Lithostratigraphy and Depositional Environments of the Bazhenovo Formation in the Koltogorsk-Nyurol’ka trench (Western Siberia)

The paper presents the results of a study of the structure and stages of accumulation of the Bazhenovo Formation in the area of the Koltogorsk-Nyurol’ka trench and adjacent positive structures of the Southeastern region of Western Siberia. Based on the use of unified lithological criteria, taking into account geophysical and paleontological-stratigraphic data, the lower four out of six members identified within the Bazhenovo Formation in the Central Region of Western Siberia were traced in the studied area. In the Southeastern region, members 1-4 constitute the entire Bazhenovo Formation, which corresponds here to most of the Volgian-lower part of the Ryazanian. At the end of the Middle Volgian time, an increase in the supply of clay material to the study area is noted due to the proximity of this area to sediment sources and some decrease in global sea level. The depositional environments of the Bazhenovo Formation in the Southeastern region were suboxic during the Middle Volgian and changed to anoxic in the Late Volgian-first half of the Ryazanian. The development of suboxic conditions in the second half of the Ryazanian led to the cessation of the accumulation of Bazhenovo deposits in the studied area.