Walker J. Facies models, Canada, 1992

Подождите немного. Документ загружается.

. -.

14.

CARBONATE

EVAPORITE MODELS

269

by organic matter (e.g., trilobites,

clams, crinoids). Table

lists the more

important modern

carbonate

compo-

nents and their fossil equivalents.

The sedimentology of organism

remains should not be equated with

their paleoecology. Evolution of the

"modern" carbonate-producing com-

munity was gradual and took place in

the Mesozoic. The shallow water

Paleozoic

brachiopod1echinoderm1

tabulate and rugose corallstromato-

poroidlred algal assemblage was re-

placed by a green and red calcareous

algaelscleractinian coral1mollusc1

foraminifera1 consortium. Many sur-

vivors from the Paleozoic era, such as

the crinoids, stromatoporoids (scle-

rosponges), brachiopods and bry-

ozoans, are still with us, but are

abundant only in deep or cool, or less

saline settings. Interestingly, these

refugees (together with barnacles) pro-

duce most of the sediment on many

modern cool water platforms.

But organisms are not just sediment

producers. Grasses and microbes of

various sorts are efficient sediment

trappers, binders and stabilizers.

lchnofauna burrow through the sedi-

ment, moving and sorting particles, in-

gesting, processing and excreting

sediment, forming grains (peloids) and

diagenetically altering fine particles. A

boring flora and fauna excavates into

grains and hard substrates, and in the

process breaks down the carbonate

host and produces-sediment. Large

skeletons form elevations above the

seafloor and create new environments.

The carbonate platform is truly a

living thing and, as the biosphere has

evolved, its character has varied

through geologic time.

Evaporitic environments are rarely

completely lifeless, although hyper-

salinity does reduce organism diversity

to low levels. Carbonate-secreting or-

ganisms are eliminated early in brine

concentration so that almost all subse-

quent precipitation is inorganic. In the

initial stages, however, microbes thrive

within the brine and on the sediment

bottom. Evaporitic sediments can be

greatly modified by this addition of

organic material, or by the binding and

trapping activities of benthic microbial

mats.

Climate

Evaporites are more sensitive to cli-

mate than any other sediment type.

They provide unequivocal evidence of

arid, but not necessarily warm, condi-

tions (they occur in Antarctic playa

lakes). Nevertheless, almost all an-

cient evaporites formed in warm cli-

mates because, when the atmosphere

is undersaturated, more evaporation

Table

The sedimentary aspect of modern warm and cool water carbonate components and their counterparts in the fossil record.

Modern, Warm Water Modern, Cool Water

Ancient Counterpart Sedimentary Aspect

Corals

ABSENT

Corals, Stromatoporoids, Large components of reefs and

Stromatolites, biogenic mounds

Coralline Sponges,

Rudist Bivalves

Bivalves, Bivalves, Red Algae,

Remain whole or break apart into

Red Algae, Echinoderms Red Algae, Brachiopods,

several pieces to form sand and

Brachiopods, Cephalopods, gravel size particles

Echinoderms, Trilobites

Barnacles

Gastropods, Gastropods, Gastropods

Whole skeletons that form sand

Benthic Foraminifera

Benthic Foraminifera Benthic Foraminifera

and gravel size particles

Green (Codiacean)

Red Algae Red Algae

Phylloid algae, Crinoids and Spontaneously disintegrate upon

Bryozoans other echinoderms, death to form many sand-sized

Bryozoans particles

Ooids, peloids ABSENT Ooids, peloids Concentrically laminated or

micritic sand sized particles

Planktonic Foraminifera, Planktonic Foraminifera,

Planktonic Foraminifera, Medium sand-sized and smaller

Coccoliths, Pteropods

Coccoliths (post-Jurassic), particles in basinal deposits

Styliolinids

Encrusting Foraminifera,

Encrusting Foraminifera, Red

Red Algae, Renalcids, Encrust on or inside hard

Red Algae, Bryozoans

Algae, Bryozoans,

Encrusting Foraminifera, substrates, build up thick

Serpulid Worms Bryozoans deposits or fall off upon death

to form sand grains

Dasyclad Green Algae ABSENT Dasyclad Green Algae Spontaneously disintegrate upon

death to form lime mud

Cyanobacteria and Cyanobacteria and Cyanobacteria and other Trap, bind and precipitate fine-

other calcimicrobes other calcimicrobes calcimicrobes (especially

grained sediments to form mats

pre-Ordovician)

and stromatolites or thrombolites

270 JAMES, KENDALL

occurs at higher temperatures.

Aridity is also prevalent in

moun-

tain-bounded basins at any latitude,

because the mountains act as rainfall-

snowfall traps and barriers, resulting in

basin-floor, rain-shadow deserts,

continental areas in low to middle

latitudes isolated from the ocean,

where temperatures are high and

humidity low, creating high evapora-

tion rates capable of generating brines

that can even precipitate potash salts,

and 3)

regions adjacent to cool, up-

welling oceanic currents

which gen-

erate cool onshore winds with high

humidity but low moisture content.

These winds warm as they move

inland, lowering their humidities and

promoting evaporation.

The general climatic setting also

partly controls the rate at which

ter-

rigenous clastic sediments are deliv-

ered to a shelf or basin by

fluvial and

eolian processes. This siliciclastic

influx can partially or completely

suppress the production of carbonate

1) reducing water transparency,

2) clogging the feeding and/or respira-

tory apparatus of sessile benthic or-

ganisms and/or, 3) increasing nutrient

and particulate organic content of the

water. Reefs, in particular, can grow in

nutrient-poor environments (Chapter

17). An increase in nitrates and phos-

phates can lead to displacement of

coelenterates by filamentous algae

and a dramatic increase in bioeroders

(Hallock and Schlager, 1986). Climate

also controls the amount and rate of

evaporation or rainfall, and thus the

composition of seawater, in shallow,

partially restricted basins on or along-

side continents. Furthermore, climate

controls the "storminess" of the ocean

and whether high energy is constant

or episodic.

Oceanography

The ocean is inexorably linked to the

atmosphere and climate, but there are

inherent attributes which directly affect

carbonate productivity.

Light penetration

Light penetration, which controls the

phototrophs, varies with water depth,

latitude (controls light refraction into

the water) and water clarity. The last is

markedly reduced where turbidity

is high as a result of clastic runoff,

particularly on west sides of

tropicall

subtropical oceans, or upwelling and

high surface productivity above outer

parts of platforms on eastern sides

of oceans (Zeigler

et al.,

1984).

Carbonates have restricted distribu-

tions at these sites.

Water temperature

In open ocean situations there is a

clear temperature stratification. Warm

to cool surface waters extend down to

-50 or -100 m, below which there is a

decrease in temperature (thermo-

cline). The deep ocean below about

1000 m is close to zero degrees. A

similar stratification can be present in

enclosed basins and shallow seas, but

the temperature in many of them, es-

pecially if estuarine circulation is de-

veloped, is roughly the same from

surface to seafloor.

A water temperature of about

20°C

partitions carbonates in modern shal-

low seas into a warm water,

low-lati-

tude realm and a cool- to cold-water

mid- to high-latitude realm. Carbon-

ates of all types are produced on

warm water platforms but such set-

tings are dominated by direct precipi-

tates and particles from phototrophic

organisms. The photic zone is about

70 m deep in most places and it is in

this zone that carbonate production is

highest, being greatest in the upper

10-20 m. On modern

cool water plat-

forms

the photic zone is of lesser im-

portance because carbonates are

generated almost entirely by

nonpho-

totrophic organisms; the only impor-

tant phototrophs are coralline algae.

The rates of sedimentation also seem

to be much lower. Sediments which

accumulate in deeper, cooler water

parts of modern warm water platforms

are much like those at higher latitudes.

Whereas warm and cool water plat-

forms have been differentiated in

Cenozoic and Mesozoic sequences,

their recognition in Paleozoic and

Precambrian carbonates, although

claimed, is less certain.

Water circulation

Unrimmed shelves and ramps have

free circulation and open marine con-

ditions are the norm. Fine-grained

carbonates, suspended by storms

and by organism activities, may be

carried from where they form to

quieter or deeper environments. Rim-

med shelves commonly have poorer

circulation and fine-grained sediment

transport is curtailed. Carbonates are

typically muddy, even in shallow

environments subject to frequent

high-

energy storms. Furthermore, poor

water circulation results in departures

from oceanic water composition.

Freshwater inflow into restricted

shelves causes brackish conditions

nearshore, or even bottom anoxia

where persistent density stratification

results. In arid climates, restricted

shelves become hypersaline. For

evaporites to form, however, the

degree of restriction required is

extreme (Lucia, 1972;

Kendall, 1988),

to the extent that large evaporite de-

posits are confined to basins that

become isolated or nearly isolated

from the open sea. Evaporative losses

in such basins are not offset by inflow,

and the water level drops below sea

level; a process termed

evaporative

drawdown.

Some interpretations of

ancient, widespread and shallow

cra-

tonic seas (Shaw, 1964; Irwin, 1965)

conclude that poor water circulation

was the norm and resulted primarily

from salinity variation, tidal and other

currents being suppressed by friction.

Oxygenation

Because well-oxygenated waters are

essential for the growth of skeletal in-

vertebrates, any decrease in dissolved

oxygen values reduces the diversity

and then the abundance of such or-

ganisms in a predictable way. Partial

to complete anoxia can be induced by

1) a stratification of the water column

by pronounced temperature

and/or

salinity layering, reducing or arresting

vertical mixing, or

a dramatic in-

crease in nutrient supply to surface

waters resulting from increased runoff

or upwelling of deep ocean waters.

Salinity

Increasing salinity reduces biotic diver-

sity (Chapter 15) (although this com-

monly is difficult to differentiate from

the effects of fluctuating salinity) and

above

40%0 most invertebrates dis-

appear; calcareous algae, however,

continue to be sediment producers for

a time. Occasionally, tolerant skeleton-

producing invertebrates thrive in sa-

linities too high for calcareous

mud-

producing algae. These shells may

accumulate in enormous numbers, as

in Shark Bay (Western Australia). This

----

14.

CARBONA

tVAPUKlT-

serves as a reminder that not all hy-

persaline carbonates are muddy. The

amount of carbonate generated by

direct precipitation, however, in-

creases with increasing salinity and

the sediments grade insensibly into

fine-grained evaporitic carbonates.

Tectonics

The tectonic setting controls the rate

and style of subsidence and the nature

of the foundation upon which a car-

bonate platform nucleates or on which

evaporites accumulate. The effect of

antecedent topography is most obvious

during the early stages of rifting.

Carbonate platforms nucleate on horsts

and/or evaporites accumulate in gra-

bens. On newly riffed, passive conti-

nental margins the rate of subsidence

is predictable. Any changes in these

rates occur slowly. lntracratonic basins

have the same subsidence style and

rate as mature passive margins. Thus,

the space for sediment production and

accumulation has been similar in these

settings throughout geologic time.

Platforms and ramps on the inboard

side of foreland basins, toward the

craton, have similar subsidence rates,

but the style is more irregular and less

predictable. Both subsidence and uplift

occur as the basin responds to

litho-

spheric flexure. Carbonate structures

and evaporite accumulations in

strike-

slip basins and/or on thrust complexes

are affected by unpredictable episodic

and dramatic subsidence.

Paleotectonic setting also deter-

mines the nature of the adjacent hin-

terland and therefore, in part, the rate

of terrigenous clastic sediment supply

to the shelf or basin. Too much clastic

sediment will "drown out" the carbon-

ate or evaporite contribution. Finally,

because platforms and basins are on

moving tectonic plates, they may drift

out of latitudes where

carbonatelevap-

orite accumulations are favoured.

Carbonate platforms may move out of

a low-latitude, warm water belt and the

sediments change to those dominated

by cool water invertebrates, with lower

accumulation rates. Alternatively, car-

bonate deposition may be replaced by

terrigenous clastics as the platform

moves into a more humid climatic belt.

Similarly, evaporitic basins may be

transported away from areas of nega-

tive water deficits so that evaporite

accumulation ceases and either

ter-

rigenous clastics or carbonates take

their place.

Carbonate platforms are most fre-

quent along passive continental mar-

gins as

small banks atop horsts on

newly rifted crust,

huge shelves

along mature margins,

giant off-

shore banks, which continue to grow

and coalesce as antecedent riff-blocks

are buried, and 4) atolls around

oceanic volcanoes. They are also fami-

liar as

shelves around the margins

of, and as banks within, shallow

in-

tracratonic basins. Although not as

common on convergent margins, they

do form irregular banks and shelves

encircling fragmented and

obducted

tectonic slivers. Ramps are prevalent

as the initial or foundation stages of

rimmed platforms or as structures

in their own right during geological

periods when there were few large

skeletal organisms to build reefs and

form

rim. They occur most frequently

in intracratonic basins or along the

inner, craton side of foreland basins.

Apart from climatic control, large

evaporite deposits only form in 1)

hydrologically closed sedimentary

basins in which outflow is less than

inflow,

originally deep depressions,

or areas which subside rapidly during

evaporite accumulation, and

set-

tings starved of terrigenous clastic

sediment. Three types of basins are

particularly susceptible to restriction

and therefore potential sites of evap-

orite precipitation (Kinsman, 1974).

lntracratonic basins are long-lived,

with active subsidence histories of

tens of millions of years. Evaporite de-

position tends to be episodic and su-

perimposed so that evaporites are

sandwiched between other sediment

types. Although aerially extensive, the

evaporites are typically thin (tens to

hundreds of metres). If the evaporites

are marine-derived, calcium sulphates

are over-represented relative to halite.

Some basins accumulate nonmarine

evaporites. Evaporitic intervals are

episodic and superimposed. Facies

consist of shallow water and mud flat

sequences (with both shelf and

basin-

central distributions) but episodes of

basin starvation, together with subsi-

dence, may generate depositional

basins with several hundred metres of

relief, in which "deep water" evaporites

can accumulate.

Divergent plate margin basins occur

within continental riff valleys, juvenile

actively spreading oceans, and failed

rifts (aulacogens). Evaporites can be

thick, exceeding

km in many in-

stances. They are generally composed

of halite but also may contain

MgSO,

deficient potash salts. Such composi-

tions suggest the influence of hydro-

thermal fluids, either as the source of

the evaporites or substantially modi-

fying the seawater source (Hardie,

1990). Where continued spreading

has generated wide oceans, evaporite

deposition essentially ceases and the

older evaporites now occur as linear

belts along the newly formed passive

continental margins.

Remnant ocean basins at conver-

gent plate boundaries, under appro-

priate conditions of restriction and

climate, can change into enormous

and deep evaporite basins. Desic-

cation of these basins, under condi-

tions of intense evaporative loss, may

create shallow water environments

several kilometres below world ocean

level. These are situations that have

no parallel today.

AUTOSTRATIGRAPHY

Once carbonate and/or evaporite pro-

duction begins in earnest the sedi-

ments accumulate by vertical and/or

lateral accretion. Vertical accretion

typifies carbonate and evaporite sys-

tems, in that sediments are produced

in place and so, with time, inexorably

build up toward sea level, (or the local

brine level in a desiccated evaporite

basin) even though they may never

reach it. Shallowing-upward is inherent

in the carbonate system and is com-

mon in evaporite settings. Lateral ac-

cretion of carbonates occurs because

the factory is too shallow to retain the

sediments or because they are piled

up against a shoreline, forming a

pro-

grading sedimentary wedge. Whereas

both lateral and vertical accretion will

occur on a carbonate shelf, vertical ac-

cretion is probably more typical on car-

bonate banks (because there are few

highs around which strandline facies

can nucleate) and in large evaporite

basins. Walther's Law is clearly appli-

cable to lateral accretion but may not

always apply during vertical accretion,

because new environments may be

created simply by shallowing or re-

stricting water movement.

exposed,

carbonate and evaporite platforms will

272 JAMES, KENDALL

be subject to diagenesis in the mete-

oric realm, the nature and extent of

which depends upon both intrinsic

(mineralogy, previous history, porosity)

and extrinsic (climate, time) factors

(James and Choquette, 1990).

Since carbonate and evaporite pro-

duction and accumulation are predom-

inantly shallow water phenomena and

the controls are dynamic, the deposi-

tional systems are always in imminent

danger of being shut down. For car-

bonates this occurs by intersecting, or

being intersected by a critical interface

that limits deposition. Such interfaces

are 1)

airlwater (exposure and shut-

down of all carbonate production), 2)

base of photic zone (the demise of

phototrophic organisms), 3) base of the

zone of wave abrasion (above which

bottom currents and wave action may

lead to erosion and cementation),

minimum zone (extinction of all

higher plants and animals), 5) thermo-

cline (water is too cold for most car-

bonate-producing organisms),

pyc-

nocline (the salinity is too great for

most organisms). These interfaces

cause the bounding discontinuities and

are responsible for the conspicuous

packaging of carbonate facies succes-

sions. Changes in the water balance

within evaporitic settings are commonly

fast, and result in equally abrupt ver-

tical changes in evaporite mineralogy

and brine depth (and consequent eva-

porite facies). This may also lead to

cessation of evaporite deposition or its

sudden appearance.

ALLOSTRATIGRAPHY

Space for accumulation must be

created in order for deposition to con-

tinue, even if sediment is being pro-

duced at a rapid rate. For carbonate

systems, this accumulation space is

that space available for deposition

between the

seafloor and any critical

interface. This is where the combined

effects of the controls come into play

in a dynamic way. The principal vari-

ables that determine the size of this

space through time are the rates of 1)

subsidence, 2) eustasy,

sedimenta-

tion (carbonate, evaporite and

silici-

clastic), and

for evaporitic systems,

brine-level suppression below sea

level. Excellent discussions of the

interaction of these variables in car-

bonate systems are found in Kendall

and Schlager

(1981), Tucker and

Wright

(1990), Goldhammer et a/.

(1 990) and Einsele et a/. (1 991),

whereas evaporite systems are dis-

cussed in Kendall (1988).

The concepts of seismic and se-

quence stratigraphy were developed

using the principles of terrigenous

clastic sedimentation (Vail

eta/., 1977;

Van Wagoner et al., 1988). Carbon-

ates and evaporites have only recent-

ly been interpreted using these tech-

niques (Sarg, 1988; Tucker, 1991).

There is currently no consensus as to

the response of sedimentation to

eustasy largely because the same

principles of sedimentation do not gen-

erally apply to both terrigenous

clas-

tics and carbonates/evaporites. Again,

it is the same problem -there is no in

situ factory for terrigenous clastics.

Eustatic changes affect terrigenous

clastic systems by controlling accumu-

lation space and hence shifting the

locus of sedimentation from one site to

another. Eustatic changes in car-

bonate and evaporite systems deter-

mine the health or very existence of

the sediment factory.

Such differences affect the dynam-

ics of sedimentation, and thus

allo-

stratigraphy. To illustrate this point we

generalize and contrast sedimentation

on a terrigenous clastic shelf and

basin with that on a rimmed carbonate

shelf and basin, during a long-term

(third-order) fluctuation in relative sea

level with superimposed short-term

(fourth- and fifth-order) perturbations

(Fig. 5). A cautionary note

these

are end members and if similar com-

parisons are made between

terrige-

nous clastic shelves and carbonate

ramps or open shelves, the differ-

ences are less pronounced.

Evapor-

ites are not restricted to either setting

but have been placed mostly in the

carbonate example because they are

commonest there.

Lowstand systems tract

Terrigenous clastics.

Sediments

bypass the exposed shelf and are de-

posited in deep water on the slope and

basin margin. Sand-rich submarine

fans, fed from

fluvial systems which

cross the shelf, form at the base of

slope during early stages of sea level

fall. Later phases are characterized by

initial filling of incised valleys on the

shelf and the formation of mud-prone

lowstand wedges on the slope and on

top of earlier submarine fans.

Carbonates.

Exactly the opposite

occurs. The carbonate factory is shut

down, or confined to a small fringing

shelf on the slope. The adjacent slope

and basin are thus starved of car-

bonate sediment, except for planktonic

carbonate, and then only in Jurassic

and younger settings. A contrasting

view (Jacquin et al., 1991) suggests

that the narrow shelf is a source of

much sediment and deep water sedi-

mentation is active. Offshore banks or

shelves bordering a carbonate hinter-

land are subject to subaerial

diagen-

esis, the intensity of which depends

upon climate, relief (extent of sea level

fall), time of exposure and original

mineralogy, resulting in a karst sub-

aerial unconformity bounding surface.

When shelves boraer a landmass of

terrigenous clastic sediments, al-

though subaerial diagenesis is com-

mon, they are also likely to be ve-

neered by prograding

fluvial deposits

that can spill over the shelf edge, cre-

ating a terrigenous clastic

lowstand

fan or wedge. This results in alter-

nating carbonate and terrigenous

clastic sediments on the shelf and in

the basin (Chapter 3).

Evaporites.

Basin-central evapor-

ites, often of great thickness, are char-

acteristic when basins become iso-

lated or greatly restricted by lowered

sea level. The extent of water-level

drop is greatly magnified by evapora-

tive drawdown. Brines in basins that

experience only partial

drawdown

rarely reach halite saturation and are

characterized by marginal gypsum

wedges and basinal, deep water lami-

nated

gypsumlevaporite carbonates

(Tucker, 1991). Basins

subject to near-

ly complete desiccation have shallow

water mud flat evaporites on the basin

floor. Such deposits are typically domi-

nated by halite and can contain some

of the more soluble potash salts.

Because evaporite deposition occurs

in near isolated environments, the ele-

vation of the basin rim (relative to sea

level), rather than the elevation of the

depositional site, is of primary impor-

tance, While slight drops in sea level

can allow basins to desiccate, slight

rises may drown basins and quickly

eliminate evaporite deposition alto-

gether. Evaporite accumulation also

causes

onlap of the basin flanks, unre-

lated to sea level rise.

Transgressive systems tract

Terrigenous clastics.

Incised valleys

on the shelf are filled and estuarine

sedimentation accompanies flooding.

There is a rapid landward shift in

facies belts, little widespread shelf de-

position and the slope and basin are

largely sediment starved, culminating

in a maximum flooding surface.

Carbonates.

In contrast, this period

of long-term relative sea level rise

allows the carbonate factory to oper-

ate at optimum capacity, slowed only

by short periods of incipient drowning.

Migration of an oxygen-minimum zone

onto the platform can also cause peri-

odic shutdown. Progressive addition of

new accumulation space results in the

infilling of previous topography, accu-

mulation of thick, subtidal-dominated

sediment packages and growth of

reefs. Amalgamated

subtidal units are

14.

CARBONATE

EVAPORITE MODELS

273

common. Vertical sediment accretion

or facies backstepping is the typical

mode of sedimentation. Reefs, which

have the greatest potential of all car-

bonates to track sea level rise

(because of their relatively high growth

rates), can build exceptionally high

(stratigraphically thick) reliefs. The

amount of off-platform sediment trans-

port depends upon the effectiveness

of the rim and the rates of carbonate

sediment production. Prograding car-

bonate

andlor evaporite tidal flats do

not generally

reach the shelf edge and

can be stranded during short-term sea

level changes. A maximum flooding

surface can cap the transgressive

systems tract. The condensed section

develops when parts of the platform

fall so far below the base of the photic

zone that carbonate production is

reduced dramatically or stops entirely.

LOW

ALLUVIAL

PLAIN

COASTAL

PLAIN

EVAPORITE

Figure

Diagrams illustrating the general sedimentary response of terrigenous clastic

versus carbonate-evaporite shelves to long-term (third-order) changes in sea level. Shaded

sequences formed during the indicated positions of sea level.

Evaporites.

the rim is continuous

and well developed it can be a barrier

to water circulation. Short-term

fourth-

or fifth-order sea level falls partially to

completely restrict movement of

marine waters onto the shelf, in-

creasing the potential for shelf

evapor-

ites. Subtidal evaporites can develop

only on the platform during these

short-term, sea level falls, and will

generally. be confined to

onshelf or

onbank basins.

Highstand systems tract

Terrigenous clastics.

Early phases

encompass eustatic rise and stillstand

while later phases record eustatic

still-

stand and the beginnings of sea level

fall. There is general widespread shelf

deposition. Aggradation typifies early

stages and is usually succeeded by

pronounced progradation. Depending

upon the rate of sediment supply, the

top of this systems tract can contain

extensive

fluvial and alluvial facies.

Slope and basin sedimentation in-

crease dramatically during late stages.

Carbonates.

Carbonate and terrige-

nous clastic settings are most alike

during this situation. The rate of addi-

tion of new accumulation space is

low and under many circumstances

periods of deposition alternate with

roughly equal periods of exposure.

The carbonate factory shrinks and

becomes localized. During early stages

antecedent depositional topography on

the shelf is filled in. Reefs reach sea

level and begin to expand laterally.

Prograding tidal flat caps may periodi-

cally reach the shelf edge. Relatively

short episodes of subaerial exposure

result in moderate diagenetic change

to carbonates.

During late stages, because the

platform is essentially at mean sea

level, the factory operates only inter-

mittently, when flooded due to short-

term changes in sea level. The main

factory is at the shelf margin, pro-

ducing a "shelf margin systems tract"

(Sarg,

1988).

Since accumulation

space on the platform top is small,

peritidal deposition is common. Few

reefs develop and biostromes or sand

shoals are the norm. The platform

exports much sediment, especially

from the margin facies, resulting in

coarse-grained sediment-gravity flows

and thick prograding or forestepping

wedges of slope sediment. Sediments

274 JAMES, KENDALL

on the platform top are intensively

altered by long periods of meteoric di-

agenesis.

Evaporites.

Prograding alluvial and

strandline terrigenous clastic facies

can form isolated saline lakes on allu-

vial plains and salinas behind beach

ridges. The reduced accumulation

space and propensity for poor circula-

tion on the carbonate platform top is

ideal for thin shelf and peritidal evap-

orite accumulation. Diachronous units

of sabkha and salina evaporites may

be widespread. Under exceptional

conditions of a rimmed platform, short-

term falls in sea level may result in

large hypersaline lagoons or shelves

within which

subtidal evaporites form.

If the climate is arid and rimmed

shelves are markedly restricted, wide-

spread thin

subtidal evaporites

develop surmounted by thick peritidal

evaporites.

CYCLICITY

The vertical, cyclic repetition of car-

bonate

and/or evaporite facies is a

hallmark of carbonate platforms and

evaporite basins (Wilson, 1975;

Kendall and Schlager, 1981; Algeo

and Wilkinson, 1988). Carbonate

cycles tend to record deposition in pro-

gressively shallower environments

(shallowing-upward); evaporites may

display vertical changes in mineralogy

indicating increasing salinity

(brining-

upward) or, less commonly, de-

creasing salinity (freshening-upward).

Cycles are m-scale to 100-m scale

and their universality means that the

mechanisms are either inherent in the

system (autostratigraphic) or the plat-

form is responding to some outside

control (allostratigraphic) that has

been present since Precambrian time,

or both. The drive to produce a shal-

lowing-upward, brining-upward or

freshening-upward sediment package

by either lateral or vertical accretion is

inherent in the systems; what is much

more difficult to determine is why and

how numerous packages are stacked

in a variety of different ways (Chap-

ters 15, 16, 18, 19).

Thin evaporite units probably form in

time intervals too short to have been

markedly influenced by sea level

changes. On the other hand, thicker,

>0.1 km, evaporites are invariably

compound units, commonly markedly

cyclic. The delicate balance of influx,

reflux and brine residence time that

controls evaporite deposition is partic-

ularly sensitive to changes in sea level

in marine-fed basins and evaporite

shelves. It is therefore not surprising

that sea level changes have been

seen as controls of evaporite basin

cyclicity. Climate, however, also con-

trols the water balance in evapor-

ite environments. Climate changes

alone, or in combination with sea level

changes, are also possible causes of

evaporite cyclicity. Continental eva-

porite~, particularly those forming in

basins above sea level, will obviously

respond only to climate (and tectonic)

changes.

SUMMARY

Carbonate and evaporite facies, while

controlled to a large degree by eu-

stasy, are also defined and deter-

mined by the nature of the in situ

sediment factory. They are therefore

fundamentally different from terrige-

nous clastic sediments. This theme

pervades the following chapters on

carbonate facies models. Evaporite

facies are treated in a single chapter.

This is because they are differentiated

on the basis of sedimentary features,

and so independent of geographic

setting and associated facies, carbon-

ate or terrigenous clastic. Identification

of environmental facies depends more

on the stratigraphic relationship of

evaporites to other facies and their

distributions relative to depositional

basins.

ACKNOWLEDGEMENTS

This introduction is much improved by

the helpful and constructive comments

C,A. Cowan,

R.G.

Walker, P.-A.

Bourque, M. Coniglio and G.M.

Harwood.

REFERENCES

Basic sources of information

Bathurst, R.G.C., 1975, Carbonate sedi-

ments and their diagenesis: Amster-

dam, Elsevier, 658 p.

Contains good summaries of carbonate

components and their modern environ-

ments of deposition.

Crevello, P.D., Wilson, J.L., Sarg, J.F. and

Read, J.F., eds., 1989, Controls on car-

bonate platform and basin development:

Society of Economic Paleontologists

and Mineralogists, Special Publication

44,405 p.

Numerous articles on the response of

different carbonates of various ages to

intrinsic and extrinsic regulatory param-

eters

a good casebook.

Franseen, E.K., Watney, W.L., Kendall,

C.G.St.C. and Ross, W., eds., 1991,

Sedimentary modelling: computer simu-

lations and methods for improved pa-

rameter definition: Kansas Geological

Survey, Bulletin 233, 524 p.

Numerous studies modelling different

carbonate depositional systems.

Kendall, A.C., 1988, Aspects of evaporite

basin stratigraphy, in Schreiber, B.C.,

ed. Evaporites and hydrocarbons: New

York, Columbia University Press, p. 11-

65.

Contains sections dealing with basinal

models, the inapplicability of Walther's

law, and relations of evaporites to sea

level changes.

Melvin, J.L., ed., 1991, Evaporites,

petroleum and mineral resources:

Amsterdam, Elsevier, 556 p.

Includes several excellent and exhaus-

tive papers on non-marine, sabkha and

shallow-water evaporites, some with ex-

cellent bibliographies.

Read, J.F., 1985, Carbonate platform

facies models: American Association of

Petroleum Geologists, Bulletin, v. 69,

1-21.

succinct and important summary of

the different types of platforms and

ramps, with many examples

the best

recent synthesis.

Schlager, W., 1981, The paradox of

drowned reefs and carbonate platforms:

Geological Society of America, Bulletin,

92, p. 197-21 1.

One of the first papers to critically look

at the controls of platform development,

an important place to start.

Scholle, P.A., ~ebod, D.G. and Moore,

C.H., eds., 1983, Carbonate deposi-

tional environments: American Asso-

ciation of Petroleum Geologists, Memoir

33, 708

beautiful synthesis of all carbonate

(and some evaporitic) depositional envi-

ronments, illustrated with colour pho-

tographs and diagrams

a critical

source book for any student.

Schreiber, B.C., ed., 1988, Evaporites and

hydrocarbons: New York, Columbia

University Press,

475

Compilation of papers covering evap-

orite stratigraphy and most aspects of

marine-derived evaporites.

14.

CARBONATE

EVAPORITE MODELS

Warren, J.K., 1989, Evaporite sedimen-

tology: Englewood Cliffs, Prentice Hall,

285 p.

Probably the best place to start; impor-

tant because it discusses models for the

origin of large-scale shelf and basinal

evaporites.

Wilgus, C.K., Hastings, B.S., Posamentier,

H.W., Ross, C.A. and Kendall,

C.G.St.C., eds., 1988, Sea level

changes: an integrated approach:

Society of Economic Paleontologists

and Mineralogists, Special Publication

42,407 p.

Collection of papers emphasizing con-

ceptual basis of sequence stratigraphy

plus numerous case studies of sea level

change from the geological record.

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

ologic history: Berlin, Springer-Verlag,

471 p.

The single most important source of in-

formation on carbonate deposits of all

ages, including thought provoking syn-

theses.

Other references

Algeo, T.J. and Wilkinson, B.H., 1988,

Periodicity of mesoscale Phanerozoic

sedimentary cycles and the role of

Milankovitch orbital modulation: Journal

of Geology, v. 96, p. 313-322.

Dean, W.E. and Schreiber, B.C., eds.,

1978, Notes for a short course on

marine evaporites: Society of Economic

Paleontologists and Mineralogists, Short

Course No. 4, 395 p.

Einsele,

G., Ricken,

and Seilacher, A.

eds., 1991, Cycles and events in stratig-

raphy; Berlin, Springer-Verlag, 955 p.

Geldsetzer, H.H.J., James, N.P. and

Tebbutt, G., eds., 1988, Reefs; Canada

and adjacent areas: Canadian Society

of Petroleum Geologists, Memoir 13,

775 p.

Ginsburg, R.N. and James, N.P., 1975,

Holocene carbonate sediments of conti-

nental shelves, in Burke, C.A. and

Drake, C.L., eds., The geology of conti-

nental margins: New York,

Springer-

Verlag, p. 137-155.

Goldhammer, R.K., Dunn, P.A. and Hardie,

L.A., 1990, Depositional cycles, com-

posite sea-level changes, cycle stacking

patterns, and the hierarchy of strati-

graphic forcing: examples from alpine

Triassic platform carbonates: Geological

Society of America, Bulletin, v. 102, p.

535562.

Hallock, P. and Schlager, W., 1986,

Nutrient excess and the demise of coral

reefs and carbonate platforms: Palaios,

v. 1, p. 389-398.

Hardie, L.A., 1986, Stratigraphic models for

carbonate tidal-flat deposition, in

Hardie, L.A. and Shinn, E.A., eds.,

Carbonate depositional environments,

Part 3: Tidal flats: Colorado School of

Mines Quarterly, v. 81, p. 59-73.

Hardie, L.A., 1990, The roles of rifting and

hydrothermal

CaCI, brines in the origin

of potash evaporites, an hypothesis:

American Journal of Science, v. 290,

p. 43-106.

Irwin, M.L., 1965, General theory of epeiric

clear water sedimentation: American

Association of Petroleum Geologists,

Bulletin, v. 49, p. 445-459.

Jacquin, T., Vail, P.R., Arnaud-Vanneau,

A,, Arnaud, H. and Ravenne, C., 1991,

Systems tracts and depositional

systems in a carbonate setting: a study

of continuous outcrops from platform to

basin on the scale of seismic lines:

Marine and Petroleum Geology, v. 8,

p. 122-1 39.

James, N.P. and Mountjoy, E.W., 1983,

Shelf-slope break in fossil carbonate

platforms, in Stanley, D.J. and Moore,

G.T., eds., The shelf-slope break; crit-

ical interface in continental margins:

Society of Economic Paleontologists

and Mineralogists, Special Publication

33,

p. 189-206.

James, N.P. and Choquette, P.W., 1990,

Limestones

the meteoric diagenetic

environment, in Mcllreath, I.A. and

Morrow, D.W., eds., Diagenesis:

Geoscience Canada Reprint Series 4,

p. 35-74.

Kendall,

C.G.St.C. and Schlager, W., 1981,

Carbonates and relative changes in sea

level: Marine Geology, v.

44,

p. 181-212.

Kinsman, D.J.J., 1974, Evaporite deposits

on continental margins: 4th Symposium

on Salt, Cleveland, Ohio, Northern Ohio

Geological Society, v.

1, p. 255-259.

Lucia, F.J., 1972, Recognition of evaporite

carbonate shoreline sedimentation, in

Rigby, J.K. and Hamblin, W.K., eds.,

Recognition of ancient sedimentary en-

vironments: Society of Economic

Paleontologists and Mineralogists,

Special Publication 16, p. 160-191.

Nelson, C.S., ed., 1988, Cool water

carbonate sediments: Sedimentary

Geology, v. 60, 367 p.

Postamentier, H.W.,

Jervey, M.T. and Vail,

P.R., 1988, Eustatic controls on

clastic

deposition

conceptual framework, in

Wilgus, C.K., et al., eds., Sea level

changes; an integrated approach:

Society of Economic Paleontologists

and Mineralogists, Special Publication

42,

109-124.

Read, J.F., 1985, Carbonate platform

facies models; American Association of

Petroleum Geologists, v.69, p.1-21.

Reading, H.G., ed., 1986, Sedimentary en-

vironments and facies: Oxford,

Blackwell Scientific Publications, 615 p.

Sarg, J.F., 1988, Carbonate sequence

stratigraphy, in Wilgus, C.K., etal., eds.,

Sea level changes; an integrated ap-

proach: Society of Economic Paleon-

tologists and Mineralogists, Special

Publication 42, p.

155-182.

Shaw, A.B., 1964, Time in stratigraphy:

McGraw-Hill, New York, 365 p.

Sonnenfeld, P., 1984, Brines and

evapor-

ites: Orlando, Academic Press, 613 p.

Tucker, M.E., 1991, Sequence stratigraphy

of carbonate-evaporite basins: models

and application to the Upper Permian

(Zechstein) of northeast England and

adjoining North Sea: Journal of

Geological Society of London, v. 148,

p. 1019-1036.

Tucker, M.E. and Wright, V.P., 1990,

Carbonate sedimentology: Oxford,

Blackwell Scientific Publications, 482 p.

Vail, P.R. and Todd, R.G., 1981, North Sea

Jurassic unconformities,

chronostratig-

raphy and sea-level changes from

seismic stratigraphy: Petroleum geology

of the Continental Shelf of Northwest

Europe, Proceedings, p. 216-235.

Vail, P.R., Mitchum, R.M. Jr., Todd,

R.G.,

Widmier, J.M., Thompson, S., Ill,

Sangree, J.B., Bubb, J.N. and Hatlelid,

W.G., 1977, Seismic stratigraphy and

global changes in sea level, in Clayton,

C.E., ed., Seismic stratigraphy

appli-

cation to hydrocarbons: American

Association of Petroleum Geologists,

Memoir 26, p. 49-212.

Van Wagoner, J.C., Postamentier, H.W.,

Mitchum, R.M., Vail, P.R., Sarg, J.F.,

Loutit, T.S., and Hardenbol, J., 1988, An

overview of the fundamentals of se-

quence stratigraphy and key defini-

tions, in Wilgus, C.K., et

a/., eds., Sea

level changes; an integrated approach,

Society of Economic Paleontologists

and Mineralogists, Special Publication

42,

p. 39-46.

Zeigler, A.M., Hulver, M.L.,

Lottes, A.L. and

Schmachtenberg, W.F., 1984,

Uniformi-

tarism and palaeoclimates: inferences

from the distribution of carbonate rocks,

in Brenchley, P., ed., Fossils and

climate: New York, John Wiley, p. 3-25.

Shallow Platform Carbonates

Andre Desrochers, Ottawa-Carleton Geosc~ence Centre,

Department of Geology, University of Ottawa, Ottawa, Ontario

KIN

6N5

INTRODUCTION

The shallow illuminated seafloor is the

place where carbonate is fixed most

rapidly by plants and animals and

where carbonate particles precipitate

most easily. While material may be ex-

ported from this factory to form tidal

flats or slope deposits, much also

stays in place, to be preserved as

what are euphemistically called

sub-

tidal limestones. The focus of this

chapter is the origin, composition,

facies and stratigraphy of these plat-

form sediments.

The dynamics of this environment

are outlined in Figure

The starting

point (upper left) is the carbonate

factory where the nature of the sedi-

ment is dictated by evolution, organism

biology, climate, salinity, water depth,

and

clastic sediment influx. Once

formed, the fate of the carbonate sedi-

ments is dictated by wind waves, tides,

storms, and large-scale oceanic cur-

rents. The balance between sediment

production and sediment transportation

determines the

growth potential of the

platform.

Carbonate platforms are dynamic

depositional systems because fluctua-

tions in sea level and variations in the

rate and style of subsidence cause

ever-changing environmental

condi-

Figure

Diagram summarizing the main factors that control the formation of carbonate successions on platforms.

278 JONES, DESROCHERS

tions (centre).

drop in sea level may

lead to siliciclastic sediment influx or

karsting of previously deposited car-

bonates. Karst topography can control

platform morphology during subse-

quent flooding. The vertical accumula-

tion of platform sediments (right) is

governed by the interplay between

carbonate growth potential, the rate of

change in relative sea level, and plat-

form morphology.

The first section of this chapter out-

lines factors that control sediment pro-

duction and distribution in the car-

bonate factory. The second presents

platform facies models and uses

modern and ancient examples to illus-

trate variations that can be expected in

each type. The third part examines the

factors that determine carbonate accu-

mulation on platforms through time.

The fourth illustrates the response of

platforms to the dynamic changes that

occur through time.

THE CARBONATE FACTORY

Location

The main environmental factors which

determine the health of the carbonate

factory are outlined in Chapter

14.

carbonate factory can form in any part

of the world where shallow water,

marine conditions exist.

fully func-

tional factory will only be established,

however, in areas isolated from exten-

sive siliciclastic influx. The broad conti-

nental shelf stretching from North

Carolina southward to the Florida

Keys, for example, is the site of both

siliciclastic and carbonate sedimenta-

tion (Fig. 2). Siliciclastic sedimentation

occurs in the north because numerous

rivers shed detrital sediment onto the

shelf. Carbonate sedimentation occurs

south of Miami mainly because

silici-

clastic sediments are absent. The Be-

lize, West Florida (Fig. 2), and

Queens-

land shelves are variants of this

general scheme because they have

siliciclastics on the inner shelf and car-

bonates on the outer shelf.

The areal dimensions of a car-

bonate factory are governed by the

size of the platform and extent of

siliciclastic sedimentation. Because

benthic phototropic organisms are

the

source of most carbonate sediment in

warm water factories, the vertical di-

mension is determined by the depth of

the photic zone, usually

100

Information regarding cool water car-

bonate factories is sparse because

they have not received as much study

as their warm water counterparts. The

cool water factory seems to be effec-

tive to depths of

350

m (or more)

because most sediment is derived

from skeletons of nonphototrophic or-

ganisms. Depths below the photic

zone on and

.around warm water plat-

forms are characterized by cool water

and pelagic carbonate sedimentation.

Of particular importance to the con-

struction of

subtidal platform facies

models and their interpretation in the

rock record are sediment composition,

the relationships between sediment

and environment and the distribution

of fossilized benthic organisms.

Peloidal lime sands

Peloidal lime muds

Figure 2 Map showing regional facies distribution on the Bahama Bank and the coasts

Florida. Modified from Wanless and Dravis (1989).

OOlDS

SKELETAL

I-)

AGGREGATES

PELLET

-----

Green CALCAREOUS

Red ALGAE

CORAL

(hermatypic)

MOLLUSC

FORAM (benthonlc)

ANIMAL

BRYOZOA

BARNACLE

OTHER (echinoderm,

ostraad, sponge

spicule, worm tubes,

ahermalypic corals

1111

May be important or dominant component

Figure

Comparison of foramol and chlorozoan carbonate assemblages. Modified from

Lees and Buller

(1 972).

Sediment composition

and can be divided into a

foramol

and

On a global scale, modern carbonate a

chlorozoan

assemblage (Fig. 3). The

sediments are largely controlled by foramol assemblage, composed of

temperature and salinity (Chapter 14) benthic foraminifera, molluscs,

barna-

Figure 4

Distribution and origin of bioclasts across Florida Bay and Florida Shelf. Note that

the sediment is derived from a diverse array of organisms. Modified from

Ginsburg (1956).

Dominant

Minor A =Aragonite

lmporfant

LMC

Low

magnesium calcite

HMC = High magnesium calcite

Figure

Distribution of main groups of animals and plants through the Paleozoic (P),

Mesozoic (M), and Cenozoic (C). The fact

that the different groups of animals and plants

have skeletons formed of different mineralogies means that there are substantial differences

between the Paleozoic and

Mesozoic/Cenozoic sediments and cool versus warm water sedi-

ments (compare with Figure

3).

Adapted from Wilkinson (1982).

cles, bryozoa, and calcareous red

algae characterizes cool water set-

tings. Recent studies of modern and

ancient cool water shelf carbonates

have highlighted the importance of

bryozoa and the term

bryomol

has

been applied to these distinctive sedi-

ments (Nelson

al.,

1988). Converse-

ly, the chlorozoan assemblage, dom-

inated by hermatypic corals and

calcareous green algae in addition to

the foramol components, occurs in

warm water areas where water temper-

ature is above 18°C. Foramol and

chlorozoan assemblages develop in

places where salinities range between

32 and

4W0. Elevated salinities in trop-

ical regions cause the demise of corals

but not calcareous green algae. Thus,

chloralgal

association usually occurs

at salinities of

>40%0. Ooids and aggre-

gates are restricted to the chlorozoan

assemblage. Pellets are more common

in the chlorozoan assemblage than in

the foramol association.

Analysis of carbonate sediment from

any platform, such as the Florida Shelf

(Fig. 4), shows that it is largely skeletal

material derived from plants

and/or

animals that secrete skeletal material

with ,a specific composition

(Scholle,

1978). Such precipitation is governed

by the metabolic processes of the or-

ganisms and so, even under similar

environmental conditions, different or-

ganisms will precipitate low magne-

sium calcite (LMC), high magnesium

calcite (HMC), or aragonite. The

chloro-

zoan and chloralgal assemblages (Fig.

are mostly aragonite and HMC. The

foramol and bryomol assemblages are

composed of HMC and LMC. Evolu-

tion becomes an important considera-

tion because different groups of or-

ganisms, each with its characteristic

mineralogy, were important at different

times in geological history (Fig.

5).

These mineralogies are variably pre-

served in the fossil record; arago-

nite fossils are commonly dissolved

whereas HMC and LMC components

are usually preserved.

Lithofacies

Although initial sediment composition

is determined to a large degree by

skeletal architecture, resultant

litho-

facies are also determined by the

overall energy spectrum and the effec-

tiveness of sediment binding. In

general terms, except for platform