POTASSIUM-ARGON DATES OF THREE PLEISTOCENE INTERGLACIAL BASALT FLOWS FROM THE SIERRA NEVADA, CALIFORNIA

Potassium-argon dates of 90,000 ± 90,000 years and 60,000 ± 50,000 years on the basalt in Sawmill Canyon of the Sierra Nevada, California, that underlies a moraine correlated with the Tahoe Glaciation and overlies pre-Tahoe till show that the Tahoe is probably less than 100,000 years old. Pre-Tahoe basalt near Sonora Pass gives a date of 150,000 ± 30,000 years. These data support the correlation of the Tahoe with early Wisconsin. Tahoe Glaciation in the southernmost Sierra Nevada seems to have been no less extensive than pre-Tahoe glaciation. The basalt that forms the Devils Postpile, believed by Matthes (1960) to have been extruded between his El Portal (Sherwin?) and Wisconsin glaciations, gives a date of 940,000 ± 160,000 years.