7.1: Evaporites
Evaporite minerals and the sedimentary rocks they comprise form from precipitation of mineral from a saline solution. Generally the fluid becomes supersaturated through evaporation and this is easiest to achieve in arid environments.
Evaporite Minerals
Although previously discussed as a stand-alone group, carbonate minerals - particularly calcite (CaCO 3 ) and dolomite (CaMg(CO 3 ) 2 ) - are also considered evaporite minerals and are some of the first ones to form as concentrations in the brine increase. Trona (Na 2 CO 3 ·NaHCO 3 ·2H 2 O) forms in freshwater evaporite settings and is mined as a source of soda ash which is used in the manufacture of glass, paper, detergents, and other chemicals.
Gypsum (CaSO 4 •2H 2 O) and anhydrite (CaSO 4 ) are two of the most common evaporite minerals and are members of the sulfate mineral class. The two are closely associated and with increased burial depth gypsum commonly loses the water and transforms into anhydrite. The process can reverse itself if anhydrite returns to suitable conditions in the presence of water. They look very similar, but anhydrite is slightly harder. The two often occur together (sometimes with small amounts of clastic mud) in nodular masses and have a distinctive "chicken wire" texture. Barite (BaSO 4 ) and celestite (SrSO 4 ) are much less common sulfates that occasionally occur as secondary minerals formed from the interaction of gypsum/anhydrite with barium- or strontium-enriched fluids.
Halite (NaCl) is the most common halide evaporite mineral and is familiar to anyone who has taken an intro geology lab and was brave enough to lick the samples. Potassium and magnesium salts are late formed minerals that are less common but economically important. Examples include sylvite (KCl), carnallite (KClMgCl 2 ·6(H 2 O)), kieserite (MgSO 4 ·H 2 O), and polyhalite (K 2 Ca 2 Mg(SO 4 ) 4 ·2H 2 O).
Although rare, some evaporite minerals are economically important when found in significant quantities. They include borax (Na 2 B 4 O 5 (OH) 4 ·8H 2 O) which is used in laundry and cleaning products and nitre (KNO 3 ) which is used for making gunpowder and cleaning products.
Evaporite Precipitation
Evaporites form in situations where brines form because the amount of water lost through evaporation is greater than water supplied by rainfall, recharge via rivers, and/or communication with a larger body of water. They can form in a variety of marine, marginal marine, and nonmarine environments that meet these conditions. The minerals listed above don't randomly or simultaneously precipitate, but rather they are formed in a predictable sequence based on the composition of the brine.
| Mineral | Percent of the original brine remaining when the mineral starts to precipitate (water depth if you started with 1,000 m) | Thickness of this mineral that would form if you started with 1,000 m of sea water and evaporated it completely |
| Calcite | 50% (500 m) | 0.10 m |
| Gypsum | 15% (150 m) | 0.61 m |
| Halite | 10% (100 m) | 13.30 m |
| K and Mg salts | 5% (50 m) | 2.99 m |
| Total evaporite thickness | 17 m |
Table \(\PageIndex{1}\): The evaporite sequence that one could expect if you started with 1,000 m of seawater and evaporated it completely (data from https://www.alexstrekeisen.it/english/sedi/evaporites.php ).
Evaporite environments
Evaporites can form in a variety of marine and terrestrial environments as long as the rate of evaporation exceeds the rate of recharge. In general, we can lump the spectrum of environments into three main families: platform evaporites, basin-wide evaporites, and non-marine evaporites:
Platform evaporites model. In this model, evaporites of modest thickness and distribution occur in marginal marine settings where seawater is able to breach a topographic barrier and flood low-lying coastal environments (ex: sabkhas ). Evaporite facies are commonly interbedded with shallow water clastics and carbonates.
Basin-wide evaporites. In this model, evaporites accumulate in areas where the crust is low-lying and/or subsiding (ex: early stages of a rift basin or the Mediterranean Sea) and seawater breaches or seeps through a barrier (ex: Straight of Gibraltar) providing a mechanism for a thick succession of evaporites to form along with associated clastic and carbonate deposits. There are no basin-wide deep water evaporites today; the best modern analog might be the Dead Sea and the easiest to visualize ancient examples are the early phases of a rift basin or the Mediterranean Sea which has a thick evaporite succession in the subsurface that formed when the basin was periodically isolated from the Atlantic Ocean between 5 and 6 Ma during the "Messinian Salinity Crisis".
Non-marine evaporites. Non-marine environments include a spectrum of environments including playa lakes , deserts, and various closed basins. As with platform evaporites they are likely to be of limited thickness and distribution. Given that many of them are sourced from non-marine waters they can also have mineralogy that is similar to, but a bit different than marine evaporites.
Marine Evaporite Models
If you need 1,000 meters of seawater to make a 17 m thickness of evaporites, how can we possibly explain the presence of evaporite successions that are tens, hundreds, or perhaps even a few thousand meters thick? If it was a single event, it would require an impossible depth of water and would be recorded as a single coherent evaporite succession. It turns out that thick evaporites successions are composed of cyclic packages of evaporites that record a complex history, from which three possible explanations for these thick successions emerge:
Deep-water/deep-basin model. In this case you have a deep body of water filling a deep topographic basin. Because evaporation and recharge are both ongoing, the water remains deep but you are are still able to create a brine of sufficient concentration to cause crystals to settle out from the water column and/or be transported downslope to deeper parts of the basin.
Shallow-water, shallow-basin model. In this model, relatively shallow basins are flooded with modest amounts of water and evaporites form as that water body dries up. Any given event results in only a modest thickness of evaporites, but subsidence allows the process to keep going through time so that thick successions can accumulate.
Deep-basin, shallow-basin model. A deep basin is separated from the open ocean by a barrier. That barrier periodically overflows allowing water to partially flood the basin. Evaporation of water from that event causes a modest thickness of evaporites to form; repeated periodic overflow allows for a thick succession to eventually fill the basin.
Salt Diapirs
The halite, gypsum, and anhydrite that make up the overwhelming majority of evaporite deposits are less dense than most other rocks and they can flow and deform plastically if given enough time (that's especially true when they are buried). This combination of properties explains why evaporite deposits are so prone to deformation and commonly form wall or plume-shaped salt diapirs that appear to rise through and deform adjacent sediments. Older literature attributed the flow of evaporites primarily to density differences, but more recent literature shows that differential loading (often by the progradation of sediments from basin margins) initiates diapirism. Once loading begins and evaporite withdrawal commences in the subsurface, it allows for rapid sediment accumulation on the diapir flanks and deformation of adjacent sediments (think of the sediment like a rolling pin that is rolling over a tube of toothpaste (the evaporites). Because evaporites are easily dissolved, these features are only rarely exposed at the surface - some of the best known examples are in arid deserts (ex: Kavir Desert of Iran) or rapidly eroding coastal exposures.
Readings and Resources
- Geological Digressions, Mineralogy of carbonates; Sabkhas - https://www.geological-digressions.com/2020/01/16/mineralogy-of-carbonates-sabkhas/
- Geological Digressions, Mineralogy of evaporites: Marine basins - https://www.geological-digressions.com/2020/03/19/mineralogy-of-evaporites-marine-basins/