Maps, Folds, Stereonets, and Simple Fabric Analysis

John Weber
Grand Valley State University
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This analysis involves analyzing structures shown on beautiful, rich, detailed, and well-prepared outcrop bedrock geology of a real region. Students construct a number of stereonets step-by-step, and the map-scale simple fabrics come to life. Students also prepare an accurate cross-section using data from the stereonet analysis.

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I use this exercise in our required undergraduate structural geology course. I first did the exercise as a graduate student at Northwestern University, where E. Tim Whitten, the map's creator taught.

Skills and concepts that students must have mastered

Students must come into this exercise knowing how to plot lines, planes, and poles on stereonets, and with some very basic descriptive knowledge about folds.

How the activity is situated in the course

I use the exercise as a stand-alone lab to prepare students for our weekend field trip to study the descriptive and kinematic history of fabrics in the Baraboo synform, Wisconsin.


Content/concepts goals for this activity

From doing the exercise students: 1) begin realizing that good field maps contain a wealth of data, and 2) realize that small- and large-scale structures "live" together in the Earth in a very systematic way.

Higher order thinking skills goals for this activity

From doing the exercise students: 1) gain high-level stereonet anaysis skills, and 2) can relate data from maps to cross-sections to stereonets.

Other skills goals for this activity

Description of the activity/assignment

After learning how to plot planes, lines, and poles on stereonets, how to describe folds, and what simple fabrics are (see attached file), students analyze the simple fabrics of this real region using the rich data that are plotted on a beautiful outcrop geologic map. In the analysis, they compare the geometry of large- and small-scale structures using step-by-step instructions and multiple stereonet analyses, draw an accurate cross-section using data from the stereonets, and work out a sequence of events and geologic history.

Determining whether students have met the goals

I work with students one-on-one as they are doing the analysis so that they don't end up with mis-plotted stereonets that don't tell them anything. I also help them to use the macrofold axial planes they determined from their stereonet analysis to draw accurate cross-sections. Once I have them on track, it takes very little effort to evaluate their work. I leave them to write up the sequence of structural events and geologic history on their own.

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