2 April 2012

Castner Marble

Posted by Callan Bentley

The oldest rock we saw on my post-InTeGrate field trip (led by Josh Villalobos of El Paso Community College) was the Castner Marble, apparently 1.2 billion years young, a metamorphosed limestone with pronounced compositional layering. It’s exposed in the Franklin Mountains of west Texas.

As far as the layering is concerned: Is it bedding, or is it foliation?

Some areas show waviness in the layering:

This could be soft sediment deformation, or maybe it’s folding:

In addition to that, in some places, little scattered dark red porphyroblasts make themselves obvious:

You probably will not be shocked to learn that these are garnets. In some spots, they get quite large:

But the more pronounced characteristic is the compositional layering…

I first saw this layered marble in the hallway display cases of the UTEP geology department:

There was also a nice “bookend” made of the same stuff in the meeting room where the Geo2YC advisory group met for the InTeGrate meeting itself:

Back out in the field, I found a clue as to the true nature of the compositional layering. Check this out:

That’s a flat-pebble conglomerate — the signature, presumably, of ripping up of semi-cohesive slabs of limy mudrock in the Mesoproterozoic sea, due to some extra-energy boost. The meteorological and sedimentological equivalent of a Red Bull.

So! We have an answer – – the layering is indeed primary sedimentary layering: it’s bedding!

In another spot, I saw some stuff that looked like soft sediment deformation, or maybe bioturbation (yes, I remember that it’s Mesoproterozoic, and therefore ostensibly there were no macroscopic animals capable of producing such bioturbation…)  Again, this was confined all to one layer…

We also saw a tabular mass of breccia, cross-cutting the (now, apparently) sedimentary bedding; perhaps it is an injectite of some sort? Or a slightly dilational fault breccia? Josh has his left hand on it in this shot:

Zooming in on details, like the big tabular block at upper right:

Zooming back out a bit,

…and back in for a final close-up:

A lovely little Texas scene with the ancient rocks (some fresh, some weathered) and the modern plants:

Now, what’s this thing?

One possibility is that it’s a dropstone:

The sedimentary layers curl up over the top of this thing, like Von Karman vortices:

Or… something else? I’d love to hear a more sensible interpretation of this odd outcrop…

A last look at a class Castner Marble outcrop:

I’m so pleased that Josh led this trip, and that I got to go!