Geology of the Femern Bælt area between Denmark and Germany

Geological and geotechnical investigations in the Femern Bælt area were undertaken from 1995 to 2010 (Rambøll Arup JV 2011) in preparation for the fixed link between Lolland in Denmark and Fehmarn in Germany. As a result, new data have been acquired on the stratigraphy and distribution of the deposits and the major structures and tectonic influence on the layers close to the surface. Previous investigations of Cretaceous–Palaeogene deposits on southern Lolland (Fig. 1) were limited due to lack of outcrops and borehole data. Two deep boreholes and geophysical surveys (1952– 1953) revealed: (1) the presence of a salt diapir at Rødbyhavn, (2) upper Maastrichtian chalk 29–143 m below Quaternary deposits and (3) an erosional window in the Palaeogene cover. Boreholes to the east of Rødbyhavn (1992–1994) revealed the sediment distribution on southern Lolland and showed that Cretaceous and Palaeogene deposits are cut by several NW–SE-orientated faults. This paper presents a summary of lithostratigraphic and biostratigraphic investigations and a brief description of the geological development in the area.


Biostratigraphy and lithostratigraphy
A chronostratigraphic, biostratigraphic and lithostratigraphic correlation is presented in Fig. 2. The lithologies and selected marker fossils reported in this article are as follows (see also Fig. 3): Tor Formation equivalent (Campanian-Maastrichtian) -The Tor Formation of the North Sea was originally described by Deegan & Scull (1977) as homogeneous white or grey chalk, bioturbated pelagic chalk, laminated chalk and redeposited chalk.The Femern Baelt area chalk is slightly indurated, with <5% flint nodules.
Danian limestone (lower-middle Danian) -comprises fossiliferous limestone, rich in echinoderm, bryozoan and bivalve fragments and is recognised as a thin layer on Lolland (DGU core 241.213).
Ølst Formation (lower Ypresian) -The Ølst Formation (Heilmann- Clausen et al. 1985) is a grey to almost black, sandy, silty and mainly non-calcareous clay, interbedded with layers and laminae of black or grey volcanic ash.
Røsnaes Clay Formation (middle Ypresian) -The Røsnaes Clay Formation (Dinesen et al. 1977;Heilmann-Clausen et al. 1985) comprises red, brown and yellow-brown, very finegrained, plastic marine clay with glauconitic beds at the base and greenish volcanic ash layers throughout.Lillebaelt Clay Formation (upper Ypresian -lower Lutetian) -The Lillebaelt Clay Formation is a green-grey, non-calcareous, very fine-grained plastic clay containing concretions, with red-brown clay beds in the lower part (Dinesen et al. 1977;Heilmann-Clausen et al. 1985).

Geological development
Lolland is located to the south of the Ringkøbing-Fyn High, in the German Basin.During the Late Cretaceaous, fully marine conditions characterised the Danish area including the northern highs and chalk of the Campanian -upper Maastrichtian Tor Formation equivalent was deposited.Danian limestone and Lellinge Greensand have recently been found in situ on southern Lolland and are documented here for the first time.The Danian limestone is c. 2 m thick.Danian nannofosssils and microfossils are also found in small limestone clasts within the Selandian deposits, indicating that Danian sediments were eroded and partially redeposited at the end of the Danian and probably also in the earliest Selandian.
The lower Selandian glauconitic and conglomeratic deposits of the Lellinge Greensand rest on Danian sediments on Sjaelland, especially in the Copenhagen area (Stouge et al. 2000).Clasts, microfossils and nannofossils from the lower Selandian also indicate deposits from the Lellinge Greensand in the Femern Baelt area.The Lellinge Greensand is at least 12-13 m thick; the top is eroded and covered by Quaternary deposits.In DGU core 241.213 the Lellinge Greensand is found 138 m below ground surface.The encountered glauconitic deposits from the Lelllinge Greensand rest on Danian limestone in a small subsided fault block, demonstrating that the lowermost Selandian was probably deposited in the area and subsequently eroded.
The early-middle Paleocene sea was more restricted than in the Cretaceous and several highs bordered the marine area (Clausen & Huuse 2002).The Danian was dominated by erosion, rising sea level characterised the early Selandian and erosion dominated again during the late early Selandian.These two erosional episodes may have resulted in sediment removal, but it is also possible that the area was subaerial during part of the Danian-Selandian.Continued erosion or nondeposition characterised the upper Selandian and therefore deposits from the Kerteminde Marl Formation (up to 100 m thick on Sjaelland) are not found in the area, probably due to uplift during this time.
Clastic marine sedimentation resumed during the upper Selandian sea-level rise, and the North Sea, Denmark and the German Basin formed a partly enclosed shelf area.
The remainder of the Palaeogene (AEbelø, Holmehus, Ølst, Røsnaes Clay and Lillebaelt Clay) formations mainly consist of fine-grained, plastic clay but there were many lithological shifts during this time.Deposition occurred in a relatively deep marine basin, although proximal to the shore.The changes in depositional conditions were related to changing circulation patterns in the North Sea and shifts in clay mineral provenance and periods of regression and non-deposition also occurred (Heilmann- Clausen et al. 1985).Intense volcanic activity caused by the opening of the North Atlantic resulted in ash deposition during the late Paleocene and Eocene.Quaternary glacial activity eroded, faulted and folded the Palaeogene sediments resulting in their present distribution (Fig. 4).The new biostratigraphic studies indicate that the Røsnaes Clay Formation is strongly folded with repeated stratigraphy, indicating movements along faults.The youngest formation in the area, the Lillebaelt Clay Formation, occurs only as floes in the glacial sediments.On land, the Palaeogene clay seems to be undisturbed towards the north whereas disturbances increase towards the south.

Conclusions
The recent biostratigraphic study has provided important information about the pre-Quaternary deposits under the c. 18 km wide Femern Baelt, imperative to planning the construction of the Femern Baelt fixed link.The Cretaceous-Palaeogene Tor Formation equivalent, Danian limestone, Lellinge Greensand, AEbelø, Holmehus, Ølst, Røsnaes and Lillebaelt Formations have been identified in multiple boreholes using multidisciplinary biostratigraphy.This information, coupled with physical rock properties (Rambøll Arup JV 2011) allows a geological cross-section of the Femern Baelt to be established and demonstrates the complex nature of the depositional and structural history of the area.In addition, in situ Danian limestone has been discovered for the first time, along with Lellinge Greensand on Lolland in DGU core 241.213.The new data from >500 m fully cored boreholes provide an excellent basis for future detailed biostratigraphic, sedimentological and basin analysis of the Palaeogene deposits in the area.

Fig. 1 .
Fig. 1.A: Map of Denmark and northern Germany showing the proposed location of the fixed link across the Femern Baelt.B: Geological map of the Femern Baelt region (after Håkansson & Pedersen 1992).

Fig. 2 .
Fig.2.Chrono-, bio-and lithostratigraphy of the Campanian-Eocene in Denmark.Note, not all formations have been found in the investigated area.