RECONNAISSANCE GEOLOGY ALONG THE CENTRAL AXIS OF THE SIERRA MADRE OCCIDENTAL IN CHIHUAHUA, MEXICO

Mexican national highway 16 from Chihuahua City to Hermosillo, Sonora, provides the major access route across the northern Sierra Madre Occidental. As the central axis of the Sierra Madre Occidental volcanic field is approached, the highway to Copper Canyon splits away to the southwest until the two roads are separated by 85 kms where they reach the canyon country to the west. The triangular area thus defined lies on the central axis of the Sierra Madre Occidental volcanic field, and a picture of the regional geology of this area is beginning to emerge. Both highways traverse or pass by numerous calderas (documented or proposed). It seems plausible that, like the central axis of Colorado’s San Juan Volcanic field, there may be no area that is not within at least one central axis caldera.

Copper Canyon at Divisadero slices 1400 m into the central axis volcanic sequence, exposing six major ignimbrite formations. The oldest unit is at least one kilometer thick, suggesting that it lies within its source caldera. The second and fourth oldest ignimbrites are the only two at that section with a plag. + pyr. pheoncryst mineralogy. The sources for these ignimbrites are not known, but an enormous thickness of pyroxene-bearing ignimbrite forms much of the Sierra Gasachic where a caldera has been postulated. The third unit from the base has been correlated with a well-exposed, resurgent caldera near San Juanito, and the fifth oldest unit correlated with a resurgent caldera in the Sierra Manzanita. All of these calderas lie along the Copper Canyon highway. The source for the capping unit has not yet been established, but large and abundant lithic fragments argue for a nearby source caldera. Previous mapping along the Chihuahua-Hermosilla highway to the north has documented the Tomochic caldera complex, the source for the Vista and Rio Verde Tuffs. These units are not found at Copper Canyon, but they are found sandwiched between the canyon’s two youngest units where they are all exposed around the flanks of the San Juanito caldera. Copper Canyon, then, exposes a section that spans the region’s known stratigraphic record.