A quick (re)view: glacier motion
Processes of Glacial Erosion and
Sedimentation
Ice surface
UF
UF
US
UF
US
UD
Jerome Lesemann
Geological Survey of Canada
Only ice
deformation
GEO 2334 – GEG 3306
Sept. 21, 2012
Sliding
rock bed
Sediment A horizon
bed
B horizon
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Glacier Hydrology
Creep
Bed Slip
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Bed Deformation
Subglacial erosion processes
• Abrasion, plucking, crushing, comminution
Joel Harper, U Montana
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Bedrock abrasion
Abrasion and plucking
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Debris-rich ice
Subglacial erosion processes
• Abrasion, plucking, crushing, comminution
Shaw, J.
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Shaw and Haldorsen (1972)
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Glacial sediment pathways
Deposition by ice: Till
• Sediment ‘storage’ (short/long term) in the bed
• Sediment deposited by the direct action of
ice.
• Sediment transport in:
– Active vs. passive ice
– Subglacial, ~ englacial, proglacial
» Ice
» Bed deformation
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Bullet-shaped boulder
Till is a diamicton
• Poorly sorted and massive (generally!)
Piotrowski and Kraus (2001)
T. Shaw
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Subglacial proglacial transport
and deposition
Ice marginal shearing
• Sediment transport through:
– Ice pathways
– Shearing (accompanied by melt-out near
margins)
– Meltwater transport
Larsen et al. (2010)
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Icelandic sandur
Subglacial proglacial transport
and deposition
Discharge (m3.d-1 x 10-3)
• Outwash plains and sandur (pl. sandar)
• Sand, gravel, muds (assuming availability)
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Google Earth
Subglacial proglacial transport
and deposition
Sandur characteristics
• Complex topography
• ‘Flashy’ discharges
– terraces
– moraines
– lateral fan elements
– multiple input points
Marren (2005)
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Maizels (1999)
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Sandur morphology
Sandur morphology
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Eskers
R-channel (Switzerland)
Are glaciofluvial sediments
necessarily proglacial
deposits?
Esker (Keewatin)
photo credit: Jan Aylsworth
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Eskers
• Channelized flow
Is subglacial meltwater flow
necessarily channelized?
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Fulton (1995)
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Subglacial deposition and
deformation of glaciofluvial
sediments, Denmark
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Lesemann et al. (2010)
Lesemann et al. (2010)
Episodic subglacial depositiondeformation
Hydraulic erosion under glaciers
• Water does more than move sediment
under glaciers!
• Highly effective erosional agent
– Cavitation
– Abrasion (corrasion)
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Lesemann et al. (2010)
Cavitation
Cavitation marks
• At high flow velocities (> 10-12 m.s-1) and low
pressures, dissolved gasses in water form bubbles
that violently implode when transported to areas of
higher pressure
Rings
Crum (1979)
photo credits: John Shaw
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Cavitation near hydraulic jumps
Cavitation in spillways, Glen Canyon dam
• 3 ft. thick reinforced concrete
• 20-25 ft bedrock
Less than 3-4 days!
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Photo: US Army Corp of Engineers
Abrasion (aka corrasion)
Photo: US Army Corp of Engineers
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What factors determine the rate
of meltwater abrasion?
Wear of clasts and bed by bed-material
load.
Important in the formation of bedrock
channels and erosion marks (incl.
potholes).
Sediment properties – size, hardness, concentration
Substrate character – hardness
Flow properties – velocity, turbulence (Re > 2500)
Channel planform – constrictions, bends
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photo credit: Tracy Brennand
GLACIAL EROSIONAL LANDFORMS
Products of hydraulic erosion in
glaciers
• S-forms
Lecture 9:
10:Glacial
GlacialLandforms
Landforms
OBJECTIVES
(+ tunnel valleys)
-describe P-forms & flutes
(rock drumlin)
Georgian Bay, Ontario
Photo: John Shaw
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Abrasion forms
Abrasion forms
Abraded, striated surface
Rat tails
Abrasion tends to plane rock surfaces even variable rock type
Origin of s-forms
Erosion forms
•
S-forms, crag-and-tail forms, crescentic furrows, as well as hairpin scours,
are explained easily by horseshoe vortices TURBULENCE
Is this the result of abrasion ?
Glacial grooves
• Considered to form
by glacial abrasion
• Need positive
feedback to deepen
initial depression
• abrasion tends to plane;
this erosion has relief
(differential erosion)
-This a sculpted form
Kor et al. (1990)
Flow characteristics and Reynolds
numbers
flow velocity * obstacle diameter (enhances turbulence)
Re =
Water
Ice
100 m
100 m
1. Basic processes likely scale with the ice
sheet
•
•
fluid viscosity (retards turbulence)
Obstacle
diameter
How does this translate to glaciers
and ice sheets?
Flow
velocity
Fluid
viscosity
Reynold’s
number
5 m/s
1.6 x 10-6 m/s
1.79 x 10-6
8.79 x 1010
2.8 x 108
2.0 x 10-16
Generation of turbulence behind obstacles occurs at Re > 1000
Ice thickness and Normal pressure on bed
Dimensions of hydrologic system
2. Produce the record of Quaternary (and
other) glaciations
•
•
•
Sediments
Landforms
Landscapes
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An example: s-form fields
Variety of s-forms in a field
Variety of s-forms.
photo credit: John Shaw
Superimposed hierarchy of s-forms.
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French River, ON (photo credit: John Shaw)
Plaster-cast of erosion marks formed by water in a flume
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Boulder lag/deposit (?)
Extent of s-form fields
Kor et al (1991)
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Percussion fractures
French River, ON (photo credits: John Shaw)
French River field ~70 km wide
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Inferred flow properties for
French River s-form field
Sediment-laden turbulent meltwater sheet
flow
– D = 0.3 – 2.0 m
– v ~ 5 – 10 m.s -1
– w ~ 70 km
– d ~10 m
debated
Q ~ 4 – 7 x 107 m3.s-1
– Sufficient to empty Lake Superior in ~20 days!
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• How do we know the LIS and CIS extents?
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Paleo-ice sheet reconstructions
What are these landforms?
5 km
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Fulton (1995)
Are they erosional or depositional?
Paleo-ice sheet reconstructions
• Why does that matter?
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DRUMLIN GROUPS
Spindles and drumlins
s
Spindle
drumlins (S)
with crescentic
depression (C)
Drumlins of
various types:
• classic
• spindle
• parabolic
Drumlins may occur
in organized groups
C
s
fluting
s
C
C
fluting
[Mollard, 3.9]
Beverley Lake, NWT
Shaw, 1996
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Hypotheses of streamlined landform genesis
Meltwater vs. deforming sediment?
Erosion by turbulent meltwater
Erosion by debris streaming
What are the implications for reconstructions
of ice sheet processes?
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