Sediment Transport

Question 1

What is Shear Stress?

Answer 1

Shear stress is a constant, it is the drag of a given surface.
Gravitational & friction forces that aggregate in the sediment

Question 2

What is Critical Shear Stress

Answer 2

Movement where shear forces overcome restrictive forces

Question 3

What is Critical Shear Velocity?

Answer 3

Velocity required to impose sediment movement

Question 4

When does sediment deposition occur?

Answer 4

When force of gravity is greater than the forces generated by currents responsible for sediment movement

Aka currents velocity not strong enough to keep particles suspended

Larger grains get deposited at higher speeds

Question 5

What does the erosion curve depict?

Answer 5

Shows the velocity required to lift an already deposited sediment

Fine sediments need as much velocity as large ones to erode because they have a higher surface area which causes higher cohesion & impermeability

Question 6

What is Bedload Transport?

Answer 6

Movement by rolling, sliding, hoping over seabed. Slower speeds than suspended sediments

Question 7

When is sediment suspended/transported?

Answer 7

When critical shear is overcome
Aka shear forces overcome restrictive forces

Question 8

List the sediment grain sizes in order of smallest to largest?

Answer 8

Clay, Silt, Sand Granules, Pebbles

Question 9

Describe sediment transport on the continental shelf, what influences sediment distribution patterns? What are the three sediment types present on the shelf?

Answer 9

The inner shelf is more attenuated by waves therefore sediments are generally course and terrigenous

The distribution pattern of sediments on the shelf results from a balance of:
i. Processes of Sediment Supply + Distribution Processes

Storms can cause increased planktonic mortality due to increased wave height and increased erosion, which decreases light in the water column, decreasing organic production

Terrigenous, bioclastic and authigenic sediments are present on the shelf

Question 10

On the Continental Shelf what will sediment distribution look like if you have a High Supply + Low Distribution of sediments?

Answer 10

Shelf covered in thick sedimentary deposits
Eg. Gulf of Mexico

Question 11

On the Continental Shelf what will sediment distribution look like if you have a Intense Supply + Very High Distribution of sediments?

Answer 11

Sediments redistributed across shelf, forming thin deposits, with sediment load being transferred to the rise

Eg. Portuguese Northern Shelf near Douro River

Question 12

On the Continental Shelf what will sediment distribution look like if you have a Moderate Supply + Very High Distribution of sediments?

Answer 12

Few sediments accumulate on shelf leaving rocky substrate uncovered

Eg. Roca Cape Portugal

Question 13

On the Continental Shelf what will sediment distribution look like if you have a Low Supply + Low-Moderate Distribution of sediments?

Answer 13

Conditions for the development of carbonate sediments. When terrigenous input is low then biogenic inputs can dominate

Eg Bahamas

Question 14

At low sea levels what happens to sediment supply on the slope?

Answer 14

Sediment supply is very intense

Question 15

What happens to the shelf morphology at high sea levels?

Answer 15

More sediment is deposited on the shelf, less on the slope and rise

A phase of smoothing morphology.

Question 16

What is the main type of sediment accumulated on the slope and rise?

Answer 16

40% terrigenous

Question 17

What is a Contour Current?

Answer 17

Currents flowing along depth contours
Common on the slope and the rise, not usually seen on the abyssal plains

Question 18

What are the three main processes that influence the sediments on the continental rise

Answer 18

Sedimentary Transverse Movements: Mass movements

Turbidity Currents: Very common on canyons. When slopes are high erosion potential of these currents is high. When the slow and eventually stop, they result in an abyssal fan

Contour Currents: common on rise and slope. Result on erosion on the side the current acts on and deposition on the other side (contourite)

Vertical Sedimentation: both terrigenous and biogenic sedimentation through the water column. Mostly terrigenous, carbonate and siliceous oozes on the rise.

Question 19

How to submarine canyons transfer sediments from the shelf to the abyssal plains? How does sea level effect this?

Answer 19

Act as sedimentary conduits

Mass movements such as turbidity currents common due to steep slopes

Low Sea Level: sediment load is higher and mass movements act more efficiently, this results in intense modelling of canyons

High Sea Level: lower sediment load therefore less mass movements and less canyon modelling

Question 20

What is required to cause a turbidity current?

Answer 20

Intense sediment supply + formation of deposits in unstable equilibrium + Steep Slopes = Turbidity Current

Question 21

How do turbidity currents affect coarse sediments in the deep oceans?

Answer 21

Turbidity currents are the only way of significant transfer of coarse particles (like sand) to the deep sea where often only fine particles occur

Question 22

What is the general accumulation rates in the Abyssal Plains?

Answer 22

Very low accumulation rates, on the scale of mm/year

Question 23

Where are the thickest layers of sediments in the Abyssal Plains?

Answer 23

On older crusts far from ridges and trenches

Question 24

What is coprolite particles and how to they effect settling velocity?

Answer 24

Coperlite particles are biological feaces and they stick to fine sediments floating in the water column increasing the overall rate of sinking

Question 25

What are the sediments in the abyssal plains composed of?

Answer 25

Terrigenous (mostly clays), Biogenic (Calcareous, and siliceous Ooze), Authigenic

Question 26

In the abyssal plains, where do red clays dominate?

Answer 26

Red clays dominate in the deepest areas >4km where there is low siliceous productivity, and it is beyond CCD

Question 27

where do terrigenous sediments dominate?

Answer 27

Slope and Rise particularly near big river inputs

Question 28

In the abyssal plains, where do Carbonate Ooze dominate?

Answer 28

In the deep seas above the CCD where there is high coccolithophore and Foraminifera productivity

Question 29

In the abyssal plains, where do Siliceous Ooze dominate?

Answer 29

In the deep seas, are latitudinally derived. Where there is high productivity of diatoms and Radiolaria. Particularly in upwelling regions