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Durham University

Department of Geography

Departmental Research Projects

Publication details

Kilfeather, A.A., Ó Cofaigh, C. Dowdeswell, J.A. van der Meer, J.J.M. & Evans, D.J.A. Micromorphological characteristics of glacimarine sediments: implications for distinguishing genetic processes of massive diamicts. Geo-Marine Letters. 2010;30:77-97.

Author(s) from Durham

Abstract

Glacimarine diamicts are produced by diverse processes, and genetic differentiation is often problematic
using macro-sedimentological criteria alone. Micromorphology offers a potentially helpful tool in such investigations. Macroscopically massive diamict samples of known glacimarine origin, from the Polar North Atlantic,
Antarctica and north Irish Sea, were prepared for micromorphological
analysis to (1) identify microstructures
unique to different modes of sedimentation and (2) interpret
genetic processes from those structures. The samples comprised examples of debris-flow, iceberg-turbate and suspension settling deposits from late Quaternary glacier influenced marine environments: tidewater glacier, sub- or pro-ice shelf and continental slopes in front of ice stream termini. Results show two significant features of debrisflow
sediments: a bimodal grain fabric of near-horizontal and -vertical grains, and laminated clay and silt coatings on
sand and pebble grains. Coatings are best developed in sediments with finer grain-size distributions and in debris flow sediments which have had relatively long run-out distances on trough-mouth fans, suggesting continuous
rotation of grains in a buoyant, turbulent aqueous environment.
This is significant because it precludes debris-flow delivery by plug flow. The micromorphology of iceberg
turbate has not been described previously. It contains structures similar to those described in tills, so that unambiguous identification of these sediments seems unlikely based on micromorphological criteria alone.
Suspension sediments range from fine-grained massive diamicts containing microfossils to more heterogeneous
coarser sediments characterised by abrupt textural variations, from ice-distal and ice-proximal glacimarine environments respectively. The ice-proximal sediments contain fine
vertical lineations marking the trajectories of dropstones through wet matrix. These dropstone tracks have not been reported in previous studies.

Department of Geography