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, Ice-walled-lake plains in North America and Europe--description, genesis, and paleoglaciological implications

Journal article
Authors Mark D. Johnson
Published in GFF
Volume 126
Pages 120
Publication year 2004
Published at Department of Earth Sciences
Pages 120
Language en
Keywords ice-walled-lake plains geoomorphology Quaternary
Subject categories Earth and Related Environmental Sciences, Quaternary geology

Abstract

Ice-walled-lake plains (IWLPs) are conspicuous but little-known landforms associated with the broad bands of hummocks that mark the southern margins of the former ice sheets in Europe and North America. They are circular to irregular, flat-topped hills up to 50 m in relief and up to a few kilometers wide surrounded by glacial hummocks. Laminated lake sediment up to tens of meters thick underlies the centers of many IWLPs, and coarser wave-washed, deltaic, and debris-flow sediment is typically present around their edges, often forming a rim ridge. These lake plains provide evidence for the final disintegration of the ice and the distribution of debris from the melting ice. IWLPs and hummocks formed where supraglacial debris slumped down ice slopes into open thaw lakes in a zone of stagnant ice. Some debris was reworked by supraglacial streams or in the ice-walled lakes. Some IWLPs, especially those in Alberta, Minnesota, and southern Sweden seem to contain more till-like material than lake sediment, indicating a continuum from IWLPs with little to abundant lake sediment. Paleoclimatic, as well as geologic, evidence shows that sediment in IWLPs was derived from supraglacial sources. At the end of the last glaciation, permafrost allowed ice-walled-lakes to persist in stagnant ice long after the active ice margin receded. Paleoclimatic evidence from North Dakota shows that ice-walled lakes persisted into the early Holocene, indicating that stagnant ice could only have survived with a thick cover of supraglacial debris. Although this supraglacial theory has been challenged in recent years, we are unaware of any evidence that indicates a substantial amount of subglacial material being squeezed up into the ice-walled lakes, nor does a subglacial squeezing hypothesis explain the existence of these lakes long after the climate warmed. IWLPs occur in areas where the marginal zone of the glacier contained abundant supraglacial debris. Compressive flow and freezing-on near the ice margin, enhanced by permafrost or readvances of ice into areas of stagnant ice, likely contributed to the accumulation of thick masses of debris-rich ice that yielded thick supraglacial debris as the ice melted. Paleoclimatic, topographic, and glacial-geologic evidence indicates that the southern margins of the great ice sheets advanced up adverse slopes, into areas of permafrost, and often at relatively high ice velocities, all of which enhanced thrusting and freezing-on of subglacial debris and its transport to englacial and supraglacial positions. The hummocky moraine surrounding the ice-walled-lake plains formed when the supraglacial debris was let down as the last ice melted.

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