8 August 2012

What those geologists were looking at

Posted by Callan Bentley

Yesterday, I showed you a scene of geologists (including me) clustered around some (presumably interesting) outcrop. I asked what you thought we might be looking at. Howard Allen, a denizen of this part of the globe, immediately identified the scene as that of the downstream end of the Athabasca Glacier.

Several people guessed that we were looking at “glacial striations,” and that’s a good guess. Indeed there were plenty of those to be seen…

Some of these striae had transverse or lunate side scratches – a kind of hybrid chattermark / striation?

In most places, the stratification was more apparent (this is a knob of limestone bedrock we were examining), and we could see bedding. In places, the bedding was improved by folding in a rather kinky style:

We also looked across a (dry) drainage coming off the glacial terminus to an outcrop beyond:

…and we can zoom in to see more details…

Check it out: What do you see?

Here are a few items that I noticed:

The real attraction, though, were some exceptional pressure-solution seams (stylolites). Some of these were classic EKG-style (or seismogram-style?):

Others were fairly low-relief seams, which in places offset white veins of calcite:


Here’s another vein cross-cut by a stylolite, showing apparent “offset”:

But this isn’t actually “offset” – these aren’t small faults.

Instead, what’s happening is that the gray limestone (and the white calcite veins it hosts) have in places been completely dissolved away.

My favorite example of this phenomenon is this one:

Same outcrop, with the camera repositioned so that the bottom edge of the photo is parallel to the stylolites:

These calcite veins are not actually offset – they are dissolved, along with the rock that once hosted them. To reconstruct the volume of the rock that has been removed, you can use these calcite veins as strain markers, and break the rock out (along the stylolites) to align the veins along their presumptive former trend and thereby estimate the volume of rock that has gone into solution:

P sol, that’s what those geologists were looking at: evidence of rock dissolving under high pressures. Weird stuff.