Upper Cenomanian foraminifers and biostratigraphy of northern district of Western Siberia

The foraminiferal assemblages and lithologyof 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 inthe lithological characteristics of the enclosing rocks: the lower zone Saccammina micra - Ammomarginulina sibirica and the upperzone Trochammina wetteri tumida - Verneuilinoides kansasensis. Beds with Gaudryinopsis nanushukensis elongatus were distinguishedin the bottom of the upper zone. The Saccammina micra - Ammomarginulina sibirica Assemblage of the Upper Cenomanian lower zoneof the studied borehole sections differs in the species composition from the superjacent one. The lowermost beds of this zone contain theprimitive foraminifers of the genera Rhabdammina, Psammosphaera, Saccammina, Hyperammina, and others. The presence ofprimitive forms is indicative of the beginning of the Late Cenomanian Boreal transgression, which did not reach the latitudinal flow ofthe Ob River. The changes in the species composition of the foraminiferal assemblages and lithological characteristics of the UpperCenomanian rocks derive from the fluctuations of depth in the progressing transgression. The significant diversification of facies (thealternation 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 detectedenclosing well-preserved foraminiferal assemblages of the upper zone. In the subjacent deposits of the Uvatskian Horizon, within thestudied borehole sections, the insufficiently preserved foraminiferal tests were encountered having coarse-grained quartz-siliceous testwalls. This kind of foraminifers are found for the first time from sections of the Uvatskian Horizon top (Upper Cenomanian) on theterritory of Western Siberia. Their investigation and development of more detailed stratigraphy of the Upper Cenomanian are of greatscientific and practical importance, because the industrial hydrocarbon pools are confined to these deposits. In this paper eleven speciesare 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 (FamilyBuliminidae) and Heterohelicida (Family Heterohelicidae). All described taxa are of great importance in the subsequent determinativeworks for the study of foraminifers and biostratigraphy of the Cenomanian deposits of Western Siberia.

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