114
mats of micro-organisms (Gerdes et al. 1985;
Reineck and Gerdes 1996) and, particularly in
supratidal marshes, plants able to withstand the severe environmental conditions further protect the
muddy sediments from erosion.
As a result of the periodic emergence of the mudflats
during low tides, the fine-grained sediment at the
surface loses water by evaporation and can becomes
more solid than freshly deposited mud covered permanently by water.
The Holocene mudflats ofthe North Sea accumulated at an
average sedimentation rate of 1 to 2 rnmIa. Over periods of
months, however, the mudflats may grow much faster
(about 5 to 10 rnmImonth), but vertical aggradation is frequently interrupted by erosional events (Reineck 1980;
Anderson et al. 1981). Long-term rapid upbuilding is limited by tidal range and basin subsidence. In supratidal areas, which are flooded only during spring tides or storms,
the rate of deposition decreases considerably.
Tidal marsh sediment along the Delaware river near
Philadelphia accumulated at the rate of 004 mrn/a during
the last 2 ka, prior to the modem colonization of this area
(Orson et al. 1990).
When muds are deposited under comparatively lowenergy conditions, they are little affected by subsequent channeling and thus contain only small portions of cross-bedded sands and sand bars characteristic of the intertidal and subtidal zones.
Mud deposition in the mudflats generates a striking contrast between mudflats along protected co asts
and beach or foreshore sands along open coasts
(Sect. 3.1). On sandy beaches the finer-grained particles are sorted out and carried into deeper water,
whereas in tidal flats the fine-grained material is deposited near the coast slightly below or above the
mean water line (Fig. 3.11a). This may occur even in
medium to high energy tidal environments (wave and
current action).
Tidal Channel Sediments
The slowly starting subsequent ebb current usually
carries only part of the mud, previously transported
landward, back into deeper water. This current in
turn will reach high velocities, as soon as the mudflats are drained and the seaward-flowing water becomes confined to the creek system. The channels
are therefore kept clean of fine-grained sediments,
and erosion along their banks is very common.
In this way small slides are produced which are
reworked by the channelized ebb or flood currents,
forrning elasts and pebbles of mud. These are fairly
resistant and often preserved in the sedimentary record.
Chapter 3 Coastal and Shallow Sea Sediments
Tidal channels, particularly those in sand-dominated tidal flats, tend to migrate laterally. A special
feature of these channels is lateral accretion bedding
(or "longitudinal" cross-bedding; cf. Fig. 3.13), in
contrast to normal, transverse cross-bedding, generated by current ripples and dunes within the channels. The thickness of the lateral accretion beds approximates the depth of the channel wh ich rnay range
from a few decimeters to several meters. The main
channels increase in width and depth seaward. Due
to the changing direction of flood and ebb currents,
the ripple- and dune-related types of small- and
large-scale cross-bedding often display bidirectional
orientation of subsequent sets of larninae, a feature
which is referred to as herringbone stratification (cf.
Fig.3.13).
Another interesting and diagnostic feature of tidal
channel deposits is the large sandwaves (often 1 to 2
m high), wh ich may indicate not only the diurnal or
semi-diurnal tidal cycle, but sometimes also the effects of the fortnightly variation from neap tide to
spring tide (neap-spring cycles; e.g. Visser 1980;
Oost et al. 1993; Fig. 3.12). Such features were
found in modem and ancient sediments of different
ages. The sandwaves grow with the dominant current
(this often being the ebb current), but are also affected by the weaker opposite current (reworked
fore sets and migrating smaller ripples).
During slack water at high water stand, clay and
silt-sized sediment (flocs and pellets) can settle on
top of sandy foresets and particularly in ripple
troughs in tidal channels and on sandflats. The mud
forms characteristic mud drapings, which partly resist later erosion. In shallow water channels, the accumulation of coarse shells and shell debris, derived
mostly from eroded parts of the tidal flats, is common.
In the subtidal zone, tidal currents can produce
large tidal sand ridges and sand sheets (cf. Sect.
3.3.3).
Sandflats and Mixed Sand-Mudflats
An irregularly and indistinctly bounded zone seaward
of the mudflats and in the neighborhood of the tidal
channels is occupied by sandflats and mixed sandmudflats. Sand transported along the channels can
spill over onto the flats and form either ± pure sand
sheets (sandflats) or thin sand layers alternating with
mud (mixed sand-mudflats). In favorable cases,
laminae accumulated during flood and ebb currents
form tidal rhythmites. More frequently, the surfaces
of both types of tidal sediments are modified and
reworked by current and wave action of chan ging
intensity. Current ripples may be superimposed by
wave ripples, and the internal structures of mixed
mats of micro-organisms (Gerdes et al. 1985;
Reineck and Gerdes 1996) and, particularly in
supratidal marshes, plants able to withstand the severe environmental conditions further protect the
muddy sediments from erosion.
As a result of the periodic emergence of the mudflats
during low tides, the fine-grained sediment at the
surface loses water by evaporation and can becomes
more solid than freshly deposited mud covered permanently by water.
The Holocene mudflats ofthe North Sea accumulated at an
average sedimentation rate of 1 to 2 rnmIa. Over periods of
months, however, the mudflats may grow much faster
(about 5 to 10 rnmImonth), but vertical aggradation is frequently interrupted by erosional events (Reineck 1980;
Anderson et al. 1981). Long-term rapid upbuilding is limited by tidal range and basin subsidence. In supratidal areas, which are flooded only during spring tides or storms,
the rate of deposition decreases considerably.
Tidal marsh sediment along the Delaware river near
Philadelphia accumulated at the rate of 004 mrn/a during
the last 2 ka, prior to the modem colonization of this area
(Orson et al. 1990).
When muds are deposited under comparatively lowenergy conditions, they are little affected by subsequent channeling and thus contain only small portions of cross-bedded sands and sand bars characteristic of the intertidal and subtidal zones.
Mud deposition in the mudflats generates a striking contrast between mudflats along protected co asts
and beach or foreshore sands along open coasts
(Sect. 3.1). On sandy beaches the finer-grained particles are sorted out and carried into deeper water,
whereas in tidal flats the fine-grained material is deposited near the coast slightly below or above the
mean water line (Fig. 3.11a). This may occur even in
medium to high energy tidal environments (wave and
current action).
Tidal Channel Sediments
The slowly starting subsequent ebb current usually
carries only part of the mud, previously transported
landward, back into deeper water. This current in
turn will reach high velocities, as soon as the mudflats are drained and the seaward-flowing water becomes confined to the creek system. The channels
are therefore kept clean of fine-grained sediments,
and erosion along their banks is very common.
In this way small slides are produced which are
reworked by the channelized ebb or flood currents,
forrning elasts and pebbles of mud. These are fairly
resistant and often preserved in the sedimentary record.
Chapter 3 Coastal and Shallow Sea Sediments
Tidal channels, particularly those in sand-dominated tidal flats, tend to migrate laterally. A special
feature of these channels is lateral accretion bedding
(or "longitudinal" cross-bedding; cf. Fig. 3.13), in
contrast to normal, transverse cross-bedding, generated by current ripples and dunes within the channels. The thickness of the lateral accretion beds approximates the depth of the channel wh ich rnay range
from a few decimeters to several meters. The main
channels increase in width and depth seaward. Due
to the changing direction of flood and ebb currents,
the ripple- and dune-related types of small- and
large-scale cross-bedding often display bidirectional
orientation of subsequent sets of larninae, a feature
which is referred to as herringbone stratification (cf.
Fig.3.13).
Another interesting and diagnostic feature of tidal
channel deposits is the large sandwaves (often 1 to 2
m high), wh ich may indicate not only the diurnal or
semi-diurnal tidal cycle, but sometimes also the effects of the fortnightly variation from neap tide to
spring tide (neap-spring cycles; e.g. Visser 1980;
Oost et al. 1993; Fig. 3.12). Such features were
found in modem and ancient sediments of different
ages. The sandwaves grow with the dominant current
(this often being the ebb current), but are also affected by the weaker opposite current (reworked
fore sets and migrating smaller ripples).
During slack water at high water stand, clay and
silt-sized sediment (flocs and pellets) can settle on
top of sandy foresets and particularly in ripple
troughs in tidal channels and on sandflats. The mud
forms characteristic mud drapings, which partly resist later erosion. In shallow water channels, the accumulation of coarse shells and shell debris, derived
mostly from eroded parts of the tidal flats, is common.
In the subtidal zone, tidal currents can produce
large tidal sand ridges and sand sheets (cf. Sect.
3.3.3).
Sandflats and Mixed Sand-Mudflats
An irregularly and indistinctly bounded zone seaward
of the mudflats and in the neighborhood of the tidal
channels is occupied by sandflats and mixed sandmudflats. Sand transported along the channels can
spill over onto the flats and form either ± pure sand
sheets (sandflats) or thin sand layers alternating with
mud (mixed sand-mudflats). In favorable cases,
laminae accumulated during flood and ebb currents
form tidal rhythmites. More frequently, the surfaces
of both types of tidal sediments are modified and
reworked by current and wave action of chan ging
intensity. Current ripples may be superimposed by
wave ripples, and the internal structures of mixed