Brussels Hill, Door County, Wisconsin: An Enigmatic Area of Disturbed Bedrock

Brussels Hill in southern Door County, Wisconsin, is a localized area of faulted and brecciated bedrock in a region of otherwise undeformed lower Silurian dolostone that has been tentatively interpreted as an impact crater. The area of disturbed rock coincides with a nearly circular, 2-km-wide, flat-topped hill that stands about 40 m above the surrounding landscape. Glacial till and polished, striated surfaces on the brecciated bedrock indicate that the disturbance predated the last ice advance and suggest that the hill may have been even higher before glaciation. Anomalous, apparently intrusive bodies of glauconite-bearing sandstone within the brecciated dolostone were likely derived from Cambrian strata, which are normally 400 m in the subsurface in the area. A 103-m drill core into the center of the disturbance recovered 70 m of brecciated dolostone and shale, much of it vesicular, but with no features diagnostic of shock metamorphism. The lowest 33 m of the core sampled the upper Ordovician Maquoketa Formation, which appeared to be subhorizontal and only locally disrupted. This is puzzling, given that material from greater depth must have been brought up through this level. The fact that an area of such fragmented rock stands as a topographic high is also hard to explain. Brussels Hill may have been the site of supersonic impact, but an endogenous process such as violent fragmentation in the upper part of a diatreme cannot be ruled out.