THE ORIGIN OF DETRITAL SHOCKED QUARTZ GRAINS FROM THE CENTRAL UPLIFT OF THE ROCK ELM IMPACT STRUCTURE, WISCONSIN, USA

Viridis Miranda¹ and Aaron J. Cavosie^1,2

viridis.miranda@upr.edu

¹Department of Geology, University of Puerto Rico

²Department of Applied Geology, Curtin University, Australia

Shocked quartz has been used as an indicator for the presence of impact structures because it records shock deformation features. One microstructure in quartz that records meteorite impact deformation is planar fractures (PFs), as these structures only form in high pressure (5-10 GPa) conditions created during a meteorite impact as the shock waves passes through rock. Shock microstructures such as planar fractures (PFs) and associated feather features (FFs) were previously found in detrital quartz grains from modern sediments that were collected within Plum Creek, a small stream eroding the Rock Elm impact structure in western Wisconsin. Rock Elm is a ~6.5 km diameter Ordovician impact structure in sedimentary target rocks [1], where shocked quartz [2] and reidite [3] have been reported. While prior studies demonstrated the presence and transport of detrital shocked quartz in Plum Creek, they did not address the transport mechanism of detrital shocked grains from the central uplift of the Rock Elm impact structure to Plum Creek. Documenting the source of the detrital shocked quartz grains is important in this case, as Holocene glacial deposits occur in the area. This study focused on searching for detrital shocked quartz grains from modern alluvium in a small unnamed tributary stream that drains the central uplift and flows west into Plum Creek. Using transmitted light microscopy, 46 detrital shocked quartz grains were identified with planar fractures (PFs), which are essentially identical to features identified previously from bedrock in the central uplift (Mt. Simons sandstone) and alluvium collected in Plum Creek. These results support the hypothesis that detrital shocked quartz grains in Plum Creek originate from exposed rocks in the central uplift of the Rock Elm impact crater.

This work was supported by NSF and the NASA Astrobiology program.

[1] Cordua 1985, Geology. [2] French et al. 2004, GSA Bulletin. [3] Cavosie et al. 2015 Geology.