Hi Faith,
There was a lot in your post, but I'm just going to focus on one small point:
Faith writes:
My problem is with the enormous time frame. The idea that ANYTHING could sit still for 50 million years is simply preposterous to my mind. How can ANYTHING "subtle" happen in a 50-million year period? Hurricanes alter seacoasts and beaches, tornados move tons of stuff from here to there, one good rain causes mudslides all over California that rearrange local landscapes drastically, not subtly, and destroy houses; all in one year; but the redwall limestone stays in place for 50 million years even in the phase where it's quietly sedimenting away and not yet lithifying?
You use the example of the Redwall Limestone layer of the Grand Canyon, so I'm going to focus on that. This layer represents what was once a shallow sea. We can tell by the makeup of the sediments that it was a shallow sea, and it persisted for a long time, perhaps longer than 10 million years. We know that it existed about 340 million years ago by radiometric dating.
The catastrophes that are so devastating on land often go unnoticed under water. The hurricanes, tornados, rains and mudslides that you mention have impacts that are felt primarily on land. Hurricane Andrew devastated southern Florida in 1992, causing billions of dollars worth of property damage, yet if you had visited the ocean floor some miles off into the Atlantic after that hurricane you would have found no sign of it at all.
The redwall limestone layer of the grand canyon does not happen to include the shoreline area of this shallow sea. The massive amounts of sediments that would have been recorded near shorelines after major storm and flood events have no effect far away from shore. Out in the middle of this quiet sea sediments slowly accumulated undisturbed eon after eon.
There are many examples of the accumulation of sediments over time - every ocean is another example. The floor of the Atlantic ocean forms at the mid-oceanic ridge at the rate of about 10 cm per year. The ridge goes pretty much north/south through the center of the ocean - it goes right through Iceland, accounting for its volcanic activity. Sea floor near the ridge is very young, while sea floor near the American, European and African continents is very old. Sea floor cores taken of sediment near the ridge are very shallow before hitting rock, while sedimentary cores taken near continents are deepest. This is because sediments gradually accumulate over time, and in general the oldest sea floor will have the greatest amount of deposited sediment (it also depends upon sediment sources, of course).
The ultimate fate of all sea floor is subduction beneath continents (continental crust is lighter than sea floor crust, so sea floor always subducts beneath continents). While there are continental rocks in some places that are billions of years old, there is no sea floor anywhere in the world older than 200 million years. The only record we have of seas and oceans older than that is those that were pushed up by tectonic forces and became part of continents, such as large areas of Arizona. The Grand Canyon is unique in that it provides visibility to the layers that underlie Arizona, but it is important to keep in mind that these layers extend for miles and miles in all directions, you just can't see them.
--Percy