Controlling Factors of Organic Enrichment in the Shuijingtuo Formation in the Lower Cambrian of the Chengkou Area,Sichuan Basin
[Objective]The Lower Cambrian Shuijingtuo Formation black shale is an important source rock and shale gas exploration target in the Sichuan Basin.However,the enrichment mechanism of organic matter in this series of black shale is still unclear.[Methods]Here,we reported total organic carbon(TOC)contents,sedimentary microfacies,major and trace elements,and pyritic framboid size distributions in the lower part of Shuijingtuo Forma-tion in the Chengkou area,northeastern Sichuan Basin on the Upper Yangtze Platform,providing new evidence for reconstructing the paleogeography,paleoenvironment,and main controlling factors of organic matter enrichment.The results show that the black shale succession in the Shuijingtuo Formation developed five sedimentary microfacies which related to the deep-water shelf slope environment.Productivity index(BaXS,P/Ti)and continental input index Ti,Al indicate that the sedimentary environment of black shale in the Shuijingtuo Formation is at a low level of paleo-productivity,and the continental input is relatively stable and has no obvious correlation with TOC.Therefore,paleo-productivity and continental input are not the main controlling factors of organic matter enrichment in the black shale of the Shuijingtuo Formation.The redox-sensitive element index(U/Th,UEF,MoEF)and pyrite framboid size distribu-tions show that the organic matter content is positively correlated with the redox index,indicating that the organic matter enrichment in Shuijingtuo Formation is mainly controlled by the change of redox conditions of the water column.[Conclusions]Based on the above understanding,it can be inferred that the organic matter enrichment of the black shale of the Shuijingtuo Formation was controlled by the fluctuation of redox conditions of the bottom water caused by the large-scale transgression event during the stage 2 to stage 3 Cambrian(529-514 Ma),and the organic matter enrichment model was proposed here.
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