How does cap rock form on top of a salt dome?

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

Cap rock forms primarily through the dissolution of salt from the top of a salt core when it is exposed to circulating, shallow groundwater. As the salt—which typically contains minor amounts of anhydrite—dissolves, it leaves behind a residue of insoluble anhydrite.1

Over time, this residual anhydrite undergoes chemical alteration to form gypsum, calcite, and sulfur. Because this process requires shallow water circulation, the presence of cap rock on deeply buried salt domes indicates that those structures were once located near the surface before being subsequently buried. The resulting cap rock is often layered, typically featuring a lower anhydrite zone, a middle transitional zone containing gypsum and sulfur, and an upper calcite zone.1