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EART23202 Interpreting the stratigraphic record > Continental clastic environments > Flashcards

Continental clastic environments Flashcards

1
Q

Give six examples of continental clastic environments

A

Rivers, lakes, glaciers, desert, volcanos, soils

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2
Q

What processes operate in rivers?

A

Fluvial (the action of a river) and alluvial (related to rivers but outside the channel, e.g. floodplains, deltas)

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3
Q

What process operates in lakes?

A

Lacustrine (the action of a lake)

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4
Q

What processes operate in glaciers?

A

Moraine (material left by a moving glacier), outwash (sediment carried by water from a melting glacier)

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5
Q

What process operates in deserts?

A

Aeolian (wind)

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6
Q

What processes operate around volcanoes?

A

Pyroclastics, debris flows

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7
Q

Describe source-to-sink

A

Holistically describes a complete sediment routing system using integrated data from multiple sources

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8
Q

What is the ultimate sink for sediment?

A

Deep sea, but much doesn’t make it there

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9
Q

What are the two end members for sources in source-to-sink?

A

Typical passive/mixed systems, typical small/large tectonically active systems

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10
Q

Why is source-to-sink useful?

A

Improve understanding of landscape/seascape evolution in 3d, thinking across disciplines, improves predictability in ancient systems

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11
Q

What do fluvial systems do?

A

Deliver the majority of terrestrial sediment, organic carbon, and pollutants to the coast

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12
Q

Why is there so much focus on fluvial systems?

A

Provide water, power generation, fishing, can cause flooding

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13
Q

Define river

A

A large conduit for the flow of water and sediment

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14
Q

When does there need to be net subsidence?

A

To preserve non-marine strata in the rock record
i.e. to be in a sedimentary basin

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15
Q

Give three examples of distributive bodies

A

Alluvial, fluvial, and megafan processes

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16
Q

How do distributive systems build?

A

Radially through repeated channel avulsion events

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17
Q

Describe a channel avulsion event

A

The rapid abandonment of a river channel and the formation of a new one

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18
Q

How are palaeorivers classified?

A

Meandering or braiding
May also be ‘flashy’ (ephemoral)

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19
Q

Describe fluvial overbank

A

Deposits accumulated as a result of overbank flooding and avulsion
An important part of the palaeoenvironmental record

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20
Q

What are the three modes of sediment transport in rivers

A

Dissolved load/wash load, suspended load, bedload

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21
Q

Describe dissolved load/wash load in rivers

A

Ions in solution
Pollution

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22
Q

Describe suspended load in rivers

A

Fine particles (sand, silt, clay)
Turbulent eddies pick up and carry upward (if velocity>settling velocity)

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23
Q

Describe bedload in rivers

A

On/near bed
Rolling or bounding (saltating)

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24
Q

Describe the relationship between suspended load and bedload transport with flow strength

A

Suspended load and bedload transport increase rapidly with flow strength
This is a non-linear relationship

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25
Q

What are the two main kinds of rivers?

A

Bedrock rivers and alluvial rivers

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26
Q

Describe bedrock rivers

A

Part of the bed is bare rock that the river has eroded into
Generally in upper reaches of rivers

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27
Q

What increases erosion in bedrock rivers?

A

Steeper slopes and the presence of tools (sediment)

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28
Q

Describe alluvial rivers

A

The bed consists of sediments (alluvium, river-associated sediment)
Generally in downstream reaches of rivers

29
Q

What forms at the exit of drainage basins?

A

Alluvial fan deposition systems
These are distributive

30
Q

What sediment processes are combined in alluvial fan depositional systems?

A

Debris flows, hyperconcentrated flows, fluvial channels, sheet floods

31
Q

What processes produces the composite cone of an alluvial fan?

A

Lobe-switching

32
Q

Describe depostion at an alluvial fan

A

Radial sediment dispersal pattern with decreasing grain size and gradient down slope

33
Q

What are the two end member types of alluvial fans?

A

Debris-flow dominated alluvial fan and stream-flow dominated alluvial fan

34
Q

Describe a debris-flow dominated alluvial fan

A

Small and steep catchments, they have high magnitude/low frequency events and common debrite lobe features

35
Q

Describe a stream-flow dominated alluvial fan

A

‘Wet’ fan that receives annual rain
Avulsion and migration of rivers dominate
Development of soil

36
Q

Give three examples of recognition criteria for ancient alluvial fans

A

Absence of marine fauna, evidence of subaerial emergence (e.g. desiccation cracks), and unidireciton to radial palaeocurrents

37
Q

Define perennial rivers

A

Those that cut through the soil profile, removing fine particles

38
Q

What are the four main types of perennial fluvial channel in planform?

A

Straight, braided, meandering, anastomosing

39
Q

Define a braided fluvial channel

A

Degree of channel subdivision by large migrating bedforms
Complex multichannel systems of low sinuosity

40
Q

Describe the characteristics of a braided river

A

Bedload-dominated (gravel/sand), high gradient, dynamic and rapidly changing

41
Q

Give three examples of origins of braiding

A

Variable (seasonal) discharge, bank erosion, high width:depth

42
Q

Describe the dynamics of braided rivers

A

Braid bars migrate downstream and sub-channels shift laterally
Overall downstream accretion

43
Q

When do multistorey belts (sheets) form?

A

When braided rivers are prevents from sweeping laterally over time because they are confined within valleys

44
Q

Give two examples of recognition criteria for low sinuosity braided rivers

A

Low preservation of fine-grained overband facies (bc of lateral mobility of channels), palaeocurrent between high and low discharge periods

45
Q

Describe a meandering fluvial channel

A

Planform deviations from straight channel
Sinuous shape

46
Q

Describe the overbank of a meandering river

A

Strength is needed to stop it washing away
Usually comes from clay and flora

47
Q

Give three distinctive facies characteristics of meandering rivers

A

Fining-upward patterns, lateral accretion surfaces, mix of channel and overbank deposits

48
Q

Give four examples of influences on meandering (sinuous) river channels

A

Sediment load (mixed load), channel slope (relatively low gradient), bank cohesion, external forcing

49
Q

What causes meandering?

A

Secondary helical flow

50
Q

Describe secondary flow cells

A

Spiral (helical) flow, flowing outward at the surface and inward at the bed
Causes erosion on the outside of beds and deposition on inside of beds

51
Q

Describe lateral accretion surfaces in meandering river channels

A

Main deposition is at point bar (inner bend)
Migration of the point bar is lateral with a downstream component

52
Q

Describe scroll bars

A

Record bend migration in a lateral and downstream direction

53
Q

Give four characteristics of in-channel facies of meandering rivers

A

Channel bed lags, lateral accretion units, fining upwards, abandoned channels

54
Q

Give four characteristics of floodplain facies of meandering rivers

A

Levees (few m), overbank silts/peats/coals, crevasse-splay deposits, palaeosols

55
Q

Give four recognition criteria for high sinuosity, meandering palaeochannels

A

Channel belts, lateral accretion surface, high preservation of fine-grained overbank facies, sheet-like channel sandstone bodies with low width:depth

56
Q

Describe an anastomosing fluvial channel

A

A more permanent distributive channel subdivision into smaller channels

57
Q

Describe fluvial deposits

A

Usually of relatively low textural and compositional maturity

58
Q

Give two examples of facies as recognition criteria for ancient fluvial deposits

A

Erosive-based coarse-grained facies associations dominated by tractional, current-produced sedimentary structures with unidirectional current
Fine-grained facies association with evidence of emergence (overbank: flood plain and levee)

59
Q

Describe features of fluvial deposits used to recognise ancient ones

A

Lack of marine fauna
Freshwater body and trace fossils may be present

60
Q

Define a channel bar

A

Elevated region of sediment that has been deposited by the flowing water

61
Q

What causes channel bar formation/migration?

A

Flow convergences causes scour
Flow divergence causes depostion

62
Q

Give three aspects of channel bar growth

A

Dune amalgamation, lateral accretion, causes flow deflection

63
Q

Describe channel bar growth/formation

A

Bars grow and migrate at high stage
Bars are dissected and reworked at low stage
They are dominated by cross beds

64
Q

When does a crevasse splay form?

A

When a flow breaks its levees and deposits sediment on the flood plain

65
Q

What are crevasse deposits evidence of?

A

Rapid depostion

66
Q

What impact can crevasse splays have?

A

Impact on downstream delta lobe

67
Q

What are the end member fluvial facies models (Miall)

A

Low sinuosity braided river
High sinuousity meandering river

68
Q

Give two examples of deposit characteristics of rivers with extremely variable discharge
i.e. sub-humid, semi-arid seasonal tropics

A

Complex internal architecture that can lack macroform elements
Abundance of sedimentary structures formed under high flow stage (planar lamination)

EART23202 Interpreting the stratigraphic record (11 decks)

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