Week 10: Subglacial processes and landforms Flashcards
What happens as ice streams move from land to sea?
Hard to soft bed landforms
Hard = erosional/shaping of rock Soft = sediment needed --> soft-bedded landforms
Erosional to depositional landforms
Subglacial and englacial debris production
Erosion/transport/deposition processes produce subglacial sediment
FROM SUBSTRATE BENEATH THE ICE STREAM
Types of subglacial and englacial debris production
Plucking and bedrock communition
Bulk freeze-on (net adfreezing)
Apron entrainment
Marginal thickening/stacking
= debris conveyer
Bulk freeze-on (net adfreezing) =
debris frozen onto bed
Freezing front penetrates into subglacial sediment layer = adds onto base of glacier
Apron entrainment =
glacier runs over pre-existing sediment in front of it and incorporates it
Marginal thickening/stacking =
compress sediment and fold = thicken
Thrusts through further compression = stacks
Debris conveyer
Net advection of debris to sub-marginal zone
Up ice –> down ice
Potential 10s of km
Erratics
Good indicator of impact of advection on system as a whole
= rock/boulder different from surrounding rock, there due to glacier
Plucking and bedrock communition
- Cannibalisation
- = BR raft detachment by plucking/attenuation
- “glacitectonite” - Communition
- = BR fracture/pulverisation
- “till” i.e. homogenous material
Continuum of plucking and bedrock communition
Fractured BR –> pulverised BR –> mono-lithological diamicton (Hiemstra et al 2007)
Glacitectonite =
formed by subglacial plucking/shearing/crushing local BR
Vertical continuum: Undisturbed BR Faulted/sheared BR Brecciated/pulversied Diamicton with erratic clasts
Shearing =
Deformation through discrete shear/strain events
Indicates dry environment undergoing high degree of lateral shear stress
Glacier bed mosaic
Frozen –> thawed –> sliding –> deforming zones
The deforming bed =
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
Deformation of till is evidence that…
Ice can move through a layer of sediment that is deforming laterally as ice moves from up-ice to down-ice
Shear stress is … towards the ice-bed interface
exponential
and therefore failure also - van der Wateren 1995
Brittle deformation =
displacement along shear planes
Ductile deformation =
mobile/viscous
Strain ellipse
Shows nature of deformation i.e. folding/stretching/pinching of substrate
Low shear strain
Sediment folded and faulted with progressive shear and attenuation = tectonic laminations and detached boudins
Some primary structures e.g. folds may be preserved
Attenuation =
reduction
High shear strain
High levels of shear sediment homogenised
All structures gone
How was deforming bed theory found?
Researching how ice streams were moving so fast in Antarctica (Alley 1986, 1987)
What is deforming bed theory
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)
Boulton and Hindmarsh deforming bed theory experiment 1987
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
Three types of failure
- 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 - NEWTONIAN VISCOUS MATERIAL
- strain rate linearly proportional to shear stress
- continuous rate of deformation - NON-LINEAR VISCOUS MATERIAL
- e.g. ice
- rate changes
Effective pressure field in deforming bed is a function of 2 elements:
- Ice weight pushing down on sediment
2. Water pressure in sediment pushing back up
How do you reduce effective pressure?
Increase PWP
As ice weight pushing down on sediment = PWP pushing back up, effective pressure approaches 0
What happens as effective pressure approaches 0?
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
Effective pressure equation
Pe = Pi - Pw
Effective pressure in soft sediments
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
Breidamerkurjokull experiment 2
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
Interconnected feedbacks with effective pressure
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
Strain partitioning =
deformation moves away from ice-bed interface and into bed when strain interface moves into bed
Is deformation style viscous or plastic?
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
Why is subglacial drainage important?
Determines PWP and therefore till movement/viscosity
Sub-till and intra-till stratified sediments =
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)
Sliding and deforming bed mosaic
Related to temporal and spatial variation in subglacial effective pressure (Boulton et al 2001)
Position of deforming spots changes spatially/temporally = overprinted signatures
Ice sheet scale subglacial landscapes and landforms - what can they tell us?
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
Where is eroded material ultimately deposited?
Towards margin of glacier
Submarginal zone =
Where net thickening of sediment is
= most subglacial landforms are here
Subglacial landforms =
longitudinal/transverse accumulations of sediment deposited beneath active ice
Genesis linked to subglacial drainage, deforming bed and basal sliding
BEDFORM CONTINUUM CONCEPT
Meltwater forms =
eskers
Complex forms =
crevasse squeeze ridges
Elongate forms =
lineations
How are lineations classified?
According to length/height/elongation ratio (ER = length/width)
Transverse forms =
ribbed (rogen) moraine
All usually form // to ice flow except rogen moraines
Approaches to studying subglacial landforms
1) MORPHOMETRY
- map/measure dimensions
- Smalley and Unwin 1968
2) SEDIMENTOLOGY
- look inside
- Shaw et al 2000
3) MODELLING
- numerically
- Hindmarsh 1998
Esker =
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)
How do eskers form?
Channels cut up into ice
Water moves through
Ultimately fills with sediment where river used to flow
Marks course of subglacial channel
Esker characteristics
Sorted/non-sorted sands/gravels
Deposited sub, en, supra glacially
Stoarrar et al 2014: 10s m high, 100s m wide, 1000s m long
Eskers; how does material get deposited in a high pressure subglacial tunnel?
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
Main types of esker
Arren and Ashley 1994:
TUNNEL FILLS
ICE CHANNEL FILLS
SEGMENTED
BEADED
Concertina
Tunnel fill eskers
En or sub
Ice channel fill eskers
Supra
Segmented eskers
Tunnel fills with pulses of sediment infilling channels as discharge pressure fluctuates during pulsed glacier retreat
Beaded eskers
Subglacial fans deposited intermittently during pulsed glacier retreat
Concertina esker
Knudsen 1995
Linked to surging glacier
Surges = crevasses = multi fractures
Glacier stops = water holds up in crevasses = deposits weird zigzags
Crevasse-fill ridges =
few m high ridges of till
Often overly flutes
How do crevasse-fill ridges form
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
Flutes =
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
Where are flutes found?
In glacier forelands
How do flutes form?
- 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 - Plough boulder through deforming bed
- = stuck into basal ice
- pushes sediment sideways as moves across bed
= 2 ridges either side
Where have drumlins recently been found?
Growing under Antartica
King et al 2007
Smith et al 2007
Drumlins =
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
“The drumlin problem”
- Diverse location and arrangement in fields
- Differing shapes/morphology
- Range of internal sediment types
- Cross-cutting drumlin forms
- Trigger mechanisms on some parts of glacier bed not others
Equifinality =
the same end result from different mechanisms
Possible theories for drumlin formation (+reference)
- Subglacial till deformation
- Boulton 1987 - Catastrophic subglacial meltwater floods
- Shaw et al 1989 - Erodent layer hypothesis
- Eyles et al 2016 - Instability theory
- Stokes et al 2016
Drumlins; subglacial till deformation
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
Key of subglacial till deformation theory
Even without resistant cores instabilities develop in deforming till
Hindmarsh 1998, Clark 2010
Drumlins; catastrophic meltwater floods
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?
Drumlins; erodent layer hypothesis
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
Drumlins; instability theory
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
What are mega-scale glacial lineations?
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
Where have MSGL been found?
First in early 1990s - Clarks 1993
Recently under Rutford ice stream W Antarctica (radar/seismic data) - King et al 2009
How do MSGL form?
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!!!)
Ribbed (rogen) moraine
= 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
Where are ribbed moraines found?
Far up ice
Mainly found in core areas
- linked to frozen core areas +/ slow ice velocity?
Examples of ribbed moraine
Cover extensive areas of the Laurentide, Fennoscandian and Irish Ice Sheet
How do ribbed moraines form?
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
Bedform continuum
Drumlins, MSGLs, flutes and moraine are all related
Stokes et al 2013
Aario 1977
Rose and Letter 1977
Rose 1987
Ely et al 2015
