Analysis of gravity data from the Cortaro Basin area, Pima County, Arizona

Abstract

A site for potential ground-water reserves lies in Cortaro Basin just north of Tucson, Arizona where ground water flowing northward from the Tucson Basin exits through Cortaro Narrows. Information concerning the nature of the subsurface, including any possible obstructions to flow, would aid in future land planning. Land subsidence is also related to bedrock topography. In the Basin and Range province, gravity analyses have been useful in delineating the amount of basin fill and the alluvium-basement contact. Data from several wells drilled in the Cortaro region by the city of Tucson were used in the study. These wells had been geophysically logged, and the formation-density logs were used to determine the in-place bulk density of subsurface formations. The resulting density contrasts, which were used for modeling, are therefore more accurate than is common in gravity studies done in the Tucson area. An integrated geophysical approach, using subsurface geophysical well logging and surface gravity and aeromagnetic data revealed that Cortaro Basin is a deep, well-defined subsurface basin separate and distinct from the Tucson Basin and that concealed bedrock defines the edges of the basin where it is not bounded by mountains.