Using cosmogenic 3He to quantify bedrock channel erosion rates on the Mooi River, South Africa

Because bedrock channels establish boundary conditions for hillslope erosion, the rate of bedrock river incision is central to controlling the rate of landscape evolution in non-glaciated post-tectonic landscapes. Bedrock incision governs the delivery of baselevel change across catchments and controls the patterns and rates of denudational unloading and oceanward sediment flux. Interpreting these interactions requires study of rivers across a wide range of tectonic, climatic, and lithological settings. Most research has focused on river incision in humid, high relief, active orogens characterized by fast uplift. The Earth’s far larger areas of drier, lower relief, tectonically quiescent landscapes remain poorly investigated. In these non-glaciated, ‘post-orogenic’ settings, lithologies that are resistant to fluvial erosion can retard the upstream transmission of baselevel change, with headward-migrating knickpoints commonly slowing or ‘stalling’ on resistant outcrop. Landscape development upstream of these resistant lithologies may become partly decoupled from downvalley changes, so that while older topography persists in the upper catchment, younger topography develops in the lower catchment as hillslopes adjust to ongoing river incision. These concepts pose challenges to traditional landscape development models but remain to be tested with well-constrained field data. To examine the lithological controls on baselevel change in a post-orogenic landscape, we investigate bedrock river incision rates on the South African Highveld using cosmogenic 3He. The Highveld has not undergone large-scale uplift during the Quaternary and is characterized by numerous resistant dolerite sills and dykes (barriers) that have intruded weaker sedimentary rocks. In channel long profiles, highly resistant dolerite intrusions with higher resistance than the surrounding sandstone create local base levels to which the reach upstream is adjusted. In the upstream sandstone reaches, large floodplain wetlands form. The stability of the floodplain wetlands is dependent on river incision rated through the dolerite. Herein, we present preliminary results from cosmogenic 3He measurements on dolerite channel bed surfaces eroded by fluvial processes on the Mooi River in South Africa. Nine samples were collected on a ~150 m reach from a dolerite channel bed close to the transition from sandstone underlying the wetland sediments upstream. Denudation rates range from 1.6-4.5 cm/kyr. While there is variation across the small area, the spatial pattern of denudation rates reflects the expected local sample specifics and downstream trends. Lower elevation samples on the channel bed have higher erosion rates while samples at higher elevations that have not recently undergone significant erosion have correspondingly lower erosion rates.