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Genesis of hummocks found in tunnel valleys: an example from Horda, southern Sweden

Journal article
Authors Gustaf Peterson
Mark D. Johnson
Sandra Dahlgren
T. Passe
H. Alexanderson
Published in Gff
Volume 140
Issue 2
Pages 189-201
ISSN 1103-5897
Publication year 2018
Published at Department of Earth Sciences
Pages 189-201
Language en
Links dx.doi.org/10.1080/11035897.2018.14...
Keywords Tunnel valley, glaciofluvial corridor, Subglacial meltwater corridor, hummock, glaciofluvial, meltwater, Subglacial, esker, southwestern baltic basin, subglacial meltwater flow, ice-sheet, northwest-territories, smaland peneplain, last glaciation, lake, drainage, sediments, till, deglaciation
Subject categories Geology

Abstract

In the south-central sector of the former Fennoscandian Ice Sheet, imprints of the sub-glacial hydrological system are present as glaciofluvial corridors, formed by glacial meltwater at the ice-bed interface during the BOlling-AllerOd warm period. Many of these are interpreted as tunnel valleys and are commonly characterized by hummocks on their valley floors. Contemporary ice sheets produce increased amounts of meltwater as a consequence of global warming, and occasionally it is observed that meltwater is suddenly released from supra- and subglacial lakes, suggesting a highly dynamic subglacial hydraulic system. Studies of the imprints and deposits from such systems on formerly glaciated terrain can expand our knowledge of ice-sheet response to increased meltwater production. Here, we study sediments exposed in two hummocks within the tunnel valley at Horda, south Sweden. One of the investigated hummocks is superposed by a small esker. This hummock consists of a diamict interpreted as a subglacial traction till, observed to be overlain by esker sediment. A second hummock displays deformed sediment at its base, which is glaciotectonically intercalated with above-lying diamict, a sub-glacial traction till. The sub-till sediments, interpreted as proglacial outwash, were deformed by overriding ice. The sediment was dated using optically stimulated luminescence (OSL), inferring a late MIS 4 or early MIS 3 age, congruent with other observations of sub-till sediments in south Sweden. The investigated hummocks on the floor of the Horda tunnel valley are interpreted to have been formed by sub-glacial fluvial erosion simultaneous with tunnel valley formation, most probably during the latest deglaciation of the area.

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