The Upper Contact Unit of the Sudbury Igneous Complex in the Garson region: Constraints on the depth of origin of a peak ring at the Sudbury impact structure

Observational and logical arguments are presented for the lithology formerly named the Garson Member of the Onaping Formation being the clast-bearing, fine-grained, chilled Upper Contact Unit (UCU) of the Sudbury Igneous Complex (SIC) in the Garson region of the Sudbury impact structure. It differs considerably, however, from the UCU in the North Range of the SIC with respect to the character of its clasts. Namely, the clasts are essentially monomict (quartzites), much larger (up to 100 m across), and much more abundant (up to 80% in places). These differences indicate a different source than "fallback" material for the clasts in the UCU in the Garson region. Their character requires a "coherent," singular source that was topographically above the SIC melt pool. Such a source would correspond to that of an emergent peak ring of fractured target rocks. The clasts are identified as Huronian Mississagi quartzite, which is estimated to have been at a nominal depth of 7.5 +/- 2.5 km at the time of impact. This provides a constraint on the depth of origin of the peak ring. This depth estimate is closest to the lower depth estimate from current numerical models of Sudbury and the similar-sized Chicxulub impact structures.