Geological Survey of Denmark and Greenland Bulletin 20, 2010, 31–34 31 Following the proposal of the offshore Anholt wind-farm project with an energy capacity of 400 megawatt in the Kat- tegat, southern Scandinavia, an evaluation of the geotechni- cal properties of the subsurface of the area is required. As a first step to map the seabed geology the Geological Survey of Denmark and Greenland (GEUS) conducted a geophysical survey (Leth et al. 2009) which, together with cone penetra- tion tests and data from boreholes, lead to a greater under- standing of the geological architecture and development of the 144 km2 survey area (Figs 1, 2). Methods We used a multibeam echo-sounder for detailed mapping of the bathymetry, and shallow seismic equipment and coring to map the shallow seabed geology including the distribution and thickness of the main geological units. A combination of two seismic devices (chirp and sparker systems) was chosen to ensure good penetration and high resolution. The sparker system provides data from the sea floor down to about 45 m into the seabed with a vertical seismic resolution in the order of 50 cm, while the chirp system provides high-resolution seismic data from the upper 5–10 m of the seabed with a ver- tical seismic resolution at decimetre scale. A side-scan sonar was used for mapping of surface sediments. At seven sites, boreholes were made to a depth of 40 m and selected inter- vals sampled. Cone penetration tests were carried out at the same sites (Fig. 2) and surface sediment samples collected for biological studies. Geological setting and seabed The Kattegat region is located in the transition zone between the Fennoscandian Shield and the Danish Basin (Fig. 1), and studies of the pre-Quaternary surface morphology show that the NW–SE-trending anticlinorium follows the trend of Late Quaternary geology of a potential wind-farm area in the Kattegat, southern Scandinavia Jørgen O. Leth and Bernhard Novak BH-09 BH-01 BH-05 BH-02 56°33´N 11°15´E 2 km 14.5–15.0 15.0–15.5 15.5–16.0 16.0–16.5 16.5–17.0 17.0–17.5 17.5–18.0 18.0–18.5 18.5–19.0 19.0–19.5 Depth (m) Fig. 2. Bathymetry of the survey area. The stars show sites with combined coring and cone penetration tests. The dashed line shows the position of the profile in Fig. 3. Sweden Jylland Sjælland Kattegat Precambrian and Palaeozoic Lower Cretaceous Upper Cretaceous Triassic Tertiary Jurassic 10°E 57°30´ 56°N 13°E 50 km ? Study area Lysegrund Fennoscandian Shield Danish Basin Fig. 1. Pre-Quaternary geology of the Kattegat. Redrawn and simplified from Lykke-Andersen et al. (1993). © GEUS, 2010. Geological Survey of Denmark and Greenland Bulletin 20, 31–34. Open Access: www.geus.dk/publications/bull 3232 dextral wrench faults that have repeatedly affected the Fen- noscandian Border Zone (Lykke-Andersen et al. 1993). The latest major tectonic event was an inversion episode which started in the Late Cretaceous. After that inversion the Kat- tegat became an area of non-deposition and net-erosion until net-sedimentation was resumed in the Saalian. At that time the basin floor of the Kattegat was characterised by strongly undulating relief controlled by large- and small-scale struc- tures in the pre-Quaternary basement. The survey area can be divided into several subareas, based on bathymetry and seabed sediment types (Fig. 2). The northern part of the survey area is smooth, with sand and silt. The central and southern parts show ridges with a relief of more than 1 m. It is suggested that the ridges were formed by waves at a time of lower than present sea level (Leth et al. 2009). The area with the highest density of gravel and boul- ders corresponds to the shallowest part of the central survey area. The boulders are generally located in arc-like, NW– SE-striking, narrow structures found mainly in the southern and western parts of the survey area (Figs 1, 2). Depths over 18 m characterised by sand and silt with pebbles occur near the western and eastern margins of the survey area. Pre-Quaternary strata – Unit PQ The pre-Quaternary surface is a regional erosional uncon- formity with high amplitude seismic reflection. The reflec- tor has been mapped throughout the area down to the limit of the penetration of the sparker system at c. 60 m below sea level (b.s.l.; Figs 3, 4D). The pre-Quaternary surface lies deeper than 60 m b.s.l. in the eastern part of the area. Two boreholes penetrated several metres of pre-Quater- nary silty, fine sand. Analysis of palynomorphs in two sam- ples yielded an Upper Cretaceous age (K. Dybkær and E. Sheldon, personal communication 2009). Glacial deposits – Unit GL The glacial deposits have been divided into two subunits, GL1 and GL2. The lower subunit GL1 is found in the central and southern parts of the survey area. It shows a character- istic medium- to low-amplitude, parallel, wavy and chaotic seismic pattern. The parallel or wavy pattern is interpreted as representing undisturbed sorted and layered sediments, whereas the wavy or chaotic pattern is interpreted as repre- senting glacially dislocated sediments. A unit with a similar seismic character as GL1 has been found in various parts of the Kattegat, and has been referred to the Late Saalian, Eemian and Middle Weichselian (Vangkilde-Pedersen et al. 1993). Marine and glaciogene sediments of these ages have been recorded in sediment cores from other parts of the Kat- tegat region (Larsen et al. 2009). The transition to seismic subunit GL2 is sharp in the southern part of the survey area, whereas it is more gradual in the central part. Internally GL2 shows a medium- to high- amplitude, seismic facies pattern that is chaotic, mounded and channelled. Low-angle oblique reflectors cutting through the whole subunit are interpreted as large-scale, glaciotectonic deformation structures. Data from sediment samples show that the subunit mainly consists of sand with poorly and well- sorted layers of clay, silt and gravel. A high density of cobbles and boulders is seen where GL2 crops out on the sea floor, as confirmed by surface samples (Fig. 4A). A unit showing a similar seismic pattern and with similar deposits from the Lysegrund area (Fig. 1) has been interpreted as subglacial and glaciofluvial deposits. The GL2 subunit probably cor- responds to the M3 unit at Lysegrund, which consists of ice- margin sediments deposited during the retreat stage of the Main Advance (Novak 1996). The seismic pattern, facies as- sociation, unit morphology, sea-floor character and lithology all suggest that GL2 represents similar ice marginal deposits. 2 km GL2 LG1 LG2 PQ GL1 Fault H2 GL2 GL1 LG1 H1 Gas No Data North South LG2 BH-05BH-01BH-02BH-09 40 50 60 D ep th ( m b el o w s ea l ev el ) 30 20 PQ Fig. 3. Schematic model of the seismic units based on a N–S-trending section along UTM 634000 mE (WGS84). The unit names refer to descriptions in the text. The boreholes (BH-01, BH-02, BH-05 and BH-09) are located at distances from 400 m to 1.5 km from the profile (Fig. 2). 33 Late glacial deposits – Unit LG The distribution of the late glacial seismic unit LG is gov- erned by the morphology of the underlying glacial surface (Fig. 4C). Its maximum thickness is 45 m. Towards the south the depressions in the glacial surface are characterised by shallow channels and small basins, and it is possible to cor- relate these depressions to a system of elongated fault-related basins in the pre-Quaternary surface SW of Anholt (Binzer & Stockmarr 1994). The seismic unit LG is subdivided into two subunits, LG1 and LG2, with a gradational boundary. LG1 shows an exter- nal apron or mound morphology with internal composite mounds as well as a hummocky, shingled, parallel reflection pattern. At its base, LG2 shows an onlap-downlap, draping style. Upwards it shows a gradually decreasing amplitude and a more pronounced semi-transparent, parallel seismic facies. In general, LG2 terminates upwards into an erosional un- conformity. A seismic ‘blacking out’ area internally in LG2 indicates gas content in a discrete level associated with a pro- nounced reflector that probably represents a sealing clay layer. Data from boreholes show that unit LG consists of a fining-upward sequence with sand and gravel at its base and layers of clay with sand and silt laminae towards its top. The characteristic seismic expressions of unit LG have also been recorded from other parts of southern Kattegat. For instance, Jensen et al. (2002) reported two stages in a Late Glacial unit located in elongated fault-related basins and suggested that the two stages are related to re-activation of normal fault ac- tivity in the elongated depressions in the period from 15 to 13.5 calendar ka BP. Holocene deposits – Unit H The transition from the late glacial to the Holocene unit H is seen as a shift to high-amplitude reflectors. Truncation of the rhythmic parallel facies of LG2 is succeeded by mounds, hummocky oblique and sub-parallel reflector patterns in the Holocene subunit H1. This subunit was previously referred to the Late Glacial (Leth et al. 2009), but after reassessment of the abrupt changes in the seismic signature and its distri- bution we conclude that the subunit is of early Holocene age. In the southern part of the survey area, 400–600 m wide channels orientated WSW–ENE and SSW–NNE are filled with Late Glacial and early Holocene (H1) deposits. The base level of H1 in these channels is around 22 m b.s.l. H1 is de- posited above the truncated LG2 unit and its distribution is confined by the older LG basins. The same base level is found in wider areas in the north and is likewise unconformable to the underlying LG unit. A significantly deeper channel crosses the central survey area with internal seismic struc- tures that indicate a unidirectional flow from west to east (H2; Fig 3). The base of this channel is generally at 26–28 m b.s.l. Locally the channel widens to 1000 m. Organic-rich sediments of Holocene age representing a lowstand at 35 m b.s.l. are well-known from the Kattegat area. After a fluvial event west of Lysegrund the water level stabilised at 34 m b.s.l. (Novak & Björk 1998). Lagoonal de- posits overlying truncated, rhythmic, Late Glacial clay-sand layers are found south-east of the survey area (Bennike et al. 2000; Novak & Pedersen 2000). In a major area these sediments are mostly found between 35 and 24 m b.s.l. and have been dated to the early Holocene (Bennike et al. 2000). DA CB 25–30 m 30–35 m 35–40 m 40–45 m 45–50 m 50–55 m 55–60 m no data 5 km 56°33´N 11°15´E Gravel-pebbles 25–100% boulders Sand-pebbles 1–25% boulders Sand and silt, pebbly solitary boulders Sand and silt 26–28 m 24–26 m 22–24 m 20–22 m 18–20 m <18 m 20–25 m 25–30 m 35–40 m >45 m 30–35 m 40–45 m Fig. 4. Maps showing seabed sediments and three seismic stratigraphic levels. A: Seabed sediment types. B: Depth of base Holocene. Black line indicates fault. C: Depth to the top of the glacial deposits. The dashed line shows the disturbed–undisturbed GL1 interface (see text). D: Pre-Quaternary surface morphology. Black line indicates fault. 3434 Lagoonal sediments at 18 m b.s.l., found 30 km east of the study area, were also dated to the early Holocene (Novak & Pedersen 2000). At most sites H2 only represents a veneer of fine- to coarse-grained sand with gravel, occasionally with silt and clay laminae as well as shell fragments of marine molluscs. However, in one of the samples marine shells and organic material are found at 9 m below the sea floor. It is suggested that the Early Holocene transgression reached a level high enough to submerge the survey area prior to 9.9 calendar ka BP. A channel in the central part of the survey area drained towards the east. Structural features In the central survey area, reflectors in the sparker profiles indicate listric normal faults dipping south-wards (Fig. 5). A 2.9 km long, E–W-striking sea-floor lineament (Fig. 4B, C) represents the top of the headwall scar, and the fault has a significant signature through the whole Quaternary pack- age. The location and strike of the faults follow the struc- tures in the pre-Quaternary basement (Binzer & Stockmarr 1994), which indicates that they represent re-activations of old faults. Final remarks Based on geophysical data, boreholes and cone penetration tests we have documented that the geological architecture of the study area is very complex. The pre-Quaternary (Late Cretaceous) basement is overlain by two glacial, two Late Glacial and two Holocene subunits. The widespread Late Glacial deposits that are up to 45 m thick are dominated by fine-grained sediments, in some areas with gas. The distri- bution of the Holocene deposits indicates the presence of channels that drained into the deeper part of the Kattegat during the early Holocene. The data described here are of great importance to the geotechnical evaluation prior to the planned foundation of windmills. Acknowledgements We thank Energinet.dk for permission to use the above data and to publish the geological results from the Anholt wind-farm project. References Bennike, O., Jensen, J.B., Konradi, P.B., Lemke, W. & Heinemeier 2000: Early Holocene drowned lagoonal deposits from the Kattegat, southern Scandinavia. Boreas 29, 272–286. Binzer, K. & Stockmarr, J. 1994: Geological map of Denmark, 1:500 000. Pre-Quaternary surface topography of Denmark. Danmarks Geolo- giske Undersøgelse Kortserie 44, 10 pp., 2 maps. Jensen, J.B., Petersen, K.S., Konradi, P., Kuijpers, A., Bennike, O., Lemke, W. & Endler, R. 2002: Neotectonics, sea-level changes and biological evolution in the Fennoscandian Border Zone of the southern Kattegat Sea. Boreas 31, 133–150. Larsen, N.K., Knudsen, K.L., Krohn, C.F., Kronborg, C., Murray, A.S. & Nielsen, O.B. 2009: Late Quaternary ice sheet, lake and sea history of southwest Scandinavia – a synthesis. Boreas 38, 732–761. Leth, J.O., Alhamdani, Z., Novak, B., Barzani, S.M. & Hindrichsen, C. 2009: Anholt offshore wind farm. Marine geophysical investigations. Danmarks og Grønlands Geologiske Undersøgelse Rapport 2009/45, 411 pp. Lykke-Andersen, H., Knudsen, K.L. & Christiansen, C. 1993: The Qua- ternary of the Kattegat area, Scandinavia: a review. Boreas 22, 269–281. Novak, B. 1996: En maringeologisk undersøgelse af kvartære lag på Lyse- grund, sydlige Kattegat, Danmark. Geologisk Tidsskrift 2, 21–25. Novak, B & Björck, S. 1998: Marine seismic studies in southern Kattegat, with special emphasis on longitudinal bars and their possible relation- ship to the drainage of the Ancylus Lake. GFF 120, 297–306. Stock- holm: Geological Society of Sweden. Novak, B. & Pedersen, G.K. 2000: Sedimentology, seismic facies and stratigraphy of a Holocene spit-platform complex interpreted from high-resolution shallow seismics, Lysegrund, southern Kattegat, Den- mark. Marine Geology 162, 317–335. Vangkilde-Pedersen, T., Lykke-Andersen, H. & Lind, G. 1993: Dislocated Quaternary deposits in southeastern Kattegat – a glacial or gravitation- al phenomenon? Boreas 22, 329–336. 20 40 60 Tw o -w ay t ra ve l ti m e (m se c) NN SSN S 100 m MultipleMultipleMultiple 15 30 45 D ep th (m ) Fig. 5. Part of the N –S-orientated seismic profile AW039 showing a fault that dips to the south. A ridge is seen along the top of the fault. Note the subsidence of the seabed above the fault. Authors’ address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: jol@geus.dk