2 July 2014
Fault breccia in the Helderberg Group, Corridor H
Posted by Callan Bentley
Here’s a breccia that Dan Doctor and I found in a tabular zone within the Helderberg Group (Devonian limestones) in one of the massive new roadcuts along Corridor H.
Is it a fault breccia or a sedimentary breccia? The breccia was bedding parallel, which suggests it could be just another bed, but it’s so darn coarse and angular (unlike the rest of the Helderberg) that we were skeptical. Indeed, pulling out a chunk (the sample you see above) and examining the bottom (the view you see below), brought additional evidence into consideration:
Those are slickensides! They are most prominent in the vein calcite patch on the left side. Zoom into this macro GigaPan by my student Robin Rohrback and check it out.
It’s a fault breccia. The fault happens to be parallel to bedding (because bedding planes are planes of weakness in sedimentary rocks), but it’s not actually a bed itself.
Also, the chert clasts seem a lot more fractured than I would expect to see in a sedimentary breccia (chert is pretty tough stuff). But that might just be confirmation bias on my part. I dunno if it would be a reliable indicator or not.