Recent Data on the Upper Cretaceous Stratigraphy and Foraminiferal Assemblages of southeastern Western Siberia

Foraminiferal assemblages have been recovered from the newly drilled boleholes ZN-1, ZN-2, ZN-3, N-15, N-33 and E-145 (in the suburbs of Tomsk and Seversk), thereby providing the possibility for distinguishing upper Turonian, Santonian, Campanian, and lower Maastrichtian deposits on the southeastern margins of Western Siberia. Numerous species of agglutinated foraminifera, widely distributed throughout Western Siberia in the lower Santonian Ammobaculites dignus - Pseudoclavulina admota Assemblage were recognised from the lowermost sample (356.0 m) of the borehole ZN-1 sequence, considered as the type section in common with the borehole N-15 section. Higher in the same sequence (354.0 m), along with sparse agglutinated quartz-siliceous forms, ten species of calcareous benthic taxa were distinguished that are characteristic for the lower Santonian Gavelinella infrasantonica Zone of the Kazakh and East European provinces. The sections recovered by boreholes ZN-1 (340.0 m) and N-15 (342.0-330.0 m) were dated as late Santonian. Characteristic species recorded there correspond to the upper Santonian Gavelinella stelligera Zone of the Kazakh and East European provinces. The assemblages distinguished from boreholes ZN-1 and N-15, indicate a Santonian age for the most part of the Slavgorodskian Horizon. The West-Siberian Batizysiphon vitta - Recurvoides magnificus Zone (upper strata of the Slavgorodskian Horizon) is assigned to the lower Campanian Substage established in borehole N-15 at a depth interval of 279.0-273.0 m. The characteristic calcareous species recorded there allow this zone to be correlated to the lower Campanian Gavelinella clementiana Zone of the Kazakh and East European provinces. The Cibicidoides primus Zone (the base of the Gankinskian Horizon) is correlated with the upper Campanian Cibicidoides voltzianus Zone of the Kazakh Province and to the Globorotalites emdyensis - Brotzenella monterelensis Zone of the East European Province by means of the characteristic species. The lower Maastrichtian Spiroplectammina variabilis - Gaudryina rugosa spinulosa Assemblage was distinguished in boreholes ZN-1 (275.4-272.5 m) and N-15 (229.4-223.2 m). The West-Siberian zone named after this assemblage is correlated with the lower Maastrichtian Brotzenella complanata - Angulogavelinella gracilis Zone of the East European Province. Marine facies containing calcareous tests correspond to a relatively deep-water facies of the central district of Western Siberia (the Ob-Irtysh Interfluve), containing agglutinated quartz-siliceous forms, the age of which is here refined.