File:Radiolarian ribbon cherts (Franciscan Complex, Lower Jurassic-Lower Cretaceous; southern Marin Peninsula, San Francisco Bay, California, USA) 1 (15710831057).jpg

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Radiolarian ribbon cherts in the Mesozoic of California, USA.

These spectacular rocks are tectonically tilted layers of chert. The dark brick-red color is from hematite (Fe2O3 - iron oxide). Weathering of these chert beds results in a ribbon-like appearance, hence the term ribbon chert. Chert is a siliceous sedimentary rock, composed of cryptocrystalline quartz (SiO2). These particular cherts have abundant microfossils of radiolarians, a group of marine, unicellular, non-photosynthesizing protists that make tiny skeletons of opal (SiO2·nH2O). Burial and diagenesis of radiolarian-rich sediments results in the formation of radiolarian cherts. These rocks are part of the Franciscan Complex (Jurassic) and were deposited on an ancient deep seafloor. The rocks got scraped from the top of a subducting slab of oceanic crust (the Farallon Plate) along coastal California.

Locality: roadcut in the southern Marin Peninsula, Golden Gate National Recreation Area, north of San Francisco, western coast of California, USA
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Source Radiolarian ribbon cherts (Franciscan Complex, Lower Jurassic-Lower Cretaceous; southern Marin Peninsula, San Francisco Bay, California, USA) 1
Author James St. John

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This image was originally posted to Flickr by James St. John at https://flickr.com/photos/47445767@N05/15710831057 (archive). It was reviewed on 6 December 2019 by FlickreviewR 2 and was confirmed to be licensed under the terms of the cc-by-2.0.

6 December 2019

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Date/TimeThumbnailDimensionsUserComment
current03:57, 6 December 2019Thumbnail for version as of 03:57, 6 December 2019960 × 615 (408 KB)Ser Amantio di Nicolao (talk | contribs)Transferred from Flickr via #flickr2commons

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