KRIEGERS FLAK OWF GEOPHYSICAL SURVEY ... - Energinet.dk

Intended for 

Energinet.dk 

Document type 

Geophysical Survey Results 

Date 

August, 2013 

Clients Ref.: 

1000/12 

Project 

KRIEGERS FLAK & HORNS REV 3 – GEO INVESTIGATIONS 2012 

KRIEGERS FLAK OWF 

GEOPHYSICAL SURVEY 

RESULTS

KRIEGERS FLAK OWF 

GEOPHYSICAL SURVEY RESULTS 

Revision 4 

Date 15/08/2013 

Made by JS, HEH, LGG, NLR 

Checked by RUBJ, JRV, PPL 

Approved by UTN 

Description Geophysical Survey Results 

Clients Ref.: 1000/12 

Project Kriegers Flak & Horns Rev 3 - Geo Investigations 2012 

Ref 

KF_OWF_Geophysical_Survey_Results_v4.docx 

Ramboll 

Hannemanns Allé 53 

DK-2300 Copenhagen S 

Denmark 

T +45 5161 1000 

F +45 5161 1001 

www.ramboll.com

KRIEGERS FLAK OWF GEOPHYSICAL SURVEY RESULTS 

CONTENTS 

1. Summary 1 

2. Introduction 2 

3. Field Survey 3 

4. Geodesy 4 

4.1 Horizontal Datum 4 

4.2 Vertical Datum 4 

5. Geological History 5 

6. Bathymetry 7 

7. Seabed Features 9 

8. Targets 11 

8.1 Infrastructure 12 

9. Marine Archaeology 13 

10. Sub-Bottom Geology 15 

10.1 Holocene Units 15 

10.2 Pleistocene Units 17 

11. Hazards 21 

11.1 Shallow Gas/Organic Deposits 21 

11.2 Faults 22 

11.3 Soft Channel Infill 22 

12. References 23 

KF_OWF_Geophysical_Survey_Results_v4.docx


LIST OF ABBREVIATION 

BAT 

Bathymetry 

BKS 

Backscatter 

BLK 

Data Blanking area (Hazard) 

C 

Cretaceous Chalk 

CAD 

Computer Aided Design 

Ci 

Cretaceous Internal Reflector 

DBS 

Depth Below Seabed 

DVR90 Danish Vertical Reference 1990 

EEZ 

Exclusive economic zone 

ELV 

Elevation 

FT 

Flow Till 

GEUS Geological Survey of Denmark and Greenland 

HAZ 

Hazard 

HF 

Holocene Freshwater 

HM1 Holocene Marine 1 

HM2i Internal reflector within underlying Holocene Marine 2 

HM2 Holocene Marine 2 - Undistinguishable base 

HM2+3 Holocene Marine 2 and 3 

Hz 

Hertz 

IHO 5E 1a IHO Standards for Hydrographic Surveys (5 th Edition, 

2008), Special Publication No. 44, 1a order survey 

KF 

Kriegers Flak 

KHz 

Kilohertz 

Km 

Kilometre 

KMS The Danish National Survey and Cadastre 

KP 

Kilometer Point 

LT 

Lower Till 

M 

Metre 

MBES Multibeam Echo Sounder 

MV 

Motor Vessel 

nT 

Nano Teslas 

QA/QC Quality Assurance / Quality Control 

OWF Offshore Wind Farm 

SBF 

Sea Bed Features 

SBP 

Sub Bottom Profiler 

SVP 

Sound Velocity Profile 

UT 

Upper Till 

TRK 

Track 

TWTT Two Way Travel Time 

UTC 

Co-ordinated Universal Time 

UTi 

Intermittent Internal horizon within Upper Till unit 

UTM Universal Transverse Mercator 

WGS84 World Geodetic System 1984 



1 

1. SUMMARY 

A full suite of hydrographic and geophysical seabed surveys have been conducted across the entire 

Kriegers Flak Offshore Wind Farm (KF OWF) site. Although the geophysical pre-investigation area 

encompasses areas that not all will be subject for KF OWF development, including for example a sand 

abstraction area in the central part of the site, the geophysical survey area will from hereon in the 

report be referenced as the KF OWF site. 

Variations in seabed morphology can be observed from the bathymetric and side scan sonar datasets. 

The seabed sediment type has been established from the hydrographic and geophysical records, 

ground truthed by a series of grab samples. 

Infrastructure and seabed obstructions have been identified using all of the available datasets, one 

cable and three wrecks have been identified in the area. In addition a number of boulders have been 

observed. 

The water depths across the site range from 12m in the central and north eastern section of the site 

deepening at the perimeter of the site to a maximum depth of 33m in the far south of the site. The 

seabed surface is dominated by sand, with till exposures in the eastern and western part. 

The sub-bottom geology has been interpreted, and a geological model for the area has been 

established, which fits the dataset and all available other information examined. 

The major geological units identified at Kriegers Flak OWF are summarised in Table 1-1. The depths are 

derived assuming a constant acoustic velocity in the sediments of 1,900 m/sec. 

Table 1-1 Identified Geological Units 

Unit Description Depth to Base 

(m below seabed) 

Holocene Marine 1 (HM1) Blanket cover of loose fine-medium sand 0-2 

Holocene Marine 2 (HM2) 

Loose fine-medium sand, showing progradation to 

the west 

0-8 

Holocene Marine 3 (HM3) 

Fine-medium sand within a spit, likely to be more 

compacted than HM1 and HM2 

0-11 

Flow Till (FT) 

Soft sands, clay and boulders less consolidated 

than the underlying tills from which they derive 

0-15 

Upper Till (UT) 

Diamict of predominantly clay with gravel, cobbles 

and boulders but also occurrences of silt and sand 

0–26 

Lower Till (LT) 

Diamict of predominantly clay with gravel, cobbles 

and numerous boulders 

0-100 

Cretaceous Chalk (C) Chalk Undetected 



2 

2. INTRODUCTION 

Energinet.dk contracted GEMS Survey Limited to conduct a hydrographic and geophysical seabed 

survey within the Kriegers Flak region and also along an associated cable route ashore. 

The Kriegers Flak Offshore Wind Farm (KF OWF) site survey area is a shallow region (12-33m) covering 

approximately 250km² in the southern Baltic Sea located 15km east of the Danish island Møn. The 

bathymetric high called Kriegers Flak is divided by the EEZ offshore borders of Denmark, Germany, and 

Sweden. The survey has been conducted in the Danish sector only bounded by the neighbouring 

German sector along the south west site limit and the Swedish sector along the north eastern site limit. 

Figure 2-1 shows an overview of the site. A total of 2,750km of survey grid is included in the survey. 

Survey lines are performed with a line spacing of 100m. North south cross lines are performed with a 

spacing of 1,000m. 

In addition, a survey was performed of the preliminary planned subsea cable route, from the northern 

survey boundary of the KF OWF site to the beach just south of the city Ishøj, which is located 

southwest of Copenhagen. The main corridor is 75km long, of which 9km are shallower than 10m water 

depth and 2km are within the 5-m water depth contour. This survey area includes approximately 

640km of survey lines. A 900m corridor has been surveyed with a line spacing of 100m. Only the 

results from the survey of KF OWF site are included in this document. 

Figure 2-1 Kriegers Flak OWF Survey Area and Cable Route 

Unfortunately GEMS Survey went into administration on 4 December 2012, during the processing and 

reporting phase of the project. Ramboll DK has been tasked to complete the outstanding data 

processing and finalise the data deliverables and reporting. 

This Geophysical Survey Results Report provides a simplified overview of the geophysical datasets, and 

is targeted at a wide audience, who may not have previous experience of geophysical surveys. 



3 

3. FIELD SURVEY 

The geophysical survey at Kriegers Flak OWF was conducted from 29 August to 14 October 2012, with 

the survey vessel MV Aquarius. Before the commencement of the survey operations, the acquisition 

parameters were optimised during a period of equipment testing which followed the vessel 

mobilisation. Survey line data were acquired running east-west across the site with a line spacing of 

100m. Perpendicular (north-south) survey lines were performed with a spacing of 1,000m. Further 

details of the survey and the following data interpretations can be found in the supporting site specific 

reports as follows: 

Kriegers Flak OWF - Mobilisation Report (2013), Ref. 1 

Kriegers Flak OWF – Operational Survey Report (2013), Ref. 2 

Kriegers Flak OWF - Acquisition Parameter Optimisation Report (2013), Ref. 3 

Kriegers Flak OWF - Processing Optimisation Report (2013), Ref. 4 

Kriegers Flak OWF - Interpretation Optimisation Report (2013), Ref. 5 

Kriegers Flak OWF - Interpretation Survey Report (2013), Ref. 6 

The geophysical survey at KF OWF site was carried out utilising the following equipment: 

 

 

 

Positioning system (C-NAV 3050M, LD2S-G2, F180R+) 

Global Acoustic Positioning System (IXSEA GAPS system) 

Heading and Motion Sensors (F180R+ and Hydrins) 

Sound velocity probes (Mini SVP, RapidSV, CTD+ V2) 

 

 

 

 

 

 

 

 

 

Multibeam Echo Sounder (R2Sonics 2024, 400 kHz dual head transducer system) 

Side Scan Sonar (Edgetech 4200 MP 300/600 kHz) 

Gradiometer data (Geometrics G882 transverse magnetometer array) 

Grab Sampler (Van Veen grab sampler) 

Pinger System (Hull-mounted 4x4 MASSA pinger array) 

Sparker Sub-Bottom Profiler(GeoResources 6kJ Sparker) 

Mini-Airgun Sub-Bottom Profiler (10in³ I/O mini airgun) 

Reflection seismic multichannel streamer (48 ch. spaced 2.5m) 

Refraction seismic (4x10 in³ airguns and streamer with 96 ch. spaced 12.5m) 

Based on available records from GEMS Survey, all equipment accuracies were verified to comply with 

the requirements of the IHO Standards for Hydrographic Surveys (5 th Edition, 2008), Special 

Publication No. 44, Ref. 7, abbreviated to as IHO 5E 1a. 

According to Ref. 7, the general requirements to an IHO 5E 1a order survey for the surveyed area with 

water depths between 12m and 33m are defined as: 

 

The maximum allowable total horizontal uncertainty within a 95% confidence level shall be 

better than 5-7m for water depths of respectively 12-33m. 

 

The maximum allowable total vertical uncertainty within a 95% confidence level shall be better 

than 0.5-0.7m for water depths of respectively 12-33m. 

Cubic features larger than 2m shall be detected in water depths up to 33m. 

Based on available records from GEMS Survey plus Ramboll DK’s assessments during processing, 

interpretation and presentation of the hydrographic and geophysical data, it is most likely that in 

general the acquired, processed interpreted and presented hydrographic and geophysical data are 

meeting the IHO 5E 1a requirements. 



4 

4. GEODESY 

4.1 Horizontal Datum 

To generate a regular gridded coordinate system (easting and northing) across the site, a project 

geodesy is assigned. The client specification for the site assumes a global spheroid of the shape 

depicted by the WGS84 model and the assigned zone in which the work is conducted is UTM32N. Any 

co-ordinates stated within this report are referenced to this geodesy. 

4.2 Vertical Datum 

The vertical reference system for the survey is DVR90, which is represented by the DKGEOID02 geoid 

model. All bathymetric and sub bottom horizon elevations stated in this report are relative to DVR90. 



5 

5. GEOLOGICAL HISTORY 

Except for the lowermost geological unit, Cretaceous Chalk (C) that was deposited in an 

intracontinental sea during the Late Cretaceous, the geology of Kriegers Flak is the result of Pleistocene 

glaciations and the basin history of the Baltic Sea Basin during the Late Pleistocene and the Holocene. 

The site area was covered by ice streams a number of times during the Elsterian, Saalian and 

Weichselian glaciations. The ice streams and their melt-water with associated debris eroded the chalk 

surface and deposited glacial till and glacigenic (deglaciation and melt-water) sediments. The glaciers 

also caused glaciotectonic deformation of older glacial/glacigenic deposits and the Cretaceous 

basement. Saalian glacial till and deglaciation deposits are known from outcrops in the southern part of 

the island of Zealand and the island of Møn (e.g. Houmark-Nielsen, 1994, Ref. 8; Damholt, 2005, Ref. 

9) west of the site. 

Saalian (and possibly also older) glacial and glacigenic deposits and (possibly dislocated) Eemian 

marine clay form part of the Lower Till unit (LT). This unit probably also includes some Weichselian 

glacial and glacigenic deposits, while at least the youngest Weichselian glacial and glacigenic deposits 

are included in the Upper Till unit (UT). 

Following the Eemian a fall in the global sea-level caused regression in the site area in the Early 

Weichselian. During deglaciation isostatic depression caused by the weight of the ice-sheet allowed an 

inflow of marine waters into the Baltic Sea Basin. In the Swedish part of Kriegers Flak an up to 10m 

thick partly marine, partly lacustrine clay succession, the Kriegers Flak Clay was deposited (Anjar et 

al., 2010, Ref. 13; Anjar et al., 2012,Ref. 12). The clay that includes thin beds of gyttja and peat is 

overconsolidated by later ice load (Klingberg, 1998, Ref. 14). The clay may be present in situ or 

dislocated in the Lower Till unit and/or in the Upper Till unit. Blanking caused by gas/peat/gyttja 

recorded in the Upper Till unit may originate from the Krigers Flak Clay. 

During the Late Weichselian the site area was overrun by at least three ice-streams, partly separated in 

time by periods of ice-free conditions (Kjær et al., 2003, Ref. 15; Houmark-Nielsen & Kjær, 2004, Ref. 

16). These ice-streams again caused erosion, glaciotectonic deformation and deposition of glacial and 

glacigenic deposits. In the site area deposits from the three ice-streams are probably included in the 

Upper Till unit. 

In the site area glaciotectonic deformation probably caused the overall geometry of the area with 

glacial highs (formed by Lower Till unit) and intermediate lows filled with Upper Till unit, late glacial 

and Holocene deposits. However, it is unclear whether this glaciotectonic deformation should be 

ascribed to the first Late Weichselian ice stream (Weichselian Main Advance) or an older ice stream. A 

multistage deformational history is also possible. 

It is not clear if deglaciation took place under aquatic or subaerial conditions. However, during and 

following deglaciation the site area was subject to redeposition, possibly including solifluction under 

periglacial conditions causing the formation of the Flow Till unit (FT) that covers the irregular glacial 

topography in lower parts of the site area. Alternatively the Flow Till unit was deposited primarily as a 

deglaciation deposit during deglaciation under aquatic conditions (in the early Baltic Ice Lake). In both 

cases the unit will comprise normal consolidated diamicton with interbedded lenses and stringers of 

sorted materials. 

Hereafter the site area was subject to the complex late glacial and Holocene sea-level and lake-level 

history of the Baltic Sea Basin. This history is synthesized in Björck (1995), Ref. 17 with minor 

corrections in e.g. Björck et al. (2008), Ref. 18 . Following deglaciation the area was covered by the 

Baltic Ice Lake, which transgressed the area c. 12 thousand years ago. 

The Early Holocene saw a new transgression, the Ancylus Transgression c. 9.5 thousand years ago 

reaching levels similar to those of the Baltic Ice Lake transgressions. The highstand lasted c. 300 years 

and was followed by c. 1,000 years of dry land until the area c. 8 thousand years ago was transgressed 

by the Littorina Sea that reached levels near present day sea-level c. 7 thousand years ago 

(Christensen, 1995, Ref. 19). 



6 

During all the above mentioned transgressions, highstands and regressions the glacial highs in the site 

area was eroded by waves and currents and the eroded material was transported and redeposited in 

coastal and basinal environments including spits and barriers forming the lower part of the Holocene 

Marine 2&3 unit (HM 2+3) that upwards evolve into a marine accumulation platform. In the glacial 

highs all units above Lower Till unit were removed by erosion. 

Recent marine processes have deposited the uppermost Holocene Marine 1 unit (HM 1) that covers 

large parts of the site, however, in some areas only as a thin veneer. 

A schematic diagram of the identified geological formations, representative for all of the site locations, 

is shown in Figure 5-1 where the horizons are shown with their assigned colouration. The depths are 

derived assuming a constant acoustic velocity in the sediments of 1,900 m/sec. 

Figure 5-1 Schematic diagram of the sub-seabed interpreted geological formations 

This schematic diagram forms the basis of a geological model established for the area. 



7 

6. BATHYMETRY 

The water depths across the Kriegers Flak OWF site range from 12m (755806E, 6106862N) in the 

central and north eastern section of the site deepening towards the perimeter of the site to a maximum 

depth of 33m (755117E, 6099435N) to the far south of the site. A distinct shoaling can be observed 

where outcrops of glacial till are observed, in particular to the west and east of site centre. At the 

centre of the site the water depths remain shallow and large accumulation of sediment can be observed 

overlying the glacial basement. Figure 6-1 show an overview of the bathymetry at Kriegers Flak OWF. 

The bathymetry across the Kriegers Flak OWF site has been acquired using a Multi-Beam Echosounder. 

Based on Ramboll DK’s assessments during the processing, interpretation and presentation of the 

hydrographic data, it is most likely the bathymetric data meets the IHO 5E 1a required accuracy as 

outlined in section 3. 

Figure 6-1 Bathymetry of the KF OWF site 

Circular seabed depressions have been identified across the site, approximately 30m wide and 1m 

deep, which may be collapsed structures but are most likely to be seabed scars from dredging activity, 

as the Kriegers Flak proved to be a valuable resource of sand for construction. Figure 6-2 shows the 

bathymetric data across an area to the south of the site where several circular depressions can be 

observed. 


KRIEGERS FLAK OWF GEOPHYSICAL SURVEYY RESULTS 

8 

Figure 6-2 

Bathymetric 

Circular Depressions, image centred on 750,350E 6,100,800N 

Seabed ridges are observed around the perimeter of the Kriegers Flak OWF site associated with the 

steeper slopes towards the boundaries of the site. These ridges are most m prominent towards the west 

of the site. Figure 6-3 

shows the bathymetric c data and profile across the ridges. It is possible 

that 

these features are a consequencee of down slope seabed sediment slumping opposed to large 

scale 

bedform migration. 

Figure 6-3 

Bathymetryy at Sedimentt Ridges toward the southwest of the site, centred 742,400E 6,100,300N 



9 

7. SEABED FEATURES 

The seabed features for Kriegers Flak OWF are derived from a collaboration of all of the geophysical 

datasets. The following seabed sediments have been identified across the site; 

 

 

 

 

 

 

 

Medium Sandy FINE SAND (predominantly fine sand (0.063–0.2mm with 5-20% medium sand 

(0.2-0.6mm)) 

Fine Sandy MEDIUM SAND (predominantly medium sand (0.2–0.6mm) with 5-20% fine sand 

(0.063-0.2mm)) 

Coarse Sandy MEDIUM SAND (predominantly medium sand (0.2–0.6mm) with 5-20% coarse 

sand (0.6-2.0mm)) 

Medium Sandy COARSE SAND (predominantly coarse sand (0.6–2mm) with 5-20% medium 

sand (0.2-0.6mm)) 

Sandy GRAVEL (predominantly gravel (2–60mm) with 5-20% sand (0.063-2.0mm)) 

Glacial Till 

Glacial Till with Numerous Boulders 

Figure 7-1 shows an overview of the derived seabed features for the Kriegers Flak OWF site. 

Figure 7-1 Seabed Features for Kriegers Flak OWF 

Grab samples has been collected at 73 locations across the Kriegers Flak OWF site and has 

subsequently been analysed by the Geological Survey of Denmark and Greenland (GEUS). The results 

from the sampling have been used to ground truth the side scan sonar interpretation and derive the 

seabed features seabed sediment information. 

The predominant seabed geology at Kriegers Flak OWF is Fine Sandy MEDIUM SAND, although the area 

is characterised by the outcrop of glacial till at the seabed. The glacial till outcrops are primarily located 

in the eastern and western side of the site. The majority of the megaripples observed across the site 

have a transport direction of northeast or southwest. 



10 

No seabed vegetation were identified across the area in the side scan sonar and bathymetric records, 

hence no areas of biological interest were identified. 



11 

8. TARGETS 

Targets lying on the seabed have been identified in the records of the side scan sonar and bathymetric 

as well as gradiometer (consisting of two magnetometers). The side scan sonar and bathymetric 

sensors identifies targets by emission and reflection of sound waves, whereas the gradiometer detects 

targets of ferromagnetic origin. 

Based on Ramboll DK’s assessments during the processing, interpretation and presentation of the side 

scan sonar plus bathymetric as well as gradiometer data, it is most likely the positions of the 

interpreted targets meets the required IHO 5E 1a accuracy as outlined in section 3. 

The following classifications have been used when identifying targets: 

 

 

 

 

 

 

Linear Sonar Targets 

Sonar Targets 

Boulders 

Depression 

As found wreck 

Magnetic Anomaly 

Targets with dimensions greater than ½ m in any direction have been picked, however due to higher 

resolution of the various sensors close to the vessel; some minor boulders close to the vessel have 

been picked, as well. 

Targets which obviously did not relate to geology based on their sonar appearance were classified as 

Man-Made. This could be features which are long and thin or square in shape and otherwise if not 

possible to classify, they were classified as unidentified Sonar or Linear Sonar Targets depending of 

shape and distribution. Seabed features with sonar appearance as a boulder were classified as 

Boulders. 

A total of 4,250 targets in total were identified in the side scan sonar and bathymetric records. 16 

targets are potentially of man-made origin and all others have been assigned as boulders, which are 

dominant in the record, some of which measure up to 10m in diameter. 

A total of 935 magnetic targets in total were identified across the Kriegers Flak OWF site. 3 targets are 

a coinciding with the locality to the 3 observed wrecks (see section 9) and 46 are associated with 

seabed infrastructure (see section 8.1). 685 targets are classified geological and all (201) other 

anomalies are unclassified/unidentified. It is likely that in many cases the anomalies are a function of 

the ferrous properties of the observed boulders. This interpretation is supported by the fact that the 

areas of high density of Magnetic Anomalies (Figure 8-1) coincide with areas classified as Glacial Till 

with Numerous Boulders (Figure 7-1). 

The anomalies associated with infrastructure denote the location of the Baltic Cable, which was clearly 

detected to the west of the site running north-south. These detections help validate the other 

geophysical records and enable the derivation of an ‘as found’ position for the infrastructure. 

Figure 8-1 provides an overview of targets picked on the side scan sonar and bathymetric as well as 

gradiometer data at the Kriegers Flak OWF site. 



12 

Figure 8-1 Identified targets at the KF OWF site 

8.1 Infrastructure 

The Baltic Cable has been identified in the far west of the site, running north-south. This has been 

identified as trenched / buried from the side scan sonar and the gradiometer data sets. 

As found cable positions have been generated from the available datasets, including the observation of 

from the geophysical data. The location falls close to the locations digitised from Admiralty charts. 

Figure 8-2 provides an overview of the identified cable. 

Figure 8-2 Identified cable within the KF site 



13 

9. MARINE ARCHAEOLOGY 

Three wrecks which may be of archaeological significance within the KF OWF site have been identified 

in the geophysical records. The three wrecks have been identified in the bathymetric dataset and in the 

side scan sonar datasets. 

Figure 9-1 provides an overview of the location of the wrecks. 

Figure 9-1 Location of identified wreck at Kriegers Flak OWF, shown database wrecks are digitised from 

Admiralty charts 

Figure 9-2 to Figure 9-4 provide images of the three wrecks, as recorded in the bathymetric and side 

scan sonar data. 

Figure 9-2 Bathymetric and Side Scan Sonar Dataset of identified wreck (ID 7343), (751949E, 6100981N) 


KRIEGERS FLAK OWF GEOPHYSICAL SURVEYY RESULTS 

14 

Figure 9-3 

Bathymetric 

and Side Scan Sonar Dataset of identified wreck ( ID 7351), (748785E, 6103749N) 

46m 

Figure 9-4 

Bathymetric 

and Side Scan Sonar Dataset of identified wreck ( ID 7358), (747369E, 6099134N) 

Excluding the three wrecks no clearly identifiable features 

have been observed across the site. The vast 

majority of targets have been identified as boulders. 13 ’ unidentified’ ’ sonar targets, which most likely 

do not represent geological boulders, have been observed. 



15 

10. SUB-BOTTOM GEOLOGY 

The geological units identified in the sub-bottom profiler datasets are listed below: 

HM1 

HM2i 

HM2 

HM2+3 

FT 

UTi 

UT 

Horizon A Internal 

Horizon B Internal 

LT 

C 

Holocene Marine 1 unit 

Internal Reflector within Holocene Marine 2 unit 

Holocene Marine 2 (Undistinguishable Base) unit 

Holocene Marine 2 and 3 (as HM2 base not observed) unit 

Flow Till unit 

Intermittent Internal horizon within Upper Till unit 

Upper Till unit 

Intermittent Internal horizon within Lower Till unit 

Intermittent Internal horizon within Lower Till unit 

Lower Till 

Cretaceous Chalk unit 

In the following sections the geological units are illustrated with maps of depths below seabed of 

bottom surfaces for Holocene units in Figure 10-1 to Figure 10-2 and for Pleistocene units in Figure 

10-7 to Figure 10-9 including description of each geological layer. 

The uncertainty when calculating the depths to the various geological units is, based on industry 

experience and assessments during the processing, interpretation and presentation, estimated to be 

approximately ± (1m + 5% of the depth). 

10.1 Holocene Units 

Holocene units are sedimentary deposits of the more recent of the two epochs of the Quaternary 

Period, beginning at the end of the last Ice Age about 11 thousand years ago. In this case, however, 

late glacial waterlain deposits from the Baltic Ice Lake about 13 thousand years ago are included in the 

Holocene units. 

The Holocene is sub-divided into three sub-units designated Holocene Marine 1 unit (HM1), Holocene 

Marine 2 unit (HM2) and Holocene Marine 3 unit (HM3), although an additional reflector within HM2 

(HM2i) have been identified. 

The upper Holocene Marine 1 unit (HM1) comprises loose, unconsolidated marine sand which 

provides a thin blanket cover across much of the site. The depth below seabed of the base of the HM1 

unit is shown in Figure 10-1. It thickens across site centre to a maximum of 2.4m. The unit is no longer 

observed in the geophysical record where the till units approach the seabed, i.e. over the glacial highs. 

However, HM1 is still likely to be present in most cases as a thin veneer. 

The underlying Holocene Marine 2 unit (HM2) is also observed to thicken across site centre, 

influenced by the relief of the exposed tills on the perimeter of the Kriegers Flak area of shoaling. The 

base of this unit has been included with that of Holocene Marine 3 unit (HM3) observed further 

east, where the HM2 unit is believed to have provenance. To the east the HM3 sediments dominate the 

record and a spit can be observed, along with some evidence for progradation to the east in the 

profiles. To the contrary in the west a clear progradation to the west is observed and here the HM2 

sediments dominate the record. The progradation of the HM2 unit results in the formation of a bank / 

slope to the west of the site. 

The exact boundary between the two units HM2 and HM3 is undistinguishable in the record, and the 

units are consequently mapped together as Holocene Marine 2&3 unit (HM2+3). The HM2+3 unit 

has a thickness of up to c. 11m in the site centre, whereas it is missing over the glacial highs. The 

topography of the base of the HM2+3 unit is rather gentle (Figure 10-2) due to the fact that most 

irregularities in the glacial topography was smoothed by the underlying Flow Till unit and probably also 

by initial transgressive erosion prior to deposition of the HM3 unit. Typical example of HM1, HM2 and 

HM3 are seen in the seismic sections in Figure 10-3. 

It is the HM2 and HM3 units which have previously proven to be a valuable sand resource for 

construction. Evidence for dredging can be observed in the northwest of the site and toward the north. 



16 

Figure 10-1 Interpreted depth below seabed (DBS) in m of Holocene Marine 1 (HM1) unit 

Figure 10-2 Interpreted depth below seabed (DBS) in m of the Holocene Marine 2 & 3 (HM2+3) unit 

Figure 10-3 Representative of the Holocene Units, Seismic Section, Line 6104900 



17 

10.2 Pleistocene Units 

Pleistocene units are sedimentary deposits from the geological epoch which lasted from about 2.6 

million to 12 thousand years ago, spanning the world's recent period of repeated glaciations. 

The Pleistocene is at the site sub-divided into three geological units. The base of the lowermost glacial 

unit, Lower Till (LT), which also is the top of the Cretaceous Chalk unit, is primarily picked from the 

multichannel sparker data set whereas the rest of the units primarily are picked from the pinger data 

set. 

The Flow Till unit (FT) is present in the record infilling the irregular/undulating till unit which lies 

beneath. The base of the unit is consequently highly irregular (Figure 10-7) and the unit is missing 

above the glacial highs. Unit thickness varies between 0m and c. 9m and variations mainly follow the 

topography of the base. The unit that comprises diamict, and probably also sorted sediments was 

deposited during and after the last deglaciation of the area and is consequently less consolidated than 

the underlying tills that have been subject to ice preloading. An example of the Flow Till unit is seen in 

Figure 10-4. 

The Upper Till unit (UT) unit is in contrast to the FT unit and is clearly depicted in the geophysical 

record. The unit is likely to comprise glacial till of predominantly clay with gravel, cobbles and boulders 

but also occurrences of silt and sand including those derived from melt water. The base of the unit is 

undulating and irregular, representing an old glacial landscape formed by glaciers and comprising the 

underlying Lower Till. The top of the larger hills have been eroded by later aquatic erosion but the 

depth below seabed map in Figure 10-8 newer the less show a clear pictures of this glacial landscape 

with large (now eroded) hills, large depressions and smaller elongated ridges. The thickness of the unit 

ranges from 0m to c. 23m and thickness variations both reflect the irregular base, the infilling 

geometry of the unit and the erosive top in areas of formerly large glacial hills. 

The Internal Reflector within the UT (UTi) is concentrated within the topographic lows of the LT 

relief, and cannot be observed extensively across the entire site. The reflector is prominent and 

therefore likely to represent a change in sediment. Taking in to consideration that the UT unit is a 

diamict the unit underlying the UTi horizon could be representative of a lens of sediment of more 

uniform grain size. Given the rather high stratigraphic position within the Pleistocene deposits the 

Kriegers Flak Clay is not the most likely candidate but it can’t be complete rules out. An example of the 

Upper Till internal reflector is seen in Figure 10-5. 

The Lower Till unit (LT) directly overlies the top of the Cretaceous Chalk. The Lower Till unit can be 

observed exposed at the seabed towards the perimeter of the Kriegers Flak and this is clearly seen in 

the seabed features datasets (Figure 7-1) through the coarsening of sediments and increased number 

of boulders. As mentioned above the top of the unit forms an old glacial landscape although the tops of 

the larger hills have been removed by later erosion. Overall the base is a gentle, sub-horizontal surface 

but in this surface a network of larger and smaller channels are cut, the deepest of them more than 

50m deep (Figure 10-9). 

The age of the channels is unknown but they are probably filled with sediments comparable to those of 

unit LT and channels and channel fill are consequently included in unit LT. 

Outside the channels the unit is generally c. 10-30m thick but over the channels the thickness is up to 

90m and the total thickness range is 0-90m. Outside the channels variations in thickness are controlled 

by the unit top, the old glacial landscape. 

The unit comprises glacial till and meltwater deposits from a number of ice stream advances and it 

includes Eemian marine clay and probably also the Kriegers Flak Clay. Both of these clay units are 

probably present as dislocated rafts and lenses rather than as uniform in situ layers. 

An internal reflector within the LT unit (Horizon A) is intermittently observed across the site, in 

particular where the Holocene sediment cover is thin. When observed it is generally of a low amplitude 

and not easily discerned in the record. The horizon does mark a slight reduction in the intensity of the 

bounding units. 



18 

A second internal reflector within the LT unit (Horizon B) is intermittently observed across the site, 

within isolated pockets where the Holocene sediment cover is thin and in particular towards the 

Swedish Sector to the east of the site. When observed it is generally of a low amplitude and not easily 

discerned in the record. The horizon does mark a slight increase in the intensity of the bounding units. 

Examples of both the Horizon A and B are seen in Figure 10-4 and Figure 10-6. 

It is unclear whether the Eemian marine clay or the Kriegers Flak Clay are connected the two internal 

reflectors or if the reflectors represent e.g. old glacial landscapes from different ice stream advances. 

Below the Lower Till unit an internal reflector is evident within the Cretaceous unit. The reflector is not 

at sufficient depth to correlate with the base of the Cretaceous Chalk, while the unit is expected to be 

at least 200m thick. 

Figure 10-4 Representative of the Glacial Units, Seismic Section, Line 6101100 

Figure 10-5 Representative of the Glacial Units with the internal Upper Till unit, Seismic Section, Line 6103600 



19 

Figure 10-6 Representative of the Horizon A and Horizon B. Seismic Section, Line 6104400 

Figure 10-7 Interpreted depth below seabed (DBS) in m of the Flow Till (FT) unit 

Figure 10-8 Interpreted depth below seabed (DBS) in m of the Upper Till (UT) unit 



20 

Figure 10-9 Interpreted depth below seabed (DBS) in m of the Lower Till unit (LT) 



21 

11. HAZARDS 

11.1 Shallow Gas/Organic Deposits 

Areas with an accumulation of soft organic matter, including peat/gyttja and potential gas are likely to 

be observed as data blanking areas in the datasets. A distinct area of data blanking and several 

isolated occurrences have been observed in the sub-bottom data 2 km north of site centre. The data 

blanking is present in the Upper Till unit; this is likely to be an accumulation of gas which has migrated 

from an organic rich unit below. Alternatively blanking can be caused by gyttja and peat in Kriegers 

Flak Clay found in situ or dislocated within the Upper Till unit. The top of the blanking, interpreted at 

the shallowest depths have been digitised. In some instances seismic ‘phase reversal’ is evident, 

another indicator of organic deposits and / or gas presence. 

Figure 11-1 shows depth below seabed (DBS) in m of the Hazard blanking unit (potentially hazardous 

area). 

Figure 11-1 Depth below seabed (DBS) in m of Hazard Blanking Unit 

Figure 11-2 and Figure 11-3 show seismic profiles of the blanking and phase reversal effects. It is 

possible that diffuse gas could be evident at shallower depths. 

Figure 11-2 Data Blanking Hazard (Multichannel Sparker Seismic Section, Line 6106300) 



22 

Figure 11-3 Phase Reversal Hazard (Multichannel Sparker Seismic Section, Line 6104300) 

11.2 Faults 

Evidence of faulting has been observed deep within the Cretaceous Chalk, and predominantly towards 

the west of the site. Examples of observed faulting are shown in Figure 11-4. 

Figure 11-4 Cretaceous Chalk Unit Faulting (Multichannel Sparker Seismic Section, Line 6168600) 

11.3 Soft Channel Infill 

No clearly defined near seabed channels are evident in the site, although the base of the Upper Till unit 

does yield undulations associated with glacial deformation, which may give rise to localised variations 

in sub-bottom geotechnical properties. 



23 

12. REFERENCES 

Ref. 1 Ramboll, 2013; Kriegers Flak OWF – Mobilisation Report- Kriegers Flak and Horns Rev 3 – 

Geo Investigations 2012 

Ref. 2 

Ref. 3 

Ref. 4 

Ref. 5 

Ref. 6 

Ref. 7 

Ref. 8 

Ref. 9 

Ref. 10 

Ref. 11 

Ref. 12 

Ref. 13 

Ref. 14 

Ref. 15 

Ref. 16 

Ref. 17 

Ref. 18 

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Ramboll, 2013; Kriegers Flak OWF - Operational Survey Report, Kriegers Flak and Horns Rev 

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Ramboll, 2013; Kriegers Flak OWF - Acquisition Parameter Optimisation Report, Kriegers 

Flak and Horns Rev 3 – Geo Investigations 2012 

Ramboll, 2013; Kriegers Flak OWF - Processing Optimisation Report, Kriegers Flak and 

Horns Rev 3 – Geo Investigations 2012 

Ramboll, 2013; Kriegers Flak OWF - Interpretation Optimisation Report , Kriegers Flak and 

Horns Rev 3 – Geo Investigations 2012 

Ramboll, 2013; Kriegers Flak OWF – InterpretationSurvey Report, Kriegers Flak and Horns 

Rev 3 – Geo Investigations 2012 

IHO, 2008; IHO Standards for Hydrographic Surveys, Edition 5, February 2008 - Special 

Publication No 44 

M.Houmark-Nielsen, 1994; Late Pleistocene stratigraphy, glaciation chronology and Middle 

Weichselian environmental history from Klintholm, Møn, Denmark." Bulletin of the 

Geological Society of Denmark, 41, 181-202 

T.Damholt, 2005; Møns geologi - en præsentation af den geologiske litteratur om Møn; 

Rapport til Natur- og Geologigruppen, Pilotprojekt Nationalpark Møn. Netpublication (in 

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M.Houmark-Nielsen, 1999; A lithostratigraphy of Weichselian glacial and interstadial 

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J.Anjar, N.K.Larsen, S.Björck, L.Adrielsson & H.L.Filipsson, 2010; MIS 3 marine and 

lacustrine sediments at Kriegers Flak, southwestern Baltic Sea. Boreas, 39, 360-366 

F.Klingberg, 1998; A Late Pleistocene marine clay succession at Kriegers Flak, westernmost 

Baltic, southern Scandinavia. Journal of Quaternary Science, 13, 245-253 

K.H.Kjær, M.Houmark-Nielsen & N.Richardt, 2003; Ice-flow patterns and dispersal of 

erratics at the southwestern margin of the last Scandinavian Ice Sheet: signature of lalaeoice 

streams. Boreas, 32, 130-148 

M.Houmark-Nielsen, J.Krüger & K.H.Kjær, 2005; De seneste 150.000 år I Danmark, 

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S.Björck, T.Andrén & J.B.Jensen, 2008; An attempt to resolve the partly conflicting data and 

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26 

C.Christensen, 1995; The Littorina transgressions in Denmark. In: Man and Sea in the 

Mesolithic (ed. A. Fischer). Oxbow Monogr., 53: 15-22.

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