Quaternary meandering evolution and architecture of a point bar in the Mun River on the sandstone-dominated Khorat Plateau from northeastern Thailand

Abstract

The Mun River, the main right bank tributary of the Mekong River, meanders and flows eastwards across the flat and sand-dominated low-lying plain of the Khorat Plateau, northeastern Thailand. The Mun River expresses its evolution with a spectacular trace of its meander landforms on the floodplain surface. This paper reveals (for the first time) the three-dimensional shallow subsurface sedimentary architecture inside a part of a sandy point bar deposit of the Mun River based on ground penetrating radar (GPR) in comparison with outcrop observation from a road cut along the GPR line survey. As a result, we recognized subsurface scroll bars, where the expansion, rotation and transformation types corresponded well with the sedimentary structures exposed on the road cut. Dominant GPR radar facies included hummocky/wavy, group of fitful waves, shingled, curved down pattern, 15–45 degree of reflection angle, sigmoid, sets of en-echelon or sigmoid, concaved upward and isolated trough shape, parallel, horizontally continuous layers and parabolic facies. These GPR facies possibly inferred the sedimentary structures of trough cross-bedded, channel-fills, side bar deposit, erosion surface of channel or chute cut-off, secondary channel scours and fills, horizontal sand-bedded and sheet flood sand. Both GPR facies and lithofacies revealed the meandering evolution of a bar-parallel and bar-transverse mid-channel bar with small braided channels on the bar top and showed the lateral migration of the point bar.