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 107

Table 1. Parameters indicating A/S change (increasing or decreasing) used to analyse the Statfjord Group in the Statfjord
Field; their relative importance and degree of applicability to the Statfjord succession.

Parameter indicating A/S change Importance Degree of applicability

1 Changes in the degree of palaeosol maturation High High

2 Variations in the thickness of preserved palaeosol profiles High High

3a Variations in the type and relative proportions of

preserved fluvial sandstone lithofacies

High High

3b Variations in the type and relative proportions of

preserved fluvial sandstone facies associations

Moderate Moderate

4 Variations in the degree of channel belt thicknesses

(1) and width (2) compared to single channel fills

High 1: High to 2: Low

5 Variations in the amount of fine-grained deposits

preserved in the succession

High under moderate to
low subsidence rate

High under moderate to
low subsidence rate

6 Frequency of occurrence and characteristics of (erosion)

surfaces between facies associations on a depositional
profile

Moderate Moderate

7 Variations in long-term avulsion frequency coupled

to variations in channel stability and location and
variations in the width of the floodplain

Moderate Low

3 Variations in the type and relative proportions
of preserved fluvial sandstone lithofacies and
facies associations indicating changes in chan-
nel style in response to upstream changes in
discharge and/or sediment load.
4 Variations in channel belt thicknesses and
width as compared to the average thickness and
width of single channel fills. This gives an
indication of changes in vertical and lateral
sandstone connectivity related to changes in
accommodation generation with respect to the
rate of sediment flux and water discharge. In
cases where the rate of deposition was (very)
high (with a sufficient rate of accumulation
space generation) two types of (combined)
responses are expected to occur: 1) much higher
avulsion frequencies can be expected (Bryant
et al ., 1995; Postma, 2014) which will lead
to the (partial) removal of floodplain fines by
erosion and a denser stacking of channels result-
ing in a higher connectivity and 2) higher pres-
ervation completeness (as expressed by the
preservation of barform tops) of individual
channel facies associations. In such cases,
systematic channel planform style changes may
indicate longer-term (>20 ka) extrinsically
driven variations in hydrodynamic flow condi-
tions during deposition.
5 Variations in the amount of fine-grained depos-
its preserved in the succession indicating
changes in accommodation generation with
respect to the rate of sediment delivery.
6 Frequency of occurrence and characteristics of
(erosion) surfaces between facies associations

within channel belts and on the overall deposi-
tional profile, such as the basal surfaces of local
individual channel fills, both in the coarse
(sand-filled channels) and well as the fine (mud
pellet-filled channels) sediment fraction. Key
issues include recognising the differences
between, for example, erosion related to i) down
cutting of a single channel or a distributary, ii)
deep thalweg or a confluence scour, or iii) a
channel belt creating a fluvial valley.
Alternatively, the erosion may be the product of
floodplain-wide degradation.
7 Variations in long-term avulsion frequency
(Stouthamer & Berendsen, 2000, 2007; Postma,
2014) coupled to variations in channel stability
(higher aggradation rates are expected to result
in a higher avulsion frequency; lower aggrada-
tion rates are expected to result in increased
channel stability and incision) and location and
variations in the width of the floodplain.

Trends in A/S change

It is emphasised that it is not so much the absolute
value of A/S but the A/S rate of change at a par-
ticular time scale that is important. Steady state
A/S can occur when either A or S or both (but
independently) are unsteady and fluctuate with a
short periodicity randomly around an average
(steady) value at short time scales so that the aver-
age over a longer time scale is constant. It can also
be the result of steady dynamic allocyclic forcing
(constant A/S) as a result of the autostratigraphic
response (Muto et al ., 2007). No indication has