Geology Reference
In-Depth Information
extensive structures of combined-flow and storm genesis)
and high intertidal-flat deposits containing numerous
small tidal-channel fills (Greb and Archer
1995
). The
succession is also considered to be deposited on an
open-coast intertidal flat over the distributary-mouth
or estuarine-channel bars.
of wave action with clear seasonal variations from
tide-dominated into wave-dominated morphodynamic
conditions, (4) developing few tidal channels on the
bare flats except complex tidal-creek systems in the
adjacent salt-marsh land, (5) exhibiting clear intertidal
zonations of a coarsening seaward trend, and (6) con-
taining abundant combined-flow and wave-induced
structures and bedding.
It is noteworthy that there is a significant difference
between the muddy and sandy open-coast tidal flats.
The muddy classification tends to have an accretional
convex-up profile with the coarsest sediment near the
mean lower water springs, and develop a cyclic pro-
gradational succession consisting of a lower half of
subtidal coarsening-upward succession and an upper
half of a fining-upward intertidal-to-supratidal succes-
sion. The sandy type usually has an erosional concave-
up profile with the coarsest sediment near the mean
high water, and develops a coarsening-upward retro-
gradational succession. Typical hummocky cross-
stratification (HCS) of short wavelength can be
common in the sandy open-coast tidal-flat deposits,
whereas not present in muddy open-coast tidal-flat
deposits. The vertical succession of sandy open-coast
tidal flats generally has higher abundance of storm-
generated beds volumetrically than that of muddy
open-coast tidal flats. A spectrum of coastal morpho-
dynamic settings is therefore reorganized to change
from the wave-influenced, tide-dominated muddy
open-coast tidal flats, the wave- and tide-dominated
accretional sandy open-coast tidal flats, wave-domi-
nated erosional sandy open-coast tidal flats, to wave-
dominated tidal beaches.
The open-coast tidal-flat deposition is far from
complete (or continuous) due to reworking by complex
physical processes. The well-developed sand-mud
couplets do not represent the continuous deposition of
tidal cycles in the vertical stacking succession. The
preservation potential of strata is very low in terms of
preservation rates of couplets and a general decreasing
trend of sedimentation rates over different time scales.
So the basic stratigraphic tenet is highlighted that
deposition by shorter-time cyclic processes (e.g., tide)
is highly reworked by successive longer-time cyclic
processes (e.g., seasonal alternations of wave climate).
Rapid continuous deposition with the neap-spring
cycles is generally exempted from the open-coast tidal
flats, except the relatively protected setting like the
estuarine or distributary channels.
9.8
Summary
Tidal flats occupy a large section of the world's unshel-
tered shoreline, especially along the coast receiving
large volumes of terrigenous fine sediments from riv-
ers that build up broad and gentle shelf deposits. Open-
coast tidal flats have received increasing interest
because of their importance in global environmental
issues posed by rising sea level, decreased sediment
fluxes linking to river damming, and increasing human
usage, also providing a modern analogue for fossil
facies interpretation.
Large tidal range favors but is not a prerequisite
for tidal-flat development. Sediment supply and the
magnitude of wave exposure are two key controlling
factors of tidal-flat morphology and sedimentology.
Open-coast tidal flats develop in wide environments,
ranging from partly exposed embayments and estuar-
ies to highly exposed deltas and coastal plains. The
commonest open-coast tidal flats are principally com-
posed of mud, extensively distributing along the tide-
dominated mega-deltas and their adjacent chenier
plains. Most of the world's largest river deltas are tide
dominated or under significant tidal influence. Littoral
currents carry the resuspended sediment from the
deltas, downdrift for tens to hundreds of kilometers
along the coast to nourish tidal flats. The longest
stretches of open-coast tidal flats are of this type,
including the muddy coast along the East China and
the Guianas. Sandy open-coast tidal flats majorly
develop in the open-mouth estuaries and the adjacent
strand plains, where large tidal ranges usually occur
owing to tide amplification by typical coastal mor-
phology, like narrow and shallow straits or funnel-
shaped estuaries.
Open-coast tidal flats bare some common features
to distinguish them from other coastal environments.
These features include: (1) developing broad and gen-
tle flats without significant morphological break along
the shore-normal profile, (2) fronting an open sea or ocean
without barriers, (3) exposing to different magnitudes
