GEOLOGICAL AND GEOPHYSICAL STUDIES OF THE HEADWATERS OF THE SAN PEDRO RIVER IN NORTHERN SONORA, MEXICO

The San Pedro River flows northward from Sonora, Mexico into Arizona's Gila River system forming part of a north-south riparian corridor that provides habitat for more than 100 resident and 250 migratory bird species. Ground water in the basin is used extensively on both sides of the border, and evaluation of the sustainability of the regional cross-border aquifer requires reasonable estimates of water use, recharge rates, sediment transmissivity and storage coefficients. Without an understanding of the three-dimensional basin framework and the constituent lithologic materials and their distribution within the catchment system, few of the essential modeling parameters can be estimated with any certainty. Aerial photographs, satellite imagery, and recent geologic mapping were used to delineate the boundaries between bedrock and alluvium over large uniform-appearing dissected bajadas and underlying bedrock pediment surfaces. Interpretations of bedrock and alluvial-fill geometry indicate that a significant portion of the catchment area in Mexico is underlain by bedrock composed of highly indurated Cretaceous sedimentary, volcanic, volcano-sedimentary, and granitic intrusive rocks. The region underlying the headwaters was affected by Basin-and-Range deformation that yielded an array of northwest-trending normal faults. The lateral extent of faulting and the resulting shape of the intervening basin were affected further by the presence of shallow intrusive rocks that form part of the pediment surface and underpin the prominent ranges that border the basin. Later north-north east structures further segmented the basin's subsurface geometry. Aeromagnetic surveys, an extensive network of vertical electrical soundings covering much of the central part of the basin, and strategic VLF surveys allow depth to buried bedrock to be estimated along with the location and thickness of clay layers that are confining units within the aquifer system. The data and interpretations suggest that the subsurface basin structure is more complex than previously recognized. In general, two hydraulically connected sub-basins are identified: one in the southern part of the study area and one in the northern part. Several shallow basin areas are suitable habitat for important wetlands occurrences.