Clay mineral analysis of sediments in the Changjiang delta plain and its application to the late quaternary variations of sea level and sediment provenance

Clay mineral analysis was conducted on the fine-grained sediments of a 20-meter core of Late Quaternary deposit in the south-central portion of the modern Changjiang delta plain to understand and characterize paleoenvironmental depositional conditions. Four clay mineral suites (Zones I to IV) were recognized in the core. The lowest level (Zone I, Late Pleistocene) of kaolinite (48%) and illite (45%) is representative of a pedogenic formation of alluvial environment. The next upper suite (Zone II, Early Holocene) of kaolinite (44%) and chlorite (22%) is related to an erosional coastal environment associated with an encroaching sea level. The suite (Zone III, Early to mid-Holocene) overlying Zone II incorporates the addition of smectite (6%), the abundance of which is closely linked to the topographic change and the relative sea level fluctuation in the study area. The uppermost suite (Zone IV, Late Holocene) of illite (53%) and smectite (7%) accumulated during the period of major deltaic progradation and presents a change of sediment source from the more local western highlands to the regional Changjiang River basin. Significant variations in the clay mineral signatures support concepts of morphologic change from a subaerially exposed area to the present depression-centered southern Changjiang delta plain during the Holocene as the delta margin was affected by sea level rise and sediment supply and source.