26 Hydrogeology (Posters)

Sunday, 9 October 2011
9:00 AM - 6:00 PM
Hall C

RE-EVALUATING THE STRATIGRAPHY, STRUCTURE, AND HYDROGEOLOGY OF THE MIOCENE NOGALES FORMATION, UPPER SANTA CRUZ BASIN, SOUTHERN ARIZONA AND NORTHERN SONORA, MEXICO

Paper number: 26-9
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Although the main aquifers of the Upper Santa Cruz basin have been reported to consist mostly of Miocene to Pleistocene alluvium (upper basin-fill aquifer), the Miocene Nogales Formation (lower basin-fill aquifer) may have greater hydrologic significance in the basin than previously known. Well data indicate the basin contains significant thicknesses of the Nogales Formation in the subsurface, and the formation locally forms the basin-fill aquifer along the Santa Cruz River at the US-Mexico border. The Nogales Formation is poorly studied, and our preliminary mapping suggests it may require significant stratigraphic revision. The formation was originally defined to consist of lower, middle, and upper informal members, but these members are difficult to trace across the basin because of rapid lateral facies changes, and the fact that exposures of individual members in stratigraphic sequence are rare. The lower member was described as tuffaceous conglomerate and sandstone, and tuff; the middle member as distinctly bedded tuffaceous sandstone and conglomerate, and tuff; and the upper member as volcanic conglomerate.

Our new mapping in the Rio Rico and Nogales 1:24,000-scale quadrangles resulted in identification of a previously unrecognized massive claystone unit that appears to be transitional with and above the lower member of the formation. The claystone may be hydrologically significant as a local confining unit in the basin, and represents a lacustrine or marsh facies. In addition, the claystone has some thin beds that contain angular pumice fragments, indicating initiation of syn-depositional volcanism and rapid burial. The upper part of the Nogales Formation contains abundant pumice clasts, volcaniclastic sediments, and air-fall and ash-flow tuff, and corresponds to increased episodes of volcanism upward through the formation. We are currently analyzing volcanic samples for geochronology to determine a more precise age for the formation, and the source of the volcanism. Basin and Range syn-extensional faulting deformed all rocks of the Nogales Formation indicated by abundant intraformational fractures and faults, and indurated gravels of the lower member have abundant sub-vertical, open fractures which may enhance transmissivity.