GEOL 325 Lecture 4: Carbonates
GEOL 325: Stratigraphy & Sedimentary Basins
University of South CarolinaSpring 2005
Professor Chris KendallProfessor Chris KendallEWS 304 EWS 304
[email protected] [email protected] 777.2410777.2410
An Overview of CarbonatesAn Overview of Carbonates
Precipitated Sediments & Sedimentary Rocks
Precipitated Sediments & Sedimentary Rocks
An Epitaph toAn Epitaph to
Limestones & DolomitesLimestones & Dolomites
GEOL 325 Lecture 4: Carbonates
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 are the product of the creation, transport, deposition, and diagenesis of detritus and solutes derived from pre-existing rocks.
Sedimentary rocks are the product of the creation, transport, deposition, and diagenesis of detritus and solutes derived from pre-existing rocks.
Sedimentary rocks are the product of the creation, transport, deposition, and diagenesis of detritus and solutes derived from pre-existing rocks.
GEOL 325 Lecture 4: Carbonates
Sedimentary Rocks Detrital/Siliciclastic Sedimentary Rocks
conglomerates & brecciassandstonesmudstones
Carbonate Sedimentary RocksCarbonate Sedimentary Rockscarbonatescarbonates
Other Sedimentary Rocksevaporitesphosphatesorganic-rich sedimentary rockschertsvolcaniclastic rocks
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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!
GEOL 325 Lecture 4: Carbonates
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!
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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!
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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)
GEOL 325 Lecture 4: Carbonates
WHITINGWHITING
LIME MUDLIME MUDACCUMULATESACCUMULATES
ON BANK, OFF BANK ON BANK, OFF BANK & TIDAL FLATS& TIDAL FLATS
GEOL 325 Lecture 4: Carbonates
Carbonate Bio-physico-chemical GrainsCarbonate Bio-physico-chemical Grains
Ooids Grapestones and other intraclasts Pellets Pisolites and Oncolites
GEOL 325 Lecture 4: Carbonates
Calcitic & Calcitic & AragoniticAragonitic
OoidsOoidsGreat Great SaltSaltLakeLake
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
Trilobite RemainsTrilobite Remains
Ostracod RemainsOstracod Remains CalcispheresCalcispheres
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
StylolitesStylolites
Tangential (A) flattened (B) concavo-convex (C) sutured (D) (after Bruce Railsback)
Intergranular contacts as seen in thin section
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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)
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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
GEOL 325 Lecture 4: Carbonates
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