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.
What is the difference between horizontal parallel lamination and planar parallel lamination?
Planar parallel lamination may be inclined whilst horizontal parallel lamination implies that it is horizontal
If laminations have sand sized grains they are created by what?
High velocity current of water (upper plane bed)
Is silts/clays/mud are planar laminated how did they form?
Settling through suspension (slow/stationary water)
Can ripples form in gravels?
No, only sand
How fast do ripples form?
Very fast
What are the 2 sides of ripples? Discuss these. Draw a diagram and label these on.
The stoss and lee sides. The stoss side is the gentle upstream side whilst the lee side is the avalanche slope side.
How do you identify the angle of climb?
There will be 2 lines to choose from, the angle of climb is the shallowest line (less steep).
What are the 3 types of angle of climb? Which is the least important one? Discuss the 2 most important ones.
- Sub-critical
- Critical
- Super-critical
- is the least important one as it as it doesn’t exist for long.
Sub-critical occurs when sedimentation is low and one ripple erodes the back of the previous one.
Super-critical occurs when angle of climb is steeper than stoss slope of ripple. Entire ripple shape preserved along with internal lamination. Sand is accumulating fast
Can you get dunes in and out of water?
Yes, however water dunes will be considerably smaller in size.
Is it possible to get ripples on the back of dunes?
Yes it is
How is cross bedding created in relation to dunes? What causes it to be curved?
As dunes migrate cross bedding is created. How curved the cross bedding is, is a function of how curved the crest line is of the dune.
How is planar cross bedding created?
Can get multiple sets of dunes stacking on top of one another (cosets)
Discuss antidunes
Antidunes are rarely preserved, they require the highest velocities to form. It is a dune like structure where cross bedding is migrating upsteam, a turbulence burst as it migrates causes it to collapse.
Discuss the 2 types of reversing flow and their frequencies.
Wave action: High frequency
Tide action: Slow frequency
What is the difference between current and symmetrical ripples? how are they created?
Current ripples are asymmetrical and are generated by waves in shallow water.
Symmetrical ripples are symmetrical and produced by waves in shallow water.
Discuss herringbone cross stratification, what environment is it created in?
In the bottom part there is cross bedding, the top part shows the flow has changed direction. Tidal environment.
When are flaser, wavy and lenticular bedding created? Order them with increasing mud.
Created when there is alterations between flow velocity, so changing between bedload transport and settling from suspension. There is a mixture of sand and mud particles.
Mud draped ripples, flaser, lenticular.
Name the environment where flaser bedding is most common
Tidal environment
Wave generated cross bedding is important in interpreting what kind of environment?
Shallow marine environment
The depth at which waves disturb sediment (wavebase) is equal to what?
Wavelength
Wavelength is equal to what?
Wavebase, The depth at which waves disturb the sediment
Do longer or shorter wavelengths penetrate deeper into the water column?
Longer wavelengths penetrate deeper into the water column
When do you get friction effects occurring?
When the bottom of the sea is shallower than the wave wants to act. This causes waves to go around in ellipses instead of circles causing waves to break onto the shore
Coarse grained layer of sediment with mud deposited on top is evidence for what?
A transgression signature
Discuss “combined flow bedform”
Simple oscillations back and forth with a superimposed unidirectional current
Discuss Hummocky cross stratification. How do you see HCS in cross section. What size of sand does it form in?
Get a mound (hummock) down into a trough (swail) and then back up into another hummock.
In cross section you see low angled truncations of cross bedding.
Forms in fine-med grained sand, not coarse grained.
HCS and SCS suggest what kind of environment?
Wave dominated marine shoreface
discuss SCS (swaley cross stratification)
in shallower water the top of the hummock tends to get eroded so you only see the swails that are left behind.
In shallow marine fairweather conditions what structures may you expect to see? How deep are the waves disturbing sediment?
Planar lamination, large scale high angle stratification, small scale wave ripple cross stratification, mud.
Waves dont disturb sediment very deep as wavelength is short.
Why is there no HCS or SCS in shallow marine fairweather conditions?
Waves have too short of motion, sediment constantly reworked
In deeper marine storm conditions what structures may you expect to see? How deep are the waves disturbing sediment?
Planar lamination, large scale high angle stratification, SCS, HCS, mud.
Waves disturb sediment deeper as wavelength is larger
In very deep water where waves act what should you expect to see?
interbedding of sands and muds, in very deep water it is very rare that you get all mud and no sand.
what kind of flow do asymmetrical ripples require?
Unidirectional
