The Lake Bosumtwi meteorite impact structure, Ghana—A magnetic image from a third observational level

Self Cite

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: Geophysical Results From Drilling and Loggingsupporting

confidence: 88%

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

Self Cite

102

“…Thus, the same effect can be accomplished by reducing the intensity of NRM and increasing susceptibility or vice-versa, but reasonable limits on the amount of each are imposed by magnetic mineralogy. In our case, the higher values of magnetic susceptibility required by the model mean that the effective susceptibility (k eff = k(1 + Q), where k is the magnetic susceptibility, Q is the Koenigsberger ratio of remanent to induced magnetization, and k eff is the effective susceptibility), is high and therefore our model is consistent with the high values of remanent magnetization observed by Plado et al (2000), Kontny et al (2007), Morris et al (2007), and the improved 3-D model by Ugalde et al (2007a).…”

Section: Magnetic Modeling Of the Cratersupporting

confidence: 88%

Results of pre‐drilling potential field measurements at the Bosumtwi crater

Danuor

Menyeh

2007

Abstract— Gravity and magnetic measurements were carried out at the Bosumtwi crater to determine the geophysical signature of the crater. Land gravity data was acquired at 163 locations around the structure and on the shore of the lake. The separation between the gravity stations was 500 m for radial profiles, but 700–1000 m along roads and footpaths that ran parallel to the lake's shore. Additionally, a marine gravity survey was carried out along 14 north‐south and 15 east‐west profiles on the lake. Magnetic data was also acquired along 14 north‐south profiles on the lake. In all marine surveys, the line spacing was 800 m, and navigation was provided by a Garmin 235 Echo Sounder/GPS. The gravity signature of the crater is characterized by a negative Bouguer anomaly with an amplitude of about −18 mgal. Using the seismic results as constraints, the gravity model obtained indicates the central uplift at a depth of 250 m. The negative anomaly is the contribution of the gravity deficiencies due to fractured and brecciated rocks in the rim area and below the crater floor, the impact breccias within the crater, and the sedimentary and water infilling of the lake. Magnetic modeling yielded a model for the causative body, which is located north of the central uplift: the model has a magnetic susceptibility of 0.03 S.I. and extends from a depth of 250 to 610 m. The causative bodies have been interpreted as impactites.

“…Our study contributes to rock magnetic and magnetic mineralogy data of the drilled lithologies, which provide data for the interpretation of the magnetic anomaly patterns and the magnetic borehole measurements (see also Elbra et al 2007; Ugalde et al 2007aUgalde et al , 2007b. Such data contribute to the understanding of the formation of magnetic minerals and the mechanisms creating and modifying magnetic properties like the remanent magnetization and magnetic susceptibility during impact-related processes.…”

Section: Introductionmentioning

confidence: 70%

Petrography and shock‐related remagnetization of pyrrhotite in drill cores from the Bosumtwi Impact Crater Drilling Project, Ghana

Kontny

Elbra

Just

et al. 2007

Abstract-Rock magnetic and magnetic mineralogy data are presented from the International Continental Scientific Drilling Program (ICDP) drill cores LB-07A and LB-08A of the Bosumtwi impact structure in order to understand the magnetic behavior of impact and target lithologies and their impact-related remagnetization mechanism. Basic data for the interpretation of the magnetic anomaly patterns and the magnetic borehole measurements as well as for new magnetic modeling are provided. Magnetic susceptibility (150-500 × 10 −6 SI) and natural remanent magnetization (10 −3 -10 −1 A/m) are generally weak, but locally higher values up to 10.6 × 10 −3 SI and 43 A/m occur. Sixty-three percent of the investigated rock specimens show Q values above 1 indicating that remanence clearly dominates over induced magnetization, which is a typical feature of impact structures. Ferrimagnetic pyrrhotite is the main magnetite phase, which occurs besides minor magnetite and a magnetic phase with a Curie temperature between 330 and 350 °C, interpreted as anomalous pyrrhotite. Coercive forces are between 20 and 40 mT. Brecciation and fracturing of pyrrhotite is a common feature confirming its pre-impact origin. Grain sizes of pyrrhotite show a large variation but the numerous stress-induced nanostructures observable by transmission electron microscopy (TEM) are assumed to behave as single-domain grains. We suggest that the drilled rocks lost their pre-shock remanence memory during the shock event and acquired a new, stable remanence during shock-induced grain size reduction. The observed brittle microstructures indicate temperatures not higher than 250 °C, which is below the Curie temperature of ferrimagnetic pyrrhotite (310 °C).