Geochronology and Mammalian Biostratigraphy of Middle and Upper Paleocene Continental Strata, Bighorn Basin, Wyoming

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Abstract
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 (2σ) 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.

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