Preliminary Geologic Map of Chiricahua National Monument, Cochise County, Arizona, with Digital Geologic Map Data

Du Bray, Edward; Pallister, John

Chiricahua National Monument contains one of the thickest and most complete sections of the Rhyolite Canyon Tuff, an ash-flow tuff erupted from the Turkey Creek caldera. The caldera, whose northern margin is a few kilometers south of the Monument, is a mid-Tertiary (27 Ma) volcanic depression that formed during eruption of the Rhyolite Canyon Tuff and partial evacuation of an underlying rhyolitic to dacitic magma chamber (Marjanierni, 1969; Pallister and others, 1990; du Bray and Pallister, 1991). The caldera is deeply eroded; volcanic and shallow plutonic levels are exposed. Shortly after eruption of the Rhyolite Canyon Tuff, the central part of the caldera was intruded and domed, and the structural margin was intruded by dacite porphyry. Dacite porphyry was erupted from a ring dike within the structural margin and accumulated as a series of thick lava flows within the caldera moat, between the central resurgent dome and the caldera wall. One of these lava flows either flowed over a low area of the caldera wall, or was erupted outside the wall, and flowed down a paleovalley into the area of the National Monument. An erosional remnant of this lava flow is now exposed atop Sugarloaf Mountain. A series of rhyolite lava flows, small-volume tuffs, and sedimentary rocks were then deposited in the caldera moat, along the southern margin of the map area. Erosion has modified and partly inverted the topographic expression of the caldera. The topographically high caldera rim was located within what is now Pinery Canyon. Erosion-resistant moat lavas now form the high terrane south of Pinery Canyon. Similarly, the high terrane of the National Monument was formerly a paleobasin where outflow facies Rhyolite Canyon Tuff accumulated to great thickness and welded densely. The resulting basin-filling tuff is more resistent to erosion than surrounding basement rocks and now forms a high-standing block.