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I’ll admit it: I spent more time in the field this fall mapping a trench for my B.Sc. thesis than was actually necessary. Part of the reason was that I wanted to make sure I collected quality field data for the project, but another part of it was that I enjoyed the excuse to get out there into the near wilderness and spend some time alone. However, now that a respectable layer of snow is blanketing southeastern Manitoba, I suppose it’s time to face reality and start treating some of that data.

The first task? Take my structural measurements – foliations, lineations, joints, and veining – and plunk them into a stereonet. For my joints I’d already done this by hand a few weeks ago (mainly to see how they looked), but for ease of use I opted for GEOrient – stereonet software which is free for academic use. Using software is a treat because you simply export a spreadsheet of your structural measurements into a tab delimited .TXT file from Excel or Open Office, and import it in GEOrient, which itself is an easy task.

Unfortunately, while GEOrient does a nice job of plotting your points on the stereonet, it’s not very robust when it comes to the visual display of pole contours. Consider this plot, for example:

GEOrient foliation plot

It does an excellent preliminary plot, but I notice two things: 1) visually it needs to be retouched A LOT, and 2) there’s a potential problem of the 8% contour continuity across the great circle. The first is no real problem, and no real criticism of the software itself – it aims to plot data, not to be a state-of-the-art graphics package. Things can easily be touched up with other graphics programs such as Illustrator, as exemplified by the final version:

Final version using Illustrator

The second, however, is a potential problem, and I’m definitely going to talk to my advisor. In the original, the contour interval crosses the great circle on the NE quadrant, but does not in the SW quad. In the final version I’ve gone ahead and manually traced it. Additionally, the retraced contours were smoothed in Illustrator to provide a better presentation.

However, this potentially raises another issue: I’ve essentially tampered with the data. By extending and smoothing contours, I’ve taken a representation of the plotted data and altered in a manner that looks better and “makes sense”. That said, I’ve only “tampered” with the data if the original GEOrient plots were correct in the first place. Pole positions are certainly correct – it’s quantitative strike/dip data – but the automated contouring is where the trickiness comes in. Although the final stereonet looks pretty slick from a design standpoint, I have to wonder if the contouring can be accepted with a degree of confidence. I’m not sure it can – I’ll have to give it some thought. I think I’ll also try other software packages, as well as hand plots, to compare results between the two.

Ultimately, from a practical standpoint, a stereonet plot can overcome these minor issues with ease. From my final contours I can confidently assert a general, preferred orientation of my foliations. Yes, there is some scatter, and yes, there are some odd things going on, but this is geology after all – and just a first treatment of the data. I’m thinking the next step will be to break down foliations between lithologies in the trench to see if there are distinct generations, as well as potentially identify different structural sub-domains throughout the linear extent of the outcrop.

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