7/08/2008

Snowball Earth Strata near Kogelfontein

Some time ago, in the middle of an igneous field trip no less, we were out on the coast of Namaqualand near the Kogelfontein igneous complex. The Kogelfontein rocks are Cretaceous, which to a South African is pretty much Yesterday, or in other words, that piddly small part of the geologic time scale that nobody bothers to learn. Did I mention that my students don't learn the Phanerozoic part of the time scale? And similarly, I never learned the other 80% of the time scale? Remind me to chalk that up on my list of SA-USA translation problems in English. I do feel like I've been accepted to some degree, with all my idosyncracies, because people don't correct me anymore when I say "Al-U-Min-Um"**. But I digress... it is a blog after all.

Anyway we took a detour to see this incredible outcrop of interbedded black turbiditic shales and clean, beautiful white marble. Yes, shales (sedimentary) and marble (metamorphic, or should I just say recrystallized).
This assemblage, Neoproterozoic* in age, is a key assemblage in the Snowball Earth story - a story better told by other outcrops in other places, perhaps, but there's evidence for it here if you accept it as such. The story goes that at 600Ma (give or take), the earth experienced a total glaciation - most of the continental landmass at the time was at high latitudes, close to the poles, and as earth cooled and entered the glacial period the continents were completely covered by ice sheets. If you're familiar with the concept of albedo - the reflectivity of the earth - you will know that rocks will absorb light and heat from the sun and re-emit it all as heat, warming the atmosphere. Ice, on the other hand, will reflect the majority of that energy back to space, and will not do the nice turn-light-into-heat action that bare land can do. The bare land bit is important here, because this is hundreds of millions of years before plants, or fungi, or any of those great heat collectors working for us today emerged onto land. So a planet with its land mass covered in ice is a very cold planet indeed, and has lost its means for warming itself from the sun's heat.

As the story goes, tectonics was still chugging away under all that ice, warming the oceans from mid-ocean ridge volcanoes and supporting tiny islands of bacterial life. The combined effect of the geothermal heat and the CO2 from volcanoes and life was eventually enough to break the cycle of cooling, and the planet warmed again. As land began to see the sun, the warming sped up and the oceans warmed up very quickly.

One of the fascinating things about CaCO3, that is marble, that is the most common mineral form of CO2, is that it is reversely soluble - that is, it dissolves more easily in cold water than in hot. That's the opposite of most everything, sugar, salt, most other minerals... So when the oceans were cold, there was a lot of CO2 dissolved in them. When the oceans warmed up quickly, all that CO2 was no longer stable in the dissolved state and CaCO3 - limestones - precipitated on the seafloor all over the world. We usually associate limestones with warm shallow places - coral reefs, etc. - but in the late-Neoproterozoic warming, limestones were forming everywhere, even in the very deep sea. Enter the outcrop at Kogelfontein.

The black shales here are deep water deposits (again, or so the story goes) that are very rarely found in association with limestones, anywhere in the rock record. Here they are together, repeated at least a couple of times: a shale, a limestone, a shale, a limestone. Or is it?

Here's trusty TA Duane pointing out the reason for the repetition. The reddish rounded lump he's standing on is the long thin hinge of an isoclinal bedding parallel fold - the axial plane of the fold lies in the plane of bedding.
The fold is ultimately doubly-plunging - it rounds off at both ends like a big sausage (that would be wors in local parlance) but there are others along strike. And in between, some beautiful evidence of tectonic interference with the stratal succession: shear foliations in otherwise sugary marbles, and strange little cuspate-lobate structures on ptygmatic folds (not sure what that folded bed is). That ZA 50-center is the size of a US quarter.


Here's a shot down the axis where a quartz-rich bed is desperately trying to maintain its radius of curvature in spite of the drag:


So the moral of the story is: earth warmed up, and here we are. Whether these marine seds actually provide anything more than circumstantial evidence for Snowball Earth and catastrophic warming of the oceans is still under debate. The other moral of the story is: don't measure section in an accretionary complex***.


*Neoproterozoic is about 1Ga -> Cambrian, for those who share my timescale bias problems.
** as opposed to "Al-Yu-Min-E-Um"
***I don't really know if it's an accretionary complex. It's supposed to be an along-strike equivalent to the Malmesbury Group down here in Cape Town. But that's an axe to grind for another post.

7 comments:

Carlo said...

Good Job! :)

Silver Fox said...

Are the black shales as deformed as the marbles? Is their any evidence of slippage along the marble-shale contacts?

Julie said...

I just wanted to tell you that I thought of you as I ascended over Stone Mountain, in Georgia (granite rock) on a sky lift...and wondered what all you would be able to tell about it if you were there! It is amazing how different bloggers pop into your mild at different times! LOL.

CJR said...

Wow, that's pretty cool... my inner palaeomagician would like to know, what's the metamorphic grade of the shales?

Ishmael said...

Pretty intersting, especially the reverse solubility of the marble!

I wish I remembered more geology.... I bailed on it when I thought my only job back here in Alaska would be working for Big Oil, which I had found out was evil about that time.

But I sure love the concepts and the vocabulary..... Maybe there's a continuing ed class on local geography I can take at Kodiak College this fall....

Oh! There's a question for you over on my blog....

Fault Rocks said...

Thanks Julie! Just been to a strange land full of Stone Mountain type inselbergs - sans confederate heroes though.

CJR - low greenschist, come on by and I'll show you some.

silver fox - oddly enough it looks like the marbles took the beating, perfect bedding in the shales with no shearing, although the marble/shale contacts were a bit ground up.

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