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.