25 July 2011

Swift Dam

Posted by Callan Bentley

Dave Lageson of Montana State University and I ran into each on our respective field trips sometime last year outside of the Two Medicine Dinosaur Center in Bynum, Montana, and he told me that I had to go check out the Swift Dam area, a little ways northwest of the town of Depuyer. So, a year later, finding myself in Depuyer (as one does), I finally did.

One thing I saw there were fantastic exposures of Mississippian coral heads:

But the real attraction was the asymmetric anticline exposed at the mountain front. All those coral heads are part of a massive slab of limestone that has arched up and slid over top of more mushy Cretaceous shales and sandstones. Here’s a views of the structure, dramatically exposed in three dimensions, looking south:

Below it are pulverized and squished up Cretaceous shale/sandstone layers (view is to the east):

[gigapan of the folded Cretaceous strata here]

As a result of all this tectonic shoving, the limestone has fractured, and dissolution has highlighted those fractures:

(Note the wedding ring there? Nice, eh?)

The most striking fractures to my eye were those that were extensional in nature, on the outer edge of the folded limestone slab. They trend the same as axial plane cleavage, as this view to the west shows:

It’s like watching a massive Mississippian breaker coming at you while you’re knee deep in the Cretaceous shallows, eh? And your surfboard is nowhere in sight!

Zooming in closer to see the pervasive extensional fractures, parallel to axial plane cleavage:

I love the streams of water providing a vertical level for your eye — somewhat unbelievably, the dam’s spillway channels excess water right over the nose of this fold!

Here’s a gigapan of this amazing scene:

A quick iPad sketch of this place, rendered as a cartoon, is here:

Here’s the spillway up top:

And a view of the fold from the south, looking north-ish, and featuring Swift Dam itself for scale:

Note how the layers tilt over towards steeper angles towards the right (east). You can click that last one to make it bigger, or here’s a gigapan version of (part of*) this same image:

* The gigapan ran out of batteries 2/3 of the way through. Grumble grumble…

The deformation isn’t limited to the mountain front. Here’s a syncline that caps a hill top to the east (reminds me of another from Patagonia):

And here’s the mountain north of the dam exposure, part of the next thrust complex up in the stack (and therefore probably older than the one exposed at Swift Dam itself):

Lighting is different because I shot this one the next morning, so it’s lit from the east in the “golden hour” light. Perspective is to the north. You see a plunging syncline and underneath it an apparent overtuned anticline, a mirror image to the situation at Swift Dam itself, vergence to the east. Interestingly, there are two apparent axes to these folds at right angles to one another, probably a trick of the perspective, but perhaps it’s something structurally complex?

Anyhow, I’m glad I visited. Lily and I spent a lovely evening here, in spite of someone horking our bag of cooling beers from the river. You should stop in, if ever you find yourself in Depuyer with a hankering for in-your-face structure and paleontology.