Abstract

This paper examines the sedimentology of a series of deposits associated with catastrophic floods caused by subglacial volcanic eruptions in southern Iceland. A simple model is proposed for the interpretation of the sediments in terms of changes in flow characteristics of the fluid-sediment mix. A range of sediment types characteristic of these flood flows is identified, the predominant one being thick, massive, homogeneous, poorly structured, black pumice, fine gravels, with < 3 percent silt + clay (Type A), capped by thin, cross-bedded (Type B1) and horizontally bedded (Type B2) sand and gravel units. Less extensive sediment types include inversely graded units (Types C3, C4); deformed sand and gravel beds (Type D); matrix-supported, poorly sorted, heterogeneous mix with >65 percent silt + clay (Type E); and single lithology boulder deposits (Type F). These flood deposits alternate with, or are channeled into, more 'normal' sandur deposits, comprising heterogeneous, poorly sorted, clast-supported, imbricated, rounded cobble and pebble gravels (Type G). The dominant sediment sequences are interpreted as representing hyperconcentrated fluid-sediment mixtures, with dispersive stresses acting to create massive, poorly sorted, nongraded or inversely graded sediments, the latter characterized by large surface boulders. The vertical changes in sediment type reflect significant changes in flow conditions during transport and subsequent deposition, with an initial massive flood surge followed by flow deceleration, sediment 'freezing' and dewatering, producing a more fluid series of flows. The more fluid phases were associated with extensive erosion and channelization, cutting deep scour channels marked by residual streamlined hummocks, boulder lags, and fields of 'megaripples', leading downstream to a 'washed' sandur plain. The paper examines some of the problems associated with the application of paleoflow models for determining former flow dynamics. The models adopted suggest that most of the flows occurred as relatively mobile, low viscosity flows, with little internal resistance to shear. While sediment concentrations are estimated to have reached about 50 percent by weight, with maximum yield strengths between 2.9 x 10 3 and 4.2 x 10 5 dn cm (super -2) , maximum viscosities ranged between only about 2 and 8 x 10 2 poises. A range of paleovelocity models applied to the flood sediment indicates flow rates of between about 4 and 13 ms (super -1) , with peak discharges reaching a maximum of 3.3 x 10 5 m 3 s (super -1) . Dating of the deposits by 14 C dating and tephrochronology indicates that at least 8 major sediment-producing jokulhlaups occurred over a 4,000-year period, between c 4500 yr BP and 1357 AD. Hence, although relatively rare, the effects of these floods still dominate the proglacial landscape, probably producing > 85 percent of the total thickness of proglacial sandur sediments.

You do not currently have access to this article.