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 collide with and closely approach one another during movement. ^[1] This is a fundamentally different support mechanism from turbidity currents (which rely on turbulence) and liquefied flows (which rely on escaping pore water). In grain flow, the grains essentially support each other: each collision transfers momentum and generates a force that pushes neighbouring grains apart, maintaining the dispersed state needed for movement.

Flow Mechanism and the Dispersive Pressure

Grain flow begins when cohesionless sediment has been piled up by traction processes - wind, wave, or current - until it exceeds the critical angle of repose, the maximum stable slope angle for that particular grain assembly. ^[1] The angle of repose is not a fixed universal value - it depends on how the grains are packed and on their shape, and it tends to be highest in deposits made of angular grains with low sphericity, because angular grains interlock more strongly. ^[1] Once the slope angle exceeds the repose angle for a given sediment, the internal shear stresses imposed by gravity overcome the sediment’s internal shear strength and avalanching begins immediately. ^[1]

The key force that keeps grains dispersed during the avalanche is the dispersive pressure - a force perpendicular to the plane of shearing that tends to push and spread grains outward from one another as the mass moves. ^[1] The relationship between shear stress (T) acting on the grains and the dispersive pressure (P) is expressed as: ^[1]

T / P = tan α ^[1]

where α is the angle of internal friction. ^[1] This formula has a direct practical implication: the minimum slope on which grain flow can sustain itself in air is around 30°, whereas under water the required slope may be even steeper. ^[1] The steep-slope requirement is one of the main reasons grain flow is geologically restricted compared to turbidity currents, which can operate on slopes of just 1°.

Although grain collisions are the primary dispersal mechanism, pore fluid can play a secondary supporting role: upward flow of pore fluids as grains settle, or the buoyancy provided by a dense mud matrix, may assist in keeping grains dispersed under some conditions. ^[1] Grain flow is similar to liquefied flow in many respects and may grade into those flows; the important distinction is that grain flow can operate in air as well as under water. ^[1]

Occurrence

Grain flow is most familiar on the lee slopes of sand dunes, where sand is driven over the crest by wind and avalanches down the slip face - a process responsible for the steep, planar foresets seen in cross-bedded aeolian sandstones. ^[1] Underwater grain flows have been directly observed in submarine canyons, where cohesionless sand avalanches down steep canyon walls. ^[1] Grain flows over the floors of Norwegian fjords have reportedly broken submarine telephone cables. ^[1]

Overall, grain flow is thought to be of limited geological significance on its own because of the steep slopes required to sustain it, although it may accompany turbidity currents on less steep slopes, moving beneath but independently of the turbidity current above. ^[1] Deposition is abrupt: when the slope angle decreases enough that the dispersive pressure can no longer keep the grains moving, the entire mass freezes en masse almost instantaneously. ^[1]

Deposits

Grain-flow deposits are massively bedded with little or no internal lamination, and grading is generally absent except for the possibility of reverse grading near the base. ^[1] Reverse grading - coarsening upward rather than the normal fining upward - is thought to develop during grain flow because smaller particles filter downward through the gaps between larger particles while everything is still dispersed, a process called kinetic sieving. ^[1]

A key diagnostic feature of grain-flow deposits is their thinness. A single grain flow event in any environment cannot produce a sandstone bed thicker than a few centimetres for sand-size grains, and deposits are commonly less than about 5 cm thick. ^[1] This thickness limit follows directly from the physics: grain flow is self-quenching. As soon as the dispersion freezes at the base, the mobile layer above is cut off and the whole event ends. There is no mechanism for sustained, prolonged sediment supply analogous to the body of a turbidity current, so individual grain-flow events leave only thin, massive slabs.

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