Named for Geronimo Head in the Superstition Mountains, 4 3/4 mi northeast of Siphon Draw, Pinal Co, AZ in the Basin-and-Range province. Intertongues with Dogie Spring Member (new) of Superstition Tuff (new). Lies within the 375-sq-km area in the interior of the Superstition cauldron. Occurs in the Goldfield and Superstition Mountains. Overlies unconformably the 24-m.y.-old Siphon Draw Member (new) of Superstition Tuff. Intruded by the informally designated 20.1 +/-1.2-m.y.-old rhyolite and quartz latite lavas. Is stratigraphically older than Canyon Lake Member (new) of Superstition. Is 430 m thick at the Geronimo Head and thicker in the Goldfield Mountains. Consists of white to pale-yellow rhyolitic ash flows and breccias. The flows vary from partially welded to nonwelded, and are generally zeolitized. Abundant lithic inclusions of dacite and basalt. The base of the Geronimo Head is younger than the 24-m.y.-old Siphon Draw Member. Uppermost ash flow of Geronimo Head dated as 16.3 +/-0.5 m.y. by K-Ar method on sanidine. Geologic map; columnar section. Miocene age.

Source: GNU records (USGS DDS-6; Denver GNULEX).