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Chapter 6: Carbonate Sedimentary Rocks

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. Calcite. Dolomite.

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Chapter 6: Carbonate Sedimentary Rocks

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  1. Chapter 6: Carbonate Sedimentary Rocks

  2. 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

  3. Calcite Group Iceland Spar CaCO3 Rhodochrosite MnCO3 Siderite FeCO3 Smithsonite ZnCO3 Magnesite MgCO3

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

  5. Aragonite Group Aragonite CaCO3 Strontianite SrCO3 Cerussite PbCO3 Witherite BaCO3

  6. 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

  7. 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.

  8. Ooid Aggregate grain (Grapestone)

  9. Microcrystalline calcite (Micrite) versus Sparry calcite

  10. Classification of Carbonate Rocks

  11. Limestone classification based on textures

  12. 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)

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

  14. 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

  15. Calcite versus Aragonite

  16. Dolomite Classification and Variation

  17. 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)

  18. Sabkha Environment

  19. Carbonate Diagenesis

  20. Carbonate Diagenesis continued…

  21. 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.

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