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 554

inversion anticlines, halokinetic uplift and basin
margins steepening. Tectonically active areas
initiated mass movements of chalk, resulting in
the deposition of allochthonous facies into the
adjacent basins (i.e. Van der Molen et al ., 2005).
Examples of allochthonous chalk facies have been
commonly described from the North Sea region
(Watts et al ., 1980; Hatton, 1986; Kennedy, 1980,
1987a, 1987b) and many other European localities
(see Bromley & Ekdale, 1987 and references
therein). These studies have suggested slope
failures to be the main cause of re-sedimentation.
A study from the Danish offshore sector (Esmerode
et al ., 2008) has suggested that chalk mass
movements could be triggered by bottom currents
that destabilised submarine slopes by erosion.
The allochthonous facies produced by mass move-
ments include deposits of slides, slumps, debris
flows and turbidity currents.

This argillaceous interval at the base of the
Ekofisk Formation has been informally referred to
as Ekofisk dense zone or Ekofisk tight zone (Perch-
Nielsen et al ., 1979; Brewster et al ., 1986; D'Heur
1986; Surlyk et al ., 2003). No truncation has been
observed in the seismic sections at this level;
however local downlap terminations have been
noted in structural lows.
As a seismic unit, the Tor Formation (Fig. 3) has
a variable thickness in the study area, showing
marked thinning towards the basin margins and
local inverted structures, such as the Lindesnes
Ridge. The reflectivity of this seismic unit is gen-
erally good, although the seismic facies are highly
varied, ranging from strong parallel reflections
to chaotic packages. The Tor Formation can be
divided into two seismic sequences separated by
the horizon picked on strong negative amplitude
trough (dark-green horizon in Fig. 3). This horizon
shows features indicating a sequence boundary,
such as truncation of the underlying seismic
reflections (Fig. 3) and onlap or downlap by reflec-
tions of the overlying sequence. This inferred
mid-Tor Formation sequence boundary has been
dated to the boundary between the lower and
upper Maastrichtian and is interpreted to have
been caused by a eustatic sea-level fall (Bramwell
et al ., 1999). The two seismic sequences of the Tor
Formation are readily distinguishable in basinal
areas where continuous sedimentation formed a
relatively thick Maastrichtian succession. Where
the Tor Formation thins onto the Lindesnes Ridge,
the two sequences are more difficult to recognise
due to their reduced thickness and the formation's
irregular seismic aspect.
The lower sequence has seldom been cored due to
its poor reservoir quality. Therefore, facies interpre-
tation of this sequence is based on well logs and
seismic characteristics. The high frequency, lateral
continuity and weak amplitude of seismic reflec-
tions are interpreted to indicate autochthonous
pelagic chalk sediments deposited under tectonically
quiet conditions. However, local onlap terminations
and chaotic reflections are observed within this
sequence and might indicate the presence of gravity
flow deposits (cf. Van der Molen et al ., 2005).
The reflectivity in the upper sequence varies
from moderate to strong and seismic reflections
are generally parallel to slightly discontinuous
(Fig. 3); however, local mound-like features and
chaotic packages are also observed. The channel
that is the focus of this study is located in the
upper sequence and its top corresponds to the

Stratigraphy of the Tor Formation

The stratigraphic nomenclature used by this study
(Fig. 2) corresponds to the Chalk Group subdivision
of Bailey et al . (1999) that comprises, in ascending
order, the Hidra, Blodøks, Narve, Thud, Magne,
Tor and Ekofisk formations. The bulk of the Tor
Formation is of Maastrichtian age. Its thickness in
the study area averages ~ 300 m, but locally reaches
500 m. The formation is generally characterised
by chalk with low clay content, as indicated by
gamma ray logs from wells and petrographic stud-
ies (Bramwell et al ., 1999).
The lower boundary of the Tor Formation
(Fig. 2; violet horizon in Fig.3) is a significant
unconformity that is expressed seismically by
erosional truncation on uplifted highs and wide-
spread depositional onlap (Bramwell et al ., 1999).
In the Valhall Field, core data show the uncon-
formity to be developed as extensive hardgrounds,
condensed sections and omission surfaces
(Kennedy, 1980, 1987a; Farmer & Barkved, 1999;
Sikora et al ., 1999).
The upper boundary of the Tor Formation with
the Ekofisk Formation (Fig. 2; light-green horizon in
Fig. 3) is also an unconformity surface with a strati-
graphic hiatus identified from nannofossils and
foraminifera in several wells in the study area (Bailey
et al ., 1999; Lottaroli & Catrullo 2000). This hiatus at
the base of the Ekofisk Formation is usually repre-
sented by hardground and flint passing upwards
into a condensed chalk succession (~10 m to 20 m
thick) with high clay content (5 wt % to 20 wt %).