Walker J. Facies models, Canada, 1992

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362 CONIGLIO, DIX

facies of proximal turbidites forming

small coalescing

"fans", and 3) a basin

facies of interbedded periplatform

ooze and thinly bedded fine-grained

turbidites (Fig. 18). Due to the narrow

width of this seaway, platform-derived

sediment effectively dilutes pure

pelagic sediments such that there is

no discrete pelagic facies.

There is no concentric facies pattern

in Exuma Sound. Instead, a single

Quaternary debris sheet covers much

of the basin floor. Yet, gullied slopes

are present and turbidites become

finer toward the basin centre. Episodic

and spatially discrete occurrences of

debrites should be expected within

closed seaways enclosed by normal

sequences of interbedded ooze and

turbidites (Mullins,

1983)' as found in

ancient slope sequences

(e.g., Cook

etal., 1972).

Subsurface facies associations

Modern slopes

comparison of short (<I2 m) cores

from north of Little Bahama Bank and

west Florida demonstrates consider-

able variation in facies associations,

both laterally and vertically, related to

the different geological settings and

slope evolution (Fig. 19). Deep sea

drilling along the northern margin of

Little Bahama Bank, however, docu-

ments spatial and temporal facies vari-

ations to subsurface depths of 300 m

(middle to late Miocene age; Austin,

Schlager etal., 1986). Even though

the margin has been prograding since

the early Miocene, and has produced

a seaward-thinning periplatform

wedge overlying pelagic carbonates, it

is clear that the same environments did

not simply undergo seaward trans-

lation with bank-margin progradation

(Harwood and Towers, 1988).

Ancient slopes

Vertical successions in ancient car-

bonate slopes typically contain disor-

ganized, unpredictable arrays of

coarse- and fine-grained facies, such

as debrites, the hallmark of carbonate

slope deposition, which punctuate se-

quences of fine-grained suspension

deposits. Naturally, local slope mor-

phology and sediment budget signifi-

cantly influence the type of sediment

that accumulates. Thus certain slopes,

or parts of slopes may be turbidite-

dominated or characterized by beds of

periplatform ooze punctuated by

debrites. Systematic thickening-upward

sequences indicative of prograding de-

positional lobes or other attributes of

submarine fan deposition (discussed

below) are not common.

Early diagenesis and

cyclic sedimentation

Early diagenetic processes are impor-

tant in both modern and ancient car-

bonate slope deposits. Bored and

mineralized hardgrounds, nodular

chalks and limestones, and shallow-

burial concretionary layers all attest to

early lithification with fragmented beds

commonly occurring as clasts within

debrites. In modern systems, the pres-

ence of fine-grained and abundant

aragonite establishes a strong diage-

netic gradient facilitating early lithifica-

tion. In addition, the alteration of

Mg-calcites appears to contribute suffi-

cient Mg, along with seawater diffu-

sion, to establish shallow-burial

dolomitic carbonates along some

slopes (Dix and

Mullins, 1992).

Aragonite cycles in modern slopes

Aragonite cycles, or alternating arago-

nite-rich and aragonite-poor layers, are

common to both Bahama and Florida

slopes, as well as other modern

periplatform settings. Gardulski et

a/.

986) describe Bahamian cycles as

being platform-driven with aragonite

percentages directly related to produc-

tivity in shallow water platform environ-

ments. In contrast, the greater depth of

the margin along the open west Florida

shelf effectively limits the seaward

extent of the shallow water carbonate

factory, and hence slope deposition is

dominated by pelagic settling. Gardulski

et al. (1 986) considered the west Florida

cycles as pelagic-driven, linked to

planktonic-oceanographic dynamics.

Cycles in the Bahamas are asym-

metric fluctuations of high-Sr aragonite

(derived from the adjacent platforms),

and correlate well with oxygen isotope

stratigraphy indicating an association

with climatically driven interglacial and

glacial sea level changes (Fig.

20a).

Elevated aragonite content coincides

SHELF

I-SLOPE~~~~~,~+OUTER

SHELF--(

E~L

WINNOWED

PELAGIC

OOZE

SANDS

HARDGROUNDS

DISTAL

DO~LAP

ESCARPMENT

Figure

16 Slope adjacent to deep open shelf margin off westem Florida.

Bathymetry and

surficial facies. Contours in metres. Location is shown in Figure 1. Modified from

Mullins et

a/.

(1988b).

Schematic cross-section illustrating geometry and major facies. Note lenticular geom-

etry

of youngest slope sequence thins landward due to onlap and thins abruptly seaward along

the distally-steepened slope. Figure not drawn to scale. Modified from

Mullins etal. (1988b).

18. CARBONATE SLOPES

with maximum platform productivity

and highstand shedding (Droxler and

Schlager, 1985). There is considerable

debate as to the reasons for lowered

aragonite percentages. Droxler

el a/.

(1 983, 1988) maintain that, in addition

to decreased aragonite production due

to platform exposure, aragonite was

also lost through

seafloor dissolution.

The timing of the Bahama minima coin-

cides well with carbonate minima in the

Pacific, Indian, and North Atlantic

oceans, some of which can be shown

to be due to carbonate dissolution.

Boardman et

a/.

(1 986) suggest that the

variation in percentages marks a

purely autocyclic control resulting from

alternate flooding and exposure of the

shallow water carbonate factory.

West Florida cycles (Fig.

20b) are

out of phase with the Bahamian cycles,

and are not asymmetric. Maximum

aragonite percentages are not always

associated with climate extremes, and

aragonite is a low-Sr variety character-

istic of planktonic pteropods rather

than platform-derived sediment. Ele-

vated percentages of aragonite,

Mg-

calcite, dolomite, and insoluble residue

coincide with glacial periods as deter-

mined by oxygen isotope stratigraphy.

These cycles are considered to be a

product of increased planktonic pro-

ductivity and dilution by hemipelagic

clays. There is no evidence of dissolu-

tion to account for the depleted arago-

nite intervals.

third, slightly different cyclic rela-

tionship occurs off the northern

Bahamas (Dix and

Mullins, 1988). The

Plio-Pleistocene cycles of carbonate

content and high-Sr aragonite are neg-

atively covariant;

i.e., intervals of high

aragonite correspond to decreased

total carbonate content, due to in-

creased terrigenous

clastic content.

Such increased deposition of clay

would occur if bottom current strengths

decreased between platforms with

rising sea level during the

glacial-to-in-

terglacial transition (Brunner, 1986).

Limestone-shaleJmar1 cycles

in ancient

slopes

Of all the various fine-grained facies

found in ancient carbonates slopes,

none is more visually striking than the

thick, monotonously interbedded lime-

stone-shale (or marl) sequences which

can extend for hundreds of metres

along strike with little or no change in

bed thickness (Fig. 8). Recent studies

suggest that this limestone-shale as-

sociation is largely the product of shal-

low-burial diagenesis

(e.g., Hallam,

1986; Ricken, 1986; Coniglio and

James, 1990). Most thin-bedded lime

mudstones were deposited from dilute

turbidity currents or from suspension

settling. Lithification of the limestone

seems to have taken place by early

dissolution of carbonate from the

Table

Comparison of Bahamian (rimmedplafform) and west Florida (open platform) slopes. Based on Mullins et al. (19886).

Bahamas

West

Florida

Morphology

gullies are important in upper bypass slope

smooth and gentle; lacks gullies; no upper bypass

slope

slope becomes more gentle in a seaward

slope becomes more steep in a seaward direction

direction

Influence of line source on

strongly controlled

shelf margin position of Loop Current eliminates

sediment dispersal

potential control by line source

Turbidites and debris flows major contributor to lower slope and basin margin

relatively minor contributor to slope

facies

Slides important important

Pelagic sedimentation

most abundant along upper bypass slopes and

dominant depositional process

in regions not diluted by sediment gravity flows,

winnowing by bottom currents, or carbonate

compensation depths

Cyclicity

aragonite cycles are platform-driven with peak

aragonite cycles controlled by higher pteropod

aragonite abundances during warm interglacial

productivity during glacial intervals

intervals

Sediment composition bank-derived Mg-calcite and aragonite are

calcite from planktonic foraminifera and coccoliths

important is most important

Diagenetic potential early submarine cementation encourages

forma-

no oblivoius source for slope-derived clasts; fine-

tion of slope-derived clasts during slope failure

grained nature of sediment reduces permeability

of sediment discouraging submarine cementation

Distribution of slope facies

variety encouraged by segmentation of banks

main facies belts with Loop Current as major

and basins control

Seismic facies numerous seismic facies simple configuration

364 CONIGLIO. DIX

SEA LEVEL

80lTOM

MARGINAL

CURRENT

ESCARPMENT

LITHOHERMS

PERIPLATFORM SANDS

BASIN

FAClES

WASHED

PERIPLATFORM

SANDS

SEA

LEVEL

CHAOTIC

SLIDES

COARSE

TURBlDlTESlDEBRlS

FLOWS

FlNE

PERIPLATFORM

TURB

/PELAGIC

OOZES

SEA LEML

HARDGROUNDS (PERIPLATFORM)

NODULAR

PERIPLATFORM

OOZE

SOFT

PERIPLATFORM

OOZE

WEST

FLORIDA

argillaceous interlayers and reprecipi-

tation in the carbonate layers. This

precipitation also cemented local

grainy turbidites and formed nodules

to layers of diagenetic lime mudstone.

Later pressure solution during burial

usually enhances all limestone-shale

or limestone-marl contacts.

Such limestone-shale cycles are

characteristic of many Paleozoic slope

carbonates and, although partially dia-

genetic, the initial depositional signal of

carbonate sediment alternating with

argillaceous sediment may be a re-

sponse to the same controls that pro-

duce aragonite cycles on modern slopes.

Figure

Facies associations of open

seaways of the northern Bahamas. a)

Example of periplatform sands and

litho-

herms influenced by the bottom-sweeping

Florida Current. Modified from

Mullins and

Neumann (1979).

Chaotic slide facies

changing downslope into gravity flows and

pelagic ooze. Modified from

Mullins (1983).

c) Diagenetic slope reflecting decreasing

current strength passing from hardground

through nodular ooze to ooze with in-

creasing water depth. Note that the upper

slope north of Little Bahama Bank (Fig.

5b)

clearly displays these current-controlled

facies. Modified from

Mullins

etal.

(1980b).

FAClES

MODELS

Earlier attempts to model carbonate

slope facies have highlighted three

common elements,

1) the consider-

able spatial and temporal facies varia-

tion,

the propensity for carbonate

sediments to accumulate along the

slope from a line source or a multitude

of feeder channels, and

the differ-

ence between siliciclastic and car-

bonate slope sedimentation in res-

ponse to sea level fluctuations.

Mcllreath and James (1 979; 1984) es-

tablished two basic models

bypass

and depositional (Fig. 21)

based on

relief between platform and basin and

SEA LEVEL

PERIPLATFORM

OOZE

COARSE

TURBlDlTES

FlNE

TURBlDlTES

PERIPLATFORM

OOZE

Figure

Facies associations of closed

seaways of the Bahamas. Based on

Schlager and Chermak (1979).

NORTH OF LllTLE

UPPER SLOPE

BAHAMA BANK

LOWER SLOPE

ARGILLACEOUS OOZE

OOZE (UNDIFFERENTIATE

Figure

Shallow subsurface slope facies from west Florida and north of Little Bahama Bank. Diagram is based--on parts of figures in

Mullins

et al.

(1 984) and Mullins

et a/.

(1 988a).

---

18. CARBONATE SLOPES 365

the nature of the platform margin

facies. Cook

a/.

(1 972) took an al-

ternate approach by considering the

wedge of sediment underlying slopes

as forming an apron adjacent to the

shallow water platform, and distin-

guishing two end members by the posi-

tion of the apron top relative to the

shallow water margin. Facies and

facies associations were not a prime

criterion for distinguishing slope types.

Only one facies, however, was set

aside: the debris sheet model was

used to explain extensive

megabrec-

cias resulting from collapse of a plat-

form margin. Using the apron-platform

ARAGONITE

2--

4-6

10-

ISOTOPE

STAQES

geometry, Mullins and Cook (1 986) re-

cently proposed a facies classification

using nomenclature similar to that used

for terrigenous sedimentary gravity

flow facies (Mutti and Ricci Lucchi,

1978). This approach emphasizes sim-

ilarities and differences between

silici-

clastic fans and carbonate aprons.

Carbonate slope facies models

based primarily upon geometry of the

slope deposits and the adjacent plat-

form margin appear to offer the most

malleable models, in which the numer-

ous possible facies and facies associa-

tions play an important complimentary,

but secondary role.

-8

ARAGONITE

('A,

PDB)

Figure 20 Aragonite abundance and oxygen isotope composition cycles if the shallow sub-

surface. Aragonite cycles from the Bahamas (a) appear asymmetric whereas those from

west Florida (b) show less asymmetry. See text for details. Diagram is based on parts of

figures in Droxler

a/. (1

983) and Gardulski et

a/.

(1 986).

Carbonate aprons

Aprons are wedge-shaped to lenticular

bodies that generally thicken towards

the platform margin. Facies belts closely

parallel the platform margin. Three

types of apron-platform geometries are

recognized,

1) rimmed platform slope

aprons, 2) rimmed platform base-of-

slope aprons, and 3) open platform

slope aprons.

Rimmed platform slope apron

Carbonate slope aprons extend

without break from the basin along

gentle

(<4") gradients up to a shallow

water platform margin (Fig.

22a). This

is similar to what has been regarded

as the depositional slope model

(Mcllreath

and James, 1979; 1984). A

line source of platform-derived sedi-

ment originates from numerous chan-

nels dissecting shoals, reefs and

islands along the platform margin.

Downslope sediment transport along

such carbonate aprons is predomi-

nantly via unchannelized sheet flows.

Because gravity deposits may be initi-

ated anywhere along the slope, corre-

lation of these deposits will be fraught

with problems except for megabreccias

resulting from sea level change or

seis-

mically induced failure of platform

margin's

(e.g., James and Stevens,

1986). Turbidites and debris sheets are

common but do not develop sys-

tematic vertical sequences; instead,

they are characterized by randomly

distributed broad sheets of carbonate

gravity flow facies interbedded with

suspension deposits. General coars-

ening- or fining-upward trends can,

however, result from progradation or

retrogradation of the adjacent platform

margin (Fig. 23). Slope aprons are not

described from modern oceans be-

cause modern platforms typically

possess steep

forereef escarpments,

and in places upper slope escarp-

ments, which are a function of glacially

induced Quaternary sea level fluc-

tuations. Ancient examples include the

Devonian of Alberta, the

Siluro-

Devonian of Nevada, and the Permian

of west Texas (Cook, 1983).

Rimmed platform

base-of-slope apron

A base-of-slope apron forms down-

slope from the platform-slope break

along relatively steep

(>4") margins

(Fig.

22b). Sediment is supplied from a

366 CONIGLIO, DIX

line source which typically takes the

form of a series of closely spaced

gullies transporting shallow water and

slope-derived sediment through some

portion of the upper to middle (bypass)

slope. Sediment emerging from a

gullied slope would initially follow a dis-

persal path outlining a series of small,

interleaved fan-shaped deposits while

broadly defining platform margin-par-

allel facies belts. As with carbonate

Proterozoic of Northwest Territories

(Grotzinger, 1986).

Carbonate submarine fans

Carbonate submarine fans appear to be

rare in the rock record, but several have

been described from the Paleozoic

(Cook, 1983; Cook and

Mullins, 1983),

Mesozoic (Ruiz-Ortiz, 1983; Wright and

Wilson, 1984; Watts and Garrison,

1986; Cooper,

1989), and Tertiary

(Gokten, 1986). No modern-day exam-

ples have been documented.

Ancient carbonate fans are inter-

preted to have been built from a point

source, similar to their siliciclastic

"channel-fegding-loben (Chapter 13)

counterparts, developing at the base

of a slope fed from a major submarine

channel (Fig.

22d). Facies associations

and types are also similar (Fig.

24).

carbonate systems, inner fan facies

slope aprons, systematic vertical se-

quences are not expected. Prominent

DEPOSITIONAL

MARGIN

facies to be expected at the top of a

LEVEL

base-of-slope apron would be coarse

debrites and talus, especially if a steep

escarpment is present. Good exam-

ples of modern base-of-slope aprons

occur along several margins in the

northern Bahamas

(Mullins and Cook,

1986), off Belize (James and Ginsburg,

1979) and Jamaica (Land, 1979). There

are several well-described ancient

ex-

WOW

REEFS AN- SAND SHOALS

SEA LEVEL

amples as well, including the Middle

Cambrian of the Canadian Rocky

Mountains (Mcllreath,

1977), the Cam-

bro-Ordovician of western Newfound-

land (James and Stevens,

1986), and

the Cretaceous of Mexico (Enos, 1977).

Open (unrimmed) platform apron

Because the platform margin is below

depths at which the carbonate factory

operates, sediments forming aprons

adjacent to such shelves are likely to

be dominantly pelagic in origin (Fig.

22c). Mass-wasting deposits include

pelagic turbidites and debrites, the

latter of which can contain clasts

derived from the platform margin or

fragmented beds of early lithified

pelagite. The generally finer grained

nature of the deeper platform margin

sediment implies that submarine ce-

mentation (and therefore production of

clasts) may not be as important as

along shallower rimmed platform

margins. Pre-Mesozoic aprons adja-

cent to open platform margins will be

highly argillaceous.

Both modern and ancient examples

of carbonate aprons adjacent to open

platform margins (distally steepened

ramps) are rare, although in the latter

case, this more likely reflects the diffi-

culty in differentiating between rimmed

shallow water platforms and open plat-

forms in the rock record. Modern slopes

adjacent to open shelves include west

Florida and Banco de Campeche,

Mexico. An ancient example is the early

PEUOlC

CARBONATES

cum

[BYPASS

MARGIN

]

SHALLOW

REEFS

AN-

SAND

BmML8

SEA

LEVEL

PERIPLATFOR*--)

SHALLOW

BASIN

SHALLOW

REEFS AN- SAND SHOMS

SEA

LEVEL

aClLLIED

BYPASS

PERIPUTF~RI OOZE

PER1

SLOPE

RARE

TUWIBIMTES

CUT

FHA

STRUCTURES

TOE

SLOPE

DEEP

BASIN

CLAYS

PERIPLATFOFM OOZE

DEBRITES

TURBIDKES

Figure

Facies models for depositional and bypass margins. ~odified from James and

Mountjoy

983).

----

---

include debris flow conglomerates and

coarse proximal turbidites deposited in

feeder channels and associated with

submarine slides. Mid-fan deposits

comprise thinning- and fining-upward

strata deposited as braided distribu-

tary channels. Thickening- and coars-

ening-upward turbidite sheets are

inferred to represent outer fan-lobe

sheets. Thin-bedded laterally contin-

uous turbidites may represent fan

fringe, basin plain, or interchannel

facies

(Mullins and Cook, 1986).

Sediment disperses in a radial fashion,

yet if numerous fans develop along a

slope also fed by a line source, the

distinctive fan geometry and facies

succession may be blurred.

In the previous older submarine fan

models (Walker,

1979), suprafan lobes

are a major structural component.

However, reappraisal of siliciclastic

facies associations based on multi-

channel seismic facies analysis

(Chapter 13) illustrates that

channel-

feeding-lobe fan systems do not char-

acterize a general fan model. In light

of this, future investigations of car-

bonate submarine fan systems need

to be cognizant of channel-levee com-

plexes, stacked thick sandy turbidites,

synsedimentary deformed sediment,

SLOPE APRON

18. CARBONATE SLOPES

and debrites. Carbonate submarine

fan systems may prove to be more

common in the rock record than is

generally recognized.

SEA LEVEL CHANGES AND

SEQUENCE STRATIGRAPHY

The sequence anatomy of carbonate

slopes, comprising allostratigraphic

units and the lithologic succession are

difficult to predict because shifting sea

level causes so many changes in the

ambient physical, biological and chem-

ical conditions of deposition. Further-

more, the relative importance of these

parameters on sedimentation often

changes as base level shifts. Se-

quence anatomy can be influenced,

however, without any change in sea

level, for example, through

seafloor

dissolution due to rise of chemical

oceanographic boundaries, erosion or

sedimentation related to variations in

bottom-sweeping currents, or changes

in the tectonics of the hinterland as it

affects terrigenous sediment input into

coastal areas (Schlager, 1991).

Despite this complexity, the manage-

ment of a slope above the common car-

bonate mineral lysoclines and com-

pensation depths when the adjacent

platform is flooded should be directly

BASE-OF-SLOPE

OPEN PLATFORM SLOPE

SUBMARINE FAN

DEEPER WATER SHALLOW3 WATER

UPPER FAN

SHALLOW

WATER

Figure 22

Block diagram facies models for a) carbonate slope apron, b) carbonate base-of-

slope apron, c) carbonate deep open platform slope, and d) carbonate submarine fan (a and b

are modified from

Mullins and

Cook,

1986).

controlled by production and off-plat-

form transport of shallow water car-

bonate sediment (Wilson, 1975; Ken-

dall and Schlager, 1981

;

James and

Mountjoy, 1983). The greatest rates of

slope aggradation and progradation

from off-platform transport of mud and

coarser detritus appear to be associ-

ated with platform flooding, establishing

conditions for highstand shedding when

sedimentation rate matches or exceeds

the rate of relative sea level rise, or

when the shallow water carbonate

factory's production rate is maximized.

Exposure

A drop in sea level to below the shal-

low, rimmed paleoplatform margin

creates the potential for

new platform

to grow on the upper part of the old

slope, although constant erosion can

prevent any significant platform devel-

opment on top of steep slopes. This

new platform will have a relatively small

area for shallow water sediment pro-

duction. Consequently, calcitic pelagic

sediment in Mesozoic and Cenozoic

sequences may predominate due to re-

duction in the input of platform-derived

aragonite and Mg-calcite whereas Pale-

ozoic slopes become sediment starved,

resulting in an increase in the relative

amount of shale, the default sediment.

During lowered sea level, the ex-

posed and lithified platform margin will

be weakened by physical and chem-

ical processes, potentially leading to

collapse and accumulation of sediment

gravity flow, slide and talus de-

posits. Whether sufficient sediments in

carbonate systems accumulate to gen-

erate recognizable

lowstand systems

tracts

analogous to siliciclastic lowstand

fans (Sarg, 1988) is debatable. Along a

pericontinental shelf margin siliciclastics

can

prograde across the shelf and

down the new slope, thereby estab-

lishing a terrigenous

clastic lowstand

systems tract. From sequence strati-

graphic studies of relatively pure car-

bonate systems, however, the

lowstand

systems tract appears to be either thin

or absent (Fig. 25; Eberli and Ginsburg,

1989; Pomar, 1991; Yose, 1991). In

contrast,

Masetti

etal.

(1 991) have sug-

gested that the greatest rate of slope

progradation in Triassic platforms of

northern

Italy occurred during sea level

lowstands. The problem of highstand

versus

lowstand slope deposition

remains a key area for research.

CONIGLIO, DIX

Because of greater water depths at

the margin, the sedimentary regime on

open platform slope aprons remains

unaffected by small sea level fluctua-

tions that would otherwise intermit-

tently expose and flood a shallow,

rimmed platform margin. Large-scale

sea level drops, however, particularly if

the platform margin is in relatively

shallow wate'r, may impart a clear

shallow water, platform-derived car-

bonate contribution to the apron.

Flooding

With a gradual rise in sea level, such

that the rate exceeds sediment pro-

duction, the

lowstand margin and plat-

form facies backstep, onlapping the

previously exposed platform and

forming a transgressive systems tract

(TST; Fig. 25). Superimposed on this

overall rise and landward

onlap may

be short intervals of rapid rise followed

by stillstands during which carbonate

platforms can catch-up.

of prolonged

duration, aggradational sequences

can develop. Submarine cementation

tends to be common in the lime mud-

rich lithofacies that characterize the

platform margin because of the longer

time available for pore-fluid migration

and cement precipitation (Sarg, 1988).

Any mass wasting from such margins

will produce slope aprons character-

ized by abundant mud-rich intraclasts.

As the rate of relative sea level rise

approaches sediment production rates,

the maximum flooding surface (MFS) is

attained with decreasing

onlap (Fig.

25). Condensed sequences or hard-

grounds may form on the platform

during this peak in

onlap. When sedi-

ment production rates exceed sea level

rise, or during a stillstand, the high-

stand systems tract (HST) prograding

sigmoid/oblique clinoforms develop

and

downlap onto the older transgres-

sive systems tract (Fig. 25). Submarine

cementation tends to be minor and

gravity flows derived from such sys-

tems will reflect the uncemented, grain-

rich and mud-poor lithofacies that

characterize the platform margin.

Highstand shedding should promote

greater mass wasting where slopes

become overloaded, or margins be-

come oversteepened. The most vigor-

ous off-platform transport may occur,

however, during early flooding of the

platform margin, decreasing during

subsequent increased water depth

much of the platform surface is placed

below wave base

(e.g., Brooks and

Holmes, 1989). Rapid outbuilding of

the margin forming a highstand sys-

tems tract may lead to

alte,rnating de-

velopment of accretionary and bypass

margins. In fact, evolution from an ac-

cretionary to bypass margin may

repeat itself many times if oversteep-

ening of the bypass margin gives way

to collapse and lower gradients

(Schlager and Ginsburg, 1981).

Drowning

Sediment production on the platform

can be temporarily or permanently shut

down during transgression, creating a

starved slope. Such drowning conjures

up images of the platform lying in the

murky depths due to a rapid rise in rel-

ative sea level related either to eustasy

or tectonics (Schlager, 1981). Cer-

tainly, rates of shallow water sediment

production and off-platform transport

appear to decrease with increasing

water depth over the platform. But,

LOCALIZED TALUS DEPOSIT

COMPOSED OF PLATFORM

MARGIN BOULDERS AND

INFILTRATED SAND

POLYMlCTlC DEBRITE FROM

COLLAPSE OF TALUS CONE AT

BASE OF BY-PASS SLOPE

LIMESTONE CHIP DEBRITE

4-.

FROM DISAGGREGATION OF

SLIDE MASS FROM UPPER

SLOPE SETTING

+PROXIMAL COARSE

TURBlDlTES

INTERBEDDED WITH SUSPENSION

DEPOSITS

eCALCARENITE CONTOURITES

rCLASSICAL BOUMA TURBlDlTES

MUDDY CONTOURITES

DISTAL TURBIDITES) FORMED BY

STRONG CURRENTS NEAR BOTTOM

OF SLOPE

NODULAR AND CONTINUOUSLY-BEDDE

SUSPENSION DEPOSITS; SOME EARLY

DlAGENETlC REMOBILIZATION OF

CARBONATE

Figure

Generalized hypothetical sequence of slope facies resulting from progradation

of the adjacent shallow water platform.

18. CARBONATE SLOPES

progradational stacking of slope se- below those commonly considered for

quences can still be generated adja- the carbonate factory (0-20 m; Glaser

cent to platforms which sit at depths and Droxler, 1991).

Since water

quality,

in addition to

water depth, may play an important

role in controlling platform growth

potential, locally retarded shallow

water carbonate production

and/or

---

LITHOLOGY INTERPRETATION

high rates of off-platform transport

MASSIVE AND THICK

AMALGAMATED

LARGE CHANNEL

LIMESTONES

WITH

SOME FILLS

EROSION SURFACES

may lead to diminished accumulation

rates along some margins and to in-

cipient drowning (Dominguez

etal.,

1988). Excessive nutrients (Hallock

and Schlager,

1986), derived from

either upwelling or across-platform

transport of lagoon waters, can inhibit

carbonate production. Thus, drowning

THICK BEDDED MASSIVE AND

SMALL

AMALGAMATED LIMESTONES

CHANNEL

ZONE

WITH

CHANNELLING

can occur during formation of a trans-

gressive systems tract in very shallow

water. Tectonic plate motion and a

platform's latitudinal change through

successive water masses, with chang-

ing temperature, salinity and/or water

MEDIUM BEDDED GRADED

TRANSITION

LIMESTONES

WITH

SMALL-SCALE CHANNELLING

TO MID FAN

clarity characteristics, can form a long-

term control on carbonate productivity

and resulting platform aggradation

(Davies

et al.,

1989). No sea level

PROGRADATION

MEDIUM BEDDED

GRADED LIMESTONES

BASIN PLAIN-FAN

TRANSITKIN

change is required for these nutrient or

tectonic influences.

Drowning in deep or shallow water

should lead to a starved or condensed

sequence characterized by low rates

of mostly pelagic deposition (in

post-

Paleozoic strata) with possible devel-

HEMlPELAGlC MUDS

THIN BEDDED LIMESTONES

AND MUDSTONES

WITH

THIN GRADED AND INTERCHANNEL

SAND

opment of phosphatic hardgrounds.

Subsequent sequence boundaries

within overlying pelagic strata will be

parallel or subparallel or mounded to

the contact with underlying slope car-

bonates, although bottom currents

may keep some areas devoid of pel-

agic sediment. Hemipelagic

terrige-

nous muds or siliceous ooze may be

the prevalent sediment type on the

drowned platform and slope in

pre-

Figure

Summary log of major lithofacies in Jurassic carbonate submarine fan se-

quence at Peniche, Portugal. Modified from Wright and Wilson (1984).

Mesozoic systems.

SYSTEMS TRACTS

HST

HIGHSTAND

TST

TRANSGRESSIVE

LST

LOWSTAND

MFS

MAX.

FLOODING

SURFACE

SHALLOW PLATFORM

FACIES

PERIPLATFORM CARBONATE SUSPENSION

PERIPLATFORM

GRAVITY

FLOWS

SUSPENSION

PELAGIC CARBONATES, DISTAL

DEPOSITS

CARBONATE

TURBlDlTES

HEMlPELAGlC

CLAYS

Figure

Geometry of carbonate se-

quences with generalized facies associa-

tions. Platform exposure leads to a thin,

condensed

lowstand systems tract (LST?) or

hiatal surface. Rising sea level creates a

backstepping transgressive systems tract

(TST) which progressively onlaps the previ-

ously exposed platform. Condensed, thin se-

quences may occur along the slope. With

greater sediment production than sea level

rise, a highstand systems tract (HST)

pro-

grades out over the maximum flooding

surface (MFS), creating a sequence of

downlapping strata. Modified from Sarg

988).

370

CONIGLIO, DIX

SUMMARY

In the first edition of

Facies

Models,

Mcllreath and James

979)

under-

scored the need for future refinement

of their models to include more study

of modern carbonate slopes in the

Bahama region and beyond. Over the

last decade, considerable research

has resulted in a greater understanding

of many previously unstudied, rela-

tively pure carbonate slopes in the

northern Bahamas-Florida region, to-

gether with parts of the Caribbean, as

well as the Pacific and Indian Oceans.

Across-slope drill transects begun by

the Ocean Drilling Program in

1985

ushered in a new era of research. The

choice of the northern Bahamas for

the inaugural Leg

101

was in partial

recognition of the need for better un-

derstanding of the facies architecture

of modern carbonate slopes. Recent

ODP transects off northeast Australia

provide a new vantage point from

which to investigate the dynamics of

basin-to-platform transitions in mixed

siliciclastic-carbonate depositional

systems. Future research investigating

open shelves within different oceano-

graphic systems will also, no doubt,

provide further understanding of slope

sedimentation and development along

this alternate type of carbonate

margin.

The variety of facies, slope morpholo-

gies,

internal stratigraphy, and sedimen-

tary processes characterizing even a

single slope illustrate that carbonate

slopes continually adjust toward a

dynamic equilibrium with a host of inter-

related climatic, oceanographic,

sedi-

mentological, tectonic, and sea level

eustatic factors. Past attempts to model

slopes provided snapshots of numerous

facies associations which characterize

the variability of this depositional system

and the various geometric configura-

tions of carbonate slopes. Recognizing

the dynamic interrelationship between

numerous parameters affecting slope

facies, our approach has been to under-

score their inherent spatial and temp-

oral variability. This knowledge allows

us to better understand observed facies

associations and temper predictions of

lateral and vertical facies development

in both modern and ancient carbonate

slopes.

ACKNOWLEDGEMENTS

The preparation of this manuscript was

made possible by an NSERC operating

grant to MC. Much of that part of the

manuscript written by GRD was pre-

pared while at the Department of Geo-

logical Sciences, University of British

Columbia. Noel James provided pho-

tographs as well as encouragement

and stimulation throughout. We thank

Stewart and the Ocean Drilling

Program for providing photographs.

Kunert assisted with drafting of several

diagrams.

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