3/19/2008

The Sole Dolomite (SOUL; DO LO MITE!)

Yah, um, non-geologists might want to take me off your bookmark list for a little while. I think that leaves only testy trifarina to read my blog. Sorry mom, people pictures coming up here at some point! Anyway,

"THE NAUKLUFT" is a big beautiful nappe complex - a klippe on the plains, west of the highlands of Namibia - the last erosional remnant of a mountain range half a million years old. The rocks are even older than that. They are thrusted on top of each other - like a deck of cards spread out on a table and swept into a stack by a southward-moving hand. At the base of the stack is the master fault, along which the rock layers slid, according to previous estimates, something like 50-80km from their place of origin.

This master fault can be seen for miles around the Naukluft - it cuts a sharp, continuous swath across an otherwise convoluted terrane. It's easy to pick out the strange, massive, yellow dolomitic fault rock. Here you see the yellow "sole dolomite" crosscutting the footwall rocks of the Nama Group (gray-blue limestones) and layered brown carbonates of the hanging wall Damara group.

Here's Jodie and Pride on top of the sole dolomite - it has nice sharp contacts top and bottom. In many ways, this fault would be a simple case to study - if the fault rock wasn't made of dolomite (CaMg(CO3)2). Dolomite is maybe the weirdest common mineral there is. Old marine rocks are often made of dolomite - but modern ones pretty much never are. So the theory goes that limestones (CaCO3) which form in the ocean can be altered to dolomite by Mg-rich sea water. Only nobody's been able to make this happen in the lab - so the how/where/why part of the story is still an open question...

Here's an example of a meta-evaporite deposit - a place where dolomite HAS been observed to form in modern environments. Evaporites are salt layers formed when water evaporates (duh) - such as in saline basins in the western US. In the past when the Straits of Gibraltar have been closed by the northward motion of Africa, huge thick layers of salt have been deposited in the Mediterranean sea. This example is from the Nama Group, under the master fault. So a first order question - were these dolomites eroded by fault motion, and ground up to make the sole dolomite? Those white blebs are albite pseudomorphed after evaporite minerals - really cool.
Answer: Um, maybe. probably not. Meta-evaporites are a pretty rare component of those footwall rocks. Ordinary marine dolomites are pretty common in the hanging wall. But either way, there is plenty of ordinary dolomite around... to find its way into the fault zone. One standing hypothesis is that when the nappe complex was moving south, it traveled over the top of a modern (at that time) evaporite basin, and the mineral salts injected like a slurry into the fault. I don't feel too comfy with this hypothesis. For one - evaporites have a lot of minerals in them that aren't dolomite - and the quantity of those minerals reported in the Sole Dolomite by the proposers of this idea is ... a lot different... than what some more recent studies have found. For another... I suspect that if somebody took a closer, more quantitative look at the depth of the fault, the behaviour of slurries, and the deformation of the footwall rocks ... one might find that this is kinematically impossible. But, I confess, I haven't done this.... yet.

The sole dolomite has two main parts - either one might be absent at any given point - but basically there is the "massive dolomite" and the "gritty dolomite". The massive is just that - a slab of crystalline dolomite. No clasts, no banding, no bedding, no structures of any note. Literally featureless. Except for one teeny tiny planar vein of silica that Ben found - and helped me sample - at the Lemeonputz section, which is incidentally, the site of reintroduction of rhinos to the Naukluft Park. The gritty dolomite is composed of amazingly spherical, smooth rounded clasts - of dolomite, mostly, and bits of other things (granites, quartzites, evaporites, etc). Both were once thought to be sedimentary layers, when the thrust faults were thought to be gravity slumps (dip be damned!) back in the 50s-60s when a German group did some structural mapping in the area. Here's Jodie, pointing out the massive dolomite (beige, level of Jodie's head) and the gritty (yellow, where Jodie is pointing). The gritty dolomite crosscuts earlier mylonitic fabrics in the uppermost footwall, as you can see here at the "type locality" outcrop, and yes I got that sample!


There are some amazing features in the gritty dolomite... which i and some of the previous workers believe to be a fluidized cataclasite (regardless of how the dolomite itself was introduced into the fault zone!). Very thin, delicate veins are sharply cut by microfaults... these must also cut the gritty matrix, but they are often invisible away from the veins! This is reminiscent of some features we have seen at Pasagshak Point, Kodiak... which we also didn't make much sense of but I will ask some of the team if they have done any more work on it...

Suspended in the gritty dolomite matrix are large clasts - both of recycled gritty matrix, and also of "massive crystalline" fault dolomite. This one is wrapped or coated in some dense hard mineral - maybe silica? I will be able to tell when I get my thin sections made, when I get my rocks from Jodie, when they are cleared to cross the border... Do you know how many permits and stamps and things you need to move rocks across borders in Africa! I swear they're the only things that can't cross.

As if we needed proof of granular origin and high fluid pressures: gritty dolomite injection features, 12m below the fault in the footwall limestones. So. Gorgeous.



So much more to do here, more to talk about... need to go back to the Naukluft! More geology posts ahead.

5 comments:

Erin K. said...

AWESOME! I"m going to be stealing these images (with credit of course!) if I ever teach intro again.

Your mom sounds like mine, 'so are there any pictures of you?' No mom, focus on the rocks.

Trifarina said...

That's really disturbing you can't just take rocks without bizarre permits. Is it because of economic minerals and such?

Fault Rocks said...

yes the economic minerals. if somebody finds a diamond somewhere (which happens!) the government sure wants to know. also the question of blood diamonds.

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