Ordovician sedimentary evolution of the northeastern margin of the South China Block and its paleogeographic implication

The South China Block is an essential area for global Ordovician research, characterized by a "platform-slope-basin" paleogeography that provides an excellent setting for investigating how different geological conditions influence sedimentary evolution. However, existing Ordovician paleogeographic models for the South China Block are relatively outdated and lack detailed sedimentary facies analysis. In addtion, current facies zone division are rough, and sedimentological research on the block's northeastern part remains insufficient, which collectively hinders a comprehensive understanding of its Ordovician sedimentary evolution and paleogeographic reconstruction. To address above issue, this study examines the sedimentary evolution of carbonate rocks from the northeastern margin of the South China Block (Chuzhou, Anhui Province). A total number of seventeen carbonate microfacies types were identified, which were grouped into seven facies associations representing seven distinct sedimentary environments. The evolution of these sedimentary environments in the study area can be divided into three phases: an inner ramp environment in the Early Ordovician, a middle ramp environment mainly in the Middle Ordovician, and an outer ramp environment in the Late Ordovician. The sedimentary evolution indicates an overall rising sea-level trend, with two apparent rises in the late Floian and the middle Dapingian. From the Dapingian to the Katian age, the study area did not exhibit sedimentary characteristics of a platform depositional environment, but instead dominated by deep-water sedimentary facies. Evidence for this deep-water setting-including increasing water depth, rising conodont diversity, and a shift in biogeographic provinces from shallow to deep water-demonstrates that the area differed significantly from previous classifications as a platform facies.