Structures in the quartzite along the coast of Hermanus

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The coastal town of Hermanus, South Africa, is a nice place to have a cup of coffee and take in the morning light…

But as the caffeine gets the old pattern-recognition centers fired up in the brain, and before long you start to notice stuff. Consider this view, for instance:

A lovely coastal view… but did you see the cross-bedding?

These are quartzites of the Table Mountain Group, the same ones I highlighted at Cape Agulhas.

Looking the opposite direction down the coast (east), you can see these beds and cross-beds overprinted by a steeply-dipping joint set:

And what’s up with this steeply dipping surface?

It’s steeper than the cross-beds, and repeated down the coast a short distance: Could this be a small “ramp” fault (blue)?

Or maybe just a series of parallel moderately-seaward-dipping fractures, tectonically insignificant…?

We also saw some veins of quartz that cut across the quartzite layers…

Annotated version: Did you see the birds (quail, I think??)?

Quartz veins like these form due to brittle failure of the rock and dilation (opening) of the resulting fractures. Hydrothermal solutions rich in dissolved silica deposit milky quartz in the resulting void space.

But not all deformation has been brittle! Just as with the folded quartzite at Cape Agulhas, the cliffs of Hermanus have a place where you can see some ductile deformation of these quartz veins:

You can make that one bigger by clicking on it. Here’s an annotated version thereof:

Let’s zoom in to a few portions of this scene…

First off, that central spot where the veins get all loopy:

The orientation of the veins and the folds which deform them both speak of “left side up” kinematics (left is north in this view; right is south):

Another spot shows a set of en echelon quartz veins that have been truncated by a fault:

This is a vertical outcrop. The movement on this fault is not apparent to me – the “footwall” block doesn’t show the vein-halves that appear on the “hanging wall” block. Without these markers, we’re only guessing at the kinematics. It could be that the upper block moved towards our perspective, or away from our perspective, but this isn’t a simple bit of dip-slip movement.

Same thing here, with the additional characteristic that the fault itself has also become a quartz vein by dilating and being filled in with milky quartz precipitation.

On a nearby horizontal outcrop, I saw some similarly oriented fractures that showed significantly less dilation:

Some were definitely quartz veins – look in the shadowy area at lower left, and just above & to the right of it.

So I guess this is the same thing, en echelon tension gashes, though the veins are much thinner (some only appear to be fractures, with minimal vein-accomodating dilation) and closely spaced:

A lovely spot to clamber about in the early part of the day, checking out signatures both clear and mysterious in these rocks.

0 thoughts on “Structures in the quartzite along the coast of Hermanus”

  1. I’m some what new to the geology world and love reading your blog to expand my knowledge. Usually I have to look up certain words to make sure I have a clear understanding of your explanations (no offense to you, just new and trying to learn more).
    So I was wondering why there would still be cross bedding structure left over from the tectonic event that would have changed it from a sandstone to a quartzite? Wouldn’t the structure be destroyed from the metamorphic event?

    Thanks,
    -Laura

    Reply

    • Laura,
      Good question! I’m using “quartzite” in the vernacular sense here – I think that sensu stricto, these would be quartz sandstones. But a lot of geologists, including those I talked to in South Africa, switched freely and loosely back and forth between sedimentary and metamorphic rock names. (Elsewhere, I heard a schist described as a “shale,” for instance.)
      That being said, primary sedimentary structures CAN survive metamorphism if the conditions are right. I’ve seen metamorphosed graded beds, metamorphosed pillow basalts, metamorphosed cooling columns, and even metamorphosed cross-bedding in a genuine quartzite. So it can happen, yep.
      Thanks for reading,
      Callan

      Reply

  2. I think I know the answer to the one-sided vein array along the fault – see Griffith, W.A., A.J. Rosakis, D.D. Pollard, and C.-W. Ko, 2009. Dynamic rupture experiments elucidate tensile crack development during propagating earthquake ruptures, Geology, 37, 795–798, doi: 10.1130/G30064A.1.

    Really cool!

    Reply

  4. Thank you Callan. I have been searching and searching for some clear photos of coastal quartzite and yours are superb; by far the best for my needs – not to rip off; just to use as descriptive inscription.

    Reply

  6. Thanks for posting this. I have been writing several pieces on South Africa while I’m living here, and since I wouldn’t even qualify as an armchair geologist, I have to seek out websites like yours to better understand the science behind the beauty around me. I will be returning for more.
    FYI those are guinea fowl (the birds).
    🙂

    Reply

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