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4. ENVIRONMENTAL ANALYSIS ,., 4.7 GEOLOGY,SOILS,AND SEISMICITY 4.7 GEOLOGY, SOILS, AND SEISMICITY 4.7.1 INTRODUCTION This chapter addresses issues regarding geology, surface soils, and seismicity as they relate to the development of the proposed RMC Pacific Vernalis Quarry operation. In general,this chapter ~ focuses on the affect that the local geology and the seismic conditions would have on a large aggregate quarry and the proposed reclamation of that quarry. The setting portion of this chapter _ presents existing conditions by describing the topography,regional and local geology, site soils, local aggregate resources,and the seismic setting. The setting section also describes various geologic and seismic hazards that could be present as existing conditions or could be realized ` after project implementation. The Regulatory Background section describes the pertinent federal, state, and local laws related to the geologic and seismic considerations of an active surface mining operation. The Impacts and Mitigations section of this chapter defines the significance criteria and presents discussion of the impacts and where necessary,applicable mitigation measures. 4.7.2 SETTING TOPOGRAPHY The proposed RMC Pacific Vernalis Quarry operation would be located in the northern San Joaquin Valley, south of the Sacramento Delta and west of the San Joaquin River. The topography of the San Joaquin Valley is flat, with elevations ranging from a few feet to a few hundred feet above mean sea level(msl). The project site ranges from 159 feet above msl in its northeast comer to 211 feet above msl in its southwest corner. The topographic gradient is approximately 40 feet per mile(0.5 degrees), decreasing in elevation toward the northeast. The ground surface is relatively flat with no prominent relief or landforms with the exception of Hospital Creek,located in the southwest comer of the site;the concrete-lined Delta-Mendota Canal,located less than 160 feet west of the project site;and the California Aqueduct,located 2,200 feet west of the project site. GEOLOGY The regional geology is influenced by the Great Valley, a topographically-dominant northwest- trending valley approximately 50 miles wide and 400 miles long that formed between the Coast Range Mountains to the west and the Sierra Nevada Mountains to the east. Alluvial fan deposits, + which consist of unconsolidated to semi-consolidated gravel, sand, silt, and clay,underlie the project site.' Below these surficial deposits are more consolidated valley fill rocks,underlain further by marine sedimentary rocks of undivided sandstone,siltstone,shale,and conglomerate. Alluvial fan sediments deposited by erosion of the sandstone and siltstone rocks in the Coast Ranges underlie the area west of the San Joaquin River,near the City of Tracy. A unit of r. l Alluvial and alluvium refers to deposits of clay,silt,sand,and gravel deposited by a stream or running water. RMC Pacific Vemelis QnaRY Hiving and Reclamation Project 4.7-1 FSA/203015 Administnmaw Draft DR March 2004