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Fluvial-Eolian System Interaction on a Water Table-Controlled Lower Delta Plain, Skeidarársandur, Iceland

Abstract

Wet eolian dune systems characterized by bed forms undergoing active construction and with interdune depressions that lie at or close to the water table are widespread on the lower delta plain of Skeidarársandur, southern Iceland. The largest eolian dune complex on the delta plain covers an area of 80 km2 and is characterized by four distinct landform types: (i) spatially isolated eolian dunes on a fluvial braid plain, (ii) extensive areas of damp and flooded interdune flat with small fluvial channels, (iii) minor eolian dune fields composed of assemblages of bed forms with simple morphologies and small, damp interdune corridors, (iv) major eolian dune fields composed of assemblages of complex bed forms up to 8 m high and floored by older eolian dune deposits that are themselves raised up to 4 m above the level of the surrounding wet delta plain. The morphology of each landform type reflects a range of styles of interaction between eolian and fluvial processes that operate coevally on the delta plain. Based on observations from a series of trenches, the geometry, scale, orientation and facies composition of sets of strata preserved in the cores of the eolian dunes, and their relation to adjoining interdune strata, have been analyzed to explain the temporal behavior of the dunes in terms of their mode of initiation, construction, pattern of migration, style of accumulation and nature of preservation. The following interpretations are made: (i) seasonal and longer-term flooding-induced changes in water table level have caused episodic expansion and contraction of the wet interdune ponds; (ii) most of the dunes are currently undergoing active construction and migration and, although availability-limited because of the high water table, substantial eolian transport occurs during winter months when sand can be blown across the frozen delta plain without being trapped by surface moisture; (iii) bed forms in the larger dune fields have grown to a size such that formerly damp interdune flats have been reduced to dry enclosed depressions and dry eolian system accumulation via bed-form climb is on-going; (iv) despite regional uplift of the proximal delta plain in response to glacial retreat and unloading since ~1900, on-going subsidence of the distal delta plain due to sediment compaction is progressively placing the deposits below the water table and is both enabling the accumulation of wet eolian systems and increasing the likelihood of long-term preservation.