Generic autogenic behaviour in fluvial systems: lessons from experimental studies - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 14

Careful mapping of fan architecture and recon-
struction of the hinterland degradation may per-
mit a complete evaluation of fan history. Yet, age
control in these coarse clastic environments is
often not sufficient to determine details about the
periods of autogenic processes in these coarse
clastic systems (e.g. Nemec & Postma, 1993; Mack
et al . 2008) and much more field work in combi-
nation with improved age control of the rock
record is needed to confirm the findings of experi-
mental studies.

delta does not exist. Over the past 5 kyr, the
Mississippi River avulsed only four times and the
Red River avulsed twice in the southern Lower
Mississippi Valley, as found by Aslan et al . (2006).
Relocation of the Mississippi River eastward
shifted local base level and led to the Red River
avulsion. Mississippi and Red River avulsion
occurred primarily through channel reoccupa-
tion. Aslan et al . (2006) argue that gradient advan-
tages did not necessarily lead to the avulsions,
although gradient advantages along the Mississippi
River floodplain are widespread.
Fielding et al . (2006) found climate change to be
a primary control on the Holocene Burdekin delta
architecture, inhibiting a shorter avulsion period
by facilitating extreme variability of discharge.
They suggested further that more frequent avul-
sion may also have been facilitated by the length-
ening of the delta-plain channels as the system
progrades seaward, which would increase the
accumulation space rapidly. The Mfolozi River
Floodplain (South Africa) is characterised by
avulsions in the floodplain head that occur pri-
marily during extremely infrequent large flood
events, where the stream flow capacity of the
channel is insufficient (Grenfell et al . 2009).
Other factors that control avulsion more indi-
rectly have been forwarded from the study of
natural examples, including neotectonics, subsid-
ence, substrate composition, sinuosity changes
and human activities (Fisk, 1952; Schumann,
1989; Schumm et al ., 1996; Jones & Harper, 1998;
Smith et al ., 1998; Stouthamer & Berendsen, 2000)
and meander bend properties (Kleinhans, 2010).
Schumm et al . (1996) describe how reductions
in channel gradient caused by increased channel
sinuosity lead to decreased sediment transport
capacity and avulsion (e.g. Makaske, 2001). It is
challenging to relate the control of all these factors
back to the control they have on aggradation
rate and to relate that to frequency in autogenic
behaviour. Yet, future stratigraphic studies in
ancient fluvial and delta settings should be
conducted to evaluate the significance of the
change in aggradation rate on channel stacking
density and connectedness further. That evalua-
tion of change in aggradation rates is more
practical than applying sequence stratigraphical
principles in surface and subsurface stratigraphic
analysis is shown by Moscariello (2003) and
Martinius et al . (this volume) in correlating strati-
graphic sequences in the Triassic-Jurassic Statfjord
Formation (Norwegian Sea).

Low-gradient river systems

No doubt the most detailed avulsion reconstruc-
tions come from the Rhine-Meuse delta complex.
Although a complete review of these works is
beyond the scope of this paper, it is worthwhile
to point out how much fast and slow changes
relative to equilibrium time affect the avulsion
frequency.
From a detailed reconstruction of the Holocene
avulsion history of the whole Rhine-Meuse delta
Stouthamer & Berendsen (2000, 2001, 2007) deter-
mined quantitative values for the avulsion param-
eters, avulsion frequency, avulsion duration and
inter-avulsion period. In the Rhine-Meuse delta
the number of coeval channels is related to avul-
sion frequency supported by high resolution age
control that helps to analyse beginning and end-
ing of river activity. The data show that instanta-
neous and gradual avulsions were almost equally
important in the Rhine-Meuse delta, with two
dominant avulsion styles: (1) regional avulsion:
the new channel followed an entirely new course
and (2) avulsion leading to reoccupation of exist-
ing channels. In the case of reoccupation, they
found two possibilities: the new channel reoccu-
pied (a) its previous channel (local avulsion) or
(b) a pre-existing channel. Stouthamer et al . (2010)
found that, over the Holocene time scale, avulsion
style was related to aggradation rate and coastal
evolution. Initial high avulsion frequencies
decreased with dropping rates of sea-level rise
(dropping rates of accumulation space) from 9000
years ago until ~ 3000 years ago. Thereafter avulsion
frequency increased again (1.89 avulsions/100 yrs)
during an inferred period of increased delivery
of fine sediment and slightly altered discharge
regimes (Stouthamer et al ., 2011), which increased
back barrier aggradation rates significantly.
For the Mississippi delta, a database on avulsion
frequency like that available for the Rhine-Meuse