STRUCTURAL CHARACTERISTICS OF A POSSIBLE IMPACT SITE NEAR BRUSSELS, DOOR COUNTY, WISCONSIN

Brussels Hill is a notable topographic high and an anomalous area of disturbed bedrock in the otherwise flat-lying Silurian dolostone in southern Door County, WI. It stands as much as 40 m above the surrounding landscape and has a diameter of ca. 3 km. The local host rocks belong to the Mayville and Burnt Bluff dolostone units of the lower Silurian series, but unusual sandstone lenses also occur in quarry exposures at the summit of the hill. Based on the presence of glauconite, this sandstone is most likely Cambrian, requiring exhumation of >400 m. The bedrock in the quarry is fragmented at every scale and characterized by large masses of breccia, variably oriented megablocks, and discontinuous fold structures. Bedding on the surrounding flanks of the hill shows a radially inward dipping pattern. Although recent joint-controlled caves are known in this part of Door County, karst or paleokarst activity would not have resulted in the features found at Brussels Hill. Neither extensive dissolution cavities nor geopetal infillings are observed, and karstic collapse cannot account for the presence of sandstone apparently derived from much lower in the stratigraphic sequence. There is little evidence by which to determine the age of this structural disturbance, other than the constraints of the Silurian host rock, and the presence of glacial striae and till on top of the brecciated rock. This study, which complements another focused on microstructural features, presents a three-dimensional depiction of the internal geometry of Brussels Hill. While an impact origin seems the most plausible interpretation, questions remain. First, if the sandstone bodies represent the central uplift of a crater, the estimated amount of exhumation is larger than expected for the diameter of the disturbance. Second, the geomorphic prominence of Brussels Hill is puzzling; why does an area of pervasively fractured bedrock stand as a topographic high in a recently glaciated landscape? We speculate that Brussels Hill may represent the eroded remnant of a once-higher central uplift of a much larger crater.