VOLCANOSTRATIGRAPHIC AND HYDROGEOLOGIC STUDY TO SITE A WELL IN BASIGORABO, SIERRA TARAHUMARA, MEXICO

Many Tarahumaran villages near Creel, Chihuahua, Mexico, lack sufficient potable water. The Jesuit Mission in Creel, Chihuahua, Mexico, has been drilling wells in villages near Creel but many of the wells are unproductive. The villages are situated in the Sierra Madre Occidental volcanic field, which is one of the largest ignimbrite provinces in the world, yet has mostly been mapped only at the reconnaissance level and virtually no work has been done on its hydrogeology. Utah Valley University students and faculty conducted field work in October 2009 as part of an ongoing hydrogeologic study to aid the Mission’s well-drilling program. The focus of the research presented here is the volcanostratigraphy and fracturing near the village of Basigorabo, which has requested a well. Prior to our work, the area around Basigorabo had only been mapped as undifferentiated 29 Ma Divisadero Tuff. Our work shows Basigorabo sits on a plateau at 2408 m elevation that is underlain by a flat – lying, strongly welded and flattened crystal-rich tuff that is the base of a major cooling unit. The base of the welded tuff is at approximately 2390 m elevation, and it is underlain by another cooling unit that contains moderately welded pumice-rich tuff with minimal flattening at its top, grading downwards into tuff with moderately flattened pumice at 2347 m. The base of this flat-lying unit is not locally exposed. The tuffs in the area contain very low matrix permeability and fractures are necessary for a viable aquifer. The welded tuff at the base of the upper cooling unit is moderately fractured along a stream that follows a lineament visible in aerial photographs. Four fracture sets were measured near the stream; the dominant set has a density of 14 fractures per 4 m and is oriented parallel to the lineament (123/85 SW). Fracture density decreases rapidly to 1 fracture per 4 m 100 m from the lineament. The less welded tuff in the upper portion of the lower cooling unit contains no visible fractures. The strongly welded tuff along the lineament may form a viable aquifer if it is sufficiently thick. Our results also suggest that where strongly welded tuffs within the Divisadero Tuff are cut by fracture zones they are potential aquifers, whereas the less welded, upper portions of cooling units are unlikely to host fractured aquifers.