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

湄公河是一条主要河流，但人们对其地质历史知之甚少。这项研究将消除人们对柬埔寨较低水平的猜测。重要的是，这项研究将提供有关主要河流为何突然通过“海盗”过程改变航道的一般性数据。为了破译海盗的过程和时间，化石木材被用作一种天然示踪剂，以确定湄公河是否在第四纪改变了航道。柬埔寨北部的湄公河在冲积河流阶地之间被切割。尽管第四纪的低基准海平面可能解释了湄公河在越南的深深切割，但与古海岸的距离不利于柬埔寨北部的基地控制。该地区在第四纪是稳定的，因此构造不控制切割。相反，第四纪气候变化和流量/泥沙负荷的调整需要仔细研究。柬埔寨境内的湄公河沿岸有两个梯田。100米阶地的年代约为600Ka，40米阶地的年代约为20Ka BP。高阶地切割不连续，低阶地宽50公里，平整连续。高阶地为含水岩石，河流为大砾石，低阶地为粘土，覆盖砂砾，呈辫状河沉积。现代的河流在岩石上有沙子，没有砾石。因此，这条河在切割时戏剧性地改变了特征。地质研究推测，在梯田的上游，湄公河以前向东流经Sarvene镇旁边的一个现存的大山谷(现在几乎是干涸的)，然后由一个现在被一条主要支流--东南部的江河占据的山谷重新回到现代河道上。另一种选择是，湄公河保持了自己的航道，就像今天一样。因此，保存在湄公河和湄公河交汇处阶地中的切割和沉积历史是了解第四纪河流发生了什么的关键。详细的研究将解释泥沙负荷的变化&气候在调节负荷、河道变化和后续河流平面图中的作用。概念验证访问显示，40米的阶地砾石层向下系统变薄，但在3-S汇合处的砂砾层不成比例地厚，那里的砾石显然来自东南河谷。厚厚的沉积物表明，要么是来自湄公河的沉积物的持续灾难性输入(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/序列，将使用卫星获取的DEM和地面测量准确绘制梯田的高程图。将对阶地沉积序列的详细沉积记录进行解释，以解释砾石层下游变薄的过程&河流样式随时间的变化。暴露在沙坑中的沉积物将在关键层位使用发光测年进行测年，这样就可以清楚地知道是否同时发生了切割或沉积。重要的是，梯田的砾石中含有一块块的化石木材。这种木材来自各种针叶树/硬木，这些针叶树/硬木化石存在于侏罗纪露头岩石中，这些岩石被侵蚀/运往河流系统。虽然这些岩石是在湄公河沿岸发现的，但关键的一点是，有些树木化石只在Sarvene镇附近的seKong山谷才知道，而另一些则来自泰国东北部(靠近湄公河)。因此，通过将梯田的木材类型组合与泰国和东南亚谷地露头的木材进行比较，我们可以知道梯田中是否有木材顺着丝冈而下。今天，Sarvene的溪流非常小&如果在湄公河阶地中发现了大量来自Sarvene的化石木材，这是一个强有力的迹象，表明湄公河最初是在第四纪期间流经Sarvene&顺着Se Kong山谷而下的。使用化石木材作为示踪剂来了解一条主要河流的航道变化是一种新颖的方法，就像应用计算机断层扫描(CT)来确定各种木材样本的3D结构和属一样。

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.