Browsing by Subject "Eocene"
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Item Conceptualizing vertebrate faunal dynamics : new perspectives from the Triassic and Eocene of Western North America(2013-05) Stocker, Michelle Renae; Bell, Christopher J., 1966-Conceptualizations of actual biological patterns as preserved in the fossil record must accommodate the results of biotic and abiotic drivers of faunal dynamics. However, those conceptualizations also may reflect cognitive biases resulting from foundational philosophical stances. Whether fossils are conceptualized as the remains of biological entities or as geological objects will affect both taxonomic identifications and secondary inferences derived from those identifications. In addition, operational research bias centered on relativistic views of ‘importance’ of particular components (i.e., taxonomic or skeletal region) of the assemblage results in preferential documentation of some taxa and marginalization of others. I explored the consequences of those specific cognitive and operational biases through examination of Triassic and Eocene faunal assemblages in western North America. For the Triassic I focused on taxonomic and systematic treatments of Paleorhinus, a group of phytosaurs important for the establishment of biochronologic correlations. Specimen-level reexamination of Paleorhinus supported a restricted usage of Paleorhinus as a clade, dissolved a biochronologic connection between terrestrial and marine deposits, and indicated a prior compression of the early part of the Late Triassic as a result of previous conceptualizations of species. I reexamined the Otis Chalk tetrapod assemblage in light of new specimens and modern phylogenetic frameworks. My examination supported a restricted usage of the Otischalkian for biochronologic correlation of the Late Triassic, and emphasized the importance of apomorphic character-based specimen examinations in conjunction with detailed lithostratigraphy prior to the development of biochronologic schema. For the Eocene I focused on undocumented terrestrial reptiles from the late Uintan fauna of West Texas. Specifically I discovered new taxa and new geographic occurrences of amphisbaenians and caimanine crocodylians. The amphisbaenians represent the southernmost record of the clade in the North American Paleogene, and, when combined with other amphisbaenian records, document that the clade responded to late Paleogene climatic changes in ways different from the inferred mammalian response. The new taxon of caimanine crocodylian represents a new geographic and temporal record of that clade. That new record indicates that the biogeographic range of extant caimans represents a climate-driven restriction from a formerly more expansive range, and suggests that the previous geographic and temporal gap in paleodistribution data is related to sampling biases and is not a solely a biological phenomenon. These data indicate that reliable characterization of vertebrate faunal dynamics requires open acknowledgment and appropriate documentation of cognitive and operational biases that affect interpretations of paleontological data.Item Depositional and diagenetic processes in the formation of the Eocene Jackson Group bentonites, Gonzales County, Texas(2011-12) Michaelides, Michael Nicholas; Kyle, J. Richard; Gardner, James; Heister, Lara; Serenko, ThomasBentonite clays are exposed in Paleogene strata stretching over 650 km parallel to the Texas coastline. This study focuses on a white and blue and a yellow and brown commercial Ca-montmorillonite bentonite near the city of Gonzales, Gonzales county, Texas. The deposits have stratigraphic ages of Late Eocene (~36.7 - 32.7 Ma). The bentonites in these deposits have varying colors, purities and brightness affording them diverse industrial uses. The distribution and geologic character of the high purity white and blue bentonite suggests that the deposit represents an accumulation of volcanic ash in a secondary tidal channel during the ash-fall event. A low rate of terrigenous clastic sedimentation and rapid accumulation of fresh ash were critical to the formation of high purity clay. The lower purity yellow and brown bentonites appear to have a fluvial origin marked by higher rates of detrital sedimentation and episodic accumulation of clay and ash. The bentonite and associated strata were studied using optical microscopy, SEM, XRD and REE analyses to constrain their textural, mineralogic, and chemical character. vii Eocene pyroclastic volcanism is well documented from sources in southwestern North America, specifically in the Sierra Madre Occidental (Mexico), Trans-Pecos (Texas) and Mogollan-Datil (New Mexico) volcanic fields. Projected Eocene wind patterns support this region as a potential source for the Gonzales bentonites. A comparison of the trace and REE fingerprints of the white and blue bentonites and the yellow and brown bentonites with data available for Late Eocene volcanics in the North American Volcanic Database provides a couple of potential matches. The strongest potential match for the Late Eocene bentonite protolith is described as a sample of silicic tuff with an age range of 32.2 – 30.6 Ma, located in the southern Mexican state of Oaxaca. While the trace and REE match is strong, the tuff is somewhat young compared to the Jackson Group sediments. In addition, the sample location is due almost directly south of the Gonzales deposits, rather than the western location expected for a Gonzales bentonite source. The other potential matches are located in New Mexico, and the Mexican state of Chihuahua. These potential matches only have 6 REE available for comparison, and require further investigation. Many Paleogene volcanic units in southern North America are undocumented with regard to REE data or precise absolute ages. As additional geochemical analyses become available for a more extensive suite of Paleogene volcanic units, stronger matches with Gulf of Mexico Basin bentonites are expected to emerge.Item Paleogeographic evolution during the Eocene Upper Wilcox in the Houston embayment with consideration of the Yoakum Canyon fill(2015-05) Conwell, David Andrew; Steel, R. J.; Olariu, Cornel; Mohrig, DavidThe Eocene Upper Wilcox clastic wedge represents the second major pulse of terrigenous material into the Gulf of Mexico basin. Recent interest in the Wilcox has been reinvigorated by the drilling of the Baha prospect in 2001, and the associated discovery of 2.5 billion barrels of producible oil in deepwater Wilcox aged turbidite deposits. To better characterize and understand the deepwater deposits, research in the delivery systems that transported sediment from the Laramide uplift to the deep Gulf of Mexico is required, with a special focus on the Wilcox shelf margin. This study incorporates over 300 well logs, and outcrop to analyze the Upper Wilcox shelf deposits in the Houston Embayment. The area of this study extends from the outcrop belt in the north-northwest of the embayment down into the subsurface to the limit of down dip well control, around 150 km to the south-southeast. From west to east it extends from Gonzales County to Grimes County, around 200 km. In the Houston embayment the Upper Wilcox was previously interpreted as mainly being fluvial deposits that prograded across the relatively stable substrate provided by the underlying delta complexes of the Lower Wilcox. Previous authors have asserted that the Yoakum Canyon (Middle Wilcox) in the southwest of the field area had its entire 3,000’ filled with prodelta muds prior to the progradation of the Upper Wilcox. However, the present work shows that upper reaches of the Yoakum Canyon were filled by the sandstone units of the Upper Wilcox. Paleogeography maps, generated by differentiating between marine and terrestrial log signatures identify five distinct sequences in the Upper Wilcox. Each sequence shows linear sand trends across the shelf, generally in a north to south direction. A majority of sand was deposited in sequences three and four, through aggradation in the east and progradation in the west over the Yoakum Canyon region. The corresponding shorelines for sequences one through four remain largely pinned along the inherited shelf edge of the Lower Wilcox in the east, whereas the shorelines strongly prograde in the southwest over the Yoakum Canyon. With this new interpretation, that the Yoakum Canyon was not completely filled at the time of Upper Wilcox deposition, it is possible that the canyon delivered 2.86-7.15*106t/yr of sediment to the deepwater reservoirs. Outcrop measurements of cross strata, taken in Bastrop County, confirm the fluvial well log interpretation and provided a base for the volume calculations.Item Provenance of the south Texas Paleocene-Eocene Wilcox Group, western Gulf of Mexico basin : insights from sandstone modal compositions and detrital zircon geochronology(2009-08) Mackey, Glen Nelson; Milliken, Kitty L.; Horton, Brian K.; Steel, Ronald J.Sandstone modal compositions and detrital zircon U-Pb analysis of the Paleocene-Eocene Wilcox Group of the southern Gulf Coast of Texas indicate long-distance sediment transport primarily from volcanic and basement sources to the west, northwest and southwest. The Wilcox Group of south Texas represents the earliest series of major post-Cretaceous pulses of sand deposition along the western margin of the Gulf of Mexico (GoM). Laramide basement uplifts have long been held to be the provenance of the Wilcox Group, implying that initiation of basement uplifts was the driving factor for this transition from carbonate sedimentation to clastic deposition. To determine the provenance of the Wilcox Group and test this conventional hypothesis, 40 thin sections were point-counted using the Gazzi-Dickinson method to determine sandstone composition and 10 detrital zircon samples were analyzed by LA-ICP-MS to determine U-Pb age spectra for each of the sampled areas. Modal data for sand grain populations suggest mixed sources including basement rocks, magmatic arc rocks and subordinate sedimentary rocks for the Wilcox Group. Zircon age spectra for these sandstones reveal a complex grain assemblage derived from older sediments and crystalline rocks ranging in age from Archean to Cenozoic. Sediment was primarily derived from Laramide uplifted crystalline blocks of the central and southern Rocky Mountains, the Cordilleran arc of western North America, and arc related extrusive and intrusive igneous rock of northern Mexico. Comparisons of Upper and Lower Wilcox zircon age spectra show that more arc related material was deposited in the Lower Wilcox, whereas more basement material was deposited in the Upper Wilcox.