USGS Home
Contact USGS
Search USGS

Geologic Framework

Project Home Geology Geophysics Remote Sensing Geochemistry Publications Gallery

The Upper Santa Cruz River Watershed, including the Tucson ground water storage basin and sub-basins to the south, are critical resources for the human populations and industries in the region. Consequently, understanding what potential substances are introduced naturally into the regional aquifer system becomes critical. Thus, we begin here by asking an important question.What non-anthropogenic aspects of the geologic terrains of south-central Arizona are effective in determining ground water underflow and water quality?

Fractures within rocks and porous rock formations are the pathways for ground water underflow, and the chemical nature of the minerals that compose the rocks in contact with the water largely determine the water quality. In general, in south-central Arizona, rocks in which the original minerals remain fresh do not contribute deleterious compounds or elements to the waters flowing through them. However, the region contains several large metallic ore deposits that may be natural sources of potentially toxic concentrations of heavy metals and associated compounds to the ground water system. The risk of metal uptake by ground water is greatest where the deposits are situated directly along the flow paths followed by the fluids, especially in altered rocks prone to the production of acidic ground waters.

To understand shallow and deep ground water chemistry, and predict specific places where ground waters may be a problem, requires a knowledge of the regional geology and rock chemistry, and the nature of regional ground water flow paths and how they are connected. This summary of the regional geologic history and ore deposits of particular importance within the Santa Cruz River drainage basin provides a useful framework for the other studies within the project.

An Historical Perspective

The geology of south-central Arizona records nearly two billion years of Earth history. Rocks that are 1.8 to 1.4 billion years old record the geologic processes that occurred at the western margin of the North American Plate as it converged on another crustal plate. During the next billion years or more sediments were alternately deposited on and eroded from a "passive" (geologically stable) continental platform. The last 260 million years has seen renewed, intermittent movements along or near a convergent plate margin. Each of these three distinct geologic environments has produced rocks and structural settings that are important factors in understanding the present evolution of the Santa Cruz River drainage basin.

Rocks that are 1.8 to 1.4 billion year old belong to the Proterozoic Era. In south-central Arizona they are composed of volcanic, plutonic, and sedimentary types, but are not extensively exposed at the surface in the Santa Cruz River drainage system. However, geophysical and geochemical information indicates these rocks are present as a thick "basement" underneath the younger rocks exposed at the surface. Once formed, these old rocks were largely eroded throughout the remainder of the Proterozoic Era, eventually forming a continental platform of low topographic relief. However, during the following Paleozoic Era the platform was a passive marine environment and the site of deposition of carbonates, evaporites (mainly gypsum) and quartz-rich sediments.

The region once again became an active continental margin at the end of the Paleozoic Era, approximately 245 million years ago. Between 215 and 180 million years ago, from the LateTriassic to Jurassic Periods, an arc of volcanoes lay along the south-central Arizona continental borderlands. The volcanoes grew in response to the westward movement of the North American Plate where it converged with a separate plate of oceanic rocks moving eastward. The heavy mass of the oceanic plate caused it to plunge (subduct) beneath the lighter continental plate and to move deep into the Earth's interior. Such movements resulted in melting of the Earth's interior and the rise of the hot materials into and onto the Earth's' surface. Slow cooling of this material within the Earth's crust formed coarse-grained igneous rocks called "plutons", whereas faster cooling at the surface produced volcanic rocks that built volcanoes. The geography of this volcanic front is interpreted by some geologists to resemble the present-day landforms of central America. The chemical compositions of the volcanic and plutonic rocks formed at this time were very similar to those of Proterozoic age that were produced in a similar geologic setting. Thus, the chemical interactions between surface and ground waters are similar for all of these rock types.

More Geology

For more information, contact Byron R. Berger.