Watermass architecture of the Ordovician-Silurian Yangtze Sea (South China) and its palaeogeographic implications

During the Late Ordovician Hirnantian Ice Age, the South China Craton experienced large changes in climate, eustasy, and environmental conditions, but their impact on the watermass architecture of the Yangtze Sea has not been thoroughly evaluated to date. Here, we reconstruct the salinity-redox structure of the Yangtze Sea based on five Upper Ordovician-lower Silurian shale successions representing a lateral transect, from a deep-water area of the Inner Yangtze Sea (IYS; Shuanghe section) across the shallow Hunan-Hubei Arch (Pengye, Jiaoye, and Qiliao sections) to the relatively deep-water Outer Yangtze Sea (OYS; Wangjiawan section). Carbon-isotope ( δ ¹³ C _org ) profiles show that the Guanyinqiao Bed (recording peak Hirnantian glaciation) thins and is less completely preserved at sites on the flanks of the Hunan-Hubei Arch than in deeper-water areas to the SW and NE, reflecting bathymetric influences. Watermass salinities were mainly marine at Shuanghe and brackish at the other four study sites, with little variation between Interval I (pre-glaciation), Interval II (Hirnantian glaciation), and Interval III (post-glaciation). Redox proxies document mainly euxinia at Shuanghe and Wangjiawan and suboxia at the other sites during Interval I, with shifts toward more reducing (mostly euxinic) conditions at most sites during Intervals II and III, which shows that all study sections were deep enough to remain below the redoxcline during the glacio-eustatic lowstand. Two features of the Shuanghe section mark it as being unusual: it alone exhibits fully marine salinities, implying greater proximity to the open ocean than for the other four sites, and it exhibits an especially large shift toward more reducing conditions during Interval III (i.e., the post-Hirnantian transgression), implying greater water depths. These features are difficult to reconcile with the standard palaeogeographic model for the Ordovician-Silurian South China Craton, which is characterized by a geographically enclosed and restricted IYS and a more-open OYS, arguing instead for the SW end of the IYS having been connected to the global ocean and the OYS having been a restricted oceanic cul-de-sac. A review of sedimentologic and facies data for the IYS region suggests that our re-interpretation of the Ordovician-Silurian palaeogeography of the South China Craton is viable, although further vetting of this hypothesis will be needed.