A Facies-scale Chemo-lithostratigraphic Composite Profile of Del Rio Claystone Through Austin Chalk Deposition, Late Cretaceous, Central Texas, USA PDF Download
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Author: Kyle Christopher Gabb
Publisher:
ISBN:
Category :
Languages : en
Pages : 188
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Late Cretaceous deposition (Comanche/Gulfian Series) across the San Marcos Arch of central Texas is characterized by both aluminosilicate- and carbonate-dominated mudrock successions. Twelve drill cores were recovered from the 2-acre construction site of the AT&T Executive Education and Conference Center on The University of Texas at Austin main campus. Thirteen detailed chemo-lithofacies were defined through a combination of visual description and elemental analysis utilizing energy-dispersive X-ray fluorescence (ED-XRF). Elemental analysis was undertaken at a 0.25-ft sampling interval to provide a record of facies-scale variability. A composite profile from two cores was created by identifying the boundary between the top of the Buda Limestone and bottom of the Eagle Ford Group and then splicing the records together, thus maximizing the length of the composite core to 174 ft and incorporating the Del Rio Claystone, Buda Limestone, Eagle Ford Group, and Austin Chalk formations. Lithofacies within the four formations were defined based on Dunham’s (1962) carbonate and Potter’s (1980) shale classifications. Important sedimentary structures and mineralogical compositions were identified and added as modifiers to the facies description. The shifts between the carbonate formations and the clay-rich formations are caused by changes in sea level, productivity, anoxia, and reworking by bottom currents. The Buda Limestone and Austin Chalk are identified to range in composition from limestone to marl, as the matrixes of the formations are composed of alumo-siliceous mud. Covariance of silicon with aluminum and titanium (proxies for detrital inputs) suggests that the silica is dominantly of terrigenous origin, rather than of biogenic origin. The enrichment of redox elements (molybdenum and vanadium) indicates the occurrence of anoxia during deposition of the Eagle Ford Group. The increased presence of volcanic ash beds within fossil-rich section of the Eagle Ford Group indicates that nutrients supplied by volcanism led to increased primary productivity, triggering depleted oxygen levels and anoxic bottom waters. Whereas sea level was likely the driving force that caused changes within the Del Rio Claystone, Buda Limestone, and Austin Chalk, productivity and anoxia are the drivers of transitions within the Eagle Ford Group.
Author: Kyle Christopher Gabb
Publisher:
ISBN:
Category :
Languages : en
Pages : 188
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Book Description
Late Cretaceous deposition (Comanche/Gulfian Series) across the San Marcos Arch of central Texas is characterized by both aluminosilicate- and carbonate-dominated mudrock successions. Twelve drill cores were recovered from the 2-acre construction site of the AT&T Executive Education and Conference Center on The University of Texas at Austin main campus. Thirteen detailed chemo-lithofacies were defined through a combination of visual description and elemental analysis utilizing energy-dispersive X-ray fluorescence (ED-XRF). Elemental analysis was undertaken at a 0.25-ft sampling interval to provide a record of facies-scale variability. A composite profile from two cores was created by identifying the boundary between the top of the Buda Limestone and bottom of the Eagle Ford Group and then splicing the records together, thus maximizing the length of the composite core to 174 ft and incorporating the Del Rio Claystone, Buda Limestone, Eagle Ford Group, and Austin Chalk formations. Lithofacies within the four formations were defined based on Dunham’s (1962) carbonate and Potter’s (1980) shale classifications. Important sedimentary structures and mineralogical compositions were identified and added as modifiers to the facies description. The shifts between the carbonate formations and the clay-rich formations are caused by changes in sea level, productivity, anoxia, and reworking by bottom currents. The Buda Limestone and Austin Chalk are identified to range in composition from limestone to marl, as the matrixes of the formations are composed of alumo-siliceous mud. Covariance of silicon with aluminum and titanium (proxies for detrital inputs) suggests that the silica is dominantly of terrigenous origin, rather than of biogenic origin. The enrichment of redox elements (molybdenum and vanadium) indicates the occurrence of anoxia during deposition of the Eagle Ford Group. The increased presence of volcanic ash beds within fossil-rich section of the Eagle Ford Group indicates that nutrients supplied by volcanism led to increased primary productivity, triggering depleted oxygen levels and anoxic bottom waters. Whereas sea level was likely the driving force that caused changes within the Del Rio Claystone, Buda Limestone, and Austin Chalk, productivity and anoxia are the drivers of transitions within the Eagle Ford Group.
Author: Christopher K. Hendrix
Publisher:
ISBN:
Category :
Languages : en
Pages : 127
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The Upper Cretaceous Cenomanian Buda Formation and Coniacian-Campanian Austin Chalk Group of Texas and northern Mexico are calcareous mudrock (chalk) successions deposited on the drowned lower Cretaceous Comanche platform. Investigations covering depositional systems, lithofacies, and diagenesis have been completed over the years, but no up-to-date chemostratigraphic studies have been conducted on either the Buda or Austin Chalk units. This investigation has supplemented sedimentological analysis with high-resolution X-ray-fluorescence (XRF) geochemical data. Two methods were employed to define chemofacies from an XRF dataset to test whether geochemical data can enhance description of the lithofacies and add insight to the lithostratigraphic analysis. Buda and Austin units are composed of four lithofacies in the subsurface of south-central Texas: (1) calcisphere globigerinid wackestone, (2) calcisphere globigerinid mud-dominated packstone, (3) calcareous mudstone, and (4) argillaceous calcareous mudstone. Observations of elemental curves plotted versus depth identified the alternation of calcite-rich chemofacies with dolomite-, clay-, and pyrite-rich chemofacies. Multiple iterations of hierarchical cluster analysis applied to the XRF dataset revealed seven chemofacies: (1) Ca-enriched, (2) Mg-enriched, (3) Al-enriched, (4) S-enriched, (5) P-enriched, (6) Zn-enriched, and (7) Ni-enriched. Neither chemofacies method is capable of defining lithofacies, but each can be used to substantiate mineralogical variability at the sub-lithofacies scale. The XRF-curve-based chemofacies identified large-scale oscillations in mineralogical composition while the HCA chemofacies identified mineralogical variability at the resolution of XRF sampling. Chemolithofacies are defined as lithofacies that are additionally characterized by chemofacies data. Delineation of the chemolithofacies is achieved by applying geochemical (and in this case mineralogical) descriptors to the lithofacies units based on the geochemical data. Ca-enriched calcisphere globigerinid wackestones and packstones are the dominant chemolithofacies of the Buda Formation and Austin Chalk Group. Calcisphere globigerinid wackestones, calcisphere globigerinid mud-dominated packstones, calcareous mudstones, and argillaceous calcareous mudstones enriched in dolomite, clays, and pyrite make up the other chemolithofacies. Variability in the chemolithofacies is attributed to rock units of more mixed mineralogy as a result of intermittent, sustained periods of increased extrabasinal sediment delivery to the shelf.
Author: Susan Davis Hovorka
Publisher:
ISBN:
Category : Chalk
Languages : en
Pages : 60
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Author: Kolin Beam
Publisher:
ISBN:
Category : Chalk
Languages : en
Pages : 82
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Author: Mark Allan Miller
Publisher:
ISBN:
Category : Geology
Languages : en
Pages : 162
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Author: Michael Douglas Fairbanks
Publisher:
ISBN:
Category :
Languages : en
Pages : 119
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Heightened industry focus on the Upper Cretaceous (Cenomanian-Turonian) Eagle Ford has resulted from recent discoveries of producible unconventional petroleum resource in this emerging play. However, little has been published on the facies and facies variabilities within this mixed carbonate-clastic mudrock system. This rock-based study is fundamental to understanding the controls, types, and scales of inherent facies variabilities, which have implications for enhanced comprehension of the Eagle Ford and other mixed carbonate-clastic mudrock systems worldwide. This study utilizes 8 cores and 2 outcrops with a total interval equaling 480 feet and is enhanced by synthesis of thin section, XRD, XRF, isotope, rock eval/TOC, and wireline log data. Central Texas Eagle Ford facies include 1) massive argillaceous mudrock, 2) massive argillaceous foraminiferal mudrock, 3) laminated argillaceous foraminiferal mudrock, 4) laminated foraminiferal wackestone, 5) cross-laminated foraminiferal packstone/grainstone, 6) massive bentonitic claystone, and 7) nodular foraminiferal packstone/grainstone. High degrees of facies variability are observed even at small scales (50 ft) within the Eagle Ford system and are characterized by pinching and swelling of units, lateral facies changes, truncations, and locally restricted units. Facies variability is attributed to erosional scouring, productivity blooms, bottom current reworking, and bioturbation. At the 10-mile well spacing scale and greater, the data significantly overestimates intra-formational facies continuity but is successful in defining the following four-fold stratigraphy: The basal Pepper Shale is an argillaceous, moderate TOC, high CGR and GR mudrock. The Waller Member is a newly designated name used in this study for an argillaceous and foraminiferal, high TOC, massive mudrock with a generally moderate CGR and GR profile. The Bouldin Member is a high energy, carbonate-rich (foraminiferal), low TOC, low and variable CGR but high GR zone. Finally, the South Bosque Formation is an argillaceous and foraminiferal, moderate TOC, massive and laminated mudrock with a moderate CGR and GR signature. GR logs alone are inadequate for determination of facies, TOC content, depositional environment, and sequence stratigraphic implications. Using integrated lithologic, isotopic, and wireline log data, cored wells in the study area are correlated across the San Marcos Arch. Geochemical proxies (enrichment in Mo, Mn, U, and V/Cr) indicate that maximum basin restriction occurred during deposition of the Bouldin Member. Bottom current activity influenced depositional processes and carbonate sediment input was driven by water column productivity. These primary controls on Eagle Ford stratigraphy and character are independent from eustatic fluctuation, rendering classical sequence stratigraphy unreliable.
Author: Russell Elliot Hinote
Publisher:
ISBN:
Category : Geology
Languages : en
Pages : 0
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Six fossil communities were recognized in the Del Rio Formation based on Q-mode cluster analysis of 158 samples collected from 14 Del Rio measured sections in central through north Texas. Using this method of analysis, samples were clustered into six distinct groups on the basis of the faunal similarity of fossil assemblages contained in the samples. Each group of samples contained a set of recurrent fossil assemblages, and the taxa in each set represented the composition of a fossil community. The six Del Rio communities maintain consistent stratigraphic positions in the formation throughout the area. The vertical succession of fossil communities corresponds to lithologic variations in the formation which represent changes of environmental conditions that occurred during deposition. Examination of the relative abundance, morphological features, and inferred living habits of the taxa in the six fossil communities aided interpretation of the environmental conditions which controlled the composition and distribution of each community. The classification of gross community feeding habits and substrate niche preferences gave additional clues to environmental conditions, and the relative diversity of taxa in the communities gave an indication of environmental favorability. Environmental changes occurring during the deposition of the Del Rio were interpreted to have resulted from a minor marine regression which probably began during deposition of the upper Georgetown Limestone. Community A in the basal Del Rio reflects an unstable environment caused by the influx of terrigenous clay at the beginning of Del Rio deposition. Community B, dominated by epifaunal, suspension-feeding taxa with adaptations for survival on a soft substrate, indicates high influx of clay, shallowing of water, and gradual restriction of open marine circulation during deposition of the lower Del Rio. Maximum regression probably occurred during the deposition of the middle Del Rio. Community C indicates restricted conditions were greatest at this time and substrate fluidity was high. Community D in the upper Del Rio reflects a gradual transgression which resulted in deeper water and more open circulation of water. However, the presence of many taxa with adaptations for living on a soft substrate indicates clay influx was still relatively high. Supply of clay diminished and became episodic at the close of deposition of the Del Rio. The restricted, soft-substrate fauna of Community E, which lived during periods of clay influx, alternates with the more diverse fauna of Community F, which lived during times of little or no clay influx. Deposition of the Del Rio ended with the last influx of clay.
Author: Arthur Wordsworth Cleaves
Publisher:
ISBN:
Category : Geology
Languages : en
Pages : 458
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A 70-foot interval from the middle part of the Glen Rose Limestone (Lower Cretaceous) has been sampled at 35 localities in central Texas for the purpose of reconstructing vertical and lateral changes of depositional environment on the San Marcos Platform. A marker unit, the Corbula Interval, crops out in the center of the stratigraphic section. The middle Glen Rose was deposited as a mosaic of shoal-water lithotopes in a broad lagoon behind the Gulf Coast Reef Trend. Over part of the Platform the sea was sufficiently shallow to permit the development of local offlap sequences. As a result, intertidal and supratidal units comprise a significant proportion of many local facies successions. In Blanco and Hays counties there are two distinct patterns of vertical facies succession. Closer to the Llano Uplift (Blanco County) 3 to 5 offlap cycles are seen in the 70-foot interval. These involve a gradational trend from subtidal through supratidal facies. Each cycle is bracketed by sharply-defined bedding planes. The cycles are regressional and result from the progradation of carbonate mud flats into a shelf sea. Further to the east and more distant from the Llano Uplift (Hays County) the facies tract lacks the imbricated succession of regressive cycles. Subtidal units comprise the bulk of the section. The difference in the vertical facies pattern for the two areas may result from their relationship to the ancient shoreline. Because the Llano Uplift was emergent during deposition of the middle Glen Rose, the outcrops closest to the Uplift contain abundant evidence of tidal flat sedimentation. Mud mounds and small islands adjacent to land may have served as nuclei for the development of local offlap sequences. To the southeast (Hays County) the shelf sea may have been slightly deeper and probably lacked the nuclei necessary to initiate these sequences. One complete cycle, the Corbula Cycle, crops out in both areas and may record a brief period of emergence for most of the San Marcos Platform
Author: John Curtis Wendte
Publisher:
ISBN:
Category : Geology
Languages : en
Pages : 186
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Author: Ryan Lee Harbor
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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The Eagle Ford is a well-known source rock for both sandstone (Woodbine) and carbonate (Austin and Buda) hydrocarbon reservoirs in East and South Texas. Recent discoveries have demonstrated that source rocks, such as the Eagle Ford, are capable of producing significant volumes of gas and oil. At the same time, variations in well producibility indicate that these rocks, like conventional reservoirs, display considerable geological heterogeneity. Yet, only limited research has been published on the subsurface stratigraphy and character of Eagle Ford facies. Understanding the types, controls, and distribution of these heterogeneities requires in-depth rock-based studies. In order to characterize Eagle Ford facies, 27 cores from 13 counties were investigated for rock textures, fabrics, sedimentary structures, and fossil assemblages. These studies were supported by light and electron microscopy as well as analysis of elemental chemistry and mineralogy. Regional subsurface stratigraphic correlations and facies distributions were defined using wireline logs calibrated from core studies. In South Texas, the Eagle Ford Formation was deposited during a second-order transgressive/regressive cycle on the flooded, oxygen-restricted Comanche Shelf. Nine depositional facies consisting predominately of organic-rich, fine-grained (5.0 % TOC) to coarser-grained (3.05 % TOC) fabrics were identified. Facies developed in low-energy environments episodically interrupted by higher-energy, event sedimentation (current winnowing, cohesive and non-cohesive density flows, and turbidity flows). Locally, these rocks show evidence of early diagenetic recrystallization of calcite. Concurrent water anoxia and organic matter preservation persisted locally into later Austin deposition, resulting in formation of a three-fold division of the Cenomanian-Coniacian Eagle Ford Formation. Common facies of lower and upper Eagle Ford members include (1) unlaminated, fissile, clay- and silica-rich, organic-rich mudrocks, (2) laminated, calcareous, organic-rich mudrocks, and (3) laminated, foraminifera- and peloid-rich, organic-rich packstones. The transitional Eagle Ford member consists of highly-cyclic (1) ripple-laminated, organic-rich wackestone (cycle base) and (2) burrowed, organic-lean lime wackestones (cycle top). Transitional Eagle Ford facies developed in oxygen-restricted, basinal depositional environments as distal equivalents to burrowed, foraminiferal lime wackestones of the Austin Formation. Facies complexities in the Eagle Ford stem from complicated and interrelated processes of sediment production and distribution, diagenesis, and water column chemistry. Integrated core studies shed light on both controls of facies formation and their spatial distribution. These findings provide a framework for upscaling the fine-scale, heterogeneous character of shelfal Eagle Ford mudrocks; thus allowing development of predictive models into the distribution of key reservoir properties in the subsurface.
