Central Region Mineral Resources Science Center
M. E. Gettings and B. B. Houser, U.S. Geological Survey, Tucson, Arizona
The thickness, distribution, and character of alluvial sediments that were deposited in the subbasins of the upper Santa Cruz structural basin in southeastern Arizona during the late Tertiary provide important constraints on ground-water availability of the area. Two basin-fill units are recognized; the middle to late Miocene Nogales Formation and an unnamed Pliocene unit termed upper basin fill. Porosity and saturated bulk density for the two units were estimated using a combination of down hole gravimeter data from nearby bore holes in similar sediments, grain density measurements of cuttings, and surface gravimetric profiles. The calculated porosity of the Nogales Formation is 16% giving a saturated bulk density of 2.32 g/cc; the calculated porosity of the upper basin fill is 21% giving a saturated bulk density of 2.24 g/cc. These values agree qualitatively with the better induration of the Nogales Formation observed in outcrop and the lower yields of water wells completed in the Nogales Formation compared to poorer induration and higher yields of the upper basin fill.
The complete Bouguer gravity anomaly map shows that four main subbasins make up the upper Santa Cruz structural basin. From north to south, these are the Amado, Tubac, Rio Rico, and Nogales subbasins. The Sopori subbasin (west of the Amado Subbasin) underlies Sopori Wash, a tributary of the Santa Cruz River.
More exact locations and shapes of these subbasins, thickness of upper basin fill, and depth to bedrock were estimated using a procedure involving interpolation of
This procedure indicates that the subbasins are shallow and contain significant thicknesses of the Nogales Formation. For example, the Amado subbasin is estimated to contain about 300 meters of upper basin fill and about 700 meters of Nogales Formation. The Sopori subbasin contains about 50 meters of upper basin fill and 650 meters of Nogales Formation. The small thickness of upper basin fill in the subbasins is, in some cases, a function of tectonic uplift and erosion following deposition, but in other cases, the subbasins may have experienced only limited extension and subsidence after deposition of the Nogales Formation.
A number of previously unrecognized faults are identified and known faults are extended using reconnaissance geologic mapping, study of driller's logs, interpretation of aerial photographs and thematic mapper satellite images, and inspection of contoured gravity and aeromagnetic anomaly data. Most faults that segment the main Santa Cruz basin and shape the boundaries of the subbasins are apparently pre-existing faults that have been reactivated by Basin and Range extension. One of these faults, the Mt. Benedict fault, controls the location of the Santa Cruz River as it crosses the basin and affects the gradient of the flood plain. The effects of this fault and of other faults within the basin fill on ground-water movement should be investigated.
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