102
4 TRANSPORTATION AND SEDIMENTATION
Fig. 4.15. Cross-section through a standing body of water with a channel discharging fluid from the left.
(A) When the entering fluid is less dense than that of the standing body of water it will flow out adjacent to
the surface. (B) When the situation is reversed, for whatever reason (temperature, salinity or turbidity), it
will flow down beneath the less dense fluid. This is a density flow.
is of great interest to geologists (Fig. 4.15). Turbidity currents are widely believed to
be a major process for the transportation and deposition of a significant percentage of
the world's sedimentary cover. The concept of the turbidity flow was introduced to geology by Bell (1942). The process was originally invoked as an erosional agent capable
of scouring the submarine canyons on delta and continental slope margins. Later it was
claimed as the generator of what used to be termed flysch deposits (Kuenen and Migliorini, 1950). This facies, typical of geosynclinal t r o u g h s - now termed zones of subduction -- is characterized by thick sequences of interbedded sand and shale. The sands
have abrupt bases and transitional tops, and they tend to fine upward. Sands of this type
are often genetically termed "turbidites" (see Sections 5.3.3.3 and 6.3.2.9.1).
This section begins by briefly discussing the hydrodynamics of turbidity flows. The
evidence for turbidity currents in the real world is then described, and this is followed
by a discussion of the parameters of ancient turbidites. The hydrodynamics of density
flows have been studied for a number of years. Attempts to equate the various physical
parameters that govern density flows have been based on both theoretical and experimental grounds. The conditions under which a turbidity current will flow down a slope
may be expressed thus:
S1 + 82 = ( d 2 - da)g.h.a,
where $1 and S 2 a r e the shear stress between the turbid flow and the floor beneath and
the fluid above; d 2 and dl are the density of the turbidity flows and of the ambient fluid
4 TRANSPORTATION AND SEDIMENTATION
Fig. 4.15. Cross-section through a standing body of water with a channel discharging fluid from the left.
(A) When the entering fluid is less dense than that of the standing body of water it will flow out adjacent to
the surface. (B) When the situation is reversed, for whatever reason (temperature, salinity or turbidity), it
will flow down beneath the less dense fluid. This is a density flow.
is of great interest to geologists (Fig. 4.15). Turbidity currents are widely believed to
be a major process for the transportation and deposition of a significant percentage of
the world's sedimentary cover. The concept of the turbidity flow was introduced to geology by Bell (1942). The process was originally invoked as an erosional agent capable
of scouring the submarine canyons on delta and continental slope margins. Later it was
claimed as the generator of what used to be termed flysch deposits (Kuenen and Migliorini, 1950). This facies, typical of geosynclinal t r o u g h s - now termed zones of subduction -- is characterized by thick sequences of interbedded sand and shale. The sands
have abrupt bases and transitional tops, and they tend to fine upward. Sands of this type
are often genetically termed "turbidites" (see Sections 5.3.3.3 and 6.3.2.9.1).
This section begins by briefly discussing the hydrodynamics of turbidity flows. The
evidence for turbidity currents in the real world is then described, and this is followed
by a discussion of the parameters of ancient turbidites. The hydrodynamics of density
flows have been studied for a number of years. Attempts to equate the various physical
parameters that govern density flows have been based on both theoretical and experimental grounds. The conditions under which a turbidity current will flow down a slope
may be expressed thus:
S1 + 82 = ( d 2 - da)g.h.a,
where $1 and S 2 a r e the shear stress between the turbid flow and the floor beneath and
the fluid above; d 2 and dl are the density of the turbidity flows and of the ambient fluid