3.1 Beach and Shoreface Sediments
Shoreline Migration
and Vertical Sediment Successions
The facies zones of the shoreface migrate back and
forth permanently in response to the momentarily
active wave regime. As a result, vertical sequences
record frequent variations between neighboring facies zones, described above. They may include the
internal structures of sand bars and rip channel fillings (Figs. 3.lb and 3.3c).
Seaward prograding shorehnes generally show a
tendency for the sands to coarsen upward. Simultaneously, bioturbation structures are less readily preserved due to the permanent erosion and redeposition
of sand in the upper shoreface zone. Transgressive
shoreface sediments display the opposite vertical development with a fining-upward sequence.
Gently sloping beaches create a wide belt of
shoreface sands, whereas steeply sloping shorelines
only produce a narrow belt of coastal sands. Then the
transitional facies between the swash-backwash zone
and deeper water may be largely absent.
The Sand Budget of the Beach-Shoreface Zone
An aspect of considerable interest is the sand budget
of the total beach-shoreface zone of a particular area.
It is fed by river input, cliff erosion, and longshore
sand transport, and it is reduced by longshore transport to other areas, los ses to coastal eolian sand
dunes, and sand transport into deeper water by storm
action (see below) or via submarine canyons beginning in the shoreface zone (cf. Sect. 5.4).
Apart from these long-term gains and losses, the
sand budget of a given shoreline section appears to
remain fairly stable under a defined wave regime.
Although the beach-shoreface sands are permanently
in motion and often migrate landward and seaward
with the seasons, their volume remains approximately constant, at least for a limited, geologically
relevant time period.
This mayaiso be the case during rising and falling sea levels. In this instance, a major part of the coastal sand belt
ean mi grate either landward or seaward without leaving
behind many relics in deeper water or in emerged coastal
areas (cf. Sect. 7.3).
On the other hand, observations and measurements at
several present-day coastlines have revealed that
longshore currents are capable of transporting enormous volumes of sand (up to several hundred thousands of cubic meters per year). For this reason, sand
accretion in coastline sections protected against
strong wave attack is common and may proceed fast.
Some modem shorelines, consisting of sandy beach
ridges (cheniers), have migrated roughly I mJa seaward during the last few thousands of years. Accre99
tion of coastal sand may take place directly on the
mainland coast or in front of barrier islands (Fig.
3.2e). Thus, high lateral sand supply may generate a
wide beach and shoreface sand belt within a geologically short time period.
3.1.3 Storms and Storm Deposits (Tempestites)
In storm-dominated shallow-marine environments,
the "normal" shoreface processes described so far, as
well as processes in deeper regions of the inner and
possibly outer shelf, are strongly affected and modified by rare storm events. Storms can deposit sandy
material in two different zones:
- In the supratidal or backshore zone (supratidal
storm layers, cf. Sect. 3.2.2).
- In deeper water beyond the shoreface zone
(tempestites).
Tempestites are sheet-like sand and mud beds of considerable lateral extent. Here, this type of storm deposits is discussed further.
Tempestites have been deseribed from many loeations and
geologieal time periods. The processes involved have been
reviewed, <:.g., by Allen (1982), Aigner (1995), Morton
(1988), Snedden et al. (1988), Duke (1990), Nummedal
(1991), Seilacher and Aigner (1991), Myrow and Southard
(1996).
Storm Generation and Storm Action
Exceptionally heavy storms may affeet the sedimentary processes not only of the coastal and shore face
zones but also those of the inner and possibly outer
shelf. Strong storms develop under special climatic
and geographie conditions. Tropical storms (hurrieanes) are initiated in low-latitude regions within the
trade wind belt. They travel westward and are deflected toward the poles by Corioli's forces.
Extratropical storms are common in zones of midlatitudinal atmospheric circulation along polar fronts.
They migrate eastward, are more consistent in their
direction, and can reach the same magnitudes as tropical storms. A third mechanism produeing landwarddirected storms are the monsoons during the summer
season.
In viewing the geologie reeord, one ean assurne that tropica1 storms prevailed during warm periods exhibiting gentle
temperature gradients between the poles and the equator,
whereas extratropieal storms were more frequent and important during cooler periods with steep temperature gradients.
Monsoons are ereated when 1arge landmasses establish
intense summer lows and winter highs. The summer season
is eharacterized by strong onshore winds carrying large
Shoreline Migration
and Vertical Sediment Successions
The facies zones of the shoreface migrate back and
forth permanently in response to the momentarily
active wave regime. As a result, vertical sequences
record frequent variations between neighboring facies zones, described above. They may include the
internal structures of sand bars and rip channel fillings (Figs. 3.lb and 3.3c).
Seaward prograding shorehnes generally show a
tendency for the sands to coarsen upward. Simultaneously, bioturbation structures are less readily preserved due to the permanent erosion and redeposition
of sand in the upper shoreface zone. Transgressive
shoreface sediments display the opposite vertical development with a fining-upward sequence.
Gently sloping beaches create a wide belt of
shoreface sands, whereas steeply sloping shorelines
only produce a narrow belt of coastal sands. Then the
transitional facies between the swash-backwash zone
and deeper water may be largely absent.
The Sand Budget of the Beach-Shoreface Zone
An aspect of considerable interest is the sand budget
of the total beach-shoreface zone of a particular area.
It is fed by river input, cliff erosion, and longshore
sand transport, and it is reduced by longshore transport to other areas, los ses to coastal eolian sand
dunes, and sand transport into deeper water by storm
action (see below) or via submarine canyons beginning in the shoreface zone (cf. Sect. 5.4).
Apart from these long-term gains and losses, the
sand budget of a given shoreline section appears to
remain fairly stable under a defined wave regime.
Although the beach-shoreface sands are permanently
in motion and often migrate landward and seaward
with the seasons, their volume remains approximately constant, at least for a limited, geologically
relevant time period.
This mayaiso be the case during rising and falling sea levels. In this instance, a major part of the coastal sand belt
ean mi grate either landward or seaward without leaving
behind many relics in deeper water or in emerged coastal
areas (cf. Sect. 7.3).
On the other hand, observations and measurements at
several present-day coastlines have revealed that
longshore currents are capable of transporting enormous volumes of sand (up to several hundred thousands of cubic meters per year). For this reason, sand
accretion in coastline sections protected against
strong wave attack is common and may proceed fast.
Some modem shorelines, consisting of sandy beach
ridges (cheniers), have migrated roughly I mJa seaward during the last few thousands of years. Accre99
tion of coastal sand may take place directly on the
mainland coast or in front of barrier islands (Fig.
3.2e). Thus, high lateral sand supply may generate a
wide beach and shoreface sand belt within a geologically short time period.
3.1.3 Storms and Storm Deposits (Tempestites)
In storm-dominated shallow-marine environments,
the "normal" shoreface processes described so far, as
well as processes in deeper regions of the inner and
possibly outer shelf, are strongly affected and modified by rare storm events. Storms can deposit sandy
material in two different zones:
- In the supratidal or backshore zone (supratidal
storm layers, cf. Sect. 3.2.2).
- In deeper water beyond the shoreface zone
(tempestites).
Tempestites are sheet-like sand and mud beds of considerable lateral extent. Here, this type of storm deposits is discussed further.
Tempestites have been deseribed from many loeations and
geologieal time periods. The processes involved have been
reviewed, <:.g., by Allen (1982), Aigner (1995), Morton
(1988), Snedden et al. (1988), Duke (1990), Nummedal
(1991), Seilacher and Aigner (1991), Myrow and Southard
(1996).
Storm Generation and Storm Action
Exceptionally heavy storms may affeet the sedimentary processes not only of the coastal and shore face
zones but also those of the inner and possibly outer
shelf. Strong storms develop under special climatic
and geographie conditions. Tropical storms (hurrieanes) are initiated in low-latitude regions within the
trade wind belt. They travel westward and are deflected toward the poles by Corioli's forces.
Extratropical storms are common in zones of midlatitudinal atmospheric circulation along polar fronts.
They migrate eastward, are more consistent in their
direction, and can reach the same magnitudes as tropical storms. A third mechanism produeing landwarddirected storms are the monsoons during the summer
season.
In viewing the geologie reeord, one ean assurne that tropica1 storms prevailed during warm periods exhibiting gentle
temperature gradients between the poles and the equator,
whereas extratropieal storms were more frequent and important during cooler periods with steep temperature gradients.
Monsoons are ereated when 1arge landmasses establish
intense summer lows and winter highs. The summer season
is eharacterized by strong onshore winds carrying large