperature of the water at various pozas in the basin may reflect the source of the water via regional and/or karst flow. The Cuatro Cienegas water chemistry is mainly evaporation controlled, but there are other important processes involved, including the dissolution and precipitation of various mineral phases. In order to understand the variability within each flow system, a reaction pathway and mixing geochemical model has been developed for each system using PHREEQC. These baseline data provide a framework for necessary future hydrologic studies in the basin.
PATTERN VARIABILITY IN NATURAL DUNE FIELDS
By
Ryan Cotter Ewing, M.S. Geo.Sci.
The University Of Texas-Austin, May 2004
Supervisor: Gary Kocurek
113 pages, 42 references
A comprehensive analysis of sand dune spacing, crest length, defect density and orientation is done in four dune fields in North America and Africa as part of a larger project investigating complexity and paleodynamics of natural dune fields. Measurements taken from Digital Orthophoto Quarter Quadrangles and Landsat ETM+ imagery of White Sands, Algodones, Agneitir, and Namib dune fields have been used in a cumulative distribution analysis. This statistical method allows for identification of different populations within the data and provides a means for comparing spacing, crest length and defect density within and between these dune fields. This method shows that in simple dune fields a single log-normally distributed population of dune spacing, crest length, and defect density occurs. In all dune fields that display compound and complex morphologies multiple log-normally distributed populations of dune spacing, crest length and defect densities occur. The trends observed in the statistical analysis show a decrease in variability as the dunes increase in spacing and crest length, and decrease in defect density. The trends of the dune crests also become more constrained to a narrower orientation as the spacing and crest length increase and defect density decreases. The trends, when used in conjunction with predictions from current models of dune formation and dating of sand dunes show a better organized pattern in the older and larger dunes supporting self-organizing computational models of dune formation. Multiple populations of dune spacing and defect density found in this study are interpreted to indicate the presence of different generations of sand dunes, created under discrete climatic events.
Return to listQUANTITATIVE SEISMIC GEOMORPHOLOGY OF GABUS AND BELANAK FIELDS, WEST NATUNA BASIN, INDONESIA
By
Mochammad Fachmi, M.S. Geo Sci
The University of Texas at Austin, 2003
Co-Supervisors: William L. Fisher and Lesli J. Wood
74 pages, 25 references
Fluvial and deltaic processes, architecture, and quantitative relationships between form, fill, and process are well understood from over a century of study in modern fluvial geomorphic systems. These historical understandings, based on modern systems study, are applied to analyze the morphology of similar, ancient depositional systems as imaged in three-dimensional seismic data FROM THE West Natuna Basin, Indonesia.
The principal objectives of this research are to 1) analyze the major architectural elements such as channel width (W), meander wavelength (Lm), radius of curvature (Rc), meander belt with (B), and sinuosity (Si), that comprise the Miocene- through Holocene-age fill in the West Natuna Basin, 2) examine the morphology and character of the fluvial architectures that form the primary reservoir units in the basin and collect morphometric data on those elements, and 3) analyze those data for their interrelationships, relate the architecture of the systems to well spacing and drainage radius, and develop models for predicting reservoir character and performance in the basin.
Proportional slicing reveals that the Miocene-Holocene sequence comprises fluvial systems, shore zone systems, and shelf systems. The fluvial systems include straight river, low-sinuosity river, high-sinuosity river, anastomosing river, and braided river. There is no consistency of the channel axis, the axis repeatedly changes from an east-west orientation into a north-south orientation within the interval. The shore zone system is represented by the appearance of prograding strandplain systems. The shelf systems were identified from a very flat and uniform amplitude map.
From the morphometric measurements of 69 channels, channel width (W) ranges from 45 to 2,174 m, meander belt width (B) ranges from 243 to 8,750 m, meander wavelength (Lm) ranges from 540 to 18,450 m, radius of curvature (Rc) ranges from 119 to 4,635 m and sinuosity (Si) ranges from 1.0 to 3.4.
Several predictive relationships can be concluded from examination of the morphometrics of the WNB fluvial systems that can aid in resolving uncertainty in subsurface reservoir character [original ending-ed.]
GEOPHYSICAL INVESTIGATIONS OF SUBGLACIAL LAKES VOSTOK AND CONCORDIA, EAST ANTARCTICA
Irina Filina, Ph.D
The University of Texas at Austin , 2007
Supervisor: Donald D. Blankenship
The subjects for this study are two subglacial lakes – Vostok and Concordia – located in East Antarctica . Lake Vostok is the largest known subglacial lake on Earth. Melting and freezing at the ice-water contact are known to occur in both lakes. These internal processes are important subjects for numerical modeling. The precise knowledge of the lake‘s bathymetry and the distribution of unconsolidated sediments at the bottom of the lake are required boundary conditions for such modeling. The ultimate goal of this research was to develop 3D bathymetry models and to establish the distribution of unconsolidated sediments for both lakes.
Joint interpretation of airborne gravity and seismic data was performed for Lake Vostok , revealing that the lake is hosted by consolidated sedimentary rocks. The modeling shows that Lake Vostok consists of two sub-basins: a larger, deeper one with water thickness exceeding 1000 m in the south and a shallower one with a water thickness of about 250 m in the north. The resulting 3D model has a substantially better correlation with seismic data than two previous models.
Lake Concordia appears to be significantly shallower with water thicknesses not exceeding 200 m for all possible host rock densities. Since the lake is relatively shallow, the sediment layer cannot be resolved. A similar pattern of freezing and melting was observed in Lake Concordia and Lake Vostok : the deeper part of the lake lies under thinner ice and is dominated by the freezing of water at the ice bottom, while in the shallower part of the lake the overlying thicker ice melts.
The analysis of seismic data in four different locations over Lake Vostok revealed the presence of unconsolidated sediments at the bottom of the lake. The sedimentary layer appears to be thicker (up to 400 m) in the northern basin, while its thickness does not exceed 300 m in the southern one.
Four different sedimentation mechanisms were considered to explain how such a thick sedimentary layer was deposited in Lake Vostok under glacial conditions. The estimates show that none of the mechanisms considered is capable of depositing the observed sedimentary layer, revealing the pre-glacial origin of Lake Vostok .
FLUVIAL-DELTAIC AND SHALLOW-MARINE SEISMIC GEOMORPHOLOGY OF THE WEST NATUNA BASIN:
IMPLICATIONS FOR RESERVOIR ARCHITECTURE AND BASIN EVOLUTION
Jed McKay Flint, MSGeoSci
The University of Texas at Austin , 2006
Supervisor: Lesli J. Wood
Quantitative seismic geomorphology and morphometric analysis of modern and paleo-landforms are powerful tools for extracting data about geologic history, processes, and fill architecture of terrestrial and marine basins. This study of the hydrocarbon-rich West Natuna Basin of Indonesia combines 3,154 km 2 of high-quality 3D seismic and analysis of 15 well logs to explain how 3D reservoir shape, size, and distribution evolve and are preserved within fluvial-deltaic and shallow-marine environments. The study also addresses how these characteristics vary in response to changes in tectonic regime and base level.
We made statistically robust, morphometric measurements of channel form and character using PC software, and where well logs have penetrated channels, we determined lithology and calculated sand percentages. The area has a variety of channel morphologies, ranging in sinuosity from 1 to 4.67 and varying in width from 30m to 4458m. Geometries range from single thread to wide, multithread. Crevasse splays and interdistributary creeks and lakes lie adjacent to channels. Sand percentages vary greatly between element types, and many multikilometer-scale elements show complex accretionary architectural composition.
The result of combining morphometric measurements with lithologic characterization is a predictive model that strengthens the relationship between channel form and sediment content. Architectural and sedimentologic changes have been assessed in the context of the basin's tectonic, climatic, and sea-level history. Results of this study apply not only to development of the study area, but to explaining how reservoir shape, size, and distribution evolve and are preserved within fluvial-deltaic and shallow-marine environments.
DEPOSITIONAL PERIODICITY AND THE HIERARCHY OF STRATIGRAPHIC FORCING IN THE TRIASSIC CARBONATES OF THE DOLOMITE ALPS, N. ITALY
Robert Murchison Forkner, PhD
The University of Texas at Austin, 2007
Supervisor: Mark Cloos
460 pages, 174 references, 2 color plates, 11 tables
The Dolomite Alps of northern Italy are a classic field locality in the development carbonate stratigraphic theory. Included in the many discoveries rooted in the geology of the Dolomites is the concept of a hierarchy of stratigraphic forcing in the Alpine Triassic. The hierarchy states that carbonate sedimentation is dominantly a record of eustasy, resulting in organized stacking patterns, and that these stacking patterns reflect the interplay between low frequency (1-10 my) eustatic cycles and their component bundled high-frequency (100 & 20 kyr) eustatic cycles.
The overall aim of this study is to further investigate the validity of the hierarchical model after recent dating of Anisian and Ladinian successions called the Milankovitchian periodicity and/or allocyclicity of the cyclic series into question. The study was completed using four sub-studies, 3 based on data collected in the field and a fourth based in cycle theory and computer modeling. First, it can be shown that allocyclic forcing exists in the Anisian Ladinian platforms of the Dolomites by comparing the stratigraphic sections measured from 2 time-equivalent, independent carbonate platforms, the Latemar and Mendola Pass. Second, computer modeling of Anisian Ladinian carbonate platform stratigraphy using Milankovitchain solar insolation as a proxy for high-frequency eustasy shows that both pure Milankovitch forcing and mixed Milankovitch/sub-Milankovitch forcing will produce synthetic carbonate platforms with stratigraphic successions comparable to those of the Anisian Ladinian platforms of the Dolomites. Third, it can be shown that the while the Norian Dolomia Principale (a regional carbonate shelf) was affected by differential subsidence, megacycles systematically increase in their number of component cycles from 2-3:1 in the eastern Dolomites (updip) to 5-6:1 in the western Dolomites (seaward). In conclusion, the concept that carbonate platform stratigraphy is a record of an interplay between eustasy, subsidence, and sedimentation is upheld, while the validity of Milankovitchian forcing acting on all Alpine carbonate cycles is questioned. Instead, cyclic carbonates with sub-Milankovitch periodicities were common in the early and mid-Triassic, while cycles with Milankovitchian periodicities were common in the late Triassic.
THE EVOLUTION OF A LOWER CRETACEOUS CARBONATE PLATFORM WITHIN A DIVERGENT MARGIN SETTING: THE CUPIDO FORMATION, NORTHEASTERN MEXICO
by
Tina Rayna Foster, M.S.Geo.Sci.
The University of Texas at Austin, 2003
Supervisor: Robert K. Goldhammer
226 pages, 127 references, 8 plates
The Lower Cretaceous Cupido Platform of the western Gulf of Mexico provides an example of passive margin platform development which is disrupted by the syndepositional movement of salt in the subsurface. This study focused on the recognition, formation, probable local and global controls, and the contribution of cyclicity to the development of carbonate platform architecture during a greenhouse period in a divergent margin setting. The Cupido Formation may provide an outcrop analogue of high resolution sequence stratigraphy for its subsurface equivalent, the Sligo Formation of Texas.
A depositional model encompassing nine lithologic subfacies was created for the platform interior of the Cupido Formation from the analysis of measured sections exposed near Monterrey, Mexico. The model consists of a shallow rimmed platform with a stable grainstone shoal acting as the shelf crest in place of the actual rudist dominated shelf margin. This model is further supported by the recognition of a shoal complex within the subsurface Sligo Formation located just above mobile evaporites which formed antecedent topography for the Mesozoic carbonate platforms.
A variety of peritidal and subtidal meter-scale cycles were identified based on their position within the platform interior. One-dimensional stacking pattern analysis, including facies proportions and thickness trends of the high frequency cycles, resulted in the recognition of depositional sequences. Depositional sequences with similar trends and system tract proportions aided in the development of a lower order supersequence. The Cupido Platform was accordingly divided into a highstand with progradational basinal turbidites, fore-slope, reef, and aggradational platform interior facies and a transgressive systems tract comprised of retrogradational platform interior facies.
Two-dimensional analysis of the measured sections was accomplished using thickness variations and the presence of extensive, shallow marine evaporites. Using the second order sequence boundary as a datum two additional depositional sequences were recognized in the landward portion of the platform. The deposition of these sequences occurred simultaneously with the deposition of the La Pena Shale as it drowned the shoal complex in the later stages of the lower order transgression. These retrogradational geometries in the Cupido Platform are supported by landward thickening in the transgressive portion of the Sligo Formation.
The most pervasive controlling factor throughout the formation of the Cupido Platform is its deposition during a long term greenhouse period. The greenhouse climate resulted in the retrogradational geometries of the shelf, in the absence of high frequency sequences, and the moderation of sealevel amplitude. The eustatic frequency of cyclicity is disturbed by the presence of large scale syndepositional faults between the shoal complex and shelf margin, which resulted in the dramatic increase of accommodation space available for sedimentation. The development of these growth faults is related to the movement of basin wide evaporites, as suggested by seismic interpretations of the Sligo Formation.
PHYSICAL MODELING OF EVAPORITE-DETACHED SALIENTS
by
Christine Ren é Zúñiga Fox , M. S. Geo. Sci.
The University of Texas at Austin, 2005
Supervisors: Randall A. Marrett and Bruno C. Vendeville
The use of scaled physical modeling of thin-skinned fold-and-thrust belts allows for the isolation of key parameters in the development of convex-toward-the-foreland folds and thrusts. Unlike salients formed in natural settings, scaled physical models can be monitored through time, with structures dissected and analyzed at the completion of the experiment. Conducting strain and displacement vector analysis of individual model stages yields quantitative insight into the kinematics of salient development.
I constructed twenty-one scaled physical models in order to study the processes leading to the formation of salients and extrapolate the models to natural examples. Thus, variation of model rheologies and boundary conditions test the influence of stratigraphic parameters on the formation of salients in thin-skinned fold-and-thrust deformation belts.
The physical models created for this study reveal that the pre-collisional foreland, lateral, and hinterland evaporitic décollement boundaries exert great influence on the degree of fold and thrust curvature across the salient, the presence of vertical-axis rotation during salient formation, and the geometry of the structural trend line patterns of folds and trusts within the salient. Brittle/viscous models showed two distinct patterns of fold curvature, local curvature near the décollement pinch-out, and distributed curvature above the evaporitic décollement, dependent on the lateral and hinterland boundaries of the décollement. Fold axial traces initiated with localized curvature above the lateral pinch-out, while structures maintained linear axial traces above the central décollement layer in models with lateral boundaries perpendicular to convergence direction of a linear indenter. However, I found that vertical-axis rotation of the folds and thrusts did occur. Conversely, fold axial traces initiated with distributed along strike curvature in models with a pre-collisional hinterland protruding décollement. Fold axial traces in viscous/viscous models initiated with localized or distributed along strike curvature that became more convex with progressive deformation. Vertical-axis rotation also occurred in these models.
EVOLUTION OF THE BRAIN IN THEROPODA (DINOSAURIA)
by
Jonathan William Franzosa, Ph.D.
The University of Texas at Austin, 2004
Supervisor: Timothy B. Rowe
357 pages, 155 references, 1 CD-ROM
The description of natural and artificial endocasts has become routine since the work of Tilly Edinger. Most workers write little more than simple descriptions of isolated specimens because endocasts are rare, and scattered throughout the world. Harry Jerison took the next step and broadly compared endocasts in detail, using them for evolutionary studies of brain size and intelligence. In those studies, Jerison concluded that dinosaurs (excluding birds) had the brain size expected for "reptiles" of their size. However insightful, many equations and assumptions Jerison used are now questioned. The goals of this study are to continue the comparative work of Edinger and Jerison, and to employ more modern techniques and information than was available when Jerison's studies were conducted, to either verify or overturn his hypotheses.
Most of this study was completed using the Department's High Resolution X-ray Computed Tomographic (CT) Scanner, which allowed digital endocasts to be made from intact braincases. This increased the number of taxa available for analyses to 18. These endocasts are described in detail, allowing 14 characters to be identified and scored. Several analyses are done, one using just the examined taxa and characters, and three in which the characters are added to existing data matrices. These analyses showed the 14 characters contain useful phylogenetic information, and one of the phylogenies had a polytomy resolved owing to the addition of the characters.
The endocast characters also show similarities in brain architecture between pterosaurs and birds, and indicate that an avian-style brain evolved gradually through the theropod lineage. The character transitions that show the gradual evolution in the cerebral hemispheres, optic lobes, floccular lobes, and other brain features in the sequence of studied endocasts form the basis for testing inferences about theropod biology, behavior, and intelligence. Trends concerning theropod sensory and prey catching ability are tested and supported, indicating that theropods became more dependent on sight, and less dependent on smell, over time, and developed better hand-eye coordination, agility, and balance for capturing smaller prey, whereas using Encephalization Quotients for intelligence determination is disputed. This hypothesis testing is one of the strengths of digital endocasts.
FACIES HETEROGENEITY, PLATFORM ARCHITECTURE AND FRACTURE PATTERNS OF THE DEVONIAN REEF COMPLEXES, CANNING BASIN, WESTERN AUSTRALIA
Edmund Locke Frost III, Ph.D.
The University of Texas at Austin, 2007
Supervisor: Dr. Charles Kerans
Carbonate facies patterns and stratal architecture are the product of the complex interaction of internal (e.g., reef-building biota, sediment production) and external drivers ( e.g., tectonics, climate, and relative sea level). In the Canning Basin of Western Australia, many of these drivers are in flux across the Late Devonian Frasnian–Famennian (F–F) boundary and significant variations in reef-building biota and long-term accommodation are observed. This study documents the evolution of the Canning Basin's Devonian reef complexes across the F–F boundary and proposes a new interpretation of the stratal architecture and paleobathymetric profile of the Famennian. Data presented here demonstrate the evolution of a shelf-crest system in the Famennian, with beds expanding basinward and the reef growing in water depths of approximately 5-15 m. The paleobathymetric profile of the Famennian described by this study represents a departure from the well-documented barrier-reef system of the Frasnian. Digital outcrop models help capture the heterogeneity of the Famennian system and allow for characterization of the Devonian reef complexes across the F–F boundary.
Syndepositional fractures are a ubiquitous feature of high-relief, reef-rimmed carbonate systems and these features exert a profound influence on many facets of platform evolution. This study documents strong variability in syndepositional fracture patterns as a function of lithofacies and depositional setting and evidence for the temporal evolution of the mechanical properties of the Devonian reef complexes is presented. A statistically significant relationship is documented between syndepositional fracture development and variations in stratigraphic architecture, approximated here by platform-margin trajectory. This relationship implies a significant stratigraphic control on syndepositional deformation in carbonate platforms and suggests that external drivers are not required to generate early fractures in high-relief carbonate platforms.
GEOCHRONOLOGY OF HEIMEFRONTFJELLA, EAST ANTARCTICA WITH IMPLICATIONS FOR GRENVILLIAN AND PAN-AFRICAN OROGENESIS
by
Roy Westin Fuller, M.S.
The University of Texas at Austin, 2003
Supervisor: James N. Connelly
104 pages, 50 references, 2 plates
Heimefrontfjella, a mountain range in East Antarctica, records a complex, multi-phase geologic history that previous workers have suggested contains evidence of both ca. 1100 Ma (Grenvillian) and ca. 500 Ma (Pan-African) aged tectonic events. This study characterizes and further constrains these events by utilizing field and petrographic observations to produce a relative time framework. This framework is constrained by twelve new U-Pb zircon, U-Pb monazite, U-Pb titanite, and K-Ar amphibole dates.
Grenvillian igneous intrusions comprise orthogneissic rocks (I 1 ), dated at 1102+5/-3 Ma and foliated megacrystic granite to diorite (I 2 ), dated at 1099+13/-6 Ma, 1097+6/-4 Ma, and 1086+6/-4 Ma. Grenvillian metamorphism and deformation occurred in two episodes. The first phase was intense migmatization of I 1 orthogneissic rocks (D 1 /M 1 ), constrained, based on field observations, to have intruded during an interval between I 1 and I 2 intrusion. A second phase of deformation and metamorphism (D 2 /M 2 ) is dated between 1064 to 1010 Ma and occurred at granulite-grade conditions, as indicated by orthopyroxene in granitic rocks. D2/M2 is directly constrained by metamorphic zircon that crystallized at ca. 1064 Ma, and zircon and monazite overgrowths that range in age from 1010 to 1020+/-25 Ma.
Voluminous mafic dikes (I 3 ), for which absolute ages were not obtained, are inferred to postdate, based on petrography and field observations, the above-described Grenvillian tectonic features, but predate Pan-African deformation.
Deformation and metamorphism during the Pan-African period (D 3 /M 3 ) produced penetrative fabrics and discrete shear zones at amphibolite grade metamorphic conditions. A range-wide right-lateral shear zone, the Heimefront Shear Zone, developed at this time. The age of Pan-African deformation and metamorphism is directly constrained by monazite dates of 503+/-1 Ma, 501+/-1 Ma, and 500+/-2 Ma, amphibole dates of 544+/-22 Ma and 528+/-22 Ma, and a titanite date of 515+/-6 Ma.
The above framework is permissive of the theory presented by previous workers that Heimefrontfjella was contiguous with the Namaqua-Natal orogenic belt in Africa during Grenvillian TimesNewRoman, associated with Rodinia convergence. The ca. 500 Ma ages together with field and petrographic observations, confirm that Heimefrontfjella was part of a Pan-African orogenic belt associated with convergence of Gondwanaland, either as an extension of the Mozambique Belt in Africa that resulted from collision of the Indo-Antarctic plate with West Gondwana, or as a later orogenic belt associated with collision of the East Antarctic Plate with West Gondwana.
MODELING REFLUX DOLOMITIZATION
by
Shawn M. Fullmer, M.S.
The University of Texas at Austin, 2005
Supervisor: F.J. Lucia
The process of reflux dolomitization is commonly invoked to explain dolostone occurrence and distribution. The time required for this process to take place is not well understood. Volumetric flow rates are the dominant control on the time requirement. Published platform scale numerical models of reflux require unrealistically high permeability values and reaction efficiencies to explain the mapped dolomite volume. Approaching the problem at the high-frequency-cycle scale allows tighter constraints on the variables and a simpler geologic setting to investigate the permeability and reaction efficiencies required to form a dolostone unit.
The high-frequency cycle used as a model in this study is located near the top of the Upper Glen Rose Formation. It is an upward shallowing, Albian age cycle that consists of a subtidal mud-dominated packstone overlain by an evaporitic tidal-flat cap. The dolostone extends 1.5 meters down from the cycle top. Permeability values reflecting conditions at the onset of dolomitization were reconstructed by, 1) obtaining current porosity and permeability values from outcrop samples, 2) approximating porosity loss due to burial diagenesis, 3) interpreting pre-dolomitization rock fabrics from thin sections, and 4) calculating permeability using the uncompacted porosity and rock-fabric information. The data was entered into a variable density flow model and the results suggest that the time required to form this 1.5 m dolostone unit is between 300 and 1,000 years using a harmonic mean permeability of 260 md (2.6 x 10 -13 m 2) and dolomite reaction efficiencies between 10-40%. These more realistic permeability and efficiency values are much lower than the 100 – 10,000 Darcy’s (10 -11 – 10 -9 m 2) permeability and 100% reaction efficiency values used in current published reflux models.
CENOZOIC TECTONICS OF THE NICARAGUAN DEPRESSION, NICARAGUA, AND MEDIAN TROUGH, EL SALVADOR, BASED ON SEISMIC REFLECTION PROFILING AND REMOTE SENSING DATA
Justin O'Leary Funk, M.S. Geo. Sci.
The University of Texas at Austin , 2007
Supervisors: Drs. Paul Mann and Kirk McIntosh
Lakes Nicaragua and Managua are the two largest lakes in Central America and cover a combined area of ~9000 km 2 of the presently active Central America volcanic front (CAVF). As part of the Subduction Factory focus area of the U.S. National Science “Margins” program, we acquired ~1925 km of shallow geophysical data over Lakes Nicaragua and Managua in May, 2006, to establish their late Quaternary structural and stratigraphic history and to better constrain regional models for active tectonics in western Nicaragua and El Salvador. In order to investigate regional, upper crustal deformation resulting from forearc sliver transport and/or slab rollback of the Cocos plate, I took a multidisciplinary approach by integrating these new data with: relocated earthquake epicenters; earthquake focal mechanisms; high-resolution digital topography from the NASA Shuttle Radar Topography Mission (SRTM); published global positioning system (GPS) vectors showing recent plate motions; onland geologic maps showing bedrock geology; previous maps of lake bathymetry and bottom sediment types; a previously unpublished, regional aeromagnetic data set; and multichannel seismic reflection profiles from the Gulf of Fonseca and Pacific Ocean. Both lakes and the Gulf of Fonseca occupy the Nicaraguan depression, which is an elongate, asymmetrical, 40-100-km-wide depression extending 342 km across the length of Nicaragua , across the 57-km-wide Gulf of Fonseca, and 225 km into the neighboring country of El Salvador where it is called the Median trough. Profiles across the Nicaraguan depression using subbottom profiles and high-resolution SRTM topographic data indicate that the basin structure is a highly asymmetrical half-graben bounded to the southwest by northeast-dipping faults with basement shallowing to the northeast. Depth to basement in the lakes area is uncertain due to the lack of deep-penetration multichannel seismic data and exploration wells. Depth to basement in the Gulf of Fonseca is also not known but does contain an overlying asymmetrical wedge of low-velocity sediments based on multichannel seismic velocity models.
New multichannel seismic, subbottom profiler, and bottom core data from the 2006 NicLakes study have improved bathymetric, bottom sediment, and recent fault maps for both lakes that can be compared to previous, speculative models for the lake's geology and tectonics. Previous work using only subaerial observations from the Nicaraguan depression - Median trough have proposed three different models for the formation of the depression and its active, seismogenic transverse faults: 1) Neogene normal faulting parallel to the length of the depression induced by slab rollback of the underlying subducted slab of the Cocos plate beneath Nicaragua and El Salvador; the zone of extension is proposed to have migrated from the northeast to the southwest from Miocene to the present; 2) Neogene to present right-lateral strike-slip faulting parallel to the volcanic front and offset locally by pull-apart basins, or centers of fault-bounded extension 3) Negoene to present right-lateral strike-slip faulting parallel to the volcanic front that is accommodated by left-lateral transverse or “bookshelf faults” at high angle to the volcanic front. The 2006 data set combined with the many preexisting data sets listed above have been integrated into three regional structural cross sections. These sections show the asymmetrical half-graben structure for the Nicaraguan depression-Median trough. The area of greatest subsidence and footwall uplift is in the Lake Nicaragua area; in the southeast; the area of least subsidence/footwall uplift is in the Gulf of Fonseca in the northwest. I interpret this structural pattern as a time-transgressive basin opening with the oldest extension pre-Miocene) starting in the southeast and migrating to the northwest. GPS data indicates that this earlier phase of intra-arc rifting is now being modified by arc parallel shear related to the northwestward transport of the Central American forearc sliver.
CALC-SILICATE ALTERATION AND CU-AU MINERALIZATION OF THE DEEP MLZ SKARN, ERTSBERG DISTRICT, PAPUA, INDONESIA
Laurel Michelle Gandler, M. S.
The University of Texas at Austin , 2006
Supervisor: J. Richard Kyle
The Deep Mill Level Zone Cu-Au skarn deposit is developed in an Upper Cretaceous to Lower Tertiary siliciclastic and carbonate succession adjacent to the 3-Ma Ertsberg pluton in the Central Range of New Guinea. The DMLZ is the deepest explored part of the Ertsberg East Skarn System that consists of the surface Gunung Bijih Timor (GBT) deposit at 4200 m elevation, through the Intermediate Ore Zone (IOZ), Deep Ore Zone (DOZ), Mill Level Zone (MLZ) and Deep Mill Level Zone (DMLZ) at 2900 to 2600 m. The DMLZ is hosted by mixed assemblages of siliciclastic and dolomitized carbonate strata that produce Mg-rich skarn assemblages. Alteration can be divided into types based on dominant lithology as 1) forsterite-diopside skarn, 2) anhydrite skarn, 3) massive sulfide, 4) magnetite skarn, 5) hornfels, 6) marble and 7) endoskarn within the Ertsberg Diorite.
Samples distributed over 300 m of vertical depth were examined petrographically and 23 were selected for chemical studies. Forsterite is the dominant olivine phase that ranges from Fo 89-100 Fa 0-3 Te 0 from 11 grains . Forsterite is commonly altered to serpentine and is associated with magnetite, anhydrite and phlogopite. Magnetite skarn contains olivine that is more Fe-rich. Clinopyroxene compositions range from Di 73-99 Hd 1-26 Jo 0-1 from 18 grains. Diopside is locally altered to tremolite and is associated with anhydrite, talc, and carbonate minerals. Garnet compositions range from Ad 43-99 Gr 0-54 Py 0-5 from 13 mineral grains. Garnet in hornfels shows compositional zoning due varying Fe and Al concentrations. Periclase after dolomite is present locally. Secondary Fe- and Mg-rich carbonate phases are common as disseminated crystals or rims on earlier carbonate crystals.
Cu-Au mineralization consists of bornite and chalcopyrite predominantly within magnetite and anhydrite skarn; pyrite and pyrrhotite are locally present. The dominant prograde skarn assemblages are controlled by protolith composition, notably the relative abundance of quartz, dolomite and calcite within the host Waripi Formation. Isochemical metamorphism of mixed assemblages of quartz and dolomite produce the greatest change in mineralogical composition and decrease in mineral volume, forming forsterite-diopside dominant skarn assemblages. Magnetite is commonly associated with forsterite-diopside mineralization and is associated with Cu-Au mineralization. Structural features created weakness zones that created fluid pathways responsible for the formation of the Ertsberg East mineralizing system. Magnetite skarn was introduced during the metasomatic fluid phase, and these fluids were also responsible for the emplacement of Cu and Au. Economic Cu and Au mineralization was locally controlled by host lithology.
UPPER MANTLE SEISMIC STRUCTURE BENEATH THE CENTRAL RIO GRANDE RIFT AND BENEATH EASTERN MEXICO AND THEIR IMPLICATIONS
Wei Gao, Ph.D.
The University of Texas at Austin , 2006
Supervisor: Stephen P. Grand
I present new images of the P- and S-wave seismic structure of the upper mantle beneath the central Rio Grande rift and surroundings and beneath eastern Mexico . The upper mantle structure is determined using traveltime delays and seismic waveforms.
The P- and S-wave tomography beneath the rift is derived through the LSQR algorithm. Tomography shows large seismic anomalies (up to 5% and 8% variation in P- and S-wave respectively) in the shallow mantle. Beneath the rift and Mount Taylor lies the seismically slow shallow mantle that primarily caused by higher temperature with possibly a small amount of partial melt. The mantle deeper than 150-km beneath the rift is not anomalously slow. A fast anomaly near 500-km depth beneath the central Colorado Plateau is probably the trailing edge of the Farallon slab. A slow seismic anomaly to the east of that anomaly at 400-km depth appears to connect to the slow shallow anomaly beneath the Mount Taylor . Beneath the Great Plains a narrow fast structure is imaged throughout the upper mantle from 200 to 600-km depth. They may form small-scale convection beneath the central rift and surrounding region.
The P and S velocity models beneath eastern Mexico are derived through waveform inversion of triplicated P and S waves using a conjugate gradient algorithm. The optimal models have discontinuities of 6.2% and 7.3% at 410-km depth and 3.3% and 6.3% at 660-km depth for P and S respectively. A common feature of the models is a low velocity zone above the 410-km discontinuity, which may be due to partial melt induced by water release from the transition zone. The overall jump in velocity at 410-km is also larger than in previously published models. Another feature is that P-wave data require a small discontinuity at 490-km depth and S-wave data require an additional discontinuity at 600-km depth. This may be a thermal anomaly due to a flat lying slab or might reflect a phase change in the transition zone.
Also, I present a new technique for regional-scale joint inversion of body and surface waves and show its strengths using synthetic and real datasets.
Return to listIMPACT OF FLUID EVOLUTION ON GARNET GROWTH IN PELITIC ROCKS FROM HARPSWELL NECK, MAINE
Joshua Garber, B.S.
The University of Texas at Austin, 2008
Supervisor: William Carlson
Complex and varying compositional zoning patterns in garnets in pelitic schists from Harpswell Neck, Maine have been attributed by various authors to equilibrium growth and coalescence of separately nucleating neighboring crystals, to overprint zoning modulated by prograde thermal effects, and to overprint zoning during nearly isothermal growth in the presence of fluids of variable composition. To help resolve this controversy, detailed field observations of the relationships among the structure and timing of multiple vein generations and garnet growth were combined with microchemical analysis of fluid inclusions in vein quartz and garnet.
Regional structures record shortening in NNE-trending tight folds and subsequent NNE shearing along kilometer-scale strike-slip faults. Three generations of quartz veins were discriminated in the field on the basis of their relationship to these structural features and cross-cutting relationships. Garnet growth is interpreted to have begun during the emplacement of the first generation of veins and to have continued during the emplacement of the second. Fluid inclusions in garnet are too rare and small for accurate analysis, but fluid inclusions in quartz veins related to garnet growth revealed two separately preserved fluids, one H2O-rich and one CO2-rich.
In the regional structural context, patterns of mineral inclusions in garnet reveal that nucleation of garnet porphyroblasts occurred during folding, with growth and some continued nucleation continuing during subsequent strike-slip shearing. Nucleation and early garnet growth is interpreted as synchronous with a dense CO2-rich population of fluid inclusions, whereas later garnet growth appears to have been related to the H2O-rich fluid trapped in later inclusions. Thus, the anomalous zoning patterns seen in Harpswell Neck garnets may be attributed to a distinct change in metamorphic fluid composition and elemental solubility. In this scenario, the early CO2-rich fluid enhances solubility for elements like Ca and Y but inhibits the diffusion of Mg, Mn, and Fe, whereas the late H2O-rich fluid is responsible for equilibrating all of these elements at the garnet rims.
Return to listURBAN EFFECTS ON GROUNDWATER RECHARGE IN AUSTIN, TEXAS
by
Beatriz Garcia-Fresca, MSGeoSci
The University of Texas at Austin, 2004
Supervisor: John M. Sharp, Jr.
175 pages, 157 references, 1 plate
Cities and urban populations are growing at a high pace, but groundwater remains an underutilized resource in most urban areas. The general impacts of urban development on groundwater include overexploitation; subsidence; decreasing quality; salt-water intrusion; disruption of ecosystems; variations in the local climate; properties of the soil; natural drainage network; and the quantity, quality, and location of both recharge and discharge. The shallow urban underground is an intricate network of tunnels, conduits, utilities, and other buried structures comparable to a natural karstic system, except that "urban karst" is generated much faster. Urbanization also introduces new sources of water, resulting in an increase of groundwater recharge. These sources include irrigation of parks and lawns, leakage from water mains and sewers, and infiltration structures.
The areal extent of Austin, Texas, has grown steadily since 1885 but has increased five-fold since the 1960's. The difference between the amount of tap water treated in the City of Austin and the amount of sewage that arrives in the wastewater treatment plants (or excess urban water), represents the amount of urban water potentially available for recharge. A water balance shows that about 7% of the treated drinking water is estimated to be lost to leaks from the distribution network and 5% to leaks from sewers. The rest of the excess urban water is used in irrigation of parks and lawns, some of which will be evapotranspired and some will turn into recharge. Smaller fractions are recharged in septic tanks and other designed infiltration devices. Direct recharge from rainfall has decreased as a result of the introduction and expansion of impervious pavements, from 53 mm/a under preurban conditions to 31 mm/a in the year 2000. However urban sources of recharge contribute an average of 85 mm/a of excess urban water, resulting on an urban recharge of 63 mm/a, and a total recharge rate that could equal 94 mm/a.
Several hydrogeochemical parameters were tested as tracers of urban recharge in Austin. Chlorination by-products (trihalomethanes) were found in high concentrations in tap water and in low concentrations in wastewater. However, they were not detected in either surface water courses or groundwater. δ 15N is a commonly used indicator of leakage from sewers, but unusually low values were obtained. Finally, 87Sr/ 86Sr of dissolved strontium shows a strong trend that can be related to the degree of urbanization over the Barton Springs segment of the Edwards aquifer. Values of this ratio from the lesser urbanized wells indicate groundwaters close to equilibrium with the limestone, while samples from the more urbanized wells show higher values, which are closer to those of tap water.
Return to listSTRATIGRATIGRAPHIC ARCHITECTURE AND BASIN FILL EVOLUTION OF A PLATE MARGIN BASIN, EASTERN OFFSHORE TRINIDAD AND VENEZUELA
E milio José Garciacaro, M.S. Geo Sci.
The University of Texas at Austin , May 2006
Supervisors: Paul Mann and Lesli Wood
Eastward migration of the Caribbean plate relative to the South American plate is recorded by a 1100-km-long foreland basin which is oldest in the west (Maracaibo basin, 65-55 Ma) and youngest in the east (Columbus basin, eastern offshore Trinidad, 15-0 Ma). Regional transpression has caused lithospheric loading and flexure along the northern margin of South America creating a large foreland basin area which propagated from west to east as the Caribbean plate moved eastward relative to the South American plate. I have integrated 775 km of deep-penetration 2D seismic lines acquired by the 2004 BOLIVAR survey, 325 km of 1975 GULFREX seismic data, 8,000 km2 of industry 3-D seismic data, and published industry well data from offshore eastern Trinidad .
Interpretation of seismic sections tied to wells reveals the following fault chronology: 1) middle Miocene thrusting along the Darien ridge related to highly oblique convergence between the Caribbean plate and the passive margin of northern South America; continuing thrusting and transpression in an oblique foreland basin setting through the early Pleistocene; 2) early Pliocene-recent low-angle normal faults along the top of the Cretaceous passive margin; these faults were triggered by oversteepening related to formation of the downdip, structurally and bathymetrically deeper, and more seaward Columbus basin; large transfer faults with dominantly strike-slip displacements connect gravity-driven normal faults that cluster near the modern shelf-slope break and trend in the downslope direction; to the south no normal faults are present because the top Cretaceous horizon has not been oversteepened as it is adjacent to the foreland basin; 3) early Pliocene-Recent strike-slip faults parallel to the trend of the Darien ridge and accommodate present-day plate motions.
Active mud diapirism in the Columbus basin is widespread and is related to overthrusting and loading of upper Miocene-lower Pliocene age mud. Analysis of the 3-D seismic data reveals the presence of extensive gravity-flow depositional elements on the Columbus basin deepwater area, characterized by mass-transport deposits at the base, turbidite frontal-splay deposits, leveed-channel deposits, and capped by fine-grained condensed-section deposits. Deep basin wells drilled in recent years have proven that turbidites were transported into the Columbus basin deepwater during the Plio-Pleistocene. Analysis of these well results suggest that a deeper oil charge is present within the Columbus basin deepwater area. The primary uncertainty for this variable hydrocarbon system is whether fault or diapiric pathways connect the petroleum charge at depth with shallower reservoir rocks.
GEOCHEMICAL EVOLUTION OF GROUND WATER IN THE BARTON SPRINGS SEGMENT OF THE EDWARDS AQUIFER
Bradley Dean Garner, M.S. Geo. Sci.
The University of Texas at Austin, 2005
SUPERVISOR: Jay L. Banner
The water quality in a karst (limestone) aquifer changes over time, making the application of traditional hydrogeologic principles difficult or impossible. This research’s goal was to advance the understanding of the Barton Springs segment of the Edwards aquifer within and around Austin, Texas. This was accomplished by analyzing time - series water - quality data from long, medium, and short time scales. Analysis provided insights into direction of ground - water flow, sources of spring discharge, and mixing of geochemically distinct waters in the aquifer. The results of this research are of interest because of the aquifers role as a drinking water supply, its role as a habitat for the endangered Barton Springs salamander ( Eurycea sosorum ), and for its central role in creating the popular Barton Springs Pool.
Twenty - six years of water - quality data were compared against contemporaneous streamflow and spring discharge rates to evaluate ground - water connection to surface - water processes. Fifteen of 26 wells in this dataset showed a correlation between these measurements. Ion ratios of Mg/Ca, SO 4 /Cl, and Na/Ca showed that active ground - water processes included dilution by recently - recharged surface water, inconguent dissolution, and mixing with water from a saline zone and an underlying aquifer. Four wells were shown to intersect major flowpaths, and five wells were shown to intersect minor flowpaths.
Major ion and Sr isotope data collected over two years from four karst springs ( Main, Eliza, Old Mill, and Upper Barton Springs) provided insight into water flow in the aquifer. Main and Eliza were fed by ground water from the same flowpath(s) in the aquifer, as their geochemical compositions were indistinguishable. Old Mill received 4–9 percent of its water from a saline zone, as shown by elevated ion concentrations and a quantitative mixing model. Upper Spring obtained some of its water from an isolated subbasin in the aquifer, as indicated by radiogenic 87 Sr/ 86 Sr values measured in this subbasin. Oxygen and hydrogen isotope values indicated that ground water was well - mixed over year or longer timescales.
Oxygen isotope samples collected from the springs following a rainfall event showed how stormflow recharge flows to the springs. A hydrograph separation using showed an immediate increase in spring discharge following rainfall but a 12 - hour delay before storm water reached the spring. This suggested an advancing front of storm water that expelled pre - storm water from the karst conduits. Discharge of pre - storm ground water was reduced by up to 44 percent after rainfall, suggesting that stormflow pressurized the karst conduit system and reduced gradients between the aquifer matrix and conduits. Specific conductance was also an effective and inexpensive tracer of stormflow, on the basis of its strong correlation (r 2 =0.96) to oxygen isotope values. Resource managers and scientists may be interested in these findings, as the potential for contamination of this spring system is increased after large rainfall events.
Return to listCHARACTERIZATION OF TRANSPORT PROPERTIES IN GRANITIC ROCK FRACTURES WITH SKINS
Terence Travis Garner, PhD
The University of Texas at Austin, 2007
Supervisor: John M. Sharp, Jr.
191 pages, 149 references, 11 tables
Hydraulic properties of fracture skins in granitic rocks from three climatically different field sites show that fracture skins can increase permeability, compared to unaltered granite, through microfractures or weathering rinds, or decrease it through the effects of mineral nucleation or precipitation and growth of biological organisms. In comparable granitic crystalline rocks, fracture skins formed in more arid climates have a higher porosity than fracture skins formed in more humid climates. Granite fractures have been collected from three field sites Elberton, Georgia, Fredericksburg, Texas, and Eyre Peninsula, South Australia. Fracture skins in Elberton are predominantly surface coatings of dust films, clay infillings, organic growths, and infillings of iron precipitates. Town Mountain Granite skins are dominated by iron banding, weathering rinds, and surface coatings of pyrolusite. The Caica Granites from the Eyre Peninsula have similar fracture skins to the Town Mountain Granite, but skin thicknesses are greater. Modeling studies with fracture skins demonstrate that the most important hydraulic properties are skin porosity, diffusion coefficients, and retardation. Through combined laboratory analyses and field investigations, the variations in transport properties of rock matrix and fracture alteration zones are documented. Transport properties of fracture skins in granitic rocks with surface coatings enhance fracture transport and alteration zones of more porous skins attenuate transport. A new laboratory method is employed to measure diffusion coefficients using laser ablation ICP-MS. Diffusive transport is shown to be preferential to grain boundaries and zones of mineral cleavage planes. Tracer tests show channeling to be a significant factor in breakthrough results on the field scale. Channels on the field scale are imaged using high frequency GPR providing an image of preferential flow paths in a fracture.
Return to list3-D SEISMIC EVALUATION OF FAULT CONTROL ON QUATERNARY SUBSIDENCE PATTERNS, RATES, AND RELATED SURFACE MORPHOLOGY IN SOUTHEASTERN LOUISIANA
Terra Jane George, MS. GeoSci.
The University of Texas at Austin, 2008
Supervisor: David Mohrig
168 pages, 69 references, 11 tables
Throughout the past century, the Louisiana coastline has been deteriorating at an alarming rate. Fault induced subsidence has been suggested to be one of the likely drivers of this land loss. I directly addressed the control of faulting on Quaternary delta subsidence using subsurface architectural data.
I carried out a systematic process for evaluating fault movement on individual faults in the subsurface by using an industry grade 3-D seismic survey to evaluate the interaction of complex fault patterns and shallow substrate (< 1s or 1,000m). Using amplitude data, as well as a coherency algorithm, horizons and faults were mapped in ~1400km2 of data in southeastern Louisiana. Measurements of fault displacement were obtained by determining time offset of synchronous horizons. Down-to-the-basin and counter-regional growth faulting was observed. Both styles of faulting are associated with movement of Jurassic-age salt deposits and display measurable offset at depths as shallow as ~.450s (roughly 380m). Fault displacements increase roughly linearly with burial depth. The displacement data together with well-log data provide a tentative Quaternary rate for the normal faulting of 0.1mm• yr-1 to 1mm• yr-1.
I have assessed the possible control of subsurface faulting on surface topography using co-registered maps of fault traces and orthophoto quadrangles. This analysis has revealed a number of instances where faulting affects modern surface morphology. Often, the up-thrown sides of growth faults are capped by marshland, whereas the downthrown blocks are drowned by ~.7m of surface water (Snedden et al., 2007). Further, the influence of faulting extends to the modern planform of the lower Mississippi River, as data indicate that growth faults control the position of a major bend near the river’s outlet.
Return to listLATE CENOZOIC SEDIMENTATION AND TECTONICS OF THE BADA, DANDERO-MAHABLE AND ABDUR AREAS, NORTHERN DANAKIL DEPRESSION, ERITREA, AFRICA
Berhane Negassi Ghebretensae, MS
The University of Texas at Austin, 2002
SUPERVISOR: Richard T. Buffler
161 pages, 72 refs, 0 plates
Three sub-basins (Bada, Dandero-Mahable and Abdur) in the northern Danakil Depression of Eritrea were the focus of sedimentologic, stratigraphic and structural investigations. In the Bada area two mainly siliciclastic deposits informally called Ramude and Bada sequences were identified. The Miocene-Pliocene (?) Ramude sequence consists of conglomerates, lithic feldspathic arenites, siltstones and mudstones with a characteristic red color. The Ramude sequence was deposited in alluvial fan and more distal floodplain/overbank and lacustrine environments.
The overlying Pleistocene ? Bada sequence consists of conglomerates, sandstones, siltstones and mudstones plus minor bioclastic carbonates consisting of gastropods, ostracods and foraminifers. Environments of deposition of the Bada sequence range from fan delta to brackish lacustrine. Faults cutting basic sills and making contact between the two sequences indicate subsidence.
Sediments in the Dandero-Mahable area consist of conglomerate-sandstone, sandstone, and mudstone-sandstone lithofacies. They were deposited in alluvial fan, braided stream and ephemeral lacustrine environments. They are characterized by "couplets" of sandstone with mudstone-siltstone, which also alternate with gypsum. Gastropods and ostracods occur as fine calcareous beds.
In the Abdur area, the lithostratigraphy consists of the Buri sequence, the Abdur Volcanics, and the Abdur Reef Limestone. The Buri sequence consists of a siliciclastic-carbonate mixture of mudstones, limestones, sandstones, and conglomerates and in places ash and diatomite. They were deposited in fluvial, lacustrine and shallow marine environments. The age of the Buri sequence, which has been deformed and faulted, ranges from 0.90 Ma to 0.72 Ma. The Abdur Reef Limestone overlies unconformably the Buri Sequence and consists of a limestone complex of biostroms, bioherms and coral reef terraces. It has been dated at 0.125 Ma +/- 0.007 and was deposited during the latest Pleistocene transgression of the sea during the last interglacial (isotope stage 5e). Two samples of the Abdur volcanics were dated as 0.44 and 1.27 Ma, respectively.
Return to listSPATIAL ARRANGEMENT OF DEFORMATION BANDS AND PROCESSES OF FORMATION WITHIN POROUS SANDSTONE: ARCHES NATIONAL PARK, UTAH
Timothy David Gibbons, MSGeoSci
The University of Texas at Austin , 2006
Supervisor: Randall Marrett
Individual deformation bands within porous sandstones reduce permeability and form barriers to fluid flow. A network of deformation bands can destroy reservoir connectivity and impose compartmentalization. Consequently, the spatial arrangement of deformation bands affects fluid flow within sandstone reservoirs, but it is unknown whether deformation bands occur at random locations or whether they follow systematic patterns.
This study, undertaken at Arches National Park , examined the spatial arrangement of deformation bands in the Moab Member of the Jurassic Entrada Sandstone. Field observations of this unit accurately quantified spatial relationships between deformation bands across 6 orders of magnitude (1 cm to 10 km). A 9.4-km-long scanline was constructed within the Moab Member, to measure spacing and orientation for individual deformation bands (N = 1467). Correlation count methodology was used to determine the spatial arrangement of deformation bands.
Correlation count analyses show that deformation bands within the Moab Member are natural fractals (self organized) at short length scales (cm to m). Although the spatial arrangement of deformation bands also is non-random at long length scales (m to km), spatial arrangement is not fractal (externally organized). Down-plunge projection shows that at long length scales, regions of abundant deformation bands correspond with hinge zones of subtle tectonic folds.
The fractal nature of deformation bands, evident at short-length scales, may be attributed to overall strain hardening during deformation band development. Deformation bands interact with one another due to strain hardening, and this results in spontaneous self organization. At long-length scales, deformation band occurrence is controlled by lateral variation in strain magnitude due to other deformation processes. Deformation bands are non-random in space, but the spatial organization is externally imposed.
AGE AND CHARACTERIZATION OF THE RED HILLS PORPHYRY
COPPER-MOLYBDENUM DEPOSIT AND ITS RELATIONSHIP TO THE CHINATI MOUNTAINS
CALDERA, PRESIDIO COUNTY, TEXAS
Amy K. Gilmer, M. S. Geo. Sci.
The University of Texas at Austin, 2001
Supervisor: J. Richard Kyle
The Red Hills deposit in Presidio County, Texas, is the easternmost porphyry copper-molybdenum system in southwestern North America. The Red Hills intrusion is located approximately 1 km south of the 32 Ma Chinati Mountains caldera, and it has been postulated that the two may be genetically related. The quartz-monzonite porphyry intruded upper Permian limestones, sandstones, and siltstones of the Ross Mine and Mina Grande Formations, as well as sandstones and limestones of the Cretaceous Presidio Formation. Four distinct alteration types are recognized within the intrusion: potassic, phyllic, propylitic, and argillic. Mineralization within the sericitized stockwork porphyry consists of chalcopyrite, chalcocite, and molybdenite within quartz-pyrite veins. Sphalerite-bearing garnet skarn occurs as small replacement pods in the sedimentary rocks along the periphery of the intrusion.
Fluid inclusion studies of quartz veins associated with phyllic alteration indicate that the fluids associated with this stage had a mean temperature of 399 °C ± 58 °C based on measured homogenization temperatures. Scanning electron microscopy (SEM) combined with energy dispersive analysis (EDA) confirmed the presence of halite, sylvite, chalcopyrite, hematite, pyrite, and bixbyite daughter minerals within fluid inclusions.
Estimated salinities for the fluid inclusions from quartz veins associated with phyllic alteration range from 33 to 47 wt. % NaCl equiv. The high salinities of these fluids suggest a magmatic source. Measured homogenization temperatures and salinities for boiling assemblages were utilized to estimate pressure at the time of trapping and to provide constraints on their depth of formation. Pressure estimates based on contemporaneous trapping of immiscible fluids indicate a range of formation pressures from 20 to 30 MPa, corresponding to depths of formation of 2 to 3 km.
New U/Pb zircon age data and Re/Os molybdenite age data from the intrusion yield ages of 64.2 ± 0.2 Ma and 60.2 ± 0.3 Ma, respectively, indicating that the intrusion and mineralization are distinctly older than all other Tertiary magmatism (48 to 17 Ma) in Trans-Pecos Texas. The new ages also indicate that the Red Hills intrusion predates the 32 Ma formation of the Chinati Mountains caldera. Geochemical data from the porphyry further suggest no direct relationship to the Chinati Mountains magmatism. The Laramide age for the Red Hills porphyry makes it contemporaneous with the majority of the porphyry copper systems (75 to 54 Ma) in Arizona, New Mexico, and northern Mexico.
Return to listSEISMIC AVO RESPONSE TO VARIATIONS IN SANDSTONE RESERVOIR PROPERTIES
by
Sharon Lee Goehring, M.S.
The University of Texas at Austin, 2005
SUPERVISOR: Robert Tatham
Successful exploration for hydrocarbon reservoirs requires understanding seismic response for different sandstone lithologies. Today, many new targets lie within shale-rich environments such as mud-rich deltaic systems, on continental slopes or other basins with shaly mobile substrates. To produce reliable predictions, any ambiguities created by changing sandstone reservoir properties must be reduced. As both lithology and pore fluids significantly influence seismic response, correctly modeling prospects requires methods which can discriminate between these separate effects.
Evaluating amplitude variation with offset (AVO) permits identification of hydrocarbon reservoirs by their anomalous seismic response from background trends. This thesis tests empirical relationships between velocities and rock properties, effective media equations and Backus-averaging under several shaly-sandstone geometries ranging from pore-space clays to laminated sands and shales to successfully predict AVO response. Algorithms using the Hashin-Shtrikman equations for rock-strengthening pore-bridging clays and Backus-averaged laminae where shales reduce average acoustic impedance for the reservoir accurately modeled deepwater seismic data from the Gulf of Mexico. These two methods produce statistically significant models which reflect the seismic AVO response for varying sandstone properties.
CORE STUDY AND SEQUENCE STRATIGRAPHIC ANALYSIS OF THE DESMOINESIAN BARKER CREEK ZONE, TABLE MESA AND RATTLESNAKE FIELDS, SAN JUAN COUNTY, NM
Alfred Dean Gomez, MSGeoSci
The University of Texas at Austin, 2003
SUPERVISORS: Richard Buffler & William Fisher
162 pages, 50 references, 8 color plates in pocket
Over 750 ft (230 m) of slabbed Pennsylvanian (Desmoinesian (Barker Creek zone)) core and 150 thin sections, from the Table Mesa and Rattlesnake fields, San Juan County, New Mexico, were investigated to determine facies, depositional environment, sequence stratigraphy and hydrocarbon reservoir development. The fields are located on the Four Corners Platform low-angle shelf system, southeast of the Paradox Basin. Facies that were interpreted from the core are; non-skeletal packstone-grainstone, skeletal packstone-grainstone, chaetetid boundstone, porous algal grainstone-boundstone, calcareous siltstone, nodular wackestone-packstone, mud-rich algal mudstone-wackestone, oil-rich mudstone-wackestone, and black laminated mudstone. These facies are placed within a depositional environment model block diagram that was developed for the study area. Basinal and slope facies are interpreted to represent low to moderate relatively deep deposits and include the black laminated mudstone, oil-rich mudstone-wackestone, calcareous siltstone and nodular wackestone-grainstone. Overall the shoal water bank and local topographic high facies are the shallowest facies belts and represent moderate to high energy deposition, which include the non-skeletal, skeletal, chaetetid, porous algal, calcareous siltstone, and mud-rich algal facies. Proximal shelf facies are interpreted as moderately shallow moderate to high energy deposits. A Barker Creek zone sequence stratigraphic model was developed for the study area. Facies were packaged into eight high-frequency sequences based on depositional energy and base-level interpretations. High-frequency sequences were packaged into two high frequency sequence sets (lower and upper Barker Creek zone) using sequence stratigraphic stacking pattern and correlation tools. The lower and upper Barker Creek zones are each comprised of one transgressive and one highstand systems tract. Systems tracts are interpreted using aggradational, progradational, and retrogradational facies stacking patterns. Highstand systems tracts are subdivided into early and late highstand components in order to further determine the best timing for productive carbonate development within the study area. The sequence stratigraphic model indicates that the Barker Creek zone was dominated by highstand depositional conditions and that the thickest accumulations of carbonates occurred during late highstand. Reservoir analysis indicates that the porous algal facies is the primary, and best developed, hydrocarbon reservoir for the two fields, and was deposited during late highstand conditions along the shallow water bank facies belt.
SENSITIVITY OF P-P, SH-SH AND P-SV SEISMIC
REFLECTIVITY TO PARTIAL GAS SATURATION
Carmen Teresa Gómez, M.S. Geo. Sci.
The University of Texas at Austin, 2005
SUPERVISOR: Robert Tatham
P-P seismic reflectivity has been widely used as a tool to discriminate fully-water saturated rocks from fully gas saturated rocks. The sensitivity of seismic reflectivity to partial gas saturation, however, is still a topic of discussion and research.
Partial gas saturation occurs when the fluid filling the pore space of a rock is a mixture of gas and some liquid, such as oil or water. At some given pressure and temperature conditions, only a limited amount of gas can be dissolved in brine or oil. If more gas is available than the maximum that can be dissolved, then it is assumed that the excess gas will be distributed as gaseous bubbles. The scale of mixing of these gas bubbles and the liquid phase (oil or brine) will depend on rock permeability, viscosity of the fluids, capillarity forces, pore connectivity, fluid density contrast, and buoyancy of the gas. The effective P-wave seismic velocity will be higher for the coarse-scale mix than for the fine-scale mix of fluids. In the case of homogeneous fluid distribution, very low gas saturation may produce the same seismic response as a commercial gas accumulation (“fizz water” effect). That is why it is important to analyze the sensitivity of seismic reflectivity to the different properties of the rock and pore fluids to examine the real probabilities of different techniques to characterize gas saturation.
From sensitivity analyses, using Zoeppritz seismic reflectivity, I observe that PP reflectivities are more sensitive to density and P-wave velocity contrast for near-offsets and more to Poisson’s ratio at the far-offsets. The effects of density and P-wave velocity in P-P reflectivity follow each other very closely for most small angles of incidence. Thus, it is not possible in principle to separate the two effects. On the other hand, P-SV reflectivities appear to be nearly equally sensitive to velocity contrast, density contrast and Poisson’s ratio of the gas sand at near offsets. Sensitivity of P-SV reflectivity to density contrast is larger at angles of incidence between (30 and 60 degrees), in particular for fluid-fluid interfaces. Therefore, this far-offset range is where independent density information may be obtained by inversion of P-SV amplitudes. For SH-SH reflectivity, I find that the effect of density contrast is stronger in the 30 to 50 degree range of incidence angles.
Inversion techniques involving P-SV data using partial stacks over varying offsets seem promising to discriminate partial gas saturation. This is because they take into account the fact that the density effect on reflectivity separates from the seismic velocity effect only at mid- to large- offsets. Using larger offsets implies that approximations to Zoeppritz reflectivities, such as Aki and Richards (1980), are less accurate for these larger offsets ( > 35 degrees), in particular for P-S reflectivity. However, higher order approximations for P-S seismic amplitudes are impractical, since they are not linear with respect to seismic velocity and density contrasts (Larsen, 1999).
There is still not a generally accepted seismic reflection method to discriminate commercial gas concentrations from “fizz water” (low gas saturation) concentrations; however from the sensitivity analyses and the study of all the last techniques available, I conclude that the use of P-SV and SH-SH AVO, in addition to P-P AVO, is essential to understand this problem. Using good well calibrated multicomponent seismic data will be also a key element to solve this puzzle.
Return to list STRATIGRAPHIC AND STRUCTURAL ANALYSIS OF THE NEOGENE SEDIMENTS OF THE OFFSHORE PORTION OF THE SALINA DEL ISTMO BASIN, SOUTHEASTERN MEXICO
Pedro Tomás Gómez-Cabrera, Ph.D.
The University of Texas at Austin, 2003
Supervisors: William L. Fisher, Martin P. A. Jackson
274 pages, 208 references
Southeastern México has been affected by regional and local tectonic events. Regional tectonic events are the Gulf of México opening and the lateral movement of micro-plates on the Pacific margin. The local tectonic events are related to salt tectonics. Autochthonous Jurassic salt serves as the detachment level for the main compressional event in the late Miocene. Jurassic salt was allochthonously emplaced in the late Miocene, then partially displaced by a huge quantity of terrigenous sediments during the Plio-Pleistocene.
This research is a study of the main geological processes that have influenced the structural and stratigraphic evolution of the Neogene sediments in the offshore portion of the Salina del Istmo basin known as the Marbella area.
Owing to data availability, the project was divided into regional and local studies. The regional study is based on 2D multi-channel seismic reflection data, and the local study is based on a 3D seismic streamer survey.
Structural analysis in the regional study permits the recognition of four buried fold belts (Agua Dulce, Catemaco, Marbella, and Marbella Norte) trending roughly NE. These fold belts are the result of tectonic convergence in the pacific margin during late Miocene. The Agua Dulce and Marbella Norte fold belts are separated by an enormous salt withdrawal basin called the Pescadores basin. The Pescadores basin is bounded on the north by a spectacular stepped, counter-regional structure. Beyond the Pescadores basin, a salt mini-basin area is recognized in the upper continental slope. Another important structural element is the Sal Somera canopy in the southern part of the study area.
Sedimentation-rate analysis, based on isochore mapping in the local study area, indicates that from SB-2.4 to SB-2.6 Ma, deposition rate peaked with a maximum of 7.5 mm/yr.
Regional and local structural restorations show that, in general, the maximum allochthonous salt mobilization was during the Plio-Pleistocene because of the huge quantity of sediments that the Chiapas massif and the Sierra de Chiapas supplied to the basin. A geohistory plot indicates rapid basin deepening at 2.4 to 2.6 Ma, which is attributed to a major influx of sediments and allochthonous salt evacuation.
Return to listCHRONOLOGY OF CENOZOIC TECTONIC EVENTS IN WESTERN VENEZUELA AND THE DUTCH ANTILLES BASED ON INTEGRATION OF OFFSHORE SEISMIC REFLECTION DATA AND ONLAND GEOLOGY
David Luke Gorney. M. S. Geo. Sci.
The University of Texas at Austin, 2005
Supervisor: Paul Mann
Newly acquired BOLIVAR seismic reflection data from offshore western Venezuela and the Dutch Antilles are combined with existing geologic and geophysical data sets to examine the complex chronology of tectonic events affecting the onshore Falcon basin and the adjacent offshore basins. This study also describes the seismic acquisition, processing, and interpretation methods applied to the BOLIVAR seismic reflection data and used in this study.
Three tectonic phases are constrained using these data: 1) Paleogene back-arc opening of the 3 to 6 km thick Falcon-Bonaire basin is the initial tectonic phase that occurred along east-west striking normal fault systems that have locally been inverted by later tectonic phases. These normal faults control the oldest depositional sequences and parallel the trend of the Bonaire basin. 2) northwest-striking normal faults crosscut these older normal faults and form deep submarine rifts that contain up to 4 km of sedimentary fill, forming deep water channels between the Dutch Antilles islands. Offshore well data and the age of onshore sediments in the Falcon basin indicate that this second phase rifting occurred in the late Oligocene through the early Miocene. 3) inversion of the Falcon basin commenced during the middle Miocene; this inversion phase is reflected in the present-day pattern of east-northeast-trending fold belt that can be traced over 200 km in the Falcon basin; a second fold-thrust belt (La Vela) can be traced over a distance of 175 km parallel to the Falcon coast; restoration of imbricate thrusts seen on seismic lines perpendicular to the Falcon coast indicates a minimum of 7 km of northeast-southwest directed thin-skinned shortening.
PHYLOGENETIC INTERPRETATION OF EGGS AND EGGSHELLS AND ITS IMPLICATION
FOR PALEOGNATHAE PHYLOGENY
Gerald Grellet-Tinner, M. S. Geo. Sci.
The University of Texas at Austin, 2001
Supervisor: Timothy Rowe
Our current knowledge of theropod phylogeny is based on skeletal morphology. Historically, paleontologists have valued data from skeletal elements as a source of characters for phylogenetic analysis. Eggs form under genetic control and should contain information regarding the evolutionary history of the group. Therefore, eggshells consisting of complex biopolymers and ionic crystals could be used in the same phylogenetic context as osteology. To date, no one yet has tried rigorously to classify eggs and eggshells in a phylogenetic context using cladistic analysis as a tool to establish an ancestor to descendant relationship.
My examination of previous literature revealed problems such as interpretation of eggs as trace fossils by previous researchers, their classification in a parataxonomic scheme, their misdiagnosis due to poor structural observations and their problematic association with parent lineage. By proposing to test the phylogenetic value of eggs and eggshells, I seek to address and resolve the above-mentioned problems. The choice of Paleognathae (ratites plus Tinamidae) was appropriate because the monophyly of this group has already been established based on osteological and molecular information, and also this group is well manageable by the number of taxa it contains. However, there are controversial aspects in the interrelationship of its members, and in the questionable assignment of Australian Dromornithidae to this clade.
My attempt to test the phylogenetic value of eggs (inclusive of egg morphology and eggshell structure) was based on the same precepts than any usual skeletal morphological based phylogenetic analysis. Characters and character states were searched, evaluated, scored in a matrix, and then run by a computer algorithm. The resulting trees were compared with existing molecular and skeletal based cladograms. In the process I had to modify and/or create new terms to describe oological characters.
My results support Paleognathae monophyly, and demonstrate Dromornithidae affinity with Neognathae. Minor differences exist among all the existing paleognath phylogenetic hypotheses, but these disparities could reasonably be argued for each analysis. Furthermore, oological characters permit the differentiation of groups at a generic level, thus validating the use of eggs and eggshells as a source of evolutionary characters.
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ALLUVIAL ARCHITECTURE AND TECTONIC SETTING OF THE MANGAS CONGLOMERATE, TYRONE MINE AREA, GRANT COUNTY, NEW MEXICO
by
Joy Deann Griffin, MSGeoSci
The University of Texas at Austin, 2001
Supervisor: Richard T. Buffler
.
The Mangas conglomerate is a Plio-Pleistocene syntectonic, extensional basin fill of the Mangas half-graben. The study area is located in the Mangas Valley in southwestern New Mexico at the juncture between the Rio Grande rift, the Mogollon-Datil volcanic province, and the Basin and Range. The study area straddles the Continental Divide on the eastern side of the Big Burro Mountains and includes excellent exposures of the conglomerate, making it an ideal location to study rift sedimentation.
The Mangas half-graben forms a northwest-trending, down-dropped block between the Big Burro Mountains to the southwest and the Little Burro Mountains to the northeast. Erosion of these surrounding highlands during the late Neogene, probably Plio-Pleistocene, provided the source for as much as 370 m of Mangas conglomerate. The Mangas conglomerate is divided into the lower Mangas and the upper Mangas units. The division is based on provenance of the sediments and color. The lower Mangas was deposited in a large graben system, as sediment was eroded from the Big Burro Mountains and the Silver City Range to the north. Uplift of these Ranges controlled sedimentation patterns, creating punctuated influxes of matrix-supported, angular, poorly sorted, poorly stratified sands and conglomerates as basin margin alluvial fans. The processes controlling deposition of the fans were debris flows, streamfloods, and sheetfloods.
After deposition of the lower Mangas, northwest-trending Basin and Range structures were reactivated. Uplift of the Little Burro Mountains along the northwest-trending Mangas fault bisected the large graben in which the lower Mangas was deposited, forming a local half-graben (Mangas half-graben). The upper Mangas was syndepositional with movement along the Mangas fault. Localized, erosional fans cut into the lower Mangas along the footwall of the half-graben. The localized nature of deposition of the upper Mangas led to very poorly sorted, angular clasts deposited by debris flow processes. Paleosols identified in the upper and lower Mangas record periods of inactivity or nondeposition on the fans. A later episode of dip-slip movement postdating deposition is recorded by northeast-trending faults associated with the Burro Chief fault that offset both the upper and lower Mangas.
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LONG TERM SUBSIDENCE MONITORING USING SYNTHETIC APERTURE RADAR INTERFEROMETRY
Krishnavikas Gudipati, MSGeoSci
The University of Texas at Austin, 2003
Supervisor: Clark R. Wilson and Sean M. Buckley
78 pages, 36 references
This study is aimed at applying radar interferometry to investigate urban land subsidence. A least square method is applied in retrieving a deformation sequence from a database of Interferometric SAR images over Phoenix, Arizona. With the least squares method, short term interferograms can be used to generate a chronologically ordered sequence of deformation and its evolution.
Synthetic Aperture Radar Interferometry (InSAR) is a satellite based radar technique developed over the past two decades and has been widely applied in various research fields related to earth sciences. It has been successfully applied in topographic mapping, earthquake studies, monitoring volcano deformation, and more recently for land surface deformations for subsidence monitoring. The most important source of error in InSAR is temporal decorrelation (i.e. loss of information due to differences in backscattered signal between the two images). The Phoenix metropolitan area in Arizona lies in broad alluvial valleys or basins surrounded by mountainous terrain of consolidated sedimentary rocks. The permeable alluvial valleys are capable of storing large quantities of groundwater. Groundwater pumping resulting from rapid urbanization and increases in agricultural and industrial water usage has caused land subsidence in the region. In all three subsidence features were studied located in the cities of Peoria, Glendale and Scottsdale.
InSAR features associated with the subsidence show larger signatures with progressive time spans. Deformation images obtained as a time series indicate the presence of non-linear trends in subsidence rates. At Peoria and Glendale, subsidence rates showed a seasonal variation and the subsidence was highly correlated with the declining water table levels. Decline in subsidence rates in was seen at Scottsdale between 1996 and 2000.
Return to listTEMPORAL AND SPATIAL CONTROLS ON CAVE WATER AND SPELEOTHEM CALCITE ISOTOPIC AND ELEMENTAL CHEMISTRY, CENTRAL TEXAS
Amber Lee Guilfoyle, M.S. Geo. Sci.
The University of Texas at Austin , 2006
SUPERVISOR: Jay L. Banner
The isotopic and elemental composition of speleothem calcite precipitated from dripwaters in caves have been used to elucidate changes in karst groundwater flow paths and changes in climate such as shifts in vegetation and rainfall patterns. Given that recent studies have shown seasonal isotopic and elemental changes in cave dripwater chemistry, then these changes are most likely reflected in the precipitated calcite. This affirms the need to study the environmental controls on the dripwater isotopic and elemental chemistry on seasonal time scales, since ultimately these are environmental controls on the precipitated speleothem calcite. In this study, physical, chemical, and meteorological variations at 13 drip-water and pool sites in two cave systems in the Edwards aquifer of central Texas are used to delineate the controls on the evolution of cave waters. The chemical evolution of these cave waters can be accounted for by variations in calcite precipitation, soil geochemistry, flow paths, and degassing of CO 2 driven by seasonal overturn of the cave atmosphere.
Calcite precipitated on glass substrates placed under three of the thirteen dripwaters in this study was analyzed for its C and O isotopic composition in order to assess 1) seasonal variations in the isotopic composition and 2) the extent to which precipitation occurs in isotopic equilibrium with its host dripwater. Each winter season, calcite growth rates increase, which corresponds to changes in the C and O isotopic composition of the calcite: higher d 13 C and d 18 O values during winter and lower d 13 C and d 18 O values during summer. Some calcite samples were found to precipitate out of C and O isotopic equilibrium, with maximum departures of 1.3‰ with respect to carbon and 1.4‰ with respect to oxygen. The results show that d 13 C and d 18 O in the modern speleothem calcite: 1) can vary sub-annually without change in vegetation or climate, and 2) reflect in the majority of cases precipitation out of isotopic equilibrium with the host dripwater that precipitates the calcites.
COMPLETE ANISOTROPIC ANALYSIS OF THREE COMPONENT SEISMIC DATA RELATED TO THE MARINE ENVIRONMENT AND COMPARISON TO NINE COMPONENT LAND SEISMIC DATA
Jason Ethan Gumble, Ph.D.
The University of Texas at Austin , 2006
Supervisor: Robert H. Tatham
To facilitate the transfer of knowledge from one arena to another, a comparison between nine component, controlled polarization shear-wave land seismic data and three component mode converted seismic shear-wave data was made. The continuing movement of petroleum exploration into the offshore arena and the increasing number of aging offshore oil fields may necessitate the use of shear wave data for full characterization of reservoir properties for finding, characterizing, and prolonging the life of the oil fields through the description of anisotropy as an analog for rock properties. Operationally limited to the use of compressional seismic sources in the marine environment, mode-converted shear-wave data recorded by three component seismic receivers must be exploited to arrive at full characterization of anisotropy.
Effective media theory, seismic modeling, and post stack analysis including four component matrix rotations and numerous other tools developed on land were adapted and used offshore. A full understanding of anisotropy, shear wave birefringence and polarization was required to evaluate the similarities and differences between direct shear waves and mode converted shear waves. The investigation of seismic anisotropy related to shear waves began with the Cymric Oil field in California , progressed out into shallow waters of the Gulf of Mexico to the Teal South Oil Field, and finished in the Williston Basin, Saskatchewan, Canada. It has been revealed that not only can lessons be learned from the historical practice of nine component seismic analysis on land and applied to the marine environment, but that also the reverse is true. The use of full azimuth, full offset data and prestack analysis was developed and has served to extend present day anisotropic analysis beyond its current limitation of 1D application. Pre-stack converted shear-wave layer stripping analysis techniques developed include extraction of a azimuthal and offset variant time-shifts for birefringence correction, as well as the analysis of azimuthal and offset variance in amplitudes and reflected polarization. A numerical comparison between controlled source polarization, direct shear-waves and uncontrolled source polarization, mode converted waves, led to a correction for apparent polarization effects on the direct shear-waves and a correction for incomplete acquisition geometry for mode-converted waves.
HIGH RESOLUTION SEQUENCE STRATIGRAPHY OF THE CRETACEOUS
WOODBINE FORMATION, HENDERSON AND NAVARRO COUNTIES, TEXAS
By
Russell Royden Hamman, M. S. Geo Sci.
The University of Texas at Austin, 2001
Supervisor: William L. Fisher
The terrigenous clastic Woodbine Formation is one of several progradational
intervals in the East Texas Basin. It is also a prolific hydrocarbon producing
horizon that accounts for almost 80% of the cumulative oil production from
the East Texas Basin.
Sediments comprising the Freestone Delta Member of the Woodbine Formation
average 650 feet thick in Henderson and Navarro Counties. Within the study
area, four marine shale bounded parasequence sets have been identified based
on changes in sand body stacking patterns and log signatures. These parasequence
sets are each built from at least two parasequences as determined through
facies mapping. Each parasequence consists of three principal sandy facies
building blocks with unique log signatures and orientations: fluvial or
distributary channel fills, channel mouth bars, and strandplains. The fluvial
and distributary channel-fill sand bodies are dip oriented and show a upward
fining signature. The channel-mouth bar facies is also dip oriented but
displays a coarsening and then upward fining log signature. Strike-oriented
sand bodies with uniform to upward coarsening vertical trends compose the
strandplain facies. The channel-mouth bar facies is typically rare in wave
dominated delta systems like the Woodbine but serves to define the sediment
supply axes for each parasequence.
Two dip-oriented sediment supply axes controlled sediment deposition.
Theses axes have distinct depositional histories that are reflected in vertical
and lateral depositional system changes throughout the Woodbine Formation.
The lowermost parasequence recorded a sand-rich shoreface environment that
evolved into a proximal fluvial setting due to a presumed decrease in accommodation
space. Above this constructional fluvial and shoreface interval, the sedimentary
system retrograded to shelf and distal deltaic depositional systems in the
upper, middle Woodbine Formation. However, rejuvenated progradation of
strandplains and associated channel fills occurred in the uppermost parasequence
set.
Sediment supply varied
between northwestern and northeastern axes in the lower and middle Woodbine
but was exclusively a northwestern source in the uppermost parasequence set.
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LATE QUATERNARY SEISMIC STRATIGRAPHY AND STRUCTURE OF THE WESTERN INSULAR SHELF MARGIN OF PUERTO RICO
Martín Hanzlík-Valentín. M.S. Geo. Sci
The University of Texas at Austin , 2006
Supervisor: Paul Mann
The modern insular shelf and slope of western Puerto Rico is characterized by reef carbonate and siliciclastic sedimentary environments controlled by the complex interplay of tectonic arching and faulting, vigorous erosion of the elevated island of Puerto Rico, fluvial incision and sedimentation pulses, reef growth, and rapid late Quaternary eustatic sea-level changes. For this study, I interpret 725 km of high-resolution, single-channel seismic lines that were collected over the western insular shelf and slope by the RV Isla Magueyes in 2000 to better understand tectonic, erosional, and eustatic controls on late Quaternary history.
The seafloor geology of the western shelf and slope area of Puerto Rico has been well studied and mapped mainly from grab and short cores collected by geoscientists at the U.S. Geological Survey and at the University of Puerto Rico at Mayagüez. Because of low tectonic uplift rates from the western coast of Puerto Rico , the onland late Quaternary sedimentary history is not recorded in Quaternary coastal outcrops. The results of my seismic interpretation and correlation with multi-channel seismic data collected in 1972, reveal four main units defined by characteristic stratal reflection terminations and seismic facies. These units include: Unit 1 (basement) - a gently folded and faulted basal section correlated to the Oligocene-early Pleistocene carbonate shelf of Puerto Rico; deeper penetration, industry multi-channel seismic lines show that these rocks are deformed in a broad east-west-trending arch; Unit 2 (lowstand systems tract, LST) - chaotic channel fill deposits filling deep (~ 30 meter) incisions formed during the Last Glacial Maximum about 20 ka; Unit 3 (transgressive system tract, TST) - poorly stratified deposits truncating the top of Unit 2 and deposited during early Holocene transgression of the shelf margin; Unit 4 (highstand system tract, HST) – late Holocene, highly stratified deposits related to aggradation as the Holocene transgression began to slow. The base of unit 4 is a downlap surface interpreted as a maximum flooding surface likely formed about 8 ka. East-northeast-striking faults are observed that offset the late Quaternary units in three separate zones off the west coast of Puerto Rico . Because of a lack of wells and long cores from the shelf and slope area, age estimates for the four units are based on correlations with sea-level curves derived from dated coral samples in the Caribbean and western Atlantic region. All four units are deformed by faulting that should be considered active and possibly hazardous for the rapidly developing west coast of Puerto Rico . In one area, a large, late Holocene slump (~ 0.016 km 3 ) is mapped using seismic, sidescan sonar and bathymetric data. Onland continuations of these faults are likely, but have not been identified due to cultural overprint of natural scarps on late Quaternary floodplains.
ANALYSIS OF FRACTURE CLUSTERING USING THE WAVELET TRANSFORM: AN EXAMPLE FROM THE MARBLE FALLS LIMESTONE
Christopher Matthew Hare, M.S. Geo.Sci.
The University of Texas at Austin, 2002
Supervisor: Randall Marrett
Characterizing the spatial arrangement of fractures is currently of interest when attempting to solve a range of geologic problems, such as fluid flow through fractured aquifers and petroleum reservoirs. Realizing that fractures can significantly affect the magnitude of a rock's permeability and control the preferred orientation of fluid flow, understanding the geometry of fracture patterns within a rock volume is necessary in order to correctly characterize the often heterogeneous behavior of the system.
Fracture data analyzed in this project were collected at Pedernales Falls State Park, near Johnson City, TX from the Marble Falls Limestone. The scanline data from Pedernales Falls consist of kinematic aperture and position measurements for 999 calcite filled veins, over a scanline length of approximately 59 meters that was oriented perpendicular to the strike of the subvertical fractures. Although the aperture values follow a power-law, the spacing values are best fit by a log-normal distribution and suggest a nearly random spatial pattern.
Current methods of spatial analysis of fracture data are limited in that the sequence of fracture spacing values is often ignored by the actual analysis. This project applies techniques of wavelet analysis in a new manner in order to test the hypothesis that the fractures at Pedernales Falls State Park are arranged spatially into non-random clusters. Analysis of the Pedernales data shows that clusters exist at multiple scales and suggests that the larger fracture clusters are made of smaller fracture clusters, and so on. This multi-scaled arrangement provides a basis for the proposal of a fractal distribution of the fracture clusters at Pedernales Falls. The multi-scaled pattern of the fracture clusters is readily analyzed by wavelet techniques which take into account position (i.e. translation along the scanline), as well as the scale of observation (i.e. the dilation of the analyzing wavelet). This new method of analysis allows for both the size and position of the fractures to be analyzed simultaneously, something that was previously unattainable using traditional techniques.
Return to listDEFORMATION OF BLOCKS AND MATRIX IN FRANCISCAN MELANGE, SAN SIMEON, CALIFORNIA: IMPLICATIONS FOR MELANGE GENESIS
Daniel F. Harrington, M.S. Geo. Sci.
The University of Texas at Austin, December, 2001
Supervisor: Mark Cloos
Superior exposure of blueschist-bearing Franciscan subduction melange exists along 4 km of seacliffs south of San Simeon, CA. The melange's chaotic fabric, inclusions of variegated lithologies, and generation and emplacement have been alternately ascribed to dominantly sedimentary or tectonic processes in previous studies.
The melange "matrix" is a fine-grained (<20 micron) black metapelite, and bears a foliation which is generally parallel to the margins of elongate melange blocks and dips ~60° to the northeast. The foliation is defined by a pervasive network of anastomosing scaly cleavage planes.
Greywacke inclusions are lenticular, and display pinch-and-swell and boudinage geometries and high (4 -12+) aspect ratios from outcrop to thin-section scale. The greywacke is 50-70% clay-sized material ("groundmass", comprising detrital matrix and authigenic clays). Detrital quartz, plagioclase, and rare K-feldspar are surrounded by groundmass, and are rounded and optically appear relatively unstrained. SEM-CL analysis reveals cryptic fracturing in quartz grains in excess of that apparent in quartz grains from a local trench-slope basin greywacke.
Mafic greenstone and blueschist inclusions display similar pinch-and-swell and boudin shapes, with aspect ratios as high as ~20. Inclusions of both types contain small (1 mm - 3 cm) pieces of relatively undeformed rock set within fine-grained (<0.3 mm) cataclastic material which is concentrated along intra-inclusion fractures, inclusion edges, and necks. Cataclastic texture is more pervasive in smaller inclusions and in those with higher aspect ratios.
Observations are consistent with a tectonically-driven process of melange formation. The majority of strain was accommodated by flow in the underconsolidated pelitic matrix, separating and dispersing included blocks. Internal block deformation was accommodated by particulate flow (grain-boundary sliding and rotation) in the greywacke groundmass and by cataclastic flow in greenstone and blueschist. Particulate and cataclastic flow mechanisms are manifestations of high fluid pressure conditions that were maintained throughout the deformational history. Cataclasis was concentrated along block margins, and was induced by interaction with the ductile matrix, and assisted by chemical alteration of crystalline material. The scaly cleavage in the matrix formed late, after the blocks were chaotically mixed, during offscraping and final dewatering. Prior to offscraping, shear strains within the subduction channel that caused distortion and dispersion were on the order of 1000s.
The grain-scale textures which indicate particulate flow in the greywacke and cataclastic flow in the crystalline inclusions are preserved in the melange at San Simeon because the material has remained unheated and untectonized since its emplacement; heating in excess of ~200°C would have obliterated most of the textures described in this study.
Return to listSTRATIGRAPHIC AND STRUCTURAL CONTROLS OF PERMIAN CARBONATE-HOSTED SILVER (PB-ZN) MINERALIZATION, SHAFTER, PRESIDIO COUNTY, TEXAS
Jennifer Adair Head, M.S.
The University of Texas at Austin, 2002
Supervisor: J. Richard Kyle
The Presidio deposit at Shafter, Texas, produced 35 million ounces of silver from 1882 to 1942, along with lead and minor gold. The production and estimated reserves of 20 to 30 million ounces makes this manto-type replacement deposit the largest silver deposit in Texas. The complex structural history of the Shafter area caused the host Permian Mina Grande limestones to be fractured repeatedly along different trends. Fractures that appear to control mineralization are related to the following: Mid-Tertiary Chinati Mountains caldera uplift, regional N-NW Texas Lineament and W Chalk Draw fault structures, and NE structures that have no regional equivalents.
The host Mina Grande carbonates can be divided into the Massive, Pseudobreccia and Basal facies. The Massive facies that lies directly below the Permian-Cretaceous unconformity is a completely dolomitized unit in which no original structures or fabrics are apparent. The Pseudobreccia facies consists of a zone in which "clasts" of original limestone occur in a matrix of orange stained dolomite due to partial dolomitization of the strata. The Basal facies is an unaltered limestone (wackestone to packstone) or only slightly dolomitized unit that forms the unit directly overlying the Ross Mine Formation.
The Mina Grande has had a complex diagenetic history that sufficiently prepared the rocks for hydrothermal mineralization. The history starts with the deposition of Mina Grande limestone as forereef facies, wackestone to packstone with abundant crinoid, fusulinid, bryozoa and brachiopod fragments. The limestones then underwent early marine and freshwater cementation (Stage-I calcite), followed by burial and early dolomitization (Stage-I dolomite). The burial also caused compaction producing micro-fractures and neomorphism of the limestone. Post-Permian uplift resulted in karstification of the Mina Grande with associated dedolomitization and dolomite dissolution. Subsequent burial during the early Cretaceous produced a second stage of dolomite (Stage-II dolomite). A second stage of calcite was also precipitated during the reburial (Stage-II calcite). During this time the karst cavities created during post-Permian uplift were filled with clay. Laramide- and Chinati Mountains caldera- related tectonic activities in late Cretaceous to middle Tertiary fractured the Mina Grande. These fractures and diagenetic porosity zones provided conduits for hydrothermal solutions that deposited calcite, quartz and Ag-Pb-Zn sulfides (Stage-III calcite and quartz). Late stage meteoric water infiltration resulted in oxidation of the sulfide ore to produce the present silver deposit. A post-oxidation calcite was then precipitated in remaining pores and fractures.
However, not all locations within the Mina Grande have had extensive porosity development. Compared to the Presidio and Shafter deposit areas, the Mina Grande Formation to the west (e.g. Perry Mine Hill) does not have significant porosity development in the form of karst and/or the altered Massive and Pseudobreccia facies. These locations are composed of the lower porosity Basal facies. This lack of extensive porosity development in the form of karst or the Massive and Pseudobreccia facies may explain why these locations do not host any stratabound ore units like the Presidio and Shafter deposits. The development of karst and/or the Massive and Pseudobreccia facies with the higher porosity is the controlling factor for the localization of stratabound deposits. In locations lacking sufficient porosity mineralization is confined to fractures.
Structural trends in the Shafter area control the distribution of mineralization. The Mina Grande fault zone and associated en echelon faults control the distribution of the mineralized zones within the Presidio Mine. Ore fluids rose from depth along these faults and migrated into the fractured Mina Grande. Both the faults controlling mineralization and the mineralization itself may be the result of a large igneous intrusion at depth. A complex diagenetic history of the Mina Grande allowed for sufficient preparation creating channels for fluid flow through karst features and the creation of pore space. This preparation along with the highly fractured nature of the Mina Grande limestone focused the hydrothermal mineralization and subsequent oxidation. Within the Presidio/Shafter deposit, structures within the Mina Grande formation, including the Mina Grande fault and en echelon faults, appear to have served as the conduits for fluid flow from depth. Well-developed Massive and Pseudobreccia facies (oriented N60E beneath the Cretaceous unconformity) with high karst-related porosity controlled where the fluids migrated into the formation and therefore the locations of the replacement deposits.
Return to listRESERVOIR TRENDS AND EXPLORATION POTENTIAL OF THE EL VADO SANDSTONE MEMBER OF THE MANCOS SHALE, NORTHWESTERN NEW MEXICO
Tiffany Setare Hedayati, M.S. Geo. Sci.
The University of Texas at Austin, 2008
Supervisor: Lesli J. Wood
The El Vado Sandstone Member (late Coniacian) of the Mancos Shale is considered by many to be an important unconventional hydrocarbon resource target (Ridgley, 2001) in the San Juan Basin, located in northwestern New Mexico. The shelf sandstones of the El Vado were deposited along the western margin of the Western Interior Cretaceous Seaway. The El Vado is part of a transgressive-regressive wedge of rock that overlies a large, older transgressive wedge which contains the Tocito Sandstone (the largest oil producer in the basin). Despite its productivity in parts of the basin, the nature, origin, and distribution of the El Vado Sandstone is poorly understood.
Using well logs and core data, the subsurface has been mapped to illustrate the distribution of the El Vado Sandstone across its extent in the San Juan Basin. The El Vado is reflected in logs as a thick (~ 120 foot) interval of low gamma ray and high resistivity. The gamma ray log character reflects highly laminated sand-to-silts interbedded with shales. A total of 13 cross sections were constructed based on the high density of well log control. Isopach maps were then generated for the entire interval and individual cycles of the El Vado. Regional net sand and net-to-gross maps, determined from 104 digitized well logs, highlight the regional distribution of sand content.
Two regions in the basin were examined in outcrop, the northeast and the southeast. In the northern outcrops, the El Vado consists of five cycles of similar thickness (~ 4 m each) separated by marine shales with cycles becoming progressively sandier upward. To the south, the El Vado Sandstone appears to transition from a shelf deposited sandstone into a lower shoreface sandstone.
Examining the El Vado Sandstone in the subsurface and in outcrops helped the author to improve the understanding of the nature and distribution of the sands throughout the basin.
Return to listHYDROGEOLOGY OF HETEROGEONOUS ALLUVIUM IN THE LEONA AQUIFER, CALDWELL COUNTY, TEXAS
Lloyd Harrison Hemphill, M.S. Geo. Sci.
The University of Texas at Austin, 2005
Supervisor: John M. Sharp, Jr.
The Leona aquifer is an important, but overlooked, water resource in Central Texas. The Quaternary Leona Formation occurs as several isolated alluvial deposits at the margins of the Edwards Plateau. Each of these deposits forms an aquifer. One of these aquifers is located near Lockhart, Texas. This aquifer is recharged by infiltration of precipitation and is discharged by numerous springs and seeps. Additional sources of discharge are evapotranspiration and cross-formational flow into the Wilcox aquifer. The saturated thickness at this location varies seasonally but is rarely greater than 3 m (10 ft).
Groundwater flow in an aquifer of this scale is influenced by its heterogeneous nature. This research identified seven different facies in the Leona Formation and the underlying Wilcox Group. These divisions were based on sediment classification, lithology, and sedimentary structures. The Leona Formation is covered by sandy and silty clay soil and caliche. Each of these facies has different hydraulic properties.
Many empirical relationships between grain size distribution and hydraulic conductivity (K) have been discussed in the literature. Equations developed by Hazen, Slichter, Terzaghi, Beyer, Saurbrei, and Kozeny were used to estimate hydraulic conductivity. Hydraulic conductivity was also measured in the laboratory with constant and falling head permeameters. Hydraulic conductivity of the Leona aquifer varies seven orders of magnitude. Hydraulic conductivity varies up to four orders of magnitude within a single facies due to small-scale differences in grain size distribution and degree of cementation. The arithmetic mean of hydraulic conductivity in vertical profiles through the Leona aquifer ranges from 0.013 cm/sec (37 ft/day) to 0.14 cm/sec (397 ft/day).
Water quality is a concern for many unconfined shallow alluvial aquifers, including the Leona aquifer. Elevated nitrate levels indicate contamination resulting from agricultural landuse. Nitrate concentration in the Leona aquifer ranges from 4 ppm nitrate as NO 3 to greater than 70 ppm nitrate as NO 3. These concentrations are significantly greater than those observed in the Wilcox aquifer.
The U.S. Geologic Survey computer code MODFLOW was used to create a groundwater model of the Leona aquifer. In the best simulation, specific yield was 0.1 and horizontal hydraulic conductivity was 0.058 cm/sec (164 ft/day). The simulated hydraulic conductivity is an order of magnitude less than observed in gravel pit outcrops. Modeled recharge was 9 percent of annual precipitation in 2003 and 20 percent of precipitation in the first six months of 2004. Five hypothetical wells were placed in the model to examine the effects of pumping on the aquifer. Wells pumped for 61 days at 0.04 l/sec (0.6 gpm) cause insignificant drawdown while wells pumped at a rate of 3.5 l/sec (55 gpm) cause up to 0.55 m (1.8 ft) of drawdown. Natural drainage of the aquifer caused the water table to decline 0.8 m (2.6 ft) over this same period. MODPATH simulations using this groundwater model indicate an average residence time in the aquifer of 13 years and a maximum residence time of 70 years.
ANALYZING DEEP-WATER NEAR SEAFLOOR GEOLOGY WITH CHIRP SONAR SUB-BOTTOM PROFILES:
GREEN CANYON GULF OF MEXICO
Jaime Hernandez. M.S. Geo. Sci.
The University of Texas at Austin , 2006
Supervisor: William Fisher
The study area is located on the continental slope in the Green Canyon deep-water area of the Gulf of Mexico . This area is being investigated by the Bureau of Economic Geology as part of several active gas hydrate studies across the area. The chirp sonar profiles used in my study were collected with an Autonomous Underwater Vehicle (AUV) utilizing a frequency-modulated seismic (sonar) source that emitted a 2 to 8 kHz sweep (chirp) frequency signal (wavelengths less than 2 meters ). The recording time is limited to about 50 milliseconds , with time zero occurring at the altitude of the AUV about 50 meters above the seafloor. The signal images to about 40 meters below the seafloor, and profiles are as long as 5 km . An interpretation of deep-water, near sea-floor geology has been conducted using both chirp sonar profiles and multibeam bathymetry.
Seismic reflections from within the shallow sediments are caused primarily by contrasts in density, rather than acoustic velocity. Reflections were successfully simulated using a model with a constant velocity of 1560 m/s and densities of 2.1 g/cc for sand, 1.4 g/cc for mud, and 1.7 g/cc for silty sand.
The chirp sonar profiles imaged near-seafloor geology at nearly a meter scale and allowed for detailed interpretation. The interaction of soft sediment deformation, creep movement and neo-tectonic activity related to gas expulsion controlled the actual topography of the sea-floor.
The geologic time represented in the 50 milliseconds of chirp data recorded with the AUV, which is about 40 meters of depth, is approximately 0.050 Ma . B.P., consistent with a depositional rate of 0.8 meters per 1000 years . Reflection patterns are interpreted to be related to fluctuations in sea level. High reflectivities (density contrasts) are interpreted to be deposited during the last sea level low stand, and low reflectivities are interpreted as sedimentation during the last high stand. Sediments deposited during the low stand are proximal while others are distal, deposited in deeper water environments.
Shallow structures observed in the chirp sonar profiles are mainly related to gas mobilization mechanisms, which shaped the topography of the seafloor in conjunction with soft sediment deformation and creep movement.
The geomorphic features are related in some cases to gas expulsion zones such as pockmarks and mud volcanoes. Other structures are related to soft sediment deformation and creep mobilization. Soft sediment deformation is confined to the deepest part of the minibasin, while the other features are not depth dependent. Highly deformed intervals at the bottom of the sequence seem to control subsequent sedimentation.
VERTICAL RESOLUTION OF SHEAR-WAVE VELOCITY INVERTED FROM PRE-STACK P-WAVE SEISMIC AMPLITUDE AND OFFSET MEASUREMENTS: NUMERICAL SENSITIVITY ANALYSES APPLIED TO THE ONADO FIELD, EASTERN VENEZUELAN BASIN
Rodolfo Carlos Hernández, M.S.Geo.Sci.
The University of Texas at Austin, 2008
Supervisor: William L. Fisher; Co-Supervisor: Carlos Torres-Verdín
Pre-stack P-wave seismic amplitude and offset inversion was conducted on a data set synthesized for the Onado oil field located in the Eastern Venezuelan Basin. The objective was to quantify the vertical resolution of the inverted shear-wave velocity (Vs) as a function of angle of incidence. Post- and pre-stack seismic P-wave data were used to extract the wavelets necessary to simulate seismic amplitude measurements via Zoeppritz’ equations. Two sets of synthesized data were used in the sensitivity analysis. The first set consisted of three different angle-range stacks comprising single-angle stacks (2°, 3°, 4°,…, 24°, 25°), three narrow angle-range stacks ([2°-4°], [4°-6°],..., [22°-24°], [24°-26°]), and five wider angle-range stacks ([2°-6°], [6°-10°],..., [18°-22°], [22°-26°]). The second set incorporated partial-angle stack models comprising an increase of maximum angle ([2°-3°], [2°-4°],..., [2°-24°], [2°-25°]), and an increase of the maximum in every five angles ([5°-10°], [5°-15°],..., [5°-30°], [5°-35°]).
Pre-stack inversion was performed on all the synthetic data sets using commercial simultaneous inversion algorithms. Vs inverted logs were derived from data synthesized at a key well and correlated with the actual measured Vs logs for the sensitivity analysis. Detailed analysis shows a general trend in the vertical resolution with increasing maximum angle for all stacks, with a progressive increase in vertical resolution (effective angle interval) from the lowest angle to a maximum range between 15° and 20° (critical point), and a subsequent decrease for farther angle data available (non-effective angle interval). This general trend enabled the selection of the largest angle to be included in the pre-stack inversion analysis. Partial-angle stacks entail a progressive gain in vertical resolution within the effective interval of incidence angles. Angle-range stacks, on the other hand, entail a more variable behavior with local lows. The difference between both data sets is attributed to the fact that the information borne by partial-angle stacks increases with an increase of angle, whereas angle-range stacks include information specific to only the associated angle interval.
PEGMATITE-CONTAINING SHEAR ZONES: A MECHANISM FOR THE FORMATION OF ISOLATED FELDSPAR AUGEN MEGACRYSTS AT WILLIMANTIC DOME, CENTRAL CONNECTICUT
Katherine Anne Herrell, MS
The University of Texas at Austin, 2002
Supervisors: Sharon Mosher, William Carlson
Large, isolated feldspar augen (12.0 to 0.5 cm) in quartzofeldspathic shear zones can be the product of intense disaggregation of pegmatites. Such augen are commonly interpreted as original porphyroclasts or porphyroblasts. The pegmatite-disaggregation mechanism of forming isolated feldspar augen is documented by pegmatites that underwent extension and thinning under amphibolite facies conditions in a low-angle ductile normal fault at the basement-cover contact of the Willimantic dome, eastern Connecticut. Field relations illustrate a laterally progressive disaggregation and boudinage of pegmatite layers. Intact pegmatites containing large feldspar augen surrounded by recrystallized pegmatite gradually disassociate, forming strands of floating feldspar augen connected by very thin layers of recrystallized quartzofeldspathic material parallel to the dominant foliation. The final separation and attenuation of these thin connective layers results in completely isolated, individual feldspars that mimic the geometry of a porphyroblast or porphyroclast with a recrystallized tail. Thin-section observations indicate that these "isolated" feldspars are still connected by slip surfaces and micro-scale lenses of recrystallized quartzofeldspathic material. These slip surfaces provide conduits for the migration of fluids that promote grain softening and recrystallization of the feldspars. Feldspar grains within single sheared pegmatite strands vary dramatically in their degree of deformation and recrystallization. Largely undeformed, intact crystals with little to no recrystallization occur alongside oblate, completely recrystallized grains. Less-deformed augen commonly contain kink bands or bent lattices that have localized partial to complete rotational recrystallization. Other augen are rimmed by rotationally recrystallized grains and deformation-induced myrmekite. The degree of internal deformation and recrystallization that each grain displays relates directly to its original orientation and the amount of locally present quartz. This mechanism for the formation of large, isolated feldspar augen through fluid-assisted pegmatite disaggregation may be common and should be considered when studying other shear zones. Such augen can be distinguished from porphyroclasts or porphyroblasts by their large size, aggregate character, lack of inclusion trails, and relict connections between grains.
Return to listCONTROLS ON SCALES OF CHEMICAL EQUILIBRATION DURING METAMORPHISM: INSIGHTS FROM GARNET ZONING PATTERNS
James Dwight Hixon, M.S.Geo.Sci.
The University of Texas at Austin, 2006
Supervisor: William D. Carlson
Garnets in metapelites from Harpswell Neck, Maine display atypical compositional zoning patterns that reveal the effects of unusual fluid compositions on rates of intergranular diffusion and scales of chemical equilibration during garnet growth.
At Harpswell Neck, pelitic rocks metamorphosed to 525-575 °C and 0.2-0.3 GPa contain chlorite + biotite + muscovite + garnet + quartz + plagioclase + ilmenite ± graphite ± pyrrhotite ± pyrite ± chalcopyrite. Garnet crystals display a wide variety of unusual patterns of compositional zoning. Mn is commonly concentrated in multiple isolated patches in the interior of garnet; this effect is superimposed on the more common tendency for Mn to partition into garnet cores during early stages of growth. Fe and Mg exhibit patterns of compositional zoning inverse to those of Mn. Ca concentration is slightly elevated or flat in garnet interiors and increases in concentric annuli near garnet rims. Y concentrations are highest in garnet cores and in concentric annuli near garnet rims.
Relative rates of intergranular diffusion can be interpreted from these garnet compositional zoning patterns. Isolated, patchy zoning of Mn, Fe, and Mg is interpreted as overprint zoning, indicating very slow rates of intergranular diffusion for these elements. In contrast, Ca and Y display patterns of higher core concentrations and concentric rings, which indicate that these elements achieved rock-wide chemical equilibrium. The observed combination of zoning patterns and inferred length scales of diffusion are not compatible with prior models in which temperature is the dominant factor controlling relative rates of intergranular diffusion.
The composition of intergranular fluid affects the solubility of elements during metamorphism, thereby exerting a fundamental control on rates of intergranular diffusion and scales of chemical equilibration. To explain observed garnet compositional patterns, the intergranular fluid would need to produce high solubility for Ca and Y and low solubility of Mn, Fe, and Mg. Limited data on solubility suggest that a fluid poor in Cl- and F-, but rich in SCO42-, is likely to have these effects; such a fluid might be generated by de-sulfidation of pyrite to pyrrhotite, or oxidation of pyrite and pyrrhotite during metamorphism, or both. Direct analysis of fluid inclusions within garnet using laser ablation inductively coupled mass spectrometry could provide a further test of this hypothesis.
In contrast to prior studies in which thermal effects dominate, this work provides evidence that variations in the composition of intergranular fluids can be important determinants of the scales of chemical equilibrium during metamorphism. The patchy zoning patterns observed in garnet from Harpswell Neck are caused by fluids of unusual composition: these fluids yield low solubility and thus slow intergranular diffusion for Mn, Fe, and Mg, but high solubility and thus rapid intergranular diffusion for Ca and Y.
Return to listESTIMATING THE SEDIMENT BUDGET FOR THE BARTON SPRINGS SEGMENT OF THE EDWARDS AQUIFER
Amy Elizabeth Hobbs, M.S. Geo. Sci.
The University of Texas at Austin, 2003
Supervisor: John M. Sharp
238 pages, 55 references
Increasing urbanization on the Barton Springs segment of the Edwards aquifer is causing concern for the survival of the Barton Springs Salamander. Construction and urbanization generate higher soil erosion rates and the increased mobile sediment may enter the aquifer causing elevated turbidity and hydrophobic contaminants. Therefore, estimating the amount of sediment entering the Edwards aquifer is necessary to evaluate the current aquifer sediment loading.
To estimate the sediment budget for the Edwards aquifer, sediment load input is estimated from suspended sediment concentrations (SSC) in the six watersheds recharging the Barton Springs segment of Edwards aquifer, and sediment load output is computed from Barton Springs turbidity and spring discharge measurements. Due to equipment and time constraints, only one watershed was monitored for SSC. Bear Creek, a rural watershed crossing the Edwards recharge zone, is sampled for SSC using two ISCO automated samplers installed next to stream discharge gaging stations. During the June-July 2002 storm event, suspended sediment samples are extracted at 1 hour to 1.5 hour intervals for the first part of the storm. The storm hydrograph for the June-July 2002 storm event has four distinct discharge peaks, but the second discharge peak damaged both automated samplers and the downstream stream gage. Thus, suspended sediment concentration data was only collected from the automated samplers for the 1st hydrograph peak.
The SSC for the other three storm hydrograph peaks is estimated using least square regression to relate stream discharge to SSC. Because the downstream gage was damaged during the flood, least squares regression is only calculated for the upstream ISCO sampling site # 2 stream discharge time series. To compute the aquifer input sediment load, the suspended sediment concentration time series was multiplied by the Bear Creek recharge rate. Based on U.S. Geological Survey stream loss studies, Barton Springs discharge records, and other Edwards aquifer discharging springs, Bear Creek contributes 13% of the total recharge to Barton Springs. To find the total sediment load entering the Barton Springs segment of the Edwards aquifer. Bear Creek's sediment load aquifer input was divided by 0.13.
To compare the aquifer's sediment load input to the sediment load output, the sediment load at Barton Spring is calculated. Turbidity measured from a probe in the main spring cave orifice is correlated to suspended sediment concentration to estimate the sediment load discharging from the springs. For the total storm period from June 29, 2002 to July 4, 2002, over 550,000 kilograms of sediment entered the Edwards aquifer and only 10,751 kilograms of sediment was discharged at Barton Springs.
A one-year sediment budget from August 1, 2001 to July 31, 2002 is also computed to examine sediment fluxes over a longer period of time. Historic U.S. Geological Survey suspended solid data collected from five of the study area six watersheds are related to stream discharge. The sediment load is calculated for each watershed based on USGS average daily stream discharge data. Barton Springs sediment output is estimated from average daily turbidity records and spring discharge. For the 8/1/01 to 7/31/02 time frame, the aquifer input is 4,511,619 kilograms and the aquifer output is 65,055 kilograms. Based on computations for the June-July 2002 storm event sediment budget and for the one-year sediment budget (8/1/01-7/31/02), there is a positive net storage of sediment in the Edwards aquifer.
Return to listMCMC ALGORITHM, INTEGRATED 4D SEISMIC RESERVOIR CHARACTERIZATION AND UNCERTAINTY ANALYSIS IN A BAYESIAN FRAMEWORK
Tiancong Hong, Ph.D.
The University of Texas at Austin, 2008
Supervisor: Mrinal K. Sen
191 pages, 139 references, 6 tables
One of the important goals in petroleum exploration and production is to make quantitative estimates of a reservoir’s properties from all available but indirectly related surface data, which constitutes an inverse problem. Due to the inherent non-uniqueness of most inverse procedures, a deterministic solution may be impossible, and it makes more sense to formulate the inverse problem in a statistical Bayesian framework and to fully solve it by constructing the Posterior Probability Density (PPD) function using Markov Chain Monte Carlo (MCMC) algorithms. The derived PPD is the complete solution of an inverse problem and describes all the consistent models for the given data. Therefore, the estimated PPD not only leads to the most likely model or solution but also provides a theoretically correct way to quantify corresponding uncertainty. However, for many realistic applications, MCMC can be computationally expensive due to the strong nonlinearity and high dimensionality of the problem. In this research, to address the fundamental issues of efficiency and accuracy in parameter estimation and uncertainty quantification, I have incorporated some new developments and designed a new multi-scale MCMC algorithm. The new algorithm is justified using an analytical example, and its performance is evaluated using a nonlinear pre-stack seismic waveform inversion application. I also find that the new technique of multi-scaling is particularly attractive in addressing model parameterization issues especially for the seismic waveform inversion.
To derive an accurate reservoir model and therefore to obtain a reliable reservoir performance prediction with as little uncertainty as possible, I propose a workflow to integrate 4D seismic and well production data in a Bayesian framework. This challenging 4D seismic history matching problem is solved using the new multi-scale MCMC algorithm for reasonably accurate reservoir characterization and uncertainty analysis within an acceptable time period. To take advantage of the benefits from both the fine scale and the coarse scale, a 3D reservoir model is parameterized into two different scales. It is demonstrated that the coarse-scale model works like a regularization operator to make the derived fine-scale reservoir model smooth and more realistic. The derived best-fitting static petrophysical model is further used to image the evolution of a reservoir’s dynamic features such as pore pressure and fluid saturation, which provide a direct indication of the internal dynamic fluid flow.
Return to listSTOCHASTIC TOMOGRAPHY AND GAUSSIAN BEAM DEPTH MIGRATION
Chaoshun Hu, PhD.
The University of Texas at Austin, 2008
Supervisors: Paul L. Stoffa and Kirk McIntosh
Ocean-bottom seismometers (OBS) allow wider angle recording and therefore, they have the potential to significantly enhance imaging of deep subsurface structures. Currently, conventional OBS data analysis still uses first arrival traveltime tomography and pre-stack Kirchhoff depth migration method. However, using first arrival traveltimes to build a velocity model has its limitations. In the Taiwan region, subduction and collision cause very complex subsurface structures and generate extensive basalt-like anomalies. Since the velocity beneath basalt-like anomalies is lower than that of high velocity anomalies, no first-arrival refractions for the target areas occur. Thus, conventional traveltime tomography is not accurate and amplitude constrained traveltime tomography can be dangerous. Here, a new first-arrival stochastic tomography method for automatic background velocity estimation is proposed. Our method uses the local beam semblance of each common-shot or common-receiver gathers instead of first-arrival picking. Both the ray parameter and traveltime information are utilized. The use of Very Fast Simulated Annealing (VFSA) method also allows for easier implementation of the uncertainty analysis. Synthetic and real data benchmark tests demonstrate that this new method is robust, efficient, and accurate.
In addition, migrated images of low-fold data or data with limited observation geometry like OBS are often corrupted by migration aliasing. Incorporation of prestack instantaneous-slowness information into the imaging condition can significantly reduce migration artifacts and noise and improve the image quality in areas of poor illumination. Here I combine slowness information with Gaussian beam depth migration and implement a new slowness driven Gaussian beam prestack depth migration. The prestack instantaneous slowness information, denoted by ray parameter gathers p(x,t), is extracted from the original OBS or shot gathers using local slant stacking and subsequent local-semblance analysis. In migration, we propagate both the seismic energy and the principal instantaneous slowness information backward. At a specific image location, the beam summation is localized in the resolution-dependent Fresnel zone where the instantaneous-slowness-information-related weights are used to control the beams. The effectiveness of the new method is illustrated using two synthetic data examples: a simple model and a more realistic complicated sub-basalt model.
Return to listFLOW DYNAMICS OF THE PITCHSTONE PLATEAU OBSIDIAN FLOW IN YELLOWSTONE NATIONAL PARK, WY
Casey Huff, B.S.
The University of Texas at Austin, 2008
Supervisor: James Gardner
Extremely large volume rhyolite flows are known to exist, but have never been witnessed, and so little is known about how such massive, high viscosity lava flows erupt. To better elucidate their behavior, this project focused on the 70 km3 Pitchstone Plateau obsidian flow, erupted 72 ka from the Yellowstone caldera. Estimates of viscosity were combined with 3D size and number data to investigate the kinetics of groundmass crystallization and estimate flow dynamics of the using a suite of samples collected relatively evenly from near vent to the flow front. To constrain viscosity and kinetics of groundmass crystallization of the magma, we investigated the pre-eruptive storage conditions of the magma. The temperature was between 700-740°C in the chamber, and volatiles dissolved in glass inclusions contain up to 3.5 wt. % water. Groundmass glass water contents of ~0.13 wt. % water show that the melt degassed completely, which led to an increase in viscosity from <107 Pa s in the chamber to 1011.6-1012.2 Pa s during extrusion. Both the vol. % crystallinity and total microlite volumes per volume (Vtotal microlites / Vsample ) show two increasing trends within the flow. The minimum amount of crystallization within the flow was 0.16 % at the vent and the maximum was 3.37 % crystallization at the end of the flow. Crystal size distributions of pyroxene microlites showed two trends of coarsening with distance from the vent. For each trend the mode was between 2-4 µm closer to the vent, and decreased to between 0-2 µm distal the vent. Feldspar morphology also changed with distance from the vent. Closer to the vent, feldspars showed more tabular, equant shapes whereas at the end of the vent feldspars were non-tabular. The microlite patterns led us to conclude that microlite growth and nucleation are controlled by magma ascent rate in the conduit, and that these two trends probably represent two pulses of ascent within the conduit.
Return to listFACIES ARCHITECTURE OF THE MANZANILLA FORMATION, TRINIDAD AND TOBAGO, WI
Georgia Vernelle Huggins, MA
The University of Texas at Austin, 2007
Supervisor: Ron Steel
81 pages, 43 references, 1 table
In studying the Late Miocene-Early Pliocene Manzanilla Formation of Trinidad the interaction between tectonics and sedimentation was examined using the depositional facies and sandbody architectures (processes and sub-environments) during a crucial time in Trinidad’s geological history. The formation records the interaction between the southwards thrusting of the Caribbean Plate onto northern Trinidad and the sediment influx of the paleo–Orinoco delta system from the southwest.
The Manzanilla Formation was deposited in the northern of two basins on the island. This 1,500m thick succession consists of the San Jose Calcareous Siltstone, the Montserrat Glauconitic Sandstone and the Telemaque Sandstone members, which were deposited in a tidally influenced basin. This contrasts the time equivalent south Trinidad sub-delta systems of the Columbus Basin, which suffered open ocean swell and the dominance of waves.
The dispersal patterns of the sub-delta deposits of the Manzanilla Formation, the role of syn-sedimentary tectonics in the growth of its deltas, the delta regime, architecture and the reservoir properties of the sandstones of the Manzanilla Formation was studied.
Preliminary results suggest: (1) the Manzanilla sub-delta deposits were dispersed from the paleo- Orinoco delta in the west-southwest, into a relatively protected (Central and Northern Range uplifts) Northern Basin, (2) the distal deltaic deposits were interbedded with coarser –grained material from fans of the tectonically active Northern Range, (3) the Northern Basin was tectonically stable during the L. Miocene – E. Pliocene, and (4) the Central Range had a smaller uplift at this time, which created southerly ‘protection’ for the delta system and caused the Manzanilla deltas to be tidally dominated. (5) The tidal nature of this reservoir is the key to understanding hydrocarbon distribution.
Return to listBIOFUEL CLEAN DEVELOPMENT MECHANISM (CDM): A FINANCIAL FERTILIZER TO EXPAND BIOFUEL MARKET
In-Chul Hwang, MA
The University of Texas at Austin, 2008
Supervisor: Charles G. Groat
120 pages, 84 references, 11 table
Over recent years, world biofuel market has been rapidly expanding with strong governmental support such as biofuel mandates and tax incentives. Among many driving forces, agricultural interest has been a dominant driver behind current biofuel boom. But at the same time, it can constrain the further growth of burgeoning biofuel industry. Steep rise in food demand caused by population growth and improving standard of life will constrain growth potential of biofuel since it competes with biofuel for existing cropland and pastureland. Second-generation biofuel such as cellulosic ethanol or algae can provide a fundamental solution to various problems of current first-generation biofuel.
This thesis seeks to identify the world biofuel market potential and constraints on world biofuel market expansion and explores possible solutions to the constraints. Especially this thesis focuses on the applicability of Clean Development Mechanism (CDM) to biofuel projects as a solution to reduce financial burden of second-generation biofuel projects and thus contribute to expanding biofuel market.
Return to listPALEOGENE TO RECENT TECTONIC AND PALEOGEOGRAPHIC EVOLUTION OF THE CARIACO BASIN, VENEZUELA
Martha Alexandra Jaimes Carvajal, M.S.
University of Texas at Austin, 2003
Supervisors: William Fisher and Paul Mann
94 pages, 64 references, 16 plates
The Cariaco basin region, located offshore of north-central Venezuela, includes the Ensenada de Barcelona shelf, the Tortuga-Margarita platform and the Cariaco basin, the largest and deepest fault-bounded late Neogene basin within the South American-Caribbean right-lateral strike-slip plate boundary. Four active fault systems are recognized in the Cariaco basin region: 1) east-west striking San Sebastian-el Pilar right-lateral strike-slip fault system, expressed as a narrow sea floor valley overlying a negative flower structure that diagonally crosses the Cariaco basin; 2) east-west striking Tortuga-Margarita fault system that exhibits mainly down-to-basin, oblique-slip throw; 3) east-west striking Ensenada de Barcelona normal-fault system,, which displays a preferential downthrown to the south; and 4) northwest-striking, right-lateral Urica fault zone, located on the Ensenada de Barcelona shelf. The Urica fault is a lateral ramp fault that forms the western edge of thrusts and folds of the Serrania del Interior fold-thrust belt.
Cenozoic convergence between the Caribbean arc and the South American passive margin was highly oblique, and diachronous with migration from west to east during Eocene to Recent time. Oblique convergence in the Cariaco basin began in the Early Miocene and continued migrating eastward along major right-lateral, strike- slip fault systems. Five tectonosequences record the interplay of tectonic and sedimentation in the Cariaco basin: 1) a ~ 0.6 km-thick Paleogene tectonosequence formed when a Cretaceous-Caribbean volcanic arc rifted to form the Tortuga and Margarita basins; 2) a ~1.65 km-thick Lower Miocene tectonosequence was controlled by a fold-thrust belt developed in the Ensenada de Barcelona shelf; 3) a 1.2 km-thick Middle-Upper Miocene tectonosequence formed by a period of extension associated with tectonic loading and thrusting in the south; and a major basin, the Cariaco pull-apart basin initiated because of misalignment between the east-west striking, right-lateral San Sebastian and El Pilar strike-slip faults; 4) a ~0.6 km-thick Pliocene tectonosequence was controlled by a period of transtension, along the San Sebastian-El Pilar right-lateral fault system; the Ensenada de Barcelona shelf subsided to form a deep basin bounded by a horst structure in the north and was filled by northward-progradational delta complexes; 5) a 1.8 km-thick Pleistocene to Recent tectonosequence formed during growth and expansion of the rhomboidally-shaped Cariaco pull-apart basin; the large expansion of the basin is attributed to mass wasting and continued filling by south-sourced progradational delta complexes.
NEW PERSPECTIVES ON PLEISTOCENE BIOCHRONOLOGY AND BIOTIC CHANGE IN THE EAST-CENTRAL GREAT BASIN: AN EXAMINATION OF THE VERTEBRATE FAUNA FROM CATHEDRAL CAVE, NEVADA
Christopher Nathan Jass, Ph.D.
The University of Texas at Austin , 2007
Supervisor: Christopher J. Bell
The interaction between climate, environments, and mammalian faunas during the late Pleistocene-Holocene has been studied intently over the last several decades. Cave deposits play an important role in our understanding of these complex interactions, but they are especially significant for our understanding of the faunal history of the Great Basin . In order to develop a deeper time perspective on mammalian faunal change, I began a project that integrated several elements necessary for identifying and interpreting biotic change in the Great Basin of the western United States . These elements included development of a framework for understanding the importance of cave deposits for the paleontological record, collection of a mammalian fauna that pre-dates the terminal Pleistocene, identification of that fauna in the midst of shifting taxonomic paradigms, and evaluation of the fauna in the context of previous regional biogeographic models.
I utilized data from the FAUNMAP database to evaluate the significance of the contribution that cave deposits make to the Pleistocene mammal record. Caves do provide unique faunal data in addition to contributing a high percentage of the individual species records for late Pleistocene mammals.
Fieldwork was conducted at Cathedral Cave , NV , in order to assess a fauna that was thought to predate the late Pleistocene-Holocene transition. In excess of 30,000 identifiable fossils were recovered in an excavation area that was roughly 1.5 x 2 x 0.7 m. Prior to fieldwork in 2003, age estimates for the fauna were between 750 ka to 850 ka. New chronologic analyses suggest a more recent age ( = 146.02±2.584 ka to 151.2±4.4 ka) that extends the known chronologic distributions of several taxa and alters previously established biochronologic frameworks for the Pleistocene. This work also calls into question previous age assignments for portions of Smith Creek Cave .
Individual faunal identifications were made using a conservative data-reliant approach in order to minimize geographic assumptions and render an independent data set useful for broad biogeographic analyses. Although the faunal data presented here do not explicitly support or refute regional biogeographic models, they do indicate that patterns of faunal change can be found even when species-level identification are not achieved.
RESIDUAL MIGRATION VELOCITY ANALYSIS IN THE PLANE WAVE DOMAIN: THEORY AND APPLICATIONS
Junru Jiao, PhD
The University of Texas at Austin, 2001
Supervisor: Paul L. Stoffa
This dissertation addresses velocity depth model building using residual migration velocity analysis in the plane wave domain. The criterion used for residual migration velocity analysis is that the results of migration with the correct velocity-depth model should reveal the same geologic structure on common image gathers (CIG's). That is, the events on the CIG are horizontally aligned since they represent the image of the same subsurface position obtained at different angles. Use of an incorrect velocity-depth model in migration causes misalignment of events in a CIG, i.e., the events on the CIG exhibit residuals. By analyzing the residuals on the CIG, we can derive the depth and the velocity corrections and thus obtain a corrected velocity depth model.
I first discuss the kinematics of seismic wave propagation and explore pre-stack depth migration in the plane wave (t, p) domain. Then, I derive the exact one, two-, and three-dimensional residual migration equations in the depth-p domain after pre-stack depth migration. To perform interval velocity analysis, a suite of velocity corrections is tested to do residual migration but only one gives the best image. The combination of this velocity correction and the original migration velocity improves the velocity model. The two main advantages of the new method are that it derives interval velocities directly and is computationally very efficient because only a top down residual migration is needed instead of top-down pre-stack depth migration. Next, I apply the new method to both the synthetic and real seismic data. The synthetic data examples show that the 2D method gives a better residual migration result than the 1D method when strong dips are present but the 1D equation also works well for 2D models when the dip angles are small. After getting a new velocity depth model, one can use the new model to perform a complete residual migration which gives much better CIG's and stacked sections than those without residual migration. Alternatively, we can also use the new model to migrate the data again and then repeat the residual velocity analysis for another iteration. The number of iterations depends on the initial model and the precision required. In the field data example, a reasonable model was obtained after only four iterations.
Return to listMONETIZATION OF STRANDED NATURAL GAS IN THE UNITED STATES UNCONVENTIONAL RESOURCE PLAYS: DEVELOPING A GREEN INITIATIVE IN THE FORT WORTH BASIN BARNETT SHALE
Andrew Stephen Johnson, M.A.
The University of Texas at Austin, 2008
Supervisor: Charles C. Groat
World natural gas reserves are abundant, estimated at approximately 6,400 trillion cubic feet (Tcf) at the end of 2006. Much of this global supply of natural gas is considered stranded due to its location that is distant from consuming markets. The biggest challenge for exploration and production companies is to determine an economically feasible way to monetize this gas. These companies must determine whether it is economic to build a pipeline, or whether liquefied natural gas (LNG) is the best way to bring this gas to market. Exploration and production companies in the United States, unlike those in other countries, are in an advantageous position to monetize stranded gas through the development of unconventional resource plays.
The Barnett Shale in the Fort Worth Basin has progressed from a sub-economic unconventional resource play in the 1990s to the largest producing onshore natural gas field in the U.S. today. As production in the Barnett has crossed over the 2.6 Bcf / d threshold, drilling activity has extended beyond the Core, Tier 1, and Tier 2 counties to areas where the gas is considered stranded due to a constrained gas pipeline take-away capacity. Until the sufficient pipeline infrastructure is developed, advances in technology have enabled for implementation of small-scale LNG systems on well sites that provide a very attractive option for the monetization of stranded gas. Small-scale LNG systems are the first step towards a fundamental change to the utilization of LNG instead of diesel fuel that would not only enable exploration and production companies to optimally develop stranded gas reserve basins, but also provide invaluable economic, environmental, and social benefits in the future.
Return to listGEOCHEMICAL EVOLUTION OF GROUNDWATER IN THE PLEISTOCENE LIMESTONE AQUIFER OF BARBADOS
Ian Christopher Jones, Ph.D.
The University of Texas at Austin, 2002
Supervisor: Jay L. Banner
This investigation is a comprehensive study of the hydrology and hydrogeochemistry of the Pleistocene limestone aquifer of Barbados, a tropical karst island aquifer. The purpose of this research is to investigate how groundwater compositions in a tropical karst aquifer vary spatially and temporally, and determine the factors or processes responsible for these variations. These questions are addressed by evaluating spatial and temporal variation of groundwater major and trace element and isotopic compositions, and thus determining relationships between groundwater composition variations and various hydrologic and geochemical factors. These factors include: recharge processes, groundwater flow paths, soil compositions, aquifer and aquitard rock compositions, and anthropogenic inputs. This research provides insight into natural processes that influence groundwater compositions, allowing us to: (1) better understand geochemical processes taking place in an aquifer; (2) use groundwater constituents as tracers to determine seasonal and interannual variation of recharge and to estimate recharge amounts; and (3) establish relative importance of land use and recharge processes in the susceptibility of a karst aquifer to contamination.
Key results of this study include: (1) development of a method of determining the spatial and seasonal distribution of recharge, and determine interannual variations of recharge using oxygen isotopes; (2) determination that recharge has a greater influence on susceptibility of a karst aquifer to contamination than land use; and (3) spatial ad temporal variations of groundwater major and trace element and isotopic compositions reflect and respond independently to diverse processes, for example, seawater mixing and variations of the aquifer and aquitard rock compositions encountered along flow paths.
PROTEROZOIC TECTONIC EVOLUTION OF SOUTHERN LAURENTIA: NEW CONSTRAINTS FROM FIELD STUDIES AND GEOCHRONOLOGY IN SOUTHERN COLORADO AND NORTHERN NEW MEXICO, U.S.A.
by
James V. Jones III, Ph.D.
The University of Texas at Austin, 2005
Supervisor: James N. Connelly
New field studies and geochronology from southern Colorado and northern New Mexico constrain the Proterozoic growth and modification of southern Laurentia. The Sangre de Cristo Mountains of southern Colorado preserve evidence for three episodes of Proterozoic magmatism, deformation, and metamorphism. Early deformation produced penetrative, NW-striking fabrics and occurred in an arc setting between 1750 – 1730 Ma. Post-Yavapai granitoid magmatism occurred at 1695 ±2 Ma and 1682 ±3 Ma and was contemporaneous with regional quartzite deposition. Deformation at 1637 ±6 Ma produced localized NE-striking, subvertical fabrics with dextral shear sense. Granitic magmatism at 1434 ±2 Ma and 1407 ±6 Ma was accompanied by NW – SE shortening between 1420 – 1412 Ma that produced subvertical, NE-striking fabrics.
Thick sequences of quartz arenite were deposited across the region between the Yavapai and Mazatzal orogenies (ca. 1.70 Ga and 1.65 Ga). New geochronology reveals that deposition occurred on exhumed, Yavapai-aged basement (1706 ±5 Ma and 1698 ±4 Ma) with detritus dominated by Paleoproterozoic sources only slightly older than the quartzites themselves. Regional quartzite sedimentation was contemporaneous with nearly continuous magmatism in the region at deeper crustal levels. The first-cycle, syn-orogenic character of quartzites contrasts with their extreme compositional maturity, requiring perhaps anomalous environmental influences that enhanced chemical weathering during deposition.
New geochronology and structural studies from the Wet Mountains, Colorado, reveal contrasting structural styles during widespread Mesoproterozoic A-type granitic magmatism. At shallower crustal levels, strongly localized deformation at 1430+5/-3 Ma produced subvertical fabrics throughout the N-striking Five Points shear zone. At deeper crustal levels, penetrative deformation accompanying granitic magmatism at 1435 ±5 Ma and 1390 ±10 Ma produced moderately- to shallowly-dipping fabrics. Regionally consistent fabric orientations and kinematics are interpreted to represent an intracontinental response to convergent tectonism, and contrasting, yet coeval, styles of deformation require a structural discontinuity in the middle crust between ca. 1430 – 1360 Ma. Weak, flowing lower crust is consistent with models for intraplate orogenesis and the development of orogenic plateaus, and the southern Wet Mountains might represent an exhumed analog for mid-crustal, low-viscosity layers inferred beneath modern intracontinental orogenic systems such as Tibet and the Altiplano.
ASSESSING CONTROLS OF CLIMATE, VEGETATION, AND SOILS ON DIFFUSE GROUNDWATER RECHARGE USING UNSATURATED FLOW MODELING
by
Kelley Elaine Keese, MSGeoSci
The University of Texas at Austin, 2005
Supervisor: Bridget R. Scanlon
Understanding climate, vegetation, and soil controls on recharge is essential for estimating potential impacts of climate variability and land use/land cover change on recharge. Recharge controls were evaluated by simulating drainage in 5-m-thick profiles using a 1-D unsaturated flow code (UNSAT-H), climate data, and vegetation and soil coverages from online sources. Soil hydraulic properties were estimated from STATSGO/SSURGO soils data using pedotransfer functions. Vegetation parameters were obtained from the literature. Long-term (1961–1990) simulations were conducted for 13 county-scale regions representing arid to humid climates and different vegetation and soil types, using data for Texas, U.S.A. Areally averaged recharge rates are most appropriate for water resources; therefore, Geographic Information Systems were used to determine spatial weighting of recharge results from 1-D models for the combination of vegetation and soils in each region. Simulated 30-yr mean annual recharge in bare sand is high (51–709 mm/yr) and represents 23 to 60% (arid–humid) of mean annual precipitation (MAP). Adding vegetation reduced recharge by factors of 2 to 30 (humid–arid) and soil textural variability reduced recharge by factors of 2 to 11 relative to recharge in bare sand. Vegetation and soil textural variability both resulted in a large range of recharge rates within each region; however, spatially weighted, long-term recharge rates were much less variable and were positively correlated with MAP (r 2=0.85, vegetated sand; r 2=0.62, variably textured soils). The most realistic simulations included vegetation and variably textured soils, which resulted in recharge rates from 0.2 to 118 mm/yr (0.1-10% of MAP). Mean annual precipitation explains 80% of the variation in recharge and can be used to map recharge.
HYDROGEOLOGIC SIGNIFICANCE OF DEPOSITIONAL SYSTEMS ON GROUNDWATER QUALITY AND WATER RESOURCES IN THE CISCO GROUP, UPPER PENNSYLVANIAN AND LOWER PERMIAN FACIES, ROLLING PLAINS, TEXAS
Katherine S. Kier, M.S. Geo. Sci.
The University of Texas at Austin, 2004
SUPERVISOR: Alan R. Dutton
121 pages, 54 references
Sixteen alternating carbonate and terrigenous depositional systems compose the Cisco Group (Upper Pennsylvanian, Lower Permain), part of the Eastern Shelf of the Midland Basin, of North-Central Texas. This study examines the role that the complex hydrogeology of the Cisco Group plays in determining the large heterogeneity in groundwater quality and resources across much of the Texas Rolling Plains.
A recognized problem in the area is the natural pollution of groundwater and surface water from salt springs and seeps from the Upper Pennsylvanian and Lower Permian. Whereas formational dip is west, toward the Midland Basin, the prevalent direction of groundwater flow is east, following the regional slope of the southern Great Plains. Discharge of brine from the Midland Basin and from salt-dissolution zones affects salinity in shallow aquifers and rivers across the Rolling Plains.
This project integrates geologic, hydrochemical, and hydrologic data in cross sections and plan view, and uses PHREEQC for geochemical analysis. Cross sections of the Cisco stratigraphy resolve vertical and lateral heterogeneity and indicate a strong hydrostratigraphic control on groundwater quality. A total of 654 water quality samples from Cisco formational units were extracted from the Texas Water Development Board (TWDB) groundwater database. TDS concentrations are less than 3000 mg/l in 93 percent of these samples. Sixteen Cisco saline water samples from the USGS produced water database are used in PHREEQC as an end member for geochemical mixing modeling. Mixing models confirm the influence and interaction of deep saline brines with freshwater end members in small quantities (~ ≥5 percent). Water quality is found to be controlled by: (1) mixing of locally recharged freshwater and saltwater moving eastward from the Eastern Basin and (2) reacting recharge water with formation minerals (mainly silicates and carbonates).
Although groundwater demands for all or parts of the six county study area are not expected to exceed supply needs, water demands for the neighboring Dallas-Fort Worth Metroplex are projected to increase up to 40 percent over the next fifty years. In order to provide for Texas’ growing population, options such as desalination are feasible for this study area in order to maximize the yield of groundwater supplies and should be incorporated into regional water plans.
Return to listTHRUST KINEMATICS IN THE LOWER CONGO BASIN,
DEEPWATER SOUTHERN GABON
Richard E. Kilby, M.S. Geo. Sci.
The University of Texas at Austin, 2005
CO-SUPERVISORS: Martin P. A. Jackson and Mark Cloos
Fold-and-thrust belts detach on Aptian salt along the toe of much of the Atlantic passive margin off West Africa. However, little is known about the timing and kinematics of this gravity driven shortening. For this study these topics have been investigated using a 3D seismic volume in the Anton Marin and Astrid Marin lease blocks of the Lower Congo Basin. A 27 km long by 14 km wide study area straddles the transition between updip extension and downdip thrusting. Distal structures comprise thrust faults regularly spaced at 3-4 km. Their strong seaward vergence is attributed to decoupling over thin salt. The salt became thinned by expulsion into passive diapirs before thrusting began. Thrust kinematics are analyzed using regional structural restoration, physical models and a new adaptation of a graphical technique originally designed for normal growth faults.
Thrust kinematics and salt tectonics are recorded by thickness variations in synkinematic sediments. These variations were quantified by measuring time-thickness differences in nine stratigraphic intervals along 64 traverses across six thrust faults. Fault activity indices (time-thickness ratios across thrust faults) identify the timing and intensity of slip for a specific fault. Comparison of fault activity indices both along strike and between different faults indicates that thrusting initiated in the distal part of the study area in the Albian and migrated landward in a break-back sequence. Distal thrusting ended when proximal thrusting culminated in the Late Cretaceous or early Paleogene. Structural restoration of a regional seismic line shows similar timing and corroborates the break-back thrust sequence.
While break-back thrust sequences appear to be commonly recognized, few detailed investigations exist of the phenomena, apart from the present study. Results of scaled physical experiments recreate a similar break-back sequence to those observed in seismic profiles and documented with the fault activity index and structural restoration. During shortening, distal structures tightened, and the underlying salt layer thinned as salt was fed into diapirs. Thinning of salt restricted salt flow and increased local basal resistance. At the same time, buttressing downdip became increasingly effective as the distal thrust belt thickened. The break-back sequence formed as younger thrusts initiated updip from the distal buttress.
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THERMAL, CHEMICAL AND STRAIN ANALYSIS OF A GRANITIC TECTONITE FROM THE WESTERN SALINIAN BLOCK, MONTEREY COUNTY, CALIFORNIA
Brian D. Knight, M.S. Geo. Sci.
The University of Texas at Austin, 2001
Supervisor: Mark Cloos
A 30-40 meter-wide zone of mylonitic quartz diorite was discovered in the Santa Lucia Range in seacliffs along the Pacific Ocean. When studied together with adjacent garnetiferous quartz-plagioclase veins, a detailed thermal, chemical, and tectonic history was developed for western Santa Lucia Range that has implications for the correlations between the Salinian block and the Sierra Nevada.
Petrographic analysis of the mylonite reveals that quartz deformed by crystal-plastic processes, while plagioclase occurs as cracked grains. Hornblende and biotite within the undeformed pluton are altered to chlorite and epidote in the shear zone. These microstructures and mineral assemblages are consistent with deformation under middle greenschist-facies conditions (300-450 °C). Rf/ff strain analysis, used to examine the strain geometry of deformed quartz grains in eight mylonitic samples, indicates the strain fabric ellipsoids have aspect ratios between 1.3 and 2.8. Whole-rock and trace element geochemical analyses for seven mylonitic and three undeformed samples were also obtained in order to study chemical changes that occurred during mylonitization. Calculation of systematic changes in the gain or loss of elements was done by normalizing concentration data to Al2O3, which was selected as a reference frame due to its low solubility in most metamorphic environments. Element mobility data reveal that the mylonites experienced bulk volume gains of 7 to 21% that were accommodated by major gains in SiO2 and K2O and minor losses in CaO and MgO relative to the protolith. These chemical changes are consistent with growth of quartz and k-feldspar in the mylonite at the expense of plagioclase, hornblende, and biotite in the protolith. The large volume gains indicate an influx and cooling of a potassic, silica-saturated fluid along the relatively permeable mylonite zone. The model proposed in this study suggests that the mylonite zone formed in the hanging wall above a major fault that juxtaposes granitic plutonic rocks above highly metamorphosed sedimentary rocks of the Sur Series.
Garnetiferous quartz-plagioclase veins, dated by the Rb-Sr method, yield an isochron age of 106 ± 11 Ma. In addition to detailed chemical and petrographic study of the veins, four garnet-biotite pairs were analyzed on the electron microprobe and Fe-Mg partitioning relationships yield temperatures between 700-800°C. This indicates that vein formation was contemporaneous with amphibolite- to granulite-facies metamorphism in the Santa Lucia Range. Vein origin is interpreted to be due to an influx of CO2-rich fluids through fractures, which locally dehydrated the wall rocks.
Existing age data was used to develop a cooling history for the region. The data suggest that mylonitization occurred between 80 and 72 Ma, contemporaneous with rapid cooling associated with the onset of low-angle subduction off the California margin. Also, two new fission-track ages of 52 Ma (zircon) and 2 Ma (apatite) indicate that the study area has a somewhat different Cenozoic cooling history than the rest of the Salinian block. This is explainable if the Palo Colorado fault, a strike-slip structure mapped within the Salinian block, has a significant component of dip-slip movement. On a larger scale, the lithotectonic evolution of the mylonite zone in the Santa Lucia Range is proposed as correlative to rocks with similar ages, cooling histories, and textures in the Tehachapi Mountains of the southernmost Sierra Nevada.
Return to listFRACTURE ORIENTATIONS AND RELATIONSHIPS IN THE HUECO TANKS SYENITE, EL PASO COUNTY, TEXAS
Jennifer Nicolle Knowles, B.S.
The University of Texas at Austin, 2008
Orientations and relationships of fractures at Hueco Tanks, El Paso County, Texas, can provide further evidence toward relative timing of tectonic events affecting the Hueco Bolson in Trans-Pecos Texas. This study concludes that the 32.3 Ma Hueco Tanks intrusion was emplaced during the transition from residual Laramide compression to Basin and Range extension, recording stress orientations through fracture response in the rock mass. Three prominent fracture sets are evident at Hueco Tanks. The first prominent set, fracture set one, comprises continuous, large-scale fractures trending NE-SW across West, East, and East Spur Mountains. Fractures of set one are consistent with stresses resulting from weak compression that lasted until ~ 32 Ma. Field relationships suggest that fracture set one is the oldest. Fracture set two consists of continuous, shallow dipping, exfoliation sheets which are characteristic of unloading and decompression during exposure. Fractures of set two commonly terminate at fractures of set one, suggesting that these fractures are younger. Fracture set three consists of fractures localized in a linear zone on the eastern half of East Mountain and northeast East Spur. These fractures are oriented NNW and NNE and dip steeply west, constituting a dominant N-S trend. This localized set terminates at fractures of set two, and are interpreted as younger.
Observations from flow features and from fracture behavior suggest that rather than a stock, Hueco Tanks has a laccolith-like morphology, with North Mountain serving as the magma conduit, for the more sill-like masses that comprise West, East and East Spur Mountains.
Return to listCHARACTERIZATION OF THE CUNNINGHAM SAND IN THE SOUTHEAST BRADLEY SPRINGER UNIT OF THE GOLDEN TREND: IDENTIFICATION OF A DELTAIC SYSTEM
Robert Lee Koch Jr., M. S. Geo Sci
The University of Texas at Austin, 2002
Supervisor: William L. Fisher
The Southeast Bradley Springer Unit, (SEBSU) is a mature field that was discovered in 1953 along the western margins of the Golden Trend in Oklahoma. The reservoir is the early Pennsylvanian Cunningham Sand of the Springer Group, which is comprised of six individual sand units, three upper sands and three lower sands. The trap for the SEBSU was formed by an erosional truncation at the base of the Atokan Unconformity caused by the uplift of the Nemaha Ridge to the east. The truncated sand was then onlapped by the Deese Group creating a seal.
The level of knowledge of the geology of the SEBSU during development in the 1950's is not known but, but it can be assumed that the interpretations are based on a theoretical framework of that time. This led to a simple layer-cake reservoir model that is inconsistent with the reservoir architecture. Presently the unit is under tertiary development and it has become apparent that the lack of geologic understanding of the reservoir architecture is the cause of numerous production problems. This project consists of defining the depositional environment and the framework of the Cunningham reservoir. The depositional framework is a controlling factor for the reservoir distribution and lateral continuity. Understanding these factors can improve the recovery from the reservoir.
The deltaic model for the reservoir was identified using a suite of data that included two cores, well logs, existing reservoir maps, and well files. The characteristics seen in the two reservoir cores were consistent with deltaic deposits. The lower three sands of the Cunningham typically exhibit a progradational log signature which is characteristic of deltaic deposits. The upper three sands have an abrupt signature characteristic of reworked deltaic deposits. Log correlations and cross sections display lateral facies changes from a fining-upward distributary channel out into delta front deposits. Isopach maps for the lower three sands display a dip orientation that coincides with the distributary channel orientation. The thickest sand, found in the northeast portions of the unit, appeared to be oriented in the down-dip direction, to the west and southwest. This emphasized the main deltaic axis created by the distributary channel prograding onto the shelf, and thinner delta front sands surround the distributary. The isopach map of the upper most sand displays a strike-oriented trend characteristic of wave reworked deltaic deposits forming a coastal or shelf sand bar.
The structural map of the base of the Cunningham, shows the western portion of the SEBSU are structurally complex. While these issues were not examined in this study, the structurally complex area contributes to reservoir discontinuity and complexity.
With knowledge of the depositional model for the Cunningham sands, an accurate reservoir model can be developed to determine and predict lateral variations in the reservoir and help solve many of the production related issues.
Return to listEVALUATION OF DNA-LABELED CLAY PARTICLES FOR USE AS A GROUNDWATER TRACER
Daniel J. Krause, M.S. GEO. SCI.
The University of Texas at Austin, 2002
Supervisor: Philip C. Bennett
Tracing is an important hydrogeologic tool for characterizing aquifers and water resources planning. Metals, hydrophobic contaminants, particulates, and microbes are all present in aquifers as potential contaminating sources. Tracers can mimic contaminants thereby better describing their behavior in the subsurface. Labeling particles is a method that can be implemented to overcome problems common to traditional tracing methods. DNA has the potential for an ideal labeling material for several reasons, namely its extreme sensitivity to amplification. Sorption of DNA to a bentonite source clay was determined through a series of experiments involving the amplification of any unsorbed DNA in the supernatant. Remaining sorbed DNA was then physically removed from the clay surface to verify initial attachment. The DNA used in these experiments seems to suffer from extreme dilution therefore a sorption isotherm was not described. Moreover, PCR in native waters does not afford control over reaction necessary for robust product. The complexity of natural aquifer water chemistry made PCR inconsistent. These data suggest extensive methods development is necessary to achieve a field deployment of this tracer.
Return to listLINKING THE NAGSSUGTOQIDIAN AND RINKIAN OROGENS OF WEST GREENLAND THROUGH THE DISKO BUGT REGION BY U-PB GEOCHRONOLOGY
Adam Wesley Krawiec, M.S. Geo. Sci.
The University of Texas at Austin, 2003
Supervisor: James N. Connelly
129 pages, 48 references, 1 plate
A significant portion of the North American Craton is composed of Archean age terranes that were assembled into a stable craton during the Paleoproterozoic. Recent work in the North Atlantic region of North America has shown that parallel, paired orogenic belts that were previously thought to be distinct may, instead, be related and constitute single, wide highly asymmetrical orogens. This study aims to test a correlation of the Nagssugtoqidian and Rinkian orogenies of West Greenland based on geochronological data from the Disko Bugt region.
The Paleoproterozoic Nagssugtoqidian Orogen has been extensively studied and a detailed geochronological history has been established. The tectonic history of the Rinkian Belt remains largely unresolved due to a lack of detailed mapping and geochronological framework. The Disko Bugt region lies between the well-defined Nagssugtoqidian Orogen and lesser-known Rinkian Belt. Similarities in deformation and style in the Nagssugtoqidian and Rinkian orogens suggest a genetic link exist between the two. If deformation in the Disko Bugt region can be correlated with the Nagssugtoqidian Orogen, it would imply that the Nagssugtoqidian and Rinkian orogens comprise a single >1100 km-wide asymmetrical collisional zone, similar in extent to the present day Himalayas.
Four previously recognized domains were sampled in a N-S transect across Disko Bugt and analyzed using both thermal ionisation mass s pectrometry (TIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Samples were selected that could provide protoliths ages and the age of deformational and metamorphic events in the region.
Results from twelve samples suggest the presence of two Archean blocks in Disko Bugt, each with a distinct age and geological history. These new ages necessitate a re-examination of the previously described domains. In addition, temporal elements of the Southern Block are coeval with the basement of the Nagssugtoqidian Orogen. Whereas the Northern Block shares a similar history with the basement of the Rinkian Belt. This evidence supports the idea that the Disko Bugt harbors a suture between two Archean terranes, a required component that has yet to be confirmed in West Greenland. Paleoproterozoic deformation across Disko Bugt is coeval with that in the Nagssugtoqidian and Rinkian orogens, thus, establishing a link between the two belts.
Return to listTECTONICS AND SEQUENCE STRATIGRAPHY OF THE VENEZUELAN BASIN, CARIBBEAN SEA
Margaret Emily Kroehler, M.S. Geo. Sci.
The University of Texas at Austin, 2007
Supervisors: Paul Mann & Gail Christeson
The central core of the Caribbean plate is the 3-4-km-deep Venezuelan Basin that is underlain by late Cretaceous oceanic plateau and oceanic crust. The Venezuelan Basin is obliquely subducted to the east-southeast beneath the continental South American plate at the east-west-trending South Caribbean deformed belt (SCDB), a 50-100-km-wide wedge of accreted sedimentary rocks. I have mapped the structure and sequence stratigraphy of five late Cretaceous to recent sedimentary sequences overlying a 230,000 km 2 area of the Venezuelan Basin . The dataset includes ~5900 km of 2D seismic reflection data acquired during five separate surveys from 1974-2004; these seismic data are tied to DSDP drill sites 146/149 and 150 which provide age and lithologic control on the interpreted sequences. I use these data to document older Cretaceous faults formed soon after the formation of the plateau and oceanic crust; to explain the along-strike variations in structural style of the actively subducting Caribbean plate; and to constrain the age and tectonic mechanism for the initiation of subduction along the SCDB. The earliest set of faults are normal faults, only affect rocks of Late Cretaceous age, and approximately parallel a set of seafloor-spreading magnetic anomalies described by previous workers. These normal faults are widely spaced in the area of smooth-topped lava flows making up the 15-km-thick oceanic plateau, but they are much more densely distributed in the area of abnormally thin (3-5-km-thick), rough-surfaced, oceanic crust in the eastern Venezuelan Basin. The faults are interpreted as the result of back-arc extension behind the Aves volcanic arc during the late Early to Late Cretaceous. Selective reactivation of inherited faults and new bending-related faults affecting late Cretaceous to recent sedimentary sequences strike east-west and parallel the trend of the SCDB. The age of the initiation of subduction at the SCDB has been determined based on the age of onlapping, wedge-shaped, sedimentary sequences in the Venezuelan Basin . Subduction began in the western Venezuelan Basin in the middle Eocene in the western study area, in the early Miocene in the central study area, and in the post-early Miocene-Recent in the eastern study area. The ages of subduction agree with a similar eastward younging in the age of folding and thrusting in northern South America . From this observation I infer that subduction of the Caribbean plate beneath the SCDB initiated as a backthrust response to collision between the Caribbean arc and the South American continent that began in the early Eocene and youngs in an eastward direction.
EFFECTS OF URBANIZATION ON HYDROGEOLOGICAL SYSTEMS: THE PHYSICAL EFFECTS OF UTILITY TRENCHES
By
Jason Noel Krothe, M.S. GeoSci
SUPERVISOR: John M. Sharp, Jr.
The effects of urbanization and, specifically, the construction of utility trenches can have a profound affect on groundwater flow systems and contaminant transport. Construction of sewer, water and other utility lines can greatly disturb the natural hydraulic characteristics of surficial geologic materials. Field hydraulic conductivity (K) tests indicate that trench hydraulic conductivities ranged from one to ten orders of magnitude higher than the undisturbed soils developed at sites on the Austin Chalk, Glen Rose Limestone, and the Colorado River alluvium. Using these data MODFLOW computer models simulated the effects of higher hydraulic conductivities on groundwater flow and contaminant transport. Three different trench geometries were used for the simulations: 1) parallel to groundwater flow; 2) perpendicular to groundwater flow; and 3) a hypothetical city utility system.
The parallel and hypothetical city trench geometries resulted in several significant findings on contaminant transport in urban areas. The first finding is that contaminants flowed in directions not predicted by the direction of the regional hydraulic gradient. The second finding is that the contaminants move as much as ten TimesNewRoman faster through the utility trenches than through the undisturbed soil. These findings suggest that contaminant remediation would be more difficult and would necessitate more comprehensive fieldwork before suitable remediation plans are devised. Finally simulations, 1 and 3, suggests that lateral dispersion of the plume increased with increased hydraulic conductivity of the utility trench. This increase in lateral dispersion could result in an increase in natural attenuation. Simulation 2 also indicates that if the trench is orientated perpendicular to groundwater flow no change in plume speed or lateral dispersion occurs. The lack of change to the system in Simulation 2, and the dramatic changes seen in simulations 1 and 3, suggest that trench orientation to groundwater flow is the major factor in the alteration of groundwater flow.
Return to listPARAMETER INVERSION FOR SEISMIC ANISOTROPY
Chandan Kumar, Ph.D.
The University of Texas at Austin , 2006
Supervisors: Robert J. Ferguson and Mrinal K. Sen
An important objective of reflection seismology is to transform a seismic dataset into a reflectivity image of the subsurface. Reflectivity imaging involves removal of propagation effects by creating an inverse propagation operator using an estimated macro velocity model. The accuracy of a reflectivity image depends on the accuracy of the velocity model used. Hence, the estimation of a good velocity model is crucial. Flat sedimentary layers, though laterally homogeneous over small-scale, often exhibit a simple kind of anisotropy called vertical transverse isotropy (VTI). Negligence of anisotropy in migration may result in significant deviation of an imaged reflector from its true position. Our ability to estimate anisotropic parameters using limited aperture, limited bandwidth seismic data is restricted. Several recent developments in data acquisition including more intense acquisition (e.g., 3-D, 4-D), broader bandwidths, and new survey methodologies (e.g., Ocean Bottom Seismics and vertical cables) have improved our ability in dealing with anisotropy. In light of these recent developments the objective of my research is three-fold - (i) finding better algorithms for anisotropy estimation that makes use of enhanced data acquisition, more powerful computers and graphical interfaces, (ii) understanding and re-evaluating our limitations/capabilities of anisotropy estimation in light of the new algorithms, and (iii) using the results of analysis as an aid to constrain anisotropy estimates.
The Common Focus Point (CFP) technology defines a recent method of prestack migration velocity analysis which has several advantages over other popular methods. One remarkable advantage is that the imaging error is given directly in time allowing easier model updates through inversion. The CFP technique has been used in recent past for estimating isotropic heterogeneous velocity model for geologically complex subsurface. For my research, I extend this method to anisotropic parameter inversion by suitably implementing the space-time domain version of CFP method for picking the traveltime error resulting from the incorrect guess model. I successfully implement this approach to estimate anisotropy of the shale thrust-sheets in the Canadian Foothills model. This model has targets of interest below transversely isotropic shale thrust-sheets and estimating anisotropic parameters is important for proper depth imaging of target zone. The synthetic P-mode data used for the analysis has been generated using a finite difference algorithm.
In order to quantitatively assess the advantages or limitations of CFP domain velocity analysis in estimating the anisotropic parameters, I have performed sensitivity analysis under different experimental settings like different observation apertures, layer thicknesses, tilt in symmetry axis, picking error and the use of mode converted data. The results quantitatively establish the advantage of joint inversion of P-P and P-Sv over the conventional inversion of solely P-P data in constraining the depth and anisotropic parameters. Also the constraint on parameter estimation improves with increase in angle of tilt of symmetry axis with respect to the reflector.
The CFP domain analysis also has advantages in dealing with mode converted P-Sv data as separate focusing at the receiver side and source side allows it to deal with one wave type at a time. In complex media, this helps avoid the cumbersome calculation of common conversion point (CCP) location. The CFP analysis has been applied for estimating anisotropic parameters as well as layer thickness by joint inversion of P-P and P-Sv synthetic data for the geology encountered in Blackfoot field.
ANALYSIS OF MULTICOMPONENT SEISMIC DATA FROM THE HYDRATE RIDGE, OFFSHORE OREGON
Dhananjay Kumar, Ph.D.
The University of Texas at Austin, 2005
Supervisors: Mrinal K. Sen and Paul L. Stoffa
Multicomponent seismic data can be used to derive P- and S-wave velocity structures of the subsurface, which can be used further to estimate rock and reservoir properties. Most seismic analysis methods and algorithms assume that the earth is isotropic. In many geologic situations, however, sedimentary rocks exhibit anisotropic behavior, and the isotropic assumption will introduce errors in the estimates of the elastic properties of the subsurface. With the goal of analyzing multicomponent seismic data from complex regions (which may show anisotropic behavior), I have developed new algorithms for 1) seismic modeling based on a ray-Born approximation and 2) traveltime computation in tilted transversely isotropic media based on Fermat’s principle. This new traveltime computation algorithm is tested on prestack depth migration of a physical model dataset. Such algorithms are essential for estimating subsurface rock properties in complex areas such as the Hydrate Ridge area, offshore Oregon.
I participated in the acquisition of multicomponent seismic data (summer 2002), at the Hydrate Ridge of the Cascadia convergent margin. The primary goal of the experiment was to map the gas hydrates and free gas, and to understand the mechanism of fluid migration. Gas hydrate is an ice-like substance that contains low molecular weight gases (mostly methane) in a lattice of water molecules. Gas hydrates and free-gas are generally detectable with seismic methods because the seismic velocity increases in the presence of gas hydrates, and it decreases in the presence of free-gas. My analysis results in estimates of P- and S-wave interval velocities and anisotropic parameters with the final goal of relating these parameters to the presence and quantification of gas hydrate and free gas. I performed interval velocity analysis in the τ-p (intercept time - ray parameters) domain following three main steps: 1) P-wave velocity analysis, 2) P- to S-wave (converted PS-wave) event correlation, and 3) S-wave velocity analysis. P- to S-wave event correlation is done using synthetic seismograms and traveltime tables. Seismic velocities are correlated to gas hydrate and free gas saturation using a Modified Wood equation. I find that Hydrate Ridge is heterogeneous and is weakly anisotropic (maximum of 10%) in some regions caused possibly by the hydrate veins. The P-wave velocity is more sensitive to the saturation of gas hydrates (maximum of 7% of rock volume) and free gas than the S-wave velocity. The S-wave velocity does not show an anomalous increase in the hydrate-bearing sediments. Thus, I conclude that hydrate does not cement sediment grains enough to affect shear properties. It is more likely that the hydrates are formed within the pore space in this region.
PORE PRESSURE PREDICTION USING MULTICOMPONENT PS-WAVE SEISMIC VELOCITIES: COLUMBUS BASIN, TRINIDAD W.I.
Kimberly Melissa Kumar, Geophysics MSGeoSci
The University of Texas at Austin , 2006
Supervisor: Robert J. Ferguson
I estimate overpressure in a seismic cross-section along a 12km traverse associated with a 2D/4C OBC line in the Columbus Basin , Trinidad, West Indies , where shallow gas reduces both data quality and apparent seismic velocity for P-waves, using a modified Eaton's equation for PS-waves. Pore pressure prediction using the modified Eaton's method involves velocity analysis, conversion of the stacking velocities to interval velocities via the Dix's equation, converting the interval velocity trends to pressures and mudweights, and comparison of the predictions to 3 wells surrounding the seismic line.
In the presence of shallow gas, the magnitude and areal extent of seismically derived P-wave and PS-wave velocity deviates from regional trends along the seismic line. PS-waves show a more accurate areal extent of velocity deviation due to overpressure than the P-wave, which is also affected by the presence of shallow gas. I verify my derived velocities and predicted-pressure values using sonic log data and observed pressure from 3 wells. Direct comparison between the sonic-derived velocities and the seismic-derived velocities shows that shallow gas reduces P-wave velocity, and that PS-wave velocity is less affected. Pressure prediction is verified using mudweights and formation tests from well logs and drilling reports. I find pressure predictions associated with P-waves, especially in areas of shallow gas are less reliable than for pressure predictions using PS-waves. I conclude that PS-wave velocity provides a superior map of overpressure in this region in areas with shallow gas clouds.
PETROLOGY OF THE SOUTHWEST MARGIN OF THE GRASBERG IGNEOUS COMPLEX, PAPUA, INDONESIA
Adam L. Lambert, MS GeoSci
The University of Texas at Austin , 2008
Supervisor: Mark Cloos
398 pages, 46 references, 1 table
The 3 Ma Grasberg Igneous Complex (GIC) of Papua, Indonesia is host to a super-giant porphyry copper-gold deposit. The GIC is shallowly emplaced into folded and faulted limestones that are as young as late Miocene. The Heavy Sulfide Zone (HSZ) is a pyrite-rich shell that surrounds the GIC. The HSZ grades into the Marginal Breccia. Near the surface the Marginal Breccia is overlain by the Banded Clay, a halloysite-clay rich unit. This study has determined the relationship of these units and characterized the pervasive alteration from the petrology of 588 samples collected from five drill cores and 23 outcrop samples.
The initial Dalam phase of intrusion generated ~5 m of skarn (epidote, garnet, and clinopyroxene) at the margin of the GIC below 3700 m elevation. The precipitation of magnetite in the core of the GIC, the hydrolysis of SO2, and the precipitation of pyrite in the periphery of the deposit generated abundant HCl and H2SO4. In the outer GIC, primary igneous minerals were altered to sericite, quartz, and pyrite by the acidic hydrothermal fluids. These fluids dissolved carbonate host rock and precipitated abundant pyrite. Layered dissolution infill indicates porosity was locally very high. High permeability along the margin of the GIC would have channeled hydrothermal fluid upwards.
As supporting carbonate rock was dissolved forming the Marginal Breccia, open fractures formed along the contact with the GIC. Pyrite precipitated into these fractures, forming layering in places. Unsupported pieces of igneous rock and skarn broke off and fell into openings which became filled with pyrite. This process caused most of the growth of the HSZ to progress inward, leaving rocks that were once at the margin of the GIC separated from it by lenses of massive pyrite. Minor sphalerite and galena precipitated at lower temperatures at the outer edge of the HSZ. Only trace amounts of copper sulfides are present within the pyrite-rich HSZ because nearly all the copper was precipitated in the center of the deposit. Any H2S or SO2 remaining in the upwelling fluids mixed with oxygenated meteoric waters forming concentrated H2SO4, which intensely altered volcanic sediments forming the Banded Clay.
The dissolution-enhanced permeability along the contact zone facilitated the drainage of magmatic fluids. This acted to increase the fluid pressure gradient driving pervasive infiltration between the center and the margin of the GIC. This was a significant contributing factor to the creation of the super-giant orebody. The dissolution of carbonate wall-rock caused the GIC to collapse outwards. This caused extension fracturing within the GIC that diverted fluid flow upwards and back into igneous rock. As a result some of the outermost igneous rock in the upper part of the GIC is relatively fresh, with primary groundmass orthoclase and plagioclase phenocrysts. The outward collapse of the GIC generated the flaring-upward shape that is a distinct attribute of the Grasberg system.
Return to listFATE AND TRANSPORT OF ARSENIC AND ANTIMONY IN THE EL TATIO GEYSER FIELD, CHILE
Jeffrey Thomas Landrum, MS Geo Sci
The University of Texas at Austin , 2007
Supervisor: Philip C. Bennett
El Tatio Geyser Field (ETGF), northern Chile , hosts widespread geothermal activity, with very high aqueous concentrations of arsenic and antimony, higher than any other known geothermal system. Boiling springs (86°C) discharge circum-neutral pH, Na-Cl type waters with low organic carbon. Net discharge of a stream draining the ETGF basin is approximately 10 cfs. As( III ), the dominant As species in discharge waters, rapidly oxidizes to As(V) at an estimated first order rate of 0.35 min -1 , determined in the field by first arrival of a tracer. As and Sb concentrations and speciation in hydrothermal waters, deposits, and microbial biomass are evaluated as a product of microbial metabolism, sorption to metal-oxyhydroxides, and co-precipitation. Mechanisms controlling these reactions ( i.e. cooling, evaporation, changes in redox and pH) are evaluated and modeled. Sequential extractions reveal that As, sorbs to Fe and Mn oxy-hydroxide complexes. In contrast, Sb solid phase partitioning is influenced primarily by the co-precipitation of Sb-oxide minerals with siliceous sinter (up to 2% wt. Sb). Diurnal variations occur in spring water chemistry and may cause Sb-rich laminations in siliceous hydrothermal deposits. Microbial energetics calculations and enrichment experiments suggest that microbial activity influences the mobility of As, and probably Sb in the ETGF basin by altering redox speciation and sorption to microbial biomass.
Return to listAN EVALUATION OF THE PRESSURE DEPENDENCE OF RARE-EARTH-ELEMENT CONCENTRATIONS IN GARNETS FROM THE LLANO UPLIFT
by Edward David Lane, M.S.Geo.Sci.
The University of Texas at Austin, 2003
SUPERVISOR: William D. Carlson
184 pages, 31 references, 1 CD-ROM
Application of a proposed rare-earth element (REE) geobarometer to highpressure metamorphic rocks from the Llano Uplift in central Texas yields pressures inconsistent with phase assemblages and garnet-orthopyroxene barometry. However, REE-abundance curves appear to vary systematically with pressure, suggesting that more elaborate barometers based on REE behavior may eventually be devised.
The Llano Uplift has experienced a complex metamorphic and deformational history, including an early high-pressure metamorphism and a later low-pressure heating event. The later event caused widespread resorption of the high-pressure equilibrium mineral assemblages, which greatly limits the application of quantitative geobarometers that rely on analysis of equilibrium vi assemblages. This study sought to extrapolate a new geobarometer to high pressure to increase understanding of the early metamorphic history of the Llano Uplift.
Bea et al. (1997) proposed a single-phase geobarometer based on systematic flattening of garnet REE-abundance curves with increasing pressure, using the ratio of Gd to Dy in garnet as a proxy for this effect. Their results indicate a linear correlation between this ratio and pressure for pelitic rocks in a pressure range of 0.4 to 0.9 GPa. This study tests the extrapolation of this barometer to higher pressures, using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to measure REE concentrations at interior and rim locations of centrally sectioned garnets from garnet-clinopyroxenites found at Whitt Ranch in the northern portion of the Llano Uplift and at Purdy Hill in the western portion of the Uplift, and from pelitic gneisses found at the Purdy Hill locality.
Application of the Bea et al. (1997) geobarometer to Whitt Ranch garnets yields pressures that range from approximately 0.4 to 0.5 GPa; Purdy Hill mafic garnets yield pressures of approximately 0.4 to 1.0 GPa; and Purdy Hill pelitic garnets yield pressures of approximately 0.4 to 0.5 GPa. REE-abundance curves from Whitt Ranch garnets are steeper than those from Purdy Hill mafic garnets. Whitt Ranch garnet curves show a gradual flattening effect and the disappearance of negative Eu anomalies from interiors towards rims. Purdy Hill mafic garnet vii curves show a gradual steepening from interiors towards rims and positive Eu anomalies in both interiors and rims. Purdy Hill pelitic garnet curves show initial steepening followed by flattening in heavy REE (HREE) concentrations from interiors towards rims and the presence of negative Eu anomalies in both interiors and rims.
Pressures of 0.4 to 1.0 GPa for the high-pressure metamorphism are not consistent with garnet-orthopyroxene geobarometry for the western portion of the Llano Uplift, which indicates pressures of 1.5 to 2.1 GPa (Anderson, 2001) nor with pressure estimates based on garnet-inclusion suites in Whitt Ranch rocks, which indicate a transition from upper-amphibolite- to eclogite-facies conditions. Very slow diffusion rates for REE in garnet (e.g., Van Orman et al., 2002) suggest that these inconsistencies are not diffusion effects caused by the later event, but instead represent growth zoning. However, REE-abundance curves from Purdy Hill mafic garnets are flatter than those from Whitt Ranch garnets, which suggests that flatter curves reflect higher pressures, corroborating the results of Bea et al. (1997). Flattening of curves from interiors to rims suggests that Whitt Ranch garnets grew as pressure increased, which is supported by the distribution of inclusions in garnet: interiors contain plagioclase inclusions but rims do not, which also accounts for the rimward disappearance of negative Eu anomalies. Steepening of REE-abundance curves in Purdy Hill mafic garnets may imply growth during decompression, although positive Eu-anomalies suggest viii breakdown of plagioclase during garnet growth. REE-abundance curves in pelitic garnets are more complex, and may record a change in the accessory phases present during the latter stages of garnet growth.
The ratio of Gd/Dy alone fails to capture the complex behavior of the REE over the full range of pressure commonly experienced by crustal metamorphic rocks, although the shapes of the REE-abundance curves for the Llano rocks change systematically with pressure in a manner similar to the pattern described by Bea et al. (1997).
PHYSICAL MODELING OF NORMAL FAULTS AND GRABEN RELAYS ABOVE SALT
Joël Hervé Le Calvez, Ph.D.
The University of Texas at Austin, 2001
Supervisors: Randall Marrett and Bruno Vendeville
In salt basins, thin-skinned extension is typically accommodated by graben development in sedimentary overburden and viscous flow in the evaporitic layer. This study presents a series of physical models in which grabens propagate along strike and interact in relay zones whose geometry depends on graben across-strike separation.
I first describe the basis for physical modeling, including scaling analysis. I then review various experimental designs used by previous authors to model the interaction between laterally offset faults, finally I introduce a design in which faults nucleate at predetermined locations above small initial instabilities but that permits faults to propagate freely thereafter.
I used series of systematic experiments to investigate the 3-D geometry and evolution of two laterally offset grabens interacting in a relay zone. Results allow the determination of the influence of key geologic parameters on the evolution of relay zones. The main parameter, the across-strike offset between the grabens, controls the size (along both strike and dip) of the relay zone and its structural style. Where the offset is large, fault traces curve gently toward one another and fault throw and amount of diapir rise below the graben decrease progressively toward the relay zone. Some distance from that zone, old faults cease to slip as new faults form. In the relay zone old fault segments remain active and no new faults form. Some new faults hard-link along strike with active segments of older faults. Offset grabens and faults start interacting only when they have propagated along strike far enough to overlap. Syntectonic sedimentation can suppress new-fault formation during extension. Another experiment shows that inversion of graben relays by late shortening forms reverse faults rooted at depth on the crest of underlying diapirs.
A second series of experiments, motivated by a field example in Canyonlands National Park, focuses on the genesis of graben arrays having regionally arcuate traces. Curvature is explained in terms of shear stresses between extending domains of the sedimentary cover, which is underlain by an evaporitic décollement, and lateral nonextending domains, a setting common along salt-bearing passive margins during thin-skinned extension.
Return to listTHE CONCENTRATION OF OFFSHORE DRILLING MARKETS
Jaewon Lee M.A.
The University of Texas at Austin, 2008
Supervisor: Christopher J. Jablonowski
71 pages, 36 references, 35 tables
This paper investigates the history of mergers and acquisitions in global offshore drilling markets from 1990 to 2005. Based on this history, the effect of increasing market concentration on prices (rig rates) is examined using a regression analysis. Industry concentration is measured by The Herfindahl- Hirschman Index (HHI). Two segments of the global drilling rig market are examined: jackup and semisubmersibles. There is strong evidence to support the conclusion that increasing market concentration in the jackup drilling rig market has resulted in increased prices; the evidence is weaker for the semisubmersible drilling rig market, but there appears to be a positive impact in this sector also This result is important for industry as it tries to understand the drivers for ever- increasing exploration and development costs. This result is also important for anti-trust regulators who are charged with maintaining competitive markets. A thorough analysis and formal treatment of the time series aspects of the data sets is outside the scope of this thesis and has been reserved for future analysis.
Return to listRADAR INTERFEROMETRY MEASUREMENT OF LAND SUBSIDENCE IN EL PASO, TEXAS
Erick Leuro, MSGeoSci
The University of Texas at Austin, 2004
Supervisor: Clark R. Wilson and Sean Buckley
119 pages, 46 references
This work presents the application of radar interferometry to detect land subsidence associated with water pumping in El Paso, Texas and adjacent areas. Geological and hydrological information are compared with the radar information to validate the results. An error treatment of the measurements is performed using the singular value decomposition technique.
Synthetic aperture radar interferometry is a proven remote sensing technique to detect ground deformation in a three-dimensional scale with millimetric precision. It has been applied successfully in earthquake monitoring, volcano deformation, glacier movement and aquifer compaction.
El Paso and Ciudad Juarez are located over the Hueco Bolson aquifer, an unconsolidated alluvial aquifer that consists of gravel, sand, silt and clay. Because of increased water pumping since the early 20 th century, the water table has changed and subsidence has occurred. Measurements of land subsidence are reported from the 1950s, 1970s, and 1980s. This work considers subsidence in the 1990s.
Return to listSTRUCTURAL ANALYSIS AND DETRITAL ZIRCON PROVENANCE IN THE WESTERN LLANO UPLIFT: IMPLICATIONS FOR A SOUTHERN COLLIDER
by
Jamie Sloan Fentiman Levine, MSGeoSci
The University of Texas at Austin, 2005
SUPERVISOR: Sharon Mosher
Structural and metamorphic analysis, mapping and detrital zircon geochronology was conducted within the Mesoproterozoic Valley Spring Gneiss, in the western Llano Uplift, along an 8 km section of the Llano River west of Castell, TX. Comprising granitic gneisses and pelitic schists, with volcanic, plutonic and supracrustal protoliths, the protolith to the Valley Spring Gneiss was deposited on Laurentia and records an uppermost amphibolite facies, polyphase deformation history consistent with continent-continent collision during the Grenville orogeny.
The Valley Spring Gneiss is characterized by both ortho- and paragneisses. To determine the provenance, three paragneiss samples were analyzed using laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to establish ages for 80 zircons from each sample. All three samples show a major peak at 1200-1350 Ma and a smaller Paleoproterozoic peak. One sample had a small Archean peak. All data are consistent with a Laurentian origin for the sediment, with Mesoproterozoic sediment derived locally. Kalahari and Amazonia, which have been proposed as colliders with Laurentia, are unlikely sediment sources for rocks of the Llano Uplift.
Six phases of synmetamorphic deformation have been documented. The mineral assemblage, including K-feldspar and sillimanite, indicates conditions above the second sillimanite isograd. Evidence for supersolidus conditions throughout deformation are widespread. Three types of leucosomes, indicative of partial melting, are parallel to early foliations. Pegmatites and granitic dikes are associated with late-stage folds and ductile shear zones; they increase in abundance eastward. The earliest deformation is characterized by two generations of isoclinal folds and penetrative metamorphic foliations, S 1 and S 2, which together form the dominant, northwest-striking, northeast- dipping metamorphic layering. The third phase, F 3, is characterized by tight folds that fold the S 1/S 2 foliation with an associated axial planar foliation. The F 3 fold axes plunge moderately to the southeast; axial planes are dominantly northwest-striking. Open, late generation folds (F 4 and F 5) refold earlier structures on both outcrop and map scales. F 4 and F 5 folds are northeast-plunging with northeast- to east-striking axial planes and southeast-plunging with northwest- striking axial planes, respectively. Late boudinage and shear zones containing melt are associated with an extensional D 6 phase of deformation. Granite and pegmatite intrusions are both syn- and post-tectonic as indicated by the presence or absence of the S 1/S 2 foliation, folding and boudinage.
The sequence, style and orientation of the structures in this area correlate with those within the Packsaddle Schist and the Lost Creek Gneiss of the western Uplift. Orientations, structural stacking and definition of domain boundaries differ between the eastern and western portions of the Uplift, supporting the likelihood that deformation in the eastern Uplift was controlled by collision of the exotic arc terrane present there, whereas the western Uplift, where the island arc is absent, directly records deep-seated effects of a continent-continent collision.
THE HYDROLOGIC FUNCTION OF SMALL SINKHOLES IN THE EDWARDS AQUIFER RECHARGE ZONE
by
Adrien Lewis Lindley, M.S.
The University of Texas at Austin, 2005
Supervisor: John M. Sharp, Jr.
Thin soils on Cretaceous karst bedrock typify the Edwards aquifer recharge zone in central and south Texas U.S.A. Most of the recharge occurs in major streambeds in this region, but abundant evidence of active karstic dissolution suggests that some of the recharge is occurring in the uplands. The City of San Antonio uses the karstic Edwards aquifer as its main municipal water source, and is undergoing rapid development in the uplands of the aquifer recharge zone. What is the risk to water quality of development over typical small-scale yet abundant upland karst features? This project is designed to determine more precisely what constitutes a significant recharge feature in the context of state law by directly measuring infiltration rates of typical upland karst features.
The hydrologic function of the soil and bedrock system in small sinkholes, identified by their morphological characteristics, and background areas is determined by large-scale constant head infiltration tests, microtopographic and soil thickness surveys, dye tracing and subsequent excavation of features, and imaging the subsurface with GPR.
Measurements made with a large-scale single ring infiltrometer compare typical upland karst features to paired control plots. The average infiltration per unit head for sinkholes is slightly higher than background in areas with similar soils (0.30 1/hr for sinkholes and 0.27 1/hr for background), though both are within the range for local soils. Results from infiltrometer experiments indicate the thin clay soils typical in the uplands dominates the infiltration process, yet recharge via these features under natural conditions is greater than background when ponding occurs due to their maintained microtopography. As the thin soils typical in the uplands of the Edwards aquifer recharge zone dominate the infiltration process in these small sinkholes, as shown by ring infiltrometer experiments, the risk of recharging poor quality water via these small features from developed areas sufficient to pose a threat to aquifer water quality is little greater than background.
GEOCHEMICAL EVOLUTION OF URANIFEROUS SODA LAKES IN EASTERN MONGOLIA
Benjamin Shawn Linhoff, MSGeoSci
The University of Texas at Austin, 2008
Supervisor: Philip C. Bennett
Extremely high concentrations of U were discovered in hypersaline soda lakes in eastern Mongolia. The lakes are small, shallow (<1km2, <1m), and terminal with variable salinity. The origin and fate of U in these lakes was investigated using geochemical analyses and modeling, using samples collected from lakes and lake pore waters, wells and a stream. Samples were analyzed for Sr and U isotopes, cations, trace metals, anions, total inorganic carbon (TIC), and unstable field parameters.
A representative groundwater in the field area is dilute and alkaline, with pH=7.9, 10 mmol L-1 of TIC and 5 mmol L-1 Cl-. In contrast, a representative lake water is pH~10 with TIC and Cl- each more than 1000 mmol L-1. Uranium concentrations in lake waters range from 0.24 to more than 62.5 µmol L-1, possibly making these lakes the highest naturally occurring U concentrations ever reported in a natural water. Groundwater concentrations of U range from 0.03 and 0.43 µmol L-1.
The U is natural and derived from groundwater discharging to stable closed basin lakes. Waters are concentrated by evaporation and U(VI) is chelated by CO3-2 to form the highly soluble UO2(CO3)3-4. Two sets of well waters with corresponding lake discharge waters were analyzed for U isotopes. Unnatural concentrations of 235U were tested for, the presence of which would indicate fallout-derived U. The average of four samples was 238U/235U=136±2 indicating that the U is naturally derived. δ234U between one well and its corresponding discharge lake had similar δ234U values (δ234U=837.6-858.5). The other sample pair however, revealed significant differences between the well and its discharge lake (δ 234U=303.6-1530). This suggests nuclide recoil is significantly enriching one of the lakes with 234U beyond secular equilibrium during alpha-decay of 238U in lake sediments or along the groundwater flow path. Modeled evaporation of lakes demonstrates that a U-mineral phase is likely to precipitate during evaporation.
Strontium isotopes varied in groundwaters between 87Sr/86Sr= 0.706192-0.709776 and in lakes 87Sr/86Sr=0.708702-0.709432. Cretaceous mafic rocks likely account for low 87Sr/86Sr values while Cretaceous alkaline rhyolites account for the high 87Sr/86Sr values. High concentrations of U, Na, Cl-, and K correlate to radiogenic Sr in lake waters indicating that U is sourced from local Cretaceous alkaline rhyolites.
THE AREQUIPA-ANTOFALLA BASEMENT, A TECTONIC TRACER IN THE RECONSTRUCTION OF RODINIA
Staci Lynn Loewy, Ph.D.
The University of Texas at Austin, 2002
Supervisor: James N. Connelly
The Arequipa-Antofalla Basement (AAB), an anomalous Proterozoic block along the central Andean margin of South America, may be critical to reconstruction of the Meso-Neoproterozoic supercontinent, Rodinia. Dalziel (1994) proposed that the AAB is an allochthonous block, transferred to Amazonia from northeastern Laurentia as a result of ca. 1.0 Ga collision between the two continents during the amalgamation of Rodinia.
New U/Pb geochronology and whole-rock isotope geochemistry from the AAB and northeastern Laurentia are used to test this proposed correlation. Based on U/Pb ages, growth polarity, sequence of age provinces, and whole-rock Pb isotopic signatures I conclude: (1) the AAB is a single basement block that formed through progressive growth within a larger continent, (2) neither Amazonia nor Laurentia were its parent craton, and (3) the AAB accreted to Amazonia during the ca. 1.0 Ga Sunsás Orogeny. Thus, the data do not support the proposed collision between Amazonia and northeastern Laurentia. Instead, the characteristics of the AAB are consistent with derivation from Kalahari. Moreover, critical examination of existing Pb data also reveals that the Pb signature of the isotopically distinct ca. 1.1 Ga basement of the southern and central Appalachians matches that of southwestern Amazonia. Based on these potential correlations, I propose an alternate position for Amazonia within Rodinia, adjacent to southeastern Laurentia and western Kalahari.
Evaluation of Pb isotopic signatures from juvenile rock units of large Precambrian age-provinces suggests that they were derived from two isotopically distinct mantle reservoirs. These reservoirs may be the precursors of those that exist today, a normal reservoir in the northern hemisphere and the DUPAL reservoir in the southern hemisphere. If true, the Pb isotopic composition of juvenile rocks in Precambrian age-provinces may indicate the hemisphere in which the province formed.
Coherency of Pb isotopic signatures across juvenile age provinces and the existence of two isotopically distinct reservoirs permit the use of Pb isotopes to correlate transferred terranes with potential parent cratons. If Pb isotopic compositions of juvenile Precambrian rocks are indicative of the hemisphere in which they formed, Pb isotopes may also be used to resolve ambiguity in polarity of paleomagnetic data.
Return to listREGIONAL SCALE FLOOD MODELING USING NEXRAD RAINFALL, GIS, AND HEC-HMS/RAS: A CASE STUDY FOR THE SAN ANTONIO RIVER BASIN, TEXAS
Marla Rose Knebl Lowrey, Ph.D.
The University of Texas at Austin , 2006
Supervisor: Zong-Liang Yang
This research develops a methodology and framework for regional scale flood modeling that integrates NEXRAD Level III rainfall, GIS , and a hydrological model ( HEC -HMS/ RAS ). The case study surrounds a summer storm event over the San Antonio River Basin (about 4000 square miles, 10000 square kilometers) in central Texas , USA , a region subject to frequent occurrences of severe flash flooding. The basic model design connects a rainfall-runoff model ( HEC -HMS) with a hydraulic model ( HEC - RAS ) to model unsteady state flow. The infrastructure presented in this study extends the prototype Map to Map GIS tool to a regional scale. The preliminary model system is driven by NEXRAD 4km rainfall grids. Subsequent model experiments investigate the potential to use numerical weather forecasts of precipitation to drive the rainfall-runoff model. Results demonstrate that despite some significant errors, the calibrated model is capable of producing flood forecasts comparable to observed conditions. Addressing the heterogeneous nature of basin geomorphology and hydrology may aid in improving model accuracy. To this end, the variation in river and floodplain characteristics and their relationship to hydrologic behavior are investigated, and implications for future modeling efforts discussed. The regional scale flood model and watershed study will provide a useful tool for future flood studies over the San Antonio River Basin or similar catchments, and demonstrates the potential of increasing forecast lead time by incorporating numerical weather forecasts of rainfall into the modeling system.
GLOBAL AND LOCAL CONTROLS ON DEPOSITIONAL CYCLICITY: CANTERBURY
BASIN, NEW ZEALAND
Hongbo Lu, Ph.D.
The University of Texas at Austin, 2004
Supervisors: William Fisher, Craig S. Fulthorpe
209 pages, 296 references
The eastern margin of the South Island of New Zealand is part of a continental fragment that rifted from Marie Byrd Land in Antarctica beginning at about 80 Ma. High rates of sediment supply since the middle Miocene have resulted in the preservation of a high-frequency, seismically resolvable record of depositional cyclicity in the offshore Canterbury basin. In addition, along-strike currents have influenced deposition, modifying sequence architecture while forming large sediment drifts. High-resolution multichannel seismic (MCS) data provide insights into the interplay of global (eustastic) and local controls (tectonics and current activity) responsible for continental margin depositional cyclicity and sequence architecture.
At least eleven large, elongate sediment drifts are identified within the Neogene shelf-slope sediment prism. The drifts were deposited in water depths of 300-750 m and developed in response to a northward flowing current. Their subsequent evolution is influenced by sediment supply, relative sea-level change, and seafloor morphology. Unconformities created by paleoslope erosion at the landward edges of drift moats differ from sequence boundaries by being strongly diachronous.
Nineteen regional sequence-bounding unconformities are interpreted. Correlation with oxygen isotopic records suggests that eustasy controls the timing of sequence boundaries. In contrast, sequence thickness, shelf edge stacking patterns, internal seismic facies, and slope inclination, are strongly influenced by local processes, principally the along-strike currents responsible for drift development. Sequences with conventional clinoform geometries along strike from active drifts suggest that currents might influence clinoform formation even in locations lacking seismic evidence of current reworking.
Reconstruction of Australian-Pacific relative plate motion reveals divergence in the central Southern Alps prior to ~20.1 Ma, followed by increasing average rates of convergence, with a marked increase after ~6 Ma. Sedimentation rates, calculated from sequence volumes, correlate well with convergence rates since ~11.5 Ma, indicating that tectonism has been the dominant control on sediment supply. In particular, high rates of sediment supply since 6 Ma, may reflect previously recognized increased plate convergence and Southern Alps uplift. In contrast, high sedimentation rate from ~15-11.5 Ma correlates with low convergence rates and is therefore probably a response to global climatic forcing and eustasy.
POLARIZATION ROTATION UPON REFLECTION OF DIRECT SHEAR
WAVES IN PURELY ISOTROPIC MEDIA
Eric Lyons, M.S.Geo.Sci.
The University of Texas at Austin, 2006
Supervisor: Robert H. Tatham
The re?ection process alters the polarization of a direct shear wave in a purely isotropic medium. I show that where the alteration occurs is governed by the shear re?ection amplitude versus incidence angle for the re?ecting interface. Speci?cally, I address the zero crossing in the S V - S V component's re?ectivity (where the amplitude changes sign), and how the zero crossing governs the alteration's location. The severity of the deviation of the polarization of the source from the observed polarization of the re?ection varies with the di ? erence in azimuth between source position and the receiver location. I develop a correction based on the observed shear amplitude versus incidence angle, which corrects to the equivalent of near-angle polarization for mid and large incidence angle (large o ? set) data. When the exact parameters from the model data are used to create the correction, the re?ected shear polarization matches the source polarization out to moderate o ? sets. A universal, model-independent correction does not perform as well as the exact correction, but it may be suitable for many applications where knowledge of regional geology is not complete. I also show preliminary results from applying the correction to anisotropic model data. The correction is most e ? ective for small percentages of anisotropy ( < 5%), after which the anisotropy signature of the observed polarization dominates over the isotropic re?ection distortion.
TRANSPORT AND TRANSFORMATION OF SULFUR SPECIES DURING SULFURIC ACID SPELEOGENESIS: GEOMICROBIOLOGICAL SULFUR CYCLING WITHIN LOWER KANE CAVE, WYOMING
by
Katrina Edna Mabin, M.S. Geo. Sci.
The University of Texas at Austin, 2005
SUPERVISOR: Philip C. Bennett
Lower Kane Cave , situated in the Madison Limestone of the Bighorn Basin, Wyoming, is forming via sulfuric acid speleogenesis. Groundwater containing dissolved sulfide and sulfate discharges into the cave, the carbonate cave walls are being replaced by gypsum, and the cave floor is covered with fallen gypsum and chert cobbles. Early conceptual models of sulfur cycling in Lower Kane Cave were limited to the autoxidation of sulfide to sulfate. The presence of a diverse community of microbial mats along the stream channels, which include sulfur-cycling bacteria, suggests that sulfur transformations within the cave are much more complex and are mediated by the microbial community. Using a suite of sampling and analytical methods, the biogeochemical sulfur cycle was investigated within aqueous, solid and gaseous reservoirs along the Upper Spring stream channel. Dissolved sulfide decreases quickly along the stream channel, and intermediate sulfoxy species are detected as well. Sediment within the main stream channel is low in total sulfur, in contrast to microbial mat biomass and deposits of processed organic material, which contain high concentrations of elemental sulfur and acid-volatile sulfur, respectively. No H 2S (g) is present in the boundary layer directly over the spring orifice, though H 2S (g) is present further downstream over areas occupied by microbial mats. Similarly, the flux of H 2S (g) from the stream surface is low near the spring orifice and higher over the mats. The overall loss of dissolved sulfide to these various sulfur species can be described by a bimolecular second-order rate equation, calculated using the initial dissolved sulfide and dissolved oxygen concentrations by the orifice, and a calculated second-order rate constant. The distribution of sulfur within aqueous, solid, and gaseous reservoirs in Lower Kane Cave is strongly associated with the presence of microbial mats. Sulfur is present along the stream channel in multiple oxidation states, indicating that both oxidation and reduction of sulfur species is occurring. The loss of dissolved sulfide from Upper Spring groundwater exceeds the loss due to volatilization or autoxidation, and suggests that microbial processes play a significant role in sulfur cycling within the cave.
SEASONAL WATER USAGE BY JUNIPERUS ASHEI: ASSESSMENT WITH STABLE ISOTOPES OF HYDROGEN AND OXYGEN
by
Amy A. McCole, MS Geo Sci
The University of Texas at Austin, 2004
Supervisor: Libby Stern
The recent expansion of Juniperus ashei (Ashe juniper) on the Edwards Plateau of Central Texas has prompted studies on the impact the juniper expansion has had on the hydrologic regime of the region. This study addresses Ashe juniper's impact on the water budget of the Edwards Plateau by examining seasonal variations in the juniper's water source and the juniper's effect on soil water content at the Honey Creek State Natural Area.
The study site experiences two distinct periods of seasonal rainfall. The dry season (late spring and summer) is characterized by large infrequent storm events followed by periods of drought. The wet season (fall, winter and early spring) is characterized by large intermittent storms and smaller frequent rainfall events.
Stable isotopes of oxygen and hydrogen from precipitation, soil water, Ashe juniper xylem water, and groundwater were compared to identify the Ashe juniper water source. These data, combined with soil water content, identify different routes of soil water loss and soil water conservation as a function of vegetation cover. The USGS Bowen ratio measurements of evapotranspiration (ET), coupled with the isotope data and soil water content data, are used to partition evaporative and transpirative water loss from the terrestrial system.
Based on a mass balance calculation, a minimum of 60% of the Ashe juniper's water was obtained from groundwater over the dry season, while over the wet season, on average, only a maximum of 30% of the Ashe juniper's water was drawn from groundwater. The soil water contents, along with groundwater, soil water, and Ashe juniper xylem water δ 18Ο values, indicate that the presence of Ashe juniper can appreciably reduce groundwater resources both by lateral roots intercepting potential recharge under wet conditions and direct uptake of groundwater by deep roots under dry conditions. Additionally, Ashe juniper usage of soil water at the beginning of the grass growth season (early spring) may decrease herbaceous productivity by drawing water from beneath the grasses. Ashe juniper canopy rainfall interception is another process by which terrestrial system water is lost to the atmosphere. The isotope and soil water content data suggest that juniper canopy interception is an important route of terrestrial water loss over the wet season. Over the dry season, little evidence is seen of juniper canopy interception. Rather, the data suggest the juniper conserves soil water by limiting evaporation beneath its canopy.
ET flux measurements are separated into evaporation from the soil, evaporation from the juniper canopy, and juniper transpiration using measured ET, precipitation, soil water content, their respective δ 18Ο values, and juniper xylem water δ 18Ο values in a mass balance calculation. This unique approach shows promise for quantifying the individual components of ET water loss from the terrestrial system. More reliable results may be found when water vapor δ 18Ο values are measured and sampling occurs over shorter time intervals.
A FIELD STUDY RECOGNIZING REDEVELOPMENT OPPORTUNITIES FOR SOUTH WESLACO FIELD, HIDALGO COUNTY, TEXAS
Andrew Dax McDavid, M.A.
The University of Texas at Austin , 2006
Supervisor: William L. Fisher
South Weslaco Field is a mature, non-associated gas field located in southeastern Hidalgo County , Texas . As of December, 2005 South Weslaco Field has produced over 151 billion cubic feet of gas from multiple Frio reservoirs contained by the 6329.15 acre South Weslaco Gas Unit (SWGU). The discovery well was drilled 1938 by the Atlantic Refining Company. Since its discovery, a total of 34 wells have resulted in seven dry holes and 27 natural gas producers. In 1997, 3-D seismic data was acquired and only one well has been drilled off existing 3-D data.
South Weslaco Field lies on the southern flank of the Norias Delta system and exhibits a mix of both aggradational and progradational deltaic sandstone reservoirs. A large down-to-the-coast growth fault creates a faulted anticline providing the main trapping feature in the upthrown and downthrown fault blocks. The upthrown fault block contributes 1.8% of the cumulative field production. These reservoirs are trapped in an upthrown three-way closure west of the growth fault. Over 98% of cumulative field production comes from reservoirs trapped in the downthrown fault block. Shallow reservoirs have four-way closure provided by an anticline that exhibits rollover into the resultant growth fault. With depth the structural crest migrates to the northeast and rollover becomes less evident.
This field study recognizes several future field development opportunities. Research has shown that the post 3-D development stage of South Weslaco Field is still in its infancy. The continued incorporation of the existing 3-D seismic data can generate various infill drilling opportunities to exploit remaining reserves. This report suggests that “new-pool” reserves can be added to South Weslaco Field from the combination of historically high gas prices, and advancements in reservoir stimulation. Deep reservoirs that have previously been deemed uneconomic can now produce economic flow rates with modern fracture techniques. From researching historic production methods employed by previous operators, this report also suggests that the possibility of remaining reserves in prematurely abandoned reservoirs.
The reevaluation of mature fields such South Weslaco can reveal opportunities for infield reserve growth. Large oil and gas companies often consider the incremental reserve growth uneconomic because of limited amount of upside hydrocarbon potential. However, the exploitation of remaining hydrocarbon reserves in mature fields can be profitable to smaller exploration and production companies.THE CHICXULUB IMPACT CRATER AND OBLIQUE IMPACT
Matthew A. McDonald, MSGeoSci
The University of Texas at Austin , 2006
Supervisors: Sean Gulick and Robert Tatham
Was Chicxulub an oblique (<45 degrees) impact? The answer to this question will help our understanding of the environmental consequences of the bolide that struck the Earth 65 ma in the Yucatan Peninsula . Planetary impact events, and impact simulations in the laboratory, show that oblique impacts have clear asymmetric ejecta distributions. However, the subsurface structures of the resultant craters are not well understood. From January 5, 2005 – February 19, 2005, we acquired 1822 km of seismic reflection data onboard the R/V Maurice Ewing imaging the massive (190+ km) Chicxulub impact crater to improve the understanding of subsurface crater structure and assess implications for impact obliquity. P re-crater stratigraphy outside the central basin of the Chicxulub impact crater includes packets of high amplitude reflectors on the seismic profiles, likely due to the presence of interbedded evaporite and carbonate layers. These reflective horizons are offset downward into the crater marking the post-impact slumping and formation of the terrace zone. A terrace zone forms when the overextended transient crater, which is formed by the initial impact, collapses gravitationally; large amounts of sediment, both inside and outside the transient crater, slump inward as a series of downward stepping blocks.
Can the terrace zone tell us something about oblique impact? This thesis is divided into two separate chapters addressing this question, and each will be independently submitted for publication. The first chapter presents the 3-D structure of the terrace zone based on seismic reflection data from the 2005 survey and an earlier survey done in 1996. The second chapter presents an analysis of Venusian craters addressing how oblique impacts relate to peak ring offsets. These craters are structurally comparable to Chicxulub, and the analysis implies that Chicxulub's SE peak ring offset is not related to impact trajectory. The two chapters in this thesis are mutually supportive, and suggest that the effects of oblique impact are overshadowed by other factors such as pre-impact structure and target properties. Therefore, subsurface asymmetries in the terrace zone do not appear to provide information about impact obliquity. The terrace zone, however, may help reconstruct the shape of the transient cavity and explain a concentration of ejecta toward the northwest.
DIFFUSION, CLOSURE TEMPERATURES, AND ACCESSORY MINERAL PETROGENESES IN A HIGH-TEMPERATURE CONTACT AUREOLE: REFINING THE INTEGRATION OF P-T AND T
by
Christopher R. M. McFarlane, Ph.D.
The University of Texas at Austin, 2003
Supervisors: William D. Carlson and James N. Connelly
229 pages, 119 references, 0 plates
Static heating during intrusion of the 1322 Ma Makhavinekh Lake Pluton (MLP) caused replacement of garnet in the adjacent country rocks (Tasiuyak Gneiss) by coronal assemblages of orthopyroxene + cordierite. Thermometry based on Al solubility in orthopyroxene, applied to relict garnet and neighboring orthopyroxene, preserves a temperature gradient from 700°C to 900°C at distances between 5750 and 20 m from the intrusion, reaffirming the robustness of this thermometry technique. Intracrystalline and intergranular Al zoning in M2 orthopyroxene are interpreted with the aid of numerical models for conductive heat flow in the aureole. The results document the ability of Al-in-orthopyroxene thermometry to preserve a detailed record of thermal histories in contact-metamorphic granulites.
Zircon in the Tasiuyak Gneiss formed at ~1850 Ma near the peak of the Torngat Orogen (M1 metamorphism). These M1 zircons were reheated during contact heating at 1322 Ma (M2 metamorphism). U-Pb dating of M1 zircon, using thermal ionization mass spectrometry (TIMS) and in situ sensitive high resolution ion microprobe (SHRIMP) geochronology, tested whether temperatures in the aureole exceeded the closure temperature for Pb diffusion in zircon. TIMS analyses failed to resolve appreciable resetting towards 1322 Ma even in samples that reached ~900 ºC. In contrast, high spatial resolution SHRIMP data revealed that cores of M1 zircon in samples that reached temperatures >800 ºC are significantly younger than rims. This reversal of core and rim ages implies that Pb was preferentially lost from cores. This observation is hypothesized to be the result Pb migration during high temperature recovery of lattice strain induced by higher impurity concentrations (REE, P) in cores.
While M1 zircon suffered intracrystalline Pb diffusion during contact heating, it did not react with the surrounding major mineral assemblage. In contrast, low-Y monazite inclusions in M1 garnet were consumed as they were exposed to the M2 coronas. New growth of high-Y monazite records both the age and temperature of metamorphism in the aureole. Dissolution and new growth of monazite occurred under anhydrous conditions, highlighting the potential use of monazite as a high temperature thermochronometer in granulites and ultra-high temperature rocks.
HIGH-TEMPERATURE CARBONATE REPLACEMENT MINERALIZATION, METAMORPHISM, DEFORMATION, AND INTRUSION IN THE BRYANT MINING DISTRICT, BEAVERHEAD COUNTY, MONTANA
by
James B. McGuire, M.S. Geo. Sci.
The University of Texas at Austin, 2003
Supervisor: J. Richard Kyle
162 pages, 52 references, plates, CD-Rom
The Bryant District, in the southwest Montana fold-and-thrust belt, contains many structurally and lithologically controlled high-temperature carbonate replacement Pb-Zn-Cu-Ag-Au deposits. Laramide crustal shortening prepared Cambrian and Devonian carbonate strata for fluid circulation through thrusting and folding. Thrust-controlled mineralization is present in the Lion Mountain mines, whereas fold-controlled chimney-style mineralization is present in the Cleve
