Carbonates
•Most shallow water carbonates are marine
•Most are biochemical in origin
•Require specific conditions for deposition
•Most abundant non- terrigenous sedimentary rocks (~10% of total)
Importance of carbonate rocks
•Record enormous information regarding depositional environment
•Carbonate class
•Primary & secondary texture
•geochemistry
•Record the path of evolution and the history of life
•Provide age control
•Play crucial role in carbonate chemistry and biogeochemical cycling
Modes of carbonate formation
•Physically-induced inorganic precipitation
•travertine, tufa, caliche
•Organic-induced inorganic precipitation
•Lake whiting events (marls)
•Primitive biocarbonate
•Coraline algae
•Architectural biocarbonate
•Mollusks, coral, foraminifera, coccolithophorida
Controls on Carbonate Precipitation
•Temperature
•Pressure
•Mixing/Agitation
•Light/Turbidity
•Nutrient content
•Carbonate chemistry
•Photosynthetic activity
•Sediment masking
Principle carbonate mineral phases
Increasing stability
•Aragonite - metastable
•Calcite - primary or secondary
•Dolomite - secondary recrystalization
•See text for details regarding carbonate geochemistry
Limestone classification systems
•Three most common:
•Folk [1959,1962]
•Dunham [1962]
•Embry and Klovan [1972] (aka “Modified Dunham”)
•Based on macro and microscopic features
•Crystal size and origin
•Allochemical (Allochems) - analogous to siliciclastic clasts
•Orthochemical (orthochems) - analogous to siliciclastic matrix
Limestones lacking allochems
CrystalinityCrystal size (mm)
•Extremely coarse >4.0
•Very coarse 1.0- 4.0
•Coarse 0.25- 1.0
•Medium 0.062- 0.25
•Fine 0.016- 0.062
•Very fine 0.004- 0.0016
•Aphano- or crypto-< 0.004
Limestones bearing allochems
Informal termAllochem size (mm)
•Calcirudites >1.0
•“Carbonate Conglomerates and breccias”
•Calcarenites 0.062- 1.0
•“Carbonate sandstones”
•Calcilutites <0.062
•“Carbonate muds”
Allochems
•Skeletal Components
•Cnidarians (Coelenterates)
•Bryozoans
•Brachiopods
•Molluscs
•Annelids (serpulids)
•Arthopods
•Echinoderms
•Non-skeletal
–Coated grains
•Ooids (<2mm)
•Pisoids (>2mm)
•Oncoids (irregular shape)
•Peloids
–Aggregate grains (grapestone)
–Limestone clasts (limeclasts)
Skeletal Allochems
•Cnidarians (Coelenterates)
•Bryozoans
•Brachiopods
•Molluscs
•Annelids (serpulids)
•Arthopods
•Echinoderms
Non-skeletal Allochems
•Coated grains
–Oolites (Ooids), <2mm
–Pisolites (Pisoids), >2mm
–Oncolites (Oncoids) (irregular shape)
–Peloids
•Aggregate grains (grapestone)
•Limestone clasts (limeclasts)
Pellets, Peloids, and grain aggregates
Fecal pellets
Orthochems
•Micrite - silt and clay size particles of carbonate. Microscopically translucent, dull brown; Macroscopically dull and opaque.
•Sparite - crystal clear, needle-like calcite cement ranging in size 1x10-3mm (microspar) to 0.1 mm.
•Both can occur alone, together, and as matrix in allochem bearing limestone.
Folk’s Limestone Classification
1. Sparry allochemical
–Interclastic, Oolitic, bioclastic, peloidal
2. Micritic allochemical
–Interclastic, Oolitic, bioclastic,peloidal
3. Micritic or Dismicritic
4.Biolithite (reef limestone)
Pros and cons of the Folk Classification
•Provides a means of describing the relative importance of allo- and ortho- chemical carbonate components within limestones
•Introduced concept of carbonate textural maturity
•Unfortunately, provides no means of differentiating primary from secondary cement
Grain support: The Dunham classification
•Accounts for recrystallized cement by assessment of grain support
•Cemented pore spaces in grain supported limestones implie a primary cement.
•Grains “floating” in spar cement a secondary cement formed from recrystalized micrite.
Modified Dunham Classification- Embry and Klovan (1972) add categories for coarse clasts and differentiate varieties of boundstone
•Floatstone- matrix supported with > 10% grains of > 2 mm size
•Rudstone- grain support by > 10% grains of > 2 mm size
•Boundstone
•Framestone
•Bindstone
•Bafflestone
Limestone diagenesis
•Dissolution
•Loss of aragonate, high Mg calcite
•Secondary porosity
•Styolites (Pressure solution)
•Cementation
•Replacement
Cementation - infilling of primary porosity
•Drusy - incomplete filling by first generation needle-like spar cement
•Blocky - irregular, patchy, equant cement filling remaing voids
•Rim - cement that engulfs and overgrows framework grains (syntaxial - of same optical orientation as coated grain)
Replacement
•Gradual replacement of one mineral phase by another
Examples:
•Aragonate--> calcite ---> dolomite
•Carbonate --> silica
Diagenetic environments and typical cements
Dolomite and Dolomitization
•Recrystalization to dolomite from other forms of carbonate
•More resistant to weathering than other carbonates
Dolomites forms under:
•High temperatures (>50°C)
•High Mg-Ca ratios >8 mol/mol (typical modern sea water ~5.4)
•High pH (>8) (Modern sea water pH 8.2)