diachronous, suggesting that the thermal state of the
system was not homogeneous at the onset of the postrift stage.
Three stages can be identified in the post-rift Cretaceous development of the northern North Sea: (1)
The incipient post-rift stage (Ryazanian–latest Albian)
was characterised by different degrees of subsidence.
The major structural features inherited from the synrift basin (e.g. crests of rotated fault blocks, relay
ramps and sub-platforms) had a strong influence on
the basin configuration and hence the sediment distribution. (2) In the middle stage (Cenomanian–late
Turonian) the internal basin relief became gradually
drowned by sediments. This is typical for basins where
sediment supply outpaces or balances subsidence, as
was the case in the northern North Sea. Thus, the
influence of the syn-rift basin topography became
subordinate to the subsidence pattern determined by
the crustal thinning profile, which in turn relied on
thermal contraction and isostatic/elastic response to
sediment loading. (3) The mature post-rift stage
(early Coniacian–early Palaeocene) was characterised
by the evolution into a wide, saucer-shaped basin
where the syn-rift features were finally erased. Since
thermal equilibrium was reached at this stage, subsidence ceased, and the pattern of basin filling became,
to a larger degree, dependent on extra-basinal processes (see Chap. 12).
The Lower Cretaceous shales are black, but only
locally do they form good source rocks. Conditions
subsequently became more oxidising as bottom circulation improved. In the Early Cretaceous grey to reddish oxidised shales were also deposited, which
become increasingly calcareous upwards. The Lower
Cretaceous shales (Cromer Knoll Formation) are shallow to deep marine mudstones with little sand.
In the Late Cretaceous the sea attained its transgressive maximum and clastic sedimentation almost
ceased across large areas of northwest Europe. Parts of
Scandinavia were probably also covered by the Cretaceous sea. Sedimentation was dominated by planktonic carbonate algae (coccolithoporids) which
formed a lime mud, the main component of Chalk,
though it also includes some foraminifera and
bryozoa. The main development of the Chalk was in
the Campanian and Maastrichtian, but sedimentation
continued up into the Danian of the Palaeocene, for the
most part through resedimentation of earlier Chalk
deposits which had been uplifted along the central
highs. In the Viking Graben the carbonate content
diminishes northwards and we do not have pure limestone (Chalk) facies like that in the southern and
central part of the North Sea. Instead, shales predominate, though often with a significant carbonate content.
At the close of the Cretaceous and beginning of the
Cenozoic, compressive movements were felt from the
Alpine Orogeny to the south. Part of this movement
was accommodated along diagonal fault zones, such
as the Sorgenfrei-Tornquist Zone in Scania and the
Kattegat. The Polish Basin and parts of the North
Sea area were uplifted (inversion) and eroded.
25.2.2.7 Cenozoic
The early Cenozoic rifting, break-up and onset of
seafloor spreading in the NE Atlantic gave rise to
differential vertical movements also affecting the
North Sea area. The sedimentary architecture and
breaks are related to tectonic uplift of surrounding
clastic source areas, thus the offshore sedimentary
record provides the best age constraints on the Cenozoic exhumation of the adjacent onshore areas.
Major depocentres sourced from the uplifted Shetland Platform and areas along the incipient plate
boundary in the NE Atlantic formed during Late
Palaeocene-Early Eocene times. A local source area
also existed in western Norway. Tectonic subsidence
accelerated in Palaeocene time throughout the basin,
with uplifted areas to the east and west sourcing
prograding wedges, which resulted in large
depocentres close to the basin margins. Subsidence
rates outpaced sedimentation rates along the basin
axis, and water depths in excess of 600 m are
indicated. Uplift along the Sorgenfrei-Tornquist Zone
caused erosion into top Chalk along the southeastern
flank of the North Sea basin.
Prominent ash layers of earliest Eocene age are
found throughout the entire North Sea and also further
north. Onshore in Denmark the volcanoclastic
sediments are known as “moler”. The extensive volcanism was related to the opening of the North Atlantic
and both the Eocene and Oligocene mudstones are
dominated by smectitic clays formed from the volcanic ash. These smectitic mudstones are characterised
by seismic velocities that are low, even compared to
the overlying Neogene sediments. Ash layers rich in
volcanic glass (hyaloclastites or tuff) may, however,
develop into hard layers due to diagenesis. They will
then be characterised by high seismic velocities,
25 Geology of the Norwegian Continental Shelf
615
system was not homogeneous at the onset of the postrift stage.
Three stages can be identified in the post-rift Cretaceous development of the northern North Sea: (1)
The incipient post-rift stage (Ryazanian–latest Albian)
was characterised by different degrees of subsidence.
The major structural features inherited from the synrift basin (e.g. crests of rotated fault blocks, relay
ramps and sub-platforms) had a strong influence on
the basin configuration and hence the sediment distribution. (2) In the middle stage (Cenomanian–late
Turonian) the internal basin relief became gradually
drowned by sediments. This is typical for basins where
sediment supply outpaces or balances subsidence, as
was the case in the northern North Sea. Thus, the
influence of the syn-rift basin topography became
subordinate to the subsidence pattern determined by
the crustal thinning profile, which in turn relied on
thermal contraction and isostatic/elastic response to
sediment loading. (3) The mature post-rift stage
(early Coniacian–early Palaeocene) was characterised
by the evolution into a wide, saucer-shaped basin
where the syn-rift features were finally erased. Since
thermal equilibrium was reached at this stage, subsidence ceased, and the pattern of basin filling became,
to a larger degree, dependent on extra-basinal processes (see Chap. 12).
The Lower Cretaceous shales are black, but only
locally do they form good source rocks. Conditions
subsequently became more oxidising as bottom circulation improved. In the Early Cretaceous grey to reddish oxidised shales were also deposited, which
become increasingly calcareous upwards. The Lower
Cretaceous shales (Cromer Knoll Formation) are shallow to deep marine mudstones with little sand.
In the Late Cretaceous the sea attained its transgressive maximum and clastic sedimentation almost
ceased across large areas of northwest Europe. Parts of
Scandinavia were probably also covered by the Cretaceous sea. Sedimentation was dominated by planktonic carbonate algae (coccolithoporids) which
formed a lime mud, the main component of Chalk,
though it also includes some foraminifera and
bryozoa. The main development of the Chalk was in
the Campanian and Maastrichtian, but sedimentation
continued up into the Danian of the Palaeocene, for the
most part through resedimentation of earlier Chalk
deposits which had been uplifted along the central
highs. In the Viking Graben the carbonate content
diminishes northwards and we do not have pure limestone (Chalk) facies like that in the southern and
central part of the North Sea. Instead, shales predominate, though often with a significant carbonate content.
At the close of the Cretaceous and beginning of the
Cenozoic, compressive movements were felt from the
Alpine Orogeny to the south. Part of this movement
was accommodated along diagonal fault zones, such
as the Sorgenfrei-Tornquist Zone in Scania and the
Kattegat. The Polish Basin and parts of the North
Sea area were uplifted (inversion) and eroded.
25.2.2.7 Cenozoic
The early Cenozoic rifting, break-up and onset of
seafloor spreading in the NE Atlantic gave rise to
differential vertical movements also affecting the
North Sea area. The sedimentary architecture and
breaks are related to tectonic uplift of surrounding
clastic source areas, thus the offshore sedimentary
record provides the best age constraints on the Cenozoic exhumation of the adjacent onshore areas.
Major depocentres sourced from the uplifted Shetland Platform and areas along the incipient plate
boundary in the NE Atlantic formed during Late
Palaeocene-Early Eocene times. A local source area
also existed in western Norway. Tectonic subsidence
accelerated in Palaeocene time throughout the basin,
with uplifted areas to the east and west sourcing
prograding wedges, which resulted in large
depocentres close to the basin margins. Subsidence
rates outpaced sedimentation rates along the basin
axis, and water depths in excess of 600 m are
indicated. Uplift along the Sorgenfrei-Tornquist Zone
caused erosion into top Chalk along the southeastern
flank of the North Sea basin.
Prominent ash layers of earliest Eocene age are
found throughout the entire North Sea and also further
north. Onshore in Denmark the volcanoclastic
sediments are known as “moler”. The extensive volcanism was related to the opening of the North Atlantic
and both the Eocene and Oligocene mudstones are
dominated by smectitic clays formed from the volcanic ash. These smectitic mudstones are characterised
by seismic velocities that are low, even compared to
the overlying Neogene sediments. Ash layers rich in
volcanic glass (hyaloclastites or tuff) may, however,
develop into hard layers due to diagenesis. They will
then be characterised by high seismic velocities,
25 Geology of the Norwegian Continental Shelf
615