National Association of Geoscience Teachers, Far West Section
Fall 1998 Meeting,
Jeffrey W. Tolhurst, Ph.D.
field guide is a general overview of the geologic setting along the San Andreas
fault system near the towns of Hollister, San Juan
Bautista, and Aromas in
Summary of the Geologic Setting along the San Andreas Fault near Hollister
San Andreas transform fault extends for approximately 1,200 km (740 mi) (Norris
and Webb, 1990) displacing two active spreading centers – one north of Cape
Mendocino and the other buried underneath the southern end of the Salton Trough
(see Figure 1). It is the longest fault
first began along the fault in southern
Lawson apparently first named the fault in 1895 after
Figure 1: The San Andreas fault
system extends from the Mendocino Triple Junction to just south of the
Figure 2: Evolution of the western margin of
January 9, 1857
April 18, 1906
October 17, 1989
June 28, 1992
Table 1: Historical earthquakes with magnitude > 7.0 on
Salinian block, or Salinian
composite terrane (SCT), is composed of granitic and high-grade metamorphic rocks sandwiched
between two fault systems – the San Andreas on the northeast and the Sur-Nacimiento on the southwest. Mesozoic Franciscan assemblage rocks,
comprised of well bedded distal to midfan turbidites, shaly melanges, and serpentinized peridotites, bound the elongate SCT on both sides (Ernst,
San Andreas and Calaveras faults are the major structural elements encountered
near Hollister and San Juan Bautista and divide the area into three major
structural blocks (
studies of the chronology of displacement on the San Andreas fault
the second phase the San Andreas stepped eastward and slip was transferred to
the new trace of the fault. The rate of
slip probably averaged about 8 mm/yr (0.31 in/yr) over a period of about 7 m.y. during this phase.
During the third phase the San Andreas experienced deformation, which led to bending of the fault to the point that it could no longer conduct slip around the bend. A new, stable, straight segment was formed. Slip rates along this segment were estimated by Sims to be about 33 mm/yr (1.3 in/yr) over a period of about 5 m.y.
Jeffrey W. Tolhurst, Ph.D.
Objectives: By the end of this trip participants will: 1) visit and observe several classic localities along the San Andreas and Calaveras faults near the town of Hollister (see Figure 3) that demonstrate different scales of offsets that have occurred over the past few years to 21 million years; and 2) visit an active quarry operated by the Graniterock Corporation, near the town of Aromas, to observe mining of crushed granite that outcrops along the San Andreas fault.
0.0 0.0 Begin trip – depart from
0.8 0.8 Union Pacific Railroad
Railroads played an integral part of the
history and settlement of the
Prior to December 7, 1870 the town of
1.2 0.4 Turn right onto the Highway 99 south on-ramp. Drive 2.5 miles to the Crows Landing off-ramp.
3.7 2.5 Take the Crows Landing off-ramp and turn right (south).
11.0 7.3 Land Use Patterns
As we leave the city,
agricultural land use patterns along
orchards consist primarily of walnuts and almonds, but also include cherries,
peaches and grapes. For more details of orchard farming in the
Soil types and their
specific mineral concentrations control the distribution of the crops grown. At
first we see deep-rooted plants such as trees because sandy loams located here
are the most productive soils for these plants.
The soils on the opposite side of the river do not have the high saline-alkaline chemistry (see Figure 5). From east to west, soils grades from finer materials, such as clays, into the coarser sandy loams. We will first see row crops of alfalfa, beans, or corn near the river, then mostly walnut orchards as we approach Interstate 5.
25.3 14.3 Ogden-Martin's Stanislaus Resource Recovery Facility
The plant to the right
of Interstate 5 (I-5), at the Crows Landing interchange, incinerates municipal
solid waste. It burns a total of 800
tons of garbage per day, in two furnaces generating 18.5 megawatts of
electricity, which is sold to the Pacific Gas and Electric Company (PG&E).
For details, see appendix B, “Geographic notes on the
26.0 0.7 Citrus Groves
The dark green, heavily foliated trees to the left of I-5 are citrus groves. Visible here are orange, grapefruit, and lemon trees. They are easily distinguished from the walnut trees in the area, which are taller, broader, and more expansive. Almond trees are shorter and fuller looking. The walnut and almond orchards are steadily being relocated to the flanks of the Valley away from prime agricultural soils due to urban sprawl. The citrus trees have been planted here because they grow best on these hills and slopes due to geological and meteorological conditions found here (see appendix B, “Geographic notes on the Great Valley: Citrus Trees”).
The large canal
crossing under I-5 winds southward toward O'Neill Forebay,
near the town of
30.5 0.9 Orestimba Creek
Creeks and streams on
the western side of the
Just past the rise out of the Orestimba Creek flood plain, I-5 briefly passes exposures
of the Valley Springs Formation. Its
type section lies about 140 miles to the north and west in the foothills of the
In stark contrast to
the lush green hills and mountains of late winter and early spring, dry annual
grasses offer a dry and barren backdrop to the
46.5 15.6 Take exit for Santa Nella and Highway 33. Stay to the right and follow the road through town south toward Highway 152.
48.3 1.8 Delta Mendota Canal (D.M.C.)
49.0 0.7 O'Neill Forebay
At the top of the rise outside of Santa Nella traveling south on Highway 33 just past the wildlife refuge, the Delta Mendota Canal again crosses under the road followed in turn by the California Aqueduct. These canals are transporting water from the O’Neil Forebay to points south. (See appendix B, “Geographic notes on the Great Valley Federal and State Water Projects”).
50.2 1.2 Turn right, just before the overpass, and head west on Highway 152.
52.4 2.2 Basalt Area (toll area)
The Basalt Area road leads to reservoir
access for camping, fishing and boating.
It is named for Basalt Hill lying just south of the terminus of this
road, which is an erosional remnant of a Miocene
basalt flow of the Quien Sabe
volcanics (Bennison and
others, 1991). It now acts as a cap rock
for the more easily eroded Great Valley Sequence. Most of the fill used to create the San Luis
Dam was quarried from Basalt Hill (Trefzger,
1963). The Quien
Sabe volcanics are similar
in age to the rhyolite at Pinnacles National Monument
but they were erupted nearly 200 miles apart (Norris and Webb, 1990). They are adjacent to one another today
because of the offset along the
53.4 1.0 William R. Gianelli Pumping-Generating Plant
The silver and red facility located at the base of San Luis Dam, to the left (south) of Highway 152, is the William R. Gianelli Pumping-Generating facility (for more details, see appendix B, “Geographic notes on the Great Valley: Federal and State Water Projects”).
Rocks of the Upper Cretaceous Panoche Formation underlie the plant and dam. At the dam, prior to construction, Payne (1962) mapped the layer, which contains conglomerates and thick sandstones interbedded with thin lenses of shale, as the Ciervo Formation. The foundation was built on weathered conglomerates, and the pump-release tunnels were carved through both rock strata. During construction, the depth and size of the tunnels were increased to reach more consolidated material and to allow for thicker cement linings to be poured for reinforcement near small faults found near the dam site.
55.0 1.6 Romero Visitor Center
stop) Turn left and proceed up the short
road leading to the
57.7 2.7 Ortigalita
The road crosses the Ortigalita fault, indicated by the linear embayment along
the Reservoir (see Figure 3). The high angle fault marks the boundary between
the tilted Great Valley Sequence (GVS) and the Franciscan Complex. It can be traced for approximately 60 miles
The Franciscan Complex in this area is composed primarily of graywacke sandstone and shale deposited in the accretionary prism of the subduction zone that was active on the western margin of North America in middle and late Mesozoic time. The rocks have been severely disrupted by the churning action of subduction and metamorphosed by deep burial within the crust. The metamorphic minerals found in the rocks indicate high pressure, but relatively low temperatures (the blueschist facies: some of the characteristic minerals give the rocks a distinct bluish cast).
57.8 0.1 Oak Grassland & Woodland
Having now passed the Ortigalita Fault, we see a major change in the macro-flora. Oak trees are commonly seen on either side of
the highway from here to Casa De Fruita on the other
63.5 5.7 Pacheco Pass - Elevation 1,368 feet
This pass divides the
headwaters of Pacheco Creek, which drains west into the
The pass was named for ranch owner Don
Francisco Perez Pacheco. He came to the
region in 1819 as an artillery maintenance man.
In 1824 he was given a citation for bravery while helping to put down a
revolt by the indigenous peoples and was promoted to comandante
of the guard at the Customs House in
A historical adobe structure dating from the rancho days was located in the valley now covered by San Luis Reservoir. An effort was made to move the building to safer ground, but in transit the house collapsed and was destroyed beyond repair.
63.7 0.2 Dinosaur Point
The road leads to an area where
65.7 2.0 Serpentinite Outcrop
Serpentine is exposed on the north
side of the highway. Bodies
of serpentine form from the alteration of peridotites
and other ultramafic rocks in the Franciscan Complex
and Coast Range Ophiolite. The serpentine masses are often in the form
of diapirs, which forced their way up through the
crust and along fault zones (Coleman, 1996).
As a consequence, whenever serpentine is exposed, large tectonic
displacements can be inferred (Page et. al., 1998).
Serpentinite is a rock composed of serpentine minerals such
as antigorite, lizardite and
or chrysotile, (Brooks, 1987). The term serpentine was derived from the
Latin word serpentinus meaning serpent rock by
Agricola in 1546 (O'Hanley, 1996). It alludes to the dark green splotched scaly
contains very high concentrations of magnesium (Mg) and iron (Fe) along with
other elements such as nickel, chromium, or selenium. Serpentine is relatively stable at the
surface, although it eventually breaks down in humid climates, leaving
iron-rich smectites and talc residues. In arid regions it will persist, forming
mountains and resistant outcrops.
Serpentine soils often support a population of unique, endemic, and rare plants. They normally have toxic levels of trace elements restricting the type of species that can establish themselves, but certain plants, known as hyperaccumulators, thrive in this environment. Most plants can store minute amounts of toxic materials, but plants adapted to serpentine soils have evolved the ability to concentrate nickel, cobalt, zinc, and other minerals in their foliage, in amounts up to 1% of their dried weight (Reeves, 1983). These plants tend to act like tiny mining operations, concentrating mineral resources that can be harvested and refined in useful quantities. It has been suggested that areas with serpentine soils could produce economic benefits not otherwise available through farming or ranching.
66.8 1.1 Meta-igneous Rocks
outcrops on each side of the road are a metagabbro
unit within the Franciscan Complex. They
are texturally related to other prominent peaks and hills in the area such as
The isolated nature of these bodies suggests an extrusive component (i.e. volcanic necks). As the more resistant metavolcanics were uplifted, they were differentially weathered, creating the knobs, hills and peaks on either side of the road.
74.8 8.0 Highway 152 passes the offramp to Casa De Fruita, the very epitome of the roadside tourist trap. Don’t miss the petting zoo, the cup flipper, and the train rides for the little kids!
76.7 1.9 Turn
left onto highway 156 toward the town of
81.1 4.4 At stop light
turn left onto
82.3 1.2 Hollister city limits.
Hollister was named
after W. W. Hollister, a Civil War colonel who was instrumental in the creation
of the town. Around 1855 Hollister and
the Flint-Bixby Company purchased the 35,619-acre San Justo Rancho land grant
from Don Pacheco, mentioned above. They
paid $370,000 for the property to establish better trade routes to and from the
quicksilver mines of New Idria (Myler,
1970). By November of 1868 Hollister had
proposed the layout and location of a new town and had sold some of his
holdings in that area. A group of fifty
local farmers, called the San Justo Homestead Association, named the town in
honor of its benefactor. On February 12,
84.9 2.6 Turn right onto
Stop 1: Walk to the west side of
the softball field to a bluff overlooking the Hollister area to the south and
Park Hill is a horst that has been
displaced vertically along the Calaveras fault (see Figure 5). The main trace of the fault trends along the
northwest side of the hill. If you
follow the fault trace to the northwest, another small hill is visible in the
distance. This is another horst along
the Calaveras fault. The fault cuts
along the northwest side of this hill, too.
Matthews (1997) reports the rate of creep along the Calaveras fault in
Hollister has varied through time. Town
records of sidewalk construction indicate that there was no movement along this
trace between 1910 and 1929. Then creep
averaged 8 mm/yr (0.31 in/yr) from 1929 to 1961. Between 1961 and 1967 the rate was 15 mm/yr (0.59 in/yr). Since 1979, slip at
one site in Hollister averaged 6.6 mm/yr (0.26
in/yr) and 12 mm/yr (0.47 in/yr) at another (Sims, 1989).
If you turn
toward the southwest, you will face the main trace of the
As you face south and look down
into the town of
85.1 0.2 Drive
85.5 0.4 Turn
85.7 0.2 Turn
Stop 2: [Note: Please practice geo-courtesy on this walk by staying out of people’s yards and keeping on the public sidewalks.] Walk to the corner of Locust and Fourth. The sidewalk on the south side of the street is dated 1910 and was offset 22.9 cm (9 in) by 1967 (Rogers and Nason, 1967; Matthews, 1997). The fault trace travels directly under the house. The curb, sidewalk, and stone wall have all been offset here with a right-lateral sense of motion.
Location map of the Hollister area showing USGS 7.5’ quadrangles covering field
trip area (modified from
Figure 4: Physiography
5: Map of the Calaveras fault
zone through Hollister (modified from
along Locust (north) to the alley on the north side of the garage next to the
house you just visited. The north wall
of the garage has been displaced right-laterally at least 25 cm (1 ft).
street here. The curb in front of the house shows evidence
of crustal shortening. The slabs have been buckled upward. Within a few tens of feet in this area there
is evidence of compression, tension, and shearing. Look carefully and you will see all three
where the fault obliquely crosses the sidewalk and street (see Figure 6).
street and continue north toward the base of Park Hill, the horst from Stop
1. As you turn the corner on
85.8 0.1 Drive
to the end of the block and turn right onto
85.9 0.1 Turn
86.0 0.1 Cross
86.1 0.1 Turn right onto 6th Street and park about half way down the block on the right side of the street.
Stop 3: The change in topography indicates a small scarp produced by the Calaveras fault. Curbs on both sides of the street are right-laterally displaced. The curb on the north side had a 11.4 cm (4.5 in) offset at a construction joint in 1967 (Roger and Nason, 1967; Matthews, 1997). The concrete retaining wall on the north side shows world-class right lateral displacement (see Figure 7). Photos of this wall are commonly included in physical geology text books illustrating active fault creep and resulting deformation. Return to the vehicle.
86.2 0.1 Turn
86.3 0.1 Turn
86.6 0.3 Turn
88.1 1.8 Turn right onto
88.4 0.3 Turn left onto
89.8 1.5 Intersection with
Figure 6: Photo showing different manifestations of
strain where the Calaveras fault obliquely crosses a curb and street at the
northwest corner of
92.1 2.3 Note the landslide headscarp at the watershed divide here. The drainage basin on your right (west) lies along the San Andreas fault and consists of loosely consolidated (low shear strength) Miocene sediments that have felt the effects of repeated seismic activity and heavy rainfall, as well as land use and vegetation changes due to grazing. The combination of these factors has resulted in the many shallow soil slips, slumps, and earthflows visible.
93.5 1.4 Hollister Hills State Vehicular
Recreation Area entrance. This
motorcycle and offroad vehicle (ORV) park was created
around 1980 after Howard Harris, a local geologist, rancher, and farmer, sold
the property to the State of
95.5 2.0 Calera Winery (formerly Almaden Winery). Park along the right side of the road and walk to the drainage ditch across the access road on the south side of the main warehouse.
Stop 4: (Note: The owner of Calera Winery, Pat DeRose, appreciates a phone call, (408) 636-9143, before
the arrival of curious geo-visitors).
The drainage ditch (see Figure 9) was constructed in 1948, when the
present building housing the winery was rebuilt. The
Figure 7: View east along
Figure 8: Photo shows flood damages incurred during
winter of 1997-8 along
102.6 7.1 Intersection of Cienega and Union Roads.
Turn left onto
105.0 2.4 Entrance to
106.2 1.2 Intersection with Highway 156. Turn left onto 156.
109.2 3.0 Turn left onto
Stop 5: The
109.8 0.1 90 degree right bend in road; follow this bend.
110.3 0.5 Turn right at
110.9 0.2 Turn right onto
111.0 0.1 Turn left onto
111.1 0.1 Park along the right side of the street and walk back to the edge of the bluff next to the grassy area in front of Mission San Juan Bautista.
Stop 6: The town of
Figure 9: Drainage ditch offset
by creep along the
111.3 0.2 Continue along
111.4 0.1 Turn
111.9 0.5 Slow
down and look to your left along a fence line perpendicular to the road. The fence was built across the San Andreas fault and is noticeably offset here at Nyland
Ranch due to creeping of the
113.5 1.6 Turn
114.4 0.9 Turn
115.1 0.7 Turn
left onto Highway 129 from
117.5 2.4 Cross
118.5 1.0 To
your right you will see an earth dam constructed by Graniterock
Company, which increases the capacity of Soda Lake, an old meander cutoff of
the Pajaro River (Wills and Manson, 1990). Granitrock pumps a slurry of fine grained material from the quarry into
119.6 1.1 Visible in the embankment along the right side of the road are horizontal groundwater wells installed by Caltrans to drain the hillslope of excess water. The rate of flow increased dramatically following the 1989 Loma Prieta earthquake (Jerry Allen, Graniterock Company, pers. comm., 1998).
120.3 0.7 Turn
121.4 1.1 Turn
121.8 0.4 Entrance to Graniterock’s A. R. Wilson Quarry. Follow the main road up the hill to the offices on the left side (follow signs).
Stop 7: Graniterock
Company’s Arthur R. Wilson quarry, located in the westernmost tip of
The following excerpt from the company’s promotional pamphlet gives a brief and simplified geologic history of the deposit:
“[The quarry’s] story began more
than 200 million years ago, when a great mass of molten granite began to push
up from the depths of the earth through limestone, sandstone and clay on the
bed of an ancient ocean. The granite
cooled, contracted and cracked, and was folded, broken, crushed and uplifted as
the Pacific Plate slowly drifted by the continent of
Allen (1946) initially classified the rock as quartz
diorite; Stinson and others (1983) later reclassified it as hornblende gabbro of Cretaceous age.
The Pliocene Purisima Formation and
Pleistocene Aromas Formation, which thicken and dip to the southwest (Higgins,
1989), overlie the deposit and are considered overburden. The gabbro is
bounded on the northeast by the
Figure 10: A Pacific Gas &
Electric Company drilling foreman checks out a landslide along
In 1871 civil engineers for the
Southern Pacific railroad, forging the main coastal line through
Before the rock can be mined the
overburden must be removed at the quarry site.
A 3.2 km (2 mi) long conveyor system (see Figure 12) transports the waste
rock to a site where it is placed.
Reclamation creates a buffer zone between the mine and surrounding
populated areas as the stockpiles are recontoured and
revegetated (Higgins, 1989). The underlying gabbro
is quarried and loaded into a self-propelled, mobile rock crusher, which is the
largest of its type in the world (Jerry Allen, Graniterock,
pers. comm. 1998). The crusher takes up
to 106 cm (42 in) diameter rocks and breaks them into 25.4 cm (10 in) and
smaller. Then a 1.6 km (1 mi) long
conveyor system transports the rock to a secondary and tertiary plant where it
is screened, washed, and sorted by size (see Figure 13). Water is used in the process and the plant
has its own water treatment and recycling facility. Unused fines from the filtration process are
The quarry is capable of
producing up to 3,000 tons of rock per hour and peaks during the summer. On a typical day there may by 100 railroad
cars and 400 trucks transporting aggregate from the processing plant. There are about 70 employees during the
summer and about 50 during the winter months working at the quarry. Haul trucks cost approximately $450,000; the
rock crusher initially cost $3,000,000 and today would run about 3 to 4 times
that amount. The newest loader
(purchased this year) cost $1,100,000 and typically burns about 180 gallons of
diesel fuel per day. The company is
planning to upgrade the automated truck and rail car loading system in the near
future. The contact person for tours of
the facility is Jerry Allen (831) 768-2000.
Location map showing Graniterock company’s
Arthur R. Wilson quarry,
Figure 12: Overburden conveyor system used by Graniterock Company to transport tailings 3.2 km (2 mi) from the quarry to the dump site. During reclamation, the non-native eucalyptus trees visible in this picture are removed and native vegetation (including oak trees, bunch grasses, poison oak, etc.) is planted.
Figure 13: Screening plant at the
Arthur R Wilson quarry. The building in
the upper left of the photo is the computerized truck and rail loadout structure.
The main trace of the
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