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Bosumtwi crater gives up its secrets

Bosumtwi Crater

Geoscientist Online 18 June 2007

An international programme, initiated over a decade ago, consisting of a drilling and geophysical programme at Bosumtwi Crater, Ghana, has been reported on in a 400+ page) issue of Meteoritics and Planetary Science1, reports Joe McCall.

Drilling on the structure, a 8.5km diameter, 1.07 million years old impact crater, was carried out from the surface of the weakly saline crater lake, which is only 78m deep at its deepest. The crater was excavated in rocks of the Birimian System (2.1 – 2.2Ga metasediments and metavolcanics). Fourteen cores in the sediment-fill are being subjected to palaeoenvironmental investigations, while two more were obtained by drilling below the sediments into the deepest section of the crater’s annular moat (to 540m) and into the flanks of the submerged central uplift (to 450m)2

The base of the post-impact sedimentary infill was encountered at 333m and 235m. Both holes penetrated fractured bedrock below impact breccias. Hole LB-07A reached 545m and penetrated lithic (upper part) and suevitic impact breccias with an appreciable amount of melt rock content. Quartz grains showing multiple planar shock deformation were abundant. Hole LB-08A encountered suevitic breccia and below, greywacke-dominated metasediment with suevite and granitoid dyke intercalations (the sediments were assumed to be bedrock). The suevite in cores differs from that recognised on the crater flanks. It was very porous.

Deep drilling confirmed the seismically modelled structure, with a central peak. Petrophysical core analysis provided no support for the existence of a homogeneous unit of melt breccia in the crater centre. The lack of a coherent melt sheet or any significant amounts of melt rock in the crater runs counter to the model predictions and geophysical surveys. This resents a problem for the modelling for other impact structures.

Because of my interest in tektites3, the most interesting of the 26 detailed articles is that by Koeberl et al.4 on the uppermost fall-back layer, which was penetrated in Hole LB-05 between the post-impact sedimentary fill and the impact breccias. Such preservation in core from an impact structure is extremely rare. In the top 10cm of this 30cm-thick layer are accretionary lapilli, microtektite-like glass spherules (most <1mm diameter) with smooth to rough surfaces, and shocked quartz grains. The glass particles make up 70-78% of the grains in the coarser top of the fall-back layer. The tektite-like fragments display splash forms – spheres, dumbbells and tear-drops, and may be stuck together. (Many tear-drops are derived from broken dumb-bells.) The chemical composition of individual tektite-like bodies is very uniform, and the bulk composition of a number of bodies averages to much the same composition as that of the Ivory Coast tektites and microtektites - which come from Bosumtwi - with the single exception of higher Ca values.

Studies of the Platinum Group elements have failed to find any evidence of extraterrestrial material in the breccias5, despite the fact that the Ivory Coast tektites did yield unambiguous evidence of the presence of a minute meteoritic component, in studies of the Re-Os isotope system6.

References cited

  1. Meteoritics and Planetary Science, 2007; Volume 42, Number 4-5 (April-May).
  2. Koeberl, C, Milkereit, B, Overpeck, C et al. 2007 M & PS, 42; International and multidisciplinary drilling project into a young complex impact structure: the 2004 ICDP Bosumtwi Crater drilling project - an overview M & PS, 42 (4-5); 483-511.
  3. McCall, G J H 2001: Tektites in the Geological Record: Showers of glass from the sky. Geological Society Publishing House, Bath;
  4. Koeberl, C, Brandstatter, F, Glass, B P, Hecht, L, Mader,D, Reimold, W U 2007 Uppermost fallback layer in the Bosumtwi crater, Ghana: Mineralogy, Geochemistry and comparison with Ivory Coast tektites M & PS 42 (4-5); 709-729.
  5. McDonald, I, Peucker-Ehrenbrink, B, Coney, L, Feriere, L, Reimold, W U & Koeberl, C 2007 Search for a meteorite component in drill cores from the Boseumtwi impact structure, Ghana: platimum group element contents and osmium isotope characteristics M & PS 42 (4-5); 743-753
  6. Koeberl, C, 1998 Identification of meteorite components in impactites In: Grady, M M, Hutchison, R, McCall, G J H & Rothery, D A Meteorites; Flux with time and impact effects Geological Society, London, Special Publications 140, 133-153.