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Sedimentology and Stratigraphy > (S5) Lakes > Flashcards

(S5) Lakes Flashcards

1
Q

What are the two main differences between sediment accumulation in lakes vs ocean?

A

Sediment accumulation much less in lakes, much more locally significant than in the ocean

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2
Q

What are the most common grain sizes associated with lake deposits?

A

Sand and mud

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3
Q

Aside from sand and mud, what other lithofacies are associated with lake deposits?

A

Limestones, Evaporites, Organic matter

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4
Q

What is “limnology”?

A

The study of modern lakes

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5
Q

What is the name for a the study of modern lakes?

A

Limnology

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6
Q

What are the seven common forms of depressions which can lead to lake formation?

A
  1. Tectonic subsidance
  2. Sag basins
  3. Thrust faults - damning river/stream
  4. Depression from erosion
  5. Glacial depressions - plus dams from moraines
  6. Landslides
  7. Caldera collapse
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7
Q

What do most of the depression formations to allow for lakes have in common?

A

Tectonic forces for the most part

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8
Q

How does a hydrologically open system work?

A

Balanced water in and out

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9
Q

What does a constant water level mean for ion concentration in lakes?

A

Lower concentration of dissolved ions as there is a constant influx of freshwater and outgoing water

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10
Q

When is a lake surface subject to evaporation?

A

High temperatures or dry air

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11
Q

How does a hydrologically closed system work?

A

No outflow due to high evaporation rates

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12
Q

What is another name for a hydrologically closed system?

A

Endorheic

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13
Q

What does a closed system mean for ion concentration in lakes?

A

Lakes become saline - may precipitate

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14
Q

What is the technical definition of a freshwater lake?

A

May be open or closed - low salinity - low supply of ions

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15
Q

What is the technical definition of a saline lake?

A

Hydrologically closed - perennial water bodies - dissolved/concentrated ions

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16
Q

What is the technical definition of an ephemeral lake?

A

Arid environment - temporary bodies of water existing for months/years but otherwise dry

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17
Q

What type of lakes is the majority of modern lakes?

A

Freshwater

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18
Q

Similar scale large/deep lakes were found in which time periods in the stratigraphic record?

A

Devonian/Neogene

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19
Q

What are the three main features of a freshwater lake?

A

Static, No tidal Influence, Circulation is mainly from surface winds

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20
Q

Surface winds on a freshwater lake can be expected to reach __ m s-1, especially in ___ with ___

A

30, narrow valleys, wind tunnels

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21
Q

Winds in freshwater lakes can move up to what grain size particles?

A

Silt to fine sands

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22
Q

How does oscillation effects in a freshwater lake change with depth?

A

Decrease with depth, below 10-20m there is no effect

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23
Q

What is the name for the upper mixed, oxic, warmer layer in a lake?

A

Epilimnion

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24
Q

What is the name for the lower non-mixed, anoxic, colder layer in a lake?

A

Hypolimnion

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25
Q

What is the name for a hypothetical line which separates the cold and warm layers in a lake?

A

Thermocline

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26
Q

What happens to organic matter which falls to the bed of a stable-system lake?

A

Will not be broken down due to anoxic environment

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27
Q

What is the name of a coal layer made up of plant material?

A

Detrital coal layer

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28
Q

What is the name of a coal layer which is made up of algal/bacterial material?

A

Sapropelic coal

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29
Q

How does a sapropelic layer contribute to oil/gas reservoirs?

A

Acts as a source rock

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30
Q

Why is there a lack of bioturbation in deep lake sediments?

A

Hypolimnion generally anoxic and unfavourable for life

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31
Q

What may a sediment laden river depositing into a lake form?

A

A delta

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32
Q

When can wave reworking occur in a lake environment?

A

Strong winds

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33
Q

What is an indicative factor of strong winds on a lake?

A

Sandy sediment around it’s margins

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34
Q

What is an indicative factor of weak winds on a lake?

A

Fine grained sediment around it’s margins

35
Q

What is the main cause for a poorly defined lake margin?

A

Gentle slope

36
Q

What is the name for a marshey environment around a lake?

A

Palustrine environment

37
Q

What is common in soil development in palustrine environments around a lake?

A

Calcareous nodules in soils

38
Q

What are the two main causes of clastic sedimentation transportation in deep lake environments?

A

Plume dispersal and density currents

39
Q

How does plume dispersal in deep lake environments work?

A

Plumes may remain above the thermocline - distribute sediment by wind driven currents

40
Q

How does density currents in deep lake environments work?

A

Coarser sediment on lake floor caused by turbidity currents - often grade from coarse to fine

41
Q

What would a deposit look like from a lake which had both plume dispersal and density currents?

A

Finely laminated mud alternating with thinly graded turbidites - thinly bedded succession

42
Q

Why is fine lamination particularly evident in deep lake facies?

A

Not destroyed by biogenic processes

43
Q

What is the name for annual/seasonal climate variations which are recorded in lake sediments?

A

Varves

44
Q

Briefly describe how varves form

A

Spring influx of cold sediment-laden water

Summer - organic material preserved in anaerobic conditions

45
Q

What is the expected scale of a varve?

A

mm laminae

46
Q

What is the name for the form created when mud cracks which dry out?

A

Desiccation cracks

47
Q

What is a likely cycle situation of desiccation cracks?

A

May be picked up and carried back out into the lake and deposited as clasts/chunks

48
Q

What would be a typical salinity of a saline lake?

A

5g L01

49
Q

What is the difference in water chemistry between a saline lake an marine water?

A

Dependant on the catchment lithology - lake will have a unique signature

50
Q

Name three common modern saline cations?

A

Sodium, Calcium, Magnesium

51
Q

Name three common modern saline anions?

A

Carbonate, Chloride, Sulphate

52
Q

What are the three main saline lake types according to the recognition of brines?

A

Soda lakes, Sulphate lakes, Chloride lakes

53
Q

What makes up the typical chemistry of a soda lake?

A

Bicarbonate ions + sodium carbonate minerals (trona and natron)

54
Q

What makes up the typical chemistry of a sulphate lake?

A

Magensium + calcium - precipitate sulphate minerals such as gypsum and mirabilite

55
Q

What is the chemistry of a chloride lake like? Give an example of this type of lake

A

Similar to marine - dead sea

56
Q

What organisms would be expected in a saline lake?

A

Limited variety - great abundance of blue-green algae which act as a food-chain to higher plants

57
Q

What may the resultant hydrocarbon deposits in a saline lake be like?

A

Successions of dark organic-rich shales and evaporites

58
Q

What is the main cause of an ephemeral lake formation?

A

Flash flooding

59
Q

What is the resultant succession from the drying of an ephemeral lake?

A

Mud and mudcracks exposed first - then overlain by evaporites which reflect lake water chemistry. Subsequent flooding may not dissolve evaporites and they may become quickly overlain by mud

60
Q

What is a signature succession found in ephemeral lakes?

A

Mud and evaporite couples in succession - cm to mm thick, repeated from flooding/drying events

61
Q

What is a desert rose?

A

a mass of gypsum crystals

62
Q

Where do desert roses form?

A

Within sediments around an ephemeral lake

63
Q

Evaporites may form within sediments around an ephemeral lake, in an area called an inland ____

A

Sabkha

64
Q

What does desiccation crack spacing depend on?

A

Thickness of the layer - broader cracks form in thicker layers

65
Q

What is a mud-chip/mud-flake?

A

A preserved section of mud removed from desiccation cracks

66
Q

When are desiccation cracks preserved?

A

When filled with silt/sand

67
Q

What are desiccation cracks a reliable indicator of?

A

Exposure to subaerial conditions

68
Q

What is the name for the cracks which form due to the shrinkage of clay underwater?

A

Syneresis cracks

69
Q

What do syneresis cracks look like?

A

Not polygonal but simple, straight or slightly curved/tapering

70
Q

Why does syneresis cracks not form in silt/sand and coarser materials?

A

Not cohesive

71
Q

What is another name for an overfilled lake?

A

Humid lake

72
Q

What occurs when a lake becomes overfilled?

A

Change from lacustrine to fluvial as river passes straight through lake

73
Q

What is the main difficulty in recognising lacustrine deposition in the geological record?

A

Hard to distinguish between lacustrine and low energy marine

74
Q

What main features does lacustrine and low energy marine have in common (3)

A

Ripples, Mud, Turbidites

75
Q

What is the main way to distinguish between lacustrine and low energy marine environments in the geological record?

A

Flora/fauna

76
Q

What are the four main lithologies expected in lacustrine environments?

A

Sandstone, mudstone, finegrained limestone, evaporites

77
Q

What is the main texture expected in lacustrine environments?

A

sands which are moderatly well sorted

78
Q

What is the expected bed-geometry in lacustrine environments?

A

Very thin bedding

79
Q

What sedimentary structures would be expected in lacustrine environments?

A

wave ripples + very fine parallel laminations

80
Q

What fossils are expected in lacustrine environments?

A

Algal and microbal, plus uncommon shells

81
Q

What sediment colours would be expected in lacustrine environments?

A

Variable - dark grey in deep lake deposits

82
Q

What facies associations are expected with lacustrine environments?

A

Fluvial deposits, evaporites associated with aeolian faces

83
Q

When do saline lakes have higher hydrocarbon potential?

A

When they have prolonged periods of a wetter environment to allow for organic growth

84
Q

How do evaporites aid in lake deposits being potential reservoirs?

A

Evaporites can act as a trap

Sedimentology and Stratigraphy (19 decks)

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