Palaeohydrology and sedimentary impacts of jokulhlaups from Kverkfjoll, Iceland

Jokulhlaups (glacial outburst floods) occur frequently within Iceland and within most glaciated regions of the world. The largest jokulhlaups known to have occurred within Iceland drained from the northern margin of the Vatnajokull and along the Jokulsa a Fjollum during the Holocene. However, little is known about the number, age and flow characteristics of the Jokulsa a Fjollum jokulhlaups. One source of meltwater into the Jokulsa a Fjollum is Kverkfjoll, a glaciated stratovolcano. This paper provides detailed sedimentological evidence demonstrating that jokulhlaups have routed through Kverkfjallarani and hence from Kverkfjoll. Sedimentological evidence of jokulhlaups includes valley-fill deposits and slack water deposits. Lithofacies, which are indicative of high-magnitude fluvial sedimentation, show that these deposits cannot be the result of nonjokulhlaup processes. The situation and nature of the sediments permit palaeoflow reconstructions. Fine-grained deposits within slack water deposits mark a break in jokulhlaup deposition and suggest that at least three jokulhlaups have drained through Hraundalur, the predominant valley within Kverkfjallarani. Evidence of lava overrunning 'wet' jokulhlaup deposits indicates that jokulhlaups occurred in close association with volcanic eruptions in the Biskupsfell fissure. The largest jokulhlaup was initially hyperconcentrated and subsequently became more fluid. Slope-area reconstructions indicate that the largest jokulhlaup had a probable average peak discharge of 45,000-50,000 m(3) s(-1); however, the peak discharge attenuated by 25-30% in just 25 km. These observations quantify the number, rheology, hydraulics and chronology of jokulhlaups from Kverkfjoll and hence within the Jokulsa a Fjollum. This study presents a model of jokulhlaup impacts and characteristics from glaciated volcanoes and/or within volcanic rifting zones. (C) 2004 Elsevier B.V. All rights reserved.