Carbonates overview

GEOL 325 Lecture 4: Carbonates

GEOL 325: Stratigraphy & Sedimentary Basins

University of South CarolinaSpring 2005

Professor Chris KendallProfessor Chris KendallEWS 304 EWS 304

kendall@sc.edu kendall@sc.edu 777.2410777.2410

An Overview of CarbonatesAn Overview of Carbonates

Precipitated Sediments & Sedimentary Rocks

An Epitaph toAn Epitaph to

Limestones & DolomitesLimestones & Dolomites

Lecture Series OverviewLecture Series Overview sediment production types of sediment and sedimentary rocks sediment transport and deposition depositional systems stratigraphic architecture and basins chrono-, bio-, chemo-, and sequence stratigraphy Earth history

Sedimentary rocks are the product of the creation, transport, deposition, and diagenesis of detritus and solutes derived from pre-existing rocks.

Sedimentary Rocks Detrital/Siliciclastic Sedimentary Rocks

conglomerates & brecciassandstonesmudstones

Carbonate Sedimentary RocksCarbonate Sedimentary Rockscarbonatescarbonates

Other Sedimentary Rocksevaporitesphosphatesorganic-rich sedimentary rockschertsvolcaniclastic rocks

Lecture OutlineLecture Outline How photosynthesis, warm temperatures & low pressures in shallow water

control carbonate distribution How carbonate sediment types is tied to depositional setting How most mud lime mud has a bio-physico-chemical origin Origins of bio-physico-chemical grains:- ooids, intraclasts, pellets, pisoids Separation of bioclastic grains:- foram’s, brach’s, bryozoan, echinoids, red

calc’ algae, corals, green calc’ algae, and molluscs by mineralogy & fabric How CCD controls deepwater carbonate ooze distribution How Folk & Dunham’s classifications are used for carbonate sediments How most diagenesis, dolomitization, & cementation of carbonates takes

place in near surface & trace elements are used in this determination How Stylolites develop through burial & solution/compaction

Limestones Form - Where?Limestones Form - Where?

Shallow Marine –Late Proterozoic to Modern Deep Marine – Rare in Ancient & commoner in

Modern Cave Travertine and Spring Tufa – both Ancient

& Modern Lakes – Ancient to Modern

CO2 - Temperature & Pressure Effect!CO2 - Temperature & Pressure Effect!

High temperatures, low pressure & breaking waves favor carbonate precipitation

CO2 + 3H2O = HCO3-1 + H3O+1 + H2O = CO3

-2 + 2H3O+1

Carbon dioxide solubility decreases in shallow water and with rising in temperature

At lower pressure CO2 is released & at higher pressure dissolves

HCO3-1 and CO3

-2 are less stable at lower pressure but more stable at higher pressure

HCO3-1 and CO3

-2 have lower concentration in warm waters but higher concentrations in colder waters

Calcium Carbonate - Solubilty Calcium Carbonate - Solubilty Note calcium carbonate dissociation: CaCO3= Ca+2 + CO3

-2 CaCO3 is less soluble in warm waters than cool waters CaCO3 precipitates in warm shallow waters but is increasingly

soluble at depth in colder waters CO2 in solution buffers concentration of carbonate ion (CO3

-2) Increasing pressure elevates concentrations of HCO3

-1 & CO3-

2 (products of solubility reaction) in sea water CaCO3 more soluble at higher pressures & with decreasing

temperature

Controls on Carbonate AccumulationControls on Carbonate Accumulation

Temperature (climate) -Tropics & temperate regions favor carbonate production: true of ancient too!

Light – Photosynthesis drives carbonate production Pressure – “CCD” dissolution increases with depth Agitation of waves - Oxygen source & remove CO2

Organic activity - CaCO3 factories nutrient deserts Sea Level – Yield high at SL that constantly changes Sediment masking - Fallacious!

Limestones – Chemical or BochemicalLimestones – Chemical or Bochemical

Shallow sea water is commonly saturated with respect to calcium carbonate

Dissolved ions expected to be precipitated as sea water warms, loses CO2 & evaporates

Organisms generate shells & skeletons from dissolved ions

Metabolism of organisms cause carbonate precipitation

Distinction between biochemical & physico-chemical blurred by ubiquitous cyanobacteria of biosphere!

Biological Carbon Pump Biological Carbon Pump Carbon from CO2 incorporated in organisms through

photosynthesis, heterotrophy & secretion of shells > 99% of atmospheric CO2 from volcanism removed

by biological pump is deposited as calcium carbonate & organic matter

5.3 gigatons of CO2 added to atmosphere a year but only 2.1 gigatons/year remains; the rest is believed sequestered as aragonite & calcite

Carbonate MineralogyCarbonate Mineralogy Aragonite – high temperature mineral Calcite – stable in sea water & near surface crust

Low Magnesium CalciteHigh Magnesium Calcite

– Imperforate foraminifera– Echinoidea

Dolomite – stable in sea water & near surface Carbonate mineralogy of oceans changes with time!

TROPICS

TEMPERATE OCEANS

BasinRamp

RestrictedShelf

OpenShelf

RestrictedShelf

OpenShelf

Carbonate Components – The KeyCarbonate Components – The Key Interpretation of depositional setting of carbonates is

based on Grain typesGrain packing or fabricSedimentary structuresEarly diagenetic changes

Identification of grain types commonly used in subsurface studies of depositional setting because, unlike particles in siliciclastic rocks, carbonate grains generally formed within basin of deposition

NB: This rule of thumb doesn’t always apply

Carbonate ParticlesCarbonate Particles

Subdivided into micrite (lime mud) & sand-sized grains

These grains are separated on basis of shape & internal structure

They are subdivided into: skeletal & non-skeletal (bio-physico-chemical grains)

Lime Mud or MicriteLime Mud or Micrite

Lime Mud or

Micrite

Lime Mud or

Micrite

WHITINGWHITING

LIME MUDLIME MUDACCUMULATESACCUMULATES

ON BANK, OFF BANK ON BANK, OFF BANK & TIDAL FLATS& TIDAL FLATS

Three Creeks Tidal FlatsThree Creeks Tidal Flats

Lime MudLime Mud--

OrdovicianOrdovicianKentuckyKentucky

Carbonate Bio-physico-chemical GrainsCarbonate Bio-physico-chemical Grains

Ooids Grapestones and other intraclasts Pellets Pisolites and Oncolites

Aragonitic OoidsAragonitic Ooids

After Scholle, 2003Aragonitic OoidsAragonitic Ooids

Calcitic & Calcitic & AragoniticAragonitic

OoidsOoidsGreat Great SaltSaltLakeLake

GrapestonesGrapestones

PelletsPellets

After ScholleAfter Scholle

Skeletal Particles - MineralogySkeletal Particles - Mineralogy Calcite commonly containing less than 4 mole %

magnesium Some foraminifera, brachiopods, bryozoans, trilobites, ostracodes, calcareous nannoplankton, & tintinnids

Magnesian calcite, with 4-20 mole % magnesiumEchinoderms, most foraminifera, & red algae

Aragonite testsCorals, stromatoporoids, most molluscs, green algae, & blue-green algae.

Opaline silica sponge spicules & radiolarians

Drafted by Waite 99, after James 1984)

ForaminiferaForaminifera

After ScholleAfter Scholle

ForaminiferaForaminifera

BrachiopodBrachiopod

BrachiopodsBrachiopods

Brachiopod

BryozoanBryozoan

Trilobite RemainsTrilobite Remains

Ostracod RemainsOstracod Remains CalcispheresCalcispheres

Trilobite Carapice

CrinoidCrinoid

Syntaxial

cementcement

Red Calcareous AlgaeRed Calcareous Algae

Surface Water Organic Productivity Surface Water Organic Productivity Marine algae & cyanobacteria base of marine food chain Fed by available nitrogen and phosphorus Supplied in surface waters by deep water upwelling Vertical upwelling drives high biological productivity at:

EquatorWestern continental margins Southern Ocean around Antarctica

Produce biogenous oozes

Deep Water Carbonate DepositsDeep Water Carbonate Deposits

Deep water pelagic sediments accumulate slowly (0.1-1 cm per thousand years) far from land, and include:

abyssal clay from continents cover most of deeper ocean floor– carried by winds – ocean currents

Oozes from organisms' bodies; not present on continental margins where rate of supply of terriginous sediment too high & organically derived material less than 30% of sediment

Carbonate Compensation Depth - CCDCarbonate Compensation Depth - CCD Deep-ocean waters undersaturated with calcium carbonate &

opalline silica. Biogenic particles dissolve in water column and on sea floor Pronounced for carbonates Calcareous oozes absent below CCD depth CCD varies from ocean to ocean

4,000 m in Atlantic. 500 - 1,500 m in Pacific

Siliceous particles dissolve more slowly as sink & not so limited in distribution by depth

Nutrient supply controls distribution of siliceous sediments

After James, 1984

Carbonate Cement Fabrics Carbonate Cement Fabrics

Crust or rims coat grains Syntaxial overgrowth – optical continuity with

skeletal fabricEchinoid single crystalsBrachiopod multiple crystals

Blocky equant - final void fill

Isopachus Marine CementIsopachus Marine Cement

Meniscus CementMeniscus Cement

Influx of Influx of MagnesiumMagnesium

Rich Rich ContinentalContinental

Ground Ground WatersWaters

Influx of Influx of sea watersea water

Evaporation Evaporation of mixed of mixed WatersWaters

1. 1. AragoniteAragonite2. 2. GypsumGypsum3. 3. AnhydriteAnhydrite4. 4. DolomiteDolomite5. Halite5. Haliteaccumulaaccumulatetein this in this orderorder

StylolitesStylolites

Dissolution seam(A), Stylolite (B), Highly serrate stylolite (C) Deformed stylolite (D). A few grains are shown schematically to emphasize the change in scale from the previous figure (after Bruce Railsback)

Two-dimensional cross-sectonal views of

Tangential (A) flattened (B) concavo-convex (C) sutured (D) (after Bruce Railsback)

Intergranular contacts as seen in thin section

After Bruce Railsback

Lecture ConclusionsLecture Conclusions Photosynthesis, warm temperatures & low pressures in shallow water

control carbonate distribution Carbonate sediment types indicate depositional setting Most mud lime mud has a bio-physico-chemical origin Ooid, intraclast, pellet, and pisoid grains have bio-physico-chemical origin Mineralogy & fabric separate foram’s, brach’s, bryozoan, echinoids, red calc’

algae, corals, green calc’ algae, and molluscan skeleletal grains CCD controls deepwater ooze distribution Folk & Dunham are best way to classify carbonates Most diagenesis, dolomitization, & cementation of carbonates takes place in

near surface crust & trace elements can be used in this determination Stylolites develop through burial & solution/compaction

End of the LectureEnd of the Lecture

Lets go for lunch!!!

Global Climate CyclesGlobal Climate Cycles

Global climatic cycles, referenced to geologic periods (yellow), megasequences (light purple), sea level cycles (blue), & volcanic output (dark purple). (Redrawn & modified L. Waite, 2002 after Fischer, 1984)

Frakes et al. (1992) have alternating cold & warm states ("cool" & "warm" modes) at comparable time scales to Fischer (1984) cycles but propose older portion of Mesozoic greenhouse (Middle Jurassic to Early Cretaceous) has a cool climate, & presence of seasonal ice at higher latitudes (after L. Waite, 2002)

Phanerozoic Global Climate HistoryPhanerozoic Global Climate History

Copied from Steven Wojtal of Oberlin College

CO2 - Temperature & Pressure Effect!CO2 - Temperature & Pressure Effect!

Carbonate precipitation favored by high temperatures, low pressure and breaking waves.

Solubility of carbon dioxide increases with depth and drops in temperature

CO2 + 3H2O = HCO3-1 + H3O+1 + H2O = CO3

-2 + 2H3O+1

At higher pressure CO2 dissolves & is released at lower pressures HCO3

-1 and CO3-2 are more stable at higher pressures but less

stable at lower pressures HCO3

-1 and CO3-2 reach higher concentrations in colder waters but

lower concentration at warm waters

Calcium Carbonate - Solubilty Calcium Carbonate - Solubilty Note behavior of calcium carbonate: CaCO3= Ca+2

Concentration of carbonate ion (CO3-2) is buffered by amount

of CO2 in solution Increasing pressure elevates concentrations of HCO3

-1 & CO3-

2 (products of solubility reaction) in sea water CaCO3 is more soluble at higher pressures Similar effect occurs with decreasing temperature CaCO3 is more soluble in cool waters than warm waters CaCO3 is increasingly soluble at depth in colder waters but

precipitates in warm shallow waters

Copied from Suzanne O'Connell Wesleyan College

Carbonates overview

Education

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