New U-Pb data for five of six individual shocked zircon grains from the Cretaceous-Tertiary (K-T) boundary fireball layer, Rock Creek site, south-central Saskatchewan, yield a source age of 548 ± 6 Ma. The collinear data points are variably discordant in proportion to the intensity of shock metamorphism exhibited by each zircon. The age and zircon 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 previous study gave an indistinguishable source age of 544 ± 5 Ma. These ages are in agreement with independent ages obtained from a previous study for single shocked zircons from the Chicxulub crater, Mexico, and from Beloc, Haiti. The U-Pb zircon data unequivocally support a meteorite impact origin for the global K-T boundary layer and imply that the Chicxulub crater is the sole North American target source.

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