The autostratigraphic view of responses of river deltas to external forcing: a review of the concepts - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 142

final shoreline position

final sea-level

alluvial-estuarine

nondeltaic rapid transgression

AUTOBREAK

Λ

deltaic transgression

time (s)
1470

alluvial-deltaic

AUTORETREAT

1160

10 cm

initial
sea-level

alluvial-deltaic

180

initial
shoreline
position

deltaic regression

0

R slr /Q S

Fig. 3. Shoreline autoretreat and subsequent autobreak, illustrating an autogenic non-equilibrium response to steady
sea-level rise. This river delta was built during an experimental run conducted with constant sediment supply and con-
stant relative sea-level rise. Λ is autostratigraphic length scale defined with Q s /R slr (Q s : rate of sediment supply, R slr : rate of
relative sea-level rise). See Muto (2001) for details of the experimental runs.

decrease with time (Muto & Steel, 1992, 1997;
Milton & Bertram, 1995). As a result of this,
including continuing sediment supply, the
regressing delta becomes unable to maintain its
areally expanding forward growth, despite the
steady dynamic external forcing including con-
tinuing sediment supply. Eventually, the delta
inevitably meets a critical moment at which it can
no longer retain its original sedimentation style
(i.e. autobreak). It was emphasised by Muto &
Steel (1997) that autoretreat would be delayed
in settings where there were very low rates of
sea-level rise and hastened where these rates
were higher. A good stratigraphic example where
shoreline progradational length (before transgres-
sive turnaround) can be seen to relate to degree of
aggradation in the coastal system is shown in
Fig. 4 (from Kirschbaum & Hettinger, 2004) where
regressive-shoreline transit lengths in the 'higher
accommodation' setting (upper half of diagram
with Rollins Sandstone shorelines) rarely exceed
20 km to 40 km, in contrast to the significantly
longer (60 km to 80 km) shoreline lengths of
the 'low-accommodation' Sego-Corcoran-Cozette
sandstones (lower half of Fig. 4). This of course
does not prove that the termination of individual
shoreline regressions in this part of the Western
Interior Seaway was caused by autoretreat but it
does provide a simple alternative to more com-
plex allogenic explanations. Aschoff & Steel
(2011) suggest a general tectonic explanation (ini-
tiating Laramide tectonic movements) for the

anomalous, 'low-accommodation' Sego-Corcoran-
Cozette shorelines as a group, though not for the
individual shoreline turnarounds.
Other examples arising from autogenic non-
equilibrium response include autodrowning of a
deltaic system with rising sea-level under a spe-
cial geomorphic condition (Tomer & Muto, 2010;
Tomer et al ., 2011), autoincision of an alluvial
river with falling sea-level (Muto & Steel, 2002b;
Swenson & Muto, 2007) and autodetachment of a
deltaic system with falling sea-level (Petter &
Muto, 2008). These are all global, deterministic
and non-cyclic autogenesis that inevitably occur
given a particular set of conditions of steady
dynamic external forcing.
It should be noted that the term 'accommoda-
tion' mentioned above is only in the context of
illustrating autogenic non-equilibrium responses
with the existing papers. The framework of auto-
stratigraphy itself is passive to this conventional
concept (Muto et al ., 2007), partly because accom-
modation, as space for potential sediment accu-
mulation (Jervey, 1988), can hardly be treated
quantitatively, specifically and objectively (Muto
& Steel, 2000; for special treatment see Kim et al .,
2006). Another major reason is that this concept
is based on a misunderstanding of alluvial grade,
which, in sequence stratigraphy, has been
regarded as the final stable state of the alluvial
downstream reaches of a river system that is
attained by equilibrium response to stationary
base level (e.g. Posamentier et al ., 1988). In natural