Sandstone and limestone layers

[kingwinner]

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! :xparty:

[Turtle]

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! :eek:

 

Last First: A course of study in its own right.:xparty:

First Last: The cross bedding is an artifact of the "ripples" found in sand, whether wind or water derived. The specific form of the bedding & the specifics of a sandstone in situ further delineate the origin. Sandstone deposits with no crossbedding do accrue from desert & beach (ocean and/or river) depoits by means such as flooding & tsunami.:lol: