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

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 glass- rich 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 less than or equal to 15 m and to occupy an area of ~1.5 km2. 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.