Clast fabric examination of impact-generated breccias, borehole LB-07A, Bosumtwi, Ghana

Abstract

An impact event always creates a cloud of ejecta generated through excavation of the target. Subsequent in-filling of the void by crater-fill deposits provides a record of post-impact processes. Full-core digital photographic scans of core segments from borehole LB-07 in the Bosumtwi impact crater provide a complete record of the in-fill process. The shape, orientation, and size of clasts within the impact breccia were measured using a best-fit ellipsoid approach. Clast size and variance, together with clast orientation data, suggest the impact breccias at Bosumtwi can be divided into a simple two-fold subdivision that loosely agrees with the lithological zonation of a lower monomict breccia overlain by a polymict breccia. The lower unit is characterized by a uniform and finer-grained clast size together with a uniform flat-lying clast orientation. The boundary between the two zones is defined by a sharp increase in clast size. The upper zone shows an average increase in clast size with decreasing depth, but full grain size spectrum together with increased grain size deviation suggest that this is a result of mixing between two populations with different grain size distribution. The main population of clasts shows an incremental decrease of clast size with decreasing depth. The upper zone also contains weakly defined shallowly dipping clast fabrics, which may be suggestive of horizontal transport or deposition onto an inclined surface.