Reconstruction of paleoredox conditions in the Upper Jurassic – Lower Cretaceous Bazhenov Sea (West Siberian Basin, Russia)

The Bazhenov Sea covered around 2 million km2 of the Mesozoic West Siberian Basin (WSB) during Volgian to Early Berrasian times, resulting in the deposition of highly organic-rich black shales as part of the stratigraphic unit "Bazhenov Horizon". As black shales are excellent archives for past large- to small-scale sedimentary systems, the study's objective is to develop a detailed depositional model reconstructing the transgression-regression cyclicity, paleoredox conditions, paleosulfidicity and -bioproductivity that prevailed during the black shales' sedimentation, and to infer insights into the primary forces and mechanisms favouring enhanced organic matter preservation and accumulation.

For this, elemental analysis, Rock-Eval pyrolysis and petrographic observations were carried out on a large number of core samples from seven well sections from the south-western WSB (Frolov region). Systematic inorganic and organic composition patterns reveal sequential and regional variations of sedimentary and diagenetic conditions in the study area.

The obtained data show that the sediments were deposited in a marine environment with significant contribution of authigenic minerals and subordinate detrital influence. The enrichment of redox-sensitive inorganic and organic compounds indicates reducing and periodically sulfidic conditions in the bottom and uppermost pore water columns during deposition. Sections that show evidence of anoxia or euxinic conditions simultaneously have the lowest terrigenous influence, suggesting that anoxia is closely linked to sea-level fluctuations in the course of eustatic transgression-regression cycles. The correlation of organic matter and redox proxy enrichments suggests that organic matter accumulation and preservation are mainly controlled by these paleoredox condition variations linked to regression-transgression cyclicity.