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Thread: Sandstone and limestone layers

  1. #1 Sandstone and limestone layers 
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    1) If an outcrop shows alternating layers of sandstone and limestone, what can you assume about the geologic history of that region?

    I said that the region had alternating periods of arid desert / dune-covered beach (sandstone layers) and shallow sea (limestone layers), am I correct?

    I know that limestone indicates that it was deposited when the region had a shallow sea, but I am not too sure about the sandstone part, are sandstone layers usually formed in arid desert / dune-covered beach?

    "Some light-colored layers in the Grand Canyon are composed of sandstone. Someo f these sandstone layers show cross-bedding, which is characteristic of sand dunes. Therefore, it appears that these layers were laid down when the region was either an arid desert or a dune-covered beach. The layers of limestone and shale must have been deposited when the region was covered by a shallow sea." (quote from textbook)

    This quote makes me to think that only sandstone with cross-bedding can be assumed to be formed in desert / beach environment, but those sandstone without cross-bedding can't be assumed this way...and this leads to my uncertainty above

    2) From the above quote, what can you tell about the history of the Colorado Plateau?
    I can figure out this as well...can someone explain?

    Thanks a lot!


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  3. #2  
    Forum Cosmic Wizard SkinWalker's Avatar
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    The deposition mechanism for the sand can be in the form of moving water as well as wind. It could be that the region was a shallow sea alternately affected by depositional forces like storms and mass die-offs of shellfish and the like which become part of the sediment load and deposited like sand.


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