Some Physical Properties of Suevites from the Bosumtwi Impact Crater, Ghana

Abstract

Suevite is a polymict breccia of clastic material derived predominantly from the crystalline basement. It is an impact-derived rock usually found at meteorite impact crater sites. In Ghana, suevites have been found at two locations at the Bosumtwi meteorite impact crater. The suevites are located in the Northern and Southern parts outside the crater rim. Due to the presence of different rock clasts of various sizes, the suevite exhibits physical properties that are quite different from those of other rocks such as granites, gneisses, etc. Suevites found in the North and South locations have some characteristic differences. In this paper, we report on the anisotropic behaviour of the compressional wave velocity Vp with pressure and azimuth for suevite samples collected from the North and South locations. The effect of pressure on Vp for the sample from the South is more pronounced than that from the North because of the high porosity of the sample at the South location. Also, the seismic velocity anisotropy is more pronounced in the samples from the South probably due to the distribution of rock inclusions in the matrix. Vp-minimum directions determined for some samples indicate that the Vp-minimum axes seemed to point toward the center of the crater. This supports the reasoning that after the impact, the ejected material on the ground might have assumed a preferred orientation with respect to the center of the crater. It was also found that suevite samples require higher saturation pressures (650 MPa and above) than solid rocks such as amphibolite which reaches velocity saturation at 100 MPa.

Key words: suevites, impact crater, compressional wave velocity, anisotropy, velocity saturation