Channel development in the chalk of the Tor Formation, North Sea: evidence of bottom current activity - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 578

ceased above the Tor-Ekofisk Formation boundary
at the base of the clay-rich Ekofisk dense zone
(Figs 5, 14 and 15) which marks a major basin-
scale turnover of the chalk depositional style with
the development of hardgrounds and significant
input of terrigenous clay/silt grade sediment
(Bramwell et al ., 1999).

It is possible that the erosion associated with
the base of the channel might reflect a minor
relative sea-level fall that narrowed sea gateways
and intensified bottom currents. Studies of the
Chalk Group in the German North Sea sector have
indeed indicated a climax of bottom current
activity in the late Maastrichtian (Surlyk et al .,
2008), a period of pronounced changes in global
ocean temperatures (Pucéat et al ., 2003) and
significant sea-level falls (Hardenbol et al ., 1998;
Miller et al ., 2004, 2005; Kominz et al ., 2008,
Surlyk et al ., 2008) possibly due to ice-cap growth
(Miller et al ., 2004; Bornemann et al ., 2008).
A hypothetical link between the origin of the
studied channel and the sequence stratigraphic
development of the Tor Formation postulated by
Bramwell et al . (1999) is suggested in Fig. 18.

DISCUSSION

Sequence stratigraphic implications

On seismic sections, numerous unconformities
have been recognised in the Chalk Group as
erosional truncations followed by bedding onlap
(Bramwell et al ., 1999). In the chalk successions
of the Norwegian North Sea, major unconformi-
ties have been interpreted to be produced by
tectonic uplift and subsequent erosion by gravity-
driven processes, e.g. unconformities at Top Narve
and Top Thud formations (Bailey et al ., 1999;
Hampton et al ., 2010). In other chalk successions
in the North Sea and in the Danish Basin,
prominent unconformities have been considered
by some authors as evidence of increased bottom-
current activity and sea-level fall and may have
possibly played an important role in the intensifi-
cation of bottom currents (Esmerode et al ., 2007,
2008; Surlyk & Lykke-Andersen, 2007; Surlyk
et al ., 2008). This notion would imply that
sequence bounding unconformities might have
formed in basins sharing the same bottom current
system (Faugères et al ., 1993). However, the
applicability of sequence stratigraphic concepts to
relatively deep-water successions with uncon-
formities/erosion surfaces produced by bottom
current processes remains uncertain, mainly on
account of diachronous variations in current
strength and uncertainties in sediment dating
(Faugères et al ., 1999; Stow et al ., 2002).
The North Sea was tectonically active during
the Late Cretaceous and hence there are also
reasons to believe that tectonic seismicity and
subsequent erosion by gravity-driven re-sedimen-
tation caused the formation of at least some of the
unconformity surfaces (e.g. Bramwell et al ., 1999).
Progressive uplift of the inversion zones during
the late Maastrichtian associated with lowering of
the sea-level may have further focused the flow of
bottom currents in addition to have increased
gravity-driven erosion and remobilisation of
previously deposited chalks.

Geographical location of the channel

The location and geometry of the channel suggest
that it was generated by a bottom current flowing
ESE parallel to the bathymetric contours of the
Lindesnes Ridge (Fig. 19). Interpretation of flow
towards the ESE is based on the funnel shaped
geometry of the channel, the proposed mecha-
nism of enlargement by headward retrogression
and by erosional versus depositional margins. The
channel location also suggests strong influence by
other topographic features. For example, the chan-
nel appears to have been constrained by the
Ekofisk and SE Tor anticline structures that flank
the channel along its northern margin (Fig. 19).
The WNW end of the channel can be identified
clearly from seismic data (Figs 4 and 5) and is
located in a horizontally stratified area to the
north of the Eldfisk Field. Strata in this area are
gently dipping to the NW and represent a sort of
shelfal region when compared to the deeper basin
located to the NW. Since it is considered that the
current in the channel was flowing from the
WNW to the ESE, the bottom current that formed
the channel must have originated as a current
that followed the elongate, deep basinal areas of
the Central Graben. The current is interpreted to
have flowed along the eastern margin of the
Josephine High before passing between the nar-
row sea-pathway flanked by the Albuskjell Field
in the N and a salt diapir in the south (Fig. 19).
This may have forced the current to travel upslope
and reduce its flowing section (i.e. confinement),
thereby increasing its velocity and erosive
potential. Once the current reached the flat area