NEW RADIOISOTOPIC AGE CONTROL ON LATE PALEOCENE CONTINENTAL STRATA: IMPLICATIONS FOR THE AGE OF CHRON 26N

A volcanic ash in carbonaceous shale from the northern Bighorn Basin yields the first radioisotopic age from upper Paleocene continental strata. The ash occurs in the upper part of the Plesiadapis churchilli biozone (Ti-4) of the Tiffanian land mammal age, in the Fort Union Formation, at Polecat Bench, Wyoming. The Polecat Bench sequence (sensu lato) is the reference section for most North American late Paleocene mammal zones. Twenty-three ⁴⁰Ar/³⁹Ar laser-fusion analyses of aliquots comprising 20 crystals each of optically clear sanidine yielded a weighted mean age of 59.00 ± 0.34 Ma* (2σ uncertainty; analytical and standard intercalibration contributions are ± 0.30 and 0.04 myrs, respectively). Crystals are euhedral and matrix-supported, and the ash bed showed no visible indication of reworking. The ash lies ~40 m below the base of C26N, which is placed at 57.91 Ma in the current geomagnetic polarity time scale (GPTS; Cande and Kent, 1995) based on seafloor spreading rates and radioisotopic tie points at or near the K-P and P-E boundaries. This 40 m represents ~160 k.y. based on the thickness of C26N, assuming constant rates of deposition and homogeneous compaction. Using this estimate, the expected GPTS-based age for the ash would be 58.07 Ma. The discordance between this estimate and the radioisotopic age for the Polecat Bench ash suggests that the base of C26N is roughly 1.0 m.y. older than depicted by the current GPTS. Corrected estimates using linear interpolation between the P-E boundary (55.8 Ma) and the ash place the base of C26N at ~58.88 Ma and the top at ~58.61 Ma. The older age for C26N is consistent with recent downward shifts in the K-P (Hicks et al., 2002) and P-E boundaries (Luterbacher et al., 2004), and does not fundamentally alter the duration or temporal distribution of the Tiffanian land mammal age. Additional radioisotopic ages from late Paleocene ashes are needed to test these results. *calculated relative to 28.34 Ma Taylor Creek Rhyolite standard.