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

How are moraines classified?

ENVIRONMENT OF DEPOSITION

PLAN FORM

RELATIONSHIP TO GLACIER ACTIVITY

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Moraine classification; environment of deposition

Terrestrial (deposited on earth’s surface)
Subaqueous
Supraglacial

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Moraine classification; plan form

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

2. Non-orientated/chaotic

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Moraine classification; relationship to glacier activity

Advance
Recession
e. g. +ve mass balance = advance = push moraine

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

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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Model processes for push/squeeze moraine formation

DEFORMATION/BULLDOZING

SQUEEZING

SLAB MELT-OUT

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

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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Push/squeeze moraine formation: squeezing (+reference)

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

Random to vertically inclined till fabrics

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

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

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

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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Main types of glacitectonic landform

Composite ridges

Hill-hole pair

Cupola hill

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Glacitectonic landform characteristics

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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How do glacitectonic landforms form?

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

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Glacitectonic landforms; gravity spreading model

= translation of glacier weight into lateral stresses

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Glacitectonic stress =

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

Normal stress (ice load)
Basal shear stress (glaciodynamic stress)

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Glacitectonic landforms; when does failure occur?

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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Models of proglacial thrusting

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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Latero-frontal moraines =

ice-contact ridges marking lateral and frontal snout margins

Highly susceptible to melt-out collapse and paraglacial reworking

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Ice contact fans/ramps =

asymmetrical, coalescent debris flow fans

- shallow distal slope and steep proximal/ice contact slope

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Processes in latero-frontal moraines

DUMPING

INCREASING COMPONENT OF SUBGLACIAL DEBRIS DOWN VALLEY

GLACIER RECESSION

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Latero-frontal moraines; dumping

Supraglacial debris transfer - slide, roll, flow, fall

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

Debris septa rises to glacier surface

2. Valley floor sediments reworked

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Latero-frontal moraines; glacier recession

= inset moraines, kame terraces and colluvial

Sedimentary deposits in latero-frontal moraines

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

Processes in ice-contact fans/ramps

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

Sedimentary deposits in ice-contact fans/ramps

Debris flow diamictons and intermittent incision by meltwater streams = interbedded: 1) diamictons 2) coarse stratified outwash

Relationship of debris and ice supply to moraine form

Benn et al 2003 Is this case for laters-frontal moraines and ice-contact fans/ramps = within-valley asymmetry of lateral moraines

What is within-valley asymmetry?

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

Equation for moraine asymmetry (+reference)

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

Types of supra glacial moraines

Medial moraines Hummocky moraine Controlled moraine

Medial moraine =

linear ridges at boundaries of ice masses

Hummocky moraine =

chaotic hummocks

Controlled moraine =

controlled by debris in ice

Models of medial moraine processes (+reference)

Eyles and Rogerson 1978 1. Ablation-dominant type 2. Ice stream interaction type 3. Avalanche type

Ablation-dominant type medial moraines

Melt out of englacial debris septa in ablation zone

Ice stream interaction type medial moraines

Lateral moraines merging at confluence of different glaciers

Avalanche type medial moraines

Rockfall onto glacier = discontinuous medial moraine

Hummocky moraine processes

1. DIFFERENTIAL ABLATION | 2. GRAVITATIONAL/MELTWATER REWORKING

Differential ablation in hummocky moraine

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

Gravitational/meltwater reworking in hummocky moraine

Repeated topographic reverses

Processes in controlled moraine

Inheritance of pattern of englacial debris septa i.e. linear mounds Flow bands bring ice towards surface

Where are controlled moraines often found?

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

Are controlled moraines really moraines?

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

Sedimentary deposits in medial moraines

Thin, linear boulder spread Sometimes associated with glacifluvial features but low preservation potential

Sedimentary deposits in hummocky moraines

Interbedded rubbly diamictons and contorted glacifluvial sediment Often associated with kame and kettle topography

Does the concept of equifinality apply to hummocky moraine?

YES

Sedimentary deposits in controlled moraine

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

Problem with preservation potential of controlled moraine

Most are ice-cored

Subaqueous moraines and depo-centres =

ice-contact accumulations of stratified sediment

Subaqueous moraines and depo-centres, types:

Subaqueous fans Grounding line fans N.B. in ice contact deltas = at water level Morainal banks Ice shelf moraines

Depocentre =

Where particular sediment has maximum thickness

Processes in subaqueous moraines and depo-centres

Hyperconcentrated flow Avalanching Grain flows Debris flows

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

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

Sedimentary deposits in ice-contact depo-centres

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

Sedimentary deposits in subaqueous moraines

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

Interdigitation =

interlocking

Ubiquitous =

found everywhere

Deposits in ice shelf moraine

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