THE HIGH TERRACE GRAVELS, NORTHEAST THAILAND - A REEVALUATION AND AN INTEGRATED THEORY OF THEIR ORIGIN

After reviewing the field evidence, an integrated geomorphic model for the high terrace gravels of Northeast Thailand is proposed. The hypot hesis advanced is that during the last 2 Ma the uplift of the Qinghai- Tibetan plateau and the highland chain extending into northwestern Tha iland set in motion a cascading series of extreme events; extinction o f the forest cover with uplift, exposure of weathered regolith at incr easing slope angles, high intensity rainfall (orographically induced d ownpours) and erosive surface and subsurface run off. The geomorphic r esponse was detachment failure of the weathered mantle and entrainment of material in a process system dominated by debris floods (sjels) wh ich were funnelled through the water gaps in the cuestas bounding the Khorat Plateau and projected as stream floods into the basins. The dep ositional products were the high terrace gravels containing wood fragm ents, which were subsequently petrified. During a later episode lateri sation produced a ferricrete duricrust capping the gravels. Rejuvenati on of the drainage in recent times has led to inversion of relief so t hat the gravel areas now remain as high relief features - the so calle d high terrace.