Lessons from the past
The impact that altered
a landscape
The Vredefort meteorite had a significant impact on the history of mining in
South Africa. Edited by Leon Louw
T
Shortly after formation, the Vredefort impact
structure would have had a central region of strongly
uplifted rock, a flat floor and a terraced rim. After
long-term erosion, all that can be confidently
assessed is the diameter of the central uplift.
This region of strongly upturned rock, called the
Vredefort Dome, is about 90km wide. Based on
scaling comparisons, geologists assume that the
original crater must have been several times this
diameter, with most pertinent estimates in the
250–300km range. This makes Vredefort the largest
impact structure yet discovered on Earth. It is also
the oldest confirmed impact structure identified on
Earth. Notwithstanding the almost 10km of erosion
that had occurred since impact and the fact that
the southern half of the impact structure is hidden
beneath younger sedimentary and volcanic deposits
of the Karoo Supergroup, the Vredefort Dome
retains a dramatic circular topographic morphology.
The Vredefort Mountainland is a circular crescent
of ridges and valleys some 10km wide and 100km
long, encircling a comparatively low-lying central
core of approximately 40km in diameter.
The sedimentary and volcanic rock layers
exposed in the Vredefort Mountainland were once
presumably horizontal, but they were strongly
disturbed by the impact so that most now attain
near-vertical, or even overturned orientation. They
preserve extensive evidence of the impact catastrophe,
both in outcrop and microscopically.
Pseudotachylitic breccias ranging from microscopic
veins and pods to networks up to hundreds of metres
long, are among the most prominent impact features.
Well exposed in former dimension-stone quarries,
these are only surpassed in individual volumes by the
breccias in the less eroded Sudbury impact structures
in Ontario, Canada.
Impact-melt rock, called Vredefort Granophyre, is
found only in vertical dykes, each several kilometres
long and up to tens of metres wide, that represent
downward-directed intrusions from the original
impact melt sheet that would have lain within the
— now eroded — crater. The Granophyre contains
a diverse population of rock fragments and a small
(0.2%) chemical trace of the bolide.
Shatter cones from a few up to tens of centimetres
in size, are most prevalent in the sedimentary-
volcanic rocks in the mountainland. Microscopic
evidence of shock includes PDFs in quartz and
planar fractures in zircon, as well as the presence
of coesite and stishovite, both high-pressure
polymorphs of silica. Given the size of the impact,
scientists were originally puzzled about the lack of
impact-produced rock and mineral glasses. But in
the 1990s, they recognised that large impacts impart
significant residual heat into the shocked rocks and
that, given time, this heat would have caused glasses
to recrystalize. These recrystaliized glasses can be
seen in unusual granofels rocks in the central parts of
the dome.
he Witwatersrand Basin, in
which most of South Africa’s
gold is mined, would have
looked decidedly different had
it not been for the gigantic meteorite that
hit the Earth about 2 000 million years
ago, close to Vredefort. It is believed that
the extraterrestrial meteorite was larger
than Table Mountain. The Vredefort
Dome impact crater is one of only 200
such impact structures that have been
identified on Earth. The geology in this
dome is fascinating.
What follows is a direct excerpt from
the article by Wolf Uwe Reimold and
Roger Gibson, titled “Africa’s Impact
Heritage”, which describes the formation
of this important geological structure.
The article was published in Africa’s Top
Geological Sites and was first published in
2016. The publisher is Struik.
The Vredefort Dome, south of Johannesburg, has a fascinating geology.
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MAY 2019 MINING MIRROR [35]