Till body morphology and structure related to glacier flow JOHN SHAW

BORJEAS Shaw, J. 1977 06 01: Till hody morphology and structure related to glacier flow. Boreas, Vol. 6 , pp. 189- 201. Oslo. ISSN 0300-9483.

Tills are described which occur in ridges and mounds arranged both parallel and transverse to the flow direction of the dcpositing glacier. Field localities arc drawn from the English Midlands, Western Canada, and South Victoria Land, Antarctica. The tills retain testural and structural properties associated with glacial transport, and have suffered a minimum of redistribution suhscquent to theii- i-clease from glacier ice. It is shown that ridges and mounds cannot he explained in terms of prefercntial till accretion. A n alternative mccha- nism is presented in which form and structurc arc a result of redistribution of debris in transpot-t by secondary flows in icc.

Flutings are longitudinal forms which are I-elated to helicoidal flow cells. Fabric distribu- tions, patterns of till thickness, and internal structure support the hclicoidal flow hypothesis.

Debris entrainment by Antarctic cold-based glaciers is cxplaincd by consideration of the moi-phology and sedimentology of the ice margin and the pattern of glacier flow. Deposition hy sublimation and melt-out produces an upwa succession of (1) undisturbed proglacial deposits; (2) a complcs of poorly sorted flow dc ts intcrcalatcd with sorted and stratified walcr-lain deposits; (3) foliated till with sub- rontal jointing and isolated clasts. A section shobbing this succession is described ft-oin Taylor Valley, Antarctica.

Transverse asymmctric ridges are related to till stacking by over-folding in the marginal zone of cold-bascd glaciers. Plastic deformation o f the dehris-laden ice may hc

cnhanced by incorporated salts. The folding process is illustrated by structurcs within Taylor glacier, and is used t o explain Pleistocene landforms and structures in Shropshirc, England and Taylor Valley, Antarctica.

.lohn Shaw. Drpc i r lmmt o/ Geo,yruphy. l /n ivcrs i /y ~f A l h ~ ~ r t a . Edmonton. . I I b ~ r / a . C‘unada. Senremher I976

Bodies of till commonly form upstanding relief features, or, where they a re buried beneath later deposited sedimcnts, they show undulating upper surfaces. The form is generally a result of differential till thickness and is not related to an undulating sub-till surface. Two inter- pretations have been placed on the variable till thickness. The first assumes that the initial till deposit was of uniform thickness and that subsequent differential erosion occurred. The second assumes differential accretion. ‘These two assumptions form the basis for thc ero- sional and depositional schools of thought on drumlin formation (Gravenor 1953). Differen- tial deposition, associated with post-depositional redistrihution of till, has been the most widely supported mechanism for various landform assemblages collectively termed hummocky mo- raine (Hoppe 1952; Gravcnor & Kupsch 1959; Stalker 1960; Boulton 1972). Some controversy remains as to whether the pos,t-dcpositional redistribution is hy gravitational mass move- ment or by ice-squeeze mechanisms. Neverthe- less, the contention remains that the redistri- bution occurs after a till is released from ice,

and the concept o f relief inversion commonly involves lateral translo’cation of ablation till such that zones with high volumes of cnglacial till form depressions in the deglaciated land- scape (Fig. 1).

The process of differential accretion is inti- mately associated with the concept of lodge- ment till, although the volumetric importance of this till type appears to have been over- stressed in the past. Nobles & Weertman (1971) presented an analysis of the thermal conditions required for the subglacial release of lodge- ment till. For till deposition to occur by under- melt of active ice they write:

Q + Jvtr>/kVTI (1)

where Q = geothermal heat input; v = sliding velocity; (?=shear stress acting a t the bo’ttom surface; J = mechanical equivalent of heat; v T = thermal gradient.

The average rate of till deposition is given by:

[O+Jvn-- I k L “I D = ___ ( I - c )

190 John Shaw BOREAS 6 (1977)

Flow and melt- A out t i l l

\ Basal t l l l Debris bands

B

Control led hummocky fnioi,~inP

FIR. 1. Relief inversion produced by differential abla- tion and slumping (after Boulton 1972).

Glacier bed at melting point (T,,,)

Td Ta

\ t \ t \

F i p . 2. Effects of subglacial topography on hcat flow (after Nobles Ri Wcertinan 1071).

where D=thickness of till deposited in unit time; c = volume fraction of debris within the ice; L =latent heat of fusion of ice.

Nobles & Weertman (1971) then showed that, for a wet bawd glacier, irregularitiey at the bed cause increased geothermal input a t depressions by heatflow refraction, and in-

creased thermal gradient ( v T) above high points (Fig. 2). Consequently, following equa- tion 2, rates of deposition arc highest above depressions and lowest above high points. The growth of irregularities by differential accre- tion is consequcntly inhibited by a negative feedback mechanism. High points cannot be a product of preferential basal accretion.

In view of the above discussion considerablc difficulty is experienced when confronted with differential till thicknesses in which the till carries structural attributes of englacial trans- port. A possible solution in which till is re- distributed by secondary flows superimpo'sed on downglacier flow will now be invcstigated. Longitudinal and transverse forms will he con- sidered separately.

Longitudinal features On the Albcrtan plains, large-scale flutings, with heights of the order several tens of meters and lengths o f thc order several kilometers, have been described by a number of authors (Gravenor & Menely 1958; Shaw & Freschauf 1973; Shaw 197.5). The flutings are composed mainly of till with minor thicknesses of strati- fied' sediment. The till is thicker, by a factor of up to twenty, beneath fluting ridges than beneath adiacent troughs. Alignment parallel with regional ice movement directions and streamlined surfaces imply association with active ice. In addition the till shows strong horizontal jointing, with a spacing of a few centimeters, in all but the surface horizons of modern soil formation. I t will be shown later that the horizontal jointing can be related to structures within active ice so the flutings are confidently ascribed to formation beneath active ice. A mechanism of differential accre- tion has becn shown to be untenable as an explanation of fluting ridge formation. As the till retains characteristics of englacial trans- port, two alternatives remain. Either the relief is a result of post-depositional erosion of the troughs, o r the ridges were sites of high mass of englacial debris above unit area of the glacier bed. Unlike the products of relief inver- sion processes, thesc areas of high mass remain t o form ridges after deposition. Shaw & Fre- schauf (1973) considered the kinematics of flows which could producc systematic spatial variation in the mass of englacial debris. Till

BOREAS 6 ( 1 9 7 7 ) Till body morphology and structure 191

Init ial conditions

Erosional f lut ings

Flutings formed by dtfferentlal

erosion and deposit ion

T i l l transport direct io t i

F lut ings formed by deposit ion

fo l lowing lateral t ranspor t of t l l l

T i l l in transport / /

Secot i t lary f low cell

Depositional f lut ings

Expected clast long - a x i s orient at ion

A

Ohservcd lo n t; ~ a x i 5

or ip r i t a t ion

R

T i l l

x 1,

I : ix . 3. Hypothctical models for f lut ing formation. E.xpectcd and observed clast orientation, Albcrta. Canada.

I92 John Show ROIIEAS 6 (1977)

fabric analysis provides a test o f tfic erosional hypothesis and two plausible mechanisms which produce spatial variations in mass o f till in transport. The models of flow and pre- dicted long-axes orientations are shown to- gether with composite diagrams of all long-axes orientations recorded at equivalent positions in fluting ridges (Fig. 3). 'I'he observed patterns give remarkable support for the secondary flow mechanism of till redistribution during transport.

The morphology and internal structurc of fluting ridges are explained by a hypothesis of englacial transpo'rt in the basal part of the glacier affected by secondary flow cells. As shown by Shaw (1975) the flow cells a re

I'iR. 4. Englacial debris hands separated from the basal debris-rich layer by clean ice, Taylor glacier.

necessarily short lived. The dynamics of the flow are not understood although indirect evidence supports the suggestion that the flutings were formed under diverging flow, and may be related to pressure gradients as- sociated with longitudinal or splay crevasses.

Transverse features Dehris entrainment The Taylor glacier in the McMurdo Dry Valleys, Victoria Land, Antarctica exhihits a minimum of 5 m of b a a 1 ice highly charged with dehris. The thickne$s is given as a mini- mum owing to inasking of the lowest part of

BOREAS 6 (1977) Till bod)) morphology and sfrricture 193

1

. -

3 I

I I

I I I I

t I I I

‘Drl ir is I irli l h s a l I( c y

F i g . 5. Debris entrainincnt by incorporation of thc frontal apron. Shaded arcas illustrate the sequential de- foimiafion of apron sediments.

the glacier by frontal aprons. The ice is bubbly clasts commonly shown for other basal ice and highly foliated. Debris within the basal bodies (Boulton 1971: Fig. 2) is not apparent layer is largely of granule or smaller size, but rather the foliation terminates a t clast although larger clasts a re interspersed. The boundaries. The basal layer comprises bands deflection of foliation planes around the large approximately 10 cm thick containing high

I94 John Shuw BOKEAS 6 (1977)

. I 3

.I?

. I 1

\

\, -..\

lllrlrl -- Gl<l( , P I f low

Fix. 6. 'I'iltetl lake terraces, Lower Wright glacicr.

debris concentrations (~O-SOVO b y volume). lntcrvening layers of clean ice are approxi- mately 2 cni thick. A zone o f prominent englacial debris layers is separatcd from thc basal layers by clean ice (Fig. 4). Sotrchez ( 1967) describctl the morphological and surfi- cia1 featiircs of a lohe of the Ferrar glacicr, Antarctica. Howcver, his dcscription does not include thc proininent vertical wall which is characteristic of most Antarctic glacier fronts. 1 he charactcristic morphology of an Antarctic glacier margin comprises a n irregular surface

- .

o f supraglacial stream channcls, and transverse structural depressions terminated by a 10-20 m high vertical cliff (Fig. 5). The cliff foot is masked by an apron of fallen ice hlocks, refrozen meltwater, supraglacial debris washed or fallen from the glacier surface, englacial debris, and wind-blown sediment. Considerable reworking of the accumulated debris occurs by mass-flow and running water. Blockfall is a n intermittent process and wind-drifted snow accumulates mainly in winter, consequently thc frontal apron comprises alternations of high and low debris concentrations. Crude stra- tification of limited lateral persistence is ex- pected.

Consider a t ion of the morphology d escrihed and the flow pattern a t the ice front (Holds- worth & Bull 1968) allows construction of a model of debris entrainment during advance of a cold-bared glacier (Fig. S ) . The mechanism is independent of the special thermal condi- tions required for the entrainment of debris hands proposed by Weertman (1961). The im- portant fact is that the morphology and flow pattern of cold Antarctic glaciers will lead to basal entrainment o f debris whether or not other inechanisms of debris incorporation such as shear or basal freezing occur. The model represcnts a n extension of the mechanism of formation for ice-cored moraines proposed by Hooke ( I 973).

In support of the concept of debris entrain- ment by incorporation of frontal aprons are:

F i g . 7. Steeply dipping $hear planes, Taylor glacier.

BOREAS 6 (1977) Till hodv morphology arid structiire I95

FiR. H. Folding in the lower debris- rich layer, Taylor glacier: (A) monocline, (B) disharmonic re- cumbent folds.

( I ) The bubbly appearance and foliation o f the ice. Apron materials contain a largc volume of air voida, particularly between fallen blocks, which account for the trapped air. Extension o f the crude stratification of the apron material explains the foliated naturc of the basal debri4 zone. (2) The relative sorting of debris (Souches 1967). The sorting is considered to be inherited from processes occurring on the apron. (3) The approximate conformity of the top of the basal debris zone and the height of the frontal apron. (4) Tilting of terraces formed a t the margins of ice-walled lakes in the marginal zone of the

Lower Wright glacier (Fig. 6) confirms the overturning mechanism illustrated in Fig. 5.

It is difficult to explain the upper debris hands which are separated by clean ice from the underlying dehris-rich basal ice. Several explanations are available but they are not easily tested. The basal freezing mechanism of Wcertman (1961) may have occurred, or shearing of the upper ice across slower moving, lowcr, clean and clehris-rich ice may have been activc. Alternatively, following the apron in- corporation model, a wedge of clean ice could have been incorporated in the debris zone during an extended period o f ice-block and wind drifted snow accumulation on thc frontal

BOREAS 6 (1977)

apron. Finally basal freezing as the glacier crossed a lake in the area upstream from the ice margin could account spersed between two zones

Morphological frutures The entrainment proce\s like bodies of debris-rich

for clean ice inter- of dirty ice.

gives rise t o sheet- ice in the basal ice

zone. However, the present margin of Taylor glacier reveals contortions o f the basal dirty ice associated with conipre5sive flow manifest in \teeply dipping shear planec in the upper ice (Fig. 7). Simple monocline5 are ohserved

I,'ig. 9. Taylor glacier: (A) asym- metric moraine, (B) ice foliation within ice-corcd moraine.

in upglacier positions, with disharmonic re- cumbent folds towards the glacier margin (Fig. 8). The effect of the folding is to produce stacking of debris-rich ice beneath zones of compressive shear. Progressive development of discrete shear zones during glacier retreat produces a series of asymmetric debris-rich bodies of ice (Fig. 9A). Subsequent back and downwasting of the ice front with sublimation and melt-out of ice within the debris zone produces a series of transverse asymmetric moraines which owe their origin to englacial flow pa,tterns.

The preservation of these features is prob-

BOREAS 6 (1977)

North M e t e r s A

m Ice-cored moraine ridge B" Morainlc surface sediments Fluvial surface sediments

b ' ig . 10. Lacroix moraines and cross-section

lematical as slumping of saturated t i l l causes degradation of the initial topography. How- ever, the mclt-out tills a t the surface dry ou t extremely quickly in the excessively low rela- tive humidities experienced in the Dry Valleys. and below the permafrost tahle ice is removed by extremely slow sublimation processes (Bell 1966). Consequently the cohesive properties of fine-grained materials permit stable slopes in excess o f 30 . Removal of icc from the under- lying t i l l leads t o retention of elements o f

fabric and structure inherited from the trans- port phase (Fig. 9B). As melt-out or sublima- tion tills above foliated ice reveal sub-hori- zontal jointing it is assumed that the jointing is a result o f melt-out or sublimation of foli- ated, debris-rich ice. Drying out a t the surface produces desiccation cracks in the form of vertical joints. Some rotation of clasts must occur with compaction on loss of ice. Some long-axes of clasts within the asymmetric till bodies show preferred parallel orientation (P.

198 John Shaw BORBAS 6 (1977)

~, Taylor glacier -’ /’

B

Foliated ice and ti l l

I Sorted debris

Ti l l with coluninai lo in t i

Lacustrine sediments ~ , ’ I ,J,, transported by y ‘: overriding ice ~ &

F i g . 11. Comparison of asym- metric till bodies. (A) Taylor glacier, (B) Shropshire, England.

Asymmetrical t i l l Proglacial Ice wedge Flow t i l l body with sub- lacustrine pseudornorph(7) horizontal toints sediments

Robinson & A. Palmer, personal comniunica- tion) while othcrs which truncate the foliation are often vertical. The intense plastic deforma- tion of the debris-rich basal layer may be, in part, a result of high salt concentrations in the lower layers. Ho’ldsworth & Bid1 (1968) noted an increase in the exponent of the flow law of cold ice frolm 1.6 to 4.3 in association with increases in salt concentration of the so called ‘amber’ ice. Meltwater from the Taylor glacier is commonly saline and forms salt deposits on the frontal apron (Black 1969). Incorporation of the frontal apron will cause a consequent increase in the salinity and, therefore, plasticity of the basal ice.

Asymmetric moraines with steep distal and gentle proximal slopes result from the d,efornia- tion of basal till in transport. Depositional processes include some redistribution and

so’rting by slumping and running water, hut sublimation tills within the interior of the moraine may retain fabric and other struc- tural properties inherited from the transport phase. The preservation of such features in areas of past glaciation is difficult to establish. However, asymmetric moraines, which remain ice cored, are found o n the floor of Taylor valley. The moraines are transverse to, and were formed by the Lacroix glacier, a n alpine glacier on the north valley wall. Proximal slopes are relatively gentle and distal slopes steep, although some relatively narrow mo- rainic ridges are approximately symmetrical in cross-section (Fig. 10). Although the moraines are draped by lacustrine sediments, a few exposures reveal till with sub-horizontal jointing and containing dispersed coarse clasts. Succes- sive ice-marginal stream channels a t progres-

F I X . 12. Facies, Antarctic cold-based glaciers

BOREAS 6 (1977) Till bodq' morphology and structure 199

sivclp lower elevations toward the north de- drainage zone. Fine materials are winnowed monstrate the successive northward deposition from the slump sediment leaving a relatively o f moraines (Fig. 10). Tlie retreat o f the coarse crudely stratified sediment. Continued Lacroix glacier is associated with a fall iii drainage from the slump scar leads to erosion level of Lake Bonney which occupies the in the upper parts and deposition of small Taylor valley cast of Taylor glacier. alluvial fans above the toe of slumped sedi-

A further example, from Shropshire, Eng- ments. Elsewhere sheet erosion forms a sorted land, of the preservation of transverse asyni- cover a t the apron toe. By these processes the metric moraines associated with glaciers con- apron toe comprises a complex of alternating siderctl to be partially co,ld hased was presented flow and washed sediments. The incorporation by Shaw (1971). He descrihed asymmetric pro'cess outlined in Fig. 5 illustrates how the bodies of till with the steeper slope up to SO . base of the apron retains a position at the base Tlie ti l l within the asymmetric bodies exhihits of the transported debris and suffers a mini- considerable sorting a t the base beneatli the mum of deformation. Above this complex the steep slope, and vertical jointing produces remaining apron material is attenuated. Final coltiinnar structure within the upper part of deposition by melt-out and sublimation without the till. Similar sorting and jointing is noted appreciable reworking should produce a n up- in the Taylor moraines. 'Mean dip of clast long-axcc beneath the steep face o f the Shropshire moraines was measured at 38 ' in the direction of slope of the face. The pebbles in the surface layers beneatli the Taylor mo- raine distal slopes are noted to dip steeply in the directio'n of slope. Reorientation by creep is the probable mechanism proclticing this alignment. Tlie upper surfaces of the Shrop-

wards facies succession of: Facies A: undis- turbed proglacial sediments; Facies B: a com- plex of poorly sorted, crudely stratified flow deposits intercalated with discontinuous sorted sediments; and Facies C: a body of foliated t i l l with sub-horizontal partings representing foliation, and isolated clasts. Exposures in Antarctic tills are rare, but a well exposed sequence occurs in a meltwater gully below

shire moraines show similar undulations to the Lacroix glacier. The basal part of the those of the mo'st recent Taylor moraines. succession was deposited by an advance of Preservation of the surface undulations of the Taylor glacier. An interpretative representa- Shropshire moraines may have resulted from tion of this sequence corresponds to the facies the overriding of these moraines while they model proposed above (Fig. 13). were still ice cored (Shaw 1971). A comparison o f the properties OF the Sliro'pshire and 'Taylor valley moraines is presented in Fig. 1 I . The similarity of properties of the moraines in the two areas supports a ConclLIsion of silnilar 'l'he morphology and internal composition of genesis. In this case Shaw (1971) misinterpreted Iongitudinal and transverse forms in till can the direction ice Iiiovement by suggesting he related t o flow structures in the transporting that the gently sloping face was distal. ice. Although areas of hummocky moraine are

generally ascribed to redistribution processes subsequent to till release, interpretations of

Sum mar y

~ ~ ~ ~ ~ i ~ i ~ ~ sequence of ~ ~ t ~ ~ ~ t i ~ cold-based glaciers

such relief should consider the possibility that the relief is a product of primary deposition. The degrcc of retcntion o f str~icttiral aspects

A systematic facie:; differentiation may be o f tlie ice-transport phase is the most useful applied to Antarctic glaciers (Fig. 12). The criterion for differentiating morphological fea- proglacial area is characterized by fliivial, tures of primary origin from those formed by lacustrine and eolian processes which produce secondary redistiihution pro,cesses. Fahric and sorted stratified deposits commonly including jointing patterns and degree of sorting are evaporites. Numerous processes operate on the found lo be the most effective properties frontal apron, hut reworking is dominantly by facilitating this differentiation. The model of running water and slumping. A common se- debris incorporation and facies successions may qtience of events is for saturated till to slump provide a simple field test for the identification leaving a flow scar which then acts as a o f cold-based glacier deposits.

200 .John Shuw BOREAS 6 (1977)

A Proylacial fluvial facies

B

C,

C,

Apron flow and 5liatified \edinient fiKiPs

Slightly attenuatctl haaa t i l l facies

Highly attenuated Ibasal t i l l facies

r 3

Diarriic ton. diswrstrl large clasts. Intensely developed tiorizonta I pint ,rig

~~ .

Mai \ tve dianilctloii rare stratitic 'itlon. liigh coiii entiation of lnrye ( lasts transitional conlact with overlying bed

\ \

common S i n d ~ a l e cross- Iarninatioii and horizontal stratification

I..ix. 1.1. 1ntrrp:ctation of glacigcnic sequence, Lacroix glacier, Antarctica.

BOREAS 6 (1977) Till body morphology and structure 201

Acknowledgements. - The Antarctic work was fi- nanced by a New Zealand University Grants Com- mittee grant to Drs. M. J. Selby and T. R. Hcaly. Field support from the Unitcd States Navy and pcrsonnel of Scott Base is gratcfully acknowledged. Professor J. D. McCraw kindly made facilities available at the University of Waikato.

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Black, R. F. 1969: Saline discharge from Taylor glacier, Victoria Land, Antarctica. Anturcric J . United S/ules 4 , 89~-90.

Boulton, G. S. 1971: Till genesis and fabric in Svalbard, Spitzbergen. In Goldthwait, R . P. (ed.): Till: A Symposium. 41-72. Columbus, Ohio State Univ. Press.

Boulton, G. S. 1972: Modern Arctic glaciers as depositional models for former ice shects. J . G e d . Sue. Lond. 12X, 361L393.

Gravenor, C. 1’. 1953: The origin of drumlins. A m . J . Sci. 2-51, 674-681.

Gravenor, C. P. & Kupsch, W. 0. 1959: Icc disintegra- tion features in Western Canada. J . Geol. I?, 48-64.

Gravenor, C. P. & Meneley, W. A. 1958: Glacial

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Hooke, R. LeB. 1973: Flow near the margin of the Barnes Ice Cap and the devclopment of ice-cored moraines. (3rd . SOC. Amer. Bull. X4, 3929-3948.

Hoppc, C. 1952: Hummocky moraine regions with special rcference to the interior of Norrbotten. Grogr. Ann. 34. 1-72.

Nobles, L. H. & Weertman, J. 1971: Influence of irregularities of the bed of an ice sheet on deposition rate of till. In Goldthwait, R . 1’. (ed.): Till: A Sym- posium, 117- 126. Columbus, Ohio State Univ. Press.

Shaw, J. 1971: Mechanism of till deposition related to thermal conditions in a Pleistocene glacier. J . Glociol. 10, 363-373.

Shaw, J. 1975: The formations of glacial flutings. N. Soc. New Zeulund Bull. 13, 253-258.

Shaw, J. & Freschauf, R. C. 1973: A kinematic discussion of the formation of glacial flutings. Cun. GeoRrapher 17, 19-35.

Souchez, R. A. 1967: The formation of shear moraincs: an example from south Victoria Land, Antarctica. J . Gluciol. 6 , 837-844.

Stalker, A. 1960: Ice pressed ridges and associated deposits in Alberta. Bull. Geol. Surv. Cunudu 57.