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The 1.85 Ga Sudbury impact structure is widely accepted to be the erosional remnant of a tectonized 200- to 250-km-diameter multiring basin. The Garson Member of the Onaping Formation in the Whitewater Group immediately overlies the coherent impact melt sheet—the so-called Sudbury igneous complex. It is of particular interest because it is poorly characterized, and its formational history is not well understood. The Garson Member is up to 500 m thick and is restricted locally, occurring along a 25 km strike length over the southeastern lobe of the Sudbury igneous complex. Detailed mapping and sampling of the Garson Member indicate that the dominant clast type is quartzite (>99%), with a few granite and quartz arenite clasts, and with a large range in clast shape and size. Partially annealed, decorated planar deformation features in quartz grains occur within the quartzite clasts, providing evidence for shock metamorphism. This, accompanied by the recrystallized, but equigranular nature of the groundmass, suggests that the Garson Member originated as a clast-rich impact melt rock. Geochemical analysis suggests a close relationship between the Garson Member and the so-called Onaping intrusion. The Garson Member and the Onaping intrusion, which has recently been interpreted as the remnants of the roof rocks of the Sudbury igneous complex, are located within the same stratigraphic context of the impact structure. We suggest, therefore, that the Garson Member and the Onaping intrusion share a similar origin as roof rocks to the Sudbury igneous complex, and they are, therefore, not part of the Whitewater Group but rather the Sudbury igneous complex.

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