DETRITAL SHOCKED ZIRCONS ERODED FROM THE 1.85 GA SUDBURY IMPACT STRUCTURE IN MODERN FLUVIAL AND GLACIAL SAND DEPOSITS

The presence of shock-metamorphosed minerals is a diagnostic feature used to confirm impact structures. Planar fractures (PFs) are a unique shock deformation feature in zircon, created by the shock wave passing through the target rocks at the impact site. Zircon is known for its resistance to chemical and mechanical weathering and its stability at high and low temperatures. In addition to bedrock studies, PFs have been documented in detrital zircons eroded from the 2.0 Ga Vredefort Dome in South Africa (Cavosie et al., 2010; Erickson et al., 2010).

We report detrital shocked zircons from diverse sedimentary environments within and around the 1.85 Ga Sudbury impact structure (Canada), including modern fluvial systems and Holocene glacial deposits (eskers and deltaic sands). Detrital shocked zircons were discovered in all main tributaries of the Vermillion River, a major fluvial system flowing southwest across the Sudbury structure. Vermillion tributaries include the Onaping, Rapid, Nelson, Whitson and Sandcherry Creek Rivers, and each tributary crosses the Levack Gneiss and/or Onaping Fm., rocks previously reported to contain shocked zircon. Glacial deposits (eskers and outwash deltas) occur across the entire Sudbury basin in orientations consistent with southwest-directed ice flow.

Petrographic (TL, RL) and SEM techniques (SE, BSE, CL, EDS, EBSD) were used to document shock microstructures on the exterior surfaces and interior polished surfaces of grains. Detrital shocked zircons were found in 21 samples, including 6 modern rivers, 5 deltas and 3 eskers. Of 3,978 zircons surveyed, PFs were identified in 119 (3%) grains; the highest percentage of shocked zircons was found in an esker sample (13/113, 11.5%). EBSD analysis reveals impact microtwins in 4 of 6 zircons analyzed. Zircon 46-72 (a Holocene delta) has 3 well preserved orientations of microtwins (Cavosie et al., this volume), the most known thus far from a single grain. These results are the first discovery of shocked zircons in glacial deposits, and the first detrital shocked zircons reported in North America. These results further demonstrate that shocked zircons survive post-impact thermal conditions, uplift, erosion and a variety of sedimentary transport processes, contributing to sedimentary systems up to 2 Gyr after an impact event.