The Sedimentary Sequence Recovered from the Voka Outcrops, Northeastern Estonia: Implications for Late Pleistocene Stratigraphy
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Estonian J. Earth Sci., 2007, 56, 1, 47–62 The sedimentary sequence recovered from the Voka outcrops, northeastern Estonia: implications for late Pleistocene stratigraphy Anatoly Molodkova, Nataliya Bolikhovskayab, Avo Miidela, and Kuldev Plooma,c a Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; [email protected] b Department of Geography, Moscow State University, 119899 Moscow, Russia; [email protected] b Geological Survey of Estonia, Kadaka tee 82, 12618 Tallinn, Estonia; [email protected] Received 8 August 2006, in revised form 8 January 2007 Abstract. New palaeoenvironmental and geological data, which may be integrated with the results from the neighbouring regions, were collected from two well-exposed continuous outcrops in the vicinity of Voka village, northeastern Estonia. These outcrops, situated in a klint depression – klint bay –, show an about 22 m thick stacked sequence of sandy to clayey subaqueous deposits. This succession of water-lain sediments documents the response to climate change during the late Pleistocene. On the basis of grain size characteristics, sedimentological structures, and luminescence chronostratigraphical data, the sequence is subdivided into two main units – A and B. Optical dating of 18 samples from the upper unit A shows that the unit is of middle Järva (= middle Weichselian) age (marine isotope stage (MIS) 3). Representative pollen spectra derived from 45 samples from the pollen-bearing part of unit A provide convincing evidence of noticeable changes in vegetation and climate in NE Estonia during the time period from 39 to 33 kyr BP, within which two intervals of severe climate and two relatively milder ones have been recognized. Preliminary data from the underlying unit B indicate that deposits of the last interglacial sensu lato and those of early pleniglacial age correlating with MIS 5 and MIS 4, respectively, occur here as well. Thus, the data obtained during the present study show unambiguously that in contrast with the expectations, the greater part of the late Pleistocene sequence is represented in the Voka section. No evidence was found for glacial activity during the late Pleistocene period predating the last glacial maximum. The use of the Voka event stratigraphy as a template facilitates search for correlative horizons in the neighbouring regions. Key words: last ice age, chrono- and palynostratigraphy, northeastern Estonia, Voka site, palaeoenvironmental changes. INTRODUCTION palaeoincisions could also be carved by subglacial melt- waters or formed by catastrophic outbursts of subglacial The greater part of the modern southern coastline of the meltwater from the marginal parts of the ice sheet. Gulf of Finland is bordered by the steep northern margin The location of the palaeoincisions in the northern of the Harju–Viru limestone plateau, widely known as the part of the limestone plateau, their sinuosity and gradual North Estonian Klint. The klint makes up approximately deepening downstream (i.e. towards the klint) testify a quarter of the up to 1200 km long Baltic Klint, which more likely to their pre-glacial fluvial genesis (Tavast & extends from the western coast of the Island of Öland in Raukas 1982). It is believed that later the palaeoincisions Sweden to the southern shore of Lake Ladoga (Fig. 1a). were transformed by the plucking of the subsequent It is more continuous and its relative height is greater glaciers and eroded by glacier meltwaters and/or inter- east of Lahemaa within the Viru plateau, culminating glacial rivers (ibid). Erosion played the greatest role at Ontika at 55.6 m a.s.l. East of Toila, remarkable cliffs in northeastern Estonia, where the palaeoincisions are occur at Voka (43 m) (see Fig. 1a) and Päite (41 m) aligned along the movement of the Pleistocene glaciers (Tammekann 1940). (from the north and northwest to the south and southeast, The klint and adjoining limestone plateau are inter- Raukas 1978; Tavast & Raukas 1982; Karukäpp 1996). sected by canyon- or ravine-like palaeoincisions at No great variation has been recorded in the dis- various depths. Different reasons have been set forth for tribution of fillings of the palaeoincisions of northern the formation of these palaeoincisions: (1) preglacial Estonia (Raukas 1978; Tavast & Raukas 1982). Late or interglacial river erosion, (2) glacial erosion of the Pleistocene deposits, mostly glaciofluvial and glacio- Pleistocene glaciers, (3) glacial and glaciofluvial erosion lacustrine in origin, prevail, sometimes being inter- during repeated advances and retreats of the Pleistocene stratified by thin layers of different-aged tills. The glaciers. Besides, as it was recently pointed out, the incisions may also be completely filled with till. The 47 Estonian J. Earth Sci., 2007, 56, 1, 47–62 (a) (b) Fig. 1. (a) Location map of the study area with the sites mentioned in the text and (b) scheme of the Voka Klint Bay with locations of the sections V1 and V3; 1, altitudes above sea level indicated in metres; 2, klint bay boundary; 3, outcrop; 4, 20 m contour line; 5, coastline; 6, klint. thickness of the glaciofluvial deposits in such buried Dating, Cambridge, in 1987 (see also Godfrey-Smith palaeoincisions of the Viru plateau only rarely reaches et al. 1988), seemed most suitable for their dating. The 40 m (Tavast & Raukas 1982) but is much greater in the latest developments of the IR-OSL technique by the one cutting the klint near Voka village some 34 km west Research Laboratory for Quaternary Geochronology, of Narva (Fig. 1a,b). Institute of Geology, Tallinn University of Technology Considering the bedding, stratification, grain size of (for details see, e.g., Vasilchenko et al. 2005; Molodkov sediments, and other geological indications, it was widely & Bitinas 2006; Molodkov et al. 2007) were applied to assumed that the upper visible part of the Voka palaeo- produce an absolute chronology of the events that are incision was filled mainly with subaqueous deposits assumed to have occurred during a relatively short during the Pandivere phase of the last glaciation (i.e. period: between about 12 400 conventional 14C yr BP – in subglacial environment) (Raukas & Stankowski that is the radiocarbon age of the Pandivere ice-marginal 2005) or with the deposits of an ice lake at the end of zone (see, e.g., Raukas & Stankowski 2005) – or about the late Pleistocene, when the ice margin had reached 14 370 cal (i.e. calendar or calibrated) yr BP, calculated either the Gulf of Finland or already South Finland using the calibration program by Stuiver et al. (1998), (Miidel 2003). The characteristics of the deposits exposed and 10 200–10 500 14C yr BP (or about 12 200 cal yr BP) at the Voka site are associated also with low-amplitude – the time of the drop from the Baltic Ice Lake level to deformations, possibly load casts, slumps, etc. the Yoldia Sea level as a consequence of the opening of Since no absolute dates are available for ice-dammed an outflow channel through central Sweden in the Mount lake deposits on the territory of Estonia and the ages Billingen region (Donner 1978; Stromberg 1992). of the deformations occurring therein are unknown, a However, contrary to expectations, the first results chronostratigraphical investigation was undertaken to of the IR-OSL analysis of the Voka outcrop deposits obtain new facts for elucidating these aspects of the obtained in 2002 indicated a much older age than earlier late-postglacial sedimentary history of the ice lake. Such supposed. As the preliminary absolute ages of these work proved possible during the fieldwork in the summer deposits fully contradict the existing viewpoint on the of 2001. geological history of the region, the problem is of As the deposits are devoid of macroscopic organic principal importance, necessitating further comprehensive remains, the optically-stimulated luminescence (OSL) investigation of the site. As the deposits of the palaeo- technique, introduced more than 20 years ago by Huntley incisions in northern Estonia are still insufficiently et al. (1985), and its feldspar-based infra-red version, studied, and the deposits predating the last glacial IR-OSL, first introduced by D. I. Godfrey-Smith, D. J. maximum (LGM) are very rarely well preserved in situ, Huntley, and W.-H. Chen in an unpublished report at the this can potentially provide a unique opportunity to study 5th International Specialist Seminar on TL and ESR the Järva sedimentary environment and to reconstruct 48 A. Molodkov et al.: The sedimentary sequence of the Voka outcrops, NE Estonia the palaeoenvironmental history of the region in a greater the Voka Klint Bay and the palaeoincision is summarized detail. in Fig. 2a,b. Judging from drilling data, most of the This paper reports some initial findings from a pilot section is composed of fine-grained sands, silts and till and subsequent study, aimed at elucidating the with a total thickness of at least 160.9 m in borehole sedimentary history and age of the deposits revealed 8618 (Fig. 2b), containing also layers of gravelly sand, in the Voka outcrops and preliminary examination of the coarse- to medium-grained sand and clays. essential palaeoenvironmental information documented The surficial part of the klint bay is composed of at at that site. least 15 m thick yellowish-brown fine-grained sands, silty sands, and silts. Miidel (2003) believed that this part represents the post-LGM glaciolacustrine and deltaic GEOLOGICAL SETTING deposits. Some boreholes revealed a 0.3–1.0 m thick layer of gravel, embedded either in silty sand or in fine The Voka site is located on the rather flat Viru lime- sand. stone plateau where its edge, the North Estonian Klint, The plateau, bordering the klint and the Voka Klint is broken by a depression – the Voka Klint Bay Bay, is covered by yellowish or brownish sandy clay till (Figs 1a,b, 2a). It is supposed that the Voka Klint Bay of the last glaciation.