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EART23202 Interpreting the stratigraphic record > Aeolian systems > Flashcards

Aeolian systems Flashcards

1
Q

Define aeolian

A

Processes of erosion, entrainment, transportation, and deposition by the wind

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2
Q

Why is it harder to move particles on land than it water?

A

The difference in density is greater

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3
Q

How are grains on land held together?

A

Gravity, friction, vegetations, cements, moisture

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4
Q

What forces promote entrainment of grains on land?

A

Ballistic impacts and aerodynamic lift

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5
Q

What forces prevent entrainment of grains on land?

A

Weight, friction, drag, cohesive inter-particle forces

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6
Q

How are sand grains primarily transported? What does their transport and deposition allow?

A

As a bedload
Allows the construction of dunes

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7
Q

How are silt and clays primarily transported?

A

In suspension

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8
Q

What is a deflation lag deposit?

A

Formed by the grains that are too heavy to move e.g. gravel

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9
Q

Give the three ways (or combination of these ways) that sand can be carried as a bedload

A

Rolling/creep due to drag on the bed
Saltation
Reptation

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10
Q

Describe saltation in the bedload transport of sand grains

A

Particles are temporarily lifted into the airflow, collide with other particles, introducing a chain reaction of interparticle collisions
This process is the most important

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11
Q

Describe reptation in the bedload transport of sand grains

A

Saltating particles return to the bed and impacting other particles, causing them to hop short distances downwind

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12
Q

Where are aeolian processes most active and widespread?

A

Arid, hot/cold deserts
These cover ~30% of land surface

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13
Q

Why are grains in deserts so suseptible to aeolian processes?

A

They lack vegetation (to provide strength and cover) and moisture

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14
Q

Describe the back-erg of a dunefield

A

Represents the trailing, upwind margin of the erg

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15
Q

Describe the fore-wind of a dunefield

A

Represents the leading, downwind margin of the dunefield

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16
Q

Define an erg

A

Wind-formed desert landscapes characterised by extensive sand dunes that cover the terrain

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17
Q

Give three examples of geomorphic elements that can be associated with dunes

A

Zibars, sandsheets, interdunes

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18
Q

What are the six types of sand dune?

A

Barchan, transverse, linear, star, dome, parabolic

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19
Q

What is a stabilising aeolian system?

A

Develop where physical, chemical, or biogenic agents promote aeolian system construction and accumulation

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20
Q

What are the three classifications of aeolian systems?

A

Dry, wet, stabilising

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21
Q

Describe a dry aeolian system

A

Accumulation is controlled by aerodynamic configuration
Airflow deceleration causes sediment-laden winds to deposit some of their load

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22
Q

Describe a wet aeolian system

A

Accumulation is controlled by progressive water-table rise concomitant with ongoing aeolian activity

23
Q

Where do siliciclastic coastal dunefield typically occur?

A

Along humid, mid-latitude coasts

24
Q

Where do carbonate dunefields typically form?

A

Preferentially along mid-low latitude coasts, esp those bordering productive shallow marine platforms (supply of carbonate sediment)

25
Q

How are aeolian processes inhibited in humid (inc. glacial regions) settings?

A

Vegetation and moisture

26
Q

What is aeolian loess?

A

A layer of fine, wind-blown silt and clay particles

27
Q

Where are loess deposits most typically found?

A

Areas that have experienced extensive glacial activity during past ice age
Areas where strong winds carry fine grains from exposed surfaces (e.g. glacial outwash plains, river valleys, deserts)

28
Q

What impacts do dust/sand storms have?

A

Reduced visibilty, productivty, resp diseases
Fertisation of oceans (stimulates growth of phytoplankton)

29
Q

What are the three scales of aeolian bedform?

A

Ripples, dunes, megadunes

30
Q

What are the three types of small-scale ripple structures that occur on wind-blown surfaces?

A

Impact ripples, megaripples, adhesion ripples

31
Q

What is the most widespread ripple type?

A

Wind ripples

32
Q

Describe wind ripples

A

Low relief and form preferentially form the coarser-grained fraction of sand in transport

33
Q

What are the three stages in the formation of wind ripples?

A

Initiation of development from perturbation on a bed
Impact ripple spacing determined by saltation path length
Upwind-facing stoss slope (subject to impact of infoming grains)

34
Q

What generates a grain flow sand avalanche?

A

Gravitational collapse of dry, loose sand on aeolian dune/megadune lee slopes inclined at the angle of repose (~32-36deg)

35
Q

What is an aeolian grain flow strata?

A

Thick package of multiple stacked aeolian grain flow avalanche deposits

36
Q

When does gravity-driven grain fall occur?

A

Where wind carrying clouds of saltating grains over a dune brink decelerates as it passes into the lee-side depression, resulting in a reduction in sediment carrying capacity

37
Q

Give another example of when gravity-driven grain fall can occur

A

On the apron of a dune where secondary cross-wind airflow is insufficient to form ripples

38
Q

Describe what happens when net aeolian sediment budget is positive

A

Accumulation occurs where bedforms may climb over one another at varying angles
Sets and cosets of cross strata are produced

39
Q

Describe the sets of cross strata formed when bedforms climb over one another

A

Contain smaller-scale facies units of wind-ripple, grainflow, grainfall, and plane-bedded strata
Therefore, dune elements are larger scale strata units

40
Q

Why is one bedform truncated by another?

A

They climb at low angles, the lower bedform is accumulated as the second overrides it

41
Q

Define aeolian interdunes

A

Elongated, relatively flat and sandy landforms that exist between larger sand dunes in desert and arid environments

42
Q

Describe a dry interdune

A

Water table is not in contact with depositional surface

43
Q

Describe a damp interdune

A

Capillary fringes of the water table are in contact with the depositional surface

44
Q

Describe a wet interdune

A

Water table is in contact with the depositional surface

45
Q

How do adhesion strata arise?

A

Arise from the adhesion of moving grains to a damp surface e.g. a damp interdune

46
Q

Describe adhesion strata

A

Low relief
Sub-horizontal structures with irregular surfaces

47
Q

Describe megadunes (draas)

A

Support development of superimposed dune-scale bedforms

48
Q

Describe a compound draas

A

Where a smaller dune is superimposed on a larger dune of the same morphological type

49
Q

Describe a complex draas

A

Where a smaller dune is superimposed on a larger dune of a different morphological type

50
Q

What is an aeolian sand sheet?

A

Areas of sand accumulation characterised by flat areas of impact wind ripples
Limited by sediment supply

51
Q

What is a zibar?

A

Low relief ridges and domes with gently inclined sloping surfaces

52
Q

What is required for the long-term preservation of aeolian sequences?

A

The strata is placed below a regional baseline, below which erosion does not occur

53
Q

What does subsidence allow?

A

It is the principal agent that generates accommodation and promotes preservation

54
Q

Give two examples of how aeolian strata can achieve long-term preservation

A

Accumulation occurs in a subsiding basin
Accumulation occurs below the water table and is protected from deflation

EART23202 Interpreting the stratigraphic record (11 decks)

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