Biostratigraphy and foraminifera of the Santonian-Campanian-Maastrichtian deposits of Western Siberia

On the base of the correlation of the Upper Cretaceous foraminiferal zones of the West Siberian and other provinces, the omission of the most part of the Campanian deposits from the section has been revealed. At the same time, the presence of the characteristic foraminiferal assemblages indicates the presence of the lowest (the top of the Slavgorodskian horizon) and upper (the bottom of the Gankinskian horizon) parts of the Campanian stage. The change of the transgression direction from the Santonian Boreal to the southern Maastrichtian under the action of ascending tectonic movements of the first order resulted in cropping of the Campanian deposits formed earlier. The great (middle) part of these deposits was afterwards destructed by the erosion and denudation processes. This non-evident stratigraphic disconformity between the Santonian (the Slavgorodskaya suite) and Maastrichtian (the Gankinskaya suite) deposits, corresponding to the omission of the middle part of the Campanian stage from the section, is established by the correlation of the West Siberian foraminiferal zones with those of Central Asia (the Kazakh province) and the East-European province. Straights, connecting the West Siberian basin with the adjacent seas, opened and disappeared under the action of the subordinate tectonic movements of the second order. In the Santonian time, the Mariinsk straight formed in the south-east, through which the calcareous secreted foraminifera characteristic for this age were migrating from the Kazakh to the West Siberian straight. Their combined finds together with agglutinated quartz-siliceous forms from the central district of Western Siberia affirmed the Santonian age of the most part of the Slavgorodskaya suite. Formerly, the whole Slavgorodskaya suite was dated to the Campanian [9] without considering tectonic movements, the occurrence of straights, without comparing foraminiferal assemblages from different provinces, including the assemblages of northern Kazakhstan and the East-European province. The samples from the newly bored seven holes (the outskirts of Tomsk and Seversk) yielded the representative foraminiferal assemblages, which enabled to distinguish the deposits of the Santonian, the bottom and top of the Campanian and the lower Maastrichtian within the sections of the Slavgorodskaya and Gankinskaya suites of the cognominal horizons. This investigation proves the Santonian / Early Campanian age of the Slavgorodskian horizon and Late Campanian / Maastrichtian age of the Gankinskian horizon. The author has been proving such a conclusion on the age of these horizons for many years in publications and at the Regional stratigraphic conferences on the Mesozoic, and, particularly, on the upper Cretaceous of Western Siberia. In the 2005 Regional stratigraphic scheme on the upper Cretaceous of Western Siberia [9], the whole Slavgorodskaya suite was again dated to the Campanian, and this is in disagreement with the data on foraminifera and the significance of the tectonic movements during the Santonian-Maastrichtian time.

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