HYDROGEOLOGIC FRAMEWORK AND GROUNDWATER CHARACTERISTICS, SAN BERNARDINO VALLEY, ARIZONA AND SONORA

The San Bernardino Valley is an asymmetric extensional basin located in southeastern Arizona and northern Sonora, Mexico, which experienced extensive mafic volcanism in the Cenozoic. A wildlife refuge in the basin provides habitat for endangered fish with water from flowing artesian wells. This study was performed to aid in assessing potential threats to the refuge's water supply.

The basin's major fault is on the western side in Arizona; the Pitaycachi Fault on the eastern side is the major fault in Sonora. An oblique anticlinal accommodation zone (WNW-trending) separates these systems of oppositely-dipping normal faults. WNW-trending transfer faults dissect the Arizona section of the basin. These features align with a WNW-trending regional lineament.

We interpreted basin structure using geologic maps, DEMs, lithologic logs, and geophysical data. In Arizona, the bedrock forms a half graben, downdropped on the western side by antithetic normal faults, with structural highs formed by transfer faults. The sedimentary architecture was interpreted from lithologic logs, surficial geology, data from surrounding basins, and the geologic history of the area. The heterogeneous fill, which contains numerous basalt interbeds, was deposited predominantly in alluvial-fan and alluvial-slope environments.

The basin structure does not have a large effect on groundwater flow. As most water is transmitted through paleochannel deposits that make up a relatively small portion of the aquifer, variations in thickness induced by structural highs and lows do not significantly affect T or h values. Transfer faults do not affect the head gradient; these faults likely predate the majority of fill deposition.

Basin architecture does impact flow. In the far northern part of the basin, most fill is derived from sedimentary rocks, and few basalt flows present. In this area, water chemistry is dominated by incongruent dissolution of carbonates. To the south, fill is derived primarily from rhyolitic rocks and basalt interbeds are ubiquitous. Mg²⁺ levels increase, carbonate reactions are minor, and ion exchange with clays from weathering of volcani-clastics is evident. Near the basin center, lacustrine clays are present at depth; ion exchange dominates those waters. Basalts, present only near the surface, impart high Mg²⁺ to shallow waters.