Preliminary Study of Sedimentation in

Lake Tonle Sap, the largest lake in the Indochina Peninsula, lies in central Cambodia. Unique sedimentation is expected in the lake due to drastic changes in its water area between the rainy and dry seasons. As the first step to examine the sedimentary processes of the lake, bottom sedi-ments of the lake and the Tonle Sap River, and surface soils of alluvial deposits were collected and examined in order to reveal the origin of the lake bottom sediments. As the results it becomes clear that clay minerals in the bottom sediments of the northern part of Lake Tonle Sap are derived both from surface soils on the alluvial deposits around the lake, and metamorphic and granitic rock bodies lying in the Mekong River basin. The latter is trans-ported, as suspended sediments by backward current in the rainy season, into the lake from the Tonle Sap River. The presence of marine creatures in bottom sediments of the lake suggests that the lake was closely connected with the sea during the last sea-level high stand. Furthermore, there is a strong possibili ty that annual changes due to alternating rainy and dry seasons will be recorded in the lake bottom sediments over the long geological period. Lake Tonle Sap, the largest lake in th