Sedimentary Geology Subglacial deformation associated with ...tion with the flute is more muted. A...

ELSEVIER Sedimentary Geology 111 (1997) 177-197

Sedimentary Geology

Subglacial deformation associated with fast ice flow, from the Columbia Glacier, Alaska

Jane K. Ha r t *, B a m a b y S m i th

Department of Geography, University of Southampton, Southampton, S017 1B J, UK

Received 30 January 1996; accepted 3 June 1996

Abstract

The Columbia Glacier is a large well studied tidewater glacier which has one of the highest non-surging modem glacier flow rates. Previous studies from ice drilling have tentatively suggested the presence of a deforming bed. This study examined the recently deglaciated (since 1974) foreland and concluded there were features indicative of deforming bed conditions, i.e., streamlined subglacial bedforms, squeeze-type push moraines and crevasse diapirs. It was also argued that the elongation ratio and height of streamlined subglacial bedforms can be used as a proxy for shear strain and thickness of the deforming layer, respectively, and when these two parameters are combined together they produce a complex but predictable eigenvalue pattern in the tills comprising the landforms.

Keywords: deforming bed; deforming bed till; flutes; Columbia Glacier; fast ice flow

1. Introduction

There has been a great deal of recent research concerned with understanding the causes of fast ice flow which can occur at any temporal or spatial scale (see Table 1). On a small scale, short-term velocity increases (over short time periods 1-10 years, Dowdeswell et al., 1991) in valley glaciers are known as surges. There have been many the- odes to explain this phenomenon; however recent research can be sunarnarised into two models, based on changes in subglacial water flows (Kamb et al., 1985) or the subglacial deforming bed (Clarke et al., 1984). On a larger scale, it has been shown that ice streams (which are the faster-moving parts of ice

* Corresponding author. Fax: +44 1703-593729; E-mail: jhart@ soton.ac.uk

sheets) change their velocities over longer (102 or 103 years) time periods (e.g., Ice Stream B, Antarc- tica; Shabtaie and Bentley, 1987). It has also been shown that during the Quaternary many of the large ice sheets moved over a deforming bed which led to an increase in glacier velocity (Boulton and Jones, 1979; Clarke, 1987; Boulton and Hindmarsh, 1987), but the spatial or temporal scale of this fast ice is not known. In contrast there are some glaciers and ice streams today which seem to have consistently flowed at a fast rate, these include Jakobshavn Isbrae, Greenland (Lingle et al., 1981) and the Columbia Glacier, Alaska. In this paper we look in detail at the sediments and landforms deposited by the Columbia Glacier and use these to interpret the nature of the subglacial processes associated with fast-moving ice.

0037-0738/97/$17.00 ~, 1997 Elsevier Science B.V. All rights reserved. PI IS0037-0738(97)00014-6

178

Table 1 Fast ice velocities

J.K. Hart, B. Smith~Sedimentary Geology 111 (1997) 177-197

Glacier type Glacier Area Velocity Reference (km 2 )

Non-surging valley glacier

Surging valley glacier

Antarctic ice Streams

Floating tongue tidewater glacier

Grounded tidewater glacier

Quaternary ice stream lobes

Fels Glacier, Alaska 18 70-100 rrdyr

Variegated Glacier, Alaska 20 Quiescent phase = 0-73 m/yr phase Surge phase = max. 23,000 m/yr

Eyjabakkajokull, Iceland 40 Surge phase = 10,950 m/yr

Ice Stream B 15,000 800 m/yr

Rutford Ice Stream 15,000 >400 m/yr

Jakobshavn Isbrae, 9,000 8360 rrdyr Greenland

Columbia Glacier, Alaska 1,100 Average rate = 1277-3285 rrdyr Seasonal high-velocity waves = 58,000-91,000 m/yr

Fastest advance rate = 7000 m/yr Laurentide Ice Sheet Lake 87,500 Michigan lobe

Raymond et al. (1995)

Sharp (1988a) Kamb et al. (1985) Sharp (1988b)

Whillans et al. (1987, 1993) Doake et al. (1987)

Lingle et ai. (1981)

Kamb et ai. (1994) Krimmel and Vaughan (1987)

Mickelson et al. (1981)

2. The Columbia Glacier

The Columbia Glacier is a large fast-flowing tidewater glacier (approx. area 1100 km2; Ferguson, 1992; see Table 1), that has undergone dramatic retreat since the early 1970s (Post, 1975; Leth- coe, 1987). It flows from the southern side of the Chugach Mountains in south central Alaska (Fig. 1) into Prince William Sound. It is one of the best studied tidewater glaciers in Alaska with a photographic record of the margin available since 1899 (Gilbert, 1910; Field, 1937; Post, 1975; Brown et al., 1982; Meier et al., 1985; Meier and Post, 1987; Calkin, 1988) and very detailed surveys of the glaciology (Meier et al., 1994; Kamb et al., 1994). This includes a recent study by Humphrey et al. (1993) who in- ferred a 65 cm basal deforming layer at the base of the Columbia Glacier from the bending of a drill stem used to study the basal properties.

The Columbia Glacier has been so well studied because icebergs from its tidewater margin flow over a large, partially submerged, neoglacial end-moraine into the Valdez shipping lanes. This moraine traps

most of the icebergs during low tide, but at high tide some of the icebergs are able to overtop the moraine, and there are fears that a channel may be cut into the moraine allowing much higher iceberg release.

Since the recent glacier retreat, the eastern margin and Heather Island are readily accessible to exam- ine the subglacial sediments. The area examined in this study was the eastern margin (Fig. 1) which was deglaciated between 1974 and 1986. The aim of this study was to try to understand the geomorphology associated with fast ice flow, and attempt to use the geomorphic data to suggest the nature of the subglacial environment beneath the modern Columbia Glacier.

3. The eastern Columbia subglacial surface

The area studied (Fig. lb, c) revealed a remark- ably well preserved subglacial surface, overlying a bedrock knoll. The surface is dominated by flutes, but there are also moraines and other transverse features, which we shall discuss in turn. At most of the sites, till fabric and shape analysis was carded out. For the till fabric investigations at each site a

Fig. 1. Columbia Glacier: (a) location in Alaska; (b) insert showing a detailed view of the Columbia Glacier; (c) detailed ages of the moraines; (d) schematic map of the area studied with the key sites.

J.K. Hart, B. Smith~Sedimentary Geology 111 (1997) 177-197 179

d)

Columbia Glacier

f P

Slte 6

0 o ° Esker

~ Flutes

0 Drumlin

Icebergs

Iceberg scour marks

,,dP~,,._~ Iceberg push moraines

//J Moraines

l) Large discontk~uous ridges

~ :~ Camp

100 metres l

N

180 J.K. Hart, B. Smith~Sedimentary Geology 111 (1997) 177-197

minimum of 30 clasts with an axial ratio greater than 1.5 : 1 were sampled. The initialised eigenvalues were then calculated. These eigenvalues (S1, $2 and $3) summarise fabric strength along the three principal directions of clustering (after Mark (1973) and Dowdeswell and Sharp (1986)). A fabric with no preferred orientation (weak fabric) would have equal eigenvalues, whilst a strong fabric would have a high value in the direction of maximum clustering (S1) which is usually the direction of tectonic transport, and a low value in the direction of least clustering (S3).

Shape analysis was carried out by the measurement of 50 clasts, including the measurement of lengths of the three axes and the overall roundness (very angular, angular, semiangular, semirounded, round and well rounded). The results are plotted on a triangular graph using the technique of Sneed and Folk (1958) and Benn and Ballantyne (1993) and the values for C40 (percentage of clasts with c :a axes ratio of less than or equal to 0.4) and RA (percentage of very angular and angular clasts) were calculated.

3.1. Modern-day iceberg 'push moraines'

The sea between the study site and the Columbia Glacier was filled with icebergs and bergy bits (Fig. 2). These ice blocks are moved ashore at high tide, and either left stranded or moved back down at low tide. The result of this was the production of two interesting styles of geomorphology superimposed on the older, mainly subglacial, surface. The first was iceberg scratches left by the movement of the bergs (approx. 0.5 m wide and 0.3 m deep) (Fig. 3a, b), similar to those described by Belderson et al. (1973) and Bennett and Bullard (1991); and the second were a series of iceberg push moraines (0.3 m high) (Fig. 3c, d), similar to those described by Nichols (1953) and John and Sugden (1975). These were small arcuate ridges pushed up by the icebergs, with striations on the sub-iceberg surface. The shape and style of deformation patterns associated with the iceberg push moraines were very similar to 'squeeze' push moraines (proglacial) and flutes (subglacial) associated with a deforming bed and described from many modem active glaciers (Price, 1970; Sharp, 1984; Boulton, 1987; Hart, 1995a,b).

3.2. Well preserved flutes and marginal features associated with site 1

The field sites were located on a peninsula which the ice retreated from in 1981 (Fig. lc, d). It consisted of a till-covered bedrock knoll whose surface is lineated into flutes which range in length from 0.3 m to over 300 m, most of which have a stoss- side striated clast (Fig. 4a). Associated with the flutes are tree trunks that have been overridden by the glacier and are now aligned parallel with the flutes and the ice flow direction (Fig. 4b). There are also many striated 'bullet-shaped' stones which have been described by numerous authors (Boulton, 1978; Krtiger, 1979; Sharp, 1982) which are also aligned with the ice flow direction.

Fig. 5 shows flute 1 (90 m long and 0.3 m high), and associated landforms that were investigated in detail. Fig. 6a and Table 2 show the eigenvalues and roundness results from four sites along the length of the flute. These all have very high eigenvalues (average values S1 = 0.742; $3 = 0.0803), similar to those found at other flute sites (e.g., Rose, 1989; Benn, 1994; Benn, 1995; Eklund and Hart, 1996; shown in Table 3), with S1 values all oriented close to the ice flow direction. Fig. 6b shows the elongation of the flute compared with results from Rose (1987). Roundness analysis carried out at two of the sites indicates that the flute was composed of very rounded clasts with low RA values, typical of subglacial material (Table 2).

The flute is traversed by four small transverse ridges, that we will discuss in turn. The outermost moraine (M1) was 0.8 m high and 1 m wide with an arcuate plan form, and contains clasts with a very low RA value which is typical of subglacial material. A further roundness test was carried out on a nearby end-moraine of similar dimensions but 0.6 m down beneath the surface (Mlb), which gave results very similar to those from moraine 1.

Moraine 2 was less continuous than moraine 1, and had similar dimensions. However, flute 1 was superimposed on the top of moraine 2. The shape indices of the clasts that composed moraine 2, were very similar to those in moraine 1 (Table 2). Moraine 3 overlies the flute and is of similar dimensions to the other two moraines. However, the clasts within this moraine were far more angular.


= ~i~i!i /iiiiiii~i~ =ii I~I~

Fig. 2. Photograph of the ice margin, glacier is in the background, and in the foreground is the iceberg-choked bay (photograph taken from the western edge of the peninsula studied - - Fig. lb, c, d).

Moraine 4 is slightly more complex, and the junc- tion with the flute is more muted. A fabric and roundness were taken at this site (M4a) which showed that the moraine was composed of very rounded material with slightly weaker fabric orientation than those on the main body of the flute. However, this transverse feature has two styles of longitudinal ridges associated with it. Firstly, features that are located on the down-glacier side of the ridge which are composed of very rounded clasts (M4b) and, secondly, ridges composed of very angular clasts located on both sides of the moraine 4 ridge (M4c).

3.3. Interpretation o f site 1

A number of alternative models for flute formation have been proposed. Boulton (1976), Amark (1980), Benn (1994, 1995), and Hart (1995b) have argued that flutes from when saturated subglacial sediment infills grooves in the ice base. These grooves are formed by the presence of large ob- stacles in the deforming bed (usually large clasts) and the saturated subglacial sediment flows into the low-pressure area in the lee of the obstruction. Al- ternatively, Schytt (1962) has argued that although saturated till is squeezed up into cavities in the lee of boulders, the release of pressure from the surround-

ing ice causes the till in the cavity to refreeze to the basal ice which will later melt-out to form a flute. Gordon et al. (1992) have suggested that flutes form from the preferential melting of the debris-rich basal ice in the lee of a large clast.

Although it has been shown that processes and thus fabrics in the debris-rich basal ice are very similar to those in the deforming layer (Hart, 1995b), and thus differentiation of flute-forming processes may be difficult, we suggest that flutes are more likely to form associated with deforming bed conditions, because:

(1) Most marginal subglacial observations of modern-day flute-forming are associated with a deforming layer (with no debris-rich basal ice), e.g., Breidamerkurjtkull, Iceland (Boulton, 1976; Benn, 1995); Vestari-Hagafellsjtkull, Iceland (Hart, 1995a), Turtmann Glacier, Switzerland (Roberts, 1995); Exit Glacier, Alaska (Hart, 1995b); Root Glacier, Alaska (Hart, 1996).

(2) Flow patterns derived from till fabrics show both flow into the low-pressure area, and along the direction of the flute, indicating that the till was moving with the glacier.

(3) Once the debris-rich basal ice melts, it behaves as a deforming layer (Vivian and Bocquet, 1973; Hart, 1995b), thus it loses any pattern that it had in

182 J.K. Hart, B. Smith/Sedimentary Geology 111 (1997) 177-197

Fig. 3. Effects of the modem-day icebergs: (a) and (b) iceberg scratches; (c) iceberg push moraines.


Fig. 3 (continued).

the debris-rich ice, and its final sedimentation will reflect its reorientati

184 J.K. Hart, B. Smith~Sedimentary Geology 1ll (1997) 177-197

trunk Fig. 4. Photograph of the subglacial surface: (a) flute 4 in the foreground with a fluted surface in the background; (b) tree trunks overrun by the glacier and orientated in the ice flow direction.

there were small fragments of supraglacial sediments deposited in the dump moraines and crevasse infills.

3.4. Description o f other flutes

The eastern margin of the Columbia Glacier was completely covered with flutes of all shapes and

sizes (see Fig. ld). The smallest flutes (apart from the lineations discussed above) tended to be less elongated. Two small flutes were investigated in detail (flutes 2 and 3). Flute 2 (Fig. 7a) consisted of a large clast with a 1.5 m lee-side tail. Till fabrics taken in the flute indicate a high fabric strength, and a flow which is almost parallel with ice direction.


_ f - - - - - ~ e+4%,(~ \ e •

e+4~ x l~ / ' ~ ' . ' ~ e_+4a J~ , -~ . / / ' ~ e_+4a r~'. /h / / / ~

\ I~ e+4a \ "~/I k~,,7"e+4a =v=,.~ / . ~ i

L \ - .___ .

angular g~ la j ~ l M4a N materials ~ l~

e e+_2~ ° o/~sed 1~0 M4b flute superimp ~ ~ ; ~ / ~ M4b I on moraine

" . ~ . ~ . . . ~ . y M 4 e ' ~ / / 0 , , , , 50 metres ~ e

Fig. 5. Detail of flute 1 and associated morainic landforms at site 1 (stereonets contoured using Kamb's method, eigenvalues are given in Table 2).

The stoss-side clast is oriented at 60 ° (ice flow direction 110 °) at this point. Flute 3 is a slightly larger flute (Fig. 7b) with a lower elongation ratio (Fig. 6b). However, the fabric results within the flutes indicate convergent flow and slightly lower-strength eigenvalues than flute 2.

Flute 4 is a much larger flute (over 68 m long and 2.5 m high) (Fig. 3a and Fig. 7c). This flute has medium-strength fabrics which converge near to the stoss-side clast and then diverge towards the end of the flute.

3.5. Interpretation of the other flutes

It can be seen that all the flutes have similar high eigenvalues whatever the size of the flute, but that the elongation ratio increases as the flutes grows (see Fig. 6b). We would suggest that these flutes represent different stages in flute evolution. Once a flute is initiated in the deforming layer due to the presence of a clast, a small prow is formed on the lee side of the clast (Hart, 1995a) but as time continues, so the flute grows in length.

3.6. Flutes and marginal features with more supraglacial components associated with the 1986 moraine (site 5)

Site 5 represents a more complex environment, with a greater supraglacial component (Fig. lc). A detailed map of the area is shown in Fig. 8a. The site consists of two zones delineated by small moraines composed of angular clasts, which must represent dump moraines (possibly annual). Flutes, a very low elongation ratio linear feature (without a stoss-side clast) and an irregular linear feature, are found in both areas and underlay the end-moraines. The eastern zone consists of a mostly rounded surface with many oriented and striated clasts (oriented down ice). The western zone has a far more angular surface, and contains many small pools of water (Fig. 8b) and linear ridges composed of angular clasts.

The low elongation ratio linear feature is very interesting (Fig. 8c) as it contains no stoss-side clast but is oriented down ice. Unlike the flutes which have very high elongation ratios (typically over 10), this has a very low ratio (3) (Fig. 6b). Detailed fabrics were taken of this feature (Fig. 6a, Fig. 8c and Table 2). The feature itself is oriented 95 ° , and


300-

200 -

1 0 0 -

~ 50 O . m

t - O

o~

1o

5

1-

(b)

0.2-

0.1

0 0.4

(a)

• 6a

• 4d 7b • I) M4a

8a • • 5a • 4e

4b (I • 3a

• 5d lc ~ 5b

i /

8b /

i b•qlm6b / / e~" / 4a l a /

ld • / / /

/

~ 3 b / /

/ /

/ /

/ /

/ /

/

0.5

2b 5e ZO 7a •

/ /

/

0.6 0.7

$1

c1

2a • • 5c • 5f

0.8 0.9 1.0

• Okstindan flutes o OksUndan megaflutes

o Glasgow drumlins (inside Loch Lomond moraine) Glasgow drumlins

• Glasgow megadrumlins

o Glasgow streamlined hills

f lu te f o r m • • 0

• 0 0 - ~ " "

• 0 . . ~ ' ' ' ~ A & k A~,& 0 • e A 6 ~ A

. - ' " . A~, AA t A~.a,~AA 0 ¢ • " ' " °

. . . . . . . ~t~' k,t~'~" , , , o ° C 2 • . . . - ' " • ° 'P " . . . . d r u m l i n f o r m C 4 . • ~ oq~° °

C5 0 , , 4 [ e l i i i i i l l i f i ( i f i i i [ i i i i i i i i ( I I ; I I i l l

1 5 10 100 1000 10000

long [a] axis (metres)

Fig. 6. (a) Graph showing the fabric results. The dashed line marks the boundary between (Hart, 1994): lodgement tills and deforming bed tills with a thin deforming layer (below the line); and deforming bed tills with a thick deforming layer (above the line). (b) Graph of elongation ratio against length from Rose (1987) with the Columbia results added (C1 = flute 1; C2 = flute 2; C3 = flute 3; C4 = flute 4; C5 = drumlin).


Table 2 Till fabric and roundnes,; results

Table 3 Comparative flute and drumlin data

Site & No. S 1 $3 Dir. C40 RA Sites Height Elongation S 1

1-a 0.724 0.101 110 37 0 ratio (a/b) 1-b 0.723 0.103 268 Columbia 1-c 0.793 0.040 88 30 0 flute 1 (C1) 0.3 300 0.742 1-d 0.730 0.077 284 flute 2 (C2) 0.2 1.5 0.712 l-M1 26 8 flute 3 (C3) 0.3 2.8 0.644 1-Mlb 40 12 flute 4 (C4) 1.5 28 0.635 l-M2 26 6 site 5 (drumlin) (C5) 0.4 3 0.712 l-M3 42 26 Rose (1989), A 3 6 0.679 1-M4a 0.603 0.085 233 30 0 Rose (1989), i 0.4 13 0.62 1-M4b 40 12 Benn (1994) 1 6 0.707 1-M4c 78 98 Eklund and Hart (1996) 0.3 17 0.8 2a 0.782 0.028 99 Hart (1997), LP1 1 1.8 0.593 2b 0.643 0.035 108 Hart (1997), LP2 1 2.4 0.733 3a 0.618 0.053 117 Hart (1997), LP3 2 2.6 0.667 3b 0.671 0.046 91 4a 0.711 0.097 260 4b 0.598 0.054 106

4c 0.668 0.049 93 supraglacial environment, with the hollows repre- 4d 0.615 0.093 76 4e 0.609 0.068 104 senting kettle holes and l inear angular r idges rep- 5-a 0.571 0.071 76 resenting crevasse infills. This site has far more 5-b 0.797 0.036 90 supraglacial sediments than the other sites studied 5-c 0.824 0.026 93 because it is closer to the valley walls where the 5-d 0.537 0.032 89 sediment can be seen to be located on the modern 5-e 0.684 0.031 109 5-f 0.753 0.016 98 glacier. It has been suggested by Rose (1987) that 6a 0.537 0.133 280 there is a continuum of s treamlined subglacial land- 6b 0.729 0.101 94 forms from small highly elongated flutes to larger 7a 0.728 0.021 235 less elongated drumlins, formed by similar processes 7b 0.544 0.082 324 (Fig. 6b). It has been shown by Hart (1995a) that 8a 0.548 0.068 55

these different forms can exist in a s imilar area, and 8b 0.715 0.110 114 8c 33 20 can be related to the thickness of the deforming bed. 8d 30 17 This is because although flutes form in ice groves,

so the fabrics are general ly divergent on the stoss side and the lee side. There is also a range in fabric strengths from high to low, with an average S 1 value for the stoss side of 0.731 and lee side of 0.694.

A further feature at this site is the irregular l inear feature. This contains both stratified sand and till and has some angular blocks on top. The width of this feature varied from 2 to 5 m and the height from 0.5 t o 2 m .

3. 7. Interpretation ,ff site 5

We suggest that the irregular, more angular surface of the western zone probably represents a more

the movement of till into these areas leads to the evacuation of till elsewhere, part icularly in the stoss side of the flute and the interflute areas. In this way the ice groove, core clast and deforming layer are interrelated. Fig. 9 shows that as the deforming bed thickens so there is more sediment movement and a heightening of the landform. In this way the subglacial landform represents a theoretical min imum thickness of the deforming layer.

Which reference to Fig. 6b, it can be seen that the low elongation ratio l inear feature at site 5 fits into the drumlin class. Although there have been many theories proposed for drumlin formation (see Men- zies, 1984), recently most workers have suggested that they are formed associated with the deforming bed (e.g., Menzies, 1979; Boulton, 1987; Pi-


a) Plan~e r 1.5m

e±2 e ~ trench-fabric taken

~,110 ° ' ~ e~:~,~ 1.25m cr

X-section

Io2m 1.5m

b)

1.

Plan X-section 0.48m

I

0.7m

e

c) e:t2(~ e e±4a ee±2~

e I Plan

e±2o e

e±2a

23.6m 7.8m 12.5m 55m

Fig. 7. Schematic diagrams of the other flutes in the site 1 area: (a) flute 2, (b) flute 3, (c) flute 4.

otrowski and Smalley, 1987; Boyce and Eyles, 1991; Hart, 1995a). These workers suggest that drumlins form when material flows around more competent

obstructions within the subglacial deforming layer, which can be made either of a clast of hard rock, or a more competent mass of soft sediment, such as


(a) 0

large clast orientated in

palaeo-ice flow direction _

o..~_ 0 0 0 o

~ esker

weak / / ~ \ ~ .,~ute '

~ flat sub/supra %

many rounded glacial % bullet shaped surface

clasts striations

O / Drumlin 0

0 0 20 km I i I

water pool

0 ©

0

o / faint

/

/

Fig. 8. Site 2. (a) Schematic map of the area; (b) photograph of a hollow in the eastern area; (c) schematic diagram of the drumlin; (d) photograph of the drumlin (in background with two figures resting on it) and moraine in foreground.


e e+~c r e e:E2cr e e

/ / X . ht 1.5m

e±6o" ~ e:~2.o" e~4~ e:i:2a

e ¢T (c)

sand and gravel or till. Hart (1995c) has suggested that there are three types of deforming bed drumlins: deposit ional - - that form in a manner similar to flutes; deformat ional - - that have a weak core which is deformed; and erosional - - that have a strong core. However, it is suggested that all three styles of drumlin form associated with net subglacial deforming bed erosion. The pattern of fabric flow directions indicates that this drumlin represents a depositional drumlin (i.e., it has a pattern similar to that recorded in flutes by Rose, 1989).

We suggest that the irregular linear feature represents a short subglacial esker, the stratified sand represents deposition in the conduit associated with flowing water whilst the till represents the saturated subglacial sediments infilling the conduit as water pressure dropped. The angular blocks which rest on the surface probably represent supraglacial debris.

Thus, we suggest that at this site there is more evidence for a subglacial surface with indications of subglacial deformation in the form of the flutes, drumlins and also till injections within the esker. Superimposed on these subglacial landforms is a cover of supraglacial sediment, reflecting this site's more marginal position.

3.8. Transverse fea tures

Fig. 8 (continued).

To the north of site 1 there is an area dominated by transverse features (site 6) (Fig. ld and Fig. 10). These consist of three styles: small intersecting transverse ridges and flutes, small continuous ridges, and large discontinuous ridges, which we will discuss in turn.

(1) Small intersecting transverse ridges and f lutes (feature 6). In the north of the area, in a small valley there are a series of intersecting flutes and transverse ridges. On inspection it can be seen (Fig. 10b) that the top surface of the intersection is composed of a very sandy diamicton, whilst the lower part is the silt-rich diamicton seen elsewhere in the area. The form of the flute is both in the silt-rich till and the sandy till and the flute clearly postdates the formation of the transverse feature. The fabrics in the feature (6a and 6b) (Fig. 6a, Fig. 10a, and Table 2) indicate a relatively high fabric strength in the silt- rich till, but a much weaker fabric in the sandy till and both are oriented in the ice flow direction.


resultant height of landform

: ~ - . - ' - _ , , ~ - . - - _ , , ~ . - . - - 2 , , % - . - - _ . , % - . - ' - 2 , , ~ . - . - ice

deforming layer

i - - . . i - f - _ . . / _ J - _ . . , _ r - - . . / _ J " - - - t - J " - -

resultant height ] of landform

ice

deforming layer

Fig. 9. Schemalic diagram to show the relationship between the size of flutes and the thickness of the deforming layer.

(2) Low continuous transverse ridges (feature 7). The main part of the ridges are composed of a very sandy diamicton approximately 30 cm thick. Beneath this is the silt-rich diamicton, which is also in the shape of a small anticline, but of a smaller amplitude than the shape of the ridge, and with the crest towards the proximal side of the ridge (Fig. 10b) (7a and 7b in Table 2). The fabrics at this site show a contrast to the latter site, with the stronger fabric in the sandy diamicton and the weaker in the silt-rich till. Also the mean orientation is 40 ° from the ice direction in the upper sandy unit and 49 ° in the lower till unit.

(3) Large discontinuous ridges (feature 8). To- wards the southern end on the area, there are three larger (1.5 m) moraines (Fig. 10c) also located at a small valley. These ridges are made entirely of till. A fabric was taken on the proximal and distal side of the ridge (8a and 8b). These indicated a weak fabric, oriented 40 ° from t]he ice direction on the proximal side, and a strong fabric, oriented 20 ° from the ice direction on this distal side. Roundness values taken

both in the ridge (8c) and on the sandy transverse ridge (8d) show a relatively similar high roundness value.

3.9. Interpretation of the transverse features

Sandy till was not found elsewhere in the study, so we suggest that this probably represents a fluvial deposit, that was subsequently subglacially deformed. The deformation in feature 6 was by fluting, but the compressive deformation in feature 7 is more equivocal, and could be formed proglacially a bull- dozed-type push moraines or subglacially (Rogen moraines).

Lundqvist (1989, 1995) and Hattestrand (1995) have argued that Rogen moraines can contain any glacial or non-glacial sediment, and like drumlins are a landform produced by many processes. Many workers, e.g., Lundqvist (1989) and Boulton (1987), have also demonstrated the link between drumlins and Rogen moraines. Most workers have argued that Rogen moraines form due to subglacial com-


• • 0.3m

1.5m

b) ~ e e+~

Ds

1.5m

~ q ~ . / \ ~ -. ~-.~.- ~-+'.~/ \ / oo \ .... ~c~-~..~.

/,,., oo \ / °° \

1.5m

Fig. 10. Schematic diagrams of the transverse ridges: (a) feature 6, (b) feature 7, (c) feature 8, (d) photograph of feature 8.

pression, caused by subglacial bedrock topography, ice compression or thermal changes (e.g., Solliel and S¢rbel, 1984, suggested that they formed at

the transition between warm-based and cold-based ice).

Thus we suggest that the simplest explanation is


Fig. 10 (continued).

that a localised relxeat occurred and outwash sediments were deposited. This was followed by a small readvance, which in places fluted the fluvial surface, but in others produced compressive deformation, which may have occurred either proglacially or subglacially.

4. Discussion

The sediment~¢ and geomorphic evidence from the eastern margin of the Columbia Glacier includes: flutes, drumlins, lineations, and eskers oriented parallel with ice flow; and squeeze-type push moraines, dump moraines and cavity diapirs orientated per- pendicular to ice :flow. The vast majority of these features are indicative of a deforming bed, and together they substantiate the suggestion of a deforming bed proposed fi:om the drilling by Humphrey et al. (1993).

Rose (1987) has suggested that the streamlined subglacial bedforms (lineations, flutes and drumlins) represent a continuum formed by similar processes. Many authors have suggested that the presence of subglacial bedforms indicates increased ice velocity (Boulton, 1987; Boyce and Eyles, 1991; Clark, 1994) and thus basal shear strain and that elongation ratio can be used as a proxy for this. Hart (1995a) has suggested that the height of bedforms relates to the

deforming layer thickness, since the landform (be it flute or drumlin) must be thinner or equal to the deforming bed thickness (Fig. 9).

Many authors have reported the presence of superimposed subglacial streamlined landforms (Rose and Letzer, 1977; van der Meer, 1983; Kr/iger and Thomsen, 1984; Rose, 1987, 1989; Hart, 1995a). However, there were only a few occurrences where this was observed at the Columbia Glacier (e.g., the lineations at site 1, and the reorientation at site 6). Instead, we suggest that the surface shows flutes at different stages of evolution. The flutes are mostly of a similar height, approx. 0.3 m except for the very large flute at site 5 which was formed in a hollow (and so may reflect a local thickening of the deforming layer to a minimum thickness of 1.5 m).

From the discussion above, it was suggested that both elongation ratio may be a proxy for shear strain, and height may be a proxy for thickness of the deforming layer. The values from known flutes and depositional drumlins are shown in Fig. 11 (Table 3). The interaction of these processes (shear strain and thickness of the deforming layer) also leads to complex eigenvalue results. It has been shown by Hart (1994) that as strength of eigenvalues increases with shear strain in a stable thickness deforming layer, but that as the deforming layer thickens this leads to a reduction in fabric strength.

194 J.K, Hart, B. Smith~Sedimentary Geology 111 (1997) 177-197

1000

500

c

100

v 50

~ to m 5

C2 E&H C4 Ioi ~

Benn Rose A 0.71, 0.68

C5 0,71

C.3I • LP2 oLd7 0.73 ( 0.64 , LP1

0.59 1

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Height (~ thickness of the deforming layer)

Fig. 11. Graph of elongation ratio against height for the examples shown in Table 3 (with S1 values also shown). The subglacial bedforms in this study are as follows: C1 to C5 represent landforms from the Columbia Glacier, Alaska (C1 = flute 1; C2 = flute 2; C3 = flute 3; C4 = flute 4; C5 = drumlin); Rose i and Rose A represent two flutes from Austre Okstindan Glacier, Norway (Rose, 1989) (these two flutes were chosen because they represent the flutes which were studied in the greatest detail); E and H represent a flute from Isfallsglaci~en, Sweden (Eklund and Hart, 1996); Benn represents flute A from Slettmarkbreen, Jotunheimen, Norway (Benn, 1994); LP1, LP2 and LP3 represent rock-cored (depositional drumlins) from Loch Portain, Isle of Uist, Scotland (Hart, 1997).

We suggest that this pattern is also reflected in the development of subglacial streamlined landforms. Within a deforming layer of constant thickness, the bedform will elongate over time, but if the deforming layer thickness changes, then the rate of elongation will change (become faster if the layer thins or slower if the layer thickens).

We tentatively suggest that these processes (and the corresponding response of the till fabric) can be described by four end-members: (a) thin deforming layer~low shear strain (e.g., Columbia flute 3 {C3 }); (b) thin deforming layer~high shear strain (Columbia flute 1 {C1 }); (c) thick deforming layer~low shear strain (Loch Portain 1, Isle of Uist, UK, Hart (1997) {LP1 }; (d) thick deforming layer~high shear strain (flute A, Norway, Rose, 1987) {Rose A}. These results show that bedforms formed under a thick deforming layer tend to have weaker fabric strengths, and that in both cases shear strain has little effect on eigenvalue strength (these results are very similar to those shown by Hart, 1994 for tills).

5. Conclusion

Ice drilling experiments by Humphrey et al. (1993), have suggested that one of the reasons for fast ice flow at the Columbia Glacier was the presence of a deforming bed. Our investigations of the

recently deglaciated foreland confirm these results. Evidence for the deforming bed includes:

(a) subglacial streamlined bedforms at many scales,

(b) squeeze-type push moraines which indicates the movement of material from beneath the glacier into the foreland, and

(c) crevasse diapirs, which indicate the present of saturated sediments.

We suggest that the deforming bed in most places was relatively thin (approx. 0.3 m) (which would be expected because of the marginal location of the site), and that the cumulative shear strain was very high, indicated by the presence of flutes over 300 m long. This thickness is consistent with the data collected by Humphrey et al. (1993) from a location up-glacier where the deforming layer would be expected to be thicker (Hart et al., 1990). However, in local areas, in particular in hollows in the landscape, the deforming layer was much thicker (1.5 m) and taller flutes were formed. This indicates how the deforming bed changes in response to local conditions, and so highlights a further problem of the spot sampling associated with ice cores. This irregularity of the deforming bed thickness may also provide further evidence for the presence of 'sticky spots' beneath the glacier. These were suggested by Kamb (1991) and Alley (1993) to be places beneath the


glacier where a discontinuity in the lubricating till of the deforming layer (usually a bedrock knoll) can support high basal shear stress.

Thus, we suggest that the landscape on the Columbia foreland was formed by fast ice flow associated with a deforming bed, and presumably a similar subglacial surface is present beneath the Columbia Glacier today.

Acknowledgements

The authors would like to thank Richard Waller and Kirk Martinez for field assistance and to KM for photography. We would also like to thank Nancy and Jim Lethcoe, Bob Krimmell and Charles Raymond for information about the site and the Tatitlek Cor- poration for permissJion to carry out research in the area. We would also like to thank Tim Aspden and his colleagues in the Cartographic Unit, Department of Geography for their excellent figure reproduction. JKH was funded by NERC grant GR9/991, and BS by the University of Southampton (Richard Newitt Trust, Roy Queare Bequest and the Vice-chancellor's fund).

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