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Depositional ENV Carbonates

Jun 04, 2018



Adnan Khan
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What is a Facies?



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5. Carbonates Depositional Environments 

Large-scale carbonate platforms are subdivided into distinct

depositional subenvironments, based on the dominant

depositi onal processes and the sediment types deposited:

1- Platform interiors (low energy),

2- Platform interiors (high energy) & platform margin sand shoals,

3- Reefs,

4- Slope & base of slope,

5- Offshore systems,

6- Deep seas.

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Lagoons are shallow-water depositional environments, <10m deep, protected

from strong wave action. They occur in the broad interiors of rimmed shelves or

behind inner ramp shoal belts. Where circulation is restricted, water

temperatures & salinities may become highly elevated.

* Carbonate production is typically dominated by phototrophs (i.e. organisms

dependent on high light intensity)   since Cenozic, sea-grasses.

- Patch reefs: common in deep open lagoons; Reef mounds: low relief banks 

5.1. Platform Interior (LOW ENERGY)

Bioturbation is very important. Mudstone burrows are filled with coarser material.

 Platform interiors include embayments, subtidal lagoons, beaches, and tidal flats.

Salinities in such settings are predomninatly normal marine, hypersaline,

 periodically brackish or they may vary seasonally between these states.

Lagoonal sediments are typically peloidal, comprising feacal pellets generated

 by mud ingestors and grains micritized by endolithic micro-organisms.Platform carbonate sand is commonly redistributed into the lagoons by storm.

Ooids transported into the lagoons may be cemented into small clusters in the

quiet platform interior to form grapestones.

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The innermost platform areas are commonly restricted hypersaline and support a lower

diversity biota. Some foraminifera, ostracods, algae, oncoids, calcispheres, gatsropods

and molluscs are common grain types.

** Where highly restricted, platform interiors in arid areas may become hypersaline due to

high evaporation and poor water circulation. Gastropods dominate the biota in the

subtidal zone, while microbial mats (stromatolites) occur in the shallow subtidal and

intertidal zones.

Shoreline carbonates are referred to as peritidal (term meaning ‘around the tides’; Folk,

1973): nearshore, very shallow subtidal zones, tidal flats & supratidal zones, and coastal


Peritidal rocks can reflect the following subenvironments:

Subtidal zone: permanently submerged, very shallow-water area, strongly

influenced by wave action and tidal currents {high energy env-t; coarse sediments}

Intertidal zone:  between normal- and high-tide levels, alternately flooded by

seawater and exposed, influenced by climate {low energy env-t;

dessication/fenestare, evaporite minerals, soils}

Supratidal zone:  above high-tide level, flooded only during high spring tides  &

storms, largely controlled by climate {in semi-arid /arid settings  evaporite & wind

deflation; in humid settings marshes} 

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Platform interior sandbodies  may develop along shorelines, in platform

interiors or, less commonly, from shallow offshore banks.

* Shoreline carbonate sandbodies possess similar characteristics to siliciclastic

sand accumulations in comparable settings, including barrier complexes

(shoreface-backshore, tidal inlets, deltas, strandplains, etc … 

Platform margin sandbodies  made up of bioclastic and oolitic sands occur

extensively at the margins of carbonate platforms, reflecting the dissipation of

most wave and tidal energy at such margins.* Key factors: topography, orientation with respect to dominant winds & waves,

and tidal range.

5.2. Platform Interior (HIGH ENERGY)

& Platform Margin

These are mainly carbonate sandbodies  , prominent features of high-energy

 subtidal to intertidal environments in many platform settings.

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5.3. Reefs and Carbonate Slope Deposystems

Since frame-building organisms have not always been present, reef mounds have

been and remain much more dominant  –  today, this is true except where corals can


Reefs can be also characterized on the nature of organisms which constructed them

(e.g. algae, stromatoporoids, rudists, corals); or on their morphologies (e.g. atoll, faro,

barrier and fringing reefs).

Reef growth may produce large wave-resistant reliefs creating different sub-

environments (e.g. fore reef, reef front, crest, back reef) and inducing subsequently

several processes.

Reefs are usually classified into frame-built reefs  (those that possess calcareous

framework) and reef mounds (those that lack a rigid structure).

The term ‘reef’  means any biological inf luenced carbonate accumulation, which

was large enough to have developed topographic relief above the sea f loor. Here

biological carbonate sediment production and environmental modif ication arereali zed to maximum extent.

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Three main forms of reef have been recognised in modern


Fringing reefs are built out directly from the shoreline and lack

an extensive back-reef lagoonal area.

Reef settings

fringing reefs build at the coastline

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Barrier reefs, of which the Great Barrier Reef of eastern

 Australia is a distinctive example, are linear reef forms thatparallel the shoreline, but lie at a distance of kilometres to tens

of kilometres offshore: they create a back-reef lagoon area

which is a large area of shallow, low-energy sea, which is

itself an important ecosystem and depositional setting.

Reef settings

barrier reefs form offshore on the shelf and

protect a lagoon behind them

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Patch reefs: In open ocean areas coral atolls develop on

localised areas of shallow water, such as seamounts, whichare the submerged remains of volcanic islands.

In addition to these settings of reef formation, evidence from

the stratigraphic record indicates that there are many

examples of patch reefs, localised build-ups in shallow water

areas such as epicontinental seas, carbonate platforms and


Reef settings

patch reefs or atolls are found isolated

offshore, for instance on a seamount

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Constructive processes

Destructuve processes


CementationCements constitute up to 80% of the volume

of some reefs cementstone reefs!!!


Modern corals grow best at depths   less than 100m in waters of near

normal sal ini t ies which vary little in temperature   outside the range 25-


Reefal communities were not constant throughout the geological time-scale

(e.g. Early Jurassic such organisms were absent    carbonate platformsare ramp-like).

Many reefs exhibit prominent biotic and sedimentological zonation which is

controlled by changes in wave energy, light intensity, degree of exposure

and sedimentation rate.


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 Reef Complexes (Fig. 9.50):These are large reef tracts  –  100s to 1000s of km long; e.g. those developed at

rimmed shelf margins.

- Reef crest &  reef front   are the main productive zones, extending from the

highest point on the reef (the crest ) to a point where frame construction ceases

(downward to 70-100m below sea-level)  –  Breakage is maximum at the crest bywave action and periodical subaerial erosion.

- Forereef   slope extends from the reef front   to the basin floor and is fed by

sediment derived from the reef through collapse, gravity flows, storms, etc… 

- Reef flat   is located behind the reef crest   and can be broadly divided into a

 pavement  (narrow zone lying immediately behind the crest with a water depth at

most of a few meters) and a sand apron  (extending into the platform interior;

water depth about 10m).

- Backreef lagoons  include sediments varying with water depth & degree of

shelter provided by reefal rim.

Reef Complexes

Reef Patches

Reef Mounds


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 Patch Reefs (Fig. 9.52):

These are isolated reefs which develop in shallow-water environments (e.g.

 platform interiors, inner ramps) –  from a few 10s of m to 10 km across.

- Regional variation  in the style of patch reef development reflect differences in

water depth and environmental energy.

- Availability of suitable substrates  appears to be a principle control on the

distribution of patch reef complexes.

Reef Mounds (and mud mounds):

These are by far the most abundant reefal structures in the geological record.

They are typically matrix-rich, frame-deficient, lensoid (biohermal) or tabular

(biostromal) structures.- They can be largely composed of bioclastic accumulations or carbonate mud or


- They are also subject to the same environmental controls that govern all reef



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Carbonate slope heights range from 10s to 1000s m with the slope angles from

~1 to 90°. Slope profiles are mostly concave upwards, but highly variable.

Grainy, non-cohesive mud-free sediments (e.g. carbonate sands,

conglomerates) are able to construct steeper slopes than muddy sediments.Early lithification of carbonate mud and early cementation of granular sediments

grant carbonates the possibility to build steeper slopes relatively to silicilcastics.

3 types of carbonate slopes have been identified from modern platforms: 

a- Erosional slopes  [steep; >25°

] represent exposed submarine rockwalls or slopestruncated by collapse of large sections of the platform margin,

b- Bypass slopes  [relatively steep; >10-12°] accumulate drapes of pelagic sediment,

yet they also host material from shallow-water platform edges,

c- Accretionary slopes  [low angle; <10°] are built of sediment gravity flow deposits.

The major site of deposition is the lower slope apron {mud-supported debris flows &

coarse turbidites}


On high-rel ief , steep-sided carbonate platforms, there is an abrupt transition at

the shal low-water platform margin to a transitional slope facies in which the

bulk of sediment has been re-sedimented.

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Storms produce a wide range of stratification types in offshore siliciclastic

regimes and have a major effect in carbonate and mixed siliciclastic-carbonate

sediments, particularly in ramp settings. In fact, below fairweather wave base,

storms are the dominant control on sediment movement.

* Cyclicity based on the ‘packaging’ of storm beds is often recognized.

Since burrowing is inhibited in deep waters, high preservation potential is

achieved towards distal parts of ramps, while bioturbation dominates shallower


Hummocky cross-stratification (HSC):  a form of medium- to large-scale

cross-stratification, in which the undulating and gently dipping laminae

preserve a 3-D bedform comprising large amplitude (1-5m), low relief (0.1-

0.5m) mounds and troughs.

5.4. Offshore Carbonate Deposystems

Similarly to siliciclastics settings, criteria used to characterize offshore carbonate

deposits are bed thickness, grain size, sedimentary structures, and faunas. Usually

 sediments are finely laminated muddy (finely grained) carbonates.

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5.6. Deep Seas

Deep-water sands and gravels are interpreted in terms of their transporting,depostional and postdepositional processes (after Bouma, 1962).

Deep-water sediments are distributed in 3 principles environments of deposition

(Fig. 10.1):

- basin floor,

- submarine fan,

- slope apron

Bouma Sequence: medium grained sand/mud turbidites (graded beds) with a

preferential sequence of sedimentary structure.

Originally, deep sea sediments were thought of consisting mostly of pelagic clays

and biogenic oozes deposited in quiet undisturbed floors. Later sands were also

demonstrated to occur in deep seas as a result of turbidity currents (densitycurrents). Such deposits are graded, they were called ‘greywacke’  and their

formation ‘flysch’. Now we use ‘turbidite’  for deposits that are produced by

turbidity currents.

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