Geochemical Constraints and Astronomical Forcing on Organic Matter Accumulation of Marine–Continental Transitional Shale Deposits in the Qinshui Basin during the Carboniferous–Permian Transition

During the Carboniferous–Permian transition, the paleoclimate of Earth transitioned from an icehouse to a greenhouse state, severely impacting the burial patterns of organic matter (OM). Within the Upper Carboniferous–Lower Permian Taiyuan–Shanxi formations (TY–SX) in the Qinshui Basin, North China, lie a series of marine–continental transitional organic-rich shale deposits. However, the mechanism behind the enrichment of OM in these shale deposits, formed during the icehouse–greenhouse transition, remains unclear. To address this issue, a suite of transitional shale deposits was investigated by using geochemical and astronomical cyclostratigraphic analyses. The results show that the depositional age of TY–SX spans from 293.65 to 300.64 ± 0.412 Ma. The depositional environments of these TY–SX shales tended toward oxidation, with the TY shales slightly more reducing than the SX shales. The paleoclimate of the TY–SX shales suggests warm and humid environments with comparatively strong chemical weathering intensity. Primary productivity in these TY–SX shales was estimated to be low, but the influx of terrigenous detritus was robust, driving OM accumulation through terrigenous inputs. The 405-kyr astronomical cycle had a forcing effect on the paleoenvironment variations, and the 173-kyr astronomical cycle showed a covariance with OM enrichment. The TY shales exhibited a stable and moderate sedimentation rate, with enhanced terrigenous OM input leading to OM enrichment, whereas the OM contents of the SX shales, deposited under conditions of low primary productivity and oxidation, experienced dilution of the OM content owing to detrital inputs.