A SEARCH FOR DETRITAL SHOCKED ZIRCONS FROM THE ROCK ELM IMPACT STRUCTURE IN WISCONSIN, USA

A study was made at the Rock Elm meteorite impact structure, located in eastern Pierce County in west-central Wisconsin, USA, with the purpose of searching for detrital shocked zircons. One aspect of the Rock Elm impact structure that is somewhat uncommon is that the target rocks are siliciclastic sedimentary rocks. Previous studies have documented shocked quartz at Rock Elm (French et al., 2004). The goal of this study was to test whether or not shocked zircons are present in the Rock Elm impact structure, and if documented, to use these microstructural indicators to further constrain the pressure of the impact event. Two sediment samples were collected in June 2011 from the Rock Elm impact structure. The first sample (11RE01) is a modern fluvial sediment sample collected on a point bar deposit in Plumb Creek, a small local stream that erodes the central uplift, to search for detrital shocked zircons in modern sediments. The second sample (11RE02) is a sample of friable sandstone from the central uplift, collected from a weathered outcrop of Cambrian age Mount Simon sandstone, which could represent the source of detrital shocked zircons at the Rock Elm impact structure. The SEM used was a JEOL JSM-5410LV scanning electron microscope, collecting backscattered electron images (BSE). A total of 459 grains were handpicked. During ultraviolet light imaging (both short and long wavelengths), many of the grains yielded anomalous UV colors (blues rather than yellows), making the distinction between zircons and other minerals, such as apatite, difficult. Grain morphologies ranged from euhedral grains that show little evidence of sedimentary abrasion, to anhedral grains that are highly rounded with aspects ratios of 1:1. Several occurrences of unusual planar features were documented in samples 11RE01 and 11RE02 on the surface of detrital zircon grains. These include parallel sets of ridges, and fractures sets that are locally parallel but that do not extend entirely across grains. It could not be confirmed if these planar features were in fact produced as a consequence of impact deformation. Not finding shock microstructures in detrital zircon grains suggests that the shock pressures were lower than 20 GPa.