Franciscan Assemblage or Franciscan Complex is a geologic term for a late Mesozoic terrane of heterogeneous rocks found throughout the California Coast Ranges, and particularly on the San Francisco Peninsula. It was named by geologist Andrew Lawson, who also named the San Andreas Fault that defines the eastern extent of the assemblage.
Easily identified by its red-green color and furrowed, twisted appearance, the assemblage is usually characterized as being primarily metamorphic in nature, its most well-known rocks being serpentine greenschists. However, the assemblage contains a wide range of different rocks in different stages of the rock cycle. A single outcrop may contain basalt, chert, blueschist and other rocks in addition to greenschist.
These rocks - which are also known as the Franciscan Complex, Formation, Series, or Group - include mafic volcanic rocks (basalt), many of which are altered to greenstone, radiolarian cherts, greywacke sandstones, limestones, serpentinites, shales, and high-pressure metamorphic rocks, such as blueschist. Although most of the Franciscan is Late Jurassic through Cretaceous in age (150-66 Ma), some Franciscan rocks as young as Miocene (15 Ma), and as old as early Jurassic (180-190 Ma) age are known. Following deposition, these rocks were then faulted, folded and mixed in a seemingly chaotic manner. Due to the lack of continuous exposures and the complex folding and faulting, it is impossible to use conventional methods to estimate the thickness of the assemblage. However, various arguments can be made that at least 50,000 feet (15,240 m) of sediment are present.
Franciscan rocks are thought to have formed prior to creation of the San Andreas Fault when an ancient deep-sea trench existed along the California continental margin. This trench, most of which is no longer evident, resulted from subduction of oceanic crust of the Farallon tectonic plate beneath continental crust of the North American Plate. As oceanic crust descended beneath the continent, volcanic rocks, mainly basalt, making up the lower plate, and marine sediments deposited on top of it were scrapped off and accreted (i.e., added) to the leading edge of the overriding plate. This resulted in widespread deformation with development of thrust faults and folding. Ophiolite (which weathers to serpentine), and rocks altered by high-pressure metamorphism (such as blueschist) were emplaced during this episode. Deformation and emplacement continued during subsequent creation of the San Andreas fault to result in a complex chaotic assemblage of diverse rock types that some refer to as a mélange.
Franciscan sediments contain a sparse, but diverse assemblage of fossils. The most abundant fossils by far are microfossils, particularly in the cherts, which contain single-celled organisms called radiolarians that have exoskeletons of silica. There are also in some of the shales microfossils of planktonic foraminifera that have exoskeletons of carbonate. These microfossils, by and large, indicate deposition in an open-water setting where deep-water conditions exist. Vertebrate fossils in the Franciscan are extremely rare, but include three Mesozoic marine reptiles that are shown in the table below. Again, these indicate an open-water, and therefore deep-marine setting. Although rare, a few shallow-marine fossils have been found as well, and include extinct oysters (Inoceramus) and clams (Buchia). Microfossils in the Calera Limestone member of the Franciscan exposed at the Permanente and Pacifica cement quarries also indicate a shallow-marine setting, with deposition on top of a seamount in the tropical Pacific Ocean and subsequent transport and accretion by the Pacific Plate onto the California continental margin. Thus, even though most of the Franciscan appears to have been deposited in a deep-water setting, it is a complex and diverse assemblage of rocks, and shallow-water settings, though not the norm, existed as well.
|Mesozoic Vertebrate Fossils of the Franciscan Assemblage|
Name means "fish-lizard of the Franciscan." Found in 1940 in San Joaquin County in a piece of Franciscan chert from the Coast Ranges washed into the Great Valley.
Although no significant accumulations of oil or gas have been found in the Franciscan, other opportunities have been exploited over the years. During the 19th century when gold mining was one of the main industries in California, cinnabar associated with serpentine in the Franciscan was mined for quicksilver (mercury) needed to process gold ore and gold-bearing gravels. Some of the more important mines were those at New Idria and New Almaden, and the Sulphur Bank Mine at Knoxville. The Franciscan also contains large bodies of limestone pure enough for making cement, and the Permanente Quarry near Cupertino, California is a giant open-pit mine in a body of Franciscan limestone that supplied most of the cement for building the Shasta Dam across the Sacramento River. The Rockaway Beach Quarry at Pacifica is another example of a major limestone quarry in the Franciscan.
- diagram is modified from figure 3.11, p. 74 in Irwin (1990).
- Bailey, Irwin and Jones (1964), p. 15-17.
- Bailey, Irwin and Jones (1964), p. 142-146; Blome and Irwin (1983), p. 77-89..
- McLaughlin (1982), p. 595-605.
- Bailey, Irwin and Jones (1964), p. 21-114.
- Bailey, Irwin and Jones (1964), p. 20.
- Wentworth et al. (1984), p. 163-173; Irwin (1990), p. 61-82.
- map is modified from figure 3.3, p. 62-63 in Irwin (1990).
- Bailey, Irwin and Jones (1965), p. 115-123; Blome and Irwin (1983), p. 77-89.
- Hilton (2003), p. 223-225.
- Tarduno et al. (1985), p. 345-347.
- Hilton (2003), "Appendix: Summary of the Mesozoic Reptilian Fossils of California," p. 272-273.
- Additional information on quicksliver deposits in the Franciscan can be found in the Wikipedia entries for New Idria and New Almaden, and the Sulphur Bank Mine.
- Austin, Donna (26 June 2009). "Kaiser dug for cement and hit aluminum foil". Cupertino News (newspaper - online edition). Retrieved 14 June 2013. Also see the following online anonymous article "Henry Kaiser’s Legacy Woven into Rich California Tapestry". Kasier Permanente. 26 November 2009. Retrieved 14 June 2013.
- Bailey, E.H., Irwin, W.P. and Jones, D.L. (1964). "Franciscan and related rocks and their significance in the geology of western California". California Div. Mines and Geology Bull. 183: 177 p.
- Blome, C.D. and Irwin, W.P. (1983). "Tectonic significance of late Paleozoic and Mesozoic radiolarians from the North Fork terrane, Klamath Mountains". In Stevens, C.H., ed. Pre-Jurassic rocks in western North America suspect terranes. Pacific Section of the Society of Palentologists and Mineralogists. pp. 77–89.
- Hilton, Richard P. (2003). Dinosaurs and Other Mesozoic Reptiles of California. Berkeley. University of California Press. 356 p.
- Irwin, William P. (1990). "Geology and plate-tectonic development". In Robert E. Wallace, ed. The San Andreas Fault System, California. U.S. Geological Survey Professional Paper 1515. p. 61–82.
- McLaughlin, R.J., Kling, S.A., Poore, R.Z., McDougall, K. and Beutner, E.C. (1982). "Post-middle Miocene accretion of Franciscan rocks, northwestern California". Geological Society of America Bulletin: v. 93, p. 595–605.
- Tarduno, John A., McWilliams, M., Debiche, M.G., Sliter, W.V., and Blake, M.C. (1985). "Franciscan Complex Calera limestones: accreted remnants of Farallon Plate oceanic plateaus". Nature 317. Retrieved 2013-06-14.
- 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.
- San Andreas Discussion - Tectonic Wedging
- Geology of the Golden Gate Headlands - National Park Service