Geology Reference
In-Depth Information
Fig. 9.4
Distribution of tidal flats (migrating mudbanks) along the coasts of northwest South America (Guiana) from the Amazon
River mouth to the Orinoco River Delta (After Eisma et al.
1991
; Plaziat and Augustinus
2004
)
Orinoco River Delta (Fig.
9.4
) at a macrotidal-to-
mesotidal setting (Wells and Coleman
1981
; Meade
et al.
1985
; Froidefond et al.
1988
; Kineke et al.
1996
;
Baltzer et al.
2004
). Sandy (truly) open-coast tidal flats
are generally coexisting with sandy estuarine tidal flats
(Fig.
9.2
), bordering the Holocene strand plains of var-
ied width (Reineck and Cheng
1978
; Semeniuk
1981
),
or against the erosional Late Quaternary deposits or
the rocky cliff (Thompson
1968
; Semeniuk
1981
; Yang
et al.
2005
).
Tidal flats are therefore classified into nine types in
terms of coastal morphology (Figs.
9.1
and
9.5
). They are
in turn grouped into sheltered or exposed (open-coast)
tidal flats on the basis of the magnitude of wave expo-
sure. So the open-coast tidal flats in this context include
both the truly open-coast tidal flats (i.e., coastal-plain
tidal flats) and the partially sheltered to highly exposed
tidal flats fringing the outer part of open-mouth estuar-
ies and deltas (Fig.
9.1
).
quently raising tidal range shoreward. The extreme
case to induce tidal bores takes place when tidal waves
enter a funnel-shaped embayment rapidly narrowing
and shoaling landward (Archer and Hubbard
2003
).
The world's largest tidal bore occurs in the Hangzhou
Bay, where the width of the bay decreases from
~100 km across the baymouth to <20 km along the
Ganpu transection, while the mean tidal range increases
landward from <2 m at the baymouth to over 5.5 m
near Ganpu (Fig.
9.7
). Epeiric seas and funnel-shaped
bays were considered to be more prevalent in the geo-
logic past, especially during the time of the supercon-
tinent Pangea (Archer
1998
), so macrotidal coastal
settings should be common at that time.
Tidal currents are usually rotary on the open off-
shore area, gradually changing into linear when tides
propagate into the distributary/estuarine channels or
toward the shore. Main flows of flood and ebb tides in
the estuaries and deltaic distributaries are steered
toward different directions because of the Coriolis
effect, resulting in different current strength and depo-
sitional patterns along the two different banks. In the
Hangzhou Bay, flooding flows are steered north, and
the convergent flows strengthen the current toward the
north bank, producing the erosional and narrow tidal
flats, while ebbing flows are steered south, and diver-
gent and weakening flows favor developing the depo-
9.3
Physiography and Morphology
9.3.1
Tide, Wave, and Wind Climate
Strong tidal amplification occurs when tidal waves
propagate into the broad and shallow shelf sea, conse-
