EVAPORITE:MODE OF FORMATION,CHARACTERISTIC

S AND THEIR ECONOMIC POTENTIALS

BY OLADIMEJI AKINSILE

UNIVERSITY OF ILORIN, ILORIN, NIGERIA.

OUTLINES

• DEFINITION• FORMATION OF EVAPORITE• ENVIRONMENT OF DEPOSITION• TYPES OF EVAPORITES• MAJOR GROUPS OF EVAPORITE • ECONOMIC IMPORTANCE OF EVAPORITES• CASE STUDY

INTRODUCTION• Evaporite is a name for water soluble sediment that results from

the concentration and crystallization by evaporation from an aqueous solution.

• Evaporite minerals are precipitated from a brine • The brine may be evaporated sea water or reconstructed in subsurface. • Most common evaporate minerals are:

• halite (NaCl) • anhydrite (CaSO4) • gypsum (CaSO4*2H2O) • sylvite (KCl)

• Some Common Evaporite Minerals of Sediments• Consider the progressive evaporation of 1 litre of sea water.

• In general: • gypsum begins to precipitate when the volume is reduced to 30% • halite after reduced to 10% • and Mg and K salts after 5%

• Na salts include NaBr • 1000 m of sea water completely evaporated yields 0.75 m of gypsum

and 1.37 m of salt.

TEXTURE OF EVAPORITES

• There are several textural attributes found in evaporite minerals, namely:

Acicular textures.

Equant textures.

Fibrous radial textures.

Fibrous parallel textures.

Lath shape crystals.

Aphanitic textures.

Sparse and isolated crystals.

FORMATION OF EVAPORITE• Although all water bodies on the surface and in

aquifers contain dissolved salts, the water must evaporate into the atmosphere for the minerals to precipitate. For this to happen, the water body must enter a restricted environment where water input into this environment remains below the net rate of evaporation. This is usually an arid environment with a small basin fed by a limited input of water. When evaporation occurs, the remaining water is enriched in salts, and they precipitate when the water becomes supersaturated.

Fig 1: Evaporation of water and precipitation of evaporite minerals.

EVAPORITE PROCESSES

Fig 2 : SCHEMATIC ILLUSTRATION OF PROCESSES THAT FORM EVAPORITE DEPOSIT

DEPOSITIONAL ENVIRONMENT OF EVAPORITES

There are 3 different depositional environments of evaporites which include

1 BASINS OF INTERNAL DRAINAGE: In arid regions with basins of internal drainage, rainfall in adjacent

areas is carried into the basin by ephemeral streams carrying water and dissolved ions.

The water fills the low point in the basin to form a playa lake and this lakes evaporates, resulting in the precipitation of salt such as halite, gypsum and anhydrite.

2 RESTRICTED BAYS OR SEAS: In areas where there is restricted input of fresh water or marine

water into a basin coupled with extensive evaporation within the basin, dissolved ion concentration may increase to a point where a dense concentration is formed within the surface

This dense saline water sinks within the basin and become oversaturated with salts like gypsum and halite

CONTD.

3 SHALLOW ARID COAST OR SABKHA Along shallow arid coastline where input of fresh water is rare and

evaporation increases the salinity of the marine water

This evaporation may increase the salinity of the water to a point where evaporite minerals like halite and gypsum are precipitated.

Observation from sabkha deposits show that this settings can form a substantial accumulations of sediment, covering broad area in geologically short period of time.

The total thickness of a sabkha deposit ranges from 30cm- 2m.

Depositional model for evaporite formationStanding water bodies

Fig 3:Evaporite faces patterns (a) in enclosed basins and (b) in restricted marginal-marine basins; (c) distribution of carbonate and sulphate in a modern Australian salina. (According to Warren and Kindall, 1985,

Fig 4: Depositional environment of evaporites.

Fig 5: Depositional environment of evaporites. (Kendull 1984)

CONTD

Fig 6: Evaporite deposits in Sabkha Environment

Depositional facies in the salinas of south western Australia. Source from J.K warren reprinted with permission from the American Association of petroleum Geologists Bulletin vol 69,p, 1017

CONTD

Fig 7 : Idealised sedimentary section of a sabkha deposit (adapted from shearman 1978), this section was drawn from a core, Warlingham borehole

CONTD.Based on this environment of deposition, we can deduce different types of

environment for evaporites;1. Graben areas within continental rift environments fed by limited riverine

drainage, usually in subtropical or tropical environments, example environments at the present which match this is the Denakil Depression, Ethiopia; Death Valley, California.

2. Graben environments in oceanic rift environments fed by limited oceanic input, leading to eventual isolation and evaporation examples include the Red Sea, and the Dead Sea in Jordan.

3. Internal drainage basins in arid to semi-arid temperate to tropical environments fed by ephemeral drainage, example environments at the present include the Simpson Desert, Western Australia, the Great Salt Lake in Utah.

CONTD.

4. Non-basin areas fed exclusively by groundwater seepage from artesian waters example environments include the seep-mounds of the Victoria Desert, fed by the Great Artesian Basin, Australia.

5. Restricted coastal plains in regressive sea environments, examples include the Sabkha deposits of Iran, Saudi Arabia and the Red Sea.

6. Drainage basins feeding into extremely arid environments, examples include the Chilean deserts, certain parts of the Sahara and the Namib desert

TYPES OF EVAPORITESThere are 2 major types of evaporite deposits namely:1. MARINE EVAPORITES They tend to have a thicker deposit and also have a system of

evaporation and are composed of minerals that are common in marine environments.

Marine Evaporites accumulate at a slower rates, (1.8m per thousand year, kirland et al 1999), as a result of lower ratio of surface area to water volume.

The most common ones are calcite, gypsum, anhydrite, halite, sylvite, carnalite, polyhalite, kainite and kieseite.

2. NON-MARINE EVAPORITES They are composed of mineral that are not common in marine

environments because the water from which non-marine evaporite precipitate have proportion of chemical elements different from those formed in the marine environments.

CONTD

Common non-marine evaporites include; blodite, borax, epsomite, gaylussite, glauberite, mirabilite, thenardite and trona, non-marine deposit may also contain halite, gypsum and anhydrite (thick non-marine deposits).

Examples of non-marine deposits are found in the Great Salt Lake and in the Dead Sea which lies between Jordan and Israel.

Major Groups of Evaporite Minerals

1. Chlorides- Halite, sylvite, carnalite, langbeinite, polyhalite, kainite.

2. Sulfates- anhydrite, gypsum, barite, and kieserite.

3. Carbonates- trona, calcite.

4. Nitrates - nitratite (soda niter) and niter.

5. Borates - typically found in arid-salt lake deposits plentiful in the south western US.

ECONOMIC IMPORTANCE OF EVAPORITE

• Evaporites are important economically because of their mineralogy, their physical properties in-situ, and their behaviour within the subsurface.

• Evaporite minerals, especially nitrate minerals, are economically important in Peru and Chile. Nitrate minerals are often mined for use in the production on fertilizer and explosives.

• Thick halite deposits are expected to become an important location for the disposal of nuclear waste because of their geologic stability, predictable engineering and physical behaviour, and imperviousness to groundwater.

• Halite formations are famous for their ability to form diapirs, which produce ideal locations for trapping petroleum deposits.

OTHER ECONOMIC POTENTIALS OF EVAPORITE

• Additives for food• Chief use in paper industry as fillers• To polished slabs for interior decoration • For sculpture • The making of plaster of paris for construction

Fig 8 : Distribution of Evaporite in North America showing the location of major salt and potash deposit

Sabhka vadose and phreatic zones of intertidal flats and desert plays an important role

1. Gypsum vs Anhydrite • gypsum is stable at surface temperatures and pressures

1. Halokenesis and formation of salt domes and other diapirs • form due to buoyancy contrasts between over and underlying sediments

CASE STUDY

• Facies and depositional Environment of the Holocene Evaporite in the Ras Shukeir area, Gulf of Suez, Egypt.

M.AM Aref, O.E.A Attia, A.M.A Wah.Introduction. This paper focus on the geology and facies of the holocene evaporite

deposits in the Ras Shukeir area, with the aim of reconstructing their depositional environment.

During holocene time, arid conditions prevailed over Egypt as evidenced by the occurrence of numerous evaporite deposits close to the present day shorelines, these evaporites overlie or enclosed within Quaternary gravelly hills.

CONTD

Fig 7: Geologic Map of Ras Shukeir Area

CONTD Geological Settings. The Ras Shukeir holocene evaporites are located on the western shoreline

of the Gulf of Suez, they are seperated from the sea by a 1km wide barrier ridge of sandy bioclastic limestones and mudstones of pleistocene age.

The studied evaporite rock crop out in 3 isolated hills seperated by small wadis, the evaporate rocks overlie uplifted beach terraces which contain rock fragment from surrounding quaternary gravels.

The evaporite sequence consists of gypsiferous marl at the bottom, that is overlain by laminated evaporites interbedded with cracked mudstone layers. The top is covered by unlithified gravels that have their source in the quaternary gravely hills.

CONTD• Facies and Depositional Environment: In this sense, the study of the

sedimentary units in the Ras Shukeir area revealed 2 lithofacies types.1. Clastic Lithofacies.2. Evaporites Lithofacies. EVAPORITES LITHOFACIES. Study of the syndepositional and diagenetic features of the evaporite

rocks in the Ras Shukeir area revealed the following facies.1. Regular Laminated Evaporite Facies. These facies occur at the bottom of the exposed evaporite hills, bed range

in thickness 40-65cm, in the eastern hill, a 40 cm thick bed is composed of 58 evaporite laminae with thickness of 0.2-0.5cm.

Enviroment of Deposition: the observed laminated structure, grain size and texture of the gypsum grains, as well as the structure of the algae micrite are typical of sedimentation in the subtidal lagoon environments.

CONTD

2. Chevron gypsum – algal micrite lamination facies. This facies is recorded in the central and western hills in 65-70cm thick bed. It

is characterized by thinly laminated, tightly to loosely packed cone shaped prismatic and chevron gypsum crystals that are commonly confined between thicker gypsum laminae.

Environment of Deposition: the characteristic vertical orientation of elongate and chevron gypsum crystals suggest subaqueous precitation gypsum from concentrated phases of shallow lagoonal waters.

3. Wavy algal laminated evaporites facies. This facies forms the top part of the sequence in the study area, in beds

ranging in thickness 20-40cm. It consist of regular alternation of anhydrite and lime mud laminae and the anhydrite bands vary in thickness.

CONTD Environment of Deposition: The laminated algal micrite - gypsum-

halite facies formed in a marginal marine salina that occupied a shallow depression on an arid supratidal flat.

4. Nodular and enterolithic anhydrite facies. This facies makes up the top of the section in the central and

western hills outcrop in beds ranging in thickness 10-15cm. It consists of white to dirty white, spherical to elliptical anhydrite

nodules set in a brownish, greenish mudstone matrix.

Environment of Deposition: Based on mineralogy and texture , the anhydrite nodules are of the sabhka of Arabian Gulf.

Fig 8 : Facies logs of the exposed evaporite rocks in the Ras Shukeir Area.

Fig9 :Depositional environment of the evaporite facies in the Ras Shukeir area.

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