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Rile No. 20-1008-48 KLEINFELDER January 3 , 1992 corresponding sample locations and are summarized on Plate A-9 in the appendix. REGIONAL GEOLOGY The site is located in the central part of the Great Valley geomorphic province of California. The valley is situated between the Sierra Nevada mountains on the east and the Coast Ranges mountains on the west. These mountain ranges were formed by uplifts which occurred during the late Tertiary* and Quaternary periods. The structural trough or depression in bedrock formations between the ranges has been filled with alluvial, lacustrine, and some marine sediments that attain a maximum thickness of about 30, 000 feet near the western margin. The bedrock complex is composed of metamorphosed marine sediments similar to those found in the foothills of the western Sierra Nevada and the core of the Coast Ranges. LOCAL GEOLOGY The portion of the valley in the site area exhibits a fairly complete stratigraphic section of Cretaceous, Tertiary and Quaternary deposits. The sediments deposited prior to mid- Tertiary time were in a marine environment. Changes in sea level, valley filling, and uplift resulted in deposition of sediments in a continental environment after mid-Tertiary time. These continental sediments are exposed at the surface in the site area. Near-surface sediments at the site have been deposited primarily during flood stages of the Mokelumne and Calaveras River systems, prior to the present-day flood-control systems. FAULTS AND SEISMICITY The Central Valley of California is considered to be an area of relatively low seismicity in a state which is characterized by moderate-to-high seismic activity. During the formation of the Coast Ranges and the Sierra Nevada mountains, numerous faults and shear zones developed. These faults are primarily in the Sierra Nevada foothills and the Coast Ranges; however, a few extend beneath the valley sediments. The site is not located within a special study zone as delineated by the Alquist-Priolo Special Studies Zones Act of 1972, and according to the San Joaquin County Seismic Safety Element and State Geology Map, no active or potentially active faults are known to reach the surface with the site area. Several faults beneath the valley that displace "basement" rocks and some of the overlying sediments are known from subsurface oil and gas exploration. The nearest of these buried faults are the Midland *See "Geologic Time Scale, " Plate A-12 . Copyright 1992 KLeinfetder, Inc. Page 4 of 9