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

Abstract

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.

Lateral transport of suevite inferred from 3D shape-fabric analysis: Evidence from the Ries impact crater, Germany

“…Here, the long axes of suevite melt particles are either subhorizontal or inclined. Furthermore, suevite particles in the Bosumtwi crater, Ghana, are also aligned in a subhorizontal plane, evident from analysis of drill core (Morris et al, 2007).…”

Impact structures in Africa: A review

“…360 m were obtained from the impactites and underlying basement. In addition, extensive geophysical studies were carried out, as described by Elbra et al, 2007 , Hunze and Wonik, 2007 , Kontny et al, 2007 , L’Heureux and Milkereit, 2007 , Morris et al, 2007a , Morris et al, 2007b , Schell et al, 2007 , Schmitt et al, 2007 , Scholz et al, 2007 , and Ugalde et al, 2007a , Ugalde et al, 2007b , Ugalde et al, 2007c .…”

Clast size distribution and quantitative petrography of shocked and unshocked rocks from the El'gygytgyn impact structure

“…Morris et al. () focused on suevite from a drill core of the Bosumtwi impact structure, analyzing the clast size and shape variations with a best‐fit ellipsoid approach. This allowed them to recognize two clast populations, which corresponded to the lithological zonation of a lower monomict and an upper polymict breccia.…”