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a general tendency for increase in net-to-gross
sand content from the proximal to distal part of
the fan is observed. This is most probably related
to the transition from a proximal by-pass domi-
nated setting consisting mostly of channels to a
more distal deposition dominated setting consist-
ing mostly of splays (e.g. Piper & Normark, 2001).
However, in well 25/2-6, there is an abnormal
thickness of sandstone that is interpreted to result
from sand injection. The sand in this case is
thought to have been evacuated from the area
drilled by well 25/5-1 (Fig. 13).
sand factories, able to carry, separate and clean
sand from mixed load”. It is worth noting that the
vertical sequence of facies seen in core from well
25/6-3 is strikingly similar to the lateral facies var-
iation documented in the active Congo turbidite
channel some 200 km basinwards from the canyon
(Babonneau
et al
., 2010, compare Fig. 11 herein to
their Figs 7 and 8); The massive, structureless,
medium-grained to fine-grained sand is found in
the axis of the Congo channel and the mud-silt
successions are found in the levees.
Our interpretation of leveed channels in the
Hermod Formation (Fig. 8) could imply that a
similar segregation process has produced the
homogeneous sand that typifies the Hermod
Member. The very abrupt vertical change in lithol-
ogy from massive sandstone to mudstone observed
in the wells may relate to the rapid abandonment
of discrete channels and their frontal splays giv-
ing way to deposition of deep-marine shales. This
interpretation can account for the abundance of
clean sandstone not only in channels but also in
remobilised sediments. The clean sand seen in
injectites and steep-sided mounds is most proba-
bly inherited directly from the parent bed and no
significant additional sandstone homogenisation
during remobilisation appears to have occurred.
For example, the channel drilled by well 26/4-2
consists of equally clean sandstone as the steep-
sided mound drilled by well 25/2-6.
The vertical facies succession in core from well
25/6-3 may be interpreted as a channel-levee
association, where the massive sand represents
the channel axis, the overlying dark shale repre-
sents upstream avulsion and abandonment and
the silt-mud layers represent the lateral levee spill
from an adjacent channel (Fig. 11). The very dis-
similar thickness and number of sandstone and
shale beds seen throughout the fan (Figs 4 and 13)
provide evidence of avulsion and fan-switching
generating sequences which form an apparently
random overprint on the long term allocyclic pat-
terns of initiation, growth and retreat.
Why is the Hermod Member dominated by clean
sandstone units overlain abruptly by mudstone?
The dominant lithology in all wells in the mid-
fan to outer-fan is clean sandstone characterised
by a blocky gamma-ray log pattern with smooth
and almost constant low readings and abrupt
changes to higher readings in the shale above and
below (Fig. 4). The blocky log response defines
sandstone units that are several tens of metres
thick and this petrophysical log response repre-
sents 40% to 80% of the total Sele Formation
thickness at the well locations. The thick, blocky,
sandstone units are typically separated by a mud-
stone dominated interval that is < 10 metres thick.
1 to 3 blocky sandstone beds (>10 m thick) occur
in each well, mainly in the older S1 part of the
Sele Formation (Fig. 4). The high sandstone pro-
portion and abrupt contact with overlying mud-
stone is not a peculiarity of the wells included in
the present study but a common feature of the
Hermod Member throughout the North Sea
(Brunstad
et al
., 2009).
By analogy with modern fans, it is thought that
sand and mud can become segregated during
transportation with the sand concentrated in
channels and frontal splays (Piper & Normark,
2001) and it is probable that sufficient mud was
advected by the time currents reached the outer
fan that clean sand was deposited (e.g. Peakall
et al
., 2000, p. 83).
Lopez (2001) argued that mixed load input to
the amazon submarine channels becomes segre-
gated as fines build levees and coarser material
becomes confined increasingly to the channel
axis, ultimately resulting in clean sand in the
channel and fine-grained, mostly muddy and silty
levees. The argument was re-iterated by droz
et al
.
(2003) in relation to the Zaire (Congo) submarine
fan, with leveed channels described as “powerful
IMPLICATIONS
Studies of 'seismic scale' outcrops and detailed
images from modern systems have narrowed the
gap in scale of observation between ancient and
modern submarine fans. The complexities of these
systems have now been documented in numerous
papers but the limitations imposed by seismic
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