Kwangsian Tectonic Uplift and the Termination of High-Toc (>3%) Shale Deposition in the Lower Silurian Longmaxi Formation, South China

The lower part of the lower Silurian Longmaxi Formation in South China contains widespread organic-rich shales, among which intervals with TOC >3% represent the key facies of high-quality shale development. Although the formation of high-TOC intervals has been widely discussed, their termination process and regional variation remain poorly constrained. In this study, under a unified graptolite biostratigraphic framework, we compiled TOC and paleoenvironmental data from different parts of the Yangtze Platform to investigate the termination horizons and controlling mechanisms of high-TOC (>3%) intervals The results show that the thickness of high-TOC (>3%) intervals varies markedly among regions, whereas their termination horizons display a clear diachronous pattern. On the inner shelf, including the Zheng’an, Xike, and Changning areas, high-TOC intervals generally terminate within the Cystograptus vesiculosus Biozone. In the outer-shelf Yihuang area, termination is delayed to the Coronograptus cyphus Biozone, whereas in some more distal areas farther from the Qianzhong Oldland, such as Weiyuan and Wuxi, high-TOC deposition may continue into the Aeronian. Geochemical data from multiple regions further indicate that this termination was first marked by a weakening of preservation conditions, as reflected by declines in redox-sensitive trace elements and related paleoenvironmental proxies, whereas the decrease in productivity proxies occurred slightly later. We therefore suggest that the termination of high-TOC shale deposition in the Longmaxi Formation was closely related to the stepwise uplift of the Kwangsian Orogeny. This tectonic process likely drove palaeogeographic reorganization, enhanced basin restriction, reduced open-marine replenishment, and weakened the maintenance of euxinic bottom waters, ultimately leading to the regionally diachronous termination of high-TOC (>3%) shale deposition. This study provides a new stratigraphic framework for understanding the spatiotemporal distribution of organic-rich shales in the Longmaxi Formation of South China.