Early Holocene regressive spit-platform and nearshore sedimentation on a glaciofluvial complex during the Yoldia Sea and the Ancylus Lake phases of the Baltic Basin, SW Finland

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Abstract

The depositional history of the Virttaankangas plain, a fan-like enlargement of the large Säkylänharju glaciofluvial ridge in SW Finland, is examined. The Säkylänharju ridge has been interpreted as having formed as an interlobate esker, and the morphology of the Virttaankangas plain as representing predominantly subaqueous glaciofluvial to glaciolacustrine deposition. This investigation uses new sedimentologic outcrop data, with extensive subsurface drilling, GPR surveys, seismic soundings, and gravimetric measurement data provided by groundwater investigations, to describe the stratigraphy, morphology and depositional history of the plain.

In contrast to earlier interpretations, the results emphasise the role of littoral processes in the origin of the Virttaankangas plain. The plain is interpreted as having originated during the development of a progradational sequence of spit-platform and superimposed nearshore sediments deposited during the glacio-isostatic forced regression of the Yoldia Sea and Ancylus Lake phases of the Baltic Basin (9900–9000 14C BP). Documented depositional history supports an interval of retarded regression at the beginning of the Ancylus Lake phase I (9500–9200 14C BP) in the Säkylänharju area.

Introduction

The Säkylänharju glaciofluvial ridge and its extension the Virttaankangas plain in SW Finland are generally described as an exceptionally large, complex esker. It forms the most prominent part of a 200-km long esker system, running from the Third Salpausselkä end moraine (Glückert, 1995) to the coast of the Gulf of Bothnia (Fig. 1).

Due to the glacio-isostatic rebound following the last deglaciation, the Säkylänharju ridge emerged as an isolated island from the ancient Baltic Sea, at about 9900–9000 14C BP Glückert, 1976, Glückert, 1994, Ristaniemi and Glückert, 1988. At that time, wind fetches extended a few hundred kilometres to the SW, and towards the end of the period also to the W–N. The regressive shoreline, exposure to powerful storms, and the abundant availability of glaciofluvial source material led to a wide distribution of shore features on the esker Aartolahti, 1972, Mäkinen, 1993. This implies highly intense removal and redeposition of glaciofluvial material within the littoral zone.

Up to now, interpretations regarding the genesis of the complex have been based on geomorphological and hydrogeological data without detailed sedimentological work. Previous studies have focused on the widespread and prominent ancient beach ridges flanking the complex Aartolahti, 1972, Mäkinen, 1993. Groundwater investigations within the Säkylänharju–Virttaankangas complex since the 1960s provide a remarkable source of subsurface data, which, however, has not been adequately treated or summarised in order to interpret past depositional environments. In this paper, we combine available subsurface data and new sedimentological outcrop observations with the morphology of the complex to describe its depositional history.

The Säkylänharju–Virttaankangas complex is divided into three major facies assemblages (I–III); these are used to outline the occurrence of shoreline deposits and their sediment source areas. The two glaciofluvial facies assemblages are first briefly described and interpreted. The paper then focuses on the sedimentology of the Virttaankangas plain (i.e. Facies assemblage III). The results show that the Virttaankangas plain is not a glacigenic delta/fan delta, as suggested by previous authors Glückert, 1971, Aartolahti, 1972, nor was it formed during deglaciation as stated by Kukkonen et al. (1993). Rather, the Virttaankangas plain formed by the redeposition of the esker material transported by longshore drift towards the southeastern part of the esker during the glacio-isostatically forced regression of the Yoldia Sea phase (Fig. 2). This redeposition has resulted in the development of a prograding spit-platform overlain by numerous beach ridges.

The lithofacies interpretations presented in this article contribute to our understanding of erosional and depositional processes in nearshore environments, which are difficult to examine in modern systems, especially under severe storm conditions. The depositional model describing the deposition of the plain can be applied to several other emerged glaciofluvial ridges in Finland. Furthermore, the results of this study will be applied in the planning of an artificial groundwater-supply project on the Virttaankangas plain.

Section snippets

Study area

The study area is situated in SW Finland approximately 61°00′N and 22°35′E (Fig. 1). The geomorphological setting of the study area is illustrated in Fig. 2, Fig. 3.

Postglacial shore-level displacement

After the deposition of the Third Salpausselkä ice-marginal formation during the Younger Dryas, the climate warmed up considerably, leading to the rapid retreat of the ice margin. According to Sauramo, 1923, Sauramo, 1940 glacial varve chronology, the margin of the ice sheet was situated in the Virttaankangas area about 400 clay varve years after the final drainage of the Baltic Ice Lake (BIL) to Yoldia Sea level. The final drainage of the BIL, correlated here to ca. 11 565 GRIP ice core years

Material and methods

The facies zonation of the Säkylänharju–Virttaankangas complex is based on morphology, observations of sedimentary characteristics from six pit exposures (Fig. 4), and on a number of groundwater research reports, including drilling, seismic soundings, ground-penetrating radar (GPR) surveys, gravimetric surveys, aeromagnetic measurements and records of groundwater levels and flow paths. The drill holes cover the entire study area; they consist of 57 hammer-driven drill holes (not penetrating to

Facies zonation of the Säkylänharju–Virttaankangas complex

The study area can be divided into three major facies assemblages, referred to here as (I) esker, (II) Säkylänharju–Virttaankangas glaciofluvial ridge and related fine-grained deposits, and (III) spit-platform and related shore-zone deposits of the Virttaankangas plain Fig. 4, Fig. 5.

Discussion

The Säkylänharju–Virttaankangas complex is interpreted here as having been formed in four main depositional phases: (I) time-transgressive deposition of the esker (subglacial tunnel infill superimposed by repeated subaqueous retrogradational fans); (II) deposition of the Säkylänharju–Virttaankangas glaciofluvial ridge and related fine-grained deposits (interlobate ice-marginal crevasse deposits and related glaciolacustrine sediments); (III) erosion of glaciofluvial deposits during rapid

Concluding remarks

The results of this study demonstrate that the role of glaciofluvial processes in the genesis of the Virttaankangas plain has been overemphasized in previous reports. The morphology of the plain is predominantly ruled by the development of the spit-platform sequence and superimposed nearshore deposits. The findings thus indicate that the deposition of the Virttaankangas plain did not require a marked stillstand of the ice margin during the last deglaciation; this is also supported by the lack

Acknowledgements

We would like to thank our enthusiastic fieldwork assistants Jani Kivi, Anna-Liisa Rautionmaa, and Nyrki Rautionmaa, and also Seppo Toivonen for skillful use of his tractor–excavator. My warmest thanks are due to the Rautionmaa family for accommodation and care during the fieldwork seasons. The work was financially supported by Emil Aaltosen Säätiö (the Emil Aaltonen Foundation), Jenny ja Antti Wihurin Rahasto (the Jenny and Antti Wihuri Foundation), Suomen Kulttuurirahasto/Satakunnan Rahasto

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