SEDIMENTATION AND LITHOSTRATIGRAPHY OF THE VUOSAARI MULTIPLE TILL SEQUENCE IN HELSINKI, SOUTHERN FINLAND HEIKKI HIRVAS, PETRI LINTINEN, JUHA PEKKA LUNKKA, BRITA ERIKSSON and TUULIKKI GRÖNLUND HIRVAS, HEIKKI; LINTINEN, PETRI; LUNKKA, JUHA PEKKA; ERIKS- SON, BRITA and GRÖNLUND, TUULIKKI 1995. Sedimentation and lithostratigraphy of the Vuosaari multiple till sequence in Helsinki, southern Fin- land. Bull. Geol. Soc. Finland 67, Part II, 51-64. A multiple till sequence interbedded with sorted sediments has been investigated at Vuosaari, Helsinki, Finland. The investigation was carried out using standard sedimentological procedures combined with microfossil analysis in order to determine the genesis of the exposed sediments. This evidence is used to correlate lithostratigraphically the sequence with adjacent multiple till sequences in other parts of southern Finland (south of the Salpausselkä zone). It is concluded that all three till beds at Vuosaari are of basal origin that were laid down by separate ice flow phases. In contrast two rhythmite beds between the tills are thought to have been deposited in open water. The sediments at Vuosaari may have been laid down during the Weichselian glaciation although it is also possible that the lowermost till bed represents Saalian till. Key words: till, lithostratigraphy, pollen analysis, diatom flora, deposition, correlation, Pleistocene, Vuosaari, Helsinki, Finland, Heikki Hirvas, Petri Untinen, Brita Eriksson, Tuulikki Grönlund: Geological survey of Finland, FIN-02150 Espoo, Finland. Juha Pekka Lunkka: Department of Geology, University of Helsinki, Fin- land. 52 Heikki Hirvas, Petri Lintinen, Juha Pekka Luukka, Brita Eriksson and Tuulikki Grönlund INTRODUCTION only 1-3 % of the total. This uppermost till is thought to have been deposited during the final Finland is situated close to the centre of the area deglaciation stage of the Weichselian glaciation. in which the Scandinavian ice-sheet was initiated According to till clast fabric and striae and spread outwards repeatedly during the measurements, the ice-flow direction during this Pleistocene. Glacial erosion has greatly affected stage was from 310°-320°. existing interglacial and glacial deposits. Indeed In the Porvoo and Siuntio districts (Fig. 1) two in most parts of Finland superficial clastic superimposed till beds are separated by a sand unit deposits, resting on the Precambrian bedrock, (Hirvas and Nenonen 1987, Bouchard et al. 1990). represent solely sediments left behind during the The lower till unit is compact and clay-rich (clay final deglaciation phase of the Weichselian content: 10-15 %). According to Rainio and glaciation. Lahermo (1984) comparable clay-rich till units However, there are a number of localities where in southern Finland are occasionally organic-rich organic interglacial and interstadial deposits and 14C dates obtained from this organic matter occur, often as erosional remnants (see Hirvas and have yielded a minimum age of over 45 ka. Clast Nenonen 1987, Hirvas 1991, Donner 1995). Since fabric measurements from this till and striae the interglacial and interstadial organic deposits observations on the bedrock beneath indicate that are not extensively preserved Pleistocene the lower till unit, south of Salpausselkä end stratigraphy in Finland is mainly based on till moraines, was deposited from 340°-360° (Hirvas stratigraphy. and Nenonen 1987, Kurkinen et al. 1989, The longest Pleistocene sequences are best Bouchard et al., 1990). preserved in a wide ice-divide zone in northern Rainio and Lahermo (1976, 1984) suggested Finland where interglacial and interstadial deposits that the lower till unit was laid down when ice for have been used as marker horizons for further the first time overrode underlying Eemian correlations of multiple glacigenic clastic interglacial sediments during the Weichselian sediments (Hirvas and Nenonen 1987, Hirvas glaciation. According to these authors, the high 1991). However, the stratigraphy of southern Fin- clay and organic content in the lower till was land, i.e. south of the Salpausselkä end moraines caused by the incorporation of interglacial (Fig. 1), is exclusively based on till stratigraphy sediments into this till (Rainio and Lahermo 1976, since in situ organic sediments that can provide 1984). Hirvas and Nenonen (1987) considered that biostratigraphical and/or chronostratigraphical the clay-rich lower till could have been deposited control have not so far been reported from this either during the Weichselian or during the area. previous Saalian glaciations. The lithostratigraphical scheme for southern Based on the observations in southern and Finland and its correlation to the sites elsewhere western Finland (north of Salpausselkä end in the country have been proposed by Nenonen and moraines and north of the so-called 'Central Fin- Hirvas (1987) and Bouchard et al. (1990). land Ice-Marginal Formation'; Fig. 1) Bouchard Lithostratigraphical schemes for southern Finland et al. (1990) proposed a formal lithostratigraphy have also been presented by Rainio and Lahermo for southern Finland. In their lithostratigraphy, the (1976, 1984) and Kurkinen et al. (1989). Kela Formation in the Siuntio area comprises two Hirvas and Nenonen (1987) and Bouchard et till members separated by a sand member. They al. (1990) have reported two superimposed till suggested that all three members of the Kela units with markedly different physical properties Formation were deposited during the Weichselian and clast fabric from southern Finland, south of glaciation. The lower till member representing the Salpausselkä end moraines. The uppermost unit lodgement till deposited during the first arrival of here is sand-rich, with clay fraction representing ice into the Siuntio area from the north (Bouchard Sedimentation and lithostratigraphy of the Vuosaari multiple till sequence in Helsinki, southern Finland 53 the text. The principal end moraine complexes in southern Finland are also shown (C= Central Finland Ice- Marginal Formation). 54 Heikki Hirvas, Petri Lintinen, Juha Pekka Luukka, Brita Eriksson and Tuulikki Grönlund et ai. 1990). They also suggested that the upper from the rhythmite and till units exposed at unit, the Siuntio Till was deposited by basal melt- Vuosaari. Although microfossil analyses are out and the Pikkala Sand member between the two seldom used for till material this procedure can, till units represents subglacial outwash sands. in favourable situations, give valuable information The Vuosaari sequence described here includes on the contemporaneous and reworked flora in three separate till units instead of the two sediments (Heinonen 1957, Hirvas and Nenonen previously reported. The primary aim of this paper 1987, Hirvas 1991). The samples for pollen is therefore to describe the lithostratigraphy of analysis were treated in the laboratory as described the Vuosaari sequence and present the most in Eriksson (1993). Whenever possible 200-500 probable depositional environments under which arboreal pollen grains were counted from each these sediments accumulated. The authors will sample and the results are shown in percentages also discuss the probable correlations of the of a total pollen sum obtained from each sample. Vuosaari sequence with those in adjacent areas The preparation of diatom slides followed the presented previously. conventional preparation method (e.g. Håkansson 1982). The diatom counts are listed in table 1 and shown in a diagram using sub-divisions introduced METHODS by Simonsen (1962). Conventional logging techniques were used to determine facies changes throughout the Vuosaari SITE DESCRIPTION pit and to construct a detailed graphic log of the exposure. Textural and structural properties, such Lithostratigraphy as unit thickness, the nature of bedding contacts, dominant grain-size of sorted sediments and The Vuosaari section is located in East Helsinki bedding structures of each unit were recorded. on the southern coast of Finland at 15 m a.s.l. (Fig. Clast fabric measurements of the tills were 1). The topography of the area is typical of coastal performed by measuring the dip direction of long southern Finland where bedrock outcrops, covered axes of 50 elongated pebbles. Clasts with dip partially by till, alternate with low-lying areas greater than 30° were classified as vertical. These which are covered by thick glacial and postglacial methods were used in order to obtain information clay. on the genesis of tills and sorted sediments. The Vuosaari sequence was discovered during Grain-size analyses from tills were carried out the power station construction conducted by the by conventional hydrometer and wet-sieving City of Helsinki. During the excavations a techniques and the humus content of tills was sediment sequence was well exposed in the pit determined by a calorimetric technique using a (dimensions ca. 100 m x 100 m) and it was Metrohm Spectrocolorimeter E1009. Specific possible to trace individual sediment units surface area values of the fine fractions (< 0.06 vertically and laterally throughout the exposure mm) were also measured using a method based (Fig. 2). The thickest sequence, comprising three on nitrogen sorption (Micrometrics Flow Sorb II separate till units interbedded with stratified 2300) on dry-sieved samples. These methods were sediments, was discovered in the eastern part of used to determine the properties of the tills the pit lying in a relatively narrow bedrock trough. exposed in the section. In the western part of the excavation