Geology Reference
In-Depth Information
Fig. 9.10
Block diagram showing the zonations (including
mangrove) and the shape of a shifting mudbank along the coast
of Guianas (After Plaziat and Augustinus
2004
). The intertidal
area of the largest mudbanks is about 20-40 km long (along-
shore) and 2-5 km wide
sediment and abundant shell debris during storm
climates (Fig.
9.12b-f
). Swash bars are conceivably
the embryo of chenier ridges, which potentially develop
into full chenier ridges if the processes continue for a
longer term (over tens of to hundreds of years). Older
chenier ridges are typically common on the extensive
Holocene coastal plains abutting mega-river deltas,
conceivably resulting from the long-term alternations
of erosion (producing chenier) and accretion (deposit-
ing mud) linked to the secular fluctuation of river sedi-
ment discharge or shifting of the river main channel
(Figs.
9.13
and
9.14
, Rine and Ginsburg
1985
; Liu and
Walker
1989
; Wang and Ke
1989
; Eisma
1998
).
Across the bare intertidal and subtidal flats, muddy
and sandy open-coast tidal flats tend to have different
cross-shore profiles and sediment distribution patterns.
Muddy open-coast tidal flats usually have an accre-
tional, convex-up cross-shore profile. The transition
between the intertidal and the subtidal flats is generally
smooth without a discernible relief, except those drop
into the estuarine or deltaic-distributary channels with
a higher slope (e.g., Ren
1985
; Wang and Eisma
1988
,
1990
; Frey et al.
1989
; Wang and Ke
1997
; Fan et al.
2004a
; Plaziat and Augustinus
2004
, Figs.
9.9
-
9.11
).
Sediment on the accretional flats tends to be coarsest
near the mean lower water springs, gradually fining
landward and seaward from there. The intertidal zona-
tions are roughly displayed with the lower sandy flat,
the middle mixed (sand-mud) flat, and the upper muddy
flat (Table
9.3
). The zonation pattern was clearly shown
by the surficial sediment distribution on the North
Jiangsu tidal flats, typically along the rapid deposi-
tional section from Sheyang to Dafeng (Fig.
9.11
;
Wang and Ke
1997
). On the central west coast of Korea
where the estuarine/embayment tidal flats are well
developed with mediate to high wave exposure, surfi-
cial sediment generally coarsens from fine silt/clay at
the upper intertidal flats to coarse silt/fine sand at the
lower intertidal flats (Frey et al.
1989
; Alexander et al.
1991
). It is noteworthy that the Guiana muddy tidal
flats have the finest deposition mainly composed of
silty clay (Table
9.3
), showing little trend variations in
grain size across the entire mudbank from the inter-
tidal to the subtidal zones (Fig.
9.10
, Rine and Ginsburg
1985
; Allison et al.
1995a, b
; Lefebvre et al.
2004
).
Sandy open-coast tidal flats tend to have a concave-
up cross-shore profile, commonly with very low
(<1-2 mm year
-1
) or even negative sedimentation rates
(Semeniuk
1981
; Hale and McCann
1982
; Yang et al.
2005
). The sandy lower intertidal flat is ordinarily very
gentle and broad, but the seaward end is commonly
bordered by ridge and runnel systems with significant
undulations. The ridges are commonly shore-parallel
with the crest-to-trough height of several decimeters to
