Unravelling the nature of deep-marine sandstones through the linkage of seismic geomorphologies to sedimentary facies; the Hermod Fan, Norwegian North Sea - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 663

has a high gamma ray (GR) reading and a similar
GR peak can also be seen immediately above the
un-cored but convincingly imaged channel sand-
stone in well 26/4-2.
Interpretation and discussion: The process
interpretation of massive sandstones of the type
described from well 25/6-3 is difficult. Several
alternative interpretations have been proposed
over the years for massive deep-marine sandstones
of this type (see overviews by Stow et al ., 1996;
Nemec, 1997; Baas, 2004). The possible interpre-
tations include deposition under a high-density
turbidity current (Lowe, 1982); post depositional
homogenisation caused by fluidisation (Hiscott,
1979; duranti et al ., 2002; duranti & Hurst, 2004;
Jonk et al ., 2005) and 'freezing' of a sandy debris
flow (Shanmugam, 1996; Shanmugam et al ., 1995;
Marr et al ., 2001; cf. Hiscott et al ., 1997). The mas-
sive sandstone facies is interpreted here as the
deposits of high-density turbidity currents. It is
suggested that deposition occurred in a channel,
primarily based on seismic geomorphology (Figs 9
and 10). The dark grey high GR clay directly above
the sandstone is believed to result from a channel
abandonment phase characterised by slow depo-
sition in an anoxic environment. a similar GR
response occurs on top of the channel drilled by
well 26/4-2.
The lack of tractional structures within the
sandstone could be explained as the product of
rapid fall-out from suspension with insufficient
time for development of an organised traction car-
pet (Walker, 1978). The general lack of vertical
grain-size grading may be attributed to deposition
under a sustained, steady flow, whereas the subtle
variations in grain size observed in some places
may relate to waxing and waning flow either dur-
ing one flow event or by amalgamation of deposits
from successive currents with slightly different
flow power (Kneller & Branney, 1995; Kneller &
McCaffrey, 2003). Cores with amalgamated mas-
sive sand collected from the active Congo subma-
rine channel have recently been interpreted in
terms of reworking of the sediment deposited in
the channel floor by erosion and traction processes
at the bases of turbidity currents (Babonneau et al .,
2010). The core from the Hermod Member may
also consist of a series of amalgamated deposits
from turbidity currents. This interpretation is sup-
ported by the network of channels and complex
splays that can only be understood as the products
of a series of flows travelling through feeder chan-
nels. The flows were most probably not identical

in terms of grain size, density and thickness so that
variations in these parameters could have resulted
in readjustments of the slope equilibrium profile
due to deposition and erosion (Kneller, 2003).
adjustments in the thalweg profile can also have
been triggered by avulsions or flows encountering
a structurally controlled sea floor relief (Pirmez
et al ., 2000). It is therefore likely that repeated epi-
sodes of channel floor aggradation and erosion
occurred before sand deposition ceased in such
channels. Each turbidite bed may have been
deposited with a mud cap, but this has been eroded
by subsequent currents. The last turbidity current
that travelled through the drilled feeder channels
could theoretically have by-passed and eroded the
top of previous turbidites before it deposited fines
from suspension (see Baas, 2004, p. 308, example 3).
These fines are eventually incorporated in the
dark grey shale represented by a high GR reading
directly on top of the sandy turbidite beds. The
upward transition from dark grey shale to shale
with silt laminae may represent the later overspill
from adjacent flows.
The presence of a sharp upper contact with
overlying shale may relate to a narrow grain-size
range available during deposition, but the absence
of traction structures towards the top is enigmatic.
This enigma was first discussed by Walker (1965)
who suggested that freezing of a traction carpet
could cause by-pass of overriding fines. However,
as thoroughly addressed by Baas (2004), the inter-
pretation of the sharp upper transition to shale
(Fig. 9) as the product of a turbidity current is
problematic because turbidity currents inevitably
pass through a final waning phase when bed-
forms should be produced. If beds with a massive
top are deposited from turbidity currents, it seems
more likely that the sharp upper contact resulted
from secondary processes such as fluidisation or
erosion (Baas, 2004).
an alternative interpretation is that the cored
interval in well 25/6-3 was fluidised. This inter-
pretation is supported by the recognition of
injectites in the overlying shale (Fig. 11). Post
depositional homogenisation could either occur
as 'in situ homogenisation' or 'injection of the
entire sand' (Hiscott, 1979). abrupt upper con-
tacts to shale, lack of primary depositional struc-
ture, common injectites in the shale above and
steep-sided mounded geometries are some of the
typical characteristics of injected beds (e.g. Jonk
et al ., 2005, Hurst & Cartwright, 2007 and refer-
ences therein, Huuse et al ., 2010 and references