Diatoms and Silicoflagellates from the Paleogene of the South of the Russian Plate: Implications for Biostratigraphy and Interregional Correlations

The study of several Paleogene sections in the south of the Russian Plate allowed the refinement of the taxonomic list of diatom and silicoflagellate assemblages. Using an integrated biostratigraphic approach, the stratigraphic range of the Bipalla oamaruensis diatom Zone was defined. The high degree of taxonomic diversity of the studied assemblages is associated with the shallow-water environments of the marginal part of the basin inhabited by various ecological groups of diatoms. The zone was established in the Tishki and Kasyanovka formations and, based on direct comparison, corresponds to the Bartonian–Lower Priabonian Cyrtophormis alta–Etmosphaera polysiphonia–Theocyrtis andriashevi radiolarian interval-zones. The Priabonian age of the Kasyanovka Formation is also confirmed by dinocyst data. The Bartonian-Priabonian boundary approximately coincides with the first appearance of the silicoflagellate Corbisema hexacantha.