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BIOS1013 - Environmental Geoscience > Lecture 4 - Sediments and Sedimentary Rocks > Flashcards

Lecture 4 - Sediments and Sedimentary Rocks Flashcards

1
Q

How are sediments transported?

A

Water

Ice

Wind

Gravity

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2
Q

What is the fate of most sediment?

A

Burial and conversion to sedimentary rock.

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3
Q

What is stratification?

A

The arrangement of sedimentary particles into layers.

This is also known as bedding.

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4
Q

What is a bedding plane?

A

The top or bottom surface of a bed.

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5
Q

What is chemical sediment?

A

Sediment that is precipitated from solution in water.

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6
Q

What is biogenic sediment?

A

Sediment that is composed of the fossilised remains of plants or animals.

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7
Q

What are the four sizes of particle that clastic sediment is divided into?

A

Gravel.

Sand.

Silt.

Clay.

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8
Q

How large are gravel particles?

A

> 2 mm.

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9
Q

How large are sand particles?

A

1/16 - 2 mm.

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10
Q

How large are silt particles?

A

1/256 - 1/16 mm.

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11
Q

How large are clay particles?

A

< 1/256 mm.

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12
Q

What scale can be used to prevent the use of fractions in clastic sediment?

A

The Phi scale.

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13
Q

How does sorting identify sedimentary rocks?

A

It identifies sediments in terms of the variability in the size of its particles.

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14
Q

What are poorly sorted and well sorted clastic sediments?

A

Poorly sorted - large range of particle sizes.

Well sorted - small range of particle sizes.

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15
Q

How does sorting occur in water?

A

The faster the water flows, the larger the particles that can be moved.

As the flow becomes slower, heavier and denser particles are deposited first.

Lighter particles are transported onward.

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16
Q

What are the two broad categories of non-clastic sediment?

A

Chemical sediment.

Biogenic sediment.

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17
Q

What can sorting suggest about how a rock was formed?

A

It can suggest how the clasts were transported.

Well sorted rocks indicate movement by water whereas poorly sorted rocks indicate movement by glaciers.

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18
Q

What happens when particles are transported?

A

They become more rounded and more spherical.

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19
Q

What is roundness?

A

A measure of the sharpness of a particles edges.

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20
Q

What is sphericity?

A

A measure of how closely the shape of a particle is like a sphere.

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21
Q

What can sorting, roundness and sphericity suggest about a rock?

A

It is a reflection of how the sediments were transported before being converted into rock.

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22
Q

What is graded bedding?

A

The particles are ordered in terms of size. Coarser particles are found at the bottom with finer particles being found at the top.

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23
Q

What is cross bedding?

A

Where the beds are vertical in respect to the thicker layer in which they occur.

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24
Q

What process forms cross bedding?

A

Turbulent flow during transport. This can occur in windy environments and the direction of the cross beds can determine the prevailing wind direction for when the rock formed.

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25
Q

What is lithification?

A

The overall process of creating sedimentary rock.

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26
Q

What is diagenesis?

A

The collective term for all the chemical, physical and biological processes that cause lithification.

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27
Q

What is the order of processes for diagenesis to occur?

A

Compaction

Cementation

Recrystallisation

Dissolution

Chemical alteration.

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28
Q

What is compaction?

A

The weight of an accumulating sediment forces the grains together which reduces the pore space.

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29
Q

What is cementation?

A

Substances dissolved in water start to precipitate which forms a cement that sticks particles together.

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30
Q

What is recrystallisation?

A

As sediment accumulates and pressure increases, less stable minerals are converted into more stable ones.

For example, aragonite is turned into calcite.

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31
Q

What is dissolution?

A

Some minerals are dissolved during burial.

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32
Q

What is chemical alteration?

A

In the presence of oxygen, organic remains are quickly converted to carbon dioxide and water.

33
Q

What is a conglomerate?

A

A clastic rock that is made up of rounded sediments with a large grain size.

34
Q

What is a breccia?

A

A clastic rock that is made up of angular sediments with a large grain size.

35
Q

What are sandstones, siltstones and mudstones?

A

Clastic rocks that are made from sand/silt/mudstones.

36
Q

What are the four broad categories of sedimentary structures?

A

Erosional

Depositional

Biogenic

Post-depositional.

37
Q

How are erosional sedimentary structures formed?

A

Where aqueous, sediment laden water flows over recently deposited sediments that are yet to be lithified.

These flows gouge out depressions which are subsequently infilled.

38
Q

What features do erosional sedimentary structures form?

A

Sole marks as they form on the lower bed surface.

These include:
- Flute marks.
- Groove marks.
- Gutter casts.
- Impact marks.

39
Q

What are the features of flute marks?

A

A heel shape, that has a rounded upstream end and a flared downstream end.

They have an asymmetric cross section and are deepest upstream.

They are relatively small (around 15cm long and 7cm wide).

40
Q

What can flute marks be used to show?

A

The direction of water transfer.

41
Q

What are groove marks?

A

Linear ridges on a base of sandstone beds that are formed by the infilling of a groove cut into underlying mudstone.

42
Q

What can groove marks be used to show?

A

The direction of water transfer (but in either direction).

43
Q

What are examples of depositional sedimentary structures?

A

Bedding and lamination.

Ripples, dunes and cross stratification.

44
Q

What are ripples and dunes?

A

Downstream migrating bedforms produced by a unidirectional flow. Their formation depends on the flow velocity, the flow depth and the grain size.

45
Q

Where are depositional sedimentary structures commonly found?

A

Rivers, estuaries, tidal flats and delta channels.

46
Q

How large are ripples?

A

They have a wavelength of < 30 cm and a height of < 3 cm.

If it is larger than this, it is classified as a dune.

47
Q

How can the shape of a ripple change?

A

It can go from straight to sinuous to linguoid in increasing energy environments.

Linguoid is a heel shaped indent against the direction of flow.

48
Q

How can the shape of a dune change?

A

It can go from straight, to sinuous to caternary to lunate in increasing energy environments.

Catenary is wave shaped (with sharp peaks).
Lunate is a heel shaped indent in the direction of flow.

49
Q

What are biogenic sedimentary structures?

A

Structures formed when life interacts with the substrate.

50
Q

What is a pipe rock?

A

A vertical structure in a rock. It is a trace fossil formed from an organism moving vertically in the sediment.

51
Q

What are ichnofacies?

A

A trace fossil that represents burrows and feeding structures. Their structure reflects how the animal moved through the system.

52
Q

What is bioturbation?

A

When an organism churns up the sediment and this is preserved as a trace fossil.

They can include footprints.

53
Q

What are examples of post-depositional sedimentary structures?

A

Mud cracks that form when mud dries out.

This means that the surface has been exposed sub-aerially.

54
Q

What can ancient mud cracks determine about a rock?

A

The orientation of the rock as mud cracks would have formed on a flat surface.

55
Q

How are evaporite rocks formed?

A

The evaporation of sea/lake water forms salts. Evaporite rocks can be made from these salts.

56
Q

What are examples of freshwater precipitates?

A

Sodium carbonate.

Sodium sulfate.

Borax.

57
Q

What are examples of seawater precipitates?

A

Calcium carbonate.

Gypsum.

Halite.

58
Q

What is the order in which seawater minerals precipitate?

A

Calcite

Gypsum

Halite

Bloedite

Magnesium Sulphates

Carnallite

Bischofite.

59
Q

What is bioclastic sediment?

A

Where the fossilised remains of plants and animals are broken and scattered.

60
Q

How is a deep sea ooze formed?

A

It is a fine mud formed from the exoskeletons of phytoplankton.

61
Q

What is calcareous ooze?

A

A fine grained, deep sea deposit of skeletal material that contains more than 30% calcium carbonate.

62
Q

What is siliceous ooze?

A

A fine grained, deep sea deposit of skeletal material formed mainly of silica.

63
Q

What is a stromatolite?

A

A growth structure formed from colonies of bacteria precipitating calcium carbonate in hypersaline environments.

64
Q

What knowledge did ancient stromatolites provide?

A

They showed that originally the Earth was spinning faster. This is because rings grow daily on a stromatolite and there were more than 365 rings in a year which shows that there must have been shorter days.

65
Q

What are cyanobacteria?

A

A photosynthetic bacteria that is responsible for oxygenating Earth.

There is strong fossil evidence of the first occurrence of cyanobacteria and the great oxidation event.

66
Q

What are banded iron formations?

A

BIFs are formations of alternating bands of iron rich and iron deficient rock which were produced at a time where the atmosphere was fluctuating between oxidising and reducing.

67
Q

What is the main source of phosphate rocks?

A

The precipitation of apatite from seawater.

Phosphorus rich sediments are forming today off the Western coast of Africa and South America.

68
Q

What are chert and flint?

A

A hard, very compact sedimentary rock, composed of fine grained quartz.

They occur in limestones.

69
Q

What is chalk?

A

A limestone formed from the compacted shells of minute floating organisms.

70
Q

What is limestone?

A

Limestone is the most important biogenic rock.

It is formed from calcium carbonate.

71
Q

What are limestones named after?

A

The major component of the limestone.

For example, reef limestone consists of mainly cemented reef organisms.

Shelly limestone consists of shell debris.

72
Q

What is oolitic limestone?

A

A limestone that contains small (0.5-2 mm) spherical grains of carbonate.

The grains are formed when carbonate particles are rolled backwards and forwards in a tidal environment.

73
Q

How is coal formed?

A

Plant remains accumulate to form an almost pure organic sediment called peat.

This peat is compressed until it eventually becomes coal.

74
Q

What happens when carbon is buried?

A

Atmospheric oxygen increases. This is because the plants have not decomposed which reduces oxygen.

75
Q

What happens when carbon is burned?

A

Atmospheric oxygen decreases as this reacts with the carbon to form carbon dioxide.

76
Q

What conditions cause a rock to become darker?

A

A chemically reducing environment (lack of oxygen). More carbon, which is dark coloured, gets buried in a reducing environment.

77
Q

What does lack of space between grains show?

A

That the mineral was cemented long after deposition as the pressure reduced the pore space.

78
Q

How are carbonate cements produced?

A

Fluids flowing through the sediment and precipitating carbonate minerals.

BIOS1013 - Environmental Geoscience (8 decks)

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