Abstract

Hydrodynamic processes responsible for governing steady-state water flow through glaciers are relatively well understood. However less is known about the erosional and depositional impacts of glacier outburst floods or jökulhlaups characterised by transient hydraulic processes and routeways. Despite the ubiquity of tunnel channels and tunnel valleys within formerly glaciated areas, their origin remains enigmatic. Few tightly constrained modern analogues exist for event-related subglacial fluvial erosion and deposition. At Skeiðarárjökull, Iceland, sudden-onset, high-magnitude jökulhlaups exiting overdeepened subglacial basins have generated a variety of hydro-mechanical erosional forms at and beneath the glacier bed. We present geomorphological and stratigraphic evidence for subglacial jökulhlaup erosion in form of tunnel channels and also sub- glacier-bed erosion associated with the development of hydrofracture and channel networks. The spatial distribution of subglacial erosional forms is influenced by sub- and proglacial topography and the distribution of buried glacier ice. Zones of glacier bed erosion are also associated in many cases with hydrofracture-fill and esker ridges demonstrating interconnection of subglacial and sub-glacier bed (subterranean) jökulhlaup flow pathways. These findings may help explain complexity with the sediment and landform record of formerly glaciated areas where tunnel channels are often found in association with subglacial deposits such as eskers. Subglacial meltwater storage and movement beneath modern ice sheets may be associated with similar landforms and deposits.