Quaternary incision of the lower Mekong River due to climate change and the history of major channel avulsion

气候变化导致湄公河下游第四纪切割和主河道撕脱的历史

• 批准号: NE/J012440/1
• 负责人: Paul Carling
• 金额: $ 6.46万
• 依托单位: University of Southampton
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2012
• 资助国家: 英国
• 起止时间: 2012 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FJ012440%2F1
• 关键词: Quaternary; incision; lower; Mekong; River

The Mekong is a major river but little is known of its geological history. This study will remove speculation about the lower course in Cambodia. Importantly, the study will provide generic data on why major rivers suddenly change course by 'piracy' process. To decipher the piracy process & timing, fossil wood is used as a natural tracer to see if the Mekong shifted course in the Quaternary. The Mekong in N. Cambodia is incised between alluvial river terraces. Although the low base sea levels in the Quaternary may explain deep incision of the Mekong in Vietnam the distance from paleocoast militates against base control in N. Cambodia. The region was stable in the Quaternary so tectonics did not control incision. Rather Quaternary climate change & adjustments in discharge/sediment load need scrutiny. Two terraces occur along the Mekong in Cambodia. The 100m terrace is dated ~ 600Ka & the 40m terrace is ~ 20Ka BP. The high terrace is incised & discontinuous; the low terrace is 50km wide, smooth & continuous. The high terrace is watercut rock with river boulder-gravel whereas the low terrace is clay covered by sand/gravel like braided river deposits. The modern river anabranches with sand on rock and no gravel. Thus the river changed character dramatically as it incised. Geological studies speculate that upstream of the terraces the Mekong River formerly flowed to the east in an an existing large valley (that is now mostly dry) in a loop by the town Saravene to return to the modern course by a valley now occupied by a major tributary - Se Kong River. An alternative is that the Mekong has maintained its course, as today. Thus the incision & sediment history preserved in the terraces at the confluence of Mekong & Se Kong are key to understand what happened to the river in the Quaternary. The detailed study will explain sediment load changes & the role of climate in mediating both load, river course changes & the successive river planforms. Proof-of-concept visit show that the 40m terrace gravel layers thin down system but are disproportionately thick at the 3-S confluence where it is clear that the gravel came from the Se Kong valley. The thick deposit argues either for a sustained catastrophic input of sediment from the Se Kong (unlikely) or for the Mekong having occupied the Se Kong valley.To understand the processes/sequence of events, the elevations of the terraces will be mapped accurately using satellite derived DEM & groundtruthing. Detailed sedimentary logs of the terrace sediment sequences will be interpreted for process to explain the downstream thinning of the gravel layers & the changes in river style over time. The sediments, exposed in sandpits, will be dated using luminescence dating at key horizons so it will be clear if either incision or deposition has occurred at any one time. Importantly the terrace gravels contain lumps of fossil wood. This wood comes from a variety of conifers/hardwoods fossilised in Jurassic rock outcrops which were eroded/transported down river systems. Although these rocks are found along the Mekong, a key point is that some kinds of tree fossil are only known from the Se Kong valley near the town of Saravene whilst others come from NE Thailand (close to the Mekong). Thus by comparing the mix of wood types in the terraces with the wood in outcrops in Thailand & Se Kong valley we can know if any wood in the terraces came down the Se Kong. Streams at Saravene are today very small & if substantial fossil wood from Saravene occurs in the Mekong terraces it is a strong indicator that the Mekong River had originally flowed past Saravene & down the Se Kong valley during the Quaternary period. Using fossil wood as a tracer to understand the changes in the course of a major river is novel as is the application of computed tomography (CT) scanning to determine the 3D structure and genus of the various wood samples.

湄公河是一条主要河流，但人们对它的地质历史知之甚少。这项研究将消除对柬埔寨较低球场的猜测。重要的是，这项研究将提供关于为什么主要河流在“海盗”过程中突然改变航道的通用数据。为了破译海盗活动的过程和时间，化石木材被用作自然示踪剂，以观察湄公河是否在第四纪改变了航向。柬埔寨北部的湄公河在冲积河阶地之间切割。虽然第四纪的低基础海平面可以解释越南湄公河的深切口，但距离古海岸的距离不利于柬埔寨北部的基础控制。该地区在第四纪时期较为稳定，因此构造对切口没有控制作用。第四纪气候变化和流量/泥沙负荷的调整需要仔细研究。柬埔寨湄公河沿岸有两个阶地。100m台地的年代为~ 600Ka， 40m台地的年代为~ 20Ka BP。高阶地是切割和不连续的；低台地宽50公里，平滑连续。高阶地是含河流卵石的含水岩石，而低阶地是由砂/砾石覆盖的粘土，如辫状河沉积物。现代河流的支流是岩石上的沙子，而不是砾石。因此，河流在切割过程中发生了戏剧性的变化。地质研究推测，在阶地的上游，湄公河曾经在萨拉文镇附近的一个环中向东流过一个现有的大山谷（现在大部分都干涸了），然后通过一个现在被一条主要支流——湄公河占据的山谷回到现代河道。另一种可能是，湄公河像今天一样保持着原有的路线。因此，保存在湄公河和湄公河汇合处阶地的切口和沉积历史是了解第四纪河流变化的关键。详细的研究将解释泥沙负荷的变化和气候在调节负荷、河道变化和连续河面中的作用。概念验证访问表明，40米的阶地砾石层使系统变薄，但在3-S汇合处不成比例地厚，显然砾石来自Se Kong山谷。厚厚的沉积物表明，要么是来自湄公河的灾难性沉积物持续流入（不太可能），要么是湄公河占据了湄公河流域。为了了解事件的过程/顺序，将使用卫星衍生的DEM和地面真相准确地绘制阶地的高程。阶地沉积序列的详细沉积测井将被解释为解释砾石层下游变薄和河流风格随时间变化的过程。暴露在沙坑中的沉积物将在关键的地层上使用发光测年法来确定年代，这样就可以清楚地知道在任何时候是否发生了切口或沉积。重要的是，阶地砾石中含有木材化石。这种木材来自于各种针叶树/硬木化石，这些化石是在侏罗纪岩石露头中被侵蚀/沿河流系统运输的。虽然这些岩石是在湄公河沿岸发现的，但关键的一点是，有些树的化石只在Saravene镇附近的Se Kong山谷中发现，而另一些则来自泰国东北部（靠近湄公河）。因此，通过比较梯田中木材类型的混合与泰国和Se Kong山谷露头的木材，我们可以知道梯田中的木材是否来自Se Kong。今天，萨拉文的河流非常小，如果来自萨拉文的大量化石木材出现在湄公河梯田中，这是一个强有力的迹象，表明湄公河最初在第四纪时期流过萨拉文并沿着Se Kong山谷流下。使用化石木材作为示踪剂来了解主要河流的河道变化是新颖的，应用计算机断层扫描（CT）来确定各种木材样品的三维结构和种类也是新颖的。