and the Baltic Sea, are too small to keep pace with the
attraction of the moon and the sun, and have small
tidal ranges. This is also true of lakes.
1. Tidal Channels. The infills may resemble fluvial
channels or submarine channels in that they form
fining-upwards sequences. Channels formed in
estuaries in fact are often connected to fluvial channel
systems. Tidal channels which are not part of a river
delta, however, tend to be filled with sandy sediments
from the surrounding tidal flat, because they have no
supply from land. Channels will often erode their
banks and cause them to collapse, and this may result
in the formation of intraformational conglomerates if
the sediments are slightly lithified.
Lateral migration of tidal channels may produce
typical epsilon cross-bedding which is the result of
lateral accretion of point bars in the tidal channel.
Tidal channels often contain a bed of marine fossils
at the base, and marine trace fossils. Channels on tidal
flats which are not associated with deltas (i.e. those in
estuaries) will not receive much clastic material from
land. Conglomerates and breccias in these tidal
channels will therefore typically be of the intraformational type, derived by local reworking of tidal
flat sediment. Because of their early lithification, carbonate beds in particular can be reworked to form
intra-formational conglomerates and breccias.
Tidal cycle duration is about 12 h and 25 min, with
currents switching direction every 6 h, and we sometimes find good examples of cross-bedding with opposite current directions. This is not always the case in
tidal environments, however. Some tidal channels are
dominated by ebb flow and others by flood currents.
This is because the ebb and flood often find different
dominant pathways. Bipolar cross-bedding is therefore not an essential feature of tidal channels. Storms
with opposing wind directions may also produce some
form of bipolar cross-bedding.
In a regressive sequence tidal channels will be
overlain by lagoonal sediments (Fig. 2.42).
2. Flaser Bedding. Consists of clay laminae in a
matrix of sandstone with ripple cross-lamination. The
clay occurs mainly as infill in ripple troughs, but may
also drape over the ripple crests as well. Flaser bedding forms as a result of alternating periods of currents
or wave activity, and slack water. The clay settles out
in the slack water periods in a tidal environment,
though this type of bedding may also form in other
environments where there is rhythmic sedimentation,
such as in certain fluvial environments. On tidal flats,
clay and silt will settle out at high tide to be deposited
between ripples formed by the ebb and flood currents.
Here the fine sediment may consist partly of clay
pellets (faecal pellets from marine organisms) which
settle out faster than clay-sized particles. Flaser bedding belongs to a type of structure which we get with
mixtures of sand and clay. Lenticular bedding
represents isolated laminae or lenses of sand in mud.
The inner parts of a tidal shelf often have
embayments consisting of very muddy sediments, usually with abundant bioturbation. Mollusc shells in the
mud (Fig. 2.43) are often eroded and deposited as shell
lag.
3. Tidal Bundles. The best identifying feature for
tidal environments is regular lamination consisting of
fine sand and mud, making up tidal couplets that each
represent a tidal cycle. Both modern and ancient tidal
sediments show a regular variation in the thickness of
such couplets, reflecting the energy levels of spring
and neap cycles. Regular laminations reflecting tidal
cycles are often called tidal bundles (Fig. 2.44).
2.40 Shallow Marine Shelves
The shelf extends from the nearshore environment to
the shelf edge, where there is a rather abrupt increase in
slope, usually at a depth of 200–500 m. The width of the
shelf varies considerably, and may exceed 1,000 km.
Continental shelves are generally very flat areas which
may be cut by deeper channels transporting sediments
across the shelf from nearshore or deltaic environments.
The study of sedimentation on modern continental
shelves is complicated by the fact that sea level was
more than 100 m lower only 10,000 years ago. This
means that most shelf areas have not yet reached an
equilibrium with respect to the modern environment.
Sandy shelf deposits are much more difficult to core
than muddy sediments and the commonly used gravity
corer has to be replaced by vibro-core equipment.
Most shelf areas are below the wave base for normal waves (fairweather wave base) and sedimentation
is governed largely by tidal currents and storms.
When the wind is landward, waves will usually
approach the beach obliquely, resulting in wave
refraction effects, particularly on relatively steep
beaches with high wave energy. This will produce
rip currents, where the water piling up against the
82
K. Bjørlykke
attraction of the moon and the sun, and have small
tidal ranges. This is also true of lakes.
1. Tidal Channels. The infills may resemble fluvial
channels or submarine channels in that they form
fining-upwards sequences. Channels formed in
estuaries in fact are often connected to fluvial channel
systems. Tidal channels which are not part of a river
delta, however, tend to be filled with sandy sediments
from the surrounding tidal flat, because they have no
supply from land. Channels will often erode their
banks and cause them to collapse, and this may result
in the formation of intraformational conglomerates if
the sediments are slightly lithified.
Lateral migration of tidal channels may produce
typical epsilon cross-bedding which is the result of
lateral accretion of point bars in the tidal channel.
Tidal channels often contain a bed of marine fossils
at the base, and marine trace fossils. Channels on tidal
flats which are not associated with deltas (i.e. those in
estuaries) will not receive much clastic material from
land. Conglomerates and breccias in these tidal
channels will therefore typically be of the intraformational type, derived by local reworking of tidal
flat sediment. Because of their early lithification, carbonate beds in particular can be reworked to form
intra-formational conglomerates and breccias.
Tidal cycle duration is about 12 h and 25 min, with
currents switching direction every 6 h, and we sometimes find good examples of cross-bedding with opposite current directions. This is not always the case in
tidal environments, however. Some tidal channels are
dominated by ebb flow and others by flood currents.
This is because the ebb and flood often find different
dominant pathways. Bipolar cross-bedding is therefore not an essential feature of tidal channels. Storms
with opposing wind directions may also produce some
form of bipolar cross-bedding.
In a regressive sequence tidal channels will be
overlain by lagoonal sediments (Fig. 2.42).
2. Flaser Bedding. Consists of clay laminae in a
matrix of sandstone with ripple cross-lamination. The
clay occurs mainly as infill in ripple troughs, but may
also drape over the ripple crests as well. Flaser bedding forms as a result of alternating periods of currents
or wave activity, and slack water. The clay settles out
in the slack water periods in a tidal environment,
though this type of bedding may also form in other
environments where there is rhythmic sedimentation,
such as in certain fluvial environments. On tidal flats,
clay and silt will settle out at high tide to be deposited
between ripples formed by the ebb and flood currents.
Here the fine sediment may consist partly of clay
pellets (faecal pellets from marine organisms) which
settle out faster than clay-sized particles. Flaser bedding belongs to a type of structure which we get with
mixtures of sand and clay. Lenticular bedding
represents isolated laminae or lenses of sand in mud.
The inner parts of a tidal shelf often have
embayments consisting of very muddy sediments, usually with abundant bioturbation. Mollusc shells in the
mud (Fig. 2.43) are often eroded and deposited as shell
lag.
3. Tidal Bundles. The best identifying feature for
tidal environments is regular lamination consisting of
fine sand and mud, making up tidal couplets that each
represent a tidal cycle. Both modern and ancient tidal
sediments show a regular variation in the thickness of
such couplets, reflecting the energy levels of spring
and neap cycles. Regular laminations reflecting tidal
cycles are often called tidal bundles (Fig. 2.44).
2.40 Shallow Marine Shelves
The shelf extends from the nearshore environment to
the shelf edge, where there is a rather abrupt increase in
slope, usually at a depth of 200–500 m. The width of the
shelf varies considerably, and may exceed 1,000 km.
Continental shelves are generally very flat areas which
may be cut by deeper channels transporting sediments
across the shelf from nearshore or deltaic environments.
The study of sedimentation on modern continental
shelves is complicated by the fact that sea level was
more than 100 m lower only 10,000 years ago. This
means that most shelf areas have not yet reached an
equilibrium with respect to the modern environment.
Sandy shelf deposits are much more difficult to core
than muddy sediments and the commonly used gravity
corer has to be replaced by vibro-core equipment.
Most shelf areas are below the wave base for normal waves (fairweather wave base) and sedimentation
is governed largely by tidal currents and storms.
When the wind is landward, waves will usually
approach the beach obliquely, resulting in wave
refraction effects, particularly on relatively steep
beaches with high wave energy. This will produce
rip currents, where the water piling up against the
82
K. Bjørlykke