Origin and significance of calcareous concretions within glacial outwash in the Tempo Valley, north-central Ireland |
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| Authors: | JASPER KNIGHT |
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| Institution: | School of Environmental Studies, University of Ulster, Coleraine, Co. Londonderry, Northern Ireland, BT52 ISA, UK |
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| Abstract: | In the Tempo Valley, north-central Ireland, flat-lying calcareous concretions are found within massive sand beds of glaciofluvial outwash which were deposited during the late Weichselian deglacial cycle. ( c . 17 000 14 000 BP). Morphologically, concretions (< 20 cm long. I I cm wide, 3 cm thick) arc bladcd to disc-shaped, lie parallel to sediment bedding planes, and have long axes aligned perpendicular to ice retreat direction. Internally, concretions are massive to framework-supported and were formed as calcareous cements precipitated from solute-rich groundwatcrs. Concretions, related to steep groundwater gradients and vigorous groundwater circulation cells, were formed proglacially at the base of a thin (1.5-2.0 m deep) layer of discontinuous permafrost as meltwater was actively expelled from the retreating ice margin. The presence of a discontinuous permafrost substrate can also help explain regional-scale geomorphic patterns indicating stagnation zone retreat of the Tempo Valley ice mass. |
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