Depositional Conditions of Shale Lithofacies During the Late Ordovician–Early Silurian in the Upper Yangtze Area, SW China: Responses to Sea-Level Changes

The shale lithofacies shows obvious heterogeneity during the Late Ordovician-Early Silurian in the Upper Yangtze area, SW China. To elucidate the Wufeng-Longmaxi shale lithofacies' formation mechanism, we conducted high-resolution cyclostratigraphic analysis and geochemical studies of the W2 well. The findings indicate significant disparities in the sedimentation rates of the Wufeng-Longmaxi Formations, with both units recording the similar to 1.2-Myr-long obliquity cycles and the similar to 405-kyr-long eccentricity cycles period. Notably, the similar to 1.2-Myr-long obliquity cycles predominantly influenced sea-level fluctuations by mediating latitudinal heat distribution between high and low latitudes during the Late Ordovician-Early Silurian. Additionally, leveraging the sedimentary noise model and core log data, we identified five distinct sequences (SQ1 similar to SQ5) based on reconstructed sea-level fluctuations. During the transgressive system tract (TST1) of SQ1, siliceous shale lithofacies association (S) was prominently developed. In the subsequent highstand systems tract (HST1) of SQ1, shifts in the depositional environment facilitated the enhanced development of calcareous shale lithofacies association (C). During the SQ2 period, the swift elevation of sea-levels facilitated the pronounced proliferation of siliceous shale lithofacies association (S). During the SQ3 and SQ4 period, lower sea-levels coupled with significant terrigenous contributions resulted in the well-developed mixed shale lithofacies association (M). Throughout the SQ5 period, intensified bottom currents and substantial clay material deposition favored the development of the argillaceous shale lithofacies association (CM) developed as the predominant lithofacies. Ultimately, our research reveals that the heterogeneity of shale lithofacies is a multifaceted response to a series of changes in the paleoredox environment, paleoproductivity levels, and terrigenous influx, all driven by sea-level fluctuations forced by astronomical orbits. Paleoredox conditions dictate biosilica's degradation and preservation, influencing sediment composition. Paleontogenic types and productivity levels set the sediment's mineral composition. Meanwhile, terrigenous input crucially impacts the accumulation of clay and clastic materials in the sediment.