THE FILLS AND STRATIGRAPHIC SEQUENCES IN THE QIANTANGJIANG INCISED PALEOVALLEY, CHINA

The tide-dominated Qiantangjiang Estuary can be zoned into three secti ons: upper normal river section, middle estuary funnel, and lower estu ary mouth. Strong rejuvenation and incision of the Qiantangjiang River formed a type I sequence boundary and a paleovalley in the last glaci ation, During the subsequent transgression and highstand, the incised paleovalley was filled and flooded, forming a complete lowstand-transg ression-highstand sedimentary sequence including sequence boundary, lo wstand systems tract, transgressive systems tract, maximum flooding su rface, and highstand systems tract. The fluvial-channel sandy gravels and gravelly sands at the bottom of the Qiantangjiang incised paleoval ley are composed of early lag sediments deposited during river incisio n, and later aggradational sediments resulted from baselevel rise whos e demarcation is the transgressive surface. The aggradational sediment s are distinct from the lag sediments in their aggradational paraseque nce assemblage, fining-upward sequence, and relatively later formation . The formation and evolution of the Qiantangjiang Estuary can be divi ded into four stages: Last Glaciation (20,000-15,000 yr B.P.): formati on of incised paleovalley. (2) Early postglacial transgression (15,000 -7500 yr B.P.): filling of the paleovalley. (3) Maximum transgression (7500-6000 yr B.P.): formation of the bay. (4) Highstand period (6000 yr B.P. to present): evolution of the estuary. Qiantangjiang River sed iments contribute only a small part of the enormous volume of deposits trapped in the Estuary, whereas the adjacent Yangtze River is an indi rect supplier of major sediments. The specific characteristics of the sedimentary sources in the estuary result in its unique sediment patte rn, i.e., sandy gravels, gravelly sands, fine sands, and muddy sands f rom upstream to downstream, which is obviously different from the norm al coarse-fine-coarse sediment distribution pattern in estuarine areas .