EXTENDED DISTRIBUTION OF NATAL GROUP WITHIN SOUTHERN KWAZULU-NATAL, SOUTH AFRICA: IMPLICATIONS FOR SEDIMENT SOURCES AND BASIN STRUCTURE

Abstract

The southern margin of the basin that contains the Ordovician (~490 Ma) Natal Group has long been defined by the “Dweshula palaeo-topographic high” located near Port Shepstone in southern KwaZulu-Natal, South Africa. Six recently discovered outcrops of Natal Group rocks occurring up to 30 km south of the Dweshula palaeo-topographic high have prompted a re-evaluation of the basin morphology. These exposures consist of maroon-coloured arenaceous and argillaceous sediments attaining a maximum thickness of ~46 m, with a mean of 15 m, which are correlated with the Eshowe Member of the Natal Group. The unconformable contact with the overlying Devonian Msikaba Formation is preserved in four outcrops and provides additional evidence that the Msikaba Formation is younger than the Natal Group. The Natal Group lithologies are dominated by laterally variable arkosic arenite, quartz wacke and siltstone, which appear to have been deposited as proximal channel and overbank sediments in a fluvial, braided alluvial plain environment. The extreme mineralogical and textural immaturity of the lithologies suggests a proximal derivation from the underlying Mesoproterozoic granitoids and gneisses of the Natal Metamorphic Province (NMP). Localized granitic-boulder conglomerate and reworked sediment drapes of granitic origin, also derived from NMP granitoids, are preserved directly beneath the unconformable contact with the Msikaba Formation. These localised units that separate the two successions represent deposition of NMP clasts by flood flow during the ~100 Myr erosional hiatus that separates the fluviatile Natal Group from the shallow marine Msikaba Formation.