EXTREME DISTAL TRANSPORT OF DETRITAL SHOCKED ZIRCONS IN THE ORANGE RIVER, SOUTH AFRICA: 2000 KM FROM THE VREDEFORT DOME TO THE ATLANTIC COAST

High pressure deformation from meteorite impact produces shocked minerals in target rocks, that when eroded, create sediments that retain diagnostic impact deformation microstructures [1]. Detrital shocked minerals, such as zircon, can be transported long distances by fluvial processes. Shocked zircons have been documented in modern alluvium from the Vaal River, >750 km downriver from the Vredefort Dome impact structure in South Africa [2]. Here we investigate extreme distal transport of detrital shocked zircons in the Orange River, the largest fluvial system in South Africa.

Alluvium samples from the Orange River (n=5) and beach sand from the Atlantic coast of South Africa (n=3) were surveyed to search for Vredefort-derived shocked zircons. Heavy minerals were concentrated using heavy liquids and a Frantz magnetic separator. A total of 9,136 zircons were handpicked and mounted on SEM stubs for BSE analysis of exterior grain surfaces to search for diagnostic shock microstructures. Preliminary results confirm the presence of two shocked zircons in beach sand at the Orange River mouth on the Atlantic coast (2/2745 grains, 0.07 %), and one shocked zircon (1/3174 grains, 0.03%) from a sandbar in the Orange River, 15 km upriver from the Atlantic coast. The shocked zircons were identified by the presence of diagnostic planar fractures.

No shocked grains were found in 1502 zircons from 3 beach sand samples collected south of the Orange River mouth (0/170, 0/549, and 0/783), precluding an origin from long shore drift. These results support an initial report that shocked zircons survive ~2000 km of transport in the Vaal-Orange river system [3]. Moreover, the confirmation of a shocked zircon 15 km upriver from the Atlantic coast unequivocally demonstrates their transport to the coast in the Orange River.

Support for this work was provided by the NSF (EAR-1145118).

References: [1] Cavosie et al., 2010 GSA Bulletin. [2] Erickson et al., 2013 GCA. [3] Erickson et al., 2012 LPSC.