Volcanology of the Gross Brukkaros Field, southern Namibia

Gross Brukkaros is a volcano-shaped inselberg rising 600 m above the Nama plain in southern Namibia. It evolved in several steps starting with the intrusion of a laccolithic body which caused uplift and radial fissuring of the overlying rocks, Cambrian, Permocarbon iferous and Upper Cretaceous Kalahari sediments as well as Jurassic dolerite sills. On the radial fissures more than 100 carbonatite dykes and 74 carbonatite diatremes were emplaced. The eruption from these dykes and diatremes caused a mass deficiency in the laccolith and subsidence of its roof, i.e. a downsag caldera formed collecting thick reworked sedimentary and pyroclastic debris. The carbonatite diatremes point to phreatomagmatic explosive activity because of occurrence of only non-vesicular carbonatite ash grains and lapilli and because the hydrogeology was conducive to such explosive activity: a near surface groundwater table is indicated by unconsolidated Kalahari sediments and the free groundwater table of the caldera lake of Gross Brukkaros.

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