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

Self Cite

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.

Section: Discussionsupporting

confidence: 52%

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

Ugalde

Morris

Pesonen

et al. 2007

Self Cite

“…9). Recent integrated analysis of gravity and borehole petrophysical data and modeling has shown laterally varying physical properties at Bosumtwi, which are related to impact cratering processes (Ugalde et al 2005(Ugalde et al , 2007. Our reconstruction results (Fig.…”

Section: Reconstruction Of the Original Crater Reliefmentioning

confidence: 74%

“…9) compacted relatively more, as it was underlain by a more porous impact breccia unit, whereas the central uplift/high and the eastern depression subsided relatively less, as they were underlain by less porous, melt-rich breccia related to structural uplift (Fig. 9) (Ugalde et al 2005(Ugalde et al , 2007.…”

Section: Reconstruction Of the Original Crater Reliefmentioning

confidence: 99%

Post‐impact structural crater modification due to sediment loading: An overlooked process

Tsikalas

Faleide

2007

Abstract-Post-impact crater morphology and structure modifications due to sediment loading are analyzed in detail and exemplified in five well-preserved impact craters: Mjølnir, Chesapeake Bay, Chicxulub, Montagnais, and Bosumtwi. The analysis demonstrates that the geometry and the structural and stratigraphic relations of post-impact strata provide information about the amplitude, the spatial distribution, and the mode of post-impact deformation. Reconstruction of the original morphology and structure for the Mjølnir, Chicxulub, and Bosumtwi craters demonstrates the long-term subsidence and differential compaction that takes place between the crater and the outside platform region, and laterally within the crater structure. At Mjølnir, the central high developed as a prominent feature during post-impact burial, the height of the peak ring was enhanced, and the cumulative throw on the rim faults was increased. The original Chicxulub crater exhibited considerably less prominent peakring and inner-ring/crater-rim features than the present crater. The original relief of the peak ring was on the order of 420-570 m (currently 535-575 m); the relief on the inner ring/crater rim was 300-450 m (currently ∼700 m). The original Bosumtwi crater exhibited a central uplift/high whose structural relief increased during burial (current height 101-110 m, in contrast to the original height of 85-110 m), whereas the surrounding western part of the annular trough was subdued more that the eastern part, exhibiting original depths of 43-68 m (currently 46 m) and 49-55 m (currently 50 m), respectively. Furthermore, a quantitative model for the porosity change caused by the Chesapeake Bay impact was developed utilizing the modeled density distribution. The model shows that, compared with the surrounding platform, the porosity increased immediately after impact up to 8.5% in the collapsed and brecciated crater center (currently +6% due to post-impact compaction). In contrast, porosity decreased by 2-3% (currently −3 to −4.5% due to post-impact compaction) in the peak-ring region. The lateral variations in porosity at Chesapeake Bay crater are compatible with similar porosity variations at Mjølnir crater, and are considered to be responsible for the moderate Chesapeake Bay gravity signature (annular low of −8 mGal instead of −15 mGal). The analysis shows that the reconstructions and the long-term alterations due to post-impact burial are closely related to the impact-disturbed target-rock volume and a brecciated region of laterally varying thickness and depthvarying physical properties. The study further shows that several crater morphological and structural parameters are prone to post-impact burial modification and are either exaggerated or subdued during post-impact burial. Preliminary correction factors are established based on the integrated reconstruction and post-impact deformation analysis. The crater morphological and structural parameters, corrected from post-impact loading and modification effects, can be used to better const...

“…Recent refraction and reflection seismic surveys Scholz et al 2002;Danuor 2004;Ugalde 2006;Ugalde et al 2007) (see traces of the seismic lines in Fig. 1) and airborne high-resolution total magnetic field, electromagnetic field, and gamma radiation measurements (Plado et al 2000;Pesonen et al 2003) helped to resolve the subsurface structure of the Bosumtwi crater in great detail (Fig.…”

Section: Introductionmentioning

confidence: 99%

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

Deutsch

Luetke

Heinrich

2007

Abstract-The 1.07 Myr old Lake Bosumtwi impact crater in Ghana was drilled within the framework of the International Continental Scientific Drilling Project (ICDP). Hole LB-08A, drilled into the outer flank of the central uplift and with a total depth of 451 m, yielded 215.71 m of impact-related rocks. This paper summarizes observations of the lithological logging on core LB-08A. Between a depth of 235.6 and ~260 m, the section consists of a melt-bearing allochthonous, polymict, and mostly clast-supported impact breccia. Down to ~418 m, the section comprises a rather uniform unit of metagraywacke alternating with phyllite to slate (lower greenschist facies); few (par-) autochthonous impact breccia bodies and rare impact dike breccias are present. The lowermost part of the section contains several centimeter-to decimeter-thick melt-bearing breccia dikes in country rocks identical to those occurring above. Omnipresent fracturing was mapped in a qualitative manner. Most prominent shock effects in the uplifted target rocks comprise planar fractures and deformation elements in quartz and polysynthetic twinning in carbonate minerals; the maximum shock pressure as evidenced by quartz is below 26 GPa. The allochthonous breccias occasionally contain a few vol% of melt particles. Suevites occur outside the crater rim, carrying diaplectic crystals, coesite, and ballen quartz as well as true melt glasses and a variety of lithic clasts, among those spectacular staurolite-rich mica-schists. The recorded shock level in the uplifted target rocks is lower than expected and modeled. Shock recovery experiments with analogue carbonaceous graywackes at 34 and 39.5 GPa yielded nearly complete transformation of quartz into diaplectic glass. We therefore exclude a specific shock behavior of the soft, fluid-rich target material (carbonaceous graywackes, shales, slates) in core LB-08A as the prime or only reason for the melt deficit and the generally low shock levels recorded inside the Lake Bosumtwi impact crater.