Geology and geochemistry of shallow drill cores from the Bosumtwi impact structure, Ghana

Abstract

Abstract-The 1.07 Ma well-preserved Bosumtwi impact structure in Ghana (10.5 km in diameter) formed in 2 Ga-old metamorphosed and crystalline rocks of the Birimian system. The interior of the structure is largely filled by the 8 km diameter Lake Bosumtwi, and the crater rim and region in the environs of the crater is covered by tropical rainforest, making geological studies rather difficult and restricted to road cuts and streams. In early 1999, we undertook a shallow drilling program to the north of the crater rim to determine the extent of the ejecta blanket around the crater and to obtain subsurface core samples for mineralogical, petrological, and geochemical studies of ejecta of the Bosumtwi impact structure. A variety of impactite lithologies are present, consisting of impact glassrich suevite and several types of breccia: lithic breccia of single rock type, often grading into unbrecciated rock, with the rocks being shattered more or less in situ without much relative displacement (autochthonous?), and lithic polymict breccia that apparently do not contain any glassy material (allochtonous?). The suevite cores show that melt inclusions are present throughout the whole length of the cores in the form of vesicular glasses with no significant change of abundance with depth. Twenty samples from the 7 drill cores and 4 samples from recent road cuts in the structure were studied for their geochemical characteristics to accumulate a database for impact lithologies and their erosion products present at the Bosumtwi crater. Major and trace element analyses yielded compositions similar to those of the target rocks in the area (graywacke-phyllite, shale, and granite). Graywacke-phyllite and granite dikes seem to be important contributors to the compositions of the suevite and the road cut samples (fragmentary matrix), with a minor contribution of Pepiakese granite. The results also provide information about the thickness of the fallout suevite in the northern part of the Bosumtwi structure, which was determined to be ≤15 m and to occupy an area of ~1.5 km 2 . Present suevite distribution is likely to be caused by differential erosion and does not reflect the initial areal extent of the continuous Bosumtwi ejecta deposits. Our studies allow a comparison with the extent of the suevite at the Ries, another well-preserved impact structure.

Petrography, geochemistry, and alteration of country rocks from the Bosumtwi impact structure, Ghana

“…The average compositions of the various rock types are given in Table 4, together with the average composition of Ivory Coast tektites (with data from Koeberl et al 1997Koeberl et al , 1998Boamah and Koeberl 2003) and upper continental crust rocks (Taylor and McLennan 1985).…”

“…9. Harker variation diagrams for metasedimentary lithologies, granite, suevite, and melt/glass fragments in suevites compared to the average values for Ivory Coast tektites (with data from Koeberl et al 1997Koeberl et al , 1998Boamah and Koeberl 2003).…”

“…10. Chondrite (C1)-normalized rare earth element (REE) patterns for the various sample groups (shale-phyllite, granite, meta-graywacke, and suevite), as well as averages for these groups and average of the Ivory Coast tektites (with data from Koeberl et al 1997and Boamah and Koeberl 2003. Normalization factors from Taylor and McLennan (1985).…”

“…This correlation is mainly based on similarities in geochemical and isotopic compositions of target rocks and tektites, as well as similarities in the ages of the impact melt from suevites and of the Ivory Coast tektites. Boamah and Koeberl (2003) carried out detailed petrographic and geochemical studies on suevites from shallow drill cores obtained to the north of the crater. Results of structural and geological mapping of the Bosumtwi crater rim were reported by Reimold et al (1998).…”

The Lake Bosumtwi meteorite impact structure, Ghana—Where is the magnetic source?

“…In early 1999, a shallow drilling program to the north of the crater rim was undertaken to determine the extent of the ejecta blanket around the crater and to obtain subsurface core samples for mineralogical, petrological, and geochemical studies of ejecta (Boamah and Koeberl 2003). A variety of impactite breccias were found, including impact glass-rich suevite and several types of lithic breccias (Boamah and Koeberl 2003).…”