Bobtail Quartz Latite Member of the Gem Hill Formation
Bobtail Quartz Latite Member of the Gem Hill Formation
Miocene
Volcanic and hypabyssal intrusive rocks located between the town of Mojave and the Garlock fault. Consists of rhyolitic or quartz latitic dikes and volcanic plugs intruded into the Gem Hill Formation and extrusive flow breccias intercalated with Gem Hill Formation sedimentary strata (Dibblee, 1958, 1967). Commonly porphyritic (up to 30%) with plagioclase, sanidine, and quartz; locally flow-laminated (Dibblee, 1958, 1967). Only one occurrence within map area, which is undated. May be correlative with the volcanic complex at Soledad Mountain southeast of map extent, for which McCusker (1982) reports K-Ar ages ranging from 21.5 ± 0.8 Ma to 16.9 ± 0.7 Ma. Unconformably overlain by the late Miocene to early Pliocene Horned Toad Formation (May and others, 2011).