Malmesbury Group and the SeaPoint Contact

The Geological Society of South Africa, Western Cape Branch held its end-of-year field trip on Saturday 8 Jan. I have been working on this blog post since then but got distracted writing a shorter (less slangy) summary for the GSSA WCB newsletter. So sorry but here it is finally. It was a walk through the Tygerberg terrane of the Malmesbury Group up to the lit-par-lit contact with the Cape Granite at Sea Point, Cape Town. Now don't give up yet, I'll tell you what all that means in a minute. The leaders were Prof. Alex Kisters (structure) from U. Stellenbosch and my dear friend Dr. John Rogers (sedimentology) from good ol' UCT.

Zircon is a great little mineral that grows during igneous or metamorphic heating in rocks, and it takes in all the Uranium. After the hot period ends, the uranium starts to decay, like a little stopwatch in the rock. This makes it possible to date the igneous or metamorphic heating in a rock. If that rock then erodes, dumping sediment into an ocean basin, the zircon's other special feature comes into play - it's darn hard. It survives all that tossing around and when you collect the sediment millions or billions of years later, it's still intact, tiny stopwatch ticking away. Even if the sed has been through another heating event, it's possible to find some of the little grains which didn't reset their clocks. So you can say, the age of the youngest zircon stopwatch in the sediment (or detritus) is an older boundary on the age of the sedimentary deposit. See? cool.

Anyway, the Malmesbury Group is a huge pile of shale-siltstone-sandstone deposited in the latest preCambrian times. It is therefore not fossiliferous in the least, which makes it somewhat more difficult to determine exactly how old it is. Some people have picked detrital zircons out of it that give 3 age groups: 2 billion years, ~1.5 billion years, and 545 million years. This means the part of the Malmesbury where these data were collected was deposited after 545Ma.

John got us started off with an orientation at Three Anchor Bay. John can be counted on for all kinds of maps, rocks, artifacts, old theses, any type of "visual aid" one could possibly wish for on a field trip!

The sedimentary structures came on fast and furious. You would have to hike miles and miles across Kodiak to find a tenth of the good stuff we saw in less that a mile of strolling. Here is a gorgeous bed that I (and Saranne Cessford, thereby earning me some credibility) interpreted as a rip-up bed: that is, a bed of sandstone, minding its own business on the seafloor, got torn up by a mudflow! which ripped it into blocks, later to settle out in the mud.


Mega version of the same: a true Olistostrome! Here full big sandstone beds have been tumbled and broken in a massive mudflow. This one is only about 4m thick but imagine if it were an order of magnitude thicker - that's where it starts getting really complicated to tell if a melange (or mix of rocks) is made by sedimentary processes or tectonic processes. Ask me later why that even matters, it's a whole nother question.

OK now for the real fun: Name That Structure (NTS). We used to play this game as undergrads, when we were first learning structures... Thinking, no doubt, that this was a rookie pursuit and we would soon run out of structures we couldn't identify. WRONG.

Something weird happening here, not immediately obvious. It has to do with bedding-cleavage intersection, but the bedding is not... normal. is it caused by the cleavage? or is the cleavage wrapping around some weird bedding features? Two pictures and then my theories. First outcrop photo: vertical joint surface normal to bedding strike, cleavage is vertical, bedding dips ~50° to your right. Second photo, cleavage barely right-dipping, bedding is left-dipping, and the big limpet is ~2cm long axis.

Theories (and I should say here that initial impressions split quite neatly between sedimentary and structural geologists):
1. Scallopy bedding - these are ripple marks with sand lags into the troughs. Cleavage later wrapped around them.
2. Structural feature - less well defined but since the wavelength is seriously perfectly regular, as well as the amplitude, and these only occur in the fold hinge - maybe they are some kind of disharmonic folding.
3. Rayleigh-Taylor discontinuities, with top planed off by subsequent turbidity currents, overprinted by pressure solution cleavage which intensifies within R-T "intrusions".

Obviously option 3 is all me and I can't blame anybody else for it. However I'm currently 60% for option 1. Opinions? How come nobody comments on my blog? And is it really true that the Japanese geologists have a petrol-rockblade-skillsaw which would be the perfect sampling solution to this kind of problem?

We get closer and closer to the contact with the granites and little dikes (dykes; ZA) seem to emerge everywhere - dikes which are folded with the pressure solution cleavage - serious compression (ductile!) during the granitic intrusion - leaving NO EVIDENCE whatsoever in the absence of active strain markers. Isn't that beautiful?

Here we have John showing Alex's sketch of the contact. The cross section Alex drew is visible to the naked eye in the profile of Lion's head which we could see from the road. Really nice place to see into the rocks. In the foreground you can see my pal Kirsten from Dresden. She runs the new ICPMS (inductively coupled plasma mass spectrometer) at work, that is, unless Eskom randomly shuts off power to random neighborhoods at random times during the day for 2-2.5 hr intervals. Oh wait that's exactly what's going on! It's called "load sharing". Maybe Ken Lay faked his own death and has found a new liberal state to punish with Enron style rolling blackouts! Or maybe not. but I digress...
Now we come to the best part. As you can see in the hillside of Lion's Head, above, we were walking from the black shales toward the edge of a big granite batholith. The contact runs right through Lion's Head and the 403Ma sandstones unconformably overlie it. As we approached the contact there were more and more little granite dikes, then bigger, then we started seeing evidence of macroscopically ductile flow in the black shales as well as the pinkish granites:
The layering is parallel to the contact, as if the granitic magma squeezed its way up hundreds of tiny parallel cracks into the black shales. There is a bit of a metamorphic aureole but really it's only a few 10's meters wide of spotty hornfels - not too hot and not too big. That's a good piece of evidence to suggest that instead of one big giant bubble of hot magma, which would have really cooked the rock around it, the whole body of granites might have arrived slowly over a long period, through tiny cracks. Kind of like ants in my kitchen... one then 10 then 1000...

The big square crystals you see are feldspars that crystallized in the magma chamber at depth and were carried up with the migrating magma. You can see in this photo that some of them seem not to be in the pink granite, but are actually surrounded with black shales. How is that done? Alex gave an explanation based on Norm Sleep's (Stanford) theory using a really elegant fluid pressure model of one of these tiny cracks. Here's Alex with his explanatory sketch, which didn't photograph, so here's my attempt to recreate it:
Anyway all the parallel dikes create an overall gradational contact where there are thicker and more frequent dikes until one crosses the contact and then it's all granite. This is called lit-par-lit or bed-by-bed.

so the question that remains: did the shale melt? or just deform viscously (but very slowly) under high temperature?



There are some conspicuously NOT FLUID looking blocks in there.