Beneath the Tucson Valley

Have you ever wondered what lies beneath the pavement and neighborhoods in Tucson? We have some idea from all the water wells drilled in the Valley, but they barely scratch the surface. The deepest and most complete picture comes from a 12,556-foot deep well drilled by Exxon in 1972. Those were the days of the great “overthrust” play in oil exploration. (See an Arizona Geological Survey publication “The Great Southwestern Arizona Overthrust Oil and Gas Play.”) That Exxon well penetrated granite bedrock at a depth of 12,001 feet but never found the “overthrust.” A complete well log and interpretation can be found in USGS Scientific Investigations Report 2004-5076.

Between the surface and the granite bedrock, the sediments within the Tucson Valley record at least 145 million years of geologic history. The Tucson Valley was formed by crustal extension beginning about 25 million years ago. At first, there was low-angle “detachment” faulting (like fanning out a deck of cards) which pulled apart the rocks. One of those detachment faults (now completely inactive) crops out along the south edge of the Santa Catalina Mountains in the Foothills area. Later on, the rocks could no longer be stretched and faulting changed to steep mountain-front faults that provided the extraordinary depth to bedrock within the forming valley.

Following is an interpretation of the geological history of the Tucson Valley based on the Exxon well.

The bottom of the Exxon hole is in Precambrian granite (probably 1.4 billion years old), which in many places in Arizona is overlain by Paleozoic marine sediments deposited 541-251 million years ago (mya). These rocks do not occur in the Exxon hole, however, so they must have been eroded away prior to valley formation. This indicates that what is now a valley was then a highland.

Beginning in latest Jurassic time (ca. 150mya) and continuing through the Cretaceous (145-65mya), northeast-southwest extension created the Bisbee Basin into which the marine sediments of the Bisbee Group were deposited. (The Bisbee Basin was part of an inland sea which for a time, connected the Gulf of Mexico with the Arctic Ocean.) The Exxon hole penetrated these rocks at depths between 10,026 to 12,001 feet. The Bisbee Group rocks, sandstone, limestone, and conglomerate, are so named because they were first described from outcrops near Bisbee, AZ. In Bisbee, these rocks form the mountains at an elevation of 5,000 feet, but in Tucson they are two miles beneath the surface.

There may have been some erosion along a sea shore as evidenced by coastal plain deposits. In mid-Tertiary time alluvial fan deposits indicate that surrounding mountains were eroding. These deposits are interspersed with lava flows and lake deposits. A violent volcanic eruption 26 million years ago deposited an ash bed in the basin (8,500 to 9,000 feet in the hole). By this time, crustal extension was deepening the basin and accelerating erosion of the surrounding mountains and filling the basins with alluvial fan material, i.e., rocks and soil. The topmost part of this interval contains sandstones and siltstones interpreted to represent deposits from a braided stream.

At 2,980 feet in the hole, there is a sharp boundary between the upper unconsolidated and undeformed alluvial sediments and the denser, highly faulted basin fill below which indicates a change in tectonic style.

The upper 1,200 feet of the valley contains unconsolidated gravels derived from alluvial fans that contain the aquifers from which we pump part of our water supply. There are deeper aquifers as yet unexploited, but the water in deeper aquifers becomes laden with dissolved salts and metals. There are several volcanic ash beds between 1,150 and 1,350 feet. Below 2,000 feet are remnants of playa lakes with deposits of gypsum.

The graphic is a false-color landsat image of the Tucson Valley. Notice the linear, northeast-trending structures on the right side of the picture. These are large folds called synforms or synclines in the Catalina-Rincon Mountains metamorphic complex. These synforms coincide with the deepest parts or sub-basins of the Tucson Valley.

And now you know what lies beneath our feet.

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