末次冰期最盛期以来长江扬中段古河谷的沉积环境

Sedimentary environment of the Yangtze River paleo-valley in Yangzhong reach since the LGM

选择长江扬中段古河谷作为研究对象,探讨末次冰期最盛期以来的沉积环境.搜集整理了扬中—泰州长江大桥地质勘探钻孔28个,根据钻孔资料,绘制了长江古河谷的地质剖面示意图.在其中2个钻孔采集了粒度样品、磁化率样品,分别用Mastersizer 2000激光粒度仪、Bartington MS2磁化率仪进行了分析;在4个钻孔采集了14C年代样品,利用液体闪烁计数仪进行测定,得出了27个年代数据.结果表明:扬中地区长江古河谷约-61 m以上沉积为末次冰期最盛期以来的沉积;末次冰期最盛期、晚冰期早期(约12 000 a B.P.),河流滞留沉积与溯源堆积形成了厚约24 m的河床相砂砾层;-23^-37 m为12 000 a B.P.-8 000 a B.P.时的河漫滩相沉积,沉积物主要为粉砂及粉砂质黏土,局部细砂;8 000 a B.P.-4 000 a B.P.为河口湾环境,形成河口湾-浅海沉积,厚度为-17^-23 m,沉积物主要为粉砂夹粉砂质黏土;-17 m以上为4 000年以来的河口坝、汊道河床沉积及三角洲形成以来的河床、河漫滩沉积.

The Yangtze River incised-valley in the Yangzhong reach was selected as the research object to study the sedimentary environment since the Last Glacial Maximum.Cores of 28 geological exploration boreholes of the Yangzhong-Taizhou Yangtze River Bridge were collected and sorted out.Based on the drilling data,the stra-tigraphical cross-section of the Yangtze River incised-valley was drawn.In two of the boreholes,grain-size samples and magnetic susceptibility samples were collected,and then analyzed respectively by using Mastersizer 2000 laser particle size analyzer and Bartington MS2 magnetic susceptibility meter;samples for 14 C dating were also collected in four of the boreholes and then were tested by liquid scintillation counter in State Key Laboratory of Lake Science and Environment affiliated to Nanjing Institute of Geography and Limnology,Chinese Academy of Sciences.At last,27 age data were obtained.It is concluded that the top 6 1-m deposition on the Yangtze River incised-valley in Yangzhong region belongs to the Last Glacial Maximum.The riverbed gravel layer,24 m in depth,was formed in the Last Glacial Maximum and the Early Late Glacial （about 12 000 a B.P.）due to the lag deposits and headward accumulation of river.Floodplain deposits developed during 12 000 -8 000 a B.P. lies at about -23^-37 m depth with the sediments mainly of silt and silty clay,partly fine sand.During about 8 000-4 000 a B .P.the Yangtze River incised-valley in Yangzhong region had presented estuary environment and developed estuarine-shallow marine deposits,-17 ^-23 m in depth,and the main components of sedi-ments are silt mixed with silty clay.About -17 m above there are mouth bar and braided riverbed deposit since 4 000 years before and the riverbed,floodplain deposits since the formation of deltas.