Surprise! It’s the Surprise Canyon Formation2 units ago we saw the Temple Butte formation; a unit th

Surprise! It’s the Surprise Canyon Formation2 units ago we saw the Temple Butte formation; a unit that makes its home in space created by erosion of the underlying units. The Surprise canyon formation tells a similar story.Here you see the Surprise Canyon formation marked by the Park Service in one of its traditional habits. On top of the Redwall Limestone, a channel has been cut by erosion. This channel created space where the mixture of siltstones, sandstones, limestones, and dolostones of the Surprise Canyon formation could be deposited.The top of the Redwall is an erosional unconfority. Once this thick limestone was deposited, the waters receded, allowing erosion to begin. The structure of the Surprise Canyon testifies to that. Like the Temple Butte formation below, it is deposited in channels, and like the Temple Butte formation it is up to 3 times as thick in the western part of the park where the area was closer to the ocean and at lower elevations.There’s a bit of a stratigraphic mystery here. The Redwall was deposited in fairly deep, tropical waters. Suddenly on top of it there is an erosional boundary, meaning the rocks appear to have gone from deep, quiet water to above-water almost in no time.This is a trick of geologic time that sediments play. When ocean waters recede, sediments still are often deposited, but receding waters expose those sediments to erosion. It’s entirely possible that shallow marine and beach sediments were deposited on top of the Redwall limestone at some point, but because the waters kept receding, those sediments just washed away. All that is left is the thick Redwall Limestone, with channels eroding into the top of it. Altogether, about 20 million years passed between deposition of the Redwall and deposition of the Surprise canyon. That’s plenty of time for sea level to decrease, for the rocks to erode, and for sea level to increase again, producing this sequence.Also in this image you see a “breccia pipe”. This structure is another result of erosion of the Redwall. When limestones erode, they often dissolve and can form caves and Karst topography. Later rocks deposited on top of a cave? Sounds like a setup for a cave in to me.-JBBImage credit: http://www2.nature.nps.gov/GEOLOGY/parks/grca/age/index.cfmMore http://3dparks.wr.usgs.gov/coloradoplateau/lexicon/surprisecanyon.htmPrevious 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/724792024248511https://www.facebook.com/TheEarthStory/posts/725410850853295https://www.facebook.com/TheEarthStory/posts/726153457445701https://www.facebook.com/TheEarthStory/posts/726938514033862https://www.facebook.com/TheEarthStory/posts/727461423981571https://www.facebook.com/TheEarthStory/posts/727462763981437https://www.facebook.com/TheEarthStory/posts/727463347314712 -- source link

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