Geochemical Characteristics And Organic Matter Accumulation Of Late Ordovician–Early Silurian Shale In The Upper Yangtze Platform,and Implications For Paleoenvironment

The Ordovician–Silurian transition was a critical interval in Earth history owing to considerable changes in biosphere,climate and environment.It is marked by marine mass extinction,large-scale glaciation,sea level change,extensive volcanism,ocean anoxic event and widespread deposition of organic-rich black shale.The Upper Yangtze Platform of South China preserve a complete stratigraphic record of Ordovician–Silurian transitional sequence and has been the subject of considerable biological,climatic,and paleogeographic study.In addition,the Upper Yangtze Platform contains several organic-rich shales and thus is the major region for the shale gas exploration and development.Among them,the Wufeng–Longmaxi Formation shale is the main strata for shale gas exploration in China and currently attracts much interest.Thus,the study on the mechanism of organic matter accumulation and depositional environment for the Upper Ordovician to Lower Silurian strata in the Upper Yangtze Paltform not only can provide evidence for “sweet spot” prediction,but also provide important insight for the Ordovician–Silurian climatic and paleoenvironmental change.In this study,four continuous Upper Ordovician to Lower Silurian shale section from the Upper Yangtze Platfrom,China were sampled.We try to reconstruct paleoenvironmental conditions(terrigenous flux,redox condition,producitivity,and watermass restriction)during the deposition of the Wufeng and Longmaxi formations and discuss the mechanisms of organic matter accumulation in these deposits by intergration of total organic carbon(TOC),major and trace element concentrations,carbon isotope geochemistry and zircon U-Pb dating.This study also investigate carbon isotope perturbations during Ordovician–Silurian transition and their relationships to climatic environmental changes.The major conclusions are summarized below.(1)The high-precision geochemical profiles of Wufeng–Longmaxi Formationshale were established and the depositional environment ofWufeng–Longmaxi Formation was disscussedThe TOC content of the Wufeng–Longmaxi Formation shale significantly changed.The Wufeng Formation displays relatively high(avg.3.2%)but significantly variable TOC values ranging from 0.12% to 6.0%.The overlying lower Longmaxi Formation is characterized by the highest TOC contents(avg.4.4%).The upper Longmaxi Formation is characterized by low and relatively constant TOC with mean value of 1.6%.In the organic-rich interval,the major elment SiO2?CaO and trace element iMo,U,V,Ni,Co,Cr and Ba were obviously enriched,but the Al2O3,Fe2O3 andTiO2 are depleted.The concentration of redox-sensitive trace elements(Mo,U and V),DOPT and covariation of Mo-U suggest that Wufeng Formation accumulated under predominantly anoxic conditions.The lower Longmaxi Formation were deposited under strongly anoxic to euxinic conditions,whereas organic-poor intervals of the upper Longmaxi Formation accumulated under suboxic conditions.P,Barium excess(Baxs)and biogenic silica(Siexcess)values indicate high paleoproductivity throughout the entire depositional sequence,but organic matter accumulation was predominantly controlled by preservation.(2)The mechanisms of organic matter accumulation was established for theWufeng and Longmaxi Formations shale,respectively.The main controlling factors of the organic matter accumulation were different for the Wufeng and Longmaxi Formation shale.Mo–U covariation and Mo/TOC values reveal the Longmaxi Formation accumulated under moderately restricted conditions.However,during the Late Ordovician,the growth of the Gondwana ice sheet led to the lowering of sea level.At the same time,tectonism transformed the Upper Yangtze Sea into a silled basin.The reduced water depths and tectonically isolated nature of the basin resulted in severe watermass restriction.Such extreme restriction effectively blocked watermass exchange with the adjacent global ocean thereby reducing the degree of oxygenation,wHICE together with enhanced saline stratification,promoted anoxic conditions favorable for the preservation of organic matter.During deposition of the lower Longmaxi Formation,as sea level rose,deepening of the basin,sediment starvation,and increased levels of nutrient recycling contributed to the enhanced preservation of organic matter.A subsequent reduction of sea level coeval with accumulation of the upper Longmaxi Formation lead to water ventilation,wHICE together with dilution by increase in terrigenous sediment flux led to the reduction in TOC content.(3)Abundant volcanic ash layers are identified from the Upper Ordovician to Lower Silurian in Upper Yangtze Platfrom and zircon U-Pb dating was performed for the tHICEer ash layers.This provides a new method to study the climatic and paleoenvironmental change during Late Ordovician to Early Silurian.We identified several thin bentonite layers in both the Wufeng and the Longmaxi formations,wHICE indicate that South Chian experienced multiple volcanic events during the Ordovician–Silurian transitional period.A volcanic ash bed(bentonite)from Xingwen section was used to U-Pb dating and yields a U-Pb zircon age of 440 ± 3 Ma,w HICE confine the termination age of the deposition of the Longmaxi Formation organic-rich shale.This age is consistent with Rhuddanian–Aeronian global boundary age and thus provides accurate isotope age for this boundary.In addition,the age from Longmaxi bentonite can be correlated with the age from Scotland and Southeastern Alaska,probably indicating they represent the worldwide eruption of vocanism at the Rhuddanian–Aeronian boundary.This provide important insight for the study of climatic and environmental change during this period.(4)Carbon isotope curve for the Upper Ordovician to Lower Silurian strata was established.The negative carbon isotope excursion in the Lower Silurian can be regarded as a defining characteristic of the Rhuddanian Ocean Anoxic event.The Ordovician–Silurian transition was also characterized by perturbations to global carbon cycle.Following the well-studied Hirnantian positive excursion,the ?13Corg decreases and displays negative excursion in OM-rich shale interval of the Lower Silurian Longmaxi Formation.This negative shift in ?13Corg can be correlated globally,and thus represents the perturbation to global carbon cycle.The negative excursion coincides with the deposition of lower Longmaxi organic-rich shale as well as anoxic to euxinic condition.Thus,we propose that the negative ?13C excursion in the Lower Silurian was a response to Rhuddanian oceanic anoxia event(R-OAE).This event was terminated by a positive excursion in OM-poor interval,wHICE is confined to 440 ± 3 Ma at the Rhuddanian–Aeronian boundary and can be correated with the recognized the Aeronian positive carbon isotope.Thus,the integration of carbon isotope stratigraphy and U-Pb age provides a useful tool for the regional and global stratigraphic correlation of Late Ordovician and Early Silurian strata.(5)The release of volcanic CO2 from extensive volcanic activity was responsible for the negative carbon isotope excursion and climatic and paleoenvironmental change during Early Silurian.Carbon isotope perturbations during Late Ordovician to Early Silurian coincides with change of depositional environment.According to the U-Pb age,the duration of negative ?13Corg excursion during R-OAE lasted about 3 Myr and was associated with.The long-lived negative carbon isotope excursion indicates a substantial and global addition of isotopically light carbon into the ocean-atmosphere reservoir,possibly linked to the release of volcanic CO2.The widespread of volcanic ash layers in the Upper Yangtze Platform provide direct evidence for the volcanic eruption.The release of volcanic CO2 might raise global temperatures,leading to the transgression.With the sea level rise,the ocean was anoxic,leading to the wide depositon of organic-rich shale.The enhanced burial of organic carbon during R-OAE could remove the substantial 12 C from the carbon reservoir and lower atmospheric pCO2,resulting in subsequent positive ?13C excursion and global cooling at the Rhuddanian–Aeronian boundary.