Scanning electron microscopy of Pleistocene tills in Estonia

WILLIAM C. MAHANEY AND VOLLI KALM

BOREAS

The depositional environment of glacial sediments canbe determined by analysis of microtextures on quartzgrains (Krinsley & Takahashi 19621- Margolis &Krinsley 1974; Whalley & Krinsley 1974; Krinsley &Trusty 1985; Krinsley & Marshall 1987). The directstudy of active subglacial deposition is extremelydifficult (Hubbard & Sharp 1989), being limited to avery few warm alpine glaciers and over time spans atbest of a few years (see Vivian & Bocquet 1973;Vivian 1975). No one has studied grains depositeddirectly by glacial ice, but there are a few SEM studiesof grains from moraines (Krinsley & Doornkamp1973; Mahaney et al. 1988), from supraglacial drift(Mahaney et al. l99l), and from glacial grains in loess(Mahaney & Andres l99l; Smalley & Glendinningl99l). The problems involved in observing the processof subglacial deposition and collecting the sampleshave led to the use of particle size and sand clastorientation from oriented blocks (Mahaney et al.1989) to infer glacial deposition without the benefit ofdirect observation. As a result the inference of aglacigenic origin is based on a combination of sedi-mentary parameters such as particle size distributions(Mahaney 1978; Haldorsen 1982), sand or pebblea-axis orientation (for example, Mahaney 1990b;Catto 1990), SEM microfabrics (Mahaney et al.1989),and microtextures (Krinsley & Marshall 1987; Ma-haney et al. 1988; Mahaney 1990a, b, 1994).

Many previous SEM studies were carried out usingsamples from end moraines (Mahaney et al. 1988)which contain quartz grains emplaced by subglacial

Mahaney, W. C. & Kalm, V. 1995 (March): Scanning electron microscopy of Pleistocene tills in Estonia.Boreas, Yol. 24, pp. 13-29. Oslo. ISSN 0300-9483.

Tills lrom four Pleistocene glaciations were recovered from drill cores in Estonia and subjected to particle sizeand microtexture analyses by Scanning Electron Microscope (SEM). All tills were deposited by thickcontinental ice-sheets following the transport of, at most, several hundred kilometers during four Fennoscan-dian glaciations. The main problem is to determine if the type and range of microtextures present on the grainsurfaces are diagnostic of transport in continental ice. The frequency of occurrence of microtextures includingfractures, abrasion, and relief features are used to test the ability of continental ice to damage quartz particlesemplaced as till. The range olquartz dissolution and presence ofcoatings on grains are also used to reconstructthe paleoenvironment that existed prior to transport as well as to estimate diagenetic effects that occurredlollowing emplacement. The available data indicate a high degree of reworking ol quartz grains from oneglaciation to another. While the shapes and microtextures of grains from source rocks are not known, the greatrange ol fracture and abrasion microfeatures, and high frequency of occurrence on grains in all tills, indicatethat glaciers are effective crushing agents. An increase in the prevalence of chemically etched grains from olderto younger tills suggests that some grains (c. 50%) escape crushing, either because of preservation in the ice andlack ol grain-to-grain contact, or as a result of massive reworking ol weathered grains lollowing interglacia-tions.

Lltilliam C. Mahaney, Geomorphology and Pedology Laboratory, Atkinson College, York Uniuersity, 4700 KeeleStreet, North York, Ontario, Canada M3J 1P3; Volli Kalm, Tartu Uniuersity, Institute of Geology, VanemuiseStr. 46, Tartu, Estonia, EE 2400; receited 12th January 1994, accepted 26th September 1994.

deposition (including meltout), as well as presumablyby mass wasting, surface meltout and thrusting mech-anisms. In order to test the full extent of the effect ofcryostatic pressure on quartz grains transported at thesubstratum/ice contact zone in a glacier, it is impor-tant to sample lodgement till (ground moraine) em-placed behind end moraines, to recover tills fromboreholes through a succession of lodgement tills ofdifferent ages and/or to study till recovered from basallayers in ice cores (Tison et al. 1993).

While it is impossible to know the precise shape ofparticles at source rock locations and the rnicrotex-tures they carry, it is possible to assume on the basisof previous work on mechanical release of quartzfrom cave walls (Mahaney & Sjoberg 1993), fromcrystalline bedrock (Mol6n 1993), and from cirquewalls above existing cirque glaciers (Mahaney et al.1991), that the dominant microtextures are fracturefaces with clean breaks across one side of individualgrains. Minor subparallel fracture lines are also com-mon on grains with subangular to subrounded shapes.No conchoidal fractures, steps, troughs, gouges, per-cussion cracks and/or abrasion microtextures havebeen found on source rock samples, although somesamples show considerable chemical etching whichsuggests that mechanical release works more emcientlyin rocks weakened by chemical weathering. Certainlyparticles introduced to transport in glaciers must beconsidered elastic inhomogeneities within differentparts of the ice body, which are affected by variablestresses. Because the upstream origin of particles and

14 William C. Mahaney and Volli Kalm

fragments is unknown it is impossible to determine theprecise distance of transport, making it necessary touse maximum distances. Particles forming inhomoge-neous inclusions in glaciers are probably acted uponby stick-slip stresses at the glacier sole, sufficient tocause fracture by propagation of pre-existing cracks,some of which may result from mechanical release ofsource rocks and lattice failure (Tison et al.1993).

In this study we present new information from theanalysis of microtextures on quartz sand grains recov-ered from tills in boreholes of south-eastern Estonia.These deposits are considered to have been emplacedby subglacial deposition by ice in direct contact withunderlying [thick] till bodies under moderate to highhydrostatic pressure in the ablation zone of largecontinental ice-sheets, with thicknesses estimated at c.1500+ m and possibly as much as 2000m (Aseyev1974; Raukas 1988; Holmlund & Fastook 1993).Thus, for the most part the ice would have been close

BOREAS 24 (t995)

to or at the pressure melting point when the grainswere emplaced, which increases the chance of grain-to-grain contact.

Field area

The boreholes are located near Tartu in south-easternEstonia (Fig. l). The slightly undulating or hum-mocky landscape is characteristic of the field area. Thesites were selected in areas known to contain thick(14-60 m) sequences of Pleistocene tills (Fig. 2). Ac-cording to conditions of deposition the structure,composition and thickness of the Pleistocene coversediments (mainly tills) vary a great deal. In SouthEstonia five till horizons can be recognized (Raukas1978; Liivrand l99l; Kajak et al. 1990).In some cases,particularly in buried pre-Quaternary valleys, till bedsare separated by organic-rich clay, silty and sandy

TBOREHOLE SITE & NO.

1 Aio - 5662 Korvek0lo - 5283 Rongu - 7

4 Volguto-ll,l45 Aokre- 156 Komeri - 580

ESTONIA

Flg. 1. Borehole location map.

Aakre I 5 Valguta I I VElgulE 1 4 REngu 7

e7-164

P,7-1

BOREAS 24 (1995)

Var

LEGEND

ffi -soit layer

ffi ttwiattt ltaty

Nl -v"rour"Till (\,br)

%'Yfi8w853i3""ffi| -upp.rugandiriit (uu)

NN uoa.ugandi (MU)

WrtA -t*t"rugsndi Tiil (LU)

I1m?6t[:ffiPffi -ro*rDainava (uD)

ffi o"r*ian sandstone (Dev)

lTll;:;:iil {5#1il*flHj,5m"**n"

El 8?-to5 -sampling site and no.

Fig. 2. Slratigraphy of tills recovered from seven boreholes in south-eastern Estonia.

Table 1. Correlation table for the investigated samples.

SEM of Pleistocene tills 15

Ais 566 K6rvekijla 528

a7-13,4

Var

a7-79

e-7^Fs .

&l

t0-

UD

a7-46

20-

Dev

Lat

1.L7.

8?-'t45

LUIt."a7-t42

a1-5Q

50-

87-t44

LUi

E 40-a;o6

=Eo

:o DU-oo

BoreholeSite

No. 7Rongu

No. 15

AakreNo. 1lValguta

No. 14

ValgutaNo. 566Aia

No. 580Kamer

No. 528Korvekiila

Latvia till(late Weichsel)

Varduva till(early Weichsel)

Merkine/interglaciation

Upper Uganditill (late Saale)

Lower Uganditill (early Saale)

Butenai/interglaciation

Upper Dainavatill (Elster)

87 -92

87 95

87 137

87- 138

87,140

87 142

87 t448'7 145

87 '79

87 80

87 9987 102

87 -10487 106

87 113

87 -11487 116

87- I 19

87 -122

87-5087 52

87 86

16 William C. Mahaney and Volli Kalm BOREAS 24 (1995\

Flg. 3. Geological map olEstonia simplified from: Geological Map of the Soviet Baltic Republics. Scale (1978) l:500000, Leningrad,Aerogeologija. Editor in Chief A. Grigelis.

deposits of interglacial or interstadial origin (Liivrandl99l). The studied till beds can be correlated withLatvia (late Valdai; late Weichselian), Varduva (earlyWeichselian), upper Ugandi (Warthe), lower Ugandi(Saale, Drenthe), and upper Dainava (Elster) glacia-tions (Table 1) (see Aseyev 1974; Bowen 1981; Ehlerset ql. 1984 and Velichko & Faustova 1986, for discus-sions of the Estonian, European and Russian glacialsequence). The Pleistocene deposits are underlain bythick (5-8 m) outcrops of middle Devonian (Fig. 3)sandstone that is quartz-rich, slightly indurated, andnearly unconsolidated.

Bedrock considerations

The underlying bedrock in the study area is silt andsandstone of middle Devonian age (Fig. 3). To thenorth, less than 30 km away, Devonian rocks give wayat the surface to dolomite and limestone of Silurianand Ordovician age, covering a belt about 50 km inwidth, mainly covered with Pleistocene drifts. To thenorth of the dolomite and limestone belt a narrowband of lower Ordovician and Cambrian sandstones.

siltstones and clays outcrop along the coast and underthe Gulf of Finland (Winterhalter et al. 1981). Cam-brian-Ordovician siltstones and sandstones are ex-tremely rich (80-90%) in quartz (Viiding et al. 1983).Ordovician carbonate rocks are mainly represented byvarious limestones and marlstones (P6lma 1982),whereas the Silurian beds are composed of dolomiticcarbonate rocks, marls, clays and argillites (Jiirgenson1988). Devonian sandstones, particularly those ofArukiila formation underlying Pleistocene deposits insouthern Estonia, are again rich in quarlz - up tol5-90o1' (Viiding et al. l98l). Accordingly, as pre-dicted by Dreimanis & Vagners (1969), the amount ofqtattz is highest (average 75-18% in fine sand) inupper Weichselian till on Cambrian-Ordovician andDevonian sandstone outcrops (Fig. 2), decreasing to45-58% in the same till on Ordovician-Silurian car-bonate bedrock (Raukas 1978). Distribution of quartzin five studied till beds in south-eastern Estonia isas follows: upper Dainava till - 86%; lower Uganditill - 77%; upper Ugandi till - 68%; Varduva till -75nh; Latvia tlll - 75% (Raukas 1978; Kajak et al.1eeo).

BOREAS 24 ( 1995)

Table 2. Principal correlation ol stratigraphical units discussed intext based on: Ehlers. J., Meyer, K.-D., Stephan, H. J., 1984;Velichko, A. A., Faustova, M. A., 1986; Liivrand, E., 1991; andRaukas and Gaigalas. 1993.

lUestern Europe East European Plain Estonia *n and subformation)

otroo

EgooJ

tate

5 N.4iddt€

o_= e o,ly

0c0oa

-c3!!A

/oldoi

JSTASnKOV(Lole Voldoi) totviq

LegosciemsMiddle Voldoi

9pnels(Eoilv Voldoi) Vorduvo

Eem Mikulino Merkine

ooo

oaoEE:

Worthe(Soole2.3)

o-E (Treene?o

Drenlhe(Soole l)

oqECE+:;;

ugondi

upper Ugondi

Russio (Odrnlsovo) vliddle Ugond

Dniepr Lower Ugondi

Holslein tikhvin Butenqi

Elstera

q5(

i!q

A

Byelo-russio

Oko LithuonicUppet

Cromer Belovezhy€

Dzukija

Vilnius

*In Estonia raC dates are available only from Legasciems (:middleWeichsel) between 3l 200 and 39 700 yr BP (Kajak et al., 1981).TLdates are available from Latvia till (43 000 yr), Varduva till (65 000,75000 and 100000 yr) and upper Ugandi till [53000 and 216000yr (Kajak et al.,1981)1.

Thus, the origin of the quartz in southern Estoniantills is principally from the Precambrian crystallinerocks of the Fennoscandian Shield and from the sand-stone exposures along the southern coast of the Gulfof Finland (45-58% of quartz in till on quartz-freeOrdovician and Silurian carbonate bedrock areas)with lesser amounts (15 30%) of quartz from Devo-nian sandstones.

MethodsThe till samples were collected from the centersof cores and they were analyzed for particlesize distributions following ASTM procedures out-lined by Day (1965). The samples were wet-sieved toremove sands (2000-63pm); the fines (<63pm)were subjected to sedimentation and the percent siltand clay were determined by a hydrometer. The sandswere oven dried, sieved, and the coarse (2 /ijrm-500 pm) fraction was subsampled; original subsamplesand replicates were studied under the light micro-scope, and individual grains (mainly quartz) wereselected as randomly as possible for detailed studyby SEM. The samples were coated with carbonand analyzed on a JEOL-840 SEM with energy-dispersive spectrometry following procedures outlinedby Mahaney (1990a). The fine sands (63-250pm)were subsampled, sprinkled on a stub with a mi-

SEM of Pleistocene tills l7

crospatula, coated with carbon, and analyzed in thesame manner.

Stratigraphy

The principal correlation of stratigraphic unitsdiscussed in the text is presented in Table 2.The Eastern Baltic terminology (Raukas & Gaigalas1993) is used throughout with reference to EastEuropean and W. European stratigraphic namesas required. The till succession of seven boreholesis presented in Table I and Fig. 2. No datesare available from sampled boreholes, but thetill exposed at the surface is almost of Latvia (lateWeichselian) age. In some cases, for example atValguta (No. 14) and R6ngu (No. 7), older tills nearthe surface are covered with glaciofluvial or glaciola-custrine deposits of late Weichselian age. The lateWeichselian (Latvia) glacial model proposed byRaukas (1991) includes development of glaciation:slow oscillatory advance (25 000-20 000 years) fol-lowed by a rapid growth of glaciers (over l0 000-5000 years), and then by their disintegration at c.10 000 BP. The late Weichselian cooling maximumabout 20 000 years ago was followed by gradual cli-matic warming and the territory of South Estonia wasfreed of the continental ice between 13 000 and12250 BP (Raukas l99l).

The ages of the pre-Latvia tills are based on super-position, while conclusions about climatic change wereestimated from earlier palynological investigations ofassociated interglacial sediments, from three studiedexcavations and boreholes ( KSrvekiila- 528, Valguta -14, R6ngu-7).

In the Kdrvekula site Holsteinian (Butenai) lacus-trine deposits are present in borehole No. 528 (Fig. 2).The Holsteinian age of these deposits is established bypalynological investigations (Liivrand l99l). Thus,the underlying till is a minimum of Elster (upperDainava) age or older.

In the Valguta-I4 borehole (Fig. 2), silty-clayeyperiglacial deposits occur between the two lowermosttill beds. Based on palynological investigations ofthese deposits, this site has been suggested as aninterstadial type site for the middle Ugandi subforma-tion in Estonia and correlated with the middle Ugandi[(Shklov) Odintsovo] interglaciation and its analogueson the East-European Plain (Kajak et al. 1976). Ac-cording to this conclusion the lower and upperUgandi tills are separated from each other in the fieldarea. Liivrand (1974, 1991) questioned the Valgutastratigraphic assignment of the intermorainic depositsto the middle Ugandi and based on pollen analysis(no raC dates) assigned them to the middle Weichse-lian. Since the intermorainic deposits under discussionare considered to be reworked material (Liivrand

Rge(ln lal'.110,000

25,000

55.000

18 William C. Mahaney and Volli Kalm

UEiIAL <z@qp -AEnISENT TIW TEST 3rI4ElEOrr€iTllot

^raLYsrS

BOREAS 24 (t995\

Fig. 4. L A. Grain size distribu-lion lor Upper Dainava till.

o.oa Fig. 4. n B. Grain size distribu-tion lor lower Ugandi till.

!o

rao

Itoiol$9€je

!op

JtVf ANALNB

$€v€ ANILBtS

G U rta zt It r.tltE{troiTi ciloE tqr (rrciof,!l

uEntll <2@o,. -REm€s€iT TtvE t€sl SATPLE

sEotrEitattd AtALYsts

90

2ao

is9lsg.o:s3rc

to

6! 312 ri6 7! !.9 t.9!

tEiTrontH GRAcE SCALE (rrmof,slo.2a o.l?

BOREAS 24 (t99s)

Fig. 4. Z C. Grain size distribu-tion for Upper Ugandi till.

Fig. 4. l1 D. Grain size distribu-tion lor Varduva till.

SEM of Pleistocene tills l9

lAEFrll <2OOqr -R€Pl€s€lI T|VE ifsl S^laL€s€0rl€{t^trot atarYsrs

lr.2 156 za 39 t.95

,€{TtoFTX CFADf SCALE {rtCROtS)

l^fil^l <zooqp -REnfsEXtArlE TEST $IPLEsE0tr:xt^loi Ataltsts

.4 .t +3 .7 .! -t .tO 1|

s 5e rtr6 z! i9 r.t5

fENtroiTi ci^of sgG (rrciois)

90

zao

i@9150!.o

\"o

J€V: ANAIBIS

.r i2 i! +taIll

to

t!o

isIls!.o1$!o

t0

je

1/ -'/...'

t,f

t-t'

,,/.,/

/'.,. VARDUVA TItt

87-95

-

87-99 -------a7-02 ...'.......'..87-t38 ----

o.or Lm@

20 William C. Mahaney and Volli Kalm

J:YC IiALEI3.t +2 .l .a rl rc

lEDr[xnid Ai^lYs€

.7 .a .9 rro .rr

BOREAS 24 (t99s)

Fig. 4. J E. Grain size distribu-tion for Latvia till.

uEl& <a&q -tfnE3fiT ttE t€gr ut[E

l1-to

rao

ioIlrc3€j5!rc

oot L&@ 6 3U rtrt Za a9 t.tl

rENrroir{ ci^oE 3qG (ttcioNsl

1974, 1991), occur beneath three different till beds at adepth of 24-25 m, and yield highly controversial TLdates (Kajak et al. 1981; Liivrand 1991), we followhere the official stratigraphic schemes of Kajak et al.(1976) and Raukas & Gaigalas (1993).

The Eemian (Mikulian) age of the organogenicdeposits at the R6ngu site (core RSngu-7) was firmlyestablished more than half a century ago (Orviku,1939). On the basis of the pollen assemblage zonesthe continental interglacial deposits of the R6ngusite correlate well with the marine sediments inthe Prangli site (North Estonia) and with otherEemian sites in northern Europe (Liivrand l99lRaukas 1991). In general, the Eemian interglacial bedsat the RSngu site are covered by upper Weichseliantill; only in a few cores (R5ngu-7 included) do glacio-lacustrine or glaciofluvial deposits cover the inter-glacial beds.

The age-correlations of the pre-Weichselian tills inthe other boreholes, as well as in South Estonia ingeneral, are based mainly on conclusions from petro-graphic/mineralogical, geochemical and textural char-acteristics from each of the till horizons (Raukas1978; Kajak et al.1990).

Consequently, in this study the pre-Weichselian sed-iments (assuming no hiatuses) include tills from theWarthe and Drenthe substages of the Ugandi (Saale)

o.! o.rt

Table 3. Mean @ values for particle size distributions

Percentile

Sample 25th 50th 75th mean phi

Latviatill

Varduvatill

UpperUganditill

LowerUganditill

UpperDainavatill

87 -7987 8087 -9287 128

87,137

87 9587 -9987 -10287 138

87 140

87 10487- 10687- I l387 t1487 -14287 508'.7 -5287 11687 -14487 -14587 8687-lr987 122

5.'/ 3.95.8 4.06.4 4.36.1 3.95.8 3.9

9.8 6.96.9 3.45.2 3.66.5 4.1

7.9 4.8

7.1 4.87.1 4.75.3 3.06.8 4.1

6.5 4.3

8.5 6.28.5 6.37.6 4.57.3 4.55.8 2.8

2.42.32.51.5

2.3

4.50.52.42.22.4

2.52.20.81.3

2.3

4.23.91.92.2o.7

3.53.84.04.03.5

6.52.73.1

3.54-0

4.84.72.84.14.1

6.06.53.94.02.0

3.01.20.8

5.9 3.1

3.5 1.6

4.0 1.6

0.30*0*

*t*(r"

'/tZ,:'

./.tg

Iv / LATVIA TILI.

87_79

-

87-80 -------87-92 ........'.....

87 - r37

* Actual value is less than 0.

BOREAS 24 (t995) SEM of Pleistocene tills 2l

,Frg. 5. Samples of upper Dainava till: ! A. Extensively fractured and unweathered quartz grain with deep narrow curved troughs(arrows). n B. Extensively fractured quartz grain with a higher number of adhering particles and arc-shaped and linear steps 20-40 pmin width. n C. Extensively fractured quartz grain with very sharp edges. n D. Enlargement of semi-void wedge-shaped crater with lreshmultiple fractures. ! E. Quartz grain with very complex history showing older fractured surlace weathered by quartz dissolution, freshersurface in center, surlace to right shows deep troughs r 30 pm across and *30 pm deep (arrow - t) as well as abraded surfaces on top(arrow - a) and on right flank which is subrounded. n F. Typical sharp-edged, fractured and abraded quartz grain with minor chemicaletching.

22 William C. Mahanev and Volli Kalm BOREAS 24 (1995)

Frg. 6. Samples of lower Ugandi till: ! A. Fractured quartz grain (center) surrounded by weathered grains of unknown composition.Semi-void wedge-shaped craters abound on this surface (arrows) with preweathered edge (top) and abraded side (right). tl B. Quirtz withsubrounded edges, older fractured and weathered surlace (bottom), fresh fracturing (center to left) and deep troughs (20 30 1rm across)il lppg {gh], ! C. Sharp-edged quartz grain with multiple linear lracture lines, abrasion on right side (center) ani weathering on upp"iright. ! D. Sharp-edged quartz with multiple fracture lines and abraded surface (lower right). ! E. Sharp-edged quartz grain-with otierfractured weathered surface on top. ! F. Multiple fracture lines and deep troughs (center and right) on-quaitz particle.

BOREAS 24 (t995) SEM of Pleistocene tills 23

Frg. 7. Samples ol upper Ugandi till: n A. Sharp-edged quartz grain with deep trough (20 pm wide) down the center. ! B. Enlargementof trough in A showing weathered surface to right. n C. Blunt-edged quartz grain (enlargement in n D. showing extensive preweatheringprior to transport). ! E. Three quartz grains showing minor lracturing and varying degrees ol preweathering with little fresh fracturing.D F. Preweathered quartz grain with lresh fractures on top.

24 William C. Mahaney and Volli Kalm BOREAS 24 (t995)

Frg. 8. Samples of Varduva ti[: n A. Sharp-edged quartz grain with multiple lractures and irregular grooves (arrows). D B. euartz grainsshowing mainly old weathered surfaces that survived transport. ! C. Extensively fractured and abiaded quartz nearly all weatherJd.Samples of Latvia till including: ! D. Fresh fracturing of old weathered quartz surface. D E. Sharp-edged, fresh fraitured quartz grainwith minor lattice failure in upper right. n F. Sharp-edged older quartz grain partly affected by weatheiing.

BOREAS 24 (1995)

glaciation and Sangaste (Elster) glaciation. Thus, thetill record spans the whole of the Brunhes and possi-bly part of the Matuyama chrons. It therefore datesfrom the upper middle Pleistocene, the bottommostbeds comprising an erosional unconformity with De-vonian sandstones.

Results and discussion

Particle size

We studied the particle size distributions (Figs. 4A-E)for the five till beds in the succession with the objec-tive of attempting to determine if increases in sand(especially coarse sand) are accompanied by differ-ences in the type and range of microtextures presenton the surface of quartz grains. Overall the lower till(upper Dainava till) contains the highest amount ofsand, which was probably derived from contact withthe local sandstone bedrock. In general, the upperDainava till also contains greater amounts of verycoarse (2-l mm) sand. Nearly all the tills have linearparticle size curves indicating a heterogeneous distribution of particles from very coarse sand to clay. Twoexceptions occur within the lower Ugandi till (samples87-50 and 87-52) which are poorly sorted, do notcontain very coarse sand and relatively small amountsof coarse and medium sand. This distribution of parti-cle sizes coupled with sand lenses on tops of pebblesmay indicate meltout till rather than lodgement till,although Haldorsen (1981) argued that meltout till isusually coarser. All major investigators (Haldorsen1982; Haldorsen & Shaw 1982; Dreimanis 1989) ofmeltout till agree that a low degree of compaction,and lenses of sorted sediments of subglacial meltouttill, indicate the presence of meltwater during deposi-tion. It is also possible that better sorted tills maysimply reflect the underlying bedrock or unconsoli-dated sediments.

Mean phi values were calculated (Table 3) to deter-mine the center of gravity for each particle size curve(Folk 1968). These data confirm a general upwardfining sequence in the succession. Within each till unit,mean phi variations allow discrimination of tills witha higher percentage of coarse particles. For example,the lower Ugandi till sample 87 -145 is the coarsestand sample 81-52 is the finest. Also, lower Ugandi tillcontains the greatest range of mean phi values of thesubsamples studied. The younger tills, with the excep-tion of Varduva till, show considerably less variationin mean phi values. The overall impression gainedfrom the mean phi trends is that above upper Dainavatill there is a general reworking of grains which tendsto produce a more uniform sediment.

Anomalous particle size curves (e.g. sample 87 -95in Varduva till, 87-113 in upper Ugandi till, and87-50 and 81-52 in lower Ugandi till) that have

SEM of Pleistocene tills 25

different shapes (Figs. 4B-C) also have different meanphi values. In some cases these curves may result frombedrock or substrate influences (samples 87 95 and87-ll3). In other cases (e.g. samples 87-50 and 87-52) anomalous curves may represent sorting duringdeposition. It is possible that both of these samplesrepresent the fines removed from meltout till anddeposited in a pre-Pleistocene deep valley. This pro-vides a range of particle size distribution possibilitiesthat ultimately may be analyzed by scanning electronmlcroscopy.

Scanning electron microscopy

The most common microtextures on quartz grainsfrom till samples, ranging in age from lower to upperPleistocene, proved to be subparallel linear and con-choidal fractures (Figs. 5-8). Curved and straightgrooves and troughs (Figs. 5.A, 5E, 68, 7A and 8A)abound on samples from all tills; grooves are shallowabrasion features ( < 5 pm deep), whereas troughs aredeeper ( >5 pm deep). Both features are accompaniedby fractures which are usually linear (Fig. 6C) in formand common on the flanks of grooves and troughs.Sharp angularity (Figs. 5C, 5F, 6D and 8E) on theedges of grains are also common. Relief varies acrossindividual grains in a single till and increases, as thedegree of fracturing increases, reaching a maximumon well fractured grains. Indeed some grains ofqrLartz, dolomite and plagioclase feldspar appear tohave relatively little relief, very minor fracturing, andblunt edges (Fig. 7C), which may represent mechani-cal weathering of source rock. Presumably thesegrains either sojourn in the glacier without sufferingdamage during transport, or they undergo transportover relatively short distances.

Crescentic gouges and arc-shaped steps, while notas common on grains in these samples, are thought torelate to abrasion (gouges) and fracturing (steps)(Mahaney et al. 1988). On some grains, craters ofvarious kinds probably represent minor low velocitycollisions. Mechanically-upturned plates and latticeshattering (Fig. 8E) may signal relatively high cryo-static pressure that affects flawed qtaftz particles inparticular. Likewise, abrasion of portions of grainsurfaces (Figs. 5E, 6A, 6C and 8C) is thought to berelated to high cryostatic pressure, possibly furthernorth in the ice-sheet.

Dissolution etching (Figs. 5E, 6A-C, 7B-E, 88and SD-F), as one of the prime weathering microfea-tures, is the extreme form of weathering common onquartz grains in almost all the tills studied. In general,it occurs on grains that are fractured, and later weath-ered. Often dissolution etching is located on the oldfractured surfaces of qaartz, which suggests that itsformation is assisted by the fractured, and henceweakened, quartz surfaces which are later weathered.Preweathered surfaces are noted where dissolution

26 William C. Mahaney and Volli Kalm BOREAS 24 ( t995)

Flg. 9. Quantitative summary ol the lrequency ol orcurrence ol different microtextures on quartz grains in the Estonian Pleistocene tillsequence.

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etching is later freshly fractured and sometimesabraded. Weathered surfaces signal soft weathering offractures and sometimes abrasion surfaces withoutpronounced etching taking place. Overprinted grainsare very complex and usually show two distinct de-grees of fracturing separated by weathering (Fig. 5E);in extreme cases overprinted grains may show threestages of fracturing separated by degrees of weather-lng.

Other microfeatures, such as adhering particles(Fig. 5B), are considered typcial of glacial grains.Despite cleaning by sonification, many grains carry awealth of adhering particles which convey importantinformation regarding glacial grinding (Smalley 1966)and relative mineral ratios (quartz/amphibole, quartzffeldspar) of material ground up in the glacial mill.Edge rounding and V-shaped percussion cracksprovide information on meltwater transport (Krinsley& Marshall 1987), either as meltwater grains that weresubsequently reworked and/or meltwater grains insubglacial cavities.

Quantitatiue summary

All the microtextures reported on glacial grains(Krinsley & Doornkamp 1973;' Mahaney et al. 19881'Mahaney et al. 1991) are used in the analysis reportedhere. A compilation of the frequency of occurrence ofdifferent microtextures is shown in Fig. 9. Of the 1500grains studied in detail, the histograms show thatsubparallel linear and conchoidal fractures dominatealong with curved and straight grooves and deeptroughs. Fracture faces are rare and confined to theupper Dainava till (see Mahaney et al. 1991, for adiscussion of fracture faces inherited from mechanicalweathering). Relief is generally on the high side as aresult of the severe fracturing of most grains.

Crescentic gouges and arc-shaped steps, long con-sidered common on grains transported by continentalice (Krinsley & Doornkamp 1973; Mahaney et al.1988), proved relatively rare in this study. Linear stepsproved somewhat more common, but deep troughsare more numerous on the majority of quartz grainsstudied in detail. Abrasion, which is common on twoout of every five grains studied, probably relates tocryostatic pressure, pressure melting temperature, andthe degree to which grains come into contact with oneanother (Mahaney et al. 1988). Lattice shatteringeffects are present on relatively few grains while me-chanically-upturned plates were not observed.

Preweathered surfaces, weathered surfaces, dissolu-tion etching, precipitation features and overprintingmicrofeatures occur relatively infrequently on a smallpercentage of grains in older tills; in younger tills thispercentage is seen to rise. Overall the presence ofadhering particles is more or less correlated with thedegree of fracturing and hence glacial grinding and itis high in all till subgroups.

SEM of Pleistocene tills 27

The presence of edge rounding and V-shaped per-cussion cracks tends to support the hypothesis thatsome grains were transported by water (Krinsley &Doornkamp 1973; Krinsley & Marshall 1987). Theslight increase in the frequency of occurrence of edgerounding and V-shaped percussion cracks over timemight be related to an increase in warm-based ice andincreased meltwater transport both subglacially andsuperglacially.

S t ra t igraphi c imp I i ca t ions

The dominant trend from the SEM analysis is theslow but steady increase of quartz grains showingincreased dissolution etching, preweathered surfaces,weathered surfaces, precipitation coatings and over-printing with age. The weathered grains in the upperSangaste till are of considerable importance becausethey provide information on previous interglacial peri-ods [e.g. the Cromerian complex of interglacialsnamed by Godwin ( 1956)1. Many of these grains showcoatings of Fe and Al that were largely unstudied, andprobably suggested either interglacial climatic influ-ences (with pronounced increases of moisture actingover short periods of time, or lesser increases actingover longer periods) or incorporation of preweatheredsand from the underlying bedrock. A preliminaryanalysis of quartz grains from three bedrock outcropsshowed considerably few weathered grains (<10%).

The degree of reworking of fractured and weatheredgrains in the sequence is striking so that by the time ofthe late Ugandi glaciation the frequency of preweath-ered and weathered qrJartz grains nearly doubles. Bythe time of the last glacial maximum, it increasesnearly fourfold, or close to one half an order ofmagnitude. Clearly quartz grains tend to stay in theglacial system, becoming reworked from one glacia-tion to the next. This is rather similar to the conclu-sions reached from the geochemical analysis of asuccession of tills at the Wellsch Valley site,Saskatchewan (Hancock et al. 1988); namely thateach succeeding glaciation reworks pre-existing mate-rial that is chemically distinct from the underlyingbedrock.

ConclusionsThick continental ice inflicted considerable damage onquarlz sand size grains emplaced during five glacia-tions (including two stades of the Weichselian Glacia-tion). Particle size distributions suggest that most tillsare lodgement in origin; a few samples are bettersorted indicating either water transport or local effectsreflecting the grain size of the underlying bedrock.

Scanning electron mciroscopy shows that subparal-lel linear and conchoidal fractures, straight and curvedgrooves and troughs dominate on the samples studied,

28 William C. Mahaney and Volli Kalm

which indicate that they are the prime microfeaturesproduced by basal transport in continental ice. Mostgrains are angular, display relatively high relief, andsharp edges especially in the older tills. Crescenticgouges, arc-shaped and linear steps are not very com-mon in these samples. Lattice shattering and mechani-cally-upturned plates also are not very common in thesuite of samples studied and may relate to a lack offlawed quarlz lattices. Preweathered and weatheredclasts increase from older to younger tills, possibly asa result of interglacial climate influences and rework-ing of grains during successive glaciations. The quanti-tative summary provides an overall view of the rangeof microtextures and the degree to which they changeover time.

Acknowledgemenls. - We thank the York University Minor Re-search Fund for financial support for the SEM analysis. Theresearch was undertaken with support from The Institute of Geol-ogy (to W.C.M), Estonian Academy of Sciences, Tallinn, and theGeological Survey ol Estonia (to V.K.). We thank Aleksis Drei-manis (University of Western Ontario) for critically reviewing adraft of the manuscript. We are deeply indebted to Anto Raukas(Estonian Academy ol Sciences) and Jaap Van Der Mere (Univer-sity of Amsterdam) for critical reviews of the manuscript. We thankDavid Hinbest and G. Mahaney for their invaluable assistance inthe laboratory.

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