Autogenic process change in modern deltas: lessons for the ancient - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 160

limited extent and it is difficult to interpret facies
variation in comparison with modern deltas
that extend tens of kilometres. Turronian Ferron
Delta in Utah is another delta where some parase-
quences have been interpreted to change the
depositional processes, between fluvial and wave,
within the same parasequence (Dewey & Morris,
2004; Li et al ., 2011).
In summary, there are increasing numbers of
descriptions of ancient delta deposits that reflect
process changes along the same outcrop belt over
distances from a few hundred metres (Wall Creek
Member) to a few kilometres (Chimney Rock
Tongue) (Fig. 7). The delta process changes can
represent either allogenic or autogenic strati-
graphic responses to external forcing conditions
but where the delta deposits are laterally continu-
ous along the same outcrop belt and occur over
short distances within the same parasequence, an
autogenic interpretation is preferred. Sedimentary
facies that change laterally as well as vertically
within the same parasequence (Fig. 7) reflect dom-
inant process changes within a single allogenic
unit. It is difficult to distinguish whether any given
process change is spatial or temporal but based on
comparison with modern systems both may occur.

respectively, with no significant change in pro-
cess. For the Danube and Mekong deltas, waves
have a stronger influence as the delta prograded
into more open and deeper water (Dan et al., 2009;
Tamura et al., 2012). The transition from one type
of delta to another happened relatively fast and
over a short distance. However, the transition
occurred at different times along depositional
strike because of differences in sediment supply.
In the modern deltas described in this study
(Danube, Mississippi, Mahakam) the differences
between coeval lobes happens because the strength
of fluvial discharge, and/or wave and tidal regime,
varies along depositional strike. These lateral,
along depositional strike, regime changes occur
over distance of kilometres to hundreds of kilome-
tres and depend mainly on the locus and size of the
main fluvial discharge at the shoreline (Figs 3, 4
and 6). The depocentre location in fluvial-
dominated deltas is controlled by avulsion of the
main 'trunk' channel (e.g. Mississippi example) or
by the unequal distribution of discharge between
the main distributaries (e.g. Danube example).
The autoretreat process in deltas is an inbuilt
autogenic response, an inevitable shoreline turna-
round from regression to net transgression during
steady external forcing (Muto & Steel, 1992). The
shoreline turnaround is explained by the ever-
extending area on the delta that needs to be filled
with sediment to maintain regression with even a
slight rise of sea-level (Muto, 2001). Because of this
eventual decrease in sediment flux to the regressing
shoreline, we suggest that different segments of the
expanding shoreline will be increasingly unable to
maintain fluvial domination (Fig. 8), resulting in dif-
ferent types of process dominance on the delta front.
A conceptual framework of three main auto-
genic delta process changes has been developed
(Fig. 8) using process change observations of
Holocene deltas. The autoretreat and transgressive
part of the shoreline in Fig. 8 used Mississippi for
the first case and then conceptional estuary mod-
els for the two other cases. The first is a change
from an initial fluvial-dominated delta to one
that becomes wave-dominated and then to a
wave-dominated estuary (such as the Atchafalaya
Bay or west of modern Mississippi Delta on
Fig. 3) during the subsequent transgression
(Fig. 8A). This case is well exemplified by
different growth stages of the Mississippi and
Danube deltas (Fig. 8A). The second case is where
an initial fluvial-dominated delta has a reduced
sediment supply and turns into a tide-dominated

DISCUSSION

Models for autogenic delta process changes

The Holocene delta examples presented are clas-
sified as 'mixed-influence' (Fig. 1A) which reflects
the mixture of processes that dominate parts of
the same delta. However, in order to understand
delta evolution and architecture, the delta classifi-
cations (Galloway, 1975; Ainsworth et al ., 2011)
should also consider temporal (Figs. 1B and C) as
well as spatial changes.
Various Holocene delta examples show that
process changes have happened extremely rapid
(hundreds to thousands years), the transitions
occur over relatively short distances (100s of
metres to kilometres) and are laterally extensive at
the scale of the delta system or of its smaller com-
ponents (Figs. 3 to 6). These changes are autogenic
because the allocyclic controlling factors are in a
steady state (i.e. developed during uniform rates
of sea-level change and constant sediment supply
rate). For example, tidal conditions in the case of
inner Mekong Delta and fluvial conditions in the
case of the inner Danube Delta persisted during
progradation of the delta for 200 km and 30 km