Jwaneng South Structure, Botswana: a New 1.3 km Diameter Buried Cenozoic Impact Crater Discovered by Airship-mounted Gravity Gradiometer

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, about 15 km S of Jwaneng. The target was generated on bouguer gravity data obtained with an airship-mounted full tensor gravity gradiometer. The circular anomaly also appears on airborne EM, ground gravity and CSAMT, images. The structure was investigated by 9 diamond drill holes, which revealed a sedimentary rock sequence, consisting of evaporitic lacustrine carbonates with plant fossils, overlain by sedimentary breccias and sandstones of the Kalahari Group (maximum thickness ~300m), filling an apparent circular bowl-shaped depression about 1.3 km in diameter. Below the sedimentary rocks is a <60m thick allogenic breccia complex consisting of a series of breccias with granitic fragments, which pass downwards into brecciated mafic rocks (which resemble Karoo dolerites geochemically) and then unbrecciated granites (of the 2785 Ma Gaborone Granite Complex). No kimberlites or any other similar intrusive rocks were encountered in the boreholes. Petrographic studies show that the rocks have suffered intense shock eformation (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, 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. Jwaneng South is the first impact structure on earth to have been discovered using airborne gravity surveys.