Thank you for posting this dedicated thread. I hope we can have a good discussion!
Up front I have to make a disclaimer: although I am an MSc geologist, and before I retired worked for 35 years in the oil & gas industry, I don’t have any particularly deep knowledge of the local geology of the interior USA. My comments are therefore going to be based more on general geological concepts and on analogs I do know better from other places. I think there are others on here who have more direct knowledge of the Grand Staircase, and I hope they may chip in and correct me in I get it wrong in the local details.
There are of course many areas in the world where the sedimentary rock layers are quite flat and very extensive. Generally, these areas were continental shelves during prolonged periods of high sea level, such as during most of the Jurassic and the Late Cretaceous (caused by factors such as the climate, and high oceanic spreading rates). Underlying the sedimentary rocks is what we call ‘basement’, much older or even primary continental crust that forms large areas (cratons) that have remained largely stable thoughout geological history. In other places these cratons are currently exposed at the surface, e.g. the Canadian Shield and the Baltic Shield. Here, it has been buried over time by younger sediments.
In essence, these were large shallow (‘epicontinental’) seas. Sediment input would be from one or more land areas bordering these seas, and of course some of the formations are carbonates that were formed in-situ through organic action. Clastic sediments on continental shelves move around a lot because of waves and currents, and will get ‘smeared out’ over large areas not in the immediate vicinity of the input points. This can all be observed in modern seas.
A short duration Flood model has a much bigger problem in explaining these extensive regular beds, because there simply won’t have been the time necessary to distribute the sands over such large areas, nor the time and amount of organisms necessary to form thick carbonate layers in-situ. Also, given the extreme assumptions about the state of the planet during the Flood, what with continents moving thousands of kilometers in a month, the energy and turbulence in the waters would be so high (if it hadn’t all boiled away, but that is another discussion) that extensive regular beds simply woudn’t be formed (let alone organic carbonate beds). These type of deposits need a fairly calm environment or else the sediment would remain in suspension.
There is more to be said about the fossils we find in these sediments - many are bottom dwellers. How would they survive being buried by kilometers of sediment coming down on them in just one month? The ones living on the seabed at the start of the flood woud perish, and nothing would be alive in there for the rest of the time. Yet, we find such fossils abundantly throughout the column. These are marine fossils so they would not have been transported there from the land. Moreover, there is exquisite vertical sorting throughout - not by size or weight, but by morphology. What caused this sorting, in the conditions assumed during the Flood?
Which brings us to another problem: how deep would the water column have to be to accommodate the sediments of this entire column all at the same time? You are talking many kilometers. Where did all that water come from, and where did it go? Where is it now?
Ther is a lot more, but this will do for now. Over to you.