10-Be Chronology of Late Pleistocene Outburst Floods in the Upper Arkansas River Basin, Colorado

During the Last Glacial Maximum (LGM), several eastward-flowing glaciers originating in the Sawatch Range, CO expanded into the upper Arkansas River valley. One of these glaciers, located in the Clear Creek valley, dammed the Arkansas River impounding a large lake. At least four coarse-grained (boulders >4 meters) flood terraces in the Arkansas River basin attributed to ice-dam breaches have been identified down-valley of the Clear Creek tributary. Here, we use 10-Be dating to constrain the age of the youngest two terraces (herein referred to as upper and lower). 10-Be ages of 19.3 ± 0.8 and 20.9 ± 1.0 ka on two boulders from the upper terrace provide an age for the upper terrace and a maximum age for the lower terrace. Two additional boulders from the upper terrace and two boulders from the lower terrace are currently being processed and will provide additional chronological constraint. Preliminary results indicate that flood boulders on the upper terrace are in broad agreement with the older mode (22.0 ± 1.4 ka, n = 5) of a bimodal 10-Be age distribution of clasts from a single-crested Pinedale moraine in neighboring Pine Creek valley. The younger mode in this distribution (15.2 ± 0.9 ka, n = 7) suggests that ice was near its LGM maximum extent as late as 15.2 ka and may correlate with the younger flood terrace. Additionally, we sampled four boulders and one cobble from a single-crested, left-lateral Pinedale moraine in the Clear Creek valley to further assess the connection between moraine abandonment age and flood terrace age. 10-Be ages of large flood boulders on flat terraces may serve as better sample types than moraine clasts for determining the glacial history of the upper Arkansas River basin.