The Events Across The Ordovician-Silurian Transition And The Organic Enrichment Of Black Shales In The East Of Sichuan Basin

The Late Ordovician-Early Silurian transition was a critical interval in Earth's history,marked by mass extinction,the Gondwana Glaciation and volcanic events.The black shales of Wufeng and Longmaxi formations in South China are the main strata for shale gas exploration at this stage.The presence of many bentonites indicated that the multi-stagevolcanisms had been happened in South China at this time.Understanding the effects of the glaciation and volcanisms on the organic matter accumulation can provide theoretical guidance for the next phase of the shale gas exploration.In this study,six continuous Wufeng-Longmaxi formations shale section and Huadi No.1 well from the east of Sichuan Basin are sampled.Selecting the black shales and bentonites as the research object,and We try to determine sedimentary sequences,sedimentary facies and evolution process of lithofacies paleogeography,and systematically analyse the mechanisms of organic matter accumulation(terrigenous flux,redox condition,producitivity)in these deposits and the time and origin of the volcanism by intergration of field investigation,major and trace element concentrations,and zircon U-Pb dating.This study also investigates the climate,the volcanic activities during Ordovician-Silurian transition and their relationships to the enrichment of organic matter.The major conclusions are summarized below:(1)According to petrological characteristics,sedimentary structures and fossils,the Wufeng,Guanyinqiao and Longmaxi formations can be divided into shore facies,tidal flat,shoal,shallow water shelf and deep water shelf facies,which verifyinga shallow water deposition between the two sets of black shales in Wufeng and Longmaxi formations.(2)Fluctuations in CIA values reflect that the climate shifted from warm to cold and then to warm again during the end Ordovician-early Silurian time period,which provides the geochemical records evidence for glaciation at the end of the Ordovician in South China.(3)Zircon U-Pb dating was performed for the bentonites and yielded the U-Pb zircon ages from 440.4±5.6Ma to 448±2Ma,which suggest that volcanic eruptions in South China lasted for at least 8 Ma across the O-S boundary.Application of various chemical discrimination diagramsimply that bentonites were mostly from middle-acid rocksand possibly derived from an island arc environment.The volcanic ash was probably related to the subduction and closure of the Qinling Ocean on the northern border of Yangtze Plate in the Early Paleozoic because of the fact that the number and thickness of the bentonites gradually decreased from the North to South and a new understanding of the tectonic nature of South China(4)The TOC variations from Wufeng to Longmaxi formation showed a high-low-high mode,and the redox indices(V/V+Ni,V/Cr,Ni/Co)implied that the water experienced an anoxic-oxic-anoxic process.The productivity(Cu,Babio,Sibio)and terrigenous inputs indices indicated higher productivity and terrigenous inputs in black shales of Wufeng and Longmaxi,when compared with those of Guanyinqiao formation.The correlations between the TOC in the sediments and redox indexes,productivity and terrigenous inputs,we found that the proportion of controlling organic matter enrichment,the redox indexes occupied first place,followed by productivity and terrigenous inputs the least.(5)From the correlations between the bentonites and the TOC,productivity and the redox conditions,we found that maybe frequent volcanic activities had dual effects on organic-rich shale:on one hand,volcanic ash provided a sufficient supply of nutrients,which triggered high marine productivity;on the other hand,volcanic ash prevented more sunshine coming into the ocean,and the extremely anoxic environment increased and led to the more death of many living things,and the enhanced theburial amount and preservation of organic matter.(6)Although both Wufeng and Longmaxi formations developed the same black shales,the formation mechanisms were different.The black shales in Wufeng formation were formed by the compressive tectonic setting and the volcanic activities,nevertheless,the black shales deposited in Longmaxi formation produced by the collisional conditions,volcanic eruption and melting glaciers...