Erosion of a granite inselberg, Gross Spitzkoppe, Namib Desert

Namibia, with its passive margin setting, long-term tectonic stability, and long-lasting arid climate is a typical granitic landscape characterized by numerous dominating inselbergs. The Namib has been the focus of quantitative studies on the overall rates of bedrock exhumation, escarpment retreat, and sediment generation, transport, and deposition. Results from these studies indicate steady bedrock erosion rates ranging between 1 and 5mmkyr^-1 over the last 10⁵-10⁶yr. This rate was determined from samples collected mostly from small inselbergs with low relief. How fast the large and "classic" inselbergs, which dominate the Namib Desert landscape, erode, generate sediment, and contribute to the overall sediment supply has yet to be systematically examined. We document erosion rates of the Gross Spitzkoppe (GS), one of the largest inselbergs in the Namib Desert, Namibia, based on measured concentrations of ¹⁰Be in samples collected from bedrock, boulders, and surface grus. The results suggest a slow lowering rate (1-2mmkyr^-1), while the overall slope processes deliver sediment to the base of the cliff with 5-6×10⁵atomsg^-1 quartz. This concentration is equivalent to an average erosion rate of ~8mmkyr^-1, 2-3 times faster than the bedrock lowering rate. Thus, the inselberg has been eroding by cliff retreat. The concentration of cosmogenic isotopes in slope sediment is controlled by exposure time on the slope, weathering of exposed bedrock, and weathering of subsurface bedrock. Sediment continues to accumulate cosmogenic isotopes while being transported on the flat pediments surrounding the inselberg. Within 1-2km from the base of the GS, the concentration doubles to ~10×10⁵atomsg^-1 quartz. The increase is achieved by dosing due to exposure and mixing with highly dosed sediment eroded from near subsurface bedrock. From this area, grus is transported through a network of low gradient channels to the Atlantic Ocean, ~100km away. Cosmogenic isotope concentration in sediment increases to ~30×10⁵ atoms of ¹⁰Beg^-1 quartz, which is 3-4 times higher than that measured in the grus close to the GS and is controlled by the erosion rate of stable bedrock throughout the entire drainage basin. Dosing due to exposure during transport obviously contributes to the overall measured concentration in the sediment as it approaches the ocean.