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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
What is the name for a hypothetical line which separates the cold and warm layers in a lake?
Thermocline
26
What happens to organic matter which falls to the bed of a stable-system lake?
Will not be broken down due to anoxic environment
27
What is the name of a coal layer made up of plant material?
Detrital coal layer
28
What is the name of a coal layer which is made up of algal/bacterial material?
Sapropelic coal
29
How does a sapropelic layer contribute to oil/gas reservoirs?
Acts as a source rock
30
Why is there a lack of bioturbation in deep lake sediments?
Hypolimnion generally anoxic and unfavourable for life
31
What may a sediment laden river depositing into a lake form?
A delta
32
When can wave reworking occur in a lake environment?
Strong winds
33
What is an indicative factor of strong winds on a lake?
Sandy sediment around it's margins
34
What is an indicative factor of weak winds on a lake?
Fine grained sediment around it's margins
35
What is the main cause for a poorly defined lake margin?
Gentle slope
36
What is the name for a marshey environment around a lake?
Palustrine environment
37
What is common in soil development in palustrine environments around a lake?
Calcareous nodules in soils
38
What are the two main causes of clastic sedimentation transportation in deep lake environments?
Plume dispersal and density currents
39
How does plume dispersal in deep lake environments work?
Plumes may remain above the thermocline - distribute sediment by wind driven currents
40
How does density currents in deep lake environments work?
Coarser sediment on lake floor caused by turbidity currents - often grade from coarse to fine
41
What would a deposit look like from a lake which had both plume dispersal and density currents?
Finely laminated mud alternating with thinly graded turbidites - thinly bedded succession
42
Why is fine lamination particularly evident in deep lake facies?
Not destroyed by biogenic processes
43
What is the name for annual/seasonal climate variations which are recorded in lake sediments?
Varves
44
Briefly describe how varves form
Spring influx of cold sediment-laden water | Summer - organic material preserved in anaerobic conditions
45
What is the expected scale of a varve?
mm laminae
46
What is the name for the form created when mud cracks which dry out?
Desiccation cracks
47
What is a likely cycle situation of desiccation cracks?
May be picked up and carried back out into the lake and deposited as clasts/chunks
48
What would be a typical salinity of a saline lake?
5g L01
49
What is the difference in water chemistry between a saline lake an marine water?
Dependant on the catchment lithology - lake will have a unique signature
50
Name three common modern saline cations?
Sodium, Calcium, Magnesium
51
Name three common modern saline anions?
Carbonate, Chloride, Sulphate
52
What are the three main saline lake types according to the recognition of brines?
Soda lakes, Sulphate lakes, Chloride lakes
53
What makes up the typical chemistry of a soda lake?
Bicarbonate ions + sodium carbonate minerals (trona and natron)
54
What makes up the typical chemistry of a sulphate lake?
Magensium + calcium - precipitate sulphate minerals such as gypsum and mirabilite
55
What is the chemistry of a chloride lake like? Give an example of this type of lake
Similar to marine - dead sea
56
What organisms would be expected in a saline lake?
Limited variety - great abundance of blue-green algae which act as a food-chain to higher plants
57
What may the resultant hydrocarbon deposits in a saline lake be like?
Successions of dark organic-rich shales and evaporites
58
What is the main cause of an ephemeral lake formation?
Flash flooding
59
What is the resultant succession from the drying of an ephemeral lake?
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
What is a signature succession found in ephemeral lakes?
Mud and evaporite couples in succession - cm to mm thick, repeated from flooding/drying events
61
What is a desert rose?
a mass of gypsum crystals
62
Where do desert roses form?
Within sediments around an ephemeral lake
63
Evaporites may form within sediments around an ephemeral lake, in an area called an inland ____
Sabkha
64
What does desiccation crack spacing depend on?
Thickness of the layer - broader cracks form in thicker layers
65
What is a mud-chip/mud-flake?
A preserved section of mud removed from desiccation cracks
66
When are desiccation cracks preserved?
When filled with silt/sand
67
What are desiccation cracks a reliable indicator of?
Exposure to subaerial conditions
68
What is the name for the cracks which form due to the shrinkage of clay underwater?
Syneresis cracks
69
What do syneresis cracks look like?
Not polygonal but simple, straight or slightly curved/tapering
70
Why does syneresis cracks not form in silt/sand and coarser materials?
Not cohesive
71
What is another name for an overfilled lake?
Humid lake
72
What occurs when a lake becomes overfilled?
Change from lacustrine to fluvial as river passes straight through lake
73
What is the main difficulty in recognising lacustrine deposition in the geological record?
Hard to distinguish between lacustrine and low energy marine
74
What main features does lacustrine and low energy marine have in common (3)
Ripples, Mud, Turbidites
75
What is the main way to distinguish between lacustrine and low energy marine environments in the geological record?
Flora/fauna
76
What are the four main lithologies expected in lacustrine environments?
Sandstone, mudstone, finegrained limestone, evaporites
77
What is the main texture expected in lacustrine environments?
sands which are moderatly well sorted
78
What is the expected bed-geometry in lacustrine environments?
Very thin bedding
79
What sedimentary structures would be expected in lacustrine environments?
wave ripples + very fine parallel laminations
80
What fossils are expected in lacustrine environments?
Algal and microbal, plus uncommon shells
81
What sediment colours would be expected in lacustrine environments?
Variable - dark grey in deep lake deposits
82
What facies associations are expected with lacustrine environments?
Fluvial deposits, evaporites associated with aeolian faces
83
When do saline lakes have higher hydrocarbon potential?
When they have prolonged periods of a wetter environment to allow for organic growth
84
How do evaporites aid in lake deposits being potential reservoirs?
Evaporites can act as a trap

Sedimentology and Stratigraphy (19 decks)

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