长江水下三角洲沉积特征与发展趋势

长江水下三角洲是一个以径、潮流相互作用为主的沉积体。野外调查、分析资料证明,在三角洲前缘,沉积物由灰、黄色细、粉沙组成,分选较好。构造见波痕交错层理等,另夹似丘状交错层理风暴沉积层。由于受径流影响,有孔虫、超微属种、个数稀少,随着离河口距离增加,水体加深,前三角洲中沉积物颗粒变细,以粉沙质泥占优,分选变差。有机炭含量较高,潜穴发育。水平、波状层理中,经常出现风暴浊积层。有孔虫属种增至30种,个数几千枚。超微含量以大洋桥石为主,>50％。前三角洲向海,颗粒变为泥质沙,分选差。波痕交错层理。有孔虫属种多于40种,个数上万枚。它反映了三角洲一陆架过渡的浅海沉积环境特点。由于科氏力、径、潮流及苏北沿岸流的作用,长江水下三角洲沉积主体目前正逐步偏向东南,这导致北部不断受改造、废弃。

Sedimentary Features and Evolutionary Trend of Subaqueous Changjiang River Delta

The subaqueous Changjiang River delta has its own distinctive depositional processes and products. It is known that the yellow silty fine sand is dominant in the delta-front and the species and numbers of foreminifera are scarce. In the subsurface strata, the Hummocky-Like sequence reflecting storm-generated sedimentation appears as a thinner interbed within thicker sediments layers, having small-scale wavy bedding obviously deposited under fair weather condition. In contrast, the darkish grey silty clay, more abundant microfauna, higher salinity and higher organic content is prevalling in the prodelta. The Graded Turbidite sequence probably representing another storm mechanism of aimutaneous sedimentation occurs repetitiously. Because of inberaction of Coriolis, in and out tidal currents, the northern distributary of Changjiang River is now being abandoned, that leads directly to the destruction of northern subaqueous delyta; the southern part of the delta, however, is gradually progradating southeastern ward as revealed by shallow seismic profiles and sediments.