Lithologic response to OAE2 across the drowned Comanche Platform, south Texas

ABSTRACT The Eagle Ford Group is a mudrock system deposited during the Late Cretaceous across a flooded carbonate platform (referred to as the Comanche Platform in south and central Texas) that rimmed the Gulf coast region of the southern United States. During the Cenomanian–Turonian the Comanche Platform was situated at the confluence of the Western Interior Seaway and the Gulf of Mexico. Bathymetric features on the Comanche Platform include: 1) the San Marcos Arch, which separates the East Texas Basin to the northeast from the Maverick Basin further to the southwest, 2) isolated fault-bound troughs including the Karnes, Atascosa, and Gonzales Troughs, and 3) structural remnants of the down-dip Aptian through middle Cenomanian Stuart City and Sligo Reef margins. Collectively, bathymetric features and oceanographic circulation patterns resulted in a complex depositional network that affected sediment source input, bottom water redox potentials, and carbonate productivity. The onset of Oceanic Anoxic Event-2 (OAE-2) at the Cenomanian–Turonian boundary (ca. 93.5 Ma) coincides with the transition from lower Eagle Ford (LEF) to upper Eagle Ford (UEF) deposition and is marked by a positive ~2–3‰ carbon isotope excursion measured from bulk organic matter. OAE-2 also reflects a transition from anoxic organic-matter rich mudrocks typical of the LEF to oxic, carbonate-rich, and organic-matter poor mudrocks typical of the UEF. As part of a regional study of the Eagle Ford Group, 42 core have been characterized using high resolution X-ray fluorescence core scanning coupled to lithofacies, biostratigraphy, volcanic ash dating, and stratigraphic wireline log analysis. In south Texas, onset of LEF deposition at ca. 96.25 Ma is dominated by compositionally banded TOC-rich argillaceous–calcareous mudstones to wackestones deposited under anoxic bottom water conditions (elevated concentrations of Ni, Cu, Zn, Mo, and V). These TOC-rich anoxic mudstones are interbedded with diagenetic calcite cemented limestones and concretions. Importantly, anoxic conditions and maximum TOC preservation occurred in the LEF approximately 2–4 million years prior to the onset of OAE-2. At the top of the LEF, initiation of OAE-2 coincides with an increase in clay content, decrease in TOC content, decrease in redox-sensitive trace elements Mo and V, and increase in bioturbation, all of which are indicative of an increase in oxidation of bottom water. Not all cores preserve the 2–3‰ carbon isotope excursion associated with OAE-2, indicating that either erosion or non-deposition occurred during the UEF, likely the result of ocean current reworking, clinoform deposition during the high stand system of the UEF, or the result of bathymetric geometries. ‘Recovery’ from OAE-2 is marked by a decrease in carbon isotope values measured from bulk organic matter back to values typical of LEF, but the lithofacies that dominate the UEF are more oxygenated, contain less TOC, and are more carbonate-rich than observed in the LEF. Collectively, integrated core data from the Comanche Platform in south Texas demonstrate that oxic bottom waters persisted during OAE-2, which is different from the Tethys Ocean that experienced increased TOC-preservation and anoxic bottom water conditions contemporaneous with the OAE-2 carbon isotope excursion.