TIMING AND EXTENT OF LATE-WISCONSIN GLACIATION IN THE ALBION RANGE, SOUTH-CENTRAL IDAHO

The Albion Range in south-central Idaho supported two separate centers of alpine glaciation during the late Pleistocene, at Cache Peak (10,399’, 3170 m) and Mt. Harrison (9265’, 2824 m). Its position northwest (i.e., upwind) of Pleistocene Lake Bonneville suggests that the range may provide a crucial control for proposed lake effects on glaciers in the Wasatch and Uinta ranges. During the local late-Wisconsin glacial maximum, valley heads adjoining Cache Peak and Mt. Harrison fed several cirque glaciers and two small valley glaciers, which reached lengths of 2-3 km. The valley glaciers left complex recessional moraines and ice-stagnation deposits; these deposits impound five small lakes. In addition, each valley preserves small terminal moraines related to a late-glacial readvance in the region. Reconstructions of the late-Wisconsin maximum glaciers indicate that they had equilibrium-line altitudes (ELAs) of ~8900’ (2710 m) below Cache Peak and ~8440’ (2570 m) below Mt. Harrison. By comparing modern climatic conditions at the Wisconsin ELAs with those of modern glacier ELAs (e.g., Leonard, 1989), we estimate that the local climate was up to 8°C cooler in the summer, or ~3000 mm/yr water-equivalent wetter. A combination of 5.5°C cooler and 620 mm/yr w.e.-wetter provides the shortest path into the envelope of modern glacial conditions. Analyses of 25.7 m of sediment cores collected from five lakes in the two glaciated valleys help constrain the timing and character of deglaciation in the region. Most cores bottomed in diamicton underlying laminated inorganic silts related to deglaciation of the sites. Holocene sediments are finer-grained (silty clay dominant), with organic contents varying from ~15-30%. All cores contain a 2-6 cm-thick light grey tephra (Mazama ash; 6850 ¹⁴Cyr B.P.; 7500 cal yr. B.P). Cores from lakes below the small innermost (and youngest) moraines in each valley record a pulse of low-organic laminated silt associated with a late-glacial (?) advance that formed the moraines. Ongoing detailed sedimentologic and radiocarbon analyses of the cores will allow us to refine these findings.