Boggs S. Principles of Sedimentology and Stratigraphy

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386

Chapter 11 I Carbonate and Evaporite Environments

Figure 11.17

Idealized facies in a typical

modern, mature coral reef

with a well-developed reef

framework. [After Long

man, M. W., 1981 , A

process approach to recog

nizing facies of reef com

plexes, in To omey, D. F.

(ed.), European fossil reef

models: Soc. Econ. Paleon

tologists and Mineralogists

Spec. Pub. 30, Fig. 1 0, p.

23, repri nted by permis

sion of SEPM, Tulsa, Okla.]

Reef Deposits

We cannot discuss all types of modern reefs and reef facies here; however, we will

examine the zoning and facies development of high-energy, platform-margin reefs

as a general model for high-energy ef environments. Figure 11.17 illusates

schematically the principal facies subdivisions of a platform margin reef. Note

that the reef consists of a central core, the reef framework, which grades seaward

into the reef slope, and a loose accumulatio 0f eef debris called the fore-reef

talus. The nearly flat, uppermost, shalowest part of the reef is called the reef flat,

which grades landward into back-reef skeletal (coralgal) sands and subtidal la

goonal deposits. Reefs are divided physiographically into fore-reef, reef-front,

reef-crest, reef-flat, and back-reef zones (Fig. 11.168).

Figure 11.17 also shmvs the types of carbonate materials typically formed 

different zones of the reef. The words rudstone, bafflestone binds tone, and frame

stone  Figure 11.17 are terms used by Embry and Klovan (1971) as modifations

of Dunham's (1962) limestone classification (see Table 6.3). Floats tone and rud

stone are unbound carbonate grains, more than 10 percent of which are more th

2 mm in size; floatstones are mud supported; and rudstones are grain supported.

Bafflestones

are carbonate components bound together at the time of deposition

stalked organisms that trapped sedent by acting as baffles. Bindstones were

bound during deposition by encrusting and binding organisms such as encrusting

foraminifers and bryozoans, and framestones were bound by organisms such as

corals, which build a rigid framev.rork structure.

These different carbonate facies represent variations in ater enel·gy, domi

nant sedimentation processes, and types of organisms in each zone of the reef.

Water energy is highest on the reef crest, which also contains the highest percent

age of framework constituents (framestones). As water energy decreases toward

both the fore reef and the back reef, the percentage of framework constients also

decreases. Note that overall the framework component of reefs is commonly

much smaller than the volume of nonframework constituents. Longman (1981)

compares the structure of reefs to that of an apple, which has a central core, or

framework, surrounded by the much larger edible fruit. The nonframework

fraction of reefs consists of organisms such as echinoderms, green algae, and

molluscs, which do not build framework structures, bioclasts broken from the

reef by wave activity, and, in lower-energy zones of the reef, some lime mud.

The fore-reef talus slope and back-reef coralgal sand zone are made up entirely

...

Distal talus

(stone, grainstone)

Reef framewo

(lramestes}

baftleste

11.4 Organic Reef Environments

387

of nonframework cotituents that consist principally of reef-derived bioclasts.

Relatively few organisms live in these zones.

Low-Ener



Reef Facies

The facies of modern, high-energy, platform margin-type reefs thus coist funda

mentally of a central framework core composed largely of corals and coralline algae;

the oue grades seaward through a zone of rubbly fare-rf talus to dr water

lime muds or shales and landward through back-reef coralga� sands to finer grained

lagnal dosits. This model serves reasonably well for high-energy reefs devel

o  most settings; however, some reefs form under much ower energy mndi

tions. Low-energy ree do not develop the characteristic zong of hgh-ergy

reefs and tend to be circular to elliptical in plan view. Organisms growing on such

reefs are dominated by the more delicate, branching forms (Fig. U.l6A). Some

low-energy reefs do not contain the typical reef structure deribed above hut are

constmcted simply of carbonate sands d muds built by organisms that are very

similar to reef-type organisms in composition (e.g., James, 1984c). Other low-energy

buildups are composed largely of nonreef-type organisms. They consist of motmd

shaped piles of skeletal fragments and I or bioclastic lime muds rich in skeletal or

ganisms and minor amounts of organic boundstone. As mentioned, these structes

are called reef mounds or simply monnds (e.g., James d Bourque, 1992).

Ancient Reefs

Reef De

osits

Reefs

as they appear in the fossil record differ in some important respects from

me reefs. first, we commonly see cient reefs only in vertical exposures. We

serve a o-dimensional limestone by composed of different components

formed at diffent times (James, 1983). Thus, we may not be able to detect all of

the facies zones displayed by modern reefs. The nature of the flank and inter-reef

facies exposed  vutcrop obviously may differ depending upon whether the ver

tical exposure cuts through the reef to expose a cross section from fore reef to back

reef or a logitudinal section numing parallel to the reef crest (e.g., Fig. 11.9). 

many cases, only the massive reef core is clearly exposed, as in Figure 11.18. The

rf structure and nature of reef facies also depend upon the kind of reef (e.g., bar-

Figure 11.18

The Resolution Reef (Pennsylvanian) exposed near Minturn, Colorado. A. Distal view

showing the biohermal shape of the reef core, enclosed in finer sediment. B. Close-up

view of the front of the massive reef core.

388

Chapter 11 I Carbonate and Evaporite Environments

rier reef, fringing reef, patch reef). An additional factor that can further complicate

recognition of ancient reef facies is diagenesis, which may cause selective dolomi

tization or solution that can obliterate or destroy parts of the reef complex.

Depending upon eir age, ancient reefs and mounds may also differ marked

ly from modem ree and mounds in terms of the dominant organisms that make

up these suctures. The hermatypic corals that dominate modem coral reefs first

appeared in the Mesozoic and thus are not components of older reefs. Older reefs

and mounds are dominated by other kinds of organisms, such as tubiphytes (prob

lematic microstructures of possible algal o gin), sponges, bryozoans, and microbes

(e.g., cyanobacteria) (Fig. 11.19). See Fagerstrom (1987), Stanley and Fagerstrom

(1988), and Kiessling, Fliigel, and Golonka (1999, 2002) for additional infoation

about reef organisms and the evolution of reef ecosystems rough time.

Because many kinds of ancient reefs, with different kinds of reef-building or

ganisms, are present in the sedimentary record, it is difficult to generalize about

e structure and makeup of ancient reefs. Still, some effort in at direction is re

quired. Figure 11.20 is a highly generalized sketch of an idealized ancient reef en

closed in nonreef deposits. The core framework consists of skeletal bindstones and

rudstones. The specific kinds of skeletal components depend upon e age of the

reef. These core sediments grade seaward into the forereef facies, consisting in

large part of rudstones and floatstones broken from the framework by wave ac

tion. The reef core grades landward into a high-energy, skeletal sand apron con

sisting also largely of material broken off the reef core. In turn, skeletal sands

grade into a back-reef facies deposited under quieter water conditions. This facies

may contain skeletal sands and muds, pelleted muds, and micrites, and stromato

lites may be common. Evaporites may also be present in some back-reef facies, re

ecting restricted water circulation in the back-reef environment. Reef growth

eventually terminates because of drowning of the reef or possibly, in some cases,

burial by siliciclastic muds. The topography of the ancient reef is commonly visi

ble benea is covering of younger sediment, as illustrated in Figure 11.20.

Occuence of Ancient Reefs

Reefs of some te are present in Phanerozoic (post-Precambrian) carbonate rocks

of most ages (e.g., see the compendium volume edited by Kiessling, Flugel, and

Golonka, 2002). Although carbonate-secreting organisms were not present during

the Precambrian, Precambrian carbonate buildups composed of stromatolites have

been reported from various localities in Nor America, Europe, Africa, and Aus

tralia. Phanerozoic rocks contain reefs composed of calcium carbonate-secreng or

ganisms. Many of these reefs were built by framework-constructing or encrusting

organisms. Reef development was not uniform throughout geologic time, howev

er, and ree are much more abundant in some parts of the rock record than in oth

ers. Figure 11.19 graphically depicts the distribution of reefs and mounds through

time and also shows e major kinds of organisms that were responsible for reef

buildg at dierent times, as previously mentioned. Figure 11.19 is based on a

database of 2470 ree compiled by Kiessling, Flugel, and Golonka (1999). These au

thors also show the geographic distribution of reefs of various ages and discuss

reef type, dimensions, and environmental setting. For an update of ese data, e

Kiesslg (2002). These variations in reef dist bution and abundance through time

reflect times when carbonate pduction on reefs flourished and times when efs

were in crisis and carbonate production declined (e.g., Flugel and Kiessling, 2002).

11.5 MIXED CARBONATE  SILICICLASTIC SYSTEMS

To avoid possible confusion, the carbonate depositional enviroents described in

the preceding sections are discussed as if only carbonate sediments are deposited

nme

(Ma)

100

150

200

250

300

350

400

450

500

550



- Reels and reel mounds

- -Mud mounds and biostromes Number of reefs

11.5 Mixed Carbonate-

Silicic

lastic Systems

389

OOthers

IWorms

 Bryozoans

i Pelecypods

�Corals

�Siliceous Sponges



Coralline sponges

 Tubiphes

BAigae

• Microbes

Figure 11.19

Distribution of reefs through

Phanerozoic time showing both

reefs and mud mounds as well

s the dominant organisms that

make up the reefs and mounds.

"Others" refers to brachiopods,

pelmatozoans, and foraminifers.

The numbers 1-7 refer to seven

major cycles of Phanerozoic reef

building. Starred lines indicate

times of mass extinctions.

Curves to the left show the cu

mulative number of reefs and

mounds in each time interval,

and horizontal bars on the right

depict the cumulative number

of reefs in which a particular fos

sil group is dominant. [From

Kiessling, W., E. Flgel, and j.

Golonka, 1999, Paleoreef maps:

Evaluation of a comprehensive

database on Phanerozoic reefs:

American Association Petroleum

Geologists Bulletin, v. 83, Fig.

13, p. 1576. Reproduced by

permission.]

 these systems.  fact, mixed carbonate and siliciclastic sediments are present in

my stratigraphic successions. Such deposits are referred to as mixed carbonate

siliciclastic successions or carbonate-clastic transitional successions. Carbonate

and siliciclastic sediments can x owing to lateral facies mixing (spatial varia

tions in environments) that produces lateral interfingering of carbonate and clastic

sediment. Mixtures may also result from sea-level change and/or variations in

sediment supply, which cause vertical variations in the stratigraphic succession of

facies, called temporal variability (Budd and Harris, 1990). Thus, siliciclastic facies

may occur in a lateral interfingering relationship (see Chapter 12) with carbonate

fades or as distinct interbeds within carbonate successions. Carbonate-siliciclastic

390

Chapter 11 I Carbonate and Evaporite Environments

Figure 11.20

Back reef

(packstones, wackestones,

Skeletal sand apron

stromatolites; evaporites

(rudstones, grainstones, packstones)

in some settings}

Idealized, schematic represen

tation of an ancient barrier reef

deposit illustrating typical reef,

fore-reef, and back-reef facies.

Based in part on the Upper

Devonian Swan Hills Reef

Buildup, central Alberta, Cana

da (Viau, 1983).

Basin facies

(lime/siliciclastic

mud)

Fore reef

(rudstones, floatstones,

bindstones)

transitions are known  a variety of environen, including coastal d itmer

shelf, middle- and outer-shelf (including reef), and the slope-to-basin environments

(Doyle and Roberts, 1988; Lomando and Harris, 1991). They may occur  temperate

as well as tropical shelf environments (e.g., Haywick, Carter, and Henderson, 1992).

11.6 EVAPORITE ENVIRONMENTS

General Statement

As discussed in Chapter 7, evaporite deposits are composed dominantly of halite

(NaCl) and the sulfate minerals gypsum (CaS042H20) and anhydrite (Ca4).

Evaporites are deposited under climatic conditions where evaporation losses ex

ceed precipitation (rain and snow). They are forming today  both nonmarine

d marine environments. The sites of modern evaporite deposition are very lim

ited in size, however, compared to many an dent evaporite basins that were gjgan

tic. To illustrate, one of the larger mern evaporite deposits occurs  Lake

Macleod, weste Australia, which is about 100 X 50  in size and contains up

to 9 m of Holocene evaporites. By contrast, the Messinian (Miocene) Mediter

ranean basins extended over an area of about 2400 X 600  and have evapori

fills up to 2 km thick (Kendall d Harwood, 1996). Although no mode analogs

for these giant evaporite basins exist, we can nonetheless ga important insight

into ancient evaporite environments by

studying modern environments.

Modern Evaporite Environments

Nonmarine Environment

Figure 7.5 (Chapter 7) illustrates the principal environments in which evaporites

are forming today. They are present in a variety of small-scale continental settings

such as springs, desert dunefields, and soils (Smoot and Lowenstein, 1991). They

11.6 Evaporite Environments

391

pically form and are most important, however, in closed basins characterized by

playas (continental sabas) that contain an ephemeral lake or salt pan, common-

ly surrounded by saline mudflats (Kendall, 1992). Evaporite minerals are deposit-

ed in salt pans when ephemeral water evaporates and also displacively and as

cents within adjacent mudflat sediment. In a few continental settings where in-

termontaine basins are fed by perennial streams, perenal saline lakes may exist

(e.g., Great Salt Lake, Utah). Nonmarine evaporite deposits may contain minerals

such

as borax, epsomite, and trona (see Chapter 7) that are not common in marine

dosits; however, gypsum, anhydrite, and halite tend to dominate nonmarine

deposits as they do in marine deposits.

Shallow Mane Envinment

Modem marine evaporites are forming in two principal kinds of settings: coastal

sabkhas and salinas (e.g., Handford, 1991; Warren, 1991). Marine sabas are

coastal supratidal mudats. Evaporite minerals do not precipitate from standing

water but instead form displacively within saba sediments, consisting of carbon

ates and/or siliciclastic deposits, in a capillary zone above a saline water table.

Water lost by evaporation is replaced by downward seepage of storm-dven sea

water or by interflow of groundwater from continental sources. As illustrated by the

sabas of the Arabian Gulf (Fig. 11 .21), evaporite minerals that cstallize within

e sabkha sediment are mainly gypsum, anhydrite, and carbonates. Algal mats

(stromatolites) commonly form in associated intertidal to supratidal sediments.

AND MUDS WITH

HARDGROUNDS (1)

TERTIARY ROCK

JUST ABOVE HWM

MIDDLE SABKHA

HALITE CRUST

(9)

SUPRATIDAL ANHYDRITE POLYGONS )

-�i



, ANHYDRITE

GYPSUM-CARBONATE MUSH (6)

LAYER (8)

LARGE GYPSUM

CRYSTALS (4)

Figure 11.21

Schematic representation

of vertical and lateral facies

relationships in sabkhas of

the Arabian Gulf.

HWM = high-water mark.

[From Warren, j. K., and G.

St.

C. Kendall, 1985, Com

parison of sequences

formed in marine sabkha

(subaerial) and salina (sub

aqueous) settings-Mod

ern and ancient: Am.

Assoc. Petroleum Geolo

gists Bull., v. 69, Fig. 2, p.

1015, reprinted by permis

sion of AAPG, Tulsa, Okla.]

392

Chapter 11 I Carbonate and Evaporite Environments

Figure 11.22

Environmental setting and

typical evaporite facies in

southern Australia salinas.

[From Warren, ]. K., and G.

St. C. Kendall, 1985, Com

parison of sequences

formed in marine sabkha

(subaerial) and salina (sub

aqueous) settings-Modern

and ancient: Am. Assoc. Pe

troleum Geologists Bull., v.

69, Fig. 4, p. 1017, reprint

ed by permission of AAPG,

Tulsa, Okla.]

Other modern shallow-marine evaporites are forming in a few marine-fed

coastal lagoons and salt pans called salinas. Salinas occur in depressions on

sabkhas, between coastal dunes, on deltas, or in tectonic downwarps behind

coastal barriers. Modern salinas are particularly common in southe and weste

Australia (e.g., Fig. 11.22), but they are present also around the margins of the

Med

iterranean, Black, and Red Seas. Salinas differ from sabkhas particularly

that evaporite deposits precipitate primarily from surface brine rather than within

sediment. Gypsum is the most common mineral in salina deposits; however, some

salinas such as Lake Macleod, Australia, contain abundant halite.

Deep-Water Environment

Wa ter depth in most modern salinas is quite shallow, a few meters at most. The

Dead Sea, lying between Israel and Jordan, provides a single modern example of a

deeper

water evaporite environment. The Dead about 15-20 km wide and 80

long, is fed by the Jordan River. The southe basin of the Dead Sea is shallow,

but

the northern basin has

a brine depth of more than 200 m. Clayey silt is the pre

dominant

sediment deposited in the basin; however, evaporitic minerals such as

gypsum, halite, and aragonite are also forming (Garber, Levy, and Friedman,

1987}. Although called a sea, the Dead Sea is actually a large, closed, perennial

saline lake that lies in a rift valley more than 400 m below sea level, and the com

position of its brine differs considerably from that of sea water (Kendall and Har

wood,

1996). Thus, although a deeper water evaporite basin, the Dead Sea is not a

CARBONATE FRINGE

GYP

ITE 111)

EOUAN

GYP

ARENITE (10)

LAMINATED

GYPSARENITE (9)

11.6 Evaporite Environments

393

modem analog for large central-basin, mane evaporite deposits (discussed below)

that formed in the geologic past.

cient Evaporite Environments

Nonmarine Environment

Bause the sites of modern continental evapote deposition are rather small on

the whole, geologists have commonly assumed that ancient nonmarine deposits

 also quite small. Hardie (1984) takes excepon to this genal perception. He

maintains

that nonmarine evaporite deposits can be as large or as thick as marine

evaporites, and cites several examples of Tertiary nonmarine salt deposits in Cali

fornia and Nevada, that are 200-600 m thick, to support his view. Still, few large

ancient noarine evaporite deposits are known, and relavely few nonmarine

de posits of any size have been reported compared to the number of putative an

cient marine evaporite deposits.

The nonmarine versus marine origin of ancient evaporites is determined

mainly on the basis of the kinds of fossils in and the sedimentology of associated

nonevaporite facies. For example, nonmarine evaporites may be associated with

continental

redbeds, alluvial fan sediments, or desert dune deposits. An example

of this kind of association is illustrated by the Wilkins Peak Member of the Green

River Formation (Eocene), Wyoming, shown in Figure 8.26 (Chapter 8). Mineralo

 may also provide clues to nonmarine origin because certain evaporite minerals

appear to form only under nonmarine conditions (Chapter 7). Other putative an

cient nonmarine evaporite deposits include the sediments of the Tajo Basin

(Miocene), Spain, filled with more than 1500 m of siliciclastic sediments and asso

ciated

evaporites; evaporites associated with fluvial and lacustrine siliciclastic

sediments in the Newark Basin (Triassic-Jurassic) of the northeaste United

States; and parts of the Permian Rotliegendes Formation of Europe (Smoot and

Lowenstein, 1991).

Mari ne Environment

As mentioned, the great bulk of ancient evaporites and the truly large evapote

deposits appear to be of marine origin. Some ancient marine evaporites were

probably deposited in settings similar to modern coastal sabkhas and salinas. On

the other hand, many ancient evaporites appear to have formed in broad shelf en

vironments or within marine basins for which there are no good modern analogs.

us, the nature of these ancient environments must be deduced from the rock

record itself. Consequently, considerable controversy has arisen with respect to

the nature of these environments, and the question of deep- vs. shallow-water

origin is particularly troubling. In any case, it now appears that ancient marine

evaporites likely accumulated in three kinds of settings: small coastal settings

similar to modern sabkhas and salinas; wide, basin-margin shelves or platforms

(Fig. 11 .23A); and extensive basin-central enviroents (Fig. 11.238).

Basin-margin evaporites likely formed in vast expanses of evapotic lagoons

and mudflats, possibly extending tens of thousands to hundreds of thousands of

kilometers, throughout which brine depths did not exceed a few meters (Kendall

and Harwood, 1996). The evaporites consist of anhydrite (originally gypsum)

and/or halite, which may be interbedded with carbonates and siliciclastic sedi

ment. Cyanobacteal mats may have been present in less saline parts of the set

ting. For example, the San Andreas Formation (Permian) of the Palo Duro Basin,

Texas, contains more than twenty cycles consisting of anhydritic mudstone over

lain by a regressive succession of carbonate, anhydrite, and bedded halite. These

cycles range from 1 to 100 m ick and are traceable over more than 10,000 2,

with only minor changes in thickness and facies (Hovorka, 1987).

394

Chapter 11 I Carbonate and Evaporite

Environments

Figure 11.23

A. Shelf Evaporites

�iO's -1000's km----+

�'

displacive (in mudflat

environments)

B. Basin-Central Evaporites

� i O's- 1OOO's km----+

1. Shallow basin, shallow water

emeent

basin flanks

2. Deep basin, shallow water (desiccated basin)

brine pan and

evaporitic flat

Principal kinds of ancient marine evap

orite environments (excluding small

scale coastal sabkhas and salinas). [After

Kendall, A. C., 1992, Evaporites, in

Walker, R. G., and N. P. james (eds.),

Facies models-Response to sea-level

change: Geological Association of

Canada, Fig. 5, p. 380, reproduced by

permission.]

Deep basin, deep water (brine-filled basin)

brine level

deep-water

laminites

Basin-central evaporites accumulated in subsiding basins that may also have

ranged from tens to thousands of kilometers across (Fig. 11 .238). Some basins had

very little relief, and evaporites accumulated from shallow bodies of brine or

wiin saline mudflats (shallow-basin, shallow-water model; Fig. 11.2381). S



lar shallow-water evaporites could form from shallow bodies of brine that existed

within basins having much greater relief, which were subjected to substantial

evaporative drawdown (deep-basin, shallow-water model; Fig. 11.2382). Finally,

deeper water evaporites formed in basins having substantial relief (tens t

hun

dreds of meters) and which were largely filled wh brine (deep-basin, deep

water model; Fig. 11.2383). Deep-water evaporites appear to be especially

characterized by fine, even, widely distributed laminations (see Fig. 7.3). They may

also include gravity-displaced deposits. The Castille Formation (Permian), Texas

and New Mexico, is a classic example of a laminated, deep-water evaporite 

which individual lamina about 2 mm thick can be traced for a distance of 113 km

(Dean and Anderson, 1978).

As noted in Chapter 7, some kind of barrier must be present between evapo

rative basins and the open ocean to provide partial restriction of ocean-water cir

culation into the basins (see Fig. 7.6). Brine levels in these basins can fluctua 

response to changes in rates of sea-water inow, outflow, and evaporation. Thus,

deep bass can alteate between deep-water evaporite deposition and shallow

water/mudflat evaporite deposition. Also, deep basins can eventually fill up with

evaporites and be converted into shallow basins.

o of the most impressive ancient marine evaporite deposits are the Permi

an Zechstein evaporites and the Miocene Messinian evaporites. The Zechstein