North Seattle Community College's
PHYSICAL GEOLOGY 101
Designer:  Tom Braziunas

Instructor:  Gwyneth Jones

glacier.jpg (49711 bytes)
Glacier National Park, Montana
Photo by Paul Carrara
USGS Public Domain Photograph

GEOLOGIC STRUCTURE EXERCISE

@2002 -- The information contained in this document is copyrighted.
No reproduction may be made without prior approval from the author (Dr. Tom Braziunas).

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I. An Introduction

During this past week we learned some terminology for the varieties of geologic structures that complicate the "rock record".   Such geologic structures include a variety of folds (bends), faults (breaks) and unconformities (gaps) in rock strata.  Normal faults, reverse faults, overturned anticlines, angular unconformities -- these are terms we have now become familiar with.

Our emphasis last week was to understand the correct sequence of events which created a particular assemblage of rock layers and structures at a particular outcrop.  This week we hope to understand how to reconstruct a much broader interpretation of the geologic story for a region.  What type of tectonic stresses caused the particular structures which we see, how can we relate ("connect the dots between") this particular rock exposure ("cliff or rock outcrop") with another rock exposure some distance away, and how can we derive a plate-tectonic scale interpretation to all this?

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II. Strikes and Dips:

A geologist reports important observations in the field through geologic symbols.  The broad geologic cross-sections we have seen were usually constructed through extrapolation of scattered surface information from a few outcrops.  We must understand the meaning of the "strike and dip" of rock layers in order to know how to "connect the dots" from one road cut to another.

The strike is the direction of a line formed by the intersection of a rock layer and a horizontal plane, in other words, the direction you would need to walk (if you were standing on a flat surface) in order to stay in contact with a particular rock layer. The dip is the angle between the inclined rock stratum and a horizontal plane.  In other words, the dip is the direction that a ball would roll down a particular rock layer if it could. Strikes and dips can apply to rock layers and to other features such as faults or unconformities.  These measurements are simply ways to orient a particular feature relative to the Earth's surface.

Strikes and dips do NOT necessarily relate to the slopes and contours of the surface topography.  For example, look at the mountain cliffs behind and to the left of St. Mary Lake in Montana as shown in the photograph below.  Do you see the layering?  

stmary.jpg (42049 bytes)
St. Mary Lake, Montana
Photo by Paul Carrara
USGS Public Domain Photograph

Let's also say that we are looking due North.  It appears that the strata dip slightly to the west.  Do you see this?  This dip has nothing to do with the angle of the cliffs or the ridge along the top of the mountain face.  The dip of a layer (or other structure) is measured relative to a horizontal plane.  The strata in the photograph appear to dip about 10o to the west.  We would state that the dip is 10o W (10 degrees West).  This angle can be measured using a protractor, if you have one.

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III. Faults and Footwork

The correct identification of a fault allows us to say a great deal about the stresses which created it and the plate tectonic dynamics under which it occurred.  Normal faults occur in divergent (extensional) crustal settings while reverse faults occur in convergent (compressional) crustal settings.  In order to determine whether a fault is normal or reverse, we have to understand which way the "footwall" and "hanging wall" slid past one another.  AND in order to know this, we have to correctly identify what is the "footwall" and what is the "hanging wall."  THIS IS CRITICAL!!

An important concept to grasp is that "footwall" and "hanging wall" are terms relative to a particular fault.  A section of a rock exposure can be a "footwall" for one fault but can be a "hanging wall" for another one.  For example, we can see two faults in the photograph below (Figure 9-11 in our textbook):

  Click on the image to enlarge the view
Figure9-11.jpg (487992 bytes)

Notice that the darkest sedimentary layer appears to be broken into three pieces (we can't really see the piece of it to the right but we can imagine it is just beyond the photograph).  The section of rock in the middle is the "footwall" for the fault to its left.  This same section is also the "hanging wall" for the fault to its right.  It is all relative!  The "footwall" and "hanging wall" are terms used in reference to a particular fault (and can change for another fault).  

In order to fully understand this terminology, I recommend that you watch the short videos on faults which are available from the main lab web page.  In these rather amateurish videos, I try to introduce the footwall/hanging wall concepts visually using the "Bread Loaf of Science."  If nothing else, you might find these short clips amusing and appetizing.

Once you have studied the videos and read the textbook's explanation of "footwalls" and "hanging walls", let's proceed below.  Notice the first fault in the diagram, I have placed a "foot" on the footwall side of the diagram.  The pointy side of the intersection of the fault with the bottom of the diagram (the "acute angle") is the toe.  Do you see this?  Thus the block of earth to the left -- the foot with this toe (which is also marked in red) --  is the footwall.  The other block to the right must be the hanging wall.

 

Figure 1.  The similar rock layers have been identified with letters (A, B, C, D, and E) on each side of the fault plane.  

Please answer the following questions:

Question 1. Has the hanging wall moved up, down, or laterally relative to the footwall?
     a. up
     b. down
     c. laterally

Question 2.  Does this mean the fault is a normal, reverse, or strike-slip?
     a. normal dip-slip fault
     b. reverse dip-slip fault
     c. lateral strike-slip fault

Question 3. Is this fault the result of compressional, extensional or shear stress?  
     a.  compressional stress
     b. extensional stress
     c. shear stress

Question 4.  What type of plate boundary is it likely to be associated with?  
     a. convergent
     b. divergent
     c. transform

Question 5. In what direction is the "strike"?  
     a. north
     b. south
     c. east
     d. west

 WB01512_.gif (115 bytes)  Here's another imagined fault below:

Figure 2.  The similar rock layers have been identified with letters (A, B, C, D, and E) on each side of the fault plane.  

Question 6. Has the hanging wall moved up, down, or laterally relative to the footwall?
     a. up
     b. down
     c. laterally

Question 7.  Does this mean the fault is a normal, reverse, or strike-slip?
     a. normal dip-slip fault
     b. reverse dip-slip fault
     c. lateral strike-slip fault

Question 8. Is this fault the result of compressional, extensional or shear stress?  
     a.  compressional stress
     b. extensional stress
     c. shear stress

Question 9.  What type of plate boundary is it likely to be associated with?  
     a. convergent
     b. divergent
     c. transform

Question 10. In what direction is the "strike"?  
     a. north
     b. south
     c. east
     d. west

Question 11. In what direction is the "dip"?  
     a. north
     b. south
     c. east
     d. west

WB01512_.gif (115 bytes)   Here's a third imagined fault:

Figure 3.  The similar rock layers have been identified with letters (B, C, D, and E) with an intrusion A. 

Question 12. Has the hanging wall moved up, down, or laterally relative to the footwall?
     a. up
     b. down
     c. laterally

Question 13.  Does this mean the fault is a normal, reverse, or strike-slip?
     a. normal dip-slip fault
     b. reverse dip-slip fault
     c. lateral strike-slip fault

Question 14. Is this fault the result of compressional, extensional or shear stress?  
     a.  compressional stress
     b. extensional stress
     c. shear stress

Question 15.  What type of plate boundary is it likely to be associated with?  
     a. convergent
     b. divergent
     c. transform

Question 16. In what direction is the "strike"?  
     a. north
     b. south
     c. east
     d. west

Question 17. In what direction is the "dip"?  
     a. north
     b. south
     c. east
     d. west

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IV. Folds and Foliation:

Now let's take a look at the structures we call folds.  These "ductile" changes in rock layers take place at great depths and under slow stresses that continue during long periods of time  With uplift and erosion, we get to see these folds eventually exposed at the Earth's surface.   Here's an imagined fold -- exposed somewhere (I guess) on that island in "Jurassic Park" because an Apatosaurus is walking across it:

Figure 4.  The similar rock layers have been identified with numbers 1-7.

Question 18. Report the direction of dip of rock strata "5" according to its orientation on the "tail side" of the dinosaur.  Report the direction of dip of rock strata "5" according to its orientation on the "head side" of the dinosaur.  
     a.  tail-north, head-south
     b. tail-west, head-east
     c. tail-east, head-west
     d. tail-south, head-north

Question 19.  What is the strike of all the beds?
     a.  north
     b. south
     c. east
     d. west

Question 20. What type of fold is this?
     a. symmetric anticline
     b. symmetric syncline
     c. asymmetric anticline
     d. asymmetric syncline

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V.  Geologic Structures in the Ugab Valley:

Please visit the Lower Ugab Valley in Namibia.  If you can't make it in person, then click on the image below.  

WB01512_.gif (115 bytes)  Click on the image to enlarge the view

ugab.jpg (3879671 bytes)

Figure 5. Beautiful spot to park our land rover and explore!   We see such folding and faulting -- and it doesn't take a geologist to appreciate the Earth stresses involved!  Of course, our friends and/or family (along for the ride) expect us to provide some interpretation of exactly what we are seeing.

Question 21. What type of fold is this?
     a. symmetric anticline
     b. symmetric syncline
     c. asymmetric anticline
     d. asymmetric syncline

Notice the fault that runs through the cliff face all the way from bottom to top.  It is pretty obvious where it cuts through the thicker light-color rock units on the lower one-third of the outcrop but it is a little harder to see above that.  Follow this fault all the way to the top of the outcrop by placing a ruler along it and looking for the offset in layering.  The fault plane is very straight. 

WB01512_.gif (115 bytes)  For help in visualizing the fault plane, click on the enhanced photograph shown by the link here.  The white line shows the fault.

Question 22.  What type of fault is this?
     a. normal dip-slip fault
     b. reverse dip-slip fault
     c. lateral strike-slip fault

Question 23. Is this fault the result of compressional, extensional or shear stress?  
     a.  compressional stress
     b. extensional stress
     c. shear stress

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VI.  Geologic Block Diagrams and Geologic Maps:

We should now be aware that sometimes the topography of the land hints at what the underlying structures are -- and  many times surface topography can be misleading as well!   Careful "strike and dip" measurements are the tools which allow us to separate what the landscape looks like above from what the shape of the geology is underneath.  As we've said, these measurements allow us to "connect the dots" between exposures to put together a full story of the geology underfoot (sometimes with the ultimate goal of knowing where to drill for oil).  Let's practice connecting the dots!  

Imagine that the sketch below is a simple "geologic map" (just like a road map) representing 20 miles by 20 miles in distance.  When you are at the center of the map, you are standing next to a rock exposure (a road cut) which shows a beautiful sandstone layer striking northeast/southwest and dipping 30 degrees to the southeast.  Note how the symbol is written on the map.

Figure 6.  Map view of an oriented sandstone layer.

Question 24.  Here's a "3-dimensional" puzzle to solve!  You need to tell your friends where they can stand on this beautiful sandstone if they can only be at one of the four corners of the map.  At which corner, will they get a close-up view of the sandstone? 
     a.  northwest
     b. northeast
     c. southwest
     d. southeast   

Question 25.  Now, which corner would your friends get a view of the sandstone if they dug deep enough through the upper layers of soil/rock? 
     a.  northwest
     b. northeast
     c. southwest
     d. southeast        

Figure 7.  Map view of an oriented sandstone layer.

Question 26.  At which corner, will they get a close-up view of the sandstone? 
     a.  northwest
     b. northeast
     c. southwest
     d. southeast  

Question 27.  Now, which corner would your friends get a view of the sandstone if they dug deep enough through the upper layers of soil/rock? 
     a.  northwest
     b. northeast
     c. southwest
     d. southeast   

Figure 8.  Map and side view of several sedimentary layers.

 

Question 28.  What direction is bed D dipping in the figure above?
     a.  east
     b.  west
     c.  east and west
     d.  bed D is not dipping

Question 29.  What direction is bed G dipping in the figure above?
     a.  east
     b.  west
     c.  east and west
     d.  bed G is not dipping

Question 30.  What geologic structure is our friendly geologist walking across?
     a.  normal dip-slip fault
     b. reverse dip-slip fault
     c. anticline
     d. syncline
     e. anticline     

 

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