PALEOCENE FORAMINIFERAL ASSEMBLAGES AND BIOSTRATIGRAPHY OF WESTERN SIBERIA

The Danian and Selandian assemblages of Western Siberia are described in the paper. Beds containing assemblages of the stable systematic composition most widespread laterally were distinguished as foraminiferal zones. The uppermost beds of the Gankinskaya Suite are composed of low-lime and, in places, silica-like clays, and the Brotzenella praeacuta Zone was established in the beds transitional to the Talitaskaya Suite. We confine these beds to the Danian Stage (the Lower Palaeocene). In this zone calcareous secreted benthic foraminifera are mainly encountered along with agglutinated forms. The zone is traced in the areas of the greatest subsidence of the Mesozoic-Cenozoic deposits, mainly in depressions. In these continuous sections of the Cretaceous-Palaeogene transition, the Danian deposits prevented from decomposition were established containing the above assemblage. In the Danian shallow facies (settlement Beryozovo, the east Tym sections, the meridional part of the Ob Basin - Priobye), another foraminiferal assemblage - Bathysiphon nodosarieformis - Glomospira charoides - is distributed. It is predominated by the primitively arranged forms along with more complicated ones. Frequently, these are the Late Maastrichtian relic species. The overlying Selandian (the Middle Palaeocene) foraminiferal assemblages are confined to the Talitskaya Suite. The Ammoscalaria friabilis Assemblage most widely occurs here and is mainly represented by agglutinated quartz-siliceous forms. Beds with this species were confined to the Ammoscalaria friabilis Zone correlatable in the east with beds containing Cyclammina coksuvorovae. The Cibicidoides proprius Assemblage is known from the sections of the Trans-Urals, the Omsk Depression, and also from the southeastern areas within the lower and middle beds of the Talitskaya Suite; species composing it are characteristic for the Selandian (Island Zealand, Denmark). The author has thoroughly studied the previously known section of Borehole 1-r (the Novo-Loginovskaya Area, the Omsk Depression). A single Danian-Palaeocene foraminiferal assemblage has previously been established by Freiman within the rocks of the depth interval 607.45-602.55 m. The present author re-investigated foraminiferal assemblages and was the first to establish not one but two assemblages of different ages: the lower is the Danian Brotzenella praeacuta Assemblage from the zone of the same designation (607.45-602.55 m; 2 m from the end of the core) and the upper is the Selandian Cibicidoides proprius Assemblage from the Ammoscalaria friabilis Zone (607.45-602.55 m; 2 m from the beginning of the core). The transition from the Danian zone of the Lower Palaeocene to the Selandian zone of the Middle Palaeocene is traced within this rock depth interval in the central district of Western Siberia. It should be recorded that the Selandian assemblage studied from the Borehole 1-r section of the Novo-Loginovskaya Area is mainly composed of the secreted calcareous foraminifera, and its systematic composition is indicative of their confinement to the shallow or coastal facies of the West-Siberian Basin. The correlation of Palaeocene foraminiferal zones of Western Siberia by the planktonic and benthic is summarized in the Table with the regard of Berggren's schemes (1995). The Danian and Selandian species are illustrated by four palaeontological plates.

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