NEW EVIDENCE OF SYNCHRONEITY BETWEEN DEGLACIATION OF THE UINTA MOUNTAINS AND THE FALL OF LAKE BONNEVILLE

Field-based reconstructions of Pinedale-age glaciers in the Uinta Mountains show that equilibrium line altitudes (ELAs) rose ~600 meters from west to east across the glaciated part of range, and that the steepest rise in ELA occurred in the westernmost 80 km.  The ELA gradient across the Uintas is greater than the regional east-west ELA gradient documented in previous studies, and suggests that, given a uniform temperature depression, precipitation in the western Uintas was enhanced relative to eastern (and downwind) parts of the range during the Last Glacial Maximum (LGM).  Previous studies along with new LGM-glacier reconstructions presented here suggest that Lake Bonneville provided moisture to valleys in the western Uintas; this inference is supported by a recently developed chronology of glacial deposits in the south-central Uintas.  The youngest of thirteen acceptable ¹⁰Be cosmogenic surface-exposure ages of moraine boulders indicates that glaciers in the Lake Fork and Yellowstone canyons maintained their maximum extents until as late as 16.1 ± 0.9 ka (based on a ¹⁰Be production rate of 5.1 atoms g SiO₂^-1 yr^-1 at high latitude and sea level scaled for site elevation and latitude), up to ~2000 years later than glaciers in the Wind River range and the Colorado Rocky Mountains.  Calendar-year-corrected radiocarbon ages from Oviatt (1997) and more recent studies indicate that Lake Bonneville fluctuated about its hydrologic maximum until ca. 16 ka and possibly later.  Despite potential uncertainties in comparing radiocarbon and cosmogenic ¹⁰Be exposure ages, deglaciation in the south-central Uinta Mountains and the hydrologic fall of Lake Bonneville from the Provo shoreline were approximately synchronous.  Furthermore, the steep ELA gradient provides evidence for a causal relationship between Lake Bonneville and glaciers in the western Uinta Mountains.