A Comparative Structural and Petrographic Study of Probable Canadian Meteorite Craters

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Abstract
The characteristics investigated to date of ten circular structures in the Canadian Shield have supported their origin by meteoritic impact.Seven, ranging in diameter from 3 to 20 km, have a simple crater form modified by erosion. The underlying rocks have properties where known, which include: Low density. Low, uniform magnetic intensity. Fracturing, alteration and deformation of the regional gneisses intensifying towards the center with the development of a coarse, essentially monomict, autochthonous breccia. Overlying of the latter within the crater by highly mixed, partly vitrified allochthonous breccia. This layer has coalesced and largely recrystallized to a lava‐like matrix in the larger craters. Unrecrystallized fragments in all the breccias showing textures comparable with artificially produced shock metamorphism. The three largest craters (30 km diameter) show a first order modification of the simple crater form due to a central uplift of the basement gneisses. The properties outlined above are found in the annulus surrounding the central uplift. Gneisses in the latter decrease in megascopic deformation towards the center but show evidence microscopically of shock deformation. Examples of this family of common characteristics are mainly drawn from Brent (4 km diameter) and the two Clearwater Lake craters (20 and 32 km).