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shelf rudist mounds (Wilson, 1975).

Most deep water mounds are similar

to aggrading reefs in their develop-

ment and response to sea level

change. They are composed of facies

that developed below, near, or slightly

above storm wave base. Unlike reefs,

where sea level changes may have

caused sharp boundaries and disconti-

nuities between facies, sea level

changes in mounds will be detectible

only by changes in the normal facies

succession of a given mound type, a

feature not detectible in seismic pro-

files. For instance (Fig.

34), assuming

that A-B-C is a complete

shallowing-

17. REEFS AND MOUNDS 341

upward facies sequence of a mound,

any break or reversal in the sequence

will indicate sea level change. The

succession ABCBA indicates slow

transgression, ABCA rapid transgres-

sion; AC rapid sea level fall.

Figure 34 illustrates, starting from a

depth-zonation model of deep water

biogenic mounds (that of Fig.

18), how

mounds responded to sea level

changes and the vertical successions

that resulted in various systems tracts.

Reef growth strategies (Fig. 29) also

apply to mounds. In TST situations

mounds were in a keep-up or catch-up

mode. Under conditions of slow sea

THE

MODEL

SEA

LEVEL

~AsE"c!~..eRGE.??.ETAZoAN

~~~

(LW

Figure

Sketch illustrating response of depth-zoned biogenic mounds to sea level

changes and the resulting vertical succession in various systems tracts.

catch-up,

give-up,

shallow-up,

reef; letters

correspond to facies in Figure

18.

level rise the upper surface of the

mounds kept pace with sea level rise

and there was no facies change. If sea

level rise was relatively rapid most

mounds were in

catch-up situation

and there was a reversal of the vertical

sequence. The deeper mounds be-

came give-up, but start-up mounds de-

veloped on shallower parts of the

slope. In an EHST situation the

mounds were in a keep-up mode and

developed normal depth-controlled se-

quences. Shallower mounds may have

acted as foundations for reefs. LHST

mounds showed the complete

depth-

zoned vertical sequence and ultimately

were capped by prograding marginal

reefs or isolated aggrading reefs.

GEOLOGICAL HISTORY OF REEFS

AND MOUNDS

(Fig. 35; Table 1)

Introduction

The history of Phanerozoic reefs and

mounds has recently been reviewed

by James and Macintyre

(1985),

Sheehan (1985), Fagerstrom

987),

Copper, (1 988) and Talent (1 988).

Descriptions of Precambrian buildups

can be found in Geldsetzer

a/.

(1 988) and Grotzinger

989).

Archean (Beginnings)

The oldest recognizable buildups are

late

Archean (2.5-2.7 Ga) stromatolitic

mounds occurring as atoll-like build-

ups around subsiding volcanoes or

linear belts on early cratonic crust. If

there are earlier structures they have

been rendered largely unrecognizable

by metamorphism.

Proterozoic

(First extensive reefs and mounds)

The formation of cratons flooded by

shallow seas during earliest Pro-

terozoic

time coincided with the ap-

pearance of diverse buildups. Reefs

and mounds were constructed by

stro-

matolites that grew across the environ-

mental spectrum. Extensive barrier

reefs, fringing reefs and pinnacle reefs

developed on carbonate platforms and

patch reefs and mounds grew in

ter-

rigenous clastic-dominated settings.

Latest Proterozoic reefs are poorly

documented but some spectacular

buildups (Fig. 1) (Aitken, 1988) illus-

trate construction by a transitional

community of stromatolites and

cal-

cimicrobes (?calcareous algae). They

have a distinctly Phanerozoic

architec-

JAMES, BOURQUE

ture with cavities, internal sediments

and fibrous cements.

Early Cambrian

Early

Qrdovician (Age of uncertainty)

The very earliest (Tommotian) Cam-

brian buildups are skeletal mounds

built by calcimicrobes and

archaeocy-

athans, together with an ever-in-

creasing fauna of other skeletal

invertebrates. Archaeocyathans, how-

ever, disappeared at the end of early

Cambrian time. Late Cambrian build-

ups are once again simple

stroma-

tolitic/thrombolitic or calcimicrobial

mounds

(Kennard and James, 1986),

much like older late Proterozoic

buildups. In early Ordovician these

mounds were gradually populated by

an evolving biota of sponges, cal-

careous algae and primitive corals.

Middle Ordovician

Devonian

(Spectacular stromatoporoid reefs)

This period is the acme of Paleozoic

reef development, coinciding with the

appearance of bryozoans, rugose and

tabulate corals and stromatoporoids.

While Ordovician structures were built

by a variety of organisms, Silurian and

Devonian buildups were constructed

primarily by stromatoporoids forming

the full spectrum of reefs and mounds.

The stromatoporoids inhabited most

shallow

subtidal environments while

tabulate and colonial rugose corals

were more common in turbid and

deeper water settings. Calcimicrobes

were particularly prominent in Dev-

onian buildups.

Toward the end of Devonian time

there was an extinction of

shallow-

marine animals on a huge scale. Most

stromatoporoids, rugose corals, tabu-

late corals and brachiopods disap-

peared resulting in the demise of this

successful ecosystem.

Mississippian

Middle Triassic

(Many mounds)

This period in earth history is charac-

terized by few large skeletal metazoa,

and all buildups are mounds, generally

on slopes and ramps and on the inner

parts of platforms. Mississippian struc-

tures are commonly skeletal to mud

mounds with variable proportions of

fenestellid bryozoa, sponges or

codi-

acean and dasycladacean algae and

locally spectacular pelmatozoan-rich

flanking beds. Most Pennsylvanian

and early Permian mounds are rich in

the phylloid algae

Archaeolith-

ophyllum, Eugoniophyllum

and

Ivan-

ovia

andlor the enigmatic platy organ-

ism

Paleoaplysina.

These organisms

are augmented and then replaced by

tubular foraminifera, stromatolites, the

?calcimicrobe

Tubiphytes,

and a wide

variety of calcareous inozoan and

sphinctozoan sponges in middle and

late Permian buildups. Remarkably,

this fauna seems to have passed

through the Permian extinction rela-

tively unscathed, and even though

there are no earliest Triassic reefs,

Middle Triassic buildups are virtually

identical to those of the late Permian

(Stanley, 1988). This Permian hold-

over fauna was largely extinguished

by an extinction event at the end of

Middle Triassic time.

Late Triassic

Early Jurassic

(First modern reefs)

This is when reefs become "modern-

ized" due largely to the evolution of

scleractinian corals, new forms of

coralline and codiacean algae and a

wide variety of endolithic organisms.

Late Triassic reefs and mounds were

built primarily by scleractinian corals,

calcisponges, and calcareous algae.

They formed buildups across the envi-

ronmental spectrum, including dis-

PERIODS BUILDUPS MAJOR BIOTIC ELEMENTS

STROMATOLITES

f--

Extinction Events

Figure

An idealized stratigraphic column representing geological time and illustrating

periods when there were only mounds and those times when there were both mounds and

reefs. Numbers indicate different associations of reef- and mound-building biota;

scale

change.

17. REEFS AND MOUNDS

343

placed low-latitude terranes. This com-

munity was terminated by the

end-

Triassic mass extinction event and

although corals again became domi-

nant reef-builders, calcisponges never

regained their previous importance.

Earliest Jurassic time was a protracted

period without significant reef growth.

Although a wide spectrum of struc-

tures similar to living reefs and

mounds characterizes late Jurassic

time, the two commonest types are 1)

shelf-margin ramp and barrier reefs

dominated by corals

andlor stromato-

poroids and a wide spectrum of acces-

sory taxa, and

deep water slope

and basin sponge-algal mounds below

normal wave base and near lower

limits of the photic zone. The reefs

grew as patches on wide platforms, as

belts of patch reefs near shelf margins

and as caps on pre-existing deep

water mounds.

Cretaceous

(Rudist bivalve mounds)

The Jurassic reef-building community

persisted into early Cretaceous time,

but was gradually augmented and

locally replaced by rudist bivalves. Early

Cretaceous buildups at the seaward

shelf edge consisted of deeper

coral-

algal communities (?below wave base)

and shallower caprinid-radiolitid rudist

communities. Interior shelf basins were

bordered by caprinid-dominated build-

ups. Middle Cretaceous buildups were

dominated by rudists forming mounds

of low-diversity, loosely constructed, es-

sentially unbound bafflestone with

coherence due to interlocking and

co-

cementation. The structures were rarely

large, mostly in the form of mounds

stacked upon one another and sepa-

rated by layers of bioclastic sediment,

much of which was produced by fre-

quent destruction of unbound

frame-

Table 1

Studies of reefs and mounds in specific geological periods, mainly since

1980

(see Fig.

35).

Period Reference

Living

Acevedo

eta/. (1989); Acker

Stearn (1990); Bosence et

a/., (1985); Callender et a/. (1990); Cortez

Risk (1985);

Eliuk (1991);

Ginsburg

Schroeder (1973); Gaus

Macintyre (1989); James

Ginsburg (1979); Johnson

Risk (1987); Messing et a/. (1990); Mullins et a/. (1981);

Van Wagoner

etal. (1990).

Holocene

Chappell

Polach (1976); Choi

Ginsburg (1982);

Davies

Montaggioni (1 985); Davies

Hopley (1 985);

Davies et

a/. (1985); Longman (1981); Macintyre (1988);

McNeill (1 988); Montaggioni (1 988); Neumann

Macintyre (1 985); Tudhope (1 989); Wanless

Tagett

(1 989).

8. Cenozoic

Bernecker

Weidlich (1990); Crame (1980); Dabrio et al.

981); Esteban (1 979); Geister (1 980); Pedley (1 979);

Polmar (1991); Riding etal. (1991).

7. Cretaceous

Camion et a/. (1988); Kaufman

Johnson (1988); Scott

(1 988); Scott (1 990); Scott et

a/. (1 990).

6. Triassic-Jurassic

Crevello

Harris (1984); Flugel

Steiger (1981); Frost

(1 981); Palmer

Fursich (1 981); Schafer (1 984); Stanton

Fliigel (1989).

5. Mississippian-Triassic

Bolton et al. (1982); Bridges

Chapman (1988); Brown

Dodd (1 990); Cranston (1 990); Davies

Nasichuk (1 990);

Davies et

a/. (1988); Flugel

Reinhardt (1989); Hileman

Mazzulo (1977); Lees

Miller (1985); Lees et a/. (1985);

Reid

Ginsburg (1986); Tedesco

Wanless (1989).

4. Ordovician-Devonian

Bourque

Gignac (1983); Bourque et al. (1986);

Burchette (1981); Copper

Brunton (1991); Moore (1988);

Playford (1980); Shaver

Sundermann (1989); West

(1 988).

Cambro-Ordovician

Barnaby

Read (1990); James (1981); James

Gravestock (1990); Pratt

James (1982); Rees et al.

(1989);

Rowland

Gangloff (1988); Toomey

Nitecki

(1 979); Webby (1 984).

Proterozoic Aitken (1988); Grotzinger (1989).

Archean Grotzinger (1 989).

works by storms. Late Cretaceous shelf

margin buildups, particularly

post-

Cenomanian, consisted of rudist-coral

communities in which radiolitids and

hippuritids were more abundant than

nodular to hemispherical and planar

corals.

Cenozoic

(Return of modern reefs)

The terminal Cretaceous event led to

the extinction of rudists and most

Cenozoic reefs were, like living reefs,

constructed by scleractinian corals and

calcareous algae. Paleocene reefs are

not common but a major Eocene radia-

tion of new hermatypic coral genera re-

sulted in extensive reef development.

The acme of Cenozoic reef growth

appears to have been the Oligocene

with subsequent plate rearrangement

and climatic deterioration destroying

some habitats (Mediterranean) and iso-

lating others (Caribbean, Indo-Pacific).

Finally, a paradox

living reefs are

atypical of the Cenozoic! During early

Pleistocene time quickly growing,,

branched corals such as the

Ac-

roporidae, Poritidae and Seriatop-

oridae evolved rapidly, dominating

most shallow water environments.

Thus, the Cenozoic shallow water reef

corals with shapes much like those of

stromatoporoids and stromatolites,

were replaced by these newly evolved,

highly branching forms.

ACKNOWLEDGEMENTS

This manuscript draws heavily on our

research supported by the Natural

Sciences and Engineering Research

Council of Canada.

REFERENCES

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Information

Choat,

J.H.,

et al., 1988, Proceedings of

the Sixth International Coral Reef Sym-

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These proceedings and those of pre-

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Fagerstrom, J.A., 1987, The evolution of

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344

JAMES,

BOURQUE

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

Tebbutt, G.E.,

1988,

Reefs, Canada

and Adjacent Areas: Canadian Society

of Petroleum Geologists, Memoir

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A gold mine of information! A total of

115

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Paleozoic and early Mesozoic reefs or-

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Harris, P.M.,

1983,

ed., Carbonate

buildups: a core workshop: Society of

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di-

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Heckel, P.H.,

1974,

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Hopley, D.,

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Although concentrating on the Great

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C.H., eds., Carbonate depositional envi-

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345-440.

An overview of the sedimentology of

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Toomey, D.F., ed.,

1981,

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found in James,

1983, 1984

and James

and Macintyre,

1985.

See Table

for se-

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_--

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18.

Carbonate Slopes

Mario Coniglio, Department of Earth Sciences, University of Waterloo,

Waterloo, Ontario

N2L

3G1

George

Dix, Department of Earth Sciences, Carleton University,

Ottawa, Ontario

5B6

INTRODUCTION

Carbonate slopes encompass a suite of

environments which pass seaward from

shallow water, sunlit platform margins

to the dark, deep, ocean basins. Ac-

cordingly, they are variably affected by

changing seawater chemistry, tempera-

ture, pressure and biota with depth.

Slopes also bear witness to extreme

scales of sedimentary processes, from

episodic and catastrophic margin col-

lapse to quiet and persistent rain of

fine-

grained sediment. These environments

are fundamental links in the carbonate

facies spectrum. Their deposits are

often the only record of the history of

carbonate platforms that are now

hidden, eroded, tectonized, dolomitized

or metamorphosed. They contain a

more complete sedimentary record of

sea level change than adjacent, periodi-

cally exposed platforms as exemplified

by the detailed record of Quaternary cli-

matic variation. Finally, carbonate

slopes are host to base metal and hy-

drocarbon deposits and have acted as

conduits through which

metalliferous

and petroleum-rich fluids have migrated

to platform host rocks (Cook,

1983;

Eberli,

1988;

Dix and Mullins,

1992).

Figure

Bathymetry of Bahamas-Florida region and Banco de Campeche area. Stippled areas are shown in detail in Figures

(north of

Little

Bahama Bank), Ila (Straits of Florida), and 16a (west of Florida), respectively. Line

A-A'

is shown in Figure 13. Contours are in metres.

Modified from Perry (1

984).

350 CONIGLIO, DIX

Study of modern carbonate slopes

has lagged behind research on shallow

water and pelagic carbonates. Prior to

1985, the structure, stratigraphy and

shallow subsurface lithology of this

realm came from short

(42 m) piston

cores, dredge hauls, bottom-sediment

grab samples, seismic-reflection pro-

files, side-scan sonar, and underwater

photographs supplemented by a few

direct observations from submersibles.

Carbonate slopes and platforms were

initially excluded from the Deep Sea

Drilling Project. Since 1985, however,

the Ocean Drilling Program has pro-

vided a wealth of new seismic, pore

water, and core data from across-slope

transects of carbonate and mixed

silici-

clastic-carbonate slopes. Two areas

with extensive drilling now include the

northern Bahamas (Leg 101

;

Austin,

Schlager et al., 1986, 1988) and the

Great Barrier Reef, Australia (ODP Leg

133 Scientific Party, 1991

;

Davies,

McKenzie et al., in press).

The best studied carbonate slopes in

today's oceans are those in the nor-

thern Bahamas-Florida region (Fig.

I),

augmented by more local studies in

Belize, Jamaica, Grand Cayman, and

several atolls in the Pacific and Indian

oceans. These slopes provide good

actualistic models with which to inter-

pret ancient carbonate slopes. Unfortu-

nately, they are all essentially pure

carbonate depositional systems, in

contrast to many ancient slope de-

posits which include a large per-

centage of terrigenous

clastic rocks.

Whether this disparity is important

awaits research associated with recent

deep-ocean drilling off northeast

Australia along the largest modern

peri-

continental mixed carbonate-siliciclastic

platform margin (Davies, McKenzie

al., in press). Actualistic modelling is

further hampered because

Cenozoic

and Mesozoic carbonates are unlike

older sediments in terms of platform or

pelagic sediment mineralogy

(Wilkin-

son, 1979),

although a few ancient

submarine carbonate fans have been

documented, no modern equivalents

have been described, and 3) until re-

cently, slopes along modern carbonate

ramps were poorly documented.

In this chapter we examine the prod-

ucts and processes that characterize

carbonate slopes and discuss a va-

riety of facies models.

fundamental

precept throughout is that the geo-

metry and preserved sedimentary

record of carbonate slope deposits are

inextricably linked to the evolution of

the adjacent shallow water platform or

ramp. Actualistic input into the models

is drawn mainly from the Bahama-

Florida region. Data from ancient

slope successions are used to temper

and refine the actualistic bias, and

bring to light aspects of slope sed-

imentation that now can only be

viewed dimly through a curtain of bur-

ial diagenetic effects, tectonic defor-

mation or paleoceanographic con-

ditions that are not replicated in

today's oceans.

SLOPE TYPE

AND

MORPHOLOGY

The shape, orientation, size and facies

of a carbonate slope are linked to a

variety of intrinsic and extrinsic factors

(Fig. 2), their interplay leading to im-

portant differences when compared to

siliciclastic slopes (Table 1).

Slope

types

Differences in slope gradient are influ-

enced by sediment budget (gain

versus loss)

and/or locus

deposi-

tion. Depositional or accretionary

slopes demonstrate net accumulation

(Figs.

3,4). The locus of sedimentation

can be 1) along the upper slope, with

sediment thickness decreasing sea-

ward, or 2) distal from the platform

edge, in which case the bulk of sedi-

ments bypass the upper slope. Indi-

vidual slopes may have bypass (in-

cluding erosional) and depositional

segments. For example, the northern

slope off Little Bahama Bank has a

steep upper portion eroded by vig-

orous turbidity currents and a gentle

lower segment, where products of

mass wasting come to rest, explaining,

at least in part, the typical concave

profiles of some carbonate slopes.

The upper part of the southwest Florida

slope, in contrast, is the site of

abun-

CONTROLS ON CARBONATE SLOPE DEVELOPMENT

PLATE MOTION

CLIMATE

EXTRINSIC

OCEANOGRAPHY

SlLlClCLASTlC INPUT

LOCAL AND

OVERALL

GRADIENT

INTRINSIC

TRANSPORT

AND EROSION

OXYGENATION

COMPENSATION DEPTHS

LYSOCLINES

SETTING

DISSOLUTION

CEMENTATION AND

AND

BlOEROSlON BIOCONSTRUCTION

Figure

Extrinsic and intrinsic controls on carbonate slope development.

18.

CARBONATE SLOPES

dant sedimentation and intermittent

characterized by truncated reflectors

can be reworked by bottom currents or

gravity flow transport but the lower

and relatively steep gradients (Fig.

dissolved. Slide masses and large fallen

slope is the site of erosional gullies

3).

Such slopes adjacent to active-

blocks that accumulate within cones of

and sediment bypassing.

ly producing, shallow water platforms

talus at the base of erosional and

Erosionalslopes illustrate net sedi-

are also bypass in nature, with sedi-

bypass slopes may act as local dams

ment loss. On seismic profiles, they are

ment accumulating at the base where it

preventing finer grained sediment from

Table

Comparison of carbonate and siliciclastic slopes.

Carbonate Slopes Sillciclastic Slopes

Nature of sediment platform, pelagic, hemipelagic pelagic and hemipelagic

turbidity currents occur during shallow flood-

turbidity currents occur during lowstands

ing of adjacent platform

Response to platform emergence

shutdown carbonate factory,

pelagics domi-

slope erosion and offlap, more sand to slope

nate periplatform ooze

Response to platform submergence

drowned margin facies contribute to slope

stranded sand bodies, fine-grained sedi-

ments prevail

Sediment dispersal pattern

point source rare; more common line source

large submarine canyons form point source

from numerous shallow gullies in upper slope

with resultant formation of submarine fans

Constructions

base of slope aprons (erosional or bypass

submarine fans common

slopes), slope aprons (depositional slopes)

next to rimmed

open platforms; rare sub-

marine fans

Success of actualistic

faCieS models

difficult to apply to Triassic and older slopes

application is relatively straightforward

and slopes with significant terrigenous com-

ponent

Slope angles

relatively high (avg.

5-15"),

increases with

relatively low (avg.

3-6"),

does not increase

height with height

Slope profiles concave slightly convex or straight

Response to to upper

slope erosion reefs can repair damaged margin

lack repair mechanism

Potential for oversteepening

high -as slopes become steeper they

low

lack of lithificiation results in slumping

become more easily cemented in marine envi- that effectively prevents oversteepening

ronment (positive feedback)

Submarine cementation common, allows oversteepening with resultant uncommon

catastrophic collapse

Pressure solution

tends to remove primary sedimentary

struc-

not significantly affected

tures on beds tops and bottoms such as sole

marks and diagnostic ichnofossils

penetrative clast contact fabrics in conglom-

less likely to show pressure solution

inter-

erates may result from pressure solution

penetration depending on clast lithology

Affect of calcite and aragonite

dissolution can occur on lower slopes,

low-

dissolution does not occur

compensation depths

ering

seafloor and increasing topographic

gradient and potential for failure

Diagenetic beds

beds can be created by diagenetic unmixing

diagenetic beds do not occur except locally

of carbonate

in carbonate-rich parts of some successions

Nodular beds

common (primary and diagenetic)

common primary (isolated ripples, load

casts, etc.) rare diagenetic nodules

(concre-

tations) in carbonate-rich parts of some suc-

cessions