CHICXULUB IMPACT EJECTA AND MODELS OF EJECTA EMPLACEMENT IN LARGE CRATERS

The Chicxulub crater in Mexico is the only large terrestrial impact crater with well-preserved proximal and distal ejecta. Proximal ejecta deposits are known from drill cores inside and near the rim. Inside the crater are 100-300 m of silicate melt-rich suevite, but only about half is ejecta. Outside the rim are ~500 m of ejecta with ~150 m of silicate melt-rich suevite overlying melt-poor, carbonate and anhydrite-rich suevite. The melt-rich suevite extends <150 km from the crater center. The ejecta blanket is exposed in numerous outcrops of the Albion Formation near the Mexico/Belize border, 330-360 km from the center of Chicxulub, and at Armenia in central Belize, 470 km from Chicxulub. The Albion Formation is composed the spheroid bed and diamictite bed. Spheroid bed exposures are 1-5 m thick and contain altered silicate melt, accretionary lapilli, and pebble-sized carbonate clasts in a fine-grained calcite matrix. The spheroid bed was deposited on a weathered Cretaceous karst land surface. Diamictite bed exposures are 8-17 m thick and contain altered silicate melt and carbonate clasts up to 9 m in a carbonate matrix. At Armenia the spheroid bed is overlain with a limestone conglomerate containing shocked quartz. Distal ejecta are comprised of three main constituents: microtektites, microkrystites, and shocked mineral grains. Microtektites are droplets of impact melt (<5 mm) that are aerodynamically shaped, vesicular, and typically altered to clay. Microtektites are found almost exclusively in North America and the Caribbean region, where they were deposited on the terminal Cretaceous surface at terrestrial sites, but are found re-deposited in slumps in marine settings close to the crater. Microkrystites are spherules of crystalline silicates (<0.5 mm) interpreted as vapor condensation droplets. They are usually altered, but most contain relict Ni-rich spinels that formed at high temperatures. Microkrystites occur globally in K-T boundary sections and in North America they overly the microtektite layer. Shocked mineral grains are distributed globally at mid-latitudes, and are associated with the microkrystites. Given the complexity of Chicxulub deposits, a variety of depositional models based on computer simulations and volcanic analogues are needed to explain ejecta emplacement in large craters.