GLACIATION ON THE FISH LAKE AND AQUARIUS PLATEAUS, SOUTH-CENTRAL UTAH

The High Plateaus of Utah include six separate mountain ranges that supported glaciers during the Pleistocene. The Fish Lake and Aquarius Plateaus (Boulder Mountain) are the two easternmost ranges on the High Plateaus that have clear evidence of glaciation. On the Fish Lake Plateau three transection glaciers headed near the Fish Lake Hightop (3546 m) and several smaller cirque constrained glaciers formed around the Hightop massif. Cosmogenic ³He exposure ages for an older moraine at Jorgenson Creek range from 79–159 ka (n=4) and suggest deposition during the penultimate glaciation (Bull Lake age, MIS 6). Multiple boulder exposure ages (n=20) from four younger moraines yield a strong relative probability peak at 20.8 ka. ELAs for the Fish Lake Plateau LGM age paleoglaciers range from 2950–3190 m. Boulder Mountain, which is located 50 km SE of Fish Lake, is a high elevation plateau (maximum elevation = 3450 m; mean elevation of mountain top = 3303 m), that hosted a true ice cap with several outlet glaciers during the LGM. Cosmogenic ³He exposure ages for three outlet glacier (W, N and NE) terminal moraines range from 17–24 ka with a broad probability peak between 19–22 ka (n=27). Two small moraines 5 km up drainage of the Fish Creek (NE) terminal moraine yielded exposure ages ranging from 15–17 ka (n=12). Exposure ages of polished bedrock on the mountain top range from 25–124 ka (n=16) with the older ages closest to the center of the mountain top and younger ages towards the spillover locations at reentrants. Estimating the ELA for the Boulder Mountain ice cap and outlet glacier system is difficult but it likely ranged from 3250–3400 m. Glaciers on the Fish Lake Plateau and Boulder Mountain responded quite differently to climate changes at the end of the LGM. At Fish Lake, both transection and cirque glaciers retreated about 0.5–1 km before depositing recessional moraines at 15–17 ka. On Boulder Mountain the NE outlet glacier system retreated some 5 km up drainage before stabilization (or re-advance) and deposition of two small moraines between 15–17 ka. These changes suggest an ELA rise to above ~3300 m after the local LGM (~20±2 ka), which effectively shut off glaciation on Boulder Mountain but only caused minor retreat at Fish Lake, and then an ELA lowering at 15–17 ka to ~3100 m.