DRILL CORE LB-08A, BOSUMTWI IMPACT STRUCTURE, GHANA: PETROGRAPHIC AND SHOCK METAMORPHIC STUDIES OF MATERIAL FROM THE CENTRAL UPLIFT

Abstract

During a recent drilling project sponsored by the International Continental Scientific Drilling Progam (ICDP), two boreholes (LB‐07A and LB‐08A) were drilled into the crater fill of the Bosumtwi impact structure and the underlying basement, into the deep crater moat and the outer flank of the central uplift, respectively. The Bosumtwi impact structure in Ghana (West Africa), which is 10.5 km in diameter and 1.07 Myr old, is largely filled by Lake Bosumtwi. Here we present the lithostratigraphy of drill core LB‐08A (recovered between 235.6 and 451.33 m depth below lake level) as well as the first mineralogical and petrographic observations of samples from this core. This drill core consists of approximately 25 m of polymict, clast‐supported lithic breccia intercalated with suevite, which overlies fractured/brecciated metasediment that displays a large variation in lithology and grain size. The lithologies present in the central uplift are metasediments composed dominantly of fine‐grained to gritty meta‐graywacke, phyllite, and slate, as well as suevite and polymict lithic impact breccia. The suevites, principally present between 235.6 and 240.5 m and between 257.6 and 262.2 m, display a fine‐grained fragmental matrix (about 39 to 45 vol%) and a variety of lithic and mineral clasts that include meta‐graywacke, phyllite, slate, quartzite, carbon‐rich organic shale, and calcite, as well as melt particles, fractured quartz, unshocked quartz, unshocked feldspar, quartz with planar deformation features (PDFs), diaplectic quartz glass, mica, epidote, sphene, and opaque minerals). The crater‐fill suevite contains calcite clasts but no granite clasts, in contrast to suevite from outside the northern crater rim.