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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.

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Presentation on theme: "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."— Presentation transcript:

1 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 1

2 (mostly) AIR FLOW 1) BEDLOAD Rolling – 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 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. 2

3 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 3

4 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 4

5 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 5

6 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 6

7 SEDIMENT GRAVITY FLOWS 1)Grain flows 7 Deposits: migrating bedforms – dunes, ripples

8 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 8

9 SEDIMENT GRAVITY FLOWS T = k + Bingham viscosity T = internal shear stress k = yield strength Bingham viscosity = ‘rigidity’ Yield stress must be exceeded for flow to occur Velocity profile = ‘plug’ profile, bordered by zones of high shear stress. 9 2) Debris flows

10 SEDIMENT GRAVITY FLOWS Structure: Shearing at base, deformation of underlying sediments. Centre of classic debris flow moves like a rigid plug Massive, (very) poorly sorted, random fabric 10 2) Debris flows

11 SEDIMENT GRAVITY FLOWS 1)Debris flows Carbonate debris flow – Palaeocene, Kirikkale, Turkey 11

12 SEDIMENT GRAVITY FLOWS 12 2) Debris flows

13 SEDIMENT GRAVITY FLOWS 3) Liquefied flows Form 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 13

14 SEDIMENT GRAVITY FLOWS 3) Liquefied flows Liquefied sand Water Resedimented sand 14

15 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 15

16 SEDIMENT GRAVITY FLOWS 4) Turbidity flows Velocity, U h given by; U h = 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 16

17 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 fixedmoving 17

18 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 18

19 SEDIMENT GRAVITY FLOWS Summary Grain flowDebris flowLiquefied flowTurbidity flow Grain collision Matrix strength & buoyancy Buoyancy Turbulence 19


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