What is an evaporite?

An evaporite is a salt rock that was originally precipitated from a saturated surface or nearsurface brine in hydrologies driven by solar evaporation. This simple definition encompasses a wide range of chemically precipitated salts and includes alkali earth carbonates. Some workers restrict the term evaporite to salts formed at the earth’s surface via solar evaporation of hypersaline waters. To underscore the highly reactive nature of evaporites in the sedimentary realm I think of such evaporites as primary, that is, precipitated from a standing body of surface brine and retaining crystallographic evidence of the depositional/hydrological process set where they formed (e.g. bottom-nucleated or current-derived textures). Outside of a few Neogene examples, there are few ancient evaporite beds with textures that are wholly and completely “primary.”

Most in the subsurface exhibit “secondary” (burial-related) textures, while remnants that have been uplifted back to the surface show “tertiary” (exhumation-related) overprints. We can contrast the solar-driven set of precipitative processes with other surface and subsurface water-loss mechanisms that can form bodies and beds of mineral salts, with the same chemical compositions as rocks typically called evaporites. And yet, if applying the term evaporite in its strictest form (i.e., a response to solar-driven evaporation), this second group of mineral salts/sediments are not “true” evaporites; rather, they are mineral salts that crystallized in response to various cryogenic, hydrothermal and burial processes.