Lithostratigraphic and petrographic analysis of ICDP drill core LB-07A, Bosumtwi impact structure, Ghana

Abstract

Lithostratigraphic and petrographic studies of drill core samples from the 545.08 m deep International Continental Scientific Drilling Program (ICDP) borehole LB-07A in the Bosumtwi impact structure revealed two sequences of impactites below the post-impact crater sediments and above coherent basement rock. The upper impactites (333.38-415.67 m depth) comprise an alternating sequence of suevite and lithic impact breccias. The lower impactite sequence (415.67-470.55 m depth) consists essentially of monomict impact breccia formed from meta-graywacke with minor shale, as well as two narrow injections of suevite, which differ from the suevites of the upper impactites in color and intensity of shock metamorphism of the clasts. The basement rock (470.55-545.08 m depth) is composed of lower greenschist-facies metapelites (shale, schist and minor phyllite), meta-graywacke, and minor meta-sandstone, as well as interlaminated quartzite and calcite layers. The basement also contains a number of suevite dikelets that are interpreted as injection veins, as well as a single occurrence of granophyric-textured rock, tentatively interpreted as a hydrothermally altered granitic intrusion likely related to the regional pre-impact granitoid complexes. Impact melt fragments are not as prevalent in LB-07A suevite as in the fallout suevite facies around the northern crater rim; on average, 3.6 vol% of melt fragments is seen in the upper suevites and up to 18 vol% in the lower suevite occurrences. Shock deformation features observed in the suevites and polymict lithic breccias include planar deformation features in quartz (1 to 3 sets), rare diaplectic quartz glass, and very rare diaplectic feldspar glass. Notably, no ballen quartz, which is abundant in the fallout suevites, has been found in the within-crater impact breccias. An overall slight increase in the degree of shock metamorphism occurs with depth in the impactites, but considerably lower shock degrees are seen in the suevites of the basement rocks, which show similar features to each other. The bulk of the suevite in LB-07A appears to have been derived from the <35 GPa shock zone of the transient crater.