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 escapes from between the grains as they attempt to settle downward under gravity, or by pore water that is forced upward by injection from below. ^[1] The distinction from turbidity currents matters: turbidity currents are dilute enough that turbulence alone keeps particles suspended, whereas liquefied flows are highly concentrated dispersions in which the grains are so closely packed that the escaping interstitial water itself becomes the support mechanism.

How Liquefaction Occurs

The trigger for liquefaction is the sudden disruption of grain-to-grain contact in loosely packed, cohesionless sediment - sand being the most common example. ^[1] A single shock - an earthquake tremor, a wave impact, a sudden loading event - can cause the grains to briefly float in their own pore fluid, collapsing the sediment framework and transforming what was a solid bed into something that behaves like a dense, mobile slurry.

A second pathway to liquefaction is fluidization: if a fluid is introduced at the base of a body of cohesionless sediment and continues to be injected upward until it forces the grains apart, those grains become supported by the rising fluid rather than by each other. ^[1] Whether triggered by shock or by fluidization, the result is the same: the sediment loses its internal shear strength and begins to behave like a high-viscosity fluid that can flow quite rapidly even on gentle slopes as low as 3°. ^[1]

Flow Duration and Deposition

Liquefied flow cannot be sustained indefinitely. The flow persists only as long as the grains remain dispersed in the fluid - the moment the grains settle back into grain-to-grain contact, the moving layer freezes and stops. ^[1] This freezing does not happen all at once throughout the flow. Instead, it begins at the base, where a layer of settled grains grows progressively thicker and migrates upward through the still-dispersed mass at a rate controlled by how fast the particles can settle. ^[1]

In thick flows carrying fine-grained sediment, this settling process can take on the order of hours, which means that a liquefied flow may travel a meaningful distance before it completely freezes - short by the standards of turbidity currents, but potentially geologically significant. ^[1] As deposition proceeds, pore water is squeezed upward through the settling grains and escapes at the surface. This upward migration of water is the direct cause of the distinctive fluid escape structures that mark liquefied-flow deposits. ^[1]

Some liquefied flows accelerate down a slope enough to become turbulent, at which point they effectively transform into turbidity currents. ^[1] This flow transformation illustrates a broader point: the categories of sediment gravity flow - liquefied, turbidity, grain, debris - are not rigid types but points on a spectrum, and flows can shift from one to another as their velocity and concentration change during transport.

Deposits

Liquefied-flow deposits are typically thick, poorly sorted sand units. ^[1] Their defining features are the fluid escape structures produced by pore water forcing its way upward through the settling sediment mass: dish structures (concave-upward lamination artefacts formed where rising water deflects settling grains sideways), pipes (narrow, near-vertical conduits through which water escaped rapidly), and sand volcanoes (mound-shaped eruptions of water-vented sand at the sediment surface). ^[1] These structures are diagnostic: wherever they appear in ancient sandstones, they indicate that deposition occurred rapidly from a highly concentrated suspension in which excess pore water had to escape upward as the sediment compacted.

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