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.