3 January 2013
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.