COSMOGENIC 10BE AND 36CL DATING OF GLACIAL MORAINE SEQUENCES IN THE TAYLOR RIVER VALLEY, SOUTHERN SAWATCH RANGE, COLORADO

Cosmogenic ¹⁰Be was measured in seven samples collected from the tops of large (>1.5 m) surface boulders along the crests of two terminal moraine complexes of the Pinedale-equivalent Taylor River Glaciation. Zero-erosion ¹⁰Be ages corrected for altitude, latitude, and topographic shielding vary between 15.1 and 19.5 ka. (Note that these numbers are likely to change slightly with continued data reduction.) Measurements of ³⁶Cl in three samples yielded ages well within this range. Age ranges are interpreted here, as they have been elsewhere, to represent the interval of moraine construction. Using an assumed rock-erosion rate of 1 mm ka ^-1 yields ages of between 15.3 and 19.9 ka. This assumption is based on comparison of the ¹⁰Be and ³⁶Cl measurements, and studies of erosion rates measured on similar granitic lithologies in regions of comparable climates and climate histories. Additional corrections for shielding by seasonal snow cover indicate that the latter ages may be as much as five percent too young. Thus deposition of the terminal moraine complex in the Taylor River Valley began as early as 20.8 ka and continued to as late as 16.0 ka. These dates also imply regional synchrony of the last glacial maximum (LGM) in the middle Rocky Mountains. Ice retreat from the LGM position presumably began ~16.0 ka ago. A preliminary ¹⁰Be age suggests the glacier retreated ~3.2 km to the second of twelve distinct recessional moraines by 14.5 ka, corresponding to a mean retreat rate of 3.9 m a ^-1. A boulder sampled on the ninth recessional moraine yielded a ¹⁰Be age of 11.7 ka, possibly marking this position as the glacier’s stillstand or readvance in response to the Younger Dryas cooling event.