COSMOGENIC 10BE EXPOSURE-AGE LIMITS ON TERMINAL MORAINES IN THE WESTERN WASATCH MOUNTAINS, UTAH, USA

Prominent terminal moraines from the Pinedale Glaciation at the western Wasatch Front have been known for more than a century. Clear morpho-stratigraphic relationships between shoreline features of Lake Bonneville and the distal slopes of terminal moraines at the mouths of the Little Cottonwood and Bells Canyons indicate that glacier ice reached the Wasatch Front prior to the Bonneville highstand (ca. 18.3 to 17.4 cal. ka). Although the radiocarbon-based chronology of Lake Bonneville is well established, the chronology of glaciation in the Wasatch Mountains is relatively imprecise due to a lack of direct numerical age limits on glacial deposits. To address this issue, cosmogenic ¹⁰Be surface exposure dating was applied to boulders at the tops of two different terminal moraines in the western Wasatch Mountains.

Based on constant production models used in version 2.2 of the CRONUS-Earth exposure age calculator, mean cosmogenic ¹⁰Be exposure ages from the ice-proximal sector of the right-lateral moraine at the mouth of the Little Cottonwood Canyon and from the right-lateral moraine in the American Fork Canyon are 15.7 ± 0.7 ka (n = 7) and 15.7 ± 1.6 ka (n = 10), respectively. These ages suggest that glaciers in the Wasatch Mountains occupied their terminal moraines after the Bonneville highstand. This finding appears contrary to morpho-stratigraphic observations of glacial and lacustrine features at the Wasatch Front, but these observations are from ice-distal sectors of terminal moraines whereas our exposure ages are from ice-proximal sectors of the moraines. A revised chronology of the Pinedale Glaciation in the western Wasatch Mountains is proposed to explain established morpho-stratigraphic observations and the exposure ages reported here. In this scenario, ice constructed the distal sector of the terminal moraine in the Little Cottonwood Canyon prior to the Bonneville highstand, and then persisted there or retreated and then readvanced to the ice-proximal side of the moraines at about 15.7 ± 0.7 ka. This interpretation is consistent with some Latest Pleistocene glacial chronologies from elsewhere in the U.S. Rocky Mountains, and suggests that the start of deglaciation in the Wasatch Mountains was approximately synchronous with the hydrologic decline of Lake Bonneville.