Lithological controls on bedrock river incision and valley formation in the eastern South African Interior

Abstract:

Resistant bedrock outcrop can exert strong control on river long profile adjustment, upstream transmission of base level fall, and valley development, particularly in post-orogenic settings. Though resistant dolerite (diabase) outcrop in river beds provides a key control on fluvial landscape dynamics in the South African interior, the rates and patterns of change have remained poorly constrained. Cosmogenic ³He measurements made in this study are the first known estimates of river incision and valley development rates in South African dolerite and yield new insight on valley evolution.

Bed incision and valley development rates on three short reaches of the Klip River, Mooi River, and Schoonspruit vary as a function of local geomorphic context though cross-sectional patterns of valley denudation are broadly similar among the sites. Channel bed incision occurs primarily through hydraulic plucking of joint-bounded dolerite blocks and minor subaerial weathering with rates varying from ~12-254 m/My. Subaerial denudation on the low-relief valley tops varies from ~20-50 m/My and the valley side denudation rates vary from ~11-12 m/My. At each of the study sites, channel bed and valley top denudation rates are approximately in balance for long intervals of time but valley side denudation rates are negligible. Valleys are roughly in topographic steady state, maintaining cross-sectional depth and width while lowering in pace with the broader landscape. The steady state condition can be interrupted by transient periods of enhanced channel bed incision that deepen the valley.

In this post-orogenic setting, enhanced bed incision is typically accomplished by knickpoints that arise from a variety of intrinsic factors including enhanced rates of block plucking (owing to variation in weathering and structure) and incision into underlying weaker lithologies. Knickpoints correlate closely with resistant dolerite outcrop in the study area, and are long-lived (commonly > 10⁵ years) features of river long profiles with positions, morphologies, and retreat rates that are unrelated to uplift timing and magnitude. The results demonstrate the critical role of river incision through resistant lithologies as a driver of fluvial landscape dynamics.