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Ii r 4.2 GEOLOGY, SOILS, AND SEISMICITY j i INTRODUCTION The geologic, soils, and seismic conditions at the project site present important controls on the feasibility of the proposed mining and reclamation plans. The geologic setting of the area provides the opportunity for sand and gravel deposition and, therefore, aggregate mining. The transport and deposition of high-quality aggregate along Hospital Creek result I� from a complex fluvial system that is.influenced by active tectonics and geologic structure. I� The same system is responsible for the deposition of fine-grained overbank deposits on y which valuable agricultural soils are developed. Extraction of aggregate from, and reclamation of, the deep mining pits proposed by the j project would result in permanent modification of the topography of the project site. Disruption and replacement of surface soil throughout most of the project site would occur. This section of the EIR describes the existing geologic, soils, and seismic conditions of the ! project site. The impact analysis was conducted based on review of published geologic reports and maps for the region, site reconnaissance, site-specific, reports prepared by consultants, and geologic data provided by the proponent (Kleinfelder, 1997). The geologic data were critically reviewed by the EIR preparers and were! found to be generally !' consistent with appropriate geotechnical methods and standards. rL li SETTING " Regional Geology . i li The project site is located on the western margin of the San Joaquin Valley, the central portion of the Great Valley Geomorphic Province of California. The Valley is a large structural trough formed between the Coast Ranges to the west and the Sierra Nevada to the east. The Valley is filled with a thick sequence of sedimentaryrocks and sediments that range from Upper Jurassic age (150 million years old) marine rocks through modern alluvial deposits.. This sequence of sedimentary rocks was apparently deposited on igneous and metamorphic basement' rocks of the Sierran structural block. The Valley is bounded to the west in the area of the project site by the low mountains of the Diablo Range. These mountains were formed by uplift and eastward tilting of Cretaceous Great Valley Group and younger Tertiary sedimentary rocks that overlie Franciscan basement bedrock. The western margin of the Valley represents a major I! geologic boundary between the Coast Range structural block to the west and the Sierran structural block to the east. This regional boundary, typically referred to as the Coast 4 '"Basement"rock is a general geologic term for the deepest known bedrock. 4 Draft Environmental Impact Report 4-5 ER-96-3 ' �I