Geology Reference
In-Depth Information
forming within or along the active distributaries of
the rivermouth estuary and comprising muddy, sandy,
to heterolithic sediments (Fig.
7.5
; Chen et al.
1982
;
Dalrymple
2010
). For rivers discharging large sedi-
ment loads, such tidal ridges accrete vertically and
horizontally, and ultimately merge to form shallow,
intertidal fl ats. These fl ats eventually become emer-
gent and vegetated to form new delta-plain environ-
ments. In this way the growth of tidal ridges marks the
incipient stage of delta-plain progradation and is a
defi ning process in tide-dominated deltas (Allison
et al.
2003
) .
The sedimentary facies that characterize the tide-
infl uenced distributaries comprise laminated to thinly
bedded sand-mud alternations with tidal signatures,
although these are not always well preserved or statis-
tically defi nable (Dalrymple et al.
2003
) . Due to the
saltwater intrusion into distributary and tidal channels,
marine to brackish fauna (e.g. molluscs, foraminifera
and ostracods) can be found >100 km upstream of the
shoreline. Foraminifera transported by fl ood tides are
recognized even further upstream, presumably trans-
ported during low river discharge and high astronomi-
cal tidal conditions. However, such patterns are
expected to be temporally and spatially variable in
complex delta systems, where differences in discharge
among active and abandoned distributary may strongly
affect onshore transport distances for marine-derived
particles.
Along the distributary channel margins, inclined
sand-mud alternations are reported from channel slope
to tidal fl ats, which are termed inclined heterolithic
stratifi cation (IHS) (Choi et al.
2004
) . Rhythmic climbing-
ripple cross-lamination and neap-spring cycles may
also be associated with IHS (Choi
2009
) . These
distributary-channel deposits contain well-sorted fi ne
silt to clay, often derived from near-bed fl uid muds
(e.g. Fly River; Ichaso and Dalrymple
2009
) . These
sediments with high accumulation rate and large sedi-
ment supply can provide indirect evidence of river del-
tas in the rock record, although they do not necessarily
distinguish them from tide-dominated estuaries unless
other indicators, such as a progradational stacking of
facies, can also be recognized.
Muddy tidal fl ats are one of the most important
components of tide-dominated deltas. The typical sed-
iment facies of this environment comprises sand-mud
alternations with fl aser, lenticular and wavy lamina-
tions or bedding, especially close to the river mouth
where sedimentation rates are high and bedding is well
preserved (Reineck and Singh
1980
). Bidirectional
features of sand-layer stacking and cross-laminations,
and mud-drapes or double mud-drapes, indicate tidally
infl uenced deposition. These sand-mud layers are basi-
cally controlled by cycles of fl ood-slack-ebb-slack
tidal currents, where slack periods produce the draping
muds and fl ood and ebb currents form planar to ripple-
laminated sand layers. However, neap-spring tidal
cycles are not often recorded in the laminations
(Dalrymple and Makino
1989
), as much of the record
is destroyed by bioturbation, waves, storms, and other
events (Fan and Li
2002
; Fan et al.
2004,
2006
) . From
the subtidal to intertidal zones, these sediment facies
typically show an upward-fi ning and thinning succes-
sion. The thicker and coarser layers in the lower inter-
tidal zone result from more mud settling from the water
column at slack tide and stronger currents during fl ood
and ebb for sand transport. The migration of tidal chan-
nels and creeks across tidal fl ats may also generate a
typically fi ning-upward and thinning-upward succes-
sion (e.g., Gulf of Papua, Walsh and Nittrouer
2004
) .
Toward the top of the succession in the upper intertidal
zone, plant rootlets and peat/peaty sediments become
common and refl ect transition to a vegetated delta-
plain facies with subaerial soil formation (Allison et al.
2003
). In tropical to subtropical areas woody man-
groves dominate these environments, with tree roots,
leaves, and other plant fragments forming peats and
organic-rich sediments.
Alternating sand-mud layers also commonly occur
within subtidal shoals that form on the delta-front
platform and likely represent the incipient phase of
channel-mouth bar formation. In the Amazon and
Ganges-Brahmaputra deltas these deposits are inter-
bedded or interlaminated sand and mud that are formed
under the strong infl uence of tides, especially the
neap-spring cycle (Jaeger and Nittrouer
1995
; Michels
et al.
1998
). The daily tidal exchange is not typically
recorded, though, either not being formed or not pre-
served. The sand layers within the delta-front platform
develop through erosion and bedload transport during
spring tides, whereas muddy layers are produced under
relatively low-energy conditions during neap tides. In
case of the Gulf of Papua shelf of the Fly and Kikori
deltas, the delta-front platform (topset) shows massive
mud with laminated sandy mud, interbedded mud and
sand, and bioturbated sandy mud (Dalrymple et al.
2003
; Walsh et al.
2004
). Some of these thick mud sets
