Walker J. Facies models, Canada, 1992

Подождите немного. Документ загружается.

19. EVAPORITES

porosity between zoned halite crystals

or in dissolution cavities (Fig.

17B).

Inclusion-rich and -poor layers in

chevron halite crystals (Fig. 17C) form

from brines with different degrees of

supersaturation. Thus they record

rapid changes in brine concentration.

This only occurs in small brine vol-

umes, implying chevron halites are

shallow water precipitates.

Detrital halite

Detrital halite is probably more impor-

tant than published studies suggest; it

is particularly susceptible to

recrystal-

lization. When preserved it is com-

posed of fragmentary surface-grown

hopper crystals and small cubes that

may represent overgrown hoppers,

crystals precipitated during brine

mixing, or reworked material from

bottom-growing crusts. Detrital halite

commonly is ripple marked, exhibits

cross bedding, and includes other

de-

trital material. Karcz and Zak (1987)

suggest that hydraulic behaviour of

halite in brine is similar to that of the

quartzlwater system. Crystal growth

continues after deposition and the

de-

trital origin is easily obscured.

Detrital halite dominates higher

energy environments, situations

where crystals are subject to constant

bottom movement, and crust develop-

ment is prevented (Weiler

a/.,

1974). Constant motion promotes

halite precipitation onto grain surfaces

with the formation of halite ooids.

Potash salts

Less is known about origins of potash-

magnesia salts. The primary nature of

sylvites in varved sylvinites is indi-

cated by the intimate association with

halite-preserving subaqueous textures

(Lowenstein and Spencer, 1990).

Sylvite layers were crystal cumulates

that crystallized as a result of surface

brine cooling.

Wardlaw (1972) de-

scribed crusts of bottom-grown

carnal-

lite, interbedded with layers of detrital

halite. The presence of tachyhydrite in

these evaporites, a mineral that can-

not survive exposure to the atmo-

sphere, indicates evaporite deposition

was entirely subaqueous.

Most potash-magnesia salts, how-

ever, are diagenetic replacements of,

or additions to, earlier halite or sulphate

deposits. All textures and structures

may be diagenetic, but less altered

beds may still exhibit surprisingly

well-

preserved depositional features.

Lowenstein (1 988) describes primary

textures in halite and anhydrite (origi-

nally gypsum) from potash ore beds of

the Permian Salado Formation of West

Texas-New Mexico. In the Salado and

the Prairie Evaporite, sylvite and some

carnallite are early diagenetic cements

that fill intercrystalline porosity and so-

lution cavities created in brine pan

halite crusts (Fig.

23;

Lowenstein and

Spencer, 1990). These can be matched

with Recent occurrences from brine

pans. Here carnallite cements are pre-

cipitated as warm, potassium-rich

surface brines descend below the

saline pan surface during desiccation

phases. Cooling of such brines causes

them to become supersaturated with

respect to carnallite.

Figure

Thin section photograph of halite cubes surrounded by void-filling carnallite

cement (dark grey) and

mudstone (black); McNutt Potash zone, Salado Formation

(Permian), New Mexico. Photo courtesy T.K. Lowenstein.

Figure

Thin section photograph of

organic-rich calcite (dark) and anhydrite

(light) laminae from the Permian Castile

Formation. Photo courtesy

W.E.

Dean.

394 KENDALL

DEEP WATER EVAPORITES

occur, however, and record short-term

Deep water evaporites are controver-

sial. Almost every deposit identified as

a deep water evaporite has also been,

at some time, interpreted as having

formed in a shallow water or supratidal

setting. For some people, evaporite

basins may have been deep enough

to sink battleships in, but for others the

floors of the same basins would have

been barely moist.

Because there are no suitable

modern analogs for deep water evap-

orite~, identification of these facies is

based entirely upon ancient examples.

Nucleation and crystal growth occurs

at the brine surface and crystals settle

through the brine column as a pelagic

rain to form cumulate deposits. Some

gypsum and halite, however, grow on

the bottom. Theoretically, evaporite

crystals might also precipitate within

stratified brines during diffusive mixing

of the different layers (Raup, 1982).

The distinctive features of deep

water evaporites reflect the relatively

large size and volume of the brine

body. Deep water evaporites are

usually identified by their vertical and

lateral continuity. Large bodies of brine

do not fluctuate in composition in re-

sponse to short-term changes be-

cause the large volume of brine acts

as a buffer. Regular interlaminations of

minerals of different solubility do

variations in the salinity of surface

brines. The majority of inferred deep

water evaporites are laminar to thin

bedded, and individual laminae are

traceable for long distances: they may

be basin wide. Stratigraphic units com-

posed of laminar evaporitic carbon-

ates, sulphates and halite may extend

over thicknesses of tens to hundreds

of metres, and are correlatable later-

ally over whole basins (tens to hun-

dreds of kilometres). It is this vertical

and lateral constancy that is one of the

main arguments for the presence of a

substantial body of brine.

Turbidites and mass flow deposits

composed entirely, or in part, of re-

sedimented evaporites may also be

emplaced within deep water environ-

ments. Originally formed in shallower

parts of a basin, their lateral transport

implies the existence of relief in the

basin and deposition within the deep

water setting.

Sedimentary processes

Depositional processes in deep water

evaporitic environments are believed

to have been essentially similar to

those in perennial salt lakes. Most pe-

rennial saline lakes more than a few

metres deep are density or tempera-

ture stratified, with denser brine at the

bottom (Neev and Emery,

1967;

-25

km-

2km-

Figure

Anhydrite pseudomorphs of

bottom-grown gypsum crystal crusts with

draped anhydrite-calcite laminae. Swallow-

tail twinning evident in former crystals at

arrows. Greater part of gypsum now re-

placed by nodular anhydrite. Lower

Anhydrite member of Castile Formation,

west Texas.

Figure

Correlation of carbonate, organic matter and anhydrite laminae, all enterolith-

ically folded (slumped?): Ratner Member of Prairie Evaporite,

Alberta. Photo courtesy

Figure

Laminated deep water anhy-

G.R. Davies. drite-halite, Paradox Formation, Utah.

19. EVAPORITES 395

Spencer, 1982). Ancient deep water

evaporites are similarly considered to

have been precipitated from stratified

brine systems. This has important im-

plications:

the denser bottom brine

protects previously precipitated evap-

orite~ from dissolution during

flood-

ingldilution events, 2) stratification

prevents surface brines from reaching

the lake bottom, so that growth of

bottom crusts is prevented or cur-

tailed, and 3) bottom brines become

anoxic, allowing organic matter to be

preserved

and/or allowing bacterial re-

duction of sulphate to proceed.

Deep water evaporite facies

Laminar sulphates and carbonates

Laminar sulphate (originally gypsum),

either alone or in couplets or triplets

with carbonate

and/or organic mate-

rial, is the most common deep water

evaporite facies (Figs.

24,

25).

passes vertically up or down into

laminar carbonates that are probably

also of evaporitic origin. Laminar

sul-

hate

mav also Dass verticallv into

laminar halite, or contain intervals of

resedimented material. Laminae are

thin, usually only 1-2 mm thick, and

are typically bounded by smooth flat

surfaces. Within short vertical se-

quences laminae tend to be of near

uniform thickness, and individual

laminae are traceable over long dis-

tances (tens to hundreds of kilome-

tres).

Laminar deposits are commonly in-

terpreted as being seasonal or annual

increments

varves. It has never

been conclusively shown that evap-

orite laminae are truly annual, but it is

commonly stated that no other hypoth-

esis explains all the features of these

evaporites. Evaporitic carbonate

laminae in Searles Lake and the Dead

Sea may be seasonal, but are not

annual; only one lamina is deposited

every three or four years.

Laminar carbonates and sulphates

record deposition in a brine body

whose bottom was unaffected by

wave and current action. Such stag-

nant, permanently stratified bodies of

brine need not, however, be particu-

larly deep. Well-developed brine strati-

fication can dampen wave motion at

shallow depths, leading to

false im-

pression of depth in the bottom sedi-

ments. The Castile Formation has

almost universally been recognized as

a deep water deposit, deposition

having occurred from brines hundreds

of metres deep. Yet even here identifi-

cation of former crusts of bottom-

grown gypsum crystals (Fig.

26),

suggests deposition may have oc-

curred from brines only tens of metres

deep (Kendall and

~a&ood, 1989).

Laminated and bedded halite

Deep water halite is difficult to recog-

nize because most examples have

suffered recrystallization. Even so, in-

ferred deep water halite is invariably

finely laminated (Fig. 27; also

Czapowski, 1987) and contains

anhy-

drite-carbonate laminae similar to

those of deep water laminated

sul-

phates. Even finer lamination within

salt layers occurs, defined by varia-

tions in inclusion content. The fine-

ness and perfection of this lamination

indicates the original halite must have

been fine grained and probably repre-

sents accumulations of cumulate crys-

tals. Salt layers and laminae of

this type have been traced for

kilome-

tres (Anderson

et a/.,

1972; Schreiber

eta/.,

1973).

Some presumed deeper water

halites are composed of large bottom-

nucleated crystals

(Nurmi and Fried-

Aw*qw"

*""

man, 1977; Beyth, 1980; Fig. 28). A

Figure

Bottom-growth halite from

deeper water setting is suggested by

Paradox Formation, Utah. Cubic termina-

the clear, inclusion-free character of

lions of clear halite crystals are outlined by

Figure

Graded clastic anhydr~te bed

the

halite

crystals,

and

absence

overly~ng laminae of anhydrite (white). The with white and dark bituminous clasts over-

absence of dissolution at the top of halite

lying dark laminated anhydrite (with other

of dissolution surfaces associated with

crusts indicates deposition was from layers having a finely clastic texture).

anhydrite laminae. Instead, anhydrite

waters deep enough to allow crusts to Zechstein-1 anhydrite, West Germany.

buries

and

defines

the

termina-

escape the effects of brine-dilution events

From Schlager and

Bolz (1977); photo-

tions of the bottom crusts. Bottom

(responsible for the anhydrite precipitation). graph supplied by Shell Oil Company.

growth of halite in deeper water set-

KENDALL

tings has yet to be satisfactorily ex-

plained. Possibly much of the facies

was deposited in only a few tens of

metres of brine, but this cannot be

assumed and does not explain the

deep water halite of the Dead Sea.

Here halite growth may be due to

density flows of very concentrated

brines from brine pans in the shallow

southern sub-basin, or may form by

the mixing of these brines with less

concentrated Dead Sea brines. Other

halite may have precipitated from a

subsurface brine that enters the Dead

Sea along faults through older under-

lying salts.

Gravity-displaced evaporites

Clastic evaporite intervals within deep

water laminated evaporites are slump,

mass flow, density current or turbidity

current deposits. Their presence is

probably the best indication of a large

body of brine being present during de-

position. They also imply the exis-

tence of unstable slope conditions in

the basin. Gypsum and anhydrite tur-

bidites (Figs. 29, 30) are similar to

their nonevaporite equivalents. Some-

times entire Bouma sequences are

present, but most beds are composed

only of graded units, or possess poorly

developed parallel laminae in their up-

permost parts (Schlager and

Bolz,

1977). Mass flow deposits are repre-

sented by breccias

composed.of clasts

of reworked sulphate (Fig.

31), either

alone or with carbonate fragments.

They are intimately associated with

units affected by slumping.

Resedimented halite deposits are

rarely identified. Their nature is only

clearly established when they contain

entrained nonevaporite clasts

(Czapowski, 1987).

EVAPORITE FACIES MODELS

Facies models have been formulated

for modern evaporitic settings: conti-

nental playa basins and marginal

marine environments (coastal sab-

khas

salinas). They have also been

deduced for ancient deep water

basins. However, the presence of the

same depositional facies in different

geographic settings, and the vast

extent of many ancient evaporites

(with no modern parallel), makes it

difficult to use geography-based mod-

els directly for ancient evaporites.

Distributions of presumed marine

evaporites in depositional basins are

believed to be equally important.

Facies models can be devised for

continental, basin-marginal (shelf)

and basin-central deposits. The need

to establish a nonmarine origin before

continental evaporites are positively

identified is obvious, but this is not

always easy. Many evaporites now

considered marine could have been

affected, to varying degrees, by non-

marine inflow.

The term

basin

requires definition.

Tectonic and depositional basins

must be distinguished. Evaporites are

usually only preserved in structural

basins but may have been formed in

depositional basins or on wide plat-

forms that had no central depression.

Basin is used here only for deposi-

tional basins.

Krumbein and Sloss (1963) identi-

fied basin-central and basin-marginal

evaporite distributions, and this

aspect was discussed by Warren

(1 989). All present-day marine-fed

evaporite environments (sabkhas and

salinas) are basin-marginal; we have

no modern representative of

marine

basin-central evaporites, although

similar depositional patterns occur in

larger salinas. (Almost all continental

evaporites are basin central.) The im-

portant difference between these set-

tings is that basin-central evaporites

occupy the entire depositional basin,

whereas shelf evaporites pass later-

ally into normal-marine strata else-

where. An intervening preserved

barrier facies (or a nonsequence

marking the former presence of a

barrier) occurs between evaporite

and open-marine facies on shelves.

For basin-central evaporites it is the

entire exposed rim of the basin that

forms the necessary restricting bar-

rier. Thus basin-central evaporites

should be represented on basin rims

by nonsequences (Kendall 1988). In

basin-margin (shelf) settings the solu-

bility of evaporites increases away

from the basin centre, whereas in

basin-central evaporites, this increase

Figure

Mass flow deposit from Z1

Anhydrite, composed of rounded blocks of

anhydrite (probably reworked fragments of

Figure

Amalgamated gypsum tur-

gypsum crusts from a shallow water plat-

bidites (now anhydrite), some of which

form) and associated with turbidites and

display pronounced grading.

Z1 Anhydrite

mm-laminated anhydrites. Near

Walken-

(Permian Zechstein), S Harz (Germany);

ried,

S-Harz, Germany. Photo courtesy G.

scale in cm. Photo courtesy G.M. Harwood. Harwood.

19. EVAPORITES 397

may occur toward the basin-centre or

away from a confining barrier. Finally,

basin-central evaporites are

lowstand

system tract deposits, filling the entire

basin and forming when sea level

drops below basin rims, whereas

basin-marginal evaporites are compo-

nents of transgressive and (especially)

highstand system tracts. However, if

the barrier occurs at the shelf edge,

almost the entire shelf may be

evapor-

itic and evaporite-accumulating. There

are no modern equivalents of such

wide evaporitic shelves.

CONTINENTAL EVAPORITES

The basic model

The basic facies model for continental

evaporites is a closed basin with a

shallow groundwater table and a

more-or-less centrally located playa

lake. Continental evaporites of any

substantial volume are confined to

the central parts of these basins, par-

ticularly in association with salt pans

and playa lakes, and their sur-

rounding saline mud flats. These

areas are the lowest parts of the

drainage basins, environments char-

acterized by almost horizontal and

largely vegetation-free surfaces of

fine-grained sediments (Fig. 4A).

Where the groundwater table inter-

sects the surface at the deepest parts

of the basin, a saline lake or a salt

pan forms and is concentrically sur-

rounded by saline and dry mud flats

(Fig.

32).

The latter may grade into

sand flats, with or without alluvial

fans. These trap most coarse detritus

and only the finest material reaches

the basin centre. Apart from

sheet-

wash flow during storms, water circu-

lation is generally confined to the

subsurface, although ephemeral or

perennial streams may cross playas

to feed ephemeral or perennial lakes.

Evaporites accumulate in playa

basins where groundwaters discharge.

Many playas, however, have water

tables so deep that no discharge

occurs, and these playas have smooth,

hard and dry surfaces. They contain

little evaporite and none accumulates.

An association with alluvial fan and

ephemeral stream deposits, eolian

sediments,

redbeds and lacustrine

carbonates all suggest nonmarine

origins. Evaporites precipitated exclu-

sively from continental groundwaters

are not commonly recognized in the

rock record. This rarity (particularly of

pre-Tertiary examples) may reflect the

ephemeral nature of many continental

evaporite basins and of evaporites in

the depositional environment. Many

evaporites move upward at the same

rate as sediment accretion, and so are

Older

evaporites

saline

mudflat

deposits

Figure

Schematic block diagram showing depositional framework for the continental

evaporite (playa complex) model. Modified from Eugster and Hardie

(1975).

nonaccumulative. Thus many evapo-

rative environments leave no record in

the form of evaporite deposits. Their

former presence, however, can be

deduced from the presence of crystal

moulds and the modification of depo-

sitional structures.

Variations from the basic model

The climate and groundwater compo-

sition (discussed elsewhere), geolog-

ical setting and the groundwater

source are the main factors causing

variations from the basic facies model.

They also determine the type, amount

and distribution of the evaporites in

continental settings.

Geoiogic/Geographic setting

Five main settings are recognized

(Smoot and Lowenstein,

1991), each

characterized by different arrange-

ments and associations of environ-

ments.

1. Rain-shadowed deep basins

bounded by mountains (intermontane

basins), like those of the desert and

semi-desert basins of the western

U.S.A., central British Columbia, the

Andes, some East African rift valleys

and the Tibetan Plateau, are charac-

terized by an alluvial fan

saline pan

association. Basins are fringed by allu-

vial fans and contain salt-encrusted

ephemeral saline lakes ringed by mud

flats and sand flats. Inflow into the pan

is confined to groundwater influx and

aperiodic storm runoff. An ancient

example is the saline pan deposits of

the Wilkins Peak Member (Eocene

Green River Formation, Green River

Basin, Wyoming).

Intermontane basins fed by peren-

nial streams constitute an important

variant. The stream produces a perma-

nent lake. Examples of this alluvial fan

perennial-stream

perennial saline

lake association are the shallow Great

Salt Lake of Utah and Lake Urmia of

Iran (both surrounded by mud flats),

and the deeper and narrower Dead

Sea of Israel, where alluvial fans enter

the lake directly. Beds of cumulate

nahcolite and halite in the Green River

Formation of the Piceance Basin

(Colorado) accumulated in a perennial

saline lake, unlike the situation in

neighbouring Wyoming.

Wide shallow basins are domi-

nated by ephemeral stream flood-

plains and dune fields. Large areas

398 KENDALL

become aperiodically inundated by

ponded floodwaters that slowly evap-

orate, precipitating a saline crust.

Complete desiccation gives rise to a

salt pan (with an area perhaps only

1/100th of that originally inundated)

fringed by a saline mud flat or wide dry

mud flats. The Lake Eyre Basin of

Australia is the largest example of this

ephemeral-stream

dune field

saline

pan

association. Ancient examples

include part of the Lower Permian

Rotliegendes of northern Europe, and

Devonian halites associated with

redbeds in the Lower Elk Point Group

of western Canada.

4. Lake Chad (north-central Africa)

is a shallow but freshwater lake sur-

rounded by eolian dune fields with

small interdune saline pans and saline

mud flats. The lake is fed by perennial

rivers. It is the only Recent example of

perennial stream

perennial lake

dune field

association.

Saline pans, or perennial lakes di-

rectly fed by groundwater occur in old

channel systems, hollows on glacial

sediments, and in karst depressions.

The preservation potential of these de-

posits is low.

Groundwater source

In the basic model groundwaters move

radially from the hinterland, converging

toward the basin centre (which also

marks the hydrographic low

point of the

basin). Flow is assumed to be hori-

zontal and shallow subsurface. This

produces a concentric pattern of in-

creasing groundwater salinity and a

"bull's eye" pattern of salt deposition

(more saline salts in the centre, Fig.

33). However, when the deepest part of

the basin floor is not centrally located,

or when groundwater enters the basin

predominantly from one side, this ideal

pattern is disturbed and becomes

asymmetric.

There is a tendency for less mineral

segregation to occur in river-fed than in

groundwater-fed playa systems. Brine

compositions in lakes fed by perennial

streams are less affected by prior pre-

cipitation of less saline salts (in periph-

eral playa flats). River waters retain

their carbonate and sulphate content,

and low-solubility salts precipitate within

the lake. In solely

groundwaterlflood-

water-fed pans, less soluble con-

stituents are retained on surrounding

flats, and pan salts are dominated by

more soluble salts, and can be

monomineralic.

Extrinsic controls on continental

evaporite facies

Except for basins that might be inun-

dated or isolated, continental evaporites

are unaffected by sea level changes.

The most important extrinsic control

is climate. The history of present-day

and many ancient playa lake com-

plexes has been one of alternating wet

and dry conditions, with corresponding

transgressive, freshened,

nonevaporite-

precipitating lakes, and regressive

(shrinking) saline lake or saline pan

stages. Variations between arid and

less arid conditions cause replacement

of perennial lakes by salt pans or even

dry mud flats, lake levels to rise and fall,

lake margins to expand and contract,

the amount of clastic sediment to vary,

and water chemistry to change.

During prolonged episodes of aridity,

groundwater tables may be lowered

such that ephemeral lakes (saline

pans) drain, convert to dry mud flats

and perhaps are encroached upon by

aeolian dunes. Lessened aridity, on

the other hand, is marked by partial or

complete dissolution of earlier-formed

salts, by deposition of basal transgres-

sive conglomerates and beach de-

posits over former playa flat deposits,

and by deposition of nonsaline

lacus-

trine sediments, amongst which oil

shales may be conspicuous.

The effects of climate change are

not straightforward in perennial lakes

that occupy topographically subdi-

vided depressions. Different

sub-

basins may develop opposite sedi-

mentary sequences. Where lakes

have asymmetric water supply and a

lowered brine level (such as Great

Salt Lake in Utah) the sub-basin still

fed by a perennial river becomes less

saline. This is because the inflow, al-

though diminished, now dilutes a

smaller volume of lake brine. In con-

trast, sub-basins now cut off from

inflow, desiccate and become more

saline. These opposite sequences are

caused by the same environmental

change, a lowered water level, pro-

duced by increased aridity.

Increased sediment supply, caused

by tectonism and climatic changes,

dilutes evaporite accumulation. Salt

pan and perennial lake deposits (com-

posed of salt crusts) are thereby re-

placed by clastic muds containing

dis-

placive evaporite crystals.

BASIN-MARGINAL (SHELF)

EVAPORITES

Models for coastal sabkhas, marginal

marine salinas and for ancient large

evaporites are discussed.

Coastal sabkhas

These supratidal areas (Fig. 4C) are

described in Chapter 16. They may

merge insensibly landward with con-

tinental sabkhas (playas), and land-

ward parts of coastal sabkhas are

affected by continental groundwaters.

Thus evaporites typical of nonmarine

settings

(e.g., trona) can be emplaced

within marine-derived sediments.

Coastal sabkhas are products of

both depositional and diagenetic pro-

cesses, the most important being the

emplacement of early diagenetic cal-

cium sulphate, less commonly of

halite. Indigenous sediments reflect

the offshore sediment mosaic, but

may contain substantial amounts of

detrital sediment from the hinterland.

Offshore sediments are washed over

the sabkha surface during storms that

episodically inundate seaward parts.

Groundwaters beneath sabkhas

become more concentrated toward

sabkha interiors, and all but the very

seaward and landward margins may

be halite saturated. Concentration

occurs by evaporation from the capil-

lary fringe and by dissolution of

earlier-formed evaporites (particularly

halite). Evaporative losses are replen-

ished by downward seepage of

storm-

driven floodwaters and/or by gradual

intrasediment flow, fluxing either from

the seaward margin or from a conti-

nental reservoir that affects landward

parts of the sabkha (Patterson and

Kinsman, 1981). The water table in-

clines seaward but brine migration is

slow. Movement is limited by the low

permeability of the predominantly

muddy sediments and by permeability

barriers such as buried algal mats

(Bush, 1973) and cemented layers

(McKenzie

et al.,

1980). These subdi-

vide the sabkha sedimentary prism

into hydrologic zones. Vertical move-

ment of groundwaters only occurs

where these barriers are broken.

Sabkhas that widen as a result of

coastal progradation have a charac-

teristic sedimentary sequence (Figs.

19. EVAPORITES

14, 34) consisting of subtidal sedi-

ments (commonly restricted lagoonal)

at the base; intertidal sediments, in-

cluding cyanobacterial mats and ce-

mented crusts; and a capping

supra-

tidal deposit with abundant displacive

gypsum

anhydrite). Displacive

gypsum grows within intertidal and

sub-

tidal sediments once these are located

beneath the sabkha environment.

Variations in sabkha sequences

1. Diastrophic control. Shallowing-

upward sequences terminated by

marine sabkhas can form from three

different events, simple progradation,

eustatic falls in sea level, or brine

level drops caused by evaporative

drawdown. The first two also affect

nonevaporite sequences and Chapter

16 discusses the different hypotheses

for shallowing-upward carbonate suc-

cessions, of which evaporite-capped

successions form only a variant. Two

problems arise when successions are

interpreted simply as progradational

events; one also relates to

nonevap-

orite successions, the other is intrinsic

to the sabkha environment.

Present-day sabkhas are protected

from the full impact of onshore storms

by beach barriers or offshore islands.

This protection allows the sabkha to

prograde. Once the lagoon behind the

barrier becomes filled with sediment,

further progradation requires forma-

tion of another barrier, seaward of the

first, behind which progradation can

be renewed. Widespread cycles com-

posed of fine-grained sediments and

capped by sabkhas thus should not

accrete laterally in a uniform manner,

but do so in a series of jumps, as off-

shore bars develop into barriers. Wide

sabkha sequences should contain

remnants of these barriers. The entire

sediment body should be an

arid-

zone equivalent of a chenier plain, an

environment described by

Picha

(1978) from Kuwait, by West et al.

(1979) from the Mediterranean coast

of Egypt, and by Warren and Kendall

(1985) from Abu Dhabi, but which

still appears not to have been docu-

mented in the ancient.

Many ancient evaporites interpreted

to have been generated in sabkha en-

vironments overlie open marine car-

bonates, rather than restricted lagoonal

deposits. Here we must assume either

that 1) barriers were absent and mud

flats were protected from wave attack

by the extensiveness and shallowness

of the offshore environment, or

the

sabkha interpretation of the evaporites

is incorrect (see below).

Figure

Saline mineral zonation in playas. A) Yotvata Sabkha (Israel), after Arniel and

Freidman (1971). B) Deep Spring Lake, California, after Jones

(1965), Ca

calcitelarago-

nite, Dol

dolomite, Gay

gaylussite, Th

thenardite, Bu

burkeite.

The second problem concerns the

preservation potential of wide

sab-

khas. These need low-gradient, stable

shelves, locations where active lateral

flow of groundwaters is uncommon.

Arid environments are subject to con-

tinual evaporation losses and, unless

this water loss is replenished, the

supratidal environment dries out and is

subject to wind erosion. Wide sabkhas

thus require a regional groundwater

flow system that allows the water table

to rise as the sabkha progrades. With-

out such a groundwater system, the

upper evaporite-bearing sediments in

the areas beyond seawater

flood-

recharge would be entirely removed

by deflation. Warren and Kendall

(1 985) interpret dolomitized

subtidal

sequences, with erosive upper sur-

faces and nodular anhydrite after

dis-

placive gypsum, but lacking overlying

supratidal units, as having formed in

this way. A seaward-dipping gradient

to the sabkha groundwater surface

would also cause additional sediment

(mainly eolian) to be deposited as

the brine surface rises, but requires

the presence of a large neighbouring

landmass with sufficient relief to act as

a recharge area for the groundwater

system. For many widespread evap-

orite units, interpreted as sabkhas,

this requirement has not be substanti-

ated. Neither have the thick eolian

supratidal sections of more landward

parts of these sabkha complexes been

identified. For these reasons the inter-

pretation of many widespread

evapor-

ites as sabkha deposits is suspect.

Sediment emergence, with the for-

mation of widespread exposure sur-

faces and the overprinting of formerly

subaqueous sediments by mud flat

processes, can occur by relative falls

in water level independent of any sedi-

ment progradation. Falls may be due

to global external events

(e.g., glacia-

tions) or to increased restriction and.

subsequent evaporative drawdown.

No obvious criteria exist to distinguish

cycles generated by sediment

progra-

dation from those that reflect episodes

of evaporative drawdown.

Nature of the host sediment. This

determines the permeability and hence

the amount of drainage in the sabkha

sediment, the further evolution of

sab-

kha brines, and the subsequent com-

pactional history of the evaporite de-

400 KENDALL

posit. Impermeable sediments inhibit

brine

reflux and, by curtailing downward

seepage of floodwaters, extend the

width of the area affected by flood re-

charge. Carbonates (particularly arago-

nite) in host sediments are of major

importance. Their dolomitization re-

leases calcium that reacts with

sul-

phate in groundwaters to form addi-

tional gypsum and anhydrite. This in-

creased sulphate precipitation and

dolomitization reduces the sulphate and

magnesium content of brines in sabkha

interiors to low levels and causes

mag-

nesite (precipitated earlier) to dissolve.

Reflux of brines capable of dolomitizing

deeper-lying carbonates causes

gypsum precipitation in these sedi-

ments. Extensive sulphate growth in

subtidal carbonates between sabkha

evaporites may obscure evidence of

the cyclic nature of an evaporite deposit

and create a single thick, composite

unit. In noncarbonate sediments,

dolomitization is absent, the sabkha

brines retain 60-70 per cent of their

sul-

phate, much less gypsum is precipi-

tated, and brines remain

magnesium-

rich and so magnesite remains stable.

The sulphate-rich brines formed in

non-

carbonate sabkha sediments react with

earlier-formed gypsum to form

poly-

halite (Holser, 1966).

Differences in sediment coherency

dictate subsequent compactional his-

tory. Lithified or coherent. sediments

preserve pseudomorphs after gypsum

crystals or halite crystal moulds. Com-

pressible sediments (particularly

organic-rich varieties,

e.g., cyanobac-

terial mats), on the other hand, allow

gypsum and anhydrite nodules to

grow, to coalesce and compact, per-

haps even to form sluggy or

even-

layered anhydrites (Shearman and

Fuller, 1969; Mossop, 1974).

Nature of the offshore water

body.

Commonly this is normal marine

to slightly hypersaline and well below

gypsum saturation. Thus

subtidal and

intertidal sediments beneath sabkha

evaporites are bioturbated and ske-

letal-rich, with cyanobacterial mats (if

present) confined to upper intertidal

environments. Where sabkhas border

more saline water bodies, sediments

beneath sabkha evaporites are lami-

nated (burrowing biota absent) and

cyanobacterial mats may extend down

well into

subtidal environments (brows-

ing biota absent). Where offshore

waters precipitate and preserve

gypsum, the sabkha sequence forms

the uppermost unit of a subaqueous

unit. Subaqueous evaporites are

transported onto the sabkha by

storms, there to form

clastic beds of

gypsum, anhydrite or even halite

debris. Such sabkha sequences would

be largely composed of such beds

and might be difficult to distinguish

from shallow subaqueous evaporites.

Marginal marine salinas

Relatively small marginal marine

salinas occur above sea level in de-

pressions on sabkhas, between coas-

tal dunes, or in ephemeral stream del-

tas or fan deltas. Those lying below

sea level (Fig.

4B)

are of greater im-

portance in terms of size, persistence

and the thickness of evaporite that can

be deposited and preserved. They

occur in tectonic depressions and be-

hind coastal barriers. Marine inflow

INTERTIDAL

1-2m

may be via minor inlets through the

barrier, surface flow across a

supra-

tidal flat, or by passage through the

barrier, discharging into the salina as

springs, brine sheets or as seepages.

Salinas are covered by water for brief

or long periods (ephemeral and peren-

nial salinas), but in both cases evap-

orite deposition is primarily by precipi-

tation from a surface brine, rather

than by precipitation within a sedi-

ment. Salinas associated with wide,

high-relief barriers differ markedly from

sabkhas in that they are unaffected by

tidelstorm surges. They therefore lack

transported marine sediment (unless

this is wind-introduced).

Salinas can be large and compa-

rable with small- to medium-sized

ancient evaporite basins,

e.g., the

Ranns of Kutch (India), 30,000 km2

(Glennie and Evans,

1976), Lake

MacLeod (Western Australia), 2,000

km2 (Logan, 1987).

Salinas and continental playa lake

Anhydrite diapirs and

contorted layers in sandy

dolomite. Erosion surface

at top, and older surface

buried after brine-level rise

chicken- wire anhydrite

after gypsum-crystal mush

Organic cyanobacterial mat

with mudcracks

displacive

gypsum crystals

grades down to burrowed

cemented crust

lagoonal burrowed

lime mud

Figure

Simplified vertical section through sabkha sequence at Abu Dhabi.

---

19. EVAPORITES

complexes are similar. Both contain

subaqueous and subaerial subenvi-

ronments in which evaporites accumu-

late. These may be arranged in similar

fashions and depositional processes

and products are comparable. They

commonly consist of an ephemeral or

perennial saline lake located in the

lowest parts of a depression, sur-

rounded by saline mud flats that pass

outward into peripheral dry mud flats.

Variations in salina sequences

The variation in mineralogy and facies

within salina evaporites is controlled by

factors similar to those controlling con-

tinental playa evaporites. Climate con-

trols the maximum salinity to which

brines can be concentrated, but inflow-

reflux ratios more commonly determine

the concentrations actually reached.

These ratios are determined by the

presence or absence of surface inflow,

the nature (width and permeability) of

marginal

shallow water

facies

DEEP WATER

DEEP BASIN

MODEL

the barrier, the hydraulic head between

sea level and brine level in the salina

(these all determine inflow rates), size

of the salina (controls losses by evapo-

ration), and the effectiveness of basal

aquitard (controls brine seepage

losses). These are discussed in the

section

Basin-central evaporites.

Ancient widespread

shelf evaporites

Many evaporites are composed of

shallow water

and/or mud flat facies,

but must have accumulated on a scale

and in settings different from those oc-

curring today. The main problem with

interpreting these evaporites is to de-

termine whether they were

1) gener-

ated during progradation of marginal-

marine evaporitic environments com-

parable with those of the present day

(in which case they are diachronous),

they formed simultaneously in

evaporitic settings significantly larger

brine level

S.L.

deposits

(prograding supratidal

sediments)

brine level

SHALLOW WATER

shallow water

SHALLOW BASIN

subaerial sediments

MODEL

(periodic overflow)

continental

inflow seepage

inflow

SHALLOW WATER

DEEP BASIN

Sill

MODEL

shallow water

subaerial evaporites

Figure 35

Depositional models for basin-central evaporites.

than any today. This is a major unre-

solved problem for many large evapor-

ites: often the problem simply has not

been addressed. Detailed stratigraphic

study of some evaporites suggests the

second explanation can be correct.

These evaporites must have formed in

vast expanses of evaporitic lagoons

and mud flats, reaching tens to hun-

dreds of thousands of square

kilome-

tres in extent, over which brine depths

were only a few metres deep (or even

shallower). Episodically they became

emergent. Tidal influences were

minimal; deposition was affected more

by storms that created laminated

clastic gypsum sands. Storms re-

worked subaqueous sediments from

shallow evaporitic lagoons and trans-

ported the resultant carbonate and

clastic evaporite sands onto neigh-

bouring flooded evaporitic flats.

Cyanobacterial mats were ubiquitous

in evaporitic flat and shallow sub-

aqueous environments where brines

had salinities below halite saturation.

Stratiform evaporite units,

thick, of mixed subaerial and shallow

water evaporites were generated in

these settings. The lateral persistence

of thin beds over large areas with only

minor changes in thickness, mineral-

ogy or facies indicates they formed on

broad, flat shelf areas, areas which

could be affected by rapid transgres-

sions. Other widespread evaporites are

more variable laterally, accumulated in

a mosaic of smaller environments, and

may have been diachronous. Both

types of evaporite commonly form the

upper parts of shallowing-upward

cycles with underlying open-marine or

restricted-marine carbonates. These, in

areas of continued subsidence, may be

vertically stacked to form thick sedi-

mentary packages (Chapter

16).

Landward they may interfinger with

continental

siliciclastics.

Internal characters of shelf evapor-

ites suggest they were deposited on

mud- and evaporitic-flats, or subaque-

ously in shallow lagoons. Mud- and

evaporite-flat sediments form shoal-

ing-upward cycles but, according to

Warren (1989) are not easily corre-

lated across a platform because of

limited continuity of individual facies.

They are deposited as a facies mosaic

of saline and dry mud flats that sepa-

rate local brine pans. Here brine pans,

saline and dry mud flat environments

KENDALL

may be comparable in size and facies

to modern coastal marine equivalents,

however, the lateral extent of the eva-

porite itself may have no modern

counterpart. Host sediments may be

fine-grained

clastics and carbonates,

or composed of detrital gypsum.

Much evaporite, previously inter-

preted as widespread sabkha deposits,

could have been deposited in salinas.

Relicts of subaqueously formed

gypsum are abundant in many "classic"

sabkha anhydrites

the Upper San

Andres Formation (Warren and

Kendall, 1985) of the Texas Panhandle,

the Miocene of

Iran (Purser, 1979), and

the Permian Bellerophon Formation of

northern Italy (Hardie,

1986), the

Jurassic Arab evaporites and overlying

Hith Anhydrite of the Arabian Gulf

(Mitchell

a/.,

1988), large parts of the

lower Buckner anhydrites of the

U.S.A

Gulf Coast (Mann, 1988), and Miss-

issippian basin-marginal anhydrites of

the Williston Basin (Fig.

14;

Lindsay

and Roth, 1982).

Widespread areas of shallow sub-

aqueous evaporite precipitation also

have no modern counterpart, al-

though the facies developed can be

matched in perennial coastal salina

and continental playa lakes. These

evaporites are also commonly cyclic

and composed of shallowing-upward

units

2-50

m thick, underlain by trans-

gressive open-marine sediments, and

terminated by thin sabkha sequences.

Units can be traced over distances of

tens to hundreds of kilometres,

e.g.,

San Andres Formation of west Texas

(Elliott and Warren,

1989), Yates

Formation anhydrites of west Texas

(Crawford and

Dunham, 1982), and

the Ferry Lake Anhydrite of east

Texas (Loucks and

Longman, 1982).

In the absence of a recent analog,

it is difficult to account for evaporites

deposited in continuous shallow brine

bodies. Comparison with the large

salina of Lake

MacLeod (Western

Australia), however, suggests that

those apparently composed of sub-

aqueous laminated gypsum may have

been deposited on evaporite flats.

Other evaporites, however, such as

bottom-grown gypsum or cumulate

halite, must have been deposited sub-

aqueously. Those composed of large

gypsum crystals indicate deposition in

relatively stable bodies of brine, this

facies being confined to deeper

Figure

Upper Red River (Herald Fm.) evaporite cycle, Saskatchewan. A) Inferred dry

mud flat deposits with cm-bedding (storm-flood deposits

?),

desiccation cracking.

Nodular

mosaic anhydrite after bottom-grown gypsum crystal.

laminated

anhydrite

laminated

dolomite

DRY

GYPSUM

EVAPORITE

FLATS

modular

mosaic

anhydrite

thin bedded

laminated

dolomite

FIG

SHALLOW

BRINE

36B

GYPSUM

CRUSTS

DEEPENING

UPWARD

CYCLE

DRY

CARBONATE

MUD

FLATS

carbonates

burrowed marine

Figure

Schematic diagram of Upper Red River (Herald Formation) evaporite cycle,

Saskatchewan, showing both "brining-up" and deepening-up character.