GEOS 304 Lab 11: Introduction to Fold and Thrust Belt
Geometry and Balanced Cross-Sections
Spring 2002
Name:________________
Introduction
Fold and thrust belts (FTBs) are some of the best studied compressional structures in the
world, largely because of their potential to contain significant oil and gas reserves. Wellknown examples include the foothills of the Canadian Rockies, The Appalachians, the
Sevier FTB of Wyoming and Utah and the Himalayas. Driven by the search for oil and
gas, many of the techniques that geologists use to create cross-sections were developed in
the Alberta Foothills of the Canadian Rockies. A key idea among these techniques is the
concept of structural balance. In its simplest form, this is the idea that for a cross-section
to be even possibly correct, you must be able to restore the beds in it to their original flatlying geometry without creating or destroying rock. In other words, you need to be able
to go from the deformed state shown in your cross-section to the original depositional
geometry through geologically reasonable steps.
Your Mission
Construct a balanced cross-section through the Wildcat FTB along the line A-A’ based
on the geologic evidence given.
What to Turn In Next Week
1. A completed, balanced cross-section with an aerial projection (60 pts)
2. A restored cross-section showing pin lines, locations of future faults and restored bed
lengths (25 pts)
3. A calculation of the amount of shortening across the Wildcat FTB (5 pts)
4. Answers to the following questions (5 pts each)
A. Does your shortening calculation depend on where you put the pin line? Why or
why not?
B. This fault has what is called a ramp-flat geometry. In some locations the fault
surface is “flat” (parallel to bedding) and in other places it “ramps” (cuts up through
bedding at a more or less constant angle). This geometry is very typical of large
thrusts. Why do you think this happens? Hint: think about what the different units
might be (i.e. shale, sandstone, etc.) and then try to relate the mechanical strengths of
those rocks to what a major fault might do in them (ramp or flat).
Other Useful Information
Typical structures of FTBs: Thrust faults, fault bend folds, fault-propagation folds, folded
thrusts and growth structures (see figures in Davis & Reynolds).
Balanced cross-sections: The basic idea behind balancing a section is to create a realistic
(though not always correct) 2-D representation of the sub-surface structural geometry. In
order for a cross-section to be realistic, you must be able to retro-deform it, that is to
restore it to the original flat-lying sedimentary stratigraphy without creating any gaps or
overlaps in the rock.
Cross-section construction: also refer to Davis and Reynolds p 669-679.
1. Assemble available data. This includes geologic maps, stratigraphic sections, and
sub-surface (well, seismic, gravity, etc.) data. Examine the map and try to get a
general idea of the region’s structure.
2. Choose where to draw your cross-section. Create a topographic profile (with no
vertical exaggeration) and transfer data gathered in step 1. This includes locations of
lithologic contacts, fold hinges, fault traces, and any subsurface data. Remember to
correct for apparent dips and carefully consider how to project data onto the line of
section.
3. Begin to build the section downwards. Used pencil!!! Draw in the eroded part of the
cross-section (the aerial projection). Use dashed lines for the aerial projection.
Remember that you have to honor all the geologic data. Make sure that you maintain
constant bed thickness. It is also important to keep in mind how the section will
eventually restore.
4. Create a pin line and restore your section. The pin line should be through an
undeformed part of your cross-section. Show where the faults will be using dashed
lines. If you can not restore your cross-section (i.e. bed lengths are not constant,
faults do not have reasonable geometries, etc.) then go back to step 3.
5. Calculate the amount and percentage of shortening accommodated by your crosssection. This is very similar to calculating the stretch of an object. Amount of
shortening is final length minus original length (lf-lo). Percent shortening is final
length minus original length divided by original length multiplid by 100.
(((lf-lo)/lo)*100)