Litho- and chemostratigraphic response and recovery from OAE2 in the Maverick Basin of the drowned Comanche Platform in south Texas

ABSTRACT The CS-1 core from Maverick County, Texas preserves a 600 ft section of Late Cretaceous Buda Formation (67 ft), the Eagle Ford Group (431 ft), and the Austin Chalk (103 ft). A nearly continuous section of lower (172 ft) and upper (259 ft) Eagle Ford Group members is preserved, providing an excellent opportunity to study depositional features and geochemical signatures before and after OAE2. The CS-1 core comes from the Maverick Basin, an intra-platform basin with increased subsidence and accommodation relative to other parts of the Comanche Platform. The Comanche Platform is located near the southern terminus of the Western Interior Seaway and the northern terminus of the proto-Gulf of Mexico, and was flooded during the marine transgression that characterized the Cenomanian with maximum flooding occurring near the end of the Cenomanian. Oceanic Anoxic Event-2 (OAE2) is recorded in the CS-1 core by a positive 3.8‰ carbon isotope excursion (δ13Corg) measured from bulk organic matter. The onset of OAE2 is coincident with the transition from the lower to upper Eagle Ford member and the Cenomanian–Turonian Stage boundary (ca. 93.8 Ma). OAE2 also reflects a transition from anoxic/euxinic organic-matter rich mudrocks typical of the lower Eagle Ford to dysoxic, carbonate-rich, organic-matter poor mudrocks typical of the upper Eagle Ford member. In the CS-1 core, the lower Eagle Ford member is dominated by globigerinid-rich wackestones and mudstones with abundant coccolith-rich peloids that have present-day total organic carbon (TOC) values that range from 3.0–6.5 wt%. These wackestone-mudstones are commonly interbedded with diagenetic calcite concretions. Other lithofacies include laminated and variably-cemented planktonic skeletal packstone–grainstones. The depositional system preserved in CS-1 defines an anoxic bottom water environment with deep-water renewal from the open ocean demonstrated through elevated concentrations of molybdenum and vanadium. The transition across OAE-2 and into the upper Eagle Ford member is reflected by a sharp decrease in concentrations of molybdenum and uranium, signaling a relatively oxygenated depositional environment. Based on the lithologic facies and chemostratigraphic data from CS-1, the deposition of the lower Eagle Ford member in South Texas occurred with maximum TOC preservation occurring ca. 96.25 Ma during LEF and 2–4 Ma prior to the onset of OAE2. Bottom water oxygenation that occurred during OAE2 within the Maverick Basin and elsewhere across the flooded Comanche Platform of South Texas demonstrates a decoupling from the global ocean, oceanic anoxia, and organic carbon burial trends occurring elsewhere during OAE2.