wave energy and abundant mud, and a limited
amount of sand.
The composition of beach sediments varies greatly,
depending on the materials available and the grade of
mechanical and chemical breakdown of the minerals.
Quartz sand is the most widespread because quartz is
the most stable of the minerals we find in most beach
sediments. But on volcanic islands, which consist
largely of basalt, there is no quartz, and we get sand
derived from basalt or volcanic glass. Carbonate sand
is formed locally from the broken skeletons of
carbonate-secreting organisms. This does not only
happen in tropical regions, e.g. the Bahamas, but also
along coasts with cold climates. In many parts of
Norway, for example, the beach sand consists very
largely of carbonate sand from molluscs, barnacles,
bryozoans and calcareous red algae.
Beach profiles are formed by wave power acting on
the coast and depend on a number of different factors:
1. The composition of the available sediments.
2. Wave power
a. Average wave height
b. Size and frequency of storms
c. Angle between the commonest orientation of the
waves and the beach.
3. Tidal range.
4. Vertical profile off the beach.
5. Supply of sediment from land or along the coast.
The morphology of the beach zone and nearshore
areas is the result of interaction of various features
related to wave activity. As waves approach land,
they will be affected by friction against the bottom.
The depth at which this starts depends on wave height
and length. When the depth becomes less than about
half the wave length, friction against the bottom will
be great, the circular wave motion (orbit) will be
distorted and oscillatory sediment transport will affect
the seabed, producing ripple marks. The depth at
which this occurs is called the wave base. Dunes are
formed of sediments which are deposited when the
waves break, and are at the same time the reason for
the waves breaking precisely there. There is thus an
interaction between the wave regime and the bottom
geometry in beach sediments. In addition to breaking
on the foreshore itself, we often find that waves break
at two or three places offshore, and at each of these
places we find a sand bar.
In the French Riviera (e.g. Nice) the beaches are
often full of rounded pebbles with less sand. This is
because they are near mountains supplying coarse
clasts and also close to a shelf edge. Sand is then easily
eroded from the beaches during storms while the
coarse clasts remain.
2.37 Prograding Beach and Barrier
Sequences
The progradation of a beach will produce a characteristic coarsening-up sequence (Fig. 2.39) which will
obey Walther’s Law of facies succession. The vertical
sequence will represent the environments from the
shelf to the shoreline. The transition between shelf
mud and fine-grained sandstones with ripples may
represent the wave base. Isolated sand layers in mud
may have been deposited near the storm wave base,
whereas the transition to continuous fine-grained sand
may represent the fairweather wave base. The thickness of the sequence from the fairweather wave base to
the foreshore is an expression of the wave energy at
the coastline when the sequence was deposited.
Bioturbation occurs in the lower part of this
sequence. Below the wave base it usually takes the
form of horizontal feeding traces, and in the lower and
middle shorefaces, where there is relatively high wave
energy, as vertical traces (Skolithos facies). As erosion
and reworking intensify, the preservation potential of
bioturbation is reduced and it becomes less frequent.
The formation of sand bars and erosion surfaces on the
upper shoreface results in cross-bedding, usually
trough cross-bedding, representing flow in the upper
part of the lower flow regime.
In the breaker zone there is an upper flow regime,
producing a planar facies which in vertical section will
appear as very low-angle cross-bedding. On the beach
we often have a beach bar which is flooded only during
storms, and a depression behind it called a runnel,
which helps to drain the backshore area. Because the
exposed beach is a rich source of sand, the wind will
tend to blow sand from the beach and redeposit it as
aeolian dunes, usually where it is trapped by vegetation. Aeolian sediments therefore often cap ancient
beach profiles, but the aeolian dunes may also be
eroded and not be preserved in the geological record.
2 Introduction to Sedimentology
77
amount of sand.
The composition of beach sediments varies greatly,
depending on the materials available and the grade of
mechanical and chemical breakdown of the minerals.
Quartz sand is the most widespread because quartz is
the most stable of the minerals we find in most beach
sediments. But on volcanic islands, which consist
largely of basalt, there is no quartz, and we get sand
derived from basalt or volcanic glass. Carbonate sand
is formed locally from the broken skeletons of
carbonate-secreting organisms. This does not only
happen in tropical regions, e.g. the Bahamas, but also
along coasts with cold climates. In many parts of
Norway, for example, the beach sand consists very
largely of carbonate sand from molluscs, barnacles,
bryozoans and calcareous red algae.
Beach profiles are formed by wave power acting on
the coast and depend on a number of different factors:
1. The composition of the available sediments.
2. Wave power
a. Average wave height
b. Size and frequency of storms
c. Angle between the commonest orientation of the
waves and the beach.
3. Tidal range.
4. Vertical profile off the beach.
5. Supply of sediment from land or along the coast.
The morphology of the beach zone and nearshore
areas is the result of interaction of various features
related to wave activity. As waves approach land,
they will be affected by friction against the bottom.
The depth at which this starts depends on wave height
and length. When the depth becomes less than about
half the wave length, friction against the bottom will
be great, the circular wave motion (orbit) will be
distorted and oscillatory sediment transport will affect
the seabed, producing ripple marks. The depth at
which this occurs is called the wave base. Dunes are
formed of sediments which are deposited when the
waves break, and are at the same time the reason for
the waves breaking precisely there. There is thus an
interaction between the wave regime and the bottom
geometry in beach sediments. In addition to breaking
on the foreshore itself, we often find that waves break
at two or three places offshore, and at each of these
places we find a sand bar.
In the French Riviera (e.g. Nice) the beaches are
often full of rounded pebbles with less sand. This is
because they are near mountains supplying coarse
clasts and also close to a shelf edge. Sand is then easily
eroded from the beaches during storms while the
coarse clasts remain.
2.37 Prograding Beach and Barrier
Sequences
The progradation of a beach will produce a characteristic coarsening-up sequence (Fig. 2.39) which will
obey Walther’s Law of facies succession. The vertical
sequence will represent the environments from the
shelf to the shoreline. The transition between shelf
mud and fine-grained sandstones with ripples may
represent the wave base. Isolated sand layers in mud
may have been deposited near the storm wave base,
whereas the transition to continuous fine-grained sand
may represent the fairweather wave base. The thickness of the sequence from the fairweather wave base to
the foreshore is an expression of the wave energy at
the coastline when the sequence was deposited.
Bioturbation occurs in the lower part of this
sequence. Below the wave base it usually takes the
form of horizontal feeding traces, and in the lower and
middle shorefaces, where there is relatively high wave
energy, as vertical traces (Skolithos facies). As erosion
and reworking intensify, the preservation potential of
bioturbation is reduced and it becomes less frequent.
The formation of sand bars and erosion surfaces on the
upper shoreface results in cross-bedding, usually
trough cross-bedding, representing flow in the upper
part of the lower flow regime.
In the breaker zone there is an upper flow regime,
producing a planar facies which in vertical section will
appear as very low-angle cross-bedding. On the beach
we often have a beach bar which is flooded only during
storms, and a depression behind it called a runnel,
which helps to drain the backshore area. Because the
exposed beach is a rich source of sand, the wind will
tend to blow sand from the beach and redeposit it as
aeolian dunes, usually where it is trapped by vegetation. Aeolian sediments therefore often cap ancient
beach profiles, but the aeolian dunes may also be
eroded and not be preserved in the geological record.
2 Introduction to Sedimentology
77