• Source: Franciscan Complex

The Franciscan Complex or Franciscan Assemblage 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 western extent of the assemblage.
The Franciscan Complex is dominated by greywacke sandstones, shales and conglomerates which have experienced low-grade metamorphism. Other important lithologies include chert, basalt, limestone, serpentinite, and high-pressure, low-temperature metabasites (blueschists and eclogites) and meta-limestones. Fossils like radiolaria are found in chert beds of the Franciscan Complex. These fossils have been used to provide age constraints on the different terranes that constitute the Franciscan. The mining opportunities within the Franciscan are restricted to deposits of cinnabar and limestone.
The outcrops of the formation have a very large range, extending from Douglas County, Oregon to Santa Barbara County, California. Franciscan-like formations may be as far south as Santa Catalina Island. The formation lends its name to the term describing high-pressure regional metamorphic facies, the Franciscan facies series.




Geologic history



The Franciscan Complex is an assemblage of metamorphosed and deformed rocks, associated with east-dipping subduction zone at the western coast of North America. Although most of the Franciscan is Early/Late Jurassic through Cretaceous in age (150-66 Ma), some Franciscan rocks are as old as early Jurassic (180-190 Ma) age and as young as Miocene (15 Ma). The different age distribution represents the temporal and spatial variation of mechanisms that operated within the subduction zone. Franciscan rocks are thought to have formed prior to the creation of the San Andreas Fault when an ancient deep-sea trench existed along the California continental margin. This trench, the remnants of which are still active in the Cascadia and Cocos subduction zone, 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, ocean floor basalt and sediments were subducted and then tectonically underplated to the upper plate. This resulted in widespread deformation with the generation of thrust faults and folding, and caused high pressure-low temperature regional metamorphism. In the Miocene, the Farallon-Pacific spreading center reached the Franciscan trench and the relative motion between Pacific-North America caused the initiation of the San Andreas Fault. Transform motion along the San Andreas Fault obscured and displaced the subduction related structures, resulting in overprinting of two generations of structures.




Description



The units of the Franciscan complex are aligned parallel to the active margin between the North American and Pacific plates. The Franciscan Complex is in contact with the Great Valley Sequence, which was deposited on the Coast Range Ophiolite, along its eastern side. The type area of Franciscan rocks in San Francisco consists of metagraywackes, gray claystone and shale, thin bedded ribbon chert with abundant radiolarians, altered submarine pillow basalts (greenstone) and blueschists. Broadly, the Franciscan can be divided into two groups of rocks. Coherent terranes are internally consistent in metamorphic grade and include folded and faulted clastic sediments, cherts and basalts, ranging from sub-metamorphic to prehnite-pumpellyite or low-temperature blueschist (jadeite-bearing) grades of metamorphism. Mélange terranes are much smaller, found between or within the larger coherent terranes and sometimes contain large blocks of metabasic rocks of higher metamorphic grade (amphibolite, eclogite, and garnet-blueschist). The mélange zones in the Franciscan usually have a block in matrix appearance with higher grade metamorphic blocks (blueschist, amphibolite, greenschist, eclogite) embedded within the mélange matrix. The matrix material of the mélanges are mudstone or serpentinite. Geologists have argued for either a tectonic or olistostormal origin. In the northern Coast Ranges, the Franciscan has been divided into the Eastern, Central and Coastal Belts based on metamorphic age and grade, with the rocks younging and the metamorphic grade decreasing to the west. The Franciscan varies along strike, because individual accreted elements (packets of trench sediment, seamounts, etc.) did not extend the full length of the trench. Different depths of underplating, distribution of post-metamorphic faulting, and level of erosion produced the present-day surface distribution of high P/T metamorphism.




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Fossils


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.




Economic importance


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 and Great Valley Group 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, the Sulphur Bank Mine at Clearlake Oaks, and the Knoxville Mine (cf. McLaughlin Mine) and others 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 Quarry in Pacifica is another example of a major limestone quarry in the Franciscan.




See also


California landslides
Farallon Plate – Ancient oceanic plate that has mostly subducted under the North American platePages displaying short descriptions of redirect targets
Great Valley Sequence – Group of late Mesozoic formations in the Central Valley of California
Subduction – A geological process at convergent tectonic plate boundaries where one plate moves under the other
Accretionary prism




Notes






References


Bailey, E.H.; Irwin, W.P.; 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.
Berkland, J. O., Raymond, L. A., Kramer, J. C., Moores, E. M., & O'Day, M. (1972). "What is Franciscan?". AAPG Bulletin, 56(12), pp. 2295a-2302.
Blake, M.C.; Howell, D.G.; Jones, David Lawrence (1982). "Preliminary tectonostratigraphic terrane map of California". Open-File Report. doi:10.3133/ofr82593.
Blome, C.D.; Irwin, W.P. (1983). "Tectonic significance of late Paleozoic to Jurassic radiolarians from the North Fork terrane, Klamath Mountains, California". In Stevens, C.H. (ed.). Pre-Jurassic rocks in western North America suspect terranes. Pacific Section of the Society of Paleontologists and Mineralogists. pp. 77–89.
Ernst, W. G. (1970). "Tectonic contact between the Franciscan Mélange and the Great Valley Sequence-Crustal expression of a Late Mesozoic Benioff Zone". Journal of Geophysical Research. 75 (5): 886–901. Bibcode:1970JGR....75..886E. doi:10.1029/JB075i005p00886.
Hamilton, W. (1969). "Mesozoic California and the underflow of Pacific mantle". Geological Society of America Bulletin, 80(12), pp. 2409-2430.
Hilton, Richard P. (2003). Dinosaurs and Other Mesozoic Reptiles of California. Berkeley. University of California Press. 356 p. ISBN 978-0-520-23315-7.
Hsü, K. Jinghwa (1968). "Principles of Mélanges and Their Bearing on the Franciscan-Knoxville Paradox". Geological Society of America Bulletin. 79 (8): 1063. doi:10.1130/0016-7606(1968)79[1063:POMATB]2.0.CO;2. ISSN 0016-7606.
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. pp. 61–82.
McLaughlin, R.J.; Kling, S.A.; Poore, R.Z.; McDougall, K.; Beutner, E.C. (1982). "Post-middle Miocene accretion of Franciscan rocks, northwestern California". Geological Society of America Bulletin. 93 (7): 595–605. Bibcode:1982GSAB...93..595M. doi:10.1130/0016-7606(1982)93<595:pmaofr>2.0.co;2.
Mulcahy, Sean R.; Starnes, Jesslyn K.; Day, Howard W.; Coble, Matthew A.; Vervoort, Jeffrey D. (2018). "Early Onset of Franciscan Subduction". Tectonics. 37 (5): 1194–1209. Bibcode:2018Tecto..37.1194M. doi:10.1029/2017TC004753.
Tarduno, John A.; McWilliams, M.; Debiche, M.G.; Sliter, W.V.; Blake, M.C. (1985). "Franciscan Complex Calera limestones: accreted remnants of Farallon Plate oceanic plateaus". Nature. 317 (6035): 345–347. Bibcode:1985Natur.317..345T. doi:10.1038/317345a0. S2CID 4350067.
Turner, F. J. (1981). Metamorphic petrology: Mineralogical, field, and tectonic aspects. Hemisphere Publishing Corporation. OCLC 5894059.
Wahrhaftig, Clyde (1984). A Streetcar to Subduction and Other Plate Tectonic Trips by Public Transport in San Francisco. doi:10.1029/SP022. ISBN 0-87590-234-0.
Wakabayashi, John (1992). "Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction, Franciscan Complex, California". The Journal of Geology. 100 (1): 19–40. Bibcode:1992JG....100...19W. doi:10.1086/629569. S2CID 140552742.
Wakabayashi, John (2011). "Mélanges of the Franciscan Complex, California: Diverse structural settings, evidence for sedimentary mixing, and their connection to subduction processes". Mélanges: Processes of Formation and Societal Significance. Geological Society of America Special Papers. Vol. 480. pp. 117–141. doi:10.1130/2011.2480(05). ISBN 978-0-8137-2480-5.
Wassmann, Sara; Stöckhert, Bernhard (2012). "Matrix deformation mechanisms in HP-LT tectonic mélanges — Microstructural record of jadeite blueschist from the Franciscan Complex, California". Tectonophysics. 568–569: 135–153. Bibcode:2012Tectp.568..135W. doi:10.1016/j.tecto.2012.01.009.
Wentworth, C. M.; Blake, M. C. Jr.; Jones, D. L.; Walter, A. W.; 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.




External links


San Andreas Discussion - Tectonic Wedging
Geology of the Golden Gate Headlands - National Park Service
Geology of San Francisco and Vicinity
The Geology of the Franciscan Complex in the Ward-Creek-Cazadero Area, Sonoma County, California by Rolfe C. Erickson

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