July 13, 2012

The Geology of Gnoem Gnoemskloof near Robertson, Cape Fold Belt, South Africa

Posted by Evelyn Mervine

On our way to Gnoem Gnoemskloof!

Last weekend my geologist husband Jackie and I visited the Robertson area of South Africa. On Saturday morning we visited a lovely little place called “Gnoem Gnoemskloof” (pronounced like “Noom Noomskloof”) at the invitation of Francie, a woman who emailed me to ask if I could tell her more about the rocks on her family’s property. Jackie and I spent a couple of hours with Francie and her family looking at the various rocks on the property, which is nestled in the Breede River Valley.

The Breede River Valley is located in the Cape Fold Belt, and there’s a great description of the Breede River Valley’s geology (focused on how the geology affects vinticulture, but still a great article) located here. Jackie and I still need to take a look at a local geologic map, but we think that the sandstones exposed at Gnoem Gnoemskloof are part of the Cape Supergroup, specifically the Witteberg Group.

All of the rocks exposed at Gnoem Gnoemskloof are sandstones. If these sandstones do belong to the Witteberg Group as we suspect, then they are early Ordovician (490-439 million years) to early Carboniferous (363-290 million years)  in age. Originally deposited in a flat-lying, shallow marine environment, the sandstones were subsequently folded during the formation of the supercontinent Pangea (~300 million years ago). The folding is quite complex in places, leading to some beautiful scenery in the Robertson area.

Some of the beautifully folded sandstones at Gnoem Gnoemskloof.

Tilted and folded sandstones at Gnoem Gnoemskloof.

Sandstones at Gnoem Gnoemskloof... next to the fantastic swimming hole!

More sandstone terrain at Gnoem Gnoemskloof.

The sandstones exposed at Gnoem Gnoemskloof are fairly uniform. For the most part, they are comprised of pure quartz although in places they also contain a small amount of mica which makes them sparkly. One thing that surprised Francie and her family is the variety of colors found in the sandstone. The sandstone can be a white, slightly gray color (close to the original color of the sandstone) or a darker tan color or a rusty red color (caused by the oxidation of iron in the sandstone). The outside of the sandstone can also sometimes be dark black, probably due to surface weathering / precipitation of material similar to desert varnish.

Before Jackie and I visited, Francie and her family thought that perhaps the different colored rocks were different types of rocks. Jackie and I explained that all the rocks were the same type of rock: sandstone. Originally, most of the sandstones would have been similar in color (a white/gray color). However, post-depositional oxidation/reduction processes and weathering processes have changed the original color of many of the sandstones. Some of the sandstones also contain quartz veins, which formed when hot fluids circulated through the sandstones and re-precipitated some of the original quartz grains as quartz veins. Some sandstones contain dark black patches or nodules which are iron-rich concretions also formed during secondary alteration.

Here are some examples of the different colors found in the sandstones at Gnoem Gnoemskloof:

Sandstones of many colors #1.

Sandstones of many colors #2. You can see quartz veins (bright white) and iron-rich (black) bands in this picture.

Sandstones of many colors #3. Here you can see a rusty red, oxidized weathering zone and a lighter-colored, less-weathered interior.

Sandstones of many colors #4. The upper rock has black, iron-rich areas and the lower rock has a coarse (big crystals) quartz vein.

Sandstones of many colors #5. This sandstone contains several fine (small crystals) quartz veins.

Sandstones of many colors #6. This sandstone had a thin black coating on one side-- perhaps something similar to desert varnish?

I think the lesson to be learned from the above photos is to never identify a rock by its color– or at least not by its color alone. Many rocks, such as sandstones, come in several colors. Geologists must use several features– texture, morphology, grain size, hardness, reaction with acid, taste (yes, taste), and many others– to identify a rock. Color is one feature used to identify a rock– but only with caution since the colors of rocks and minerals often vary due to the presence of trace elements and can also be modified through weathering and reduction/oxidation processes.

Although the sandstones at Gnoem Gnoemskloof come in many colors and have also been folded and uplifted, some of the original sandstone features such as ripple marks, cross-bedding, and even trace fossils have been preserved in places.

An exposure of sandstone. Along this exposure we observed ripple marks and trace fossils.

Ripple marks, with hammer for scale.

Cross-bedding, with my handsome husband Jackie for scale.

Jackie explains cross-bedding to Francie's husband and the rest of us (out of the picture).

A closer view of some cross-bedding. Look at how the red color (from iron oxidation) makes the cross bedding stand out! Neat, isn't it?

Trace fossils, located about 2 meters away from the ripple marks shown above.

We found the trace fossils particularly interesting, but neither Jackie nor I are fossil experts. Therefore, I sent the fossil pictures through to fellow geoloblogger Tony Martin, an ichnologist who blogs at Life Traces of the Georgia Coast. Tony identified the trace fossils as, “Zoophycos ( ‘witchbroom’ pattern) and probably Scolicia (tube with meniscae).”

Here’s an annotated version of the picture above:

Trace fossils, with labels.

I asked Tony to write up a little something about the Zoophycos trace fossil, and here’s what he wrote:

An All-Too-Brief Diagnosis of Zoophycos

Anthony Martin, @Ichnologist

Zoophycos is a beautiful, complex trace fossil with a wide geologic range, from the Lower Cambrian Period to the Holocene Epoch. It serves as the namesake of the Zoophycos ichnofacies, an assemblage of invertebrate trace fossils that mostly represent sediment feeding in deeper, quiet-water marine environments. But this trace fossil also occurs in shallow-water (shelf) deposits from the Paleozoic Era. I’ve encountered it in Ordovician shallow-marine rocks of Ohio-Kentucky and Carboniferous prodelta rocks of Kentucky. So for it to be in Ordovician prodelta rocks of South Africa is probably not too unusual, but might be worthy of further investigation by ichnologists who work specifically with Zoophycos. (Surprisingly, it’s more than one person!)

How was Zoophycos made, and what made it? That’s been the subject of lots of discussion among ichnologists, but everyone agrees that it is a systematic feeding trace, in which its maker repeatedly probed into the sediment and shifted its position each time, making “lobes.” These lobes then overlap and spiral in one direction or another, making what some researchers have described as a “Chinese hat” arrangement. Because of its long geologic range, it obviously has not been made by a single species of animal, but rather reflects an identical behavior expressed by many species over the past 500+ million years. These trace fossils were most likely made by a marine worm, and the best candidates so far are sipunculids, also known as peanut worms.

Thanks for the information, Tony! Fascinating! If you’re ever in South Africa, we’ll have to take you down to Gnoem Gnoemskloof to have a look at the trace fossils in person.

Here’s a few more pictures of the trace fossils:

Another view of the trace fossils.

Yet another view of the trace fossils.

And yet another view of the trace fossils.

Francie, thanks so much for inviting us out to Gnoem Gnoemskloof! We had a lovely time checking out your beautiful property and your rocks. I hope that Jackie and I were able to help you better understand the geology of your property. We’ll have to come back in the summertime to take a dip in your swimming hole!