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Glaciers and Glaciation > Week 11: Ice marginal sedimentation and landforms (moraines) > Flashcards

Week 11: Ice marginal sedimentation and landforms (moraines) Flashcards

1
Q

How are moraines classified?

A

ENVIRONMENT OF DEPOSITION

PLAN FORM

RELATIONSHIP TO GLACIER ACTIVITY

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2
Q

Moraine classification; environment of deposition

A
  1. Terrestrial (deposited on earth’s surface)
  2. Subaqueous
  3. Supraglacial
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3
Q

Moraine classification; plan form

A
  1. Linear/orientated (i.e. ice flow // or transverse)

2. Non-orientated/chaotic

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4
Q

Moraine classification; relationship to glacier activity

A
  1. Advance
  2. Recession
    e. g. +ve mass balance = advance = push moraine
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5
Q

Ice marginal moraines - push/squeeze moraines = (characteristics)

A

proglacially constructed ridges <10m characterised by:

  • <25% glacitectonised structures
  • saw-tooth plan form (pectin in snout)
  • seasonal deposition of active ice = annual push moraines
  • stable glacier margins = large moraine complexes where stacked on top of one another
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6
Q

Model processes for push/squeeze moraine formation

A

DEFORMATION/BULLDOZING

SQUEEZING

SLAB MELT-OUT

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7
Q

Push/squeeze moraine formation: deformation/bulldozing (+reference)

A

Shaw 1984

Sediment bulldozed as glacier pushes forwards

Orientation of clasts related to emplacement due to ice moving over sediment
- till fabrics record variety of processes e.g. folding/overriding/debris flow/ice slope colluvium

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8
Q

Push/squeeze moraine formation: squeezing (+reference)

A

Price 1970

= extrusion of saturated sub-marginal till due to weight of overlying ice

Summer process in poorly drained areas around snout
Glaciers can’t go through poorly drained material = poorly drained material goes through them!!!

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9
Q

Evidence for squeezing as a formation model for push/squeeze moraines

A
  1. Random to vertically inclined till fabrics

2. Saw-tooth form (b/c till squeezes up into marginal longitudinal crevasses)

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10
Q

Push/squeeze moraine formation: slab melt out (+reference)

A

Kruger 1993, Matthews et al 1995

Then further investigated by Evans and Hiemstra 2005

Seasonal cycle of:

1) winter freeze-on, detachment and transport of till slab
2) summer melt-out of freeze slab

N.B. Kruger more realistic

In temperate mid-latitude locations

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11
Q

Evidence for slab melt out as a formation model for push/squeeze moraines

A

Multiple till slabs

Strong till fabrics with no evidence of microscale shearing (fine grained sediment gets moved around instead)

Abundance evidence of microscale porewater escape

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12
Q

Main types of glacitectonic landform

A

Composite ridges

Hill-hole pair

Cupola hill

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13
Q

Glacitectonic landform characteristics

A

Proglacially fold and thrust structures

Much larger in scale than push moraines

High % pre-existing sediment formed by thrust

Till carapace = smooths over

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14
Q

How do glacitectonic landforms form?

A

Low strength proglacial sediment + high glacier stresses = proglacial compression/thrusting/folding

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15
Q

Glacitectonic landforms; gravity spreading model

A

= translation of glacier weight into lateral stresses

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16
Q

Glacitectonic stress =

A

lateral stress due to lateral displacement of subglacial materials in response to:

  1. Normal stress (ice load)
  2. Basal shear stress (glaciodynamic stress)
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17
Q

Glacitectonic landforms; when does failure occur?

A

FAILURE when glacitectonic stress > shearing resistance

Small cohesion
High Pw
- Pw approaches Pi = total glacitectonic stress approaches 0
= movement along thrust planes/elevation of thrust blocks by compression

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18
Q

Models of proglacial thrusting

A

Croot 1988 = composite ridge construction by surging glacier, Iceland

Mulugeta and Kooi 1987 = squeeze box
- fold dip increases back towards ice

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19
Q

Latero-frontal moraines =

A

ice-contact ridges marking lateral and frontal snout margins

Highly susceptible to melt-out collapse and paraglacial reworking

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20
Q

Ice contact fans/ramps =

A

asymmetrical, coalescent debris flow fans

- shallow distal slope and steep proximal/ice contact slope

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21
Q

Processes in latero-frontal moraines

A

DUMPING

INCREASING COMPONENT OF SUBGLACIAL DEBRIS DOWN VALLEY

GLACIER RECESSION

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22
Q

Latero-frontal moraines; dumping

A

Supraglacial debris transfer - slide, roll, flow, fall

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23
Q

Latero-frontal moraines; increasing component of subglacial debris down valley

A
  1. Debris septa rises to glacier surface

2. Valley floor sediments reworked

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24
Q

Latero-frontal moraines; glacier recession

A

= inset moraines, kame terraces and colluvial

25
Q

Sedimentary deposits in latero-frontal moraines

A

Boulton and Eyles 1979 = “supraglacial morainic till”

Interbedded:
1) mass flow diamictons
2) outwash and glacilacustrine deposits (due to ponding between moraine/snout)
= STRATIFIED MORAINES

26
Q

Processes in ice-contact fans/ramps

A

Debris flow fans prograde out from debris-charged snout onto valley floors

27
Q

Sedimentary deposits in ice-contact fans/ramps

A

Debris flow diamictons and intermittent incision by meltwater streams
= interbedded:

1) diamictons
2) coarse stratified outwash

28
Q

Relationship of debris and ice supply to moraine form

A

Benn et al 2003

Is this case for laters-frontal moraines and ice-contact fans/ramps

= within-valley asymmetry of lateral moraines

29
Q

What is within-valley asymmetry?

A

Larger lateral moraines found on valley sides with larger free face areas

30
Q

Equation for moraine asymmetry (+reference)

A

Benn 1989

IM = Ms/Mf

Ms = mean cross sectional area of moraines on debris mantle-dominated slopes

Mf = mean cross sectional area of moraines on free face dominated slopes

31
Q

Types of supra glacial moraines

A

Medial moraines

Hummocky moraine

Controlled moraine

32
Q

Medial moraine =

A

linear ridges at boundaries of ice masses

33
Q

Hummocky moraine =

A

chaotic hummocks

34
Q

Controlled moraine =

A

controlled by debris in ice

35
Q

Models of medial moraine processes (+reference)

A

Eyles and Rogerson 1978

  1. Ablation-dominant type
  2. Ice stream interaction type
  3. Avalanche type
36
Q

Ablation-dominant type medial moraines

A

Melt out of englacial debris septa in ablation zone

37
Q

Ice stream interaction type medial moraines

A

Lateral moraines merging at confluence of different glaciers

38
Q

Avalanche type medial moraines

A

Rockfall onto glacier = discontinuous medial moraine

39
Q

Hummocky moraine processes

A
  1. DIFFERENTIAL ABLATION

2. GRAVITATIONAL/MELTWATER REWORKING

40
Q

Differential ablation in hummocky moraine

A

Due to uneven sediment cover e.g. concentration debris (dirt cone)

Graphs (diagram) show that max. ablation occurs with 0.5-1cm thick debris layer

41
Q

Gravitational/meltwater reworking in hummocky moraine

A

Repeated topographic reverses

42
Q

Processes in controlled moraine

A

Inheritance of pattern of englacial debris septa i.e. linear mounds

Flow bands bring ice towards surface

43
Q

Where are controlled moraines often found?

A

Downwasting sub-polar and polar snouts and in permafrost terrains i.e. glacier not entirely removed

44
Q

Are controlled moraines really moraines?

A

Often glacier ice looks like it has melted out but is actually below the surface

If it has melted out completely they wouldn’t be preserved

45
Q

Sedimentary deposits in medial moraines

A

Thin, linear boulder spread

Sometimes associated with glacifluvial features but low preservation potential

46
Q

Sedimentary deposits in hummocky moraines

A

Interbedded rubbly diamictons and contorted glacifluvial sediment

Often associated with kame and kettle topography

47
Q

Does the concept of equifinality apply to hummocky moraine?

48
Q

Sedimentary deposits in controlled moraine

A

Discontinuous

Chains of linear hummocks
- low amplitude ridges to undulatory rubble veneer

49
Q

Problem with preservation potential of controlled moraine

A

Most are ice-cored

50
Q

Subaqueous moraines and depo-centres =

A

ice-contact accumulations of stratified sediment

51
Q

Subaqueous moraines and depo-centres, types:

A

Subaqueous fans

Grounding line fans
N.B. in ice contact deltas = at water level

Morainal banks

Ice shelf moraines

52
Q

Depocentre =

A

Where particular sediment has maximum thickness

53
Q

Processes in subaqueous moraines and depo-centres

A

Hyperconcentrated flow

Avalanching

Grain flows

Debris flows

54
Q

How can sediment be reworked in subaqueous moraines and depo-centres?

A
  1. Glacier oscillation (glacitectonic deformation)
  2. Iceberg scouring and dumping
  3. Mass movements
55
Q

Sedimentary deposits in ice-contact depo-centres

A

e.g. ice-contact subaqueous fan –> delta continuum

Coalescent sand and gravel interbeds

Fining distally

= rippled sands, locally cut and filled by active channels and slumps

56
Q

Sedimentary deposits in subaqueous moraines

A

e.g. morainal banks or grounding zone wedges (GZW)

= interdigitation of subglacial till and subaqueous outwash at a stable grounding line

Glacitectonic disturbance is ubiquitous

57
Q

Interdigitation =

A

interlocking

58
Q

Ubiquitous =

A

found everywhere

59
Q

Deposits in ice shelf moraine

A

Freeze-on and onshore pushing of glacimarine/glacilacustrine sediments by floating glacier margin
= horizontal moraine

Glaciers and Glaciation (29 decks)

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