Mesozoic fluvial deposits on a rifted continental margin near Vaalputs, Namaqualand, South Africa

Abstract

Scattered outcrops of faulted, silicified and kaolinitised (saprolitised) sediments comprising conglomerate and cross-bedded sandstone, form prominent mesas and inliers near Banke, south of Vaalputs, the national radioactive waste disposal site in Namaqualand. Although their geomorphologic expression is similar, these sediments are lithologically and texturally distinct from the equally silicified soils, diatreme infills and hillside screes of the palaeo-weathering profile type commonly occurring between the farm Banke and the Vaalputs site. Sedimentary and structural features identified in this study show that the Dasdap sediments may represent an alluvial fan sequence with a source area in the west, and deposited on a continental margin marked by relatively steep topographic gradients. The Dasdap beds could not be dated conclusively, but there is evidence their deposition was likely coeval with the emplacement of melilititic and kimberlitic diatremes in the Late Cretaceous. The robust fluvial system inferred by the Dasdap sediments is compatible with the humid climate responsible for the siliceous saprolites widely associated to the erosional African surface across southern Africa. Subsequent to these events, widespread faulting and jointing of the sediments provide striking evidence for tectonic reactivation of the continental margin of southwestern Africa in the Cenozoic. On the basis of these distinguishing features, the Dasdap sediments may deserve the formation status.