Rubidium-Strontium Age of the Bosumtwi Crater Area, Ghana, Compared with the Age of the Ivory Coast Tektites

102

Abstract-The Bosumtwi impact crater in Ghana, arguably the best-preserved complex young impact structure known on Earth, displays a pronounced rim and is almost completely filled by Lake Bosumtwi, a hydrologically closed basin. It is the source crater of the Ivory Coast tektites. The structure was excavated in 2.1-2.2 Gyr old metasediments and metavolcanics of the Birimian Supergroup. A drilling project was conceived that would combine two major scientific interests in this crater: 1) to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago, at a near-equatorial location in Africa for which very few data are available so far, and 2) to obtain a complete record of impactites at the central uplift and in the crater moat, for ground truthing and comparison with other structures.Within the framework of an international and multidisciplinary drilling project led by the International Continental Scientific Drilling Program (ICDP), 16 drill cores were obtained from June to October 2004 at six locations within Lake Bosumtwi, which is 8.5 km in diameter. The 14 sediment cores are currently being investigated for paleoenvironmental indicators. The two impactite cores LB-07A and LB-08A were drilled into the deepest section of the annular moat (540 m) and the flank of the central uplift (450 m), respectively. They are the main subject of this special issue of Meteoritics & Planetary Science, which represents the first detailed presentations of results from the deep drilling into the Bosumtwi impactite sequence. Drilling progressed in both cases through the impact breccia layer into fractured bedrock. LB-07A comprises lithic (in the uppermost part) and suevitic impact breccias with appreciable amounts of impact melt fragments. The lithic clast content is dominated by graywacke, besides various metapelites, quartzite, and a carbonate target component. Shock deformation in the form of quartz grains with planar microdeformations is abundant. First chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and Mg, but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values. Core LB-08A comprises suevitic breccia in the uppermost part, followed with depth by a thick sequence of graywacke-dominated metasediment with suevite and a few granitoid dike intercalations. It is assumed that the metasediment package represents bedrock intersected in the 484 C. Koeberl et al.

Section: Brief Summary Of the Geology Of Bosumtwimentioning

confidence: 99%

An international and multidisciplinary drilling project into a young complex impact structure: The 2004 ICDP Bosumtwi Crater Drilling Project—An overview

Koeberl

Milkereit

Overpeck

et al. 2007

102

“…Early work describing the formation of the Bosumtwi structure proposed both cryptoexplosive (Rohleder 1936;Junner 1937) and meteorite impact (MacLaren 1931) mechanisms for its origin. A large body of subsequent geological, geophysical, and geochemical work has clearly demonstrated its impact origin (e.g., Littler et al 1961;El Goresy 1966;Lippolt and Wasserburg 1966;Schnetzler et al 1966Schnetzler et al , 1967Kolbe et al 1967;Chao 1968;Jones et al 1981;Jones 1985aJones , 1985bKoeberl et al 1998;Reimold et al 1998;Plado et al 2000;Boamah and Koeberl 2002Scholz et al 2002;Wagner et al 2002;Pesonen et al 2003;Artemieva et al 2004;Dai et al 2005; multiple sources in this issue).…”

Section: Introductionmentioning

confidence: 99%

“…1) (Barnes 1961;Lippolt and Wasserburg 1966;Schnetzler et al 1966Schnetzler et al , 1967Glass 1969;Durrani and Khan 1971;Glass and Zwart 1979;Jones 1985a;Koeberl et al 1996aKoeberl et al , 1997Koeberl et al , 1998Dai et al 2005). Remote sensing, geophysical, and drill core studies demonstrate that the Bosumtwi crater, which is buried beneath up to 294 m of post-impact lacustrine sediments, is a complex impact structure characterized by an annular moat filled with over 135 m of lithic and suevitic impact breccia and by a 130 m high, 1.9 km wide, faulted central uplift overlain by about 25 m of melt-bearing impact-related units ( Fig.…”

Section: Introductionmentioning

confidence: 99%

Shock‐metamorphic petrography and microRaman spectroscopy of quartz in upper impactite interval, ICDP drill core LB‐07A, Bosumtwi impact crater, Ghana

Morrow

2007

Abstract-Standard and universal stage optical microscope and microRaman spectroscopic examination of quartz from the upper impactite interval of the International Continental Scientific Drilling Program (ICDP) Lake Bosumtwi crater drill core LB-07A demonstrates widespread but heterogeneous evidence of shock metamorphism. In the upper impactite, which comprises interbedded polymict lithic breccia and suevite from a drilling depth of 333.4-415.7 m, quartz occurs as a major component within metasedimentary lithic clasts and as abundant, isolated, single-crystal grains within matrix. The noted quartz shock-metamorphic features include phenomena related to a) deformation, such as abundant planar microstructures, grain mosaicism, and reduced birefringence; b) phase transformations, such as rare diaplectic quartz glass and very rare coesite; c) melting, such as isolated, colorless to dark, glassy and devitrified vesicular melt grains; and d) secondary, post-shock features such as abundant, variable decoration of planar microstructures and patchy grain toasting. Common to abundant planar deformation features (PDFs) in quartz are dominated by -equivalent crystallographic planes, although significant percentages of and other higher index orientations also occur; notably, c(0001) planes are rare. Significantly, the quartz PDF orientations match most closely those reported elsewhere from strongly shocked, crystalline-target impactites. Barometry estimates based on quartz alteration in the upper impactite indicate that shock pressures in excess of 20 GPa were widely reached; pressures exceeding 40-45 GPa were more rare. The relatively high abundances of decorated planar microstructures and grain toasting in shocked quartz, together with the nature and distribution of melt within suevite, suggest a water-or volatile-rich target for the Bosumtwi impact event.

“…Once thought of as a volcanic caldera (Junner 1937), a number of geochemical tests and the presence of planar deformation features (e.g., Koeberl 1994b; Koeberl and Reimold 2005;Schnetzler et al 1966Schnetzler et al , 1967 have confirmed that the crater is an impact structure. The crater has a complex form with a slightly pronounced outer ring 20 km in diameter surrounding the crater rim, which is 10.5 km in diameter, and with a central uplift 1.5 km in diameter .…”

Section: Introductionmentioning

confidence: 99%

Hydrothermal alteration in the Bosumtwi impact structure: Evidence from 2M₁‐muscovite, alteration veins, and fracture fillings

Petersen

Newsom

Nelson

et al. 2007

Abstract-Drill-core samples from the Bosumtwi impact structure (1.07 Myr old and 10.5 km in diameter) in Ghana exhibit mineralogical evidence for post-impact hydrothermal alteration. Nine samples of drill core obtained through the 2004 International Continental Scientific Drilling Project (ICDP) were studied, including an uppermost fallback layer overlying impactite breccias, and partly deformed massive meta-graywacke bedrock. The petrographic study revealed alteration veins containing secondary sericitic muscovite (comparable to 2M 1 -muscovite) crosscutting original bedding in meta-graywacke and forming a matrix between clasts in impactite breccias. X-ray diffraction (XRD) shows that these impactite samples are rich in 2M 1 -muscovite, consistent with post-impact fluid deposition and alteration. Optical analysis indicates the presence of a pre-impact stratiform chlorite in meta-graywacke samples and a secondary alteration chlorite occurring in all samples. Secondary illite was detected in upper impactites of drill core LB-08A and samples containing accretionary lapilli. The lower temperature constraint for the hydrothermal event is given by 2M 1 -muscovite, secondary chlorite, and illite, all of which form at temperatures greater than 280°C. An absence of recrystallization of quartz and feldspar indicates an upper temperature constraint below 900 °C. The presence of alteration materials associated with fractures and veins in the uppermost impactites of drill cores LB-07A and LB-08A indicates that a post-impact hydrothermal system was present in and adjacent to the central uplift portion of the Bosumtwi impact structure. A sample containing accretionary lapilli obtained from drill core LB-05A exhibits limited evidence that hydrothermal processes were more widespread within the impactites on the crater floor.