Controlled moraines: origins,characteristics and palaeoglaciological implications |
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| Authors: | David J.A. Evans |
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| Affiliation: | 1. Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10944, USA;2. Department of Earth Sciences, Climate Change Institute, University of Maine, Orono, ME 04469, USA;3. Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA;4. GNS Science, Private Bag 1930, Dunedin 9054, New Zealand;5. Department of Geosciences, University of Oslo, 0316 Oslo, Norway;6. Department of Earth and Planetary Sciences, University of California, Berkeley, CA 95064, USA;7. Antarctic Research Centre, School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand;1. Lamont–Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA;2. Aix-Marseille Université, CNRS-IRD-Collège de France, UM 34 CEREGE, Aix-en-Provence, France;3. Institute of Geological Sciences, University of Bern, Switzerland;4. Department of Earth Sciences and Climate Change Institute, University of Maine, Orono, ME 04469, USA;5. Insitut für Teilchenphysik, Eidgenössische Technische Hochschule, Zürich, Switzerland;6. Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA;7. Earth and Planetary Science Department, University of California – Berkeley, Berkeley, CA 94720, USA;1. Departments of Arctic Geology and Arctic Geophysics, University Centre in Svalbard, 9170 Longyearbyen, Norway;2. Department of Physical Geography and Quaternary Geology, Stockholm University, 106 91 Stockholm, Sweden;3. Bolin Centre for Climate Research, Stockholm University, 106 91 Stockholm, Sweden;4. Department of Geography and Sustainable Development, University of St Andrews, KY169AL, St Andrews, Scotland, UK;5. School of Geography, Queen Mary University of London, E14NS, London, UK;6. Department of Geography, College of Science, Swansea University, SA2 8PP, UK;1. Durham University, Department of Geography, Lower Mountjoy, South Road, Durham DH1 3LE, UK;2. Stockholm University, Department of Physical Geography and Quaternary Geology, and Bolin Centre for Climate Research, 106 91 Stockholm, Sweden;3. Simon Fraser University, Department of Earth Sciences, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada |
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| Abstract: | ![]()
Controlled moraines are supraglacial debris concentrations that become hummocky moraine upon de-icing and possess clear linearity due to the inheritance of the former pattern of debris-rich folia in the parent ice. Linearity is most striking wherever glacier ice cores still exist but it increasingly deteriorates with progressive melt-out. As a result, moraine linearity has a low preservation potential in deglaciated terrains but hummocky moraine tracts previously interpreted as evidence of areal stagnation may instead record receding polythermal glacier margins in which debris-rich ice was concentrated in frozen toe zones. Recent applications of modern glaciological analogues to palaeoglaciological reconstructions have implied that: (a) controlled moraine development can be ascribed to a specific process (e.g. englacial thrusting or supercooling); and (b) controlled moraine preservation potential is good enough to imply the occurrence of the specific process in former glacier snouts (e.g. ancient polythermal or supercooled snouts). These assumptions are tested using case studies of controlled moraine construction in which a wide range of debris entrainment and debris-rich ice thickening mechanisms are seen to produce the same geomorphic features. Polythermal conditions are crucial to the concentration of supraglacial debris and controlled moraines in glacier snouts via processes that are most effective at the glacier–permafrost interface. End moraines lie on a process–form continuum constrained by basal thermal regime. The morphological expression of englacial structures in controlled moraine ridges is most striking while the moraines retain ice cores, but the final deposits/landforms tend to consist of discontinuous transverse ridges with intervening hummocks, preserving only a weak impression of the former englacial structure. These are arranged in arcuate zones of hummocky moraine up to 2 km wide containing ice-walled lake plains and lying down flow of streamlined landforms produced by warm-based ice. A variety of debris entrainment mechanisms can produce the same geomorphic signature. Spatial and temporal variability in process–form relationships will lead to the sequential development of different types of end moraines during the recession of a glacier or ice sheet margin. |
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