Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Sedimentology and Stratigraphy > (S3) Deserts > Flashcards

(S3) Deserts Flashcards

1
Q

What are katabatic winds?

A

Strong, cold air masses moving down mountain slopes/off edges of ice masses

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is lift force proportional to?

A

Velocity of flow and density of medium

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What speed wind is required to lift quartz grains up to 0.5 mm diameter?

A

30 m s-1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Aeolian processes can only generally carry quartz grains up to __ mm in diameter

A

0.5 mm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is the technical definition of a desert?

A

less than 250 mm of precipitation per year

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the technical definition of semi-arid?

A

Between 250 - 500 mm of precipitation per year

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is an erg?

A

An accumulation of aeolian blown sand

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is often lacking for aeolian transportation to be prominent?

A

Vegetation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is sand-blasting?

A

Erosive effect removing and polishing surface

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the name for a clast with two faces polished smooth?

A

Zweikanter

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the name for a clast with three faces polished smooth?

A

Dreikanter

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is a common feature of zweikanter and dreikanter?

A

Angled edges between the faces

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is desert varnish?

A

Iron/Manganese Oxides from long exposure to oxidising conditions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What three common facies are often found in deserts?

A

Ephemeral lakes, sand dunes, alluvial fan deposits

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the difference between grain collision in air vs water?

A

Grain collision in air much more effect than water due to waters cushioning effect

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Name the three common features found in aeolian deposits

A

Quartz - other softer materials destroyed on impact
Small grains (<0.5 mm diameter)
Frosted surface

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

How can we see the surface frosting of an aeolian grain?

A

Scanning Electron Microscopy (SEM)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What textural maturity is usually found in aeolian deposits?

A

Angular due to chipping impact - may be rounded if suspended for a long time

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What happens to grains close to the transport threshold?

A

Rolled as bedload - creating ripples/dunes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What is the winnowing effect?

A

Sorting of grain sizes - removes finer particles leaving more coarse grained remaining

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Give an example where aeolian features were inherited one or more cycles ago

A

An aeolian sandstone eroded by water where sand grains retain their original aeolian features

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What would be the name for the most expected sedimentary sample?

A

Compositionally mature, texturally mature, quartz aranite

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What are the three groups of aeolian bedforms?

A

Ripple field, dune field, draa field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What scale are ripples?

A

mm to cm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

What scale are dunes?

A

10’s cm to 10’s m

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What scale are draas?

A

> =100 m

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

How do grains move to form ripples?

A

Saltation - one bounces and lands to hit another into suspension. Irregularities form pile ups.
Coarser grains remain while finer grains winnowed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

How does the sand become graded in a ripple?

A

Inversely graded - finer sand with coarser sand on top

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

Cross-lamination is more common in ___ ripples than ___ ripples

A

sub-aqueous ripples than aeolian ripples

30
Q

What is a typical wavelength of an aeolian dune?

A

3 - 600 m

31
Q

What is the typical heigh of an aeolian dune?

A

10cm to 100m high

32
Q

What process occurs on the stoss side of an aeolian dune?

A

Saltation which may form superimposed ripples

33
Q

What process occurs on the lee slope of an aeolian dune?

A

Grain flow/avalanche

34
Q

When does cross bedding occur in a dune?

A

When there is a net accumulation of sand

35
Q

What process occurs on a dune in periods of high wind?

A

Sand is in temporary suspension then fallout occurs on the lee side which mixes in with the grain flow

36
Q

What are the three features of a transverse dune?

A

Planar cross-beds
Straight crested, perpendicular to wind
Abundant sand supply needed

37
Q

What are the four features of Barchan dunes?

A

Less sand supply needed
Lunate structures
Arculate slip faces
Trough cross bedding

38
Q

What are the two features of linear (seif) dunes?

A

Two prominent wind directions of 90 degrees

Cross-bedding on both sides of the dunes

39
Q

What are the two features of star dunes?

A

Slip faces in multiple directions

Cross bedding displaces variablity

40
Q

Cross beds are most commonly formed on dunes of what size range?

A

1m - 20m

41
Q

Large scale cross-bedding is seen more frequently in __ dunes than ___ dunes

A

sub-aqueous than desert

42
Q

Ergs which are viewed at high altitudes are often what?

A

Draas - with dunes/ripples superimposed

43
Q

What is a typical wavelength of a draa?

A

100’s m to km’s

44
Q

Which of the formations can draa’s form?

A

Star, linear and transverse

45
Q

Barchan dunes may have faces with up to ___ degrees variablity?

A

45

46
Q

How is wind direction expressed?

A

The direction in which the wind blows from - i.e. westerlies from the west

47
Q

What may the stratagraphic succession of a desert facies look like?

A

It may contain dunes, ephemeral lakes and rivers, and alluvial sands all in susession

48
Q

What is the dominant factor in determining distribution and extent of sandy deserts?

A

Climate - Arid conditions required, i.e. no water or veg

49
Q

What is usually required for an erg formation?

A

Local/regional depression

50
Q

What does a high water table do to dune processes?

A

Wet interdune sand is stable and thus not picked up by aeolian processes - accumulation of sand

51
Q

How does a fall in the water table effect a desert environment?

A

Usually results in net erosion

52
Q

What is a common feature in erg formations in warm sub-tropical regions?

A

Dry, offshore wind patterns, on western side of continents where easterlies have lost moisture crossing easterly continent

53
Q

What is the primary reason for higher aeolian transportation during an ice age/glacial?

A

More extreme pole-equator pressure variation, higher winds

54
Q

Give an example of extensive permian aeolian deposis

A

Northern Europe, where there was the gondawana glaciation in the Southern Hemisphere

55
Q

When may an erg be a relic within a modern desert?

A

Was once very active - such as in the Pleistocene, now inactive

56
Q

What gives the yellow colouring of desert sand?

A

Goethite or Iron Hydroxite (Fe(OH)x)

57
Q

What results from the oxidation of goethite and what colour is it usually?

A

Iron Oxide (Haematite), Strong red colour

58
Q

What colours are unlikely to be found in the minerals within a desert setting and why?

A

Green, grey, black

Usually reduced iron oxide and dark organic material which are unlikely in such a dry and air exposed environment

59
Q

What are red beds?

A

strong successions of red sandstone/mudstone

60
Q

Why is haematite presence and the red colour found in red beds not necessarily indicative of an oxidising continental environment? (3)

A

Not all desert deposits are red
Colour result of oxidation after deposition
Sediments deposited in other environments can also be deep red - e.g. deep marine mud

61
Q

Why are aeolian sandstones some of the best reservoirs for oil?

A

Highly sorted and rounded, gives excellent porosity and permeability
Units 10’s to 100’s m thick
Mud/evaporites can act as a trap

62
Q

Why is there little or no potential for formation and preservation of hydrocarbons?

A

Strong oxidising conditions

63
Q

What is the typical lithologies of aeolian deposits?

A

Sand and silt only

64
Q

What is the typical mineralogy of aeolian deposits?

A

Mainly quartz, rare examples of carbonates and other materials

65
Q

What is the typical texture of aeolian deposits?

A

Well to very well sorted silt to medium sand

66
Q

Are fossils often preserved in aeolian deposits?

A

Rare in desert dune - occasional vertebrate bone

67
Q

What is the typical bed geometry of aeolian deposits?

A

Sheets or lenses of sand

68
Q

What is the typical sedimentary structures found in aeolian deposits?

A

Large scale dune cross-bedding + parallel strata in sands

69
Q

How is paleocurrent of the aeolian deposits often determined?

A

Dune orientation from x-bedding giving wind direction

70
Q

What is the typical colours found in aeolian deposits?

A

Yellow to red, due to iron hydroxides and oxides

71
Q

What is the typical facies associations of aeolian deposits?

A

Alluvial fans, ephemeral river/lake facies in deserts, beach deposits/glacial outwash facies

Sedimentology and Stratigraphy (19 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions