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.
26-537: 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
52-486: A building material and a sculptural material across many eras and societies. Its oldest known uses date to the early third millennium BCE , in Egypt's early dynastic period . Its wide use in sculpture and vessels is thought to have been due to its fine grain size and resistance to fracturing, making it suitable for fine detail and intricate shapes. Aside from its structural uses, greywacke stone (or molds taken from it)
78-658: 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. Although no significant accumulations of oil or gas have been found in
104-481: 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
130-659: Is a texturally immature sedimentary rock generally found in Paleozoic strata . The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume. The origin of greywacke was unknown until turbidity currents and turbidites were understood, since, according to the normal laws of sedimentation , gravel , sand and mud should not be laid down together. Geologists now attribute its formation to submarine avalanches or strong turbidity currents. These actions churn sediment and cause mixed-sediment slurries, in which
156-432: Is another example of a major limestone quarry in the Franciscan. Greywacke Greywacke or graywacke ( German grauwacke , signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale ), dark color, and poorly sorted angular grains of quartz , feldspar , and small rock fragments or sand-size lithic fragments set in a compact, clay -fine matrix. It
182-615: Is difficult to characterize mineralogically, it has a well-established place in petrographical classifications because these peculiar composite arenaceous deposits are very frequent among Silurian and Cambrian rocks, and are less common in Mesozoic or Cenozoic strata. Their essential features are their gritty character and their complex composition. By increasing metamorphism , greywackes frequently pass into mica- schists , chloritic schists and sedimentary gneisses . The term "greywacke" can be confusing, since it can refer to either
208-849: Is sometimes calcareous . Greywackes are abundant in Wales , the south of Scotland , the Longford-Down Massif in Ireland and the Lake District National Park of England ; they compose the majority of the main Southern Alps that make up the backbone of New Zealand . Both feldspathic and lithic greywacke have been recognized in Ecca Group in South Africa . Greywackes are also found in parts of
234-444: Is valuable to practitioners of traditional motion picture miniature photography , because due to its unusually mixed nature, it remains looking natural when portraying a wide range of miniature scale ratios, from 1:1 to as high as 1:600. Prehnite-pumpellyite facies The prehnite-pumpellyite facies is a metamorphic facies typical of subseafloor alteration of the oceanic crust around mid-ocean ridge spreading centres. It
260-660: The Eastern Desert east of the Nile . They were an early object of geological study in Britain where the Geological Society was founded in 1807, and excited much public interest in geology. Greywacke was interesting because it was found in many places in Britain and its occurrence in particular places was evidence of the pattern of geological strata that had been laid down. Greywacke stone has been used as
286-531: 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
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#1732772779022312-879: 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
338-582: 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
364-570: 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
390-522: 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
416-408: 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 ,
442-600: 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. The units of the Franciscan complex are aligned parallel to
468-731: 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,
494-404: 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
520-399: The finer beds associated with them. Their component particles are usually not very rounded or polished, and the rocks have often been considerably indurated by recrystallization , such as the introduction of interstitial silica . In some districts, the greywackes are cleaved, but they show phenomena of this kind much less perfectly than the slates. Although the group is so diverse that it
546-420: The immature (rock fragment) aspect of the rock or its fine-grained (clay) component. Greywackes are mostly grey, brown, yellow, or black, dull-colored sandy rocks that may occur in thick or thin beds along with shales and limestones . Some varieties include feldspathic greywacke , rich in feldspar , and lithic greywacke , rich in other tiny rock fragments. They can contain a very great variety of minerals ,
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#1732772779022572-495: The principal ones being quartz, orthoclase and plagioclase feldspars, calcite , iron oxides and graphitic, carbonaceous matters, together with (in the coarser kinds) fragments of such rocks as felsite , chert , slate , gneiss , various schists , and quartzite . Among other minerals found in them are biotite , chlorite , tourmaline , epidote , apatite , garnet , hornblende , augite , sphene and pyrites . The cementing material may be siliceous or argillaceous and
598-444: The resulting deposits may exhibit a variety of sedimentary features. Supporting the turbidity origin theory is the fact that deposits of greywacke are found on the edges of the continental shelves , at the bottoms of oceanic trenches , and at the bases of mountain formational areas. They also occur in association with black shales of deep-sea origin. As a rule, greywackes do not contain fossils , but organic remains may be common in
624-444: 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. Franciscan sediments contain
650-598: 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
676-564: The term describing high-pressure regional metamorphic facies , the Franciscan facies series . 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
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