Page 213 - Sedimentology and Stratigraphy

200 Clastic Coasts and Estuaries

Fig. 13.1 Reflective coasts are usually
erosional with steep beaches and a

narrow surf zone. Dissipative coasts may
be depositional, with sand deposited
on a gently sloping foreshore.

up a deposit. The sources of this sediment are from the
marine realm, either terrigenous clastic detritus
reworked from other sources or bioclastic debris.
The terrigenous material ultimately comes from riv-
ers, with a small proportion of wind-blown origin and
from direct erosion of coastlines. This sediment
is brought to a depositional coastline by tidal, wind-
driven and geostrophic currents (11.4) that trans-
port material parallel to shorelines or across shallow
seas. Wind-driven waves acting obliquely to the
shoreline are an important mechanism of trans-
port, creating a shore-parallel current known as
longshore drift. Shallow seas are generally rich
Fig. 13.2 An erosional coastline: wave action has eroded in fauna, and the remains of the hard parts of
the cliff and left a wave-cut platform of eroded rock on these organisms provide an important source of bio-
the beach. clastic material to coastlines.
The form of a depositional coastline is deter-
mined by the supply of sediment, the wave energy,
rocks sprayed with seawater can play an important
thetidal rangeand theclimate. Climateexerts a
role in breaking up the material. Material accumulates
strong control on coasts that are primarily sites of
at the foot of cliffs as loose clastic detritus and occa-
carbonate and evaporite deposition, and these
sionally as large blocks when whole sections of the cliff
environments are considered in Chapter 15.
face are removed. Cliff erosion may result in wave cut
Along clastic coastlines a beach of sandy or grav-
platforms (Fig. 13.2) of bedrock eroded subhorizon-
elly material forms where there is a sufficient sup-
tally at beach level. Wave action, storms and tidal
ply of clastic sediment and enough wave energy to
currents will then remove the debris as bedload, as
transport the material on the foreshore. The form
suspended load or in solution. This contributes to the
of the beach, and the development of barrier sys-
supply of sediment to the marine environment, and
tems and lagoons, is dependent on whether the
away from river mouths can be an important source coastline is in a micro-, meso- or macrotidal
of clastic detritus to the shallow marine realm.
regime. Sea-level changes also strongly influence
coastal morphology. In the following sections the
13.1.2 Depositional coastlines processes related to beach formation are first consid-
ered, followed by a description of the morphologies
A coastline that is a site of accumulation of sediment that can exist in wave-dominated and tidally influ-
must have an adequate supply of material to build enced coasts.