Gross Brukkaros - the unusual intracaldera sediments and their magmatic components

Gross Brukkaros, Namibia, is a broad (10 km diameter) dome structure in Nama Group country rocks, with a crater-shaped central depression. Formation of the dome structure has recently been attributed to a shallow intrusion, which subsequently became partly depleted during volcanic activity in the Brukkaros Volcanic Field, causing cauldron subsidence. The subsiding depocentre became filled by a sedimentary sequence corresponding to alluvial fan delta/lacustrine environments. This paper gives a detailed petrographical and chemical description of the Brukkaros sediments and their lithic and mineral constitutents. The main sources for the Brukkaros sediments are shales, cherts, sandstones, and quartzites from Karoo Sequence and Nama Group rocks as well as Karoo dolerites. A contribution of aeolian deposits (early Kalahari?) is also identified. Metasomatic-hydrothermal alteration and cementation of the Brukkaros rocks can be shown to have operated already during sedimentation. -Authors