THREE-DIMENSIONAL HYDROGEOLOGIC FRAMEWORK MODEL OF THE RIO RICO AND NOGALES 7.5' QUADRANGLES, UPPER SANTA CRUZ BASIN, SOUTHERN ARIZONA

We developed a three-dimensional hydrogeologic framework model of the Rio Rico and Nogales 7.5’ quadrangles to investigate groundwater resources in the upper Santa Cruz basin, southern Arizona. Analysis of the model resulted in defining the geometry and thickness of the basin fill and alluvial aquifer system which provides water to Nogales and surrounding communities of southernmost Arizona. The model software enables viewing subsurface data interactively in three-dimensional space, with the ability to sequentially slice through the modeled geology to better understand the internal complexities of the basin geometry, structure, stratigraphy, and hydrology. We constructed the model based on integration of geologic maps and cross sections; digital elevation data; geophysical data including aeromagnetic, gravity, and transient electromagnetic data; and shallow water well data. The model consists of 3 layers which include a lowermost confining unit modeled as bedrock, and contains Jurassic monzonite, Cretaceous sedimentary, volcanic, and plutonic rocks, and Tertiary volcanic and plutonic rocks. The middle layer consists entirely of the Miocene Nogales Formation, which contains moderately fractured volcaniclastic sandstone, conglomerate, siltstone, and claystone. New hydrogeologic investigations indicate that parts of the Nogales Formation may be productive aquifers and a potential deeper source of groundwater for the area. The uppermost model layer includes late Miocene to early Pleistocene basin-fill and Quaternary alluvium combined, and although the basin-fill has good to excellent hydraulic properties, most of the water currently produced by the City of Nogales, AZ, has been from shallow Quaternary alluvium. Eighteen faults were modeled that may affect how water migrates through the model area. Some of the faults have displacements greater than 500 meters to expose the Jurassic bedrock as a central core (Mount Benedict horst block) within the model area. The Mount Benedict fault is the primary fault modeled, and it forms the east side of the horst block, extends across the entire model area, and controls the course of the Santa Cruz River from the Rio Rico Arizona area, southward into Sonora, Mexico.