The Franciscan Complex is the name
given to the series of rocks that make up the majority of the Coast
Ranges that are on the east side of the San Andreas Fault. The
individual rock types that are found in the Franciscan Complex
include greywacke sandstone, siltstone and argillite, basalt or greenstone with units of
greenstone, ribbon chert, limestone, serpentinite, blueschist,
amphibolite, and eclogite. This by itself is a mouthful of rock
types, but on top of that, areas of the Franciscan Complex were
fractured, faulted, and mixed together in a jumble of rock. Thse
areas are given the term melange, which is French for mixture or
blend.
This melange resulted from the way that the Franciscan Complex
formed. These rocks formed at the plate boundary between the
ancient Farallon Plate and the North American Plate. Some 200
million years ago, the Farallon Plate was subducting beneath the
North American Plate. The Farallon Plate was an oceanic plate, so
it had mostly deep ocean rocks that included clays and siliceous
sediments made up of the shells of radiolarian organisms, but also
some limestone from the tops of seamounts (mostly composed of
foraminiferal ooze) and basaltic sea floor (from the spreading
center).
This process of one plate sliding under the other did not go
smoothly. Periodically, pieces of the the Farallon Plate would be
scraped off and added to the edge of the North American Plate in
what is called an accretionary wedge. This process repeated
multiple times creating layers in the accretionary wedge.
Additionally, tectonic forces fractured and folded the layers in
the parts of the wedge mixing up the rocks into a melange.
Eventually, about 12 million years ago, the Farallon Plate was
completely subducted under the North American Plate near San
Francisco, ending the formation of the Franciscan Complex here (the
Juan de Fuca Plate is the last remnant of the Farallon Plate that
is not yet subducted). The plate boundary changed to the transform
boundary between the Pacific and North American Plates we see
today.
In the figure above, the part
called the Franciscan subduction complex would later become the
California coastline where we see this melange. The Great Valley
sequence is about where the San Joaquin Valley is and the Volcanic
chain is now the Sierra Nevada Mountains.
The cliffs at the south end of Rodeo Beach show this melange.
Within the cliffs are a variety of rock types. Without even knowing
what the rock types are, one can see that there are rocks with
different colors and textures right next to each other.
Logging questions:
- The text "GC3BNFC The Franciscan Melange at Rodeo Beach" on the
first line
- The number of people in your group.
- How many rock types can you see based on color?
- What are the relationships between the rock types, layers,
folds, or what?
- How do the rock types in the cliffs relate to The
Rainbow Sands of Rodeo Beach?
The above information was compiled from the
following sources:
- William P. Elder, Geology of the Golden Gate
Headlands, National Park Service, Golden Gate National Recreation
Area,from, Stoffer, P.W., and Gordon, L.C., eds., 2001, Geology and
Natural History of the San Francisco Bay Area: A Field-Trip
Guidebook, U. S. Geological Survey Bulletin 2188, p.
61-86.