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The geology and regional correlation of the Proterozoic Okwa Inlier, western Botswana

Precambrian Research
Publication Date
DOI: 10.1016/0301-9268(92)90104-v
  • Earth Science


Abstract The small Okwa inlier exposes four unconformity-bounded Proterozoic lithostratigraphic assemblages in the Kalahari region of central southern Africa, where the Precambrian geology is otherwise largely obscured by Phanerozoic strata. The inlier provides the only exposures in the Okwa basement, a structural block between the Archaean Kaapvaal-Zimbabwe Craton and the Late Proterozoic Damara orogen. Felsites and sericitic quartzites of the Okwa basement complex were intruded by varied granites between 2.1 and 2.0 Ga ago. Deformation of the granites to form gneisses at ∼ 1.8 Ga (D 1) was accompanied by metamorphism of pre-tectonic mafic dykes in the epidote amphibolite facies. In the Mid-Proterozoic, felsic tuffs and red sandstones of the lower Okwa group were intruded by dolerite sills, and a contemporary northeasterly dyke-swarm intruded the basement granitoids. A fracture cleavage was imposed on the lower Okwa group during folding and greenschist/low amphibolite facies metamorphism (D 2) at ∼ 1.15 Ga. The red-bed sediments of the upper Okwa group suffered only very mild deformation (D 3). The youngest assemblage is assigned to the Late Proterozoic Nama Group. It mainly consists of gently dipping, fine-grained sandstones. The Okwa basement complex is confirmed as a segment of Eburnian-aged crust which was accreted to the Archaean craton at about 1.8 Ga, probably adjacent to epicratonic sediments allied to the Magondi Supergroup of Zimbabwe or to the Olifantshoek Sequence of South Africa, although such sediments are nowhere exposed in the Okwa sector. The lower Okwa group is thought to be equivalent to volcano-sedimentary sequences such as the Sinclair Group of Namibia. The upper Okwa group can be correlated either with immediately pre-Damaran units such as the Auborus Formation of the Sinclair area, or with the Nosib and Ghanzi Groups in the early Damara Supergroup. The occurrence of Nama sediments overlying the Okwa basement seems to confirm their suspected presence beneath the Karoo Supergroup in the Ncojane-Nosop Basin of western Botswana.

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