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 109

(A) (B)

Mature paleosol

Less mature paleosols
and floodplain materials

Fig. 5. Palaeosol maturation and accommodation/sediment supply (A/S) relationships. In the stratigraphic analysis of
fluvial successions, the degree of maturation of palaeosols developed in floodplain mudstone deposits and their relation
with underlying and overlying palaeosols needs to be taken into account. Decreasing palaeosol maturation values (indi-
cated by the colour transition from green to yellow) stratigraphically upward from a mature caliche horizon suggests
increasing A/S values (A). Increasing palaeosol maturation values (indicated by the colour transition from yellow to green)
suggest decreasing A/S values (B). White dots in the brown-coloured interval indicate caliche in a mature palaeosol.

two trends defined herein. However, here, trends
are defined based on a number of criteria that
define changing A/S conditions through time
(although stratigraphic intervals typified by
increasing and decreasing A/S values were also
mapped by Martinsen et al ., 1999). Deposition
and preservation of sediment operate at different
rates and time scales. Different time scales of
interacting controlling mechanisms (tectonic and
climatic) result in a hierarchy of cyclicity and
related sequence stratigraphic surfaces formed by
fluvial processes.

Berendsen (2000, 2007) suggest that avulsion fre-
quencies may decrease or increase during periods
of decreased aggradation depending on the time
scale and that avulsion locations are primarily
related to allogenic factors interacting over time
and space. Flume studies on the other hand sug-
gest that avulsion frequencies increase during
increased aggradation rate and vice versa (Bryant
et al ., 1995; Postma, 2014). This is here interpreted
to be possibly related to the fact that these flume
studies primarily address the effects of changes in
aggradation rate over longer time scales. Individual
avulsions are commonly locally occurring events
and, given the above mentioned factors, do
not necessarily have sequence stratigraphic cor-
relation value. Stouthamer & Berendsen (2007)
reported a short-term autogenically controlled
avulsion frequency for the Rhine-Meuse delta of
500 yr and an allogenic controlled long-term trend
at a time scale of approximately 10 4 yr. Given this
behaviour and the uncertain nature of factors con-
trolling avulsions in continental fluvial systems,
it is therefore important to recognise single avul-
sion products in core and exclude these from
usage in stratigraphic correlation. Although prob-
lematic, single avulsion products may be recog-
nised by higher preservation completeness of an
individual sandstone body as expressed by the
preservation of barform tops of individual chan-
nel facies associations. The occurrence of avul-
sion products in a stratigraphic unit bounded by
correlatable surfaces can be used for analysis of
environmental and stratigraphic change. An
example of these principles of correlation in flu-
vial successions is shown in Fig. 7.
An expansion surface (Dahle et al ., 1997;
Martinsen et al ., 1999) may develop during

Long-term A/S change versus short-term
autogenic processes

The stratigraphic analysis of changes in A/S value
can only be applied to sediment stored in the
net accommodation 'window' (see definition in
Appendix I). It cannot be applied to sediments
preserved in the real-time accommodation
'window' (see definition in Appendix I) because
of the temporary nature of sediment storage.
Sediment deposited in the real-time accommoda-
tion 'window' as a result of avulsions can only be
permanently preserved when the channel(belt)
stays away long enough from the avulsion site for
the sediment to subside into the net accommoda-
tion 'window' (Fig. 6). For example, avulsions on
the Holocene Rhine-Meuse lower delta plain are
both autogenically as well as allogenically con-
trolled and are associated with periods of aggrada-
tion (Stouthamer & Berendsen, 2000, 2007).
Avulsion frequencies occur at various time scales.
Shorter periodicities are likely to be autogenically
controlled whereas longer periodicities appear to
be controlled by allogenic factors. Stouthamer &