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This study examines the extent to which observed large-scale stage variations are reflected in the proglacial landform and sedimentary record of the November 1996 jökulhlaup, Skeiðarárjökull, Iceland. Discrimination of rising from falling flood stage landforms and deposits is usually based upon the interpretation of the geomorphic and sedimentary record. Sedimentary successions in proglacial environments have been interpreted on the basis of vertical sedimentary characteristics which are then linked to the flood hydrograph. Most research has considered efflux within channels active on both rising and falling flow stage where the resultant morphology and sedimentology are the product of the temporal variability of both water and sediment flux. Spatial segregation of rising and falling stage proglacial outwash during the November 1996 jökulhlaup provided a superb opportunity to examine the role of flow stage in the creation and preservation of distinctive proglacial jökulhlaup landforms and deposits. Rising stage deposits contained finer, more poorly sorted sediment than found on falling stage successions and erosional surfaces. Rising stage deposits showed upward-coarsening successions, characteristic of progressive supply of coarser-grained sediment with stage increase, compatible with previous models of rising stage sedimentation. Some rising stage successions however, showed few signs of large-scale grading, and instead contained repeated cycles of sedimentation, recording individual sedimentation pulses. Distinctive upward-coarsening successions on a waning stage outwash fan were generated by sediment reworking and winnowing. The presence of an upward-coarsening succession alone is clearly not diagnostic of rising stage deposition. Conduits occupied by flows on both rising and falling flow stages were characterised by initial rising stage fan deposition followed by falling stage dissection and exhumation of ice blocks and rip-up clasts deposited on the rising flow stage. Rising stage deposits contained both single upward coarsening successions as well as successions consisting of stacked upward-coarsening and normally-graded units. Where waning stage flows were routed through a single conduit, high sediment efflux and aggradation rates were maintained late into the waning stage. Winnowing and sediment starvation resulted in progressive bed coarsening from matrix-supported gravels to clast-rich armour. This study illustrates the geomorphic and sedimentary significance of major within-jökulhlaup sediment reworking and ice-margin erosion over distances of 10^2-10^3 m. Ill-defined erosional, streamlined terraces reflect exhumation on the flood waning stage. This landform and sedimentary succession could easily be confused with the product of fluvial depositional and erosional cycles operating over longer timescales associated with more sedate rates of glacier retreat within former proglacial areas.
Author(s): Russell AJ, Knudsen Ó
Editor(s): I.P. Martini, V.R. Baker and G. Garzon
Publication type: Book Chapter
Publication status: Published
Book Title: Flood and megaflood deposits: recent and ancient examples
Year: 2002
Pages: 67-83
Series Title: International Assocication of Sedimentologists Special Publication
Publisher: Blackwell Science
Place Published: Oxford
URL: http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0632064048,descCd-tableOfContents.html
Notes: IAS Special Publication no. 32
Library holdings: Search Newcastle University Library for this item
ISBN: 9780632064045
