9/13/2008

Ptygmatic Folds in the Prince Albert Shales

The first exercise I do with the students when we begin working in Laingsburg is to have them run around just outside the field station and find a fold. Luckily this is not so hard as the Prince Albert Formation here is wrinkled like a raisin. I have some theories about this.
Sorry for the crap outcrop photo - should have taken the advice about
"morning vs. afternoon shots".

The Prince Albert Formation is early Permian in age and directly overlies the Dwyka Group diamictites dating from the Carboniferous-Permian Gondwanide glaciation. The paleo-latitude is about 70°S. I have a massive pet peeve about people calling the Dwyka Group "tillites" because that term clearly implies terrestrial origin and the rocks are very clearly submarine. The first person to describe the rocks (Alex du Toit in 1929) may not have known this but the very detailed work of JNJ Visser in the 70s and 80s made very clear the glacio-marine origin. If there's any doubt, please see: dropstones everywhere throughout the sequence. So don't say "Dwyka Tillite" to me unless you want to hear more about that. But I digress! I'll return to the Dwyka in future posts because I'm finding it more and more fun. Rock flour is weird.

Anyway, the Prince Albert Formation overlies the mudcracked glacial muds at the top of the Dwyka. It has a few meters of grayish pink shales/siltstones, followed by a few more meters of chestnut brown shales/siltstones, and then a few more meters yet of black shales/siltstones with blue phosphate nodules (and at least one 8-cm thick black chert bed, discovered by my students this year! Shout out to Klipdrift Group!). I'm being vague about thickness because nearly everywhere we see the Prince Albert in the Laingsburg area it is majorly folded and faulted and we actually have no clear idea how thick it should be. Looks like it was the preferential site for accomodating strain in the stratigraphic sequence. I would guess it is related to the big hard massive Dwyka diamictite below, and the relatively deformable Ecca Group turbidites above - those two strain very differently and the Prince Albert had to squish around to make up the difference. That would be the technical term of course. You see - this is how I check whether my students are plagiarising my blog.

Anyway, you can see in the photo above that some beds make nice concentric folds and others make parallel folds (see different shapes within the little syncline on the right). On this local scale, the thin yellowish ash beds flow into the hinges to accommodate the different shapes of adjacent folded beds. I think the Prince Albert Formation on a regional scale is doing the same thing the yellow ash beds are doing on this outcrop scale - flowing toward the hinges.

Maybe you need a refresher on folding mechanisms? Check this one out.

Do you see the amazing cleavage refraction?

Our class rep, also known as "the ginger kid" (in his own mind at least)
is impressed by the faulted folds.


The style of folds varies between the pinkish grey, chestnut and black members of the Prince Albert, I think as a function of viscosity variation in the strata. In the basal pinkish-grey member, there are lots of fine clayey shales and a few "siltstone" beds which are pretty hard and quartz cemented. This results in a thin stiff layer in a low viscosity matrix - the necessary condition for... (drum roll please)... PTYGMATIC FOLDS!






3 comments:

Silver Fox said...

Oooh, neat ptygmatic folds! Great photos of those, especially the sideways one - kind of unusual.

What is cleavage refraction? Something about the faulting?

kes said...

I second the yay for ptygmatic folds! Saw the set in Little Poleta for the first time this summer and I'm jealous of those. Great picture!

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