Lower Pilarcitos Creek Groundwater Basin Study - Coastside ...

The bedrock has been affected by folding and faulting but the overlying
unconsolidated formations have experienced only minor deformation (Geoconsultants,
June 1987). Geoconsultants (June 1987) compiled a geologic map of the coastal plain
area between El Granada and Cañada Verde based on Brabb and Pampeyan (1983)
and Lajoie et al. (1974).
Purisima Formation. The Lower Pilarcitos Creek groundwater basin is underlain
by a thick section of Tertiary age Purisima Formation. In 1984, an exploration boring
(5S/5W-30H) drilled by Half Moon Bay Properties near Balboa Boulevard revealed that
the Purisima Formation occurs to depths of at least 1,031 feet (elevation of 1,011 feet
below mean sea level [msl]).
Figure 14 shows a generalized east-west cross section of the Half Moon Bay
terrace. Note that the vertical exaggeration is only five times. The top of the Purisima
Formation ranges between about 45 feet below msl beneath the coastline to about 60
feet above msl in the low foothills east of Half Moon Bay and along Highway 92. The
overlying unconsolidated sediments form a relatively thin veneer on top of the Purisima
Formation.
The undivided Purisima Formation is of Pliocene and upper Miocene age and is
predominantly gray and greenish-gray to buff fine-grained sandstone, siltstone, and
mudstone (Brabb and Pampeyan, 1983). The Purisima Formation is considered nonwater
bearing and is readily recognized by drilling contractors. Where groundwater has
been encountered in the Purisima Formation, water quality is usually poor with elevated
concentrations of chloride, iron, manganese, and total dissolved solids (TDS)
(Kleinfelder, April 1988).
Marine Terrace Deposits. The marine terrace deposits overlying the Purisima
Formation form the main aquifer in the Lower Pilarcitos Creek groundwater basin. The
sediments were deposited in shallow water on old wave-cut marine terraces that are
now slightly warped and elevated above sea level (Lajoie et al., 1979). The marine
terrace deposits are weakly consolidated, moderately weathered, well-sorted to poorlysorted
sand and gravel and range from 30 to 60 feet thick (Brabb and Pampeyan, 1983).
Surface outcrops of marine terraces occur east of Half Moon Bay and onlap the Purisima
Formation. The hydrologic properties of the Marine Terrace deposits are described in
detail in the subsequent section, Marine Terrace Hydrologic Properties.
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