Sarah Mager & Sean Fitzsimons
Department of Geography, University of Otago, Dunedin, New Zealand
Abstract
The processes of erosion and entrainment for dry-based glaciers
are poorly understood. Until recently it had been assumed
that dry-based glaciers were largely incapable of basal erosion
because low basal temperatures create an environment which
strictly limits the availability of water and excludes or
limits the possibility of sliding. However, observations in
the Dry Valleys, Victoria Land, Antarctica have identified
morainic ridges adjacent to small cold-based glaciers. This
paper presents data on the analysis of entrained sediment
extracted from four small alpine glaciers in the Dry Valleys.
The approach of this study is to examine whether sedimentary
signatures of entrained basal debris can be used to elucidate
processes of debris entrainment and transportation.
The main techniques employed in this study are particle size,
shape and roundness analysis of sediments from the glacier
substrate, different basal ice facies and proglacial deposits.
The results show that samples from basal ice tend to
have sedimentary signatures very similar to parent characteristics
with regard to roundness and shape, but there were significant
differences in mean particle size and an increase in the number
of modes in particle size distributions. These characteristics
suggest that some crushing of the entrained debris may have
occurred. However, the absence of more angular particles
in the basal ice is not consistent with this interpretation.
An alternative interpretation is that increased modality
of the particle size distributions has been caused by abrasion
and that a "textural maturity" in roundness and shape is achieved
very quickly over a short transport path. Samples from
blocks of sediment entrained in basal ice which retained their
parent sedimentary and bedding structures were also analysed.
In some instances "tails" of material extended downstream
from sediment blocks. Examination of these "tails" revealed
no significant change in clast shape or roundness with increasing
distance from the source. There was, however, a decrease
in mean particle size and an increase in the number of modes
within the particle size distributions.
Reference
Mager, S. 1996. Sediment transport in dry-based glaciers
: an assessment of sedimentary signatures imprinted by glacial
transportation. Unpublished BSc (hons) dissertation, in Geography,
at the University of Otago, Dunedin, New Zealand. 78 pp.
Mager, S. and Fitzsimons, S. 2001. Sediment transport in
dry-based glaciers: an assessment of the sedimentary signatures
imprinted by glacial modification [Abstract]. Presented at:
2001 Geography A Spatial Odyssey, Joint Conference of the
New Zealand Geographical Society and the Institute of Australian
Geographers in Dunedin, in February 2001.
