Depositional Environment of the Lower Atco Formation, Austin Chalk Group, West Texas

Date
2022
Authors
Gillespie, Kyle William
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Abstract

The lower Atco Formation of the Austin Chalk Group is exposed in outcrop from a road cut along US 90 in Langtry, Texas northwest of Del Rio. Handheld spectral gamma ray profile was collected in order to correlate with wireline logs within west and central Texas. Elemental geochemistry (Ti, Si, K, Zr, Cu, Zn, P, Ba, V, Pb, Fe, and Mn) data was obtained using X-ray Fluorescence (XRF) spectrometry to investigate the facies variation and environmental systems of the Austin Chalk Group in west Texas. With a distally steepening carbonate ramp model that is usually accepted for the Austin Chalk Group, lateral facies variation changes over long distances. Only the mid and outer ramp facies are observed in the Langtry measured section. Whereas the Cenomanian-Turonian boundary is observed as an unconformity at the boundary between the Austin Chalk Group and the Eagle Ford Shale in Bexar County, continuous deposition is described within the measured section at Langtry, Texas. Trace elements concentrations lead to an interpretation that detrital influx influenced rhythmic deposition between clean wackestone and argillaceous marl lithologies. Nutrient proxies display productivity sourcing from surface and bottom water currents (i.e., upwelling currents). The lower Atco is correlated using gamma ray measurements from west to central Texas. The data presented in this research helps identify 3rd-order depositional cycles in the stratigraphic framework of Late Cretaceous chalks and fill a correlation gap for the Austin Chalk Group across Texas.

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Keywords
Austin Chalk, Carbonate, Depositional environment, West Texas, X-ray Fluorescence, XRF
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Department
Geosciences