PLEISTOCENE GLACIATION ON THE FISH LAKE PLATEAU, UTAH: RECESSIONAL LGM MORAINE CHRONOLOGY AND NUMERICAL MODELLING OF PEAK TO RECESSIONAL LGM ICE CAP AND CIRQUE GLACIERS

The Fish Lake Plateau (FLP) is located in central Utah and is capped with Tertiary volcanic rocks from the Marysvale volcanic center. Previous work by our group determined cosmogenic ³He exposure ages of pyroxene bearing andesite boulders atop prominent moraines around the FLP Hightop. These data include moraines from transection or ice-cap outlet type glaciers as well as cirque constrained glaciers. Recalculated exposure ages (LSDn) of published and unpublished ³He data from inferred LGM-age moraines at Rock Canyon, Pelican Canyon, Jorgenson Creek, Tasha Creek, Sevenmile, Mt Marvine, and the Potholes sites yield ³He ages of 14-42 ka (n=30). Removing three older outliers (>35 ka) and using the P-CAAT tool (Dortch et al., 2022 QG), produces relative probability curves with a prominent Gaussian peak at 20.7 ka and a smaller secondary peak at 16.9 ka. The smaller peak is in line with many other recessional moraine records in the Western US. Here, we mapped and sampled possible recessional moraines from sites around the FLP. Recently available 1-m DEM data aided in that effort; however, the recessional moraine record around the FLP is spotty and not well developed. The most obvious recessional moraines were at Rock Canyon, where three boulders yielded new ³He ages of 14.6, 16.8 and 16.8 ka. At Pelican Canyon and Tasha Creek, recessional moraine scraps yielded ages that overlap with the outboard LGM moraines (19-21ka). At Sevenmile a prominent ridge 0.4 km upvalley from the terminal moraines yielded four ages ranging from 14.9 to 20.6 ka with three ages ≥19.5 ka suggesting quick retreat from the terminal position well within the peak LGM period. The features at Rock Canyon are by far the best in terms of clear and identifiable recessional moraines as well as exposure age data similar to other regional sites. We also simulate the LGM and recessional Fish Lake ice cap/valley glaciers using a glacier model that represents ice extent in x, y, and z using the shallow-ice approximation, basal sliding, and a representation of the surface energy balance for melt and sublimation. We reproduce the mapped maximum ice extent of the ice cap to estimate MIS 2 paleoclimate. We also investigate how the threshold response of the ice cap-plateau system to deglacial climate change combine to produce the recessional pattern of moraines around the FLP.