Chapter 3 - Geology and Landforms of the Mekong Basin

Abstract

The Mekong flows in a pan-shaped basin shaped by regional geology. The upper basin in China is a steep narrow valley and its geometry is determined primarily by Himalayan orogeny. The drainage basin widens south of the Chinese border in Lao PDR and Thailand but remains mountainous. A large part of the Mekong Basin comprises pre-Tertiary metamorphosed terrestrial and marine sedimentary rocks associated with suture zones, and intruded granites. Downbasin from Vientiane to Savannakhet, a shallow Quaternary alluvium, mainly fine to coarse sand with some clay and gravel, covers Paleozoic and Mesozoic rocks in the valley of the Mekong. Mesozoic sandstones and evaporites reappear in the Mekong valley further south beyond this Quaternary cover, locally overlain by shallow alluvium. In Cambodia, the basin widens to about 500 km with an alluvium of variable thickness as surface cover. Permian carbonates, Triassic sedimentary rocks, and Neogene basalts form the divides, and with granite emerge from underneath the alluvial cover elsewhere to form low isolated hills. The Mekong Lowland stretches north–south along the river from the northern mountains to the delta. The lowlands widen from Vientiane as the river angles cross-basin toward the eastern divide marked by the northern Annamite Range.