The Fills and Stratigraphic Sequences in the Qiantangjiang Incised Paleovalley, China

Abstract

ABSTRACT The tide-dominated Qiantangjiang Estuary can be zoned into three sections: upper normal river section, middle estuary funnel, and lower estuary mouth. Strong rejuvenation and incision of the Qiantangjiang River formed a type I sequence boundary and a paleovalley in the last glaciation. During the subsequent transgression and highstand, the incised paleovalley was filled and flooded, forming a complete lowstand-transgression-highstand sedimentary sequence including sequence boundary, lowstand systems tract, transgressive systems tract, maximum flooding surface, and highstand systems tract. The fluvial-channel sandy gravels and gravelly sands at the bottom of the Qiantangjiang incised paleovalley are composed of early lag sediments deposited during river incision, and later aggradational sediments resulted from baselevel rise whose demarcation is the transgressive surface. The aggradational sediments are distinct from the lag sediments in their aggradational parasequence assemblage, fining.up. ward sequence, and relatively later formation. The formation and evolution of the Qiantangjiang Estuary can be divided into four stages: Last Glaciation (20,000-15,000 yr B.P.): formation 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 sediments contribute only a small part of the enormous volume of deposits trapped in the Estuary, whereas the adjacent Yangtze River is an indirect supplier of major sediments. The specific characteristics of the sedimentary sources in the estuary result in its unique sediment pattern, i.e., sandy gravels, gravelly sands, fine sands, and muddy sands from upstream to downstream, which is obviously different from the normal coarse-fine-coarse sediment distribution pattern in estuarine areas.