Formation of subglacial till under transient bed conditions: deposition, deformation, and basal decoupling under a Weichselian ice sheet lobe, central Poland

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Formation of subglacial till under transient bed conditions:deposition, deformation, and basal decoupling under aWeichselian ice sheet lobe, central Poland

J . A. PIOTROWSKI*, N. K. LARSEN*, J. MENZIES� and W. WYSOTA�*Department of Earth Sciences, University of Aarhus, C.F. Møllers Alle 120, DK-8000 Arhus C,Denmark (E-mail: [email protected])�Department of Earth Sciences, Brock University, St Catharines, ON L2S 3A1, Canada�Institute of Geography, N. Copernicus University, Sienkiewicza 4, PL-87-100 Torun, Poland

ABSTRACT

A multi-proxy approach involving a study of sediment architecture, grain size,

grain roundness and crushing index, petrographic and clay mineral

composition, till fabric and till micromorphology was applied to infer

processes of till formation and deformation under a Weichselian ice sheet at

Kurzetnik, Poland. The succession consists of three superposed till units

overlying outwash sediments deformed at the top. The textural characteristics of

tills vary little throughout the till thickness, whereas structural appearance is

diversified including massive and bedded regions. Indicators of intergranular

bed deformation include overturned, attenuated folds, boudinage structures, a

sediment-mixing zone, grain crushing, microstructural lineations, grain

stacking and high fabric strength. Lodgement proxies are grooved intra-till

surfaces, ploughing marks and consistently striated clast surfaces. Basal

decoupling by pressurized meltwater is indicated by undisturbed sand

stringers, sand-filled meltwater scours under pebbles and partly armoured till

pellets. It is suggested that the till experienced multiple transitions between

lodgement, deformation and basal decoupling. Cumulative strain was high, but

the depth of (time-transgressive) deformation much lower (centimetre range)

than the entire till thickness (ca 2 m) at any point in time, consistent with the

deforming bed mosaic model. Throughout most of ice overriding, porewater

pressurewashigh, in thevicinity of glacier floatationpressure indicating that the

substratum, consisting of 11 m thick sand, was unable to drain subglacial

meltwater sufficiently.

Keywords Basal decoupling, bed deformation, lodgement, subglacial pro-cesses, till.

INTRODUCTION

Warm-based ice sheets moving over soft sedi-ments interact with their beds in complex feed-backs influencing extent and timing of glaciationsand thus affecting global environmental changes(MacAyeal, 1993; Clark, 1994; Clark et al., 1999).Fundamental to constraining these interactions isthe knowledge of the ice movement mechanism,for past glaciations typically reconstructed fromthe sedimentary record such as the nature of tills.

After the recognition that unconsolidated bedmaterial subjected to shear stressmay bemobilizedand act as a traction carpet (Boulton, 1986), muchresearch has been devoted to subglacial deforma-tion as an ice movement mechanism (Alley et al.,1986; Alley, 1991; Hicock & Dreimanis, 1992;Benn, 1995; Boulton, 1996; Menzies, 2000; vander Meer et al., 2003). However, despite multi-faceted work involving field, laboratory andtheoretical studies, there is still no consensusregarding the depth, extent and continuity of bed

Sedimentology (2006) 53, 83–106 doi: 10.1111/j.1365-3091.2005.00755.x

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deformation, and the criteria to distinguish bedsthat have been subjected to high strain from thosethat have not (Paterson, 1994; Murray, 1997;Piotrowski et al., 2001). One major question isthe role of ploughing in sediment deformation(Tulaczyk, 1999; Tulaczyk et al., 2001) – candragging of clasts embedded in basal ice againstthe substratumor the rugged base of the ice itself beresponsible for most deformation previouslyattributed to pervasive bed deformation within amobile layer?The alternative hypothesis, glacier sliding over

the ice-bed interface (Lliboutry, 1979) has gainedsupport through documentation of thin layers ofsorted sediments intercalated with basal tills(Brown et al., 1987; Piotrowski & Tulaczyk,1999; Munro-Stasiuk, 2000), attributed to hydrau-lic lifting of ice because of water pressures eleva-ted to the vicinity of ice floatation level. Suchlayers not only indicate sheets of water under theice, but through their preservation also a limited,if any, deformation. A compromise is a model ofvariable bed conditions resulting in the presenceof stable and deforming spots that change theirshape and depth in time and space during iceadvance. This ice-bed mosaic model (Piotrowskiet al., 2004; see also Alley, 1993; Fischer & Clarke,1997; Piotrowski & Kraus, 1997; Smith, 1997;Vaughan et al., 2003; Evans & Hiemstra, 2005)may account for multiple switches from sliding todeformation events and back as often recorded inspatial transitions of till facies.Ice movement is coupled with sediment redis-

tribution processes, which are often interpretedfrom the properties of tills. Although geneticclassifications of tills (e.g. Dreimanis, 1988) havefor many years guided the research, some recentstudies question the established criteria to iden-tify till facies (Bennett et al., 1999) and theprocesses associated with the formation of sub-glacial tills (van der Meer et al., 2003). Here, adetailed sedimentological account of a till sectionat the border of one major Weichselian ice streamsouth of the Baltic Sea, the Vistula Lobe ispresented. The section is exceptional in that itreveals unequivocal evidence of bed deformationand other active-ice features which, in combina-tion with petrographical, micromorphologicaland till fabric data enable constraint of the icemovement mechanism and till formation proces-ses. The purpose of this paper is to demonstratethat, consistent with the bed mosaic model, theice sheet moved by a transient combinationof thin-skinned bed deformation, ploughing,lodgement and basal sliding, the relative import-

ance of which was influenced by the ability of thebed to dissipate porewater pressure. This is afollow-up of Larsen & Piotrowski (2003) who gavedetailed statistical treatment of till fabrics in thissame section.

STUDY AREA

The gravel pit (K2) at Kurzetnik is located on theupper terrace of the Drweca River valley in north-central Poland, about 200 km NW of Warsaw(Fig. 1). The area lies within the range of theWeichselian glaciation, which reached the maxi-mum extent about 50 km to the south ca 20 ka BPduring the Leszno phase (Kozarski, 1995; Marks,2002; Wysota et al., 2002). The stratigraphicalframework of the area was established by Churski(1966) who, in a nearby gravel pit (K1), found aSaalian till overlain by fluvial deposits and theEemian gyttja, covered in turn by glaciofluvialdeposits and a Late Weichselian till. In K2,Wysota (2000, 2001) found a similar successionbut without the Eemian deposit. Worth noting inthe context of this study is the substantial thick-ness of ca 11 m of the sandy–gravelly outwashsediments underlying the Weichselian till. Thisoutwash is a braided river deposit, partly dissec-ted by large-scale erosional scars filled withGilbert-type deltas. Larsen & Piotrowski (2003)subdivided the Weichselian till into three super-posed units, characterized by a highly uniformand strong fabric indicating ice flow from the NW.Based on some structural characteristics of thetill–outwash contact, it was suggested that theglacier moved over a thin deforming bed.Local pre-Quaternary bedrock consists of Mio-

cene and Pliocene sand, silt and clay. Followedup-ice to the NW, Cretaceous sandstones occur inthe Vistula River valley, succeeded by Palaeozoicand Mesozoic carbonates (chalk, limestone, dolo-mite and marl) and non-carbonates (sandstone,mudstone and shale), finally ending with Pre-cambrian igneous rocks of the ScandinavianShield. Occurrence of these formations (Fig. 1;Znosko, 1968; Pettersson, 1997) is relevant topetrographic composition of the till studied inthis paper.The exposed section is ca 20 m long, 3 m high,

and it stretches approximately east–west with theview direction approximately to the south(Fig. 2). This orientation is offset by ca 45� withrespect to the ice flow direction, providing a goodopportunity to study glaciodynamic structuralelements. Exposed is the entire Weichselian till,

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the topmost part of the underlying outwash andthe overlying post-glacial Drweca River sedi-ments which will not be considered here.

METHODS

In the course of fieldwork, sedimentologicalmapping of the till and outwash sand wasconducted. Special care was devoted to structuralactive ice elements such as folds, shear planes,ploughing marks, ribbs and flutings; inclusions ofsorted sediments in the till; the nature of contactsbetween lithologically different sediments andthe appearance of boulder surfaces.

Three vertical profiles (P1–P3; Fig. 2) throughthe till and the outwash sand spaced at 8 and5Æ5 m intervals were selected for detailed ana-lysis. In each profile, segments of 30 cm (length)by 20 cm (height) separated by approximately10 cm gaps were designated for till fabricmeasurements (Larsen & Piotrowski, 2003) andlaboratory analysis including grain-size distribu-tion, clast roundness, crushing index, petro-graphic composition, clay mineralogy and tillmicromorphology. Till fabric was also measuredin eight additional places at the till base outsidethe profiles.Till fabric measurement was performed at each

site on 30 elongated clasts with a-axes between

500 km

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eden

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Germany Poland

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Sand-, silt-andclaystone

Limestone, marl,clay limestone,chalk, dolomiteand dolomite marl

Crystalline rock

Poland

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lll

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Morainic plateau

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Floodplain

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Gravel pit

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River Kurzetnik

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Kurzetnik

Gotland

Fig. 1. Location of the study site in gravel pit K2 at Kurzetnik, and major pre-Quaternary bedrock types in therelevant area of Poland and north of it. The overview map shows the extent of the Weichselian ice sheet in northernEurope.

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0Æ7 and 5Æ6 cm long. Eigenvectors (V1, V2, V3) andeigenvalues (S1, S2, S3) were calculated accordingto Mark (1973). Grain-size distribution of < 2 mmfraction was determined by a standard sieve andpipette analysis, and petrographical analysis wasperformed on a 2–4 mm fraction whereby contentof stable (igneous rocks, sandstone, quartz, chert)and unstable (limestone, dolomite, marl, chalk,calcareous concretions) components was deter-mined according to Kronborg (1986). In theanalysis, individual components in the stablegroup sum up to 100% in order to eliminatepossible influence of weathering on interpret-ation, whereas the content of unstable lithologiesrefers to the sum of all components analysed.Clay mineralogy was analysed using standardX-ray diffraction technique (Eslinger & Pevear,1988). These parameters were determined to gaininsight into glacial mixing and homogenizationprocesses in relation to bedrock provenance of tillcomponents.Clast roundness (Powers, 1953), roundness

index (RA) (Matthews, 1987; Benn & Ballantyne,1994) and the crushing index (CI) (Olsen, 1983)were determined to obtain a semi-quantitativeestimate of strain and comminution. The RAindex is given as the percentage of angular andvery angular grains. The CI index is calculated asthe percentage of subrounded, rounded and well-rounded grains with fresh physical breakagezones and sharp edges. Statistical errors relatedto different granulometry (Krinsley & Doornkamp,1973) and lithology (Szabo & Angle, 1983;

Mazzullo & Ritter, 1991) were minimized byrestricting the analysis to quartz grains in the2–4 mm size range. Crushed grains are found invarious environments and they cannot alone beused as an indicator of a specific depositionalenvironment. Here, crushed grains serve as aproxy of the energy level within sediment, whichin turn helps constrain the sediment deformationintensity.Undisturbed and oriented samples for till

micromorphology were collected in the threeprofiles (Fig. 2) using aluminium Kubienaboxes. In the laboratory, the samples were air-dried, then impregnated with acetone-basedpolyester resin, and finally cut and mountedon glass (Murphy, 1986). A mammoth 5 · 8 cmvertical thin section was made from each sam-ple parallel to the ice flow direction, giving 22sections in total. Thin-section analysis wascarried out using a petroscope and a standardpetrographic microscope. As a result of the highcarbonate content, only S-matrix combiningplasma (< 30 lm) and skeleton grains(> 30 lm) was analysed, whereas plasmic fabric(arrangement of plasma) and skelsepic plasmicfabric (arrangement of plasma combined withskeleton grains) (van der Meer et al., 2003;Menzies et al., in press) were not detected. Inorder to quantify the horizontal and verticalvariability of microstructures, 17 · 23 mm largeareas of each thin section considered represen-tative were selected for this study and analysedusing 40· magnification.

K307

K306

K305K304K303K302K301

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Fluvial sand

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Fig. 2. Section at Kurzetnik with major sediment units, profiles P1–P3, and sampling sites. Ice flow was from theright and into the page. Note 2· vertical exaggeration.

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SEDIMENT FACIES DESCRIPTION

Distinction of the geological units is based hereon visual examination of the section applyingsedimentological and structural criteria. A unitis defined through a distinct set of characteris-tics absent elsewhere in the section. It may beseparated from another unit by both sharp andtransitional contacts. The units were prelimi-narily distinguished by Larsen & Piotrowski(2003) and are now confirmed by more detailedstudy.

Outwash sand: undeformed

At the base of the section, there is well-sorted(98% sand) medium- to coarse-grained, occasion-ally gravelly sand (Fig. 2). The sand is distinctlybedded with both trough (Fig. 3A, bottom) andplanar cross-stratification. Individual sets are upto ca 30 cm thick (Fig. 3B, bottom), with foresetsdominating and topsets preserved in only a fewplaces. Bottomsets, typically enriched in coarsergrains at the transition from foresets are welldeveloped in the lower part of the sand. Occa-sionally, parallel stratification occurs (Fig. 3C,bottom). Minute, centimetre-scale current ripplesare found within the cross-stratified sand. Thesand is undisturbed throughout its exposedthickness and all directional structures indicatepalaeo-current flow from NW to SE.In accordance with Wysota (2000), the sand is

interpreted as a braided river deposit that accu-mulated in the proximity of the advancing icesheet. It represents the topmost, undisturbed partof the ca 11 m thick outwash succession.

Outwash sand: deformed

Upwards in the succession, there appear minor,subhorizontal shear planes along which intactsediment packages are slightly (millimetre–cen-timetre) displaced. Further up, the horizontaldisplacement increases and ductile deformationsin the form of rooted folds occur. These gradeinto overturned, attenuated folds and hooks(Fig. 3B and D) with visible sediment displace-ment in the range of up to 20–30 cm. The foldsare asymmetrical with upper limbs flattened andoriginal sedimentary structures heavily distor-ted, yet discernible. Occasionally, allochthonouswedges of sand bounded by shear planes arefound within the folded succession (Fig. 3D). Alldeformation structures show displacement to theSE.

Within a few centimetres further up, folds giveway to faint subhorizontal layering and lamin-ation (Fig. 3A and C). Here, the individual strataare typically 1–5 cm thick, have diffuse upperand lower contacts and can be traced horizontallyfor up to ca 2 m before they disappear. Sand inthese layers is non-graded and lacks any visiblesedimentary structures. Closer inspection revealsthat the sand layers are superposed, extremely flatlenses stretched in the same direction as the foldsand hooks below. In a few isolated places, thehorizontal layering is absent and the folded sandis directly capped by a till. Typically, any struc-tures within the sand seem to disappear towardsthe base of the overlying till giving an appearanceof increasing sediment homogenization. Thethickness of the deformed sand varies betweenca 0Æ2 and 0Æ4 m. Grain size is the same as in theundeformed sand below.Transition from the undeformed sand to the

zone of brittle deformation, then ductile defor-mation and finally to subhorizontal tectoniclayering covered with a thin structureless horizonindicates upward-increasing sediment strain andmixing caused by ice overriding. This is furthersubstantiated by the uniform direction of defor-mation, which corresponds to the fabric in theoverlying till (see below).

Till unit A

The sand grades upwards into till within a2–3 cm thick zone of mixing where grain sortingrapidly decreases giving rise to a diamictictexture. The till appears coarse-grained and veryheterogeneous with up to 25 cm large bouldersrandomly dispersed in silty clay matrix (Fig. 3E).Its thickness is typically between ca 10 and20 cm. Till A is light-brown in colour andgenerally macroscopically structureless, butsmudges and thin laminae of sorted sedimentare occasionally found close to the underlyingsand. Based on the grain size, Larsen & Piotrowski(2003) suggested that the till is a mixture of local(sand) and far-travelled material released from theice sole, especially its fine-grained matrix and theboulders which are absent in the underlyingoutwash.About 10 m to the right of the exposed

section shown in Fig. 2, till A contains a large(ca 2Æ3 · 0Æ7 m), elliptic pod of sand apparentlyderived from the underlying outwash (Fig. 3F).The pod contains stratified, tilted sand cut by aseries of centimetre-scale faults. Some ductiledeformation is visible around the outer part of

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the pod. On the right-hand (up-ice) side of thepod is a partly detached tongue of sand indica-ting its counterclockwise rotation, and on theleft-hand (down-ice) side there is a tail of sandstretching from the pod and wedging out awayfrom it, suggesting flow of till around the

pod and erosion of its edges. The pod isinterpreted as an augen structure formedunder unidirectional strain caused by the over-riding ice sheet.The position of till A on top of the heavily

deformed sand coupled with the transitional

A

C D

E F

B

Fig. 3. Details of the deforming bed at the bottom of the sediment succession. Ice movement from right to left andinto the page. Location of pictures in Fig. 2. (A–C) Transition from undeformed sand (bottom) through deformedsand, till unit A to till unit B (top). (D) Transition from deformed sand (bottom) through till unit A to till unit B (top).Note overturned drag folds at the till A base. (E) Coarse-grained, stony till A (centre of the photograph) as a trans-itional unit between sand and till B above. (F) A boudin within till unit A consisting of fine-grained, stratified anddeformed sediment. Note a roll-out feature on the right-hand (up-ice) side suggesting counterclockwise rotation and atongue on the left-hand (down-ice) side suggesting stretching and attenuation of the pod. The site is located ca 10 mto the right of section shown in Fig. 2.

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contact indicating stress-generated grain diffu-sion and mixing of the bed material with newmaterial released from the ice, suggests that thisunit is a deformation till. It was generated in thetop of an up to ca 0Æ6 m thick deformation zonepartly encompassing the underlying sand (Larsen& Piotrowski, 2003, fig. 9). This is consistent withstrain profiles elsewhere (Boulton & Jones, 1979;Boulton & Hindmarsh, 1987; Benn, 1995; Iversonet al., 2003; Kjær et al., 2003) showing highestdisplacement immediately below the ice sole.The deformation zone as exposed in Kurzetnik isone of a few places where pervasive deformationof the subglacial bed can be demonstrated unequi-vocally, based on straightforward structural dataas above.

Till unit B

Till A is covered by a structurally different, up toca 1Æ5 m thick till B. The contact between theseunits may be sharp (Fig. 3C and E) or gradualwith a few-centimetre-thick transition zone(Fig. 3D), and till B has a distinctly lower gravelcontent. It is slightly darker than till A.Till B is macroscopically massive at the bot-

tom, with an exception of some isolated smallboudins and heavily attenuated lenses of sand.Further up, about 30 cm from its base, horizontalsand stringers appear, the frequency and lengthof which increase upwards to reach a maximumclose to the contact with the next overlying tillunit. The sand stringers are typically <1 cm andnot >2 cm thick, with the maximum measuredlength of 3Æ6 m. Some of the sand layers seem tobe bundles of very thin individual laminae(Fig. 4A), whereas others are single layers. Wherestones intervene with the stringers, the latter arein most cases downwarped (Fig. 4B–D), but somedrapes on top of the stones were also observed(Fig. 4E). In virtually all cases, the undulationsof the sand layers are symmetrical around thestones, i.e. no indices of lateral strain werenoticed. Sand in the layers is ungraded andmacroscopically structureless. Abundance of thesorted layers in the upper part of the till causesits distinctly bedded appearance there. This isfurther enhanced by reddish-brown, horizontal,clayey smudges occasionally containing aggre-gates of brecciated clay.The sand layers make it possible to separate

out blocks of till to study the morphology of theintra-till surfaces. This reveals a whole range ofminute ice-flow parallel features (strike ca 120�),first noticed at this site by Larsen & Piotrowski

(2003). Commonly, the till surfaces reveal agroove-ridge, fluted morphology (Fig. 5A and B)with relief amplitude below ca 1 cm. Individuallineaments are up to about 1 m long, they mayvary in width slightly, and typically have axialratios of over 10:1. In numerous instances, sandlayers were noticed draping the lineaments andmimicking their shapes. Some till surfaces reveallines of stones consisting of several pebbles(Fig. 5C), the long axes of which are oriented inaccord with the ice-flow direction. In manycases, flutings have cobbles on their ice-proximalends, similar to those known from large-scaleglacial flutings elsewhere. The cobbles are fre-quently striated on their upper surfaces whereasthe undersides are unabraded. Another commongroup of features are centimetre-scale ploughingmarks behind cobbles (Fig. 5D). All these struc-tures are remarkably well preserved between thinsand layers.Throughout till B, heavily weathered granite

and gneiss boulders occur (Fig. 4F and G). Theweathering is so strong that they disintegratebetween the fingers. However, the boulder mater-ial is not mixed with the surrounding till, andcontacts are intact and sharp. Because clay min-eralogical composition of the till does not showany weathering (see below), it is likely that theboulders were weathered already prior to glacialtransport and deposition.An intriguing feature is sand pockets found

under some boulders (Fig. 4H). The pockets areflat, confined to an area immediately beneath thestones and not connected to any other sortedsediment bodies in the till such as the sandstringers mentioned above.These structural characteristics indicate both

active ice action and basal decoupling during theformation of till B. Ice-parallel lineaments, similarto those described by Ehlers & Stephan (1979) andShaw (1987, 1994), are interpreted as ploughingfurrows left by pebbles projecting from the basalice and dragged along the bed (Clark & Hansel,1989; Jørgensen & Piotrowski, 2003), and as mini-flutings formed due to till accretion in pressureshadows behind lodged pebbles (Rose, 1989;Benn, 1994a; van der Meer, 1997b). Sand stringersare seen as a result of ice lifting from the bedfollowed by sediment washing and sorting in thinwater cavities, subsequently filled with sand. Thisexplanation supports the models of e.g. Brownet al. (1987); Piotrowski & Tulaczyk (1999) andMunro-Stasiuk (2000) who also interpreted tillsintercalated with sorted sediment as evidence ofhydraulic lifting of a glacier by pressurized basal

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AA

C D

E

G H

F

B

Fig. 4. Sedimentary features of till unit B. (A–E) Sand stringers characteristic for the upper part of the till. Notesymmetrical down-warping below (B, C, D) and coating above (E) stones. Sand layers are interpreted as in situsedimentation in shallow cavities at the ice/bed interface. (F, G) Heavily weathered but intact boulders in till matrix,indicative of a melt-out process. (H) A boulder with sand pocket below interpreted as a meltwater scour. Coin forscale in all pictures.

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meltwater. The alternative explanation is that,tectonic layering is discarded. It may be discardedbecause there is a lack of unidirectional distur-bances of sand layers in contact with boulders andbecause the sharp contacts with the surroundingtill indicate a lack of strain-induced diffusivemixing (Weertman, 1968; Piotrowski & Tulaczyk,1999). Further, the substantial thickness of tillseparating the stringers from the outwash sandbelow the till precludes the outwash sand frombeing an immediate source for the stringers.Suggestion of the temporary basal cavities is alsoconsistent with sand pockets under boulders,which probably formed as outwash infill of scourscreated by meltwater erosion under bouldersprojecting from the basal ice (Munro-Stasiuk,2000). The heavily weathered but intact boulderssuggest passive deposition by melt-out from adebris-rich basal ice.Ploughing marks present on intra-till surfaces

document sediment deformation, the depth ofwhich can approximately be constrained. Tulac-zyk (1999) showed that plastic deformation of till

around a ploughing clast may affect till to adepth of ca 2Æ7–4Æ5 times the clast diameter.Average diameter of clasts with ploughing marksin this study is about 3 cm, giving a maximumdeformation depth of ca 13Æ5 cm. This corres-ponds roughly to the typical spacing of theundisturbed sand stringers, which can be aslow as ca 5 cm, however. It may therefore beconcluded that the depth of deformation causedby ploughing was somewhere between ca 5 and13 cm at most, apart from the lower part of thetill where it cannot be constrained because of themassive till structure.

Till unit C

The upper boundary of till B is marked by asudden disappearance of sand stringers andcolour change of the matrix. Unit C is an up to1 m thick, macroscopically massive, grey till withrare sandy smudges at the base. The lack ofstructural elements makes it difficult to constrainits origin based on field observation alone.

AA

C D

B

Fig. 5. Lineaments at till surfaces excavated at till/sand stringer contacts indicating lodgement and ploughing. Icemovement from right to left. Location of pictures in Fig. 2. Note the fluted upper LS surface in (A) and (B), line ofimbricated stones on the upper surface in (C), and a ploughing mark behind a stone preserved on till underside in (D)(just above the coin). Coin for scale in all pictures.

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GRAIN-SIZE DISTRIBUTION AND GRAINMORPHOLOGY

The undeformed and deformed sand has similartextures with weight peaks in the fine-sand range(Fig. 6). Within the till succession, the coarsest-grained unit is till A with 80% sand, 14% silt and6% clay, whereas tills B (58%, 26% and 16%)and C (61%, 27% and 12%) have similar compo-sition. Furthermore, till A has about 10% moregravel than the overlying units, and the relativelyhigh spread of measurements indicates its spa-tially variable, immature texture. Grain-size

distributions in tills B and C are almost the same(Fig. 6).Grain roundness (RA) is fairly uniform through-

out the entire sediment sequence with the contentof angular and very angular grains between ca 4%and 14% (Fig. 7). No systematic variations invertical profiles occur. CI is also relatively stablethroughout the vertical profiles (Fig. 7). Anexception is the significant rise in CI from un-deformed to deformed sand by ca 16% implyingthat strain was accompanied by crushing ofgrains.

PETROGRAPHIC AND CLAY MINERALCOMPOSITION

Three dominant components of the stable petro-graphic fraction are igneous rocks (typically over60%), sandstone and quartz (Fig. 7). In theunstable spectrum, limestone, dolomite and marldominate with typically ca 30–40%. In somesamples, traces of chert, chalk and concretionswere found. Clay minerals are primarily smectiteand illite with roughly equal shares amounting toca 85% of the composition, whereas kaolinite andchlorite make up the rest. Worth noting is auniform distribution of the analysed components.All till units have approximately the same com-position and there are no systematic verticaltrends within any of these units. Furthermore,the outwash sand differs from the tills only by aslightly higher content of igneous rocks. In sum,the petrography and clay mineralogy can be seenas a well-mixed combination of far-travelled andlocal components, little affected by post-deposi-tional weathering.

TILL FABRIC

Thirty fabric measurements in all three till unitstreated statistically and interpreted by Larsen &Piotrowski (2003) revealed that no conclusionscan be made about till facies origin from the fabricdata alone. Here, the vertical variability of fabricorientation and strength is focused on. All meas-urements reveal a very high clustering of fabricsexpressed by the mean S1-value of 0Æ876 (rangebetween 0Æ736 and 0Æ971), and very low variabil-ity of mean directions (V1 eigenvectors) withstandard deviation of just 6� (Fig. 8A). A plot ofintegrated fabric strengths (Fig. 8B) shows thatthe strength increases from till A to the lower partof till B, then drops towards the bottom of till C

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igh

t[%]

0·001

1 0·1 0·01

0

10

20

30

We

igh

t[%]

0·001

2

-1 0 2 6 8 10

Sand Silt Clay

Till unit Cn = 7

Till unit Bn = 12

Till unit An = 3

Deformed sandn = 3

Undeformed sandn = 3

[mm]

[phi] 4

Fig. 6. Frequency distributions of clay-to-sand grainsizes in the sediment units. Whiskers give standarddeviation.

92 J. A. Piotrowski et al.

� 2005 International Association of Sedimentologists, Sedimentology, 53, 83–106

Page 11

Smec

tite

Kaolin

iteChlo

rite

Illite

010

2030

4050

6070

8090

100%

010

2030

4050

6070

8090

100%

010

2030

4050

6070

8090

100%

Concr

etion

Chalk

Limes

tone

, clay

limes

tone

,

dolom

itean

dm

arl n

= 1

56

n =

142

n =

126

n =

140

n =

150

n =

236

n =

159

n =

168

n =

204

n =

187

010

2030

4050

%

n =

104

n =

156

n =

115

n =

149

n =

173

n =

159

n =

166

n =

123

n =

144

010

2030

4050

%

n =

147

n =

286

n =

149

n =

211

n =

142

n =

146

n =

144

n =

146

n =

141

010

2030

4050

%

Igne

ous r

ock

Chert

Quartz

Sands

tone

Def

orm

ed s

and

K30

1

K30

2

K30

3

K30

4

K30

5

K30

6

K30

7

K30

8

Und

efor

med

san

d

010

2030

4050

6070

8090

100%n

= 2

49

n =

317

n =

267

n =

239

n =

213

n =

319

n =

296

n =

255

n =

281

n =

256

Def

orm

ed s

and

K20

1

K20

2

K20

3

K20

4

K20

5

K20

6

K20

7

Und

efor

med

san

d

010

2030

4050

6070

8090

100%n

= 2

23

n =

314

n =

296

n =

328

n =

307

n =

238

n =

280

n =

223

n =

195

010

2030

4050

6070

8090

100%n

= 2

01

n =

442

n =

258

n =

337

n =

193

n =

234

n =

189

n =

250

n =

211

Def

orm

ed s

and

K10

1

K10

2

K10

3

K10

4

K10

5

K10

6

K10

7

Und

efor

med

san

d

Angula

r

Round

ed

Subro

unde

d

Suban

gular

010

2030

4050

6070

8090

100%n

= 5

5

n =

61

n =

42

n =

60

n =

41

n =

73

n =

70

n =

55

n =

55

n =

56

010

2030

4050

6070

8090

100%n

= 4

0

n =

60

n =

68

n =

65

n =

66

n =

45

n =

63

n =

60

n =

40

010

2030

4050

6070

8090

100%n

= 3

5

n =

78

n =

49

n =

60

n =

32

n =

51

n =

33

n =

64

n =

39

RA

010

20%

010

20%

010

20%

CI

010

2030

4050

%

010

2030

4050

%

010

2030

4050

%

Fig.7.Summary

ofpetrographicaland

mineralogicalcomposition,grain

roundness,grain

roundness

index(RA)and

crush

ingindex(CI).Sample

location

givenin

Fig.2.

Subglacial soft-bed processes and deposits 93

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Page 12

and then increases again. It is noted that theturning points in this pattern are slightly offsetwith respect to till unit boundaries.Taken together (Fig. 9), the exposed sediment

succession reveals low parameter variability withsome distinct changes at the bottom and topcontacts of till unit A (grain-size distribution) andbetween the undeformed and deformed sand(crushing index).

TILL MICROMORPHOLOGY

Figure 10 shows examples of thin sections withmapped structural elements of the S-matrix ineach thin section, Fig. 11 is an overview over allstructural elements in all sections and Fig. 12 is a

synthesis of laterally integrated distribution ofthese elements in the till succession, justified by asmall site-to-site variability of element frequencyat the same depths in tills.

Single and multiple-direction lineations(sl and mdl)

Single lineations are aligned elongated skeletongrains and multiple-direction lineations are twoor more crosscutting single lineations (Menzieset al. in press), believed to represent discreteshear zones (e.g. Dewhurst et al., 1996; Hiemstra& Rijsdijk, 2003). Both single and multiple-direction lineations are present in Kurzetnik.The average length of both types of lineations is< 5 mm, but they can be up to 15 mm long (e.g.Fig. 10A). They occur most frequently in till B(Fig. 12). The lineations are often superimposedon other types of microstructures (turbate struc-tures and till pellets), without displaying anyvisible signs of lateral displacement (e.g. K306 inFig. 11).

Edge-to-edge crushing (ee)

Direct contact between two or more grains, whereone or all of the grains are fractured, is termededge-to-edge crushing (Menzies et al., in press).The number of edge-to-edge crushings dropssignificantly from till unit A to B and remainsapproximately constant further up (Fig. 12). Insome cases, broken-off microfragments are foundnear the parent grains, indicating in situ sub-glacial crushing (Hiemstra & van der Meer, 1997).In Kurzetnik, many of the grains are onlyfractured internally, so that it cannot be ruledout that also processes other than subglacialdeformation may be responsible for the fractures.However, the close relationship between graincontacts and internal radiating fractures suggeststhat subglacial shearing may be the most likelyexplanation.

Grain stacking (gs)

Stacks of more than five closely spaced grains,grain stacking, occur in all parts of the tillsuccession with no significant vertical change inquantity (Fig. 12). They primarily consist of sand-sized grains and are < 5 mm long. Typically, thegrain stacks are oriented at high angles withrespect to the major stress direction, i.e. almostvertically (Figs 10 and 11). Grain stacks, analog-ous to grain bridges at larger grain sizes, support

N

V1 direction

Standard deviation

Mean V1 direction

n = 30A

B

0·76 0·8 0·84 0·88 0·92 0·96

Fabric strength (S1)

0

0.5

1

1.5

2

2.5

Dep

th (

m)

1

2

3

Till unit A

Till unit B

Till unit C

Fig. 8. Synthesis of till fabric data. (A) Mean directions(V1 eigenvalues) of all 30 fabric sites (30 stones mea-sured at each site) showing directional uniformity ofthe data; (B) Laterally integrated fabric strength (S1

eigenvalues; dots) through the till succession. Note thehigh strength of the fabric and its inconsistent trends inthe vertical profile interpreted as being a result ofvarying strain.

94 J. A. Piotrowski et al.

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Page 13

stress during sediment deformation (Hooke &Iverson, 1995; Iverson et al., 1996).

Turbate structure (tu)

Turbate structures are circular arrangements ofskeletal grains with or without a core stone(Hiemstra & Rijsdijk, 2003). They are also knownas circular, milky-way, galaxy or rotation struc-tures. They form in response to velocity gradientswithin a ductilely deforming sediment (e.g. vander Meer, 1997a). In Kurzetnik, turbate structuresare 2–5 mm in diameter and in most caseswithout core stones (Fig. 10B). There is anincrease in the number of observed turbate struc-tures from till unit A to B, and then a slight dropupwards to C (Fig. 12).

Till pellets (tp)

Till pellets can be subdivided into three types: (i)pellets of till similar to the surrounding material,(ii) pellets similar to the surrounding materialwith plasmic fabric and (iii) pellets of differentcomposition to the surrounding material with orwithout plasmic fabric (van der Meer, 1993). Tillpellets originate by reworking of allochthonousand autochthonous sediments (van der Meer,1987; Carr et al., 2000). In Kurzetnik, till pelletsare made of till both similar and dissimilar to the

surrounding material (Fig. 10E) and as they lackany plasmic fabric they are type 1 and 3 pellets.Type 3 pellets consist of silt and sand or slightlydarker diamicton, which is more fine-grainedthan the surrounding material. There is a distinctpeak of till pellet frequency in the upper, beddedpart of till B (Figs 11 and 12). Till pellets aretypically located within sand stringers or atcontacts between sand stringers and till, and theyare often covered with a thin layer of sandcreating armoured pellets (Fig. 10F).

Domains (do)

Zones of similar sediment that can be differenti-ated from the surrounding sediment are calleddomains (Menzies, 2000). Subhorizontal domainsare often interpreted as shear zones (van derMeer, 1996), whereas more diffuse domains havebeen attributed to other processes (e.g. Menzies &Zaniewski, 2003). Frequency of domains culmi-nates in the middle of till B and to a lesser degreein till C (Fig. 12). They can be subdivided intotwo groups. The first comprises subhorizontal tillbands (Fig. 10C) and the second consists ofmillimetre- to a few-centimetre-thick subhorizon-tal sand stringers embedded in the till (Fig. 10Dand F). There is no significant difference in the S-matrix assemblage between the two domains, butin the sand stringers little evidence of internal

0 20 40 60 80 100(%)

0 20 40 60(%)

0 20 40 60(%)

0 20 40 60 80(%)

0 20 40(%)

Till unit C

Till unit B

Till unit A

Deformed sand

Undeformedsand

RA CIlimestone, clay limestone,

dolomite and marl

chalk

concretion

igneous rockquartz

sandstone

chert

smectiteillite

kaolinite

chlorite

sandsiltclay

V1 Grain-size <2 mmRoundness &crushing ind.

Claymineralogy

Fine-gravel(unstable)

Cla

ySi

ltSa

ndG

rave

lD

iam

ict

f m c f c

Fluvial sand

0

1

2

3

Depth (m)Fine-gravel

(stable)

Fig. 9. Laterally integrated structural, petrographical and mineralogical data from all samples. V1 is the meaneigenvector in till fabrics. Note the largely uniform distribution of the measured parameters throughout thesuccession.

Subglacial soft-bed processes and deposits 95

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Page 14

K301 x

x

x

x

x x

x

xx

x

x

xx

x xA

BK106

x

x x

x

x

x

x

x

x

xx

x

xx

K303

x

x x

x

x

x

xx x

x

C

DK304

x

xxx

xx

x

x

x

Fig. 10. Examples of vertical micrographs in plane light and interpretations of till unit A (A), till unit C (B) and tillunit B (C–F). Ice flow direction from left to right. View field 17 · 23 mm; see Fig. 2 for location. Note that image (F)showing ‘armoured’ till pellets within a sand stringer is from sample K104, but from a different area than the oneused to quantify S-matrix (Fig. 11). Further explanation in the text.

96 J. A. Piotrowski et al.

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Page 15

deformation is present. Only a few single linea-tions crosscut the sand stringers.

Interpretation

Microstructures reveal evidence of both ductile(turbate structures) and brittle (single and mul-tiple-direction lineations and edge-to-edge con-tacts) deformation in the till succession. Themultiple-direction lineations and long subhori-zontal single lineations, most likely producedby subglacial shear (Menzies et al., in press),are consistent with other indications of beddeformation presented above. The lower con-tent of S-matrix in till unit A compared withthe rest of the till may be related to the morecoarse-grained nature of this unit, which couldhamper the development of certain microstruc-

tures because of more frequent grain collisionsthere.Ductile deformation structures appear to pre-

date the brittle ones, which indicates a change inthe style of till deformation as expected if thezone of highest stress migrates upwards duringsubglacial till accretion. Moreover, as till isemplaced and immobilized, porewater wouldmigrate out and the till when re-stressed wouldact in a brittle style. It is suggested that grainstacks developed during a transition from ductileto brittle deformation to stabilize the effect ofshear deformation as envisaged by Hooke &Iverson (1995). This process could have beenfacilitated by a drop of porewater pressure and, inconsequence, increase of intergranular friction.The co-existence of till pellets and sand strin-

gers in till unit B indicates that they originated

K204

x

x

x

xx

x

x

x

x

xx

x

E

FK104_1 thin sand layer

Lineations (sl and mdl) Turbate structure (tu)

Domains (do) Till pellets (tp)

x

Grain stacking (gs)

Edge to edge crushing (ee)Sample numberK207

Fig. 10. Continued

Subglacial soft-bed processes and deposits 97

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Page 16

K101

x xx

xx

x

x

x

x x

x

x

xx

x

xx

x

xx

x

x x

x

xx

K103

x

x

xx

x

x

x

x

xx

x

xx

x

x

x

x

x

x

x

x

xx

x

x

x

x

x

x

x

x

K105

x

x

xx

x

x

x

x x

xx

K107

x

x

x

x x

x

x

K102

x

x

x

x

x

xx

x

x

x

x

x

x

x

xx

x

x

x

x

x

x

x

K106

x

x x

x

x

x

x

x

xx

x

xx

K201 xxx

x

xx

x

x

xx

x

xx

xx

xx

x

x

xx

x

x

xx

x x

x

K203x

xx

x

x

x

x

x

x

x

xx

x

K204

x

x

x

xx

x

x

x

x

xx

x

K205

x

x

x

xx

x

x

x

x

x

x

x

x

x

x

x

K206 x

x

xx

x x

x

x

x

x

x

xx

x

xx

x

x

x

x

x

K207

xx

x

x x

x

x

Lineations (sl and mdl) Turbate structure (tu)

Domains (do) Till pellets (tp)

x

Grain stacking (gs)

Edge to edge crushing (ee)Sample numberK207

Poor impregnation

K301 x

x

x

x

x x

x

xx

x

x

xx

x x

K302

x

x

x

xx

x

x

x

K303

x

x x

x

x

x

xx x

x

K304

x

xxx

xx

x

x

x

K305

x

x

x

x

x x

x

xx

K306x

x x

x

x

xx

x

xx

x

x

K307 x

xx

x

x

xx

x

x

x

x

x

x

x

x

x

xx

xx

xx

K308

K104

K202

Fig. 11. S-matrix elements observed in 22 vertical micrographs from profiles P1 (right-hand column), P2 (middle),and P3 (left-hand column); see Fig. 2 for location. Ice-flow direction from left to right, view field 17 · 23 mm.

98 J. A. Piotrowski et al.

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Page 17

during the same event. It is suggested that the tillpellets represent rip-up clasts which formed insubglacial cavities concurrently with the depos-ition of sand stringers. This would explain thedifferent provenance and armoured character ofsome of them, and the lack of shear deformation/destruction that would be expected if the envi-ronment was affected by intergranular strain. Thisis particularly obvious in the case of till pelletsoccurring at the contact between sand stringersand the till. Coupled with micro- and macro-structures indicating shear in close proximity tothe pellets, it can be concluded that strain wasrelatively low and/or depth of the deformationzone was shallow. It is noted that the inferredgeneration of the bedded till involving multiplebasal decoupling corresponds to the model of vander Meer (1987) who studied banded till se-quences containing till pellets in Switzerland andin The Netherlands.Finally, it is noted that the lack of upward

increase of microstructures indicative of subgla-cial shear suggests that the entire till successionwas not subject to pervasive deformation at thesame time.

DISCUSSION

The sequence of events

Compilation of together the structural and texturaldata leads to a reconstruction that begins with iceoverriding and deforming the top of outwash

sediments (Fig. 13A). Primarily, the ductile styleof deformation indicates high subglacial pore-water pressure, despite the high drainage capacityof the bed consisting of about 11 m thick sand.This suggests that deformation occurred close tothe ice margin where large volumes of surfaceablation water may reach the bed and cause itsoversaturation. As englacial conduits rarely reachdeeper than about 200 m into the ice sheet(Fountain & Walder, 1998), this phase of deforma-tion probably happened at an early stage ofoverriding. Subsequently, formation of till Acommenced. Presence of both local and far-travelled material, together with some deforma-tion structures at its base and a gradual transitionfrom the deformed sand beneath indicate that thistill was formed by a combination of lodgement(sensu Dreimanis, 1988) and deformation(Fig. 13B). Deformation of sand below continuedalso after some till was emplaced, as indicated bythe diffuse, mixed sand–till interface.Formation of till B was a more complex process

resulting primarily from lodgement and plough-ing, giving a distinct imprint of glaciodynamiclineaments (Fig. 13C). Intergranular deformationcannot be excluded, especially in the lower partof the till, given its similarity in texture, fabric,petrography and mineralogy to the unequivocallydeforming till A. As till B thickened, waterdrainage from the ice base into the outwash sandin the bed became progressively less efficient,leading to basal decoupling by ponded waterwhen glacier flotation point was reached(Fig. 13D). This happened repeatedly in the

sl mdl ee gs tu tp do

Till unit C

Till unit B

Till unit A

Fig. 12. Quantity and types ofS-matrix elements from till micro-graphs laterally integrated from thethree profiles (Fig. 2). Circles markprofile P1, diamonds profile P2, andsquares profile P3. sl, single linea-tions; mdl, multiple direction line-ations; ee, edge to edge crushing; gs,grain stacking; tu, turbate structure;tp, till pellets; do, domains. Themean is represented by a thick greyline.

Subglacial soft-bed processes and deposits 99

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Page 18

Por

ewat

erpr

essu

rein

crea

se

Dep

th o

fde

form

atio

n

Bul

k hy

drau

lic c

ondu

ctiv

ity d

ecre

ase

time

P ~Pw i

P ~Pw i

P ~Pw i

P >Pw i

Undeformed sand

Deformed sand

Till unit A

Till unit B(lower part)

Till unit B(upper part)

Flutings andploughing marks

Hydraulic decoupling, flushingand deposition of outwash sandin the cavity

ice

Sand stringers

till unit CIce flow direction

A

B

C

D

E

Till

acc

retio

n

waterdrainage

Fig. 13. Time-transgressive formation of the till succession at Kurzetnik reconstructed from all proxies studied.Major processes are bed deformation (A, B), lodgement with ploughing (C), and basal decoupling and washing (D).Transitions between these processes were probably caused by variations in local porewater pressure. During tillaccumulation the bed was deforming in a thin, patchy zone that moved up parallel to till accretion. Details in thetext.

100 J. A. Piotrowski et al.

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Page 19

upper part of till B, as shown by frequent layers ofsand there. Spacing of these sand layers con-strains the thickness of a possible deforming bedto a few centimetres. Furthermore, the intactnature of the sand layers suggests that thediamicton immediately above them may be ofthe subaquatic rain-out type lowered from thefloating ice base onto the sand layers and pro-tecting them from subsequent deformation. Littlecan be said about till unit C, but by analogy withthe macroscopically more massive parts of tills Aand B it can be suggested that lodgement anddeformation were the dominant processes(Fig. 13E).

Till fabric signature as a proxy of deformation

Closely spaced till fabric measurements spreadover the entire till thickness facilitate theexamination of fabric evolution and thus sedi-ment strain during the ice overriding. Numerousresearchers have tried to infer till deformationintensity from fabric pattern, and some haveconcluded that a high degree of deformation ismarked by relatively low S1 eigenvalues, typic-ally less than ca 0Æ65 (Dowdeswell & Sharp,1986; Hicock, 1992; Hart, 1994). However, pro-cesses involved in the formation of specificfabric pattern are often poorly constrained andseldom supported by independent data, beingtypically interpreted from the geological recordleft by past glaciers. Studies on modern glaciersare no less problematic because of the inacces-sibility of subglacial beds. Therefore, rather thancomparing present results with the data of pastglaciations, laboratory experiments on till defor-mation are preferred which, conducted underwell-defined conditions, yield first-hand data onsediment fabric signature in relation to strainmagnitude. Such experiments, initiated byHooyer& Iverson (2000) using a ring-shear device, yieldinvaluable information on till rheology and struc-ture evolution that, despite all limitations anduncertainties of laboratory simulation, mayserve to constrain soft sediment behaviour underglaciers.Hooyer & Iverson (2000) demonstrated that

fabric in tills subjected to pervasive shearing inlaboratory experiments is high (S1 eigenvalues0Æ78–0Æ87), and particles attain stable position at ashear strain of ca 2Æ0. No further significant fabricmodification was noticed up to the termination ofshearing at a strain of ca 370. The mean S1

eigenvalue of the unequivocally deformed till Aat Kurzetnik is ca 0Æ88. It is difficult to determine

the strain of this till, but it can be estimated forthe underlying deformed sand with partly pre-served sedimentary structures. There, stretchingthe folds back to the original position by ca 55 cmover the depth of ca 11 cm of the material gives astrain of ca 5 (under a simplifying assumption ofuniform shearing this means that a total of ca30Æ2 cm3 of sand was advected through eachmetre of width of the deforming bed). Gradualtransition from the sand to till A with parallelincrease of sediment mixing and homogenizationshows that strain in till A was greater than in thesand. It is concluded that strain > 5 correspondsto S1 � 0Æ88, which is consistent with Hooyer andIverson’s suggestion that subglacial deformationleads to strong rather than weak fabric. Noevidence of clast re-orientation perpendicular toice-flow at higher strain is found (Carr & Rose,2003).Laterally integrated fabric strength at Kurzetnik

(Figs 8B and 9), shows an increase of S1 to ca 0Æ92in the lower part of till B, then a drop to ca 0Æ8 atthe bottom of till C, and again an increase to themaximum of ca 0Æ96 at the top of till C. Usinglaboratory tests as reference, this trend mayrepresent varying strains during till generationin Kurzetnik. Furthermore, the decrease of tillstrength in the upper part of till B may be linkedwith the postulated frequent basal decouplinginferred from the sand stringers and some micro-structures. Decoupling would stop till shearing,and thus temporarily halt fabric evolutiontowards higher clustering. It is noted that theS1-value spread in the profile is relatively smalland the values very high, much higher than themean of 0Æ54 determined by Hooyer & Iverson(2002) for the till of Des Moines lobe of theLaurentide ice sheet, for which shear strain lessthan ca 2Æ0 was inferred.Ploughing marks and respective clasts pre-

served in the till suggest that till fabrics resultpartly from strain caused by clasts projecting fromthe ice sole, dragged against the soft bed beforethey were lodged. Tulaczyk (1999) showed thatplastic deformation around a ploughing clast mayaffect till to a depth of ca 2Æ7–4Æ5 times the clastdiameter. As clasts associated with ploughingmarks in till B are typically up to a few centi-metres in diameter, their influence could havebeen substantial up to a depth of around 15 cm.On the contrary, fabric disturbance in the imme-diate vicinity of a clast, as may be expected, wasnot noticed in any of the 30 measuring sites. It isthus difficult to evaluate the role of ploughing intill fabric arrangement in Kurzetnik.

Subglacial soft-bed processes and deposits 101

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In studies of a modern basal till of Bre-idamerkurjokull in Iceland, Boulton (1979), Boul-ton & Hindmarsh (1987) and Benn (1994b, 1995)noticed a two-tieed structure with dilated, heav-ily deformed upper part (A) and more compactlower part (B) with brittle deformation confinedto shear planes. The two tills had different fabricsignature with till B characterized by stronger a-axis clustering. It is noted that the Kurzetnikrecord yields a different picture with fabricorganization increasing upwards within thebottom part of the succession, consistent withprediction of plastic (March) rotation.The distribution of fabric pattern can be used

as a means of constraining the depth of defor-mation during till accretion. It is assumed thatstrain rate in a deforming bed increases upwardsto reach a maximum just below the ice sole, asis known from some modern glaciers (Boulton &Hindmarsh, 1987; Iverson et al., 2003). There-fore, if the till succession at Kurzetnik wasdeforming throughout its entire thickness, asystematic upward increase in till fabricstrength should be observed. As this is not thecase, the maximum deformation depth is con-strained by the two parts of till with increasingS1-values, which is ca 0Æ5 m at most in eachcase.

Strain magnitude

The suggestion of a thin deformation zone is alsosupported by the lack of vertical trend in grainroundness and the crushing index, both of whichcan be used as proxies of glacial comminution(Dreimanis & Vagners, 1971; Boulton et al., 1974).The only significant jump in CI, from undeformedto deformed sand, can be explained by shearing ofpure sand without the cushioning effect of fines(Sammis et al., 1987) giving strong stress concen-tration at grain edges. Experiments on sedimentcomminution (Haldorsen, 1981, 1983; Iversonet al., 1996) show that silt is produced underhigher strain than crushed sand grains which,under favourable conditions, should lead to amulti-modal grain-size distribution with a dis-tinct peak in the silt fraction (e.g. Piotrowski,1992). Lack of such peak in the granulometriccurves from Kurzetnik tills would then indicate arelatively low strain. However, there may not be astraightforward relationship between shearingintensity and comminution, because Tulaczyket al. (1998) reported very little shearing-relatedcomminution under Ice Stream B, whereas Hiem-stra & van der Meer (1997) documented abundant

occurrence of crushed grains in a pervasivelydeformed till.Petrographic and mineralogical composition,

which is the same in the till succession and theunderlying sand, indicates that interpreting ahigh degree of glaciodynamic sediment homo-genization and thus deformation of tills withuniform composition may be misleading. Datafrom this study show that homogenizationalready took place in the glaciofluvial environ-ment and the resulting, well-mixed materialserved as an important source for till components.Co-existence of features indicating material

release from a sliding base of the glacier, e.g.ploughing, deformation and decoupling, suggeststhat large parts of the Kurzetnik till can beconsidered a hybrid lodgement/deformation till(see also Benn & Evans, 1996) with certaincontribution of waterlaid material. Most proxiessuggest that deformation depth during till accre-tion was relatively shallow, typically within acentimetre range and not more than a few tens ofcentimetres. The deformation zone moved time-transgressively upwards as till accretion pro-ceeded, affecting a thin layer of till immediatelybeneath the ice sole whereas deeper till remainedlargely stable. This protected such features asheavily weathered boulders, sand stringers withdrape structures, sand pockets under stones,boulders with striated upper surfaces, andploughing marks inherited from the ice-bedcontact from destruction, mixing or re-orientationby shear (e.g. Weertman, 1968; Paterson, 1994, p.170). Despite the shallow depth of deformation,the total strain was high. This strain, however, isa cumulative effect of time-transgressive defor-mation distributed over the entire ice advance,which means that at any given point of timedeformation depth was much less than the wholetill thickness. This in turn indicates that asubstantial volume of glacial debris was transpor-ted englacially to be, after release from the icesole, subjected to advection in the deforming bed.A similar mode of thin-skinned deformationspread over time was suggested by Larsen et al.(2004) for another area of last glaciation, inDenmark.Sedimentary data suggest multiple switches

between different modes of till formation andglacier movement reflecting a mosaic of sub-glacial conditions changing in time and space inresponse to varying glaciological parameters, inparticular water pressure. Such a mosaic was alsoinferred elsewhere for the Pleistocene ice sheets(e.g. Krzyszkowski, 1994; Benn & Evans, 1996;

102 J. A. Piotrowski et al.

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Piotrowski & Kraus, 1997; Knight, 2002; Jørgen-sen & Piotrowski, 2003; van der Meer et al., 2003;Larsen et al., 2004) and recently elaborated in afield data-constrained model by Piotrowski et al.(2004).Deformation of till requires porewater pressure

of at least 90–95% of the ice overburden pressure(Paterson, 1994, p. 169), and basal decouplingwill only occur if water at the ice-bed interface ispressurized to at least the ice overburden pres-sure. Evidence from Kurzetnik indicates, there-fore, that subglacial water pressure was highthroughout the period of ice advance, fluctuatingbetween a condition of water-pressure inducedice-lift (hydraulic lifting and sediment washingwithin a thin meltwater layer) and a condition ofice contact with the base (basal coupling anddeformation). High subglacial water pressure wasinferred from many Pleistocene locations (e.g.Shoemaker, 1986; Brown et al., 1987; Piotrowski& Kraus, 1997; Cutler et al., 2000; Breemer et al.,2002; Fisher & Taylor, 2002), but Kurzetnik maybe exceptional in that the site is underlain by atleast 11 m of regionally extensive outwash sandwith high hydraulic transmissivity. This suggeststhat even in areas with high drainage capacity,water excess at the ice sole is likely, with possibleconsequences for generating subglacial channelsand tunnel valleys such as those abundantlyoccurring in northern Poland.

CONCLUSIONS

The investigated site provides evidence of com-plex ice-bed interactions leading to till formationand deformation. Most of the till succession withlargely uniform petrographic and mineralogicalcomposition but different structural properties isinterpreted as a hybrid lodgement/deformationtill. Till formation occurred under water pressuresin the vicinity of ice overburden pressure, result-ing in temporary basal decoupling by thin waterlayers. Gradual transition from sand to till con-sisting of a zone of upward-increasing strain andsediment mixing lends support to the idea thatpervasive deformation leads to granular diffusionand homogenization across lithological boundar-ies (Weertman, 1968; Piotrowski & Tulaczyk,1999; Hooyer & Iverson, 2000). In accordancewith well-constrained laboratory experiments, thevery high fabric strength (average S1 eigenvalue ca0Æ88) indicates high strain of the till (above ca 5).Vertical variations in fabric strength are inter-preted as a signature of different strains that the

till has experienced. The data, in particular thepartial preservation of sedimentary structures,ploughing marks, weathered and unrotatedstones, and lack of systematic trend in fabricstrength in the profile show that deformation,although resulting in high cumulative strain, wasfocussed in a thin (typically centimetre range)zone of the deposits immediately under the icesheet which zone moved time-transgressivelyupwards as till accretion proceeded. It followsthat substantial volumes of the morainic materialwere transported englacially, whereas the mater-ial advected in the subglacial traction carpet wasvolumetrically less significant. Ice movement wasby a combination of bed deformation and basalsliding with ploughing, the relative importance ofwhich varied in the course of glaciation depend-ing on the water input-controlled degree of basalcoupling. Presence of thick (> 11 m) outwashsand under the till coupled with the inferredwater pressures in the vicinity of glacier floatationpoint lends support to the suggestion thathydraulic transmissivity of beds overridden byPleistocene ice sheets was typically insufficient toevacuate all meltwater from the ice base asgroundwater flow.

ACKNOWLEDGEMENTS

JAP and NKL thank the technical staff at thesedimentology laboratory at Aarhus Universityfor sample processing, and JM thanks CandyKramer and John Taylor for technical help inthe micromorphology laboratory at Brock Univer-sity. The following research grants are acknow-ledged: SNF grant no. 21-00-0410 and AarhusUniversity grant to JAP, and COWI grant to NKL.We thank the landowners for granting access totheir property. The manuscript benefited fromconstructive reviews by Colm O Cofaigh andDoug Benn.

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