Walker J. Facies models, Canada, 1992

Подождите немного. Документ загружается.

CONIGLIO,

DIX

Hallam, A.,

1986,

Origin of minor limestone-

shale cycles: climatically induced or dia-

genetic: Geology, v.

14,

609-612.

Hallock,

and Schlager, W.,

1986,

Nutrient excess and the demise of coral

reefs and carbonate platforms: Palaios,

389-398.

Harris, P.T., Davies, P.J. and Marshall,

J.F.,

1990,

Late Quaternary sedimenta-

tion on the Great Barrier Reef conti-

nental shelf and slope east of

Townsville, Australia: Marine Geology,

94,

55-78.

Harwood, G.M. and Towers, P.A.,

1988,

Seismic sedimentologic interpretation of

a carbonate slope, north margin of Little

Bahama Bank,

Austin, J.A., Jr.,

Schlager, W.,

a/.,

Proceedings, ODP,

Scientific Results, Leg

101,

College

Station, Texas, p.

263-278.

Heath, K.C. and Mullins, H.T.,

1984,

Open-

ocean, off-bank transport of fine-grained

carbonate sediment in the northern

Bahamas,

Stow, D.A.V. and Piper,

D.J.W., eds., Fine-grained sediments:

deep water processes and facies:

London, Blackwell Scientific Publi-

cations, p.

199-208.

Hine, A.C., Locker, S.D., Hallock, P. and

Mullins, H.T.,

1991,

Strongly contrasting

modes of slope failure and erosion

along the carbonate margins of three

open seaways; Northwest Nicaraguan

Rise: American Association of Petro-

leum Geologists, Bulletin, v.

75,

595-

596

(abstract).

Hiscott, R.N. and James, N.P.,

1985,

Carbonate debris flows, Cow Head

Group, western Newfoundland: Journal

of Sedimentary Petrology, v.

55,

735-

745.

Hubert, J.F., Suchecki, R.K. and Callahan,

R.K.M.,

1977,

The Cow Head Breccia:

sedimentology of the Cambro-Ordovician

continental margin, Newfoundland,

Cook, H.E. and Enos, P., eds., Deep-

water carbonate environments: Society

of Economic Paleontologists and

Mineralogists, Special Publication

25,

125-1 54.

James, N.P. and Ginsburg, R.N.,

1979,

The

seaward margin of Belize barrier and

atoll reefs: International Association of

Sedimentologists, Special Publication

191

James, N.P. and Mountjoy, E.W.,

1983,

Shelf-slope break in fossil carbonate

platforms

an overview,

Stanley,

D.J. and Moore, G.T., eds., The

shelf-

break critical interface on continental

margins: Society of Economic Paleon-

tologists and Mineralogists, Special

Publication

33,

189-206.

James, N.P. and Stevens, R.K.,

1986,

Stratigraphy and correlation of the

Cambro-Ordovician Cow Head Group,

western Newfoundland: Geological

Survey of Canada, Bulletin

366, 143

Jansa, L.F.,

1981,

Mesozoic carbonate plat-

forms and banks of the eastern North

American margin: Marine Geology, v.

44,

97-1 17.

Jenkyns, H.C.,

1986,

Pelagic environments,

Reading, H.G., ed., Sedimentary envi-

ronments and facies: London, Blackwell

Scientific Publications, p.

343-397.

Kendall, C.G.St.C. and Schlager, W.,

1981,

Carbonates and relative changes in sea

level: Marine Geology, v.

44,

181 -21 2.

Kenter, J.A.M.,

1990,

Carbonate platform

flanks: slope angle and sediment fabric:

Sedimentology, v.

37,

777-794.

Krause, F.F. and Oldershaw, A.E.,

1979,

Submarine carbonate breccia beds

depositional model for two-layer, sedi-

ment gravity flows from the Sekwi

Formation, (Lower Cambrian),

Macken-

zie Mountains, Northwest Territories,

Canada: Canadian Journal of Earth

Sciences, v.

16,

189-199.

Land L.S.,

1979,

The fate of reef-derived

sediment on the north Jamaican island

slope: Marine Geology, v.

29,

55-71.

Land L.S. and Moore, C.H., Jr.,

1976,

Deep

forereef and upper island slopes, north

Jamaica,

Frost, S.H., Weiss, M.P.

and Saunders, J.B., eds., Reefs and

related carbonates

ecology and sedi-

mentology: American Association of

Petroleum Geologists, Studies in

Geology

53-67.

Ludvigsen, R.,

1989,

The Burgess Shale:

not in the shadow of the Cathedral

Escarpment: Geoscience Canada, v.

16,

51 -59.

Masetti, D., Neri, C. and Bosellini, A.,

1991,

Deep-water asymmetric cycles and

progradation of carbonate platforms

governed by high-frequency eustatic os-

cillations (Triassic of the Dolomites,

Italy): Geology, v.

19,

336-339.

~c~rossan, R.G.,

1958,

Sedimentary

boudinage structure in the Devonian

lreton Formation of Alberta: Journal of

Sedimentary Petrology, v.

28,

316-

320.

Mcllreath, I.A.,

1977,

Accumulation of a

Middle Cambrian, deep-water limestone

debris apron adjacent to a vertical sub-

marine carbonate escarpment, southern

Rocky Mountains, Canada,

Cook,

H.E. and Enos, P., eds., Deep-water

carbonate environments: Society of

Economic Paleontologists and Mineralo-

gists, Special Publication

25,

113-

124.

Mcllreath, I.A. and James, N.P.,

1979,

Carbonate slopes,

Walker, R.G., ed.,

Facies Models: Geological Association

of Canada, Geoscience Canada Reprint

Series

133-149.

Middleton, G.V. and Hampton, M.A.,

1976,

Subaqueous sediment transport and de-

position by sediment gravity flows,

Stanley, D.J. and Swift, D.J.P., eds.,

Marine sediment transport and environ-

mental management: New York, John

Wiley and Sons, p.

197-21 7.

Mitchum, R.M.,

1978,

Seismic stratigraphic

investigation of west Florida slope, Gulf of

Mexico: American Association of Petro-

leum Geologists, Studies in Geology

193-223.

Morse, J.W. and Mackenzie, F.T.,

1990,

Geochemistry of sedimentary carbon-

ates: Developments in Sedimentology

48,

Amsterdam, Elsevier,

707

Mullins, H.T.,

1983,

Modern carbonate

slopes and basins of the Bahamas,

Cook, H.E., Hine, A.C. and Mullins,

H.T., eds., Platform margin and deep

water carbonates: Society of Economic

Paleontologists and Mineralogists, Short

Course

12,

4-1

4-138.

Mullins, H.T., Gardulski, A.F., Hinchey, E.J.

and Hine, A.C.,

1988b,

The modern car-

bonate ramp slope of central west

Florida: Journal of Sedimentary Petro-

logy,

58,

273490.

Mullins, H.T., Gardulski, A.F. and Hine,

A.C.,

1986,

Catastrophic collapse of the

west Florida carbonate platform margin:

Geology, v.

14,

167-170.

Mullins, H.T., Gardulski, A.F., Hine, A.C.,

Mellillo, A.J., Wise, S.W., Jr. and

Applegate, J.,

1988a,

Three-dimen-

sional sedimentary framework of the

carbonate ramp slopes of central west

Florida: A sequential seismic strati-

graphic perspective: Geological Society

of America, Bulletin, v.

100,

514-533.

Mullins, H.T., Heath, K.C., Van Buren, H.M.

and Newton, C.R.,

1984,

Anatomy of a

modern open-ocean carbonate slope:

northern Bahama Bank: Sedimentology,

31, p.141-168.

Mullins, H.T. and Neumann, A.C.,

1979,

Deep carbonate bank margin structure

and sedimentation in the northern

Bahamas,

Doyle, L.J. and Pilkey,

O.H., eds., Geology of continental

slopes: Society of Economic Paleon-

tologists and Mineralogists, Special

Publication

27,

165-1 92.

Mullins, H.T., Neumann, A,, Wilber, R.J.

and Boardman, M.R.,

1980b,

Nodular

carbonate sediment on Bahamian

slopes: Possible precursors to nodular

limestones: Journal of Sedimentary

Petrology, v.

50,

11 7-1 31.

-. .

- -

18.

CARBONATE SLOPES

373

Mullins, H.T., Neumann, A., Wilber, R.J.,

Hine, A.C. and Chinburg, S.J.,

1980a,

Carbonate sediment drifts in northern

Straits of Florida: American Association

of Petroleum Geologists, Bulletin,

64,

p. 1701-1717.

Mutti, E. and Ricci Lucchi, F., 1978,

Turbidites of the northern Apennines:

Introduction to facies analysis: Inter-

national Geology Review, v. 20, p.

125-

166.

Nardin, T.R., Hein, F.J., Gorsline, D.S. and

Edwards, B.D., 1979, A review of mass

movement processes, sediment and

acoustic characteristics, and contrasts

in slope and base-of-slope systems

versus canyon-fan-basin floor systems,

Doyle, L.J. and Pilkey, O.H., eds.,

Geology of continental slopes: Society

of Economic Paleontologists and

Mineralogists, Special Publication 27,

p. 61 -73.

ODP Leg 133 Scientific Party, 1991, A

chronicle of carbonate platform develop-

ment: Geotimes, April 1991, p. 22-26.

Paull, C.K., Freeman-Lynde,

R., Bralower,

T.J., Gardemal, J.M., Neumann, A.C.,

D'Argenio, B. and Marsella, E., 1990,

Geology of the strata exposed on the

Florida Escarpment: Marine Geology, v.

91,

p. 177-1 94.

Perry, R.K., compiler, 1984, Bathymetry of

the Gulf of Mexico and the Caribbean

Sea: American Association of Petrol-

eum Geologists, Scale 1 :3, 289, 263.

Pickering,

K.f., Hiscott, R.N. and Hein,

F.J., 1989, Deep marine environments:

London, Unwin Hyman, 416 p.

Pomar, L., 1991, Reef geometries, erosion

surfaces and high-frequency sea level

changes, upper Miocene reef complex,

Mallorca, Spain: Sedimentology, v. 38,

p. 243-270.

Read, J.F., 1985, Carbonate platform

facies models: American Association of

Petroleum Geologists, Bulletin, v. 69,

p. 1-21.

Ricken, W., 1986, Diagenetic bedding, a

model for marl-limestone alternations:

New York, Springer-Verlag, 210 p.

Rudolph, K.W. and Lehmann, P.J., 1989,

Platform evolution and sequence

stratigraphy of the Naturna platform,

South China Sea,

Crevello, P.D.,

Wilson, J.L., Sarg, J.F. and Read, J.F.,

eds., Controls on carbonate platform

and basin development: Society of

Economic Paleontologists and Mineral-

ogists, Special Publication 44, p.

353-

361.

Ruiz-Ortiz, P.A., 1983, A carbonate subma-

rine fan in a fault controlled basin of the

Upper Jurassic,

Betic Cordillera, south-

ern Spain: Sedimentology, v. 30, p.

33-

48.

Sarg, J.F., 1988, Carbonate sequence

stratigraphy,

Wilgus, C.K, etal., eds.,

Sea level changes: an integrated ap-

proach: Society of Economic Paleon-

tologists and Mineralogists, Special

Publication 42, p. 155-1 81.

Schlager, W., 1981, The paradox of

drowned reefs and carbonate platforms:

Geological Society of America, Bulletin,

v. 92, p. 197-21 1.

Schlager, W., 1991, Depositional bias and

environmental change

important

factors in sequence stratigraphy: Sed-

imentary Geology, v. 70, p. 109-130.

Schlager, W. and James, N.P., 1978,

Low-

magnesian calcite limestones forming at

the deep-sea floor, Tongue of the

Ocean, Bahamas: Sedimentology,

v. 25, p. 675-702.

Schlager, W. and Chermak, A., 1979,

Sediment facies of platform-basin tran-

sition, Tongue of the Ocean, Bahamas,

Doyle, L.J. and Pilkey, O.H., eds.,

Geology of continental slopes: Society

of Economic Paleontologists and

Mineralogists, Special Publication 27,

p. 193-208.

Schlager, W. and Ginsburg, R.N., 1981,

Bahama carbonate platforms

The

deep and the past: Marine Geology,

44, p. 1-24.

Schlager, W. and Camber,

O., 1986,

Submarine slope angles, drowning

un-

conformities, and self-erosion of lime-

stone escarpments: Geology, v. 14,

p. 762-765.

Scholle, P.A., Arthur, M.A. and Ekdale,

A.A.,

198313, Pelagic environment,

Scholle, P.A., Bebout, D.G. and Moore,

C.H., eds., Carbonate depositional envi-

ronments: American Association of

Petroleum Geologists, Memoir 33,

p. 620-691.

Stoakes, F.A., 1980, Nature and control of

shale basin fill and its effect on reef

growth and termination: Upper Devo-

nian Duvernay and lreton formations of

Alberta, Canada: Bulletin of Canadian

Petroleum Geology, v. 28, p. 345-410.

Stow, D.A.V. and Shanmugam, G., 1980,

Sequence of structures in fine-grained

turbidites: comparison of recent deep-

sea and ancient flysch sediments:

Sedimentary Geology,

25, p. 23-42.

Twichell, D.C., Parson, L.M and Paull, C.K.,

was 1990, Variations in the styles of

erosion along the Florida escarpment,

eastern Gulf of Mexico: Marine and

Petroleum Geology, v. 7, p. 253-266.

Walker, R.G., 1979, Turbidites and associ-

ated coarse clastic deposits,

Walker,

R.G., ed., Facies Models: Geological

Association of Canada, Geoscience

Canada Reprint Series 1, p. 171 -1 88.

Watts, K.F. and Garrison, R.E., 1986,

Sumeini Group, Oman-evolution of a

Mesozoic carbonate slope on a south

Tethyan continental margin: Sedimentary

Geology, v. 48, p.

107-168.

Watts, K.F. and Blome, C.D., 1990,

Evolution of the Arabian carbonate plat-

form margin slope and its response to

orogenic closing of a Cretaceous ocean

basin, Oman,

Tucker, M.E., Wilson,

J.L., Crevello, P.D., Sarg, J.R. and

Read, J.F., eds., Carbonate Platforms

facies sequences and evolution:

International Association of

Sedi-

mentologists, Special Publication 9,

p. 291 -323.

Wilkinson, B.H., 1979, Biomineralization,

paleoceanography, and the evolution of

calcareous marine organisms: Geology,

7, p. 524-527.

Wilson, J.L., 1975, Carbonate facies in ge-

ologic history: New York,

Springer-

Verlag, 471 p.

Wright, V.P. and Wilson, R.C.L., 1984, A

carbonate submarine-fan sequence

from the Jurassic of Portugal: Journal of

Sedimentary Petrology, v. 54, p.

394-

412.

Yose, L.A., 1991, Part 2: Sequence stratig-

raphy of mixed

carbonate/volcani-

clastic slope deposits flanking the

Sciliar

(Schlern)-Catinaccio Buildup,

Dolomites, Italy,

Bradner, R., Fliigel,

E., Koch, R. and Yose, L.A., The

northern margin of the

Schlern/Sciliar-

RosengartenlCatinaccio Platform,

Guide-book Excursion A: Dolomieu

Conference on Carbonate Platforms and

Dolomitization, p. 17-39.

19.

Evaporites

Alan C. Kendall, School of Environmental Sciences, University of East Anglia,

Notwich NR3

7TJ

England

INTRODUCTION

Evaporites are chemical sediments

precipitated from brines. They may

form anywhere there is a substantial

deficit in the water budget due to

evaporation. The feedstock for most

large evaporite deposits is believed to

have been seawater, but groundwa-

ter~ are the sources for many evapor-

ite~ in continental settings. Carbonates

are commonly interlaminated with the

more soluble materials and should

also be considered, when they are

precipitated by similar processes, as

evaporites.

A diverse group of minerals occur in

evaporites, but most are rare to minor

components. The more important min-

erals are shown in Table 1. The diver-

sity of marine-derived evaporite min-

erals is more limited. Dissolved consti-

tuents in modern seawater are domi-

nated by sodium and chloride, and only

12 components have concentrations

greater than 1 ppm. The proportions of

major ions in seawater are constant

and apparently have been since late

Precambrian times. This constancy im-

plies that seawater evaporation fol-

lowed similar pathways in the past.

Evaporites are formed in a wide

range of depositional settings, from

soils to lacustrine environments on

continents, and from the supratidal to

the deep subaqueous realm in the

marine setting. Evaporite minerals

may crystallize in these environments

at the

brinelair interface or, less com-

monly, within the brine itself, on the

floor of a brine pool, in brine-soaked

sediments as displacive or incorpora-

tive crystals, or as efflorescent crusts

in the capillary fringe of subaerially

exposed settings. The crusts have low

preservation potential. Consequently,

evaporite deposits can be described

as being

1) subaqueous accumula-

tions of surface-nucleated

crystals,

crystal cumulates,

2) subaqueous

bottom precipitates, or

crusts,

3) dia-

genetically

emplaced

intrasediment

precipitates,

which may replace, dis-

place or incorporate nonevaporite or

previously formed evaporite host sedi-

ments, or

clastic accumulations

evaporite particles. After deposition,

evaporites are subject to many early

diagenetic processes that 1) alter the

original mineralogy and sediment

fabric to varying degrees (including

complete obliteration of original char-

acteristics), or 2) lead to complete

removal by dissolution. Thus many

evaporites are not strictly primary pre-

cipitates: they are diagenetic minerals

emplaced within other sediments, or

are diagenetic replacements of true

primary precipitates.

In this chapter the overall controls on

evaporite precipitation are discussed; a

new classification of evaporite facies

emphasizing sedimentary features

rather than geographic settings is intro-

duced; and facies models for conti-

nental, basin marginal and basin

Table

Common evaporate minerals. From Hardie

(1984).

anhydrite CaSO,

leonhardtite MgS04.4H,0

aphthitalite (glaserite)

K,SO,.(Na,K)SO,

leonite MgSO4.K2S0,4H2O

antarcticite

CaCI2.6H,O

loewite 2MgSO4~2Na,SO4~5H,O

aragonite CaCO,

magnesium calcite (Mg,Ca,,)CO,

bassanite

CaS04.1/,H,0

mirabilite Na2S04.1 OH,O

bischofite

MgCI,.6H20

nahcolite NaHCO,

bloedite (astrakhanite)

Na,SO4.MgS0,.4H2O

natron Na,CO,.l OH,O

burkeite

Na2C0,.2Na,CO4

pentahydrite MgS0,.5H20

calcite CaCO,

pirssonite CaC03.Na,C0,.2H,0

carnallite MgCI,.KCI.6H20

polyhalite 2CaS04.MgS04.K,S0,.2H,0

dolomite

CaCO,.MgCO,

rinneite FeCI2.NaCI.3KCI

epsomite

MgS04.7H,0

sanderite MgS04.2H,0

gaylussite

CaC03.Na,C0,.5H,0

schoenite (picromerite) MgS04~K,S04~6H,0

glauberite

CaSO,.Na,SO,

shortite 2CaC0,. Na,CO,

gyps'"-''

CaS04.2H,0

sylvite KC I

halite

NaCl

syngenite CaSO,.K,SO,~H,O

hanksite

9Na$0,.2Na,C03.KCI

tachyhydrite CaCl2.2MgC1,.1 2H20

hexahydrite

MgSO,.GH,O

thenardite Na,SO,

kainite MgSO,.KCI.l '/,H20

thermonatrite Na2C03.H,0

kieserite

MgSO,.H,O

trona NaHC03.Na2C03.2H,0

langbeinite

2MgS04.bS04 van'thoffite MgS0,.3Na,S04

KENDALL

central evaporites are presented.

CONTROLS ON EVAPORITE

PRECIPITATION

Conditions necessary for the formation

and preservation of substantial evap-

orite deposits are stringent, thus the

major controls of evaporite deposition

need to be included in any treatment

of dynamic facies models. The type of

evaporite produced is a product of the

degree of brine concentration, the

brine composition, and the hydrology

and climate of the depositional site.

Brine concentration

The order in which evaporite minerals

precipitate is controlled by their rela-

tive solubilities, the least soluble min-

erals precipitating first. Calcium

carbonate begins to precipitate from

seawater concentrated

1.8

times;

gypsum begins precipitating at

3.8

times seawater concentration, and

halite only when the brine reaches a

concentration

10.6

times that of sea-

water

(McCaffrey et a/.,

1987).

Mag-

nesium sulphate first appears at brine

concentrations of about

times sea-

water, whereas a

times concentra-

tion is needed to precipitate potassium-

bearing phases. The composition of an

ancient marine-derived evaporite

deposit is therefore, in part, a measure

of the brine concentration reached at

the depositional site. This is partially

controlled by climatic factors.

brine and air are at the same tem-

perature, water can only be evapo-

rated when its vapour pressure ex-

ceeds the partial pressure of water in

the atmosphere

its relative humidity;

Kinsman,

1976a).

The vapour pres-

sure of a brine varies with the chem-

ical activity of water in that brine, itself

a function of the brine's ionic strength

or salinity. The mineralogy of evapor-

ite~ is therefore strongly controlled by

climatic variables. Low humidities are

required to evaporate brines to the

concentrations needed to precipitate

minerals of high solubility,

e.g., if the

relative humidity of air over an evap-

orite basin exceeds

76%,

brines can-

not be concentrated by evaporation to

halite saturation. Over oceans, mean

relative humidities are near

loo%,

whereas at low-latitude coasts, hu-

midities fall to

70-80%.

Humidities low

enough to allow primary potassium

and magnesium salts to precipitate

(below

67%)

are confined to regions

distant from oceans. These salts will

not occur in marginal marine evaporitic

environments. Today, the only signifi-

cant region of potash salt precipitation

is in the Qaida'm Basin of western

China, a region

1500

km from the near-

est ocean, in an orographic desert.

Brine composition

Compositions of continental waters

are more variable than seawater, so

that a greater diversity of evaporite

minerals form when they are evapo-

rated. As each mineral phase is precip-

itated during evaporation, it removes

components from solution, so modify-

ing the remaining brine composition.

Evaporation pathways of brines are

largely determined by the initial

Ca:[HCO,

CO,] ratio, and the

Ca:SO, ratio (Eugster and Hardie,

1978).

The first ratio determines

whether the remaining brine will

become alkaline and depleted in

calcium, or will become neutral and de-

pleted in carbonate species. The

second ratio, at the stage of gypsum

saturation, determines whether a brine

will become depleted in calcium or

sul-

phate. Waters with Ca

CO,

HCO,,

upon evaporation precipitate alkaline

carbonates, whereas those with Ca

CO,

HCO, may evolve into brines

that precipitate sodium sulphates.

Mineralogical, geochemical and

Figure

Comparison of evaporite compositions expected from seawater evaporation

(I),

compared with average compositions of

(2)

Zechstein evaporites,

(3)

"average" evaporites

(dominantly from cratonic basins;

riftduvenile oceanic evaporites would be more halite domi-

nated), and

(4) anhydrite-dominated sequences. Data modified from Borchert and Muir

(1964).

19. EVAPORITES

sedimentological criteria for discrimi-

nating between marine and nonmarine

evaporites have been proposed by

Hardie (1984) and Smoot and

Lowen-

stein (1991). Most controversial are

Hardie's (1990) suggestions that the

commonest potash salts were precipi-

tated from hydrothermal brines or

from evaporated seawater modified by

mixing with these brines. Salts com-

posed of

halite-sylvite-carnallite

tachyhydrite

bischofite (the mag-

nesium

sulphate-poor evaporites)

cannot be derived from seawater evap-

oration. Mahy of these evaporites

occur in rift and transtensional

strike-

slip basins, locations where there are

upwelling hydrothermal CaCI, brines,

rich in

Na+,

K+,

and Ca+ but poor in

Mg+ and SO;'-. These brines, driven

upward thermally, may enter closed or

partially closed basins, there to mix

with surface waters. Evaporation of

these mixed waters will precipitate

magnesium

sulphat,e-poor evaporites.

Magnesium sulphate-poor evapor-

ite~ also occur in cratonic sag basins

where rising deep-seated hydrothermal

brines are not expected. However, the

hydraulic head, produced during the

desiccation of deep evaporite basins,

may cause underlying groundwaters to

migrate upward and enter the basin

(Kendall, 1989). In this way

pre-ex-

isting CaCI, brines or newly formed

CaCI, waters, generated during

dolomitization of limestones, enter the

basin and modify surface waters in the

same manner as Hardie proposed.

Basin

hydrology

All models of evaporite deposition

involve a consideration of basin hy-

drology. Evaporite depositional sys-

tems are dynamic and open, and must

be considered in terms of inter-related

water and salt budgets (Logan, 1987).

lnflow of water and salts

Large volumes of seawater are re-

quired to form evaporite deposits. A

single batch of seawater, even in a

deep basin, cannot produce any signif-

icant deposit.

Basin restriction

Although large volumes of water are

required, the depositional site must not

be too open or brines will constantly

be diluted; evaporation is unable to

prevent dilution from unrestricted

surface-water inflow. Evaporite basins

must therefore be restricted or isolated

from the sea. The ratio between the

surface area of the basin and the

cross sectional area of any inlet must

EVAPORATION RATE

HIGH

LOW

...

Long

Residence Time

Moderate

Long

Residence

Time

Low

Brine

Level

High

Brine

Level

Salinity

High

Salinity

Moderate

(to

High

I-

Figure

2 Models of isolated evaporite basins. Evaporative drawdown occurs in response

to low influx rates (relative to evaporative losses). Deep water evaporites only form in basins

with low rates of

reflux and evaporation; all others experience significant desiccation.

However, with low rates of evaporation, long time periods are necessary to concentrate the

entire brine volume in the basin to the point where evaporites accumulate.

be at least >106:1 for gypsum to form,

and for halite, the inlet must be at least

eight orders of magnitude smaller than

the basin area (Lucia, 1972).

All marine-derived evaporite se-

quences are deficient in the more solu-

ble phases (Fig. 1). Thus the brine

components forming the missing

more-

soluble phases must have been ex-

ported from the basin. The size of any

inlet to a halite-precipitating basin,

however, is so small that there must, in

reality, be complete surface disconnec-

tion of the basin from the open ocean

(Lucia, 1972). In such isolated basins

there is no opportunity for surface

reflux

of remaining brines back to the sea.

Short

Residence

Time

Low

Brine

Level

Salinity

Low

lnflow rates into isolated basins

Disconnected but marine-fed basins

must gain their water by seepage

through permeable barriers, and may

receive a significant proportion of their

water from continental sources. But

seepage through barriers is unable to

offset evaporative losses in the basin.

The loss of brine volume, and a con-

sequent lowering of the basin brine

level is termed evaporative

drawdown

(Maiklem, 1971). When evaporation

losses exceed inflow, nothing can

prevent basins from desiccating.

Preservation of a perennial brine in

basins (required for "deep-watern

evaporites) necessitates

1) episodic

seawater input into the basin over the

barrier, 2) a reduction in the evapora-

tion rate, or

additional seepage

inflow from the basin periphery or from

the basin floor.

AAA

--HIGH

Short

Residence

Time

Low

Brine

Level

Salinity

Low

Brine reflux in isolated basins

Loss of the more soluble components

in isolated basins must be by

reflux

through the basin floor. It is unreason-

able, however, to expect that signifi-

cant brine

reflux can occur over the

main part of an evaporite basin floor. If

it were permeable enough to allow

wholesale brine export, then this same

permeability should also allow upward

seepage of groundwater. Brines in the

basin thus would be constantly diluted

by this upward seepage, and brines of

any significant concentration could not

be generated. As fast as they form,

they would sink away through the per-

meable basin floor, or be diluted by

inflow. Thus a major prerequisite for

evaporite deposition in basins is the

presence of a basal aquitard, one

cap-

LOW

\-HIGH

378 KENDALL

able of confining brines within basins

(Logan, 1987). Such a basal seal in

large basins is unlikely to be every-

where complete. Porous parts are

potential sites for brine

reflux or for

upward seepage of groundwaters into

the basin.

Inflow-outflow ratios in basins

Surface brines can only occur where

losses from a basin (evaporation

brine reflux) are more or less balanced

by inflow through the barrier

basin-

floor seepage inflow, rainfall and addi-

tions of continental surface waters). If

inflow exceeds water losses, the basin

fills and becomes more dilute: if losses

exceed inflow, the basin desiccates.

The balance between losses and

inputs also determines the salinity of

basin brines (Fig.

2).

Brine salinity is

partially a function of the residence

time of water in the basin (Logan,

1987). With low

reflux rates, residence

times are long, and evaporation may

be able to concentrate brines to high

concentrations. When air humidities

are high, however, brines cannot be

concentrated beyond a certain point.

When residence times are short (high

reflux rate), brines spend insufficient

time in the basin to be concentrated to

high salinities, before being exported

from the basin. Regardless of the

aridity of the climate, only evaporite

minerals of relatively low solubility

(carbonates, gypsum) are precipitated.

Neither influx nor

reflux rates will be

stable during a basin's history. Sea

level changes increase or decrease

the hydraulic head between ocean and

the basin brine level, so increasing or

decreasing the amount of seepage

inflow. Sediment aggradation causes

brine levels to rise, reducing hydraulic

head and rates of seepage influx.

Sediment aggradation is also associ-

ated with lateral overstep of basin

flanks, expanding the area for pos-

sible brine

reflux. Sites of influx and/or

reflux may also become choked by

precipitates or sediment, progres-

sively reducing or stopping flow.

Variations in the inflow-reflux ratio

cause changes in the volume of

surface brine retained within a basin

and the evaporite mineral being pre-

cipitated. Such changes may, how-

ever, occur without any change in

climate.

EVAPORITE

FAClES

Today, the relatively small area of con-

tinental crust with shallow seas limits

evaporite deposition to continental

lake basins and arid shorelines. This

CONTINENTAL EVAPORITES

SAUNE

PAN

ERENNAL SALINE LAKE

COASTAL EVAPORITES

SABKHA

COASTAL SALINA

barrim

tsdh

nudnlr

SAUNA (EPHEMERAL)

EVAPORITE FACIES ASSOaATED

FAaES

UmIDm

Blra-pan

anpailk

tat

MudUat

Rmdd

sh.Yov-ubaqwout

...

.;.

...

, ,

SandUat

Cj:',

Olgl&fn,

Iln rmdfiat .nvlmnrmnt.l

'm-mtr'

iLunimr).

SALINA

(ERENNAU

Figure

Modern evaporite depositional environments; diagram courtesy of

C.R.

Handford. Evaporites form in non-marine interior basins

with playa lakes, salt pans and mud flats (continental sabkhas), coastal supratidal mud flats (marine sabkhas) and marine-fed, coastal

lagoons and salt pans (salinas).

has not been the case in the past.

Evaporite sequences are generated

in a variety of modern geographic set-

tings (Fig.

4). They grade into each

other, are affected by similar deposi-

tional processes, and can produce

similar facies. Identification of the geo-

graphic setting of ancient evaporites

thus may depend more upon associ-

ated facies than upon 'internal charac-

teristics.

The absence of Recent deep

basinal and widespread

shallow-

marine evaporitic settings (Fig.

means that ancient evaporites of this

type are identified by comparing their

facies with those of

ephemeral and

permanent playa lakes and with small

marine-fed salinas. In the absence of

Recent analogs, models explaining the

distribution and extent of these ancient

facies (particularly those that cover

hundreds of thousands of square

kilo-

metres, exceed several kilometres in

thickness, or which directly overlie

oceanic basement) must be deter-

mined by deductive reasoning coupled

with information about the

physico-

chemical behaviour of brine systems.

One of the most important environ-

mental parameters to be determined

for these ancient evaporites is the

depth of brine during their deposition.

Brine

depth

Schreiber

a/.

(1976) recognized

three main environmental settings for

subaqueous evaporites (Fig. 6).

Figure

A) An interior basin: view of

Death Valley, California with alluvial fans

flanking mountains in background; salt pan

in middle-distance (about

km wide), and

saline mud flat (with efflorescent crusts) in

foreground.

A salina: aerial view of part

of Lake

MacLeod basin with perennial lake

(about

km wide) fed by seepage through

barrier (part of seepage face in foreground)

and evaporite flats in background. C) A

coastal sabkha: oblique view of

Abu

Dhabi sabkha. Supratidal sabkha in fore-

ground; intertidal algal belt is the dark zone

(1-2

km wide); seaward of this is the

lagoon

(20

km across), barrier islands and

open Persian Gulf in far distance. Photo

courtesy

R.K.

Park.

380

KENDALL

Identification was based on sedimen-

tary structures. Criteria used include

structures indicating wave and

current activity, identifying high-energy

intertidal and shallow

subtidal environ-

ments,

algal structures (in the ab-

sence of wave- and current-induced

structures), believed to indicate a

deeper environment, but one still

within the photic zone, and 3) wide-

spread, evenly laminated sediments,

lacking evidence of current and algal

activity (but associated with

gravity-

displaced sediments) characterizing

the deepest, subphotic environment.

There are problems with the use of

these criteria.

Stromatolites may grow in pro-

tected, shallow water settings and the

absence of current structures from

algal-bearing sediments is not neces-

sarily a criterion of greater depth. It

may also be impossible to discriminate

between planar stromatolites and

organic films, formed by a passive

Figure

Evaporite depositional environ-

ments, in addition to continental and

coastal settings, represented in the rock

record. For key see Fig.

Figure

Main physical environments of

subaqueous evaporite deposition and the

chief sulphate and halite facies present in

each. Modified from Schreiber

et a/.

973).

SHELF EVAPORITES

Barris

Sea

10's

1000's km

CII

l,,",

AII

AA,,llll,,.~~Al\M

various arrangements of mdflatlevaporite flat

shallow subaqueous evaporites

BASIN-CENTRAL EVAPORITES

emergent

10's

1000's km

basin flanks

various arrangements of

mdflatlevaporite flat

shallow subaqueous evaporite

SHALLOW-WATER

SHALLOW-BASIN

SHALLOW-WATER

DEEP BASIN

(Dessicated Basin)

flanldng

evaporite shelves brine level

deep-water

evaporite

DEEPWATER

DEEP BASIN

(Brine-filled basin)

-C-

-ALGAL

LAMINATED

WlTH

CROSS-BEDS

AND

RlPRES

CHEVRON HKlTE

LAYERS

COMPOSED

HAUTE RAFTS

CRYSTWE

LAMNATED

WITE

DEWS

FLOWS

EVEN m-LAMNATEO

TURBlDFTES

19. EVAPORITES

accumulation of organic detritus,

which have no depth significance.

Widely correlatable lamination is not

unique to deep water environments.

Similar lamination occurs in evaporites

deposited in subaerial, but ephemer-

ally flooded, environments (see Figure

18 later in this chapter)

Dissolution surfaces and

bottom-

grown crusts may also be used to

assess depositional depth in

evapor-

ites. An abundance of dissolution sur-

faces implies each water influx into the

depositional site dissolved part of the

previously precipitated evaporite. Thus

each flooding event diluted the entire

brine column (Adams, 1969; Warren,

1982). This can only happen when

brine volumes are small, implying

shallow brine depths. Despite the

shallow water origin (5-10 m) of some

evaporites lacking dissolution sur-

faces, the presence or absence of

these surfaces reflects a feature of

fundamental importance. Environ-

ments affected by dissolution events

are composed of repeatedly dissolved

and reprecipitated salts, whereas

those of deeper environments are not

subjected to such recycling.

Continued growth of evaporite crys-

tals on the sediment surface requires

that they be in contact with supersatu-

rated brines. Evaporative concentration

leading to supersaturation, however,

only occurs at the air-brine interface. In

deep bodies of stratified brine, descent

of supersaturated brines will be

pre-

Table

Evaporite facies.

MUD FLAT EVAPORITES

formed by displacive evaporite growth in pre-existing sedi-

ments.

DRY MUD FLAT

FACIES:

evaporites non-accumulative, former presence re-

vealed by modifications to primary depositional and desiccation structures.

SALINE MUD FLAT FACIES:

evaporites persistent* as crystalslnodules; few

sedimentary structures survive in host sediments.

SHALLOW WATER EVAPORITES

formed by precipitation andlor sedimentation from

shallow surface brines (usually

m deep) which are permanent or ephemeral

features of the environment.

EVAPORITE FLAT FACIE3

laminated clastic evaporites precipitated and

deposited from unconfined ephemeral brine- and flood-sheets+. Dissolution

features usually absent or subdued because of cyanobacterial coatings and

frequency of flooding. Transitional into

BRINE PAN (EPHEMERAL SALINE LAKE) FACIES:

evaporites precipi-

tatedldeposited from ephemeral but confined bodies of brine. Laminated

detrital evaporites

andlor alternations of evaporite crusts and saline mud

flat detrital layers (with displacive evaporite crystals). Well-developed disso-

lution surfaces and patterned ground phenomena. Cyanobacterial mats in

less saline settings. Transitional into

SHALLOW PERENNIAL LAKE FACIES:

evaporites depositedlprecipitated

from permanent shallow brines. Evaporite crusts with flood-introduced de-

trital layers; dissolution surfaces in shallower parts; buried crystal termina-

tions in deeper. Cyanobacterial mats in less saline shallower settings.

Transitional into

DEEP WATER EVAPORITES

precipitated or emplaced in brines generally deeper than

20-40

m (but deep-water-appearing evaporites may occur in brines only a few

metres deep). Characteristically they lack dissolution surfaces and bottom

crusts.

BASIN FLOOR FACIES:

sediments with fine, even, and widely distributed

laminations.

BASIN MARGIN FACIES:

basin floor laminatedlshallow water crusts, with

resedimented evaporites

turbidites, slumps and mass flow deposits.

Evaporites may later be removed by undersaturated groundwaters, making distinction

from dry mud flat facies difficult.

Growth of displacive evaporites within evaporite flat sediments superimposes saline mud

flat characteristics.

vented by the density stratification, and

bottom-grown crusts should not form.

Even when brine stratification is

absent, as it is for brief periods in the

present-day Dead Sea (Steinhorn,

1985; Anati

et al.,

1987), there is no

drive to transport supersaturated

brines to the deep basin floor. They do

not differ substantially in density and

should mix with the main part of the

brine long before reaching the bottom.

Advection of supersaturated brines

probably affects brines down to a

salinocline or thermocline (20-40 m).

Where the basin floor lies at shallower

depths it should be affected by

ad-

vecting brines, and bottom crusts could

potentially grow. Unless

supersatura-

tion can be achieved by means other

than surface evaporation, evaporite

crusts should record precipitation in

brines only a few tens of metres deep.

Three main groups of evaporite

facies are here recognized (Table

2),

mud flat

facies in which evaporites ac-

cumulate within pre-existing sediments

in episodically flooded, subaerial envi-

ronments, 2)

shallow water

facies, typi-

cally characterized by evaporites

containing dissolution

surfaces/bottom-

grown crusts, and

deep water

facies

characterized by the presence of lami-

nations and gravity-displaced sedi-

ments, and lacking indications of

shallow water environments. Deep

water evaporites may, however, be de-

posited in only a few tens of metres of

brine, whereas many shallow water

evaporites are deposited on evaporitic

flats that only become flooded during

storm surges or particularly high tides.

Since here all depositional events are

subaqueous, the interpreted environ-

ment is so identified, even though the

depositional site may be subaerially

exposed for most of the time.

MUD

FLAT EVAPORITES

These evaporites grow in occasionally

flooded, but essentially subaerially

exposed, saline mud flats around

playa lakes and salina ponds, and in

supratidal flats or coastal sabkhas.

Here there is an imbalance between

evaporative outflow and seepage

inflow such that brine tables lie within

the sediment. Flooding events, pro-

duced by storms, spring tides or wind

set-up, punctuate the history of these

areas and are the chief depositional

agents. The brine table usually occurs

KENDALL

less than a metre below the surface so

that all sediments reside within

phreatic or capillary zones.

Sedimentary processes

Saline mud flats are equilibrium defla-

tion-sedimentation surfaces with a to-

pography controlled by the brine table

and its gradients. Wind removes partic-

ulate material that has dried out on the

flats, but this can be inhibited by cyano-

bacterial mats or efflorescent crusts.

Disruption of upper sediments by efflo-

rescent crust growth creates materials

sensitive to wind action and formation

of clay dunes. Gypsum crystals, precip-

itated in uppermost sediments, may

concentrate as lag deposits or be swept

together as gypsum dunes.

The shallow water table allows

evaporation to occur with concomitant

concentration of pore fluids. Thus sal-

ine mud flats are sites of brine forma-

tion regardless of the salinities of the

inflowing groundwaters. The brine type

and the mineralogy of the evaporites

that precipitate from them are, how-

ever, highly dependent upon the chem-

ical composition of the ground- and

floodwaters. Continuing evaporation

generates a pronounced groundwater

concentration gradient away from input

sites (commonly toward the basin

centre in playas, and away from the

sea in coastal sabkhas). Progressively

more saline minerals can be precipi-

tated and preserved in these directions.

Little, if any, evaporite precipitation

occurs from surface brines; instead

evaporites grow within soft, brine-

soaked sediments to form intrasedi-

ment evaporites (Fig.

7A), or at the

surface, by evaporation of groundwater,

as ephemeral efflorescent crusts (Fig.

78). Precipitation results from brine

evaporation from the capillary fringe, or

by reactions between the brine and the

host sediment (including previously

formed evaporites). Host sediments are

typically siliciclastic or carbonate muds,

but may be sands or earlier precipitated

evaporites.

Sediment is transported onto mud flat

surfaces episodically: by spring tide or

storm-backup sheet floods in coastal

sabkhas, or by storm-fed sheets in

playas downslope (or downwind in

playas). These sediment additions are

only preserved where the brine table

rises a corresponding amount. Other-

wise the extra sediment dries out and is

deflated. Where flood-introduced sedi-

ments persist, the episodic nature of

depositional events imparts a lamina-

tion. This is continuously disrupted and

destroyed between flooding events by

the growth and dissolution of ephem-

eral evaporite crystals and crusts, and

by episodes of surficial drying that

cause extensive and multiple mud-

cracking.

Efflorescent crusts of saline minerals

accumulate on mud flats during ground-

water discharge and evaporation (Figs.

7B, 8), or by the evaporation to dryness

of ponded floodwaters. Because evapo-

ration is both rapid and complete,

crusts include metastable and highly

soluble salts. Rain and floodwaters dis-

solve the more soluble constituents to

form concentrated but chemically

"simple" brines. Ultimately such brines

may reach basin-central ponds or brine

pans.

Thicker saline crusts are similar to

those of salt pans but are surficial and

ephemeral features. Growth of evap-

orite crystals causes volume increase

and the formation of salt blisters (Fig.

8A), salt-thrust polygons (Fig. 8B,C);

or, when affected by rain, highly irreg-

ular dissolution surfaces

with consid-

erable relief (Fig.

9).

Even salts that survive dissolution

by floodwaters and become buried are

ephemeral when underlying ground-

waters are undersaturated. Upward

movement of undersaturated brine,

caused by evaporative losses near the

Figure 7

Recent mud flat evaporites. A) Displacive lenticular gypsum crystals forming

"gypsum mush" layer, Abu Dhabi sabkha. Photo courtesy

B.C.

Schreiber.

Ephemeral

halite crust with "pop-cornm-like growths

(10

cm across): Howz-e-Soltan salt lake, Iran.

Although halite forms the most conspicuous surface evaporite, gypsum is the only phase ac-

cumulating within the sediment. Photo courtesy

Fayazi.