The Impact of Tectonic and Environmental Changes on Facies, Depositional Geometries, and Sequence Architecture within the Austin Chalk Group, South-Central Texas




Cooper, John Robert

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The Late Cretaceous Austin Chalk Group in south-central Texas has vertical and lateral variations in thickness and facies resulting from irregular paleotopography and multiple 3rd-order sea-level cycles. This study investigates the impact of the San Marcos Arch and its tectonic evolution during the Coniacian to early Campanian on facies and sequence architecture within the Austin Chalk Group. Outcropping sections of the Austin Chalk Group along the margin of the Edwards Plateau in south-central Texas stratigraphically transition between inner and outer ramp environments. This switch is recorded in vertical stratigraphic succession is separated by discontinuity surfaces and condensed deposits. Inner ramp facies consist of Thalassinoides -burrowed bivalve wackestone to packstone with minor occurrences of grainstone. Outer ramp facies consists of Planolites-burrowed foraminiferal chalk and marl with fine skeletal debris. Condensed facies associated with transgressive re-working are characterized by glauconitic packstone with high concentrations of coarse skeletal debris and lithoclasts. Gamma ray profiles of measured outcrops in Bexar County are correlated to subsurface wireline logs of wells drilled along the outcrop and subsurface trend, in order to construct a stratigraphic framework across the San Marcos Arch. This stratigraphic framework is biostratigraphically well constrained and correlated to the Austin Chalk type section in Travis County. The integration of elemental (Mn, Ti, Zr, Si, K, P, Cu, Zn, Ba, Fe, V, Pb) and delta13C isotope geochemical data within this sedimentological and stratigraphic framework of the succession in Bexar County enables the identification of three 3 rd-order depositional sequences within the Austin Chalk Group. Thickness maps of each depositional sequence illustrate the tectonic evolution of the San Marcos Arch and the role of tectonics during deposition of the Austin Chalk Group. Syndepositional tectonic movement plays an active role in the geometry of Austin Chalk depositional sequences and should be taken into account for sequence stratigraphic models developed in chalk depositional settings.


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chalk, depositional geometries, hardgrounds, sequence stratigraphy, tectonics