29 September 2017

Friday folds: soft sediment deformation in thin sections of MTD sandstone

Posted by Callan Bentley

For your Friday fold delectation, I offer you these GIGAmacro images of thin sections of the exquisite mass transport deposit (MTD) above the Spechty Kopf Diamictite (latest Devonian) in the exceptional roadcut just east of the Alleghany Front along Corridor H in Hardy County, West Virginia. This was a package of interbedded sand and mud that collapsed into deeper water en masse, deforming internally as it went. This is soft-sediment deformation, not post-lithification tectonic deformation. We dubbed the folded sandstone bodies “ploudins,” as they were sort of half “pillow” (as in ball & pillow soft sediment deformation) and part “boudin” (as in boudinage). Explore the ploudins’ internal structure these enormous images of the slides under plane and cross-polarized light. Search for graded bedding, cross-bedding, folding, faulting, organic debris (carbonized plant scraps), and maybe even a zircon or two. These images are Flash-based, so if you don’t have Flash, you can’t see them. Enable Flash on your browser, or click the “link” after each one to go to a non-Flash source page.

Here’s an annotated “cheat sheet” to guide your eyeballs on the first one:

These slides were prepared by John Weidner, a former math professor and a dedicated volunteer in our department.

Here’s a GigaPan of slide JW46 under cross-polarized light:
Link 2.19 Gpx GIGAmacro by Robin Rohrback & Callan Bentley

Here’s a GigaPan of slide JW46 under plane-polarized light:
Link 2.71 Gpx GIGAmacro by Robin Rohrback

Here’s a GigaPan of slide JW45 under cross-polarized light:
Link 2.10 Gpx GIGAmacro by Robin Rohrback

Here’s a GigaPan of slide JW45 under plane-polarized light:
Link 3.22 Gpx GIGAmacro by Robin Rohrback

My former student Maddy Rushing and I have been working on a characterization of this outcrop which we plan to present at GSA in Seattle next month. We’re using these sort of images as well as some other new digital toys to more fully describe and interpret the outcrop, in particular the Spechty Kopf Diamictite, which appears to be glaciogenic. But once you throw in plant fossils, changing oxidation states, and a mass transport deposit, it all starts to look a wee bit more complicated. Another volunteer in our department, Sarah Yun, has just completed a new suite of thin sections of the diamictite’s massive and laminated members. I can’t wait to show them off! If you’re available, Maddy’s talk on this work is Monday at 4:55pm in Bryan Turner’s and Shannon Dulin’s mud rock session. Please come!