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A RESEARCH CARRIED OUT

BY

AGBAJE TITUS MAYOWA

Email address: feelmayor@gmail.com

AT THE UNIVERSITY OF ILORIN, KWARA STATE, NIGERIA

EVAPORITE: MODE OF

FORMATION,

CHARACTERISTICS AND

ECONOMIC POTENTIAL

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TABLE OF CONTENT

INTRODUCTION………………………………………………………………………………………………3

CLIMATE RELATIONSHIP WITH EVAPORITE……………………………………………………..4

FORMATION OF EVAPORITE……………………………………………………………………………5

DEPOSITIONAL ENVIRONMENT OF EVAPORITE……………………………………………….6

EVAPORITE: GENERAL SCALE AND FACIE MODEL…………………………………………….9

MARINE AND NON MARINE EVAPORITE………………………………………………………….13

ECONOMIC POTENTIAL OF EVAPORITE……………………………………………………………16

REFERENCE …………………………………………………………………………………………………….17

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INTRODUCTION

Evaporites include a wide range of chemical precipitates that form at the Earth’s surface or in near

surface environments from brines concentrated by solar evaporation in restricted basins. They are Sea

water chemistry, mineral succession of isothermic evaporation of sea water. Evaporites are Lateral and

vertical distribution of facies driven by relative sea water level changes. Typical sedimentary

environment: sub aquatic, sub aerial -sabkhas, shelf margin -lagoon, salina, isolated basin, and playa.

Evaporites occur in rocks of most ages, including the Precambrian, but they are particularly common in

Cambrian, Permian, Jurassic and Miocene successions. We have marine deposits of evaporite and non

marine deposits of evaporite.

Table 1, the common marine and non marine evaporites

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CLIMATE RELATIONSHIP WITH EVAPORITE

As water evaporates and solutes become more concentrated, the rate of evaporation slows because of

the increasing density and surface tension of the brine. Because very saline waters are denser than fresh

water, a higher proportion of the infrared of incoming sunlight refracts back into the water and causes

the internal temperature of the water to rise as high as 35-55oc. The high temperature permits

evaporation to continue even in moderately humid regions. At high elevations (lower atmospheric

pressure) or in very windy areas, the evaporation process accelerates. Additionally, halophylic bacteria

commonly live in saline water (Colwell et al., 1979) coloring it pink or red. This causes the water

temperature to rise 3-6oc above similar waters without these bacteria.

In regions where relative humidity is above 65%, halite may form, but is preserved with difficulty

(Kinsman, 1976). When the humidity falls below 65%, halite forms and is preserved. Low relative

humidity throughout the year (below 35%) is needed to continue evaporation beyond halite, to form

and preserve potassic and magnesian salt precipitates. Such saline waters and the resulting salts are

strongly hygroscopic and diurnal change in humidity is commonly enough to dissolve most

potassium/magnesium salts form (carnallite, MgCl2.KCl.6H20 and sylvite, KCl being the most common).

However, polyhalite, which is a syngenetic mineral, is somewhat more stable

(2CaSO4.MgSO4.K2SO4.2H20; Pierre 1983; Peryt & Pierre, 1994). Settings arid enough to precipitate

Potassic and Magnesian salts are rare, occurring either at very high elevations and/ or within orographic

shadows developed in hot and arid regions. At least one study (Braitsch, 1964) suggests that in order for

carnallite (MgCl2.KCl.6H20) to precipitate, water temperatures must be considerably elevated, in the

range of 41-47oc.

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FORMATION OF EVAPORITES

Evaporate minerals may form where the rate of evaporation of a water body is greater than the total

water inflow, leaving a concentrated mineral residue. There are three critical controlling factors in

evaporative mineral formation and accumulation: initial ionic content (and ratios), temperature and

relative humidity.

Although 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 oversaturated. The crystallization of these salts ultimately leads to the formation of evaporites.

Fig 1, formation of evaporites

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DEPOSITIONAL ENVIRONMENT OF EVAPORITES

There are different depositional environments of evaporites. We have;

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 lake evaporates, resulting in the precipitation of salt such as halite, gypsum and

anhydrite.

Fig 2, Model for evaporite deposition in an intracratonic basin, where eustatic sea-level changes are a

major control. After Clark & Tallbacka (1980)

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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

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

setting can form a substantial accumulation of sediment, covering broad area in geologically short

period of time.

Fig 3, Depositional Environment of Evaporites

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Fig 4, Principal Depositional Environments of Evaporites

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EVAPORITES: GENERAL SCALE AND FACIES MODEL

Saltern and mudflat evaporites, along with slope and basin deposits, construct three interrelated

settings in either continental or marine-fed basins: continental evaporites, platform evaporites, and

basinwide evaporites.

Basinwide evaporites

Basinwide evaporites are thick, basin-filling units (often >50m thick) of deep water/shallow water

evaporite deposits containing textural evidence of many different depositional settings, including

mudflat, saltern, slope and basin. Basinwide evaporites have no modern in scale and diversity and are

often described as “saline giants”. They constitute the sedimentary fill in many ancient evaporite basins,

such as the Delaware Basin of west Texas, the Zechstein Basin of Europe and the Late Miocene

Messinian Subbasins in the Mediterranean.

Fig 5, location and age of some of major basinwide evaporite deposits (after Kendall 1992)

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Basinwide evaporites were deposited in three main settings (Fig. 6; Kendall 1992):

• Deep water-deep basin

• Shallow water-shallow basin

• Shallow water-deep basin

For the further run away of this contribution some short remarks only to the deep water-deep basin.

Deep water-deep basin evaporites have basins centers dominated by “deepwater” evaporites composed

mostly of finely laminated salts, where individual laminar grouping can be correlated over wide areas

Fig 6, Basinwide evaporite settings after Kendall 1992

Basin slopes are dominated by reworked Saltern evaporites deposited as slumps and turbidites.

Seawater flows into the basin as a perennial seepage; if the basin remains isolated, the evaporite unit

can fill the basin to a water level just below that of the supplying ocean. The basin fill starts off,

dominated by deepwater laminated and resedimented salts that pass up section into shallow-water

Salterns, mudflats, or continental playas.

A good example of deeper water deposition, occur in the basin-centre deposits of the north and south

Zech stein basins of NW Europe.

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FACIES MODELS

Deep water Evaporite Facies

In this environment, the brine is at or near saturation with respect to gypsum and/or halite. Crystal

growth probably occurs mainly at the air-water interface and crystals settle through the water column

as pelagic rain. Regular interlaminations of minerals of different solubilities (calcite and gypsum with or

without halite) reflect variations in brine influx, annual temperature or evaporation rate. Some calcium

sulphate may grow within the upper layers of the bottom sediment and some salt may be precipitated

during the mixing of brines in stratified water body.

Evaporite turbidites and mass-flow deposits derived from shallower water carbonate and evaporite

accumulations may also be emplaced within this environment. The depth of water in which “deep

water” evaporites accumulate is difficult to determine. However where turbidities (composed of basin-

marginal materials) occur at the basin centre, the centre to basin-margin distance combined with a

minimal 10 slope suggests a minimum depth. A minimum water depth can also be obtained by observing

the relation of basinal evaporites to topographic elevations.

Fig 7, Depositional setting and characteristics of deep water evaporites (after Schlager and Bolz 1977)

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Shallow water evaporite facies

Deposition of shallow water evaporites occurs in brines that were at or near saturation with respect to

gypsum or halite and in environment that may have been subjected to strong wave and current action

causing sediment scour, transport and redeposition.

Water depth may range from few centimeters to 20 meters. In fact many evaporites considered

Subaqueous may have been deposited on Evaporitic flats that only became flooded during storm surges

or particularly high tides. Evaporites precipitation may occur at the air water interface, at the sediment-

water surface or beneath the sediment surface and varying amounts of continental and marine-derived

sediments may be periodically transported into the Evaporitic environment.

Fig 8, Deposition of shallow water evaporite facie. Above after Hardie 1970 and below Vai et al 1977

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MARINE DEPOSIT OF EVAPORITE

The depositional record formed within a sabkha setting is defined by the presence of a sedimentary

profile developed adjacent to a water body in arid setting. The matrix may be carbonate, mixed or

siliclastic and can be emplaced by wind and/or water. The evaporites accumulate as part of a soil profile

in the upper phreatic and vadose zones. Gypsum, anhydrite and halite are the most common evaporite

minerals, although continental influx may cause other minerals to form.

Sabkha accumulations are commonly thin sequences (30 cm to 1-2 cm), with each cycle topped by

truncated wind or water –cut surfaces. Stacked, repeated sequences are not uncommon in the rock

record; however, very thick, massive, regular beds of nodular sulphate do not represent a sabkha

accumulation.

Fig 9, Cross section views of Abu Dhabi Sabkha, illustrating different stratigraphic presentations. (A) By

Butler et al 1982 and (B) by Kirkham 1997

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Fig 10, Sedimentary section of a Sabkha adapted from Shearman 1978

Whether the simple model (Shearman 1978) or the complex model (Kirkham 1997), when a single

sabkha cycle is complete, it is composed of three basic components:

• At the base, it has laminar, intertidal algal/bacteria mats with intercalated muds (commonly

containing displacive, lenticular gypsums).

• In the middle (the capillary zone if the soil profile), it contains displacive nodular anhydrite or

gypsum with coalescent sulphate nodules and enterolithic sulphate layers in a matrix composed

of carbonate, siliclastic or mixed sands and muds.

• At the top, there is a truncation surface cut either by deflation or storm action.

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NON MARINE DEPOSIT OF EVAPORITE

In most non marine accumulations (saline pans and associated mudflats), the same morphologies

develop as in marine settings, but they may have a different range of mineralogy. Non marine

evaporative environments can become very concentrated but the chlorinity may be relatively low. For

this reason, certain burrowing organisms can live in some of these lakes bottoms. Another unexpected

aspect of at least one nonmarine setting is the interfingering of coal with evaporites. Nury and Schreiber

1997 documented that the fresh end of an elongate graben-filling lake has a brackish-water marsh

(terrestrial marsh plants, now coal) which passes laterally into micritic carbonates and then into gypsum

beds with no erosion surfaces.

Therefore, evaporites apparently can accumulate contemporaneously with considerable organic matter

(that becomes coal) in a single water body, along a salinity gradient in a shallow lake.

Fig 11, Idealized section across a salt lake with associated mudflat by Salvany 1997

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ECONOMIC POTENTIAL OF EVAPORITE

• Evaporites are economically useful in the petroleum system because of their ductility, thus

suitable as cap rock. Other economic importance are linked to cement production, preservation

and seasoning of food, drying agents and in the production of sulfur from sulfuric acid.

• 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 behavior, and

imperviousness to groundwater.

• Halite formations are famous for their ability to form diapirs which produce ideal locations for

trapping petroleum deposits.

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REFERENCES

Braitsch, O., (1971). Salt deposits, their origin and composition. Springer, Berlin.

Butler, G.p., Harris, P.M., Kendall, C.G., and St. C. (1982). Recent evaporites from the Abu Dhabi coastal

flats. In: Depositional and Diagenetic Spectra of Evaporites, SEPM Core Workshop, 3, 33-64.

Colwell, R.R., Litchfield, C.D., Vreeland, R.H., Kiefer, L.A., and Gibbons, N.E. (1979). Taxonomic studies

of red halophillic bacteria. Int. J. Sys. Bacter., 29, 379-399.

Hardie, L.A., and Eugster, H.P., (1971). The depositional environment of marine evaporites: a case for

shallow clastic accumulation. Sedimentology, Vol. 16.pp. 187-220.

Kendall A. C. (1992). Evaporites, Facies Models: Responses to sealevel change. Geological Association of

Canada: 375-409.

Kinsman, D.J.J., (1976). Evaporites: relative humidity control of primary mineral facies. J. Sedim. Petrol.,

46, 273-279.

Kirkham, A., (1997). Shoreline evolution, Aeolian deflation and anhydrite distribution of the Holocene,

Abu Dhabi. Geoarabia, 2, 403-416.

Pierre, C., (1983). Polyhalite replacement after gypsum at Ojo de Liebre Lagoon (Baja Californial,

Mexico): an early diagenesis by mixing of marine brines and continental waters. In: Sixth International

Symposium on Salt (Ed. By B.C. Schreiber and L.Harner, Vol. 1, pp. 257-265. Salt Institute , Alexandria,

VA.

Salvany, J.M., (1997). Continental Evaporitic sedimentation in Navarra during the Oligocene to Lower

Miocene. (Ed by G. Busson and B.C Schreiber), pp. 397-411. Columbia University press, New York.

Schlager W., Bolz H. (1977). Clastic accumulation of sulphate evaporites in deep water. Journal of

Sedimentary Geology 47: 600-609.

Sheraman, D.J., (1978). Halite in sabkha environments. In: Marine Evaporites (Ed. By W.E. Dean and B.C.

Schreiber), SEPM Short Course, 4, 30-42.

Tucker M. E. (1991). Sequence stratigraphy of carbonate-evaporite basins; models and application to

the Upper Permian (Zechstein) of Northeast England and adjoining North Sea. Journal of the Geological

Society of London 148: 1019-1036.

Vai, G.B., and Ricchi Lucchi, F., (1977). Algal crusts autochthonous and clastic gypsum in a cannibalistic

evaporite basin, a case history from the Messinian of Northern Appenines. Sedimentology, Vol.

24.pp211-244.