The intention of the map and section is not geologic accuracy or authority but to give an idea of the complexity and richness of the Franciscan Formation (also known as the Franciscan Complex) and to relate the Geologic Flags to Bay Area geology in a large area proximate to 555 Beach St, San Francisco, the location of the flag pole where the flags have been installed (see map and images above) for the Spring of 2004. The patterns used on the Flags and corresponding designs indicating the rocks and structures on the map were derived from a larger geologic indexing and identification system for nearly all rock types, see the drawing: ÒStudy: Geology Flags - Basic Rock Types/Structures,Ó John Roloff, 2004. The anthroturbation elements (human built architecture and structures) represented by the top most flag (reinforced concrete) and recent (Quaternary) surficial deposits (primarily soils, sands and alluvium) are not shown on the map for reasons of clarity as most of the urban areas of map are covered by these materials.
Geologists understanding of the Bay Area geology and the Franciscan Complex has evolved considerably since the acceptance of the theory of plate tectonics since the 1960Õs and its role in the accretion and emplacement of marine sediments, volcanics and crust that comprise the outer coast ranges of California.
It is now seen that the Franciscan Complex of the California Coast Ranges was formed as a subduction complex related to east-dipping subduction along the western North American plate margin from the Late Jurassic through the Early Tertiary, this occurred over a period of at least 140 million years (Wakabayashi, 1992a). The Franciscan is part of an 800 km long assemblage of similar rocks along the west coast of North America. The main rocks of the Franciscan Complex are predominately marine sedimentary rocks such as sandstone and shale with lesser amounts of marine basaltic rocks and chert as well as serpentinite, a hydro thermally altered ultramafic (low silica, high iron and magnesium) oceanic crust. MŽlange zones that contain exotic blocks of other Franciscan rocks in a sheared shale or clay matrix often separate more coherent thrust sheets. In regions that are not significantly altered by Cenozoic deformation, Franciscan rock units, including mŽlanges, comprise a stack of nappes (terranes, e. g., Alcatraz Terrane) similar to structures that are used to describe the European Alps (Seiders, 1991; Wakabayashi, 1992, 1999). Apparent accretion ages of the nappes show them to be younger structurally downward, which is a reverse of the normal depositional order of younger rock on top of older rocks. This structure suggests emplacement as a result of progressive underplating or offscraping in the subduction zone trench during the process of subduction (Seiders, 1991; Wakabayashi, 1992). Some of the sedimentary (graywacke sandstone, radiolarian cherts) and volcanics (basalts) have been metamorphosed by the heat and pressure of subduction process, some at depths of 25 to 30 km, or more before being revealed at the surface by uplift and erosion (Wakabayashi, 1999); meta-graywacke, meta-cherts and meta-basites (greenstones) are the corresponding metamorphic names. The active subduction process, which still continues north of the Mendocino Triple Junction off the Mendocino coast to the north, ended in the Bay Area about 25 million years ago with the formation of the San Andreas fault (Konigsmark, 1998).
General and Cited Professional
References:
Alt, D. and Hyndman, D., 2000, Roadside Geology or Northern and Central California, Mountain Press, Missoula, MT.
Bailey, E., W. Irwin, D. Jones, 1964, Franciscan and Related Rocks, Bulletin 183, CA Division of Mines and Geology.
Elder, W., Geology of the Golden Gate Headlands, National Park Service, Golden Gate National Recreation Area, PDF file: http://www.nps.gov/prsf/ geology/geoguide.pdf
Konigsmark, T., 1998, Geologic Trips, San Francisco and the Bay Area, GeoPress, Guala, CA.
McPherson, G., Phipps, S. and Grossman, J., 1990, Diverse Sources for Igneous Blocks in Franciscan MŽlanges, California Coast Ranges: Journal of Geology, v. 98, p. 845-862.
Maruyama, S., Liou, J. and Seno, 1989, Possible Depth Limit of Underplating by a Seamount: Tectonophysics, v. 160, p. 327-339.
Pestrong, Raymond, Professor, Geology, San Francisco State University, e-mail exchange.
Seiders, V., 1991, Conglomerate Stratigraphy and Tectonics in the Franciscan Assemblage of Northern California and Implications for Cordilleran Tectonics: U. S. Geological Survey Open File Report, OFR 91-50, 21 p.
Schlocker, J, 1974, Geology of the San Francisco North Quadrangle, California, Geologic Survey Professional Paper, 782, Washington D.C.
Wahrhaftig, C., 1984, A Streetcar to Subduction, American Geophysical Union, Washington, D.C.
Wakabayashi, J., 1992a, Nappes, Tectonics of Oblique Plate Convergence and Metamorphic Evolution related to 140 Million Years of Continuous Subduction, Franciscan Complex: Californian Journal of Geology, v. 100, p. 19-40.
Wakabayashi, J., 1992b, Metamorphism and Tectonic Origin of Franciscan Metabasites and a Field Trip Guide to Three Localities in the San Francisco Bay Area, in Schiffman, P., D. Wagner, ed., Field Guide to the Geology and Metamorphism of the Franciscan Complex and Western Metamorphic Belt of Northern California, California Division of Mines and Geology, Special Publication 114.
Wakabayashi, J., 1999, The Franciscan Complex, San Francisco Bay Area: A Record of Subduction Complex Processes in Wagner, D., Graham, S., ed., Geologic Field Trips in Northern California, California Division of Mines and Geology, Special Publication 119.
Maps:
The geologic map and section on this sheet were compiled and interpreted from several different maps and related research. The maps are:
California Division of Mines and Geology, Geologic Map of the San Francisco-San Jose Quadrangle, California, 1:250,000, Map 5A, Regional Geologic Map series.
Wahrhaftig, C., A Streetcar to Subduction, American Geophysical Union, Washington, D.C.,1984, several different maps and diagrams.
USGS Topographic Map, San Francisco Quadrangle, 1:100,000 series, 1978.
USGS Geologic Map of the San Francisco North Quadrangle, San Francisco and Marin Counties, California, Plate I, Professional Paper 782, 1974.