The sedimentology of river confluences
The sedimentology of river confluences
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河流交汇处的沉积学
DOI：
10.1111/sed.12504
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发表时间：
2018
期刊：
影响因子：
3.5
通讯作者：
Sambrook Smith G
中科院分区：
文献类型：
--
作者：
Sambrook Smith G
河道汇流是大型河流网络中的关键节点，但令人惊讶的是，人们对其时空演变知之甚少。此外，由于汇流与垂直冲刷有关，而垂直冲刷通常延伸到平均河道深度的几倍，因此与这种冲刷相关的沉积物在岩石记录中应该具有很高的保存潜力。矛盾的是，这种冲刷很少被观察到，而且人们对它们的保存和沉积学解释知之甚少。本研究详细介绍了一个基于物理的地貌动力学模型的结果，该模型被应用于模拟大型河流交汇处的演变和冲积结构。模型内的边界条件被定义为接近孟加拉国恒河和贾穆纳河的交界处，模型的输出得到在该交界处收集的地球物理数据的补充。数值模拟揭示了几种不同类型的沉积填土，它们与沙洲的地貌动力学行为、辫子沙洲下游的汇流冲刷、弯道冲刷和主要交汇冲刷有关。与现有的主要是定性的概念模型比较表明，这些模型都不能简单地应用，尽管每种模型的元素在数值模拟产生的矿床和地球物理数据中都很明显。模拟冲刷沉积物的特征随河道迁移引起的改造程度的不同而变化，这是目前汇流沉积学概念模型中没有充分考虑的因素。因此，主要交界处冲刷的冲积结构以普遍存在的剥蚀面和最厚的沉积层为特征。辫状坝和弯道冲刷下游的汇流冲刷可以保留一些大型的单独冲刷，但这些位置的特点是平均冲刷厚度低于主要交汇处冲刷，并且缺乏大范围的冲刷面。与上面突出显示的任何特定冲刷类型无关的沉积区域记录了最薄的沉积环境。冲刷冲积结构的这种多样性可能在某种程度上解释了古代交界处冲刷的悖论，即虽然岩石记录中可能有大量的大型冲刷，但很少有报道。目前的结果勾勒出汇流沉积学的可能范围，并将为认识和解释古代河流记录中的这些沉积提供新的工具。
Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterized by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterized by lower average set thickness compared to major junction scour and by a lack of large‐scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record.
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