The Dominion Group
It is thought that the sediments of the Dominion group were deposited about 3.07 billion years ago, in a rift valley that was formed when the crust under the craton began to stretch. This depression was then flooded by an igneous intrusion from the mantle. Some layers of the Dominion Group contain gold deposits, which have been mined with minor success.
Not much else about the Dominion Group is known, as this all happened a very long time ago and there are few places where the rocks are well-preserved. In the Vredefort area, for example, the Dominion Group is characterised by a thin layer of lava that is only exposed at one or two locations.
After the Dominion Group was laid down, parts of the Kaapvaal craton subsided and any low lying basins were inundated with water. This formed a large, shallow sea across much of the continent and large scale deposition of sediments began. At this point, it is useful to take a closer look at the role water plays in the sedimentation process.
The Witwatersrand Supergroup
Basically, water transports eroded fragments away from the source rock. The speed of the water usually determines the distance the fragments will travel. When rivers hit larger bodies of water (such a lake or the sea), the flow slows down considerably and many of the heavier particles drop down and sink to the bottom.
Finer particles, such as sand and mud, remain suspended in the water for longer and tend to be deposited further away from the shore. The reason why I’m telling you all this fascinating stuff will become apparent later.
So, about 2.97 billion years ago, several layers of sedimentary rock were laid down in the shallow sea that covered much of the present-day South African interior. This is called the West Rand Group and it created several interesting topographical features, such as the eponymous Witwatersrand ridge which runs between Germiston and Krugersdorp.
Sometime later, the craton rose up a little and the land buckled into a series of ridges and basins. The rivers now ran from the highlands in the north and west into a series of lakes and depressions, where deposition was more rapid.
The rocks formed during this period are called the Central Rand Group. Together, the West Rand Group and Central Rand Group are called the Witwatersrand Supergroup.
A Geological Mystery
The Central Rand Group was rather special, however, because heavy grains of gold were periodically deposited on the river beds, along with the gravel.
This created the richest gold field on Earth, containing nearly half of the planet’s gold. Later, these gold-bearing conglomerate rock layers (called reefs) would be mined with spectacular success. The original source of these gold particles, however, remains something of a geological mystery.
Around 2.7 billion years ago, the Kaapvaal Craton collided with the Zimbabwe craton to the north. This created a mammoth mountain range that rose up along what is now called the Limpopo Mobile Belt.
It also resulted in a great flood of lava that came pouring out of cracks in the cratons, which covered the rocks of the Witwatersrand Supergroup. These lavas form the bottom layer of the Ventersdorp Supergroup.
The Transvaal Supergroup
Moving swiftly ahead, about 2.5 billion years ago, the Kaapvaal Craton subsided again and became virtually submerged by an expansive inland sea.
This triggered another round of heavy sedimentation and marks the beginning of the Transvaal Supergroup. This also coincides with the end of the Archaean and the start of the Proterozoic era, which lasted until 570 million years ago.
The Transvaal Supergroup is an interesting bunch of rocks. The lowest layer, called the Black Reef Formation, contains traces of gold. Above that is the Chuniespoort Group. This layer contains evidence of some of the earliest life on Earth – photosynthesising cyanobacteria.
OK, it doesn’t sound so impressive, but these microbial critters released oxygen as a waste product and may have been responsible for increasing the levels of free oxygen in the atmosphere. The oxygen from the cyanobacteria also reacted with the sea water to create vast quantities of iron oxides, which resulted in substantial iron and manganese deposits that we mine today.
The bacterial colonies grew in shallow water, sort of like coral reefs, and absorbed carbon dioxide from the water. This caused calcium carbonate to precipitate, which then covered the gooey layer of bacteria. The bacteria proceeded to re-grow on top of the calcium layer and the process was repeated over and over again.
This formed dome-shaped bacterial colonies, which subsequently become fossilised into what we call stromatolites. Examples of these petrified stromatolites have been identified in the Vredefort area and have been specified as an outlying part of the World Heritage Site.
The calcium carbonate also formed a layer of rocks called the Malmani dolomites. These are important because the dolomite was subsequently eaten away by ground water that percolated along cracks in the rock, to form a series of caves. Later still, these caves would become a final resting place for early human ancestors, such as Mrs. Ples and Little Foot.
The famous fossil finds in the Cradle of Humankind, Sterkfontein, Makapansgat and Taung have all come out of this limestone layer. It has even been suggested that the Vredefort impact may have had a hand in creating the cracks that gave rise to caves, and thus helped us solve the riddle of human evolution.
The Bushveld Igneous Complex
After the deposition of the Transvaal Supergroup, there was a rumbling from down below. Around 2.06 billion years ago, a series of explosive eruptions disturbed the peace of the cyanobacteria and covered the Kaapvaal Craton with a layer of rocks and ash.
Subsequently, another plume from the mantle pushed up a vast amount of igneous material which forced its way through lateral planes in the sedimentary Transvaal rocks, but did not reach the surface.
This subterranean magma chamber cooled slowly and formed a remarkable series of layered rocks called the Bushveld Igneous Complex. It extends across much Southern African interior and is the largest example of its kind in the world. The Complex also contains some of the planet’s largest deposits of platinum, chromium, copper, vanadium, titanium and nickel.
The Vredefort Bolide
Then came a big surprise. 2 billion years ago, the Vredefort bolide came hurtling down towards Earth. It was several kilometers wide and travelled at extremely high velocity towards the centre of the Kaapvaal Craton. The impact was unimaginable.
The surface rocks were vaporised and the deeper crust was turned inside out, giving modern geologists a rare glimpse into the ancient past. The Vredefort bolide literally made a big impression.
After the impact, new rocks would continue to form in Southern Africa, and all the geological processes described above are still continuing today.
However, a complete account of the sub-continent’s geology is beyond the scope of this book. Instead, we must now turn away from the ground and look to the skies, for a blow by blow account of the greatest ‘single energy release event’ the world has ever seen.
By David Fleminger