Sedimentary basins

The majority of the geological resources and raw materials that mankind is utilizing, such as oil, gas, coal, drinking water, geothermal warm water, minerals, building materials etc. is situated in sedimentary basins. The formation and development of the basins through geological time, their content of geological materials and valuable raw materials, and the distribution and quality of these are therefore very important technical and scientific research themes.

A sedimentary basin is a confined area on Earth, where subsidence of the base of the basin has occurred through geological time accompanied by deposition of sediments of various types and composition. The form of a basin area can vary from elongated, irregular to oval of a few kilometers to more than 1000 km in diameter, and the basin is often bounded by large fracture zones (fault systems) toward crystalline basement or older metamorphic rocks. Sedimentary basins can exist for time periods up to several hundred million years. Their generation, development and disappearance is related to plate tectonic movements, and when they are compressed due to collision of the tectonic plates, the basin-fill of sedimentary layers is deformed and folded forming mountain belts. The layers can then be studied in natural outcrops, road cuts etc. exposing the beds.

Examples of sedimentary basins include The Danish Basin that stretches from Bornholm in the east across most of the Danish onshore area including the Danish waters and further westward, where the basin combines with the North Sea Basin. The Danish Basin is filled with erosion material (gravel, sand, silt and clay) primarily sourced from the wearing down over time of the Scandinavian Shield basement. The basin-fill also includes kilometers of chemical precipitated salt and carbonate formed by deposition of microscopic skeleton remains from marine algae (coccoliths). The thickness of sedimentary layers in a basin may add up to many kilometers, e.g. more than 10 km as in the Danish Basin and the North Sea Basin.

The oldest rocks in Denmark is the basement, which underlies the entire country at various depths down to 10-15 km, whereas it rises over the sea and is exposed on Bornholm. In The Danish Basin, geological layers formed in many different depositional environments during the latest approx. 500 million years overlie the basement. These layers contain evidence about the shifting position of sea and land, climate changes, the development of the life of the fauna and plants, and indications of how the Earth’s crust has moved through time.