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Geology 370: Glacial Geology

GLACIAL DEBRIS TRANSPORT

A. Introduction

• Supraglacial debris sources:

  • freeze-thaw weathering of valley walls

  • avalanches

  • wind-blown sediments (e.g. volcanic ash)

• Subglacial debris sources:

• abraded and plucked material

• Two transport zones: high and low

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B. High Level Debris Transport

Image credit: ©Bruce Molnia, Terra Photographics. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images

• Supraglacially derived debris

• Medial moraines

  • confluence of two lateral moraines

  • debris plume downglacier from rock walls or nunataks

• Supraglacial debris characteristics

• debris thickness

• angular, coarse clasts unaltered by transport

 

Tasman Glacier, New Zealand 2001

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C. Low Level Debris Transport

• Sources

• Basal debris concentration

• Basal transport zones

  • zone of traction

  • zone of suspension

• Basal debris characteristics

• highly altered by transport

• abrasion increases roundness; crushing increases angularity

  • terminal roundness

• faceted & striated clasts

• particle fabric may develop: elongated particles aligned with ice flow direction

• more fine-grained than supraglacial debris

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D. Transfer Between Low- and High Level Transport

• Occurs at terminus

• Shear planes or thrusts

  • most common in glaciers with mixed basal thermal regimes

• Thrusts affect debris structure

• warm based glaciers - few thrusts; most transport is high level

• mixed regime glaciers - well-developed thrusts; thick englacial & subglacial debris concentrations

• cold based glaciers - dominated by high level transport

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E. Summary

• High & low level transport

• High level transport: little change; angular, coarse clasts, no fabric

• Low level transport: much change; sub-rounded, more fine-grained clasts, may develop a fabric