The Cenomanian–Turonian Eagle Ford Group of South Texas: Insights on timing and paleoceanographic conditions from geochemistry and micropaleontologic analyses

• The micropaleontology study found 13 biostratigraphic datums in the Eagle Ford.
• Lower Eagle Ford marls were deposited under anoxic to euxinic conditions.
• Radiolaria-rich limestones were deposited under suboxic to anoxic conditions.
• Eagle Ford organic-rich shales were deposited prior to the onset of OAE2.
• Eagle Ford marl–limestone cycles were a product of enhanced oceanic mixing.

The interbedded organic-rich marls and limestones of the Eagle Ford Group of South Texas were deposited on the shelf of the Central Texas Platform during the upper Cenomanian through the Turonian. Micropaleontological analyses of thin-sections and residue samples from subsurface cores taken in Atascosa and Karnes counties yielded 13 biostratigraphic datums, which were integrated with calcareous nannofossil datums, stable carbon isotopic and bulk inorganic geochemical analysis to gain an understanding of the paleoceanographic conditions of the study area and their relationship to regional and global events. The paired thin-section/residue sampling was necessitated by poor recovery of planktonic foraminifera in washed samples, which were, however, abundant in most thin-sections. Lower Eagle Ford marls were deposited under euxinic conditions produced by a stratified water column. Periodic enhanced water column mixing increased oxygen levels and surface-water primary productivity, which promoted calcite diagenesis and generation of limestones containing calcispheres and radiolaria. An oxygenated bottom water mass entered the basin in the uppermost Cenomanian prior to the onset of Oceanic Anoxic Event (OAE) 2, causing a spike in benthic foraminiferal diversity and decreasing preservation of organic matter. Although the lower portion of the upper Eagle Ford is age-equivalent to OAE2 as indicated by positive shift in δ13Corg, it contains less organic matter and evidence of more oxygen than the lower Eagle Ford, so deposition of the Eagle Ford black shales is not contemporaneous to OAE2.