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      The Vredefort Structure.

                Misconceptions and Facts - Dr.   Joe Mayer

                                                                                                                      

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From what vantage point can the Dome Structure be best observed?

Considering the size of the Structure (Figure 1), it is clear that nobody would be able to see the Vredefort Dome in its entirety from any point on the ground.  Not at Venterskroon nor at any point along any of the hiking trails.  To view the Structure in totality, one needs to be at some point in space.  Refer to the satellite imagery of the Vredfort Dome on the opening page of this website.

 

 

Although the south-eastern part of the Dome is concealed, we know that it exits below the sediment cover. This is based not only on borehole information, but more convincingly so on data from satellite aeromagnetic imagery.  The latter confirm the near-circular ground plan for the entire Structure (Figure 2)

 

 

 

 

 

Fig. 2 Aero-magnetic survey confirming that the Vredefort Structure has a near circular ground plan. The fact that the ground plan is slightly           elliptical can possibly be ascribed to some degree of post-impact deformation that the Structure had suffered.

 

 

If the Vredefort Dome is a meteorite impact structure, why a dome?

If the Vredefort Dome originated because of an impact, why is the result described as a dome structure? The reason for this is that the picture we presently see at the impact site resembles that of a decapitated (eroded) geological dome. To understand how and why this picture is interpreted as a dome we need to consider the development of a normal geological dome and the effect of erosion.

 

Imagine a succession of horizontal-lying beds (formations) being pushed up at some point from below. The forces from inside the earth pushing upwards, shape the succession of formations in to a symmetrical dome. Now take a horizontal cut through this dome at some lower level (simulating the action of vertical erosion).  The result will be a structure showing up as a number of concentric rings due to the transgressive cut over the successive  superimposed formations contained in the dome. All of the formations showing up as concentric circles will be dipping outwards away from the centre of the structure. The lowest or oldest formation, however, will show up on the erosion plane as a circular core, whereas each successive higher formation will progressively form a ring further out around the core.

 

 

Because the Vredefort Structure (Figure 1) partly conforms to this picture, it has been regarded as a dome.  It differs, however, from the ideal symmetrical dome in that the formations in the north-western sector of the Structure do not dip outwards but inwards towards the centre of the Structure. The reason for this is that beds in that sector are, in fact, inverted or overturned.   Figure 3 is a diagrammatic northwest to southeast cross-section through the Structure, explaining this situation.

 

 

 

 

 Fig.3 A diagrammatic section (not to scale) from northwest to southeast through the Vredefort Structure to illustrate why the formations of the           north-western sector are dipping inwards towards the centre of the Structure (Pink = ancient granite, Brown = Westrand Group, Yellow           = Central Rand Group, Green = Ventersdorp Supergroup, Blue = Transvaal Supergroup.

 

A question that may be asked at this point is how do geologists tell if the strata in the north-western sector of the Vredefort Structure are really inverted and not merely right way-up strata dipping south-eastwards?

 

Sediments, containing “fossilised” ripple marks provide the key (Figure 4).   Ripple marks of the “oscillatory type” are caused by the to and fro movement of shallow water near the edge of a lake. Such ripples will show ripple ridges which are symmetrical in profile and characterised by sharp-pointed ridges separated by wider ripple valleys showing smooth rounded profiles (Figure 4, picture on the left).  A “negative” of ripple structures will appear on the undersides of beds i.e. the ripple ridges, in contrast, will be wide rounded ridges separated by narrow v-shaped ripple valleys as illustrated in Fig 4 (picture on the right-hand side). Therefore, seeing a “negative” of ripple marks on the apparent upper sides of beds, confirms that the beds are inverted.

 

 

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Figure 4. Ripples on upright beds left; ripple casts (negative) on overturned beds right

 

  

Now that it is understood why, what we see in the root zone of the Vredefort impact  resembles a dome structure,  we need to review the mechanism of its formation again in order  to compare it to the formation of a normal geological dome.

 

To reiterate we need to give an overview again of the events following the Vredefort impact.  It has already been mentioned that the meteorite impact resulted in a crater of 100 km in diameter being formed on surface. The collision, first of all, sent a primary shock, comprising high-speed shock waves, vertically down into the earth.  The impact event was followed by a rebound response from inside the earth, causing a central plug in the floor of the crater to undergo an upheaval of some tens of kilometres.  The timeframe for this action is possibly debatable.

(Note similar features of central upheaval are exhibited by craters on our neighbouring planets, Figure 5).

 

 

 

Figure 5.  Impact craters on the surface of Venus.  Note some of the craters showing central elevated areas.

 

 The action of upheaval, of a part of the crater floor, results in the upward folding of deep-seated layered formations around the central plug.  A long period of erosion then follows which exposes the up-folded strata, revealing a structure which resembles that of an eroded geological dome.

 

Considering terminology here, the term “Dome” for an impact structure is technically incorrect.  The reason for this is because a dome roof never existed for such a structure but a crater formed in its place. To distinguish between structures of different origins but similar appearances in eroded plan profiles, the   term “astrobleme” (meaning scar left by a falling star) has been suggested for those of impact origin.

 

If this is so, why has the name “Vredefort Structure” stuck and why not rather call the Vredefort Structure the “Vredefort Astrobleme”? In this particular case the long history of human involvement with the Structure seems to be the important reason.  The Vredefort Structure has been considered an eroded geological dome for more than a century.  The fact that it derived its name from the name of the town Vredefort (Parys at the time had not been established yet). This is an indication of how long the name “Vredefort Dome” has been on the tongues of the geological fraternity and now passed on to the man in the street.

 

About a decade ago (the late nineties) there was still some resistance to the meteorite impact hypothesis explaining the Structure. Not long ago shock features such as shatter cones, which pervade the collar rocks of the Vredefort Structure,   were still either ignored or explained in terms of crustal stress by proponents of the endogenic hypothesis (origin by forces from inside the earth).  The acceptance of the Structure to be an astrobleme is a fairly recent one, but seeing that the term “Dome” has been deeply engraved it will remain as a name for the Structure.

 

 

                                                                                                                      

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