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!
Thank-you for presenting this blog in this fashion, as if the reader is along on the trip with your curiosity and learning. I enjoyed reading it, and exploring the photos with you.
Interesting rocks! I wonder how much pressure solution has gone on here. Some of that wavy “bedding”(?) almost has a stylolitic look to it (e.g. the lower half of photo 4), with apparent small-scale truncation features. I don’t see any “classic” stylolites, but with a carbonate this old (and full of garnets!) I’d be very surprised if there wasn’t some pressure solution going on at some point. Perhaps the compositional layering has resulted in differential pressure solubility–are those buff coloured layers sandy? Maybe some of the layers have been condensed by pressure solution and others less so or not at all. Idle speculation. It would definitely be fun to spend some time gawking at this exposure!