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 143

GR RES

?

?

?

50 m

~ 80 km

Fig. 4. Shoreline progradational patterns along eastern Book Cliffs, near Pallisades, Colorado (from Kirschbaum & Hettinger,
2004). The thin yellow-coloured shorelines of Sego/Corcoran (lower quarter of diagram) and Cozette (group of shorelines
just below the middle of the diagram) members prograded rapidly and extensively, in contrast to the aggradational and
short shoreline lengths of the overlying Rollins sandstone (see Aschoff & Steel, 2011 for details). Associated thicknesses of
coastal plain deposits (green) reflect differing aggradational styles and suggest that individual shoreline turnarounds may
well be non-equilibrium autogenic responses to steady external forcing. Red colours reflect transgressive estuarine valleys.
The difference in external forcing between the two groups of shorelines was suggested to be tectonic by Aschoff & Steel
(2011). GR = gamma ray and RES = resistivity.

fluvio-deltaic systems having moving bounda-
ries, the alluvial grade can be attained and sus-
tained only during base level fall and never
functions to limit alluvial aggradation (Muto &
Swenson, 2005, 2006). Even with falling sea-
level, alluvial rivers can aggrade steadily without
valley incision if a particular set of geomorphic
conditions is available (Petter & Muto, 2008). In
terms of this recent notion of alluvial grade,
which plays a key role in the autostratigraphy of
river deltas, the accommodation concept meets a
serious self-contradiction that sediment accumu-
lates even if there is no accommodation (i.e. even
if the space is part of 'anti-accommodation'; see
Muto & Steel, 2000). In autostratigraphy, the
accommodation concept is not useful to explore
the understanding of responses of river deltas to
sea-level changes.

non-equilibrium response (Fig. 2). The autore-
treat-autobreak process is unavoidable if sea-level
continues to rise at a constant rate. For a deltaic
shoreline to migrate in a particular constant direc-
tion and not to experience autoretreat-autobreak,
R slr is required to decrease inversely proportional
to the square-root to cubic-root of time, otherwise
Q s is required to increase in proportion to the
square to cubic of time (Muto & Steel, 2002a).
Such unsteady dynamic external forcing would
illustrate allogenic non-equilibrium response.
A clear example of allogenic non-equilibrium
response is allogenic grade (Muto & Swenson,
2005), the graded state of any alluvial river
attained and sustained under unsteady dynamic
external forcing. Fig. 5 shows a longitudinal pro-
file of a one-dimensional deltaic clinoform built
with sea-level fall decelerating at a rate inversely
proportional to the square-root of time. Although
an alluvial river on the topset of the prograding
clinoform has always existed, it has simply
extended basinward without significant deposi-
tion and erosion to sustain the grade. If sea-level
falls at a constant rate in the same geomorphic
condition, the river cannot attain grade but inevi-
tably undergoes an irreversible change from aggra-
dation to degradation (Swenson & Muto, 2007).

ALLOGENIC NON-EQUILIBRIUM
RESPONSES

The second type of non-equilibrium response is
where steady stratigraphic configuration can
result from unsteady external forcing. This type of
stratigraphic response is referred to as allogenic