Impact breccia in the Precambrian of Ontario, Canada.

This outcrop consists of a clast-rich impact pseudotachylite. This is the Sudbury Breccia, which formed during one of the largest impact events in Earth history. The Sudbury Impact Structure is located in southeastern Ontario. It is ovoid in shape, about 37 miles long by 19 miles wide, but it was originally ~circular, having been laterally compressed by tectonic deformation. Before erosion and structural deformation, the structure may have been between 60 and 120 miles across. The impacting object is estimated to have been about 6 miles in size. The impact event itself occurred at 1.85 billion years ago. At that time, this part of Ontario was ocean. Computer modeling indicates that the splash wave generated by the Sudbury Impact may have reached about 30 miles high and generated a ~300 feet high megatsunami that radiated away from the target area for up to ~1800 miles. The impact punched a hole down into underlying mantle rocks.

Abundant evidence indicates that the Sudbury, Ontario area was indeed the site of an impact long ago. This includes impact breccias, shattercones, and extensive syn-formational sulfide mineralization. The sulfides are rich enough for mining to occur. In fact, the Sudbury Mining District targets the # 1 most productive nickel deposit on Earth.

At this outcrop, the clasts in the Sudbury Breccia are principally pieces of Cartier Granite, a 2.6 billion year old igneous intrusion. The dark-colored matrix is cryptocrystalline impact pseudotachylite, which formed by melting and rapid cooling.

Geologic unit: Sudbury Breccia, Paleoproterozoic, 1.85 Ga

Locality: Crab Lake South outcrop - Route 144 roadcut, ~200 meters south of parking area for Crab Lake, south-southeast of the town of Cartier, Sudbury Impact Structure, Ontario, southeastern Canada (46° 40’ 28.51 North latitude, 81° 32' 34.43" West longitude) (= stop 34 of Rousell et al. (2009) - A field guide to the geology of Sudbury, Ontario. Ontario Geological Survey Open File Report 6243, p. 165.)

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