Autogenic process change in modern deltas: lessons for the ancient - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 150

Pontén & Plink-Björklund, 2009). Ancient delta
deposits are only rarely interpreted in terms of
mixed energy (Gani & Bhattacharya, 2007;
Carvajal & Steel, 2009) and the criteria used to
recognise transitions between process regimes
are not always clear. Using observations and
published studies on modern and Holocene del-
tas, models of autogenic process change from flu-
vial to wave and to tide process dominance are
suggested in this study. Although the models are
based on modern deltas they should also be gen-
erally valid for ancient delta deposits. Two main
points that are emphasised here are (1) that pro-
cess change in deltas are commonly autogenic
responses (changes in stratigraphy or morphol-
ogy that happen despite constant forcing of the
external variables, see Muto et al . 2007) and (2)
that process change most often happens (though
not always) at short time scales (tens to thousands
of years) and over short distances (kilometres to
10s of km).

nevertheless the more local ones such as shoreline
morphology, position of the main sediment rout-
ing and accommodation (Ainsworth et al ., 2008).
The process changes in Holocene deltas have been
described in terms of transitions from one type to
another (McManus, 2002) and were presented as a
'dynamic' application of Galloway's classification
but no link with the autogenic responses was
made. Holocene process changes of the deltas
described below are also shown in Fig. 1B but
here we emphasise that these are autogenic pro-
cess changes.
The fluvial, wave or tide processes dominate the
delta shoreline at a given time or at a given loca-
tion but the process dominance can change in
time, as the delta evolves, or in space along the
same shoreline. The process changes in deltas
have been averaged in the delta classification
(Galloway, 1975) and this is useful when the entire
delta complex is described but does not address
the architectural complexities at lobe level and
smaller scale. Realisation of the multiple possibili-
ties of the process dominance, or influence, led to
a classification scheme with 22 delta types
(Aisworth et al ., 2011). This detailed classification
divides the ternary classification into multiple
fields within which deltas are ascribed by their
dominant process and two levels of subordinate
process. For example, the Mitchell River Delta in
Australia is described as a tide-dominated, fluvial-
influenced, wave-affected delta. Despite improved
understanding that comes with such a classifica-
tion, it still does not address the dynamics of the
delta system in space and time.

Delta process changes

The tripartite classification of deltas (Galloway,
1975) based on morphology (Fig. 1A) and domi-
nant processes (fluvial, wave, tidal) is widely used
because it is simple to understand and easy to
apply. The same classification is used here to sep-
arate delta types or segments of deltas. Although
not explicit within the fluvial-wave-tide process
classification, there are other controlling factors
inherent to the classification. For example, river
discharge, yearly hydrograph variability and grain
size distribution will contribute to the fluvial
regime (e.g. Orton & Reading, 1993) whereas coastal
morphology, shelf bathymetry, basin area and
shape have some control on wave and tidal regimes
(Orton & Reading, 1993; Ainsworth et al ., 2008). In
his delta process classification article, Galloway
(1975) suggested the tendency for the regressive
part of a sequence to be fluvial-dominated whereas
the back-stepping or transgressive parts tend to be
wave-dominated.
The implications of sea-level change on delta
type was also discussed by Porebski & Steel (2006)
and by Yoshida et al . (2007) who argued that tidal
processes tend to be more common in highstand
shorelines and inner-shelf deltas, whereas low-
stand deltas, especially those at the shelf edge,
tend to be wave-dominated. Although some of
these indirect generalities are likely to hold, the
most direct controls on process dominance are

Autogenic responses

An understanding of autogenic response accom-
panies the realisation that not all changes seen in
rivers and deltas are equilibrium responses (i.e. to
changes in A/S ratio). Accommodation (A) and
supply (S) ratio was defined as a key descriptor
for the behaviour of the deltaic shoreline but the
two terms are linked in a non-linear manner and
generate complex shoreline behaviour (Muto &
Steel, 1997; Muto et al ., 2007). Some changes hap-
pen without changes in the external variables A
and S and these are non-equilibrium or autogenic
responses (Muto & Steel, 2014). The reference
to external variables A and S is for an entire
river-delta system, for example the sediment sup-
ply (S) delivered by a river is constant but its
delivery might vary laterally along the delta