Geology Reference
In-Depth Information
outer platform to high wave energy and tidal-current
acceleration (Figs.
7.5
and
7.6
). Structures on this
outer portion of the delta-front platform include fi ne to
medium-scale bedding with wave ripples, hummocky
and trough cross-stratifi cation and frequent sharp-
based erosional contacts formed by storm-wave scour.
Subaqueous dunes are also occasionally reported from
this zone of the delta-front platform (Gagliano and
McIntire
1968
; Kuehl et al.
1997
) . Overall tidal signa-
tures are not well developed in these deposits despite
the strong cross-shelf tidal currents, because of gener-
ally lower sedimentation rates and frequent bed resus-
pension by waves.
At water depths below fair-weather wave base
(~5-30 m), sedimentary facies of the delta-front slope
are characterized by a coarsening-upward succession
of alternating sand and mud deposits (e.g., Changjiang,
Mekong, Ganges-Brahmaputra) or laminated to biotur-
bated muds (e.g., Gulf of Papua, Amazon). Individual
bedding units often comprise graded (upward fi ning)
and fi nely laminated sand-silt layers with sharp basal
contacts, such as in the Ganges-Brahmaputra (Michels
et al.
1998
) and Changjiang deltas. Ripples are also
found on the seabed of the delta front of the Changjiang
(Chen and Yang
1993
). However, clear tidal signatures
are not always present in the delta-front slope sedi-
ments of tide-dominated deltas, because tidal currents
are not usually well-developed this far offshore.
Similarly, prodelta sediments even further offshore are
often highly bioturbated and intercalated with silt
stringers and thin shell beds. The shell beds result pri-
marily from storms, which may also transport coarser-
grained sediments to the prodelta. In contrast to the
prevalent tide-dominated facies formed in the delta-
plain distributaries and the adjacent intertidal to sub-
tidal delta-front platform, the delta-front slope to
prodelta environments are mostly infl uenced by waves,
ocean currents, and storms.
local variability in lithology, structure, and stratal
relationships. In deltaic settings where accretion rates
are relatively high, facies associations record delta pro-
gradation and lobe development that typically occurs at
timescales of 10
1
-10
3
years . For tide-dominated deltas
the most frequently described facies association is that
of the lower delta plain, which captures the advancing
deltaic shoreline and subtidal to supratidal transition
(Allison et al.
2003
; Harris et al.
1993
; Hori et al.
2002a,
b
;
Ta et al.
2002
; Dalrymple et al.
2003
) . As described
from numerous delta-plain systems, the facies association
comprises an 8-10 m thick, fi ning upward succession
starting with sandy, cross-stratifi ed subtidal shoals,
which grade into heterolithic intertidal mud-sand cou-
plets and are capped by a rooted mud-dominated
supratidal soil (Fig.
7.7
).
Other facies associations that have been described
for tide-dominated deltas include tidal bars, tidal gul-
lies and channels, incised distributary channels, and
the subtidal shelf (Fig.
7.7
; Davies et al.
2003
;
McCrimmon and Arnott
2009
; Tänavsuu-Milkeviciene
and Plink-Björklund
2009
). The tidal-bar facies asso-
ciation is variably described as a fi ning-up or coars-
ening-up succession of cross-stratifi ed sand with
bidirectional fl ow indicators and inclined planes that
is very similar to, if not the same as, the portion of
the delta-plain facies association (Fig.
7.6b
). The
difference between the upward-fi ning and upward-
coarsening descriptions is likely related to their
proximity to the active distributary mouth, the fi ning-
up example being more proximal to the rivermouth
and receiving abundant sediment to make a rapid
transition from subtidal to vegetated intertidal
setting, whereas the coarsening-up succession may
be a more wave-tide dominated downdrift littoral
deposit. The tidal gullies and distributary channels are
regularly described as fi ning-up , current-rippled to
planar-bedded deposits with a sharp, often incised,
lower contact. However, the most characteristic features
of these facies associations is the regular occurrence
of mud clasts that refl ect the local reworking of shal-
low intertidal and supratidal delta-plain deposits as
channels migrate, avulse, and incise (Fig.
7.6c
;
Dalrymple et al.
2003
; Davies et al.
2003
; Tänavsuu-
Milkeviciene and Plink-Björklund
2009
) . On aver-
age, though, tidal channels are relatively laterally
stable (e.g. Fagherazzi
2008
) and so the muddy delta-
plain deposits that cap tidal-channel sands are
commonly preserved in the upper stratigraphy of the
subaerial delta clinothem.
7.4.3
Facies Associations
Because many factors can infl uence the formation of
stratigraphic sequences over 10
3
-10
5
years, it is also
useful to consider mesoscale facies associations that
characterize the various subenvironments of tide-
dominated deltas (Fig.
7.6
; Gani and Bhattacharya
2007
; Heap et al.
2004
). A facies association is a group
of sedimentary facies that are typically found together
and defi ne a particular environment, but also allow for
