THE EXPRESSION OF THE CENOMANIAN-TURONIAN OCEANIC ANOXIC EVENT IN NORTHERN MEXICO

The Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2) was an intense episode of accelerated global change induced by a significant perturbation of the carbon cycle. It has been interpreted as an interval of widespread organic-carbon burial under conditions of expanded marine anoxia. However, recent investigations have documented the development of oxic-suboxic conditions in the southern gateway of the Western Interior Seaway during this event caused by the inflow of boreal water mass. Although this oceanographic change has been reported as far south as northern South America, its record in Mexico is still unknown. Here, we study the record of the Cenomanian-Turonian Eagle Ford Formation recovered from a core from the Sabinas basin (northeast Mexico). Sedimentological, petrographic and geochemical analyses were performed in order to better understand the timing and nature of redox conditions during the OAE 2 interval. The sediments deposited before the anoxic event consist of a limestone-black shale succession with thin layers of bentonite and bearing common inoceramid shells and pyrite nodules. Moreover, it is poorly bioturbated (Bioturbation Index-BI: 0-1), rich in organic matter (Total Organic Carbon-TOC: up to 6%) and shows high concentrations of redox- (U, Mo, V) and productivity-sensitive trace elements (Cd, Ni, Cu). Conversely, the OAE 2 time-equivalent interval is characterized by highly bioturbated limestone beds (BI: 2) and records an overall decrease in organic matter (TOC: 0-1%) and trace elements content. These data suggest that anoxic-eutrophic conditions prevailed in the basin during late Cenomanian time, and improved bottom water oxygenation occurred during OAE 2.