112
Supratidal zone above the mean high water line.
- Intertidal zone between the mean high and mean
low water lines.
Subtidal zone below the mean low water line.
The higher the tidal range, the more water has to be
transported from deep water into the coastal zone and
back again during one tidal cycle. Consequently, high
tides are associated with strong tidal CUITents flowing
onshore and offshore. They often reach velocities of
1 to 2 m/s, with local values of up to 4-8 m/s. In tidal
flats, their maximum velocities comrnonly develop in
the middle of the flood or ebb period, when the rise
or fall of sea level is most pronounced (cf. Fig.
3.l1b). At other locations, for example in special
tidal channels or at the heads of estuaries, the tidal
current maxima and minima may occur earlier or
later in relation to the tidal cycle.
Coastal Morphology and Tidal-Influenced
Environments
The hydrographie regime of tide-influenced coastal
areas is also affected significantly by the presence or
absence of baITier islands accompanying the coastline. The relationship between coastal morphology
and tidal amplitude is sumrnarized in Table 3.1 and
Fig. 3.1 Ob-d.
Table 3.1. Tidal-influenced environments
Tidal range
Coastal morphology
- Micotidal
<2m
Long barrier islands, few
inlets
Short barrier islands with
- Mesotidal
2-4m
numerous inlets, ebb and
flood tidal deltas
Small or missing islands,
- Macrotidal
>4m
estuaries with subtidal
ridges
Apart from tidal action and coastal morphology, the
sedimentary processes in tidal flats are influenced by
wind-generated waves. Therefore it has become common for sedimentologists to subdivide coastal areas
into three subenvironments:
- Wave-dominated shorelines: beaches, microtidal
baITier islands and cheniers.
- Mixed wave-tide injluenced shorelines: mesotidal
baITier islands with tidal in lets and ebb and flood
tidal deltas.
Chapter 3 Coastal and Shallow Sea Sediments
- Tide-dominated shorelines: tidal flats, estuaries
and associated sand ridges.
This subdivision is based on the assumption that
wind waves are the dominant coastal process when
the tidal range is less than 2 m. Features such as
beaches, sand spits, and long baITier islands are controlled mainly by wind-generated waves (cf. Sect.
3.1). In contrast, tidal ranges in excess of 4 mare the
dominant influence on coastal areas and their sediments, although wind waves also play some part. In
this chapter, both tide-dominated and mixed wavetide influenced environments will be discussed.
3.2.2 General Characteristics of Tidal Sediments
Tidal Environments, Provenance and Distribution of
Sediments
Modem tidal environments can be subdivided into
three zones (from higher to lower elevated areas):
- Mudflats.
- Mixed sand-mudflats.
- Sandflats.
Mud and sand refer to grain-size fractions, which
may consist of both silicic1astic and/or carbonate material. The three zones display distinctive characteristics which can be easily recognized in ancient sedimentary rocks. Tidal sediments are therefore of outstanding interest for paleoenvironmental and
paleogeographic reconstructions.
Besides numerous special articles, several textbooks present summeries and some special books deal entirely with
this topic (e.g., Ginsburg 1975; Friedman and Sanders
1978; Reineck and Singh 1980; Klein 1985c; Elliott
1986a; Boer et al. 1988; Reinson 1992; Flemming and
Bartolomä 1995; Clark et al. 1999).
Tidal flat sediments may have different sourees: input by rivers, cliff erosion, and reworked material
from the offshore sea bottom. The proportion of
mudflats, mixed sand-mudflats, and sandflats also
depends on the availability of mud and sand in the
source areas. Along coastal plains and near muddominated rivers, muddy tidal flats tend to prevail. In
front of coastal mountain ranges delivering large
quantities of sand into a tidal regime, sandier flats
and subtidal sand ridges are prominent.
An ancient example of the latter situation is the Miocene
(Burdigalian) marine molasse in the northem foreland basin of the Alps (Homewood and Allen 1981). It can be assumed that sandy material usually forrns narrower tidal
flats than finer grained muds, which better withstand erosion and reworking (see below).
Supratidal zone above the mean high water line.
- Intertidal zone between the mean high and mean
low water lines.
Subtidal zone below the mean low water line.
The higher the tidal range, the more water has to be
transported from deep water into the coastal zone and
back again during one tidal cycle. Consequently, high
tides are associated with strong tidal CUITents flowing
onshore and offshore. They often reach velocities of
1 to 2 m/s, with local values of up to 4-8 m/s. In tidal
flats, their maximum velocities comrnonly develop in
the middle of the flood or ebb period, when the rise
or fall of sea level is most pronounced (cf. Fig.
3.l1b). At other locations, for example in special
tidal channels or at the heads of estuaries, the tidal
current maxima and minima may occur earlier or
later in relation to the tidal cycle.
Coastal Morphology and Tidal-Influenced
Environments
The hydrographie regime of tide-influenced coastal
areas is also affected significantly by the presence or
absence of baITier islands accompanying the coastline. The relationship between coastal morphology
and tidal amplitude is sumrnarized in Table 3.1 and
Fig. 3.1 Ob-d.
Table 3.1. Tidal-influenced environments
Tidal range
Coastal morphology
- Micotidal
<2m
Long barrier islands, few
inlets
Short barrier islands with
- Mesotidal
2-4m
numerous inlets, ebb and
flood tidal deltas
Small or missing islands,
- Macrotidal
>4m
estuaries with subtidal
ridges
Apart from tidal action and coastal morphology, the
sedimentary processes in tidal flats are influenced by
wind-generated waves. Therefore it has become common for sedimentologists to subdivide coastal areas
into three subenvironments:
- Wave-dominated shorelines: beaches, microtidal
baITier islands and cheniers.
- Mixed wave-tide injluenced shorelines: mesotidal
baITier islands with tidal in lets and ebb and flood
tidal deltas.
Chapter 3 Coastal and Shallow Sea Sediments
- Tide-dominated shorelines: tidal flats, estuaries
and associated sand ridges.
This subdivision is based on the assumption that
wind waves are the dominant coastal process when
the tidal range is less than 2 m. Features such as
beaches, sand spits, and long baITier islands are controlled mainly by wind-generated waves (cf. Sect.
3.1). In contrast, tidal ranges in excess of 4 mare the
dominant influence on coastal areas and their sediments, although wind waves also play some part. In
this chapter, both tide-dominated and mixed wavetide influenced environments will be discussed.
3.2.2 General Characteristics of Tidal Sediments
Tidal Environments, Provenance and Distribution of
Sediments
Modem tidal environments can be subdivided into
three zones (from higher to lower elevated areas):
- Mudflats.
- Mixed sand-mudflats.
- Sandflats.
Mud and sand refer to grain-size fractions, which
may consist of both silicic1astic and/or carbonate material. The three zones display distinctive characteristics which can be easily recognized in ancient sedimentary rocks. Tidal sediments are therefore of outstanding interest for paleoenvironmental and
paleogeographic reconstructions.
Besides numerous special articles, several textbooks present summeries and some special books deal entirely with
this topic (e.g., Ginsburg 1975; Friedman and Sanders
1978; Reineck and Singh 1980; Klein 1985c; Elliott
1986a; Boer et al. 1988; Reinson 1992; Flemming and
Bartolomä 1995; Clark et al. 1999).
Tidal flat sediments may have different sourees: input by rivers, cliff erosion, and reworked material
from the offshore sea bottom. The proportion of
mudflats, mixed sand-mudflats, and sandflats also
depends on the availability of mud and sand in the
source areas. Along coastal plains and near muddominated rivers, muddy tidal flats tend to prevail. In
front of coastal mountain ranges delivering large
quantities of sand into a tidal regime, sandier flats
and subtidal sand ridges are prominent.
An ancient example of the latter situation is the Miocene
(Burdigalian) marine molasse in the northem foreland basin of the Alps (Homewood and Allen 1981). It can be assumed that sandy material usually forrns narrower tidal
flats than finer grained muds, which better withstand erosion and reworking (see below).