IMPACT CRATERING RESEARCH GROUP

Impact of meteorites, asteroids, and comets is the most widespread surface-modifying process on planetary bodies in the Solar System. Southern Africa?s cratering record has been the subject of research by Roger Gibson and Sharad Master, in close collaboration with a number of institutions in Europe and North America. The ICRG?s expertise has also been applied to a number of other impact craters in Africa and North and South America. These multidisciplinary investigations, involving remote sensing, geophysical, geological, mineralogical, geochemical and geochronological techniques have been aimed at identification of possible new impact structures, verification of their origin, and detailed analysis of the geological structure and rock deformation in such crater structures.

An added responsibility of the ICRG is the dissemination of information about impact cratering. To this end, members are involved in radio and TV talk show interviews, field tours and public lectures. Over the last 18 months a particular focus in this regard has been on the Vredefort Dome World Heritage Site where Roger Gibson has been involved in the development of information pamphlets and audiovisual presentations as well as the design of a geological interpretation centre at Vredefort.
For more information contact roger.gibson@wits.ac.za

Aims of Research

Research in the ICRG is aimed at:

• Improving the terrestrial - and especially the African - impact cratering record.
• Contributing to the understanding of the process and the global effects of impact cratering.
• Identifying the structural geological and rock deformation aspects of complex impact structures.

Recent activities

Frank Wieland (Structural analysis of the collar of the Vredefort Dome, 2007) and Martin Tuchscherer (Description of the Yaxcopoil-1 impactites and basement, Chicxulub Crater, Mexico, 2008) and Louise Coney (Description of the LB-07A ICDP drillcore from Lake Bosumtwi, Ghana, 2009) obtained their PhDs recently. Paula Ogilvie (Metamorphic studies in the Vredefort Dome) is currently writing up her PhD. Lauren Jolly (impactite succession) and Gabrielle Townsend (pre-impact history of the target rock lithologies) are completing their MSc studies on the USGS-ICDP Eyreville-B drillcore through the Chesapeake Bay crater, with funding from the NRF Unlocking the Future research focus area. Roger Gibson is currently involved with several students from the Humboldt-Universitat and Uwe Reimold and Ulrich Riller (McMaster University, Canada) with studies of the pseudotachylitic breccias in the Vredefort Dome and with Carl Anhaeusser in evaluating enigmatic breccias in the Setlagole area of North West Province as possible impact remnants. A possible new complex impact structure buried beneath the Kalahari Basin in Botswana is also being investigated with Gordon Cooper and Susan Webb.

Much of 2008 was spent in preparation for the Large Meteorite Impacts and Planetary Evolution IV Conference that was held in the Vredefort Dome in August. Nearly 100 delegates attended and Roger Gibson and Uwe Reimold are currently editing a Geological Society of America Special Paper volume related to the conference. The excursions to the Vredefort Dome were particularly well attended and an updated, expanded guide to the geology of the Vredefort impact structure has been published by the Council for Geoscience. Roger is also leading the development of a geological exhibition for the newly-built Visitor Centre in the Vredefort Dome World Heritage Site.

Research continued on the shocked and thermally metamorphosed rocks of the Vredefort dome with Dr. Alex Deutsch (University of Munster) and Prof. Gary Stevens (University of Stellenbosch) as part of Paula Ogilvie?s PhD. The Lake Bosumtwi and Chesapeake Bay projects involve wide-ranging collaboration with Uwe Reimold and his co-workers in Berlin, Christian Koeberl and students at the University of Vienna, and Wright Horton and Mick Kunk at the USGS (Reston). First results of a palaeomagnetic investigation of the Vredefort dome with Ms. Johanna Salminen (PhD student) and Dr. Lauri Pesonen (University of Helsinki) have been published.


Conferences attended by ICRG members in 2007/8

Recent Publications

• Gibson, R.L. & Reimold, W.U. 2005. Shock pressure distribution in the Vredefort impact structure, South Africa. In: Kenkmann, T., H? F. & Deutsch, A. Large Meteorite Impacts and Planetary Evolution III. Geological Society of America Special Paper, 384, 329-349. ICRG Contrib. 71.
• Reimold, W.U. & Gibson, R.L. 2005. ?Pseudotachylites? in large impact structures. In: Impact Tectonics (eds. Koeberl, C. & Henkel, H.) Impact Studies, Vol. 6, Springer, Heidelberg, pp. 1-53. ICRG Contrib. 55
• Reimold, W.U., Koeberl, C., Gibson, R.L. & Dressler, B.O. 2005. Economic mineral deposits in impact structures and their geological settings. In: Impact Tectonics (eds. Koeberl, C. & Henkel, H.) Impact Studies, Vol. 6, Springer, Heidelberg, pp. 479-552. ICRG Contrib. 66.
• Gibson, R.L., Reimold, W.U. & Lana, C. 2005. Comment on: New PGE and Re/Os-isotope data from lower crustal sections of the Vredefort dome and a reinterpretation of its ?crust-on-edge? profile. South African Journal of Geology, 108, 309-313. ICRG Contrib. 90.
• Tuchscherer, M.G., Reimold, W.U., Koeberl, C. & Gibson, R.L. 2005. Geochemical and petrographic characteristics of impactites and Cretaceous target rocks from the Yaxcopoil-1 borehole, Chicxulub impact structure, Mexico: implications for target composition. Meteoritics and Planetary Science, 40, 1513-1536. ICRG Contrib. 94.
• Hayward, C.L., Reimold, W.U., Gibson, R.L. & Robb, L.J. 2005. Gold mineralisation within the Witwatersrand Basin, South Africa: evidence for a modified placer origin, and the role of the Vredefort impact event. In: I. McDonald, A.J.
• Boyce, I.B. Butler, R.J. Herrington & D.A. Polya (eds.) Mineral Deposits and Earth Evolution. Special Publication of the Geological Society of London, 248, 31-58. ICRG Contrib. 78.
• Wieland, F., Gibson, R.L. & Reimold, W.U. 2005. Structural analysis of the collar of the Vredefort dome, South Africa ? significance for impact-related deformation and central uplift formation. Meteoritics and Planetary Science, 40, 1537-1554. ICRG Contrib. 72.
• Reimold, W.U. & Gibson, R.L. 2005. Meteorite Impact! The danger from Space and South Africa?s mega-impact, the Vredefort Structure. Chris van Rensburg Publications, Johannesburg, 319 p.
• Reimold, W.U. & Gibson, R.L. 2006. The melt rocks of the Vredefort Impact Structure - Vredefort Granophyre and pseudotachylitic breccias: implications for impact cratering and the evolution of the Witwatersrand Basin. Chemie der Erde, 66, 1-35.
• Lana, C., Gibson, R.L., Reimold, W.U. 2006. The Broodkop shear zone, southeast Vredefort dome (South Africa): a Dominion-related extensional feature? South African Journal of Geology, 109, 265-278.
• Wieland, F., Reimold, W.U. & Gibson, R.L. 2006. New observations on shatter cones in the Vredefort impact structure, South Africa, and evaluation of current hypotheses for shatter cone formation. Meteoritics and Planetary Science, 41, 1737-1759.
• Armstrong, R.A., Lana, C., Reimold, W.U. & Gibson, R.L. 2006. SHRIMP zircon age constraints on Mesoarchean crustal development in the Vredefort dome, central K pv l craton, South Africa. In: Reimold, W.U. & Gibson, R.L. Processes on the Early Earth. Geological Society of America Special Paper, 405, 233-253.
• Tuchscherer, M.G., Reimold, W.U., Gibson, R.L., De Bruin, D. & Spath, A. 2006. Major and trace element compositions of melt particles and associated phases from the Yaxcopoil-1 drill core, Chicxulub impact structure, Mexico. Meteoritics and Planetary Science, 41, 1361-1379.
• Coney, L., Gibson, R.L., Reimold, W.U. & Koeberl, C. 2007. Lithostratigraphic and petrographic analysis of ICDP Borehole LB-07A from the Bosumtwi Impact Structure, Ghana. Meteoritics and Planetary Sciences, 42, 569-589.
• Coney, L., Reimold, W.U., Gibson, R.L. & Koeberl, C. 2007. Geochemistry of impactites and basement lithologies from ICDP Borehole LB-07A, Bosumtwi Impact Structure, Ghana. Meteoritics and Planetary Sciences, 42, 667-688.
• Salminen, J., Pesonen, L.J., Reimold, W.U., Donadini, F. & Gibson, R.L. 2009. Paleomagnetic and rock magnetic study of the Vredefort impact structure and the Johannesburg dome, K pv l Craton, South Africa - Implications for the apparent polar wander path for the K pv l Craton during the Mesoproterozoic. Precambrian Research, 168, 167-184.
• Horton, J.W. Jr., Gibson, R.L., Reimold, W.U., Wittmann, A., Gohn, G.S. & Edwards, L.E. 2009. Geologic column for the ICDP-USGS Eyreville B core, Chesapeake Bay impact structure: Impactites and crystalline rocks, 1,095-1,766 m. Geological Society of America Special Paper, in press.
• Townsend, G., Gibson, R.L., Horton, J.W., Jr., Reimold, W.U., Bartosova, K. & Schmitt, R.T. 2009. Geochemical and petrographic analysis of the crystalline basement and megablock sections of the Eyreville-B drillcore, Chesapeake Bay impact structure. Geological Society of America Special Paper, in press.
• Gibson, R.L., Townsend, G., Horton, J.W., Jr. & Reimold, W.U. 2009. Pre-impact tectonothermal evolution of the crystalline target in the ICDP-USGS Eyreville-B core, Chesapeake Bay impact structure. Geological Society of America Special Paper, in press.
• Schmitt, R.-T., Bartosova, K., Reimold, W.U., Koeberl, C., Mader, D., Wittmann, A. & Gibson, R.L. 2009. Geochemistry of impactites and crystalline basement-derived lithologies from the ICDP-USGS Eyreville A and B drill cores, Chesapeake Bay impact structure, Virginia/USA. Geological Society of America Special Paper, in press.
• Lieger, D., Riller, U. & Gibson, R.L. 2009. Generation of fragment-rich pseudotachylite bodies during central uplift formation in the Vredefort Impact Structure, South Africa. Earth and Planetary Science Letters, in press.
• Mohr-Westheide, T., Reimold, W. U., Riller, U. & Gibson, R.L. Pseudotachylitic breccia and microfracture networks in Archean gneiss of the central uplift of the Vredefort Impact Structure, South Africa, South African Journal of Geology, submitted.