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

Reconstructing drainage evolution has important implications for constraining long-term source-to-sink configurations. Furthermore, the analysis of ancient landscapes can support research in geomorphological concepts such as steady state and landscape evolution modelling. Techniques such as cosmogenic dating and morphometric analysis have rarely been applied to investigate the long-term drainage evolution of systems draining southern South Africa. This study focusses on the large-scale antecedent Gouritz catchment, Western Cape. Integrating provenance, cosmogenic and geomorphological (planform / morphometric indices) evidence indicates the trunk rivers are Cretaceous or older in age (i.e. principal topography of Mesozoic age). The trunk rivers fed huge volumes of sediment offshore during large-scale Mesozoic exhumation of southern South Africa with remnant coeval deposits in onshore extensional basins. However, there is a mismatch of onshore exhumation and offshore deposition and material is now found on the Falkland Plateau; separating source-to-sink by 6000 km. During exhumation, large scale pediments formed that grade to individual base levels and should be taken as individual features, not ‘surfaces’ correlated across the continent. A second phase of pediment evolution occurred in the mid-Cenozoic, dissecting the pediments and eroding small catchments into the Cape Fold Belt. These smaller order streams are strongly affected by the tectonic grain of the fold belt, whereas the trunk rivers are not, which is shown by variation in morphometric indices. The lack of correlation between catchment properties and denudation rates indicate the system has decoupled and that allogenic factors are now dominant. Due to the low rates of denundation, weathering is currently the rate limiting factor. However, during the early evolution of the catchment, tectonic activity was more dominant than the present day. The ancient catchment is in geomorphic steady-state, and highlights the need for further research into long-term landscape evolution, and linkage to offshore depositional records.