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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.
A substantially modified history of the last two cycles of Lake Bonneville is proposed. The Bonneville lake cycle began prior to 26,000 yr B.P.; the lake reached the Bonneville shoreline about 16,000 yr B.P. Poor dating control limits our knowledge of the timing of subsequent events. Lake level was maintained at the Bonneville shoreline until about 15,000 yr B.P., or somewhat later, when catastrophic downcutting of the outlet caused a rapid drop of 100 m. The Provo shoreline was formed as rates of isostatic uplift due to this unloading slowed. By 13,000 yr B.P., the lake had fallen below the Provo level and reached one close to that of Great Salt Lake by 11,000 yr B.P. Deposits of the Little Valley lake cycle are identified by their position below a marked unconformity and by amino acid ratios of their fossil gastropods. The maximum level of the Little Valley lake was well below the Bonneville shoreline. Based on degree of soil development and other evidence, the Little Valley lake cycle may be equivalent in age to marine oxygenisotope stage 6. The proposed lake history has climatic implications for the region. First, because the fluctuations of Lake Bonneville and Lake Lahontan during the last cycle of each were apparently out of phase, there may have been significant local differences in the timing and character of late Pleistocene climate changes in the Great Basin. Second, although the Bonneville and Little Valley lake cycles were broadly synchronous with maximum episodes of glaciation, environmental conditions necessary to generate large lakes did not exist during early Wisconsin time.
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