Mesozoic and Cenozoic basin configurations in the North Sea - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 437

The main depocentres belonging to sequence
CSS-5 are in the southern Central Graben area
(with thickness > 500 ms TWT), in the southern
Viking Graben area (>500 ms TWT) and in the
southern part of the Norwegian-Danish Basin
(> 500 ms TWT).
In the southern Viking Graben area sequence
CSS-5 reveals internal seismic reflections dipping
to the east. Sequence CSS-5 is characterised by a
parallel to sub-parallel seismic reflection pattern
in the southern Central Graben area, however, with
onlap relations against the Mid-North Sea High.
The depocentre in the Norwegian-Danish Basin
is located south-west of the CSS-4 depocentre,
including a basinward shift in onlap onto the
CSS-4 sequence. Here, CSS-5 is characterised by
SW dipping clinoforms, located south-west of the
Oligocene sequences. Correlation of sequence
CSS-5 to onshore Denmark reveals that it is time-
equivalent with the Billund and Vejle Fjord for-
mations (Rasmussen et al ., 2010).
Sequence CSS-6 is mainly recorded in the
southern parts of the study area (Fig. 13), although
it thins to less than 100 ms TWT across the Coffee-
Soil Complex and the Norwegian-Danish Basin. In
the north-eastern part of the Norwegian-Danish
Basin CSS-6 is truncated by glacial erosion in
Pliocene and Pleistocene times. The sequence
thins to below seismic resolution in the northern
North Sea and for this reason is not recorded
within the Viking Graben area.
Sequence CSS-6 can be subdivided into two
sub-sequences, namely CSS-6.1 and CSS-6.2.
These are separated by an onlap surface, where
the sub-sequence CSS-6.2 onlaps sub-sequence
CSS-6.1 in the Coffee-Soil Fault Complex area.
The upper bounding surface of sequence CSS-6 is
characterised by an amplitude anomaly across the
study area, with onlap relations from the overly-
ing CSS-7 sequence in the south-western parts of
the Norwegian-Danish Basin.
The Central Graben area was the main depo-
centre during deposition of the sub-sequences
CSS-6.1 and CSS-6.2 (Fig. 13B). The CSS-6.1 sub-
sequence has its maximum thickness in the
north-eastern part of the Central Graben area and
pinches out towards south-west. The CSS-6.2
sub-sequence is of constant thickness throughout
the Central Graben area but thins towards the NE
and SW, across the Coffee-Soil Fault Complex
and the Mid-North Sea High (Figs 9 and 13).
The CSS-7 sequence is time-equivalent to the
Utsira Formation in the central and northern North

Sea and the Molo and Kai formations in the
Norwegian Sea (e.g. Eidvin et al ., 2007). The base
of the CSS-7 sequence corresponds to the mid-
Miocene unconformity (e.g. Huuse & Clausen,
2001), which is also present onshore Denmark
(base sequence E in Rasmussen, 2004). Onlap
patterns are recorded in the south-western part of
the Norwegian-Danish Basin, against top sequence
CSS-6 (Fig. 2 K). An amplitude anomaly is recorded
within the lower part of sequence CSS-7, which
acts as a downlap surface for the upper part of the
same sequence in the Central Graben area. Two
main depocentres of mid-late Miocene age are
observed in the study area: in the southern Viking
Graben area (>300 ms TWT) and in the Central
Graben area (>1000 ms TWT; Fig. 13C). These are
separated by the Jæren High.
In the Viking Graben area, sequence CSS-7 is
represented by the Utsira Formation. The seismic
reflectors onlap against the East Shetland Platform
in the west and downlap the CSS-6 sequence in the
east, at the Utsira High area. In the Central Graben
area, sequence CSS-7 downlaps onto the underly-
ing CSS-6 sequence, thins across the Ringkøbing-
Fyn High and Coffee-Soil Fault Complex and
finally becomes truncated in the southern part of
the Norwegian-Danish Basin by glacial erosion
in Pliocene and Pleistocene time. The sequence
onlaps the Mid-North Sea High at the south-western
margin of the Central Graben area. Within the
Central Graben area, sequence CSS-7 is character-
ised by SW dipping clinoforms (Fig. 9).
Interpretation: The Miocene sequences reveal a
further development of the Oligocene trend
regarding the infilling of the North Sea Basin.
A lowering of the relative sea-level at the tran-
sition between sequence CSS-4 and CSS-5 is
indicated by the SU at top CSS-4. A subsequent
transgression is indicated from the onlapping pat-
terns in the same area (Fig. 2). Sequence CSS-5 in
the depocentre in the southern Viking Graben area
was sourced from the East Shetland Platform in
the west. Within the depocentre in the Norwegian-
Danish Basin, the clinoform orientation reflects
a continuing progradation from South Norway.
The lower part of sequence CSS-6, sub-sequence
CSS-6.1, was transported towards the SW from its
source area in mainland Norway. In contrast
sequence CSS-7, the Utsira Formation, was mainly
sourced from the west (East Shetland Platform),
although in the north-eastern parts it also received
sediments from southern Norway (e.g. Gregersen
et al ., 1997; Eidvin et al ., 2007; Gregersen &