Franciscan Assemblage

Franciscan Assemblage Stratigraphic range: late Jurassic to Cretaceous
Roadcut showing Franciscan chert in Glen Canyon Park, San Francisco
Type	varied; primarily metamorphic (low grade), but also sedimentary, igneous and high-grade metamorphic
Underlies	various
Overlies	basement; Coast Range Ophiolite in some areas
Lithology
Primary	schist (incl. serpentinite), sandstone, basalt
Other	shale
Location
Region	California Coast Ranges, northern Transverse Ranges
Country	United States
Type section
Named for	San Francisco, California

Franciscan Assemblage or Franciscan Complex 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 eastern extent of the assemblage.

Easily identified by its red-green color (sometimes dark blue) and furrowed, twisted appearance, the assemblage is usually characterized as being primarily metamorphic in nature, its most well-known rocks being serpentine schists. However, the assemblage contains a wide range of different rocks in different stages of the rock cycle. A single outcrop may contain basalt, chert, and other rocks in addition to schist.

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 the term describing high-pressure regional metamorphic facies, the Franciscan facies series.

These rocks - which are also known as the Franciscan Complex, Formation, Series, or Group - include mafic volcanic rocks (basalt), many of which are altered to greenstone, radiolarian cherts, greywacke sandstones, limestones, serpentinites, shales, and high-pressure metamorphic rocks, such as blueschist. Although most of the Franciscan is Late Jurassic through Cretaceous in age (150-66 Ma), some Franciscan rocks as young as Miocene (15 Ma), and as old as early Jurassic (180-190 Ma) age are known. Following deposition, these rocks were then faulted, folded and mixed in a seemingly chaotic manner. Due to the lack of continuous exposures and the complex folding and faulting, it is impossible to use conventional methods to estimate the thickness of the assemblage. However, various arguments can be made that at least 50,000 feet (15,240 m) of sediment are present.