Petrophysical and paleomagnetic data of drill cores from the Bosumtwi impact structure, Ghana

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).

Section: Introductionmentioning

confidence: 70%

“…Surprisingly, melt rocks are only represented by the suevite unit in drill core LB-07A and some suevite dikes in drill core LB-08A (see Koeberl et al 2007). Furthermore, pyrrhotite rather than Fe oxides is the main carrier of the magnetic properties (this work; Elbra et al 2007).…”

Section: Introductionmentioning

confidence: 93%

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

Kontny

Elbra

Just

et al. 2007

“…Although the drilling yielded unequivocal evidence of the three anticipated upper layers (Coney et al 2007;Ferrière et al 2007), it did not penetrate the hypothesized highly magnetic body in the structure. Instead, drilling went through a weakly magnetic breccia with only minor amounts of impact-melt fragments (Coney et al 2007;Ferrière et al 2007;Elbra et al 2007;Kontny et al 2007;Morris et al 2007). The borehole magnetic logging revealed distinct shortwavelength magnetic anomalies in the bottom part (Morris et al 2007).…”

Section: Introductionmentioning

confidence: 99%

The Lake Bosumtwi meteorite impact structure, Ghana—Where is the magnetic source?

Ugalde

Morris

Pesonen

et al. 2007

Abstract-The Bosumtwi impact structure (Ghana) is a young and well-preserved structure where a vast amount of information is available to constrain any geophysical model. Previous analysis of the airborne magnetic data and results of numerical simulation of impact predicted a strongly magnetic impact-melt body underneath the lake. Recent drilling through the structure did not penetrate such an expected impact-melt rock magnetic source. A new 3-D magnetic model for the structure was constructed based on a newly acquired higher-resolution marine magnetic data set, with consideration of the observed gravity data on the lake, previous seismic models, and the magnetic properties and lithology identified in the two International Continental Scientific Drilling Program (ICDP) deep boreholes. The new model contains highly magnetic bodies located in the northeast sector of the structure, not centered onto the drilling sites. As in previous models, higher magnetization than that measured in outcropping impactites had to be assigned to the unexposed source bodies. Integration of the new model with the borehole petrophysics and published geology indicates that these bodies likely correspond to an extension to the south of the Kumasi batholith, which outcrops to the northeast of the structure. The possibility that these source bodies are related to the seismically identified central uplift or to an unmapped impact-melt sheet predicted by previous models of the structure is not supported. Detailed magnetic scanning of the Kumasi batholith to the north, and the Bansu intrusion to the south, would provide a test for this interpretation.

“…However, it was possible to extend the data set due to an exchange of data between the working groups at the University of Leoben, Austria; the Division of Geophysics, Department of Physical Sciences, University of Helsinki, Finland (see also Elbra et al 2007); and the GeologischPaläontologisches Institut, Ruprecht-Karls-Universität Heidelberg, Germany (see also Kontny et al 2007). As the three groups used similar methods but examined different features of the magnetic properties, the data exchange was important.…”

Section: Sampling and Laboratory Investigationsmentioning

confidence: 99%

“…Bulk density and porosity were obtained as standard petrophysical parameters and are reported in Elbra et al (2007). Unfortunately, the electrical and elastic properties could not be measured for all of the samples.…”

Section: Sampling and Laboratory Investigationsmentioning

confidence: 99%

Characterization of the log lithology of cores LB‐07A and LB‐08A of the Bosumtwi impact structure by using the anisotropy of magnetic susceptibility

Schell

Schleifer

Elbra

2007

Abstract-Petrophysical data are commonly used for the discrimination of different lithologies, as the variation in mineralogy, texture, and porosity is accompanied by varying physical properties. A special field of investigation is the analysis of the directional dependence (anisotropy) of the petrophysical properties, which can provide further information on the characteristics of the lithologies, due to the fact that this parameter is different in the various rock-forming and rockchanging processes, e.g., deformation or sedimentation.To characterize the rocks in drill cores LB-07A and LB-08A, which were drilled into the deep crater moat and central uplift of the Bosumtwi impact structure, Ghana, samples were taken for the study of petrophysical properties. In the present work the magnetic properties of these samples were determined in the laboratory. The results are discussed in relation to the various lithologies represented by this sample suite.The shape and degree of magnetic anisotropy, in combination with the magnetic susceptibility, proved useful in distinguishing between the different lithologies present in the drill cores (polymict lithic breccia, suevite, shale component, and meta-graywacke). It was possible to correlate layers of high (shale component), intermediate (graywacke, polymict lithic breccia), and low (suevite) anisotropy degree with the lithostratigraphic sequences determined for cores LB-07A and LB-08A. The shape of the anisotropy showed that foliation is most dominant within the shale component, whereas lineation is more pronounced in the meta-graywacke and polymict lithic breccia. An overall increase of the anisotropy degree was observed from core LB-07A towards core LB-08A. Thus magnetic anisotropy data provide a useful contribution towards an improved petrophysical characterization of the lithostratigraphic sequences in drillcores from the Bosumtwi impact structure.