Lecture 9 - subglacial processes and landforms Flashcards
Alley et al (1986)
ice sheet behavior and instability depends on the dynamics of the large ice streams which drain it
up stream of Ice stream B (WAIS) glacial till deformation is primary mechanism of movement
if till isn’t deforming the velocity must arise from sliding of ice over a rigid substrate, lubricated by water, but this sufficient water layer doesn’t exist here, and permeable sed allows drainage
Benn and Evans (1996)
continuum of bed strengths can be recognized in deforming strata
deformable sediments are important influence on G dynamics
subsole deformation = pimrary mechanism for maintaining fast flow of some ice streams and outlet glaciers (e.g. Columbia river)
shaw (1989) proposed the Drumlins could result from large subG floos
Benn and Evans (1996) deformation till
Deformation till usually homogenized diamictic material formed by glacially induced shear of subsole materials
3 types of deformation till
- A = truncated
- B = Distorted but not truncated
- Communition (in situ crushing and abrasion)
high velocities during surging glaciers active phase thought to be due to rapid till deformation
Boulton et al (2001)
soft bedded modern gs = defo is significant contributor to movement
deforming till has the capacity to absorb strain –> protects the strata below
drumlins found almost everywhere on beds of former ice sheets: reflects the widespread occurrence of sediment deformation below them: implications for coupling of ice sheet flow and bed properties
concepts of movement = ice flow (internal defo and basal sliding) and deformable bed (new)
- -> bed no longer regarded as passive substratum, now part of a coupled system
- -> production and distribution of tills and features are important parts of the coupling
sediment transport rate important for determining till forming capacity: in steady state must be equal to erosion and depo rates
Boutlon et al (2001) hydrology coupling
SubG system mechanic behavior: also determined by subG water pressure regime/drainage
basal hydraulic systems can play vital role in controlling the coupling between the G and till defo processes
Evans and Hiemstra (200%
sediment availability increases at the margin
transport of subglacial sediment results in excavation of over deepening in sub marginal settings
conveyor belt transport of sediment leads to the marginal thickening of till sequences and sediment excavation of up glacier regions
Evans and Hiemstra (2005) deformation till
mosaic of subG processes will overprint sedimentary and structural signatures on till sequences, or A/B horizon till deformation characteristics
SubG tills = deposited when enG debris releases from active or stagnant Gs in a subG position
most important till producing mechanisms in subG settings = melt out, lodgment and deformation –> net advection of subglacial till to the margin
tills large scale architecture: deforming and strongly coupled to G
- coupling ensures erosion occurs in acc area where G is accelerating
Clark (2010)
subG landforms arise from self-organisation in the coupled flow of ice, sediment and water
different subG features = flutes, drumlins, MSGL and ribbed/rogen moraines
- might be genetically related
Smalley and Unwin (1968)
proposed that drumlins might result from simultaneous subG erosion and deposition if a layer of lubricating sediment existed and ‘flowed’, conducing geomorphic work
Clark (2010) Drumlins
most explanations cannot explain how drumlin formation started in the first place
drumlin formation instability theory
- the ubiquity and patterns of bedforms well explained by naturally arising flow instability in the subG system
- shaped mostly in glacial sediment and generated by the activity of overriding ice flow
drumlin formation requires the bed to be deforming in order to generate bumps and deform them
Clark (1993)
Landsat imaging shows large scale pattern of streamlining associated with former phases of ice flow
- assemblage includes drumlins and megaflutes
King et al (2009)
most large ice sheet discharge via fast flowing ice streams
MSGL are evidence that highly elongate reforms are characteristics of fast flowing regions in ice sheets
- association between fast ice flow and saturated deformable sediments
King et al (2009) MGSL hypothesis
meltwater hypothesis - high discharge, turbulent subglacial water flows erode soft beds into mega-lineations BUT evidence for minimal thickness of meltwater at ice bed interface
groover ploughing hypothesis - keels of ice (created by passage over hard bedrock upstream ) plough furrows in subglacial sediment to create erosional lineations
Stokes at al (2013)
instability theory
- coupled flow of ice and till causes spontaneous formation of relief in the till surface
- model predictions of bedroom height and length are consistent with observations
drumlins record key information relating to ice sheet flow history - ice sheet flow direction and changes through time
some drumlins have obvious core, but others do not, how do their bumps begin to form in the first place?
- unstable system - when a positive feedback act to amplify small disturbance –> natural variations may become larger
Dunlop and Clark (2006)
ribbed moraine = large subG formed transverse ridges
- cover extensive ares of the former Laurentide, Scandinavian and Irish ice sheets, likely to exist beneath Antarctic ice sheet
no body knows the correct formation:
formed by thrust stacking of debris rich basal ice, followed by ice sheet stagnation and passive melt-out
fisher and shaw argue they could result from sub G megafloods
some favor a deforming bed mechanism
two step formational hypothesis
- argue that G processes remoulded pre-existing ridge structures
- disagreement about the origin of pre-cursor ridges:
- drumlins and then change flow direction (Boulton, 1987)
- or ice cored moraines in an ice marginal setting (Moller 2006)
