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4 TRANSPORTATION AND SEDIMENTATION
triggered by the descent of cold polar waters into ocean basins in lower latitudes. Here
they rotate according to Coriolis force, clockwise in the Northern Hemisphere, anticlockwise in the Southern Hemisphere. These geostrophic currents not only transport
mud in nepheloid layers, giving rise to muddy contourites, but are also responsible for
the traction carpet from which are deposited the sandy contourites (Stow and Lovell,
1979; Stow, 1985).
The third main type of suspension deposit occurs where turbid flows enter bodies of
water with no significant density difference. This situation, termed "hypopycnal flow"
(Bates, 1953), allows a complete mixing of the two water masses. Fine material then
settles out of suspension from the admixture of water bodies. The term hemipelagite
is applied to massive deep marine muds whose absence of evidence of current action
suggest that they have settled out of suspension. (They are, however, sometimes bioturbated.) Hemipelagites grade into the red clays and biogenic muds of the true oceanic
oozes, sometimes termed "pelagites." In nearshore environments sedimentation is accelerated where muddy freshwater mixes with seawater. The salts cause the clay particles to flocculate and settle more rapidly than if they were still dispersed.
It is easy to establish that a fine-grained deposit settled out of a subaqueous suspension. It is virtually impossible to determine whether the transporting mechanism was the
last gasp of turbidity current, a nepheloid layer, or a hypopycnal flow. The end product
of all three processes is a claystone with varying amounts of silt. These sediments may
be massive or laminated due to vertical variations in grain size or chemical composition.
Silt and very fine sand ripples may sometimes be present testifying to occasional traction current activity. Other sedimentary structures include slumps, slides, and synaeresis cracks. The slides and slumps occur because suspension deposits can form on slopes
that are inherently unstable. Sedimentation continues until a critical point is reached
at which the mud slides and slumps downslope to be resedimented from suspension in
a more stable environment. This may completely disturb the lamination of superficial
sediments. Deeper, more cohesive muds may retain their lamination though this may
have been disturbed into slump and slide structures due to mass movement downslope.
On level bottoms spontaneous dewatering of clays leads to the formation synaeresis
cracks (see Section 5.3.5.4), which are themselves infilled by more mud.
4.3 EOLIAN PROCESSES
At the beginning of this chapter it was pointed out that eolian and aqueous transportation and sedimentation shared many features. This is because both processes are essentially concerned with the transportation of a granular solid in a fluid medium. Gases
and liquids both lack shear strength and share many other physical properties. Eolian
processes involve both traction carpets and suspensions (dust clouds). Turbidity flows
are essentially unknown except in volcanic gas clouds termed nudes ardentes. These are
masses of hot volcanic gases with suspended ash and glass shards. These masses move
down the sides of volcanoes at great speed. The resultant deposit, termed an "ignimbrite," may have been formed at a sufficiently high temperature for the ash particles to
be welded together (Suthren, 1985).
4 TRANSPORTATION AND SEDIMENTATION
triggered by the descent of cold polar waters into ocean basins in lower latitudes. Here
they rotate according to Coriolis force, clockwise in the Northern Hemisphere, anticlockwise in the Southern Hemisphere. These geostrophic currents not only transport
mud in nepheloid layers, giving rise to muddy contourites, but are also responsible for
the traction carpet from which are deposited the sandy contourites (Stow and Lovell,
1979; Stow, 1985).
The third main type of suspension deposit occurs where turbid flows enter bodies of
water with no significant density difference. This situation, termed "hypopycnal flow"
(Bates, 1953), allows a complete mixing of the two water masses. Fine material then
settles out of suspension from the admixture of water bodies. The term hemipelagite
is applied to massive deep marine muds whose absence of evidence of current action
suggest that they have settled out of suspension. (They are, however, sometimes bioturbated.) Hemipelagites grade into the red clays and biogenic muds of the true oceanic
oozes, sometimes termed "pelagites." In nearshore environments sedimentation is accelerated where muddy freshwater mixes with seawater. The salts cause the clay particles to flocculate and settle more rapidly than if they were still dispersed.
It is easy to establish that a fine-grained deposit settled out of a subaqueous suspension. It is virtually impossible to determine whether the transporting mechanism was the
last gasp of turbidity current, a nepheloid layer, or a hypopycnal flow. The end product
of all three processes is a claystone with varying amounts of silt. These sediments may
be massive or laminated due to vertical variations in grain size or chemical composition.
Silt and very fine sand ripples may sometimes be present testifying to occasional traction current activity. Other sedimentary structures include slumps, slides, and synaeresis cracks. The slides and slumps occur because suspension deposits can form on slopes
that are inherently unstable. Sedimentation continues until a critical point is reached
at which the mud slides and slumps downslope to be resedimented from suspension in
a more stable environment. This may completely disturb the lamination of superficial
sediments. Deeper, more cohesive muds may retain their lamination though this may
have been disturbed into slump and slide structures due to mass movement downslope.
On level bottoms spontaneous dewatering of clays leads to the formation synaeresis
cracks (see Section 5.3.5.4), which are themselves infilled by more mud.
4.3 EOLIAN PROCESSES
At the beginning of this chapter it was pointed out that eolian and aqueous transportation and sedimentation shared many features. This is because both processes are essentially concerned with the transportation of a granular solid in a fluid medium. Gases
and liquids both lack shear strength and share many other physical properties. Eolian
processes involve both traction carpets and suspensions (dust clouds). Turbidity flows
are essentially unknown except in volcanic gas clouds termed nudes ardentes. These are
masses of hot volcanic gases with suspended ash and glass shards. These masses move
down the sides of volcanoes at great speed. The resultant deposit, termed an "ignimbrite," may have been formed at a sufficiently high temperature for the ash particles to
be welded together (Suthren, 1985).