Impact breccia in the Precambrian of Ontario, Canada.

This is an outcrop of dark-colored, carbon-rich impact breccias of the Onaping Formation. These rocks 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, Sandcherry Member rocks are exposed - they have been interpreted as fall-back impact breccias and autoclastic impact breccias (Rousell et al., 2009). Many lithic fragments in this impact breccia are rimmed by thin layers of fine-grained material. Such grains are termed "accretionary lapilli". Some sulfides were observed at this site.

Geologic unit: Sandcherry Member, Onaping Formation, lower Whitewater Group, Paleoproterozoic, 1.85 Ga

Locality: High Falls outcrop - roadcut on the western side of Route 144, just north-northwest of High Falls overlook (= Jackson Lookout) along the Onaping River, northwest of the town of Phelans & west of town of Dowling & south-southeast of the town of Onaping, Sudbury Impact Structure, Ontario, Canada (46° 35’ 26.91" North latitude, 81° 23’ 01.65" West longitude) (= stop 24 of Rousell et al. (2009) - A field guide to the geology of Sudbury, Ontario. Ontario Geological Survey Open File Report 6243, p. 156.)

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