CHEMOSTRATIGRAPHY OF UPPER ALBIAN (LOWER CRETACEOUS) COMANCHE SHELF MARGIN

This study targets the Lower Cretaceous Upper Albian Fort Lancaster and Devils River formations on the Comanche Shelf in south Texas. In order to conduct facies, biostratigraphic, and cyclostratigraphic research, two outcrop sections were measured and sampled in detail. In the nearly 70m-thick Pandale section, analysis of outcrop and thirty-five thin sections allows the definition of four lithofacies associations, four deepening-upward and nine to ten shoaling-upward depositional cycles, revealing a depositional environment of slightly restricted inner-shelf. Particularly, in the nodular Burt Ranch Member at the base of this section, a micrite-rich wackestone interval distinctively shows positive δ¹³C excursion and relatively low taxa diversities. Bracketed by basal Upper Albian ammonite biozonation, this interval possibly corresponds to the depositional records of OAE 1c. In the nearly 28m-thick Pecos River Hwy 90 bridge section, based on outcrop and nineteen thin sections, five lithofacies associations are described, three to four deepening-upward and seven shoaling-upward depositional cycles are defined, and the depositional environment is interpreted as high-energy shelf-margin. In addition to these high-frequency cycles two subaerial exposure surfaces in both sections suggest relative sea-level fall. By ranges of biostratigraphically significant benthic foraminifera and bivalves, these cycles are further correlated with the six third-order depositional cycles of the north Texas Trinity River Composite section within a mixed carbonate-siliciclastic intra-shelf basin setting, revealing different sea-level histories between different depositional settings upon the Comanche Shelf. Seventeen diagenetic elements of cementation and porosity data have revealed sequences of diagenetic events associated with early marine (eogenetic), burial (mesogenetic), and subaerial (telogenetic) diagenetic environments. Two different paragenetic sequence models are constructed for the two outcrop sections, further reflecting their different depositional environments within the Comanche Shelf.