Thomson, Olivia A.

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    Detrital shocked minerals derived from the 1.85 Ga Subdury impact structure
    (2012) Thomson, Olivia A.; Cavosie, Aaron J.; College of Arts and Sciencies - Sciences; Santos, Hernán; Moser, Desmond; Department of Geology; Morales, Betsy
    The Earth preserves an incomplete geologic record of meteorite impact structures. Identification of detrital shocked minerals in the sedimentary record provides diagnostic evidence that can advance documentation and identification of ancient impact events whose craters have long since eroded. Thus far, detrital shocked minerals have only been described in alluvial deposits in South Africa. To evaluate other sedimentary environments for their potential to preserve detrital shocked minerals, I investigated ~6,000 sand grains from modern and Holocene sand deposits at the 1.85 Ga Sudbury impact basin in Ontario, Canada for the presence of shock microstructures in zircon using scanning electron microscopy (SEM). In addition, I surveyed ~20,000 quartz grains in thin section for the presence of detrital shocked quartz. Modern fluvial systems investigated include the Vermilion River and its main tributaries, the Rapid, Nelson, Sandcherry Creek, Onaping, and Whitson rivers, and also the Spanish river below its confluence with the Vermilion. Holocene glacial sands investigated include eskers and outwash deltas that occur across the entire Sudbury basin. Detrital shocked zircons were found in each of the different sedimentary deposits (21 of 27 samples), including six modern rivers and eight Holocene glacial deposits (five deltas and three eskers). 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. Of the 3,978 zircons surveyed by SEM, impact generated planar fractures were identified in 118 grains (3%); the highest percentage of shocked zircons was found in a delta sample (8/28, 29%). Detrital shocked quartz was identified in Holocene glaciofluvial sands on the western side of the impact basin, and not identified elsewhere. Each river crosses the Levack gneiss and/or the Onaping Formation, rocks previously reported to contain shocked zircon and shocked quartz; these are the most likely source rocks for the detrital shocked minerals. Shocked zircon-bearing glacial eskers and outwash delta deposits occur across the entire Sudbury basin in orientations consistent with the derivation of the shocked minerals from the same bedrock units. These results are the first confirmation of detrital shocked minerals in glacial deposits, and also the first detrital shocked minerals reported in North America. In addition to detrital shocked minerals reported from the Vredefort Dome, these results from the Sudbury impact structure 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. The results also identify a new reservoir for the preservation of detrital shocked minerals, glaciofluvial sand deposits (eskers and outwash deltas).