IMPACT-INDUCED SHATTER CONES OR PERCUSSION MARKS ON QUARTZITES OF THEWITWATERSRAND AND TRANSVAAL SUPERGROUPS

Crescentic fractures resembling impact-produced shatter cones occur in stream-bed and palaeo-channel exposures on quartzites of the Klapperk op Member (Timeball Hill Formation, Transvaal Supergroup) in the Mpuma langa Province and the Orange Grove Quartzite Formation (Witwatersrand Supergroup) from the southeastern sector of the Vredefort Dome. They are described and compared with percussion marks resulting from boulde r impact during rapid flow in a fluvial setting. No impact-related sha tter cones were observed in the Orange Grove Quartzite and other litho logies from this portion of the collar around the Vredefort Dome. Furt hermore, evidence for sedimentary palaeo-environments in the immediate vicinity of this cone-fractured exposure strongly suggests that these particular cone fractures in the Vredefort impact structure formed by boulder impact in an ancient river bed, rather than as a result of me teorite impact-induced shock deformation. An origin by boulder impact must also be favoured for the cone fractures in stream-bed exposures o f Klapperkop Quartzite along the southeastern perimeter of the Bushvel d Complex, in the Mpumalanga Province. To date no concrete evidence in favour of an impact origin of the Bushveld Complex has been recorded. Our observations draw attention to a degree of similarity between met eorite impact-produced shatter cones and percussion marks formed by bo ulder impacts during fluvial transport. This also highlights the need to establish definitive criteria for the distinction between percussio n marks and shatter cones.