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 may come from Cambrian or Ordovician strata that lie 100s of meters below the surface. Although we have not found shatter cones in the host dolostones or planar deformation features in the sandstone blocks, both rock types have microstructures that may reflect shock metamorphism at lower peak pressures. Some beds in the local Silurian sequence are vuggy, with cm-size cavities lined with euhedral dolomite crystals. In thin sections of samples from Brussels Hill, the infilling crystals have unusual concentric fractures, suggesting tensile spallation radially toward the interior of the cavities. These could record a high amplitude pressure wave passing through a rock with large open pore spaces. The sandstone blocks contain both rounded and highly angular grains, some with unusual concave margins unlikely to represent primary sedimentary features. A puzzling aspect of Brussels Hill is that even though it is underlain by pervasively fragmented rock and lies in an area that was glaciated only 15,000 yrs ago, it stands as a topographic high. Although Brussels Hill does not have definitive characteristics of an eroded impact structure, we conclude that an impact origin best explains its formation.