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 radiolarian 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[1] 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.
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Prehistoric tools
Archaeological recovery of Franciscan chert prehistoric tools from Native American sites in the western USA acknowledges the use of Franciscan material as a diagnostic indicator of rock source and thus the presumption of ancient trading patterns among tribes. For example finds of Franciscan chert in Central California suggest extensive trade between tribes of the San Francisco Bay Area with Chumash peoples.[2]
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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.
- Hogan, C. M. 2008 Morro Creek, ed. by A. Burnham [1]
External links
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