Walker J. Facies models, Canada, 1992

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19. EVAPORITES 383

surface, dissolves the buried rem-

nants of the salt crust and

reprecipi-

tates it at the new surface. Further

away from the input sites, however,

groundwaters become increasingly

more saline and calcium sulphate, or

even halite, may become stable within

the sediment. Minerals in surface

crusts thus do not usually reflect the

character of evaporite minerals pre-

served in underlying sediments. Many

fine-grained, dolomitic

redbed se-

quences probably represent deposits of

these evaporitic, but essentially

non-

evaporite-preserving, environments.

Sedimentary facies

host sediments

Two types of mud flat are distin-

guished in playas (Hardie et

a/., 1978;

Smoot and Lowenstein, 1991) and the

same concepts can be adopted for

salinas (Handford, 1991) and for

marine-marginal sabkhas. Saline mud

flats lie closer to saline lakes, salt

pans, salina ponds or the sea, where-

as dry mud flats occur in more distal

locations and may pass laterally into

coarser sand flats.

Dry

mud

flats

Depositional and desiccation struc-

tures (Figs. 10, 11) are generally pre-

served. Mud and sandy laminae form

during and after flooding but these

become partially disrupted by mud

cracks, sheet cracks and the growth

and subsequent dissolution of saline

minerals. Dry mud flats are covered

with efflorescent saline crusts between

flooding events. When saline crusts

begin to dissolve in floodwaters, intro-

duced coarser sediment is trapped in

depressions on the dissolving crust.

Upon burial and complete dissolution

of saline crusts, these coarser sedi-

ments are preserved as irregular

coarser-grained patches (sand-patch

fabric, Fig. 1 1 C).

Slowly aggrading dry mud flats are

composed of more massive sediments

(Castens-Seidell, reported in Hardie,

1986; Smoot and Lowenstein, 1991).

Here, most of the sediment deposited

during flooding events subsequently

blows away in the wind, only material

deposited in mud cracks survives.

Figure

Surface features of salt pan, Death Valley, California.

"Puffy-ground" (small

Upon drying

Out#

the

surface

salt blisters) on dry mud flat marginal to salt pan (photograph

across in foreground).

covered by new mud cracks which are

Thin salt crust with polygonal fracture system and growth of efflorescent salt in fractures. C)

able to trap new sediment during

Salt-crust polygons with upthrust edges (photograph

across) in foreground. subsequent floods. Repeated mud

384 KENDALL

cracking and sediment filling, together

with growth and dissolution of halite

crystals, generates massive

mud-

stones.

Saline

mud

flats

These mud flats are composed of sed-

iments in which lamination and desic-

cation structures have been destroyed

by the growth of abundant

intrasedi-

ment evaporite crystals. Floods cause

partial dissolution of previously precipi-

tated evaporites and sediment col-

lapses into the cavities formed.

Repeated episodes of these pro-

cesses produces a chaotic mix of

evaporite crystals within an

unlarni-

nated mud matrix (Figs.

12,

13A).

Mud

flat evaporite facies

lntrasediment evaporite crystals grow

by incorporating sediment (trapped as

inclusions within the growing crystals),

or displacively, by pushing aside the

sediment, or by both processes

(Fig.

13A). The amount of evaporite

emplaced within sediments varies

enormously (Fig.

12).

Saturation of groundwaters with

respect to calcium and magnesium

carbonates occurs quickly. This

causes precipitation of

1) calcite

cement or caliche layers in

coarse-

grained sediments, such as alluvial fan

sands and gravels surrounding playas,

micron-sized high-Mg calcite and

protodolomite soft muds, or 3)

traver-

tines and pisolitic caliche when precip-

itation occurs at peripheral springs. In

arid coastal settings, seawater also

quickly becomes carbonate saturated

in the intertidal zone, causing forma-

tion of cemented crusts.

lntrasediment gypsum crystals most

commonly exhibit a discoidal

hemi-

pyramidal habit (flattened normal to the

c-axis, Fig. 7A) and may be complexly

twinned to form rosettes (desert roses).

Gypsum grows displacively within

~i~~~~

~~~k~dl~ upthrust polygons,

par-

Figure

Recent mud flat sediments. A) Flaser-bedded clastic playa sediment with clay

tially

dissolved

form

highly irregular,

0.5

drapes

cm coin for scale).

Contorted gypsum and red mud; note truncated gypsum

relief. Devil's ~~lf course, ~~~th valley,

layers marking former playa surface

cm coin for scale), Bristol Dry Lake (California);

USA. photo courtesy C.R. Handford.

p---

19. EVAPORITES 385

cyanobacterial mats or fine-grained

sediments (sometimes forming crystal

mushes), or grows poikitically within

sandy sediments, where it occurs as

large, lenticular crystals. Gypsum pre-

cipitation causes calcium depletion in

groundwaters, and the increased

MgICa ratio of brines may promote

dolomitization of pre-existing aragonite

and precipitation of magnesite.

Dolomitization of aragonite also re-

leases strontium that precipitates as

celestite.

Anhydrite has a more restricted dis-

tribution in mud flats and is confined to

the capillary zone. It may replace

earlier gypsum (Butler

et a/.,

1982), or

be newly formed. Nucleation of anhy-

drite requires a seasonal high tempera-

ture above

35"C, and is only preserved

where mean annual temperatures

exceed

20°C (Kinsman, 1976a). It

forms discrete nodules and bands of

coalesced nodules (Fig.

14), some of

which may take the form of

ptygmatic

(enterolithic) layers. Nodules grow by

host-sediment displacement, and re-

placement and dilution of the host sedi-

ment mav occur to such an extent that

this is relocated to internodule areas or

primary anhydrite. Composite anhy-

drite nodules are remnants of gypsum

crystal clusters, and massive anhydrite

forms from gypsum mush layers. Such

soft anhydrite layers become deformed

into enterolithic folds and microdiapirs

with continued anhydrite growth

(Fig. 15).

Displacive anhydrite growth is not

the only mechanism for generating

nodules. Primary gypsum nodules

occur (West

etal.,

1979), and gypsum

and anhydrite regularly alter backward

and forward, dependent upon temper-

ature changes and flooding or rainfall

events (Gunatilaka

et al.,

1985). Much

nodular and bedded nodular anhydrite

in Recent and ancient evaporites has

replaced gypsum-pan (salina) de-

posits, rather than being displacive or

replacive after displacive gypsum

(Hardie, 1986).

The addition of displacive gypsum

and anhydrite in mud flat sediments

raises the sediment surface. If the

water table fails to rise a correspond-

ing amount, the upper parts of the

sediment dry out and blow away.

Deflation exposes anhydrite and

gypsum at the surface and truncates

layering and other structures present

within these sediments

(Figs.10,

14B, 15).

Displacive halite in modern playa

sediments has been described (Gomitz

and Schreiber, 1981

;

Handford, 1982)

but it is uncertain if precipitation oc-

curred beneath shallow brines, or within

mud flat capillary zones. Crystals com-

monly grow preferentially along cube

corners and edges (especially when

they are precipitated from highly super-

saturated brines), forming cubes with

hopper-like pyramidal hollows on each

face. In extreme cases,

skeletallden-

dritic halite crystals are generated (Fig.

13b). Sediments with abundant dis-

placive halite cubes (Fig. 12) are

termed

haselgebirge

(Arthurton, 1973).

Halite also occurs as salt crusts on

coastal sabkha surfaces, as veneers

around grains in the upper part of the

capillary zone, and as solid cubes in

sands. Halite is not an accumulative

phase here, but dissolves diurnally in

morning dew, is blown away, or dis-

solves in floodwaters.

Figure

Ancient dry mud flat dolomites.

Centimetre-bedded silty dolomites with erosive bases and sediment-filled moulds after dis-

placive halite hopper crystals (arrowed). 6) Silty dolomite, originally laminated but now highly disrupted by superimposed generations of desic-

cation cracks and

(?)

repeated growth and dissolution of evaporites. Both from Paradox Formation (Pennsylvanian), Utah. C) Herald Formation

(Ordovician of Saskatchewan) dolomites with irregular patches of coarser sediment (sand patch fabric), believed to have accumulated on top of

former saline crusts, filling depressions created during floodwater dissolution.

386 KENDALL

scour, transport and redeposition.

Most facies probably form in brines

less than

m deep. Cyanobacterial

activity is important in more protected

environments and many sediments

are subject to periodic drying.

Most of our original understanding of

these evaporites came from interpreta-

tions of ancient examples (Hardie and

Eugster, 1971; Schreiber et al., 1973;

Vai and Ricci Lucchi, 1977; Garrison et

a/., 1978). More recently studies of arti-

ficial salt ponds

(Busson, 1982; Orti

Cabo and Busson, 1984) and of Recent

salinas (Arakel, 1980; Kushnir, 1981;

Perthuisot, 1981

;

Warren, 1982;

Logan, 1987) have confirmed older in-

terpretations and suggested others.

Shallow water evaporites occur in

shallow perennial and ephemeral playa

lakes and in marine-fed salina ponds,

these being typically surrounded by

mud- or evaporite-flats. Ephemeral

saline lakes are also termed salt pans.

They grade insensibly into mud flat-like

environments where evaporites accu-

mulate as detrital sediments or by

surface precipitation. These evaporite

flats are occupied for brief episodes by

unconfined brine sheets, usually only a

few centimetres deep.

Three present-day shallow water

evaporite environments can be recog-

nized: perennial saline lakes, salt

pans, and evaporite flats. Evaporites

with shallow water features also occur

in ancient deposits where they form

laterally extensive deposits unlike

those of Recent examples. Some of

these may form by lateral accretion in

depositional environments similar to

those of the present, whereas others

may require explanations that are

drastic departures from present-day

settings (see sections Basin-marginal

(shelf) evaporites and Basin-central

evaporites).

Sedimentary processes

Perennial saline lakes persist for tens

to thousands of years without drying

up. They require substantial perennial

inflow

a large river, an inlet or a

highly permeable barrier. During wet

periods/seasons they may greatly

expand, flooding surrounding flats.

Figure

Upward passage from red silty and anhydritic mudstone with abundant dis-

Evaporation at the lake surface gen-

placive halite (haselgebirge) to similar sediments with lesser amounts of halite crystals (with

erates

'Oncentrated

surface

brines

hopper morphology) and bedded halite with thin anhydrite and clay laminae. Sequence

which saline minerals

and

marks a change from a saline mud flat to a salt pan deposit. Lotsberg Salt

(M.

Devonian),

grow. Brines and crystals sink but,

Alberta.

unless the entire brine column is

satu-

rated, the crystals dissolve before

reaching the lake bottom. This dissolu-

tion progressively increases brine con-

centrations until crystals no longer

dissolve and they begin to accumulate

on the lake floor.

The minerals precipitating from

surface waters are determined by the

chemical composition of the inflow, the

evaporation/inflow ratio, and the pre-

vious history of the lake. If evaporation

just slightly exceeds inflow, only min-

erals of low solubility (carbonates) will

be precipitated. For more soluble min-

erals to be precipitated, the

evapora-

tion/inflow ratio must be higher, and

bottom brines must be dense enough

to support the surface brine for suffi-

cient time to allow it to become super-

saturated with respect to the saline

mineral. When bottom brines are not

dense enough, surface brines sink

before supersaturation is reached.

Lakes experiencing a progressive

reduction in their inflow exhibit a pro-

gressive change from precipitating low

to high solubility salts. But lakes af-

fected by short term decreases in

inflow may display no change in pre-

cipitation because the change in the

evaporation/inflow ratio is not matched

by a sufficient change in the density of

the entire lake brine.

Dissolution of the uppermost evap-

orite deposits in shallow perennial

19.

EVAPORITES

lakes occurs during freshening events

(storms, wet seasons) when mixing of

the entire brine column is complete.

Wind turbulence causes mixing, and

during strong persistent winds there

may be wind setup where brines are

driven or kept

upslope. Migration of

lake margins for tens of kilometres is

controlled by changes in wind condi-

tions, and brine sheets may be discon-

nected from a lake to move over

neighbouring mud- or evaporite-flats.

Wave action is limited by restricted

depth

and/or fetch. Maximum expendi-

ture of wave energy occurs at lake

margins where evaporite crystals may

be continuously moved and abraded

as they

grow.

This forms rippled sands

and gravels composed of rounded

crystal clasts.

Crystal growth at brine surfaces pro-

duces small, thin and platy or needle-

like crystals or brine-displacing, hollow

hopper-shaped crystals. Crystals may

coalesce into floating rafts. Crystals

and rafts float on the brine surface,

held up by surface tension, until they

grow large enough to sink, or are

spilled by waves. They can be trans-

ported downwind to accumulate at

shorelines, but most accumulate on

lake floors forming loosely packed

layers of cumulate crystals.

Carbonates may also be chemically

or biochemically precipitated in

surface waters to produce sediments

similar to those of freshwater lakes

(Dean and Fouch 1983; Eugster and

Kelts 1983; Allen and Collinson,

1986). They form carbonate units be-

tween evaporites during episodes of

lake dilution, or form seasonal partings

between evaporite laminae.

Crystal crusts precipitate on lake

bottoms from well-mixed supersatu-

rated brines. They overgrow cumulate

crystals or older crusts, or nucleate on

stable substrates

(e.g., cyanobacterial

mats; Fig. 16). Crusts deposited at

greater depths are protected from dis-

solution during flood events by a

density stratification developed

between the inflow and the bottom

brine, and become draped by detrital

or chemical laminae. Those in shal-

lower parts are partially dissolved

during freshening events and may be

difficult to distinguish from saline pan

deposits (see below).

Shallow lakes with bottom brines at

or below gypsum saturation support

luxuriant cyanobacterial mats (Fig. 16).

Gypsum and carbonates precipitate in

and upon the mats.

Salt pans are occupied by ephemeral

bodies of brine that form when inflow

temporarily exceeds outflow.

retum to

normal evaporative conditions causes

such lakes to shrink and disappear.

Repeated floods generate sequences

Figure

Displacive halite crystals.

With zonally arranged sediment inclusions. Arrows

(Permian) of west Texas; photo courtesy

C.R.

Handford.

Skeletal halite (now markel

dolomite layers, Lake Alma anhydrite (Ordovician) of Saskatchewan.

mark erosion surface truncating halite crystals

dark pyritic films) displacing storm-sheet

KENDALL

composed of thin salt crusts (of halite,

trona, gypsum or

mirabilitelthenardite)

separated by crystal-rich mud partings

and dissolution surfaces (Fig. 17).

During floods, shallow brackish lakes

form, and fine-grained sediments, in-

troduced by floodwaters, are deposited

as thin layers. Floodwaters also dis-

solve

efflorescent saline crusts on

surrounding flats, and

previously

precipitated salt crusts of the salt pan.

After floods, lake waters concentrate

by evaporation. At first they precipitate

cumulate crystals, but eventually crys-

tals grow on the lake floor as crusts. If

several minerals are precipitated they

exhibit a "bull's-eye" distribution

pattern, with the most soluble minerals

confined to the centre of the lake and

precipitated just prior to complete lake

desiccation. Newly exposed salt crusts

have high porosities with crystal inter-

stices occupied by saturated brine. Salt

surfaces are kept moist by evaporative

draw and by precipitation of dew on hy-

drophilic salt surfaces during cold

nights. The evaporation rate falls to

values as low as 11170th of the rate

from standing bodies of the same

brine, so the brine level rarely drops

more than a few metres beneath the

surface. Nevertheless brines continue

to concentrate by evaporation, with

development of overgrowth cements

on salt crystals, precipitation of more

soluble minerals in pore spaces, and

growth of displacive crystals within

mud layers. Continued crystal growth

causes crust expansion and formation

of polygonal fractures with overthrust

edges (Fig.

8C).

Preferred brine evap-

oration in fractures (Fig.

8B)

causes lo-

calized precipitation of efflorescent

crusts. This widens cracks and con-

tinues the upturn of polygon margins to

form relief up to

cm on the pan

surface that traps eolian dust.

Because each flood introduces new

salts (from surrounding flats and the

hinterland), less salt in the pan is dis-

solved than was precipitated during

preceding lake concentration stages.

Each salt layer is largely composed of

material that has been repeatedly dis-

solved and reprecipitated.

Gypsum pans differ from more

saline salt pans. During floods gypsum

is more commonly reworked than dis-

solved, and accumulates as crystal

lags and intraclasts at the base of mud

layers. Also after mud layers are de-

Figure

Sabkha cycles in Frobisher Evaporite (Mississippian of Saskatchewan). A)

Laminated microdolomites (probably hypersaline lagoon) overlain by cyanobacterial mat

containing anhydrite pseudomorphs of vertically oriented lenticular gypsum crystals, and

nodular mosaic anhydrite.

Upper part of sabkha cycle shown in (A) with inclined rows of

nodular mosaic anhydrite (after subaqueous gypsum), truncated by erosion surface and

overlain by laminated micrites of succeeding lagoonal member. Nature of anhydrite suggests

deposition partly occurred in a salina on the sabkha surface.

posited, they are colonized by cyano-

bacterial mats. Their contorted sur-

faces are the template for later

gypsum precipitation as surface crusts

and as cements that grow beneath the

blistered surfaces. Surfaces are exag-

gerated by continued gypsum crystal

growth that buckles, domes and frac-

tures the layers even more.

Evaporite flats are only occupied

ephemerally by brine sheets. These

are only a few centimetres thick but

may cover areas of several hundred

square kilometres. They form as out-

flows from saline lakes, either season-

ally, when lowered evaporation rates

allow lakes to expand, or episodically,

when wind stresses detach brine

sheets. Evaporite flats are also af-

fected by flood sheets: ephemeral

events caused by lower salinity

inflows. Floods cause dissolution of

earlier evaporites. This material

repre-

cipitates as the floodwaters evaporate,

with the formation of laterally persis-

tent thin beds or laminae (Fig.

18),

some traceable over the entire extent

of the flooding, perhaps hundreds or

thousands of square kilometres.

Evaporite flats occur in essentially

the same geographic settings as mud

flats but probably form in environ-

ments with greater frequency of

flooding and an abundant supply of

detrital evaporite.

Cyanobacterial mats are important

during deposition of thin layered to

laminated gypsum on evaporite flats.

Brine sheets, upon evaporation, pre-

cipitate crusts composed of tiny

aci-

cular crystals. These are reworked by

later brine or flood sheets to form

de-

trital laminae, the main features of

which may be controlled by cyano-

bacterial mats. Mats collect and bind

evaporite sediment and, as the flood

subsides, the coarser load is deposited

as a traction layer or as a settle-out to

form a normally graded lamina. The

cyanobacteria grow through this,

reestablish themselves on the surface,

and protect the underlying sediment

from erosion during the next flood.

Shallow water evaporite facies

An abundance of shallow water clastic

textures and structures, coupled with

the presence of desiccation features,

crystal crusts, and cyanobacterial mats

makes identification of well-preserved

shallow water evaporites relatively

straightforward. Difficulties arise when

depositional features are lost or ob-

scured by later diagenesis, or when

these sediments are modified by

dia-

genetic overprinting, as when the de-

positional environment desiccates and

becomes subject to mud flat pro-

cesses.

Laminated sulphates

Shallow water laminites consist of

current-deposited carbonate micrite

and clastic silt- and sand-sized gypsum

crystals/cleavage fragments in reverse-

and normally graded laminae. Crystals

originally grew as bottom crusts that

were later broken and reworked, or

were precipitated brine-surface crystals

that sank. All particles eventually

become overgrown by cement, and

laminae are converted into interlocking

gypsum crystal mosaics.

Cross bedding, ripple-drift bedding

(Fig.

19),

basal scoured surfaces and

rip-up clasts testify to environments

with periodic high-energy events such

as floods and storms. Reverse-graded

laminae probably form when an up-

ward segregation of coarser particles

occurs in highly concentrated, flowing

sand sheets. These occur in extremely

shallow waters during storm surges,

characteristic of evaporite flats. Shallow

water deposition is also demonstrated

by the presence of micritic, organic-rich

stromatolites; by bird or dinosaur foot-

prints; or by fossil brine shrimp or their

fecal pellets.

Gypsum laminites altered to

anhy-

drite rarely provide sufficient evidence

for precise environmental reconstruc-

tion.

Gypsum crusts

This facies occurs as crusts and su-

perimposed beds of vertically stand-

ing, elongate and commonly swal-

low-tail twinned crystals (Fig.

20),

less

than a centimetre to over five metres

in height. Crystals are commonly

eu-

Figure

15 Surface trench about

deep in Abu Dhabi sabkha with diapiric

layers of anhydrite (after gypsum); upper

sediment layer truncating anhydrite

is a

storm-washover and eolian carbonate-

Figure 16

Rubbery, wrinkled cyanobacterial mat encrusted with gypsum crystals,

1-2

rnm

clastic unit. Photo courtesy

R.K.

Park.

long; Hyeres salt lagoons,

France. Photo courtesy

G.M.

Harwood.

'pJO4pUeH 'H.3 Asaunoo OlOqd '~Jdel 41~0~6

al!leq Jeep pue 'qo!~-uo!sn~ou! pall!k-au!Jq

suo!ieuJalle ~I!M ~epho al!Ieq uoJAaqo a16u!s

I!elaa (3

'MEIPJEM

'3'~

Aq paueol uo!pas u!ql

.eu!Luel al!~pAque ue Aq paleounJ1 ale JawJoj aql 'slelsh3 Gu!~~!~-P!oA 'Jealo pue 'qo!~-uo!snpu! 'Apnop 6u!u!eluoo ~aAel al!le4 uo~~aq:, q6no~ql

uopas u!q1

.ueMayoleyses 'al!~ode~3 a!J!eJd JaMol 'al!~pAque

aeu!wal qi!~ al!leq uo~~aqo lo stsn~=)

.ai!leq ued-au!~g

arn6ij

hedral, and define a vertical palisade

Larger gypsum crystals form in

envi-

clastic gypsum layers or crusts com-

fabric or radiating upward-conical

ronments of constant and active brine

posed of centimetre or smaller crys-

clusters. Crystals are separated by

flow: brine ponds that are drained

tals. Internal lamination and included

micritic carbonate or fine-grained

during wet seasons only develop

cyanobacterial mats indicate crystals

gypsum; or secondary overgrowth

produces an interlocking crystal

mosaic. Other crystals exhibit bizarre

growth and twinning patterns and suffer

crystal-splitting to generate palmate

clusters of subparallel subcrystals (Fig.

21;

see Orti-Cabo and Shearman,

1977). Twinning occurs at a variety of

angles, probably because of organic

matter incorporation along curved

crystal faces; the greater the impurity,

the more obtuse the twinning angle.

Sometimes only one twin arm grows.

The crystals contain faint lamination

(Fig.

21), defined by carbonate and

anhydrite inclusions, which pass

through the crystalline beds, parallel

to bedding. Inclusions may parallel

crystal faces, recording successive

growth stages, or may be more

planar, defining dissolution surfaces.

Many crystals have invaded

cyano-

bacterial mats (Fig. 22B).

Figure 20 Coarsely crystalline gypsum facies. A) Palisades of gypsum crystals (scale bar

cm long), Miocene of Sicily. Photo courtesy

B.C.

Schreiber.

Layered anhydrite with

pseudomorphs after gypsum crystals, Otto Fiord Formation (Pennsylvanian), Ellesmere

Island (scale bar divisions in cm). Photo courtesy N.C.

Wardlaw.

Figure

Thin-bedded to laminated

clastic gypsum from evaporite flat; Lake

MacLeod (Western Australia). Beds are lat-

erally persistent across basin and record

precipitation events after successive

floods.

Figure

Laminated anhydrite with

minor disseminated dolomite defining lami-

nation, ripples, minor cross stratification

Figure

Swallow-tail twinned gypsum crystal (25 cm across) with dissolution surfaces at

and scoured surfaces. Poplar Beds arrow. Inset is a cleavage plane surface of the same crystal with inclusion-defined growth

(Mississippian) of Saskatchewan.

layers. Photo courtesy B.C. Schreiber.

392 KENDALL

grew poikilitically, enclosing surficial

veneers of sediment. Episodes of

slight undersaturation create minor

dissolution surfaces that truncate crys-

tals. Renewed precipitation, however,

occurs on the etched surfaces, burying

them within the crystal (Fig. 21).

Beds composed of gypsum crystals

may be difficult to identify when con-

verted to anhydrite. During burial they

are replaced by mosaic anhydrite

(Rouchy, 1976; Loukes and

Longman,

1982; Kendall and Harwood, 1989)

which can closely resemble, and be

confused with displacive nodular anhy-

drite. When the original gypsum crys-

tals grew and poikilitically enclosed

carbonate laminae associated with

planar dissolution surfaces (Fig. 21)

the nodular anhydrite retains such

lamination and can easily be mistaken

for a replacement of laminated gyp-

sum. Gypsum crusts may also be re-

placed by halite, sylvite or

polyhalite;

the crystals being outlined by anhy-

drite. Such replacements only occur in

the shallowest of brine ponds where

there is a great temperature contrast

between the brine layer and gypsum

crystals beneath the pond floor.

Coarse clastic gypsum

Gypsum sands and pebbly sands,

composed of worn gypsum cleavage

fragments with variable amounts of

carbonate and other materials, may

locally be abundant, but rarely consti-

tute major rock units. They do, how-

ever, indicate that gypsum can be

transported and deposited in the same

manner and environments as other

clastic sediments, so long as the water

body is gypsum saturated. These

sands exhibit structures indicative of

current or wave activity, or may be

penecontemporaneously disturbed and

contain load casts. Clastic gypsum

occurs as shoestring sands or as sand

sheets representing channel, beach,

offshore shoal or spit deposits, or it

may occur as beds between layers of

laminar gypsum or gypsum crusts.

Halite crusts

There are detailed descriptions of

halite crusts from the ancient

1973). Various halite growth habits

are observed but the most important

are chevron halite (Fig. 17) and crusts

composed of cornet-shaped crystals.

Inclusions in crystals are concentrated

into layers parallel to cube faces (Fig.

17C), so that, in crystals with corners

uppermost, the zoning appears as

chevrons with upwardly pointing

apices.

Most upper surfaces of halite layers

are either crystal growth faces (inter-

ruption in growth caused only by tem-

porary and slight brine undersaturation:

characteristic of perennial lakes), or

truncation surfaces associated with

cavities (recording more extreme ep-

isodes of brine undersaturation and ha-

lite dissolution; characteristic of saline

pans). Each layer is composed of two

types of halite: an inclusion-zoned

halite, and a clear halite cement filling

(Wardlaw and Schwerdtner, 1966;

BrOd~10

and Spencer$ 987)9

Figure

A) Anhydrite after displacive gypsum crystals that grew in mud-cracked stroma-

Recent salt pans (Shearman, 1970;

tolitic carbonate. Each gypsum crystal is now a small, angular anhydrite "nodule", Souris

Lowenstein and Hardie, 1985) and

River Formation (Upper Devonian), Saskatchewan.

Anhydrite after gypsum crystals that

from experimental studies (Arthurton,

grew within cyanobacterial mat,

Souris River Formation, Saskatchewan.