Geohydrologic reconnaissance of the Imperial Valley, California

Abstract

The Imperial Valley occupies a broad lowland in the southern, wider part of the Salton Trough section of the Basin and Range physiographic province.The trough is a landward extension of the depression filled by the Gulf of California, from which it is separated by the broad, fan-shaped subaerial delta of the Colorado River.Much of the land surface is below sea level, and the valley drains northwestward to the Salton Sea, which was 232 feet below mean sea level in 1968.The Imperial Valley is bordered by the Chocolate Mountains on the northeast, the Peninsular Range of Baja California and southern California on the southwest, and the Salton Sea on the northwest; it is contiguous with the Mexicali Valley in Mexico on the southeast.The Salton Trough, which evolved during Cenozoic time, is a structural as well as a topographic depression in which the surface of the basement complex lies thousands to tens of thousands of feet below the basement-complex surface in the bordering mountains.The basement complex is composed of plutonic rocks of early and late Mesozoic age which intrude Mesozoic and older metamorphic rocks.The Salton Trough is traversed by the San Andreas fault system.Development of the trough involved both folding and warping as well as faulting; much of the folding is related to movement along the major faults.Structural relief caused by folding, faulting, and warping is inferred to exceed 14,000 feet.The pre-Tertiary basement complex of the trough is overlain by a thick sequence of predominantly nonmarine sedimentary rock that ranges in age from Eocene to Holocene.The Cenozoic rocks beneath the south-central part of the Imperial Valley probably are more than 20,000 feet thick.Rocks as old as Eocene crop out in the bordering mountains, but none of the fill in the central part of the trough appears to be older than about middle Miocene, and most of it is Pliocene and younger.The sedimentary and volcanic rocks of Eocene and Miocene age that are exposed in the mountains are moderately to strongly deformed and are semiconsolidated to consolidated.Consequently, these rocks constitute an insignificant part of the ground-water reservoir.A marine unit, the Imperial Formation of late Tertiary (Miocene or Pliocene) age, is extensively exposed in the western part of Imperial Valley, but apparently it was not penetrated in an oil test hole 13,443 feet deep in the central part of the valley.The Imperial Formation is overlain by a thick heterogeneous sequence of nonmarine deposits.Some of the deposits were derived locally, but most were brought in by the Colorado River.Generally, the river deposits consist of silt, sand, and clay, as contrasted with the locally derived deposits of coarse sand and gravel near the margins of the valley.The last major marine invasion of the Salton Trough is probably represented by the Imperial Formation.Subsequent incursions of the Gulf of California appear to have been minor and of short duration.samples analyzed contained more than 2 mg/1 fluoride.Concentrations of dissolved solids apparently increase to the east.Test well LCRP 8, perforated from 135 to 560 feet, yielded water that contained about 2,000 mg/1 dissolved solids; two test wells a few miles farther east yielded water containing about 5,000 mg/1 dissolved solids.'The members, as Christensen described them, were assigned names, but since these names have no official status dnd do not fully correspond with the units of Woodring (1932), they are not indicated here.