The ICDP Lake Bosumtwi impact crater scientific drilling project (Ghana): Core LB‐08A litho‐log, related ejecta, and shock recovery experiments

Abstract-In an attempt to identify the type of projectile, 14 samples from the Bosumtwi crater in Ghana were analyzed for platinum group element (PGE) concentrations by nickel sulfide fire assay inductively coupled plasma-mass spectrometry (ICP-MS). The majority of the samples come from the impactite material recovered by cores LB-07A and LB-08A, which were drilled by the International Continental Scientific Drilling program (ICDP). One sample originates from the fallback material found at the contact between the impactite and the overlying lake sediment in core LB-05B. No clear signature of a meteoritic contamination was identified in the 13 impactite samples. The target rock apparently dominates the PGE contribution in the impactites. These results agree with the PGE concentrations reported for the suevites collected at the crater rim and in other parts of the Bosumtwi ICDP cores. However, based on Cr and Os isotopic signatures, a meteoritic component could be present in the sample of fallback material, supporting the reports of the existence of meteoritic material in the Ivory Coast tektites. Further analyses of the fallback material from the Bosumtwi drill cores should confirm (or not) this first result.

Section: Impactite Samples From Lb-07a and Lb-08a And Methodologymentioning

confidence: 99%

“…1) of these two cores has been established by Koeberl et al (2007aKoeberl et al ( , 2007b. The petrography and shock characteristics of the impactites are described in detail in accompanying papers (Deutsch et al 2007;Ferrière et al 2007aFerrière et al , 2007bKarikari et al 2007;Morrow 2007).…”

Section: Introductionmentioning

confidence: 99%

Platinum group elements provide no indication of a meteoritic component in ICDP cores from the Bosumtwi crater, Ghana

Goderis

Tagle

Schmitt

et al. 2007

“…Therefore, the more realistic approach of stacking the thin sheets one below the other, leaving some space in between them, would first require more space (>1000 m, taking into account 1000 m for the 200 thin sheets, plus 60-100 m of host rock considering 0.5-1 m between each sheet), and secondly the computed magnetic response of such a geometry would still be smaller than the observed field. Furthermore, this alternate model would extend from 200 to 1200 m depth, which is not feasible according to the borehole logs (Coney et al 2007;Deutsch et al 2007;Ferrière et al 2007), and also would mean a block of 4000 × 4000 × 1000 m of highly magnetic material, which was not encountered by the deep ICDP boreholes.…”

Section: New Magnetic Modeling Synthetic Modelingmentioning

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.

“…A detailed description of the geology is given by Koeberl and Reimold (2005). Lithological descriptions of the rocks recovered by the ICDP deep drilling are given by Coney et al (2007), Ferrière et al (2007), Deutsch et al (2007), and Koeberl et al (2007).…”

Section: Regional Geologymentioning

confidence: 99%

Integrated 3‐D model from gravity and petrophysical data at the Bosumtwi impact structure, Ghana

Ugalde

Danuor

Milkereit

2007

Abstract-The Bosumtwi impact structure of central Ghana was drilled in 2004 as part of the International Continental Scientific Drilling Program (ICDP). A vast amount of geoscience data is available from the pre-site surveys and the actual drilling phase. A 3-D gravity model was constructed and calibrated with the available data from the two ICDP boreholes, LB-07A and LB-08A. The 3-D gravity model results agree well with both the sediment thickness and size of the central uplift revealed by previously collected seismic data, and with the petrophysical data from the LB-08A and LB-07A core materials and the two borehole logs. Furthermore, the model exhibits lateral density variations across the structure and refines the results from previous 2.5-D modeling. An important new element of the 3-D model is that the thickness of the intervals comprising polymict lithic impact breccia and suevite, monomict lithic breccia and fractured basement is much smaller than that predicted by numerical modeling.