The Lake Bosumtwi ,

i Department of Geology, University of Ghana, Legon Accra, Ghana MICHAEL BACON J DAVID A. HASTINGS" Geological Survey Department, P.O. Box M80, Accra, Ghana

ABSTRACT ter from north to south and 10 km from REGIONAL GEOLOGY east to west. In most places, the rim is 250 The l-m.y.-old Bosumtwi Crater, Ghana, to 300 m above the lake, whose greatest The crater was excavated in Precambrian has a nearly circular shape with a rim deph is about 80 m (Junner, 1937). Early Lower Birimian rocks (~ 2,000 m.y. old) diameter of 11 km north-south and 10 km workers proposed a cryptovolcanic origin that trend northeast-southwest and dip east-west. It is surrounded by a circular de- (Rohleder, 1936; Junner, 1937), but an steeply northwest and southeast (Fig. 1). pression and an outer ridge of diameter 20 origin by impact (Maclaren, The west and northwest sides of the lake are km. Polymict averaging at least 20 1931) has recently been supported by a cut in phyllites, graywackes, feldspathic m thick with clasts as much as 5 m long large body of evidence. quartzites, and pebbly grits; the rest are occur on the outer ridge, and the crater rim A rock that closely resembles pumiceous predominantly argillaceous phyllites. Upper shows in situ shattered rock. Patches of tuff has been shown to be — it gives Birimian metamorphosed basalts and suevite have been found in the circular de- a Rb/Sr age of about 2,000 m.y., which is pyroclastic rocks occur to the southeast in pression north and south of the crater. similar to the country rocks (Schnetzler and the Obuom Range and just reach the south- Analogy with better-known craters others, 1966), and it contains (Lit- east corner of the lake. Small granite intru- suggests that Bosumtwi has a central uplift tler and others, 1962), nickel- spherules sins, probably connected with the rising to 200 m beneath the lake floor. An (El Goresy, 1966), (Chao, granite, crop out around the north, west, aeromagnetic anomaly of amplitude 50 1968), melted ilmenite, and zircon decom- and south sides of the lake, the largest at nanotesla (nT) over the northern half of the posing to baddeleyite (El Goresy, 1968). I'epiakese on the northeast side of the cra- lake is interpreted as due to a layer of mag- However, shatter cones have not been ter. There are a few dikes of dolerite, am- netized fallback beneath the lake found apart from a dubious early report phibolite, microgranite, and intermediate sediments. The normal polarity of the (Rohleder, 1934). rocks. Sedimentary rocks of the Precamb- breccia shows that the crater was formed The suggestion that the tek- rian Tarkwaian System outcrop to the during the normal Jaramillo event of 0.97 tites were produced by a meteorite impact southeast of the Obuom Range (Moon and to 0.85 m.y. ago, which agrees with the at Bosumtwi (Barnes, 1961; Cohen, 1963) Mason, 1967; Woodfield, 1966). magnetic stratigraphy of the related Ivory has been confirmed by their K/Ar and Coast microtektites. A regional gravity sur- fission-track ages, which are very similar to CRATER MORPHOLOGY vey indicates a negative Bouguer anomaly the Bosumtwi suevite (Zahringer, 1963) over the crater. There is some geochemical Gentner and others, 1964, 1967; Fleischer The watershed of the internal drainage evidence that the meteorite was an iron, and and others, 1965; Durrani and Khan, basin was previously thought to mark the its mass and energy are suggested as about 1971). These give an age of about 1 m.y. for rim of the crater (Junner, 1937; Saul, s 19 3 10 tons and 3 xlO joules or 7.3 x 10 the origin of the Bosumtwi crater and the 1969). Figure 2 shows that, although the megatons. Ivory Coast and microtektites. The watershed maintains a fairly constant dis- Rb/Sr age of the tektites is the same as for tance of 1 to 2 km from the lake over most INTRODUCTION the Bosumtwi and country rocks of its circumference, it is as much as 5 km (Schnetzler and others, 1966; Lippolt and from the lake in the south. The center of the Lake Bosumtwi is on the northwest side Wasserburg, 1966; Kolbe and others, lake is therefore displaced by about 1.5 km of the Obuom Range about 30 km south- 1967). The similarity in chemistry between north of the center of the drainage basin. east of Kumasi, Ghana (Fig. 1). It lies in a these three groups has been shown for This displacement of the lake center might nearly circular depression 11 km in diame- major and minor elements (Gentner and be explained by a tilting of the crater to the others, 1967; Cuttita and others, 1972), Rb north since its formation, but this cannot be and Sr (Schnetlzer and others, 1966; Lip- the case, as the late Tertiary peneplain in * Present addresses: (Jones) Esso Exploration polt and Wasserburg, 1966; Kolbe and which the depression was excavated has a (Europe-Africa) Inc., St. Clements House, others, 1967), Ba and the lanthanides very gentle southward dip (Brash, 1962). Church Street, Walton on Thames, Surrey KT12 (Schnetlzer and others, 1967), radioactive For most of the circumference of the lake, 2QL, England; (Bacon) Department of Geology, elements (Rybach and Adams, 1969), oxy- Chelsea College, 271 King Street, London W6 the watershed does seem to represent the 9LZ, England; (Hastings) Eros Data Center, gen iotopes (Taylor and Epstein, 1966), and crater rim, because the slope from the lake Sioux Falls, South Dakota 57198. lead isotopes (Wampler and others, 1969). shore to its crest is fairly uniform, although

Geological Society of America Bulletin, Part I, v. 92, p. 12-349, 6 figs., 1 table, June 1981.

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steeper in the west and southwest. On the south side, the steep slope rises from a shelf of lake sediments, but its crest has a similar height to the crater wall elsewhere (Fig. 2). This crest is separated from the watershed by an area of irregular topography; it is proposed that it represents the southern rim of the crater. This gives the crater a north- south diameter of 11 km and a nearly circular shape (Fig. 2). The precipitous crater wall is interrupted by a 50- to 100-m-wide terrace, 125 m above lake level, which was probably cut by the lake when it stood at its highest level and overflowed at the lowest point on the rim (Talbot, 1976). Junner (1937) showed that the crater is surrounded by a ring- shaped depression that varies in width from 1 to 5 km and has an annular drainage pat- tern (Fig. 2). Outside this depression is a ridge (the "main" ridge) 30 to 80 m above it, which can be traced from the Obuom Range south of the lake around its western and northern sides as far as the Anum River. The outer edge of the main ridge forms an escarpment that is most noticeable on the southwest side and which maintains a fairly constant distance of 10.5 km from the center of the lake (Junner, 1937). The narrowest part of this depression is in the south. If the crater rim is as suggested, then the distance between the rim and the main ridge is 3.5 to 5 km. Johnson and others (1964) described ridges around the Ries crater, Germany, at 1.4 R and 1.9 R, where R is the radius of the crater. These two ridges may be analo- gous with ridges at the Lake Bosumtwi cra- ter. If the average diameter of the crater is 10.5 km and the radius of the main ridge is 10 km, then the main ridge is at 1.9 R. The watershed in the south is at a varying dis- tance from the lake center, but for 2 km it is fairly constant at about 7.5 km from the center. This might be a segment of an "in- ner" ridge at about 1.4 R. Figure 1. Geological map of the Bosumtwi Crater area. Inset: location map. CRATER EJECTA

Outcrops of breccia around the lake have that occurs in the circular depression and rock types, usually up to 1 m and, rarely, as been described by Junner (1937), Wood- on the outer ridge. They attributed these much as 5 m long. It occasionally shows field (1966), and Moon and Mason (1967). breccias to rock having been shattered in subhorizontal bedding and is found princi- They can be divided into three groups. The situ without much relative displacement, pally on the outer ridge. There is a notice- first group consists in any one exposure of those nearer the lake having been formed at able coarsening of the size of the clasts only one rock type and can often be seen a greater depth where lateral movement toward the lake. This is clearly analogous grading into unbrecciated rock. Moon and was inhibited. This group is probably the with the Bunte Breccia of the Ries (Pohl and Mason (1967) divided this group into two equivalent of the "autochthonous fractured others, 1977). Its thickness is unknown, but types: (1) a shattered rock in which the rocks" (Dence, 1968) or "fractured zone" Woodfield (1966) stated that it crops out on blocks were displaced very little relative to (Innes, 1961) seen in other craters. the outer ridge in the banks of streams that one another, found on the crater rim; and The second group crops out along stream meander between hills that average 20 m in (2) a coarse rock in a fragmentary matrix valleys as angular blocks of many different height, a minimum estimate of its average

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thickness. It may be similar to the Bunte The third group are rocks equivalent to Ata stream, between the rim and the Breccia which averages about 50 m thick the Ries suevites which have been found watershed. The suevite is a massive purple- (Horz and others, 1977). At one locality, north of the crater in the Buonim stream, in gray rock with fragments as much as a few this allochthonous breccia may be seen the depression between the rim and the centimetres long, ranging from fresh angu- overlying the autochthonous breccia. outer ridge, and south of the crater in the lar phyllite through powdery white rocks to

Figure 2. Topographical map of the Bosumtwi Crater area showing crater rim, watershed, and outer ridge. Contours at 200-ft intervals.

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yellow-brown vesicular glass. Its distribu- in the southern part of the outer ridge there MAGNETIC DATA tion appears to be very patchy, and the are fault breccias that dip northward at low maximum thickness seen is 12 m (Junner, angles; he suggested that these were con- An aeromagnetic survey flown in 1960 by 1937), compared with 25 m for the suevite nected with other fault breccias around the Surveys Ltd for the Ghana Geolog- outside the Ries Crater (Pohl and others, outer ridge and form a ring-shaped shatter ical Survey Department covered the crater 1977). The area shown covered by suevite zone that dips inward toward the lake. area (Fig. 3). Flight lines were oriented by Woodfield (1966) and Moon and Mason Woodfield (1966) and Moon and Mason north-south with a mean spacing of 500 m (1967) is about 1.4 km2, but this estimate (1967) did not confirm these observations, and the mean terrain clearance was 200 m. may be inaccurate, as the distribution of but it is significant that inward-dipping In general, magnetic anomalies over the suevite is very poorly known. thrust faults are a well-known feature of Lower Birimian have northeast trends and Junner (1937) stated that in some places impact craters (Pohl and others, 1977). low amplitudes, typically 20 nT. The Upper

Figure 3. Aeromagnetic map of the Bosumtwi Crater area. Contours at 10-nT intervals. A-A' is line of profile in Figure 4.

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Birimian and Tarkwaian rocks to the

southeast of the crater have anomalies with crater the same northeast trend but larger center amplitudes of as much as 200 nT. Km 0 10 Over the northern half of the lake, there is an east-west negative anomaly about 5 •20 •20 km long with a subsidiary positive anomaly to the north, creating a peak-to-peak

amplitude of 50 nT. This differs from nor- magnetic mal Lower Birimian anomalies in trend and anomaly nT amplitude and may have an origin related -20 -20 to the crater formation. It resembles the anomalies of up to 300 nT at the Ries crater -40 L and 75 nT at Gosses Bluff, which are as- J-40 sociated with layers of fallback breccia (Pohl and others, 1977; Milton and others, 1972). The Bosumtwi anomaly may well have a similar origin, but because any brec- cia is likely to be very variable in magneti-

zation and thickness, an infinity of models 200 200 would be possible by varying these parame- ters independently. However, we may get some idea of the likely form of a breccia 400 400 body by analogy with more thoroughly depth explored craters elsewhere. 600 006 0-05 -003 -0-11 004 0'60 0-85 0-46 010 0-14 -005 600 Craters with diameters between 3.5 and magr istisat ion A/m 30 km have central uplifts surrounded by rings of breccia (Dence and others, 1977). 800 800 10 Published reconstructed cross sections show 1-0 magnetisation that the crest of the central uplift is about A/n 650 m below the original rim at Deep Bay 0 (diameter 9.5 km), 450 m at Nicholson Lake (12 km) and 400 m at East Clearwater (16 km) (Dence, 1968; Dence and others, -V0 -1-0 •20 >20 1968). A central uplift might therefore be calculated expected at Bosumtwi with its crest 550 m below the rim or 200 m below the lake bed. magnetic Such an uplift, if present, would not be anomaly visible because it would be buried beneath nT -20 -20 the lake sediments. Similarly analogy with these craters suggests that fallback breccia at Bosumtwi would form a ring as much as -40 -40 200 m thick between 1 and 3.5 km from the center. Sediment is now accumulating on B the lake floor at a rate of about 0.5mm/yr (D. A. Livingstone, 1980, written com- mun.). This rate is unlikely to have been constant over the past 1 m.y. but if it is taken as an average, then there should be about 500 m of sediment overlying the breccia in the deeper parts.

Figure 4. Aeromagnetic profile along line A-A' (Fig. 3). A: model with a body of variable magnetization, 200 m thick at 500 m depth; section, histogram of magnetiza- tion values and comparison of calculated and observed anomalies. B: model with a body of uniform magnetization and vari- able thickness at a depth to top of 500 m; section and comparison of calculated and observed anomalies.

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Figure 4A shows a model in which a models of Figures 4A and 4B give satisfac- which the breccia acquired its magnetiza- horizontal breccia layer of variable mag- tory fits to the anomaly with RMS misfits of tion, argue against the impact being caus- netization and 500 m depth to top is uni- 1.5 nT and 1.9 nT, respectively, the mag- ally related to the magnetic reversal at the formly 200 m thick, but in Figure 4B, the netization values of up to 0.85 Aim and beginning of the Jaramillo event (Durrani depth to top and magnetization are con- 0.89 Aim are well within the range of mea- and Khan, 1971). stant but the thickness varies. In making sured magnetizations of breccias at other these models, an apparent northeasterly craters — up to 7.0 Aim at the Ries and GRAVITY DATA regional trend over the Lower Birimian of 4.3A/m at Gosses Bluff. Models of the same about 2 nT/km was removed from the type as in Figure 4B using a range of depths Gravity data have recently been collected profile before interpretation and the direc- to the top of the breccia body show an in- around the lake as part of a regional survey tion of magnetization was assumed to be creasing RMS misfit with depth, the limit of of southwest Ghana, performed by Hast- the same as the Earth's present field. The acceptability being 700 m. These models ings for the Geological Survey Department anomaly is not sufficiently elongated for a suggest that the breccia forms a lens about 2 of Ghana. Figure 5 shows a Bouguer two-dimensional interpretation to be km wide at a depth of a few hundred metres anomaly map of the lake area. The sur- strictly valid, especially in view of the dif- just north of the center of the lake. The rounding gravity field is complex. The elon- ference in strike direction between the posi- weak extension to the south could be a thin gated positive anomaly southeast of the tive and negative anomalies, but a full layer of breccia draped over the central up- lake is due to the southwest-northeast- three-dimensional interpretation seems lift. The different trends of the negative and trending Upper Birimian metavolcanic pointless in view of the lack of any definite subsidiary positive anomalies are suggestive rocks in the Obuom Range; the broad low geological constraint on the body shape. of a concentric form, and the two small to the east is attributed to the Banso Gran- Therefore, magnetizations of the blocks anomalies in the southern part of the lake ite, and the low in the northwest corre- were first found by a two-dimensional ma- could represent small patches of breccia. sponds to the outcrop of the Kumasi Gra- trix method (Bott 1967), and these mag- The aeromagnetic anomalies are therefore nite. netizations were then checked by three- compatible with the predicted ring of brec- Impact craters generally have negative dimensional forward modeling, assuming cia, but the breccia would be nearer the cen- Bouguer anomalies, greatest over the center that the cross section remained unchanged ter and much thicker in the north. and falling off radially to near zero at the over a strike length of 4.9 km centered on The models require that the causative edge, due to the reduction in density of the the profile. No attempt was made to fit the body is normally polarized, indicating that underlying rocks by brecciation (Innes, wings of the anomaly remote from the the crater was formed during a period of 1961; Dence and others, 1968; Pohl and causative structure, where the amplitude is normal polarity. Presently available data on others, 1977), but complex craters may only a few nanotesla, and uncertainties of the age of the crater are summarized in have a superimposed central high caused by observation and possible overlap by other Table 1. They suggest an average age in the uprise of denser rocks in the central uplift anomalies would make the significance of range 0.85 to 1.10 m.y., during the reversed (Sweeney, 1978). In our case, there are no the fit doubtful. A preliminary test showed Matuyama epoch, but the requirement of gravity observations over the lake and very that for a model of the dimensions consid- normal polarity restricts the age to the few within the crater rim, but the map does ered here the Root Mean Square (RMS) dif- normal Jaramillo event from 0.89 to 0.95 suggest a low beginning about 8 km from ference over the profile A—A' (Fig. 3) be- m.y. (Cox, 1969). This is supported by the the center and increasing to about —4 mgal tween a two-dimensional and a three- observation that Ivory Coast microtektites at the lakeside. However, if the lake water is dimensional model of strike length 5 km are found near the base of the Jaramillo assigned a depth of 80 m and the sediments was 2.2 nT, after adjustment of the mag- event in cores from the Atlantic (Glass and a thickness of 500 m with a density of 2.1 netization value to make the peak-to-peak others, 1979). The facts that the mic- gm/cc, then calculations assuming an aver- amplitudes of two- and three-dimensional rotecktites occur above the base of the age crustal density of 2.7 gm/cc and cylin- calculations the same. Therefore any fit Jaramillo event and that normal polarity drical symmetry for the structure show that with an RMS deviation of less than 2.2 nT already existed during the 1,000 yr or so the gravity effect of the lake water and sed- was regarded as acceptable. The two immediately following the impact during iments alone would be about —5 mgal at

TABLE 1. AGE DATA FOR THE BOSUMTWI CRATER

Material Age Technique Source (m.y.)

Tektites 1.3 ± 0.2 K/Ar Zahringer (1963) 1.2 ± 0.1 Suevite glass K/Ar Gentner and others (1964) 1.4 ± 0.2 Tektites 0.7 - 1.4 Fission track Fleischer and others (1965) Suevite glass 0.2 - 1.5 Tektites 1.1 ± 0.1 K/Ar Tektites 1.02 ± 0.1 Fission track? Gentner and others (1967) Suevite glass 1.04 ± 0.2 Fission track Microtektites 1.09 ± 0.20 Fission track Gentner and others (1970) Microtektites 0.88 ± 0.13 Fission track! Durrani and Khan (1971) 0.86 ± 0.06 Fission trackj

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the edge. It would therefore be premature to those of the suevite glasses, but the Ivory where D|( is the rim diameter in kilometres speculate about the gravity effects of brec- Coast tektites have 100 to 150 ppm (Cuttita and E is the kinetic energy in joules, for cra- ciated rocks beneath the lake. and others, 1972). These high values may ters with Dli > 2.4 km. Taking the rim also represent meteoritic material. diameter of Bosumtwi as 10.5 km gives 3 X THE METEORITE El Goresy (1966) found metallic 1019J or 7.3 x 103 megatons for the spherules in the Bosumtwi suevite. His mic- Bosumtwi meteorite. Assuming it was a Regional geochemical sampling of stream roprobe analysis of one specimen gave NiFe meteorite with a density of 8 gmicc sediments by the Ghana Geological Survey 95.4% Fe, 5.2% Ni, and 0.6% Co, similar and a velocity of 24.6 km/sec (the RMS im- showed Ni concentrations of 0 to 20 ppm to the kamacite of some iron . pacting velocity of known Earth-crossing over the Lower Birimian rocks and 0 to 10 Palme and others (1978) suggested a frac- asteroids; Shoemaker, 1977), then it would ppm over the granites. Values of as much as tionated iron meteorite on the basis of the have had a mass of about 108 tons and a 80 ppm occur inside the outer ridge, espe- concentrations of siderophile elements in an diameter of about 300 m. cially on the southern and northeastern Ivory Coast tektite. The available evidence Kieffer and Simonds (1980) stated that sides (Moon and Mason, 1967; Woodfield, thus points to an iron meteorite for the im- during initial contact of meteorites with the 1966). This anomaly may represent pacting body at Bosumtwi. Dence and ground, a jet of molten target material meteoritic material. Ni concentrations in others (1977) stated that the energy of a equal to several meteorite masses is shot out the Lower Birimian rocks (5 to 50 ppm) meteorite is given by: from the interface, and this might be the 5 3X4 (Moon and Mason, 1967) are similar to Dr = 1.96 X 10- E" source of tektites. It is noteworthy that the present estimate of the mass of the Ivory Coast microtektites is 2 x 107 tons (Glass and others 1979), only one order of mag- nitude less than predicted from the mass of the meteorite.

CONCLUSIONS

The meteoritic origin of the Bosumtwi crater and its genetic relationship to the Ivory Coast tektites have previously been convincingly demonstrated by a large body of chemical, petrographical, and geo- chronological data. In this paper some little-known information has been brought together; the significance of this informa- tion in relation to the meteorite impact theory has not been stressed previously. In particular, the magnetic anomaly over the lake has been interpreted as an indication of a layer of fallback breccia. The meteorite impact origin is compatible with all that is known about the crater. A cross section of Bosumtwi, based on all of the information Figure 5. Bouguer gravity anomaly map of the Bosumtwi Crater area showing gravity considered in this paper, is given in Figure stations; contours in mgals. 6.

Figure 6. Hypothetical cross section across Bosumtwi Crater along line B-B' (Fig. 2).

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ACKNOWLEDGMENTS Gentner, W., Lippolt, H. J., and Muller, O., Geochimica et Cosmochimica Acta, v. 42, 1964, Das Kalium-Argon-Alter des p. 313-323. Bosumtwi Kraters in Ghana und die The authors are grateful to the Geologi- Pohl, J., Stoffler, D., Gall, H., and Ernston, K., chemische Beschaffenheit seiner Glasser: 1977, The Ries impact crater, in Roddy, cal Survey Department of Ghana for per- Zeitschrift für Naturforschung, v. 19A, D. J., and others, eds., Impact and explo- mission to publish the geophysical data and p. 150-153. sion cratering: New York, Pergamon Press, to M. R. Dence and H. C. Halls for con- Gentner, W., Kleinmann, B., and Wagner, G. A., p. 343 -404. structive comments on the manuscript. 1967, New K-Ar and fission track ages of Rohleder, H.P.T., 1934, Uber den Fund von Ver- impact glasses and tektites: Earth and griesungserochein und Drucksuturen am Planetary Science Letters, v. 2, p. 83-86. Kesselrand des Kryptovulkanischenungen REFERENCES CITED Gentner, W., Glass, B. P., Storzer, D. and Bosumtwi: Zentralblatt Mineralogie, Wagner, G. A., 1970, Fission track ages and Geologie und Palaeontologie, Abt. A., no. Barnes, V. E., 1961, Tektites: Scientific Ameri- ages of deposition of deep-sea microtek- 10, p. 316-318. can, v. 205, November, p. 58-65. tites: Science, v. 168, p. 359-361. 1936, Lake Bosumtwi, Ashanti: Geographi- Bott, M.H.P., 1967, Solution of the linear inverse Glass, B. P., Swincki, M. B., and Zwart, P. A., cal Journal, v. 87, p. 51-65. problem in magnetic interpretation with 1979, Australasian, Ivory Coast and North Rybach, L., and Adams, J.A.S., 1969, The application to oceanic magnetic anomalies: American tektite strewn fields: Size, mass radioactivity of the Ivory Coast tektites and Royal Astronomical Society Geophysical and correlation with geomagnetic reversals the formation of the Bosumtwi crater Journal, v. 13, p. 313. and other Earth events: 10th Lunar and (Ghana): Geochimica et Cosmochimica Brash, H. 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A., Zahringer, J., 1963, K-Ar measurements of tek- El Goresy, A., 1966, Metallic spherules in Moss, J. J., Sedmic, E.C.E., Vanson, J., and tites: International Atomic Energy Author- Bosumtwi crater glasses: Earth and Plane- Young, G. A., 1972, Gosses Bluff impact ity, Vienna; Radioactive dating, p. 289- tary Science Letters, v. 1, p. 23-24. structure, Australia: Science, v. 175, 305. 1968, The opaque minerals in impactite p. 1199-1207. glasses, in French, B. M., and others, eds. Moon, P. A., and Mason, D., 1967, The geology 0 Shock metamorphism of natural materials: of Vi field sheets 129 and 131, Bompata Baltimore, Mono Book Corp., p. 531—554. S.W. and N.W.: Ghana Geological Survey MANUSCRIPT RECEIVED BY THE SOCIETY Fleischer, R. L., Price, P. B., and Walker, R. M., Bulletin 31. JANUARY 24, 1980 1965, On the simultaneous origin of tektites Palme, H., Janssens, M. J., Takahashi, H., An- REVISED MANUSCRIPT RECEIVED OCTOBER 31, and other natural glasses: Geochimica et ders, E., and Hertogen, J., 1978, Meteoritic 1980 Cosmochimica Acta, v. 29, p. 161-166. material at five large impact craters: MANUSCRIPT ACCEPTED NOVEMBER 18, 1980

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