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as various types of forced regression deposits in a
tidally infl uenced setting (Van Wagoner et al.
1991
;
Yoshida et al.
1996
) . Willis and Gabel (
2001,
2003
)
have since argued that the Sego Sandstone actually
represent the tidal channels and inner shelf sand sheet
of a tide-dominated delta system, which incised into its
own muddy delta-front platform and prodelta deposits
during progradation. Such a mud-incised succession of
progradating tidal channel deposits has also been
described from the Miocene-age record of the Ganges-
Brahmaputra delta (Fig.
7.6d
; Davies et al.
2003
) .
With only modest variation this general succession of
an upward-coarsening subaqueous-delta unit overlain
by an upward fi ning subaerial-delta unit has been
documented in many of the world's modern tide-
dominated delta systems, including the Ganges-
Brahmaputra (Allison et al.
2003
), Mekong (Ta et al.
2002
), Changjiang (Hori et al.
2001
) , and Fly (Harris
et al.
1993
; Dalrymple et al.
2003
) . Such similarity
suggests that this stratigraphic succession may be a
useful tool in distinguishing tide-dominated deltas in
the rock record (Willis
2005
). Local variation in the
tide-dominated delta succession has been recognized
in the Mekong system, which has become increasingly
wave infl uenced in the late Holocene and shows an
upward-coarsening succession ending in wave-swept
foreshore to aeolian beach-ridge deposits (cf. Fig.
7.6b
,
lower profi le; Ta et al.
2002
) . In the Mahakam delta,
alongshore heterogeneity in stratigraphic successions
arises from the greater fl uvial infl uence relative to tidal
reworking (Gastaldo et al.
1995
) .
7.5
Stratigraphy
7.5.1
Stratigraphic Successions
Deltas are defi ned as discrete shoreline deposits formed
where rivers supply sediment more rapidly than can be
redistributed by basinal processes (Elliott
1986
) ; thus
shoreline advance is essential for distinguishing them
from estuaries, which also occur at river mouths but are
transgressive depositional systems. As defi ned, deltas
are regressive prograding to aggrading systems (Boyd
et al.
1992
; Dalrymple et al.
1992
) . Therefore deltaic
successions will overall shallow upward, ideally includ-
ing facies associations from prodelta, delta-front slope,
delta-front platform, and delta-plain environments, in
ascending order (Fig.
7.7
; Dreyer et al.
2005
) .
In tide-dominated deltas that support a compound
clinothem with prograding subaerial and subaqueous
deltaic units, the idealized stratigraphic succession can
be subdivided into two major intervals (Fig.
7.7
). The
lower portion shows an upward-coarsening facies suc-
cession from the prodelta to delta-front slope and outer
platform deposits that is marked at its top by sharp-
based wave and current scours. This lower interval is
overlain by an upward-fi ning succession of prograding
deposits from the inner delta-front platform and shoal-
ing to subaerial delta-plain facies. The upper interval is
most typically represented by the delta-plain facies
association (see Sect. 7.4.1), but may also include local
sub-environments such as tidal channel bars or estua-
rine distributary associations. Within the overall deltaic
succession, the coarsest and most well-sorted deposits
typically occur in the boundary zone between the
delta-front platform and slope, and secondarily in the
prograding, distributary-mouth channel bars (Coleman
1981
; Hori et al.
2001,
2002b
; Dalrymple et al.
2003
;
Tänavsuu-Milkeviciene and Plink-Björklund
2009
) .
7.5.2
Delta Progradation
The rate of delta progradation can strongly infl uence
the delta facies succession. As the subaerial delta pro-
grades basinward, the tidal distributary channels can
incise up to 20 m into the delta-front platform deposits,
and a relative rise of sea level (e.g., commonly through
subsidence) is important in order to preserve topset
deposits of the outer delta-front platform. The Ganges-
Brahmaputra and Mahakam deltas are examples of
such progradational and aggradational deltas that dis-
play a largely continuous and conformable Holocene
succession from prodelta to delta-plain facies
(Goodbred et al.
2003
; Storms et al.
2005
) . If distribu-
tary channels are stable relative to delta progradation,
a delta succession will form as described above.
However, if the lateral migration of distributaries is
fast relative to delta progradation, then much of the
delta-front facies will be replaced by distributary-
channel fi ll, which is thought to occur in the Fly river
delta (Dalrymple et al.
2003
) .
7.5.3
Role of Sea-Level Change
Sea-level change can also force environmental changes
that may appear similar to delta progradation in the
stratigraphic record. During periods of sea-level fall
