Hummocky moraine: sedimentary record of stagnant Laurentide Ice Sheet lobes resting on soft beds |
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| Institution: | 1. Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland;2. Laboratory of Ion Beam Physics, ETH Zurich, Zurich, Switzerland;1. Scottish Association for Marine Science, Oban PA37 1QA, UK;2. British Geological Survey, The Lyell Centre, Edinburgh EH14 4AP, UK;3. Department of Geography, University of Durham, Durham DH1 3LE, UK;1. Department of Geography, University of Sheffield, Sheffield S10 2TN, UK;2. School of Geosciences, University of Aberdeen, Aberdeen AB24 3UF, UK;3. Department of Geography, Durham University, Durham DH1 3LE, UK;4. Department of Geological Sciences, Stockholm University, Stockholm SE-106 91, Sweden;5. Department of Earth Science, University of Bergen and Bjerknes Centre for Climate Research, Bergen N-5020, Norway;6. School of Geography and Environmental Sciences, Ulster University, Coleraine BT52 1SA, UK;7. Department of Earth and Environmental Sciences, The University of Rochester, Rochester, NY 14627, United States;1. British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA Scotland, UK;2. Physical and Environmental Sciences, University of Toronto, Scarborough, ON M1C 1A4, Canada;3. Geological Survey of Finland, P.O Box 97, F1-67101 Kokkola, Finland;4. Biological and Environmental Sciences, University of Stirling, Stirling, FK9 4LA Scotland, UK |
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| Abstract: | Over large areas of the western interior plains of North America, hummocky moraine (HM) formed at the margins of Laurentide Ice Sheet (LIS) lobes that flowed upslope against topographic highs. Current depositional models argue that HM was deposited supraglacially from stagnant debris-rich ice (`disintegration moraine'). Across southern Alberta, Canada, map and outcrop data show that HM is composed of fine-grained till as much as 25 m thick containing rafts of soft, glaciotectonized bedrock and sediment. Chaotic, non-oriented HM commonly passes downslope into weakly-oriented hummocks (`washboard moraine') that are transitional to drumlins in topographic lows; the same subsurface stratigraphy and till facies is present throughout. These landforms, and others such as doughnut-like `rim ridges', flat-topped `moraine plateaux' and linear disintegration ridges, are identified as belonging to subglacially-deposited soft-bed terrain. This terrain is the record of ice lobes moving over deformation till derived from weakly-lithified, bentonite-rich shale. Drumlins record continued active ice flow in topographic lows during deglaciation whereas HM was produced below the outer stagnant margins of ice lobes by gravitational loading (`pressing') of remnant dead ice blocks into wet, plastic till. Intervening zones of washboard moraine mark the former boundary of active and stagnant ice and show `hybrid' drumlins whose streamlined form has been altered by subglacial pressing (`humdrums') below dead ice. The presence of hummocky moraine over a very large area of interior North America provides additional support for glaciological models of a soft-bedded Laurentide Ice Sheet. |
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