Petrographic studies of “fallout” suevite from outside the Bosumtwi impact structure, Ghana
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چکیده
available online at http://meteoritics.org 1761 © The Meteoritical Society, 2006. Printed in USA. Petrographic studies of “fallout” suevite from outside the Bosumtwi impact structure, Ghana Daniel BOAMAH† and Christian KOEBERL* Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria †Present address: Geological Survey Department, P.O. Box M80, Accra, Ghana *Corresponding author. E-mail: [email protected] (Received 14 January 2006; revision accepted 06 August 2006) Abstract–Field studies and a shallow drilling program carried out in 1999 provided information about the thickness and distribution of suevite to the north of the Bosumtwi crater rim. Suevite occurrence there is known from an ~1.5 km2 area; its thickness is 15 m. The present suevite distribution is likely the result of differential erosion and does not reflect the initial areal extent of continuous Bosumtwi ejecta deposits. Here we discuss the petrographic characteristics of drill core samples of melt-rich suevite. Macroscopic constituents of the suevites are melt bodies and crystalline and metasedimentary rock (granite, graywacke, phyllite, shale, schist, and possibly slate) clasts up to about 40 cm in size. Shock metamorphic effects in the clasts include multiple sets of planar deformation features (PDFs),Field studies and a shallow drilling program carried out in 1999 provided information about the thickness and distribution of suevite to the north of the Bosumtwi crater rim. Suevite occurrence there is known from an ~1.5 km2 area; its thickness is 15 m. The present suevite distribution is likely the result of differential erosion and does not reflect the initial areal extent of continuous Bosumtwi ejecta deposits. Here we discuss the petrographic characteristics of drill core samples of melt-rich suevite. Macroscopic constituents of the suevites are melt bodies and crystalline and metasedimentary rock (granite, graywacke, phyllite, shale, schist, and possibly slate) clasts up to about 40 cm in size. Shock metamorphic effects in the clasts include multiple sets of planar deformation features (PDFs), diaplectic quartz and feldspar glasses, lechatelierite, and ballen quartz, besides biotite with kink bands. Basement rock clasts in the suevite represent all stages of shock metamorphism, ranging from samples without shock effects to completely shock-melted material that is indicative of shock pressures up to ~60 GPa.
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