The active temperate glacial landsystem: a model based on Breiðamerkurjökull and Fjallsjökull, Iceland

Abstract

Accurate interpretations of ancient glaciated terrains rely heavily on our knowledge of process–form relationships in contemporary glacierized basins. A landsystems model for temperate, actively receding glaciers is presented based upon Breiðamerkurjökull and Fjallsjökull, Iceland. Historical documentation, maps and/or aerial photography documenting recession since 1903 provide a unique series of “snapshots” of the evolving glacial geomorphology at these snouts. This documentation is employed in association with sedimentological investigations to assess the evolution of sediment–landform assemblages at active temperate glacier margins, using the wealth of geomorphological and sedimentological information produced during the recent recession of Breiðamerkurjökull and Fjallsjökull. Three depositional domains are recognized: (1) areas of extensive, low amplitude marginal dump, push and squeeze moraines derived largely from material on the glacier foreland and often recording annual recession of active ice; (2) incised and terraced glacifluvial forms, such as recessional ice-contact fans and hochsandur fans, and simple and complex, anabranched eskers and small areas of pitted outwash; (3) subglacial landform assemblages of flutes, drumlins and overridden push moraines located between ice-marginal glacifluvial depo-centres. The lack of supraglacial sediment in active temperate glaciers like Breiðamerkurjökull and Fjallsjökull generally precludes the widespread development of chaotic hummocky moraine. The hummocky terrain previously termed “kame and kettle topography” has mostly evolved by melt-out into a complex network of anabranched eskers over the period 1945–1998 or actually comprises pitted or kettled outwash (sandar). The tills across the foreland were emplaced by subglacial deformation and lodgement, and comprise materials derived from pre-existing stratified sediments in addition to localized abrasion of rock surfaces and patches of lake sediments. Till sequences thicker than 2 m have been constructed by the sequential plastering of till layers onto stratified sediments and bedrock. Because this stacking is a sub-marginal process, it is suggested that complex till sequences similar to those observed at Breiðamerkurjökull/Fjallsjökull may be employed in the reconstruction of ancient glacier margins. Additionally, the geomorphology of the active, temperate landsystem at east Breiðamerkurjökull may contain subtle surge signatures, verifying the historical record of small surges by this part of the glacier. This illustrates the danger of employing landform–sediment associations from restricted study areas (e.g. parts of landsystems) as representative process–form models for glaciated terrains.