62nd Annual Meteoritical Society Meeting 5034.pdf

THE TSWAING CRATER - ONE OF THE WORLD'S BEST PRESERVED, AND MUSEUM PLANS FOR THE SITE. D. Brandt, Department of Geology, Private Bag 3, P.O. WITS, 2050, , e-mail: [email protected]

The Tswaing Crater: Some of the world's best Geophysical work and modelling has shown that the examples of simple bowl-shaped impact craters are found in the crater is a near-surface disturbance, as well as, the absence of southern African crater record. These include the Roter Kamm magnetic volcanic material which may have been thought to crater in Namibia and, one of the worlds best preserved craters: exist below the crater-fill material [8]. the Tswaing Crater. Tswaing (Sotho for "place of salt") is A comparison of the geological structures at the situated on our doorstep, 40 kilometres to the north-northwest of Tswaing crater with those of other well studied impact craters , in the Province and in the Bushveld Igneous (e.g. Meteor Crater in Arizona) showed that the structure of the Complex. Tswaing Crater is exactly what could be expected at a Until 1991, the crater was largely believed to be [4]. These studies are all in agreement with an volcanic in origin eg. [1]; [2]. The fortuitous coincidence of the impact origin for the crater. spatial occurrence of the crater with occurrences of various Museum Plans for the Site: Only two other impact volcanic-type alkaline intrusions, was the main argument in craters, worldwide, have museums, making the Tswaing Crater favour of a volcanic origin for the crater. In 1991, however, a Museum the third ever, and the only one in Africa. Towards the core drilled to 200 metres in the centre of the crater, provided end of 1997, the Development Master Plan for the Tswaing definitive evidence for an impact origin of the crater [3]. The Museum was approved by the Tswaing Forum and other core showed 90 metres of crater-fill sediments overlying 100 stakeholders. This plan provided for the establishment of metres of (fragmented rock), which grades downwards various components, namely an environmental education centre, into unaffected, solid granite [3]. It was in this core, between 90 a cultural and crafts centre, and the museum building. All these and 150 metres depth, that many diagnostic shock metamorphic components will be integrated and interlinked with regard to effects, which are uniquely associated with impact-deformed functional layout, use, design etc. Staff of the Department of rocks, were found. Geology have been involved in all phases of the museum Geological mapping of the crater and the surrounding development to date and are now actively involved in exhibit region showed that intrusions of a volcanic nature and of the planning and design. same composition as those occurring at the crater, occur in the References: [1] Wagner P.A. (1922) Geol. Surv. S. Afr. entire region and are not confined to the crater [4]. In addition, Mem., 20, 136 pp. [2] Feuchtwanger T. (1973) B.Sc. Hons. the intrusions at the crater have been fractured by what Proj., Wits Univ., 41 pp. [3] Reimold W.U. et al. (1991) LPS presumably was the cratering event, suggesting that they pre- XXII, 1117-1118. [4] Brandt D. & Reimold W.U. (1995) S. Afr. date the crater. Further research, using various dating techniques J. Geol., 98(3), 287-303. [5] Partridge T.C. et al. (1993) [5]; [6], showed that the cratering event occurred approximately Palaeogeog., Palaeoclimat., Palaeoecol., 101, 317-337. [6] 200 000 (0.2 million) years ago and that the volcanic rocks Storzer D. et al. (1993) LPS XXIV, 1365-1366. [7] Brandt D. et occurring at the crater are 1300 million years in age [7]. These al. (1996) S. Afr. J. Geol., 99(3), 293-297. [8] Brandt D. et al. results show that the intrusions at the crater are related to the (1994) Meteoritics, 29(3), 379-384. much older Pienaars River Alkaline Complex and Pilanesberg related volcanism.