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 638

of sand supply in upslope basins and secondly to
full retreat of the deltaic delivery systems to the
basin-margin, leaving the remainder of the basin
in a sediment-starved state.

drainage basin flood events, a prolonged interval
of time is required for the formation of a storey.
In such a case, the alternating predominance of
channel storeys to lobe storeys and hemipelagic/
pelagic drapes may correspond to longer-term
changes in flood frequencies in the drainage basin
and thereby point to climatic cyclicity. In real world
examples, lobe storeys probably form from single,
exceptionally catastrophic, floods as well as via a
series of floods of variable magnitude and frequency.
These variations have the potential of leading to
variable stacking patterns (e.g. vertical, compensa-
tional, or shingled) of the component beds.

Bed, compound beds and lobe storeys

At the other end of the spectrum of stratigraphic
resolution, individual beds and compound beds
('bed elements') probably reflect single major
floods in the alluvial and fluvio-deltaic hinter-
land, at least during the progradational stages of
fan development. The origin of lobe storeys is
more speculative. However, the amalgamated
character of the central lobe packages, coupled
with their down-lobe and across-lobe storey
feathering into layered packages of sandstone
beds of inferred hyperpycnitic affinity and inter-
bedded turbiditic mudstones, suggests either 1)
prolonged but rather irregular delivery, or 2)
repeated but frequent sediment delivery from
hyperpycnal flows. An individual lobe storey is
therefore interpreted to either represent a single
major flood event of fluctuating intensity, or a
series of frequent floods in the drainage basin. In
that respect, it is interesting to note that the
inferred dimensions of (and thus volume of) sedi-
ment within a single lobe storey is of the same
order or slightly larger than that of deltaic mouth
bars related to single flooding events (e.g. Fielding
et al ., 2005). If a single flood were able to deliver
all of its sediment load to the fronting deepwater
areas, larger architectural elements would form,
as additional sediment would be incorporated
into the flow due to: 1) flow erosion into former
delta-front sediments (e.g. Deptuck et al ., 2008);
and 2) via conduit flushing en route to the receiv-
ing downslope basin (Piper & Normark, 2009).
The similar thickness of channel storeys and
lobe storeys, combined with the facies characteris-
tics of the channel storeys points to channel-fills
originating from a combination of erosion, trac-
tional deposition and lag aggradation during lobe
construction and a later 'backfilling' stage, poten-
tially reflecting waning flow conditions. The latter
was probably promoted by the 'backstopping' effect
of the depositional lobe topography at the channel
'mouth'. It is accordingly argued that architectural
elements from single beds to lobe storeys (Fig. 14),
in principle, represent climatic signals, either sea-
sonal or periodic outsized flooding events. If, on
the other hand, a channel storey to lobe storey
reflects deposition during a series of recurring

Lobe storey set and lobe complexes

Avulsion of the site of active lobe deposition
probably resulted from upfan lateral channel
shifting or avulsion, in turn probably induced by
(near-) complete channel-plugging from preced-
ing flows or flow events. As a corollary, lateral
migration may be favoured in instances where
the bed or lobe storey produced during the pre-
ceding depositional event(s) were of insufficient
height to promote channel backfilling. Lateral
migration and short distance avulsion would
have led to lateral shingling or compensational
stacking of channel storeys and lobe storeys,
respectively. The resultant stack of lobe storeys
is referred to herein as 'lobe storey sets' or 'lobe
complexes'. Borehole dipmeter data suggest that
these lobe storey sets or lobe complexes were
constructional features with significant deposi-
tional relief and topography to form an undula-
tory seafloor. Such a depositional topography
would have had the potential to promote back-
filling of the submarine fan distributary system
and, with time, avulsion. Higher order avulsion,
leading to longer distances lateral shift in the site
of active fan deposition, resulted in the forma-
tion of a new channel storey to lobe storey set or
complex. In either case, the stacking of bed, bed-
sets and lobe storeys to form channel storey and
lobe storey sets and complexes are interpreted to
represent autogenic processes on the fan surface
and should be regarded as various types of 'self-
organising' processes common to and intrinsic
to any type of channel-fed depositional sub-
environment (e.g. Piper & Normark, 2001).
Lobe storeys should ideally feather out laterally
and distally through turbidite to hemipelagic
mudstones. With increasing distance from the
active site of lobe deposition, the mudstones