Abstract

The young mountain ranges of northeast Papua have sharp crests and very steep side slopes mantled by shallow weakly weathered soils. Rock debris from slope denudation and stream corrasion is rapidly transported by streams and is deposited as alluvial fans and plains and littoral deltas and beaches. The sediments range from coarse conglomerate in some fans to clay in freshwater and some tidal swamps. The present study has concentrated on the fine sand fractions which have about 4 percent of quartz and 1 percent opaque minerals. Of the remainder one-half is rock fragments in various stages of weathering, one-quarter is unstable light minerals (mostly plagioclase), and one-quarter is unstable heavy minerals (mostly ferromagnesian minerals). These sediments have a dioritic and gabbroic chemical composition and are classed as plagioclass-rich litharenites. It is concluded that the terrestrial part of the sedimentary cycle is extremely rapid in the active orogenic portions of the humid tropics. Only the beach sands have reached a mature stage (with sigma phi under 0.5), but the grains are mostly angular.

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