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OCR A LEVEL GEOLOGY > Sedimentary rocks > Flashcards

Sedimentary rocks Flashcards

1
Q

What are 7 minerals found in sedimentary rocks?

A

Quartz
Plagioclase Feldspar and K Feldspar
Biotite and Muscovite Mica
Calcite
Gypsum
Halite
Haematite

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2
Q

What is diagenesis?

A

All the process that are acting on sediment

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3
Q

What is lithification?

A

A subset of diagenesis, includes:
Cementation (Mineral sticking grains together)
Compaction (Grains compressed together)

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4
Q

What is weathering and erosion?

A

Weathering - The break up of rocks “IN SITU” where the rock structure is gradually weakened
Erosion - The wearing away of rock by “Moving agents” such as wind, ice, water etc

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5
Q

What are the three types of weathering?

A

Biological
Mechanical/Physical
Chemical

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6
Q

Name 3 types of biological weathering

A

Animal burrowing
Roots
Organic acids

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7
Q

Name 3 types of physical weathering

A

Freeze thaw - Where water gets into cracks of rocks. This water then freezes and expands, widening the cracks
Thermal expansion - When rocks are hot, they expand. When they are cooled they contract. This causes cracking
Pressure release - Rock on top is weathered causing the rock underneath to expand

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8
Q

Name 5 types of chemical weathering

A

Oxidation
Carbonation
Hydration
Hydrolysis
Solution

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9
Q

What are the 4 modes of transportation of sediment?

A

TSSS
Traction
Saltation
Suspension
Solution

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10
Q

What is the Hjulstrom curve?

A

Shows the relationship connecting current velocity and grain size in order to figure out when a sediment will get deposited.

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11
Q

What does maturity describe?

A

How much a rock has been changed/ how long a rock has been through the rock cycle.

Mature = Well sorted, Rounded
Immature = Poorly sorted, Angular

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12
Q

Describe the rock cycle.

A

Starting with Igneous rocks:
1. Igneous rocks get ejected from volcano
2. These get eroded and weathered
3. Get transported and deposited underwater
4. Goes through lithification
5. Sedimentary rocks created
6. Some rise due to uplift, rest get metamorphosed
7. Metamorphic rock gets partially melted
8. Magma accumulates in chamber
9. Crystallisation
10. Repeat

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13
Q

What are 4 clast shapes?

A

3 axis all about equal = Sphere
2 axis equal, 1 shorter = Rod
2 axis equal, 1 longer = Disc
3 axis unequal = Blade

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14
Q

What are the 6 roundness descriptions?

A

Very angular
Angular
Sub-Angular
Sub- Rounded
Rounded
Well-rounded

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15
Q

What is the phi scale?

A

Is a sediment particle size scale (-2 -> 8)
-2 being very coarse sand (largest sediment size)
8 being clay (smallest sediment size)

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16
Q

What do the following terms mean?
Rudaceous
Arenaceous
Argillaceous

A

Rudaceous = sediment size is >2mm (Pebbles)
Arenaceous = Sediment size is 0.0625 - 2mm (Sands)
Argillaceous = Sediment size is <0.0625mm (Silts, clays muds)

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17
Q

What are the 9 sedimentary rocks we have to know?

A
  1. Conglomerate (Rounded pebbles)
  2. Breccia (Angular pebbles)
  3. Orthoquartzite (Quartz grains and cement, a sand)
  4. Desert Sandstone (Millet seed with iron oxide cement, a sand)
  5. Arkose (Sand with >25% K Feldspar)
  6. Greywacke (Immature, mixed sand)
  7. Clay (Plastic)
  8. Shale (Layered)
  9. Mudstone (Not layered)
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18
Q

What controls affect the shape and overall look of a sedimentary rock?

A
  1. The transport agent (Wind, Water, Ice) affects the roundness, sorting and size
  2. Transport time (Longer = more round and smaller)
  3. Rock/Mineral affects shape
  4. Energy of environment (Higher = better sorted)
  5. Climate affects the transport agent and weathering rate/type.
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19
Q

What is compaction? And how can it affect muds and sands?

A

-As sediments accumulate on top of eachother, the pressure increases, which compacts the sediment underneath. This causes grains to become closely packed and reduces porosity.

-Muds and sands can then reduce their thickness by 80% when compacted due to their minerals becoming aligned.

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20
Q

Explain pressure dissolution

A

Is most commonly seen in coarse, grained rocks.
Minerals dissolve more easily when they are affected by stress. So when put under pressure, the minerals recrystallise or removed by groundwater. This causes the two rocks two become merged and produce an irregular grain contact.

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21
Q

How are plants compacted and turned into coal?

A

Plant matter dies and falls into a place with a lack of oxygen such as a swamp.
Anaerobic bacteria then turns the plant into peat.
Lignin, waxes and resins are preserved such as amber
Peat is then compacted and water is then expelled due to increasing pressure, reducing volume and increasing the proportion of carbon

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22
Q

How are sandstones cemented?

A

Sandstones have a higher permeability than muds.
When groundwater containing dissolved minerals in solution flow through the pore spaces they precipitate which binds the grains together.
An example of this is desert sandstone where sands have been cemented by haematite (iron oxide)

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23
Q

Name 4 cemented rocks

A

Orthoquartzite - Quartz cement from pressure dissolution
Fossiliferous limestone - Calcite cement from solution of carbonate shells
Desert sandstone - iron minerals cement grains together
Greywacke - Clay matrix from the breakdown of feldspar.

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24
Q

How are limestones cemented?

A

-Composed of aragonite which then stabilise into calcite.
-Pore spaces usually filled with calcite, forming sparite.

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25
Q

Name 8 sedimentary structures

A

Cross bedding
Graded bedding
Imbrication
Salt Pseudomorphs
Ripples
Desiccation cracks
Flute casts
Barchan dunes

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26
Q

Explain how cross bedding is formed

A

Sand grains are moved by wind/river. The sand is then deposited to form dunes and create a slope at a maximum of 37 degrees. Sand blown over the dune add to the dunes length or make a new dune on top which creates a cross bed. Is a good way up and paleo current indicator

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27
Q

Explain how a barchan is formed

A

Wind blows sand. When deposited it builds up creating a C shape in the desert. Sand blown over the top then bounces off the edge to add to the dune similar to cross bedding. This then happens over a long period of time and creates a C shape in the desert that will slowly move. The end of the C is the direction of wind flow so is a good paleo-current indicator.

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28
Q

Explain how ripple marks form

A

Ripple marks from as sand grains are transported by saltation in high energy conditions. Symmetrical ripples are formed by oscillating currents such as beaches (tidal zone going in and out).
Asymmetrical ripples are formed by currents that travel in 1 direction such as wind/rivers.
Are good paleo-current and environment indicators

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29
Q

Explain the formation of graded bedding

A

The largest, heaviest particles are found at the bottom of the bed, finer at the top. They form when the energy levels of water drop and sediment is deposited. Is a good way up and paleo environment indicator

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30
Q

Explain how desiccation cracks form

A

Form in clay rich environment. Loss of water due to evaporation causes mud to contract forming polygonal shaped blocks, with a v shaped crack in between. These v shapes then are filled in by sediment and preserved. These are good way up and paleo environment indicators.

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31
Q

Explain how salt pseudomorphs form

A

Cubic halite crystals grow at the surface of a bed due to evaporation out of solution. These cubes are embedded into sediment and then dissolved back into water. However, the shape of the salty block is left and filled in by sediment preserving it. Good paleo environment indicators.

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32
Q

Explain the formation of imbrication

A

Pebbles are rolled along a stream bed and then pile up against eachother, inclined toward downstream.

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33
Q

Define facies

A

Includes all the characteristics of a sedimentary rock that are produced by its environment of deposition and allow it to be distinguished from rock deposited on an adjacent environment.

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34
Q

Define lithofaceies and biofacies

A

lithofacies = includes all physical and chemical properties
biofacies = include all the paleological chracteristics

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35
Q

What does the theory of uniformitarianism state?

A

The past is key to the present. Present day processes would have operated in the same way now as they did in the past.

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36
Q

Define polymictic, oligomictic and monocytic conglomerates

A

Polymictic = A coarse grained rock containing clasts of many rock types
Oligomictic = A coarse grained rock containing clasts of a few rock types
Monocytic = A coarse grained rock containing clasts of a single rock type

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37
Q

What are 4 sedimentary rocks/minerals found in deserts?

A

Desert sandstone - very well sorted, mature, very well rounded
Wadi conglomerates - poorly sorted immature
Alluvial fan arkose - feldspar rich, coarse
Playa lake evaporites

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38
Q

What are wadis?

A

River channels in desert regions in which flash flood occur very occasionally

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39
Q

What are 2 ways sediment is transported in a desert?

A

Wind
Flash floods

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40
Q

What are alluvial fans?

A

An alluvial fan is a triangle-shaped deposit of gravel, sand, and even smaller pieces of sediment, such as silt. Look like deltas and form at the end of flash floods

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41
Q

Define eustatic and isostatic sea level change

A

Eustatic = Sea level change due to melting of ice/freezing of water, thermal expansion/contraction of water
Isostatic = Relative sea level change due to the land rising/descending caused by glaciers

42
Q

Define transgressive and regressive

A

Transgressive = Sea level rise due to melting of ice/thermal expansion (WARMER CLIMATES)
Regressive = Sea level fall due to isostatic rebound/water freezing/thermal contraction (COLDER CLIMATES)

43
Q

What sedimentary rocks are found in glaciers?

A

Till
Fluvio-glacial sands and gravel
Angular clasts
Erratics
Boulder clay

44
Q

What is alluvium?

A

Fluvial sediments such as clays, sands, silts etc

45
Q

How do rivers migrate?

A
  1. River flow is slower on the inside of meanders so material is deposited
  2. This creates a point bar of sand/clay with conglomerate at the bottom.
  3. The outside bend gets eroded and the river will do this over and over again
  4. Will migrate in the direction of the outside bend of the meander
46
Q

What is channel lag?

A

Coarse sediment left in a channel bed.

47
Q

What is a braided stream?

A

Wide, shallow channels with small islands made of gravel and sand causing multiple streams to re-route around them

48
Q

Name 5 types of limestone

A

Oolitic
Fossiliferous
Crinoidal
Reef
Shelly

49
Q

What are ooliths and how do they form?

A

Small particles of lime that form when water is saturated with lime and precipitates out due to the tropical climate small tidal ranges so that they are not destroyed. Once they reach a certain size, they settle and become cemented together, forming oolitic limestone

50
Q

What is fossiliferous limestone?

A

Biologically formed limestone composed of fossil fragments stuck together either by a micrite matrix or sparite cement

51
Q

What is crinoidal limestone?

A

Limestone that contains fossilised crinoid stems sections.

52
Q

What is reef limestone?

A

Fragments of fossilised coral cemented together

53
Q

What is shelly limestone?

A

Fragments of fossilised bi-valves and brachiopods cemented together

54
Q

How is chalk formed?

A

Biologically formed. Composed of cocoliths (single celled algae). White = pure calcium carbonate.

55
Q

What are the two classifications used to identify sedimentary rocks?

A

Folk
Dunham

56
Q

Define sparite and micrite

A

Sparite = crystallised calcite
Micrite = lime mud

57
Q

What do we call limestones with:
Fragments
Ooliths
Fossils
Pellets
Using the folk classification

A

Fragments = Intasparite, intamicrite
Ooliths = oosparite , oomicrite
Fossils = biosparite , biomicrite
Pellets = pelsparite, pelmicrite

58
Q

Using the dunham classification, what do we call sedimentary rocks with little to no grain, but supported by mud

A

Mudstone = less than 10% grain
Wackestone = More than 10% grain

59
Q

Using the dunham classification, what do we call sedimentary rocks with lots of grain and lacks of mud

A

Packestone = Some mud, lots of grain
Grainstone = no muds lots of grain

60
Q

What is stokes law used for?

A

The way viscosity affects the settling velocity of particles:

It shows that the terminal settling velocity of small spheres in a fluid are inversely proportional to tits viscosity and directly proportional to the density difference between the fluid and soil, the radius of the spheres and gravity.

61
Q

How can cross bedding show current velocity?

A

Small scale cross bedding formed from ripples show lower velocity than large scale trough cross bedding caused by movement of dunes.
Furthermore, horizontal stratification with current lineation can show rapid velocity, although normally horizontal bedding would show little to no current velocity.

62
Q

What is flocculation of clay particles?

A

Where clay particles stick to eachother which have higher settling velocities
Clay particles have negative charges, which attracts cations absorbed (added) into the surface of the clay.
Charged clay particles then repel eachother.

63
Q

What is a turbidity current?

A

High-Velocity current that flows down gentle gradients because sediment disperses within it makes it denser than sea water. These currents are triggered by earthquakes or slope instability.

64
Q

What are turbidites?

A

An upward fining deposit deposited from a turbidity current.

65
Q

What is the Bouma sequence?

A

An idealised sequence of sediments and sedimentary structures seen in a turbidite deposit.
1 = top of sequence
5 = bottom of the sequence

  1. Shale with parallel laminations in mud - Means no current
  2. Fine sandstone and siltstone with parallel laminations in the sand and silt - Means low energy current and suspension settling, changes in the current energy cause laminations.
  3. Sandstone (greywacke) with cross bedding, and climbing ripples - Sufficient energy to carry sand by saltation, climbing ripples form when deposition exceeds rate of migration, but energy insufficient to cause erosion of ripples.
  4. Coarse, then medium sandstone (greywacke) with flat bed (meaning high velocity), graded bedding and sole marks including flute casts and tool marks - Sufficient energy to carry sand grains by traction. Sole structures occur on the base (pits or groves) and act as moulds for sediment deposited above, when filled producing clasts.
  5. Coarse bed of pebbles and conglomerate in a sandy matrix with graded bedding, rip up clasts and eroded base - shows high energy, coarsest grains in suspension, erosion of base.
66
Q

What are climbing ripples?

A

A series of cross-laminae formed by superimposing migrating ripples. They form when deposition exceeds the rate of migration of ripples.

67
Q

What are tool marks?

A

Impressions made on the surface of soft sediment by the dragging or bouncing of an object (tool) in a current.

68
Q

What are rip-up clasts?

A

Pieces of shale or mudstone eroded by a current containing suspended sediment. They are preserved when the current deposits its sediments.

69
Q

How does a turbidity current occur?

A
  1. Sediment brought to the sea by rivers accumulates on the continental shelf
  2. If the sediment at the edge of the slope becomes unstable and loses shear strength, it will begin to move down slope under the force of gravity
  3. Rapid sedimentation from storm waves and earthquakes can cause the material to become unstable
  4. Once the sediment is falling, it is denser and has low friction so it flows with high velocity down the slope
  5. Then will transport huge volumes of clastic material, depositing it in submarine fans on the continental rise or in ocean trenches and possibly abyssal plains if there is fast movement
70
Q

What are biogenic deposits?

A
  1. Are calcareous and siliceous oozes (clay containing >30% biogenic skeletal material made of silica)
  2. When planktonic organisms die, their shells sink to the sea floor to be preserved as microfossils
  3. The composition of the ooze is dependent on the composition of the organism
  4. Below the CCD the oozes will dissolve
  5. Oozes accumulate where the rate of deposition is greater than the rate of solution (due to carbonic acid)
71
Q

What does Walther’s law of facies state?

A

A vertical sequence of facies will be the product of a series of depositional environments which lay laterally adjacent to each other.

72
Q

What sections are deltas split up into?

A

The delta top, the delta front, the prodelta

73
Q

What are top sets?

A

The uppermost horizontal layers of delta commonly consisting of channel sandstones, coal and seat earth

74
Q

What is seat earth?

A

A sandy/clay-rich fossil soil found beneath a coal seam. It represents the soil in which coal forming plants grew and frequently contains carbonised traces of plant roots.

75
Q

What are foresets?

A

The inclined layers formed on the delta front, commonly consisting of cross-bedded sandstones

76
Q

What are bottomsets?

A

The lowest horizontal layers of a delta, commonly consisting of shales.

77
Q

What is bioturbation?

A

the disturbance of sediment by the activities of organisms.

78
Q

How do deltas form?

A

Deltas form where a river flows into a sea or lake, and deposits its load of sediment.

79
Q

What must the environment be like for deltas to form?

A

There must be little wave or tidal action to allow sediment to build up at the mouth and not be washed away

80
Q

What causes the creation of distributaries in deltas?

A

Deposition of sediment causes the river channels to be blocked so the river switches course.

81
Q

What sequence do deltas produce?

A

Cyclothems

82
Q

Why do deltas produce cyclothems?

A

Cyclic sedimentation from rivers

83
Q

What does a deltaic sequence look like?

A

1 = top
5 = bottom

  1. Coal formed on the delta top (due to dead plant matter)
  2. Plant roots in the seat earth (the soil the plants used)
  3. Cross-bedded sandstone formed on the delta front (will fine towards the bottom)
  4. Marine shale containing bivalve fossils
  5. Marine limestone containing brachiopod fossils
84
Q

Why does coal form in deltas?

A

Most of the flora is in swamps, so when the plant matter dies, anaerobic respiration takes p[lace increasing the chance of the plant material being preserved.

84
Q

What is the delta top?

A
  1. Is dominated by distributaries with swamps, bays or flood plains in between.
  2. The sediments deposited here are called top sets, usually coarse grained sands and gravels deposited in the distributary channels, as well as channel sandstones containing cross bedding
  3. Is above sea level
  4. Will contain the seat earth
84
Q

What is the delta front?

A
  1. As the river meets the sea, the coarsest sand grains are deposited first and the finest silt last.
  2. Deposition occurs on the delta front
  3. Is under the sea
  4. As the front of the delta advances into the sea, a vertical succession forms
  5. Is basically the slope under the water from the delta
  6. Contains the foresets
85
Q

What is the pro delta?

A

Deposition takes place in low-energy deeper water at the bottom of the delta front
These sediments are called bottom sets
They consist of clays and slits which are thinly bedded and lack sedimentary structures
They lithify to form shales which may contain marine fossils

86
Q

What is the word used to describe the movement of deltas

A

Progradation (prograding deltas)

87
Q

What type of sequence are deltaic sequences?

A

Coarsening upwards sequence

88
Q

What causes the cyclothems (deltaic sequence) to be repeated many times ?

A

Marine transgressions and regressions
-Changes in sea level
-Local isostatic changes
-Changes in position of delta lobes due to migration of channels
-Changes in rate of sedimentation

89
Q

Explain how we can use modern day deltas to see what deltas were like in the carboniferous?

A

The presence is the key to the past (uniformitarianism)
The way deltas are now, will be the same as how they were in the carboninferous

90
Q

Explain the delta formations in the UK in the Carboniferous and how they formed

A

Lower (359 Ma)
1. Caledonian mountain chain was eroding rapidly
2. This was then carried by rivers down to the shoreline
3. This meant deltas were prograding south due to the sediment supply
4. Swamp forests were common on the delta tops
5. The water was clear warm sediment free and shallow with coral reef limestone

Middle (330 Ma)
Same conditions
Swamp forests and deltas were prograding south still

Late (300 ma)
Same conditions
Shoreline moved south, due to change in sea level
This meant that swamp forests were covered over the North of the UK
Deltas prograding south but started more south than before

91
Q

What are levees?

A

Are naturally formed raised river banks from when the river floods, and sediment is deposited.

92
Q

What are crevasse splays?

A

Where sediment builds up between tributaries or when sediment leaks out of a river.

93
Q

Why are deltas great examples to explain diachronicity

A

They show lots of lateral variation
Diachronous beds “cut across time” eg millstone grit of Scotland is much older in Wales/South England but are of the same rock type

94
Q

What creates BIFs?

A
  1. Cyanobacteria started to photosynthesise, the atmosphere was mainly CO2 and CH4
  2. The oceans contain large amounts of dissolved iron
  3. Conditions were reducing
  4. The main by product from photosynthesis was 02
  5. This 02 bonded with the dissolved oxygen to create iron oxide
  6. This then precipitates out of the ocean and fall to the sea bed
  7. Layers of iron oxide lay there and are a global phenomenon
95
Q

What are BIFs?

A

Banded Iron Formations
Layers of iron oxide

96
Q

In what period were the BIFs created?

A

The pre-Cambrian during the Archean era.

97
Q

What are stromatolites?

A

Cyanobacteria that produce layered structures from precipitated limestone

98
Q

In what conditions do arkoses form?

A

High energy erosion

99
Q

In what conditions do greywacke form?

A

Landslides underwater movement of sediment rapidly

OCR A LEVEL GEOLOGY (12 decks)

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