temporal changes in subglacial meltwater activity: field evidence from the late devensian in the...

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Sedimentary Geology 160 (2003) 291–307

Temporal changes in subglacial meltwater activity: field evidence

from the late Devensian in the north of Ireland

Jasper Knight*

Glacial Research Group, School of Biological and Environmental Sciences, University of Ulster, Coleraine, Co. Londonderry,

Northern Ireland, BT52 1SA, UK

Abstract

Temporal changes in meltwater abundance, distribution and characteristics (controlling subglacial processes and ice sheet

dynamics) can be inferred from subglacial sediment successions. Field evidence for changes in subglacial meltwater

characteristics over time is presented from two sites (Doonan, Drummee) near a former late Weichselian (Devensian) ice centre

in the north of Ireland. On a macroscale, both sites investigated show subglacial diamicton overlying glacially planated bedrock

platforms. In more detail, primary sedimentary structures and facies variability show a complex relationship between

depositional processes and meltwater characteristics at the ice/bed interface (IBI). Sedimentary evidence suggests sediment

transport and deposition took place by low-viscosity subglacial slurries (mobile sediment–meltwater admixtures), which are

part of a continuum between the processes of subglacial sediment deformation and subglacial meltwater flooding. Subtle

changes in meltwater abundance and distribution at the IBI controlled slurry rheology, mechanisms of particle support and

detailed sediment depositional processes.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Temporal changes; Subglacial meltwater activity; Late Devensian

1. Introduction water production over refreezing by basal ice at the

Subglacial meltwater availability, distribution and

mobility can exert a strong influence on the dynamics

of temperate (warm-based) and polythermal (variably

warm- and cold-based) ice sheets, and can be partly

inferred from subglacial morphological and sedimen-

tary evidence preserved in the geological record

(Menzies, 1989; Shaw, 1996). In a subglacial environ-

ment, free meltwater potentially comes from a number

of different sources including (1) an excess of melt-

0037-0738/03/$ - see front matter D 2003 Elsevier Science B.V. All right

doi:10.1016/S0037-0738(03)00088-5

* Fax: +44-28-7032-4911.

E-mail address: [email protected] (J. Knight).

pressure melting point, (2) frictional/strain heating, (3)

englacial and supraglacial sources and (4) meltwater

or porewater that is moving through or contained

within subglacial sediments (Paterson, 1994). Here,

the term ‘meltwater’ refers to free water present under

hydrostatic pressure at the ice/bed interface (IBI), and

‘porewater’ refers to the water, irrespective of source,

held interstitially within the subglacial sediment pile.

Collectively, these waters and their movement are

potentially an active geomorphic agent, generating

subglacial morphological and sedimentary signatures

(Shaw, 1996).

Fig. 1 identifies some of these potential signatures

which are associated with a range of glacial sedimen-

s reserved.

Fig. 1. Diagram illustrating the continuum of sedimentary processes, structures and characteristics of the deformation and meltwater flood

models. Sediment slurries (italics, double arrow) cover a range of flow types including mass flows, debris flows and fluidal flows.

J. Knight / Sedimentary Geology 160 (2003) 291–307292

tary processes. These processes vary along a contin-

uum between grain-on-grain ‘lodgement’ directly into

the substrate and in a relatively water-poor environ-

ment, to low-viscosity mass- and fluidal-flows devel-

oped in a water-abundant environment at the IBI.

Sediments associated with these processes are very

diverse and include water-washed (sand and gravel)

layers of varying thicknesses and dimensions, includ-

ing stringers, lenses and interbeds (Fig. 1) which

directly record meltwater flow at the IBI. Other sedi-

ments may include any poorly sorted and poorly

organised sediment admixture (diamicton) which has

a low viscosity and high porewater content. The

process of subglacial sediment deformation has often

been invoked in the formation of subglacial diamic-

tons (Hart and Boulton, 1991; Boulton et al., 2001). In

the literature, the umbrella term ‘deformation’ refers

to a unit or package of subglacial sediment which is

moving under the effects of glaciotectonic strain

(Boulton, 1987). Use of the term ‘deformation’ does

not specify a single sedimentary process, and makes

no assumptions on the porewater content or grain size

spectrum of the sediment package (which together

control its viscosity).

Debate on the role of subglacial water in the

generation of geomorphic and sedimentary signatures

has focused around two main approaches (Menzies,

1989) which can be viewed as end-members along

this process continuum (Fig. 1).

(1) Models invoking pervasive subglacial sediment

deformation consider that high porewater pressure

within the sediment pile maintains the sediment in a

soft, potentially deformable state, and that relatively

little free meltwater is present at the IBI (Boulton et

al., 1974; Boulton and Hindmarsh, 1987). According

to this model, water-washed layers are formed infre-

quently at the IBI and may be destroyed or incorpo-

rated by later tectonic deformation (Hart and Boulton,

1991). (2) Subglacial meltwater flood models consider

that most subglacial water activity takes place at the

IBI, and that water moves through both organised and

disorganised networks, channels, sheets and films

which may be of regional extent (Sharpe and Shaw,

1989; Rains et al., 1993; Shaw, 1994a, 1996). In

meltwater flood models, meltwater flow takes place

semi-independently of substrate type, sediment rheol-

ogy or porewater variations within subglacial sedi-

ments, and is maintained by high hydraulic pressure at

the IBI. Evidence for subglacial meltwater flood

events includes a range of net erosional and net

depositional forms on different scales, from melt-

water-cut bedrock channels to interbedded sand and

gravel layers with preserved internal sedimentary

structures (Shaw, 1996; Pair, 1997).

The two above models take differing views of the

role played by meltwater and porewater variations in

generating morpho-sedimentary signatures at the IBI.

The deformation model suggests that these waters are

relatively passive and that water fluxes are relatively

small, whereas the subglacial flood model suggests

that meltwater is an active subglacial geomorphic

agent. Neither the deformation nor the meltwater

J. Knight / Sedimentary Geology 160 (2003) 291–307 293

flood model, however, satisfactorily addresses melt-

water processes both within the substrate and at the

IBI (Menzies, 1989). Some theoretical considerations

(Jenson et al., 1995; Piotrowski, 1997) and evidence

from late Pleistocene (Piotrowski and Kraus, 1997;

Piotrowski and Tulaczyk, 1999) and modern ana-

logues (Boulton et al., 1974, 2001) suggest that water

is important at both the IBI and within subglacial

sediments, and can occur at different scales and

different times during ice sheet evolution (Boulton

and Hindmarsh, 1987). It is likely, however, that

sedimentary and meltwater-related processes taking

place at the IBI lie somewhere between these per-

ceived end-members (Muller, 1982; Ruszczynska-

Szenajch, 1982; Menzies, 1989), which should be

tested against field evidence. Resolving this problem

is important for producing accurate numerical models

and geologically based reconstructions of past ice

sheets.

In order to examine the role of subglacial melt-

water more widely, this paper looks at temporal

changes in subglacial meltwater distribution and char-

acteristics, as can be reconstructed from up-sequence

changes in sediment successions. This paper has two

main aims: (1) to describe the subglacial structures

and sediments at two sites near the centre of the late

Weichselian (Devensian) ice sheet in the north of

Ireland; and (2) to evaluate up-sequence (temporal)

changes in IBI processes and subglacial meltwater

distribution at these sites. An interpretation involving

sediment deposition by slurries or flows of varying

viscosity (including ‘deformation’ processes) can

account for sedimentary structures and facies relation-

ships, and describe the nature of meltwater fluxes both

within the bed and at the IBI at these sites.

2. Regional glacial setting

During the late Devensian glaciation, ice flowed

outwards from linked lowland dispersal centres in the

north of Ireland towards marine margins located on

the Irish continental shelf and in the Irish Sea (Fig.

2a). Drumlinisation occurred in a number of dated

millennial-scale episodes between f 16.6 and 13.814C ky BP (McCabe and Clark, 1998; Knight et al., in

press). These drumlinisation episodes involved fast

ice flow onto the continental shelf, the shaping of

subglacial bedforms inland and transport of sediment

to ice margins building moraines and outwash spreads

(McCabe, 1993; Knight and McCabe, 1997a). Field

geomorphic and sedimentary evidence is presented

from two sites (Doonan, Drummee) near the former

ice sheet centre in the Omagh Basin, north–central

Ireland (Fig. 2b).

3. Field evidence

3.1. Doonan

The exposure is located on the crest of a bedrock-

cored, diamicton-covered ridge in the western Omagh

Basin (Figs. 1 and 2). The exposure (50 m long, 10 m

high) comprises a single massive diamicton facies

(Dmm facies; sensu Eyles et al., 1983) which overlies

a glacially smoothed bedrock platform (Fig. 3). In this

area, ridges (up to 1800 m long, 450 m wide and 30 m

high) are aligned perpendicular to regional westerly

ice flow and are interpreted as subglacial Rogen or

ribbed moraines (Knight and McCabe, 1997a). Bed-

rock comprises flat-bedded and vertically jointed

Devonian Old Red Sandstone, but orientation of the

ridge on which the exposure is located follows the

strike of an adjacent Tertiary dolerite dike which

underlies part of the ridge axis. Carboniferous lime-

stone beds outcrop 4 km to the south.

(1) The bedrock platform has a sharp planar to

undulating upper surface which is marked by a

number of styles of erosional detail. Non-

connected whaleback ridges are present at some

locations on the bedrock surface (Fig. 3a). These

ridges (0.8–1.5 m high, 5–6 m long and 1–3 m

wide) are symmetric in long profile and have

ridge axes aligned 150–330j (n = 5). Overall

ridge shape does not follow bedrock-jointing

patterns (Fig. 4A). The whalebacks show a

hierarchy of crosscutting ice-erosional forms.

Whaleback flanks are steep (< 50j) and show

shallow erosional scours which cut across bed-

rock joints (Fig. 4A). On whaleback flanks,

striations are present both within and outside

these scours. The striations are short (< 10 cm

long), curved, occasionally cross-cutting, are

flank-parallel and follow the overall shape of

Fig. 2. (a) Location map showing late Devensian ice flow vectors in the north of Ireland (after McCabe, 1993) and location of the study area

(inset). (b) Generalised glacial geomorphology of the study area and the location of the studied sites (Doonan, Drummee) and ice flow vectors.


Fig. 3. (a) Drawn section of the exposure at Doonan. (b) Generalised sediment log and clast fabrics measured at Doonan. The two clast fabric

data and bulk sediment samples were taken from the same locations (arrowed).


the bedform (Fig. 4A). Whaleback crests are

generally smooth and planar. Microrelief is

provided by shallow grooves (up to 1 cm deep,

30 cm long, 2 cm wide and spaced 8 cm apart)

which are aligned 100–280j. Striations on

whaleback crests are mainly short scratches

aligned towards 270j (n = 7). The crests of some

whalebacks show plucked, angular fractures

which are aligned parallel to joint planes (Fig.

4B). The hard rock substrate between the whale-

backs is generally planar, bedding plane-parallel,

and shows minor incised erosional features (s-

forms; Kor et al., 1991). These s-forms are linear,

have a ridge and furrow morphology and are

aligned consistently 120–300j (n = 4). The ridgesare up to 4 m long, 0.4 m high and 1.0–1.6 m

wide and disappear towards the margins of the

whalebacks. Striations are present at all locations

on the bedrock platform but are found mainly on

higher-relief surfaces. Striations on this surface

are generally long (50–60 cm long), straight and

symmetric and aligned 120–300j.(2) The overlying diamicton (8–10 m thick) is

texturally variable, vaguely planar-bedded and

is laterally consistent in geometry across the

exposure. The bedrock/diamicton contact is not

Fig. 4. (A) The bedrock surface features at Doonan showing whaleback with striations and s-forms. In all photos the trowel is 28 cm long. (B)

The bedrock surface at Doonan showing the plucked crest of the whaleback.


seen clearly. The diamicton is matrix-dominant

throughout (< 90%) and contains dispersed sub-

to edge-rounded pebbles and cobbles (< 25-cm

diameter) which are striated across all surfaces.

Discontinuous sorted sandy stringers and lenses

(< 20 cm long, millimeter scale in thickness) are

present throughout. These are flat-lying in the

profile, and may be deformed by loading from

above. No glaciotectonic shears are identified.

Up-sequence, clast structures change from dis-

organised matrix-supported clusters (Fig. 5A,B)

to better-organised, flat-lying and discontinuous

clast lines (< 5 m long) which occasionally de-

fine bed boundaries (< 1 m apart). Clast clusters

Fig. 5. (A) Chaotically organised channel-fill sediments within diamicton at Doonan, showing clast clustering and clast-supported granules (top

left). (B) Annotated sketch of a.


composed of openwork massive granules are

found associated with poorly defined upward-

going water escape structures (< 60 cm high, 5

cm wide) and infilled channel-shaped structures

(1.2 m wide, 1 m deep). These channel-shaped

structures have long axes aligned north–south,

generally sharp lateral and upper margins and are

infilled by clast-dominant sediments comprising


chaotically organised and clustered pebbles and

isolated boulders which are surrounded by an

aligned, granular matrix (Fig. 6A,B). Clasts are

often aligned in single layers parallel to channel

margins. In the middle of the sediment succes-

sion, clasts dispersed within the diamicton are

mainly (70%) flat-lying and are associated with a

flat-lying horizon of clasts held in a cemented

calcite matrix (Fig. 7). This horizon is present

discontinuously across the length of the expo-

sure. Bulk sediment samples removed from near

the bottom and top of the sediment succession

(samples A and B, respectively, 4 m apart

vertically) are similar in both grain size charac-

teristics and clast lithology. The matrix is sand-

dominant and contains components from all

grain size classes (Fig. 8). Most clasts are local

Old Red Sandstone (64.2–65.3%) with other

clasts derived from the south (Carboniferous

limestones; 16.3–17.8%), the east (igneous rocks

Fig. 6. (A) Infilled channel structure within diamicton at Doonan showing

Annotated sketch of a.

from the NE Omagh Basin; 10.2–10.7%) and the

north (schist and quartzite from the Sperrin

Mountains; 7.1–8.0%). Despite a similar com-

positional make-up, clast fabrics taken from the

bulk sample locations are rather different (Fig.

3b). Sample A, lowermost in the profile, has a

poorly developed fabric and relatively low S1value. Sample B, from near the top of the profile,

shows a bimodal clustering and a higher S1value.

3.2. Depositional environment of sediments at

Doonan

The exposure at Doonan records an upward

decrease in the availability of free subglacial meltwater,

with concomitant changes in diamicton depositional

processes (Fig. 3b). The conformable relationship

between the bedrock surface and overlying diamicton

beds, their similar directional signatures and identical

rotated clast. Note the sharp upper contact of the channel infill. (B)

Fig. 7. Lateral discontinuity of the concreted horizon within the diamicton profile at Doonan (arrowed).


clast composition throughout the sediment profile

suggests that the bedrock was eroded and sediment

deposited during the same west- or northwest-going ice

event.

Bedrock surfaces showing s-forms (Figs. 4A,B)

have been attributed to a range of IBI processes

including ice scouring, the movement of water-satu-

rated subglacial diamicton, pressurised subglacial

meltwater and meltwater–ice or meltwater–sediment

mixtures (Dahl, 1965; Gjessing, 1966; Sharpe and

Fig. 8. Graph of matrix (63–2000 Am) grain-size distribution for

samples from Doonan and Drummee. Sample locations are

indicated by arrows from the clast fabric plots shown in Figs. 3b

and 9, respectively.

Shaw, 1989; Kor et al., 1991; Pair, 1997). At

Doonan, the presence across the bedrock platform

of meltwater scours and furrowed s-forms suggests

that erosion by free subglacial meltwater was an

important initial process. The variable orientation of

meltwater scours around whaleback flanks indicates

adaption of flow to pre-existing subglacial topogra-

phy and, with reference to whaleback size, may

suggest that the meltwater layer was on the order of

a few tens of centimeters thick (cf. Pair, 1997).

Striations found within some meltwater scours, and

plucking on whaleback crests, may record a later

period of abrasion associated with ice/bed recoupling

(Gjessing, 1966) due to thinning of the IBI meltwater

layer.

This initial event sequence and related changes in

IBI meltwater are supported by diamicton character-

istics low down in the sediment profile. Here, poor

clast organisation, presence of vertically aligned clast

long axes, low fabric eigenvalues and presence of

water escape structures suggest chaotic sediment

release by meltout from the basal ice zone (cf. Eyles

et al., 1982; Paul and Eyles, 1990) and sediment

transport by low-viscosity slurries, promoting clast

organisation into chaotic clusters (Hampton, 1975).

The clast fabric shows the low S1 and high S3 values

typical of ‘glacigenic sediment flow’ (Dowdeswell

and Sharp, 1986), but fabric spread better resembles


‘deformation till’ (Hicock et al., 1996). Abundant

porewater during and following sediment release

and accumulation may have been maintained by low

lateral water pressure gradients. This is evidenced by

the poor clast fabric, small vertical water escape

structures and the presence of discontinuous sandy

washes and stringers which suggest limited drainage

organisation at the IBI. The development of channel-

shaped features, which are infilled with clustered and

aligned clasts separated by a coarse granular matrix,

suggests incision and filling by free-flowing melt-

water/sediment admixtures at the IBI. Such subglacial

incision into soft substrates has been previously

described from the European (Piotrowski et al.,

1999) and Laurentide ice sheets (Klassen and Hughes,

2000). Small changes in sediment concentration

within meltwater, or changes in porewater volume

(pressure) within subglacial sediment, can also lead to

marked changes in sediment rheology (Clarke, 1987).

Flow characteristics of these sediment admixtures can

be deduced from sedimentary structures preserved

within the infilled channels cut into the host diamicton

beds. In these channels, chaotic clast clusters and

aligned clasts attest to sediment transport in cohesive

slurries in which particle support is maintained by a

combination of turbulent suspension, high porewater

pressure and grain–grain dispersive pressure (Han-

vey, 1992; Mulder and Alexander, 2001). Matrix

strength is not an important particle support mecha-

nism in such sand-dominated sediments (Shanmugam,

1996). The presence of clast clusters and clasts

aligned parallel to channel margins suggests organ-

isation of this grain-size component, in which larger

clasts are swept to lateral margins of the slurry and

deposited by frictional effects, or form a high-density

traction or plug-flow deposit within the slurry flow

body (Hampton, 1975; Major and Iverson, 1999). The

shallow channels were actively cut during passage of

these flows and parallel to ice flow direction. The

vertically aligned boulder in the centre of the channel

infill, surrounded by a ring or aureole of smaller

aligned clasts (Fig. 6A), is similar to some rotation

structures formed within tabular diamicton bodies by

glaciotectonic deformation (e.g. Menzies and Malt-

man, 1992). Its presence within an infilled channel,

and associated with structures formed from viscous

slurries, does not support formation wholly by gla-

ciotectonic deformation. A more likely explanation is

that this outsized boulder was carried by turbulent

sediment mixing (grain–grain interaction) within a

high-density slurry or flow, which allowed the clast to

move about during transport. Sediment ‘freezing’ and

preservation of internal sedimentary structures,

including the boulder aureole, were caused by rapid

dewatering through the dominantly sand matrix dur-

ing flow cessation (Muller, 1982; Postma et al., 1988).

This model for the transport and rotation of outsized

clasts is qualitatively similar to modes of transport in

subaerial and subaqueous debris flows (e.g. Coussot

and Meunier, 1996). In a meltwater-abundant subgla-

cial environment, glacial driving stress is likely an

important mechanism of generating highly pressured,

high-density and turbulent flows (Hanvey, 1992;

Shaw, 1994b, 1996).

Above this level in the exposure profile at Doonan,

which is only 1–2 m thick, clast lines and flat-lying

clasts at the top of the section suggest deposition of

diamicton by lodgement (Kruger, 1979), which is

supported by high S1 and low S3 values typical of

‘lodgement’ till (Dowdeswell and Sharp, 1986;

Hicock et al., 1996).

In this sediment profile, the upward transition from

meltout to lodgement processes, and the organisation

of diamicton structures, denotes a change in subglacial

meltwater abundance and behaviour (cf. Krzyszkow-

ski, 1994). There are two possible reasons for these

hydrological changes. First, a thickened sediment pile

may have provided accommodation space for pore-

water transfer at certain layers within the sediment

profile (the A horizon of Boulton et al., 1974),

especially in relatively sandy sediments. Changes in

porewater transfer rates lead to changes in sediment

viscosity (Hampton, 1975; Pierson, 1981). Here, for

example, sediment dewatering (from low-viscosity

slurries associated with the process of sediment melt-

out from basal ice) was succeeded by meltwater

organisation into subglacial channels developed on

this hardened, dewatered sediment. In turn, this was

the ‘harder’ substrate into which clasts were later

lodged (Kruger, 1979; Hicock, 1992), observed in

the uppermost part of the exposure. Second, faster-

flowing ice may have increased strain in the sediment

pile and driven sediment dilation, deformation and

meltwater expulsion (Boulton et al., 1974, 2001).

Tulaczyk (1999) suggests that sediment deformation

occurs to the greatest depth (0.1 m) in sandy diamic-


tons. The absence of glaciotectonic shears also sug-

gests that the strain was accommodated by deforma-

tion.

3.3. Drummee

This exposure is located in a disused Carboniferous

Lower Limestone quarry excavated in a rock-cored

drumlin. Regionally, the diamicton cover thins rapidly

to the west and north (Chapman, 1970), and drumlins

(up to 720 m long, 470 m wide and 35 m high) are

generally separated by meltwater channels or inter-

drumlin lakes. The exposure (100 m long, 15 m high)

comprises a single matrix-dominant diamicton (Dmm

facies) (Figs. 9 and 10). Diamicton beds conformably

overlie the sharply planed bedrock surface which is

bedding plane-parallel and occasionally has a stepped

profile.

The limestone beds show a shallow (5j) dip to the

southwest. The bedrock surface is characterised by

isolated ridges (up to 6 m long, 2 m wide and 30 cm

high) which are aligned 120–300j. Occasionally

linear striations (oriented 140–320j) are present

across ridge crests. The contact between the diamicton

and bedrock is not clearly exposed.

The overlying sediment consists of poorly defined,

laterally continuous and flat-lying beds (1–3 m thick)

of massive, homogenous and matrix-dominant

Fig. 9. Sedimentary log and clast fab

(< 85%) diamicton. The beds generally dip shallowly

(10j) towards the northwest. The diamicton matrix

component ranges from fine to coarse sand (Fig. 8).

Laterally continuous flat-lying sandy stringers (< 2 m

long, < 1 cm thickness) are present throughout.

Stringers have sharp, compacted margins and may

be deformed. Clasts are exclusively of local Lower

Carboniferous limestone, are mainly cobbles and

boulders (< 60 cm diameter) and are edge- or sub-

rounded or bulleted with striations present across all

clast faces. Clasts are commonly organised into dis-

continuous lines (< 10 m long) which demarcate bed

boundaries (Fig. 10A). A clast fabric from the lower

part of the profile (3 m above the bedrock surface) has

weakly bimodal clustering and a medium S1 value

(Fig. 9).

In the uppermost part of the succession, diamicton

beds are more poorly defined. A channel-shaped

structure (3 m deep, 5 m across), whose axis is aligned

generally ice-flow parallel at 150–330j, is present atthe top of the succession near the drumlin crest (Fig.

10B). The channel margins are cut into the host

diamicton beds, and are paved with clasts which

follow the channel outline. The channel is infilled

by concentrations of wholly locally derived monoli-

thologic pebbles and boulders. These are variably

clast-supported to openwork and form clusters (up

to 1-m diameter). Clasts within these structures may

ric of the Drummee exposure.

Fig. 10. (A) Diamicton beds showing subhorizontal clast lines at Drummee (arrowed). (B) Channel-shaped structure infilled with dipping and

clast-supported boulders at Drummee. Note the contrast between the clast-dominant channel infill and matrix-dominant host diamicton, and the

sharp channel margins (arrowed). Spade is 1 m long.


be chaotically arranged or show long axes which dip

consistently to 330j. Clasts are also aligned parallel

to, and demarcate, the outer margins of the channel

which are broadly symmetric (Fig. 10B). At the top of

the section, up to half of clasts of all sizes show in situ

shattering and fracturing.

3.4. Depositional environment of sediments at

Drummee

The exposure at Drummee records an upward

increase in the organisation of subglacial meltwater,

from its accommodation within the sediment pile, to


channelisation at the IBI. Directional signatures

within the diamict beds are similar to those on the

underlying bedrock surface; therefore, it is assumed

that both were formed during the same northwest-

going ice advance.

The sharply smoothed and laterally continuous

bedrock surface, which follows limestone bedding

planes, attests to subglacial abrasion, areal scouring

and formation of small ridges (cf. Lliboutry, 1994).

The general absence of features such as s-forms

suggests that meltwater was not freely available, or

was confined to a thin, discontinuous sheet which did

not permit organisation into channelised networks

(Kor et al., 1991; Pair, 1997). Such general absence

of meltwater at the IBI may also have promoted

plucking of bedrock blocks along joints and bedding

planes, leading to the stepped profile of the platform

surface (Sugden et al., 1992).

On the exposure scale, the lateral continuity and

tabular, flat-lying nature of the diamicton beds, and

presence of clast lines and bulleted and striated clasts,

suggest deposition by active subglacial lodgement

(Kruger, 1979, 1984). A clast fabric from within the

diamicton beds shows eigenvalues typical of ‘glaci-

genic sediment flows’, although they are also near the

‘lodgement’ domain (Dowdeswell and Sharp, 1986)

(Fig. 9). Fabric spread suggests ‘lodgement’ or ‘defor-

mation’ as the dominant process (Hicock et al., 1996).

A lodgement interpretation is supported by the

absence of free meltwater indicators on the bedrock

surface, and the presence of sandy stringers which

interbed with the diamicton facies (e.g. Eyles et al.,

1982). Clast lines, dipping clasts and vertical pillars

within diamict beds further up-profile (Fig. 10B)

suggest that sediments were deposited from viscous

slurries in which clasts are left as residuals by the

movement of the finer matrix component (Hicock,

1991, 1992; Tulaczyk, 1999). Deposition by viscous

slurries is also evidenced by the flow-parallel b–c

plane dip of some clasts (e.g. rollers, which can be

rafted by such viscous slurries; Postma et al., 1988).

The flat-lying and laterally continuous diamicton beds

across the drumlin crest, and continuity of diamicton

internal structures across the exposure, suggest that

the IBI was of subdued relief during accumulation of

the sediment pile, thus the sediment layers were

possibly deposited as unconfined sheets. Similar

mechanisms of sediment behaviour are thought to be

important in maintaining fast ice flow beneath ice

streams (e.g. Alley et al., 1989; Knight et al., 1999).

These mechanisms may also be important in the

formation of drumlins (Hanvey, 1992; Knight and

McCabe, 1997b), and this supports some previous

drumlinisation models in which low driving stress is

maintained by bedload slurries (Boulton, 1987). A

viscous (sediment-rich) slurry interpretation may also

help explain the absence of dewatering and other

meltwater/porewater structures during cessation of

sediment movement and subsequent freezing (Hamp-

ton, 1975; Muller, 1982; Menzies and Maltman,

1992). The presence of an infilled channel near the

drumlin crest has been observed in other subglacial

settings (e.g. McCabe and Dardis, 1994; Piotrowski et

al., 1999). Here, the channel margin is clearly ero-

sional and the orientation of the channel axis is

consistent with other flow indicators at the site. This

suggests that the entire sediment succession (diamic-

ton and channel cutting and infilling) was formed

during a single unidirectional ice flow event. Clast

organisation within the channel infill (both chaotically

arranged and imbricated clast clusters), and the

steeply incised nature of the channel, suggests very

rapid and unstable incision and contemporaneous

infilling by sediment slurries (Hanvey, 1992; Piotrow-

ski et al., 1999). Degree of clast organisation within

these sediment slurries depended on sediment rheol-

ogy (porewater content) and concentration of larger

clasts (determining clast/clast interactions). Sediment

slurries of variable viscosity are therefore likely to

have cut and filled the channel feature under high

hydraulic pressure and/or glacial driving stress. Larger

clasts were transported and deposited as a result of

increased clast interaction and lower porewater pres-

sure during waning flow stages (Hampton, 1975;

Pierson, 1981; Postma et al., 1988; Major and Iverson,

1999).

4. Depositional environments at the ice/bed

interface

Sedimentary structures, facies relationships and

clast macrofabrics at Doonan and Drummee suggest

that sediments at these locations were deposited,

generally, by processes of lodgement and deforma-

tion, respectively. However, at both sites, it is inferred


that these generalised sediment depositional processes

were accompanied by up-sequence changes in melt-

water/porewater availability (Fig. 11). This is seen in

the up-sequence changes in diamicton characteristics

and contemporaneous channel-fill structures, which

suggest deposition from sediment slurries of variable

viscosity, while IBI processes were in transformation

between ‘lodgement’ and ‘deformation’. A model for

sediment pile evolution at the described sites integrat-

ing these components is presented in Fig. 11 and is

discussed below.

4.1. Subglacial hydrological processes

Field evidence at the two sites shows contrasting

changes in subglacial meltwater activity during accu-

mulation of the sediment pile (Fig. 11). At Drummee,

the principal location of the meltwater flux changed

over time, from interstitially within the sediment pile

to channelisation at the IBI. This contrasts with the

situation at Doonan where subglacial meltwater

became increasingly accommodated within the sedi-

ment pile over time following initial channelisation

at the IBI (Fig. 11). A renewed period of channeli-

sation may suggest temporarily increased meltwater

Fig. 11. Schematic diagram showing up-sequence (temporal) changes in

Drummee. At Doonan, there is a decrease in free IBI water over time, wher

for explanation.

supply. Also at Doonan, the limited presence of

water-escape structures, and absence of associated

features such as clastic dikes, suggests low lateral

and moderate vertical porewater pressure gradients,

and a fairly constant ice driving stress. However, the

intra-diamicton concretions found at this site record a

period of high lateral porewater pressure gradients

(Lamothe et al., 1983; Fairchild and Spiro, 1990)

which may be related to vigorous drainage through

the sand-dominant diamicton matrix. This switch

between hydraulic regimes (from low to high lateral

hydraulic gradients; Fig. 11) may indicate a linked

change in level of ice activity (from low to high) and

water source (from water produced as a by-product

of subglacial sediment meltout at the IBI, to calcite-

rich porewater or groundwater circulating within

subglacial sediments). It also shows that as the sedi-

ment pile accumulated, it provided a permeable

(sand-rich) accommodation space for water move-

ment within the bed rather than at the IBI. The

general absence of glaciotectonic deformation at

these sites (compare with those observed by Hart et

al., 1990; Menzies and Maltman, 1992; Lachniet et

al., 2001 in fine-grained diamictons) may be due to

the sandy diamicton matrix which does not generally

hydrological processes at the ice/bed interface (IBI) at Doonan and

eas at Drummee, there is an increase in IBI water over time. See text


favour the build-up of high porewater pressure and

permit sediment dilatancy (Menzies and Maltman,

1992; Tulaczyk, 1999).

4.2. Subglacial sedimentary processes

Sedimentary evidence at Doonan and Drummee

suggests that there is a close relationship between

sediment deposition processes and meltwater involve-

ment in those processes (Figs. 3, 9 and 11). For

example, at both sites, release of sediment/meltwater

mixtures by meltout of basal ice leads to generally

rapid local sedimentation, poorly developed fabrics,

high porewater content and changes in vertical and

lateral water pressure gradients (e.g. Paul and Eyles,

1990; Krzyszkowski, 1994). At Drummee, the ab-

sence of water-escape structures and concreted layers

(despite the underlying limestone bedrock) may be

due to the effectiveness of IBI drainage (as evidenced

by the infilled channel structure) in lowering water

pressure. Water escape structures and channel fills at

Doonan, present in the lower portion of the diamicton

succession, record an overall increase in porewater

confining pressure, perhaps as a result of the ‘cap-

ping’ by a dense lodgement-style diamicton (Fig. 3).

The sedimentary successions at both sites suggest

deposition by sediment slurries. This process is anal-

ogous to subaerial and subaqueous debris and grain

flows which produce similar sedimentary structures

(e.g. Carter, 1975; Coussot and Meunier, 1996; Major

and Iverson, 1999; Mulder and Alexander, 2001).

Clast lines and clusters, and stronger fabrics, record

deposition by slurries which may have been organised

into discrete channels or flows of varying thicknesses

(e.g. Fig. 10B). The resulting sediments (Figs. 3, Figs.

5–7, 9 and 10) are similar in texture and sedimentary

structures to those associated with ‘deforming beds’

(sensu Hart et al., 1990). The rheology of both

‘deforming beds’ and debris flows/sediment slurries

is strongly controlled by interstitial water content

(Menzies and Maltman, 1992; Boulton, 1996; Tulac-

zyk, 1999). Rather than a sediment package respond-

ing uniformly to a poorly defined process of

‘deformation’, it is likely that, in many cases, the

mechanism of sediment transport and deposition is by

sediment slurries or flows of variable viscosity in

which different-sized sediment grains are supported

by and change position within the flow body during

its activity (e.g. Hampton, 1975; Pierson, 1981;

Coussot and Meunier, 1996). This mechanism can

account for the range of ‘deformation’ structures,

including clast lines and rotated clasts, observed

within matrix-dominant diamictons (Hart et al.,

1990; Hicock, 1991).

Often, sediment slurries and subglacial deforma-

tion deposits show similar internal sedimentary

structures and have a similar geometric relationship

to adjacent sediments (e.g. Hart et al., 1990; Hart

and Boulton, 1991). It is therefore likely that some

subglacial diamictons previously identified as

‘deformation tills’ may therefore be better classified

as sediment slurry deposits formed under relatively

low driving stress (Carter, 1975). The concept of

subglacial sediment slurries is also useful because

it can act as a crossover between the subglacial

‘deformation’ and meltwater flood models (Fig. 1).

Sediment slurries include key components of both

end-member models: differential particle/sediment

package movement (deformation model) and genetic

relationship to IBI meltwater (meltwater floods

model).

5. Conclusions

(1) Detailed field evidence from two sites in north–

central Ireland illustrates the complex genetic

relationship between inferred processes of sedi-

ment deposition and the presence and character-

istics of meltwater both at the IBI and in the

immediately subjacent sediment. The field evi-

dence does not fit to either a subglacial deforma-

tion model or a subglacial meltwater floods

model. These models can be considered as end-

members along a continuum related to changes in

sediment viscosity (the ratio of sediment to

porewater) which may change over time and

space.

(2) On the basis of sedimentary structures and facies

relationships, deposition at both sites is interpreted

to have taken place mainly from subglacial

slurries. Slurry rheology was controlled by the

precise nature of the sediment–water admixture,

including sediment grain size. These slurries may

have operated both as unconfined and confined

(channelised) flows.


(3) An interpretation of sedimentary structures and

facies relationships based on sediment slurries can

successfully incorporate components of both

subglacial deformation and subglacial meltwater

flood models. The presence of water-escape

structures and chaotically organised clast clusters

indicates active porewater migration, dewatering

and flow freezing and exchange of water between

the IBI and subjacent sediment.

Acknowledgements

I thank J.C.G. Schwan, A.J. van Loon and J.

Vandenberghe for critical comments on the paper,

which was handled editorially by A.J. van Loon.

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