Virtual Landscapes of Texas

Table of Contents

1. Sedimentary record of Cretaceous and Tertiary salt movement, East Texas basin : Times, rates, and volumes of salt flow and their implications for nuclear waste isolation and petroleum exploration
   1. Introduction
   
   
   2. Data Base
   
   
   3. Early History Of Basin Formation And Filling
   
   
   4. Geometry Of Salt Structures
   
   
   5. Evolutionary Stages of Dome Growth
   
   
   6. Pillow Stage
   
   
   7. Geometry of Overlying Strata
   
   
   8. Geometry of Surrounding Strata
   
   
   9. Depositional Facies and Lithostratigraphy
   
   
   10. Diapir Stage
   
   
   11. Geometry of Surrounding Strata
   
   
   12. Depositional Facies and Lithostratigraphy
   
   
   13. Postdiapir Stage
   
   
   14. Geometry of Surrounding Strata
   
   
   15. Depositional Facies and Lithostratigraphy
   
   
   16. Holocene Analogs
   
   
   17. Formation Of Subtle Petroleum Traps
   
   
   18. Patterns Of Salt Movement In Time And Space
   
   
   19. Group 1 Diapirs: Pre-Glen Rose Subgroup (pre-112Ma)
   
   
   20. Group 2 Diapirs: Glen Rose Subgroup To Washita Group (112 to 98 Ma)
   
   
   21. Group 3 Diapirs: Post-Austin Group (86 to 56 Ma)
   
   
   22. Initiation And Acceleration Of Salt Flow
   
   
   23. Overview Of Dome History
   
   
   24. Rates of Salt Movement And Dome Growth
   
   
   25. Propositions
   
   
   26. Proven Propositions
   
   
   27. Unproven Propositions
   
   
   28. Simplified Propositions
   
   
   29. Distinguishing Between Syndepositional And Postdepositional Thickness Variations
   
   
   30. The Problem
   
   
   31. Structural Evidence
   
   
   32. Sedimentological Evidence
   
   
   33. Methodology
   
   
   34. Distinguishing Between Regional and Salt-Related Thickness Variations
   
   
   35. Calculating Volumes of Salt Mobilized and of Salt Lost
   
   
   36. Rates Of Dome Growth
   
   
   37. Net Rates of Pillow Growth
   
   
   38. Net Rates of Diapir Growth
   
   
   39. Gross Rates of Diapir Growth
   
   
   40. Growth Rates And Strain Rates
   
   
   41. Implications For Waste Isolation
   
   
   42. Conclusions
   
   
2. Illustrations
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   6. FIGURE 1. Location map showing the East Texas Basin, Gulf Coast Basin, inland salt-diapir provinces, and salt domes and massifs. (After Martin, 1978.)
   
   
   7. FIGURE 2. Index map of the East Texas Basin showing cross-section lines and locations of logged wells used in interval isopach mapping.
   
   
   8. FIGURE 3. Stratigraphic column of the East Texas Basin. Rightmost column shows the duration of isopach intervals used in this report. Mapped horizons were selected on the basis of reliable regional subsurface correlation rather than on exact equivalence with group boundaries. Geochronology based on van Hinte (1976 a) and van Eysinga (1975). (After Wood and Guevara, 1981.)
   
   
   9. FIGURE 4. Schematic north-northwest – south-southeast cross sections showing evolutionary stages in formation of the East Texas Basin and adjoining Gulf of Mexico (not to scale). Intervening area lies just south of the present Sabine Arch. Arrows indicate inferred thermally induced isostatic movement of the crust. (From Jackson and Seni, 1983 a.)
   
   
   10. FIGURE 5. Schematic block diagrams of facies around salt structures, showing relation between salt flow and sediment accumulation during early evolution of the East Texas Basin. (A) Initiation of salt flow in Late Jurassic, 150 to 137 Ma ago. (B) Initiation of group 1 diapirs in Late Jurassic – Early Cretaceous time, 137 to 115 Ma ago. (After Jackson and Seni, 1983 a.) See fig. 29 for later evolution, labeled (C) and (D).
   
   
   11. FIGURE 6. Isometric block diagrams of salt structures in the East Texas Basin showing three-dimensional configuration of structure contours on top of Louann Salt or, where salt is absent, on top of pre-Louann basement. (A) Northwest view (Jackson and Seni, 1983 a). (B) Northeast view. Constructed by isometric projection and incremental translation of contours, following Lobeck (1924, p. 138-142).
   
   
   12. FIGURE 7. Structure map on top of the Louann Salt or on top of the pre-Louann basement, showing the four salt provinces in the East Texas Basin; seismic control and line of cross section A-A' are indicated at the bottom of the figure.
   
   
   13. Figure 8. Schematic stages of dome growth in the East Texas Basin showing typical lithologic and thickness variations in strata above and around the salt structures during (A) pillow stage, (B) diapir stage, and (C) postdiapir stage.
   
   
   14. Figure 9. Mapped areas of stratal thinning in five isopach intervals over the crests of salt pillows in East Texas Basin.
   
   
   15. Figure 10. Southwest-northeast cross section and map of Hainesville Dome, East Texas Basin, showing the structure of surrounding strata. Lower Cretaceous strata onlap, offlap, and pinch out around the dome. Both syndepositional and postdepositional erosion were active. Dome growth evolved from pillow stage to diapir stage between the end of Washita time and the end of Eagle Ford time. Cross section and map from interpretation of seismic data after Loocke (1978).
   
   
   16. Figure 11. Map of primary peripheral sinks in East Texas Basin based on isopach maps of four stratigraphic units. Strata in primary peripheral sinks thickened mainly by salt flow from areas updip of the salt pillows; subordinate lateral flow was into the growing pillows.
   
   
   17. Figure 12. Map of net sandstone, Paluxy Formation, central and northern East Texas Basin. Dip-oriented trends of net sandstone bypass topographic highs over Van, Hainesville, and Hawkins pillows. Syndepositional erosion associated with major fluvial axes on the eastern and western flanks of the Hainesville pillow may have aided diapirism there at the expense of the Van and Hawkins pillows.
   
   
   18. Figure 13. Cross section D-D', Paluxy Formation, across Van, Hainesville, and Hawkins salt structures, northern part of the diapir province in the East Texas Basin. Decreased thickness and sand percent over each structure indicate that fluvial systems bypassed topographic swells over these salt structures in Paluxy time, as shown in the inset map.
   
   
   19. Figure 14. Schematic cross sections showing the inferred evolution of salt structures from (A) original salt layer, through (B and C) pillow stage, (D) diapir stage, and (E) postdiapir stage. (Modified from Trusheim, 1960.)
   
   
   20. Figure 15. Map of secondary peripheral sinks, East Texas Basin, based on isopach maps of five stratigraphic units. Only sinks thickened greater than 50 percent with respect to regional thickness are shown. The actual area affected by salt withdrawal is much greater than the secondary peripheral sinks shown here (compare with figs. 17, 18, 19, and 20).
   
   
   21. Figure 16. Cross section U-U', pre-Pecan Gap Chalk (Upper Cretaceous) strata, in a secondary peripheral sink, Hainesville Dome area. Stratigraphic section is up to 215 percent thicker than surrounding sediments, indicating massive flow of salt into the dome; however, lithic variations are minor. Location of cross section is given in figures 2 and 17.
   
   
   22. Figure 17. Isopach map of Lower Taylor Formation – Austin Group, Hainesville Dome area, northern part of the East Texas Basin. Axial trace (approximately along cross section U-U') of the secondary peripheral sink intercepts Hainesville Dome.
   
   
   23. Figure 18. Isopach map of Paluxy and Walnut Formations, central East Texas Basin, showing thickened strata in saltwithdrawal basins around Brooks and East Tyler Domes. Thin areas overlie Van, Hainesville, and Hawkins salt pillows.
   
   
   24. Figure 19. Isopach map of Washita Group, central East Texas Basin, showing thickened strata in salt-withdrawal basins around Mount Sylvan, Steen, and East Tyler Domes.
   
   
   25. Figure 20. Isopach map of Glen Rose Subgroup, central East Texas Basin, showing thickened strata in salt-withdrawal basins near La Rue, Brushy Creek, and Boggy Creek Domes. These domes are flanked by a large secondary peripheral sink indicating rapid dome growth during Glen Rose deposition. The Fairway Field (diagonal pattern) is located in reef and reefassociated facies on an elevated saddle between the withdrawal basins.
   
   
   26. Figure 21. Cross section Z-Z', Glen Rose Subgroup, through Brushy Creek Dome and near La Rue and Boggy Creek Domes, East Texas Basin. Glen Rose strata thicken in the secondary peripheral sinks, and reef facies occur in the James Limestone on an elevated saddle between sinks. Location of cross section is given on figures 2 and 20.
   
   
   27. Figure 22. Cross section X-X' and maps of isopach, net sandstone, and structure, Paluxy Formation, northern part of the East Texas Basin. Effects of both salt pillow movement (represented by Van, Hainesville, and Hawkins pillows) and diapiric salt movement (represented by Brooks and East Tyler Domes) are shown. The sand body in the withdrawal basin around East Tyler Dome (on cross section and on net-sandstone map) was isolated from the sandstone feeder system between Hainesville and Hawkins pillows by a subtle structural saddle east of Steen Dome (structure map). This saddle was also a topographic high in Paluxy time (isopach map).
   
   
   28. Figure 23. Map of tertiary peripheral sinks in the Wilcox Group around domes and uplifted areas over domes, southern part of the East Texas Basin. Subsidence in sinks affected greater areas than did uplift over domes. Strata are well preserved in withdrawal areas and are poorly preserved in uplifted areas.
   
   
   29. Figure 24. Sand-percent map, Wilcox Group, southern part of East Texas Basin. Eight diapirs in this area lie in interaxial areas containing relatively low percentages of sand.
   
   
   30. Figure 25. Cross section B-B', Wilcox Group, Bethel Dome area (cross section located on figs. 2 and 24). Four channel-fill sandstones 15 m (50 ft) thick occupy the tertiary peripheral sink east of Bethel Dome. Five of the six sandstones pinch out over the diapir.
   
   
   31. Figure 26. Cross section A-A', Wilcox Group, Oakwood Dome area (cross section located on figs. 2 and 24). The tertiary peripheral sink contains sand-rich facies. A paleotopographic mound over the dome deflected Wilcox fluvial systems, so that thinned strata over the crest of the dome comprise mud-rich, floodplain facies.
   
   
   32. Figure 27. Relation between above-dome topography and surficial sand distribution, Upper Texas Gulf Coast. (A) Map of shallow salt domes and surficial sand and mud, Beaumont – Port Arthur area. Coastal diapirs are preferentially located along the margins of dip-oriented sand belts or in muddy, interaxial areas. Histograms (B and C) show percentage of surface over domes covered by sand and local relief, respectively. Abundant surficial sand (including eolian) blankets most of the Upper Texas Gulf Coast in the Bay City – Freeport area. Domes in this anomalously sandy area account for about half the domes in the 75- to 100-percent-sand class. (Histograms after Fisher and others, 1972,1973; McGowen and others, 1976. Map after Fisher and others, 1973.)
   
   
   33. Figure 28. Location map showing the distribution of extrusion Hormuz salt plugs in the Persian Gulf and surrounding area. Geologic map shows Yas Island Dome, off the Trucial Coast, United Arab Emirates. The Island is flanked by coral-algal reefs and carbonate sand and gravel. Areas designated "salt" include other rocks mantled by erosional debris from the salt plug. Bathymetry and distribution of Holocene carbonate sediments in the Persian Gulf are strongly controlled by salt diapirism. The seafloor between Yas Island Dome and Jebel Dhana Dome, which is on the mainland, also contains course carbonates clastics and patch reels. Farther offshore from Yas Island, rim synclines are expressed as topographic depressions on the seafloor in which carbonate mud and muddy sand are accumulating (Yas Island Dome map modified from Purser, 1973; regional map modified from Kent, 1979.)
   
   
   34. Figure 29. Schematic block diagrams of facies around salt structures showing the relation between salt flow and sediment accumulation during late evolution of the East Texas Basin. (C) Climax of group 2 diapirism in the Early Cretaceous, 115 to 98 Ma. (D) Deceleration of diapirism in the Tertiary, 56 to 48 Ma. (After Jackson and Seni, 1983b.) See figure 5 for earlier evolution, labeled (A) and (B).
   
   
   35. Figure 30. Schematic cross section through a mature diapir showing typical facies variations and potential petroleum traps (numbered) in siliciclastic fluvial, deltaic, and slope depositional systems and in carbonate and siliciclastic shelf depositional systems. Location of traps 1 through 9 and sand-body geometry in siliciclastic systems deposited during postdiapir stage are from Halbouty (1979). Various trpas include: (1) combination trap in sandstone in anticline over crest of dome; (2) braben fault trap over dome; (3) porous cap rock; (4) stratigraphic trap in flank sandstone pinch-out; (5) structural trap beneath overhang; (6) strucutral trap uplifted and buttressed against salt stock; (7) unconformity trap; (8) fault trap downthrown away from salt stock; (9) fault trap downthrown toward salt stock; (10) combination trap in sandstone from updip pinch-out of porous facies in peripheral sink; (11) fault trap in sandstone over turtle structure; (12) fault trap in sandstone in peripheral sink; (13) stratigraphic trap in sandstone from domeward pinch-out of porous facies in peripheral sink; (14) combination trap in sandstone at crest of turtle structure; (15) unconformity trap in sandstone over crest and flanks of precursor pillow; (16) unconformity trap in carbonates from enhanced porosity over crest and flanks of precursor pillow; (17) combination trap in carbonates from pinch out of enhanced porosity zone on distal flanks of precursor pillow; (18) structural trap in carbonates over crest of turtle structure; (19) combination trap in carbonates from enhanced porosity due ot paleotopography over turtle structure; (20) combination trap in carbonates from enhanced porosity due to paleotopography over rasied saddle between peripheral sinks; (21) combination trap in carbonates from enhanced porosity near dome due to paleotophography butressing against salt stock; (22) combination trap in carbonates over crest of salt stock from enhanced porosity due to paleotopography and structure.
   
   
   36. Figure 31. Three groupings of domes in the East Texas Basin based on the timing of stages of salt movement. Only the final evolutionary stage of the oldest diapirs (group 1) is preserved in Glen Rose and younger stratigraphic units. Where shown, pre-Glen Rose history is based on interpretation of available seismic sections.
   
   
   37. Figure 32. Map of three age groups of salt diapirs in the diapir province (dashed line), East Texas Basin. Patterned areas around diapirs correspond to outer limits of secondary peripheral sinks. Note gradual migration of group 2 subgroups A, B, and C toward group 3 areas. The Mexia-Talco Fault Zone defines the northern and western margins of the basin and marks the approximate updip limit of the Louann Salt.
   
   
   38. Figure 33. Regional isopach map of the Glen Rose Subgroup, East Texas Basin. Large secondary peripheral sinks in the central part of the basin indicate that La Rue, Brushy Creek, and Boggy Creek Domes grew diapirically during Glen Rose time.
   
   
   39. Figure 34. Regional isopach map of the Paluxy and Walnut Formations, East Texas Basin. The Walnut Formation is an offshore, time-equivalent fades of the Paluxy Formation. Small secondary peripheral sinks around Brooks and East Tyler Domes are in the central part of basin; large, thin areas formed in the northern part of basin over the crest of growing salt pillows like Hainesville Dome.
   
   
   40. Figure 35. Regional isopach map of the Lower Taylor Formation and Austin Group, East Texas Basin. A large secondary peripheral sink around the diapiric Hainesville Dome dominates the northern part of the basin.
   
   
   41. Figure 36. Technique for calculating the volume of a salt-withdrawal basin.
   
   
   42. Figure 37. Dome-growth-rate curve calculated on the basis of compacted sediments (bottom line) (fig. 47) and curve calculated on the basis of decompacted sediments, East Texas Basin (top line). At the youngest stage of dome growth, the two curves differ by only 5 percent.
   
   
   43. Figure 38. Percentage thickness change (Δh%) around selected diapirs and pillows in the East Texas Basin. Diagrammatic insets show methods of calculating Δh%.
   
   
   44. Figure 39. Fold-shape analysis of Ramsay (1967, p. 359-372). (A) Thickness parameters of a folded layer (fold I) in profile. (B, C) Fundamental fold classes on the t`/α a and T`/α a graphs showing plots for fold I (fig. 39A) and folds II and III (fig. 40). Curves marked II it and III it are based on isochore thickness rather than on Tα so do not represent the true fold shape.
   
   
   45. Figure 40. Cross sections (folds II and III ) of model domes WD-4 ( II ) and WD-6 ( III ) of Dixon (1975, his figs. 6A and 21 A). The models were constructed to simulate growth of gneiss domes and salt domes, respectively. The symbol μ represents the viscosity of a particular layer. Dip isogons are shown in the left-hand figures; isochore thicknesses (IT) are shown in the right-hand figures. (Isochore thicknesses are plotted as curves II it and III it in fig. 39.)
   
   
   46. Figure 41. Measurement of isochore thickness parameters in model domes. (A) Changes in thickness up the flanks of a pillow at dips of α = 0, 5, 10, and 25 degrees. Percentage thickness change is [(IT α – IT0)/IT0]X 100. (B) The effect of increasing flank dip of a pillow. Even though orthogonal thickness, tα, decreases up the flank, the isochore thickness, ITα, increases in the same direction. (C) Thickness variations in an ideal model, whose maximum dip of 10 degrees on the flanks of the rim syncline is the same as the maximum dip in East Texas rim synclines. (D) Measurement of thickening in a rim syncline whose maximum limb dip is greater than 10 degrees. Percentage thickness change is [(IT10 – ITo)/IT0]x 100.
   
   
   47. Figure 42. Histograms showing frequency distribution of thickness changes measured (A) in rim synclines of model domes (14 layers) and (B) over model pillow crests (11 layers). Models are illustrated in Dixon (1975, his figs. 6A and 21 A) and Ramberg (1981, his figs. 11.13, 11.17, 11.38, 11.46, 11.58, 11.80, 11.81, 11.83, 11.93, and 12.1). (A) The median thickness change of 1.5 percent thickening is identical to the maximum thickening theoretically possible at this limb dip in a fold formed by buckling, as calculated by the equation T'α. = sec α. Maximum thickening is 7 percent. (B) Increased thinning with increased steepness of pillow flanks is reflected by the decreasing minimum curve; maximum thinning at 10 degrees dip is -8 percent. The mean and median curves are skewed rightward with increasing dip because of the geometric effect illustrated in figure 41B.
   
   
   48. Figure 43. Comparison of axial-trace positions in vertical cross sections through 14 of the 16 East Texas Basin salt domes (Bullard and Whitehouse Domes have a measurable sink at only one level) (top figure) and 23 model domes (illustrated in Parker and McDowell, 1955, their fig. 21; Dixon, 1975, his figs. 2B, 3B, 4B, 5B, 21A, 21B, and 21C; Ramberg, 1981, his figs. 11.2, 11.13F, 11.16, 11.17, 11.19, 11.25, 11.32, 11.38, 11.39, 11.45, 11.50, 11.58B, 11.93, 11.95, and 12.1B). Axial traces of primary, secondary, and tertiary sinks (measured from maps) of the East Texas diapirs migrate progressively closer to the diapirs through the evolutionary stages of growth. In contrast, axial traces in the model-dome cross sections curve away from the dome as they ascend; one unrepresentative model is marked by asterisks.
   
   
   49. Figure 44. Cross sections showing the relation between structural attitude of strata flanking diapirs and the geometry of (A) Hainesville, (B) Grand Saline, and (C) Keechi Domes. Unpierced strata arch over diapirs having gently dipping margins, like Keechi Dome. Structurally based methods of measuring dome growth, such as stratal uplift per time (reflected in increasing dip with depth) are useful where dome flank strata are not pierced by the dome. But the growth rates of Grand Saline and Hainesville Domes could not be measured accurately by a similar structural technique because flank strata have been pierced and are not uplifted around these domes. Dome uplift has been accommodated by faulting along the contacts of the salt stocks. (After Wood and Giles, 1982.)
   
   
   50. Figure 45. Methods of calculating net and gross rates of dome growth, and applications, assumptions, restrictions, and advantages of each method. (A) Net pillow growth equated with sediment thinning. (B) Net diapir growth equated with sediment thickening. (C) Gross diapir growth calculated by dividing volume of salt moved by the maximum cross-sectional area of the diapir neck.
   
   
   51. Figure 46. Histograms of thickness and rate of regional sediment accumulation of major stratigraphic units in the East Texas Basin from 112 to 56 Ma ago. Rate of sediment accumulation equals vertical thickness divided by duration of each unit. Crooked horizontal line connects modes of sediment accumulation rates and reveals a systematic decline in accumulation rates over time. Salt-influenced values of increased thickness and rate form tails on the right side of each histogram.
   
   
   52. Figure 47. Cumulative-probability curves of sediment-accumulation rate in the East Texas Basin. Declining regional accumulation rates are evidenced by displacement of curves of younger units to the left. The increase in slope of the curves of younger strata compared with older strata reflects less variable sediment accumulation rates. Salt-induced skewness is clearly evident by decreasing slopes above the 95th or 98th percentile (stippled zone). These thickest parts of each unit accumulated in peripheral sinks. Salt-induced thinning over pillows is not apparent at the lower parts of curves in this diagram because of the small number of post-Glen Rose pillows compared with post-Glen Rose diapirs. Wilcox data are omitted because the Wilcox Group is restricted to the southernmost part of the basin.
   
   
   53. Figure 48. Contour map showing sample grid spacing and standard deviation of sediment-accumulation rate for the Lower Taylor Formation and Austin Group. Secondary peripheral sinks around Hainesvilleand Bethel Domes exhibit rates of sediment accumulation three standard deviations more variable than regional values, indicating the high degree of local variability induced by salt flow. Compare with the isopach map in figure 35.
   
   
   54. Figure 49. Histogram of the volumes of salt-withdrawal basins and the volumetric rates at which salt-withdrawal basins were filled in the East Texas Basin. The rate and volume of sedimentary fill in salt-withdrawal basins are equal to the rate and volume of salt flow, respectively.
   
   
   55. Figure 50. Histogram of the volumes of salt-withdrawal basins around individual diapirs of basins formed since 112 Ma ago (early Glen Rose time) in the East Texas Basin.
   
   
   56. Figure 51. Net rates of salt-pillow growth, East Texas Basin, calculated by equating pillow-crest uplift with (A) the rates of sediment thinning over the crests of pillows (compare with fig. 54) and (B) the rates of sedimentation in primary peripheral sinks. Rates calculated by sediment thickening always exceed those calculated by sediment thinning (see fig. 45). Both graphs show that pillows grew fastest during the Early Cretaceous.
   
   
   57. FIGURE 52. Maximum net rates of dome growth from 112 accumulation and net rates of dome growth. This reflects to 56 Ma ago for Oakwood and Hainesville Domes, the absence of thick secondary peripheral sinks in calculated by equating the rise of the diapir crest with the stratigraphic units younger than 112 Ma, which indicates rate of sediment accumulation in secondary (Hainesville the absence of large volumes of salt flow and rates of Dome) and tertiary (Oakwood Dome) peripheral sinks. dome growth roughly equivalent to mean rates of regional Mean regional rate (±1cr) of sediment accumulation sediment accumulation. Therefore, the period of greatest (dashed line) is shown. Volumetric rate of sedimentary diapiric growth for Oakwood Dome predated 112 Ma ago filling of salt-withdrawal basins (crosshatched area) is (see figs. 31 and 32). (B) In contrast, the younger inferred to be equal to the volumetric rate of salt flow into Hainesville Dome is a group 3 diapir that exhibits a large the salt structure. Volumetric rate was calculated by difference between mean rates of regional sediment dividing planimetered volumes of salt-withdrawal basins accumulation and net rates of dome growth. Hainesville (fig. 36) by the duration of the isopach interval. Dome is surrounded by the largest secondary peripheral (A) Oakwood Dome is a group 1 diapir that exhibits sink in the East Texas Basin. The sink was filled from 86 to little difference between regional rates of sediment 56 Ma ago (see figs. 4 and 10).
   
   
   58. Figure 53. Maximum net rates of dome growth (solid line represents the growth rate of the most rapidly growing diapir during deposition of any given stratigraphic unit) and mean rate of regional sediment accumulation in the East Texas Basin from 112 to 56 Ma ago. Dome-growth rates of group 2 diapirs peaked in the Early Cretaceous during a time of high regional sediment accumulation. In contrast, subsequent growth of group 3 diapirs, associated with the growth of Hainesville and Bethel Domes in the Late Cretaceous, was accompanied by low rates of regional sediment accumulation.
   
   
   59. Figure 54. Residual rates of dome growth for 16 East Texas salt domes calculated by subtracting mean rate of regional sediment accumulation from net rate of dome growth. Residual rates of dome growth are independent of regional sediment-accumulation rates. Even with removal of high rates of regional sedimentation (compare fig. 53), most domes grew fastest during the Early Cretaceous.
   
   
   60. Figure 55. Gross rates of dome growth in the East Texas Basin calculated by dividing volume of salt moved by the maximum cross-sectional area of the diapir neck. Relatively rapid vertical growth rates greater than 300 m/Ma (1,000 ft/Ma) are estimated for several domes by this approach.
   
   
   61. Figure 56. Hypothetical gross and actual heights of some post-Glen Rose diapirs in the East Texas Basin. Hypothetical gross heights were calculated by dividing the volume of the salt-withdrawal basin by the maximum cross-sectional area of the diapir neck. Actual diapir heights above the post-Glen Rose Subgroup are shown for comparison. Hypothetical gross heights exceed actual heights of Hainesville, Bethel, and Steen Domes because crests of these domes have been continually removed by dissolution or extrusion. In the case of Brooks, Mount Sylvan, and Oakwood Domes, the hypothetical gross diapir height is less than the actual height of the salt column above the top of the Glen Rose Subgroup. This is because growth rates during the postdiapir stage were too low to be measured given the contour interval used in this method.
   
   
   62. Figure 57. Comparison of published net and gross rates and methods of dome growth with those of the present study. The shapes of the growth-rate curves for East Texas diapirs (this study and Netherland, Sewell and Associates, 1976) are similar to those of North Louisiana diapirs (Kumar, 1977), and estimates by different methods yield values within the same order of magnitude. Estimated dome growth rates based on youngest strata decline exponentially and converge to a low value of about 20 m/Ma (66 ft/Ma). A similar terminal growth rate for domes in the North German Zechstein Salt Basin was estimated by Jaritz (1980).