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Glaciers and Glaciation > Week 10: Subglacial processes and landforms > Flashcards

Week 10: Subglacial processes and landforms Flashcards

1
Q

What happens as ice streams move from land to sea?

A

Hard to soft bed landforms

Hard = erosional/shaping of rock
Soft = sediment needed --> soft-bedded landforms

Erosional to depositional landforms

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2
Q

Subglacial and englacial debris production

A

Erosion/transport/deposition processes produce subglacial sediment

FROM SUBSTRATE BENEATH THE ICE STREAM

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3
Q

Types of subglacial and englacial debris production

A

Plucking and bedrock communition

Bulk freeze-on (net adfreezing)

Apron entrainment

Marginal thickening/stacking

= debris conveyer

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4
Q

Bulk freeze-on (net adfreezing) =

A

debris frozen onto bed

Freezing front penetrates into subglacial sediment layer = adds onto base of glacier

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5
Q

Apron entrainment =

A

glacier runs over pre-existing sediment in front of it and incorporates it

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6
Q

Marginal thickening/stacking =

A

compress sediment and fold = thicken

Thrusts through further compression = stacks

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7
Q

Debris conveyer

A

Net advection of debris to sub-marginal zone
Up ice –> down ice
Potential 10s of km

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8
Q

Erratics

A

Good indicator of impact of advection on system as a whole

= rock/boulder different from surrounding rock, there due to glacier

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9
Q

Plucking and bedrock communition

A
  1. Cannibalisation
    - = BR raft detachment by plucking/attenuation
    - “glacitectonite”
  2. Communition
    - = BR fracture/pulverisation
    - “till” i.e. homogenous material
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10
Q

Continuum of plucking and bedrock communition

A

Fractured BR –> pulverised BR –> mono-lithological diamicton (Hiemstra et al 2007)

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11
Q

Glacitectonite =

A

formed by subglacial plucking/shearing/crushing local BR

Vertical continuum:
Undisturbed BR
Faulted/sheared BR
Brecciated/pulversied
Diamicton with erratic clasts
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12
Q

Shearing =

A

Deformation through discrete shear/strain events

Indicates dry environment undergoing high degree of lateral shear stress

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13
Q

Glacier bed mosaic

A

Frozen –> thawed –> sliding –> deforming zones

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14
Q

The deforming bed =

A

Where unfrozen substrate/till forms a layer below the ice/bed interface and deformation can occur throughout
- can be component of ice movement/sliding/creep

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15
Q

Deformation of till is evidence that…

A

Ice can move through a layer of sediment that is deforming laterally as ice moves from up-ice to down-ice

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16
Q

Shear stress is … towards the ice-bed interface

A

exponential

and therefore failure also - van der Wateren 1995

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17
Q

Brittle deformation =

A

displacement along shear planes

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18
Q

Ductile deformation =

A

mobile/viscous

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19
Q

Strain ellipse

A

Shows nature of deformation i.e. folding/stretching/pinching of substrate

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20
Q

Low shear strain

A

Sediment folded and faulted with progressive shear and attenuation = tectonic laminations and detached boudins
Some primary structures e.g. folds may be preserved

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21
Q

Attenuation =

A

reduction

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22
Q

High shear strain

A

High levels of shear sediment homogenised

All structures gone

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23
Q

How was deforming bed theory found?

A

Researching how ice streams were moving so fast in Antarctica (Alley 1986, 1987)

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24
Q

What is deforming bed theory

A

Bed = max. 2m deep, most 30-50cm
Two tier shear strain movement layers moving at different rates = A and B horizon tills
(Boulton and Hindmarsh 1987, Boulton et al 2001)

= soft substrate beneath glacier may undergo permanent strain in response to applied stresses of glacier ice = sediment deformation
(Boulton 1986, Boulton and Hindmarsh 1987)

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25
Q

Boulton and Hindmarsh deforming bed theory experiment 1987

A

Tunnel to bed of Breidamerkurjokull, outlet of Vantajokull SE Iceland
Segmented rods placed and excavated a few days later
Saturated till (high PWP)

A-horizon = upper 0.5m (90-95%) of glacier motion
- intergranular DUCTILE shear

B-horizon = brittle deformation

Max. displacement towards ice-bed interface

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26
Q

Three types of failure

A
  1. PERFECTLY PLASTIC MATERIAL
    - doesn’t deform until yield stress (100kPa) = instant deformation
    - rate of subsequent deformation (strain rate) independent of any further stress applied
    - failure along discrete shear plane
    - relates to brittle shear
  2. NEWTONIAN VISCOUS MATERIAL
    - strain rate linearly proportional to shear stress
    - continuous rate of deformation
  3. NON-LINEAR VISCOUS MATERIAL
    - e.g. ice
    - rate changes
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27
Q

Effective pressure field in deforming bed is a function of 2 elements:

A
  1. Ice weight pushing down on sediment

2. Water pressure in sediment pushing back up

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28
Q

How do you reduce effective pressure?

A

Increase PWP

As ice weight pushing down on sediment = PWP pushing back up, effective pressure approaches 0

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29
Q

What happens as effective pressure approaches 0?

A

Accelerated deformation in deforming bed
Till highly pressurised and saturated = dilated
Liquifies = particles in sediment forced apart by water in pore space
Behaves in ductile manner

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30
Q

Effective pressure equation

A

Pe = Pi - Pw

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31
Q

Effective pressure in soft sediments

A

Glaciers underlain by soft sediments behave differently to those resting on hard rigid bed

Low basal shear stresses
= flow higher rate
= develop lower profile
–> grow/decay rapidly

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32
Q

Breidamerkurjokull experiment 2

A

Boulton et al 2001

  • excavated trenches and buried transducers in front of mini-surge
  • elevated PWP in till
  • increase strain rates until threshold basal Pw reached
  • decreased strain rates

= rise/fall of dilatant (deforming) horizon

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33
Q

Interconnected feedbacks with effective pressure

A

STICK

  • start to add water = PWP + stress rises
  • some lateral deformation/basal sliding (SLIP)
  • +water = till dilate
  • upper unit deforms
  • deformation exaggerates just as PWP begins to fall
  • ice recouples strongly to bed = period of most deformation
  • PWP drops off completely and ice becomes stiff as particles lock

i.e. “stick slip motion with increasing PWP then recoupling and deformation as PWP drops

34
Q

Strain partitioning =

A

deformation moves away from ice-bed interface and into bed when strain interface moves into bed

35
Q

Is deformation style viscous or plastic?

A

Boulton and Hindmarsh 1987: Breidamerkujokull till strain patterns = non-linearly viscous material

Iverson et al 1997/1998: lab experiments = perfectly plastic

Engelhardt and Kamp 1998, Truffet et al 1999/2000, Tulaczyk et al 2000: subglacial measurements = plastic failure at vertically migrating levels

Evans et al 2006, O Cofaigh et al 2007: consistent with above, observations of subglacial tills from paleo-ice sheet beds

36
Q

Why is subglacial drainage important?

A

Determines PWP and therefore till movement/viscosity

37
Q

Sub-till and intra-till stratified sediments =

A

non-deformed water laid sediment deposited in films at ice-bed interface

Supports evidence for basal sliding - get pockets of water that cause ice to slide

Juxtaposed undeformed, sub-till stratified sediments and deformed till substrate contacts (Piotrowski et al 2006)

38
Q

Sliding and deforming bed mosaic

A

Related to temporal and spatial variation in subglacial effective pressure (Boulton et al 2001)

Position of deforming spots changes spatially/temporally = overprinted signatures

39
Q

Ice sheet scale subglacial landscapes and landforms - what can they tell us?

A

Large-scale time-transgressive patterns of erosion/deposition by ice sheet over soft bed

History of deposition at site reflects migration of depositional/erosional zones during advance/retreat

40
Q

Where is eroded material ultimately deposited?

A

Towards margin of glacier

41
Q

Submarginal zone =

A

Where net thickening of sediment is

= most subglacial landforms are here

42
Q

Subglacial landforms =

A

longitudinal/transverse accumulations of sediment deposited beneath active ice

Genesis linked to subglacial drainage, deforming bed and basal sliding

BEDFORM CONTINUUM CONCEPT

43
Q

Meltwater forms =

A

eskers

44
Q

Complex forms =

A

crevasse squeeze ridges

45
Q

Elongate forms =

A

lineations

46
Q

How are lineations classified?

A

According to length/height/elongation ratio (ER = length/width)

47
Q

Transverse forms =

A

ribbed (rogen) moraine

All usually form // to ice flow except rogen moraines

48
Q

Approaches to studying subglacial landforms

A

1) MORPHOMETRY
- map/measure dimensions
- Smalley and Unwin 1968

2) SEDIMENTOLOGY
- look inside
- Shaw et al 2000

3) MODELLING
- numerically
- Hindmarsh 1998

49
Q

Esker =

A

principal landform created by meltwater; ridge deposited beneath ice
Infillings of ice walled river channels
Sinuous ‘elongate ridges of glacifluvial sand and gravel’ (Benn and Evans 1998)

50
Q

How do eskers form?

A

Channels cut up into ice
Water moves through
Ultimately fills with sediment where river used to flow
Marks course of subglacial channel

51
Q

Esker characteristics

A

Sorted/non-sorted sands/gravels

Deposited sub, en, supra glacially

Stoarrar et al 2014: 10s m high, 100s m wide, 1000s m long

52
Q

Eskers; how does material get deposited in a high pressure subglacial tunnel?

A

Would think that slow deposition of material would constrict channel and increase flow velocity = entrain BUT

  • eskers form during sudden blockage of tunnel +/ rapid drop in water discharge i.e. at tunnel exit
  • sediment deposited as fan back up into tunnel
  • deglaciation = waves of sediment deposited at margin
    = beads of different depositions deposited time transgressively as glacier margin moves back

Burke et al 2008 = can be v quick e.g. <24 hours

53
Q

Main types of esker

A

Arren and Ashley 1994:

TUNNEL FILLS

ICE CHANNEL FILLS

SEGMENTED

BEADED

Concertina

54
Q

Tunnel fill eskers

A

En or sub

55
Q

Ice channel fill eskers

A

Supra

56
Q

Segmented eskers

A

Tunnel fills with pulses of sediment infilling channels as discharge pressure fluctuates during pulsed glacier retreat

57
Q

Beaded eskers

A

Subglacial fans deposited intermittently during pulsed glacier retreat

58
Q

Concertina esker

A

Knudsen 1995

Linked to surging glacier

Surges = crevasses = multi fractures

Glacier stops = water holds up in crevasses = deposits weird zigzags

59
Q

Crevasse-fill ridges =

A

few m high ridges of till

Often overly flutes

60
Q

How do crevasse-fill ridges form

A

Material squeezed up crevasses (lower P) which penetrate to base
Crevasse squeeze ridges
Related to glacier surges (Sharp 1985)

Shows presence of deforming bed
Low preservation potential

61
Q

Flutes =

A

streamline bedform composed of glacial till

Hart 1995 = some may be erosional

10s m long, 10s cm-few m high (low amplitude)

Low preservation potential

62
Q

Where are flutes found?

A

In glacier forelands

63
Q

How do flutes form?

A
  1. Boulton 1976: “sediment deforming into the low pressure cavity down-ice from an obstacle”
    - boulder stuck at ice bed
    - ice streams around = cavity
    - till squeezed into cavity
    - height and width due to initiating boulder
  2. Plough boulder through deforming bed
    - = stuck into basal ice
    - pushes sediment sideways as moves across bed
    = 2 ridges either side
64
Q

Where have drumlins recently been found?

A

Growing under Antartica

King et al 2007

Smith et al 2007

65
Q

Drumlins =

A

inverted spoon shape

Long axes record ice flow direction

Vast drumlin fields of 1000s

Range of sediment; stratified/deformed

Can cross-cut/morph
= possible evidence for bedform continuum theory

66
Q

“The drumlin problem”

A
  1. Diverse location and arrangement in fields
  2. Differing shapes/morphology
  3. Range of internal sediment types
  4. Cross-cutting drumlin forms
  5. Trigger mechanisms on some parts of glacier bed not others
67
Q

Equifinality =

A

the same end result from different mechanisms

68
Q

Possible theories for drumlin formation (+reference)

A
  1. Subglacial till deformation
    - Boulton 1987
  2. Catastrophic subglacial meltwater floods
    - Shaw et al 1989
  3. Erodent layer hypothesis
    - Eyles et al 2016
  4. Instability theory
    - Stokes et al 2016
69
Q

Drumlins; subglacial till deformation

A

Stiffer areas e.g. coarse grained within deforming layers = cores
Enhanced deformation takes place around
Cores can uproot/migrate

:) Good for drumlins with cores of rock/till/stratified sediment

:( Preservation potential controversial

70
Q

Key of subglacial till deformation theory

A

Even without resistant cores instabilities develop in deforming till
Hindmarsh 1998, Clark 2010

71
Q

Drumlins; catastrophic meltwater floods

A

Resemble fluvial bedforms from turbulent flow

Floodwater scours ice sheet base then infilled with stratified sediment
Floodwaters excavate materials from between resistant cores = drumlins as erosional remnants = explains stratified cores

Applies to flutes and drumlins

BUT: Clarke et al 2005
= where does water come from?

72
Q

Drumlins; erodent layer hypothesis

A

Drumlinisation leaves no substantial stratigraphic record because primarily erosional process
Cuts an unconformity across pre-existing bed materials

Recent idea

Soft bed/hard bed transition type model; ice moulded and streamed across bedrock hard terrain then moves through soft bed areas = erodes and mould sediment beneath

73
Q

Drumlins; instability theory

A

Mobile bed with different viscosity/rheology to ice above

A) Ice and sediment allowed to deform and sliding can occur at till interface

B) Subglacial systems prone to development of along-flow instability = waveforms (bedforms) at ice till surface

C) emerge as drumlins

Growing bumps through wave instability rather than core of rheological contrast

74
Q

What are mega-scale glacial lineations?

A

MSGL

6-100km long, 200-1300m wide, 200-500m apart

Exceptional length related to ice velocity

Produced by ice streams

Like longer/thinner drumlins

Huge ER over 10:1

75
Q

Where have MSGL been found?

A

First in early 1990s - Clarks 1993

Recently under Rutford ice stream W Antarctica (radar/seismic data) - King et al 2009

76
Q

How do MSGL form?

A

Clark 1993 = fast ice flow

Clark et al 2003 = related to streamlining of deforming bed or groove ploughing

Shaw et al 2009 = meltwater origin (where is the water from!!!)

77
Q

Ribbed (rogen) moraine

A

= sub glacially formed transverse ridges
Occur far up ice
Represent thermal boundary from cold to warm ice

10m high, 100s m wide, 1000s m long

Various sediment and evidence for stratification and deformation

In association with other subglacial bedforms

78
Q

Where are ribbed moraines found?

A

Far up ice
Mainly found in core areas
- linked to frozen core areas +/ slow ice velocity?

79
Q

Examples of ribbed moraine

A

Cover extensive areas of the Laurentide, Fennoscandian and Irish Ice Sheet

80
Q

How do ribbed moraines form?

A

Aylsworth and Shilts 1989, Bouchard 1989 = shearing/stacking of subglacial sediment

Fisher and Shaw 1992 = subglacial megafloods

Hattestrand and Kleman 1997 = fracturing theory based on jigsaw pattern

Frozen till sheet switches to warm based ice and fractures due to extensional flow

81
Q

Bedform continuum

A

Drumlins, MSGLs, flutes and moraine are all related

Stokes et al 2013

Aario 1977

Rose and Letter 1977

Rose 1987

Ely et al 2015

Glaciers and Glaciation (29 decks)

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