Sedimentary Structures

Question 1

Sedimentary Structures

Answer 1

How the sediment gathers/groups itself as the sediment is being transported

• depends on flow process, velocity and grain size
• the graph shows structures formed by flow in a unidirectional
• Unidirectional current flow (-> only flows in one direction) forms specific sedimentary structures related to the velocity of flow

Question 2

Antidunes

Answer 2

• Form at high velocities
• Flow is almost erosional but mostly deposits sediment acros all grain sizes (on the boundary of where it is eroding and where it is depositional
• Antidunes are a highly mobile bed
• Whole flow involved so that the dunes mimic the form of surface of a standing wave
• may migrate upstream or form behind obstacles
• Sand mirrors the surface of the water, getting steeper and steeper until it collapses. The starts to regrow (happens because close to boundary of erosion
• low preservation in the rock records because highly unstable and high velocity

Question 3

Upper Plane Bed

Answer 3

• fast, often shallow flow
• streaky look to it, streaky linearities that are parallel to the flow
• Upper plane bed laminations and current lineation’s - mobilises the whole bed so that all the grains are moving but only 1-2 grains deep
• can be generated by wind as well

Question 4

Clast Imbrication

Answer 4

• In coarser grainsizes, bedload transport causes clasts to stack against each other as they creep downstream

Question 5

Dunes

Answer 5

• form at mid-high velocities
• Coarser grain sizes only
• height = 0.05-10 m
• Wave length = 0.6 -100’s m
• Dunes vary in size with flow depth and flow velocity, not grain size
• Cross-section through dunes shows cross-bedding
• Dunes make up the majority of the flow - affecting the top surface of the water still.
• how deep the water the flow is decides how big the dunes will be
• Erose in the stoss, accumulate in the lee
• water/air above slightly mimics the shape of the dunes
• crest is an active location of deposition
• Ripples and dunes can co-exist, with slight difference in grainsize or flow velocity
• Can have dunes in tidal estuaries (transportation due to tides). Line records will show the change in tides. Even currents that aren’t constant can exist long enough to form dunes in the record.

Question 6

How Dunes Migrate:

Answer 6

• Dunes form as a chain. The shallow side faces upstream and known as the stoss. The steep side faces downstream and is known as the lee. Crest is the top
• Water or wind flow hits the bottom of the stoss, eroding it slightly. Grains are deposited up the slope, until they accumulate at the crest. When the crest gets too steep, it collapses resulting in cross beds (represent the lee slope)
• the sand collapses downwards but the water/wind goes over the top until it hits the next dune, where the process is repeated.
• there may also be a back eddy where sediment will be transported back to the lee side

Question 7

Why are Martian dunes so large?

Answer 7

• Lack of obstacles? -> Makes wind stronger?
• Less gravity? Affects avalanches on the lee face (angle of repose could be different)
• Less dense atmosphere so not reworked by smaller winds?
• Deeper depth of flow (Key)
• Dry so water doesn’t hold the sediments together (makes all of the sediment available to move)

Question 8

Ripples (Currents):

Answer 8

• Finer grain sizes only
• Ripples and dunes are distinct
• With changing velocity or grain size there is an abrupt change in bedform, not gradual
• Certain velocities -> fine grains form ripples, larger ones form dunes
• Ripples form in low flow velocities and fine grain sizes
• Asymmetric in form
• Move by erosion on stoss side. Deposition at crest and avalanching down leeside (similar to dunes)
• Height <0.4m
• Wavelength <0.5m
• Ripples indices = wavelength/height = 10-40
• Ripple size varies with grain size (not flow depth or flow velocity) - correlates to ripples being smaller and forming in a range of grain sizes
• ripples look similar to the dune formation but don’t affect the overlying flow in the water/air. Only occur in the boundary layer (bed roughness area)
• Straight crested ripple form 2-dimensional tabular cross-laminations. Tabular = planar lee face = straight crests
• Sinous crested ripples form 3-d trough cross-laminations. Trough cross bedding = sinuous crests
• How wide your flow is will decide if it is sinuous or straight. Width of flow will control amount of friction on sides -> narrow flow - more friction on sides. Velocity can also affect it
• Need overall deposition to preserve ripples - greater rate of deposition means more ripple form preserved. Beds aren’t straight = angle of climb -> the steeper the angle, the faster the rate of accumulation. Angle of climb can help you understand how fast the sediment is accumulating