| Abstract | The Chi River which has the watershed area of about 49,000 km2 and the length of 442 km is one of the main river in the Northeast of Thailand. Within the Khon Kaen area, the Chi River Basin was classified into 11 geomorphological units, namely, hills, pediments, colluvium, strath terrace, alluvial terrace, subcatchment alluvial plain, low terrace, lakes and swamps and/or solution sinks, earthen mounds, meandering belts, and entrenched meander rivers. The present Chi and its tributary, Phong Rivers are underfit streams. The Quaternary sediments in the area were deposited on top of the weathered Khorat Group bedrock. They are comprised of the residual soils, gravel beds, laterites, colluvium, red and yellow losses, swamp and lake deposits, middle terrace deposits, low terrace deposits, flood deposits, meandering belt deposits, and sheet-wash deposits. Alluvial stratigraphy revealed ancestral fluvial system, now buried under the present landscape. The paleo-valley was dissected into the older deposits and valley fills was very complex. Five major litho genetic units of the Chi River's alluvium can be subdivided, namely, swamp and lake deposits, alluvial deposits associated with middle and low terraces, flood deposits, and meandering belt deposits; with an average thickness of 16 m. The swamp and lake deposits overly the red and yellow loess. It can be distinguished by the presence of high amount of carbonaceous matter. Hematite/goethite are the most abundant heavy minerals and kaolinite is the major clay mineral. Middle terrace deposits are contemporaneous with the swamp and lake deposits. They are distinguished on the basis of the yellow-brown color, ubiquitous motJJM^k“and common occurrence of zones of manganese-oxide nodules, coatings, and concretions. Two cycles of fining-upward sequence ?ere observed. The fining-upward sequence consists of the lower point bar lateral accretion and the upper floodplain vertical accretion. The point bar facies is always coarser than the floodplain facies and usually starts from coarse sand to gravelly sand at base with gradual fining upward. Hematite/goethite are the most abundant heavy minerals while kaolinite is the major clay mineral of the point bar facies but for floodplain facies, montmorillonite is more dominant. The low terrace deposits overly the middle terrace deposits and are contemporaneous with the flood deposits and lag deposits of the meandering belt. This unit was distinguished from other units by its uniform gray color, high silt content (for floodplain facies), occurrence of common but dispersed thin manganese-oxide coatings or fine oolites, and fine fragments of wood charcoal. Heavy minerals of the low terrace deposits are dominated by ilmenite/leucoxene and anatase. Their clay minerals are mainly kaolinite while quartz is the major mineral for silt fraction. The flood deposits represent a deposition of sediments by flash flood. The meandering belt deposits occurred along meandering belt levees of the Chi and its tributaries. Heavy minerals of this unit are dominated by ilmenite/leucoxene. Their clay minerals are mainly kaolinite and montmorillonite. After the deposition of loessial soil (= 18,000 yrs. B.P.), the climate changed from cold and dry to more humid. At about 6,000 - 5,000 yrs. B.P., the swamp and lake deposits were formed and at about the same time, the middle terrace alluvium was deposited. The middle terrace deposits were exposed to subaerial processes of diagenesis and subaerial erosion before burial by the low terrace deposits. The climate became humid again at about 3,000 - 2,000 yrs. B.P., and the low terrace sediments were deposited. Evidence from flood |