NEW GEOLOGIC MAPPING IN THE UPPER SANTA CRUZ BASIN, SOUTHERN ARIZONA; ESTABLISHING THE FOUNDATION FOR MODELING THE BASIN HYDROGEOLOGY

New geologic mapping in the upper Santa Cruz Basin, southern Arizona, has provided new insights into the hydrogeology of the basin. Our objectives focused on mapping alluvial aquifer units because existing published regional maps lacked detail for those units. Our mapping also identified newly recognized faults and fractures which may control ground water flow in the basin, and it formed the basis for producing a 3-D hydrogeologic model to more fully understand the basin subsurface hydrostratigraphy and structure.

Geophysical studies define a series of subsurface alluvial sub basins in the upper Santa Cruz Basin ranging from 500 to 800 m in depth. The sub basins are generally filled with three hydrogeologic units. The lowest and thickest unit includes moderately fractured sandstones of the Miocene Nogales Formation, with maximum thicknesses of about 1 km. These thick sequences of the fractured sandstones combined with porosity, hydraulic conductivity, and fabric analyses of sandstone samples from the formation, indicate the Nogales Formation sediments may be potential deep sources of groundwater in upper Santa Cruz Basin. Late Miocene to early Pleistocene basin-fill deposits overlie the Nogales Formation and have maximum thicknesses of 150 m. Early Pleistocene to Holocene surficial deposits form the uppermost hydrogeologic unit, and were deposited mostly within deeply incised axial valleys of the Santa Cruz River and major tributaries including Nogales Wash and Sonoita Creek. The surficial deposits have maximum thicknesses of about 30 m.

Our new geologic map includes a network of cross sections, illustrating the depth, geometry, and structure of the alluvial sub basins forming the basin aquifer system. Several north-south oriented sections were constructed along the axes of the Santa Cruz River, and Nogales Wash inner valleys, to show the geometry of the sub basins, and subsurface bedrock divides separating the basins. A series of east-west section tie lines intersect the north-south sections and show the deepest parts of each sub basin. This design allows for visual interpolation in three dimensions across the cross section grid, to define the geometry and thickness of the aquifer units, and visualize changes in basin depth and structure across the sub basins forming the upper Santa Cruz basin aquifers.