Sedimentation and stratigraphy at Eyjabakkajökull—An Icelandic surging glacier

QUATERNARY RESEARCH 24, 268-284 (1985)

Sedimentation and Stratigraphy at Eyjabakkajtikull-An Icelandic Surging Glacier

MARTIN SHARP

Department of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, United Kingdom

Received July 6, 1984

A model for sedimentation by surging glaciers is developed from analysis of the debris load, sedimentary processes, and proglacial stratigraphy observed at the Icelandic surging glacier, Eyja- bakkajokull. Three aspects of the behavior of surging glaciers explain the distinctive landform- sediment associations which they may produce: (a) sudden loading of proglacial sediments during rapid glacier advances results in the buildup of excess pore pressures, failure, and glacitectonic deformation of the overridden sediments: (b) reactivation of stagnant marginal ice by the down- glacier propagation of surges is associated with large longitudinal compressive stresses. These induce intense folding and thrusting during which basal debris-rich ice is elevated into an englacia1 position in a narrow marginal zone. As the terminal area of the glacier stagnates between surges, debris from this ice is released supraglaciahy and deposited by meltout and sediment flows; Cc) local variations in overburden pressure beneath stagnant, crevassed ice cause subglacial lodgement tills, which are sheared during surges, to flow into open crevasses and form “crevasse-fill” ridges. 0 1985 University of Washington.

INTRODUCTION EYJABAKKAJOKULL

A significant number of contemporary glaciers experience “surges” (sudden rapid advances or pulses of motion which are apparently unrelated to climatic changes; Meier and Post, 1969), and it has been suggested that parts of some Pleistocene ice sheets may have surged (Wright, 1973). As yet, however, there are no accepted geo- morphic or sedimentologic criteria for the identification of former surges in the geological record. If, however, evidence from glacial geology and geomorphology is to be used as a bsis for the reconstruction of glacial and climatic history, it is important to be able to distinguish between the effects of glacier surges and climatically induced glacier fluctuations. The present study, conducted at the Icelandic surging glacier Eyjabakkajokull, was stimulated by the de- sire to determine whether and why contemporary surging glaciers deposit distinctive landform-sediment associations.

Eyjabakkajokull drains the northeastern part of the Vatnajokull ice cap (Fig. l), de- riving part of its nourishment from the main ice cap and part from accumulation basins in a mountain complex to the east of the main glacier tongue. Three distinct ice streams combine to form this tongue, which is IO km long and 4 km broad at its snout, and which presently lies at an altitude of 700 m. The area around the glacier is un- derlain by basalts and interstratified volcanic sediments.

Eyjabakkajokull has a well-documented history of surging, with advances recorded in 1890, 1931, 1938, and 1972 (Thorar- insson, 1943; Todtmann, 1960; Williams, 1976). During the 1972 surge the glacier advanced nearly 3 km at rates of over 30 m day-’ (Williams, 1976). The 1938 and 1972 surges seem to have affected only the central and eastern flow units, while the western part of the glacier advanced 250 m

268 0033-5894/85 $3.00 Copyright 0 I985 by the University of Washington. All rights of reproduction in any form reserved.

AN ICELANDIC SURGING GLACIER 269

FIG. I. Map showing the landform-sediment complexes at Eyjabakkajokull as they were in lY67. A = sandur; B = fluted lodgement tills with crevasse-fill ridges: C = chaotic, hummocky topography: D = medial moraines; E = ridges of glacitectonically thrust sediments; F = lobate complexes oi ridges of glacitectonically deformed sediments outside the limits of ice advance. Ice margin positions from 1890, 1931, 1938, 1945, 1967. and 1972 are shown, as are the locations of the eight stratigraphic sections shown in Figure 8. Inset: Location map of Eyjabakkajokull.

in 193 I. The eastern part of the glacier re- treated about 600 m between 1890 and 1935 (Thorarinsson, 1943) and 610 m between 1938 and 1953 (Todtmann, 1953). Aerial photographs taken in 1967 indicate con- tinued recession, a pattern which was probably not interrupted until the 1972 surge which took the glacier margin to within 1.5 km of the limit of the 1890 advance. Stag- nation and thinning of the glacier terminus since 1973 has destroyed the intense cre- vassing developed during the 1972 surge.

DEBRIS LOAD OF EYJABAKKAJOKULL

Methods of Debris Description

Two-kilogram samples of the <32-mm fraction of sediments were subjected to particle size analysis by overlapping wet sieve and hydrometer methods. Samples were classified texturally on the basis of the position in which they plot on a gravel-sand- silt and clay ternary diagram, using the classification of Lawson (1979. p. 86). Graphical statistical measures (Folk and

270 MARTIN SHARP

Ward, 1957) were calculated from the size data. Lithology, Powers roundness (Pow- ers, 1953), Zingg shape (Zingg, 193.5), and occurrence of striations on clasts coarser than 32 mm were determined in the field.

Volcanic Ash Fragments of dark volcanic glass and ve-

sicular ejecta, mixed with varying amounts of fme silt, occur in discrete bands and dispersed throughout the foliated glacier ice. They also occur in dispersed form in the clear ice layers which separate debris bands in the basal parts of the glacier. Total debris concentrations in the foliated ice range from 0.008 to 0.085 g liter-t in coarse bubbly ice, from 0.032 to 1.95 g liter-’ in coarse clear ice, and from 0.059 to 0.318 g liter-’ in clear basal ice. This sediment consisted of poorly to very poorly sorted (pi = 1.52 2 0.40 +), negatively to very negatively skewed (pi = -0.28 * 0.08) sandy silt, and silty sand (E = 4.17 & 0.43 4). It appears to be incorporated by aeolian ac- tion at the upper surface of the glacier.

Medial Moraines Two prominent medial moraines emerge

onto the surface of Eyjabakkajokull on ei- ther side of the central flow unit within 3 km of the snout, The sediments of the central moraine contained clasts of black basalt, orange, yellow, and green agglomer- ates, and pale green and brown tuffs. Many clasts were heavily oxidized and shattered by weathering, only 20% were striated and they were generally very angular with a high proportion of disks and blades (Fig. 2). Texturally the sediment was poorly to extremely poorly sorted (mi = 3.10 5 1.13 +), negatively to very negatively skewed (pi = -0.38 ? 0.13) sand, gravelly silty sand, sandy gravel, and gravel (E = 0.86 & 1.33 +) (Fig. 2). The debris of the western moraine was lithologically different, containing clasts of black and red- boled basalt, orange and brown agglomer- ates, and a red tuff, many of which had thick weathering rinds. Clasts were again

very angular, but there were fewer blades and more rods and spheres than in debris from the central moraine. Of the total number of clasts, 46% were striated (Fig. 2). This sediment was very poorly sorted (pi = 3.30 * 0.23 +), very negatively skewed (pi = -0.38 & 0.06) sandy gravel and gravelly silty sand (m = -0.32 ? 0.26 4) (Fig. 2).

The coarseness of these sediments, their low silt and clay content, low frequency of striated clasts, the angularity of the clasts, and the relatively high proportion of disks and blades are all characteristic of a debris assemblage which has been little modified by crushing and abrasion (Boulton, 1978). The debris is probably supraglacially derived, and has passed through the glacier along englacial and supraglacial transport paths. Although the moraines are not linked to their source by continuous debris outcrops, tracing back along flowlines suggests that they are derived subaerially from two nunataks in the upper glacier.

Englacial Fluvial Sediments

Three types of fluvial sediment were identified in englacial and supraglacial positions. An englacial meltwater conduit par- tially unroofed by ablation was found to contain stratified, moderately well sorted (Eli = 0.79 +), negatively skewed (Ski = - 0.298) silty sand (Mz = 3.69 +). Particles were primarily volcanic ash fragments, suggesting that these sediments were derived by the englacial reworking of ash by meltwater. Coarser, crudely bedded gravel deposits which melted out to form long ice- cored ridges were apparently derived by reworking of medial moraine sediments, to which they were lithologically similar. Clasts were rather angular, but predomi- nantly spherical, perhaps indicating selective transport of spherical particles in englacial conduits (Fig. 2}. Sediments consisted of very poorly sorted (SDi = 3.08 $), negatively skewed (Ski = - 0.185) gravelly sands (Mz = -0.29 +) (Fig. 2).


a b CENTRAL MEDIAL

100

WESTERN MEOIAL 100, !

ENGLACIAL FLUVIAL-LATERAL

u

1 ENGLACIAL FLUVIAL-FRONTAL 100,

BASAL DEBRIS-LATERAL

EASAL DEERIS-FRONTAL 100,

b ! I

FIG. 2. Characteristics of englacial sediments from Eyjabakkaj6kull. (a) Proportion of striated clasts: S = striated, N = non-striated. (b) Zingg shape: S = spheres, R = rods, D = disks. B = blades. (c) Powers roundness: VA = very angular, A = angular. SA = subangular, SR = subrounded. R = rounded. WR = well rounded. (d) Particle size composition.

More extensive beds of massive sand and disks (Fig. 2). The sediment was very gravel up to 3 m thick crop out englacially poorly sorted (Eli = 3.10 $1, very nega- and supraglacially at the western extremity tively skewed (Ski = -0.39) sandy gravel of the frontal margin of the glacier. These (Mz = - I .85 4) (Fig. 2). sediments were unfrozen, but lay on ice Todtmann ( 1960) records the deposition slopes which dipped upglacier at angles be- of outwash gravels over dead ice in the area tween 0’ and 30°. This debris included where this gravel now crops out and 1967 clasts of basalt, olivine basalt, and olivine aerial photographs show that at that time gabbro, and blocks of peat. Clasts were the area was isolated from the main body markedly more rounded than those in of the glacier and consisted of hummocky gravels derived from the medial moraines topography surrounded by outwash chan- and there were fewer spheres and more nels and glacifluvial sediments. It therefore

272 MARTIN SHARP

appears that these supraglacially deposited gravels were overridden and reincorporated into the glacier during the 1972 surge.

3asally Derived Debris

A series of debris-rich layers trending approximately parallel to the glacier margin crop out on the glacier surface in a 150-m- wide zone around the snout of Eyjabakka- jokull (Fig. 3). Individual debris-rich layers were up to 15 cm thick, had debris concentrations between 1.29 and 276.45 g liter-t, and dipped upglacier at angles of up to 60° (Fig. 3). The characteristics of debris entrained in these layers varied with position around the glacier snout. Along the western lateral margin the layers contained fresh,

unweathered clasts of basalt and olivine basalt. Sixty-eight percent were striated and they were more rounded than clasts from the medial moraines (Fig. 2). The sediment consisted of very poorly to extremely poorly sorted (pi = 4.16 * 0.56 $), POS-

itively to very negatively skewed (pi = -0.25 & 0.25) gravelly silty sand and sandy silt (m = 1.75 2 1.66 +) (Fig. 2) that appeared as abraded rock and mineral fragments in a matrix of rock flour under the microscope.

Along the frontal margin of the glacier the layers contained clasts of basalt, olivine basalt, olivine gabbro, and a yellow and green tuff, indicating a source akin to that of the supraglacial gravels, a relationship further supported by the roundness of the

Ice Flw -

b CI 10 20 30 40 50 Distancdm~

FIG. 3. (a) Outcrop of arcuate debris-rich layers at the margin of Eyjabakkaj6kull in 1981. (b) Section parallel to ice flow direction at the western margin of Eyjabakkaj6kull showing the major debris- bearing structures.


clasts (Fig. 2). Eighty-one percent were striated, however, compared to only twenty-eight percent in the gravels. The debris consisted of extremely poorly sorted (pi = 4.53 * 0.15 +), negatively t0 very negatively skewed (pi = -0.41 ? 0.11) gravelly silty sand (RZ = 0.38 & 0.76 4) (Fig. 2), and was thus coarser and more negatively skewed than the debris cropping out along the western margin.

The debris from these layers was finer grained, more poorly sorted, more in- tensely striated, and less angular than that from the medial moraines, and it contained a greater proportion of rods and spheres. These characteristics indicate comminution of debris by abrasion and crushing as it passes through the zone of traction at the glacier sole (Boulton, 1978). Incorporation of outwash sediments overridden by the glacier during the 1972 surge may explain the roundness of clasts and the existence of a distinct gravel mode in sediments from the frontal debris-rich layers (cf. Slatt, 1971).

Although the debris-rich layers were prominent features at Eyjabakkajokull in 1981 and are clearly visible in 1945 aerial photographs, they are virtually absent in 1967 aerial photographs. As the glacier had receded 1.7 km between 1945 and 1967, I suggest that layers which developed in a narrow zone around the glacier margin during the 1938 advance were destroyed by ablation between 1945 and 1967, and that the layers seen in 1981 had formed during the 1972 surge. Such sequences of stacked englacial debris bands could form by net accretion of ice onto the glacier sole by the refreezing of basal meltwaters (Weertman, 1961) or by the shearing of thrust faulting of a thin basal layer of debris-rich ice in a region of compressive flow (Goldthwait, 1951). Although a number of authors have disputed the validity of the thrust/shear hy- pothesis (Weertman, 1961: Hooke, 1968), thrust faulting with displacement rates ap- proaching 0.1 m hr ’ was observed near the margin of Variegated Glacier, Alaska,

during its 1982-1983 surge. The initiation of these thrust faults was associated with longitudinal compressive strain rates of 1.75 x 10e6 sect’ which developed as the surge front propagated into thin stagnant ice at the glacier margin. Debris-rich basal ice was observed to be moving upward into the glacier along the plane of these thrust faults (Sharp and Anderson. in prepara- tion).

Evidence for a thrust-fault origin for debris-rich layers at Eyjabakkajokull comes from the contrasting characteristics of debris from the debris-rich layers and the in- tercalated clear ice layers. Debris from the clear ice layers consists of fragments of volcanic glass and ejecta while there is a total absence of the abraded sand grains and rock flour which are typical of the debris- rich layers. This contrast is difficult to explain in terms of a regelation origin for the whole sequence of debris-bearing ice, since regelation would be unlikely to repeatedly entrain alternate bands of diamict-like debris and volcanic ash. Even an enhanced regelation process operating in subglacial cavities during surges (Clapperton, 1975) would not easily explain the lateral conti- nuity (over 1 km) of some individual debris- rich layers since subglacial cavities are likely to be of limited lateral extent. Thrusting, resulting in the intercalation of debris-rich ice from the glacier sole with glacier ice, seems much better able to explain the observed properties of the debris- rich layers. It must, however, be stressed that thrusting is not responsible for the ini- tial entrainment of the debris, merely for its subsequent redistribution.

A number of vertical debris dikes crop out along the margins of the glacier. They form rectilinear networks orientated normal and subparallel to the glacier margin, and are consistently aligned parallel to and continuous with “crevasse traces” (Hambrey and Milnes? 1977) on the glacier surface. The sediment consists of

274 MARTIN SHARP

very poorly to extremely poorly sorted (mi = 4.51 & 0.33 +), negatively to very negatively skewed (pi = - 0.38 & 0.11) gravelly silty sand (RZ = 0.99 2 0.72 4) which is texturally indistinguishable from diamictons (believed to be lodgement tills) exposed at the glacier margin. The origin of these debris dikes is discussed more fully elsewhere (Sharp, in press). The pattern and morphology of the dikes and the fabric and composition of their constituent sediment suggests that they are formed by the intrusion of material derived from the di- latant upper horizon of sheared subglacial diamictons into open crevasses as the stagnant glacier sinks into its bed after a surge.

SEDIMENTS AT THE GLACIER MARGIN Having described the debris transported

by Eyjabakkajokull, we now consider how it is released from the glacier and deposited. Sediments are described using the lithofacies codes of Eyles et ul. (1983).

Lodgement Till Along the lateral margins of Eyjabakka-

jokull, there are extensive exposures of massive, clast to matrix-supported diamicton (Facies Dcm and Dmm), which form a continuous sheet approximately 1 m thick which can be traced laterally beneath the glacier. The upper surface of this sediment is fluted (Boulton, 1976) and crossed by steep-sided ridges which appear to be the melted out remnants of englacial crevasse fillings. Many large clasts exposed on the surface of the sediment have a stoss and lee form (Sharp, 1982a) and consistently orientated striae on their upper surfaces. Others are associated with lee-side “till wedges” (Boulton, 1976) and grooves in the sediment surface immediately upglacier from them, features indicative of lodgement by ploughing into the sediment surface (Kruger, 1979). Pebble fabric is well developed, with a maximum parallel to the local ice flow direction indicated by the orientation of flutes (Fig. 4). Where the diamicton directly overlies bedrock it may be intruded

b d

FIG. 4. Pebble fabrics from (a) lodgement till and Cb- d) sediment flow deposits at Eyjabakkajhkull. 2-D vector magnitudes of the fabrics are (a) 65.3, (b) 2.4, (c) 17.3, and (d) 24.8%. Arrows indicate the direction of ice flow (a) and sediment flow (b-d).

into joints, but it more commonly overlies Holocene peat and silt. Contacts with underlying sediments may be sharp, but there is often evidence for shear mixing and deformation of layering in the overridden sediments. Both diamicton and underlying sediments tend to be overconsolidated and show subhorizontal fissility.

The texture, pebble fabric, and sedimentary structures characteristic of this diamicton suggest that it was deposited subglacially from actively sliding ice, and probably therefore by lodgement. It is not clear at what stage in the surge cycle deposition occurred. Both the diamicton and the underlying silt and peat have been subjected to subglacial deformation (Boulton and Jones, 1979) and this seems most likely to have occurred during surges when subglacial water pressure was probably high and the glacier was actively sliding over the area. The diamicton lacks the remolded upper horizon which is characteristic of deformed lodgement tills elsewhere in Iceland (Boulton and Dent, 1974; Sharp, 1984). If such a horizon formerly existed at Eyjabak- kajokull it may have been destroyed by consolidation beneath stagnant ice between


surges and by intrusion into open crevasses to form crevasse fillings (Sharp, in press).

Meltout Tili

Debris from ash bands, englacial debris- rich layers, and crevasse fillings is released onto the upper surface of the glacier in a narrow marginal zone by the in situ melting of the enclosing ice. Much of this debris is rapidly remobilized and disaggregated by sediment flows (Lawson, 1979), but where the preexisting debris cover is sufficiently thick, debris may be released under con- lining conditions which inhibit mixing and deformation. As the interstitial ice melts out the sediment undergoes limited collapse, causing increased particle packing and minor readjustments of grain contacts. Maximum thicknesses of 1 .l m of supraglacial meltout till were recorded at Eyja- bakkajokull. Depending upon whether or not it incorporates material derived from several different sources, the sediment is a matrix-supported massive or stratified diamicton (Dmm, Dms). Ash bands are a frequent cause of stratification and the diamicton, which is largely composed of basally transported debris, is often capped by up to 0.4 m of medial moraine debris. Uneven melting of the buried ice surface produces gentle folding and small-scale faulting of the banded structure. Meltout till lacks the subhorizontal fissility characteristic of sediment which has undergone consolidation beneath an ice overburden. Sub- dued ridges on the surface of outcrops of meltout till mark the outcrop of individual debris bands.

Sediment Flow Deposits

Sediment flows (Lawson, 1979) are responsible for reworking material released onto the glacier surface. They mix debris from different sources and spread it across a 150-m-wide zone around the glacier margin. Sediment flows at Eyjabakkajokull had water contents between 16.3 and 22.2% by weight, values which would characterize them as Type II or Type III flows in Law-

son’s (1979) classification (Table 1; Fig. 5). The low shear strengths, small thicknesses. and rapid motion of these flows. the frequent collapse of the plug zone, and the occurrence of channelization and fan deposition support this interpretation. There were significant relationships (.JJ < .Ol ) between flow water content and the C$ median grain size and silt and clay content of flows (Fig. 5). An increase in water content was associated with an increase in silt and clay content and a decrease in median grain size, suggesting that the competence of the flows decreased with increasing water content.

Flow deposits were very poorly to extremely poorly sorted (Eli = 4.06 2 0.66 $), very negatively skewed (Ski = -0.35 ? 0.20) sandy gravels, gravelly silty sands, and sandy silts (E = 0.97 ? 1.57 $) (Fig. 6). The coarser deposits were better sorted than the finer ones (Fig. 6). Depletion in tines relative to source material presumably reflects selective deposition of coarse particles and selective removal of silt and clay in flowing meltwater (Lawson, 1979). Pebble fabrics of flow deposits showed no significant preferred orientation and were much weaker than lodgement till fabrics (Fig. 4: Sharp, 1982b). They did, however, show a preferred sense of a-axis dip parallel to the depositional surface? and there were a significant number of steeply dipping par- titles.

Deposits of lower water content flows included discontinuous pods and lenses of sediment derived from distinctly different sources. Sinkage of larger clasts to the base of higher water content flows produced 1 wo distinct sedimentary units: an upper one with a poorly sorted matrix and deficiency of coarse clasts (dewatered unit: Lawson, 1981; Fig. 7), and a lower one relatively en- riched in the coarser fractions (basal transport unit; Lawson, 1981). Both units were remolded by shear during flow and lost all structural and fabric properties of their source sediments. Many flows at Eyjabak- kajokull were, however, so thin that coarse

TABL

E 1.

PRO

PERT

IES

OF

12 A

CTIV

E SE

DIME

NT

FLOW

S AT

EYI

ABAK

KAJ~

KULL

1 2

3 4

5 6

7 t3

9

10

11

12

Wate

r co

ntent

(wt

%)

16.3

17

.1

18.0

19

.1

19.2

19

.2

19.3

19

.4

20.6

21

.2

21.9

22

.2

Bulk

wet

dens

ity

(g c

mp3

) 2.

4 2.

1 2.

2 2.

0 2.

2 2.

0 2.

1 2.

0 2.

0 2.

1 2.

1 2.

1 M

edian

gr

ain

size

(+)

-1.8

-1

.6

-0.5

-1

.3

-0.1

-0

.5

-0.3

-0

.4

0.1

-0.1

0.

2 0.

3 La

rges

t cla

st in

sam

ple

(mm

) 38

.0

33.0

28

.0

36.0

39

.0

34.0

21

.0

27.0

26

.0

37.0

28

.0

22.0

M

ean

surfa

ce s

hear

stre

ngth

(k

g cm

m2)

0.

0 0.

01

0.01

0.

01

0.01

0.

0 0.

0 0.

0 0.

01

0.0

0.0

Flow

spe

ed (

cm s

et-‘)

10

.0

7.0

7.0

12.0

20

.0

3.0

10.0

5.

0 5.

0 10

.0

Flow

dep

th (

cm)

2.5

15.0

8.

0 5.

0 12

.0

20.0

10

.0

15.0

8.

0 Fl

ow g

radie

nt

(deg

rees

) 20

.0

25.0

12

.0

30.0

25

.0

50.0

15

.0

20.0

20

.0

Flow

len

gth

(m)

8.0

2.5

10.0

2.

5 Pl

ug p

rese

nt

? X

X X

X X

X Ch

anne

lized

?

X X

X X

x x

Fan

depo

sition

?

X X

X x

x 5


lobate flows also produced erosional con- GEOMORPHOLOGY AND tacts where bulldozing occurred at the front STRATIGRAPHY OF THE of the flow. Other contacts were sharp and PROGLACIAL AREA conformable, though some showed signs of deformation in response to loading by flow The six principal landform types found in deposition. Flow deposits are resedi- the proglacial area of EyjabakkajGkull (Fig. mented, stratified, clast- to matrix-sup- 1) combine to form three glacigenic land- ported diamictons (Des(r), Dms(r)). form-sediment complexes:

FIG. 7. Sequence of sediment flow deposits at the western margin of Eyjabakkaj6kull. The section below the finger shows silt-clay drape, dewatered unit deticient in coarse clasts, and basal shear unit.

278 MARTIN SHARP

(2) Broad Ridges of Glacitectonically Deformed Sediments

Three ridges up to 25 m high were apparently produced by the 1890, 1931, and 1938 advances (Todtmann, 1960). These ridges have a thin surface veneer of massive or crudely bedded gravel (Facies Gm) and resedimented diamicton (Dms(r), Des(r)) which is thickest on the proximal slopes of the ridges. This veneer lies unconformably over the main body of the ridge which consists of peat, silt, sand, and gravel (Fl, Sm, Sh, Gm) (Sites 2 and 3, Fig. 8). The internal structure of the ridges shows evidence of intense compression normal to the ridge axes. Beds have been tilted and dip upgla- tier at angles of 15’ to 30”, and there is extensive crumpling and small-scale folding of layering in the silt and peat. Croot and Garton (1982) suggested that beds in the vicinity of the 1890 ice margin had undergone 66% horizontal shortening, while further upglacier parts of the same beds which had been overridden by the glacier had undergone 300% horizontal extension as a result of the overburden pressures beneath the ice. A major thrust fault in the core of one

3

ridge (Fig. 9) originates at the contact between cohesive peat and silt and underlying sand and gravel. Low-angle slip faults indicate movement of material away from the localized uplift associated with the thrust. Downvalley from the crest of the 1890 thrust ridge are seven lobate complexes of ridges which decrease in height from 25 to 0.2 m with increasing distance from the glacier (Fig. 1). These ridge complexes relate to the glacier tongue splitting up into a number of discrete sublobes as it flowed between a series of rock knobs. The ridges are asymmetrical with a steeper distal limb, and individual segments are of limited lateral extent. Most have a turf cover, but those in the westernmost complex are ve- neered with up to 1 m of gravel (Croat, 1978).

Thrusting of the silt and peat probably requires the development of high pore pressures and strength reduction at the contact with the underlying sand and gravel aquifer. This would be favored by the very rapid rates of loading associated with the advance of a surging glacier over its proglacial area (Matthews and Mackay, 1960), by the high rates of meltwater production which

FIG. 8. Stratigraphy of eight sections from the proglacial area of EyjabakkajGkull. Locations of the sections are shown on Figure 1. Sediments are described using the lithofacies code of Eyles et u/. (1983).


F 3~. 9. Section through a ridge of glacitectonically thrust sediments at Eyjabakkajtikull. Note ove1 .a11 upglacier tilt of the sediments, the central thrust fault. and the thin veneer of glacially depo\ sedi ments on the crest of the ridge. Photograph by J. C. Gemmell.

the ited

occur beneath a glacier moving at surging velocities (Weertman, 1969), and by the in- filling of subglacial drainage conduits with deforming sediments (Clarke er u/., 1984). The juxtaposition of materials of contrasting permeability in the overridden sediments would also assist by producing a highly anisotropic groundwater regime un- favorable to both lateral and vertical dissi- pation of pore water pressures. The association of glacially thrust ridges with con- fined aquifers was also noted by Moran et 01. (1980).

Development of high pore water pressures in the subglacial sediments would re- duce their strength and allow them to un- dergo shearing deformation and horizontal extension. This would be translated into horizontal compression in the vicinity of the ice margin where there must be a zone of transition from those parts of a bed which are undergoing subglacial deformation and those parts which lie beyond the glacier margin and are undeformed. In areas which were rapidly crossed by the ad- vancing ice front this compression would

be short lived, but in the vicinity of the final ice-margin position reached at the end of the surge compression would be manifested by horizontal shortening and vertical thick- ening of sediments and by the formation of glacially thrust ridges. Formation of these thrust ridges creates a zone of localized uplift at the glacier margin, and the lobate ridge complexes formed outside the ice margin seem to form by gravity spreading of peat and silt away from this uplift (Croat and Garton, 1982). Individual ridges within these complexes may then be viewed as the sutiace expression of the slices of an im- bricate thrust sheet (Fig. IO).

Belts of chaotic, hummocky topography, some of which are still ice cored, occur upglacier from the thrust ridges, particularly where medial moraine sediments crop out (Fig. 1). Sedimentary sequences in these areas (Sites 4 and 5, Fig. 8) are thicker and more complex than on the thrust ridges themselves. They consist of interstratitied

280 MARTIN SHARP

a

b

FIG. 10. Model for sedimentation by Eyjabakkajokuh. (a) Immediately after a surge: 1. Lobate complexes of ridges formed by gravity spreading of peats and silts away from the region of uplift; 2. Ridges of glacitectonicahy thrust peats and silts; 3. Normal faults in sands and gravels in the core of the ridge; 4. Water escape structures in overridden sediments; 5. Outwash gravels; 6. Ridge composed of resedimented diamictons; 7. Englacial debris bands raised from the bed along thrust faults: 8. Debris dikes on the glacier surface; 9. Sediment flows: 10. Subglacial lodgement tills; 11. Crevasse- fill ridges. (b) Early quiescent phase: 12. Formation of upraised marginal rim of debris-covered ice by differential ablation. (c) Late quiescent phase: 13. Chaotic hummocky ice-cored topography; 14. Crevasse-fill ridges on fluted lodgement till surfaces (15) exposed by glacier retreat.

massive gravel (Gm), resedimented diamicton (Dms(r), D&r), and Dcm(r)), and massive, vertically jointed diamicton (Dmm) units, which may be meltout tills (Site 5). These sediments were evidently deposited supraglacially, but they may overlie massive subglacially sheared diamicton (Dmm(s)) and organic sand @h(s)) at depth (Site 4). The resedimented com- ponent of the sequence includes materials derived from both medial moraines and the glacier bed, and these often show in- terbedded contacts (Site 4). These hummocky areas appear to have evolved from marginal zones containing stacked englacial

debris bands which developed during pre- vious surges. The association of hummocky areas with the outcrop of medial moraine debris suggests that this is important in ini- tially reducing ablation rates to levels at which debris can accumulate on the ice surface without immediately being reworked.

(3) Fluted Till Surfaces with Crevasse-Fill Ridges Fluted till surfaces crossed by crevasse-

fill ridges occur upglacier from the hummocky areas, particularly along the lateral margins of the glacier (Fig. l), where they are exposed by the retreat of clean ice from


within the debris-covered marginal zone. Sedimentary sequences in these areas consist of thin, massive, sheared diamictons (Dmm(s), Dcm(s)) overlying sheared lay- ered organic sands (Sh(s)) and lacustrine silts (Fl, Fm(s)) (Sites 6, 7, 8; Fig. 8). The subtill sediments at Site 6 show a variety of water escape structures which indicate liq- uefaction of the basal massive sand unit (Sm) and its fluidized intrusion into the overlying silts (Fig. 11). This reflects the development of high pore fluid pressures in the sand layer, probably in response to loading of a sequence composed of materials of contrasting permeability. An origin related to ice loading is suggested by the fact that the structures cut out immediately beneath the lower surface of the diamicton.

SUMMARY

The observed landform-sediment complexes at Eyjabakkajokull are produced by a number of processes, the occurrence of which is closely linked to the surging behavior of the glacier. These include: (1) extensive glacitectonic deformation of over-

ridden sediments in response to the development of high pore water pressures during rapid glacier advances; (2) formation of englacial debris bands in a narrow zone around the glacier margin, a result of thrust faulting of basal debris-rich ice in a zone of intense longitudinal compression developed in thin ice near the glacier snout: (3) overriding and reincorporation of stagnant debris-covered ice during surges; (4) formation of a debris-covered marginal zone as a result of ablation of stagnant ice containing englacial debris bands during the quiescent phase between surges; (5) isola- tion of this marginal zone from the main body of the glacier as a result of the differential ablation of clean and dirty ice, and evolution by meltout and sediment flow processes into a belt of chaotic, hummocky. and kettled topography in which supraglacially deposited sediments predominate: (6) formation of crevasse fillings by the intrusion of sheared subglacial sediments into open crevasses as the stagnant but heavily crevassed glacier sinks into its bed during the early part of the quiescent period be-

FIG. 1 I. Site 6. Water escape structures reflecting fluidized intrusion of basal sands into overlying silts as a result of sudden loading by the glacier advance.

282 MARTIN SHARP

tween surges; and (7) exposure of fluted, subglacially deposited diamictons as overlying clean ice retreats away from the debris-covered marginal zone.

While it is clear that no one of these processes is necessarily unique to surging glaciers, the occurrence of this particular com- bination of processes is clearly related to the alternation of periods of rapid motion and/or advance with periods of stagnation which occurs in all surging glaciers. It is therefore the total assemblage of landforms and sediments, indicative of processes oc- curring at all stages of the glacier’s activity cycle, which may assist in the recognition of paleosurges in the geologic record. In particular, the arrangement of landforms and sediments into three broad belts around the glacier margin reached at the end of a surge may be a feature characteristic of surging glaciers. With increasing distance upglacier these belts are dominated, re- spectively, by the effects of (a) glacitectonic deformation; (b) supraglacial sedimentation; and (c) subglacial sedimentation (Fig. IO).

DISCUSSION

Clayton and Moran (1974, Fig. 9) have developed a model for sedimentation by lobes of the late Wisconsin Laurentide Ice Sheet in the Prairies region of North America which shows an identical spatial arrangement of landforms and sediments to that produced by Eyjabakkajokull, Clayton and Moran attribute the thrusting of bedrock and elevation of basally derived debris to an englacial position to the existence of a marginal zone frozen to the glacier bed. In such an area debris is entrained by the refreezing of basal meltwaters and thrusting of bedrock results from the development of high pore pressures where the drainage of these meltwaters through a subglacial aquifer is impeded by a surface layer of frozen sediments. At Eyjabakkajokull, however, both of these processes occurred during surges and in marginal positions

which were only reached by considerable advances of the glacier. Observed rates of advance require that the glacier was sliding over its bed and it is highly unlikely, therefore, that the marginal zone could have been frozen to the bed.

If Clayton and Moran’s interpretation of the processes responsible for the depositional landscapes of the Prairies is correct, then the similarity between those landscapes and that produced by Eyjabakka- jokull is sufficient to bring into question the assumption that Iandform-sediment associations can provide a reliable basis for the reconstruction of glacier thermal regime (Boulton, 1972; Eyles and Slatt, 1977). Dis- tinctive processes associated with surging may also influence the depositional landscapes produced by glaciers, and in this re- spect it is pertinent to note that many of the lobes of the Laurentide Ice Sheet, from which Clayton and Moran’s model is developed, may have surged (Wright, 1973; Teller and Clayton, 1982). Successful interpretation of depositional landscapes clearly requires that attention be paid to the rate and chronoIogy of glacier fluctuations as well as to landform-sediment associations.

ACKNOWLEDGMENTS This work was carried out during tenure of a NERC

Studentship in the Department of Geography, Univer- sity of Aberdeen, Scotland. Dr. C. M. Clapperton and Dr. D. E. Sugden provided constructive and helpful supervision, and P. Sharp, J. C. Gemmeil, and J. J, MacNeih provided invaluable field assistance. Grants from the Royal Geographical Society and nu- merous other trusts and companies assisted with their expenses. I thank S. Kennedy for particle size anal- yses and D. E. Lawson for the computer program used to contour stereo nets. The Icelandic National Research Council gave permission to work at Eyja- bakkajokull, and Bragi Gudmundsson of Egilsstadir made it possible for me to visit the glacier. Dr. P L. Gibbard and Dr. D. E. Sugden made constructive criticisms of an earlier draft.

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Sedimentation and stratigraphy at Eyjabakkajökull—An Icelandic surging glacier

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