Unraveling the Cretaceous-Paleogene boundary event across the Gulf of Mexico—High-resolution Rayon reef section, Valles-San Luis Potosi platform, Mexico

The Cretaceous/Paleogene boundary, marked by the Chicxulub impact ejecta layer has a global distribution, with distal, intermediate and proximal boundary sections of distinct mineralogy, geochemistry and stratigraphy. Distal sections show a thin double-layer of spherulites and clay, enriched in platinum group elements and impact minerals. Proximal boundary sections around the Gulf of Mexico-Caribbean Sea are thicker, with clastic tsunami and debris flow deposits and the fireball layer in between the spherulitic and clay layers. Impact-induced disturbance and stratigraphic gaps have limited the resolution in unveiling the event sequence. Here we present a study on a recently discovered reef section in a carbonate platform, ~ 1,050 km away across the Gulf of Mexico. The boundary is marked by a black clay with shock quartz, glassy spherules, metallic particles, broken zircons, gypsum crystals with fragmented spherules and platinum group elements (Ir, Os, Pd). Fossils in the black clay are absent and only fossil evidence at the bottom is charred matter, covered by lignite mixed with spherules. Corals and pelecypod shells are incrusted by spherules. The lower unit is formed by cream-colored sandstones of a rudist-coral reef environment with variable thickness and the upper unit by sandstones with few gastropods and fragmented shells. The Rayon reef in the Valles-San Luis platform provides a high-resolution proximal record of Chicxulub impact in the Gulf of Mexico.