Gary Nichols. Sedimentology and Stratigraphy(Second Edition)

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1990; Jones & Desrochers 1992). Wave-dominated

shorelines may have a beach ridge that confines a

lagoon or a linear strand plain attached to the coastal

plain. Ramps with mesotidal regimes will show a mix-

ture of beach barrier, tidal inlet, lagoon and tidal-flat

deposition. Agitation of carbonate sediment in shallow

nearshore water results in a shoreface facies of carbo-

nate sand bodies. Skeletal debris and ooids formed in the

shallow water form bioclastic and oolitic carbonate

sand shoals. Benthic foraminifers are the principal com-

ponents of some Tertiary carbonate ramp successions.

The mid-ramp area lies below fair-weather wave

base and the extent of reworking by shallow-marine

processes is reduced. Storm processes transport

bioclastic debris out on to the shelf to form deposits

of wackestone and packstone, which may include

basin

carbonate mudstone

open shelf

grainstone/packstone

Rimmed carbonate

shelf

reef

boundstone

forereef slope

rudstone and redeposited

limestone

restricted shelf

mudstone/wackestone

Carbonate ramp

inner ramp lagoon

wackestone/mudstone

inner ramp

packstone/wackestone

outer ramp

mudstone/wackestone

deep water carbonate

mudstone

inner shelf

grainstone/packstone

Non-rimmed carbonate shelf

slope

redeposited limestone

outer shelf

packstone/wackestone

inner ramp barrier bar

grainstone shoals

mid-ramp

redeposited limestone

Fig. 15.15 Generalised facies

distributions on carbonate platforms:

(a) ramps, (b) non-rimmed shelves and

238 Shallow Marine Carbonate and Evaporite Environments

hummocky and swaley cross-stratification (14.2.1). In

deeper water below storm wave base the outer ramp

deposits are principally redeposited carbonate mud-

stone and wackestone, often with the characteristics

of turbidites. Redeposition of carbonate sediments is

common in situations where the outer edge of the

ramp merges into a steeper slope at a continental

margin as a distally steepened ramp. Homoclinal

ramps have a consistent gentle slope on which little

reworking of material by mass-flow processes occurs

(Read 1985). In contrast to rimmed shelves reefal

build-ups are relatively rare in ramp settings. Isolated

patch reefs may occur in the more proximal parts of a

ramp and mud mounds are known from Palaeozoic

ramp environments.

Carbonate ramp succession

A succession built up by the progradation of a carbo-

nate ramp is characterised by an overall coarsening-

up from carbonate mudstone and wackestone depos-

ited in the outer ramp environment to wackestones

and packstones of the mid-ramp to packstone and

grainstone beds of the inner ramp (Fig. 15.16)

(Wright 1986). The degree of sorting typically incre-

ases upwards, reflecting the higher energy conditions

in shallow water. Inner ramp carbonate sand deposits

are typically oolitic and bioclastic grainstone beds

that exhibit decimetre to metre-scale cross-bedding

and horizontal stratification. The top of the succes-

sion may include fine-grained tidal flat and lagoonal

sediments. Ooids, broken shelly debris, algal mate-

rial and benthic foraminifers may all be components

of ramp carbonates. Locally mud mounds and patch

reefs may occur within carbonate ramp successions.

On shelves and epicontinental seas where there

are fluctuations in relative sea level, cycles of carbo-

nate deposits are formed on a carbonate ramp. A

sea-level rise results in a shallowing-up cycle a few

metres to tens of metres thick that coarsens up

from beds of mudstone and wackestone to grain-

stone and packstone. A fall in sea level may

expose the inner ramp deposits to dissolution in kars-

tic subaerial weathering (6.6.3) (Emery & Myers

1996).

15.4.2 Non-rimmed carbonate shelves

Non-rimmed carbonate shelves are flat-topped

shallow marine platforms that are more-or-less

Fig. 15.16 Schematic graphic log of a carbonate ramp

succession.

Types of Carbonate Platform 239

horizontal (Fig. 15.16), in contrast to the gently dip-

ping morphology of a carbonate ramp. They lack any

barrier at the outer margin of the shelf (cf. rimmed

shelves) and as a consequence the shallow waters are

exposed to the full force of oceanic conditions. These

are therefore high-energy environments where carbo-

nate sediments are repeatedly reworked by wave

action in the inner part of the shelf and where rede-

position by storms affects the outer shelf area

(Fig. 15.17) (James 2003). They therefore resemble

storm-dominated clastic shelves (14.2), but the depos-

its are predominantly carbonate grains. Extensive

reworking in shallow waters may result in grain-

stones and packstones, whereas wackestones and

mudstones are likely to occur in the outer shelf area.

Coastal facies are typically low energy tidal-flat depos-

its but a beach barrier may develop if the wave energy

is high enough.

15.4.3 Rimmed carbonate shelves

A rimmed carbonate shelf is a flat-topped platform

that has a rim of reefs or carbonate sand shoals along

the seaward margin (Fig. 15.16). The reef or shoal

forms a barrier that absorbs most of the wave energy

from the open ocean. Modern examples of rimmed

shelves all have a coral reef barrier because of the

relative abundance of hermatypic scleractinian corals

in the modern oceans. Landward of the barrier lies a

low-energy shallow platform or shelf lagoon that

is sheltered from the open ocean and may be from a

few kilometres to hundreds of kilometres wide and

vary in depth from a few metres to several tens of

metres deep.

Distribution of facies on a carbonate

rimmed shelf

In cases where the barrier is a reef, the edge of the

shelf is made up of an association of reef-core, fore-reef

and back-reef facies (15.3.2): the reef itself forms a

bioherm hundreds of metres to kilometres across.

Sand shoals may be of similar extent where they

form the shelf-rim barrier. Progradation of a barrier

results in steepening of the slope at the edge of the

shelf and the slope facies are dominated by redepos-

ited material in the form of debris flows in the upper

part and turbidites on the lower part of the slope.

These pass laterally into pelagic deposits of the deep

Fig. 15.17 Schematic graphic log of a non-rimmed carbo-

nate shelf succession.

240 Shallow Marine Carbonate and Evaporite Environments

basin. The back-reef facies near to the barrier may

experience relatively high wave energy resulting

in the formation of grainstones of carbonate sand

and skeletal debris reworked from the reef. Further

inshore the energy is lower and the deposits are

mainly wackestones and mudstones. However, ooidal

and peloidal complexes may also occur in the shelf

lagoon and patch reefs can also form. In inner shelf

areas with very limited circulation and under condi-

tions of raised salinities the fauna tends to be very

restricted. In arid regions evaporite precipitation may

become prominent in the shelf lagoon if the barrier

provides an effective restriction to the circulation of

seawater.

Rimmed carbonate shelf successions

As deposition occurs on the rimmed shelf under

conditions of static or slowly rising sea level the

whole complex progrades. The reef core builds out

over the fore reef and back-reef to lagoon facies overlie

the reef bioherm (Fig. 15.18). Distally the slope de-

posits of the fore reef prograde over deeper water

facies comprising pelagic carbonate mud and calcar-

eous turbidite deposits. The steep depositional slope

of the fore reef creates a clinoform bedding geo-

metry, which may be seen in exposures of rimmed

shelf carbonates. This distinctive geometry can also

be recognised in seismic reflection profiles of the

subsurface (22.2) (Emery & Myers 1996). The asso-

ciation of reef-core boundstone facies overlying fore-

reef rudstone deposits and overlain by finer grained

sediments of the shelf lagoon forms a distinctive facies

association. Under conditions of sea-level fall the reef

core may be subaerially exposed and develop karstic

weathering, and a distinctive surface showing evi-

dence of erosion and solution may be preserved in

the stratigraphic succession if subsequent sea-level

rise results in further carbonate deposition on top

(Bosence & Wilson 2003).

15.4.4 Epicontinental (epeiric) platforms

There are no modern examples of large epiconti-

nental seas dominated by carbonate sedimen-

tation but facies distributions in limestones in the

stratigraphic record indicate that such conditions

have existed in the past, particularly during the

Jurassic and Cretaceous when large parts of the

Base of slope

pelagic mudstone

and carbonate

turbidites

10s metres

Fore-reef slope

deposits

Reef front rudstone

breccia

Reef boundstone

LIMESTONES

mud

wacke

pack

grain

rud &

bound

Rimmed carbonate shelf

Scale

Lithology

Structures etc

Notes

Fig. 15.18 Schematic graphic log of a rimmed carbonate

shelf succession.

Types of Carbonate Platform 241

continents were covered by shallow seas (Tucker &

Wright 1990). The water depth across an epicon-

tinental platform would be expected to be variable

up to a few tens to hundreds of metres. Both tidal

and storm processes may be expected, with the latter

more significant on platforms with small tidal ranges.

Currents in broad shallow seas would build shoals of

oolitic and bioclastic debris that may become stabi-

lised into low-relief islands. Deposition in intertidal

zones around these islands and the margins of the

sea would result in the progradation of tidal flats.

The facies successions developed in these settings

would therefore be cycles displaying a shallowing-up

trend, which may be traceable over large areas of the

platform.

15.4.5 Carbonate banks and atolls

Isolated platforms in areas of shallow sea surrounded

on all sides by deeper water are commonly sites of

carbonate sedimentation because there is no source of

terrigenous detritus. They are found in a number of

different settings ranging from small atolls above

extinct volcanoes to horst blocks in extensional basins

and within larger areas of shallow seas (Wright &

Burchette 1996; Bosence 2005). All sides are exposed

to open seas and the distribution of facies on an iso-

lated platform is controlled by the direction of the

prevailing wind. The characteristics of the deposits

resemble those of a rimmed shelf and result in similar

facies associations. The best developed marginal reef

facies occurs on the windward side of the platform,

which experiences the highest energy waves. Carbo-

nate sand bodies may also form part of the rim of the

platform. The platform interior is a region of low

energy where islands of carbonate sand may develop

and deposition occurs on tidal flats.

15.5 MARINE EVAPORITES

Evaporite deposits in modern marine environments

are largely restricted to coastal regions, such as eva-

porite lagoons and sabkha mudflats (15.2.2 &

15.2.3). However, evaporite successions in the strati-

graphic record indicate that precipitation of evaporite

minerals has at times occurred in more extensive

marine settings.

15.5.1 Platform evaporites

In arid regions the restriction of the circulation on

the inner ramp/shelf can lead to the formation

of extensive platform evaporites. On a gently sloping

ramp a sand shoal can partially isolate a zone of very

shallow water that may be an area of evaporite pre-

cipitation; the subtidal zone here often merges into a

low-energy mudflat coastline. Shelf lagoons behind

rims formed by reefs or sand shoals can create similar

areas of evaporite deposition, although the barrier

formed by a reef usually allows too much water cir-

culation. Evaporite units deposited on these platforms

can be tens of kilometres across (Warren 1999).

15.5.2 Evaporitic basins (saline giants)

Evaporite sedimentation occurs only in situations

where a body of water becomes partly isolated from

the ocean realm and salinity increases to supersatura-

tion point and there is chemical precipitation of

minerals. This can occur in epicontinental seas or

small ocean basins that are connected to the open

ocean by a strait that may become blocked by a fall

in sea level or by tectonic uplift of a barrier such as a

fault block. These are called barred basins and they

are distinguished from lagoons in that they are basins

capable of accumulating hundreds of metres of eva-

porite sediment. To produce just a metre bed of halite

a column of seawater over 75 m deep must be evapo-

rated, and to generate thick succession of evaporite

minerals the seawater must be repeatedly replenished

(Warren 1999).

Deposition of the thick succession can be pro-

duced in three ways (Warren 1999) each of which

will produce characteristic patterns of deposits

(Fig. 15.19).

1 A shallow-water to deep-basin setting exists where

a basin is well below sea level but is only partly filled

with evaporating seawater, which is periodically

replenished. The deep-water setting will be evident if

the basin subsequently fills with seawater and the

deposits overlying the evaporites show deep marine

characteristics such as turbidites.

2 A shallow-water to shallow-basin setting is one in

which evaporites are deposited in salterns but contin-

ued subsidence of the basin allows a thick succession

to be built up. The deposits will show the character-

istics of shallow-water deposition throughout.

242 Shallow Marine Carbonate and Evaporite Environments

3 A deep-water to deep-basin setting is a basin filled

with hypersaline water in which evaporite sediments

are formed at the shallow margins and are redeposited

by gravity flows into deeper parts of the basin. Nor-

mally graded beds generated by turbidites and poorly

sorted deposits resulting from debris flows are evidence

of redeposition. Other deep-water facies are laminated

deposits produced by settling of crystals of evaporite

minerals out of the water body. As a basin fills up, the

lower part of the succession will be deeper water facies

and the overlying succession will show characteristics

of shallow-water deposition.

Deep-basin succession can show two different pat-

terns of deposition (Einsele 2000). If the barred basin is

completely enclosed the water body will gradually

shrink in volume and area and the deposits that result

will show a bulls-eye pattern with the most soluble

salts in the basin centre (Fig. 15.20). In circum-

stances where there is a more permanent connection

a gradient of increasing salinity from the connection

with the ocean to the furthest point into the basin will

exist. The minerals precipitated at any point across

the basin will depend on the salinity at the point and

may range from highly soluble sylvite (potassium

Fig. 15.19 Settings where barred

basins can result in thick successions

of evaporites.

evaporites redeposited

from shallow water

into deeper water

direct precipitation

of evaporites

perennial

seepage

shallow water

evaporites

subsiding basin

sill

shallow water

evaporites

sill

periodic overflow

weak

seepage

sill

periodic overflow

seepage

Shallow water  shallow basin setting

Shallow water  deep basin setting

Deep water  deep basin

Marine Evaporites 243

chloride) at one extreme to carbonates deposited

in normal salinities at the other. If equilibrium is

reached between the inflow and the evaporative loss

then stable conditions will exist across the basin and

tens to hundreds of metres of a single mineral can be

deposited in one place. This produces a teardrop

pattern of evaporite basin facies (Fig. 15.20).

Changes in the salinity and amount of seawater in

the basins result in variations in the types of evaporite

minerals deposited. For example, a global sea-level rise

will reduce the salinity in the basin and may lead to

widespread carbonate deposition. Cycles in the deposits

of barred basins may be related to global sea-level fluc-

tuations or possibly due to local tectonics affecting the

width and depth of the seaway connection to the open

ocean. Organic material brought into the basin during

periods of lower salinity can accumulate within the

basin deposits and be preserved when the salinity

increases because hypersaline basins are anoxic.

There are no modern examples of very large,

barred evaporitic basins but evidence for seas preci-

pitating evaporite minerals over hundreds of thou-

sands of square kilometres exist in the geological

record (e.g. Nurmi & Friedman 1977; Taylor 1990).

These saline giants have over 1000 m thickness of

evaporite sediments in them and represent the

products of the evaporation of vast quantities of

seawater. Evaporite deposits of latest Miocene

(Messinian) age in the Mediterranean Sea are evi-

dence of evaporative conditions produced by partial

closure of the connection to the Atlantic Ocean. This

period of hypersaline conditions in the Mediterranean

evaporation increases

across the semi-isolated

basin

sea

Teardrop pattern

halite

gypsum

calcium carbonate

Bulls-eye pattern

sea

halite

gypsum

calcium

carbonate

evaporation of entire

water body in an isolated

basin

Fig. 15.20 (a) A barred basin, ‘bulls-eye’ pattern model of evaporite deposition; (b) a barred basin ‘teardrop’

pattern model of evaporite deposition.

244 Shallow Marine Carbonate and Evaporite Environments

is sometimes referred to as the Messinian salinity

crisis (Hsu

1972).

15.6 MIXED CARBONATE–CLASTIC

ENVIRONMENTS

The depositional environments described in this

chapter are made up of ‘pure’ carbonate and evaporite

deposits that do not contain terrigenous clastic or

volcaniclastic material. There are, however, modern

environments where the sediments are mixtures of

carbonate and other clastic materials, and in the

stratigraphic record many successions consist of mix-

tures of limestones, sandstones and mudstones. These

typically occur in shallow-marine settings. The

changes from carbonate to non-carbonate deposition

and vice versa are the result of variations in the

supply of terrigenous clastic material and this is in

turn determined by tectonic or climatic factors, or

fluctuations in sea level.

Climate plays an important role in determining

the supply of sand and mud to shallow marine

environments. Under more humid conditions, the

increased run-off on the land surface results in more

sediment being carried by rivers, which are them-

selves more vigorous and hence deliver more sedi-

ment to the adjacent seas. A change to a wetter

climate on an adjacent landmass will therefore result

in increased deposition of sand and mud, which

will suppress carbonate production on a shelf. Alter-

nation of beds of limestone with beds of mudstone

or sandstone may therefore be due to periodic clim-

atic fluctuations of alternating drier and wetter con-

ditions. However, other mechanisms can also

cause fluctuations in the supply of detritus from the

continent to parts of the shelf. Tectonic uplift of

the landmass can also increase the sediment supply

by increasing relief and hence the rate of erosion.

Tectonic activity can also result in subsidence of

the shelf, which will make the water deeper across

the shelf area: a relative sea-level rise will have the

same effect. With increased water depth, more of the

shelf area will be ‘starved’ of mud and sand, allowing

carbonate sedimentation to occur in place of clastic

deposition. Fluctuations in sea level (which are

described in more detail in Chapter 23) may therefore

result in alternations between limestone and mud-

stone/sandstone deposition.

Carbonate deposits can co-exist with terrigenous

clastic and volcaniclastic sediments under certain

conditions. Deltas built by ephemeral rivers in arid

environments may experience long periods without

supply of debris and during these intervals carbonates

may develop on the delta front (Wilson 2005), for

example, in the form of small reefs that build up in

the shallow marine parts of ephemeral fan-deltas

(Chapter 17). Time intervals between eruption epi-

sodes in island arc volcanoes (12.4.2) may be long

enough for small carbonate platforms to develop in

the shallow water around an island volcano, giving

rise to an association between volcanic and carbonate

deposition (Wilson & Lokier 2002).

Characteristics of shallow marine carbonates

. lithology – limestone

. mineralogy – calcite and aragonite

. texture – variable, biogenic structures in reefs, well

sorted in shallow water

. bed geometry – massive reef build-ups on rimmed

shelves and extensive sheet units on ramps

. sedimentary structures – cross-bedding in oolite shoals

. palaeocurrents – not usually diagnostic, with tide,

wave and storm driven currents

. fossils – usually abundant, shallow marine fauna

most common

. colour – usually pale white, cream or grey

. facies associations – may occur with evaporites, asso-

ciations with terrigenous clastic material may occur

Characteristics of marine evaporites

. lithology – gypsum, anhydrite and halite

. mineralogy – evaporite minerals

. texture – crystalline or amorphous

. bed geometry – sheets in lagoons and barred basins,

nodular in sabkhas

. sedimentary structures – intrastratal solution brec-

cias and deformation

. palaeocurrents – rare

. fossils – rare

. colour – typically white, but may be coloured by

impurities

. facies associations – often with shallow marine

carbonates