Equatorial weathering, landform development and the formation of white sands in north western Kalimantan, Indonesia

The morphology and geochemistry of weathering profiles in Miocene granodiorites from the coastal lowlands of NW Kalimantan, Indonesia (1°N lat.), indicate long-term weathering rates, landform development and mechanisms for the formation of `white sandsʹ found widely in valleys and depressions. Samples are analysed from convex hill, piedmont glacis, and valley slope profiles. All profiles above the saturated zone are strongly gibbsitic, but kaolinite minerals are dominant below the water table. The profiles are interpreted as expressions of the perhumid Equatorial climate (rainfall 3000 mm y−1) acting within a dynamic regime of landscape lowering during the Neogene, the current profiles being largely Quaternary (0.4–0.6 ka). Possible effects of Quaternary tectonic and environmental changes are considered, including those caused by a Late Pleistocene drier climate when there was greater water table fluctuation and surface erosion. Landform evolution is a form of dynamic etchplanation with a long-term rate of landscape and weathering profile lowering of ca. 50 mm ka−1. Within this regime some of the `white sandsʹ may be residual weathering products locally redistributed by colluvial or alluvial processes. They originated in the podzolising environment of the weathering profile, and later experienced post depositional (diagenetic) changes within the hydromorphic regime of the valley floor.