CHICXULUB CRATER SOURCE FOR SHOCKED ZIRCON CRYSTALS FROM THE CRETACEOUS-TERTIARY BOUNDARY-LAYER, SASKATCHEWAN - EVIDENCE FROM NEW U-PB DATA

New U-Pb data for five of six individual shocked zircon grains from th e Cretaceous-Tertiary (K-T) boundary fireball layer, Rock Creek site, south-central Saskatchewan, yield a source age of 548 +/- 6 Ma, The co llinear data points are variably discordant in proportion to the inten sity of shock metamorphism exhibited by each zircon. The age and zirco n shock features establish a strong temporal and genetic link, with K- T distal ejecta from the fireball layer at the Berwind canyon site in the Raton basin, Colorado, where texturally similar zircons from a pre vious study gave an indistinguishable source age of 544 +/- 5 Ma. Thes e ages are in agreement with independent ages obtained from a previous study for single shocked zircons from the Chicxulub crater, Mexico, a nd from Beloc, Haiti. The U-Pb zircon data unequivocally support a met eorite impact origin for the global K-T boundary layer and imply that the Chicxulub crater is the sole North American target source.