3. Continental Clastic Environments Flashcards

(33 cards)

1
Q

What are examples of continental clastic environments?

A

1) Rivers - fluvial, alluvial
2) Lakes
- Lacustrine
3) Glacial - moraine, outwash
4) Desert - aeolian
5) Volcanic - Pyroclastic, debris flow
6) soils
7) Natural, urban and in-between

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2
Q

What is source to sink

A
  • the complete sediment routing system
  • a holistic approach
  • intergration of multiple data sources
  • focus on link between segments/depositional environments
  • link between processes and stratigraphy
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3
Q

Why is source to sink usefull?

A
  • improve understanding of landscapes and seascape evolution in 3D
  • encourage thinking across disciplines
  • improve predictability in ancient systems
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4
Q

Why are fluvial systems important?

A
  • deliver vast majority of terrestrial sediment, organic carbon, and pollutants to the coast
  • provide water
  • flooding
  • power generation
  • fishing
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5
Q

define river

A

a ‘large’ conduit for the flow of water and sediment

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6
Q

define fluvial

A

processes and bedforms relating to rivers

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7
Q

define alluvial

A

processes and deposits related to rivers but occuring outside of the channel itself (floodplains, deltas)

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8
Q

What are the three modes of sediment transport in rivers?

A

1) Dissolved load
2) Suspended load
- Fine particles (sand,silt,clay)
- Turbulent eddies pick up, carry upward if vel. > settling vel.
3) Bedload
- On/near bed: rolling, bouncing (‘saltating’)

Suspended and bedload transport increase rapidly with flow strength (nonlinear relationship)

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9
Q

What are the two main types of rivers?

A

1) Bedrock rivers
- Part of the bed is bar rock, which the river has eroded into cutting down
- Generally in upper reaches of rivers
2) Alluvial rivers
- Bed consists of sediment (has a floodplain)
- Downstream reaches

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10
Q

Describe bedrock river

A
  • Part of the bed is bar rock, which the river has eroded into cutting down
  • Generally in upper reaches of rivers
  • erosion rate depends on slope
  • presence of sediment ‘tools’ (clasts) increases erosion
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11
Q

What is an alluvial fan depositional system?

A

They form at the exit of a drainage basin, has a radial sediment dispersal pattern and decreasing grain size and gradient downslope

Mix of sedimentary processes
- Debris flow
- Hyperconcentrated flows
- Fluvial channels
- Sheet floods

Fan is build of lobes and lobe-switching processes produces a composite cone

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12
Q

Describe a debris-flow dominated alluvial fan

A
  • small and steep catchments
  • High magnitude/low frequency events
  • Common debrite lobe features
  • big flash flood
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13
Q

describe a stream-flow drominated alluvial fan

A
  • ‘Wet’ fan receive annual rains
  • Avulsion and migration of rivers dominate
  • Soil development
  • better sorted
  • finer grained
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14
Q

What is the recognition criteria for ancient alluvial fans

A
  • Ancient fan deposits located adjacent to, and tilt towards, normal faults at basin margins
  • Upward changes in grain-size and facies reflect cycles of growth/shrinkage(allogenic) and/or lobe switching(autogenic)
  • Absence of marine fauna and immature texture
  • Evidence of subaerial emergence: palaeosols and desiccation cracks
  • Unidirectional to radial paleocurrents
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15
Q

Main types of (perennial) fluvial channel

A

Straight
Braided
- degree of channel subdivision by large migrating bedforms
Meandering
- Planform description of channel deviation from straight
Anastomosing
- More permanent distributive channel subdivision into smaller channels

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16
Q

Recognition criteria for ancient fluvial deposits

A

Deposits usually of relatively low texturally and compositional maturity
- Erosive-based,coarse grained facies associations (palaeochannel belts) dominated by tractional, current produced sedimentary structures with unidirectional paleocurrent patterns
- Fine grained facies associations (overbank: levee and floodplain deposits) include evidence of emergence

Marine fauna are lacking, but freshwater body and trace fossils may be present

17
Q

What are sinuous river channels and what influences them?

A

Sinuous = shape
Meandering = process

Influences:
- sediment load: mixed load
- channel slope: rel. low gradient
- bank cohesion
- external forcing

18
Q

What are the distinctive facies characteristics of sinuous/meandering rivers?

A
  • Fining-upward patters
  • Lateral accretion surfaces
  • Mix of channel and overbank deposits
19
Q

What causes meandering?

A

Secondary helical flow
- spiral flow (helical cells)
- outward at the surface, inward at the bed

Erosion on outside of bends
Deposition on inside of bends

20
Q

What are lateral accretion surfaces?

A
  • Main deposition in meandering fluvial channels is point bar on the inner bend
  • migration of the point bar is lateral with a downstream component
  • fining-up, and >1m thick, with inclined bedding (5-15degrees)
  • small-scale structures on inclined beds indicating orthogonal flow
21
Q

What are scroll bars

A

record bend migration in a lateral and downstream direction
seen on mars and detectable using seismology

22
Q

Describe an in-channel facies from a meadering river

A
  • channel bed lags
  • lateral accretio units
  • finig upwards
  • abandoned channels
23
Q

describe a floodplain facies

A
  • levees (a few metres)
  • overbank silts, peats/coals
  • crevasse-splay deposits
  • palaeosols
24
Q

What is the recognition criteria for high sinuosity, meandering palaeochannels?

A
  • low relief basal erosion surfaces - migration leads to channel belts
  • tractional structures that record a sytematic reduction in flow velocity upwards or laterallt through the channel fill: lateral accretion surface
  • relatively high preservation of fine-grained overbank facies
  • sheet-like channel sandstone bodies with relatively low width:depth
25
What are braided rivers?
* Complex multichannel systems of low sinuosity * Bedload dominated - gravel or sand * High gradient * Dynamic and rapidly changing
26
What are the origins of braiding?
* Variable discharge (seasonal) * Slope/discharge & grainsize * Sediment supply * Bank erosion * High width:depth ration
27
Describe bar formation/migration
* Flow convergence causes scour * Flow divergence causes deposition * Bar growth - Dune amalgamation - Lateral Accretion - Causes flow deflection * Bars grow and migrate at high stage * Dissected and reworked at low stage * Dominated by cross-beds
28
How is there a record of (sandy) braided river dynamics?
* Braid bars migrate downstream whilst sub-channels shift laterally: **downstream accretion** * Deepest channels produce composite basal master erosion surface * Remnants of braid bars and channel-fills are preserved between the internal erosion surfaces * Braided rivers tend to sweep laterally with time, unless they are confined within valleys, to form **multistorey channel belts (sheets)**
29
What is the recognition criteria for low sinuosity braided rivers?
* High relief, composite erosion surfaces resulting in sheet-like channel belts * Widespread planar and trough cross bedding, commonly coarse-grained * Palaeocurrent variance between high and low discharge periods * Low preservation of fine-grained overbank facies: lateral mobility of channels
30
What is preserved from a floodplain?
* **Levees:** low relief, fine and thin away * **Overbank fines:** sediment storage * **Palaeosols:** climate, organic traps, roots * **Crevasse splay deposits**
31
Describe fluvial channel crevasse and avulsion
Aulsion is the rapid abandonment of a river channel and the formation of a new river channel Crevasse channel and splay - thinner than parent channel Crevasse seposits: evidence of rapid deposition Impact downstream on dleta lobes
32
What are the summary end-members for fluvial facies models?
**Braided River:** at low flow stage. Deposits are dominated by **downstream accreation**. Deposits from **low sinuosity**, braided rivers are commonly wider & thicker than meanering rivers **High sinuosity meandering perennial river**. Sand sheets comprise **lateral accretion** dominated deposits (point bars) sperated by **extensive floodplain** mudstones and splay sands
33
What are the facies models for the sub-humid and semi-arid seasonal tropics?
Rivers with **extremely variable discharge**, have distinctive deposit characteristics: 1. erosionally based channel-fill that exhibit complex lateral facies changes, 2. complex internal architecture that may lack macroform elements 3. an abundance of sedimentary structures formed under high flow stage (planar lamination)