The Franciscan Complex is dominated by
greywackesandstones,
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 Franciscan Complex is an assemblage of metamorphosed and deformed rocks, associated with east-dipping
subduction zone at the western coast of North America.[6] Although most of the Franciscan is
Early/Late Jurassic through
Cretaceous in age (150-66 Ma),[7] some Franciscan rocks are as old as early
Jurassic (180-190 Ma) age and as young as
Miocene (15 Ma).[8] The different age distribution represents the temporal and spatial variation of mechanisms that operated within the subduction zone.[9] 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
Californiacontinental 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 floorbasalt and sediments were subducted and then tectonically underplated to the upper plate.[10] This resulted in widespread deformation with the generation of
thrust faults and
folding, and caused high pressure-low temperature regional metamorphism.[10] 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.[11]
Description
The units of the Franciscan complex are aligned parallel to the active margin between the
North American and
Pacific plates.[12] The Franciscan Complex is in contact with the
Great Valley Sequence, which was deposited on the
Coast Range Ophiolite, along its eastern side.[13][14] 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.[15] 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).[10] 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.[16] The matrix material of the mélanges are mudstone or serpentinite. Geologists have argued for either a tectonic or
olistostormal origin.[17] 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.[18][19][10] 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.[9][10]
Gallery
Ribbon Chert of the
Marin Headlands Terrane, exposed at Marshall's Beach, San Francisco.
Pillow structures preserved in
greenschist-facies metamorphosed basalts of the FranciscanComplex, Black Sands Beach, Marin Headlands, California. Field of view is approximately 2 m wide.
Folded blue metacherts with
glaucophane-rich layers, exposed in outcrops on Kayak Beach,
Angel Island, northern San Francisco Bay.
Sheared block-in-matrix fabric composed of serpentinite blocks in serpentinite matrix, exposed on Perles Beach,
Angel Island, Marin County, California. Pencil for scale. The strong anastomosing
foliation is folded, sub-vertical in the lower part of the photo and more gently dipping in the upper part of the photo.
Shale matrix
mélange of the Franciscan Complex at Marshall's Beach, San Francisco, California. Sandstone blocks (light grey) contain white mineral veins. Dark grey shale matrix displays strongly foliated anastomosing scaly fabric. Geologist John Wakabayashi for scale.
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
planktonicforaminifera that have exoskeletons of
carbonate. These microfossils, by and large, indicate deposition in an open-water setting where deep-water conditions exist.[20] Vertebrate fossils in the Franciscan are extremely rare, but include three Mesozoic marine reptiles that are shown in the table below.[21] 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).[20] 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.[22] 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 Complex
Name means "
fish-lizard of
California." Found in 1935 in Stanislaus County in a piece of Franciscan chert from the Coast Ranges washed into the Great Valley.
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 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.[24] The
Rockaway Quarry in
Pacifica is another example of a major limestone quarry in the Franciscan.
^Bailey, Irwin and Jones (1964), Franciscan and related rocks and their significance in the geology of western California. California Division of Mines and Geology, v. 183 p. 15-17.
^Irwin, William P. (1990). Wallace, Robert E. (ed.). "Geology and plate-tectonic Development". The San Andreas Fault System, California-U.S. Geological Survey Professional Paper. 1515: 61–82.
^Irwin, William P. (1990). Wallace, Robert E. (ed.). "Geology and plate-tectonic development". The San Andreas Fault System, California. U.S. Geological Survey Professional Paper. 1515: 74.
^
abcdeWakabayashi, John (1992-01-01). "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.
ISSN0022-1376.
S2CID140552742.
^Wentworth et al. (1984), p. 163-173; Irwin (1990), p. 61-82.
^Wassmann, Sara; Stöckhert, Bernhard (2012-09-28). "Matrix deformation mechanisms in HP-LT tectonic mélanges — Microstructural record of jadeite blueschist from the Franciscan Complex, California". Tectonophysics. Chaos and Geodynamics: Melanges, Melange Forming Processes and Their Significance in the Geological Record. 568–569: 135–153.
Bibcode:
2012Tectp.568..135W.
doi:
10.1016/j.tecto.2012.01.009.
ISSN0040-1951.
^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.
ISSN2156-2202.
^Turner, Francis J. (1981). Metamorphic petrology: mineralogical, field, and tectonic aspects (2d ed.). Washington: Hemisphere Pub. Corp.
ISBN0-07-065501-4.
OCLC5894059.
^Wahrhaftig, Clyde (1984). A Streetcar to Subduction and Other Plate Tectonic Trips by Public Transport in San Francisco. Washington, D. C.: American Geophysical Union.
doi:
10.1029/sp022.
ISBN0-87590-234-0.
^Wakabayashi, John (August 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, Geological Society of America, pp. 117–141,
doi:
10.1130/2011.2480(05),
ISBN978-0-8137-2480-5
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.
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.
Turner, F. J. (1981). Metamorphic petrology: Mineralogical, field, and tectonic aspects. Hemisphere Publishing Corporation.
OCLC5894059.
Wahrhaftig, Clyde (1984). A Streetcar to Subduction and Other Plate Tectonic Trips by Public Transport in San Francisco.
doi:
10.1029/SP022.
ISBN0-87590-234-0.
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.
S2CID140552742.
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).
ISBN978-0-8137-2480-5.
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.