Litho-, bio- and chemostratigraphy across the Cenomanian/Turonian boundary (OAE 2) in the Vocontian Basin of southeastern France

The Oceanic Anoxic Event 2 (OAE 2) was one of at least two large oceanic anoxic events during the Cretaceous Period, and it is characterized in the stratigraphic record by double positive peaks in the δ13C curves of carbonate and organic carbon. Since the positive δ13C shift is believed to have resulted from massive burial of isotopically depleted organic carbon generated globally in anoxic oceanic environments, the positive δ13C shift tend to coincide with widespread deposition of organic-rich sediments. In the case of OAE 2, however, the organic-rich horizons do not accord exactly with the two intervals with positive δ13C shifts at many sites such as Pueblo, Tarfaya, Eastbourne, and Gubbio (e.g., Tsikos, H., Jenkyns, H.C., Walsworth-Bell, B., Petrizzo, M.R., Forster, A., Kolonic, S., Erba, E., Premoli Silva, I., Baas, M., Wagner, T., Sinninghe Damesté, J.S., 2004. Carbon-isotope stratigraphy recorded by the Cenomanian–Turonian Oceanic anoxic event: correlation and implications based on three key localities. Journal of the Geological Society, London 161, 711–719).The Thomel Level of the Cenomanian/Turonian (C/T) boundary in the Vocontian Basin (southeastern France) accumulated during the OAE 2, and it is marked by frequent intercalations of black shale and organic-rich marl. Detailed litho-, bio- and chemostratigraphy analyses across the C/T boundary in the Vocontian Basin indicate that the δ13Ccarbonate profile fluctuation across the Thomel Level corresponds well with changes in lithofacies, total organic carbon (TOC), and calcium carbonate (CaCO3) abundance. Positive δ13C excursions within the OAE 2 correlate with basal and middle parts of the Thomel Level that are characterized by high TOC, low CaCO3, and predominantly contain organic-rich sediments such as black shale and dark gray marl. On the other hand, negative δ13C intervals within the OAE 2 correspond with lower and upper parts of the Thomel Level that consist of marly limestone and limy marl with low TOC and high CaCO3 values. Since the fluctuation of redox conditions during Thomel Level deposition in the Vocontian Basin coincides well with the global carbon burial rate indicated by its δ13C profile, the Thomel Level could be an ideal candidate for the reference section of the OAE 2.