Geoscience Reference
In-Depth Information
Advances in experimental techniques allow for
more complete tests of hypotheses related to sen-
sitivity of transport systems to imposed frequency
of external forcing. Experiments with a wide range
of frequencies in the basinal forcing will provide a
fundamental understanding of origin of cyclic
sedimentation in natural systems and thus a way
to better decouple environmental signatures from
rock records.
CONCLUSIONS
0.5 m
1 The fluvial autogenic sediment storage and
release process occurs naturally and periodi-
cally changes transport rates in fluvial systems.
Channel patterns alternate between strong
channelisation and sheet flow to modulate flu-
vial sediment transport processes. The charac-
teristic time for this internal alternation is
scaled with the sediment supply rate, basin
length, flow depth and width, as shown in
equation (1).
2 Moving boundaries, such as the shoreline and
alluvial-bedrock transition, records internal
variability. Experiments with base-level
change produce increase in the variability in
the shoreline migration rate for base-level rise
and decrease in the variability of the shore-
line migration rate for base-level fall. The
variability in the alluvial-bedrock transition
shows an opposite trend to the base-level
forcing.
3 Lateral tectonic tilting lengthens the autogenic
channel time scale by decreasing the channel
lateral migration rate. An experiment with a
lateral subsidence variation slows down the
fluvial reorganisation process and induces
long-term cyclic sedimentation without time-
varying allogenic controls.
4 Cyclic changes in basinal forcing at a frequency
less than the characteristic relaxation (equilib-
rium) time in equation (2) were significantly
blurred during the stratal development in
sedimentation.
5 Future projects should be specifically designed
to measure quantitatively the fluvial autogenic
processes and investigate associated strati-
graphic products under precisely controlled
boundary conditions (tectonic subsidence rate,
sediment and water supplies etc.). The level of
contribution of autogenic processes to strati-
graphic development almost certainly varies
Fig. 6. A sliced dip section across the footwall and hang-
ingwall in XES 08. The blue and top yellow layers were
developed during the 4 th stage with two tectonically quies-
cent periods.
the XES 05 experiment but with cyclic varia-
tions of the fault slip rate in time. Unlike the
XES 05 experiment, the XES 08 experiment did
not develop clear cyclic patterns for tectonic
variation in the strata (Fig. 6). The 4 th stage of the
XES 08 experiment was composed of three sub-
stages with two tectonically quiescent periods in
the first (16 hours) and last (24 hours) stages and
a middle tectonic stage. The middle tectonic
stage was applied for 6 hours with a slip rate
almost double that of the other stages, both in
the XES 05 and 08 experiments (Straub et al .,
2009). However, the 6 hour duration (cf. 65 hours
modified autogenic channel time scale in XES
05) was too short to develop a topographic low
significant enough to be recorded in the final
deposit and the two 16 and 24 hour non-tectonic
stages had long enough duration (compared to
the 13 hour autogenic frequency in XES 05 non-
tectonic stage) to reorganise the tectonically
deformed surface.
A numerical rice-pile model in Jerolmack &
Paola (2010) demonstrated a damping of cyclic
sediment (rice grains) input signal in the output
flux at the downstream end. Cyclic input applied
at the upstream end, with a time period less than
a characteristic relaxation time, is shredded
through the rice-pile transport system, unless the
magnitude of the perturbation is very large (Fig. 4
in Jerolmack & Paola (2010)).
Stratigraphic products attributed to periodic
changes in base-level, tectonic movement and
sediment supply should be better examined.
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