Presentation on theme: "Sediment Gravity Flow We have discussed two classes of flows that can move particles: Simple dry mass wasting; and conventional fluid flow. A third flow."— Presentation transcript:
Sediment Gravity Flow We have discussed two classes of flows that can move particles: Simple dry mass wasting; and conventional fluid flow. A third flow class, wet or fluid-assisted mass wasting, has properties of both.
Grain flows-occurs when cohesionless sediments (ex. Dry sand) moves downward under the pull of gravity. Although there is air or water trapped between the grains, it merely acts as a lubricant and does not actually propel the grains.
Grain flow The most familiar examples of grain flow are sand avalanches. They occur whenever a sand dune becomes steeper than the angle of repose (25°-30 °).
Grain flow Grain flows also occurs in the deep sea, where sand avalanches down the sides of steep submarine canyons.
Fluidized Sediment Flows Concentrated dispersions of grains supported by pore water in between. They begin to flow when something increases the pressure on the interstitial pore water, turning the once-firm sand into a soupy liquid, ex. quicksand. Quicksand - grain shift and settle past one another; liquefaction. A person should not continue to sink (as in a swimming pool) beyond your point of buoyancy. To get out swim as you would in a pool. Lie flat.
Fluidized Sediment Flows Liquefaction occurs in others instances, for example in shockwaves sent through saturated soil by an earthquake.
Mudflows and Debris flows Mudflows are composed of a slury-like mass of liquefied mud that move downhill under the force of gravity. If there are large particles, ranging up to boulder size, they are known as Debris flows. Mudflows and Debris flows are most common in steep mountain canyons during heavy rainstorm.
Mudflows and Debris flows These flows have the consistency of wet cement and can move as fast as water in a flash flood.
Sequence of aerial photograph of a debris flow moving along a canyon bottom.
Mudflows and Debris flows Some beds exhibit reverse (inverse) grading, with the coarsest material at the top and the finer at the bottom. The pressure push the largest grains to the top, when stop they settle.
Turbidity Currents Turbidity Currents are gravity flows in which the sediment is supported by upward turbulence of the fluid within the flow. It is trigger by gravity. The mass of suspended sediment is denser than water. Typically produces normal graded bedding.
Sedimentary structures of the four major types of sedimentary gravity flow deposits.