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?