Structural Geology Lab - Cross Section Exercise

Cross section from http://geomaps.wr.usgs.gov/parks/olym/ftstop10.html.

Motivation: For me there is something immensely satisfying in constructing a geologic map. They are a combination of observation, scientific logic and analysis, inferences, gut-feelings, and creativity. They represent a long-standing contribution to geology. Other geologists have expressed similar feelings. In a deformed terrane a crucial effort in the process of map making is the drafting of cross sections. It can be considered a test of the map pattern (most geologic maps have a significant component of interpretation since extrapolation between outcrops is necessary) and often leads to revision of the geologic map. Cross sections also allow for better conceptualization of the 3-D geometry of the structures, and thus aids in understanding the kinematic history of the area. For example, tectonic vergence (transport direction, often evident in the asymmetry) is immediately discernible in cross section, but less so from a map view. Also, if estimates of shortening are to be made drafting of a balanced cross-section is necessary, as will be described later in the course. Importantly, cross-sections suggest what exists in the firmament beneath our feet and beyond our vision - a 3-D extrapolation. They are also used as input into other models (e.g. of fluid migration), and the results are only as good as the cross section. We will also discuss balanced cross sections later in the course. Typically cross sections are produced in a variety of software environments these days, and often feed into 3-d models (e.g. USGS site modeling structures in Italy - http://pubs.usgs.gov/of/2005/1428/dedonatis2/index.html)

There are three possible maps to create cross sections from that I have developed over the years (two associated with the Appalachians and one with Spitsbergen), and I will indicate to you which one you should do. Some have a cross section displayed, but of limited extent. While this cross section can serve as a guide, you can and should be able to do better, and your cross section will be for the specific given line.

You should first spend time becoming familiar with the geology depicted on the map. What patterns are due to folds, what patterns due to topography? What types of faults exist? Are there any major unconformities? What type of stratigraphy is present? If not colored, adding colors often helps in understanding the map pattern. If you can identify a plunging fold axis, by looking obliquely at the map in the direction of the fold axis, you can get a crude cross section view (if topography is at a small enough scale that it doesn't influence the map expression to much).

A cross section line should be chosen to show a maximum amount of information, i.e. it should cross major faults and fold structures, and should be perpendicular to the structural 'grain'. Mark the cross section line on your map. Your cross section should include the following (and should be totally self explanatory); vertical and horizontal scale (note section lines), extrapolation of the structures into the air with dashed lines, extrapolation to a reasonable depth (think of any constraints you might be able to place on this), cardinal directions (top of map is north), and an explanatory key.

Several questions will confront you as you proceed. What colors to use? What fold style to use (angular, curved, parallel, similar, etc.)? The map itself will give you important clues to the structural style. How much does the map information constrain your interpretation? What dip to give to faults with little indication of dip? How far to extrapolate down to?

As far as steps in cross section drafting the following is a suggestion:

Ask yourself as you critique and revise your map - is the cross section a) geologically reasonable, b) consistent with the map?

Graph paper is very useful as the cross section base. Using Illustrator or inkscape makes the job easier in that they have guiding grids that can be used.

Be Neat! Don't hesitate to come to me with questions. You have two weeks to complete this one.

One of the three possible maps from which you will be constructing your cross section. Adobe Illustrator and or Inkscape file templates are available at Blackboard.


Apparent dip in a cross section

If you have a cross section that is perpendicular to the strike of a planar feature (e.g. bedding plane or fault), then its inclination in that cross section is the true dip. However, given that often strikes vary in an area, then one can expect that for some of the structure the cross section will not be perpendicular to some of the strikes. In that case, the inclination in the cross section view is something less than the true dip and is called the apparent dip. In the extreme case of a strike parallel to the cross section line the feature appears horizontal (an apparent dip of 0 degrees). If one considers the acute angle between the cross section and a given strike then the tan(apparent dip) is equal to the tan(true dip) * sin(acute angle between strike and cross section line). The table and graph above capture that relationship for 10 degree increments and can be used for estimation of apparent dips for your cross section construction.


 

The above is a simplified cross section of the north end of the Rio Grande rift in southern Colorado. Source of cross section: http://www.nps.gov/grsa/resources/geo_cross_section.htm

Link to Maryland geologic cross sections that can give you a feel for the structural style in Appalachian fold-thrust belt.

2013 - HDM