LATEST PLEISTOCENE LAKE SHORELINES AND GLACIAL CHRONOLOGY IN THE WESTERN BASIN AND RANGE PROVINCE, USA - INSIGHTS FROM AMS RADIOCARBON DATING OF ROCK VARNISH AND PALEOCLIMATIC IMPLICATIONS

Accelerator mass spectrometry radiocarbon dating of organic matter in the basal layer of rock varnish provides a minimum surface exposure age for the underlying landform. Varnishes 14C ages indicate that the latest Pleistocene highstand of Lake Lahontan was abandoned before 12.7 ka, Searles Lake before 13.6 ka, Lake Manly before 13.0 ka, and Lake Mojave before 11.6 ka. The maxima of the Tioga glaciation at Pine Creek in the eastern Sierra Nevada, California, and the Middle Creek glaciation in the adjacent White Mountains of California-Nevada was just before 18-19 ka; major recessional moraines were deposited in these ranges before 12-13 ka. Penecontemporaneous glacial retreat and high levels in closed-basin lakes cannot be explained by glacial melt water, but probably reflects higher snowlines and more precipitation. A relatively minor glacial advance, perhaps related to the "Younger Dryas" in Europe, may be penecontemporaneous with a slight transgression in lake levels about 10-11 ka in the western Basin and Range, suggesting a temporary return to cooler or wetter conditions, or both at the termination of the Pleistocene. © 1990.