K-Ar Mineral Age of Ash Bed in Pico Formation, Ventura Basin, California: ABSTRACT

Abstract

A widespread volcanic ash layer, 1-4 inches thick, in the Pliocene Pico Formation (Wheelerian microfaunal stage) has been dated radiometrically. Twenty-five-pound samples of ash were taken from each of two localities. Each sample yielded about 4 grams of sanidine and 1 gram of biotite. These minerals are unaltered and show euhedral grain boundaries; some sanidine crystals have rims of glass. The glass matrix, comprising 95 per cent of the sample, is isotropic and apparently unaltered. Radiometric ages follow. Table All ages were run at Shell Development Company except the biotite from Ventura River, which was run by John Obradovitch, United States Geological Survey, Denver, Colorado. The mineral ages are consistent within the range 8.4-9.7 m.y. The glass ages are discordant, apparently because of argon leakage; the glass is therefore useless in radiometric dating despite its unaltered appearance. The upper part of the Pico is divided into two molluscan zones, a lower warm-water zone, and an upper zone containing a molluscan assemblage resembling that living today in Pacific Northwest waters. The temperature shift from warm to cold previously was correlated with the onset of Pleistocene glaciation and, because beds containing the cold-water mollusks are folded, the age of folding (Coast Range orogeny) traditionally has been considered to be intra-Pleistocene. R. F. Meade has shown that the base of the cold-water molluscan zone is just below the ash bed near South Mountain; it is possible that the molluscan shift is not caused by glaciation but by a pre-Pleistocene change in oceanic current patterns accompanying a general late Tertiary cooling. The new radiometric age and the invalidation by Durham, Jahns, and Savage (1954) of mammalian evidence for a Pleistocene age of folding suggest that both the molluscan temperature shift and Coast Range orogeny occurred in Pliocene time. End_of_Article - Last_Page 486------------