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Siliciclastic, Chemical, Pedogenic and Organic Sediments in Contemporary Rift Environments |
Dept of Geography, Birkbeck College, University of London, London WC1E 7HX, UK
Dept of Geology, Royal Holloway & Bedford New College, Egham Hill, Egham, Surrey TW20 0EX, UK
The Plio-Pleistocene sediments of the Lake Turkana basin are the product of episodic deposition interspersed with long periods of reworking. Their stratigraphy is complex and includes a wide range of lithotypes from fine lake clays to extensive coarse gravel sheets. The current phase of landscape evolution is dominated by fluvial incision and slope retreat under semi-arid conditions. Rates of sediment derivation are high, with measured values of up to 1600 t km–2 a–1, and a catalogue of evidence of surface processes (rainsplash pillars, mobile soil mantles, etc.) illustrates the erosional dynamism of an area that currently receives a rainfall of only 300 mm a–1. Detailed process studies reveal the ten-fold decline in infiltration capacity of silty regoliths that results from impaction of the surface by the first fall of a rain season. They also explain the rapid onset of overland flow which only accounts for between 2 and 7% of stormwater but which ravages the soil, causing erosion that ranges from 0.2 to 0.5 kg m–2 a–1. Shallow gradient slopes are shown to export fine material (D50 = 0.2 mm) and to concentrate coarse fragments > 4 mm as a surface lag. But studies on steeper gradient slopes where the regolith is typically a talluvium, and where processes are gravity-dominated, sliding or rolling movements indicate a direct relationship between particle size and downslope movement towards the stream system. Despite the variable importance of different processes on different type-slopes, a clearcut relationship emerges between slope gradient and erosion rate that allows us to model slope retreat and landscape evolution. A volumetric analysis gives an age to the present basin landform of 0.11 Ma. Analysis of sequential links in the sediment transport train permits us to match clastic materials deposited in the accumulation zones of rift valley basins (riverine floodplain, lake delta, beach) with those of the erosional source areas.
