Abstract
Landscape evolution studies using cosmogenic nuclides have been conducted in South Africa for the last two decades. The Vredefort Dome, which forms part of the world’s largest and oldest meteorite impact crater, has previously undergone extensive studies relating to its age, geology and geomorphology. However, no studies pertaining to the rate at which the rocks in the Vredefort Dome are being eroded are documented. The Vaal River flows in mixed bedrock-alluvial terrain through the Vredefort Dome, particularly in the Parys area, contributing towards the erosion and therefore landscape evolution in the dome. The character of the river is anabranching in the granitoid area, but then flows through a relatively narrow canyon in quartzites forming the collar of the dome. This study adopts the use of the cosmogenic nuclides 10Be and 26Al extracted from purified quartz samples, to investigate the erosion rates in the river and adjoining areas for both granitoid and quartzite bedrock along the Vaal River. The results of the study reflect that the erosion rates along the river for the quartzite and granitoid samples are ~ 3.08 and ~3.05 m/Ma respectively. Among the sampled quartzites, some indicate much lower apparent erosion rates, and this related to where they were sampled, with reference to their elevation above the river level (referred to as ridge quartzites). These quartzite samples were mainly collected from old, now elevated strath terraces, and in the logarithmic plot of 26Al/10Be ratios vs [10Be], they mainly plot on the constant exposure curve rather than on the steady erosion one. If the cosmogenic data on them are interpreted as yielding exposure ages (the preferred interpretation), then these range from 0.23 to 0.79 Ma. Among the sampled granitoids, there are samples (taken at a distance from the river bed) that, plotted in the same diagram, indicate constant exposure rather than erosion. Apparent surface exposure for these yielded an exposure age range of 0.27 to 0.37 Ma. It was expected that the granitoids would have erosion rates that are relatively faster than those of the typically more resistant quartzites. However, erosion rates in the river bed are equal for the two lithologies. This can be explained by the fact that the river is anabranching in the granitoids, changing its trajectory from time to time, while in the quartzites it is confined to a narrow, more permanent channel. If the river bed erosion in the quartzites were slower than in the granitoids, we would expect that a dam would have resulted in the central part where the granitoid rocks are found, but there is no evidence of this ever having happened. This points to a self-regulating mechanism. Determining the erosion rates allows for further understanding of anabranching in rivers, understanding the erosion mechanisms that are shaping the Vredefort Dome and v furthermore, contributes towards the database on erosion rates and landscape evolution in southern Africa.
M.Sc. (Geology)