PLEISTOCENE GLACIATION OF THE SOUTHERN HALF OF BOULDER MOUNTAIN, UTAH: EXPOSURE AGES OF MORAINE BOULDERS AND NUMERICAL MODELLING OF THE LGM ICE CAP

Boulder Mountain (BM) is located in south-central Utah. The mountain “summit” is actually a 180 km² upland plateau with elevations of 3250-3449 m. BM is capped with Tertiary volcanic rocks derived from the Marysvale caldera. The summit of BM had an ice cap during the LGM with ice flowing radially outward and spilling off the mountain as outlet glaciers at several reentrants. Smaller cirque constrained glaciers formed under the summit cliff. Previous work determined cosmogenic ³He exposure ages of pyroxene bearing andesite boulders atop prominent moraines in the W, N and NE reentrants. Recalculated exposure ages (LSDn) from those locations are LGM in age (18-22 ka), with two recessional moraines yielding ages of 14-17 ka. We found no evidence of MIS 4 or MIS 6 glacial deposits around the N half of the mountain. Here, we show new mapping and cosmogenic ³He exposure age data from the S half of the BM ice cap system. New sites with clear terminal moraines include the SW outlet glacier at Row Lakes, the S drainages of W and E Boulder Creeks, the Oak Creek cirque glacier, and the Donkey Creek outlet glacier to the E. Cosmogenic ³He ages of moraines in the S are also 18-21 ka with a mean of 20.1 ka. The Oak site had two ages from a lateral moraine in the MIS 4 range (57-71 ka). A possible morainal ridge downstream of the E Boulder Creek LGM age moraine has ages from MIS 3–6. Similarly, a bouldery deposit at Sawmill Point, high above W Boulder Creek has one 3He age in the MIS 4 range. These new data indicate that the whole of the BM ice cap likely advanced and reached its maximum extent at approximately the same time during the peak LGM / MIS 2. The MIS 4–6 ages suggest a more extensive record of pre-LGM glaciation in the S half of the mountain than in the N. We also simulate the LGM BM ice cap 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. The model shows that outlet glaciers flowing down E and W Boulder Creeks drained the largest portion of the ice cap. Using the glacier model, we constrain MIS 2 climate and explore the response of the ice cap to transient deglacial climate scenarios. The narrow elevation range for the equilibrium line altitude to intersect the plateau leads to a highly thresholded response of the BM ice cap to rising temperatures.