Paleontologic description of deposits of the Abalakskaya and Bazhenovskaya suites on the territory of the Shirotnoye Priobie(Western Siberia)

The present paper considers the results of our studies at the Abalakskaya and Bazhenovskaya suites where the Callovian and Late Jurassic associations of ammonites, foraminifers, spores and pollen were found. The deposits of these suites were investigated from a number of borehole sections situated on the territory of the Shirotnoye Priobie. The combination of zonal scales created from different groups of the fauna and the flora was used to divide the studied stratigraphic interval of deposits into stages and substages. Such kind of complex investigation on diverse groups of fossil organisms considerably extends the possibility for a more detailed zoning of sediments of the Abalakskaya and Bazhenovskaya suites dated to the Callovian and Late Jurassic ages. The data were obtained on the areal extent of the investigated species of fossil organisms within the stated territory. The Callovian and Upper Jurassic deposits of the borehole sections on the territory of the Shirotnoye Priobie contained foraminifera, ammonites, spores and pollen. The boundary between those stages was traced by paleontological finds. The deficit of the factual material prevented the Kimmeridgian Stage from being characterized and zoned in detail. The substantiation of the stratigraphic position of the lower and upper boundaries of the Bazhenovskaya Suite in the section is reported on the base of the results of the characteristic biocomplex assemblage analysis, along with logs (more often, the radioactive logging). In solving problems on establishing the Jurassic - Cretaceous boundary, the data on the taxonomy and character of changes in the foraminiferal fauna at this boundary are of great interest, because the evolution of this group of organisms is a basis for constructing stratigraphic schemes of unexposed areas and correlation charts of oil-and-gas prospecting boreholes. In this connection, the data are presented on establishing the Volgian Stage by the fauna and foraminifera, and the Volgian-Berriasian transition was characterized by bioassociations. Thus, the zonation of the deposits of the Abalakskaya and Bazhenovskaya suites has been conducted on the basis of the zonal ammonite and foraminiferal scales together with palynological complexes found in bottom sediments. The complex of microfaunal and palynological approaches combined with the litho-and cyclostratigraphy enhances the resolving power of the stratigraphic basis, which, in its turn, is necessary for paleogeographical reconstructions, geological mapping, mineral search and exploration. The investigation will result in further detailed paleogeographical reconstructions elucidating the history of the formation of sedimentary basins on the territory of the Shirotnoye Priobie

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