Multimethod radiometric age for a bentonite near the top of the Baculites reesidei Zone of southwestern Saskatchewan (Campanian–Maastrichtian stage boundary?)

Abstract

A 15 cm thick bentonite at the top of the Baculites reesidei Zone in Cruikshank Coulee north of Herbert in southwestern Saskatchewan yielded biotite, sanidine, plagioclase, and zircon. Rb–Sr analyses give a mineral isochron with an age of 72.5 ± 0.2 Ma (± 2σ). U–Pb analyses of zircon produced concordant 206Pb/238U and 207Pb/235U ages of 72.4 ± 0.4 Ma (± 2σ) and 72.6 ± 0.4 Ma (± 2σ), respectively. Laser 40Ar/39Ar dating of sanidine and biotite revealed ages of 72.5 ± 0.2 Ma (± SE) and 72.6 ± 0.2 Ma (± SE), respectively. If the Campanian–Maastrichtian stage boundary is near the top of the Baculites reesidei Zone, then the overall average age of 72.5 ± 0.4 Ma dates the stage boundary.