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The Holocene Great Belt connection to the southern Kattegat, Scandinavia: Ancylus Lake drainage and Early Littorina Sea transgression CARINA BENDIXEN, JØRN B. JENSEN, LARS O. BOLDREEL, OLE R. CLAUSEN, OLE BENNIKE, MARIT-SOLVEIG € SEIDENKRANTZ, JOHAN NYBERG AND CHRISTIAN HUBSCHER

Bendixen, C., Jensen, J. B., Boldreel, L. O., Clausen, O. R., Bennike, O., Seidenkrantz, M.-S., Nyberg, J. & H€ ubscher, C.: The Holocene Great Belt connection to the southern Kattegat, Scandinavia: Ancylus Lake drainage and Early Littorina Sea transgression. Boreas. 10.1111/bor.12154. ISSN 0300-9483. Late- and postglacial geological evolution of the southern Kattegat connection to the Great Belt was investigated from high-resolution seismic data and radiocarbon-dated sediment cores in order to elucidate the Ancylus Lake drainage/Littorina Sea transgression. It was found that glacial deposits form the acoustic basement and are covered by Lateglacial (LG) marine sediments and postglacial (PG; Holocene) material. The LG deposits form a highstand systems tract, whereas the PG deposits cover a full depositional sequence, consisting of a lowstand systems tract (PG I), transgressive systems tract (PG II; subdivided into three parasequences) and finally a highstand systems tract (PG III). PG I sand deposits (11.7–10.8 cal. ka BP) are found in a major western channel and in a secondary eastern channel. PG II (10.8–9.8 cal. ka BP) consists of estuarine and coastal deposits linked to an estuary located at the mouth of the channels. Both channels drained fresh water from south to north. The PG III, that is younger than 9.8 cal. ka BP, represents the threshold marine flooding at the southeastern branch of the palaeo-Great Belt channel. At 9.3 cal. ka BP, fully marine conditions were established, shortly before the flooding of the threshold to the northern part of the Great Belt. These early Holocene spits and sand bars are preserved as features on the present seabed, probably as a result of the rapid sea-level rise that led to back-stepping of the early Holocene palaeo-coast system. This study shows no evidence of major erosion or delta deposition linked to the emptying of the Ancylus Lake, which suggests that continuous water flow from the south characterized the area, without any major drainage event of the Ancylus Lake impacting the southwestern Kattegat. Carina Bendixen ([email protected]), Jørn B. Jensen and Ole Bennike, Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 København K, Denmark; Carina Bendixen and Lars O. Boldreel, Department of Geosciences and Natural Resource Management, Geology Section, University of Copenhagen, Øster Voldgade 10, 1350 København K, Denmark; Ole R. Clausen and Marit-Solveig Seidenkrantz, Center for Past Climate Studies, Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, 8000 Aarhus C, Denmark; Johan Nyberg, Geological Survey of Sweden, Villav€agen 18, Uppsala, Sweden; Christian H€ubscher, Geophysics, University of Hamburg, Bundesstr. 55, D-20146 Hamburg, Germany; received 27th March 2015, accepted 7th October 2015.

Kattegat is a relatively protected marine basin, located in the transition zone between the Baltic Sea and the North Sea. It forms an important gateway between the enclosed Baltic Sea and the North Sea. A number of studies have shown that this connection was active in parts of the Late Weichselian and the Holocene (e.g. Bergsten & Nordberg 1992; Majoran & Nordberg 1997). Owing to its relatively protected environment, Kattegat has formed a depositional basin during the late Quaternary (e.g. Lykke-Andersen et al. 1993; Seidenkrantz 1993a; Gyllencreutz et al. 2006), which allows the recording of a significant part of the last glacial and postglacial history, and in particular its development as a gateway between the Baltic Sea and the North Sea. This is especially true for the southern Kattegat, where the sediments has the potential of recording all significant changes in the outflow from the Baltic Sea. The southern Kattegat is thus an ideal site for the study of outflow of water from the Baltic Sea, in particular through the largest waterway, the Great Belt (Fig. 1A). During the last few decades, the Late- and postglacial development of the Kattegat region has been DOI 10.1111/bor.12154

investigated with regard to sedimentological and environmental changes seen from the perspective of sediment core studies, foraminiferal studies and shallow seismic interpretation (e.g. Seidenkrantz 1993a; Seidenkrantz & Knudsen 1993; Conradsen 1995; Bennike et al. 2000; Novak & Pedersen 2000; Jensen et al. 2002a; Bendixen et al. 2013). In particular, the relationship between the Ancylus Lake and the Kattegat region has been greatly debated (e.g. Bj€ orck 1987; Jensen et al. 1999; Bennike et al. 2000, 2004; Lemke et al. 2001; Bj€ orck et al. 2008). The Ancylus Lake conundrum, which has been debated for decades, is the path of the drainage of the freshwater Ancylus Lake. Did it occur through Mecklenburg Bay and Fehmarn Belt, along the eastern side of Langeland and out through the Great Belt to Kattegat, i.e. the Dana River, through Oresund or through Lake V€anern, central Sweden (Andren et al. 2011)? The transgression of the Ancylus lake is supposed to have been caused by differential uplift, leading to the raising of Ancylus Lake above sea level and new outlet thresholds therefore could be eroded (Berglund et al. 2005; Bj€ orck et al. 2008). Recent studies by e.g. Bj€ orck et al. (2008) and © 2015 Collegium Boreas. Published by John Wiley & Sons Ltd

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Carina Bendixen et al.

BOREAS

A

640 000

Norway

Sweden

6 263 000 Øresund

Denmark Great Belt

Germany

Study area

B

572019

572009

572016

572014 572012

572011

572015

572022

10 km Depth (m) 40

641 000

671 000

C

Fig. 2

6 263 000

a

d c b Fig. 5

10 km

6 233 000

Fig. 1. Location map of the study area in UTM coordinates, zone 32. A. Regional map of Scandinavia with the location of the study area in southern Kattegat, Denmark. Arrows mark the Great Belt and Øresund straits. For high-resolution bathymetry see Hell et al. (2012) and Baltic Sea Bathymetry Database (2015). B. Close-up of the study area where the seismic profiles included in the database used for this study are marked by lines and core locations are shown as grey stars. The geographical positions of the cores are given in Table 2. The thick black lines illustrate key seismic lines of pinger profile 162D, boomer profile 572017 and sub-bottom profile 004 shown in Figs 2, 4 and 5, respectively. The black star is the location of core 8537 studied by Nordberg & Bergsten (1988). C. Bathymetry of the study area with thick black lines illustrating the key seismic lines (pinger profile 162D – Fig. 2, boomer profile 572017 – Fig. 4 and sub-bottom profile 004 – Fig. 5) and core locations are shown as grey stars. The black star is the location of core 8537 studied by Nordberg & Bergsten (1988). a, b = elongated ridges; c = estuary/channel; d = channel.

Bj€ orck (2008) have discussed the drainage path and suggested a solution of a ‘calm’ drainage model through the German–Danish area; however, there has not been a conclusive description of how the latest lake phase of the Baltic Sea, the Ancylus Lake, interacted with the marine southern Kattegat during the early Holocene before the Littorina transgression. The present study was based on both newly acquired seismic data and existing seismic data (pinger, sparker, boomer and parametric sub-bottom profiler), vibrocore data and 14C age determinations of marine shells and terrestrial plants from the vibrocores (Fig. 1B, C). The Lateglacial and Holocene sediments within the Baltic Sea region are often classified according to the so-called Baltic Sea stages: Baltic Ice Lake, Yoldia Sea, Ancylus Lake and Littorina Sea. However, sequencestratigraphical approaches have previously been used to interpret seismoacoustic profiles and sediments cores and have proven to be an effective method (Bennike et al. 2000, 2004; Moros et al. 2002). Virtasalo et al. (2014) proposed the incorporation of the allostratigraphical and lithostratigraphical approach when interpreting data from the Baltic Sea region owing to its relevance for regional setting. Therefore, in this paper, it was decided to establish a sequencestratigraphical classification for the interpreted seismic data set together with sediment cores and 14C ages and afterwards relate the classification to the so-called Baltic Sea stages. The overall objective of this paper was to describe and discuss the early Holocene depositional environment in the southern part of Kattegat, at the entrance to the Great Belt and Baltic Sea in order to illustrate the geological evolution of the area. The primary aim was to reconstruct the changing environment and the interaction between the Baltic Sea and southern Kattegat during the early Holocene lowstand and subsequent Holocene marine transgression. The secondary aim was to identify based on the seismic interpretation, if possible, how the drainage of the Ancylus Lake occurred and how the drainage affected the southern Kattegat region.

Geological setting The study area in the southern part of Kattegat is located at the northwestern edge of the SorgenfreiTornquist Zone, which is an old crustal weakness zone that has been reactivated several times (Mogensen & Korstg ard 2003). During the Upper Cretaceous and the Cenozoic a number of tectonic inversions phases in the Sorgenfrei-Tornquist Zone have been suggested to have occurred (Ziegler 1990; Vejbæk & Andersen 2002), caused by an interaction of the Alpine orogeny and the opening of the North Atlantic ocean (Nielsen et al. 2005, 2007). The dating of the latest tectonics is, however, owing to the Quaternary erosion, the subject

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The Holocene Great Belt connection to the southern Kattegat, Scandinavia

of a lively discussion (Japsen et al. 2007; Nielsen et al. 2009), which is beyond the scope of the present study. This location also places the Kattegat at the rim of the Fennoscandian shield where repeated ice sheets were formed, resulting in a complex Pleistocene development during the last glacial cycle. The Quaternary strata in the region include Eemian and Weichselian deposits (e.g. Seidenkrantz 1993a,b; Seidenkrantz & Knudsen 1993; Klingberg 1996; Knudsen et al. 1996). During the last deglaciation of the region following the Weichselian glacial maximum, major melting phases were interrupted by stillstands and re-advances of the ice margin until approximately 17 cal. ka BP (Houmark-Nielsen & Kjær 2003) when the ice sheet had fully retreated from the area (e.g. M€ orner 1969; Lagerlund & Houmark-Nielsen 1993; Houmark-Nielsen 2008). Prior to the last glacial cycle, a deep basin existed in the Kattegat (Lykke-Andersen et al. 1993; Seidenkrantz 1993a); this has been gradually filled by sediments at least since the Eemian (Houmark-Nielsen 1987; Seidenkrantz 1993a,b; Seidenkrantz & Knudsen 1993). Consequently, major parts of the Kattegat area are characterized by thick successions of Late Weichselian and Holocene glacifluvial and glaciomarine sediments (M€ orner 1969; Bergsten & Nordberg 1992; Seidenkrantz 1992, 1993a; Gyldenholm et al. 1993; Conradsen & Heier-Nielsen 1995; Knudsen et al. 1996; Gyllencreutz et al. 2005; Erbs-Hansen et al. 2011); these sediments are unevenly distributed (M€ orner 1969; Bergsten & Nordberg 1992). Owing to ice-load during the Late Weichselian (M€ orner 1969; Bergsten & Nordberg 1992), a generally high relative sea level existed in the southern Kattegat when the ice margin melted away, despite the still-low global eustatic sea level at that time (Fairbanks 1989; Peltier 2002; Siddall et al. 2003). A global eustatic sea level rise followed at the end of the Weichselian glaciation, but the faster glacio-isostatic rebound of the crust resulted in a fall of the relative sea level in the Kattegat and Baltic Sea region. The regression reached a lowstand at a minimum of 30–40 m b.s.l. (b.s.l. = below present sea level; Bennike et al. 2000; Jensen et al. 2002b). When the eustatic sea-level rise surpassed the isostatic rebound, the relative sea level began to rise (M€ orner 1969; Bennike et al. 2000), which in the southern Kattegat was dated to about 11.4 cal. ka BP (Jensen et al. 2002b). During the deglaciation, the isostatic rebound caused a temporary isolation of the Baltic Sea region from the Kattegat and the North Sea at least twice. This allowed the formation of two lake stages within the Baltic region: the Baltic Ice Lake and the Ancylus Lake (e.g. Bj€ orck 1995; Andren et al. 2002, 2011). The Baltic Ice Lake existed from 16 to 11.7 cal. ka BP (Bj€ orck 1995). During the early stage of the Baltic Ice Lake that existed from 10.7 to 9.8 cal. ka BP (Andren et al. 2011), the main drainage pathway for the glacial

3

meltwater may have been via Øresund (Agrell 1976; Bj€ orck 1981, 1995). As the isostatic uplift of the outlet area in Øresund exceeded the eustatic sea-level rise, the Baltic Sea region became fully isolated from the sea and damming of the glacial lake was complete (Andren et al. 2011). The final drainage of the Baltic Ice Lake resulted in a lowering of relative water level in the Baltic basin by about 25 m, by drainage through the south-central part of Sweden (Bj€ orck et al. 1996; Jakobsson et al. 2007) prior to the onset of the Holocene, which is dated to about 11.7 cal. ka BP (Walker et al. 2009). The Baltic Ice Lake stage was succeeded by the marine Yoldia Sea phase that lasted from 11.6 to 10.7 ka BP; the relative sea level was characterized by a regression during the late younger Yoldia Sea phase (Andren et al. 2011). As the isostatic rebound in south-central Sweden continued, the relative sea level fell and the Baltic Basin became isolated once more and the Ancylus Lake formed. The Ancylus Lake stage, when a freshwater lake filled much of the present Baltic Sea basin, is dated to 10.7–9.8 cal. ka BP (Bj€ orck 1995; Brenner 2005; Bj€ orck et al. 2008). During the early part of this stage, the Ancylus Lake drained via south-central Sweden, but later the drainage part shifted to the Danish–German straits (Andren et al. 2011). Based on interpretations of shallow seismic profiles and sediment cores Novak & Bj€ orck (1998) suggested that the drainage of the Ancylus Lake occurred rapidly at the end of the Ancylus Lake phase and took place over a few years via southern Kattegat. Subsequently, the relative sea level in the southern Kattegat was low, with the initial Holocene transgression resulting in the formation of a barrier-lagoon system. The lagoon sediments are found at depths of 24– 35 m below present sea level and are dated to between 10.5 and 10.3 cal. ka BP (Bennike et al. 2000). The subsequent marine transgression of the lagoon system took place at approximately 9.3 cal. ka BP (Bennike et al. 2000). The first sign of brackish water conditions in the Great Belt was seen at about 9.4 cal. ka BP (Jensen et al. 2002a) and on Lysegrund, southeast of our study area, a spit-platform complex developed at 10.8–8.9 cal. ka BP (Novak & Pedersen 2000), during a period of rapid transgression and abundant sediment supply. Marine conditions in Aarhus Bay were seen at 8.7 to 8.0 cal. ka BP (Jensen & Bennike 2009) and in the central Baltic Sea the first marine influence took place at about 8.5 cal. ka BP (Berglund et al. 2005). This regional marine inundation marks the beginning of the Littorina transgression, and the increasing relative sea level resulted in, amongst other changes, alterations to the hydrographical conditions of the Kattegat region (Nordberg & Bergsten 1988), and led to deposition of marine mud/gyttja in the deeper parts of the Kattegat.

4


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Material and methods The material for this study consisted of seismic data and vibrocores collected on a number of cruises from various vessels. Various seismic instruments, including pinger, sparker, boomer and parametric sub-bottom profiler (Table 1). Differential global positioning systems (DGPS) or global positioning systems (GPS) with accuracies of 1–5 m were used for navigation. The seismic data show a variety of instruments with specific resolutions of the subsurface. The pinger data (4 kHz) have a resolution of 0.3 m, whereas the highfrequency Innomar parametric sub-bottom profiler (6–10 kHz) data have a resolution of 0.05–0.10 m. Additionally, the boomer data (0.8–16 kHz) have a resolution of 0.3 m and the sparker data (0.5–1 kHz) have a resolution of 0.5 m. The seismic data were processed using the software PROMAX (Table 1) and the seismic data were interpreted using PETREL software, with a two-way travel-time conversion to metres using a velocity of 1500 m s1. The seismic database for this study is shown in Fig. 1B. Depositional sequence stratigraphy is used as an interpretation technique to describe termination of reflections, unconformities and internal reflection patterns (Mitchum et al. 1977) of seismic data, and subdivision in systems tracts and parasequences (Fig. 2) are used to establish chronologies, study depositional environments, relative sea-level changes and shoreline movements (Hunt & Tucker 1992; Posamentier & Allen 1993; Catuneanu 2006; Catuneanu et al. 2011). Eight vibrocores (for location see Fig. 1B, C; for geographical coordinates see Table 2) were selected to study the sedimentary features of pronounced changes and therefore to interpret the seismic data with respect to lithology, age determination and depositional environment. Furthermore, this information was useful for correlation of the seismic reflections. The cores were split, described and samples were collected for radiocarbon dating. The samples were collected from the centre of the cores in order to avoid down-core contamination. The cores were well preserved, with primary sedimentary structures such as laminated layers visible.

For radiocarbon dating of shells of marine molluscs and remains of land plants, the tandem accelerator at the AMS 14C Dating Centre, Aarhus University, was used. The 14C ages were calibrated using the INTCAL13 curve (terrestrial sample) or the Marine13 curve (marine samples; Reimer et al. 2013); they are reported as years before present (BP = AD 1950; Table 2). Krog & Tauber (1974) suggested a DR = 0, i.e. a standard reservoir age of 400 years, for the region, whereas quite a variety of values have been suggested for the modern reservoir age, as studies have shown that the DR value may have varied during the Holocene and from region to region in the larger Kattegat area (e.g. Olsen et al. 2009; Lougheed et al. 2013; Philippsen et al. 2013; 14CHRONO Marine Reservoir Database 2015). The primary cause for this is the fossil carbon from reworking of calcareous-rich sediments combined with the large freshwater influx, with little or no reservoir age. Additionally, higher reservoir ages in some areas close to coastal carbonate-rich bedrock (Lougheed et al. 2013) are suspected to be caused by the influence of groundwater rich in carbonate ions and depleted in 14C. This uncertainty is further accentuated through the infaunal habitat of some species of marine bivalves that live as sediment-feeders and which may thus incorporate more old carbon in their shells than is the case for other taxa. This may apply to Macoma balthica (Mangerud et al. 2006). Redeposition of material can also yield too old ages and therefore it is important to consider whether the material dated here could have been reworked. However, the shells used for dating were well preserved, suggesting that they were probably not redeposited. Currently, the uncertainties regarding a potential deviation from the standard marine reservoir age may not be resolved further, and we therefore chose DR = 0 for the current study. Detailed data on the bathymetry of the region were provided by The Danish Geodata Agency (Fig. 1C). The bathymetric data set was compiled from archived data that consist of old analogue measurements and modern digital data from recent echo sounding surveys (Hydrographic surveying 2015).

Table 1. Overview of the seismic database. The data were collected during seismic cruises. Vessel

Year

Acoustic instruments

Frequency

Processing

HNLMS ‘Tydeman’ RV ‘A. v. Humboldt’ MV ‘Laura’

1989 1997–1999 2010

Pinger EDO-western model, 515A system EG & G uni-boom system Marine Multi-Tip Sparker – Geo-source 200

4 kHz 0.8–16 kHz 0.5–1 kHz

RV ‘Ocean Surveyor’

2013

Marine Multi-Tip Sparker – Geo-source 800

0.5–1 kHz

RV ‘Alkor’

2013

SES-2000 medium parametric sub-bottom profiler SES-2000 medium parametric sub-bottom profiler

6–10 kHz 8 kHz

– Bandpass filtering Bandpass filtering Kirchhoff’s migration KF-filtering Deconvolution Migration – –


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NW

5

20

30

40 m b.s.l.

SE

20

30

40 1 km

m b.s.l.

NW 20 H

PG III

PG III PG II.2

30

PG II.1 LG

PG I

LG GAS

LG LG

GAS GAS

GAS

40 m b.s.l.

SE 20 572014 572011 572012

PG II.2 PG II.1

572015

b

c

PG II.3

PG II.2

PG III

30

PG II.1

PG II.1

LG LG

WG

40

GAS 1 km

m b.s.l. Sequence boundary Bundle-wise built out

1, 2, 3

Parasequence Parasequence number

PG LG WG

Postglacial Lateglacial Glacial

Holocene Pleistocene Pleistocene

m b.s.l.

Metres below present sealevel

Highstand systems tract Transgressive systems tract Lowstand systems tract

Fig. 2. Selected part of pinger profile 162D. The upper profile shows a seismic section from NW to SE and the lower section illustrates the sequence stratigraphical interpretation of lowstand systems tract, transgressive systems tract, highstand systems tract and sequence boundary. The location of the profile is shown in Fig. 1B and C. The interpreted eastern spit system is marked with b and the approximate location of the estuary is marked by a and c as illustrated in Fig. 1C. The positions of selected cores (572014, 572011, 572012 and 572015) are illustrated.

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Table 2. Radiocarbon dates of shells of marine molluscs and plant remains. Asterisk marks terrestrial sample. Core

N. lat.

E. long.

Water depth (m)

Laboratory no.

Species1

Depth b.c.t2 (m)

Age (14C a BP)

Calibrated age (a BP)3

Sequence

572009

56°31.2680

11°34.5390

29.2

572011

56°27.4410

11°39.3600

27.9

AAR-4533 AAR-4534 AAR-4060* AAR-4061

3.65 4.95 5.97 1.50

10 31080 10 41080 11 12070 923535

11 11 12 10

PG I PG I LG PG II.2

572012

56°27.4020

11°40.0230

28.1

AAR-4535 AAR-4062

5.63 0.60

10 05090 852055

10 929–11 162 9066–9237

PG I PG III

572016

56°27.8080

11°29.6860

25.6

572019 572022

56°34.6530 56°18.1270

11°25.6890 11°10.5510

32.4 23.3

M. M. B. C. M. M. B. C. C. L. M. M. M.

5.25 3.35 4.87 2.82 5.07

914575 960080 996090 10 60075 867090

9792–10 078 10 365–10 582 10 801–11 065 11 739–12 157 9094–9504

PG II.2 PG II.3 PG II.1 PG II.1 PG II.3

AAR-4063 AAR-4536 AAR-4537 AAR-4064 AAR-4065

edulis edulis nana edule balthica edulis crenatus edule edule littorea balthica edulis edulis

185–11 313–11 903–13 001–10

474 677 073 147

1

Full names are: Mytilus edulis, Betula nana, Macoma balthica, Balanus crenatus, Cerastoderma edule, Littorina littorea. Below core top/sea floor. Calibration is according to the INTCAL13 data set (terrestrial sample) and the Marine13 data set (marine samples; Reimer et al. 2013).

2 3

Results and interpretation The modern-day bathymetry in our study area shows a pattern of elongated ridges (marked a and b; Fig. 1C) and channels (marked c and d; Fig. 1C) with a dominant SW–NE direction. The average water depth increases towards the NE from about 15 m to more than 40 m (Fig. 1). The elongated ridges and the channels do not reflect the present-day hydrographical conditions in southern Kattegat (Myrberg & Lehmann 2013); instead the bathymetry represents a palaeo-river mouth in a coastal setting where the coast prograded towards the north (Figs 3, 5). The well-preserved palaeogeography is key to understanding the development of the area between the Great Belt valley and the southern Kattegat basin, in combination with interpretations of the seismic profiles, the vibrocores and the age determinations. Sequence stratigraphy was used to divide the seismic data into sequences, bounded by unconformities and internal parasequences. The sequences comprise Weichselian glacial deposits, probably till (WG), followed by a highstand, probably representing glaciomarine Lateglacial deposits (LG). We excluded WG deposits from our sequence stratigraphical analysis, as these deposits are related to glacial depositional environments and not controlled by changes in relative sea level. The WG is bounded by an upper erosional unconformity (Fig. 3). This unconformity separates the glacigenic sediments below from the marine deposits above. Owing to a Lateglacial relative sea-level fall that resulted in a lowstand, the Lateglacial deposits are bounded by an upper erosional unconformity. During the subsequent sea-level rise, postglacial (PG) deposits accumulated. The rapid Holocene sea-level rise resulted in overstepping of the palaeo-coast system, which is preserved in the present-day bathymetry. The preserved system includes partly infilled valleys that

continue north of the Great Belt valley; the valleys are bounded by spit-like elongated ridges. The valleys gradually widen northwards to become an enclosed basin between the prograding spit-like elongated ridges, possibly creating an estuary. In some areas, the presence of shallow gas makes the interpretation of the seismic data difficult because of the masking effect of the gas (Figs 2, 4; Hovland & Judd 1988). Glacial (WG) The seismic characteristics of the lower boundary of the WG deposits were not identified during this study because of the limited penetration of the seismic recordings. The upper boundary is formed by a pronounced erosional unconformity (Fig. 2). The internal reflection pattern is chaotic and in some places it was impossible to identify internal reflection patterns owing to the masking effect of gas bubbles in shallower layers (Fig. 5) or because of limited seismic penetration (Figs 2, 5). In the eastern part of the study area, where the deposits occur at shallow depths, the minimum thickness is approximately 15 m (Figs 2, 5) but the thickness varies over the study area where the deposits are also located beneath the seismic penetration. Previous studies by e.g. Houmark-Nielsen (1987) showed that the unit consists of glacial till. A Weichselian age can be assumed as Eemian sediments are found at a lower stratigraphical level in various sites around the greater Kattegat region (e.g. Seidenkrantz 1993b; Houmark-Nielsen & Kjær 2003). Lateglacial (LG) The seismic characteristic of the LG upper boundary is truncation of the underlying reflections and this is


BOREAS

W

7

E

572014

1 km

0

572016

572015

PG II.3

25

10.1 cal. ka BP PG III

572011

572012

10.5 cal. ka BP PG II.3 9.2 cal. ka BP PG II.2 PG II.2

10.9 cal. ka BP

30

PG II.1 PG II.1

Sand

PG I

Clay Silt

Sand

Clay Silt

Sand

Clay Silt

PG II.2

9.9 cal. ka BP

35

Symbols Marine mollusc (few) Marine molluscs (few to many) Plant remains Stones

m b.s.l.

Sand

Clay Silt

Sand

Clay Silt

LG Highstand system tract Transgressive systems tract Lowstand system tract

11.0 cal. ka BP

Lithology Sand Silt Clay and Silt Clay gyttja

Structures Sand gyttja

Laminated

Organic material Heterolitic clay with sand Heterolitic clay with silt

Homogenous

Weakly laminated

Fig. 3. Correlation of sedimentological logs from a transect of sediment cores from west to east close to the pinger profile 162D shown in Fig. 2. The succession is divided into systems tracts and a sequence boundary separates the LG highstand sediments from the PG sediments. The boundary between PG II.1 and PG II.2 to the east originates from the interpretation of pinger profile 162D (Fig. 2). Details for the core can be found in Fig. 3 and geographical coordinates and radiocarbon ages are in Table 2. For seismic line and core location see Fig. 1C; for legend see Fig. 2.

interpreted as an erosional unconformity. The internal reflection pattern consists of pronounced continuous parallel reflections conformably draping the WG upper boundary (Fig. 2). In general, the LG deposits cover the deeper parts of the WG depressions in the Kattegat region and in the study area the thickness of the deposits is 10–20 m (Fig. 2), but can be up to 100 m (Jensen et al. 2002b). In general, the lithology of the LG deposits changes from laminated clay in the deeper parts to interlayered fine sand and clay in the upper part. The coarsening upward is probably a result of a continuous moderate regression until the eustatic sea-level rise surpassed the glacio-isostatic rebound in the early Preboreal (M€ orner 1969; Jensen et al. 2002b). Shallow water sediments from the youngest part of the sequence were sampled by core 572009 (Fig. 7). In this sediment core, laminated silt with some plant remains is overlain by finegrained sand with no structures. A single sample of bark fragments of Betula nana was 14C dated to 13 cal. ka BP (Table 2). In earlier papers, the deposition of fine-grained Lateglacial marine sediments has been

shown to take place during highstand periods (M€ orner 1983; Nielsen & Konradi 1990; Richardt 1996). Postglacial (PG) Postglacial sediments reach a thickness of about 12 m in channels (Figs 2, 4) and their thickness varies over the rest of the study area. The postglacial sediments have been divided into three separate units based on changes in internal reflection patterns. PG I. – PG I has a limited distribution in the study area and is mostly found in the deeper basin and in the channels that have eroded into the underlying LG sediments. The lower boundary is uneven and truncates the underlying sediments; it corresponds to the sequence boundary of the LG sequence. In general, the upper boundary is conformable with the sediments above (Figs 2, 4, 5). The internal reflection pattern in the southernmost proximal channel area reflects asymmetrical progradational infill that downlaps on the LG surface (Fig. 4). In the deeper

8


BOREAS

SSW

NNE

25 30 35 40 1 km

45 m b.s.l. NNE

SSW

572016

25

PG II.3 PG II.2

PG II.1

30 35

PG I Gas

1 km

LG

40 45 m b.s.l.

Fig. 4. Selected part of boomer profile 572017. The upper profile shows a seismic section from SSW to NNE and the lower section shows the sequence stratigraphical interpretation. The profile shows the westerly spit progradation (marked with a on Fig. 1C), which occurred during the period interpreted as the transgressive systems tract. The core 572016 penetrated into PG I, PG II.1 and PG II.3. Details for the core can be found in Fig. 3 and geographical coordinates are in Table 2. For seismic line and core location see Fig. 1C; for legend see Fig. 2.

northernmost, more distal part, the channels are less pronounced and only partly filled in as wedges on the western rims of the channels (Fig. 2). Sediments of postglacial age were recovered in several sediment cores (Figs 3, 6) and typically consist of sand with some cobbles and pebbles. In core 572009, the lowermost 0.5 m above the sequence boundary, dominated by alternating sand and gravel deposits and laminated silt, is interpreted as the initial transgression phase. Four samples of shells of marine molluscs were dated to 11.7–10.8 cal. ka BP (Table 2). The sediment infill of the incised valleys is interpreted as lowstand systems tract deposits that accumulated in a transitional zone between a southernmost channel-dominated area and a northernmost basin-dominated initial estuary mouth. PG II. – PG II covers PG I across the entire study area. PG II shows a complicated internal reflection pattern that indicates three development phases, interpreted to be coastal parasequences (1–3), separated by flooding surfaces, on the seismic profiles (Figs 2, 4, 5) and clo-

sely related to the earlier-mentioned bathymetric pattern of elongated ridges (a and b; Fig. 1C) and channel depressions with a dominant SW–NE direction (c and d; Fig. 1C). The PG II consists of a complex depositional system and has therefore been subdivided into a southwestern, a central and an eastern area. In the southwestern, most proximal channel area (a; Fig. 1), PG I is covered by PG II sediments, which are divided into three parasequences. The internal seismic characteristic for PG II parasequence 1 (PG II.1) is parallel reflections to the northeast and small wedge shaped attached to the rim of PG I to the southwest (Fig. 4). It is approximately 2 m thick. The section with parallel reflections consists of laminated clay and silt, whereas the wedge consists of weakly laminated sand and silt with few pebbles and marine shells dated to about 10.5 cal. ka BP (core 572016; Fig. 3, Table 2). This is interpreted to represent the initial coastal development of the early transgressive systems tract. PG II parasequence 2 (PG II.2) in Fig. 4 is a wedgelike structure that is displaced northeastward and


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SSW

9

NNE 20

25

30

1 km m b.s.l. SSW

NNE 20

WG

PG III

25

PG II.3

PG I

PG II.2 PG II.1

30

Gas 1 km

m b.s.l.

Fig. 5. Selected part of Innomar parametric sub-bottom profile 004 (6–10 kHz). The upper profile shows a seismic section from SSW to NNE and the lower section shows the sequence stratigraphical interpretation. The profile shows the easterly spit progradation (marked with b on Fig. 1C), which occurred during the period interpreted as the transgressive systems tract. A stacked reflection pattern is illustrated in PG II.2 with stippled lines. For seismic line location see Fig. 1C; for legend see Fig. 2.

partly covers PG II.1. The seismic profile 572017 (Fig. 4) shows an internal sigmoidal north-northeastward prograding reflection pattern that in combination with the bathymetry (Fig. 1) shows that PG II.2 represents the development of a sand bar in front of the entrance to the palaeo-Great Belt channel. Beneath the prograding reflection pattern, few parallel reflections can be interpreted. PG II parasequence 3 (PG II.3) in Fig. 4 is interpreted as a wedge deposit deposited on top of PG II.1, prograding north-northeastwards and overlying PG II.2.

On the bathymetric map the PG II.3 wedge forms a palaeo-coastal rim at the entrance to the palaeo-channel. The sediments consist of sand and gravel in the lowermost part, upwards changing into mediumgrained sand separated by a thin silt layer (core 572016; Fig. 6). The development history of the three parasequences reflects the early transgressive coastal development (PG II.1), followed by development of a sand bar (PG II.2) and the successive back-stepping of the coast (PG II.3).

10

Carina Bendixen et al. SW

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572022 0

25

10 km

572016

PG III

PG II.3 9.3 cal. ka BP

PG II.3

10.5 cal. ka BP

Sand

30

Clay Silt

572009

NE

572009

10.9 cal. ka BP 30

PG II.1 PG II.1

m b.s.l.

Sand

Clay Silt

PG I 11.3cal. ka BP

572019

11.5 cal. ka BP

LG

Sand

Clay Silt

35

11.9 cal ka BP

Sand

Clay Silt

13.0 cal. ka BP

m b.s.l.

Fig. 6. Correlations of sedimentological logs in a S–N profile. The sediments have been divided into system tracts and a sequence boundary separates the LG highstand sediments from the PG sediments. The sequence boundary is only identified in the two cores furthest northeast. For core locations see Fig. 1B and C; for geographical coordinates see Table 2; for legend see Figs 2 and 3.

In the central area (c; Fig. 1C) PG II sediments cover PG I and LG deposits (c; Fig. 2). In the central part, only two PG II parasequences have been identified (PG II.1 and PG II.2). PG II.1 is characterized by subparallel internal reflections that drape LG and PG I inside and outside the channels (Fig. 2). PG II.1 subparallel reflections consist of up to 10-m-thick laminated clay with a small content of silt (core 572011; Fig. 3), indicating a more basin-dominated area. PG II.2 is separated from PG II.1 by a distinct lower reflection and the internal reflection pattern is characterized by subparallel reflections. The upper boundary shows reflection truncation and is interpreted as an unconformity. The sediments are fine-grained from upwards-fining silt to layered silt and clay. Radiocarbon dating of marine molluscs shows ages of 10.1– 9.9 cal. ka BP. The basin-dominated sediments have been exposed to erosion and are locally missing. The laminated clay

with a small content of silt has a high content of marine molluscs as identified in core 572011 (Fig. 3) and may represent overwash flooding events as proposed by Bennike et al. (2000). In the eastern margin of the study area (b; Fig. 1C), PG II has a wedge-shaped structure that has been divided into three parasequences similar to the southwestern area. PG II.1 is characterized by an initial wedge, weakly northeastward progradation and downlapping on glacial till, LG or PG II.1 deposits (Figs 2, 5). PG II.2 is characterized as a prograding sequence that shows an internal reflection pattern characterized by a pronounced stacked progradation (dotted lines; Fig. 5). The characteristic stacked bundled wedges are indicative of a possible tidal depositional environment (Dalrymple et al. 2011; Davis 2011). PG II.2 sediments were recovered in core 572015 (Fig. 6) and the sediments consist of laminated clay and silt. PG II.2 and PG II.3 are separated by a flooding surface in the


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southeast (Fig. 2) and this surface is identified in core 572015 as a lithological change into silt and clay with a high content of marine molluscs. In PG II.3 the internal reflection pattern shows subparallel, high-amplitude reflections (Figs 2, 5) with an upper erosional boundary truncating the reflections. PG II.3 sediments were recovered in core 572015 (Fig. 6) and the sediments consist of laminated silt and clay with a high content of marine molluscs changing into laminated clay and silt. The combination of seismic cross-sections (pinger profiles 162D and sub-bottom profile 004; Figs 2, 5) with an internal northeastward prograding reflection pattern and the northward elongated ridges on the bathymetry (Fig. 1C) shows that parasequences 1 to 3 represent the development of palaeo-bar systems at the possible estuary mouth of the funnel-shaped entrance to the palaeo-Great Belt channel. The possible estuary mouth system is found between the Great Belt palaeo-fluvial system and the Kattegat marine basin (c; Fig. 1). The mapped bar system represents marginal spits and central tidal sand banks, which were partly preserved during a time when the relative sea level continued to rise and the palaeo-coast line stepped back. PG II is interpreted as transgressive systems tracts during a sea-level rise. As the sea level rose over a threshold in the northern Great Belt, heavy erosion of the PG II units took place. This has resulted in the absence of parts of PG II.2 and PG II.3 within the central, more basin-dominated area. Five samples of marine mollusc shells were 14C dated and yielded ages of PG II ranging from 11.9 to 9.1 cal. ka BP (Table 2; Figs 3, 6). Dating of marine conditions in the southern Kattegat palaeo-Great Belt channel at 9.3 cal. ka BP (core 572022; Fig. 6; Table 2) just north of the northern threshold to the Great Belt marks the timing of the marine ingression in this area.

11

PG III. – PG III is found throughout the study area except for the top of the sand bars towards the northwest. The lower boundary of PG III is characterized by erosion and truncates the underlying sediments creating an unconformity (Figs 2, 4, 5), which is interpreted as a transgressive surface. The upper boundary is the present sea floor. In the more shallow parts of the study area, the deposits show semi-transparent parallel internal reflection patterns of low amplitude (Fig. 2). Sediments were recovered in five sediment cores (Figs 3, 6) with silty clay in the deeper parts grading into sand in the more shallow parts of the study area (Figs 3, 6). In the southern part of the study area, mud and clay are found in the channels (Fig. 7). PG III is interpreted as highstand systems tracts and forms the youngest sediments in the region following the flooding of the northern Great Belt threshold.

Discussion The Lateglacial (LG) sediments are widespread in the southern Kattegat and are found both in the basin areas and in deeply eroded channels, as the sediments drape the surface of the underlying till (Fig. 2). This was probably caused by an initial relative high sea level (local highstand), caused by glacio-isostatic depression from the Weichselian ice sheet. This occurred at a time when the global eustatic sea level was low as a result of the still-extensive presence of ice sheets and glaciers (Fairbanks 1989; Peltier 2002). The highstand was followed by a regression and significant erosion of the LG deposits occurred. This resulted in the development of an unconformity, which forms the sequence boundary to PG I (Figs 2, 4). This is illustrated in the idealized SSW–NNE profile (Fig. 7). The fine-grained LG deposits (13.0 cal. ka BP; Table 2) found in the northernmost part of the

SSW

NNE 572022

20 572016

PG III PG II

PG I

LG

30

PG II 572009

WG

LG

PG II

40

PG I 1 km

50 m b.s.l.

Fig. 7. Constructed idealized SSW–NNE profile showing the distribution of the interpreted sequences in the transition area between the Great Belt channel and the southern part of the Kattegat. For core locations see Fig. 1B and C; for geographical coordinates see Table 2; for legend see Figs 2 and 3; for location of profile see Fig. 8.

12


study area (core 572009; Fig. 6) below the overlaying sequence boundary indicate that the regression had already reached close to its maximum lowstand level in the Younger Dryas. The highstand systems tract deposits consist of fine-grained sediments (Jensen et al. 2002b). The unconformity is most significant where erosional channels are found (Fig. 4), reaching a maximum lowstand erosion depth at 30– 40 m b.s.l. during the earliest Holocene, as documented by previous studies (e.g. Bennike et al. 2000; Jensen et al. 2002b). During the early transgression in the Kattegat, sand above the LG silt and clay is interpreted as lowstand PG I sediment (core 572009; Fig. 6), deposited within erosional channels during coastal marine conditions. Shells of marine molluscs were dated to 10.8–11.7 cal. ka BP in cores 572011 and 572009 (Figs 3, 6; Table 2). The sediments consist primarily of sand with few cobbles and pebbles, as recorded in vibrocores 572011 and 572009 (Figs 3, 6), interpreted to be more coastal deposits. The distinct difference in the internal reflection pattern of PG I suggests the presence of a primary western channel with more pronounced flow and a secondary eastern channel during the initial transgression. Gradually, the eustatic sea-level rise surpassed the diminishing isostatic rebound and a relative sea-level rise in Kattegat resulted in the deposition of PG II estuarine and coastal deposits. South of the estuary freshwater channels formed, a sandy spit developed inside the estuary to the southwest and sand bars and a silty spit formed at the mouth of the estuary, towards the north (Figs 1, 2, 4, 5). Elongated ridges developed parallel to the flow of the palaeo-channels creating sand bars and spits located parallel to the channel inlets and with northwards internal progradation. The initial formation of the spits was dated to 10.9 cal. ka BP on shell material from vibrocore 572016 (Table 2; Fig. 3). The southwestern sandy spit is characterized by progradation of wedges and back-stepping, which are indications of rising sea level (Fig. 4). The eastern spit shows a stacked internal pattern within PG II.2 (Fig. 5), which may indicate an environment with tidal influence. This is also supported by the morphology of the estuary, which is mouth-shaped, as well as by the bar and spit distribution. Bennike et al. (2000) suggested that the fossil foraminiferal faunas could reflect a tidal-affected area; however, they did not find evidence of tidal influence in the sedimentary structures. A modified version of the tidal model for the Kattegat and Skagerrak regions shows a tidal amplitude up to approximately 1 m (Uehara et al. 2006; K. Uehara, pers. comm. 2014), and support the hypothesis of a palaeo-Kattegat with tides. The PG II estuary developed in the period 10.8– 9.8 cal. ka BP, which was a period of great impor-

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tance for the Ancylus Lake stage of the Baltic Basin. Many previous studies have suggested that the relative shore level of the Ancylus Lake dropped significantly after its maximum highstand at about 10.3 cal. ka BP (e.g. Bj€ orck 1995; Bennike & Jensen 1998; Jensen et al. 1999). Novak & Bj€ orck (1998) suggested that there was a dramatic drainage event of the Ancylus Lake into the Kattegat via the Dana River within a few years. The present study is located in the crucial parts of the palaeo-channel but no signs of large-scale erosion or delta deposits, indicating major drainage, have been observed. Early Holocene deposits in the Great Belt also lack any indication of a rapid lowering of the water level of the Ancylus Lake (Bennike et al. 2004). Hence, it appears that if the drainage of the Ancylus Lake occurred via the Dana River through the Great Belt into southern Kattegat, it must have occurred as a noncatastrophic continuous flow, supporting the noncatastrophic flow scenario (Bj€ orck et al. 2008). The division of the transgressive systems tract into three parasequences (PG II.1, PG II.2 and PG II.3) is also seen in the vibrocores as lithological changes. In core 572011 (Fig. 3) a change from clay and silt to silt deposits is recorded. This is interpreted as the transition between PG II.1 and PG II.2, which is also identified in the seismic section of pinger profile 162D (Fig. 2). Furthermore, core 572015 (Fig. 3) shows a change in lithology from clay and silt to silt deposits, which is interpreted as the transition between PG II.2 and PG II.3, also identified in the seismic sections of pinger profile 162D and sub-bottom profile 004 (Figs 2, 5). The flooding surfaces that separate the parasequences of PG II and changes in lithology are a possible indication of variations in the sea level during the overall transgression during the Holocene. The general trend for the early Holocene is a rising sea level, but fluctuations may be occurring on a regional scale. The interpreted parallel reflections below the prograding reflection pattern in the western spit deposits may be a result of the regional sea-level fluctuations. This may indicate a short period with stillstand before the prograding of the spit continued. It is possible that the flow regime in the branches of the palaeo-Great Belt channel changed from PG I deposition to PG II deposition, so that the latter flow is more even with no primary and secondary system. The transgression continued and sea-level rise led to back-stepping of the coast, which is seen in the spit deposits as a back-stepping wedge (Figs 4, 7) and a back-stepping coastline. Shells of the marine bivalve Mytilus edulis from marine sand from sediment core 572022 have been dated to 9.3 cal. ka BP (Table 2; Fig. 6). This age means that the transgression had reached the southwesternmost part of the study area at this time. As sea level continued to rise, the sea flooded the northern Great Belt threshold, where marine water

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could flow into the Great Belt and further into the Baltic Sea. This resulted in major erosion of sediments in the study area, which is seen as the erosional upper boundary of the PG II. This erosional surface is interpreted as the transgressive surface separating the transgressive systems tract from the highstand systems tract (Figs 2, 5). The flooding of the threshold and continued rise of the relative sea level resulted in the environment changing into a more basin-like system. This is seen in the lithology: in the northern part of the study area silt is overlain by clay with a small content of silt (core 572011; Fig. 3) and in the southern part of the study area, sandy sediments are overlain by mud and clay (core 572022; Fig. 6). In seismic sections, PG III is characterized by low-amplitude parallel reflections, which indicates finer-grained material. The erosive effect of the threshold flooding is especially significant in the southeastern branch of the palaeo-Great Belt channel within the estuary deposits (Fig. 2). In that area, parts of PG II.2 are missing and there is no evidence of PG II.3 (Fig. 2). Nordberg & Bergsten (1988) described a change in lithology from clay with sand to clay with shells in core 8537, at a depth of 2.0 m, which was dated to approximately 8.8 cal. ka BP. This sediment core was taken from a location within the present study area (black star; Fig. 1). The authors interpreted this change as a gap in the stratigraphy. The change in lithology corresponds to the findings of the present study; it may correspond to the transition between the transgressive systems tract and highstand systems tract and represents an erosional transgressive surface. Nordberg & Bergsten (1988) also reported a change in the benthic foraminiferal fauna and they interpreted the faunal change to be caused by a change in the current system. As the northern Great Belt threshold was flooded, marine water entered the Baltic region and water from the Baltic Sea was mixed with water in the Kattegat region. This mixing probably led to changes in the fauna and the local current system. In sediment core 572011, which is located north of Nordberg & Bergsten’s (1988) core, clay and silt deposits are identified as representative of the highstand systems tract. Opening of the Øresund strait occurred sometime between 8.5 and 8.0 cal. ka BP (Bj€ orck et al. 2008) and had an additional influence on the fauna and the local current system of the Kattegat. Fully marine conditions first occurred in Aarhus Bay at 7.7–9.0 cal. ka BP (Jensen & Bennike 2009; P. Rasmussen, pers. comm. 2015), in the Great Belt at approximately 8.1 cal. ka BP (Bennike et al. 2004), in Mecklenburg Bay at about 8 cal. ka BP and in the Arkoina Basin at about 7.2 cal. ka BP (R€ oßler et al. 2011). These ages indicate a transgression pathway from southern Kattegat via the Great Belt and into the Arkona Basin (R€ oßler et al. 2011).

Land Spits Sand bars Sea River Core locations

13

671 000

641 000 Fig. 7

6 263 000

10 km

9.9 cal. ka BP

6 233 000

Fig. 8. Palaeogeographical map of the study area (Fig. 1) at approximately 9.9 cal. ka BP. The palaeoreconstruction stretches over the entire study area, including a complete estuary system with back barrier basin, sand bars and spits.

The bathymetrical data show spits and sand bars (a and b; Fig. 1C) and channels with a dominant SW– NE direction (c and d; Fig. 1C). These features do not reflect the present-day current system in the area. These morphological features formed during the early Holocene, during relative sea-level rise that led to back-stepping of the early Holocene palaeo-coast and flooding of the threshold in the northern Great Belt. Based on the data presented in this study, an idealized profile is constructed for the Lateglacial and Holocene development (Fig. 7) and a map of the study area showing the geography at approximately 9.9 cal. ka BP (Fig. 8) is generated. The palaeogeographical reconstruction (Fig. 8) shows the coastal environment at the final stage of the estuary’s development, during the early Holocene transgression, at approximately 9.9 cal. ka BP. It illustrates two palaeo-branches of the palaeo-Great Belt freshwater channel flowing into the Kattegat and the development of spits parallel to the inlets as well as sand-bar development parallel with the outflow from the western inlet channel. The enclosement of the area by the spits created an estuary. The presence of sand bars indicates a coastal environment possibly influenced by tides. The palaeogeographical reconstruction shows that the present bathymetry reflects the situation of the palaeogeographical map at 9.9 cal. ka BP.

Conclusions Interpretation of newly acquired seismic data and existing seismic data, vibrocore data and 14C age determinations of marine shells and terrestrial plants from

14


the vibrocores have led to a new understanding of the early Holocene depositional environment in the southern part of Kattegat in relation to the drainage of the Ancylus Lake and following the Littorina transgression. •

•

• •

•

•

•

Postglacial (PG – Holocene) sediments represent a full depositional sequence that includes lowstand systems tract (PG I), transgressive systems tract (PG II) and highstand systems tract (PG III). In addition, PG II is interpreted as three parasequences separated by flooding surfaces (PG II.1, PG II.2 and PG II.3). Spit development (PG II) enclosed an estuary and a stacked, bundled internal reflection pattern in PG II.2 (Fig. 5) indicates an environment with tidal influence and regional variations in the rising sea level during the early Holocene. The estuary existed in the period 10.8–9.8 cal. ka BP, which is synchronous with the drainage of the Ancylus Lake. The study shows that if drainage of the Ancylus Lake occurred via the Dana River into the southern Kattegat, it must have been characterized by a continuous flow as there is no evidence recorded in the study area of major erosion or delta deposition, which would indicate a catastrophic drainage event. A continuous Holocene sea-level rise resulted in back-stepping (landwards retreat) of coastal systems, and flooding of the northern Great Belt threshold and major erosion occurred. The age of the marine bivalve Mytilus edulis from core 572022 of 9.3 cal. ka BP (Table 2; Fig. 6) shows that the transgression had reached the southwesternmost part of Kattegat at that time. The present-day bathymetry reflects the early Holocene palaeomorphology of spits and sand bars. This morphology is preserved in the present-day seabed because of the rapid sea-level rise. The preservation of the old morphology is a unique opportunity to reconstruct the palaeoenvironmental development of the area around 9 cal. ka BP, including a complete estuary system with back barrier basin, sand bars and spits (Fig. 8).

Acknowledgements. – We thank the captains and crews of RV ‘Ocean Surveyor’, RV ‘Alkor’, RV ‘Alexander von Humboldt’ and MV ‘Laura’ for practical assistance during the work at sea. This paper is published with the permission of the Geological Survey of Denmark and Greenland and the work was financed by Geocenter Denmark through the DAN-IODP-SEIS project, including the Geological Survey of Denmark and Greenland, University of Copenhagen and Aarhus University. Funding was also provided through the Danish Council for Independent Research, Natural Sciences project (DFF12-126709/FNU, OCEANHEAT). Schlumberger is thanked for its university grant licence for PETREL at the Department of Geosciences and Natural Resource Management, Geology section, University of Copenhagen. We further wish to thank Svante Bj€ orck, Richard Gyllencreutz, Helge W. Arz and Jan A. Piotrowski for constructive and useful comments on the manuscript.

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