Upper Cretaceous zonal stratigraphy of the West Siberian Plain based on foraminifera

Investigations on Late Cretaceous foraminifera and their significance for the biostratigraphy of the West Siberian Plain are presented in this paper. Correlation with data on bivalves has allowed the erection of a revised division of the Upper Cretaceous deposits into local biostratigraphical units that are correlatable with the chronozones of the general stratigraphical scale. Twelve local foraminiferal zones have been established through the section. We have designated type sections for the Lower and Upper Coniacian zones of the plain (boreholes 1r, settlement of Kamyshlovo, and 23k, Beryozovo). The rank of the Beryozovskian Horizon (a regional stage), widely known previously, was promoted to Superhorizon (a regional stratigraphical unit). The Cenomanian-Santonian foraminiferal assemblages differ considerably from those of the Campanian-Maastrichtian, and are similar to coeval assemblages from other provinces within corresponding palaeobiogeographical realms, i.e., Arctic and Boreal-Atlantic. Twenty-two assemblages have been established. They vary in their composition laterally according to facies. Zones and beds containing these assemblages are well based on benthic forms and correlate with separate stages of the General Stratigraphical Scale of Russia.