100
quantities of moisture inland. The best present-day exampie is the monsoon belt of Asia along the margin of the Indian and Pacific Oceans.
Onshore winds generate not only large waves with a
deep wave base, but also surface currents driving a
net mass flux of water toward the coast (Fig. 3.4a).
The setup of water along a wide section of the coast
is enhanced by low barometrie pressure, abundant
rainfall, converging shorelines, and broad, shallow
shelves. The excess water volume can flow back either through rip currents (carrying some sediment in
suspension) along the surface andJor by bottom currents which may carry sediment in higher concentrations.
Bottom return flows in the surf and shoreface
zones tend to be partially channelized and may produce surf channels. Unconfined, nonchannelized bottom currents on the inner shelf, generated by
downwelling water masses, are referred to as
geostrophic currents. These are also deflected by
Corioli's fore es and therefore flow obliquely away
from the coastline. In regions with high tides,
geostrophic flows can be augmented by tidal ebb currents.
Tsunami waves, generated by submarine earthquakes, volcanic eruptions, or huge rock falls and submarine slides,
have very large wave lengths which cause extremely high
waves in shallow water and along some coastlines. These
waves inundate lowlands and produce backflows of high
velocity. The tsunami backflow, in particular if it becomes
channelized, has a high capacity to erode and transport
coarse-grained material into relatively deep water. The
resulting "tsunamiites" resemble in some ways the
tempestites. Theyare described in special articles (e.g. Sediment Geol 104, 1996).
Storm waves and storm-induced geostrophic currents
operate simultaneously and thus cause a combined
flow system (Fig. 3.4b). Back-and-forth oscillation of
the ground wave is superimposed on the quasi-steady
bottom current. The net shear stress imparted on the
sea floor may erode and move sediment during one
half of the wave stroke, but be insufficient to do so
during the other half. The shoreface and parts of the
inner shelf are dominated by wave-induced oscillatory shear, while the deeper environments are controlled mainly by steady, obliquely offshore or almost
shore-parallel currents.
Fig. 3.4. a Wind-induced surface currents, deflected
landward by Coriolis forces, setup of storm flood,
and generation of rip currents and geostrophic bottom currents (return flow). b Close-up of a showing
the generation of combined flow in a section across
the shoreface and inner shelf, simplified. c Swaley
Chapter 3 Coastal and Shallow Sea Sediments
It appears that shore-normal current directions measured at
the base of storm sand beds reflect the dominant influence
of wave-induced near-bottom flow during the storm peak,
whereas internal structures such as cross bedding frequently deviate from this direction. They largely result
from the geostrophic current component of combined flow.
Shore-parallel rippled tops may be caused by subsequent
waves approaching the shoreline at nearly right angles.
Some graded sand beds on the shelf are generated by riverfed density underflows.
Characteristics of Tempestites
Bedforms and Facies Patterns
Storms frequently erode seaweed and shells of
bottom-dwelling fauna of the foreshore region and
accumulate them landward of the normal beach or
barrier zone as supratidal or backshore storm beds.
These have a low preservation potential and are not
described further here.
Seaward of the beach, storm-generated bed forms
show a distinct trend from the surf zone into deeper
water (Figs. 3.4 and 3.5):
- The surf zone and upper shoreface are controlled
by fairweather wave action which usually destroys
the fingerprints of previous storms. Megaripples, flat
swash larnination on offshore bar crests, trough
cross-stratification and planar or low-angle swash
larninae are the dominating structures.
- The rniddle slioreface may preserve some structures
formed during storms, such as flat, nearly horizontal
bedding or low-angle swaley cross-stratification (Fig.
3.4c). Coarse particles often form lags; mud is sorted
out and deposited in deeper water; bottom life is limited to a filter-fee ding infauna.
- On the lower shoreface (5 to 20 m water depth), the
stress imparted by both flow components (oscillating
water and geostrophic currents) may stir up both
sand and mud, which are redeposited as a graded bed
at the same location or nearby (proximal tempestite;
Fig. 3.4d). At the peak of storm action, irregular
scours and hollows form at the sea bottom (pot and
gutter casts) which are later filled with the coarsest
siliciclastics or bioclasts available. These casts and
tool marks sometimes display bipolar or multi-directional current action. The typical internal sedimentary
structure produced by the combined flow regime is
low-angle hummocky cross-stratification (RCS).
This normally occurs on top of a graded division
and hummocky cross-stratification (HCS) , rniddle
shoreface, muddy interbeds are missing.
d Siliciclastic tempestite, lower shoreface. e Distal
tempestite, sandy-silty. (Based on Dott and Bourgeois 1982; Wp.lker et al. 1983; Aigner 1985, and
others)
quantities of moisture inland. The best present-day exampie is the monsoon belt of Asia along the margin of the Indian and Pacific Oceans.
Onshore winds generate not only large waves with a
deep wave base, but also surface currents driving a
net mass flux of water toward the coast (Fig. 3.4a).
The setup of water along a wide section of the coast
is enhanced by low barometrie pressure, abundant
rainfall, converging shorelines, and broad, shallow
shelves. The excess water volume can flow back either through rip currents (carrying some sediment in
suspension) along the surface andJor by bottom currents which may carry sediment in higher concentrations.
Bottom return flows in the surf and shoreface
zones tend to be partially channelized and may produce surf channels. Unconfined, nonchannelized bottom currents on the inner shelf, generated by
downwelling water masses, are referred to as
geostrophic currents. These are also deflected by
Corioli's fore es and therefore flow obliquely away
from the coastline. In regions with high tides,
geostrophic flows can be augmented by tidal ebb currents.
Tsunami waves, generated by submarine earthquakes, volcanic eruptions, or huge rock falls and submarine slides,
have very large wave lengths which cause extremely high
waves in shallow water and along some coastlines. These
waves inundate lowlands and produce backflows of high
velocity. The tsunami backflow, in particular if it becomes
channelized, has a high capacity to erode and transport
coarse-grained material into relatively deep water. The
resulting "tsunamiites" resemble in some ways the
tempestites. Theyare described in special articles (e.g. Sediment Geol 104, 1996).
Storm waves and storm-induced geostrophic currents
operate simultaneously and thus cause a combined
flow system (Fig. 3.4b). Back-and-forth oscillation of
the ground wave is superimposed on the quasi-steady
bottom current. The net shear stress imparted on the
sea floor may erode and move sediment during one
half of the wave stroke, but be insufficient to do so
during the other half. The shoreface and parts of the
inner shelf are dominated by wave-induced oscillatory shear, while the deeper environments are controlled mainly by steady, obliquely offshore or almost
shore-parallel currents.
Fig. 3.4. a Wind-induced surface currents, deflected
landward by Coriolis forces, setup of storm flood,
and generation of rip currents and geostrophic bottom currents (return flow). b Close-up of a showing
the generation of combined flow in a section across
the shoreface and inner shelf, simplified. c Swaley
Chapter 3 Coastal and Shallow Sea Sediments
It appears that shore-normal current directions measured at
the base of storm sand beds reflect the dominant influence
of wave-induced near-bottom flow during the storm peak,
whereas internal structures such as cross bedding frequently deviate from this direction. They largely result
from the geostrophic current component of combined flow.
Shore-parallel rippled tops may be caused by subsequent
waves approaching the shoreline at nearly right angles.
Some graded sand beds on the shelf are generated by riverfed density underflows.
Characteristics of Tempestites
Bedforms and Facies Patterns
Storms frequently erode seaweed and shells of
bottom-dwelling fauna of the foreshore region and
accumulate them landward of the normal beach or
barrier zone as supratidal or backshore storm beds.
These have a low preservation potential and are not
described further here.
Seaward of the beach, storm-generated bed forms
show a distinct trend from the surf zone into deeper
water (Figs. 3.4 and 3.5):
- The surf zone and upper shoreface are controlled
by fairweather wave action which usually destroys
the fingerprints of previous storms. Megaripples, flat
swash larnination on offshore bar crests, trough
cross-stratification and planar or low-angle swash
larninae are the dominating structures.
- The rniddle slioreface may preserve some structures
formed during storms, such as flat, nearly horizontal
bedding or low-angle swaley cross-stratification (Fig.
3.4c). Coarse particles often form lags; mud is sorted
out and deposited in deeper water; bottom life is limited to a filter-fee ding infauna.
- On the lower shoreface (5 to 20 m water depth), the
stress imparted by both flow components (oscillating
water and geostrophic currents) may stir up both
sand and mud, which are redeposited as a graded bed
at the same location or nearby (proximal tempestite;
Fig. 3.4d). At the peak of storm action, irregular
scours and hollows form at the sea bottom (pot and
gutter casts) which are later filled with the coarsest
siliciclastics or bioclasts available. These casts and
tool marks sometimes display bipolar or multi-directional current action. The typical internal sedimentary
structure produced by the combined flow regime is
low-angle hummocky cross-stratification (RCS).
This normally occurs on top of a graded division
and hummocky cross-stratification (HCS) , rniddle
shoreface, muddy interbeds are missing.
d Siliciclastic tempestite, lower shoreface. e Distal
tempestite, sandy-silty. (Based on Dott and Bourgeois 1982; Wp.lker et al. 1983; Aigner 1985, and
others)