Stratigraphy and microfaunal characteristics of the Tavdinskaya suite of Western Siberia

Over the course of many years, theTavdinskaya suite has been studied in order to ascertain the lithology and stratigraphy of this part of the marine Palaeogene of WesternSiberia. Numerous borehole sections have been studied in Transuralia and especially in the southern part of Western Siberia (southwardthe latitudinal flow of the Ob River). At that time the basin was connected with southern seas through the straights in Turgay andnorthern Urals. Since the beginning of the Oligocene, the northern part of Western Siberia and the whole Arctic Region were exposed tothe upward tectonic movements. This resulted in the shallowing and separation of the West-Siberian and Canadian basins and wideningof the straight between Greenland and Norway. As it has previously been stated [2], calcareous secreted foraminifers migrated to theWest Siberian Basin from the Atlantic Ocean via the shallowed warmed Arctic Basin and farther through the straight in the northernUrals. This may be evidenced by the systematic composition of foraminifers wherein European and American species of benthos andplanktonic forms are present. There are no similar foraminifers in the Eocene and Oligocene of Kazakhstan. The most characteristicdistinguished planktonic forms are as follows: Paragloborotalia postcretacea (Mjatliuk), P. anguliofficinalis (Blow), Globanomalinamicra (Cole), G. barbadoensis (Blow), Globigerina ouachitaensis Howe et Wallace gnaucki Blow et Banner, G. angustiumbilicataBolli, G. ampliapertura Bolli, G. officinalis Subbotina praesentata Podobina. Besides the planktonic forms, the characteristic calcareousbenthos species were encountered: Cibicidoides pseudoungerianus (Cushman), Nonionellina oligocenica (Cushman et McGlamery), N.оvata (Brotzen), Evolutononion decoratum (Cushman et McGlamery), E. advenum (Cushman) and elphidiids. The Early Oligoceneforaminiferal assemblage of Western Siberia characterizes the Cibicidoides pseudoungerianus - Protononion decoratum Zoneoccupying the most part of the section of the Tavdinskaya suite. One of the recently studied sections of this suite, wherein the EarlyOligocene foraminifers were distinguished, is the borehole of Section 8 (depth 159.35-87.60 m) of the Russian-Polyanskaya Areasouthward Omsk on Kazakhstan borderline [2]. The systematic composition of foraminifers, the rate of their preservation andlithological characteristics of rocks are indicative of their dwelling in the shallow coastal southern part of the West Siberian basin. Morecomprehensive information on the systematic composition of the foraminiferal assemblages studied previously and their comparisonwith those of other regions, including the Rupelian typical section of Belgium, has been presented in other publications [1, 2]. In theTavdinskaya suite, the underlying quartz-siliceous agglutinated species gave way to the predominating calcareous secreted forms. Thisis recorded by us both in the West Siberian Province and in the Canadian one, thus indicating the general change of physical-andgeographicaland bionomical conditions in the whole Arctic area.

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