Abstract

Fossil pelecypods, Pecten (Amusium) lompocensis Arnold and Pecten (Lyropecten) crassicardo Conrad, were discovered in rhyohtic tuff and breccia of the Obispo Formation, San Luis Obispo County, California. The potassium-argon age of plagioclase obtained from the middle of the tuff is 20.9 ± 1.5 × 106 years. The age of plagioclase in the Morro Rock andesite-dacite volcanic neck is 23.5 ± 1.8 × 106 years.

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