Biostratigraphy of the Campanian–Maastrichtian Boundary Deposits of the Eastern Part of the European Palaeobiogeographic Province (Section Lesnaya Respublika) by Microfauna

New data on the taxonomic composition and stratigraphic distribution of planktonic and benthic foraminifera and ostracods in the Campanian and Maastrichtian of the Middle Volga region (Lesnaya Respublika section, Saratov) are presented. The understanding of the compositions of microfauna assemblages has been considerably expanded. Based on the results of biostratigraphic analysis, biostratons were identified in the rank of beds and zones: for benthic foraminifera, the Assemblage Zone Neoflabellina praereticulata/N. reticulata (LC19) of the eastern part of the European Palaeobiogeographic Province, for planktonic foraminifera – Beds with Laeviheterohelix glabrans, for ostracods – Beds with Eopajienborhella marssoni–Cytherelloidea levigata–Cythereis zygopleura varia. All the identified units cover boundary deposits; it was not possible to establish criteria for the Campanian and Maastrichtian subdivision in this section. Different sedimentation conditions were reconstructed for the upper part of the Ardym Formation, the Nalitovo Formation, and the basal part of the Lokh Formation: two associated with a cooling period and one with a warmer period. The dependence of the preservation of microfauna with skeletons of different chemical composition on pH variations in sediment was revealed.