Remote sensing, field studies, petrography, and geochemistry of rocks in central Zambia: no evidence of a meteoritic impact in the area of the Lukanga Swamp

Abstract

The Lukanga Swamp in central Zambia has previously been proposed as the site of a large (52 km diameter) impact structure on the basis of alleged observation of shock-diagnostic planar deformation features (PDFs) in quartz of breccia from the southern margin of the swamp. The southern margin of the swamp, marked by the Nyama Dislocation Zone, consists of quartzite and silicified meta-siltstone, shale, sedimentary quartz breccia, and fault breccia. Structures and textures in the meta-sediments are syn-sedimentary, whereas fault breccia displays tectonic fabrics. In thin section, quartz in sedimentary quartz breccia displays widely spaced, randomly oriented, subparallel, non-planar fluid inclusion trails, which were earlier misidentified to represent decorated PDFs formed by meteoritic impact. Siderophile element abundances (<105 ppm) in the rocks from around the swamp are normal for quartz-rich crustal rocks, and there are no relative enrichments in the breccias compared to other rocks to suggest extraterrestrial contributions. Aeromagnetic data do not reflect an impact crater signature. We analyzed regional structural and seismic data in an effort to account for the development of the swamp. From the structural synthesis it appears that reactivation of movements along the Nyama and Kapiri-Mposhi Dislocation Zones may have led to the development of the Lukanga Swamp, probably during the Cenozoic era.