GROUND WATER Non-Water Bearing and Form the Base of the Santa Clara–Calleguas Basin
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GROUND WATER non-water bearing and form the base of the Santa Clara–Calleguas Basin. Although these rocks are not an The Santa Clara–Calleguas drainage basin is part important source of ground water, the erosion and of the tectonically active Transverse Ranges subsequent deposition of these rocks are the source of physiographic province. The mountains are composed the unconsolidated deposits that form the Santa Clara– of a variety of consolidated marine and terrestrial Calleguas ground-water basin. The sedimentary rocks sedimentary and volcanic rocks of Late Cretaceous of Cretaceous age are exposed in the Topatopa through Quaternary age. The subbasins of the Santa Mountains north of the ground-water basin and in the Clara–Calleguas Basin are filled with a mixture of Simi Hills and Santa Susana Mountains south of the consolidated and unconsolidated marine and terrestrial basin (California State Water Resources Board, 1956, coastal deposits of Tertiary and Quaternary age. These pl. 10). These rocks are generally non-water bearing basin-fill sediments and consolidated rocks form a except within the poorly cemented and fractured complex set of aquifer systems that have been the sandstones in the hills near Simi Valley (Turner, 1975, primary source of water supplies since the early 1900s. p. 3). Agriculture has been the main user of ground water, The consolidated Tertiary sedimentary rocks and in recent years public supply and industry have underlie most of the ground-water basin and compose become significant users of ground water. The the surrounding mountains and hills. These rocks are geohydrology of the basin is discussed in detail in predominantly marine in origin and are nearly reports by California Department of Public Works impermeable except for the slightly permeable (1934), California Department of Water Resources sandstones and within fracture zones. Some of these (1954, 1958, 1974a,b, and 1975), California State Miocene formations contain oil and tar sand beds, Water Resources Board (1956), Mann and Associates natural gas, and related methane and brines. The Pico (1959), and Turner (1975). The reader is referred to Sandstone of Pliocene and Pleistocene epochs these reports for a more complete description of the underlies the unconsolidated deposits throughout most geohydrology of the Santa Clara–Calleguas Basin. of the ground-water basin and crops out in the mountains on the north side of the Santa Clara River Valley (California State Water Resources Board, 1956, Geologic Framework pl. 10). These rocks are also considered to be of low permeability and non-water bearing. For this report, the lithologic units mapped by Volcanic rocks and related intrusive rocks of Webber and others (1976), Dibblee (1988, 1990a,b, Miocene age underlie parts of the southern Oxnard 1991, 1992a,b,c,d), and Dibblee and Ehrenspeck Plain, South Pleasant Valley, and Santa Rosa Valley (1990) in the Santa Clara–Calleguas Basin and subbasins (figs. 7 and 8D,E). Although these rocks are surrounding area were grouped into two general considered non-water bearing, they have been categories: (1) upper Cretaceous and Tertiary bedrock, developed for water supply where alluvial deposits are and (2) Quaternary unconsolidated deposits. The absent, such as in the Santa Rosa Valley subbasin. outcrop pattern of these combined units is shown in These volcanic and intrusive rocks also crop out in the figure 7A and their stratigraphic relations are shown in Santa Monica Mountains along the southern and figure 7B. southeastern boundaries of the ground-water basin (California State Water Resources Board, 1956, pl. 10) Consolidated Rocks and in the offshore submarine canyons along the The upper Cretaceous and Tertiary consolidated southwestern boundary of the basin (Kennedy and rocks include sedimentary, volcanic, igneous, and others, 1987, pl. 2A). metamorphic rocks. These rocks are virtually 28 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Unconsolidated Deposits than 2,000 ft in the Santa Clara River Valley near The Quaternary unconsolidated deposits consist Ventura (Dibblee, 1992a,b,c,d) and consist of a series of the Santa Barbara Formation (Weber and others, of relatively uniform fine-grained sand layers 100 to 1976), the Las Posas Sand (Dibblee, 1988, 1990a,b, 300 ft thick separated by silt and clay layers 10 to 20 ft 1991, 1992a,b,c,d; Dibblee and Ehrenspeck, 1990), the thick. The upper part of San Pedro Formation consists San Pedro Formation (Weber and others, 1976), and the of lenticular layers of sand, gravel, silt, and clay of Saugus Formation (Weber and others, 1976; Dibblee, marine and continental origin. The continental fluvial 1988, 1990a,b, 1991, 1992 a,b,c,d), all of the silt, sand, and gravel deposits within the upper part of Pleistocene epoch, and unconsolidated alluvial and the San Pedro Formation are referred to as the Saugus fluvial deposits of the Pleistocene to Holocene epoch. Formation by Dibblee (1988, 1990a,b, 1991, In the Santa Clara–Calleguas Basin, the unconsolidated 1992a,b,c,d) and Dibblee and Ehrenspeck (1990). deposits are grouped together into the upper-aquifer These deposits reach a maximum thickness of more system and the lower-aquifer system (fig. 7B). than 5,000 ft in the Piru subbasin in the Santa Clara The Santa Barbara Formation, mapped by River Valley (Dibblee, 1991). The sand and gravel Weber and others (1976), overlies consolidated Tertiary layers range from 10 to 100 ft thick and are separated rocks in most of the ground-water basin and consists of by silt and clay layers that generally are 10 to 20 ft marine sandstone, siltstone, mudstone, and shale. The thick. The Santa Barbara and San Pedro Formations are thickness and lithology of the formation varies absent in the Santa Rosa Valley subbasin east of the considerably throughout the basin, but the formation is San Pedro Fault and in the South Pleasant Valley thickest, more than 5,000 ft, in the Ventura area (Yerkes subbasin southeast of the Bailey Fault. In the eastern and others, 1987). The formation is of low permeability part of the Santa Rosa Valley subbasin and in the and generally contains water of poor quality eastern part of the South Pleasant Valley subbasin, throughout most of the basin (Turner, 1975) and, recent alluvial and terrace deposits were deposited therefore, is not considered an important source of unconformably on the marine shale and sandstone beds ground water. In the East Las Posas Valley subbasin, of the Santa Margarita Formation (Late Miocene) or the Santa Barbara Formation contains layers of sands rest unconformably on the Conejo Volcanics (Middle and gravels that are an important source of water to Miocene). For this study, the Santa Margarita wells in areas where younger unconsolidated deposits Formation in the Santa Rosa Valley subbasin is are absent or are unsaturated. The coarse-grained grouped with the unconsolidated sediments of the section of the Santa Barbara Formation in the East Las lower system. During the Pleistocene epoch, major Posas Valley subbasin is commonly referred to as the changes in sea level resulted in cycles of erosion and “Grimes Canyon” member (California Department of deposition (Dahlen, 1992). The sequence of deposits Water Resources, 1956). above the erosional unconformities typically starts with The Santa Barbara Formation and the lower part a basal conglomerate that is laterally extensive, of the San Pedro Formation mapped by Weber and relatively more permeable than the underlying others (1976) consist of shallow marine sand and deposits, and a potential major source of water to wells gravel beds that were indicated as a separate formation, perforated in these deposits. These coarse-grained the Las Posas Sand, by Dibblee (1988, 1990a,b, 1991, layers of fluvial and beach deposits are interbedded 1992a,b,c,d) and Dibblee and Ehrenspeck (1990). with extensive fine-grained layers. These deposits reach a maximum thickness of more Ground Water 29 30 Simulation ofGround-Water/Surface-Water Flowinthe SantaClara–Calle A 119°30' 119°15' 119° 118°45' Lake Piru T 4 Fillmore A' N 34° 22' Santa Paula 30" Pitas Point ? Fault Shoreline B' Clara ? E Santa Ventura Creek T 2 Offshore N g Oxnard uas Ground-Water Basin,VenturaCount D' Plain ElRio C' ? Camarillo McGrath Oxnard Fault Calleguas A Thousand Oaks 34° ? B 07' Area 30" not mapped E' 010MILES Pacific Ocean C 010KILOMETERS y , California ? ? D R22W R18W Figure 7. A, Generalized surficial geology of the Santa Clara–Calleguas ground-water basin and extents of layers in the numerical ground-water flow model, and B, stratigraphic column and related aquifer designations of geologic units by source and aquifer-system model layers, Ventura County, California. (Modified from Weber and others, 1976). The Late Pleistocene and Holocene deposits are EXPLANATION unnamed, consist of relatively flat-lying marine and UNCONSOLIDATED DEPOSITS continental unconsolidated deposits, and are regionally grouped into the upper system of water-bearing Upper-Aquifer System – deposits (fig. 7B). These deposits, which were derived Alluvium (Shallow aquifer) from local sources and from the Santa Clara River and Submarine shelf deposits Calleguas Creek, were deposited unconformably on the Holocene (Oxnard aquifer) older unconsolidated deposits and contain basal Upper Pleistocene deposits conglomerates that are laterally extensive and produce (Mugu aquifer) substantial ground-water supplies. In the Mound and Lower-Aquifer System – Oxnard Plain subbasins, the basal zones are overlain San Pedro Formation with fine-grained deposits of low permeability. Alluvial Quaternary (Upper and Lower Hueneme and Fox Canyon aquifers) Undifferentiated and fluvial sand and gravel deposits with interbedded Submarine slope outside Santa Clara- fine-grained deposits of the Holocene epoch Cenozoic Pleistocene Calleguas Basin Undifferentiated unconformably overlie the Late Pleistocene deposits. Santa Barbara Formation The basal deposits of the Holocene epoch consist of (Grimes Canyon) gravel and sand, which are overlain by fine-grained BEDROCK (Consolidated or deposits throughout most of the Santa Clara River non-water bearing) Valley and Oxnard Plain subbasins.