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Sedimentology > Week 6 > Flashcards

Week 6 Flashcards

1
Q

Causitive controls =

A

Allocyclic

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2
Q

River system consequences =

A

Autocyclic

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3
Q

Autocyclic controls

A

Internal to system
Self regulation
Often caused by allocyclic

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4
Q

River systems =

A

Main transportation method for sediment across land surface

e.g. frontal sides of orogens = highest sediment yields 10,000 t/km2/yr

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5
Q

River system types

A

Meandering

Braided

Straight

Anastomosing

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6
Q

Graded river profile; concept

A

Trying to reach equilibrium (never actually ereached)
Reacts to stimuli = auto cyclic changes

  • gradient
  • channel width
  • roughness
  • discharge
  • sediment calibre (long axis)
  • velocity
  • depth
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7
Q

Base level in rivers =

A

Lake level
Nickpoint
SL

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8
Q

Braided rivers characteristics

A 
High energy multi-thread channels
Steep valley gradient <0.5'
Large variable discharge
Bedload transport
Sandy-gravel bed rivers
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9
Q

Channel lag deposits =

A

Accumulation of coarse material

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10
Q

Braided river bedforms

A

Armouring

Bedload

Scours

Dunes

Braid bars

(No vegetation if active)

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11
Q

Armouring =

A

Bed surface coarsened relative to subsurface

  • graded bedding
  • pebble clusters
  • pebble imbrication
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12
Q

Braid bars - channel bar

A

2 channels

Eddy = scour + deposit
= nucleation point

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13
Q

Braid bars - margin/bank bar

A

More restive and doesn’t move as frequent as channel bar
“Slug”
= accretion units with large scale cross stratification

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14
Q

Braid bars sediments

A

Fine upwards
Coarser in middle
Palaeocurrent indicators ALTHOUGH eddies can confuse?!

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15
Q

Classic log of braided river =

A

Coarse
Little variation
Lots of bars
Blocky profile

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16
Q

Meandering rivers characteristics

A

Single channel
High sinuosity

Selective:
- bank erosion
- point bar deposition ("scroll bars")
- meander cut off
- avulsion
= MIGRATE
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17
Q

What do meandering rivers require?

A

Bank full discharge for helicoidal flow

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18
Q

Helicoidal flow and point bars

A

OUTSIDE BEND
Upper flow regime sedimentary structures
Dunes/trough cross bedding

INSIDE BEND
Lower flow regime
Lateral accretion deposits/point bar 10-20m thick

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19
Q

Crevasse splay =

A

Levee breached
= splay of sand onto floodplain

Well sorted ~mature

= PROCESS OF GETTING SAND ONTO FLOODPLAIN!!!

20
Q

Meandering river classic log =

A

Fining upwards

21
Q

Alluvial fan =

A

“Semi conical, downstream fining sediment accumulation predominantly of alluvial origin and breccias/conglomerates”

22
Q

How do alluvial fans form?

A

Due to a decrease in transporting capacity and horizontal flow expansion
= slope <10’
= <100km2

Source: mass wasting

  • gravity flows
  • rock falls
  • debris flows
  • mud flows
23
Q

Bajada =

A

Coalesced alluvial fans, especially along faulted mountain fronts where accommodation spaced used = move further

24
Q

COLLUVIAL

A

Coarse grained, close to source

Dominated by mass-movement

= talus cones, avalanche boulder tongues, debris flow fans

25
Q

ALLUVIAL

A

10-20km away from mountain front

Dominated by ephemeral +/ permanent streams

= debris flow AND streamflow deposits

26
Q

Alluvial fan nomenclature

A

Colluvial

Alluvial

Fluvial

Glaciofluvial

27
Q

FLUVIAL

A

10s-100s km away from mountain front

Dominated by ephemeral +/ permanent streams

= debris flow AND streamflow deposits

28
Q

GLACIOFLUVIAL

A

Streamflow discharge derived from glaciers

29
Q

Stream power

A

Important for alluvial environments
- especially arid/semi-arid

Rivers inefficient transporting 2-3%
Alluvial fans 10-20%

30
Q

Controls on fan size

A
  1. Drainage area

2. Climate/geology

31
Q

Occurrence of alluvial fans

A

Where TECTONIC UPLIFT = appropriate topography:

  • mountain/thrust fronts
  • fault scarps: rift basins
  • valley sides
  • arid-humid climate

e.g. extensional tectonics like hanging walls - size due to drainage

32
Q

Physical processes in alluvial fans

A
  1. FLOWS emerge, diverge, infiltrate, stream power dissipates = lobe
  2. Radial shifts of feeder channel = AVULSION
  3. Periodic TILTING along faulted margin
    - increase slope = INCISION/ENTRENCHMENT
    - decrease slope = ACCRETION/DEPOSITION
33
Q

Types of flow in alluvial fans

A

DEBRIS

STREAM FLOODS

SHEET FLOODS

34
Q

Debris flow =

A

High density, high viscosity
Fine grained
Proximal

35
Q

Stream floods =

A

Low velocity

Usually confined to channels

36
Q

Sheet floods =

A

Shallow, extensive
Sharp bases
Laterally extensive several 100s m
Distal

37
Q

Why is it hard to unpick whether a signal is tectonic/climatic?

A

Different signals can give same sedimentary outputs

e. g. angular clast
- fault reactivation
- arid environment

e. g.2 large fan
- lots of accommodation space
- humid/wet environment

38
Q

Sedimentary structures along an alluvial fan

A

Massive gravel

Gravel and planar cross beds

Planar and trough cross beds

Trough cross beds

39
Q

Alluvial sediments

A

Poorly sorted
Large clasts
Matrix supported

40
Q

Humid alluvial fans

A

Sheet flow/stream
Shallow
Expanse further

41
Q

Semi arid alluvial fans

A

Debris flow
Smaller
More restricted

42
Q

Mega/fluvial fans

A

10^3-10^4 km2
Extreme low slop 0.01-0.1’

e.g. Kosi Megafan, Himalayas

43
Q

Recognition of alluvial fans

A

Radial (+palaeocurrents)

Abundance of g flow deposits

Adjacent to mountain/high area

Prograding - coarsening upwards sequence

Channels smaller and more abundant downstream

Poor internal stratification

Rapid lateral grain size change

Debris flow/stream dominated processes

Root traces, mud cracks, ripples in distal

44
Q

Ancient alluvial fans =

A

Still there after mountain eroded away = gives indication of what was there before

(Whereas present = obvious b/c next to current topography)

45
Q

Pebble clusters =

A

Obstacle clast and collection of stoss and lee side particles

46
Q

Pebble imbrication =

A

In highly turbulent flows
Pebbles kept in suspension = preserve angle to flow
Best in platy/discoidal pebbles

47
Q

What factors change as a fluvial system evolves from a typical meandering system to a braided system?

A

Increasing stream power and grain size

Decreasing drainage basin area, discharge regularity and sorting

Sedimentology (9 decks)

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