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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
ALLUVIAL
10-20km away from mountain front Dominated by ephemeral +/ permanent streams = debris flow AND streamflow deposits
26
Alluvial fan nomenclature
Colluvial Alluvial Fluvial Glaciofluvial
27
FLUVIAL
10s-100s km away from mountain front Dominated by ephemeral +/ permanent streams = debris flow AND streamflow deposits
28
GLACIOFLUVIAL
Streamflow discharge derived from glaciers
29
Stream power
Important for alluvial environments - especially arid/semi-arid Rivers inefficient transporting 2-3% Alluvial fans 10-20%
30
Controls on fan size
1. Drainage area | 2. Climate/geology
31
Occurrence of alluvial fans
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
Physical processes in alluvial fans
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
Types of flow in alluvial fans
DEBRIS STREAM FLOODS SHEET FLOODS
34
Debris flow =
High density, high viscosity Fine grained Proximal
35
Stream floods =
Low velocity | Usually confined to channels
36
Sheet floods =
Shallow, extensive Sharp bases Laterally extensive several 100s m Distal
37
Why is it hard to unpick whether a signal is tectonic/climatic?
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
Sedimentary structures along an alluvial fan
Massive gravel Gravel and planar cross beds Planar and trough cross beds Trough cross beds
39
Alluvial sediments
Poorly sorted Large clasts Matrix supported
40
Humid alluvial fans
Sheet flow/stream Shallow Expanse further
41
Semi arid alluvial fans
Debris flow Smaller More restricted
42
Mega/fluvial fans
10^3-10^4 km2 Extreme low slop 0.01-0.1' e.g. Kosi Megafan, Himalayas
43
Recognition of alluvial fans
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
Ancient alluvial fans =
Still there after mountain eroded away = gives indication of what was there before (Whereas present = obvious b/c next to current topography)
45
Pebble clusters =
Obstacle clast and collection of stoss and lee side particles
46
Pebble imbrication =
In highly turbulent flows Pebbles kept in suspension = preserve angle to flow Best in platy/discoidal pebbles
47
What factors change as a fluvial system evolves from a typical meandering system to a braided system?
Increasing stream power and grain size Decreasing drainage basin area, discharge regularity and sorting

Sedimentology (9 decks)

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