三角洲沉积模式的新认识

New Understanding of Delta Depositional Model

水槽实验和三角洲现代沉积考察发现,三角洲前缘中并不发育水下分流河道,三角洲平原上的分流河道延伸到湖(海)平面处就消失了,三角洲前缘的砂体通常呈席状和扇状,而非条带状,这些都与传统的认识有所不同。扇三角洲沉积模式与河流三角洲有很大不同,不能套用河流三角洲的沉积模式来研究扇三角洲。洪积扇和扇三角洲平原上均发育2种地貌单元,即河道及河道间,两者都为砂砾岩沉积,并没有沉积泥岩的环境。与砂砾岩互层的泥岩不是在洪积扇沉积时沉积的,而是洪积扇退缩时期的扇前和扇间沉积,或是洪积扇废弃时期的沉积。至于扇三角洲前缘,主要是河道和片流带入的砂砾沉积,河道带入的砂砾形成河口坝,片流带入的砂砾形成"水下片流浅滩",两者连片形成席状砂砾岩,与扇三角洲平原砂砾岩连为一体。

Through flume experiment and investigation of modern deltas,it is found that in front delta,underwater distributary channels are not formed,the distributary channels of delta plain disappear immediately once extending to the lake（sea） level;front delta sand bodies are usually sheet-shaped and fan-shaped,instead of ribbon shaped.These are different with the traditional understandings of them.The study suggests that depositional model of fan deltas differs widely from that of river deltas.On alluvial fans and fan delta plain develop two types of topography units： channel and inter-channel,both of which are sandy-conglomeratic deposits.There is not environment for shale or mudstone deposition.The mudstone deposit interbeded with sand-conglomerate is actually formed at pre-fan and inter-fan during regression of alluvial fan or when the alluvial fan is abandoned.The deposits of front fan deltas are dominated by sandy-conglomeratic deposit from channel flow and sheet flow.Sand-conglomerates from channel flow form mouth bars,and those from sheet flow form underwater sheet flow shoal,both of which are connected together to shape sheet-like sandy-conglomeratic deposits,and further connected with those of fan delta plain.