William S. Cordua

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

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), ξ{11\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\overline{2}\) \end{document}2}, and r / z {10\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\overline{1}\) \end{document}1}. 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.

Rock Elm structure, Pierce County, Wisconsin: A possible cryptoexplosion structure

The Rock Elm structure is a nearly circular feature in western Wisconsin flat 44°43′N, long 92°14′W) in an area of otherwise little-deformed Cambrian and Ordovician shallow-marine sediments. The structure has a ring boundary fault with at least 50 m of vertical displacement, a sediment-filled ring basin, and central denial uplift. The ring basin fill consists of a shale-sandstone sequence of probable Ordovician age not found outside of the structure. Blocks of deformed Lower Ordovicisin carbonates are generally in angular unconformity beneath the basin fill but locally are in fault contact against basin-fill rocks. The central dome consists of conglomeratic sandstones, tentatively identified as the Upper Cambrian Mount Simon Sandstone, that have been uplifted more than 250 m. Although the deformation of the rocks is unusual for the region, there have been no shock-metamorphic features yet found associated with the structure. The Rock Elm structure has the essential features of a cryptoexplosion structure as described by Dietz and by McCall. Conclusive evidence is lacking as to whether the Rock Elm structure formed as a result of a bolide impact, explosive volcanic event, or other localized violent geologic event.

Wisconsin's Elusive Wavellite

T Late Cambrian sandstones in Wisconsin are known for their frac sand* resources but not for much of interest to mineral collectors. Yet in 1972, two U.S. Geological Survey geologists, Harry Klemic and Mary Mrose (1972), described a wavellite occurrence in these sandstones between the towns of Merrillan and Black River Falls in Jackson County. In the intervening forty-five years wavellite has not been reported elsewhere from the many outcrops of Cambrian sandstone throughout Wisconsin and adjacent Minnesota. This is despite extensive field work by many geologists (Klemic and Ohlson 1973; Odom 1978; Mossler 1992; Mahoney et al. 1997; Aswasereelert, Simo, and LePain 2008). Through the years, I have done field work in the Merrillan area to further document the occurrence and try to explain why wavellite seems to be unique to that region. Specimens can still be found there by diligent collectors.