Abstract
40Ar/39Ar dating of a bentonite associated with a cold methane seep deposit in the upper Campanian Baculites compressus Zone of the Pierre Shale in South Dakota yields an age of 73.79 ± 0.36 Ma. This is in close agreement with the previously published age of 74.05 ± 0.39 Ma for this zone (Obradovich, 1993) and nearly identical to an unpublished age of 73.70 ± 0.13 Ma for the same zone, both samples of which are from a more proximal locality near the base of the Bearpaw Shale in Montana, as recalculated relative to a Fish Canyon standard age of 28.201 Ma. We also report dates for two bentonites from the upper Campanian Exiteloceras jenneyi and Didymoceras stevensoni zones.
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