Organic matter accumulation on the Dalong Formation (Upper Permian) in western Hubei, South China: Constraints from multiple geochemical proxies and pyrite morphology

Organic-rich black shales in the Upper Permian Dalong Formation are considered excellent source rocks in the western Hubei Basin, South China. However, the mechanisms of organic matter (OM) accumulation remain controversial. Furthermore, the evolution of primary productivity and ocean hypoxia during the Late Permian in the western Hubei Basin is unclear. In this study, we discuss the paleoclimate, paleoceanography, paleoenvironment and the mechanisms of OM accumulation based on geochemical characteristics and pyrite framboid data from drillcore H 1 in Ensi, western Hubei Province. Black shales from the Dalong Formation have high total organic carbon (TOC) contents (2.8-9.3%), while the mudstone samples contain low TOC contents (0.48-1.96%). The weak chemical weathering indicates paleoclimate conditions were relatively cold during black shale deposition, whereas rapid warming occurred during gray mudstone deposition, which may be connected with the Late Permian Changhsingian global climate event, Redox indices, namely, V, U, and Mo concentrations, and the relationship of U-EF and Mo-EF combined with pyrite framboid data suggest that an anoxic to euxinic environment predominated during black shale accumulation. In contrast, oxic to hypoxic marine conditions pervaded the western Hubei Basin during the deposition of mudstone. The primary productivity indices (Cu/Al and Nixs) suggest that high paleoproductivity occurred in the Late Permian, especially during periods of transgression. However, declining primary productivity with enhanced intensity of chemical weathering may indicate that biological extinction was associated with a rapid increase in temperature during the Late Permian. Negative/positive relationships between the TOC contents and Baxs and Nixs values demonstrate that sulfate was consumed by a higher OM input and OM accumulation was mainly controlled by high primary productivity. The study implies that ocean hypoxia and high primary productivity played important roles in OM accumulation. The depositional model shows that high primary productivity possibly resulted from weathering of volcanic ash from land during transgressions and that intense degradation of OM resulted in an euxinic environment.