Mid-Cretaceous Oceanic Anoxic Events in the outer Eastern Carpathians: biostratigraphic and chemostratigraphic events

The mid-Cretaceous times are characterized by the presence of the most numerous Oceanic Anoxic Events (OAEs) in the whole Mesozoic, reflecting the occurrence of superplumes, associated with high ocean crust formation rates and increased volcanism (i.e., Larson, 1991; Jenkyns, 2010). In general, the Romanian Carpathians, and especially the Eastern Carpathian bend, enclose significant successions of a deep-water setting, deposited during mid-Cretaceous times. The investigated successions are mainly composed of black and dark-grey shales, grey marls, and sparse cm-thick radiolarites. Based on the calcareous nannofossil biostratigraphy, the studied successions cover the late Albian-Cenomanian interval, encompassing the UC0 up to UC3a biozones. Within the studied successions, the isotope δ13Corg show several fluctuations. The oldest peak is situated in the UC0 nannofossil zone, across the Albian-Cenomanian boundary interval, starting slightly below the LO (last occurrence) of the nannofossil Hayesites albiensis. The youngest recorded peak is placed in UC3a nannofossil subzone and encloses the FO (first occurrence) of the nannofossil Lithraphidites acutus. We assume that the oldest positive excursion identified correspond to the Albian-Cenomanian Boundary Event (ACBE), while the youngest one is most probably the chemostratigraphic overprint of the MCE (mid-Cenomanian Event). Both depositional intervals that contains the ACBE and MCE are characterized by the deposition of rich-organic black shales.