湘西北慈利地区二叠-三叠纪之交沉积与氧化还原状态演化OA

The Cili section in northwestern Hunan is situated within the Xiakou-Lichuan Gulf along the northern margin of the middle Yangtze carbonate platform.During the Permian-Triassic transition,it represented a deep-water land shelf depositional environment in front of the platform,providing new evidence for revealing the evolution of marine ecosystems under different environment in this period.Based on an analysis of sedimentary microfacies and fossil assemblages,combined with pyrite framboid size data,this study reconstructs the paleo-depositional environment and redox condition evolution of the Cili section from late Permian to early Triassic.The results indicate that the section successively developed the Wujiaping formation,Dalong formation,and Daye formation from late Permian to early Triassic.The Wujiaping formation is dominated by foreslope deposits,containing brachiopod,echinoderm,and foraminifer assemblages.The Dalong formation was deposited in a restricted deep-water shelf setting with weak hydrodynamics,characterized by spherical radiolarians and siliceous sponge spicules.A significant biotic extinction event occurred at its top,accompanied by a shift to an inner deep-water shelf environment.The Daye formation mainly consists of mudstone and argillaceous limestone,interpreted as having been deposited in an outer deep-water shelf setting where fossils are rare,with only occasional bivalves observed.Pyrite framboid sizes reveal that oxygen-poor conditions began to develop at the top of the Wujiaping Formation and intensified gradually,reaching anoxic to euxinic states at the extinction boundary.Overall,sea level in the study area underwent a"rise-fall-rise"cycle across the Permian-Triassic transition,with oxygen levels decreasing during sea-level fall.In summary,the deep-water shelf facies in the Cili area during the Permian-Triassic mass extinction event was jointly controlled by sea-level fluctuations and marine redox conditions.