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Primarily composed of carbonate minerals, carbonate rocks demonstrate 2 main
types
1. Limestone - composed of calcite or aragonite
2. Dolostone - composed of the mineral dolomite (CaMg(CO3)2).

Carbonates are responsible for Karst landscapes and caves environments due to their
solubility in slightly acidic water.

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Limestones are mostly composed of calcium carbonate in the form of either calcite
and aragonite. Limestones are commonly (but not exclusively) composed of the
skeletal fragments of organisms. Of all sedimentary rocks limestone makes up about
10%.

Aragonite is in the orthorhombic system. Almost all mollusk shells and corals
(ScleracLnia) use aragonite for their hard parts. Aragonite is thermodynamically
unstable and alters into calcite over Lme. At 380-470°C aragonite changes into
calcite.

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Calcite is in the trigonal-rhombohedral system but rhombohedra are generally rare.
The two common forms of calcite are low magnesium calcite and high magnesium
calcite. Organisms that secrete Low Mg Calcite include brachiopods, trilobites and
echinoderms (sea urchins, starfish, crinoids etc).

In order of stability low magnesium calcite is the most stable form of calcium
carbonate in limestones followed by high magnesium calcite and then aragonite.
Over Lme most limestones will be altered to Low Mg Calcite.

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CaCO3 can also be precipitated inorganically with the the type of calcite being
deposited in the oceans varying over Lme. In “calcite seas” low-magnesium calcite in
the main precipitate. In “Aragonite seas” aragonite and high-magnesium calcite is
precipitated. Calcite seas were common during the Early Paleozoic and the Middle to
Late Mesozoic. Middle Paleozoic and Cenozoic Lmes (including today) are generally
Aragonite seas. Calcite seas generally form during Lmes when the Earth was warmer
than today with Aragonite seas more common during cooler periods.

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Calcite seas tend to be common when seafloor spreading is acLve and global
greenhouse climate condiLons prevail. The reason: increased cycling of seawater
through hydrothermal vents…… As the seawater passes through the hot basalLc
ocean crust calcium rich minerals are converted into magnesium-rich clays via
hydrothermal metamorphism. This reduces the Mg/Ca raLo of the sea water
returning to the oceans favoring the producLon of calcite over aragonite. The
associaLon of calcite seas with greenhouse condiLons also relates to increased
spreading rates which would also be related to increased volcanism and increased
CO2 delivery to the atmosphere. Warm condiLons and warmer oceans also favour the
precipitaLon of calcite over aragonite.

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These are the basic chemical steps of the precipitaLon of calcite – an important part
of the carbon cycle.

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In addiLon to the catalyzing acLvity of organisms, the concentraLon of dissolved CO2
is a very important factor in calcium carbonate precipitaLon. Reduced CO2
concentraLon means you have lower producLon of carbonic acid (which dissolves
calcite) and higher CaCO3 precipitaLon. CO2 concentraLon is effected in 3 principle
ways:
1. At lower pressures CO2 is released from water. As a result lower pressure
(shallower condiLons) will be more conducive to the precipitaLon of calcite.
2. CO2 has lower concentraLon in warm waters.
3. Increased water agitaLon leads to diffusion of CO2 to the atmosphere, making
condiLons more hospitable to CaCO3 deposiLon
Overall therefore, high temperatures, lower pressure & breaking waves favor
carbonate precipitaLon

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AccumulaLon and precipitaLon of CaCO3 below 5000 meters is uncommon. The
depth below which it cannot accumulate is referred to as the Carbonate
CompensaLon Depth (CCD), which varies between 3000-5000m depending on water
temperature. At depth more CO2 is dissolved in the water making the ocean more
acidic (carbonic acid – see equaLon on figure above) which dissolves the CaCO3.

The posiLon of the sea bed relaLve to the CCD therefore will determine which type
of sediment can be deposited. If above the CCD calcareous oozes can form which
when lithified form a type of fine grained limestone we know as chalk. It is possible to
find calcareous oozes in oceans deeper than the CCD if a layer of clay or siliceous
ooze is deposited on top of calcareous oozes that form on a ridge that is above the
CCD. As the ocean floor moves away from the ocean ridge it cools and falls below the
CCD (see diagram) – any sediment that accumulated at the ridge (above the CCD) and
capped by the clay or siliceous oozes will be protected in the deeper ocean.

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Modern limestones are mostly deposited in reefs, carbonate sand bodies close to
reef and in lagoons. In the past (especially during warm greenhouse periods)
limestones were more common due to warmer condiLons and greater area of
shallow shelf seas.

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Allochems are equivalents of grains in clasLc sediments. Material between the
allochems is equivalent to the matrix or cements in clasLcs.

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Skeletal grains are any biological component of a limestone. The character of the
skeletal grains with vary depending on the environment AND geological period of
deposiLon of a rock.

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For example crinoids (stalked echinoderms) were very important components of
Devonian and Carboniferous shallow marine segngs and significant limestone
producers. They are much less important today and generally restricted to deep
marine segngs in relaLvely low numbers. You can see some of the important reef
producers in this figure - no, you don’t have to learn all of this figure for the exam.

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a) Ooids = < 2 mm in diameter and spheroidal. They are referred to as "coated”
sedimentary grains meaning they are concentrically layered. They are most
commonly composed of calcium carbonate, but can also be formed by iron or
phosphate-based minerals.
Environment of deposiLon - usually in shallow tropical seas (eg Bahamas or Persian
Gulf). Pisoids are ooids >2 mm in diameter.

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Ooid form as concentric layers of material are precipitated around a nucleus such as
a shell fragment or grain of sand. Today, most ooids are composed of aragonite. Most
ancient ooids are composed of calcite forming either by direct precipitaLon during
periods of calcite seas or via the alteraLon of ooids that were originally aragonite.
The formaLon of an ooid is thought to require the elevaLon of the saturaLon of
calcium carbonate increases… this can occur when water is warm and / or agitated
which will drive off CO2 and help precipitate calcite.

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b) Oncoids - spherical concentrically layered coated grains formed by photosyntheLc
cyanobacteria that grow around a nucleus such as a shell fragment. Oncoids, like
stromatolites are layered associaLons of cyanobacteria but form spherical bodies
rather than columns or domes of stromatolites. As oncolites form from
photosyntheLc cyanobacteria they form within the phoLc zone (depth to which
sunlight can penetrate to the ocean floor). Oncolites rarely exceed 10 cm in diameter.

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c) Peloids are thought to be the fecal pellets (poop) of organisms. If they occur in high
concentraLons and become a liple squashed they can give the impression of a rock
composed enLrely of micrite (carbonate mud).

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d) Intraclasts are irregularly-shaped allochems. Form by erosion of parLally lithified
sediment which is then re-deposited as irregularly shaped grains (syndeposiLonal).

e) Limestones may also contain non carbonate rock fragments, quartz and clays but
collecLvely these must be less than 50% of the rock if it sLll to be classified as a
limestone.

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Micrite = ”CaCO3 mud” composed of clay sized crystals. In todays oceans micrite
commonly forms as calcareous algae skeletons breakdown. The exact mode of
formaLon of ancient micrite is uncertain.

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We will deal with limestone cements in more detail during the lecture on diagensis.

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Developed by Robert L. Folk – uses a descripLon of the three main components of
carbonate rocks: allochems (grains), matrix (mostly micrite), and cement (sparite). A
two part naming system is used – the first name refers to the grains and the second
refers to the matrix or cement. The folk scheme is commonly used when describing
thin secLons of carbonate rocks.

You can have more than one type of allochem named in the Folk classificaLon. EG:

• Oolites + Fossils + Spar matrix = Oo bio sparite = Oobiosparite.

• Pellets + Oolites + Fossils + Micrite matrix = pel oo bio micrite = Peloobiomicrite.

The system can also be used if there is both matrix and cement EG:
Fossils + Spar matrix + Micrite matrix = bio spar micrite = Biosparmicrite

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The Dunham system classifies using deposiLonal textures. Rocks are divided into four
main groups based proporLons of allochems and whether or not they were originally
in contact with each other. If not originally in contact = mud-supported. If originally in
contact = grain supported. As the Dunham scheme deals with texture rather than
grain composiLon it is more useful when describing hand samples.

- Mudstone: <10% grains supported by a lime mud.

- Wackestone: >10% grains, supported by a lime mud.
- Packstone: contains lime mud, but is grain supported.
- Rudstone: coarse limestones supported by grains >2 mm.
- Grainstone: lacks mud, and is grain supported.
- Boundstones: original components have been bound together arer deposiLon
such as in a coral reef

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Limestone deposiLon has a complex and varied history. Will here provide a VERY
brief overview of some of the environments of deposiLon of limestone and some of
the main limestone types that characterize them.

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Although carbonates can form in a number of terrestrial environments, we will
mostly concentrate on the lacustrine environments here.
i. Inorganic carbonates: limestones (micrites) precipitated by loss of CO2 due to
evaporaLon (water warms) in a shallow environment (low pressure). Fresh water
entering into a saline lake can also help precipitate carbonates. If the water is
agitated, ooids may also form.
ii. Algal carbonates. Can form when you get blooms of calcareous algae in a lake
but also may form in stromatolites as you can see in the image above
iii. Ooids and skeletal sands. Fragments of algae or larger carbonate secreLng
organisms like mollusks.

A common facies papern in carboante precipitaLng lakes would be stromatolite

“reefs” in shallow areas (with ooids and skeletal sands in the more agitated area)
passing into finer micrites in the deeper / quieter segngs

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Ancient marine carbonate environments were much more extensive than they are
today. In general though carbonate facies paperns can be divided into a broad
papern of shore (Ldal flat in the above) – platorm or shelf (both deep and shallow)
– basin.

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Intermipently covered with water with week currents and wave acLon. Sediments
are micrite dominated and may contain pellets. The sediments in these environments
may exhibit fenestrae, gaps within the sediment that may represent desiccaLon and
shrinkage of material with the sediment or the trapping of gas bubbles. These spaces
are then filled wither with sediment or cement. Mud cracks may also be a feature of
these sediments.

The fossils in these environments will generally be of restricted diversity as Ldal

environments are a stressful environment to inhabit given daily changes in water
level and potenLally salinity. In part due to this (as there will be fewer grazing
organisms) stromatolites and algal mats may be common. In very arid environments,
intense evaporaLon may permit the development of evaporites in this environment.
In addiLon, this evaporaLon may draw water through the sedimentary pile and cause
the dolomiLzaLon of some of the calcium carbonate (See later).

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Similar lithologically to the previous environment but without evidence of exposure
to the air. Green calcareous algae are the main sediment contributors in this
environment. Such lagoonal limestones are common in the geological record.

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Occur in areas of high Ldal acLvity and current / wave acLon. Depth of deposiLon
generally less than 10m forming beaches and shoals composed of ooids and rounded
skeletal fragments.

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These areas are generally poorly developed today when compared to the geological
past when extensive shallow (epeiric) seas where more common. Modern examples
would include the Yucatan shelf and the eastern Gulf of Mexico. Carbonates in this
environment are deposited in standard salinity / well oxygenated condiLons. The
sediments change in character from more grainstone towards the shoreline to more
micrite rich ocean-ward. These sediments are commonly heavily bioturbated. The
skeletal components are diverse. Patch reefs may be common, parLcularly towards
the outer shelf area.

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Reef is a term that is used in many contexts. For us we will consider a reef to be a
topographic wave resistant structure. Reef rocks are commonly classed as
biolithotes, framestones and bindstones and may contain considerable primary
porosity in the form of large caviLes represenLng the complex structure of the
framework of a reef. These caviLes may be filled with debris and cement. This
potenLal porosity and permeability makes them great petroleum reservoirs.

In front of this, bioclasLc material broken off from the reef will be deposited down
the reef as talus. In this area rudstones and floatstones may be found associated with
slump structures. It is also in this area that carbonate turbidites may be generated
that pass down the fore-reef and into the basin.

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Today corals and coralline algae are the most common reef producing organisms. In
the past the main reef producing organisms have been very different. During the
Precambrian and Cambrian stromatolites were the main reef producers.
Stromatoporoids ( a kind of sponge), rugose and tabulate corals are characterisLc of
lower paleozoic reefs… see the figure earlier in this lecture for more informaLon.

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Reefs can roughly be broken down into 3 zones:

1. Fore Reef and reef crest: The steep slope of the fore reef can be almost verLcal
with organisms construcLng the reef at the reef crest. Tallus will fall from the reef
and be deposited downslope someLme in the form of turbidites.

2. Reef Flat: No more than 1 – 2m of water depth, Major area of growth of reef
organisms

3. Back Reef: Will receive some debris from the reef flat and pass into lagoon
deposits

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The following taken from: hpp://www.nps.gov/gumo/naturescience/
geologicformaLons.htm
“Through much of the early and middle Permian much of modern-day New Mexico
and Texas occupied the western edge of Pangaea near the equator. A vast ocean
surrounded Pangea, but a narrow inlet, the Hovey Channel, connected the ocean
with the Permian Basin, an inland sea which covered parts of what is now northern
Mexico and the southwestern United States. The Permian Basin had three arms: the
Marfa, Delaware, and Midland Basins. The middle arm (the Delaware Basin)
contained the Delaware Sea which covered and area 150 miles long and 75 miles
wide over what is now Western Texas and Southeastern New Mexico. For several
million years the Capitan Reef expanded and thrived along the rim of the Delaware
Basin unLl events altered the environment criLcal to its growth approximately 260
million years ago. The outlet connecLng the Permian Basin to the ocean became
restricted and the Delaware Sea began to evaporate faster than it could be
replenished. Evaporites began to precipitate out of the vanishing waters and drir to
the sea floor, forming thin, alternaLng bands of mineral salts and mud.”

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Pelagic sedimentaLon takes over in these systems. Too deep for significant benthic
(bopom dwelling) organisms to thrive. May get some tallus and trubidites closer to
the reef front. Lower limit of deposiLon of sediment set by the CCD.

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Modern carbonate pelagic organisms are pteropods, coccoliths and formainifera.

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Good example of extensive pelagic limestone producLon: Cretaceous chalk seas.
Perfect for pelagic carbonates: warm climaLc condiLons, shallow seas flooded
conLnents providing extensive shelf environments far from any clasLc sources.

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Dolomite: calcium magnesium carbonate CaMg(CO3)2. Rocks composed of dolomite =
dolostones. The proporLon of dolomite in a sediment will determine its overall
classificaLon:

Limestone: 0 – 10% dolomite in the rock

DolomiLc limestone: 10 – 50% dolomite
CalciLc Dolomite: 50 -90% dolomite
Dolomite: 90 – 100% dolomite

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Although the possibility of primary sedimentaLon of dolomite has been proposed it is
mostly regarded as forming as the alternaLon of lime (CaCO3) sediments during early
diagenesis or the alteraLon of limestones in deep burial segng. In this image,
diamond-shaped crystals of dolomite have grown arer deposiLon, during diagenesis.
They replace the fine calcium carbonate mud (dark material in the photo) that makes
up the rest of the rock. Because of the coarse nature of dolomite crystals much of the
original textures of limestones are lost during ‘dolomiLzaLon.’

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