Study Guide for First Midterm Exam - Geo 4602 - Kleinspehn

Physical Properties of Detrital Sediment

1c) What is the relationship between grain size and weathering regime?  What is the relationship between grain size and parent material?  What is the relationship between grain size and transport mechanism?

2c) What techniques does one use to define the grain-size distribution of sediment?  Of sedimentary rock?  Can you explain how the techniques work?

3c) What factors control grain-size distribution?  How does grain-size distribution relate to rate of deposition?  How does grain-size distribution related to weathering processes?

4c) If you were to be working in the field, what would you write in your field notebook to describe the grain size of sedimentary rock that you were observing?  What is the difference between the mean grain size and the median grain size?  How do they relate to d₉₀ ?

5c) What are the three parameters that are used to describe grain shape, and why are three parameters necessary?  What is the distinction between the form of detrital grains and roundness of detrital grains?  What controls grain form?  What controls grain roundness?  Grain sphericity?

6c) Under what conditions would you predict sediment to be deposited with an isotropic fabric?  How is an isotropic fabric related to sorting?   What are the controls on the development of preferred orientation of grains or clasts in detrital sediment?  Why would imbrication develop instead of a preferred orientation parallel to bedding, e.g. why do clasts dip upstream?  

7c) How can imbrication be used to interpret the position of ancient shorelines? 

Bedforms I and Sediment Transport

1d) What are surge deposits?  Can you give examples?  Explain how they are formed.

2d) Can you write the continuity equation, define all the terms and explain the relations expressed by the equation?

3d) Why is volume concentration of sediment in the bed a factor in calculating the change in bed height?

4d) Which factors could increase or decrease in order to initiate erosion of the sediment bed?

5d) What is the critical shear stress?  What controls critical shear stress?    How does it generally relate to grain size?  When does this relation break down?  Why?

6d) What is the general relationship between sediment transport rate and applied shear stress?

7d) Could you draw a diagram illustrating the basic force balance that controls applied shear stress?

8d) What is the drag coefficient?  How does it influence grain transport?

9d) How is bedload different from suspended load?  What controls whether sediment is transported as bedload or suspended load?

10d) Can you draw a cross section showing a typical profile of concentration of sediment per unit volume of fluid (c) from the bed upwards through the water column? How would it differ if there were no bedload, but only suspended sediment?

11d) Why are bedforms generated in silt/very fine sand or larger grain sizes?

12d) What are the 3 primary controlling variables for bedform development?  Is one variable more important in controlling the resultant bedforms?

13d) Why is in not valid to extrapolate the bedform stability fields derived from flume experiments directly to bedforms preserved in rock?

Bedforms I continued 

1e)  How does one distinguish between 2D ripples and 3D ripples?

2e) Over which grain-size ranges do ripples form?   Over what grain-size range do dunes form?  How does one distinguish between ripples and dunes?  

3e) Which bedforms develop in gravel?

4e)  What is the relationship between flow depth and ripples?  Flow depth and dunes?  Flow depth and antidues?

5e)  Are there grain sizes over which upper plane bed conditions do not develop?

6e)  What is the difference between parallel lamination and plane-parallel lamination?  What is the difference between planar lamination and cross-lamination?  What is the difference between cross-stratification and cross-bedding?

7e) How do bedforms relate to Froude number?

8e)  If you observe bedding surface in an outcrop that displays both 3D ripples and parting lineation, what could you infer about flow conditions at the time of deposition?

9e) If you were standing on a bridge looking down at a modern river channel that had active antidunes migrating on its bed, what would the surface of the water in the river channel look like?

10e)  What causes cross-stratification to develop?

11e) When you observe cross-stratification in the geologic record, what fraction of the original bedform height are you likely observing?  On what basis is your estimate made?

12e) Why are antidunes rarely preserved in the geologic record?

13e)  If ripples or dunes are migrating on a bedding surface and flow conditions suddenly drop below the critical shear stress, what happens to the bedforms? 

Bedforms II

1f)  If you were observing an outcrop or core, what criteria would you use to distinguish between ripples developed in water versus those developed by the wind?

2f) Why does saltation dominate bedload transport by the wind?  How does saltation relate to the geometry and distribution of sediment grain size in wind ripples?

3f) What are differences between dunes developed in water versus those developed by the wind?

4f) Do upper-flow plane bed conditions exist for transport by wind?

5f) How does the critical shear stress of a grain differ between transport in water versus transport in air?

6f) Under what conditions would the wind transport gravel?  What bedforms do you think would develop?

7f) What causes bedforms to climb?  Do climbing bedforms develop in water?  Do climbing bedforms develop in air?

8f) What is the relationship between bedform climb and the continuity equation?

9f) What controls bedform migration speed?  What controls the rate of vertical sediment accumulation?

10f) Can you give examples of depositional settings in which one would expect the flow to change directions?

11f) If the flow changes directions, how do bedform re-equilibrate to the new conditions?

12f) If the flow changes speed, how do bedforms re-equilibrate to the new conditions?  Would the resultant sedimentary structure be different if the flow increased its speed versus a flow that slowed down?

13f) Which variables control the development of bedforms under oscillatory flow (waves), and how do those variables compare to the variables that control bedform development under unidirectional currents?

14f) Under what conditions do waves produce dunes on the bed surface?  Can waves generate upper-flow plane bed conditions?

15f) How is the wave base defined?  What happens on the bedding surface for water that is deeper than the wave base?  

16f) Can you write an equation for the boundary between "deep-water waves" versus "shallow-water waves", i.e. how are the two distinguished?

17f) How does the bottom orbital diameter relate to the wavelength of the waves?

18f) If you had to calculate the ripple index for ripples on a bedding surface, what would you measure?  What is the name for a ripple that has a ripple index of 1?

19f) What criteria can one use to distinguish between ripples developed in under unidirectional flow in water versus those developed by waves?

20f) Can you give examples of depositional settings in which one would expect a combination of unidirectional currents and waves?

21f) What is hummocky cross-stratification (HCS) and how does it develop?  What criteria does one use to recognize HCS in the field? What criteria does one use to recognize HCS in a core?  

22f) If you were diving in a lake or ocean and were looking down on a bedding surface that had HCS on it, what would it look like?

23f)  How would one distinguish cross-stratification produced by 3D dunes under unidirectional flow versus HCS?

24f) Do waves generate plane beds with parting lineation?  If so, what conditions are required?

Additional Primary Sedimentary Structures

1g)  What is the difference between a primary versus secondary sedimentary structure?

2g) What types of sedimentary structures are useful to infer a relative rate of deposition?  What would one need to know to infer a quantitative rate of deposition for an ancient deposit?

3g)  How do the driving forces differ for transport of sediment over bedforms versus transport of sediment by mass flows?

4g) What conditions need to be met to initiate motion of a mass flow of sediment?

5g) What is the difference between matrix versus cement in a sedimentary rock?

6g)  Two non-marine strata occur adjacent to one another in an outcrop.  The underlying bed is matrix-supported conglomerate and the overlying bed is clast-supported conglomerate.  Both beds are derived from the same source area.  What processes would control whether the deposit was matrix-supported or not?

7g) What processes generate normal grading?  Are these processes restricted to subaqueous setting or can normal grading develop in aeolian settings?

8g)  What processes generate inverse (reverse) grading?  Are these processes restricted to subaerial settings or can inverse grading develop in subaqueous deposits?

9g)  What processes generate inverse (reverse) to normal grading?

10g) Under what circumstances can graded bedding be used reliably to determine the stratigraphic upward direction?

11g)  What kinds of outcrops best expose sole marks?  How would they be recognized in sediment cores?

12g)  How does one distinguish flute casts from scour marks from groove marks?

13g)  Give examples of 'tools' that could generate groove marks or bounce marks in the geologic record?

14g)  In which depositional settings would one predict flute casts to develop?  What conditions have to be met?  In which depositional settings would one predict groove marks to develop?  Are the settings similar for scour marks?  In which ranges of grain sizes do these structures develop?

15g) Which sedimentary structures are useful to interpret the unique azimuth for the flow direction at time of deposition and which structures provide a paleocurrent direction with 180° uncertainty?

Non-biogenic Secondary Sedimentary Structures

5h) What is the relationship between rate of deposition and the tendency to develop density instabilities leading to soft sediment deformation?

6h) In cross-section, how would you distinguish mudcracks vs. sandstone dikes vs. vertical burrows vs. fluid-escape pipes in a rock sequence?

7h) What is the size range of pockmarks and where do they typically form?  How would they appear in outcrop?

8h) What conditions need to be met to generate a density instability in sedimentary strata?

9h) Among fluid-escape pipes, dish structures, pockmarks, sediment volcanoes, flame structures, load casts and sedimentary dikes, which can develop once the sediment is lithified?

10h) How does one distinguish a dish structure from a load cast?

11h) Of the different types of deformed bedding discussed in lecture, which ones would you expect to be absent from a seismically active region?

Geobiology and Biogenic Sedimentary Structures