Gross Brukkaros (Namibia)—an enigmatic crater-fill reinterpreted as due to Cretaceous caldera evolution |
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| Authors: | T Stachel V Lorenz I G Stanistreet |
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| Institution: | (1) Institut für Geologie, Universität Würzburg, Pleicherwall 1, D-97070 Würzburg, Germany;(2) Department of Geology, University of the Witwatersrand, P. Bag 3, 2050 Wits, South Africa;(3) Present address: Dept. of Geology & Applied Geology, G12 8QQ Glasgow, UK |
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| Abstract: | At Gross Brukkaros a central depression has developed within domed Nama Group sediments and has functioned as a local depocenter, with a primary fill deposited during the Cretaceous and a small secondary fill by alluvial fans during the Tertiary and Quaternary. The diameter of the entire structure is about 10 km and that of the central depression is about 3 km. Within this depocenter the sedimentary sequence consists mainly of debris-flow and mudflow deposits, with minor intercalations of fluviatile (braided channel) sediments and fossiliferous lacustrine deposits. The sedimentary system represents a set of coalesced subaerial fans which formed a fringing sedimentary apron along the margin of the depocenter. This sedimentary apron passed distally and centrally into a permanent lake, which was characterized by a fluctuating water level. Facies transitions observed are typical of those described from modern and ancient fan delta systems. Contact relationships show the Gross Brukkaros sediments to be about the same age (Upper Cretaceous) as the surrounding carbonatitic volcanism. An Upper Cretaceous age is also consistent with the plant fossil association recently recognized within the lacustrine beds of Gross Brukkaros. We attribute the genesis of the dome structure to the shallow intrusion of a laccolith-shaped, strongly alkaline to carbonatitic magma body. Subsequent depletion of the reservoir due to volcanic activity around and in(?) Gross Brukkaros led to subsidence resulting in the development of the Gross Brukkaros depocenter. Differences between Gross Brukkaros and the general caldera model consist of a radially oriented dike pattern and the formation of the caldera by downsagging rather than cauldron subsidence, as derived from the absence of ring faults and ring dikes. The first (radial dikes) may be attributed to comparatively strong initial doming; the latter (lack of ring faults) to the small size of the caldera, its incremental subsidence, and finally the sedimentary wall rocks instead of a rigid crystalline crust. |
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| Keywords: | caldera crater sediments debris-flow alluvial fan delta Gross Brukkaros |
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