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F PRE-TANK REPLACEKE9T Site Assessment Work Plan 6131 Pacific Ave, Stockton, CA Aepfa P"Olect No. 90.020 Pape 9 cretaceous sandstones and shales overlay pre-Cretaceous ultra basic igneous rocks, granitic rocks, and metamorphosed sedimentary and volcanic rocks. Sediments originating from the Sierra Nevada and the_ Coast Ranges were deposited in a marine environment during the Cretaceous to the Tertiary. Interruptions in the pre-Pleistocene r» sedimentary depositional cycle occurred several times as volcanic i , activity, uplift, faulting and/or folding altered the region. The Site is situated near the axis of the San Joaquin trough. Recent Alluvium and the Victor Formation overlay the Laguna Formation. The Laguna Formation consists of clay and sand units and unconformably overlays the andesitic sediments of the Mehrten Formation. From the Tertiary to the present, the depositional environment has been alluvial. several thousand feet of unconsolidated silty, sandy and gravelly sediments lie within the basin (DWR 1967) . 2.9.1 Mehrten Formation The Miocene/Pliocene Mehrten Formation is composed of andesitic sand and sandstone units derived from Sierran andesitic_ flows and agglomere,tes interbedded with conglomerate tufaceous siltstone and claystone horizons. Beneath the center of the San Joaquin Valley the f , upper surface of the Mehrten Formation is 800 to 1000 feet below the surface. The formation outcrops along the eastern margins of the rz.A valley, dips toward the west and is nearly flat lying near the center of the valley. The unit thickens to approximately 600 feet at its western margin (DWR 1967) . !a 2.9.2- Laguna Formation � The Pleistocene Laguna Formation unconformably overlays the Mehrten Formation. The Laguna Formation consists of heterogeneous fluvial continental s{:diments of nonvolcanic sands and gravels. Included within the F'o::-mation is the Arroyo Seco gravel, a thin unit south of the MokelumnF River consisting of weathered sand and cemented gravels. The Laguna Formation outcrops along the eastern margins of the valley, ( ' dips toward the west and is nearly flat lying beneath the center of the valley. The unit thickens in a westerly direction to approximately 1000 feat beneath the Lodi area (DWR 1967) . y 2.9.3 Victor Formation Recent Alluvium The Victor Formation/Recent Alluvium occurs near the center of the valley and' is generally coarser grained than the underlying Laguna - Formation. The formation consists of heterogeneous deposits of sand and gravels with discontinuous lenses of silt and clay. The Formation dips gently and thickens to the west. At its western margin the ;� Formation is approximately 100 feet thick (DWR 1967) .