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

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

heim

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:

belem

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

heim

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.

Enjoy!

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.

0 thoughts on “Fossils as strain markers: the boudinaged belemnites of the Swiss Alps”

  1. Declan points out that this simple calculation is not a perfect technique:

    Regarding your calculation, that is the original Badoux (1964) approach (also in Ramsay 1967). However, it significantly underestimates the strain in the bulk of the rock. For example, it assumes no stretching beyond the end fragments. Hossain improved the method by measuring from the centers of the last gaps, not the edges of the last fragments. But the best estimate of rock strain is given by Ferguson’s sequential strain reversal, sometimes it’s 50% more that the Badoux method.

    It would also be worth pointing out to readers that belemnites record only stretching, not shortening. Unlike veins, they do not fold and then boudinage, so they are most useful when the entire bedding plane is in the field of elongation throughout. If a belemnite starts off in the shortening field, it will rigidly rotate unit the line of no infinitesimal strain and then start to boudinage, so it will record only the latter part of the strain history.

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  2. Callan, do you know the location those belemnites came from? I’m over in the Rhone Valley near Martigny in a few days time and would love to have a look around
    Thanks Linda

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    • Unfortunately, I don’t seem to have that information handy. I just looked in the places I thought I might find it, and I couldn’t locate it. Bummer! Good luck tracking them down.

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  4. For future reference Linda (and anyone else interested) they most likely came from a slate quarry at Leytron, in Valais – not far from Martigny. My (dim) memory of a brief visit on a student trip was of some scree/spoil on the north side of the valley at Leytron, accessed via a minor road. 46.192131, 7.209487 on Google Maps (roughly).

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