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.

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

(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.

2

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

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

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


5

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


1) Grain flows

6


1) Grain flows

7

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


2) Debris flows

8


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.

9

2) Debris flows


Structure:

Shearing at base, deformation ofunderlying sediments.

Centre of classic debris flow moves like a rigid plug

Massive, (very) poorly sorted, random fabric

10

2) Debris flows


1) Debris flows

Carbonate debris flow – Palaeocene, Kirikkale, Turkey

11


12

2) Debris 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

13


3) Liquefied flows

Liquefied sand

Liquefied sand

WaterWater

Water

Resedimented sand Resedimented sand

14


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


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

16


4) Turbidity flows

1929 Newfoundland Earthquake

Resulting turbidity flow cut telephone cables on Atlantic sea bed

Patterns of motion around turbidity head

fixed moving

17


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


Summary

Grain flow Debris flow Liquefied flow Turbidity flow

Grain collision Matrix strength & buoyancy

BuoyancyTurbulence

19

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.

Documents

debris flows

aerial flows

friction grain flows

efficient grain flows

slope sediment gravity

sediment gravity flows

subaqueous flows

larger grains