Late Pleistocene sedimentation and landf orm development in western Kalimantan (Indonesian Borneo)

Abstract

Widespread Quaternary alluvial sediments occur around the coastal margins of western Kalimantan. These strongly podzolised 'white sands' occur as major alluvial bodies that may be 15-20m higher than Holocene/ contemporary floodplains inland but converge and pass beneath them towards a near coastal hinge line. It is presumed that the sediments continue off-shore and correlate with previously documented 'Alluvial Complex' sequences on the submerged areas of Sundaland. Microscopic and SEM examination of quartz indicates minimum wear on most grains, and the alluvial sediments can be distinguished clearly from those of contemporary beach environments. Their extent, morphology and sedimentary characteristics indicate rapid aggradation within coastal catchments, and some formations appear to be low angle, wet alluvial fans. Radiocarbon assay indicates ages greater than 40000 RC yr BP for the sediments with 2 finite dates of 54200 +3400/-2400 RC yr BP and 51000 +2100/-1700 RC yr BP. It is concluded that they were laid down during low sea levels and by rapid erosion and deposition possibly caused by increased seasonality of rainfall, decreased total precipitation and associated ecological changes. Neogene tectonic movements may also affect their disposition.