The Vaal River is unique in being the only major river flowing across an asteroid impact crater – the largest such crater at that.
The Vaal crosses into the Dome via a macrofault in the mountains, and goes out via another such fault. These faults splay out radially from the centre of the Dome and are an indication that a massive impact must have happened here. The Vaal was born on the Karoo system (a sedimentary sea that came much later than the asteroid impact and covered the old Dome underneath). As the river eroded the karoo sediments it re-exposed the underlying structures and hence it is what is called a “superimposed” river. It came down on top of older features.
The river is vividly illustrated in our videos on You Tube:
The Vaal is one of the world’s very old water courses now occupying what is arguably among the oldest of river basins, stretching back to the Witwatersrand Basin 3 billion years ago. The remarkable story of river systems and the Dome has been neglected in most literature about the crater. Graeme Addison has written books about the world’s rivers and is a keen paddler, so naturally he set about researching the origins of the Vaal and its relationship with the Dome.
The most interesting discovery was the link between the asteroid blast, the river and the gold wealth of South Africa. Without the interaction of the Vaal River with the crater structure, South Africa might not have enjoyed the gold wealth that it enjoys today. Gold, fire and water are intimate companions in the story of the crater.
Golden heritage of fire and water
The Vaal and its many ancestors have drained the central portions of what is now Southern Africa for billions of years. Before the Vredefort blast occurred, rivers were carrying gold dust into the central Witwatersrand basin, creating a great ocean in the middle of a supercontinent. Not for nothing was the Witwatersrand named “the ridge of white waters”. To the north lies the Limpopo drainage and to the south the Vaal watershed.
Mountains around the Witwatersrand lake, particularly in the north or Limpopo belt, must have contained gold – though how it got there in such quantities remains a planetary mystery. The mountains were high and steep, sending strong rivers down into the lake where they dropped their deposits of pebbles and powder gold (“placer gold”) in conglomerates that formed in the river deltas. When the ocean dried up what was left was the solid strata of what we call the Witwatersrand Supergroup today.
The strata lay some 7-12 km thick in the Witwatersrand Basin and as time went on, pressure and heat hardened most of the strata into crystalline quartzites and softer shales. These strata were lying horizontally, not standing up nearly vertically as they do now. The impact flipped the strata upwards and buried some of the gold-bearing rock very deeply. The gold appears to have been concentrated by hot fluid, moving through pores and cracks, in deepdown rock “reefs”.
About 2 billion years ago – some 700 million years after the formation of the Witwatersrand – the Vredefort meteorite hit the Earth here. It blew a hole in the crust and capsized the Wits strata, turning them on edge along the “collar” of the Dome. The gold-bearing strata were capsized along with the other strata. Further away towards the edges of the crater, gold-bearing strata were driven well below the surface as the landscape buckled and collapsed. The meteorite did not bring the gold but had a lot to do with its preservation.
If the meteorite had not hit here, the flat-lying strata of the Witwatersrand basin would mostly, by now, have been eroded by rivers and carried away into the sea. In other words, we have the impact to thank for South Africa’s modern gold wealth. The drawings on Page 7 show how the strata were bent or fractured by the impact, and how today what remains of the Witwatersrand Supergroup only breaks the surface along the Reef and at the Vredefort Dome, while the rest lies well below the surface. This is why mining operations at places like Carletonville have to go so deep to reach the gold – some of the shafts at Western deeps are nearly 4km deep! The very much eroded Bergland is what remains of these strata closer to the Dome core, standing mainly in the northwestern collar.
The Vaal now runs through the Bergland, across the granite terrain of the Dome, and back through the Bergland, along impact-related faults in the structure. Long after the impact, the Dome was covered by the Karoo Supergroup. Eventually the Vaal and its tributaries like the Klip and Suikerbosrand rivers, carried awaymuch of the Karoo sedimentary overlay to expose the much older Dome below. Thus the Vaal superimposed itself on the Dome below.
The Vaal is much younger than the Dome, about 10% percent its age in fact. The asteroid strike took place about two billion years ago, while the Vaal was born with the Karoo system which is “only” some 230 million years old. The original surface of the crater, which was possibly about 360km across, has long since been eroded away, and much of it has been reburied by newer formations. For a long period of hundreds of millions of years after the blast, the crater was steadily eroded by river systems and finally it was drowned under the Karoo sea. Heavy sedimentation completely covered the crater and its central Dome under about 10km of Karoo strata.
But then the Vaal and Orange Rivers began to remove the Karoo. The Vaal is thus a superimposed river that has come down from above and cut into the much older Dome feature lying buried below. What we can see of the crater today is the remaining structure on the northwestern side, while in the southeast it is still partly buried under Karoo layers. Before it found the Dome, the Vaal was already an ancient river (as rivers go) and it was mature and slow-flowing, snaking across the Karoo plain with big meanders. The river has kept these meanders but has now steepened and is undergoing a process of “younging” (getting younger) as it forms rapids, islands and small canyons through the Dome granite.
Look at it this way: rivers originally deposited the Wits and Karoo basins, then the impact distorted the landscape driving the gold deep down, and finally rivers exposed the remnants of the Vredefort structure along with the gold embedded in the even more ancient Witwatersrand strata. The rivers, the impact, and our gold wealth are all intimately connected.
– Graeme Addison