Icelandic jökulhlaup impacts: Implications for ice-sheet hydrology, sediment transfer and geomorphology

Russell et al. Icelandic jökulhlaup impacts

Icelandic jökulhlaup impacts: implications for ice-sheet hydrology,

sediment transfer and geomorphology

Andrew J. Russell1*, Matthew J. Roberts2, Helen Fay3,

Philip M. Marren4, Nigel J. Cassidy5, Fiona S. Tweed6 & Tim Harris6

(1) School of Geography, Politics and Sociology, Daysh Building, University of Newcastle, Newcastle upon Tyne,

NE1 7RU, UK

(2) Research Section, Physics Department, Icelandic Meteorological Office (Veðurstofa Íslands), Bústaðavegur 9,

Reykjavík , IS-150, Iceland.

(3) Department of Geography, Lancaster University, Lancaster, LA1 4YW UK

(4) School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa.

(5) School of Earth Sciences & Geography, Keele University, Keele, Staffordshire, ST5 5BG, UK

(6) Department of Geography, Staffordshire University, College Road, Stoke-on-Trent, Staffordshire, ST4 2DE, UK

Abstract

Glaciers and ice sheets erode, entrain, and deposit massive quantities of debris. Subglacial

meltwater fluxes exert a fundamental control on ice dynamics and sediment transport budgets.

Within many glacial systems outburst floods (jökulhlaups) constitute high magnitude, high

frequency meltwater fluxes, relative to normal ablation controlled discharge. This paper presents a

synthesis of research on recent Icelandic jökulhlaups and their geomorphological and sedimentary

impact. We identify jökulhlaup impacts within subglacial, englacial and proglacial settings and

discuss their wider significance for ice sheet hydrology, sediment transfer and geomorphology.

Because jökulhlaups erode, deposit, and re-work sediment simultaneously, they usually cause

significant glaciological and sedimentological impacts. Jökulhlaups that propagate as subglacial

flood waves often produce widespread hydromechanical disruption at the glacier base. Recent

Icelandic jökulhlaups have been recognised as highly efficient agents of sediment subglacial

sediment re-working and glacial sediment entrainment. Models of jökulhlaup impact therefore need

to encompass the sub- and englacial environment in addition to the proglacial zone where research

has traditionally been focussed. Most jökulhlaups transport sediment to proglacial sandar, and often

directly to oceans where preservation potential of their impact is greater. Proglacial jökulhlaup

deposits form distinctive sedimentary assemblages, coupled with suites of high-energy erosional


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landforms. Our study of modern jökulhlaup processes and sedimentary products may be useful for

the interpretation of meltwater processes associated with Quaternary ice sheets.

Keywords: Jökulhlaups, Ice sheets, Subglacial erosion, Englacial and Subglacial deposition,

Sediment transfer, Proglacial, Outwash, Iceland.

*Corresponding author. Tel.: +44-(0)191-222-6951; fax: +44-(0)191-222-5421.

E-mail address: [email protected]

1. Introduction & aims

Numerous cataclysmic glacial outburst floods (jökulhlaups) are known to have drained from

Quaternary ice sheets in North America (e.g. Bretz, 1923; Baker, 1973; Waitt, 1984; Kehew &

Lord, 1987; Teller et al., 2002; Clague et al., 2003) and Siberia (Baker et al., 1993; Rudoy and

Baker, 1993; Carling et al., 2002). Although there is unequivocal evidence of the drainage of large

proglacial ice-dammed lakes, there is considerable debate about the occurrence and dynamics of

giant subglacial sheet floods in Canada (e.g. Shaw, 1989, 1994; Brennand and Shaw, 1994;

Brennand et al., 1996). Periodic coupling of subglacial drainage and underlying groundwater

systems is believed to exert significant controls on ice sheet dynamics, recognisable through

distinctive sedimentary deposits and landforms (Boulton et al., 1995, 2001; Piotrowski, 1997a).

Permeable glacier beds are thought to supply large volumes of meltwater to the groundwater system

(Boulton et al., 1995; Piotrowski, 1997a, b). High water pressures within subglacial aquifers are

believed to be responsible for hydrofracturing and liquefaction of the glacier substrate (Boulton and

Caban, 1995; Rijsdijk et al., 1999). Boulton et al. (1995) suggested that subglacial aquifers were

capable of transmitting all meltwater generated by ice sheets, but Piotrowski (1997a, b) argued that

groundwater flux alone was incapable of instantaneously discharging large volumes of meltwater.

Instead, Piotrowski suggested that excess meltwater accumulation at the glacier bed results in

occasional jökulhlaups, which excavate subglacial sediments to produce tunnel valleys. All of the


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above research themes involve large-scale storage and release of meltwater under conditions of

varied hydraulic pressure and routeway characteristics.

In Iceland, the combination of high precipitation rates, frequent subglacial eruptions and

enhanced geothermal heat flux generates frequent jökulhlaups (Björnsson, 1992, 2002; Einarsson et

al., 1997). This makes Iceland a prime location for the study of jökulhlaups and their resultant

deposits and landforms. The presence of large temperate glaciers in Iceland, combined with an

abundance of relatively friable volcanic sediment, allow enormous sediment fluxes to the North

Atlantic (e.g. Stefánsdóttir et al., 1999). Repeated jökulhlaups have built vast outwash plains or

‘sandar’ which form much of Iceland’s southern coast (Maizels, 1991; Nummedal et al., 1987).

Although the significance of Icelandic jökulhlaups for terrestrial sediment transport and geomorphic

change has been recognised for centuries, only recently have processes associated with modern

jökulhlaups been examined in detail (e.g. Russell & Knudsen, 1999a; Roberts et al., 2000, 2001).

In particular, jökulhlaups at Skeiðarárjökull in 1996 and Sólheimajökull in 1999 have added greatly

to our understanding of jökulhlaup processes within both ice-marginal and proglacial zones (Russell

and Knudsen, 1999a; Roberts et al., 2000a, b). A growing number of palaeo-hydrological and

palaeo-glaciological studies now consider models derived from the study of modern jökulhlaups

(e.g., Cutler et al., 2002; Fisher and Taylor, 2002; Shaw, 2002). For example, the November 1996

jökulhlaup from Skeiðarárjökull has been used as a modern analogue for subglacial sheet- flow

(Shaw, 2002). There is a need to capitalise on our knowledge of modern jökulhlaups as a tool for

providing geophysical models of floodwater hydraulics, sedimentation and geomorphic impact that

can be used as an analogue for former ice sheet processes.

This paper presents a synthesis of research on two recent Icelandic jökulhlaups and

summarises a number of key topics, pertinent to the reconstruction of Quaternary ice sheet

processes. Specifically, we identify jökulhlaup impacts within subglacial, englacial and proglacial


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settings and discuss their wider significance for ice sheet hydrology, sediment transfer and

geomorphology.

2. Recent jökulhlaups

In the following section we describe relevant background to the 1996 and 1999 jökulhlaups.

A volcanic eruption beneath the Vatnajökull ice cap began on September 30, 1996 (Guðmundsson

et al., 1997). Over the next month, 3.8 km3 of meltwater travelled subglacially into the Grímsvötn

subglacial lake until it reached a critical level for drainage (Björnsson, 1997, 2002). The resulting

jökulhlaup began in Skeiðará, the most easterly river draining Skeiðarárjökull (Fig. 1), on the

morning of November 5, and reached a peak discharge of 45-53 x 103 m3s-1 within 14 hours

(Björnsson, 1997; 2002; Snorrason et al., 1997; 2002). The 1996 jökulhlaup attained peak flow

conditions much sooner than any previously monitored jökulhlaup from Grímsvötn (Björnsson,

1997; 2002; Roberts et al., 2000a) (Fig. 1a). Floodwater burst from multiple ice-roofed vents and

fractures, which densely perforated the entire 23 km wide ice margin (Russell & Knudsen, 1999a;

Roberts et al., 2000a, b, 2002; Waller et al., 2001) (Figs. 1b, c).

Intense hydrothermal activity in the upper ice catchment of Sólheimajökull on 17 – 18 July

1999 gave rise to an immediate volcanogenic jökulhlaup (Fig. 2). Aerial reconnaissance on the 18

July identified the source of the flood as a 40 - 50 m deep and 1 - 1.5 km wide ice cauldron, with a

volume of 20 x 106m3, located about 1 km inside the subglacial rim of the Katla caldera

(Guðmundsson et al., 2000). Steep bedrock terrain in the area of the cauldron (Björnsson et al.,

2000) implies that floodwater could not have accumulated hydrostatically and that floodwater

drained immediately down-glacier. The peak discharge of the resulting jökulhlaup decreased

markedly downstream from an average of 4.4 x 103 m3s-1 immediately in front of the western outlet

within 250 m of the glacier margin (Russell et al., 2002), to an average of 1.7 x 103 m3s-1 distally at

the road bridge (Sigurðsson et al., 2000) (Fig. 2).


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3. Sub- and englacial jökulhlaup routing

During the November 1996 jökulhlaup, floodwater emerged initially from the extreme

eastern part of Skeiðarárjökull at the Skeiðará outlet, at c. 07:10 on 5 November 1996 (Snorrason et

al., 1997, 2002; Roberts et al., 2000a, b, 2001; Roberts, 2002). By 10:47, individual conduit outlets

were visible along fractures in the Gígjukvísl area. By 16:30, ice-marginal and supraglacial

jökulhlaup outlets had developed across the entire 23 km margin of Skeiðarárjökull (Figs. 1b and

4a). Surface outlets on the western flank of Skeiðarárjökull were probably active for a maximum of

1 hour. The duration of supraglacial outbursts became progressively less as the subglacial flood

wave moved westwards (Russell and Knudsen, 1999a). By darkness on the 5 November (c. 17:00),

discharge from all supraglacial outlets had ceased, apart from increasing flow from two isolated

fractures, 0.4 km due west of the Gígjukvísl rising stage outlet (Roberts et al., 2000). At the same

location, approximately 17 hours later on the following day, a large supraglacial ice-walled canyon

had been incised into the glacier margin (Roberts et al., 2000; Russell and Knudsen, 1999a, b;

Russell et al., 2001) (Figs. 1, 4a and 6).

During the c. 6 hour duration of flooding at Sólheimajökull, numerous high-capacity

supraglacial and ice-marginal outlets developed (Russell et al., 2000, 2002; Roberts et al., 2002;

2003) (Figs. 2c and 3). The jökulhlaup burst initially through the western lateral margin at a

distance of c. 4 km from the snout (Roberts et al., 2000a, b) (Fig. 2c). Floodwater exiting the

western flank of the glacier drained into two former ice-dammed lake basins, which rapidly filled

and drained catastrophically during the flood (Roberts et al., 2002, 2003) (Fig. 2c). Water draining

from the upper temporary lake flowed ice-marginally, but drainage from the lower temporary lake

was subglacial (Roberts et al., 2002, 2003). The lower temporary ice-dammed lake was completely

choked with over 106 m3 of fragmented ice that was excavated hydraulically from the ice margin,

probably within minutes, as floodwater rapidly filled the valley (Roberts et al., 2003) (Fig. 2c).


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The progressive development of supraglacial and ice-marginal outlets indicates that

floodwater affected large areas of the terminal regions of both glaciers. The majority of the

supraglacial outlets were active for minutes to hours, suggesting that they operated in response to

large, short-term increases in basal hydraulic pressure. Evidence suggests that as the hydraulic

competence of ice-marginal outlets adjusted to ambient basal water pressure, discharge from

supraglacial outlets ceased, and flow from ice-marginal conduits increased.

Both jökulhlaups were characterised by sudden spatial and temporal shifts in floodwater

routing. The progressive development of jökulhlaup outlets across Skeiðarárjökull suggests that

floodwater was forced to migrate westwards due to the insufficient discharge capacity of ice-

marginal conduit outlets. The propensity for fracture-type outlets at Skeiðarárjökull decreased with

increasing distance to the west of the Gígjukvísl catchment (Figs. 1b and c). This implies that the

rate of increase in basal hydraulic pressure was lower in the zone between the Rivers Gígjukvísl and

Súla. Jökulhlaup outlets in this area consisted mainly of retro-fed moulins, crevasses and englacial

conduits, which indicate that although ice overburden pressure was exceeded, the threshold for

fracture outlet formation was not reached.

The potential for changes in floodwater routing is greatest during the rising stage of

jökulhlaups due to the inability of pre-existing sub- and englacial drainage to react immediately to

exponential increases in water input (Bindschadler, 1983; Clarke, 1994; Cutler, 1999). Given short

flood durations, sub- and englacial drainage may not achieve hydraulic equilibrium. Multiple

jökulhlaup outlets and within event outlet evolution are perhaps some of the most distinctive

characteristics of both recent jökulhlaups. Complex temporal phasing of jökulhlaup discharge from

numerous outlets adds considerable complexity to resulting proglacial morphology and

sedimentology.

4. Jökulhlaup outlet types


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4.1. Ice fractures

During the period between ice-dam failure and emergence of the 1996 jökulhlaup at the

glacier margin (~10. 5 h), pronounced fracturing and uplift of Skeiðarárjökull occurred (see Fig. 5

of Roberts et al., 2000b). About 6 km down-glacier from Grímsvötn, glaciostatic uplift was inferred

at ~10 m across a flood tract =1-km-wide (Björnsson, 1997). Within 1-km of the edge of the snout,

estimates of flood- induced surface displacement (normal faulting) from stereo-paired aerial

photographs, taken on November 5, suggest vertical uplift of the order of 2–5 m at several

locations. Additionally, line-of-sight observations from the nearby community of Skaftafell confirm

that the long-profile of the snout swelled markedly during the first 9 h of the jökulhlaup. In <9 h,

jökulhlaup outlets spread rapidly westwards across the 23-km-wide Skeiðarárjökull snout (Roberts

et al., 2000b). Before the formation of high-capacity subglacial outlets, turbid floodwater emerged

from the glacier surface at successive locations along the snout. Supraglacial outbursts persisted for

minutes to an hour. However, before being replaced by conduits, efflux from early and late rising-

stage fracture configurations towards the centre of the glacier-margin lasted =2 h (Roberts et al.,

2000b).

According to Mandl and Harkness (1987), intrusive hydrofracturing will occur at the bed of

a brittle material when hydraulic pressure exceeds overburden and a component of its tensile

strength. Once these two thresholds are exceeded, there will be a near- instantaneous, vertical

growth of a fluid-driven crack toward the glacier surface (Hubbert and Willis, 1957; Geertsma and

de Klerk, 1969; Pollard and Holzhausen, 1979; van der Veen, 1998; Rist et al., 2002).

Hydrofracturing operates by a process of continuous hydraulic action, which acts perpendicularly to

the fracture walls, levering the surrounding material apart. Hydraulic waves propagating rapidly

within subglacial flood circuits can induce water hammer events due to high pulses in water

pressure (Kavanaugh and Clarke, 2000). Brittle features at the glacier base provide an englacial

opening that can be hydrofractured apart, thus allowing floodwater to race toward the fracture tip


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with each successive split. Sediments preserved in relict hydrofractures record instantaneous

fracture widths of 0.001 – 2 m (Ensminger et al., 2001; Roberts et al., 2001) (Figs. 4 and 5). The

magnitude of the local equipotential gradient and the presence and tensional strength of preexisting

structures within englacial ice determines the planform strike and dip of supraglacial fracture

outlets. In zones of lower potential gradient jökulhlaup hydrofractures either slant up-glacier

perpendicularly or obliquely to the plane of the glacier surface (Roberts et al., 2002b), although

their routing will be influenced by pre-existing structural weaknesses at both micro- and macro-

scales (Daneshy, 1978). Decreasing fluid pressure within hydrofractures as they progresses towards

the ice surface will heighten the importance of structural routing controls (Roberts et al., 2000b).

Skeiðarárjökull and Sólheimajökull exhibited characteristic patterns of supraglacial

fracturing (Figs. 4 and 5). Fracturing at shallow ice thicknesses showed clear signs of vertical

displacement relative to the up- and down-glacier sides of the fracture. Conversely, fracturing at

deeper ice thicknesses occurred without widespread surface displacement. Shallow ice fracturing

was most likely initiated by hydraulic jacking, i.e. the pressure of basal floodwater was able to lift

local sections of ice margin (Iken et al., 1983). At Skeiðarárjökull, networks of dry fractures

commonly assumed a ‘spider’ pattern; planimetrically, this consisted of a closed polygon, with

linear fractures radiating out from each corner (Fig. 4a). This type of fracturing is most likely a

product of highly localised basal hydraulic jacking, which effectively sheared a column of ice out of

the glacier. Concentric rows of dry fractures to the North of the supraglacial ice-walled canyon at

Skeiðarárjökull suggest localised tensional shear by ice subsidence, and are similar to fractures

observed in zones of rapid ice collapse (Malthe-Sørenssen et al., 1998). Localised fracture patterns

consistent with hydraulic jacking may therefore be diagnostic of subglacial jökulhlaup drainage

routeways.


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4.2. Supraglacial embayments

During the November 1996 jökulhlaup arcuate ice-surface depressions or ‘embayments’

developed higher on the surface of Skeiðarárjökull at intervals along ice flow transverse fractures,

created by the mechanical removal of fragments of glacier ice, already heavily fractured by high

water pressure during the November 1996 jökulhlaup (Roberts et al., 2001; Waller et al., 2001).

Although Waller et al. (2001) suggested that localized ice collapse above large up-glacier dipping

fractures resulted in normal faulting; Roberts et al. (2001) reported that ice removal occurred

instead in a zone of diffuse fractures that stem from a main or ‘arterial’ fracture (Figs. 4a-d).

Roberts et al. (2000a, b) reported that flow was vigorous from these supraglacial outlets for only the

first few hours of the jökulhlaup, after which flows concentrated at point sources along fractures

and at progressively lower elevations along the glacier margin. The main Gígjukvísl outlet, located

at the lowest elevation along the glacier snout and having the lowest hydraulic potential received

progressively greater amounts of jökulhlaup water. Roberts et al. (2000b) suggest that

hydrofractures generated early in the jökulhlaup allowed higher stage flood flows to carve a large

supraglacial ice-walled channel (Figs. 1c, 4a, and 6). The glacier fractures developed and witnessed

during the initial rising stage of the 1996 jökulhlaup provided a template for the highly irregular

geometry of the ice-walled canyon system that developed on the late rising flow stage (Roberts et

al., 2000b).

4.3. Supraglacial ice-walled canyons

The distinctive ice-walled channel that developed during the November 1996 jökulhlaup at

Skeiðarárjökull, provided the first opportunity for detailed analysis of processes governing the

origin and post flood modification of such a feature. The ice-walled canyon extended supraglacially

for over 700 m from the active pre-jökulhlaup glacier margin (Russell et al., 2001) (Figs. 4a and 6).

The canyon, locally in excess of 50 m in depth and between 100-300 m wide, resulted from the


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removal of ~ 5 x 106 m3s-1 of glacier ice by floodwaters (Russell et al., 2001) (Fig. 6a). The

unusual geometry of the ice-walled channel has played a major role in controlling the sedimentary

architecture of channel- fill deposits (Russell & Knudsen, 1999b; Russell et al., 2001; Cassidy et al.,

2003) (Fig. 11a).

5. Ice block release during jökulhlaups

Widespread break up of glacier ice and abundant hydraulic release of ice blocks constitutes

one of the most distinctive jökulhlaup impacts (Fig. 12). During the 1996 jökulhlaup, ice blocks

were released from 5 main sources: (i) large-scale tunnel collapse, (ii) ice margin flotation or

‘hydraulic jacking’ where the pressure of basal flood water lifted sections of the ice margin causing

ice blocks to break off, (iii) formation of supraglacial fracture outlets associated with hydraulic

jacking (Roberts et al., 2000a, b); (iv) undercutting of the margin by flood water causing ice cliff

collapse; and (v) entrainment from ‘dead’ ice in the proglacial zone. The location and degree of ice

fracturing is therefore an important control on the size of ice blocks and amount of ice transported

in a jökulhlaup (Fay, 2002a). Surprisingly large volumes of ice were removed from ice-cored

moraines and ice covered outwash sediment during the November 1996 jökulhlaup (Russell &

Knudsen, 1999a; Russell et al., 1999; Knudsen et al., 2001). November 1996 jökulhlaup flows

undercut buried glacier ice over a distance of several kilometres within the Gígjukvísl channel. The

presence of large masses of ‘dead’ glacier ice, seemingly detached from the active glacier margin,

has several implications for jökulhlaup impact. Undercutting of older ice-cored topography allows

large blocks to be entrained and transported to relatively distal locations. Jökulhlaups with relatively

sedate rising flow stages are associated with less glacier hydrofracturing and consequently the

release of lower ice block volumes. For exponentially rising jökulhlaups with a low propensity for

glacier hydrofracturing and ice-marginal break up, entrainment of buried ice dominates ice-block


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production and subsequent proglacial impacts. Retreating, debris-laden glacier margins ensure that

subsequent jökulhlaups flow through increasing wider tracts of dead ice.

6. Jökulhlaup erosion of subglacial sediments

During the 1996 jökulhlaup at Skeiðarárjökull, highly turbid floodwaters issued from

supraglacial fracture outlets. Given the absence of an alternative source of fine-grained sediment,

the initial sediment- laden appearance of supraglacial discharge is attributed to the purging of fine-

grained diamicton from the glacier bed at the front of the flood wave. Englacial jökulhlaup deposits

emplaced during the November 1996 jökulhlaup contain many rip-up clasts comprising sheared

glacial diamicton and stratified glaciofluvial sediment (Roberts et al., 2001; Waller et al., 2001)

(Fig. 4b). Rip-ups or intra-clasts within englacial and ice proximal proglacial jökulhlaup deposits

are diagnostic of subglacial mechanical excavation (Russell and Knudsen, 1999a; Russell et al.,

2001; Roberts et al., 2001) (Figs 7a and b). The rip-ups are contained within frozen englacial

jökulhlaup deposits, often within fractures seemingly too narrow to have allowed their passage

(Roberts et al., 2001; Waller et al., 2001). The presence of frozen rip-up clasts composed of basal

ice and diamicton indicate direct mechanical erosion of the glacier bed (Waller et al., 2001) (Fig.

4b). Similarly, at Sólheimajökull, high silt concentrations in supraglacial jökulhlaup deposits and

the presence of numerous rip-up clasts was attributed to reworking of subglacial till deposits

(Roberts et al., 2001, 2002a).

The initially high sediment concentration of the November 1996 jökulhlaup at

Skeiðarárjökull (Snorrason et al., 1997, 2002), coupled with the occurrence of copious numbers of

rip-ups composed of stratified glaciofluvial sediment and organic material within rising-stage

proglacial outwash, suggests that most excavation took place during the rising stage of the

jökulhlaup. By contrast, sedimentological evidence shows that the Gígjukvísl outlet maintained a


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high sediment flux throughout the jökulhlaup. Here, large numbers of rip-ups continued to be

supplied into the jökulhlaup waning stage. Although most rip-ups comprise stratified glaciofluvial

sediment, large numbers of turf clasts were brought up containing soil, wood and tephra layers.

Radiocarbon dating of logs and shrub twigs suggest that all the excavated sediment is early

Holocene in age (Gomez pers. comm.). Skeiðarárjökull is known to have advanced to its present

position during the last millennium, providing ample opportunity for over-riding of former

proglacial sediments. Although the exact extent of over-ridden sediments is not known, it is highly

probable that excavated sediments are derived from erosion of a subglacial surface, which extends

up to 10 km from the glacier snout (Björnsson et al., 1999). After the 1996 jökulhlaup, a slightly

sinuous, 0.5 km wide trench extended 10 km up-glacier obliquely from the head of the Gígjukvísl

ice-walled canyon (Tweed et al., 2001). The presence of highly supercooled meltwater means that

the ice-surface depression cannot be explained by the collapse of a thermally eroded conduit. Smith

et al. (2000) and Magilligan et al. (2002) estimate that 50-96 x 106 m3 of sediment was deposited in

the immediate proglacial area. At a minimum, this figure is volumetrically equiva lent to ten 10 km

long, 50 m wide and 10 m deep subglacial channels at Skeiðarárjökull. There is a compelling case

for major subglacial excavation of unconsolidated sediment and the creation or enhancement of at

least one major subglacial jökulhlaup tunnel channel (cf. Piotrowski, 1994, 1997a; Shaw, 1994).

7. Sub- and englacial jökulhlaup sedimentation

7.1. Eskers

Eskers are typically sinuous, steep-sided, sharp-crested ridges of glaciofluvial or

glaciolacustrine sediment, traceable through deglaciated landscapes over distances of = 0.01 to =

100 km (e.g. Gorrell and Shaw, 1991; Warren and Ashley, 1994; Brennand and Shaw, 1996;

Brennand, 2000). Although esker ridges have many different possible origins, the most common is

by fluvial deposition where sedimentation occurs within pre-existing sub or englacial conduits


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(Banerjee and McDonald, 1975) and where the physics of sub- and englacial water movement

explains their spatial occurrence (Shreve, 1985). Supraglacial ice-walled or subaerial channels have

also been hypothesized from the Quaternary sedimentary record (Warren and Ashley, 1994). High

energy flow conditions generated within confined ice-walled channels result in successions

dominated by high energy lithofacies. Skeiðarárjökull and Sólheimajökull, Iceland provide the only

examples of eskers known to be deposited during jökulhlaups. Within the sedimentary record

relatively few eskers have been attributed to jökulhlaups (e.g. Schulmeister, 1989; Brennand, 1994;

Russell, 1994), perhaps due to the difficulties of distinguishing magnitude and frequency regimes

within a high-energy depositional environment.

Englacial eskers were emplaced at several locations within Skeiðarárjökull by November

1996 jökulhlaup flows (Russell et al., 2000) (Figs. 8a,b). Eskers display ascending and descending

englacial trajectories towards the glacier margin and are clearly associated with proglacial

jökulhlaup deposits (Fig. 8). The location of esker termini match, exactly those of conduits

observed on the waning flow stage. The 1999 jökulhlaup at Sólheimajökull exited the glacier via a

number of outlets, the largest of which was on the western margin of the glacier (Roberts et al.,

2000b, 2002, 2003; Russell et al., 2000, 2002; Sigurðsson et al., 2000) (Fig. 3a). Russell et al.

(submitted) point out the presence of a wide subglacial esker leading to the western outlet of the

July 1999 jökulhlaup. The esker ridge comprises a wide subglacial cavity filled with boulder-sized

sediment.

It is clear that recent Icelandic jökulhlaups have been responsible for the formation of both

sub and englacial eskers extending commonly for 102 m in a down flow direction and 101 m in

width. Clark and Walder (1994, p. 305) remarked that esker dimensions commonly far exceed those

quoted for R-channels (~10-m in diameter) created by what they considered to be, the largest

observed jökulhlaups. Clark and Walder (1994) therefore assumed that, eskers could not form

wholly within any single event. However, large eskers (widths > ~10m) are known to have formed


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in ice-marginal locations during individual, high-magnitude jökulhlaups at Skeiðarárjökull and the

presence of a 150-200 m deep linear collapse trough on the surface of Skeiðarárjökull leading from

Grímsvötn subglacial lake points to the development of a >100 m diameter subglacial conduit.

Restoration of R-channels symbolizes a switch from pervasive subglacial erosion to localized

subglacial deposition; hence in case of the November 1996 jökulhlaup at Skeiðarárjökull, R-channel

sedimentation was confined to the later stages of jökulhlaups and within routeways with access to

transportable sediment. For glaciers overlying unconsolidated sediments, sustained waning-stage

sediment evacuation ensures that sediment builds up rapidly within R-channels. In addition, if

hydraulic supercooling occurs, then sedimentation rates may increase further (Shreve, 1985). Esker

formation by subsequent melt-out of englacial sediment is highly improbable in the case of the

November 1996 or July 1999 jökulhlaups. Instead, widespread glacier hydrofracturing and

mechanical erosion of sections of the glacier bed supplied large volumes of coarse-grained sediment

to englacial jökulhlaup conduits. Deposition occurred simultaneously within the conduit over

considerable distances exceeding 100 m. It is clear that eskers do form during jökulhlaups and that

they can be used to provide important information about sub- and englacial flow conditions and

sediment fluxes.

7.2. Fracture fills

Observations of high-magnitude, sediment-charged supraglacial meltwater discharges

during the November 1996 Skeiðarárhlaup and the July 1999 Sólheimahlaup graphically illustrated

the potential for meltwater to transport sediment to high elevations within glaciers (Roberts et al.,

2000a, b, 2001; Roberts 2002). Although previously hypothesized for normal glacier hydrological

conditions (Kirkbride and Spedding, 1996) there had been a scarcity of observations of englacial

sediment entrainment processes. After the November 1996 jökulhlaup, large sediment-stained

patches could be seen over large parts of the surface of Skeiðarárjökull, down-glacier of major


15

supraglacial outburst locations (Russell and Knudsen, 1999a) (Figs 1b,c, 4a and 10a). As well as

supraglacial deposition, large volumes of sediment were deposited within both pre-existing and

jökulhlaup-generated fractures (Roberts et al., 2000a, 2001, 2002; Waller et al., 2001). Englacial

deposition consisted predominantly of fluvially-sorted and -bedded sand and gravel, with occasional

structureless units deposited within vertical, dyke- like structures (Roberts et al., 2001; Waller et al.

2001) (Figs. 4b,d and 5a,b,d). Primary sedimentary structures commonly included planar bedding,

ripple-drift cross-lamination, cross-bedding and clast imbrication. Sedimentary architecture was

dominated by the three dimensional geometry of the fracture/conduit system, which was often

highly complex (Roberts et al., 2001; Roberts 2002) (Figs. 4d and 5a). Sedimentary macroforms

included large mushroom-shaped sedimentary structures broadly analogous to sand volcanoes

(Roberts et al., 2001) (Fig. 5c). Roberts et al. (2001) suggested that englacially-deposited

mushroom-shaped structures were the product of deposition around a vertical jet of water entering

an englacial cavity (Figs. 5c and 10b). Unlike true sand volcanoes, that are associated with the

escape of fluidized material, englacial deposition took place both on the stoss and lee-sides of a

radially expanding bedform (Roberts et al., 2001). Stoss-side depositional angles commonly

increase from moderate (~45º) to vertical and over-vertical planes within a single depositional unit

(Fig. 10b). Such angles of repose are completely inconsistent with the escape of fluidized sediment

from a standard sand volcano (Fig. 10b). Roberts et al. (2001, 2002) and Roberts (2002) attributed

high angled stoss-side bedding to a process of rapid freeze-on of englacial sediment load to the

conduit margins. The rapid freeze-on of meltwater to conduit margins occurred via the process of

glaciohydraulic supercooling (Lawson et al., 1998; Evenson et al., 1999; Roberts et al., 2001,

2002a). During the 1996 jökulhlaup, sudden elevation of meltwater from the base of a deep

subglacial trough beneath Skeiðarárjökull resulted in floodwater exiting the glacier below its

pressure-determined freezing point (Fig. 10a). The fact that all observed November 1996 englacial

jökulhlaup sediments were frozen within a temperate glacier near sea level supports the hypothesis


16

of sediment accretion by supercooled discharge (Fig. 10a). Although the preservation potential of

primary englacial sedimentary structures is low, November 1996 jökulhlaup deposits are preserved

at the margin of Skeiðarárjökull near the glacier bed, where ice was thinner (Roberts et al. 2001)

(Figs. 9a,b). The role of high magnitude jökulhlaups to freeze large volumes of sediment englacially

has major implications for palaeo-glacier sediment flux and consequently the terrestrial and marine

sedimentary record (Roberts et al., 2001, 2002; Roberts, 2002).

8. Proglacial jökulhlaup impacts

8.1. Relationship between hydrograph type and flow rheology

Hydrograph shape has traditionally been linked to jökulhlaup ‘type’ such that storage-

release jökulhlaups are normally associated with sedate rises to peak discharge and modest

sediment concentrations (Maizels, 1991, 1993, 1997; Maizels and Russell, 1992). Similarly, non-

storage release volcanogenic jökulhlaups have been associated with sediment concentrations high

enough to generate hyperconcentrated flow cond itions (e.g. Maizels 1997). Here we use known

hydrograph shapes and sediment fluxes during recent Icelandic jökulhlaups allows us to assess the

relationship between hydrograph shape and flow rheology. Despite the short duration of rising flow

stages of the November 1996 and July 1999 jökulhlaups, flows within the proglacial zone were

predominantly fluvial. At Skeiðarárjökull, deposition of entire bed load layers took place at the bed

of a highly turbulent fluidal flow as a non-Newtonian grain flow or traction carpet (Russell and

Knudsen, 1999a, 2002; Fay 2001, 2002a). Cassidy et al. (2003) interpreted crude polymodal matrix-

supported stoss-side beds deposited within the supraglacial ice-walled canyon as the product of

localised rapid suspension sedimentation within a predominantly turbulent fluidal flow. Cassidy et

al.’s interpretation is consistent with observed flow characteristics during the November 1996

jökulhlaup as well as low overall sediment concentrations inferred from within event sediment

sampling (Snorasson et al. 2002) and a consideration of sediment budgets (Smith et al., 2000;


17

Magilligan et al., 2002). Although many of the November 1996 jökulhlaup units at Skeiðarárjökull

display a-axis parallel fabrics generally thought to be associated with high sediment concentrations,

they are associated with the stoss-sides of large-scale fluvial bed and bar forms where deposition

occurs without flow separation. At Sólheimajökull, very poorly sorted, polymodal matrix-supported

units were deposited as an expansion bar at a subglacial tunne l mouth (Russell et al., 2002) (Fig.

3a). At this location, high stream powers combined with relatively low flow depths and high

velocities generated a locally hyperconcentrated flow of debris clasts and ice blocks.

8.2. Role of glacier fluctuations and proglacial topography in determining the geomorphological

and sedimentological impacts of jökulhlaups

Fluctuations of glacier margins exert a direct control on the nature of proglacial aggradation

or incision (Maizels, 1979; Marren, 2002), thus the sequencing of jökulhlaups with glacier

fluctuations and will directly affect their morphological and sedimentological impacts. During

jökulhlaups, the relative position of a glacier margin and its moraines determines whether ice-

proximal backwater conditions may occur. Retreat of a glacier from a prominent moraine system

often produces a ‘trench’ or topographic low between the glacier and the moraine system. This

topographic low is often occupied by proglacial lakes or minor fluvial systems that flow parallel to

the glacier margin before breaking through the surrounding moraines (Klimek, 1972, 1973;

Bogacki, 1973; Galon, 1973a, b). Older sandur surfaces represent abandoned, unconfined

jökulhlaup fans and ice-marginal river courses, which develop during periods of glacier retreat and

proglacial trench development (Maizels, 1991; Gomez et al., 2000). The November 1996 jökulhlaup

in-filled an ice-marginal lake occupying the proglacial trench within minutes (Russell and Knudsen,

2002a; Knudsen et al., 2001a; Cassidy et al., 2003).

Skeiðarársandur during the 20th Century provides an example of the differences in

sedimentation patterns during sandur aggradation and incision phases. In descriptions of the 1934


18

and 1938 jökulhlaups, water is known to have drained directly from the glacier onto the sandur, and

across most of the proglacial area of Skeiðarársandur. The withdrawal of the ice margin from the

Little Ice Age moraines at Skeiðarársandur caused drastic changes in the routing and dynamics of

jökulhlaups. The 1954 jökulhlaup flowed mainly parallel to the glacier, whilst the 1996 jökulhlaup

was confined to the three main river systems, and only coalesced to occupy the entire width of the

sandur in the most distal portion (Russell and Knudsen, 2002b). Boothroyd and Nummedal (1978)

presented a model, based partly on Skeiðarársandur, which highlighted proximal-distal lithofacies

variations on a sandur displaying a fan- like geometry. In the wake of the November 1996

jökulhlaup, Russell and Knudsen (1999a) identified the presence or absence of a proglacial trench

as a major control on sandur sedimentology (Fig. 17). Russell and Knudsen (2002b) documented

the response of the various channels across Skeiðarársandur to the 1996 jökulhlaup pointing to the

fact that the greatest geomorphic impacts were associated with relatively new channels cutting

through the moraine belt, such as the Gígjukvísl channel (Russell et al., 1999). Based on the

preservation of jökulhlaup sediments at depth, Marren et al. (2002) suggest that the Skeiðará river

has been ‘hardened’ to the relatively frequent high magnitude jökulhlaups.

Little attention has focussed on the interaction between the proglacial topography and

jökulhlaup flow dynamics. Russell and Knudsen (1999a) highlighted the role of proglacial

topography in generating within-jökulhlaup backwater effects with implications for downstream

sediment and ice block concentrations. At the height of the November 1996 jökulhlaup, 60-100 x

106m3 of water was temporarily stored in a backwater (slackwater) lake upstream of the Gígjukvísl

moraine constriction (Russell and Knudsen 1999a). Channel constrictions on the rising stage

allowed backwater conditions to prevail throughout much of the proglacial trench. Observed

erosion of channel constrictions resulted in progressive lowering of backwater levels during the

jökulhlaup. Backwater effects could act as a major control on jökulhlaup sedimentation where the

active glacier margin was flanked by older high elevation ice-contact deposits (Russell and


19

Knudsen, 1999a). However, the potential of backwater controls to dominate processes and patterns

of sedimentation was greatly reduced as the flood progressed. Reduction in local base levels and

increases in flow energy associated with the removal of backwater effects are expected to result in

fan reworking and incision. The backwater effects acted to reduce the transport of coarse-grained

sediment from the glacier margin (Russell and Knudsen, 1999a, 2002a; Gomez et al., 2002). Rising

stage deposition into backwater lakes resulted in the formation of relatively flat-topped, radial deltas

showing rapid down-fan decrease in ice block size (Russell and Knudsen, 2002a) (Fig. 13a).

Ground Penetrating Radar (GPR) surveys of November 1996 jökulhlaup deposits within the main

proglacial trench reveal progradational sedimentary architecture overlain by shallower angled

structures indicative of aggradation (Cassidy et al., 2003) (Fig. 11b). The GPR profile shows a

transition from lower sub-horizontal to prograding foreset reflections. Sub-horizontal bedding is

thought to represent and earlier phase of jökulhlaup deposition with flows obliquely into the section

(Cassidy et al., 2003). Foreset beds are consistent with deposition from flow exiting ice-walled

canyon the late rising and waning stage (Cassidy, et al., 2003). Such a succession is compatible with

a transition of deposition from backwater dominated to free flowing jökulhlaup flow. As local

backwater controls were removed during the jökulhlaup, there was a greater potential for earlier

rising stage sediments to be reworked and transported further down the fluvial system. Several

cycles of storage and release can be identified for the Gígjukvísl channel system during the

November 1996 jökulhlaup as various backwater lakes were created and destroyed. Slackwater

sediments typically comprised: horizontally-bedded sands (Sh), ripple cross- laminated (Sr),

climbing ripples (Scr) and laminated silts (Fl). Although extensive, proglacial slackwater

successions were typically less than 3 m in thickness.


20

8.3. Variations of jökulhlaup stage and routing

On Skeiðarársandur, the morphology of ice-contact November 1996 jökulhlaup outwash

fans is highly varied, depending upon jökulhlaup outlet morphology and pre-jökulhlaup proglacial

topography (Russell & Knudsen, 2002a). Rising stage-dominated fan morphology is controlled by

the presence of topographically-controlled backwater lakes (Fig. 13a). Flat-topped, steep-sided fans

bear similarities to jökulhlaup deltas graded to temporarily-raised lake levels (Shakesby, 1985;

Russell, 1994; Russell & Marren, 1999) (Fig. 13a). Ice blocks completely buried by rising stage

flow result in the post-jökulhlaup formation of circular kettle-holes (Fig. 14). Some of the largest

ice blocks are only partially buried and give rise to kettle-holes surrounded by an obstacle scour

mark or ‘kettle-scours’ (Fay, 2001, 2002a). Rising stage deposits contained both single upward

coarsening successions as well as successions consisting of stacked upward-coarsening and

normally-graded units. The Gígjukvísl rising stage fan was well preserved due to efflux from the

supraglacial ice-walled channel, which also maintained backwater conditions through the

jökulhlaup waning stage. Other November 1996 jökulhlaup fans at the Western Skeiðará were

deposited into a topographically-controlled backwater zone, partially infilling pre-jökulhlaup lake

basins.

At the Gígjukvísl, waning-stage flows were routed through a single conduit and high

sediment efflux and aggradation rates were maintained late into the waning stage (Figs. 4a and 14).

Absence of backwater conditions during the jökulhlaup waning-stage resulted in the creation of

outwash fans built up by the aggradation of individual sheet- like layers (Russell and Knudsen,

1999a; 2002). Where a fan is subject to high sediment fluxes on the rising and waning-stage,

aggradation can continue until the very end of the jökulhlaup, thus allowing surface coarsening and

shallow channelisation to occur (Figs. 13b,d). The waning-stage Gígjukvísl fan had a uniform

surface gradient and graded directly into the main jökulhlaup channel parallel to the glacier margin

(Figs. 12 and 14). Fan morphology was dominated by two major channels between large shoals of


21

deeply embedded ice blocks (Figs. 12 and 14). Waning-stage erosion of sediment within the

upstream supraglacial ice-walled canyon was accompanied by aggradation of the outwash fan

surface. As such, the overall geometry of this fan was well preserved (Figs. 12 and 14). Later

waning stage fan reworking was limited to aerial scouring and winnowing of sediment rather than

major, localised fan incision. Lack of major incision of the waning stage fan is due to high sediment

fluxes being maintained through the waning flow stage due to upstream erosion within the ice-

walled canyon (Russell and Knudsen, 1999b; Fay, 2001, 2002a). Winnowing and sediment

starvation result in progressive bed coarsening from polymodal matrix-supported gravel to clast-

rich armour (Fig. 13d).

Multiple stage outwash fans at the Western Gígja, Sælhúsakvísl and Skeiðará are all

characterised by rising stage aggradation and major waning stage incision, creating a distinctive

heavily-dissected fan morphology (Figs. 13c, e). Migration of the main Gígjukvísl outlet during the

1996 jökulhlaup allowed spatial segregation of rising- and waning-stage flow. Although backwater

conditions and sediment flux strongly influence ice-contact jökulhlaup deposit morphology, these

factors are controlled by flow stage.

The morphological and sedimentological record of a single jökulhlaup from a large glacier

is highly varied due to the presence of numerous outlets of varying size active during discrete

periods of the jökulhlaup. The main morphological and sedimentological characteristics of ice-

contact jökulhlaup fans associated with different flow stages are illustrated by the schematic

diagrams in Figure 13. Conduits occupied by flows on both rising and falling flow stages are

characterised by initial rising stage fan deposition, followed by falling stage dissection and

exhumation of ice blocks and rip-up clasts (Figs. 13d,e). Backwater-controlled and non-backwater

controlled outlets subject to prolonged falling stage flows will be heavily dissected as sediment

fluxes decline. Such erosion removes the finest grain sizes but leaves large rip-up clasts, boulders

and ice blocks.


22

8.4. Geomorphological and sedimentological impacts of ice blocks during jökulhlaups

Ice blocks are grounded in locations where flow deceleration and expansion occur in

association with areas of relatively high elevation (Russell, 1993; Maizels, 1997; Fay, 2001). Such

areas include: (1) pre-flood topographic high points; and (2) backwater locations where depositional

surfaces are graded to lake levels and associated with relatively low flow depths and velocities.

During a jökulhlaup, the largest ice blocks will be transported in routeways characterised by high

discharge, velocity, depth and stream power (Russell, 1993; Fay, 2001). High rates of jökulhlaup

sediment deposition may partially or totally bury blocks further preventing their re-entrainment.

In general, ice-block size decreases down-sandur reflecting the progressive inability of the

flow to move the larger ice blocks as slope gradient and stream power decrease (Russell, 1993;

Maizels 1995; Fay, 2001). Within confined channels, ice-blocks decrease in size in a down flow

direction (Russell and Knudsen, 1999a; Fay, 2001). On a more local scale, however, decrease in

ice-block size is non-uniform. Non-uniform decrease in down-fan ice-block size occurs because: (1)

smaller ice blocks are arrested in the lower velocity wake of larger grounded ice blocks while large

ice blocks bypass these zones to ground further downstream; and (2) small ice blocks released on

the waning limb of a flood ground in areas occupied by larger blocks grounded earlier in the flood.

Ice-blocks form clusters when a single, dominant grounded ice-block obstacle clast arrests, floating

ice blocks of a smaller or similar size causing them to ground in the stoss or lee of the obstacle. The

fact that ice blocks float means that flow depth and velocity are crucial controls on the formation of

ice-block clusters.

Fay (2001, 2002b) observed that during the 1996 jökulhlaup a hummocky morphology

formed where waning stage deposition occurred in areas of high stranded ice block densities and

little waning stage erosion (Fig. 14). On unconfined outwash fans, hummocky topography forms on

the distal lower portions of the fans subject to lower velocity, (Fay, 2002b). Scour around ice-block


23

clusters captures erosive waning stage flows creating chute channels. Waning stage duration, rate of

discharge decrease and substrate characteristics control the occurrence and intensity of chute-

channel incision (Russell, 1993; Fay, 2001). Scour around ice-block clusters not only leads to the

formation of chute channels but consequently controls the within-channel routing of waning stage

flow (Russell, 1993; Fay, 2001, 2000a).

In the aftermath of the November 1996 jökulhlaup, Fay (2001) identified and classified a

wide range of ice block obstacle marks within coarse-grained sediment of up to boulder size (Fig.

15). The nature of ice-block obstacle scour is determined by (a) obstacle size, (b) flow velocity, (c)

flow depth, (d) flow rheology, (e) nature of the substrate, (f) timing of ice-block grounding (i.e.

scour time), (g) distribution of already grounded ice blocks (number and how closely spaced), (h)

ice-block shape and (i) ice-block flotation (Fig. 15). Flow depth is an extremely important limiting

factor as ice blocks are often not fully submerged by water. In such cases ‘kettle-scours’ develop by

partial burial of large ice blocks and scour during the flood followed by in situ ice-block melt after

the flood recedes (Fay, 2002a). In locations subject to high jökulhlaup discharge and high sediment

flux, where little reworking occurs, rapid sediment deposition occurs in the lee of grounded ice

blocks to form aggradational ice-block obstacle shadows (Fay, 2002a) (Figs. 15 and 16). These

shadows exhibit an anticlinal-shaped structure, deposited by lee-side eddies. Upstream-dipping

antidune stoss-side strata are deposited around ice blocks during jökulhlaups in locations

characterised by high discharge and high sediment flux and very little waning stage erosion (Fay,

2002a) (Fig. 16). Steep downstream-dipping beds and low-angle upstream-dipping beds are

associated with antidune-washout and antidune trough fill (Fig. 16).

8.5. Post-jökulhlaup geomorphological and sedimentological impacts of ice blocks

Kettle holes have been reported from many jökulhlaup channels (Jewtuchowicz, 1971,1973;

Churski, 1973; Galon, 1973 a, b; Klimek, 1973; Nummedal et al., 1987; Maizels, 1992, 1995, 1997;


24

Molewski, 1996; Russell and Knudsen, 1999a, 2002b; Olszewski and Weckwerth, 1999; Russell et

al., 2001b; Fay, 2002b). Kettled or pitted sandur is proglacial outwash in which numerous kettle

holes have formed from the melt out of partially or completely buried ice blocks leaving hollows

when the ice melts (Clarke, 1969; Maizels, 1977). During jökulhlaups, large ice blocks are often

progressively buried by aggrading sediments, smaller ice blocks, however, can be incorporated

within a sediment rich flow allowing deposition simultaneously with flood sediments (Klimek,

1972; Russell and Knudsen, 1999a; Fay, 2002b).

Kettle holes may be steep-walled or inverse-conical or in character. A kettle hole’s collapse

sequence is controlled by the physical properties of the sediment in which it forms (Fay, 2002b).

Inverse-conical kettles form due the melt of partially or totally buried ice blocks and develop by

sediment slumping and avalanching down the kettle walls. Steep-walled kettles form as overlying

sediment collapses into voids created by the in situ melt of completely buried ice blocks (Fay,

2002b). Deep kettles with steeply-dipping or overhanging walls form, often through sudden roof

collapse, in coarse matrix-support sediments or in entirely fine-grained sediments. Steep-walled

kettle holes may form over small buried blocks, or over larger, buried ice blocks which melt

irregularly. Steep-walled kettles can develop into inverse-conical kettle holes by slide or avalanche

of sediment into the kettle hollow. The spatial distribution of steep-walled and inverse-conical kettle

holes is controlled by the distribution of buried ice and these kettle holes tend to form in glacier

proximal locations that experienced high jökulhlaup sediment concentrations.

Kettle holes and obstacle marks may possess raised ice block meltout diamicton rims

(rimmed kettles) (Fuller, 1914; Thwaites, 1926, 1935; Maizels; 1992, Branney and Gilbert, 1995;

Olszewski and Weckwerth, 1999; Fay, 2002b) and/or diamicton mounds in the base of the

depression. Diamicton mounds may also be deposited directly on the streambed (Maizels, 1992).

Following the 1996 jökulhlaup, isolated conical sediment mounds formed in slackwater locations

where small ice blocks were stranded on fine sediments and aeolian processes were active (Fay,


25

2002b). The mounds were composed of flood sediments in the lower part and ice-block melt out

diamicton in the upper part. The ice block melt out diamicton protected the stream bed beneath

while erosion around the diamicton by wind deflation produced the mound.

9. Jökulhlaup sediment transfer

Scouring of unconsolidated sediment from beneath Skeiðarárjökull during the 1996

jökulhlaup illustrates the sediment scavenging-potential of subglacial floodwater (Russell and

Knudsen, 1999, 2002a). Due to the residence time of volcanogenic floodwater in Grímsvötn, no

primary eruption products from the 1996 eruption were detected in floodwaters from

Skeiðarárjökull (Maria et al., 2000). Hence, bedload and suspended sediment load came from

fluvial erosion of older volcaniclastic material and overridden glaciofluvial sediments in storage

along the subglacial flood route (Stefánsdóttir et al., 1999). Based on field measurements of

sediment concentration, Snorrason et al. (2002) inferred that 1.8 x 1011 kg of suspended sediment

load was conveyed in rivers draining floodwater from Skeiðarárjökull. From satellite radar

interferograms, Smith et al. (2000) estimated 7.3 x 107 m3 of ice-proximal bedload deposition

during the 1996 jökulhlaup. These values give a total sediment yield of = 3.1 x 1011 kg (̃ 1.8 x 108

m3) and a mean flux of 1.8 x 106 kg s–1 over the 47 h flood duration equivalent to 0.07% average

sediment concentration by volume. Averaging 1.8 x 108 m3 of sediment over the putative area of

glacier bed impacted by floodwater (~5.8 x 108 m2), implies 0.3 m (1.8 x 10–3 mm s–1) of subglacial

erosion for a single jökulhlaup.

Nummedal et al. (1987) estimated that jökulhlaups account for 92% of all sediment

transported across Skeiðarársandur. Smith et al. (2000) and Magilligan et al. (2002) used SAR

interferometry to estimate net erosion and deposition during the November 1996 jökulhlaup to

suggest a sandur wide net volume gain of 31-38 x 106 m3. Net erosion and deposition does however


26

vary considerably depending upon location. Most deposition (50-96 x 106 m3) occurs within the

immediate proglacial area as defined by the belt of abandoned outwash surfaces and ice-cored

moraines. By contrast, there is considerable jökulhlaup channel erosion within the main outlet

channels through the moraine belt and in the area immediately downstream of the moraine belt

(Russell et al., 1999; Smith et al., 2000; Knudsen et al., 2001). Snorrason et al. (1997, 2002)

indicate that suspended sediment concentrations decreased progressively from 121 to 5 g L-1 within

the Skeiðará River during the November 1996 jökulhlaup, pointing to progressive sediment

exhaustion on the (~ 20 hour) rising flow stage. Snorrason et al. (1997, 2002) estimate that 180 x

106 tons of suspended sediment was transported by the 1996 jökulhlaup, stressing it is likely that

this figure severely underestimates the total amount of sediment transported during this event.

Using seismic reflection surveys, Guðmundsson et al. (2003) provide the first systematic

attempt to calculate the thickness and volume of sediment on Skeiðarársandur. Based on a total

reconstructed sediment volume of 100 km3, Guðmundsson et al. (2003) estimated Holocene sandur

growth at a minimum of 1 km3/century and equated this to five 1996-equivalent jökulhlaups per

century. Guðmundsson et al.’s study provides clear evidence that jökulhlaups dominate sediment

supply to the sandur. It is therefore highly probable that the majority of the Skeiðarársandur

sedimentary succession comprises jökulhlaup related units echoing the conclusions of sedimentary

studies on the sandur (Russell & Marren, 1998; Marren, 2002).

10. Discussion

The location and characteristics of sub- and englacial jökulhlaup flow is subject to marked

spatial and temporal evolution (Roberts et al., 2000). Within-event jökulhlaup routing through a

large ice mass may evolve or change over timescales of days to months and involve changes in

outlet location over distances of 101 – 102 km. Each jökulhlaup routeway may acquire a local


27

signature in terms of sediment supply, conduit evolution and flood duration. High water pressures

during the onset of high-magnitude jökulhlaups allow significant supraglacial outflow and the

temporary filling of ice-marginal, ice-dammed or moraine-dammed lake basins (Roberts et al.,

2003). The sedimentary record of such basins may record the early stages of high magnitude

jökulhlaups. Hydrofracturing of glacier ice during the initial stages of jökulhlaups adds temporary

hydraulic circuits to the glacial drainage system increasing the rapidity of the jökulhlaup rising

stage.

During the 1996 and 1999 jökulhlaups meltwater travelled as a subglacial wave underneath

large areas of Skeiðarárjökull and Sólheimajökull. The exact nature of this wave is inferred to be in

the form of a short-lived sheet flow after which flow collapsed into a lower pressure channelised

network of sub and englacial channels. Geomorphic evidence suggested that the 1999 jökulhlaup

exited Sólheimajökull at several widely spaced locations (Roberts et al., 2003a), implying a

subglacial flow at peak discharge covering the entire glacier bed to an average water thickness of

2.6 m. During the rising stage of the 1996 jökulhlaup from Skeiðarárjökull ~108 m3 of floodwater

was in subglacial transit, providing a mean subglacial water depth of ~1.7 m over ~575 km2

(Björnsson, 1998). The dynamics of recent jökulhlaups imply that localized tracts of the glacier bed

are subject to short- lived sheet-flow conditions during cataclysmic jökulhlaups (Shoemaker, 1992a).

When the 1996 jökulhlaup neared peak discharge, changes in vent location and geometry signified

the progressive abandonment of a pressurised sheet flow and the restoration of high-capacity

channelised drainage (Björnsson, 1998; Roberts et al., 2000b). Channelised subglacial flow is the

most stable means of transmitting large volumes of meltwater (Shoemaker, 1992a, 2002), especially

if flood channels are embedded in unconsolidated sediments (Walder, 1994; Walder and Fowler,

1994). For floodwater flowing subglacially through over-deepened glacier basins, hydraulic

supercooling allows rapid accretion of sediment to fracture and conduit walls by freeze-on, thereby


28

throttling the flow and maintaining high water pressures within a distributed system up-glacier of

the reverse slope of the over deepening (Roberts et al., 2001; 2002) (Figs. 10a,b).

High-magnitude jökulhlaups draining through large glaciers would be expected to drain

initially via linear fracture outlets, and when efflux becomes increasingly focused, point-source

efflux causes the formation of ice embayments and subsequently large ice-walled channels.

Progressive focusing of efflux on major outlets would generate major ice calving, thus releasing ice

blocks to the proglacial area. Head ward excavation and extension of large supraglacial ice-walled

channels may create significant supraglacial accommodation space for sedimentation. Progressive

development of ice-walled channels results in back-filling of supraglacial outlets resulting in

distinctive bar stoss-side accretion (Fig. 11a). Upstream-dipping strata within ice-walled channels

are likely to be associated with downstream dipping beds associated with progradation in the

proximal proglacial zone. All outlet types are associated with en- or supraglacial deposits. Where

floodwater ascends from the base of an over-deepened basin or to high elevations via high-angled

hydrofractures, jökulhlaup sediment will be deposited by supercooled meltwater flows providing a

frozen record of the jökulhlaup.

Jökulhlaups are highly effective at eroding large volumes of sediment from the glacier bed.

Mechanical erosion of the substrate indicates the paucity of thermal erosion and would be an

indicator of rapid jökulhlaup onset within an ice sheet environment. Late Pleistocene tunnel valley

genesis is a contentious topic (O’Cofaigh, 1996), and a jökulhlaup hypothesis has survived for

decades (Shaw, 2002). Evidence of significant subglacial erosion during the 1996 jökulhlaup is

valuable for testing hypotheses of tunnel valley genesis. For the first time, the jökulhlaup

hypothesis can be tested at an appropriate scale using actual glaciological data, rather than palaeo-

glaciological inference.

We have shown that it is possible for both large scale englacial and supraglacial eskers to

develop in their entirety during single jökulhlaups. The definition of the term ‘esker’ may also need


29

to be broadened to include primary fluvial deposition associated with the infill of complex fracture

networks and englacial voids. Although the preservation potential of eskers in en- and supraglacial

settings is lower than that for their subglacial counterparts, there are many examples of englacial

sediment and landform preservation (e.g. Price, 1969; Roberts et al., 2001). Preservation of

supraglacial sedimentary successions has also been widely invoked to form kame terraces (e.g.

Brodzikowski and van Loon, 1991; Benn & Evans, 1998). Subglacial jökulhlaup eskers identified at

Skeiðarárjökull show evidence of hyperconcentrated flows suggesting extremely rapid deposition

within tunnels allowing them to become rapidly sediment choked and thereby encouraging the

development of anastomosing systems of R-channels.

Modern Icelandic jökulhlaups deposited enormous volumes of sediment that froze

instantaneous ly within hydrofractures and pre-existing crevasses and voids within the glacier.

Sedimentary structures diagnostic of accretion by supercooled flows may help in the identification

of former jökulhlaup deposits. Jökulhlaup sediments can also be deposited within glaciers without

the need for hydraulic supercooling.

Reconstructions of jökulhlaup rheology can provide important information on both sediment

supply and the nature of the flood hydrograph. The shape of the hydrograph is strongly dependent

on whether the flood is a storage-release or a direct release event, which in turn, is largely

controlled by glaciological factors. The presence of locally hyperconcentrated flows implies a ready

supply of sediment due to high rates of subglacial sediment erosion and removal. High rates of

sediment transfer by sediment-laden flows play a large part in the rapid construction rates of sandar

in jökulhlaup-prone areas, and produce distinctive proglacial geomorphology. Often, well-

developed braided river channels are locally suppressed, as normal flows are forced to incise

through flood deposits, and form single channel rivers through the flood impacted areas.

Glacier margin fluctuations play a key role in determining sandur stratigraphy as they

determine exactly where sedimentation on a sandur will take place. Advancing glaciers encourage


30

aggradation, and jökulhlaups that take place during advancing periods will be accompanied by

widespread sedimentation in the proximal and medial zones of the sandur, forming local sediment

stores (Fig. 17). The proximal incision that usually accompanies glacier retreat creates a basin that

can form a sediment trap during jökulhlaups, again creating a local sediment store (Fig. 17).

However, downstream of the proximal incision zone, there is usually a hinge point where a

transition to net aggradation occurs, as sediment eroded from upstream is re-deposited. In a

jökulhlaup situation, where the proximal basin created by incision acts as a sediment trap, the hinge

point is usually a point of flow constriction, and is therefore associated with large-scale erosion as

the constricting channel is widened. The Gígjukvísl channel during the 1996 jökulhlaup on

Skeiðarársandur is a good example of this (Russell et al., 1999). Consequently, a zone of

unconfined deposition occurs downstream of the hinge point, as eroded sediment is re-deposited. In

a jökulhlaup situation this deposition is likely to take the form of large braid bars scaled to the flood

channel dimensions (Marren et al., 2002). Therefore in both jökulhlaup and non-jökulhlaup

situations, the glacier margin fluctuation history will determine the exact pattern of sediment

deposition and storage on the sandur. The hydrological magnitude and frequency regime of the

sandur (flood or non-flood dominated) will determine the actual character of the sediment that is

stored in these depositional zones. The location, nature and distribution of packages of proglacial

sediment can therefore provide important information on glacial hydrology and glacie r dynamics.

A second important implication of glacier margin fluctuations is that over medium to long

time scales, fluctuations in glacier margin position are related to changes in glacier mass balance.

There is a close correspondence between proglacial annual average and peak discharges and glacier

size (Clague, 1975). The whole character of flood impacts will change with changes in glacier mass

balance. As ablation related discharges increase the contrast between ‘normal’ and extreme events

may decrease. Alternatively, where floods are from the drainage of ice-dammed lakes, a positive

glacier mass balance will result in larger, more stable lakes that will produce large but relatively


31

infrequent floods that mat have more significant geomorphological effect. A negative mass balance

will result in smaller, less stable lakes tha t produce frequent small floods; consequently, proglacial

channel systems may be able to withstand the geomorphic effects of high-frequency, low-

magnitude jökulhlaups (Tweed and Russell, 1999).

Where proglacial topography constricts the flow of floodwater from the glacier margin,

backwater lakes can be formed. The size of the backwater lake is controlled by the size of the

jökulhlaup. The temporary, backwater lakes trap sediment of sand size and upwards depositing

deltaic sedimentary sequences some of which are reworked by waning stage flows (Fig. 17).

Rising stage deposits characteristically contain finer, more poorly sorted sediment than

found in falling stage successions and on erosional surfaces. Backwater-controlled sedimentation

results in delta-like foresets containing large amounts of sediment ranging from sand to gravel in

size. Rising stage deposits in the western Gígjukvísl show upward-coarsening successions,

characteristic of progressive supply of coarser-grained sediment with increased stage (Fig. 17).

This is compatible with models of rising stage sedimentation proposed by Maizels (1991, 1993,

1997) and Maizels and Russell (1992). The western Gígjukvísl rising stage successions however,

show few signs of large-scale grading, and instead contain repeated cycles of sedimentation

recording individual sedimentation pulses representing the passage of near bed sediment pulses

(Todd, 1989; Sohn, 1997). Distinctive coarsening upward successions on the Gígja waning stage

fan were generated by upstream sediment reworking together with the transition of deposition from

suspended to bedload (Fig. 13d). Waning stage upward coarsening successions are not represented

in existing models of jökulhlaup sedimentation. Indeed, upward coarsening successions within

models presented by Maizels (1989a, b, 1991, 1993, 1997), Russell and Knudsen (1999a) and

Russell and Marren (1999) all represent deposition from rising stage flows. It is now clear however

that the presence of an upward-coarsening succession alone is not diagnostic of rising flows stage

deposition. Careful examination of the abundance of the sand and fine-gravel matrix through the


32

upward coarsening succession should allow waning stage bed coarsening by winnowing to be

distinguished from rising stage sediment transport capacity increases. Rising and falling stage

sedimentary successions will be preserved on outwash fans subject to prolonged high sediment

fluxes. Such fans will also display less spatial variability of surface texture than found on the other

fan types.

During the November 1996 jökulhlaup sediment reworking and ice-margin erosion took

place over distances of 102-103 m (Fig. 6a). Ill-defined erosional, streamlined terraces reflect

incision on the flood waning stage (Figs. 13c,e). This landform and sedimentary succession could

easily be confused with the product of fluvial depositional and erosional cycles operating over

longer timescales associated with more sedate rates of glacier retreat within former proglacial areas

(e.g. Thompson and Jones, 1986).

Knowledge of processes responsible for the formation and pattern of ice-block related

features within jökulhlaup channels, provides a remarkable insight into a process-form relationship

that is truly diagnostic of jökulhlaups within terrestrial environments. Detailed knowledge of the

associations between ice blocks and resultant bedforms and lithofacies enables the identification of

jökulhlaups, involved in the release and transport of ice blocks within both modern and ancient

terrestrial proglacial environments. Since obstacle marks, kettle holes, isolated conical mounds and

ice-block diamicton rims and mounds are essentially formed on the stream bed, their identification

within the stratigraphic record can be used to recognise individual palaeofloods thereby enabling

reconstruction of jökulhlaup magnitude and frequency regime. Ice-block obstacle tails provide

information on flow velocity and flood depth (Russell, 1993). Where deposition occurs in the lee of

ice block obstacles, shadow sediments denote maximum grain size transported at the time of

shadow deposition. Since ice-block obstacles are not fully submerged during jökulhlaups, obstacle

tail height, whether depositional or erosional in nature, provides a minimum flow depth at the time

of formation. Ice-block obstacle tails also record waning- limb flow direction (Russell, 1993). Fay


33

(2001, 2002a) interpreted large upstream dipping beds around the margins of kettle-scours as the

product of supercritical flow conditions, associated with transitional Froude numbers (0.8 - 1.2).

Upstream dipping beds associated with flow around large proglacial ice blocks can be used to

reconstruct former flow depths and velocities (Fig. 16). Sediment exposures developed after the in

situ melt of the ice blocks within kettle scours may therefore provide an insight into jökulhlaup

processes. The depth of kettle-scours provides a minimum thickness of a single jökulhlaup

depositional unit. Ice blocks may enhance the preservation of jökulhlaup deposits. For example, the

deposition of obstacle shadows and the development of hummocky meltout topography produces a

higher amplitude jökulhlaup deposit surface than would be formed in the absence of the ice blocks.

Localised scour around ice blocks may generate complex cut and fill structures which may reduce

the thickness and lateral continuity of jökulhlaup deposits.

11. Conclusions and wider implications

Observations from Skeiðarárjökull and Sólheimajökull provide an insight into the dynamic

response of glaciers to widespread zones of negative effective-pressure. We have illustrated the

potential for jökulhlaups to erode, transport and deposit sediment within sub-, en- and terrestrial

proglacial environments. High magnitude jökulhlaups with rapid linear rises to peak discharge have

left their signatures within most parts of the glacial & proglacial system. Supercooled jökulhlaup

discharge has the potential to entrain large volumes of sediment into former ice sheets.

From the sediment budgets reviewed and presented in this paper, it is apparent that

jökulhlaups are a dominant part of the meltwater magnitude and frequency regime of ice sheets. In

terms of geomorphological work, reflected by volumes of sediment transported, jökulhlaups are

highly significant events. Our review of recent findings from the study of two high-magnitude

jökulhlaups indicates that jökulhlaups are responsible for the bulk of sediment within the vast

sandur of Iceland.


34

Marine records of late-Pleistocene sediments reveal a series of enormous influxes of

meltwater and iceberg-rafted detrital carbonates and drop-stones (Heinrich, 1988; Colman, 2002).

Periodic and abrupt releases of floodwater from reservoirs beneath the unstable piedmont lobes of

the Laurentide and Fennoscandinavian ice sheets (Shoemaker, 1999) possibly generated gigantic

ice-freighted jökulhlaups (e.g. Blanchon and Shaw, 1995; Bond et al., 1992; Shaw and Lesemann,

2003). To account for transient, cataclysmic late-Pleistocene jökulhlaups, and to some extent,

glacial landscape-genesis within former Pleistocene flood tracts (Shaw, 2002), it is necessary to

invoke subglacial sheet-floods (Shoemaker, 1992a, b, 1999). Notwithstanding sediment

preservation potential issues, glacier-hydrological processes during the 1996 and 1999 jökulhlaups

provide a useful modern-day analogue for the likely process-response of terrestrial Pleistocene ice-

lobes to jökulhlaups.

Acknowledgements

We acknowledge fieldwork grants from: The Earthwatch Ins titute (AJR, MJR, PMM, NJC, HF, and

FST), The UK Natural Environment Research Council (GR3/10960, AJR), (GR3/12969, AJR and

FST) and NERC (GT04/97/114/FS, HF). We thank Ragnar Frank Kristjánsson for supporting our

research within Skaftafell National Park. We would like to thank both anonymous reviewers for

their constructive comments on this manuscript and Dave Butler and Jay Fleisher for editing this

paper.


35

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Figure captions

Fig. 1 Location of Skeiðarárjökull and Skeiðarársandur within Iceland and in relation to

Vatnajökull ice cap. (a) The 1996 eruption site (Gjálp) and the jökulhlaup source

(subglacial lake Grímsvötn) are indicated. The 1996 jökulhlaup drained subglacially

over a distance of 50 km before exiting the snout of Skeiðarárjökull. (b) The 1996

jökulhlaup drained the glacier snout via multiple sub- and englacial outlets, inundating

large tracts of the glacier surface and proglacial area. Jökulhlaup drainage from the

glacier snout was impeded by moraine ridges and high elevation inactive sandur

surfaces, resulting the in the development of lateral flow, parallel to the ice front in the

Gígjukvísl channel system. (c) Aerial photograph of waning stage jökulhlaup flows

within the Gígjukvísl channel system at 12:00 on November 6th 1996. The pre-flood

glacier margin is indicated by the dotted white line. The location of supraglacial outlets

and the supraglacial ice-walled canyon are indicated on both (b) and (c).


57

Fig. 2 (a) Location of Mýrdalsjökull ice cap in Iceland. (b) Location of Sólheimajökull relative

to the Mýrdalsjökull ice-cap, Katla subglacial volcano and surrounding outwash plains.

(c) Map of the lower portion of Sólheimajökull showing the location of July 1999

jökulhlaup outlets. Subglacial and subaerial jökulhlaup routeways and temporary storage

locations are indicated. Jökulhlaup waters ascended through > 400 m thickness of glacier

ice via hydrofractures (Roberts et al., 2000; 2003). Rapid jökulhlaup onset temporarily

filled the upper and lower ice-dammed basins (Roberts et al., 2000; 2003).

Fig. 3 The July 1999 jökulhlaup exited Sólheimajökull at a number of locations, depositing

coarse-grained, ice-contact outwash (see Fig. 1c for location). (a) Oblique view of ice-

contact jökulhlaup deposits in front of the largest subglacial outlet on the western side of

the snout of Sólheimajökull. Boulders up to 13 m in diameter are deposited chaotically


58

together with ice blocks up to 20 m in diameter. Grain size decreases visibly

downstream (arrow indicates flow direction). (b) Westerly view across the snout of

Sólheimajökull showing multiple flood paths from the glacier. Discharge from the

western outlet (lower left) over-printed evidence of flow from the smaller central and

eastern outlets.

Fig. 4 (a) Aerial photograph taken during the waning stage of the 1996 jökulhlaup shows the

location of the supraglacial fracture outlet (boxed) in relation to the pre-flood glacier


59

margin (broken white line) and the supraglacial ice-walled canyon. A large spider

shaped fracture can be seen to the east of the supraglacial ice-walled canyon. At the

centre of the fracture system a block of ice has been displaced downward. (b) Frozen

sedimentary infill of a laterally extensive, up-glacier dipping ‘arterial’ fracture (Roberts

et al., 2000, 2001). As well as frozen bedded sands and silts, rip-up clasts are found.

These rip-ups comprise basal ice and frozen deforming bed sediments which have been

mechanically removed from the glacier bed approximately 200 m below. (c) View over

fracture outlet area in 1998 after ~ 30 m of post-jökulhlaup ice surface ablation (Waller

et al., 2001). The linear ‘arterial’ fracture is exposed within the amphitheatre shaped ice-

surface depression (broken white line). The location of the main fracture section (b) and

a section through a complex fracture network (d) are provided. A circle encloses a group

of figures for scale. (d) A complex network of branch fractures is in- filled mainly by

sands. Sedimentary structures indicate that meltwater flow ascended locally through the

fracture network but also flowed laterally.


60

Fig. 5 Englacial jökulhlaup deposits at Skeiðarárjökull, although having a low preservation

potential provide an intriguing insight into fluvial sedimentary processes within

pressurised fracture networks. Englacial sedimentation is thought to be enhanced by

highly supercooled meltwater, allowing sediment accretion to vertical ice walls (Roberts

et al., 2001, 2002). Sediment emplaced in photographs (a), (b) and (d) would have been

at least 50 m below the surface of Skeiðarárjökull in November 1996. (a) Hydrofractures

commonly bifurcate as they propagate upwards forming a complex network of branches.

In this case meltwater has deposited progressively finer sediment within a series of

branch fractures. Ice axe is 70 cm in length. (b) Sedimentation also occurs within

englacial voids dissected by hydrofractures. The presence of coarse-grained sediment

also indicates that hydrofracture networks had to be sufficiently dilated to allow the ir

transit through the glacier (Roberts et al., 2001). (c) Typical upward-fanning or

‘mushroom’ structures are seen within shallower ice near the Súla vents on the western

margin of Skeiðarársandur. These beds are accreted by hydraulically supercooled and


61

sediment charged meltwater (see Fig. 10b for a schematic diagram or this process). (d) A

medium-sized fracture is packed with frozen fluvial sediment displaying a variety of

sedimentary structures including: cross-stratification, planar and wavy bedding.

Sedimentary structures indicate that accretion was initiated along the flanks of the

fracture and that there were several phases of sedimentation.


62

Fig. 6 (a) Oblique aerial view of supraglacial ice-walled canyon at Skeiðarárjökull taken in

July 2002. The supraglacial ice-walled canyon was mechanically eroded in less than 17

hours supplying over 6 x 106 m3 to Skeiðarársandur (Russell and Knudsen, 1999a,b;

Roberts et al., 2000; Russell et al., 2001). The head of the canyon extends nearly 1 km

up-glacier from the glacier snout. The englacially-fed supraglacial canyon channel

conveyed a peak flow of ~20,000 m3s-1, with sufficient stream power to carry boulder-

sized sediment in suspension. Ice-surface lowering between 1996 and 2002 of

approximately 60 m has brought about topographic inversion with the 4- 10 m thick

canyon sediment- fill currently protecting underlying ice from ablation. The pre-1996

jökulhlaup ice-margin is indicated by the broken white line. The heavily kettled

Gígjukvísl waning stage outwash fan can be seen in the distance beyond the ice-contact

slope. The location of the two Ground Penetrating Radar profiles presented in Figure 11

is indicated. White arrow indicates location and direction of view for Figures 6b and c.

(b) Proximal to distal view of the supraglacial ice-walled canyon in April 1997 from the

location indicated in Figure 6a. Note people in the middle distance and jeep (circled) for

scale. At peak discharge the canyon was filled to almost bank-full capacity. (c) Proximal

to distal view of supraglacial ice-walled canyon in July 2000 from the same view point

at Figure 6b. Ablation has resulted in approximately 30 m of ice-surface lowering and

the development of the canyon floor as a ridge.


63

Fig. 7 Rip-up clasts or intra-clasts are commonly found on and within ice-proximal at

Skeiðarárjökull (Russell and Knudsen, 1999a; 2002). Given the relatively scarcity of

boulder-sized sediment at Skeiðarárjökull, rip-up clasts are commonly the largest clasts

found within 1996 jökulhlaup deposits. (a) Three 2 – 3 m diameter rip-ups are located in

a cluster on the stoss-side of a bar located within the supraglacial ice-walled channel.

Broken lines indicate the location and extent of each rip-up. (b) This 2 m diameter rip-up

composed of stratified and deformed sediment provides evidence of hydro mechanical


64

erosion of subglacial material. Rip-ups of similar size and composition have been found

within englacial fracture-fills (Waller et al., 2001; Roberts et al., 2001) and within

subaerial jökulhlaup deposits (Russell and Knudsen 1999a,b).


65

Fig. 8 (a) Down-glacier view of the large englacial esker which feeds the head of the

supraglacial ice-walled canyon. The esker ridge joins the supraglacial ice-walled canyon

at the exact location of a zone of violent up welling observed during the jökulhlaup

waning stage. Within the first few hours of the November 1996 jökulhlaup, englacial

drainage collapsed from a complex hydrofracture network into series of large englacial

conduits. This photograph was taken 2 km up-glacier of the November 1996 ice-margin

position (indicated by broken line). (b) Esker ridge (crestline indicated by broken line)

on the surface of Skeiðarárjökull in August 1998. This esker was deposited englacially

during the November 1996 jökulhlaup and is clearly associated with proglacial

jökulhlaup channels and the former ice-contact slope (indicated by dotted line).


66


67

Fig. 9 A rectilinear network of November 1996 jökulhlaup fracture-fill ridges is preserved in

the Sælhusakvísl area of Skeiðarárjökull. (a) A ridge in the foreground provides primary

sedimentary structures. The broken white line in the top right marks the location of the

glacier margin during the November 1996 jökulhlaup. (b) The rectilinear ridge network

matches the hydrofracture pattern observed in the overlying glacier after the 1996

jökulhlaup (Roberts et al., 2001).

Fig. 10 (a) Schematic diagram of Skeiðarárjökull showing the large scale geometry of major

‘arterial’ fracture outlets fills within Skeiðarárjökull. Jökulhlaup- induced

hydrofracturing allowed meltwater to ascend even steeper adverse gradients resulting in

highly supercooled meltwater discharge (Roberts et al., 2002). (b) On a smaller scale,


68

sediment- laden supercooled meltwater ascending into an englacial void or surface

crevasse is known to produce a vertical jet which expands to create a series of high-

angles sedimentary structures (Roberts et al., 2001). The following sequence of events is

proposed: (i) initial discharge into the void allowing sedimentation to take place

laterally; (ii) - (iii) deposition takes place surrounding the vertical jet allowing accretion

on both stoss and lee sides of the lateral deposits (iv); stoss side accretion is dominant

and bedding commonly accretes into ‘over-vertical’ or fan- like geometries. Accretion

within the feeder vent rapidly chokes the flow.


69

Fig. 11 Ground Penetrating Radar (GPR) profiles were obtained within the ice-walled canyon

and its associated proglacial outwash fan (see Figure 6a for locations). (a) GPR profile


70

and sedimentary interpretation for the ice-walled canyon and proximal Gígjukvísl fan

area. A set of large-scale (10 m amplitude) upstream dipping surfaces are found at depth

within the supraglacial ice-walled canyon. These features represent distal to proximal

accretion on the stoss-side of a series of a channel-scale bedform during progressive

head ward expansion of the ice-walled canyon (Cassidy et al., 2003). The top 2 – 3 m of

the succession consists of lower-angled upstream dipping proximally and foresets within

the distal portions of the ice-walled canyon (Cassidy et al., 2003). These upper deposits

reflect a later stage of deposition from shallower flow. (b) This GPR profile was taken

on the distal portion of the Gígjukvísl waning stage fan (see Fig. 6a for location). The

GPR profile shows a transition from sub-horizontal to prograding foreset reflections.

Sub-horizontal bedding is thought to represent and earlier phase of jökulhlaup deposition

with flows obliquely into the section (Cassidy et al., 2003). Foreset beds are consistent

with deposition from flow exiting ice-walled canyon the late rising and waning stage

(Cassidy, et al., 2003). Stacked, large-scale, trough-shaped reflectors between 1964 and

2044 m represent a confluence zone between flows exiting the double embayment in a

northwest to southeast direction and those flowing along the axis of the proglacial trench

in an east to west direction (see Fig. 12).


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Fig. 12 Panorama of the Gígjukvísl waning stage fan taken in April 1997 showing the location

of the supraglacial ice-walled canyon and one of a series of large linear hydrofracture


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outlets. Flow radiating from the ice-walled canyon is confluent with an east to west

draining flow in the foreground. Large numbers of ice blocks are deeply embedded

within jökulhlaup deposits and have generated large-scale obstacle marks. The

development of numerous kettle holes between 1997 and 2000 indicated the presence of

significant numbers of buried jökulhlaup-transported ice blocks.

Fig. 13 Schematic diagrams illustrating some of the main morphological and sedimentological

characteristics of ice-contact jökulhlaup fans associated with different flow stages. (a)

Rising stage deposition into a topographically-controlled backwater lake. Rapid rises in

local base level combined with rapid reductions in stream power allow the progradation

of radial delta- like deposits. (b) Absence of backwater conditions during the jökulhlaup

rising stage results in the creation of an outwash fan built up by the aggradation of

individual sheet- like layers. Fans such as that in front of Gígjukvísl ice-walled canyon

are subject to high sediment flux on both rising and falling flow stage can aggrade until

the very end of the jökulhlaup when surface armouring and shallow channelisation

occur. (c) Heavily dissected non-backwater controlled outwash due to declining

sediment flux on the prolonged falling flow stage. (d) Ice-contact fan at the Western

Skeiðará was deposited and heavily dissected during the November 1996 jökulhlaup (see

Fig. 2b for location). (e) Prolonged waning stage flow on the Gígjukvísl fan resulted in a


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transition from polymodal matrix-rich units to clast-supported units brought about by the

increasing dominant ce of bedload transport prior to deposition (Russell and Knudsen,

2002).

Fig. 14 Oblique aerial photograph of Gígjukvísl fan. Large-scale obstacle marks and kettle holes

are deposited in linear clusters which radiate from the supraglacial ice-walled channel.

The melt out of large numbers of partially buried ice blocks within close proximity of

each other creates zones of hummocky topography. Linear ice block clusters generated

two main channels across the Gígjukvísl fan.


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Fig. 15 Morphology and general composition of a range of ice-block obstacle marks formed

during jökulhlaup flow. Type 1: kettle-scours with both proximal and lateral scour and

tail formed by partial or total burial and subsequent partial exhumation. Type 2: semi-

circular obstacle marks formed by partial exhumation of partially buried ice blocks on

the margins of the main channels. Type 3 and Type 4: obstacle marks characterised by a

proximal and lateral scour crescent, a ridge stoss-side of the scour crescent and a large

aggradational tail whose structure is anticlinal. Truncation of anticlinal-shaped bedding

produces the flat-topped morphology and armoured nature of Type 4 obstacle marks.

Type 5: Obstacle marks <5 m in diameter with fine-grained gravel in the immediate lee

of the scour hollow and coarse gravel lag characterising the distal part of the tail.

Formed by waning stage deposition followed by late waning stage erosion. Type 6:

entirely erosional obstacle marks. Formed by total exhumation of buried ice blocks or by

scour around ice blocks grounded on the late waning stage (Fay, 2001).


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Fig. 16 (a) Schematic diagram showing the formation of antidune stoss-side strata around an ice

block. (i) Flow becomes supercritical just down flow of the ice block, generating an

upstream-migrating standing wave. (ii) Plan view of the upstream-migrating standing


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wave. (iii) Antidune stoss beds are deposited from the upstream-migrating surface wave.

(iv) 'Washout': with increasing bedform height, flow on the upstream side slows and

deepens increasing the rate of sedimentation until the antidune is no longer stable and

collapses. Downstream-dipping strata are produced by rapid migration of asymmetrical

bed waves immediately after the surface wave breaks. Associated with antidune

collapse, water stored upstream of the antidune is released eroding the antidune strata

and filling the antidune trough with low angle back set beds. (v) Resultant section after

the flood has receded (Fay, 2001). (b) Upstream-dipping gravel beds deposited on the

side of a large deeply embedded ice block. View is from the location of the ice block

outwards into the flow. Lower angled upstream dipping beds on- lap the higher-angled

strata indicating lower stage deposition (adapted from Fay, 2002a). Upstream dipping

beds represent antidune stoss-side sedimentation within a localised zone of supercritical

flow around the ice block.


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Fig. 17 Schematic diagram showing how vertical sedimentary characteristics might vary

depending on whether jökulhlaup waters were confined by moraine ridges (a) and (b) or

were free to spread out from the ice-margin (c- f). Note increased spatial variability of

sedimentary successions associated with the moraine-confined conditions where

backwater conditions act as a major control on sedimentation at different times during

the flood. Unconfined jökulhlaup successions are divided into locations where waning-

stage sediment flux decreases (a) and where sediment flux remains high even on the

waning flow stage (b). In the moraine-confined scenario, backwater conditions result in

much finer grained deposits downstream of the backwater zone (d). Sedimentary


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successions associated with ice-contact proglacial fans may also vary depending upon

degree of backwater ponding or whether deposition is into shallow fast flows (c) or into

sluggish deep flows (f). Successions within supraglacial ice-walled channels may show

considerable sedimentary variability.

Icelandic jökulhlaup impacts: Implications for ice-sheet hydrology, sediment transfer and geomorphology

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