Eocene clocks agree: Coeval 40Ar/39Ar, U-Pb, and astronomical ages from the Green River Formation

Abstract

U-Pb ages of zircon from the Firehole and Analcite ash beds in the Eocene Green River Formation (Wyoming, United States) are indistinguishable from 40 Ar/ 39 Ar ages of sanidine after adjusting the latter to the astronomically calibrated age of 28.201 Ma for the Fish Canyon sanidine standard. Six of nine zircon analyses from the Firehole ash yield concordant ages in an overlapping cluster and give a weighted mean 238 U- 206 Pb age of 51.66 ± 0.20 Ma (full external 2σ uncertainty), indistinguishable from the recalibrated 40 Ar/ 39 Ar age of 51.40 ± 0.25 Ma (full external 2σ uncertainty), which is adjusted to eliminate samples with low radiogenic 40 Ar*. Significant 238 U- 206 Pb age scatter likely reflects some combination of minor inheritance, Pb loss, and possibly magma chamber residence. Seven of eight analyses of Analcite ash zircon yield a weighted mean 238 U- 206 Pb age of 49.23 ± 0.13 Ma (full external 2σ uncertainty), whereas the sanidine 40 Ar/ 39 Ar age is 49.24 ± 0.18 Ma (full external 2σ uncertainty). Calibrating Green River Formation 40 Ar/ 39 Ar ages to the 28.201 Ma age for Fish Canyon sanidine permits the first direct comparison of specific Green River Formation strata to the astronomical solution for Early Eocene insolation. This comparison supports the hypothesis that periods of fluvial deposition coincided with minima in long and short eccentricity, and that periods of lake expansion and evaporite deposition correspond to eccentricity maxima.