Geological And Geochemical Characteristics Of The Lower Cambrian Strata,and The Evolution Of Palaeoenvironment And Development Of Source Rock During Arly Cambrian In Jiangsu And Anhui Areas

During terminal Ediacaran to early Cambrian,animals on earth diversified rapidly in several million years,which is called "Cambrian explosion" and is one of the most significant evolution events during the geological history.Ediacaran and Cambrian sedimentary successions are well developed in the Yangtze Platform,South China,and are fossiliferous,providing an excellent opportunity for studying the environmental and biological evolution during this stage.Numerous studies on the strata in the past 20 years revealed valuable information about animal activities,redox environments,and biological processes in the ocean during this stage.However,the research area has been focused on the western Yangtze Platform,yet little work has been carried out on the lower Cambrian strata in the eastern Yangtze Platform due to a lack of well-preserved sections,limiting our understanding of the stratigraphic framework,palaeoenvironmental condition,and source rock distribution in this region.In this work,we carried out systematic petrological and geochemical investigations of the lower Cambrian strata in the eastern Yangtze platform with core and rock debris samples collected from four drilling wells,including Well WN2 and Well XY1 in Ningguo county,Anhui Province,as well as Well K2 in Kunshan city and Well DS1 in Wuxi city,Jiangsu Province.High-resolution carbon isotope analysis results reveal that carbon isotope curves in the research area are comparable to the global Cambrian carbon isotope curve.BACE,SHICE,CARE,and MICE excursions can be identified in the Hetang Formation in Ningguo area,demonstrating the age of the strata possibly ranges from Cambrian Fortunian to early Stage 4.AECE and ROECE can be identified in the Dachenling Formation overlying the Hetang Formation,supporting that its age could range from middle to terminal Cambrian Stage 4.MICE,AECE,and ROECE excursions can be recognized in the Mufushan Formation in Kunshan area,revealing a sedimentary age from late Cambrian Stage 3 to terminal Cambrian Stage 4.On the other hand,CARE,MICE,and ROECE excursions can be identified in the Well DS1 section in Wuxia area,indicating that this region possibly accepted no deposition during Cambrian Fortunian and Stage 2.Based on systematic nitrogen isotope study and compilation of nitrogen isotope data across the Yangtze Platform,we proposed a comprehensive evolution model of the ocean redox structure,nitrogen cycle,and animal in the ocean from terminal Ediacaran to early Cambrian Stage 4.During terminal Ediacaran,the chemocline in the ocean was possibly low and ?15N values in the strata are generally higher than 2 ‰,revealing an aerobic nitrogen cycle in both the shallow and deep water regions in the ocean.During Cambrian Fortunian,the chemocline in the ocean became shallow because of transgression,leading to decreasing ?15N velues in the deep water region in the ocean.During middle Cambrian Stage 2,the nitrogen cycle in the shallow water region became dominated by N2 fixation due to low N:P ratio in the water column.With limited consumption and increased terrestrial input,nutrients as well as other substances such as SO42-and Mo could have accumulated and increased to a high level in the water column.During late Cambrian Stage 2,intense upwelling of anoxic deep water stimulated by hydrothermal activities replenished abundant NH4+ to shallow water region and terminated the N-limitation environment.The flourishment of biological activites in the nutrient-enriched envrioment led to significant release of O2 and expanded oxygenation toward the basin.During early to middle Cambrian Stage 3,a nutrient-enriched oxic water environment was established in the shallow area,which could have stimulated the rapid diversification of animals during this stage.During late Cambrian Stage 3,?15N values in the mid-depth water region decreased,possibly resulting from participation of Fe or V-based nitrogenase.During Cambrian Stage 4,?15N values in the shallow area were higher than 3‰ and increased in the mid-depth area,demonstrating expanded oceanic oxygenation.Systematic petrological and mineralogical studies were carried out on the Hetang and top Xijianshan formation in the Well WN2 section to figure out the compositional characteristic and occurrence of Ba-rich hydrothermal minerals.The result shows that Ba-rich silicates are common in the Xijianshan and Hetang formation,and can be divided into Ba-poor K-feldspar,Ba-rich K-feldspar,and cymrite based on composition and occurrences.Incorporation of NH4+in the Ba-poor and Ba-rich K-feldspar leads to no-stoichiometric compositions from electronic microprobe analysis.Cymrite and Ba-rich K-feldspar in chert were possibly formed by Ba-rich hydrothermal fluids released to the seafloor,while Ba-rich K-feldspar in other rocks was formed by hydrothermal fluids infiltrated into the strta when un-consolidated.On the other hand,Ba-poor K-feldspar was formed by ionic exchange between hydrothermal fluids and K-feldspar primarily contained in the rock.No evident impact of the hydrothermal fluids on the concentration of redox-sensitive trace elements such as V,Ni,Mo,and U can be identified,yet a strong positive Ba-Eu correlation occurs in the samples due to enrichement of both Ba and Eu in the hydrothermal fluids.Combined with previous studies and personal work,we discussed the palaeogeographic framework in the research area and the result demonstrated the Chaohu area in the northwest and the Wuxi-Kunshan area in the northeast were possibly topopraphically high and source rock was not developed,while the Dongzhi-Huangshan-Kaihua region was topopraphically low,with thick source rock sgements containing high TOC contents.On the other hand,petrology,organic geochemistry,and reservoir property investigations of the Hetang Formation in the Ningguo area reveal that the strata are proper for the formation of shale gas reservoir,especially the carbonaceous mudrock segment in the upper Hetang Foramtion.