A new age for the initiation of an overflowing Lake Gunnison and the regressive phase of Lake Bonneville

Abstract An ∼0.2-km-long gravel spit (1398 m above sea level) at Sunstone Knoll in the Sevier basin, Utah, prograded into Lake Gunnison, a shallow lake in the Sevier basin that overflowed northward into the Great Salt Lake basin during the regressive phase of Lake Bonneville. Six radiocarbon dates for Anodonta shells and one optically stimulated luminescence age, which overlaps with the uncertainty range of the radiocarbon dates, yield an age for spit development and therefore, the initiation of Lake Gunnison overflow, at ∼15.5 cal ka. This age is older than the age of a larger spit 8 m lower in elevation that ended its progradation in Lake Gunnison about 12 cal ka. Strontium isotope ratios of the Anodonta shells from Sunstone Knoll (0.71049, 0.71059, 0.71064) are within the range of values for Lake Gunnison. The new date from Sunstone Knoll is consistent with cosmogenic dates from the Provo shoreline for the initiation of the regressive phase of Lake Bonneville (about 70 m higher than the spit). The major climatic shift, which caused the lake water budget and hydrology to change from overflowing while the Provo shoreline was forming to closed-basin conditions during the regressive phase, occurred by about 16.5 cal ka.