A complex sequence of Oligocene-age volcanic and volcaniclastic rocks form a major volcanic center in the Picacho area of the southeasternmost Chocolate Mountains, Imperial County, California. Basal-volcanic rocks consist of lava flows and flow breccia of trachybasalt, pyroxene rhyodacite, and pyroxene dacite (32 m.y. old). These volcanic rocks locally overlie fanglomerate and rest unconformably on pre-Cenozoic basement rocks. They are intruded and overlain by rhyodacitic to rhyolitic plug domes, lava flows, volcaniclastic deposits, and locally thick unwelded and welded rhyolite ignimbrite. South and southeast of a prominent arcuate fault zone in the central part of the area, the rhyolite ignimbrite (26 m.y. old) forms a major ash-flow sheet. In the southwestern part of the Picacho area the rhyolite ignimbrite interfingers with and is overlain by dacite flows and laharic breccia derived from a probable cone complex, the deeply eroded center of which is marked by a dacite plug that forms the southern half of Picacho Peak. The rhyolite ignimbrite and the dacite of Picacho Peak are overlapped by lava flows and breccia of pyroxene andesite (25 m.y. old) that locally rest on pre-Cenozoic basement rocks. The rhyolite ignimbrite has a minimum volume of 40 km3 and is not associated with a collapse cauldron or major subsidence features.
The volcanic rocks of the Picacho area form a slightly bimodal volcanic suite consisting chiefly of silicic volcanic rocks with subordinate andesite. The lavas have an alkali-lime index of 61, show no iron enrichment, and define a slightly potassic volcanic suite. Late Miocene augite-olivine basalt contains normative hypersthene but is most similar in major-element abundances to transitional alkali-olivine basalt of the Basin and Range province.
Normal separation faults in the Picacho area trend northwest and north parallel to major linear mountain ranges in the region. The areal distribution of the 26-m.y.-old rhyolite ignimbrite and the local presence of megabreccia and fanglomerate flanking probable paleohighs suggest that the ignimbrite was erupted over irregular topography controlled by northwest- and north-trending probable basin-range faults. These relations date the inception of faulting in south-easternmost California at pre-26 and probably pre-32 m.y. ago. A transition to basaltic volcanism in the area is dated at 13 m.y. ago and is significantly later than the previously cited date of transition to fundamentally basaltic volcanism in the southwestern United States.