Autogenic process change in modern deltas: lessons for the ancient - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 152

shoreline due to channel avulsions. The resulting
variability of deposits under such conditions is
considered as autogenic responses because there
is no unsteadiness in the external forcing (to the
given river-delta system). Furthermore, these
autogenic responses in system morphology or in
stratigraphy can be stochastic (e.g. channel avul-
sions or lobe-shifting and their stratigraphic
responses) or deterministic. The latter response,
exemplified by delta autoretreat (Muto & Steel,
1992) is of particular importance because it can be
extracted from stratigraphic data (method shown
by Muto & Steel, 2002), thereby allowing the
residual stratigraphic response to be interpreted
as 'allogenic', a result of change in the rate of the
external forcing variables. This is the essence of
'autostratigraphy', as advocated by Muto et al .
(2007). Delta autoretreat was first demonstrated
from numerical modelling (Muto & Steel, 1992)
and flume experiments (Muto, 2001) but did
not include process changes associated with the
autogenic turnaround of the shoreline from regres-
sion to net transgression. The present study argues
that during shoreline autoretreat there is com-
monly an associated process change.
Based on observations of Holocene delta stud-
ies, we suggest that process change is common in
deltas and is also associated with the autoretreat
process. The data collected suggest that deltas
commonly become wave-dominated during the
turnaround from regression to transgression, dur-
ing shoreline autoretreat.

before the inception of most Holocene deltas
(Stanley & Warrne, 1994). However, despite some
local Holocene variability, sea-level rise (Fig. 2) and
sediment supply can be considered constant over
this period for a given delta system. The Holocene
can therefore be considered a period of 'steady forcing'
with deltas showing primarily autogenic responses.

Autogenic progradation of modern deltas

Holocene delta formation

Most modern deltas started to prograde about
7.5 ka ago (Stanley & Warne, 1994) when eustatic
sea-level rise rate decreased from 11 mm/yr to
about 2.5 mm/yr (Fairbanks, 1989, fig. 2). Many
deltas (Danube, Mekong, Godavari, Rhone,
Mississippi) started to prograde when they were
far inland from their present shoreline positions,
at the head of embayments. They built multiple
lobes as the deltaic system prograded across the
pre-existing shelf. These deltas were fed by rela-
tively large rivers and were able to prograde even
during slight sea-level rise. The examples pre-
sented below focus on process changes as reflected
in delta morphology and on the controls on these
changes. The temporal and spatial scale of changes
in modern deltas has been described herein and
the observations compared with outcrop exam-
ples of delta process changes.

Fluvial-dominated to wave-dominated process
change: modern examples

Why are Holocene deltaic changes attributed
to autogenic responses?

For fluvial-dominated to wave-dominated pro-
cess change, the examples of the Mississippi and
Danube deltas are instructive. The Mississippi
Delta has had an extensively studied evolution
(Fisk et al ., 1954; Frazier, 1967; Coleman, 1988;
Tornqvist et al ., 1996; Roberts, 1997, 1998). The
Mississippi Delta started to form, like most large
deltas, around 7.5 ka when the shoreline was
about 300 km landward of the present shoreline.
The delta is composed of multiple lobes (Frazier,
1967) and is considered the type example for
fluvial-dominated cases, with multiple coeval
distributary channels (Galloway, 1975; Coleman
& Wright, 1975). Despite the fluvial dominance
of the last lobe (Plaquemines-Modern), it is prob-
able that the same lobe after abandonment will
turn into a wave-dominated delta and then into a
shoreface and a shoal (Penland et al ., 1985). The

External forcing during the last 7.5 ka of Holocene
time is considered conducive to autogenic
responses because sea-level rose at a relatively
constant rate of about 2.5 mm per year (Fairbanks,
1989, fig. 2) and also climate (a proxy for sediment
supply) did not commonly undergo dramatic
changes. There are variable rates of relative sea-
level rise because deltas occur in tectonic set-
tings with different subsidence rates. Sediment
supply also probably varied at decadal to centen-
nial scales linked to global climate patterns
because the deltas are in different climatic zones
(Syvitski et al ., 2003). Goodbred & Keuhle (2000)
demonstrated a high supply peak in the Ganges-
Brahmaputra delta because of monsoon intensifi-
cation during the early Holocene (11 ka to 7 ka BP)