5 September 2012

Structures in the quartzite along the coast of Hermanus

Posted by Callan Bentley

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.