Franciscan Assemblage

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Roadcut showing Franciscan chert rock in Glen Canyon Park. The remarkable folding of the stacked layers indicates the tectonic forces that lifted up the coastal mountain ranges, and which warped the originally planar layers of this rock into the fantastic shapes they now present. The chert itself in this area is rich with fossils of radiolarian creatures. ©2007 Eric A. Schiff.

The Franciscan Assemblage is a geological term for an accreted terrane of heterogeneous rocks found on and near the San Francisco Peninsula. It was named by geologist Andrew Lawson who also named the San Andreas Fault which bounds the Franciscan Assemblage.

Also known as the "Franciscan Formation," "Franciscan Series," "Franciscan Group," "Franciscan assemblage," or "Franciscan Complex," it includes altered mafic volcanic rocks (greenstones), deep-sea cherts, greywacke sandstones, limestones, serpentinites, shales, and high-pressure metamorphic rocks, all of them faulted and mixed in a seemingly chaotic manner.

It forms the major component of the Pacific Coast Ranges of California.

Wentworth and others (1984) interpreted the juxtaposition of the Franciscan Assemblage and the section consisting of the Coast Range ophiolite and the Great Valley sequence to have happened through landward movement of the Franciscan Assemblage as a tectonic wedge.

[edit] References

Wentworth, C. M., Blake, M. C. Jr., Jones, D. L., Walter, A. W., and Zoback, M. D. 1984. Tectonic wedging associated with emplacement of the Franciscan assemblage, California Coast Ranges. In Blake, M.C., ed., Franciscan geology of northern California. Pacific Section, Society of Economic Paleontologists and Mineralogists, Field Trip Guidebook 43, p. 163-173.