A gravity station network was established over the Tucson Basin on a nominal one mile spacing for the purpose of determining major hydrologic boundaries. Gravity data were supplemented by magnetic, seismic, and borehole geophysical data. Gravity data reveal a marked linearity of pattern which in general correlates with structural trends previously noted in the area. Based on surface geological data, radiometric dating, and the present survey, it is suggested that the Tucson basin began to form in mid-Tertiary time under tensional stresses. As the circum-basin mountain blocks rose, the Tucson basin was gradually filled by the Pantano formation and its equivalents. Around the margins these were alluvial fan deposits which graded laterally into each other and into playa deposits in the central basin. During the Pliocene Epoch this pattern of internal drainage was disrupted and Pantano deposition ended with the inception of a period of erosion. Deposition of later sediments coincided with external drainage and throughflowing streams. The late-Tertiary and Quaternary sediments probably have been faulted to a minor degree by continuing movement along basement structures or by differential compaction over basement topography. The extent to which this affects groundwater gradients is a function of the percentage of coarse material in the aquifer. Further complications in development of these upper aquifers may be due to igneous dikes and to buried glide blocks of relatively impermeable sediments. The Pantano aquifers are probably broken into relatively small, autonomous hydrologic units by near-vertical fault systems. During this study two new applications of geophysical data were used to analyze basin hydrology. Using Gauss' Theorem the total mass deficiency of the basin was determined and from this a total water content of 129 cubic miles was calculated. The second application involved the use of a borehole density log to calculate the aquifer storage coefficient and revealed a probable value of 0.05, much lower than previously estimated.

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