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EART27201 Sedimentary rocks and fossils > Carbonate sedimentary rocks > Flashcards

Carbonate sedimentary rocks Flashcards

1
Q

How are biogenic sediments formed?

A

Compounds that have been metabolised by organisms

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2
Q

What are the two types of biogenic sediments?

A

Skeletal material (mainly carbonates, some silicates and phosphates) and soft parts

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3
Q

How are chemical sediments formed?

A

Evaporites. They are precipitated directly from the atmosphere or water without aid.

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4
Q

What types of minerals transcend biogenic and chemical processes and what characterises the group?

A

Carbonates, characterised by a bicarbonate ion (CO3^2-)

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5
Q

Where can metal species in ocean water originate from?

A

Weathered continental and mid ocean ridge processes

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6
Q

What are the three most common carbonates to precipitate from ocean water?

A

Dolomite, magnesium calcite (low-high Mg), aragonite

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7
Q

Aragonite is metastable and how is it kept in form?

A

Kept in form by metabolic processes

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8
Q

What does precipitation depend on?

A

Availability of Ca^2+

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9
Q

What does dissolution depend on?

A

CO2 concentration

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10
Q

Give three examples of how CO2 can be removed from water?

A

Increasing water temperature, decreasing water pressure, and agitation of water

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11
Q

Are carbonate producer heterotrophs or autotrophs?

A

They can be either

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12
Q

Why do both autotrophs and heterotrophs live in shallow water?

A

Because autotrophs require light and heterotrophs consume autotrophs

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13
Q

What factor is a prerequisite for biogenic activity?

A

Light

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14
Q

Why does most carbonate productivity occur in the top 15m of water?

A

Because carbonate producers can be heterotrophs or autotrophs, tending towards shallow, lighter water.

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15
Q

What can also affect where carbonate productivity occurs and why?

A

Turbidity increase results in more suspended mud and silt, reducing light penetration.

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16
Q

Are autotrophs or heterotrophs more dominant in carbonate production? Give a reason why this is.

A

Autotrophs dominate. Carbonate production is higher at low latitudes (inside 20C winter isotherm), which is dominated by autotrophs.

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17
Q

What factor can have a strong influence on autotrophs?

A

Temperature (can affect rates of all biochemical reactions)

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18
Q

What oceans are dominated by heterotrophic carbonate production?

A

Outside the 20C winter isotherm (at higher latitudes)

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19
Q

Describe what happens to ocean water to increase salinity

A

Evaporation rate is higher than freshwater input. This concentrates ions, meaning more Ca is available. Concentrates other metals, increasing salinity. Organisms lose water by osmosis because of their water concentration gradient

20
Q

Give two examples of conditions that increase ocean salinity?

A

Low rainfall and high temperatures and being poorly connected to open oceans

21
Q

Why does increased salinity lead to highly specialised autotrophs dominating carbonate production?

A

Water with high salinity provides poor conditions for heterotrophs. The solubility of CO2 is reduced, favouring carbonate precipitation (by highly specialised autotrophs)

22
Q

Name three carbonate producing associations

A

Chorozoan, chloragal, foramol

23
Q

Describe the chlorozoan association

A

Coral reefs, red and green algae. T>15C. Normal salinity 32-40%.

24
Q

Describe the chloragal association

A

Algae and microbial mat. T>15C. Higher salinity. e.g. Mediterranean ocean and Gulf of Arabia.

25
Q

Describe the foramol association

A

Foraminifera molluscs, echinoderms, bryozoans, ostracoda. (non-reef forming). T<15C. Normal salinity (32-40%). e.g. around the British Isles.

26
Q

Where is carbonate productivity highest and what associations are formed here? Why?

A

On the ocean-facing side of the upwelling of cold open ocean waters. This is where Chlorozoan reefs form because there is the highest nutrient supply.

27
Q

What are the four things that make up a carbonate?

A

Grains, matrix, cement, and pore space

28
Q

What are carbonate grains also known as?

A

Allochems

29
Q

What is the difference between the production of the grain and matrix of carbonate rocks compared to those of clastic rocks?

A

The grain and matrix of clastic rocks are produced by weathering, those of carbonate rocks are produced by an organism producing a skeleton.

30
Q

What are the two origins of carbonate grains?

A

Skeletal (bioclasts) and non-skeletal

31
Q

Name three categories of skeletal originating carbonate grains

A

Fossil fragments, encrusters or reef-builders, and planktonic, nektonic, and benthic

32
Q

What are encrusters/reef-builders?

A

Organisms whose main phase of life is attached to the substrate and remains in place after death and are buried/encrusted by younger reef-builders, attaching to the dead organism rather than the substrate, building a reef over generations. Most abundant modern reef-builder is coral.

33
Q

Compare the carbonate production of planktonic and nektonic organisms

A

Planktons can produce large amounts of carbonate, whereas nektons aren’t major carbonate producers

34
Q

How are non-skeletal carbonates produces?

A

Chemically generated.

35
Q

Name the three categories of non-skeletal carbonates

A

Coated grains, peloids, and grain aggregates

36
Q

What are ooids and pesoids?

A

Where micrite has precipitated chemically and the grain nucleus rolls in an oscillating water current and picks up layers of micrite. Mostly marine origins. Spherical.

37
Q

What is the difference between ooids and pesoids?

A

Ooids are smaller than 1mm in diameter and pesoids are greater than 1mm in diameter.

38
Q

What are oncoids?

A

Where micrite is precipitated by algal and microbial coatings on the outside of grains. Irregular layers of micrite are precipitated directly out of the water onto the cortex (layers are bound by the microbial mat, not chemical precipitation). Formed in shallow marine water with a less regular current.

39
Q

What are peloids?

A

Structureless grains, possibly formed by the ingestion and excretion of calcite. 100-500mm diameter.

40
Q

What are grain aggregates?

A

Grains formed by the binding and cementation of preexisting grains that then behave as one coherent grain.

41
Q

Describe how carbonate grains can behave as clastic rocks

A

They can be intracastic: reworked and partially lithified local material, within the basin, or extraclastic: lithified material (any rock type) from outside the immediate depositional area/basin

42
Q

Describe the composition of carbonate rock matrix

A

Composed of carbonate mud or micrite (can be from disaggregated peloids). Formed by the abiogenic (direct precipitation) of microscopic carbonate crystals from super saturated water (whitings).

43
Q

What factor is not relevant in the classification of carbonate rocks and why?

A

Transportation is irrelevant because they are formed close to the place they will be buried.

44
Q

What do carbonate grain size and shape depend on and what are they independent to?

A

Depend on the growth of the organism and are independent from the environment energy or the grains transport.

45
Q

What is used in the classification of carbonate rocks?

A

Mode of life of skeletal grain. Genetic origin of non-skeletal grain. NOT TRANSPORT.

EART27201 Sedimentary rocks and fossils (10 decks)

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