in trough cross-bedding the laminae have a rounded
surface which is concave in the downstream direction.
The foreset lamination may form a relatively sharp
angle with the underlying bed, or may have a more
tangential contact. The latter is typical of troughshaped sets.
Aeolian dunes may be many metres high, and their
cross-bedding will then be correspondingly large
(Fig. 2.15).
Whatever the size of the bedform, ripple movement
provides the basic mode of migration. Seabed survey
profiling records clearly show smaller ripples climbing
the stoss sides of larger ones (so-called “megaripples”,
i.e. small dunes), which in turn are climbing the stoss
sides of sandwaves.
Cross-sections through dunes show “large-scale
cross-stratification”, which is often referred to as
“cross-bedding”. This can be observed in real time
on seabed video recordings made during periods of
strong tidal current flow, where gradual forward
movement of “megaripples” is seen, caused by sand
cascading down the lee side after reaching the crest.
“Plane beds” (upper stage) may form when the
shear stress against the bed exceeds the values which
produce dunes. In cross-section we only see planar
lamination, which is an internal structure, but on the
bedding surface we may see very small ridges, which
define a lineation called primary current lineation,
parallel to flow.
At even higher velocities in relatively shallow
water, standing waves may produce antidunes when
the Froude number exceeds 0.8. The antidunes which
are produced when standing waves are in phase with
the bedforms develop resulting in low-angle crosslamination which dips up-current.
Both ripples and dunes are formed through sand
being transported along the bottom and deposited in
sloping strata on the lee side of the structure. In consequence they always have dipping laminations
(foreset beds) which may lie at an angle (angle of
repose) of up to 35
to the surface of the bed, though
such high angles are rather rare. Current ripples in plan
view may be straight, or form curved patterns (sinuous
crests). Ripples with a symmetrical cross-section
(symmetrical ripples) are formed by waves as a rule.
Asymmetrical ripples are formed by a predominantly
unidirectional current and their steeper side faces
downstream. Ripples with a high sinuosity are also
asymmetrical in most cases.
Tongue-shaped (linguoid) ripples have a very high
sinuosity and asymmetry and are usually formed in
shallower water or under higher velocities than
straight-crested types. Wave ripples in particular
may split laterally into two ripples. This is called
bifurcation. In intertidal zones ripples formed at
high tide may be eroded at low tide, and the crests
become flattened. When the tidal flat is submerged at
high tide it may also be below the normal wave base,
resulting at slack water in deposition of clay which
tends to collect in the ripple troughs. Current ripples
with thin lenses of clay between them constitute
flaser bedding. Wind may generate waves moving
in different directions, particularly at very low
water, so that we find two or more sets of ripples at
an angle to each other (interference pattern). In most
cases each bed with current ripples represents a sort
of equilibrium with deposits reflecting current
patterns. Isolated sand lenses in clay are called lenticular bedding.
Normally current ripples form completely horizontal beds. In some cases, however, we find examples of
current ripples appearing to climb downstream in relation to the horizontal plane. They form several sets of
cross-laminated beds delimited by erosion boundaries,
but with small internal erosion planes. These are called
climbing ripples and are due to sedimentation taking
place so rapidly that, in contrast to normal ripples,
there is no equilibrium between erosion and sedimentation. Climbing ripples are therefore typical of
environments with rapidly declining flow velocity
and consequently a high rate of sedimentation.
Sandwaves are large-scale transverse bedforms,
generally 2–15 m high, with a wavelength of
150–500 m. They may be formed at flow velocities
of 65–125 cm/s. Cross-stratification may be symmetrical or asymmetrical on both sides of the sandwave,
depending on the relative strength of the opposing
currents.
At velocities of less than 1 m/s sand ribbons may be
deposited – longitudinal bedforms developed parallel
to the currents. Sand ribbons are typical of subtidal
environments (20–200 m), and they may be up to
20 km long, 200 m wide and less than a metre thick.
Relatively low-energy environments (<50 cm/s)
are characterised by sand patches and mud. The sand
forming the patches probably only moves during
storms. Lateral structures are typically current ripples
and small dunes.
52
K. Bjørlykke
surface which is concave in the downstream direction.
The foreset lamination may form a relatively sharp
angle with the underlying bed, or may have a more
tangential contact. The latter is typical of troughshaped sets.
Aeolian dunes may be many metres high, and their
cross-bedding will then be correspondingly large
(Fig. 2.15).
Whatever the size of the bedform, ripple movement
provides the basic mode of migration. Seabed survey
profiling records clearly show smaller ripples climbing
the stoss sides of larger ones (so-called “megaripples”,
i.e. small dunes), which in turn are climbing the stoss
sides of sandwaves.
Cross-sections through dunes show “large-scale
cross-stratification”, which is often referred to as
“cross-bedding”. This can be observed in real time
on seabed video recordings made during periods of
strong tidal current flow, where gradual forward
movement of “megaripples” is seen, caused by sand
cascading down the lee side after reaching the crest.
“Plane beds” (upper stage) may form when the
shear stress against the bed exceeds the values which
produce dunes. In cross-section we only see planar
lamination, which is an internal structure, but on the
bedding surface we may see very small ridges, which
define a lineation called primary current lineation,
parallel to flow.
At even higher velocities in relatively shallow
water, standing waves may produce antidunes when
the Froude number exceeds 0.8. The antidunes which
are produced when standing waves are in phase with
the bedforms develop resulting in low-angle crosslamination which dips up-current.
Both ripples and dunes are formed through sand
being transported along the bottom and deposited in
sloping strata on the lee side of the structure. In consequence they always have dipping laminations
(foreset beds) which may lie at an angle (angle of
repose) of up to 35
to the surface of the bed, though
such high angles are rather rare. Current ripples in plan
view may be straight, or form curved patterns (sinuous
crests). Ripples with a symmetrical cross-section
(symmetrical ripples) are formed by waves as a rule.
Asymmetrical ripples are formed by a predominantly
unidirectional current and their steeper side faces
downstream. Ripples with a high sinuosity are also
asymmetrical in most cases.
Tongue-shaped (linguoid) ripples have a very high
sinuosity and asymmetry and are usually formed in
shallower water or under higher velocities than
straight-crested types. Wave ripples in particular
may split laterally into two ripples. This is called
bifurcation. In intertidal zones ripples formed at
high tide may be eroded at low tide, and the crests
become flattened. When the tidal flat is submerged at
high tide it may also be below the normal wave base,
resulting at slack water in deposition of clay which
tends to collect in the ripple troughs. Current ripples
with thin lenses of clay between them constitute
flaser bedding. Wind may generate waves moving
in different directions, particularly at very low
water, so that we find two or more sets of ripples at
an angle to each other (interference pattern). In most
cases each bed with current ripples represents a sort
of equilibrium with deposits reflecting current
patterns. Isolated sand lenses in clay are called lenticular bedding.
Normally current ripples form completely horizontal beds. In some cases, however, we find examples of
current ripples appearing to climb downstream in relation to the horizontal plane. They form several sets of
cross-laminated beds delimited by erosion boundaries,
but with small internal erosion planes. These are called
climbing ripples and are due to sedimentation taking
place so rapidly that, in contrast to normal ripples,
there is no equilibrium between erosion and sedimentation. Climbing ripples are therefore typical of
environments with rapidly declining flow velocity
and consequently a high rate of sedimentation.
Sandwaves are large-scale transverse bedforms,
generally 2–15 m high, with a wavelength of
150–500 m. They may be formed at flow velocities
of 65–125 cm/s. Cross-stratification may be symmetrical or asymmetrical on both sides of the sandwave,
depending on the relative strength of the opposing
currents.
At velocities of less than 1 m/s sand ribbons may be
deposited – longitudinal bedforms developed parallel
to the currents. Sand ribbons are typical of subtidal
environments (20–200 m), and they may be up to
20 km long, 200 m wide and less than a metre thick.
Relatively low-energy environments (<50 cm/s)
are characterised by sand patches and mud. The sand
forming the patches probably only moves during
storms. Lateral structures are typically current ripples
and small dunes.
52
K. Bjørlykke