UPPER CENOMANIAN FORAMINIFERS AND BIOSTRATIGRAPHY OF NORTHERN DISTRICT OF WESTERN SIBERIA

The foraminiferal assemblages and lithology of the Upper Cenomanian deposits of the northern district of Western Siberia were studied from seven borehole sections of the Van-Eganskaya Area. Two foraminiferal zones were established by changes in the composition of detected foraminiferal assemblages and in the lithological characteristics of the enclosing rocks: the lower zone Saccammina micra - Ammomarginulina sibirica and the upper zone Trochammina wetteri tumida - Verneuilinoides kansasensis. Beds with Gaudryinopsis nanushukensis elongatus were distinguished in the bottom of the upper zone. The Saccammina micra - Ammomarginulina sibirica Assemblage of the Upper Cenomanian lower zone of the studied borehole sections differs in the species composition from the superjacent one. The lowermost beds of this zone contain the primitive foraminifers of the genera Rhabdammina, Psammosphaera, Saccammina, Hyperammina, and others. The presence of primitive forms is indicative of the beginning of the Late Cenomanian Boreal transgression, which did not reach the latitudinal flow of the Ob River. The changes in the species composition of the foraminiferal assemblages and lithological characteristics of the Upper Cenomanian rocks derive from the fluctuations of depth in the progressing transgression. The significant diversification of facies (the alternation of relatively deep-water and shallow facies) was observed not only in the Uvatskian Horizon section (Upper Cenomanian) but also laterally, especially in its uppermost beds. That is why in some sections, the marine facies of the dark gray clays were detected enclosing well-preserved foraminiferal assemblages of the upper zone. In the subjacent deposits of the Uvatskian Horizon, within the studied borehole sections, the insufficiently preserved foraminiferal tests were encountered having coarse-grained quartz-siliceous test walls. This kind of foraminifers are found for the first time from sections of the Uvatskian Horizon top (Upper Cenomanian) on the territory of Western Siberia. Their investigation and development of more detailed stratigraphy of the Upper Cenomanian are of great scientific and practical importance, because the industrial hydrocarbon pools are confined to these deposits. In this paper eleven species are originally described, of which one species and 5 geographical subspecies are new. They are assigned to orders Ataxophragmiida (families Trochamminidae and Ataxophragmiidae), Rotaliida (families Discorbidae and Anomalinidae), Buliminida (Family Buliminidae) and Heterohelicida (Family Heterohelicidae). All described taxa are of great importance in the subsequent determinative works for the study of foraminifers and biostratigraphy of the Cenomanian deposits of Western Siberia.