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 551

of cores and seismic data (Kennedy 1987a, 1987b;
Skirius et al ., 1999). Re-sedimented chalks occur
in several areas of the Central Graben, notably
where inverted or halokinetic structures were
active (Perch-Nielsen et al ., 1979; Hardman &
Kennedy, 1980; Kennedy, 1987a, 1987b; Watts
et al ., 1980; Hardman, 1982; Brewster & Dangerfield,
1984; Brewster et al ., 1986; Hatton, 1986; Nygaard
et al ., 1993; Bramwell et al ., 1999; Farmer &
Barkved, 1999; Van der Molen, et al ., 2005).
More recent studies of seismic data from the
Danish and German offshore sectors, as well as in
the Paris Basin, have identified stratal geometries
in the Chalk interpreted to have been produced by
contourite bottom currents capable of producing
both erosional and constructional features (Lykke-
Andersen & Surlyk, 2004; Surlyk & Lykke-Andersen,
2007; Esmerode et al ., 2007, 2008; Surlyk et al .,
2008; Esmerode & Surlyk, 2009). The presence of
ridges, furrows and channels in chalk successions
has also been described in the extensive cliff
exposures of Etretat, northern France (10 m to
60 m deep, < 1 km wide channels; Quine & Bosence,
1991; Lasseur, 2007; Lasseur et al ., 2009). Channels
have in addition been observed in seismic data
from the offshore sector of the UK (Evans &
Hopson, 2000; Evans et al ., 2003), the Netherlands
(Van der Molen, 2004) and Denmark (Back et al .,
2011). Quine & Bosence (1991) interpreted the
channels observed at Etretat to be the product of
submarine erosion by tidal or oceanic currents
during lowering of relative sea-level. A similar
interpretation is favoured for the UK examples,
with shallowing suggested to have been induced
by tectonic uplift (Evans & Hopson, 2000; Evans
et al ., 2003). The Danish channel examples are
interpreted to have been formed by bottom cur-
rents (Esmerode et al ., 2007) or alternatively by
gravity flows (Back et al ., 2011).
The existing evidence from well cores, seismic
datasets and outcrop studies indicates that a range
of processes operated and interacted in the European
Chalk Sea. Pelagic deposition was the principal
'background' process, while re-deposition by mass-
flows and reworking or preferential deposition by
bottom currents played an important erosional
and depositional role due to the creation of sea
floor topography during tectonic uplift (Cartwright,
1989; Vejbæk & Andersen, 2002) and the varying
spatial pattern of epicontinental seawater circula-
tion. The Chalk Sea occupied Europe for nearly
35 Ma and hence a detailed stratigraphic knowl-
edge is required to reconstruct palaeogeographic

evolution. No detailed and regional stratigraphic
synthesis at the scale of the North Sea currently
exists. Current understanding of palaeogeographic
development has not been developed yet and
changing internal dynamics therefore relies on a
patchwork of local case studies such as this one.
The present study focuses on the Maastrichtian
Tor Formation in a central area of the Norwegian
North Sea sector, where both pelagic and gravity-
driven depositional processes as well as bottom
currents are recognised to have occurred. In this
area, a large channel (up to 5 km wide, ~30 km
long) has been identified within the Tor Formation.
Identification of the channel based on well and
seismic data was first published by Bramwell
et al . (1999). The channel has been drilled by the
2/4-12 well in 1975 and by the 2/8-15 well in
1995. The present study interprets the depositional
characteristics and evolution of the channel struc-
ture utilising a regional 3D seismic dataset, well
logs, biostratigraphic data and core material from
wells within and close to the channel. A regional
merged 3D seismic cube (PGS Megasurvey) with
line spacing of 12.5 m for both in-lines and
crosslines has been used. The dominant frequency
of the seismic dataset at the studied depth is about
26 Hz with vertical resolution of ~ 40 m. The lateral
resolution is around 200 m. The seismic data were
calibrated with a number of wells (2/7-4, 2/7-14,
2/4-12, 2/4-A-8, 2/4-A-6 and 2/8-15) provided
with gamma ray, neutron porosity, sonic and
density logs. Cores from wells 2/4-12 and 2/4-A-8
were logged and described. Detailed core descrip-
tions and core photographs for wells 2/7-4, 2/7-
14, 2/4-A-6 and 2/8-15 were also available and are
presented below.
The methodology used in this study is a seismic
stratigraphic analysis integrated with well logs
and core description. Key seismic horizons were
selected for interpretation based on their useful-
ness in defining a series of seismic reflection
packages marked by truncation or onlap relation-
ships. Amplitude and coherency attribute maps
were used to highlight the position and lateral
extent of the channel. Once identified on seismic
attribute maps, the channel was interpreted on the
basis of 3D seismic data as a seismically defined
body. The well data were integrated by using
synthetic seismograms to calibrate the well logs
and formation markers. Dating and lateral correla-
tion of the seismic horizons have been verified by
numerous wells with biostratigraphic data. The
selected boreholes have been correlated using