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Funnel-shaped channel planform geometry: the
funnel-shaped geometry of the channel suggests
gradual retrogression by a progressive headward
retreat towards the WNW (Fig. 16). Erosion of the
chalk ooze gradually produced a scour on the sea
floor and failure of the unstable chalk ooze at the
channel head. As the ESE-flowing current was
funnelled into the channel, it tended to increase
its velocity and erosive capability, thus keeping
the channel active and open until the widest point
at the ESE end of the channel where erosive
capability again decreased.
Channel orientation: the channel is orientated
parallel to the palaeobathymetric contours of the
Lindesnes Ridge, suggesting that it was generated
by bottom currents flowing parallel to this major
palaeotopographic feature. The broad southwards-
trending curve of the channel path towards its ESE
end is interpreted to have been controlled by sea
floor palaeotopography, notably by the position of
the Lindesnes Ridge and the SE Tor anticlinal struc-
ture located respectively to the S and NE. These
topographic highs were already prominent features
on the chalk sea floor during the late Maastrichtian
(Bramwell
et al
., 1999) and most likely constrained
the course of the channel during its development.
The asymmetric geometry of the channel
cross-section in its central and eastern segments
(Fig. 5, Sections C, D and E) is interpreted to be
indicative of a current flowing towards the ESE as
interpreted in other modern and ancient contour-
ite channels in the northern hemisphere (e.g.
Akhmetzhanov
et al
., 2007; Surlyk
et al
., 2008).
The asymmetric cross-sectional morphology of
this and other contourite channels has been
attributed to lateral asymmetry in the flow erosion
capability due to deflection of the current by the
Coriolis force. This leads to greater erosion on the
right side of the current flow and lesser erosion on
the left side (northern hemisphere; cf. Surlyk
et al
., 2008).
In the case of the channel studied, it is inter-
preted that the Coriolis force gradually deflected
the current towards the right side (southern side)
of the channel. Lateral and vertical velocity
gradients created a helicoidal flow where sedi-
mentation preferentially occurred on the left side
of the channel and erosion on the right side.
The channel does not appear to have been
formed by erosional processes alone. In seismic
cross sections, the channel appears as a scour with
sharp margins and without any sort of lateral lev-
ees (Fig. 5, Sections A and B). However, if formed
solely by erosion, the margins of the channel
would be unstable unless the chalks were already
lithified as hardgrounds during erosion (cf.
Esmerode & Surlyk, 2009). If this would have been
the case and erosion was the main mechanism for
the formation of the channel, the bottom current
would have necessarily been very powerful in
order to erode such a deep scour (~100 m deep)
into lithified chalk sediments. Alternatively, the
steep flanks of the channel may result from a
persistent aggradation of the channel margins,
while the axial zone was dominated by sediment
bypass and erosion alternating with periodic
accumulation of pelagic chalk. Stabilisation of the
channel flanks during progressive aggradation
probably occurred through the development of
hardgrounds during periods of non deposition
(cf. Quine & Bosence, 1991).
After an initial erosive phase, sediment bypass
and periodic pelagic deposition predominated in
the channel; currents persisted along the channel
axis, whilst aggradation continued at the channel
margins. The absence of allochthonous chalk
facies in the lower channel-fill unit indicates that
the channel margins remained gentle enough to
avoid collapsing. The channel was thus a
low-relief feature defined by differential sedimen-
tation rate and the growth of its margins.
A similar mode of channel formation has been
suggested for the chalk of the Paris Basin (Esmerode
& Surlyk, 2009) and the chalk of Etretat, Normandy
(Quine & Bosence, 1991). At Etretat, the channels are
up to 1 km wide and 60 m deep and have been
produced by bottom currents through sediment
erosion, winnowing and bypass combined with
alternating deposition and cementation of the
channel surface represented by the formation of soft-
grounds and hardgrounds. Those channels resemble
the present one in terms of both geometry (width,
depth, inclination of margins) and channel-fill
succession of autochthonous to allochthonous chalk
shared by the channel and its margins (Fig. 17).
Phase 2: Channel infill
The initial erosive and constructive phase was
followed by a phase of channel filling with mainly
allochthonous chalk, derived probably from the
channel flanks and the surrounding topographic
highs, such as the Lindesnes Ridge and the SE Tor
structure. A dominant southern provenance from
the Lindesnes Ridge for the material has been
identified by Sikora
et al
. (1999) based on
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