Where is my sink? Reconstruction of landscape development in southwestern Africa since the Late Jurassic

Quantifying the rates and timing of landscape denudation provides a means to constrain sediment flux through time to offshore sedimentary basins. The Late Mesozoic evolution of drainage basins in southern Africa is poorly constrained despite the presence of several onshore and offshore sedimentary basins. A novel approach has been developed to calculate the volume of material eroded since the Late Jurassic at different time steps by constructing structural cross-sections and extrapolating thicknesses of eroded material. Using different assumptions, the calculated volumes of material eroded from southwestern Africa range from 2.52 × 106 km3 (11.3 km of vertical thickness removed) to 8.87 × 105 km3 (4.0 km of vertical thickness removed). For the southward draining systems alone, the calculated removal of 7.81 × 105–2.60 × 105 km3 of material is far greater than the volumes of sediment recorded in offshore sedimentary basins (268 500 km3). Reconstruction of the drainage systems using geomorphic indicators and clast provenance of the Uitenhage Group, as well as extrapolated surface exposure ages, indicate the southern draining systems were active from the Late Jurassic with coeval activity in axial and transverse drainage systems. The calculated volumes are tied to published apatite fission track (AFT) dates to constrain the changes in exhumation rate through time (using multiple scenarios), which indicate a significant amount of Early Cretaceous exhumation (up to 1.26 × 106 km3, equivalent to 5.70 km of vertical thickness). For the first time, this has permitted long-term landscape evolution to be used to support the interpretation that some of the ‘missing’ sediment was deposited in sedimentary basins on the Falkland Plateau as it moved past southern Africa during the Early Cretaceous. This implies that in this instance, the sinks are separated from their source areas by ~ 6000 km.