The seabed sediment map is the result of the compilation of all available information on the nature of the seabed. The map based primarily on seismic and acoustic data combined with sediment samples. The majority of data has been acquired during mapping surveys for sand and gravel resources requested by the government since the early 1970s. Furthermore, data from the habitat mapping programs, data acquired in the North Sea by the Coastal Authority and data originating from offshore windfarm mapping and other private sectors have been used.
GEUS stores and distributes geological seabed data acquired by GEUS or third parties. Sediment data is available via the Jupiter-database while seismic/acoustic data is available via the Marta-database.
What does the map show
The sediment map shows the spatial distribution of the seabed sediments in the Danish waters. The given sediment classification represents the major sediment type of the upper 0.50 m of the seabed. Due to the uneven data distribution, the classifications are based on interpolations between data points together with the knowledge of the geological history the water depths and seabed morphology. Furthermore, available biological information (distribution of algae, eelgrass, mussel beds etc.) has been included as background information.
There is no systematic seabed mapping plan for the Danish waters. The seabed data was acquired during different projects conducted for the public sector or for private enterprises. Some also collected during EU funded projects. Therefore, we cannot claim full coverage datasets in producing the seabed sediment map. The final sediment map has different confidence level that varies with respect to the quality and coverage of the surveyed areas. Despite its pragmatic nature, looking at the survey lines and the sampling points published in the GEUS MARTA database can give an indication about the uncertainty of the sediment classification. Areas where you find dense survey lines and ground truth sampling points, have relatively higher confidence of the sediment classification that those areas with sporadic or scattered data points. This needs always to be remembered before making any further use of the data.
We recommend the end user to contact us if in doubt of the confidence of their area of interest.
The sediment classification expresses the sediment type of the upper 0.50 m of the seabed. Each sediment class is defined based on the specific grain size distribution. However, “till” is a mixed sediment of clay, sand and gravel, and ”sedimentary rocks” express the presence of deposits with an age of several million years and different histories.
The sediment classes are:
- Till: Mixed sediment type of glacial origin. Often covered by a thin layer of sand, gravel, boulder and/or sandy mud washed out of the till.
- Sand, gravel and pebbles: Mixed sediments of more than 0.50 m thickness. Lag sediments covering till, meltwater deposits or fossil coastal deposits.
- Sand: Homogeneous layer of loose, well-sorted sand. Often combined with ripples and/or sand waves due to current or wave action.
- Sandy mud/muddy sand: A mixed sediment type composed of variable content of sand and mud. Deposited at the rim of basins or as a thin cover layer in erosion areas.
- Mud: Soft and fine-grained sediment with more than 10% fine organic matter and less than a few percent coarser material. Very high water content. Often with shells and plant remains. Related to accumulation and basin areas in the inner Danish waters.
- Quaternary clay: Marine, meltwater or lake deposits of clay. Often laminated with sand/silt and/or peat layers, in some cases covered by few cm of lag sediments (sand, gravel or pebbles). The deposit is often related to the Yoldia Clay (Kattegat), The Baltic Icelake (The Baltic Sea) or Holocene clay (The North Sea).
- Sedimentary bedrock: Outcropping Mesozoic deposits around Bornholm, and outcropping chalk in Øresund and offshore chalk cliffs (e.g. NE North Sea and Djursland).