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Maps are sections

Little of my late 1960s undergraduate coursework was any use in my career as a geologist. The exception was the time we spent in the field mapping outcrops, and a senior staff member impressed on us that ‘a map is a section’. Over beer late one night it occurred to me that a geological map is a section not just in 3D space, but it also represents a slice in geological time. That seed idea lay dormant for a few years but then I found a way to build a career and a company — Midland Valley — out of it.

What makes a geologist, and what we are mostly paid for outside of academia, is the ability to tell a story about the geological evolution of an area, and to make maps and sections of the earth’s crust at a scale of a few tens of metres to a few kilometres or sometimes tens of kilometres. Mapping and section-drawing skills are at the heart of this, irrespective of whether you are working in engineering, mining, hydrocarbons, or environmental management. In my view, mapping and section building are the core geological skills.

Throughout my career, first as an academic and then in industry, I have been astonished at how poor our skills are at constructing cross sections and maps, relative to other things we do as a profession such as geochemistry or geophysics. Much of what passes for predictive cross sections, while they may honour the data, make no more sense than an Escher staircase. A typical specimen looks good in 2D but makes no sense in 3D let alone in 4D as it has evolved through geological time. If we are to look after our planet more effectively, given our need to exploit it to maintain our societies, understanding the subsurface is more important than ever.

A map really is a section. Any geologist should be able to draw an accurate section across the map that is compatible with it and the 3D object it depicts. The key is to know the stratigraphy so you know what’s up and what’s down. Then work out the plunge of any structure depicted and the dips of the major structural panels. Don’t forget that rule of first-year class — if the surface dips up the valley it will ‘V’ up the valley. Then the key trick is to place yourself so that you are looking down the plunge and then the map will project onto the section. Try this by slicing up a cardboard tube with a sharp knife. Once you’ve mastered this concept you should be able to take a map and sketch a cross section across it in a matter of minutes.

If you are looking at a section plane that is not perpendicular to plunge, commonly a strike or oblique section, all the dips will be apparent dips and less than true dip. A lot of geologists get this wrong, especially when drawing faults. If, for example, your section crosses a normal fault perpendicular to plunge then you will see the maximum dip of the fault say 65–70 degrees. If your next section is perpendicular to the first section, the same fault could appear nearly horizontal (see Learn about seismic). All of this uses standard projection and graphic techniques that tend to be relegated to undergraduate mapping classes and then forgotten. Powell (1992) probably provides the most comprehensive guide with numerous exercises ranging from the simple to fiendishly complex.

Using any starting point from a conventional map, it will usually be possible to build a number of seemingly plausible sections. Using a synthetic dataset, Bond et al. (2012) showed that professional geoscientists come up with a wide range of interpretations and only a few arrive at the ‘correct’ solution. She also showed that those who checked the section by thinking about how the geology developed were three times more likely to be correct than those who did not.

Here are some of the quick checks of geometry and geological sense that I use when drawing sections. This is a process known as ‘balancing’:

  • Do bed lengths add up across your section?
  • Can you jigsaw your model back together and understand how the folding might be associated with faulting?
  • If you see dip changes across faults, check whether this is due to a shaped fault or if the displacement across the fault is oblique to the section.
  • Check that fault displacements vary systematically.
  • Check that area or volume changes are compatible with your understanding of the geological history.
  • Is there an alternative hypothesis that could help explain the geology?
  • Is your model compatible with your understanding of the regional picture? Discrepancies need to be resolved.
  • Single sections across strike-slip faults or shear zones won’t balance in 2D. On the other hand, make sure you don’t just turn everything into a strikeslip deformation zone!

References
Bond, C, R Lunn, Z Shipton, and A Lunn (2012). What makes an expert effective at interpreting seismic images? Geology 40 (1), 75–78, DOI 10.1130/G32375.1.
Powell, D (1992). Interpretation of geological structures through maps: an introductory practical manual. Longman Scientific and Technical. 176 p.

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