Evaporites

Evaporites are all deposits composed of minerals that originally precipitated from saline solutions concentrated by solar evaporation — rock salt being the most familiar example. ^[1] Their high solubility and susceptibility to deformation mean that most ancient evaporite beds have been diagenetically or secondarily altered during burial, so that virtually no completely primary evaporite beds older than about 25 million years survive intact. ^[1]

Evaporites occur in rocks of most ages including the Precambrian, but they are especially common in Cambrian, Permian, Jurassic, and Miocene successions; although their total volume is far less than that of carbonate rocks, individual deposits such as the Miocene Messinian of the Mediterranean region reach thicknesses exceeding 1 km. ^[1] Evaporites form under both marine and nonmarine conditions, but marine evaporites are generally thicker and more laterally extensive and are of greater geologic interest. ^[1]

Composition and Classification

Evaporite deposits are composed dominantly of halite (rock salt), anhydrite, and gypsum; approximately 80 minerals have been reported from evaporite deposits overall, but only about a dozen are common enough to be considered important evaporite rock formers. ^[1]

Excluding carbonate minerals (which are not true evaporites), the most common minerals of marine evaporites are the calcium sulfates gypsum and anhydrite, followed by halite, then the potash salts (sylvite, carnallite, langbeinite, polyhalite, kainite), and finally the magnesium sulfate kieserite; marine evaporite minerals can be grouped into chlorides, sulfates, and carbonates. ^[1] Gypsum is more abundant than anhydrite in modern evaporite deposits, but anhydrite is more abundant in ancient deposits due to diagenetic alteration of gypsum to anhydrite over time. ^[1]

Nonmarine evaporites are characterised by minerals not common in marine evaporites, because nonmarine waters typically have more bicarbonate and magnesium and little or no chlorine compared to seawater; nonmarine evaporite minerals include bloedite (Na₂SO₄·MgSO₄·4H₂O), borax [Na₂B₄O₅(OH)₄·8H₂O], epsomite (MgSO₄·7H₂O), trona [Na₃H(CO₃)₂·2H₂O], and others, though nonmarine deposits may also contain anhydrite, gypsum, and halite. ^[1]

Gypsum and Anhydrite

Calcium sulfates are deposited dominantly as gypsum, which can be altered to anhydrite — and anhydrite can form pseudomorphs after gypsum — even while the sediments remain in their depositional environment. ^[1] Upon burial to a few hundred meters, gypsum is dehydrated to anhydrite, and this loss of water is accompanied by a 38 percent decrease in solid volume; because of this rapid dehydration, most ancient calcium sulfate deposits are composed of anhydrite. ^[1] After uplift and exposure to low-salinity surface waters, anhydrite can be hydrated back to gypsum with an accompanying increase in volume; these volume changes distort original depositional structures and textures, explaining why many calcium sulfate deposits display distorted fabrics. ^[1]

Nodular and Chickenwire Anhydrite

Three fundamental structural groups of anhydrite are recognised on the basis of fabric, bedding, and distortion: nodular, laminated, and massive. ^[1] Nodular anhydrites are irregularly shaped lumps of anhydrite partly or completely separated from each other by a salt or carbonate matrix; chickenwire structure is a specific variety consisting of slightly elongated, irregular polygonal masses of anhydrite separated by thin dark stringers of carbonate or clay minerals. ^[1]

The formation of nodular anhydrite begins with displacive growth of gypsum in carbonate or clayey sediments; these gypsum crystals then alter to anhydrite pseudomorphs that continue to enlarge by addition of Ca²⁺ and SO₄^2– from an external source, growing displacively into anhydrite nodules. ^[1] Chickenwire anhydrite forms when the growing nodules ultimately coalesce and interfere, pushing most of the enclosing sediment aside so that what remains forms the thin stringers between nodules; the only requirement for nodular anhydrite formation is that crystals grow in mud in contact with highly saline brines — conditions met in both sabkha and deeper-water settings. ^[1]

Laminated Anhydrite

Laminated anhydrites consist of thin, nearly white, anhydrite or gypsum laminations alternating with dark gray or black laminae rich in dolomite or organic matter; individual laminae are commonly only a few millimetres thick and rarely reach 1 cm. ^[1] Many laminae are remarkably uniform with sharp planar contacts and can be traced laterally for more than 100 km; in some successions hundreds of thousands of laminae compose vertical sequences hundreds of metres thick, as in the Permian Castile Formation of southeast New Mexico and west Texas. ^[1] Alternating light and dark pairs have been interpreted as annual varves reflecting seasonal changes in water chemistry and temperature, though they could equally represent cyclic changes of longer duration. ^[1] The lateral persistence of laminated evaporites — indicating uniform depositional conditions over wide areas — is taken as evidence that they precipitated in quiet water below wave base, in either a protected shallow-water area or a deeper-water environment. ^[1] In some modern sabkha deposits, continued lateral growth and coalescence of anhydrite nodules creates a demand for space; the resulting lateral pressures cause layers to become contorted, forming ropy bedding or enterolithic structures. ^[1]

Massive Anhydrite

Massive anhydrite lacks perceptible internal structures, is less common than nodular and laminated varieties, and little is known about its formation; it is suggested to form by evaporation at brine salinities of approximately 200 to 275 parts per thousand, just below the salinities at which halite begins to precipitate (compared to normal seawater at 35 parts per thousand). ^[1]

Halite

Halite forms as crusts (presumably in shallow water) and as very finely laminated deposits that may reach thicknesses of as much as 1,000 m; laminated halite deposits commonly include anhydrite-carbonate laminae, and dark inclusion-rich laminae may alternate with light, inclusion-poor laminae. ^[1] Originally formed halite crystals can be chevron, coronet, or hopper shaped; halite that has been recrystallised and subjected to movement (salt flowage) may instead display highly interpenetrating, centimetre-size crystals, and halite deposits may also show sedimentary structures such as ripples and cross-bedding. ^[1]

Evaporation Sequence

When ocean water is evaporated, evaporite minerals are precipitated in a definite sequence: minor quantities of carbonate minerals begin to form when the original volume of seawater is reduced to about one-half; gypsum appears when the volume is reduced to about 20 percent; halite forms when the water volume reaches approximately 10 percent of the original; and magnesium and potassium salts are deposited when less than about 5 percent of the original volume remains. ^[1] Precipitation of gypsum at the 20-percent stage increases the Mg/Ca ratio in the residual water, which is why dolomite is commonly found in association with evaporites in ancient successions. ^[1] The same general sequence occurs in natural evaporite deposits, although natural deposits contain a greater proportion of CaSO₄ (gypsum and anhydrite) and a lesser proportion of Na-Mg sulfates than the laboratory sequence predicts. ^[1]

Depositional Settings

Modern evaporite deposits accumulate in a variety of subaerial and shallow subaqueous environments: subaerial settings include coastal and continental sabkhas (salt flats) and interdune environments; shallow subaqueous settings are mainly saline coastal lakes called salinas. ^[1] No modern example of a deepwater evaporite basin is known (with the possible exception of the Dead Sea), yet geologists believe that many of the thick, laterally extensive ancient evaporite deposits did accumulate in deepwater basins, since the Permian Zechstein of the North Sea area, for example, exceeds 2 km in thickness — a volume that could not be produced from a closed seawater column without a mechanism for continuously renewing the supply of marine water into a partially isolated basin. ^[1]

Master UPSC Geology Optional

Ex-ONGC Geologist & Rank Holder

Learn the exact analytical answer-writing patterns needed for UPSC Optional from an AIR 2 & AIR 25 holder.

1-on-1 Personalized Mentorship

Elite Batch (Strictly 10 Seats)

Targeted Strategy for AIR 1-100

Bilingual Conceptual Lectures

Join Us

Offline in Delhi