23 January 2012

An ancient delta at the Three Rondawels?

Posted by Callan Bentley

Here’s a look at what you see if you go to the Three Rondawels viewpoint above the Blyde River Nature Reserve in northern South Africa:

A lovely scene. The three mega-hoodoos on the left are the eponymous “rondawels” (pronounced ron-da-vulz), which is the Afrikaans word for a round hut. These erosional remnants are more or less cylindrical and of the same dimensional ratio as the huts, so this name seems moderately apt. However, something besides the scenery caught my eye…

If you zoom in to that quartzite cliff-face opposite, you’ll see this:

That looks to me like an ancient delta’s progradational sequence: the horizontal deeper-water bottomset beds overlain by the foreset beds, gently sloping at the subaqueous angle of repose off to the right (indicating the direction the delta built out towards), and finally topped off by the horizontal shallow-water topset beds. What do you think?

Recall that this is the typical form the internal anatomy of a delta takes on:

The strata here are of the Wolkberg Group, a Paleoproterozoic sedimentary sequence. If I’m right about the delta interpretation, then the source of the sediments (upstream for the river feeding the delta) was to the northeast. Nowadays, you can’t go too far to the northeast before you run out of African continent. I’m not sure what the continental configurations were like in the Paleoproterozoic, but I wonder at the source of this sediment. The quartzite composition implies that it would have had plenty of time to mature before deposition, and that implies a mighty long river system, which implies a distant source for the sediment. Where did this sand start off? What mountains shed it into that putative ancient long river? Which continent would this sand call “home”?

These are the questions that go through my mind when looking out at the same scene that the tourist next to me might simply call ‘pretty’… It’s a weird thing to look at the world through geology-colored glasses.