A DETAILED CHRONOLOGY OF THE LAST DEGLACIATION DEVELOPED FROM 10BE DATING OF MORAINES AND BEDROCK IN THE UPPER ARKANSAS RIVER VALLEY, COLORADO ROCKY MOUNTAINS, U.S.A

Numerous studies have dated Pinedale terminal moraines across the western United States; however, few locations have precisely dated moraine sequences and bedrock transects spanning the last deglaciation. Thus, data in support of whether the last deglaciation was forced by global-scale mechanisms, such as increasing CO₂ concentrations, or regional factors, like changing moisture content or insolation, remains elusive. Here, we present a new and detailed chronology of mountain glaciation developed from ¹⁰Be dating in the upper Arkansas River valley, Colorado, to shed light on the dominant mechanism(s) that forced the onset and rate of deglaciation in this region. We use our growing database of ¹⁰Be ages to reconstruct the glacial history of the last deglaciation using an exceptionally well-preserved moraine sequence in Lake Creek valley, and multiple upvalley polished bedrock outcrops in the adjacent Clear Creek valley. ¹⁰Be ages from boulders on the Pinedale terminal moraines in Lake Creek valley average ~21.8±0.3 ka (n=14; Schweinsberg et al., 2016), which adds to previously published Pinedale terminal moraine ages of ~23.6±1.4 ka (n=5), 20.5±0.2 ka (n=3), and 16.6±1.0 ka (n=7) in adjacent valleys (Young et al., 2011). The rare preservation of multiple (>10) recessional moraines located between the Lake Creek valley Pinedale terminal moraine and a previously published upvalley bedrock sample (~15.8±0.4 ka; Young et al., 2011) affords an unusual opportunity to develop a detailed chronology of deglaciation. We will present new ¹⁰Be ages from three recessional moraines in this valley system, which will supplement one ¹⁰Be age from a recessional moraine boulder that dates to ~16.2±0.2 ka. Additionally, we will present new ¹⁰Be ages from glacially sculpted bedrock upvalley from the previously dated Pinedale terminal moraine in Clear Creek valley to constrain the timing and rate of glacier retreat. This growing chronology will lead to an improved understanding of the timing and pattern of Pinedale glaciation and deglaciation in the upper Arkansas River valley, CO, where 69 ¹⁰Be ages now exist or are being processed from moraines and upvalley bedrock transects.