My colleague John has a great Honors student who is doing a project out at Cape Hangklip. On Saturday the three of us went out there to scope the field area and see if we could find some of the outcrops described by geologists who worked the area previously. Cape Hangklip is a beautiful peak east across False Bay from the Cape Peninsula. It is composed of the same Table Mountain Group rocks, but more deformed than on Table Mountain intself. On top of Table Mountain, relatively conformably overlying the Peninsula Sandstone which forms its ramparts, is the Pakhuis Tillite.
Donna and John and myself spent Saturday hiking around Cape Hangklip with the goal of finding the similar strata as are exposed on Table Mountain, and chasing down an old mystery John found in the literature about their true origins. Here are Donna and John entering the park where we started our hike.
We had a long slow hike up the pass (east to west in the Cape Hangklip google earth image above) and crossed a lot of quartz-cemented breccias. By the top of the pass we still hadn't uncovered our quarry so we did a little bush wacking looking for it. We had a report from (other) John and Roy that was easy to find, just past the waterfall. So we went way off trail following the sound of this waterfall. Donna didn't say but I think she has some climbing experience.
The creeks run red here due to the pH affect from the tannins and other organic acids from the rich fynbos soils. These then acidic creeks run over Fe-Mn cemented breccias in the Peninsula formation - I don't know about their origin just yet - but their timing might help shed light on the Pen. Fm. history. My other colleague "John" [Andy! More "Johns" than Women!] has a student looking at the geochemistry of these cements on Table Mountain. I'm hoping to get up there to do some field work with him in Skeleton Gorge as soon as possible. Here is the Fe-Mn breccias weathered out along the slope:
Donna and I looking down this CRAZY CLIFF to Betty's Bay. Her family has a place out there so she can get in a lot of hours on this mountain in relative comfort. Here we're seated on the precipice at the head of the gorge, on what appears to be a crushed damage zone with most fractures running actually perpendicular to the gorge axis, and roughly vertically dipping. Cementation of this damage zone might be responsible for creating this nick point. There's a big waterfall below.
While climbing around and down one side of that gorge, we could see across to the other. See how there are nice concentric anticlines with crush zones where the synclines should be? Rad! Is that evidence of ramp thrust propogation? [Note to students: if you are snooping on my blog, you better be able to answer this question. Rick will help you.]
Now, in my previous life in North America I think we were rather strict about using the word "tillite" in that "tills" are sediments deposited directly by ice. Sedimentary rocks which suggest that ice was around but which are deposited in water or other means, we wouldn't have called a till (when lithified, "tillite"). If till is reworked and deposited by a river, it would contain glacially formed clasts, but it would have more sorting and sed structures of a river or delta, this is called a sandur. I already took note at Laingsburg that the Dwyka Tillites (these are amoung the famous deposits which link South Africa to other parts of Gondwana) some of which are really glaciomarine basinal sediments with dropstones. Are there any ice people out there who would care to get me off my high horse about the word "till"? CAn I bring myself to accept this flippant usage of a geological term? Anyway I dgress. Here is a faceted quartz pebble in the Cape Hangklip Member. These pebbles are the strongest line of evidence for ice processes in this section.
The Peninsula Fm. is a thick sequence of very very clean quartz sands, medium to coarse grained, with massive bedding and abundant crossbedding ( trough, herringbone, etc...) with common pebble beds and lenses with the pebbles nearly 100% vein quartz and chert. Sorting is near perfect. Now low greenschist-facies metamorphosed with abundant quartz cements, all non-quartz material is strongly aligned fibrous sericite in interstices, fibers are bedding-parallel. OK, simple mineralogy, simple microstructre, everything is explicable so far. Two models exist for its deposition: braided fluvial (maybe sandur?) and shallow marine. At Cape Hangklip there is a diamictite of fine quartz silt matrix with random assortment of quartz pebbles, basically no bedding. It is a lens a few km long within the upper portion of the Peninsula Fm. So the main question of Donna's thesis will be to determine the origin of this "Cape Hangklip Member", interpret its sedimentary origins, and compare it to the Pakhuis Tillite from Table Mountain. It is so similar in fact that there is a question of whether the CHM is just a repetition of the Pakuis/Peninsula contact by faulting. Here's the diamictite at Cape Hangklip:
I won't go into detail about the controversy because that's Donna's job! Suffice it to say that so far I have the opposite opinion to my colleague and I think that's best for Donna because she can make up her own mind without two advisors pushing her in the same direction. Finally, here is a blurry picture of a beautiful pair of orange-breasted sunbirds in the garden birdbath: