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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

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2
Q

What is lift force proportional to?

A

Velocity of flow and density of medium

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3
Q

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

A

30 m s-1

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4
Q

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

A

0.5 mm

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5
Q

What is the technical definition of a desert?

A

less than 250 mm of precipitation per year

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6
Q

What is the technical definition of semi-arid?

A

Between 250 - 500 mm of precipitation per year

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7
Q

What is an erg?

A

An accumulation of aeolian blown sand

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8
Q

What is often lacking for aeolian transportation to be prominent?

A

Vegetation

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9
Q

What is sand-blasting?

A

Erosive effect removing and polishing surface

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10
Q

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

A

Zweikanter

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11
Q

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

A

Dreikanter

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12
Q

What is a common feature of zweikanter and dreikanter?

A

Angled edges between the faces

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13
Q

What is desert varnish?

A

Iron/Manganese Oxides from long exposure to oxidising conditions

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14
Q

What three common facies are often found in deserts?

A

Ephemeral lakes, sand dunes, alluvial fan deposits

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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

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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

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17
Q

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

A

Scanning Electron Microscopy (SEM)

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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

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19
Q

What happens to grains close to the transport threshold?

A

Rolled as bedload - creating ripples/dunes

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20
Q

What is the winnowing effect?

A

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

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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

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22
Q

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

A

Compositionally mature, texturally mature, quartz aranite

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23
Q

What are the three groups of aeolian bedforms?

A

Ripple field, dune field, draa field

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24
Q

What scale are ripples?

A

mm to cm

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25
What scale are dunes?
10's cm to 10's m
26
What scale are draas?
>=100 m
27
How do grains move to form ripples?
Saltation - one bounces and lands to hit another into suspension. Irregularities form pile ups. Coarser grains remain while finer grains winnowed
28
How does the sand become graded in a ripple?
Inversely graded - finer sand with coarser sand on top
29
Cross-lamination is more common in ___ ripples than ___ ripples
sub-aqueous ripples than aeolian ripples
30
What is a typical wavelength of an aeolian dune?
3 - 600 m
31
What is the typical heigh of an aeolian dune?
10cm to 100m high
32
What process occurs on the stoss side of an aeolian dune?
Saltation which may form superimposed ripples
33
What process occurs on the lee slope of an aeolian dune?
Grain flow/avalanche
34
When does cross bedding occur in a dune?
When there is a net accumulation of sand
35
What process occurs on a dune in periods of high wind?
Sand is in temporary suspension then fallout occurs on the lee side which mixes in with the grain flow
36
What are the three features of a transverse dune?
Planar cross-beds Straight crested, perpendicular to wind Abundant sand supply needed
37
What are the four features of Barchan dunes?
Less sand supply needed Lunate structures Arculate slip faces Trough cross bedding
38
What are the two features of linear (seif) dunes?
Two prominent wind directions of 90 degrees | Cross-bedding on both sides of the dunes
39
What are the two features of star dunes?
Slip faces in multiple directions | Cross bedding displaces variablity
40
Cross beds are most commonly formed on dunes of what size range?
1m - 20m
41
Large scale cross-bedding is seen more frequently in __ dunes than ___ dunes
sub-aqueous than desert
42
Ergs which are viewed at high altitudes are often what?
Draas - with dunes/ripples superimposed
43
What is a typical wavelength of a draa?
100's m to km's
44
Which of the formations can draa's form?
Star, linear and transverse
45
Barchan dunes may have faces with up to ___ degrees variablity?
45
46
How is wind direction expressed?
The direction in which the wind blows from - i.e. westerlies from the west
47
What may the stratagraphic succession of a desert facies look like?
It may contain dunes, ephemeral lakes and rivers, and alluvial sands all in susession
48
What is the dominant factor in determining distribution and extent of sandy deserts?
Climate - Arid conditions required, i.e. no water or veg
49
What is usually required for an erg formation?
Local/regional depression
50
What does a high water table do to dune processes?
Wet interdune sand is stable and thus not picked up by aeolian processes - accumulation of sand
51
How does a fall in the water table effect a desert environment?
Usually results in net erosion
52
What is a common feature in erg formations in warm sub-tropical regions?
Dry, offshore wind patterns, on western side of continents where easterlies have lost moisture crossing easterly continent
53
What is the primary reason for higher aeolian transportation during an ice age/glacial?
More extreme pole-equator pressure variation, higher winds
54
Give an example of extensive permian aeolian deposis
Northern Europe, where there was the gondawana glaciation in the Southern Hemisphere
55
When may an erg be a relic within a modern desert?
Was once very active - such as in the Pleistocene, now inactive
56
What gives the yellow colouring of desert sand?
Goethite or Iron Hydroxite (Fe(OH)x)
57
What results from the oxidation of goethite and what colour is it usually?
Iron Oxide (Haematite), Strong red colour
58
What colours are unlikely to be found in the minerals within a desert setting and why?
Green, grey, black | Usually reduced iron oxide and dark organic material which are unlikely in such a dry and air exposed environment
59
What are red beds?
strong successions of red sandstone/mudstone
60
Why is haematite presence and the red colour found in red beds not necessarily indicative of an oxidising continental environment? (3)
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
Why are aeolian sandstones some of the best reservoirs for oil?
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
Why is there little or no potential for formation and preservation of hydrocarbons?
Strong oxidising conditions
63
What is the typical lithologies of aeolian deposits?
Sand and silt only
64
What is the typical mineralogy of aeolian deposits?
Mainly quartz, rare examples of carbonates and other materials
65
What is the typical texture of aeolian deposits?
Well to very well sorted silt to medium sand
66
Are fossils often preserved in aeolian deposits?
Rare in desert dune - occasional vertebrate bone
67
What is the typical bed geometry of aeolian deposits?
Sheets or lenses of sand
68
What is the typical sedimentary structures found in aeolian deposits?
Large scale dune cross-bedding + parallel strata in sands
69
How is paleocurrent of the aeolian deposits often determined?
Dune orientation from x-bedding giving wind direction
70
What is the typical colours found in aeolian deposits?
Yellow to red, due to iron hydroxides and oxides
71
What is the typical facies associations of aeolian deposits?
Alluvial fans, ephemeral river/lake facies in deserts, beach deposits/glacial outwash facies

Sedimentology and Stratigraphy (19 decks)

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