Carbonate Sedimentary Rocks

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Carbonate Sedimentary Rocks

Chapter 6

Carbonate Sedimentary Rocks

Introduction

Carbonate Mineralogy

Formation of carbonate sediments

Carbonate petrology

Diagnesis

Summary

Introduction

Carbonates cover 7% land surface Economic Importance

Greater than 50% oil and gas reservoirs worldwideare contained in carbonate rocks 70% giant oil fields in Cretaceous rudistid reefs (Middle East) In Western Canada, carbonate-hosted petroleum deposits

are primarily found in Devonian reefs

Carbonate deposits host Mississippi Valley type (Pb-Zn) ore deposits

Major economic importance as industrial "mineral"(agriculture stone, cement)

Most carbon on earth is stored in carbonatesediments, therefore plays a major role in theglobal carbon cycle


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Carbonate minerals include calcite,magnesite, dolomite, siderite, ankerite,

and others

Modern carbonate sediments consistprimarily of aragonite, but include somecalcite and dolomite


Calcite can contain several percent Mg

Low-magnesian calcite (calcite) < 4%MgCO3

High-magnesian calcite > 4% MgCO3 Dolomite CaMg(CO3)2 different

crystal structure than high-magnesiancalcite

MnCOMnCO33RhodochrositeRhodochrosite

SrCOSrCO33StrontianiteStrontianiteCaMn(COCaMn(CO33))22KutnohoriteKutnohoriteFeCOFeCO33Siderite,Siderite,

BaCOBaCO33WitheriteWitheriteCa(Mg,Fe)(COCa(Mg,Fe)(CO33))22AnkeriteAnkeriteMgCOMgCO33MagnesiteMagnesite

CaCOCaCO33AragoniteAragoniteCaMg(COCaMg(CO33))22DolomiteDolomiteCaCOCaCO33CalciteCalcite

formulaformulamineralmineralformulaformulamineralmineralformulaformulamineralmineral

AragoniteGroupAragoniteGroup

(orthorhombic)(orthorhombic)

Dolomite GroupDolomite Group

(hexagonal)(hexagonal)

Calcite GroupCalcite Group

(hexagonal)(hexagonal)

Carbonate Minerology


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Modern oceans favour precipitation ofaragonite or high-mag calcite

Early Plaeozoic and middle-lateCenozoic favoured precipitation of (low-mag) calcite, because of lower Mg/Caratios at those times

Carbonate ReactionsH2O + CO2 + CaCO3 Ca

2+ + 2HCO3

Carbon dioxide dissolved in waterproduces carbonic acid, which in turndissolves calcium carbonate (eitheraragonite or calcite)

However, when bicarbonate and calciumare in sufficient supply, calcium carbonatecan precipitate

Carbonates and Biology Extraction of Ca+2 and HCO3- from seawater to

precipitate skeletal carbonate (grains)

Photosynthesis removes CO2 from water andincreases pH, which in turn promotes calciumcarbonate precipitation

Decay of soft tissues can increase alkalinity,increasing pH and promoting carbonateprecipitation

Feeding/sediment ingestion reshapes sediment,generating pellets

Bacterial activity catalyzes calcium carbonateprecipitation


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Formation of CarbonateSediments

Environment must be just right

Depth, temperature, salinity, clastic influx,

nutrients, etc.

Goldilocks window (Goldhammer et al.1990)

Generally marine environments

Most carbonate sediments are producedbiologically or by biochemical mediation

Global Distribution of Reefs

Boggs 2001


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Carbonates and Salinity

Jones and Desrochers 1992

Minerology of

SkeletalOrganisms

Scoffin 1987


Four types of carbonate particles: Precipitates direct or biologically

mediated precipitation (e.g., ooids)

Bioclasts shells, tests, spicules etc. ofinvertebrates, microbes and algae

Peloids microcrystalline carbonate,generally agglutinated feces ordiagenetically altered grains

Lithoclasts fragments of consolidated orlithified sediment


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SedimentProduction

Controls on distribution of carbonates:

Environmental tolerances of skeletonproducing organisms

Concentration of necessary components(Ca+2, CO3-2 )

Dilution of sediment production by

siliciclasticsAbundant mud can shut down carbonate-

secreting organisms (e.g., filter feeders)


Sediment composition is fundamental incharacterizing the depositional environment

Grain size not necessarilya function of hydraulicregime Commonly is, but some carbonates behave like clastic

sediments (e.g., grainstones, carbonate turbidites)

Platforms, banks formed in situ: self-generatingand self-sustaining

Temporal and spatial style of accumulationdepends upon nature of the sedimentsthemselves



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Marine Environments

CalcareousAlgae


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Molluscs

Forams

Bryozoans


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Stromatolites

Crinoids

Carbonate Production through Time

Boggs 2001


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Carbonate rocks contain many of the sametypes of sedimentary structures present in

siliciclastic rocks E.g., cross-bedding, cross-lamination, parallel

lamination, trace fossils, etc.

Other structures include stromatolites,teepee structures, etc.


Dunes of oolitically coated peloids

Cross-bedded Pleistocene grainstones


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Petrology of Carbonate Rocks

Grains (Skeletal)

Mud (Micrite)

Cement


Calcite is present in at least threetextural forms in a limestone

Carbonate grains

Carbonate mud

Sparry calcite


Carbonate grains aggregate particlesor grains that may have undergonemechanical transport before deposition

Allochems

Silt size or larger

Skeletal particles, ooids, etc.


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Carbonate mud (Microcrystalline calcite)

Modern environments generally needle-

shaped aragonite crystals

Ancient limestones have similar-sizeparticles made of calcite

May also include some siliciclastic grains(clays, quartz, etc.)

Micrite


Sparry Calcite

Relatively large crystals of calcite, generally0.02 -> 0.1 mm

White in hand specimen or in plane-polarized light

Product of diagenesis

Pore-filling cement Replacement


Mineralogy plays a small role inclassification -> most carbonates aremonomineralic

Mineralogy used to distinguish dolomite(dolostone) from limestone or carbonatesfrom non-carbonates


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When studying carbonate rocks, we firstdistinguish them on the basis of chemical

composition, then focus on texture Different from siliciclastic sedimentary rocks

Sedimentary Rocks

ClasticChemical/

Biochemical

Composition

Particle Size(Carbonates)

Particle Size

Composition

Sedimentary Rocks

ClasticChemical/

Biochemical

Composition

Particle Size(Carbonates)

Particle Size

Composition


Dunham (1962; and modifications byEmbry and Klovan, 1972)

Based on depositional texture:

Grain packing and relative abundance ofallochems to micrite

Depositional binding of grains

Carbonate ClassificationModified Dunham


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Folk (1959, 1962)

Based on relative abundance ofallochems, micrite, sparry calcitecement

Carbonate Classification (Folk)

Coral Boundstone or Framestone


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Crinoidal Packstone

Crinoidal Wackestone


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Oolitic Grainstone

Gastropod Packstone

Mudstone (micrite)


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Dolomite/dolostone

Composed of > 50% of the mineral

dolomite

Abundant from Precambrian to Holocene

Some are obviously diagenetically alteredlimestones (see Diagenesis)

Origin of fine-grained dolostones remainselusive dolomite problem


Dolomite/dolostone

Several models for early-formed dolomite

Hypersaline model (sabkhas, etc.)

Mixing-zone model

Seawater model

Boggs 2001


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Diagenesis

After deposition, carbonate sedimentsare subjected to a variety of diagenetic

processes Changes in porosity, mineralogy, chemistry

Carbonate minerals more susceptible todissolution, recrystallization,replacement than most siliciclasticminerals

Diagenesis

Carbonate minerals may experiencepervasive alteration of mineralogy

E.g., aragonite->calcite, dolomitization

These changes can alter or destroyoriginal depositional textures

Porosity may be reduced or enhanced

Carbonate Cements


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Summary

Calcite, aragonite and dolomite aremost common carbonate minerals

Environmental conditions need to bejust right for deposition of carbonatesediments Salinity, temperature, water depth, etc.

Most carbonate sediments producedbiologically or by biochemical mediation

Summary

Limestones consist primarily of grains(allochems), micrite and sparry calcite

Four types of carbonate grains:lithoclasts, skeletal particles,precipitates, peloids

Modified Dunham classification uses

(primarily) relative proportion of grainsand micrite

Summary

Dolostone (dolomite rock) consists of>50% dolomite

Different origins possible

Diagenesis can dramatically affectmineralogy, porosity, texture ofcarbonate rocks

Carbonate Sedimentary Rocks

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