REVISED CHRONOLOGY OF LATE PLEISTOCENE GLACIERS, WASATCH MOUNTAINS, UTAH

During the late Pleistocene, glaciers extended beyond the mouths of Little Cottonwood and Bells Canyons on the western flank of the Wasatch Mountains. Till from these glaciers is in contact with lacustrine sediments from Lake Bonneville, a large pluvial lake that also existed during the late Pleistocene. Previous glacial chronologies have been based primarily on the stratigraphic relation between the till and the lake sediments, relying on the well-constrained radiocarbon chronology of Lake Bonneville. Using this approach, and one radiocarbon age that provides a maximum limiting age of the till, the OIS 2 glaciers have been previously interpreted to precede the highstand of Lake Bonneville by 4,000-5,000 years.

New stratigraphic exposures reveal that the till is interfingered with, or deposited on the lake sediments, suggesting that the glaciers were at their maximum extent either contemporaneous with, or post-dating the Bonneville highstand. In addition, geomorphic evidence suggests that the ice advanced beyond the canyon mouths at least once after the lake dropped below the level of the Bonneville highstand. New ¹⁰Be exposures ages from boulders on the crests of the two youngest moraines at Little Cottonwood Canyon support the interpretation of a more recent glacial advance. ¹⁰Be exposure ages indicate a glacial advance at approximately 16.9 ± 0.4 to 15.2 ± 0.4 ¹⁰Be ka (mean of 15.9 ± 0.7 ¹⁰Be ka), a time during which the lake was at the Provo stillstand, an open-basin phase occurring after the lake receded from the Bonneville highstand. This revised glacial chronology, coupled with the Lake Bonneville data, suggests that climate conditions in the eastern Bonneville Basin were cold and/or wet as late as 15.2 ± 0.4 ¹⁰Be ka, and that the glaciers and lake responded synchronously to changing climate conditions.