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

New data on the taxonomic composition and stratigraphic distribution of planktonic and benthic foraminifers and ostracods in the Campanian and Maastrichtian of the Middle Volga region (Lesnaya Respublika section, Saratov) are presented. The understanding on the compositions of the microfauna assemblages are significantly expanded. Based on the results of biostratigraphic analysis, the biostratons are identified in a rank of Beds and Zones: the Neoflabellina praereticulata/N. reticulata (LC19) Assemblage Zone of the eastern part of the European paleobiogeographic province for benthic foraminifers, Beds with Laeviheterohelix glabrans for planktonic foraminifers, and Beds with Eopaijenborhella marssoni–Cytherelloidea levigata–Cythereis zygopleura varia for ostracods. All these subdivisions cover the boundary deposits; it is not feasible to establish the criteria for the subdivision of the Campanian and Maastrichtian in this section. Different sedimentation conditions are reconstructed for the upper part of the Ardym and Nalitovo formations and the basal part of the Lokh Formation: two of them are associated with cooling and one is related to warming. A correlation between the preservation of microfauna with skeletons of distinct chemical composition and pH variations in the sediment is revealed.