Geology Reference
In-Depth Information
Fig. 9.22
Schematic model
for shoreline evolution cycles
in the Guiana coast. Serious
erosion takes place at the
interbank phase (
top panel
).
It is succeeded by leading
edge mudbank deposition
(
second panel
). The accretion
continues till the passage of
the leading edge to reach the
maximum progradation (
third
panel
). And erosion takes
place again by the swing of
the trailing edge (
bottom
panel
). Note that there is a net
coastal plain growth with each
mudbank-interbank cycle
(After Allison and Lee
2004
)
between the coastline and the direction of wave prop-
agation accounts for the different behaviors of the
respective mudbanks. Consequently, the mudbanks in
Guiana are shorter than those in Surinam, and their
behaviors are more erratic than the latter (Eisma et al.
1991
; Augustinus
2004
).
plain of the Bohai Bay, five in the North Jiangsu coastal
plain, and eight in the Changjiang Delta (Fig.
9.13
).
The oldest chenier in the Changjiang Delta and the
North Jiangsu coastal plain was dated 6,460 year BP
and 6,160 year BP, respectively. The two chenier plains
were therefore assumed to begin to develop since the
mid-Holocene maximum flooding period, but have
afterward been interrupted several times to produce
other cheniers.
The chenier formation represents an episode of ero-
sion, whereas the intervening mud deposition represents
a period of coastal progradation. The development of
chenier plains is generally presumed to mainly result
from sediment starvation by the distributary channel
switching (Lees
1992
). The lower reach of the Huanghe
switched several times between debouching into the
Bohai Bay and the Yellow Sea, accounting for the forma-
tion of the cheniers on the western coastal plain of the
Bohai Bay and the North Jiangsu coastal plain (Liu and
Walker
1989
; Wang and Ke
1989
). Each chenier on the
southern coast of the Changjiang Delta was formed when
the Changjiang main channel switched north and away
from the south bank. The same mechanism was also
9.4.1.4 Megacycles (Hundreds
to Thousands of Years)
Mega-scale coastal development is referred to the geo-
logical evolution of the Holocene coastal plains over a
time scale of centuries to millennia. It is generally
addressed through using cores or large-scale coastal
morphological features. Chenier ridges are typical and
extensive features on coastal plains near the mouths of
rivers (typically large rivers), excellent indicator for
the ancient coastline location and evolution (Meckel
1975
; Liu and Walker
1989
; Wang and Ke
1989
; Lees
1992
). They demarcate the flat coastal plains with
coastline-parallel ridge series, mainly composed of
sand and shell debris, a few decimeters to meters higher
than the surround mudflat/marsh deposits. There are
generally four chenier ridges along the western coastal
