Geology Reference
In-Depth Information
to achieve the continuous rapid deposition (Tessier
1993
; Greb and Archer
1995
; Fan and Li
2002
; Fan
et al.
2004a, b
; Dalrymple
2010
). Cyclic tidal rhyth-
mites are therefore quite limited in their spatial and
temporal distributions throughout the geological time
(Davis et al.
1998
).
Noncyclic tidalites are actually much more widely
distributed than the cyclic tidal rhythmites, considering
that most of tidal flats are not exclusive from wave
impacts. The exposure to wave increases from tidal
flats fringing open-mouth estuaries, deltas, to coastal
plains, which are all referred as open-coast tidal flats in
this context (Fig.
9.1
). The recent studies highlight that
the interaction of tides and waves is major mechanism
for sediment transport and morphological and strati-
graphic formation on open-coast tidal flats (Allen and
Duffy
1998
; Li et al.
2000
; Fan et al.
2004a, b, 2006
;
Lee et al.
2004
). Moreover, some sandy open-coast
tidal flats can be wave dominated (Yang et al.
2005
;
Dalrymple et al.
2006
). The strata of open-coast tidal
flats are characterized by containing a mixture of tide-
and wave-generated sedimentary structures with a gen-
eral increase in the thickness ratio of storm deposition
and tidal deposition as the wave exposure increases. It
should therefore undoubtedly lead to misidentification
of the open-coast tidal-flat deposition, if the pure tide-
depositional criteria were employed for facies inter-
pretation (Dalrymple
2010
).
Open-coast tidal flats remained less studied until
the current century, although they are much more
widely distributed than the sheltered tidal flats along
the world coast, and their importance for both modern
and ancient tidal sedimentology has been highlighted
since the mid-1970s (Klein
1975
; Wang
1983
; Ren
1985
; Wells et al.
1990
; Fan et al.
2004a
; Yang et al.
2005
). Several reasons account for the situation. Open-
coast tidal flats are majorly composed of fine-grained
sediment with mud domination, which is usually
considered less economically important for resource
exploitation. Muddy open-coast tidal flats are princi-
pally distributed along the coasts of South and East
Asia, Oceania, and South America, receiving gigantic
volume of fluvial fine sediment (Figs.
9.2
-
9.4
), which
are less accessible to Western European and North
American. Although Chinese scientists began pilot
studies of tidal flats in the early 1960s (e.g., Wang
1963
; Li et al.
1965
), most of their research results were
published in Chinese (see a review given by Shi and
Chen
1996
), also unreadable to the western scientific
community. Sandy exposed tidal flats are common
along the open coast neighboring small river mouth,
nourished by riverine sediment principally composed
of coarse grains (coarse silt and sand), like the central
west coast of Taiwan (Reineck and Cheng
1978
), the
southwest coast of Korea (Yang et al.
2005
), the east
and northwest coast of India (Mukherjee et al.
1987
)
and the northwest coast of Australia (Semeniuk
1981
).
They are also common in parts of Western Europe and
North America, like the coast of the southwestern
English Channel, the Irish Sea, and the Strait of Georgia
in Canada (Hale and McCann
1982
; Deloffre et al.
2007
; Yang et al.
2008a
). The sandy open-coast tidal
flats lie at a transitional ground between the tide-domi-
nated muddy open-coast tidal flats and wave-dominated
shorefaces, and their affinity to wave dynamics and
shoreface morphology makes them easy to be misinter-
preted as tidal beach. All of these may account for their
having been much less studied until the present decade
(Yang et al.
2005, 2008a
; Dalrymple
2010
).
There has been a rapid increase in publications on
open-coast tidal flats since the beginning of the new cen-
tury. The topics cover all fields including hydrodynam-
ics, sediment- and morphodynamics, sedimentology, and
stratigraphy, especially those in China and Korea (Li
et al.
2000, 2005a
; Chang and Choi
2001
; Fan
2001
; Fan
and Li
2002
; Fan et al.
2001, 2002, 2004a, b, 2006
; Kim
2003
; Yang et al.
2005, 2006a, b, 2008a, b
; Dalrymple
et al.
2006
; Gao
2009
; Wang et al.
2009
). In these
studies, open-coast tidal flats have been highlighted as a
major coastal setting, significantly different from the
well-known sheltered tidal flats and tidal beaches. This
chapter attempts to summarize these research progresses
on open-coast tidal flats, cultivating a general sedimen-
tary model for future comparison studies of the systems.
9.2
Depositional Systems
Tidal flats are low-relief environments typically flank-
ing the coast of broad shelf with marked tidal rhythms
(Fig.
9.2
). Macrotidal conditions undoubtedly favor the
development of extensive tidal flats, but they are also
common in mesotidal to microtidal coasts (Eisma
1998
).
Sediment supply (source, flux, and size) and the magni-
tude of exposure to waves are two other key factors
controlling tidal-flat morphology and sedimentology.
