Geochemical Response Of Biological Source And Sedimentary Environment Of Sinian-Cambrian Shale

The Sinian-Cambrian is a special transition period in the geological history,which is accompanied by many important geological events,such as the breakup and recombination of ancient continents,global great oxidation events,biological explosion,ocean oxygenation events and so on.The Sinian-lower Cambrian strata were widely distributed in the Yangtze Platform,which provide a guarantee for the study of the biological source of organic matter,paleocean environment and organic matter enrichment mechanism in this special period.Well drilling in the Sichuan Basin reveals the shale deposits of the Cambrian Maidiping and Qiongzhusi Formations with a huge thickness,while the Doushantuo Formation is mainly located in peripheral area of the Sichuan Basin such as the Changkou and the Zunyi.Previous studies on these three sets of shales mainly focused on conventional geochemical analysis(include total organic carbon,pyrolysis,fundamental molecular geochemical analysis).In this study,the drilling core and outcrop samples from the Ziyang-Weiyuan,the Gaoshiti-Moxi,the Zunyi and the Chengkou area were used to analyze.Based on observation of fossil morphology by scanning electron microscopy,biomarkers composition,elemental geochemical parameters and stable carbon isotope compositions of kerogen,the biological source of organic matter,sedimentary environment and organic matter enrichment model of the Sinian Doushantuo Formation and the Cambrian Maidiping and Qiongzhusi Formations were determined.A flourishing community of marine micro-organisms can be visualized during the Late Neoproterozoic to Early Cambrian in the Sichuan Basin: algaes(include chlorophytes,pelagophyceae,dinoflagellates,diatoms or their ancestors),sponges and cyanobacteria proliferated in a shallow sea euphotic zone,while some anoxygenic phototrophic ?-Proteobacteria proliferated in the bottom anoxic water.In addition,rhizobia and some methanotrophs with negative carbon isotopes also contributed to the organic matter of these three sets of shales.The redox sensitive element parameters and molecular biomarkers showed that the Maidiping Formation located in the DeyangAnyue aulacogen was deposited in a strong restricted marine environment.The arid climate condition results in the increase of water salinity,which eventually cause stratification of water and the bottom water in an anoxic setting.Due to the limited nutrients and high salinity of water,the reproduction of organisms is restricted.Therefore,the paleoproductivity of the Maidiping Formation is not high and the main factor affecting the organic matter enrichment is the preservation condition.When the Qiongzhusi Formation began to deposit in the studying area,the Yangtze Platform underwent a largescale transgression,and the ocean brought abundant nutrients,which increase the primary productivity significantly and the high producticity is considered to be the main factor caused the high organic matter abundance in the Qiongzhusi Formation.The isochronous stratigraphic framework of the Cambrian Maidiping and Qiongzhusi Formations of the Sichuan Basin and the Yangtze Platform was established based on the analysis of the organic carbon isotope and marker bed correlation.The results show that the Maidiping Formation in the Sichuan Basin corresponds to the Zhujiaqing in the Xiaotan section,the Kuanchuanpu Formation in the Shatan section,the Yanjiahe Formation in the Aijiahe section and the Liuchapo Formation in the Daotuo,Longbizui and Yanwutan sections.The Qiongzhusi Formation is corresponding with the Shiyantou,Guojiaba,Shuijingtuo,Jiumenchong and Niutitang Formations.