EVIDENCE OF DETRITAL SHOCKED ZIRCONS IN CENOZOIC FLUVIAL TERRACES FROM THE VAAL AND ORANGE RIVERS, SOUTH AFRICA

Provenance studies have used microstructural and isotopic data to demonstrate that detrital shocked minerals in modern sediments in the Vaal River, South Africa originate from the Vredefort Dome impact structure. Detrital shocked minerals have also been reported in Pleistocene fluvial deposits (river terraces) of the Vaal River at distances up to 500 km from the Vredefort Dome (Cintron et al., 2011). Here we report occurrences of detrital shocked zircons in Pleistocene and Pliocene fluvial deposits at distal locations in the Vaal and Orange Rivers that extend the record of detrital shocked minerals eroded from the Vredefort Dome further across southern Africa.

The Vredefort Dome is the oldest and largest impact structure on Earth with an age of 2.02 Ga. The structure, located in the Kaapval craton, is believed to have a 300 km original diameter, but only 90 km diameter is presently exposed. Shocked minerals have been reported in a variety of rocks at the Vredefort Dome, in modern sediments within the structure (Cavosie et al., 2010) and at distal locations in the Vaal River (Erickson et al. 2013 GCA). In this study, we report detrital shocked zircons in a Pleistocene age Vaal River terrace, the Rietputs Fm., near the confluence with the Orange River at Douglas, 760 km downriver from the Vredefort Dome. In addition, we report the first detrital shocked mineral from the Orange River, found in a Pliocene fluvial terrace ~40 km downstream from the Vaal confluence, approximately 800 km from the Vredefort Dome.

Impact-generated planar fracture (PF) deformation microstructures were documented using backscattered electron (BSE) imaging with a scanning electron microscope (SEM); detrital shocked zircons with closely-spaced PFs visible on exterior grain surfaces were identified in each of the two samples. The detrital shocked zircons are medium-sized (up to ~500 μm), subhedral to anhedral, and many are rounded grains with no identifiable crystal faces. The lowest abundance of shocked zircons occurred in the Pliocene terrace on the Orange River, which yielded a single shocked zircon (1/154). The highest abundance was found in the Rietputs Fm. at Douglas, where 8 of 77 grains (10%) were shocked. These results further demonstrate the longevity of detrital shocked minerals in a long-lived cratonic fluvial system over time.