Geologic Mapping of a Virtual Landscape II - Three River Hills

Mark Helper, The University of Texas at Austin, helper@jsg.utexas.edu

This exercise is based on the Virtual Landscapes Geological Mapping activity developed by Jacqueline Houghton and the University of Leeds Virtual Landscapes team

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Summary

This second virtual mapping exercise builds on the first (Geologic Mapping of a Virtual Landscape), but contains a more complicated geological puzzle to solve. This virtual landscape is also much larger, with a base map that does not show outcrops. In the interest of time, only ~85 outcrops in northwest corner of the landscape are mapped. The pop-up virtual compass and GPS instrument are again there for navigation, but this time students discover and plot the outcrops on a base map rather than simply navigating to them. As before, the goals include a completed geologic map and stratigraphic column. The additional challenges are a cross section and a brief write-up of the geologic history. The latter takes the form of chronologically ordered, sequential list of events (e.g. deposition, metamorphism, uplift, intrusion, faulting, folding, etc). The map, stratigraphic column, cross section and geologic history can be completed in three days with video conferencing sessions that include team breakout groups to address key questions and observations, test hypotheses and produce sketch cross sections.

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Context

Audience

This virtual mapping exercise replaced the second of three weekend mapping exercise when the Spring 2020 COVID-19 pandemic prevented taking students to the field. It is one of six weekend field exercises for a required, sophomore-level, Introduction to Field and Stratigraphic Methods, an introductory field class for geoscience majors. Students have not yet taken a course in structural geology so concepts of overturned folds, overturned bedding, and thrust faults are introduced as part of the exercise.

Skills and concepts that students must have mastered

Students at this point have used a compass to measure strike/dip and plunge/trend, know how to read a topographic map, and can write and understand sedimentary rock descriptions. They have also learned to plot GPS coordinates on a gridded base map. They completed a similar but far simpler virtual mapping exercise (Lighthouse Bay) prior to this exercise that did not require a cross section or geologic history. That exercise focused on use of strike lines and the rule of V's as tools for mapping rock units through areas of no exposure, and on systematically building a map and stratigraphic column from outcrop notes and sketches. This exercise introduces new concepts of overturned vs. upright bedding, mapping and interpreting stratigraphic omission and repetition, cleavage and cleavage bedding relationships (including intersection lineations), mapping an axial trace, and mapping and recognizing faults. These concepts are introduced in the context of the exercise during the first two days.

How the activity is situated in the course

This is the second of two virtual mapping exercises conducted over a weekend after students have completed the first one and three lab exercises on interpreting geologic maps and another on constructing cross sections. These were all completed during the second half of the semester and were accompanied by lectures on these topics. This activity is entirely appropriate for a field camp mapping exercise for students who have had a structural geology class.

Activity Length

This activity takes 3 days.

Goals

Content/concepts goals for this activity

  1. Inferring the presence, and mapping the location, of faults on the basis of the repetition or omission of stratigraphy;
  2. Using limited observations (scattered outcrops) of bedding strike/dip and upright vs. overturned stratigraphy to infer the presence of an overturned fold
  3. Using cleavage-bedding relationships to understand upright vs. overturned stratigraphy
  4. Constructing an accurate cross section through folded and faulted stratigraphy
  5. Accurately plotting and extrapolating data to assemble a geologic map

Higher order thinking skills goals for this activity

  1. Design a field strategy to collect or select data in order to answer a geologic question
  2. Collect accurate and sufficient data on field relationships and record these using disciplinary conventions (field notes, map symbols, etc.)
  3. Develop an argument that is consistent with available evidence and uncertainty
  4. Communicate clearly using written, verbal, and/or visual media (e.g., maps, cross-sections, reports) with discipline-specific terminology appropriate to your audience
  5. Synthesize geologic data and integrate with core concepts and skills into a cohesive spatial and temporal scientific interpretation

Other skills goals for this activity

Working in small groups to come up with and present hypotheses to test with field/mapping observations. These groups are part of the first day's breakout sessions.

Description and Teaching Materials

This exercise is a set of instructions and teaching materials to complete a geologic mapping exercise that uses part of the University of Leeds "Three River Hills" Virtual Landscape. Further descriptions and links to other Virtual Landscape materials by the developers are posted at another TTE SERC site. The virtual landscape is accessed through a stand-alone program available through a zip file included below or downloaded at the Leeds site. Instructions for installing the program on Windows or Mac OS computers are included below, including a short video on installing an ancillary program, CrossOver, that allows the Windows version of the software to run on Mac OS Catalina. The included standalone version of the software for Macs with a pre-Catalina OS works very well as is, but requires relaxing some of the default Mac security settings. When used in April 2020, all students in a class of 54, about half using Macs (some with a Catalina OS), successfully ran the program; getting the software up and running should not be a deterrent or impediment to using this exercise.

Students can map on a paper copy of the topographic base map (PDF file), or map with the drawing tools in PowerPoint using a provided template. A short video ( in the zipped "Videos" folder below) explains how to use the PowerPoint template and the included mapping symbols to build a map.

Instructors and students should begin by reading the "Three River Hills Project Summary and Instructions" PDF, which describes installing and running the software and the procedure for completing the exercise. A zip folder contains the topographic base map and cross section template, both as PDFs and PowerPoint versions. PowerPoint teaching materials are provided in a separate zipped folder.

Technology Needs

PC or Mac with stand-alone virtual landscape program, included as part of the exercise, copied to a folder on the hard drive. Easiest to operate with a mouse, but can be done with keyboard and touchpad.

Teaching Notes and Tips

As noted in the supporting materials, "Instructions and Summary", this exercises uses only a portion of the entire virtual landscape - students can wander out of the virtual mapping area! Pop-up GPS and compass instruments allow precise navigation and plotting of data, so this should not be an issue. There is a 1-7 m mismatch of GPS elevations and contour map elevations in the SE corner of the map. To agree with the provided key, students should rely on GPS and topo. map Eastings and Northings, not on GPS elevations, to accurately plot locations of outcrops. The notebooks at the outcrops provide both written descriptions and sketches. Students should pay close attention to the sketches and incorporate information from them in their rock unit descriptions. The sketches also clearly show inverted stratigraphy on the overturned limb of a syncline. Student should think about explanations (that will become map symbols) for why metamorphic rocks rest on top of sedimentary rocks, and to test different fault/fold hypotheses with sketches and then compare those to the field observations at outcrops closest to the contact. A strain gradient is present in both footwall and hanging wall strata where metamorphic rocks ("psammites") are in contact with sedimentary rock units, as implicitly revealed in outcrops notes by the extent of cleavage/foliation development, "rodding", and attenuation of unit thicknesses, but my students did not recognize this, instead focusing on simply rock unit locations. This needs attention as a teaching topic. The proper use of an axial trace symbol also need attention - students need to understand why axial traces are rarely ruler straight, that they separate overturned from upright outcrops or beds that dip in different directions, that they show a plunge direction, etc.

There are approximately 85 outcrops to map within the indicated area. This part of the exercise, which will include compiling notes from the field books at each outcrop, plotting the outcrops and the strike and dip measurements at each, can take up to 6 hours. As indicated in the teaching materials, part way through this process students can be organized into breakout teams to address questions that require mapping hypotheses to resolve - What is the stratigraphy? Where are there stratigraphic busts (omissions, repetitions)? How can these busts be explained? Sketch cross sections can be introduced at this point as a way to arrive at hypotheses and test them, as can generic map patterns for folds and faults. A second get-together can be used to teach concepts of axial plane cleavage, overturned bedding and overturned folds (see teaching materials).

Assessment

Student maps, cross sections, stratigraphic unit descriptions and a geologic history are evaluated relative to the provided answer keys. Map scores can include 20% for presentation details (neatness, clarity, symbol use) and 50% for proper placement of outcrops, plotting data correctly, and extrapolating contacts using rule of Vs and strike line approximations. The remaining 30% can be for interpretation of stratigraphic omission or repetition (i.e.placement of faults and a fold axial trace) and use of proper symbols to do so. The cross section template makes it relatively easy to complete a cross sections that matches the key. If it is not done as a separate exercise, it needs to match their own map (50% of score) and honor the stratigraphy and data on their map (50%). The stratigraphic column with unit descriptions should show the metamorphic rocks below the sedimentary units, with unit descriptions that incorporate both the notebook written descriptions and sketch information.

References and Resources

Houghton JJ; Lloyd GE; Robinson A; Gordon CE; Morgan DJ (2015) The Virtual Worlds Project: Geological mapping and field skills, Geology Today, 31, pp.227-231. doi: 10.1111/gto.12117.

An accompanying SERC page with other Virtual Landscapes materials by Jacqueline Houghton and the University of Leeds developers.



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