 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; October 2002; v. 30; no. 10; p. 939–942; 4 figures. 939 Pronounced central uplift identified in the Bosumtwi impact structure, Ghana, using multichannel seismic reflection data Christopher A. Scholz* 204 Heroy Geology Laboratory, Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA Tobias Karp Institut fu ¨ r Geowissenschaften, Abteilung Geophysik Otto-Hahn-Platz 1, 24118 Kiel, Germany Keely M. Brooks 204 Heroy Geology Laboratory, Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA Bernd Milkereit Department of Physics, University of Toronto, 60 St. George Street, Toronto, Ontario M5S 1A7, Canada Philip Y.O. Amoako Ghana Geological Survey, P.O. Box M80, Accra, Ghana Justice A. Arko Institute of Mining and Mineral Engineering, University of Science and Technology, Kumasi, Ghana Figure 1. Location, bathymetry, drainage, and multichannel seismic reflection track line map for Lake Bosumtwi. Outer boundary shows limit of Bosumtwi drainage basin, and dashed line shows elevation of former outlet. Inset shows crater location within Africa and relative to Ivory Coast tek- tite field. Bathymetric contour interval is 10 m. Dark lines show profile locations, and shaded line shows location of Figures 2 and 3. ABSTRACT The Lake Bosumtwi impact structure is the youngest and best- preserved complex terrestrial impact crater and serves as an im- portant reference site for the study of cratering processes. Because the impacting body struck continental crystalline target rocks and not a submerged sedimentary platform, no significant backwash processes have modified the crater morphology. Not only may Bos- umtwi contain the best-preserved central uplift structure on Earth, but it is the most accessible relatively large, young crater in the solar system generated in a large gravity field. There is a well- established link between the Lake Bosumtwi impact structure and the Ivory Coast tektite field, and the lacustrine sediments within the crater contain a unique 1 m.y. record of paleoclimate in the continental tropics south of the Sahel. Eight profiles of marine-type multichannel seismic reflection (MCS) data were acquired from the 8-km-diameter, 75-m-deep lake that fills much of the crater. These were augmented by wide-angle seismic data acquired with ocean-bottom hydrophones. MCS data reveal a well-defined central uplift near the northwest-central part of the lake and a maximum postimpact lacustrine sediment thickness of 310 m. The central uplift structure has a diameter of 1.9 km and a maximum height of 130 m above the annular moat inside the crater. An intermediate velocity layer (3200 m/s) beneath the lacustrine sediment is inter- preted as fallback breccia or a breccia-melt horizon. The measured apparent depth of the crater (d a ) is 500 m, implying a slightly higher aspect ratio for the structure than predicted from published empirical relationships. The Bosumtwi structure is a small complex crater that deviates slightly from trends predicted from classical scaling laws, perhaps because of the effects of a large gravity field. Keywords: Lake Bosumtwi, impact cratering, terrestrial impact struc- tures, multichannel reflection seismology, West Africa, paleolimnology. INTRODUCTION Meteorite impact cratering was a formative process during the early evolution of the terrestrial planets and played an influential role in the development of Earth’s surface, atmosphere, and biosphere throughout Earth history (e.g., Grieve et al., 1995). Observations of craters on the other terrestrial planets and Earth’s Moon have provided abundant data on crater morphology and on impact-structure scaling relationships (e.g., Croft, 1985; Cintala and Grieve, 1994), but ground- truth verification of impact materials and processes is possible so far only from examination of Apollo Moon mission samples and from the study of terrestrial impact craters. With a rim diameter of 10.5 km and an age of 1.07 Ma (Koeberl et al., 1997), the Bosumtwi impact struc- ture, Ghana, is arguably the youngest and best preserved of the 95 terrestrial impact structures 6 km in diameter (Fig. 1). In addition, *E-mail: cascholz@syr.edu. it is one of a relative few young craters formed in a hard crystalline target rather than in a sedimentary platform, as is the case with the Zhamanshin structure (Garvin and Schnetzler, 1994). Although the Zhamanshin structure is similar in size (14 km diameter) and age (0.87 Ma), Bosumtwi’s strong target rocks are more similar in rheology to those on the Moon and the other terrestrial planets. Martian cratering processes in particular are poorly understood, because one-third of the large impact structures there are without central peaks (C. Schnetzler, 2001, personal commun.). Bosumtwi may be especially useful for de- ciphering Martian impacts, both having developed in a large gravity field. Because the impacting body struck a subaerial site in the conti- nental interior, rather than a submerged continental shelf, no profound backwash effects distorted the structure following the impact, as was likely the case in many submarine impacts (e.g., Morgan et al., 1997; Poag et al., 1999).