Formation of kettle holes following a glacial outburst flood (jôkulhlaup), Skeiôarârsandur, southern Iceland

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1 Tile Extremes ofthe Extremes: Extraordinary i'loads (hoceediims of a symposium hold al Reykjavik. Iceland. July 2000). IAI IS l'util, no Formation of kettle holes following a glacial outburst flood (jôkulhlaup), Skeiôarârsandur, southern Iceland HELEN FAY School of Earth Sciences and Geography, Keele University, Keele, Staffordshire ST5 SBG, UK ggd25(?7>.csci.keele.ac.uk Abstract The 1996 jôkulhlaup on Skeiôarârsandur, southern Iceland, involved the transport of ice blocks released from the glacier margin. The morphology, sedimentology and spatial distribution of kettle holes and other ice-blockrelated features, which developed post-flood in this proglacial fluvial system, were examined. Four types of phenomena are described and explained: kettle chains orientated both parallel and transverse to the principal palaeoflow direction, hummocky topography, steep-walled and inverse-conical kettle holes, and conical sediment mounds. Key words kcltlehole; jôkulhlaup; ice block; Iceland; glacier; sandur; morphology; sedimentology BACKGROUND AND AIMS Although kettle holes formed by the melt of buried ice have been reported from many present-day proglacial environments (e.g. Russell & Knudsen, 1999) very little research has focused on the impact of ice blocks on outwash plains following jôkulhlaups (Maizels, 1992; Branney & Gilbert, 1995). This may have led to in invalid palaeoglaciological reconstruction. For example, widespread kettled topography of former outwash plains interpreted as "pitted" or "kettled" sandur formed by the passive decay of glacier ice (e.g. Thwaites, 1926) may actually be indicative of a dynamic jôkulhlaup origin. The November 1996 jôkulhlaup onto Skeiôarârsandur provided an opportunity to study a suite of post-jôkulhlaup features related to ice blocks. The aim of this paper is to describe and explain the morphology, sedimentology and spatial distribution of post-jôkulhlaup ice-block related features. This study will provide an improved understanding of jôkulhlaup impact. This, in turn, will allow improved identification, and palaeoreconstruction of historical deposits, in particular, in distinguishing palaeojôkulhlaup landscapes from landscapes dominated by non-fluvially driven processes, such as those created by the passive decay of buried glacier ice. THE NOVEMBER 1996 JÔKULHLAUP AND FIELD METHODS On 30 September 1996, a volcanic eruption began beneath the Vatnajôkull Glacier in southern Iceland (Fig. 1). The resultant jôkulhlaup on 5 November 1996 transported large numbers of ice blocks up to 45 m in diameter onto Skeiôarârsandur. Information on jôkulhlaup flow was obtained from video film taken on 5 and 6 November 1996, oblique video film taken on 7 November 1996 and from sedimentary

2 206 Helen Fay /////^s Seeluhusskvis! Location of hummocky topography in Fig. 3 Flood outlet Palaeoflow direction Flow from crevasse outlets Lake Hâôldukvisl X//A Old surface T7T^ Obstacle marks and kettle holes on higher level outwash surfaces including steep-walled and inverse-conical kettle holes \ I Hummocky topography Fig. 1 A schematic map of the field area in Iceland showing the location of the postjôkulhlaup ice-block-related features shown in Figs 2-5. logs. Post-jôkulhlaup data were collected in the field during April 1997 and the summers of 1998 and Peak jôkulhlaup discharge figures for the Gigjukvisl River were available from Russell et al. (1999). RESULTS Kettle holes have developed in lines up to c. 270 m in length parallel and transverse to the principal jôkulhlaup flow direction (Figs 1 and 2). The largest kettle hole within Skciôarmjokull Longitudinal kettle chain Longitudinal kotlk' cliniii n- i» ]> Rising stage diiuctiun -JS> Waning stafj-" rirer.tiop Fig. 2 A pattern of kettle holes in front of the main flood outlet including kettle chains parallel and transverse to the jôkulhlaup flow direction.

Formation of kettle holes following a glacial outburst flood, Skeiôarârsandur, southern Iceland 207 each chain is located at one end of the chain.

3 Formation of kettle holes following a glacial outburst flood, Skeiôarârsandur, southern Iceland 207 each chain is located at one end of the chain. Hummocky topography, composed of numerous closely spaced hollows with raised rims, can be identified in locations subjected to backwater conditions during the rising stage of the flood and in lower fan locations formed on the waning stage of the flood (Figs 1 and 3(a)). Figure 3(b) shows a section through one of the rims. The lower unit consists of structureless, clastsupported, poorly sorted medium sand and pebble-gravel. This material is consistent with the sediments on the streambed. The overlying unit, cm in thickness, consists of very poorly sorted, structureless, ungraded, silt to boulder-sized diamict. Steep-walled and inverse-conical kettle holes are found proximal to the main flood outlets where sediment flux was high (Fig. 1). Steep-walled kettle holes range from 0.5 to 11 m in diameter and have depths of up to 2.2 m. Two types of steep-walled kettles have been identified: Type (a): A shallow circular depression, formed in coarse-grained, clast supported sediments, with vertical to inward-dipping walls, whose base is a coherent block of streambed sediment (Fig. 4 (a)). Type (b): Up to 4.5 m deep circular pit, formed in coarse-grained sediments dominated by matrix support or in entirely fine-grained sediments, with steeply dipping to overhanging walls (Figs 4(b) and (c)). Type (b) kettles were often observed to form within 3 days. Inverse-conical kettles range in size from 2 to 20 m in diameter. The kettle walls are composed of disaggragated sediments and many of the kettles are unstable with material slumping or avalanching down the kettle walls. 1(a); (b) 0 5 m Fig. 3 (a) Hummocky topography distally in the Gigjukvisl River channel, (b) Section through a kettle rim showing cm of very poorly sorted diamict overlying poorly sorted gravel. (a) (0 Fig. 4 (a) Type (a) kettle hole with a coherent block of streambed sediment in the base, (b) Type (b) kettle hole with overhanging walls, (c) Type (b) kettle hole, <2 m in depth, with vertical walls.

4 208 Helen Fay Sediment mounds were observed in slackwater sediments within the Gigjukvisl River routeway (Fig. 1). The heaps are isolated and conical in shape with heights of up to 1 m and diameters up to 3 m. A section taken through a mound (Figs 5(a) and (b)) shows that the lower 75 cm is composed of cross stratified laminated to thinly bedded medium to coarse sand. This material is comparable with that of the streambed, which has been reworked by aeolian activity. The overlying 25 cm thick unit is composed of very poorly sorted, ungraded, silt to boulder-sized diamict. Fig. 5 (a) A sediment mound, (b) Section through a sediment mound showing 25 cm of very poorly sorted diamict overlying cross stratified sand. DISCUSSION AND CONCLUSIONS Longitudinal and transverse kettle chains have formed in a similar way to particle clusters where particles are entrapped in the stoss or train of a larger obstructing clast (cf. Brayshaw, 1984). Video film shows ice blocks being transported around large already grounded blocks and into their lee. Ice blocks are transported into secondary flow cells that are deflected downwards and outwards from large grounded ice blocks in the form of a "horseshoe vortex" (cf. Fay, 2002). The enhanced velocities in front and along the sides of the grounded ice block prevent the grounding of other blocks and they are transported into the lee of the ice block where they are grounded as a result of lower velocities. The chains of kettle holes orientated transverse to the principal palaeoflow direction shown in Fig. 2 were actually formed parallel to flow on the rising flood stage when flow in this location was predominantly from east to west (Fig. 2). Hummocky topography identified in backwater zones was formed by the rapid deposition of sediment around numerous closely-spaced grounded ice blocks. In more distal, lower fan locations where flow shallowed and decelerated, hummocky topography formed around numerous grounded ice blocks which acted as a focus for the rapid deposition of finer-grained sediment on the waning stage of the flood. The diamict, overlying slightly better sorted finer grained fluvial sediments, is interpreted as ice-block melt-out till (cf. Maizels, 1992). Kettles with circular raised rims resemble "rimmed" kettles described by Maizels (1992). While "rimmed" kettles are entirely composed of ice-block melt-out till, the rims described in this study are formed of fluvial sediments capped with a drape of ice-block till. Steep-walled and inverse-conical kettle holes result from subsidence over small ice blocks completely buried by rapid sediment deposition. Type (a) (Fig. 4(a)) kettle

$Formation of kettle holes following a glacial outburst flood, Skeiôarârsandur, southern Iceland 209 collapse is initiated by downsag followed by subsidence along a ring-shaped fracture (cf.$

5 Formation of kettle holes following a glacial outburst flood, Skeiôarârsandur, southern Iceland 209 collapse is initiated by downsag followed by subsidence along a ring-shaped fracture (cf. Branney & Gilbert, 1995). Type (b) kettle holes (Figs 4(b) and (c)) develop through progressive subsidence along a steeply outward-dipping ring fracture or by the formation of a cavity from ice-block melt under a coherent roof of sediment, followed by sudden roof collapse (cf. Branney & Gilbert, 1995). Cavities form due to the more competent sediments of kettle hole Type (b), in this case owing to a fine grain size or matrix support. The inverse-conical kettle shape develops as a result of slide or avalanche of material down the kettle walls along peripheral fractures (Branney & Gilbert, 1995). The upper diamict unit in the sediment mounds is interpreted as ice-block melt-out till (cf. Maizels, 1992). This layer, deposited from small ice blocks (1-3 m) grounded during slackwater conditions, protected the fluvial slackwater sediments beneath from aeolian reworking. As wind deflation scoured around the till patch, till rolled/slid down the sides of the exposed fluvial sediments further protecting the sediments and forming a residual conical mound. The principal controls on the formation and spatial distribution of the studied features are as follows: (a) the grounding of a single, dominant ice-block obstacle clast controls the formation and distribution of longitudinal and transverse kettle hole clusters, (b) the number of ice blocks, flow sediment concentration, flow velocity and ice-block sediment concentration control the formation and distribution of hummocky topography, (c) flow conditions, ice-block size, and wind deflation control the formation and spatial distribution of conical mounds, (d) the physical properties of the host sediment and ice-block size control the development of steep-walled and inverse-conical kettle holes, (e) flow sediment concentration and ice-block size control the spatial distribution of steep-walled and inverse-conical kettle holes, so that small ice blocks are buried by rapid sediment deposition in glacier-proximal locations where sediment flux is high. The identification of controls on the formation and distribution of ice-block related features contributes to the knowledge of fluvial processes occurring during jôkulhlaups and therefore has important implications for the palaeohydraulic reconstruction of jôkulhlaups in modern and ancient fluvial systems. Acknowledgements The author wishes to thank the Natural Environment Research Council (award reference: GT04/97/114/FS), Dr A. J. Russell, Hazel Magrath and Paul Reid. REFERENCES Branney, M. J. & Gilbert, J. S. (1995) Ice-melt collapse pits and associated features in the 1991 lahar of Volcan Hudson, Chile: criteria to distinguish eruption-induced glacier melt. Bull. Volcano! 57, Brayshaw, A. C. (1984) Characteristics and origin of cluster bedforms in coarse-grained alluvial channels. In: Sedimentology of Gravels and Conglomerates (ed. by E. H. Koster & R..1. Steel), Can. Soc. Petrol. Geof, Memoir no. JO, Calgary.

6 210 Helen Fay Fay, H. (2002) Formation of ice block obstacle marks during the November 1996 glacier-outburst Hood (jôkulhlaup)..skeiôarârsandur, southern Iceland. In: Flood ami Mega/iood Processes and Deposits (ed. by P. I. Martini, V. R. Baker & G. Garzon). int. Assoc. of Sedimentology Special Publ. no. 32. Blackwell Science, Oxford, UK. Maizels, J. K. (1992) Boulder ring structures produced during jôkulhlaup flows-origin and hydraulic significance. Ann. 74A, Russell, A. J. & Knudsen, Ô. (1999) Controls on the sedimentology of the November 1996 jôkulhlaup deposits, Skeiôarârsandur, Iceland. In: Fluvial Sedimentology VI (ed. by N. D. Smith,.1. Rogers & A. G. Plint), Int. Assoc. Sedimentology Special Publ. no. 28. Blackwell Scientific, Oxford, UK. Russell, A. J., Knudsen, Ô., Maizels,.1. K. & Marren, P. M. (1999) Channel cross-sectional area changes and peak discharge calculations for the Gigjukvisl during the November 1996 jôkulhlaup, Skeiôarârsandur, Iceland. Jôkull 47, Thwaites, F. T. (1926) The origin and significance of pitted outwash. J. Geol. 34, Geogr.

Controls on the development of supraglacial flood water outlets during jôkulhlaups

The Extremes of the Extremes: Extraordinary Floods (Proceedings of a symposium lieid at Reykjavik, Iceland. July 2000). IAHS Publ. no. 271. 2002. 71 Controls on the development of supraglacial flood water