Micropalaeontological biostratigraphy and palaeoecology of the Middle Eocene of the Lower Volga region, southwest Russia

A biostratigraphical study of the Eocene interval, based on dinoflagellate cysts and calcareous nannofossils, was performed in two boreholes from the Lower Volga region (Yergeni Upland), southwest Russia. Dinoflagellate cyst zonation of the interval allowed six zonal subdivisions to be recognised: Upper Ypresian: (1) Areosphaeridium diktyoplokum; Lutetian succession: (2) Costacysta bucina, (3) Phthanoperidinium regale–Phthanoperidinium clithridium, (4) Enneadocysta arcuata, (5) Phthanoperidinium cornutum–Phthanoperidinium distinctum; Bartonian succession: (6) Dracodinium rhomboideum. The last two zones are reliably correlated with the calcareous nannofossil zones CNE12 and CNE14. The analysis of microbiota composition, palaeoecological analysis of dinoflagellate cyst assemblages, and palynofacies analysis allowed the identification of substantial cyclical changes in sedimentary environments in the marginal shelf zone of the Peri-Tethys during the Middle Eocene.