Scientific Investigations Report 2007–5251
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5251
Version 2.0, June 2013
The Coachella Valley is the northernmost extent of the Salton Trough, which is the landward extension of a ridge/transform fault system (the East Pacific Rise) of the Gulf of California (McKibben, 1993). Near the end of the Miocene epoch, a spreading center separating the western Farallon plate from the eastern Pacific plate was obliquely subducted under the North American continent (McKibben, 1993). The modern Gulf of California and the Salton Trough formed about 12 million years ago during a period when subduction ceased and when the formation of an inland belt of east-west extension, alkali basalt volcanism, and crustal-spreading-induced subsidence and basin sedimentation began (McKibben, 1993). Prior to about 6 million years ago, the shear zone constituting the principal tectonic boundary between the Pacific and North American plates appears to have shifted about 250 km (155 mi) inland into this belt, initiating the formation of the modern Gulf of California and the Salton Trough (McKibben, 1993). As the Salton Trough opened, it was filled with sediment from the delta of the Colorado River. The river has been building its delta from the east into the trough since about 5 million years ago, and sedimentation has apparently kept pace with the crustal-spreading induced subsidence (McKibben, 1993). The relation between subsidence that has occurred on a geologic time scale, and vertical land-subsidence changes measured during this study are unknown.
The Coachella Valley is filled with as much as 3,700 m (12,000 ft) of sediments; the upper 610 m (2,000 ft) are water-bearing (California Department of Water Resources, 1979). In this report, the water-bearing deposits are referred to as the aquifer system, which consists of a complex unconsolidated to partly consolidated assemblage of gravel, sand, silt, and clay of alluvial and lacustrine origins (fig. 2). Sediments tend to be finer grained (contain more silt and clay) in the southern part of the valley than in the northern part because of the greater depositional distance from mountain runoff and from lacustrine deposition from ancient Lake Cahuilla. In the southern Coachella Valley, the aquifer system consists of, from top to bottom: a semiperched zone that is fairly persistent southeast of Indio; an upper aquifer; a confining layer; and a lower aquifer (California Department of Water Resources, 1964, 1979).
The near-surface semiperched zone overlies the upper aquifer southeast of Indio and consists of silts, clays, and fine sand. The semiperched zone is as much as 30 m (100 ft) thick and generally is an effective barrier to deep percolation (California Department of Water Resources, 1964, 1979). The upper aquifer is present throughout the Coachella Valley and consists of unconsolidated and partly consolidated silty sands and gravels with interbeds of silt and clay. In general, the upper aquifer is 45 to 90 m (150 to 300 ft) thick. The aquifer is unconfined except where it is overlain by the semiperched zone, southeast of Indio. In the southern Coachella Valley, the upper aquifer is separated from the lower aquifer by a confining layer of silt and clay that is 30 to 60 m (100 to 200 ft) thick. The lower aquifer is the most productive source of ground water in the southern Coachella Valley; it consists of unconsolidated and partly consolidated silty sands and gravels with interbeds of silt and clay. The top of the lower aquifer is about 90 to 180 m (300 to 600 ft) below land surface. Available data indicate that the lower aquifer is at least 150 m (500 ft) thick and may be as much as 600 m (2,000 ft) thick (California Department of Water Resources, 1964, 1979).
Geologic structures in the Coachella Valley have a marked influence on the occurrence and movement of ground water. The principal structural features of Coachella Valley are faults and fault-related drag and compressional folds. The most notable fault system is the northwest-trending San Andreas Fault Zone that flanks the eastern side of the valley (fig. 2). Although movement within the San Andreas Fault Zone is predominantly right lateral (across the fault, movement is to the right), vertical displacement has downdropped the southwest block (California Department of Water Resources, 1964). The faults have either juxtaposed consolidated rocks against partly consolidated or unconsolidated water-bearing deposits or displaced preferential flow paths in the partly consolidated or unconsolidated water-bearing deposits. This juxtaposition and displacement, in conjunction with cementation, compaction, and extreme deformation of water-bearing deposits adjacent to faults, can create low-permeability zones that can act as barriers to ground-water flow.
U.S. Department of the Interior | U.S. Geological Survey
URL: https://pubs.usgs.gov/sir/2007/5251
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