The Evolution of Continental Weathering and Upwelling in Early Silurian and the Implications for the Organic Matter Accumulations in the Shales of Longmxi Formation

Abstract: The organic-rich shales in the Early Silurian Longmaxi Formation are the important source rocks in South China, however, the deposition mechanism of these shales is still under debate. [Objective] In order to reconstruct the climatic and oceanic environments during the Early Silurian, and investigate the influence of significantly geological events (especially the continental weathering and upwelling) on the organic matter accumulation, [Methods] this study presents the geochemical compositions of the Lower Silurian succession of the Upper Yangtze Platform (BD1 core, Butuo district). [Results] The results show that the Longmaxi period experienced a transition from weak chemical weathering intensity in early period to intense chemical weathering intensity in later period. In addition, the Longmaxi Formation in studies area is characterized by restriction ocean. This study posits that early Longmxi sediments exhibited low chemical weathering intensity and more congruent weathering, while Aeronian sediments were characterized by intermediate chemical weathering intensity and incongruent weathering. Given that the gradual enrichment of oxygen within bottom waters can be supported by the redox condition proxies, this study suggests that congruent weathering-induced high paleoproductivity in early Longmaxi is the primary factor accelerating anoxia and further the enrichment of organic matter. [Conclusion] In summary, there is obvious differences between the deposition of Longmaxi shales in different areas, and this study suggests that the weathering-induced high productivity contributed to the deposition of organic-rich shales, while the upwelling likely triggered the organic matter accumulations in Longmaxi shales in the outer shelf, instead of inner shelf.