31 August 2016

Ball & pillow and other sedimentary structures in Graafwater Formation, Table Mountain

Posted by Callan Bentley

I hiked Table Mountain yesterday, south of Cape Town, via the notoriously steep Platteklip Gorge. I detoured a wee bit on the “contour trail” which heads toward Devil’s Peak to see some fine exposures of the Graafwater Formation. It’s mainly a red shale with some green shale, and some fine sandstone. This outcrop particularly caught my eye:


Those are pretty sweet “ball and pillow” structures. Here’s the most photogenic example:


“Ball & pillow” is a type of soft sediment deformation, when fresh sand is deposited atop soft, squishy mud. The density inversion triggers a load/sag phenomenon, and the sand pooches downward while the mud squooches up alongside, folding the sand into a shape much like an up-side-down mushroom cap.

Want to explore more? I shot a GigaPan there for your edification and amusement:
Link GigaPan by Callan Bentley

I also observed rippled sands (big wavy-topped pink unit in the photo below)…


…and trace fossils (look for the little white / light-green burrows cutting across the red shale in the close-up photo below:


And then there was this feature – at first I thought it was a mudcrack (which have been documented in the Graafwater), but then I noticed it had a spiral sort of shape in 3D, and there was a smaller 3D offshoot that pokes out to the right, about halfway up. So maybe it’s a trace fossil. Anyone want to chime in on their interpretation?