Structural Geology-Spatial Thinking Exercises

Earth Science Extras

by Russ Colson

 

Rocks not Folded or Faulted

Value: 2

The picture above is of the Sioux Quartzite formation in Pipestone NM in SW MInnesota. East is to our right and slightly away from us. Which map view correctly portrays the observational data that can gathered from this picture?

 Note: Geologists use a strike and dip symbol on a map (see below) to record observational data about the orientation of rocks at the surface.

 

 

 
 
 
 

 

 

 

Take a few minutes to think about the block diagram above and complete the cross sectional view that is left blank in the front. Take note of the observational data on which this diagram is based: The geologist who created the map (the top view seen in perspective in the block diagram) measured the rocks at the surface to be horizontal (indicated by the plus in a circle), and only observed layer A at the surface, with contacts with no other rocks seen in the map area. The cross sectional view on the right must be based on other information, such as drilling core data or information about the regional stratigraphy from some nearby area. What will the front cross section look like? Do the multiple choice questions below to see if you got it right.

 Click here to take the multiple choice version of this puzzle

Value: 2

Which of the block diagrams below correctly interprets the map data to show the likely cross sectional view of the rock below the surface?

 
 
 
 

 

 

 

 

Take a few minutes to think about the block diagram above and complete the two cross sectional views that are left blank. Then do the multiple-choice question below to see if you were right.

 Click here for a multiple choice version of this problem

Value: 2

Which of the following block diagrams correctly interprets the data seen in the map view of the block diagram above?

 
 
 
 

 

  

 

Take a few minutes to think about the block diagram above and complete the two cross sectional views that are left blank. Then do the two multiple choice questions below to see if you were right.

 

Value: 2

Remembering the common sequence of seashore deposits--in which detrital sand is deposited along the higher-energy beach, mud farther offshore where the wave motion is quieter, and biochemical limestone farther offshore yet where the detrital sediments don't reach--suppose that layer D is sandstone, layer C shale, and layer B Limestone. Also assume that the rocks have not been tipped upside down. Does this sequence represent marine transgression (sea advancing over the land) or marine regression (sea retreating from the land?

 
 

 

Value: 2

Which block diagram shows the correct interpretation of the map data seen in the block diagram above?

 
 
 
 

 

Rocks with Faults

 

 

Take a few minutes to think about the block diagram above and complete the front cross sectional view that is left blank. In the map above, the dashed line represents a fault. (A fault is a break in the rock where the rock has moved.) The 'dip' symbol coming off the fault indicates the direction of dip of the fault, not of the rock layers. The orientiation of the rock layers as observed by the mapping geologist is indicated as being horizontal by the circle with a plus sign in it.

Value: 2

Based on the map view in the block diagram above, which shows the direction that the fault is dipping and gives information about which side of the fault that has moved up because of the rock layers that appear on either side of the fault (assuming that the surface is eroded to flat in comparision with the scale of movement on the fault), indicate whether this fault is compressional (reverse) or extensional (normal).

 
 

 Click here for a multiple choice question about completing the block diagram above.

Value: 2

Which of the block diagrams below correctly interprets the data seen in the map view and side cross section view in the block diagram above?

 
 
 
 

   

 

 

Take a few minutes to think about the block diagram above and complete the two cross sectional views that are left blank. In the map above, the dashed line represents a fault, and the 'dip' symbol coming off the fault indicates the direction of dip of the fault, not of the rock layers. The orientiation of the rock layers is indicated by the strike-dip symbol. This fault is not strike-slip, but which side of the fault has moved up can be inferred from the change in rock layers as one crosses the fault.

Value: 2

Considering the dip of the rocks, and assuming that the rocks have not been tilted upside down, we can infer that

 
 
 
 

Value: 2

Considering the orientation of the strike and dip symbol, we can see that the rock layers

 
 
 
 

Value: 2

Consider the rocks on either side of the fault, say in the vicinity of the fault-dip symbol. From the outcrop pattern of rocks, and an understanding of which rock layers are above and below the others (as figured out in a previous question) we can see that

 
 
 
 

Value: 2

Which of the following block diagrams correctly interprets the cross-sectional views implied by the observational data present in the map view?

 
 
 
 

 

Value: 2

Based on which direction the fault is dipping (east) and the direction of motion on the fault (east side has moved up), and considering the cross-sectional interpretation above, we see that this fault is

 
 

 

Value: 2

Now for a more complex block diagram. Which of the following faults are reverse (compressional) and which are normal (extensional)?

 
 
 
 

Value: 2

Depending on the geometry of the angle of rock tilt, the angle of the fault, the strike of the rocks and the strike of the fault, whether the fault is normal or reverse, and the erosion of the upthrown block, faults can sometimes either duplicate layers of rock at the surface or 'fault out' layers at the surface (that is, layers that are present underground do not show up at the surface of the earth). Which of the following show faulting out and which show duplication of layers at the surface?

 
 
 
 

 

Rocks with Folds

 

Value: 2

Based on the observed pattern of rock layers and the orientation of layers as indicated by strike-and-dip symbols,

 
 
 
 
 
 

 

 

Take a few minutes to think about the block diagram above and complete the two cross sectional views that are left blank. The orientiation of the rock layers is indicated by the strike-dip symbols. THen do the multiple choice question below to see if you got it right.

 

Value: 2

Which of the following block diagrams represents the best interpretation of the surface data seen in the map view above?

 
 
 
 

 

Value: 2

Remembering the common sequence of seashore deposits--in which detrital sand is deposited along the higher-energy beach, mud farther offshore where the wave motion is quieter, and biochemical limestone farther offshore yet where the detrital sediments don't reach--suppose that layer A is sandstone, layer B shale, and layer C Limestone. Also assume that the rocks have not been tipped upside down. Does this sequence represent marine transgression (sea advancing over the land) or marine regression (sea retreating from the land?

 
 
 
 

 

For our grand-finale challenge on the spatial thinking exercises, we are going to consider index fossils and faunal succession (the progressive change in organisms through time) as a means to infer whether a structural feature is an antincline or syncline. We are also going to look at a more complex situation where the fold is itself plunging, as was discussed in the lecture. Here are block diagram illustrations for the case where either an anticline or syncline are plunging. Notice that the dip of the rock layers seen on the right side of the block diagrams reflects only the plunge of the fold, not the fold itself, whereas the front cross section shows only the fold and not the plunge. Notice that the direction in which the rock outcrops 'V' in the map view depends on both the direction of plunge and whether the fold is an anticline or syncline. The red arrows mark the axis of the fold, with the arrow indicating the direction of plunge.

 

The index fossils we're going to consider are graptolites. Graptolites were colonial animals that lived in sea-weed-like apartment complexes that drifted over much of the sea and many different depositional environments during the Ordovician. Individual species had a fairly short range of existence and there was a progressive change in the type of colonies that they built, making it easy to identify what time a particular individual colony might have lived. In simplified form, the progression of body types from the early Ordoviian into the late Ordovician and early Silurian is shown below.

The Thecae are the "apartments" where individual animals lived in the colony, and the stipes are the dangly strings on which the apartments are built. The creatures drifted in the sea sort of like sea weed, resulting in their being distributed widely, a valuable characteristic of an index fossil. Notice that the progression is from more stipes that hang downward toward fewer stipes with the apartments reversed in direction and more tightly tied to a single stipe.

 

Value: 2

Based on the map pattern of the rock layers, and the index fossils seen in selected layers (and considering the plunging anticline and syncline shown above), this structure is

 
 
 
 

 

last updated 3/28/2020.   Text and pictures are the property of Russ Colson.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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