The Absolute Age of the Eifelian Tioga Ash Bed, Pennsylvania

Abstract

The Tioga ash bed, bed B of the Zeigler pit, Union County, Pennsylvania, lies in the Polygnathus costatus costatus Zone of the Eifelian Stage of the Middle Devonian Series. The 27.5-cm-thick bed contains igneous zircon and monazite crystals that have been dated by precise U-Pb techniques. Zircons are characterized by inherited Pb that occurs as barely perceptible interior regions enriched in inclusions. The zircons are discordant and do not reveal the igneous age of the bed. In contrast, high quality monazite crystals lack any evidence of inheritance or loss of Pb, and multiple analyses have overlapping $$^{207}Pb/^{235}U$$ ages of $$390.0 \pm 0.5 Ma$$. The monazite analyses are concordant or very slightly reversely discordant. The consistency of the monazite data provides a new precise estimate of age for the lower Eifelian.