Geochemical characteristics of organic-rich shale, Upper Yangtze Basin: Implications for the Late Ordovician-Early Silurian orogeny in South China

摘要

The tectonic-sedimentary environment of the Upper Yangtze Basin in China underwent significant changes from the Late Ordovician to Early Silurian. The widely distributed shale formed during this interval has recorded key information about paleo-ocean conditions. Analysis of major, trace and rare earth elements was conducted on 135 shale samples collected from five wells in the study area. The covariation of the Mo-U enrichment factors indicates that the Upper Yangtze Basin in the Wufeng stage was characterized by sedimentary differentiation. The southern basin was dominated by a reducing bottom water environment under euxinic conditions. The northern and central basins were dominated by a suboxic-anoxic environment. Up to the early Longmaxi stage, the Upper Yangtze Basin formed a relatively uniform sedimentary environment with gradually increasing oxygen content in the bottom water. The paleoproductivity index indicates that marine organisms flourished in the Wufeng to early Longmaxi stages, making an important contribution to the siliceous content in organic-rich shale; subsequently, terrestrial siliciclastics gradually dominated. Analysis of paleogeography, rare earth elements, and weathering indicates that Chuanzhong and Qianzhong uplifts in the Wufeng stage were the provenance area of the Upper Yangtze Basin, with a relatively high weathering degree. Weathering decreased in the beginning of the Longmaxi stage and then gradually increased, with the Xuefeng Uplift becoming the main provenance areas after the early Longmaxi stage. Controlled by stress transfer from the continuous syn-Caledonian Kwangsian Orogeny in eastern South China, the Upper Yangtze Basin and Xiangzhong Basin were squeezed and pushed up successively. The uplift separated the open shallow sea and formed a relatively limited deep-water shelf environment, resulting in a relatively euxinic bottom water environment, which was favorable for organic matter accumulation. The tectonic-sedimentary evolution of the Upper Yangtze Basin in the Late Ordovician to Early Silurian proposed in this study indicates that the formation of organic-enriched shale was not only related to glacio-eustatic change but also strongly controlled by tectonic activities in South China.