Sediment Gravity Flows
During fluid-flow transport, fluids (water, wind, ice) move under the action of gravity and the sediment is simply carried along with the fluid. [1] Sediment can also be transported essentially independently of fluid by the effect of gravity acting directly on the sediment. [1] In this type of transport, fluids may play a role in reducing internal friction and supporting grains, but they are not primarily responsible for downslope movement of the sediment - movement of the sediment under gravity creates the flow, and flow stops when the sediment load is deposited. [1] This reversal of the cause-and-effect relationship between sediment and fluid - in gravity flows, the sediment drives the flow, rather than the flow driving the sediment - is what makes this class of processes distinct from all fluid-flow transport.
A Spectrum of Mass Movements
Gravity mass movements can be grouped into rock falls, slides, and sediment gravity flows. [1] Rock fall involves free fall of blocks or clasts from cliffs or steep slopes. [1] Slides are en-masse movements of rock or sediment owing to shear failure that take place with little accompanying internal deformation of the mass. [1] Sediment gravity flows are more fluid types of movement in which breakdown in grain packing occurs and internal deformation of the sediment mass is intense. [1]
A spectrum of gravity movements exists, ranging from those in which sediment is moved en masse and fluids act mainly to reduce internal friction by lubricating the grains, to those in which transport is on a grain-by-grain basis and fluids play an important role in supporting the sediment during transport. [1]
Why Sediment Gravity Flows Matter
Sediment gravity flows are of particular interest because they are capable of rapidly transporting large quantities of sediment, including very coarse sediment, even into very deep water in the oceans. [1] They occur in subaerial environments - snow avalanches, pyroclastic flows, base surges, grain flows of dry sand, and volcanic and nonvolcanic debris flows - and in subaqueous environments, where they include grain flows, debris flows, turbidity currents, and liquefied sediment flows. [1]
Conditions for Initiation
Sediment gravity flows can occur only when grains become separated and dispersed to the point that internal friction and cohesiveness are sufficiently reduced to lower the strength of the sediment mass below the critical point required for gravity to initiate movement. [1]
The Four Support Mechanisms and Corresponding Flow Types
Four theoretical types of dispersive and support flow mechanisms can achieve the required reduction in internal strength: turbulent flow (turbulence), upward escape of intergranular fluid, grain interaction (dispersive pressure), and support by a cohesive matrix. [1] Four observed flow types correspond to these theoretical support mechanisms: turbidity currents, liquefied flow, grain flow, and mud and debris flow. [1] These four mechanisms of gravity transport can be thought of as members of a spectrum of sediment-gravity-flow processes, and one type may grade into another under some conditions - as when a submarine mud flow changes into a turbidity current downslope with additional mixing and dilution by water. [1]
Turbidity Currents
Turbidity currents are supported by fluid turbulence. [1] They are a kind of density current that flows downslope along the bottom of an ocean or lake because of density contrasts with the surrounding water arising from suspended sediment. [1]
Liquefied Flow
In liquefied flow, cohesionless sediment is supported by upward displacement of fluid (dilatance) as loosely packed structure collapses, settling into a more tightly packed framework; slopes in excess of 3° are required. [1]
Grain Flow
In grain flow, cohesionless sediment is supported by dispersive pressure - the result of grain interaction in an inertial (high-concentration) or viscous (low-concentration) regime; steep slopes are usually required. [1]
Debris Flow and Mud Flow
In debris and mud flows, shear is distributed throughout the sediment mass; strength is principally from cohesion due to clay content, with additional matrix support possibly from buoyancy. [1] They behave as Bingham plastics - non-Newtonian fluids that require an initial yield stress before they begin to flow. [1]
The gradient from debris flow through liquefied flow through grain flow to turbidity current represents a progressive reduction in the concentration and cohesion of the sediment-fluid mixture and a corresponding increase in the role of fluid turbulence as the dominant grain-support mechanism. Understanding which mechanism dominated in an ancient deposit is a primary goal of submarine fan stratigraphy.
Related Topics
Turbidity Current
A turbidity current is a kind of density current that flows downslope along the bottom of an ocean or lake because of density contrasts with the surrounding (ambient) water arising from sediment...
Debris Flow
Debris flows are dense, slurry-like flows composed of highly concentrated, poorly sorted mixtures of sediment and water that behave fundamentally differently from fluid flows.{/* SRC: Boggs p.39:...
Grain Flow
A grain flow is a type of sediment gravity flow in which cohesionless particles - most commonly sand - are kept dispersed and in motion not by a fluid but by the forces generated when grains...
Liquefied Flow
A liquefied flow is a type of sediment gravity flow in which the grains are held apart and supported not by turbulence - as in a turbidity current - but by the upward movement of pore water that...
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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