Geology Reference
In-Depth Information
Fig. 7.1
Map of the world's major river delta systems, with those forming tide-dominated deltas indicated (
bold type; fi lled circle
)
(Modifi ed after Hori and Saito
2007
)
input (see tide-dominated estuaries chapter), river del-
tas must receive adequate sediment from the river to
build a clinothem, which is a sedimentary deposit having
characteristic topset-foreset-bottomset morphology,
often in a sigmoidal or 'S' shape. In this way river-fed
coastal systems may be depositional, but they are not
deltaic if lacking a defi nable clinoform morphology
and progradational features. The surfaces defi ning
many deltaic clinothems are very low-gradient (<3°)
for fi ne-grained deltas and may be diffi cult to recog-
nize in core or outcrop, so other criteria discussed in
this chapter may be important in recognizing deltaic
settings from such data. In simplest terms, it is expected
that large volumes of heterolithic mud will be found
offshore of deltaic rivermouths, which should be a dis-
tinguishing character from most other river-infl uenced
settings. Inherent in this defi nition, deltaic systems
will be controlled at a fi rst order by river discharge and
fl uvial sediment load and secondarily to the rate of
reworking by marine processes, primarily waves, tides,
and coastal currents.
Although modern and ancient deltas may share a
general clinoform morphology, examples from around
the world show considerable variability in their sur-
face geomorphology, lithology, process, and response
to external forcing. To account for some of this vari-
ability, deltas are commonly classifi ed by the domi-
nant process controlling sediment dispersal, and hence
surface geomorphology (Galloway
1975
) . The end-
members in this ternary classifi cation scheme are
river-, wave- and tide-dominated delta systems, with
many examples exhibiting intermediate characteristics
that can be classifi ed as mixed-energy (Figs.
7.1
and
7.2
).
Large deltas may also comprise a composite system,
where different portions of the delta are morphologi-
cally distinct and controlled differently by fl uvial, wave,
or tidal processes (Bhattacharya and Giosan
2003
) .
More recent variations of this scheme have in addition
considered grain size (Orton and Reading
1993
) , sedi-
ment supply, and sea level (Boyd et al.
1992
) , although
the original Galloway classifi cation arguably remains
the most useful for large river deltas.
7.2
Background
7.2.1
Past Research
Although the study of river deltas was active during
the fi rst half of the twentieth century (e.g. Russell and
Russell
1939
), comparatively little research was done on
tidally dominated systems, due in part perhaps to
their large size, remote locations, and challenging
navigation. In the 1970s when delta classifi cation
models were fi rst emerging (e.g. Wright and Coleman
1971
; Galloway
1975
) , the only “tide-dominated”
end-members that had been studied in any detail
were the very small Klang-Langat delta of Malaysia
