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Geological Sciences Theses Abstracts : 1997-2000

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The Theses Abstracts from 1997-2000 are from masters and dissertations completed at the University of Texas in the field of Geological Sciences. For copyright reasons, PhD abstracts are not included here. Dissertation abstracts may be found in Dissertations and Theses: Full Text.



THREE-DIMENSIONAL PRESTACK PLANE-WAVE KIRCHHOFF DEPTH MIGRATION IN LATERALLY VARYING MEDIA

by

Faruq Eazam Akbar, Ph.D.

The University of Texas at Austin, 1997

Supervisors: Paul L. Stoffa and Mrinal K. Sen

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PETROPHYSICAL AND GEOLOGICAL CHARACTERIZATION OF DOLOMITIZED CARBONATE RAMP RESERVOIRS: SEMINOLE SAN ANDRES UNIT WELL 5309 GAINES COUNTY, WEST TEXAS

 

by

Fatma Akyurek, M.S.

The University of Texas at Austin, 1999

Supervisor: William L. Fisher

 

In order to define reservoir heterogeneities adequately, internal variability of reservoir rocks, geometry of the pore system and enclosing petrophysical framework, incorporation of stratigraphic framework, facies architecture with depositional model, and diagenetic modification need to be delineated. Such a study of Seminole San Andres Unit (SSAU) well 5309, Gaines County, West Texas was selected to understand reservoir heterogeneity scale and reservoir performance.

Complexity of carbonate pore networks is distinguished by the rance of sorting, and size of carbonate particles that result in rock fabric. Three basic rock fabric/petrophysical classes were identified using a modified Dunham's (1962) approach. These are (1) dolograinstone, (2) grain-dominated dolopackstone and medium crystalline mud-dominated fabrics (20-100 microns) and (3) fine crystalline mud-dominated fabrics (<20 microns). Dolomitization is a major effect in pore size distribution because of differences in the size, shape, and arrangement of crystals. In the mud-domininated fabrics, pore geometry is controlled by crystal size. When the crystal size increases, enhancement in permeability occurs because of an increase in particle size. Two major pore types control the petrophysical characteristics of carbonate rocks: interparticle and vuggy porosity. High permeability occurrences are associated with interparticle pore space of grainstone texture. Low permeability is related to microintercrystalline pore space of the fine crystalline (<20 microns) muddy facies. Mixed interparticle-intercrystalline pore spaces in grain-dominated and medium crystalline (20-100 microns) mud-dominated fabrics have intermediate permeability range. These rock fabric classes formed distinct porosity/permeability transforms and water saturation ranges.

Core analyses were calibrated with wireline log analyses to expand the reservoir model. Total porosity was calculated from the combination of density-neutron logs. Separate-vug porosity was delineated by using acoustic logs. Water saturation was calculated by using Archie's equation. Permeability is related to rock fabric and interparticle porosity which was calculated by subtracting separate-vug porosity from total porosity. Rock fabric data was integrated with interparticle porosity and permeability from the core analysis to form interparticle porosity/permeability transform for each rock fabric class. Water saturation is a function of particle size, sorting, and reservoir height above the free water table. Plotting water saturation against porosity can determine particle size and three rock fabric groups. The water saturation-rock fabric-porosity relationships could not be established because of high water saturation. Consequently, to calculate permeability from the log responses by using rock fabric specific interparticle porositypermeability transform, rock fabrics from the thin section description had to be used. Thus, correlations were achieved between core and log analyses.

Sequence stratigraphic framework was tied to local and regional stratigraphy within the cycle and high frequency sequence scale with the Seminole San Andres Unit 2505 and within high frequency sequence scale with the outcrops of the San Andres Formation at Lawyer Canyon, Algerita Escarpment, New Mexico to construct a 2-D reservoir model. Four fourth-order high frequency sequence boundaries were recognized through the cored section of the Seminole San Andres Unit 5309.

 

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DATA INTEGRATION FOR RESERVOIR CHARACTERIZATION: A CENTRAL ARABIAN OIL FIELD

By

Salem Gulaiyel Aljuhani, PhD

University of Texas at Austin, 1999

Supervisor: M. M. Backus

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HYDROGEOLOGY OF THE LOWER CRETACEOUS EDWARDS AND TRINITY GROUP FORMATIONS NEAR JUNCTION (KIMBLE COUNTY) TEXAS

by

Stephen Robert Allen, M.A.

The University of Texas at Austin, 1997

Co-supervisors: Clark R. Wilson, Barry J. Hibbs

This study describes ground-water flow in the Lower Cretaceous formations near Junction (Kimble County), Texas. Rock exposures were examined throughout the 150-mile study area to determine the nature and distribution of permeable features. Dominant features include nearly vertical fractures and horizontal bedding planes in carbonate rocks of the Edwards Group formations (Edwards), and coarse grained fluvial channel deposits in the underlying Hensel Sand Formation (Hensel). Static water levels were measured in over one hundred wells and contoured to reveal the existence of two separate potentiometric surfaces, one overlying the other. Preliminary, but useful estimates of transmissivity, hydraulic conductivity, and ground-water velocity were derived using specific capacities from eighty-three wells completed in both the upper (Edwards) and lower (Hensel) aquifers.

At the edge of the Edwards Plateau where the contact between the Edwards and the Hensel is exposed on the face of the erosional escarpment, ground water discharges from the Edwards aquifer through numerous low volume springs and seeps. An even greater proportion of ground-water discharge leaks from the Edwards aquifer to the underlying Hensel aquifer across a thin low permeability bed at the base of the Edwards which consists of marly, unfractured, nodular limestone. The sum of these two components of discharge is approximately equal to precipitation recharge to the Edwards.

To gain additional insight into cross-formational flow an analysis of major and minor ions, redox potential, and dissolved oxygen was conducted for twenty-one water wells which were located along three north-south (inferred flow direction) transacts. The Edwards waters were found to be a Ca-Mg-HCO3, facies; the Hensel waters, a mixed facies, This difference in hydrochemical facies was initially thought to be caused by ion evolution along flowpaths, but it more likely reflects the existence of a regional aquifer below a locally constrained aquifer. High values of dissolved oxygen and redox potential in the Edwards aquifer indicate that recharge is predominant; lower values of these parameters in the Hensel aquifer indicate that this water occurs in an intermediate or discharge zone.

To test the conceptual model of steady-state ground-water flow, a numerical model was constructed using the MODFLOW finite-difference computer code. Over one hundred trial and error simulations were executed to calculate leakage through the confining bed, discharge from springs, and discharge to the Llano River. In addition, the distribution and magnitude of focused recharge to the Edwards aquifer was calculated, estimates were made for unknown hydrogeologic parameters, and the Edwards aquifer was demonstrated to be fully perched above the Hensel aquifer.

The increased understanding of the ground-water flow regime resulting from this study will support range management activities and improve the success rate of water well drilling.

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EVOLUTION OF NEOGENE CONTRACTIONAL GROWTH STRUCTURES, SOUTHERN GULF OF MEXICO

by

Mario Aranda-Garcia, M.S.

The University of Texas at Austin, 1999

 

Supervisor: Randall A. Marrett

 Structural analysis of four contractional growth structures in the Southern Neogene Belt of Mexico was undertaken to test conceptual models of folding mechanisms for contractional deformation and to obtain structural interpretations of important oil prospects and one oil field. Based on seismic data, application of these models provides coherent explanations of structural development incorporating the geometry of the structures as well as the geometry and age of the growth strata, as shown in viable balanced cross-sections, and demonstrate a complex evolution of folding mechanisms in a 3D framework.

 Folding and faulting are important mechanisms of deformation in the brittle upper crust, and in particular are the main processes of fold-thrust belt development. Excellent examples for the geometries developed in these belts have been documented around the world. However most of them lack growth strata, due mainly to their deep levels of erosion, and for this reason their development between undeformed and final geometry is weakly supported with data. Growth strata are those units deposited during deformation, and ideally they record the entire evolution of a contractional terrain. In modern thrust belts preservation of growth strata is uncommon; marine growth sediments have better potential for preservation of internal and external features than do continental growth sediments. Some excellent examples of contractional growth structures from the southern Gulf of Mexico are used here to test fault-related folding and buckle folding models and to obtain detailed interpretations of one discovered oil field and some important prospects for oil exploration. The structures analyzed are part of the Southern Neogene Belt of Mexico a thrust belt that represents upper crustal shortening in the southern portion of the North America Plate. The Sardinero, Jujo, Tunich and Catemaco anticlines are located in the Veracruz and Campeche-Tabasco-Chiapas basins. These basins were dominantly filled by upper Eocene-Neogene marine deposits. The regional trend of the structures is NW-SE but this orientation is modified in part where the Jurassic salt is present. The basal detachments for the Sardinero and Catemaco structures are located in Paleogene mudstone units and the detachment for the Tunich and Jujo structures is in Jurassic salt.

The structures analyzed were selected because they are good examples of contractional growth structures with good seismic data sets and chronostratigraphic well control. The pre-kinematic strata of Sardinero anticline have geometric similarities with a fault-bend fold structure, but growth strata on the backlimb show a more complex evolution. The growth strata were deposited during an early buckle-folding stage between 15.5-3.5 Ma which was followed by the development of and translation over a fault ramp after 3.5 Ma.

The Jujo anticline prekinematic strata has a good fit with predictions for a fault-bend fold and the geometries of growth strata agree with the model. Structural timing from growth strata indicate folding during the mid-Miocene and Pliocene.

The Tunich and Catemaco anticlines correspond to curved detachment folds. They have apparent fault-related folding geometries at pre-kinematic levels but they were initiated by buckle folding as the geometries of their growth strata demonstrate. The age of deformation for both anticlines was between 21 to 2.4 Ma and 16.5 to 5.5 Ma respectively.

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HIGH RESOLUTION SEQUENCE STRATIGRAPHY OF THE LEONARDIAN LOWER CLEAR FORK GROUP, PERMIAN BASIN

By

Eduardo Ariza, MS

The University of Texas at Austin, 1998

 

Supervisor: William Fisher

Precise definition of chronostratigraphic units in carbonates requires the use of diverse tools, techniques and models; especially when working with subsurface information. The restricted one-dimensional character of well data demands the use of proven models that validate any interpretation made in the subsurface.

Outcrop-based, high-resolution sequence stratigraphy provides a fundamental model for subsurface characterization. The use of outcrop information to generate a sequence stratigraphy-based model that can be extrapolated to analogous subsurface stratigraphic units, is a methodology that generates accurate results. First, outcrops provide two dimensional information that allow the definition of lateral and vertical facies relationships and stratal architecture as well as important stratigraphic surfaces (i.e. sequence boundaries). Second, outcrop data provide reliable information with respect to base level changes that are expected to be similarly recorded in subsurface rocks.

The present study combines surface and subsurface data to characterize the stratigraphic framework of the Leonardian Lower Clear Fork Group (L2) in the Permian basin. This work provides insights about the nature of shallow carbonate deposition in the Permian Basin. It characterizes the stratigraphic architecture at the level of cycle sets, producing a model that can be applied regionally.

Two areas from West Texas were studied: Apache Canyon, in the Sierra Diablo, provided outcrop information, and South Wasson Field in the Northwest Platform of the Midland Basin provided subsurface data. Both areas are located in the Permian Basin which is recognized as one the most important regions in the USA, not only by its hydrocarbon potential, but also by the outcrops that are acknowledged as a world class analogs of carbonate reservoirs.

This study allows definition of a regional base-level curve demostrating that the Lower Clear Fork is made up of two high frequency sequences (HFS). The lower HFS, L2.1, is symmetric and composed of transgressive and high stand system tracts. In contrast, the upper HFS, L2.2, is asymmetric, having no record of a high stand system tract on the platform. This suggests that Lower Clear Fork platform deposition was ended by forced regression. The contact with the underlying L1 unit is characterized by a subaerial exposure represented in the platform by a karstic surface. The contact with the overlying L3.1 unit is a sequence boundary product of the forced regression that occurred during deposition of the L2.2 HFS.

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TECTONICS AND SEDIMENTATION OF THE GULF OF PARIA AND NORTHERN BASIN, TRINIDAD

by

Stephen Babb, Ph.D.

The University of Texas at Austin, 1997

Supervisors: Richard T. Buffler and Earle F. McBride

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REVISION OF THE PLEISTOCENE TO HOLOCENE WILSON-LEONARD MICROVERTEBRATE FAUNA AND ITS PALEOENVIRONMENTAL SIGNIFICANCE

by

Robin Lee Balinsky, M.A

The University of Texas at Austin, 1997

Supervisor: Ernest L. Lundelius, Jr.

The 1992-93 excavation of the Wilson-Leonard archeological site, Williamson County, Texas, has revealed a fragmentary yet diverse microvertebrate fauna comparable to other Pleistocene to Holocene microvertebrate faunas along the eastern edge of the Edwards Plateau. The site, which is located along the margin of the Brushy Creek Valley off FM 1431 near Leander, contains a long and fairly continuous well-dated sequence of fluvial and colluvial fill. Matrix for the microvertebrate analysis was collected from two 1 x 1 m pits (Units 12 and 20), at 5 and10 cm intervals and was screenwashed using 1/8 inch and 1/16 inch mesh screens. After the initial sorting process, which included separation and extraction of bone, lithics, snails, seeds, and eggshell, vertebrate specimens were identified to their lowest possible taxonomic level through comparison with modern specimens. Due to temporal and monetary constraints, emphasis and effort was concentrated on Unit 20 because of its more complete stratigraphic extent. Results have yielded 45 identifiable taxa, including 29 taxa identifiable to genus and 8 identifiable to species. Despite poor preservation and the fragmentary nature of specimens, faunal identification was sufficient to draw both paleoenvironmental conclusions and cultural information concerning the use of small animals as a food resource from the sampled taxa.

Seven rodent taxa, including the bog lemming (Synaptomys cooperi ), the yellow-faced pocket gopher (Cratogeomys castanops ), the marsh rice rat (Oryzomys palustris), the muskrat (Ondatra zibethicus ), the pine/prairie vole (Microtus sp.), the eastern pocket gopher (Geomys sp.), and the smooth-toothed pocket gopher (Thomomys sp.) no longer live in the area. The identification of these rodents from this well-dated stratigraphic context has allowed for a more refined paleoenvironmental interpretation of fluctuating wet and dry climates than was previously known from approximately 11,500 years before present to approximately 1,000 years ago. The presence of Thomomys sp. at Wilson-Leonard has also expanded the known past range of this rodent an additional 100 km westward.

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TECTONIC EVOLUTION OF BRANSFIELD STRAIT, WEST ANTARCTICA

by

Daniel Hugh Njal Barker, Ph.D.

The University of Texas at Austin, 1997

Supervisors: Ian W. D. Dalziel and James A. Austin, Jr.

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SPATIAL AND TEMPORAL VARIABILITY IN SOIL MOISTURE ON A HILL-SLOPE: RATTLESNAKE HILL, AUSTIN, TX

by

Monika S. Bartelmann, M.S.

The University of Texas at Austin, 1997

Supervisor: Mark P. Cloos

Understanding the spatial and temporal variations in soil moisture on a 100 m scale on a hill-slope is necessary to understand soil moisture patterns at single sites (km scale) that are part of larger scale studies. This study investigated 1) the near surface spatial and temporal soil-moisture variability across a 70 m by 200 m grid on a gentle hill-slope with 12 m of relief and 2) spatial and temporal variability in moisture content as a function of soil type and topography. The study used gravimetric soil moisture measurements taken over two spatially different patterns; 1) three one meter radius circles each located along the transect and 2) a 70 m by 200 m grid with a ten meter resolution on a gentle hill-slope with 12 m of relief. Sampling was done almost daily between November 8, 1996 to March 14, 1997. Geographic Information Systems (GIS) applications and geostatistical methods were used to integrate the soil moisture data. Soil moisture patterns were compared to a digital elevation map (DEM) of the site. Conclusions were: 1) spatial variability in moisture content is controlled by topography, both macro and meter-scale, which can cause up to 30 percent variation in soil moisture on a scale as small as 100 meters; 2) the standard deviation of soil moisture content within a one meter sampling plot is three percent in the sandy loam soil and two percent in the clay soil, and as such, variations of soil moisture on the study site less than this are not statistically significant; 3) temporal variations in moisture content are controlled by topography as seen by the redistribution of moisture content from high to low areas; 4) over time (3 days), differences in soil moisture content that were related to texture are considered statistically insignificant; 5) there is no single representative soil moisture content for the entire hill-slope due to variations caused by differing soil types, and position on hill-slope (topography).

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WOLFCAMPIAN (LOWER PERMIAN) PLATFORM AND BASIN CARBONATES: HUECO MOUNTAINS AND MIDLAND BASIN, WEST TEXAS
WOLFCAMPIAN (LOWER PERMIAN) PLATFORM AND BASIN CARBONATES: HUECO MOUNTAINS AND MIDLAND BASIN, WEST TEXAS
by
Jennifer Lynn Beall, M.S.
The University of Texas at Austin, 1999
Supervisor: Brenda Kirkland

The sedimentology, sequence stratigraphy, and diagenesis of a Wolfcampian (lower Permian) platform to basin carbonate succession in the Hueco Mountains (Diablo Platform) and of similar age deposits of allochthonous (resedimented) carbonates productive at Howard Glasscock field (Midland Basin) in West Texas, were studied in an attempt to determine processes generating and transporting sediment off the shelf and the timing of such sediment generation in the sea level cycle.
The Hueco Canyon Formation, a Wolfcampian (Lower Permian) platform-to-basin carbonate succession in the Hueco Mountains (Diablo Platform), comprises a complex record of deposition controlled by changes in glacioeustatic sea level. The succession consists of four unconformity-bound sequences containing basin-to-slope to shallow platform facies. The oldest three sequences were deposited during regression, and the youngest following a transgression that resulted in a backstepping of facies relative to the previous sequence. The second and third sequences in the succession (upper Wolfcampian-2 (UW-2) and UW-3, respectively) are well exposed. They record the evolution of the platform margin facies from high-energy shoal to phylloid algal buildup as the margin gradually steepened. The UW-2 sequence shows aggradational and progradational architecture. It is capped by an exposure surface marked by karstification and slumping. The UW-3 sequence consists of at least five higher frequency packages that show retrogradational, aggradational and progradational and, in the uppermost part of the sequence, strongly progradational stacking patterns.
In the Hueco section two types of resedimented carbonate occur. One type is interpreted as a rocky shoreline deposit. Karstification and margin collapse associated with this resedimented deposit indicate that it formed during a sea level lowstand. The other type of resedimented carbonate represents downslope debris flow facies. Based on the association of the resedimented material with prograding buildups, it originated during a highstand. These two types of sediment are distinguished by bedding geometry, by position in the sequence framework, and by grain and clast content.
Based on type of clast and on evidence of subaerial erosion, Lower Permian allochthonous carbonate reservoirs in the Midland and Delaware basins can also be classified into the same lowstand and highstand types. A third type of allochthonous deposit is represented by coarse grained, skeletal gravity flow deposits that form reservoir interval at the Howard Glasscock field, Howard County, Texas. This type is interpreted to have formed from debris shed from a lowstand buildup. This interpretation is based on composition of both the gravity flows and the rock encasing them. Comparison of sequences exposed in the Hueco Mountains to those of sequences defined regionally in the subsurface demonstrates the high degree of sub-seismic scale stratigraphic and facies complexity of Wolfcampian carbonate successions. Outcrop-based sequence stratigraphic models of icehouse carbonate successions are necessary for realistic subsurface interpretation of Wolfcampian platform and basin carbonate.
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NON-DARCY FLOW THROUGH POROUS MEDIA

by

James Edward Bené, M.S.

The University of Texas at Austin, 2000

Supervisor: John M. Sharp

Since its introduction, Darcy's law has been implemented as a mathematical tool that allows simple calculation and prediction of low velocity subsurface flows. However, turbulence, non-isothermal conditions, as well as other factors can create conditions where Darcy's law does not accurately describe the head and velocity distributions within a given porous matrix. Darcy's law has been widely applied to analytical and numerical modeling of fluid flow through porous matrix, regardless of the hydrogeologic setting. This study attempts to quantify the error incurred by these models through simultaneous numerical modeling of the mass continuity equation using Darcy's law as well as Forchheimer's relation. To this end, results from steady-state and transient Darcy-based and Forchheimer-based numerical models are presented in this study.

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HYDROGEOLOGICAL INVESTIGATIONS AT DIAMOND Y SPRINGS AND SURROUNDING AREA, PECOS COUNTY, TEXAS

by

Radu Boghici, M.A.

The University of Texas at Austin, 1996

SUPERVISOR: John M. Sharp, Jr.

This study presents the results of local hydrogeologic investigations at Diamond Y Springs and vicinity, a 660 mi2 (1710 km2 ) area of north-central Pecos County, Texas. The data confirm the hypothesis of Rustler aquifer waters as the chief source of flow at Diamond Y Springs today. Dissolution of halite and gypsum, base exchange, evaporation, and mixing of the two hydrochemical facies of the Rustler with recent local waters can explain the water chemistry and isotopic composition. The main controls on the regional flow pattern are: (1) the permeability contrast between the Belding-Coyanosa trough fill and the Edwards Formation, (2) the amount of cross-formational flow recharging the Edwards-Trinity (Plateau) aquifer through the Belding fault system, (3) the amount of recharge from the Rustler aquifer, and (4) the amount of groundwater pumped in the Belding-Fort Stockton and Coyanosa irrigation districts.

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3D SEISMIC STRATIGRAPHIC ANALYSIS OF SOUTH TIMBALIER BLOCK 54, GULF OF MEXICO

by

Michael Hayden Bradley, M.S.

The University of Texas at Austin, 2000

Supervisor: William L. Fisher

3D seismic interpretation techniques reveal the structural style and stratigraphy of the offshore Louisiana South Timbalier 54 oil and gas field. A crestal graben thick, located over a deep-seated salt dome, is bounded by two opposing growth faults and is segmented by related rollover structures and antithetic faulting. The Pliocene-Pleistocene sediments are stacked progradational deltaic sands that are highly channelized.

The progradational succession of the deltaic sands was imaged using a stratal slicing method. Five reference horizons were mapped over a 2.0 second seismic two-way time interval to analyze the stratigraphy. The 3D seismic volume was sliced every 10 ms using these reference horizons as constraints. Reflection amplitude maps from these slices were generated, and the resulting areal patterns of amplitude variation were interpreted as facies changes. These facies changes indicated shifting centers of deposition of the deltaic system prograding into the basin.

An ancestral submarine canyon marks the top of the analyzed interval. The canyon has been mapped throughout the volume, revealing a meandering morphology in the canyon fill. The canyon may directly related to the ancestral Mississippi River and has been identified as the La Fourche or Timbalier Channel.

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PHYLOGENETIC SYSTEMATICS AND TAXONOMY OF CROCODYLIA

by

Christopher Andrew Brochu, Ph.D.

The University of Texas at Austin, 1997

Supervisor: Timothy Rowe

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INTERPRETATION OF HIGH-RESOLUTION SEQUENCE STRATIGRAPHY AND FORAMINIFERAL ANALYSIS ON THE NEW JERSEY OUTER CONTINENTAL SHELF: IMPLICATIONS FOR LATEST PLEISTOCENE-HOLOCENE SEA LEVEL FLUCTUATIONS

by

Keith Fleming Buck, M.A.

The University of Texas at Austin, 1997

Supervisor: William E. Galloway

Foraminiferal assemblages in vibra-cored sediments within 2.5 m of the seafloor document high-resolution sea level fluctuations, revealing the complex Quaternary depositional history of the New Jersey margin. Biostratigraphic results, integrated with ongoing 2-D/3-D high-resolution seismic analyses, reveal paleoenvironments of shallow sequences beneath the outer shelf. Cluster analysis of foraminifera sampled every 20 cm (average) define four sample groups (A-D) dominated by benthic species. Group A, characterized by Cibicides lobatulus, is a mid-outer shelf assemblage. Group B is composed of shelf species typical of groups A, C and D; B is characterized by low abundances and a fairly uniform distribution of foraminifera, suggesting an increased rate of deposition. Group C, dominated by Elphidium excavatum, is an inner-middle shelf assemblage. Group D, defined by Ammonia beccarii, represents marginal marine environments.

Seismic analysis indicates that one core penetrates the flank of a buried channel, including fill and the pre-channel section. This core displays three alternations of groups C and D within the channel fill, suggesting sea level fluctuations between channel incision (as late as 45 kyrs B.P.) and channel filling (as late as 12.5 kyrs B.P.).

Elphidium faunas from the upper 5 m of the New Jersey outer continental shelf record the paleoenvironmental signature of the last transgression since Wisconsinan maximum (~18 kyrs B.P.). Fluctuations between percentages of Elphidium excavatum, Elphidium subarcticum and Elphidium articulatum demonstrate the highly transitional nature of paleoenvironments during the last sea level rise. An overall decrease in pulse percentages of E. subarcticum (high energy foraminifera, "adhesive" attachment to mineral grains) upcore may indicate an overall decrease in energy during the last transgression. Subdivision of E. excavatum into three variants, E. excavatum forma clavatum, E. excavatum forma lidoensis and E. excavatum forma selseyense, suggests higher-order cycles during this transitional interval. Plots of these three variants underscore rapid alternations from temperate/boreal to arctic environmental conditions. Alternating dominance of E. excavatum forma lidoensis (boreal) and E. excavatum forma clavatum (arctic) may correlate with glacial meltwater events (sea level rises) and glacial readvances (sea level falls), respectively.

Unconformities and/or discontinuities within cores are revealed through Integrated Paleontologic System (IPS) cosine [theta] curves of faunal assemblages. Abrupt changes in assemblages are shown by dissimilarity points, suggesting a dramatic change in paleoenvironment.

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ARTIFICIAL NEURAL SYSTEMS FOR INTERPRETATION AND INVERSION OF SEISMIC DATA

by

Carlos Calderón-Macías, Ph.D.

The University of Texas at Austin, 1997

Co-supervisors: Mrinal K. Sen and Paul L. Stoffa

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GEOMETRIC AND KINEMATIC EVOLUTION OF DETACHMENT FOLDS, MONTERREY SALIENT, SIERRA MADRE ORIENTAL, MEXICO

by

Rion H. Camerlo, M.S.

The University of Texas at Austin, 1998

Supervisor: Randall A. Marrett

 

Detailed geometric, strain, and kinematic analyses of kilometric, unfaulted, isoclinal anticlines exposed in the Sierra Madre Oriental fold-thrust belt of Mexico form the basis for testing models of detachment folding. San Juan Bautista and San Blas anticlines have cylindrical, curved, parallel fold geometries. Calcite-twin, fault, and vein extension magnitudes are small. Flattened diagenetic stylolites and clasts indicate pressure solution was locally important. Displacements and spacings of layer-parallel faults indicate that flexural-slip was important during folding. Layer-parallel strain models are inconsistent with the parallel geometries. Variable cleavage orientations, consistent fold wavelengths and moderate correlations of vein extension with layer curvature suggest some limb rotation, possibly with fixed hinges, accompanied fold development.

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FACIES ARCHITECTURE OF MIOCENE FLUVIAL-DELTAIC RESERVOIRS IN CENTRAL ORINOCO BELT (EASTERN VENEZUELA BASIN)

by

Josefa Carbón, M.S.

The University of Texas at Austin, 2000

SUPERVISOR: William L. Fisher

The Orinoco Belt region is located in the southern part of the Eastern Venezuela Basin. The Tertiary succession comprises a thick sequence of terrigenous clastic sediments, deposited in a semi-restricted basin. The major productive units are the fluvial-deltaic sandstones of the Oficina Formation (Early to Middle Miocene). These sandstones contain heavy and extra-heavy crude, and constitute one of the largest accumulations of hydrocarbons in the world.

The study area is situated at the central-eastern part of the Zuata Area, one of the four operational divisions of the Orinoco Belt. The sedimentary sequence comprises thick sandstone bodies, interbedded sandstones and mudstones, mudstones, organic-rich shales, lignites and coals, deposited in a delta plain, in lacustrine to brackish, and shallow marine environments. Data from well logs, conventional cores, and previous geological studies were analyzed to define the vertical and lateral distribution of Oficina sandstones.

Lower Oficina sandstones are mainly tidally influenced, distributary channel fill deposits, characterized by blocky log patterns, a high sandstone/ mudstone ratio, substantial thickness and lateral persistence. Amalgamation of several episodes of channel fill, along with slow subsidence, allowed both accumulation and preservation of a large amount of sands. Upper Oficina sandstones are characterized by upward-fining log patterns and a low sandstone/mudstone ratio, and they are interpreted as deposits of a mixed-load, meandering fluvial system.

Laterally persistent coal layers were used as stratigraphic markers and interpreted as peat accumulation under regional marsh environments as the result of very low sedimentation rates during transgression. Thin and more discontinuous coal layers probably represent peat accumulation in small interdistributary depressions. Resistivity cross sections show complex sand-body architectures with discontinuities caused by low-permeability facies that interfere with fluid migration. Inside major sandstone bodies, thin impermeable layers occur as the result of channel migration over the delta plain. Major barriers to flow are delta plain muds and tidal flat deposits, a consequence of channel avulsion or delta-lobe shifting.

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AGE AND ORIGIN OF THE CORONEL JOÃO SÁ PLUTON, BAHIA, BRAZIL

Christina Hansen Castellana, M.S.

The University of Texas at Austin, 1998

Supervisor: Leon E. Long

The northeastern portion of Brazil was last reworked during the Pan African-Brasiliano-age (700 to 550 Ma) collision between the Congo-São Francisco Craton and the West African-São Luis Craton. This area comprises Archean and Paleoproterozoic cratonic blocks that are scattered within a network of Pan African-Brasiliano age metasedimentary fold belts.

Two possible interpretations of the tectonic evolution of NE Brazil have been forwarded. An intercratonic model states that NE Brazil and its African extension consist of a mosaic of terranes made of cratonic blocks, island arcs, and arc basins which were assembled during the Pan African-Brasiliano orogeny. An intracratonic model states that the fold belts originated as continental extensional basins that were later involved in the Pan African-Brasiliano collisional event. This study tests these models by examining the Brasiliano-age Coronel João Sá pluton that intrudes the Sergipano Fold Belt, a small segment of an ESE-WNW collision belt that wraps around the northern margin of the São Francisco and Congo Cratons.

The U-Pb zircon age of the pluton is 627 +/- 2 Ma with clear evidence of ca. 1760 Ma - 2240 Ma inheritance. This inheritance argues for a large continental component at the source of this pluton.

Initial Sr and Nd isotopic compositions at 627 Ma indicate that magma mixing had taken place. Trace element concentrations of the hypothetical endmembers rule out the involvement of a primitive (mantle or MORB) end-member as a contributing component. The Nd composition of this pluton may be accounted for by anatectic melting of available continental sources without the necessity of invoking a Brasiliano-age mantle or MORB component. Mixing of magmas derived from at least two continental crustal endmembers better explains the isotopic and trace element data. Petrographic and chemical evidence suggests that one source could have been dehydration melting of lower continental crustal amphibolite.

These data suggest anatexis of old continental crustal material, and support an intracratonic continental basin origin for the Sergipano Fold Belt.

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GUADALUPE FORMATION, CUSIANA FIELD, LLANOS FOOTHILLS OF COLOMBIA: A SANDSTONE-DOMINATED UNIT DEPOSITED IN A SHELF SYSTEM WITH TIDAL CURRENT INFLUENCE

by

Jaime O. Castillo, M.S.

The University of Texas at Austin, 2000

Supervisor: William Fisher

The sedimentary depositional environment defines the internal architecture of the reservoir, one of the most important controls in rock quality distribution and reservoir heterogeneity. Different depositional environments form distinct architectures, which result in distinct heterogeneities and different fluid flow behavior in the reservoirs.

Previous studies in Cusiana Field assigned different depositional settings for the Upper Cretaceous Guadalupe Formation, ranging from fluvial and nearshore to open marine. This study integrates previous work and additional geological information to interpret the sedimentary conditions during the deposition of the Guadalupe in Cusiana Field.

Published regional paleogeographic studies, previous sedimentological and biostratigraphical studies, core description information, and electric well log data were analyzed and integrated using well cross sections as a main tool.

Integration of the geological data indicated that the Guadalupe Formation is a sandstone-dominated unit deposited in a shelf system with low depositional rates, and under high physical reworking by tidal currents or combination of currents with tidal influence. Similar processes persisted over a long period of time, 10 million years, generating a "condensed unit" 500 feet thick with continuous sedimentation and with no major stratigraphic breaks.

The internal reservoir architecture presents high horizontal continuity of the sandstone packages, with minor but locally important vertical flow barriers and retarding flow zones represented by relatively thin mudstone layers and very fine-grained and highly bioturbated sandstone facies. The dominant vertical anisotropy generated by the internal rock facies arrangement in combination with the strong diagenetic overprint (compactation and quartz cementation) represent the dominant controls in the fluid flow behavior in the field.

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MESOPROTEROZOIC TECTONOTHERMAL DEVELOPMENT OF SOUTHWESTERN SWEDEN: NEW AGE CONSTRAINTS USING FIELD RELATIONSHIPS AND U-PB GEOCHRONOLOGY

by

Catherine Anne Christoffel, M.S.

The University of Texas at Austin, 1997

Supervisor: James N. Connelly

The Ätran terrane of southwestern Sweden is a Mesoproterozoic, polydeformed terrane and a product of crustal growth westward away from an Archean core. Both Gothian (Labradorian) and Sveconorwegian (Grenvillian) deformations are recorded, but it is difficult to identify the effects of each deformation because the resultant fabrics are essentially parallel. Recent debate regarding the timing of regional gneiss-formation revolves around two distinct hypotheses; one assigns the bulk of the deformation to the Gothian Orogeny and the other attributes it to the Sveconorwegian Orogeny.

U-Pb ages from four lithotectonic units constrain the timing of tectonic and thermal events for the region. Primary, igneous zircons from a felsic orthogneiss yield a protolith crystallization age of 1664+/-7 Ma. The first dyke suite intruded the gneisses immediately after the earliest deformation and was then metamorphosed at 1654+/-9 Ma during the second major deformation and migmatization event. Migmatitic layers in the gneisses and mafic dykes are cross-cut by less deformed dykes of the second dyke suite that crystallized between 1426+9/-4 Ma and 1399+7/-6 Ma. The Gothian Orogeny was thus responsible for the majority of the deformation and migmatization recorded in the study area. In addition to ca. 1.4 Ga magmatism, metamorphism at 1438+12/-8 Ma resulted in new zircon growth in the mafic gneiss. This magmatism and metamorphism are interpreted to be a local manifestation of a regional inter-orogenic thermal event that may be related to ca. 1.4 Ga magmatism known elsewhere in southwestern Sweden. Sveconorwegian deformation, constrained by the undeformed third dyke suite, occurred before 946+6/-4 Ma. Metamorphic (re)crystallization of monazite (948+/-9 Ma), titanite (932+/-5 Ma and 935+/-7 Ma) and rutile (878+/-9 Ma) indicate late-Sveconorwegian static metamorphism and suggest slow cooling rates of 4.5-11deg.C/million years from 948 Ma to 932 Ma and 2.5deg.-5deg.C/million years from 932 Ma to 878 Ma during erosion and collapse of the Sveconorwegian Orogen.

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ALONG-STRIKE VARIATION OF A NON-VOLCANIC RIFTED MARGIN: A CONCURRENT MCS AND OBS INVESTIGATION OF GALICIA BANK AND THE IBERIA ABYSSAL PLAIN ALONG-STRIKE VARIATION OF A NON-VOLCANIC RIFTED MARGIN: A    CONCURRENT MCS AND OBS INVESTIGATION OF GALICIA BANK AND THE IBERIA ABYSSAL PLAIN

by

Stephen Anthony Clark, M.S.

The University of Texas at Austin, 1999

SUPERVISORS: Paul L. Stoffa and James A. Austin, Jr.

The Atlantic margin of Spain and Portugal is widely considered to be the classic non-volcanic passive continental edge. Mesozoic rifting between North America and Europe/Africa occurred in three stages, the last of which, during the Early Cretaceous, corresponded to a northward-propagating breakup of Iberia from its conjugate margin, the Grand Banks of Newfoundland. Relatively thin sediment cover and a lack of syn- and post-rift volcanism make this margin one of the best places on Earth to image clearly the Moho and deep crustal structures created by such rifting. Results from ODP Legs 149 and 173 have shown that crustal structure of the western Iberia margin is characterized by a ~100 km wide ocean-continent transition (OCT). Drilling of topographic highs indicates that uppermost crust within the OCT consists mostly of serpentinized peridotite, believed to be exhumed upper mantle.

A seismic experiment, R/V Maurice Ewing cruise 97-05, conducted in July-August 1997 by an international consortium led by Rice University, acquired MCS reflection data across and along the OCT. Simultaneous far-offset reflection and refraction data were recorded along many of the lines, with ocean-bottom seismometers (OBSs) from UTIG, ocean-bottom hydrophones (OBHs) from GEOMAR, and land stations from the universities of Madrid and Barcelona. MCS profile 9 runs north-south, parallel with and ~200 km west of the coast of Portugal. This profile characterizes the bathymetric contrast between Galicia Bank in the north (~2 km water depth) and the Iberia Abyssal Plain (IAP) in the south (~5 km water depth). Profile 9 is 270-km-long, with 10 OBS receivers located at 15-50 km spacing along its length. R/V Maurice Ewing



Along the southern half of profile 9, interpreted sediments are continuous and up to 4500 m (2.7 s twt) thick. Farther north, on southeastern Galicia Bank, sediment accumulation occurs only in isolated basins. Acoustic basement in the IAP is occasionally overlain by interpreted pre/syn-rift sediments. Strong, multiple sub-basement reflections reveal complex extensional faulting beneath the IAP. Some of these faults, occurring at 3 to 4 s sub-seafloor, may be related to a southward extension of "S," a group of bright reflectors which underlies tilted fault blocks beneath western Galicia Bank. The "S" reflector has previously been interpreted either as Moho or as an intra-crustal detachment fault, but has not yet been sampled. Ray-tracing and inversion of the travel times observed on the OBS data reveal a southward-thinning wedge of presumed extended continental crust stretching the entire N-S length of the IAP imaged, from the flank of Galicia Bank to the southern end of profile 9. A clear Moho reflection, with a P-wave velocity of ~8.0 km/s, lies at the base of this wedge, reaching depths as shallow as ~6 km below the sea floor or just ~1700 m below the top of acoustic basement. There is no evidence of substantial serpentinization of the upper mantle along profile 9, as defined by the presence of a lower crustal layer with P-wave velocities of 7.0-7.6 km/s.


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PATTERNS OF EVOLUTION AND VARIATION IN THE TAPIROIDEA (MAMMALIA: PERISSODACTYLA)

By

Matthew William Colbert, Ph.D.

The University of Texas at Austin, 1999

Supervisors: Timothy Rowe and Ernest L. Lundelius

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SEISMIC STRATIGRAPHIC ANALYSIS OF THE NORTHERN PART OF THE PATTANI BASIN, GULF OF THAILAND

by

Gundogan Coskun, M.S.

The University of Texas at Austin, 1998

Supervisor: William L. Fisher

The Tertiary Pattani Basin was opened on a pre-Tertiary basement complex under a extensional dextral transtensional shear regime created by the collision of Indian and Asian plates.

The basin is filled with predominantly nonmarine sands, shales, coals, and minor marine shales and limestones. The distribution and the type of sand-shale alternations define the seismic reflection character of depositional intervals. Because of the distinct seismic reflection properties of these lithologies, seismic reflection amplitudes and reflection patterns can be used to define lithologies, facies, and depositional environments. Using unconformity and flooding surfaces the Tertiary section was divided into seven stratigraphic intervals, which are also identifiable with their seismic reflection characteristics.

The Tertiary section was deformed by two-stage extensional deformation after deposition of Sequence V. The first stage created major north-south-trending normal faults. The second stage created NE-SW small-scale normal faults in the Tantawan area and NW-SE small-scale normal faults in the Benchamas area.

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FORWARD SEISMIC MODELING OF A SHELF-TO-SLOPE CARBONATE DEPOSITIONAL SETTING FROM OUTCROP DATA, THE ABO FORMATION OF APACHE CANYON, WEST TEXAS, AND COMPARISON TO ITS SUBSURFACE EQUIVALENT, KINGDOM ABO FIELD, MIDLAND BASIN
Bruno Courme, M.S.
The University of Texas at Austin, 1999

Co-Supervisors: W.L. Fisher and C. Kerans

The subject of this study is seismic forward modeling of outcrop data from the lower Permian Abo Formation carbonates of Apache Canyon, Sierra Diablo, West Texas. In this area, the Abo Formation is characterized by platform and slope carbonate deposits exhibiting highly progradational clinoform patterns, and is associated with an extensive paleokarst system that developed at its upper boundary. This work investigates the seismic response of these outcrops and uses these patterns to evaluate potential for imaging these signatures in subsurface seismic data. For this study I used data from the Kingdom Field, located about 120 miles northeast of the Sierra Diablo.
The seismic modeling approach used here is known as the vertical incidence technique. It relies on the definition of an impedance model based on the stratigraphic and petrophysical description of the outcrop. Velocity data were collected on the outcrop, using a hand-held sonic probe. Calibration with core-plug-based petrophysical measurements yielded a transit-time to porosity transform. By relating velocity to mapped depositional facies, an impedance model was built and calibrated to subsurface conditions.
The analysis of sonic probe data and comparison with laboratory and subsurface data shows: (1) acceptable values for transit-time measured on outcrop in comparison with laboratory measurements, (2) the differences introduced by surface versus subsurface conditions in transit-time measurements, including the influence of pore-filling fluid, (3) a good correlation between transit-time and porosity measured on outcrop, approximated by a linear transform, accounting for depositional texture as a dominant secondary factor, and (4) the need for a modification of the later transform to be applied under subsurface conditions.
Results from the outcrop model illustrate that: (1) the shelf-to-slope transition is clearly imaged at all frequencies, and (2) misinterpretation of terminations is likely to occur using frequencies typical of subsurface data. Regarding the potential imaging of paleokarst-related features and of regression-driven clinoforms, it is concluded that: (1) small-scale depressions are difficult to image, (2) large cave collapses can be resolved, with a better imaging potential at low frequency, and (3) at high frequency, clinoform patterns are not clearly resolved, but rather trigger diachronous reflections.
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OLIGOCENE TO RECENT STRATIGRAPHIC AND TECTONIC HISTORY OF THE CENTRAL SOLOMON INTRA-ARC BASIN AS DETERMINED FROM MARINE SEISMIC AND ONSHORE GEOLOGY

Shane Cowley, M.S.

The University of Texas at Austin, 1998

Supervisor: William E. Galloway

The Solomon island arc has been the focus of seismic, geologic, and marine geophysical surveys over the past 25 years since it was first proposed that the Solomon arc was the best example of an arc polarity reversal on Earth. As most arcs are assumed to have reversed polarity during their evolution, the Solomon Islands became a natural laboratory for the study of the deformational and igneous signatures of this fundamental tectonic process. The Solomon island arc is bounded to the north by the North Solomon trench, along which the Pacific plate is subducted and to the south by the San Cristobal trench along which the Australia and Solomon Sea plates are subducted. The Central Solomon intra-arc (CSB) basin overlies this zone of dual subduction which has been interpreted as polarity reversal with the North Solomon trench representing extinct or slowing subduction and the San Cristobal trench representing the site of younger, more active subduction. The Cretaceous Ontong Java Plateau (OJP), the largest oceanic plateau on Earth, collided with the North Solomon trench sometime in the Late Neogene, but the timing and consequences of this collision and its role in the subduction polarity process are poorly understood. The CSB, which developed in Oligocene to Recent time above the Solomon arc, provides a valuable sedimentary and deformational record of the events leading up to and accompanying the OJP collision and arc polarity reversal. The main objective of this thesis was to use seismic stratigraphic analysis within the CSB to document the consequences and timing of collision of the OJP.

The most complete 2D seismic grid in the CSB to date is provided by 3150 km of multi-channel seismic data collected in 1982 aboard the R/V S. P. Lee, and 300 km in 1995 aboard the R/V Maurice Ewing. Using a workstation, I analyzed these digital data and produced maps of four seismic sequences above acoustic basement in the Russell and Shortlands basins, and two sequences above acoustic basement in the Indispensible basin (all sub-basins of the CSB). During the Late Oligocene to Late Miocene, the oldest sequence, sequence Ra, accumulated in a normal fault-bounded back-arc basin and was principally derived from exposed island-arc basement on the islands of Choiseul, Santa Isabel, and Guadalcanal. Sequence Rb accumulated during the main phase of deformation caused by the collision of the OJP. This sequence and was folded above inverted normal faults along the northern edge of the Russell basin. Onshore - offshore correlations between seismic and outcrop support a Late Pliocene age for this deposition and deformation. Deformation in the Shortland basin was slightly younger indicating a diachronous collision for the OJP with northwestward progression. During the Late Pliocene and Early Pleistocene sequence Rc was derived from both sides of the closing basin. Plio - Pleistocene volcanism related to arc polarity reversal and the initiation of subduction at the San Cristobal trench formed a chain of volcanic islands on the southern side of the basin providing a source for this sediment. Shortening still occurs across the Solomon islands and the basin deforms as a large syncline parallel to the Solomon arc.

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STUDY OF ALBIAN CARBONATE ANALOGS: CEDAR PARK QUARRY, TEXAS, USA, AND SANTOS BASIN RESERVOIRS, SOUTHEAST OFFSHORE OF BRAZIL

by

Wagner Maia Cruz, Ph.D.

The University of Texas at Austin, 1997

Supervisor: William L. Fisher

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INTEGRATED RESERVOIR CHARACTERIZATION OF SUN OIL AND GAS FIELD, SOUTH TEXAS

by

Jianchun Dai, Ph.D.

The University of Texas at Austin, 2000

Supervisor: William L. Fisher

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DELINEATION OF THE GROUND-WATER FLOW SYSTEMS OF THE EAGLE FLAT AND RED LIGHT BASINS OF TRANS-PECOS TEXAS

by

Bruce Kelley Darling, Ph.D.

The University of Texas at Austin, 1997

Supervisor: John M. Sharp

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A STUDY OF SURFACE SOIL MOISTURE VARIABILITY WITHIN REMOTE-SENSING FOOTPRINTS

by

Johanna A. Devereaux, M.S.

The University of Texas at Austin, 1998

 

Supervisor: James S. Famiglietti

Soil moisture has been recognized for its integral role in the hydrological cycle at many scales. Because of its inherent variability, however, soil moisture content is difficult to quantify and represent in larger-scale hydrological models. Remote sensing may provide the primary means by which soil moisture is regionally and globally observed. Therefore, it is crucial to improve characterization of surface soil moisture variability at the remote sensing scale.

During the Southern Great Plains 1997 (SGP97) Hydrology Experiment (a large-scale soil moisture remote sensing experiment in Oklahoma), surface soil moisture (0-6 cm) was measured daily for about one month. Soil moisture measurements were taken in six, 0.64 km2 fields (the approximate footprint of the SGP97, plane-borne sensor) throughout the SGP97 region. The data were collected at 49 points on a 100-meter grid within each field.

Several transects were sampled at a higher-resolution to observe local topographic effects. Various probe calibration techniques, including in situ, slurry, and pressure plate extraction, were tested in order to understand the overall measurement uncertainty. Each field's daily distribution of soil moisture was examined to find any temporal changes in response to precipitation events and subsequent drying.

Results reveal changes in soil moisture distributions corresponding to changes in the field's average soil moisture content. Overall, the standard deviation, skewness, coefficient of variation and kurtosis generally increased with decreasing moisture content. The distribution of soil moisture progressed from negatively-skewed/non-normal under very wet conditions, to normal in the mid-range of mean moisture content, to positively-skewed/non-normal under dry conditions. By quantifying these trends at remote-sensing scales, remotely sensed soil moisture values become more useful to a wider range of earth scientists.

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ARCHITECTURE OF A SHORE-ZONE RESERVOIR SYSTEM IN BARÚA FIELD, MARACAIBO BASIN WESTERN VENEZUELA

Fé1ix Antonio Díaz, M.S.

The University of Texas at Austin, 1998

Supervisor: Noel Tyler

This thesis presents the results of an integrated reservoir characterization study of the Middle Eocene Misoa B and Paují A-10 sandstone reservoirs in Barúa field, Maracaibo Basin, western Venezuela. This study consisted of integrated geophysical and geological analyses in order to (1) characterize structurally and stratigraphically the controls on reservoir architecture and (2) identify field development opportunities for increased production and ultimate recovery.

Structurally, Barúa field is a north-south trending faulted anticline that developed as a result of Tertiary orogenesis. Source rocks are postulated to be the Cretaceous La Luna Formation, and oil production is from Middle Eocene sandstones that correspond to the Misoa and Paují Formations.

From well logs and core descriptions, the Misoa and Paují Formations can be subdivided in upward-coarsening parasequences. Depositional environments for Misoa and Paují reservoirs in Barúa field were interpreted as a barrier-island systems that prograded towards the northeast, but overall the systems show a retrogradational architecture. Primary facies of this depositional environment are barrier-core, shoreface, back-barrier, and lagoon.

Seismic amplitude maps correspond strongly to net sandstone maps, with sandstone correlating with low amplitude responses. Faulting has also created multiple compartments that have not been adequately contacted or drained. Seismic identification of sandstone distribution and faults provide criteria for selecting new infill, field extension, and exploratory wells in the area, resulting in opportunities for incremental oil recovery and reserve growth potential.

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GEOLOGIC MODELS OF SEDIMENT-BUFFERED HYDROTHERMAL VENTS : A CASE STUDY OF THE RED DOG ZN-PB-AG OREBODY, WESTERN BROOKS RANGE, ALASKA

by

David Edgerton, PhD

The University of Texas at Austin, 1997

Supervisor: J. R. Kyle

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BIOSTRATIGRAHIC AND SEQUENCE STRATIGRAPHIC ANALYSIS OF THE YEGUA FORMATION, HOUSTON SALT EMBAYMENT, NORTHERN GULF OF MEXICO

By

Qing Fang, Ph.D.

The University of Texas at Austin, 2000

Supervisor: William E. Galloway

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SEDIMENTOLOGY AND DEPOSITIONAL HISTORY OF THE MIOCENE GILA GROUP IN THE MIMBRES BASIN, GRANT COUNTY, NEW MEXICO SEDIMENTOLOGY AND DEPOSITIONAL HISTORY OF THE MIOCENE GILA GROUP IN THE MIMBRES BASIN, GRANT COUNTY, NEW MEXICO

by

Laura Lea Faulkenberry, M.S.

The University of Texas at Austin, 1999

SUPERVISOR: Richard T. Buffler


Tectonic development and basin morphology of continental rift basins influences aspects of basin sedimentation, including depositional processes, depositional environments, facies distribution, and the overall stratigraphic architecture. This research looks at these aspects of rift basin sedimentation by performing a detailed outcrop study of the Mimbres basin in southwestern New Mexico. The Mimbres basin is a north to northwest trending asymmetrical triangular-shaped half-graben on the western edge of the southern Rio Grande Rift. The basin fill consists of conglomerates and sands of the Gila Group, which was deposited between the late Oligocene and late Miocene, based on 40Ar/39Ar ages of underlying and overlying basalt flows at 27.8 + 0.45 and 6.29 + 0.18 to 6.53 +0.76, respectively. 40Ar/39Ar ages of underlying and overlying basalt flows at 27.8 + 0.45 and 6.29 + 0.18 to 6.53 +


The Gila Group is subdivided into eight facies, six of which occur as alluvial fan deposits on the basin margins and two of which occur along the basin axis. The alluvial fan facies include 1) massive oligomictic colluvium, 2) clast-supported debris flow conglomerates, 3) matrix-supported debris flow conglomerates, 4) debris flow sandstones, 5) channelized streamflood conglomerates and sandstones, 6) tabular sheetflood conglomerates and sandstones; the basin axis facies include 7) channelized, cross-stratified fluvial conglomerates, and 8) organic-rich lacustrine siltstone.
Alluvial fan deposits within half-grabens can be subdivided into footwall alluvial fans and hangingwall alluvial fans. The footwall fans in the Mimbres basin are slightly smaller aerial extent, show paleoflow from the basin margins to the basin center, and are extremely coarse. They are dominated by clast-supported debris flow conglomerates, but contain Facies 1 through 5. The hangingwall fans have a larger aerial extent, show paleoflow from the basin margin to the basin center, contain a wide range in clast sizes, and contain Facies 2 through 6. Basin axis deposits include Facies 7 and 8 that have distinctive boundaries and paleoflow directions to the north and northwest, paralleling the basin margin.

 Evidence suggests sedimentation within the Mimbres Basin is controlled by both autocyclic and allocyclic processes. Bed-scale heterogeneities and amalgamation of facies in both footwall and hangingwall alluvial fans reflect an autocyclic development, including geomorphic lobe and channel avulsion, and compensation between fan packages. Large-scale changes include upward coarsening within individual facies and an overall basinward shift of proximal over distal facies. These are best developed in the hangingwall fans and have been interpreted as being controlled by allocyclic processes. Tectonism is the most likely allocyclic control because of the continuity in the coarseness the sediments suggests a relatively constant climate. Progradation of the gross alluvial packages within the Mimbres Basin is caused by at least one overall cycle of tectonism during Gila Group deposition.

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DENSITY-DRIVEN FREE CONVECTION ACROSS LOW PERMEABILITY STRATA, SOUTH TEXAS EXAMPLE

by

Thomas Raymond Fenstemaker, M.S.

The University of Texas at Austin, 2000

Supervisor: John M. Sharp, Jr.

This study investigates the plausibility of salinity-driven free convection in sediments of the South Texas portion of the Gulf of Mexico Basin by analysis of salinity data, Rayleigh Number calculations, and numerical models. Previous studies speculated that free convection might account for high fluxes evidently required for diagenesis in the basin, but low permeability shales were calculated to be a barrier to free convection. In the study area, salinity inversions are located near the transition zone from hydropressures to overpressures. The positioning of denser brines over less saline and therefore less dense fluids provides a significant buoyancy force. Simple Rayleigh Number calculations and numerical simulations suggest that these systems are unstable near the high end of the expected ranges of shale permeability (10-15 - 10-16 m2). Numerical simulations also show that heterogeneities in fluid salinities and permeability of underlying shales are conducive to free convection. These simulations indicate that that salinity-driven free convection can occur at lower permeabilities (10-16 - 10-18 m2) than is predicted by simple Rayleigh Number calculations. In addition, this study demonstrates the inability of simple Rayleigh Number estimates to predict accurately the occurrence of free convection in heterogeneous systems. Free convection at depth in large sedimentary basins, such as the Gulf of Mexico, may be more common than hitherto expected.

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SEISMIC INTERPRETATION OF SHORE ZONE-INNER SHELF DEPOSITS IN CORPUS CHRISTI BAY SOUTH TEXAS

Carlos Juan Fimlay Paz, M.S.

The University of Texas at Austin, 2000

Supervisor: William L. Fisher

 

Oligocene shelf, distal shoreface, and barrier sandstone reservoirs of the Frio Formation have been producing significant volumes of gas for more than 30 years in the Corpus Christi Bay area. These mature reservoirs are located in structural-stratigraphic combination traps.

In this study, 280 km2 of 3D seismic data, 183 wells, core information, production data, and visualization techniques were integrated and analyzed to develop a more precise image of reservoir configurations.

Tops of Upper, Middle, and Lower Frio were interpreted and correlated into the 3D seismic volume on a sub-regional scale. Tops of five maximum flooding surfaces (MFS), designated H, I, J, K, and L, were interpreted in a more detailed correlation for a reservoir characterization study in the Corpus Channel NW field.

Coherency cubes created from seismic data provided a detailed structural interpretation of the Upper, Middle, and Lower Frio, and at the reservoir scale, for MFS H, I, J, K, and L, which bound all the reservoirs in the Middle Frio. Attribute strata slices enhanced previously published stratigraphic interpretations, giving an excellent plan view of the principal component elements of the Frio and Miocene shore zone systems. Seismic attributes (amplitude anomalies) respond both to gas content and to sand distribution trends. Visualization techniques provided a good general 3D picture of the structural setting and the reservoir configuration.

The structural pattern is interpreted as a gravity-induced, down-to-the-basin, normal growth fault system and associated faulted rollover anticlines and shale diapirs. These features typify the structural style found throughout the Cenozoic fill of the Gulf Coast Basin.

Core information was integrated with geological mapping and seismic interpretation products for characterization of the Middle Frio J-2A reservoir in the Corpus Channel NW field. A mosaic of facies, which include barrier core, inlet channels, washover fans, and more distal to inner shelf deposits compose the reservoir. An infill well is proposed based on integration of this data.

Techniques developed herein can be usefully applied to other reservoirs in the Lower and Middle Frio in the Corpus Christi Bay area.

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LOWER PERMIAN SEQUENCE STRATIGRAPHY OF THE WESTERN DELAWARE BASIN MARGIN, SIERRA DIABLO, WEST TEXAS

by

William Mills Fitchen, Ph.D.

The University of Texas at Austin, 1997

Supervisor: Richard Thurman Buffler

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THE EFFECT OF RELATIVE SEA LEVEL CHANGE ON THE DEVELOPMENT OF PHYLLOID ALGAL BIOHERMS, LABORCITA FORMATION (LOWER PERMIAN), SACRAMENTO MOUNTAINS, NEW MEXICO

by

Ian Thorkild Gordon, MA

The University of Texas at Austin, 1997

Supervisor: Brenda Kirkland George

The purpose of this project is to understand the factors that led to the development of phylloid algal bioherms in the Wolfcampian (Lower Permian) Laborcita Formation, Sacramento Mountains, New Mexico. The Laborcita Formation outcrops consist of a narrow, 27 km long strip with a maximum thickness of 240 m. It is defined as the transitional beds interfingering between the underlying Pennsylvanian Holder Formation marine carbonates, and the overlying Wolfcampian Abo Formation terrestrial red beds. The outcrops of the Laborcita Formation are generally situated oblique to depositional dip, and display dramatic lateral facies changes from terrestrial red beds in the south to spectacular open marine phylloid algal bioherms in the north. My study area encompasses the northern biohermal portion of the formation.

I measured ten sections which formed the basis for my cross section. The biohermal portion of the formation is parallel with depositional strike, and lateral facies changes are minimal. I identified four depositional environments: fluvial, restricted marine, deltaic, and open marine. The recurrent stratigraphic motif in the northern part of the Laborcita Formation consists of deltaic progradation during relative sea level highstand, aggradation of fluvial or restricted carbonate facies during lowstand, and most spectacularly, growth of phylloid algal bioherms during transgression.

The stratigraphic context and modern and Late Paleozoic analogs of the Laborcita Formation phylloid algal bioherms all suggest that their maximum development occurred during relative sea level transgression. The ubiquity of deltas and the ample evidence for syndepositional tectonism in the northern portion of the Laborcita Formation suggest that sediment supply and tectonism are the principal controls on sequence development. The Laborcita Formation bioherms occur on top of regional deltaic sand thicks. This suggests that abandoned delta lobes formed platorms suitable for bioherm development. Upwelling of reducing, nutrient-rich waters may help explain the enhanced Late Paleozoic bioherm growth in the Orogrande basin.

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PHYLLOID ALGAL BIOHERMS AND OOID GRAINSTONES: CHARACTERIZATION OF RESERVOIR FACIES UTILIZING SUBSURFACE DATA FROM THE ANETH PLATFORM AND OUTCROP DATA ALONG THE SAN JUAN RIVER, PARADOX BASIN, SOUTHEASTERN UTAH

 by

Jonas Paul Gournay, PhD

The University of Texas at Austin, 1999

 

Supervisor: Brenda L. Kirkland

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CONTINENTAL RIVER ROUTING FOR FULLY COUPLED CLIMATE SYSTEM MODELS

by

Stephen Thomas Graham, Ph.D.

The University of Texas at Austin, 2000

Supervisor: James S. Famiglietti

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THE GRENVILLE OROGENY IN WEST TEXAS: STRUCTURE, KINEMATICS, METAMORPHISM AND DEPOSITIONAL ENVIRONMENT OF THE CARRIZO MOUNTAIN GROUP

 by

Stephen Whiteford Grimes, PhD.

The University of Texas at Austin

May, 1999

Supervisor: Sharon Mosher

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MIOCENE STRATIGRAPHY AND DEPOSITIONAL FRAMEWORK OF NORTHEASTERN MARACAIBO BASIN, VENEZUELA: IMPLICATIONS FOR RESERVOIR HETEROGENEITY PREDICTION IN TECTONICALLY-ACTIVE SETTINGS

by

José Ignacio Guzmán Espinal, Ph.D.

The University of Texas at Austin, 1999

Supervisors: William L. Fisher, Noel Tyler

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PERMEABILITY STRUCTURE IN FRACTURED AQUIFERS

by

Todd Halihan, Ph.D.

The University of Texas at Austin, 2000

Supervisor: J. M. Sharp

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SYN-OROGENIC SLOPE AND BASIN DEPOSITIONAL SYSTEMS, OZONA SANDSTONE, VAL VERDE BASIN, SOUTHWEST TEXAS

by

Herbert Scott Hamlin, Ph.D.

The University of Texas at Austin, 1999

 

Supervisor: William E. Galloway

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HYDROTHERMAL ALTERATION AND FLUID EVOLUTION OF THE GRASBERG PORPHYRY CU-AU DEPOSIT, IRIAN JAYA, INDONESIA

by

Jeffrey S. Harrison, M.S.
The University of Texas at Austin, 1999
Supervisor: J. Richard Kyle

The Grasberg is a gold-rich porphyry copper deposit located at the crest of the Central Range of Irian Jaya and is the largest orebody of the prolific Ertsberg District. The deposit is hosted within a nested suite of 3 Ma quartz monzonite to quartz monzodiorite intrusions. Currently the units of the Grasberg are divided into three groups of intrusions. From oldest to youngest, these are the Dalam, the Main Grasberg Intrusion and the Kali.
The units are all porphyritic and very similar petrographically. To date, textural criteria regarding the percentage and character of phenocryst phases serve as the best manner to differentiate between units. Major-oxide and trace-element analyses of the least altered samples available for each unit show strong genetic ties between the rocks of the Grasberg and also document an evolution to slightly more evolved/fractionated compositions with the increasingly younger units.
Two distinct types of alteration affect the rocks of the Grasberg that show consistent spatial arrangements throughout the deposit. The very center of the deposit is relatively unaltered but surrounded by an alteration type which consists of pervasive magnetite+biotite+K-spar. Towards the margins of the complex, an alteration assemblage dominated by quartz+sericite becomes increasingly well-developed and overprints K-silicate alteration. These expressions of alteration are interpreted to be similar to or variations of the K-silicate and phyllic alteration styles described in other porphyries. Other frequently recognized alteration types noted in similar deposits, such as propylitic and advanced argillic alteration, are largely absent but may have been better-developed in the eroded or excavated portions of the complex.
The history of vein formation appears to have been closely related to the development of alteration at Grasberg, as the paragenetically earliest vein phases (magnetite+biotite) correspond with the earlier alteration assemblages observed in the center of the deposit. Temporally late veins are strongly associated with sericite-rich selvages suggesting that the conditions of late vein-forming fluids were similar to the fluid conditions responsible for the phyllic alteration pervasive at the margins of the complex. If fluids responsible for vein formation and the development of pervasive wall-rock alteration can be linked, than mineralizing veins appear to occur temporally between the development of the two alteration styles identified.
Alteration at Grasberg is arranged concentrically and symmetrically around the youngest Kali intrusion. Note however that pervasive alteration and high-grade Cu-Au mineralization are observed within the Kali near contacts with other lithologies. Major-oxide and trace-element analyses of altered samples, when compared to average unaltered compositions, generally show that samples affected by K-silicate alteration are enriched in Fe 2 O 3 , MgO and K 2 O, while SiO 2 concentrations were elevated overall within phyllic-altered samples. The trace-element data generally mimicked trends observed in the major-oxide data and secondary mineral abundance is believed to exert the strongest control on compositions of altered samples. The exception to secondary mineral abundance controlling the compositions of altered rocks is Na 2 O which likely reflects the relative preservation of plagioclase phenocrysts rather than an alteration phase.
Based on the phase relations between alteration minerals and fluid inclusion measurements in quartz veins associated with specific alteration types, the K-silicate alteration preserved in the center of the complex appears to have been the result of high temperature (>700°C), iron-rich oxidizing brines (~70 wt% NaCl equiv.). The zonation observed within the K-silicate alteration at Grasberg, from magnetite in the center of the deposit to biotite outwards, may have been produced by a slightly less oxidizing fluid with decreased iron activity, perhaps also at lower temperatures (~500°C), as suggested from fluid inclusion data. Phyllic alteration appears to have been produced by fluids that were increasingly cooler (200-500°C), less saline (~30-50 wt% NaCl equiv.), and more acidic. Stable isotope measurements performed on mineral separates prepared to represent major expressions of alteration suggest that Grasberg's hydrothermal system was predominantly magmatic throughout the course of its development.

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POST-STACK MIGRATION VELOCITY ANALYSIS OF THREE-DIMENSIONAL SEISMIC REFLECTION DATA

by

Alison Teagan Henning, M.A.

The University of Texas at Austin, 1997

Co-Supervisors: Thomas H. Shipley and Milo M. Backus

The objectives of this investigation were to evaluate a visual velocity analysis technique and to determine a velocity function for improved imaging of a three-dimensional seismic reflection data set. The data were collected at the northern Barbados ridge accretionary complex, located at the Lesser Antilles active margin. The specific purpose of the high resolution 3-D survey was to examine the structural development of the accretionary prism, for which accurately migrated seismic sections are essential. The interior structure of the prism was not well-resolved in the initial migrated images because of poorly known velocities.

The velocity analysis technique consisted of using seismic data movies to extract velocity information from the near zero-offset 3-D data. This technique exploits the fact that zero-offset data contain wide-angle information in the form of diffractions and discordant dipping specular reflections. Because migration is sensitive to velocity, we performed 3-D migrations of the data using a series of trial velocities to determine the velocity empirically. Seismic data movies display a sequence of frames of the same seismic section migrated at different velocities to show how the image changes with velocity.

Velocities derived from the movies were used to construct a 3-D migration velocity function. The re-migrated seismic sections provide enhanced images of steep features in the seismic data, such as faults in the prism, which improve structural interpretation of the data set. 2-D and 3-D forward modeling validated the use of the movie technique and provided estimates of the velocity sensitivity for the visual criteria used to determine the proper imaging velocity.

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INTERPRETATION AND DISTRIBUTION OF DEPOSITIONAL SYSTEMS: OLIGOCENE FRIO DEPISODE IN THE SUBSURFACE OF BURGOS BASIN, NORTHEASTERN MEXICO.

by

José Javier Hernández Mendoza, MSGeoSci

The University of Texas at Austin, 2000

Supervisor: William E. Galloway

The Frio Depositional Episode in the Burgos Basin is a major progradational wedge of Oligocene age in the Northwest Gulf Coast Basin. The Frio Depisode (FDE) is one of the most prolific Tertiary sections in the Burgos Basin in terms of gas production. Although this basin has been exploited for several decades, it lacks a reliable regional depositional framework that correctly classifies the different plays and provides a valid sedimentological model for advanced recovery techniques. This study is designed to understand and classify the FDE within a regional structural-stratigraphic context. The methodology used is not new, but this is the first time that it has been applied to this area. Nine dip oriented and three strike oriented well log cross sections were used to correlate and subdivide the FDE and to make a series of isopach, net sandstone, sandstone percentage, and log facies maps for the Upper, Middle and Lower FDE.

The Norma fluvial system supplied most of sediments of FDE in the Burgos Basin. The Norma fluvial system is located in the central-western part of the area, and consists of a bed-load channel-fill, aggradational streamplain, and floodplain facies. The large volume of clastic sediments supplied to the basin formed a deltaic sequence over undercompacted prodelta and slope muds, thus causing the development of syndepositional faults i.e., growth faults. The delta-front sediments were reworked and transported by waves and coastal currents in the strike direction southward along the ancient coast; the sediment transport formed strike elongated strandplain, coastal bar, and spit sand bodies. Based upon maps, facies successions, and core descriptions, the Reynosa delta system is proposed to be a wave-dominated to wave-modified delta system. This study challenges the barrier bar system model proposed in the 1980's as the primary depositional system for the FDE.

Part of the Middle FDE was eroded by fluvial incision beneath the Norma fluvial axis. The Conglomerado Norma was deposited as coalesced, aggradational fluvial channel deposits on this surface in the later FDE.

In the southern part of the study area the sedimentation trend is oblique in relation to the regional faulting. This divergence may be the result of the strong influence of concurrent post-orogenic extension of the adjacent Sierra Madre Oriental.

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DEPOSITIONAL ENVIRONMENTS AND DIAGENESIS OF THE EARLY CRETACEOUS COX SANDSTONE, FINLAY MOUNTAINS, TRANS-PECOS TEXAS

by

David Lee Hicks, MA

The University of Texas at Austin, 1997

SUPERVISOR: E.F. McBride

The Cox Sandstone is an Early Cretaceous sequence of mixed siliciclastics and carbonates deposited in supratidal to shallow marine settings. Deposited alone, the northeastern shoreline of the Chihuahua Trough during the Albian transgression of the Mexican Seaway, the Cox in the Finlay Mountains represents a transition from lower coastal plain to shallow marine conditions as the shoreline n-migrated to the northeast.

Cementation of the Cox Sandstone, primarily by quartz, calcite and kaolinite has been influenced by depositional environment and can be related to the burial history of the Cox and tectonism in the adjacent Chihuahua Tectonic Belt.

Reconstruction of the primary composition of the Cox Sandstone indicates that a composition of approximately Q86F7L7 has been modified during diagenesis to a present composition of Q89F3L8. Individual lithofacies have experienced varying amounts of feldspar replacement (1-7 %), suggesting that the textural character (grain size, sorting, sand geometry, etc.) of individual lithofacies has influenced diagenesis.

Burial history and pre-cement porosity of the Cox suggests quartz cementation occurred after maximum burial of 1.2 km. Conditions favorable for quartz cementation first occurred during the Late Cretaceous - Early Tertiary and may be related to the development of the nearby Chihuahua Tectonic Belt. Hot (60 - 100'C), evolved formation waters expelled from evaporates, limestones and clastics of the Chihuahua Trough were forced through the Cox, providing the necessary fluid flow for significant volumes of quartz cement. Calcite post-dates quartz but predates the igneous intrusion of the Finlay Mountains. Isotopic and petrographic data suggest variable degrees of calcite recrystallization in the Cox as a result of this intrusion. The isotopic values of the least affected cements (the heaviest [partialdiff]180 values) suggest calcite was derived from similar fluids as was quartz, namely, hot evolved formation waters or brines from within the Chihuahua Basin.

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QUANTITATIVE STUDIES OF PORPHYROBLASTIC TEXTURES

By

David Marshall Hirsch. Ph.D.

The University of Texas at Austin, 2000

Supervisor: William D. Carlson

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EVIDENCE FOR MILANKOVITCH ORBITAL FORCING IN THE CRETACEOUS UPPER GLEN ROSE FORMATION OF THE EAST TEXAS BASIN AND THE FORT TERRETT FORMATION OF THE CENTRAL TEXAS PLATFORM

by

Cory Lane Hoffman, Ph.D.

The University of Texas at Austin, 2000

Supervisor:  Brenda L. Kirkland

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DEFORMATIONAL HISTORY OF THE VALLEY SPRING DOMAIN IN THE NORTHEASTERN LLANO UPLIFT, DEVILS WATERHOLE, INKS LAKE STATE PARK, BURNET COUNTY, TEXAS

by

April Michele Hoh, M.S.

The University of Texas at Austin, 2000

Supervisor: Sharon Mosher

Structural mapping, geochemical and metamorphic analyses, and U-Pb dating of Valley Spring Domain (VSD) units within the Devil's Waterhole (DW) of Inks Lake State Park reveal that the northeastern Llano uplift was deformed and metamorphosed synchronous with the southeastern Llano uplift. No early deformation or metamorphism is recognized, as previously proposed for the DW area, in the VSD in the northeastern Llano uplift. The VSD deformation and metamorphism are consistent with the interpretation that the Llano uplift is a collisional core.

Detailed structural mapping of Devils Waterhole (DW) of Inks Lake State Park, reveals five foliations (S1-S5) and four associated fold generations (F2-F4) in migmatitic host rock and a 1253 +3/-2 Ma foliated sill. The structures are compatible in style, sequence, and orientation and are the same in the host rocks as the foliated sill.

The DW is surrounded by a deformed gneiss, the 1232 ± 4 Ma Inks Lake Gneiss (ILG). The style and sequence of the structures at DW are comparable to structures in the ILG, thus deformation in the northeastern Llano uplift is constrained to post 1232 ± 4 Ma.

Deformation at DW occurred synchronous with amphibolite to granulite facies metamorphism. The metamorphic mineral assemblage in pelitic units coupled with the presence of leucosomes parallel to S1 through S3 indicate that early structures were forming in higher temperature conditions than the later structures (S4/F4 and S5/F5). Higher temperature metamorphic conditions are recorded in DW than in the southeastern Llano uplift. U-Pb dating of metamorphic zircons from two amphibolite gneiss samples from DW yield ages of 1129± 3 Ma, which is compatible with timing of metamorphism in the southeastern Llano uplift.

Geochemical analyses of two DW amphibolite gneiss samples were compared to samples in the surrounding Inks Lake Gneiss and theVSD. The two DW samples appear to be related to two meta-igneous amphibolite gneisses from the southeastern uplift near Honey Creek (Zumbro, 1999). All samples from the VSD show a tectonic signature consistent with an arc.

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PETROLOGY AND GEOCHEMISTRY OF THE MARQUEZ SHALE SEPTARIAN CONCRETIONS, BASTROP COUNTY, TEXAS

by

Gavin Lee Hudgeons, M.S.

The University of Texas at Austin, 1999

 

SUPERVISOR: Earle F. McBride

 

Many aspects of the development of septarian concretions are still debated, including origin, septarian fracture formation, and the role of biogenic processes in their origin. This study utilizes a variety of geochemical and petrographic techniques to understand the processes which produced septarian concretions in the Eocene Marquez Shale, Bastrop County, Texas.

The Marquez Shale septarian concretions are lens or disc shaped with the long axes parallel with the plane of bedding. The long axes range in size from 0.2 to 0.4 m in diameter and are generally two times the length of the short axes. The septarian fractures are up to 1.5 cm wide in the concretion center and thin outward, rarely reaching the concretion margin. The septarian concretions are encased within lens- or disc-shaped secondary concretions whose long axes also lie in the plane of bedding. These "encasing concretions" contain no septarian fractures and differ in textural and geochemical properties from the septarian concretions.

Stable oxygen isotope data and preservation of delicate textures suggest concretion cements began precipitating within the first few meters of burial and ended at around 480 m depth. Cements in encasing concretions, which surround the septarian concretions, precipitated from 600 m to 920 m. Stable carbon isotope data suggest the cements in septarian concretion cements precipitated in the zone of sulfate reduction, and the encasing concretion cements precipitated in the zone of methanogenesis.

Two generations of septarian veins have formed. Both generations appear radial in horizontal slices, but are oriented vertically in vertical slices. Later generations crosscut earlier fractures and fossil shells. Petrographic observations and microprobe transects across the calcite fracture infills shows three optically and chemically distinct zones of calcite growth. Stable isotope data suggests the bulk of the fracture infill was precipitated in the zone of methanogenesis after the formation of the encasing concretions.

Although hints of concentric zonation are macroscopically visible, geochemical analyses, cathodoluminescence properties, and carbonate composition from across transects from the center to the margins of the concretions show no temporal trends. However, carbonate abundance decreases from the centers to the margins of the septarian concretions, suggesting that the cements precipitated outward from the centers to the margins of the concretions.

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MESOPROTEROZOIC STRUCTURAL EVOLUTION AND LITHOLOGIC INVESTIGATION OF THE WESTERN LLANO UPLIFT, MASON COUNTY, CENTRAL TEXAS

by

Brian Butler Hunt, M. S.

The University of Texas at Austin, 2000

Supervisor: Sharon Mosher

The Llano Uplift of central Texas contains the largest exposure of Mesoproterozoic rocks along southern Laurentia and is thus crucial to the understanding of orogenesis and plate reconstructions along a portion of one of the largest orogens in the world. Most of the current understanding of the Mesoproterozoic tectonic evolution of southern Laurentia comes from the southeastern portion of the Llano Uplift. To fully characterize the tectonic evolution Llano Uplift, detailed mapping is necessary in the less-studied western Llano Uplift.

The Mesoproterozoic Llano Uplift exposes mid-crustal, poly-deformed and metamorphosed schists and gneisses and abundant pre- to post-tectonic granites through an erosional window of Phanerozoic sedimentary rocks. Three lithologic groups were mapped in the western Llano Uplift, from structural highest to lowest these are the Valley Spring Gneiss (VSG), Lost Creek Gneiss (LCG) and Packsaddle Schist (PS). The VSG consists of pelitic schists and pink quartzofeldspathic schists and gneisses. The LCG is a thick, homogeneous package of medium- to coarse-grained augen granite gneiss, interpreted to be a deformed coarse-grained porphyritic pluton. The PS consists of a heterogeneous package of interlayered quartzofeldspathic gneisses and amphibolites and minor marbles. These lithologies are consistent with the PS and VSG domains described in the southeastern Llano Uplift (Mosher, 1998; Reese et al., 2000). The exotic Coal Creek Domain (CCD) of the southeastern Llano Uplift is not observed in the western Llano Uplift.

The western Llano Uplift, including the VSG, LCG and PS, records a deformational history that resulted in multiple fold generations F1-F5 and characterized by a penetrative axial planar foliation S1-S5. F2's are isoclinal folds of S0 (primary layering) and S1 that locally fold F1 axial planes and have steeply plunging and generally easterly trending hinge lines. F3 folds are locally developed, nearly collinear and coplanar with F2's, tight to open, and fold all previous structures (F1/F2) and fabrics (S1/S2). F4's are open folds with northeast-trending axial traces that occur on a regional-scale,. F5's are open to tight folds of all previous structures, with hinge lines that are primarily southeast trending and steeply plunging. S0 to S3 orientations vary from north to east dipping because of reorientation by younger folds. S4 foliations strike to the northeast and S5 foliations are northwest striking and nearly vertically dipping. Late left-lateral shearing (D6) with generally an easterly trend and boudinage affects the VSG, LCG and VSG in this study area and is commonly associated with unfoliated granite material. Four generations of intrusive granitic sills and dikes are documented and provide relative and absolute age constraints on deformation. The oldest recognized deformation (D1-D3) is constrained between 1253 +5/-3 Ma and 1126 +5/-4 Ma (Roback, et al., 1999). D4 and D5 deformation are constrained between 1126 +5/-4 Ma and 1076 ± 5 Ma (Roback, et al., 1999). Although a change in metamorphic conditions is documented to have occurred between D2 and D3, metamorphic fabrics and assemblages indicate granulite facies conditions during D1, D2 and D3. Amphibolite facies metamorphism occurred during D4 and presumably D5.

Deformation in the Llano Uplift has a similar polyphase deformational history to that recorded here for the western Llano Uplift. Deformation in the eastern Llano Uplift is similarly constrained between ca. 1238 to 1091 Ma. In addition, the youngest deformation generation (F5) can be directly correlated in orientation and timing from the western to the southeastern Llano Uplift, and is constrained between ca. 1119 and 1091 Ma in the southeastern uplift. Both the western and eastern Llano Uplift contain late shear zones and extension structures. Structural differences between the western and southeastern Llano Uplift include differences in style and orientation of all but the latest (D5 and D6) structures. In addition, dip of fabrics and, therefore, structural stacking of lithologic domains is opposite, and no mylonite zones were identified in the west.

In conclusion, the lithologic domains appear to correlated across the Llano Uplift based upon gross lithologic similarities and the tectonic evolution is similar to the well-studied southeastern Llano Uplift, though the kinematics and orientations differ.

These conclusions may require that the kinematics of deformation in the southeastern uplift were controlled by the presence of the exotic island arc terrane (CCD) whereas the kinematics of deformation in the western uplift were controlled by continent-continent collision.

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SEQUENCE STRATIGRAPHY OF THE PERMIAN DELAWARE MOUNTAIN GROUP, BASIN-FLOOR SETTING, DELAWARE BASIN, WEST TEXAS AND SOUTHEASTERN NEW MEXICO: APPLICATION OF STACKING PATTERN ANALYSIS

by

 Wade Don Hutchings, M. S.

The University of Texas at Austin, 2000

Supervisor: William L. Fisher

The Permian Delaware Mountain Group (DMG) consists of cyclic packages of sandstone, siltstone, carbonate mudstone, and organic-rich siltstone that were deposited in the Delaware Basin during Guadalupian time. Three orders of stratigraphic cyclicity are recognized in the DMG: eight low-order cycles, 32 intermediate-order cycles, and more than 50 high-order cycles. Low and intermediate-order cycles are similar in scale to composite and high-frequency sequences. All cycles are bounded by organic-rich siltstones, or carbonate mudstones that represent periods of sediment starvation in the basin.

Stacking pattern analysis combined with other stratigraphic tools can be effectively used in the unlimited accommodation setting of the DMG. One-dimensional stratigraphic analysis from a high-density, integrated well log / 3-D seismic data set from the Geraldine Ford (GF) area is effective in initiating a stratigraphic hierarchy and framework for the central and northern Delaware Basin. High-frequency cyclicity is better expressed in the distal, basin-floor setting of the Delaware Basin due to more complete stratal preservation than in proximal, slope settings.

Regional correlation of low-order cycles characterized by upward-thinning cycle stacking patterns identifies a dynamic depositional system that is linked closely to stratigraphic position within low and intermediate-order cycles. In most low-order cycles, basal intermediate-order cycles thicken basinward and upper cycles thicken slopeward in response to changing accommodation conditions on the coeval carbonate shelf. Thicker, sandstone-rich cycles dominate the lower portions (LST, ~ late HST) of low-order cycles, while thinner, more organic-rich siltstone, carbonate mudstone, and calcareous-siltstone rich cycles dominate the upper portions (TST, ~ early HST).

The Manzanita, Hegler, and Pinery intervals are limestones in the GF study area and are identified based on correlations to the PDB-04 core. These intervals along with the South Wells, Radar, McCombs, and Lamar intervals are also correlated to outcrops in the Guadalupe and Delaware Mountains. The result of this work is a favorable comparison of low-order cycles in the basin defined by observational criteria, to recent sequence stratigraphic frameworks focused on the carbonate shelf, shelf margin, and slope deposits of the Guadalupe Mountains.

In the GF study area, compensational stacking of intermediate and high-order cycles and of individual sandbodies is common. A random lateral distribution of sandbodies is also very common. The lateral continuity of sandy lithofacies is greater in dip directions than in strike directions as would be predicted. Strong correlation exists between low-order cycle tops and positive seismic reflection events in the GF 3-D survey. Reflection peaks are centered at or offset just above higher impedance, low-order cycle tops. Low relief mounding is a common element observed in the seismic data set. However, the size of mound relief is too large when compared to high-resolution log cross-sections.

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MULTICOMPONENT SEISMILOGY: BASIC CONCEPTS AND APPLICATIONS

By

Elsa Jeanneth Jaimes Romero, M. A.

University of Texas at Austin, 2000

Supervisor: Fisher, William

(Master's report--no abstract available)
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STRUCTURAL STYLES OF THE ANDEAN FOOTHILLS, PUTUMAYO BASIN, COLOMBIA

by

Juan Carlos Jiménez, M.S.

The University of Texas at Austin, 1997

Supervisor: Randall Marrett

Interpretation of seismic profiles, earthquake fault-plane solutions, radar images, and geometry of structures suggests that two different structural styles are viable alternatives for the Putumayo basin in Colombia.

An eastern domain, varying in width from 4 to 13 km, might be characterized by strike-slip faulting parallel to the Andes because it exhibits similar structures to those formed in restraining bend settings, an example is the Orito fold, the largest known oil field in the basin. Correlation of seismic reflections with wells into the Orito fold and foreland indicates a post-Miocene age for this structure. Previous interpretations of contractional dip-slip movement on Andes-parallel structures, as proposed by Portilla (1991) with faults involving basement, are also viable.

A 15 km-width western domain is interpreted as a region of foreland-dipping rocks uplifted above their regional level by wedging of pre-Cretaceous (?) rocks beneath known Jurassic rocks. Above the Jurassic rocks thin-skinned deformation occurs inside of the Mesozoic and Cenozoic sedimentary cover, also in the form of wedging. Mesozoic and Paleozoic (?) rocks were injected into of a late Cretaceous-early Paleocene unit composed of shale. The western domain is truncated to the west by a major reverse fault that places Paleozoic rocks over Mesozoic and Cenozoic rocks.

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STRATIGRAPHIC CORRELATION OF THE ENTRADA FORMATION IN NORTHEAST ARIZONA WITH EMPHASIS ON RELATIVE TIMING OF TODILTO FORMATION DEPOSITION

by

Jason Gregory John, M.S.

The University of Texas at Austin, 2000

Supervisor: Brenda L. Kirkland

Twelve Middle Jurassic outcrop localities encompassing parts of northwestern New Mexico, northeastern Arizona, and southeastern Utah were measured and correlated in an attempt to document physical or stratigraphic evidence of a connection between the Todilto sea to the southeast and an ancient marine environment that existed to the northwest. The Todilto Formation has been previously interpreted to overlie the Entrada Formation in northwest New Mexico, but its stratigraphic equivalent in southeast Utah where it is not present was uncertain.

The results show that the Entrada Formation is composed of three members throughout the study area which include, from oldest to youngest, the Middle Member, the Upper Red Member, and the Salmon Member. Cross sections and field relationships from this study show that the Todilto Formation correlates into the middle portion of the Upper Red Member of the Entrada Formation. No physical connection in the form of a seaway exists in the study area due to a paleo-high in the Red Mesa region of northeastern Arizona. The high resulted in a thinning of the Middle and Upper Red members of the Entrada Formation and acted as a western boundary to the Todilto water body. The Carmel Formation underlies the Entrada Formation and changes facies to become the Dewey Bridge Member eastward along the Arizona/New Mexico state line.

The Entrada Formation of the study area was deposited by an extension of a wet eolian system from the north whose depositional facies are controlled by regional ground water fluctuations that are related to eustatic sea level changes. Two levels of cyclicity were interpreted in the study area. Two large decimeter-scale cycles that define sequences 1 and 2 are tied into the stratigraphic framework of southeast Utah. Smaller, meter-scale cycles as a result of water table fluctuations are also observed, but its relationship to eustacy is uncertain. The Todilto Formation correlated into an overall rise in sea level during the lower half of sequence 2 (S2) in the study area. Therefore, the rising sea level may be related to the flooding of the Todilto basin.

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THE ROLE OF THE OCEAN IN THE PLANETARY ANGULAR MOMENTUM BUDGET

Thomas James Johnson, Ph.D.

The University of Texas at Austin, 1998

Supervisor: Clark Wilson

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GEOLOGIC EVOLUTION OF THE RINCÓN DE LA VIEJA VOLCANIC COMPLEX, NW COSTA RICA.

by

Kirt Anton Kempter, Ph.D.

The University of Texas at Austin, 1997

Supervisor: Daniel S. Barker

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LAND SUBSIDENCE ALONG THE TEXAS GULF COAST DUE TO OIL AND GAS WITHDRAWAL

by

Kaveh Khorzad, (M.S. Geo Sci)

The University of Texas at Austin, 2000

Supervisor: John M. Sharp Jr.

Land subsidence caused by groundwater withdrawal in the Houston-Galveston region is a well-documented phenomenon. Subsidence of up to 3m (10ft) has been calculated in the region since 1905. Hydrocarbon withdrawal is a plausible cause of subsidence where groundwater withdrawal has diminished and significant petroleum production has occurred for over 70 years.

Sixteen fields were investigated by acquiring reservoir bottom hole pressure data (BHP) near bore-hole extensometers set up by the Harris-Galveston Coastal Subsidence District (HGCSD). All reservoirs were found to be well below hydrostatic pressure; a few of them were underpressured even before production began showing a possible hydraulic connection between fields. BHP data was used in a reservoir model and a boundary clay reservoir model to calculate subsidence. Subsidence under these fields is predicted to be as high as 0.30 m (~1.00 ft) in a nine-year period or 33.33 mm yr-1 (1.3 in yr-1) at the Goose Creek field and as low as 0.007 m (0.023 ft) in a twenty-year period or 0.33 mm yr-1 (0.01 in yr-1) at the Gillock field. Elevation benchmark data used in producing a cross section of a line of oil fields show connection of subsidence bowls above fields on two scales: A smaller local scale subsidence bowl on the order of 5 km (3.1 mi) and a more regional subsidence bowl on the order of 40 km (24.9 mi).

Point estimation of the contribution to total subsidence from petroleum production and groundwater withdrawal show that the majority of the subsidence occurring presently in Harris and Galveston counties is from oil and gas withdrawal. Effects from clay equilibration caused by previous groundwater pumping were interpreted to be minimal.

Implications of this study are: 1) hydrocarbon production, although not the major contributor to most land surface subsidence in this area, is significant and 2) depressurization and subsequently subsidence from oil and gas fields may be regional, connecting neighboring fields. This is inferred from the fact that some fields were already underpressured before production began in addition to benchmark data showing connection of subsidence bowls.

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APPLYING ARTIFICIAL NEURAL NETWORK TECHNOLOGY IN RESERVOIR CHARACTERIZATION STUDIES

 

Cem Okan Kilic, M.S.

The University of Texas at Austin, 1998

Supervisor: William L. Fisher

In this study a methodology is presented to estimate critical inputs to the reservoir characterization process where direct measurements of these inputs are not available. These inputs are effective porosity, permeability to reservoir fluids, formation rock properties (Poisson's ratio and Young's modulus), and borehole shear and compressional sonic velocities.

In the calibration well, wireline logs with complete input datasets are used to train the neural network. The network is then used on an offset well with incomplete inputs to obtain missing information. The results are contrasted with traditional regression correlation techniques to demonstrate the utility of the Artificial Neural Network technology.

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NATURAL GAS ULTIMATE RECOVERY GROWTH MODELING BY PLAYS IN THE GULF COAST BASIN

by

Eugene Miryong Kim, Ph.D.

The University of Texas at Austin, 1998

 

Supervisor: William L. Fisher

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CYCLICITY AND RESERVOIR QUALITY VARIATIONS IN THE BLAIR AND ROCK SPRINGS FORMATIONS, ROCK SPRINGS UPLIFT, SOUTHWEST WYOMING

Susan Dannette Knox, M.A.

The University of Texas at Austin, 1997

Supervisor: Earle F. McBride

The Blair and Rock Springs formations on the southeast side of the Rock Springs uplift provide an opportunity to analyze clastic cycles from the Cretaceous for evidence of Milankovitch forcing. The formations also contain different facies that can be analyzed and characterized in terms of their reservoir quality, so that heterogeneities within facies and between facies within the same formation can be quantified.

I measured and analyzed 12 sections using the gamma method of Michelle Kominz. The gamma method uses least squares regression to calculate inverse sedimentation rates, [gamma], for each facies in a cycle. Gamma values obtained were negative in three sections and varied widely for the same facies from section to section, indicating that there is not an unique relationship between facies and sedimentation rate or the mean cycle duration is not constant as assumed.

Thickness and gamma-corrected sections contain periods within the Milankovitch band, primarily precession and obliquity, but also contain periods, such as 31 Ky-33 Ky and 77 Ky-78 Ky, not predicted by the theory. The presence of periods predicted by the Milankovitch theory in the uncorrected thickness series suggests that cyclic climate variations in the Interior Seaway may be responsible in part for the periodicity in the cycles. The presence of other periods, however, suggests that factors, such as delta lobe switching, marine processes, or variations in water and sediment discharge also influenced the creation of the cycles.

I classified 53 samples from 10 facies using QFRD percentages, where D represents intrabasinal, detrital dolomite. Mean QFRD for the facies range from litharenites and dolomite-bearing litharenites to dolomite-bearing, feldspathic litharenites.

Reservoir quality for each facies based on field classifications of Levorson (1967) and North (1985) are: Facies 1 is good, 2 is fair to moderate,3 is poor to fair, 4 is fair to very good, 5 is very good, 6 is fair to very good, 7 is poor to very good, and 8-10 are very good.

Early cementation preserved much of the intergranular volume (IGV). The high IGV has enhanced reservoir quality through secondary porosity. Cementation and clay introduced by bioturbation and by grain alteration has decreased reservoir quality. Porosity and permeability show considerable variation within facies because of local variations in cementation and dissolution.

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EARLY DEVELOPMENT OF THE SOUTHERN KERGUELEN PLATEAU (INDIAN OCEAN) FROM OCEAN BOTTOM SEISMOGRAPH AND MULTICHANNEL SEISMIC REFLECTION DATA

by

Lis Kathrin Könnecke, M.S.

The University of Texas at Austin, 1997

Co-Supervisors: Millard F. Coffin and Paul L. Stoffa

Cretaceous terrestrial and terrigenous sediment recovered from Ocean Drilling Program (ODP) sites on the Kerguelen Plateau (southern Indian Ocean) indicate a long history of subaerial exposure. We examine late syn- and early post-constructional processes on the southern plateau, using ocean bottom seismograph (OBS), multichannel seismic (MCS), and ODP data. Near-range OBS data are used to develop velocity-depth models for eight locations in the sedimentary Raggatt Basin. The resulting models are constrained by refracted and reflected signals, including multiples and converted shear waves, as well as by migrated MCS data. These models allow us to examine the characteristics and distribution of Lower Cretaceous lava flows, determine the extent and distribution of Albian to Coniacian/ Santonian terrestrial and terrigenous sediments over the Kerguelen Plateau, and re-examine Maestrichtian and Paleocene seismic sequences.

For the Albian/ Aptian basement complex, predominantly composed of flood basalt, we define an upper and a lower series. The upper series is characterized by a P-wave velocity of ~ 4.7 km/s, an increase of intercalated terrestrial sediment and altered flow-tops towards the top, and a decrease in flow thickness, while the lower series is marked by velocities of ~ 5.4 km/s and contains thicker flows and less sediment. Similar observations have been made for Broken Ridge, and for large igneous provinces worldwide. A volcanic center in the Raggatt Basin is characterized by relatively low seismic velocities, and dipping reflections at the flanks of Raggatt Basin have been recorded as refractions. Terrestrial and terrigenous sediment of the Raggatt Basin, immediately overlying basement, is characterized by a seismic low velocity layer with velocities ranging from 2.2 - 2.9 km/s and a thickness of more than 1100 m at the center of the basin. Nearby source regions (e.g. Banzare Bank and other elevated areas south of the basin) account for a terrestrial and terrigenous sediment volume of ~ 12,500 km3, deposited over ~ 20 my. A major shift in the depositional center of the Raggatt Basin began during Santonian to early Maestrichtian time, and concluded by early Late Paleocene time in the central basin, earlier than previously assumed.

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CONTINENTAL MARGIN DEFORMATION ANALYSIS AND RECONSTRUCTION-EVOLUTION OF THE EAST CHINA SEA BASIN AND ADJACENT PLATE INTERACTION

by

Fanchen Kong, Ph.D.

The University of Texas at Austin, 1998

Supervisors: Ian W. D. Dalziel and Lawrence A. Lawver

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SUBSURFACE MESO-SCALE STRUCTURAL GEOLOGY OF THE KUCING LIAR AND AMOLE DRIFTS AND PETROLOGY OF THE HEAVY SULFIDE ZONE, SOUTH GRASBERG IGNEOUS COMPLEX, IRIAN JAYA, INDONESIA

By

Cori Alyne Lambert, M. S.

THE UNIVERSITY OF TEXAS AT AUSTIN, May, 2000

Supervisor: Mark Cloos

Meso-scale structures were mapped ~1 km in the subsurface south of and adjacent to the Grasberg Igneous Complex, located within the Gunung Bijih Mining District. Bedding planes, faults and veins were mapped and measured in the Kucing Liar Exploration Drift (inclusive of the Kucing Liar Dewatering Drift and the Kucing Liar north offshoot) and the Amole Drift. Structural analysis reveals the following: (1) A calculated fold axis for bedding measured in the subsurface, on the southern limb of the Yellow Valley Syncline (YVS) (311 degrees/28 degrees) shows a divergence from the calculated YVS fold axis from previous data at the surface (285 degrees/15 degrees) by Quarles van Ufford (1996) of 26 degrees clockwise and 13 degrees steeper. (2) The movement direction indicated by the average trend of slickensides on bedding-plane faults is 032 degrees, consistent with the movement direction determined for regional-scale contraction as concluded by Quarles van Ufford (1996), 210 degrees-220 degrees to 030 degrees-040 degrees for the episode of folding from ~12 to ~4 Ma. (3) Strike-slip faults are the dominant structure, but sense of shear indicators are present on very few fault planes. Nonetheless, kinematics are inferred to be similar to that found by Sapiie (1998), who concluded that the GBMD was subjected to strike-slip deformation as a left-lateral Reidel shear system from ~4 to ~2 Ma. (4) The average orientation of 463 veins measured indicates an overall NW-SE extension direction, almost exactly parallel to that observed at the surface by Sapiie (1998) and Luck (1999). (5) Extended domain boundaries connecting similar structural domains along the Kucing Liar Dewatering and Amole drifts trend roughly NE-SW, and are most likely related to the presence of the SE-corner of the pull-apart zone into which the Grasberg Igneous Complex was intruded.

Chapter 2: Petrology of the Heavy Sulfide Zone, Grasberg Igneous Complex,

Irian Jaya, Indonesia

Abstract

The Heavy Sulfide Zone (HSZ) is a cylindrical zone that surrounds the Grasberg Igneous Complex (GIC) in the subsurface, below approximately 3700 m elevation. For logging classification purposes, it is defined by the presence of >20% sulfide minerals per 2-3 m-segment of core. The relationships among the HSZ, the GIC, and the surrounding carbonate country rocks have been interpreted from this petrologic study of 71 samples collected at 2900 m elevation along mine drifts on the south side of and within the GIC.The mineralogical characteristics of the HSZ are: (1) Pyrite is the dominant sulfide phase and is typically fine-grained (hundreds of microns to a few millimeters in total diameter). (2) Non-sulfide phases (depending on the location of the sample) typically consist of dolomite or calcite, with trace amounts of quartz, potassium feldspar, sericite, biotite, and chlorite. No garnet, pyroxene or other calc-silicate minerals were identified. (3) Mineralogical banding (pyrite, sphalerite and galena) at the hand-sample scale occurs in samples that are outliers to the HSZ proper. (4) Sphalerite and galena are present at the margins of the HSZ, and pyrite. Minor amounts of chalcopyrite, digenite, bornite and covellite are present in the mid- to proximal-Grasberg Igneous Complex. (5) Pb-isotopic analyses of samples from the HSZ and from banded outliers reveal a magmatic signature similar to the nearby igneous rocks of the GIC.As observed along the Kucing Liar north offshoot, the limit of marbleization as defined by an average grain size of 100 to 200 m occurs ~130 m from the GIC. As observed along the Amole Drift, the limit is more gradational, with an average grain size of ~200 mm at ~100 m from the GIC and an average grain size of ~100 mm at ~225 m from the GIC. Mineralized Type 3A marginal breccias, described at the surface by Sapiie (1998), occur between the GIC and the carbonate country rock. These breccias are evidence for marbleization pre-dating HSZ mineralization. The preferential replacement of mud clasts over marble clasts in the most peripheral samples suggests that the dominant chemical control was one of infiltration-reaction process, triggered most likely by a change in pH when the incoming hydrothermal fluids were neutralized by carbonate host rock. Banded outliers to the HSZ represent fracture pathways away from the HSZ that reopened several times.The mineralogy implies that the temperature at which this deposit formed had an upper limit of ~500 degreesC and a lower limit of ~250 degreesC. The pressure at which the deposit formed was approximately 500 bars, at the depth of the Amole and Kucing Liar Drifts. The HSZ was most likely formed by pressure-driven infiltration of hydrothermal fluids starting at temperatures of ~800 degreesC, upwards and outwards from a pocket beneath a cupola above a cooling and crystallizing stock. Cooling of outward-migrating hydrothermal fluid and contact with the carbonate country rock caused precipitation of sulfide phases in the contact zone. Highly permeable breccia zones focused flow of the hydrothermal fluids.

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HYDROGEOLOGIC PROPERTIES OF FRACTURE SKINS AND THEIR EFFECTS ON RADIONUCLIDE TRANSPORT

by

 Michael Thomas Landrum, M. S. Geo. Sci.

The University of Texas at Austin, 2000

Supervisor: John M. Sharp, Jr.

It is recognized that fractures have a strong influence on the transport of fluids, solutes, and colloids in fractured rocks. Recent interest in radionuclide transport in fractured rocks has been prompted by the search for permanent high-level nuclear waste repositories, and uncertainties about the rates at which radionuclides will travel through fractures in the event of a waste release are a primary concern. However, little attention has been paid to the mineral coatings, alterations, crushed rock, and biofilms, or "fracture skins," on fracture surfaces. Diffusion of solutes from a fracture into the matrix is recognized as an important attenuation mechanism of contaminants moving along a fracture, especially for weakly sorbing ions such as iodine and technetium. The permeability, diffusivity, and porosity of some fracture skins may differ significantly from the "unaltered" matrix. This implies that fracture skins may affect the rate of radionuclide transport through a fracture by enhancing or limiting the exchange of radionuclides between the fracture and matrix. The objectives of this study are to measure the porosity and effective diffusivity of fracture skin and rock matrix samples of the Topopah Spring Tuff, Nevada; determine if the hydrogeologic properties of the fracture skin are different from the rock matrix; and speculate how the fracture skin could affect the transport of a radionuclide, using a 2-D steady-state analytical solution for the transport of a radionuclide in a set of parallel, finite fractures.

The measured hydrogeologic properties of the Topopah Spring Tuff and fracture skins include effective diffusivity, porosity, mean pore diameter, specific surface area, bulk density, and particle density. The effective diffusivity, porosity, and mean pore diameter of the fracture skin are significantly lower than the matrix values, and the specific surface area of the fracture skin is significantly higher than the matrix value; whereas, their bulk densities and particle densities are similar.

A Fortran code was written to examine the effect that each parameter in the model, especially the fracture skin properties, has on the radionuclide concentration in the fracture. The sensitivity analysis indicates that a fracture skin is most important when it has an effective diffusivity that is lower than the matrix value and when its porosity and retardation factor are higher than the matrix value.

The effect that the calcite fracture skins might have on radionuclide transport in the Topopah Spring Tuff was investigated by comparing the analytical solutions with and without fracture skin using the properties measured in this study and from the literature. Transport of the radionuclides iodine-129, technetium-99, plutonium-239, uranium-234, neptunium-237, and americium-243 were simulated. The results of the non-sorbing iodine and technetium simulations indicate that the porosity and effective diffusivity of the fracture skins are not low enough to significantly affect radionuclide concentrations in the fractures. However, the americium simulation indicates that the fracture skins might affect radionuclide transport if there is a large contrast between the fracture skin and matrix retardation factors.

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SEISMIC AND GEOLOGICAL CHARACTERIZATION OF THE MIDDLE EOCENE MISOA FORMATION, CENTRO LAGO FIELD, MARACAIBO BASIN, VENEZUELA.

by

Pedro Jose Leon Rodriguez, M.A

The University of Texas at Austin, 1997

SUPERVISOR: Noel Tyler

Centro Lago field, in Lake Maracaibo, Venezuela, produces from Cretaceous to Miocene strata, with Eocene Misoa C reservoirs having the greatest production in the field. The middle Eocene reservoir section, the focus of this study, is approximately 1800 feet thick. Reservoir sandstones were deposited in a progradational estuarine fill system and primary facies are tidal sand ridge deposits.

A multidisciplinary approach based on the use of an interactive workstation allowed the construction of a geological model for the area. Sequence stratigraphic marker beds identified from core and well log data, in addition to the unconformities recognized on seismic sections, were identified and correlated throughout the 3D seismic volume. Interactive map analysis of the structural attributes, such as edge detection and dip and azimuth data obtained from the 3D volume, as well as the use of flattened seismic sections permitted the definition of the structural framework for Cretaceous, Paleocene and Eocene.

The middle Eocene section was divided into eight genetic units which are bounded by maximum flooding surfaces and tectonic unconformities. These genetic units have an average thickness of 150 feet. A detailed interpretation of reservoir architectures, based on seismic and well log character, was made for the defined genetic units. This interpretation was the input for analysis of seismic attributes such as amplitude and instantaneous phase.

Attribute analysis indicated a strong relationship between seismic amplitude and net oil sand. The evaluation of amplitude maps obtained for each genetic unit confirmed the accuracy of this relationship and supported the importance of 3D seismic data in reservoir characterization. Based on this relationship between amplitude and net oil sand, prospective areas for the placement of new, resource-targeted infill wells and zones favorable for recompletion are proposed.

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DEPOSITIONAL CONTROLS AND GENESIS OF THE JINDING SANDSTONE-HOSTED ZN-PB DEPOSIT, YUNNAN PROVINCE, SOUTHWEST CHINA

by

Ning Li, Ph.D.

The University of Texas at Austin, 1998

Supervisor: J. Richard Kyle

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CONTROLLED FRACTURE STUDY OF GEOLOGIC MATERIALS

by

 

Sarah Robena Lindsay, M.S.

The University of Texas at Austin, 2000

Supervisor: John M. Sharp, Jr.

Fracture surface roughness affects the hydraulic properties of materials. In geologic materials, fracture surface roughness has been qualitatively studied in relation to material properties such as grain composition and grain size. However, the relationship between fracture inducing energy and the resulting fracture surface topography has not been thoroughly investigated. It is hypothesized that a relationship exists between the force applied to a geologic material and the roughness of the resulting fracture surface.

Using the facilities at The University of Texas' Center for NonLinear Dynamics, I fractured 47 dolostone and 28 quartzite rock samples, recorded the fracture energy and characterized the resulting fracture surface roughness. The experimental procedure consisted of sizing and polishing the samples, performing the breakage experiment, and imaging the fractures by taking digital photomicrographs. The imaged fracture surface roughness was quantified using fractal analysis.

The breakage experiments had a twenty percent rate of success resulting in seven dolostone and seven quartzite experimentally clean samples. Although there is overlap in the fracture energy ranges of the two rock types, the experimental results showed that in general, the dolostone samples broke at lower fracture energies than quartzite samples. Dolostone samples display good correlation of right and left fracture surface topography whereas quartzite samples display poor correlation. Analyses to date indicate no apparent relationship between fracture energy and the fractal dimension of fracture surface roughness. This may be because of heterogeneities in the samples. Unaccounted excess fracture energy was recorded during breakage, which may result in subsurface microfracturing. However, subsurface microfracturing was only observed in the quartzite samples. The dolostone samples showed no subsurface damage.

This study establishes a methodology for controlled breakage experiments of geologic materials and the quantitative investigation of the relationship between fracture energy and fracture surface roughness. Further experimental work is necessary to thoroughly test the hypothesis.  

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POST MID-CRETACEOUS SEQUENCE STRATIGRAPHY AND DEPOSITIONAL HISTORY OF NORTHEASTERN GULF OF MEXICO

by

Qunling Liu, Ph.D.

The University of Texas at Austin, 2000

Supervisor: Richard T. Buffler

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FRACTURED RESERVOIRS: CONCEPTS AND CASE STUDIES

By

Adriano Lobo Alvarez, M. A.

University of Texas at Austin, 2000

Supervisor: William Fisher

(No abstract available--Masters Report)

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STRUCTURAL GEOLOGY OF THE GRASBERG LIME QUARRY AND AMOLE DRIFT : IMPLICATIONS FOR EMPLACEMENT OF THE GRASBERG IGNEOUS COMPLEX, IRIAN JAYA, INDONESIA

 by

Roy Burvil Luck, M.S.

Supervisor: Mark Cloos

Pt.1

The Grasberg Igneous Complex (GIC) is one of the world’s foremost copper-gold porphyry-type systems. The system is Pliocene in age and is situated at an elevation of 4000 m in the Central Range of Irian Jaya, Indonesia. Two structural regimes are noted within the Gunung Bijih Mining District - a 100 km2 Contract of Work (COW) area operated by PT Freeport Indonesia. The most apparent is a Miocene contractional episode that resulted in kilometer-scale folding in the Central Ranges, concurrent with arc-continent collision in a north-dipping subduction zone. The second, more subtle structural regime, was discovered by Sapiie (1998). It is one of strike-slip faulting, evidenced by five major strike-slip fault zones with tens to hundreds of meters of displacement, as well as ubiquitous mesoscale strike-slip faults throughout the district, including the GIC itself. This strike-slip regime lasted from ~4 Ma to ~2 Ma, and has been categorized as a left-lateral Riedel shear system.

Fieldwork was conducted in two locations - the Grasberg lime operation to the north of the Grasberg pit, and the Amole Drift. Structural analysis at the Grasberg lime operation extended the area mapped by Sapiie (1998). New data in this area is similar to that compiled elsewhere within the district. Strike-slip faulting is predominant. The pattern of north-south striking right-lateral faults and northeast-southwest striking left-lateral faults is essentially identical to that described by Sapiie along the HEAT (Heavy Equipment Access Trail) and Grasberg access roads. Similarly, the orientation of veins at the Grasberg lime operation indicates extension along an axis trending 300o/120o. Slight variations in the strike of veins and left-lateral faults observed within the Gunung Bijih Mining District are likely due to the mechanical anisotropy due to intrusion of the GIC.

Structural analyses were compiled within the GIC in the Amole Drift, at the 2900 m level. An effort was made to correlate structures in the subsurface to previously mapped structures at the surface, at 1000 m higher elevation. Strike-slip faulting is predominant in the subsurface. Dip-slip faults are concentrated in the northern portion of the GIC, north of the Late Kali Intrusion (LKI). Sense-of-slip indicators are rare in the GIC, with the result that faults are categorized only on the basis of orientation and slip direction. As is the case at surface levels within the GIC, the orientations of faults in the subsurface are domainal in character. Four fault domains (A-D, classified from south to north in accordance with Grasberg Domains 1-3 of Sapiie, 1998) are observed in the subsurface. Domain A extends within carbonate host rock from crosscut 17 to beyond crosscut 18, and features faults striking northeast/southwest (C. Lambert, 1999, personal commun.) - an orientation consistent with left-lateral faults within the district. Domain B extends from crosscut 19 to 22 (outside the margins of the GIC) and has a single population of faults striking north/south (C. Lambert, 1999, personal commun.). Between crosscuts 23 and 27, a bimodal fault population with two diverging fault strike orientations is observed (Domain C). This most complex zone is believed correlable to Grasberg Domain 2 (GD2) and coincides with the most intensive mineralization in the Grasberg. Domain D, north of crosscut 27, has a single fault population that shifts gradationally from a northwest/southeast orientation to a north/south orientation farther to the north. This domain is correlated to GD3 as mapped by Sapiie (1998), and is interpreted as a zone of faults with trajectories refracted toward the center of the GIC.

Interestingly, domain boundaries in the subsurface are correlable to the surface along planes projected parallel to the Late Kali Intrusive (LKI). The LKI is the youngest phase of the GIC. By contrast to the earlier two phases of the GIC, the LKI is tabular and dike-like. The LKI strikes 140o and dips 75o - parallel to the average orientation of faults in GD2, measured in the center of the GIC by Sapiie (1998). It is likely that this fault trend facilitated intrusion of the LKI. The boundary between domains C and D is correlated to the boundary mapped by Sapiie (1998) between GD2 and GD3 at the surface, when this boundary is shifted 100 m further to the south and projected along a plane parallel to the LKI. The boundary between domains B and C is correlable to the boundary between GD1 and GD2 when it is projected along a plane parallel to the LKI. No structural geology work has been completed in the part of the Grasberg pit overlying subsurface domains A and B because this was an area of rugged terrain prior to 1995.

Pt.2

The Grasberg Igneous Complex (GIC) contains one of the world’s foremost copper porphyry deposits. The GIC has a diameter of 1.7 km at the 4000 m level and tapers to a stock with a diameter of ~500 m at the 2500 m level. The GIC has three intrusive phases - the Dalam unit, the Main Grasberg Intrusion (MGI, age 3.06 Ma) and the Late Kali Intrusion (LKI, age 2.94 Ma). Prior to this study, a comprehensive reconstruction of the events leading to and enabling intrusion of the GIC had not been undertaken. Based on local and regional structural data, geochemical studies and petrology, a model is proposed for emplacement of the GIC - one that is consistent with the local structural geology and mineralization history. Breccia exposed along the northern and southern margins of the GIC provides important clues as to why the GIC widens towards the surface.

The GIC emanated from a parent magma chamber at 10-15 km depth, near the top of the crystalline basement of the Australian continent. Five extension mechanisms contributing to intrusion of magmas forming the GIC include: 1) elastic deformation of wallrock, 2) ductile deformation of wallrock, 3) brittle deformation of wallrock accomplished via normal faulting and extension fracturing, 4) metamorphic recrystallization of wallrock and consequent volume loss related to reduction in porosity, and 5) dissolution of carbonate host rock. The relative contributions of each extension mechanism vary temporally throughout the intrusive process, as well as vertically along the GIC.

The driving force for emplacement of the GIC is the buoyancy caused by the difference in densities between the deformed sedimentary sequence and the underlying felsic melt at a depth of 10-15 km. Extension within a dilational jog between two strike-slip faults provided space for intrusion of the GIC. At depths greater than ~5 km, closure of extension fractures and ductile distortion of wallrock account for most of the 500 m diameter of the GIC. Metamorphic recrystallization, which resulted in diminution of porosity and consequent volume loss, accounts for only tens of meters of radial accommodation space around the margins of the GIC. Nearer the surface, the decreased component of ductile wallrock deformation of wallrock is compensated by an increase in normal faulting (shear fracturing).

Sediments of the Kembelangan and New Guinea Limestone Groups were thinned by normal faulting in a pull-apart structure within a strike-slip environment. Buoyancy of magma in a parent chamber drove initial ascent of the Dalam phase of the GIC to within 1 km of the surface. As the Dalam phase ascended to the surface at shallow levels, the extended host rock was then ‘shouldered aside’ and structurally rethickened. At depths less than 1-2 km, water saturation and volatile release generated the monomict Dalam Fragmental unit, and eruptive activity resulted in formation of the polymict Dalam Volcanic unit. The Dalam conduit vented to the surface, where volcanic activity in a maar caldera setting resulted in the extrusion of volcaniclastic sediment and andesite flows. This material was mixed with Dalam Fragmental material by caldera collapse to form the Dalam Volcanic unit.

The Main Grasberg Intrusion (MGI) rose through the plastic interior of the previously intruded Dalam unit. Subsequently, collapse of the cupola roof and escape of magmatic fluids formed a chalcopyrite stockwork zone hosted by the MGI, the highest-grade ore in the GIC. Outward fluid infiltration formed the Heavy Sulfide Zone lining the margins of the GIC. Precipitation of sulfide minerals at depth increased the acidity of magmatic fluid, and fluids rising along the margins caused extensive dissolution of fractured carbonate host rock at shallower depths. Dissolution of wallrock is primarily responsible for the flaring-upward profile of the GIC to the north and south. As wallrock dissolved, the Dalam and MGI phases extended and overlying sequences of volcaniclastic sediment and andesite flows collapsed into their current, steeply-dipping profile, as opposed to their previous sub-horizontal attitude. The final phase of the GIC, the LKI, intruded the cooled center of the GIC along a fault zone trending 300°/120°.

 

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PROCESSING A HIGH RESOLUTION 3D SURVEY OF THE SHALLOW SUBSURFACE ON THE CONTINENTAL SHELF OF NEW JERSEY

by

Eddy Caturnanda Luhurbudi, M.A.

The University of Texas at Austin, 1997

Supervisors: Paul L. Stoffa and Mrinal K. Sen

A high resolution 3D single-channel seismic survey was performed off the coast of New Jersey in 1993. It is a part of an initiative intended to study the history of sedimentary deposition in the late Quaternary on the northwest Atlantic continental margin. The 3D survey consisted of 60 lines of about 3107 shot points each. Shot spacing was 2.5 m; lines were spaced 10 m apart. With a sampling interval of 0.1 ms, 3000 samples were collected for each trace. The seismic source had relatively high frequency content (500 - 3500 Hz), which can reveal geological structure in great detail. Only data within the travel time interval between the seafloor arrival and the sea surface-reflected ghost were processed. The processing sequence consists of four different steps: pre-processing, binning, correcting for semi-diurnal tidal variations, and migration. In the pre-processing step, we bandpass filtered each trace, picked the seafloor arrival time, muted unneeded signals, deconvolved the source wavelet, equalized trace amplitudes, smoothed seafloor arrivals, and manually corrected residual statics.

We remove the effect of semi-diurnal tidal variations that occured during the acquisition. Maximum variations in the tides amount to about a meter of offset. At the highest frequency of the source (3500 Hz), this offset shifts the data by about 4.5 wavelengths. The tidal correction is performed by shifting the sea surface by an amount computed from equivalent shotpoints in the cross-line direction. The average of the corrections correlate well with tidal gauge data recorded at Atlantic City, roughly 100 km to the west. This correction affects only vertical resolution.

The second correction, binning, affects the survey's lateral resolution. Due to ship wander and the survey's tight line spacing, shot points deviate from their intended positions. To produce a well-sampled grid, the data must be aggregated into bins of larger dimensions than the original shotpoint and line spacing. A bin size of 2.5m x 12.5m with 20% and 40% overlap in two horizontal directions (along-line and cross-line) to produce sufficient coverage, while preserving good lateral resolution.

Finally, the pre-processed and binned data are migrated using Stolt's method. Stolt migration is performed in the frequency-wavenumber ([omega],kx,ky) domain. In our implementation, data traces are transformed individually by 1D FFT to f([omega],x,y,), [omega]-slices are 2D-transformed to f([omega],kx,ky) and mapped to f(kx,ky,kz) before being inverse-transformed to f(x,y,kz). Individual traces are then 1D-transformed to obtain a depth image, f(x,y,z). In this study, water velocity is used as the constant migration velocity. Migration enhances the quality of the images (dip angles become steeper, for example) and makes them accurately interpretable in the depth domain.

Two submarine channels are revealed that are believed to be related to subaerial flow during the last (Wisconsinan) deglaciation. The 3D images show that these two channels are oriented S and SSE directions, respectively. The eastern channel system enters the survey area as two distinct channels, which then join together.

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FORAMINIFERAL BIOSTRATIGRAPHIC AND PALEOCEANOGRAPHIC ANALYSIS OF THE EAGLE FORD, AUSTIN, AND LOWER TAYLOR GROUPS (MIDDLE CENOMANIAN THROUGH LOWER CAMPANIAN) OF CENTRAL TEXAS

by

Jason James Lundquist, Ph.D.

The University of Texas at Austin, 2000

Advisor: Brenda L. Kirkland

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SEISMIC COUPLING ALONG THE NEW HEBRIDES TRENCH

James Edward Lundy, Master of Science in Geological Sciences

The University of Texas at Austin, 1998

Supervisor: Stephen Grand

Nearly one hundred years of seismicity in the region of the New Hebrides Trench and the Vanuatu Archipelago in the South West Pacific were analyzed to determine what portion of slip along the trench axis is seismic. The New Hebrides Trench is a region of active subduction and one known to have some anomalous geodetically determined horizontal motion rates when compared with far-field regional convergence vectors and predictions of modern global plate motion models. Analysis of averaged annual seismic slip in events related to subduction indicate that only 10 to 30% of the plate convergence is manifested as seismic slip along the majority of the trench axis despite large bathymetric features currently impinging the central portion of the arc. This suggests that although these large features are the likely cause of deformation within the overriding plate, they do not lead to significantly stronger coupling at the plate interface.

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GROUND-WATER FLOW AND SOLUTE TRANSPORT IN A FRACTURED CHALK OUTCROP, NORTH-CENTRAL TEXAS

Robert Earl Mace, Ph.D.

The University of Texas at Austin, 1998

Supervisors: John M. Sharp, Jr. and Alan R. Dutton

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GEOLOGIC AND HYDROLOGIC CONSTRAINTS ON FLUID AND HEAT FLOW IN OVERPRESSURED ROCKS OF THE RIO GRANDE EMBAYMENT, GULF OF MEXICO BASIN

by

Thomas Edward McKenna, Ph.D.

The University of Texas at Austin, 1997

Supervisor: John M. Sharp, Jr.

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HIGH RESOLUTION X-RAY COMPUTED TOMOGRAPHY (CT) OF THE SKULL OF AN EXTANT OPOSSUM (MONODELPHIS DOMESTICA) AND A COMPARISON OF ITS ONTOGENY TO SYNAPSID PHYLOGENY

by

Thomas Edward Macrini, M.S.

The University of Texas at Austin, 2000

Supervisor: Timothy Rowe

The internal cranial osteology of the skull of Monodelphis domestica, the gray short-tailed opossum, was examined and described using high resolution X-ray computed tomography (CT). In addition, the relative order of appearance of discrete character transformations of the synapsid mandible and auditory region (together a subset of the morphology of the synapsid skull examined and labeled using CT technology) was quantitatively compared in the ontogeny of M. domestica. These data were compared to the relative order of appearance of the same transformations in two phylogenies containing both extinct and extant synapsids.

The skull of an extant adult female M. domestica was digitally sliced at the University of Texas High Resolution X-ray Computed Tomography Facility in Austin, Texas. The digital slices of the skull were anatomically labeled (see included compact disc) and the braincase and internal cavities were described with respect to osteology.

Discrete character transformations from 33 characters were examined in M. domestica ontogeny and synapsid phylogeny but only a subset of those were observable and able to be analyzed in both contexts. The relative order of appearance of 22 discrete character transformations (19 characters) in the ontogeny of Monodelphis was compared to that described in two published synapsid phylogenies using Spearman's R ranked correlation tests. The characters were tested for a von Baerian correlation in the ontogeny of M. domestica and synapsid phylogenies.

A von Baerian correlation is a statistically significant correlation (positive or negative) between the relative order of appearance of character transformations in the ontogeny and a phylogeny of a taxon. Von Baerian correlations only refer to the relationship between the specific characters compared in both ontogeny and phylogeny. In addition, the effect of using two different model synapsid phylogenies, selected from the literature, was examined.

Results indicate that the relative order of appearance of character transformations of the mandible and auditory region of synapsids are statistically correlated with each other in two separate analyses involving different phylogenies. These characters form a von Baerian correlation and potentially should be weighted differently in future phylogenetic analyses. However, the null hypothesis that randomly selected characters taken from throughout the skeleton will have a von Baerian correlation needs to be tested. If this null hypothesis is rejected, then the characters of the mandible and the auditory region used in this study should be down-weighted in future phylogenetic analyses of synapsids.

Also, it is not clear whether the similar results from analyses using different phylogenies is a result of the overall similarity of these phylogenies. These phylogenies need to be compared using an alternative test to see if they are statistically different. Because they differ so much in their taxon samplings, these phylogenies cannot be compared by a Templeton's test. Potentially, von Baerian correlations could be used to test the similarity of phylogenetic hypotheses that differ in their topology and in the extent of taxon and character sampling.

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MOBILE SEDIMENTS IN A KARST AQUIFER

by

Barbara June Mahler, Ph.D.

The University of Texas at Austin, 1997

Supervisor: Philip Bennett

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NUMERICAL MODELING OF DEFORMATION AROUND ALLOCHTHONOUS SALT TONGUES DRIVEN BY SEDIMENTARY LOADING: APPLICATION TO THE GULF OF MEXICO

Ryan John Mann, M.S.

The University of Texas at Austin, 1998

Co-Supervisors: Daniel Schultz-Ela and Randall Marrett

The Gulf of Mexico contains numerous salt tongues that formed by extrusion of salt from emergent diapirs due to differential loading of the mother salt. Many subsalt hydrocarbon plays are associated with salt tongues, but geophysical resolution beneath them is extremely poor. However, suprasalt seismic resolution is generally good. Thus understanding the relationship between the suprasalt and subsalt structures is critical for reliable interpretation of subsalt regions. Numerical simulations described here indicate that significant differences in this suprasalt/subsalt relationship depend on variables such as salt tongue geometry, sedimentation rate, sediment density, and depositional style. Structural features simulated in response to differential sedimentary load accumulated over the salt tongue were compared with one another and with natural examples on 2-D seismic sections.

Some of the key features were predicted by various finite-element models of salt tongues were as follows. A normal growth fault at the head (landward edge) of a tongue associated with a seaward-migrating depocenter in the overlying sediments indicates deformation above a sublapped salt tongue with a welded feeder. Conversely, a salt tongue with a bathymetric mound containing closely spaced, extensional faults located near the head and lacking a migrating depocenter indicates a sublapped salt tongue connected to an active inclined feeder. A salt tongue having onlapping sediments and an active inclined feeder has a more localized dynamic bulge closer to the head of the tongue as compared with the sublapped tongue. Undercompacted sedimentation causes a less localized, smaller, and more persistent dynamic bulge. Steeper sedimentation (3[ring]) over a salt tongue focuses sedimentation toward the head of the salt tongue, and forms basinward-thickening growth wedges characteristic of a stepped counter-regional system. Steps along the base of a salt tongue induce multiple regions of extension in the overburden and retard the salt tongue's evolution.

After applying these guidelines from modeling, poorly imaged zones in natural examples of salt tongues can be more realistically interpreted.

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HYDROGEOCHEMISTRY OF THE PINE RIVER METHANE SEEPS: DURANGO, COLORADO

David Tucker Martineau, M.S.

The University of Texas at Austin, 1997

Supervisor: Philip C. Bennett

Residents of the Pine River Ranches community located on the northern rim of the San Juan Basin, 13 Km north of Bayfield, Colorado have expressed concerns regarding the rate of methane seepage in the community. This investigation was designed to locate the methane seepage and find the controls and rates of seepage. Methane was found in four seep trends across the Pine River Ranches. The four seep trends correlate with different coal zones in the Fruitland Formation. In the center of the seep trends soil gas concentrations of methane were found to exceed 95% with small concentrations of nitrogen, oxygen, and carbon dioxide. On the fringe of the seeps where methane, nitrogen, oxygen, and carbon dioxide are present, methanotrophic bacteria consume methane and oxygen to form carbon dioxide. At these localities the concentration of carbon dioxide can exceed 8%, 4-5 times higher than concentrations for normal root respiration. Methanotrophic bacteria are not active in the center of the seep because methane physically displaces the oxygen necessary for metabolism.

Denitrification in the areas of methane-stressed vegetation cause the soil nitrate levels to drop to less than 0.6 ppm. Normal levels of nitrate (~2.5 ppm) were found in areas of stressed vegetation with no methane present. No evidence of iron and sulfide reduction were detected.

In the center of the seeps methane flux ranged between 0.9 to 14.4 l/min/m2. Flux is higher in saturated areas such as wetlands and the riverbed lower in dry soil areasand may relate to the thickness of the unsaturated zone. As the thickness of the unsaturated zone increases, dispersion and/or diffusion of methane also increases, decreasing the specific flux. River water levels and localized precipitation are major controls of the unsaturated zone thickness. Methane flow does not appear to be effected by barometric pressure and there was no long-term temporal variation in flow during the study period. Water levels in area wells suggest that the shallow alluvial aquifer is recharging the coal aquifer, and the difference in water levels between the shallow alluvial aquifer and coal aquifer is increasing at ~0.2 m per year. Water level does change with barometric pressure in the seep area. Change in barometric pressure causes a shrinkage or expansion of methane bubbles present in the shallow alluvium aquifer, effectively converting the ëfluidí in the shallow alluvium aquifer into a compressible emulsion.

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THE NEOTECTONICS OF THE MACQUARIE RIDGE COMPLEX, PACIFIC-AUSTRALIA PLATE BOUNDARY

by

Christina Gisela Massell, M.A.

Co-Supervisors: Millard F. Coffin and Sharon Mosher

Recently acquired marine geophysical data document regional neotectonics over the Macquarie Ridge Complex, a portion of the Pacific-Australia plate boundary south of New Zealand. The most significant result of my analyses of primarily tectonic spreading fabric and fracture zones is the identification of the plate boundary along the Macquarie Ridge Complex, a relict spreading center. I interpret a ~5-10 km "Macquarie Fault Zone" between the two plates along a bathymetric high that extends nearly the entire length of the Pacific-Australia plate boundary south of New Zealand. In conjunction with teleseismic studies, the new data lead me to conclude that this is the active Pacific-Australia plate boundary. A broad zone of less intense deformation associated with the plate boundary extends ~50 km on either side of the Macquarie Fault Zone. I classify distinct segments of the plate boundary as having experienced compressional and strike-slip deformation on the basis of marine geophysical data, in contrast to teleseismic evidence that overwhelmingly suggests strike-slip motion along the entire surveyed boundary today. The southern Puysegur and McDougall segments show no evidence for past underthrusting, whereas data from the Macquarie and Hjort segments strongly suggest past compression. The present-day strike-slip plate boundary along the Macquarie Ridge Complex coincides with the relict spreading center responsible for Pacific-Australia crust in the region. My conceptual model for the transition from seafloor spreading to strike-slip motion along the Macquarie Ridge Complex explains the decreasing length of spreading axis segments and spacing between fracture zones, as well as the arcuate bend of the fracture zones to become asymptotic to the ultimately transcurrent or transpressional plate boundary.

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EVIDENCE FOR ANISOTROPY IN THE D" LAYER BENEATH ALASKA

by

Eric M. Matzel, M.A.

The University of Texas at Austin, 1996

SUPERVISOR: Stephen P. Grand

D" is a unique region at the base of the mantle, which is of considerable interest because of its influence on mantle convection and on core-mantle interactions. Typically 150 - 300 kilometers thick, it is characterized by unusually low velocity gradients, and large seismic variability. Global inversions and more detailed localized studies conclude that seismic velocities within D" vary greatly. Due to sparse data coverage and the lack of knowledge of the fine details of the deep mantle, the cause of the heterogeneity in D" remains unknown. This study considers the possibility that the velocity structure of D" is anisotropic. The data examined consist of seismograms from deep Japanese earthquakes recorded at WWSSN receiver stations in North America. The source-receiver combinations span distances of 70o-106o with associated S waves passing through D" beneath Alaska. Differential travel times of the Sab, Scd, ScS and SKS phases are used to constrain the velocity structure in D". Shear waves refracted by D" are observed beyond 72o and provide a sensitive measurement of the velocity structure in D". Beyond 89o, the vertically polarized (SV) and horizontally polarized (SH) shear waves often appear distinctly split, although, at distances less than that the components are more nearly synchronous. Near 95o, SH occurs as a double arrival, while SV remains a single arrival synchronous with the second SH arrival. We have been unable to reproduce these effects in isotropic model synthetics. Synthetics for transversely isotropic models have been computed that do match these waveforms. The anisotropy was constrained to be only within D", with a vertical symmetry axis. We conclude that these observations may be explained by an anisotropic layer in D", possibly created by the orientation of anisotropic minerals produced by convection at the base of the mantle. The Lay D" discontinuity may be due to a transition to anisotropic mantle a few hundred kilometers above the core-mantle boundary.

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A PHYLOGENETIC ANALYSIS OF THE EURYAPSID REPTILES

by

John William Merck, Jr., Ph.D.

The University of Texas at Austin, 1997

Supervisor: Timothy B. Rowe

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SQUAMATES OF THE CAMPANIAN TERLINGUA LOCAL FAUNA, BREWSTER COUNTY, TEXAS

by

Mary Stewart Miller, M. A.

The University of Texas at Austin, 1997

Supervisor: Timothy B. Rowe

The Terlingua Local Fauna is a diverse upper Cretaceous microvertebrate assemblage which includes remains of sharks, bony fish, amphibians, mammals, turtles, squamates, crocodiles, dinosaurs, and birds. The fauna is found in the upper shale member of the Cretaceous (Campanian) Aguja Formation near Big Bend National Park, in west Texas. The squamates of this fauna represent the southernmost occurrence of the group from the Western Interior Seaway, excluding two single taxon occurences in Mexico. Their description provides opportunity for comparison with well-studied northern squamates of the same age and contributes to our understanding of latitudinal differentiation among taxa bordering the Western Interior Seaway.

Sixteen squamate taxa have been identified, six of which are Squamata indet. All remains are highly fragmentary, the majority of which are dentary and maxillary fragments and isolated osteoderms. The anguimorphs include the glyptosaurine anguid, Odaxosaurus piger, Odaxosaurus sp., a xenosaur, a necrosaur (cf. Parasaniwa sp.), and a varanid cf. Paleosaniwa candensis. The scincomorphs include a large taxon with distinct teeth, and two teiids with tooth morphologies primitive to that previously known for the group. A single strongly recurved tooth in a jaw fragment indicates the presence of a snake. An indeterminate autarchoglossan is represented by rectangular, faintly keeled osteoderms.

The squamates of the Terlingua Local Fauna show a marked degree of endemism, most notably by the presence of the large scincomorph, the teiids, and the autarchoglossan with keeled osteoderms. This evidence supports the idea that there were distinct northern and southern faunas adjacent to the Western Interior Seaway during the Late Cretaceous, one that has been proposed from studies of similarly aged mammals, dinosaurs and marine vertebrates.

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AN INVESTIGATION OF LAND/ATMOSPHERE INTERACTIONS: SOIL MOISTURE, HEAT FLUXES, AND ATMOSPHERIC CONVECTION

by

Karen Irene Mohr, Ph.D.

The University of Texas at Austin, 2000

 

Supervisor: James S. Famiglietti

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LATE PENNSYLVANIAN SHALES OF NORTH-CENTRAL TEXAS: AN ASSESSMENT OF THEIR DEPOSITIONAL ENVIRONMENT

by

Margaret Ann Molineux, Jr., Ph.D.

The University of Texas at Austin, 1997

Supervisor: James Sprinkle

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RELATIONSHIP BETWEEN FRACTURE APERTURE AND LENGTH IN SEDIMENTARY ROCKS


Javier Geovanni Moros, M.S.
The University of Texas at Austin, 1999

Supervisor: Randall Marrett


Extension fractures from nine formations were measured to study the scaling relationship between fracture length and aperture. Fractures in sandstones and carbonates were studied from the surface and subsurface at observational scales ranging from outcrops, cores and thin sections. Fractures having three kinds of spatial arrangement were analyzed: isolated, en echelon and linked fractures. In each fracture, aperture was measured at regular intervals along the length of the fracture.
Fractures observed in all nine formations display aperture distributions that are best represented by a flat to irregular plateau along most of the fracture and high aperture gradients near the tips. Similar configurations were obtained by treating two or more overlapping fractures as a single amalgamated fracture, suggesting that local aperture minima are products of the linkage of paleo-single segments.
Measurements of fracture dimensions demonstrate that fractures follow power-law scaling across at least 5 orders of magnitude. A mathematical expression of the form b=gLm where b = aperture, L = length, and g and m are empirical scaling constants) provides the best model for the observed fractures. The exponent m is lithology-dependent and has a value of about 0.75 for sandstones and a value of almost 1.0 for carbonates. The same kind of expression is valid for different types of fractures such as isolated, en echelon and linked fractures. b=gLm
This study provides the basis to improve physical models of fracture growth and to predict fracture behavior through different scales. Fracture linkage appears to impose a strong character on the aperture variation along fractures, so mechanical models of fracture growth need to take linkage into account. The empirical relation between aperture and length will be useful for estimating fracture length in the subsurface, where it rarely can be measured directly.

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TEMPORAL LINKS BETWEEN CLIMATE AND HYDROLOGY: INSIGHTS FROM CENTRAL TEXAS CAVE DEPOSITS AND GROUNDWATER

by

MaryLynn Musgrove, PhD

The University of Texas at Austin, 2000

Supervisor: Banner, J. L.

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MAGNETICS OF THE PUERTO RICO TRENCH AND AEROMAGNETICS OF THE NORTH SLOPE OF ALASKA

by

Stefan Paul Muszala, M.S.

The University of Texas at Austin, 1998

 

SUPERVISORS: Paul L. Stoffa and Lawrence A. Lawver

During the summer of 1996, the R/V Maurice Ewing collected single-channel-seismic, magnetic, gravity, Hydrosweep bathymetric and HMR1 sidescan data north of the island of Puerto Rico to constrain the geologic and tectonic setting of the Puerto Rico trench area. Magnetic data from this cruise are merged with other available data in the study area, and we interpret these data with the aid of the Euler deconvolution method. The area north of the island of Puerto Rico is divided into three magnetic zones. Zone 1 is dominated by northwest to southeast trending magnetic highs and corresponding lows. Zone 2 consists of an east-west region of relatively low magnetic anomalies and occurs south of zone 1. Zone 3 is dominated by the highest magnetic values in the study area and is roughly centered under the island of Puerto Rico. The Main Ridge, a bathymetric high found within the southern wall of the trench, is continuous with a north-west, south-east trending magnetic high that has not been mapped previously and is found within magnetic zone 2. This high also corresponds to a fracture zone found on the North America plate.

Parallel to the Main Ridge, and lining up with the Fourth of July Ridge, is another previously unmapped fracture zone. Results from the 3-D Euler deconvolution are compared to those of the seismic and bathymetric data. The 3-D Euler deconvolution is able to identify the faults found in the seismicand bathymetric data and is further able to delineate some other faults and magnetic boundaries. The Septentrional Fault Zone that has been extensively mapped on land in the Dominican Republic is identified and terminates at the Mona Canyon. A separate fault, the South Puerto Rico Slope fault exists to the north of the island of Puerto Rico. Also mapped is the North Puerto Rico Slope fault and other strike-slip and thrust faults surrounding the Main Ridge. A model is proposed that incorporates the existence of strike-slip faults and the fracture zones associated with the subduction of the North America Plate. Evidence from this study support a tectonic interpretation of subduction followed by more recent strike-slip faulting.

Removing the magnetic signal of cultural noise from that produced by the underlying geology is important for getting the true earth signal from aeromagnetic data. I present a processing technique that combines the use of Very Fast Simulated Annealing (VFSA) and an equivalent source technique to help find drill-platforms and their attributes in aeromagnetic data from the North Slope of Alaska. The anomalous signal can then be removed from the data. This technique also has implications for waste and chemical contaminant sites where a common problem is determination of the number of drums at a particular waste-site and their location.

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FACIES ARCHITECTURE AND PRODUCTION OPTIMIZATION IN THE UPPER CRETACEOUS-UPPER EOCENE, OPERATIONAL UNIT K1, APIAY FIELD, LLANOS BASIN, COLOMBIA

by

Jorge Enrique Nieto, M.S.

The University of Texas at Austin, 1997

Supervisor: Noel Tyler

This integrated reservoir study of upper Cretaceous to upper Eocene reservoirs in Apiay field documents more than 57 MMSTB (million stock tank barrels) of unrecovered mobile oil in a zone (K1 operational unit) which contains 87.5 MMSTB of OOIP (original oil in place). This study proposes new recompletions and new targeted infill wells that will be necessary to produce the remaining oil in this highly prospective interval.

Apiay field, located in southwest Llanos Basin in Colombia, produces from an upper Cretaceous to upper Eocene fluvial and transitional marine system. The Apiay structure contains reserves in upper Cretaceous to lower Tertiary strata, although since its discovery in 1981, production has come exclusively from the massive fluvial sandstones of the K2 operational unit. A high production decline and the necessity of finding additional reserves from uncharacterized reservoirs provide the justification for developing a multidisciplinary approach to understand and predict facies architecture in the K1 operational unit and to evaluate its potential for additional oil.

The K1 operational unit was divided into six genetic units from detailed stratigraphic correlation and vertical seismic resolution. Interpretation of facies distribution was carried out and integrated with well log, core, seismic amplitude, petrophysical, and production behavior analyses. Facies distribution demonstrates that the lower K1 operational unit was deposited in a fluvial-dominated delta during the Cretaceous transgression. The middle K1 operational unit formed in a mesotidal shorezone system which evolved into a fluvial-dominated deltaic system in the middle part. Finally, the upper K1 operational unit was discordantly deposited above the upper Cretaceous and records braided fluvial stream sedimentation in the late Eocene.

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THE ANALYSIS OF RADAR ALTIMETER WAVEFORM REFLECTIONS OVER CONTINENTAL ICE SHEETS

 

by

 

Vannaroth Nuth, Ph.D.

The University of Texas at Austin, 1999

Supervisor: Clark R. Wilson

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AFTERSHOCK OCCURRENCE RATE DECAY FOR INDIVIDUAL SEQUENCES AND CATALOGS

by

Paul A. Nyffenegger, Ph.D.

The University of Texas at Austin, 1998

Supervisors: Cliff Frohlich and Stephen P. Grand

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MANAGING RISK WITH DERIVATIVES IN THE OIL AND GAS BUSINESS

by

Sandra Patricia Parra Redondo, M.A. .

The University of Texas at Austin, 1998

SUPERVISOR: Willem CJ. van Rensburg,

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RESERVOIR ARCHITECTURE AND RESERVE GROWTH POTENTIAL OF MIOCENE FLUVIAL-3DELTAIC DEPOSITS, BACHAGUERO FIELD, MARACAIBO BASIN

 by

Ronald Oribio Quintana, MA

The University of Texas at Austin

 

Supervisor: Noel Tyler

The methodology used in reservoir characterization studies aimed at the identification of reserve growth potential in mature oil fields requires, as a first step, the determination of reservoir architecture, that is, the stratigraphic and structural relations of genetic units and their component depositional systems. While the impact of structural discontinuities on reservoir performance is well known, facies architecture is becoming recognized as a controlling parameter in reservoir behavior, since it provides heterogeneities to the reservoir, and hence, determines the drainage efficiency of the reservoir.

The clastic Miocene section at the Bachaquero Field, located in the Maracaibo Basin, was selected for this reservoir architecture study, which was carried out through the analysis and integration of 3-D seismic, core, and welllog data.

Structurally, the most important element in the area is the Pueblo Viejo Fault System, which underwent several periods of positive tectonic inversion, controlling the sedimentation locally and leading to the development of normal, reverse, and backthrust faulting of variable orientation.

The stratigraphic section was divided into four cycles, based on core description, well-log patterns, and regional geology, and using using flooding surfaces to establish genetic divisions. This section displays primarily progradational and aggradational stacking patterns. From electric log correlations, sandstone mapping, and lateral and vertical relationships, the depositional environments are interpreted as moderately to highly heterogeneous fluvial and fluvial-dominated deltaic systems. Successive vertical movement along the Pueblo Viejo Fault led to partial and total erosion of some units and to periods of abrupt shifting of the depositional axis.

Petrographic analysis indicates sandstones are subarkosic and that diagenetic processes caused early precipitation of siderite and generation of secondary porosity by dissolution of framework grains, improving reservoir quality, especially in cycles 3 and 4.

Opportunities for incremental recovery exist in the form of infill wells on the crest of the structure where well density is low, and as field extension opportunities at the flanks of the structure in areas where the structure is shallower than what previous structural interpretations indicate.

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PREDICTION OF MACROFRACTURE PROPERTIES USING MICROFRACTURE INFORMATION, MESAVERDE GROUP SANDSTONES, SAN JUAN BASIN, NEW MEXICO

by

Orlando José Ortega Pérez, M.S.

The University of Texas at Austin, 1997

Supervisor: Randall Marrett

The study of natural microfractures is triggered by sampling problems. The sampling of subsurface macrofractures is limited to well cores, geophysical or production tools which provide scarce material for their study. A better understanding of relationships between microfractures and macrofractures can help characterize macrofractures by extrapolating observed microfracture properties to infer the properties of the macrofractures. As my study helps demonstrate, the abundance of microfractures makes this approach interesting and possibly feasible.

Fracture orientation is a fracture property that could be constant from the microscale to the macroscale. Fracture frequency could potentially be extrapolated through different scales of observation too. The extent to which these hypotheses are valid is uncertain. Furthermore, no investigations on the effect of mechanical boundaries on the validity of these extrapolations has been done previously. The objectives of this study are to test these hypotheses in sandstones of the Mesaverde Group, San Juan Basin, New Mexico. The use of a cathodoluminescence detector attached to a Scanning Electron Microscope (SEM) allowed observation of microfractures not readily visible using conventional microscopic techniques.

The orientations of microfractures reproduce the orientations recorded at the macroscopic scale from cores and outcrops when sufficient amounts of microfracture data are collected, in this case, more than 100 microfractures collected from approximately 8 mm2 of a thin section.

Microfracture size distributions are best modeled using power laws and extrapolation of microfracture frequencies to the scale of macrofractures successfully predicts the macrofracture frequencies. Fracture size distributions change at the length scale of the thickness of the mechanical layer, typically by increasing the exponent of their power-law distribution. This change suggests more homogeneity of fluid flow at large scales if the classic parallel-plate model is used to estimate the permeability of the fracture system. It might be possible to make better estimates of permeability for fracture systems once we know the fracture frequency distribution and the thickness of the mechanical layers that control the fracturing. The implications of these conclusions have impact on realistic modeling of fluid flow through fracture systems. Further research to clarify these questions is necessary.

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PHYLOGENETIC RELATIONSHIPS AMONG AMERICAN BADGERS (TAXIDIINAE) AND THE EVOLUTION OF THE BADGER ECOMORPH

by

Pamela Renee Owen, Ph.D.

The University of Texas at Austin, 2000

Supervisors: Timothy B. Rowe and Ernest L. Lundelius, Jr.

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DEPOSITIONAL FACIES ANALYSIS FROM SEISMIC ATTRIBUTES IN THE PATTANI BASIN, GULF OF THAILAND

by

Yong-Joon Park, M.S.

The University of Texas at Austin, 1999

Supervisor: William L. Fisher

 

The Pattani Basin is the largest Tertiary rift basin in the Gulf of Thailand. The Basin was formed by extensional forces caused by a strike-slip fault system. Tertiary sequences of the Pattani Basin consist of three syn-rift sequences and three post-rift sequences. This study includes structural analysis of the northern Pattani Basin, areal description of depositional facies, and integration of spatial relationships using 3-D seismic and well data.

Graben structures and basement geometry are defined from 3-D seismic structural interpretation. Fault reactivation is recognized by comparing vertical throw at different depths on individual faults. The fault structures were initiated in the early Tertiary and reactivated as recently as the Quaternary. Three-dimensional structural interpretation indicates that most faults were created by extensional stresses caused by dextral shearing of a regional strike-slip fault system.

Well log data indicate that the representative depositional facies of the studied intervals are sandy, fluvial, channel-fill facies encased in shaly floodplain deposits. Seismic responses were predicted from a synthetic seismogram using a model of dominant depositional facies. Due to large impedance differences between sand and shale, seismic responses of channel sands are distinguishable from other facies. Peak-to-trough amplitude and instantaneous frequency seismic attributes are used in depositional facies interpretation. A series of stratal surface slices were created by time-shifting the reference stratal surface.

Three intervals A, B, and C are interpreted on the successive stratal surfaces. The shallowest interval, A, is the Quaternary transgressive succession. Each stratal surface showed flow pattern variation of fluvial channel facies. Two transgressive cycles were identified in interval A. Interval B also indicated fluvial facies. Two depositional cycles were identified, and each cycle was divided into three phases. High sediment discharge rate through large channels and abrupt changes in depositional environment imply tectonic influences. Identifying depositional facies and environment change was possible for a limited area in interval C. Depositional facies successions are described by interpreting seismic attributes on the successive stratal surfaces.

 
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VEINS AND ALTERATION ENVELOPES IN THE GRASBERG IGNEOUS COMPLEX, GUNUNG BIJIH (ERTSBERG) DISTRICT, IRIAN JAYA, INDONESIA

by

Sarah Penniston-Dorland, M.S.

The University of Texas at Austin, 1997

Supervisor: Mark Cloos

The Grasberg Igneous Complex (GIC) consists of three main phases of igneous activity: the Dalam Igneous Complex, the Main Grasberg Intrusion, and the Kali Intrusion. Each contains veins revealing a history of fluid flow that has concentrated minerals of economic value.

A generalized sequence of early magnetite +/- quartz veins followed by quartz +/- sulfide/oxide veins followed by late chalcopyrite/pyrite veins is observed in the Dalam Igneous Complex and Main Grasberg Intrusion. The youngest igneous body, the Late Kali Intrusion, cross-cuts the older igneous bodies as well as their veins, and has biotite +/- quartz, quartz +/- pyrite and pyrite +/- quartz veins.

Pyrite +/- quartz veins with alteration envelopes up to 14 cm total width are found in regions of the complex that are higher in elevation and distant from the center of copper mineralization. Geochemical analyses of wall rock and alteration envelopes from eleven samples are compared to determine which components were added to the altered samples and which were removed by the fluids. Most major components were removed by fluids (Na2O, MgO, SiO2, CaO, FeO, and TiO2) along with many trace elements (Cu, Cl, Ga, Rb, Sr, Nb, Ba, Y, Zr, Eu, Yb, Tl, Au, La, Th, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Tm, and Lu). Gain or loss of K2O and P2O5 vary depending on the sample. H2O and S were added to the altered wall rock. Typical host-rock mineral assemblages include plagioclase, biotite, quartz, alkali feldspar, pyrite, chalcopyrite, and magnetite. Typical alteration envelope mineral assemblages include muscovite, alkali feldspar, pyrite, and quartz. Balanced reactions between wall rock minerals and fluid to produce alteration minerals typically involve the consumption of HCl, indicating that the altering fluids had a low pH. The alteration envelopes are believed to be the result of changes as fluids flowed through the complex, including decreasing temperature, generation of HCl by the precipitation of pyrite and chalcopyrite from copper and iron chlorides in the lower and central parts of the complex, and/or the decrease in the fluid prewall rock fluid pressure surrounding veins.

Scanned cathodoluminescence of quartz in quartz-sulfide veins reveals detailed textures on the scale of tens to hundreds of microns including concentric growth zoning and fractures. Growth textures indicate that the quartz grew into open space, so these veins remained open during infilling. Vein growth is believed to have occurred from fluids that flowed through the veins. Microfracturing occurred after the veins began to close.

Experimental studies of Cline and Bodnar (1991) applied to fluid exsolution from magma chambers are used as a basis to explain the sequence of veining. Fluid separating from a magma at low pressure (<1 kilobar) has initially low concentrations of copper, whereas fluid separating from a magma at high pressure (>=2 kilobars) has initially high concentrations of copper. Crystallization from a deep batholithic magma chamber at depths greater than 6 km with a molten stock reaching up to shallower depths (less than 3 km) can account for the changes in copper precipitation observed in the GIC system over time. Early crystallization in a stock at shallow depths led to exsolution of an early fluid that was relatively copper-poor. This resulted in early magnetite +/- quartz veins. Deeper-seated crystallization eventually generated copper-rich fluids forming chalcopyrite/pyrite veins. Finally, the latest stages of veining following the Late Kali Intrusion were relatively copper-poor due to the last fluids exsolving from the deeper copper-depleted part of the magma chamber.

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SEQUENCE STRATIGRAPHY, STRUCTURE, AND TECTONICS OF THE SOUTHERN ONTONG JAVA PLATEAU AND MALAITA ACCRETIONARY PRISM, SOLOMON ISLANDS

by

Eric James Phinney, M. A.

Supervisor: Milo M. Backus

The Ontong Java Plateau is the largest and thickest oceanic plateau on Earth and one of the few oceanic plateaus actively converging on an island arc. Previous tectonic and geochemical models for the formation of the Ontong Java Plateau and its overlying sedimentary cover are handicapped by a lack of high quality geophysical observations of the structure and stratigraphy of the plateau and a paucity of well information. In this thesis, I present new seismic velocity determinations and geologic interpretation of 2000 km of 2-D multichannel seismic data collected across the southwestern Ontong Java Plateau, North Solomon Trench, and northern Solomon Islands. Three seismic megasequences inferred to range in age from early Cretaceous to Quaternary are recognized on the basis of distinct interval velocities and seismic stratigraphic facies. I show that a high-amplitude reflector, previously identified as the top of igneous basement, is related to a late Cretaceous unconformity between two sedimentary megasequences which together define a much thicker sedimentary section on the plateau than previously recognized. I integrate seismic facies analysis, sediment thickness, and structural relationships to interpret the three megasequences into a dynamic plate tectonic framework which includes two periods of mantle plume activity at 122 Ma and 92 Ma, a period of gentle folding and uplift associated with the 92 Ma mantle plume event, middle to late Miocene faulting possibly related to arrival of the plateau at the North Solomon Trench, and recent normal faulting related to flexure and southwest-directed subduction of the plateau at the North Solomon Trench.

Previous workers have proposed several possibilities for the fate of oceanic plateaus at subduction zones ranging from complete subduction to complete accretion. I present multichannel seismic data from the Ontong Java Plateau/Solomon Islands convergent zone showing active accretion of a 7 km section of the sedimentary cover and igneous crust of the oceanic plateau. Correlation of the seismic data with satellite-derived free-air gravity data confirms the presence of a 900-km-long, 140-km-wide fold-thrust belt, named here the Malaita Accretionary Prism, composed of accreted sediments and upper crust from the plateau. I divide the Malaita Accretionary Prism into four structural domains interpreted to have formed by diachronous, southeast-to-northwest progression of the active deformation front along the North Solomon Trench. Uplift and deformation of ponded sediment across the prism and in the North Solomon Trench confirm that plateau accretion is active. Thrust faults dipping southwest from the North Solomon Trench at 10-17deg. and surficial convergent deformation imaged in the most recent, northwestern domain show that greater than 1 km of sediments and 6 km of upper crust are being actively transferred from the plateau to the Solomon Islands arc. Linear gravity trends and structural patterns indicate that plateau accretion is not simply the result of failed plateau subduction at the North Solomon Trench during the late Miocene as proposed by previous workers. Instead, gravity and seismic data suggest that the middle Pliocene through present accretion of the Malaita Accretionary Prism was activated by shallow subduction of 4-0 m.y. old crust of the Woodlark Basin and bounding bathymetric highs along the southwestern edge of the Solomon arc

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ORIGIN OF THE 1.3 GA HÄSTEFJORDEN AND URSAND INTRUSIONS, SW SWEDEN: ND EVIDENCE FOR CRUST-MANTLE INTERACTION IN THE GENESIS OF A-TYPE SUITES

by

Jennifer Evelyn Piontek, M.S.

The University of Texas at Austin, 1999

Supervisor: James N. Connelly

The Mesoproterozoic southwestward growth of the Baltic Shield in southern Sweden involved the Gothian (1.75-1.55 Ga) and Sveconorwegian (ca. 1.0 Ga) orogenies and an intervening period of episodic, typically bimodal, magmatism. Although several authors have interpreted these magmatic pulses as anorogenic (citing spatial and temporal relationships with the bracketing orogenic events), detailed geochemical and isotopic data were lacking. I have examined and characterized two supposedly anorogenic plutons from Southwest Sweden - the bimodal Hästefjorden intrusion and Ursand granite.

U-Pb ages for granites of the Hästefjorden (1334 +7/-3 Ma) and Ursand (1319+/-6 Ma) plutons indicate a lapse of ca. 215 m.y. between the end of the Gothian Orogeny and this pulse of magmatism. Although some models explain anorogenic magmatism as a time-delayed consequence of orogenesis, the greater than 215 m.y. temporal gap between orogenesis and magmatism (and subsequent magmatic suites in southwest Baltica) lies beyond the longest modeled delay of 200 m.y., making this suite truly anorogenic by any temporal definition. The geochemical signatures of the Hästefjorden and Ursand granitic phases are also consistent with those proposed for anorogenic granites, including high Fe/Mg, K2O/Na2O and Ga/Al ratios as well as high F, REE, and HFSE.

Initial eNd values for the granitic Hästefjorden samples cluster around -2.2, whereas mafic samples vary from -0.2 to -2.7. In contrast, the Ursand granite is more juvenile with eNd values ranging from 0 to +0.5. Because this signature is significantly lower than the eNd of the depleted mantle at the time the intrusions crystallized, this suggests that both the felsic and mafic magmas of the intrusions were strongly contaminated by a crustal source. The granitoids of the Transscandinavian Batholith (TIB) that lie directly to the east of the field area are the mostly likely candidate for the source of crustal contamination. Estimates of the proportion of crust to mantle material were calculated using a two-component mixing model, and range from ~5-30% crust for the Hästefjorden mafic samples, to ~20-50% for the Hästefjorden granite, and ~5-20% for the Ursand granite.

The geochronological, Nd and chemical data from this suite are consistent with models for anorogenic magmatism that involve initiation of melting by mantle upwelling and the formation of evolved mafic magmas at base of the lower crust. The evolved mafic magma may undergo extended fractional crystallization to directly produce granitic magma, or it may form a mafic underplate of the crust. This underplate could then produce A-type granitic magma by partial melting of the low-temperature differentiates of the underplate and mixing with crustal melts during renewed mantle upwelling.

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A RECORD OF CHANGING SEAWATER CHEMISTRY AND DIAGENESIS DERIVED FROM 87Sr/86Sr VARIATIONS THROUGH THE LATEMAR CARBONATE PLATFORM, DOLOMITES, NORTHERN ITALY

by

Benjamin Paul Pursell, M.S.

The University of Texas at Austin, 1997

Supervisor: Jay L. Banner

High-resolution Sr isotope stratigraphy has the potential to be a powerful tool in understanding the global processes that drive secular changes in ocean chemistry. Among the problems inherent in extracting reliable data from pre-Cenozoic carbonates are 1) lack of stratigraphic continuity, 2) poor absolute time control, 3) diagenetic alteration, and 4) siliciclastic contamination.

These problems are minimized in the isolated carbonate platform of the Latemar Formation in the Dolomites, Northern Italy. This platform provides a stratigraphically continuous 500 m section with excellent relative time constraints, a well understood sea level and diagenetic history and a minimal siliciclastic component. The platform is composed of cyclic strata of subtidal peloid packstone bases with subaerial exposure caps. These meter-scale cycles were microsampled to obtain least altered marine material as well as diagenetic phases in order to assess their effects on the marine components.

A series of petrographic and chemical criteria were used to constrain the extent of preservation versus alteration and minimize non-carbonate contaminants. Diagenetic fabrics were identified using plane light and cathodoluminescence. Trace element criteria include: 1) Sr/Mn > 5, to screen for diagenetic alteration; 2) Fe < 500 ppm, Si < 400 ppm, and Al <100 ppm to screen for siliciclastics that may add a non-marine 87Sr/86Sr component to the marine signal and; 3) Rb/Sr < 0.002 so that the correction for ingrown 87Sr is less than analytical uncertainty. Application of these criteria has refined the structure of the 87Sr/86Sr seawater curve for the Middle Triassic.

Previous sequence stratigraphic studies of this platform have shown 1) an initial sharp third order rise in sea level through the Anisian followed by, 2) a drop in the rate of sea level rise across a maximum flooding surface that marks the Anisian/Ladinian boundary, and 3) a fall in sea level of equal magnitude followed by another much smaller rise and fall. Results of this study show changes in the 87Sr/86Sr value of seawater that are coincident with these changes in sea level. Sea level highs correlate with lows in 87Sr/86Sr recorded in the Latemar Platform and sea level lows correlate with 87Sr/86Sr highs recorded in the platform. This correspondence between variations in 87Sr/86Sr and sea level history supports the idea that eustacy controlled the third order shifts in sea level through the Middle Triassic.

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EMPLACEMENT AND DEFORMATION OF LATE SYN-OROGENIC, GRENVILLE-AGE GRANITES IN THE LLANO UPLIFT, CENTRAL UPLIFT

By

 

Robert Mark Reed, PhD

The University of Texas at Austin, 1999

 

Supervisor: Sharon Mosher

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GEOMETRIC AND KINEMATIC EVOLUTION OF A COMPLETE DETACHMENT FOLD IN A NATURAL CROSS-SECTION

 

Laura Rico, M.S. Geo.Sci.

The University of Texas at Austin, 1999

Supervisor: Randall A. Marrett

The San Julian Uplift is located within the Sierra Madre Oriental, a Laramide-age thin-skinned fold and thrust belt which deformed Upper Jurassic and Cretaceous units. Tilting of a detachment fold train along the northeastern flank of the San Julian Uplift produced steeply plunging fold axes, and subsequent erosion created a natural cross section of entire folds, detachment level, and older units. One fold in this train, the San Miguel anticline, is used here to test geometric and kinematic implications of current detachment fold models and to provide an analysis of the relationship between regional-scale folding and macroscopic and microscopic deformation mechanisms.

The San Miguel anticline has a dominantly cylindrical, parallel geometry and an overall curved shape. A markedly small amount of evaporites make up the final-state decollement layer. Significant deformation was accommodated in the San Miguel anticline by the development of veins, stylolites, second-order folds, and flexural-slip horizons. Axial-parallel bedding-perpendicular veins and stylolites accommodated a significant amount of strain and indicate bending was an important folding mechanism. Second-order folding was an important mesoscale strain mechanism that produced structures on the limbs of the anticline but was entirely absent on the roof and synclinal floors. Their distribution, asymmetry, and angular geometries imply an origin as inner-arc contraction features which were later sheared, or as a result of flexural slip on the limbs of the fold. Bedding-parallel faults accommodated a significant portion of the strain in the forelimb, indicating that flexural slip was a significant mechanism in this structural domain. Calcite twin strain magnitudes are small, but indicate a general bedding-perpendicular contraction, and might represent overprinting by the later tilting event.

The strain mechanisms that accommodated fold growth were active heterogeneously throughout the fold. Thickness changes, although present, do not agree with the stratigraphic distribution predicted by the layer-parallel strain model. Significant portions of strains are best interpreted as products of hinge-related bending, and their presence in areas of low final-state layer curvature is an indication of migrating hinges. This conclusion is further supported by the limited abundance of evaporites present in the anticlinal core and underneath the synclinal floors. The non-uniform distribution of strain throughout the limbs of the fold and the correlation of high strain areas with some final-state hinges might conflict with a migrating-hinge fold model, however, so the evolution of the San Miguel anticline might be best explained by a combination of the migrating-hinge and rotating-limb models.

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GEOCHEMICAL AND TECTONIC EVOLUTION OF THE COLORADO PLATEAU MANTLE LITHOSPHERE: EVIDENCE FROM GRAND CANYON MANTLE XENOLITHS

 

by

Joyce Christine Alexis Riter, Ph.D.

The University of Texas at Austin, 1999

Supervisor: Douglas Smith

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ESTIMATING CHANGES IN TERRESTRIAL WATER STORAGE

by

Matthew Rodell, Ph.D.

The University of Texas at Austin, 2000

Supervisor: James S. Famiglietti

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TECTONIC ANALYSIS, STRATIGRAPHY AND DEPOSITIONAL HISTORY OF THE MIOCENE SEDIMENTARY SECTION, CENTRAL EASTERN VENEZUELA BASIN

by

Luís Oswaldo Rodríguez, Ph.D

The University of Texas at Austin, 1999

Supervisor: William E. Galloway

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NUTRIENT-DRIVEN COLONIZATION AND WEATHERING OF SILICATES

by

Jennifer Roberts Rogers, Ph.D.

The University of Texas at Austin, 2000

Supervisor: Philip C. Bennett

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VADOSE-ZONE GEOCHEMISTRY OF PLAYA WETLANDS,

HIGH PLAINS, TEXAS

by

Katherine Duncker Romanak, Ph. D.

The University of Texas at Austin, 1997

Supervisor: Philip C. Bennett

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COMPARISON OF TWO ARCHEAN ORTHOGNEISS SUITES IN THE PALEOPROTEROZOIC NAGSSUGTOQIDIAN OROGEN OF WEST GREENLAND

by

Michelle Allison Roth, MSGeoSci

The University of Texas at Austin, 2000

SUPERVISOR: James N. Connelly

The North Atlantic region consists of stable Archean cratons separated by a system of orogenic belts that formed during the Paleoproterozoic. The 1.92-1.72 Ga Nagssugtoqidian Orogen of West Greenland occupies a central position, linking orogens in Labrador and Baffin Island (eastern Canada) to those in northwest Scotland and the northern Baltic Shield.

The east-west trending Nagssugtoqidian Orogen is approximately 150 km wide and bounded to the north and south by forelands of Archean orthogneisses. The Orogen has been divided upon the basis of lithology, metamorphic grade, and structural style into the Southern (SNO), Central (CNO), and Northern (NNO) Nagssugtoqidian Orogens. The Nagssugtoqidian Orogen contains primarily reworked Archean orthogneisses, and in the CNO, lesser amounts of Paleoproterozoic supracrustal and magmatic rocks.

Two tectonic models have been proposed to explain the Paleoproterozoic reworking within the Nassugtoqidian Orogen: 1) an intra continental model and 2) a continent-continent (intercratonic) collisional model. The intracratonic model suggests that Paleoproterozoic deformation within the Nagssugtoqidian Orogen is due to a distal collisional orogen elsewhere in the North Atlantic region. The collisional model is currently preferred in recent literature because it best accounts for the existence of subduction-related magmatism, thrust stacking and imbrication of Paleoproterozoic and Archean rocks, and incorporation of ultramafic lenses of probable oceanic crust. Missing is evidence of distinct cratons to the north and south. Detailed mapping in the CNO identified two orthogneiss units proposed to represent parts of different cratons caught up within the orogen, now lying adjacent to the north and south of a proposed Paleoproterozoic suture. However, these crustal domains have not been adequately characterized isotopically. It is the purpose of this study to characterize these orthogneiss units in the Kuup Akua fjord area, CNO, western Greenland using field observations, U-Pb, Sm-Nd, and common Pb isotope systems.

In the field, the northern and southern orthogneiss units within the Kuup Akua fjord area are distinguished only by differences in metamorphic grade and occurrence of mafic dikes. U-Pb geochronology indicates that the two orthogneiss units crystallized at 2808 +13/-Il Ma and at ca. 2850 Ma respectively. Both ages fall within the previously determined are range for the orthogneisses in a much larger region (2.87-2.81 Ga), throughout which no significant age differences can be distinguished. Similarly, eNd data cannot distinguish between the two units, in part because of complications due to the resetting of the Sm-Nd system during 1.85 Ga metamorphism. Only common Pb isotope patterns indicate any isotopic distinction, the southern unit yielding an older errorchron age. However, this also is not considered sufficient evidence to support the existence of distinct cratonic blocks. The apparent older Pb errorchron age for the southern orthogneiss unit can also be attributed to the presence of early Archean rafts, the disturbing influence of granulite-facies metamorphism at 1.85 Ga, or some combination of these factors.

The lack of decisive evidence to support the existence of distinct cratonic blocks does not discount the intercratonic model, but future work can no longer use the claim that these units are distinct to support this model. Possible tectonic scenarios during the Paleoproterozoic explaining the lack of distinct crustal domains include: 1) Juxtaposition of two chronologically and isotopically similar cratons, and 2) rifting apart of one continuous craton at ca. 2.04 Ga followed by later re-amalgamation. The lack of distinct crustal domains can also be attributed to Paleoproterozoic intracratonic adjustment due to distal plate activity, but this scenario does not account for the presence of the Paleoproterozoic rocks within the CNO.

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ICE-DOMINATED DELTAS: THE EFFECTS OF ICE COVER ON DELTAIC SEDIMENTATION, LATE PERMIAN OF THE SYDNEY BASIN, AUSTRALIA

by

Stephen Allen Royer, M.S.

The University of Texas at Austin, 1998

Supervisor: William E. Galloway

During Late Permian time, the Sydney Basin of New South Wales, Australia was located near 70deg. S latitude and experienced a seasonally cold climate. Significant ice is postulated to have developed each winter, reducing or ceasing entirely the fluvial discharge to the marine and deltaic systems that filled the basin. Furthermore, marine pack ice effectively prevented waves from reaching the shore, resulting in low energy conditions in the deltaic environments for most of the year. These Late Permian deltas do not display the typical sediment transport processes of temperate to tropical fluvial-, wave-, or tide-dominated deltas, and are more appropriately termed ice-dominated deltas.

The ice cover had a profound effect on deltaic sedimentation, resulting in the development of depositional environments, facies, and sedimentary characteristics unique to ice-dominated deltas. Distinctive features include: 1) the occurrence of sub-ice channels, a network of subaqueous extensions of distributary channels; 2) lack of a well-defined distributary mouth-bar sand; 3) an abundance of ice-deposited sediments and glendonite, an authigenic carbonate mineral indicative of polar marine climate; and 4) preservation of angular, lithic detritus because of the dominance of mechanical weathering. The presence of a sub-ice platform, a gently dipping surface seaward of the subaerial delta plain, is inferred by: 1) sub-ice channels; 2) an increase in grain size basinward across the sub-ice platform; and 3) absence of well-defined distributary mouth-bar sands. The sub-ice platform created a zone of coarse sediment bypass and consequent detachment and basinward shift of the prograding delta front from the shoreline.

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LATE PLEISTOCENE FAUNA AND ENVIRONMENT AT ZESCH CAVE, MASON COUNTY, TEXAS

James Christopher Sagebiel, M.S.

The University of Texas at Austin, 1998

Supervisor: Ernest L. Lundelius, Jr.

Zesch Cave, located three miles west of Mason, Texas, contains the most northwestern Pleistocene fauna described from the Edwards Plateau. Zesch Cave is formed within the Pennsylvanian Smithwick Formation on the western edge of the Llano Uplift. Fossiliferous, carbonate cemented sediment crops out in the eastern corner of Zesch Cave, encroaching from a filled or collapsed adjacent cavern. The outcrop is extremely bone-rich, and may be the distal edge of a debris cone formed within the adjacent Pleistocene cavern. A diverse mammalian fauna, including at least eight extinct and nine extralimital taxa, was sealed within Zesch Cave's travertine during late Rancholabrean time. I have based my faunal and environmental assessments upon these fossils.

Unfortunately, the preservation process has removed most organic material from the bone, precluding direct dating by 14C methods. The travertine carbonate is derived from Pennsylvanian limestone, and the bicarbonate may not have equilibrated to atmospheric 14C levels, thus thwarting 14C dating of the travertine. However, the Zesch Cave Fauna includes an undoubtedly Rancholabrean mammalian assemblage.

Extralimital mammalian species within this fauna provide the most useful information. By examining modern distributions of the extralimital mammals, I have determined their responses to changes in the climate of central Texas. Factors limiting the geographic distribution of small mammals vary among taxa. Thus, the changing distribution of a particular species traces the change of some limiting environmental factor, e.g., soil, vegetation, moisture or temperature.

The method of accumulation directly affects the composition of cave faunas, and must be understood in order to correctly interpret the importance of various faunal components. Only the ungulate material at Zesch Cave shows evidence of pre-burial exposure (desiccation, soil acid etching). Most small mammal (body weight less than four kg) material probably came to this cave in the jaws or beak of a carnivore. Lower jaws and proximal elements far outnumber intact cranial material and distal elements. This is typical of small animal bones collected from carnivoran scat and owl pellets. The completeness of many small carnivoran fossils and the occurrence of coprolites attributable to them indicate that these animals resided within the cave.

This Rancholabrean assemblage of extralimital taxa suggests cooler temperatures and a wetter, more equable Pleistocene climate. Taxa including burrowing mammals (Geomys, Cynomys, Scalopus and Taxidea), extinct pronghorn (Capromeryx) and camelids (Hemiauchenia and Camelops) indicate an open grassland habitat with mixed forest brakes and well-developed soils.

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STRIKE-SLIP FAULTING, BRECCIA FORMATION AND PORPHYRY Cu-Au MINERALIZATION IN THE GUNUNG BIJIH (ERTSBERG) MINING DISTRICT, IRIAN JAYA, INDONESIA

Benyamin Sapiie, Ph.D.

The University of Texas at Austin, 1998

Supervisor: Mark P. Cloos

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A PATH INTEGRAL FORMULATION OF ELASTIC WAVE PROPAGATION

y

Robert Brian Schlottmann. Ph.D.

The University of Texas at Austin, 2000

Supervisor: Stephen P. Grand

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HYDROGEOLOGY AND LOCAL RECHARGE ANALYSIS IN THE TOYAH BASIN AQUIFER

by

Stefan Karl Schuster, M.S.

The University of Texas at Austin, 1997

Supervisor: John M. Sharp

The objective of the study is to analyze and quantify groundwater resources and spring discharges in a Trans-Pecos, Texas spring system that is home to two endangered species, the Comanche Springs Pupfish and the Pecos Gambusia. Spring discharge data for 1900-1996 show distinct declines in flow, probably in response to increased pumping demands over the past century. Groundwater levels continue to decline and improved management of groundwater resources is needed to sustain spring flow. Geochemical analyses of major ion and isotope data indicate multi-component recharge mechanisms that are poorly understood. The thesis details the hydrogeology of the spring system and presents: 1) a database of all documented 1900-1996 regional spring discharges, 2) a comprehensive detail of regional precipitation and correlation of the rainfall with spring discharges, and 3) potentiometric surface maps showing a continued trend of groundwater decline and discharge to pumpage in the Toyah Basin. Results indicate no significant correlation between annual average precipitation at Balmorhea and annual spring discharges, providing further supporting data of a dominant regional flow component. For higher elevations correlations of land surface elevations and potentiometric surfaces indicate a high percentage of the variability in the potentiometric surface is explained by variations in local topography. Lower elevation correlations indicate the presence of a regional flow component. An objective is to document the impact of local recharge on the spring system discharge. Geographic Information Systems (GIS) coverages of average rainfall, major and minor aquifers, land uses, and infrastructure provide an opportunity to observe spatial and temporal relationships among physical characteristics. The groundwater is critical for local drinking water supplies, irrigation in ranching, agriculture, and tourism development and this study seeks to provide a better understanding of the regional hydrologic system.

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SEDIMENTARY REGIMES AT THE MACQUARIE RIDGE COMPLEX SOUTH OF NEW ZEALAND: INTERACTION OF SOUTHERN OCEAN CIRCULATION AND TECTONISM

by

Catherine Lorraine Schuur, M.A.

The University of Texas at Austin, 1997

Co-Supervisors: Millard F. Coffin and Ian W. D. Dalziel

Seafloor structure at the Macquarie Ridge Complex strongly influences the intensity and circulation pattern of ocean currents south of New Zealand. Heterogeneous sedimentary environments on the Macquarie seafloor reflect the interaction of highly variable bathymetry in the Macquarie region (a pattern of ridges and deeps along the plate boundary, reactivated oceanic fracture zones and seafloor spreading fabric), with circulating water masses (east-flowing surface, intermediate and deep waters of the Antarctic Circumpolar Current and north-flowing Antarctic Bottom Water), and with deposition of terrigenous and pelagic sediment.

HAWAII MR1 sidescan sonar and bathymetry data and multi-channel seismic reflection profiles from Rig Seismic Cruise 124, combined with existing data allow us to map the distribution of sediment at the plate boundary and to observe its relationship to the basement morphology of the region. Five sedimentary (or erosional/nondepositional) provinces flank the ridge complex: 1) the northwest Macquarie hemipelagic drifts, 2) the current-modified Solander submarine fan complex, 3) the southwest Macquarie (Tasman) manganese nodule province, 4) the Emerald Basin pelagic drift province, and 5) a swath of sediment-free oceanic crust related to the 53.5deg.S passage in the Macquarie Ridge Complex.

We present a map of ocean current circulation, describe the present tectonic and oceanographic setting, and point out implications for regional sediment transport, tectonics and stratigraphy. Water leaving the southern Tasman Sea deposits hemipelagic sediment in the northwest Macquarie region. Strong bottom currents in the Solander trough rework New Zealand-derived turbidites deposited during sea level low stands creating the current-modified Solander fan. Manganese nodules form under Antarctic Bottom Water flowing north in the southwest Macquarie province. Currents deposit pelagic drifts in the Emerald Basin in the lee of the plate boundary ridge system. Opening of the 53.5deg.S passage in the Pliocene caused a major increase in strong ocean current circulation, reworking and removal of sediment from all the sedimentary provinces, and enhanced injection of suspended sediment into the Deep Western Boundary Current east of New Zealand.

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A SEISMIC SURVEY IN ANTARCTICA , PARALLEL SCHEMES FOR SEISMIC MIGRATION AND TARGET ORIENTED VELOCITY ANALYSIS

by

Vikramaditya Sen, Ph.D.

The University of Texas at Austin, 1998

 

Co-supervisors: Paul L. Stoffa and Thomas H. Shipley

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DEPOSITIONAL HISTORY AND RESERVOIR CHARACTERIZATION OF THE NORTHEAST HARDESTY FIELD, TEXAS COUNTY, OKLAHOMA

by

Sunday Kirk Shepherd, M.S.

The University of Texas at Austin, 2000

Supervisor: William L. Fisher

The Northeast Hardesty Field in the Oklahoma Panhandle was developed in the late 1950's and produces oil from the Morrow Formation. The Upper Morrow sandstones are fluvial channel fills that form prolific hydrocarbon reservoirs. However, lack of vertical and lateral continuity, grain size variation, rapid facies changes and diagenesis have hindered development projects.

The Upper Morrow "A" was deposited in an incised valley in response to fluctuating base level. The Mid-continent region was tectonically active from late Mississippian to early Pennsylvanian time. Slight tectonic fluctuations initiated changes in stream gradient. The complex style of deposition from both braided and meandering streams is a result of changes in regional structure during deposition.

Distinctions between braid bars deposited by braided streams and point bars deposited by meandering streams are made based on sedimentary structures observed in cores. Braid bars are commonly massive to cross-stratified or structureless. Common features include pebble sheets, mudstone clasts altered to siderite, and carbonized organic debris. Grain size ranges from coarse to very coarse and pebbly with gravel found at the base. These bars are generally poorly organized and poorly sorted. Point bar deposits are characterized by cross-stratified to planar or ripple lamination. Silty, micaceous drapes are common throughout the deposits. Grain size ranges from silt to medium-grained sand with pebbles found at the base. Point bars are generally well-organized and well-sorted.Deposition can be separated into three stages. Initially, braided streams incised into underlying Morrow shales and braid bars were deposited. Tectonic shifts caused the stream gradient to decrease and more stable, meandering streams deposited point bars over the braid bars or shales. Some cores show evidence of another episode of incision following a tectonic shift and subsequent gradient increase. In the final stage of sand deposition, braided streams incised into the point bars and braid bars were again deposited. Shales encase the incised valley package.

Diagenesis has severely impacted the reservoir quality of the Upper Morrow sandstones in the Northeast Hardesty area. Porosity and permeability have been reduced by cementation and compaction. Natural dissolution of chemically unstable detrital grains and authigenic cements has increased porosity and permeability in the reservoir. Production data and petrographic examination indicate the presence of clays that fill pores and clog pore throats, reducing permeability. Although the Upper Morrow "A" sands appear to be attractive reservoirs, diagenesis has drastically reduced reservoir quality.

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FACIES ARCHITECTURE AND DEPOSITIONAL HISTORY OF THE UPPER MIOCENE GENETIC SEQUENCE, SOUTH MARSH ISLAND AREA, GULF OF MEXICO

by

Nazim Ozgur Sipahioglu, M. S.

The University of Texas at Austin, 2000

SUPERVISOR: William E. Galloway

Deposition of the Upper Miocene genetic sequence, which was supplied byCentral and East Mississippi Rivers began following a flooding event datedby the Textularia W LAD, and ended with the regional flooding recorded by the Robulus E biostratigraphic top. Three depositional subsequences, which were bounded by four flooding surfaces, were recognized in the UpperMiocene of South Marsh Island Area, offshore Louisiana, Gulf of Mexico, using a 3-D seismic data set and 41 digital well logs. The interval studied is characterized by a fluvial-dominated delta that prograded the North Central Gulf of Mexico margin through Late Miocene time. The lower subsequence contains isolated prodelta - distal delta front facies with thick shale breaks. The middle subsequence lies within a series of proximal delta front and delta plain silts and sands. Thick, sharp-based, blocky incised valley fill is very characteristic at the top. The upper subsequence contains delta plain facies that grade into aggradational fluvial facies with crevasse splays. An incised valley marks the Messinian sea level fall. Syndepositional listric growth faults, which are typical of progradational margins, are abundant. Each facies association displays characteristic seismic reflection attribute and distribution. Prodelta and delta front deposits of the lower subsequence display low to medium amplitude, moderately continuous and parallel reflectors that become less continuous and subparallel towards south. Proximal delta front and plain facies observed in the middle subsequence are characterized by moderate to high amplitude reflectors, which are mostly parallel and continuous. The delta plain of the upper subsequence display high to medium amplitude reflectors that are moderately continuous and parallel to subparallel. The fluvial facies in the same subsequence typically exhibit variable to low amplitude reflectors that are discontinuous and poorly parallel.The depositional episodes were related to several sea level curves. In the long term, a regressive cycle characterized the depositional episode. In the short term, however, the oxygen isotope curve displays the best match between rise and highstand phases and the flooding surfaces.

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EVOLUTION OF HYPOGENE VEINING AND ASSOCIATED ALTERATION IN THE TYRONE PORPHYRY COPPER DEPOSIT, GRANT COUNTY, NEW MEXICO

Virginia Pon Smith, M. S.

The University of Texas at Austin, 1998

Supervisor: J. Richard Kyle

Recent deep drilling and mining exposures now make it possible to study the evolution of hypogene veining and associated alteration at the Tyrone porphyry copper deposit. Laramide (56-53 Ma) monzonitic and dioritic stocks emplaced within Precambrian granitoid rocks are host to porphyry-style copper sulfide mineralization. Petrographic examination of core and open pit rock samples utilizing cross-cutting vein age relationships indicates five major textural styles of hypogene mineralization and alteration.

Early constructive K-silicate vein assemblages containing quartz +/- biotite +/- molybdenite +/- magnetite +/- K-feldspar +/- calcite +/- minor pyrite +/- chalcopyrite are associated with minor K-feldspar and quartz + sericite (illite) + chlorite wall rock alteration. Middle-stage destructive K-silicate assemblages are dominated by coeval quartz + pyrite +/- chalcopyrite +/- sphalerite +/- trace bornite and quartz stockwork veins with texturally destructive quartz + sericite +/- chlorite alteration halos. Late vein assemblages contain vuggy quartz +/- pyrite +/- sphalerite +/- galena +/- calcite that generally lack alteration halos, but which are spatially and temporally associated with selectively pervasive and vein-controlled propylitic (chlorite +/- epidote +/- sericite +/- calcite) alteration. The above alteration styles are overprinted by late-stage argillic (smectite, kaolinite and illite) alteration.

Stable oxygen and hydrogen isotopic analyses of quartz and illite alone cannot provide a conclusive evolutionary fluid model. However, the data are consistent with a model in which the hypogene system evolved from a predominantly magmatic hydrothermal system during early constructive K-silicate alteration (18O/16O = +5 per mil (SMOW) at ~400 ºC) toward a mixed magmatic-meteoric hydrothermal system during later destructive K-silicate alteration (18O/16O = -1 to + 3 per mil, D/H = -35 to -65 per mil (SMOW) at ~300 ºC).

The close spatial association of chalcopyrite with destructive K-silicate alteration and fluid inclusion petrography suggest that chalcopyrite precipitation resulted from the dilution and cooling of copper-bearing magmatic fluids through interaction with meteoric fluids. This model is in contrast to classic genetic models that propose the bulk of chalcopyrite mineralization is related to constructive K-silicate alteration.

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INTEGRATED DIAGENETIC MODELING AND RESERVOIR QUALITY ASSESSMENT AND PREDICTION OF THE ÁGUA GRANDE SANDSTONES, EARLY CRETACEOUS, RECÔNCAVO BASIN, NORTHEAST BRAZIL

INTEGRATED DIAGENETIC MODELING AND RESERVOIR QUALITY ASSESSMENT AND PREDICTION OF THE ÁGUA GRANDE SANDSTONES, EARLY CRETACEOUS, RECÔNCAVO BASIN, NORTHEAST BRAZIL

by
Rogerio Schiffer Souza, Ph.D.
The University of Texas at Austin, 1999

Supervisor: Earle F. McBride

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THE NATURE AND CAUSES OF BACKSTOP DEFORMATION IN THE NORTHERN LESSER ANTILLES SUBDUCTION ZONE

by

John Stephen Stachowiak, M.S.

The University of Texas at Austin, 2000

SUPERVISORS: Nathan Bangs and Paul Stoffa

Investigation of deformation mechanics in the northern Lesser Antilles subduction zone has, until recently, focused primarily on the pronounced variations in mechanical properties developing across the deformation front and the toe of the accretionary wedge.As a result, the major contrast in mechanical properties that exists between the island-arc crust, which forms the backstop to the accretionary wedge, and the accretionary wedge itself, has largely gone unstudied.In particular, the role of the backstop in forearc mechanics and the fate of sediments subducting beyond the toe of the accretionary prism have not been well discerned.A seismic experiment completed in 1998 was designed to address these two issues by imaging the Lesser Antilles subduction zone landward of the outerarc high.

Analysis of the seismic reflection data reveals a seaward-dipping backstop with a morphology consisting of isolated domes and troughs (~15 km in diameter) underlying a forearc basin that spans approximately 60 km and contains up to 7 km of sediment.The localized uplift of the backstop has also caused deformation of the overlying forearc basin sediments.Structural and stratigraphic analysis of this deformation provided indirect insight into the deformation history of the backstop, showing that forearc basin sedimentation commenced prior to deformation and thus records the distribution and relative timing of the backstop deformation, uplift of the backstop has been a gradual process, uplift is currently ongoing and, deformation has recently shifted from near the center of the forearc basin to the basin's eastern margin.

The three most likely backstop uplift mechanisms are the subduction of a basement ridge, interplate friction and sediment underplating. Based on the timing of backstop uplift (gradual, continuous) and the distribution of deformation (localized doming), the former mechanisms were precluded from being the primary cause of backstop uplift. Sediment underplating, however, can account for the observed deformation. Support for underplating as a primary backstop deformation mechanism is derived from: 1. A subducting sediment analysis, which showed that, on average, ~500 m of sediment reaches the backstop, providing sufficient material for underplating to occur and 2. A mass balance calculation suggesting that only about 5-10% of the subducting sediment would need to be underplated in order to produce the observed uplift.

A model of deformation mechanics driven by sediment underplating beneath the backstop is proposed, incorporating all the foregoing observations and conclusions.The importance of this underplating model lies in its relevance to a margin type that is globally very common: convergent margins with a high sediment influx. The contribution of sediment underplating to backstop deformation probably surpasses that of subducting basement ridges, which are relatively rare and episodic. In addition, it may be as important, or more important, than interplate coupling, because of it pervasiveness. Sediment underplating plays a crucial role in the mechanics of a subduction zone that deserves further investigation at this and other convergent margins around the world.

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STRUCTURE AND KINEMATICS OF THE PLOMOSA PASS AREA, NORTHERN PLOMOSA MOUNTAINS, ARIZONA

by

Theodore Richmond Steinke, MA

The University of Texas at Austin, 1996

Supervisor: Sharon Mosher

The Plomosa Pass area of the northern Plomosa Mountains contains exposures of deformed Paleozoic and Mesozoic metasedimentary and metaigneous rocks that are part of the Maria Tectonic Belt which trends west-northwest from west-central Arizona into southeastern California. The Maria Tectonic Belt was primarily developed during Mesozoic basement-involved crustal shortening and is inferred to have been located near the Mesozoic margin of the North American craton. Detailed (1:3000 scale) geologic mapping and structural analysis of the approximately 6 km2 Plomosa Pass area reveal a complex structural history. The southwestern section of the map area is occupied by a weakly to strongly foliated crystalline igneous complex. An intrusive relationship with upper Paleozoic and lower Mesozoic metasedimentary units, as well as regional similarities, suggest that this complex be correlated with the Jurassic magmatic-arc-related Kitt Peak-Trigo Peaks super-unit. A 166.4+/-1.0 Ma crystallization age (U/Pb zircon) obtained during this study for a granodiorite sample from the igneous complex confirms this new interpretation. The Jurassic igneous complex is locally deformed by the southwest-dipping Deadman thrust mylonite zone, a heterogeneous band of anastomosing mylonitic foliation that ranges in thickness from approximately 10 m to 500 m. Structural and petrographic analysis of the mylonitic fabric indicates that it formed as a result of top-to-the-northeast, reverse-sense shear under low temperature crystal-plastic deformation conditions.

The footwall of the Deadman thrust mylonite zone contains greenschist-facies metasedimentary rocks equivalent to the upper Paleozoic section of western Grand Canyon that record multiple phases of pre-mid-Tertiary ductile deformation. Four phases of deformation (D1-D4) are identified. The first phase of deformation (D1) resulted in outcrop- to map-scale isoclinal folds (F1) of compositional layering with a continuous axial planar foliation (S1). The second phase (D2) is expressed in outcrop- to map-scale recumbent isoclinal folds (F2) that fold S1 and refold F1 folds. Extreme attenuation accompanied F2 folding in the Supai Group and Kaibab Formation. Locally, D1 and D2 structures are reoriented by a series of east-verging folds (F3) with axes trending roughly north-south. D4 deformation refolded these previous fold generations by a series of asymmetric northwest- or southeast-plunging, and in some cases, doubly plunging, folds (F4) with moderately inclined, southwest-dipping axial planes. D4 also imparted a shear kink-band fabric. Facing direction of F4 folds is variable and related to stratigraphic repetition by F2 folding, but asymmetry suggests F4 folds verge to the northeast. Geometric relationships between structures and fabrics in the Deadman thrust mylonite zone to those in the footwall indicate that D4 structures are caused by progressive, northeast-vergent shortening related to shear along the Deadman thrust mylonite zone and that minor F2 folds near the zone have been passively rotated towards the transport direction during top-to-the-northeast D4 shear. All ductile and brittle-ductile fabrics and fold phases in the footwall of the Deadman thrust mylonite zone have been truncated by the intrusion of the 20.4+1.8-2.1 Ma Mudersbach granite (U/Pb zircon, personal communication, Mark Martin, 1996). The area has been cut by the Plomosa detachment fault, a mid-Tertiary, low-angle normal fault associated with the Colorado River extensional corridor. Contrary to previous reports, pre-mid-Tertiary structures and fabrics in the Plomosa Pass area are located in the footwall block of the warped Plomosa detachment fault and did not experience mid-Tertiary hanging wall block rotation. The curved nature of the Plomosa detachment fault is most likely an original feature. Synchronous granitoid intrusions such as the Mudersbach granite interacted with detachment faulting in the Plomosa Pass area to produce areas of metasomatism, contact metamorphism, and Fe and Cu mineralization.

Lack of age constraints and uncertainty of original orientation and vergence directions of D1 and D2 deformation phases in the Plomosa Pass area make correlation to regional deformational events difficult. Style, conditions, and relative timing of D1 and D2 are consistent with pre-Laramide-age, Mesozoic deformation in the rest of the Maria Tectonic Belt. The local and regional significance of D3 structures remains unknown. The most likely candidate for regional correlation of D4 in the Plomosa Pass area is the Late Cretaceous to early Tertiary north- to northeast-directed contractional Laramide-age deformation that is regionally represented by the Mule Mountains thrust system and northeast-verging refolds of the southwest-facing Big Maria syncline. D4 structures in the Plomosa Pass area are likely part of a larger belt of Laramide-age, mostly northeast-verging, contractional deformation that extends from southeastern California into northern Mexico.

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CLIMATIC AND TECTONIC CONTROLS ON QUATERNARY EOLIAN SEDIMENTARY SEQUENCES OF THE CHOTT RHARSA BASIN, SOUTHERN TUNISIA

by

Christopher Stephen Swezey, Ph.D.

The University of Texas at Austin, 1997

Supervisor: Gary Kocurek

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PRESTACK SPLIT-STEP FOURIER DEPTH MIGRATION ALGORITHMS AND PARALLEL IMPLEMENTATIONS ON CRAY T3E

by

Mehmet Celaleddin Tanis, Ph.D.

The University of Texas at Austin, 1998

 

Supervisors: Paul L. Stoffa and Mrinal K. Sen

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HYDROGEOLOGIC CHARACTERIZATION OF FRACTURED CARBONATE AQUIFERS EMPLOYING GROUND-PENETRATING RADAR

HYDROGEOLOGIC CHARACTERIZATION OF FRACTURED CARBONATE AQUIFERS EMPLOYING GROUND-PENETRATING RADAR

by

Georgios Padelis Tsoflias, Ph.D.
The University of Texas at Austin, 1999

Supervisor: John M. Sharp, Jr.

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PALEOECOLOGY AND DIAGENESIS OF THE LOWER CAPITAN REEF SLAUGHTER CANYON, GUADALUPE MOUNTAINS, NEW MEXICO

By

Courtney Hanna Turich, M.S.

The University of Texas at Austin, 2000

SUPERVISOR: Brenda Kirkland

This study is part of a larger project to identify variations in the nature of the Capitan reef through time. The goal is to document variations with depth in faunal diversity, fabric development, and diagenetic history in the lower Capitan reef. Samples were collected from the Massive Member of the lower Capitan Formation near the Goat Cave Trail in Slaughter Canyon at 5 m intervals along a 90 m transect.

In the lower Capitan reef, sponge diversity is consistently low, but rock fabric does show depth-dependent trends. At depths 10 to 15 m below the outer shelf-reef contact, millimeter-scale laminae and aragonite botryoids dominate the samples. At depths greater than 15 m below the outer shelf/reef transition, micrite and internal sediment are dominant. On average, marine cements comprise 8% of thin sections, ranging from 0-36%. Also, fractures filled with siliciclastic material are common.

Carbon and oxygen stable isotope analysis of fossil sponges, sediment, and spar reveal a co-varying linear trend (R2=0.9). With increasing depth below the outer shelf-reef contact, 13C and 18O of sparry calcite increases (d13C= 3.55 to +3.59 and d 18O = -9.58 to 2.47. Fossil sponges, originally aragonitic, show a similar trend. Isotopic compositions of micrite and internal sediment do not vary with depth.

Meteoric fluids penetrated the reef-rock and precipitated sparry calcite. With increasing depth below the transition, rock-water interaction enriched diagenetic fluids, and isotopically heavy sparry calcite precipitated.

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THE OSTEOLOGY OF SYNTARSUS KAYENTAKATAE AND ITS IMPLICATIONS FOR CERATOSAURID PHYLOGENY

by

Ronald Stephen Tykoski, M.S.

The University of Texas at Austin, 1998

SUPERVISOR: Timothy Rowe

Our knowledge of early Mesozoic theropods is poor, limited by the fragile nature of their skeletons and the small size of many taxa. A handful of specimens provides a nearly complete picture of the osteology of the ceratosaur Syntarsus kayentakatae, and a detailed description of the skeleton is given here. The skull possesses greater pneumaticity than previously recognized in such an early theropod, and a furcula, the first recognized in a ceratosaur, is also identified. The Shake-N-Bake fossils, previously thought to represent juveniles of S. kayentakatae, are actually a new small species of ceratosaur. This conclusion is supported by extensive ossification and fusion in elements throughout the skeleton. New data gathered from S. kayentakatae, the Shake-N-Bake species, and literature are used to conduct a phylogenetic analysis of Ceratosauria. The results agree in most respects to previous hypotheses of ceratosaurid phylogeny. A monophyletic, dichotomous Ceratosauria is supported. The Shake-N-Bake species lies outside Coelophysidae, but closer to Coelophysis bauri than to Dilophosaurus wetherilli.

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DELINEATION OF REGIONAL GROUNDWATER FLOW PATHS AND THEIR RELATION TO REGIONAL STRUCTURAL FEATURES IN THE SALT AND TOYAH BASINS, TRANS-PECOS TEXAS

By

Matthew Martin Uliana, Ph.D.

The University of Texas at Austin, 2000

Supervisor, John M. Sharp, Jr.

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PALEOCENE-EARLY MIDDLE EOCENE SEQUENCE STRATIGRAPHY AND LITHOSTRATIGRAPHY IN THE RIO GRANDE RIVER VALLEY, TEXAS

By

Geoffrey William Upitis, M. S.

The University of Texas at Austin, 1998

SUPERVISOR: William E. Galloway

Paleocene through early Eocene strata in Webb and Zapata counties, Texas, can be subdivided into four 3rd order genetic stratigraphic sequences: (1) sequence A begins at the Cretaceous-Tertiary boundary and is bounded above by the lower Wilcox unconformity, (2) sequence B is bounded below by the lower Wilcox unconformity and is capped by the Tilden Shale, (3) sequence C includes strata of the middle and upper Wilcox subgroups and is bounded by maximum flooding surfaces associated with the Reklaw 1 condensed section at the top and by the Tilden Shale at the bottom, and (4) sequence D contains strata of the Queen City Formation and is bounded by maximum flooding surfaces at the base and top of the sequence. Four to six 4th order sequences were identified within each of the 3rd order sequences, and three to twelve 5th order parasequences were interpreted within the 4th order sequences.

The genetic stratigraphic sequences can be internally subdivided into progradational, aggradational and retrogradational parasequence sets delineated by minor flooding surfaces. Progradational and retrogradational packages represent alternating highstand and transgressive system tracts.

Four facies associations are defined from field and well log analysis: (1) shelf, (2) shoreface, (3) coastal plain, and (4) lagoon. The shelf facies association occurs within the most basinward extensions of the Queen City, Wilcox and Midway Formations in the subsurface, and the Laredo Formation in outcrop. Shoreface facies are the thickest and most abundant in the Middle and Upper Wilcox and also occur as massive stacked and amalgamated deposits in the Queen City Formation in the subsurface and are associated with the Indio through Laredo formations in outcrop. Coastal plain facies association and lagoonal facies are found in the lower El Pico Formation in outcrop and in the upper Wilcox and Queen City formations in the subsurface.

Depositional systems in Webb and Zapata counties are best represented in outcrop and subsurface by a series of prograding wave-dominated deltas and transgressive shore-zone systems with associated shelf, shoreface, lagoon, and coastal plain facies associations. The Lobo Member of the lower Wilcox subgroup, the upper Wilcox subgroup, and the Queen City Formation represent periods where the shoreface progradation was greatest, with minor sediment pulses occurring in the middle Wilcox subgroup.

This study provides a depositional model for Paleocene-Eocene strata of the North Western Gulf of Mexico basin, and a useful sequence stratigraphic guide for petroleum exploration in Webb and Zapata counties.

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STRUCTURE AND TECTONICS OF THE PUERTO RICO-VIRGIN ISLANDS PLATFORM AND MULTI-CONFIGURATION GROUND PENETRATING RADAR DATA

by

Jean-Paul Van Gestel, Ph.D.

The University of Texas at Austin, 2000

Supervisor: Paul L. Stoffa

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GEOMETRIC ANALYSIS OF THE REED WASH DETACHMENT FOLD TRAIN, WEST FLANK OF SAN RAFAEL SWELL, UTAH

 

Vickye Rocío Vélez, M.S.

The University of Texas at Austin, 2000

Supervisor: Randall A. Marrett

Geometric fold models and methods to calculate depth to detachment play a very important role for structural interpretations in areas with limited data and for construction of balanced cross sections. However, the applicability of these models and methods remains largely untested against well-constrained natural folds. The well-exposed Reed Wash detachment fold train located in central Utah was used in this study to test the applicability of detachment fold models and methods to calculate depth to detachment. Evaluation of the geometric and kinematic implications of detachment fold models is particularly important because detachment fold geometry is less completely constrained by concepts of balance than are other fold types, so that there are multiple end-members of equally viable evolutions.

The Reed Wash area presents a unique opportunity to study detachment folds because it contains a spectacular fold train, completely exposed from floor to roof detachment, of ~10 m amplitude folds across more than 100 consecutive anticlines. Folds at all stages of growth are present, so a picture of the fold development can be assembled. This fold train was developed in the Middle Jurassic Carmel Formation, confined within a 20 m interval characterized by intercalation of siltstones and gypsums. The detachment folds may represent the tip of a triangle zone at the leading edge of the thin-skinned Sevier fold-thrust belt.

The fold train was surveyed using a total station to obtain ~4300 control points in 3D for the fold geometry at five different horizons on 64 anticlines. In addition, 40 control points were surveyed to constrain the geometry of the detachment surface in 3D. All data were projected parallel to fold axes into a profile-plane section for geometric analysis. Qualitative field observations coupled with these data were used for quantitative analysis of the folds in cross section.

The anticlines of the Reed Wash area have consistent fold axes and layer thicknesses, indicating that the folds have a dominantly cylindrical and parallel geometry. Consequently, the layer-parallel strain model cannot explain the formation of these folds with the possible exceptions of the lower and upper decollement layers, because this model relies primarily on thickness changes of the folded layers. Interlimb angles systematically decrease from east to west and arc lengths systematically increase from east to west. The geometric fold models can be tested by assuming that these systematic lateral variations reflect the progression through time of folding. The migrating-hinge model can explain the variable arc lengths but not the systematic interlimb angle changes. The rotating-limb model can explain the variable interlimb angles but the systematic arc length changes are problematic. Additionally, the standard rotating-limb and migrating-hinge models implicitly require variable lower detachment depths, but this is inconsistent with independent observations in the Reed Wash area. Although neither migrating-hinge nor rotating-limb model can explain all observations, a hybrid model embracing both of these processes (limb rotation and hinge-migration) may best explain the evolution of the Reed Wash fold train. The two processes must have acted synchronously or alternated repeatedly, and both processes initiated early in fold growth. Limb rotation ceased as the folds became isoclinal. Hinge migration began at the outset of folding or shortly after and persisted throughout folding.

Evaluation of four methods to calculate depth to detachment shows generally high variability and different degrees of accuracy for the Reed Wash fold train. The depths to detachment estimated using the variable detachment depth method (Homza and Wallace, 1995) and the new mobile syncline method (Marrett, 1997, unpublished document) are more consistent with independent constraints on the real position of the lower detachment than are the depths obtained from the traditional and modified excess-area methods.

The traditional excess-area method yields erroneous values of depth to detachment for many individual folds in Reed Wash. The modified excess-area method generally failed at adjusting the conventional calculations to more consistent depths to detachment. The variable detachment depth method has the shortcoming of low predictive capability, because it requires a priori knowledge of the initial thickness of the decollement layer. Finally, the new mobile syncline method appears to be the most predictive method and provided reasonably accurate results, however more work is necessary to further develop and standardize this methodology.

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PETROLEUM GEOLOGICAL DATA MANAGEMENT

Master's report by

Cengiz Tolga Vur, M.A.

The University of Texas at Austin, 1999

Supervisor: William Fisher

Geologists collect large volumes and various types of data during their research from paper to digital format. The basic importance of data management is to extract information easily and correctly. There is no difference between a geologist's field book and a huge digital database system in terms of gathering and managing information. If the collected data are logically related with some certain meaning, it is always a database. In the last two decades, some commercial petroleum companies and research institutes have been trying to build large and complex digital databases to support their activities. In this report, you will see an integration of relational database management for petroleum geology applications. Today, there are many powerful programs that deal with them, but the logic is to know how to manage the geologic data from paper copy to digital. Even if there is a perfect database, before synthesizing the interpretations and geological information of any kind of geoscience disciplines, the main issue is to manage data among numerous formats, applications, and platforms. These are the biggest obstacles for the geoscientist. Recent studies show that highly trained geoscientists may spend most of their time in moving, reformatting and loading, but only 20% of that in analyzing and interpreting the data. That is why data management is an important issue in geoscience and petroleum related work. There are many commercial programs and environments. Landmark's OpenWorks environment is one of these and is used an example to show how petroleum geological data should be used properly in management systems.

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MECHANICS OF GRABEN EVOLUTION IN THE NEEDLES DISTRICT OF CANYONLANDS NATIONAL PARK, UTAH

by

Patrick Walsh, M.S.

The University of Texas at Austin, 2000

Supervisors: Randall Marrett and Daniel D. Schultz-Ela

Grabens in Canyonlands National Park provide and example of active salt-tectonic extensional deformation. Downcutting of the Colorado River into evaporites during the last 500,000 years allowed gravity spreading of a 460 m thick package of younger strata down a west-dipping regional slope. Previous workers have suggested that faults initiated and propagated either upward from the salt-sedimentary rock contact or downward from the land surface. Researchers also disagree about the geometry of faults at depth. Proposed explanations for the mechanisms of deformation include brittle plate gliding, salt flow and salt dissolution. I address these issues using new finite element models of graben mechanics and new field observations of fault and layer geometries. My numerical models may indicate how salt flow affects the evolution of the grabens and how faults initiate in the overburden. They may also predict structures that have not previously been observed. Field observations may be used to study large-scale deformational patterns and to validate predictions made by the numerical models. Many aspects of this geologic system may be used as an analog for areas of economic interest where salt influences deformation.

Finite-element model results suggest that erosion generates differential overburden loading that expels salt and causes grabens to form sequentially away from the simulated Colorado River canyon. The combined effects of differential loading and salt flow into the canyon flex the canyon walls upward, which mimics the natural Meander anticline. Upward flexure at the canyon wall combined with adjacent subsidence forms two structures with opposite senses of fault propagation directions; horst-bounding faults near the canyon propagate upward from the salt contact, while graben-bounding faults farther from the canyon propagate downward from the surface. Modeled faults are vertical from the surface down to approximately 100 m. Below that, they dip inward beneath the grabens and converge above the salt contact, as observed in cross-cutting tributary canyons in the field. The models show that continued extension of the overburden leads to the formation of reactive diapirs beneath each graben. The centers of inter-graben horsts flex downward, bounded by uplifted footwalls above reactive diapirs.

Spacing of the newly formed grabens depends on material properties of the overburden and the salt. An increase in overburden stiffness in the models decreases graben spacing. Salt models commonly assume salt to be purely viscous, and scaling arguments then predict viscosity only changes the rate of structural evolution, but not the geometry or stresses. However, the more realistic visco-elastic behavior of salt in my models demonstrates that change in viscosity indeed affects graben spacing. Both increased relaxation time and increased shear modulus increases the resulting graben spacing.

Field observations showed that multiple faults commonly bound each side of individual grabens. Surveyed points along a stratigraphic layer show more than 40 m of differential horst subsidence occurred in the most deformed area. Data suggest that during the area's evolution individual horst blocks flexed, rotated and subsided relative to their regional positions. Horst block flexures vary along strike, changing the relative asymmetry of individual graben. Anticlines that are transverse to grabens occur locally. Similarities between surveyed horst geometry points and model geometry indicate that salt flow from beneath the overburden may allow the observed horst flexures and subsidence. The local nature and the orientations relative to grabens of the transverse anticlines may indicate that they formed in response to abrupt displacement variations in graben-bounding faults.

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EMPLACEMENT OF THE IRIAN OPHIOLITE AND UNROOFING OF THE RUFFAER METAMORPHIC BELT OF IRIAN JAYA, INDONESIA

by

Richard J. Weiland, Ph.D

Supervisor: Mark Cloos

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DELINEATION OF A MESOPROTEROZOIC MAGMATIC ARC IN THE LLANO UPLIFT, CENTRAL TEXAS: EVIDENCE FROM ND ISOTOPES

by

Christopher Peter Whitefield, M.A.

The University of Texas at Austin, 1996

Supervisor: James N. Connelly

The Llano uplift of central Texas represents a metamorphic core containing the imbrication of three distinct lithotectonic units that include, from structurally highest to lowest and from south to north, the Coal Creek (CCD), Packsaddle (PSD), and Valley Spring (VSD) domains. Parts of the VSD possibly represent juvenile crustal additions to the southern margin of the North American craton and may be an extension of the Granite-Rhyolite terrane. The PSD represents an intensely deformed supracrustal sequence that was proximal to the VSD, because both domains were intruded by an apparently cogenetic suite of granitoids. The CCD consists of a weakly foliated igneous suite dominated by compositions ranging from tonalite to granodiorite that intrudes older gneisses. The northern boundary of the CCD is marked by the Sandy Creek shear zone (SCSZ), a kilometer-scale mylonite zone containing granites and amphibolites with a pervasive NW to NE trending fabric that dips to the S. A striking paradox is apparent when comparing Sm-Nd isotopic results from the CCD with data from the SCSZ and exposures to the north. The more mafic lithologic assemblage of the CCD has a more evolved initial [epsilon]Nd signature than the more compositionally evolved granitic rocks to the north that possess a juvenile [epsilon]Nd signature.

Twenty-seven Sm-Nd analyses from this study were combined with published data from Patchett and Ruiz (1989). The combined data set defines fundamental differences between the CCD and the rest of the uplift. Epsilon Nd values at the time of crystallization (initial [epsilon]Nd) from the CCD range from +1.2 to +4.1 with depleted mantle ages ranging from 1344 to 1679 Ma. Exposures to the north have a more juvenile initial [epsilon]Nd signature that ranges from +2.4 to +6.1 with depleted mantle ages ranging from 1177 to 1545 Ma. The granitoid rocks of the SCSZ yielded the most juvenile [epsilon]Nd signature found thus far in the uplift. The southern margin of the SCSZ represents the northern extent of the CCD isotopic signature. The more evolved isotopic signature of CCD implies that its petrogenesis involves an older crustal component. The origin of this older crustal component remains elusive; however, its absence from the other rocks of the uplift suggests that the CCD evolved separately from the uplift.

All of the late granitic intrusive rocks have a juvenile [epsilon]Nd signature indicating the involvement of a depleted mantle source. The granites that intrude the CCD commonly have a more evolved [epsilon]Nd signature, indicating that the granite melts incorporated portions of the country rock.

The mylonitic fabric of the SCSZ is parallel to the weak foliation exhibited within the CCD. Both fabrics appear to have been formed during the same event (D2: the first common event); however, the different lithologies of the two areas resulted in distinctly different responses to deformation. Analysis of the fabrics in the SCSZ has revealed a thrust sense of shear with top-to-the-north movement. An extensional crenulation cleavage cuts the D2 mylonitic fabric, suggesting that down-to-the-south extension followed contractional deformation.

Based upon the isotopic criteria and field evidence, the southern margin of the SCSZ represents a fundamental lithologic, structural, and isotopic boundary between the CCD and the rest of the uplift. The CCD evolved separately from the Llano uplift and its accretion is thought to have occurred along the SCSZ, when the magmatic arc overrode the southern margin of the North American craton during continental orogenesis.

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BACKGROUND VELOCITY ESTIMATION, AVO INVERSION AND FULL WAVEFORM INVERSION FOR HORIZONTALLY STRATIFIED MEDIA: A DIVIDE AND CONQUER APPROACH

by

Ganyuan Xia, Ph.D.

The University of Texas at Austin, 1997

Supervisors: Paul L. Stoffa and Mrinal K. Sen

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USE OF OUTCROP FRACTURE MEASUREMENTS TO ESTIMATE REGIONAL GROUNDWATER FLOW, BARTON SPRINGS SEGMENT OF EDWARDS AQUIFER, CENTRAL TEXAS

Christopher Kent Zahm, M.S.

The University of Texas at Austin, 1998

Supervisors: John M. Sharp and Randall Marrett

This thesis is the presentation and application of new techniques that use fracture data collected from outcrop to make estimates of regional-scale permeability, permeability anisotropy and fracture roughness in the Barton Springs segment of the Edwards aquifer of central Texas. Calculations were compared to measurements of permeability determined from well tests, permeability anisotropy derived from dye tracers and fracture roughness measurements determined from fracture surfaces at similar outcrops.

Fracture permeability was determined by collecting fracture apertures measurements along transect lines oriented perpendicular to the predominant fracture strike in four outcrop locations. Each outcrop is a large pavement of carbonate bedding-plane exposure located in the streambed of Barton Creek. The sampled apertures were used to generate frequency-size distribution plots. Segments of the data that could be fitted with a straight line and that were not affected by censoring biases were considered to be self affine and were described using power-law equations. Fractal modeling was used to estimate the permeability contributions for the censored data while the remaining data was used to calculate permeability of the effectively measured apertures. Using this technique, fracture permeability at the outcrop and regional scale was determined to be on the order of 10-10 m2. This permeability is similar to permeabilities determined from well tests in the Edwards aquifer. Estimates of permeability only considered fracture aperture and did not include the effects of connectivity, fracture roughness and/or channeling.

A newly introduced fracture anisotropy factor used outcrop measurements of fracture length and orientation to predict regional permeability anisotropy. Fracture maps were generated at scales of 4 m2, 100 m2 and 100 km2. The fractures patterns have similar attributes of fracture length and orientation at all three scales. These maps were used to develop length-weighted rose diagrams of fracture segments that quantify the distribution of the fracture orientations. The rose diagrams illustrate similar mean orientation and fracture intensity at the scales considered. Normalization of the petals of the rose diagram was done to determine the newly introduced fracture anisotropy factor (f). Using f with the cubic law, groundwater flow vectors were estimated. This technique provides a tool to predict regional groundwater flow vectors from outcrop measurements of fractures a priori to drilling wells.

Fracture roughness factor was estimated using a hydraulic aperture that was calculated from stream loss data. The fracture roughness factor may account for an average of 47% frictional losses in fluid flow and can be used in a modified cubic law to determine discharge and velocity of groundwater through fractures.

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DEPOSITIONAL MODEL AND SEQUENCE STRATIGRAPHIC FRAMEWORK FOR UPPER ALBIAN/LOWER CENOMANIAN CARBONATE RAMP, WESTERN COMANCHE SHELF, TEXAS

by

Laura Cutright Zahm, M.A.

The University of Texas at Austin, 1997

 

Supervisors: B. Kirkland-George and C. Kerans

Part 1: Facies distributions and sequence stratigraphic framework of the upper Albian/lower Cenomanian, Segovia Formation

This study documents the sequence stratigraphy of the upper Albian/lower Cenomanian (Lower Cretaceous) Segovia Formation (360 ft/110 m thick) along a transect from an inner ramp crest (Comanche Platform) to intrashelf basin (Fort Stockton Basin) in Crockett and Pecos Counties, West Texas. Undeformed, flat-lying ramp strata of the Segovia Formation were described in roadcuts extending 40 miles along Interstate-10, between Ozona and Iraan, Texas. Correlations are aided by a laterally extensive and visually distinctive clay horizon interpreted as a soil horizon. Based on the examination of the exposed outcrops, the Segovia Formation can be subdivided into six high-frequency sequences (approximately 500 ky duration) on the basis of facies distributions and cycle stacking patterns.

Sequences consist of a transgressive svstems tract (TST) and an overlying highstand systems tract (HST), which together comprise an upward-deepening followed by upward shallowing succession of cycles from below fair-weather wave base to above fair-weather wave base. Toucasid mounds, fine-grained peloidal skeletal wackestone to packstone, Thalassinoides burrowed-argillaceous wackestone, and Gryphaea Wackestone stack vertically to define meter-scale, retrogradational stacking of cycles within the TST. Cycles of the TST are interpreted to represent higher accommodation and deeper water depths on average per cycle. The ramp crest HST is dominated by current-stratified, caprinid grainstone that averages 30 ft (10 meters) thick and extends 40 miles laterally in a shelf to basin direction. Grain size, sorting, and stratification types within the shoal complex are used to define the inner ramp crest, ramp crest, middle ramp, outer ramp, and distal ramp facies tracts. The HST ramp crest grainstone contains large, in situ caprinids and represents deposition in the shallowest water depths. These facies reflect deposition within fair-weather wave base, which in turn reflects low accommodation and progradation of the ramp margin.

The sequence stacking patterns of the Segovia Formation allow a detailed understanding of facies distribution along the ramp margin trend in terms of facies continuity, geometry, and thickness throughout the evolution of the Fort Stockton intrashelf basin.

 

Part 2: Cyclostratigraphic and Ichnofacies Analysis of the Upper Albian Salmon Peak Formation, Maverick Basin, Texas

The Upper Albian Salmon Peak Formation consists of hemipelagic intrashelf basin deposits in the Maverick Basin of south Texas. It is positioned behind the main Stuart City Reef trend and surrounded landward (north and west) by high-energy carbonate ramp margin facies of the Devils River Limestone. The Salmon Peak Formation is overlain by Del Rio Shale and underlain by McKnight Formation. The top of the Salmon Peak Formation corresponds approximately to the Albian/Cenomanian boundary. The Salmon Peak is continuously cored in the International Boundary and Water Commission core ID22 from Val Verde County, near Del Rio, Texas. Integrated biostratigraphic, sedimentologic, and time-series analysis of this core allows refinement of both biostratigraphic and cyclostratigraphic histories of this Upper Albian carbonate platform-to-basin system.

The 116 meter (380 feet) thick Salmon Peak Formation consists of five lithofacies: (1) structureless mudstone, (2) sparse Skolithos burrowed mudstone to wackestone. (3) Thalassinoides or Planolites-like burrowed organic-rich mudstone to wackestone, and (4) dark laminated organic-rich foraminifera-bearing mudstone, (5) massive coarse-grained pelletal wackestone to grainstone. A three-fold compound cycle hierarchy is recognized in the ID-22 core. Facies 1-2-3 and 2-3-5 high-frequency cycles alternate with more organic-rich, starved 2-3-4 high-frequency cycles to define intermediate-scale cyclicity. Long term stacking records a lower asymmetric retrogradational-progradational cycle (middle McKnight through lower Salmon Peak) and an upper cycle dominated by retrogradation and decreased oxygenation in the Maverick Basin.

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SORPTION IN A MODEL FRACTURE SYSTEM

by

Mitchell Donald Zimmerman, M.S.

The University of Texas at Austin, 1997

Supervisor: Philip C. Bennett

Fracture "skins" are alteration zones on fracture surfaces created by chemical and physical weathering processes. These skins dramatically increase surface area where sorption can occur, as compared to the unaltered rock matrix. This study examined the sorption of organic solutes onto altered fracture surfaces in an experimental fracture-flow apparatus.

Fracture skin samples containing abundant metal oxides, clays, and organic material from the Breathitt Formation (KY) were collected in a manner such that skin surface integrity was maintained. The samples were reassembled in the lab in a flow-through apparatus that simulated ~2.7 meters of linear fracture conduit. A two tracer injection scheme was utilized with the sorbing or reactive tracer compared to a non-reactive tracer (chloride) injected simultaneously. Sorption was assessed from the ratio of the first temporal moments of the breakthrough curves and from the loss of reactive tracer mass. Sorption was evaluated as a function of flow velocity, temperature, and solute type.

Evaluation of breakthrough curves suggests dual-flow regimes, with sorbing and non-sorbing flow fields. Based on mass loss calculations, significant sorption was found for the reactive components, and sorption increased with decreasing flow rate, decreasing compound solubility, and increasing temperature. Based on moment analysis, however, there was little retardation of the center of solute mass. These data suggest that non-equilibrium sorption processes dominate, with slow desorption and boundary layer diffusion leading to extensive tailing in the breakthrough curves.

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MULTICOMPONENT SEISMIC INTERPRETATION OF THE SECOND WIND FIELD, KIOWA AND CHEYENNE COUNTIES, COLORADO

by

Helena Zirczy, M.S.

The University of Texas at Austin, 2000

Supervisor: Robert H. Tatham

 

The location of the Second Wind field within the Stateline trend in the Colorado-Kansas border region was selected for studying the resolution capabilities of multicomponent seismology for lithology and hydrocarbon discrimination. The study concentrates on the Upper Morrow fluvial and estuarine valley fill sediments lying unconformably on Lower Morrow and Mississippian carbonates. Upper Morrow thicknesses range from 100-150 feet where the erosional valley exists, and the productive, basal sandstone thickness varies from 20 to 50 feet. The structural components include post-Morrow high angle faulting, and the presence of structural depressions, which result from post-Morrowan faults that support dissolution collapse features in the Mississippian carbonate.

From the 4 km2, 3D nine-component seismic survey, P-wave and SH-wave 3D seismic reflection volumes were analyzed. A multicomponent VSP was used to assure the correct tie between the seismic data volumes and the stratigraphic tops. This is especially important for the SH-wave interpretation since shear wave sonic logs are not available. Additionally, the seismic survey area contains four wells, three of which have conventional P-wave sonic logs.

Isochron maps were made from the P and SH seismic volumes and used to calculate Ts/Tp ratios with apparent satisfactory results revealing the presence of possible gas zones, as well as changes in sand-shale lithology within the mapped interval. Detailed seismic amplitude analysis, using also horizon slices, indicates a channel trend defined with the Ts/Tp mapping. Also a possible correlation between wavelet character and qualitative lithology was defined.

Finally, a flat spot representing a gas-water contact was identified where known gas production exists, at well B, with an area of approximately 22 ac, and a second possible flat spot was identified close to a prospective zone, south of well C, with 14 ac in area.

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A STRUCTURAL, PETROLOGIC, AND GEOCHEMICAL INVESTIGATION OF THE VALLEY SPRING GNEISS OF THE SOUTHEASTERN LLANO UPLIFT, CENTRAL TEXAS

A STRUCTURAL, PETROLOGIC, AND GEOCHEMICAL INVESTIGATION OF THE VALLEY SPRING GNEISS OF THE SOUTHEASTERN LLANO UPLIFT, CENTRAL TEXAS

by

Justin Andrews Zumbro, M.S.


THE UNIVERSITY OF TEXAS AT AUSTIN
December, 1999



Supervisor: Sharon Mosher

Recent studies of the Valley Spring Gneiss (VSG) of the southeastern Llano Uplift of central Texas indicate that these gneissic rocks are dominantly of igneous origin and share a common deformational history with the supracrustal rocks of the overlying Packsaddle Schist. The results of this study further constrain the tectonic evolution of the Grenville orogenic belt along the southern margin of Laurentia.
Deformation affecting the VSG is similar in style, orientation, and tectonic transport to that of the Packsaddle Schist. The dominant fabric is S2, which is axial planar to tight to isoclinal folds of S0/S1. Asymmetric augen in mylonites reveal tectonic transport to the NE. The ubiquitous, SE-trending lineation is an intersection lineation parallel to major fold axes. S3 is a well-developed metamorphic fabric that cuts S2 at a low angle. Both S2 and S3 are traceable across the contact between the VSG and the Packsaddle Schist. In the area mapped, S4 is only locally expressed in the VSG as ESE – plunging crenulations. Broad, open, NE-verging F5 folds reorient all previous structures. The only type of structure commonly observed in the VSG that is rare in the Packsaddle Schist is a late extension in two directions producing large-scale, pervasive boudinage. The effects of boudinage are most pronounced adjacent to garnet clinopyroxenite layers.
Field mapping of a 4.5 km x 11.5 km area ~8km SE of Llano and NW of the contact with the Packsaddle Schist in Honey Creek as well as thin section analysis has distinguished five laterally extensive, mappable units of primarily intrusive (?) igneous origin in the uppermost VSG. Compositions are dominantly microcline – rich granitic gneisses with lesser amphibole, biotite , and biotite – amphibole gneisses. These gneisses are in thrust contact with the overlying Packsaddle Schist and are ~20 m.y older. At structurally deeper levels, the VSG becomes a more diverse lithologic unit dominated by metasedimentary rocks with lesser interlayered metaigneous rocks. The oldest dated unit in the uplift that records an older deformational and metamorphic history (Reese, 1995; Reese et al., in review) is laterally extensive and intruded by amphibole and felsic gneisses. Abundant late, syntectonic aplites and pegmatites cut the pervasive schistosities, but are also foliated, at least at the margins of larger bodies.
Garnet clinopyroxenites, apparently representing retrogressed eclogitic rocks, are also preserved with observable contacts. These layers parallel the gneissic layering and major compositional contacts and are extensively boundinaged. The margins of the boudins are foliated parallel to the surrounding gneissic foliation, particularly where severely retrogressed, and in some boudins, contain an older, highly oblique foliation. The nesting of these boudins with those of weakly foliated pegmatites indicates that boudinage was late syntectonic.


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