Cosmogenic 3HE Measurements Provide Insight into Lithologic controls on Bedrock Channel Incision: Examples from the South African Interior

Resistant bedrock outcrops can exert control on river long-profile adjustment, upstream transmission of base level fall, and valley development, particularly in postorogenic settings. To examine how variation in lithologic resistance impacts landscape development in the postorogenic eastern South African interior, cosmogenic 3He in pyroxene from Karoo dolerite was measured in samples from valleys of the Klip and Mooi Rivers and the Schoonspruit. The denudation rates measured from cosmogenic 3He in the Klip and Mooi Rivers and the Schoonspruit are widely variable, with channel bed denudation rates ranging from 14 to 255 m/m.yr. and valley side and top denudation rates ranging from 11 to 50 m/m.yr. Various processes of channel bed erosion occurring at grain to block scales (abrasion, plucking, subaerial weathering) result in the widely ranging channel bed incision rates. In this setting, river incision rates are restricted by moderate unit stream powers (∼20 to >50 W/m2) and by limited sediment supply, resulting in a lack of abrasive tools. In many dolerite valleys, channel bed incision is commonly slow enough for local base levels to remain essentially stable for extended periods of time (>10 k.yr.). These results suggest that in the postorogenic eastern South African interior, resistant dolerite in channel long profiles can result in spatially variable rates of channel bed incision, with implications for the patterns and tempo of wider landscape dynamics.