The boundary between Hemphillian and Blancan North American Land Mammal Ages has been elusive and equivocal. Our work identifies that boundary at about 4.9 to 5.0 Ma, based on paleomagnetic and radioisotopic dating of strata producing Blancan and Hemphillian mammals in Meadow and Spring Valleys of Lincoln County, eastern Nevada, and in the Pine Nut Mountains of western Nevada. Magnetostratigraphic study of the Panaca Formation in Lincoln County yielded a composite section with six magnetozones. The Healdsburg Tephra was identified near the top of the section in Meadow Valley, thereby placing the Lincoln County composite section relative to the Geomagnetic Polarity Time Scale (GPTS). The arvicolid rodent Mimomys, an indicator of the Blancan LMA, appears near the base of magnetic chron C3n.3r, about 4.98 Ma, in the Panaca Formation. Both Blancan and Hemphillian mammals have been reported from a stratigraphic sequence near Buckeye Creek in the Pine Nut Mountains of western Nevada. Magnetostratigraphic study of this area produced a thick (280 m) section with six magnetozones. A pumice layer near the top of magnetozone D yielded an 40Ar/39Ar date of 4.96 Ma, thereby correlating magnetozone D with Chron C3n.4n (Thvera subchron) of the GPTS. A Hemphillian rhino is recorded below the pumice layer and a Blancan bear is recorded above it. Correlation of the Lincoln County and Pine Nut Mountain sections in Nevada is consistent with placement of the Hemphillian/Blancan boundary in chron C3n.3r or the top of chron C3n.4n of the GPTS, about 4.9 to 5.0 Ma.
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