GEOPHYSICAL EXPLORATION OF THE SOUTHERN FAULT BLOCK OF THE ROCK ELM IMPACT CRATER IN PIERCE COUNTY, WI

The Rock Elm impact crater (Pierce County, WI) is a complex crater roughly 6.5 km in diameter which formed in flat-lying sandstones and carbonates deposited in a shallow-marine environment 460 million years ago in the Middle Ordovician. Since its formation, the crater experienced burial, glaciation, and erosion, leaving very few outcrops and little surface expression. Impacts into sedimentary targets and shallow-marine environments are not as well understood as those into igneous and metamorphic target materials. Thus, our work focuses on determining the subsurface geology and extent of this crater using geophysical methods. In particular, this study examines the southern slump block, a large block of uplifted Prairie du Chein Dolostone that slumped back down into the crater along its southern edge during the modification stage of crater formation.

For this project, seismic refraction and electrical resistivity surveys were executed along County Road HH just north of the Nugget Lake County Park Entrance. This area represents the possible location of the contact between the disturbed Prairie du Chein Dolostone and the Rock Elm Shale (one of two units deposited within the crater after its formation). This contact was initially mapped by Dr. William Cordua from the University of Wisconsin, River Falls, to an accuracy of +/- 100 feet. Due to the lack of outcrops at this location, the contact location is only an approximation; geophysical techniques offer a way to image the subsurface and increase our knowledge of this area in a non-intrusive manner.

Both the seismic and electrical resistivity surveys were centered at Cordua's approximated location for the contact. The seismic survey line had a total length of 120 m, and a series of forward and reverse shots were completed in order to determine if dipping interfaces were present. The electrical resistivity survey line had a total length of 275 m along which three arrays (a Wenner, Schlumberger, and reciprocal Schlumberger with IP) were run to image the resistivity of the subsurface. The data collected at Rock Elm will be used to provide constraints on the subsurface geology and better locate the contact between the Prairie du Chein Dolostone and the Rock Elm Shale.