Detailed structural analysis of the rim of a large, complex impact crater: Bosumtwi Crater, Ghana Wolf U. Reimold Dion Brandt Department of Geology, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa Christian Koeberl Institute of Geochemistry, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria ABSTRACT The 1 Ma Bosumtwi Crater in Ghana is an 11-km-diameter, presumably complex, well- preserved impact structure that is associated with the Ivory Coast tektite strewnfield. Detailed structural geologic studies along a complete traverse through the northwestern rim section indi- cated four zones characterized by distinct deformation styles from just outside of the crater rim to near the crater floor. Zone 1 is dominated by thick deposits of lithic impact breccia, intercalated in places with products of local mass wasting. Zone 2 contains inward-dipping thrust planes, conjugate radial fractures, isoclinal folding, and overturned stratigraphic sequences. Zone 3 represents a megabreccia zone, in which block size decreases upward and outward toward the rim crest. The innermost zone 4 is dominated by intense thrust faulting of multiple orientations, resulting in complex duplex- and lens-shaped bodies. These deformation styles generally corre- spond to those previously reported from the rims of simple bowl-shaped meteorite-impact craters and appear to be characteristic of impact structures in general. INTRODUCTION Bedrock and impact formations outside of the southern rim sectors. A variety of granitoid intru- The geologic structure and macrodeformation crater are only accessible along road cuts and in sions (mainly biotite or amphibole granites) was associated with some small (<2 km), well- stream beds. In January 1997 we studied a series mapped by Junner (1937) and Moon and Mason preserved, simple bowl shaped impact structures of fresh road cuts, produced in 1993 in the course (1967). In addition, we observed numerous, but (Meteor Crater [Arizona], the Odessa crater of complete reconstruction of the access road to generally <1-m-wide, dikes of biotite granitoid at [Texas], and the Pretoria Saltpan crater [South the crater lake through the northwestern crater- many basement exposures in the crater rim. Many Africa]) have been studied before (Brandt and rim section. These exposures allowed detailed of these dikes have vertical to subvertical orienta- Reimold, 1995, and references therein). In con- structural analysis along a 7-km-long traverse tions. The overall granitoid component in the trast, most larger, complex impact structures are through the crater rim (Figs. 1 and 2). Two other, region is estimated at about 2%. too deeply eroded to allow detailed structural less complete cross sections were studied along Numerous breccia exposures have been analysis of their crater rims. The young (1.07 Ma, gravel roads in the northeastern and southern mapped around the crater in the past (e.g., Junner, Koeberl et al., 1997) Bosumtwi impact crater, sectors of the rim and generally confirmed the 1937; Moon and Mason, 1967; compare Fig. 1). located at 06°32′N, 01°25′W, about 30 km south- findings along the northwest traverse. These Our investigations confirm the presence of suevite east of Kumasi in the Ashanti Province of Ghana results provide, for the first time, a detailed deposits to the north and southwest of the crater. It is still well preserved (Fig. 3A). Although the assessment of the deformation associated with is, however, not certain at this stage whether any of structure of the central part of this impact crater the crater rim of a complex impact structure. the mapped occurrences of lithic (clastic) breccia is completely obscured by Lake Bosumtwi, it represent impact breccia. Extensive sedimento- must be assumed, on the basis of the existing GEOLOGY OF THE BOSUMTWI AREA logical work is needed to establish how much of terrestrial impact crater record (cf. Grieve and A schematic geologic map of the area (Fig. 1) the reported breccias represents impact ejecta or is Pilkington, 1996) that Bosumtwi represents a shows that the crater is nearly completely filled by the result of lateritization and secondary mass- complex impact crater. At 10.5 km in diameter, it Lake Bosumtwi and talus sediments in its wasting processes in this tropical and topographi- should feature a collapsed central uplift structure environs. The rim-to-rim crater diameter is about cally varied environment. Along the northwest (Melosh, 1989). 10.5 km. The main bedrock strata are metasedi- traverse through the crater rim, Birimian meta- The origin of Bosumtwi has been controver- mentary rocks in greenschist facies belonging to graywacke and shale are the dominant rock types, sial for several decades, but since the 1960s, sev- the Birimian Supergroup, dated at 2.1–2.2 Ga besides minor occurences of shale and some dike- eral lines of evidence have supported an impact (Wright et al., 1985; Leube et al., 1990; Hirdes and pod-like granitoid intrusions. origin (cf. Jones et al., 1981; Koeberl et al., et al., 1996). To the southeast of the crater, some 1997). The Bosumtwi impact crater has also been Tarkwaian metasedimentary rocks occur as well. STRUCTURALANALYSIS associated with the Ivory Coast tektite strewn- The regional geology is characterized by a strong, Lineament analysis on both Thematic Map- field, on the basis of similarities in chemical and northeast-trending fabric with steep dips to either per 5 (TM5) Landsat and SPOT imagery con- isotopic composition, and identical ages for the northwest or the southeast. Variations in this firms the field observations that the crater is tektites and crater glasses (Koeberl et al., 1997, trend, due to folding, were observed on a local transected by numerous, generally steeply dip- and references therein). scale. Lithology at and around Lake Bosumtwi is ping radial fractures and/or faults. Pairs of No structural geologic data have been avail- dominated by metagraywackes and metasand- lineaments commonly occur in a scissors-like able so far, because of the extensive cover of the stones, but some shale and mica schist were also arrangement. Concentric lineaments are promi- whole region by rainforest and lateritic soils. observed, especially in the northeastern and nent, but not very abundant. Field observations Geology; June 1998; v. 26; no. 6; p. 543–546; 3 figures. 543 Figure 1. Geologic map of area around Lake Bosumtwi, with location of traverse (A–B) along access road to the crater lake through northwestern crater rim. Dashed line A–B: crater rim section based on observations along winding road, but projected onto a straight cross section (compare Fig. 2). strongly suggest that inward-directed slumping presumably the result of faulting in the rim. At posed that outside of the crater rim, about 1.5 km occurred, possibly along concentric and listric the inner end of the traverse, a relatively gentle from the rim crest, a shallow depression of about faults. The rim shape is complex (Fig. 2); the rim talus slope is exposed around the lake (compare 4 to 5 km width was surrounded by another slight has steep inner and outer slopes, and significant also Fig. 3A). (30–80 m) ring-shaped elevation. Our topo- depressions occur below the main rim crest— Earlier workers (e.g., Jones et al., 1981) pro- graphic studies support the presence of a wide, shallow depression in the northern and north- eastern environs of the crater. This feature could be the result of formation of a ring depression dur- ing transient cavity collapse, or of preferential erosion of impact ejecta along several stream beds, which follow the crater rim in a semicon- centric pattern. Jones et al. (1981) discussed the possibility that this ring morphology could be related to the multiple-ring features observed, for example, around the Ries Crater (Germany). However, we favor a different scenario: a ring fracture, formed during the cratering phase, led to the development of the specific drainage pattern observed around Bosumtwi Crater, which, in turn, caused the formation of a semiannular erosion channel. The terrane farther away from the crater does not display any evidence of cratering- Figure 2. Zonation of northwestern crater rim along A–B traverse (Fig. 1). Zone 1: dominantly induced structural control on the drainage pattern. ejecta breccia cover and local mass wasting. Zone 2: inward-dipping thrust planes, conjugate radial fractures, isoclinal folding, and overturned stratigraphy. Zone 3: megabreccia with upward- The presence of shatter cones at Bosumtwi, decreasing block sizes. Zone 4: dominant thrust faulting of multiple orientations, resulting in which had been suggested by Rohleder (1934), duplex- and lens-shaped bodies of pre-impact strata. could not be confirmed by us, but some curvi- 544 GEOLOGY, June 1998 Figure 3. A: View of north- ern crater-rim section from lakeshore, demonstrating radial macrofaulting and A B megablock slumping to- ward crater interior. B: Rim section zone 2; thrust plane dipping inward (to left) and cutting Birimian metagraywacke. C: Zone 2; isoclinal folding in strat- ified Birimian metasedi- mentary rocks. D: Zone 3, near highest point of rim; lithic impact breccia composed of centimeter- to decimeter-sized clasts. E: Zone 3; megabreccia with meter-sized blocks of Birimian metasedimentary rocks and (lightest-colored) 2 Ga granitoids; width of field of view = 10 m. F: C D Duplex- and lens-shaped shale and metagraywacke blocks between variably oriented thrust faults; width of field of view = 8 m. E F planar joints, partially displaying slickensides, metamorphic and chemical evidence for their or- erally smaller than 5 cm. Some larger clasts were observed. Furthermore, melt-breccia occur- igin by impact melting leads us to prefer an origin exhibit internal brecciation. Clast shapes are vari- rences were also not found, whereas abundant for these dikes by pre-impact intrusion of a rela- able, ranging from angular to well rounded.