ARCHIVED REPORTS_1993_1 - Laserfiche WebLink

Home Browse Search

L C' L n n 4.4 GEOLOGY: AND GEOTECHNICAL ENVIRONMENTAL SETTING The geological setting, hazards and constraints that are considered significant with respect to the development of the proposed project are discussed in this section. Data that have been considered in the determination of significant environmental impacts and corresponding mitigation measures are included. Geology The project site is located at the western margin of the Great Valley physiographic province, at the eastern flank of the Diablo Range, just south of Tracy. The valley is filled with Tertiary and Quaternary age clastic sediments to depths of greater than 20,000 feet. Streams flowing northeastward from the Diablo Range have developed a succession of lobate -shaped alluvial fans at the base of the mountains. Sediments deposited as recent alluvial fans at the mouth of Corral Hollow Creek, Lone Tree Creek, Hospital Creek, and others are made up of the San Luis Ranch Alluvium and the Los Banos Alluvium, which contain reworked sediments derived from older alluvial fan deposits. The alluvial fans overly a thick sequence of clastic sediments of the Pliocene and Pleistocene age Tulare Formation. A thin veneer of Holocene -aged Patterson Alluvium overlies these alluvial fans. The project site lies within a reclaimed gravel pit in alluvial fan deposits of Corral Hollow Creek. Within the alluvial fan deposits, the subsurface geology changes very rapidly aerially and with depth. Borings drilled at the site indicate that the thickness of alluvial gravels is on the order of 100 feet. Below these deposits, borings and wells encountered the Tertiary -Quaternary age clay, silt, sand, and gravel of the Tulare Formation. The California Division of Mines and Geology classifies deposits of the Corral Hollow Creek alluvial fan in the project area as Mineral Resource Zone 2 (MRZ-2). MRZ-2 areas have adequate information indicating that significant mineral deposits are present, or a high likelihood for their presence exists. This zone is applied to known mineral deposits, or where it can be demonstrated that the likelihood for occurrence of significant mineral deposits is high. The project site is located in an area of "Known Sand and Gravel Deposits" and "Operating Excavation Sites' as described in the Extractive Resources section of the Resources chapter of the San Joaquin County General Plan 2010. RMC Lonestar Industries leased the project site in 1947 with the intent to mine the site for sand and gravel. Mining activities started at the site in 1971, and continued through 1975. The gravel mining operation consisted of excavation and hauling only. Mining continued until the excavation reached depths of 30 to 40 feet. By 1975, mining activities ceased, as the mineral resources remaining in the pit had a clay and silt content of over 50%, making them uneconomic for mining. At this time, the mining of further mineral resources is not feasible. In 1986, RMC Lonestar Industries reclaimed the project site to grassland, and was released from further requirement by San Joaquin County. There is no natural top soil remaining in the abandoned quarry pit. Soil types in the site vicinity, however, include the CapayStomar-Zacharias Group. This soil group, when developed on the gentle topography of alluvial fans, is typically characterized by clayey to silty clayey texture, slow permeability, high shrink - swell potential, low erosion potential, and is moderately well drained. The regional topography, typical of the Diablo Range foothills, slopes gently toward the northeast. Prior to the commencement of gravel mining activities, the elevation of the site was approximately 165 feet MSL. Present site elevations are approximately 30 feet below this grade. iER -93-1 -56- (9-27-93)