Paleozoogeographical regionalization of Western Siberia in the Late Cenomanian (by data on foraminifera)

When establishing paleozoogeograph-ical regions of the West Siberian Province in separate ages of the Late Cretaceous, the Late Cenomanian, the natural habitat of different species and the structure of foraminiferal assemblages were taken into account. Among species of an assemblage, both local and widespread forms were established. The ratio of all these forms served as the basis for distinguishing separate districts. Benthic foraminifera are closely connected with facies, so borders of districts established on the basis of these foraminiferal assemblages often coincide with the facies borders on paleogeographical schemes constructed previously for different Late Cretaceous ages. The expansion of the marine facies in Transuralia defines the location of the Western paleozoogeographical district. In the north of the province within the top of the Uvatskaya Suite, the marine gray clays and siltstones are traced, which enclose the foraminiferal assemblages Saccamina micra - Ammo-marginulina sibirica (Lower) and Trochammina wetteri tumida - Verneuilinoides kansasensis (Upper). Based on the systematical composition of these assemblages and lithological characteristics of enclosing rocks, the northern district of the West Siberian province was established (northwards the latitudinal flow of the Ob River). The study of the benthic foraminiferal assemblages recovered from the kern of boreholes of the Van-Eganskaya area allowed the conclusions on the expansion of the Late Cenomanian Boreal transgression almost up to the latitudinal flow of the Ob River. Southwardly (the central and eastern district) the continental facies are expanded. The recent data on foraminifera from borehole sections of the south-eastern districts (borehole E-150, Seversk) suggest the existence of the Yenisei riverine Strait in the Late Cenomanian. From the section of borehole E-150 (at depth 438.5-421.4 m) the Late Cenomanian foraminiferal assemblage was recovered, which contained the relatively high-arranged forms of the Gaudryinopsis genus. The species composition of this assemblage is analogous to that from the beds with Gaudryinopsis nanushukensis elongatus of the Van-Eganskaya area (the bottom of the Trochammina wetteri tumida - Verneuilinoides kansasensis Zone). The foraminiferal assemblages from the borehole sections of the Van-Eganskaya area, from other sections of the northern and south-eastern districts are somewhat similar to those from the Canadian Province (North Canada, North Alaska). Besides new species, common species, geographical subspecies and vicariants were distinguished, thus making it possible to assign the enclosing deposits of the West Siberian Province to the Late Cenomanian.

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