Abstract

A date of 19.9 m. y. (early Hemingfordian) from a welded rhyolite tuff at the top of the type section of the Valley Springs Formation, and one of 25.7 m. y. (early Arikareean) from the basal welded rhyolite tuff in the Sonora Pass area, indicate that the Valley Springs Formation was deposited before late Miocene, and that rhyolitic volcanism in the Sierra Nevada began by earliest Miocene time.

Three dates of 8.8–9.0 m. y. (early Hemphillian) on biotite-augite latite, which stratigraphically overlies the Table Mountain latite in the Stanislaus drainage, show that the Table Mountain latite is a member of the Mehrten Formation, which contains the late Hemphillian Oakdale fauna and the early Clarendonian Two Mile Bar fauna.

A 3.5-million year date on basalt filling a canyon cut in the Chagoopa surface at Little Kern River, a 9.5-million year date on basalt from Jose Basin in Fresno County, and dates of 3.5–3.6 m. y. from basalts in the Upper San Joaquin drainage indicate that the Chagoopa surface and the Mountain Valley stage are at least as old as early Pliocene, and that the Sierra Nevada had a minimum relief of 4000 feet at that time.

Basalt beneath McGee Till on McGee Mountain gives a date of 2.6 m. y., and basalt on Coyote Flat gives a date of 9.6 m. y. Basalt beneath Sherwin Till in Owens Gorge has an age of 3.2 m. y. A rhyolite tuff and an overlying basalt in Deep Springs Valley, White Mountains, give ages of 10.9 and 10.8 m. y., respectively.

First Page Preview

First page PDF preview
You do not currently have access to this article.