A History Of Deep Time – Part 3
Prof Maarten de Wit
An ancient continent
Between 3 200 and 3 100 million years ago, the world’s oldest gold deposits formed along the north-western edge of the Barberton Mountains. Some of these are exceptionally rich and are still exploited at the Fairview and Consort mines located close to the road between Barberton and Malelane. In that same period, the Barberton rocks were extensively deformed and uplifted by plate tectonic processes similar to those operative on the present earth, and a substantial continental mass had stabilised the Mpumalanga region by 3 000 million years BC.
The hard evidence for the existence of this ancient continent comes from the escarpment to the south and west of the Barberton Mountainland,
where its rocks were engulfed by a vast volume of numerous different types of granite between 2 800 and 3 450 million years ago. These granites
were at depths of 10–30 kilometres below the surface of the crust at that time, yet today the road from Carolina to Oshoek, and beyond into Swaziland, meanders across them on a relatively flat and extensive highveld surface – a peneplain.
The age of this peneplain can be deduced from what lies directly on top of it. In the Machadodorp–Kaapsehoop region, for example, the granites
and the Barberton rocks are directly overlaid by the slightly younger series of near horizontally layered rocks that, further northward, also make up much of the great and spectacular Drakensberg escarpment. These rocks comprise the Transvaal Sequence, a flat blanket of sedimentary rocks, up to 3 000 metres thick, which formed in a shallow sea between 2 700 and 2 400 million years ago.
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TOP: A wall of pillow lava in the central region of the Barberton Mountainland.
ABOVE: Gas escape structures in a fossilised mud pool, Barberton Mountainland.
Yet, before this ancient sea could transgress across the flat granite surface, erosion must have planed off the early continent to great depths to
expose this granite close to sea level. This dates the remarkably extensive Mpumalanga peneplain to earlier than 2 700 million years BC, and as such it is the oldest mature continental erosion surface of the world. Later, this surface was exhumed again for us to traverse when the overlying Transvaal rocks were eroded in their turn.
Towards the north of Barberton, way beyond Nelspruit, as far as Graskop, and stretching across most of the Kruger National Park, the later erosion also removed rocks well below the ancient Mpumalanga peneplain to create the lowveld. The bedrock of the lowveld therefore comprises the old granites of the early Mpumalanga continent. Traversing the escarpment, near Blyde River Canyon, from Graskop to Pilgrims Rest, or from Sabie to Lydenburg, one therefore ascends from the granites of the earliest geological era of Mpumalanga directly across the ancient peneplain into the rich earth history preserved in the layers of the Transvaal Sequence.
During this journey abundant domes of fossil stromatolites may be observed. The domes are up to several tens of metres across, and are made up of cyanobacterial mats and limestone stacked on top of one another to form the earth’s earliest extensive carbonate reefs, something like the modern Great Barrier Reef of Australia, yet constructed only by primitive organisms.
By this stage in the earth’s history, bacteria had evolved into the first photosynthesisers; and their reefs were the first oxygen-producing ‘factories’ of the world, bellowing oxygen, a gas that was toxic to the existing unicellular life, into the oceans and atmosphere. Once oxygen production rates began to exceed those of the methane and carbon dioxide gases from volcanic activities, the atmosphere of the early earth gradually came to be enriched enough in oxygen for more modern life forms to emerge and thrive.
Pillow lava and pillow lava with ocelli in the Hooggenoeg Formation in the southern region of
the Barberton Mountainland.
An additional byproduct of all this oxygenation of the oceans was the production of vast volumes of iron ore, preserved in the Transvaal Sequence as Banded Iron Formations (from which South Africa now derives most of its steel products). There is keen global interest in the scientific evidence, revealed in the upper layers of the Transvaal Sequence by researchers of the University of Johannesburg, that the transition to a more modern oxygenated world apparently accelerated between 2 300 and 2 400 million years ago, but the reasons for this are still far from clear.
Further west, and higher in the sequence, the landscape is shaped by the dark rocks of the Bushveld Complex, the world’s largest magmatic intrusion
emplaced in the Transvaal rocks about 8 kilometres below the sea surface at that time, 2 060 million years ago. Stretching far across the border into Limpopo and North West provinces, this intrusion is host to the world’s greatest stock of platinum. To the south, near Uitkomst, satellites of the Bushveld intrusion are also enriched by great quantities of nickel. In other places still, tin mineralisation occurs in associated granites (later exploited by early indigenous miners).
