Carbonate Diagenisis Flashcards
What is percentage of carbonate rocks in the sedimentary record?
Around 13%
Epeiric platform is characterized by ……
Warm, shallow in land sea
What is a carbonate platform?
Term applied to any shallow marine environment where carbonates accumulate.
Facies boundaries !!!
One rock have different specifics characteristics related to the environment that was deposited.
In the fore reef talus we would find a pile of broken of corals, so we can infer that the …………and …………. will be different
Substrate and organisms
What structures can we find in a reef front?
Those are the …………of the reef
Bafflestone
Bindstone
Framestone
Core
What structures can I find in the reef crest?
Bindstone
What structures can I find in the back reef?
Bafflestone
Floatstone
What structures can I find in the sand apron part of the reef?
Rudstone and grainstone
What structures can I find in a lagoon environment?
Packstone and wakestone
Reefs have a major impact on the environment. They ……….waves, decreasing the energy of the environment.
Break or baffle
What structures can we expect to find on the fore reef talus and back reef?
Rudstone and grainstone
When tides move sediment in and out, little particles get coated with calcium carbonate and thus forming……………
Ooids
A rock formed by ooids is called……..
Oolite
Carbonate sand shoals:
Where ooids form, is a very specific environment, where shallow, warm and gently agitated waters make a perfect depositional setting for carbonates. Can we expect planar bedding in this setting?
No because we will have increase Bioturbation!!
Describe the environmental setting you would find in a lagoon?
Low energy environment
Increased nutrients
Very fine grained sediment
Increased Bioturbation
Lagoon- warm, shallow waters promote evaporation and concomitant increase in salinity- hyper-saline solution- precipitation of calcium carbonated
We can start altering the sediment as soon as it was deposited.
True/false
True
Eogenesis - immediately post deposition.
Diagenic processes:
Microbial micritization : transforms the original grains (recrystalization) or coats the grains with a micritic envelope, due to the action of……….
Cementation: process occurring when you have ………………..pore water.
Pore fluids are supersaturated with respect to cement phase
There are no kinectic factors inhibiting precipitation.
Microbes
Supersaturated or hipersaline
How do we modify salinity?
Evaporate water
Temperature
pH
What is cement?
Crystals precipitated in a fluid-filled space, growing attached to a substrate; commonly aragonite, HMC, LMC, Dolomite ( others are less common)
!!!! Precipitated as opposed to deposited!!!!!!!
Diagenic processes:
What is Neomorphism?
Transformation between one mineral and itself or a polymorph. It is thought to be a “wet” process, meaning that it is occur across a thin, interstitial fluid film.
E.g., taking a calcite and growing a bigger calcite grain.
Replacement of aragonite shells or cement by calcite.
Recrystalization of a lime mud to produce coarser crystals.
Diagenic processes:
Dissolution: occurs when fluids are…………..with respect to a given mineral. Particularly important in meteoric settings; also related to Lysoclines in deeper marine settings.
Undersaturated
What are Lysoclines?
!!!!!!
Change in salinity wit depth in oceans!!!
What are the characteristics of meteoric water?
Acidic- low pH
Under saturated with carbonates.
What is the difference between precipitation and dissolution?
!!!
Precipitation happens when the pore space is supersaturated and dissolution happens when the pore water is undersaturated.
What is the difference between non-rimmed and rimmed carbonate platforms?
Decreased energy in rimmed shelf because energy is wasted in the corals (reef).
High energy in non-rimmed platform causing completely different types of organisms and sedimentary structures to be there. Consequently you will have different Bioturbation in each setting.
Diagenetic processes:
Mechanical compaction: processes occurring under increased overburden when grains are translocated for closer packing, fractured, crushed. It results in …………. unless you have…………….
!!!!!
Decreased porosity
Early cementation
Diagenetic processes:
Chemical compaction: produced by ………. Process results in a “ ……………..” Between the adjacent grains and, ultimately, dissolution seams and ………….., surfaces that are marked by the remaining, relatively insoluble materials (clay, oxides, organic matter)
!!!!!!!!
Pressure-enhanced dissolution
Fitted fabric
Stylolites
What is Fitted fabric?
One grain intertwined to the other
As we increase mechanical and chemical compaction, with increase in pressure, water is pushed out of the porous, forming dewatering, flame, ball/pillow structures. It can happen in ……………settings too
Carbonates
Marine diagenesis:
Occurs on: …………
Shallow waters depth: where the carbonate is!!!!
The sea floor
Tidal flats
Beachs
Marine diagenesis:
Shallow waters (to shelf (200m) and slope depths:
Good potential for cementation depending on:
………
Deep waters:
High pressure, low water temperature and high partial pressure of CO2 lead to dissolution Because seawater becomes more …………..
Shallow waters: decreased porosity
Salinity
Temperature
Pressure
Deep waters: Increase porosity
Undersaturated (1st with respect to aragonite then HMC, then LMC. Acidity breaks things up increasing porosity.
Isopachous cement?
Is a cement of precipitated minerals that form rims of nearly equal thickness around the grains in a sedimentary rock during diagenesis. Common in sub aqueous environment.
The interesting is what happens to pore water. When we have waves in a high energy environment, we are not just moving water at the surface, we are moving water in and out of the……..
Pore spaces
Marine diagenetic setting:
Active shallow marine setting: wave/ tides pumping pore waters through reefs and shoals, producing: Aragonite And HMC cements forming ……………..’coats’ composed of needles,fibers, blades on grains/ skeletons
Intragranular cements, that is, within shells such as gastropods and forams.
Isophacous
Marine diagenenic:
Less active (stagnant) marine setting: modern muddy sands in relatively protected settings, characterized by: Early more localized Cementation of grains into aggregates, intragranular cements. Intense microbial...................
Microbes don’t live in places with high energy.
Micritization
Marine diagenesis:
Marine phreatic settings as in intertidal- supratidal zones such as:…….., ………., producing:
………… Cement in lower intertidal zones and meniscus cements in ……….. (Low/high) Intertidal zones ( vadose Zones)
Micrite coats
Beach rock and tidal flats
High
Marine diagenesis:
Water table separates a zone of intermittent drying & saturation, called ………
from a zone that is constantly wet (permanently fluid-saturated , called ………….
Vadose zone
Phreatic zone
Vadose zone?
Intermittent wet (above low tide)
Phreatic zone?
Constant wet (below low tide)
What is vugs?
Gaps/holes/spaces in the sediment often formed by secondary dissolution.
How do we form larger crystals in the phreatic zone?
Change in pH
Change in temperature
Change in pressure
Change in concentration of solution
The pore spaces are continuously soaked, which allows a slow and long periods of growth (large crystals)
Meteoric diagenisis:
In phreatic zone:
Cement growth is more continuous with ……………..
LMC in blocky, approximately ……………rims or pore-filling
Phreatic zone cements are ……………..to burial cement.
Larger crystals
Equant
Transitional
Meteoric diagenesis:
Vadose zone!
Calcareous soil horizons can form (calcretes, …………..soils)
LMC calcite is precipitated in distinctive forms:…………., pendant
Caliche
Meniscus
Vadoze zone: …………….wet. They form calcretes, desert pavements, early diagenesis, calcite and concrete nodule.
Periodically (dry and wet)
Phreatic zone:……………. that is continuously growing and getting significantly larger
cement
Ooids form………..cement in marine phreatic zone.
Meniscus
Unlike marine water, fresh water has very little Ca2+ and carbonate, so it becomes a little …………….
Acidic
Large crystals are indicative of………………zone
Phreatic
Once aragonite needles, transformed into LMC – this is a ……………process!
Neomorphic process
Burial diagenesis:
CaCO3 rocks enter this setting either:
Directly from the marine setting, with marine pore fluids, or-
After ……………………… where mineralogy is partly stabilized and fluids are fresh or mixed (marine-meteoric)
Meteoric diagenesis
Burial diagenesis:
Subsurface conditions include:
Higher pressure and……….
Higher ……………
Low rates of fluid flow:
- ……….
- ……….
Temperature
Salinity (but not always) very generalized statement!
Low porosity
Low permeability
Sparry calcite: is a crystalline mosaic of calcite crystals in which individual crystals are easily discernable with an optic microscope. In carbonates, sparry calcite forms as a cement during diagenesis within……………..porosity or ……………. of aragonite.
Primary
Neomorphism
Burial diagenesis: Common processes include:
Mechanical compaction
Chemical compaction e.g pressure solution producing……………. and “fitted fabric”
Cementation of typically coarse sparry calcite (LMC)
Neomorphism and recrystalization of any remaining …………and HMC
Burial ……………
Stylolites
Aragonite
Dolomitization
Dolomitization:
If you precipitate ………….., you take …….out of the system, increasing the concentration of Mg. think of Mg:Ca
Calcium carbonate
Ca2+
What is the importance of dolomite?
Common petroleum reservoir rock
Used in cement
Dolomite Isomorphs:
Substitute……. and …….. To form the Isomorphs.
Ankerite (CaFe(CO3)2)
Kutnahorite (CaMn(CO3)2)
Dolomite has a range of composition but we still call it dolomite
Iron and manganese
Dolomite Isomorphs:
Most commonly, partial substitution of iron results in………………..dolomite:
CaMgFe(CO3)2
Feroic
We call it dolostone if it made of ………..or more of dolomite.
50%
Dolomitization is the replacement of ….. With……
Ca2+ with Mg
The vast majority of dolomites are thought to have formed via………. Of pre-existing carbonate minerals.
E.g calcite and aragonite
Replacements can ( but doesn’t alway) result in …………….. of the original limestone fabric.
Replacement
Destruction
Dolomitization:
Probably a replacement. What so we need?
Pore fluids - large amounts of …… Into the system.
Get rid of aragonite and…………(lots of Ca+)
Move fluid into the system, replenishing with………
Mg
Gypsum
Mg
Dolomitization requirements
Pore fluids wit elevated Mg:Ca ratio
Evaporation of seawater(to form …….)
Precipitation of …………. and gypsum ( remove ……)
Some sort of ………..
Must be long- term as LOTS of Mg is needed to dolomitize large volumes of carbonates
Brine
Ca2+
Advection
Max enrichment with Mg is obtained with temperatures passing 60 C (really bad to reproduce on land)
According to one model:
(1) Ca2+ + Mg2+ + 2(CO3 2-). – CaMg(CaCO3)2
Dolomitization is not ……………of dolomite from sea water.
Direct precipitation
According to the replacement model:
(2) Mg2+ + 2(CaCO3)– CaMg(CO3)2+ Ca2+
Replacement of calcium with magnesium. We need max enrichment of Mg2+
Requires addition of Mg2+ and removal of Ca2+
It has been demonstrated that can happen but why does not work?
Lot easier at high temperatures past 60C but we can’t find this occurring naturally and flowing through a carbonate platform.
So temperature is the problem!!!
Meteoric diagenisis:
In phreatic zone:
Cement growth is more continuous with ……………..
LMC in blocky, approximately ……………rims or pore-filling
Phreatic zone cements are ……………..to burial cement.
Larger crystals
Equant
Transitional
Meteoric diagenesis:
Vadose zone!
Calcareous soil horizons can form (calcretes, …………..soils)
LMC calcite is precipitated in distinctive forms:…………., pendant
Caliche
Meniscus
Vadoze zone: …………….wet. They form calcretes, desert pavements, early diagenesis, calcite and concrete nodule.
Periodically (dry and wet)
Phreatic zone:……………. that is continuously growing and getting significantly larger
cement
Ooids form………..cement in marine phreatic zone.
Meniscus
Unlike marine water, fresh water has very little Ca2+ and carbonate, so it becomes a little …………….
Acidic
Large crystals are indicative of………………zone
Phreatic
Once aragonite needles, transformed into LMC – this is a ……………process!
Neomorphic process
Burial diagenesis:
CaCO3 rocks enter this setting either:
Directly from the marine setting, with marine pore fluids, or-
After ……………………… where mineralogy is partly stabilized and fluids are fresh or mixed (marine-meteoric)
Meteoric diagenesis
Burial diagenesis:
Subsurface conditions include:
Higher pressure and……….
Higher ……………
Low rates of fluid flow:
- ……….
- ……….
Temperature
Salinity (but not always) very generalized statement!
Low porosity
Low permeability
Sparry calcite: is a crystalline mosaic of calcite crystals in which individual crystals are easily discernable with an optic microscope. In carbonates, sparry calcite forms as a cement during diagenesis within……………..porosity or ……………. of aragonite.
Primary
Neomorphism
Burial diagenesis: Common processes include:
Mechanical compaction
Chemical compaction e.g pressure solution producing……………. and “fitted fabric”
Cementation of typically coarse sparry calcite (LMC)
Neomorphism and recrystalization of any remaining …………and HMC
Burial ……………
Stylolites
Aragonite
Dolomitization
Dolomitization:
If you precipitate ………….., you take …….out of the system, increasing the concentration of Mg. think of Mg:Ca
Calcium carbonate
Ca2+
What is the importance of dolomite?
Common petroleum reservoir rock
Used in cement
Dolomite Isomorphs:
Substitute……. and …….. To form the Isomorphs.
Ankerite (CaFe(CO3)2)
Kutnahorite (CaMn(CO3)2)
Dolomite has a range of composition but we still call it dolomite
Iron and manganese
Dolomite Isomorphs:
Most commonly, partial substitution of iron results in………………..dolomite:
CaMgFe(CO3)2
Feroic
We call it dolostone if it made of ………..or more of dolomite.
50%
Dolomitization is the replacement of ….. With……
Ca2+ with Mg
The vast majority of dolomites are thought to have formed via………. Of pre-existing carbonate minerals.
E.g calcite and aragonite
Replacements can ( but doesn’t alway) result in …………….. of the original limestone fabric.
Replacement
Destruction
Dolomitization:
Probably a replacement. What so we need?
Pore fluids - large amounts of …… Into the system.
Get rid of aragonite and…………(lots of Ca+)
Move fluid into the system, replenishing with………
Mg
Gypsum
Mg
Dolomitization requirements
Pore fluids wit elevated Mg:Ca ratio
Evaporation of seawater(to form …….)
Precipitation of …………. and gypsum ( remove ……)
Some sort of ………..
Must be long- term as LOTS of Mg is needed to dolomitize large volumes of carbonates
Brine
Ca2+
Advection
Max enrichment with Mg is obtained with temperatures passing 60 C (really bad to reproduce on land)
According to one model:
(1) Ca2+ + Mg2+ + 2(CO3 2-). – CaMg(CaCO3)2
Dolomitization is not ……………of dolomite from sea water.
Direct precipitation
According to the replacement model:
(2) Mg2+ + 2(CaCO3)– CaMg(CO3)2+ Ca2+
Replacement of calcium with magnesium. We need max enrichment of Mg2+
Requires addition of Mg2+ and removal of Ca2+
It has been demonstrated that can happen but why does not work?
Lot easier at high temperatures past 60C but we can’t find this occurring naturally and flowing through a carbonate platform.
So temperature is the problem!!!
Dolomitization model: Addition of both Mg2+ and CO3 2- to the system.
CaCO3 + Mg2+ + CO3 2- – CaMg(CO3)2
Eliminates the removal of Ca2+ as a requirement
What is the problem?
Kind of the same thing of number replacement . The only difference is that you don’t have to eliminate Ca2+, you can only have to cram Mg2+ into CaCO3
Dolomitization:
What are we trying to replace?
The Ca2+ and CaCO3
Dolomitization:
What are we trying to do?
We have Mg2+ and we are trying to insert in a …….. structure to make dolomite
carbonate
Dolomitization:
Where does it take place?
Carbonate platforms and in temperate climate, around 30 from the equator.
What is a Sabkhas?
Marine lake that is filled periodically (coastal lagoon)
What is Neomorphism?
Is a term used by folk to cover the combined processes of inversion ( eg. Transformation of aragonite to calcite) and recrystalization ( change in size or shape of a crystal with little or no change in chemical composition or mineralogy)
