CORRELATION BETWEEN THE TOMOCHIC AND COPPER CANYON AREAS, THE SIERRA MADRE OCCIDENTAL, CHIHUAHUA, MEXICO

A reconnaissance study of major ignimbrite sheets and their source calderas is currently underway in the large region of far western Chihuahua made accessible by a paved road to the well-known tourist attraction of Copper Canyon. The Copper Canyon area lies south of the previously mapped Tomochic caldera complex. It contains at least three calderas and a thick stratigraphic section that includes all of the major ignimbrite units exposed in the Tomochic area.

The oldest Tomochic ignimbrites, the Cascabel and San Felipe Tuffs, are exposed around the western half of a resurgent caldera located just northwest of the town of San Juanito. The caldera’s ring-fracture zone cuts the Cascabel ignimbrite. The San Felipe Tuff is exposed in erosional remnants along the caldera rim, and it is petrographically the same thick exposures forming the caldera’s resurgent dome. The middle part of the Tomochic section, the Vista and Rio Verde Tuffs, are exposed locally north and west of the San Juanito caldera and along Copper Canyon highway at the village of Bocoyna. The upper part of the Tomochic section, the Cueva and Herreria Tuffs, also extend into the San Juanito caldera area. With respect to lava rock, the area contains numerous silicic to intermediate flows at a variety of stratigraphic levels. Abundant exposures of the mafic lava rock known as SCORBA are found throughout the area interlayered with Cueva Tuff or Cueva-like soft ignimbrite.

Copper Canyon, itself, has yet to be mapped, but preliminary observations along trails and from the air reveal about six major ignimbrite sheets overlying a thick section of lava rock. An examination of available rock samples suggests that these ignimbrites may correlate with the lower and middle part of the Tomochic stratigraphic section.