Scanning electron microscopy of Pleistocene tills in Estonia

WILLIAM C. MAHANEY A N D VOLLI KALM

Mahaney, W. C. & Kalm, V. 1995 (March): Scanning electron microscopy of Pleistocene tills in Estonia. Boreas, VOI. 24, pp. I 3 .29. OSIO. ISSN 0300-9483.

Tills from four Pleistocene glaciations were recovered from drill cores in Estonia and subjected to particle size and microtexture analyses by Scanning Electron Microscope (SEM). All tills were deposited by thick continental 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 grain surfaces are diagnostic of transport in continental ice. The frequency of occurrence of microtextures including fractures, abrasion, and relief features are used to test the ability of continental ice to damage quartz particles emplaced as till. The range of quartz dissolution and presence of coatings on grains are also used to reconstruct the paleoenvironment that existed prior to transport as well as to estimate diagenetic effects that occurred following emplacement. The available data indicate a high degree of reworking of quartz grains from one glaciation to another. While the shapes and microtextures of grains from source rocks are not known, the great range of fracture and abrasion microfeatures, and high frequency of occurrence on grains in all tills, indicate that glaciers are effective crushing agents. An increase in the prevalence of chemically etched grains from older to younger tills suggests that some grains (c. 50%) escape crushing, either because of preservation in the ice and lack of grain-to-grain contact, or as a result of massive reworking of weathered grains following interglacia- tions. William C. Mahaney, Geomorphology and Prdology Laboratory, Atkinson College, York University, 4700 Keele Street, North York, Onturio, Canada M3J IP3; Volli Kalm, Tarlu University, Inslitule of Geology, Vanemuisr Str. 46, Tartu, Estonia, EE 2400: received 12th January 1994, accepted 26th September 1994.

BOREAS

The depositional environment of glacial sediments can be determined by analysis of microtextures on quartz grains (Krinsley & Takahashi 1962; Margolis & Krinsley 1974; Whalley & Krinsley 1974; Krinsley & Trusty 1985; Krinsley & Marshall 1987). The direct study of active subglacial deposition is extremely difficult (Hubbard & Sharp 1989), being limited to a very few warm alpine glaciers and over time spans at best of a few years (see Vivian & Bocquet 1973; Vivian 1975). No one has studied grains deposited directly by glacial ice, but there are a few SEM studies of grains from moraines (Krinsley & Doornkamp 1973; Mahaney et al. 1988), from supraglacial drift (Mahaney et al. 199 l), and from glacial grains in loess (Mahaney & Andres 1991; Smalley & Glendinning 1991). The problems involved in observing the process of subglacial deposition and collecting the samples have led to the use of particle size and sand clast orientation from oriented blocks (Mahaney et al. 1989) to infer glacial deposition without the benefit of direct observation. As a result the inference of a glacigenic origin is based on a combination of sedi- mentary parameters such as particle size distributions (Mahaney 1978; Haldorsen 1982), sand or pebble a-axis orientation (for example, Mahaney 1990b; Catto 1990), SEM microfabrics (Mahaney et al. 1989), and microtextures (Krinsley & Marshall 1987; Ma- haney et ul. 1988; Mahaney 1990a, b, 1994).

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

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

While it is impossible to know the precise shape of particles at source rock locations and the microtex- tures they carry, it is possible to assume on the basis of previous work on mechanical release of quartz from cave walls (Mahaney & Sjoberg 1993), from crystalline bedrock (Molen 1993), and from cirque walls above existing cirque glaciers (Mahaney et al. 199 l), that the dominant microtextures are fracture faces with clean breaks across one side of individual grains. 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 have been found on source rock samples, although some samples show considerable chemical etching which suggests that mechanical release works more efficiently in rocks weakened by chemical weathering. Certainly particles introduced to transport in glaciers must be considered elastic inhomogeneities within different parts of the ice body, which are affected by variable stresses. Because the upstream origin of particles and

14 Williarn C. Mahaney and Volli Kulm

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

I n this study we present new information from the analysis of microtextures on quartz sand grains recov- ered from tills in boreholes of south-eastern Estonia. Thesc dcposits are considered to have been emplaced by subglacial deposition by ice in direct contact with underlying [thick] till bodies under moderate to high hydrostatic pressure in the ablation zone of large continental ice-sheets, with thicknesses estimated at c. 1500-t m and possibly as much as 2000 m (Aseyev 1974; Raukas 1988; Holmlund & Fastook 1993). Thus, for the most part the ice would have been close

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

Field area The boreholes are located near Tartu in south-eastern Estonia (Fig. 1). The slightly undulating or hum- mocky landscape is characteristic of the field area. The sites were selected in areas known to contain thick (14-60 m) sequences of Pleistoccne tills (Fig. 2). Ac- cording to conditions of deposition the structure, composition and thickness of the Pleistocene cover sediments (mainly tills) vary a great deal. In South Estonia five till horizons can be recognized (Raukas 1978; Liivrand 1991; Kajak et al. 1990). I n some cascs, particularly in buried pre-Quaternary valleys, till beds are separated by organic-rich clay, silty and sandy

Fix. 1. Borehole location map

BOREAS 24 (1995)

Aakre 15 Yalguta I 1 Valguta 14 Ringu 7 Kameri - 58 0

LEGEND - -801 hyer

CItviaT11 ear) .Vaduva T11 Wr)

Metkine (=€ern) m - K lntegl depostts - Uppor Ugandi Till (UU)

-Wch Ugandi (MU)

-Lower Ugandi Till (LU) Butenai (=Hotatein) - - internlac dewat8

SEM of' Pleistocene tills 15

Table I . Correlation table for the investigated samples.

Borehole No. 7 No. 15 No. 1 1 No. 14 No. 566 No. 580 No. 528 Site Rongu Aakre Valguta Valguta Aia Karner Korvekiila

Latvia t i l l 87-92 87- 128 87-137 87 79 (late Wcichsel) 87-80 Varduva till 87-95 87-99 87- 138 (early Weichsel) 87-102 87- 140 Merkine/ interglaciation Upper Ugandi till (late Saale) Lower Ugandi till (early Saale)

87-113 87- I14 87-1 16

Butenaij interglaciation Upper Dainava 87-119 till (Elster) 87- 122

87- 104 87- 142 87- 106

87-144 87-50 87-145 87-52

87-86

16 Williutn C. Mrrhaney and Volli Kalm HOKLAS 14 ( I Y Y 5 )

~ ~

Fiy. 3. Geological map of Estonia simplified from: Geological Map of the Soviet Baltic Republics. Scale ( 1978) 1:SOO 000. Leningrad. Aerogeologi.ia. Editor in Chief A. Grigelis.

deposits of interglacial or interstadial origin (Liivrand 1991). The studied till beds can be correlated with Latvia (late Valdai; late Weichselian), Varduva (early Weichselian), upper Ugandi (Warthe), lower Ugandi (Saale, Drenthe), and upper Dainava (Elster) glacia- tions (Table l ) (see Aseyev 1974; Bowen 198 l; Ehlers et ul. 1984 and Velichko & Faustova 1986, for discus- sions of the Estonian, European and Russian glacial sequence). The Pleistocene deposits are underlain by thick (5-8 m) outcrops of middle Devonian (Fig. 3) sandstone that is quartz-rich, slightly indurated, and nearly unconsolidated.

Bedrock consirkwltions

The underlying bedrock in the study area is silt and sandstone of middle Devonian age (Fig. 3). To the north, less than 30 km away, Devonian rocks give way at the surface to dolomite and limestone of Silurian and Ordovician age, covering a belt about 50 km in width, mainly covered with Pleistocene drifts. To the north of the dolomite and limestone belt a narrow band of lower Ordovician and Cambrian sandstones,

siltstones and clays outcrop along the coast and under the Gulf of Finland (Winterhalter ct al. 1981). Cam- brian-Ordovician siltstones and sandstones are ex- tremely rich (80-90%) in quartz (Viiding et a/. 1983). Ordovician carbonate rocks are mainly represented by various limestones and marlstones (Pdlma 19821, whereas the Silurian beds are composed of dolomitic carbonate rocks, marls, clays and argillites (Jurgenson 1988). Devonian sandstones, particularly those of Arukiila formation underlying Pleistocene deposits in southern Estonia, are again rich in quartz - up to 75-90%1 (Viiding et nl. 1981). Accordingly, as pre- dicted by Dreimanis & Vagners (1969). the amount of quartz is highest (average 75-78% in fine sand) in upper Weichselian till on Cambrian - Ordovician and Devonian sandstone outcrops (Fig. 2). decreasing to 45-58% in the same till on Ordovician-Silurian car- bonate bedrock (Raukas 1978). Distribution of quartz in five studied till beds in south-eastern Estonia is as follows: upper Dainava till - 86%; lower Ugandi till - 77%; upper Ugandi till - 68%; Varduva till - 75%; Latvia till - 75% (Raukas 1978; Kajak r t a/. 1990).

SEA4 of Pleistocene tills 17

Ttrhlc, 2. Principal correlation of stratigraphical units discussed in text bawd on: Ehlers. J.. Meyer. K.-D.. Stephan. H. J., 1984; Velichko. A A , Faustova. M. A.. 1986; Liivrand. E.. 1991; and Raukas and Gaigalas. 1993.

Mikulino Merkine

Moscow Upper Ugandi

Dniepr

Holstein Likhvin Butenai

flge Iln 111W 1o.m

25.000

55,OW

122.ooo

132.000

198.000

252,000

352,000

Elster

Byelo- Cromer

Dzukija

Vilnius

*In Estonia l4C' dates are available only from Legasciems ( =middle Weichsel) between 31 200 and 39 700 yr BP (Kajak et a/ . , 1981). TL dates are available from Latvia t i l l (43 000 yr). Varduva till ( 6 5 000, 75 000 and 100 000 yr) and upper Ugandi till [ 153 000 and 216 000 yr (Kajak C I u/.. 19Sl)l.

Thus, the origin of the quartz in southern Estonian tills is principally from the Precambrian crystalline rocks of the Fennoscandian Shield and from the sand- stone exposures along the southern coast of the Gulf of Finland (45-58's of quartz in till on quartz-free Ordovician and Silurian carbonate bedrock areas) with lesser amounts ( 15-30X) of quartz from Devo- nian sandstones.

Methods The till samples were collected from the centers of cores and they were analyzed for particle size distributions following ASTM procedures out- lined by Day (1965). The samples were wet-sieved to remove sands (2000-63 pm); the fines ( < 63 pm) were subjected to sedimentation and the percent silt and clay were determined by a hydrometer. The sands were oven dried, sieved, and the coarse ( 2 mm- 500 pm) fraction was subsampled; original subsamples and replicates were studied under the light micro- scope, and individual grains (mainly quartz) were selected as randomly as possible for detailed study by SEM. The samples were coated with carbon and analyzed on a JEOL-840 SEM with energy- dispersive spectrometry following procedures outlined by Mahaney (1990a). The fine sands (63-250pm) were subsampled, sprinkled on a stub with a mi-

crospatula, coated with carbon, and analyzed in the same manner.

Stratigraphy The principal correlation of stratigraphic units discussed in the text is presented in Table 2. The Eastern Baltic terminology (Raukas & Gaigalas 1993) is used throughout with reference to East European and W. European stratigraphic names as required. The till succession of seven boreholes is presented in Table 1 and Fig. 2. No dates are available from sampled boreholes, but the till exposed at the surface is almost of Latvia (late Weichselian) age. In some cases, for example at Valguta (No. 14) and Rdngu (No. 7), older tills near the surface are covered with glaciofluvial or glaciola- custrine deposits of late Weichselian age. The late Weichselian (Latvia) glacial model proposed by Raukas ( 1991) includes development of glaciation: slow oscillatory advance (25 000-20 000 years) fol- lowed by a rapid growth of glaciers (over 10000- 5000 years), and then by their disintegration at c. 10 000 BP. The late Weichselian cooling maximum about 20 000 years ago was followed by gradual cli- matic warming and the territory of South Estonia was freed of the continental ice between 13 000 and 12 250 BP (Raukas 1991).

The ages of the pre-Latvia tills are based on super- position, while conclusions about climatic change were estimated from earlier palynological investigations of associated interglacial sediments, from three studied excavations and boreholes (Kdrvekiila- 528, Valguta- 14, Rdngu-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 by palynological investigations (Liivrand 199 1 ). Thus, the underlying till is a minimum of Elster (upper Dainava) age or older.

In the Valguta- 14 borehole (Fig. 2), silty-clayey periglacial deposits occur between the two lowermost till beds. Based on palynological investigations of these deposits, this site has been suggested as an interstadial type site for the middle Ugandi subforma- tion in Estonia and correlated with the middle Ugandi [( Shklov) Odintsovo] interglaciation and its analogues on the East-European Plain (Kajak et al. 1976). Ac- cording to this conclusion the lower and upper Ugandi tills are separated from each other in the field area. Liivrand (1974, 1991) questioned the Valguta stratigraphic assignment of the intermorainic deposits to the middle Ugandi and based on pollen analysis (no I4C dates) assigned them to the middle Weichse- lian. Since the intermorainic deposits under discussion are considered to be reworked material (Liivrand

1 X Willimi C . Mchrriey and Volli Kulni

MATERIAL <ZOO% - RLPRESLNIATIM TEST S A Y R E

$LDIMENTATIOY ANALVSIS SIEVE ANALYSIS

MATERIAL <2OOOp -REPRESENlATIVE TEST SAMPLE

SLOIMENIATION ANALYSIS SIEVE ANALYSIS

0.08 Fig. 4. J A. Grain s i x distribu- tion for Upper Dainava t i l l .

Fig. 4. U B. Grain size distribu- tion for lower Ugandi till.

HOKEAS 24 (1995) SEM of Pleistocene tills 19

F/g 4 C Grain size distribu- tion for Upper Ugandi till

99 99

99

SO

a0

ro so 40 50

30

20

10

0 0

., .a .lo .I I ,,I. .I 3 .

I 4

I O W 500 250 ' Z 5 8.3 312 ( 5 6 70 39 , 95 0 9# 0.9 02. 012 0 0 6

VENTWORTH G R L O L SCALE IUICRONSI

YAlLllAL <ZOO+ - RLWESLNTATIVL TEST SAYPLT

3 L V L ANALYSIS SEDIYENTATION ANALYSIS

W L N l W O l l H ORADL SULC (Y IClONSl

0.01 L 2000 loo0 800 tW I25 63 3U lb l 78 $9 1.95 0.W 0.49 024 0.12 0

Fig. 4. tion for Varduva til l .

1 D. Grain size distribu-

20 William C. Muhaney und Volli Kulm

WlEIUL - ILI ILSLNTAlIW TCSl S A Y U C

lCDlW.NTAno* ANALYSIS

BOREAS 24 (1995)

W I N T W O I T M ORADC S O L E iY ICIONSl tion for Latvia till.

1974, 1991), occur beneath three difierent till beds at a depth of 24-25 m, and yield highly controversial TL dates (Kajak et ul. 1981; Liivrand 1991). we follow here the official stratigraphic schemes of Kajak et al. (1976) and Raukas & Gaigalas ( 1993).

The Eemian (Mikulian) age of the organogenic deposits at the Rbngu site (core Rbngu-7) was firmly established more than half a century ago (Orviku, 1939). On the basis of the pollen assemblage zones the continental interglacial deposits of the Rdngu site correlate well with the marine sediments in the Prangli site (North Estonia) and with other Eemian sites in northern Europe (Liivrand 1991; Raukas 1991). In general, the Eemian interglacial beds at the Rdngu site are covered by upper Weichselian till; only in a few cores (Rbngu- 7 included) do glacio- lacustrine or glaciofluvial deposits cover the inter- glacial beds.

The age-correlations of the pre-Weichselian tills in the other boreholes, as well as in South Estonia in general, are based mainly on conclusions from petro- graphic/mineralogical, geochemical and textural char- acteristics from each of the till horizons (Raukas 1978; Kajak et af. 1990).

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

T d l c 3. Mean cf, values for particle si7e diqtrihutions.

Percentile

Till Sample 25th 50th 75th mean phi

Latvia 87- 79 2.4 3.5 5.7 3.9 till 87 - 80 2.3 3.8 5.x 4.0

87 92 2.5 4.0 6.4 4.3 87 128 1.5 4.0 6. I 3.9 87-137 2.3 3.5 5.x 3.9

Varduva 87 95 4.5 6.5 9.8 6.9 till 87 99 0.5 2.7 6.9 3.4

8 7 ~ 1 0 2 2.4 3.1 5.2 3.6 87-138 2.2 3.5 6.5 4.1 87-140 2.4 4.0 7.9 4.8

Upper 87-104 2.5 4.8 7.1 4.8 Ugandi 87-106 2.2 4.7 7.1 4.7 till 87-113 0.8 2.8 5.3 3.0

87-114 1.3 4.1 6.8 4.1 87 142 2.3 4.1 6.5 4.3

Lower 87- 50 4.2 6.0 8.5 6.2 Ugandi 87-52 3.9 6.5 8.5 6.3 till 87-116 1.9 3.9 7.6 4.5

87-144 2.2 4.0 7.3 4.5

Upper 87-86 0.3 3.0 5.9 3.1 Dainava 87 I19 0* 1.2 3.5 I .6

0.8 4.0 1.6 t i l l

87 145 0.7 2.0 5.8 2.8

87-122 o*

* Actual value is less than 0.

BOREAS 24 ( 1995) SEM of Pleistocene tills 21

A

C

a

F

Fig. 5. Samples of upper Dainavd till: n A. Extensively fractured and unweathered quartz grain with deep narrow curved troughs (arrows). 1 B. Extensively fractured quartz grain with a higher number of adhering particles and arc-shaped and linear steps 20-40 pm in width. .I C. Extensively fractured quartz grain with very sharp edges. 0 D. Enlargement of semi-void wedge-shaped crater with fresh multiple fractures. 0 E. Quartz grain with very complex history showing older fractured surface weathered by quartz dissolution, fresher surface in center, surface to right shows deep troughs % 30 pm across and + 30 pn deep (arrow ~ t) as well as abraded surfaces on top (arrow - a ) and on right flank which is subrounded. 0 F. Typical sharp-edged, fractured and abraded quartz grain with minor chemical etching.

22 Willium C. Muhuney und Volli Kulm BOREAS 24 ( 1995)

A

C D

Fig. 6. Samples of lower Ugdndi till: C3 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). 0 B. Quartz with subrounded edges, older fracturcd and weathered surface (bottom), fresh fracturing (center to left) and deep troughs (20--30 pm across) in upper right. fl C. Sharp-edgcd quartz grain with multiple linear fracturc lines, abrasion on right side (centcr) and weathering on uppcr right. r i D. Sharp-cdged quartz with multiple fracture lines and abraded surface (lower right). 0 E. Sharp-edged quartz grain with older fractured weathered surface on top. n F. Multiplc fracture lines and deep troughs (ccnter and right) on quarts particlc.

BOREAS 24 (1995) SEM of Pleistocene tills 23

A B

C

E F Fig. 7. Samples of upper Ugandi till: 0 A. Sharp-edged quartz grain with deep trough (20 pm wide) down the center. B. Enlargement of trough in A showing weathered surface to right. 0 C. Blunt-edged quartz grain (enlargement in 0 D. showing extensive preweathering prior to transport). U E. Three quartz grains showing minor fracturing and varying degrees of preweathering with little fresh fracturing. 0 F. Preweathered quartz grain with fresh fractures on top.

24 William C. Mrrhaney and VoNi Kalm

A

C

E

BOREAS 24 (1995)

B

D

Fig. 8. Samples of Varduva till: 0 A. Sharp-edged quartz grain with multiple fractures and irregular grooves (arrows). 0 B. Quartz grains showing mainly old weathered surfaces that survived transport. 0 C . Extensively fractured and abraded quartz nearly all weathered Samples of Latvia till including: 0 D. Fresh fracturing of old weathered quartz surface. n E. Sharp-edged, fresh fractured quartz grain with minor latticc failure in upper right. KI F. Sharp-edged older quartz grain partly affected by weathering.

BOREAS 24 ( 1995) SEM of’ Pleistocene tills 25

glaciation and Sangaste (Elster) glaciation. Thus, the till record spans the whole of the Brunhes and possi- bly part of the Matuyama chrons. It therefore dates from the upper middle Pleistocene, the bottommost beds 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 present on the surface of quartz grains. Overall the lower till (upper Dainava till) contains the highest amount of sand, which was probably derived from contact with the local sandstone bedrock. In general, the upper Dainava till also contains greater amounts of very coarse (2- 1 mm) sand. Nearly all the tills have linear particle size curves indicating a heterogeneous distri- bution of particles from very coarse sand to clay. Two exceptions occur within the lower Ugandi till (samples 87-50 and 87-52) which are poorly sorted, do not contain very coarse sand and relatively small amounts of coarse and medium sand. This distribution of parti- cle sizes coupled with sand lenses on tops of pebbles may indicate meltout till rather than lodgement till, although Haldorsen (1981) argued that meltout till is usually coarser. All major investigators (Haldorsen 1982; Haldorsen & Shaw 1982; Dreimanis 1989) of meltout till agree that a low degree of compaction, and lenses of sorted sediments of subglacial meltout till, indicate the presence of meltwater during deposi- tion. I t is also possible that better sorted tills may simply 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 upward fining sequence in the succession. Within each till unit, mean phi variations allow discrimination of tills with a higher percentage of coarse particles. For example, the lower Ugandi till sample 87-145 is the coarsest and sample 87-52 is the finest. Also, lower Ugandi till contains the greatest range of mean phi values of the subsamples studied. The younger tills, with the excep- tion of Varduva till, show considerably less variation in mean phi values. The overall impression gained from the mean phi trends is that above upper Dainava till there is a general reworking of grains which tends to produce a more uniform sediment.

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

different shapes (Figs. 4B-C) also have different mean phi values. In some cases these curves may result from bedrock or substrate influences (samples 87-95 and 87-113). In other cases (e.g. samples 87-50 and 87- 52) anomalous curves may represent sorting during deposition. It is possible that both of these samples represent the fines removed from meltout till and deposited in a pre-Pleistocene deep valley. This pro- vides a range of particle size distribution possibilities that ultimately may be analyzed by scanning electron microscopy.

Scanning electron microscopy The most common microtextures on quartz grains from till samples, ranging in age from lower to upper Pleistocene, proved to be subparallel linear and con- choidal fractures (Figs. 5-8). Curved and straight grooves and troughs (Figs. 5A, 5E, 6B, 7A and 8A) abound on samples from all tills; grooves are shallow abrasion features ( < 5 pm deep), whereas troughs are deeper ( > 5 pm deep). Both features are accompanied by fractures which are usually linear (Fig. 6C) in form and common on the flanks of grooves and troughs. Sharp angularity (Figs. 5C, 5F, 6D and 8E) on the edges of grains are also common. Relief varies across individual grains in a single till and increases, as the degree of fracturing increases, reaching a maximum on well fractured grains. Indeed some grains of quartz, dolomite and plagioclase feldspar appear to have relatively little relief, very minor fracturing, and blunt edges (Fig. 7C) , which may represent mechani- cal weathering of source rock. Presumably these grains either sojourn in the glacier without suffering damage during transport, or they undergo transport over relatively short distances.

Crescentic gouges and arc-shaped steps, while not as common on grains in these samples, are thought to relate to abrasion (gouges) and fracturing (steps) (Mahaney et al. 1988). On some grains, craters of various kinds probably represent minor low velocity collisions. Mechanically-upturned plates and lattice shattering (Fig. 8E) may signal relatively high cryo- static pressure that affects flawed quartz particles in particular. Likewise, abrasion of portions of grain surfaces (Figs. 5E, 6A, 6C and 8C) is thought to be related to high cryostatic pressure, possibly further north in the ice-sheet.

Dissolution etching (Figs. 5E, 6A-C, 7B-E, 8B and 8D-F), as one of the prime weathering microfea- tures, is the extreme form of weathering common on quartz 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 old fractured surfaces of quartz, which suggests that its formation is assisted by the fractured, and hence weakened, quartz surfaces which are later weathered. Preweathered surfaces are noted where dissolution

c L

%FR

EQ

UE

NC

YO

FOC

CU

RR

EN

CE

i=

=

e €

BOREAS 24 (1995) SEM of Pleistocene tills 27

etching is later freshly fractured and sometimes abraded. Weathered surfaces signal soft weathering of fractures and sometimes abrasion surfaces without pronounced etching taking place. Overprinted grains are very complex and usually show two distinct de- grees of fracturing separated by weathering (Fig. 5E); in extreme cases overprinted grains may show three stages of fracturing separated by degrees of weather- ing.

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

Quun f itutive sumniary

All the microtextures reported on glacial grains (Krinsley & Doornkamp 1973; Mahaney et al. 1988; Mahaney et ul. 1991) are used in the analysis reported here. A compilation of the frequency of occurrence of different microtextures is shown in Fig. 9. Of the 1500 grains studied in detail, the histograms show that subparallel linear and conchoida1 fractures dominate along with curved and straight grooves and deep troughs. Fracture faces are rare and confined to the upper Dainava till (see Mahaney et ul. 1991, for a discussion of fracture faces inherited from mechanical weathering). Relief is generally on the high side as a result of the severe fracturing of most grains.

Crescentic gouges and arc-shaped steps, long con- sidered common on grains transported by continental ice (Krinsley & Doornkamp 1973; Mahaney et a/. 1988), proved relatively rare in this study. Linear steps proved somewhat more common, but deep troughs are more numerous on the majority of quartz grains studied in detail. Abrasion, which is common on two out of every five grains studied, probably relates to cryostatic pressure, pressure melting temperature, and the degree to which grains come into contact with one another (Mahaney et al. 1988). Lattice shattering effects are present on relatively few grains while me- chanically-upturned plates were not observed.

Preweathered surfaces, weathered surfaces, dissolu- tion etching, precipitation features and overprinting microfeatures occur relatively infrequently on a small percentage of grains in older tills; in younger tills this percentage is seen to rise. Overall the presence of adhering particles is more or less correlated with the degree of fracturing and hence glacial grinding and it is high in all till subgroups.

The presence of edge rounding and V-shaped per- cussion cracks tends to support the hypothesis that some grains were transported by water (Krinsley & Doornkamp 1973; Krinsley & Marshall 1987). The slight increase in the frequency of occurrence of edge rounding and V-shaped percussion cracks over time might be related to an increase in warm-based ice and increased meltwater transport both subglacially and superglaciall y.

St ra tigruphir iniplicat ions

The dominant trend from the SEM analysis is the slow but steady increase of quartz grains showing increased dissolution etching, preweathered surfaces, weathered surfaces, precipitation coatings and over- printing with age. The weathered grains in the upper Sangaste till are of considerable importance because they provide information on previous interglacial peri- ods [ e.g. the Cromerian complex of interglacials named by Godwin (1956)l. Many of these grains show coatings of Fe and A1 that were largely unstudied, and probably suggested either interglacial climatic influ- ences (with pronounced increases of moisture acting over short periods of time, or lesser increases acting over longer periods) or incorporation of preweathered sand from the underlying bedrock. A preliminary analysis of quartz grains from three bedrock outcrops showed considerably few weathered grains ( < 10%).

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

Conclusions Thick continental ice inflicted considerable damage on quartz sand size grains emplaced during five glacia- tions (including two stades of the Weichselian Glacia- tion). Particle size distributions suggest that most tills are lodgement in origin; a few samples are better sorted indicating either water transport or local effects reflecting the grain size of the underlying bedrock.

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

28 Willicrm C. Mahaney and Vofli Kulm BORFAS 24 (1995)

which indicate that they are the prime microfeatures produced by basal transport in continental ice. Most grains are angular, display relatively high relief, and sharp edges especially in the older tills. Crescentic gouges, 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 the suite of samples studied and may relate to a lack of flawed quartz lattices. Preweathered and weathered clasts increase from older to younger tills, possibly as a result of interglacial climate influences and rework- ing of grains during successive glaciations. The quanti- tative summary provides an overall view of the range of microtextures and the degree to which they change over time.

A c ~ k ~ i ( ~ ~ i . l i , ( l g c ~ ~ ~ i i , ~ i t . ~ . We thank the York University Minor Re- search Fund for linancial support I'or the SEM analysis. The rcscarch was undertaken with support from The Institute of Geol- ogy ( t o W.C.M). Estonian Academy of Sciences, Tallinn, and the Geological Survey of Estonia ( to V.K.). We thank Aleksis Drei- manis (University of Western Ontario) for critically reviewing a draft of the manuscript. We arc deeply indebted to Anto Raukas (Estonian Acadcmy of Sciences) and Jaap Van Der Mere (Univer- sity of Amsterdam) for critical reviews of the manuscript. We thank David Hinhest and G . Mahaney for their invaluable assistance in the laboratory.

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