Biostratigraphy and foraminifera of Ipatovskian Horizon of Western Siberia

The micropaleontological investigation of the Ipatovskian Horizon of Western Siberia is directed at the precise definition of its age and stratigraphic position in the upper Cretaceous section. The Narym iron-bearing horizon, whose ores are of the industrial importance, is confined to the upper beds of the Ipatovskian Horizon. The Ipatovskian Horizon is lithologically non-uniform: in the west it is composed by grey gaize-like clays of the lower Beryovskaya Suite (about 40-60 m thick), in the central district - by light-grey siltstones and gaize-like clays of the Sedelnikovskaya Suite (up to 60 m thick), in the east - by the Ipatovskaya Suite (from 60 m thick in Pudino up to 270 m in Napas) composed by grey sands, friable siltstones and clays. Diverse foraminiferal assemblages are discovered in all rock varieties from Transuralia to Napas. Beds containing these assemblages are divided into two zones: the lower Coniacian Haplophragmium chapmani - Ammoscalaria antis Zone and the upper Coniacian Dentalina tineaformis - Cibicides sandidgei Zone. In the south-east of Western Siberia the mixed Coniacian assemblage Haplophragmium chapmani - Cibicides sandidgei has been discovered. The beds containing it (up to 20 m thick) are composed by brownish-grey siltstones which are appropriate to the rocks of the Narym iron-bearing horizon. Moreover, the colour of the tests is brownish, characteristic for the colour of foraminifera from the eastern district. The foraminiferal assemblages from the Ipatovskian Horizon of Western Siberia have been studied in different times by the author, and those from Transuralia by the author in cooperation with other researchers. The West-Siberian foraminiferal species have been compared with the reported coeval forms from Sweden and Austria along with the Coniacian tests from the Muskiki Member (Canada) donated by J. Wall to the author. The systematic compositions of the assemblages varying from west to east and from north to south are indicative of different habitat conditions for these organisms in the West-Siberian Coniacian basin. In the eastern district which is tectonically more active, the shallowing of the basin is supposed, with which at that time the formation of the Narym iron-bearing horizon was connected. Its clayey interbeds have the Late Coniacian Dentalina basiplanata - D. tineaformis Assemblage impoverished in the systematic composition. The tests are coloured brown, evidently owing to iron oxides. Two accompanying tables (Tables 1 and 2) display two lower and upper Coniacian foraminiferal zones with the characteristic species along with the names of the Coniacian assemblages from diverse districts of Western Siberia. Two plates (pls. I and II) illustrate foraminiferal species from the lower and upper Coniacian zones. Plate III demonstrates a new foraminiferal assemblage recovered from borehole S-160 (depth 353.5 m) of the south-east Western Siberia (town Seversk).

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