Geology Reference
In-Depth Information
there is a forced regression of the shoreline that drives
delta progradation and potentially downward incision.
If the drop in sea level is relatively fast compared to
the rate of delta progradation, then the succession
should shift toward a more fl uvially dominated stratig-
raphy with decreasing marine and tidal infl uence
(Bhattacharya
2006
). However, with further sea-level
fall and a narrowing of the shelf, tidal range will ulti-
mately drop and tidal energy will decrease consider-
ably relative to a growing wave infl uence. It might
therefore be inferred that tide-dominated deltas are
more generally highstand features, as adequate tidal
energy is less well developed during lowstands due to
narrow shelf widths. Indeed meso- to macrotidal con-
ditions in the modern are associated exclusively with
broad shelves or large drowned valleys and embay-
ments. Regional morphology of the continental margin
(e.g. rift settings, epicontinental seas) could maintain
tidal amplifi cation even during lowstand, though, in
such settings as the Cretaceous Western Interior
Seaway (Bhattacharya and Willis
2001
) and the Gulf
of California.
Sea-level rise following a lowstand leads to the
transgression and marine inundation of incised valleys
formed during the previous fall of sea level. Riverine
sediments are effectively trapped in these valleys to
form fl uvial and coastal plains, resulting in sediment
starvation on the adjacent shelf and the formation of a
ravinement surface and condensed section (Hori et al.
2004
; Goodbred and Kuehl
2000
) . Continued sea-level
rise and transgression of the shelf and valleys will
tend to favor tidal amplifi cation and the development
of tide-infl uenced or tide-dominated environments
(Uehara et al.
2002
; Uehara and Saito
2003
) , although
such responses are also dependent on shelf and shore-
line physiography. If sediment supply is suffi cient rela-
tive to the rate of sea-level rise, though, then these
transgressive estuarine settings will evolve into deltas
with an associated change in shoreline trajectory from
landward to seaward. When constrained within the
incised valleys, such highstand deltaic successions
typically overlie transgressive estuarine sediments
along the maximum fl ooding surface (Hori et al.
2002a,
b
;
Tanabe et al.
2006
). Where deltas have infi lled their
lowstand valley, channel avulsion and migration to
interfl uve areas will lead to delta-lobe formation
directly on the lowstand exposure surface and sequence
boundary (Goodbred and Kuehl
2000
; Ta et al.
2005
) .
In some cases, such as the Mekong and Red river
deltas, tidal dominance may wane as the delta progrades
into the estuarine embayment and coastal morphology
shifts from concave to convex, making the system
more wave-dominated as the delta lobe faces more
open ocean (Ta et al.
2005
; Tanabe et al.
2006
) .
7.6
Summary
Tide-dominated deltas are an end member of the
river-wave-tide ternary delta classifi cation and have
been studied in earnest only since the 1970s. Several
comprehensive research programs during the 1980s
and 1990s developed a sound knowledgebase on the
hydrodynamics, sediment transport and marine pro-
cesses, and strata formation in tide-dominated deltaic
settings. More recent research on modern deltas, par-
ticularly studies involving the drilling of cores and
the collection of observational data, have accelerated
our understanding of the specifi c sedimentary envi-
ronments, processes, and stratigraphic successions
found within and around tide-dominated deltaic
settings.
Today most modern tide-dominated deltas are build-
ing seaward through modestly prograding deltaplains
and more rapidly prograding muddy subaqueous cli-
nothems. The sedimentary facies within these settings
are typically, perhaps characteristically, heterolithic
and often mud-dominated (e.g. Changjiang, Fly),
although some systems may have an appreciable sand
component (e.g. Ganges-Brahmaputra). In contrast,
most sections of the rock record that have been inter-
preted as tide-dominated deltas comprise sand-domi-
nated, or alternating sand-mud, sedimentary facies.
This apparent bias toward coarse-grained ancient
examples may arise from the diffi culty of distinguish-
ing deltaic successions from other mud-dominated
sedimentary facies, many of which may lack clear
indicators of fl uvial origin due to the strong overprint
of tidal processes. The broad distances across which
many modern tide-dominated deltas develop also pres-
ent a challenge at the outcrop scale, and differences in
fl uvial sediment input (e.g., coarse vs. fi ne) may further
limit the recognition of unique facies characteristics.
In terms of human impacts, more than 200 million
people live in tide-dominated delta systems today,
ranking them among the world's most economically
and culturally important environments. In many systems
the mangroves, salt marshes, and tidal fl ats typical of
tide-dominated delta systems are threatened by human
activities. Several modern deltas are already severely
