11.8: Representative Coast Range Rocks

Serpentinite

California's state rock, serpentinite, is composed of minerals belonging to the serpentine group and is commonly found in the Coast Ranges. Forming from the metamorphism of mafic and ultramafic igneous rocks, serpentinite often appears green and scaly like a serpent for which it is named.

In some parts of the Coast Ranges, serpentinite outcomes come in a block-in-matrix form. In these locations, coherent blocks of metamorphosed mafic rocks are surrounded by serpentinite that appears to have accommodated slip around the blocks. This is visible in places like Ring Mountain in Marin County. Ring Mountain is also home to the only Mariposa Lily Population (see Minerals and Rocks).

Figure \(\PageIndex{1}\): Turtle rock on Ring Mountain in Marin County is a block of blueschist suspended in a matrix of serpentinite (covered in grass). “Turtle Rock blueschist block on Ring Mountain” by Christierowe via Wikimedia Commons is licensed under CC BY-SA 4.0.

Eclogite

Eclogite is a unique and rare metamorphic rock that represents the highest set of temperatures and pressures (Figure \(\PageIndex{2}\)). For eclogite to form, the atomic ingredients in the protolith basalt reorganize to form distinctive red- and green-colored minerals: the rich green of omphacite (a kind of pyroxene, see Minerals and Rocks), and the cranberry-red of garnet (the variety called pyrope). Eclogites are astonishingly dense, which is perhaps not surprising considering they form to be stable at pressures corresponding to depths of at least 50 km. Eclogite can be found in the Francsican complex in places like Ring Mountain in Marin County and along the coast near the town of Jenner.

Figure \(\PageIndex{2}\): Eclogite consists of the green mineral omphacite (a variety of pyroxene) and prominent red pyrope (a variety of garnet). This outcrop appears on Ring Mountain, near Tiburon, California. This work by Callan Bentley is licensed under CC BY-NC 4.0.

Blueschist

Blueschist is the rock that results when basaltic protoliths are driven deeper into the subduction zone, to ~35 to 50 km depth (Figure \(\PageIndex{3}\)). The rate of subduction is greater than the rate of thermal equilibration; as a result, the subducted basalt soon finds itself at conditions where the pressure is quite high, but the temperature is still relatively cold. These are the conditions at which the minerals lawsonite and glaucophane form. Metabasalts in the Angel Island Terrane, one of the terranes of Franciscan, have seen these conditions, and where basalt was present, it has been recrystallized to make blueschist. Outcrops of these unique blueschists can be found on Angel Island in San Francisco Bay.

Figure \(\PageIndex{3}\): Blueschist with garnets from Ward Creek, western Sonoma County, California, USA likely derived from Mesozoic-aged Franciscan Complex. Approximately 8 cm across. "Garnetiferous blueschist (Ward Creek, Sonoma County, California, USA) 2" by James St. John via Flickr is licensed under CC BY.

Radiolarian Chert

Chert is a dense, fine-grained chemical sedimentary rock that is composed of silica. While chert can be formed by a few different processes, radiolarian chert specifically is formed from an accumulation of tiny (0.5-1 mm) silica shells or “tests” of marine plankton called Radiolaria on the ocean floor (Figure \(\PageIndex{4}\)). Franciscan chert contains some tropical and subtropical forms of Radiolaria suggesting that the sediments formed much further south than their present day location. The Franciscan chert is often called ribbon chert, named for its hard silica-rich chert beds separated by soft, clay-rich shale beds. Franciscan chert is best viewed at the Marin Headlands in San Francisco, but it can also be found in other parts of the Coast Ranges such as on Mount Diablo in the East Bay Area (Figure \(\PageIndex{5}\)) and along the coastline in Redwood National Park.

Figure \(\PageIndex{4}\): A magnified piece of chert shows radiolaria as small dark spheres about the size of a pencil point (left), and a scanning electron microscope image of extracted radiolaria (right). This work by the National Park Service Public Domain.

Figure \(\PageIndex{5}\): Steeply dipping beds of radiolarian red chert of the Franciscan Complex on Mount Diablo. This is one of many locations in the Coast Ranges where Franciscan Complex cherts outcrop. "Franciscan Complex Chert on Mount Diablo (with scale bar)" by Allison Jones is licensed under CC BY-NC 4.0

Greywacke Sandstone

Approximately 90% of the Franciscan Complex is composed of greywacke, an immature, poorly sorted sandstone containing grains of angular rock fragments. It is often called a "dirty sandstone" and typically forms as a result of turbid, mud- and sand-laden underwater landslides, called turbidity flows, which occur along steep continental slopes associated with subduction zones (Figure \(\PageIndex{6}\)). These flows often result in marine sediments that grade from coarse in size at the bottom to fine at the top. Once lithified, these rocks are known as turbidites.

Figure \(\PageIndex{6}\): Turbidity currents form deep-sea sediment fans, which exhibit graded bedding. "Turbidity Current" by Elizabeth J. Hermsen and Jonathan R. Hendricks, a derivative from the original, is licensed under CC BY-NC-SA 4.0.

Greywacke sandstones of the Franciscan sometimes contain clam and ammonites from the Jurassic and Cretaceous, providing ages for the Franciscan. Outcrops of Francsican graywacke are common at Baker Beach at the southern end of the Golden Gate Bridge (Figure \(\PageIndex{7}\)), the Cliff House in San Francisco, and Salt Point State Park.

Figure \(\PageIndex{7}\): Graywacke sandstone exposed at Baker Beach in the Presidio displays small holes, a type of weathering called tafoni, which is typical for this type of rock. This work by the National Park Service is in the public domain.

Graywacke in the Coast Ranges is not exclusive to the Franciscan: Greywacke also occurs in the older Great Valley Group. Graded greywacke of the Great Valley Group can be seen at Lake Berryessa (see Great Valley Sequence).