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Dynamic Earth > Week 3 To 5 > Flashcards

Week 3 To 5 Flashcards

1
Q

How are clay minerals formed?

A

-Magma cools the mineral crystallising out becomes polymerised
- From the double chain silicates downwards, Aluminium is found in the amphibole, mica and feldspar families
- When these hydrolyse they leave behind insoluble residues of clay minerals

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2
Q

What is the structure of clays?

A
  • Clays are sheet or phyllosilicate minerals
  • Extremely fine-grained (<2micrometers)
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3
Q

What are the 2 components of clays?

A

Tetrahedral components - a layer of SiO4 tetrahedra in which 3 of the 4 oxygens are shared
Octahedral component - Aluminium in octahedral coordination with O2- and OH- ions so that Al3+ ions are sandwiched between 2 sheets of O2- and OH-.

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4
Q

What are the 2 types of clays that we have learned?

A

Kaolinte - 2 layered kandites, SiO4 tetrahedra linked to gibbsite octahedral by common O2- and OH- ions
Smectite - 3 layers, a gibbsite octahedral layer is sandwiched between 2 SiO4 tetrahedral layers
- Smectites can shrink and swell as water is adsorbed and desorbed.

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5
Q

Uses of clays?

A

Original clastic and diagenetic clays - aquifers and petroleum reservoirs
Kaolinites - ceramics and paper
Smectites- lining ponds, cat litter clumping and diarrhoea remedies

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6
Q

What happens during post depositional change?

A

-Sediment is unconsolidated
-The more rapidly it is transported and deposited, the less well sorted it will be and the more unstable it will be
- Soft sediment can be modified by the effects of gravitation and fluid movement to form soft sediment deformation structures

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7
Q

What happens at a grain/micro scale?

A

-Physical and chemical changes alter characteristics of sediment(diagenesis)
-Diagenetic processes occur at low temperatures and pressures eg Lithiification

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8
Q

What is burial diagenesis and what are the 4 different contacts?

A

Burial diagenesis: mechanical compaction and this leads to a reduction of volume and porosity
-Point contacts
-Long contacts
-Concavo-convex contacts
-Sutures contacts
These are progressive starting at point contacts and ending at sutured

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9
Q

What is cementation?

A

Growth of crystals in pore spaces
-most sediments are saturated with pore waters with which they react
- ions dissolved in one place maybe carried to another to another place where they precipitate
-Local chemistry dictates processes

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10
Q

What are common cements?

A

-Calcium carbonates
-Silica/quartz
-Clay minerals

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11
Q

What forms nodules and concretions?

A

-Calcium carbonate, silica and pyrite may all form nodules and and concretions
-Silica concretions are common in chalk where they are known as flint- a form of chert

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12
Q

What geological process forms coal?

A

Organic diagenesis

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13
Q

What is a sediment and how much of the Earths surface is made up of sedimentary rocks?

A

Sediment - solid fragmented material which is either eroded from pre-existed rock, chemically precipitated from solution or secretion from organisms
-70%

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14
Q

What are the 2 rock types?

A

Crystalline - igneous and metamorphic
Fragmentary- usually sedimentary

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15
Q

Importance of sediments?

A

-Understanding earth surface processes(palaeoenvironments,facies analysis, Earths history
- Economic resources and pollution sinks

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16
Q

What is facies analysis?

A

Facies analysis- a body of rock with distinctive characteristics that reflect the conditions of deposition

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17
Q

How is a facies analysis done?

A

-It’s done on levels of grain type to sedimentary structure to associations of structures
Results in a series of typical environmental packages or facies models eg deep marine facies or alluvial fan facies

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18
Q

What is a mud and what is a sand?

A

Mud - anything where you cannot discern individual grains with the naked eye
Sand - grains visible with the naked eye but <2mm longest axis

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19
Q

What descriptors are used to describe and classify sedimentary rocks?

A

-Sorting
-roundness
-sphericity
- Texture is the combination of all 3

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20
Q

What is the compositional classification of sediments?

A
  • 70%+ - mud
  • 15-70% - wacke
  • 0-15% - arenite
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21
Q

What features change with maturity of a sediment?

A

Mineralogy - the more mature, the higher proportion of quartz
Texture - the more mature, the more rounded the grains are as they have been abraded and rolled
Size - the more mature, the smaller the grains will be, and further they may have been transported from source and the lower the velocity of the depositing flow

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22
Q

What is a fluvial system?

A
  • They erode, deposit and transport sediment
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23
Q

Where do fluvial systems transport sediment from?

A
  • Uplands to lakes and the sea
    This is done through fans, channels, floodplains, deltas and estuaries
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24
Q

What do fluvial deposits record?

A

Geometry, flows and processes of ancient environments and link to climate, tectonics and sea level

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25
Q

What are alluvial fans and their significance to the geological record?

A

-Minor component of stratigraphic record
-However palaeoclimatically and palaeotectonically highly significant
- Indicates sharp terrestrial relief and form at significant changes in slope
-Indicative of climatic extremes
- Contains coasts form sources long eroded away

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26
Q

What is an Intramontare basin?

A

Intramontare basin
- Coalescing fans form a sediment apron
- May drain into a closed basin or river system

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27
Q

How is fan size determined?

A
  • Size and relief of the drainage area, lithology and climate
  • The bigger the fan the lower the gradient
    -The gradient of a fan depends on proportion of cohesive fines in the sediment
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28
Q

What are the 6 shapes of river?

A

-Straight
-Single channel
-Without channel bars
- Sinuous(meandering)
-Multiple channel
- With channel bars(braided)

29
Q

What are sinuous channels?

A
  • Very erosive on the outside and deposition on the inside
  • Asymmetrical
30
Q

What is a desert?

A

an area with precipitation/ evapotranspiration is <0.2 mm per year

31
Q

What is aeolian transport?

A
  • where air is the transporting medium/fluid
  • the least viscous
32
Q

What is the angle of repose of dry sand and how is the angle affected?

A

-<35 degrees due to surface area, density and friction
-the angle decreases with increasing maturity
and increases with the addition of water

33
Q

What are the 4 types of dunes?

A

-Barchan dunes -> crescent shaped dunes that face the wind
-Linear dunes -> sand ridges with long parallel crests
-Transverse dunes -> linear sand deposits that form perpendicular to the direction of the prevailing wind
-Star dunes -> dunes with multiple arms/ridges formed from multiple wind directions

34
Q

What are draas?

A

-Draas are very large aeolian landforms that posses steep and slip faces and has lower order superposed bed-forms such as ripples and dunes

35
Q

What is a loess/interdune sediments?

A

-Loess -> a sedimentary deposit composed of largely of silt sized grains that are loosely cemented by calcium carbonate

36
Q

What is an Arid basin deflation surface and what are its features?

A

-Armouring of pebbles left behind where the wind can’t pick them up
-High diurnal temperature ranges
- Precipitation varnish on grains
-Hard iron rich layer holds pebbles in place

37
Q

What are key features of an arid plain?

A

-Surface floods and the water evaporates which leaves behind marginal salt crust
- The salt crust grows vertically once there is no more space to grow laterally forming desiccating polygons

38
Q

What is a playa basin?

A

a dry, vegetation free, flat area at the lowest part of an undrained desert basin

39
Q

What is a delta?

A
  • A protrubance of the shoreline that forms when a river enters a basin whose indigenous processes, waves and tides, are unable to redistribute the sediment load
40
Q

What are key features of a delta?

A
  • They are progradational so they grow out into the sea or lake.
  • They produce coarsening -> upward cycles during progradation
  • Channel flow of sediment slows down and disperses pushing sediment away
41
Q

What are the 3 types of flow?

A

Homopycnal flow - turbulent water mixes causing early dumping of sediment
Hyperpycnal flow - turbulent water has a greater density than the receiving water body causing sediment to disperse
Hypopycnal flow - water with a lower density than the receiving body flows into the receiving body

42
Q

What are the 3 types of delta?

A

River dominates - occurs when waves and tide action is weak eg Mississippi
Wave dominated - occurs when wave strength affect the sediment flow
Tide dominated - occurs when tidal action is strong and frequent causing ‘packets’ of sediment orientated parallel to the delta flow

43
Q

What is a limestone?

A
  • Any rock >50% calcium carbonate
44
Q

How are non-clastic limestones formed?

A

They are formed by biogeochemical processes or direct precipitation

45
Q

What are the key features of the deposition of limestones?

A

-Produced and deposited in the same basin

46
Q

Key feature of limestones?

A

-they effervesce with dilute acid
-can be scratched with steel

47
Q

What is the reaction between CaCO3 and water and carbon dioxide?

A

CaCO3 + H2O + CO2 -><- Ca2+ + 2HCO3-

48
Q

What are the carbonate minerals?

A

-Calcite -> trigonal crystals

-Aragonite-> Orthorhombic crystals, only stable in warm water environments and areas close to these
-Dolomite -> trigonal crystals

49
Q

What are skeletal grains?

A
  • Grains formed by dead, carbonate secreting organisms such as bivalves, gastropods, Brachiopoda, Cora,s Echinodermata, foraminifera, Bryozoa, Porifera, Arthropoda and trilobites
50
Q

What organisms secreted calcite?

A

-Brachiopoda
-Corals
-Echinodermata
-Foraminifera
-Bryozoa
-Porifera
- Arthropoda
-Triolbites

51
Q

What organisms secreted aragonite?

A

-Bivalves(originally)
-Gastropods

52
Q

What are the non-skeletal grains and what are their key features?

A

-Peloids -> oval particles with fecal origin and shows quiet, slow water environments
-Intraclasts-> pieces of pre existed limestone becoming clasts of younger limestone
-Ooids-> little particles found in tropical environments and made from aragonite needles that are precipitated and glued together by bacterial mucus

53
Q

What are the major controls on carbonate sedimentation?

A

-Geotectonics -> dictates nature of coastal areas and will determine both hinterland topography and river drainage
-Climate-> carbonates accumulate most rapidly in the tropics and many of the important carbonate-producing organisms can only exist in warmer tropical waters
-Temperature and salinity-> ooids form when sea water is warmer than 18 degrees C and most carbonate biomineralising organisms are intolerant to fluctuations in salinity.
-Water depth-> most skeletal growth occurs in water less than 5m deep and the most productive zone is 9-15m
-Turbidity-> can be increased by the addition of suspended clay and by cutting out light turbidity inhibits production by organisms dependant on light

54
Q

What is a carbonate platform and what are the 5 types of platform?

A

-A carbonate platform is where most shallow-water carbonates are generated and deposited

-The rimmed shelf
-The carbonate ramp
-Epeiric platforms
-Isolated platforms
-Drowned platforms

55
Q

What is the rimmed shelf?

A
  • a shallow water platform bordered on its seaward margin by a pronounced break in slope
  • at the break there is a continuous rim of reefs or of carbonate sand shoals
  • gravity flows are common off the shelf-break
    Eg Great Barrier Reef
56
Q

What is the carbonate ramp?

A

-Gently sloping surface of around 1 degree
-Nearshore, wave-agitated sandy facies pass offshore into muddier, deeper water facies
- Barrier reefs are never found although small mud mounds and pinnacle reefs may form

57
Q

What is an Epieiric platform?

A

-Extensive flat, cratonic areas by shallow sea
-On their ocean ward margins they may be bounded by gentle , ramp-like slopes or by steep slopes
-Their margins may be rimmed by barrier reefs or sand shoals
-The platform itself has its own depositional environments dominated by shallow intertidal-subtidal facies
-Ancient ones were larger due to higher sea levels

58
Q

What is an isolated platform?

A

-Shallow water zones with steep sides
- The facies distribution on the platform is affected by prevailing wind and storm direction
-The Bahama bank is an example of a large isolated platform
-Atolls are a specific type of isolated platform governed by volcanic and reefal processes

59
Q

What are drowned platforms?

A

-They can be any form of platform which has been subjected to rapid relative sea level rise causing deep water facies to be deposited over shallow water facies

60
Q

What are the different deep marine structures and their key characteristics?

A

-Shelf -> 1-2 degrees, basin ward slope of 0-180m bsl and detrital sediments are found there
-Slope -> >6 degrees, 180-3200m bsl and both pelagic and detrital sediments are found there
-Rise -> 2 degrees, 3200-5000m bsl and pelagic and redistributed sediments are found there
-Abyssal plain -> 0.05 degrees, >5000m bsl, marginal fans reworked by contour currents and pelagic sediments are found there

61
Q

What is the origin and supply of sediment for deep marine facies?

A
  • Primarily produced by weathering and erosion of pre-existing rocks depending on tectonics, glacial and eustatic processes
    -Most is detrital but the rest is pelagic
62
Q

What fluids and forces acting in deep oceans?

A

-In deep water, thermohaline currents cause circulation of water
- Current velocity is an average on 2ms-1 rising to 10-20ms-1

63
Q

What are the processes by which sediment and solutes are redistributed?

A

-Resedimentation -> moves sediment downslope over the sea floor from shallow to deep water
-Debris flows-> concentrated viscous sediment dispersions which move intermittently via glacial type movement
-When the critical yield strength is exceeded at which point flow begins along the basal zone of the highest shear stress
-Loosely packed silt and sand are the most likely to move

64
Q

What are turbidity flows?

A

-Most common sediment-driven underflows in water bodies, resulting from relatively dilute suspensions of sediment
-Many are initiated by the mixing of slumps and debris flows with sea water or by currents feeding into sub-marine canyons or by storms stirring
-Some sediments maybe deposited as turbidities associated with submarine fans

65
Q

What is normal bottom current?

A

-The action of normal bottom currents on resedimented deposits eg turbidites turns them into contourites

66
Q

What is surface currents and pelagic settling?

A

-Pelagic sedimentation affects the non-marginal areas, beyond the influence of detrital processes of the deep seas
- Organic -> formed from the skeletal parts of silica and carbonate-producing organisms resident in the oceans
-Inorganic-> derived from desert dust storms, volcanic eruptions etc which forms the abyssal clays
-Their sedimentation involves the slow settling of microscopic particles in the water column

67
Q

What are authigenic processes?

A

-In areas where sedimentation rates is negligible and where the chemistry is suitable, eg round hydrothermal vents, ferromanganese nodules and crusts, umbers and ochres may form

68
Q

What are ferromanganese nodules and metaliferous deposits?

A

-Ferromanganese nodules are generally of todorokite
(Mn2+R+R2+)(Mn4+Mn2+)O6x3H2O where R= other metals
-Metaliferous deposits are frequently associated with MOR volcanism eg Troodos umbers(Mn and Fe rich) and ochres(Mn poor)

Dynamic Earth (3 decks)

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