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The Bighorn Basin of northwestern Wyoming preserves one of the most complete records of middle Paleocene to lower Eocene continental biota. The geochronology of this important interval depends partly on numerical calibration of the geomagnetic polarity time scale (GPTS), but the middle and late Paleocene parts of the GPTS have been poorly constrained radioisotopically. A new volcanic ash from the northern Bighorn Basin provides the first radioisotopic age for upper middle Paleocene strata (upper Selandian) and is an important calibration point for the base of polarity Chron C26n and the Selandian-Thanetian stage boundary in the GPTS. Sanidine from the ash has a weighted mean age of 59.00 ± 0.30 (2 σ) Ma, based on twenty-three 40Ar/39Ar bulk laser-fusion analyses. The ash corroborates the revised age estimate for the base of Chron C26n in the most recent geomagnetic polarity time scale (GPTS-04) and confirms that C26n is older than depicted in the 1995 GPTS.
We place Paleocene mammalian biozones in the Bighorn Basin into a refined geochronologic framework. These biozones are the primary basis for biostratigraphic correlation of middle and upper Paleocene continental deposits in North America. New paleomagnetic data provide synchronous tie points for correlation among three stratigraphic sections in the northern Bighorn Basin and to the GPTS. Paleomagnetic correlation, study of new fossil material, review of known faunas, and refined stratigraphy enable us to reevaluate several biozones. We define three new zones based on first occurrences of taxa: the Phenacolemur (Ti-4b), Probathyopsis (Ti-5a, revised), and Copecion (Cf-3, revised) zones. The Phenacolemur zone includes some faunas previously placed in the Plesiadapis churchilli and Plesiadapis fodinatus zones (Ti-4 and Ti-5a, respectively). The Probathyopsis zone replaces the P. fodinatus zone and the Copecion zone replaces the Phenacodus-Ectocion acme zone (Cf-3).
Seven of the biozones considered here are subdivisions of the Tiffanian land mammal age. The Tiffanian was originally typified by the Mason Pocket fauna in southwestern Colorado, but it is now much better known from faunas in the northern Bighorn Basin. Biostratigraphic evidence and new paleomagnetic data from the Bighorn Basin indicate that Mason Pocket is considerably older than was previously recognized and occurs in Chron C26r, rather than C25r. The geochronologic framework presented here helps to constrain the temporal ranges of species that occur in the middle and late Tiffanian, and it allows the Bighorn Basin record to be better compared to other faunal, floral, and paleoclimate records, both regionally and globally.