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A Structural Transect Across a part of the
Marin Headlands Terrane of the Franciscan Complex,
Southern Marin County, California
Sarah Petty
Spring 2016 Field Methods
San Francisco State University
Introduction
The Marin Headlands terrane is part of the Franciscan Complex, lying east of the
San Andreas Fault and just northwest of the Golden Gate Bridge. The Franciscan
Complex exposed in our study area of Marin Headlands consists of pillow basalt,
radiolarian chert and shale. These rocks were created at different times through the
middle Jurassic to middle Cretaceous. They were accreted to western North America in
the late Mesozoic or Tertiary as the Farallon plate was subducting under the North
American plate.
Our study was conducted over three day trips in early February 2016 and was led
by John Caskey, Ph.D. and completed by students from the San Francisco State
University Field Methods class. We mapped a 1.5 km length of road-cut along a section
of Conzelman Road below the informally named Kirby ridge. The purpose of our study
was to map and collect structural data in order to construct a geologic cross section and to
examine geometric relations between faults and folds.
This paper presents a geologic map and cross section of the study area,
macroscopic fold data from radiolarian chert units in the map area (previously collected),
and descriptions of map units and structural geology, as well as interpretations of the
possible relations between faults and folds in the map area.
Stratigraphy
The Marin Headlands Terrane contains rock types typical of the Franciscan
Complex and range in age from Middle Jurassic to Middle-Cretaceous. The oldest unit is
greenstone and was formed at the mid-ocean ridge as pillow basalt piled in layers of
underwater lava flows. The radiolarian chert was deposited on top of the greenstone over
millions of years, as the Farallon plate moved east. The sandstone and shale unit was then
deposited on top of the chert as the Farallon plate neared North America. As these rocks
entered into the Franciscan subduction zone, they were severely folded and faulted.
During the Tertiary, as subduction continued, they were accreted onto the North
American continental plate, thus becoming the Franciscan complex. The units, as
described in further detail below, and tectonic setting help us to further understand the
stratigraphy we observe today.
Pillow basalt/greenstone
The pillow basalt was formed under water at the East Pacific Rise spreading
center far west of the North American continent. Magma flowed out of the mid ocean
ridge and was rapidly cooled forming pillow structures. The dark coloration, and
aphanitic texture are due to this process as well. Hydrothermal alteration of minerals at
the spreading center gives the basalt a dark green color, the reason for the name
greenstone. It is a slope-forming unit that occupies saddles along Kirby Ridge and the
base of the unit is not exposed at any location in our mapping area.
Radiolarian Chert
The pelagic chert of the Marin Headlands is abundant in siliceous radiolarian
fossils that are silt to sand size and that are visible with a hand lens. The depositional
environment was the abyssal plain below the calcium carbonate compensation depth and
near the equator. The chert we observe is extremely folded due to being wedged against
the continental margin and the red color indicates iron oxidation. The chert is very well-
bedded in 2-10 cm thick layers alternating with thin shale beds. The chert and the
greenstone are separated by a basal depositional contact.
Sandstone and Shale
The sedimentary sandstone and shale unit is a light grey to dark grey and is poorly
bedded where exposed. It is a medium-grained, and fairly weakly cemented. These rocks
were derived from erosion of rocks on the North American continent and deposited by
submarine landslides that correspond with subduction zones along continental margins.
The sandstone-shale unit lies conformably on top of the chert unit.
Structure
Faults
There are several faults exposed in road cuts or otherwise inferred based on map
relations. The southernmost thrust is approximately 300m southwest of Kirby Peak and
places the greenstone over chert. The thrust dips moderately to the southwest. There are
prominent shear fractures in the greenstone that are subparallel to the fault contact. A
second thrust is well exposed roughly 200m southwest of Kirby Peak that places
greenstone over sandstone and shale. It dips gently to moderately to the southwest and
features a prominent shear fabric and boudinage structure in the footwall. The shear
fabric if formed by compressional stress. The boudinage structure was formed by
extensional stress. A third thrust, not exposed but inferred between greenstone and shale
in the northernmost part of the map area, places greenstone on sandstone. The fault is not
distinct, but we are able to map it because of stratigraphic relations. A fourth fault is
mapped approximately 65m northeast of the boudinage structure below Kirby Peak based
on the stratigraphic and structural relations expressed. The sandstone- shale unit should
lie conformably on the chert bed with no vertical offset. Since we see the sandstone-shale
unit lower than the chert without it being overturned, we can infer some offset has
occurred. If we found sandstone on the northeast side of the fault contact, we would find
it above the chert on Kirby Peak. We do not find it there, so we estimate a minimum
vertical offset of 30m.
Folds
The chert beds are highly and tightly folded throughout much of the map area.
Macroscopic fold data collected during a previous field trip is presented below. An open,
upright fold is well exposed just northeast of the boudinage area forming an anticline
along the exposed depositional contact between chert and sandstone units as seen in
Figure 4. Macroscopic fold data was collected from folded chert in the southwest portion
of our mapping area as well as the chert below Kirby Peak. This data is presented in the
stereoplots in Figure 8. Though the data I previously collected is somewhat incorrect, we
know that the general trend of the southernmost folded chert in our mapping area trends
southwest. The more northern section of chert trends northwest. In knowing that the folds
change from southwest to northwest trending, without being tremendously discontinuous,
we can infer that the folding and thrusting formed during the same deformational
episodes, during the time these rocks were accreted onto the North American plate.
Conclusions
In taking a closer look at the rock types we see in the Marin Headlands, and
mapping the attitudes of folds and faults in the area, we can conclude that the incredible
deformation exposed is due to the combination of plate tectonics, the Franciscan
Subduction Zone, the accretionary wedge and uplift of the area. We observe a connection
between the trends of the folds and faults solidifying our interpretation that the formation
of these structures occurred during the same deformational episode.