A Rb/Sr isochron age of 63.6 ± 0.4 Ma for bentonite biotite and sanidine at Hell Creek, Montana, dates a palynofloral change marking the Cretaceous–Tertiary boundary in west-central North America. Extensive Rb and Sr variations caused by alteration of the biotites does not seem to unduly affect the presumed crystallization age of the biotite.

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