Heavy mineral evidence of Late Pleistocene hard clay weathering in the Jianghan Basin

Extensive hard clay layers in the Middle and Lower Yangtze River Plain were formed through fluvial incision during glacial periods and subsequent subaerial weathering of paleo-interfluves. The hard clay serves as a key for investigating re-weathering processes in lowland settings. This study focuses on the hard clay and its overlying and underlying strata from borehole HL in the Jianghan Basin, Hubei. A multi-proxy approach using optically stimulated luminescence (OSL) dating, grain-size analysis, and heavy mineral analysis, was achieved, the depositional environment, infer sediment provenance was reconstructed, and the weathering characteristics recorded within the hard clay were elucidated. Results show that the hard clay was formed between 77 and 11 ka, corresponding to the Marine Isotope Stages (MIS) 5a to 2, and experienced multiple phases of deposition and pedogenesis. The hard clay is characterized by dark green, yellowish-green, or yellowish-brown colors, and the grain-size composition was dominated by silt and clay. Grain-size parameters indicate significant fine-particle enrichment due to pedogenic and weathering modification. The heavy mineral assemblage is dominated by epidote-group, amphibole-group, and opaque iron oxides. The assemblage exhibits a high degree of homogeneity, suggesting thorough sediment mixing during transport and deposition under weak hydraulic conditions that did not cause significant mineralogical sorting. Stable heavy mineral compositions indicate that the sediments in the hard clay and its adjacent strata were primarily derived from the upper Yangtze River catchment. Furthermore, the ratio of unstable to stable heavy minerals (UM/SM) reveals that the weathering intensity of the hard clay is higher than that of the overlying and underlying strata. A comparative analysis with the First Hard Clay in the Yangtze River Delta further revealed that while the weathering intensity of the HL hard clay is weaker than that of its deltaic counterpart, both are much stronger than that of modern sediments from the upper Yangtze River. This compelling evidence suggests that after being transported and deposited in the mid-lower plain, the sediments underwent more intense chemical weathering during periods of subaerial exposure. Therefore, the formation of hard clay in the Middle and Lower Yangtze River Plain is controlled by glacial-stage exposure and weathering due to sea-level fall. A clear spatial trend of increasing weathering intensity was observed from the hard clay in the Jianghan Basin to the First Hard Clay Layer in the delta. This trend is likely attributable to the differences in the duration of subaerial exposure and weathering across the plain.