现代荆江江心洲沉积

Modern Channel Islands Deposits in Jingjiang Reach of Yangtze River

江心洲与边滩和心滩的主要差别在于其表面植被发育和长期稳定。河道规模的水力强度决定了由上游江心洲向下游江心洲沉积物粒度的不断细化,砂坝规模的局部水流条件决定了江心洲表面上由粒度和岩石相类型所指示的微相具有分带性。江心洲上部层序主要是溢岸水流沉积的结果,但其厚度和分布范围却受江心洲大小的限制,与正常溢岸沉积具有不同的指相意义。江心洲的沉积过程形成(1)正常向下游加积(NDA),(2)汇流区向下游加积(DAC),(3)反向双侧积(CABB)和(4)向上加积(UA)等四个三维建筑结构主要素,由此构成了江心洲砂体的三维建筑结构。

Channel island is a kind of fluvial topographical unit that is different to channel bar and side bar in river channels. Most of them developed in anastomsed rivers. Five of the seven channel islands in Jingjiang reach of Yangtze river have been investigated. The largest one has an area of 6.17km^2 and the smallest one has an area of 0.66km . Lengthes range from 6.9km^2 to 2.70km and width from 1.98km to 0.34km. Sediments are fining downstream island by island and for every of them from the fore to the tail. There groups of lithofacies distribute at different parts of a channel island. Gravel facies including Gm, Gt, Gp are dominated at the fore of channel island; Sandy facies including St, Sr, Sp, Sn, Si are mainly developed at the middle of channel island; Fines including Fi, Fm are found at the tail of channel island Two facies-assemblage G-S-F and S-F are found vertically. The word' Upper Channel Island Sequence' has been proposed by the author to describe the stratification situated at the top of channel island. Within the upper channel island sequences, fines are 70. 17% in thickness and 62.5% in unit-numbers, sands are only 27.3% in thickness and 33.3% in unit-numbers. Merely gravel unit has been discovered in the channel island. Frequences of sedimentary structures are distributed as: ripple laminations 23.3%, horizontal laminations 35%, parallel bedding 6.5 massive bedding 8.4%, rootlite and burrows are 9.3% and 7.4% respectly. A few of tabular beddings are discovered on these sequences. Four types of three dimension (3D) architecture elements including 1) NDA. 2) DAC 3) LABB and 4) UA are concluded to constitute the 3D architecture of an channel island sandbody.