Carbonates Flashcards
Siliciclastic deposits
Allochthonous- produced outside the basin and transported in, only found on land, tells about the land and its provenance; extrabasinal
Carbonate deposits
Autochthonous- particles are made inside the basin, tell water temperature and salinity; turn into rocks rapidly (like 20 years)
Carbonate Components
- Allochems (formed elsewhere)
- Orthochems (formed in situ)
Skeletal Allochemical Components
- Bivalves -Crinoids (echinoderms)
- Gastropods - Trilobites
- Brachiopods -Foraminifera
- Bryozoans -Ostracods
- Algae -Corals
Non-skeletal Allochemical Components
- Coated Grains (Ooids and pisoids)
- Oncolites
- Peloids
- Intraclasts
- Aggregates
Ooids
-small coated grains, less than 2 mm
-have internal structure (such as a core)
2 Types
1) Tangential/Concentric
2)Radial
Tangential/Concentric Ooids
- most commonly and predominantly aragonitic ooites
- most modern ooids
- form in the Bahamas, Persian Gulf, Turks & Caicos
- form in high energy, normal marine environments
- needles of aragonite, roll back & forth, flatten
- “snow ball” precipitation
Radial Ooids
- predominantly mg-calcite
- mainly ancient ooids (great salt lake, Baffin bay, Persian gulf-modern analog)
- forms in hypersaline and low energy environments (relict in normal environ.)
Pisoids
-similar to ooids but are greater than 2 mm in size
Micrite
- microcrystalline calcite mud (lime mud)
- <4 microns
- silt & clay-sized fragments
- dull-brown to transparent
Spar(ite)
- microcrystalline cement; interlocking crystals
- microspar (0.001 mm)
- sparry cement (0.02-0.1 m)
- actually grows
Oncoids/Oncolites
-algae balls that are continually flipped over to continue creating a crust
Peloids
- internally structureless
- fecal pellets (possibly from worms)
- rod shaped & circular in diameter
- difficult to tell apart from ooids from outside
Micritized Grains (Peloids)
-initial boring is filled with micrite mud which continues until the organism is completely micritized
Grain aggregates
-ooids starts rolling, and then the cement sticks them together
Limestone clasts
-pieces of clasts are ripped up and incorporated into sed. rock
Origin of Micrite
- polygenetic (more than 1 origin)
- physical disintegration (calcareous green algaes)
- mechanical and biological erosion of carbonate sediments (sea shells, skeletons, etc.)
- direct precipitation (very rare, snowball earth & P-T boundary only 2 times- when no life)
Origin of Sparry calcite
- recrystallization and replacement (neomorphism)
- primary, pore filling cement or recrystallization of micrite
Grabau’s Classification
- based on grain-size, but is not used today
- Calcirudites = Pebbles
- Calciarenites = Sand
- Calcisiltites = Silt
- Calcilutites = Silt & mud
Folk’s Classification
- based on allochems & orthochems
- useful in describing cements but not for field work
- doesn’t tell if sparry calcite is primary (grew) or secondary (recrystallization)
Dunham’s Classification
- very useful for field descriptions
- used by almost all carbonate geologists
- names tell about energy of description
Modifications by Embry & Klovan
- added an extra row to dunham classification
- modern dunhams classification
Baffled environment
- low energy environment in a high energy area
- behind corals (where they block the energy)
- often the area where lime mud concentrates
Precipitation of CaCO3 in Sea Water
- seawater is oversaturated with respect to calcite by a factor of 6 & aragonite by a factor of 4
- doesn’t occur because dissolved Mg and organic coatings stick to crystal surfaces which blocks Ca and CO3
- Loss of organic life in Snowball Earth and P-T lead to large precipitations
Secular Variation of Carbonate Mineralogy of Seawater
- driven by seafloor spreading rates
- seafloor flushes out the Mg during spreading
Controls on CaCO3 Production
- Temperature
- Salinity
- Pressure
- Agitation
- Organic activity
- Sediment masking
Temperature and CaCO3
- higher temperatures promotes precipitation
- decreased CO2 solubility
Salinity and CaCO3
-an increase causes a decrease in CO2 solubility
Pressure and CaCO3
- low pressures promotes precipitation
- drop in pressure/partial pressure releases CO2
Agitation and CaCO3
- addition of atmospheric CO2 by aeration
- the CO2 that doesn’t want to be there
Organic Activity and CaCO3
- direct precipitation or modify the geochemical environment (ex. nanobacteria)
- Biggest driver!!
Sediment Masking and CaCO3
- silt and clay in water column results in marl (carbonate and siliciclastic mix) deposition
- sand doesn’t usually have any effect
Carbonate Productivity
- the amount of biogenic carbonate produced in shallow seas is determined by the productivity within the food chain
- relatively shallow waters with low amounts of suspended terrigenous clastic material are therefore most favorable for carbonate production
Carbonate Factory
- the shallow region of high biogenic productivity
- highest productivity is in the light saturation zone at the top of the photo zone (where light reaches)
- productivity decreases the deeper into the photo zone
- this will shift down slightly in colder water
Carbonate Factory Controls
- Evolution -Biological factors
- Climatic Zone -Temperature/Salinity
- Clastic sediment input
- Water depth -Turbidity
Carbonate Factory’s Relationship to Sea Level & Grain Type
- Grainstone must be above normal wavebase (so mud is washed out)
- Packstone is at the borderof the normal wavebase and storm wavebase
- Can only tell energy of deposition but not environment
