Antecedent fluvial systems on an uplifted continental margin: constraining Cretaceous to present-day drainage basin development in southern South Africa

Quantitative geomorphological analysis, measurement of suspended river load, and novel dating techniques provide critical information to aid development of coupled tectonic-erosion models of landscape evolution. Generally, these datasets are focussed towards highly dynamic settings that are characterised by rapidly uplifting mountains in extreme climates producing intense bedrock incision. Drainage evolution on uplifted continental margins with thin regolith, resistant bedrock, and semi-arid climate is less well constrained. One example of such a setting is southern South Africa where the rates of drainage basin denudation and sediment supply to coastal depocentres are poorly understood.
The Cape Fold Belt and the inboard Karoo Basin developed in a compressional setting during the amalgamation of Pangaea. The area then underwent extensional deformation from the Jurassic. Evidence from offshore deposition suggest that since the Cretaceous southern South Africa has been dissected by numerous bedrock rivers with tight meanders, which cut orthogonally across the structural grain of >2km high mountains that comprise highly resistant rock-types (quartzites and meta-granites), and drain into the Atlantic and Indian Oceans.
The confined routing of rivers across the highest modern topography, and incision through highly resistant bedrock, indicates that the drainage pattern is in part antecedent. Extreme examples of antecedent drainage include the rivers that cut through the Himalayas. Across vast areas of southern Africa, the main driver(s) for this inherited drainage pattern remains unclear. In the present-day, the area is generally considered to be tectonically quiescent, and an arid to semi-arid climate prevails in much of the upper catchment. This suggests that the significant bedrock
incision events occur during punctuated high magnitude, low frequency flood events rather than through more frequent, low magnitude stream discharges. However, the timing of the switch from a depositional to incisional fluvial system, the rates of bedrock incision, and the volume of material eroded throughout the evolution of the river system are all unknown. Furthermore, changes in tectonic and climatic conditions in southern Africa are poorly constrained in the Cenozoic. To reconstruct the morphological evolution of the river systems, samples will be collected from erosional surfaces and from alluvial terraces and will be dated by dosimetric and cosmogenic methods.
This studentship aims to provide new quantitative constraints on the geomorphological evolution of southern South Africa over the last ~100 Myr, including constraints on the timing of uplift and denudation of southern Africa to augment published studies from around the Atlantic and Indian Ocean margins of Africa. This research will provide critical information on the rate and volume of sediment transfer from an uplifted continental margin to the continental shelf and the deep ocean to expand existing source to sink and tectonic-erosion models of landscape evolution.
This interdisciplinary studentship will necessarily involve linkage between field and laboratory work, as well as desk-based GIS investigations. The holistic approach means that the student will be trained in fluvial sedimentology and geomorphology, detailed field mapping and sampling, and GIS techniques. Laboratory work will include luminescence and electron spin resonance dating, and cosmogenic isotope analysis.