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 572

These basal deposits are overlain by a thick
accumulation of allochthonous chalks interpreted
to have been deposited by slumps (facies DC,
Table 2), debris flows (facies PFC) and mudflows
(facies HC). Pelagic facies (BHC) are subordinate
and only occasionally present. The change from
pelagic to allochthonous mass-flow deposits is
generally sharp, but may appear to be transitional
due to substrate deformation. The thickness of the
mass-flow deposits decreases and slump deposits
become rare passing upwards in the succession.
The occurrence of slump deposits (facies DC) in
well 2/4-A8, close to the channel, may reflect
instability of the areas marginal to the channel.

part of the channel-fill succession (around 40%)
than the successions in the areas marginal to the
channel (around 30%) and are less intercalated
with other slumps (facies DC) and low density
turbidite deposits (facies HC); (3) the thickness of
intercalated slump and low density turbidite
deposits (facies DC and HC) is greater in the areas
marginal to the channel than in the upper chan-
nel-fill unit; (4) the clast size in the deposits of
the upper channel-fill unit is considerably larger,
particularly considering the large clasts observed
in well 2/8-15.
Some other aspects of the successions of the
channel-fill and in the areas marginal to the
channel are remarkably similar, namely their
range of facies and their distinct stratigraphic
bipartition, with the lower unit containing a
significant amount (~35%) of autochthonous
chalk facies. The similarity of both core facies
associations and wireline logs suggests that simi-
lar sedimentation occurred concurrently, but at
different rates, both within the channel and over
areas that were marginal to it.

Correlation of the channel-fill with the deposits
in the areas marginal to the channel

Correlation of well logs within the channel-fill is
based on the recognition of the distinctive lower
and upper channel-fill units (Fig. 14). The upper
channel-fill unit can be further subdivided into
two distinct intervals, the lower one having
distinctly lower porosity (upper unit low porosity
interval in Fig. 14) and thinner high-to-low
porosity variations compared to the rest of the
unit. This difference is interpreted to reflect an
initial phase of the channel filling, characterised
by interplay of a bottom current flowing through
the channel and gravitational mass movements
from the channel flanks.
The correlation of wells 2/7-4, 2/7-14, 2/4-12,
2/4-A8 and 2/4-A6 is based on core sedimentology,
wireline log responses and regional seismic
interpretation (Fig. 15). Biostratigraphic studies of
microfossils and nannofossil have been used as a
support for the lithological and seismic based
correlation. Correlations between the successions
of the channel-fill and those at the margins of the
channel suggest chronostratigraphic equivalence
between the lower channel-fill unit and the
autochthonous pelagic facies of the areas marginal
to the channel, as well as between the upper
channel-fill unit and the allochthonous facies of
the channel margins.
The channel scour and its infill, although
recognisable in seismic sections, apparently do
not contain facies that are markedly different
from those in the areas surrounding the channel.
The main differences observed are as follows:
(1) the channel-fill succession is thicker than in
the areas marginal to the channel; (2) debris flow
deposits (facies PFC, Table 2) constitute a greater

CHANNEL EVOLUTION

The evidence from wells indicates that the channel
developed through two main phases: an initial phase
of erosion followed by current-controlled aggrada-
tion of pelagic chalk, resulting in a steep sided sea
floor conduit and a secondary phase that involved
mainly passive infilling of the channel by gravita-
tional processes. The two phases are portrayed in
Fig. 16 and discussed in the following sections.

Phase 1: Channel development

Channel features similar to the one studied have
been described from the Chalk Group elsewhere
in NW Europe and have been interpreted to result
from a variety of possible causes: (i) the erosive/
constructive action of gravity flows passing
downslope (Back et al ., 2011); (ii) the erosive/con-
structive action of contour-parallel bottom cur-
rents (Lykke-Andersen & Surlyk, 2004; Surlyk &
Lykke-Andersen, 2007; Esmerode et al ., 2007,
2008; Surlyk et al ., 2008; Esmerode & Surlyk,
2009) and; (iii) submarine erosion due to tidal or
oceanic currents (Evans & Hopson, 2000; Evans
et al ., 2003). Recent seismic-based study of the
Chalk Group in the Danish sector (Back et al .,
2011) has suggested that an incised channel