Heavy minerals in the palaeo and modern Orange River and offshore southern Namibia

Linking the erosional record of drainage basins with the depositional record of sedimentary basins is a major challenge in geosciences. Thick diamondiferous gravel terrace deposits along the lower Orange River and their coeval downstream offshore deposits provide a rare opportunity for understating source-to-sink relationships. Two distinct gravel terrace deposits are recognised, based on clast and heavy mineral assemblage analysis at Boom, Lorelei, Sendelingsdrif, Daberas, Auchas Major, Auchas Lower and Arrisdrif, referred to as the Proto and Meso Orange River deposits. These are compared to offshore sample sites.
The Proto Orange River deposits are coarser and characterised by a dominance of Karoo Supergroup shale and sandstone clasts whereas the younger Meso Orange River gravels exhibit a banded iron formation dominance. Differences in clast assemblage between the Proto and Meso deposits are ascribed to a more powerful river system during Proto-Orange River time, driven by a changing drainage basin geomorphology, rather than reworking of older deposits or changes in clast provenance. This is accompanied by an increase in local bedrock sources, including amphibole-epidote in the heavy mineral assemblages. The fluvial and marine gravels have similar detrital heavy mineral assemblage derived from the Namaqua Metamorphic Complex and Gariep Belt. Offshore, the eastern regions show a higher amphibole-epidote content similar to that of the Meso Orange River gravel. Therefore, offshore amphibole-epidote distribution is a function of the relative amount of Namaqua Metamorphic Complex derived sediments and also decay of these two minerals over time and is neither a function of northward longshore drift nor geomorphology of the seabed or sea level.
Diamond indicator minerals derived from kimberlites do not persist into the heavy mineral assemblage of the study area. In Atlantic 1, a weak positive correlation of coarse magnetite with diamond grade for gravels that are closer to the Orange River mouth could be explained by their hydrodynamic similarities.