Fission-Track Ages of Zircons from Lonergan Creek (Shoreline Fossil Locality, Ash Hollow Formation, Upper Miocene) in Western Nebraska

A bed of vitric ash occurs 2 m above the primary fossiliferous unit at Lonergan Creek Locality in the Ash Hollow Formation on the north shore of Lake McConaughy, western Nebraska. Zircon crystals recovered from the ash were dated using the fission-track method. The grouping of youngest-age euhedral grains among the largely detrital suite is interpreted as the airfall population, with a fission- track age of 10.42 ± 2.5 Ma. Owing to uncertainties about the origin of this youngest group, the age should be interpreted as a detrital date and considered a maximum age for the ash. The Lonergan Creek Ash cannot be correlated with certainty to any other dated Great Plains ash. However, it is of similar age to an ash 37 m above the base of the type section of the Ash Hollow formation and also to the Davis Ash, lying above the Minnechaduza Fauna in Cherry County, Nebraska