A PREVIOUSLY UNRECOGNIZED IMPACT STRUCTURE AT BRUSSELS HILL, DOOR COUNTY, WISCONSIN: BRECCIATION AND SHOCK-METAMORPHIC FEATURES

Brussels Hill is an anomalous area of intensely fractured, faulted, and folded bedrock in a region of otherwise undeformed lower Silurian dolostone in Door County, WI. The area of disturbed rock coincides with a distinctive, nearly circular, flat-topped topographic high ca. 2 km in diameter, standing 40 m above the surrounding landscape and ringed by rugged tree-covered slopes. Bedding orientations vary dramatically over distances of meters. Coherent structures are difficult to discern, and fragmentation appears to have happened at multiple scales. Both mono- and polymict breccias occur, commonly as wedges that seem to have been intruded between bedding planes. Silurian dolostone is the only bedrock normally exposed in this area, but fault-bounded blocks of sandstone occur at Brussels Hill. This atypical rock likely comes from Cambrian strata that lie up to 300-400 meters below the surface. Although we have not found shatter cones in the host dolostones, we have identified shock-metamorphic planar microstructures (PMs) in the quartz grains of the sandstones. Planar fractures (PFs) typically occur in multiple parallel sets that stretch the length of the grain. In addition, these sandstones feature mechanically twinned and mosaicized grains. The age of the Brussels Hill disturbance is not well constrained but must date between post-Early Silurian and pre-Late Pleistocene as its target rocks are lower Silurian and the site lies in a recently glaciated shield area. As Brussels Hill shares many striking similarities with the central uplift of the Rock Elm impact structure in western Wisconsin—in diameter, topographic rise, stratigraphic uplift, and PMs—we conclude that Brussels Hill is likely the eroded remnant of a central peak of a larger impact structure.