Evaporite rocks
Evaporites are layered crystalline sedimentary rocks that form from brines generated in areas where the amount of water lost by evaporation exceeds the total amount of water from rainfall and influx via rivers and streams. The mineralogy of evaporite rocks is complex, with almost 100 varieties possible, but less than a dozen species are volumetrically important. Minerals in evaporite rocks include carbonates (especially calcite, dolomite, magnesite, and aragonite), sulfates (anhydrite and gypsum), and chlorides (particularly halite, sylvite, and carnallite), as well as various borates, silicates, nitrates, and sulfocarbonates. Evaporite deposits occur in both marine and nonmarine sedimentary successions.The fundamental controlling factor in the formation of evaporite deposits is climate, because the seawater can become sufficiently concentrated for precipitation to occur only if the rate of loss through evaporation exceeds the input of water. These arid environments are principally found in subtropical regions where the mean annual temperatures are relatively high but the rainfall is low. Modern marine evaporite deposits are all found in coastal settings where precipitation occurs in semi isolated water bodies such as lagoons or directly within sediments of the coastal plain, places where recharge by seawater is limited. In the past, larger areas of evaporate precipitation resulted from the isolation from the open ocean of epicontinental seas and small ocean basins.
Evaporite minerals are precipitated in order depending on what percentage of the original volume of (sea) water remains:
1) Calcite (CaCO3) e dolomite (CaMg(CO3)2).
2) Gesso (CaSO4*2H2O) e/o anidrite (CaSO4).
3) Halite (NaCl).
4) Sali di K e Mg: sylvite (KCl), carnallite (KMgCl3*6H2O), polyhalite (K2Ca2Mg(SO4)4*2H2O), kainite (KMg(SO4)2*H2O), kieserite (Mg SO4*H2O).
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The succession of salts precipitated from seawater. On evaporation CaCO3 is precipitated first. When evaporation has reduced the volume to 19% of the original amount, CaSO4 begins to precipitate; at 9.5% of the original volume, NaCL start to precipitate, and so on.
If a column of seawater 1000 m deep is completely evaporated, only 14-17m of evaporites (mainly halite) are deposited. Some evaporite successions are thick (several 100 m), too thick to be simply the result of the evaporation of a single water mass and also their composition would not result from such a single event.
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Evaporation of a seawater column and resulting evaporite deposits.
Marine Evaporites
Evaporite deposits in modern marine environments are largely restricted to coastal regions, such as evaporite lagoons and sabkha mudflats. However, evaporite successions in the stratigraphic record indicate that precipitation of evaporite minerals has at times occurred in more extensive marine settings.Evaporitic basins (saline giants)
Evaporite sedimentation occurs only in situations where a body of water becomes partly isolated from the ocean realm and salinity increases to supersaturation point and there is chemical precipitation of minerals. This can occur in epicontinental seas or small ocean basins that are connected to the open ocean by a strait that may become blocked by a fall in sea level or by tectonic uplift of a barrier such as a fault block. These are called barred basins and they are distinguished from lagoons in that they are basins capable of accumulating hundreds of metres of evaporite sediment.
To produce just a metre bed of halite a column of seawater over 75m deep must be evaporated, and to generate thick succession of evaporite minerals the seawater must be repeatedly replenished. Deposition of the thick succession can be produced in three ways each of which will produce characteristic patterns of deposits:
1) A shallow-water to deep-basin setting exists where a basin is well below sea level but is only partly filled with evaporating seawater, which is periodically replenished.
2) A shallow-water to shallow-basin setting is one in which evaporites are deposited in salterns but continued subsidence of the basin allows a thick succession to be built up.
3) A deep-water to deep-basin setting is a basin filled with hypersaline water in which evaporite sediments are formed at the shallow margins and are redeposited by gravity flows into deeper parts of the basin. Normally graded beds generated by turbidites and poorly sorted deposits resulting from debris flows are evidence of redeposition.
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Settings where barred basins can result in thick successions of evaporites. From Nichols G. 2009: Sedimentology and Stratigraphy.
Deep-basin succession can show two different patterns of deposition. If the barred basin is completely enclosed the water body will gradually shrink in volume and area and the deposits that result will show a bulls-eye pattern with the most soluble salts in the basin center. If equilibrium is reached between the inflow and the evaporative loss then stable conditions will exist across the basin and tens to hundreds of metres of a single mineral can be deposited in one place. This produces a teardrop pattern of evaporite basin facies. Changes in the salinity and amount of seawater in the basins result in variations in the types of evaporite minerals deposited. Organic material brought into the basin during periods of lower salinity can accumulate within the basin deposits and be preserved when the salinity increases because hypersaline basins are anoxic.
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bulls-eye and teardrop pattern model of evaporite deposition modified from Nichols G. 2009: Sedimentology and Stratigraphy.
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Modern barred basin: Gulf of Kara Bogaz, Caspian sea.
There are no modern examples of very large, barred evaporitic basins but evidence for seas precipitating evaporite minerals over hundreds of thousands of square kilometres exist in the geological record. These saline giants have over 1000m thickness of evaporite sediments in them and represent the products of the evaporation of vast quantities of seawater. Evaporite deposits of latest Miocene (Messinian) age in the Mediterranean Sea are evidence of evaporative conditions produced by partial closure of the connection to the Atlantic Ocean. This period of hypersaline conditions in the Mediterranean is sometimes referred to as the Messinian salinity crisis.
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Map showing location and age of some of major basin wide evaporite deposits (after Kendall 1992).
Arid lagoons
In hot, dry climates the loss of water by evaporation from the surface of a lagoon is high. If it is not balanced by influx of fresh water from the land or exchange of water with the ocean the salinity of the lagoon will rise and it will become hypersaline, more concentrated in salts than normal seawater.
An area of hypersaline shallow water that precipitates evaporite minerals is known as a saltern. Deposits are typically layered gypsum and/or halite occurring in units metres to tens of metres thick. In the restricted circulation of a lagoon conditions are right for large crystals of selenitic gypsum to form by growing upwards from the lagoon bed.
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schematic representation of an arid lagoon environment.
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Close-up view of twinned Messinian selenite crystals of the Piedmont Basin (Italy). Photo by Marcello Natalicchio
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Twinned gypsum crystals (selenite) in Cyprus near Elediou.
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Coorong saline lagoons, Australia.
Arid sabkha flats
Arid shorelines are found today in places such as the Arabian Gulf, where they are sites of evaporate formation within the coastal sediments. These arid coasts are called sabkhas; they typically have a very low relief and there is not always a well-defined beach. The coastal plain of a sabkha is occasionally wetted by seawater during very high tides or during onshore storm winds, but more important is also a supply of water through groundwater seepage from the sea. Gypsum and anhydrite grow within the sediment while a crust of halite forms at the surface. In general, anhydrite forms in the hotter, drier sabkhas and gypsum where the temperatures are lower or where there is a supply of fresh, continental water to the sabkha.
The gypsum and anhydrite grow by displacement within the sediment, with the gypsum in clusters and the anhydrite forming amorphous coalesced nodules with little original sediment in between. These layers of anhydrite with remnants of other sediment have a characteristic chicken-wire structure. Halite crusts are rarely preserved because they are removed by any surface water flows.
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schematic representation of a sabkha environment.
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Nodular anhydrite. Dukan sabkha
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chicken-wire structure in anhydrite
Nonmarine Evaporites
Evaporite deposition in the nonmarine environment occurs in closed lake, those without outlet, in arid and semiarid regions. Such lakes form in closed interior basins or shallow depressions on land where drainage is internal and runoff does not reach the sea. If water depths are shallow or, more typically, somewhat ephemeral, the term playa or playa lake is commonly used.Water inflow into closed lakes consists principally of precipitation and surface runoff, both of which are small in amount and variable in occurrence in arid regions. Groundwater flow and discharge from springs may provide additional water input, but evaporation rates are always in excess of precipitation and surface runoff. Sporadic or seasonal storms may give rise to a sudden surge of water inflow. Because closed lakes lack outlets, they can respond to such circumstances only by deepening and expanding. Subsequent evaporation will reduce the volume of water present to prestorm or normal amount; fluctuation of closed lake levels therefore characterizes the environment.
Such changing lake levels and water volumes lead to fluctuating salinity values. Variations in salinity effect equilibrium relations between the resulting brines and lead to much solution and subsequent reprecipitation of evaporites in the nonmarine environment. As a result of these complexities as well as the distinctive nature of dissolved constituents in closed lake settings, nonmarine evaporite deposits contain many minerals that are uncommon in marine evaporates e.g., borax, epsomite, trona, and mirabilite.
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Difference between marine and nonmarine evaporites.
Bibliography
• Nichols G. Sedimentology and Stratigraphy. (2009).
• John K. Warren. Evaporites Sediments, Resources and Hydrocarbons. (2006)