Geological Survey of Denmark and Greenland Bulletin 7, 2004, p 45-48 45 The present-day Storebælt (Great Belt), the waterway be- tween the islands of Fyn and Sjælland (Fig. 1), contains deep- ly incised valleys, locally more than 50 m deep, and is of crucial importance to the water exchange between the fully marine Kattegat and the brackish Baltic Sea. The role of this important gateway changed significantly during the late and post-glacial period (since 15 000 B.P.), when the Baltic Basin experienced alternating freshwater, brackish and marine con- ditions as a result of changes in relative sea level (Figs 2, 3). The importance of the Storebælt in understanding the dynamics of the Baltic Basin is reflected in the large number of studies carried out (see Bennike et al. 2004). The first detailed sedimentological and stratigraphic studies in the Storebælt area that demonstrated the presence of early Holo- cene freshwater deposits below the seabed were those of Krog (1960, 1965, 1971), who also presented the first shore-dis- placement curve for the area (Krog 1979). The BALKAT project The late and post-glacial evolution of the south-western Baltic Sea has been studied in detail during the past 15 years as part of the multi-disciplinary BALKAT project, a co-operation between the Geo- logical Survey of Denmark and Green- land (GEUS), the Baltic Sea Research In- stitute in Warnemünde, and other part- ners (Jensen et al. 2002). Acquisition and interpretation of shallow seismic data and sampling of vibrocores form the basis for sequence stratigraphic and sedi- mentological studies, which together with micro- and macropalaeontological stu- dies have resulted in detailed interpreta- tions of depositional environments. The chronology has been established by nu- merous radiocarbon dates. Initial studies focused on the Fakse Bugt and Gedser Rev region and the history of the Baltic Ice Lake (Jensen & Stecker 1992; Lemke & Kuijpers 1995; Jensen et al. 1997) and were followed by detailed studies of the Ancylus Lake stage (Jensen et al. 1999). Further studies of relative shore- level changes in the region (Bennike & Jensen 1998) and the Femer Bælt threshold (Lemke et al. 2001) showed that local ice lakes developed in front of the retreating Fennoscandian ice sheet. Two major transgressions of the Baltic Ice Lake are recorded, with maximum highstand levels of approximately 30 m and 20 m below present sea level (Figs 2, 3; Björck 1995). The Ancylus Lake transgression reached just above the southern threshold of the Storebælt, and was followed by a regression that presumably exposed large parts of the former lake bottom (Figs 2, 3). The identification of these lake stages in the Femer Bælt area show a much wider distribution than previously expected, with possible connections to the Katte- gat (Yoldia Sea, Littorina Sea) via the Storebælt (Fig. 3). Studies in southern Kattegat and the northern Storebælt region have revealed that a late Pleistocene relative sea-level The Storebælt gateway to the Baltic Jørn Bo Jensen, Ole Bennike,Wolfram Lemke and Antoon Kuijpers Geological Survey of Denmark and Greenland Bulletin 7, 45–48 (2005) © GEUS, 2005 Falster LangelandsbÊlt Germany Sweden Poland Fakse Bugt Ø resu n d 12°E 14°E 54°N 55°N 10°E 56°N 20 m 20 m 40 m 20 m Lille Bælt G edser Rev Femer Bælt Baltic Sea Kattegat 50 km S toreb æ lt Jylland Fyn Sjælland Denmark Fig. 1. Present-day general bathymetry of the south-western Baltic Sea. Location of Fig. 5 (red frame) and the northern and southern thres- holds in Storebælt (red circles) are shown. highstand was followed by a lowstand during the latest Plei- stocene (12 000 – 11 500 B.P.; Fig. 2). This was in turn suc- ceeded by the Littorina Sea transgression, which resulted in a series of back-stepping coastal deposits (Jensen et al. 2002). Recent studies under the BALKAT project have been con- centrated in the central Storebælt area and the northern and southern threshold areas in order to obtain a more detailed understanding of the interaction between the Kattegat and the Baltic Basin. The Storebælt gateway The central Storebælt area, between the fully marine Kattegat and the brackish Baltic Basin, has been influenced by drai- nage from lakes in the Baltic area and marine transgressions from the Kattegat. Both depend on the relative levels of the southern and northern thresholds. In this area the incised val- ley fills provide a unique opportunity to study the initial effects of drainage and transgression, as well as the timing of these events, that – with some delay – had a great influence on the Baltic area. During the later stages of the BALKAT project, shallow seismic data and vibrocores have been collected for the area extending from the northern entrance to the southernmost part of the Storebælt. The relative sea level changes in the southernmost Kattegat region (Fig. 2) are clearly recognised at the entrance to the Storebælt, where late glacial marine highstand sediments are cut by the Younger Dryas lowstand erosional unconformity and followed by an early Holocene succession of channel fill, estuary river mouth sediments and backstepping shoreface deposits (Fig. 4A, B; Bennike et al. 2000; Jensen et al. 2002). The northern Storebælt threshold is located in a less than 1 km wide incised valley (Figs 1, 5). Profiles north and south of the threshold (Fig. 4C, D) show that marine transgressive deposits are found in the incised valley north of the thres- hold, whereas a transitional brackish unit exists below the Littorina Sea deposits south of the threshold. Radiocarbon datings of the brackish sediments using terrestrial plant macrofossils indicate that the initial transgression of the Littorina Sea took place at about 9400 B.P. The central Storebælt incised valley (Figs 1, 4E) was formed by meltwater during the deglaciation about 17 000 B.P. (Bennike et al. 2004), and the initial fill is represented by late glacial lake sediments. The youngest late glacial unit is restricted to the channels, and is believed to be the Baltic Ice Lake extension into the Storebælt area. The late glacial sedi- ments are truncated by an erosional unconformity overlain by Lower Holocene freshwater sediments that include river and lakeshore deposits, and followed by extensive lake deposits formed in the time interval between 10 900 and 8800 B.P. (Bennike et al. 2004). Deposition of the early river deposits is coeval with the maximum level of the Ancylus Lake. The initial sign of the marine transgression in the cen- tral Storebælt area is dated to 8100 B.P. by marine shells. 46 northern threshold Ancylus Lake Baltic Ice Lake southern threshold Littorina transgression 10 30 50 –10 –30 –50 S h o re -l e ve l (m ) years B.P. 16 000 12 000 8000 4000 0 central Kattegat northern Storebælt Femer Bælt Fig. 2. Shore level changes relative to present-day sea level in central Kattegat, northern Storebælt and Femer Bælt. The levels of the thres- holds in the Storebælt are indicated. 10 500 B.P.15 000 B.P. Yoldia Sea Littorina Sea 100 km 100 km Baltic Ice Lake Ancylus Lake Ice Fig. 3. Palaeogeographical maps showing the distribution of land and sea/lake at 15 000 years and 10 500 years B.P. The threshold in the southernmost part of the Storebælt is found in a few hundred metres wide channel at about 25 m below present sea level (Figs 1, 4F, 5). However, fine-grained freshwater sediments dated to the time of maximum Ancylus Lake transgression at 10 300 B.P. brings the pre-Ancylus Lake threshold down to about 30 m below present sea level. There is no evidence of a rapid Ancylus Lake drainage (Dana River) as earlier proposed by Björck (1995). Initial marine transgression of the Storebælt Based on the recently collected data it is possible to recon- struct a palaeogeographical scenario for the initial Holocene marine transgression (10 000 – 9500 B.P.) of the Storebælt area (Fig. 5). At about 10 000 B.P. the Ancylus Lake was mainly drained by a river system located in the Storebælt area with an outlet in the southern Kattegat area (Bennike et al. 2000). In gene- ral, the drainage pathway through the Storebælt was restricted to channels less than 1 km wide. A transitional brackish estu- ary was restricted to the area immediately north-east of the northern threshold. The transgression of the Littorina Sea resulted in flooding of the northern Storebælt threshold at about 9500 B.P. and a brackish environment extended to about 20 km south of the threshold. At the same time, a large local lake developed in the central and southern part of the Storebælt area due to a ground-water level rise, related to the relative sea level rise north of the northern Storebælt threshold, and brackish and marine conditions were gradually established in these areas around 9400–9100 B.P. (Winn et al. 1998; Bennike et al. 2004). Future perspectives During the 15 years of BALKAT co-operation a unique data- base covering the late and postglacial sediments in the south- western Baltic region has been generated. The main key areas have now been studied, but further biostratigraphical inves- tigations and datings are required for Lillebælt, Øresund and parts of Kattegat. When this work is completed, a detailed model of the postglacial evolution of the western Baltic re- gion can be developed including data on palaeogeography, fauna and flora evolution, and climatic changes. This model will be of great importance for future scientific co-operation involving marine geological, archaeological, eco- logical and palaeo-climatic studies in the rest of the Baltic region. Furthermore a detailed knowledge on the palaeogeo- graphic evolution is important in locating potential sand and gravel resources. Large offshore construction works may also 47 SW NE –20 m –40 m –40 m –50 m –20 m –30 m –20 m –40 m –20 m –20 m –30 m –40 m 5 kmB EW SW NE LG W E LG LG GAS W E EW 1km northern threshold southern threshold A C D E F Gas 1km 1km 1km 1km Till Local ice lake Baltic Ice Lake Freshwater Brackish Marine highstand Marine transgression Littorina Sea P le is to c e n e H o lo c e n e Fig. 4. Profiles illustrating the influence of the northern and southern thresholds in the zone between the fully marine Kattegat and the brac- kish to lacustrine central and southern parts of the Storebælt. For loca- tion of profiles, see Fig. 5. benefit from the studies. For example, the planned Femer Bælt Link bridge requires seabed information for geotechni- cal, raw material and hydrographic evaluations, as well as for monitoring possible impacts on the environment. Acknowledgement Our friend and colleague, Wolfram Lemke, unexpectedly passed away on 21 April 2005. We wish to acknowledge his enthusiastic participation in our joint projects and his inspiring contributions to our long-standing co-operation. His premature death is a great loss to the scientific com- munity. References Bennike, O. & Jensen, J.B. 1998: Late- and postglacial shore level changes in the southwestern Baltic Sea. Bulletin of the Geological Society of Denmark 45, 27–38. Bennike, O., Jensen, J.B., Konradi, P.B., Lemke, W. & Heinemeier, J. 2000: Early Holocene drowned lagoonal deposits from the Kattegat, southern Scandinavia. Boreas 29, 272–286. Bennike, O., Jensen, J.B., Lemke, W., Kuijpers, A. & Lomholt, S.J. 2004: Late- and postglacial history of the Great Belt, Denmark. Boreas 33, 18–33. Björck, S. 1995: A review of the history of the Baltic Sea, 13.0–8.0 ka BP. Quaternary International 27, 19–40. Jensen, J.B. & Stecher, O. 1992: Paraglacial barrier–lagoon development in the Late Pleistocene Baltic Ice Lake, southwestern Baltic. Marine Geology 107, 81–101. Jensen, J.B., Bennike, O., Witkowski, A. & Kuijpers, A. 1997: The Baltic Ice Lake in the south-western Baltic: sequence-, chrono- and bios- tratigraphy. Boreas 26, 217–236. Jensen, J.B., Bennike, O., Witkowski, A., Lemke, W. & Kuijpers, A. 1999: Early Holocene history of the southwestern Baltic Sea: the Ancylus Lake stage. Boreas 28, 437–453. Jensen, J.B., Petersen, K.S., Konradi, P., Kuijpers, A., Bennike, O., Lemke, W. & Endler, R. 2002: Neotectonics, sea-level changes and biological evolution in the Fennoscandian Border Zone of the southern Kattegat Sea. Boreas 31, 133–150. Krog, H. 1960: Post-glacial submergence of the Great Belt dated by pollen-analysis and radiocarbon. Report of the International Geolo- gical Congress, XXI Session, Part IV, 127–133. Krog, H. 1965: On the post-glacial development of the Great Belt. Baltica 2, 47–60. Krog, H. 1971: The early Post-glacial development of the Storebælt as reflected in a former fresh water basin. Quaternaria 14, 85–92. Krog, H. 1979: The Quaternary history of the Baltic, Denmark. In: Gudelis, V. & Königsson, L.-K. (eds): The Quaternary history of the Baltic, 207–217. Uppsala: Uppsala University. Lemke, W. & Kuijpers, A. 1995: Late Pleistocene and Early Holocene palaeogeography of the Darss Sill area, southwestern Baltic. Quaternary International 27, 73–81. Lemke, W., Jensen, J.B., Bennike, O., Endler, R., Witkowski, A. & Kuijpers, A. 2001: Hydrographic thresholds in the western Baltic Sea: Late Quaternary geology and the Dana River concept. Marine Geology 176, 191–201. Winn, K., Erlenkeuser, H., Nordberg, K. & Gustafsson, M. 1998: Paleohydrography of the Great Belt, Denmark, during the Littorina transgression: the isotope signal. Meyniana 50, 237–251. 48 Authors’ addresses J.B.J, O.B & A.K., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: jbj@geus.dk W.L., Baltic Sea Research Institute, Seestrasse 15, D-18119 Rostock-Warnemünde, Germany. 10 000 B.P. F E D C A northern threshold southern threshold B 56°N 55°N 9 500 B.P. F E D C A northern threshold southern threshold B 56°N 55°N 11°E 11°E 25 km 25 km A B Fig. 5. Palaeogeographical maps of the Storebælt area during initial Holocene transgression. A: At c. 10 000 years B.P., before the transgres- sion of the northern threshold, the Ancylus Lake was connected to Kattegat via a river system ending in a narrow, brackish estuary. B: After the threshold was transgressed at c. 9000 years B.P., a much more exten- sive brackish water estuarine complex developed at the mouth of the river, and a major lake was formed in the central part of the Storebælt area. For legend and sections A–F see Fig. 4.