New data on Upper Cenomanian foraminifers and biostratigraphy of northern district of Western Siberia

Foraminifers andbiostratigraphy of the Upper Cenomanian of the northern district of Western Siberia were first studied with the use of materials fromseven newly bored holes on the territory of the Van-Yeganskaya area. Two foraminiferal zones (the Lower Saccammina micra -Ammomarginulina sibirica and the Upper Trochammina wetteri tumida - Verneuilinoides kansasensis) were established in the UpperCenomanian by the change in the species composition of the foraminiferal assemblages and the lithology of the enclosing rocks of sevenstudied borehole sections. The beds with Gaudryinopsis nanushukensis elongatus were traced within the lower part of the upper zone.The change in the species composition and the rocks lithology results from the depth variations in the spread of the advancing Borealtransgression. The significant difference of facies (the alternation of the relatively deep-water and shallow facies) is observed not onlywithin the section of the Uvatskian Horizon (the Upper Cenomanian), but also laterally, especially in its uppermost strata. Hence, insome borehole sections of the Van-Yeganskaya area, in the upper strata of the Uvatskian Horizon, the marine facies of dark-gray clayswere distinguished, to which the well-preserved foraminiferal assemblages of the upper zone were assigned. The similar well-preservedassemblages had previously been distinguished from the dark-grey, almost black clays of the top of the Uvatskian Horizon within theborehole sections of the Tazovskaya and Purpeyskaya areas. In the lower deposits of the Uvatskian Horizon, within the investigatedseven borehole sections of the Van-Yeganskaya area, the foraminiferal tests, not sufficiently well preserved, were discovered havingprincipally coarse-grained quartz-siliceous wall. The comparison of the Late Cenomanian foraminiferal assemblages from the WestSiberian and Canadian provinces was made within the whole Arctic paleobiogeographical Realm. Thereby, the common species,vicariants and geographical subspecies were established. Owing to this comparison, the detailed correlation and refinement of the age ofWest-Siberian foraminiferal zones and beds of the Upper Cenomanian stratons were performed. In the present paper, ten species aredescribed, eight of them are new. They are assigned to the following orders: Ammodiscida (the superfamily Haplophragmiidea - thefamilies Haplophragmoididae and Haplophragmiidae), Textulariida (the family Textulariidae). All the described species are illustratedon three paleontological plates. These taxa are of great importance for the subsequent determinative works for the study of foraminifersand biostratigraphy of the Cenomanian deposits of Western Siberia.

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