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Sedimentology > Carbonates > Flashcards

Carbonates Flashcards

1
Q

Siliciclastic deposits

A

Allochthonous- produced outside the basin and transported in, only found on land, tells about the land and its provenance; extrabasinal

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2
Q

Carbonate deposits

A

Autochthonous- particles are made inside the basin, tell water temperature and salinity; turn into rocks rapidly (like 20 years)

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3
Q

Carbonate Components

A
  • Allochems (formed elsewhere)

- Orthochems (formed in situ)

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4
Q

Skeletal Allochemical Components

A
  • Bivalves -Crinoids (echinoderms)
  • Gastropods - Trilobites
  • Brachiopods -Foraminifera
  • Bryozoans -Ostracods
  • Algae -Corals
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5
Q

Non-skeletal Allochemical Components

A
  • Coated Grains (Ooids and pisoids)
  • Oncolites
  • Peloids
  • Intraclasts
  • Aggregates
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6
Q

Ooids

A

-small coated grains, less than 2 mm
-have internal structure (such as a core)
2 Types
1) Tangential/Concentric
2)Radial

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7
Q

Tangential/Concentric Ooids

A
  • 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
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8
Q

Radial Ooids

A
  • 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.)
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9
Q

Pisoids

A

-similar to ooids but are greater than 2 mm in size

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10
Q

Micrite

A
  • microcrystalline calcite mud (lime mud)
  • <4 microns
  • silt & clay-sized fragments
  • dull-brown to transparent
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11
Q

Spar(ite)

A
  • microcrystalline cement; interlocking crystals
  • microspar (0.001 mm)
  • sparry cement (0.02-0.1 m)
  • actually grows
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12
Q

Oncoids/Oncolites

A

-algae balls that are continually flipped over to continue creating a crust

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13
Q

Peloids

A
  • internally structureless
  • fecal pellets (possibly from worms)
  • rod shaped & circular in diameter
  • difficult to tell apart from ooids from outside
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14
Q

Micritized Grains (Peloids)

A

-initial boring is filled with micrite mud which continues until the organism is completely micritized

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15
Q

Grain aggregates

A

-ooids starts rolling, and then the cement sticks them together

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16
Q

Limestone clasts

A

-pieces of clasts are ripped up and incorporated into sed. rock

17
Q

Origin of Micrite

A
  • 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)
18
Q

Origin of Sparry calcite

A
  • recrystallization and replacement (neomorphism)

- primary, pore filling cement or recrystallization of micrite

19
Q

Grabau’s Classification

A
  • based on grain-size, but is not used today
  • Calcirudites = Pebbles
  • Calciarenites = Sand
  • Calcisiltites = Silt
  • Calcilutites = Silt & mud
20
Q

Folk’s Classification

A
  • 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)
21
Q

Dunham’s Classification

A
  • very useful for field descriptions
  • used by almost all carbonate geologists
  • names tell about energy of description
22
Q

Modifications by Embry & Klovan

A
  • added an extra row to dunham classification

- modern dunhams classification

23
Q

Baffled environment

A
  • low energy environment in a high energy area
  • behind corals (where they block the energy)
  • often the area where lime mud concentrates
24
Q

Precipitation of CaCO3 in Sea Water

A
  • 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
25
Q

Secular Variation of Carbonate Mineralogy of Seawater

A
  • driven by seafloor spreading rates

- seafloor flushes out the Mg during spreading

26
Q

Controls on CaCO3 Production

A
  • Temperature
  • Salinity
  • Pressure
  • Agitation
  • Organic activity
  • Sediment masking
27
Q

Temperature and CaCO3

A
  • higher temperatures promotes precipitation

- decreased CO2 solubility

28
Q

Salinity and CaCO3

A

-an increase causes a decrease in CO2 solubility

29
Q

Pressure and CaCO3

A
  • low pressures promotes precipitation

- drop in pressure/partial pressure releases CO2

30
Q

Agitation and CaCO3

A
  • addition of atmospheric CO2 by aeration

- the CO2 that doesn’t want to be there

31
Q

Organic Activity and CaCO3

A
  • direct precipitation or modify the geochemical environment (ex. nanobacteria)
  • Biggest driver!!
32
Q

Sediment Masking and CaCO3

A
  • silt and clay in water column results in marl (carbonate and siliciclastic mix) deposition
  • sand doesn’t usually have any effect
33
Q

Carbonate Productivity

A
  • 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
34
Q

Carbonate Factory

A
  • 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
35
Q

Carbonate Factory Controls

A
  • Evolution -Biological factors
  • Climatic Zone -Temperature/Salinity
  • Clastic sediment input
  • Water depth -Turbidity
36
Q

Carbonate Factory’s Relationship to Sea Level & Grain Type

A
  • 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

Sedimentology (2 decks)

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