THE VREDEFORT RING
THE
V REDEFORT
131
RING.
By B. B. Brock, B.A.Sc., Ph.D., M.I.M.M.
[PLATE
XXIV]
ABSTRACT.
The Vredefort Ring, consisting of an overturned collar of 40,000 feet of sediments and
lava around a core of older granite, is situated at the deepest portion of a geosyncline
which differs from most of the described geosynclines of the world in that it never gave
rise to a folded mountain chain.
The outer rim of the basin discloses a set of converging faults of great displacement
which, when projected inwards under the cover of younger beds, circumscribe the granite
core. Evidence is presented to show that these faults are of extremely early age, thus of
very deep penetration, and that they have had repeated movement throughout the ages.
These faults, it is submitted, released a column of old granite which became a safety
valve to relieve the tremendous maladjustments caused by the concentrated accumulation
of sediments. A slight hade away from an original hub is postulated giving a wedging
action which would convert the vertical upthrust into a radial centrifugal pressure in the
crust, evidences of which are abundant (apart from the overturning of the collar).
The structure is thought to be the surface expression of a terrestrial process which
worked on the principle of the hydraulic jack. This inevitably demands the presence of a
liquid, at depths below the zone of possible observation. The liquid would be a basaltic
or andesitic magma, of which there is plenty of surface evidence. The other essential
requirement is a confined space containing the liquid, without which the principle cannot
operate; this is presumed to be the space into which the geosyncline subsided.
The importance of this interpretation of the structure is that it illustrates one way
in which purely vertical upward forces can operate in the earth's crust.
TABLE OF CONTENTS.
Page
INTRODUCTION
THE UNIQUE CHARACTER OF AFRICA AS A CONTINENT
RARE INSTANCES OF LATERAL COMPRESSION
EVIDENCE OF DOMINANCE OF VERTICAL MOBILITY
FAULTING WITHIN THE GEOSYNCLINE
EARLY AGE OF THE VREDEFORT FAULTS
VENTERSDORP "PLATEAU LAVA"
PROBABLE PRE-WITWATERSRAND VREDEFORT HUB
132
133
134
135
136
138
138
139
THE VREDEFORT RING AND THE GEOSYNCLINE
140
VERTICAL MOVEMENT TRANSLATED INTO HORIZONTAL
141
142
142
143
144
THE YOUNGER GRANITE
BRACHYSTRUCTURES
CONCLUSION
REFERENCES
132
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
INTRODUCTION.
The Vredefort structure, comprising a thick overturned collar of sediments
and lava encircling a core of older granite, is an unusual feature of a unique
continent. There is no necessity to describe it as it has been admirably described
and meticulously mapped by L. T. Nell and it has been a subject of controversy
for twenty five years. No"one, not even the eminent Professor Daly2, has produced
a solution that satisfies everyone.
There are many and varied theories, the high spots of which are reviewed
below:(1) Centripetal pressure was postulated by Hall and Molengraaff3 and
.received N e1'sl partial support.
(2) Nell suggests magmatic upheaval as an alternative solution.
(3) Bailey4 attempts to show how a magma can develop a centrifugal
force to account for the overturning of the strata forming the rim.
(4) Du Toit 5 suggested that the granite core behaved like a gigantic punch,
producing an asymmetric mushroom structure, the weight of which presumably
overturned the beds.
(5) Bishopp6 denies the possibility of the sediments ever having been
mushroomed because of the impossibility of elongating the beds to that extent.
His rather elaborate scheme to convert vertical pressure into horizontal centrifugal pressure tends to obscure the merit in his analysis of the resulting forces.
His reply7 to Truter's8 criticism contains a succinct summary of the essential
approach. This will be referred to again in due course.
(6) Maree 9 contributes something to the knowledge of the shape of the
granite ,under the Karroo cover, but adds little to the clarity of the tectonics.
Taking his cue from Truter, brachystructures occupy much of his attention.
(7) CIOOS10 considers that the cover was pierced and not domed. He
recognises that the tectonic movements around the edge of the basin preceded
the end of sedimentation of the Witwatersrand system, and he attaches great
importance to the fact that the site of the upheaval is where the sediments are
deepest and hitherto undisturbed.. He gives a series of diagrams showing the
upward thrust of the core turning gradually outwards as the beds are uplifted.
N ell!, after commenting on Cloos' paper, says "A complete solution of the
Vredefort problem cannot be claimed at present."
(8) Daly2, with a fine disregard for all previous theories, introduced a large
meteorite to solve his troubles. The significance of his paper is that with his
sixty years of world-wide experience he found all previous tbeories quite inacceptable.
At the centre of the sedimentary basin, which contains the very essence of
South Africa's prosperity, is the Vredefort Ring, whose origin must be under':..
stood before the subsidiary surrounding structures can be fu11y understood.
The latter will be a major concern of every gold mine in the Free State and the
Klerksdorp area. For this reason it is believed that the recording of recently,.
discovered relevant facts which might elucidate the problem is justified .. All the
THE VREDEFORT RING
133
data collected in the course of four years of intimate association with the development of the Free State and Klerksdorp areas points in the one direction, and such
theory as has been introduced into this dissertation has been necessary only to
make the picture complete.
The writer believes that the true picture of events has been obscured in
the past by (a) confusion between cause and effect and (b) a lack of appreciation
of the essential fact that we are dealing with a terrane where block faulting is
the rule, and vertical movement is far predominant over horizontal movement.
Such horizontal movements as are in evidence are of a secondary nature-the
result of certain specific vertical movements.
THE UNIQUE CHARACTER OF AFRICA AS A CONTINENT.
Africa as a continent, excluding the part north of the Sahara which is
Mediterranean in its structural relationships, is unique in that it consists largely
of an elevated plateau. This explains incidentally why Africa remained the dark
continent for so long: the difficult access to the hinterland which distinguishes
it from other continents.
The reasons for Africa's aloofness need not concern us, as long as we recognise
that character, which cannot be doubted if we look at a globe showing the present
mountain systems of the world. The major mountain systems can be broadly
divided into two groups, one centred on the Himalayas and the other on the
High Andes. The Himalayas constitute a focus from which offshoots sprawl
spider-wise over Asia, Europe, the East Indies and even Australia. Only the
extreme northern fringe of Africa is affected. The Andes-Rockies group trends
the length of the Americas. Southern Africa is about as far away from these
disturbances as it could possibly be.
The elevation of the continent has been persistent since pre-Cambrian
days; there have been no major encroachments of the sea since Transvaal times,
no batholithic intrusions on a large scale since the pre-Cambrian, few evidences
of compression, few folded mountains; plateau lavas are common; basaltic
sills, dykes and flows have been predominant since pre-Cambrian times almost to
the exclusion of acid intrusions. Such thrust faulting as exists can commonly be
seen to be a subsidiary effect of some other more important process.
Rift valley conditions are becoming increasingly apparent in the Witwatersrand-Ventersdorp rocks of the Free State gold field and the Klerksdorp area. In
the 'latter, where exploration is more advanced, the predominance of normal
faulting over thrust faulting is conspicuous. In the Free State gold field only
about one per cent of the many hundred boreholes show repetition of bedding,
and then only on a small scale. The rule is " loss of ground", hence normal faults.
A discussion of the rift valley question in general is outside the scope of this
paper beyond pointing out that, even if the Ramp hypothesis be accepted, the
Great Rift Valley system shows an overwhelming preponderance of vertical
movement over horizontal movement at the surface. This in itself-over such a
prodigious extent.:.-makes Africa unique among continents.
134
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
The abundance of dolerite and basalt demands a considerable number of
fractures which penetrate deeply enough into the crust to tap a source of basic
magma. The comparative absence of granite since pre-Cambrian days is dependent on the comparative absence of folded mountains. Both these observations
are in accordance with the fundamental premise that Africa is different from
other continents in that a very large proportion of it comes into Bucher's12
category of " a fracture zone of low mobility".
The true structural character of South and Central Africa has been largely
obscured by a number of incidental or superficial features, primarily (1) the
Karoo Basin, (2) the Kalahari sand, (3) the folded mountains of the Cape and
(4) the vast dome-like mass of older granite constituting most of Southern
Rhodesia. Where the fundanlental structure is not masked, block faulting or
rift faulting is observed to be the rule, beginning in the pre-Cambrian and
continuing (with some gaps in the record) up to the post-Karroo in southern
Africa and to the Tertiary or later in East Africa.
Vertical uplift of older granite masses is very apparent. The scale varies
from enormous terranes like Southern Rhodesia, down to domes measured in
just a few miles like Konkola Dome in the Copper Belt, an elliptical dome of old
granite with the surrounding sediments dipping quaquaversally at angles from
40 to 70 degrees. This type of structure is common throughout Africa. Whatever caused the vertical uplift, its presence cannot be denied. Grabens most
commonly occur in the inter-granite areas but are by no means confined to them.
This, then, is the setting of the area about to be discussed, and it is important
to see it in its proper setting. The principal factors are (1) a surprising vertical
mobility of masses of old granite, (2) a corresponding tendency of inter-granite
areas relatively to subside and (3) a very marked tendency to graben-and-horst
structure in the inter-granite areas.
RARE INSTANCES OF LATERAL COMPRESSION.
Real mountains must have been present round the Witwatersrand basin
in pre-Witwatersrand times to provide such an abundance of coarse sediments
for the Witwatersrand beds. The schistosity too in the remnants of the older
sediments tells of compression. In those days Africa was probably similar to
other continents in having folded mountain ranges with associated large scale
batholithic intrusions and inter-range lowlands vulnerable to the advance of a
rising ocean. The evidence of this lies in the prolific amounts of old granite,
bounding basins containing the Witwatersrand and Transvaal systems of sediments, which speak of widespread encroachments of the ocean.
Since the early pre-Cambrian days (of whose mountains only remnants are
exposed) evidences of horizontal compression are comparatively rare. Isoclinal
folds are conspicuously absent. Exceptions to this rule are local.
The Cape mountains, forming the margin of the continent hundreds of miles
to the southward of Vredefort, are the nearest approach to folded mountains
that we have. There are also some steeply inclined beds and some overthrusts
in the Waterberg series to the north. Neither of these have any bearing on the
THE VREDEFORT RING
]35
Vredefort problem. The Bushveld Complex, which is a problem in itself, is not a
normal case of mountain building. In any case it has no direct connection with
the pro blem in hand.
Truter 8 mentions intense compression near Prieska about 400 miles southwest of Vredefort and suggests that it may have a bearing on the latter structure.
In a country manifestly riddled with pre-existing faults, this compression, it is
submitted, could not persist very far laterally, particularly as the thrusts and
overfolding observed in the Prieska area are in themselves the manifestations
of the relief of that lateral pressure.
In the immediate vicinity of Vredefort there is the question of the overturning of the beds that form the collar. This is one of the features which inspired
this paper and will be dealt with in due course. The thrust faults, arranged more
or less concentrically around the " dome" (including the Potchefstroom thrust
described by Truter13 ), likewise are an incidental effect of the Vredefort movements.
EVIDENCE OF DOMINANCE OF VERTICAL MOVEMENT.
The" folding" of the Witwatersrand beds round the basin's edge does not
represent folding due to compression involved in the contraction of the earth's
crust. The inclination of the beds is largely the result of the relative upward
movement of granite hubs round the rim of the geosyncline or, alternatively,
the subsiding of the basin relative to the rim; probably both.
The Johannesburg hub, for instance, cannot have been denuded of its great
thickness of sediments and lava without having risen a comparable distance.
Alternatively, if the full thickness of sediments was never laid down near the top
of the dome, it was because the dome had already risen relative to the basin.
Either way a large vertical movement is indicated to allow of an exposed granite
surface in the pre-Ventersdorp land surface, while the lava, a short distance
away, lies conformably on top of 25,000 feet of clastics representing the whole
of the Witwatersrand system.
The large angular unconformity below the Gold Estates at Klerksdorp14
means that the uplift of the edge of the basin had already begun in Witwatersrand
times.
Unconformities, locally conspicuous near the edge of the basin, fade out
towards the centre. The step faulting away from granite hubs is thought to
date back to these times.
The underground evidence at Western Reefs shows normal faults very much
predominant. Thrust faults are very minor, confined to a mangled corner of a
fault block bounded by normal and transverse faults. Dips of bedding are
moderate, and vary from the original attitude only by tilting of fault blocks.
Folding scarcely comes into the picture. The same is predicted of the Orange
Free State goldfield.
On the evidence there is no escaping the conclusion that within the area
under consideration the predominant forces acted vertically-gravity with its
concomitant adjustments, acting on blocks of various sizes, bounded by steeply
dipping faults.
136
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
FAULTING WITHIN THE GEOSYNCLINE.
The major faults within the geosyncline can be classified into four main
groups : 1. Strike faults dipping away from the margin of the basin.
2. Strike faults dipping towards the margin of the basin.
3. Faults radial to the margin of the basin, transverse with relation to the
strike of the beds, and tangential to the Vredefort hub.
4. Thrust faults arranged concentrically around the Vredefort hub.
Where two granite hubs on the margin are quite close together, and a thrust
is directed towards the gap between them as in the case of the East Rand, the
crowding effect complicates the stresses and the fault pattern, and has caused a
number of faults which have not been fitted into the above generalised table.
(1) Normal step faults dipping away from the granite margin are strike
faults, roughly parallel to the edge of the basin. This set is typified by the
Buffelsdoorn fault and by the western fault of the Odendaalsrus graben.
(2) Normal faults, complementary to the first set, i.e. dipping towards
the margin of the basin, are typified by the Schoonspruit fault and the eastern
fault of the Odendaalsrus graben.
At Western Reefs the Buffelsdoorn type is predominant, whereas in the Vaal
Reefs area, closer to the centre of the basin, the complementary type predominates.
Both of these sets are of a very early age, and movement has been continued intermittently on them throughout the ages. N eF4 has dated them as
beginning about the time of the extrusion of the Ventersdorp lava. Borehole
evidence in the Orange Free State strongly suggests that these faults might have
begun before the end of the Witwatersrand period. The early normal fault
system being parallel to the margin of the basin, it follows that it is also roughly
concentrically arranged with reference to the Vredefort hub which is in the middle
of the basin.
(3) Transverse faults, apparently younger than the early normal faults,
are found in every mining district. Their characteristics are the steepness of
their dip, and their comparative straightness. (There are exceptions to this due
probably to later disturbance.)
In any individual mining area these faults may rightly be called transverse
faults, but from the broad viewpoint almost all these cross faults are heading
roughly for one margin or other of the Vredefort hub, that is tangentially to it.
Henceforth in this paper they will be called" Vredefort faults". The term" tangential fault" is avoided because "tangential" is usually tacitly understood
to mean relating to the earth itself. Typical faults of this group, to mention
only better known ones, are the Bank fault, the 'Vest Rand fault and the Sugar
Bush fault. There are about a dozen established, many of them as yet nameless,
and more are suspected.
(4) Thrust faults, concentric to the Vredefort structure, are typified by
the Potchefstroom fault.
THE VREDEFORT RING
137
Lest it be thought that the Vredefort faults have been arbitrarily selected
from a hodge-podge of apparently haphazard faults, it should be stated that there
are very few major faults which do not fall into any of the above categories, and
that the better known of the Vredefort faults have throws measured in thousands
of feet.
The early normal faults do not affect the Vredefort problem very intimately,
except in so far as they point to the structural trends being of a very early age,
and to the Vredefort hub being some sort of focus even during the deposition of
the Witwatersrand system.
Observations on the Vredefort faults are confined largely to the margins of
the basin~ This is in the nature of things, in view of the complications introduced
by subsequent thrust faulting in the overlying Transvaal rocks nearer the centre
of the basin.
In close proximity to the Vredefort Ring, the early faults will have been
obscured by the overturning of the beds.
The Vredefortfaults thus can never be followed from the margin of the basin
to their intersection near the centre. Their magnitude, however, and the general
-straightness of these faults makes it permissible to project them beyond their
visible limits. Faults with throws of the order of 7,000 feet (Ararat and Bank)
to 15,000 (Sugar Bush) cannot be dismissed lightly. The inward projections of
the dozen circumjacent faults shown on the accompanying plan combine to make
a very fair outline of the hub.
Evidence of actual faults bounding the older granite is not easy to find on
the ground, nor is this very surprising. At the base of the inner rim of Witwatersrand hills there is usually a hiatus of a few hundred feet without outcrops. Even
where granite itself forms a part of the innermost range of hills there is a marked
depression between it and the lowest Witwatersrand member. At one place
where outcrops of granite were observed within 100 feet of the base of the Witwatersrand system, the granite was indeed gneissic or mylonitised, with large
quartz augen smeared around with sericite. Admittedly this does not necessarily indicate faulting but, whatever hypothesis is adopted, there must have
been a great movement between the core and the collar. Nell estimates the
amount of vertical movement to be 40,000 feet.
For the greater part of the periphery of the Ring the actual contact would
seem to be well hidden beneath soil and talus at the base of the surrounding
hills, and this, it is submitted, is because of the relatively low resistance to
weathering of the mylonitised granite in contrast to both the Witwatersrand
rocks and the more massive granite of the Core.
The writer has examined only a few miles of the periphery and would venture
the opinion that the actual contact is not likely to be sufficiently exposed to prove
anything. Such exposures as have been recorded are complicated by movements
subsequent to the uplift.
The contact, however, has been mapped with sufficient accuracy that. the
straightness of segments of the periphery is noteworthy.
138
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
It might be argued that the Vredefort faults are a result of the Vredefort
upthrust, and indeed they could not avoid being influenced by that drastic
movement. One of the primary purposes of this paper is to show that the Vredefort faults preceded the upthrust of the Vredefort granite and that they were an
essential part of the process.
EARLY AGE OF THE VREDEFORT FAULTS.
The rough symmetry of the Vredefort faults within the basin of sediments
indicates a close relationship between the pattern of the faulting and the shape
of the basin, and hence with the collapse of the geosyncline due to an accumulation of sediments too great for the stability of the crust. This in itself dates the
age of the Vredefort faults as being prior to the upthrust.
Corroborative evidence regrettably is sketchy and is based to a large extent
on borehole evidence which, although highly suggestive, does not constitute
proof. Borehole data are accumulating to show that the amount of displacement
of a fault as measured by reefs in the Witwatersrand system is very commonly
much greater than the throw as measured on the base of the lava. This is so of
the Ararat fault among others in the Orange Free State. The Bank fault on the
West Wits line also shows a great variance of throw depending on the datum
used. The movement on this plane of weakness appears to have begun in the
pre-Ventersdorp, continued pre-Transvaal, and again post-Transvaal.
The thickness of the Lower Ventersdorp lava where it is protected by the
Ventersdorp Upper Sediments is observed in boreholes to vary rapidly from one
side of the Ararat fault to the other. There again is a strong suggestion that
pre-Ventersdorp faulting controlled to some degree the distribution of the lava.
This is especially the case where the lower lava is missing altogether on the
upthrow side of one of the bounding faults of the Odendaalsrus graben while
still under protective cover of the upper sediments, which have been intersected
frequently enough in this locality to be correlated as a continuous body. The
indication is that the graben was an established structural feature before the
lava was extruded.
As the data accumulate, more and more point to a very early age of faulting,
and to repeated ages of movement on the same fault. The latter feature is particularly striking in the underground observations at Western Reefs.
VENTERSDORP •• PLATEAU LAVA".
Although it is virtually certain that some of the major faulting was instituted
before the end of the Witwatersrand period, there is no doubt that this was
augmented and intensified in the Ventersdorp period by the great outpouring of
plateau lava.
The term" plateau lava" is used advisedly, because, in the absence of volcanic necks or throats, it is believed that dykes constituted the feeders. In any
case, volcanic cones are not common15 , apar~ from local evidences of accumulation
THE VREDEFORT RING
139
of volcanic debris, whereas there is an abundance of evidence to show that the
lava had been exceedingly liquid and had flowed for great distances horizontally.
The constancy of the markers of porphyritic lava, for instance, tells a convincing
story.
The characteristic of plateau lava country is block faulting. This is a worldwide rule. Not only is there a potential void created by the vacating magma, but
there is the additional weight of the lava on top of the crust. Gravity naturally
is the major force concerned, and block faulting is the result.
Examples of plateau lava country are the Deccan in India, Snake River in
WaShington-Oregon, etc. The structural aspect in such areas is the domination
of block faulting. Lateral compression does not come into the picture.
PROBABLE PRE-WITWATERSRAND VREDEFORT HUB.
Willemse16 , who made a comprehensive study of the Vredefort granite,
concluded that the primary structures in the old granite indicate that the circular
structure of the region might have been largely determined by the nature of the
Basement Complex.
By a totally different approach, the writer's observations and deductions
have also led to the idea of a pre-Witwatersrand Vredefort hub. The circular
sweep of the early normal fault system round the inner ring suggests this. Also,
there is a marked tendency of the Vredefort faults to skirt the base of the hubs
on the margin of the basin (e.g. Sugar Bush, West Rand and Vaalbank faults).
There is furthermore a habit of faults to join tangentially two such hubs. The
pre-Witwatersrand hubs of granite controlled the early topography and the limits
of the basin; no doubt they influenced or were influenced by the earliest faulting.
Faults intimately related to the earliest topography can with a certain degree of
assurance be given a correspondingly early age. Thus the Vredefort faults which
tangentially join a hub on the rim to the Vredefort hub probably had their
inception in pre-Witwatersrand days, which naturally demands a pre-Witwatersrand age of the Vredefort hub. Here, incidentally, is a convenient source of the
Dominion Reef Volcanics (here well developed).
The penetration of the Basement by the Vredefort faults is certain in any
case, but the suggested Basement origin of them adds plausibility to their
continued steepness with depth.
Here it might be mentioned that if this origin be accepted it follows that a
slight hade of these faults away from the hub is to be expected.
From the sedimentational aspect, it is perhaps significant that, although
the total thickness of the section is greatest at the centre of the basin, the lower
portion of the Lower Witwatersrand of the collar is very appreciably thinner than
near the rim of the basin, suggesting that the Vredefort hub existed as an upland
in the Basement topography.
The Vredefort hub, if it existed in pre-Witwatersrand times, obviously was
suppressed under the enormous weight of superimposed sediments, and the
Basement faults would be temporarily sealed up, later to be made use of in a
big way during the collapse of the geosyncline.
140
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
Whether or not the original Vredefort hub existed is still somewhat con~
jectural, however plausible. The Vredefort faults, however, are realities, whatever may have controlled their precise location. Without undue extrapolation
(their straightness and magnitude have been noted) together they bound the
granite plug in a roughly circular shape.
It should be noted that evidence of the Vredefort faults cannot be expected
to be found in the surrounding rim of the Vredefort hub. The early faults were
nearly vertical when the strata were horizontal, and would be rolled under with
the overturning of the beds. .All that would remain of them would be the roughly
circular boundary of the granite plug.
THE VREDEFORT RING AND THE GEOSYNCLINE.
The Vredefort Ring occupies the deepest part of the Witwatersrand geosyncline, where the thickness of sediments and lava was about 40,000 feet. The
minimum npward movement of the plug of old granite must have been this
distance and it probably was considerably more. This represents an extreme
example of the predominance of vertical movement over horizontal at the
surface of the earth's crust.
It is a well established geological fact that folded mountain chains are
normally the ultimate outcome of the collapse of a geosynclinal basin. There are
no folded mountain ranges worthy of the name that are not associated with a
geosyncline in which great thicknesses of sediments had been deposited. This,
as far as the writer is aware, is a universal law. The folding of the mountains
affords a relief to the pent-up stresses of overloading. The converse-that
geosynclines must give rise to mountain chains-is not so infallible, but in those
cases where the latter rule does not apply, such as the case under consideration,
one must look for an alternative manifestation of a relief of pent-up stresses.
In the Vredefort area we do not have to look very far for such evidence.
The granite core of the Vredefort Ring, released by tangential faults, acted as a
safety valve. The uplift of a cylinder 26 miles in diameter for a vertical distanee
of over seven miles, represents a considerable relief.
The mechanism causing these major adjustments, seems to be a fairly exact
analogy to a hydraulic press. The geosyncline represents a piston, its weight
represents the force applied, and its subsidence the stroke of the piston. The
second piston, actuated by the first, is represented by the column of rock released
by the Vredefort faults. The force of the subsid~nce of the basin under the weight
of i.ts contents is transmitted through the medium (we must presume) of a liquid
in a closed system, to lift the smaller piston with great force.
The analogy is so nearly perfect, in so far as can be judged by results alone,
that one is tempted to go further and deduce something of the nature or the
mechanism at depth. The liquid involved can only be the basaltic magma of the
substratum, and for this terrestrial hydraulic jack to work, the magma must be
in a confined space, otherw,ise there would be no building up of pressure to .the
required amount to lift the cylinder of rock through a great distance.
THE VREDEFORT RING
141
It should be pointed out at once that this upward movement of a faultbounded cylinder of old granite should not be confused with isostatic adjustment.
The only parallelism is the vertical direction. At the time of the initiation of the
movement, the specific gravity of the fault-bounded plug of bedded rocks would
be practically identical with that surrounding it. There is no reason to believe
that the isostatic equilibrium over the area of the geosyncline required any
appreciable internal compensation, certainly not enough to produce such drastic
results.
The release of the plug, it is submitted, was caused by the Vredefort faults
which originated very early in geologic history (and thus penetrated very deep
into the crust), and which developed before and/or during the outpouring of
Ventersdorp lava, when the thickness was prodigious enough to threaten the
stability of the whole basin. The early fractures thus are an integral part of the
process.
Hence it is seen that the process is one of punching or piercing and not of
doming. Bishopp 7 insisted that the term " dome" is incorrect and misleading,
and suggests the "Vredefort Ring", with which this writer has willingly complied.
VERTICAL MOVEMENT TRANSLATED INTO HORIZONTAL.
Returning now to the concentrically arranged thrust faults around the Vredefort Ring, Bishopp 7 insists: "These radial stresses were transmitted outwards
with decreasing intensity and diminishing uniformity of distribution". The
evidence of this is in the present distribution of the rocks.
The Potchefstroom thrust, Truter13 claims, was caused by a force from the
west. If this were so (Bishopp argues), the pressure, already released at the thrust
plane, would not have been fully transmitted to the inner circle of the Vredefort
structure.
Subsequent pertinent data supporting Bishopp are: (a) the Schoonspruit,
Kromdraai and Buffelsdoorn normal faults, which were in existence long before
the VredefQrt structure, pass west of Potchefstroom. These would certainly
have absorbed any thrust that came from the westwards, leaving very little
thrust for Potchefstroom. (b) Early normal fault planes dipping northwest have
been deduced from the intensive boring at Vaal Reefs. From our more intimate
knowledge of this set of faults at Western Reefs, compression is known to have
" closed up " old normal faults to a greater or lesser degree, never so great that
the thrust fault predominates (except possibly in the overlying Dolomite series).
It is an almost infallible rule-of-thumb amongst mining folk at Western Reefs
that when a raise steepens a down-throw fault is being approached. In other
words the drag of a thrust fault occurs on the plane of a normal fault. It is believed that this is a less intensive instance of what happened in the Potchefstroom
thrust where the latter was so intense as to have obscured the early history.
The pre-existing northwesterly dipping normal fault would make the centrifugal Vredefort thrust here an underthrust from the southeast, which is precisely
what Bishopp argued without the advantage of the knowledge of the early
faulting.
J
142
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
A. tendency of the Vredefort faults to hade slightly outwards from the centre
would result not in a cylinder but a gentle cone truncated at the top. There is
very little positive evidence about this because the actual fault planes are rarely
exposed. Only the A.rarat fault can be said (from borehole data) to dip away from
the hub.
Imagine the wedging effect of a gently tapered cone on a seven mile upward
journey! Imagine the horizontal thrust radiating from this process! There is
no need to look further for an explanation of the overturned beds, nor of the
horizontal thrust which is transmitted radially through the uppermost beds at
least as far as Potchefstroom, and to a lesser degree as far as Western Reefs.
The rising of the core, it is submitted, began during the deposition of the
Witwatersrand beds or the Ventersdorp lava. The rise was gradual and the plug
projecting at the surface would probably be eroded quickly but with, of course, a
lag. Between the Ventersdorp and the Transvaal periods, erosion probably
levelled off the projecting core, and the Transvaal system evidently was laid over
the top. Movement, however, continued in post-Transvaal days, and the Trans:'
vaal system was pierced and overturned in a similar manner.
The repeated movements of the uplift of the Vredefort core are no doubt
reflected in the repeated movements in the faults with which we are becoming
increasingly familiar.
THE YOUNGER GRANITE.
The younger granite, whose outcrops are situated off-centre in beds previously overtilted, obviously arrived there very much as an afterthought. It
sneaked quietly into place presumably by stoping but certainly without disturbing the high degree of regularity in the disposition of the ring, which follows
a highly organised pattern.
The lack of symmetry of the contact type of metamorphism noted by N eP
is quite in keeping with the asymmetrical distribution of the younger granite on
the surface, and it points· very clearly to an eccentric body below the surface,
and not necessarily a very large body at that. Is there any reason to go further
to explain the eccentricity of the contact metamorphism ? If a magma had caused
a seven (or more) mile uplift of a column of overlying rock it would have been
the result of a tremendous urge to reach the surface by brute force and by the
shortest possible route. Even if this were mechanically possible, it is not compatible with the insinuating habit of its "offshoots", which have used subtlety
much more than force.
The Vredefort Ring, judging by the observed structural results, was not
caused by an oblique upthrust. If the shape of the ring under the Karroo cover
is elliptical or pear-shaped, then that is the shape of the prism or column, and
there is no need to read into it an oblique force.
BRACHYSTRUCTURES.
These subsidiary folds, like the younger granite, must be mentioned since
other writers have attached great importance to them as a possible clue to the
origin of the Vredefort Ring.
THE VREDEFORT RING
143
The miniature domes and canoe-shaped synclines, mentioned by Truterll
and so conspicuous on the geological map (Sheet No. 61), are peculiar to the
Transvaal system, whereas the Vredefort Ring had its origin very much earlier.
It is submitted that the brachystructures in the Transvaal rocks are a reflection of pre-Transvaal topography which was influenced by grabens and horsts
causing basins and ranges. It is an established characteristic of a major graben
that cross faults interrupt the floor, dividing this floor itself into horsts and grabens. A graben within a graben and a horst within a horst in the old topography
would give ideal conditions for the formation of initial basins and domes in the
overlying sediments. Subsequent movements on the same faults might even have
accentuated these features. Doubtless the later thrust is an unpleasant complication in the unravelling of the story.
There is, however, some very real evidence in support of thjs hypothesis,
facts which cannot be conveniently explained by any other hypothesis advanced
to date. The fact that near Potchefstroom the Ventersdorp rocks directly
overlie Hospital Hill beds means that here was a horst elevated to a similar
degree as the margins of the basin (west of Klerksdorp, for instance) where only
the Lower Witwatersrand is present. Whether the meagre representation of
Witwatersrand beds near Potchefstroom is due to excessive erosion or to meagre
deposition is beside the point. Either way, it speaks of a large and early uplift
of that fault block. The outline of this horst corresponds to Truter's brachyanticline which extends from Welgevonden 39 to Witstinkhoutboom and beyond.
Pre-Transvaal grabens are now represented by canoe-shaped brachysynclines.
CONCLUSION.
The Vredefort Ring exists because of the following factors : 1. There is a marked tendency in this region towards vertical movement
of blocks of the earth's crust rather than lateral movements related to crustal
shortening.
2. The weight of rocks accumulated in a major geosyncline caused a large
hydraulic pressure in the substratum below the geosyncline.
3. The collapse of the geosyncline followed a concentric structural pattern.
An incipient Vredefort hub as the centre is postulated.
4. Fractures roughly radial to the margin of the basin and tangential to
the Vredefort hub released a plug, which became a safety valve.
5. The core, it is submitted, was slightly conical, translating vertical movement into a centrifugal horizontal pressure on the crust as the gently tapered
cone pierced the crust. The continued piercing effect overturned the beds.
6. The movement began when the weight of sediments became great enough
to cause collapse, and continued intermittently until after the deposition of the
Transvaal system.
Thus the Vredefort Ring structure which has been variously described as
" remarkable", "wonderful", "marvellous", "peculiar", "startling", etc. is
the result of commonplace processes in an unusual combination of circumstances.
144
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
It is, however, remarkable as a rather extreme case of the upward vertical
movement of a fault block of limited area. The controlling features are : (a) the close spacing of nearly vertical intersecting faults which penetrate
to unusual depths-to the substratum itself, and
(b) a great hydrostatic pressure related to the subsidence of the geosyncline
near whose centre these faults converge laterally.
The structure is of importance in suggesting very clearly a type of terrestrial
mechanism which has worked on the principle of the hydraulic press. From this
a very clear inference follows: the substratum below the geosyncline must
have been liquid within a confined space, or the hydraulic principle could not
work. The limits of this confined space, it is suggested, roughly coincided are ally
with the limits of the subsided syncline, with which it must be intimately related.
REFERENCES.
(1) NEL, L. T.: "The Geology of the Country around Vredefort." Geol. surv.
S. Afr.
(2) DALY, R. A.: "The Vredefort Ring-Structure of South Africa." Jour. Geol.,
LV3 1947, p. 125.
(3) HALL AND MOLENGRAAFF.: "The Vredefort Mountain Land in the Southern
Transvaal and Northern Orange Free State." Shaler Memorial Series: Verh. Akad. Wet.
Amsterdam 2e section, Deel XXIV, No.3.
(4) BAILEY, E. B.: "Domes in Scotland and South Africa: Arran and Vredefort."
Geol. Mag., LXIII, 1926, p. 481.
(5) Du TOIT, A. L.: "Geology of South Africa." 1939, p. 180.
(6) BISHOPP, D. W.: "The Geodynamics of the Vredefort Dome." Trans. geol.
Soc. S. Afr., XLIV, 1941.
(7) BISHOPP, D. W.: "Reply to discussion on the Geodynamics of the Vredefort
Dome." Proc. geol. Soc. S. Afr., 1941, p. cviii.
(8) TRuTER, F. C.: Discussion on " The Geodynamics of the Vredefort Dome."
Proc. geol. Soc. S. Afr., 1941, p. lxxiv.
(9) MAREE, B. D.: "The Vredefort Structure as Revealed by a Gravimetric
Survey." Trans. geol. Soc. S. Afr., XLVII, 1944, p. 183.
(10) CLOOS, H.: "Fortschritte in der Kartierung von Transvaal." Geol. Rdsch.,
Bd. XXVII, 1937, Heft 3/4, p. 250.
(11) NEL, L. T., TRuTER, F. C. AND WILLEMSE, J.: "The Geology of the Country
around Potchefstroom and Klerksdorp." Geol. surv. S. Afr.
(12) BUCHER, W. H.: "The Deformation of the Earth's Crust." Princeton Univ.
Press, 1933, p. 325 et seq.
(13) TRuTER, F. C.: "Observations on the Geology and Tectonics of a Portion of
the Potchefstroom District." Trans. geol. Soc. S. Afr., XXXIX, 1937.
(14) NEL, L. T.: "The Geology of the Klerksdorp-Ventersdorp Area." Geol.
8urv. S. Afr.
(15) Du TOIT, A. L.: Op/cit., p. 96.
(16) WILLEMSE, J.: "On the Old Granite of the Vredefort Region and some of its
Associated Rocks." Trans. geol. Soc. S. Afr., 1937, p. 43.
ANGLO-AMERICAN CORP.
LTD.,
44, MAIN ST., JOHANNESBURG.
TRANS. GEOL. SOC. B.A., VOL. LIlT.
PLATE
XXIV.
LEGEND
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