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。这项研究将消除对柬埔寨下游的猜测。重要的是，这项研究将提供关于为什么主要河流突然通过“海盗”过程改变航向的通用数据。为了破译海盗的过程和时间，化石木材被用作天然示踪剂，看看湄公河是否在第四纪改变了路线。湄公河在N。柬埔寨是切割之间的冲积河流阶地。虽然第四纪低基准海平面可以解释湄公河在越南的深切割，但距离古海岸的距离不利于北湄公河的基准控制。柬埔寨.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审查。柬埔寨境内的湄公河沿着有两个阶地。100 m阶地距今约600 Ka，40 m阶地距今约20 KaBP。高阶地切割不连续，低阶地宽50公里，平坦连续。高阶地为含水岩石，含河漂石-砾石，而低阶地为粘土，被砂/砾石覆盖，类似辫状河沉积物。这条现代河流在岩石上形成了一个没有砾石的支流.因此，这条河在切割时戏剧性地改变了性质。地质研究推测，湄公河上游的阶地以前流向东部，在一个现有的大山谷（现在大部分是干的），在一个循环的城镇Saravene返回到现代的过程中，现在被一个主要的支流- Se Kong河占据的山谷。另一种说法是湄公河一直保持着它的路线，就像今天一样。因此，湄公河与湄公河交汇处的阶地中保存的下切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变化和气候在调节负荷、河道变化和连续河流平面形态中的作用。概念验证访问表明，40 m阶地砾石层在系统中变薄，但在3-S汇合处不成比例地厚，很明显砾石来自Se Kong山谷。厚的存款要么支持一个持续的灾难性的沉积物输入从Se Kong（不太可能）或湄公河已经占领Se Kong valley.To了解的过程/事件的顺序，梯田的海拔将使用卫星派生DEM和地面实况精确映射。阶地沉积物序列的详细沉积记录将被解释为过程，以解释下游砾石层变薄和河流风格随时间的变化。暴露在沙坑中的沉积物将在关键层位使用发光测年法进行测年，这样就可以清楚地知道是否在任何一个时间发生了切割或沉积。重要的是，阶地砾石含有化石木材块。这种木材来自侏罗纪岩石露头中的各种针叶树/硬木，这些岩石被侵蚀/沿着河流系统运输。虽然这些岩石是沿着湄公河发现的，但关键的一点是，有些种类的树木化石只在Saravene镇附近的Se Kong山谷发现，而其他的则来自泰国东北部（靠近湄公河）。因此，通过比较梯田中的木材类型与泰国和Se Kong山谷露头中的木材，我们可以知道梯田中是否有任何木材从Se Kong下来。今天Saravene的溪流非常小，如果在湄公河阶地中发现大量Saravene的化石木材，这是一个强有力的迹象，表明湄公河最初在第四纪流过Saravene和沿着Se Kong山谷流动。使用化石木材作为示踪剂来了解主要河流的变化是新颖的，因为计算机断层扫描（CT）扫描的应用程序来确定各种木材样本的3D结构和属。

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.