The Ormen Lange turbidite systems: sedimentary architectures and sequence structure of sandy slope fans in a sediment-starved basin - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 628

into the background depositional environment.
The laterally confined nature of individual beds,
potentially suggest a more tongue-like geometry. The
co-occurrence with deformed strata and injectites
suggest both basin-floor instability as well as
post-depositional remobilisation. The presence
of sandy injectites reflects post-depositional
de-watering, suggesting initial high water content
of the sandy parent bed, in turn reflecting rapid
en masse dumping or frictional freezing of sandier
flows (Lowe, 1982).
The slump-bed association consists of metres-
thick packages of deformed, slumped and thrust-
faulted Bouma-type (base-absent or top-absent) T abcd
turbidites (Table 3; Fig. 8D). The typically coarse-
grained character of the slumped Ta and Tb divisions
suggest proximity to channelised areas, similar to
what was inferred for the central to peripheral lobe
associations (see above). In turn, this favours an
origin as channel-mouth lobes (e.g. Deptuck et al .,
2008) or alternatively, as overbank or inter-channel
splay facies (e.g. Gardner et al ., 2003).
The background sediment associations consist of
deep-marine mudstones and claystones (Table 1).
The character and texture of these 'background'
mudstones vary stratigraphically, from thoroughly
bioturbated (BI 5 to 6) green-greyish mudstones to
pin-striped laminated, virtually unbioturbated, dark
claystones. Different combinations of lithofacies,
ichnofacies and microfacies allowed a subdivision
of these fine-grained lithologies into pelagic, hemi-
pelagic, turbiditic and contouritic mudstones and
claystones (Table 1).
The green-greyish variety consists predomi-
nantly of contouritic and hemipelagic mudstones
as well as some pelagic mudstones and claystones,
deposited in a well-oxygenated, lower slope set-
ting. The contouritic variety is identified based on
its lithology, which is similar but slightly siltier/
sandier than the hemipelagic equivalent and
microfacies type which reflects a stressed sea floor
environment. The microfacies observed in the
contouritic mudstones is similar to that observed
in the turbiditic equivalents but very different to
that of the hemipelagic and pelagic facies. The
ghost sedimentary structures, suggesting an origin
as up to 10 cm-high, slowly migrating (as can be
inferred from bioturbation along foresets) ripples,
is again different to what is observed in otherwise
similar-looking turbidite beds (representing
Bouma T cd subdivisions) or deposits of hybrid-flow
origin.

The pinstriped variety, by contrast, represents a
mixture of distal turbiditic mudstones (Stow &
Bowen, 1980; Janocko, 2008) with siltstone and
sandstone interlamina alternating with bands of
hemipelagic and pelagic mudstones and claystones.
The pinstripe-bedded mudstones are interpreted
to have been deposited in physico-chemically
stressed environments, related to some combination
of reduced oxygenation and increased sedimenta-
tion rates. Notably, the changes in the character of
the background mudstones typically parallel
stratigraphic variations in the sandy facies associa-
tions or fan types. The darker, pinstriped variety
comprises the background sediment to the more
disorganised or mass-flow-influenced fan facies
associations or subtypes. Where both the sandy
fan and background facies associations change in
consort, an allogenic mechanism or change in the
basin type or its configuration is favoured as the
controlling factor(s).
Thicker intervals (or beds) of massive, dark
pelagic and subordinate hemipelagic claystones
form the caps of the sandier packages or bedsets.
These pelagic claystones represent sand-starved
conditions and accumulated during prolonged
intervals when sandy fan deposition ceased, either
due to sand trapping in upslope subbasins or to
complete sand shut-off to the basinal areas.

Springar sandstones; reservoir architectures
and stratigraphic development

The bulk of the Springar sandstone interval consti-
tutes stratified units of interbedded turbiditic sand-
stones of the peripheral and lobe fringe facies
associations, as well as rare central lobe associations
with greenish-grey mudstones of mixed turbiditic
and hemipelagic origin. Pelagic mudstones are
common only at certain intervals, within cappings
of individual bedsets. The upper part of the Springar
sandstone interval is dominated by less well-
organised sandstones of the megabed, central lobe
and channel-belt facies associations.
The lower part of the Springar sandstone interval
in the Ormen Lange area can be organised into
packages up to 10 m-thick, although they typically
do not exceed 6 metres in thickness (Fig. 9). These
units commonly form bedsets composed of tabular
and relatively extensive beds that define coarsen-
ing-to-fining and thickening-to-thinning upwards
motifs (Figs 5 and 6). The basal coarsening-upwards
and thickening-upwards trends are commonly