Cosmogenic 10Be exposure dating of Bull Lake and Pinedale moraine sequences in the upper Arkansas River valley, Colorado Rocky Mountains, USA

Many formerly glaciated valleys in the western United States preserve detailed glacial features that span the penultimate glaciation through the last deglaciation; however, numerical age control is limited in many of these systems. We report 35 new cosmogenic 10Be surface exposure ages of moraine boulders in the Sawatch Range, Colorado. Eight ages suggest Bull Lake moraines in Lake Creek (range: 132–120 ka, n = 4) and Clear Creek (range: 187–133 ka, n = 4) valleys may correlate with Marine Isotope Stage 6. In Lake Creek valley, 22 10Be ages from Pinedale end moraines average 20.6 ± 0.6 ka, and 5 10Be ages from a recessional moraine average 15.6 ± 0.7 ka, indicating that glaciers occupied two extended positions at ~21–20 and ~16 ka. The glacial extent dated to ~16 ka was nearly as great as that of the earlier glacial phase, suggesting that climate conditions in the Colorado Rocky Mountains at this time were similar to those of the last glacial maximum. Combining these moraine ages with seven previously published 10Be ages from cirque and valley-bottom bedrock reveals that the Lake Creek paleoglacier lost 82% of its full glacial length in ~1.5 ka and was completely deglaciated by ~14 ka.