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reworked during the storage and release events, in
comparison to XES 05. The high water discharge
in XES 02 also took a relatively longer period for
channelisation and led to larger degradation in
the delta top surface. More symmetrical cycles of
the wetted fraction were shown in XES 02 while
asymmetrical (positively skewed) fluctuations
were dominant in XES 05, which indicates a rela-
tively different period for the release event. XES
05 shows quicker release of sediment and a longer
period for sediment storage, whilst XES 02 shows
a very symmetric distribution of times for storage
and release events (Fig. 2).
signatures of palaeo-environmental variations. The
causes of these changes are frequently inferred from
episodic active tectonics separated by relatively
quiescent periods (e.g. Blair & Bilodeau, 1988; Dorsey
et al
., 1997), climatically controlled variation in
sediment yield (e.g. Smith, 1994), sea-level change
(e.g. Posamentier
et al
., 1988; Posamentier & Vail,
1988) and global climate changes by the Earth's
orbital cyclicity (e.g. House, 1985) and solar and
oceanic variations (e.g. Roth & Reijmer, 2005).
However, stratigraphic interpretations of high-
frequency signals have been questioned in the
rock record. It has been argued that depositional
systems might not fully record high-frequency
external changes in the stratigraphy if the time
that sedimentary systems need to respond to the
external changes is longer than the period over
which the external forcing cycles. It has also been
argued that depositional systems might exagger-
ate high-frequency autogenic signals in the stra-
tigraphy, depending on the nature of the external
forcing. A characteristic response time for depo-
sitional basins was first theoretically derived by
Paola
et al
. (1992) and Paola (2000) and implica-
tions of this scaling to natural systems and flume
analogues were further discussed in Postma
et al
.
(2008). The basin equilibrium time serves to
define what 'high frequency' is here; high-fre-
quency external controls are environmental
basin forcing that cycles over a smaller time
period compared to the basin equilibrium time-
scale. This time scale takes the following form:
Suggestions for further work
The experimental studies discussed above provide
a suite of quantitative measurements of autogenic
processes; however, more thorough investigations
are required to understand the physical links
between the sediment and water discharge condi-
tions and autogenic processes. The experimental
studies discussed above dealt with 1) the same
Q
s
/
Q
w
ratio but different absolute amounts of sedi-
ment and water (Kim & Jerolmack, 2008) and 2)
changes in
Q
w
whilst keeping
Q
s
the same (Van Dijk
et al
., 2009). However, experiments dealing with
changes in
Q
s
while keeping
Q
w
constant are miss-
ing. Additionally, 1) the current suite of experi-
ments was limited to only a few cycles of the
autogenic events; and 2) the effects of different
grain sizes, sediment mixture with multiple grain
sizes and cohesiveness have not been thoroughly
tested yet. More experiments should be done with
experimental duration sufficient enough to capture
events with ranges of
Q
s
and
Q
w
, over which the
experiment can produce enough numbers of the
autogenic events for a meaningful statistical analy-
sis, multiple grain-size mixtures and cohesiveness.
These experiments will provide insight into quan-
titative understanding of the fluvial autogenic pro-
cesses and implications to field-scale stratigraphy.
L
2
υ
T
eq
=
(2)
where
L
is the length of the depositional system
and
υ
is the diffusivity coefficient. Castelltort &
Van Den Driessche (2003) applied this basin dif-
fusional relaxation time to modern worldwide
rivers and reported that the basin response time
scale ranges from 10
4
to 10
6
years. Allen (2008)
conducted a similar analysis on large Asian rivers
and suggested that this time scale is in the range
of 10
5
to 10
6
years. Large river systems with exten-
sive floodplains therefore tend to buffer any exter-
nal variations when the external forcing period
is less than the response time scale. The basin
response time scale overlaps with the time scales
for many geological processes, e.g. Earth orbital
cycles, which calls into question the origin and
interpretations of high-frequency patterns in
stratigraphic architecture.
CONTROLS OF ALLOGENIC FORCING
ON AUTOGENIC PROCESSES
Basin response time scale and high-frequency
stratigraphic signals
Sedimentary systems are sensitive to environmental
changes. Changing architectural-stacking patterns
in stratigraphy have generally been accepted as
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