Reconnaissance Volcanic Geology of the Batamote Mountains, Ajo, Arizona

Abstract

The Batamote Mountains represent a 14-16-million-year-old eroded shield volcano complex in the Basin and Range province near Ajo, Arizona. Analysis of Landsat 7 Enhanced Thematic Mapper “Plus” data and fieldwork enabled mapping the geology at 1:24,000 scale. Volcanic deposits and field relations indicate that mid-Tertiary eruptions initially involved mild explosions from at least six vents, evidenced by near-surface intrusive units surrounded by ash, oxidized cinders, and spatter deposits. The associated lava flows partly cover the pre-Batamote basement silicic volcanic rocks. At least three subsequent effusive phases emplaced multiple flows from a minimum of six vents on the cinder cone deposits and earlier lava flows. The last volcanic event involved fire-fountains that produced several short, thin, agglutinated flows, forming the steeper summit of the volcano. Extension and normal faulting caused rocks in the central portion of the Batamote Mountains to collapse; subsequent erosion of summit material left the amphitheater morphology seen today.