Jokulhlaup deposits in proglacial areas

This paper discusses the main causes and characteristics of jokulhlaup ('glacier burst') floods, and explores the extent to which they generate depositional landform and sediment assemblages that are distinct from those of 'normal', braided river outwash ('Type I' outwash). Two main jokulhlaup outwash environments are identified: Type II outwash, produced by sudden drainage of ice-dammed lakes; and Type III, associated with drainage during subglacial geothermal activity, and distinguished by deposits resulting from high sediment concentrations and hyperconcentrated flows. In fluid flows, especially ones yielding Type II outwash, the most common deposits are large-scale expansion bars (and locally, eddy and pendant bars), and 'mega-ripples' or dunes, both forms normally composed of large-scale gravel-cobble cross-bedding, often capped by an imbricated boulder lag (a 'Type B2' lithofacies sequence). The armour is absent only where runoff decreased too rapidly to allow surface winnowing. Other jokulhlaup facies include extensive boulder beds (Type C), inverse-normally graded cobble beds (Type D5), ice-proximal debris flow deposits and deformed bedding containing diamicton clasts (Types G and H), and slack-water sediments (Type A). Type III outwash is dominated by massive, homogeneous, flood surge granules, underlain by pre-surge gravels, and capped by post-surge fluid bedforms, reflecting deposition during both the rising and falling Limbs of the flood hydrograph (Type E4). The paper demonstrates that jokulhlaups do generate distinctive assemblages of depositional landforms and sediments, and concludes with a model of the dominant lithofacies sequences and associated landforms in proglacial environments subject to jokulhlaup drainage. (C) 1997 Elsevier Science Ltd.