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EART23202 Interpreting the stratigraphic record > Continental clastic environments > Flashcards

Continental clastic environments Flashcards

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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
What are the two main kinds of rivers?
Bedrock rivers and alluvial rivers
26
Describe bedrock rivers
Part of the bed is bare rock that the river has eroded into Generally in upper reaches of rivers
27
What increases erosion in bedrock rivers?
Steeper slopes and the presence of tools (sediment)
28
Describe alluvial rivers
The bed consists of sediments (alluvium, river-associated sediment) Generally in downstream reaches of rivers
29
What forms at the exit of drainage basins?
Alluvial fan deposition systems These are distributive
30
What sediment processes are combined in alluvial fan depositional systems?
Debris flows, hyperconcentrated flows, fluvial channels, sheet floods
31
What processes produces the composite cone of an alluvial fan?
Lobe-switching
32
Describe depostion at an alluvial fan
Radial sediment dispersal pattern with decreasing grain size and gradient down slope
33
What are the two end member types of alluvial fans?
Debris-flow dominated alluvial fan and stream-flow dominated alluvial fan
34
Describe a debris-flow dominated alluvial fan
Small and steep catchments, they have high magnitude/low frequency events and common debrite lobe features
35
Describe a stream-flow dominated alluvial fan
'Wet' fan that receives annual rain Avulsion and migration of rivers dominate Development of soil
36
Give three examples of recognition criteria for ancient alluvial fans
Absence of marine fauna, evidence of subaerial emergence (e.g. desiccation cracks), and unidireciton to radial palaeocurrents
37
Define perennial rivers
Those that cut through the soil profile, removing fine particles
38
What are the four main types of perennial fluvial channel in planform?
Straight, braided, meandering, anastomosing
39
Define a braided fluvial channel
Degree of channel subdivision by large migrating bedforms Complex multichannel systems of low sinuosity
40
Describe the characteristics of a braided river
Bedload-dominated (gravel/sand), high gradient, dynamic and rapidly changing
41
Give three examples of origins of braiding
Variable (seasonal) discharge, bank erosion, high width:depth
42
Describe the dynamics of braided rivers
Braid bars migrate downstream and sub-channels shift laterally Overall downstream accretion
43
When do multistorey belts (sheets) form?
When braided rivers are prevents from sweeping laterally over time because they are confined within valleys
44
Give two examples of recognition criteria for low sinuosity braided rivers
Low preservation of fine-grained overband facies (bc of lateral mobility of channels), palaeocurrent between high and low discharge periods
45
Describe a meandering fluvial channel
Planform deviations from straight channel Sinuous shape
46
Describe the overbank of a meandering river
Strength is needed to stop it washing away Usually comes from clay and flora
47
Give three distinctive facies characteristics of meandering rivers
Fining-upward patterns, lateral accretion surfaces, mix of channel and overbank deposits
48
Give four examples of influences on meandering (sinuous) river channels
Sediment load (mixed load), channel slope (relatively low gradient), bank cohesion, external forcing
49
What causes meandering?
Secondary helical flow
50
Describe secondary flow cells
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
Describe lateral accretion surfaces in meandering river channels
Main deposition is at point bar (inner bend) Migration of the point bar is lateral with a downstream component
52
Describe scroll bars
Record bend migration in a lateral and downstream direction
53
Give four characteristics of in-channel facies of meandering rivers
Channel bed lags, lateral accretion units, fining upwards, abandoned channels
54
Give four characteristics of floodplain facies of meandering rivers
Levees (few m), overbank silts/peats/coals, crevasse-splay deposits, palaeosols
55
Give four recognition criteria for high sinuosity, meandering palaeochannels
Channel belts, lateral accretion surface, high preservation of fine-grained overbank facies, sheet-like channel sandstone bodies with low width:depth
56
Describe an anastomosing fluvial channel
A more permanent distributive channel subdivision into smaller channels
57
Describe fluvial deposits
Usually of relatively low textural and compositional maturity
58
Give two examples of facies as recognition criteria for ancient fluvial deposits
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
Describe features of fluvial deposits used to recognise ancient ones
Lack of marine fauna Freshwater body and trace fossils may be present
60
Define a channel bar
Elevated region of sediment that has been deposited by the flowing water
61
What causes channel bar formation/migration?
Flow convergences causes scour Flow divergence causes depostion
62
Give three aspects of channel bar growth
Dune amalgamation, lateral accretion, causes flow deflection
63
Describe channel bar growth/formation
Bars grow and migrate at high stage Bars are dissected and reworked at low stage They are dominated by cross beds
64
When does a crevasse splay form?
When a flow breaks its levees and deposits sediment on the flood plain
65
What are crevasse deposits evidence of?
Rapid depostion
66
What impact can crevasse splays have?
Impact on downstream delta lobe
67
What are the end member fluvial facies models (Miall)
Low sinuosity braided river High sinuousity meandering river
68
Give two examples of deposit characteristics of rivers with extremely variable discharge i.e. sub-humid, semi-arid seasonal tropics
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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