PRESERVATION OF VREDEFORT-DERIVED SHOCKED MINERALS IN 300 MA DWYKA GROUP TILLITE, SOUTH AFRICA

Shocked minerals eroded from impact structures provide an important record of ancient impact events. Microstructures such as planar fractures (PFs) in zircon and planar deformation features (PDFs) in quartz can be preserved for billions of years, despite erosion and tectonic activity. To date, shocked minerals have been documented in modern sediments near the Vredefort and Sudbury impact structures (Cavosie et al., 2010; Thomson et al., 2014), as well as in distal alluvium transported by the Vaal River >750 km from the Vredefort Dome (Erickson et al., 2013). The presence of shocked minerals in fluvial deposits indicates that sediments deposited by other transport mechanisms could also preserve records of impact. Here we report the occurrence of shocked minerals eroded from the 2.02 Ga Vredefort impact structure in glacial tillite of the 300 Ma Dwyka Group in the Karoo Basin, South Africa.

Ten tillite samples were collected from sites 15-30 km from the center of the Vredefort impact structure. These samples, from the collar and inner core region, are poorly indurated. One well-lithified diamictite was collected outside (SW) of the impact structure, 40 km from the center. Grain exteriors of 100 zircons from all but one tillite sample were surveyed by SEM (SE, BSE). Shocked zircons were identified in all samples, ranging from 9% (diamictite) to 63% (sand-rich tillite, core). Shocked zircons are characterized by 1 to >4 orientations of PFs. 100% of monazite grains from tillite in the core (n=21) were shocked, preserving up to 3 orientations of planar features. In addition to detrital shocked zircon and monazite, petrographic analysis reveals that shocked quartz is present in all samples, including the diamictite from outside the impact structure. The shocked quartz grains contain up to two orientations of decorated PDFs, consistent with quartz from bedrock at the Vredefort Dome.

These results from the Paleozoic Dwyka Group are the first report showing that detrital shocked minerals can be preserved in ancient lithified sediments. One implication of these findings is the possibility that older glacial tillite deposits in the geologic record may contain shocked minerals dispersed from ancient impact craters long since eroded.

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