Shocked zircons in the Onaping Formation: Further proof of impact origin

The Onaping Formation fills the structural basin at Sudbury, Ontario, Canada. This formation is composed of three members: a basal, coarse, mainly quartzitic breccia (Basal Member); a light-colored, heavily included, polymict middle unit (Gray Member); and a similar but dark-colored upper unit (Black Member). Two different origins were proposed for the Onaping: (1) volcanic ash-flow sheet; and (2) impact fall-back ejecta. These origins are critically discussed in a review paper coauthored by proponents of each view.