VOLCANIC STRATIGRAPHY OF THE CREEL-DIVISADERO AREA, CHIHUAHUA, MEXICO

Although the Sierra Madre Occidental (SMO) is known as the world’s largest silicic volcanic province, it remains mostly unmapped in terms of delineation of it’s ignimbrite cooling units and identification of it’s calderas. The Creel-Divisadero area is located in the central axis of the SMO. The geology there appears to resemble the central part of Colorado’s San Juan volcanic field e.g. structurally composed of overlapping resurgent calderas and stratigraphically consisting of ignimbrite sheets with stratigraphic complexities induced by caldera-related topographic relief.

Previous work in northwestern Chihuahua has documented caldera complexes at Tomochic and near San Juanito, north and northeast of the Creel-Divisadero area respectively. Ignimbrites of the Tomochic volcanic center have been traced into the upper part of the volcanic section northwest of Creel where they overlie ignimbrites erupted from calderas near San Juanito. The only previous work in the Creel-Divisadero area is a geochemical study at Copper Canyon in which rocks a short distance south of Divisadero were examined from river to rim. That work described a 1 km-thick sequence dominated by altered lava overlain by 400 meters of ignimbrite.

Work in progress on a 1,400-meter section immediately north of Divisadero shows quite a different sequence. There, the upper 700 meters consist of six different ignimbrites formations, including at least two erupted from calderas near San Juanito. The lowest of these units is a distinctive, massive white ignimbrite, as much as 300 meters thick, but which shows pronounced lateral thickness variations. The overlying ignimbrites also show significant lateral thickness variations between Divisadero and Creel. While work has just begun on the section’s lower 700 meters, the one available sample is also an ignimbrite, and the possibility that this represents an intracaldera facies is being explored. A Copper Canyon caldera would also be consistent with and would help explain the pronounced thickness variations of overlying units in the Creel-Divisadero and area.