Chapter 6 carbonate sedimentary rocks

Chapter 6: Carbonate Sedimentary Rocks... There are two main categories of carbonate rocks:... Limestone composed of primarily CaCO 3 textures Carbonate Grains• Carbonate clasts extracl

Chapter 6: Carbonate Sedimentary Rocks

There are two main categories of carbonate rocks:

Dolomite Group

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

Limestone (composed of primarily CaCO 3 ) 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.

D: Normal ooids

cemented with sparry.

E: Radial ooids

cemented with sparry &

micrite.

F: Pellets

cemented with sparry.

Aggregate grain (Grapestone)

Microcrystalline calcite (Micrite) versus Sparry calcite

Classification of Carbonate Rocks

Limestone classification based on textures

Origin of Carbonate Rocks

-Water

condition Direction of change Directed effect Effect on CaCO3

solubility

Kind of CaCO3precipitated

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

activity of

“foreign cations”

More likely to precipitate Micrite or ooidsPrinciple factors that affect inorganic precipitation of

CaCO3 in water (Table 6.4 pg 175)

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

Calcite versus Aragonite

Era Period Dominate Carbonate Mineral

Aragonite (A) + magnesian Calcite

High-(HMC) (Aragonite Sea) Mississippian

Devonian Silurian

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 non-

carbonate minerals that

accumulate as carbonate

minerals dissolve