New Students & Old Rocks

A couple weeks ago (before that trip to Namibia) I checked out the department's SUV for the weekend and we went on a "recce" (that's a reconnaissance trip) to check out some areas in the Cape Fold belt where two of my Honours students will be working for their theses this year. The two projects are really interesting and both capitalize on areas where there has been some research done in the past, but a structural approach could potentially give a lot of new insight.

We started at Pakhuis Pass, near Clanwilliam - the type section for the Ord/Dev. Pakhuis Formation. The Pakhuis Formation is a periglacial marine deposit (mostly, although there is some facies variation) which overlies the voluminous Peninsula Fm. sandstones which make up Table Mountain. At that time, our corner of Gondwana was at a latitude of about 70°S - a good place to make lots of ice and heavy d18O in the oceans confirms that this was a icey time on southern earth. The upper part of the sandstone, below the Pakhuis, has some folds & deformed bits - this could be due to drag from glaciers running over it, or to loading/soft sediment deformation from burial by the Pakhuis Fm., or by later folding during the Cape Fold belt orogeny. Here's an example of some sedimentary deformation, overturning of crossbeds:

The top of the Peninsula sandstones, at Pakhuis Pass, is a striated and compacted surface. Here are Nils and Scott in front of it. This surface is a disconformity and I could be convinced that it was striated by the passage of ice... although the overlying Pakhuis here is most likely fluvial...

This was Sila's first adventure in geotourism. He seemed to enjoy the driving, the scenery, the tour-de-force of the northern limb of the Cape Fold belt. How did he entertain himself when the rest of us took a while checking out the rocks? See for yourself:

We went south to Hottentots Holland nature preserve.... this is way "downstream" with respect to the Ordovician-Devonian paleo basin. This is also the location of the famous kloofing route through Suicide Gorge - kloofing is somewhat akin to what we used to call creek walking, with the addition of "compulsory jumps" - that is, over waterfalls which you can't avoid due to the steep canyon walls - so it is generally done only in the downstream direction. Suicide Gorge gets its name from it's unusually high compulsory jump: 15m.
In this photo you can see some sandstones in the ridge on the right - that's the Peninsula Fm. with the Pakhuis Fm. on top of it. The smooth green area in the saddle is the Cederberg shale, and overlying that, on the left, the Nardouw group sandstones. Here (and elsewhere) there is a localized fold zone at the base of the Nardouw sandstones! This has to be related to the Cape folding... since the folds are cylindrical, coaxial, sometimes kink folds... etc... they are clearly a tectonic feature. So why are they concentrated at this area in the section?

Here's all the boys on top of the Pakhuis Formation. Here, as in most of the area south of Pakhuis Pass, it's represented only by the Sneeukop Member which is a gritty, sandy, massive diamictite with randomly distributed quartzite pebbles - some of which are faceted/striated. Recent work has interpreted this as a glaciomarine diamictite - possibly sub-floating-ice shelf environment. Could be! It's not too thick (a few meters) and is regionally continuous for 10s-100s km. At Sneeukop (why did I take no photos?) it is involved in the upper-Peninsula deformation. Could this be giant load casts? Nils? take it from here. You're gonna need a nice compass and a lot of patience.
Here's a sandstone dike in the diamictite - derived from the underlying Peninsula sandstones. These are everywhere. They seem to have a preferred orientation. They have nice margin-parallel layering. They are apparently identical to the diamictite in grain size distribution (excepting the pebbles!). Paleoseismites? Catastrophic loading by the deposition of the diamictite? We may never know.

Oblique view of the diamictite bed.... very strange very cool concentric curviplanar banding features! Possibly oxidation bands - they are pinkish. Totally absent in neighboring units. Unclear how they relate to the joints which are now present.... another mystery.

to be continued...

1 comment:

Trifarina said...

Kloofing sounds like something that should only be done near the van... in case you get kloofed.