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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).
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.
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