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The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz

Bevan M. French^,1, William S. Cordua^,2 and J.B. Plescia^,3

¹ Department of Mineral Sciences, Smithsonian Institution, Washington, D.C. 20560, USA
² Department of Plant and Earth Science, University of Wisconsin–River Falls, 410 South Third Street, River Falls, Wisconsin 54022, USA
³ U.S. Geological Survey, 2255 North Gemini Drive, Flagstaff, Arizona 06001, USA

The Rock Elm structure in southwest Wisconsin is an anomalous circular area of highly deformed rocks, 6.5 km in diameter, located in a region of virtually horizontal undeformed sedimentary rocks. Shock-produced planar microstructures (PMs) have been identified in quartz grains in several lithologies associated with the structure: sandstones, quartzite pebbles, and breccia. Two distinct types of PMs are pres ent: P1 features, which appear identical to planar fractures (PFs or cleavage), and P2 features, which are interpreted as possible incipient planar deformation features (PDFs). The latter are uniquely produced by the shock waves associated with meteorite impact events. Both types of PMs are oriented parallel to specific crystallographic planes in the quartz, most commonly to c(0001), {112}, and r/z{101}. The association of unusual, structurally deformed strata with distinct shock-produced microdeformation features in their quartz-bearing rocks establishes Rock Elm as a meteorite impact structure and supports the view that the presence of multiple parallel cleavages in quartz may be used independently as a criterion for meteorite impact. Preliminary paleontological studies indicate a minimum age of Middle Ordovician for the Rock Elm structure. A similar age estimate (450–400 Ma) is obtained independently by combining the results of studies of the general morphology of complex impact structures with estimated rates of sedimentation for the region. Such methods may be applicable to dating other old and deeply eroded impact structures formed in sedimentary target rocks.

Key Words: Rock Elm (Wisconsin) • meteorite impact craters • shock metamorphism • quartz deformation • quartz cleavage • planar deformation features (PDFs)