page_title: Franciscan Complex
section_title: Description
hierarchical_section_title: Franciscan Complex / Description
caption_attribution_description: English: Typical Franciscan shale-matrix mélange with clasts of sandstone and greenstone on Marshall's Beach, San Francisco, USA. Lens cap for scale = 55 mm.
context_page_description: 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.
  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 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 the different terranes that constitute the Franciscan. The mining opportunities within the Franciscan is 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.
context_section_description: The units of the Franciscan complex are aligned parallel to 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, the Franciscan can be divided into two groups of rocks. Coherent terranes are internally consistent in metamorphic grade and includes 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 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 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.
caption: Shale matrix mélange with clasts of sandstone and greenstone on Marshall's Beach, San Francisco
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