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WATER RESOURCES TECHNICAL REPORT2.5.2 StratigraphyThe lithologic units in the Santa Clara–Calleguas Basin can be grouped into two generalcategories: (1) upper Cretaceous and Tertiary bedrock and (2) Quaternary unconsolidateddeposits. The outcrop pattern of the lithologic units is shown in Figure 2-7 and theirstratigraphic relationships are shown in Figure 2-8. These two figures are modified from theUSGS groundwater modeling study (USGS, 2003). Figure 2-8 relates the lithologic units andformations to their associated aquifer units, which are further described below. A summaryof the stratigraphic analysis by the USGS is provided below.Upper Cretaceous and Tertiary BedrockThe upper Cretaceous and Tertiary consolidated rocks include sedimentary, volcanic,igneous, and metamorphic rocks, which are virtually nonwater bearing and form the base ofthe Santa Clara-Calleguas Basin. Although these rocks are not an important source ofgroundwater, the erosion and subsequent deposition of these rocks are sources ofunconsolidated deposits that form the Santa Clara-Calleguas groundwater basin. Theconsolidated Tertiary sedimentary rocks underlie most of the groundwater basin andcompose the surrounding mountains and hills. These rocks are predominantly marine inorigin and are nearly impermeable.Quaternary Unconsolidated DepositsThe Quaternary unconsolidated deposits consist of the Pico Formation, Santa BarbaraFormation, the Las Posas Sand, the San Pedro Formation, and the Saugus Formation. All areof the Pleistocene epoch and are unnamed, unconsolidated alluvial and fluvial deposits ofthe Pleistocene to Holocene epoch. As further described below, these unconsolidatedQuaternary deposits are grouped together into an upper aquifer system (UAS) and loweraquifer system (LAS) across the Santa Clara-Calleguas Basin.Pleistocene Epoch. During the Pleistocene epoch, major changes in sea level resulted incycles of erosion and deposition. The sequence of deposits above these erosionalunconformities typically starts with a basal conglomerate that is laterally extensive,relatively more permeable than the underlying deposits, and a potential major source ofwater to wells perforated in these deposits. These coarse-grained layers of fluvial and beachdeposits are interbedded with extensive fine-grained layers.The Santa Barbara Formation overlies consolidated Tertiary rocks in most of thegroundwater basin and consists of marine sandstone, siltstone, mudstone, and shale. Thethickness and lithology of the formation varies considerably throughout the basin. It isthickest, more than 5,000 feet, in the Ventura area. The formation is of low permeability andgenerally contains water of poor quality throughout most of the basin.The lower part of the San Pedro Formation consists of shallow marine sand and gravel beds.These deposits reach a maximum thickness of more than 2,000 feet in the Santa Clara RiverValley near Ventura and consist of a series of relatively uniform fine-grained sand layers100 to 300 feet thick separated by silt and clay layers 10 to 20 feet thick.The upper part of the San Pedro Formation consists of lenticular layers of sand, gravel, silt,and clay of marine and continental origin. The continental fluvial silt, sand, and graveldeposits within the upper part of the San Pedro Formation are referred to as the SaugusW112003002SCO LW1458.DOC/ 033390002 14