Page 178 - Sedimentology and Stratigraphy

Tides 165

to this zone can also be applied at about 5000 m, shelves. In some places it may be as little as 20 m
below which the deepest parts of the oceans are called water depth but can be 50 to 200 m water depth
the hadal zone. if the shelf borders an ocean with a large fetch for
The shelf (neritic environment) can be usefully storm waves. This deeper shelf area between the fair
further divided into depth-controlled zones (Fig. 11.2), weather and storm wave bases is called the offshore-
although in this case the divisions are not defined transition zone. The offshore zone is the region
by absolute depths, but the depths to which certain below storm wave base and extends out to the shelf-
processes operate. Their range therefore varies edge break at around 200 m depth.
according to the conditions in a particular basin The activities of a number of physical, chemical and
because the depths to which tidal processes, waves biological processes are determined by water depth,
and storms affect the shelf vary considerably. The and in turn these influence the sediment accumula-
foreshore is the region between mean high water tion on the different parts of the sea floor. The following
and mean low water marks of the tides. Depending sections consider some of these processes and how they
on the tidal range (11.2.2) this may be a vertical affect depositional environments.
distance of anything from a few tens of centimetres
to many metres. The seaward extent of the foreshore
is governed also by the slope and it may be anything
from a few metres, if the shelf is steeply sloping and/or 11.2 TIDES
the tidal range is small, to over a kilometre in places
where there is a high tidal range and a gently sloping 11.2.1 Tidal cycles
shelf. The foreshore is part of the beach environment
or littoral zone (13.2). According to Newton’s Law of Gravitation, all objects
The shoreface is defined as the region of the shelf exert gravitational forces on each other, the strength
between the low-tide mark and the depth to which of which is related to their masses and their distances
waves normally affect the sea bottom (4.4.1), and this apart. The Moon exerts a gravitational force on the
is the fair weather wave base. The lower depth that Earth and although ocean water is strongly attracted
the shoreface reaches depends on the energy of the gravitationally to the Earth, it also experiences a small
waves in the area but is typically somewhere between gravitational attraction from the Moon. The water
5 and 20 m. The width of the shoreface will be gov- that is closest to the Moon experiences the largest
erned by the shelf slope as well as the depth of the fair gravitational attraction and this creates a bulge of
weather wave base and may be hundreds of metres to water, a tidal bulge, on that side of the Earth
kilometres across. In deeper water it is only the larger, (Fig. 11.3). The bulge on the opposite side, facing
higher energy waves generated by storms that affect away from the Moon, can be thought of as being the
the sea bed. The depth to which this occurs is the result of the Earth being pulled away from that water
storm wave base and this is very variable on different mass by the gravitational force of the Moon.

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Fig. 11.3 The gravitational force of the

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Sun and Moon act on the Earth and on

anything on the surface, including the

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water masses in oceans.