23 February 2012

Contorted fractures in the Contorted Bed

Posted by Callan Bentley

Today, let’s zoom in on a little something I saw at the outcrop of the Contorted Bed in downtown Johannesburg, South Africa:

What are these things? I only saw them at this one spot.

These look to me like en echelon fractures at a high angle to bedding, that were then deformed due to dextral (top to the right) shearing. But they’re really closely spaced, and they show no apparent dilation (and therefore no subsequent infilling with vein minerals).

So then maybe they are R’ Reidel shears (also implying a dextral sense of shear)? The spacing seems about the same as the two other times I’ve seen Reidel shears in the “wild”… but two isn’t much of a comparison data set. I defer to you, dear Internet…