Lec 5+6 Flashcards
Name types of transport
Unidirectional currents, waves, turbidity currents (gravity) mass flow deposits (gravity)
Are erosion and transport simultaneous?
No, erosion tends to happen first and then transport
What is the relationship between erosion and transport?
erosion occurs during the transport phase however they are not simultaneous. Erosion happens first and is followed by transport
What do depositional processes do to grain arrangement
It affects the organisation of grains, strongly influencing sorting and grain sizes. This overall creates internal grain arrangement which shows us sedimentary structures.
Discuss post depositional alteration and give examples
This occurs once sediment stops moving and comes in the form of soft sediment deformation (SSD)
Examples: Load casts, desiccation cracks, concretions (can have evaporite casts), fluidisation (flame structures), cementation, rooting or burrows
Define soft sediment deformation
When the sediment is deformed before it is lithified, it has not become cemented and the grains are still loose.
If you cannot see grains with a hand lens what type of sediment are you looking at? If you can see grains what sediment band are you looking at?
Clay/Silt, if you see grains in sand band.
Entrainment Velocity Diagram: How high does velocity need to be to get sediments into motion?
The velocity needs to be as high as the red line on the entrainment velocity curve. At the RHS of the diagram velocity needs to be higher as the grain size is large so need a larger velocity and force to carry them
Why do you need a higher velocity to move silts and clays than you do to move sand?
They need a higher velocity because cohesion forces make silt and clay harder to erode, and electrostatic forces bind grains together stopping them from moving individually.
How is sediment sorted? (with reference to entrainment velocity curve)
Sediment is sorted by grain size and grain density (hydrodynamic sorting)
What are the 2 distinct zones during transport by current of water? Discuss these.
Bedload transport and suspended bedload.
Bedload transport occurs when coarser grained sediment bounces along the bottom becoming more rounded.
Suspended bedload occurs when finer grained sediment remains in suspension and may never touch the bottom.
Settling from suspension gives what kind of sedimentary structure?
Horizontal planar lamination
Which is denser, wind or water?
Water
What is the prime difference between wind and water transport?
wind does not move particles >2mm, whereas water does.
Can you get bedload transport and suspended bedload in both wind and water transport?
Yes you can
Discuss structureless sediment
Structureless sediment is rare, sometimes structures are so large or small that you just cannot see them. Alternatively there may have been a structure that was destroyed once the sediment stopped moving, this may have been by a process like fluidisation, burrowing or formation of soils.
Why are there no lab recordings of clay sized particles, only sand sized ones?
Because we cannot scale grain size correctly in a flume without getting so small that the grains want to stick together so all experiments start with sand sized ones
What 2 things cause us to have different scales of structures?
Velocity and grain size
When sand starts to move what kind of lamination does it create? What part of the plane bed does this resemble?
Planar parallel lamination. Lower stage plane bed
Which is seen and preserved the most; the upper or lower stage plane bed?
Upper stage plane bed, lower stage rarely preserved
How is the upper stage plane bed created?
When velocity gets high enough it flattens out dunes and starts a new part which is the upper stage plane bed.
