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…
Callan, what are the beautiful blue-gray layers?
Don’t know exactly – but in general, they are layers of sediment, and there’s something different about their composition: perhaps more heavy minerals (which are often darkly colored) or perhaps more organic content (fossil carbon), or perhaps something else. But it’s a sedimentary bed.
If this is from the Contorted bed banded iron formation layer then there is a good possibility the blue is from reibeckite, the blue sodium-rich asbestiform mineral that often co-occurs with BIF.