Stratigraphic Refinement Unveils High-Resolution Shallow Marine Biotic Dynamics Immediately Preceding the Late Ordovician Mass Extinction in South China

Exceptionally well-preserved Late Ordovician successions in South China offer a globally significant archive for investigating both the Great Ordovician Biodiversification Event (GOBE) and the Late Ordovician Mass Extinction (LOME), both of which make the South China become one of the best regions in the world to investigate the GOBE and the LOME. In the border region between Zhejiang and Jiangxi provinces, i.e. the Jiangshan-Changshan-Yushan area (abbreviated as the JCY area) of East China (South China paleoplate), the late Katian carbonate deposits, formerly referred to as the Sanqushan Formation (or its equivalents), are virtually the only strata in South China preserving diverse shallow marine biotas, providing rare ecological snapshots of the final biodiversity peak of the GOBE immediately preceding the LOME. Despite their importance, the lithostratigraphic framework and age constraints of these fossiliferous rocks remain debated for decades. Traditionally, the late Katian deposits in JCY area are thought to be represented by three lithostratigraphic units, i.e. the Xiazhen, the Sanqushan and the Changwu formations. The former two are associated with the paleogeographic setting of the margin of the Zhe-Gan platform, whereas the latter corresponds to the Zhexi slope., corresponding to different paleogeographic settings, i.e. the Zhe-Gan platform, the margin of the Zhe-Gan platform and the Zhexi slope respectively. The three formations were thought to be roughly contemporaneous. Based on a critical review of integrated sedimentological and paleontological data, supplemented by new and detailed field observations, we support the interpretation that the Xiazhen Formation represents the upper portion of the ‘Sanqushan Formation’, and propose to elevate the latter to group rank to represent platform facies of this particular interval, corresponding to the slope facies Changwu Formation. The revised Sanqushan Group comprises, in ascending order, the Yaojiakeng, the Jitoushan, and the Xiazhen formations. The Yaojiakeng and the Jitoushan formations correlate more precisely with the Dicellograptuscomplexus Biozone, while the Xiazhen Formation aligns with the Paraorthograptuspacificus Biozone, both of which are late Katian in age. This refined stratigraphic framework enables high-resolution reconstruction of biotic evolution prior to the LOME, offering new insights into regional ecological dynamics and their broader global significance.