Estimating limiting age for pleistocene erosional surfaces in central Montana by uranium-series dating of associated travertines

Travertine deposits overlie the highest Number 2 surfaces in central Montana and clasts of travertine occur in gravels on lower Number 2 surfaces. Dating these travertine deposits may provide limits on the ages of surfaces that record intervals of extensive erosion during Pleistocene time. Analysis of three travertine samples from the southeast side of The Park yield an average uranium-thorium age of 73 000 ±+M 7 000 years. Another sample from the west side of The Park is 320 000 (+ 160 000, ? 70 000) years old. These results indicate that travertine deposits may have formed at several intervals. The surface beneath The Park travertine is older than about 320 000 years. Number 2 pediment gravels that contain travertine downslope from the oldest dated sample may be younger than about 320 000 years.