Geological Survey of Finland, Special Paper 46 - arkisto.gsf.fi

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Geological Survey of Finland, Special Paper 46 Antti E. K. Ojala and Jukka-Pekka Palmu Reconstruction of the geological history of the Suurpelto area is an important task because the city of Espoo has recently begun land-use planning of the area. The area has deposits of silt and clay up to 25 m thick. These fi ne-grained sediments in the Suurpelto area have accumulated since the Fennoscandian ice-sheet retreated from the area ca. 13,000 years ago, during the early history of the Baltic Sea Basin (BSB) (e.g. Björck 1995, Saarnisto & Saarinen 2001). Based on geotechnical information it is known that the composition and geotechnical properties of the Suurpelto deposits vary both vertically and horizontally, making it a challenging site for construction. Therefore, the Suurpelto project was launched in 2005 in cooperation with the city of Espoo, the Geological Survey of Finland, the University of Helsinki and Helsinki University of Technology, aiming to provide scientifi c and technical information for land-use planning and ground engineering, particularly for tasks such as construction The Suurpelto area is about 1 km wide and 2 km long and located in Espoo, on the southern coast of Finland, about 10 km west of Helsinki city centre (Figs. 1 and 2). Nowadays the Suurpelto area is situated 4 km from the coastline, lying between 5 and 10 metres above sea level (a.s.l.), and has a current land uplift rate of 2 mm yr -1 (Kakkuri 1991). It is surrounded in all directions by a rolling relief of bedrock topography with relative altitude differences of about 50 m. The bedrock is an ancient peneplain of crystalline amphibolites, gneisses and granites that slopes towards the south with a very low gradient (Laitala 1991). Quaternary deposits up to 30 m thick have fi lled bedrock fracture zones and depressions during glacial and post-glacial times. The basal Quaternary deposits consist of a thin till cover overlain by a glaciofl uvial sand deposit of approximately 2 m in thickness in the Suurpelto area. Upon these lies a section of fi ne-grained sediments, the primary target of the present investigation, that was formed during the retreat of the continental Fennoscandian ice-sheet from the area and during different phases of the BSB deposition. 148 INTRODUCTION STUDY SITE suitability and stabilization. Geophysics, geochemistry, geotechnical engineering and sedimentological methods have been applied in the Suurpelto project (Ojala et al. 2006, Palmu et al. 2006). One of the purposes of the project is to compare and combine results gained with different investigation methods, and in the longer term, to develop new methods or method combinations that could be effi ciently and economically applied to similar areas. This paper presents results from the sedimentological part of the Suurpelto project. It includes the methods of logging several physical properties of fi ne-grained sediment deposits and a highly detailed characterization of sediment lithostratigraphy. The primary objectives were to identify the main sedimentary units of the Suurpelto deposits, to characterize their composition and structure, and to construct a 3D geological model of the arrangement of these different units in the Suurpelto area. The geological history of the Suurpelto area has been closely connected with the BSB deposition during Late Weichselian and Holocene. Following deglaciation (ca. 13,000 yrs ago), transgressive waters of the Baltic Ice Lake reached their maximum altitude of 120 m a.s.l. in the Espoo area. Subsequently, the lake suddenly drained westwards to the Atlantic and the water table lowered by 26 m at around 11,600 years ago (Eronen 1990, Saarnisto & Saarinen 2001). The emergence of the Suurpelto area proceeded during the Yoldia Sea phase, with the highest sea level of about 80 m a.s.l. in Espoo area (Eronen 1990). It was followed by Ancylus Lake phase (ca. 10,700-9,000 years ago) and Litorina Sea phase (9,000-) of the BSB history. Exposure of the Suurpelto study area from underneath the regressive Litorina Sea waters began around 3,000 years ago (Hyvärinen 1999). An isolation threshold of the Suurpelto basin is situated in the SE corner, nowadays named the Lukupuro ditch, at an altitude of 6 m (Fig. 2). The Suurpelto area became isolated from the Litorina Sea of the BSB around 2,000 years ago (Hyvärinen 1999) and then gradually dried up.
Geological Survey of Finland, Special Paper 46 Sedimentological characteristics of Late-Weichselian–Holocene deposits of the Suurpelto area in Espoo, southern Finland Fig. 1. Location and Quaternary deposits of the Suurpelto area. Digital elevation model and basemap © National Land Survey of Finland, permit No. 13/MYY/07. It is important to note that during deglaciation and the different phases of the BSB the sedimentary environment in the Suurpelto area varied signifi - cantly due to several reasons. The primary variables were the discharge intensity of melt-water from the retreating Fennoscandian ice-sheet, the relative Fig. 2. Shoreline displacement in southern Finland during the last 9000 years according to Hyvärinen (1999). The study site, Suurpelto, is marked with a blue circle. Digital elevation model and basemap © National Land Survey of Finland, permit No. 13/MYY/07. water depth (sea level rise vs. glacioeustacy), the water salinity (i.e. fresh water/brackish water), and the intensity of primary productivity due to climate change and nutrient availability. Therefore, the composition and structure of the Suurpelto fi ne-grained deposits has varied through time and space. 149
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GEOLOGICAL SURVEY OF FINLAND Specia
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Johansson, P. & Sarala, P. (eds.) 2
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4 CONTENTS Preface, Peter Johansson
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Geological Survey of Finland, Speci
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Applied Quaternary research in the
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DRUMLINS IN KUUSAMO AND RANUA In Ko
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RIBBED MORAINES IN SOUTHERN FINNISH
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ued fl owing from west to east but
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Sarala, P. 2005a. Glacial morpholog
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Pleistocene deposits in southern Li
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Heavy minerals Medininkai Upland Ei
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The South Lithuanian phase of glaci
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Fig. 4. Lateral drainage channels o
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for dating. The deglaciation of nor
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In Lokka, the sheetfl ow tracks wer
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Fig. 5. A compilation of low altitu
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deposited over the fi lled channel,
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Michigan Lobe as it advanced into t
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Type IV Tunnel channels The Type IV
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Attig, J. W., Mickelson, D. M. & Cl
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