The Bosumtwi meteorite impact structure, Ghana: A magnetic model

Abstract

Abstract-A magnetic model is proposed for the Bosumtwi meteorite impact structure in Ghana, Africa. This relatively young (-1.07 Ma) structure with a diameter of -10.5 km is exposed within early Proterozoic Birimian-Tarkwaian rocks. The central part of the structure is buried under postimpact lake sediments, and because of lack of drill cores, geophysics is the only way to reveal its internal structure. To study the structure below and beyond the lake, a high-resolution, low altitude (-70 m) airborne geophysical survey across the structure was conducted, which included measurements of the total magnetic field, electromagnetic data, and gamma radiation. The magnetic data show a circumferential magnetic halo outside the lakeshore, -12 km in diameter. The central-north part of the lake reveals a central negative magnetic anomaly with smaller positive side-anomalies north and south of it, which is typical for magnetized bodies at shallow latitudes. A few weaker negative magnetic anomalies exist in the eastern and western part of the lake. Together with the northern one, they seem to encircle a central uplift. Our model shows that the magnetic anomaly of the structure is presumably produced by one or several relatively strongly remanently magnetized impact-melt rock or melt-rich suevite bodies.Petrophysical measurements show a clear difference between the physical properties of preimpact target rocks and impactites. Suevites have a higher magnetization and have low densities and high porosities compared to the target rocks. In suevites, the remanent magnetization dominates over induced magnetization (Koenigsberger ratio > 3). Preliminary palaeomagnetic results reveal that the normally magnetized remanence component in suevites was acquired during the Jaramillo normal polarity epoch. This interpretation is consistent with the modelling results that also require a normal polarity magnetization for the magnetic body beneath the lake. The reverse polarity remanence component, superimposed on the normal component, is probably acquired during subsequent reverse polarity events.

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

“…Detailed geophysical studies paved the way for the drillings (Plado et al 2000;Karp et al 2002;Scholtz et al 2002;Pesonen et al 2003). Several cores (16) were drilled in the lake sediments to extract paleolimnological and paleoclimatic data to document the environmental changes taking place over the last million years in this key region affected by West African monsoon.…”

Magnetic fields of lunar multi‐ring impact basins

“…These range from short-wavelength anomalies with a radial extent of a fraction of the transient cavity radius (e.g., those in the Haughton, Manicougan, and Lake St. Martin basins (Pohl et al 1988;Clark 1978, 1982)), to larger groups of anomalies that fill most of the transient cavity (e.g. the outer ring of magnetic anomalies in the Chicxulub basin (Pilkington and Hildebrand 1994) or the central anomaly in the Bosumtwi crater (Plado et al 2000)). The more localized anomalies have generally been ascribed to shock remanent magnetization (SRM) or chemical remanent magnetization (which should not be a factor in the highly reducing lunar environment) in the unmelted central uplift region, while anomalies that roughly fill the transient cavity region have generally been interpreted as thermal remanent magnetization (TRM) in melt-rich suevite breccias and/or impact melt rocks.…”

Search for a meteoritic component in drill cores from the Bosumtwi impact structure, Ghana: Platinum group element contents and osmium isotopic characteristics

“…Plado et al 2000;Pesonen et al 2003), undertook a shallow drilling program outside the northern crater rim of Bosumtwi. Seven holes were drilled to the north of the crater to a maximum depth of 30 m and at a distance of 2.5-8 km from the lake shore (see Boamah and Koeberl 2002.…”