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Newhall Oil Field (CDOG, 1991). Regionally, fine-grained deposits of Towsley, including
siltstone, mudstone and shale, are considered to be aquitards to groundwater flow.

Sedimentary units older than the Towsley Formation occurring in the subsurface in the
Eastern Hydrographic Subarea include the Modelo Formation, Topanga Formation, and
Paleocene-Eocene rocks (see Figure 6A, Newhall area). The Modelo and Topanga
Formations are exposed in the Santa Susana Mountains wherethey are typically comprised

of interbedded siltstone, mudstone, and fine-grained sandstone. Eocene rocks occur in the
subsurface in the Eastern Hydrographic Subarea but regionally are only exposed in outcrop
within Elsmere Canyon (Figure 12), where they are comprised of well indurated sandstone,
siltstone, and lenticular conglomerate beds (Janes, 1991). Slade (1986; 1988) collectively
considered the older consolidated and cemented Tertiary rocks of the Eastern Hydrographic
Subarea not to be part of the groundwater reservoir in the area, due to their low primary
porosity, limited storage capacity, and low potential yield. Likewise, their affiliation to much

of the oil field production at Newhall and surrounding areas (i.e, production from Modelo
beds) indicates these rocks do not contain useable groundwater.

Igneous and metamorphic crystalline basement rocks are exposed in the upper watershed

areas of the Sierra Pelona and the San Gabriel Mountains. These crystalline rocks generally
yield only small quantities of water to wells from fractures and joints (The Mark Group,
1987; after Robson, 1972). Slade (1986; 1988) did not consider these rocks to be part of the
groundwater reservoir in the Santa Clara River valley, based on their lack of primary
porosity, limited storage capacity, and low yield potential.

Sylmar Basin Subarea: Non-water bearing deposits and aquitards in the Sylmar Basin

Subarea are similar to that of the Eastern Hydrographic Subarea discussed above, and
include Tertiary sedimentary rocks and crystalline basement rocks. In the Sylmar Basin
Subarea, the main recognized non-water bearing sedimentary units are the early
Pliocene-age Repetto Formation (Towsley equivalent; Figure 6A, San Fernando Area) and
the Pico Formation (Brown, 1975). The Repetto is variable in thickness (400 to 3,000 feet)

and generally comprised of interbedded mudstone, siltstone, sandstone and conglomerate.
The Pico Formation is thinner than equivalent deposits in the Ventura Basin area (1,500 to
3,000 feet) and is a mudstone to siltstone with lenses of sandstone and conglomerate
(Brown, 1975). Crystalline basement rocks are identical to those of the Eastern

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