Page 215 - Sedimentology and Stratigraphy

202 Clastic Coasts and Estuaries

commonly form along coasts with a barrier system,
but can also be found along strand-plain coasts.
In a sedimentary succession these beach dune ridge
deposits may be seen as well-sorted sand at the top of
the beach succession (Fig. 13.6). Some preservation of
the roots of shrubs and trees that colonised the dune
field is possible, but the effect of the vegetation is often
to disrupt the preservation of well-developed dune
cross-bedding.

13.2.2 Coastal plains and strand plains

Coastal plains are low-lying areas adjacent to seas
Fig. 13.4 Foreshore-dipping and backshore-dipping strati-
fication in sands on a beach barrier bar. (Fig. 13.7). They are part of the continental environ-
ment where there are fluvial, alluvial or aeolian pro-
cesses of sedimentation and pedogenic modification.
13.2.1 Beach dune ridges Coastal plains are influenced by the adjacent marine
environment when storm surges result in extensive
Aeolian processes can act on any loose sediment flooding by seawater. A deposit related to storm flood-
exposed to the air. Along coasts any sand that dries ing can be recognised by features such as the presence
out on the upper part of the beach is subject to rework- of bioclastic debris of a marine fauna amongst depos-
ing by onshore winds that may redeposit it as aeolian its that are otherwise wholly continental in character.
dunes (8.6.1). Coastal dunes form as ridges that lie Sandy coastlines where an extensive area of beach
parallel to the shoreline and they may build up to deposits lies directly adjacent to the coastal plain are
form dune complexes over 10 m high and may stretch known as strand plains (Fig. 13.7). Along coasts
hundreds of metres inland (Wal & McManus 1993). supplied with sediment, beach ridges create strand
Vegetation (grasses, shrubs and trees) plays an plains that form sediment bodies tens to hundreds of
important role in stabilising and trapping sediment metres across and tens to hundreds of kilometres long
(Fig. 13.5). The limiting factor in beach dune ridge and progradation of strand plains can produce exten-
growth is the supply of sand from the beach. They sive sandstone bodies. The strand plain is composed of

Fig. 13.5 A beach dune ridge
formed by sand blown by the wind
from the shoreline onto the coast
to form aeolian dunes, here
stabilised by grass.