An expanded lower Eocene shelf sequence from the eastern Aquitaine Basin, SW France: biostratigraphy, biofacies, and stable carbon and oxygen isotopes

Abstract

The early Eocene is characterized by a succession of orbitally-controlled global stable carbon isotope excursions, with some being linked to climatic and related biotic perturbations. The impact of these isotopic excursions has been primarily studied in deep-sea sections under comparably stable conditions. In order to investigate the impact of global post-PETM isotopic signals on shallow marine settings, the Ypresian neritic ʻBlue Marls’ of the Corbieres (SW France) were investigated. High-resolution records of microfossil biota and stable carbon and oxygen isotopes pinpoint biostratigraphic, paleoecologic and geochemic constraints. Calcareous nannofossil biostratigraphy positions the sequence in the upper part of zone NP11, possibly ranging into basal NP12, which is conformable with larger benthic foraminifera data indicating shallow benthic zone SBZ8. This implies a time span of about 0.4 Myr and high overall sedimentation rates of about ~ 32 cm/kyr for the section. A shallowing upward trend from outer neritic to coastal settings is observed in the development of the lithostratigraphy and the microfossil assemblage. The assemblages can be subdivided in seven larger biofacies and four ostracod assemblage zones. The lower third of the section is characterized by strongly fluctuating and partly high plankton/benthos-ratios for neritic settings. A final pronounced peak in plankton occurrence is associated with strong decrease of benthic biota, suggesting anoxic conditions in the outer neritic environment. Several local negative δ13C- and δ18O-excursions can be identified in the section. The upper, most pronounced and consistent negative δ13C excursion is tentatively linked to global carbon isotope excursion K (ETM3) based on the biostratigraphic constraints.