Franciscan Cherts of Marin

These rocks, like the pillow lavas they cover, clearly formed underwater, definitely in very deep water, possibly at the Equator and during the age of dinosaurs (Marin's wet Jurassic Park?). This is an image of an exposure of these ribbon cherts in the Marin Headlands. A example of a modern environment in which the majority of Marin's cherts were formed is seen in this seismic profile collected during the voyage of the USNS Kane in 1968.

Microscopic radiolarian skeletons, are today, forming extensive sediments in the Equatorial regions - they seem to like warm water and thrive in the nutrient rich Equatorial upwelling region. Radiolarians are protozoans which form skeletons of glass (SiO2). Their skeletons are quite well preserved within the hard chert, even though there has been extensive alterations of the originally soft oozes they comprise. This scanning electron micrograph of a radiolarian actually comes from similar-aged limestones in the San Andreas Fault zone near Olema.

Some cherts resting immediately upon the pillows, have been significantly metamorphosed by hot seawater and converted into "pretty chert" or "jasper". This photo shows a thermal front within chert in close proximity to highly altered pillow lavas. The cherts close to the pillow lavas commonly have significant accumulations of metallic oxides within them, commonly manganese within Marin, but some small amounts of copper have been mined from the western slopes of Mt. Tamalpais. This photo shows interbedded manganese and chert from Red Rock Island in San Francisco Bay. All of these metallic deposits are likely ancient "black smoker" deposits, like those reported photographed from the East Pacific Rise where it enters the Gulf of California (see pillow lava page) and in the Gorda Ridge off of Northern California and Oregon.

While Ring Mountain it self does not have any good chert exposures, there happens to be abundant chert on the top of Ring Mountain. This chert was imported for the construction of the Nike missle site that occupied the ridge during early parts of the cold war.

Hypotheses on the origins of cyclic bedding in radiolarian cherts

Cushman Foundation for Foraminiferal Research material on Radiolarians


by Catherine Nigrini and T. C. Moore, Jr.
Cushman Foundation for Foraminiferal Research (no longer in print)'s lists of resources

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