FORAMINIFERAL PALEOECOLOGY AND PALEOCEANOGRAPHY OF THE CENOMANIAN-TURONIAN EAGLE FORD SHALE AT LOZIER CANYON, TEXAS

Despite its global extent, one of the most striking features of Oceanic Anoxic Event 2 (OAE2 – recognized globally by its distinctive δ¹³C signature) is the heterogeneity of its expression in disparate settings. In some locations the entire interval is preserved as black shale, while others only contain a thin bed of black shale within the excursion interval; other locations contain no black shale at all. Perhaps the best example of the latter category is the Western Interior Seaway (WIS) of the United States: peak TOC in the WIS falls after the global event. To understand why this is the case, the present study focuses on understanding the transition between the WIS and the Tethys in west Texas. We here focus on a complete outcrop of the Cenomanian-Turonian Eagle Ford Shale in Lozier Canyon, Terrell Co., Texas. The Eagle Ford at this location is divided into 5 litho-facies, which have also proven useful for paleoecological interpretation. Facies A is essentially barren of foraminifera and contains evaporate minerals at its base, indicating it was deposited in a restricted, hypersaline environment. Facies B contains a low-diversity, low-abundance assemblage of mostly planktic forams and records the highest TOC values of the section. Towards the top of the facies, benthics become more common, planktics increase in diversity, and the keeled genus Rotalipora goes extinct. These events correspond to similar trends at the Cenomanian-Turonian stratotype section near Pueblo, CO, although the extinction of Rotalipora occurs earlier at Lozier. This south-to-north diachronaity suggests the incursion of an oxygen minimum zone into the WIS from the Gulf of Mexico. Facies C, which corresponds to the OAE2 δ¹³C excursion, shows a sharp shift from a diverse planktic assemblage to one dominated by biserial taxa (“Heterohelix Shift”). The assemblage in Facies D shows increasing abundance and diversity, indicating a recovery from the previous low-oxygen conditions. Facies E contains a diverse, abundant planktic and benthic fauna. The story of Lozier Canyon is one of gradually improving conditions driven by increasing sea level and oxygen content. Imprinted on this local signal are trends that mark Lozier as being more similar to the environments WIS than to Tethys, suggesting the transition zone is further to the southeast.