冰后期长江河口段古河谷地层层序特征

末次盛冰期,由于基面大幅度下降,长江河口段形成了深切古河谷。河流携带的部分粗砂砾在河床上形成滞留沉积。冰后期海平面上升引发的海侵造成了长江古河谷的充填和河床相、河漫滩河口湾相和部分河口湾浅海相的形成。尔后的进积产生了部分河口湾浅海相及三角洲相。根据钻孔资料绘制的古河谷地质剖面图显示,冰后期海侵旋回出现在三角洲顶点到现在河口口门地区,溯河而上受海洋影响的程度逐渐减弱。古河谷的中部,靠近主泓线附近,沉积层序完整。古河谷的边缘地带,往往缺失底部的河床相砂砾层。底部的厚层河床相砂砾层是由河床滞留沉积和溯源加积堆积形成的,在古河谷沉积层序中存在着多种沉积间断。

Characristics of Post-glacial Incised-valley Stratigraphic Sequence in Yangtze River Mouth Area

During the last glaciation maximum,strong fall in base-level led to rejuvenation of the Yangtze River,and a deep incised-valley was formed.Some coarse gravel loaded by the river deposited in the river channel.The postglacial transgression caused by sea level rise led to the filling of the incised valley and formation of the river channel,floodplain-estuary,estuarine-shallow marine and deltaic stratigraphical units.From bottom to top,the middle incised valley is composed of channel,floodplain-estuary,estuary-shallow marine,and modern deltaic facies.As demonstrated by section of drilling cores in the incised valley,the post-glacial transgressive cycle appeared between the apical area of the Yangtze delta and the river mouth area,with decreasing marine influence upstream.In the middle of the incised-valley,stratigraphic sequence was perfect.The edge of the incised-valley lacked the sand-gravel bed of the river channel at the bottom.The retrogressive aggravation and the river channel deposition of the last glaciation maximum and the sea-level rise of the postglaciation accumulated sand-gravel bed of the thick river channel facies at the bottom.There exited many types of the depositional gap within depositional sequence.