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 141

and maintains a constant rate of shoreline migra-
tion in a particular direction. Furthermore, if Q s
and R slr do not change with time, the deltaic sys-
tem maintains its stratigraphic response; for exam-
ple, if sediment supply is large enough to overcome
relative sea-level rise, regression will occur and
will then be maintained as long as there is no
change in Q s and R slr . Most existing models of
genetic stratigraphy including sequence stratigra-
phy have been developed on the basis of this
hypothesis, explicitly or implicitly (Weller, 1960;
Sloss, 1962; Curray, 1964; Curtis, 1970; Vail et al .,
1977; Posamentier et al ., 1988; Galloway, 1989;
Shanley & McCabe, 1994; Neal & Abreu, 2009). It
is not that these examples are necessarily wrong,
but we believe that the assumption of equilibrium
response leads to a lack of rigor in the analysis
and can cause a misleading result, as discussed
further below.

HYPOTHESIS OF EQUILIBRIUM RESPONSE
(CONVENTIONAL STRATIGRAPHY)

DYNAMIC
EXTERNAL FORCING

STRATIGRAPHIC
CONFIGURATION

STEADY STEADY

EQUILIBRIUM
RESPONSE

UNSTEADY UNSTEADY

AUTOSTRATIGRAPHY

DYNAMIC
EXTERNAL FORCING

STRATIGRAPHIC
CONFIGURATION

AUTOGENIC NON-EQUILIBRIUM
RESPONSE

STEADY STEADY

UNSTEADY UNSTEADY

ALLOGENIC
NON-EQUILIBRIUM RESPONSE

AUTOGENIC NON-EQUILIBRIUM
RESPONSES

Fig. 2. Different views of the stratigraphic response of
a depositional system to dynamic external forcing.
Conventional stratigraphy has been developed based upon
the hypothesis of equilibrium response that to steady
dynamic external forcing, equilibrium response holds true
in general. Here any unsteady stratigraphic features are
commonly attributed to unsteady dynamic external forc-
ing. Autostratigraphy suggests two more types of responses:
autogenic non-equilibrium response (unsteady strati-
graphic configuration by steady forcing) and allogenic non-
equilibrium response (steady stratigraphic configuration
by unsteady forcing).

There are two other main types of stratigraphic
response that need to be considered along with
the equilibrium response option. The first is auto-
genic non-equilibrium response, by which steady
external forcing produces an unsteady strati-
graphic configuration (Fig. 2). A clear example of
autogenic non-equilibrium response is the autore-
treat of deltaic shorelines (Muto & Steel, 1992,
1997; Swenson et al ., 2000; see Fig. 3). Even when
all basin dynamic conditions including R slr , Q s
and upstream water discharge are kept constant,
all deltaic systems experiencing regression even-
tually become transgressed. During autoretreat,
there inevitably occurs a critical moment, the
'autobreak' of Muto (2001) (see also Parker et al .,
2008), after which the depositional system loses
its original geometry and deltaic character. This
sequential transition from early deltaic regression
to later non-deltaic rapid transgression is inevita-
ble, as long as relative sea-level continues to rise
at a constant rate.
The fundamental cause for this non-equilibrium
response in river deltas is their progressive growth
during rising sea-level. As the delta grows, an
increasing fraction of sediment (with constant Q s )
supplied by the feeder river system is consumed
for alluvial aggradation, whereby sediment
available for progradation of the delta tends to

and common assumption, as though equilibrium
response holds true in general. The corollary of
this popular assumption is that any significant
unsteady stratigraphic trends would be attributed
to unsteady dynamic external forcing (Fig. 2).
The hypothesis of equilibrium response sug-
gests that regression and transgression reflect
imbalances between the effect of relative sea-level
rise and the effect of sediment flux; and that
coastal aggradation without significant shoreline
migration reflects a balanced state between these
two controls. Alongside this, it has also been gen-
erally assumed that, given steady external forcing
by constant sediment supply (rate Q s ) and con-
stant relative sea-level rise (rate R slr ), a river delta
grows to achieve an equilibrium configuration,
produces a particular sediment-stacking pattern