Paleoenvironment And Mechanism Of Organic Matter Enrichment During The Deposition Of Wufeng-Longmaxi Formations In East And South Margin Of Sichuan Basin,South China

The Wufeng Formation-Longmaxi Formation sedimentary period coincides with the transition period at the turn of the Ordovician-Silurian.This is an important period in geological history,during which a series of major geological events occurred,including widespread orogeny,volcanic eruptions,global climate cooling,sea level fluctuations,and biological extinctions.This period was also an important period of global deposition of organic-rich black shale.The Ordovician-Silurian period in South China has a paleogeographic pattern of confined basins,which may have been more significantly influenced by regional tectonic movements and volcanic eruptions.Like the global black shale deposition during this period,the organic-rich black shale system of the Wufeng Formation also developed in South China during the Late Ordovician.Together with the organic-rich black shale of the Early Silurian Longmaxi Formation,this black shale is generally considered to be a key formation for shale gas exploration and development in South China because of its wide distribution,large thickness,high organic matter content and moderate maturity.The formation of hydrocarbon source rocks during this sedimentary period is usually considered to be closely related to the paleomarine environmental changes caused by biological extinction and climate fluctuations during this period.Therefore,it is very important to carry out the paleomarine depositional environment and organic matter enrichment mechanism of the Wufeng Formation-Longmaxi Formation in the east-south margin of the Sichuan Basin to explore the Ordovician-Silurian paleomarine depositional environment,controlling factors and biodepositional events in South China.This research selected the Chengkou area in northeast of Chongqing,the Xiushan area in southeast of Chongqing,and the Leibo-Changning area in south of Sichuan as the focus of the study.By using high-resolution sampling and testing analysis,combined with the regional geological data,the well-exposed outcrops and newly drilled cores are studied.The whole rock samples and thin section samples were observed by sedimentological methods,and the main trace elements,organic carbon content,sulfur content,organic carbon isotopes and nitrogen isotopes of the samples were tested and studied by geochemical methods,and the paleoclimatic changes during this period and their effects on the paleomarine environment were comprehensively analyzed,and the organic matter enrichment mechanisms of the Wufeng and Longmaxi formations were discussed on this basis.Following understandings were obtained:(1)The redox states of water in different regions of South China are obviously non-homogeneous.The deep-water section(Shuanghe)is anoxic in all periods except for the Guanyinqiao Formation,which is in a depleted oxygen state.The differential redox states reflected by multiple sets of strata in the study area all show fluctuating trends of paleo-oceanic variability.(2)Paleomarine productivity reconstructions from different regions of South China reveal differential productivity patterns in the study area during the Ordovician-Silurian period.The good Ba-Al correlation and high biogenic barium content across the region suggest that the South China paleo-ocean had high productivity levels in the Ordovician-Silurian.(3)The ancient ocean during the Ordovician-Silurian transition in the study area is relatively limited.During the same period,the northeast part was more open than the southeast part,and the southwest part was the most restricted.In the same area,the Longmaxi period was more open than the Wufeng period,and the Guanyinqiao deposition period was the most restricted.(4)The Ordovician-Silurian strata in the study area recorded global climate change information,which showed that the early and middle Wufeng and Longmaxi periods were warmer,and the climate in the late Wufeng period became colder and reached a peak during the Guanyinqiao depositional period.The above climate change features are well documented and regionally comparable by the CIA of the Yangzi Platform,suggesting a good depositional record of paleoclimatic changes in South China at low latitudes during this period.(5)Carbon and nitrogen isotope records from different study areas in South China show coupled variability highly correlated with climate fluctuations.The oxidizing environment in the shallow water profile(YY-1),influenced by the regional depositional environment,caused the lower part of the Wufeng Formation to have high carbon isotope values and large fluctuations.The apparent positive carbon isotope drift of the Longmaxi Formation in the open sea profile(YC-1)may be related to the massive burial of organic matter due to long-term high productivity.The strong denitrification in all studied profiles led to a large loss of bioavailable nitrogen,and nitrogen fixation caused the lower nitrogen isotope values before and after the ice age;the weakened denitrification was responsible for the positive nitrogen isotope drift during the ice age.(6)The organic matter enrichment of the Wufeng Formation-Longmaxi Formation in the Sichuan Basin benefits from anoxic water bodies,a certain level of productivity and a low deposition rate.Although the regional weights of the above factors are different,iron limitation is an important reason that may cause the accumulation of organic matter in the whole basin.