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About the Franciscan Formation

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Taken from: Hardin, D.R., 1997. California Geology. Prentice Hall, Upper Saddle River, New Jersey. 479 p.   From pages 264-265.

"Continuing detailed studies of the Franciscan have created an even more complex picture of its formation. It appears that Franciscan rocks represent not one but several, accreted terrains. These separate Franciscan blocks represent the remnants of several oceanic plates that met their end against the edge of the North America. So far, at least nine different blocks or terrains have been identified. Studies of radiolaria contained in the chert, paleomagnetic analysis of limestone and detailed studies of the mineral content of the greywacke sandstones are some of the tools that geologists are using to complete this picture. 

The area where Franciscan rocks are seen can be broadly divided into three belts. In the Coastal Belt, a northwest trending zone between Eureka and the Russian River, Franciscan rocks are mainly greywacke and shale sequences that are not disrupted as other parts of the Franciscan. The Central Belt, including the San Francisco Bay Area, is found east of the Coastal Belt and this part of the Franciscan Assemblage is a jumble of rocks. Isolated small blocks of exotic greenstone, blueschist, eclogite, chert or greywacke 'float' in a matrix off highly sheared mudstone. Geologists term this mixture a mélange, adopting a cooking term used to describe a soup that contains floating chunks of goodies. In the Franciscan mélange, the 'soup base' or matrix of sheared mudstone, and the floating chunks are blocks of more resistant rock types."


Hardin, D.R., 1997. California Geology. Prentice Hall, Upper Saddle River New, Jersey 07458.  479 p.

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