Geology Reference
In-Depth Information
bedforms (Fig.
9.25a-f
). Interfering ripples are also
frequently present, with the modification magnitude of
one group of wave ripples by another increasing sea-
ward from the upper to the lower bare flats (Fig.
9.25g-i
).
Accumulation of these small rippled beds tends to pro-
duce lenticular and wavy bedding, which is commonly
seen on the tile sedimentation and along the erosion
cliff of remnant muddy patches (Fig.
9.26
). On the
fortnightly (one neap-spring-neap cycle) tiles, 3-6-cm-
thick deposits were not found to have direct link with
neap-spring cycles in terms of lamina number and
thickness variation (Fig.
9.26e-h
), even the extrapo-
lated sediment rate reaching up to 72-144 cm year
-1
.
It is reasonably linked with the formation of rippled
laminae, lenticular and wavy bedding by waves or com-
bined wave and tide flows instead of purely tides (usu-
ally known as tidal bedding, Reineck and Singh
1980
),
and thicker and sandier laminae represent higher energy
events of larger waves or the combined flows of larger
waves and higher tides instead of purely higher tides.
The muddy open-coast tidal flats can temporarily
shift into sandy flats during the storm conditions,
developing erosion features, dunes, and storm-generated
bedding. On the Chongming Eastern Flat, erosion
by rising storm waves starts at discrete points, and the
erosion holes gradually expand to unite each other
until there are only a few isolated muddy patches on
the sandier deflated flats, following with the bedforms
growing from small ripples into large dunes (Fig.
9.25j-o
).
Storm decaying initiates to deposit first a sandy lag
layer with abundant shell debris and mud pebbles over
the erosion surfaces, and follows with a thinning- and
fining-upward succession, that both grain size and
thickness of sandy laminae decrease upward, gradu-
ally returning the normal tidal-flat thinly interlayered
deposition (Fig.
9.27
). During a storm season, previ-
ous storm deposits tend to be reformed by the follow-
ing storms, producing a single amalgamated
storm-deposited succession. For example, units b, c,
and d were deposited by typhoons Neil, Olga, and Paul,
respectively, over the typhoon season in 1999; the for-
mer two units were the remnants by subsequent storm
reworking (Fig.
9.27
; Fan et al.
2004a
). A thinning-
and fining-upward succession is therefore a storm-
related small succession consisting of a lower half of
sand-dominated layers (SDLs, storm deposition) and an
upper half of mud-dominated layers (MDLs, after storm,
normal tidal-flat deposition). The small storm-related
succession usually has approximately half-and-half
ratios of SDLs and MDLs, commonly seen in the
Changjiang Delta where a high sedimentation rate is
generally achievable with several centimeters per year
(Li et al.
2000
; Fan and Li
2002
).
The deposits of sandy open-coast tidal flats may
consist predominantly of high-energy storm deposits
with volumetrically minor amounts of tidally induced
lamination (Yang et al.
2005, 2008a
). The Baeksu
sandy flats in southwest Korea were finely explored by
Yang et al. (
2005
, 2006, 2008a, b). In summer lower
energy season, the flats are commonly veneered by mud
layer of several centimeters thick, consisting of thinly
interbedded to interlaminated sand and mud. The mud
layer can be partitioned into two to three smaller-
scale upward-fining successions, interpreted as weak
summer storm deposits (Fig.
9.28
). In winter higher
energy season, the flats turn into sandy substrate topped
by dune field. The deposits contain extensive wave-
generated parallel lamination and short-wavelength
(0.3-2 m) hummocky cross-stratification (HCS,
Table
9.3
, Fig.
9.28
), highly similar with those of
shoreface facies. Yang et al. (
2008a
) suggested that the
storm deposits on the sandy open-coast tidal flats con-
tained evidence of tidal modulation of storm processes,
in which single storm layer is composed of three dis-
tinctive rippled intervals: (1) landward-dipping, ripple
cross-lamination at the base, produced by combined
flows during rising tide; (2) symmetrical buildup of
wave-ripple cross-lamination in the middle, formed by
oscillatory wave motion at high tide when currents are
weak; and (3) seaward-dipping, ripple cross-lamina-
tion at the top, deposited by combined flows again dur-
ing falling tide. Because of limited input of fine
sediments and lower sedimentation rate, the summer
muddy laminated successions were less preserved,
leading to the strata mainly composed of winter sandy
deposits with typical HCS (Yang et al.
2005
).
It is generally concluded that deposits of open-coast
tidal flats are characterized by abundant sedimentary
features generated by waves or combined flows, mak-
ing them distinctly different from the sheltered tidal
flats. The features of tidal modulation of wave action
distinguish them from the shoreface facies. The most
extensive cyclic successions are strongly asymmetric
with only the upper half of a fining-upward cycle
(Figs.
9.27
and
9.28
), denoting the annual depositions
of seasonal prevalent large waves alternating with small
waves (Baker et al.
1995
; Li et al.
2000
; Dalrymple
et al.
2003
; Fan et al.
2004a
; Yang et al.
2005
).
