ARCHIVED REPORTS_DRAFT ENVIRONMENTAL IMPACT REPORT

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4.7 GEOLOGY, SOILS AND SEISMICITY This section describes current conditions in the project area relative to geology, soils, and seismicity; analyzes potential environmental impacts; and recommends mitigation measures to reduce significant or potentially significant impacts to less-than-significant levels. Paleontological impacts are addressed in Section 4.8, "Paleontological Resources." 4.7.1 ENVIRONMENTAL SETTING PHYSIOGRAPHIC SETTING The project area is located at the northern end of the San Joaquin Valley. Together,the San Joaquin Valley and the Sacramento Valley constitute the Great Valley of California. The Great Valley Geomorphic Province is located between the Sierra Nevada Geomorphic Province on the east and the Coast Range Geomorphic Province on the west. The Great Valley is composed of thousands of feet of sedimentary deposits that have undergone periods of subsidence and uplift over millions of years. During the Jurassic(approximately 206 million years Before Present [B.P.]) and Cretaceous (approximately 144 million years B.P.)periods of the Mesozoic era,the Great Valley existed in the form of an ancient ocean. By the end of the Mesozoic era,the northern portion of the Great Valley began to fill with sediment as tectonic forces caused uplift of the basin. Geologic evidence suggests that the Sacramento Valley and San Joaquin Valley gradually separated into two separate water bodies as uplift and sedimentation continued. By the time of the Miocene epoch(approximately 24 million years ago), sediments deposited in the Sacramento Valley were mostly of terrestrial origin. In contrast,the San Joaquin Valley continued to be inundated with water for another 20 million years, as indicated by marine sediments dated to the late Pliocene epoch(approximately 5 million years ago). Most of the surface of the Great Valley is covered with Recent(Holocene) (less than 10,000 years ago) and Pleistocene (10,000 to 1.5 million years old) alluvium. This alluvium is composed of sediments from mountains of the Sierra Nevada to the east and the Coast Ranges to the west that were carried by water and deposited on the valley floor. Siltstone, claystone, and sandstone are the primary types of sedimentary deposits. LOCAL GEOLOGY The project vicinity spans the U.S. Geological Survey(USGS)Manteca and Lathrop 7.5-Minute Quadrangles. The topography of the project vicinity is relatively flat,with elevations ranging from approximately 20 feet above mean sea level in the western portion of the project area to approximately 50 feet above mean sea level in the eastern portion of the project area(a distance of approximately five miles). Most of the sedimentary deposits in the project area accumulated in a marine environment during alternating cycles of deposition and erosion over many thousands of years. Fluctuations of glacial meltwaters in the Sierra Nevada account for the deposition of alluvial-fan deposits in the northern San Joaquin Valley, and are generally correlated with the Pleistocene-age Modesto(approximately 40,000 years B.P.) and Riverbank Formations (approximately 300,000 years B.P.). In addition,Holocene sediments are continually being deposited on the Valley Floor and in tributaries to the Sacramento-San Joaquin Delta as a result of sediment transport and deposition by rivers, streams, and creeks (Olmsted and Davis 1961,Atwater 1982,Helley and Harwood 1985, Page 1974).Near surface sediments at the Manteca wastewater treatment plant site have been deposited primarily during flood stages of the Stanislaus and San Joaquin River systems,prior to the arrival of present-day flood control systems(Kleinfelder 1997). Much of the area surrounding the City of Manteca is covered by Holocene dune sand deposits (Wagner et al. 1991). Inland dunes usually exist in the form of ridges, small hills, and knolls found on ancestral lake beds and outwash plains. Manteca WQCF and Collection System Master Plans EIR EDAW City of Manteca 4-7.1 Geology,Soils,and Seismicity