CL-36 DATING OF THE PLEISTOCENE GLACIAL RECORD IN THE ICICLE CREEK DRAINAGE, CASCADE RANGE, WASHINGTON

Surface-exposure ages of granite boulders on seven moraines record the Late Pleistocene history of a major glacier system in the eastern Cascade Range. During the four youngest advances, the Icicle Creek glacier built terminal moraines at Leavenworth and upvalley at Rat Creek. Right-lateral moraines of these and segments of four earlier advances are preserved on Boundary Butte Ridge. The youngest boulder-rich moraines are sharp-crested, but successively older moraines have lower boulder frequencies and are more degraded. Ninety-one ³⁶Cl dates cluster in seven groups: Rat Creek II, 13,000-12,000 yr; Rat Creek I, 14,000-13,000 yr; Leavenworth II, 18,000-15,000 yr; Leavenworth I, 20,000-18,000 yr; Mountain Home, 77,000-71,000 yr; pre-Mountain Home, 98,000 yr; and Peshastin, 108,000 yr. Older weathered drift (Boundary Butte moraine) likely is >= 150,000 yr old. That the greatest Late Pleistocene advance occurred at the onset of the last glaciation (marine isotope substage 5d) was not anticipated, but the timing is consistent with that of major glacier advances elsewhere in North America and the Eurasian Arctic at the onset of the last glacial age, and with a rapid, dramatic increase in global ice volume at that time.