25 April 2013

Deformation in the Lake Vermillion Formation

Posted by Callan Bentley

Today, let’s go back to the Pike Dam, where we spent some lovely moments last week, agog at the lovely graded beds and flame structures visible there. In contrast, today we want to examine the deformational structures seen elsewhere at this same outcrop. There are folds and faults and joints and more exotic fare: tension gashes and Riedel shears.

The deformation here is the youngest to affect the Vermillion District of the Superior Craton. Their orientation is apparently conjugate (at a 60° angle to) a northeast-trending regional fault, according to the field guide to this trip (Bauer, et al., 2011).

You’ll notice some of these show a left-lateral sense of shear, while others show right-lateral kinematics. So… the deformation of these rocks is complicated.

There are also more subtle features to the deformation: joint sets and very small-scale-offset faults.


I love seeing examples of fractures weathering out in positive relief – a sure sign of fluid flow through these joints, and reinforcing their walls with whatever was dissolved in those fluids (silica, probably?). Here’s an example:

Finally, a year and a half later, I’m close to finishing off my blogging coverage of this excellent field trip. One more topic is a special one – I’ll save it for tomorrow…