Page 71 - Sedimentology and Stratigraphy

Processes of Transport and Sedimentary Structures
58 Nichols/Sedimentology and Stratigraphy 9781405193795_4_004 Final Proof page 58 26.2.2009 8:16pm Compositor Name: ARaju

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Fig. 4.20 A bedform stability

diagram which shows how the type
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of bedform that is stable varies with

( . both the grain size of the sediment

and the velocity of the flow.
and for clear water at a particular temperature energy by an earthquake, landslide or similar phe-
(108C), and the boundaries will change if the flow nomenon. Tsunamis are waves produced by single
depth is varied, or if the density of the water is varied events, and these are considered further in section
by changing the temperature, salinity or by addition 11.3.2. Continuous trains of waves are formed by
of suspended load. Bedform stability diagrams can be wind acting on the surface of a water body, which
used in conjunction with sedimentary structures in may range in size from a pond to an ocean. The height
sandstone beds to provide an estimate of the velocity, and energy of waves is determined by the strength of
or recognise changes in the velocity, of the flow that the wind and the fetch, the expanse of water across
deposited the sand. For example, a bed of medium which the wave-generating wind blows. Waves gen-
sand that was plane-bedded at the base, cross-bedded erated in open oceans can travel well beyond the
in the middle and ripple cross-laminated at the top areas they were generated.
could be interpreted in terms of a decrease in flow
velocity during the deposition of the bed.
4.4.1 Formation of wave ripples

4.4 WAVES The oscillatory motion of the top surface of a water
body produced by waves generates a circular pathway
A wave is a disturbance travelling through a gas, for water molecules in the top layer (Fig. 4.21). This
liquid or solid which involves the transfer of energy motion sets up a series of circular cells in the water
between particles. In their simplest form, waves do below. With increasing depth internal friction reduces
not involve transport of mass, and a wave form the motion and the effect of the surface waves dies out.
involves an oscillatory motion of the surface of the The depth to which surface waves affect a water body is
water without any net horizontal water movement. referred to as the wave base (11.3). In shallow water,
The waveform moves across the water surface in the the base of the water body interacts with the waves.
manner seen when a pebble is dropped into still water. Friction causes the circular motion at the surface to
When a wave enters very shallow water the ampli- become transformed into an elliptical pathway, which
tude increases and then the wave breaks creating the is flattened at the base into a horizontal oscillation.
horizontal movement of waves seen on the beaches of This horizontal oscillation may generate wave ripples
lakes and seas. in sediment. If the water motion is purely oscillatory
A single wave can be generated in a water body the ripples formed are symmetrical, but a superim-
such as a lake or ocean as a result of an input of posed current can result in asymmetric wave ripples.