Architectural analysis of a volcaniclastic jökulhlaup deposit, southern Iceland: sedimentary evidence for supercritical flow

Abstract

The 1918 eruption of the glacially capped Katla volcano, southern Iceland, generated a violent jö kulhlaup, or glacial outburst flood, inundating a large area of Mý rdalssandur, the proglacial outwash plain, where it deposited ca 1 km 3 of volcaniclastic sediment. The character of the 1918 jö kulhlaup is contentious, having been variously categorized as a turbulent water flow, a hyperconcentrated flow or as a debris flow, based on localized outcrop analysis. In this study, outcrop-based architectural analyses of the 1918 deposits reveal the presence of lenticular and tabular bedsets associated with deposition from quasi-stationary antidunes and down-current migrating antidunes, and from regular based bedsets, associated with transient chuteand-pool bedforms, all of which are associated with turbulent, transcritical to supercritical water flow conditions. Antidune wavelengths range from 24 to 96 m, corresponding to flow velocities of 6 to 12 m sec )1 and average flow depths of 5 to 19 m. This range of calculated flow velocities is in good agreement with estimates made from eyewitness accounts. Architectural analysis of the 1918 jö kulhlaup deposits has led to an improved estimation of flow parameters and flow hydraulics associated with the 1918 jö kulhlaup that could not have been achieved through localized outcrop analysis. The observations presented here provide additional sedimentological and architectural criteria for the recognition of deposits associated with transcritical and supercritical water flow conditions. The physical scale of sedimentary architectures associated with the migration of bedforms is largely dependent on the magnitude of the formative flow events or processes; sedimentary analyses must therefore be undertaken at the appropriate physical scale if reliable interpretations, regarding modes of deposition and formative flow hydraulics, are to be made.

Sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications: an example from Anglesey, north Wales, UK

“…Put simply, subglacial meltwater deposits are the product of a hydrological regime that is not unique to subglacial environments, with similar regimes documented from a range of other sedimentary environments including intertidal (Cheel & Middleton, 1986), fluvial (Cheel, 1990, Røe, 1987Alexander et al, 2001) and channelised turbidity currents (Ito & Saito, 2006). Upper plane bed flow regimes have also been recognised within glacial meltwater systems including subglacial (this study ;Brennand, 1994;Fisher et al, 2003), englacial (Delaney, 2001(Delaney, , 2002 proglacial (Duller et al, 2008) and subaqueous ice-contact environments (Russell & Arnott, 2003;Hornung et al, 2007;Lang & Winsemann, 2013).…”

“…Erosional bedforms will typically develop where fluid sediment concentrations are low and / or turbulence is high. This causes helicoidal vortices to become unstable, leading to lateral erosion and the generation of erosion surfaces (Duller et al, 2008). By contrast, the semi-continuous to continuous sinusoidal bedforms at Lleiniog imply high and constant sediment concentrations, and stable helicoidal vortices that promoted net linear deposition (cf.…”

“…Proglacial glacial outburst flood deposits comprise an extensive range of sediments driven by source proximity and the shape of the flood hydrograph (Clarke, 2003;Benn & Evans, 2010). However, at a meso-to smallscale, deposits are frequently characterised by complex facies associations and bounding surface geometries which reflect marked and rapid shifts in energy regime, sediment supply and turbulence (Maizels, 1992(Maizels, , 1993(Maizels, , 1997Marren, 2005;Russell et al, 2005;Duller et al, 2008;Marren et al, 2009). This meso-to small-scale architectural and sedimentological complexity is not apparent in the sequences at Lleiniog.…”

“…Within a glacigenic context this includes subglacial drainage channels (Fisher et al, 2003) and outburst floods (jökulhlaups) debouching either subaerially (Duller et al, 2008) or onto subaqueous ice-contact fans (Lang & Winsemann, 2013). They have commonly been attributed to rapidly-aggrading stationary antidunes (Lang & Winsemann, 2010) that are supercritically climbing due to massive rates of sediment deposition under transitional dune to upper flow regimes (Cheel, 1990;Fielding, 2006).…”

“…Upper flow regime bedform development within glacial environments is likely to be strongly influence by all, or a combination of, flow confinement, elevated discharges and sediment budgets (Brennand, 1994;Duller et al, 2008). These can relate to normal background flow conditions within confined channels (ice walled or incised into the bed), or constrained / unconstrained flow associated with high-magnitude glacier outburst flood events.…”