Geology Reference
In-Depth Information
encroaches upon a low-lying coastal plain along a uni-
formly sloping surface. A reverse situation, or rather
the simultaneous loss of sedimentary facies at both
ends, appears to occur where a barrier system migrates
up against a coastal cliff or a progressively steepening
shore. Such a loss of finer-grained sedimentary facies
along the mainland coast of the Wadden Sea was first
recognised by Flemming and Nyandwi (
1994
). It was
suggested to reflect the response of the back-barrier
basin to land reclamation, the hydrodynamic energy -
especially wave action - having increased as the water
depth at high tide increased along the foot of the dike.
This interpretation subsequently received strong sup-
port from a study in which the loss of accommodation
space and the grain-size composition of tidal flats lost
to land reclamation was numerically reconstructed
(Mai and Bartholomä
2000
). Thus, contrary to intu-
ition, the widths of individual sedimentary facies
belts do not simply adjust to fit into the reduced space,
but the finer-grained ones are instead progressively
eliminated.
A similar effect is postulated to occur along cliffed
coasts or where a steepening slope obstructs normal
barrier evolution, the latter case being illustrated in
Fig.
10.29b
. The process can be conceived to continue
until the entire back-barrier depositional system has
been removed and the former barrier sand has evolved
into a perched coastal dune (Roy et al.
1994
). This
aspect in the stratigraphic evolution of barrier-island
depositional systems has received little attention thus
far. Nevertheless, some evidence favouring such an
interpretation, even though not entirely conclusive,
can be found in the literature (Curray et al.
1969
;
Belknap and Kraft
1977
; Kraft et al.
1979
. Vos and
van Kesteren
2000
).
Sea in scale and studied detail (Klein
1976
). Excellent
regional summaries covering various aspects of
Wadden Sea research, including comprehensive lit-
erature citations up to the time of publication, can
be found in Reineck and Singh (
1980
), Dijkema
et al. (
1980
), Postma (
1982
), Ehlers (
1988
), Oost
and de Boer (
1994
), Flemming and Davis (
1994
),
Flemming and Hertweck (
1994
), and Bartholdy and
Pejrup (
1994
).
Among back-barrier tidal flat systems that merit being
mentioned here are the ones off the Copper River delta
located along the Pacific coast of Alaska (Reimnitz
1966
; Galloway
1976
; Hayes and Ruby
1994
). However,
little detail on intertidal sediment distribution and sedi-
mentary structures has been published. The same, in
principle, holds for the enigmatic Ria Formosa (Algarve)
barrier island system off southern Portugal, which is
unique in the sense that it is a non-coastal plain system
backed by a steeply rising coastal cliff. The accessible
literature (Pilkey et al.
1989
; Davis
1994a
) mostly con-
centrates on the barrier islands and provides little
information on the back-barrier tidal flats. Some geo-
morphological, sedimentological and geochemical data
exist but are difficult to access (Granja
1984
; Granja
et al.
1984
; Monteiro et al.
1984
; Dias
1986
). Because
the barrier island chain gradually approaches the coastal
cliff towards the border of Spain, it could represent an
ideal case to validate the concept of progressive loss of
finer-grained sediment facies along the foot of the cliff
as the back-barrier tidal flats get narrower.
The back-barrier systems along the east and south
coasts of the North American continent are mostly of
the lagoonal and/or estuarine type with extensive salt
marshes covering the intertidally exposed parts with
no or only very narrow bare intertidal flats. These
locally display stratigraphies that are similar to more
extensive tidal flat systems such as the Wadden Sea.
Examples can be found in Leatherman (
1979
),
FitzGerald et al. (
1994
), Hayes (
1994
) and Oertel and
Kraft (
1994
).
A variety of back-barrier tidal flat system that does
not quite correspond to the classic type discussed here,
are coastal lagoons in which intertidal flats occupy a
large part of the area. Well documented examples of
this type are Willapa Bay along the Pacific coast of the
USA (Clifton et al.
1989
; Dingler and Clifton
1994
)
and Langebaan Lagoon along the west coast of South
Africa (Flemming
1977
). The main difference to clas-
sic back-barrier tidal flats is the fact that the grain-size
10.6
Modern Examples and Ancient
Analogues
10.6.1 Modern Examples
As pointed out earlier, most larger-scale back-barrier
depositional systems documented in the literature lack
substantial bare tidal flats, being instead dominated by
salt marshes or mangroves and estuarine or lagoonal
water bodies. As a consequence, there are few
documented examples of modern bare siliciclastic
back-barrier tidal flats that can match the Wadden
