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Geological footprints – time capsules

19 July 2013

www.leopard.tvFor many people a rock is only that and is a hard object against which one may stub a toe. For someone with more insight, however, rocks contain clues about the origin of the Earth. Such knowledge is of inestimable value because, unlike humans, rocks are the product of countless millions of years of change in our environment.

At first glance the Waterberg may appear to be only a mass of deep-red rocks or cliffs. Closer inspection will reveal that every rock is a conglomerate of petrified sediment and pebbles that has formed in a body of water. However, it is only when one starts to wonder about where, when and how this occurred that the full picture emerges. These rock formations of the Waterberg also show a relationship with other sedimentary geological formations that occur the Soutpansberg, Olifantshoek in the Northern Cape province and Rehoboth in Namibia and indicate a similar origin.

The conglomerate rocks of the Waterberg developed in water that flowed sluggishly. Moreover, their uniform and intense red colour indicate that the water was rich in oxygen for the entire year. Dating of these rocks indicates that they were formed some 1.9 to 1.7 million years ago in a vast inland lake on an extensive floodplain that once covered much of the ancient Cape-Vaal tectonic plate or craton. This tectonic plate, which forms much of South Africa, originated some 2.7 billion years ago as an island in the primordial oceans that once covered the entire Earth. This tectonic plate was part of the first Supercontinent Ur on the Earth and the name Ur means the oldest. The Waterberg formation stretches from Bela-Bela in South Africa north to the eastern parts of Botswana, and in South Africa east to just north of Middelburg in Mpumalanga. In the Waterberg region the sediments were up to 7 km deep. This implies that the Waterberg originated from sediments which had accumulated in the deepest part of this ancient lake.

The conglomerate rocks consist mainly of pebbles and sandstone which wholly contains iron oxide. This indicates that the water was rich in oxygen when the sediments were formed and means that the Waterberg formations were the first of their kind in the history of the Earth. The oxygen was the product of cyanobacteria which were the first known form of life on the Earth with their fossils being found in rocks of the Barberton Mountains that are 3.5 billion years old. This oxygen transformed trace elements of iron in the sediments into iron oxide. In the primordial oceans the oxygen was dissolved in the water to bind with iron to form beds of iron. In sediments which show red and white layers of rock the surplus of oxygen was only seasonal and such rocks are older than those of the Waterberg.

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The source of the sediments and pebbles which formed the Waterberg conglomerates is still unclear. The valleys of the Limpopo, Sand and Nzhelele rivers are known zones of geological faulting between the old Cape-Vaal and Zimbabwe tectonic plates. There are also several other faulting systems in this region. Some people believe that pressure between the Cape-Vaal and Zimbabwe tectonic plates once created a mountain higher than Kilimandjaro in the Limpopo Valley. This mountain then eroded totally in a wet period on Earth with the sediments accumulating in the ancient lake in which the Waterberg formations were formed. Other people believe that the sediments were formed by the weathering of the rock of the Palala Plain as large rivers once flowed to the south and south-west over the entire Cape-Vaal tectonic plate towards a giant ancient lake. This will, however, still imply that the origin of the Waterberg sediments is to the north of it. Because they are hard, these conglomerates resisted weathering when the lake dried up some 1.5 billion years ago while the surrounding land eroded. As the softer material surrounding these formations gradually eroded, the Waterberg arose from the plain as a giant, hard core to form the largest upside down rock saucer or lotolith that is known to the world.

The Waterberg has a rich history but it was formerly a much wetter place. In Northern Sotho it is therefore known as Thaba Meetsi, or the mountain of water. Based on its rich biodiversity and unique origins the Waterberg has been declared an International Biosphere Reserve. Like the needle rocks of the Nambung National Park in western Australia and the Tsingy de Bamaraha rock forest on the west coast of Madagascar, among others, the Waterberg is a geological time capsule which reflects a former wetter environment on Earth long before the appearance of humans.

The needle rocks, or Pinnacles, in the Nambung National Park near Cervantes north of Perth along the coast of western Australia only became known to most Australians in the 1960s. The area once was most probably part of a shallow estuary with a rich marine life. Countless sea shells drifted to the sea floor with time to form a thick calcrete layer. When sea levels started to drop, wind erosion formed high but unstable dunes of calcrete-rich sand along the coast. The dunes eventually migrated further inland where rain moved the calcrete to the lower levels of the dunes. Weathering of the dunes eventually allowed plants to grow there and their roots penetrated cracks in the soft calcrete and opened up root canals. When the plants later died, quartzite (sand) filled the cracks and root canals. The further erosion of the calcrete eventually left a forest of petrified quartzite needles as silent witnesses of environmental change over many centuries. Although there are also other possible explanations, they all tell the same basic tale.

www.leopard.tvThe Tsingy de Bamaraha rock forest in the Melaky region of the west coast of Madagascar covers a surface area of some 1519 km2 and has been declared a UNESCO World Heritage Site in 1990 based on its unique geography, mangroves and wildlife. The southern part of some 666 km2 forms part of the Tsingy de Bamaraha National Park which allows tourism, but the northern part of 853 km2 is a strictly protected area. The entire rock forest consists of sharp, high rock needles of calcrete which formed in ancient sedimentary deposits through wind and groundwater erosion. It is fascinating to see how easily and agilely wild animals such as lemurs move around in and over these needles which protect the wildlife against predators and humans and offers shade in hot temperatures.

When someone therefore stubs a toe somewhere against a rock, it may be interesting to delve more deeply into the origin of the rock. It will create a whole new source of knowledge and will once again show that humans have only been on the Earth for a moment of time. This means that we have a moral obligation for the conservation of the environment.

 

References

Anon 2013. Tsingy de Bamaraha Strict Nature Reserve. http://en.wikipedia.org/w/index.php?title=Tsingy_de_Bamaraha_Strict_Nature_Reserve&oldid=548461412

Anon 2013. The Pinnacles (Western Australia). http://en.wikipedia.org/w/index.php?title=The_Pinnacles_(Western_Australia)&

McCarthy, T & B Rubidge 2005. The story of Earth & life. Cape Town: Struik Nature.

Walker, C & J du P Bothma 2005. The soul of the Waterberg. Houghton: Waterberg Publishers & African Sky Publishing.

By: Prof J du P Bothma

 

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