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GL3521 > Aeolian Sediment s > Flashcards

Aeolian Sediment s Flashcards

1
Q

Name the places you can get aeolian sediments.

A

Subtropical areas that are dry, beaches, coastlines. Rain shadow areas, polar regions adjacent to glaciers, centre of large continents far away from sea.

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2
Q

Are aeolian sediments temperature dependent?

A

No, the area just need to be dry.

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3
Q

why are grains that are <80Um hard to entrain?

A

Because they are so small, electrostatic attraction make them stick together so you need a larger velocity to get them into motion.

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4
Q

How much water do you need to stop entrainment of sediment? How much moisture do you need to entrain sand?

A

5% moisture stops wind erosion.

Only need 0.6% moisture to entrain sand.

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5
Q

What can grow in dunes due to the presence of moisture and bind grains together? what part of the dune will they grow in?

A

Evaporite minerals, fungi and algae. Will grow in the low lying areas such as the interdunes and beaches.

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6
Q

what are the 2 primary modes of transportation related to aeolian sediments?

A

Suspension and saltation.

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7
Q

Discuss suspension

A

The finest grained silt sizes go into suspension, the non-settling grains get transported long distances <20Um and can be carried out of the environment. The settling grains go into suspension but drop out quickly (20-60Um) these are known as Stoess.

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8
Q

Discuss saltation

A

Saltation is when grains land from being in suspension they kick up other grains causing them to be entrained creating a positive feedback, this positive feedback means the velocity required to entrain the sand is lower which is known as impact velocity.
Impact erosion occurs as saltation causes sand to travel close to the ground in a cloud (<2cm) which causes sand blasting of grains otherwise known as ventifacts.

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9
Q

Discuss the relationship between wind and sorting. Name the different sorting groups.

A 
Wind is very good at sorting, the efficiency varies with transport distance, aeolian sand is always better sorted than the original sediment supply. 
Very course sand (2-4mm or above)
Coarse sand (0.3-1mm)
Fine-Medium sand (0.3-0.1mm)
Silt (<0.1mm)
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10
Q

Can the sand mineralogy only be quartz?

A

No

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11
Q

What could sand mineralogy be made of?

A

Evaporites, carbonates, volcaniclastics, feldspars, quartz, clay pellets

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12
Q

Discuss common aeolian sand texture. what is sand texture dependent on?

A

angular - sub angular. 8% are rounded.

Depends on source sediment and proximity.

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13
Q

What is the difference between water and wind created ripples?

A

Wind created ripples are not very high.

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14
Q

what gets left behind in relation to ripples depends on what 2 things, these two things in turn depend on …. and ….

A

What gets left behind depends on the angle of climb and if ripple slipfaces have developed, these two things in turn depend on sedimentation rate and sediment sorting

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15
Q

What side is the stoss and lee side at?

A

stoss side is upwind side, lee side is the downwind side which is steeper.

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16
Q

Discuss how stacked ripples form? What if they were water created what would their structure look like?

A

As one ripple is moving along, another is coming and stacking on top causing us to end up with stacked ripples.

If they were water created because the amplitude is higher you see the climb and internal laminations left behind.

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17
Q

Wind ripples have: ……. lamination = … rate of climb, no ……..

A

Flat lamination = low rate of climb, no slipface.

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18
Q

Draw a water ripple and a wind ripple and discuss the difference.

A

Water ripples have higher amplitudes, so you see the climb and internal laminations left behind.

Wind ripples: Are not as high, laminations created by migration of ripples but you dont see internal laminations preserved. Subcritically climbing because angle of climb is low. Flat lamination = low rate of climb, no slipface.

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19
Q

Can ripple laminations be inversely graded?

A

Yes, the finer grains are winnowed off crests and coarser grains are sheltered in trough. Tend to get bimodal grain size

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20
Q

What angle is the stoss slope usually?

What angle does sediment tend to avalanche on the lee slope/slipface

A

10-15 degrees

33/34 degrees.

21
Q

where do grainflows occur ?

A

They occur on the slipface when avalanching occurs causing grainflows.

22
Q

when do grainfall occur?

A

During grainflows some sediment is picked off the top and is suspended, it then falls out and is deposited beyond the dune toe. They are rarely preserved as they are often reworked.

23
Q

Which is less likely to be preserved, grainflows or grainfalls?

A

Grainfalls have the least preservation potential as they are often reworked.

24
Q

Draw on a diagram and discuss where grainfalls and grainflows occur.

A

On the upwind side you get wind ripples, on the slipface grainflows occur as you get avalanching of the sediment. Some sand is whipped off the top into suspension and drops out which is known as grainflows.

25
Q

Discuss some features you would find in a grainflow.

A

Thick units of crossbedding, lense shaped in cross section, poorly sorted and loosely packed, high permeability.

26
Q

Which is more permeable, grainflows or grainfalls?

A

Grainflows.

27
Q

what are the 1st and 2nd sedimentary structures you look for to identify that something is aeolian?

A

wind ripple lamination and grainflow lamination

28
Q

Why do lamination patterns vary alot in aeolian sediments?

A

Because there is a seasonal variation in the wind, it is not always perpendicular to the dune crest

29
Q

dune types can be ………. or …………., discuss these types.

A

Morphological or morphodynamic.
Morphological - Shape in plan form
Morphodynamic - Relationship of the shape of the dune to the dominant wind direction.

30
Q

what does RDD stand for?

A

Residual Drift Direction which is the average wind direction

31
Q

discuss compound and complex dunes

A

when dunes are stacked on top of one another they can either be compound or complex. Compound dunes are dunes that are all the same type stacked on top of one another, complex dunes are where one type is superimposed on top of another.

32
Q

what is the largest dune type on the planet?

A

Star dunes

33
Q

Discuss the preservation potential of star dunes

A

Star dunes will only be preserved if there are converging winds and a high amount of sand available, the highest parts of these dunes are never preserved but the lowest parts are.

34
Q

Discuss star dunes

A

Star dunes are the largest type of dune on the planet, occur when you have a large sand supply and converging winds. They are pyramidal in shape and have multiple slipfaces. The highest parts of them are not preserved but the lowest parts are.

35
Q

What is the most common dune form on the planet? what percentage do they make up of dunes globally?

A

Linear dunes, make up 50% of dunes globally.

36
Q

Discuss linear dunes. What are the two main types and give an example of them.

A

they are the most common dune form, make up around 50% of dunes globally. There are a variety of types but they are all long (20-40km), straight, parallel and regularly spaced. They form in areas with bimodal winds and are dominated by wind ripple lamination
Two main types are
- low partly vegetated ex. Simpson desert australia
- Sharp crested sinuous type. ex Sinai, East Sahara

37
Q

Discuss sand sheets

A

Areas that are lacking dunes where the wind is movign the sediment. Sediment supply is limited as the water table is too high too often. There is a limited time for dune building due to periodic flooding and sediment being too coarse. Internally complicated with wind ripples laminations and are disturbed by burrowing, rooting and soils. Make up 38% of aeolian deposits

38
Q

State the percentage of aeolian deposits that are sand sheets

A

38%

39
Q

Where do interdunes occur and what are their dominant factors

A

Occur between dune bedforms, dominant factors are height of water table and sediment supply.

40
Q

discuss the 2 classes interdunes are divided into

A

Deflationary: nothing accumulating, losing sediment
depositional: accumulating sediment. Divided into 3 subsets: wet, dry and damp.

41
Q

Discuss stokes surface

A

Is deflation to the water table, you cannot deflate lower than the water table.

42
Q

dry interdunes share similar characteristics to ….. …….

A

sand sheets

43
Q

From modern studies what do we expect to see in damp and wet interdunes?

A

Sabkhas, adhesion structures, non-aeolian facies.

44
Q

Discuss features that may be present in wet interdunes

A

SSD; rooting or burrowing.
Early cementation; gypsum halite etc.
Lots of deformation like fluid escape and bioturbation

45
Q

Discuss bedform climbing

A

As the dune migrates the lee side scour removes the stoss and interdune sediment, if this is not completely removed then you have sediment accumulating. The lee side will be preserved during accumulation

46
Q

What is the sediment budget controlled by?

Discuss positive, neutral and negative budgets.

A

Controlled if supply is cut off or sediment is too wet to transport.
Positive budget during accumulation when IN>OUT.
Neutral budget during bypass when IN=OUT.
Negative budget during erosion when IN

47
Q

Can the angle of climb vary? Discuss.

A

Yes it can vary if there is a rising water table.

48
Q

Can aeolian sediments be bioturbated?

A

Yes when sediment is damp

49
Q

Discuss mass flows in relation to aeolian sediments

A

Mass flows can occur in the form of liquefaction due to flooding in dune areas creating structureless sands. Marine flooding or large rainfall events

GL3521 (16 decks)

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