Major ignimbrites and volcanic centers of the Copper Canyon area: A view into the core of Mexico's Sierra Madre Occidental

Reconnaissance mapping along Copper Canyon highway has established ignimbrite stratigraphic relationships over a relatively large area in the central part of the Sierra Madre Occidental volcanic field in western Chihuahua, Mexico. The oldest ignimbrites are found in the central part of the area, and they include units previously mapped from north of the study area, in and around the Tomochic volcanic complex. Copper Canyon, at the southern end of the study area, exposes younger units, including the intracaldera tuff of the Copper Canyon caldera and five overlying ignimbrites. Well-exposed calderas are found near San Juanito, in the central part of the map area, and at Sierra Manzanita, to the far north. Stratigraphic evidence for yet another caldera in the northern part of the area is found in the Sierra El Comanche. The stratigraphic and limited available isotopic age data suggest that volcanism was particularly active similar to 30 m. y. ago. This reconnaissance survey also documented lava-flow lithologies consistent with previous observations from Tomochic that intermediate lavas have erupted throughout that area's volcanic history and that basaltic andesite became particularly abundant as felsic volcanism waned. The combined Copper Canyon - Tomochic area gives the first view into the core of the giant Sierra Madre Occidental volcanic field, expanding that offered by earlier reports, mostly from peripheral regions. The emerging picture is one of a dramatically thickened and stratigraphically complex volcanic section related to numerous overlapping caldera complexes, much like that documented for the core of the San Juan Mountains volcanic field, Colorado.