15/12/2023
Landscapes and Land-forms of the Vredefort Dome
(According to Gibson and Reimold in " Landscapes and Landforms of the Vredefort Dome , Exposing an old wound ")
The Vredefort Dome is a region of strongly upturned rock formations. The term " Dome " is used in a geological sense that it describes the arrangement of rock formations that illustrate by both their geometry and their relative ages , that old originally deeply buried rocks have risen in the centre , pushing aside and rotating the overlying layers.
(Figure 1 : A schematic block diagram of the Vredefort Dome indicating the arrangement of the major rock formations (Orange : Archaean Basement Complex , Yellow : The Witwatersrand Supergroup , Green : The Ventersdorp Supergroup , Blue : The Transvaal Supergroup and Grey : The Karoo Supergroup) relative to the Vaal River and the Vredefort Mountainland. The World Heritage Site is only part of this Dome.)
The Dome is almost 90 km wide , although most of the southern half lies hidden under a thin layer of younger Karoo Supergroup rocks. In the centre of the Vredefort Dome lie predominantly granitic gneisses , most of which crystallised from magmas deep below a volcanic arc , about 3.100 Ma. These coarse crystalline Archaean gneisses define the 40 km wide core of the Dome. Surrounding this core is a 20 - 25 km wide collar of metamorphosed layered sedimentary and volcanic rocks that arrange in age from 3,074 to 2,100 Ma , and that comprise the Witwatersrand , Ventersdorp and Transvaal Supergroups. The Ventersdorp Supergroup comprise metamorphosed basalt lava flows , whereas the Witwatersrand Supergroup (quartzite , metamorphosed shale , iron formations and conglomerates) and the Transvaal Supergroup (dolomite , quartzite and shale) represent sedimentary sequences. Most of the sedimentary layers have near - vertical dips and are inverted in large sections of the collar , so that they dip steeply inwards towards the centre of the Dome.
(Figure 2 : The inner collar of the Dome showing the upturned and strongly disrupted quartzite layers of the lower Witwatersrand Supergroup - the example at Kommandonek. Dips are generally steep , and directed towards the centre of the Dome)
The overturning reflects the last stage of the dome formation , when the dome collapsed downwards and outwards under its own weight. The Dome would have taken no more than a few minutes to form after the impact , but rocks in its core were uplifted by as much as 20 - 25 km , making its formation one of the most remarkable geological events in Earth's history. It is also one of the few places on Earth were an originally vertical section of Earth's crust , more than 20 km deep , is now exposed at the surface.
Another consequence of the impact was intense heating of the rocks by the energy from the shock wave , which metamorphosed and annealed the rocks. The unusual intensity and broad extend of these metamorphic changes , together with considerations of the uplift and erosional history of the region , suggests that the rocks presently exposed at the surface , were still buried as much as 8 - 10 km below the surface of the crater immediately after the impact event. The modern landscape of the Dome bears no relation to the original crater. There was an extreme amount of erosion involved in the process.
The Vredefort Dome is only partially exposed because ist southern half is largely covered by a thin layer of Karoo Supergroup sediments. The core of the Dome is dominated by gently rolling topography at an elevation of about 1,410 - 1,430 m above sea level. A few small dome - shaped hills exposing the granite bedrock rise up to 30 m above their surroundings. The Vredefort Mountainland corresponds to the inner and middle parts of the geological collar that lie between 20 and 30 km from the centre of the Dome. The highest point in the inner collar lies at Steenkampsberg , 1,658 m above sea level , bu the overall highest point is at Bakenkop , 1,676 m above sea level , north of the Vaal River in the northern collar.
(Figure 3 : The overall highest point in the Vredefort Mountainland is at Bakenkop , on the farm Buffelskloof , at a height of 1,676 m above sea level)
All ridges correspond to outcrops of quartzite (seen at Witkop) , whereas all valleys correspond to meta - shale (seen at the Booysens shale) , meta - basalt (seen in the Tygerfontein valley) and / ore meta - dolerite (seen at Rooipoortje) rocks that are more susceptible to chemical weathering.
(Figure 4 : Ridges with outcrops of quartzite which can be seen at Witkop , Koedoeslaagte)
(Figure 5 : Valleys with basaltic rocks which can be seen in the Tygerfontein valley)
(Figure 6 : Digital elevation map of the Vredefort Dome showing the 5 topographic zones , zones : a - e)
There are 5 concentric zones identifiable in the Vredefort Mountainland , beyond the core area. The zones are as follows :
A. The inner collar : (examples at Kommandonek , Donkervliet and Koedoeslaagte)
The inner collar mostly comprise of shale , ironstone , basalt and dolerite sills with numerous subsidiary thin quartzite bands. It also has sharply defined ridges of white quartzite , and narrow steep valleys with the steepest slopes directed inwards.
(Figure 7 : An example of the sharply defined ridges of white quartzite of the inner collar which can be seen at Hydocks Rust - in the centre at the background mountain range / first impact ring)
B. The homogeneous upper Witwatersrand Supergroup : (examples at Vanvuurenskloof , Buffelskloof and Rooderand)
This comprises mostly of a 3 - 4 km zone of extremely rugged topography , cut by narrow gorges that exploit faults or major fractures that cut across the strike of the rocks. It is predominantly quartzite with a single shale layer.
(Figure 8 : An example of the rugged topography , cut by narrow gorges at Koedoeslaagte. In the background is Bakenkop , the overall highest point in the Vredefort Mountainland)
(Figure 9 : The prominent quartzite with the single layer of Booysen Shale at Rooderand. Example of Shatter Cones can be observed in the Shale)
C. The meta - basalt lavas of the Ventersdorp Supergroup : (examples at Tygerfontein , Leeufontein and Kromdraai)
The Ventersdorp Supergroup has a much more subdued topography , with their weathering resulting in deep , fertile soils , and scattered rounded hills rather than ridges.
(Figure 10 : Subdued topography with scattered rounded hills rather than ridges as can be seen in the area of Leeufontein. Bakenkop can be seen in the middle at the back.)
(Figure 11 : Evidence of fertile soils can be seen at Kromdraai which supports the saying that the Tygerfontein valley is the " bread basket of the Vredefort Dome. ")
D. The dolomite of the lower Transvaal Supergroup : (examples at Grootedrift and Gatsrand)
This dolomite is poorly exposed , but in some areas its been filled with more resistant chert to form rubbly linear outcrops. This terrain , with its relatively flat topography , is known for its sinkholes , caves and water springs , with the region generally known as Gatsrand.
(Figure 12 : Rubbly linear outcrops of chert breccia's can be seen at Grootedrift)
(Figure 13 : Within the chert breccia's outcrop one will also find this cave in the Monte Christo Formation of the Cuniespoort Group (Transvaal Supergroup). The entrance has collapsed some time ago.)
(Figure 14 : The view from Bakenkop towards the Gatsrand shows the terrain with its relatively flat topography.)
E. The upper Transvaal Supergroup :
This Supergroup covers the outer limits of the Vredefort Dome and contains a similar composition as the lower Witwatersrand Supergroup. This Supergroup is outside the boundaries of the Vredefort Dome World Heritage Site.
