|
Impressive examples of glaciotectonic deformation of Weichselian deposits are exposed along part of the northern Danish North Sea coast at Rubjerg Cliff. The deformation took place approximately 30 ka ago during a major expansion of the Scandinavian ice sheet. It is likely, however, that glacial deformation of shallow deposits during this episode occurred in a much wider area than may easily be recognised from the coastal cliff exposures mentioned. Knowledge obtained from exploration wells and seismic surveys indicate that large parts of the Skagerrak - Kattegat region were affected by the advancing glaciers.
Stratigraphy established by foraminifera in exploration well materials indicated that layers comprising Eemian and Weichselian marine deposits were severely deformed in the upper 100 metres or more in the Frederikshavn City area. This area is located approximately 45 km east of the Rubjerg Cliff location.
Folding of organic rich Eemian clayey layers and overlying more sandy Weichselian deposits may explain the occurrence of many minor gas reservoirs with limited or no interconnections, as had had been found during the many attempts to exploit the gas in the 1930ies and 1940ies. Large differences in stable carbon isotopic composition of the methane gas, up to 10 per mil, found later supported the idea of minor isolated gas reservoirs being present.
Numerous seeps of old bacterial methane in the coastal areas of Kattegat east of Frederikshavn City indicate the presence of Pleistocene organic rich deposits at shallow depth, the age of methane being determined by the radiocarbon method. Ridges of Pleistocene deposits can occasionally be observed on shallow seismic profiles which also show plumes of gas extending towards the seafloor. Mapping of authogenic carbonates formed by oxidation of methane at seepage locations, are easier than mapping the seeps themselves and gives a more complete view of the extension of gas seeps.
The distribution of authogenic carbonates sites combined with areas of gas plumes obtained form shallow seismic surveys may then give an idea of the distribution of shallow organic rich Pleistocene deposits, and thereby the areas of major deformation of Emian-Early Weichselian marine sediments Shallow methane gas is present in Holocene deposits of the central and deeper parts of the Kattegat. This gas obscures any reflections on shallow seismic profiles and makes it impossible to depict the depth contours of the Pleistocene. The methane gas in the fine grained sediments of the deeper parts of Kattegat is young as was determined by radiocarbon dating. One may imagine that the distribution of relatively thick marine organic rich sediment present prior to the major extension of the Scandinavian ice sheet 30 ka ago resembles today's distribution of gas-rich sediment. Using the example of Rubjerg Cliff, we may then get a better understanding of the geological processes behind the formation of gas reservoir and gas seeps in the area
|
