New evidence favouring an endogenic origin for supposed impact breccias in Huronian (Paleoproterozoic) sedimentary rocks

Breccia bodies in argillites of the Paleoproterozoic Gowganda Formation near Whitefish Falls, Ontario have been described as Sudbury-type breccias, formed as a result of fragmentation and milling of rocks in response to an asteroid impact in the vicinity of Sudbury. Much of the evidence for this interpretation revolves around the interpretation of early “pre-impact” structures in breccia fragments as tectonic products of an hypothesized “Blezardian” orogeny. The absence of a regional unconformity in the Huronian succession, however, and widespread evidence of early “soft sediment” deformation argue against this interpretation. Irregular shapes of diabase bodies and intrusion of sedimentary veins into the igneous bodies in unbrecciated zones suggest that the sediments were unconsolidated at the time of igneous intrusion. Additional support for a soft-sediment origin of the breccias is provided by the occurrence, in “slumped” argillite bodies, of clasts of argillite that have similar characteristics to those in the breccias. All these features suggest that the brecciation was the result of interaction between hot mafic melts and water-rich, semi-consolidated sediments. Some breccias have many of the characteristics of peperites. This conclusion demands caution in ascribing an impact origin to such breccias.