Abstract

Sudbury breccias are commonly attributed to meteoritic impact at about 1.85 Ga in the vicinity of the Sudbury Igneous Complex. In the Whitefish Falls area, about 75 km southwest of Sudbury, similar breccias are widely developed in argillites of the ~2.3 Ga Gowganda Formation. There is abundant evidence of "soft sediment" deformation of the Huronian sediments in the form of complex "fault" contacts, clastic dyke intrusions, and chaotic folding. These movements appear to have been penecontemporaneous with intrusion of highly irregular diabase bodies, which are interpreted as being older than the ~2.2 Ga Nipissing diabase. Complex shapes of diabase bodies and highly irregular contact relationships between diabase and argillites, including intrusions of sediment veins into diabase, support intrusion of the diabase into incompletely consolidated sediments. These data, together with chemical evidence of mixing of diabase, argillite, and other materials in the breccia bodies, suggest that the breccias at Whitefish Falls may have formed as a result of interaction between hot mafic magma and semiconsolidated, water-rich mud, more than 350 Ma prior to formation of the Sudbury Igneous Complex and attendant phenomena that are presumed to be impact related.

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