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Earth 1A > Sediments > Flashcards

Sediments Flashcards

1
Q

Sediment

A

Any geological materials generated at the Earth’s surface

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2
Q

Clastic sediment

A

Sediment that has been separated from parent rock by errosion.
Split into terrigenous, volcaniclastic and carbonates.

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3
Q

Siliclastic sediment

A

Clastic sediment that is silicate based - pretty much all of it

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4
Q

Terrigenous sediment

A

Clastic sediment that originates from land.
e.g. constituents of mudrocks, sandstones, conglomerates

common minerals: quartz, feldspar

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5
Q

Volcaniclastic sediment

A

ashes and tuffs - ejected from volcanic erruptions

common minerals: micas, quartz, feldspar

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6
Q

Organic sediments

A

sediment from dead organisms that isn’t rich in calcium carbonate.
e.g. hydrocarbons and peat which turn to oil/gas and coal

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7
Q

Chemogenic sediment

A

Minerals precipitated inorganically

e.g. ironstones or evaporites

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8
Q

Cementation

A

Sticking together of unconsolidated sediment.
Water precipitates out different minerals, e.g. calcite, between grains.
Can begin immediately or may require burial

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9
Q

Compaction

A

Closer packing of grains due to weight of sediment above

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10
Q

Stages in Diagenesis

A

Mineralogical changes to sediment after burial

Cementation and/or compaction
Recrystallisation
Dissolution and replacement

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11
Q

Uniformitarianism

A

Assumption that observations of the present can inform us about the past i.e. environments that make characteristic structures were the same in the past

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12
Q

Sedimentary log

A

Graphical representation of vertical sections of rock, showing how sedimentary signatures change through stratigraphic section.

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13
Q

Outcrops

A

Show high resolution detail of sedimentary record

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14
Q

Boreholes

A

Show large scale, regional characteristics of sedimentary rock units, but lack high resolution.

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15
Q

Superposition

A

Generally, a succession of strata represent a sequence of depositional events

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16
Q

Walther’s law

A

Sedimentary environments are diachronous.
If 2 sedimentary units are adjacent and there’s no unconformity between them, the vertical transition between them shows a lateral transition in adjacent environments.

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17
Q

Diachronous

A

adjacent components of a sedimentary environment can be active at the same time

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18
Q

Evaporites

A

Form when a natural body of water evaporates and leaves behind the salts that were dissolved in it.

Carbonates, chlorides, sulphates

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19
Q

Erosional processes

A

Pick up grains from previously deposited sediment or from parent rock

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20
Q

Transport

A

Movement of grains by fluid or gravity

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21
Q

Deposition

A

Grains deposited in their temporary or final resting place when the fluid carrying them can no longer keep moving the grains

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22
Q

Physical weathering

A

Weakening and breaking up parent rock

e.g. freeze thaw, temperature changes, salt growth or biological intrusion like growing roots

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23
Q

Chemical weathering

A

Weakening rock by chemical changes
by solution in groundwater or brine, hydrolysis or oxidation
e.g. feldspar to kaolinite and quartz by hydrolysis; oxidation of pyroxene to magnetite and quartz; dissolution of calcite

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24
Q

Regolith

A

Unconsolidated sediment that hasn’t been transported

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25
Q

Reynold’s number

A

Determines if flow is laminar or turbulent which affects grain entrainment, transport and deposition.
Affected by viscosity, velocity and flow diameter

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26
Q

Labile Minerals

A

Weak, cleaved minerals that are readily converted to clay minerals.
In order of increasing resistance to weathering: Olivine, pyroxene, amphibole, biotitie, muscovite

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27
Q

Quartz grains

A

Polycrystalline: Different grains sutured together which go into extinction independently.
Unstrained quartz: Single crystal, uniform extinction.
Strained quartz: Single crystal, undulating extinction.
Most common in clastic rocks

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28
Q

Feldspar

A

Labile minerals, so weather to clay minerals easily

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29
Q

Lithic fragments

A

polycrystalline fragments of existing rock

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30
Q

Clay minerals

A

Sheet silicates that form during chemical weathering.

Cohesive and flocculate together as larger aggregates

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31
Q

Kaolinite

A

e.g. hydrolysis of feldspar

Often forms in warm, humid environments with acidic water

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32
Q

Montmorillonite

A

Clay mineral

Swelling clay: expands with water

Forms in moderate climates with neutral/alkaline pH or alkaline, arid conditions

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33
Q

Illite

A

Clay mineral

e.g. from biotite weathering

Forms in soils in temperate climates with acidic groundwater or in arid conditions

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34
Q

Bioclasts

A

Fragments of biological debris in sedimentary rocks

35
Q

Maturity of sandstones

A

Supermature: Contain mostly quartz with only a few lithic fragments, no labile minerals. Good sorting and rounding. e.g. shallow marine, beach and aeolian environments.
Mature: Contain mostly quartz with some lithic fragments and labile minerals e.g. fluvial environments
Immature: Contain labile minerals and lithics, 40-60% quartz. Poor sorting and rounding. e.g. glacial environments

36
Q

Properties of grains

A

Grain size: larger grains require more viscous and faster fluids to move them

Grain shape:
Roundness: a measure of surface roughness, i.e. how much attrition occurred
Sphericity: how close grains are to perfect spheres since spheres are used in most models

37
Q

Porosity

A

Dependent on grain size, shape and sorting

Where cement can be precipitated and how fluid can move through the sediment post deposition

38
Q

Stoke’s Law

A

Determines settling velocity of a particle
Depends on grain size, densities of the grains and fluid and the fluid’s viscosity
Only accurate for low concentrations of spherical grains in laminar flow

39
Q

Entrainment

A

A fluid begins to exert enough force on a grain to begin to move it

40
Q

Bedload

A

Sediment transported by rolling or saltation

41
Q

Suspended load

A

Sediment transported while suspended in a fluid

42
Q

Rolling

A

Moves only due to drag force of fluid parallel to bed

43
Q

Saltation

A

Lift generated by Bernoulli effect: Fluid mass constricted as it passes over an grain; velocity increases to conserve constant flow rate; decrease in static pressure above grain, which generates lift. If lift is greater than weight, grain is briefly lifted into the flow. If fluid velocity <= settling velocity, the grain falls back to the bed.

44
Q

Suspended load transport

A

Lift generated by Bernoulli effect: Fluid mass constricted as it passes over an grain; velocity increases to conserve constant flow rate; decrease in static pressure above grain, which generates lift. If lift is greater than weight, grain is lifted into the flow. If viscosity and fluid velocity are high enough, the bouyancy >= weight and the grain remains as suspended load

45
Q

Hjulstrom Diagram

A

http://www.coolgeography.co.uk/A-level/AQA/Year%2012/Rivers_Floods/Long%20profile/Hjulstrom.htm

Key points:
Logarithmic scale for both axes, in factors of 10
Flow velocity in cm/s from 0.1 to 1000
Grain size in mm from 0.001 to 1000
Grains of mud size are never deposited, but when they clump together due to cohesive properties, they eventually get big enough to be deposited.
Curve for erosion dips down from c. 300 to minimum of 11 (grain size 0.1mm), then grows to c. 1000
Curve for deposition starts at grain size of 0.01mm and grows to
c. 200

46
Q

Graded beds

A

Occur due to changes in fluid velocity i.e. different grain sizes are entrained and the critical settling velocity changes.

Normal grading: Fining upwards in a single bed due to decelerating fluid flow
Reverse grading: Coarsening upwards in a single bed due to accelerating fluid flow

47
Q

Change in grain size over multiple beds

A

Fining upwards in a series of beds: Decrease in fluid velocity of depositional environments
Coarsening upwards in a series of beds: Increase in fluid velocity of depositional environments

48
Q

Wackestone

A

Sandstone with more than 15% matrix

Lithic wacke: >5% lithic fragments, more lithic fragments than feldspar
Feldspathic wacke: >5% feldspar, more feldspar than lithic fragments
Quartz wacke: >95% quartz

49
Q

Mudstone

A

More than 75% matrix

50
Q

Arenite

A

Sandstone with up to 15% matrix

Quartz arenite: >95% quartz
Lithic arenite: >25% lithic fragments, more lithic fragments than feldspar
Feldspathic arenite / arkose / arkosic arenite: >25% feldspar, more feldspar than lithic fragments
Subarkose: Between 25% and 5% feldspar, more feldspar than lithic fragments
Sublitharenite: Between 25% and 5% lithic fragments, more lithic fragments than feldspar.

51
Q

Ripple Formation

A

Flow irregularities in the viscous sublayer (turbulent sweeps) make small grain clusters which cause the streamlines at the bed’s base to detach from the bed’s surface at the top of the clusters.
Where the streamline detaches and reattaches from the bed are the flow separation point and the flow attachment point.
On the stoss side of the ripple, flow contracts and on the lee side, flow expands. On the stoss side, grains roll or saltate up the ripple. On the lee side, there is an increase in static pressure due to the Bernoulli principle and sediment is deposited.
Ripples migrate downstream and previous lee sides are preserved as cross-lamination.
Ripples up to 4cm high. Wavelengths up to 0.5m
Formation is independent of flow depth.

52
Q

Viscous sublayer

A

Typically less than 1mm thick
Layer of fluid at base of the bed that is so affected by drag that the fluid velocity is low enough to maintain effectively laminar flow.

Hydraulically smooth:
Grain diameter < thickness
Ripples can form

Hydraulically rough:
Grain diameter < thickness
Ripples can’t form

53
Q

Stoss side

A

Upstream side

54
Q

Lee side

A

Downstream side

55
Q

Straight vs. linguoid or sinuous ripples

A

With time and increasing fluid velocity or depth, straight ripples become sinuous or linguoid ripples.
Straight ripples have planar sross-lamination
Linguoid and sinuous ripples have trough cross-lamination

56
Q

Climbing ripples

A

If the sedimentation rate is high enough, there is no net removal of grains from the stoss side of ripples. A migrating ripple will climb up the stoss side of the ripple in front

57
Q

Starved ripples

A

Fixed amount of sediment in a system

The rate of removal of grains from the stoss side = the rate of addition to the crests and avalanches down the lee side

58
Q

Dune formation

A

Also form by detachment of streamlines from the bed. But, related to turbulence throughout entire flow.
Dependent on flow depth since this affects the scale of turbulent eddies.
Low flow rates create straight creasted dunes with planar cross bedding.
High flow rates create sinuous creates dunes with trough cross bedding because at the flow reattachment point there are market scour pits.

59
Q

Lower plane bed

A

Form when grain diameter > 0.7mm and viscous sublayer becomes hydraulically rough i.e. ripples can’t form

60
Q

Why dunes aren’t just large ripples

A

Very few bedforms are c. 10cm high and c. 1m in wavelength. So, ripples don’t just grow until they become dunes

61
Q

Counter-flow ripples

A

If flow rate is high enough, a large roller vortex can be created on the lee side of dunes from flow separation. These vortices will cause ripples to migrate up the slope of dunes, antiparallel to the net current.

62
Q

Upper plane bed

A

Fluid velocity is high enough to entrain all grains that would have made up other bedforms. Leads to high concentrations of suspended load which dampens turbulence. No turbulence = no ripples or dunes.
Has planar lamination in cross section.
Parting lineation or primary current lineation: Streaks of different grain sizes parallel to flow. Eddies sweep into viscous sub-layer but can’t form ripples.

63
Q

Froude number

A

Supercritical flow when FN > 1

Depends on flow velocity and flow depth

64
Q

Supercritical Flow

A

Flow velocity > wave velocity

Temporary standing wave forms on the surface that eventually steepen and break upstream

65
Q

Antidunes

A

Created by supercritical flow
In phase with the water surface
Have cross bedding that dips up stream

66
Q

Lower flow regime

A

Ripples, dunes, lower plane beds are stable

67
Q

Supper flow regime

A

Antidunes and upper plane beds are stable

68
Q

Bedform stability diagrams

A

Show what bedforms are stable at a given temperature and flow depth.
Flow velocity against grain size

69
Q

Wave ripples

A

Waves are orbital motion of water molecules.
If water depth > wave base, this exponentially dies out with increasing depth due to drag.
If water depth < wave base, orbital motion becomes more elliptical and eventually becomes horizontal oscillation due to friction with bed.
Symmetrical ripples generated: Max velocity of oscillation at centre, minimum velocity at extremes. So, grains rolled from centre to edges.

70
Q

Wave base

A

Greatest depth at which wave effects are felt = half the surface wavelength

71
Q

Sedimentary facies

A

A body of rock that has a certain combination of lithological, physical and biological structures

72
Q

Depositional sedimentary environments

A

Net accumulation of sediment

Preserved in geological record

73
Q

Erosional sedimentary environments

A

Net errosion

Not preserved in geological record

74
Q

Accomodation space

A

Where sediment can accumulate
Isolated from erosional processes

Affected by changes in balance of erosion and deposition; tectonics; sea level change

e.g. deep ocean or half-graben or trenches

75
Q

Sedimentary basin

A

Topographic lows generated by subsidence

76
Q

Extensional basin

A

Form during 1st stage of rifting

77
Q

Intracratonic basin

A

Large areas of subsidence within a continental block.
Large areas but low rates and amount of subsidence.

e.g. Murzuk and Kufra basins in southern Libya.

Those that formed from extinct rifting are due to thermal subsidence. Rifting thins the crust and uplifts hot mantle. So there is a greater geothermal gradient. When rifting stops, the area cools to the stable geotherm and the crust above subsides.

78
Q

Horst

A

Sections of an extensional basin that faulted upwards

79
Q

Graben

A

Sections of an extensional basin that faulted downwards

80
Q

Passive margin basins

A

During rifting, continental crust is injected with basaltic magmas and thins. After the formation of an ocean, the transitional crust forms the continental shelf and slope - where lots of sediment settles.

81
Q

Pull-apart basins

A

Created when diverging strike-slip motion of an offset fault.

e.g. Dead Sea

82
Q

Terrestrial rift valley

A

Basin formed by extension of continental crust that’s on land
e.g. Death valley

83
Q

Maritime rift

A

Accomodation space formed by extension in oceans and seas.

e.g. Gulf of Corinth in Greece

84
Q

Peripheral foreland basin

A

Due to loading of a volcanic arc during oceanic subduction. Basin forms on the opposite side of the volcanic arc from the trench.

Earth 1A (17 decks)

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