What is a cap-rock in geology ?

Cap-rock : definition

The cap-rock is the top part of a salt dome, formed by the reaction of the minerals in the dome with infiltrated surface water, marine or continental. It includes insoluble residues from the dome (clays, sulphates, etc...), concentrated following the dissolution of salts by the infiltration water, as well as minerals resulting from complex reactions between water and salts : anhydrite often bedding, calcite, gypsum, abundant sulphur, sulphides... These reactions are favored and maintained by the presence of bacteria. The depth (between 90 and 690 m) and the shapes of the cap-rock are very variable ; their power varies from a few tens to a few hundred meters.

Cape-rock is currently the largest global sulphur deposits, intensively exploited in particular in the United States, in and near the Gulf of Mexico. They are often also very effective oil traps.