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delta and eventually into a tide-dominated estuary
during transgression (Fig. 8B). The third case is
when an initial tide-dominated delta changes
into a wave-dominated delta because of a
decrease in sediment supply or an increase in
wave regime and later into a wave-dominated (or
tide-dominated) estuary (Fig. 8C). There are two
other cases without regime change during autore-
treat. One case is when the delta is wave-domi-
nated initially and continues to be wave-dominated
until it is transgressed and turns into a wave-dom-
inated estuary. Another case is when a tide-domi-
nated delta maintains its character during
progradation until it later transforms into a tide-
dominated estuary.
The autogenic process-change model depicted
in Fig. 8 is conceptual but the regressive part of
the model is based on the observations of the
Holocene deltas studied (Figs 3 to 6). The trans-
gressive part of the model presented in Fig. 8 is
less well documented but is based on the late
Pleistocene histories of the studied deltas which
show that they initiated as the rate of sea-level rise
decrease (Fig. 2; during late Pleistocene to early
Holocene) within lowstand incised estuaries or
embayments that formed during late Pleistocene
to early Holocene transgression which subse-
quently were flooded (Coleman, 1988; Panin,
1989; Storms
et al
., 2005; Ta
et al
., 2005).
that facies changes within the same regressive-
transgressive cycle are likely to be an autogenic
responses.
Ancient deltas are commonly mapped and clas-
sified in the subsurface based on the morphology
of sand isopach maps (Galloway, 1975; Fisher &
McGowen, 1967; Busch, 1971; Xue & Galloway,
1995). With the increased seismic resolution of
dense 2-D datasets (e.g. Sydow & Roberts, 1994)
and acquisition of larger 3-D datasets, it is possi-
ble to do a seismic morphological analysis of del-
tas (e.g. Jackson
et al
., 2010, Perov & Bhattacharya,
2011) to distinguish delta process changes.
Autogenic process changes (at short time scales),
as observed during the evolution of the Holocene
deltas or in ancient deltaic deposits at outcrop, are
difficult to infer from subsurface maps. However,
the model of autogenic process changes described
above (Fig. 8) might serve as a rule for delta evo-
lution under steady forcing conditions for the
subsurface datasets that have low resolution (e.g.
seismic).
Why is it important to differentiate autogenic
and allogenic delta process changes?
The interpretation of autogenic process changes
in deltas has implications for regional stratigra-
phy and for local correlations and reservoir archi-
tecture; there is no need to infer changes in
the external variables in order to have significant
stratigraphic changes within deltaic units.
However, allogenic process changes in deltas are
also known to be common and certainly at time
scales of eustatic change (10s of ka). This was
pointed out by Porebski & Steel (2006) and
Yoshida
et al
. (2007), who noted that straight
wave-dominated coastlines had some tendency to
be associated with rising and highstands of sea-
level in the sedimentary geology literature and for
tide-influenced shorelines to be associated with
limited and protected (within bays and estuaries)
accommodation, especially during falling and
lowstands of sea-level. Allogenic process changes
of this type are probably fairly widespread later-
ally and commonly associated with the develop-
ment of large-scale flooding surfaces and sequence
boundaries, developed, say, at the time scale of
cross-shelf regressive-transgressive eustatic cycles
(1 ka to 300 ka, as shown by Burgess & Hovius,
1998 and by Muto & Steel, 2002). Autogenic
behaviour of delta system, on the other hand,
would apply at the scale of segments of a delta
Implications for ancient delta studies
It is difficult to reconstruct the plan view mor-
phology of ancient deltas using outcrops data but
delta process changes are well recorded in the
facies stratigraphy, through distinct delta-front
facies successions (Gani & Bhattacharya, 2007;
Plink-Bjorklund, 2008; Carvajal & Steel, 2009). In
the outcrop examples presented herein (Fig. 7),
the facies changes occur over short distances
(hundreds of metres to kilometres), even shorter
than inferred from modern deltas. Observations of
modern deltas strongly suggest that such process
changes in the rock record are autogenic responses.
It has been shown that delta process changes can
be allogenic responses to the unsteadiness of
external drivers (Porebski & Steel, 2006, Yoshida
et al
., 2007). However, there is not always a need
to invoke an increase (or decrease) of the rate of
sea-level change to produce the observed stratig-
raphy in different delta types, as shown by mod-
ern deltas (Figs 4 to 6). The autogenic process
change model developed herein (Fig. 8) suggests
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