The biostratigraphy of the Coniacian stage of Western Siberia (based on foraminifera and paleozoogeography)

The objective of the present paper is to substantiate the location of the Coniacian Stage within the local and regional stratigraphic subunits of Western Siberia. Having summarized the data on foraminifera, molluscs, paleozoogeography with due regard to the tectonic movements, the present author came to a conclusion on the Coniacian Age of the Ipatovskian Horizon and the suites confined to it. The foraminiferal assemblages from these suites somewhat differ in their systematic composition throughout the section, but over the lateral they have some common species. In the north-west (near settlement Beryozovo, borehole 23, northern Transuralia) within the section of the Nizhne-Beryozovskaya Subsuite, the present author has found two Coniacian assemblages: Haplophragmi-um chapmani - Ammoscalaria antis (at depth 165.0 m; Early Coniacian) and Dentalina tineaformis - Cibicides sandidgei (depth interval 164.0-150.0 m; Late Coniacian). Beds enclosing these assemblages have been established by the author as the zones with the same names. In the south Transuralia, the Late Coniacian assemblage has been revealed within the section of the Kamyshlovskaya Suite and studied by Papulov and Amon, and subsequently by Podobina and Amon. Eastwards, these stratons (the Nizhne- Beryozovskaya Subsuite and Kamyshlovskaya Suite) are correlatable with the Sedelnikovskaya Suite (the central district) and Ipa-tovskaya Suite (the eastern and south-eastern districts). The microfauna has not been revealed within the deposits of the Ipatovskaya Suite of the eastern district, except for the upper beds confined to the Narym iron-ore horizon, and the suite lowermost beds containing sporadic foraminifera. The suite is dated to the Coniacian on the base of finding the mixed (Turonian and Coniacian) foraminifera within the lower beds and its coincidence with the Sedelnikovskaya Suite as well as with the Nizhne-Beryozovskaya Subsuite characterized by the Coniacian foraminifera. The Coniacian age of the top of the suite has been also ascertained from the finds of the Den-talina basiplanata - D. tineaformis Assemblage within the greenish aleurolite interbeds of the Narym iron-ore horizon. Separate species of this assemblage are analogous to those from the Transuralian Late Coniacian Dentalina tineaformis - Cibicides sandidgei Assemblage. In the south-eastern district (the Seversk polygon near Tomsk), the Coniacian foraminifera have been discovered within deposits synchronous to the Ipatovskaya Suite (up to 50 m thick). Here, the author has established the Haplophragmium chapmani -Cibicides sandidgei Assemblage, species of which are characteristic for the Coniacian Stage as a whole. During the Coniacian time, the tectonic settings were notable for the activity in all paleozoogeographic districts of Western Siberia, and it was especially significant in the eastern part of this region. Hence, there is a great diversity of suites forming the Ipatovskian Horizon, as well as a variability of their foraminiferal assemblages.

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