27 July 2012

Friday fold: Spray Mountain Group siltstone

Posted by Callan Bentley

Today, we return to Banff National Park, to the outcrops next to the parking area for Bow River Falls…

Zoomed-in closer to the thinner layers at left:

These strata (shale and siltstone) were laid down in the quiet aftermath of the Permo-Triassic extinction, as terranes colliding with the edge of North America (far to the west) began shedding clastic sediment into the basin to the east. The lack of critters in any profusion means the beds weren’t bioturbated, and so we can still see the delicate layering of the original stratification. Primary structures such as tool marks indicate that┬á the depositional current oscillated in strength.

The strata were tilted and folded (a gentle sort of kink) during the rise of the Canadian Rockies in the late Mesozoic and early Paleogene. These strata remind me very much of the Martinsburg Formation in Virginia: deposited during the earliest phase of Appalachian mountain building, and tilted and deformed during the latest phase of that ancient event.

Happy Friday!