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Shallow-marine carbonate facies

Article in Geological Society London Special Publications · January 1985

DOI: 10.1144/GSL.SP.1985.018.01.08

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Geological Society, London, Special Publications
Shallow-marine carbonate facies and facies models
M. E. Tucker

Geological Society, London, Special Publications 1985; v. 18; p. 147-169

doi:10.1144/GSL.SP.1985.018.01.08

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 Shallow-marine carbonate facies and facies models

M. E. Tucker

SUM M A R Y : Shallow-marine carbonate sediments occur in three settings: platforms, shelves

and ramps. The facies patterns and sequences in these settings are distinctive. However, one type
of setting can develop into another through sedimentational or tectonic processes and, in the
geologic record, intermediate cases are common. Five major depositional mechanisms affect
carbonate sediments, giving predictable facies sequences: (1) tidal flat progradation, (2) shelf-
marginal reef progradation, (3) vertical accretion of subtidal carbonates, (4) migration of
carbonate sand bodies and (5) resedimentation processes, especially shoreface sands to deeper
subtidal environments by storms and off-shelf transport by slumps, debris flows and turbidity
currents.
Carbonate platforms are regionally extensive environments of shallow subtidal and intertidal
sedimentation. Storms are the most important source of energy, moving sediment on to shoreline
tidal flats, reworking shoreface sands and transporting them into areas of deeper water.
Progradation of tidal flats, producing shallowing upward sequences is the dominant depositional
process on platforms. Two basic types of tidal flat are distinguished: an active type, typical of
shorelines of low sediment production rates and high meteorologic tidal range, characterized by
tidal channels which rework the flats producing grainstone lenses and beds and shell lags, and
prominent storm layers; and a passive type in areas of lower meteorologic tidal range and higher
sediment production rates, characterized by an absence of channel deposits, much fenestral and
cryptalgal peloidal micrite, few storm layers and possibly extensive mixing-zone dolomite.
Fluctuations in sea-level strongly affect platform sedimentation.
Shelves are relatively narrow depositional environments, characterized by a distinct break of
slope at the shelf margin. Reefs and carbonate sand bodies typify the turbulent shelf margin and
give way to a shelf lagoon, bordered by tidal flats and/or a beach-barrier system along the
shoreline. Marginal reef complexes show a fore-reefwreef core--back reef facies arrangement,
where there were organisms capable of producing a solid framework. There have been seven
such phases through the Phanerozoic. Reef mounds, equivalent to modern patch reefs, are very
variable in faunal composition, size and shape. They occur at shelf margins, but also within shelf
lagoons and on platforms and ramps. Four stages of development can be distinguished, from
little-solid reef with much skeletal debris through to an evolved reef-lagoon-debris halo system.
Shelf-marginal carbonate sand bodies consist of skeletal and oolite grainstones. Windward,
leeward and tide-dominated shelf margins have different types of carbonate sand body, giving
distinctive facies models.
Ramps slope gently from intertidal to basinal depths, with no major change in gradient.
Nearshore, inner ramp carbonate sands of beach-barrier-tidal delta complexes and subtidal shoals
give way to muddy sands and sandy muds of the outer ramp. The major depositional processes are
seaward progradation of the inner sand belt and storm transport of shoreface sand out to the deep
ramp.
Most shallow-marine carbonate facies are represented throughout the geologic record.
However, variations do occur and these are most clearly seen in shelf-margin facies, through the
evolutionary pattern of frame-building organisms causing the erratic development of barrier reef
complexes. There have been significant variations in the mineralogy of carbonate skeletons, ooids
and syn-sedimentary cements through time, reflecting fluctuations in seawater chemistry, but the
effect of these is largely in terms of diagenesis rather than facies.

T h r o u g h the study of recent sediments and their the A r a b i a n Gulf and Shark Bay Western
ancient counterparts, it is possible to synthesize Australia have c o n t r i b u t e d m u c h to the under-
the facies distributions into various facies standing and interpretation o f ancient carbon-
models. These summaries of facies patterns can ates. The data f r o m these m o d e r n c a r b o n a t e
be extremely useful w h e n new sequences are e n v i r o n m e n t s go a long way towards providing
being examined; the facies models do have a the basis for useful facies models. H o w e v e r ,
predictive quality which can be i m p o r t a n t when there are three i m p o r t a n t deficiencies of the
particular facies-types are being sought, as in recent sediment record which must be b o r n e in
p e t r o l e u m a n d mineral exploration. M o d e r n m i n d w h e n trying to p r o d u c e generally appli-
m a r i n e carbonates have been studied intensively cable c a r b o n a t e facies models:
over the last two decades and, in particular, the (1) As a consequence of the Pleistocene glaci-
results of researches in the Bahamas, Florida, ation and associated m a j o r fluctuations in

147
148 M. E. Tucker

sea level over the last 1 Ma, in most areas The geotectonic context is of paramount import-
where carbonates are forming today, sedi- ance. It controls one of the prime requisites for
mentation only began 4-5000 years ago. carbonate sedimentation, the lack of siliciclastic
Thus relic topographies exert a strong material, by determining hinterland topography
control on sedimentation and in some and drainage. Geotectonics also determines the
areas (mostly low latitude, deeper-water, depositional setting, and three types are
mid- to outer continental shelves) relic distinguished and discussed later: the platform,
carbonate sediments abound. A steady- shelf and ramp. However, the setting can be
state situation with an equilibrium modified considerably, once carbonate sedimen-
between sediments and environments has tation is established. Geotectonics controls the
frequently not been attained. orientation of shorelines and platform-shelf
(2) Sea level at the present time is relatively margins and with climate this determines the
low compared with much of the geologic energy level and direction of wind-waves, storm
record. Thus there are now no extensive and tidal currents, as well as the circulation
low-latitude shallow seas (epeiric or epi- pattern and location of upwelling, nutrient-rich
continental seas) where carbonates are zones. Both geotectonics and climate control the
accumulating, comparable with the many position and fluctuations of sea-level. This is of
instances in the past of whole cratons great significance to the production of car-
being covered by knee-deep marine bonate sediment and the resulting facies mosaic.
waters. Rates of subsidence and uplift, which also affect
(3) Modern carbonate sediments are almost sea-level transgressions and regressions, and the
entirely produced by biogenic processes, location of positive and negative areas, are also
apart from ooids and possibly some determined by geotectonic factors. Climate is
lagoonal lime muds. The organism types important in terms of seawater salinity, especial-
contributing their skeletons to limestones ly where lagoons are involved. Salinity is a
have varied drastically throughout the major factor, for many organisms cannot
Phanerozoic due to changing fortunes: tolerate any deviations from stenohaline
the evolution of new groups and demise of conditions.
others. The roles played by organisms Where optimal conditions exist for the
have also changed through time; this is growth of organisms with carbonate skeletons,
particularly important when considering then it appears that the carbonate production
reef limestones. In addition, the dominant rate is fairly constant, regardless of the
mineralogic composition of organism types of organism involved (e.g. Smith 1973;
skeletons and inorganic CaCO3 precipi- Hallock 1981). Production rates determined
tates has varied through time, in response for benthic foraminifera, corals and coral-
to fluctuations in seawater-atmosphere line algae on seaward reef flats are around
chemistry. 1.5-4.5 kg CaCO 3 m -2 year -l, equivalent to a
As a result of the above three points, facies carbonate deposition rate of 0.5-1.5 mm year -1.
models for carbonates cannot be derived entirely Rates are somewhat lower in back-reef lagoons
from studies of recent carbonate sedimentary (0.1-0.5 mm year-l). Production rates can be
processes and products; essential information much higher on the reef front, 6 m 1000 years -1
has to come from the rock record. In addition, has been recorded (6 mm year-l). The point to
as alluded to above, in some instances facies note is that the carbonate production rates are
models have to allow for the evolutionary determined by ocean physico-chemistry, rather
pattern of carbonate-secreting organisms than organic-biological factors.
through time.
The important papers or compilations in the
field of carbonate facies and facies models are:
Depositional processes and facies
Irwin (1965), Shaw (1964), Purser (1973a), sequences: constant sea level
Laporte (1974), Heckel (1972, 1974), Logan Where carbonate sedimentation takes place
(1974), Bathurst (1975), Ginsburg (1975), without any change in sea level, there are five
Wilson (1975), Sellwood (1978), James (1979), principal depositional processes which lead to
Asquith (1979), Toomey (1981) and FlOgel the formation of characteristic facies sequences
(1982). (see Fig. 1).
Major controls on carbonate (1) Tidal flat progradation results largely
from deposition of shallow subtidal sedi-
sedimentation
ments on flat-marginal beach ridges and
There are two overriding controls on carbonate on the flats themselves during major
sedimentation: (1) geotectonics and (2) climate. storms. Trapping and some precipitation
 Shallow-marine carbonate facies 149

tidal flat progradation on ~ HWM

...... ll
reel progradahon at shelf ~ .... - - S L
~,nd plait . . . . . . . . . . g .... ~ , , ~ ) ..""..,~

vertical accretion of sublidal stage ! S[ stage 2 SL

carbonates, ramps pladorms.
shelves FWWB . . . . E:.:::"t .--:',':.s":?:.f. ~ ::;:',C.-:::':':-!::::
":: : ' - ' : :"

migration of (a) beach barrier hdal delta ~~------:"~"~ _SI

r'albor'ale progradalion on ramps
sand bodies and open shelves
/- (b) shoreward migration (;f : ~ ' ~ ~ L
shell and iwrndward marginal sand shoals
platform {
rnatgirls | c) offshore rnlgratlorl of ~ ~
~leeward marginal sands . . . .

re- Sedlmenlatlon ~ SL b ~ l ~ l t ''~-~"

(a} olfshore storm deb is f ow." d ty "~o .

transport especlail,f currents on shell/ ° " 2 " - - ~ ' "
On ramps platform margkTs

FIG. I. The principal depositional processes of carbonate sediments. The typical settings in which
these processes operate are also noted.

of sediment by algal mats on the flats are migration into the shelf-lagoon or plat-
important. Some carbonate (and other form interior is important in windward
minerals) can be precipitated inorganically locations, giving rise to quiet-water, below
on tidal fiats in an arid climate. The net fair-weather wave-base packstones and
result is a shallowing-upward sequence wackestones passing up into above wave-
(further discussed later) of intertidal sedi- base storm or tide-dominated grainstones.
ments overlying subtidal sediments. In On leeward margins, offshore, basinward
detail there are often variations in the transport of skeletal sands is significant
microfacies of these shallowing-upward and can lead to progradation of the
sequences, depending on the type of tidal margin itself.
flat, energy level and climate, etc. (5) Offshore storm transport and deposition
(2) Reef progradation is important at shelf- of shoreface carbonate sediment is very
breaks and platform margins and mostly important on ramps, less so on shelves
involves seaward growth of the reef over and platforms. Other resedimentation
its storm-produced talus (fore-reef slope). processes, slides, slumps, debris flows and
(3) Vertical accretion of subtidal carbonates turbidity currents, all of which may be
can take place when sediment production storm or seismically induced, are import-
rates are high. Shallowing-upward seq- ant at shelf-breaks and platform margins.
uences are produced, of deeper subtidal When there are fluctuations in sea level, either
facies giving way to shallower subtidal through eustatic or local tectonic effects, then
facies (and of course intertidal facies many more facies patterns can arise. These are
could follow naturally). discussed in succeeding sections.
(4) Migration of carbonate sand bodies is
significant in relatively high-energy Depositional settings of shallow-
locations, giving beach-barrier-tidal delta
marine carbonates
complexes, especially on ramps, and sand
shoals, especially at shelf-breaks and Shallow-marine carbonates are being and have
platform margins. Under constant sea- been deposited in a wide range of geotectonic
level beach-barrier-tidal delta complexes settings. Three basic depositional settings of
will prograde offshore if there is a good shallow-marine carbonates can be defined (see
supply of sediment (i.e. high organic pro- Fig. 2).
ductivity in the shoreface zone or abun- (1) Theplatform: a very extensive (102-104 km
dant ooid formation in the tidal deltas). wide), quite flat cratonic area covered by a
With sand shoals, their shoreward shallow (epeiric) sea. Seawards, a platform is
150 M. E. Tucker

parts of western Europe, and some of the

platform m ~lm Tertiary of the Middle East. Water depths on
102 104 km
the cratons were generally less than 5-10 m, so
that shallow subtidal to intertidal environments
shelf "~" SL -~.. he.lt~ L
dominated. The intertidal areas would have
open shelf X ummed s consisted of tidal flats many kilometres to tens
of kilometres wide. These would have developed
ramp ~ SL
extensively in the platform interiors, with
supratidal flats beyond, giving way to the
peneplained land surface where subaerial
ram;.) .---*shell
processes such as pedogenesis and karstification
would have operated. Tidal flats would also
p.alfol rn--.,, shelf
have developed around slightly more positive
areas upon the platform. Apart from local
plalform or shelf~am~; , . - - - ~ . ~ ~
shoals of skeletal sand, the subtidal would also
be a near-flat surface but probably with slightly
FIG. 2. The three major depositional set- deeper and slightly shallower areas reflecting
tings of shallow-marine carbonates, plat- pre-existing topography on the craton or the
form, shelf and ramp, along with the effects of differential subsidence.
common transitions from one to the other. It is generally accepted that the epeiric seas
had only small tidal ranges. Tidal currents
bounded by a margin which may have a gentle or would have been insignificant on the open
steep slope. (2) The shelf: a far less extensive platform, but perhaps quite strong in any
(10-103 km wide) area characterized by the channels of the broad intertidal zone. For much
presence of a distinct shelf-break, where the of the time, platforms would have been very
gradient increases dramatically into an adjacent quiet, low energy environments, with only
basin. Although often relatively flat, there can wind-wave activity. Fairweather wave-base
be substantial gradients on the shelf itself, and would have been quite shallow, less than 5 m.
many are rimmed--i.e, they have a barrier of The platform margins on the other hand would
reefs or carbonate sand shoals along the shelf- have been sites of much turbulence for much of
break, with a shelf lagoon behind. (3) The ramp: the time (e.g. Mazzullo & Friedman 1975). Tidal
a gently sloping surface, passing seawards into currents and waves from open ocean swell would
deeper and deeper water. hove been very important, especially if an abrupt
Initially, these three depositional settings are change in slope existed at the platform margin,
determined by geotectonics, but once carbonate causing all wave and current energy to be
sedimentation is established, then one type of dissipated over a short distance. Sand bodies
setting can be transformed into another, either and reefs could well have been developed along
through the natural processes of carbonate sedi- the platform margin, as occur along many shelf-
mentation itself, or through further geotectonic breaks (see later section) and these could further
effects. Omitting sea-level fluctuations, there are have reduced circulation on the platform itself.
three common patterns: (1) a ramp may develop The dominant process affecting platform sedi-
into a shelf, especially through reef growth; mentation would have been storms, their freq-
(2) a platform or shelf may develop into a ramp uency, direction and magnitude controlled by
through differential subsidence along a hinge climatic factors. Severe storms can raise sea level
line, and (3) a platform may develop into a shelf by several metres and give rise to currents
through contemporaneous fault movements reaching 1 m s-1. On a craton-sized platform,
(Fig. 2). storm winds blowing persistently from one
quadrant will pile up water in a down-wind
direction. Where normally quite shallow water
Carbonate facies patterns on exists ( < 2 m), the platform floor itself could be
platforms exposed as the sea is blown off it. Strong surges
would cross the platform after the storm sub-
Carbonate platforms are very extensive areas of sided and the sea returned to its normal level.
negligible topography. Although non-existent During storms, the platform interior tidal
today, shallow epeiric seas covered the cratons flats would be flooded and much shallow
many times during the geologic record. Examples subtidal sediment deposited upon them. In the
include the Cambrian and Ordovician of North subtidal, skeletal debris would be transported
America, the Upper Dinantian and Jurassic of and sorted during storms and post-storm surges,
 Shallo w-marine carbonate facies 151

and deposited to give grainstone beds.

Winnowed shell lags (rudstones) would be left
after the passage of storm currents and waves.
In general terms, the facies pattern of a
carbonate platform would thus consist of
skeletal-peloidal wackestones with lenticular
grainstones in the platform interior (tidal flat
deposits), skeletal-peloidal grainstones and
packstones of the shallow subtidal (above
fairweather wave-base) and skeletal packstones
and wackestones with grainstone horizons in the
deeper subtidal (below fairweather wave-base).
Below storm wave-base, skeletal wackestones
would dominate with thin beds of storm-derived
skeletal packstone-grainstone.
Under constant sea level, apart from some
aggrading of the shallow subtidal sediments
through simple skeletal carbonate production, CRUST
ZONE P ~
the dominant depositional process would be
progradation of the tidal flats. (The movement Fie. 3. The subenvironments of (a) an
of shallow subtidal sand shoals during storms active and (b) a passive tidal flat from the
could also be important.) western side of Andros Island, Bahamas.
(a) After Hardie (1977), (b) after Gebelein
et al. (1980).
Modern platform carbonates
Although there are no modern examples of passive tidal flat has virtually no tidal channels
the very extensive platforms of the past, we can and consists of broad depressions separated by
get an indication of what sedimentation must former beach ridges rising 1-2 m above normal
have been like from the studies of the interior of high water mark. The depressions are variably
the Great Bahama Bank (e.g. Shinn et al. 1969; occupied by water to form ponds with intertidal
Hardie 1977; Gebelein et al. 1980). To the west flats and algal marshes around.
of Andros Island, there occur protected tidal At their seaward margin, both types of tidal
flats and a shallow subtidal platform. Tidal flat have a low beach ridge which is constructed
range is very low (0.46 m) and wind-wave of sediment thrown up from the shallow subtidal
activity is also weak since Andros Island acts as during storms. The sediment of the present and
a barrier to the dominant and persistent north- former beach ridges is largely skeletal-peloidal
easterly trade winds. Occasional winter storms grainstone-packstone with lamination. Algal
from the west to north produce strong waves in marsh sediments are dominantly algal lamin-
spite of the shallowness of the platform (average ites with laminoid fenestrae, but skeletal-
depth 5 m). Sedimentation is largely controlled peloidal layers (up to 7 cm thick) of storm origin
by the rare storm events. are intercalated. Pond sediments are chiefly lime
Tidal flats are complex areas of many sub- mud, much of which is pelletized by gastropod-
environments: tidal channels, beach ridges annelid defecation. Surrounding intertidal flats
(hammocks), lev6es, ponds, intertidal flats, may have surficial cemented crusts, often of
areas of surficial crusts and algal marshes (often dolomite, and consist of peloidal lime mud with
freshwater). Parts of the tidal fiat are per- many irregular fenestrae (birdseyes).
manently subaqueous; other areas are exposed The shallow subtidal offshore from the tidal
for some of the tidal cycle or for certain seasons fiats is an area of peloids and skeletal grains in
of the year. To describe the fluctuations in water the shoreface zone, where affected by wind
cover, an exposure index has been introduced by waves, and skeletal peloidal lime muds below
Ginsburg et al. (1977) to indicate the percentage fairweather wave base ( > 2-5 m). Algal micritiz-
exposure of a subenvironment over a year. ation of grains is common and algal mats may
Two distinct types of tidal flat occur on the partly cover the surface. Bioturbation is
west side of Andros Island (Fig. 3): one type, ubiquitous. Skeletal-peloidal sands do occur
which can be termed an active tidal flat, is through sed!ment reworking and winnowing
dominated by tidal channels (comprising 15 T0 of during storms.
the tidal flat complex), draining ponds, inter- The sequences generated through deposition
tidal flats and algal marshes; the other type, a on the two types of tidal flat will be different
152 M. E. Tucker

ACTIVETIDAL FLAT are accumulating along the Trucial Coast,

evaporative
Arabian Gulf. In upper intertidal areas, dis-
I .!-'i~'~-~\ dOlOmite coidal gypsum crystals are being precipitated
and in the supratidal (sabkha) these are being
I~ . \ tidal replaced by anhydrite; farther landwards
I" . \ channel
deposit enterolithic anhydrite may form. In extremely
arid locations, halite (and even potash salts)
could precipitate in supratidal areas as crusts

2°2 and beds in depressions (supratidal salinas). On

a passive tidal flat, evaporites would accumulate
extensively where waters in ponds and ground-
PASSIVE TIDALFLAT waters beneath slightly higher areas were only
infrequently replenished with seawater during

~.
soit

1/ hammock
~ /
~ /
. fenestrae
algaL, algaltaminae
marsh evaporativedolomite
major storms. Lenticular gypsum and nodular
anhydrite could be expected to form within the
sediment, and bedded evaporites could pre-
_-I-/ t~pond : peloida[muds cipitate within the ponds.
I - I - , /?d O[Omite
, ( _ [ m i x i n g .... The reasons for the two types of tidal flat are
i"~_ , subtida[ thought to be related to sediment supply and
. ~ subtidal shoreline orientation with regard to winter
~ bioturbation storms. Compared with the passive flats, the
Pteistocene active flats are adjacent to shallow subtidal areas
I I I I limestone
of lower sediment production rates, and they are
FIG. 4. The sequences of an active and oriented such that they receive the full force of
passive tidal fiat from the west side of storm winds and waves (i.e. the meteorologic
Andros Island. tidal range is high). By contrast, the shoreline
fronting the passive flats is more oblique to on-
(Fig. 4). The migration of the tidal channels on coming storms (the meteorologic tidal range is
the active type will give rise to skeletal lag low) and sediment production rates appear to be
deposits and skeletal-peloidal grainstones, with higher.
cross-bedding if it is not extensively bioturbated.
Areas not affected by the channels will consist of Shallowing-upward cycles of carbonate
pond and marsh deposits (lime muds, fenestral platforms
peloidal wackestones, algal laminites) plus
prominent storm layers (thin packstones- Carbonate platforms respond dramatically to
grainstones). The passive tidal flat will consist sea-level changes. Left to their own devices, with
largely of pond and algal marsh deposits, with constant sea level and no subsidence or uplift,
few storm layers (since the marginal beach ridge platforms will build up to sea level and just
protects the tidal flat), but with no suggestion of above, through progradation of tidal fiats and
any sediment reworking by tidal channels. Zones vertical accretion of shallow subtidal sediments
of beach ridge sediments will occur within the into shallower depths. The typical sequence
tidal flat sequence. produced through sedimentation on a platform
Dolomite crusts may develop in both types of is thus a shallowing-upward sequence of subtidal
flat, but they are more likely to form intra- through to intertidal and supratidal deposits. A
clasts and edgewise conglomerates on the active relative drop in sea level will expose the platform
flat through tidal channel reworking. Poorly to supratidal-subaerial processes, namely
ordered dolomite of possible meteoric-marine sabkha evaporite precipitation if the climate is
mixing-zone origin has been reported from the arid and there is still a source of seawater, or to
shallow subsurface of the passive-type flat soil formation, such as calcrete development if a
(Gebelein et al. 1980). The occurrence of beach semi-arid climate, or karstification if more
ridges on the passive flat, with their freshwater humid. With a relative rise in sea-level, subtidal
lenses below, should lead to a wider develop- environments are widely established over a plat-
ment of mixing-zone dolomite there, compared form, with tidal flats at the distant shoreline.
to the active flat which will have mainly marine There are numerous accounts of shallowing-
to hypersaline groundwaters. upward cycles in the geologic record, demon-
Under a more arid climate than that of the strating that subtidal environments were re-
Bahamas, evaporites would precipitate in the peatedly established by the periodic flooding of
areas of high exposure index. Such evaporites platforms through transgressive events. For
 Shallow-marine carbonate facies 153

examples see Coogan (1969), Wilson (1975), 'regressive' phase tidal flat sediments (fenestral,
Ginsburg (1975) and Somerville (1979). Detailed cryptalgal peloidal wackestones) appear to
studies show that the cycles are not all the same. increase in thickness as cycles are traced
Microfacies analysis reveals differences within shorewards and may comprise the whole cycle in
one cycle when traced laterally across a plat- very proximal areas. Lateral variations are also
form, and between cycles in a vertical sequence. seen in the nature of emergence horizons at the
Frequently the cycles of one particular stage or top of each cycle: in proximal areas, palaeo-
substage of a geological period have features in karstic surfaces are usually developed (possibly
common, which are different in the cycles of a above a calcrete). These pass distally into
succeeding stage. 'sutured discontinuity surfaces', interpreted as
As an example, the recent work of Gray the product of intertidal, rather than wholly
(1981) can be cited. In the Llangollen area of subaerial, dissolution and erosion (cf. Read &
mid-Wales, shallowing-upward cycles are de- Grover 1977).
veloped in the Asbian and Brigantian stages of The lateral variations in cycle form are
the Upper Dinantian, Lower Carboniferous primarily a function of the gradient of the
(Fig. 5). Asbian sediments were deposited in the platform. The transgressions appear to have
Llangollen and Oswestry embayments, separ- been relatively rapid, and during the trans-
ated by the Berwyn High. Brigantian sediments gression, a basal bed was developed in distal to
were deposited uniformly over the whole area. medial areas. Sedimentation after the initial
When cycles are traced towards the plat- transgression was determined by depth, especial-
form interior, gradual but distinct changes ly relative to wave base (see Fig. 6). Differences
are observed in addition to a general shore- between cycles of different stages relate to the
wards thinning of each cycle. Away from the magnitude of the transgressions. For example,
open platform, the transgressive phase (a compared with Asbian cycles, those of the
below fairweather wave base, thin argillaceous Brigantian are dominated by thin-bedded below
packstone-wackestone facies), which forms the wave-base packstones-wackestones. This indi-
lower part of each cycle, gradually reduces in cates that the transgressions were more wide-
thickness. Sequences of more proximal areas spread, resulting in a greater depth of water over
tend to have shallow subtidal (above fairweather the platform.
wave base) sediments in their lower parts. The Within the Upper Dinantian sequence of mid-

Sutured drscontinu~b/
surface
Sd surface
I
some dolomite
inlraclasts
~i I F.... {ral fab.....
.:.!:.

b ostrome

-- Planar erosion ~ @ paiaeokarsl

surface Calcrele
-~-V
~ e,J co ---I--if- I

FIG. 5. Two typical shallowing-upward cycles from the Lower Asbian (Tynant) and Brigantian
(Trefor) of the Llangollen area, mid-Wales. Microfacies associations: MA. 1 calcisphere wackestone
(tidal-flat facies), MA.2 algal packstone-wackestone (below fairweather wave-base sandy mud and
muddy sand facies); MA.3 algal grainstones (shoreface, above wave-base sand facies). After Gray
(1981).
154 M. E. Tucker

•,, erosion dissolution

' -~ . . . . .

t..,-.--.0
<'~"...~'.:. ~ shoreface
:.: : ~ ' . t
belowwave~s~

ca! FI I I

z i ,'-
i ,,.-~
! ©eplh o1 she',l flOOding
E

FIG. 6. Diagram interpreting the lateral variation of a cycle in terms of depth of shelf flooding. Cycle
on left is more proximal (farther towards platform interior) whereas cycle on right is more distal.
After Gray (1981).
Wales, and elsewhere in the U.K., it is not Carboniferous shallowing-up cycles, and it
uncommon to find cycles 'missing' in proximal seems in many others in the geological record,
areas, or unrecognizeable in more distal areas. this is not seen. It appears that the shallowing is
Problems of stratigraphic correlation can result. largely due to depositional processes. Carbon-
Sedimentologically, this can be explained with ates can build up relatively quickly--modern
reference to Fig. 7. The absence of sediments of depositiona! rates of non-reef carbonates are
a particular cycle in proximal areas results from 0.5-1 m 103 years -~, and rates of tidal flat pro-
weak transgressions that did not extend land- gradation are rapid too--several km 103 years -~.
wards as far as earlier and later ones. Where this It is probable that much time is represented by
happened, the subaerial processes operated for a the palaeokarstic surfaces; 50,000 years has been
longer time (over several cycles) and very suggested by Walkden (1974). The length of
marked palaeokarstic horizons can develop each British Dinantian cycle has been variously
(multiple palaeokarsts). In more distal platform estimated as 0.2-0.5 Ma. Most shallowing-up
areas, it could happen that the prograding tidal cycles appear to be the result of episodic trans-
flats did not arrive, so that subtidal conditions gressions, involving periodic sea-level rises of
were maintained throughout. Fluctuations from only a few metres. In the Upper Dinantian of the
below to above wave base may be recognized by U.K. this appears to have happened some 30 or
careful study of microfacies (e.g. Jefferson more times.
1980), or it may be that sea-level changes were A question often asked is whether the trans-
not substantial enough on the open platform to gressions are induced by local tectonics, i.e.
cause any major modification to the sedimen- episodic subsidence of the craton, or through
tary facies. eustatic sea-level rises (see for example
There has been much discussion as to the Ramsbottom 1977; George 1978). The latter can
underlying mechanism causing repetition of the be induced by fluctuations in ocean basin
shallowing-up cycle. Once flooded, tidal flats volume through plate tectonic processes, or in
will prograde across a platform and the the volumes of glacial ice at the poles. However,
thickness of the cycle as well as the facies the former proces s, which can give a rate of sea-
sequence through the cycle will depend on the level change of 1 cm 103 years -~ is three orders
depth after the initial transgression. Tidal flat of magnitude slower than glacio-eustatic changes
progradation has been referred to as deposition- (e.g. up to 10 m 103 years -x for the Flandrian
al regression, but regression can also occur transgression) (Donovan & Jones 1979). Modern
through a eustatic sea-level fall or through slight subsidence rates can reach 2.5 m 103 years -~.
uplift of the craton. Such an external sea-level Thus, both glacio-eustacy and platform sub-
fall can be recognized by the absence or im- sidence can produce rates of sea-level rise which
poverishment of peritidal facies, but with the exceed modern carbonate production rates
development of emergence phenomena (palaeo- (maximum 1 m 103 years-~). To distinguish
karsts, calcretes, vadose diagenesis) directly between a local tectonic and a eustatic control
upon subtidal sediments. In most of the British for the sea-level rises, the lateral continuity of
 Shallow-marine carbonate facies 155

/ PROGRADATION
Lncreas~n9 duration ol emergence.~

SUBTIDAL TIDAL FLAT EMERGENT SHELF.^.stit ca'~ or~

I erosion dissolution I . . . . atiOn a n d / ~ / .::.:::~/:
....... t__ surface_ ca cret
tl._da_l ~ange ...... "~ ~ .................................. '::":"~ Karst
Superimposition

4
3
s "

2 1

FIG. 7. Model for development of shallowing-upward cycles in a broadly transgressive sequence. The
lateral variation of each cycle and the vertical variations between cycles are seen to be reflections of
the degree of flooding of each transgression. After Gray (1981).

the cycles is important. If individual cycles can 125 cm km -1) to the slope into the adjoining
be correlated from one platform to another then basin. Where shelves border oceans they are
a eustatic control is likely. However, platforms frequently sites of long-term downwarp so that
within a region may all be undergoing episodic thick sedimentary packets can develop there,
subsidence of a similar magnitude, especially if if sedimentation can k e e p pace with the
they all occur within the same overall geo- subsidence.
tectonic framework, so that shallowing-up Where starved of siliciclastic sediment,
cycles of a similar thickness and type would carbonates will accumulate on the shelf and at
occur on all platforms. These cycles would be the present time two major types of carbonate
difficult to distinguish from those of eustatic shelf are recognized: open shelves and rimmed
origin. The exact cause of repeated shallowing- shelves (Ginsburg & James 1974). Open shelves
up cycles on carbonate platforms is thus far slope gently away from the shoreline and depths
from clear. at the shelf-break are substantial (50-200 m).
Although shallowing-upward cycles char- Carbonate sediments on the outer shelf are
acterize ancient platform carbonates, there are largely relic; sedimentation was unable to keep
instances where formations consist of cycles pace with rising sea-level during the Flandrian
showing the opposite trend, of deepening up. transgression. Rimmed shelves, which are the
Intertidal fenestral, algal laminated limestones modern counterparts of many ancient carbonate
pass up into shallow subtidal facies which may shelves, are characterized by the development of
be capped by an emergence horizon, with a soil reefs and carbonate sand bodies along the shelf
and karstic solution effects, before the overlying margin (Fig. 8). Depths are shallow adjacent to
tidal flat unit of the next cycle. These the shelf-break, or the area may even be
transgressive cycles reflect times of slow to subaerial if islands have formed. The shelf
moderate sea-level rise and rapid sea-level fall. margin is a turbulent, high energy zone where
They are not common in the geologic record, oceanic waves (swell), storm waves, and possibly
but they are well known in the Alpine Triassic, tidal currents, impinge on the seafloor. Organic
where they have been called Lofer cycles productivity is highest under these conditions,
(Fischer, 1964). especially if the sea is fertile through upwelling.
Much precipitation of CaCO 3 in the form of
Carbonate shelf facies and models ooids and cements occurs along shelf margins.
Behind the rim, there is usually a shelf lagoon.
A shelf is a relatively narrow (tens to hundreds This will vary in its degree of protection from
of kilometres), but frequently laterally exten- the marginal turbulent zone, depending on how
sive, generally shallow-water depositional en- well the reefs and sands of the margin act as a
vironment, which has a well-defined margin barrier. At one extreme, a true shelf-lagoon will
(from which things can fall off!), adjacent to a exist, being a very quiet environment with poor
deeper water basin. Some shelf-margins are circulation and perhaps hypersalinities during
fault-bounded. Usually there is a rapid increase dry seasons. It will only be affected by major
in gradient from the shelf (modern shelf gradient storm events. At the other, an open shelf will
156 M. E. Tucker

BASIN I SLOPE CARBONATE SHELF

below fair weather wave base maximum wave action protected subaerial

shaLes/pelagic re-sedimented reefs and carbonate lagoonal and tidal flat supratidal
limestones carbonates sand bodies carbonates : broad belt carbonates
I

~, , , -"~P~,~ /

B
FIG. 8. (a) Facies model of shelf carbonates. After Coogan (1969). (b) Lateral facies distribution on a
carbonate shelf.

exist, subject to continuous wave and tidal The south Florida shelf is 6-35 km wide, with
motions. a shallow shelf break at a depth of 8-18 m. The
The shoreline at the inner margin of the shelf gradient on the shelf is very low, 1:1000,
may be dominated by tidal flats, especially if whereas the seaward slope initially has a
there is a significant tidal range (meso-macro- gradient of 1:40 (1V2°). The shelf, with a
tidal), or by a beach-barrier-tidal delta coastline N E - S W orientation, is subject to winter storms
if wave energy is substantial (determined by from the NE and summer trades from the SE, so
prevailing climate and coastline orientation) and that water movement is dominantly on-shelf.
tidal range low (micro-meso-tidal). Most likely The shelf-break serves to focus the wave
this would occur in an open shelf setting and the energy so that water circulation is vigorous
facies pattern would be similar to a carbonate there. Tidal range is 50-90 cm at the shelf-
ramp (see that section). break. Most carbonate sedimentation is taking
place along the seaward margin of the south
Florida shelf. Reefs dominated by corals and
Modern shelf carbonates
calcareous algae form a belt up to 1 km wide
These have been studied extensively in the along the shelf margin. Not all reefs are living,
Caribbean (the Bahamas, Florida, Belize, but this appears to be part of a pattern, of
Jamaica, etc.) and off Queensland (the Great shifting zones of active reef growth. Much
Barrier reef). The south Florida shelf can be skeletal carbonate sand occurs on the outer shelf
briefly described here as a good example of a and this can form distinct sand bodies with sand
carbonate shelf (for details see Enos & Perkins waves, tidal channels and spillover lobes. Much
1977). skeletal debris is concentrated behind the reefs
 Shallow-marine carbonate facies 157

in back-reef talus piles. Storm movement of of the reef has changed too. The term 'carbon-
reef-rubble may lead to the formation of low ate buildup' is now widely used to denote a
islands. Much reef debris is transported off carbonate body of restricted extent that
shelf, down chutes to form wedges up to 12 m possessed some topographic relief. A reef is a
thick on the shallow slope. Shoreward of the little more specific, generally denoting a buildup
shelf margin, there are patch reefs and skeletal where there is much skeletal material, some in
sand shoals and these give way to bioturbated growth position with perhaps some acting as a
muddy sands and sandy muds in the quieter framework. To some workers a certain degree of
water, inner shelf. Although sedimentation on wave resistance is necessary for the term reef to
the inner shelf is strongly controlled by a relic be applied. Although many different types of
topography of Pleistocene limestones (which reef have been recognized, two broad types are
form the Florida Keys), one feature of note is barrier reefs, occurring especially at shelf (and
the presence of seagrass-algal stabilized mud- platform) margins, and patch reefs or reef
banks on the seaward side of the Keys and mounds occurring there too, but also in the shelf
within Florida Bay (e.g. Rodriguez Bank, lagoon and perhaps on the platform. For a
Turmel & Swanson 1976). Lime mud is largely recent review see Longman (1981) and other
derived from disintegration of calcareous green papers in Toomey (1981), also James (1979),
algae. Tidal flats and mangrove swamps are James & Ginsburg (1980), Laporte (1974), Frost
irregularly developed along the shoreline and et ai. (1977), Stoddart (1969), Perkins (1975) and
around the Florida Keys. Schroeder & Zankl (1974).
The facies pattern seen on the south Florida The best-known modern shelf-marginal reefs
shelf is thus one of shelf-marginal reefs are formed chiefly by scleractinian corals and
(framestones-boundstones) and carbonate sands coralline red algae. The corals produce an
(oolitic and skeletal grainstones), giving way to organic framework to the reef and the crustose
skeletal packstones and wackestones of the algae cement, bind and hold the reef together.
protected inner shelf lagoon, with skeletal Coral growth forms and species occur in distinct
mudstones of the mud-banks, and then tidal flat zones over the reef, with strong, solid corals
facies of the shoreline. This pattern is identical growing in the most turbulent zones and more
to that of many ancient carbonate shelf leafy, delicate corals, living in slightly deeper less
sequences. agitated waters. In addition to the red algae,
Where the sea-level stand is stable for a sub- other encrusters include bryozoans, foramin-
stantial period of time, three depositional ifera, serpulids and bivalves. Other organisms,
processes operate: (1) either seaward progra- especially molluscs, echinoids and fish, find
dation of tidal flats over shelf lagoon sediments, homes within the crevices of the reef. The latter
if a protected shelf or seaward progradation of a are common with most, especially the larger
beach-barrier-tidal-delta complex, if an open, ones, being growth cavities, where the frame-
high wave energy, shelf; (2) shoreward progra- work skeletal organisms have enclosed space.
dation of shelf marginal carbonate sand bodies Small cavities abound and many of these are pro-
into the shelf lagoon and (3) seaward progra- duced by boring organisms such as lithophagid
dation of the marginal reefs, reef debris fans and bivalves, clionid sponges, polychaete worms and
sand bodies, constructively extending the shelf- endolithic algae and fungi. The cavities are
margin. frequently partially or wholly filled with internal
Tidal flat progradation generates a shallowing- sediment. The precipitation of carbonate
up unit, and this has been discussed in the cements is very important in modern shelf-
carbonate platform section. Beach-barrier-tidal marginal reefs where seawater is constantly
delta progradation is discussed in the carbonate being pumped through by wave action. Acicular
ramp section. Processes (2) and (3) are dealt with aragonite and bladed and micritic high-Mg
in succeeding sections. calcite lithify loose carbonate sediment in reef
crevices and line and fill cavities (e.g. James et
al. 1977; James & Ginsburg 1980).
Shelf margin reefs and other reef facies
Coral and algal growth can be very rapid (6 m
There is a vast literature on reefs, both 103 years -1 recorded) and in areas of low tur-
modern and ancient, and much discussion has bidity, much turbulence, abundant light and
arisen as to what defines a reef. The argu- fertile seas, such as occur along many tropical
ment arises largely because most ancient reefs shelf margins, a strong, wave-resistant ridge
cannot be compared directly with modern (reef) develops. This rises above the shoreward
reefs. Organisms contributing to reef limestones shelf lagoon where lower sedimentation rates
have changed through time so that the character prevail. Under conditions of stable sea-level, the
158 M . E . Tucker

~. ~ ' F FRAMEWORK CORALOAL

X ~ ~ ~F~-SLOPE SAND

FIG. 9. The facies of a modern shelf-margin reef complex. This model can only be applied to reefs in
the geologic record where a strong framework existed. From Longman (1981), with the permission of
the author and Society of Economic Paleontologists and Mineralogists.

1 2 3 4 5 6
SEDIMENT- SEDIMENT SEDIMENT- SEDIMENT- MIXED RIGID
TRAPPING TRAPPING TRAPPING TRAPPING MASSIVE MASSIVE
NONCAL- DELICATE BRANCHING CONICAL AND FRAMEWORK
EXAMPLES CAREOUS BRANCHING ORGANISMS ORGANISMS BRANCHING (hermatypic
ORGANISMS ORGANISMS (Ramose (some ORGANISMS corals, and
(seagrass, (fenestrate bryozoans, rudistids and (stromatoporoids, stromatoporoids
algae) bryozoans finger coraIs) Pateozoic massive tabulate,
Phy/Ioid aigae) "Gordol~'thon'" corats) and rugose corals)

TYPE
OF Mud-mounds Mud.mounds, bioherms Patch-reefs and
REEF and knoll-reefs walled-reef complexes

F1G. 10. The roles played by organisms in constructing carbonate buildups and the types of buildup
produced. From Longman (1981) with the permission of the author and the Society of Economic
Paleontologists and Mineralogists.

zone of active reef growth (the reef crest) coralgal growth can exist here. A relatively flat
progrades seawards over an apron of debris surface, around 1 m below low water mark
eroded from the reef during storms (the fore- develops behind the reef as it progrades and this
reef). A steep to overhanging wall of active reef flat gives way to further reef debris,
 Shallow-marine carbonate facies 159
A A A

generated by and washed over during storms, in ! P, PI ~'~ scleractinian corals

the back reef area. Reef rubble can be piled up A 7
'~3 d h , ~ + calcareous algae
to form islands along the shelf margin.
Where sea level rises relatively slowly, it is
,all 6 rudist bivalves
possible for reef growth to keep pace and a K
substantial vertical thickness of reef rock can be
formed. Such preferential vertical growth along
the shelf margin will lead to the development of J ~ l ~ ~ 5 scleractinian corals
J
a relatively deep shelf-lagoon, with below wave- " 4 + stromatoporoids
base sedimentation. A rapid sea-level rise is
likely to kill off marginal reefs. Reefs cannot
cope with significant falls in sea level either. ' ~ 1 1 ~ 3 sponges +
Growth could only occur in the shallow subtidal calcareous algae
on the front of the former reef. Exposure of the
reef would lead to the development of emer-
gence phenomena and the shelf itself would
become subaerial if the sea-level drop was
sufficient. Thick reef limestones do not form A
during a period of relative sea-level fall. D A 2 stromatoporoids +
A tabulate corals
Studies of the Recent have given us a well- A
defined facies model for reefs (Fig. 9): the fore-
reef zone of reef-slope proximal talus and distal
talus, the reef itself (or reef core) of reef 1 bryozoa stromatoporoids
o tabulate corals
framework and reef crest, and the back-reef
zone of reef flat and back-reef sand. This facies
model can be applied to many ancient shelf-
marginal reef complexes, but in detail there are
often departures. This is especially the case
where the reef core is concerned, since in many
instances the organisms involved did not have
the ability to produce a rigid framework and/or
encrusting and binding organisms were not A
present. In those instances, a wave-resistant reef
(i.e. a true reef) could not form. ,dh Reef Mound
Many organisms have contributed in a variety
Reef Complex
of ways to carbonate buildups through geologic
time. The organisms have varied from non-
FIG. 11. The Phanerozoic record of reef
calcareous grasses and algae through delicate mounds (patch reefs) and reef complexes,
colonial bryozoans, calcareous algae and corals, with the dominant organisms that have con-
to robust conical rudistids and rugose corals, to tributed towards formation of the latter.
massive or laminar stromatoporoids and tabulate After James (1979).
and rugose corals. The organisms' effects have
varied from simple trapping of sedimentary all these reefs can show similar facies patterns to
grains through to formation of a rigid frame- modern coralgal reefs (phase 7), with the best
work (see Fig. 10). development along shelf margins, a seaward
The record of organisms capable of producing fore-reef talus apron, and a shoreward back-reef
large, rigid, branching, massive or tabular skeletal sand facies passing into a shelf lagoon.
skeletons has not been continuous. There have In detail there will still be many differences,
been seven major phases (Fig. 11) (James 1979; principally in how the organisms behaved and
Longman 1981): (1) Middle and Upper Ordo- interacted in the reef core facies, but also in such
vician bryozoan-stromatoporoid-tabulate coral features as the degree of syn-sedimentary
reefs, (2) Silurian and Devonian stromatoporoid- cementation and the extent of b i o e r o s i o n ,
tabulate coral reefs, (3) Upper Permian sponge- especially by boring organisms.
calcareous algal reefs, (4) Upper Triassic and There are many carbonate buildups in the
(5) Upper Jurassic scleractinian coral-stromato- geologic record which were not shelf-marginal
poroid reefs, (6) Upper Cretaceous rudist reef complexes, but nevertheless had topo-
bivalve reefs and (7) Tertiary-Quaternary scler- graphic relief and often exerted an influence on
actinian coral-red algal reefs. On a broad scale, the surrounding seafloor. I n shelf environ-
160 M. E. Tucker

ments these have been referred to as bioherms, Facies mosaics around highs on shetf/ramp
major factors: depth, high size, energy lever and direction
patch reefs, reef mounds, knoll reefs and banks.
(Mud mounds are a further type but they most A B

commonly developed in somewhat deeper water

locations on ramps or slopes to platforms and
_
shelves, and so are discussed under the later
carbonate ramp section.) SL ~ .'i. " " "

Reef mounds are just as variable as marginal

reefs. Typically though, there is little framework
to a reef mound but in situ skeletons may be grainstones
common. Much of the mound normally consists D
of skeletal debris, along with carbonate mud.
Sediment trapping was the likely cause of many
reef mounds with bryozoans, crinoids, finger
corals, rudistids and some algae all able to take
this role. Around the reef mounds, which are
generally massive and lenticular, there usually
occur well-bedded limestones of skeletal debris.
Some of this would be derived from the buildup,
washed off during storms, but much is generated
in the slightly deeper off-mound waters, and FIG. 12. Facies models showing the de-
may thus have a different faunal composition. velopment of patch reefs and associated
At the present time, patch reefs are common facies on a topographic high on a shelf or
in shelf lagoons behind marginal reefs, as in the ramp (based on the work of Purser 1973b).
Caribbean for example, and they also occur on
modern carbonate ramps, such as in the Arabian much in common with those on shelves and plat-
Gulf. Modern patch reefs usually have coral forms in the Caribbean. Of interest in terms of
frameworks and calcareous algal crusts con- facies models, is that a spectrum of reef develop-
solidating the structure, just like the marginal, ment can be discerned, with four stages giving
barrier reefs. A zonation of corals also occurs four distinct facies models (Fig. 12). The initial
with more robust and sturdy forms on the upper stage is the development of carbonate sands on
part of the reef and more delicate forms around the high through enhanced carbonate skeletal
the base. Many other organism types are production on this shallow agitated area and
associated and bioerosion is just as important. winnowing out of lime mud. Sand distribution is
The patch reefs give rise to a halo of skeletal asymmetric, sedimentation taking place mainly
sand, which is frequently asymmetric, with the in a downwind direction. Stage 2 is the estab-
greater portion located on the leeward side. lishment of a reef on the windward side of the
The location of many patch reefs appears to high, giving rise to more rapid production and
be random, dependent on suitable substrates for accumulation of skeletal sand. In stage 3, reef
coral attachment and then a period of rapid growth is well advanced, producing more of a
growth to establish the buildup. The same barrier to waves and currents, so that downwind
probably applies to many ancient reef mounds, tails of reef debris from the margins of the reef
that restricted areas of suitable substrate produce a leeside lagoon. Island formation is
were colonized by the framework-building or likely from storm piling-up of reef talus. In
sediment-trapping organisms and that once stage 4, the reef has grown most or all the way
established, the mounds' growth was assured. around the topographic high to enclose a
However, it is clear that the location of some lagoon. Lime muds may accumulate here in this
patch reefs and reef mounds was determined by atoll-like stage.
slight topographic highs. These could have There are numerous descriptions of ancient
resulted from a more positive area of basement equivalents of patch reefs and it is clear that
or a storm-deposited shoal of carbonate sand. there is much variation in shape and size, as well
Some modern patch reefs have formed upon as internal structure. As an example, the Silurian
slight rises in the underlying relic topography. reefs of the Great Lakes area have been im-
Patch reefs of the Arabian Gulf have develop- aginatively divided on the basis of shape into
ed where the seafloor is locally elevated and this blue spruce, spread-eagle, mammary gland, hay-
is frequently due to salt diapirism (Purser stack and lime kiln types by Shaver (1977). The
1973b). Although the depositional setting is shape is determined by sea-level fluctuations,
basically a carbonate ramp, the patch reefs have rates of subsidence and time available for reef
 Shallow-marine carbonate facies 161

A. WINDWARD OPEN: STORM AND TIDE DOMINATED C. TIDE DOMINATED

shelf break
bankward ,~ > ~ ' - ~ reefs dee water -- - - - island g r o w t h . . . .
sediment ~.~,.~. J~ ~, ~ P ..~. ~_~ -s?~J~goon
transport _-7 / ,~"I!~(
:'-~ 9 ~ ¢ ~ --~island :3,ql - - tidal - -- -~_

skeletalpackstones ~ ---'~-~_----~ d-'~ \~ inactive "---.. uL//,~h~ ~ . . . . --

and wackestones ~ - - - - _~q.((~i~ F~Ax sand ridges
__ -- . -~ - - _ ._ -.~.,. ~ / \\ IlJll-v2 "" " ' - - ~. ~( spitLover Lobes, '
• - - - . . . . . G ,\ strong tidaL currents: ---...~ tidal channels
bankward sediment transport "-.... shelf
• _-~ ---~--_~ ~ e b b and island g r o w t h "'-..break
~.--. spillover/ ~.~ channels
--- lobe
Sand body sediment : - - - - -_ _-- flood-oriented
Sand body sediment:
oolite and skeletal grainstones sand waves oolite and skeletal grainstones

B. WINDWARD PROTECTED O. LEEWARD MARGINS /

.... shelf lagoon

..).."~~,and ......~):.--.. kk~ .'~--_-_-_-_-_-_-_~....-:,..(. : -_

offbank [[(I.." . ' .k : ... ". . . .
• • ....... : . ~:- :::.,~: : . . i ~ : . i
sediment (i:k:~:~ ~ : .\. \ . . " -- "
: ~.)i : :L~". : - \ . ' : ~ . . j : i ~ shelf break
transport LC-"~ 1" ." . ' . \ ".~. ~ - J -
~ ~-_(-~ ~='~-~~ ~ - i ~ ~ - ~ ) ~ ~ - - ~ r e e f s • . .-.~... . -- '

:::: Sand body sediment:

shelf.
~ break
. . shelf lagoon

tgrranvitP°~tlo~ ~ / - ~ , ~---- --- ( - ~ ~--- mid- slope skeletal grainstones

and packstones,
processes - / - : ~ - - ~ ~ - - saodbod,es much micritization ~ \ ~ ---~& i :(~'::

protected ---Ix"/--- - ~ " : : sandiSlandSshoals:

formeror reefs,
Sand body sediment:
skeletal grainstones, little micritization karstic bedrock

FIG. 13. Facies models of shelf-marginal carbonate sand bodies, where shelf-break orientation
relative to waves, storms and tidal currents is the controlling factor (based on work of Hine et al.
1981).
growth. There were strong trade winds in this ensures continuous turbulence along the shelf-
area during the Silurian so that reefs are margin and constant reworking of sediment and
frequently asymmetric and their debris is prefer- erosion of reefs. Ooids are frequently an
entially developed on one side (the lee) and atolls important component of shelf margin sands
with lagoons formed (Lowenstam, 1950). The since CaCO 3 precipitation is promoted by the
various facies models of Fig. 12, derived from active movement of suitable nuclei (fine skeletal
modern patch reefs can thus be applied. Silurian grains) and the warming and CO2-degassing of
platform sedimentation was relatively fast in ocean water as it comes on to the shallow shelf.
inter-reef areas so that a substantial differential There have recently been several detailed studies
topog[aphy did not develop between the reef of modern shelf-margin sand bodies on the
mounds and the surrounding seafloor. How- Bahama Platform (Hine 1977; Hine et al. 1981)
ever, some reefs grew on the slope into adjoining and from these it is clear that several types exist,
basins, where little inter-reef sediment was dependent on the orientation of the shelf-break
deposited. These reefs managed to keep up with relative to the prevailing wind direction. Distinct
the faster subsidence rate to produce relatively differences exist between leeward and windward
tall, narrow structures termed pinnacle reefs. locations, and between those of an open and
These frequently have an apron of reef debris on those of a protected aspect. Tide-dominated
the leeward side too. parts of the shelf-margin give a further type of
sand body (see Fig. 13).
Open windward shelf margins are generally
Shelf-margin carbonate sand bodies
the most turbulent and sand bodies are well-
Carbonate sands are usually generated in developed there. Reefs are also present, usual-
abundance along shelf (and platform) margins. ly a little seaward of the sand belt and these
Much of the sand is of skeletal origin, derived supply much of the sediment. The near-constant
from shelf-break reefs (if present) and from the agitated conditions, however, promote the for-
skeletons of organisms which live in the shelf- mation of ooids so that the sand is an oolitic and
margin areas. The sudden barrier that the shelf- skeletal grainstone. The dominant bedform of
slope makes to open ocean and storm waves the sand body is a linear sand wave with a
162 M. E. Tucker

wavelength of 20-100 m, oriented transverse to However, the important feature of this shelf-
the flow. Superimposed upon the sand waves are margin type is that the dominant onshore wave
smaller-scale dunes (wavelengths 0.6-6 m) on and storm currents are reflected offshore to
the sand-wave crests, and ripples (wavelengths produce strong down-slope bottom currents.
less than 0.6 m) on the flanks. The sand waves Lobes of sand are thus deposited at the shelf
are asymmetric on the seaward side, and sym- break, on the slope and at the toe of the
metric on the shallower shelf-lagoon side of the slope. Resedimentation is important as sediment
sand body, where tidal currents move the sand is transported off the shelf by grain flows, debris
backwards and forwards through the tidal cycle. flows and turbidity currents. The high energy
Storm-generated currents have cut channels location gives a rapid turnover of sediment so
across the sand body and at the ends of the that little is micritized by algae and few ooids are
channels, especially on the shelf-lagoon side, produced.
spillover lobes are developed. These are also In a tide-dominated location, linear sand
covered by sand waves, migrating into the ridges are produced parallel to ebb and flood
lagoon. The internal structures to be expected directions, usually normal to the line of the shelf
within these sands are large-scale planar cross break. Sand waves and dunes are present on the
beds, from the sand waves, and smaller-scale sand ridges and at the ends of the channels
cross beds from the dunes. Orientations will be between the ridges, especially at the lagoon end,
largely lagoonward reflecting bedform move- spillover lobes are developed. Sand is frequently
ment during major storms, but some off-shelf oolitic, as a result of the constant reworking,
orientations will also occur, especially if ebb and skeletal too if there are reefs nearby. Where
tidal currents are important. tidal currents are oblique to the shelf break, then
On windward protected shelf margins an is- lateral development of the tidal sand ridges may
land or barrier reef affords some protection to occur. Islands can be formed, especially if there
the shelf lagoon. Sand is produced in abun- is sufficient wave and storm activity to throw up
dance, from wave and storm attack on the reefs, beach ridges.
and this is deposited between the reefs and On leeward shelf margins, the dominant
island, or just off-bank from the reefs. direction of water movement is off-shelf, and

~ ~ :.'.:~,:..:.::... x ~
% 2 ~/ •'''• • "::' "
... . . .:.. -no,
% "....'.......
:>"
%
'•'.)";"
' : : !::::))..:;:
""•
'-". -..
':'?'7. • .:

: .... .•,.,•••

.- ~

I ¢ / lUppe~ Mar~s & E,,~po~,tes I ~ ~

I Leeds~ \ JUpperMaglesian L,mestone I '""""::'::'"::'::::::~'bt?':C""~ .......''"":"::::'::'
I "F ~ IMiddle Marts & Evaporites
I f Hu~ jLower ~, . . . . .
a~,~,aue:sy
SprotbrougnM'l
[ f ~ IMagreslan._ , Hampole Bedsl
I t .,I I, . l bOrrnaTtOr]. . . . I
r Sheffed ( ~L~meslone) Wetherov M i
I " ' I
LPernl~an outcro,c2.21Perm~an strala NE Enqland I

FIG. 14. An example of a facies model for a shelf-margin sand body, the upper Lower Magnesian
Limestone of Yorkshire, England. After Kaldi (1984)•
 Shallow-marine carbonate facies 163
1
CARBONATE RAMP I PERITIDAL
E]ASIN deep ramp shallow ramp [ CARBONATE ~LATFORM

be[ow fair weather wave base [wave dominated protected subaeria[

F-
I barrer island complex tagoonat to supratida[
shale/pelagic thin- bedded I ooid sand shoals tidal flat carbonates
limestone limestones [ patch reefs carbonates ± evaporites

sea level / "~-'~'~o~ ~ - ~

Fie. 15. The carbonate ramp facies model based on Ahr (1973).

the storms and waves have moved over the shelf are also distinguished. Shorewards (west) of the
itself before reaching the shelf break. Sand shoal complex, a lagoon existed and tidal flats.
bodies are not well developed in this situation. This shelf-marginal oolite body was strongly in-
On open leeward margins, sand is produced by fluenced by onshore directed trade winds
reworking of the sandy mud of the lagoon, so blowing from NE or E.
that grains are extensively micritized and peloids
are an important constituent. Sand waves may
The carbonate ramp
develop at the shelf edge and there is much off-
shelf transport of sediment. On protected The ramp was recognized by Ahr (1973) as a
leeward margins, islands are present and these major type of depositional setting for carbonate
act as a barrier to the off-shelf movement of rocks and was put forward as an alternative to
sand. The latter thus accumulates against the the carbonate shelf. Although the ramp model
islands and can cause them to enlarge. The has not been applied to ancient carbonates as
islands themselves may be reefs or sand shoals of much as the shelf and platform models, there are
a former higher sea-level stand, or the result of certainly many limestone sequences which are
karstic weathering of earlier carbonates during a best understood by reference to this model of
sea-level fall. Active reefs may develop seawards facies distribution. A carbonate ramp is a gently
of the islands since the latter will keep fine sloping surface, gradients of the order of a few
sediment from smothering them. metres per kilometre, contrasting markedly with
Carbonate sand bodies are best developed the steep slope up to a carbonate shelf, and the
along open windward shelf margins and the sand relatively flat surface of the shelf itself or a
is moved into the lagoon during major storms. platform. On a ramp, shallow-water carbonates
The net result is a sequence of lagoonal pass gradually offshore into deeper and deeper
sediments (wackestones-packstones) overlain by water and then into basinal sediments (Fig. 15).
the carbonate sands (grainstones) with There is no major break in slope which is the
shoreward-directed cross bedding. A similar characteristic feature of a shelf to basin
sequence can be produced along tide-dominated transition. In spite of this, there are often
margins if there is much lagoonward sediment similarities between the ramp facies and
transport. Thick sequences of grainstone will be nearshore open shelf facies (see earlier section).
produced if there is a substantial rate of The distinctive sediments of the inner ramp
subsidence or sea-level rise and sedimentation is are carbonate sands formed in the agitated
able to keep pace. shallow subtidal shoreface zone (above fair-
Ancient shelf-margin carbonate sand bodies weather wave base) and low intertidal. On a
have been described from a number of for- ramp, wave energy is not as intense as along a
mations. One useful example from Britain shelf margin where oceanic swell and storm
occurs in the Permian Lower Magnesian Lime- waves are suddenly confronted with a shallow
stone (now Cadeby Formation) of Yorkshire steep slope. Nevertheless, the gradual shoaling
and Nottinghamshire (Kaldi 1984). A n o r t h - of a ramp does result in relatively strong wave
south-oriented oolite shoal complex, located at a action in the shoreface-intertidal and this
break of slope is dominated by large-scale cross permits the formation of shoreline carbonate
bedding produced by sand waves (Fig. 14). sand bodies. Storm events are generally very
Spillover lobes generated by major storm events important on ramps and along with normal
164 M. E. Tucker

wind-wave activity give rise to beach-barrier with distinct lobes (spillover lobes) along their
complexes through shoreward movement of margins. Sandwaves, dunes and ripples cover
sand. Offshore storm surges are important in the surfaces of the tidal deltas.
transporting shoreface sands to the outer, The localized shoal areas which occur on the
deeper ramp. Shoreward of the inner ramp sand ramp are sites of skeletal sand accumulation
belt, lagoons and tidal flats may develop. These where these highs extend above wave base. Reefs
will be of limited extent if the ramp continues too may develop on the highs and these have
into the supratidal, but if the ramp leads up to a been discussed in an earlier section (see Fig. 12).
platform, then a very extensive lagoon-tidal flat- In fact small patch reefs do occur just offshore
supratidal area will occur behind the beach from the beach-barrier system. The slight
barrier. hypersalinity of the Gulf water prevents the
stenohaline corals from flourishing.
A modern ramp
Ramp facies
The best developed and described modern
carbonate ramp is off the Trucial Coast of the The nearshore sand belt of a ramp consists of
Arabian Gulf (Loreau & Purser 1973; Wagner & skeletal and oolitic grainstone. Peloids may be
van der Togt 1973). Here the seafloor gradually important too. Apart from compositional dif-
slopes down from sea level to a depth of 90 m in ferences, the facies developed will be identical to
the axis of the Gulf. The slope is not a smooth that of a siliciclastic beach-barrier system (see
surface; there are many local positive areas for example Elliott 1978; Reinson 1979). The
rising above the ramp surface which are struc- beach-barrier carbonates will show bedding
turally controlled, mostly being salt diapirs. The dipping at a low angle offshore (surf-swash
Trucial Coast is a mesotidal area with a tidal deposit) and onshore from deposition on the
range of 2.1 m along the shoreline, dropping to backsides of beach berms. On-shore directed
1.2 m within the lagoons. The NE-SW-oriented cross-bedding will be produced by shoreface
coast directly faces strong winds (the Shamal) megaripples and wave-ripple cross-lamination
coming from the NNW. Because of a very arid will also occur. Aeolian cross-bedding is likely
climate, and the partly enclosed nature of the through barrier-top wind-blown dune migration
Gulf, salinity (40-45 °70o)is a little higher than in (also shoreward). Rootlets and vadose diagene-
the Indian Ocean (35-37070o) and in the lagoons tic fabrics are possible in the upper barrier
it may reach 70070o. sediments and burrows may occur in the low
On the deep ramp, skeletal sandy muds are intertidal-shoreface part. Tidal deltas, which
extensively developed with bivalves and could be largely oolitic, would give rise to
foraminifera the most important sediment offshore and/or onshore directed cross-bedding
contributors. These sediments give way to from sand wave and dune migration. Herring-
carbonate sands on the shallow ramp and these bone cross bedding is possible from tidal current
form subtidal shoals and beach-barrier-island- reversals. Tidal inlet migration, resulting from
tidal delta complexes. The beach barriers are longshore current effects, will rework beach-
composed of skeletal sand reworked from the barrier sediments and give rise to a sharp-based
shoreface and ooids. There are aeolian dunes on shell lag of the channel floor, overlain by
the subaerial parts of the barriers. During major variously cross-bedded, cross-laminated and
storms sand may be carried over the barriers to flat-bedded sand of the deep to shallow channel
be deposited in washover fans on the back and migrating spit.
barrier. Behind the beach-barrier system there At depths a little greater than fairweather
occur lagoons and then extensive intertidal flats wave base, organic productivity is still high so
dissected by tidal channels and partly covered by that bioclastic limestones will be dominant.
algal mats. Landwards, are the broad supratidal Shoals of skeletal debris (grainstones) are likely
flats or sabkhas wherein gypsum-anhydrite is to be formed through reworking by storm
precipitating. In actual fact the back-barrier waves. These would have medium to small-scale
environments are particularly well developed cross-stratification, and contain winnowed
along the Trucial Coast because this area is a horizons of coarser debris (rudstones).
tectonic depression. The lagoons are connected Hummocky cross-stratification is possible here
to the open Gulf via tidal channels through the too. Migration of these skeletal sand shoals
barriers and within these inlets ooids are being would take place during major storms and result
precipitated. Ebb and flood tidal deltas have in coarsening-up, thickening-up units, a few
developed at the Gulf and lagoon ends of the metres thick, of skeletal wackestones passing up
tidal channels. The deltas consist of sand shoals into skeletal grainstones and rudstones.
 Shallow-marine carbonate facies 165

In slightly deeper areas, below storm wave Ancient ramp facies

base, skeletal packstones and particularly
Ramp facies are widely developed in the
skeletal wackestones will dominate. Fine
geological record. Ahr (1973) interpreted the
carbonate will be derived largely from shallow-
Jurassic Smackover Formation of Texas as a
water areas where fragmentation of skeletal
shallow ramp sand body. The Smackover, an
debris takes place; some lime mud will be
important oil reservoir, consists of a seaward
formed in situ. Thin graded bioclastic grain-
prograding wedge of oolite. The deeper ramp
stones and packstones will be common in the
below storm wave-base areas of the ramp, facies are peloidal wackestones and mudstones
deposited from seaward flowing storm-surge and then basinal organic-rich lime mudstones.
currents. Scoured bases, grooves, even flutes, Behind the oolitic sand belt evaporites and red
can be expected on the bases of these storm beds developed in supratidal and subaerial en-
beds, as well as a sequence of internal structures vironments. ~"amp carbonates are well repres-
indicating deposition from waning flow (flat ented in the Upper Cambrian and Middle
Ordovician of the Appalachians in Virginia
bedding to ripple cross-lamination especially).
Burrows may occur on the base and within the (Read 1980; Markello & Read 1981). Oolites
storm bed. characterize the shallow ramp and ribbon
carbonates the deeper ramp. The latter consist
Sea-level changes and subsidence rates will be
of storm-deposited shallow water carbonate
important in determining the thickness of the
sediments.
shallow ramp sand belt and also its migration
In Britain, parts of the Dinantian carbonate
direction. During a still-stand or slight sea-level
sequence of South Wales are ramp in character.
fall, a thick sand body can develop through
In the southern part, deeper water argillaceous
seaward progradation of the shore-line beach-
limestones give way to bioclastic-oolitic lime-
barrier system, if there is a continuous supply of
stones farther north. Several major phases of
sand. A slow sea-level rise can lead to the
shallowing can be recognized, where skeletal
shoreward migration of the sand belt, and the
wackestones give way to skeletal-oolitic grain-
generation of a transgressive sequence (barrier
stones. Below fairweather wave-base skeletal
sands overlying lagoonal sediments). In this
instance the low barrier sediments will consist of wackestones contain thin skeletal packstone-
washover fan sands, giving small coarsening-up grainstone beds of storm-surge origin and
cycles (lagoonal peloidal muds to the barrier thicker sequences of skeletal grainstone suggest
sands) as the fans prograde into the lagoon. As the development of sand shoals through more
with siliciclastic beach-barrier systems, rapid persistent storm reworking. Migration of these
transgression may leave little record of the shoals gives metre-thick coarsening upward
shoreface, other than a disconformity (a ravine- units. Oolite sand bodies were formed when
ment) and a basal conglomerate, produced by above wave-base depths persisted. Then shore-
surf-zone erosion. face shoals and beach barriers developed, with

Agitated lagoon Quiet lagoon

~,~. / Barrier island
" . . . . ~ T : ~ ~ ,Agitated back-barrier zone NW
~'~" ,i--.'~,,. :I::'LI-~,'
: ; . ~ ~ /---On-barrier drainage channels
\~ i:,'~ :L:.:;'I~I~: "" On-barrier ponds ~
\ \~- "~,;\/i..ii~: , ' Complex bars
\ '~ =\-~, " : .~i~-i~, Barrier inlet
\ ~\- '..:~-.:.\ :"~: Ebb-hdal delta
~ . \ ::if'. Spillover lobes

" :: : ~::: : :: : : : : :: :: : : : : : : : : : : : : "~ .-2" 2" 2"" •. ~ ' ~ ii':)':

!i:~i!'i
"F~;?!#~:-:-:~......... ~ ..... ;'4~;~!!~:::ii~:?:'.::.-~!:~-i !~::.
f: :::;:~i:::~52~:.(:::i
:::::::i-!:::? "s e:i:~i!).i;iil
:::#~.'i:::'
ii:::i}ii~:::i::ii:i!:i.!~,:-ili .n
\~ :.i i:: Tidal flats i i ~ .:-:~:':':':'~,:i':;:;;i:!!!::::::::i::~i:~.i.ii:~i:
\ l i:: i i i i : ; i i i i i : ;;;;i•••!!••i••••;••i;iii!!!i••i•••.••••.;;•••••••:•:•:•:•:•:•:•:•:•:•:•:•:•••:•:.:.:•:•:•••:•.•`••• " • • .... . . . . . . .- ....-.....-...:.~~-L~t!'~

FIG. 16. An example of a facies model for a barrier-lagoon system, probably formed on a shallow
ramp, the lower Bajocian (Jurassic) Lincolnshire Limestone Formation, east Midlands. From Ashton
(1977). The barrier gradually builds lagoonwards in response to episodic transgression.
166 M. E. Tucker

lagoons and tidal flats behind where lime and then a later drusy sparite. Stromatactis
mudstones, dolostones and cryptalgal laminites structures are now regarded as the product of
accumulated. early cementation of the carbonate mud, giving
Another major carbonate sand body develop- lithified surficial crusts, and then a degree of
ed in the mid-Jurassic of eastern England winnowing of uncemented sediment from
(Ashton 1977). The Lincolnshire Limestone beneath the crust (Bathurst 1982).
was deposited in a major N-S-oriented barrier
island-tidal delta complex and consists of C a r b o n a t e facies through time
variously cross-bedded oolite. A well developed
lagoon to the west was covered by the There are as yet no clear trends in the pattern of
barrier as it transgressed shorewards (Fig. 16). carbonate facies through time, but a few
Back barrier washover fans gave rise to generalizations can be made. Ramp, platform
coarsening up units. Supratidal barrier top, tidal and shelf grainstones, packstones, wackestones
delta and spillover lobe facies have all been and lime mudstones are similar wherever they
recognized. Unfortunately the seaward facies occur. There are great variations in the grain
are not exposed. types present, skeletal and non-skeletal, but the
processes operating were basically the same, so
that the end-products are similar. There were
Carbonate buildups on ramps, notably mud-
certain periods when particular facies were well
mounds
developed in certain areas. This is especially the
Since there is no major break of slope on a case with platform carbonates, since, generally,
carbonate ramp, reef complexes of the barrier they are best developed at times o f relatively
type, which are common at shelf-margins, do high sea-level stands, when cratons are flooded
not develop. Small patch reefs or reef mounds and shallow epeiric seas widespread. For
are common, however, and can occur on the example, during the lower to mid-Palaeozoic, a
seaward side of the barrier, in the back-barrier time of global sea-level rise, extensive platform
lagoon, and on any topographic highs on the carbonates were deposited on the North
deep ramp. These reef mounds will be similar to American and Russian cratons. The repetition
those of carbonate shelves, described earlier. of shallowing-upward units, so characteristic of
One particular type of buildup which does platform sequences, also requires special
occur on ramps is the mud mound (also called conditions of periodic sea-level rises (eustatic or
reef knoll and mud bank), composed largely of local tectonic), and these also only occurred at
micrite, usually clotted and often pelleted. certain times in the Phanerozoic (e.g. the
Mud mounds are particularly common in the Carboniferous). Major variations in carbonate
Palaeozoic and mostly occur in offshore, deep facies through the Phanerozoic did occur as a
ramp, or shelf-slope locations. They clearly had result of the changing fortunes of carbonate
some form of seafloor expression since the flank secreting organisms. The biogenic component of
beds generally dip off the mud-mound structure, limestones has thus varied considerably in
having original depositional slopes of 5-20 °. skeletal grainstones and packstones, but it is
Mud mounds do not contain any metazoan particularly in the formation of reefs and other
frame builders and this has given rise to much carbonate buildups that these variations are
discussion, posing two questions: how did the most marked. The roles played by the various
mud-mound form, and where did all the lime organisms in reef construction have varied
mud come from? Recent work by Pratt (1982) considerably, from simple sediment trapping
on many mud-mound limestones has revealed and baffling, through to frame building and
similarities between the clotted, rarely laminated
~.~
mud-mound framework and shallow-water ::::::::'.'~
...... :...~- % d "
7
• " ~
7 ~
" ~d'j. ?:'.*g>:"
• .~.:
-.
cryptalgal structures, particularly those of a
thrombolitic character (unlaminated stromato- 'arogonite threshold'
lites). Pratt (1982) thus proposed that mud -Pc I CAM I ORD ISILI DEVI CARB I PER ITRI JUR I CRET I CEN I
mounds were formed through organic binding high-Hg calcite and, tess abundantly, oragonite
by algal mats of locally produced lime mud. One
calcite; Hg content genera[ty tower,
particular structure that is common in many increasing toward 'threshotd'
mud mounds is stromatactis. This is a cavity
structure, typically occurring in families and FIG. 17. The fluctuations in the mineralogy
having a flat floor of internal sediment and an of shallow-marine carbonate precipitates,
irregular roof. Although variously filled, many ooids and cements through time. After
have an initial isopachous fibrous calcite cement Sandberg (1983).
 S h a l l o w - m a r i n e carbonate f a c i e s 167

binding (see Fig. 10). As a result of this, the 1981; Sandberg 1975, 1983). On the basis of
structure and facies distributions in ancient ooid preservation and the nature of early
buildups vary tremendously. The record of reef cements, it appears that aragonite (and high-Mg
complexes comparable to modern shelf- calcite) was the principal precipitate in the late
marginal reefs, and of reef knolls similar to Precambrian to Cambrian, Upper Carbonifer-
modern patch reefs, has been discussed in an ous to Triassic and early Tertiary to Recent, with
earlier section (see Fig. 11). calcite (MgCO 3 less than 8 mol °70) the
Another trend in carbonate facies which is precipitate in the mid-Palaeozoic and Jurassic-
apparent is the predominance of mud mounds in Cretaceous. There is also a broad trend of more
the Palaeozoic. They occur in deeper water calcite-secreting organisms in the Palaeozoic and
ramp and outer shelf-slope settings but they are more aragonite-secreting organisms in the
particularly important in Upper Ordovician to Mesozoic-Cainozoic. The fluctuations in miner-
Lower Carboniferous strata. The geologic re- alogy of marine carbonate precipitates are pre-
cord of oolite facies also is not continuous; they sumably a reflection of changes in seawater
are conspicuously absent in the Devonian of chemistry. The major factors controlling car-
western Europe, although all other carbonate bonate precipitates are M g / C a ratio and pCO 2,
facies types are well developed, and Cretaceous- with to a lesser extent, organic geochemistry,
early Tertiary oolites are also poorly represented. temperature and salinity. In fact the trend of
Finally, the record of pelagic carbonates has not Fig. 17 coincides with the global sea-level curve;
been continuous. With a few special cases, aragonite precipitates at times of low global sea-
notably the Ordovician Orthoceras limestones of level and calcite at times of high sea-level stand.
Scandinavia and the Devonian cephalopod lime- An underlying geotectonic control is thus
stones of western Europe, pelagic carbonates are implied. Two possibilities are: fluctuations in
poorly represented in the Palaeozoic. From the pCO2 due to variations in the rate of subduction
Jurassic, coccoliths and planktonic foraminifera of carbonate sediment, or fluctuations in
permitted the deposition Of ammonitico-rosso- M g / C a ratio due to variations in the rate of
type facies, as in the Alps, and chalks from the seafloor spreading.
Cretaceous.
It has recently been established that there have ACKNOWLEDGMENTS: I am grateful to Dave Gray and
been major fluctuations in the mineralogy of Mike Ashton for allowing me to quote from their
marine carbonate precipitates through the Ph.D. theses and to use their unpublished figures.
Phanerozoic (Fig. 17) (MacKenzie & Piggott

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MAURICE E. TUCKER, Department of Geological Sciences, University of Durham,

Durham DH1 3LE.

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