lecture outline: how sediments move – contrast how; 1) air/water moves grains with how; 2) gravity...
TRANSCRIPT
LECTURE OUTLINE:
How sediments move – contrast how; 1) air/water moves grains with how; 2) gravity moves grains.
1) Movement through the air; 1) bedload and; 2) suspension motion.
2) Sediment gravity flows; Grain flows; Debris flows; Liquefied flows; Turbidity flows
3) Pictures (videos hopefully) of real-life sedimentary deposits and structures formed by sediment gravity flows – mostly from Cretaceous basins in Central Turkey
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(mostly) AIR FLOW
1) BEDLOADRolling – continuous contact with surface
Saltation (saltare – to jump) grains move by a series of ballistic ‘hops’ with a steep ascent angle and a shallow descent angle.
Heights are generally 100-500 grain diameters (helped by low viscosity of air and high density contrast) but this is dependent on the substrates.
On a pebbly surface saltating grains will reach higher because of a higher rebound effect.
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2) SUSPENDED MOTION
Occurs higher than saltation, here grains are kept aloft by eddy currents.
Clay grains permanently held in suspension in AIR are called dustload. WATER – washload.
A combination of bedload and suspended motion commonly occurs - grain may be moved in suspension by an eddy current while in a saltating trajectory; INCIPIENT SUSPENSION
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Grain aggregates will transport themselves with the aid of gravity – no help from the overlying stationary fluid medium. Gravity flows must overcome the effects of friction.
Four flow types; 1) Grain flows; 2)Debris flows; 3) Liquefied flows; 4)Turbidity flows
1)and 2) can occur sub-aerially
All occur sub-aqueously
SEDIMENT GRAVITY FLOWS
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1)Grain flows
Grain/grain collisions between the flowing grains – e.g. Avalanche
Cannot overcome friction – grain flows may only occur on steep slopes that exceed the angle of slope stability
Θi = angle of intitial yield = critical slope angle at which grain flow will initiate = 40° for tightly-packed sands and 30 ° for loose sands.
Gravitational forces induce shear at the base of the pile, and the grains begin to move down slope
SEDIMENT GRAVITY FLOWS
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After slope failure – grains kept aloft above the basal shear plane. Energy supplied by grain/grain collisions.
Not efficient -Grain flows cannot be more than a few cms thick for sand-sized grain, they will not travel very far.
Reverse grading: 1) Kinetic filtering – small grains filter through the gaps between larger grains until they rest near the shear plane. 2) Larger particles move upwards through flow to equalise stress gradients
SEDIMENT GRAVITY FLOWS
1) Grain flows
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SEDIMENT GRAVITY FLOWS
1) Grain flows
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Deposits: migrating bedforms – dunes, ripples
Slurry-like flows in which silt- to boulder-sized grains are set in a fine-grained cohesive matrix
Grains are supported by the strength and buoyancy of the matrix which lubricates the grains and stops them sinking: so debris flows can occur on gentle sub-aerial and sub-aqueous slopes
Sub-aerial flows started by heavy rain (e.g. Volcanic slopes – lahars). Sub-aqueous flows initiated by earthquake shocks
Strength of a debris flow depends on the matrix cohesion properties
SEDIMENT GRAVITY FLOWS
2) Debris flows
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SEDIMENT GRAVITY FLOWS
T = k + Bingham viscosity
T = internal shear stressk = yield strengthBingham viscosity = ‘rigidity’
Yield stress must be exceeded for flow to occur
Velocity profile = ‘plug’ profile, bordered by zones of high shear stress.
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2) Debris flows
SEDIMENT GRAVITY FLOWS
Structure:
Shearing at base, deformation ofunderlying sediments.
Centre of classic debris flow moves like a rigid plug
Massive, (very) poorly sorted, random fabric
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2) Debris flows
SEDIMENT GRAVITY FLOWS
1) Debris flows
Carbonate debris flow – Palaeocene, Kirikkale, Turkey
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SEDIMENT GRAVITY FLOWS
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2) Debris flows
SEDIMENT GRAVITY FLOWS
3) Liquefied flowsForm when loosely-packed sand is shocked – this causes grains to become momentarily suspended in their own pore fluid.
Negligible friction – so flow can occur on very low slopes
Grains soon ‘settle out’ as they come into contact with their neighbours – ‘settled out’ grains + fluid move upwards through the flow
Upwards movement is not uniform – may be concentrated in pipes = FLUIDISATION
Dish and pillar structures in the flow – sand volcanoes at the surface
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SEDIMENT GRAVITY FLOWS
3) Liquefied flows
Liquefied sand
Liquefied sand
WaterWater
Water
Resedimented sand Resedimented sand
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SEDIMENT GRAVITY FLOWS
4) Turbidity flows
Density currents of a turbulent sediment and water mixture
Well developed ‘head’ and ‘tail’ regions
Slope angle of 1° needed to offset energy losses caused by friction
Initiated by slumps caused earthquake shocks
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SEDIMENT GRAVITY FLOWS
4) Turbidity flows
Velocity, Uh given by; Uh = 0.7 (Δρ / ρ) gh
Δρ = density contrast between flow and ambient fluid ρ = density of ambient fluid h = head thickness
Low Concentration flows: sediment deposition only a short distance behind the head – well-sorted, fining upwards
High concentration flows: sediment deposition followed by mass shearing and liquefied sediment deposit – poor sorting, poor grading, massive
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SEDIMENT GRAVITY FLOWS
4) Turbidity flows
1929 Newfoundland Earthquake
Resulting turbidity flow cut telephone cables on Atlantic sea bed
Patterns of motion around turbidity head
fixed moving
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SEDIMENT GRAVITY FLOWS
4) Turbidity flows
Ideal Bouma sequence of a turbidity flow
But not always ideal – especially with increasing distance from flow origin
Many sediment gravity flows are combinations of debris and turbidity flows
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SEDIMENT GRAVITY FLOWS
Summary
Grain flow Debris flow Liquefied flow Turbidity flow
Grain collision Matrix strength & buoyancy
BuoyancyTurbulence
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