3 January 2013
Inside the French Thrust
Posted by Callan Bentley
Previously, I’ve mentioned the lovely outcrop of the French Thrust in Sun River Canyon, Montana. It’s one of the locations that Pete Berquist and I take students to on our annual Regional Field Geology of the Northern Rockies class.
Here it is:
…And here is a GigaPan of the outcrop:
The view is to the south. The light-colored rocks on the right (west) are older. They are Mississippian-aged carbonates.
The darker-colored rocks on the left (east) are Cretaceous shales.
What do you see if you duck your head into the little nook formed by the more-rapidly weathering shale, right there at the fault surface itself?
Ooooh…. Interesting! Let’s zoom in a bit:
There are little chunks of the carbonate strung out in the sheared-out shale.
M = Mississippian (carbonate)
K = Cretaceous (shale)
These boudins likely began as asperities (little projections) off the bottom surface of the carbonate hanging wall, and got ripped off and tumbled and stretched as faulting progressed.
A horse! (of course). Nice drag fold, too–were you saving that for Friday?
Right! I forgot that these things were called horses!
Where’s the drag fold? I missed it…
In your third photo from the top, to the left of the “horse”, in the shale. It’s pretty well camouflaged, being shale in shale, but real, I think. I’ve emailed you an annotated copy of your photo.
For those who would like to see it, here’s Howard’s image: