The Bright Angel Shale and the Tonto PlatformUp section from the Tapeats Shale you will often find t

The Bright Angel Shale and the Tonto PlatformUp section from the Tapeats Shale you will often find this unit, the Bright Angel Shale. I said “often” find it not because it isn’t there, but because this unit is a great example of how easily shale units weather.Shales are made of fine-grained silt and clay sized particles and are often poorly held together. Their ability to erode so easily creates one of the distinguishing features of the Grand Canyon; the Tonto platform, the flat area seen in the second image. The inner portion of the Grand Canyon is supported by the strong Tapeats sandstone and the river has only cut into it in small areas. The overlying Bright Angel formation however, erodes easily and has been cut back large distances from the river, creating a flat plateau held up by the top of the Tapeats Sandstone. That platform is known as the Tonto Plateau.The Bright Angel formation is a Cambrian-aged shale and is up to 150 meters thick. The Tapeats sandstone below recorded beach environments; the overlying shale was likely deposited in much deeper, quiet water, suggesting that sea levels across the western U.S. continued to rise throughout the Cambrian.The Bright Angel Formation varies in color from greens through red, with patches of gray as well. The colors are produced by iron and minerals like glauconite. Glauconite contains reduced (2+) iron. It is deposited in marine sediments usually as a result of biological processes and gives a greenish color when present. When it gets exposed to air, the iron rusts (turns to Iron 3+), switching the color to a strong red. The units colors therefore are giving hints about how the unit was formed.A few other interesting details. There are places in Grand Canyon where the Bright Angel sits directly on top of the Great Unconformity and the Tapeats is missing; that is consistent with sea levels continuing to rise and swamping mesas and hills where the Tapeats wasn’t deposited. There are also occasional sandy layers inside the Bright Angel that look like the Tapeats; possibly deposited when debris flows carried more coarse-grained sediment out to sea from the nearshore environment. Finally, as you see here, the Bright Angel also contains a variety of fossils, including Trilobites!-JBBImage credits: http://3dparks.wr.usgs.gov/coloradoplateau/photo/tonto.jpghttps://www.flickr.com/photos/grand_canyon_nps/8227961772https://www.flickr.com/photos/grand_canyon_nps/4749628242Sources:http://3dparks.wr.usgs.gov/coloradoplateau/lexicon/brightangel.htmhttps://www2.bc.edu/~strother/gc.htmlhttp://www.grandcanyonnaturalhistory.com/pages_nature/geology/8-bright-angel.htmlhttp://www.bobspixels.com/kaibab.org/pda/rckbs.htmhttp://www.utahgeology.com/fm_brightangel.phpPrevious articles:https://www.facebook.com/photo.php?fbid=71718732167564https://www.facebook.com/photo.php?fbid=717596974968016https://www.facebook.com/photo.php?fbid=718487278212319https://www.facebook.com/TheEarthStory/posts/718917208169326https://www.facebook.com/TheEarthStory/posts/719035941490786https://www.facebook.com/TheEarthStory/posts/719534524774261https://www.facebook.com/photo.php?fbid=720485404679173https://www.facebook.com/photo.php?fbid=720916891302691https://www.facebook.com/TheEarthStory/posts/721282287932818https://www.facebook.com/TheEarthStory/posts/721455997915447https://www.facebook.com/TheEarthStory/posts/722212221173158https://www.facebook.com/TheEarthStory/posts/722332104494503https://www.facebook.com/TheEarthStory/posts/723288294398884https://www.facebook.com/TheEarthStory/photos/a.352867368107647.80532.352857924775258/723925267668520/?type=1https://www.facebook.com/TheEarthStory/photos/a.352867368107647.80532.352857924775258/724756080918772/?type=1https://www.facebook.com/TheEarthStory/posts/2080492818678418https://www.facebook.com/TheEarthStory/photos/a.352867368107647/2081921681868865https://www.facebook.com/TheEarthStory/photos/a.352867368107647/2083451208382579https://www.facebook.com/352857924775258/posts/2084958928231807/ -- source link
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