Journal of African Earth Sciences, Vol. 18, No. 4, pp. 263-295, 1994 Pergamon Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0899-5362/94 $7.00 + 0.00 0899-5362(94)00044-1 African meteorite impact craters: characteristics and geological importance CHRISTIAN KOEBERLt~ 1Institute of Geochemistry, University of Vienna, Dr.-Karl-Lueger-Ring 1, A-1010 Vienna, Austria. 2Economic Geology Research Unit, Depa~ i.ment of Geology, University of the Witwatemmnd,Johannesburg 2050, South Africa. (Received 26 October 1993 : accepted 12 May 1994) Abslzact - Geologists have realized that impact cratering is the single most important surface-forming and modifying process for the other terrestrial planets and the satellites of all planets. The recognition of impact cratering as an important geological process on earth has been rather slow. However, geologists are now realizing that giant impacts have had a determining influence on the geological and biological evolution of our planet. The study of impact craters allows important conclusions, not only about the origin mid history of our solar system and its planets, but also about a fundamentally important geological process. In addition, impact craters may have a definite economic importance as some craters have been shown to contain important mineral or oil deposits. F'dtesn meteorite impact craters have so far been identified on the African continent:. Amguid (Algeria), Aomunga (Chad), Aouelloul (Mauritania), B.P. (Libya), Bosumtwi (Ghana), Highbury (Zimbabwe), Kalkkop (South Africa), Oasis (Libya), Ouarkziz (Algeria), Roter Kamm (Namibia), Saitpan (South Africa), Talemzane (Algeria), Tenoumer (Mauritania), T'm Bider (Algeria), and V~:lefort (South Africa). This paper presents an overview of these craters, as well as a discussion of impact processes, the recognition of impact cTaters, and the geological arid economic importance of impact craters. R&nml~ - Lea g~ioguea ont maintenantr6alis~ que les crat~rea d'impact constituent le processus majeur de formation et de modification des surfaces des autres plan~,tes telluriques ainsi que de lenrs satellites. La reconnaissance des crat~res d'impact en rant que processus g~ologique important sur Terre est relativement r&'ent. Les g6alogues ont cependant maintenant accept~ clue lea impacts g~ants ont eu une influence d~terminante sur l'~'volutiong~logique et biologique de notre plan~e. L'~,tude des crat~es d'impact m~ne/~ des conclusions importantes non seulement sur l'origine et l'histoire de notre syst~me solaire et ses plan~tes rnais c~galement sur un processus g~ologique qui eat fondamental. De plus, les crat~ms d'impact petrvent avoir une importance &:onomique, en contenant parfois des mm,-',ra~i~tions ou des hydrocarbums. Quinze impacts m@~oritiques ont ~ identifi~ jusqu'A pr~ent sur le continent africain: Amguid (Alg~'ie), Aomunga (Tchad), Aouelloul (Mauritanie), B.P. (Libye), Bosumtwi (Ghana), Highbury (Zimbabwe), Kalkkop (Afrique du Sud), Oasis (Libye), Ouarkziz (Alg~ie), Roter Kamm (Namibie), Saltpan (Afrique du Sud), Talemzane (Alg~rie), Tenoumer (Mauritanie), Tm Bider (Alg~rie) et Vredefort (Afrique du Sud). Cet article p~C,sente une rue d'e~semble de ces crat~rea ainai qu'une discussion du processus d'impact, de la reconnaissance des crat~res d'impact et de l'importance g~'ologique et ~-onomique de ces crat~s. INTRODUCTION The planetary exploration program and extensive lunar research led to the recognition of the fact that The recognition of the importance of impact cratering practically all craters visible on the moon are of impact on earth has been slow in coming. The traditional thesis origin. From there, it is a logical step (that still many of geology calls upon uniformitarianism as postulated geologists were not willing to take) to accept that, over by James Hutton (1726-1797) and Charles LyeU (1797- its histor~ the earth has to have been subjected to an 1875), who laid the foundation to the view that slow, even larger number of impacts than the moon because endogenic processes lead to gradual changes in our of its larger gravitational cross-section. From geological record. Impact is an exogenic, relatively rare, observations of bodies crossing the earth's orbit, violent, and unpredictable event and initially was astronomers have by now a fairly good understanding thought to violate every tenet of uniformitarianism. The of the rate with which asteroids and comets strike the impact origin of craters on the earth (and the moon) earth (Shoemaker et al., 1990; Weissman 1990). For has, therefore, been opposed by traditional geologists example, bodies with diameters >1 km, creating craters over much of our century. The history of impact studies >10 km in diameter, collide with the earth at a frequency is, in some ways, similar to the history of accepting plate of about 4.3xl0~/year (Shoemaker et al., 1990). Our tectonics (Mark 1987 and Marvin 1990 give a historical current understanding of other planets and satellites account of impact cratering,~ with solid surfaces (i.e. Mercur~ Venus, Mars and the 263 264 C. KOEBERL satellites of the outer planets) in the solar system shows that impact is either the most important, or one of the most important, surface-forming or -modifying factors. This leads to the question, if there is such a large number of impacts on earth, where are all the impact Ao,~l_~u/ • W Oan, craters? In attempting to answer this question, one must consider several factors. On the one hand, the earth is v.nga rather unique among the terrestrial planets as its surface is actively reshaped by volcanism and tectonics (rifting, X~v.~m'ntw£ subduction, faulting, etc.) and it has an active atmosphere and hydrosphere. These processes lead to a rapid, in geological terms, obliteration of the impact record on earth, at best leaving either deeply eroded structures, or craters that are covered by later sediments. On the other hand, impact craters have not been of main research interest and therefore many structures have not yet been discovered. Nevertheless, an improved understanding of impact craters has led to the recognition of many structures in recent times. While in 1972 only about 50 confirmed impact craters were Kamm listed, the number had increased to more than 130 by 1991 (Grieve 1991), and currently (1994) stands at around 150. The currently known impact craters in Africa are Figure 1. Distribution of currently known (1994) meteorite impact shown in Fig. 1. and their basic characteristics are listed craters in Africa (see Table 1 for details). in Table 1. However, the discovery rate of impact craters in Africa lags behind that of most of the rest of the world. the target area and attenuate in its environs. It may be Dietz (1965) listed 8 structures as probable young interesting to compare the energy released by typical impact craters, all of which are now confirmed. Four of meteorite impacts to that of "normal" terrestrial those were in Africa. Today, Africa has far from a similar processes, such as earthquakes and volcanic eruptions. share of established young impact craters, even though, Events forming small impact craters (5-10 km diameter) considering cratering rate estimates (Grieve 1984,1987; release about 102"25 ergs, while formation of larger Trefil and Raup 1990), there must be numerous craters craters (50-200 km diameter) releases about 10~'3° ergs in Africa still waiting to be discovered. Considering the (French 1968; Kring 1993; B. French pers.comm. 1994). substantial importance of impact craters for geology, This compares with about 6x1023 ergs for the 1980 but also for a possible economic interest and influence eruption of Mount St. Helens (which is comparable to on the evolution of life on our planet, impact craters in the energy released by the largest U.S. nuclear device - general (and in Africa in particular) deserve more Bravo), about 102. ergs for the big 1906 San Francisco extensive study. In this paper I will summarize criteria earthquake, or the total annual energy release from the for the recognition of impact craters, give a description earth, including heat flow (which is by far the largest of the known African impact craters and conclude with component), volcanism, and earthquakes ofabout 10~ ergs a discussion of their geological importance. (French 1968; Kring 1993; B. French pers.comm. 1994). A number of criteria for the recognition and RECOGNITION OF IMPACT CRATERS confirmation of impact structures have been developed over the past decades. These criteria include: The formation of an impact crater is an almost i) crater morphology instantaneous process. Space limitations do not permit ii) geophysical anomalies me to describe the full basics of cratering mechanics. iii) evidence for shock metamorphism Contrary to some opinions, this process is fairly well iv) presence of meteorites or traces thereof. understood from theoretical and experimental These points will be briefly discussed below. It considerations (Gault et al., 1968; Roddy et al., 1977; should be noted that the impact origin of a structure Melosh 1989). Some important concepts have to be usually cannot be confirmed using a single criterion, mentioned though. It is necessary to consider the unless diagnostic shock metamorphic effects are found. enormous energy released upon the impact of a large Even then, a combination of several criteria, including meteorite, which hits the earth with a velocity between morphological observations, should be used. The about 11 and 72 km s-'. Most of the characteristics of an interested reader is urged to consult some of the impact crater are the consequence of the enormous following works for more details on various aspects of impact energy, which is instantaneously released, and, impact cratering: geological importance of impacts - in particular, the resulting shock waves that penetrate Silver and Schultz (1982), Sharpton and Ward (1990); African meteorite impact craters: characteristics and geological importance 265 Table 1.