Slow Rates of Rock Surface Erosion and Sediment Production across the Namib Desert and Escarpment, Southern Africa

Slow erosion has characterized the Namib Desert, the Namibian escarpment, and the adjacent Namibian highlands over the Pleistocene. Paired analyses (n566) of in-situ-produced Be and Al in quartz-bearing samples of bedrock primarily from inselbergs, of sediment from dry river and stream channels, and of clasts from desert surfaces reveal large inventories of these cosmogenic nuclides indicating significant landscape stability over at least the past million years. Bedrock samples (n 5 47) collected in three transects from the coast, across the escarpment, and into the highlands, show no spatial pattern in elevation-normalized nuclide abundance despite a difference in mean annual precipitation (MAP) between sample sites at the coast (MAP <25 mm yr) and those in the highlands (MAP >400 mm yr). Average model erosion rates inland of the escarpment (3.2 6 1.5, n 5 9) are indistinguishable from average rates seaward of the escarpment (3.6 6 1.9, n 5 38) indicating that rock on the pedimented coastal plain is eroding at the same rate as rock in the highlands. Sediment samples (n 5 3) from small streams suggest that the landscape as a whole is eroding more rapidly than the bedrock outcrops and that a basin in the steep escarpment zone is eroding several times faster (16 m my) than either a basin in the highlands (5 m my) or a basin in the coastal plain (8 m my). Data from large rivers (n 5 4) constrain erosion rates, averaged over 10 yrs and 10 to 10 km, between 3 and 9 m my. Small quartz clasts (n 5 12) collected from four desert surfaces record extraordinarily long, variable, and in some cases complex exposure histories. Simple Be model ages are as high as 1.8 my; some minimum total histories, considering both Be and Al and including both burial and exposure, exceed 2.7 my. As a group, the Namibian cosmogenic data do not support the model of significant and on-going escarpment retreat. The similarity of erosion rates calculated from Be analysis of fluvial sediments and longer-term (10 yr), average mass removal rates estimated by others using fission track analysis of rock suggests that Namibian erosion rates have reached a steady state and are changing little over time. At outcrop scales, the concordance of Be and Al in most bedrock samples suggests that the model of steady, uniform bedrock erosion is valid; there is no indication of intermittent burial, shedding of thick rock slabs, or stripping of previous cover. At an intermediate scale, a transect of bedrock samples north of Gobabeb demonstrates that the northern boundary of the massive Namib Sand Sea has been steady and unshifting. Similarly low cosmogenically estimated erosion rates across west and central Namibia suggest that the landscape is in geomorphic steady state, its overall appearance changing only slowly through time.