Chapter 6: Carbonate Sedimentary Rocks

There are two main categories of carbonate rocks: Calcite (CaCO 3 ) Dolomite (CaMg(CO 3 ) 2 ) Both Calcite and Dolomite will contain varying amounts of Calcium and Magnesium. Other elements make up only trace amounts. CalciteDolomite

Calcite Group Iceland Spar CaCO 3 Rhodochrosite MnCO 3 Magnesite MgCO 3 Siderite FeCO 3 Smithsonite ZnCO 3

Dolomite Group Dolomite CaMg(CO 3 ) 2 Ankerite Ca(Mg,Fe,Mn)(CO 3 ) 2

Aragonite Group Aragonite CaCO 3 Strontianite SrCO 3 Cerussite PbCO 3 Witherite BaCO 3

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. 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: CO 2 + H 2 O ↔ H 2 CO 3 (carbonic acid) H 2 CO 3 ↔ H + + HCO 3 - (bicarbonate ion) HCO 3 - ↔ H + + CO (carbonate ion) __________ H 2 O + CO 2 + CaCO 3 ↔ Ca HCO 3 - (where the CaCO 3 can be either Calcite or Aragonite)

Water condition Direction of change Directed effectEffect on CaCO 3 solubility Kind of CaCO 3 precipitated TemperatureIncreaseLoss of CO 2, increase in pH More likely to precipitate Micrite or ooids PressureDecreaseLoss of CO 2, increase in pH More likely to precipitate Micrite or ooids SalinityDecreaseDecrease in activity of “foreign cations” More likely to precipitate Micrite or ooids Principle factors that affect inorganic precipitation of CaCO 3 in water (Table 6.4 pg. 175)

Organic activity and CaCO 3 precipitation Extraction of CaCO 3 from water Growth of shells and tests Photosynthesis Removes CO 2 from water, thereby increasing pH. Decay of soft tissue Increases pH of water Feeding, sediment ingestion Reshapes sediment Bacterial activity Promotes CaCO 3 precipitation

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. Ca 2+ (aq) + Mg 2+ (aq) + 2CO (aq) = CaMg(CO 3 ) 2 (solid) 2CaCO 3 (solid) + Mg 2+ (aq) = CaMg(CO 3 ) 2 (solid) + Ca 2+ (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.