Page 212 - Sedimentology and Stratigraphy

13

Clastic Coasts and Estuaries

The morphology of coastlines is very variable, ranging from cliffs of bedrock to gravelly or
sandy beaches to lower energy settings where there are lagoons or tidal mudflats. Wave
and tidal processes exert a strong control on the morphology of coastlines and the
distribution of different depositional facies. Wave-dominated coasts have well-
developed constructional beaches that may either fringe the coastal plain or form a
barrier behind which lies a protected lagoon. Barrier systems are less well developed
where there is a larger tidal range and the deposits of intertidal settings, such as tidal
mudflats, become important. A very wide range of sediment types can be deposited in
these coastal depositional systems and in this chapter only terrigenous clastic environ-
ments are considered: carbonate and evaporite coastal systems are covered in the
following chapter. Estuaries are coastal features where water and sediment are supplied
by a river, but, unlike deltas, the deposition is confined to a drowned river valley.

13.1 COASTS wave, tide and current processes. At depositional
coastlines the gradient is normally relatively gentle
Coasts are the areas of interface between the land and and a lot of the wave energy is dissipated in shallow
the sea, and the coastal environment can comprise a water: provided that there is a supply of sediment,
variety of zones, including coastal plains, beaches, these dissipative coasts can be sites of accumulation
barriers and lagoons. The shoreline is the actual of sediment (Woodroffe 2003).
margin between the land and the sea. Coastlines can
be divided into two general categories on the basis of
their morphology, wave energy and sediment budget. 13.1.1 Erosional coastlines
Erosional coastlines typically have relatively steep
gradients where a lot of the wave energy is reflected Exposure of bedrock in cliffs allows both physical and
back into the sea from the shoreline (a reflective chemical processes of weathering: oxidation and
coast, Fig. 13.1): both bedrock and loose material hydration reactions are favoured in the wet environ-
may be removed from the coast and redistributed by ment, and the growth of salt crystals within cracks of