COSMOGENIC-EXPOSURE AGE LIMITS ON THE LAST GLACIAL MAXIMUM IN THE SOUTH-CENTRAL UINTA MOUNTAINS, NORTHEASTERN UTAH

The Uinta Mountains are an east-west trending range in the central Rocky Mountains that contain abundant evidence for late-Pleistocene alpine glaciations. We measured concentrations of terrestrial in-situ cosmogenic ¹⁰Be in six quartzite boulders on a moraine in the south-central part of the range to calculate exposure ages that provide the first age estimates of the local Last Glacial Maximum (LGM). Ages were calculated using a scaled production rate of 5.1 ± 0.3 atoms g SiO₂^-1 yr^-1; Licciardi et al. (2001) suggest that this production rate is reasonable for high elevations in the central Rocky Mountains. Cosmogenic-exposure ages of moraine boulders range from ca. 19 to 15 ¹⁰Be ka, and the weighted mean of five boulder exposure ages is 17.1 ± 1.6 ¹⁰Be ka (including propagated 1-sigma errors associated with analytical measurements and the ¹⁰Be production rate).

These exposure ages suggest that the LGM in the south-central Uintas occurred during marine oxygen-isotope stage 2, similar to other ranges within the central Rocky Mountains. However, slight variations in cosmogenic exposure ages of LGM moraines are apparent among the southern Uintas, the Wind River Range, and the Yellowstone Plateau. The range of ages in the southern Uintas is slightly younger than the range of exposure ages determined for moraines in the Wind River Mountains, and older than those on the Yellowstone Plateau. Our oldest boulder exposure age provides a minimum estimate of the onset of the local LGM, ca. 19 ¹⁰Be ka. This event may have coincided with the rise of Lake Bonneville, as suggested by Munroe and Mickelson (2002).

Given the potential for late-Pleistocene climatic variability within the Uinta Mountains, these exposure ages may not represent the age of the LGM for the entire range. Additional work is underway to determine whether the timing of the LGM varied spatially within the Uinta Range and among ranges in the central Rocky Mountains.