³HE EXPOSURE AGES FOR GLACIAL DEPOSITS, FISH LAKE PLATEAU, UTAH

The Fish Lake Hightop Plateau (2700 – 3500 m), in south-central Utah, experienced Pleistocene glaciation. Glacial moraines were mapped at four locations in the Fish Lake area: Pelican Canyon, Jorgenson Cirques, Tasha Creek, and the Seven Mile Cirques. Earlier researchers recognized moraines from two periods of glaciation and interpreted these as the results of Bull Lake (WI) and Pinedale (WII) glaciations. The WII moraines are steeply sloping (~30°) with 1-2 m boulders exposed along the sharply crested moraine ridges. WI moraines are exposed below the Jorgenson Cirques and Seven Mile Cirques and characterized by diffuse, low relief surfaces with few large boulders exposed along the moraine crests. Bathymetric analysis of Fish Lake indicates the presence of a submerged WI moraine associated with Pelican Canyon. We determined ³He exposure ages of pyroxenes from large andesite boulders exposed in prominent moraine crests at several locations around the Fish Lake basin. The exposure ages were corrected for non-cosmogenic ³He using a suite of shielded samples and for possible snow shielding. The mean exposure ages from four different sharp crested moraine sets (WII) range from 21.1±2.1 to 23.2±3.7 ka. These ages are coincident with the global LGM (21±2 ka) and similar to other cosmogenic ages of main Pinedale advances in the western United States. Two boulders on a smaller recessional lateral moraine in Tasha Creek canyon range from 15.5±1.8 to 17.0±2.0 ka and suggest deposition during a recessional standstill or small readvance during deglaciation. These ages are also similar to other cosmogenic chronologies from the western United States and suggest deposition associated with cooling related to Henrich event 1. A weathered boulder exposed on the eroded moraine set (W1) at Jorgenson Creek yielded an age of 135±4 ka. Considering the effects that boulder weathering and moraine erosion have on boulder exposure ages, we suggest that this is a minimum age for this glaciation (MIS 6).