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Mesozoic Deformation of the Franciscan Complex in the Marin Headlands
Terrane
Alexandra Allen
ERTH 620: Field Methods
Geology Department
San Francisco state University
The Marin Headlands, located in Marin County, California, directly north of San
Francisco, is a scenic highland exposing a mélange of rocks from the Franciscan Complex in
what is known as the Marin Headlands terrane, as shown in figure 1. This terrane block is one of
eight formed millions of years ago through intense tectonic activity that deformed a sequence of
depositional units called the Franciscan Complex. The three main rocks ranging from 80 to 200
million years old that make up the sequence exposed in the Marin Headlands include greenstone
altered from basalt, radiolarian chert, and alternating beds of sandstone and shale. More than 200
million years ago, during the Jurassic period, greenstone and chert were deposited in the central
Pacific Ocean, near the equator, in a mid-oceanic ridge on the Farallon plate (Elder). As the ridge
pushed the plate away, it moved northeast until it reached a latitude close to Mexico, where it
then began to subduct underneath the continental crust of the North American plate’s westernmost side during the Cretaceous period (Leech). Because the density of the mafic-basaltic ocean
crust is greater than that of the continental crust, the Farallon plate was pushed beneath the North
American plate completely, forming a marine trench in the process of subduction (Konigsmark).
As the oceanic plate scraped along the continental crust, marine sediments along the continental
margin filled in much of the trench. The stress and strain caused by the friction as these plates
slid past each other caused high grade metamorphism as well as intense deformation and folding.
The lightest material scraped off and along the plates, became wedged between the plates as part
of an accretionary wedge that was uplifted in an eastward direction by major normal faults
(Leech). Prior to the formation of the San Andreas Fault, in the Cenozoic, 30 million years ago,
the deformation caused by subduction had fractured the wedge into eight terranes in the form of
“shingles”. As the San Andreas Fault formed and extended from south to north, it pulled the
terrane blocks northward along the continental coast, rotating the blocks along the way. During
the Cenozoic period, the San Andreas transform carried the terrane all the way to the Marin
Headlands, rotating the block an estimated 90° from its origin (Elder). This specific block of
terrane made up of heavily deformed Franciscan Complex rock was again uplifted by thrust
faults to its current location and named the Marin Headlands terrane.
On the dates of February 12th and 19th of 2016, my fellow students in ERTH 620: Field
Methods in Geology at San Francisco State University and I traversed to the Marin Headlands in
Marin County, California to map and collect field data in order to understand the geologic
structures and units in the Franciscan Complex exposed in this location through cross sections
and geometric data of folds and faults. Our research site was located about 4.5 miles north-west
from the northern peninsula of San Francisco, along Conzelman Road. We analyzed and
collected data on an exposed outcrop of the complex ranging from where the road becomes oneway to the intersection of Conzelman Road and McCullough Road along the Cliffside to the
northern side of the road below Kirby Ridge. Using Bruntons, we collected a series of strikes and
dips, as well as plunge data of macro and micro folds, unit contacts, and faults while mapping
geologic units of the Franciscan complex along the way. This data will be used to make figures
for a report on the structures of the Marin Headlands, including a cross section across the three
major points on Kirby Ridge (figure 3), a stratigraphic column of the units exposed (figure 6), a
topographic map depicting the units in the sequence as well as their contacts and any faults
cutting through (figure 2), and stereonets showing the orientation of folds (figure 5). This report
will contain these figures as well as interpret the Franciscan Complex’s paleo-depositional
environment, and any relations between faulting and folds located and mapped in the sequence.
The three units of the Franciscan Complex that comprise the Marin Headlands terrane
were formed in the ocean, but different processes created the rocks, in order from oldest to
youngest: greenstone, chert, and sandstone/shale, as shown in figure 6. The rocks in this
sequence range from 200 to 90 million years old and have a maximum thickness of 15,240
meters thick (National Park Service).
Greenstone
The oldest unit, greenstone, is a highly metamorphosed igneous basalt of a yellowish-green color
with evidence of bedding with a thickness of 5-10 m. The greenstone in this location does not
have its base exposed. Pillow basalts that have an average width of 10 cm to 1 foot can be found
in some of the layers, in imbrication that show stratigraphic up, as well as the flow direction and
evidence that it was in a fluid (seawater) (Leech). When basaltic lava poured out of a midoceanic ridge volcano, it immediately came into contact with seawater that combined with the
heat from the magma, altered minerals in the basalt into chlorite and pumpellyite, creating
greenstone (Golden Gate National Recreational Area). The greenstone in the Marin Headlands is
discontinuous and because of its vulnerability to weathering, generally is slope forming where it
has been thrust above younger chert beds that form lenses. Along Kirby Ridge, as seen in figure
1 and 2, the greenstone sits along saddles along the ridge. The greenstone has been aged to be
older than 100 million years (Elder).
Radiolarian Chert
The middle-aged unit is an open ocean formed, radiolarian, red chert. The chert sits above
greenstone as a depositional contact. Radiolarians, single-celled organisms with silica shells, die
and fall to the ocean floor where they decompose and their shells remain in the form of silicate-
rich chert. This is a sedimentary rock with layers anywhere between 2-12 cm thick, and a general
thickness of 80 m with some small layers of red clay throughout. These layers are parallel to
bedding and show stratigraphic up (Leech). Because of the intense Z-folding, that resulted from
grinding against the continental margin while being transported in the terrane blocks, in the
bedding, it has been called a ribbon chert. Folds have a general orientation, striking southwest
and dipping west as shown in figure 2 and 3. The exposed chert is red due to oxidation of
silicates and very brittle. Fossils of douvilleiceras mammillatum and mantelliceras date the unit
between 200 and 100 million years ago between the Early Jurassic and Late Cretaceous
(National Park Service).
Sandstone/Shale
The youngest unit in the Franciscan Complex that is observable in the Marin Headlands is a
sandstone and shale unit. Sandstone, the most common clastic rock found in the complex, is
coarse-grained with angular grains of K-feldspar, quartz, and basalt. The sandstone is a
yellowish-brown and heavily weathered though it is well-cemented, as shown in figure 6. The
layers of sand were deposits from the continental margin of the North American plate that filled
a marine trench during subduction through turbidites and submarine landslides (Leech). Because
of the rapid flow of sediment, particles settled out of suspension with the smallest differentiating
to the top, forming shale. Shale layers are a very fine-grained rock of a greyish-black color that
are also subject to intense weathering and erosion and therefor a slope forming unit with the
sandstone (Elder). Beds of the sandstone/shale exposed in the Marin Headlands range from 1-5
m thick but the bedding has been estimated to be about 20 m thick. Shale layers are thin, between
1-5 cm thick as shown in figure 2 and 3. The sandstone/shale unit is a depositional contact sitting
above the radiolarian chert and estimated to be 100-90 million years old, deposited during
subduction of the Farallon plate in the Early Cretaceous (National Parks Service).
Faulting in the Marin Headlands is common with normal and thrust faults prominent
along Kirby Ridge as seen in figure 2. Four main thrust faults were observed along Conzelman
Road and orientations were measured for each one to better understand the source of faulting
here.
The first fault exposed along Conzelman Road, moving from the southern to northern
end, is approximately 200 meters north from the one-way road as seen in figure 2. This fault
pushes older greenstone above younger chert and is a reverse-thrust fault with the hanging wall
of greenstone moving upward relative to the footwall of red chert, as seen in figure 3. The fault is
a contact between the two units, with slick and sides along the shear zones parallel to the fault,
expressed in the greenstone. The fault itself dips steeply to the southwest, turning due west as it
moves towards the surface and higher topography. Shear fabric in the chert is dipping 50°-70°
southwest.
The second thrust located about 300 meters north of the previous fault, as shown in figure
2, is a fault contact between all three of the units in the Franciscan Complex exposed in the
Marin Headlands. This fault is also a reverse-thrust with the hanging wall of older greenstone has
been pushed above and over the younger units of sandstone/shale and chert that make up the
dropping footwall as seen in figure 3. The units dip steeply to the southwest with the chert
striking S40°E and dipping 40°SW with its layers parallel to the fault contact. Layering in the
greenstone has a strike of N20 ° W and a dip of 30 ° SW. The orientation of the fault contact
between sandstone/shale and greenstone is 176 °, 36 ° NW. Shearing is apparent in greenstone
unit.
The third fault along Conzelman Road is not exposed but is oriented about 110 meters
north from the previous, and is a vertical strike-slip fault, marking a contact between chert and
younger sandstone/shale units, shown in figure 2. Though we cannot exactly see the fault
because of heavy vegetation and topsoil, we can infer its existence due to the disrupted
continuity of the chert unit. To the northeast of the hidden contact, chert sits higher than the
sandstone/shale unit on top of the exposed ridge, striking N40 ° W. As seen in figure 3, there
must be a vertical fault to explain the movement of chert directly perpendicular to the orientation
of layering.
The fourth fault, another reverse-thrust, along this area of the Marin Headlands is another
fault contact that pushes older greenstone above younger sandstone/shale and chert units of the
Franciscan Complex. This fault is exposed at the intersection of Conzelman Road and
McCullough Road as seen in figure 2.
Folding in the Marin Headlands terrane is extensive and intense, with several categories
exposed throughout the greenstone, chert, and sedimentary units. Chert layers are the most
obviously altered beds with Z-folds and Chevron folds throughout as seen in figures 2, 3, and 6.
Most macroscopic folding is oriented southwest with Chevron folds plunging southwest on a
smaller scale. On the eastern side of the exposed ridge, folds dip only gently south
(Konigsmark). As seen in figures 4a,b, and c, orientations of macrofolds in the chert have similar
characteristics in stereonet data collected in the previous semester in a structural geology class.
Folding in sandstone/shale units is not common and not as well defined due to heavy erosion.
General folding in the Marin Headlands terrane can be attributed with synclines and anticlines
(Elder). One measured anticline between sandstone/shale and chert, located about 500 meters
from the intersection of the one-way road and Conzelman Road as shown in figure 2, is dipping
at a low angle. Plumose structures from mode 1 fractures are evident in the chert as well as
columnar joints. The axis of the anticline itself is parallel to faulting on either side with its
shorter limbs oriented northward. Synclines in the area, on average, plunge southwest, oriented
about 40 ° S25 ° W with the shorter limbs between 155 ° and 160 ° (Konigsmark). Thrusting and
folding in the area have similar orientations and cut through the same units, leading me to
believe that they are coeval and most likely formed during the late Mesozoic during intense
deformation from the movement of the Farallon plate northward. This is most likely why folds
are parallel to bedding with axis perpendicular to bedding, and oriented southwest. Some folding
does not follow the general orientation and it can be assumed that these folds are younger and
may be the result of the major normal fault in the Marin Headlands that uplifted the terrane block
to its current position.
This paper concludes that Mesozoic deformation from the movement of the Marin
Headlands terrane, composed of the Francsican Complex, resulted in intense faulting and
folding, especially in the brittle chert units. The orientations and fault data included in this report
can be attributed to the overview detailing the history of the complex structures found throughout
the Marin Headlands.
Reference Page
Elder, William P. "Geology of the Golden Gate Headlands." Usgs.gov. National Park
Service, Golden Gate National Recreation Area, Calif., 2001. Web. 16 Apr. 2016.
"Geology of the Golden Gate Headlands." Golden Gate National Recreational Area.
Nps.gov, 2016. Web. 26 Apr. 2016.
Konigsmark, Ted. "Geologic Trips San Francisco and the Bay Area." Geologictrips.com.
Geopress. Web. 26 Apr. 2016.
Leech, Mary. "Geology of the Marin Headlands and the Half Moon Bay Coast."
Sanandreasfault.org. Stanford University, 19 May 2001. Web. 26 Apr. 2016.
United States. National Park Service. "Geology Resources." National Parks Service. U.S.
Department of the Interior, 2016. Web. 26 Apr. 2016.