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transportation and sedimentation, by luis julian nuñez and ivan montero -…
transportation and sedimentation
GRAVITATIONAL PROCESSES
The force of gravity is an integral part of all sedimentary processes- aqueous, eolian, or glacial. Gravity can, on its own, however, transport sediment, but for this to acquire a horizontal component it requires some additional mechanism.
Grain Flows
The concept of the grain flow was expounded by Bagnold (1954, 1966). Grain flows are
liquefied cohesionless particle flows in which the intergranular friction between sand
grains is reduced by their continuous agitation. This is believed by some to be nonturbulent and to involve considerable horizontal shearing
Debris Flow
A debris flow is defined as a "highly-concentrated non-Newtonian sediment dispersion
of low yield strength" (Stow et al., 1996). This includes mud flows, though not all debris flows are muddy. Debris flows occur in a wide range of environments, ranging from
deserts to continental slopes
Fluidized Flows
Fluidization of a sand bed occurs when the upward drag exerted by moving pore fluid
exceeds the effective weight of the grains. When this upward movement exceeds the
minimum fluidization velocity, the bed expands rapidly, porosity increases and the bed
becomes liquefied and fluid supported, rather than grain supported.
In conclusion it must be stated that gravity-related sedimentary processes are not well
understood. They are often observed today, though few observers survive to write about
their experiences, and their deposits may be studied
AQUEOUS PROCESSES
Sedimentation from Traction Currents
Consider now one of the most important processes for transporting and depositing sediment. Traction currents are those which, as already defined, move sediment along by
rolling and saltation as bed load in a traction carpet.
Unidirectional Traction Currents
The basic approach to understanding traction-current sedimentation has been through experimental studies of unidirectional flow in confined channels. These can be made in artificial channels, termed flumes, which are now standard equipment in many geological laboratories.
Bidirectional Tractional Currents
Unidirectional currents characterize deposition in fluvial channels. In marine environments, however, traction currents are commonly bidirectional
A time-velocity graph for a single tidal cycle may be plotted as shown in the image At high and low tide the current velocity will be zero. Current velocity gradually increases and then decreases as the tide ebbs, and gradually increases and decreases as the tide floods, and so on. As current velocity approaches zero at high and low tide
Sedimentation from High-Density Turbidity Currents
The concept of density flow has already been introduced. Where two fluid bodies of different density are mixed, the less dense fluid will tend to move above the denser one.
Conversely, the denser fluid will tend to flow downward.
bidity currents. The sediments termed turbidites show the following features: They are
generally thick sequences of regularly interbedded sandstones and shales.
Sedimentation from Low-Density Turbidity Currents
Fine-grained clay and silt are seldom if ever deposited from traction currents because
they tend to be transported in suspension rather than as bed load. A certain amount of sand and silt is deposited at the distal end and waning phases of turbidity flows.
GLACIAL PROCESSES
varved clays, and the sands and gravels of fluvioglacial outwash plains. These deposits, though associated with glaciation, are actually eolian, aqueous suspension and traction current deposits, respectively.
Consider now the mechanics of glacial transport and decomposition. Ice, formed from compacted snow, moves both in response to gravity in valley glaciers and in response to horizontal pressures in continental ice sheets.
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 Sedimentation from Traction Carpets
These studies describe how sediment, blown by the wind, moves by sliding and saltation just like particles in water. Silt and clay are winnowed from the traction
carpet and carried off in dust clouds. Studies of the threshold velocity needed to commence air movement show that, as with aqueous transport, the threshold velocity increases with increasing grain size.
Quartz particles of about 0.10 mm (very fine sand) are the first to move in a rising wind. Silt and clay need velocities as strong as those for fine sand to initiate movement (Horikowa and Shen, in Allen, 1970). This is analogous to the Hjulstrom effect for the threshold of particle movement in aqueous flows.
Eolian Sedimentation from Suspension
Most eolian sediment is deposited by wind blowing over desiccated alluvium. The gravel remains behind. The sand saltates into dunes. The silt and clay are blown away in suspension. This much is known, and there is no question of the competence of wind to transport large amounts of silt and clay in suspension
It has been calculated that between 25 and 37 million tons of dust are transported from the Sahara throughout the
longitude of Barbados each year.This quantity of dust is sufficient to maintain the present rate of pelagic sedimentation in the entire North Atlantic (Prospero and Carlson, suspension in the way that mud settles on the sea floor
Loess occurs in laterally extensive layers, often of great thickness. It is slightly calcareous, massive, and weathers into characteristic polygonal shrinkage cracks
by luis julian nuñez and ivan montero
