Week 12: Glacifluvial processes and landforms Flashcards
“Chaos”
Ice-contact environments have glacifluvial sediment and meltwater processes
As meltwater evolves over time, the tunnels carrying the water are also constantly evolving/changing shape
Kames =
sand/gravel deposits
Chaotic distribution
Hummocky mounds
Ice-walled lake plains =
accumulations of sediment in larger holes created within a glacier over time
Sandur =
flat outwash plains of sand/gravel
Spatial/temporal continuum
Glacifluvial landform assemblages occupy positions on this continuum
Series of tunnels/cavities working their way through ice
Constantly depositing sediment in ‘temporary’ holes
How do drainage networks in receding glaciers evolve?
Enlarge drainage pathways
Amalgamating drainage pathways
Switching from hydrostatic to atmospheric pressure
- as ice downwashes
- from fully subglacial system to supra/en/sub
Example of evolution of a glacier drainage network
R-channel esker
(pressure change)
H-channel esker
(tunnel collapse = completely open to atmosphere)
Ice-walled channel fill
(ice surface drainage)
Kame and kettle topography
Proglacial sandur progradation
Types of esker infills
R-channel
H-channel
Esker R-channel infill
Under hydrostatic pressure = water forced to flow uphill
= hummocky long profiles going up and down slopes
Esker H-channel fill
Under atmospheric pressure = aligned with bed slope
N.B. Includes ‘valley eskers’
What causes an esker to have an undulatory long profile?
Individual cavities within ice are filled up before moving along to next segment
Possible planforms of eskers
- Single continuous ridges (uniform cross-sectional profile)
- Single ridges of variable height/width
- Single low ridges linking numerous mounds/beads
- Complex braided systems
Relationship of eskers to ice flow
USUALLY ASSUMED ALIGNED // OR SUB-// TO ICE FLOW
BUT can be transverse e.g. valley esker Oldufelsjokull
Evidence of eskers deposited in segments during ice sheet retreat
Long ridges punctuated by ice marginal features
Dispersal centre = void area with radial patterns of eskers coming from it
e.g. Keewatin divide
Equidistant tunnels, spacing out the drainage system
Become more frequent during deglaciation due to increased rates of ice margin recession and climate warming
Example of mapping eskers
20,000 mapped by Storrar et al 2014 on Laurentide Ice Sheet
Boulton et al explanation of incremental deposition of eskers (N.B. Can also be continuous)
2007: Breidamerkurjokull
- Tunnel axis remains stable
- Strong coupling between groundwater and summer meltwater
- Open/shut due to flotation/water pressure decoupling of glacier
Hooke and Fastook explanation of incremental deposition of eskers
2007: S Laurentide ice sheet
- Tadpole shape (narrow further back from margin)
- separate deposition of each segment in sub-marginal tunnel
- melt/sedimentation rates increase with increasing glacier margin surface slope
- melt rates exceeds tunnel closure rates towards margin = reduce water flow (larger tunnel) = deposition
Esker sediments
Silts to boulders/diamictons
Bedded sands and gravels
Cut and fill sequences
Cyclical patterns
Bedform/discharge changes due to:
- glacier melt patterns
- tunnel constrictions and cavities
Cut and fill =
channel cut through pre-existing sediment then deposits
Supraglacial eskers
aka interlobate ‘moraines’
Occurs where amount of supra glacial sedimentation overwhelms everything else
- initially sub/en
- evolve = supra glacial ridges
- widen = supraglacial lakes with interlobate moraines
- later stages of evolution = kame and ice-walled lake plain
Example of supra glacial esker/interlobate moraine
Harricana Moraine, Laurentide Ice Sheet 1000km long, <10km wide, <100m high
Jokulhlaup and surge eskers
If tunnel opens up into series of restricted/less restricted zone = escape of water into snout
= zig-zag eskers (crevasse infills)
What do the limbs of crevasse infills represent?
A crevasse that water surged into and deposited an esker
Jokulhlaup and surge esker example
1996 Skeidararjokull jokulhlaup
What happens at the esker tunnel/lake interface?
Sediment splays into cavities due to flotation of tunnel mouth
= beaded eskers
Classification of kames
Terraces
Hills
Plateaus/ice-walled lake plain
Kame and kettle topography =
fluvial deposition on wasting glacier surface
Guided by controlled ridges of debris-charged ice and infilling of glacier karst
Kettle hole =
Discontinuous surface due to collapse beneath
= heavily disturbed stratified sediments
Kame terraces =
ice-marginal stream beds
- normal faulting and kettle holes
- accordant surfaces
(appear as terrace from a distance)
Ice-walled lake plains (kame plateau) =
circular plateau juxtaposed with chaotic kames/eskers/hummocky moraines
Partially flooded elongate channels between
= locally disturbed lake and glacifluvial sediments
Ice-walled lake plain (kame plateau) genesis
- infills of glacier karst holes
- melting of surrounding ice
- = upstanding plateau with collapsed sides
Stable ice-walled lake plains
Debris-charged ice
Unstable ice-walled lake plains
Debris-poor ice
- if hardly any sediment a pancake of sediment is recorded in landscape = hardly noticeable
= decreases preservation potential
Sandur =
Proglacial outwash fan
Braided channel network
High discharge for short periods of time
Downstream fining
Sandur; proximal zone
Few deep and narrow channels carrying sediment in their water
Sandur; intermediate zone
Complex network of wide/shallow braids with many channels abandoned
Sandur; distal zone
Very shallow, ill defined channels only flooded by sheet flow at high discharges
How do pitted sandar form?
1) Proximal outwash buries glacier snout
- N.B. overlap with kame/kettle topography
2) Ice blocks buried on jokulhlaup sander and melted out
Relates to melting of ice
Example of pitted sandar
Myrdalssandur 1918 flood deposits
Glacifluvial successions
Miall 1977, 1978
Trollheim
Scott
Donjek
Platte
