PLEISTOCENE GLACIATIONS ON THE FISH LAKE PLATEAU, UTAH

The Fish Lake Plateau is located in south central Utah and is comprised of a group of high-elevation uplands underlain by Oligocene to Miocene aged volcanic rocks. Large areas of the Fish Lake Plateau were clearly glaciated during the Pleistocene. The Fish Lake Hightop likely hosted a small ice cap or permanent snow field that fed “outlet” glaciers that drained to the NE, ESE, and WSW. Several constrained cirques glacier complexes formed around the N and E periphery of the Hightop Plateau and isolated glaciers formed on the NW slope of Mt. Marvine and in the headwaters of U.M. Creek between Mt. Terrill, Mt. Marvine, and the U.M. Plateau. Well preserved terminal and latero-terminal moraines mark the furthest down slope advance of the last glacial event. The majority of these moraines have steep slopes (20–30°), relatively unweathered boulders, and only minimal soil development, suggesting they are Pinedale (~LGM, ~MIS 2) in age. In two locations, older accumulations of till with obvious to subtle morainal morphology are preserved down slope of the younger moraines. We determine cosmogenic 3He exposure ages for multiple boulders from the crests of six different moraines around the Fish Lake Plateau. We used a 3He production rate of 116 atoms/g/yr, scaled to each samples altitude and latitude (Lal, 1991), to calculate the exposure ages. The exposure ages were corrected for nucleogenic and mantle 3He using a suite of shielded samples and all the ages were corrected for possible snow shielding. The mean (± s.d.) exposure ages (excluding outliers) of four of the younger moraines range from 21.1±2.3 ka to 23.2±3.6 ka suggesting moraine stabilization during early Pinedale time. Exposure ages from the two older moraines are variable but somewhat suggestive of deposition during Bull Lake time (~MIS 6). ELA estimates for Pinedale age glaciers were determined using the AAR and THAR methods and range from 2950–3100 m. Comparison of modern climate conditions at the LGM ELA to a modern glacial envelope suggests that LGM summer temperatures were likely 8–12 °C colder than present and that winter snow packs were 1.5–4 times modern. More sophisticated paleoglacier modeling, additional exposure ages, and analysis of deep sediment cores from Fish Lake are forthcoming and will provide further insight into the Quaternary history of the Fish Lake Plateau.