If I might jump in here...
And slevesque doesn't need to know where the salt water came from. He needs to know that it was deposited by evaporation.
True, evaporation is the best mechanism available for the supersaturation of the elements that make up salt beds. I think this is what you are saying. While the conditions under which evaporation might occur vary, the mechanism is pretty much the same.
And yes, I can think of some other methods of producing halite from an undersaturated solution. We might use temperature changes, perhaps; or mixing of different solutions. However, I don't see these as producing massive, bedded salt formations that are chemically zoned in the way that evaporitic basins occur.
Personally, considering the size of the basin is trivial. In plate tectonics we can imagine, predict and find, restricted basins of almost any size in the geological record. It just so happens that the large basins in the southern US probably occurred along with the start of the opening of the Atlantic Ocean and, like the Afar Region salt deposits were formed, but on a colossal scale.
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