Today's post is about how geological processes have shaped the rocks at Golden Gate. All these processes acted at much larger scales than the borders of the national park, but their effects are preserved so beautifully in the park that I thought it would make a nice topic.
The scenery at Golden Gate is some of the nicest you can encounter: richly coloured sandstone cliffs and basalt, and open grasslands. The story of how this landscape developed is equally interesting.
We can divide the development of Golden Gate into two parts. First, we had the formation of the sedimentary and igneous rocks that form the spectacular scenery, and secondly, we had the uplift of these rocks (together with a large part of southern Africa) and erosion which gives us the cliffs that are present today. Let’s look at those two components individually.
1.) The formation of the sedimentary and igneous rocks – the Clarens and Drakensberg formations
The Clarens Formation is the name of the well-exposed sandstone that caps many of the hills in Golden Gate. Geologists have worked out that this sandstone was deposited in an ancient desert about 200 million years ago. How do they know this? By comparing the grains and structures preserved in the rocks to a bunch of different modern environments. For example, in the sandstones at Golden Gate, sand grains are typically less than a millimetre and have a very small range in sizes. This is what we expect of sediment that was transported and deposited by wind, as opposed to water. Another important clue are structures like the one shown in the photo below – these are called sedimentary structures and they form at the time that the sediment is deposited. If you look carefully at the middle bed, you’ll see some lines sloping from left to right. These are called cross-beds and they form by the process of dune migration (so what you’re looking at is actually an ancient dune that migrated from left to right). These cross-beds can tell you about the speed and depth of the flow that deposited them, and also the direction of flow. Really large cross-beds like the one below from Golden Gate are typical of desert environments.
What else do we know about the ancient desert at Golden Gate? Well, there’s some exciting fossils that tell us about what life was like 200 million years ago. The most important fossils to come out of Golden Gate are eggs in which the embryos of baby dinosaurs can be seen (more on this in another post!). The eggs are from a dinosaur called Massospondylus, which was a large herbivore (about 5 m long) and a fairly common animal in this ecosystem (Reisz et al. 2005). The most common large predator was Coelophysis, a 3 m long animal that hunted in packs (Kitching and Raath, 1984). And our own ancestors, early mammals were also present – a mammal called Megazostrodon has been found in rocks slightly older than the Clarens Formation on a farm near Golden Gate (Gow, 1986).
On top of the Clarens Formation, we find lava of the Drakensberg Formation. These lavas erupted 180 million years ago and the eruption was part of the beginning of the breakup of Gondwana (a large continent which contained Africa, South America, Antarctica, Australia and India). Originally, the lava would have covered much of southern Africa, but because of erosion, it’s only left in a few places – central South Africa and Lesotho, Mpumalanga and Limpopo (along the eastern margin of Kruger National Park) , and also in Namibia and Zimbabwe.
2.) The uplift of the southern African interior
The last exciting thing to happen in the interior of southern Africa (geologically speaking) happened 180 million years ago, when the Drakensburg lavas erupted. Since then, not much has happened, except that the land has been lifted up to its current altitude of 2000-4000 m by tectonic processes in the Earth. The altitude of the South African interior is strange: if you think of places with high altitudes, like the Himalayas, the Andes, the Alps, these areas have one thing in common – tectonic plates are moving together and their collision creates mountains. In southern Africa, plates aren’t colliding; in fact, the African Plate is under extension in South Africa. So why does Golden Gate, together with the Drakensberg, sit at such a high altitude, when it has no business being up there? The answer to that is in the mantle that underlies the South African crust (remember our planet has three zones: the core in the middle, the mantle in the middle and the crust on the outside). The mantle under the South African crust is very hot and buoyant, and has been since the eruption of the Drakensberg lavas (e.g. Gurnis et al. 2000). This mantle has pushed the crust up, causing elevation and increased rates of erosion in the South African interior (well represented by the topography at Golden Gate). Because of the uplift and elevation, we have had erosion over the last 180 million years, and this has led to the spectacular scenery at Golden Gate.
Gow, C.E. 1986. A new skull of Megazostrodon (Mammalia, Triconodonta) from the Elliot Formation (Lower Jurassic) of southern Africa. Palaeontologia Africana volume 26, pages 13-23.
Gurnis, M., Mitrovica, J., Ritsema, J. and van Heijst, J-H. 2000. Constraining mantle density structure using geological evidence of surface uplift rates: The case of the African Superplume. Geochemistry, Geophysics, Geosystems volume 1.
Kitching, J.W. and Raath, M.A. 1984. Fosssils from the Elliot and Clarens formations (Karoo sequence) of the northeastern Cape, Free State and Lesotho, and suggested biozonation based on Tetrapods. Palaeonologia Africana volume 25, pages 111-125
Reisz, R.R., Scott, D., Sues, H-D., Evans, D.C., and Raath, M.A. 2005. Embryos of an Early Jurassic Prosauropod dinosaur and their evolutionary significance. Science volume 309, pages 761-764.