长江水下三角洲的动力沉积

DYNAMIC SEDIMENTATION IN THE SUBAQUEOUS YANGTZE(CHANGJIANG)DELTA

2002年9—10月在长江水下三角洲获取了163个表层样,综合粒度、有孔虫、介形类、有机碳以及CaCO3含量的资料表明,研究区明显分为两部分,即长江河口现代三角洲和晚更新世末期三角洲。根据表层沉积物的粒度特征,现代水下三角洲可以分为以黏土质砂和细砂为主的前缘斜坡区(A区)、以黏土质粉砂为主的前三角洲区(B区)和以砂-粉砂-黏土为主的三角洲向陆架过渡区(C区),呈现较为明显的粒径细化趋势,有机碳与黏土含量呈现明显的正相关,CaCO3含量的变化不大,从Pejrup图可以看出现代三角洲从A区到C区的动力递减变化。晚更新世末期三角洲(D区)粒径较粗,有孔虫和介形类的氧化壳含量较高,有孔虫和介形类的丰度和含量也较高,CaCO3的含量较高,与现代三角洲形成明显的对照。对比20世纪70年代和80年代的资料,近岸物质趋向粗化,北支水下沙脊发育,现代三角洲不断向外扩展,残留砂面积减少。50多年来河口来沙的减少已经引起整个现代三角洲物质粗化,现代沉积区沉降中心面积大幅度减少。

In late September and early October 2002,163 surface samples were grabbed from the Yangtze subaqueous delta within a range of 122°~123°E and 30°50′~31°50′N. Based on analyses of grain size, foraminifera, ostracoda, carbonate, organic carbon of surface sediments, the study area can be divided into two parts, i.e. the modern subaqueous delta and late Pleistocene delta. The modern subaqueous delta can further be classified into Zone-A, Zone-B ,Zone-C,and Zone-D. Zone-A is the delta front with clayey sands and fine sands as its dominant components. Zone-B is the prodelta with clayey silt. Zone-C is the transition from prodelta to continental shelf with sand-silt-clay. From A to C, grain size tends to be finer with weakening dynamics from Pejrup diagram. There is a good positive correlation between organic carbon and clay percentage with low carbonate. Zone-D is composed of relict late Pleistocene delta sand with high contents of oxidized tests, high abundance and percentage of foraminifera and ostracoda and high percentage of carbonate. Based on comparisons with surface sediment types in 1970s and 1980s, the modern subaqueous delta is expanding seaward and mixing with the relict late Pleistocene delta sands, meanwhile,nearshore sediment is coarsening and subaqueous sand ridge is developing. The decrease of fluvial sediment towards the estuary has led to coarsening of bottom sediments in the modern delta and shrinking of modern depocenter.