PRELIMINARY HYDROLOGIC SURVEY OF THE SIERRA TARAHUMARA, CHIHUAHUA, MEXICO: AQUIFER PETROLOGY

Many Tarahumaran Indian villages in the Sierra Madre Occidental volcanic field, Mexico, lack sufficient sources of potable water. Well – drilling efforts by the local Jesuit Mission of Creel to aid the villagers have been only marginally successful. A hydrologic survey was conducted by students and faculty of the UVU Department of Earth Science to aid future drilling. Here we report on a preliminary petrologic study of rocks penetrated by 11 producing and 7 dry wells in hopes of correlating rock type to well yield as determined by other workers in our group.

The studied wells are located near Creel, in the heart of the > 300,000 km² Sierra Madre Occidental volcanic field, which is composed primarily of Tertiary silicic ignimbrites and lavas, but generally has not been mapped in detail. The region is characterized by high plateaus, ca. 2400 m elevation, cut by deep canyons. The studied wells all are situated on plateaus, and penetrate the 29 Ma Divisadero Tuff. Rock samples were examined in outcrop and hand sample in the field, and thin sections were later made and examined petrographically. We found that the Divisadero Tuff contains poorly to strongly welded lapilli tuffs. Hoodoo – forming poorly welded tuff is present in outcrop at most poor-producing wells (hydraulic conductivities K < 1x10^-7 m/s), and contains 10% lithics, 35% plagioclase, 15% biotite, and 5% quartz. In contrast, strongly welded tuff outcrops at the two highest producing wells (hydraulic conductivities K = 3.9x10^-6 and 2.8x10^-7 m/s), and one example contains 10 % lithics, 45% plagioclase, 30% biotite, 1-5% quartz, and 1-3% hornblende. Both strongly welded tuffs contain a welded matrix and lithics and phenocrysts in greater abundance and size than the poorly-welded tuffs; however, the mineralogy of the two welded tuffs differ from each other. The welded tuffs grade upwards into poorly welded tuff, which we interpret to represent individual cooling units, and the differing mineralogy of the welded tuffs indicates the presence of at least two cooling units. The association of strongly welded tuffs and higher conductivity wells raises the possibility that they form better aquifers, possibly as a result of more fracturing or less alteration to clay, and warrants further investigation.