Basin and Range

This province of southern and western Arizona is where the earth’s crust was stretched and broken by numerous faults so that mountain ranges and basins (broad valleys) were formed by the vertical displacement of large consolidated blocks of rock. From mountain top to the valley basement, the average displacement has been estimated at approximately 10,000 feet, with the valleys filled by up to 7,000 feet of gravel, sand, and silt.

The sediments or alluvial materials that fill these valley basins originate from the mountains above, and typically consist of sands and gravels produced by the weathering of granite rock. The valleys are filled with materials produced by the action of erosion and transported by rivers and streams (Figure 7). Often, impermeable geologic barriers blocked the basins from forming rivers that would drain the basin and thus created lakes. In these cases, the valley fill may include lake deposits of silt and clay, and occasionally salt. Wells completed in the granites and other rocks of the mountain ranges bordering the alluvial valleys will vary in yield, depending on the number of water-bearing rock fractures intercepted by the well borehole. Note that local geology may vary from the generalizations made above.

Figure 7: Profile of a basin and range aquifer

Major agricultural areas of the state, as well as the cities of Phoenix and Tucson, are located in the Basin and Range province. Increasing ground water pumping continues to lower water table elevations, which has resulted in land subsidence in some locations.& Because of dropping water tables and local geology, wells in these sediments may require drilling to excessive depths to reach water-bearing zones. For example, in some locations within the San Pedro Valley, domestic water wells must drill through nearly 400 feet of the St. David Clay Formation to find water-bearing sands and gravels.

Across Arizona, pockets of alluvial sands and gravel, and lenses of ancient river gravel channels now buried in clay may result in finding water where none had been expected. In addition, the depth to water and thickness of the water saturated zone of the aquifer, and aquifer permeability, will control the ability of a well to yield sufficient volumes of water.