12 December 2016

Fossils as strain markers: the boudinaged belemnites of the Swiss Alps

Posted by Callan Bentley

Don’t you hate it when plate tectonics ruins a perfectly good fossil? This is a sketch of a belemnite from the Swiss Alps:


The thing has been broken into segments, with calcite filling the gaps between the segments. What a bummer! Now we’re going to have a much harder time reconstructing the life habits of the organism that left this fossil behind… It was a squid-like thing, with an internal skeleton made of calcite, roughly shaped like a bullet. That’s the “hard part” that fossilized and is so hopelessly shattered in the example shown above:


… But wait! Perhaps there’s a silver lining to this fractured fossil. Let’s take another look:


Structural geologist Albert Heim realized that this is a terrific resource for quantifying the change in the rock’s shape. We can use the original fossil material coupled with the new vein material to get a sense of how deformed the rock containing them is.

You can measure the length of the individual dark colored fossil segments, and you can measure the overall length of the final post-boudinaged fossil, and you can compare them.

Original length, lo (black belemnite pieces only) = 82 mm

Final length, lf (black segments + white vein material in between them) = 185 mm

Now we can calculate elongation (e) and stretch (S).

e = (lf-lo) / lo = (185-82) / 82

e = 1.3

S = 2.3

So the sample was lengthened by +130% at least, resulting in a final fossil-adjacent portion of the rock that was 230% as long as it originally was. *

I bring all this up because I recently came into possession of three samples of deformed belemnites from this same site (or a similar one). My colleague Declan De Paor is retiring, and he turned over much of his structural geology rock collection to me. I’ve made GIGAmacro images of the specimens, for the sake of sharing them with you here.


Link 0.73 Gpx GIGAmacro by Callan Bentley

Link 1.24 Gpx GIGAmacro by Callan Bentley

Link 0.44 Gpx GIGAmacro by Callan Bentley

* Declan points out in a post-post email that this technique is flawed. See the details in the first comment below.