Heavy mineral composition and provenance analysis of the first hard clay on the southern wing of the Yangtze River delta

The hard clay formed during the Late Pleistocene in the Yangtze(Changjiang) River Delta serves as a crucial carrier for lowland weathering. Constraining its provenance is vital for elucidating both the mechanisms of hard clay formation and lowland weathering processes. This study focuses on the 0~10m depth interval of the ST borehole(31°07'22″N, 120°52'11″E), located on the western flank of the southern Yangtze River Delta, which includes the first hard clay layer and its adjacent strata(overlying and underlying strata). Grain-size samples were collected at 4~8cm intervals to constrain the depositional environment of the hard clay. Additionally, seven samples were selected for chronological analysis and twelve for heavy-mineral analysis. Optically stimulated luminescence(OSL) dating and TESCAN Integrated Mineral Analyzer(TIMA) analyses were employed to determine the formation age of the hard clay and characterize its heavy-mineral composition. Our results indicate that the first hard clay in the ST borehole formed between ca. 55~79ka. A sedimentary hiatus existed between the hard clay and overlying strata, suggesting post-depositional exposure and weathering. Sedimentation of the hard clay occurred under weak hydrodynamic conditions; however, homogenization during sediment transport resulted in similar heavy-mineral species and assemblages across different grain-size fractions. Lowland weathering exerted a limited influence on the total amount of extremely stable and stable heavy minerals within the hard clay, while unstable heavy minerals, such as amphibole and pyroxene, showed a pronounced decrease in abundance. The ratio of unstable to stable heavy minerals(UM/SM) reached a maximum of 6.69%in the hard clay, substantially lower than values of approximately 21%in the overlying strata and 32%in the underlying strata. Heavy-mineral assemblages in the hard clay and its adjacent strata primarily consist of amphibole, epidote, iron-bearing metallic minerals, and titanite, and closely resemble those of modern Yangtze River sediments. Based on an integrated assessment of depositional environment, diagnostic heavy minerals, and stable heavy-mineral assemblages, sediment contributions from the Xiaoshu Loess, the continental shelf, and northern Zhejiang are excluded. These lines of evidence collectively demonstrate that the Yangtze River is the primary sediment source for the first hard clay layer in the ST borehole and its adjacent strata.