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Stratigraphy Of The Ernst Member Of The Upper Cretaceous Boquillas Formation, Black Gap Wildlife Management Area, Brewster County, Texas
White, Robert Alan
White, Robert Alan
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2020
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Abstract
The time-equivalent Boquillas Formation and Eagle Ford Shale (EFS) were deposited on the South Texas Shelf in the Late Cretaceous (Cenomanian-Turonian) during a time of widespread marine transgression. The two formations consist of organic-rich shales and marls interbedded with calcareous limestones that vary laterally and vertically in thickness. The organic-rich shales and marls in the lower part of the EFS have been major targets for unconventional production in south Texas and the East Texas Basin since 2008. The goal of this study was to characterize the depositional environment of the Ernst Member in the Black Gap area of Brewster County and potentially correlate the lithological, geochemical, and mechanical data gathered with the proximal and distal Ernst Member and Eagle Ford studies. The Ernst Member of the Boquillas consists of alternating layers of low angle cross stratified to laminated grainstones, laminated argillaceous wackestones, and bentonites. The rocks were deposited below storm-wave base in a commonly anoxic/euxinic environment that contained rarely oxygenated bottom waters. Intermediate and surface-waters were commonly oxygenated and sustained life, which eventually settled as calcite and was deposited along with detrital clay material and lime-mud. Additionally, the system was influenced by bottom water currents that constantly reworked and deposited pelagic carbonate and clay material. The Ernst Member in the Black Gap area is composed of four separate chemofacies. The section can further be divided into five zones based on lithostratigraphic data paired with major and trace elemental trends. Zones A and B in this study are correlated to the organic-rich zones being produced in south Texas. Detailed correlations can be made between the Ernst Member and other Boquillas/Eagle Ford sections over moderate distances, such as 30 to 50 miles (48.3 to 80.5 km), and units in Ernst Tinaja and Hot Springs in BBNP can be correlated northeastward to the Black Gap Wildlife Management area. A large sampling interval of every six inches to a foot (15-30 cm) for obtaining XRF, spectral gamma ray (SGR), and mechanical data appears to be adequate to correlate between sections with moderate to large distances between them. Additionally, broad geochemical correlations can be made from Black Gap Wildlife Management area to the Langtry area in Val Verde County and to the northwest portion of the Maverick Basin in Maverick County.
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Geological Sciences