SEDIMENTARY CHARACTERISTICS OF THE YANGTZE RIVER'S PALEOVALLEY IN NANJING SINCE THE LAST GLACIATION MAXIMUM

At the last glacial maximum, deep incised-valley of the Yangtze River was developed due to the strong fall of the sea level. Nowadays the incised-valley fill in the Yangtze River Nanjing section is divided into three depositional rhythms from coarse to fine sediments based on the sedimentary particle size. During the last glacial maximum, the sedimentary sequence was from gravel to coarse and medium sands as the deep incised-valley channel was narrow and the river flew swiftly. During the post-glacial period, the flow velocity reduced as the channel widened gradually and the sediment ranged from gravel,coarse sand, medium sand to fine sand. During the Holocene, the flow velocity further reduced as the riverbed further widened. The sediment was, in consequence,coarse, medium, fine and powdery sands. Great thickness of the fine sandy layer is detected in the research work. The evidence shows that during the middle of the Holocene, the river channel swung several times according to the sediment with fine sand and sandy clay interlayers formed on both flanks. In this paper, the incipient velocity and average velocity of flow are reckoned and the sedimentary environment is identified in terms of the sedimentary particle size grading in different stages. The riverbed stratigraphic configuration and the sedimentary characteristics in different stages reflect the climate changes and environmental evolution during the last glacial maximum, the post-glacial period and the Holocene.