Franciscan Complex

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Credit: Geology, soils & wine quality in Sonoma County, California W.H.(Terry) Wright Franciscan Complex: The oldest bedrock occurs on steep slopes and high hills and mountains in northern Sonoma County. Called the Franciscan Complex from exposures near San Francisco, it is an eclectic collection of different rock types that date back as far as 150 million years but can include rocks as young as 40 million years. Mostly of oceanic origin, the Franciscan Complex includes sea-floor marine sediments along with iron-rich igneous volcanic, plutonic, and metamorphic rocks. These rock types originated far offshore and became mixed together by faulting at a fundamental plate tectonic geologic boundary called a subduction zone, where the ocean floor tectonic plate dives under the continental edge. One result of complex interactions at subduction zones is “mélange”, the French word for mixture, complex rocks with textures resembling “rocky road” ice cream. Franciscan Complex mélange is particularly well-exposed along the Sonoma County Pacific coast and on high ridges between Bodega Bay and Gualala. These igneous rocks and the associated sandstones and conglomerates are part of the ocean floor that was scraped off onto the continent during subduction; an ophiolite complex designated part of the Great Valley Sequence. Rock distribution in the Franciscan Complex is random, as are the soils that come from their breakdown. Nutrient ratios vary greatly, with a natural emphasis on high magnesium and low potassium occurrences. The recent land rush to find quality vineyard sites in Sonoma County has resulted in vineyard development of slopes on Franciscan Complex rocks and their overlying soils. The varied soils in these areas present new and sometimes formidable challenges for vineyard development and management. The best areas for development will be on soils developed from marine sediments, primarily greywacke sandstone and shale. Places to avoid are areas of Franciscan mélange with serpentine and other high magnesium-content soils, commonly with high clay content. .

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Unstable Slopes in the Franciscan Complex Terrane: Lessons Learned from Urban Quarry Slopes in the San Francisco Bay Area
UNSTABLE SLOPES IN THE FRANCISCAN COMPLEX TERRANE: LESSONS LEARNED FROM URBAN QUARRY SLOPES IN THE SAN FRANCISCO BAY AREA Ted M. Sayre1 & John M. Wallace2 1Cotton, Shires & Associates, Inc., Consulting Engineers and Geologists, 330 Village Lane, Los Gatos, CA 95030 ([email protected]) 1Cotton, Shires & Associates, Inc., Consulting Engineers and Geologists, 330 Village Lane, Los Gatos, CA 95030 ([email protected]) Abstract: As the need for urban living space intensifies, an increasing number of former rock quarry sites have become prime real estate for residential living areas. This change in land use can result in inappropriate and potentially catastrophic consequences when residential structures are placed in close proximity to unstable quarry slopes that were not adequately investigated prior to development. Examples of this have occurred on the San Francisco peninsula where recent slope failures along several quarry faces serve as a testament to the variability of the Franciscan Complex terrane and the unique set of failure mechanisms associated with a given Franciscan rock type, underscoring the importance of a thorough geologic assessment prior to development. In two presented case studies, residential developments were adversely impacted by recent rock slope failures. Incomplete characterization of site geologic conditions and inadequate identification of critical slope failure mechanisms led to unsatisfactory setbacks and/or ineffective mitigation designs. The Knockash Hill development in San Francisco is underlain by thinly-bedded “ribbon” chert of the Franciscan Complex that was previously mined for roadway aggregate. The steep quarried slope, located immediately above new residential units, presented an excellent geologic exposure of bedded chert that displayed adversely oriented discontinuities and weak shale interbeds.
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DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Review of the Great Valley sequence, eastern Diablo Range and northern San Joaquin Valley, central California by J. Alan Bartow1 and TorH.Nilsen2 Open-File Report 90-226 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1990 , Menlo Park, California 2Applied Earth Technologies, Inc, Redwood City, California ABSTRACT The Great Valley sequence of the eastern Diablo Range and northern San Joaquin Valley consists of a thick accumulation of marine and nonmarine clastic rocks of Jurassic to early Paleocene age deposited in a forearc basin that was situated between the Sierran magmatic arc to the east and the Franciscan subduction complex to the west. In the western part of the basin, the sequence rests conformably on the Jurassic Coast Range Ophiolite or is faulted against the structurally underlying Franciscan Complex. Beneath the eastern San Joaquin Valley, the sequence unconformably onlaps igneous and metamorphic rocks of the Sierran magmatic arc. The sequence generally thickens westward to as much as 8-9 km in the Diablo Range, where it is unconformably overlain by late Paleocene and younger strata. The stratigraphy of the Great Valley sequence has been the subject of much work, but problems, particularly nomenclatural, remain. Lithostratigraphic subdivisions of the sequence have not gained widespread acceptance because of the lenticularity of most sandstone bodies, abrupt fades changes in subsurface and outcrops, and the lack of detailed subsurface information from closely spaced or deep wells.
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Bigbig Sursur
CalCal PolyPoly -- PomonaPomona GeologyGeology ClubClub SpringSpring 20032003 FFieldield TTriprip BigBig SurSur David R. Jessey Randal E. Burns Leianna L. Michalka Danielle M. Wall ACKNOWLEDGEMENT The authors of this field guide would like to express their appreciation and sincere thanks to the Peninsula Geologic Society, the California Geological Survey and Caltrans. Without their excellent publications this guide would not have been possible. We apologize for any errors made through exclusion or addition of trip field stops. For more detailed descriptions please see the following: Zatkin, Robert (ed.), 2000, Salinia/Nacimiento Amalgamated Terrane Big Sur Coast, Central California, Peninsula Geological Society Spring Field Trip 2000 Guidebook, 214 p. Wills, C.J., Manson, M.W., Brown, K.D., Davenport, C.W. and Domrose, C.J., 2001, LANDSLIDES IN THE HIGHWAY 1 CORRIDOR: GEOLOGY AND SLOPE STABILITY ALONG THE BIG SUR COAST, California Department of Conservation Division of Mines & Geology, 43 p. 0 122 0 00' 122 0 45' 121 30 Qal Peninsula Geological Society Qal G a b i Qt la Field Trip to Salina/Nacimento 1 n R S a A n L Big Sur Coast, Central California I g N qd A e S R Qt IV E Salinas R S a lin a s Qs V Qal 101 a Qs Monterey Qc lle Qt Qp y pgm Tm Qm Seaside pgm EXPLANATION Qt Chualar Qp Qt UNCONSOLIDATED Tm pgm SEDIMENTS Qp Carmel Qal sur Qs Qal Alluvium qd CARMEL RIVER Tm Qal Point sur Qs Dune Sand Tm Lobos pgm 0 S 0 36 30 ie ' r 36 30' pgm ra Qt Quaternary non-marine d CARMEL e S terrace deposits VALLEY a Qal lin a Qt Pleistocene non-marine Tm pgm s Qc 1 Tm Tula qd rcit Qp Plio-Pleistocene non-marine qd os F ault Qm Pleistocene marine Terrace sur sur deposits qd Tm COVER ROCKS pgm qd Tm Monterey Formation, mostly qm pgm qm pgm marine biogenic and sur pgm clastic sediments middle to qdp sur qd late Miocene in age.
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