Lab 11: Structural Analysis Using Stereonets

Lab 11: Structural Analysis Using Stereonets ESCI 331 – Geologic Structures and Map Interpretation Northeastern Illinois University Dr. Genet Ide Duke

“The Graveyard Lab”

I. Purpose

This is a “big-city” simulation of making a geologic map and interpreting the structure of a small area. The simulation setting is the Bohemian National Cemetery on Foster Avenue in North Chicago, abutting Northeastern Illinois University to the west. • To practice measuring strike and dip with the Brunton compass. • To practice making maps with strike and dip symbols. • To practice plotting planes on the equal-area stereonet. • To interpret patterns on the stereonet.

II. Procedure:

Part 1 – Field work  Measure the strike and dip of the headstones in your assigned section of the Bohemian National Cemetery near Foster Avenue. You will work in groups and measure different areas of the south side of the cemetery. Record the strikes and dips in your field notebook. (Note: Use the flat tops of the headstones. If there is a rounded top, use the flat base of the headstone from which to measure strike and dip.)  Make a sketch map in your field notebook of the headstones in the area you are studying. Plot the strikes and dips of the headstones on the map of your study area as though you were putting strikes and dips correctly on a geologic map.

Part 2 – Lab work  Make a map of the strikes and dips that you measured in the cemetery. (Re-check your precise numbers for strike and dip, and plot those. Don’t just trace over your sketch map in your field notebook, because you may have been a bit sloppy in the field.)  Remember to use north arrow and scale. (Remember, this is not exactly a perfect simulation of geologic bedding, we will be able to detect structural patterns that may reveal drainage patterns over the ground and under the surface between the graves.)  Plot the planes (strike and dip of headstones) on tracing paper overlaid on the equal area stereonet.  Is there an intersection? If there is, determine the trend and plunge of that intersection. If not, is there a pattern that you can detect? What interpretations can you make?  On a different sheet of tracing paper, plot the poles to the planes. To plot the pole, rotate the great circle of the plane so its strike is N-S, then count 90 degrees along the equator passing through the middle point of the stereonet. The point you arrive at represents a line perpendicular to the plane you started with, i.e. the pole. What patterns do you see, if any? Describe them. What interpretations can you make?