LITHOFACIES AND CHEMOSTRATIGRAPHY OF THE UPPER CRETACEOUS BUDA FORMATION AND AUSTIN CHALK GROUP, SOUTH TEXAS, USA

Lithologic and bulk geochemical data from cored intervals of the Upper Cretaceous Buda Formation and Austin Chalk Group reveal periods of high biological productivity and scarce preservation of organic matter on the drowned south Texas shelf. Given their temporal relationship to the opening of the Western Interior Seaway, the Buda and Austin successions contained in cores from the Maverick Basin and Comanche Shelf of the ancestral Gulf of Mexico demonstrate the effects of tectonic, physiographic, and paleoceanographic influences through time and space. High-resolution (2 inch) energy-dispersive X-ray fluorescence (XRF) data are paired with a hierarchical clustering method to quantify and characterize the elemental composition of the cores. These geochemical data are incorporated into a lithostratigraphic and petrographic framework to better understand paleoceanographic conditions during the Late Cretaceous. The absence of redox-sensitive trace metals and presence of heavily bioturbated lithofacies are in accord with a well-oxygenated environment of deposition. High calcium values (30-40%) correspond to the coccolith-dominated wackestone lithofacies that are characteristic of the formations. Although these biogenic phases dilute the signatures of detrital elements, thin argillaceous seams reveal intermittent sediment inputs of primarily kaolinitic mineralogy delivered to deeper water, most likely during storm events. Pyritization is evident in core of the Buda Formation and more prevalent in the underlying Del Rio Clay. The unconformable Buda-Eagle Ford contact is identifiable lithologically by finely laminated globigerinid packstones/argillaceous wackestones and geochemically by a sharp increase in elemental proxies for organic preservation and oceanic dysoxia/anoxia.