Foraminiferal and nannofossil paleoecology and paleoceanography of the Cenomanian–Turonian Eagle Ford Shale of southern Texas

The Upper Cretaceous of central Texas is dominated by a broad, shallow carbonate platform called the Comanche Platform that occupies an important gateway between the epeiric Western Interior Seaway (WIS) of North America to the open-marine Gulf of Mexico/Tethys. We investigated the Cenomanian–Turonian Eagle Ford Shale on and adjacent to the Comanche Platform to determine whether the Eagle Ford Shale has an affinity with the Western Interior, and if so, determine where the transition from Western Interior to open ocean is located. We were also interested in the relationship, if any, between the organic-rich facies of the Eagle Ford and Oceanic Anoxic Event 2 (OAE2). Our work is based on quantitative foraminiferal population counts and associated sedimentary particles (including inoceramid prisms, sand, glauconite, and pyrite grains), calcareous nannofossil assemblages, carbon isotopes, and total organic carbon (TOC) from three sites across a range of paleowater depths: an outcrop in Lozier Canyon in Terrell County, west of Langtry, TX, an outcrop at Bouldin Creek outside of Austin, TX, and Swift Energy's Fasken A #1H core in Webb County, TX. The highest TOC in the Eagle Ford occurs before the onset of OAE2 (6% at Lozier Canyon, 7% at both Bouldin Creek and the Fasken Core) and then declines steadily through the rest of the section (except for a small increase at the end of OAE2 at Lozier and a fairly large post-OAE2 increase at Bouldin Creek). Nannofossil paleoproductivity indicators (%Zeugrhabdotus and %Biscutum) track TOC at Bouldin Creek and in the Fasken Core and display similar trends to those observed in the Western Interior Seaway to the north. Benthic foraminiferal abundances increase as TOC decreases and the lithology shifts from laminated black shale to bioturbated light gray shale; low-oxygen-tolerant infaunal Neobulimina spp. appear first and gradually increase in abundance; epifaunal benthics appear soon after. This oxygenation trend continues through the OAE2 interval and the upper Eagle Ford contains a diverse epi- and infaunal benthic foraminiferal assemblage and macrofossil assemblage. This trend of decreasing TOC and nannofossil paleoproductivity indicators coupled with increasing benthic foraminiferal abundance and diversity (and seafloor oxygenation) corresponds to rising sea level. Based on foraminiferal and nannofossil events, TOC trends, and changes in lithology, both platform sites have a strong affinity with the Western Interior, wi