SEDIMENTARY CHARACTERISTICS OF THE YANGTZE RIVER'S PALEOVALLEY IN NANJING SINCE THE LAST GLACIATION MAXIMUM
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摘要: 末次冰期盛冰期海平面大幅度下降,长江发育古深槽。根据沉积物的颗粒状况,南京段古河谷的充填可以分为3期明显的由粗到细的沉积韵律:末次盛冰期深切古河谷,河床窄陡,沉积物颗粒粗,为卵砾石到中砂、粗砂;冰后期河床较宽,沉积物为砾石、粗砂到中砂、细砂;全新世,河流进一步展宽,沉积物为粗砂、中砂到细砂、粉砂,细砂沉积厚度很大。全新世中期河床有数次左右摆动,两侧形成了细砂-砂质黏土互层的沉积。根据不同时期沉积物的颗粒级配情况,推算出各时期河流的起动流速和平均流速,验证不同时期的沉积环境,认为剖面的深切河槽是局部深切的结果。各期河床形态和沉积物的特征,反映了末次盛冰期、冰后期、全新世的气候变化和环境演变。Abstract: 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.
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