A quantitative approach to the evolution of the central Walvis Basin offshore NW-Namibia

Rifting and breakup of Westgondwana in the Late Jurassic/ Early Cretaceous initiated the formation of the South Atlantic and its conjugated pair of passive continental margins. The Walvis Basin offshore NW-Namibia is an Early Cretaceous to recent depositional centre with a typically wedge-shaped postrift sedimentary succession covering an area of 105000km. A 2D model transect across the central Walvis Basin and adjacent onshore areas is used as a case study to investigate quantitatively the denudational history of the evolving passive margin and the related contemporaneous depositional postrift evolution offshore. The database for both the onshore and offshore part of the model traverse is well constrained by own field work, published data as well as by seismic and well data supported by samples. The ultimate goal of this project is to present an integrated approach towards a quantitative link between surface processes and internal processes in terms of a mass and process balance. The Namibian passive margin is characterised by long-lived denudational uplift resulting in the development of a gently inclined coastal plain seperated from the elevated hinterland by the Great Escarpment. In the onshore part of the study area, the exposed stratigraphic record comprises Late Proterozoic to Cambrian metasedimentary rocks and intrusive complexes of the Damara Group that are unconformably overlain by shallow marine to lacustrine sediments of the Karoo Supergroup. With a significant hiatus, these are overlain by rift-related continental debris flow, fluvial, and aeolian deposits that interfinger with the base of the Early Cretaceous Etendeka Group volcanic sequence. The Albin Ridge is a unique NNW-SSE-striking morphological feature preserved in NW-Namibia with sedimentological and tectonic implications that can be directly related to South Atlantic rifting. Rifting and oceanic onset in the evolving South Atlantic propagated from S to N, leading to seafloor spreading in the Walvis Basin with the magnetic anomaly M4, at the latest. Margin evolution is influenced by intense magmatism associated with the Tristan da Cunha mantle plume thus forming a textbook example of a volcanic passive margin. Rift-related features as well as mid to lower crustal levels are strongly overprinted by basaltic magmatism. This observation is supported by vp-modelling at a lithospheric scale, by the recognition of seaward dipping reflector wedges, by subsidence analysis as well as by reduced basal heat flow inferred from maturity modelling of the basin fill. A wide basaltic continent-ocean-transition zone also corresponds to the observed gravity signature of the margin. Stratigraphic remnants combined with implications from apatite fission track analysis are used to reconstruct the thickness of erosion and the denudational history of the onshore part of the model traverse quantitatively. A maximum thickness of 5km of erosion in the coastal plain can be deduced which is suggested to be the most probable estimation as well. An amount of 44000km of clastic detritus has been supplied to the adjacent basin segment during passive margin evolution. The depositional history of the offshore postrift sequence is reconstructed from the depthconverted seismic section ECL 89-41. The interpretation of seismic marker horizons calibrated with high resolution biostratigraphic well data reveals an amount of 58000km of sediment deposited in the offshore part of the model transect since breakup. Reconstructing the postrift denudational and depositional history, respectively, depicts a broad qualitative correspondence, yet the mass balance calculations indicate a volumetric misfit of 25%. This observation is supported by petrographic and geochemical provenance studies as well as by stratigraphic basin modelling which reveal a misfit of even 50%. It is suggested that only half of the sediment, at the most, encountered offshore can be attributed to nearby volcanic sources; a substantial proportion of the detritus has been transported into the model traverse by coastparallel currents that may have already been established during early margin evolution. Provenance considerations indicate that the Etendeka volcanic plateau has already been eroded below its base until the Maastrichtian; this is supported by a major denudation peak in the Late Cretaceous. Basement subsidence, basal heat flow, global sea level changes, and sediment supply rates inferred from the denudational history are incorporated into a stratigraphic basin model. The final basin model is not only capable of supporting the volumetric misfit in the order of 50% that is already indicated by provanance analysis but also of reproducing the geometry and stratigraphic architecture observed offshore. Stratigraphic modelling therefore provides physical explanations of main sequence