ater Westside Ground-Water
Basin. Review and analysis of stratigraphic relationships provided a basis for delineating the
distribution and characteristics of the formations that are targeted in the ground-water basin for
water supply purposes. This aspect (stratigraphy) is essential for interpreting subsurface data
(e.g., logs and drillers reports) which may be sparse in some areas or ambiguous due to inherent
limitations in well data.

On the matter of seawater intrusion, offshore topography and geology provide additional insight
into the relationship of the ocean to the ground-water basin and possibly explains the lack of
evidence for intrusion in response to the pumping depression which exists in the main part of the
Westside Basin.

Subsurface cross-sections created for this investigation depict the lake-aquifer system by
combining each of the aspects cited above and discussed herein (i.e., regional geology,
stratigraphy, and offshore topography and geology). The cross-sections provide an interpretation
of the physical setting based on available data and information, and therefore represent the
physical aspect of the system conceptualization being developed in this investigation.


Regional Geology

General regional geology of the study area is summarized from previously published reports
including Schlocker (1974), Brabb and Pampeyan (1983), Helley and others (1979), Yates and







Conceptualization of the Lake Aquifer System
Westside Ground-Water Basin

10
others (1990), Phillips and others (1993), Bonilla (1971), and other reports. The four major
geologic units exposed in the study area are: Mesozoic Franciscan Complex; Pleistocene Merced
Formation; Pleistocene Colma Formation; and Pleistocene to recent dune sands (Figure II-1).
The minor, although wide-spread, units of recent alluvium along stream channels, and landslides
are largely not shown on the geologic map (Figure II-1) because of the map scale.

The Mesozoic Franciscan Complex consists of well to moderately consolidated, highly deformed
marine sandstone and shale, and is considered non-water bearing bedrock. The Pleistocene
Merced Formation is a weakly consolidated, marine to non-marine fine sandstone, siltstone and
claystone, and moderately deformed. The Pleistocene Colma Formation is composed of
unconsolidated to weakly consolidated fine sand to sandstone with interbedded clay beds,
deposited in non-marine fluvial, beach, estuary and dune environments, and is largely un-
deformed. The Pleistocene to recent dune sands consist of unconsolidated to weakly
consolidated fine sand to sandstone of dune field origin.

The major structural features of the region consist of the San Bruno fault, the San Andreas Fault
System, and the Serra fault (Figure II-1). The San Bruno fault was originally defined by Lawson
in 1895 (Yates and others, 1990) to explain the lack of Merced Formation exposures to the
northeast. The location, age, and activity of the San Bruno fault are largely unknown, but may
have been inactive since Upper Merced time. The San Andreas Fault System is an active, right
lateral strike slip fault with the west side moving northward relative to the east side.

The Serra fault has been recently extended northward to the ocean (Barr, 1999), and is believed
to be a steeply southwest dipping reverse fault with the west side up-thrust relative to the east
side. The fault is poorly exposed and trends along the eastern edge of the Merced Formation
surface exposure (Figure II-1). North of the Olympic Club, the Merced Formation and the Serra
fault is covered by dune sands.








Conceptualization of the Lake Aquifer System
Westside Ground-Water Basin

11
Stratigraphic Descriptions

Mesozoic Franciscan Complex consists of highly deformed, well consolidated sandstone, shale
and other rock types exposed in the low hills to the northeast. The surface of the bedrock slopes
southwestward and is covered by younger sedimentary deposits. Beneath Lake Merced and
southward to Daly City the bedrock appears to descend rapidly from about 600 feet below the
ground to depths of 3,000 feet below the ground (Phillips and others, 1993), possibly reflecting
offset along the San Bruno fault.

The Merced Formation is considered to be largely of Pleistocene age and has a measured
thickness of over 5,000 feet in exposure along the sea cliffs (Clifton and Hunter, 1987, 1991).
The lower Merced Formation, approximately 4,000 feet thick, from the San Andreas Fault to
Thornton Beach consists of thick-bedded sequences of marine shelf deposits of fine sandstone
and siltstones. The lower Merced appears to be moderately deformed with northwest dips of 45
to 70 degrees.

The middle Merced is a 600-foot sequence of thinner bedded near shore, marine to beach,
estuary, and non-marine dune and fluvial deposits of fine sandstone, siltstone, and mudstone. An
ash bed near the top of the sequence has been dated at 400,000 yr b.p. (before present). These
beds are moderately dipping to the northeast at 25 to 45 degrees with some evidence of folding
and steeper dips near the Serra fault near the Olympic Club.

The upper Merced is about a 500-foot sequence from the Fort Funston area to the Great
Highway. The unit consists of a sequence of thinner bedded beach, dune, estuary, and fluvial
deposits of weakly consolidated fine sandstone with some gravel and mudstone beds. The unit is
only moderately to slightly deformed with decreasing northeast dips from 20 to 5 degrees. The
beds were originally excluded from the Merced Formation (Hall, 1966, Hunter and Clifton,
1982), but were included in the Merced Formation by Clifton and Hunter in1991.

The Colma Formation appears to have originated as a surficial map unit for the low relief and
slope covering deposits east of the Merced Formation exposures (Bonilla, 1971). The unit is
defined as consisting of fine-grained sand with some clay, silt and gravel beds of fluvial,







Conceptualization of the Lake Aquifer System
Westside Ground-Water Basin

12
floodplain, alluvial fan and dune sand origin. Separation of the Colma from underlying beds in
the subsurface by borehole information appears to be difficult.

Dune sands also appears to be a surficial map unit based on relict dune topography and cross-
bedding exposures north of Lake Merced and dune features west of Lake Merced. The unit
consists of fine-grained sands with some clay soil horizons. The age of dune sand may range
over the uppermost Merced Formation, Colma Formation, and up to recent dune deposits.
Again, separation of dune sands from the Colma Formation is difficult from borehole
information.

Offshore Topography and Geology

One aspect of the study area which has implications on the connection of the Westside Basin
with the ocean is offshore topography and geology (Green and Kennedy, 1989). On the ocean
side of the Westside Basin study area, offshore topography is a gentle westward sloping plain
with ocean water depths reaching only 60 feet at 2 miles offshore; 100 feet at 8 miles offshore,
and; 300 feet at 25 miles offshore, which marks the continental shelf to steeper continental slope
to abyssal depths. This continental shelf is underlain by a thick sequence of Quarternary
sedimentary deposits complicated by the San Andreas fault (two miles offshore) and other
offshore faults, including a possible extension of the Serra fault. Somewhere offshore is
probably the continuation of the Merced and Colma Formations.

During the Pleistocene glaciations, sea levels fell to 300-400 feet below present sea level,
exposing a wide coastal plain. Across this coastal plain, flowed the fresh water Sacramento-San
Joaquin River draining the Central Valley and the Sierra Nevadas. At the base of the Middle
Merced, the Sacramento-San Joaquin River is believed to have flowed out the Colma-Merced
Valley prior to later moving to the Golden Gate during the last glaciation (70,000-10,000 yr b.p.,
Helley and others 1979). Presumably, non-marine deposits were laid down across the central
plain and possible fresh ground-water flushed saline water from marine deposits for a distance
offshore. An indication of this process may be the fact that the USGS Fort Funston monitoring
well test hole did not encounter saline water in the lower Merced to a total depth of 1,800 feet.








Conceptualization of the Lake Aquifer System
Westside Ground-Water Basin

13
The last glacial sea-level low stand ended at about 15,000 years ago and sea level has risen to the
present. Helley and others (1979) show shorelines at 15,000 to 12,000 feet west of the Farallon
Islands, and at 9,000 years still about 5 miles offshore (see Figure 12 of citation). This indicates,
along with the gentle continental slope, that possibly a fresh-water ground-water system could
extend for a considerable distance offshore, up to several miles.

Geologic Cross-Sections of Lake Merced-Aquifer System

The subsurface configuration of geologic units beneath the study area was evaluated by a series
of cross-sections using water well drillers reports and especially the use of geophysical electrical
logs (Figure II-2). Cross-Section A, B, and C (Figures II-3, 4, and 5) show correlation of
subsurface units in a we