132 Sea Level Processes and Effects of Sea Level Change
Waves can also be responsible for producing certain types of sediments, mechanically as well as chemically. Polished beach shingles and calcareous oolites are
examples. Calcareous oolites are characteristic for unusually warm and saline environments (Chap. 3.7.2). Apparently they grow only within the zone agitated by waves
and tides.
5.2.2 Tides and Storm Action: the Intertidal Zone. The tides, those long waves
made by the Moon's daily passage, raise and lower the sea level along the coasts by
a few centimeters or by many meters every day. The tides originate in a complicated
interplay between the rotation of the Earth-Moon system about its common center,
the System's rotation about the sun, the Earth's spin, and the topography of the floor
of oceans and seas. The tides which we witness at the coast are large rotational waves
radiating from points of no motion situated in the central areas of the ocean basins.
The amplitudes of the waves are highest close to the coast (Fig. 5.4).
Thus, tides are strictly tied to astronomical forcing and are shaped by basin morphology. If the distribution of their frequencies and amplitudes can be reconstructed
for the geologic past, we can obtain important clues about changes in the Earth-Moon
system, and about basinal morphology.
Here, we are concerned with the "indicators" of tidal action in a general sense. Of
course the classic sea level indicator facies is represented by the whole of the deposits
of the intertidal zone, which is alternately submerged and exposed by the tides. We
have already introduced one such type of broad intertidal, the "wadden" (Fig. 4.17).
Biological features will be discussed in Sections 6.2-6.6.
Tidal flats can be associated with river deltas as in the Bay of Bengal and at the
mouth of the Amazon River, or with estuaries, as in the Gulf of Korea or around the
North Sea. Characteristic small-scale features of these sediments are flaser- or lenticular bedding, bimodal ("herringbone") cross-bedding, a variety of erosional and
bioturbational markings, channel lag deposits, and graded laminae.
Rain imprints and desiccation cracks may also occur. On a larger scale, a landward
decrease of grain sizes due to decreasing current activities and time of water cover
marks a contrast to normal offshore conditions.
Intertidal flats and adjacent areas are subject to rapid changes in the conditions of
sedimentation. Peak winter storms wreak havoc along the coastal lowlands of the
North Sea from time to time. In 1362, an enormous storm, the "Great Man Drowning", raised the sea level by about 6 m along the Friesian Coast, where Germany
borders with Denmark. The storm flood created new access for the tides into the salt
marshes and moors lying behind natural barriers, inland from the intertidal flats. Soon
the tides brought mud which accumulated on top of the marshes and the peat, producing a storm layer. To the people living there, this geologically common event meant
destruction of homes and pastures. In 1634, another fierce storm produced widespread flooding, destroying villages and killing thousands. The present coastal geography of the area is essentially a product of those two storms. A faint indication of
plowed fields can be seen to this day on certain intertidal flats of the region. Some of
the land has been reclaimed since, aided by the influx of sediment from the wadden,
and by the building of dikes.
Waves can also be responsible for producing certain types of sediments, mechanically as well as chemically. Polished beach shingles and calcareous oolites are
examples. Calcareous oolites are characteristic for unusually warm and saline environments (Chap. 3.7.2). Apparently they grow only within the zone agitated by waves
and tides.
5.2.2 Tides and Storm Action: the Intertidal Zone. The tides, those long waves
made by the Moon's daily passage, raise and lower the sea level along the coasts by
a few centimeters or by many meters every day. The tides originate in a complicated
interplay between the rotation of the Earth-Moon system about its common center,
the System's rotation about the sun, the Earth's spin, and the topography of the floor
of oceans and seas. The tides which we witness at the coast are large rotational waves
radiating from points of no motion situated in the central areas of the ocean basins.
The amplitudes of the waves are highest close to the coast (Fig. 5.4).
Thus, tides are strictly tied to astronomical forcing and are shaped by basin morphology. If the distribution of their frequencies and amplitudes can be reconstructed
for the geologic past, we can obtain important clues about changes in the Earth-Moon
system, and about basinal morphology.
Here, we are concerned with the "indicators" of tidal action in a general sense. Of
course the classic sea level indicator facies is represented by the whole of the deposits
of the intertidal zone, which is alternately submerged and exposed by the tides. We
have already introduced one such type of broad intertidal, the "wadden" (Fig. 4.17).
Biological features will be discussed in Sections 6.2-6.6.
Tidal flats can be associated with river deltas as in the Bay of Bengal and at the
mouth of the Amazon River, or with estuaries, as in the Gulf of Korea or around the
North Sea. Characteristic small-scale features of these sediments are flaser- or lenticular bedding, bimodal ("herringbone") cross-bedding, a variety of erosional and
bioturbational markings, channel lag deposits, and graded laminae.
Rain imprints and desiccation cracks may also occur. On a larger scale, a landward
decrease of grain sizes due to decreasing current activities and time of water cover
marks a contrast to normal offshore conditions.
Intertidal flats and adjacent areas are subject to rapid changes in the conditions of
sedimentation. Peak winter storms wreak havoc along the coastal lowlands of the
North Sea from time to time. In 1362, an enormous storm, the "Great Man Drowning", raised the sea level by about 6 m along the Friesian Coast, where Germany
borders with Denmark. The storm flood created new access for the tides into the salt
marshes and moors lying behind natural barriers, inland from the intertidal flats. Soon
the tides brought mud which accumulated on top of the marshes and the peat, producing a storm layer. To the people living there, this geologically common event meant
destruction of homes and pastures. In 1634, another fierce storm produced widespread flooding, destroying villages and killing thousands. The present coastal geography of the area is essentially a product of those two storms. A faint indication of
plowed fields can be seen to this day on certain intertidal flats of the region. Some of
the land has been reclaimed since, aided by the influx of sediment from the wadden,
and by the building of dikes.