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3 <br /> Geologically,the Great Valley is filled to great depths by marine and continental sediments,which <br /> are the result of millions of years of inundation by the ocean and erosion of the rocks that form the <br /> surrounding mountains. The Recent age Great Valley deposits are represented primarily by the <br /> alluvial, flood and delta plains created by the Sacramento and San Joaquin Rivers and their <br /> tributaries. The region persisted as a lowland or shallow marine embayment during the entire <br /> ' Cenozoic and until at least the late Mesozoic (Norris &Webb, 1990). <br /> The Great Valley is an asymmetrical synclinal trough with its axis off center to the west. The San <br /> Joaquin Valley is an asymmetric structural trough with a broad,gently inclined, and little deformed <br /> east flank and a relatively narrow west flank. <br /> ' Two major faults, the Stockton and White Wolf faults, cut perpendicular to the axis of the valley. <br /> The Stockton fault is located approximately one mile north of the Site and the White Wolf fault is <br /> ' located at the southern end of the valley. The Stockton Arch,a broad structure that is bound on the <br /> north by the Stockton fault but has a poorly defined southern limit, separates the San Joaquin and <br /> Sacramento sedimentary basins. The Stockton Arch does not have appreciable structural relief,but <br /> it did have an influence on sedimentation. Cenozoic deposits in the San Joaquin Valley thicken <br /> southeastward from 800 meters over the western part of the Stockton Arch to over 9,000 meters in <br /> the southern part of the Valley (USGS, 1991). <br /> The valley fill consists of an up to a 9 kilometer thick sequence of Jurassic to Holocene aged marine, <br /> lacustrine, fluvial, and eolian sediments. The valley fill rests on a westward-tilted block of <br /> crystalline basement composed of Sierra Nevada plutonic and metamorphic rocks under the eastern <br /> part of the valley and mafic and ultramafic rocks under the central and western parts of the valley <br /> (USGS, 1991). <br /> 3.3 Regional Hydrogeology <br /> ' The Great Valley aquifer system is formed primarily of sand and gravel with significant amounts of <br /> silt and clay,all of which have been eroded mainly from older rocks at the boundaries of the valley. <br /> Little groundwater flows through the extensive deposits of consolidated marine and mixed marine <br /> and continental sediments that overlie the crystalline basement rocks. The marine sediments usually <br /> contain saltwater or brine,but near the northwestern,western, and southeastern margins of the San <br /> Joaquin Valley, some fresh water is withdrawn from these deposits (USGS, 2002). <br /> The environments in which the continental sediments were deposited varied, but most were <br /> deposited in a fluvial environment; however, the deposits contain some lacustrine beds. Locally, <br /> volcanic rocks and dune deposits are part of the aquifer system(USGS, 2002). <br />