Sedimentary Facies Evolution and Oncoidal Development Conditions of Wujiaping Formation of Upper Permian in Hongxing Area,East Sichuan
Widespread development of microbiolites or microbial-associated deposits is a common sedimentological response to the aftermath of mass extinctions.Recent studies have suggested that the end-Middle Permian extinction was the sixth largest extinction event in geological history,but the record of microbial deposition after the extinction is still lacking.In this study,the oncoidal limestone at the bottom of Wujiaping Formation of Permian in Well HY3,Hongxing area,East Sichuan,was investigated in detail,and the sedimentary facies evolution of its related strata was also analyzed.From the top of Maokou Formation to the bottom of Changxing Formation in Well HY3,5 sedimentary facies(assemblages)and 11 sedimentary microfacies are identified.The vertical evolution of sedimentary facies in Wujiaping Formation indicates three third-order sea level change cycles where the overall water depth gradually increases.The oncolite is only found in the mid-late transgression stage of the first sedimentary cycle of Wujiaping stage.Based on the characteristics of core and lamination,the oncoids of Wujiaping Formation can be divided into four types.With the increase of sedimentary water depth,the type parameters,diameter and density of oncoids present three distinct development stages from the bottom upward.The changes of these parameters indicate that suitable hydrodynamic conditions are one of the key factors for the development of oncoids.In addition,the development of oncoids may have been influenced by biological extinction and terrigenous inputs.The oncoidal horizon corresponds to the late Middle Permian extinction period,and the sharp decline of epigenetic biodiversity provides sufficient ecological space for the development of oncolite induced by microbes.In the adjacent strata above the oncoidal limestone or in similar water depth conditions,the terrigenous input may be enhanced due to the closer proximity to coal,resulting in the disappearance of oncolite.
Late PermianWujiaping Formationsedimentary microfaciesoncoidbiological extinctionsea level changesedimentologyshale gasgeobiology
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