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)