9/30/2008

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.

8 comments:

Trifarina said...

You know what injectites mean don't you?! MONEY!

Funny how this glacial rock flour keeps surfacing in my life this week. Collect some and we can brew up some yarn!!!

BrianR said...

oooh ... injectites!

From the photos you show and the descriptions it sure seems like that's what those coarse-grained 'bodies' are.

I'm second-author on a paper about some rather large (~60 m wide) conglomerate injectites we documented in Patagonia (see blog post about that here). It was in an entire AAPG Memoir all about clatic injectites ... mostly subsurface examples, but a handful of outcrop examples as well.

The texture of these things are indeed distinctive ... I'm not aware of a proper term ... they always looked kind of 'ropey' to me.

Regarding internal organization, we observed more muddy matrix on the edges and more sandy matrix in the interior of the dikes. We also saw a 'fining' trend in one intrusion that cut up through the stratigraphy.

In terms of the detailed processes related to the emplacement of these things, I don't think we yet have a good handle on it. Our paper was very descriptive and didn't go into a lot of detail re mechanics.

Would be a great thesis project to look into it in more detail.

snowflyzone said...

The best part about spanning two disciplines is that I can claim either one at any point in time.

Because you've called me out, I have a clarifying question about your injectites. Are they composed of layers that have one set of waveforms with each layer having a different set of superimposed waveforms (the braided texture) on them? Or am I reading this wrong? You know you're just itching to whip up a quick illustrator sketch. Also, do the injectites form sub-glacially, or are they post-retreat?

Trifarina said...

Wait a minute! Carter is claiming she's not a geologist? Unfair labeling practices.

Anonymous said...

I've seen something like this at the other end of the Karoo sedimentary sequence up in the Freestate on the Lesotho border - a sandstone dyke intruding up into the basalts. Much the same mechanism I expect: Had a couple of good discussions on that outcrop and I have still failed to convince the owner of the farm - a non-geologist.

Fault Rocks said...

snowfly:
1. yes
2. neither - they are subaqueous. supra-aqueous ice (bergs? floating sheet?) dropped the d-stones.

Did I just come up with a good band name? OK it's not great.

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