Sam2, that’s not really the point. You can ask your local orthodox geologist about the necessary preconditions required to instantly fossilize large salt formations to withstand thousands of years of erosion (not to mention the fallout from an entire city being blown up right nearby). The point is; why assume it still exists?
Also, and again, I’m by no means a scientist, but I would assume salt formations that form slowly, over thousands of years, one grain at a time, are more likely to fossilize and harden than something that instantaneously turns into salt. I always imagined a consistency not unlike a chunk of salt that you sometimes get in table salt containers. But this second paragraph is all just my assumptions.