Rapid and selective clast weathering on an alluvial fan, eastern Karoo, South Africa

A small alluvial fan formed on the surface of a dry infilled dam in January 2010 as a result of a high-magnitude rainfall and flood event. Gravel and boulder-size clasts in the fan comprised dolerite, mudstones and fine-grained sandstones. Within a month of formation some mudstone gravels had disintegrated due to weathering processes. The fan was investigated four years later and the weathering of different lithologies was categorised and quantified. Almost half of the weathered clasts were completely weathered forming small heaps or spreads of particles < 4 mm in size. The majority of these were mudstones. Dolerites, except for those with signs of weathering prior to transport, were almost all unaffected. Rapid weathering of mudstones which underlie small areas of the local slopes helps explain how fine grained materials are transferred from slopes to the valley bottom gullies. The considerable thicknesses of colluvial and alluvial deposits that have developed in the Holocene and now mantle the footslopes and infill valley bottoms, are composed of dolerite and occasional sandstone clasts and fine-grained material derived from rapid and selective weathering of mudstones and sandstones.