Walker J. Facies models, Canada, 1992

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280

JONES, DESROCHERS

margin facies, lithofacies on rimmed

Bafflestone and bindstone

material may be incorporated if it is

platforms are muddy while those on

Plants can induce deposition through

transported into the area by storms.

open, unrimmed platforms are grainy.

baffling and binding. They also provide

Such facies are easy to recognize in

ideal substrates for calcareous epi-

modern warm and cool water settings,

Mudstone

biontic organisms. Although plants typ-

because the plants are obvious (Fig.

Calcareous mud in warm water set-

ically root in muds and sands, coarser

7).

Their recognition in ancient succes-

tinas comes from the breakdown of

green calcareous algae (e.g., Penicil-

/us, Halimeda; Fig.

6),

inorganic pre-

cipitation from seawater,

and/or from

the disintegration of large skeletal parti-

cles into their smallest crystallographic

unit. These muds accumulate in quiet

water areas that are not affected by

tidal

and/or strong oceanic currents.

Such habitats are found in deep water

shelf areas below wave base or in the

lee of islands and shoals. Conversely,

calcareous mud can accumulate in

moderate- to high-energy areas if it is

bound into place by cyanobacterial

mats, sea grasses like

Thalassia, or

trees such as mangroves. Recognizing

low-energy as opposed to

moderate-

or high-energy mudstone facies

diffi-

cult because they are lithologically

similar. They are best differentiated on

the basis of faunal

composition/diver-

sity and sedimentary structures.

~ra~onitic muds-are rare or absent

in cool water settings because the

or-

Figure

General view of lagoonal seafloor covered with skeletal sand and mud with nu-

ganisms that produce the aragonite

merous green calcareous algae (Halimeda), each about

cm high. Water is about

are scarce. ~~d

these settings

deep. Frank Sound, Grand Cayman Island, West Indies.

formed from the bioerosional break-

down of benthic skeletons or the accu-

mulation of pelagic ooze.

Wackestone and packstone

Wackestones and packstones are

transitional between low-energy

mud-

stones and high-energy grainstones in

warm water settings. They are not

common in cool water settings be-

cause of the overall lack of mud. Such

sediments generally accumulate on

warm water platforms where current

activity has been insufficient to remove

the mud. As such they tend to be

located away from the edges of plat-

forms or on deeper parts of ramps

where there is some protection. In

areas of moderate to high energy they

are organically bound.

Many wackestones and packstones

are formed of pellets that reflect the

activity of burrowing

(e.g., ghost

shrimps)

and

grazing

(e'g''

sea

cucum-

Figure

General view of Thalassia grass which is stabilizing sand and mud in the inner

hers)

animals'

cemented

early*

such

part of a shallow lagoon in water about

m deep. The mound is about

cm across and

fecal

pellets

are

preselved

and become

was formed from sediment expelled by the burrowing shrimp Callianassa. The dark-coloured

the dominant constituent of resultant

sediment is material that has been brought up from sediments below the surface where con-

wackestones or packstones.

ditions are reducing.

15.

PLATFORM CARBONATES

$ions is difficult because the organic

lissue of plants is lost following their

rapid decay. Examples of fossilized

Thalassia

grass and/or mangrove roots

are the exception rather than the rule.

Poor sorting of sediment, or the presence

identifiable mound-like structures may

provide some clues. Knowledge of the

associated

fauna/flora, however, may

the best evidence because some taxa

are known to be specific to

seagrass

meadows or mangroves. Gastropods and

foraminifera are particularly useful

because certain forms live only on grass

blades or submerged tree roots (Jones

and Hunter, 1990).

Bindstones produced through the trap-

ping action of algal mats may be easier to

recognize because they are generally

laminated. Conversely, burrowing

organ-

Figure

General view of shelf sands in

m of water that have been extensively biotur-

bated by burrowing shrimp

(Callianassa).

Such burrowing activity will homogenize the sedi-

ment and completely destroy other original sedimentary structures. Hammer handle is

long.

Figure

Boulders from a rimming reef prograding over lagoonal sands. Most clasts are

coral heads that are transported during storms. Large clasts in the foreground are

cm in

diameter. East Sound, Grand Cayman Island.

isms

(e.g., shrimp, crabs; Fig. 8) may

remove all trace of such laminations as

they homogenize the sediment.

Grainstone and rudstone

Grainstones in cool water settings are

well-washed skeletal sands derived

from bryozoa, molluscs, brachiopods,

and foraminifera. Further information

on the depositional setting of such

grainstones can be deduced from the

bioclasts. Skeletal morphology has

been used to define environmentally

sensitive subfacies in the widespread

bryozoa-rich facies that characterize

cool water platforms (Nelson

a/.,

1988; James and Bone, 1991).

Warm water grainstones, formed

of bioclasts, ooids,

and/or peloids,

usually occur in high-energy areas,

such as shoals and beaches. In some

cases high-energy conditions are con-

stant

(e.g., tides and/or oceanic waves

and currents). In other instances the

environments are only periodically

subjected to storm-induced

high-

energy conditions which remove the

mud but leave coarser-grained sedi-

ment behind. Haloes of gravels and

boulder-rich sediments (rudstones) are

common around patch reefs or behind

barrier reefs (Figs. 9, 10). Such sedi-

ment may be generated catastrophi-

cally during storms or through the

everyday breakdown of the reefs by

bioerosion.

Storm

deposlts

Storms can quickly and radically alter

sediment distribution on any part of the

platform that is above storm wave base.

Although their effect on shallow water

carbonates is the most obvious it must

be remembered that the base of

stom-

generated waves may reach tremen-

dous depths. Nelson

a/.

(1 982) noted

that storm waves produced current ve-

locities of up to 11

cmlsec at depths

down to

100

m on the Three Kings

Plateau, northern New

Zealand.

Storm waves and currents cause ex-

tensive remobilization of sediments and

basinward transport of sands and

muds. This leaves beds of

grainstone-

packstone with distinctive sedimentary

structures that will be interbedded with

the fairweather, bioturbated

wacke-

stone-mudstone (Figs. 11, 12). Hum-

mocky cross-stratified grainstone and

graded grainstone-packstone are two

types of tempestite that commonly form

282 JONES, DESROCHERS

between fairweather and storm wave

base. Tempestite beds can, however,

range from interbedded

grainstone-

mudstone to amalgamated grainstone

depending upon water depth, frequency

and intensity of storms, and proximity to

the source area or shoreline (Aigner,

1985; Sami and Desrochers, 1992).

Other storm-generated features in

tem-

pestites include scoured bases, swaly

cross stratification, grading, gutter

casts, interference ripple caps, shell

co-

quinas and condensation horizons (see

Kreisa, 1981).

Storm deposits can be recognized

by evaluating geomorphologic, strati-

graphic, and biostratinomic evidence.

Geomorphic evidence includes the for-

mation of

offbank spillover lobes and

onbank sand lobes (Aigner, 1985)

which appear to develop after the

passage of hurricanes over shallow

water areas (Ball, 1967; Hine, 1977).

Stratigraphic evidence includes

sharp-

based skeletal layers that commonly

overlie erosional surfaces and

fining-

upward sequences (Aigner, 1985).

Biostratinomic evidence includes ex-

cellent preservation of articulated

shells despite movement from their

original life position. This occurs

because the shells are rapidly buried by

storm-transported sediment and there

is little opportunity for postmortem

breakdown by biological and physical

processes. Storms will commonly trans-

port or move large masses of skeletal

material from their place of growth.

Thus, coral heads can be transported

for considerable distances before being

deposited elsewhere on the platform.

This process may be accelerated if the

coral basal attachment had previously

been weakened by bioerosion.

Distribution

biota

Animals and plants are sensitive to

their surrounding environment. The

biota will change in concert with varia-

tions in light level, oxygen content,

water temperature, sedimentation rate,

water depth, substrate condition, water

turbidity, and food availability across a

carbonate platform

(Heckel, 1972;

Pickerill and Brenchley, 1991). This

dependence of the biota on the envi-

ronment is important because it

pro-

pertaining to the different benthic com-

munities and their distribution provide

vital clues regarding depositional con-

ditions. On modern rimmed platforms

Thalassia

banks grow on the inner

shelf whereas patch reefs and mobile

sands occur on the outer shelf. Such

faunal zonations across shelves have,

for example, also been broadly out-

lined for the Permian of the Southern

Alps

(Flugel, 1981a) and the Triassic

of Northern Calcareous Alps

(Flugel,

1981 b).

More detailed community analysis

can help elucidate paleogeography.

Boucot (1

975), for example, developed

Figure

Coarse sands and gravels on outer part of shelf. Ciast at upper right is

long. Water is about

1.5

deep.

valuable

the

Figure

lnterbedding of thin, sharp-based tempestite units and thicker, normally bedded

carbonate factory that Cannot be ob-

lime mudstones. Lens cap

(55

mm in size) is located on packstone-filled depression

tained from lithofacies

alone.

between two hummocks of a HCS tempestite unit with a gradational, intraclastic base.

At the broadest scale, information

Bescie Formation (Lower Silurian), Anticosti Island, Quebec.

15.

PLATFORM CARBONATES

Figure

A graded tempestite unit with gradual transition from intraclastic coarse-grained

pensated by

increase

the

grainstone up into laminated fine-grained grainstone overlain by nodular bedded mudstones.

number of individuals of the species

Note sharp scoured base. Pencil is

cm long. Bescie Formation (Lower Silurian), Anticosti

that survive. Similarly, the biota Can

Island, Quebec. provide information about the consis-

Calcareous Foraminifera

Agglutinated Foraminifera

Calcisponges,

Hyalosponges

Calcareous worm tubes

Figure

Effect of salinity on modern plants and animals. This diagram clearly shows how a decrease or increase in salinity will severely

limit the diversity of the fauna and flora. Modified from

Heckel

(1972).

JONES, DESROCHERS

tency of the substrate; for example, it

is the biota that usually leads to the

recognition of hardgrounds and

firm-

grounds (Fig. 14).

PLATFORM FACIES MODELS

Platform morphology determines sub-

tidal facies distribution. In most cases,

however, the poor exposure of ancient

successions or the lack of seismic pro-

files precludes an assessment of orig-

inal platform morphology. Thus, the

challenge is often to use the facies ar-

chitecture to interpret platform type.

This challenge is accentuated by the

difficulty that can be encountered in

translating the interpreted environ-

mental parameters into a precise de-

positional setting! Geography is crucial

because it is the spatial relationship

between lithofacies and/or biofacies

that usually provides the vital informa-

tion. In the following section general

facies attributes are outlined first and

then amplified by specific modern and

ancient examples.

Unrimmed shelves

Unrimmed shelves are swept by

onshore waves because there is no

barrier along their seaward margin.

Such shelves range from ramps where

slopes are relatively uniform (a few

metreslkm or <I0) and merge gradually

into a relatively shallow basin (Ahr,

1973; Read, 1983, 1985) to open

shelves (distally steepened ramp of

Read, 1985) where there is an in-

crease in gradient in the outer, deep

shelf area (Ginsburg and James, 1974)

before it merges with the deeper,

oceanic basin. The nature of the shelf

edge may vary along strike. The West

Florida Shelf, for example, has a ramp

profile without a major break in slope

(-1-2") in its northern portion; a distinct

shelf edge with concave-up, outer

slope profile (up to 12") in its central

part; and a steeper but similar profile

(-5") in its southern portion.

Modern, unrimmed shelves (Fig. 15)

are characterized by

a seafloor, 10

to 300 km wide, gently sloping off-

shore from a continental area,

facies belts

variable width that

closely parallel bathymetric contours,

3) gradual transition of facies belts

from inner, shallow shelf to outer,

deep shelf to basin,

-4)

high-energy,

carbonate sands in the wave-

andlor

tide-agitated, inner shelf (above fair-

weather wave base), 5) skeletal

muddy sands to muds in quiet, deeper

outer shelf (below fairweather wave

base) that are only periodically af-

fected by storms, 6) no continuous

reef trends, and 7) localized patch

reefs and sand shoals.

Holocene, warm water

Modern, warm water, unrimmed plat-

forms are the sites of low or moderate

energy processes controlled by waves,

local tides

and/or oceanic currents.

High-energy conditions may be in-

duced by hurricanes. Modern exam-

ples include the ramp off the Trucial

Coast (Fig. 16) and the open shelves

of West Florida and Campeche

(Fig. 17). Much of the sediment on

these shelves is not in equilibrium with

present-day shelf hydrodynamic pro-

cesses. Sands on the West Florida

Shelf, for example, formed in water

about 5 m deep during the early

Holocene, are now stranded near the

shelf edge in water 80 to 100 m deep.

The Persian (Arabian) Gulf, a typical

foreland basin, is a shallow, asym-

metric sea with its deep axis

(400 m)

near the steeper Iranian coast

(Fig. 16). The north side receives

ter-

rigenous sediment from the moun-

tainous lranian coast. Almost pure

carbonate sediment accumulates on

the Arabian side because there is little

siliciclastic material being brought into

the area. Sediments off the Trucial

Coast accumulate on a ramp

(c35 cm/

km). A transect perpendicular to strike

crosses

back ramp with microbial

intertidal flats that pass landward into

broad evaporitic sabkhas and

skeletal-

pelletal sands to pelleted lime muds in

protected coastal lagoons, 2) a shal-

low ramp with high-energy

skeletal/

oolitic sand shoals, beach-barrier

systems and coral reefs, 3) a deep

ramp that is transitional from

aggre-

OMINANT

TYPE

Colonial Corals

Calc. Worm Tubes

Other Brachiopods

Pelmatozoans

Figure

Substrate preferences for modern animals and plants. Modified from Heckel

972).

gatelskeletal sands dominated by mol- marls (>20% terrigenous material) of

luscs and foraminifera to skeletal

the deeper axial zone that contain

muddy sands (mud-supported)

domi-

coccoliths. Sediment distribution is

nated by mollusc debris, and 4) a

controlled by NW winds (shamals) that

gradual transition into bivalve-rich generate waves several metres high

torm

Wave

Base

Figure

Facies models for unrimmed shelves

(A)

warm water ramps, and

(B)

cool water

open unrimmed shelf (high energy).

Figure

Distribution

facies on the ramp off the Trucial Coast in the Persian Gulf with

summary of conditions that control sediment formation. Modified from Purser

(1973,

1983).

that move sediment to depths of

Local tidal currents are less important.

The biota is influenced by water circu-

lation, especially in the restricted

lagoons where stagnation leads to ele-

vated salinities

(>70%0). Although the

biota in open gulf waters (salinities

-40-50%0) resemble the typical chloro-

zoan assemblage, the coral fauna is

less diverse and codiacean algae are

absent.

The West Florida and Campeche

shelves, on opposite sides of the Gulf

of Mexico (Fig.

17), have contrasting

styles of sedimentation. The Gulf of

Mexico is a partially enclosed,

mi-

crotidal, low-energy pericontinental

sea that is essentially wind dominated.

Although SE to NE waves associated

with the Trade Wind Belt prevail, most

energy comes from waves crossing

the deep waters of the open gulf.

Shelf sedimentation is controlled by

waves and, to

lesser extent, by

tides. Hurricanes, however, radically

modify sedimentation.

Facies on the West Florida Shelf

(Fig. 17) which parallel the shoreline

include 1) inner shelf

quartz/molluscan

sands,

outer shelf coralline algal

sands,

shelf edge oolitic, peloidal,

and lithoclast sands (mainly relict),

and

slope planktonic foraminiferal

oozes with up to

per cent clay min-

erals. Landward of the shelf edge,

small carbonate buildups are covered

by branching and massive corals,

bry-

ozoans, and

Halimeda.

The facies ar-

ctiitecture on the Campeche Shelf

differs from that on the West Florida

Shelf (Fig. 17) because there are no

siliciclastic sands on the inner shelf,

and isolated organic buildups occur at

the shelf edge on top of antecedent

highs.

Ancient, warm water

The Jurassic Smackover Formation in

the southern United States accumu-

lated on a ramp characterized by ooid

grainstones, packstones, and sand-

stones in the nearshore belt (Moore,

1984). Further offshore,

peloidallbio-

clastic wackestones and foraminiferal

mudstones were deposited in a deeper,

lower energy setting. The deepest

water areas were covered by

organic-

rich, laminated mudstones. Other

examples of ancient ramps include

those in the Upper Cambrian to Middle

Ordovician carbonates of the

Appala-

JONES, DESROCHERS

chians in Virginia (Read,

1980;

Markello and Read,

1981)

and possibly

the Lower Carboniferous of South

Wales (Tucker and Wright,

1990).

Holocene, cool water

Modern, unrimmed platforms in cool

water settings include the relatively

low-energy ramp of the Gulf of Gabes

in the Mediterranean (Fig.

18)

and the

high-energy, swell-dominated Rottnest

Shelf off western Australia (Fig.

19).

The Gulf of Gabes and its northern,

shallower extension which extend for

200

km along the east coast of Tunisia,

have a gently seaward-dipping

seafloor

with the

m depth contour situated

up to

km offshore (Fig.

18).

The

main depositional belts, parallel to

shoreline and bathymetric contours,

are

a wave-agitated inner shelf

above wave base at

quiet, low-energy outer shelf in water

m deep, and

a deeper

shelf in water more than

100

m deep.

The wave-agitated, inner shelf is

rimmed landward by a low-relief rocky

coast with local sabkhas and salinas.

Except for a narrow nearshore band of

quartzose sands, the inner shelf sedi-

ments are bioclastic sands distributed

in a complex mosaic of sedimentary

facies, especially in areas of sea grass

meadows. The sediment is locally

muddy because of baffling and stabi-

lization by these sea grasses. Coarser

and better sorted bioclastic sands

occur where the sea grass is absent or

in areas exposed to waves. Shelf sedi-

ments are coralline algae, foramini-

fera, molluscan and bryozoan debris,

and locally, abundant

Halimeda

plates.

The mixture of these bioclasts sug-

gests that this area is transitional

between foramol- and chlorozoan-type

sediments.

The wave-dominated Rottnest Shelf

lies on a passive continental margin

that flanks a continental area of low

relief and sluggish drainage (Fig.

19).

Bryozoans and calcareous red algae

dominate along with molluscs, fora-

minifera, echinoderms, pteropods,

os-

tracods, and brachiopods. The inner

shelf, up to

100

m deep, is veneered by

wave-rippled cross-stratified sand. The

outer shelf,

100-1 70

m deep, is covered

bioturbated bryozoan sand to sandy

mud and the upper continental slope

(>I70

m) is blanketed by muddy plank-

tonic carbonate.

Ancient, cool water

with identifying ancient temperate

Examples of ancient carbonate

suc-

water carbonates. One example of this

cessions that formed on temperate,

type of succession is mid-Cenozoic

unrimmed platforms are rare, probably

carbonates of the Eucla Platform in

because of the problems associated

southern Australia (James and Bone,

Tidal

current:

microtidal conditions

Waw

current:

mainly

generated

open gulf, affects most of shelf

Tidal

current:

microtidal conditions

(<<

Figure

Comparison of facies distribution on the unrimmed West Florida Shelf (upper

right) and the Campeche Bank (lower right) and summary of conditions that control sediment

formation.

MI*

about

37%.

wrnnu:

ral.tlvdy

low

ndd

wmnt:

rnicrolkid

mn&d

(-1.8

Wlnd:

atrang,

p.rdmant

wind8

with

wnn

Sldlian

current

Planktonic

mud

Blodulk

und

Figure

Facies distribution in the Gulf of Gabbs, Tunisia and summary of conditions that

control sediment formation. The open shelf is mantled by sediments that are a mixture of

cool and warm water components. Modified from Purser

(1983).

Uthoskel Grainstone

Cl1m.t.: Mediterranean dimate with cool winter and warm

dry

summer.

Evaporation

precipitation.

Wlnd:

dominantly onshore.

but

wind-driven waves of

mim

imporlaw.

Wave regime: mainly SW swell waves with wave

bas@

50 m;

mme

winter and

rare

cyclonic storm waver.

Water

depth:

gently sloping shell with gradient of 2 to

dm; divided into wide.

flat

inner zone and steeper outer zone extending

shen break at 170 m.

Water temperature: 16'lo 20°C

ndal

reglme: miuolidal

Clrculatlon: oool subtropical water masses

Indian Ocean generate minor

bottom currents along outer shell and upper continental dope.

1991). The limestones are largely

skeletal grainstones with bioclasts

derived from bryozoa, bivalves,

bra-

chiopods, and some corals. Facies

include a lower cross-bedded and an

overlying thin-bedded facies that prob-

ably represent subaqueous dunes and

interdunes that originated in water

more than 70 m deep. This example is

interpretable because the rocks can

be compared directly to the modern

cool water carbonate sediments off-

shore. Interpretations of older carbon-

ates are more equivocal because it

has proven difficult to decouple the

effects of evolution and environment.

Nevertheless, Brookfield

988) sug-

gested that Trenton limestones in the

Middle Ordovician of southern Ontario

were probably deposited in such a

setting. The

brachiopod-bryozoa-

crinoid-dominated faunas in these

limestones are strikingly similar to the

benthic biota on modern temperate

shelves in the Southern Ocean.

Rimmed shelves

Rimmed shelves differ from open

shelves because reefs, sand shoals

and/or islands located along their

seaward margin effectively reduce

connection between the open ocean

and shelf. The type and distribution of

facies is related to shelf depth (Fig. 20).

Shallow water shelves have

grass-

Figure

Facies distribution on the Rottnest Shelf, Western Australia and

covered sands and muds on their

summary of conditions that control sediment formation. Modified from Collins

inner

parts

and

skeletal

sands

and

988).

patch reefs on their outer parts. Deep

water shelves, with lagoons up to

m deep, are generally floored by

muds. Steep-sided reefs, surrounded

by haloes of reef talus, can form on

the outer shelf whereas molluscan

sands can develop in shallower,

nearshore areas.

Holocene

The Florida (Fig. 21), Belize (Fig. 22),

and Queensland shelves are modern

examples. The Florida Shelf is covered

by carbonate sediments whereas the

Belize and Queensland shelves are

mantled by siliciclastic and carbonate

sediments.

comparison between the

Florida and Belize shelves illustrates

the effect of water depth on facies dis-

tribution.

The Florida Shelf, with water less

than 20 m deep, is covered by

bio-

Figure

Facies models for warm water (A) and cool water

(B)

rimmed

genic sediment (Fig.

21)

with most of

shelves. The latter is modified from Sellwood

(1986).

the mud being derived from the break-

288 JONES, DESROCHERS

down of calcareous algae such as

Halimeda

and

Penicillus.

Patches of

corals along the shelf edge are sepa-

rated by areas of loose sediment. The

outer shelf, subjected to high-energy

conditions generated by onshore

waves, is either bare Pleistocene lime-

stone or rippled sand. Patch reefs and

sessile organisms occur throughout.

The inner shelf is covered by

fine-

grained sediment that is inhabited by

Thalassia,

algae (e.g.,

Halimeda,

Penicillus),

molluscs, coralline algae,

echinoids, and small corals. Burrowing

worms, molluscs, and crustaceans ho-

mogenize the sediment. Carbonate

banks

(e.g., Tavernier and Rodriguez

banks) occur inboard over depres-

sions in the underlying Pleistocene

limestone basement.

The Belize Shelf facies (Fig. 22)

comprises

an inshore siliciclastic

belt, 2) a central trough-shaped lagoon

ranging from

m deep in the north to

m deep in the south,

pinnacle

reefs and atolls (faroes) in the shelf

lagoon, and

a barrier reef that re-

stricts circulation across the shelf.

Nearshore siliciclastic facies of

quartz-

ose sands are derived from onshore

Pleistocene deposits. Sedimentation in

the shelf lagoon is controlled by water

depth and salinity. Fine-grained sedi-

ments, accumulating in deeper parts

, ,

coM

fmnts move south.

Molluscan-foraminifera1

Foraminifera1 marl

Sand

rich in

Halimeda

in north,

cmtyr in south. Cold air fmm north In October to January.

Average of 1 hurricane every

years.

Wlnd:

predominantly from eant

Ocmn current: Predominantly from east due to Caribbean current.

Water

depth:

m on shelf, shallowest in north

(40

Shdf Mrrgln: rimmed by

10 km wide barrier

(<3

m water depth), with

coral reefs and islands along entire length. Steep on oceanward dde.

S.llnlty: Generally

35.7%.

36.1% well

mlxed

depth8 of 30 -50

Decreases to north and wuth due to influxes

freshwater. Slightly higher

Figure

Facies distribution on a segment of the Florida Shelf

Figure

Facies distribution on the Belize Shelf and summary of

near Key Largo and summary of conditions that control sediment

conditions that control sediment formation. Modified from Purdy

formation. Modified from Enos

977).

(1974), Ginsburg and James (1974).

15. PLATFORM CARBONATES

of the lagoon where energy levels are

lowest, display two main trends. First,

in an west-east direction, the car-

bonate content changes from 30 per

cent nearshore to 90 per cent near the

barrier reef. Second, in a north-south

direction, reflecting increasing water

depth, there is a gradual change in

dominant bioclasts from

~~olluscs and

foraminifera in the north, to

breen algal

(Halimeda)

plates in the centre, to

foraminifera

(Gypsina)

tests in the

south. Coarse sediment shed from the

pinnacle reefs and atolls accumulates

as leeward blankets. The barrier reef

rim has rubble on the crest and an

apron of sand on its landward side.

Ancient

The Dachstein Limestone (Late

Triassic) in the northern calcareous

Alps, contains numerous examples of

rimmed shelves. Such shelves, with

rims of reefs or oolitic grainstone

shoals (Schafer and

Senowbari-

Daryan, 1981), encompass a complex

set of facies that parallel the shoreline.

Other examples of rimmed platforms

occur in the Devonian of the Canning

Basin, Australia (Playford, 1980); the

ZONE

ZONE Y ZONE

LOW

ENERGY

LOWENERGY

EVAPORES UNDWARD QRAINSTONES FlNE-GRAlNED BASIN

MUDSTONES (DOLOMITIZED) SEAWARD SEDIMENTS

Figure

Facies model for an epeiric shelf (from Irwin,

1965).

Figure

Facies models for carbonates on warm water

(A)

and cool water

(B)

banks.

Upper Cambrian through Devonian

strata of Nevada (Tucker and Wright,

1990); and the Devonian of western

Canada.

Epeiric

shelves

The lack of modern epeiric seas means

that ancient examples must be used to

model the depositional characteristics

of these vast shallow water

(40 m)

systems. The rocks present a dilemma

because it is difficult to explain how

such sediments could all be formed in

'normal' seawater. Why are evaporites

generally lacking from such shelves?

lrwin (1965) suggested that epeiric

shelves were quiet, shallow water

areas not influenced by tidal activity.

He divided them into,

zone

for the

open ocean below wave base, 2) zone

for a narrow high-energy zone that

was under the influence of tidal ex-

change, and

zone

for the platform

beyond the effects of tidal exchange

(Fig. 23). This model assumed that

tidal currents were not maintained

across the platform because of the

dampening effect of shallow water.

Thus, epeiric shelves were viewed as

enormous areas of restricted circula-

tion, affected only by wind-induced

waves. Storm activity strongly influ-

enced sediment and biota distribu-

tions. Storms caused piling of water in

a downwind direction and when the

storm had finished, water surged back

across the platform, creating currents

capable of transporting and sorting

skeletal material.

Ginsburg (cited in Pray, 1981) pro-

posed three models that might permit

the requisite seawater bathing of

epeiric shelves,

prograding coastal

complexes, an autocyclic model with

shoreline-fringing banks prograding

across the open shelf, 2) banks and

seaways formed because the shelf is

dissected by channels that allow water

circulation, and

wind flushing, a

model that is essentially the same as

that suggested by

lrwin (1965). Pratt

and James (1985) suggested that

epeiric shelf deposition was controlled

by tidal activity and storms. They argued

that tides were not dampened and

thus viewed an epeiric shelf as a

shallow, tidally swept sea character-

ized by scattered islands and banks of

varying sizes. Although shallow

subtidal

sediments dominate the sequence,

each island and bank gave rise to its