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

Abstract

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.