Dwykacious Injectites

The famous Dwyka "Tillites". As previously ranted on this blog, these are not lithified tills! They are glaciomarine. As proof, I offer the drop pebble. Isn't he cute. (top of photo is stratigraphic top)

The Dwyka Group contains one formation, the Elandsvlei Fm., making it the city and county of San Francisco of geologic Groups. Sorry if that's too corny for you. The whole package is matrix-supported, laminated and massive diamictites. The massive ones are ridge-formers in the field area and the laminated ones are valley-formers. They are informal called "coarse" for the ridge formers and "fine" for the valley formers but I actually think the difference is in cementation rather than grain size, possibly having to do with more abundant clays in the "fine" laminated units prohibiting silica circulation. The matrix is glacial rock flour, a very fine sediment (quartz/felspar ground down to clay-size particles) which is unique to glacial erosion.

Anyways, along some of these facies-boundaries we have channel sands. These are often called "eskers" but they are not true eskers as this is not a ground moraine.
Super TA Nic perched on the stratigraphic top of one of these channel sands - the dip is to the left on the photo at about 40*S and Nic is sitting on the top of the deepest part of the channel. Original vertical thickness is about 2m and the sand body tapers to the foreground and background (these represent the edges of the channel). This particular channel deposit has a nice coarse, well-rounded conglomerate around the edges - like a gravel bar? The matrix of the conglomerate is greenish-gray rock flour, resembling the rock into which the channel cut.

Here's where things get even better (and by "better" I mean "more structurally interesting"). See that long spindly (~15cm thick) sandy arm reaching gently UPSECTION from the sandstone channel? IT'S A SANDSTONE INJECTITE! If you look carefully at the photo (click to enlarge if you need to) you will see that the lamination in the gray-green matrix is going roughly across the photo while the dyke cuts upsection (and up-photo) to the left.

Injectites usually form when a porous, saturated sediment is overlain by a less porous sediment. The overlying rock acts as a seal and doesn't allow the water to escape from the porous sediment. Pressure increases as the sediments are buried and eventually the porous rock can become very "overpressured", with the trapped water in the pores carrying the weight of the rocks above. This is an unstable state and can only persist as long as the overlying seal rock can withstand the pressure! Eventually, the sealing rock fails - usually along a planar or curviplanar fracture - this occurs when the overpressure reaches a greater magnitude than the weight of the rock, or some kind of disturbance (earthquake, passing landslide or debris flow) triggers the failure. The high pressure fluid/sediment mixture escapes its former captivity by injecting outward and upward along the fractures. When the pressure is released, the water is free to move off but the sediment is left behind in the fractures, forming "injectites" or "sandstone dykes".

The injectites are found in a particular stratigraphic horizon in the Dwyka Group (2c/3f contact for those of you in the know) where small sandy bodies are common. The sand is coarse, well-sorted and nearly pure quartz (C. Herbert pers. comm. last week in the computer lab). Injectites are curviplanar with roughly parallel surfaces (although they sometimes undulate out of phase). Thickness varies from about 25cm to 3cm in the several examples Nic and I stumbled upon while looking for faults fortuitously discovered. The outer surfaces of the injectites are very smooth and polished. They are now quartz cemented and weather out relative to the finer-grained, less well-cemented rock flour matrix.

The outer wall surfaces of the injectites have a very distinctive texture - I don't know if it has a name, but it's something like flute marks but sort of braided looking.... Can anyone help me out here? Has this been described before? Perhaps my dear friend the "former" geologist can help.
Have I ever told you that you can't un-geologist yourself? It's like finding out about santa claus carter. Your world has rocks in it and that makes you different... forever.
Eh hem, excuse me. Anyway, the anastomosing flutes are about a centimeter to 3cm in wavelength, with high amplitude (~0.5x wavelength) and vary quite a bit in length. Sources say that the famous Panoche Hills injectite complex in central California may show similar clastic-dike-margin-textures... but on a larger scale...

The geometries fo the injectites can be rather complex - they are even sorted with coarsening towards the center - reflecting increased flow velocities with distance from the conduit walls. Here's Nic again sitting on what is either a) a folded injectite or b) the complex branching/intersection of multiple injectites - Somebody should find out!
What can these surface textures tell us about viscosity and velocity of injectites, strength of sediments, and fluid pressure in the ancient sub-glacial-icey seas? Somebody should find out! Who should that somebody be? I'm hoping one of my future honours students....

In case you doubt that these sandstone channels are submarine, I present you... the drop-boulder. I'm sorry, so sorry for this but each time I look at this picture I think of Cornwallis.


More on the rock chip bug

Throughout my life as a geologist I have known a lot of folks who are torn between geology and biology... SuperNaturalists? Geobiogists? "Slug-cutters"? That one's from Andy Duncan who has a rivalrous relationship with zoologists who vie for his barstool.

One of these friends ended up sawing the heads off frozen timber wolves and grinding up extinct bird parts and putting them in a geology machine. Another ended up shipwrecked somewhere in the Inside Passage communing with Orcas and "dropped out" to go to vet school, via cattle ranching. Another dropped geology altogether (or so she says) and become a kayak guide and forester (but there are still a disproportionate number of rocks at her house). Anyway we've got one here, not sure what adventures she will get up to in the future but she right away sent me another example of the rock chip bug. He has a name! And it is Pamphagidae!

This little guy was caught in the grey limestones of the Cango Inlier near Outshoorn in the Cape Fold Belt. (Recall that I recently described his long-lost cousin in this post). The astute student also notes that she's seen them in suits matching red weathered dolerite in the Karoo. Looks like it's a pan-south-african bug.

So thank you to the sharp-eyed geobiogist who sent me this photo ( I make it a practice not to post students' identifying details unless they explicitly authorize me to, UCT geologists will know who she is anyway).


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!


Carried away at the Pizza Place

This is what can happen if you mark cross sections and eat pizza and there are crayons on the table. Particularly if you keep thinking about what you wish the cross sections would look like...


A word about ecotones in the Klein Karoo

OK I have never pretended to be a biologist in spite of a growing fixation on succulents, but there are a few concepts from biology that just seem REALLY IMPORTANT and stuck with me somehow.

The first is the "ecotone" - basically the line marking the edge of the area where a species is successfully living (actual definition is more systemic) - so maybe i don't know the right term but I'm trying to say, the edge of the world for a particular species.

For example, anybody ever driven the Extraterrestrial Highway? Otherwise known as NV state route 375? It was laid out to skirt the Joshua tree ecotone - literally - on one side (south) of the highway there is a semi-geometric grid of Joshua Trees as far as the eye can see. On the other side.... none.

Why do I like this idea so much? It's pleasantly mathematical - as in calculus plus binary - somehow asymptotic to one, then zero. It has a pleasing physics sensation like quantum mechanics - can't see the line when you're looking at it? And in a geological sense - to which it is often correlated, any way - it reminds me of the brittle-ductile transition. A hard and distinct theoretical boundary - but more often than not, shifted or curved by strain rate, temperature, moisture, other mysterious factors.... easy to describe in the abstract, hard to pinpoint in the field. LOVE IT LOVE IT LOVE IT***.

This absolutely beautiful, Georgia-OKeefeish, [absolutely cuter than any foram] pink Aloe plant reminded me of the ECOTONE concept. Succulent people - please name it ??? This beauty and her blushing sisters were found on one hill, in one mapping area, facing one way, on one formation. Literally about 50m3 of real estate, bounded by changes in slope, strike or lithology. Beyond that - another subspecies of different colour, size or shape. To a girl from the California Chaparal this explodes my little mind. Even the plants here are on geologic time.... and the geology? on astronomic time I think. Billions instead of millions. I know I left nothing for scale but that aloe is about a meter across.

Two formations down section - we found this little beast. He is in the lower Prince Albert siltstones, which are pink. OK this section is often madly folded and thickened but the stratigraphic thickness is about 10m. Maybe 15m. Above that: Dark brown siltstones and shales. Below: green glaciomarine diamictites. Literally this guy crosses geologic boundaries and his whole life plan for camoflage is over.
I'm desperately sorry I don't have a photo, but THERE IS A DARK BROWN ONE EXACTLY LIKE HIM IN THE NEXT UNIT UPSECTION. again, a perfect match to the chippy opally silty rocks.

*** You know what i got a lot of crap for recently? "ILOVEITILOVEITSOMUCH".
Turns out this is just more evidence that Americans are flakey. but you know what? IDO FRICKIN LOVE IT SO MUCH. It's that great to be here and see these things. No time for post-colonial understatement.

It's that Field Mapping time of year again

Hey folks I'm back from Laingsburg again with the 2nd year field trip. Every year it seems to get better and better as the sedimentologist and I iron out the plan and I get to know the field. This year's class seemed particularly keen and that makes it even more fun of course. And since they've let me know they found this blog, I'll say they were also clever and good looking, and don't forget to turn in your course evaluations kids. Anyway.

Here's the now traditional "first fold photo" (see 2007 first fold photo). Always a bit of a step up learning about trend and plunge and remembering strike and dip. This year I had an extra prac during 2nd year structure and we used it to do some mock mapping exercises. Seemed like it helped. We were forced by the weather to do things a bit differently this year because it snowed/sleeted on our first day of instruction. What the hey? this is not why I moved to Africa, to field map under ice. At least, not Quaternary ice...

Anyway I'm going to try to arrange some more inclement weather for next year because that was the only day the students complained about the mapping conditions. Awesome.

I digress for a moment to show you a cute tiny angulate tortoise. What could be cuter?

We return to customary annual photos with the "giant pencil cleavage fencepost" installment for 2008 (see 2007 giant pencil cleavage fencepost). This beauty makes use of the Laingsburg Formation turbidite sands.
Sunrise on the last morning in camp:
Time to rise and shine!
more hard core geology posts to come!