SHATTER CONES AND FOSSILS: EVIDENCE ON THE FORMATION AND AGE OF THE BRUSSELS HILL WISCONSIN IMPACT SITE

Brussels Hill is a recently discovered (2010) impact site located in Door County, Wisconsin. Although not well exposed, evidence indicates that the hill preserves the remnants of a post early Silurian impact. This evidence includes the overall structure of the hill, the character of its deformation, its lithic composition which is dominated by brecciated carbonates from several early Paleozoic stratigraphic units and shatter cones. The hill is circular, about 2.4 km in diameter, and rises up to 60 meters above the surrounding area. Bedrock exposures at Brussels Hill are predominantly composed of highly disturbed strata which is in sharp contrast with the undisturbed, well bedded, and flat lying Silurian dolomites found throughout the region. The hill center is relatively flat topped with few rock exposures although a small quarry is present. This central area is ringed by a broad and heavily wooded band or rim of irregular terrain with natural exposures of large irregular dolomitic blocks of breccia, mounds of blocks, and limited bluffs of the same material. These rock exposures are characterized by abundant brecciation (megabreccias, polymictic breccias, monomictic breccias, breccia dikes), huge megaclasts, exotic lithologies with evidence of impact melt rocks some showing “burned” rims, and decarbonization, along with extensively tilted and deformed strata. A wide variety dolomites dominate the rock types found on Brussels Hill. The majority cannot be dated but are lithologies similar to those found in local Silurian and Ordovician units. A single small clast has produced late Ordovician conodonts while more common blocks containing either the brachiopods Virgiana or Pentamerus suggest an early Silurian age. Rare unfossiliferous sandstones occur which are similar to those found in late Cambrian or early Ordovician rock units of the area but otherwise are undated. Collectively these features strongly suggest the presence of impact breccias and an impact origin for the hill. Shatter cones have been found as rare clasts within breccia deposits indicating the Brussels Hill structure is an impact site. Overall the hill exhibits evidence for excavation and ejection stages of crater formation that included elastic waves producing fracturing, brecciation, and faulting with crater-fill and ejecta blanket deposits.