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't figure out this as well...can someone explain? Thanks a lot! Please Register or Log in to view the hidden image!
This is not my field, as you well know, but I have recently read some information which pertains to the first part of your question. Sandstone and limestone. I don't believe that sandstone requires an arid environment. The sediments can be laid down under water as well. I came across this while reading into the Chicxulub event (the bolide impact at the KT boundary which is conjectured to be the final nail in the dinosaurs' coffin.) There are three geologic layers associated with this event. First is a layer of spherules (conjectured by some to be formed from molten debris raining down to earth). Third is an iridium layer (the classic proof of bolide impact). These are the two layers commonly referred to when speaking of Chicxulub. But, there is also a second layer sandwiched between the two in certain areas. A sandstone layer. There are multiple ways of interpreting this geologic evidence. The mainstream interpretation is that the Chicxulub hit. Caused massive amounts of debris to erupt from the crater and come raining down to earth to form the spherule layer. The sandstone layer was then deposited by tidal waves. The iridium layer came last. The more controversial interpretation is that several hundred thousand years came between the spherule and the iridium layer. But that's another story. The point is that the sandstone layer was deposited underwater.... I think. Reading what I just wrote, it seems that the tidal wave deposited sediments onto land to form the sandstone layer, but I recall clearly that it was underwater. Especially since part of the controversial interpretation was evidence of plankton fossils embedded in the layers... Ah. Here we go. Underwater. For sure. Limestone is organic base, yes? I think that this would be key to the answer your teacher was looking for. Oh. And links to the Chicxulub stuff. http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dino_trans.shtml http://geoweb.princeton.edu/people/faculty/keller/chicxulub.html
Kingwinner- Your answer is essentially correct. The formation of sandstone requires sorting, which is basically the removal of sediments that are either larger or smaller than sand-sized grains. Well sorted sand is generated in moderately high-energy environments- in the case of desert wind can transport the sand grains, but not heavier material and will blow away the lighter material. In marine environments, waves supply the energy necessary to winnow out the smaller material. Once you get beyond the wave base, finer grained (siltstone, shale) sediments and limestones will be deposited. As sea level rises and falls (transgression and regression), shorelines and deeper water will migrate back and forth, especially in the case of shallow seas. As sea levels fall, sandstone will be deposited on top of previously deposited deeper water shales and limestones. As sea levels rise, the limestones/shales will be deposited on top of the previously deposited sandstone. As per my remarks above, probably a shallow transgressive/regressive sea.
Aye, seems right enough. IIRC, the red sandstone conglomerate around Arbroath in the East of Scotland has a red sandstone matrix, with a mixture of other rocks init, because it was formed from erosion debris coming off the CAnadian shield a few hundred million years ago.
