Week 11: Ice marginal sedimentation and landforms (moraines) Flashcards
How are moraines classified?
ENVIRONMENT OF DEPOSITION
PLAN FORM
RELATIONSHIP TO GLACIER ACTIVITY
Moraine classification; environment of deposition
- Terrestrial (deposited on earth’s surface)
- Subaqueous
- Supraglacial
Moraine classification; plan form
- Linear/orientated (i.e. ice flow // or transverse)
2. Non-orientated/chaotic
Moraine classification; relationship to glacier activity
- Advance
- Recession
e. g. +ve mass balance = advance = push moraine
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
Model processes for push/squeeze moraine formation
DEFORMATION/BULLDOZING
SQUEEZING
SLAB MELT-OUT
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
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!!!
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)
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
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
Main types of glacitectonic landform
Composite ridges
Hill-hole pair
Cupola hill
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
How do glacitectonic landforms form?
Low strength proglacial sediment + high glacier stresses = proglacial compression/thrusting/folding
Glacitectonic landforms; gravity spreading model
= translation of glacier weight into lateral stresses
Glacitectonic stress =
lateral stress due to lateral displacement of subglacial materials in response to:
- Normal stress (ice load)
- Basal shear stress (glaciodynamic stress)
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
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
Latero-frontal moraines =
ice-contact ridges marking lateral and frontal snout margins
Highly susceptible to melt-out collapse and paraglacial reworking
Ice contact fans/ramps =
asymmetrical, coalescent debris flow fans
- shallow distal slope and steep proximal/ice contact slope
Processes in latero-frontal moraines
DUMPING
INCREASING COMPONENT OF SUBGLACIAL DEBRIS DOWN VALLEY
GLACIER RECESSION
Latero-frontal moraines; dumping
Supraglacial debris transfer - slide, roll, flow, fall
Latero-frontal moraines; increasing component of subglacial debris down valley
- Debris septa rises to glacier surface
2. Valley floor sediments reworked