Page 183 - Sedimentology and Stratigraphy

170 The Marine Realm: Morphology and Processes

bring a lot of sediment into suspension near the sea generate flow of the denser fluid beneath the less
floor and the mixture of sediment and water moves as dense water. Cold surface water descends at high
a gravity-driven underflow across the shelf, from shal- polar latitude, sink points, and these water masses
lower to deeper water. The deposits of these storm then move around the oceans as thermo-haline bot-
processes are referred to as tempestites: there is tom currents (Stow 1985). The water that is moved
further discussion of the processes and products of from the polar regions is replaced by warm surface
storm-dominated shelves in Chapter 14. waters and this sets up a circulation system that
transports water thousands of kilometres in the
world’s oceans. Geostrophic currents are wind-
11.3.2 Tsunami driven currents related to the global wind systems,
which result from differences in air mass tempera-
Tsunami is the Japanese for ‘harbour wave’ and tures combined with the Coriolis force (6.3). The pat-
3
4
refers to waves with periods of 10 to 10 seconds tern of ocean currents is shown in Fig. 11.10.
that are generated by events such as subsea earth- The effects of these currents on sedimentation are
quakes, large volcanic eruptions and submarine land- most noticeable in deeper waters (16.4) as their effects
slides. In the past such waves were sometimes in shallower water are often masked by the influences
incorrectly called ‘tidal waves’, but their origins of tides, waves and storms. Thermo-haline currents
have no connection with tidal forces. These events are typically weaker than storm and tidal currents but
can set up a surface wave a few tens of centimetres are of larger volume. They mainly move clay and silt
amplitude in deep ocean water and a wavelength of in suspension and very fine sands as bedload.
many kilometres. As the wave reaches the shallower Thermo-haline currents are also important in the dis-
waters of the continental shelf, the amplitude is tribution of nutrients in the oceans. Bottom currents
increased to ten or more metres, producing a wave move nutrients from colder regions to areas where
that can have a devastating effect on coastal areas upwelling occurs and the nutrient-rich waters reach
(Scheffers & Kelletat 2003). the surface. As a consequence, these areas of upwel-
The effects of a tsunami are dramatic, with wide- ling are regions of high organic productivity and can
spread destruction occurring near coasts, both near result in deposits rich in biogenic material.
the source of the wave and also anywhere in the path
of it, which can be thousands of kilometres across an
ocean. They also have a serious impact on shallow 11.5 CHEMICAL AND BIOCHEMICAL
marine environments causing disruption and redepo- SEDIMENTATION IN OCEANS
sition of foreshore and shoreface sediments. It has
been suggested that beds of poorly sorted debris con- The most important chemical and biochemical sedi-
taining a mixture of deposits and fauna from different ments in modern seas and ancient shelf deposits
coastal and shallow marine environments may form are carbonate sediments and evaporites, and in the
as a consequence of tsunami (Pilkey 1988). It may be oceans plankton generate large quantities of carbo-
possible to distinguish them from ordinary storm nate and siliceous sediment. In addition there are
deposits by their larger size, but in practice it may be other, less abundant but significant chemical and
difficult to show that a deposit is generated by a biochemical deposits.
specific mechanism.

11.5.1 Glaucony and glauconite
11.4 THERMO-HALINE AND
GEOSTROPHIC CURRENTS The term glauconite is commonly used by geologists
to refer to a dark green mineral that is found quite
Currents that are driven by contrasts in temperature commonly in marine sediments. In correct usage the
and/or salinity are called thermo-haline currents. use of this term should be restricted to a potassium-
Cold water is dense and will sink relative to warmer rich mica, which has the mineral name glauconite,
water, and seawater is denser if the salinity is greater because this is in fact only one member of a group of
than normal: these temperature and salinity contrasts potassium and iron-rich phyllosilicate minerals that