The Franciscan Complex or Franciscan Assemblage is a

geologic Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth s ...

term for a late

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or " sutured" to crust lying on another plate. The crustal block or fragment preserves its d ...

of heterogeneous

rock Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wale ...

s found throughout the

California Coast Ranges The Coast Ranges of California span from Del Norte County, California, Del Norte or Humboldt County, California, south to Santa Barbara County, California, Santa Barbara County. The other three coastal California mountain ranges are the Trans ...

, and particularly on the

San Francisco Peninsula The San Francisco Peninsula is a peninsula in the San Francisco Bay Area that separates San Francisco Bay from the Pacific Ocean. On its northern tip is the City and County of San Francisco. Its southern base is Los Altos and Mountain View, ...

. It was named by geologist

Andrew Lawson Andrew Cowper Lawson (July 25, 1861 – June 16, 1952) was a Scots-born Canadian geologist who became professor of geology at the University of California, Berkeley. He was the editor and co-author of the 1908 report on the 1906 San Francisco ...

, who also named the

San Andreas Fault The San Andreas Fault is a continental Fault (geology)#Strike-slip faults, right-lateral strike-slip transform fault that extends roughly through the U.S. state of California. It forms part of the tectonics, tectonic boundary between the Paci ...

that defines the western extent of the assemblage. The Franciscan Complex is dominated by

greywacke Greywacke or graywacke ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and Sorting (sediment), poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size Lith ...

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

s and conglomerates which have experienced low-grade

metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...

. Other important lithologies include

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

serpentinite Serpentinite is a metamorphic rock composed predominantly of serpentine group minerals formed by serpentinization of mafic or ultramafic rocks. The ancient origin of the name is uncertain; it may be from the similarity of its texture or color ...

, and high-pressure, low-temperature metabasites (

blueschist Blueschist (), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and similar rocks at relatively low temperatures () but very high overburden pressure, pressure corresponding to a depth of . The b ...

s and

eclogite Eclogite () is a metamorphic rock containing garnet ( almandine- pyrope) hosted in a matrix of sodium-rich pyroxene ( omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisit ...

s) and meta-limestones. Fossils like

radiolaria The Radiolaria, also called Radiozoa, are unicellular eukaryotes of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ect ...

are found in chert beds of the Franciscan Complex. These fossils have been used to provide age constraints on the different

s that constitute the Franciscan. The mining opportunities within the Franciscan are restricted to deposits of

cinnabar Cinnabar (; ), or cinnabarite (), also known as ''mercurblende'' is the bright scarlet to brick-red form of Mercury sulfide, mercury(II) sulfide (HgS). It is the most common source ore for refining mercury (element), elemental mercury and is t ...

and limestone. The outcrops of the formation have a very large range, extending from

Douglas County, Oregon Douglas County is one of the List of counties in Oregon, 36 counties in the U.S. state of Oregon. As of the 2020 United States census, 2020 census, the population was 111,201. The county seat is Roseburg, Oregon, Roseburg. Oregon Geographic ...

Santa Barbara County, California Santa Barbara County, officially the County of Santa Barbara (), is a County (United States), county located in Southern California. As of the 2020 United States census, the population was 448,229. The county seat is Santa Barbara, 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 Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

at the western coast of North America. Although most of the Franciscan is Early/Late Jurassic through

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

in age (150-66 Ma), some Franciscan rocks are as old as early

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

(180-190 Ma) age and as young as

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

(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

when an ancient deep-sea trench existed along the

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental marg ...

. This trench, the remnants of which are still active in the Cascadia and Cocos subduction zone, resulted from

subduction Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second p ...

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...

of the Farallon tectonic plate beneath

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...

of the North American Plate. As oceanic crust descended beneath the continent,

ocean floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

and sediments were subducted and then tectonically underplated to the upper plate. This resulted in widespread deformation with the generation of

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. I ...

s 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 North America is a continent in the Northern and Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Caribbean Sea, and to the sou ...

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 Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than and are too small to ...

and

, thin bedded ribbon

with abundant

ns, altered submarine pillow basalts ( greenstone) and

s. 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 Jadeite is a pyroxene mineral with composition Na Al Si2 O6. It is hard (Mohs hardness of about 6.5 to 7.0), very tough, and dense, with a specific gravity of about 3.4. It is found in a wide range of colors, but is most often found in shades ...

-bearing) grades of metamorphism. ''

Mélange In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically con ...

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 Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose ...

, and

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

-blueschist). The mélange zones in the Franciscan usually have a block in matrix appearance with higher grade metamorphic blocks (blueschist, amphibolite,

greenschist Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as Chlorite ...

, 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 Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...

decreasing to the west. The Franciscan varies along strike, because individual accreted elements (packets of trench sediment,

seamount A seamount is a large submarine landform that rises from the ocean floor without reaching the water surface (sea level), and thus is not an island, islet, or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly a ...

s, etc.) did not extend the full length of the trench. Different depths of underplating, distribution of post-metamorphic faulting, and level of

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

produced the present-day surface distribution of high P/T metamorphism.

Gallery

File:Ribbon chert of the Marin Headlands Terrane, exposed at Marshall's Beach, San Francisco.jpg, Ribbon Chert of the Marin Headlands Terrane, exposed at Marshall's Beach, San Francisco. File:Pillow Basalts.jpg, Pillow structures preserved in

-facies metamorphosed basalts of the Franciscan Complex, Black Sands Beach, Marin Headlands, California. Field of view is approximately 2 m wide. File:Blue folded metacherts with glaucophane-rich (ashy?) layers.jpg, Folded blue metacherts with glaucophane-rich layers, exposed in outcrops on Kayak Beach, Angel Island, northern San Francisco Bay. File:Sheared Serpentinite.jpg, 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 In mathematics (differential geometry), a foliation is an equivalence relation on an topological manifold, ''n''-manifold, the equivalence classes being connected, injective function, injectively immersed submanifolds, all of the same dimension ...

is folded, sub-vertical in the lower part of the photo and more gently dipping in the upper part of the photo. File:Shale matrix melange.jpg, Shale matrix

mélange In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically con ...

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 A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

. The most abundant fossils by far are

microfossils A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...

, particularly in the cherts, which contain single-celled organisms called

radiolarians The Radiolaria, also called Radiozoa, are unicellular eukaryotes of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ecto ...

that have

exoskeleton An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...

s of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

. There are also in some of the shales microfossils of

planktonic Plankton are the diverse collection of organisms that drift in water (or air) but are unable to actively propel themselves against currents (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they pro ...

foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

that have exoskeletons of

carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...

. These microfossils, by and large, indicate deposition in an open-water setting where deep-water conditions exist.Bailey, Irwin and Jones (1965), p. 115-123; Blome and Irwin (1983), p. 77-89. 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 Oyster is the common name for a number of different families of Seawater, salt-water bivalve molluscs that live in Marine (ocean), marine or Brackish water, brackish habitats. In some species, the valves are highly Calcification, calcified, a ...

(''Inoceramus'') and

clams Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...

(''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.Tarduno et al. (1985), p. 345-347. 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 Gold mining is the extraction of gold by mining. Historically, mining gold from Alluvium, alluvial deposits used manual separation processes, such as gold panning. The expansion of gold mining to ores that are not on the surface has led to mor ...

was one of the main industries in California,

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 New Almaden, known in Spanish language, Spanish as Nueva Almadén, is a historic community and former mercury (element), mercury mine in the Capitancillos Hills of San Jose, California, located at the southwestern point of Almaden Valley, San Jo ...

, 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 A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...

, and the Permanente Quarry near

Cupertino, California Cupertino ( ) is a city in Santa Clara County, California, United States, directly west of San Jose, California, San Jose on the western edge of the Santa Clara Valley with portions extending into the foothills of the Santa Cruz Mountains. The ...

is a giant

open-pit mine Open-pit mining, also known as open-cast or open-cut mining and in larger contexts mega-mining, is a surface mining technique that extracts rock or minerals from the earth. Open-pit mines are used when deposits of commercially useful ore or ...

in a body of Franciscan limestone that supplied most of the cement for building the Shasta Dam across the

Sacramento River The Sacramento River () is the principal river of Northern California in the United States and is the largest river in California. Rising in the Klamath Mountains, the river flows south for before reaching the Sacramento–San Joaquin River D ...

. The Rockaway Quarry in Pacifica is another example of a major limestone quarry in the Franciscan.

Notes

References

* *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. * * * * Hamilton, W. (1969). "Mesozoic California and the underflow of Pacific mantle". ''Geological Society of America Bulletin'', ''80''(12), pp. 2409-2430. * * * * ** * * ** ** ** ** *

External links

San Andreas Discussion - Tectonic Wedging

Geology of the Golden Gate Headlands - National Park ServiceGeology of San Francisco and Vicinity

The Geology of the Franciscan Complex in the Ward-Creek-Cazadero Area, Sonoma County, California
by Rolfe C. Erickson {{Authority control Terranes Cretaceous California Jurassic California Upper Jurassic Series Geologic formations of California Geology of Monterey County, California Geology of San Francisco Geology of San Luis Obispo County, California Geology of San Mateo County, California Geology of Santa Barbara County, California Geology of Ventura County, California Metamorphic complexes Natural history of the California Coast Ranges Petrology Structural geology Geology of Marin County, California