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GROUND WATER IN THE CENTRAL VALLEY, CALIFOnNIA A9 merous names by petroleum geologists (Park and Wed- texture change in the continental deposits overlying dle, 1959,pl.3;Sacramento Petroleum Association, 1962, predominantly marine rocks and deposits in about 1,000 figs.6,7, 10,20,and 27,and table 2). In places the marine ; mi2 of the San Joaquin Valley. deposits provided the source material for the overlying In this report and in chapter D(Williamson and others, continental deposits that form the freshwater aquifers of 1985), the base of the aquifer system is taken as coinci- the valley. Generally, the marine deposits contain saline dent with the base of the post-Eocene continental depos_ water, some of which has migrated into adjacent and its. This is not strictly true in the southeastern San overlying freshwater aquifers. Joaquin Valley where sandy marine beds underlying the In a few places in the San Joaquin Valley, the marine continental deposits contain freshwater and are hydro- rocks and deposits have been flushed of saline water and logically part of the aquifer system. The thickness of the contain freshwater, which they yield to wells. In the aquifer system, based largely on a thickness map of Sacramento Valley, no marine deposits were reported as post-Eocene continental deposits prepared by R.W. Page yielding freshwater to wells,although Olmsted and Davis (U.S. Geological Survey, written commun., 1981; Page, (1961, p. 134)reported that marine rocks were flushed of 1974), is shown in figure 8. The thickness of the aquifer connate water locally. The marine rocks, then, provide system averages about 2,400 ft and increases from north very little freshwater in the Central Valley. to south, with a maximum thickness of more than 9,000 ft Continental deposits of post-Eocene age overlie the near Bakersfield. However, the contact between conti- marine deposits and contain most of the freshwater nental and the underlying marine deposits is not always aquifers in the Central Valley.An important contribution certain because the two types of deposits interfinger in to the quantification of storage capacity in the aquifer some places,particularly near the south end of the valley. system was made when Page(1983)successfully mapped DeLaveaga(1952, p. 102) suggested that the continental s,e5s ?/flip--tib ��.� � � ,.�� _. ••r= 'ter�� 'shy_.._yam- �•''��{► - W_ +i�.....+r--�_ r• Y ► ik- AL � A - - �' z o /�• ti 1 1+'� � til L I",WN, - f oILnr41_e wirunur v[¢TIGwL 87fAGGERA TION FIGURE 6.—Generalized oblique view(northward)of part of Central Valley structural trough.Coast Ranges lie to the east;Sierra Nevada to the west.(From`Geology Illustrated"by John S. Shelton. Copyright 1966,W.H. Freeman and Company. Used with permission.)