Geology of the fresh ground‐water basin, Central Valley, California, with texture maps and sections

The Central Valley of California, which is about 400 miles long and averages about 50 miles wide, comprises about 20,000 square miles. Geologically, the valley is a large asymmetric trough that is bounded by granitic, metamorphic, and marine sedimentary rocks of pre-Tertiary age. The trough has been filled with as much as 6 vertical miles of sediment in the San Joaquin Valley and as much as 10 vertical miles of sediment in the Sacramento Valley; these sedi- ments range in age from Jurassic to Holocene.

Some volcanic rocks and deposits crop out in the valley, but of those only the Tuscan Formation in the northeastern part of the Sacramento Valley is of major importance to the fresh ground-water basin.

Post-Eocene continental rocks and deposits contain most of the fresh water in the Central Valley; they crop out virtually over the whole valley and in most places overlie or contain saline water at depth.

Continental rocks and deposits of Tertiary age include the Mehrten Formation. The Mehrten is of great importance to the fresh ground-water basin of the Central Valley and yields large quantities of water to wells.

Although continental rocks and deposits of Tertiary and Quaternary age compose a number of formations and informal deposits, in total they constitute the major aquifer of the Central Valley. In most places, similarity in sediment type between the continental deposits and some underlying rocks and deposits and even between separate units of continental rocks and deposits makes mapping of subsurface geologic contacts difficult if not practically im- possible. In this respect, a unit that can be mapped on the surface is difficult to delineate in the subsurface, and, although such a unit can be called an aquifer, it merges with similar units in the subsurface to form a major, widespread aquifer.

Continental rocks and deposits of Tertiary and Quaternary age include (1) the Kern River Formation; (2) the Laguna Formation; (3) the Tulare Formation; (4) the Tehama Formation; and (5) a number of younger formations, as well as some informally named deposits. These deposits and formations also include lacustrine and marsh deposits, which are much thicker and more extensive in the San Joaquin Valley than in the Sacramento Valley. These continental rocks and deposits generally crop out as wide belts along the flanks of the Central Valley and range in thickness from 0 foot along the flanks to about 15,000 feet in the extreme southern part. In general, the rocks and deposits consist of a heterogeneous mixture of generally poorly sorted clay, silt, sand, and gravel; in some places they consist of more consolidated sediments, such as mudstone and sandstone. Yields to wells from these rocks and deposits, except the lacustrine and marsh deposits, range from about 20 gallons per minute to 4,500 gallons per minute.

Lacustrine and marsh deposits crop out in the San Joaquin Valley but not in the Sacramento Valley. Beneath Tulare Lake bed in the San Joaquin Valley, the lacustrine and marsh deposits, which constitute a thick plug of clay and silt from which lenses of clay and silt emanate at irregular intervals, are in places more than 3,600 feet thick. The lens mapped as the E Clay in the San Joaquin Valley is the most extensive lacustrine clay in the entire Central Valley and includes the Corcoran Clay Member of the Tulare Formation. More recent mapping has shown that the E Clay in the extreme southern part of the valley is shallower than previous reports had indicated; therefore, in this report the clay is referred to as the modified E Clay. Perhaps the expansion of lakes and the resulting deposition of extensive clays in the San Joaquin Valley occurred principally because of a large downwarping basin beneath an area known as Tulare Lake bed, and perhaps a similar expansion and deposition did not occur in the Sacramento Valley because a similar downwarping basin probably had not developed there.

Continental deposits of Quaternary age crop out chiefly along the major rivers and streams of the valley as well as other low-lying areas; the deposits include river deposits, flood-basin deposits, and sand dunes. River deposits, including channel and flood-plain depos- its, are considered to be the most permeable deposits in the valley; in general, they are not tapped by wells. Flood-basin deposits consist largely of fine-grained beds that restrict the vertical movement of water. Sand dunes are not considered important aquifers because they generally lie above the water table.

The large, asymmetrical, northwestward-trending structural trough of the Central Valley is the principal structure controlling the occurrence and movement of ground water in the area. Because the flanks of the valley are higher than its axis, recharge from tributary rivers and streams has caused heads in the ground water along the flanks to be higher than those along the axis. Therefore, the overall ground-water movement in the Central Valley is from the flanks toward the axis and from there toward the delta area. Secondary structures in the valley, such as arches and faults, also influence the occurrence and movement of ground water.

As it is used in this report, “texture” means the proportion of coarse-grained to fine-grained sediment in sedimentary rocks and deposits. In the Central Valley, most of the deposits for which data are available generally contain no more than 40 to 60 percent of coarse-grained sediment, where coarse-grained sediment includes clayey and silty sand and gravel. Texture columns, maps, and sections, in depth intervals of 300 feet, show that the alluvial deposits of the Central Valley are a heterogeneous mixture whose character ranges over short distances and depths from chiefly fine-grained to chiefly coarse-grained and vice versa. Nevertheless, some areas are underlain chiefly by fine-grained sediment and others by coarse-grained sediment; sediments of like size in an area indicate that sources and depositional environment probably were similar for long periods of time.