San Francisco geology: a diversion from AGU

Serpentinite block at Marshall Beach (SW side of the Golden Gate Bridge). The road with folded chert can be seen on the far (north) side of the bridge.

Serpentinite block at Marshall Beach (SW side of the Golden Gate Bridge). The road with folded chert can be seen on the far (north) side of the bridge.

This year I attended the American Geophysical Union fall meeting in San Francisco. This meeting does not offer organized field trips the way that GSA and other conferences with a strong focus on geology do. However, nice outcrops can easily be found near the city, and even though I am not particularly familiar with the geology of the area, renting a bike one day before the meeting was a good decision that I recommend to any geologist who is up for some local hard rock geology.

The closest localities are found on the north and south sides of the Golden Gate Bridge. I started at the north side and found the light to be good during the first part of the day there. The southwest side is great in the evening light.

Iron and manganese-rich chert and related sediments at Conzelman Road.

Iron and manganese-rich chert and related sediments at Conzelman Road.

Crossing the Golden Gate Bridge takes you to the Golden Gate National Recreation Area. I  Just after crossing the bridge (west side), go left onto Conzelman Road. As you wind your way up the road there are nice exposures of folded chert layers several places, both before “Battery Spencer” and farther up the road.

Folded chert.

Folded chert, Conzelman Road. Some quite interesting patterns have formed locally due to layer-parallel shear/slip. Flexural slip is an important mechanism during folding of such layered rocks where the mechanical contrast between the chert layers and thinner shale layers is very high.

The cherts are oceanic (pelagic) sediments of the Franciscan Complex, a rather chaotic assemblage of Jurassic-Cretaceous rocks that were crumbled and sheared during accretion onto the N American continental margin in the pre-San Andreas Fault days when this margin was more of a “standard” subduction zone.

Parts of the Franciscan Complex appear as chaotic in the sense that blocks and fragments of different rock types occur together in what is referred to as mélange. There may be both depositional and deformational components to a mélange, but clearly deformation played an important role during the formation of these rocks. Typically blocks of serpentinite, amphibolite, chert and sandstone occur intermixed. Several of these rocks can be seen on the San Francisco (south-west) side of the Golden Gate Bridge along the bluffs of Marshall Beach (Battery to Bluffs Trail). This area is also affected by landslides, which also give the rocks a chaotic (“melangy”) appearance.

Melange

Melange, Marshall Beach. Blocks of serpentines are most prominent in the picture.

Ultramafics and serpentinites commonly show nice vein systems, and also nice slickensides. You will find them here as well, as shown in the picture. Once serpentine forms in fractures, the rock easily deforms, and this typically happens repeatedly throughout its lifespan. 

Slickensides formed on slip surface in serpentinite.

Slickensides formed on slip surface in serpentinite.

I am sure there are other nice hard-rock localities around, and feel free to comment on that below. With a car the selection of nice outcrops will of course be significantly larger. Here are a couple of resources if you want more information about the geology of the area: http://pubs.usgs.gov/bul/2195/b2195.pdf

http://www.sanandreasfault.org/Geology%20of%20the%20Golden%20Gate%20Headlands%20Field%20Guide.pdf

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About Haakon Fossen

Professor of structural geology, University of Bergen. Author of book Structural Geology, published at Cambridge University Press
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One Response to San Francisco geology: a diversion from AGU

  1. Sowreh says:

    it was so nice! thank you so much!

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