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.

Cerro do Jarau is a prominent, ~13.5-km-wide, circular landform rising >200 m above the plains of the “pampas” in southern Brazil. The name (meaning Jarau hills) comes from the prominent crests of silicified sandstones, which form a semiring of elevated hills in the northern part of the structure. The origin of this structure has been debated for decades, and varied suggestions of its formation include either endogenous tectonic processes or large meteorite impact. However, no conclusive evidence to support either hypothesis has been presented to date. This structure was formed in Mesozoic volcano-sedimentary rocks of the Paraná Basin and consists of the Jurassic-Cretaceous Guará (sandstones), Botucatu (sandstones), and Serra Geral (basalts) formations. The Botucatu Formation sandstones are intensely silicified and deformed, and were subject to radial and annular faulting. Our investigations at Cerro do Jarau identified the occurrence of parautochthonous monomict lithic breccia and polymict breccias resembling suevite and striated joint surfaces resembling crude shatter cones in sandstones and basalts. In addition, our first mineral deformation studies show the presence of rare planar features in quartz clasts in polymict breccias. The identification of these features at Cerro do Jarau, for the first time, is suggestive of an impact origin for the structure. If confirmed by further investigation of possible shock features, Cerro do Jarau would become the sixth known impact structure in Brazil, as well as the fifth basalt-hosted impact structure on Earth.

The Carswell structure in the western Athabasca Basin, northern Saskatchewan (Canada), has previously been interpreted as an eroded impact structure with a minimum diameter of ~36 km, the outer margin of which is broadly defined by an outer ring of sediments composed of the only algal reefs observed in the Athabasca Group, the Carswell Formation. This ring surrounds an 18-km-wide uplifted basement core composed of gneiss units of Archean to Paleoproterozoic age that display shatter cones, planar deformation features (PDFs), pseudotachylyte veins, and impact melts and breccias (Cluff melt sheet, Cluff breccias) indicating that pressures and temperatures locally exceeded 60 GPa and 1500 °C. Detailed analysis of the basement–Athabasca Group contact from field and drill core samples indicates that shock features are not present in the Athabasca Group sediments in direct contact with the highly shocked basement gneisses. This pattern is inconsistent with a post–Athabasca Group age for the impact. Moreover, our study has revealed PDF-bearing quartz grains in basal units of the Athabasca Group 14 km south of the southern edge of the basement core (outside the estimated outer ring). The new proposed model suggests that the impact event is of pre-Athabasca (Proterozoic) age and that it produced a multiring structure that controlled the paleogeography of the Athabasca Group units in the western part of the basin. The model is well supported by basin analysis and gravity data. The Carswell Formation is the result of algal reefs building on peak-ring–related seamounts at the end of Athabasca Group deposition. The overturned bedding observed locally adjacent to the basement core is interpreted as the result of gravity-driven readjustment of the central uplift.