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During regional exploration for diamondiferous kimberlites by De Beers Botswana in 2006, a pronounced circular geophysical anomaly, ∼1 km in diameter (now called the Jwaneng South Structure), was discovered at ∼24°46'E, 24°42'S, ∼15 km S of Jwaneng. The circular anomaly appears on airborne gravity gradiometer and EM, and ground gravity and CSAMT, images. The structure was investigated by 9 diamond drill holes laid out on E-W and N-S lines. The drilling revealed a sedimentary rock sequence, filling an apparent circular bowl-shaped depression. Below the sedimentary rocks are a series of breccias with granitic fragments, which pass downwards into brecciated mafic rocks and then granites. No kimberlites or any other similar intrusive rocks were encountered in the boreholes.
The main lithologies found in the boreholes are basement rocks consisting of granites (of the 2785 Ma Gaborone Granite Complex), overlain by mafic lavas (of the 2782 Ma Lobatse Gp); a variety of different breccias, and a sedimentary sequence (probably Kalahari Group) consisting of sandstones, minor evaporitic carbonate rocks, sedimentary breccias and conglomerates, and silcretes near surface. Four breccia types occur: (1) autochthonous authigenic (in situ) breccias; (2) allogenic heterolithic "fallback" breccias (3) breccia dykes; and (4) sedimentary breccias. Near the base of the sedimentary succession overlying the allogenic breccias, sedge-like plant fossils have been found in lacustrine evaporitic carbonate rocks, indicating a Cenozoic, and most probably Miocene or younger, age (M. Bamford, prelim. data). Drilling data shows that the structure is 1.3 km in diameter, has a bowl shape (no rim) with maximum depth of ∼300m, and an allogenic breccia lens filling it which is ∼60m thick.
Petrographic studies show that the rocks have suffered intense shock deformation (although impact-diagnostic PDFs have not yet been found). They have been completely shattered into angular fragments in the authigenically brecciated basement, and into subangular to subrounded shapes in the allogenic breccias, which have a sandy matrix. Individual mineral grains show features found in known impact structures- including mosaicism, deformation bands and lamellae in plagioclase and alkali feldspars, and cleavage in quartz. Macroscopic indicators of shock include "gries"-textured breccias, in which very closely-spaced intersecting sets of fractures break up the rock into lozenges; and multiply-striated joint surfaces, formed by intersecting shock waves, analogous to shatter cones. The disposition of fractured basement, allogenic breccias, breccia dykes, and sedimentary fill, as well as the geophysical characteristics, closely resemble those found in other buried impact structures developed on granitic targets (Tswaing, Kgagodi, Brent), and modelling points to the origin of the structure due to hypervelocity impact by a meteorite (or asteroidal fragment), with a diameter, if chondritic, of ∼100m.
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