2 Die Küste, 74 ICCE (2008), 1-17
and to the East by the western coasts of Scandinavia. It opens into the Atlantic Ocean to the
North, via the English Channel to the Southwest, and it has a free connection to the Baltic
Sea via the Skagerrak to the East. The sea-bottom of the relatively shallow southern North
Sea off the Dutch and German coast shows no distinctive morphological variation. It is com
posed of young Pleistocene and Holocene soft-rock deposits (Zeiler et ah, 2008).
2.1 Palaeozoic to Palaeogene Development
The basement of the North Sea basin consists of more than 543 million years old Pre-
cambrian crystalline rocks and faulted metamorphic rocks of Palaeozoic age (for stratigraphy
see Table 1). This basement rises towards North and continues in the Norwegian Mountains
and the Scottish Highlands. In the North Sea basin, the top of this basement is located in
depths of 8-10 km. It is overlaid by terrestrial, lagoonal and liminic deposits into which coal
seams are partly incorporated. At the beginning of the Perm Period (Tab. 1), the continental
basement was strongly influenced by volcanic activity splitting it into several parts. During
the Upper Rotliegend, mainly arid conditions prevailed. Thick series of terrestric sandstones
were deposited, which later functioned as reservoir rocks for the economically used North
Sea gas. During Zechstein (Perm period), the sedimentary basin was periodically flooded
from North-west. As a consequence of repeated transgressions and high evaporation rates,
series of siliclastic rocks, carbonates and salt strata, the latter of up to 1,200 m in thickness,
were deposited. (Richter-Bernburg, 1972). The maximum thickness of deposits can be
found in the south-western-most part of the North Sea basin (Fig. 1).
Tectonic activity and changing shallow marine and terrestrial conditions persisted
throughout the Mesozoic period (251-65.5 million years BP). In general, the sinking ten
dency of the North Sea basin as action at a distance of continental drift, continued, but
‘Block-and-Graben’ tectonics led to some relative movements withpartiallly sinking but also
uplifting tendencies. Already at the end of the Triassic period, the sediment load on top of
the older salt strata started to deform the salt deposits by thermo-mobilisation, resulting in
an early development of salt dome structures (Halocinesis). Later, as a consequence of on
going salt tectonics and partly other factors leading to vertical dislocations, sills and deep
anoxic basins were formed to later become the source areas for the development of hydro
carbons. The mobility of the salt continues until today. In the subsurface of the North Sea,
the Northern German und Dutch lowlands, and below the most south-western part of the
Baltic Sea, salt diapers in elongated form (Fig. 2), up to 100 km in length, exist. They expand
vertically from a depth of up to 8,000 m to the present surface (Ziegler, 1990). An obvious
example for salt tectonics is the island of Helgoland. Here the characteristic red sandstones
were uplifted by more than 4,000 m (Schmidt-Thome, 1987). Nowadays, not only the
troughs along the salt structures are of economic interest as potential reservoirs for hydro
carbons (e.g. Mittelplate oil field) but huge artificial caverns in the saltdomes can be used for
the storage of energy sources such as oil, gas or compressed air and C0 2 as well.
In the German part of the North Sea basin, the sedimentation history of the older
Caenozoic period (Palaeogen, 65.5-23 million years BP) is characterised by shallow marine
silica-clastic deposits. The subsidence, which already started during the Triassic period
(Table 1), continued, giving the North Sea sedimentary basin its typical shape of a shallow
bowl in which organic-rich sediments were subsided to depths, which allow the formation
of oil and gas (Teichmuller et ah, 1979).