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      The Vredefort Structure.

                 Misconceptions and Facts - Dr.  Joe Mayer


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What evidence can be presented pointing to an impact origin for the Vredefort Dome?



Certain features which are ascribed to high-speed shock occur in the collar formations of the Vredefort Structure and have now been generally accepted as evidence for an impact origin for the Vredefort Structure.  These are: 


   Megascopic[1] evidence  

v     Shatter cones

v     Densely spaced megascopic  planar fractures in quartzite rocks

v     Pseudotachylite and pseudotachylite breccias  


  Microscopic evidence  

v     High-pressure polymorphs (related forms) of the mineral quartz and quartz deformation lamellae.  



Megascopic evidence  

Shatter cones  



 Penetrative downward-moving shock-wave fronts, due to the primary shock caused by the meteorite impact, were responsible for the formation of unique megascopic structures, called shatter cones.  Shatter cones are found in various rock types of the collar rocks of the Vredefort Structure but are particularly common in the quartzite formations of the collar.  Although referred to as cones these omnipresent structures are mostly seen only as segments of cones.  The segments may represent parts of a full cone varying from less than 20° up to 180° of the full cone. 

Characteristic of the cone segments are striations on the cone surface which radiate from the apex of the cone towards its base, showing patterns referred to as “splaying” or “horse tailing” (Figure 6).

[1]  Megascopic = Can be seen with the naked eye as opposed to microscopic



Figure 6.  The author sitting next to a large shatter cone (Division of Geology Northwest University).    

                   Note the distinct striations on the cone segment. Apex of cone is towards the bottom



Shatter cones segments are found representing different sizes of cones. The cone in Figure 7 can easily be held in the palm of one’s hand.  Although it appears to be a complete cone it is in fact a composite cone consisting of the combined segments of different cones. 






Figure 7. A composite shatter cone in basite found by Professor A.A. Bisschoff.



Only one discreet cone, at this point in time, has ever been discovered in the collar rocks of the Vredefort Structure (Figure 8). 





 Fig.8. A unique and rare discreet shatter cone found in the rocks of the Vredefort Structure, 

          discovered by the author and colleague Hans Albat.



Densely spaced megascopic planar fractures in quartzite  



These features, as illustrated in Figure 9, are widely found in the quartzite rocks of the Vredefort Structure in association with shatter cones. Their geometric relationship to shatter cones has been statistically demonstrated by colleague Hans Albat and the author.  From this we conclude that the planar fractures too are of a shock origin.  




Figure 9. Traces of a single set of densely spaced planar fractures in quartzite. 

               More often two sets of such planar fractures are found  intersecting in an obtuse angle of 100°(Note pencil for scale)



Where these planar fractures are present in beds containing pebbles, they too pass through pebbles (Figures10).  






Figure 10. A chert pebble showing planar fractures (small insert photo shows fractures on same pebble annotated).



Pebbles being objects with definite shapes, reveal that movement occurred on the planar fractures. This is because small displacements of about 1mm along one side of the fracture in relation to the other side occur so that small steps are noticeable on the pebble surface.


During the rebound shock, after the impact, deep-seated strata were disturbed  by being tilted out of their horizontal position.  In addition they were also subjected to severe shock vibrations which caused them to partly fracture in some localities.  High temperatures and high gas pressures were generated at these sites resulting therein that in places part of the fractured rock material was  ground to powder and part  converted to melt.  The powder and melt  invaded  the spaces between broken rock fragments.  Upon cooling they formed the  dark glassy material which cements the broken rock. Some of this material intruded as veins, over short distances, into cracks in the strata. This is the origin of the pseudotachylite breccia patches (Figure 12) and veins observed, in certain areas, in the Vredefort Mountain land.




Fig.12. Pseudotachylite  in Parys granite, geological hammer shows scale  - ( illustration  after  Truswell 1977)[4]


[4] TrusweII , J.F. (1977). The geological evolution of South Africa. Purnell,

Cape Town. 218 pp.  


Microscopic evidence

  High-pressure polymorphs (related forms) of the mineral quartz and   quartz deformation lamellae


Another important effect that the primary shock wave had had during its passage, is that it exerted albeit momentarily exceptionally high pressures on some of the buried quartz minerals.  The composition of quartz is of course silica (chemical formula SiO

2).  This caused some of the quartz grains of the Witwatersrand rocks to transform to high-pressure forms of SiO2 namely the minerals coesite and stishovite (Figure11).






Fig. 11.  Diagram showing pressures at which coesite and stishovite are formed at relatively low crustal temperatures. 



Martini (1978; 1991)[2] discovered coesite and stishovite in the Witwatersrand quartzites of the Vredefort Structure.  The loading by 17 kilometres of strata cannot account for pressures high enough to form coesite and stishovite.  It is therefore logical to conclude that the high-speed shock, due to the meteorite impact, was responsible for the extra pressure needed to convert some quartz grains in Witwatersrand quartzites to coesite and stishovite in the Dome area.  It may also be mentioned here that  Carter[3], as early as 1965, concluded  from microfabric studies in quartz grains  of certain quartzites from the collar of the Structure, that these frabrics resulted  from the passage of a shock wave, generated by meteorite impact.

[2] Martini, J.E.J. (1978). Coesite and stishovite in the Vredefort Dome, South Africa. Nature, 272, 715-717.

Martini, J.E.J. (1991). The nature, distribution and genesis of the coesite and stishovite associated with the pseudotachylite of the Vredefort Dome,   South Africa. Earth Planet, Sci. Left., 103, 285-300.  


[3] Carter, N.L. (1965). Basal quartz deformation lamellae – a criterion for recognition of impactites.  Amer.J.Sci.,263, 786-806.




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