Applying accommodation versus sediment supply ratio concepts to stratigraphic analysis and zonation of a fluvial reservoir - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 121

Graded floodplain profile

Real time accommodation space

Sea level = ultimate base level

Geomorphic level of erosion: stratigraphic reference (base) level

Real time accommodation space

Sea bed

Net accommodation space

Datum

Fig. 14. Position of the stratigraphic reference, or base level, in alluvial and fluvial depositional systems and its relation
with sea-level and the sea bed.

the physical location of the equilibrium profile in
the alluvial subsurface rock record (i.e. core) and
how it changes with time and location. Factors that
influence sedimentation and erosion in alluvial
plain and fluvial environments include variations
in (i) dispersive sediment transport in response to
the stream power, (ii) rate of sediment supply,
(iii) grain size of sediment delivered and (iv) com-
paction (Thorne & Swift, 1991; Schumm, 1993).
The boundary between the aggradational and degra-
dational river states is the graded state (Davis, 1902;
Mackin, 1948) and Swenson & Muto (2005) showed
that the graded state is linked to sediment supply
and the parameters discussed above.
From studies on modern river systems it is clear
that no universally accepted set of criteria exists
for determining whether all or part of a river sys-
tem is in equilibrium and the 'average' character
of a stream is uncertain from observation at a spe-
cific geographic location (Knighton, 1998). It is
therefore almost impossible to determine whether
an ancient stream was in equilibrium, in fact it is
most likely that the system was not in equilibrium
at the point in time of preservation as each chan-
nel is subject to short-term fluctuations and long-
term evolutionary tendencies (Knighton, 1998).
The co-occurrence of different channel types in
specific stratigraphic intervals, for example, could
point to a state of disequilibrium or non-equilib-
rium of streams at medium (10 3 yr to 10 4 yr) to long
(> 10 5 yr) time scales.
It is proposed here to use the lower boundary of
real-time accommodation or upper boundary of
net accommodation (lower limit to erosion) as the
stratigraphic reference level or fluvial base level
(Figs 13 and 14). In areas not directly affected by
laterally migrating fluvial channels or channel
belts, the upper boundary of net accommodation
(lower limit to erosion) at a specific time scale can

in some cases be genetically coupled to the most
mature palaeosols (Weissmann et al ., 2002;
Bennett et al ., 2006). However, in other cases large
interfluve areas can have a complex nature with
co-existing lakes, small rivers and aeolian dunes
in addition to areas experiencing, for example,
gully erosion (Gibling et al ., 2005; Tandon et al .,
2006). In such cases, the active upper zone of the
interfluve area lies within the envelope of short-
term accumulation. The stratigraphic reference
level (the upper boundary of net accommodation
equivalent to the lower limit to erosion) may in
such cases be picked at a laterally extensive cali-
che layer formed at a depth relatively unaffected
by short-term erosion and deposition (the base of
the envelope of short-term accumulation). The
upper limit of real-time accumulation space (or
temporary deposition) is not considered a valid
choice for the stratigraphic reference level because
the upper limit to deposition will not be preserved
systematically in the stratigraphic record. Note
that the concept of stratigraphic base level was
originally defined as the equilibrium surface that
'describes the direction in which a stratigraphic
system is likely to move, toward sedimentation
and stratigraphic preservation or sediment bypass
and erosion' (Barrell, 1917; Sloss, 1962; Wheeler,
1964; Cross, 1988). Blum & Törnqvist (2000) noted
that the concept of stratigraphic base level is really
the same as the upper limit of accommodation.
They considered that it has outlived its usefulness
and should be abandoned.
The nature of the relatively most mature palaeo-
sols depends on the location of the alluvial system
with respect to the global (and zonal) system of
convective atmospheric circulation ('Hadley' sys-
tem controlling both temperature and humidity)
and the local climatic regime (controlled by local
geographical factors). For example, in a wet-dry