Jokulhlaup Deposits in Proglacial Areas

JOKULHLAUP DEPOSITS IN PROGLACIAL AREAS

JUDITH MAIZELS Department of Geology and Geophysics, Grant Institute, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, U.K.

Abstract -- This paper discusses the main causes and characteristics of j6kulhlaup ('glacier burst') floods, and explores the extent to which they generate depositional landform and sediment assemblages that are distinct from those of 'normal', braided river outwash ('Type l' outwash). Two QSR main jrkulhlaup outwash environments are identified: Type II outwash, produced by sudden drainage of ice-dammed lakes; and Type III, associated with drainage during subglacial geothermal activity, and distinguished by deposits resulting from high sediment concentrations and hyperconcentrated flows. In fluid flows, especially ones yielding Type II outwash, the most common deposits are large- scale expansion bars (and locally, eddy and pendant bars), and 'mega-ripples' or dunes, both forms normally composed of large-scale gravel-cobble cross-bedding, often capped by an imbricated boulder lag (a 'Type B2' lithofacies sequence). The armour is absent only where runoff decreased too rapidly to allow surface winnowing. Other j6kulhlaup facies include extensive boulder beds (Type C), inverse-normally graded cobble beds (Type D5), ice-proximal debris flow deposits and deformed bedding containing diamicton clasts (Types G and H), and slack-water sediments (Type A). Type III outwash is dominated by massive, homogeneous, flood surge granules, underlain by pre-surge gravels, and capped by post-surge fluid bedforms, reflecting deposition during both the rising and falling limbs of the flood hydrograph (Type E4). The paper demonstrates that jrkulhlaups do generate distinctive assemblages of depositional landforms and sediments, and concludes with a model of the dominant lithofacies sequences and associated landforms in proglacial environments subject to jrkulhlaup drainage. © 1997 Elsevier Science Ltd

INTRODUCTION This paper first examines the hydrologic and hydraulic characteristics of jrkulhlaup flows that control the J6kulhlaup deposits, ranging from mega-scale ripples, morphology and sedimentology of j6kulhlaup deposits dunes and boulder bars to thick, stacked sequences of in subaerial proglacial fluvial environments; then sum- hyperconcentrated sandur deposits, occur in many marises the main processes of deposition during jrkulh- proglacial environments. However, the criteria for laups, and reviews the morphological and recognising j6kulhlaup deposits as such, and for differ- sedimentological characteristics of the resulting jrkulh- entiating them from non-flood outwash deposits, are only laup deposits. Finally, a classification of distinctive poorly defined, hence their significance in aiding the jrkulhlaup lithofacies is proposed, together with a interpretation of palaeo-environmental changes during framework for the palaeohydraulic and palaeoenviron- deglaciation has often been underplayed. Glacial floods mental interpretation of different jrkulhlaup sediment can occur on a variety of time-scales, lasting for hours or sequences. days, and recurring on annual, decadal or longer time- scales. 'Glacier burst' floods or jrkulhlaups exhibit a wide variety of flow characteristics, marked not only by HYDROLOGIC AND HYDRAULIC CONTROLS contrasts in flow magnitude, but also in the form, duration ON JOKULHLAUP DEPOSITION: FLOW and complexity of the flood hydrograph. In addition, the CHARACTERISTICS sediment concentration of the flood can change signifi- Causes of Jiikulhlaups cantly during different stages of flow. However, the impact of a jrkulhlaup on the proglacial environment, and J6kulhlaups may be generated by a number of on the depositional sequence in particular, reflects not different mechanisms, the most common of which is only these hydraulic criteria but also the nature of the failure of an ice dam which has acted to impound geomorphic constraints on the flood and the sensitivity of an ice-marginal lake. J6kulhlaups may be triggered the proglacial landscape to change. by a number of other mechanisms, including water 793 794 Quaternat T Science Reviews." Volume 16

Geology Topography Climate

Glacier / ice sheet /behaviour Sediment I_ I Flood Mechanism of Reservoir -4 availability ~~ routeway flood generation characteristics

Channel Flood Flood gradient magnitude frequency

Sediment /I F,ood I I Flood concentration I I I Hydrograph / ~ L timing

Fluvial hydraulics

Flowtype F L~ Extentof Erosive ~_~ Erosive and regime flooding capacity landforms

Preservation Sediment Spatial L Vertical potential type distribution sediment i of flood sequence + sediment thickness Longterm l stratigraphy

FIG. I. Simplified model of the factors controlling flow and sediment characteristics of j6kulhlaup floods (modified from Maizels and Russell, 1992).

overflowing an ice dam; rupture of subglacial, englacial Physical Controls on Jiikulhlaup Characteristics and supraglacial water bodies; sudden breaching of a moraine or unconsolidated rock dam; rock falls or The main controls on j6kulhlaup flow characteristics ice avalanches on to the glacier; periods of extreme ice and their potential geomorphic impact are illustrated in melt; prolonged and heavy rainstorm events; glacier the framework model (Maizels and Russell, 1992) (Fig. 1). surges; and periods of subglacial geothermal activity These controls are in turn dependent on the larger-scale or eruptions of subglacial volcanoes containing crater regional factors of geology, topography, climate, and lakes (e.g. Rist, 1983; Rc~thlisberger and Lang, 1987; glacier or ice-sheet behaviour. The mechanism of Church, 1988; Costa, 1988b) drainage and the reservoir characteristics largely control J. Maizels: Jrkulhlaup Deposits in Proglacial Areas 795 the magnitude, frequency and timing of jrkulhlaups, phically through failure of its ice dam. Flood flows while the form of the hydrograph as it is propagated reached peak discharges of 21×106 m 3 s -l, boundary downstream also reflects the morphology and gradient of shear stresses of up to 104 N m 2 and stream powers as the flood routeway. Timing of the flood depends on the much as 2.5x105W m -2 (Bretz, 1926; Baker, 1973a, source of the flood waters, as well as on the frequency of Baker, 1978; Baker and Bunker, 1985; Baker and Costa, flooding, and on rates of ice sheet or glacier ablation, lake 1987). Even larger floods appear to have occurred in the filling rates, and ice marginal fluctuations. Altay Mountains, Siberia, where Late Pleistocene lakes The availability of sediment to the flood waters affects were impounded behind tributary glaciers extending into the sediment concentration of the flood which, when the Chuja valley (Baker et al., 1993; Rudoy and Baker, combined with hydrograph and flood power character- 1993; Carling, 1996a). istics, controls the hydraulics and flow regime of the The nature of the j6kulhlaup hydrograph can also be flood along its routeway -- i.e. the degree of turbulence, reflected directly in vertical variations within the viscosity, transport capacity, boundary shear stress and sedimentary profile. Jrkulhlaup hydrographs exhibit a flood power -- and both the spatial and temporal extent wide range of forms (e.g. Church, 1988; Costa, 1988b), to which the j6kulhlaup exhibits fluid, hyperconcentrated with rising and falling limbs extending over periods of or viscous debris flow conditions. The spatial extent of hours to months (e.g. Bjrrnsson, 1988). The flood flooding also depends on the nature of the flood routeway, hydrograph is largely controlled by the mechanism of thereby also affecting the distribution of erosion and flood generation (Fig. 1). In the case of lake drainage, deposition by flood waters, and the preservation potential which is the most common source of jrkulhlaups, of these features. flooding occurs through breaching of an ice dam, and invasion of englacial or subglacial tunnels by meltwaters. The flood hydrograph then becomes dependent on the Flow Characteristics lake and dam characteristics and the rate of tunnel Magnitude, frequency and power of flows expansion which, in simple situations, are amenable to The magnitude, frequency and power of j6kulhlaup numerical modelling (Spring and Hutter, 1981; Clarke flows are directly reflected in the grain size, extent, and Mathews, 1981; Clarke, 1982; Clarke et al., 1984; thickness and sedimentary structures of the resulting Craig, 1987). deposits. In order to infer the palaeoenvironmental history A typical jrkulhlaup hydrograph exhibits a prolonged of the deposit, it is essential to improve understanding of rising limb and a rapid recession curve, reflecting the the control exerted by hydraulic conditions over the gradual expansion of the tunnel routeway until the lake resulting sedimentary record. basin has completely drained, or partially drained down to Peak flow magnitude depends largely on the volume of the level of the tunnel inlets. For example, the drainage of water that has been stored and on the rate at which it can an ice-dammed lake on the western margin of the drain through the glacial drainage system (Clague and Greenland ice-sheet produced a steady rising limb Mathews, 1973). Jrkulhlaup discharges commonly ex- extending over 18 hr before reaching a peak of ceed 'normal' ablation-controlled flows by several orders 100 m 3 s l, and returning to pre-flood levels within only of magnitude. Modern examples come from Baffin Island 12 hr (Russell, 1989; see Fig. 2A, B). where Church (1972) recorded jrkulhlaup flows from Many hydrographs also exhibit marked irregularities sudden lake drainage of ca 200 m 3 s -I, compared with during the course of the j6kulhlaup. During drainage from more normal flows of ca 20 m 3 s -l, while on the an ice-dammed lake, ice tunnels can become temporarily Icelandic sandurs, increases of between 3 and 5 orders blocked by ice jams, which may partially or completely of magnitude have been estimated for j6kulhlaups shut off flood flows, until pressures rise sufficiently to generated by subglacial geothermal activity (e.g. Bj6rns- allow meltwater to break through. At Strandline Lake, son, 1988; Maizels, 1991). Jrkulhlaups can therefore Alaska, for example, Sturm et al. (1987) recorded five contribute a significant proportion (e.g. 10-15%; see shut-offs of flow lasting over 40 hr. RCthlisberger and Church, 1972; Russell, 1989) of the total annual melt- Lang (1987) also demonstrate that highly irregular flood water runoff from a glacierised catchment. hydrographs can be generated by extreme melt rates and Estimates of palaeohydraulic parameters for Pleistocene storm precipitation affecting Alpine glacier catchments. and more recent jrkulhlaups have been based on a wide Irregularities in j6kulhlaup discharge may also arise from range of numerical modelling techniques. Unfortunately, a differential rates of expansion of the ice tunnel network, review of these methods is not within the scope of this from tapping of englacial or subglacial storages, or from paper, but details may be found in Baker (1973a), Church complex routing paths along the flood channelway. (1978), Costa (1983), Maizels (1983, 1987, 1989a), Jrkulhlaups produced by different mechanisms are Carling (1996b), and many others. characterised by distinct hydrograph forms. Thorarinsson Some of the greatest flood magnitudes, powers and (1957), working in Iceland, for example, where jrkulhla- boundary shear stresses estimated for j6kulhlaup flows ups occur in response to a wide range of mechanisms, occurred at the end of the last glacial. Some of the largest showed that while the hydrographs produced during freshwater floods on Earth were those generated during sudden drainage of ice-dammed lakes could extend over repeated Late Wisconsinan draining of glacial Lake periods of days and weeks, those produced from volcano- Missoula, in Washington State, which drained catastro- glacial events could reach peak flows of >105 m 3 s J in 796 Quaternar~, Science Reviews: Volume 16 (A)

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FIG. 2. (A) Valley sandur at Kangerlussuaq, west Greenland, during normal summer ablation flows, looking downstream. (Photo with permission of A. Russell.) (B) Same valley sandur during peak of j6kulhlaup flow in July 1987 at a discharge of 1080 m 3 s -1. (Photo with permission of of A. Russell.) J. Maizels: J6kulhlaup Deposits in Proglacial Areas 797 only 4-5 hr (Fig. 3A, B). Volcano-glacial jrkulhlaups to a concomitant increase in shear stress, flood power, may also exhibit significant variations in flow during the transport and erosive capacities. By contrast, where flows event, reflecting (permanent or temporary) damming of are able to expand on to unconfined sandur plains, flow flood routeways with lava or ash, or changes to the depths can decrease rapidly downstream, leading to a subglacial volcanic topography. dissipation of flood power and a reduction in erosive impact. Where flows are temporarily impounded behind a Magnitude of sediment yields rock or moraine constriction, deeper quieter waters allow upstream sedimentation, while the downstream flood Sediment concentrations in jrkulhlaups can be ex- wave may be significantly attenuated, and peak flows and tremely high and can significantly affect the dynamics suspended sediment loads reduced. This variety of and type of flow (see below), and hence the geomorpho- possible topographic conditions generates a wide range logical impact of the flood. Suspended sediment con- of depositional environments, each one characterised by a centrations recorded in jrkulhlaup events have reached as different kind of deposit. For example, point bars may much as 70,000 kg s -l (3.5% sediment concentration) in accumulate only in the bends of channels and bedrock an Icelandic jrkulhlaup flowing from a sediment-rich routeways, eddy bars at tributary mouths, pendant bars volcanic catchment (e.g. Tomasson, 1974; Tomasson et downstream of obstacles, and expansion bars only where al., 1980) to only 4500 mg 1-1 from a Greenland flows open out from canyon or gorge sections on to plains jrkulhlaup flowing in a sediment supply-limited, gneissic or broad valley floors (e.g. Baker, 1973a, 1984; Russell, channel (Russell, 1992). The high sediment concentra- 1992; O'Connor, 1993). tions of many jrkulhlaups reflect the normally abundant sources of glacially-derived (and in some cases, volcani- cally-derived) sediment, which is introduced into highly Pattern of deglaciation turbulent, powerful flows capable of transporting large The nature, rate and direction of deglaciation can have volumes of material. an impact on the amounts of both water and sediment Sediment inputs to jrkulhlaup flows are particularly released from the glacial system. Changes in the water high in areas of rapid deglaciation (e.g. Hammer and balance of the source ice mass affect the volumes and Smith, 1983), and where there are high influxes of rates of water release to the meltwater drainage system, sediment from the valley sides or walls of the flood while fluctuations in the position of the ice margin can routeway, or during volcanic eruptions. Sediment inputs control the drainage history of any ice-dammed lakes (e.g. to the flood are also likely to be highest during the first Teller, 1990; Maizels, 1997). Similarly, actively moving flood in a sequence, or when flood events are separated and eroding ice generates larger volumes of transported by prolonged periods of time, allowing any depleted debris than stagnating ice, suggesting that maximum supplies to be built up again before the next flood. High sediment supplies to glacial meltwater systems are likely sediment inputs may also be associated with periods of to occur during periods of active ice advance. However, high rainfall, snowmelt or slope instability in the during glacier retreat large volumes of glacial sediment catchment (e.g. Gurnell, 1987). can be released and exposed to subaerial processes, Jrkulhlaups are capable of transporting a wide range of providing proglacial river systems with a virtually sizes of material, ranging from silts and clays to large unlimited sediment supply. Indeed, most sediment boulders, in some cases reaching 10-15 m in diameter transported by meltwater streams is derived not by direct (e.g. Baker, 1973a), as well as transporting numerous release from the ice into the channel but from reworking large ice blocks over long distances, until they become of the channel boundaries (e.g. Hammer and Smith, 1983; stranded on higher surfaces as flows begin to wane Fenn, 1987). Much j6kulhlaup sediment therefore appears (Fig. 3B, and see below). to be derived from reworking of earlier outwash deposits, Jrkulhlaup sediment transport normally accounts for a or from freshly exposed debris-rich ice, or, in the case of very high proportion of the total sediment load removed volcanogenic floods, from fresh inputs of extruded annually from the catchment. Church (1972) estimated igneous material. that jrkulhlaup flows removed up to 90% of total annual Large-scale vertical sedimentological trends within sediment yields of a Baffin Island catchment, compared interstadial or interglacial outwash deposits have been with two outburst events removing ca 50% of annual widely interpreted as reflecting changes in meltwater sediment yields in an Alpine catchment (Beecroft, 1983; discharge associated with the advance or retreat of the ice Gurnell, 1987). margin. For example, large-scale upward coarsening sequences capped by till have been described from many Topographic controls on jrkulhlaup flows Quaternary deposits (e.g. Miall, 1980, Miall, 1983; Ehlers The topographic nature of the flood routeway has a and Grube, 1983), while large-scale upward-fining major effect on the type of flow and resulting deposits sequences may also develop during ice retreat, in since it determines the zones of flow constriction, response to increasingly distal flows, lower gradients expansion, separation and ponding, each controlling the and downstream fining of outwash sediments. However, nature of the resulting depositional environment. Where both recessional and advance outwash sequences may flow is confined between valley walls or within a rock-cut also exhibit the reverse vertical trend from that expected. channel, flow depths are considerably increased, leading For example, a number of upward-fining outwash 798 Quaternary Science Reviews: Volume 16

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FIG. 3. (A) Aerial view of Sula sandur, south Iceland, subject to j6kulhlaups from Vatnaj6kull, looking north to the ice cap during peak meltwater flows. (B) Aerial view of Gigjukvisl, Skeidararsandur, south Iceland, during the waning stage of the November 1996 j6kulhlaup. Note the dense accumulations of ice blocks on the expansion bar, and scattered ice blocks in the channel. View is northeastwards towards Vatnaj6kull (see Russell et al., in prep.). J. Maizels: J6kulhlaup Deposits in Proglacial Areas 799 sequences are overlain by glacial till (e.g. Reed et al., relatively few detailed examples of fans or outwash 1965; Vincent, 1984), while upward-coarsening or coarse deposits generated by j6kulhlaups. lag-capped outwash deposits have been recorded from Maizels (1991, 1993a) distinguished three dominant many retreat outwash deposits (e.g. Costello and Walker, outwash types, based on her analysis of sediment 1972; Dawson and Gardiner, 1987). Such variations in sequences from the Icelandic sandurs. The classification vertical grading reflect a wide range of local glacioby- allows differentiation of outwash type according to the drologic and channel processes, rather than changes in nature of the dominant flood regime and sediment inputs runoff regime related solely to glacier fluctuations. The of the meltwater system (see Table 1). role of high magnitude flood events is particularly critical Type I sandurs are 'classic' sandurs, defined as those in the accumulation of long-term glaciofluvial sediment which are mainly produced by 'normal', ablation-related sequences, since floods affect the degree to which earlier seasonal meltwater flows, commonly operating within a flood and non-flood deposits are preserved in the braided river environment, with no exceptional inputs of sequence. Most deglaciation vertical sediment sequences sediment, and with regular reworking of proglacial are therefore likely to be more complex than those channel sediments. In proximal zones, sediment sequences suggested by traditional interpretations, since they are dominated by clast-supported, massive, imbricated probably reflect numerous periods of aggradation and gravels associated with the development and migration of erosion, multiple floods, and varying inputs of water from longitudinal bars (Miall's Gm lithofacies; and Trollheim different flow populations, and of sediment from different (Miall's 'GI') and Scott (Miall's 'GII') outwash types sources (see discussion in Maizels, 1997). Most potential (Miall, 1978; Rust, 1978). In more distal areas, linguoid for improved understanding of long-term sedimentologi- bars, overbank accretion and channel fill deposits lead to cal sequences lies in vertical facies modelling, where the formation of sandy cross-bedded lithofacies (Miall's sediment sequences can be related to changes in hydraulic Donjek (GIII), South Saskatchewan (SII) and Platte (SII) and sediment supply conditions (e.g. Maizels, 1993b, braided river profiles)). Numerous studies of Type I 1994). outwash have been carried out from both modern and Pleistocene braided proglacial environments (e.g. Boot- hroyd and Ashley, 1975; Church and Gilbert, 1975; Miall, FRAMEWORK FOR DESCRIPTION OF 1977, 1978; Boothroyd and Nummedal, 1978; Rust, 1978; JOKULHLAUP DEPOSITS Smith, 1985; Brodzikowski and van Loon, 1991). These will not be discussed any further in this paper. Type II outwash is dominated by j6kulhlaup drainage Classification of Outwash Types from ice-dammed or subglacial lakes, i.e. by 'limnogla- Glacial outwash represents a particular form of alluvial cial' floods (Fig. 2A, B). These sandurs are characterised sedimentary environment, comprising humid-glacial fans, by large-scale, coarse-grained bedforms, but no systematic coarse-grained braided and fine-grained low sinuosity modelling or analysis of the universal diagnostic criteria river systems in which water and sediment are largely for identifying j6kulhlaup outwash has yet been com- derived from a glacial source. Many of these glacial pleted. This paper goes some way towards these environments are subject to both 'normal' and 'cata- objectives. strophic' runoff events, the latter assuming variable Type III outwash is dominated by 'volcano-glacial' stratigraphic and geomorphic significance according to j6kulhlaup drainage, such as that which affects large areas rates of deposition and erosion, in turn reflecting the of the Icelandic sandurs (Fig. 3A, B), and that generates runoff and sediment supply regimes, and the climatic and distinctive suites of lithofacies sequences. These are tectonic settings. Unfortunately, at present there are discussed in more detail below.

TABLE 1. Dominant lithofacies sequence of three characteristic outwash types (after Maizels, 1991, 1993a) Sandur or Dominant lithofacies Secondary lithofacies Dominant lithofacies Secondary lithofacies Dominant meltwater outwash sequence sequence types types regime and depositional type environment Type I A4 A, G Gmi, Sh, Suf, Sr, Gm, Sp, Gp, Gt, Sd, Normal seasonal St, Guf, FI Fd, Fe, Fro, Fs meltwater flows in braided fiver environment Type II B2 B, C, E, F Guc, Guf, Gxp, Gh, Sh, Suf, Go, St, Sp, J6kulhlaup drainage Gxt, Blg Gh, F1 from ice-dammed or subglacial lake ('limno- glacial' floods) Type Ill E4 E3, C, D1-D5, El, GRIn, GRx, GRh, Guf, Suf, Gt, Gp, St, Sp, J6kulhlaup drainage E2, F, G, H Gm, Gx, Gh, Sm, Sh, FI, GRch, GRo, during subglacial Suc, Guc GRni, GRd, GRuc, volcanic eruption GRmb, GRmp, GRmc, ('volcano-glacial' Go, Bm floods) See Table 2 and Table 3 for keys to lithofacies sequences and types. 800 Quaternary Science Reviews: Volume 16

TABLE 2. The main lithofacies types characteristic of j6kulhlaup and non-j6kulhlaup outwash (after Maizels, 1991, 1993a) Boulders (>256 mm) Bi Imbricated boulders Blg Boulder lag Bm Massive boulder bed Bx Large-scale boulder cross-bedding Bsl Single lithology boulder deposit Gravels (8-256 mm) Gh Horizontally bedded gravels Gm Massive, clast-supported crudely bedded gravels Gmi Massive clast-supported, imbricated gravels Gni Normal-inversely graded gravels Go Openwork gravels Gp Small-scale planar cross-bedded gravels Gs Matrix-supported gravels Gsi Matrix-supported, imbricated gravels Gt Small-scale trough cross-bedded gravels Gu Matrix-supported, unsorted gravels Guc Upward-coarsening gravels Guf Upward-fining gravels Gxp Large-scale planar cross-bedded gravels Gxt Large-scale trough cross-bedded gravels Granules (2--8 ram) GRch Channelled massive granules GRd Deformed granule bedding GRm Massive, homogeneous granules GRmb Massive, pseudo-bedded GRmc Massive, with isolated clasts GRmp Massive, with pebble stringers GRo Openwork granules GRruc Repeated upward-coarsening cycles GRruf Repeated upward-fining cycles GRt Trough cross-bedded granules GRuc Upward-coarsening granules GRuf Upward-fining granules GRx Cross-bedded granules Sands (0.06~3--2 mm) Sd Deformed sands Sh Horizontal/plane-bedded sands Sm Massive sands Sp Plane cross-bedded sands Sr Ripple cross-laminated sands St Trough cross-bedded sands Suc Upward-coarsening sands Suf Upward-fining sands Sx Cross-bedded sands Fines (<0.063 mm) Fd Deformed laminated silts and clays FI Laminated silts and clays Fm Massive silts and clays, or silts Fs Silts and clays with load structures Fe Laminated silts and clays with erratic dropstones

Lithofacies Types and Lithofacies Profiles gravels (G); granules (GR); sands (S); and, fines (F). The main sedimentary structures associated with each Using Miall's framework of lithofacies description, lithofacies type are summarised in Table 2 (see Maizels, modified by Maizels (1991, 1993a), five main lithofa- 1991, 1993a for more detailed discussion). cies types can be recognised from j6kulhlaup deposits Maizels differentiated flow environments within the according to dominant grain size: boulder beds (B); Icelandic sandurs according to the sedimentary character- J. Maizels: J6kulhlaup Deposits in Proglacial Areas 801

TABLE 3. Classification of lithofacies types and sequences in j6kulhlaup outwash deposits (modified from Maizels, 1991, 1993a)

Lithofacies Characteristic structures Dominant Secondary Examples Classification in sequence lithofacies lithofacies Maizels, 1991, types types Outwash type Location Reference Maizels, 1993a

Stratified A Horizontally laminated/ Gmi, Sh, Suf, Gm, Guf, St, Sp, II A3 A4 Missoula Baker, 1973a, b; CI, C2, G bedded, intercalated F1 Gp, Gt, Sr, Sd, floods Waitt, 1980, with small-scale cross- Fm, Fs, Fd, Fe 1984, 1985; bedding, clast-supported Atwater, 1984, sands and gravels 1986 A l Upward-fining II A4 Chuja and Rudoy and Kuray basins, Baker, 1993 Siberia A2 Upward-coarsening A3 Cyclic-normal grading A4 Cyclic-normal grading IlI A3 Myrdalssandur, Maizels, 1993a with coarse interbeds Iceland and coarse lag deposits

Large-scale cross- II BI Kuray dunes, Carling, 1996a bedded gravels and Siberia boulders B I Large-scale cross- Gxp, Gxt, Blg Suf, Go II B1 Rocky Clague, 1975 bedding Mountain Trench B2 Large-scale cross- Gxp, Gxt, Gxuf, Bx II B2 Lake Bonneville Malde, 1968; bedding with coarse lag Gxuc flood Jarrett and Malde, 1987; O'Connor, 1993 B3 Upward-fining large- Gxp, Gxt, Gxuf St, Sp, Suc II B2 Missoula floods Baker, 1984 scale cross-bedding B4 Upward-coarsening Gxp, Gxt, Gxuc Bx, Gh II B2 Platovo dunes, Carling, 1996a large-scale cross- Siberia bedding B5 As B3, with coarse lag Gxp, Gxt, Gxuf, Bx, Gh II B2 West Greenland Russell, 1992 Blg and in prep. B6 As B4, with coarse lag Gxp, Gxt, Gxuc, Bx, Gh II B3 Tippecanoe fan, Fraser and Blg Wabash valley Bleuer, 1988

C Boulder deposits C 1 Large-scale, crudely Go II C 1 Porcupine Thorson and F horizontally bedded valley, Yukon/ Dixon, 1983; boulder deposits Bi, Blg, Bsl, Bhl Alaska Thorson, 1989 C2 Gravel and boulder Gh accumulations with boulder lag

Massive D Massive graded III D Icelandic Maizels, 1989a, C sediments, matrix-poor sandurs b, 1991, 1992, 1993a D1 Normal grading Gmi, GRuf, Suf Sh, F1 D2 Cyclic normal Gmi, GRuf, Suf Sh, F1 D3 Normal-inverse Suc, GRuc, Gmi D4 Inverse Gmi, Suf, GRuc Sh, FI D5 Inverse-normal Guf; GRuc, Gmi Gh, GRh, Suf, Suc, GRo

istics of successive vertical lithofacies profiles. A varying associations with stratified sediment. Type H modified classification system is adopted here (sum- represents sediments which have been subjected to marised in Table 3) with a view to presenting a model of structural deformation. diagnostic j6kulhlaup profiles at the end of the paper. Eight types of vertical lithofacies profile have been Flow Types and Lithofacies Sequences distinguished: Types A to C represent stratified j6kulhla- up sediment profiles, while Types D to G represent The relationships between hydraulic conditions and profiles dominated by structureless, massive deposits with fluvial lithofacies types are relatively well established, 802 Quaterna#:v Science Reviews: Volume 16

TABLE 3. continued.

Lithofacies Characteristic structures Dominant Secondary Examples Classification in sequence lithofacies lithofacies Maizels, 1991, types types Outwash type Location Reference Maizels, 1993a

Massive homogeneous, IEI Souris spillway Kehew, 1982: with bedding at top and/ Kehew and or base Lord, 1986, 1987: Kchew and Clayton. 1983 E1 Massive, with cross- GRIn, GRx, GRch, Big, GRo bedding at the top GRt, Gxp, Gxt E2 Massive with horizontal GRIn, GRh, Go, Sh, F1. Fs 11 E2 Wabash valley Fraser and bedding at the top Gin, Guf, Suf Bleuer, 1988 E3 Massive with cross- GRin, GRx. GRch, GRo, Sh. 111 El-3 Icelandic Maizels, 1989a, bedding near top, Gxp, GRt, F1, Fs sandurs 1989b. 1991, capped with horizontal GRh 1992, 1993a bedding E4 Massive with basal GRh, Gm, GRch, Sh, Gs. Ill E4 Myrdalssandur, Maizels, 1991, gravels Gh, GRm. FI, Fs, Big, Gxp, South Iceland t993a and capping of cross- GRx, GRt, Gxt bedded GRo and horizontally bedded units

F Massive, poorly I11 FI-F4 South Icelandic Maizels, 1989a, A structured/no bedding sandurs b, 1991, 1992, structures, matrix-poor 1993a FI [Indistinct beddingl II F5 Mid-continental Kehew. 1982; spillway Kehew and channels, Lord, 1986, North America 1987; Kehew Sm and Clayton, GRIn, Gin 1983 F2 Decreased amount o1 Bm internal Oms F3 bedding Blg F4 [No bedding] F5 Massive poorly II F5 Baldakatj delta, Elfstr6m, 1983, structured with coarse N. Sweden 1987, 1988 lag

G Massive, structureless, Gs, Gu, Dms Gsi 111 G Icelandic Maizels, 1989a, E matrix-rich sandurs b, 1991, 1992, 1993 a

H Deformed bedding GRd, Sd Fs, Gs, Gu I11 H Icelandic Maizels, 1989a, D sandurs b, 1991, 1992, 1993 a especially for sand bedforms. However, the hydraulic strength of the flow. These theologic changes allow conditions associated with deposition from deep, fast, hyperconcentrated and debris flows to transport not only highly turbulent floods, especially those containing large amounts of sediment, but also to transport large relatively high sediment concentrations and transporting boulders and ice blocks through the effects of buoyancy large boulders, are more poorly understood. In addition, and dispersive stresses. The different flow types produce the terminology is often confused, and largely derived distinctive landform and sediment assemblages. from non-glacial environments. Most j6kulhlaup flows are highly turbulent, with sediment concentrations well below the threshold of Turbulent fluid flows 40% for heavy sediment suspensions or 'hyperconcen- Flows with low sediment concentrations (normally trated' grain flows. The initial stages of a j6kulhlaup may <40% by weight, <20% by volume) generally exhibit be characterised by sediment-laden flows in which turbulent characteristics, allowing the development of concentrations exceed ca 80%, resulting in non-Newtonian stratified bedforms associated with the downstream debris or mud flows (Costa, 1984, 1988a, b; Smith, 1986). migration of tractive load, grain-by-grain movement. High sediment concentrations act to dampen flow These processes can produce a range of sedimentary turbulence, and increase the bulk density and yield structures associated with turbulent fluid flow, in which J. Maizels: J6kulhlaup Deposits in Pvoglacial Areas 803

T3 __ T4 T5 Magnetic north m T6 0 100 200 ! ! i

T1 T2 GR

Stage recorder

T5 T6 TI~ ~ !;i:i; Exhumed terrace e Vegetation //// Pond bi T2 T3 ~ Activechannels ss Mega-ripples Slope breaks ~ Normal flows ~ Obstacle marks ~ Chute channels

FIG. 4. Morphological map of 'Sandur l', Kangerlussuaq, west Greenland, illustrating the main zones of deposition and scour in response to repeated limno-glacial j6kulhlaups of ca 1080 m ~ s i (from Russell, submitted). fluid shear induced by the boundary varies logarithmi- (Bingham) or high viscosity, non-Newtonian (pseudo- cally through the vertical profile. At subcritical Froude plastic) behaviour. Lower viscosity flows may form a numbers (FRI) dunes become washed out to between 1.01 and 1.3 g cm 3 (Costa, 1984), boundary form plane-bedded coarse sands, and imbricated gravel, shear can be transmitted through the fluid-sediment mix pebble and cobble sheets. J6kulhlaup flows, although as a dispersive pressure, such that inter-grain collisions characterised by extremely high discharges, often remain cause the coarser grains to move to zones of lower shear, subcritical, reaching supercritical conditions only locally i.e. to the edges of the flow. This mechanism can promote in constricted bedrock reaches and along steep sections of inverse grading of sediments, with larger clasts found as the channel. 'rafted' surface and lobe-edge boulders and as isolated, outsized clasts, and absence of cross-stratification (e.g. Macroturbulent flow conditions Pierson, 1981; Lajoie, 1984; Postma, 1986; Smith, 1986). Sediments deposited by hyperconcentrated flows tend to Macroturbulent flow conditions arise during very deep, be more poorly sorted than water flood sediments (Trask high gradient flows which can lead to the development of sorting coefficients of 1,0 to 1.6, compared with 1.8 to secondary circulation, Flow separation, and the growth 2.7, respectively; Costa, 1984). Openwork gravel se- and collapse of vortices around obstacles and in the shear quences can also develop, especially on foreset slopes, zone along irregular channel boundaries, especially large- where high energy traction currents prevent the deposi- scale dunes (Matthes, 1947; Baker, 1973a, 1984). Matthes tion of fines and deposit short lenses of matrix-free (1947) defined large-scale turbulent structures as 'kolks'. granules or gravels. Where flows are more turbulent, The upward vortex activity is characterised by intense shearing may produce crude sub-parallel laminations, a energy dissipation and steep pressure and velocity crude fabric, imbricated clasts, and basal erosional scours gradients, acting to generate extremely powerful hydrau- and flutes. Deposits may form extensive sheets across the lic lift forces. Although macroturbulence is still not fully whole sandur plain (Maizels, 1993a), but with increasing understood, it is clear that it can lead to the transport of sediment concentrations and flow cohesiveness deposits boulders in suspension. The resulting deposits include a may form steep fans, lobes or plugs (Lawson, 1982; variety of boulder deposits, including boulder levees and Maizels, 1989a). berms along former shear zones, such as channel margins. Debris flows Hyperconcentrated flows The terminology used to describe debris flows is Once the yield strength has been exceeded, laminar tortuous, and largely derived from non-glacial environ- flows may assume either low viscosity, Newtonian ments. Here, debris flows are defined as those containing 804 Quaternao, Science Reviews: Volume 16 A I I 14 21 (m) 01 (b) (d) (a) (c) (e) .0.25 I Gm S~p 20°dip 264 ° -0.50 ! ? Sp(pe) p(pe) 30/35°dip 312 ° -0.751 ~ ,~ 30°dip 331 o -1.00 Sp 262 ° Sp 25Odip 305° Sp(Pe) (m) 35°dip 268 ° -1.25 25°dip 239 ° ~,-~.

-1.50 25°dip 275 ° Sp 25°dip 300 ° ~Sp Sp(Pe) 30°dip 10 °

-1.75 7°dip 354 ° ~ Sp(pe) 25Odip 300 ° Sp 5°dip 228 ° -2.00 ~,~ 35°dip 318 o -2.25

(a) (b) (c) (d) (e)

0 Sh 0.25 $h -. 0,50 Gm ~_~ 20°dip 230 ° Gm Sp/ep "'" ~:l~0Odip~- 280 ° (m) 0.75 Pe) 1.00 1.25 Magnetic bearing 1.50

5 14 19 25 (m) FIG. 5. Vertical sedimentary profiles of sections on 'Bar M' of 'Sandur l', Kangerlussuaq, west Greenland, in response to repeated limno-glacial j/Skulhlaups of ca 1080 m 3 s-~. See Fig. 4 for location of Bar M. (A) Proximal edge of distal limno- glacial bar, (B) Mid-bar location between several chute channels (from Russell, submitted).

sediment concentrations of between 70 and 90% by weight and decaying rapidly downstream to form distal flood (ca 47-90% by volume), possess bulk densities of 1.8- regime fluvial deposits (e.g. Lawson, 1982; see below). 2.3 g cm -3, and exhibit non-Newtonian behaviour (Costa, 1984). Water and sediment move at the same velocity, following the Coulomb-viscous model, in which particles Composite .flows are supported primarily by sediment cohesion and buoyant A single flood event may well exhibit a range of flow forces, as well as dispersive stresses and structural support conditions, and hence produce a range of associated from intergrain contacts. All sediment sizes may be depositional sequences that vary both spatially and present, resulting in a very poorly sorted deposit (sorting temporally. Common temporal changes include the coefficients of between 3.6 and 12.3). On flow cessation, occurrence of fluid 'intersurge' flows (e.g. Pierson, debris flows may 'freeze' en masse to produce massive, 1981) in which fluid bedforms alternate with more poorly sorted, structureless, non-graded, matrix-supported massive or inversely graded, coarse elastic sediments deposits. Such highly viscous, cohesive sediments deposited by the main flood 'surge'. A surge is defined commonly form distinct lobate forms, with lobe height, here as a turbulent, hyperconcentrated flow in which morphology and maximum clast sizes related to strength of sediment and fluid move en masse. The 'freezing' of the debris, bedslope, and grain concentration (Johnson, sediments as water and/or sediment supply diminish can 1970; Costa, 1984; Nemec and Steel, 1984). Debris flows result in reverse grading, rafted isolated large clasts, and require relatively high gradients (e,g. 0.045-0.310; Costa, laminations due to late-stage shearing during deposition 1984) to sustain motion, and hence occur only locally in (Carter, 1975; Postma, 1986). J6kulhlaups may also proglacial areas, normally confined to ice-proximal sites comprise pre-surge, surge and post-surge stages, with J. Maizels: JSkulhlaup Deposits in Proglacial Areas 805 the main flood surge characterised by hyperconcentrated Each bar type is characterised by distinctive sediment flows, with watery flows occurring in the early and late associations. stages of the flood (e.g. Harrison and Fritz, 1982; Pierson Expansion bars are associated with deposition in areas and Scott, 1985, 1988a, b; Maizels, 1991, 1993a). Rapid of flow deceleration, especially at the junction between deposition of debris flow sediments releases excess water steep, confined reaches and wide, low gradient reaches. from a breakout point, leading to transformation down- Many large bars have been used as indicators ofjrkulhlaup stream into hyperconcentrated streamflow or fluid deposits outwash, while large-scale cross-bedding has been (e.g. Lawson, 1982). Dewatering of proximal hypercon- interpreted as representing the foreset bedding of centrated flow deposits can also lead to distal incision of migrating bar forms (e.g. Allen, 1982). Expansion bars outwash by fluvial 'runout' flows. formed the largest type of bar deposit generated by the Lake Bonneville flood which drained along the Snake River course (Malde, 1968; O'Connor, 1993). The bars are MORPHOLOGY AND SEDIMENTOLOGY OF up to 5 km long, and exhibit steep foresets (lithofacies J()KULHLAUP DEPOSITS Gxp; see Table 2) and trough and fill structures (Gxt) Major Depositional Landforms capped by up to 3 m of horizontally bedded winnowed, armoured, imbricated boulders up to 4 m in diameter The largest depositional landforms that may be (Blg), forming a distinctive 'B2' lithofacies sequence produced by jrkulhlaups include outwash fans, valley (Table 3). Longitudinal bars, also located downstream of sandurs and sandur plains. The coastal plains of southern constrictions, were found to extend for some 3 km parallel Iceland (Fig. 3A, B), parts of southwestern Alaska, and with the main channel. Downstream of 'Mile 460' gravel western Greenland (Fig. 2A, B) and Baffin Island, for bars are littered with boulders up to 3 m in diameter, and example, contain some of the most extensive sandurs and reach up to 90 m in height (Jarrett and Malde, 1987). valley sandurs and many, if not all, are subject to periodic The internal stratigraphy of bar deposits that accumu- jrkulhlaup events generated by a range of different lated during the Bonneville Flood also reflects deposi- mechanisms (e.g. Church, 1972; Fahnestock and Bradley, tion at different stages of the jrkulhlaup, namely, 1973; Boothroyd and Ashley, 1975; Maizels, 1991; during the main flood peak followed by waning stage Russell, 1992). Pleistocene outwash plains were more aggradation. O'Connor (1993) describes bar deposits extensive and bounded the margins of many segments of from parts of the flood routeway which exhibit large- the Laurentide and Eurasian ice sheets, where ice- scale foresets (Gx), with individual sets 0.25-1.5 m dammed lake drainage was often catastrophic (e.g. thick, and trough-and-fill structures (Gxt) tens of metres Kehew and Lord, 1987). Jrkulhlaup landscapes exhibit wide. This unit is overlain by 1-3 m of coarser-grained a wide variety of large-scale erosional forms as well as but better sorted, horizontally bedded gravels containing depositional features, such as the hundreds of stream- imbricated boulders up to 10 m in diameter, forming lined, residual hills, longitudinal grooves and giant a Type B2 vertical profile (Table 3). This sequence potholes, scours and plunge pools that characterise the was interpreted by O'Connor (1993) as representing Channelled Scablands of the Columbia Plateau, Wa- rapid downstream accretion of large-scale bar fronts shington State (Baker, 1973a, Baker, 1973b), and the during the main highly turbulent flood wave, followed complex systems of huge spillway channels that lead by rolling and winnowing of bar top material to form away from all the major glacial lake basins bounding the an armoured surface of imbricated cobbles and boulders southern edge of the Laurentide Ice Sheet (e.g. Kehew during recession flow. and Lord, 1987). The main depositional zones contain a Type B2 profiles have also been described from range of macro bedforms (bars) and meso bedforms expansion and point bars on a west Greenland valley (dunes), as well as 'washed' sandur spreads and boulder sandur subject to repeated j6kulhlaup events. The bars are lags; pitted or kettled outwash and fields of ice-block composed of large-scale sand and gravel cross-bedded obstacle marks; steep, lobate and hummocky proximal units overlain by coarse cobble-boulder lag (Fig. 2A, B, fans; and a variety of distinctive sediment assemblages Figures 4-6; and see Russell, 1992, and Russell, (see summary in Maizels, 1995). submitted). The sequence was attributed to the downstream migration of gravel bars during peak flows, followed by high energy sheet flows which initiated Macroscale Bedforms: Bar Forms winnowing of the bar surface to create a coarse cobble- The terminology used to describe bars is confusing and boulder lag. ambiguous, especially since bars may be depositional or Pendant bars are deposited in zones of localised flow erosional in origin, and reflect formation during a single expansion or deceleration, particularly downstream of flood event or reworking during numerous flow events. obstacles which lie within the main flow path. Large The terminology used here follows that adopted by Baker scale pendant bars have been identified in uniform steep (1973a) for the Missoula flood deposits. gradient channels of the Washington scablands drained The medium-scale depositional landforms associated by the Missoula floods. Here, pendant bars are up to with jrkulhlaup outwash include expansion bars, pen- 3.2 km long and extend up to 30 m above the valley dant bars (downstream of bedrock obstacles), point bars floor (Baker, 1973a). Similarly, along the route fol- and eddy bars, as well as streamlined erosional bars. lowed by the Lake Bonneville flood in Idaho, narrow 806 Quaternao, Science Reviews: Volume 16

FIG. 6. Large-scale, imbricated boulder deposits forming a point bar along a j6kulhlaup routeway at the edge of the west Greenland ice sheet, and associated with j6kulhlaup discharges of ca 1080 m 3 s 1. Material is derived from erosion of gneissose bedrock from a gorge section lying a short distance upstream (see also Russell, 1992: Russell, submitted). Flow was from right to left. steep crested pendant bars up to 2 km long, bounded adopted for these features, and they are also known by a by deep scour troughs, were identified by O'Connor variety of terms including giant 'current ripples', °meg> (1993). ripples', 'whaleback dunes' or 'gravel waves'. The scale Eddy bars are formed at the mouth of tributary valleys of these features is largely dependent on flow depth and where backwater eddies are created in the tributary flow, size of material available for entrainment. leading to deposition of sediments derived from two Giant dunes formed in zones of flow expansion during different sources: from tributary flow and from marginal catastrophic drainage of Lake Missoula, where over 100 flow in the main channel. Eddy bars contain a wide 'giant ripple' trains have been identified, most of them variety of sediment sizes and structures, with interfinger- concentrated on proximal ends of riffles (Pardee, 1942; ing between poorly sorted boulder gravels, cross-bedded Baker, 1973a, b, 1984). The dunes exhibit huge gravel granule gravels, graded sand-silt layers and laminated foreset beds comprising alternating beds of granules, silts. The foresets indicate a range of flow directions, for, gravels and pebbles, with the stoss slopes mantled with an while most foresets dip away from the main channel armour of cobbles and boulders up to 1.5 m diameter (a (especially when deposited by j6kulhlaup flows), others Type B2 profile: Table 3). The ripples exhibit heights and dip towards the main channel, reflecting fluctuations in wavelengths of ca 15 m and 150 in, respectively, and backwater flows, eddies and back-flow currents (Baker, were associated with maxirnum j6kulhlaup velocities 1984). Eddy bars can extend upstream to form sequences exceeding 30 m s l and discharges of nearly of slack-water deposits (see below). 14x10 ~ m 3 s 1; Baker, 1973a, b, 1984: Craig, 1987), In addition, streamlined boulder bars can also be Large-scale dunes have also recently been identified as formed by erosion of earlier outwash deposits during the result of cataclysmic j6kulhlaup drainage during waning stage j6kulhlaup flows. On Myrdalssandur, for failure of an ice dam in the Chuja River valley in the example, bars up to 4 km long and mantled with a variety Altay Mountains, Siberia (Baker et al., 1993: Rudoy and of kettle forms, were formed by waning stage erosion into Baker, 1993). Carling (1996a) has studied the morphol- sheet deposits covering the whole sandur (Fig. 7: Maizels, ogy, sedimentology and palaeohydraulic implications of 1992, 1993a). six major dunefields in the Chuja valley, the largest (the Kuray dunefield) extending over a 24 km tract. Accord- ing to Carling, the dunes comprise unimodal, openwork, coarse gravel locally filled with small amounts of sand or Meso-scale Bedforms: Giant Dunes granule matrix. Median b-axis particle sizes average The most distinctive large-scale bedforms that are 35 mm, while cobbles and boulders up to 0.5 in diameter produced during catastrophic j6kulhlaup drainage are are common. In addition, large tabular blocks over 2 m in large-scale dunes. No standard terminology has been diameter occur on the stoss or crests, either as isolated J. Maizels: J6kulhlaup Deposits in Proglacial Areas 807

i~ 'i ¸ ~¸

FIG. 7. Hummocky surface of erosional bar on volcano-glacial outwash, Myrdalssandur, south Iceland. The j6kulhlaup occurred in 1918 AD, and generated hyperconcentrated flows up to l0 6 m 3 s ~. The bar is mantled with gravel dunes, 'rimmed kettles' and 'till-fill' kettles. Flow was from right to left. Note Katla ice cap in background, and Land Rover for scale. blocks or in clusters, surrounded by horseshoe scour thick (Gxp) (unit 4) (Fig. 9), generally exhibiting normal hollows. grading within each set. The basal sands and gravels (unit Giant dunes have also been identified on the bars and 1) were interpreted as pre-flood braided outwash flood terraces produced by j6kulhlaups from Neoglacial associated with formation of longitudinal and transverse Lake Alsek, Yukon (Clague and Rampton, 1982). The bars (Type A1 profile). By contrast, the cross-bedded lunate and long-crested dunes are composed of pebble, gravels (unit 4) were considered to represent large-scale cobble and boulder sized material. They exhibit wave- equilibrium bedforms, exhibiting tangential basal con- lengths up to 200 m long, and heights of >5 m, while tacts, in turn associated with the rapid accretion of 3- erratic boulders were found to have been rafted in ice dimensional dunes, with gravel transported in at least blocks for many kilometres downstream. Smaller scale intermittent suspension past the point of flow separation. 'megaripples', up to 0.5 m high and 10.5 m wavelength, Fraser and Bleuer (1988) argue that the discrete upward are found on Solheimasandur in southern Iceland, coarsening and thickening of the trough infill sediments associated with volcano-glacial j6kulhlaup discharges of (unit 3) at the base of the flood gravels would have ca 10 4 m 3 s J(Fig. 8; cf. Fig. 7; Maizels, 1989a, b). required several days of slowly increasing flow depth, Sediment sequences containing large-scale cross- velocity and sediment availability before reaching the bedded structures related to an extreme flood event have flood peak, which generated the main large-scale dune been described by Fraser and Bleuer (1988) from the Late systems. The j6kulhlaup is likely to have declined rapidly Wisconsinan Wabash valley in Indiana, which carried from flood peak since set thickness declines upward, meltwater drainage from the Lake Erie basin. These reflecting lower flow depths and sediment fluxes as the sequences correspond to Type B3 profiles. At the channel aggraded. The absence of a significant decline in Tippecanoe Fan, where meltwater that had been stored grain size upwards reveals that flow velocities remained in disintegrating ice to the north was rapidly released, the fairly constant until waning stage flows, when some local sequence comprises (1) basal cobble gravels exhibiting reworking took place to produce fine-grained trough crude horizontal bedding and grading up into cross- cross-sets at the top of the sequence. Elsewhere, sand bedded sands and gravels, overlain by (2) a boulder lag content increases upwards. The full sequence therefore and (3) a series of troughs infilled with an upward resembles a Type AI profile (unit 1) overlain by a Type coarsening and thickening sequence of successive sets of B3 profile (see Table 3), in which the characteristics of climbing ripples, plane beds, festoon trough sets, through both the rising and falling limbs of the flood hydrograph to pebbles and cobbles (St, Suc, Guc, Gt, Gh). The main could be inferred from variations in the grain size and part of the sequence comprises up to 30 m of large-scale scale of sedimentary structures through the vertical cross-bedded cobble gravels forming megasets up to 5 m profile. 808 Quaternary Science Reviews: Volume 16

FIG. 8. Pumice granule 'mega-ripples' on the slopes of a glacio-volcanic j6kulhlaup channel, Skogasandur fan, south Iceland. The flood occurred ca 1500 BP, reaching a peak discharge of ca 8000 m3 s l (Maizels, 1989a). Flow was from left to right in the picture.

Slack-water Deposits rhythmites are composed of laminated silt and clay couplets, 1-2 cm thick, which exhibit internal normal Where a tributary valley lies at a high angle to the main grading from basal very fine sand to silt. The rhythmites flood channel, the valley can act as a sediment trap during are interbedded with flood beds, 10-20 cm thick, which backwater flooding to produce distinctive sediment suites also exhibit clear upward fining grading. In proximal known as 'slack-water sediments' (Baker, 1973a, 1983; zones, the basal horizon comprises graded structureless Patton et al., 1979) or 'backflood deposits' (Atwater, gravel, overlain successively by horizontally laminated 1984). These sediments can form upstream extensions of very coarse to medium sand; ripple cross-laminated and eddy bar deposits or merge with glaciolacustrine facies climbing ripple cross-laminated coarse to fine sand; and where the main valley remains ponded for long periods of parallel or horizontally laminated very fine sand and silt. time. The elevations of successive flood deposits within There is some evidence of load casting, deformation and slack-water sequences have been widely used in recon- slumping, and ice-rafted glacial erratics. This sequence struction of the hydraulic gradient and palaeodischarges closely resembles a typical Bouma sequence deposited by of associated j6kulhlaups (e.g. see methodology in a marine turbidite, suggesting that these slack-water Kochel and Baker, 1988; Baker et al., 1983; O'Connor sediments represent rapid deposition from dense sediment and Webb, 1988). gravity flows during flood surges up the backwater The most detailed analyses of slack-water deposits tributary valleys. A distinctive feature of these flood beds have been undertaken in tributary valleys of glacial Lake is that they become finer grained and thinner upvalley, Columbia, which formed the main receiving basin of with cross-beds dipping upvalley, reflecting the direction floods from glacial Lake Missoula during successive of flood surges away from the main channel (Baker, Pleistocene glaciations, and particularly during the last 1973a, b; Patton et al., 1979; Baker and Bunker, 1985). glacial phase from ca 15.5 to 13.35 ka BP (Baker and The interpretation of slack-water deposits is not Bunker, 1985; Waitt, 1985). Atwater (1984), Atwater straightforward, however, since rhythmites may not (1986) identifies two types of slack-water deposit from necessarily represent a single flood event. Many j6kulhla- the Sanpoil tributary: slack-water rhythmites or varves ups exhibit multiple peaks and the irregularities of the (Type A3 profiles; Table 3) and flood beds (Type A4 flood routeway can act to modify successive flood waves profiles; also named 'Touchet Beds'). The slack-water to produce a highly complex hydrograph, such that a J. Maizels: J6kulhlaup Deposits in Proglacial Areas 809

Graphic Percent Interpretation log composition Boulder Deposits 50 lO0 Many j6kulhlaup channelways are distinguished by the presence of boulder fields and pavements, 'washed' 14 O 0 °..."'. Explanation O~ °0 * o..... outwash and till surfaces, boulder lags, armoured 3 0 e°'.'l '.. O0 *,' =m '. Cobbles boulder-strewn terraces, and boulder erratics. Boulder Food deposits C @ ° 0 '. deposits have been recorded from many of the spillway formed under 12 0 0 °l" *" • ".' • channels which carried j6kulhlaup meltwaters from 0 0 * P, ° ''' conditions Coarse pebbles of deep, the Lake Agassiz basin (Matsch, 1983; Teller and powerful flow Fine pebbles Thorleifson, 1987), Lake Nipigon (Teller and Thorleif-

10 son, 1983) and Lake Wisconsin (Clayton and Attig, Granules 1987) where palaeoflood discharges ranged from ) 0 ,el" • .'. ". 0 • • Q. • 3x103 to 105 m 3 s -]. Boulder bars are also found in the Sand Missoula and Lake Bonneville flood routeways (Baker,

Silt and clay 1973a; O'Connor, 1993), on the Icelandic sandurs (Fig. 7; Maizels, 1992), on west Greenland j6kulhlaup bars (Fig. 6), and form boulder lags within j6kulhlaup Outwash - fan deposits sediment sequences (e.g. Fraser and Bleuer, 1988). 6 formed under conditions Extensive j6kulhlaup boulder deltas described from of episodic flow northern Sweden exhibit discrete boulder lobes, stream- in a proximal ice setting lined scour marks, residual till remnants, and incised channel networks (Elfstr6m, 1987). Boulder berms, linear accumulations and clusters of large imbricated

Valley - train deposits boulders, are also found along the margins of many formed under cobble bars, forming the edge of the zone of flow 2 ~ conditions separation during j6kulhlaup floods (e.g. Russell, of episodic flow in a distal setting 1992, Russell, submitted).

FIG. 9. Stratigraphic sequence at Attica, Indiana, showing Kettle Holes and Ice-block Obstacle Marks internal structures and grain-size distribution of j6kulhlaup deposits and underlying valley-train and outwash-fan sediments A major feature of glacier burst floods is the transport resulting from rapid drainage of water stored in the disintegrat- of large ice blocks through the proglacial meltwater ing Laurentide ice margin to the north (from Fraser and Bleuer, system and subsequent deposition on to bars within the 1988). outwash zone. Two types of associated small-scale landforms can develop, depending on flow conditions, local bar topography and bar and ice block sedimentol- single lake drainage event may be represented in the ogy. The most common feature is kettle holes, sometimes sedimentary record by more than one rhythmite horizon forming pitted sandur areas where dead ice-marginal ice (Baker, 1973a; Baker and Bunker, 1985; Maizels, 1997). has been buried and gradually melted out to form an Nevertheless, making assumptions about the frequency of irregular topographic surface. However, most pitted varve formation and flood bed occurrence allowed Waitt outwash may well result from the melting of huge (1985) to suggest that over 40 floods occurred during the numbers of ice blocks transported by j6kulhlaup flows last glacial phase, i.e. about every 20 to 60 years over a (Russell et al., submitted). Large, buoyant blocks of ice 3000-4000 year period, but with flood frequency are transported downstream (especially by hyperconcen- increasing towards the end of glaciation as the ice sheet trated flows; see 6. below) into zones of decelerating and thinned. By contrast, Atwater (1984), Atwater (1986) expanding flow where they become stranded on areas of identified at least 89 graded flood beds between 2000 to maximum elevation, namely bar heads and channel 3000 rhythmite beds. Arguing that each represented a margins. Where debris-rich ice blocks have melted out, separate flood from glacial Lake Missoula over the period bar deposits may be covered with kettle holes exhibiting 13.35-15.55 ka BP, he found that backflooding occurred debris rims or may be filled with till to form mounds every 35-55 years, with flood magnitude decreasing with (Figures 7-10; Maizels, 1992). Where flow continues increasing frequency of flood occurrence. Varve fre- around the ice block, streamlined scour or obstacle marks quency increased in the middle of the stratigraphic can develop (Fig. 11). Russell (1993) found, for example, sections, indicating that while flood frequency declined that during the waning stages of a recent j6kulhlaup in (from every 25-40 years to every 50-55 years), flood west Greenland, ice blocks were deposited along the magnitude increased during the middle part of the glacial edges of main channels and on the stoss sides of the main phase, when the ice sheet was thicker. The final stages of bar units of a valley sandur. As in the case of normal rock ice thinning and lake drainage were associated with obstacles, Russell demonstrated that the size of ice blocks j6kulhlaup recurrence intervals reduced to only every 1-2 entrained and transported by j6kulhlaups is related to the years (Baker and Bunker, 1985). transport capacity and depth of flow. The relationships 810 Quaternary Science Reviews: Volume 16

Type 1: 'Normal' kettle hole Type 2: 'Rimmed' kettle Glacio-volcanic J6kulhlaup Outwash

Glacio-volcanic j6kulhlaup outwash exhibits a distinctive suite of vertical lithofacies sequences, reflecting high OoO o ~'b oC) o sediment concentrations during peak flows, and marked changes in sediment characteristics associated with changes in flow conditions and sediment fluxes during Type 2: 'Crater' kettle Type 4: 'Till-fill' kettle the course of the j6kulhlaup. Although numerous examples of volcanic fluvial deposits have been recorded (Lajoie, 1984), the few glacio-volcanic examples available suggest that many similar features are present. Many of the huge sandurs of south Iceland, for

.. o.. o • • o . -o --_- example, are of glacio-volcanic j6kulhlaup origin. The o o,:,o - o -,,o -ooo 0 0 0 ~::~ °0¢~o ~ ° 0 j6kulhlaups are characterised by extremely large volumes FIG. 10. Classification of j6kulhlaup kettle-hole forms shown of meltwater (up to 6 km~), much of which is stored in in cross-section, based on debris content of the original ice subglacial craters prior to catastrophic drainage (Thorar- bh)cks, ranging from clear ice blocks (Type I) to debris- insson, 1957; Bj6rnsson, 1988). The glacio-volcanic saturated ice blocks (Type 4) (from Maizels, 1992). sandurs largely comprise a massive, black pumice granule lithofacies at least 8 m thick, with very little variation in median grain size (0.75-2.83 mm) and percentage fines (below 3.5%) through the vertical sequence (Fig. 12A; found for the ice blocks closely reflected relationships Jonsson, 1982: Maizels, 1989a, Maizels, 1989b, Maizels, that have already been established between (i) the size 1991, Maizels, 1992, Maizels, 1993a, Maizels, 1995). and shape of normal rock obstacles, and (ii) flow depth This type of massive granular outwash is defined here as a and flow direction (e.g. Allen, 1984). Ice-block obstacle Type F vertical profile (Table 3; redesignated from 'Type marks are also characterised by distinctive sedimentol- A' in Maizels, 1991, Maizels, 1993a). Five sub-groups of ogy, as coarser grained material accumulates in the Type F profiles, from F5 to FI, can be distinguished, shadow ridge downstream of the obstacle, while fines are exhibiting a progressive increase in internal structure, the deposited in its immediate wake. presence of boundary laminations, and improved sorting.

FIG. 11. Ice-block obstacle marks in a zone of flow expansion on a lateral bar of the Gigjukvisl River, Skeidararsandur, south Iceland, as it exits the confines of Neoglacial moraine deposits. These features formed during j(~kulhlaup l]ows probably generated from sudden drainage of Grimsv6tn. Flow was from right to left. J. Maizels: J6kulhlaup Deposits in Proglacial Areas 811

These profiles are interpreted as representing hyper- into contact with bed obstacles such as rock or till concentrated grain flow deposits containing varying protuberances, older substrate deposits, or ice blocks, proportions of coarse clasts entrained from along the leading to deceleration and rapid dewatering, while flow path, with the continuum from F5 to F1 profiles marked deformation of bedding can occur during rapid representing an increase in fluid flow and boundary shear deposition and settling of the flood deposit. and traction transport of grains. They are regarded as forming during peak flow or surge conditions when sediment concentrations were at a peak. Deposits were SPATIAL VARIABILITY IN LITHOFACIES rapidly 'frozen' in situ following rapid dewatering of the DISTRIBUTIONS highly porous pumice medium. Proximal-distal Changes The most widespread sequence forming the Icelandic glacio-volcanic sandur plains is Type E4 (Fig. 12B). Proximal-distal variations in sedimentology are nor- These typically comprise up to 5 m of the massive mally highly distinctive in proglacial fluvial systems. granule lithofacies underlain by crudely bedded gravels, Type I and Type II sandurs (Table 1) are characterised by and overlain by up to 3.5 m of trough cross-bedded strong downstream fining associated with a transition granules, capped by thin, horizontally bedded sands and from crudely horizontally bedded, proximal gravel beds granules. Type E4 profiles are interpreted as representing associated with longitudinal bars to more distinctly plane- a four-stage j6kulhlaup event comprising pre-surge and cross-bedded sand and gravel beds produced by reworking of sandur gravels, an hyperconcentrated flood migrating sand bars and fluvial dune and ripple systems in surge, and post-surge waning stage flows associated with distal areas (e.g. Boothroyd and Ashley, 1975; Boothroyd rapidly migrating dunes, followed by shallow sheet flows and Nummedal, 1978; Miall, 1978; Rust, 1978). Bed (Maizels, 1989a, b, 1991, 1992). thickness also declines markedly downstream. A range of graded granule lithofacies also occurs Marked contrasts in sedimentology were identified by within the Icelandic glacio-volcanic sandurs (Type D Russell (1992), Russell (submitted) over a 1 kin-long lithofacies sequences; Table 3). Normally graded granules valley sandur in west Greenland which is subject to (Type D1) are interpreted as a waning stage deposit repeated j6kulhlaups (Fig. 2A, B). The proximal zone is associated with declining competence; repeated, normally characterised by boulder berms, cobble sheets, poorly graded cyclic sequences (Type D2) are interpreted as sorted channel fills, composite lateral accretion deposits, representing deposition during the waning stages of erosional channels, and ice-block grounding structures. successive flood waves of a single j6kulhlaup; normal- Vertical lithofacies sequences exhibit distinct Type H3 inverse (Type D3) and inversely graded lithofacies (Type profiles, with Gp and Sp lithofacies overlain by massive D4) are interpreted as hyperconcentrated grain flow gravels, and capped by an imbricated cobble lag (Fig. 5). deposits in which dispersive stresses promoted the This sequence is interpreted as representing bar front upward migration of the coarser clasts towards the migration during each successive j6kulhlaup, followed by surface. In both the latter types, the whole sediment unit waning stage winnowing. The distal area is divided into was moving as a single mass, and was deposited rapidly an upstream free-flow zone, and a downstream backwater on dewatering. zone. In the former, lateral and downstream migration of On steeper, ice-proximal slopes the glacio-volcanic sand and gravel bars generated numerous Gp units sandur deposits may contain clay-rich admixtures of separated by reactivation surfaces, and overlain by sand matrix-supported, heterogeneous clasts, interpreted as and gravel sheets and local fields of megaripples. As the debris flow deposits (Type G deposits; Table 3; and see bars reach the deeper water of the backwater zone, planar Eyles and Miall, 1984). These were probably generated cross-stratification increases in scale. Within the back- by high inputs of sediment from newly expanded, debris- water zone, bars become lobate with tabular foresets and rich glacial tunnelways occupied by the rising flood wave steep planar cross-stratification associated with bar fronts, (cf Jackson, 1979). However, because sandur slopes are deltas or lobes forming at the mouths of chute channels relatively low (generally below 0.03), and meltwater incised into the bar front. Coarse-grained lags result in volumes so high, debris flow deposits are rare beyond the distinctive Type B2 profiles. ice-proximal zone. Glacio-volcanic outwash (Type III sandurs) may A number of sandur sites also contain highly deformed exhibit ice-proximal lobes or fans deposited by debris sediments (Type H profiles; Table 3), locally exhibiting or hyperconcentrated flows, and dissected by later stage steeply dipping crude bedding planes, folds, overfolds fluid flows cutting deep flood channels. More distal zones and faults. Large masses of diamicton and compacted are characterised by more fluid flows, leaving areas of gravels may be included along rising shear planes and 'washed' sandur, streamlined residual bar forms bounded incorporated into higher lenses of the deposit. Imbrication by erosional channels and terrace sequences, and fields of is distorted. Type H sediments are interpreted as dunes, kettle holes and hummocky 'till-fill' kettles j6kulhlaup deposits that have been disturbed during (Fig. 7). Downstream fining and thinning occurs mostly over-riding by, or convergence with, subsequent pulses within the fluid flow lithofacies of the multistage outwash of flood flows, by entrainment of eroded bed, bank or ice- deposits, while the hyperconcentrated flow lithofacies, rafted materials, or by melting of incorporated ice masses. representing the main flood surge, shows minor fining Such situations arise where sediment-rich flows come and thinning out downstream, but with some increased 812 Quaternao' Science Reviews: Volume 16

(A)

(B) FIG. 12. (A) Massive, homogeneous pumice granules that form the bulk of the glacio-volcanic sandur deposits of south Iceland. This is taken from a typical Type E vertical profile (see Table 3). (B) Vertical section ca 8 m high exhibiting a typical Type E4 vertical lithofacies profile characteristic of the glacio-volcanic sandurs of south Iceland (from Maizels, 1991). Basal bedded pre-surge gravels are overlain by thick, massive Type F flood surge granules, capped by trough cross- bedded and horizontally bedded post-surge granules. J. Maizels: J6kulhlaup Deposits in Proglacial Areas 813 development of internal laminations (cf Scott, 1988a, outwash comprises massive inverse-normally graded Scott, 1988b; Maizels, t995). cobble or boulder gravels (Type D5 profile), reflecting deposition during both the rising and falling limbs of the j6kulhlaup hydrograph. Lateral Variations Where flows become increasingly hyperconcentrated Strong lateral variation in vertical lithofacies types (Type III outwash), deposits may form ice-proximal lobes occurs within sandurs where j6kulhlaup flows have and fans, with more fluid sheets spreading across encountered a range of different boundary conditions. downstream sandur plains. Sediment sequences become On the west Greenland valley sandur (Type II outwash), dominated by Type E4 profiles, in which the record of the for example, Russell (1992), Russell (submitted) identified full hydrograph may be preserved. Thin, pre-surge basal finer-grained lateral zones where bedding associated with gravels incorporating rip-up clasts of soil and older thin sheet gravels and trains of megaripples dip away from outwash, are overlain by thick, massive, hyperconcen- the main channel (Figures 4 and 5). Some lateral sites were trated surge gravels or granules. These are truncated by characterised by upward fining sequences associated with post-surge trough cross-bedded units representing rapidly vertical bar growth (Type A1 profile). On Myrdalssandur aggrading and migrating dunes, capped by waning stage, (Type IlI outwash), Maizels (1993a) identified distinctive horizontally laminated sands and silts. Downstream and vertical lithofacies sequences associated with zones of lateral variations in flow conditions lead to limited ponded water; expanding, constricted and converging deviations from this characteristic sequence. Most flows; flows across, and emerging from, a partially buried commonly these take the form of normal-inversely graded lava field; and flows around bedrock obstacles. (Type D3 profiles) and inversely graded (Type D4 profiles) gravels or granules associated with more cohesive flows. MODELS OF PROGLACIAL J()KULHLAUP At high sediment concentration runoff may also take OUTWASH DEPOSITS the form of ice-proximal debris flows, leading to localised deposition of lobes and tongues of massive, matrix- It is clear from this review that the contrasts in supported, heterogeneous gravels, including erratics, sedimentology between j6kulhlaup outwash and 'normal' former ice blocks, flow structures and rip-up clasts (Type braided river outwash are so great that distinctive models G profile). Similarly, outwash deposits containing of j6kulhlaup outwash deposits need to be firmly deformed bedding, erratics, rip-up clasts, and masses of established. diamicton (Type H profile) result from over-riding of successive flood waves or entrainment of erodible substrate materials. Finally, fine-grained sands and silts Lithofacies Types can accumulate locally from suspension, or as turbidity Building on earlier work by a number of authors, currents, in zones of slack water, in backwater reaches including Maizels (1989a, b, 1991, 1993a, b), a model is and areas of temporarily ponded water. These sediments presented here that represents the relationship between include both finer-grained rhythmite horizons and inter- the different j6kulhlaup outwash deposits described in bedded coarser-grained 'flood beds' (profile Types A3 this paper and (l) the nature of the flow (horizontal axis), and A4, which can be found in all outwash types). and (2) the sizes of sediment available (vertical axis) (Fig. 13; modified from Maizels, 1989b) In turbulent and macroturbulent fluid flows of Types I Landscape Characteristics and II outwash, the most common deposits are those of expansion bars, and locally of eddy and pendant bars. Landscape models for Types II (Fig. 14) and III Expansion bars in Type II j6kulhlaup outwash are j6kulhlaup outwash (Fig. 15) indicate some of the normally composed of large-scale gravel-cobble cross- relationships so far established between landforms, bed- beds associated with downstream migration of the forms and sedimentary sequences for j6kulhlaup depos- accreting bar front, sometimes capped by an imbricated its in proglacial areas. The landscape model for Type boulder armour layer, to form a Type B2 vertical profile II outwash (limno-glacial j6kulhlaup outwash; Fig. 14) (Table 3). J6kulhlaup flows transporting high sediment highlights the formation of large-scale expansion bars loads can mantle outwash plains and proximal bar downstream of a constricted channel zone, such as a deposits with large-scale gravel dunes, also composed lake drainage spillway. The bar deposits are composed of large-scale cross-bedded units. According to the rate of of upward coarsening, crudely bedded gravels, cobbles decrease in flood discharge, dune sequences may or may and boulders, mantled with an imbricated boulder not be capped by a coarse-grained armour layer (Type B2 lag, waning stage sand and silt horizons, or distal ripple and B 1 profiles, respectively). trains, and bounded by large-scale cross-bedding at Type II j6kulhlaup deposits characteristically contain the bar front. Kettle holes and pitted outwash character- large percentages of cobble and boulders, and can form ise parts of the bars, together with ice block obstacle extensive boulder fields, boulder pavements, crudely marks and chute channels. Trains of giant dunes horizontally bedded, imbricated boulder beds, or boulder represent zones of deeper, faster water transporting high lobes (Type C profiles). In coarse sediments, j6kulhlaup loads of sand and gravel, and associated with large- 814 Quaternary Science Reviews." Volume 16

LAMINAR< ~TURBULENT CHARACTERISTICS 100%¢ HIGH ¢(70%) SEDIMENT CONCENTRATION (40%) LOW. :~(0%) OF

FLUID - SEDIMENT NON - NEWTONI~INGHAM ,,,NEWTONIAN DFLUIDAL MIX COHESIVE(~., D COHESIONLESS VISCOUS, HIGH YIELD STRENGTH~:~ NON -VISCOUS, LOW YIELD STRENGTH

FLOW TYPE DEBRIS FLOWS ~____r_HYPERCONCENTRATED GRAIN FLOWS.~FLUID FLOWS

DOMINANT BUOYANCY CLAST- SUPPORT YIELD STRENGTH DISPERSIVE STRESS l/~ TURBULENCE MECHANISM

HYPERCONCENTRATED SURGE DEPOSITS I FLUWALBEDFORMS I A1 POSTSURGE I LAMINATED ...... -'- ~q ~'~ ~ 1 INTERSURGE A ]lm .... - ~ . .--~ DEPOS TS ~ i I/ "".'.'":"" ":'.";".-'., .'.:':~ E3 E4 ~I 1 \ F- :":'~':::~ .'-':::":: ~, ~ ~ ~ IlL c , G ::."::!:."

.- -T:i. il &ss,vE I ':' :.": 4 / ,". o',...'...~'. - ~, ~, POORLY STRUCTURED I • " ~". " ,", :- . • " , l~'~ "o"..;i : [~O " ";I NON-GRADED I I "" "" "":l "'1 . • .: • ; ~--:k::'..% /'/ ~ '. ' .~7~)'-',' ~| WELLSORTED I " 'L" " "": :"/--i"+ :l , ./7 \ ,';'"..'.<~}'~'" "~ SURGE DEPOSITS ~k "'~ i~ :~[ . \ I PRE- SURGE DEPOSIT~'e~r'~(~" "(~ [--::'~':.:~k'~ c f ~ t ~SmVE, v~: ~r~q ,[~ll~ql~ ~:.:o.~;~ r~ MATRIX - RA r~_oo#~-~ ,~) ~,,-~ -~^ '.1~:1 > ~ s-P ..... u-~ 1~6 ' • ,'.-~.:"'~' A1 .. o ~ NON~RADED ~. :..O. ~L°..,~', I °.," "f'~ ....I ]..."C." o .I POORLY '.J.- .~° .~ (~': .'. "L " ~i~'C';'~ I ['-~"~.{ °1[ ' L ¢ / ~ LJ- F" 0'.': °'" "J SORTED . ~'~."o..ooo., 1. • ~%xil~i],:~ ,~.'~.~

/ - k - i:, , ~~ o, .' • :.,~' ~ ,~.:.~1 ~;~c.~.,.~.. c f eel ~'~'.:0' ' "" "'°° °" °' ~ ~"(~"" ~ G~NO~D LY INVERSELY POORLY "' S ' GRADED D4 D3 A1 S~D £3 FLUVIAL POORLY • ~ "' GRAVELS SORTED '0 •°o;#s~.. v ..... +i i":"+ °'°°~

".;-i-:-. I...... _.:_ ~. b;O* 41i~ •~%:.-o .~( c f >,, RISING ) A1 FLOW SEQUENCE ,o, KEY c f ~ SILTS I I :.".'-."~:::+;~,.. SANDS SEDIMENT SIZE GRAVELS COBBLES+ BOULDERS f c SEDIMENT SIZE

FIG. 13. Vertical lithofacies profiles of outwash deposits, classified according to the characteristics of the fluid-sediment mix and relative availability of sediment (horizontal axis) and size distribution of the source sediments (vertical axis). c = coarse-grained materials: f : fine-grained materials• See Table 3 for key to profile notation (modified from Maizels, 1993a). J. Maizels: J6kulhlaup Deposits in Proglacial Areas 815

FIG. 14. Model of valley sandur development in an area of limno-glacial jrkulhlaup drainage downstream of a spillway channel, generating expansion, eddy and pendant bars, trains of megaripples, boulder lags, and large-scale cross-bedding. Key to notation: (1) expansion bar; (2) pendant bar; (3) megaripples or dunes; (4) slack-water deposits; (5) large-scale dune cross-bedding with reactivation surfaces; (6) large-scale bar front cross-bedding; (7) imbricated boulder lag; (8) channel fill deposits; (9) small-scale ripples; (10) chute channels and lobes; (11) kettle holes and kettle fills; (12) ice-block obstacle marks; and (13) wash limit on adjacent valley-side slopes.

FIG. 15. Model of outwash development in an area of volcano-glacial j6kulhlaup drainage, generating lobate fans, streamlined residual hummocks, and hyperconcentrated flood sequences (modified from Maizels, 1991). Key to notation: (1) stacked sequences of multistage massive granular sediments, especially Type E4 sequences (see Table 3 and text); (2) terraced boulder deposits; (3) high level, abandoned sandur surface exhibiting thin gravel horizon and braided palaeochannel networks; (4) 'washed' sandur; (5) lobate fan deposited by hyperconcentrated jrkulblaup flows; (6) incised j6kulhlaup channel with streamlined residual hummocks, boulders and megaripples; (7) hummocky distal j6kulhlaup deposit; (8) streamlined, hummocky, erosional bars mantled with rimmed and till-fill kettles; (9) incised jrkulhlaup channel; (10) incised active meltwater channel; (11) streamlined erosional bars, wash limits and scattered boulders and dune forms downstream of bedrock obstacles. 816 Quaternary Science Reviews: Volume 16 scale cross-bedded units. Eddy bar deposits become REFERENCES finer grained upstream in tributary valleys where they become intercalated with slack-water sediments, while Allen, J.R.L. (1982) Late Pleistocene (Devensian) glaciofluvial outwash at Bane-y-Warren, near Cardigan (West Wales). pendant bars downstream of bedrock obstacles exhibit Geological Journal 17, 3147. streamlined tail deposits. Allen, J.R.L. (1984) Sedimentao, Structures: Their Character The landscape model for Type III outwash (volca- and Physical Basis, Vol. 2. Elsevier. no-glacial j6kulhlaup outwash; Fig. 15) indicates that Atwater, B.F. (1984) Periodic floods from Glacial Lake the more proximal sandur zones are commonly asso- Missoula into the Sanpoil Arm of Glacial Lake Columbia, ciated with debris lobes or hyperconcentrated fan northeastern Washington. Geology 12, 464467. Atwater, B.F. (1986) Pleistocene glacial-lake deposits of the deposits incised by flood channels leaving streamlined Sanpoil River Valley, northeastern Washington. U.S. Geolo- residual hummocks, boulder fields, and trains of large gical Survey Bulletin 1661, 39 pp. ripples or giant dunes. Aggradational bar forms are Baker, V.R. (1973a) Paleohydrology and sedimentology of Lake indistinct or absent, and instead comprise extensive Missoula flooding in Eastern Washington. Geologieal SocieO, areas of 'washed' gravel sandur or finer sediments of America Special Palter 144, 73 pp. dissected by fluid waning flows into erosional bar Baker, V.R. (1973b) Erosional forms and processes for the catastrophic Missoula floods in eastern Washington. In forms. These are mantled with fields of kettle holes, Fluvial Geomorphology, ed. M. Morisawa, pp. 123-148. including 'rimmed' or 'till-fill' kettles to form a pitted Allen and Unwin. or hummocky surface, and bounded by incised channels Baker, V.R. (1978) Paleohydraulics and hydrodynamics of and terraces. The deposits comprise stacked sequences scabland floods. In The Channelled Scabland: Comparative of multistage granular sediments deposited during single Planeta O' Geology. Field Col!ference Guidebook, eds V.R. flood hydrographs, and separated by boulder or gravel Baker and D. Nummedal, pp. 59 79. Columbia Basin, Washington, DC. lags. Baker, V.R. (1983) Paleoflood hydrologic analysis from slack- water deposits. Quaternary Studies in Poland 4, 19-26. Baker, V.R. (1984) Flood sedimentation in bedrock fluvial systems. In Sedimentology of Gravels and Conglomerates, eds E.H. Koster and R.J. Steel, pp. 87-98. Canadian Society of Petroleum Geologists, Memoir 10. SUMMARY AND CONCLUSION Baker, V.R., Benito, G. and Rudoy, A.N. (1993) Paleo- hydrology of Late Pleistocene superflooding, Altay Moun- This review demonstrates that j6kulhlaups do produce tains, Siberia. Science 259, 348-350. distinctive assemblages of landforms and sediment Baker, V.R. and Bunker, R.C. (1985) Cataclysmic late sequences, associated with high stream powers and coarse Pleistocene flooding from Glacial Lake Missoula: a review. sediment supply. 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Lawson, D. (1982) Mobilization, movement and deposition of Miall, A.D. (1983) Glaciofluvial transport and deposition. In active subaerial sediment flows, Matanuska Glacier, Alaska. Glacial Geology, ed. N. Eyles, pp. 168-183. Pergamon Press. Journal of Geology 90, 279-300. Nemec, W. and Steel, R.J. (1984) Alluvial and coastal Maizels, J.K. (1983) Palaeovelocity and palaeodischarge conglomerates: their significant features and some comments determination for coarse gravel deposits. In Background to on gravelly mass-flow deposits. In Sedimentology of Gravels Palaeohydrology, ed. K.J. Gregory, pp. 101-139. John Wiley and Conglomerates, eds E.H. Koster and R.J. Steel, pp. 1-31. & Sons. Canadian Society of Petroleum Geologists Memoir 10. Maizels, J.K. (1987) Modelling of paleohydrologic changes O'Connor, J.E. (1993) Hydrology, hydraulics, and geomorphol- during deglaciation. Geographie Physique et Quaternaire 40, ogy of the Bonneville Flood. Geological Society qfAmerica 263-277. 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Smith, G.A. (1986) Coarse-grained nonmarine volcaniclastic Thorson, R.M. (1989) Late Quaternary palaeofloods along the sediment: terminology and depositional process. Geological Porcupine River, Alaska: implications for regional correla- Society of America Bulletin 97, 1-10. tion. US Geological Survey Circular 1026, 51-54. Smith, N.D. (1985) Proglacial fluvial environments. In Glacial Thorson, R.M. and Dixon, E.J. (1983) Alluvial history of the Sedimentary Environments, eds G.M. Ashley, J. Shaw and Porcupine River, Alaska: Role of glacial-lake overflow from N.D. Smith, pp. 85-134. Society for Economic Paleontolo- northwest Canada. Geological Society of America Bulletin 94, gists and Mineralogists, Short Course Lecture Notes No 16. 576-589. Spring, U. and Hutter, K. (1981) Numerical studies of Tomasson, H. (1974) Grimsvatnahlaup 1972, mechanism and j6kulhlaups. Cold Regions Science and Technology 4, 227- sediment discharge. Jokull 24, 27-39. 244. Tomasson, H., Palsson, S. and Ingolfsson, P. (1980) Comparison Sturm, M., Beget, J. and Benson, C. (1987) Observations of of sediment load transport in the Skeidara j6kulhlaups in j6kulhlaups from ice-dammed Strandline Lake, Alaska: 1972 and 1976. Jokull 30, 21-33. implications for paleohydrology. In Catastrophic Flooding, Vincent, J.-S. (1984) Quaternary stratigraphy of Canada- A eds L. Mayer and D. Nash, pp. 79-94. Allen and Unwin. Canadian contribution to IGCP Project 24. Geological Survey Teller, J.T. (1990) Volume and routing of Late-Glacial runoff of Canada Paper 84-10, 87-100. from the southern Laurentide ice sheet. Quaternary Research Waitt, R.B. (1980) About forty last-glacial Lake Missoula 34, 12-23. j6kulhlaups through southern Washington. Journal of Geol- Teller, J.T. and Thorleifson, L.H. (1983) The Lake Agassiz- ogy 80, 653-679. Lake Superior connection. In Glacial Lake Agassiz, eds J.T. Waitt, R.B. (1984) Periodic j6kulhlaups from Pleistocene Teller and L. Clayton, pp. 261-290. Geological Association Glacial Lake Missoula -- new evidence from varved of Canada Special Paper 26. sediment in northern Idaho and Washington. Quaternary Teller, J.T. and Thorleifson, L.H. (1987) Catastrophic flooding Research 22, 46-58. into the Great Lakes from Lake Agassiz. In Catastrophic Waitt, R.B. (1985) Case for periodic, colossal j6kulhlaups from Flooding, eds L. Mayer and D. Nash, pp. 121-139. Allen and Pleistocene glacial Lake Missoula. Geological Society of Unwin. America Bulletin 96, 1271-1286. Thorarinsson, S. (1957) The j6kulhlaup from the Katla area in 1955 compared with other j6kulhlaups in Iceland. Jokull 7, 21-25.