Why do shallow salt domes often contain cap rock?

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

Shallow salt domes often contain cap rock because it develops through the dissolution of salt from the top of the salt core when it is exposed to circulating, shallow water.1

As the salt dissolves, it leaves behind a residue of insoluble anhydrite. Over time, this residue is altered into other minerals, including gypsum, calcite, and sulfur. Consequently, cap rock is typically found on domes that are currently near the surface or were positioned in the shallow water zone at some point in their geological history before being buried.1