Carbonate sedimentary rocks form from the precipitation of minerals from water through chemical or biochemical processes. They include limestones composed of calcite and dolostones composed of dolomite. Carbonate rocks are classified based on their components, including allochems like skeletal grains and non-skeletal grains, and orthochems like micrite and spar. Robert Dunham's classification system categorizes carbonate rocks based on depositional texture as mudstones, wackestones, packstones, or grainstones, indicating the energy of the depositional environment.

1. Classification of sedimentary rocks-
Autochthonous sediments: Carbonate
sedimentary rocks
SUBMITTED BY: SUSHANT ADHIKARI
ROLL NO-19
MSC GEOLOGY, 1ST SEMESTER

2. CONTENT OVERVIEW:
• Introduction to sedimentary rock
• Types of sedimentary rocks
• Autochthonous sediments
• Carbonate sedimentary rocks
• Components
• Classification
• Limestone diagenesis and porosity evaluation
• Dolomite diagenesis
• Importance of carbonates rocks

3. INTRODUCTION
•Sedimentary rocks are formed when sediment is
deposited out of air, ice, wind, gravity, or water flows
carrying the particles in suspension.
•The most important geological process that leads to
the creation of sedimentary rocks are erosion
,weathering, dissolution , precipitation and
lithification.
•Figure.1- Showing process of formation of
sedimentary rocks.

4. TYPES OF SEDIMENTARY ROCKS
•Sedimentary rocks can be organized into two categories.

6. Autochthonous sediments
•Autochthonous is a term that refers to sediments that are
found in the same place where they were formed or in a
location very close to its site of deposition.
•In simple word the deposited rock have no any
displacement from the deposited area.
•Autochthonous sediments are produced locally and
include biogenic sediment such as carbonate (e.g. shell,
foraminifer and coral fragments) and silica (e.g. sponge
spicules and diatoms).
Figure.2- A conceptual model for the
allochthonous and autochthonous sources for
the high altitude lacustrine sediments.

7. Carbonate sedimentary rocks
•Chemical/biochemical sedimentary rocks originate by precipitation of minerals from
water through various chemical or biochemical processes.
•Carbonate rocks are intrabasinal in origin
•Biochemical process includes the accumulation of fossils, the activity of organisms, and
other inorganic processes – all involving dissolved carbonates and water.
•They are distinguished from siliciclastic sedimentary rocks by their chemistry, mineralogy,
and texture.

8. CON’T
•The carbonate rocks make up 10 to 15% of sedimentary rocks. They largely consist of two types
of rocks and these two are most abundant Carbonate rocks.
a) Limestones which are composed mostly of calcite (CaCO3) or high Mg calcite [(Ca,Mg)CO3],
and
b) Dolostones which are composed mostly of dolomite [CaMg(CO3)2]
•They are present in many Precambrian assemblages and in all geologic systems from the
Cambrian to the Quaternary.
a. Precambrian-Paleozoic - Dolomite
b. Mesozoic and Cenozoic Carbonates –Limestone
•Another common carbonate rock, containing a mixture of fine-grained calcite and terrigenous
mud, is marl.

9. Minerology of carbonate Sediment
•The elemental chemistry of carbonate rocks is dominated by
calcium ( Ca 2+), magnesium ( Mg2+), and carbonate (C032 -)
ions.
•Calcium and magnesium are present in both limestones and
dolomites; however, magnesium is a particularly important
constituent of dolomites
•Calcium carbonates occurs in two mineral i.e.
•i)Aragonite and ii) Calcite
•Aragonite crystallizes in the orthorhombic crystal system, while
Calcite is rhombohedral. Calcite forms an isomorphous series
with magnesite (MgCO3).

10. • The principal carbonate minerals are the calcium
carbonates, calcite and its unstable polymorph
aragonite; and dolomite, calcium magnesium
carbonate. Modern carbonate sediments are
composed of both aragonite and calcite.
• Only calcite, the more stable variety, occurs in
lithified limestones. Dolomite does not occur as a
biogenic skeletal mineral.

12. Components
There are two components of carbonate rocks
1. Allochemicals-
An allochems is a carbonate particle that was formed outside of the depositional area and
transported in, hence a carbonate "clast.“
2. Orthochemicals
Carbonate sediments that form within the depositional area represent the rock cement or
matrix
Orthochems binds allochems together and lithify the sediments

13. Allochems
Allochems are subdivided into two kinds.
1. Skeleton (Biogenic Grains)
2. Non Skeleton
1. Skeleton
The skeleton components includes all the bioclast of a carbonate –secreting invertebrates .These
invertebrates includes Foraminifera's, Mollusca, Gastropods .

14. Figure-3 Showing typical thin section and appearance of
skeletal components

15. 2. Non skeleton
Four major types on Non skeleton grains are
recognized
a) Various coated grains (ooids, Piiods, Oncoids)
b) Peloids
c) Clumped or aggregated grains(lumps, grape
stones) and
d) Limestone clasts(limeclasts)
Figure.4- Showing principal non skeletal components

16. a) Various coated grains
Ooids (Oolith)
Size range- 1-5mm(diameter)
Ooid are coated carbonate grains that contain a
nucleus of some kind-a shell fragment, pellet, or
quartz grain-surrounded by one or more thin
layers or coatings (the cortex) consisting of fine
calcite or aragonite crystals.
Ooids form where strong bottom currents and
agitated-water including tidal sand bars or tidal
deltas between barrier islands conditions exist
and where saturation levels of calcium
bicarbonate are high.
modern ooids -aragonite
ancient ooids -calcite.
Ooids with asymmetric coatings and
superficial oolites form in quiet water.
Figure.5- Oolith in thin section.

17. Pisolites, oncoids, and oncolites are enveloped
by irregular layers. All these grains are
frequently larger than ooids and commonly
are over a centimeter in diameter.
Pisolites form by the precipitation of calcium
Carbonate around nuclei trapped in sediment
within the vadose zone of soils or marine tidal
flats
Oncoids form on the surface of intertidal and
supratidal flats where Carbonate precipitates
from salt water spray and marine flood
waters.
A rock composed of oncoids is termed an
"oncolite."

18. Figure.6- showing Pisoids and oncoids in thin section

19. b)Peloids
Fine grained (0.1-0.5mm) sand to silt sized
clast of microcrystalline carbonate which
lacks coherent internal structure
Peloids are formed in shallow marine low-
energy platform carbonate settings
Most common peloids (pellets) are fecal
pellets of waste matter generated by such
organisms as fish and shrimps
Others are produced by the micritization of
other kinds of allochems: ooids, oncoids,
interclass
Figure.7- Showing Fecal pellets

20. c)Clumped or aggregated grains(lumps, grape stones)
Grains aggregates are formed when carbonate particles such
as ooids and peloids attached to one another.
Figure.8- showing grape stone in thin section
d)Limestone clasts(limeclasts)
fragments of earlier-formed limestone, mostly
intraclasts from a local source.
Figure.9- showing limeclasts in thin section
d)Limestone clasts(limeclasts)
fragments of earlier-formed limestone, mostly
intraclasts from a local source.

21. Figure.10- Showing Depositional Environment of ooids and non skeletal
materials

22. 1. Micrite
Size-0.03-0.04 mm in diameter
Folk (1 959) proposed the contraction micrite for microcrystalline calcite, a term that has been
universally adopted to signify very fine grained carbonate sediments
Orthochemicals
• Calcium carbonate mud occurring as
a matric
• Translucent under microscope, dull
brown
• produced form biochemical ooze or
attrition of shells

23. 2. Spary calcite or spar
•Size- 0.02-0.1mm
•Spar is carbonate cement
•Crystals of spar are generally coarser than micrite
•Under the microscope, spar is crystal clear without the hazy brownish cast of micrite
Neomorphism- the various diagenetic processes of recrystallization and replacement ,including
changes in minerology- is very common in carbonate rocks.
Yesterday’s micrite can become today’s spar and vice versa

24. Figure.11- showing micrite and spar

25. Classification
Three classification schemes are in common use by those who work on carbonate rocks
a) A very simple but often useful scheme divides limestones on the basis of grain size into
calcirudite (most grains >2mm), calcarenite (most grains between 2mm and 62mm) and
calcilutite (most grains less than 62mm
b) The classification scheme of R.L. Folk( Folk’s Classification)
Based mainly on composition, distinguishes three components:
I. the grains (allochems),
II. matrix, chiefly micrite and
III. cement, usually drusy sparite.

26. •An abbreviation for the grains (bio —skeletal
grains, oo —ooids, pel —peloids, intra —
intraclasts) is used as a prefix to micrite or
sparite, whichever is dominant. Terms can be
combined if two types of grain dominate, as
in biopelsparite or bio-oosparite.
•Another term introduced by Folk’s is
biolithite, referring to a limestone formed in
situ, such as a stromatolite or reef-rock; and
dismicrite, referring to a micrite with cavities
(usually sparfilled).
•Folk also indicated that the textual structure
of limestone had matured.
•Folks Classification is used only in lab.
Figure.12- showing Classification of limestones based on composition

27. Figure.13- showing Classification of limestones based on
composition and textural parameters and implied depositional
conditions

28. C). The classification of R.J. Dunham ( Dunhams Classification)
Classification based on depositional texture
The three criteria used to define the original Dunham classes were:
•the supporting fabric of the original sediment
•the presence or absence of mud (the fraction <20 μm in size
•Evidence that the sediments were organically-bound at the time of deposition.

29. Based on these criteria defined four classes.
Mudstone- a mud-supported carbonate rock containing <10% grains. Generally
indicates calm water and apparent inhibition of grain-producing organisms (low-energy
depositional setting).
Wackestone- a mud-supported carbonate rock containing >10% grains. Generally
indicates calm water and restriction of grain-producing organisms (low-energy
depositional setting). In cases where grains are exceptionally large, Embry and Klovan
(1971) designated these carbonates “floatstones.”
Packstone- a grain supported fabric containing 1% or more mud-grade fraction. Embry
and Klovan (1971) designated these carbonates “rudstones.”
Grainstone- a grain-supported carbonate rock with <1% mud. hey generally are
deposited in moderate- to high-energy environments, but their hydraulic significance can
vary

30. Later two additional classes were added within this scheme:
Boundstone- Carbonate rocks showing signs of being bound during deposition (Dunham,
1962). Embry and Klovan (1972) further expanded the boundstone classification on the basis
of the fabric of the boundstone.
i)Framestone – Organism builds a rigid framework
ii)Bindstone- organism encrust and bind loose sediments together
iii)Bafflestone- the organisms do not form a framework or bind the sediments together
but provide protected areas for the sediment to accumulate by baffling the currents.
Boundstones generally are deposited in higher energy environments, where currents can
provide nutrients to the organisms that form the boundstone, as well as carry away waste
products.
Crystalline carbonates: Carbonate rocks that lack enough evidence of depositional texture to
be classified. Extensive dolomitization commonly obliterates the original depositional texture.

31. Figure.14- showing Classification of limestones based on depositional textural

32. Limestone diagenesis and porosity evaluation
Diagenesis is the hardening of loose sediment into sedimentary rock, so in the case of
carbonate sediments – skeletons that make up carbonate sediments
The diagenesis of carbonates involves many different processes and takes place in near-surface
marine and meteoric environments, down into the deep-burial environment.
Six major processes can be distinguished:
a. Cementation,
b. Microbial micritization,
c. Neomorphism,
d. Dissolution,
e. Compaction and
f. Dolomitization

33. a . Cementation
Major diagenetic process
Infilling of primary voids in or between particles, or
of solution cavities by chemically precipitated
cements.
Sediments compact together in this way and result
in the formation of Carbonate rocks.
i) Early diagenetic cement formation ("cement A")
At the expense of metastable carbonate minerals
(e.g., aragonite in skeletal grains)
By evaporation of pore water rich in carbonates, in
the supratidal zone
On the ocean floor
ii) Late diagenetic cement formation ("cement B")
Cement formed after sediment consolidation or
after compaction
Figure.15-Showing Principal kinds of cements that
form in carbonate rocks during diagenesis

34. b. Microbial micritization
The formation of micrite by the boring into skeletal carbonate particles by
cyanobacteria(blue-green algae), and the subsequent precipitation of micrite within the
borings.
c. Neomorphism
This term was introduced by Folk (1965: 21) as a "comprehensive term of ignorance" for all
mineral transformations in which the mineral either remains intact or is converted into a
polymorphous mineral.
◦ i) Coalescive neomorphism
◦ Larger crystals grow at the expense of smaller crystals ("aggrading" neomorphism, or small crystals grow
within a large crystal ("degrading" neomorphism)
◦ ii) Transformation (inversion) of aragonite to calcite by solution and in-situ precipitation in an aqueous
environment.
◦ Homoaxial transformation
◦ Heteroaxial transformation:

35. iii)Recrystallization (Folk's terminology, 1965!)
Growth of unstrained crystals at the expense of strained
crystals of the same mineral, and increasing pressure and
temperature conditions (metamorphism)
d) Dissolution
Processes in which carbonate is selectively dissolved, e.g.,
in clay seams and secondary porosity occurs.
e) Compaction
Compaction takes place during burial, resulting in a closer
packing of grains, their fracture and eventual dissolution
where in contact. Compaction are of two types.
Compaction
Mechanical Chemical
Figure.16-Showing recrystallization that
destroyed fossil fragments

36. •Mechanical compaction in grainy sediments
leads to a closer packing of the grains and a
rotation of elongate bioclasts towards the
plane of the bedding
•Chemical compaction is the result of
increased solubility at grain contacts and
along sediment interfaces under an applied
stress.
Due to chemical composition three
kind of textures are developed
fitted fabrics, stylolites and pressure-
dissolution seams
Figure.17- Showing Irregular boundary (arrow)
between two echinoderm fragments formed as a
result of pressure solution (chemical compaction)

37. f) Dolomitization
Dolomitization is a major alteration process for many limestones and the dolomite, CaMg(CO3)2, may
be precipitated in near-surface and burial environments.
Dedolomitization
Dolomite may be replaced by calcite to produce limestone again. This calcification process is
referred to as dedolomitization and predominantly takes place through contact with
meteoric waters.
Silification
Like dolomitization, can take place during early or late diagenesis. It takes the form of
selective replacement of fossils or the development of chert nodules and layers.

38. Three major diagenetic environments are
distinguished
a) Marine
Diagenesis takes place on and just below
the sea-floor in both shallow and deep
water, and in the intertidal–supratidal zone
b) near-surface meteoric
Near surface diagenetic can affect a
sediment soon after it is deposited if there
is shoreline progradation or a slight sea-
level fall, or it may operate much later when
a limestone is uplifted after burial.
c) Burial
Begins at a depth below the sediment
surface of tens to hundreds of meters, that
is, below the zone affected by surface
processes
Figure.18- Showing Carbonate diagenetic environments

39. Porosity evaluation
•Porosity in carbonate rocks, most commonly limestones and dolostones, is of great importance
to study since around half of world’s hydrocarbon reserves are made up of dolomite and
limestone.
•The porosities of Holocene carbonate sediments are very high: *40–75% and these higher values
are common in micritic limestones.
•High porosities are associated in the deep water facies that are mainly oozes and these have
both inter-and intra-particle porosities (Schlanger and Douglas 1974).
•The porosity-permeability relation in carbonates may or may not be linear
•Archi’s scheme (based on qualitative evaluation of texture and porosity)
•The Choquette-Pray scheme (utilizes depositional and diagenetic changes in the rock),
•the Lucia scheme (works on inter-relationship between porosity, permeability and the particle
size) etc

40. Carbonates possess both primary and secondary
porosities, which reduces with progressive burial
leading to increasing rigidity of the rock.
Primary Porosity includes
1. framework porosity, formed by rigid carbonate
skeletons such as corals, stromatoporoids and algae,
especially in reef environments;
2. interparticle porosity in carbonate sands,
dependent on grain-size distribution and shape;
3. porosity in carbonate muds provided by fenestrae
(birdseyes) and stromatactis
Figure.19 –Showing Porosity in Carbonates.

41. Secondary porosity includes:
1. Moulds, vugs and caverns formed by dissolution of grains and rock, commonly through
leaching by meteoric ground watersr, but also by basinal (connate) water
2. Intercrystalline porosity produced through dolomitization
3. fracture porosity, formed through tectonic pressures, and through collapse and brecciation
of limestone as a result of dissolution
Primary porosity, and also secondary, is commonly facies controlled.

42. Table –Showing Summary of the Main Diagenetic Processes in Carbonate Rocks and Their
Effects on the Amount and Type of Porosity

43. Dolomite diagenesis
Dolomite is a rhombohedral mineral, CaMg(CO3 ) 2
Dolostone is the appropriate term for a rock composed of that mineral.
Carbonate rocks are divided on the basis of dolomite content into:
limestone 0–10% dolomite
dolomitic limestone 10–50% dolomite
Calcitic dolomite 50–90% dolomite
dolomite (dolostone) 90–100% dolomite

44. Dolomitization process by which limestone is altered into dolomite when limestone comes into
contact with magnesium-rich water, the mineral dolomite, calcium and magnesium carbonate,
Dolomite forms in super saline environment where Mg : Ca ratios exceed this value. It is
noteworthy, however, that dolomite may form at the expense of calcite for Mg: Ca ratios less
than 1:1 if the salinity is very low (Folk and Land, 1974).
Two main types of dolomite Primary and Secondary .
Primary Dolomites
Those which are formed at the time of deposition.
Secondary Dolomites
Secondary dolomites are defined as those that are obviously of post depositional origin. This is
clearly shown by the way in which such dolomites have an irregular distribution, discordant to
bedding and cross-cutting sedimentary structures

45. The origin of dolomites and dolomitization models
There is still much debate and argument over the origin of dolomite, particularly concerning the
pervasive dolomitization of extensive limestone platforms.
Ancient dolomite five wide-ranging classes of dolomitization models are presently existing which
are given below:
a. Evaporative Dolomitization
b. Seepage-reflux Dolomitization
c. Mixing-Zone Dolomitization
d. Burial Dolomitization
e. Seawater Dolomitization

46. a. Evaporation dolomite
Dolomite is in fact formed in high intertidal supratidal and sabkhas environment.
Dolomitic that formed in the supratidal environment are precipitated from evaporated sea
water.
Early formation of gypsum and aragonite resulting to a sophisticated Mg/Ca proportion of pore
water to enhance the formation of dolomite.
b. Seepage-reflux dolomitization
This process comprises the formation of dolomitizing solutions over vaporization of lagoon
water or tidal flat pore water besides then the succession of these solutions into nearby
carbonate rocks.

47. c. Mixing zone dolomitization
This type of dolomite formed by the mixing of seawater with
the fresh water. The source of water may be rainwater .
d. Burial dolomitization
Burial dolomitization involves prime mechanism which is the
dewatering of basinal mud rocks due to compaction and
removal of Mg-rich fluids into neighboring shelf edge.
e. Seawater dolomitization
Seawater itself can also be a source of dolomite because it
contains the sufficient amount of Mg ions with little
modification if a good pumping process exists.
Figure.20- Models for seawater dolomitization of
limestones, all basically different ways of pumping
seawater through a carbonate platform
 The widely accepted hypothesis of dolomitization is that limestone is transformed into dolomite by
the dissolution of calcite followed by dolomite precipitation.

48. Figure.21 –Showing Models of dolomitization, illustrating the variety of
mechanisms for moving dolomitizing fluids through the sediments

49. Importance of Carbonates rocks
1. Marine environments,
2. Palaeoecological conditions and the evolution of life form.
3. Agriculture,
4. Industrial purposes ,
5. Act as a reservoir rocks for more than 1/3 of the world’s petroleum reserves