African Meteorite Impact Craters: Characteristics and Geological Importance

Home , Aouelloul crater, Bouguer (Martian crater), BP Structure, Campbell (lunar crater), Carpenter (crater), Christian Koeberl

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. The Known African Meteorite Impact Craters

Name Country Latitude Longitude Diameter Age (Ma) Ref. (km)

Amguid Algeria 26005 ' N 04023' E 0.45 <0.1 [1] Aorounga Chad 19006' N 19015 , E 12.6 0.01 [6] AoueUoul Mauritania 20015' N 12041 . W 0.36 3.1+0.3 [1] B.P. Stucture Libya 25019 ' N 24020 ' E 2.8 <120 [1] Bostumtwi Ghana 06032 ' N 01025 , W 10.5 1.1+0.2 [1] Highbury 71mbabwe 17005 ' S 30°09' E 15-25 <1800 [7] Kalkkop South Africa 32°43 ' S 24°26 ' E 0.64 <5 [2] Oasis Libya 24035 ' N 24024 ' E 11.5 <120 [1] Ouarkzi7 Algeria 29000 ' N 07033' W 3.5 <70 [1] Pretoria Salt-pan South Africa 25°24 ' S 28005 , E 1.13 0.2 [3] Roter Kamm Namibia 27046 ' S 16018 ' E 2.5 3.7+0.3 [4] Talemzane Algeria 33°19' N 04°02' E 1.75 <3 [1] Tenoumer Mauritania 22°55' N 10°24' W 1.9 2.5+0.5 [1] Tin Bider Algeria 27°36' N 05007' E 6 <70 [1] Vredefort South Africa 27000' S 27030' E 180-300 2002.+_52 [5]

Data from: [1 ] Grieve (1991); [2] Reimoldeta/. (1993a); [3] Koeberl et ai. (1994b); [4] Koeberlet ai. (1993a); [5] Therriault et al. (1993), Walraven et al (1990); [6] Becq-Girdaudonet al. (1992); [7] Master et ai. (1994). shock metamorphism- French and Short (1968), St6ffler crater depth, or about one-tenth of the transient (1972, 1974); cratering mechanics - Gault et al. (1968), diameter) plus the amount of downward displacement Roddy et al. (1977), Melosh (1989); impact melts and of the target rocks (Gault et al., 1968; Roddy et al., 1977; glasses - Dence (1971), St6ffler (1984), Koeberl (1986), Grieve 1987; Melosh 1989). The transient cavity is Bou~ka (1993). Some aspects are briefly described in unstable leading the crater walls to collapse. Impact the next sections. breccia fills the crater leading to the morphology shown schematically in Fig. 2a. In larger crater structures the CRATER MORPHOLOGY cavity floor is unstable and rises rapidly to form a central uplift. Slumping of the rim may lead to terracing Evidence of an appropriate crater form is a key and a generally flatter morphology (i.e. a lower depth/ criterion for the initial identification of an impact diameter ratio) than for simple craters (Fig. 2b). The structure. A fundamental distinction between craters central peak or peak ring structure contains severely of meteoritic and volcanic origin is that impact is a shocked material and is often more resistant to erosion surface process that produces circular, shallow, and than the rest of the crater. In old eroded structures it rootiess structures. Small structures (on eerth < 4 km [< 2 lan in sedimentary targets] in diameter) have a bowl- may, thus, be the only remnant of the crater (see the shaped cross section and are called simple craters (Fig. section on the Oasis structure). Later modification of 2a). Structures >4 km in diameter show a complex the crater may result not only in the erosion of the morphology (Fig. 2b). All craters have an outer rim and structure, but may also be caused by tectonic show some crater infill (i.e. brecciated and/or fractured deformation, yielding truncated or non-circular rocks, impact melt rocks; Fig. 2a, b). In complex craters, structures (e.g. the Sudbury impact structure in Canada; a central structural uplift, consisting either of a central Pye et al., 1984). peak or of one or more central peak ring(s), exposes basement rocks uplifted from considerable depth (up GEOPHYSICAL ANOMALIES to several kin). During theinitial phases of crater formation a deep Geophysical studies are essential in the identification cavity, called the "transient crater", is developed. The and study of impact craters (Pilkington and Grieve depth of the transient cavity is the sum of the excavation 1992). A number of crater structures were only located depth (which is approximately one-third of the transient because of the discovery of geophysical anomalies. This 266 C. KOEBERL

a associated with impact structures can be highly variable. Such magnetic patterns may not be , D r unambiguous. Some larger structures may show high- amplitude anomalies due to remanently magnetized target rocks. Ground penetrating radar is a relatively new technique for the study of (covered) ejecta in or around smaller impact craters (Grant and Schultz 1993). Simple crater For a review of the geophysics of impact craters, see Pilkington and Grieve (1992).

SHOCK METAMORPHISM

b During impact, a supersonic shock wave is generated I D r I and propagates into the target rock. If rocks are compressed at pressures above their Hugoniot elastic limit, irreversible (structural) changes occur in the minerals and rocks. The Hugoniot elastic limit (HEL) is, broadly speaking, the maximum stress above which Complex crater " " " plastic, or irreversible, distortions occur in the solid medium through which the compressive wave travels; see compilations by Roddy et al., 1977; Melosh 1989. The HEL is on the order of about 5-10 GPa for most • Shockedbreccia minerals and rocks. The only known ~atural process A Unshockedbreccia leading to such high shock pressures that exceed the W Impact melt [] Imp~em-ta HELs is impact cratering. In stark contrast, endogenlc F-tattered bedrock metamorphism of crustal rocks rarely exceeds temperatures of 1200°C and pressures of 2 GPa. Shock Figure 2. Schematic cross section of (a) simple and (b) complex pressures and temperatures during impact may reach, meteorite impact craters. Note that complex craters are shallower depending on the magnitude of the event, many 100 and may contain a continuous impact melt sheet. They also show a structural uplift which exposes basement rocks from greater depth. GPa and several 1000°C, which may lead to the (Koebefl and Sharpton 1992.) superheating of matter without vaporization. It is important to realize that shock compression is not a holds true especially for craters that are deeply eroded thermodynamically reversible process and the or are covered by later sediments and that, therefore, Hugoniot equations conserve mass, momentum, and do not have a direct surface expression. Important energy, but not entropy (see review by Melosh 1989). examples of sediment-covered craters include the 65 Some of the structural changes in minerals and rocks Ma Chicxulub impact structure in Yucatan, Mexico due to shock metamorphism are uniquely characteristic (-300 km in diameter, Sharpton et al., 1993), the 36 km of the high pressures and extreme strain rates associated in diameter Manson crater in Iowa, USA (Hartung et with impact. Static compression yields different a/., 1990), several craters in Russia, and the economically products. The formation conditions for shock important Ames structure in Oklahoma, USA metamorphic products are relatively well understood (Carpenter and Carlson 1992; see section on economic (H6rz 1968; French and Short 1968; Gratz et al., 1992; significance). Huffman et al., 1993). The shock pressure regime, up to Geophysical anomalies in craters include gravity about 100 GPa, has been experimentally calibrated with anomalies, which are usnally negative for simple craters laboratory shock experiments with most rock-forming due to the presence of a breccia lens and fractured minerals (see references listed above, and St6ffler 1972, bedrocks with reduced density in comparison to the 1974). target rocks. For complex craters, the signature may be A number of these diagnostic shock features are more complicated and varied, often forming a gravity listed in Table 2. Planar deformation features (PDFs) high over the central uplift surrounded by an annular are usually best developed in quartz, but occur also in gravity low. Seismic studies, especially reflection feldspar minerals or olivine (for examples see Figures seismic surveys, provide important details on the in French and Short 1968; Alexopoulos et al., 1988; or subsurface structure of craters. Indeed, the initial Fig. 17c). They are generally accepted to be diagnostic discovery of the only confirmed underwater impact for shock (French and Short 1968; St6fxqer 1972, 1974; crater, the 45 km-diameter Montagnais crater on the Alexopoulos et al., 1988; Sharpton and Grieve 1990). continental shelf about 200 km south of Halifax, Nova PDFs are only one of many features characteristic for Scotia (Canada), which is covered by 510 m of marine shock. Others include diaplectic glass (an amorphous, sediments, was due to seismic studies done during oil isotropic phase preserving the crystal habit and exploration. In addition, the magnetic anomalies sometimes planar features, but formed without melting; Table 2. Features Diagnostic of Pmgm-~ive Shock Metamorphism

Pressure Range (GPa) Features Target Characteristics Feature Characteristics

2-30 Shatter cones Best developed in homogeneous, fine-grained, Conical fracture surfaces with subordinate massive rocks striations radiating from a focal point

5-4.5 Planar fractures and Highest abundance in crystalline rocks; Found Sets of extremely straight, sharply defined Planar deformation in many rock-forming minerals; e.g., quartz, parallel lamellae; occur often in multple asets features (PDFs) Feldspar, olivine, and zicron. with specific crystallograpic orientations.

30-40 Diaplectic glass Most important in quartz and feldspar (e.g., Isotropization through solid-state tmmfonnation maskelynite from plagioclase). under preservation of crystal habit as well as primary defects and sometimes planar features. Index of refraction lower than in crystal but higher than in fusion glass. Q

15-50 High-pressure polymorphs Quartz polymorphs most common: Coesite, Recognizable by its crystal parameters, Stishovite; but also Ringwoodite from olivine, confirmed usually with XRD or NMR; and others. abundance influenced by post-shock temperature and shock duration; Stishovite is temperature-labile. D. 45->70 Mineral melts Rock-forming minerals (e.g., lechatelierite from Contrary to diaplectic glass, complete quartz) transformation into glass by ~ion of a mineral. o

>60 Rock melt Best developed in massive silicate rocks Occur Impact melts are either glamy (fusion glasses) as individual melt bodies (moonto m size) or as or crystalline; of macroscopically homo- a coherent melt sheets, up to 1000km3. geneous, but microscopoically often heterogeneous composition.

70-140 Impact diamond From carbon present in target rocks; rare. Hexagonal form; usually very small but occasionally up to nun-size.

Data from: Alexopoulos et al. (1988),French and Short (1968), Sharpton and Grieve (1990),St6ffler (1972,1974). 268 c. KOEBERL a stage intermediate between crystalline and normal >1700°C) or baddeleyite (the thermal decomposition glassy phases), high-pressure polymorphs (e.g. coesite, product of zircon forming at a temperature of about stishovite), or mineral or rock melts (impact glass) 1900°C). Some impact melts may have been formed by fusion of minerals or of the complete target superheated, without being vaporized, to temperatures rock. PDFs are <1-3 mm thick, extremely straight of 10 000°C or higher. Depending on the initial (planar) zones spaced at about 2-10 mm and are often conditions of formation, and the cooling history, impact filled with glass. They are usually arranged in the form melts may either be quenched to form impact glasses, of one or multiple sets per host grain and composed of or, if cooled slower, usually form very fine-grained numerous, strictly parallel features that may extend impact melt rocks. Due to their metastable nature, through the whole grain. PDFs are best studied using a impact glasses may devitrify after some time, universal stage (or spindle stage), or by transmission depending also on the post-impact conditions. Impact electron microscopy (TEM; Gratz et al., 1992; Leroux et glasses are found at numerous, usually younger, impact al., 1994). They often occur in multiple sets parallel to craters, including several African craters. Such glasses certain crystallographic orientations, of which have a chemical composition that is indistinguishable especially the {0001} or c (basal), {1013} or ca, and from that of the target rocks (i.e. they may have a {1012} or 7r orientations of quartz-hosted PDFs are sedimentary composition). For example, their distinct impact-diagnostic and their relative abundances are Rb-Sr isotopic composition, which is identical to that used to calibrate shock pressure regimes (Robertson et of the target rocks, and distinct from that of any al., 1968; H6rz 1968). The occurrence of diagnostic shock intrusive or volcanic rocks, can be used to infer an features is by far the most important criterion for impact origin (e.g. at the Tenoumer crater, French et al., evaluating the impact origin of a crater structure, 1970). particularly when several aspects of the range of Impact glasses have very low water contents (about progressive shock metamorphic effects (Table 2) can be 0.001 - 0.05 wt%), often show rather inhomogeneous identified. chemical compositions, sometimes preserve shocked On a macroscopic scale, the occurrence of shatter minerals from the target rocks, contain high- cones is widely believed (but not universally accepted) temperature decomposition phases (such as to be characteristic of impact (Dietz 1968; Milton 1977). baddeleyite), and may show indications for the The formation of these features, which have been admixture of a small meteoritic component. More duplicated in explosion crater experiments, is details on various aspects of impact melts and glasses dependent on the target rock and the shock regime. A are discussed by E1 Goresy et al. (1968), Dence (1971), good lithological indicator for impact may be a layer of St6ffler (1984), Koeberl (1986, 1992) as well as in the fragmental breccia found as crater fill or overlying a sections on the Aouelloul, Bosumtwi, and Saltpan possibly raised, partially brecciated, and up- or craters. overturned rim. This ejecta breccia may display the inverted stratigraphic sequence of the target area, as the REMNANTS OF THE METEORITIC PROJECTILE youngest target rock on top of the target sequence will be ejected and deposited first, followed by older target Although the presence of meteorites at a crater is, of rocks. A suite of various breccia types can be generated course, definitive proof for origin by meteorite impact, by impact (St6ffier and Grieve 1994): monomict or impact physics and erosion prevent the wider use of polymict breccias consisting of : this criterion. First of all, the meteoritic impactor i) cataclastic (fragmental) (projectile, or bolide) is also subjected to a very high ii) suevitic (fragmental with a minor component of pressure shock wave, which leads to almost instant melt fragments) vaporization of most or all of the projectile. Only during iii) impact melt (melt breccia with a minor dastic the impact of smaller objects, with lower impact velocity component) breccias. due to atmospheric retardation, does a small percentage Whether all these breccia types can be identified at a of projectile material survive. The cut-off is somewhere particular impact site depends on factors such as the around a I - 1.5 km crater diameter, but even at craters size of the crater, the composition of the target area around I km diameter (e.g. Meteor Crater, Arizona) only (Kieffer and Simonds 1980) and the level of erosion a few percent of the original projectile mass are not (Roddy et al., 1977; H6rz 1982; Grieve 1987). Another good indicator for shock metamorphic vaporized due to the partial breaking-up of the processes is the occurrence of impact glass. At pressures projectile prior to impact. The other complication is the in excess of about 60 GPa, rocks melt to form impact low resistance of meteorites towards erosion. Under melts. The high temperature of the melts is produced normal terrestrial conditions, stony meteorites survive by shock waves, which generate temperatures far erosion for only a few thousand years, while iron beyond those common in the earth's crust (or in volcanic meteorites may resist maybe ten times longer. Thus, eruptions), as evidenced by the presence of high- only a minor fraction of all meteorite craters, namely temperature minerals such as lechaterlierite (a melt the very young and small ones, can be expected to glass which forms from pure quartz at temperatures contain meteoritic fragments. African meteorite impact craters: characteristics and geological importance 269

METEORITIC COMPONENT IN IMPACT where 187Os/I~Os and ~Re/l~Os are the present day CRATERS: RE-OS ISOTOPES ratios, (187Os/1~:)s)1 is the initial ratio, ~ is the decay constant and t is the age of the rock. The mass balance of impact melting and vaporization shows that, in addition to the meteorite being essentially l I /'1 completely vaporized upon impact, a large volume of / target rocks (several orders of magnitude larger than 100 °'@" / that of the projectile) is melted and even partly // / vaporized. Thus, all impact-derived rocks, such as • / / breccias, impact melts, and impact glasses, have a 10 / / composition that is predominantly derived from the 0 • / f:h / target rocks. A possible minor meteoritic component is I:l~O // extremely difficult to differentiate from the terrestrial 1.0 / ,,,b composition. Only a few characteristic elements, which < A/ 00 are much more abundant in meteorites than in / • • / AA /•/ A Ocean Floor terrestrial crustal rocks, may be used as tracers for a 0.1 / -- Basalts meteoritic component. The platinum group elements A Canadian Shiek (PGEs, e.g. Ir, Os, Pt, Pd) are several orders of magnitude • Brent more abundant in most meteorites (i.e. chondrites and Rochechouart 0.01 • E. Clearwater iron meteorites) than in crustal rocks. Chondritic 0 Siiiiksjgrvi / meteorites contain, for example, on the order of 400- / I"IKalkkop 800 ppb Ir or Os, while the average crustal Ir and Os I / I I I I abundances are on the order of 0.02 ppb (Taylor and 0.01 0.1 1.0 10 100 McLennan 1985). The addition of only 0.1 wt% of a Ir (ppb) chondritic component to a terrestrial crustal rocks would result in an addition of ~0.4 ppb Ir to the Figure 3(a). Diagram of Au versus Ir contents of terrestrial and extraterrestrial materials, after Palme (1982). The Ir and Au contents background value. An enrichment of PGEs (usually Ir) of some impact melt rocks and breccias from several impact craters in impact melts or breccias may, thus, provide good are shown, plotting near the meteoritic side of the diagram. The Ir/ evidence for a meteoritic component (Morgan et al., Au ratio chosen for the meteoritic line is for ordinary chondrites (type 1975; Paime et al., 1978; Palme 1982). Figure 3a shows H) and represents an extreme value; most chondritic meteorites have how the concentrations of Au and Ir in impact melts or ratios closer to the terrestrial line. However, because of the mobility of gold, the classification of rocks as cosmic vs. terrestrial is breccias can be used to distinguish between a cosmic ambiguous; this is shown by the value for a breccia from the Kalkkop and a terrestrial signature. However, Au is a rather crater, which plots in between the two fields, while, in fact, it does mobile (and volatile) element, which can easily be contain a cosmic component (see Fig. 3b). enriched in various rocks. In addition, some particular target rocks (e.g. mafic or ultramafic rocks, or target areas containing mineralized zones) may also yield I.~ . . , . , . , . , , ' ' ' , ' ' " ' elevated PGE abundances (see section on the Bosumtwi crater). Thus, while elevated PGE abundances may be 0.80 / ~ ..... good evidence for a meteoritic contribution, they are not unambiguous and need to be evaluated in 0.60 ,, comparison to the indigenous content of PGEs in the © // • ...-"~"'- Breccia-2 target rocks. ~.. 0.40 , -~" . A new tool, which allows a more definite _~ ,, ..-'~Br~a-4 identification of a cosmic component, is the study of Re and Os isotopes (Koeberl and Shirey, 1993). The Re- 0.20 ~"~--Breccla-3 Os isotopic system is based on the [5-decay of ~tRe to ~"~ Chondrit~s and ironmeteorites ~STOs (half-life = 42.3-3:1.3 Ga). Re and Os have a different 0.00 .... ' .... ' , ' ' ' ' ' ' ' ' 0.0 5.0 10.0 15.0 20.0 geochemical behavior during the formation of crustal rocks which are formed by the partial melting of mantle 187Re/188Os rocks. Os is highly compatible and remains in the Figure 3(b). l~Os/lasOs vs. leTRe/leeOs diagram for rocks from the residue, but Re is moderately incompatible and is, Kalkkop crater, South Africa. The data array for meteorites is shown therefore, enriched in the melt. Thus, crustal rocks have in the lower left part of the plot. The field defined by target rock high Re and very low Os concentrations. Due to the samples (sandstone and shale) is in the upper right, showing values that are typical for the upper continental crust. The four samples of high Re concentrations, the abundance of 1~Os in crustal suevitic breccia (from different depths in the drfllcore) plot in a mixing rocks increases significantly with time. The growth of field between a meteoritic component and target rocks (from which 187Os from the decay of 187Re can be described by the breccias are derived). Breccia-3 (112.7 m) contains about 10-times normalizing to a non-radiogenic Os isotope: higher Os abundance than the target rocks and has a near-meteorific 187Os/1~Os ratio, clearly indicating the presence of a meteoritic component. Such mixing relations can be used to help constrain the lS7Os/1~Os = (lSTOspSSOs)i + [18rRe/l~)s](e~ - 1) origin of a crater structure (after Koeberl et al., 1994a.). 270 C. KOEBERL

Rocks from the terrestrial mantle have a low present- component is evident and clearly shown in Fig. 3b. day '87Os/'~Ds ratio of about 0.13, and meteorites also This method enables not only the discrimination have low '~ZOs/'~)s ratios of about 0.11 to 0.18 ('"7Os/ between a crustal and meteoritic source of any Os (PGE) '~Ds=0.95-1.5) (Walker and Morgan 1989; Horan et al., enrichments, but also permits quantification of such a 1992). As Os is much more abundant than Re in component (Koeberl and Shirey 1993; Koeberl et al., meteorites (and in the mantle), the '~ZOs/'~Ds ratio in 1994a,c). The presence of a small meteoritic component such rocks changes only very slowly with time. The in breccias or melt rocks is very good evidence for the crustal 'aOs/'~)s ratio, on the other hand, increases impact origin of such rocks (and the structure where rapidly with time, because Re abundances are several they are found). The study of Re-Os isotopes may be orders of magnitude higher than Os abundances. used as a new and important diagnostic tool for Isotopic ratios of crustal rocks depend on age and assessing the impact origin of crater structures (Reimold elemental abundances. '~ZOs/'~)s ratios of about 0.67 et al., 1993; Koeberl et al., 1994a), which could, in some to 1.61 (with an average of about 1.2) are thought to be aspects and for certain crater structures, rival the representative of currently eroding upper continental diagnostic capacity of shock metamorphic effects. crust (Esser and Turekian 1993). The absolute abundances of Os as well as the 'r/Re/ CONFIRMED METEORITE CRATERS IN AFRICA '~Os and '~ZOs/'=Os ratios in meteorites are thus distinctly different from those in old crustal target rocks. Amguid, Algeria Impact melt rocks, glasses, or breccias consist This crater was first recognized from an aircraft in predominantly of terrestrial target rocks, mixed with a 1954 and is described in detail by Lambert et al. (1980). usually very small (<1%) meteoritic component. The 450 m diameter crater is exposed in Lower However, the difference in absolute Os abundances Devonian sandstones. It has an elevated rim up to 50 between meteorites and crustal target rocks favors m high and is filled with very bright and fine-grained detection of small amounts of a meteoritic component. compacted eolian silts (Fig. 4a). In cross-section the This concept has been used for the study of Cretaceous- crater shows sandstone beds with a dip that becomes Tertiary (K-T) boundary clays (Luck and Turekian 1983). progressively steeper in the upper part of the wall, with The first application of the Os isotopic system to impact overturning of the upper sandstone layers at the NNW craters was attempted by Fehn et al. (1986), but low and SSE parts of the rim (Fig. 4b). The crater is precision and the complicated laboratory method surrounded by a nearly continuous ejecta blanket up prevented its wider use. to about 100 m from the crater rim. The near-perfect The recent development of the negative thermal preservation state of the crater led Lambert et al. (1980) ionization mass spectrometry technique (Creaser et al., to estimate an age of up to 0.1 Ma. Petrographic 1991) made a broader application for impact crater examination of samples from the crater wall showed studies possible. This method allows the determination the existence of up to three sets of PDFs in quartz grains, of abundances and isotopic ratios of Os and Re at the thus confirming the impact origin of the crater (Lambert low abundance levels found in target rocks and in et al., 1980). impact-derived rocks, while using relatively small amounts of material. An example for the use of this Ouarkziz, Algeria method is given in Fig. 3b, which also shows its This crater (earlier called '~I'mdouf") is a severely advantage over using only elemental abundances. eroded ring structure with a diameter of 3.5 km and is Abundances and isotopic compositions of Re and Os superimposed on a fault structure, which is part of the were measured in target rock and impact breccia local geology (Fig. 5). The structure has rarely been samples from the Kalkkop crater, South Africa (Koeberl visited, and only one geological study is available (Fabre et al., 1994a). On an Ir vs. Au diagram (Fig. 3a), most et al., 1970). The crater is set in Carboniferous limestones breccias would fall close to the terrestrial line, while and shales of the Upper Vis~en and Lower Namurian only one (from 89.15 m depth, which is shown in the formations which are upturned at the crater rim. The Figure) shows an indication of a cosmic component. rock formations also show fractu~g and folding at the However, using the Re-Os isotopic system, the rim, and the central area is largely covered by alluvium. abundances and isotopic ratios of Re and Os in the Fabre et al. (1970) report on the presence of breccias and target rocks are typical for old continental crust, while "planar features" in quartz from the inner part of the the breccia samples have Os contents up to 10 times rim. The crater consists of three concentric zones: an higher than the target rocks, as well as low '~ZOs/'~Os outer zone with outward dipping concentric faults, an ratios. Such values are incompatible with old inward dipping zone, and in the centre, brecciated continental crustal values. The higher Os content vertical dipping beds of a concentric uplift. Its age is together with the lower 'SZOs/'~3s ratios demonstrate not well constrained. It occurred after the formation of that the breccia contains up to 0.05% of an the Mesozoic peneplain and is pre-Tertiary (Fabre et al., extraterrestrial component. The mixing relationship 1970). Clearly, this crater deserves a more thorough between the target rocks at Kalkkop and a meteoritic study. African meteorite impact craters: characteristics and geological importance 271

/ ," Slumped rim Sandstone t L~ co~'~, \ alluvial and % eolian deposi Eon. dep~ 'iI t I / t l I I I I I ! I I IA I I

\ ?.:.:..... \ ~"% I : \ l .~::: % I ~. I

"% I ! I "'"- l/ 100 m "-----... /--- I I

b Overturned strata

Unconsolidated X~ _~ "~Upturned Centre!

~...... ~-~j sandstone ~ True crater floor t s o moto i

I Figure 4. Amguid crater, Algeria. (a) Schematic map of the crater, showing the infill with fine-grained eolian deposits; the dashed line shows the extent of the continuous ejecta blanket;, the structure on the right (and in the lower left comer) is a riverbed. (b) Schematic cross section through half of the crater, showing the original transient cavity ("true" crater floor) which is now filled with impact debris and later sediments (compare Fig. 2a); overturned strata are characteristic of impact craters. (Lambert et al., 1980).

Talemzane, Algeria a meteoritic origin of the crater was proposed (Karpoff This simple bowl-shaped crater has a diameter of 1953). A more detailed investigation of the structure by 1750 m and is locally also known under the geographic Lambert et al. (1980) yielded definitive evidence for an name "Daiet E1 Ma/idna". Following a first visit on the impact origin. The crater is emplaced in Senonian or ground in 1951 and an investigation from the air in 1952, Eocene limestones and has an elevated rim up to 70 m 272 C. KOEBERL

Figure 5. Ouarkziz crater,Algeria. The crater (as indicatedby the arrows) is superimposed on a fold structure trending NW-SE. In this and all followingSpace Shuttlephotographs, north is up. Space Shuttle photograph 41C-31-1032. above the crater floor. In cross-section (Fig. 6), the crater clay and limestone formations. Lower Cretaceous shows strongly fractured upturned and locally sandstones are exposed in the central part of the overturned limestone beds at the rim, with large blocks structure, about 500 m above their normal stratigraphic of ejected limestone scattered along the outer rim position, which indicates the presence of a structural (Lambert et al., 1980). Breccia dikes occur at the crater central uplift. The stratigraphy of the concentric wall, and detrital or reworked monomict breccia is limestone ridges is very complex and the strata are found at the crater floor near the rim. Quartz is rare In intensely folded. No definitive macroscopic evidence the breccias, because limestone is the main target rock, (e.g. shatter cones) for an impact origin was found. but some quartz grains showing characteristic PDFs However, a petrographic study of the central sandstone have been found (Lambert et al., 1980). The age estimate showed its brecciated nature, and quartz grains with of <3 Ma is based on the level of erosion (Lambert et al., up to 8 sets of PDFs have been found (Lambert et al., 1980). 1981), providing positive evidence for shock metamorphism. The unusual structure of the crater, the T'm Bider, Algeria lack of brecciation and macroscopic evidence for shock The structure (also known as "radema/t"), is about metamorphism in a major part of the crater, as well as 6 km in diameter, appears as a series of at least three the presence of considerable ductile deformation, may concentric annular ridges at 2, 3.5, and 6 km, be due to its formation in soft target rocks that are partly respectively, and is set in Lower to Upper Cretaceous dominated by day. The age is not well known and is

/ Upturned autochthonous limestone Large blocks of ejected~~limestone---, / Breccia veins Mixed breccia /-~ Mixed breccia I Detrital breccia li ,, I~~ Recent alluvial crater filling I

Autochthonous F- Hi~lyfractured /,/ ~, I I limestone ~ Br~cc]ated / /- "". . I / 100 ] Scale (meters) I True crater floor I / 100 200 300 400 ' 0 ' ' ' ' Centre

Figm'e6. Schematiccross section of Talerazane crater, Algeria. The 1.75-kin-diametercrater has a raised rim with upturned target limestone (after Lambertet al., 1980). African meteorite impact craters: characteristics and geological importance 273 given only as younger than the target rocks. The lack B.P. crater). Multiple sets of planar elements were of allochthonous crater fill materials may indicate detected in quartz grains from orthoquartzite (French erosion to the crater floor. A more detailed field study et al., 1974). A few samples of a glass-bearing seems warranted. microbreccia were found at the crater, containing fragments of brownish, partly devitrified glass with B.P. Structure, Libya sandstone fragments and shocked quartz grains (French This structure was first described by Martin (1969) et al., 1974). As with the B.P. structure, its age is only and is named after the B.P. Exploration company. It constrained to post-date the target rocks, which are also consists of two eroded and discontinuous rings of hills. Nubian sandstones. The inner ring is about 2 km in clian~eter with an average relief of 30 m, while the outer ring has a diameter of Libyan Desert Glass about 2.8 km and a maximum relief of about 20 m. The Libyan Desert Glass (LDG) is an enigmatic natural rocks at the centre of the structure show intense jointing. glass found in an area of about 6500 km z between sand The rocks exposed are from the Early Cretaceous dunes of the south-western corner of the Great Sand Nubian Sandstone Formation and include sandstone, Sea in western Egypt, near the Libyan border. The first siltstone, and quartz conglomerate. Medium- to coarse- to report about LDG were Fresnel (1850) and Clayton grained orthoquartzite yielded highly shocked quartz and Spencer (1934). The glass occurs as centimetre to grains with multiple sets of PDFs (French et al., 1974). decimetre-sizecl irregular and strongly eroded blocks. The geology of the structure has been studied in some Its fission track age has been determined at around 29 detail (Underwood 1975, 1976; Underwood and Fisk Ma (Storzer and Wagner 1977). LDG is very silica-rich 1980), but no geochemical study of the crater rocks is at about 96.5-99 wt.% SiO 2, and shows a limited available so far. Such a study is particularly desirable variation in major and trace element abundances because of a possible association of the B.P. or the Oasis (Barnes and Underwood 1976; Fudali 1981; Weeks et structure with the occurrence of Libyan Desert Glass al., 1984; Koeberl 1985). Although the origin of LDG is (see below). Again, the age is poorly constrained, being still debated by some workers, an origin by impact given only as postdating the target rock age. seems most likely. There are, however, some differences to "ela~ical" impact glasses. Evidence for an impact Oasis, Libya origin includes the presence of schlieren, partly digested This eroded crater, named after the Oasis Oil mineral phases, lechatelierite (a high-temperature company, has a dlanaeter of about 11.5 km but the most mineral melt of quartz), baddeleyite (a high prominent part is a central ring of bills, about 5.1 km in temperature breakdown product of zircon, Kleinmann diameter and 100 m high, sun'ounded by an annular 1969; Storzer and Koeberl 1991), and the possible depression (Fig. 7). The structure is exposed in the same existence of a meteoritic component (Murali et al., 1989; rocks as the B.P. structure (about 85 km south of the C. Koeberl unpublished data). Although there is a

Figure 7. Oasis crater, Libya.The ring structure (composedof discontinuous hills) on the left side of the image has a diameter of about 5.1 km and constitutes the central uplift, while the crater is about 11.5km in diameter and is visible as a slightly darker area surrounding the central ring (as indicated by the arrows). SpaceShuttle photograph 51B-S2-2577. 274 c. Ko~n~L similarity of LDG trace element composition with sequence. No unequivocal mineralogical evidence of Nubian sandstone in general (Murali et al., 1988), no shock metamorphism has yet been described for the specific comparison has yet been made with material crater rocks. However, planar fluid inclusion trails from either the B.P. or Oasis structures. (presumably remnants of PDFs) in the form of distinct sets in quartz grains were recently found in Zli Aouelloul, Mauritania sandstone from the crater rim (Koeberl and Reimold in The AoueUoul impact crater has a diameter of about prep.). The crater is filled with a poorly sorted sandy 390 m and is situated in the Adrar region of the western silt which is overlain by well-sorted windblown sand. Sahara Desert in Mauritania (Fig. 8a,b). It was A gravity study indicated a maximum sedimentary fill discovered from the air in 1938 and first visited on the thickness of about 23 m, underlain by a breccia lens ground in 1950 (Monod and Pourquie 1951). The crater extending to a maximum depth of 130 m (Fudali and is exposed mainly in Ordovician Zli sandstone, with Cassidy 1972). Slightly different dimensions based on some Oujeft sandstone present. Some parts of the rim, gravity data (i.e. about 100 m for the depth of the breccia which rises 15 to 25 m above the surroundings (53 m lens) are given in Grieve et al. (1989). above the crater floor), show an overturned rock Numerous fragments of a dark glass, which, in view

F'~gure8. Aouelloulcrater, Mauritania. (a) Verticalaerial photograph (westis up); (b) obliqueview. Photographs courtesy Prof. Th. Monod, Paris. Africanmeteorite impact craters: characteristicsand geologicalimportance 275 of absence of any indication of volcanic rocks or activity, some enrichments in the glass (Chao et al., 1966b; was taken as key evidence for an origin by impact, have Koebefl and Auer 1991), which was used to suggest that been found mainly outside the crater rim, and, to a the crater was formed by an iron meteorite projectile lesser degree, on the inner slope (Campbell-Smith and (Morgan et al., 1975). The high-temperature impact Hey 1952; Chao et al., 1966b). Some of the glasses were origin of the glass is shown by the presence of found to contain microscopic Ni-rich (9 wt%) iron lechatelierite and baddeleyite, which are diagnostic of spherules which may be related to the meteoritic an impact origin (El Goresy 1965; E1Goresy et al., 1968). projectile (Chao et al., 1966a). The glass is somewhat The age of the crater was determined by K/Ar and inhomogeneous, showing abundant schlieren of fission track dating of impact glass to be 3.1+0.3 Ma different chemical composition (Fig. 9a) and partly (Storzer and Wagner 1977; Fudali and Cressy 1976). digested quartz and feldspar grains (Fig. 9b). The composition of the glass is similar to that of the Zli Tenoume~; Mauritania sandstone, but a few, mainly siderophile, elements show This almost perfectly circular crater has a diameter

Figure 9. Impactglass (sampleAOL-21) fromAouelloul crater, Mauritania. (a) Microphotographof schlieren and partly meltedmineral grains (predominantlyquartz); dark areasare of brownish colorand enrichedin Fe (plane polarized light; width of image: 2.2 ram); (b) Back-scatteredelectron (BSE) microphotograph showing the inhomogeneityof the glass and partly digested quartz crystals (darker gray);scale bar = 1 mm. 276 C. KOEBERL

of about 1.9 km and is located in the Western Sahara in rock is not identical to that of the gneissic and granitic Mauritania, almost 400 km to the NW of the Aouelloul basement rocks and seems to incorporate a component crater. The crater is excavated in a peneplained surface derived from rare amphibolite veins found in the of Precambrian gneisses and granites, which are gneissic terrain (Fudali 1974). Some Rb-Sr isotope data covered by a thin veneer of young (probably Pliocene) show that the glasses are indeed melted basement rocks sediments. The crater is filled with unconsolidated (French et al., 1970). However, no detailed trace element sediments and has a present depth of about 100 m. The or isotope studies have been made to date. The age of depth to the base of the post-impact sediments is the crater was determined to be 2.55:0.5 Ma from K/Ar estimated to be 200 - 300 m based on geophysical measurements of the melt rock (French et al., 1970). measurements (Fudali and Cassidy 1972; Grieve et al., 1989). A swarm of small "dikes" supposedly containing Aorounga, Chad "rhyodacitic lava", which are intrusive into concentric The circular depression of Aorounga, which appears fractures of the crater and outcrop just outside the crater on some geological maps, has a diameter of 12.6 km rim, were originally taken as evidence for a possible and is situated in Northern Chad, about 110 km volcanic origin of the structure (Monod and Pomerol southeast of the Emi Koussi volcano in the Tibesti 1966). Massif. It has first been studied in a photogeological However, French et al. (1970) found up to eight investigation of Gemini, Apollo, Landsat, and aerial different sets of PDFs in quartz grains from obviously photographs by Roland (1976), who suggested an origin strongly shocked inclusions of granitic basement rocks either as a granite diapir or an impact crater, but in the "lava", showing that the crater is of impact origin. concluded that a diapir is the more likely explanation. The "lava" is in fact a rapidly quenched impact melt The structure was mentioned by Grieve et al. (1988) as rock, consisting of a fine-grained intergrowth of a possible impact crater. A recent French expedition to plagioclase laths and pyroxene crystals up to 50 mm in the area succeeded in collecting a few samples from length in a matrix of brownish glass (French et al., 1970). the structure, which show evidence of multiple sets of The melt rock also contains lechatelierite (diaplectic PDFs (Becq-Giraudon et al., 1992). The host rock of the quartz glass) inclusions. The composition of the melt crater is a fine-grained, well-sorted, slightly carbonate-

Figure 10. Aoroun~a, Chad. This structure has a diameter of 12.6 km and is situated about 110 km southeast of the Emi Koussi volcano in northern Chad. It comprises a central uplift,surrounded by two concentric ring s[n'uctureswith a topographic reliefof about 100 m. Northeast is up; Space Shuttle photograph STS43-75-~. African meteorite impact craters: characteristics and geological importance 277 bearing sandstone of Upper Devonian age. 1966; Kolbe et al., 1967). The crater is almost completely The structure has an outer and an inner ring wall, filled by Lake Bosumtwi, which has a maximum depth both rising about 100 m above the mean level of the of about 80 m, with the crater rim rising about 250 - 300 surrounding plaiu, and which are separated from each m above the lake level (Fig. 11c). The structure has been other by a depression of uniform width (see Fig. 10). A known since late last century, but its origin was the somewhat eccentric central uplift is located in the subject of a controversy (Jones 1985b). While Mar'laren central depression. The ring wall.q consist of steeply (1931) thought the crater to be of impact origin, outward dipping strata, with dips of 40 ° - 50 ° in the Rohleder (1936) preferred an endogenic explanation. outer, and 80° in the inner wall (Becq-Giraudon et al., However, outcrops of suevitic breccia were found 1992). Some breccia, containing some fine-grained clasts around the crater (Jones et al., 1981), and the high- with a fluidal texture, was found on top of the inner pressure quartz modification coesite (Littler eta/., 1961) rim wall (Becq-Giraudon et al., 1992). Becq-Giraudon et as weU as Ni-rich iron spherules and baddeleyite, the al. (1992) estimated the age as young as 12 000 to 3 500 high-temperature decomposition product of zircon, years old, which seems too young, considering the discovered in vesicular glass from the crater rim (El obvious erosional state of the structure (Fig. 10). Goresy 1966; El Goresy et al., 1968), are supporting an Unfortunately, the availability of samples is extremely impact origin for the structure. The composition of melt limited, and a civil war currently (1994) raging in in the suevite is shnilar to that of the basement rocks northern Chad prevents more detailed field studies (J.- (Jones 1985a). Siderophile element contents of some F. Becq-Giraudon pers. comm. 1994). melt rocks do not show any obvious extraterrestrial component (Palme et al., 1981). Some general Bosumtwi, Ghana geophysical studies of the area are available (Jones et The almost circular Bosumtwi crater in Ghana has a al., 1981), but no detailed geophysical measurements rim to rim diameter of 10.5 km (Fig. 11a) and is exposed of the crater itself have been made. An assessment of in 2 Ga old lower greenschist facies metasefliments of the structure of the crater (Fig. 11c) is, thus, largely the Lower Bh'imian Group (Fig. 11b; Schnetzler et al., hypothetical (Jones et al., 1981).

Figure 11. Bosumtwi crater, Ghana. (a) The 10.5-kin-diameter crater is filled with a lake and is clearly vis~le on this Landsat TM photograph. 278 C. KOEB~:Rt.

i/ ! b t GHANA t~ / I / ) Kumasim • ~, lO30,W l°20'W ~ B~t~ -

6°30'N

Watershed 1.o N __~.~ ~o~ruo~, _~ ~m ~ s O.

1.OE Sediments 0 L 2 1.5 Breccia I km km

Figure 11 Bosumtwi crater, Ghana Co) General geological map of the crater area (after Jones et al., 1981); (c) Inferred schematic cross section of the crater, based on the interpretation of geological and geophysical data (after Jones et al., 1981). African meteorite impact craters: characteristics and geological importance 279

Ivory Coast tektites country rocks around Bosumtwi (Chamberlain et al., Tektites are natural glasses occurring on earth in four 1993). strewn fields: Australasian, Ivory Coast, Central Glass from the Bosumtwi crater as well as Ivory Coast European, and North American. Tektites occur in three tektites have been dated with a variety of methods different forms (Muong Nong-type, splash-form, and (Gentner et al., 1964; Lippolt and Wasserburg, 1966; ablation shaped) on land (e.g. O'Keefe 1963), and as Storzer and Wagner 1977). The preferred ages are nu'crotektites predominantly in deep sea cores (Cassidy 1.03i0.11 Ma for Bosumtwi glass (fission track age), and et al., 1969; Glass et al., 1979). Geochemical arguments 1.05+0.11 Ma and 1.10~0.10 Ma (fission track and 4°Ar- have shown that tektites have been derived by 39Ar ages, respectively) for the Ivory Coast tektites hypervelocity impact melting from terrestrial upper (Koeberl et al., 1989b). A recent Re-Os isotopic study crustal rocks (Taylor 1973; Koeber11986,1992). Tektites has provided evidence for the presence of up to 0.6% from the Ivory Coast (Cote d'Ivoire) have been known of an extraterrestrial component in Ivory Coast tektites since 1934 (Lacroix 1934), while microtektites, which and demonstrated that the Re-Os isotopic are related to the tektites found on land by having very characteristics of the tektites and Bosumtwi impact similar chemical composition and age, have been found glasses can be derived by mixing of metasedimentary in deep sea sediments off the coast of western Africa target rocks from Bosumtwi and a meteoritic (Glass 1968, 1969). The geographical distribution of component (Koeberl and Shirey 1993). This study also microtektite-bearing deep sea sediments can be used revealed that some target rocks contain higher to define the extent of the strewn field (Glass and Zwart abundances of siderophile elements (e.g. Os), which 1979; Glass et al., 1979, 1991). would make it difficult to uniquely identify a meteoritic The Bosumtwi crater was suggested to be the source component in the Bosumtwi impact glasses and Ivory crater of the Ivory Coast tektites, because the tektites Coast tektites, as already suspected by Jones (1985a). and the crater lithologies have the same Rb-Sr and Sta- However, the study of Re and Os isotopes allows to Nd model ages (Schnetzler et al., 1966; Shaw and distinguish between Os having a terrestrial crustal Wasserburg 1982), a similar chemical composition signature, and Os having an extraterrestrial (meteoritic) (Schnetzler et al., 1967; Jones 1985a), as well as similar signature. While there is little doubt about the impact isotopic characteristics (e.g. Schnetzler et al., 1966; Shaw origin of the crater and the connection to the Ivory Coast and Wasserburg 1982). ALso, tektites and Bosumtwi tektites, the geophysical characteristics of the crater and impact glasses have the same age (Koeberl et aL, 1989b). the detailed petrography of rocks from the crater remain The Ivory Coast tektites have large negative end ValUes poorly known and should be investigated in the future. of about -20, which are typical for old continental crust, yielding mantle extraction Sm-Nd model ages of about Highbury, Zimbabwe 1.9 Ga. This is in agreement with the whole rock Rb-Sr The Highbury structure is a near-circular feature of ages of the rocks around the Bosumtwi crater which about 15 km in diameter, which has first been noted in range from 1.9 to 2.1 Ga (Schnetzler et al., 1966; Kolbe et 1985 on Landsat imagery by German geologists; al., 1967). Figure 12 shows that the oxygen isotopic however, no follow-up study or publications resulted. characteristics of the tektites are almost In 1993, an expedition (including the author) was indistinguishable from those of the sedimentary mounted to investigate several crater-like structures in Zimbabwe, including Highbury. This study resulted in 80 the recognition of the Highbury structure as an impact I .... I .... I .... I .... I .... I .... I .... crater (Master et al., 1994). The country rocks in the area 75 are arkoses and metadolomites of the Deweras Group, which are flanked to the east and west by the Striped 70 Slates and Mountain Sandstone members of the Nyagari Formation of the Lomagundi Group (Master 65 et al., 1994). The ring structure can be seen on Landsat t~ imagery (Fig. 13), showing some contrast with the r/l•c2 60 highly vegetated Striped Slates of the Lomagundi Group to the east and west of the structure. The feature 55 [] has a diameter of about 15 km. However, the abrupt

~'i i i I I i i i I I i i i i i i i i I i i I i I i i i i I i i i termination of the Mountain Sandstones to the 50 northwest and west, and some hints from the space 8 9 10 11 12 13 14 15 images, suggest that the structure might be as large as 8 80 25 km in diameter. Some brec~as were found in the central area, which Figure 12. Correlation between silica content and the oxygen isotopic may represent the remnant of a central uplift. Quartz composition (81eO) for Bosumtwi crater target rocks (open squares- granodiorites, asterisks-metasediments, filled cirde-microgranite), crystals from samples from the central uplift area show Ivory Coast tektites (triangles), and Bosumtwi impact glass (stars) planar fluid inclusion trails similar to those found at (after Chamberlain et al., 1993). the Vredefort structure in South Africa. However, some 280 C. KOEBERL

Figure 13. The Highburyimpact structure in Zimbabwe. The diameterof the structure, as indicatedon the photograph (arrows), is about 15 kin. However,there is someevidence that the crater maybe 25 km in diameter.This structure is deeplyeroded. The linear featurein the lower right comer of the image is the Great Dike, Zimbabwe. Space Shuttle infrared photograph STS36-92-28. quartz grains also show bona fide PDFs, and pockets of of melting; clasts therein are generally devoid of shock almost unaltered glassy material were found as well metamorphic features; these melt rocks are usually (Master et al., 1994). These samples will be of great value assumed to be locally derived by friction melting during to determine the age of the structure, which is, so far, impact or tectonic processes (Reimold 1991; not well constrained. The crater postdates the north- Magloughlin and Spray 1992).] The crater was formed south trending thrust faults of Magondi (1.8 Ga) age, in a two-layer target: an upper layer of Gariep but seems to be offset in the southeastern sector by metasediments (schist, marble, + quartzite and dextral wrench faults, which are thought to be of late sandstone), overlying the crystalline basement of the Irumide (ca 1 Ga) age (Master et al., 1994). Namaqualand Metamorphic Complex. The basement was also heavily intruded by coarse-grained quartz Roter Kamm, Namibia veins and quartz- and quartz-feldspar pegmatites. The 2.5 km diameter, near-circular Roter Kamm Melt breccias found at the crater were derived mainly crater in southern Namibia, which has an obvious 150 from metasedimentary target rocks, with rarely m high rim, is located in the Namib Desert, about 80 detected contributions from pegmatite and granite/ km due north of Oranjemund (Fig. 14). An impact origin granodiorite. Three varieties of melt breccias can be for this crater structure was first proposed by Dietz defined as: (1965) and FUdali (1973) and was confirmed in the late i) "schistose" 1980s by a detailed geological study of the crater and ii) quartzitic melt breccias the discovery of clasts in impact melt breccias with iii) "true" impact melt breccias. characteristic shock deformation features indicative of All these melt breccia types are petrologically and shock pressures up to 30 GPa (Reimold et al., 1988; chemically heterogeneous (Reimold et al., 1994; Reimold and Miller 1989, Koeberl et al., 1990a). Rock Degenhardt et al., 1995). Most of the breccias, including deformation in basement rocks exposed along the crater the "true" impact melt breccias, may have been rim include severe jointing and cataclastic, mylonitic produced in situ and not from a coherent impact melt and pseudotachylitic breccias. [Pseudotachylites are body (Reimold et al., 1994). A few of the impact melt very fine-grained or glassy rocks containing evidence breccias contain basement rock clasts displaying shock African meteorite impact craters: characteristics and geological importance 281

geophysical data (Fudali 1973), which give some depth estimates, a future drillirtg project at the crater, as well as detailed geophysical investigations, including ground probing radar and possibly a reflection seismic survey, will be of extreme importance.

Vredefort, South Africa The Vredefort structure (or Vredefort Dome) is situated about 120 km SW of Johannesburg (Fig. 15a,b). Its origin has, for a long time, been the subject of a controversy. Daly (1947) first proposed that it was formed by large-scale meteorite impact. This proposal was later renewed by Dietz (1961) and Hargraves (1961). The structure consists of a central core of mainly granitic gneiss terrane with a diameter of about 45 km. About three fourths of the core are surrounded by a collar, which is formed by a series of upturned and overturned strata including 3.07 Ga Dominion Group metavolcanics, 2.7-3.0 Ga Witwatersrand metasedlments and Ventersdorp lavas, and 2.25-2.5 Ga rocks from the Transvaal Sequence (Reimold 1993). Vredefort is the type locality for pseudotachylite, which is commonly developed in many Vredefort lithologies. Geophysical studies show a semi-annular magnetic anomalyin the inner part of the structure, and a positive concentric Bouguer gravity anomaly in the central area of the Vredefort Dome, indicating the presence of denser material below (Antoine et al., 1990; Corner et al., 1990). The controversy regarding the origin of the Vredefort structure has recentlybeen reviewed by Reimold (1993). The study of Vredefort is complicated by the fact that it is a very old (2.0 Ga, Walraven et al., 1990), large, and Figure 14. Roter Kamm crater, Namibia. The excellent preservation deeply eroded structure, which cannot be directly state of the crater is clearly visible. Northeast is up. Space Shuttle compared to younger and smaller craters. Despite some photograph 61C-40-001. open problems (Reimold 1993) the evidence for impact is compelling. Shatter cones, which record a shock effects, such as planar deformation features in quartz environment (Dietz 1968; Milton 1977), are abundant or diaplectic quartz glass (Reimold and Miller 1989). at Vredefort (Hargraves 1961; Dietz 1961). Planar The presence of unusual quartz pebbles at the crater microdeformationfeatures were found in quartz grains rim, which have been formed after the impact event, from Vredefort and have been interpreted as being was interpreted to be evidence for a post-unpact phase produced by impact shock (Carter 1965,1968; Grieve et of hydrothermal activity (Koeberl et al., 1989a). The al., 1990; Leroux et al., 1994). Their origin has been presence of large vesicles filled with hydrothermal somewhat controversial (Lilly 1981). The features, mineral assemblages in some schistose breccias and however, do not have the complete set of other petrographic and chemical data support the crystallographic orientations normallyassociated with hypothesis of an impact-induced hydrothermal event shock-produced planar deformation features in quartz. in the crater area (Koeberl et al., 1990b; Reimold et al., Grieve et al. (1990) admitted that the PDFs at Vredefort 1994). The age of the Roter Kamm crater has recently are abnormal, but attributed this to post-shock been shown to be 3.7+0.3 Ma by ~°Ar-~Ardating of glass recrystani~ation of quartz. The presence of the high- from an impact melt rock (Koeberl et al., 1993a). The pressure quartz polymorphs coesite and stishovite have limited outcrop, the heterogeneityof some target rocks, been documented (Martini 1978, 1991; McHone and the scarcity of true impact melt breccias (containing Nieman 1988). glass and shock metamorphosed clasts) and the A very fine-grained and homogeneous rock, extensive sand cover of the crater area make surface occurring as dikes at the contact between core and collar studies of the Roter Kamm crater very difficult. This and within the basement gneisses, is called the Vredefort crater is very well preserved and one of the few craters Granophyre. It was interpreted as an impact melt rock on earth which will allow the determination of that was injected into the basement (French et al., 1989; important morphometric data (i.e. the exact internal French and Nielsen 1990), although this view was not structure; Grieve 1993). Although there are some universally accepted (Reimold et al., 1990). If there was 282 C. KOEBERL

Figure 15. Vredefort structure, South Africa. (a) large-scale view: the Vredefort Dome is in the centre of the image, with the Vaal river winding from the lower left part of the image to the upper right. This view shows the whole Witwatersrand Basin. Space Shuttle photograph S08- 35-1294. any doubt as to an impact origin of Vredefort, recently previously assumed. This would make Vredefort one obtained results should settle this question. A new of the largest impact structures recognized on earth, on study of the mid-amphibolite facies metamorphism in a par only with the 200-250 km Sudbury structure in the collar of the Vredefort Dome, which has been Canada (Grieve et al., 1991) or the ~300 km diameter thought to be supportive of an endogenic model, Chicxulub multi-ring impact structure in Mexico indicates that the metamorphic event was unrelated to (Sharpton et al., 1993). More work is necessary to the crater-forming event, supporting the impact model understand the details of the remnant of this huge (Gibson et al., 1994). Rhenium-osmium isotopic studies impact structure. show very good evidence for the presence of a small meteoritic component in some Vredefort Granophyre Saltpan, South Africa The Salt-pan (also termed Pretoria Saltpan, or samples (Koeberl et al., in prep.). Zoutpan; also: Tswaing - Place of Salt) crater has a In addition, a detailed TEM study of the diameter of 1.13 km and is located about 40 km NNW characteristics of the planar microdeformations of Pretoria. The interior of the structure consists of a revealed the presence of mechanical Brazil twin fiat crater floor that is partially covered by a highly lamellae in the basal plane (Leroux et al., 1994). Such saline lake (Fig. 10a). The maximum elevation of the lamellae have only been found as quartz growth crater rim over the present crater floor is 119 m, and features and as deformation effects in impactites, or in the rim rises up to 60 m above the surrounding plains. statically deformed rocks that were subjected to The crater was formed about 220,000 years ago (based pressures of more than about 2 GPa. In addition, other on fission track dating; Storzer et al., 1993; Koeberl et characteristic PDF orientations were identified in the al., 1994b) in crystalline basement of 2.05 Ga (Walraven TEM study, which should finally put the discussion et al., 1990) Nebo granite of the Bushveld Complex (Fig. about anomalous PDFs to rest. A recent interpretation 16b). Some lamprophyre, trachyte, and minor of the geological structure of the Vredefort Dome carbonatite occur in sparse outcrops along the inner rim suggested a diameter of 180-300 km (Therriault et al., wall and in the region surrounding the structure (Brandt 1993), which means that the structure is larger than and Reimold 1993). Africanmeteorite impact craters: characteristicsand geologicalimportance 283

Figure 15 Vredefortstructure, South Africa(b) The centreof the pictureshows the brightercore, in the northwesternpart surrounded by the darker collar, which are made up by the rugged hills of the VredefortMountain Land. SpaceShuttle photograph STS27-33-56Z.

The origin of the Saltpan crater has been debated for breccia, classified as unconsolidated suevitic impact a long time (for a historical account of reports on the breccia, contains numerous shock-metamorphosed Pretoria Saltpan see Levin 1991). Wagner (1920, 1922) quartz, feldspar and biotite grains, besides glass and discussed the findings of a dolomitic breccia, thought melt breccia fragments (Fig. 17a-c), confirming the to be of volcanic origin, which led to wide acceptance impact origin of the Saltpan crater (Reimold et al., 1991, of the hypothesis of a volcanic origin. Results of a 1992b). The breccia layer is underlain by strongly gravity survey were interpreted also to support a fractured and locally brecciated (monomict cataclastic volcanic origin (Fudali et al., 1973). However, it has been breccia) granite, which is occasionally intercalated with recently shown that the occurrence of volcanic rocks is narrow layers of sandy breccia to a depth of 161 m. The not restricted to the crater area and is part of a regional amount of solid granite increases gradually with depth, volcanic event that took place at about 1.2 - 1.4 Ga until drilling was stopped at a depth of 200 m in (Brandt and Reimold 1993). Apparently, the undeformed granitic crater floor (Fig. 16c). well-preserved crater and the Late Proterozoic regional The chemical composition of the bulk breccia and volcanism are unrelated. impact glass fragments is very similar to that of average A meteorite impact origin for the Saltpan, first basement granite, but the impact glass fragments show suggested in 1933, was largely based on morphological considerable enrichments of Mg, Cr, Fe, Co, Ni, and Ir observations (Rohleder 1933), and was supported by (Koeberl et al., 1994c). This can be explained by later structural and fission-track work (Milton and admixture of <10% of a chondritic meteorite Naeser 1971). In 1988, a borehole was drilled to finally component. High Ir concentrations (-100 ppb) found settle the question regarding the origin of the crater, in sulfide spherule samples complement the Ir and to study the uninterrupted paleoenvironmental abundances in the impact glasses (which are lower than record preserved in the crater lake sediments (Partridge those of the spherules, but still elevated compared to et al., 1993). The drillcore revealed an internal crater the contents in the target granite) and indicate some stratigraphy (Reimold et al., 1992b) comprising about fractionation during impact. Re-Os isotopic studies of 90 m of crater sediments that are underlain by at least the target granite show very low Os abundances of 53 m unconsolidated granitic breccia (Fig. 16b, c). This about 7 ppt and high 'STOs/ISSOs ratios of about 0.72, 284 C. KOEI)ERL

~y~,/Fragmental granite breccia Fra-mental -ranite breccia ~,,--~ " ~~:~-- Inferrtxt post-impact profile ~)~ - -u so ~ X~ Present profile / --~ •'" " ..... ~ ~.,,Talus / ~ v', ~.~y> : o ~llOO ~ /Colluvium /Borehole Saline lake , //~,o . | r/" '1 ~ N. I.dJ.U~ -1o5o /7"~'_~'=~---~--~--~------~-~---.------) -~a~-." ~ \ ,, Colluvium "1ooo [] Saline muds Colluvial fan [ E~] Carbonate rich sediments ~'~'/ ..-r. " • - i "" ~" [] a,d oo, s

. Fractured Granite " / \ , , . -85o Granite/ / /'yfl~ \A'~ ~'~aranitebrecoa

- 1O0 200 300 400 500 6(}0 700 8(K) 900 1000 ~ i o() 1200 1300 1400 15(X} I I t I I I I I I I ---J I I I I -4 ~ Meters -]'2"3 ~~ Gravity Profile j ...... ~

Figure 16. Salt'pan crater, South Africa. (a) Oblique aerial view from the west (courtesy D. Brandt); (b) Cross section of the crater as deduced from the analysis of drill core samples and geophysical measurements (gravity data after Fudali et al., 1973; geology after Reimold et al., 199"2b). which are typical for old continental crust (compare Fig. consists of sandstone, some mudstones, shales, and rare 3b). In contrast, the breccia samples have much higher chert lenses collectively belonging to the Beaufort Os abundances (-80 ppt) and lower '87Os/l~Ds ratios Group of the Karoo Sequence. The drill-core from the of about 0.205, which dearly indicates the presence of 1940s (Blignault et al., 1948; Haughton et al., 1953) is a meteoritic component (Koeberl et al., 1994c). unfortunately no longer available. In mid-1992, a new vertical bore-hole was sunk into Kalkkop, South Africa the centre of the crater. The core has reached a depth of The Kalkkop structure has a diameter of 640 m and 151.8 m, where drilling was temporarily suspended is located in the Eastern Cape Province of South Africa. (Reimold et al., 1993). The top 89.3 m consist of crater First studies of the geological setting and subsurface sediment, namely a finely laminated limestone. The geology (Blignaudt et al., 1948; Haughton et al., 1953) limestone is underlain by breccia (Fig. 18b) which is did not reach any conclusions regarding the origin of very similar to glass-bearing, suevitic, breccia known the structure. The possibility of an origin by meteorite from other impact craters. The breccia contains impact was mentioned by Dietz (1965) and Reimold et abundant day-mineral inclusions, presumably resulting al. (1992a), who, however, noted that not enough from alteration of glass fragments, as well as some small information was available to decide this question. glass particles. The dast population varies considerably Morphologically the structure is a circular feature with with depth over a scale of meters and consists mostly a light interior limestone fill that is conspicuous in aerial of sandstone and mudstone or shale. Shale clasts are photographs (Fig. 18a). The basement of the structure often highly fractured and break easily. A large number African meteorite impact craters: characteristics and geological importance 285

Depth(m) concluded that they were not of impact origin. The same 80- conclusion was obtained by Lambert et al. (1981) for Foum Teguentour (26o15' N, 02025' E, 8 km diameter) and Mazoula (28024' N, 07o49 ' E, 0.8 km diameter). A meteorite impact origin has been proposed for a 90- ~iiiiiii~i~iiii number of other structures, but, mainly due to the lack "//.'.'v/.' of detailed (or, sometimes, any) studies, no conclusions have yet been reached. One of the best candidates is Temimichat Ghallaman in Mauritania (24 ° 15'N, 09 ° 10(I- ~ 39'W), about 150 km NE of Tenoumer. The crater has a Granitic "sand" diameter of about 750 m and is slightly hexagonal, •[.•(suevitic breccia) i.~-~-i-~.i.i. Pebbles/fragments < 3cm similar to the Saltpan or Meteor craters. A gravity study o oo [=~ Pebbles/fragments> 3cm indicates that the crater is rather shallow compared to • .. • ...... '. • - - .../... Fractured granite normal impact structures (Fudali and Cassidy 1972). 110- ::~:~:i:iii:i Solid granite No definitive evidence for shock metamorphism has a a a Mylonitc yet been found, but future studies are dearly necessary. Denaeyer and G6rards (1973) considered the possibility that some crater-like structures in Rwanda might be of 120- ":':':':':':': impact origin, but did not find any supportive evidence. • //.-.- - -

• /...... /. Some breccias, which were claimed to contain shocked quartz, were reported from the Lukanga Swamp, a ...... j: depression in central Zambia with a diameter of about 130- ~ 52 km (Vrana 1985), but the structure has not been studied any further. Also in Zambia, the Bangweulu Basin was proposed as a 150 km diameter multiring impact basin (Master 1993). In addition, it was ~!:i:o::o suggested that the 800-km-fliameter Bangui magnetic 140- ~.~.~.~.~.~.] anomaly in Central Africa may be the remnant of a huge and very old impact structure (Girdler et al., 1992). This, "//.'/v.'." however, is highly speculative. Another hotly contested suggestion is the interpretation of the Bushveld 150 Complex as being of impact origin (Hamilton 1970; Figure 16 Saltpan crater, South Africa (c) Boreholestratigraphy (after Rhodes 1975; Elston 1992); however, so far no evidence Reimold et al., 1992b). for shock metamorphism has been found (French 1990). Approximately 3.2 Ga spherule beds in the Barberton of quartz crystals (as well as some feldspar grains) from Greenstone Belt, South Africa, were suggested to be of the breccia show diagnostic PDFs with up to 6 impact origin, largely based on their petrographic orientations per grain. As mentioned before, the structure and their considerable PGE enrichments abundances and isotopic compositions of Re and Os (Lowe and Byerly 1986; Lowe et al., 1989; Kyte et al., demonstrate that the suevitic breccia contains an 1992). Such spherule beds are, however, unknown from extraterrestrial component (cf Fig. 3b), and confirm the any confirmed impact structure on earth, and are impact origin of the Kalkkop crater (Reimold et al., 1993; distinctly different from any spherule deposits at the Koeberl et al., 1994a). Cretaceous-Tertiary boundary, or from (usually very rare) glassy spherules found within the breccias of a Proposed, Doubtful, And Discounted Structures few impact craters. In addition, the absolute A number of crater structures on the African concentrations of PGEs found to be associated with continent have been proposed to be of impact origin, some of the Barberton spherule layers are far too high but, for various reasons, have not, or not yet, been to be accounted for by a meteoritic source (up to, and accepted as confirmed impact craters, while others have even above, the equivalent of 100% of a chondritic since been discounted. For example, it was long thought meteorite component!). Recent detailed mineralogical that the Richat and Semisiyat structures in Mauritania, and geochemical studies have shown that the PGEs are which are composed of a striking succession of always associated with sulphide minerMiTations, and concentric rings of bills, may be of impact origin. A that all spherules are composed of secondary minerals, detailed field and petrographic stud)~ however, showed which exhibit a growth pattern that might be mistaken no evidence for meteorite impact, and it was concluded for chondrule-like quench textures (Koeberl et al., that they are eroded domes formed by uplift (Dietz et 1993b). In addition, Ni-rich Cr-spinels, which were al., 1969). Lambert et al. (1980) investigated the E1 thought to be supportive of an impact origin (Byerly Mouilah (33o51 ' N, 02o03' E, 4.5 km diameter) and Aflou and Lowe 1992), have compositions and iron oxidation (34o00 . N, 02o03 . E, 3x5 km) structures in Algeria and states unlike any known meteoritic (or impact-related) 286 C. KOEBERL

Figure 17. Saltpan crater, South Africa. (a) Photomicrograph of quartz clasts (white), mottled sericitized feldspar, and impact glass particles of various shapes (spherules, ropes, droplets, fragments) from a suevitic bn~ccia (90.7 rn depth; 2.2 mm wide, Koeberl et al., 1994b); (b) PDFs in quartz grains from breccia at 102.0- 103.5 m depth; width: 355 mm, parallel nicols (courtesy W.U. Reimold). spinels (Koeberl and Reimold 1994). These spherules be that humanity engages in scientific research because are, therefore, most likely not associated with any it is curious and wants to increase its knowledge and impact event and the source of the enrichments of derive satisfaction out of the increased knowledge. This various elements must be sought for in hydrothermal is more or less the same reason why we engage in art alteration processes. or cultural activities. The history of science is full of examples that were of absolutely no practical use at the SIGNIFICANCE OF IMPACT CRATERS time (or even hundreds of years later), but without knowledge of which our human culture would be so Scientific Importance much poorer. For those who think that pure knowledge A very common question is, why should we study is not enough, I have a few other answers. impact craters? A number of different answers can be Impact is probably the most important and given to this question. A philosophical answer would fundamental process in the solar system (Shoemaker Africanmeteorite impact craters: characteristicsand geologicalimportance 287

Figure 17 Saltpan crater,South Africa (c) PDFs in quartz grain from granite breccia23A, collectednear the northern rim; widtl~ 0.7 mm, parallel nicols(courtesy W.U. Reimold).

1977). The earth and all other planets were formed by Tertiary sediments and thus has no surface expression. the accretion of small bodies, with a succession of Numerous other buried craters of various sizes are impacts leading to the accumulation of larger planetary certainly still to be detected on earth. How many of bodies. In the earlier history of the solar system, impacts them affected evolution as severely as Chicxulub dida were much more common than now and led to should be subject of future research. In addition, it is catastrophic resurfacing processes on most solid bodies, important to realize that what happened in the past can including the earth and the moon.As mentioned before, happen at any time again. Impacts can have disastrous impacts continue to be the most important surface- consequences for our civilization, as there is a I in 10 forming and -modifying process for most terrestrial 000 chance that a large asteroid or comet, with about 2 planets and satellites. By studying impact craters and km diameter, may collide with the earth during the next processes, we can thus learn something very century, disrupting the ecosphere and erasing a large fundamental about the history and evolution of the percentage of humanity (Chapman and Morrison 1994). bodies of our solar system. Even a relatively small impact can be devastating: a Another aspect is the influence of impacts on the meteoritic projectile (iron or stony meteorite) of 250 m geological and biological evolution of our own planet. diameter (impact energy approximately equivalent to While geologists used to confine themselves to the 1000 megatons) would produce a crater of about 5 km preconceived limitations of internal processes to explain in diameter. Such events happen on earth about every the evolution of the earth, we are beginning to 10 000 years, and would devastate about 104 km2, locally appreciate that impacts played a larger role than we disrupting civilization (Chapman and Morrison 1994). have realized before. A concrete example is the discussion about the causes for major biological Economic Significance extinctions. After more than one decade of intense It may be surprising to hear that impact craters can debating for and against impact, which also led to much have an economic importance. A number of workers fruitful research, it is now commonly accepted that an have addressed this question (Sawatzky 1975; Donofrio enormous impact event occurred at the Cretaceous- 1981; Reimold and Dressler 1990; Masaitis 1992). In Tertiary (K-T) boundar~ 65 Ma (see papers in Silver addition, they can be used for other types of scientific and Schultz 1982; Sharpton and Ward 1990, which study. For example, a somewhat important aspect is the document the geological and biological aspects of this use of crater lake deposits to study the paleoc-limatic debate). Just very recently, the "culprit" has been record (e.g. at Lake Bosumtwi, Talbot and Delibrias identified in form of the Chicxulub multi-ring impact 1980; or at the Saltpan crater, Partridge et al., 1993). basin, which is almost 300 km diameter (Sharpton et The unique morphology of impact craters, as well as al., 1992,1993). This 65 Ma crater is the largest currently structural and lithological changes in the target rocks known crater on earth and escaped detection for so long due to their origin, may lead to economically important only because it is completely covered by about I km of accumulations of mineral resources. For example, a 288 C. KOEBERL

(b)

Figure 18. Kalkkopcrater, SouthAfrica. (a) Verticalaerial photograph, showing the bright circularlimestone filling of the structure (courtesyW. U. Reimold);(b) Drillcore sample of suevitic breccia from 133.9 m depth, diameter5 cm (courtesyW. U. Reimold). number of craters are associated with hydrocarbon structure in Alaska is associated with important gas reservoirs. The Red Wing Creek crater in North Dakota, reserves (Kirschner et al., 1992). In Africa, the existence USA, is about 200 Ma, has a diameter of 9 km, and of a gas pool due to decomposing organic matter in the contains significant oil reserves trapped in the crater lake sediments has been proposed underneath the structure (Brenan et al., 1975; Sawatzky 1977; Donofrio Bosumtwi crater in Ghana (Jones 1983). 1981). A number of other probable or confirmed impact Some crater structures are associated with significant craters (e.g. Viewfield or Eagle Butte, both in Canada) mineralizations. For example, the Sudbury impact contain either oil or gas. Another important oil- structure (Ontario, Canada) hosts the largest Ni-Cu producing crater, which was recently identified to be mineralization in the world, as well as important PGE of impact origin, is the 15 km diameter Ames crater in mineralizations, which are a direct result of the impact Oklahoma, USA (Hamm and Olsen 1992; Roberts and event (Grieve et al., 1991). It has also been speculated Sandridge 1992; Carpenter and Carlson 1992). This that the nature of some of the Witwatersrand gold structure hold an estimated total of about 20 million mineralization is a consequence of the Vredefort impact barrels of crude oil. The newly recognized Avak impact event (Reimold 1994). The Ternovka crater (Ukraine) African meteorite impact craters: characteristics and geological importance 289 hosts economic deposits of ferruginous quartzites, the indicates that, even taking only the presently known Boltysh (Ukraine) impact crater hosts several million (low) number of small craters in Africa into account, tons of oil shales, the Carswell Lake crater (Canada) there should be at least five craters in the 10-50 km hosts important uranium deposits, while impact- diameter range in Africa. It may well be craters of this produced diamonds are extracted from rocks at the size that are of particular economic interest. In addition, Popigai structure in Siberia (Masaitis 1992). Impact considering recent estimates for current cratering rates breccias have also been used as building materials. For on earth (Grieve 1984,1987; Trefil and Raup 1990), it is example, numerous buildings in N6rdlingen, German~ very likely that we have, at present time, only and the Rochechouart Castle in France are constructed recognized a small percentage of all craters (no matter with suevite (a glass-bearing impact breccia) from the which size) on the earth. Such an estimate suggests that Ries and Rochechouart craters, respectively. The we may only have discovered about 10% of all impact exploration of impact craters may well yield craters present in Africa. Furthermore, considering the economically interesting results. percentage of the area of Africa compared to the rest of the earth's continents, Africa is underrepresented regarding the number density of impact craters by a CONCLUSIONS AND OUTLOOK factor of 2-3 (considering the craters known per unit area in other continents). In this review of African impact craters, I tried to In conclusion, I hope that by the time this paper is summarize our knowledge about the individual crater published, at least one or two new craters will have structures on this continent, as well as to put them into been added to the list of African meteorite craters. (This a scientific and economic context. It is dear from the is exactly what has already happened in the interval discussion above that our knowledge of meteorite between the time when the initial manuscript was craters in Africa, in general, and that of most specific written, and the time when the revision was completed.) craters, in particular, is still sparse. Some craters, for Remote sensing will certainly play a major role for example, the Salt-pan crater have been studied in great identifying impact craters in the future. Researchers detail, while research on others has been, to say the least, should also be cautioned, though, not to overinterpret meagre. Most craters have been studied in the field and available data. I have pointed out that there are a petrological investigations on crater rocks led to their number of criteria that have to be fulfilled before an identification as impact craters. However, detailed impact origin can be accepted for a given crater. structural, geological, petrological, geochemical, or Circularity of a structure, or the presence of fractured geophysical studies are missing for a surprisingly large rocks, is not enough. It is, therefore, important to number of structures. Drill cores are available for only consider the importance of impact craters in the three (South) African impact craters. Clearly, further education of geologists. The study of impact craters is investigations of all these African craters should provide not an esoteric pursuit for the few, but an integral part a wide field of study for interested geologists. of our understanding of the geological evolution of our However, not only the study of known impact craters planet. is of importance. Table 1 and Fig. 1 demonstrate that there must be huge gaps in our current knowledge of Acknowledgements meteorite craters present in Africa. First of all, there is I am grateful to the University of the Witwatersrand, no good physical reason why impact craters in Africa Johannesburg, for a visiting research fellowship during should occur mainly in southern Africa and in certain which this paper was written, and to C.R. Anhaeusser North African desert areas. Surely there are still and T.S. McCarthy for the invitation to work at EGRU unrecognized impact craters in most other African and the Department of Geology, University of the countries, particularly in the centre of the continent. The Witwatersrand. I furthermore thank the organizers of history of crater discoveries shows that it took quite the 16th Colloquium on African Geology in Swaziland, some time to find some structures, or, if they were especially R. Maphalala and M. Mabuza, for the known, to correctly deduce their origin. Furthermore, invitation to present this paper as a keynote address. the size distribution of the known African meteorite This study was supported by the Austrian Fonds zur craters is highly suspicious. Almost all African craters, F6rderung der wissenschaftlichen Forschung, Project with only two exceptions, are smaller than about 15 km P9026-GEO. I especially appreciate the efforts of my diameter (see Table 1). friend and colleague W.U. Reimold, who has reviewed If we take the size distribution of all craters known this paper and made numerous suggestions for worldwide (Grieve 1991) and compare it with the improvement, and with whom I have had the pleasure number of the presently known African craters, we to collaborate on many aspects of the study of African immediate see that (with one exception) no impact impact craters. Furthermore, I am grateful to S. Master craters more than about 15 km are known in Africa - and D.J. Robertson for comments, and especially to B. which is physically implausible, and only reflects our French and R. Grieve for very helpful and detailed poor knowledge of African craters. Such a calculation reviews and comments on this paper. 290 C. KOEBERL

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