Chapter 6 carbonate sedimentary rocks

Chapter 6: Carbonate Sedimentary Rocks


There are two main categories of carbonate rocks:
•

Calcite (CaCO3)

•

Dolomite (CaMg(CO3)2)

Both Calcite and Dolomite will contain varying amounts of Calcium
and Magnesium. Other elements make up only trace amounts.

Calcite

Dolomite



Calcite Group

Iceland Spar CaCO3
Rhodochrosite MnCO3
Siderite FeCO3

Magnesite MgCO3

Smithsonite
ZnCO3

Dolomite Group

Dolomite CaMg(CO3)2

Ankerite Ca(Mg,Fe,Mn)(CO3)2


Aragonite Group

Aragonite CaCO3

Strontianite SrCO3

Cerussite PbCO3

Witherite BaCO3


Limestone (composed of primarily CaCO3) textures
Carbonate Grains
• Carbonate clasts (extraclasts & intraclasts--Lithoclasts)
•Extraclast: derived from older limestone located outside the depositional
environment.
•Intraclast: derived from seafloor, adjacent tidal flats or a carbonate beach
•Lithclast: a nonspecific term used when the distinction between extra &
intraclast cannot be made.

• Skeletal particles

• Ooids
• Peloids
• Aggregate Grains
Matrix cements: either sparry calcite or micrite


A: Rounded
clasts cemented
by sparry calcite.
B: Angular
clasts in micrite.
C: Fossiliferous
limestone with
sparry cement.
D: Normal ooids
cemented with
sparry.
E: Radial ooids
cemented with
sparry &
micrite.
F: Pellets
cemented with
sparry.


Ooid

Aggregate grain (Grapestone)

Microcrystalline calcite (Micrite) versus Sparry calcite


Classification of Carbonate Rocks


Limestone classification based on textures


Origin of Carbonate Rocks
Limestone:
CO2 + H2O ↔ H2CO3 (carbonic acid)
H2CO3 ↔ H+ + HCO3- (bicarbonate ion)
HCO3- ↔ H+ + CO32- (carbonate ion)
__________
H2O + CO2 + CaCO3 ↔ Ca2+ + 2HCO3(where the CaCO3 can be either Calcite or Aragonite)


Principle factors that affect inorganic precipitation of
CaCO3 in water (Table 6.4 pg. 175)
Water
condition

Direction of
change

Directed effect

Effect on

CaCO3
solubility

Kind of
CaCO3
precipitated

Temperature

Increase

Loss of CO2,
increase in pH

More likely to
precipitate

Micrite or
ooids

Pressure

Decrease

Loss of CO2,
increase in pH

More likely to
precipitate

Micrite or
ooids

Salinity

Decrease

Decrease in
More likely to
activity of
precipitate
“foreign cations”

Micrite or
ooids



Organic activity and CaCO3 precipitation
•Extraction of CaCO3 from water
Growth of shells and tests
•Photosynthesis
Removes CO2 from water, thereby increasing pH.
•Decay of soft tissue
Increases pH of water
•Feeding, sediment ingestion
Reshapes sediment
•Bacterial activity
Promotes CaCO3 precipitation

Ceno-zoic

Era

Period

Dominate Carbonate
Mineral

Neogene-Quaternary

A + HMC
(Aragonite Sea)

Paleogene

Mesozoic

Cretaceous

Low-magnesian
Calcite (LMC)
(Calcite Sea)

Jurassic

Triassic
Permian
Pennsylvanian

Aragonite (A) + Highmagnesian Calcite
(HMC)
(Aragonite Sea)

Paleozoic

Mississippian
Devonian
Silurian

Ordovician

Cambrian

Low-magnesian
Calcite (LMC)
(Calcite Sea)

Calcite versus Aragonite


Dolomite Classification and Variation


Dolomite formation:
The Dolomite problem….
Scientists have not yet been successful in the laboratory in
precipitation perfectly ordered Dolomite (50% Calcium and 50%
Magnesium) at the normal temperatures and pressures of the Earth’s

surface.

Ca2+(aq) + Mg2+(aq) + 2CO32-(aq) = CaMg(CO3)2(solid)
2CaCO3(solid) + Mg2+(aq) = CaMg(CO3)2(solid) + Ca2+(aq)


Sabkha Environment


Carbonate Diagenesis


Carbonate Diagenesis continued…



Stylolites: a pressure-solution
feature common in carbonate
rocks. These features are often
associated with clay minerals
and other fine-size noncarbonate minerals that
accumulate as carbonate
minerals dissolve.