Sedimentary Environment And Geochemical Characteristics Of The Lower Cambrian Black Rock Series In Northern Guizhou

The lower Cambrian black rock series are widely distributed in South China and Tarim basin,but the controlling factors and distribution characteristics of organic-rich mudstone and shale are still under debate.The Lower Cambrian black rock series,including black shales,siliceous and carbonate,were organic-rich sediments in Yangtze Block,which covered on the dolomites of Dengying Formation or cherts of Liuchapo Formation.In this study,we collected samples of the Niutitang Formation(or its time-equivalent)from eight typical profiles of different sedimentary environments(Inner platform facies,Slope facies and Basin facies)and carried out integrated petrology,mineralogy and geochemical analyses.The results suggest that different sedimentary environments are characterized by different redox conditions during the early Cambrian.Sediment lithologies of the inner platform are mainly black shales with some thinly bedded phosphatic rocks and cherts or phosphate/chert nodules.The black shales are thick and have high abundance of organic matter.Trace elements(V,Ni,Mo and U)of black shale from the inner platform facies are significantly enriched,which indicate dysoxic-anoxic environment with ephemeral dynamic sulfidic water masses.The thickness of the upper slope black shales is small,but organic matter abundance is the most high.Trace elements(V,Ni,Mo and U)enrichments are low in comparable with the other settings,thus suggesting oxic-dysoxic conditions.The majority of the upper slope facies is deposited in oxic state with instant euxinia developed.Basin facies strata are composed of interbedded black chert and black siliceous shale,which are thin in thickness and have low organic matter abundance.The enrichment factors of V and U are higher than those from the inner platform,but for Ni and Mo,their EF values are relatively lower.The majority of the basinal strata were deposited in anoxic conditions.The characteristics of rare earth elements show that the cherts located basin facies were formed by seafloor hydrothermal.In conclusion,the thickness,organic matter abundance and enrichment of trace elements of the lower Cambrian black rock series in northern Guizhou are mainly controlled by the belts of different sedimentary environment,different redox state and seafloor hydrothermal.