NEW CONSTRAINTS ON THE TIMING OF LATE PLEISTOCENE GLACIATION AT TWIN LAKES IN THE UPPER ARKANSAS RIVER VALLEY, COLORADO: INITIAL RESULTS

We will present a new chronology of a late Pleistocene moraine complex in the Upper Arkansas River valley developed from cosmogenic ¹⁰Be surface-exposure dating. This research was stimulated by a new geologic map of the Granite 7.5’ Quadrangle covering Lake and Chaffee counties, Colorado, in which the surficial deposit interpretations have recently been revised based in part on newly acquired LiDAR data. This revised geologic map affords the opportunity to evaluate past glacial events in greater detail than previously possible. In particular, the map delineates a discrete moraine position of unknown age (Qtpo; “Pinedale older”) that lies between Bull Lake (Qtb) and Pinedale (Qtpy; “Pinedale younger”) moraines in Lake Creek valley near Twin Lakes. Only one previously obtained ¹⁰Be age of 19.7±0.8 ka constrains the age of the younger Pinedale moraine enclosing Twin Lakes, and there are no ages on older moraine positions. Thus, the primary objective of this study is to apply ¹⁰Be dating to develop a detailed chronology of the three mapped moraine positions enclosing Twin Lakes. These deposits provide the opportunity to reconstruct a glacial history extending as far back as the penultimate glacial maximum (Marine Isotope Stage 6) and spanning the last deglaciation. Results from this research will provide direct age control for multiple glacier maxima including a previously undated map unit, and help to define the broader Pinedale and Bull Lake glacial history in the Upper Arkansas River valley. Lastly, outcomes from this study will contribute towards elucidating broader-scale patterns of Pinedale glaciation in the western United States and provide insight on climatic and nonclimatic factors that influenced Pinedale terminal moraine records in this region.