Geology Reference
In-Depth Information
Fig. 9.29
Sedimentation on the observation tiles (40-cm wide
and 40-cm long) visited by regular intervals spanning from a half
day to a month or longer. The field monitoring experiment was
carried in 2002 with six tiles fixed on the middle intertidal-flat
surface at the same time, and each tile was scheduled to visit and
redeploy at different time intervals spanning from a half day to
six months (
a
). However, the bi-monthly and semiannual tiles
were taken away by waves after 6-7-week deployment, so no
record was available from these two tiles. Photos (
b
-
e
) showed
examples of sedimentation on the tiles. There are two sand-mud
couplets on the semidiurnal and daily tiles (
b
and
c
) and seven
couplets on the fortnightly and monthly tiles (
d
and
e
). The
dashed line marks the couplet boundaries. The ballpoint pen is
14 cm long, and pointing toward the sea (After Fan et al.
2004a
)
to form their own couplets. In the period from May 4
to June 4 in 2002, there were cumulatively 55, 13, and
7 couplets observed on the daily, fortnightly, and
monthly tiles, respectively (Fan et al.
2004a
). Compared
to the maximum of 120 couplets potentially deposited
by 60 semidiurnal tides in the period, the preservation
rates of couplets were 45.8%, 10.8%, and 5.8%,
respectively, for the daily, fortnightly, and monthly
intervals. It is indicated that the preservation rate of
couplets decreases rapidly as time intervals increase
(Fan et al.
2004a
). The same conclusion has been
achieved from the tile observation experiment in 1999
at the Nanhui Mudbank (Table
9.4
, Fan and Li
2002
).
Based on the core study, Li et al. (
2000
) further extrap-
olated that the preservation rate of couplets could be
lowered to 0.2% over a 100-year scale. Also, the sedi-
mentation rates calculated over different time intervals
decrease exponentially as the time intervals increase
(Table
9.4
, Fan et al.
2001
).
The problem of the low spatial resolution of
the preservation potential studies by in situ measure-
ments can be effectively solved by numeric modeling.
A forward modeling approach has been employed to
simulate the preservation potential of tidal-flat deposits
on the North Jiangsu coast (Fig.
9.11
, Gao
2009
). The
results showed that the preservation potential was the
highest over the upper part of the intertidal flat and
the lower part of the subtidal flat, and the lowest near
the mean sea level and the mean low water springs
(Fig.
9.30
). Also, the preservation potential decreased
as the tidal flats prograded seaward. The slope of tidal
flats has significant influence on the preservation poten-
tial, in that the minimum value is approximately four
times greater for the slope scenario of tan E = 0.5 × 10
-3
than that of tan E = 1.0 × 10
-3
(Gao
2009
). The simula-
tion result of the preservation potential is comparable
with those from field experiments in the Changjiang
Delta (Li et al.
2000
; Fan
2001
; Fan and Li
2002
).
The lower preservation rates indicate that the inter-
tidal-flat deposition is riddled with various diastems.
The incompleteness of tidal-flat deposition has been
explored in detail along the 4-m-thick intertidal-flat
deposition in the Changjiang Delta (Fan et al.
2002
).
The 4-m-thick strata were extrapolated to deposit in
