The Danian and Selandian calcareous benthic foraminiferal assemblages and biostratigraphy of Western Siberia

Danian and Selandian successions in western Siberia are rich in calcareous benthic foraminifers. The assemblages with a constant systematic composition that have the widest lateral occurrence are distinguished as foraminiferal zones. The Brotzenella praeacuta Zone was established in the low-carbonate, clayey beds that form the transition from the Gankinskaya Suite to the Talitskaya Suite. These transitional beds are assigned to the Danian. The calcareous benthic foraminifers in this zone occur mainly together with agglutinated forms. The zone is traced along the areas of the largest subsidence of the Mesozoic-Cenozoic basin, principally in depressions. The transition from the Cretaceous to the Palaeogene is represented by a continuous succession, in which the Danian deposits escaped from erosion and were found to contain the B. praeacuta assemblage. In Danian rocks with a shallow-marine facies (near Beryozovo and in the Ust'-Tym depression, Fig. 1), the Bathysiphon nodosarieformis - Glomospira charoides assemblage occurs. It is dominated by primitive forms in addition to more advanced tests (mainly Late Maastrichtian relic species). The overlying Selandian (Middle Palaeocene) foraminiferal assemblages are assigned to the Talitskaya Suite. The Ammoscalaria friabilis assemblage is widely spread and is represented mainly by agglutinated quartz-siliceous forms. Beds with these species are attributed to the Ammoscalaria friabilis Zone, which can be correlated with beds in the East containing Cyclammina coksuvorovae. The Cibicidoides proprius assemblage is known from the lower and middle beds of the Talitskaya Suite in Transuralia, the Omsk Depression and more south-eastern areas; the species of this assemblage are characteristic of the Selandian.