Detailed structural analysis of the rim of a large, complex : Bosumtwi Crater,

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 that is associated with the 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 , 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 -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 east of 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 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 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. We lithic (clastic) breccias were observed. None of tively hot felsic magma, which incorporated interpret this material as a mixture of lithic (frag- the >60 crater-rim samples collected by us dis- Birimian country rocks and locally melted these mental) impact breccia and locally accumulated plays characteristic shock-metamorphic defor- biotite (volatile)–rich xenoliths. products of secondary mass-wasting processes mation. However, numerous metasedimentary or At more than 60 stations, structural observa- along the steeply dipping outer-rim slopes. granitic clasts in suevite samples exhibit shock tions were obtained along traverses through the In zone 2, brittle deformation is ubiquitous. effects, such as single or multiple sets of planar crater rim, in a series of distinct zones, which we This regime consists mainly of highly fractured deformation features (PDFs) or the presence of will discuss in detail for the northwesterly rocks, which commonly exhibit conjugate radial diaplectic quartz glass, that are characteristic for traverse: Exposures in zone 1 (Fig. 2) are domi- fractures. Quartz veining occurs along many at >10 to <35 GPa. Much nated by occurrences of breccias composed fractures and joints of numerous orientations of the clastic component in melt fragments from mainly of shale (at individual outcrops, >90 and—together with some quartz-filled extension suevite was derived from basement granites. vol%) and fine- to medium-grained metasand- gashes—presumably resulted from pre-impact Granitoid intrusions into the crater rim often stone and metagraywacke, with minor (<10 deformation and hydrothermal activity. Meta- exhibit, in microscopic patches, granophyric or vol%) schist. Most clasts are strongly weathered. graywacke and metasandstone are the dominant spherulitic textures. However, lack of shock- Clasts may be as wide as 1.5 m, but they are gen- lithologies, but shale (up to several meters wide)

GEOLOGY, June 1998 545 was also noted in some exposures. Some shale COMPARISON WITH DEFORMATION Gold Fields of South Africa Limited. Supported by the contains significant amounts of carbonaceous STYLES AND ZONES IN SIMPLE Austrian Fonds zur Förderung der wissenschaftlichen Forschung, Projects P09021-GEO and START Y-58 matter. Outward-thrusting faults (Fig. 3B) typi- BOWL-SHAPED IMPACT CRATERS (Koeberl), and by a Foundation for Research Develop- cally dip inward at 40°–45°. Orientation of bed- Structural observations have been reported ment of South Africa grant (Reimold). Walter Alvarez ding is highly variable; it is generally inward dip- from the crater rim of the 1.13-km-diameter provided a much appreciated review. ping at angles between 20° and vertical, but at Pretoria Saltpan (Tswaing) meteorite crater in several locations it is outward dipping or even South Africa (Brandt and Reimold, 1995) and REFERENCES CITED Brandt, D., and Reimold, W. U., 1995, The geology of overturned. Symmetric gentle and open folds from the rims of the Odessa (Texas) and Meteor the Pretoria Saltpan impact structure and the sur- were noted, but symmetric close and isoclinal (Arizona) impact craters (Shoemaker, 1960; rounding area: South African Journal of Geology, (Fig. 3C) folds are more common. Faulting Shoemaker and Eggleton, 1961). In the cases of v. 98, p. 287–303. exploits less competent shale layers, resulting in both bowl-shaped and complex impact struc- Gault, D. E., Quaide, W. L., and Oberbeck, V. R., 1968, Impact cratering mechanics and structures, in bedding-parallel slip. Slickensides are preserved tures, ejecta breccia covers the top and outer zone French, B. M., and Short, N. M., eds., Shock on some fault planes in metasedimentary rocks of the crater rim. In the lowermost inner zone, metamorphism of natural materials: Baltimore, and generally confirm upward and outward inward-dipping bedding orientations prevail, Maryland, Mono Book Corporation, p. 87–99. thrust movements. whereas upper parts of the inner slopes are char- Grieve, R. A . F., and Pilkington, M., 1996, The sig- On top of the rim, in zone 3 (Fig. 3D), ejecta acterized by highly variable bedding orientations, nature of terrestrial impacts: AGSO Journal of Geology and Geophysics, v. 16, p. 399–420. breccia occurs. It consists of generally angular including overturning. On inward-dipping fault Hirdes, W., Davis, D. W., Lüdtke, G., and Konan, G., blocks of metasandstone and metagraywacke planes, outward-directed thrusting is indicated. In 1996, Two generations of Birimian (Paleo- (with minor shale) of <60 cm clast sizes. Recent contrast, a deformation zone 2—as documented proterozoic) volcanic belts in northeastern Côte mass wasting, for example, in the form of rock- at Bosumtwi—has not been observed in these d’Ivoire (West Africa): Consequences for the “Birimian controversy”: Precambrian Research, falls and debris slides, is related to the steep smaller structures. Megablocks have not been v. 80, p. 173–191. slope in this zone. Descending farther down the observed in small craters, and anticlines are typi- Jones, W. B., Bacon, M., and Hastings, D.A., 1981, The inner slope of the rim, rock fragments increase in cally observed in small craters in the middle parts Lake Bosumtwi impact crater, Ghana: Geological size to tens of meters (Fig. 3E). Such mega- of the inner rim. Society of America Bulletin, v. 92, p. 342–349. blocks are surrounded by relatively finer-grained Impact structures larger than Bosumtwi (e.g., Junner, N. R., 1937, The geology of the Bosumtwi caldera and surrounding country: Gold Coast breccia (clast sizes to tens of centimeters). It is the 24-km-diameter Ries Crater, Germany) are Geological Survey Bulletin, v. 8, p. 1–38. possible that some megablocks in the lower parts known to have well-developed terrace systems Koeberl, C., Bottomley, R. J., Glass, B. P., and Storzer, of this zone may still be in situ. Most along the outer rim. This feature of large impact D., 1997, Geochemistry and age of the Ivory megablocks are faulted and display internal craters is consistent with our observation of Coast tektites and microtektites: Geochimica et blocks that apparently slumped inward along Cosmochimica Acta, v. 61, p. 1745–1772. brecciation, on a centimeter-to-decimeter scale. Leube, A., Hirdes, W., Mauer, R., and Kesse, G. O., In the lower parts of this zone, megablocks com- steeply oriented fault planes. 1990, The Early Proterozoic Birimian Super- monly have bedding dips toward the inner crater group of Ghana and some aspects of its associ- and, often, are separated by dike-like zones of CONCLUSIONS ated gold mineralization: Precambrian Research, finer-grained clastic breccias. Main lithologies We have demonstrated that similar deformation v. 46, p. 136–165. Melosh, H. J., 1989, Impact cratering—A geologic proc- represented in this zone are metagraywacke, styles characterize crater rims of small (simple, ess: New York, Oxford University Press, 245 p. minor shale, and an appreciable amount of pod- bowl-shaped) and larger (complex) impact struc- Moon, P. A., and Mason, D., 1967, The geology of 1/4° and dikelike granitoid intrusions. tures. In particular, outward-directed thrust fault- field sheets nos. 129 and 131, Bompata S.W. and Zone 4 is characterized by inward-dipping ing and intense folding characterize the middle to N.W.: Ghana Geological Survey Bulletin, v. 31, upper parts of impact crater rims. In the light of the p. 1–51. (~50°) shear zones and faults, which appear to be Rohleder, H. P. T., 1934, Über den Fund von related to thrusting. Vertically oriented blocks generally accepted sequence of impact-cratering Vergriesungs-Erscheinungen und Druckstruk- were also noted. Bedding-parallel faults, dipping stages (Melosh, 1989), the following generaliza- turen am Kesselrand des kryptovulkanischen at ~45° to the south-southeast are common in this tion can be made: Most observed deformation Bosumtwi-Sees, Ashanti: Zentralblatt für Min- zone. Breccia lenses as much as 6 × 2 m in size structures appear to be related to the cratering eralogie, Geologie, und Paläontologie, Jahrgang 1934, p. 316–318. were noted. The irregular orientations of target- (compression and excavation) phase of the impact Shoemaker, E. M., 1960, Penetration mechanics of high rock blocks are attributed to complex thrust fault- event. For example, fold structures such as those velocity , illustrated by Meteor Crater, ing as well as block rotation in this lower part of reported here from Bosumtwi Crater and earlier by Arizona: International Geological Congress, 21st, the crater rim. Duplex- and lensoid-shaped (often Brandt and Reimold (1995), have been discussed Copenhagen, Det Berlingske Bogtrykkeri, v. 18, by Gault et al. (1968) as the result of initial p. 418–434. pinching and swelling) blocks (Fig. 3F) are abun- Shoemaker, E. M., and Eggleton, R. E., 1961, Terrestrial dant in this zone. Normal and antithetic faulting, outward-directed motion, during which rocks are features of impact origin, in Milo, M. D., ed., Pro- related to slumping along the crater rim during first driven horizontally outward and then ceedings of the geophysical laboratory/Lawrence the late modification stage of the cratering event deflected upward toward the surface. Only the Radiation Cratering Symposium, p. A1–A27. (Melosh, 1989), was noted along this steep and antithetic normal faulting observed primarily on Wright, J. B., Hastings, D. A., Jones, W. B., and Williams, H. R., 1985, Geology and mineral re- unstable inner slope. Small-scale faults and folds, the inner-rim slopes of the crater seems to be sources of West Africa: London, George Allen & possibly related to an earlier (either pre-impact related to postcratering modification of the highly Unwin, p. 38–45. regional or early impact—i.e., shock compres- unstable rim. sional) phase of deformation, occur within large Manuscript received November 17, 1997 upturned or rotated blocks. Some of these blocks ACKNOWLEDGMENTS Revised manuscript received March 19, 1998 The support of C. E. Oduro, Director of the Geo- Manuscript accepted March 20, 1998 are intensely fractured (shattered). Toward the logical Survey of Ghana, and G. O. Kesse, former lowermost inner part of the rim, blocks of all Director of this institution, and the field assistance of regional lithologies are highly fractured and K. Atta-ntin were instrumental to the success of our sheared and locally internally brecciated. The field study. Landsat imagery was kindly provided by bedding attitudes observed in this strongly de- formed zone typically dip gently (<15°) inward.

546 Printed in U.S.A. GEOLOGY, June 1998