A NEW 10BE CHRONOLOGY OF LATE PLEISTOCENE MORAINES AND GLACIAL OUTBURST FLOOD TERRACES IN THE UPPER ARKANSAS RIVER VALLEY, COLORADO

We used cosmogenic ¹⁰Be exposure dating to derive a chronology of Pinedale glaciation and deglaciation in Twin Lakes, Clear Creek and Pine Creek valleys in the upper Arkansas River basin, Colorado. At their Pinedale maximum extent, valley glaciers emanating from Clear and Pine Creek valleys dammed the Arkansas River. Glacier retreat led to outburst floods that deposited two boulder-rich terraces along the Arkansas River downstream of Pine Creek. Our chronology stems from 31 ¹⁰Be ages from moraine boulders, flood boulders and glacially-sculpted bedrock. The oldest Pinedale ice limit is apparently preserved in the Pine Creek valley dating to 22.4±1.4 ka, and ¹⁰Be ages from Clear Creek valley suggest ice initially retreated from its Pinedale maximum extent at 19.3±0.2 ka. This age of 19.3 ka correlates with the age of the upper flood terrace which dates to 19.2±0.1 ka, suggesting that the Clear Creek glacier may have acted as the main ice dam. ¹⁰Be ages also indicate that following a Pinedale maximum extent culminating at ~22.4 ka in the Pine Creek valley, ice re-occupied the same moraine at or before 15.8±0.4 ka. This surprisingly young age is supported by ages of 15.6±0.4 and 15.3±0.3 ka from glacially-sculpted bedrock upvalley of the moraine, which constrain timing of deglaciation in the lower Pine Creek valley. Interestingly, the age of the lower terrace of 17.8±0.6 ka is not expressed in the moraine records. Nonetheless, deposition of the lower flood terrace requires that glaciers were near their Pinedale maximum and damming the Arkansas River extent at that time. ¹⁰Be ages from glacially-sculpted bedrock inboard of the range-front end moraines reveal valley deglaciation spanning from ~15.5 ka to ~13 ka. Taken together, the new chronology indicates that Pinedale maxima culminated asynchronously at 22.4±1.4, 19.2±0.2, 17.8±0.6, and 15.8±0.4 ka, but that deglaciation took place relatively synchronously in all valleys between ~15.5 and ~13 ka.