and microtexture analyses by Scanning Electron Microscope (SEM). All tills were deposited by thick continental ice-sheets following the transport of, at most, ...
Scanning electron microscopy of Pleistocene tills in Estonia WILLIAM C. M A H A N E Y A N D VOLLI K A L M
BOREAS
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 Fennoscandian 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 interglaciations. 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.
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. 199l), 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 sedimentary 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; Mahaney 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 mechanisms. 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 important to sample lodgement till (ground moraine) emplaced 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 microtextures 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. 199l), that the dominant microtextures are fracture faces with clean breaks across one side of individual grains. Minor subparallel fracture lines are also common on grains with subangular to subrounded shapes. No conchoidal fractures, steps, troughs, gouges, percussion 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 inhomogeneous 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 recovered 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
Fix. 1. Borehole location map
to or at the pressure melting point when the grains were emplaced, which increases the chance of grainto-grain contact.
Field area The boreholes are located near Tartu in south-eastern Estonia (Fig. 1). The slightly undulating or hummocky 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). According 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
SEM
BOREAS 24 (1995)
Yalguta I 1
Aakre 15
Valguta 14
of'Pleistocene
tills
15
Kameri - 58 0
Ringu 7
LEGEND -801hyer
CItviaT11 ear) .Vaduva T11 Wr)
m --
Metkine (=€ern) K lntegl depostts Uppor Ugandi Till (UU)
-Wch Ugandi (MU)
-Lower Ugandi Till (LU)
--
Butenai (=Hotatein) internlac dewat8
Table I . Correlation table for the investigated samples.
Borehole Site
No. 7 Rongu
Latvia till (late Wcichsel) Varduva till (early Weichsel) Merkine/ interglaciation Upper Ugandi till (late Saale) Lower Ugandi till (early Saale) Butenaij interglaciation Upper Dainava till (Elster)
No. 15 Aakre
No. 1 1 Valguta
87-92
87- 128
87-95
87-113 87- I14 87-1 16
87-119 87- 122
No. 14 Valguta
No. 566 Aia
No. 580 Karner
87-137 87-99 87-102
87- 138 87- 140
87- 104 87- 106
87- 142 87-144 87-145
No. 528 Korvekiila 87 79 87-80
87-50 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) glaciations (Table l ) (see Aseyev 1974; Bowen 198l; Ehlers et ul. 1984 and Velichko & Faustova 1986, for discussions 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). Cambrian-Ordovician siltstones and sandstones are extremely 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 predicted 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 carbonate bedrock (Raukas 1978). Distribution of quartz in five studied till beds in south-eastern Estonia is 86%; lower Ugandi as follows: upper Dainava till till - 77%; upper Ugandi till - 68%; Varduva till 75%; Latvia till 75% (Raukas 1978; Kajak r t a/. 1990). -
-
SEA4 of Pleistocene tills 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. flge
Iln 111W 1o.m 25.000 55,OW
122.ooo Mikulino
Merkine 132.000
Moscow
Upper Ugandi 198.000 252,000
Dniepr 352,000 Holstein
Likhvin
Butenai
Elster
ByeloCromer 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 till (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 sandstone 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 Devonian sandstones.
Methods The till samples were collected from the centers of cores and they were analyzed for particle size distributions following ASTM procedures outlined 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 mm500 pm) fraction was subsampled; original subsamples and replicates were studied under the light microscope, 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 energydispersive spectrometry following procedures outlined by Mahaney (1990a). The fine sands (63-250pm) were subsampled, sprinkled on a stub with a mi-
17
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 glaciolacustrine 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) followed by a rapid growth of glaciers (over 100005000 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 climatic 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 superposition, while conclusions about climatic change were estimated from earlier palynological investigations of associated interglacial sediments, from three studied excavations and boreholes (Kdrvekiila- 528, Valguta14, Rdngu-7). In the Kdrvekula site Holsteinian (Butenai) lacustrine deposits are present in borehole No. 528 (Fig. 2). The Holsteinian age of these deposits is established by palynological investigations (Liivrand 1991). 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 subformation in Estonia and correlated with the middle Ugandi [( Shklov) Odintsovo] interglaciation and its analogues on the East-European Plain (Kajak et al. 1976). According 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 Weichselian. Since the intermorainic deposits under discussion are considered to be reworked material (Liivrand
1X
Willimi C . Mchrriey and Volli Kulni MATERIAL
