辫状河沉积构型研究进展:沉积演化与构型特征

Depositional models of braided rivers:characteristics of sedimentary evolution and architecture

辫状河沉积是陆相沉积盆地的重要组成部分,是油气、铀等矿产资源的重要载体。近30年来,辫状河研究在沉积演化与构型特征等方面取得长足发展,因此系统梳理现有研究成果并展望未来发展方向具有重要的理论意义与工业价值。综合辫状河研究进展并结合笔者及团队的多个研究实例,根据辫状河沉积内部结构与地貌演化速率分为稳定型与游荡型辫状河,两者地貌特征无显著区分标志,形成与演化过程基本相似,可划分为4个发育阶段:水流扰动形成横向朵状(叶状)单元坝,河流汇聚形成初始河道与坝尾沉积,单元坝生长、拼接形成复合坝,河流动态平衡阶段。但是两河型的内部构型存在显著区别,稳定型辫状河心滩内部由一系列横向较为稳定的垂积体与落淤层组成;游荡型辫状河心滩频繁被河道冲刷,导致辫状河道沉积与心滩之间无明显界限、形成“泛连通体”。目前,针对辫状河沉积构型的研究已十分丰富,通过河流动力学与沉积学交叉学科研究辫状河沉积构型模式的主控因素、建立不同沉积条件下多样化的沉积模式,并探究多样化沉积模式的演化过程与成因机理已成为当前的研究难点、热点与发展趋势。

Braided river deposits are important components of continental sedimentary basins,and the important reservoirs for hydrocarbon,uranium and other mineral resources.In the past three decades,the study of braided rivers has made great progress in the deposit features,evolution and sedimentary architecture.It has high theoretical significance and industrial value to review previous researches and to forecast the development direction.Generally,braided rivers can be divided into relatively stable and classical ones,according to the variety of sedimentary architecture and the evolution rate of landform.There is no significant distinction between geomorphic features of the two kinds of braided rivers.Their formation and evolution processes are basically similar,mainly consisting of four developmental stages:(i)formation of transverse unit bars with limited relief from an initially featureless bed;(ii)channel development around bars and in some cases dissecting transverse unit bars;(iii)formation of relatively simpler compound bars;and(iv)amalgamation of these simpler compound bars into more complex compound bars.Nevertheless,characteristics of their inner architecture show significant differences.The mid-channel bars in relatively-stable braided rivers are usually composed of a set of vertically accretionary sandbodies and falling silt layers.In terms of the classical braided rivers,erosion/incision surfaces and chutes are in a heavy density.There are no clear boundaries between the bars and channels,and so the bars and channels are stacked on each other and forming a pan-connected sandbody.A large number of studies on the sedimentary architecture of braided rivers have been performed,which shows a growing trend of studying the various sedimentary models of braided rivers,and of exploring their major controls,evolution process and formation mechanism by integrating fluvial dynamics and sedimentology.