Tufa is so Weird and Cool

TUFA is porous carbonate (CaCO3) rock precipitated in the terrestrial environment by surface water. It can form in almost any area, shape or form where water containing calcium (usually dissolved from contact with rocks) mixes with some source of carbon dioxide (either CO2-rich waters or from air or plants). It is the same reaction by which carbonate crystallizes in caves, forming stalactites, stalagmites, etc. CaCO3 has the odd characteristic of being reversely soluble, that is, more soluble in cold water than in hot. So cool groundwater or stream water which warms up on the surface can result in the precipitation of tufa.

I was lucky enough to see the tufa towers in Mono Lake, California from a kayak with the UCSC summer field course, 2005. Those are formed when groundwater coming down from the Sierra Nevada flows up along fissures into the lake bed and mixes with the saline lake water. Those deposits are 100s-1000s of years old and make clusters of towers foamy, porous, chalky carbonate a few meters high around the lake bed springs.

Now, in contrast, I present the tufa deposits of the Naukluft. Here's a closeup on a tufa boulder found in Waterkloof. You can see in the lower part of the photo that the tufa crystal form resembles moss and the tufa may have precipitated directly onto (and aided by) plants growing around the spring. There is also a cavity filled with layered, dense, agate-like CaCO3. This is mammillary travertine, commonly found in caves. The width of this photo is approximately 10 cm. For an idea of exactly how much tufa we are talking about, here's a look at the modern stream channel in Waterkloof. Yup, that is all tufa. Tufa cemented gravels and boulder beds, layers of moss tufa, dense, non-porous tufa, tufa terraces with delta-like foreset tufas, perched tufa-gravel beds cemented to the bedrock canyon walls. It's the Willie Wonka Tufa Factory, with a rainbow river of tufa.

Here we are in Die Walle, where a giant boulder of Tufa is trapped in a crystalline cement of tufa, and then incised by the modern river which may or may not be precipitating tufa in its modern gravels.

And now for the kicker:
Any one outcrop in the Naukluft seems to be bigger than all the tufa in Mono Lake combined. I believe this photo shows the 80m-tall Klein Blasskopf tufa cascade described by Heather Viles et al. (Sedimentary Geology 2007), who put forth a nice model for the formation of the tufa 'barages' or 'cascades'. (It's nice to know what these are called because I was calling them tufa megacakes or tufa edifi in the field.) One attribute Viles et al. didn't discuss is the fact that the 4 truly giant tufa cascades are roughly in line with one another. This line happens to be the southern edge of a great big E-W trending valley that cuts across the center of the Naukluft, where most drainages are N-S trending. This correlates with the slope break - cause or effect? The slope break itself might be enough to perturb the hydrogeologic regime and cause tufa formation - but what if a fault or fracture system caused the slope break? And what if that fracture system was also a conduit for ground water to reach the surface? Just sayin. Better find out, that's why I keep getting all these free flights to Windhoek.

Final thoughts - sure, there's a lot of carbonate in the rocks around here. But there's more carbonate in various parts of Nevada which I have visited, with nice kloof-like mountain gorges where tufa could precipitate at nick points and mossy waterfalls. But nowhere in the American west have I seen anything approximating an 80m TUFA BATTLE STAR. Is that just me? Or is there something really cool going on to explain all that tufa?


Sampling Waters in the Naukluft

For an arid environment, there sure was a lot of water around. Apparently there has been a lot more rain than usual; that is why the area was greener and buggier than anyone had seen in recent memory.

Water resource characterization is actually the main motivator behind this whole effort; my nominal (and periferal) role is to determine what role (if any) ancient or recent faults and fractures are playing in water exchange between the surface and the subsurface.

Most of the water resource extraction in the Naukluft Mountains, and pretty much all of it on the surrounding farms, is accomplished by "borehole". Many have a real working windmills - These usually pump directly into troughs for herd animals (and wild animals) to access.

Although these are often the easiest source for ground water samples, there are a few issues with them. For one, the boreholes are usually lined with some kind of pipe - usually metal - this will affect the geochemistry of the water if it sits in the well a while. Second, they are drawing all the time (whenever the wind blows). This helps with the first concern, as it means water doesn't reside in the borehole too long. However, it means the water level in the borehole might not be in equillibrium with the water level in the surrounding formation - and we want to know the water level of the regional/local water table. If the windmill pumps water out of the well, it takes some time for the water to flow from the rock formations back into the bore holeand during this time, the water level we measure will be too low. This ranger at Zais helped us open the cap under the windmill so we could drop the water level meter down.
We found a few boreholes, such as this one, which had solar panels apparently to drive pumps. This one looked like it was installed to keep a shower ready to go at this outpost on Die Walle farm. Alet is checking the shower tank for water... empty. Kate is getting ready to check the borehole for water... sealed.
The coolest samples come from the rock springs and seeps. Here's Kate suspended by her toes on a slippery algae-covered cliff wall, catching tiny drops of water as they emerge from the rock. We also sampled the surface water such as the stream Kate is trying not to fall into - this will all give a picture of what underground water looks like, what surface water looks like, and how the streams are fed during the dry season... Due to the high visibility of travelling thunder storms, Jodie and the students were able to chase down a few and collect several samples of rain water as it fell out of the sky... this is a great data point to have because it will tell us what the water looks like when it enters the Naukluft. I wish I had a picture of Jodie in a sudden deluge, holding a giant green bucket to shield her head from the huge rain droplets... so pleased with her cold rain sample... but as I was watching from the warmth of the dry bakkie the photos look like a rainy sheet of wet window. hm.

I have to say, Kate was really the workhorse of the water sampling team. She climbed walls and dove to the bottom of clear spring pools. Here she is after sampling the spring-fed creek in Waterkloof. You can see the wall of tufa behind her - more on the tufa deposits in a future post. Anyway it suggests that both surface and groundwater flow through Waterkloof were much greater in the past than today. How far back is the past? We don't know.
Here's Ben in the gorge at Die Walle, entertained by the struggling students climbing the walls to capture tiny drips of water! Behind him in the cliff you can see the reason for this canyon's name, Die Walle: the falls. Standing under that wall, you could see shoots of droplets literally spraying a meter or more out from the wall in the blackened area. A big pool at the bottom feeds the creek running out of the gorge.

We tested the water onsite for dissolved oxygen, pH, temperature, conductivity, eH, and total dissolved solids... Kate and Chris Harris have already started isolating the CO2 dissolved in the water to look at the carbon and oxygen isotopes. She will also analyse the water molecules themselves for hydrogen isotopes. She will also look at the isotope signatures of the rocks in the area, and the carbonate rocks that precipitate in these waters...These will hopefully be able to tell her about the chemical interaction between rocks and water, and possibly help trace the pathways by determining which rocks have more contact with water before it flows to seeps or springs.

Shane and Alet at U. Stellenbosch will find out about those total dissolved solids - what are they? What cations and anions are present, and do they come from rocks, from the rain itself, from biological processes, or human pollution? Does the water get saltier as it flows underneath the ground from the mountains out toward the plane, and the farmers there with their bigger ranches and multiple boreholes?

Pride from U.Nam will be putting it all together, as they say, he will be using MODFLOW to build a 3D picture of the Naukluft groundwater in space and in chemistry.

The big questions we all will work on together:
- If the farmers say some springs are drying up, is that due to over use of a static reservior? Or is there something fundamentally changing in the water system of the Naukluft?
- From raindrop to tap, how does the water evolve and change in chemistry? Is it always safe for humans and animals to use? if the water regime of the area is changing, will this affect the chemistry and safety of the water, as well as quantity?
- How much new water demand can the system support?
- Why are the rocks so damn cool?
Thank you.


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.


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...


Scenes from the Naukluft

In no particular order:

1. sunrise, looking west you can see the thin line of red along the horizon which is the edge of the Namib sand sea:
2. shady respite from the sun in waterkloof:
3. social weaverbirds
4. thunderstorm moving across the plain
5. butterflies everywhere
6. big sky and fluffy clouds, Naukluft mountains on the horizon
7. little boy watches as his father opens the well for us at Zais farm8. donkey carts are primary transportation for many Nama people
9. gray limestone: footwall. yellow blocky dolomite: fault rock.
10. springbok along the road

more details to come later...