VOLCANOSTRATIGRAPHY OF THE DIVISADERO TUFF IN THE SIERRA TARAHUMARA NEAR ESCUELA SAN ELIAS, CHIHUAHUA, MEXICO

The Sierra Madre Occidental Volcanic Field in northwestern Mexico is one of the largest ignimbrite provinces in the world. The Jesuit Mission in Creel, Mexico has been drilling wells for Tarahumara Indian villages located on the volcanic field, but many of these wells have been unproductive. The wells sited by the mission were predominantly sited using water-witching techniques, such as dowsing rods. Previous work in the area consists only of reconnaissance mapping, in which the region was mapped as one undifferentiated 29 Ma Divisadero Tuff. Utah Valley University students and faculty traveled to Creel as part of an ongoing hydrogeologic study to aid the Mission’s well-drilling program. In order to determine which potential well sites may be productive, we mapped the volcanostratigraphy of the area around a productive well and spring near Escuela San Elias. The well is at 2460 m elevation and penetrates flat-lying, moderately welded and flattened tuff. The productive spring is at 2400 m elevation along fractures in strongly welded and flattened crystal-rich tuff. The strongly welded tuff grades upwards into the moderately welded tuff at the elevation of the well. Above the well, the tuff grades into moderately welded, slightly flattened tuff at a maximum elevation of 2498 m. We interpret the strata as a single cooling unit at least 98 m in thickness. The 50 m deep well reaches from the moderately welded central portion of the unit into the strongly welded base. Because the Divisadero tuff has very low matrix permeability, fracturing is considered to be a requirement for a viable aquifer. Fractures are abundant near the well, averaging over 9 fractures per meter, and the spring is located on highly fractured tuff. These observations suggest that zones of fracturing in and near the strongly welded bases of cooling units are viable locations for wells.