Geochemistry of Some Large Canadian Craters

Abstract

PETROGRAPHIC criteria have been applied to large and complex structures containing rocks of igneous appearance in attempts to prove an impact origin of these structures^1,2. Discovery of alkaline and carbonatitic rocks in the Brent crater³, widely accepted as a holotype of “fossil” impact sites, throws serious doubt on petrographic criteria of origin. We have therefore examined four large craters on the Canadian shield by geochemical methods. The locations and diameters of these structures are as follows–East Clearwater (56° 05′ N., 74° 45′ W.), 14 miles; West Clearwater (56° 10′ N., 74° 45′ W.), 19 miles; Carswell (58° 27′ N., 109° 30′ W.), 24 miles; Manicouagan (51° 25′ N., 68° 30′ W.), 38 miles. All four structures⁴ are roughly circular basins having a depth less than 1 per cent of the diameter, and with a concentric central prominence. The basin is almost circular at West Clearwater and Carswell quadrate at East Clearwater and octagonal at Manicouagan. Rocks of igneous appearance outcrop on the edges of the central prominence and (except at Carswell, in the lowest parts of the basin. Volumes of igneous rocks range from less than 0.01 miles³ at Carswell to more than 20 miles³ at Manicouagan. Both dyke-like and sheet-like masses are present–the latter greatly predominant in volume. The lower parts of the sheets are rich in inclusions, and locally consist of unusual breccias (suevite) which are also found as dykes.