| Abstract: | In both sand quarry faces and boreholes the Old Alluvium of Singapore is a matrix supported pebbly sand with minor beds of better sorted sands and clays. The beds can be grouped into four textural classes (pebbles, coarse sand with fine pebbles, medium to coarse sand, clay, and silt), each with characteristic sedimentary structures. The deposit appears to be the proximal facies of an ancient braided river alluvium of possible Pleistocene age. This widespread but scattered alluvium is found both on land and offshore on the Sunda Shelf in Southeast Asia. It is believed to have been deposited during low sea levels. In Singapore, the mineralogy of the grains forming the Old Alluvium suggests a mixed provenance of granitic and low-grade metamorphic origin. The volume and freshness of the deposited material indicate an environment of considerable relief, seasonal rainfall, and extremely active erosional processes, conditions considerably different from the current ones. The Old Alluvium was probably deposited by seasonal rivers which experienced periodic large floods. Such regional conditions could have prevailed over much of Southeast Asia at the time of deposition of the Old Alluvium. |
