Discordance of erosional tempos: a non-linear and scale dependent evolution in the Orange River basin (Southern Africa)

Abstract

Various erosion rate monitors have been exploited to evaluate the denudational history of the Orange River basin from the Mesozoic to the present. Extrapolation back in time from contemporary sediment loads is hazardous, even throughout the Holocene, and low temperature thermochronometry is unable to provide constraints on the recent cooling history, so that a gap cannot be bridged between a Cretaceous period of significant post-rift denudation and a recent acceleration of the human impact. In the first period the role of tectonics may be viewed as the driving force whereas cropland soil losses have dramatically increased through gully erosion during the last century. This paper highlights the limitations of comparing denudations rates over a long period of time as measurements of current processes appear irrelevant for interpreting long-term landscape evolution. Two ways of acquiring an understanding of landforms are to be considered as they bear evidence of an irreducible disparity between the tectonic and anthropic imprints.