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The Lake Bosumtwi Impact Crater, Ghana The Lake Bosumtwi impact crater, Ghana 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 meteorite impact (Maclaren, The west and northwest sides of the lake are km. Polymict breccias 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 suevite — 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 coesite (Lit- east corner of the lake. Small granite intru- suggests that Bosumtwi has a central uplift tler and others, 1962), nickel-iron spherules sins, probably connected with the Kumasi rising to 200 m beneath the lake floor. An (El Goresy, 1966), shocked quartz (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 breccia 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 Ivory Coast 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 tektites 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 suevites 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. 342 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/6/342/3434270/i0016-7606-92-6-342.pdf by guest on 27 September 2021 LAKE BOSUMTWI IMPACT CRATER 343 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.
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