North Seattle Community College's
GEOLOGIC STRUCTURE EXERCISE
@2002 -- The information contained in this document
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?
Each question along the way is worth one point. Use the "Week 8 -- Lab Homework Part 1" form to submit your answers. TOTAL POINTS POSSIBLE = 25.
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 roadcut 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?
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. As explained in your lab manual, 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). Use your protractor on your computer scene to verify that you understand this -- and review page 27 of our lab manual.
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):
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 this 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.
The similar rock layers have been identified with letters (A, B, C, D, and E) on each side of the fault line. Please answer the following four questions:
Question 1 (1 point). Has the hanging wall moved upward or downward relative to the footwall -- and does this mean the fault is a normal, reverse, or strike-slip?
Question 2 (1 point). Is this fault the result of compressional, extensional or shear stress? What type of plate boundary is it likely to be associated with? Why?
Question 3 (1 point). In what direction is the "strike"? Is it north, south, east or west -- and why?
Question 4 (1 point). Report the dip of this fault in the way described above.
Here's another imagined fault below:
Question 5 (1 point). Has the hanging wall moved upward or downward relative to the footwall -- and does this mean the fault is a normal, reverse, or strike-slip?
Question 6 (1 point). Is this fault the result of compressional, extensional or shear stress? What type of plate boundary is it likely to be associated with? Why?
Question 7 (1 point). Report the dip of this fault in the way described above.
Here's a third imagined fault:
Question 8 (1 point). Has the hanging wall moved upward or downward relative to the footwall -- and does this mean the fault is a normal, reverse, or strike-slip?
Question 9 (1 point). Is this fault the result of compressional, extensional or shear stress? What type of plate boundary is it likely to be associated with? Why?
Question 10 (1 point). Report the dip of this fault in the way described above.
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 uplisft 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:
Question 11 (1 point). Is this an anticlinal fold or synclinal fold? Why?
Question 12 (1 point). Report the dip of rock strata "5" according to its orientation on the "tail side" of the dinosaur. Report the dip of rock strata "5" according to its orientation on the "head side" of the dinosaur. Do we consider this a symmetric fold, an overturned fold, or what? Explain.
Let's look at a photograph of an actual fold:
Here's a fold we've seen earlier (during our homework on Metamorphic Rocks). We are looking down on a fold -- north is toward the top of the photograph. The fold itself will not tell us much about the direction of the stress applied. Was it north/south or was it west/east? We really can't tell because we have such a limited view. Is what we can see really part of an overturned fold?
We really need to look at the "foliation" of the rocks in order to determine the direction of the stress. The "foliation" is a part of the metamorphic alteration of the rock layers. Remember that this alteration is something that affects the mineral composition, orientation, and layering within the rocks -- it is not the same as the folding of the rock layers. Some of your rock specimens (in your lab kit) show foliation -- they certainly don't show folding because they are such small specimens!
It is important to understand what foliation is ("schistose" or "phyllitic" texture) as opposed to major folding of rock stata. Do you know the difference?
Look at the differences by clicking on the enhanced photographs here and comparing the folding (in red in the top photograph) with the foliation (in blue in the bottom photograph).
Question 13 (1 point): Explain the difference in foliation and folding as shown in the two enhanced photographs. Why are the foliation lines different from the folding curve?
Question 14 (1 point): Foliation is obviously quite different from folding. What does foliation tell us about the direction of the stress? From what direction did the stress occur and why?
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 (taken from the Second Edition of our textbook by Chernicoff and Fox).
Click on the image to enlarge the view
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 15 (1 point). What type of fold is this? Be specific (is it asymmetrical, overturned, recumbent, synclinal, antisynclinal, etc.).
Question 16 (1 point). The folding is so extreme that it is difficult to decide which rock layers are the younger ones and which are older. What sedimentary structures could we look for to help us decide which layers are younger and which are older?
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 line is very straight.
For help in visualizing the fault line, click on the enhanced photograph shown by the link here. The white line shows the fault.
Question 17 (1 point). Walk directly across the road from your rover. At that location on the outcrop, measure the dip of that very thick light-color sedimentary layer near the bottom of the cliff (labeled as "C" in the enhanced photograph). What is the dip direction and angle? Assume that we are facing north when we look at the photograph.
Question 18 (1 point). Follow this same layer (C) to where it curves and reaches the top of the outcrop. What is its dip direction and angle at the very top of its exposure?
Question 19 (1 point). What type of fault is this (normal, reverse, thrust)? Why?
Question 20 (1 point). Say that our land rover is 12 feet (4 meters) long. Using our cool vehicle as the scale, measure the offset that has occurred (in meters) along the fault. Place a ruler along the fault line as it cuts through that lower thick light-color sedimentary layer labeled "D" in the enhanced photograph. How much offset is there?
Question 21 (1 point). Carefully follow the fault to the top of the outcrop (used the enhanced photograph). Look for the patterns in the layers on each side of the fault to see which ones match up. What is the approximate offset (in meters) along this fault line based on the shift in the layer marked "A"? What about based on the layer marked "B"? If this the same amount of offset as you determined for Question 20? If not, what could be an explanation for the difference?
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.
Question 22 (1 point). 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(s), will they get a close-up view of the sandstone? (northwest, northeast, southwest, and/or southeast) Why?
Question 23 (1 point). Same question for the geologic map below. At which corner(s), will they get a close-up view of the sandstone? Why? At which corner will your friends be able to dig deep into the earth and reach that beautiful sandstone layer? At which corner will your friends never be able to see that sandstone layer in the earth beneath them? Why?
Question 24 (1 point). Given the strike-and-dip information shown on the geologic block diagram below (each sedimentary layer is labeled A through J), what geologic structure is our friendly geologist walking across? Why?
With this basic understanding, let's revisit the Grand Canyon and take a closer look at the information recorded on a piece of a geologic map. The geologist (in this case, John H. Maxson) has compiled his observations in great detail on this map.
Question 25 (1 point): Take a close look at the map below. Find a strike and dip symbol, describe where it is on the map and explain what it tells you about the orientation of that rock unit.
(Click on the "thumbnail" image above or, alternatively, click here)