Composition and depositional processes of Cretaceous -Tertiary impact deposits in Belize and southeastern Mexico

Emily Burns, University of Rhode Island


The Cretaceous-Tertiary Chicxulub impact crater in Yucatán, Mexico is the result of a large extraterrestrial impact into shallow seawater on a carbonate bank, an environment unique to Earth in the solar system. To better understand the sequence of events during and after the impact, the stratigraphy, petrography, and geochemistry of three impact-related deposits at the Cretaceous-Tertiary boundary (KTB) on Yucatán were examined to determine their composition and depositional processes. The deposits studied are two normally graded carbonate breccias near Guayal and Bochil villages in southeastern Mexico and a clay-rich layer at the base of the continuous ejecta blanket at Albion Island, Belize. The geochemistry and mineralogy of the clay fraction of all three deposits show that it is largely smectite, different from the clay fraction of the local carbonates analyzed in this study, and chemically consistent with altered impact glass found at other KTB sites. All three deposits appear to be mixtures of varying proportions of altered glass and carbonate debris. The latter is different in composition at each site, supporting the hypothesis that it is largely local material gouged up during ballistic sedimentation or sliding of the ejecta after landing. The chemistry of the clay fraction of the basal layer at Albion Island suggests that it is jetted melt from early in the impact that reached the site before the continuous ejecta blanket. The percentage of the clay component in the graded deposits in Mexico increases with stratigraphic height. A spike in Ir and other metal concentrations at the very top of the graded deposits is due to fallout of impact dust containing extraterrestrial material. The Mexican deposits' structure suggests that they formed as sediment gravity flows of debris at the edge of the solid ejecta blanket, and that tektites were mixed into them during transport. The location and nature of the ejecta deposits suggest an unusually extensive ejecta blanket, probably due to wet conditions, with associated sediment gravity flows at its edges. Accretionary lapilli found at Guayal are largely replaced by diagenetic chert. They may represent the location of the 100° isotherm in the impact vapor cloud. ^

Subject Area

Geology|Biology, Oceanography|Geochemistry

Recommended Citation

Emily Burns, "Composition and depositional processes of Cretaceous -Tertiary impact deposits in Belize and southeastern Mexico" (2007). Dissertations and Master's Theses (Campus Access). Paper AAI3298365.