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O R I G I N A L P A P E R

Martin Bak Hansen Holger Lykke-AndersenAli Dehghani Dirk Gajewski Christian Hu bscherMorten Olesen Klaus Reicherter

The MesozoicCenozoic structural framework of the Bay of Kiel area,western Baltic Sea

Received: 20 October 2004 / Accepted: 21 April 2005 / Published online: 12 July 2005 Springer-Verlag 2005

Abstract A dense grid of multichannel high-resolutionseismic sections from the Bay of Kiel in the westernBaltic Sea has been interpreted in order to reveal theMesozoic and Cenozoic geological evolution of the

northern part of the North German Basin. The overallgeological evolution of the study area can be separatedinto four distinct periods. During the Triassic and theEarly Jurassic, EW extension and the deposition ofclastic sediments initiated the movement of the under-lying Zechstein evaporites. The deposition ceased dur-ing the Middle Jurassic, when the entire area wasuplifted as a result of the Mid North Sea Doming. Theuplift resulted in a pronounced erosion of Upper Tri-assic and Lower Jurassic strata. This event is markedby a clear angular unconformity on all the seismicsections. The region remained an area of non-deposi-tion until the end of the Early Cretaceous, when the

sedimentation resumed in the area. Throughout theLate Cretaceous the sedimentation took place undertectonic quiescence. Reactivated salt movement is ob-served at the Cretaceous Cenozoic transition as a resultof the change from an extensional to compressionalregional stress field. The vertical salt movement influ-enced the Cenozoic sedimentation and resulted in thin-skinned faulting.

Keywords Seismic interpretation North GermanBasin Basin evolution Mesozoic Cenozoic

Introduction

Over the years, several seismic studies have been carriedout in the region around the western Baltic Sea forexample the EUGENO-S (EUGENO-S working group1998), BABEL (BABEL working group 1991, 1993),DEKORP-BASIN (e.g. Meissner and Krawczyk 1999;Krawczyk et al. 1999), and POLONAISE 97 (e.g.Guterch et al. 1999; Grad et al. 1999) projects. Theseprojects were carried out to obtain a better under-standing of the deep-rooted tectonic structure of theregion. Poprawa et al. (1999) did a study on the tectonic

evolution of the Baltic Basin based on a subsidenceanalysis from boreholes. Detailed studies of the sedi-mentary cover were carried out in the adjacent North-east German Basin (Scheck and Bayer 1999; Bayer et al.1999;Kossow et al. 2000; Kossow and Krawczyk 2002;Scheck et al. 2003a; Scheck et al. 2003b) and in theNorthwest Germany and the German North Sea Sector(e.g. Baldschuhn et al. 2001), but until now no or verylittle systematic and localised research has been carriedout to investigate the post-Permian structural frame-work of the south-western Baltic Sea.

In order to investigate the post-Palaeozoic structuraland depositional evolution of the western Baltic Sea,

which corresponds to the northern part of the NorthGerman Basin and the transition towards the BalticShield (Fig.1), the Universities of Aarhus (Denmark)and Hamburg (Germany) have joined forces completingseven marine geophysical surveys in the area between1998 and 2004 in the frame of the Baltseis and theNeoBaltic (Hu bscher et al.2004) projects. During theseresearch cruises, a dense grid of high-resolution seismicdata were acquired together with potential field data(gravity and magnetic) along the ship track. This paperpresents the results of the seismic interpretation of the

M. B. Hansen (&)

A. Dehghani

D. Gajewski

C. Hu bscherInstitute of Geophysics, University of Hamburg,Bundesstrasse 55, 20146 Hamburg, GermanyE-mail: bak-hansen@dkrz.deTel.: +49-40-428382970Fax: +49-40-428385441

H. Lykke-Andersen M. OlesenDepartment of Earth Sciences, University of Aarhus,Finlandsgade 6-8, 8200 Aarhus N., Denmark

K. ReicherterInstitute of Geology and Palaeontology, Hannover University,Callinstrasse 30, 30167 Hannover, Germany

Int J Earth Sci (Geol Rundsch) (2005) 94: 10701082DOI 10.1007/s00531-005-0001-6

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Bay of Kiel sub-area of this regional study. For thispurpose, a dense grid of 96 seismic sections and theinformation from four explorational wells (Kegns-1,Sllested-1, Rdby-1 and Rdby-2) were used (Fig.2).

Geological framework

The North German Basin (NGB) (Fig.1), which is a

sub-basin of the Southern Permian Basin (Ziegler 1990;Scheck and Bayer 1999), belongs to a series of relatedCarboniferous-Permian intracontinental basins extend-ing from the southern North Sea to Northern Poland(Kossow et al. 2000), referred to as the Central Euro-pean Basin System (CEBS). The Ringkbing-Fyn High(RFH) is a series of east-west striking basement highsrunning across the North Sea and the Danish mainland(Clausen and Pedersen 1999) that bounds the NGB tothe north. The southern flank of the RFH is more or lessidentical to the path of the Caledonian DeformationFront (CDF). To the south, the NGB terminates at theVariscan Deformation Front (VDF) and to the east at

the Teisseyre-Tornquist Zone (TTZ). Furthermore, twoapproximately EW striking structural lineaments crossthe NGB: The so-called Trans European Fault (TEF)and the Elbe Lineament (EL) further to the south.

The initial main phase of thermal subsidence in theNGB began in Early Permian and continued until theMiddle Triassic (Van Wees et al. 2000). The lowermostdeposits consist of CarboniferousPermian volcanics andaeolian, fluvial and shallow-lake sediments overlain byUpper Permian (Zechstein) evaporites (Scheck and Bayer1999). The Zechstein evaporite sequence developed as a

result of a series of marine transgressions into the intra-continental basin from the north after a period of terres-trial condition (Ziegler 1990; Kossow et al. 2000). TheTriassic succession is characterised by a Lower Triassicred-bed clastic sequence (Buntsandstein), deposited rap-idly under terrestrial conditions and overlain by marinecarbonates (Muschelkalk) (Kossow and Krawczyk 2002).A sea level drop during the Late Triassic (Keuper) led torenewed terrestrial sedimentation (No ldeke and Schwab1976). The Triassic succession was overlain by a LowerJurassic sequence, deposited under shallow-marine con-ditions (Gearhart Geo Consultants 1985). The subsidencepattern of the NGB was interrupted by a period of uplift

from the Middle JurassicEarly Cretaceous (Ziegler 1990;Underhill 1998). As a result of the uplift, parts of theLowerJurassic and Upper Triassic sedimentary sequenceswere eroded (Kossow et al. 2000). Sedimentation resumedtowards the end of the Early Cretaceous (Albian) with thedeposition of a terrestrial clastic sequence followed byUpper Cretaceous marine marls and carbonates, indi-cating a period of tectonic quiescence and rising sea level(Kossow and Krawczyk 2002). Towards the end of theLate Cretaceous andduring thePalaeogene, the NGB wasinverted as a result of several inversion pulses related tothe Alpine Orogeny (Ziegler 1990; Krauss 1994). TheCenozoic succession shows a facies pattern of terrestrial

and shallow-marine clastic sediments deposited understrong influence of halokinesis (Kossow et al.2000).

Database

High-resolution multichannel seismic data

During 19982000, the University of Aarhus (AU) on-board R/V Dana acquired parts of the high-resolutionmultichannel seismic sections used in this study as part

Fig. 1 Location of the study area and the main structural elementsof the region (after Krauss 1994; Vejbk1997; Bayer et al. 1999;Clausen and Pedersen1999)

Fig. 2 Map of the study area, showing the positions of the seismicsections and the deep wells used in this work. The positions of thesalt structures are based on Vejbk (1997), Clausen and Pedersen(1999) and this study. The positions of the sub-salt faults are fromVejbk (1997)

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of the BaltSeis project (survey names: DA98, DA99,DA00). The seismic equipment for these three surveysconsisted of a 70 cubic inch, 100 bar sleeve gun clusterfired every 12.5 m, and a 48-channel streamer (300 mlong) with a hydrophone group separation of 6.25 m.This configuration gives a common mid point distanceof 3.125 m. Source and receivers were towed 3 m belowthe sea surface.

The data acquisition was continued by the Universityof Hamburg in cooperation with AU between 2001 and2003 onboard R/V Alkor (AL 185, AL 225) and R/VHeincke (HE 172) (survey names: AL01, HE02, AL03)as a part of the NeoBaltic project. During the HE 172cruise, the same source configuration was used, but thistime a 96-channel streamer (600 m long) with the samehydrophone group separation was used.

On the 2003 AL 225 cruise, a single 105 in3, 140 bar,G-gun, fired every 12.5 m, was used, along with the 96-channel streamer. The data were recorded digitally on allfive cruises and sampled every 1 ms, with a recordinglength between 2 s and 3 s. The penetration is in theorder of 1.52 s two-way travel time (TWT) for all the

profiles. The vertical resolution of the seismic data isbetween 8 m and 10 m, whereas the horizontal resolu-tion is in the order of 2025 m.

Well data

The stratigraphic report of the offshore Kegns-1 explo-ration well (Gearhart Geo Consultants1985) and strati-graphic information from the three onshore explorationwells, Sllested-1, Rdby-1 and Rdby-2 (Fig.2) (Nielsenand Japsen1991), were used for correlating the differentinterpreted horizons on the seismic profiles.

Stratigraphic interpretation

Using the data from the offshore exploration well Ke-gns-1 (Gearhart Geo Consultants1985), situated in thenorthwestern corner of the survey area (Fig. 2), it waspossible to date sixmajor horizons traced across theentiresurvey. These are: Base Middle Triassic (BMT), BaseUpper Triassic (BUT), Base Jurassic (BJ), Mid JurassicUnconformity (MJU), Base Upper Cretaceous (BUC)and Base Cenozoic (BC). It was only possible to trace theTopZechstein (TZ)horizonon a few sections.In addition,some internal reflectors within the Upper Triassic, Upper

Cretaceous and the Cenozoic successions were used forthe structural and depositional interpretation. A strati-graphic table is presented in Fig.3.

Sedimentary sequences (Fig. 3)

Lower Triassic (Buntsandstein)

This sequence, bounded by the TZ and BMT horizons,consists of fluvial to lacustrine sediments and playa-lake

Fig. 3 Stratigraphic table showing the dominant lithologies,average thicknesses and the ages of the major horizons interpretedin the northern part of the North German Basin. The thicknessdata are from Nielsen and Japsen (1991).TZTop Zechstein,BMTBase Middle Triassic,BUTBase Upper Triassic, BJBase Jurassic,MJU Mid Jurassic Unconformity, BUC Base Upper Cretaceous,BCBase Cenozoic

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deposits (Scheck and Bayer 1999). Due to the limitedpenetration on the seismic sections, the TZ reflection inthis succession is only well defined on a few profiles.

Middle Triassic (Muschelkalk)

This sequence comprises carbonates deposited duringtransgressive to shallow-marine conditions (GearhartGeo Consultants1985), bounded by the BMT and BUTreflectors. This is the oldest sedimentary sequence that istraceable around the entire survey area.

Upper Triassic (Keuper)

This sequence is a succession of limnic-fluvial and playa-type sediments (Scheck and Bayer1999). The base of thissequence is defined by the BUT horizon. In the centralparts of the study area, the Upper Triassic sequencetruncates at the MJU horizon, while in the marginal areasthe BJ reflector is its upper boundary. In addition, twointernal reflectors were also traced within this succession.

Jurassic

Three successions of Jurassic sediments were interpretedin different places within the survey area, but unfortu-nately none of them are located in areas with deep wellcontrol. The BJ reflector bounds the base of the se-quence and upward it is truncated by the MJU horizon,in accordance with an Early Jurassic age. According toKossow et al. (2000), the Lower Jurassic sequence in theNorth German Basin consists of interbedded marinemud and sandstones.

Lower CretaceousThis is a thin succession of homogenous thickness,traceable over the entire study area. The MJU and BUCreflectors bound the sequence. The base marks a majortransgression of Albian age from non-deposition toshallow marine conditions (Gearhart Geo Consultants1985; Kossow et al. 2000). The sequence consists ofcalcareous sediments, primarily red marls.

Upper Cretaceous

This sequence is bound by the BUC and BC reflectorsand consists of chalk sediments deposited during shal-

low marine becoming open marine conditions. The se-quence is successfully mapped over the entire study area.The upper boundary marks an unconformity (GearhartGeo Consultants1985; Kossow et al. 2000). Two inter-nal reflectors within this succession were also tracedthroughout the survey area.

Cenozoic

This sequence, which is basally bound by the BC horizonand is present at the seafloor, consists of brackish marine

clay-silt sediments (Scheck and Bayer 1999) of Tertiaryage, deposited under strong influence of salt movement(Kossow et al. 2000). The succession consists of theQuaternary glacial deposits at the top. Three internalhorizons within the sequence were also traced.

Seismic stratigraphy

Figures4,5 and 6 show examples of interpreted seismicsections in the study area (for location see Fig. 2). Allthree profiles show that the Mesozoic and Cenozoicstrata were deposited under the influence of salt move-ment. Furthermore, it can be seen that an angularunconformity (MJU) at depths between 500 ms and1,000 ms separates the Cretaceous and Cenozoic suc-cessions from older strata. In the central part of the Bayof Kiel, it is the Upper Triassic strata that truncates atthe MJU horizon (Fig.4 and 6), while a succession ofLower Jurassic sediments is preserved in the eastern partof the study area (Fig.5).

Geological evolution

Methods

The interpretation of the depositional and structuralevolution of the Bay of Kiel is based on the created time-structure maps (Fig.7, 8), time-isochore maps (Fig.913) and the restored sections of seismic profile HE02-33(Fig.14). The time-structure maps show the present dayvertical depth in two way travel time (TWT) to thespecific surfaces. These were used to map out the posi-tion of important salt structures and to identify the

trends of different fault systems. The time-isochore mapsshow the present day vertical thickness in TWT of givensedimentary sequences bounded by two specific surfaces.These maps were used to determine periods of activefaulting, salt movement and the trends of local palaeo-depocenters. Generally, for both types of map, only thefault trends that can be traced from line to line acrossthe survey have been included.

The restoring method used in Fig. 14is a simple 2Dmodel backstripping, with the assumption that all sedi-mentary layers are deposited horizontally. In this model,neither volumetric proportions, nor compaction, havebeen taking into consideration.

Because of the limited penetration of the seismic data,it has not been possible to map the Top Rotliegendsurface underneath the Zechstein evaporites, and theTop Zechstein surface has only been traceable on few ofthe seismic sections. Thus, it is not been possible to giveany volumetric estimate of the degree of salt movementin the study area. Because of the limited mapping of theTop Zechstein horizon on the HE02-33 line, the back-stripping is made with the assumption that the LowerTriassic sequence (Buntsandstein) is deposited with analmost homogenous thickness across the seismic section.

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Present-day structures

The BMT surface (Fig. 7) displays a gentle dip towardsthe south and southwest towards the centre of NGB

from basin margin in the north. The general dip of thesurface is disturbed by several anomalies, interpreted tobe the result of halokinesis. In the western part, threeanomalies are seen where the surface is elevated to be-tween 500 ms and 900 ms TWT.

These three updomed areas will subsequently be re-ferred to as Salt Pillow A, B and C (Fig.2). Besidesthese, two other salt structures will also be referred in theinterpretation below, the Kegns Pillow (Clausen andPedersen1999) and the Fehmarn Pillow (Nielsen1998).The positions of these salt structures are shown on

Fig.2 along with the sub-salt fault trends (Vejbk1997).

Two different fault systems cut the BMT surface: onestriking NS crossing Salt pillows A and B, and onestriking E-W on top of Salt pillow C. A maximum dis-

Fig. 4 Time-migrated seismicsection HE02-33 (see Fig. 2 forlocation) and interpretedsection crossing Salt Pillow B inthe southwestern corner of theBay of Kiel. The section showsthat the fault system above thesalt structure developed thin-skinned as a result ofhalokinesis.TZTop Zechstein,BMTBase Middle Triassic,

BUTBase Upper Triassic,IUT1 and 2 Internal UpperTriassic 1 and 2, MJUMidJurassic Unconformity, BUCBase Upper Cretaceous, IUC 1and 2 Internal UpperCretaceous 1 and 2, BCBaseCenozoic, IC 1 and 2 InternalCenozoic 1 and 2

Fig. 5 Time-migrated seismic section (compilation of line DA98-29and line DA99-18) and interpreted section (see Fig. 2 for location)of an E-W directed line across the Bay of Kiel. The generaldistribution reveals how the angular Mid Jurassic Unconformityseparates the Triassic and the Jurassic successions from the

Cretaceous and the Cenozoic successions. The entire Mesozoicand Cenozoic succession have been deposited under the influence ofhalokinesis. TZTop Zechstein, BMT Base Middle Triassic, BUTBase Upper Triassic, IUT1 and 2 Internal Upper Triassic 1 and 2,BJ Base Jurassic, MJU Mid Jurassic Unconformity, BUC BaseUpper Cretaceous,IUC 1 and 2 Internal Upper Cretaceous 1 and 2,BCBase Cenozoic, IC 1 and 2 Internal Cenozoic 1 and 2

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placement of more than 200 ms TWT on the N-Sstriking fault system is seen on top of Salt Pillow B. Themaximum displacement of the EW striking fault systemis seen to be a little more than 100 ms TWT.

As with the BMT surface, the MJU surface (Fig. 8)also shows a general dip from the north-northwest to-wards the south-southeast. The surface dip is disturbed bya few anomalies in the western part of the study area.Again these are interpreted to be a result of salt with-drawal from the deeper areas, into Salt Pillows A and B.Another clear anomaly is seen in the eastern part of theregion around the Fehmarn Salt Pillow. The shallowestparts of the MJU surface (less than 400 ms TWT) is foundon top Salt Pillow B and the Fehmarn Salt Pillow, whilethe deepest part (more than 1300 ms TWT) is found in thesoutheastern corner towards the centre of the basin.

The MJU surface is cut by the same N-S trendingfault system as the BMT surface (Fig. 4, 5and 7). Themaximum displacement is more than 100 ms TWT ontop of Salt Pillow B. The EW striking fault system onthe BMT surface is seen to terminate underneath thissurface (Fig.6), showing that the active faulting within

this system ceased before the Albian Transgression. OnFig.6, it is seen that one fault in the EW striking sys-tem cuts the MJU surface with a minor displacement,but this has only been detected on this particular seismicsection and is therefore not considered as a general faulttrend in the post MJU sediments.

Early Triassic (Buntsandstein)

Because of the limited resolution of the Top Zechsteinreflector it is difficult to conclude on the general distri-

Fig. 7 Time-structure map ofthe Base Middle Triassic (BaseMuschelkalk) surface in msTWT. Contour interval is100 ms. Wells referred to in thetext are also shown. The orangecolour shows the shallowestpart of the surface on top ofSalt Pillow B whereas thepurple colour shows the deepestpart in the southwestern corner

Fig. 6 Time-migrated seismic section DA98-22 (see Fig.2 forlocation) and interpreted section crossing Salt Pillow C in thenorthern part of the Bay of Kiel. The section reveals how most ofthe individual faults of the fault system on top of the salt structureterminate at the Mid Jurassic Unconformity. TZ Top Zechstein,BMTBase Middle Triassic, BUTBase Upper Triassic, IUT1 and 2Internal Upper Triassic 1 and 2, BJ Base Jurassic, MJU MidJurassic Unconformity,BUCBase Upper Cretaceous,IUC 1 and 2Internal Upper Cretaceous 1 and 2, BCBase Cenozoic,IC 1 and 2Internal Cenozoic 1 and 2

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bution of the Buntsandstein sequence in the Bay of Kiel.Figure5 shows a thickening of the succession in atwestwards direction, without any thinning anomaly onthe northern flank of Salt Pillow B. The same trend isseen across Salt Pillow C (Fig.6). Based on theseobservations, it is supposed that the Lower Triassicterrestrial red beds were deposited in the Bay of Kielduring a period of relative tectonic quiescence. Thewestward thickening is a result of greater subsidence

towards the centre of the NGB southwest of the studyarea.

Middle Triassic (Muschelkalk)

The time-isochore map of the Middle Triassic (Fig.9)shows an overall thickness of between 100 ms and300 ms TWT, with an average of between 200 and

Fig. 8 Time-structure map ofthe MJU surface in ms TWT.Contour interval is 100 ms.Wells referred to in the text arealso shown. The dark orangecolour shows the shallowestparts of the surface on top ofSalt Pillow B and the FehmarnPillow whereas the blue colourshows the deepest part in thesouthwestern corner

Fig. 9 Time-isochore map ofthe Middle Triassic(Muschelkalk) succession in msTWT. Contour interval is50 ms. Wells referred to in thetext are also shown. Theorangecolour shows the thinnest partof the succession on top of SaltPillow B whereas the light greencolour shows the thickest parts.Thethick grey line around SaltPillow B marks the area where

the succession has been exposedto later erosion

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250 ms. The higher degree of subsidence away from thebasin margin is seen as a general thickening trend to-wards the centre of the basin in the southwards direc-tion. The three thickness anomalies observed over theKegns Pillow and Salt Pillow A and B provide a clearindication that the initial vertical salt movement intothese structures began during the Middle Triassic. Thisis supported by the slight thinning of the Middle Triassicsuccession over Salt Pillow B in the BUT restored sec-tion (Fig.14a).

It should be mentioned that the thin succession ontop of Salt Pillow B is the result of later erosion, (seeFig.14d), and not due to a more accelerated halokinesisin this particular structure.

The randomly distributed four areas of thickerMiddle Triassic successions around Salt Pillow A and Bthan on top of the structures, gives no specific indicationof the direction of the salt movement. It is supposed thatthe Muschelkalk sequence was deposited under constantsubsidence and under the influence of salt movementinitiation in the western part of the Bay of Kiel.

Late Triassic (Keuper)

Due to the erosion of the Upper Triassic sequence(Fig.10), in the western part of the study area, it has notbeen possible to describe an overall distribution patternfor this succession. In the areas without erosion, thick-nesses of more than 300 ms TWT are seen, with anexception on top of the Fehmarn Pillow. Within the areaof erosion, the thickness varies from zero on top of SaltPillow B to more than 450 ms TWT in the southern partof the Bay of Kiel. The thinnest successions are found on

top of Salt Pillows A, B and C and the thickest succes-sions in the local depocentres between the salt structures.

The more abrupt thickness variation of the successionaround the EW striking fault system on top of SaltPillow C shows that this system developed during theLate Triassic.

Even though the Upper Triassic succession was latersubject to erosion (Fig. 4,5and6), the general present-day thickness variation throughout the study area showsthat the sequence was deposited under the influence ofvertical salt movement from the areas with a thick suc-cession into the salt structures. This interpretation issupported by the clear thinning of the succession boundby the internal Upper Triassic horizons on top of SaltPillow B (Fig. 14).

By comparing the position of the depocentres withthe position of the salt structures, it can be seen thatthe evaporites presumably moved in a preferred eastwest direction into the structures an indication for aneast-west main stress direction. This is supported by thedirection of the eastwest striking fault system overSalt Pillow C. This fault system developed as a result

of localised northsouth directed thin-skin extensionover the structure, resulting from the upward saltmovement.

Early Jurassic

According to Nielsen and Japsen (1991), the Jurassicsediments recovered from the four deep wells in thestudy area are all of Early Jurassic age and the upperboundary succession is characterised by an erosionalsurface of presumably Middle Jurassic age. Figure 11,

Fig. 10 Time-isochore map ofthe Upper Triassic (Keuper)succession in ms TWT. Contourinterval is 50 ms. Wells referredto in the text are also shown.Thered-orange coloursshow thethinnest parts of the successionon top of Salt Pillow A, B andC whereas the dark green colourshows the thickest parts in therim synclines between the saltstructures. The thick grey linemarks the area where thesuccession has been exposed tolater erosion

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therefore only shows the present day thickness variationof the Lower Jurassic sequence in the Bay of Kiel.

Lower Jurassic sediments are preserved in two areasin the western part of the region, and in the entireeastern part. On Fig.11, it is seen that the two EarlyJurassic packages in the western part of the study areaare located in rim synclines between salt structures. Thisshows that the centre of the uplifted area is located tothe west of the study area, due to the higher degree oferosion in this direction. The preservation of LowerJurassic sediments in the rim synclines between the saltstructures also suggests that this succession was depos-ited under the influence of halokinesis. This is deducedusing the assumption that the erosion occurred more orless horizontally (Fig.14b).

Middle JurassicEarly Cretaceous

Due to the absence of Middle and Upper Jurassic andLower Cretaceous sediments, it can only be concludedthat the study region was an area of non-deposition

during these time intervals. It cannot be determinedwhether the upward salt movement, detected during theLate Triassic and Early Jurassic, continued throughoutthe Middle and Late Jurassic or if it ceased at the onsetof the erosional event. The reason for this is that theMJU flattened section (Fig.14b) reflects the basingeometry at the end of the period of non-deposition.

Cretaceous

The Cretaceous succession (Fig.12) generally thickenstowards the centre of the NGB in the south to south-

west, which again is interpreted as the result of greatersubsidence in the central part of the NGB. Minor vari-ations are observed on top of the Fehmarn Pillow andSalt Pillow A, while more pronounced thickness varia-tions are seen on the top of Salt Pillow B and in the areasto the south and to the west of the structure. Thesevariations are the result of halokinesis.

On all the interpreted seismic sections (Fig. 4, 5and6), it is seen that the thin Lower Cretaceous successionwas deposited with a very uniform thickness throughoutthe Bay of Kiel. This observation is supported by theBase Cenozoic restored section of the HE02-33 line(Fig.14c). This shows that the sedimentation resumed inthe Bay of Kiel during a period of tectonic quiescence.

Figure14c also shows that the thinning over SaltPillow B only can be recognised in the upper succession,bound by the IUC1 and the BC horizons, of the UpperCretaceous sequence. Thus, it can be concluded that thetectonic quiescence detected at the time of resumedsedimentation continued towards the end of the LateCretaceous where several salt structures were reactivatedin the study area.

Cenozoic

Like the previously described successions, the distribu-tion of the Cenozoic sediments also shows an overallthickening trend towards the centre of the basin in thesouth to southwest (Fig13). Two pronounced thicknessanomalies are seen on top of the Fehmarn Pillow and ontop of Salt Pillow B. On the very top of both structures,the Cenozoic succession is actually absent. Furthermorea minor anomaly is seen on top of Salt Pillow A. In

Fig. 11 Time-isochore map ofthe Jurassic succession in msTWT. Contour interval is50 ms. Wells referred to in thetext are also shown. The mapshows that Jurassic sedimentsare preserved in the eastern partof the study area and in twopockets in the western part

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addition to the southwestern corner, the thickest suc-cessions are found in the rim synclines between theKegns Pillow, Salt Pillow A and Salt Pillow B andsoutheast of Salt Pillow B. More abrupt thickness vari-ations are detected around the N-S striking fault systemcrossing Salt Pillow A and B, showing that activefaulting took place within this system during the Ceno-zoic. The thickness anomalies show that the reactivated

vertical salt movement observed towards the end of theLate Cretaceous in the structures must have continuedthroughout the Cenozoic Era (Fig.4,5,6,13and14d).

On the isochore map of the Cenozoic sequence(Fig.13), it can be seen that the depocentres in the rimsynclines between the salt structures reflect a WNWESE striking trend. This indicates that the main stressdirection at this time was SSWNNE. The strike

Fig. 12 Time-isochore map ofthe Cretaceous succession in msTWT. Contour interval is50 ms. Wells referred to in thetext are also shown. Theorangecolour shows the thinnest partof the succession on top of theFehmarn Pillow whereas the

green colour shows the thickestparts in the southwestern corner

Fig. 13 Time-isochore map ofthe Cenozoic succession in msTWT. Contour interval is50 ms. Wells referred to in thetext are also shown. The redcolour shows the thinnest partsof the succession on top of theSlat Pillow B and the FehmarnPillow and the white colourwhere the succession is absent.Thepurple colour shows thethickest part in thesouthwestern corner

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direction of the fault system above Salt Pillow A and B(Fig.7, 8 and 13) also supports this. This fault systemdeveloped as a thin-skinned extension on top of thestructures as a result of the reactivated vertical saltmovement in the region (Fig.14d). Due to the poorlydefined reflection pattern within the Cenozoic successionon the seismic profiles and the limited description of theCenozoic sediments in the reports from the explorationwells in the area (Gearhart Geo Consultants 1985;Nielsen and Japsen 1991), it can not be determinedwhether the vertical salt movement ceased sometimeduring the Cenozoic Era or if it is still ongoing at thepresent day.

Discussion and conclusions

The results of this seismic interpretation of the Bay ofKiel show that the post-Palaeozoic sedimentary depo-sition took place during two different periods of gener-ally continuous subsidence in the northern part of theNGB. These periods were separated by a period ofpronounced uplift and erosion from the Middle Jurassic

and towards the end of the Early Cretaceous. Thedeposition during both periods of subsidence took placeunder the influence of halokinesis controlled by the re-gional stress field. The observed sedimentation during

Fig. 14 Structuralreconstruction of the seismicline HE02-33 (Fig. 4) (see Fig. 2for location).adThese parts ofthe figure shows the geometricreconstruction of the section. bThis part of the figure showsthat the initial salt movement

took place during the LateTriassic followed bypronounced erosion.dThis partof the figure shows that the saltmovement was reactivatedduring the Cenozoic andcoursed the thin-skinnedfaulting above the saltstructure.TZTop Zechstein,BMTBase Middle Triassic,BUTBase Upper Triassic,IUT1 and 2 Internal UpperTriassic 1 and 2, MJUMidJurassic Unconformity, BUCBase Upper Cretaceous, IUC 1and 2 Internal Upper

Cretaceous 1 and 2, BCBaseCenozoic, IC 1 and 2 InternalCenozoic 1 and 2

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the Early and Middle Triassic without major tectonicactivity is confirmed by the observations by others in theadjacent areas of the Bay of Kiel (e.g. Scheck and Bayer1999; Kossow and Krawczyk2002; Scheck et al. 2003a).

The initiation of vertical salt movement on the tran-sition between the Middle and Late Triassic creating NS trending depocentres and salt structures correlateswith the onset of a regional EW directed extension.This extension is indicated by accelerated subsidenceand basement-affected normal faulting in the NStrending Central Graben and Horn Graben in the NorthSea area, the nearby Glu ckstadt Graben west of thestudy area, and the development of the RheinsbergTrough to the southeast of the Bay of Kiel (Ziegler1990;Fisher and Mudge1998; Scheck et al. 2003b). The EWdirected regional extension during the Late Triassic andEarly Jurassic agrees with the assumption that the EWstriking fault system on top of Salt Pillow C (Fig. 6, 7and10) developed thin-skinned as a result of vertical saltmovement, and not as a basement controlled structure.

The Middle Jurassic to Early Cretaceous period ofuplift and erosion removed parts of the Lower Jurassic

and Upper Triassic successions within the study area,can be correlated with the development of the CentralNorth Sea Dome due to plutonic activity (Ziegler 1990,Underhill 1998). The uplifted area, as a result of theplume, was centred upon what was to become the NorthSea rift triple junction (Underhill 1998). This explainsthe higher degree of erosion in the westwards directionin the Bay of Kiel towards the centre of the dome.Others have reported the adjacent Northeast GermanBasin to be an area of non-deposition during the Middleand Upper Jurassic and Lower Cretaceous (Kossowet al. 2000; Kossow and Krawczyk 2002; Scheck et al.2003b), which is also seen on the Ringkbing-Fyn High

to the north (Ziegler1990). This correlates well with theobservations from this study area.

The resumed sedimentation towards the end of theEarly Cretaceous and the continued subsidence withoutmajor tectonic activity during the Late Cretaceous isconfirmed by regional scale observations (e.g. Ziegler1990).

The reactivated vertical salt movements observed onthe Cretaceous Cenozoic transition were initiated due toa change in the regional stress field from extensional tocompressional due to the onset of the Alpine Orogenesis(Ziegler 1990; Gemmer et al. 2003). This explains theWNW-ESE striking long axial trend of the depocentres

between the salt structures during the deposition of theCenozoic succession (Fig.13). The direction can becompared with the direction of the folds observed in theNortheast German Basin by Scheck et al. (2003a). TheNNESSW directed compressive stress field also con-firms that the NS striking fault system upon Salt PillowA and B (Fig.4,5,7,8,13,14d) developed thin-skinnedas a result of halokinesis.

Acknowledgements The authors would like to thank Per Trinham-mer for his technical assistance during the data acquisition, Egon

Nrmark for the processing of the Baltseis surveys and his assis-tance during the processing of the NeoBaltic surveys, Ole RnClausen for the fruitful discussions and Kathryn Brookes for herconstructive comments. The Baltseis surveys were funded by theDanish Natural Science Research Council (grant no. 970176,9802129 and 9901810). The NeoBaltic project is funded by theGerman Research Foundation (DFG) grant no. HU698/7-1 in theframe of the Special Research Project 1135. The authors would liketo thank two anonymous reviewers for their help in improving themanuscript.

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