Evaporites are non-clastic, or chemical sediments, created through the precipitation of dissolved salts from water. They most frequently occur at the site of a former large water body such as a lake or landlocked sea, on coastal plains (sabkha zones), or where rivers feed very arid desert areas. As the water involved slowly evaporates the salts become more concentrated and at well-defined concentrations they begin to recrystallise. They are different to the more conventional clastic sedimentary rocks which include mudstone, siltstone and sandstone in being soluble in water and much more mobile under pressure.
These rocks form an economically important group of minerals. Sylvinite (a mixture of potassium chloride, sodium chloride and clay minerals) and halite (rock salt) are extracted by Cleveland Potash Ltd. from their mine at Boulby, near Staithes. The sylvinite lies within Permian strata over a kilometer below the surface. It is a little under 290 million years old and was laid down when a shallow ancient sea, dubbed the Zechstein – and which occupied an area between Northern England, through the North Sea Basin, to Poland – underwent several cycles of evaporation and transgression. Sea water comprises ~3.5% dissolved salts of which the bulk is sodium chloride or common salt. Evaporation yields successively limestone, anhydrite (calcium sulphate), halite and finally potassium and magnesium salts. Anhydrite was formerly mined and processed at Billingham on Teesside.
Evaporite deposits can flow under pressure producing salt-domes which disturb the strata through which the pass and internally exhibit complex folding. They also retain heat and may be a target for future geothermal energy projects.