DEFINITION OF EVAPORITE

Evaporite sediments include salts precipitated

from aqueous solutions, typically associated with the evaporation

of desert lake basins known as playas, or the evaporation

of ocean waters trapped in restricted marine basins

associated with tectonic movements and sea level changes.

They are also associated with sabkha environments along

some coastlines such as along the southern side of the Persian

(Arabian) Gulf, where seawater is drawn inland by capillary

action and evaporates, leaving salt deposits on the surface.

Evaporites are typically associated with continental

breakup and the initial stages of the formation of ocean basins.

For instance, the opening of the south Atlantic Ocean about

110 million years ago is associated with the formation of up to

3,280 feet (1 km) of salts north of the Walvis–Rio Grande

Ridge. This ridge probably acted as a barrier that episodically

(during short sea-level rises) let seawater spill into the opening

Atlantic Ocean, where it would evaporate in the narrow rift

basin. It would take a column about 18.5 miles (30 km) thick

of ocean water to form the salt deposits in the south Atlantic,

suggesting that water spilled over the ridge many times during

the opening of the basin. The evaporite forming stage in the

opening of the Atlantic probably lasted about 3 million years,

perhaps involving as many as 350 individual spills of seawater

into the restricted basin. Salts that form during the opening of

ocean basins are economically important because when they

get buried under thick piles of passive continental margin sediments,

the salts typically become mobilized and intrude overlying

sediments as salt diapirs, forming salt domes and other oil

traps exploited by the petroleum industry.

Salts may also form during ocean closure, with examples

known from the Messinian (Late Miocene) of the Mediterranean

region. In this case thick deposits of salt with concentric

compositional zones reflect progressive evaporation of

shrinking basins, when water spilled out of the Black Sea and

Atlantic into a restricted Mediterranean basin. So-called closing

salts are also known from the Hercenian orogen north of

the Caspian Sea, and in the European Permian Zechstein

basin in the foreland of the collision.

As seawater progressively evaporates, a sequence of different

salts forms from the concentrated brines. Typically,

anhydrite (CaSO4) is followed by halite (NaCl), which forms

the bulk of the salt deposits. There are a variety of other salts

that may form depending on the environment, composition

of the water being evaporated, and when new water is added

to the brine solution and whether or not it partly dissolves

existing salts.

See also DIAPIR; PLAYA; RIFT; SEDIMENTARY ROCKS.

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