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What evidence shows that deeply buried salt domes with cap rock were once closer to the surface?

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salt dome Figure 1: Interrelationships of salt structures (see text)

The evidence that deeply buried salt domes were once closer to the surface lies in the presence of cap rock atop the salt core.1

Cap rock is generally believed to form through the dissolution of salt by circulating water, a process that occurs in the shallow subsurface zone. As the salt at the top of the core dissolves, it leaves behind a residue of insoluble minerals—primarily anhydrite—which subsequently alters into gypsum, calcite, and sulfur.1

Because this chemical transformation requires the presence of circulating water typically found at shallow depths, the existence of this cap rock on a dome that is now deeply buried indicates that the structure must have been positioned near the surface at some point in its history before being subsequently buried by further sedimentation.1