Carbon Isotope Secular Curve for the Upper Cretaceous Chalks in the South Texas Maverick Basin Showing the Expression of the OAE-2 and OAE-3 and Associated Lithofacies and Chemostratigraphy

The Upper Cretaceous oceanic anoxic events (OAEs) 2 and 3 are recorded in a ~950’ cored interval in the South Texas Maverick Basin that covers the Del Rio (Early Cenomanian) through the upper Austin Chalk (Early Campanian) interval. The dominant lithofacies in this interval is deeper water marly chalk to chalky marl. A continuous inorganic δ13C isotope curve acquired along the full length of the core distinctly displays both OAEs. OAE-2 occurs in a 100 ft interval of the Eagle Ford, with the isotope curve showing a rapid positive increase in δ13C from -1‰ to 2‰ followed by a gradual drop back to -0.7‰. The Austin Chalk OAE 3 occurs over a much broader interval of approximately 300 ft in which the isotope curve shows a sharp positive increase -0.5‰ to ~1.5‰ δ13C and a rapid drop back to 1‰. The associated lithology in the Eagle Ford is organic-rich, laminated chalky marl and in the Austin Chalk there is a gradation from interbedded organic-rich chalky marl up into marly chalk. The δ13C‰ curve in South Texas matches the composite reference δ13C‰ curve from the Upper Cretaceous English chalk, and nearly all the events noted in the English chalk are recognizable in the South Texas isotope curve. The OAE-2 appears as a shorter term event, whereas the OAE-3 appears as a long-lasting event. Notably, neither OAE shows any abrupt or distinct changes in lithofacies, but the XRF analyses of trace elements reflect distinct chemical changes going into OAE-2.