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.

Chicxulub is the only known impact structure on Earth with a fully preserved peak ring, and it forms an important natural laboratory for the study of large impact structures and understanding of large-scale cratering on Earth and other planets. Seismic data collected in 1996 and 2005 reveal detailed images of the uppermost crater in the central basin at Chicxulub. Seismic reflection profiles show a reflective layer ~1 km beneath the apparent crater floor, topped by upwardly concave reflectors interpreted as saucer-shaped sills. The upper part of this reflective layer is coincident with a thin high-velocity layer identified by analyzing refractions on the 6 km seismic streamer data. The high-velocity layer is almost horizontal and appears to be contained within the peak ring structure. We argue that this reflective layer is the predicted coherent melt sheet formed during impact, and it may be comparable with the unit known as the Sudbury Igneous Complex at the Sudbury impact structure. The Sudbury Igneous Complex, interpreted as a differentiated impact melt sheet, appears to have a similar scale and geometry, and an uppermost lithological sequence consisting of a high velocity layer at the top and a velocity inversion beneath. This comparison suggests that the Chicxulub impact structure also contains a coherent differentiated melt sheet.