Contemporary Evidence for Subglacial Bed Deformation ELERI EVANS.

Contemporary Evidence for Subglacial Bed Deformation

ELERI EVANS

Contemporary Evidence for Subglacial Bed Deformation

• Introduction

• Two contrasting approaches • Research Aims

– Features produced by bed deformation• Primary Glacigenic Deposits

• Subglacial Associations

– Research into current processes• Methodologies

• Research Conclusions

• Problems

• Conclusion

– Future Direction

Introduction

Subglacial Deformation occurs where:

Sediments beneath a glacier may undergo permanent strain in response to stresses imposed by the ice.

The shear stress at the bed > yield strength of the bed material.

Sediment yield strength depends on porewater pressure (water supply as a result of meltwater).

Processes Involved in Bed Deformation

• Subglacial behaviour depends on:ice sheet velocity, basal shear stress, pore-water pressure, effective pressure and the geotechnical properties of the subglacial material.

• Deforming movement can be through ploughing, shearing and sliding

Research Aims

To understand the interactions between subglacial material and the glacier, with reference to basal flow and deformation.

To establish the complex bed topography under glaciers.

Research Aims cont…

To determine the extent and dynamics of ice sheets that were present during the last glaciation.

To map and interpret deglaciated bedrock and sediments to develop understanding of bed deformation processes.

To understand the mechanisms associated with porewater pressure in relation to glacier bed deformation.

Features Produced – Primary Glacigenic Deposits

• Great variety - Result from combination of lodgement, deposition and melt out.

• Examined as evidence of subglacial deformation.– Lodgement till

– Glacitectonite

– Deformation till

– Melt-out till

– Intermediate varieties

Features Produced – Subglacial Associations

• Subglacial facies associations – complex, highly variable, between types of sediments.

• Rheologic superposition – successive formation of tills with different deformational histories

• Cofaigh and Evans 2001

• Bedforms – related to substrate morphology, local stress variations, sediment supply, ice flow and sediment deformation histories.

These Include:• Flutings • Rogen moraine • Crevasse-fill ridges • Subglacial volcanic

forms

• Drumlins (megaflutings)

• Lee-side cavity fills• Eskers

Flutings, Breidamerkurjokull, in Benn and Evans 1998

Research Examples

• Classification of deformed materials based on sediment properties (Benn and Evans 1996).

• Boulton et al., (1996) studied the role of subglacial mud in glacier surges.

Hart and Smith (1997) studied bedforms of Columbia Glacier, Alaska.

Hart and Rose (2001)

Research Examples cont…

• Ice-blocks frozen into surrounding diamict (Bennet et al., 2003).– Evidence that basal processes remain active at

subfreezing conditions.

• Till microfabrics (Chaolu and Zhijiu, 2001).– Strong particle and void microfabrics.

Research into Current Processes - Common Techniques

• Markers• Drilling – boreholes• Glacier remote sensing• Shear testing

• Chemical analysis of ice• Time-lapse photography• Caves and tunnels

• Till - granulometry, particle morphology, clast form, micromorphology.

– G.S. Boulton in 1979

Research Into Current Processes

Direct observation and instrumentation:

– Radio-Echo Sounding profiles (Gades et al., 2000)

– 3D radar array-processing (Moran et al., 2000) –High resolution, short-pulse, ground-

penetrating radar (GPR)

Research Into Current Processes

– LVDTs – linear variable displacement transducers, precision dial gauges and strain markers. (Fitzsimons et al., 2000)

– Tethered stake apparatus (Engelhardt and Kamb, 1998).

a) No shear in till

b) Presence of shear

Current Research Techniques

• Modelling Techniques

– Finite-element ice-flow model with a Coulomb failure criterion within the basal till layer (Truffer, Echelmeyer and Harrison, 2001)

– Viscous model of till rheology to describe gravitational loading instabilities in glacial sediments (Hindmarsh and Rijsdijk, 2000)

Overall Conclusions from Research

Locally, highly variable basal ice flow can occur at the base of temperate glaciers.

Deformed till tends to have strong particle and void microfabrics.

Till can be treated as a Coulomb-plastic material.

Problems with Research

• Models strongly influenced by ruling hypotheses and untested assumptions.– Models only approximations

• High clutter in radar surveys.

• Till based flow law hard to determine.

• Data acquisition can affect study area.

• Limited knowledge of processes.

Conclusions

• Possible Future Direction of Research

– Thermal Processes.• Porewater freezing, ice infiltration, ice melting.

– Mechanics of individual processes or phenomenon.

– Local scale processes.– Models based on accurate ‘assumptions’.

References:• Benn, D.I., and Evans, D.J.A., (1996) The interpretation and classification of

subglacially-deformed materials. Quaternary Science Reviews v.15 pp.23-52.

• Bennett, M.R., Waller, R.I., Midgley, N.G., Huddart, D., Gonzalez, S., Cook, S.J. and Tomio, A., (2003) Subglacial deformation at sub-freezing temperatures? Evidence from Hagafellsjokull-Eystri, Iceland. Quaternary Science Reviews 22, 915-923.

• Chaolu, Y and Zhijiu, C. (2001) Subglacial deformation: evidence from microfabric studies of particles and voids in till from the upper Urumqi river valley, Tien Shan, China. Journal of Glaciology 47.

• Cofaigh, C.O., and Evans, D.A., (2001) Deforming bed conditions associated with a major ice stream of the last British ice sheet. Geology v. 29; no. 9 pp. 795-798.

• Engelhardt, H. and Kamb, B., (1998) Basal sliding of Ice Stream B, West Antarctica. Journal of Glaciology 33, 83-98.

• Fitzsimons, S.J., Lorrain, R.D., Vandergoes, M.J., (2000). Behaviour of subglacial sediment and basal ice in a cold glacier. In: Maltman, A.J., Hubbard, B., Hambrey, M.J. (Eds.(, Deformation of Glacial Materials. Geol. Soc. Special Publication No. 176, London, pp. 181-190.

References cont…• Hart, J.K (1995) Suglacial erosion, deposition and deformation associated

with deformable beds. Progress in Physical Geography v 19 no.2 pp.173-191.

• Hart, J., and Rose, J. (2001) Approaches to the study of glacier bed deformation. Quaternary International 86 pp.45-58.

• Hart, J.K. and Smith, B., (1997) Subglacial deformation associated with fast ice flow, from the Columbia Glacier, Alaska. Sedimentary Geology 111, 177-197.

• Moran, M.L., Greenfield, R.J., Arcone, S.A., Delaney, A.J., (2000) Delineation of a complexly dipping temperate glacier bed using short-pulse radar arrays. Journal of Glaciology 46.

• Nolan, M., and Echelmeyer, K., (1999) Seismic detection of transient changes beneath Black Rapids Glacier, Alaska, USA: techniques and observations. Journal of Glaciology 45 no.149.

• Truffer, M., Echelmeyer, K.A., and Harrison, W.D., (2001) Implications of till deformation on glacier dynamics. Journal of Glaciology 47.