LACUSTRINE BASAL AGES FOR THE LAST GLACIAL-INTERGLACIAL TRANSITION IN THE UINTA MOUNTAINS, UTAH

Sediment cores extracted from 20 lakes in the Uinta Mountains of northeastern Utah constrain the timing of the last deglaciation and events during the glacial-interglacial transition. Each of the lakes is upstream from an LGM terminal moraine, and six are impounded by recessional moraines. All cores were retrieved by a percussion corer driven to the point of refusal, and all penetrated inorganic silty clay beneath gyttja. Organic material from as deep as possible within each core was submitted for AMS ¹⁴C dating. Material analyzed included terrestrial macrofossils, bulk sediment, pollen concentrates, charcoal, and daphnia ephippia. No radiocarbon reservoir effect was observed when ages on bulk sediment were checked against terrestrial material. Radiocarbon ages were converted to calendar years BP using the IntCal09 calibration curve in Oxcal 4.2. Given the stratigraphy observed in the cores, these calibrated basal ages are considered close limits on the timing of the local deglaciation and lake formation. There is no clear pattern of basal age vs. elevation, although younger ages generally correspond with greater elevation rise above downvalley terminal moraines. Rates of glacier terminus rise, calculated from ¹⁰Be cosmogenic surface-exposure ages for terminal moraines, and ¹⁴C ages for lakes at higher elevations, range from 70 to 100 m/ka, similar to values obtained via the same approach for the Wind River Range and the Colorado Front Range. Many of the basal ages fall within the Bølling/Allerød interval, matching regional evidence of rising temperatures, increasing aridity, falling pluvial lake levels, and glacial retreat at this time. These basal ages are also consistent with cosmogenic surface-exposure ages determined for striated bedrock (Refsnider et al., 2008, QR) and some cirque-floor moraines (Marcott, 2011, PhD dissertation) in the western Uinta Mountains. However, about half of the lakes, including all of those dammed by moraines, have basal ages that cluster early in the Younger Dryas interval, and in a second cluster ~11.5 ka BP. In one case, cores taken on either side of a moraine ridge bisecting a lake basin constrain moraine formation to between 16.7 and 12.6 ka BP. These younger ages raise the possibility of local cirque glacier advances during the Younger Dryas in the Uinta Mountains.