Chapter 6: Carbonate Sedimentary Rocks

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Chapter 6 Carbonate Sedimentary Rocks
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• 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
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Calcite Group
Iceland Spar CaCO3
Rhodochrosite MnCO3
Siderite FeCO3
Smithsonite ZnCO3
Magnesite MgCO3
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Dolomite Group
Ankerite Ca(Mg,Fe,Mn)(CO3)2
Dolomite CaMg(CO3)2
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Aragonite Group
Aragonite CaCO3
Strontianite SrCO3
Cerussite PbCO3
Witherite BaCO3
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• 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

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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.
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Ooid
Aggregate grain (Grapestone)
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Microcrystalline calcite (Micrite) versus Sparry
calcite
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Classification of Carbonate Rocks
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Limestone classification based on textures
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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)
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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 activity of foreign cations More likely to precipitate Micrite or ooids
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• 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

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Calcite versus Aragonite
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Dolomite Classification and Variation
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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 Earths surface. Ca2(aq)
Mg2(aq) 2CO32-(aq) CaMg(CO3)2(solid) 2CaCO3(s
olid) Mg2(aq) CaMg(CO3)2(solid) Ca2(aq)
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Sabkha Environment
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Carbonate Diagenesis
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Carbonate Diagenesis continued
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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.