The geology of the Brandberg-Abahuab area, South West Africa

Abstract

Abstract provided by author:

The Karoo episode commenced during Late-Carboniferous times. The Dwyka tillite was followed by the deposition of shale, sandstone and limestone with a total thickness of 150 m. These sediments do not correspond lithologically to the sedimentary units of the Karoo System in other parts; neither do they include any index-fossils, The name Doros Formation is therefore suggested for this sequence. The Cave Sandstone Stage, which is aeolian in origin, marks the closure of the Karoo sedimentary cycle. This was followed by the intrusion of Karoo dolerite, and the eruption of Karoo basalt and rhyolite

Two plutons of post-Karoo age are present in the area. The Doros Complex, which is gabbroic in composition, is situated in the centre of area 2014C. Five periods of intrusion are distinguished, of which only one exhibit signs of magmatic differentiation. This differentiation advanced in an undersaturated, alkaline direction and the final product presently exposed, is aegirine bostonite. The contact between the complex and the surrounding rocks is partly concordant, partly discordant and always sharp. Permissive emplacement of the magma, with slight updoming of the Karoo sequence is postulated. The dolerite sill in the vicinity of the complex represents the lateral continuation of this ethmolitic body

The Brandberg pluton marks the end of the Karoo episode. It is granitic in composition and five intrusions have been identified. These are all similar in composition and no magmatic differentiation during the emplacement of these rocks is postulated. The Brandberg granites are however the product of magmatic differentiation of Karoo basalt and an extrusive phase in the form of rhyolite presently capsthe Karoo basalt north of the Brandberg. The mode of emplacement was very similar to that at Doros, although signs of forceful injection are present. The mechanism for the formation of cone sheets and ring dykes was probably operative during the emplacement of the pluton