Abstract

Burnt Mountain is the name attached to a series of low, dark-coloured hills, dominantly composed of carbonaceous shale, siltstone, sandstone and various ferruginous rocks belonging to the Permian Verbrande Berg Formation, Ecca Group, Karoo Supergroup, in the Huab Basin, northwestern Namibia. At this site, which owes its name to the “burnt” appearance of the rocks, the Ecca Group rocks form a 150 m high mesa, capped with aeolian sandstone of the Cretaceous Twyfelfontein Formation, Etendeka Group. Reinvestigation of the outcrops at Burnt Mountain revealed that the exceptional 600 m-wide outcrop of the Verbrande Berg Formation at this site is the result of a north-south normal fault with a downthrow to the west. This fault forms one of a series of comparable north-south-striking structures in the Huab Basin that are linked to the opening of the proto-Atlantic Ocean. In the lower part of the Verbrande Berg Formation, where the carbonaceous shale is enriched in carbon, lenses of ferruginous fossiliferous carbonate rocks (FFC) with abundant plant fossils, as well as large (m-sized), flattened, ferruginous concretions are developed. Small concretions (cm-sized) as well as bedding-parallel segregations of ferric oxyhydrate minerals (largely hematite and goethite) are common in the FFC and are interpreted to be sulphide-derived. In the middle part of the Verbrande Berg Formation, a prominent unit (PU) of a medium-bedded, micro-crystalline, ferruginous and porous rock (FPR) is interbedded with the carbonaceous shale and sandstone. The FPR, which contains altered relicts of glass particles and possibly accretionary lapilli spheres, is interpreted to be the leached and ferruginated product of coarse-grained vitric lithic lapilli tuff and tuff, emplaced on an active swamp surface. The associated volcanic activity apparently was long-lived and culminated in the deposition of the PU. The ferrugination of the FPR is attributed to interaction of the nascent volcaniclastic rocks with acid bog water, augmented by oxidation products of early diagenetic sulphide that formed in the FPR on burial. Prolific development in places of Liesegang structures in the FPR is ascribed to fault-related fracturing and the infiltration of oxygenated meteoric water, prior to the intrusion of dolerite dykes that are here tentatively associated with the Etendeka volcanism. The positive identification of volcaniclastic rocks in the Verbrande Berg Formation in the Huab Basin corresponds to the occurrence of tuffs, at the same stratigraphic level, in comparable rocks in the Aranos Basin in southern Namibia as well as in the western parts of main Karoo Basin in South Africa. The FPR is the main source of the dark-coloured hematitic clasts (“clinker-like” rock) which contribute to the “burnt” appearance of the Burnt Mountain.

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