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autogenesis that is considered primarily in
autostratigraphy.
Whenever a depositional system is considered,
it must have a distinct spatial extent with a bound-
ary to its outside. Any action by the outside upon
the depositional system is regarded as external
forcing. Two kinds of external forcing can be dis-
tinguished: static and dynamic. In the case of river
deltas, static external forcing includes stationary
base level and unchanged basement topography;
dynamic external forcing includes base level
change and basin tectonics. Sediment and water
supply is also part of dynamic external forcing
since it involves input of material from the out-
side of the system. In the present context, it hardly
makes sense to presume a situation under which
no sediment (and no water) is supplied to the dep-
ositional system and thus no strata accumulate
there. In this sense, any active depositional sys-
tem cannot be free from either dynamic external
forcing or static external forcing. What matters
with an active depositional system is whether the
dynamic external forcing is steady (i.e. rate con-
stant) or is unsteady (i.e. rate variable). From this
viewpoint, the term autogenic should refer to the
origin of stratigraphic or geomorphic features that
arise as responses from both steady dynamic
external forcing and static external forcing (Fig. 1).
Conversely, the term allogenic refers to strati-
graphic or geomorphic responses generated as a
result of the dynamic external forcing being
unsteady. It should be noted, contrary to some
popular usage, that the terms autogenic and allo-
genic are therefore best used to refer to strati-
graphic responses and not as adjectives for the
main controls themselves.
One might claim that a perfect sediment-bypass
(and water-bypass too) system, where neither
aggradation nor degradation occurs in spite of
continuing sediment supply, could be regarded as
being effectively free from dynamic external forc-
ing. The state of such a system, in case of river
deltas, would be grade. However, the graded state
of an alluvial-deltaic river having moving bounda-
ries is possible only when relative sea-level drops
in a particular pattern depending on basin topog-
raphy (Muto & Swenson, 2005, 2006; Petter &
Muto, 2008). The attainment of grade rather
reflects significant exertion of dynamic external
forcing.
THE HYPOTHESIS OF EQUILIBRIUM
RESPONSE
Two primary factors controlling regression and
transgression in deltaic settings are sediment sup-
ply to the basin and relative sea-level rise or fall
(basin subsidence + base level fluctuation). An
open question to this proposition is how these
two factors function in building the stratigraphic
architectures of river deltas. With respect to this
question, there is a long-standing notion, here
referred to as 'the hypothesis of equilibrium
response'. Equilibrium response is a type of
response by which steady external forcing gives
rise to steady stratigraphic configuration or steady
sedimentary behaviour of the depositional system
(Fig. 2). In fact, equilibrium response is physically
possible but limited only to a special circum-
stance (Muto & Swenson, 2006). Nonetheless, the
hypothesis of equilibrium response is a popular
CONVENTIONAL STRATIGRAPHY
AUTOSTRATIGRAPHY
STRATIGRAPHIC
PRODUCT
EXTERNAL
FORCING
STRATIGRAPHIC
PRODUCT
FORCING
INTERNAL
AUTOGENIC
STATIC
STEADY
DYNAMIC
EXTERNAL
AUTOGENIC
ALLOGENIC
UNSTEADY
DYNAMIC
ALLOGENIC
Fig. 1.
Usage of the terms autogenic and allogenic in terms of the relationship between forcing and resulting stratigraphic
products. In conventional stratigraphy, autogenic and allogenic products correspond to internal and external forcing of the
system, respectively. However, in the autostratigraphic view, autogenic refers to the origin of stratigraphic or geomorphic
features that arise as responses from both steady dynamic external forcing and static external forcing, whereas allogenic
refers to stratigraphic or geomorphic responses to unsteady dynamic external forcing.
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