Bentonites associated with coal seams at the North American Cretaceous–Tertiary (K–T) boundary in the Red Deer Valley of Alberta, in the Frenchman Valley of Saskatchewan, and near Hell Creek, Montana, were dated by the K–Ar, Rb–Sr, and U–Pb methods. Based upon palynological stratigraphic correlation and the presence of a sharp iridium anomaly, the three sampled bentonite horizons all occur less than 1 m above the established K–T boundary. There is evidence of possible systematic errors giving dating results outside nominal statistical error values. Despite possible constant errors, the weighted mean of nine average dates is calculated as 64.3 ± 1.2 Ma (2σ standard deviation). An unweighted averaging produces a grand mean age of 64.4 ± 1.2 Ma.

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