Boggs S. Principles of Sedimentology and Stratigraphy
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376
Chapter 11 I Carbonate and Evaporite Environments
Figure 11.6
0
10
Te xture
Sand (0-1 0% mud)
Sand (0-50% mud)
Mud (>50% mud)
Carbonate content
Sediment map of South Florida Bay area, an example of a modern rimmed carbonate shel
showing the distribution of carbonate sediment by grain size on the shelf platform. [After
Sellwood, B. W., 1978, Shallow-water carbonate environments, in Reading, H. G. (ed.),
Sedimentary environments and facies, Fig. 10.21 A, p. 281, reprinted by permission of Else
vier Science Publishers, Amsterdam. Originally after Ginsburg, R. N., and N. P. James, 1974,
Holocene carbonate sediments of continental shelves, in Burk, C. A., and C. l. Drake (eds.),
The geology of continental marings, Fig. 23, p. 150, Springer-Verlag, New York.]
skeletal grainstones and packstones at the base upward to reefal and coral-bearing
deposits that in are capped by nonskeletal grains tones similar to mode sed
imts in the interior of the Bahama Banks (Manfrino and Ginsburg, 2001).
Note the general progression of facies on the rimmed shelves (Fig. 11.6, 11.7)
from reef buildups and shelf-edge sands on the higher energy outer shelf to car
bonate muds and muddy carbonate sands on the lower energy middle and inner
shelves. By contrast, most of the open-shelf, carbonate ramp in Figure ll.S is cov
ered by carbonate sand deposits at depths less than about 100 m, mixed, on this
shelf, with some terrigenous quartz sands.
Examples of Ancient Carbonate Shelf Successions
Isolated Plaos
Examples of carbonate sediments that may have formed in ancient settings similar
to all of the platform types illustrated in Figure 11.2 have been reported in the
published literature. For example, early to middle Tr iassic carbonates in the
Dolomite Alps of northern Italy are thought to represent deposition on isolated
platforms much like the modern-day Bahama Banks (Bosellini, 1991). These an
cient carbonates consist of flat-lying successions up to 800 m thick composed of
meter-scale, cyclic, peritidal carbonates with teepee structures. These deposits
probably formed within the interiors of isolated carbonate platforms under moder
ately
low energy, open to restricted, shallow, subtidal conditions. Platform-interior
sediments grade in a seaward direction to peloidal skeletal grainstones/pack
stones and algal boundstones with sponges, which were deposited on the higher
energy platform margin.
-
�Reefs
®
0 20
km
B
Reefs
�
D
Coralgal
D
Oolitic
D
Grapestone
D
Ooid shoals
Pellet lime mud
�
Lime mud
Rimmed Shelves
11.2 Carbonate Shelf (Nonreef) Environments
377
Atlantic
Ocean
N
'
'2o
o
- _ - - - �
t
,
�
I
I
\
I
/
5
0
Figure 11.7
Carbonate sediment distribution on
an isolated carbonate platform, the
Great
Bahama Banks. Figure A
shows the positions of the major
banks and channels in the Bahama
area. [After Geblein, C. D., 1974,
Guidebook for modern Bahaman
plaform environments: Geological
Society of America Annual Meeting,
1974, Fig. 18, p. 22.] Figure B
shows sediment distribution on a
portion of Great Bahama Bank sur
rounding Andros Island. [After Sell
wood, B. , 1978, Shallow-water
carbonate environments, in Read
ing, H. G. (ed.), Sedimentary envi
ronments and facies. Fig. 1 0.21 B, p.
281 , reprinted by permission of Else
vier Science Publishers, Amsterdam.
Originally after E. G. Purdy, 1963,
Recent calcium carbonate facies of
the Great Bahama Banks: jour. Geol
ogy
, v. 71, Fig. 1, p. 473.]
e Permian (Guadalupian) carbonate deposits of the Delaware Basin, western
Texas and southern New Mexico, have often been cited as an outstanding example
of an ancient rimmed shelf deposit (e.g., Wa rd, Kendall, and Harris, 1986; Saller et
a!., 1999). These deposits extend throughout an area of about 100,000 km
2
and are
completely encircled by a reef margin, the Capitan and Goat Seep reefs (Fig.
11.9). Outer-shelf deposits make up a belt about 10 to 15 km across, dominated
by shallowing-upward cycles of lagoonal mudstone and wackestone, capped by
laminated anhydrite. The outer-shelf belt grades abruptly to an inner lagoonal to
378
Chapter 11 I Carbonate and Evaporite Environments
Figure 11.8
Ooid shoal in shallow water
of the Great Bahama Bank.
Photograph courtesy of the
National Oceanic and At
mospheric Ad ministration
(NOAA). Downloaded from
the Internet 4/30/04.
Figure 11.9
Bold cliffs of massive Capitan
Limestone (ref) that formed
the rim of a huge Permian
carbonate platform. The plat
form (shelf) carbonates ex
tend toward the back of the
photograph; deeper-water,
basinal sediments are present
in the foreground. Photo
graph
courtesy of Gregory
Retallack.
0
·
�·
·
marginal-coastal playa facies about 40 km wide, composed of intercalated
dolomite mudstones and siltstones, and evaporitic units up to 7 m thick. Behveen
the reef and shelf-lagoonal deposits is a belt of carbonate sand bodies, 800 m to 3
wide, consisting
of a coarsening-upward succession of cross-laminated to struc
tureless fine-grained carbonate grainstones.
Ramps
Carboniferous limestones of southwestern Britain provide an example of an
cient carbonate ramp deposit (Burchette, Wright, and Faulkner, 1990). The ramp
11.3 Slope/Basin Carbonates
379
succession forms a wedge that begins with outer-ramp carbonate muds and bio-
clastic limestones containing abundant crinoid and brachiopod remains. Reef
mounds with relief up to 200 m also formed in the outer-ramp setting. Outer-
ramp deposits give way in a shoreward direction to bioclastic sand bodies of the
mid-ramp and finally to ooid sand bodies laid down as shoals or beaches in the
nearshore area.
Epeiric Plaorms
The great bulk of carbonate sediments formed throughout geologic time have
probably been deposited on epeiric platforms (Fig. 11.2). Such platforms were
widespad at various times, such as the late Precambrian (e.g., China), Cambro
Ordovician (e.g., North America; Middle East), Mississippian, Triassic-Jurassic
(e.g., weste Europe), Permian, and Tertiary (e.g., Middle East) (Wright d
Burchette, 1996; Tucker and Wright, 1990). Storms and hurricanes were likely
dominant processes on these broad shelves; however, tidal processes may also
have been important (e.g., Pratt and James, 1986). Shelf carbonate facies are char
acterized by distinctive suites of largely normal marine organisms and carbonate
xtures that are generally muddy, alough lithofacies types range from lime
mudstones, wackestones, grainstones, and packstones to stromatolitic bound
stones and patch-reef boundstones. Bedding of shelf carbonates is variable and
lens- or wedge-shaped layers are common, although some shelf carbonate beds
may be laterally extensive. Carbonates are commonly interbedded with thin shale
beds. Sedimentary structures include cross-bedding in lime-sand units, extensive
bioturbation structures and burrows, and flaser and nodular bedding.
Many epeiric deposits appear to be dominated by shallowing-upward cyclic
successions that range from a few tens of meters to hundreds of meters thick.
Many successions begin with a high-energy carbonate sand or conglomerate unit
followed upward progressively in the depositional succession by sediments de
posited in the lower energy, subtidal, open-marine shelf; intertidal zone; suprati
dal zone; and possible nonmarine environment. Some depositional cycles appear
to have ended with deposition of evaporites. Such a succession is basically regres
sive (progradational); however, because rates of carbonate sedimentation com
monly exceed rates of basin subsidence or sea-level rise, sediments also build
upward toward sea level. Sediment is thus deposited in progressively shallower
water as the sediment surface acctes toward sea level, generating the shallowing
upward successions (e.g., James, 1984b). lntraplatform basins, with water depth
commonly
less than 100-200 m, can form within epeiric platforms. During sea
level highstands, water within these basins may be stratified-oxygenated at the
top but suboxic to anoxic at the bottom. Sea-level lowstands may lead to isolation
of the basin and onset of evaporitic condions (Wright and Burchette, 1996).
Repetition of large-scale shallowing-upward successions may be largely the
result of repeated episodes of rapid sea-level rise, ooding the carbonate platform,
followed by periods of standstill during which shallowing-upward successions
develop, e.g., Wilkinson (1982). Osleger and Read (1991) suggest that meter-scale
cyclicity is the result of Milankovich-forced sea-level oscillations (see discussion of
stratigraphic cycles in Chapter 12), with a cyclicity on the order of 20,000-40,000
years. Several additional examples of ancient carbonate platform deposits are dis
cussed in Alsharhan and Scott (2000) and Zempolich and Cook (2002).
11.3 SLOPE/BASIN CARBONATES
Although we tend to think of carbonate sedimen as sictly shallow-water de
posits, as mentioned, deeper-water carbonates have been identified in several areas
of e mode ocean, such as the slope and adjacent basin floor around the Bahama
380 Chapter 11 I Carbonate and Evaporite Environments
Figure 11.10
Platform. They have also been reported from many Phanerozoic-ge stratigraphic
successions. As shown in Figure 11 .3, carbonate sediments are generated primarily
on the she. No important source of carbonate sediments exists within deep water
except that provided by the rain of calcareous pelagic organisms. Therefore, with
the exception of calcareous oozes, carbonate sediments deep water are derived
from the shelf by transport processes that include storm waves, rbidity currents,
debris and grain flows, slumping, slidg, and rock falls. Carbonate sediment de
posited on the slope and basin by these processes generally consists of bioclastic
debris and limestone blocks derived from the talus slopes off reef fronts. Also, sed
iments may be transported downslope from carbonate sand shoals or •ime-mud
deposits on the platform margin. Modem examples of carbonate slopes have been
reported from the northe Bahamas-Florida region as well as from BeHze, Jamica,
Grand Cayman, the northeast Australian coast, and several atolls the PacHie and
Indian Oceans (e.g., Coniglio and Dix, 12). Modem slope carbonates consist
mainly of pure carbonates, in contrast to many ancient slope deposits that clude
a large percentage of terrigenous clastic rocks.
Three types of carbonate slopes are recognized (Fig. 11 .10): erosional (steep
slope angle), by-pass (moderate slope angle), and accretionary (low slope angle).
Only accretionary slopes are sites of significant carbonate deposition, although
minor amounts of sediments may be deposited on by-pass slopes. Erosional and
by-pass slopes mainly serve as conduits by which carbonate sediment moves fm
shallow to deeper water. Several kinds of carbonate sediments can be deposited
on slopes. Periplatform oozes consist of fine sediment swept off the shelf mixed
Principal kinds of carbonate slopes, based on examples from
the Bahama Platform. [After Schalger, W., and R. N. Gins
burg, 1981, Bahama carbonate platforms-The deep and the
past: Marine Geology, v. 44, Fig. 10, p. 15. Reproduced by
permission.]
11.3 Slope/Basin Carbonates
381
with foraminifers and coccoliths tha settle from the water colwnn. Ta �us deposits
and debrltes are ca rbonate breccias and conglomerates derived from shaUow
water. Turbidites ar� the carbonate equivalents of siliciclastic turbidites.
o basic models for accretionary (depositiona•) carbonate slopes have been
proposed: sfo aprons and submarine fans (fig. H.ll). Carbonate aprons are dis
tinguished by having a line source or multiple sources that feed sediment seaward
through closely spaced gullies, gerterating wedge-shaped aprons of sediment
(e.g., MUtllins and Cook, 1986). Apror\s may develop on the slopes of both rimmed
and unrimme platforms. S1ope aprons, which xtend without break from the
basin up to the sallow-water margin, putatively develop along rimmed plat
forms where sopes are less than -4°. Such aprons have not been described from
the modern ean; however, ancient examples have been reported. Open-platform
slop�s are similar in form to slope aprons but develop on unrimmed platforms.
Base-of-slope aprons (Fig. 11.11A) form along rimmed platforms downslope from
the platform-slope break on steeper slopes ranging between 4 and 15°. diment
bypasses the uppr slope and is transferd to its foot by way of numerous gullies.
Proximal sediments in the uppermost part of the apron consist of talus, debrites,
thick turbidites, and riplatform oozes. Sediments in the distal apron consist
mainly bf finer grained turbidites.
Carbonate submarine fans (Fig. 11.118) are similar in form to the siliciclastic
submarine
fans described in Chapter 10. Sediment is supplied to fans from a sin
gle point source through a major channel that may bifurcate in its lower reaches to
A. Base-of-Slope Apron
B. Submarine Fan
Basin
F
Shallow-water
platform-main
facies
Shallow-water
platform-main
facies
Feeder
channel
Figure 11.1 1
Schematic block diagrams illustrating models for two
fundamental kinds of carbonate slopes: slope aprons and
carbonate submarine fans. A. Shows the kind of apron
that develops along rimmed platforms where slopes
range between �4 and 1 5°. B. Shows an idealized car
bonate submarine fan. (A. after Mullins, H. T., and H. E.
Cook, 1986, Carbonate apron models: Alternatives to the
submarine fan model for paleoenvironmental analysis
and hydrocarbon exploration: Sedimentary Geology, v.
48, Fig. 24, p.66. B. After Coniglio, M., and G. R. Dix,
1992, Carbonate slopes, in Walker, R. G., and N. P. james
(eds.), Facies models: Response to sea level change: Geo
logical Association of Canada, Fig. 22a,b, p. 367. Repro
duced by permission.]
382
Chapter 11 I Carbonate and Evaporite Environments
generate different sediment lobes. Commonly, coarsest sediment is deposited
the upper fan and sediment becomes finer in more distal parts of the fan. No mod
ern carbonate submarine fans are known; however, a few examples of ancient fans
have been reported (see reviews in Tucker and Wright, 1990, and Coniglio and
Dix, 1992).
11.4 ORGANIC REEF ENVIRONMENTS
As mentioned, the outer shelf of many rimmed platforms is characterized by near
ly continuous carbonate reefs that constitute an effective barrier to wave move
ment across the shelf. Reefs may also be developed as fringing masses along the
shoreline or as isolated patches within the inner shelf. Reefs constitute a unique
depositional environment that differs greatly from environments in other parts of
the shelf. They have been studied intensively for years; however, discussion of
reefs has long been plagued by confusion over the precise meaning of the term
reef. Carbonate workers have been unable to agree on whether to restrict use of
the term reef to carbonate buildups or bioherms that have a rigid organic frame
work or core, built of colonial organisms, or to extend the definition to include car
bonate buildups of other types that do not have a rigid-framework core. The word
biohe is a nonspecific term used for lenslike bodies of organic origin that a
enclosed in rocks of different lithology or character; a bioherm may or may not
have a rigid internal organic framework The term carries no connotation of the in
teal structure or composition of the lens. By contrast, a biostrome is a tabular
body of carbonate rock such as typically forms in nonreef platform environmen.
Wilson
(1975) uses the term carbonate buildup for a body of locally formed, later
ally restricted, carbonate sediment that possesses topographic relief, without
gard to the inteal makeup of the buildup. In this book, I follow the usage of
Longman (1981, p. 10), who defines a reef as "any biologically influenced buildup
of carbonate sediment which affected deposition in adjacent areas (and thus dif
fered to some degree from surrounding sediments), and stood topographically
higher than surrounding sediments during deposition." Most reefs, defined
this way, are built by larger organisms that are capable of thriving in energetic en
vironments.
Modern Reefs and Reef Environments
Depositional Setting
Most mode reefs (e.g., Fig. 11.12) form in shallow water. The most striking a
the linear reefs located along platform margins, commonly called barrier reefs.
These reefs are more or less laterally continuous, and the reef trend may extend for
hundreds of kilometers-for example, the Great Barrier Reef of Australia, which
runs for some 1900 along the eastern shelf of Australia (Fig. 11.13). In a few
mode localities where shelves are very narrow, linear reefs are located hard up
against the shoreline, with no intervening lagoon, and thus are called fringing
reefs. Isolated, doughnut-shaped reefs called atolls occur around the tops of some
Pacific
seamounts that rise out of deeper water. These reefs form an outer wave
resistant barrier that encloses a shallow lagoon. Faro reefs are ringlike (atoll-like)
structures that form within lagoons or on atoll margins. Small isolated reef masses
commonly referred to as patch reefs, pinnacle reefs, or table reefs occur along
some shelf margins or scattered on the middle shelf. Flat-topped table reefs may
also form in deeper water (Fig. 11.14).
Mounds are structures built by smaller, commonly delicate and/ or solita
organisms, possibly aided by inorganic processes, in tranquil settings (e.g., James
11.4 Organic Reef Environments
383
Figure 11.12
Modern coral reef. Photo
graph courtesy of National
Oceanic and Atmospheric Ad
ministration (NOAA) . Down
loaded from the Internet
4/30/04.
Figure 11.13
The Great Barrier Reef off the
coast of Queensland, Australia.
Photograph courtesy of Nation
al Aeronautics and Space Ad
ministration (NASA).
Downloaded from the Internet
5/4/04.
and Bourque, 1992; Monty et al., 1995) either shallow or deeper water. Microbial
mounds are built by stromatolites/thrombolites and calcimicrobes (microbes ca
pable of mediahng carbolate precipitation}. Skeletal mounds are composed of
reef-building organisms (see below) pfus calcareous algae, broyzoans, sponges,
ahermatypic hexacorals, and some kinds of brachiopods and bibalves. Mud
mounds are formed by lime mud (inorganic?) accumulations with various
amounts of fossils. Mounds range in size frm small structures (1-5 m high) to gi
gantic edifices that may reach 1 m high (e.g., We ndt et al., 1997).
384
Chapter 11 I Carbonate and Evaporite Environments
Figure 11.14
Schematic representation
of the principal kinds of
reefs. Based on Tucker, M.
E., and V. P. Wright, 1990,
Carbonate sedimentology:
Blackwell Scientific Publica
tions, Fig. 4.86, p. 192.
Figure 11.15
Reef Organisms
We tend to think of all reefs as coral rfs (e.g., Fig. 11.12); however, many organ
isms in addition to corals can contribute to the formation of reefs. These organisms
clude iue-green algae (cyanobacteria), coralline red algae, green algae, encrust
ing foraminifera, encrustg bryozoa, sponges, and molluscs (e.g., fig. H.l5 ).
the geologic past, reef-building organisms also included some now-exinct groups
such as the archaeocyathids, stromatoporoids, fenestellid bryozoans, and rudistid
clams. Nonetheless, corals are certainly dominant stituents of modern reefs,
and tw types of corals ane recognized. The principal oorals in shallow-water reefs
are hem1atypic (zoanthelae} hexacorals. Hermatypic corals cany out a symbiotic
relationship with several kinds of unicellular organisms, mainly algae, referred to
collectively as zooxanfhellae. These algae ive or between the living cells of the
corals and aid them in gag energy by roducing photosynthetic products
(Cowen, 1988). They may also facilitate the press of secreting calcium carnate
by removing C02 from the ssues during photosynthesis. Because the zooxan
thellae require sunlit waters, hermatypic corals are restricted to living in ·very shal
low water. Ahermatypic {azooxantheUae) corals lack the symbiotic relatidnship
(or do not require it) and are not restricted to shallow water (e.g., Martin Willison
Corals
Stromatoporoids
Red algae
Stromatolites
Bryozoans
Phylloid algae
Sponges
Codiacean algae
Seagrasses
Crinoids
Some common organisms that
act as frame-builders, sediment
contributors, bafflers, binders,
and precipitators in reefs and
mounds. The thickness of the hor
izontal bars indicates relative im
portance. [After Tucker, M. E., and
V. P. Wright, 1990, Carbonate
sedimentology: Blackwell Scientif
ic Publications, Fig. 4.88, p. 194.
Reproduced by permission.)
11.4 Organic Reef Environments
385
et al., 21). Some coral species can apparently have life strategies ranging from
zxantheUate-hermatypic to azooxanthellate-ahermatypic (Best, 2001). They are
one of the principal organisms today that form carbonate buildups in deeper
water. Thei distribution ranges fmm shallow wateF tQ water depths exceeding
20 m (Stanley and Cairns, 1988). Differences in the attributes of shallow- and
deep-water corals are explored by Hatcher (21).
Some reef-building organisms such as corals and stromatoporoids are im
portan frame-builders (Fig. 11.15), which construct wave-resistant cores of reefs.
Others, such as crinoids and codiacian algae (e.g., Halimeda), whose skeletal ele
ments may disintegrate into smalleF fragments, re important sediment contribu
tors. Bafflers are organisms such as seagrass that provide a protective baffle
against currents and thus generate a localized, low-energy envronttrnf i which
fine sediment can accumulate. Binders such as cyanacteria (which form stro
matolites) trap and bind sediment, and precipitators are primarily microbes, such
as cyanobacteria, that help mediate precipitation of carbonate muds. The relaive
iprtance of some common organisms as frame-builders, sediment contributors,
bafflers, binders, and precipitators is illustrated in Figure 11.15.
The grmvth forms of reef-building organisms may range from delicate branch
ing forms to globular or massive structures (Fig. 11.16A). The form of the organ
isms is closely related to the water energy over the reef and, thus, varies over
di{ferent parts of the reef (Fig. 11.168). Organisms that live in low-energy parts of
the reef tend to have delicate branching or platelike forms. Those living in higher
energy zones of the reef develop hemispherical, encrusting, or tabular forms that
better able to withstand strong wave action.
0
Groh Forms
��
Delicate, branching
-
Thin, delicate, plate-like
. .
Globular, lbous,
calumnar
Robust. dendroid,
branching
·-
Hemispherical, domal,
irregurar, massive
-
-
Encrusting
--
- Tabular
Reef
�
Crest
-
s
ack
Ree
t
-
t
Ree
t
_
T
I
_
I
_
I
Flat
··j
.
�
.
.
Globular
Environment
Wave Energy
Sedimentation
low
high
low
low
moderate
high
moderate-high
moderate
moderate-high
low
Intense
low
merate
low
Reef Front--Fore Reef-•
Waves and swells
Figure 11.16
Growth forms of reef-building organisrm.
A. Principal kinds of rowth forms an
their relationship to water energy. B. Dis
tribution of growth forms across a typical
reef. [A. after James, N. P., 1983, Reef en
vironment, in Scholle, P. A., D. C. Bebout,
and C. H. Moore ( eds.), Carbonate depo
sitional environment: Am. Ass. Petrole
um Geologists Mem. 33, fig. 59, p. 374.
B. after James, N. P., 1984, Reefs, in Walk
er, R. G. (ed.), Facies models, 2nd ed.:
Geoscience Canada Reprint Ser. 1. Fig. 9,
p.
233, reproduced by permission of Geo
logical Association of Canada.]