Walker J. Facies models, Canada, 1992

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258

WALKER

SEQUENCE BOUNDARY

HST

HIGHSTAND SYSTEMS TRACT

LST

-LOWSTAND S.T.

bf basin floor fan

sf slope fan

low lowstand wedge

prograding complex

Figure 38 Submarine fans in a sequence-stratigraphic context. This diagram is a combina-

tion of similar diagrams published by Vail (1987) and Posamentier and Vail (1988). Diagram

shows the conceptual development of a fan within one sequence

(i.e., the result of one cycle

of sea level fluctuation). SB

sequence boundary, TS

transgressive surface, CS

con-

densed section (or maximum flooding surface), and cs

coastal sandstones. None of the

modern fans discussed in this chapter has a thick

lowstand wedge (with downlapping

shallow-marine Highstand Systems Tract deposits) on top of the "slope fan" (channel-levee

complex, which does

not

occur on the slope). See text for further discussion of this contro-

versial model.

LATERAL MIGRATION

CHANNELS

NEL SANDS CUT DOWN

OGRADED SHEET SANDS

STACKED CHANNELS

Figure

Interpretation of sheet sandstones and overlying channelized sandstones in

Frigg Fan, North Sea. From

McGovney and Radovich (1985).

-5

LA60 SOFIA LENS CERRO TOR0

FORMATION

Figure

Schematic cross section through a coarse "Lago Sofia" lens in the Cretaceous

Cerro Torro Formation, southern Chile. From Winn and

Don (1979).

]E:/

Figure

(right) Stratigraphic section through Cretaceous rocks at Wheeler Gorge,

California. Base of section is faulted (Santa Ynez Fault), and overlain by a small slump. The

lower part of the section consists of

250

m of mudstones. CM

channel margin deposits, LV

ww-

levee. Arrows indicate thinning- and fining-upward facies successions.

13. TURBlDlTES AND SUBMARINE FANS 259

1979), and the early Cretaceous

Gevaram canyon of Israel (7 km wide,

1000 m deep; Cohen, 1976). Of these,

only the Czechoslovakian example

contains appreciable sandstone and

conglomerate (up to 300 m) in the

canyon fill; the others are

mudstone

filled.

The Yoakum canyon appears to

have formed by retrogressive slump-

ing at the shelf break (Dingus and

Galloway,

1990), as has been sug-

gested for the canyons and feeders of

the Plio-Pleistocene Mississippi

fan. By contrast, the cutting of the

Meganos canyon has been related to

Paleocene fault movements (Fischer,

1979, p. 7). Both the Czechoslovakian

canyons and the Gevaram canyon

appear to be submerged

fluvial valleys

further developed and enlarged by

marine processes. It would now ap-

pear that the initiation of all of these

canyons except

Meganos is directly

related to a lowering of relative sea

level. The canyons appear to have

widened and deepened whilst they

acted as conduits for turbidity currents,

but because of relatively steep axial

gradients, little turbidite sand or gravel

remains in the canyon. The canyons

are all

mudstone filled (except for the

CHANNEL

LEVEE COMPLEX

-------

----

-----------------

- -

-----------

- - -

- -

BASIN

FLOOR

---------------=-----------------

---

----

-----------

------------

-------_-__

Figure

Interpretation of the Wheeler Gorge section in terms of the models of Figure 37.

WG indicates the vertical profile of Wheeler Gorge. Circled numbers suggest the order of

channel-levee system development, and uncircled numbers 1-3 indicate the stacked chan-

nels at Wheeler Gorge.

lateral accretion and

avulsion. Thus the thinning-and

fining-upward levee sequences are interpreted to reflect lateral migration of the channel

away from Wheeler Gorge. Channel-levee systems 2 and

are necessary in order that the

levee sequences of system

can build on top of the stacked channel deposits of system 1.

No horizontal scale is implied; systems

and 3 could equally well be on opposite sides of

system

and system

could be a mirror image of what is shown, on the left of Wheeler

Gorge. Stipple indicates sandstone and conglomerate within channels.

lower parts of the Czechoslovakian

canyons), presumably during a rise of

relative sea level. The size of the

canyons suggests some turbidity

current scouring (not just retrogressive

slumping), and hence the model pre-

dicts submarine fan sandstone and

conglomerate deposits at the ends of

the canyons. For various reasons,

these deposits have not yet been dis-

covered.

Channel-levee systems

Channels seen on the surfaces of

modern fans in mid-fan areas average

about 40 m deep, but aggraded HAD

thicknesses of channel-fill facies range

from about 60-530 m (Table 3). Levee

and

overbank deposits of modern fans

have similar thicknesses, and widths of

tens of kilometres. In the geological

record, they are probably made up of

laterally continuous and extensive

thin-

bedded turbidites.

In the literature, the interpretation that

most closely approaches the model of

channel-levee systems developed in

this chapter is the Eocene Frigg Fan

(subsurface, North Sea;

McGovney and

Radovich, 1985). The interpretation is

based on seismic lines, well logs and

cores, and shows four radiating

finger-

like sandbodies (Fig. 39) 4-5 km wide

and 10-20 km long. The stacks of chan-

nelized sandstones occur along the

central parts of the fingers, and vary

from 20-200 m in thickness. with thin

shale partings 2-5 m thick,

chan-

nelized sandstones cut into older

sheet-like sandstones that have a much

greater lateral extent, and show very

gently mounded seismic reflectors. In

the interpretation (Fig.

39),

it is notable

that the channel sands are stacked in

the upcurrent position, but are shown

as thinner and laterally migrating in the

downcurrent position (as in the modem

Rhone and Mississippi fans, Figs. 30,

31).

In outcrop, the scale of channel-levee

systems makes them difficult to recon-

struct. Many of -the finer-grained facies

may be obscured by cover, and the

section may be broken up by folds and

faults. The best reconstruction is prob-

ably that of Winn and

Don (1979), from

Late Cretaceous rocks of southern

Chile. They summarise their obser-

vations in a cross section that shows

Figure

Slumped slope deposits in the Carboniferous Gull Island Formation of County

two superimposed channel-levee

Clare, western Ireland.

systems, with a total aggraded channel

WALKER

thickness of about 300 m (Fig. 40). The

channel deposits consist of sandstones

and conglomerates. Thinner-bedded

turbidites make up the levee deposits,

and the proposed width of the

channel-

levee system is about 20 km. Some of

the thinner-bedded turbidites occur in

slumped horizons up to about 15 m

thick; reconstructed directions of

slumping tend to be mostly eastward,

whereas regional paleoflow in the chan-

nels is southward. This suggests the

possibility that the slumped horizons

represent levee failure (cf., Rhone and

Amazon fans).

There is also one superb vertical

section through a channel-levee system

in the Cretaceous Wheeler Gorge

section in

California (Walker, 1985; Fig.

41). In Figure 41, the section is trun-

cated at the base by the Santa Ynez

Fault. A basal slump is overlain by a

fine-grained section about 250 m thick,

dominated by

1 -cm-thick graded silt-

stones

there are about 6000 such

beds present. The depositional en-

vironment appears to have been very

quiet and monotonous, far from the

action of large turbidity currents (the

"basin floor" of Figure 41). Stacked

channels overlie the basin floor de-

posits extremely abruptly, suggesting a

lowering of relative sea level and rapid

progradation of a channel-levee sys-

tem. Channels 1 and 2 are both con-

glomeratic at the base and fine upward

through pebbly and massive sand-

stones. The sandstones of channel 2

are overlain by thin-bedded turbidites

(CM in Fig.

41), characterized by

bedding

lenticularity and CCC turbidites

(Fig.

5).

These features, along with the

stratigraphic position, suggest a

channel margin (CM, Fig.

41).

Channel

has an erosive base, and a spectac-

ular example of a collapsed channel

wall with stratified blocks similar to

those in Figure 15. The bulk of the fill is

unstratified conglomerate, but there is a

fining upward at the top through

massive sandstones into classical

tur-

bidites. The two levee successions

(LVI and LV2) both fine upward, from

classical turbidites into mudstones.

Succession

LV2

(Fig.

41)

is dominated

by multiple horizons of soft-sediment

slumping within a thin-bedded turbidite

facies (Fig. 13). The levee interpretation

is based particularly on the association

with coarse channelized deposits, and

e of deformation within

thin-bedded CCC turbidites.

The scale (1 00-m channel stack,

80-

m-thick levees) invites comparison with

channel-levee systems of the Amazon

(Fig. 29) or Rhone Fans (Fig. 30). The

interpretation of Figure 42 shows how

guidance from the channel-levee fan

model developed in this chapter (Fig.

37) can suggest predictions from one

new piece of information (the Wheeler

Gorge section). Note that the use of a

Rhone- and Amazon-based model sug-

gests that channel-levee systems must

shingle at the same stratigraphic level

as the Wheeler Gorge channels before

thick levee deposits

(LV1 in Fig. 42)

can spread across the channels at a

higher stratigraphic horizon.

Other channel-levee interpretations

in the literature commonly refer to

sandy systems of limited lateral extent,

and with channel depths of only a few

tens of metres (e.g., Cretaceous

Chatsworth Formation of California;

Advocate and Link, 1981). Alternatively,

there are interpretations of shifting and

nested channel and channel-margin

facies 20-40 m thick, where the

in-

terbedding of sandstones and mud-

stones may respond acoustically as

an HAD facies (Upper Miocene

Capis-

trano Formation, California, Walker,

1975b). It is possible that these exam-

ples, or systems like them, represent

thalweg channels within a much larger

canyon (Figure

28), rather than indi-

vidual channel-levee or channel-margin

systems.

Sheet-like turbidites

There are many examples of appar-

ently unchannelized, sheet-like

tur-

bidites in the geological record. In some

cases, individual beds can be traced for

kilometres or tens of kilometres, im-

plying very smooth basin floors (Enos,

1969; Hesse, 1974; Ricci Lucchi and

Valmori, 1980). The topography of

modern fans shows that smooth,

low-

gradient depositional areas exist 1) on

the lower fan, and 2) on the flatter

parts of levees some distance from the

levee crest. Limited data from modern

fans suggest that the lower fan setting

would contain much sandier turbidites

than on the distal parts of levees. It is

important to develop criteria for distin-

guishing the two settings, in order to

make better predictions of lateral facies

changes, and in order to place the de-

posits correctly

within a fan stratigraphy

(the lower fan sandy sheet-like facies

would commonly be older than the

distal levee facies).

In the geological record, classical

tur-

bidites in apparently unchannelled set-

tings commonly occur in thicken-

ing-upward successions. These have

been related to the progradation of de-

positional lobes (Mutti and Ricci Lucchi,

1972), although this interpretation must

now be questioned because 1) deposi-

tional lobes are not common on modern

fans, and 2) some thickening-upward

successions might be related to fluctua-

tions of relative sea level (Mutti, 1985). It

is not known whether thickening-upward

successions occur in the lower sandy

parts of modern fans. Conversely, depo-

sitional lobes in the geological record

can only confidently be identified by their

external form, not by the presence of

thickening-upward successions. There

are few good examples where the ex-

ternal form can be defined. One

example is the Cretaceous Forbes

Formation, California (Weagant,

1972),

where the stack of sandy lobes and in-

terbedded mudstones is about 1000 m

thick. Individual lobes are up to 60 m

thick and 10 km in diameter in the

Grimes gas field. The lobes appear to be

irregularly stacked, and are separated

by blanketing mudstones that partly trap

gas in the reservoirs.

Slumpldebris flow

facies

Although there are many examples of

slumps and debris flows in the geolog-

ical record, there are very few that ap-

proach the scale of the chaotic and

acoustically transparent layers on

modern fans (many tens up to about

200 m). One of the best examples of a

slumped slope facies is in Penn-

sylvanian rocks on the coast of western

Ireland, where there is an almost con-

tinuously exposed section up to 550 m

thick (Martinsen, 1989; Fig. 43). The

styles of deformation include mud

slumps 0.5 to 15 m thick (with com-

posite thicknesses to about 50 m) and

mud and sand slides involving sedi-

ment thicknesses up to about 60 m.

Large-scale levee failures, similar to

those of the Amazon and Rhone fans,

have not been identified in the geolog-

ical record, although the slump in the

Waitemata Group (Fig. 14) is

possible

analogy. On a smaller scale, 15

thick

slumps of thin-bedded turbidites, pos-

sibly of levee origin, occur in

Creta-

....

. -.

---

13.

TURBlDlTES AND SUBMARINE FANS

261

ceous rocks of southern Chile (Winn

and

Dott,

1979).

CONCLUSIONS

Submarine fan studies are presently

state of flux. The seismically derived

information from modern fans con-

cerning facies geometry

and

internal

stratigraphy cannot yet be applied very

easily

the geological record, where

the scale of observation is commonly

very much smaller. Conversely, fea-

tures

the geological record cannot

always be related to parts of modern

fans.

lnformation is gradually becoming

available to

link

fan development with

fluctuations of relative sea level, but

there

urgent need for precise

dating within modern fans. lnformation

from smaller sand-rich fans

needed

to complement the data from large fans

such

the Amazon, Mississippi

and

Rhone. Finally, tectonic setting clearly

influences fan development

major

ways; however, there are too few

well-

studied fans

each tectonic setting to

allow any models to be formulated.

REFERENCES

Basic sources of information

Bouma, A.H., Normark, W.R. and Barnes,

N.E. (eds.), 1985, Submarine fans and

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A well-organized compendium of infor-

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DSDP

Leg 96.

Pickering, K.T., Hiscott, R.N. and Hein, F.J.,

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This is the latest textbook

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This volume appeared in January, 1992,

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the Pacific.

Normark, W.R.,

1970,

Growth patterns of

deep-sea fans: American Association of

Petroleum Geologists, Bulletin, v.

54,

21 70-2195.

Normark, W.R.,

1978,

Fan valleys, channels

and depositional lobes on modern sub-

marine fans: characters for recognition of

sandy turbidite environments: American

Association of Petroleum Geologists,

Bulletin, v.

62,

912-931.

Normark, W.R., Piper, D.J.P. and Hess,

G.R.,

1979,

Distributary channels, sand

lobes, and mesotopography of Navy sub-

marine fan, California Borderland, with

applications to ancient fan sediments:

Sedimentology, v.

26,

749-774.

Normark, W.R., Piper, D.J.W. and Stow,

D.A.V.,

1983,

Quaternary development of

channels, levees and lobes on middle

Laurentian Fan: American Association of

Petroleum Geologists, Bulletin, v.

67,

1400-1 409.

O'Connell, S., Stelting, C.E., Bouma, A.H.,

Coleman, J.M., Cremer, M., Droz, L.,

Meyer-Wright, A.A.,

Normark, W.R.,

Pickering, K.T. and Stow, D.A.V.,

1985,

Drilling results on the lower Mississippi

Fan,

Bouma, A.H., Normark, W.R.

and Barnes, N.E., eds., Submarine fans

and related turbidite systems: New York,

Springer Verlag, p.

291-298.

Picha, F.,

1979,

Ancient submarine canyons

of Tethyan continental margins,

Czechoslovakia: American Association of

Petroleum Geologists, Bulletin, v.

63,

67-86.

Piper, D.J.W. and Normark, W.R.,

1981,

Re-

examination of a Miocene deep-sea fan

and fan-valley, southern California:

Geological Society of America, Bulletin, v.

82,

1823-1830.

Piper, D.J.W., Stow, D.A.V. and Normark,

W.R.,

1984,

The Laurentian Fan; Sohm

Abyssal Plain: Geo-Marine Letters, v.

141-146.

13.

TURBlDlTES AND SUBMARINE FANS

263

Piper, D.J.W., Shor, A.N. and Hughes Clark,

J.E.,

1988,

The

1929

"Grand Banks"

earthquake, slump, and turbidity current,

Clifton, H.E., ed., Sedimentological

consequences of convulsive geological

events: Geological Society of America,

Special Paper

229,

77-92.

Pilkey, O.H.,

1988,

Basin plains; giant sedi-

mentation events,

Clifton, H.E., ed.,

Sedimentologic consequences of convul-

sive geologic events: Geological Society

of America, Special Paper

229,

93-99.

Posamentier, H.W. and Vail, P.R.,

1988,

Eustatic controls on clastic deposition

sequence and systems tract models,

Wilgus, C.K.,

et al.,

eds., Sea-level

changes: an integrated approach:

Society of Economic Paleontologists and

Mineralogists, Special Publication,

42,

109-1 24.

Posamentier, H.W., Jervey, M.T. and Vail,

P.R.,

1988,

Eustatic controls on clastic

deposition

conceptual framework,

Wilgus, C.K.,

et al.,

eds., Sea-level

changes: an integrated approach: So-

ciety of Economic Paleontologists and

Mineralogists, Special Publication

42,

109-124.

Ricci Lucchi,

and Valmori, E.,

1980,

Basin-

wide turbidites in a Miocene, over-sup-

plied deep-sea plain: a geometrical

analysis: Sedimentology, v.

27,

241-

270.

Stelting, C.E. and DSDP Leg

Shipboard

Scientists,

1985,

Migratory characteristics

of a mid-fan meander belt, Mississippi

Fan,

Bouma, A.H., Normark, W.R. and

Barnes, N.E., eds., Submarine fans and

related turbidite systems: New York,

Springer-Verlag, p.

283-290.

Uchupi, E. and Austin, J.A., Jr.,

1979,

The

stratigraphy and structure of the

Laurentian Cone region: Canadian Jour-

nal of Earth Sciences, v.

16,

1726-

1752.

Vail, P.R.,

1987,

Part

seismic stratigraphy

interpretation procedure,

Bally, A.W.,

ed., Atlas of seismic stratigraphy:

American Association of Petroleum

Geologists, Studies in Geology no.

27,

1-10.

Walker, R.G.,

1965,

The origin and signifi-

cance of the internal sedimentary struc-

tures of turbidites: Proceedings of the

Yorkshire Geological Society, v.

35,

32.

Comparison of turbidite structures with

those made in flume experiments, fol-

lowing the publication of the Bouma se-

quence in 1962.

Walker, R.G.,

1973,

Mopping up the turbidite

mess,

Ginsburg, R.N., ed., Evolving

concepts in sedimentology: Baltimore,

The Johns Hopkins Press,

1-37.

philosophical history of the turbidite

concept, based on the ideas of Thomas

Kuhn. Read the paper to understand

the title!

Walker, R.G.,

1975a,

Generalized facies

models for resedimented conglomerates

of turbidite association: Geological

Society of America, Bulletin, v.

86,

737-

748.

Defines descriptive parameters, and

shows how four models were constructed

using those parameters.

Walker, R.G.,

1975b,

Nested submarine

channels at San Clemente, California:

Geological Society of America, Bulletin,

86,

91 5-924.

Walker, R.G.,

1978,

Deep water sandstone

facies and ancient submarine fans:

models for exploration for stratigraphic

traps: American Association of Petroleum

Geologists, Bulletin, v.

62,

932-966.

submarine fan model of the channel

depositional lobe type, influential in its

time, but now obsolete because it ignored

external controls, especially sea level

fluctuations.

Walker, R.G.,

1985,

Mudstones and thin-

bedded turbidites associated with the

Upper Cretaceous Wheeler Gorge con-

glomerates, California: a possible chan-

nel-levee complex: Journal of Sedi-

mentary Petrology, v.

55,

279-290.

Introduces the idea of "CCC turbidites",

and relates levee features to thick chan-

nelled conglomerates. This example is

on the scale

a single channel-levee

system in the Rhone Fan (see Droz and

Bellaiche, 1985).

Weagant, F.E.,

1972,

Grimes gas field,

Sacramento Valley, California,

King,

R.E., ed., Stratigraphic oil and gas fields.

American Association of Petroleum

Geologists, Memoir

16,

428-439.

Weimer, P.,

1989,

Sequence stratigraphy of

the Mississippi Fan (Plio-Pleistocene),

Gulf of Mexico: Geo-Marine Letters, v.

185-272.

Excellent documentation of the seismic

facies of the seventeen Mississippi fan

sequences; the paper is a

goldmine of

information.

Weimer,

P.,

1990,

Sequence stratigraphy,

facies geometries, and depositional his-

tory of the Mississippi Fan, Gulf of

Mexico: American Association of

Petroleum Geologists, Bulletin, v.

74,

425-453.

Winn, R.D., Jr. and Dott,

R.H.,

Jr.,

1979,

Deep-water fan-channel conglomerates

of Late Cretaceous age, southern Chile:

Sedimentology,

26,

203-228.

14.

Introduction to Carbonate and

Evaporite Facies Models

Noel

James, Department of Geological Sciences, Queen's University,

Kingston, Ontario K7L 3N6

Alan C. Kendall, School of Environmental Sciences, University of East Anglia,

Norwich

NR4

7TJ England

THE SEDIMENTS ARE BORN,

NOT MADE

This deceptively simple phrase encap-

sulates the main theme of carbonate

and evaporite sedimentation. It also

highlights the differences between

such deposits and the terrigenous

clastic facies described in preceding

chapters. Terrigenous clastic sedi-

ments are made primarily by the dis-

integration of parent rocks and are

transported to the depositional envi-

ronment. Once there, patterns of tex-

ture and fabric are impressed upon

them by the hydraulic regime. The sig-

natures of such facies are in their sedi-

mentary structures and grain-size

variations (Table 1). Carbonate and

evaporite sediments are "born" as pre-

cipitates or skeletons within the depo-

sitional environment. For carbonate

sediments, this attribute has profound

consequences, namely 1) sediment

composition is fundamental in charac-

terizing the depositional environment,

2) grain size variations need not signal

changes in hydraulic regime,

large

structures such as platforms are pro-

duced entirely by sediments formed in

place, they are self-generating and

self-sustaining,

the temporal and

spatial style of accumulation depends

upon the nature of the sediments them-

selves. Consequences for evaporites

kclude: 1) mineralogy reflects the type

of water being evaporated and the

salinity of bottom brines,

many sedi-

ments are never transported but grow

place, at or below the sediment

surface, and 3) most are shallow water

(40 m) or pelagic deposits.

carbonate sediments and facies

have become better understood over

Ihe

last two decades (Bathurst, 1975;

Wilson, 1975; Scholle et al.,

1983), at-

tention has shifted to the dynamics of

sediment accumulation. Compilations

for evaporites have appeared more re-

cently (Dean and Schreiber, 1978;

Sonnenfeld, 1984; Schreiber, 1988;

Warren, 1989; Melvin, 1991). The

li-

brary of modern and ancient examples

has grown to the point where true

comparative sedimentology can now

be practised, revealing previously un-

recognized, universal themes in sedi-

mentation. Nevertheless, as illustrated

in the following chapters, consensus

has yet to be reached on many funda-

mentals of sediment dynamics.

Dynamic stratigraphy demands that

the input distilled from modern and

ancient environments should be as

fully understood as possible. Problems

arise for some carbonates because the

ecology of extinct rock-forming orga-

nisms may not be fully known. These

difficulties are even more prevalent

for evaporites because many have no

Holocene analogue.

In the following chapters the facies

are considered first and then placed in

an allostratigraphic context. In many

instances evaporites are a minor, but

nevertheless important, component of

other facies models,

e.g., interdune

evaporites in eolian facies, coastal

sabkhas in carbonate or

siliciclastic

peritidal facies. However, when evap-

orite~ and evaporitic carbonates

fill

entire basins, or evaporites form later-

ally extensive shelf deposits, they war-

rant separate facies models (see

Table

in Chapter 19). This intro-

duction summarizes themes impor-

tant to all carbonate and evaporite

accumulations.

CARBONATE

PLATFORM

AHiF

BANK

Figure

sketch showing the different

geometries of platforms and their termi-

nology.

scale is intended.

SOME TERMINOLOGY

Words used to describe carbonate and

evaporite accumulations vary greatly,

as do their meanings (Ginsburg and

James, 1975; Wilson, 1975; Read,

1985; Geldsetzer et al., 1988). Al-

though they can form across the envi-

ronmental spectrum, the thickest car-

bonates

accrete- as platforms while the

thickest evaporites form as basinal de-

posits. To maintain continuity, the fol-

lowing section defines the terms used

in this book.

Platforms

A carbonate platform (Fig.1) is a large

edifice formed by the accumulation of

sediment in an area of subsidence.

Most such structures have a flat top,

possess steep sides and can be

several kilometres thick and extend

over many hundreds of square

kilome-

tres. A carbonate shelf is a platform

tied to an adjacent continental land

mass. This hinterland is a potential

source of terrigenous clastic sediment,

fresh water, terrigenous organic matter

and nutrients. A craton many hundreds

to thousands of kilometres across is

called an epeiric platform when

covered by shallow seawater. A car-

bonate bank is an isolated platform

surrounded by deep ocean water and

cut off from terrigenous clastic sedi-

ments. (Note that here "bank" is not

synonymous with mound

a smaller,

reef-like carbonate body that may lack

definitive evidence of an organic frame-

work, see Chapter 17). A carbonate

atoll is a specific type of bank devel-

oped on a subsiding volcano. Similar

banks, unrelated to volcanoes, are

termed faros. Atolls and banks can be

so dominated by reefs that their geo-

logical expressions are termed reef

complexes or carbonate buildups.

The platform edge is critical.

rimmed platform (Fig.

has a seg-

mented to continuous rampart of reefs

andlor lime sand shoals along the

266 JAMES, KENDALL

margin that absorbs ocean waves.

Modern warm water platforms are gen-

erally rimmed because corals are proli-

fic and construct large reefs along the

edges of shelves and banks. By ab-

sorbing waves and swells and dissipat-

ing storm energy, the rim generates a

variety of lower energy environments. It

also confines the movement of coarse-

grained sediment to the

lagoon/shallow

platform and can potentially restrict

water circulation, increasing the possi-

bility of evaporites. Accumulation space

is limited by sea level and carbonate

facies are strongly differentiated.

An unrimmed or open platform

(Fig.

2),

is one in which there is no bar-

rier. Unrimmed platforms occur today

on the leeward side of large tropical

banks and are the norm in all cool

water settings.

ramp is an unrimmed

shelf that slopes gently basinward at

angles of less than 1 degree. Near-

shore, wave-agitated facies grade into

deeper water, low-energy deposits

and there is no discernable break in

slope. Because oceanic waves and

swells can sweep directly onto and

across the shallow

seafloor of open

shelves and ramps,

1) the energy level

of most shallow water environments is

high, 2) nearshore facies are complex,

sediment can easily be transported

into deep water,

only the nearshore

zone can keep pace with sea level

rise, and 5)

subtidal accumulation

space will be controlled as much by

the depth of wave abrasion as by sea

level (Chapter 15). Since unrimmed

platforms and ramps are affected by

the same physical processes as ter-

rigenous clastic shelves (Chapters

11,

12), they have similar facies. What

sets them apart is the continual in

place production of carbonate sedi-

ment and early cementation, which

constantly alters the nature of the sea

floor.

Lagoons and widespread shallow

platforms may become restricted and

hypersaline. Widespread shelf evapor-

ites that pass basinward into open

marine sediments are confined to

rimmed platforms (Fig.

3).

The absence

of restrictive barriers on unrimmed plat-

forms limits evaporites to marginal

sabkha and associated salina deposits.

In situations where the entire deposi-

tional basin is evaporitic, however,

evaporitic shelves may occur without

platform edge barriers. Theoretically,

evaporite ramps could occur around

evaporite basins; however, not one has

been confidently identified. This is

probably because rapid evaporite ac-

cumulation is restricted to shallow

brines. Evaporites are either confined

to marginal shelves built on the shal-

lowest parts of older ramps, or they

form on the deepest parts, when the

adjacent basin desiccates.

Basins

Deep water environments are only sig-

nificant repositories for carbonate sed-

iment in post-Jurassic time, when

pelagic calcareous microorganisms

evolved. These planktonic foramini-

fera, green algae and tiny gastropods

live in the upper parts of the water

column and upon death sink to the

seafloor as a pelagic rain (Fig. 2).

Older basinal sediments are primarily

terrigenous clastic or siliceous, with

Table

Differences between terrigenous clastic, carbonate and evaporite sediments.

Terrigenous Clastic Carbonate Evaporite

Climate is no constraint, sediments occur

Most sediments occur in shallow, warm

Most sediments occur in shallow-water or

worldwide. water environments. mud flat environments.

Sediments are both terrestrial and Sediments are mostly marine.

Sediments occur only in restricted

marine. terrestrial and marine environments.

Grain-size reflects hydraulic energy of the

Grain-size reflects the size of skeletons and

Crystal size reflects nucleation and growth

environment. precipitated grains. rate, or diagenetic alteration.

Mud indicates settling from suspension.

Mud commonly indicates prolific growth of

Fine

carbonateslsulphates indicate rapid

organisms that produce tiny crystals.

precipitation.

Currents and waves form shallow-water Many sand bodies form by localized

Shallow-water sand bodies are rare.

sand bodies.

physicochemical or biological production

of carbonate.

Environmental changes are induced by

Environmental change can be induced by

Environmental change is induced by

widespread changes in hydraulic

localized buildup of carbonate, without

changes in basin dynamics.

regimen. change in hydraulic regimen.

Sediments remain unconsolidated in the

Sediments are commonly cemented on the

Sediments are commonly cemented or

depositional environment. seafloor. form crystal crusts in the depositional

environment.

Periodic exposure does not alter the

Periodic exposure results in intensive dia-

Periodic exposure results in growth of

sediments. genesis. intrasediment evaporites or wholesale

dissolution.

Walther's law applies to most deposits. Walther's Law applies to many, but not all,

Walther's law applies to few deposits.

deposits.

14. CARBONATE

EVAPORITE MODELS 267

smncwmo

CARB

Figure

sketch of rimmed and unrimmed platforms illustrating the position of the car-

bonate factory and the main directions of sediment transport. Note that open ocean waves

and swells impinge on open shelves and ramps but are absorbed at the edge of the rimmed

platform. Offshore banks are the same as these shelves and can have rimmed (generally

windward) and unrimmed (generally leeward) margins.

'ONTINENTAL

COASTAL

SALINA.

SABKHA.

SHELF

SALINE

PAN

CONTINENTAL

SHELF EVAPORITES

BASIN

FLANKING

EVAPORITE

SHELF

~~~&~!~~

HIGH

BRINE

LEVEL

Figure

sketch illustrating locations of the evaporite factory and resulting facies in con-

tinental, shelf and basin-central locations. Continental playa evaporites can be extremely

large. Basin-central evaporites form as basin-margin shelves and deep water deposits when

brine level is high and as shallow water deposits when brine level is low.

the only carbonate input coming from

shallow platforms.

All large and thick evaporites occupy

depositional basins that suffered par-

tial or complete desiccation during

isolation from the sea. Such

evaporite

basins

can be flanked by

evaporitic

shelves,

or alternatively, earlier car-

bonate shelves that become emergent

during evaporitic episodes (Fig.

3).

Evaporites cease to form during periods

of flooding and the same basins be-

come open- or restricted-marine envi-

ronments.

THE SEDIMENT FACTORY

Carbonates

The

carbonate factory

(Fig.

is the

shallow, illuminated seafloor. Particles

of all grain sizes are born here, either

crystallizing as skeletons or precipi-

tating directly out of seawater. Sedi-

ments mostly remain in place forming

widespread

"subtidal" deposits (Chap-

ter 15) or reefs and mounds (Chapter

17). Much of the abundant fine frac-

tion is resuspended periodically and

piles up as muddy peritidal flats

against highs on the platform and

along the shoreline (Chapter 16). Fine

sediment is also swept seaward where,

together with sediment gravity flows

originating at the margin, it accumu-

lates on the slope and on adjacent

basin margin (Chapter 18). Regard-

less of facies, it is the factory that is at

the core of carbonate deposition. All

carbonate facies and carbonate

stratigraphy depend on the "health" of

this production unit.

Evaporites

The

evaporite factory

(Chapter 19) can

operate in both terrigenous

clastic and

carbonate settings (Fig.

3).

Evaporites

similarly accumulate most rapidly in

shallow water conditions. Export of

sediment from this evaporite factory

does occur, but it is substantially less

than in carbonate settings because so

much of the sediment is lithified ini-

tially, or quickly becomes cemented.

Environments (see Table 2 in Chapter

19) range from continental lakes to

coastal salinas and sabkhas to

shelf-

wide complexes. Evaporites can also

develop in shallow- and deep-water

basin centres.

The advantage of modelling evap-

orite systems is that deposition occurs

by physico-chemical precipitation.

268 JAMES, KENDALL

The original overall composition and

mineralogy of an evaporite provide in-

formation about the type of water

being evaporated, the brine tempera-

ture and salinity, and the degree of

basin isolation. The disadvantages

occur because, 1) most early models

were chemical in nature

and/or based

upon few examples, 2) evaporites

exposed in outcrop (uncommon) are

much altered, and subsurface core is

limited, 3) no areas of present-day

evaporite deposition compare in size

with many ancient basins (Fig. 4),

4) changes in depositional conditions

are rapid, profound and commonly

result in superimposed facies, making

original environmental recognition diffi-

cult,

characteristically, new environ-

ments destroy and replace older ones,

rather than moving laterally, so that

associated facies are unreliable when

interpreting poorly preserved deposits,

and

evaporites are susceptible to

wholesale, postdepositional change

that can remove all primary features.

THE

CONTROLS

For carbonate production and sedi-

ment accumulation to be at a maxi-

mum, the environment must be just

right; not too deep, not too shallow;

not too warm, nor too cold; not too

fresh, but not too saline either; not

too much terrigenous clastic sediment;

not too many nutrients but neither

too few (the Goldilocks Window of

Goldhammer et

a/., 1990). A similar

depositional "window" controls evap-

orite accumulation. Water inflow must

not be too much, nor too little: brine

loss

(reflux) must be allowed to occur,

but must not be too great; evaporation

rates must be high, but terrigenous

clastic influx must be low. The prin-

cipal regulators are climate, oceanog-

raphy (including basin

and/or shelf

restriction) and tectonics, but for most

carbonates, organism biology is the

major factor.

Organism biology

Most carbonate sediments are pro-

duced biologically or by biochemical

mediation. There are four sorts of car-

bonate particles: 1) precipitates

those grains formed by direct or bio-

logically mediated precipitation of

calcium carbonate,

e.g., ooids and

some lime mud;

bioclasts

the cal-

ANCIENT EVAPORITE BASINS

(

DEAD

SEA

)

LAKE MACLEOD

KARA

BOGUZ

TRUCIAL

COAST

RECENT

EVAPORlTlC

ENVIRONMENTS

Figure

Comparison of size (aerial extent) of Recent (right) and ancient (left) evaporite

settings.

careous sheaths, shells, tests and

spicules of the myriad of sessile, bur-

rowing, drifting or swimming inverte-

brates, microbes and algae that live in

shallow and deep water across the

environmental spectrum; 3) peloids

grains of microcrystalline carbonate,

generally agglutinated or cemented

feces or diagenetically altered grains;

and 4)

lithoclasts

fragments of con-

solidated, hardened or lithified sedi-

ment. Precipitates around, and skel-

etons of, phototrophic organisms are

particularly important because they

rapidly produce enormous amounts of

sediment. Such organisms are mostly

photosynthetic microbes and algae, or

invertebrates such as living stony

corals, giant clams and big forami-

nifers, which have symbiotic

pho-

totrophs (light-dependent microorgan-

isms) in their tissue. These symbionts,

by taking over metabolic functions,

allow otherwise small- or modest-sized

invertebrates to precipitate large cal-

careous skeletons, but restrict such

organisms to the photic zone. This

bargain, large size for light depen-

dence, restricts carbonate deposition

to very specific settings.

We rely heavily upon observations

from modern environments of deposi-

tion to interpret ancient sedimentary

sequences and to construct facies

models. This approach works, and is

seen to work, because the composi-

tion of most sedimentary particles has

remained the same through time; a

quartz sand grain or an ooid is the

same in the Pleistocene, Permian or

Precambrian. Because organisms have

changed with time, it is difficult, at first

glance, to compare modern and spe-

cific ancient carbonate facies. This

tends to intimidate those who are

beginning the study of carbonates,

but it should-not. Carbonate-secreting

organisms in the rock record, when

viewed as sediment producers, do

have living counterparts, although they

may not even belong to the same

phylum. Thus "carbonates are like

Shakespeare; the plays remain the

same, only the actors changen. This

is because, despite the numerous

groups of organisms with hard parts,

there are only two ways in which these

hard parts are arranged either as

whole, rigid skeletons

(e.g., foramini-

fers, snails, corals) or as numerous in-

dividual segments held together in life