Back to the desert: Diamictite strain analysis explained

After a week of tromping around Virginia’s Piedmont backcountry, it was time to get back to get back to my metadiamictite from the Oman desert. The fresh air was a nice reset and I was able to come back to the McG TC with a crisp get-it-done drive.

First order of business: cut more sample slabs. My Gladiator colleagues needed to cut their samples so I was able to tag along to the saw room and slice my rocks. Because my samples are a conglomerate with a silty/muddy matrix, they cut very easily, and even fall apart along fractures at times.  The Gladiators’ rocks were as tough as the Gladiators themselves –because of their igneous composition– and taught a valuable lesson of patience with the rock saw. Where the saw would leave banded marks on the slabs because it would jam or need to be adjusted, we could then take the samples to the grinder to polish the surface and make them more presentable. The diamictite didn’t need to buff out any marks, but I polished a piece for the experience and found that it took far less time than the Gladstone rocks for the same reason that it cut easier.

After I cut enough surfaces, I photographed the samples and imported the pictures into Adobe Illustrator. Some of my samples are larger than others, which allows me to cut more slabs. This week’s 2 samples yielded 18 faces that all ended up looking similar to this one from 18GB5 when I was finished with them:

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Whether you’re like Chuck and think it looks like a sick (ill) fish, or like me and think it looks like a sick (rad) fish, hopefully you can appreciate the beauty of this sample. The dark matrix contrasts nicely with the clasts and my chosen sea foam green tracings, and has a smattering of various lithologically varied casts.

This slab was cut in a way that we believe exposes the YZ axes (intermediate and maximum shortening) so I expect the clasts here to be less deformed than those measured from a slab that runs perpendicular to this plane. Although the clasts don’t all look to have been deformed the same amount (because of their differing lithologies) the majority appear to have a long axis oriented in the same general direction: angled more or less 30° to the right of the frame. This is important when trying to orient the sample in real space; more about that later in the summer.

The first step to strain analysis is figuring out what you’re looking at. I begin by numbering the grains and identifying the lithology of each in my notes. Most I can use a hand lense with, but sometimes I need to head to the microscope for the tricky ones. This will help me later with my hypothesis, as I anticipate the sedimentary clasts (siltstones, sandstones, mudstones, diamictites) will have deformed more readily than the igneous and quartz clasts I’m finding.

After everything has a name and a number, I can trace the clasts and find the Y and Z axes. The Y is the longest part of the clast from end to end, and the intersection of the Z axis should be representative of the center of the clast, and at right angles to the Y.I then measure the length of the axes and the phi angle of relative to 0.These values all get shipped to The Spreadsheet (Chew, 2003). This tool is invaluable, as it performs the mathematical component of the analysis for an entire slab in a fraction of a second, whereas it would likely take me the better part of an hour for 5 clasts.

I am finding that not all of my samples exhibit the same strain ratio, suggesting that my study site may have enjoyed heterogenous strain. This was not wholly unexpected, especially because I have samples that were collected kilometers apart and from different units, but does add an additional layer of complexity to my project. I am curious to see what my next week’s results say about the Ghubrah Bowl’s tectonic history.

Note: This post is for the week of June 10-15, 2019