HYDROGEOLOGIC CHARACTERISTICS OF THE MIOCENE NOGALES FORMATION: POTENTIAL NEW DEEPER SOURCE OF GROUNDWATER IN THE UPPER SANTA CRUZ BASIN, SOUTHERN ARIZONA

Population growth, declining annual recharge to alluvial basins, and near depletion of groundwater produced from shallow alluvial deposits in the Nogales, Arizona area, has increased the demand to explore for deeper alluvial groundwater resources in this part of the upper Santa Cruz River basin. Results from new investigations focusing on hydrogeologic characteristics of the Miocene Nogales Formation suggest that parts of the formation may yield potential new deeper sources of groundwater. Much of this work is based on detailed geologic mapping and analysis of structural characteristics of the formation in the Rio Rico and Nogales 1:24,000-scale quadrangles accompanied by detailed depth to basement maps generated from geophysical datasets. We analyzed tuffaceous sandstone samples of the formation for total and effective porosity and for saturated hydraulic conductivity. Total- and effective-porosity values generally are from 20 to 40 percent, and saturated hydraulic-conductivity values (ranging from between 10^-5to 10^-3 cm/s (4 and 57 cm\d) are higher than typical for sandstones, and extend into the silty sand category. Thin-section analyses from sandstone samples show that most framework grains are derived from erosion from local Jurassic monzonite and Tertiary volcanic-rock sources. Matrices of most samples consist of disaggregated pumice fragments, but some contain predominantly silt and clay. Pore space consist of moldic, intercrystalline, and fracture porosity. About one third of the samples have fracture-induced porosity, which corresponds to variably intense open-spaced fracturing common in outcrops throughout the area. Brittle deformation at all scales occurred during mid-Tertiary Basin and Range extension, which appears to have been contemporaneous with deposition of the formation. Faults and fractures predominantly are north- to northwest-trending and regional in extent, and may significantly influence groundwater flow in the upper Santa Cruz basin by enhancing permeability and flow rates attributable to porosity characteristic intrinsic to the sedimentary rocks themselves.