The Marysvale region, located in the High Plateaus of south-central Utah (Fig. 1), was the site of extensive and prolonged volcanism from middle to late Cenozoic time. The area contains flows, breccias, and pyroclastics some 3,000 m thick covering approximately 1,000 km2. The volcanics range in composition from basalt to rhyolite and in age from 30 m.y. to at least 7 m.y. old.
The general geology and volcanic stratigraphy of the Marysvale area has been studied by Callaghan and co-workers (Callaghan, 1939; Callaghan and Parker, 1961a, 1961b, 1962a, 1962b; Willard and Callaghan, 1962) and more recently by Steven and Cunningham (Steven and others, 1977; Cunningham and Steven, 1977). The volcanic stratigraphy of the surrounding southern High Plateaus has been described by Rowley and Anderson (Rowley, 1968; Anderson and Rowley, 1975; Rowley and others, 1975), but correlations of volcanic sequences of rocks between the High Plateaus and the Marysvale area remain problematic. The reader is referred to Steven and others (1977) for a comprehensive discussion of the Marysvale stratigraphy.