Investigating the autogenic process response to allogenic forcing: experimental geomorphology and stratigraphy - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 133

accounted well for the difference in variability in
the shoreline migration rate observed during the
falling and rising rims of the sinusoidal base-level
cycle (see Fig. 11 in Kim et al . (2006a)). In the
model, the magnitude of the slope increase within
a single modelling time step during the storage
events are constrained by the supplied sediment
discharge; and thus the slope increment is smaller
for a larger delta. A fraction of the topset slope
averaged over the total experiment (i.e. 1% to 4%
in XES 02) was given to limit the total range of the
slope fluctuation in the model. The modelling
results show that autogenic event signatures (i.e.
variability in shoreline migration rate) differ
according to the direction of base-level change
even though the autogenic process size (i.e. the
range of topset slope fluctuation between maxi-
mum and minimum threshold slopes) is assumed
to be constant. Base-level fall enhances the sedi-
ment release process, forcing the shoreline sea-
ward migration, but regression is inevitable in this
setting, thus diminishing the effect on varying the
shoreline migration. However, a sediment release
event during overall transgression easily reverses
the direction of shoreline migration because the
delta front develops over the shallow submerged
topset surface. In the Kim et al . (2006) study, it is
clear that the delta geometry plays a role in the
footprint of the shoreline autogenic fluctuation
during base-level rise and fall but it is not clear if
the event size (angle between threshold maximum
and minimum topset slopes) varies by the direc-
tion of the base-level change due to the lack of
high-res (in time) topographic scans. The follow-
ing experiments address in part how autogenic
event size varies during base-level rise and fall.
A research team in the ExxonMobil Upstream
Research Company conducted a series of experi-
ments using a sediment mixture with a polymer
that improves deposit cohesiveness (Hoyal &
Sheets, 2009; Martin et al ., 2009b). This sedi-
ment mixture restricts both channel sidewall
erosion and channel widening, thus allowing for
relatively stable, distributary channel networks
to form, in contrast to the typical braided system
in other experiments discussed in this paper.
Martin et al . (2009b) produced a cohesive delta
in an experiment with two stages: stage 1 with-
out base-level rise; stage 2 with base-level rise. In
the second stage, the rate of base-level rise was
kept constant at a rate designed to nearly main-
tain the same size of the delta top surface (i.e.
strongly aggrading delta with minor progradation).

The base-level rise in the second stage forced a
two-fold increase in fluvial deposition and caused
a two-fold increase in frequency of the fluvial
autogenic process compared to the first stage.
Roughness of the shoreline along the experiment
was also measured and characterised, thus show-
ing statistical saturation at an opening angle
θ = 16.5º (distributary lobes are scaled with 2 θ ).
These shoreline roughness and lobe scales are
consistent across both the delta progradation
(with no base-level rise) and aggradation stages
(with base-level rise), supporting the two-fold
increase in the autogenic channel time scale due
to enhanced fluvial deposition of the supplied
sediment. High-resolution topographic data are
still missing in this experiment, which might
allow for detecting changes in the threshold
slopes due to base-level forcing in more detail.
However, the consistent roughness in the shore-
line across the two stages of constant base-level
and linear rise of base-level hints that no major
changes in the autogenic event size occurred due
to the base-level control.
The migration of the upstream end of the exper-
imental deposit in these experiments (XES 02 and
Martine et al . experiment) is restricted due to the
vertical tank wall. As a result, the upstream
boundary at the transition between alluvium and
bedrock exposure (henceforth alluvial-bedrock
boundary) could not migrate freely. Kim & Muto
(2007) presented a series of experiments that
allowed for free migration of the upstream end of
the deposit. An isolated delta free from the tank
walls developed over a sloped non-erodible base-
ment in each of their experiments. The basement
slope was set at a higher slope than the steepest
delta topset slopes so that sediments bypassed the
exposed upstream bedrock surface. Either con-
stant base-level rise or fall was applied over the
last half of an experiment after the first half of sta-
tionary base-level. The overall trend of the varia-
bility change in the shoreline migration rate
during base-level rise and fall is in good agree-
ment with the previous results: that is maximised
when the mean shoreline migration direction is
against the base-level change, but minimised if
they are aligned. Thus there is no difference in the
process for sediment internal buffering by allow-
ing a free moving alluvial-bedrock boundary.
However, the amplitude of the autogenic signal in
migration rate of the alluvial-bedrock transition
increases during base-level fall and diminishes
during base-level rise, which is the opposite trend