Franciscan Assemblage
Encyclopedia
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
term for an accreted terrane
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
of heterogeneous rock
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
s found on and near the San Francisco Peninsula
San Francisco Peninsula
The San Francisco Peninsula is a peninsula in the San Francisco Bay Area that separates the San Francisco Bay from the Pacific Ocean. On its northern tip is the City and County of San Francisco. Its southern base is in Santa Clara County, including the cities of Palo Alto, Los Altos, and Mountain...
. It was named by geologist Andrew Lawson
Andrew Lawson
Andrew Cowper Lawson July 25,1861- June 16,1952 was a 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 Earthquake which became known as the "Lawson Report"...
who also named the San Andreas Fault
San Andreas Fault
The San Andreas Fault is a continental strike-slip fault that runs a length of roughly through California in the United States. The fault's motion is right-lateral strike-slip...
which bounds the Franciscan Assemblage.
Also known as the "Franciscan Formation," "Franciscan Series," "Franciscan Group," "Franciscan assemblage," or "Franciscan Complex," it includes altered mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
volcanic
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...
rocks (greenstones), deep-sea radiolarian cherts, greywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s, limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
s, serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
s, shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s, and high-pressure metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s, all of them faulted
Geologic fault
In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the Earth's crust result from the action of tectonic forces...
and mixed in a seemingly chaotic manner.
It forms the major component of the Pacific Coast Ranges of California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
.
Wentworth and others interpreted the juxtaposition of the Franciscan Assemblage and the section consisting of the Coast Range ophiolite and the Great Valley sequence to have happened through landward movement of the Franciscan Assemblage as a tectonic wedge.
Paleofauna
| Vertebrates of the Franciscan Assemblage | ||||||
|---|---|---|---|---|---|---|
| Genus | Species | Location | Member | Abundance | Notes | Images |
Ichthyosaurus Ichthyosaurus Ichthyosaurus is an extinct genus of ichthyosaur from the Early Jurassic of Europe . It is among the best known ichthyosaur genera, with the Order Ichthyosauria being named after it... |
I. californicus |
Means "fish-lizard of California California California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area... ." |
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I. franciscanus |
Means "fish-lizard of the Franciscan Assemblage." |
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Plesiosaurus Plesiosaurus Plesiosaurus was a genus of large marine sauropterygian reptile that lived during the early part of the Jurassic Period, and is known by nearly complete skeletons from the Lias of England... |
P. hesternus |
Means "one who is near to being a lizard Plesiosaur Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods... of the West coast West Coast of the United States West Coast or Pacific Coast are terms for the westernmost coastal states of the United States. The term most often refers to the states of California, Oregon, and Washington. Although not part of the contiguous United States, Alaska and Hawaii do border the Pacific Ocean but can't be included in... ." |
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