The structure is about 180 km across, one of the largest impact structures so far identified with confidence on Earth. That chemistry could have been generated by melting together a mixture of the sedimentary rocks in the region. The igneous rock under Chicxulub contains high levels of iridium, and its age is 65 Ma, exactly coinciding with the K-T boundary. On top of the igneous rock lies a mass of broken rock, probably the largest surviving debris particles that fell back on to the crater without melting, and on top of that are normal sediments that formed slowly to fill the crater in the shallow tropical seas that covered the impact area. Well-known impact craters often have tektites associated with them as well as shocked quartz and tiny glass spherules.
Tektites are larger glass beads with unusual shapes and surface textures.
They are formed when rocks are instantaneously melted and splashed out of impact sites in the form of big gobbets of molten glass, then cooled while spinning through the air. The clay is overlain by a layer of turbidite, submarine landslide material that contains large rock fragments. Some of the fragments look like shattered ocean crust, but there are also spherical pieces of yellow and black glass up to 8 mm across that are unmistakably tektites. The black tektites formed from continental volcanic rocks and the yellow ones from evaporite sediments with a high content of sulfate and carbonate. Above the turbidite comes a thin red clay layer only about 5-10 mm thick that contains iridium and shocked quartz.