长江三角洲顶部冰后期地层的沉积特征

Sedimentary features of postglacial strata at the top of Changjiang River Delta

钻探证实，镇江市谏壁镇长江北岸高桥镇北部埋藏硬粘土层，之上为冰后期松散沉积物。沉积物主要为粉砂、粘土质粉砂、砂泥互层，中部局部夹含细砾中细砂，顶部见淤泥质粘土；夹数十层植物炭屑层；以水平纹理为主，局部见小型交错层理、波状层理。表明冰后期以来该地处于水动力较弱的河湖环境，为多期湖沼相沉积夹汉道河流相沉积，局部有海相沉积物加入。根据碳化植物碎屑^14C年龄，推测冰后期底界年龄约13000aBP，海侵到达该地的时间约为9000aBP，海侵最高位时间约为6500aBP。冰后期平均沉积速率约为4．9m／ka，湖沼相3．4～6m／ka，河流相10～15m／ka。对全新统的底界年龄和海相层的埋藏深度进行了讨论。

It is confirmed by borehole data that the sediments above the buried hard clay layer in northern Gaoqiao town, Zhenjiang city, north bank of Yangtze River, are postglacial loose sediments. Sediments are mainly silt, clayey silt and sand intercalated with mud （sandmud）; the medium-fine sand with fine gavel is found in the middle part and the puddly clay at the top. There are tens of carbonized plant fragment interbeds in sediments. The lamination types of postglacial strata are primarily horizontal lamination and locally smallscale cross lamination and wave lamination, indicating weak hydrodynamic fluviolacustrine deposition environment since postglacial period and sedimentary facies dominated by multi- phase lake basin bog facies intercalated with limnetic facies and branch fluvial facies locally with marine sediments added in early stage. According to ^14C ages of carbonized plant fragments, it is inferred that the boundary age between Pleistocene and postglacial strata is about 13 000 aBP,the transgression arrived at 9 000 aBP and the high est transgression position appeared at about 6 500 aBP. The deposition rate of postglacial strata is about 4.9 m/ka, that of limnetic facies about 3.4-6 m/ka and that of fluvial facies about 10-15 m/ ka. The boundary age of Holocene and the buried depth of marine layer are also discussed.