Sediment Transport Flashcards
What is Shear Stress?
Shear stress is a constant, it is the drag of a given surface.
Gravitational & friction forces that aggregate in the sediment
What is Critical Shear Stress
Movement where shear forces overcome restrictive forces
What is Critical Shear Velocity?
Velocity required to impose sediment movement
When does sediment deposition occur?
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
What does the erosion curve depict?
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
What is Bedload Transport?
Movement by rolling, sliding, hoping over seabed. Slower speeds than suspended sediments
When is sediment suspended/transported?
When critical shear is overcome
Aka shear forces overcome restrictive forces
List the sediment grain sizes in order of smallest to largest?
Clay, Silt, Sand Granules, Pebbles
Describe sediment transport on the continental shelf, what influences sediment distribution patterns? What are the three sediment types present on the shelf?
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
On the Continental Shelf what will sediment distribution look like if you have a High Supply + Low Distribution of sediments?
Shelf covered in thick sedimentary deposits
Eg. Gulf of Mexico
On the Continental Shelf what will sediment distribution look like if you have a Intense Supply + Very High Distribution of sediments?
Sediments redistributed across shelf, forming thin deposits, with sediment load being transferred to the rise
Eg. Portuguese Northern Shelf near Douro River
On the Continental Shelf what will sediment distribution look like if you have a Moderate Supply + Very High Distribution of sediments?
Few sediments accumulate on shelf leaving rocky substrate uncovered
Eg. Roca Cape Portugal
On the Continental Shelf what will sediment distribution look like if you have a Low Supply + Low-Moderate Distribution of sediments?
Conditions for the development of carbonate sediments. When terrigenous input is low then biogenic inputs can dominate
Eg Bahamas
At low sea levels what happens to sediment supply on the slope?
Sediment supply is very intense
What happens to the shelf morphology at high sea levels?
More sediment is deposited on the shelf, less on the slope and rise
A phase of smoothing morphology.
What is the main type of sediment accumulated on the slope and rise?
40% terrigenous
What is a Contour Current?
Currents flowing along depth contours
Common on the slope and the rise, not usually seen on the abyssal plains
What are the three main processes that influence the sediments on the continental rise
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.
How to submarine canyons transfer sediments from the shelf to the abyssal plains? How does sea level effect this?
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
What is required to cause a turbidity current?
Intense sediment supply + formation of deposits in unstable equilibrium + Steep Slopes = Turbidity Current
How do turbidity currents affect coarse sediments in the deep oceans?
Turbidity currents are the only way of significant transfer of coarse particles (like sand) to the deep sea where often only fine particles occur
What is the general accumulation rates in the Abyssal Plains?
Very low accumulation rates, on the scale of mm/year
Where are the thickest layers of sediments in the Abyssal Plains?
On older crusts far from ridges and trenches
What is coprolite particles and how to they effect settling velocity?
Coperlite particles are biological feaces and they stick to fine sediments floating in the water column increasing the overall rate of sinking
What are the sediments in the abyssal plains composed of?
Terrigenous (mostly clays), Biogenic (Calcareous, and siliceous Ooze), Authigenic
In the abyssal plains, where do red clays dominate?
Red clays dominate in the deepest areas >4km where there is low siliceous productivity, and it is beyond CCD
where do terrigenous sediments dominate?
Slope and Rise particularly near big river inputs
In the abyssal plains, where do Carbonate Ooze dominate?
In the deep seas above the CCD where there is high coccolithophore and Foraminifera productivity
In the abyssal plains, where do Siliceous Ooze dominate?
In the deep seas, are latitudinally derived. Where there is high productivity of diatoms and Radiolaria. Particularly in upwelling regions
