COSMOGENIC 10BE EXPOSURE DATING OF EROSIONAL LANDFORMS OF THE PROVO SHORELINE, LAKE BONNEVILLE BASIN

Lake Bonneville was the largest paleolake in the Great Basin of the southwestern United States during the Late Pleistocene. The rise and fall of the lake during the interval ca. 30-10 ka is represented by well-preserved sediments and landforms throughout the lake basin and is temporally limited by a well-developed radiocarbon chronology. After Lake Bonneville reached its highest shoreline elevation, failure of the lake threshold resulted in a catastrophic flood and a rapid 125-m drop in lake surface elevation to the Provo shoreline at ca. 18.0 ka, where the lake likely overflowed for 2-3 kyr before commencing its final regression. While the time when the lake dropped to the elevation of the Provo shoreline (1440-1455 m asl) is well established, the duration over which the lake occupied the shoreline is not. Erosional landforms formed in Prospect Mountain Quartzite along the Provo shoreline are suitable targets for cosmogenic ¹⁰Be exposure dating and can help to refine the chronology of the overflowing phase of Lake Bonneville. Cosmogenic ¹⁰Be exposure ages of wave-cut notches and platforms in the Silver Island Range and Table Mountain (the northern, main body of Lake Bonneville) and in the Cricket Mountains (Sevier body) are generally consistent with the existing radiocarbon chronology of the Provo shoreline, but with some important differences. Surfaces at the edge of the wave-cut platforms have exposure ages that predate the occupation of the Provo shoreline by 2-10 kyr, suggesting that these surfaces have some inherited ¹⁰Be. Surfaces of wave-cut notches and along wider wave-cut platforms have exposure ages that fall within the overflowing phase of Lake Bonneville when the Provo shoreline was occupied. The oldest exposure ages of wave-cut platforms near the Provo shoreline range from 18.5-18.0 ka, consistent with radiocarbon age limits on the Bonneville flood, but we cannot rule out the possibility of inherited ¹⁰Be from these surfaces. The youngest exposure ages of the wave-cut platform along the Provo shoreline have a mean of 16.5 ka, possibly indicating an earlier onset of the regressive phase of Lake Bonneville than previously thought. Continued development of a cosmogenic chronology of the Provo shoreline will help to refine the exposure age limits reported here.