Tuesday, January 22, 2013

Modern analogues: an a-ha moment

This photo is from my first experiences snorkeling and my first time using an underwater camera. It was in 2006 and I was on vacation (in Fiji) from my job as a geophysicist. While I was in the water I was more focused on seeing this strange (to me) new world and learning how to capture it on film a memory card than thinking about carbonate environments and deposition.

When I looked back at the photos a few years later while I was logging carbonate cores for a class, this one caused an "a-ha" moment. As in, "a-ha, now I understand boundstone."

There are several classification systems for carbonate rocks. As a student, I always found carbonate rocks confusing and complicated and intimidating. I still do, but, now that I've had a the opportunity to snorkel on three different reef systems (the fringing reefs of the Fijian islands, the Belize Barrier Reef and the Great Barrier Reef) it is a bit easier for me to visualize how limestones form.

Boundstone is part of the Dunham classification of carbonate rocks. It's a term for carbonate rocks that are bound at deposition. And in this picture, you can see how that happens. These two speckled butterflyfish (Chaetodon citrinellus) are swimming around coral that is being filled in by sand, shell fragments, and other carbonate detritus. It's being bound in place.

If you want to take it a step further, you could use the Embry and Klovan classification, in which case I think this might be framestone (I hope there are carbonate sedimentologists reading this who can correct me in the comments if I'm wrong).

I am actually fascinated by the whole carbonate classification process. The two dominant classification systems were developed by Dunham and Folk (I met Bob Folk very briefly once, and he was very kind). But their classifications weren't detailed enough for the rocks that Embry and Klovan were looking at, so they expanded the Dunham classification, particularly of boundstone (Embry and Klovan break boundstone up into bafflestone, framestone, and bindstone). I've also been lucky enough to have several conversations with Ashton Embry, and his enthusiasm for geology and knowledge about the Canadian Arctic are amazing.

I think one of the most important things about modern analogues is that they remind us that what we see in the rock record is often formed by processes not instantaneous events. As the space around the coral above is filled in, whether or not it will keep growing up and extending will depend on the relative sea level.

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