Uppermost impact fallback layer in the Bosumtwi crater (Ghana): Mineralogy, geochemistry, and comparison with Ivory Coast tektites

available online at http://meteoritics.org Uppermost impact fallback layer in the Bosumtwi crater (Ghana): Mineralogy, geochemistry, and comparison with Ivory Coast tektites Christian KOEBERL1*, Franz BRANDSTÄTTER2, Billy P. GLASS3, Lutz HECHT4, Dieter MADER1, and Wolf Uwe REIMOLD4 1Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria 2Mineralogisch-Petrographische Abteilung, Natural History Museum, Burgring 7, A-1010 Vienna, Austria 3Department of Geology, University of Delaware, Newark, Delaware 19716, USA 4Mineralogy, Museum for Natural History, Humboldt University in Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany *Corresponding author. E-mail: [email protected] (Received 01 October 2006; revision accepted 01 January 2007) Abstract–In 2004, an International Continental Scientific Drilling Program (ICDP) drilling project at the Bosumtwi impact crater, Ghana (10.5 km in diameter, 1.07 Myr old), was performed to study the sediments that fill the lake as well as the underlying impactites. In one (LB-05) of 16 cores drilled into the lake sediments, the zone between the impact breccias and the post-impact sediments was penetrated, preserving the final, fine-grained impact fallback layer. This ~30 cm thick layer contains in the top 10 cm “accretionary” lapilli, microtektite-like glass spherules, and shocked quartz grains. Glass particles—mostly of splash form less than 1 mm size—make up the bulk of the grains (~70–78% by number) in the coarser size fraction (>125 m) of the top of the fallback layer. About one-third of all quartz grains in the uppermost part of the layer are shocked, with planar deformation features (PDFs); almost half of these grains are highly shocked, with 3 or more sets of PDFs. K-feldspar grains also occur and some show shock deformation. The abundance of shocked quartz grains and the average shock level as indicated by the number of sets of PDFs, for both quartz and K-feldspar, decrease with depth into the layer. The well-preserved glass spherules and fragments are chemically rather homogeneous within each particle, and also show relatively small variations between the various particles. On average, the composition of the fallback spherules from core LB-5B is very similar to the composition of Ivory Coast tektites and microtektites, with the exception of CaO contents, which are about 1.5 to 2 times higher in the fallback spherules. This is a rare case in which the uppermost fallback layer and the transition to the post-impact sediments has been preserved in an impact structure; its presence indicates that the impactite sequence at Bosumtwi is complete and that Bosumtwi is a very well-preserved impact crater.In 2004, an International Continental Scientific Drilling Program (ICDP) drilling project at the Bosumtwi impact crater, Ghana (10.5 km in diameter, 1.07 Myr old), was performed to study the sediments that fill the lake as well as the underlying impactites. In one (LB-05) of 16 cores drilled into the lake sediments, the zone between the impact breccias and the post-impact sediments was penetrated, preserving the final, fine-grained impact fallback layer. This ~30 cm thick layer contains in the top 10 cm “accretionary” lapilli, microtektite-like glass spherules, and shocked quartz grains. Glass particles—mostly of splash form less than 1 mm size—make up the bulk of the grains (~70–78% by number) in the coarser size fraction (>125 m) of the top of the fallback layer. About one-third of all quartz grains in the uppermost part of the layer are shocked, with planar deformation features (PDFs); almost half of these grains are highly shocked, with 3 or more sets of PDFs. K-feldspar grains also occur and some show shock deformation. The abundance of shocked quartz grains and the average shock level as indicated by the number of sets of PDFs, for both quartz and K-feldspar, decrease with depth into the layer. The well-preserved glass spherules and fragments are chemically rather homogeneous within each particle, and also show relatively small variations between the various particles. On average, the composition of the fallback spherules from core LB-5B is very similar to the composition of Ivory Coast tektites and microtektites, with the exception of CaO contents, which are about 1.5 to 2 times higher in the fallback spherules. This is a rare case in which the uppermost fallback layer and the transition to the post-impact sediments has been preserved in an impact structure; its presence indicates that the impactite sequence at Bosumtwi is complete and that Bosumtwi is a very well-preserved impact crater. INTRODUCTION AND GEOLOGICAL SETTING The Bosumtwi impact structure in south-central Ghana is associated with the Ivory Coast strewn field, one of the four known tektite strewn fields (e.g., Koeberl et al. 1997). Bosumtwi is a well-preserved complex impact structure (centered at 06°30 N, 01°25 W) and is situated about 32 km southeast of Kumasi, the capital of the Ashanti region of Ghana. The crater has a distinct, steep rim with elevations of up to 300 m above present-day lake level and is almost completely filled by Lake Bosumtwi, which is 8 km in diameter. The crater is surrounded by a slight and irregular circular depression, as well as an outer ring of minor topographic highs with a diameter of about 20 km (Jones et al. 1981; Reimold et al. 1998; Wagner et al. 2001). The Bosumtwi impact crater was excavated in lower greenschist facies metasediments (metagraywacke, quartzitic metagraywacke, metatuffs, phyllites, shales, and schists) of the 2.1–2.2 Gyr old Birimian Supergroup. Rocks to the southeast of the crater contain altered basic intrusives (Birimian metavolcanics) in addition to metasediments. Clastic Tarkwaian sediments occur further to the east and southeast, and are thought to have been formed by the erosion of Birimian rocks. A detailed review, describing all aspects of Bosumtwi, and a new geological map were recently published by Koeberl and Reimold (2005). The Bosumtwi structure is one of the best-preserved 710 C. Koeberl et al. terrestrial meteorite impact structures, with proximal ejecta in the form of suevite and other impact breccia deposited outside the crater rim, to a distance of about 1 crater radius. The crater is particularly important because of its association with the Ivory Coast tektites, which were first reported by Lacroix (1934) from an 80 km wide area in Ivory Coast (Côte d’Ivoire) territory. Microtektites were found in deep-sea cores off the coast of West Africa (Glass 1968, 1969) and related to the tektites found on land. These microtektites are up to one millimeter in size and show a variety of shapes, mostly spherical shapes, droplets, tear-drops, dumbbells, and fragments of particles of these respective shapes. The geographical distribution of microtektite-bearing deep-sea cores has been used to determine the extent of the strewn field (e.g., Glass and Zwart 1979; Glass et al. 1979, 1991), and the microtektite abundance (number/cm2) and size distribution was used to accurately predict the size of the source crater located at Lake Bosumtwi (Glass and Pizzuto 1994; Glass et al. 1991). Tektites are now known to have formed during hypervelocity impacts on Earth and to represent melts of surficial, predominantly sedimentary, precursor rocks of upper crustal composition (see, e.g., Koeberl 1994; Montanari and Koeberl 2000, and references therein). Several lines of arguments were used to conclude that Bosumtwi is most likely the source crater of the Ivory Coast tektite strewn field. These include similar chemical compositions (Schnetzler et al. 1967; Jones 1985) and similar isotopic characteristics of the tektites and rocks found at the crater (e.g., Schnetzler et al. 1966; Lippolt and Wasserburg 1966; Shaw and Wasserburg 1982; Koeberl et al. 1998), as well as the similar ages of tektites and Bosumtwi impact glasses (e.g., Gentner et al. 1964; Storzer and Wagner 1977). Precise fission track and 40Ar-39Ar step-heating dating on both Ivory Coast tektites and Bosumtwi impact glass established a reliable age of 1.07 ± 0.05 Myr for the Bosumtwi impact event and the tektites (Koeberl et al. 1997), and the magnetostratigraphically determined age of the Ivory Coast microtektite layer also agrees with the age of the Bosumtwi crater, providing a firm basis for the link between the Bosumtwi impact and the tektite-forming event.