197
STRATIGRAPHIC FEATURES AND TECTONICS
STRATIGRAPHIC FEATURES AND TECTONICS
OE
PORTIONS
OF
BECHUANALAND AND GRIQUALAND WEST.*
By D. J. L. Visser, D.Se.
(Published by permission of the Honourable the Minister of Mines.)
[PLATES XXVII-XXX).
ABSTRACT.
This paper deals with the geology of portions of Bechuanaland and Griqualand West,
situated on the eastern edge of the Kalahari in the Northern Cape Province.
The normal geological succession is described in brief, and the anomalous relationships found along and to the west of the Gamagara rand are treated more fully. The
brecciated equiyalents of the upper portions of the Dolomite and of the lower and upper
portions of the Lower Griquatown stage are described separately; their close association
with post-Matsap earth movements is indicated and a post-Matsap age and a tectonic
origin assigned to them~
The area is divided structurally into three belts: an intensely folded and overfolded western belt; a central belt of low-angle compressional faults, of which six are
described, and a gently folded eastern belt, in which marginal reverse and tear-faulting
have taken place.
The nature and age of the earth movements are discussed briefly, and their relation
to the mineral deposits of the area indicated.
CONTENTS.
Page
L -INTRODUCTION TO THE AREA
A. Summary of previous work in the area
B. Location of area
C. Topography and its relation to structure
II.-GEOLOGY OF THE AREA
••
A. General statement
B. The Transvaal system
1. The Dolomite series
2. The Pretoria series
(a) The Lower Griquatown stage
(i) The banded ironstone zone
(ii) The banded jasper zone
(iii) The tillite zone
(b) The Ongeluk stage
(0) The Upper Griquatown stage
C. The Waterberg system
1. The Gamagara series
2. The Matsap series
(a) The Lower Matsap stage
(b) The Hartley Hill stage
(0) The Upper Matsap stage
198
198
199
200
201
201
203
203
204
204
204
204
206
206
206
207
207
209
209
210
210
* The substance of this paper was submitted, under the title" Observations on the
Tectonics and Stratigraphy of Portions of Bechuanaland and Griqualand West," in fulfilment of the requirements for the degree of D.Sc., University of Stellenbosch, in 1939,
This paper presents the above in rewritten and abridged form.
198
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
Page
D. The Ca,taclastic rocks
1. The Siliceous breccia
2. The Blinkklip breccia
3. The origin of the brecci~s
E. The Karroo system
F. The superficial deposits
G. The igneous intrusions
1. Diabasic rocks related to the Ongeluk volcanics
2. Basic intrusive rocks other than Karroo dolerites
3. Dolerites and related rocks of Karroo type
4. Kimberlite pipes ..
IlL-TECTONICS OF THE AREA
A. General statement
B. The intensely folded western belt
C. The thrust-faulted central belt
1. General remarks ..
2. The Maremane thrust
3. The Gamagara thrust
4. The Vlakfontein thrust
5. TheAucampsrust thrust
6. The Matsap Hills thrust
7. The Lynputs thrust
8. Further cases of possible overthrusting
9. The associated normal faulting
D. The gently folded eastern belt
1. The folding
2. The associated marginal faulting
E. The orientation of dykes and fracture zones
F. The nature and age of the diastrophic movements
IV.-THE RELATION OF THE MINERAL DEPOSITS OF THE AREA TO THE 'E ARTH
MOVEMENTS
211
211
211
213
215
215
215
216
216
216
216
217
217
217
219
219
219
223'
226
234
238
244
246
246
247
247
248
249
250
VI.-AcKNOWLEDGMENTS
250
250
251
251
251
252
253
VIL-LIST OF REFERENCES
253
1.
2.
3.
4.
The asbestos deposits ..
The barite occurrence
The iron ores ..
The manganese ores ..
V.-THE GEOLOGICAL HISTORY OF THE AREA
I.-INTRODUCTION TO THE AREA.
A.
SUMMARY OF PREVIOUS WORK IN AREA.
The earliest accounts of the structure and of some of the rocks are found in
the writings of the travellers, Burchell (18, pp. 6-7),* Lichtenstein (18, p. 6),
and Moffat (18, pp. 64-65). 'Stow (18, pp. 68-77, and 19) was, however, the first
to establish the general geological succession. Some of his observations are
truly remarkable. Thus, he gave an accurate description of the disturbed
Griquatown beds on a hill called Ramaje's kop, north of Danielskuil, lying in
hollows in undisturbed Campbell Rand beds, and looked for a cause of the
crumpling in horizontal pressure from the west. In the rounded contours of the
hills, as seen from a distance, he saw signs of glacial (Dwyka) erosion.
*·The figures in brackets refer to the list of references given at the end of the paper.
STRATIGRAPHIC FEATURES AND TECTONICS
199
A. W. Rogers (14, 15 and 17) surveyed portions of the KUl'uman, Hay,
Gordonia, Barkly West and Prieska divisions in 1904 and 1905, and gave the
first systematic account of the stratigraphy and structure of the area. Although
generally agreeing with Stow, he correctly placed the Ongeluk lavas in the
Griquatown series, and ascribed the origin of the Blinkklip breccia to slumping
of the Lower Griquatown beds into hollows, dissolved out of the underlying
Dolomite, under the influence of gravity, and perhaps earth nlOvements which
started the collapse. Stow had regarded this rock as being of detrital origin.
The strip of Matsap (Gamagara) beds forming the present Gamagara rand, was
regarded by Rogers as a fault outlier which had first been depressed along a
normal fault (his Paling fault) and had subsequently become elevated by movement in the reverse direction along the same plane.
Geological interest in the area was roused by the discovery of manganese in
1922, by the work of Hall (11) on the manganese deposits in 1925, and by Wagner's
investigation (23) of the iron ores somewhat later. Although those publications are
primarily of an econonlic nature, they, nevertheless, attracted attention to the
complex geologic structures of the manganese fields, about which there was
already considerable difference of opinion. To obtain a proper understanding
of the mineral deposits in relation to these unusual structur~l features, N el
undertook a detailed geological survey of the Gamagara rand and the adjoining
country in 1927-1928(13). In general, he upheld Rogers' views, and his conclusions may be summarised as follows: (i) the group of ferruginous conglomerates, shales and quartzites building the Gamagara rand is, for reasons of
its isolation, termed the Gamagara series, and regarded as a local facies of the
Matsap series; (ii) the complicated structure observed along the Gamagara
rand is due to pre-Matsap faulting, to post-Matsap faulting and to local subsidence of the lowermost beds into solution cavities in the underlying dolomite;
(iii) to the west of the Gamagara rand overthrusting has taken place, bringing
in succession, Dolomite, Lower Griquatown beds and Ongeluk lavas on to the
Gamagara quartzites from south to north; (iv) the manganese ores were formed
by metasomatic replacement of the basal Gamagara beds and the Siliceous
breccias, where they lie on the dolomite.
During the winter months of 1936, 1937 and 1938, the writer remapped,
on the scale of 1 : 74375 (500 Cape roods to the inch), portions of the area
originally covered by Rogers and Nel. Particular attention was given to the
general tectonics of the area and the complicated structures characterising the
western portion. This portion of the work forms the basis of the present paper.
B.
LOCATION OF AREA.
The area under consideration, some 6,000 square miles in extent, forms part
of the divisions of Kuruman, Hay, Gordonia and Barkly West. It is bounded
on the west and east by longitudes 22°30'E. and 23°30'E., and on the north
and south by latitudes 27°30'S. and 29°S. and is covered by geological sheets
173 (Oliphants Hoek), already published, 174 (Postmasburg) and 175 (Griquatown), the last two being in course of preparation. Of this area, the writer
mapped the portion west of longitude 22°50'E., while in the neighbourhood of
200
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
Postmasburg, his mapping extended up to the Gamagara rand and across the
flat country to the north-east and south-east of the township. In addition, use
was made of opportunities to examine the areas to the east of longitude
22°50'E., which were mapped by the writer's colleagues, the late Mr. P. R.
Botha, and Drs. L. G. Boardman, F. C. Truter and B. Wasserstein, so that the
following account is based on the writer's personal acquaintance of the area as
a whole. The accompanying map (Plate XXVII) was compiled from the largescale maps of the writer and his colleagues, but is greatly generalised.
c.
TOPOGRAPHY AND ITS RELATION TO STRUCTURE.
The country forms a portion of the inland plateau of South Africa, with
heights of about 4,400 feet above sea-level. The relief is generally low, without,
however, being monotonous like that of the Kaap plateau to the east. The
landscape is, characteristically, of the inselberg type, with rounded or sharp:
crested peaks and ridges projecting from the sand-covered flats. This feature
becomes more noticeable westwards, towards the Kalahari.
An outstanding feature is the arcuate shape of the ranges of hills. In the
east the Kuruman hills and the Asbestos mountains form a conspicuous arc,
convex to the east. North of Postmasburg, the Klipfontein hills, a series of relics
of brecciated banded ironstones, form a similar arc on a smaller scale. In the
western portion of the area the ridges of Upper Griquatown and Lower Matsap
beds form similar arcs, with the general pattern of which the rounded hills on
Makganyene, Aucampsrust and Wolhaarkop and the Matsap hills farther south
are roughly in alignment. The arcuate ridges and mountain ranges are parallel
to the conspicuous Langeberg in the west, which displays intense asymmetrical
folding and overfolding.
The only exception to this general rule is the Gamagara rand in the central
portion of the area, which is remarkably straight and trendsnearly north-south.
This rand approximately coincides with the axis of the Maremane anticline.
The strata building the rand were probably deposited in a fairly straight troughlike basin eroded out along the crest of the anticline, and the whole tilted to the
west during the post-Matsap orogenic period (1, pp. 82-83).
In addition to the arcuate shape of the mountain ranges, their development
has been further controlled by the composition and structure of the rocks··of
which they are built. The Asbestos mountains and Kuruman hills, built of
resistant banded ironstones and jaspers, form the flanks of two major synclines.
The contact between the rocks of the tillite zone and the Ongeluk volcanics is
nearly always occupied by longitudinal valleys, while within these synclines the
Ongeluk lavas build rounded hills. These form the highest ground between
Postmasburg and Griquatown. In the south-western portion of the area, the
banded jaspers forming the Matsap hills have been gently arched. The rounded
hills on Wolhaarkop, Aucampsrust and Makganyene are due to a similar arching
of the resistant banded jaspers. The Lower Matsap ridge and the Langeberg,
on the other hand, are built of hard quartzites which show intense folding and
steep dips to the west.
STRATIGRAPHIC FEATURES AND TECTONICS
201
The valley pa.tterns are naturally dependant on the disposition of the hills,
and longitudinal valleys are most conspicuous. There are also many transverse
branch valleys, cut at right angles to the strike. In the areas occupied by Lower
Griquatown beds and Ongeluk lavas, two more sets of valleys, roughly at right
angles to each other and inclined at 45° to the strike of the beds, are developed.
They may be well observed to the north of Groenwater and, to a smaller extent,
north-west of Makganyene. Their directions conform to those of crush zones
and shear planes in the Lower Griquatown and Ongeluk beds, and small-scale
tear faulting along such planes has apparently determined the positions of those
transverse and oblique branch valleys.
Attention should also be drawn to remnants of a Karroo topography, as
evidenced by scattered remnants of glacial pavements, the rounded contours of
some of the hills, typical U-shaped valleys (22, pp. 13-14) and valleys filled with
Dwyka shales in the south-western portion of the area. Along the Gamagara
rand and in the Klipfontein hills, these traces of Dwyka glaciation have proved
useful in determining the time of slumping and of manganese mineralisation
(1, pp. 92-100).
'
The larger present-day drainage channels, the Gamagara loop in the north
and the Postmasburg-Matsap loop in the south, bear no direct relation to the
general tectonic plan of the area; instead, they seem to have been superimposed
on a post-Matsap and probably also on a post-Karroo topography.
n.-GEOLOGY OF THE AREA.
A. GENERAL STATEMENT.
The solid geology of the area comprises a great thickness of sedimentary
beds, but includes three volcanic groups and some minor intrusives in the form
of sheets, dykes and pipes. According to age, these rocks fall into four groups,
separated by unconformities, viz., rocks belonging to the Transvaal, Waterberg
and' Karroo systems and superficial deposits of recent accumulation. The
Karroo beds are nearly always covered by superficial accumulations and are not
shown on the accompanying map. The bulk of the rocks found exposed in the
area, therefore, belong to the Transvaal and the Waterberg systems. The
former is probably late Proterozoic to early Oambrian in age, while the latter
may be of early Palaeozoic age.
The succession within the Transvaal system is conformable throughout,
beginning with a great thickness of dolomitic limestones below and passing
upwards through the Lower Griquatown banded ironstones and jaspers and the
tillite, the Ongeluk volcanics and the great thickness of sediments and volcanic
rocks of the Upper Griquatown stage. It should be pointed out here that marked
lithological differences exist between the rocks belonging to the upper division
of the Transvaal system, i.e., the Pretoria series, in Griqualand West and those
of the Transvaal. Whereas in the type area around Pretoria quartzites are
responsible for bold surface features, they are sparingly developed and inconspicuous in this area. On the other hand, the banded ironstones and jaspers
which form but a small portion of the Pretoria series in the Transvaal, become
strikingly prominent in Griqualand West. The volcanic rocks also assume
greater dimensions in the latter area.
202
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
The following table gives the stratigraphical sequence:System.
Series.
Stage.
Recent.
Character.
Thickness.
rock
Sand,
rubble, terrace
gravels, surface
limestone.
From a few
up to a few
hundred feet.
Post - cretaceous?
Igneous
Intrusions.
Kimberlites.
I
PostKarroo.
Karroo.
Dolerites.
Dwyka.
Tillite and upper
shales.
50-100 feet.
Post - Matsap and preKarroo.
Diabasic
dykes.
Post - Mat- Catasap and pre- clastic.
Karroo.
I
Brecciated banded cherts, jaspers, banded
ironstones and
Gamagara rocks.
From a few
up to 100 feet.
Upper
Matsap
Purple quartzites, grits, conglomerates and
shaly rocks.
Considerable;
from 4,000 to
5,000 feet.
Hartley
Hill.
Basal conglomerate, lavas, breccias, tuffs, green
and purple
quartzites.
± 2,000 feet.
Lower
Matsap.
Qua r t zit e s,
shales and limestones.
3,000 feet.
Basal conglomerate, shales and
quartzites.
From a few
up to 1,200
feet.
Jaspers, cherts,
1 i m est 0 n e s,
qua r t zit e s,
shales and lavas.
± 5,000 feet.
I
I
I
Waterberg.
Matsap.
Gamagara.
I
I
I
I
Upper
Griquatown.
I
Diabasic sills.
Ongeluk.
Lavas with minor
tuffs, cherts and
jaspers.
Lower
Griquatown.
Banded
iron- 2,500 feet.
stones, jaspers,
tillite, mudstones
and quartzites.
Pretoria.
Transvaal.
I Dolomite.
I
± 3,600 feet.
Dolomitic lime- 5,000 feet.
tones and cherts. I
I
STRATIGRAPHIC FEATURES AND TECTONICS
B.
1.
203
THE TRANSVAAL SYSTEM.
THE DOLOMITE SERIES.
The upper portions of this series, composed of bluish-grey dolomitic limestones with frequent intercalated chert bands near its contact with the overlying banded ironstones, are exposed in three areas: in the north-eastern
corner of the map, south of Kuruman; in the south-eastern portion, in the neighbourhood of Griquatown, and north of Postmasburg, where it occupies the fiat
country capped by relics of Blinkklip and Siliceous breccia. In the neighbourhood of the Chee beacon several fault outliers are found in the banded ironstones. West of the Gamagara rand several larger and smaller outcrops, bearing
a fault relationship to the under- and overlying rocks, have been mapped.
In the eastern portion of the area, where the succession is normal, the Dolomite is succeeded conformably by the banded ironstones of the Lower Griquatown stage, and dips westwards at an angle of seldom more than 10°. The same
relationship holds to the north-east of Postmasburg, but the dip is 12-15° to the
south-east near Postmasburg, and 2-5° to the east farther north. West and
north-west of Postmasburg, the conformable relationship holds no longer. Near
Beeshoek the Dolomite is capped by Siiiceous and Blinkklip breccias, while the
Klipfontein hills north of Postmasburg are built of the same rocks. From Doornfontein northwards to beyond Lohathla, the basal members of the Gamagara
series, inclined to the west, rest on the Dolomite, which here is nearly horizontal
or gently inclined to the east. From about five miles south and south-east of the
Gamagara beacon, Siliceous and Blinkklip breccias once more rest on the Dolomite.
West of the Gamagara rand the Dolomite is not continuously exposed, but
is restricted to some small, isolated areas. On Doornfontein and farther north
.on Paling and Magoloring, it rests on the westward-dipping Gamagara quartzites
and is badly shattered along the contact. About half a mile west of the Gamagar a rand on Doornfontein the underlying Gamagara rocks are exposed in a
window in the overlying Dolomite, which is succeeded directly by the uppermost members of the Lower Griquatown stage. On Magoloring, the Dolomite
is followed by intensely folded and fractured Gamagara quartzites. In the
:r;J,orth-western corner of Magoloring, a lens-shaped mass of dolomite, about
25 feet thick at the ,utmost and very much fractured, is sandwiched between
tillite underneath and jaspers belonging to the upper portion of the banded
jasper zone above, the entire succession dipping westwards at 12° (Fig. 3, p. 229).
Three miles to the south, the Dolomite is exposed in the core of a small dome.
On the northern and southern sides fractured banded jaspers, succeeded conformably by the tillite, lie on the dolomite, but on the eastern and western
sides, along the crest of the elongated dome, the banded jaspers are absent
and the tillite lies on the dolomite. The latter· is intensely brecciated, and
along the contact with the tillite the t'Yo have been milled up together to form
a mylonite in which sliced pebbles from the tillite and angular fragments of
dolomite are cemented in an earthy matrix which has been partly manganised.
The contact is a low-angle fault which, in this locality, has caused the elimination of at least 2,500 feet of banded ironstones and jaspers.
204
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH A.FRICA
In the Aucampsrust and W olhaarkop areas the Dolomite is exposed in the
cores of two elongated domes. The dome structures are actually displayed in
the overlying rocks; the Dolomite dips persistently eastwards, at 5° on Aucampsrust and at 10-15° on Kameelhoek and Welgevonden. The overlying rocks are
Siliceous breccias and members of the Gamagara series, and the angular unconformity is very distinct along the western flanks of the donles. (Figs. 4, p. 231,
and 6, p. 235.)
The Honingkrans outcrop, on the crest of a low rand some 2 miles west
of the homestead, is of particular interest because it involves some complex
folding and faulting. On the eastern side of the rand, Ongeluk lavas are exposed;
then follow about 50-60 feet of banded jaspers, very much fractured and turned
black. Next comes a thickness of 20-30 feet of dolomite, which is overlain directly
by the tillite, and then more Ongeluk lavas. The whole succession dips westwards at 12°. The dolomite is intensely sheared and fractured, and along the
contact with the overlying tillite mylonitised rock similar to that found on
Magoloring is developed. Along the contact of the dolomite with the underlying banded jaspers a dolomite-jasper breccia, about one foot thick and similar to the rock described above, is found. The contacts of the dolomite with the
over- and underlying rocks are clearly low-angle compressional faults, and they
are discussed fully on pages 238 and 240 .
2.
THE
PRETORIA SERIES.
This . group of rocks, formerly referred to as the Griquatown series (16) is
characterised by the thinly bedded, fine-grained, ferruginous and siliceous nature
of its sedimentary members, and the prominence of volcanics. It is divided into
a Lower Griquatown stage, an Ongeluk or Middle Griquatown stage and an
Upper Griquatown stage.
(a) The Lower Griquatown stage.-In the eastern portion of the area this is
fully developed and follows conformably on the dolomite. It is divisible into
three useful stratigraphic units, viz., a banded ironstone zone, 1,000 feet thick,
following on the topmost cherts of the Dolomite series, a banded jasper zone,
1,500 feet thick, and a tillite zone, seldom more than 50 feet thick, at the top.
(i) The banded ironstones are thinly laminated and consist of alternating
yellowish -brown, more siliceous, and red to dark brown, more ferruginous bands.
Occasional almost black highly magnetic bands are present. About 100 feet·
above the contact with the dolomite lies the asbestos horizon.
This zone is exposed along the entire eastern flank of the Dimoten and
Ongeluk-Witwater synclines, where it dips westwards at 5-10°, and along part
of the western flank, where, to the south-east, east and north-east of Postmasburg,.
it dips eastwards at 2-10°. Its brecciated counterpart, referred to as the Blinkklip
breccia, is found as outliers capping the Klipfontein hills and also in a few
localities immediately east of the Gamagara rand.
(ii) The banded jaspers are light to dark brown in colour, largely siliceous,.
more thickly bedded and with less pronounced and more irregular banding~
An occasional band of blue soda-amphibole is present, e.g., south-west of
Wolhaarkop. The asbestos horizon on Blackridge also falls within this zone~
STRATIGRAPHIC FEATURES AND TECTONICS
205
An arenaceous facies, representing a stage between banded jaspers and quartzites, is locally developed, e.g., east of Lucasdam and in the hills east of Camelthorns. In the last-named locality two bands of yellowish-weathering limestone, individually a foot thick and 3 feet apart, are exposed. Their position is
some 300 feet below the tillite zone.
In the eastern portion of the area the full thickness of banded jaspers is
exposed within the Dimoten and Ongeluk-Witwater synclines. The contact
with the underlying banded ironstones (i) is gradational. In the western portion
of the area the greatest thickness of banded jaspers is seen in the Matsap hills,
where the beds are folded into a broad, low anticline, pitching to the north. Along
the eastern flank the dip is 10-25° to the east, except in the neighbourhood of
Mastap, where they have been overfolded locally. Along the western flank the
dips are to the west at about 25-30° near the northern extremity, 15-25° in the
neighbourhood of Blackridge, and up to 45° in the vicinity of Camelthorns.
The outlying hills of banded jaspers on Vaalwater, Kaffirkop and Witboom are
probably continuous with this mass underneath the cover of superficial deposits,
and may link up in the same way with the exposures farther north.
In the, Wolhaarkop-Aucampsrust area and to the west thereof a small
portion, representing not more than the upper quarter or fifth of the banded
jasper zone, is exposed. South of Wolhaarkop the banded jaspers crop out along
the nose of a southward-plunging dome; they grade downwards into Siliceous
breccias which rest unconformably on the dolomite. The tillite rests on the
jaspers conformably along the south-eastern flank of the nose, the dips being
east and south at 10-15°. On the south-west side a thin sheet of Gamagara
quartzites overlies them with only a slight angular unconformity.
The sharp ridge extending from this point northwards to Lucasdam is also
built of 200-300 feet of banded jaspers, succeeded conformably on the western
side by the tillite,while Ongeluk lavas are exposed in the valley on the eastern
side.
Along the western boundary of Aucampsrust a fair thickness of banded jaspers, dipping westwards at 25°, is exposed. Along the northern boundary the
thickness is considerably less, where the beds fringe the nose of a dome-shaped
structure with a northward pitch. Tillite is found on the east and north sides,
in which directions the beds dip at low angles.
A further elongated dome-shaped mass of banded jaspers is exposed on
Makganyene and Kouwater. Dips nowhere exceed 20°. Windows in which
Gamagara quartzites are exposed are seen in a valley eroded along the anticlinal
axis. The actual thickness of banded jaspers present cannot be more than 200
feet. The isolated outcrops on the adjacent portion of lVlagoloring have already
been referred to on page 203.
Banded jaspers, usually finely fractured, are also found along the western
flank of the Gamagara rand, e.g., on Magoloring (Fig. 1, p. 222) and farther north
on Lomoteng and Bishop. Near the Gamagara valley, on Parson, another small
outcrop has been mapped. The jaspers rest on westward-dipping Gamagara
quartzites and are succeeded directly by the Ongeluk lavas.
206
TRANSACTIONS OF THE GEOLOGIC SOCIETY OF SOUTH AFRICA
(iii) The tillite zone. The rocks of this zone are sandwiched conformably
between the thick mass of banded jaspers below and the massive andesitic lavas
above, and, despite their comparative thinness, are remarkably persistent.
Outcrops are somewhat isolated, but are spread over almost the entire area and
serve as a valuable, and in some places the only marker horizon for unravelling
the structure.
The tillite proper is greenish-grey when fresh, and is compo~ed of pebbles
and rounded rock fragments of varying size scattered through an ash-grey clayey
matrix. Outcrops are usually strewn with chert and quartz pebbles, which seldom measure more than an inch across the longest diameter, though, occasionally,.
bigger boulders are found. Striated and facetted pebbles are by no means rare.
The zone usually begins with a thin band of feldspathic quartzite, which may,.
however, be partly or wholly replaced by mudstones. Then follows the tillite
proper and above it some more mudstones containing scattered pebbles. The
mudstones are calcareous to some extent.
The best exposures are in the Dimoten syncline, below the Dimoting beacon.
Excellent exposures are found in the south, a few miles west of Griquatown,.
and on Magoloring (Fig. 1, p. 222) and on the southern portion of Vlakfontein
(Fig. 4, p. 231), west of the Gamagara rand.
(b) The Ongeluk Stage.-Composed of a great thickness of competent, massive
andesitic lavas of remarkably consistent texture and composition, this stage
occupies the troughs of the two great synclines in the eastern part of the area.
West of the Gamagara rand it is also well represented. North-west and north of
the Makganyene-Kouwater hills the lavas form hilly country and are present
up to the base of the Gamagara rand. Between the Gamagara rand and the
Wolhaarkop-Aucampsrust-Makganyene hills the lavas fill the so-called Vlakfontein syncline. West of the latter group of hills they crop out in the valley
running southwards from Lynputs through Lucasdam. The isolated exposures.
in the valley on the western portions of Kameelhoek and W olhaarkop have
already been mentioned.
West of the Matsap hills the lavas are seldom exposed, but have been
found in wells. On the western portions of Matsap and Cone (Fig. 9, p. 241) and
farther south on Camelthorns and Dingle (Fig. 11, p. 243) the lavas also occur
in wells, and together with the tillite and banded jaspers, indicate the presence of
three small synclines, two of which have been overfolded from the west.
Crush zones, occupied by quartz-epidote rock, traverse the lavas. Northwest of Makganyene their direction is N.E.-S.W., conforming to the direction
followed by a few dolerite dykes in the neighbourhood. South of Matsap they
run in a N.W.-S.E. direction. They can be traced for a few hundred yards and
seeln to be related to the post-Matsap tectonic disturbances.
(c) The Upper Griquatown Stage.-Such strata are confined to the western
portion of the area and make a series of low hogback ridges which form a
distinct arc, from a point north-west of Coxjn the north as far as Floradale in
the south. Only the more resistant members, the jaspers, cherts and quartzites, make good outcrops; the softer shales and volcanic rocks are seen only in
dongas or in wells or where sandwiched between more resistant beds.
STRATIGRAPHIC FEATURES AND TECTONICS
207
There is a distinct variation in thickness as well as in lithological characters
along the strike, so that no two sections will appear exactly alike (13, p. 29).
From measurements at various points along some thirty miles, the following
composite section was established.
9. Massive amygdaloidallavas and tuffs, very much altered, but probably
also of andesitic composition, with thin shale and quartzite partings.
8. White quartzites with some interbedded shales.
7. Highly micaceous and much cleaved shales, seldom exposed.
6. White to pink, fine-grained quartzites, flaggy quartzites, ferruginous
shales and some intercalated limestone bands with chert lenses.
5. White and brownish finely-banded to massive bedded cherts, much
jointed, slickensided and ofte~ brecciated. A. limestone band is found in this
horizon on Honingkrans, Lucasdam and for a few miles farther south.
4. Green amygdaloidallava.
3. Greyish and brown chert with interbedded limestone.
2. Massive red jaspers, shot with white vein quartz.
1. Banded ironstones and banded jaspers, consisting of thin, alternately
red (more siliceous) and brown to black (more ferruginous) bands.
In addition to the brecciation and slickensiding observed in the brittle cherts
and jaspers, the shales also show signs of deformation. They have been overfolded on a small scale and a fracture cleavage, inclined at 35° to the bedding
planes, developed. Strike jointing, inclined at varying angles to the planes of
cleavage, and dip jointing are also developed; their combined effect is to cause'
the shales to break up into elongated slabs with polygonal cross-sections.
Unless duplication of the Upper Griquatown rocks has taken place through
obscure folding and faulting, their thickness must be considerable, which, as
calculated in the Lucasdam neighbourhood, amounts to about 5,000 feet, of which
the upper 2,000 feet are lavas and tuffs.
C.
THE
W ATERBERG
SYSTEM.
Rocks correlated with the Waterberg system of the Transvaal, crop out
prominently in the western portion of the area. They are largely of sedimentary
origin, laid down in comparatively shallow water, but also include a volcanic
group. They are divided into the Gamagara series and the Matsap series.
Reasons for this division will be given below (p. 209).
1.
THE GAMAGARA SERIES.
In the type locality, the Gamagara rand, the series consists, from below
upwards, of a rather coarse, ferruginous (basal) conglomerate, a thickness of
brownish, ferruginous and grey aluminous shales, and a variable thickness,
ranging from a few up to several hundred feet, of pinkish and purplish quartzites. They have presumably been deposited unconformably on the Dolomite
along the central portion of the rand, and on the Lower Griquatown stage at
the northern and southern extremities of the rand. They strike nearly northsouth over a distance of about 40 miles and dip west at 10-25°.
208
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
West of the Gamagara rand the outcrops are more interrupted. Exposures
in windows in the Dolomite on Doornfontein (p. 203) and in the banded jaspers
on Makganyene (p. 205), and the outliers on the Dolomite on Magoloring (p. 203)
have already been mentioned. On .Aucampsrust, the quartzites, only a few feet
thick with thin lenses of manganised shale and conglomerate at the base, rest
partly on banded jaspers, partly on siliceous breccia and partly on the dolomite
itself. This anomalous relationship is discussed again on pages 230 and 232. On
Kameelhoek and the northern portion of Wolhaarkop thin .conglomerates, shales
and quartzites rest on a sheet of Siliceous breccia which separates them from the
dolomite. On the south-western portion of Welgevonden and the adjoining
portion of Wolhaarkop outliers of the quartzites rest on fractured banded
jaspers and on Ongeluk lavas. Farther to the north on Jenkins quartzites are
exposed in windows in the overlying lavas, in other words, as fault inliers. On
Witboom a lens-shaped body of quartzites, a few feet thick, is sandwiched
between slightly disturbed banded jaspers below and intensely brecciated jaspers
above. The quartzites exposed on the eastern flank of Kaffirkop (Fig. 9, p. 241)
apparently rest on red jaspers tentatively correlated with the Upper Griquatown
stage, whereas on Vaalwater the underlying rock is without doubt Ongeluk
lavas. The quartzites here are overlain by Lower Griquatown banded jaspers,
which are badly brecciated along the contact and locally rolled up with the
quartzites. Slickensides developed along or immediately above the contact
indicate a low-angle movement of the banded jaspers from west to east.
From ·east of the Matsap beacon Gamagara quartzites are exposed almost
continuously up to four miles south of the Langkloof beacon. They show a flat
anticlinal structure, and where the underlying Lower Griquatown banded
jaspers are exposed, no angular unconformity is apparent. The basal conglomerate is seldom seen, but where it is exposed, as east of the Matsap beacon, it is
very coarse, the pebbles of chert and banded jasper being up to 12 inches in
length. Usually the pebbles are but 1-3 inches long. In the poort west of
Matsap its thickness is about 15 feet (14, pp. 191-192). South-west of the Langkloof beacon a thickness of at least 25 feet is exposed; the quartzites in this
neighbourhood measure about 200 feet Q4, p.190). On Hartebeeshoek, a domeshaped mass of quartzites, with overthrust banded jaspers, occurs on the downthrow side of a post-Matsap normal fault with a throw of at least 300 feet
(Fig. 10, Section Q-Q').
The isolated quartzite outcrops to the north-east and north of Oamelthorns
show a change in strike from north-south to east-west. .They link up with the
larger mass of quartzites flanking the nose of a northward-pitching anticline on
Camelthorns and the adjoining farms (Fig. 11, p. 243). .Along the north-eastern
flank the dips range from 12-30° to the east and north-east, but along the western
flank the dips are steeper, up to 45°. On Dingle the quartzites are once more well
exposed, dipping to the west at a varying angle. In this area the contact between
the quartzites and the underlying banded jaspers also appears conformable,
but the overriding banded jaspers on the western side show signs of a strong
cataclastic deformation along the contact. The quartzites have also suffered
from this deformation, but to a smaller extent.
STRATIGRAPHIC FEATURES AND TECTONICS
209
In the south-western corner of the map, on Bushridge, quartzites dipping
at low angles to the west are exposed. Outcrops of red banded jaspers which
may be assigned to the Upper Griquatown stage, have been found among the
dunes, about a mile to the east, and the presence of the underlying Ongeluk
volcanics has also been recorded in wells in the same locality. In this neighbourhood, as at Kaffirkop, the Gamagara beds seem to have been deposited on rocks
of the Upper Griquatown stage.
As the solution of structural problems is dependant on correct correlation,
it will be pertinent at this stage to discuss briefly the stratigraphical position of
the Gamagara series. Rogers assigned the Gamagara beds to the Matsap series
without reservations and described the succession along the Gamagara rand as
a fault outlier of Matsap beds (14, pp. 191-194, and 15, pp. 51-52). Du Toit
regarded them as a special siliceous facies of the Lower Griquatown stage, but
gives no reasons for this correlation (9, p. xxxii) . ..Because of this difference of
opinion, 'N el examined both possibilities and suggested the name Gamagara
series for this group of sediments (13, pp. 29-30 and pp. 44-45). Boardman
(1, pp. 22-23) has given tlie question further consideration, and the conclusion
of both these authors is that the Gamagara series is younger than themembers
of the Transvaal system, and was laid down unconformably on an erosion
surface of these rocks, which at the time of deposition of the Gamagara series
must have been gently folded. The writer's own findings in the western portion
of the area agree with this conclusion. The correlation of the Gamagara series
with the lower members of the Matsap series seems justified on lithological grounds.
In the south-western part of the area, somewhat beyond the confines of the map,
the disposition of the Gamagara beds and the upper members 'of the Matsap
series seem to indicate a normal sedimentary succession.
2.
THE MATSAP SERIES.
Composed largely of bedded arenaceous rocks, this series builds two arcuate
mountain chains, convex to the east, in the western portion of the area. The
strike varies, from N.10oW.~S.10oE. in the neighbourhood of Olifantshoek
through nearly N .-S. west of Lyn puts and in the vicinity of Magaikwa and
Andriesfontein beacons, to N.25°E.-S.25°W. in the south-western portion of
the area. The dip is steeply to the west. Strong folding and overfolding from
the west are diagnostic features. A- threefold division may be made as follows:(c) An Upper Matsap stage:
quartzites, grits and conglomerates, and
shales.
(b) A Hartley Hill or Middle Matsap stage: basal conglomerate, lavas,
breccias, tuffs and interbedded quartzites.
(a) A Lower Matsap stage:
quartzites, conglomerates, shales and lime-
stones.
(a) The Lower Matsap Stage.-On account of the dominantly quartzitic
nature and high dip of the beds, it forms a sharp ridge stretching from northeast of Olifantshoek to Floradale in the south, with a few isolated exposures
farther'to the south-west. In the type area east of Olifantshoek the lower
210
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
half of the stage consists of alternate bands of fine-grained greyish-white quartzite, thin greenish shales and bluish limestones. The upper half consists of
pinkish, slightly coarser-grained sheared quartzites. The dip is to the west at
55°, but local vertical dips are common. The total thickness is about 3,000
feet.
Some six miles S.S.E. of Olifantshoek the lower zone disappears, to
reappear on the south-western portion of Floradale. The central portion of the
ridge is indeed composed entirely of quartzites of the upper zone, which here
contain a few lenticular bands of conglomerate and some thin shale bands.
An overfolded anticlinal structure is revealed in sections exposed in some of
the deep transverse kloofs (Fig. 8, Section F-F'). The significance thereof,
taken in conjunction with the presence of some outliers of Hartley Hill lavas
on the eastern side of the rand, is discussed on pages 244 and 246.
(b) The Hartley Hill Stage.-North-east and east of Olifantshoek this stage,
which is largely volcanic, is exposed in the limbs of a small syncline which pitches
to the south. The dips are low to the east and south on the western and northern
sides respectively, but steep to the west on the eastern side (Plate XXVIII,
Fig. 1).
A thirty-foot conglomerate band lies at the base and rests conformably on
the Lower Matsap quartzites. Then follow several hundreds of feet of greenish
amygdaloidal andesitic lavas. About halfway up in the succession a fifty-foot
band of greenish quartzite is found interbedded. Above this band mostly
greyish-green breccias and dark greyish tuffs are found. Near the top of the
succession, two bands of purple quartzite, altogether 300 feet thick, are interstratified with the tuffs and lavas. They are very reminiscent of the Upper
Matsap quartzites, except for the presence of abundant angular fragments of
volcanic rock. This fact, together with the nature of the basal conglomerate,
points to the settling of pyroclastic rock-material in the basin in which the
Matsap sediments were accumulating. These lavas might have been erupted
subaqueously.
From a point about five miles S.S.E. of Olifantshoek, outcrops of the lavas
are few and far between, although they have been located as far south as the
neighbourhood of Paardekloof. A few small outcrops of undoubted Hartley
Hill lavas on the eastern side of the Lower Matsap ridge, on Lynputs, are of
interest, in connection with the overfolding of the Lower Matsap beds, in proving
the presence of an overthrust at the base of the above-named ridge.
(0) The Upper Matsap Stage.-The Langeberg and its foothills are built of
a great thickness of thickly bedded, medium-grained purple quartzites belonging
to this stage. Pebble washes and lenticular bands of grit, passing into smallpebble conglomerates, are frequent. West of tbe Langeberg the quartzites
become finer-grained and whitish, and bands of a micaceous shale are also
found. Cross-bedding is present commonly and ripple marking occasionally
(Plate XXIX, Fig. 1). Asynlmetrical folding, overfolding from the west and
strong shearing are further diagnostic features (Plates XXIX, Fig. 2, and XXX,
Fig. 1). The dip of the strata is usually steeply towards the west.
STRATIGRAPHIC FEATURES AND TECTONICS
D.
THE
211
CATACLASTIC ROCKS.
The name Cataclastic series has been suggested by Dr. F. C. Truter for the
group of brecciated ferruginous and siliceous rocks which, though often represented by small and isolated outcrops, are of widespread occurrence in some parts
of the area, particularly along the Gamagara rand, the dolomite peneplain to
the east and the Aucampsrust-Wolhaarkop area to the west.
North and north-east of Postmasburg the Klipfontein hills are built of
detached remnants of breccia which cap the Dolomite and are in turn overlain
by remnants of Gamagara beds. Immediately east of the Gamagara rand, and
overlain on the western side by Gamagara beds, breccias are found south -east,
east and north-east of Beeshoek. From north of Doornfontein to beyond
Bishop, no breccia is found along the eastern flank of the Gamagara rand, and
the Gamagara beds rest directly on the Dolomite. From north of Bishop up to
the Gamagara loop breccias are once more found along the eastern slope of the
Gamagara rand, and for some distance eastwards. On Aucampsrust, a thin
sheet of breccia separates the overlying Gamagara beds from the underlying
Dolomite, and along the western flank of the dome, its contact with the Dolomite is a distinct angular unconformity. On Kameelhoek, vVelgevonden and
Wolhaarkop the sheet of breccia is very thin and locally absent. Near the
Wolhaarkop beacon, however, a thickness of about 100 feet is present, and the
breccia passes upwards gradationally into finely fractured banded jaspers, the
banded ironstones being entirely absent.
The cataclastic rocks may be divided into two groups, viz., the Siliceous
breccia and the Blinkklip breccia.
1. The Siliceous breccia, a term suggested by Truter to include all the
breccias referred to by Nel (13, pp. 69-71) as chert breccias, is indeed composed
mainly of fragments of chert derived from the upper portions of the Dolomite
series. Locally, however, other siliceous rocks, mainly Gamagara quartzites and
silicified Gamagara shales, enter. South-east of Beeshoek, Truter has found
silicified Gamagara shales forming a large portion of the Siliceous breccia and
grading into the unbrecciated silicified shales (21, p. lviii). Boardman has
described similar instances from the Beeshoek manganese quarries (1, p. 39),
and also of Gamagara quartzites incorporated in and forming the bulk of the
Siliceous breccia in the Klipfontein hills east of Manganore station (ibid., pp.
37-38). On the road from Kameelhoek to Welgevonden, the writer has found a
breccia, in which sub angular and rounded blocks of Gamagara quartzite, intensely
fractured and partly recrystallised, could still be recognised.
Where the Siliceous breccia is overlain by the Blinkklip breccia, fragments
of banded ironstones have found their way into the former, and the contact is
not sharp but gradual. The gradational contact between the Siliceous breccia
and the fractured banded jaspers on Wolhaarkop has already been noted.
2. The Blinkklip breccia takes its name from a small hill with a bouldery
peak about three miles north-east of Postmasburg, known locally as Gatkop
and referred to by Rogers (15, pp. 37-38) as Blinkklip Kop. It was from an
examination of the breccia at this and neighbouring localities that Rogers got
the idea that the rock was a slump breccia.
~12
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
The Blinkklip breccia is intimately associated with and normally overlies
the Siliceous breccia. It is therefore distributed over the same area occupied by
the latter, but in the Aucampsrust-Wolhaarkop area no material that can be
described as true Blinkklip breccia has been found. The reason for this will be
apparent when the composition of the Blinkklip breccia is considered presently.
The Blinkklip breccia is exposed at its best in the arc-shaped Klipfontein
range of hills. Along the Gamagara rand it is found only east of Beeshoek and
farther north at the Gamagara beacon, and at a few isolated points to the north
thereof. Its absence in the intervening stretch is probably due to erosion of the
parent banded ironstones in pre-Matsap times and the deposition of the Gamagara sediments directly on the Dolomite.
The composition of the Blinkklip breccia is quite simple; it is the brecciated equivalent of the basal portions of the Lower Griquatown beds, and consists
of a heterogeneous aggregation of angular fragments, chips and occasional
slabs, of banded ironstones of all sizes, cemented together firmly by silica and
haematite. From this statement it is clear why in the Wolhaarkop environment,
where a few hundred feet of the upper portion of the highly siliceous banded
jasper zone is exposed, no true Blinkklip breccia has been developed.
During the large-scale survey of the dolomite flats east of the Gamagara
rand, an area in which the greatest development of Blinkklip breccia was known
to exist, it transpired that the amount of actual breccia was indeed rather small.
Around Beeshoek beacon, 1·5 miles east of th~ little village, the bandedferruginous rocks, resting on the Siliceous breccia, are brecciated for the lowermost
15-25 feet. The remainder of the rocks shows fracturing, folding and overfolding,
while the topmost portion is practically undisturbed and horizontally disposed.
Boardman (1, p. 42) has shown that in the Klipfontein hills the disturbance dies
away within 100 feet from the base. At Gatkop the best exposures of Blinkklip
breccia are at the base of the little hill, just above the sheet of Siliceous breccia
which separates it from the Dolomite. In this rock the fragments of banded
ironstone, which are relatively small, are jumbled about in the most haphazard
fashion and are cemented firmly by iron oxides. The remainder of the mass of
banded ironstones has been broken into larger slabs and blocks in which contorted folding, overfolding to the east, telescoping effects and slickensiding are
observable. The whole mass has undoubtedly been disturbed, but the movement
indicated seems to have been of a horizontal nature and directed from the west.
A word about the nature of the floor of the breccias, in particular of the
Siliceous breccia, may not be out of place here. It is usually somewhat undulatory, and in some localities extremely irregular (1, p. 36) due to sagging of the
breccia into solution cavities in the underlying dolomite. That much, if not all
of the breccia existed at the time of slumping is shown by the nature of the
tumbled-in mass, which usually consists of fragments of loose chert, quantities
of residual soil, and chunks of dislodged breccia, intermixed and lying beneath
the main mass of breccia. On the south-eastern side of Gatkop this main nlass of
breccia is seen, on closer inspection, to consist of a previously-formed breccia,
portions of which can be traced into the sub-horizontally disposed sheet of
Siliceous breccia, underlying the Blinkklip breccia, and which has been broken
up later into larger and smaller blocks and fragments and recemented by
specularite.
STRATIGRAPHIC FEATURES AND TECTONICS
213
Considering, however, the nature of the breccia floor in general, it appears
to be more regular than is evident at first sight. Viewed from the highest point
in the Klipfol1tein hills; the floor is seen to slope gently in a northerly and north·
westerly direction towards the Gamagara valley. Towards the south-west and
south there is a similar inclination of 2-3 0 in the direction of Beeshoek and Postmasburg. East of the Klipfontein hills the floor slopes to the east. The floor is
thus very gently arched, and bears an unconformable relationship to the Dolomite below, which is more steeply inclined to the south-east and east.
'
In the Klipfontein hills the Blinkklip breccia is in many places overlain by
Gamagara beds. Where the latter rest on the breccia, they are intensely
puckered; where they lie on the less-disturbed banded ironstones, they usually
occupy small basin-shaped hollows. Where the Blinkklip breccia is missing and
the Gamagara beds transgress on to the Siliceous breccia, they have in most cases
been inextricably mixed with the latter. Boardman has mapped and described
numerous instances where Gamagara beds are infolded with the Blinkklip and
Siliceous breccias, and overfolded from the west (1, pp. 83-84). (See also Fig. 5,
Section J -K, eastern part.)
3. The Origin of the Breccias.-When Stow (19) first described the Blinkklip
breccia, he expressed the opinion that it was of detrital origin. Many years later,
Rogers came to the conclusion that it was formed by the collapse of Lower
Griquatown beds under the influence of gravity, and aided, perhaps, by earth
movements, into solution cavities in the dolomite. He ruled out the possibility
of earth movements alone being the cause of the brecciation because he could
detect no sign of thrusting in the localities where the breccia was found
(14, pp. 178-179). N el upheld Rogers' views, but added that although the
Blinkklip breccia rests on the dolomite, its shattered and disturbed condition is
not reflected in the latter, except for a few feet of brecciation along the immediate
contact. He noted, further, that the absence of conspicuous shear planes and
of triturated rock like mylonite seems to exclude thrust faulting as a factor in
the formation of the breccia (13, p. 24). Du Toit, however, hinted at the larger
part possibly played by earth movements (9 and 10). Still, the above writers
stressed the importance of collapse under gravity as a factor, in fact, the principal
f'actor, in the formation of the breccia.
During the present survey Truter was the first to note the causal relationship between the- breccias and the post-Matsap earth movmnents, and definitely
assigned to them a tectonic origin. Along and east of the Gamagara rand,
Boardman has investigated this problem in detail, while in the AucampsrustWolhaarkop area, the writer was entrusted with this task, and we have
unreservedly come to the conclusion that the post-Matsap tectonic disturbances
have played at least a major part in the formation of the breccias.
In the description given above, it has been pointed out how at various
localities Gamagara beds have become involved in the brecciation, and how both
the breccias and the overlying Gamagara beds have been involved in the postGamagara folding. In addition, the following aspects should be stressed. The
breccias always rest on dolomite which shows signs of brecciation along the
contact. The basal portion of the breccias, the Siliceous breccia, is an extremely
fine-grained, structureless mass, becoming progressively coarser upwards and
214
pa~sing
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
gradationally into the Blinkklip breccia, where overlain by the latter.
The Blinkklip breccia, similarly fine-grained at its base, also becomes coarser and
less disturbed upwards and passes into gently folded banded ironstones. Moreover, Boardnlan has recorded the presence of east-west zones and sheet-like
bodies of actual mylonite, fine-grained and brick-like in colour and texture,
within the Blinkklip breccia, and the writer has observed similar sheet-like bodies
of mylonite in the Siliceous breccia on Wolhaarkop. In this locality the Siliceous breccia passes through finer and coarser stages from the base upwards
into a finely-fractured banded jasper at the top. The disposition of the breccias
into definite sheet-like bodies, of which only erosion relics have survived into the
present, and the general evenness of their floor are factors militating against an
origin through gravity collapse.
Slickensiding is frequently observed in both the breccias and the overlying
Gamagara beds. Where it has been studied east of, along and west of the Gamagara rand, it indicates a movement at low to moderately high angles from west
to east in both these rock groups.
Having indicated in the above paragraphs the importance of the post-Matsap
earth movements in the formation of the breccias, it remains to examine how far
slumping has been responsible for the brecciation. Boardnlan has given this
problem detailed consideration, and as the writer's own views are in full agreement with his, the following is a summary of his conclusions.
Theoretically, there could have been three periods of slumping:(i) During the post-Transvaal erosion cycle, in late Proterozoic times. The
Gamagara beds have, as noted previously, been deposited unconformably on an
erosion surface of rocks of the Transvaal system, and its basal member has been
derived mainly from debris of the Lower Griquatown stage. Now, if slumping
and brecciation of the rocks of this stage had taken place prior to the deposition
of the Gamagara series, traces of the Siliceous and Blinkklip breccias, which, as
we see them to-day, are hard, resistant rocks, should have found their way into
the Gamagara basal conglomerate, as pebbles. .At present the slopes of the
hills capped by the Siliceous and Blinkklip breccia are littered with blocks and
rounded fragments of these rocks. Nel (13, p. 33) states that" the parent rock
of much, if not all, of the pebbles in the basal conglomerate or breccia is the
Blinkklip breccia," and though this does not necessarily imply that pebbles of
Blinkklip breccia as such should be present in the conglomerate, their undoubted
presence could not be verified during the subsequent survey (22, p. 29). The
evidence for pre~Matsap slumping and brecciation is therefore at the most
indefinite.
(ii) During the Dwyka period of glacial erosion in late Carboniferous times.
During this period it was cold and the land was covered by ice, so that subsurface solution of the dolomite was probably at a standstill.
(iii) During the post-Karroo cycle of erosion which extends into the present.
The evidence to hand in the Klipfontein hills, for example, indicates that the
bodies of manganese ore, the Siliceous and Blinkklip breccias, the Gamagara
beds and the Dwyka glacial pavements on the latter have all been involved.
Most, if not all, of the slumping that can be observed along the manganese fields
to-day has, therefore, taken place during this period.
STRATIGRAPHIC FEATURES AND TECTONICS
215
To recapitulate: (i) the close association between the Blinkklip and the
Siliceous breccia indicates a genetic relationship and a common origin; (ii) the
direct relationship between the breccias and the crustal movements from the
west in post-Matsap times points to a tectonic origin of the breccias; (iii) the
participation of Gamagara beds in the earth nlovements and the incorporation
of Ganlagara quartzites and shales in the breccias indicate a post-Matsap age
and a tectonic origin for the latter; (iv) collapse, under gravity, of the breccias
and overlying rocks subsequent to their fornlation, is indicated.
E.
THE KARROO SYSTEM.
Rocks belonging to the basal member, the Dwyka series, are present in the
area, but seldolll exposed. On Plaatjiesdam, some greenish, thinly laminated
Upper Dwyka shales are exposed at the homestead, beneath a capping of surface
limestone. Half-a-mile east of the homestead the underlying tillite was found
in a well at a depth of 80 feet. In the neighbourhood of Matsap the presence of
Upper Dwyka shales has been proved by numerous wells sunk through the surface limestone capping, and they appear to fill a narrow trough-like depression
stretching from Vaalwater northwards to Witboom and probably beyond. In
places the shales are highly gypsiferous.
Mention has already been made of the rounded contours of some of the
mountain- and hilltops, a phenomenon which has already been attributed to
glacial action in Karroo times. In the north-eastern portion of the area the
origin of some typical U-shaped valleys has been ascribed to the same cause.
N ear the northern termination of the Gamagara rand and in the Klipfontein
hills, Boardman has found and described smoothed and polished pavements of
glacial origin (22, p. 14, and 1, pp. 92-93). He has used this information advantageously to date the period of slumping and the period of manganese deposition.
F.
THE SUPERFICIAL DEPOSITS.
Over larg~ areas the rock formations are covered by recent deposits of sand,
rock rubble, gravels and surface limestone. Coarse detrital deposits are found
only at the foot of the mountains, as talus slopes or as local outwashes of scree.
Gravels are equally sparingly developed, being found as a thin capping on the
surface limestone terraces along the Postmasburg loop. Surface limestone is
more common. South-west of Beeshoek and west of the Postmasburg loop, it
builds a fine terrace which stretches southwards as far as Witboom. In the
neighbourhood of l\Iatsap, it forms a thin cover on Ongeluk lavas and Dwyka
shales. North-east of Postmasburg and along the Gamagara loop east of the
Gamagara rand it caps the Dolomite. Around Griquatown it is alsb found on
the Dolomite. Sand is by far the most abundant covering material, especially
in the northern and western portions. The total thickness of superficial deposits
is usually measurable in tens of feet, but in the north and west the sand cover
is in places 300-400 feet thick.
G.
THE IGNEOUS INTRUSIONS.
Sill-like, dyke-like and pipe-like bodies of intrusive rock are found scattered
over the area, but are abundant only in the eastern portion and extremely rare in
the strongly-folded western belt. They belong to at least three and probably
four distinct periods of intrusion and nlay be subdivided as follows : -
216
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
1. Diabasic Rocks related to the Ongeluk Volcanics.-These are in the fornl of
sills or thin sheets injected along the stratification planes of the older rocks,
and seem to favour the contact zone between the Lower Griquatown beds and
the Ongeluk volcanics. Thus, the Groenwater sill is found along the contact of
the banded jaspers and the Ongeluk lavas, whereas the Aarkop sill on Magoloring
is in the lavas, about 150 feet abov,e the contact with the Lower Griquatown beds
(Fig. 3, and Fig. 4, Section E-E'). The rocks are more crystalline and coarser
in texture than the Ongeluk lavas and appear to be somewhat more basic.
Chemical analyses have, however, shown no appreciable differences in composition between the lavas and the intrusive rocks, and the latter are therefore
considered to be a related and slightly younger intrusive phase of the Ongeluk
volcanicity.
2. Basic Intrusive Rocks other than Karroo Dolerites.-They are confined to
the western folded strip, a fact which may perhaps account for their intense
alteration. They are found as narrow discontinuous dykes, intrusive into all
rock formations from the Ongeluk lavas upwards, and they have been emplaced
more or less vertically, either parallel or at right angles to the strike of the beds.
They are medium-grained, greenish-grey diabases and have been intensely altered.
Rogers (15,p. 63) regarded the dykes intrusive into the Hartley Hill lavas east of
Olifantshoek as r,elated to the latter. Similar dykes have been found intrusive
in the Upper Matsap quartzites, so that they are at least post-Upper Matsap in
age. They differ fronl the dolerites of Karroo type in that their presence could
not be detected magnetonletrically, even where traverses were nlade over the
exposed dykes. The Karroo dolerites of the area, on the other hand, give strong
magnetic anomalies. On these grounds they are for the time being classed as
post-Matsap, but pre-Karroo in age.
3. Dolerites and Related Rocks of Karroo Type.-In the western folded belt
rocks belonging to this group are rare and seldom well exposed. In the central
and eastern portions they are abundant, being found in three more or less separate
areas: On the dolomite flats north of Postmasburg, in the Ongeluk-Witwater
syncline, and on the portion of the Kaap plateau east of Griquatown.
The rocks have a close lithological resemblance to the Karroo dolerites, with
which they are correlated. Related varieties which are distinctly gabbroic, have
also been found and described (22, pp. 48-49).
The most favoured directions, along which the longest and most continuous
dykes are found are N.-S., N.N.E-S.S.W., N.E.-S.W., and E.S.E.-W.N.W.
There are shorter, more discontinuous dykes running E.N.E.-W.S.W., E.-W.,
S.E.-N.W., and S.S.E.-N.N.W. How these directions are related to the postMatsap earth movements will be discussed later on.
In the Langeberg dykes of this group, emplaced at right angles to the
direction of strike of the quartzites, are responsible for some of the larger poorts
through the mountains, e.g., Bergenaarspad (Plate XXVIII, Fig. 2) and
Padkloof.
4. Kimberlite Pipes.-A number of pipes, some of which are extremely
small, have been opened up as diamond mines in the area. Near Postmasburg
the Postmasburg and West End pipes are found. A smaller pipe is situated
STRATIGRAPHIC FEATURES AND TECTONICS
217
farther north in the Klipfontein hills. On Makganyene two pipes have been
opened up, while farther north, on Roscoe, another small pipe has been tested
for its diamond content. The old Peise:rton diamond mine is situated to the southeast of Postmasburg. Nel has described the pipes in detail and has produced
evidence to show that they are at least post-Karroo in age (13, pp. 50-52).
IlL-TECTONICS OF THE AREA.
A.
GENERAL STATEMENT.
A. characteristic structural feature of the area is the arrangement of its
mountain ranges in parallel arcuate belts. The western belt is an intensely folded
mountainous region, comprising the Langeberg and its foothills. The eastern
gently-folded belt if of moderate relief and is characterised by longitudinal,
transverse and oblique faulting along the eastern margin, at or near the northern
and southern extremities. The strike faults and the principal fold -axes are
parallel to the curving mountain ranges.
In striking contrast with the above, the central belt is one of low relief.
It includes the north-south trending Gamagara rand, which is remarkably
straight, but displays a certain amount of curving in the central part, which
corresponds to the disposition of the Klipfontein and the A.sbestos hills to the
east and the Langeberg and its foothills to the west. Furthermore, this belt is
characterised by extensive thrust-faulting, and herein lies, to a, large extent,
as Ti'uter has stated (22, p. 50), the key to the tectonics of the region. The close
parallelism of the arcuate mountain ranges, the principal fold-axes and the
strike faults suggest that they are .related to the post-Matsap earth movements
from the west.
The subdivision of the area into three structural belts, as sketched above,
and the order in which they will be treated below, does not, however, suggest a
similar chronological sequence in the structural evolution of the area. This will
become apparent from the descriptions given below, and will be outlined in detail
in the last chapter.
B.
THE INTENSELY FOLDED WESTERN BELT.
Within this belt are included the rocks of the Upper Griquatown s~age and
the entire Matsap series. The sedimentary members of these two groups bear
testimony to intense crustal deformation in post-Matsap tinles. Not only do the
beds dip steeply to the west, but they have also been extensively folded, overfolded from the west, contorted locally and sheared on a grand scale. The main
structural features are parallel to the Langeberg, which strikes N.200W.S.200E. in the north, north-south in the central portion of the belt, and
N.25°E.-S.25°W. in the south.
The Upper Griquatown beds show no folding, except for overfolding on a
small scale. Powerful deformation is evident from the steep westerly dips,
the brecciation and slickensiding in the brittle jaspers and cherts, the development of fracture cleavage in the shales and the strong shearing in the lavas.
In the Lower Matsap beds similar steep dips, approaching 90° in places,
and shearing and fracturing are prevalent; but whereas in the northern portion
of the belt the beds are apparently not folded, folding and overfolding may be
observed in the central portion, where the lower zone of quartzites, shales and
218
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOU-TH AFRICA
linlestones is not exposed. This section of the Lower J\Iatsap rand consists of a
sharp-crested anticlinal ridge lying to the west of the broader, higher portion of
the rand in which the quartzites are overfolded tow~rds the east (Fig. 8,
Section F-F').
East of Olifantshoek, where the Hartley Hill lavas are fully developed, they
have moderated the folding slightly. The lavas have been bent into a low anticline pitching to the south and partly eroded away on the western ~ide, the 3,xis
being now situated in the sand-filled valley east of Olifantshoek. On the eastern
side of the anticline lies an asymmetrical brachysyncline, the trough of whi~h is
occupied by Upper Matsap quartzites. The anticline and the syncline both fade
out into the contorted Upper Matsap quartzites to the south. Farther to the
south, the lavas are seldom exposed, but where they do crop out, they are much
sheared.
The thickly-bedded Upper Matsap quartzites display a series of asymmetricaJ but fairly open folds in the area to the north-west of Olifantshoek (Plate
XXX, Fig. 1). Boat-shaped synclines and dome-shaped anticlines, arranged en
echelon, are quite characteristic. Along the western flank of the Langeberg, and
notably to the south-west of Olifantshoek, the dips of the beds become very
steep, and the folds are closely compressed and isoclinal. A well-developed set of
joints, at right angles to the bedding planes, is present locally and may easily be
mistaken for a pseudo-dip to the east at a low angle.
In the neighbourhood of the Magaikwa and Andriesfontein beacons, a series
of pitching synclines and anticlines, the latter overfolded to the east and intensely
sheared in the cores, is developed. Some of the overfolds have probably passed
into small overthrusts, as, for example, east of Magaikwa.
From Andriesfontein southwards overfolding is evident all along the Langeberg, the dips being generally fairly low along the western flank and steep along
the eastern flank. Shearing is, however, so pronounced that the bedding planes
are more often than not entirely obliterated. Basin structures, overfolded on
the western side, may be observed east of Andriesfontein, at Bergenaarspad and
at Paardekloof. South of Paardekloof the folding is isoclinal on the east side
of the mountain and strongly contorted in the crestal portions. In the transverse kloofs west of the Heknaar beacon, two overfolds with a squeezed syncline
in between may be observed. A section across the mountain range at this point
(Fig. 12, Section T-T') gives a good idea of the type of folding generally
,encountered.
It is evident that the strongest folding has taken place in the central portion
of the folded belt, the intensity decreasing towards the north and the south.
This fact finds a reflection in the areas lying due east of this central portion of
the folded belt, e.g., the intensely-folded, bow-shaped central portion of the
Gamagara rand, the Klipfontein hills and the saddle-anticline connecting the two
great synclines in the eastern gently-folded belt. Taken together, these facts
serve to indicate that the focal point of the tangential stresses responsible for
the crustal deformation of the area lay somewhere to the west of the central
portion of the area, or more or less due west of the Magaikwa beacon, and also
point to a genetic relationship between the intense folding in the western belt
.and the structures encountered farther east.
STRATIGRAPHIC FEATURES AND TECTONICS
C.
219
TIIE1- THRUST-FAULTED CENTRAL BELT.
General Remarks.-Whereas in the western belt the rock beds have
yielded to deformational stresses by folding, overfolding and shearing, these
phenomena are of secondary importance in the central belt, where low-angle
faulting has taken place on a grand scale. The structural problems of this belt,
which stretches from the Gamagara rand to the foot of the Langeberg, are mainly
centred around the anomalous relationships existing between the rocks of the
Gamagara series and the older rocks belonging to the Transvaal system, which
the former normally overlie unconformably. Nel was the first to recognise and
accept thrust faulting as a factor responsible for some of the anomalous relationships observed along the western flank of the Gamagara rand, and probably,
also, on A.ucampsrust and Kameelhoek, where he observed somewhat similar
structures (13, p. 57). During the remapping of the area in 1936-1938, it became
clear that low-angle compressional faulting in post-Matsap times ~ad taken
place on a more extensive and intensive scale than was ·surmised by Nel. It
also became evident that the brecciated ferruginous and siliceous rocks, localised
as they are to the three inliers of dolomite in the central portion of the area,
and of post-Matsap age, as has been proved on previous pages, are causally
related to the low-angle compressional faulting.
On the accompanying map the writer has indicated six thrust-faults, marked
T.F.1 to T.F.6. They have been named from localities in which they are best
exposed and will be described under those headings in the succeeding paragraphs.
2. The Maremane Thrust (T.F.1).-This thrust-fault runs east of the
Gamagara rand, the thrust-plane following the contact between the Dolomite,
which forms the floor of the thrust, and the overlying rocks. In the Klipfontein
hills, in the neighbourhood of Postmasburg, east of Beeshoek and east of the
northern part of the Gamagara rand, in the neighbourhood of the Gamagara
valley, the thrust-plane lies between the Dolomite and the Siliceous breccia.
A.long the Gamagara rand, between Doornfontein and a point a few miles north
of Lohathla, the thrust follows the contact between the Dolomite and the Gamagara beds. Here the plane is inclined at a low angle to the west. In the neighbourhood of Postmasburg the inclination is in a southerly and south-easterly
direction, whereas to the east of Gamagara trigonometrical beacon it is gently
to the north. The thrust plane is thus slightly up-arched, and seems to conform
roughly in shape to the pre-Matsap Maremane anticline.
The presence of a thrust-plane between the Dolomite building the Maremane
flats and the overlying basal members of the Pretoria and the Gamagara series
has, up to the present, been a much debated question. Truter was inspired
with this idea when he studied the inextricable intermixture of silicified Gamagar a shales and quartzites with the Siliceous and Blinkklip breccias on the
eastern flank of the Gamagara rand on Olynfontein, south of Beeshoek, as well
as the nature, extent and distribution of the Blinkklip and Siliceous breccias
on this and the adjoining farms. Boardman has produced further proof that the
breccias have originated as a result of horizontal movement, from west to east,
of brittle-banded ironstones belonging to the basal portion of the Lower Griquatown stage and cherts belonging to the upper portion of the Dolomite series,
along a floor of dolomite. He goes so far as to state that" the zone of movement
1.
220
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
thus created can be regarded as in fact a secondary thrust-plane, but it is purely
local and subsidiary to the main break (the Gamagara thrust, described farther
on) which passed above, at no great distance as can be inferred from the low
angle of the thrust-plane" (1, p. 87). This" secondary" thrust-plane must, therefore, also exist between the Dolomite and the Gamagara basal conglomerate and
shales, along the eastern side of the Gamagara rand. Of this, he finds proof on
the farm Macarthy, north of Lohathla, where, along the crest of the Gamagara
rand, a small syncline of Gamagara beds has been overfolded from the west
and has passed into a thrust of small dimensions along the anticlinal axis
(22, p. 55). Boardman's explanation is that the overriding mass along the Gamagara thrust, the plane of which is now exposed along the western flank of the
Gamagara rand, caused a sympathetic drag of the rocks forming its floor in an
eastward direction across the underlying dolomite. The overfolding on the
eastern side of the Klipfontein hills and elsewhere along and east of the Gamagara
rand, in which the Blinkklip and Siliceous breccias and the Gamagara beds have
been involved, has resulted from the same movement.
N ow, the writer's view, which is in accord with Truter's original idea, is
that this secondary thrust-plane of Boardman's is, in fact, a major thrust, the
first to have been initiated in this area. An exhaustive search through the
geological literature has disclosed no instance in which secondary thrusting has
taken place below a larger thrust as a result of drag exercised by the overriding mass upon its floor. Thrust-faults usually follow one upon the other in
order o( relative age. Where a piling up of strata, or imbrication, has taken
place, this has preceded the main thrusting, as Cadell (4) and his successors have
proved, and it is followed by the major thrust, the plane of which lies at the base
of the smaller structures. The presence of a minor thrust on the eastern side
of the Gamagara rand, below a major thrust on the western side of the rand, would
therefore be an unusual feature. The writer pictures the trend of events in postTransvaal times as follows : Prior to the deposition of the Gamagara and Matsap beds, a geosynclinal
trough was created in the region of the present Langeberg. In the region of the
Maremane anticline, to use Rogers' term, the rocks belonging to the Transvaal
system suffered a westward tilt and were slightly arched at the same time.
Erosion then set in and stripped the overlying rocks until the Dolomite was laid
bare along the crest of the arch, which was studded with relics and arcuate
festoons of banded ironstone cappings. In this way a relatively incompetent
stratum, the fissile banded ironstone zone of the Lower Griquatown stage, was
exposed between the great thickness of massive dolomite below and the thicklybedded, massive jaspers and great thickness of unbedded and competent
Ongeluk lavas above, thus creating a weak zone along which strata would yield
most readily to crustal deformation. At the same time the Gamagara and
Matsap beds were being deposited to the west; the former as shallow-water,
offshore deposits upon the gently-tilted rocks belonging to the Transvaal
system, and the latter in the deeper portions of the geosyncline, which gradually
became shallower as it filled up. By this sedimentation, the load on the competent strata overlying the incompetent banded ironstones was increased.
STRATIGRAPHIC FEATURES AND TECTONICS
221
At the close of the period of Matsap sedimentation powerful compressive
forces set in from the west and were applied more or less in the central portion
of the area under discussion. They caused strong folding in the Matsap beds,
culminating in overfolding towards the east, shearing and thrusting on a small
scale, and resulted in a piling-up of rock beds in the eastern portion of the
geosyncline. Ooncurrently herewith, the Maremane anticline and the two completnentary synclines to the north-east and south-east, which were probably also
in existence already, became accentuated. With pressure from the west being
maintained, the rocks finally yielded by low-angle faulting along the basal
portion of the incompetent stratum of banded ironstones and thinly-bedded
cherts, and the mass rode forward bodily along a floor of massive dolomite.
The conditions pictured here are similar to those described by O. W. Hayes in
the case of the Rome and Oartersville overthrusts in the southern Appalachians.
(Quoted by Ohamberlin and Miller (4, p. 4).)
The dolomite floor itself suffered a certain amount of fracturing and brecciation, but the overriding brittle cherts and banded ironstones fared the worst.
Along the sole of the thrust they were minutely fractured, granulated and
locally pulverised. Higher up, brecciation became progressively coarser, passing
by degrees through layers in which bent, puckered and contorted slabs of banded
ironstones have been tumbled about and telescoped, into slightly folded and,
later, undisturbed banded ironstones. ,Outlying relics of banded ironstone and
chert were caught up in the movement, propelled forward and brecciated in the
same way. Frontal portions of such outliers were caught along the thrust-floor,
tucked in underneath the mass and the latter overfolded. Wherever overlying
Gamagara beds were caught up in the forward movement, they were inextricably
ground up with the brittle cherts and banded ironstones along the thrust-plane,
and in this process the quartzites and already silicified shales suffered most.
Where the Gamagara beds rest directly on the Dolomite, there is very little
sign of cataclastic deformation. Along the central portion of the Gamagara
rand the shales have been compressed into a series of isoclinal folds dipping
steeply to the west, but on account of the soft, smooth and yielding nature of
the basal beds, they would tend to slide easily along the floor of dolomite without
showing any effect of deformation except some local drag-folding. The absence
of mylonite or sheared rock along this portion of the thrust-plane should therefore not be regarded as proof against overthrusting. Some overseas examples
may well be cited here in support of this statement.
In the northern Rocky mountains of Wyoming, where Oarboniferous limestones have been thrust over Oretaceous shales for at least twenty-two miles,
the zone of crush breccia above the thrust-plane is stated to be notably thin at
nlost points. The soft shales below the major thrust-plane, while much
crumpled in places, are nearly horizontal and undisturbed over considerable
areas (7, p. 53).
In Newfoundland a series of argillaceous sediments compose the overriding
mass of a thrust involving a horizontal displacement of some six miles. They
are, on the whole, less deformed than anywhere else in the region, where they
have been intensely folded, and are not appreciably sheared within twenty-five
feet of the fault (6).
\----':~-
I
..
~~~=s2'~=2~~£69~~~~
;;',-, . . /Z",\
', -,/.'-'
I
I
"1,1
/1
:'~ ~ I \ I~
\
\
l~
t-:>
t-:>
~
~
m.
,
':'// I".7 .o,' •• "• ~.":
'- \-:"" I
L~ "
;7~A-
~ -=:\-----J.4~\
~-
~
1'/:::':
>
a
r-:.
"'',I' .~'".'
9'1' S:s ¥~~~~tc&=y=~~I:::r
. . V,,/~
,g-i
~I':'
'rlT~~LIJ
. ~~ ._:,:
/1
~ ~
~
CLII-:-r-,--Lr-c;I;=L:J:::J
.'
...,...-..-..J-
,/ \
,\;I~ oJ
>!
\
~
b~J \-·c-~e:-::"'::=::E=l..L.-L
a'\'~~~
~
!====-~
\:
MAG 0 L- 0 R I N G'
TIN
\
'-- ___...~ _ - f~
\
0\
/
L
r
-f''='='--'''-./1
-s-~j\ :43'-=:7
\ ...:
J " I, ",,"--~'-$
,- t.
I/
I ~ t:
1/ -=-... 6b, ,------==r
1,'41'~{,-,
/'
,,'="=',
l
}
J
;/}
~
m.
o
I:I;j
~
\\~~~~
\:,;'
~
o
o
o
0
0
:-1/-,:::
~\~\..!'-I
Ongeluk volcanics
T,II,'e
Lower Griquatown beds
} Pretoric
serres
WATERBERG
SYSTEM
TRANS VAAL
SYSTEM
~
o
~
o
S
a
>
t1
UJ.
o
Dolomite series
8trj
Dolerite dyke
~
1
~
o
I:I;j
m.
I
o
I
S
~
~
~>
FIG. l.-·The geology of portion of central Gamagara rand. (After L. G. Boardman.) Generalised. and T.F.l inserted by writer.
STR.ATIGRAPHIC FEATURES AND TECTONICS
223
In a recent publication on the structure of the central portion of the Sawtooth range, forming the eastern front of the Rocky mountains in north-western
Montana, where Devonian and Carboniferous limestones have repeatedly been
thrust over Jurassic and Cretaceous shales, no mention is made of any deformation of either the former or the latter along the thrust-plane. Where intense
imbrication of the limestones has taken place above the major thrust-planes,
thin bands of shale have been caught up between the splinters of limestone and
have acted as a lubricant along the thrust-planes (8).
As the overthrust mass rode forward, the rapidly increasing frictional
resistance along the thrust-plane finally arrested the forward movement, but as
the compressive stresses from the west were being maintained, the rocks yielded
to further overthrusting along a higher horizon and at a somewhat shallower
level of the earth's crust.
The horizontal displacement along the Maremane thrust does not seem to
have been very great. Boardman (1) reckons that a heave of! to t mile is quite
sufficient to account for the drag-folding, brecciation, granulation and mylonitisation observed along the thrust-plane. Now, if the present arched shape of
the thrust-plane be taken as indicative of the shape of the pre-Matsap Maremane
anticline, with which it conforms roughly, it is at once apparent that the axial
line of this arch is some two miles to the east of the axis of the present Marmnane
anticline, which runs along the Gamagara rand. The writer is therefore prepared
to accept a movement of at least that amount along the Maremane thrustplane.
West of the Gamagara rand, on Magoloring, there are two outliers of Gamagara quartzite resting on fractured dolomite, the latter having been thrust over
the Gamagara beds forming the rand itself. These two masses of quartzite
have been so intensely fractured, that no bedding planes can be discerned, but
they have seemingly been thrown into a series of isoclinal plications dipping
steeply to the west. The writer does not doubt the fact that the Dolomite here
exposed is portion of the floor of the Maremane thrust, raised to its present level
west of the Gamagara rand by the younger Gamagara thrust. They, therefore,
represent two thrust" Klippen," isolated by the Gamagara thrust from the
main overthrust mass lying to the east of the latter. (Fig. 1, and Fig. 2,
Section B-B'.)
3. The Gamagara Thrust (T.F.2).-The presence of this thrust-fault along
the western side of the Gamagara rand suggested itself to N el (13, p. 57), when
he found rocks belonging to the Transvaal system resting on the younger Gamagara beds at angles of 30° or less, along the western side of the rand. Boardman
made a further study of these anomalous relationships along the northern portion
of the Gamagara rand in 1936 and summarised them as follows: "(i) Rocks of
the younger (Gamagara) series lie at the same altitude as and occupy a position
between older rocks (belonging to the Transvaal system) which they normally
overlie unconformably. (ii) The older rocks on either side of the younger rocks
lie at the same level, although they belong to stratigraphical horizons more than
3,000 feet apart" (22, p. 53). The relationships between the older and the
younger rocks on the eastern side of the Gamagara rand have been described
above; those on the western side will now be examined.
224
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
~est
East
A
A'
8'
8
St,ructure sections across central Gamagara rand.
After L. G.Boardman, with T.F.l inserted by writer.
b
Cataclastic Rocks.
t3t Lower Griquatown Beds.
wg Gamagara Series.
t2 Dolomite Series.
At3 Ongeluk Volcanics.
Horizontal scale :-250 C.Roods = 1 inch.
Vertical scale exaggerated.
c
c'
---
Sections across northern and central portions of Makganyene area.
At3 Ongeluk Volcanics.
t3t Lower Griquatown Beds.
t3
Tillite.
t2 Dolomite Series.
Horizontal scale :-250 'C.Roods = 1 inch.
Vertical scale 4x.
FIG. 2.
On the southern portion of Doornfontein and on the western portion of
Paling, farther to the north, the Dolomite rests on Gamagara quartzites dipping
westwards at 10-20°. The Dolomite is intensely brecciated along and for a small
distance above the contact; the quartzites are fractured in smaller degree, and
locally appear as windows in the Dolomite (Fig. 5, Section J -K). On the
STRATIGRAPHIC FEATURES AND TECTONICS
225
southern portion of Magoloring the same relationship holds, although the contact
is not visible and the beds dip steeply (40-50°) to the west (Fig. 2, Section B-B').
On the northern portion of Magoloring and farther northwards on Lomoteng, a
thin sheet of banded jaspers, belonging to the upper portion of the banded
jasper zone, rests on the Gamagara quartzites, the entire succession dipping
westwards at 20-25°. The jaspers have been strongly fractured, slickensides
indicating a movement from the west at a low angle; the underlying quartzites
show few signs of disturbance. On the north side of the transverse kloof cutting
into the Gamagara rand on Magoloring, the overthrust sheet of banded jaspers
has carried along a small but complete thickness of Gamagara beds, ranging
from the conglomerate at the base to quartzites on top, originally deposited
unconformably on the jaspers. Locally, the underlying jaspers have been
eliminated through the low-angle faulting, and the Gamagara basal conglomerate
has been let down on to the underlying Gamagara quartzites forming the thrust
floor. (Fig. 1, and Fig. 2, Section A-A'.) Boardman has found that in this
locality the pebbles of the conglomerate have been sliced in step-like or rotational
fashion and the clayey matrix slickensided, but as a whole, the conglomerate
has suffered less deformation than the brittle banded jaspers (1, pp. 28 and 64).
On Bishop and the adjoining portion of Macarthy to the north, banded
jaspers lie against the westward-dipping Gamagara quartzites. The thickness
present must be small, for only a short distance to the west the Ongeluk lavas
are exposed. This fact will be considered more fully under the succeeding heading. The jaspers have undergone considerable fracturing without, however,
having passed into actual breccias. Boardman (22, p. 56) also notes that slickensided surfaces, passing into shattering, are locally so conspicuous in the quartzites that the bedding is obscured. This shattered quartzite has a dark purplish
colour and can be seen to finger into the undisturbed white quartzite underneath.
It no doubt represents a mylonitised facies of the latter.
On Jenkins and northwards towards the Gamagara valley, Ongeluk lavas
are exposed adjacent to the Gamagara quartzites, which dip to the west,
apparently underneath the former. About a mile west of the Gamagara rand
on Jenkins, two outcrops of Gamagara quartzite protrude through the Ongeluk
volcanics. They are regarded as fault inliers or windows of Gamagara beds,
and testify to the low inclination of the thrust-plane, which has evidently been
slightly folded in this locality.
About a mile south of the Gamagara loop a small patch of Lower Griquatown banded jaspers is exposed, surrounded on nearly all sides except the
eastern, by Ongeluk lavas, under which the jaspers seem to dip. On the eastern
side they rest on westward-dipping Gamagara quartzites. The tillite, which is a
remarkably persistent bed, is absent, and this seems to indicate that here the
thrusting has been somewhat complex. Further allusion to the relationships in
this locality will be made under the succeeding heading.
The anomalous stratigraphical relationships enumerated above are readily
explained on the assumption of low-angle compressional faulting along the
western margin of the Gamagara rand, the contact between the Gamagara
quartzites and the overlying rocks belonging to the Transvaal system marking
the trace of the thrust. Evidence for such low-angle movement in an easterly
direction along the fault plane has also been produced.
226
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
North of the Gamagara loop the trace of this thrust is obscured by a cover
of sand. About fifty miles farther north, near the junction of the Gamagara
loop and the Kuruman river, and beyond the confines of the accompanying
map, lies the Black Hock, recently mapped and described by Boardman
(3, pp. 51-6D). In 'this small outcrop of banded ironstones hehasfoundevidence,
in the form of slickensides, small low-angle faults and sub-horizontal crush
zones, indicating crustal movements from the west, and he suggests a possible
contemporaneity with the post-Matsap tectonic disturbances. The possibility
that the post-~atsap thrust-faults may extend into this area is suggested,
although positive proof is lacking. The writer's opinion is that if the post-Matsap
thrust-faults do extend towards the Black Hock, the thrust-fault that Boardman suggests in the first of his sections through the Black Hock (ibid., p. 56) is
most likely to be the extension of the Gamagara thrust.
South of Beeshoek, the trace of the Gamagara thrust is once more obscured
by a cover of surface limestone. Farther to the south, on the farms Waterboersdam and Lake Warren, there are exposures of Gamagara quartzites and
Lower Griquatown, banded jaspers, Indicating a possible extension of the thrust
in that direction. On the Cape Geological Sheets 45 (Postmasburg) and 41
(Griquatown), Hogers shows an inlier of Lower Griquatown beds in the Ongeluk
volcanics, bounded on the east by a fault which is probably of the same nature,
i.e., normal, as his Paling fault along the Gamagara rand. Boardman's
remapping of the area has indicated the presence of a triangular patch of Ga~a­
gara quartzites between two hogback ridges of banded jaspers, which rest on
the quartzites and dip to the west at 6-8°. On the eastern side the quartzites
rest on Ongeluk lavas. A short distance to the east a small cake of banded
jaspers, dipping eastwards at 4 0, rest on the Ongeluk volcanics, the contact being
a low-angle fault. This low-angle fault also separates the banded jaspers from
the Gamagara quartzites at the base of the two hogback ridges to the west, and
the thrust-plane has once more been curved into a gentle arch.
4. The Vlakfontein Thrust (T.F.3).-Barely half-a-mile west of the trace
of the Gamagara thrust-fault there is evidence of further overthrusting at a
slightly higher level. The presence of this thrust, which will, for the sake of
convenience, be referred to as the Vlakfontein thrust, is based on the following
evidence : -
On the southern portion of Doornfontein, a few hundred yards west of the
Gamagara rand, the tillite is found resting on the Dolomite which forms the overthrust (allochthonous) block of the Gamagara thrust. The tillite is somewhat
shattered, blackish, due to a later infiltration of manganese oxides, and milled
up with the dolomite along the contact plane, which is clearly a thrust-fault
dipping westwards at 2-3°. On the northern portion of Doornfontein and the
adjoining portion of Paling, four small outliers or " Klippen" of tillite have been
found on the Dolomite, bearing the same relation to the latter as on the southern
portion of Doornfontein. The fact that in these localities a thickness of 2,500
feet of banded ironstones and jaspers, normally present between the Dolomite and
the tillite, has been eliminated, gives an indication of the extent of the horizontal movement which must have taken place along this thrust-plane.
STRATIGRAPHIC FEATURES AND TECTONICS
227
On the northern portion of Vlakfontein and the adjoining portion of Magoloring, Ongeluk lavas crop out within half-a-mile of the Gamagara rand (Fig. 2,
Section B-B'). On the central portion of Magoloring a thin sheet of banded
jaspers, succeeded normally to the west by the tillite and the Ongeluk volcanics,
is found resting on Gamagara' conglomerates, shales and quartzites forming part
of the allochthonous block of the Gamagara thrust (Fig. 1). The banded jaspers
are strongly fractured and brecciated along the contact, similar, to the overriding banded jaspers along the Gamagara thrust, only a few hundred yards to
the east. This second overthrust l;lheet of banded jaspers has .also carried along
a small basin-shaped outlier of Gamagara beds, deposited unconformably on it.
It is apparent from this description and from that of the Gamagara thrust in
the same locality (p. 225) that the field relationships of the various rock groups
are very complicated indeed. Boardman has, 'by mapping on a large scale,
.succeeded admirably in grouping together the banded jaspers and overlying
Gamagara beds belonging to the separate overthrust masses. From this it became
clear the during the Vlakfontein overthrusting the rocks involved, including the
plane of the Gamagara thrust, were folded, resulting in an anticlinal bulge in
the floor of the Vlakfontein thrust. The plane of the latter, which took part jn
the folding, sliced across the anticlinal bulge, cut through the entire thickness
of the Gamagara overthrust block, into the floor of the latter before curving
back upwards. This is illustrated in Figure 2, Section A-A'.
North of the locality described above the Vlakfontein thrust forks into two
separate thrusts. Along the eastern branch the tillite has been shoved across a
small patch of Gamagara quartzites lying at the foot of the Gamagara rand.
The trace of the western fork is found along the Magoloring-Lomoteng boundary
fence. The banded jaspers and tillite, here forming a low ridge, have been thrown
into a series of gentle anticlinal and synclinal folds, with an overfolded anticline
running along the western flank of the ridge. On Lomoteng, this overfold has
passed into a small thrust. Along the eastern flank of the hill, however, the
tillite, following conformably on banded jaspers at the base, dips at 15° underneath banded jaspers exposed near the crest of the ridge. Along the contact,
which is taken as the thrust-plane, there are signs of fracturing and brecciation,
particularly in the overriding jaspers, but the deformation has been slight. The
thickness of the overthrust mass of banded jaspers, which is followed normally
by the tillite only a few yeards west of the thrust-fault, is barely 50 feet.
Farther north, the trace of the thrust is obscured by sand. On Bishop and
Macarthy, the close proximity of the Ongeluk volcanics to the Gamagara rand
has been noted (p. 225), indicating that only a small thickness of banded jaspers
is actually present, whereas the tillite, which is a remarkably persistent bed,
seems to be absent. This indicates the presence of a thrust-fault-in the
writer's opinion the northward extension of the Vlakfontein thrust-between
the banded jaspers and the Ongeluk volcanics on the farms named. On.J enkins,
it apparently merges into the Gamagara thrust, but the contact between the
Ongeluk volcanics and the underlying banded jaspers exposed a mile south of
the Gamagara valley on Parson, is evidently also a thrust-fault. This second
thrust, lying above the Gamagara thrust, is shown on the geological section
accompanying Sheet 173 (Oliphants Hoek).
228
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
There are indications that the Vlakfontein thrust extends southwards,
though a direct connection cannot be established owing to the presence of a
cover of surface limestone. On Witboom a thin lens-shaped sheet of Gamagara
quartzites is sandwiched between banded jaspers above and below, the succession
dipping westwards at 5-15°. There is no apparent angular unconformity between
the quartzites and the underlying jaspers and the contact seems to be an unconformable sedimentary one. Along the contact with the overlying jaspers the
quartzites have been crushed and altered into a tough glassy-looking purplish
rock, the alteration extending downward for 3-5 feet and fading out gradually.
The overlying banded jaspers have been granulated and intensely brecciated
near the contact, resembling the unferruginised Blinkklip breccia in places.
The most intense alteration is within thirty feet from the contact, which is
taken as the thrust-plane. Upwards the breccia grades through strongly plicated
and contorted layers into the undisturbed beds dipping gently to the west.
(Fig. 8, Section N-N').
On Kaffirkop, to the east and south of the beacon, banded jaspers, dipping
westwards at 1.2-15°, have been thrust over massive, pinkish Gamagara quartzites. Intense granulation of the former for a few feet above the contact was
observed, but for the rest the jaspers have only been folded and puckered within
the contact zone (Fig. 8, Section 0-0').
On Vaalwater, the Gamagara quartzites are exposed in an embayment in the
thinly-banded jaspers of the Lower Griquatown stage. They are nearly horizontal, and are overlain by the banded jaspers, the dip of the entire succession
becoming westwards along the western flank of the little hill. Along the contact
of the quartzites with the jaspers the former have been crushed and locally
recrystallised into a tough, glassy-looking purplish rock. The banded jaspers
have, over a distance of 20-25 feet above the contact, undergone intensive
brecciation. The rock has a strong outward resemblance to the Blinkklip breccia
and stands out as bold black hills, followed higher up and to the west by less
disturbed jaspers. Near the contact thin lens-like bands show intense granulation, and thin sheets of quartzite have been caught up in the overriding banded
jaspers and inextricably milled up with the latter (Fig. 9, and Fig. 10,
Section P-P').
West of the Gamagara rand the Vlakfontein thrust-plane dips westwards
at 2-3°. On Magoloring, it has been shown that the overthrusting was accompanied by gentle folding, and that the thrust-plane itself became folded. This
simultaneous folding and overthrusting is a characteristic feature of the area
west of the Gamagara rand, and the reappearance of the Vlakfontein thrustplane at the surface in the Wolhaarkop, Aucampsrust and l\lakganyene hills is
therefore by no means puzzling. How this conclusion is arrived at will be set
forth below.
The low westward dips of the banded jaspers and the tillite above the
Vlakfontein thrust-plane, west of the Gamagara rand, have already been noted.
Along the eastern flanks of the Aucampsrust and Makganyene hills, the banded
jaspers and tillite dip to the east at 5-10°. The intervening stretch of flat country, which is occupied by Ongeluk lavas, is therefore a. shallow sYIl(~linc, the
229
STRATIGRAPHIC FEATURES AND TECTONICS
o
o
o
o
o
o
o
o
o
o
1 N
Onge/uk volCanics
Tillite
Lower Gr;quotown beds
DolomIte series
/
I~IDo/erites of Kortoo
~
~ DIabase
~.ongeluk
....r
b
related
lavas
o
FIG. S.-The Geology of Makganyene and Adjoining Farms.
type
/0
1 f<.1iIe
;
230
TRANSACTIONS OF THE' GEOLOGICA:L SOCIETY OF 'SOUTH AFRICA
Vlakfontein syncline of Rogers (14, p. 188). It measures slightly more than
two miles across at the widest point on Vlakfontein, but extends for at least
t'e n miles in a north-south direction. Whereas probably the full thickness' of
tillite is' present along its western limb, the thickness of banded jaspers separating
the tillite from the dolOIp.ite below is very variable, ranging from nothing on the
western portion of Magoloring, where the tillite rests on dolomite, to about
300 · feet at the utmost in the Wolhaarkop neighbourhood, though usually
considerably below that figure. The stratigraphic relationships indicate that
the dolomite exposed on the western portion of Magoloring, Aucampsrust andr
Kameelhoek-Wolhaarkop represents portion of the floor or autochthonous
block of the Vlakfontein thrust, while the overlying strata represent the alloch..;
thonous block. In the Wolhaarkop, Aucampsrust and Makganyene hills the
floor of the thrust is actually exposed in a number of windows formed through
partial erosion of the folded overthrust block. The detailed relationships along
these outlying portions of the thrust-plane will now ·be considered.
About a mile east of the Magoloring-Kouwater boundary fence, the Dolomite is exposed on the crest of a small dome-shaped hill. It is overlain by
tillite on the west and east and by remnants of banded jaspers, succeeded
normally by tillite, on the north and south (see p. 203). While no dips can be
observed in the intensely fractured dolomite, the plane of the thrust appears to
be gently curved, and the roof rocks display a broad dome structure in sympathy
therewith. Locally, they have furthermore been thrown into small synclinal
and anticlinal folds. (Fig. 3, and Fig. 2, Section D-D') ..
The dome structure referred to above is continued southwards across
Makganyene. In the core of the dome erosion has laid bare Gamagara quartzites, overlain on all sides by banded jaspers. The quartzites have been crushed
and turned into a glassy-looking purplish rock near the contact with the overriding banded jaspers, which are locally at least 100 feet thick. The latter have
been finely granulated along the contact and pass upwards through more and
less brecciated varieties into the normal banded or thickly-bedded jaspers,
dipping gently to the west and to the east. The Gamagara quartzites represent
remnants-originally laid down on the Dolonlite or Lower Griquatown beds and
which have probably taken part in previous horizontal movement (Fig. 7
Section E-E').
The dome structures, as revealed by the Lower Griquatown beds, are continued on Aucampsrust, Kameelhoek and Wolhaarkop. The Makganyene dome
pitches to the south at its southern end, while the Aucampsrust-Wolhaarkop,
dome pitches to the north at its northern end, the two being connected by a
saddle-syncline filled with Ongeluk lavas.
The Dolomite is exposed on Aucampsrust in four oval-shaped areas elongated in a north-south direction. In all the outcrops it has an eastward dip'
of 5°, and is overlain either by a thin sheet of manganiferous breccia, by manganised Gamagara conglomerates and shales, or directly by the quartzites. On
the eastern side of the Dolomite outcrops, the quartzites dip to the east at 2-3°,
and locally at the same angle as the underlying Dolomite. On the western side,
near the Aucampsrust-Lucasdam boundary fence, the Gamagara beds dip westwards at 5-8° (Fig. 4). On Kameelhoek, Welgevonden and Wolhaarkop the
,
.
~-~ :~~~
.
\,
HONINGKRANS®
\:,.'..
-~\:
\
..
~
{o~'\
\~... ~~
I'
~ ~.
I
I
•
I
•
..:.
00
,',. . .
" . .
......
:-
I
")., /
,''a'q
,0,
."
7---"'---
LUCASDAM
v- \' _ _ _ _ _ _ - - - - - - - - - - -
• __ ......,.._,,'~..
c:J Superficiol deposits
ll>-
I
~
~
--~~'::..t
__ ~
...-_-+-_~__,._."'T""'
i':Ij
~,
/ ......~~~I_§~
t=t
II
G,omagorQ ,series
...
.',.
,.
'I
I
•.
. . .\. \ .
\
00
~
TIN
...
I
. .'I,: . .-
--'
1-3
~,
~r'
•
If
• '.oj •
t
•• J
'.:.
~.~
:;J.--
;Jr..l-..r..L...~,....
Dolomite series
O.
~tr.l
I /.
Lower GriqoQ/own beds
~
1-3
~
."...
o
Ongeluk. volcanics
Til/ite zone
Il>-
~.
•
,~~:-~ ... ,
Cotoc/qstic rocks
t;1
o-I,.!,'-'f
", \~
__ ' 0
~
.
.,-d__
,
[:~u.~i~.:t::~~
~
\ ·0·.
~
t
...
tr.l
a
1-3
..........
o
.... ......
~
H
a
..............
....
..
00
-"""
..,, .....
l ' Mile
•
o
0
,- -,
,,
.'
l . . .IG. 4.-·The geology of Aucampsrust and adjoining farms.
~
I-'-
232
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
Dolomite occupies a larger oval-shaped area and dips to the east at 12° and to
the south-east at 15°, in the south-eastern portion. Along the eastern margin
of this oval-shaped area the Dolomite is overlain by remnants of nearly horizontal Gamagara beds, separated from the Dolomite by a thin layer of partly
manganised siliceous breccia. Farther south the Gamagara quartzites transgress on to the Lower Griquatown banded jaspers. Along the western margin of
the Dolomite area a fairly continuous, but rather thin sheet of Gamagara beds,
dipping at 5-10° to the west, rests on the eastward-tilted edges of the Dolomite.
A thin sheet of Siliceous breccia separates the two locally (Fig. 6).
Now it has been postulated that in the two areas under discussion a small
anticline similar to the pre-Matsap Maremane anticline was formed and that
there, as along the present Gamagara rand, the Dolomite was laid bare by erosion
and subsequently covered unconformably by Gamagara beds (1, p. 28, and 3,
p.53). There are a number of factors militating against this postulate. On the
south-western portion of Vlakfontein the banded jaspers, disposed in a broad
anticline with a northward pitch, are followed conformably by the tillit'e. About
i-mile east of the Van Druten beacon they are overlain by Gamagara quartzites,
both being nearly horizontally disposed. About 1 to 2 miles south-east of the
beacon the Gamagara quartzites are again seen to overlie the banded jaspers
which here have a low dip to the south-west, south and south-east, the m~ss
being roughly dome-shaped. About two miles south-east of the beacon the
quartzites have transgressed on to the tillite, which dips at 5° to the east. The
contact between the Gamagara quartzites and the Lower Griquatown banded
jaspers and tillite is an unconformable sedimentary contact; yet within i-mile
of the tillite exposures the Gamagara quartzites rest on the Dolomite. The
question is, what has happened to the 2,500 feet thickness of banded ironstones
and jaspers between the two points mentioned ~ (See Fig. 4).
About two miles south of the above-mentioned beacon, the Gamagara
quartzites rest on westward-dipping banded jaspers. Some two miles north of
of Kameelhoek homestead the same relationship may be observed. Northeast and east of Wolhaarkop banded jaspers, brecciated and intenselY'puckered,
lie between the Gamagara beds and the Dolomite (Fig. 6). The intense brecciation and granulation of the jaspers in the neighbourhood of Wolhaarkop has
already been described (p. 214).
The relation of the Dolomite to the overlying Gamagara and to the Lower
Griquatown beds, of which only the upper 200-300 feet are represented, would
fit with a powerful overthrust, and the field evidence is to the effect that in this
area the Gamagara beds were deposited unconformably only on the upper
portions of the banded jasper and tillite zones. During the overthrusting the
beds were thrown into a series of dome- and basin-shaped folds. Along the crests
and the western flanks of the domes the resistance along the thrust-plane would
naturally hav;e been greatest. This resulted in the cutting out of the bang.ed
jaspers and the tillite along the central parts of the dome structures and the
letting down of the Gamagara beds on the dolomite thrust-floor, so that they
also became involved in the horizontal movement from west to east (see p. 211).
That this thrust is the Vlakfontein thrust has been indicated on p. 230.
West
East
G
H
o
0
9
c
~( ~(:~'_?!:::-?1 13
0
0
H
o
0
Cataclastic Rocks.
Gamagara Series.
Ongeluk Volcanics.
Tillite.
Lower Griquatown Beds.
Dolomite Series.
Horl. scale :-250 C.Rds. 0= 1 inch.
Vert. scale 5x.
Section J-K after L. G. Boardman.
with T.F.1 inserted by writer.
East
West
b
wg
At3
t3
t3t
t2
00
8
~
~
...
1-3
Q
~
~
...
a
71 3
!olj
l;Ij
~
~
~
00
East
West
J
o
o
0
0
0
oAI3
~
~
t::;j
8
0
0
l;Ij
a
8
o
...a
~
East
West
00
J
FIG.
5.-Structure sections across Aucampsrust area and eastwards.
~
~
~
234
TR,ANSACTIONS' OF THE GEOLOGICAL SO'C1ETY OF SOUTH AFRICA
On the southern portion of Vlakfontein duplication of the banded jasper
and tillite zones has been brought about by three high-angle thrusts (Fig 5,
Section I-J). This imbrication, although antecedent to, is an integral part of,
the main sliding at a lower level (4, and 5, p. 10),.
On Magoloring, a similar imbrication in the banded jaspers has been noted
(Fig. 2, Section A-A', western part). It was accompanied by intense brecciation
above the small thrust-planes and, as a result of subsequent fer ruginis ation, has
given rise to small bodies of high-grade iron ore.
The amount of crustal shortening along the Vlakfontein thrust is probably
considerable. If one assumes an initial dip of 3° along the western limb of the
pre-Matsap Maremane anticline, which seems reasonable, it can then be calculated that the tillite, now exposed near the crest of the anticline, must originally
have been some nine mil~s farther to the west. At least that amount of horizontal movement is therefore indicated.
5. The Aucampsrust Thrust (T.F.4).-This thrust has been followed from
the northern portion of Magoloring as far south as the south-western corner of
Wolhaarkop, where it is cut off by the younger Matsap Hills thrust.
In the north-western corner of Magoloring, the following succession is encountered in a traverse from east to west across a low hill. At the base are banded
jaspers, followed by the tillite. Then, at the foot of a small scarp of banded
jaspers, a lens-shaped body of Dolomite is exposed, which partly overlies the
tillite mentioned above and is, in turn, overlain, together with the tillite not
covered by it, by the banded jaspers forming the scarp. The entire succession
dips to the north-west at 12°. North-west and south-east of the hill the Ongeluk
lavas crop out. The relationship between the Dolomite and the underlying tillite
on the one hand and the overlying banded jaspers on the other is a thrust in either
case, and the structure has apparently developed from an overturned fold in
which all the beds forming the floor and the roof of the Vlakfontein thrust had
been involved, i.e., the Dolomite, the relatively thin overthrust sheet of banded
jaspers, the tillite and the Ongeluk volcanics. The stretched underlimb of the
overturned anticline finally passed into a thrust (Fig. 3, and Fig. 2, Section 0-0').
The supposition that such overfolding did take place is strengthened by the
stratigraphical relationships along the western flank of the Makganyene dome
and the adjoining portion of Kouwater. Here the tillite is found preserved in
two synclinal basins, both overfolded on the western side. Slight cataclastic
deformation is evident along the contact between the tillite and the banded
jaspers overlying it on ~hewestern side. Between the two synclines of tillite
the overlying banded jaspers extend as a distinct, though thin, sheet for some
distance up the flank of the dome, the contact with the undedying jaspers being
step.;.llke and marked by puckering and fracturing, particularly in the overlying
sheet. The contact described marks the plane of the thrust, and the amount of
sliding along this plane does not seem to hav;e ,been so extensive as in the case
of the older thrusts already d'e scribed (Fig. 7, 'Section E-E').
South of Makganyene the trace of the thrust disappears underneath a cover
of superficial deposits, but it' reappears on the south-eastern portion of Honingkrans. About It miles north of the Van Drutrn beacon a thin sheet of Gamagara quartzites, resting horizontally with an unconformable sedimentary contact
235
STRATIGRAPHIC FEATURES AND TECTONICS
.. :),
Af· ::,:1
,'\','
.,
.'('.
VI.,
. ·'1
,
I·· .,,'/
,,. . .,
,
:.
'1· • -;,
:/~25
-,
,
,
.....
\
,~
,,'.
...
'.
,/:'
, ... ' .
'
•
....
. .:/
•
-, / • KAMEELHOEK
,,/
,'..
,'~5"· •• •• r'
,'..
~
•
l
,{I' ••••
II,
t.\~)
\,~
••
,... . . .
••}
Y
Iff
7
A.....
It .,:./
...... . ; , ' , ' .,/.../,
, "
.....: ... ~/ ~-."
t")
'0-1
;,'<-:,
" . . .. ~ I
~·o • ••. :
~
I
I
·/~'
/1
C:'
~
~
I
:~~r-~-r~~~~~-L~-J:'II~~
SIr--P-r--L-,-J...-..,.-...l-.,---I.,..j,
M :94
T N
c:=J
Superficial deposIts
t::;:J;:-:;~~:;J Cotae/ostic rocks
E3 Gamagaro series
Onge/uk volcanICS
Tillite zone
Lower G"quOlOwn beds
Dolomite series
o
;
FIG. 6.-The Geology of W olhaarkop and Environs.
1 Mile
r
236
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
on banded jaspers, is overlain by a sheet of banded jaspers, about 50-100 feet
thick, which, because of its nearly horizontal disposition, presents a scarp-like
face on the northern, eastern and south-eastern sides of the hill on which the
beacon stands. About t mile north-east of the beacon the jaspers rest on tillite;
! mile east of the beacon they rest on banded jaspers which a. few yards to the
south are overlain unconformably by Gamagara quartzites. The contact
between the over- and underlying banded jaspers was, after a careful search,
found to be marked by a thin zone of brecciation and granulation. South-east
and south of the beacon the jaspers rest on Gamagara quartzites dipping northwest and west at 2-5°. A description of this contact need not be repeated here.
(I:?ee Fig. 4.)
East of the" nek " lying t mile east of the Van Druten beacon, is another
conspicuous flat-topped hill, capped by a sheet of banded jaspers, some 50 feet
thick. They rest on jaspers in the" nek," overlie Gamagara quartzites on the
western and southern sides of the hill, more jaspers on its eastern side and the
tillite on the north-eastern side (Fig. 4 and Fig. 5, Section H-I). Brecciation
and puckering of the jaspers and slight crushing of the quartzites may be
observed in the contact zone, which is the plane of the thrust.
,Approximately two miles south of Van Druten, along the AucampsrustLucasdam boundary fence, the thrust-plane is again exposed, where stronglycontorted banded jaspers override the westward-dipping Gamagara quartzites.
After having crossed the fence into Aucampsrust the quartzites peter out and the
banded jaspers underlying the quartzites to the north forin the floor of the thrust.
The thrust-plane follows a slight depression in the banded jaspers, along which
the rocks show intense fracturing and plication.
Some two miles north of Kameelhoek homestead, the banded jaspers are
again seen to override the westward-dipping Gamagara quartzites. At this
point the bandedjaspers wedge out between the thrust-plane on the east and the
Ongeluk volcanics, which succeed them conformably 'on the west. Farther to
the south the trace of the thrust therefore lies at the foot of the low ridge of
Gamagara beds, while Ongeluk lavas, exposed at random a few hundred yards
to the west of the ridge, form the roof of the thrust (Fig. 7, Section L-L').
On the western side of W olhaarkop a sheet of almost black, highly ferru.,.
ginised and somewhat puckered banded jaspers, not more than 40-50 feet thick,
is found. It rests on finely-fractured banded jaspers to the south-west of the
beacon, and on Siliceous breccia to the north-west. It dips to the west at
55-70° and is followed conformably at the foot of 't he hill by the tillite, and the
latter in turn by the Ongeluk volcanics. The plane of contact between this
sheet of ferruginised banded jaspers and the underlying fractured jaspers and the
Siliceous breccia is the thrust-plane (Fig. 6, and Fig. 7, Section M-M').
The intense brecciation observed on Wolhaarkop now becomes apparent
for the following reasons : (i) the close proximity of two planes along which movement had taken
place;
(ii) the strong folding concurrent with the overthrusting, as evidenced by
the local steepening of the thrust-planes;
fast
West
--_ .. - ........ _--- --
E
----
'--
Ul
~
Section across Makganyene dome.
~
~
L'
L
1-3
t-1
~
~
~
S
a
i'Y'.>/
;;:,/
~
l;j
~
Section across northern part of Wolhaarkop dome.
Wolhaarkop
1-3
q
~
Ul
~
~
t:::I
1-3
l;j
a
1-3
0
~
t-1
a
Ul
Section across southern part of W olhaarkop dome.
t3 Tillite.
K Karroo dolerit-e.
b
Cataclastic Rocks.
t3t Lower Griquatown Db Diabase related to the
wg Gamagara Series.
Beds.
Ongeluk volcanics.
At3 Ongeluk Volcanics.
t2 Dolomite Series.
Horl. scale :-500 C. Roods = 1 inch.
Vertical scale 4x.
FIG. 7.
t:--:l
~
~
238
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
(iii) the inability of the Dolomite to take part to the same degree in the
sharp folding and the resultant creation of a powerfully discordant contact
which aided further large-scale brecciation. .Along this discordant contact the
Dolomite itself was liable to intense fracturing. Proof of such brecciation to
a depth of some 80 feet has been found in the borehole on .Aucampsrust (2, pp.
30 and 34).
6. The Matsap Hills ThrUi~t (T.F.5).-West of the Wolhaarkop-.Aucampsrust-Makganyene hills evidence of further overfolding, followed by overthrusting,
is to hand. This is notably the case in the central portion of the belt, between
Lynputs and Wolhaarkop, where the uniformity of the flat country occupied
by Ongeluk volcanics is broken by a ridge of banded jaspers, presenting a steep,
scarp on the eastern and a gentle slope on the western side. South of W olhaarkop·
the outcrop of the thrust is obscured for some distance underneath superficial
deposits, but it emerges in the Matsap hills, north-west of Matsap, and is continued in a southerly direction to beyond the southern limits of the map.
N ear the northern and on the southern boundaries of Lynputs, banded jaspers, dipping 30° and 20° tu the west respectively, form low north-south ridges.
Ongeluk lavas on the eastern side are overlain by banded jaspers, whereas on
the western side they overlie the jaspers in the normal, conformable way. The
structure appears to be the result of overfolding,followed by a breaking of, and
sliding along the attenuated underlimb of the overfold (Fig. 8, Section F-F').
On Honingkrans, the structure is similar, but somewhat more complicated.
The complete succession has been given on page 204, and the following further
remarks are relevant:(i) The contact between the Ongeluk volcanics exposed on the eastern side
of the hill and the overlying banded jaspers is similar to that described on Lynputs, and implies a thrust-fault developed from an overfold.
(ii) The dolomite exposed along the crest of the ridge can only be a port jon
of the Dolomite series, for such a thickness of dolomitic limestones as represented
here is nowhere present in the Pretoria series.
(iii) The intensely fractured and sheared nature of the contact between the
dolomite and the underlying banded jaspers, and the presence of mylonite'
between the dolomite and the overlying tillite indicate the presence of a lowangle fault both below and above the dolomite, so that actually three low-angle
faults are present within this small width of outcrop.
(iv) From the frequent exposures of dolomite to the west of the Gamagara.
rand, it is evident that only a thin veneer of rocks belonging to the Pretoria
series, and in particular the Lower Griquatown stage, is present in this central
structural belt. In the writer's opinion the peculiar and seemingly complex
structures on Honingkrans have originated somewhat as follows : (i) Overfolding followed by overthrusting along the stretched underlimb
of the overfold. In these movements the rocks forming both the floorand the roof of the Vlakfontein thrust, as well as the thrust-plane
itself, were involved.
West
/,,~-~---
East
,
\
\
///
\
F'
F
w.
H
I:d
>-
~
Section across northern portion of Lynputs.
Q
I:d
>Iot;j
N'
N
::I:!
H
a
!:rj
t;j
o
>H
0
o
0
0
0
~
NW.-SE. section across Witboom.
o
----,,,
,
0'
w.
0"
>~
t:::l
H
t;j
a
H
0
~
H
aw.
rc
Dw
wmu
Fwm
Section. across Matsap area.
wml"Lower Matsap Beds. t3 Tillite.
wg . Ga~agara Series.
t3t Lower Griquatown
t3m Upper Griquatown
Beds.
Beds.
t2 Dolomite Series.
At3 Ongeluk Volcanics.
Horl. scale :-500 C. Roods = 1 inch.
Vertical scale 4x."
FlO. 8.
Surface limestone.
Dwyka Series.
Upper Matsap Beds.
Hartley Hill lavas.
l:-:l
co
eo
240
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
(ii)
A renewal of movement, with continued tangential pressure from the
west, along the portion of the Vlakfontein thrust-plane now lying
above the new-formed thrust. This existing plane appears to the writer
to be the most likely one along which further sliding would take place
subsequent to the overfolding and the breaking of the underlimb of the
overfold. Through this renewal of movement, the portion of the banded
jasper zone present above the Vlakfontein thrust-plane was cut out
and the tillite let down on to the Dolomite. The final state of affairs,
as now seen on the surface, is represented in Figure 5, Section G-H.
This depends largely on the depth to which erosion has proceeded. It
becomes clear from the section that at a higher erosion level no dolomite might have been exposed, whereas at a lower one the banded
jaspers on the eastern side of thehill might have been cut out.
On Lucasdam, next to the road to Beeshoek, Ongeluk lavas are exposed on
the eastern flank of a low" bult." They are overlain by banded jaspers, somewhat fractured and puckered, dipping westwards at 45°. The jaspers wedge out
towards the south between the Ongeluk lavas on the east and the tillite which
overlies them conformably on the west. The thrust follows the contact between
the lavas and the jaspers (Fig. 4).
On the western portions of Kameelhoek and Wolhaarkop the banded jaspers
are 200-300 feet thick, dip at 20-40° to the west and form a prominent ridge,
steep and scarp-like on the eastern side, where it overlooks a valley in which
Ongeluk lavas are exposed, and pr.esenting gentle dipslopes on the western side,
where the tillite succeeds the jaspers conformably. The thrust runs along the
base of the ridge (Fig. 6, and Fig. 7, Section L-L', western part).
In the south-western corner of Wolhaarkop the banded jaspers have been
thrust over Gamagara quartzites which have been deposited on gentle dipslopes
of banded jaspers lying to the east and north -east. Where the ridge built of the
overthrust mass of banded jaspers ends farther south, Ongeluk volcanics are
exposed and are clearly overlain by the banded jaspers, the contact being the
thrust-plane.
East of the Matsap beacon the trace of the thrust-fault can once more be
located fairly closely. Coarse conglomerates belonging to the Gamagara series
are exposed in this neighbourhood and are overlooked by prominent hills of
banded jaspers on the west, the dip of the latter being to the west at a low angle.
At one spot a sheet of banded jaspers, dipping to the south-west at 15°, rests on
the conglomerates. The jaspers are somewhat fractured close to the contact,
which is the plane of the thrust.
On the western portion of J\fatsap the Lower Griquatown beds have been
inverted, so that the tillite, exposed on the eastern slope of a hill, dips westwards at 20°, underneath the banded jaspers. In the sand-filled valley to the
east the Ongeluk lavas have been found in a well. The inversion is therefore
due to overfolding. Some two miles south of the tillite outcrop the Ongeluk
lavas were seen in a donga, at the base of an escarpment of banded jaspers. The
topmost portions of the lavas visible are only 4 or 5 feet vertically below the
lowermost banded jaspers exposed. There is no trace of the tillite and this seems
,
--,
........
--,
,'fO---.
i=/J. HlY/~4
• '"
-:"
:
"
. /';/
'.'
\
, ',:,r;~-~~,:
/
'I
j''''
';-
"
//
,~~ , O ; ~r.
..... ,
-.........
t===;'
,
,.---.J -
----.),
_ __F_5
\-;::f
- (
'-7 I...---!
'i1:
~
~
- __
~_r
/"-"".-.
I
~
J--.,J
t-:.
A
\
!
,I
~
,"
.,:
~
~--I
~/
;'
~
i~O
..,"
'-A-r
_--.~,
:
"!)
/
/
,""
..
~/
xI
.,--. , "''/
/--:-'
t---' .
~-
~
.....
~
," . '
I
I·
~
."
P>
It5' ' /
~
.....
: ~ .)j
'~
(fj;;'
,/ ~
/
I
Q
KAFFIRKOP
~
po..
~
w
~
~
~
,
~--.
,!
-!
,"/
./
./
~
/
" ~y
/'
Y
~
CJ
Superficial deposits
Q
~
Gomogoro series
o
~
.....
tr.:I
~
Q
00
Upper Griquotown
bed~
Ongeluk volcanICS
1,/
Tillite
:;',
'"
,
:•• 7
~
rn
po..
,,'
/"
~
t;;
I
VAALWATER
rn
~
P>
-'
,:' '. ,,:/
I
,"
>
;t=:::;:~"
/
I
'
:}'/
:
~~ :It-6Hay 35---1
I
!
I
j::;;'
f
,~..,~
r:;;: ~'-----'
,
/
... .
'-:: .. .
I
\ ~~~''~---~--:
r'/}
'
'
/
\'...../if-=:
,
,,\
,
L~
CONE
I
,
I~--';';'-"~''-------I
"..
".'~'
2j',
//
"
,,'
) /,' \
,/~' : ' , :, '
• k.,'
,'.,/ ~
~
" ",,'"
~'"'7' MATSAP
lavos
"
\K,'.Il
",
I
... .,' ( '
.,
,,""
.....,'
/
--
fO';' •
\
Kaffirko'pn ",.,.-;
\
I
/,'
\ • • •,,' "'"
.,
'---,
~
: , exposed In donga
:':'/
",,~.-:: \~/
I
:,,..... J"'"
I ,/
: , 'j , \
,'. 'L\
• ''~Ongelul.
'_.'
-
~'\I
-, •
" . ',:l
~
,,"
-_-?,,,,,,, "
/-'.'
.. ·A·"':::-"
:'~,,~:/
:.
~r
.
\
.~
,...J!
,' .. :~'
,-1
l .... --"
• , Lower Griquotown beds
,'~I
:II!'--
~-v'
L....
11,
_ _ _'
'+-
-----7'- I
:-:-~-I~"'~"~~'
~
"
.
®
o
1 Mile
;
, ..J::-- ~
,
FIG. 9.-The geology of the country around Matsap.
t-:l
~
~
242
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
to indicate that the overfoldmentioned has passed into a thrust, the trace of
which runs nlore or less along the base of the banded jasper hills (Fig. 9, and
Fig. 8, Section 0-0', western part).
West
East
p
pi
Section across portion of Vaal water and westwards.
Section across Hartebeeshoek, north of Langkloof trig. beacon.
RI
R
t3t
=-=--=~.:..:.;IWg
-,--_: --:.= - - -_.
Section across northern portion of, Camel thorns arp.a.
wg Gamagara. Series.
t3 Tillite.
At:3 Ongeluk Volcanics.
t3t Lower Griquatown Beds.
Hor!. scale :-500 C. Roods== 1 inch.
Vertical scale 4x.
Fig. 10.
Farther south the thrust-plane is exposed some 3t miles west of Vaalwater,
where a thin capping of banded japsers is found on the Gamagara quartzites.
(Fig. 10, Section P-P', western part.) The relationships along the contact are
the same as found on Vaalwater, although the disturbances are not of
the same intensity. In the neighbourhood of Langkloof the same relationships hold. On Hartebeeshoek the Gamagara quartzites, over which the
banded jaspers have been thrust, have been thrown into a small dome-shaped
fold elongated in a N .E.-S.W. direction. Exposures of portions of the overthrust
sheet of banded jaspers along the flanks of this elongated dome indicate that the
thrust-plane has been curved in the same waJ and that the folding continued
after the main sliding had taken place. (Fig. 10, Section Q-Q'.)
243
STRATIGRAPHIC FEATURES . AND TECTONICS
(.
,'.
:::;~:,
CAMELTHORNS
T N
,;- .... -..... ,
: .. -',
\,
1
... .
I
,. • • •
: :
,. . .
''''"1
-\
J •• '
r.I'~\.\.
,.:-
1 • • _
! .', \.. !
...'
:-..~.\, :~
:. .. -'
~ ... :
~
~,~~
o
SuperfiCial deposits
~ Gamogara
series
Tillite
Lowe; Griquotown beds
o
1 Mile
E=~===:==~!========S'
FIG. 11.-The geology of Camelthorns and adjoining farms.
244
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
West of the trace of the thrust-plane the banded jaspers forming tue
allochthonous block dip gently to the west, but have locally been thrown into
low anticlines and shallow synclines. At Blackridge the folding is of a more
intense nature, and the beds are overfolded and locally contorted. Within this
zone of overfolding the Blackridge asbestos mines are situated.
To the north -east of Oamelthorns the thrust-plane is seen to swing first to
the west, then to the north around the nose of a northward-pitching anticline,
and then back in a southerly direction. Along the north-western and northern
flanks of the anticline the dip of the overthrust sheet and of the underlying
Gamagara quartzites is low, as much as 30° locally. Along the wester!). flank it
increases up to 45 ° due west of the Venter beacon, where the thickness of Ganlagara quartzites along the thrust-floor is small. Farther to the south the dip is
very low once more and locally reversed, e.g., on Dingle, as a result of minor
folding in which the thrust-plane itself was involved. (Fig. 11.)
The pitching anticline, mentioned above, extends southwards into Dingle,
where it fades out owing to a pitch to the south and the development towards
the south-west of a complementary brachysyncline, filled with Ongeluk lavas,
which are not exposed but have been located in wells. On the west-central
portion of Oamelthorns the anticline is overfolded towards the east. The tillite
is exposed in one place along the western limb and dips steeply westwards,
underneath the banded jaspers. The sand-covered flat to the east is therefore
a syncline, filled with Ongeluk lavas, which have been encountered in a few
wells (Fig. 12, Section S-S').
The overfolding dies out towards the north and the south. North of the
Venter beacon its probable continuation is marked by a small, secondary thrust,
post-dating the main thrust. Thereby a narrow strip of banded jaspers belonging to the roof of the main thrust has on the downthrow side of the minor thrust,
near its southern end, become wedged between Gamagara quartzites forming
. the floor. Near the northern end, on the'other hand, a narrow strip of Gamagara
quartzites on the upthrow side has become wedged between banded jaspers.
(Fig. 12, Section R-R'.) Brecciation along the thrust-plane is very slight in the
case of the underlying quartzites, and is limited to the lowermost 20 feet of over,thrust banded jaspers.
A characteristic feature therefore of the Matsap Hills thrust is the folding
and overfolding which preceded the thrusting and the further folding and to a
smaller extent overfolding which succeeded the thrusting and caused the folding
of the thrust-plane itself.
7. The Lynputs Thrust (T.F.6).-It has been shown that in the neighbourhood of Olifantshoek the Lower Matsap beds may be subdivided into a lower
zone composed of quartzites, shales and limestones, and an upper zone of pinkish,
sheared quartzites (p. 210). The fact that along the central portion of the Lower
Matsap ridge, i.e., in the neighbourhood of Lynputs, only the rocks belonging
to the upper zone are represented and that they are overfolded towards the
east, has also been noted (p. 210). Furthermore, a few remnants of somewhat sheared Hartley Hill lavas, normally found on the western side of the Lower
Matsap ridge, have been found on the eastern side and at the base of the ridge,
on Lynputs (p. 210). On the south-western portion of Floradale, quartzites,
East
West
s·
5
Matsap Hills Anticline
Camelthorn:l
Syncline
U1
L ~~'-
Sand
o
0At3
0
0
0
0
0
Section across southern portion of Camelthorns area and eastwards.
b~~:L~;~'~:: -:-~~~-~~:::
~
pj
II>
~
~
S
a
~
P>
~
~w.
T
P>
~
I::;;
~
a
8
o
~
Section across Langeberg at Heknaar trigonometrical beacon.
wmu Upper Matsap Beds.
wg
Gamagara Series.
At3 ) Ongeluk Volcanics.
t3
Tillite.
t3t
Lower Griquatown Beds.
Horizontal scale :-500 C. Roods == 1 inch.
Vertical scale 4x.
FIG.
H
a
fA
12.
t:-:>
~
~
246
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
shales and limestones belonging to the lower zone of the Lower Matsap stage,
are exposed. They dip at 40-5(J° to the west and are overlain by a mass of
whitish, crushed quartzite practically devoid of bedding planes. The plane of
contact, which is clearly a thrust, inclines at a low angle to the west.
It is therefore apparent that along the central portion of the Lower ~:Iattap
ridge the beds have been overthrust subsequent to the overfolding. Thereby
the contact between the Hartley Hill lavas and the underlying quartzites
belonging to the upper zone of the Lower Matsap stage, has suffered an eastward
displacement, as far as the contact between the Upper Griquatown beds and the
overlying rocks belonging to the lower zone of the Lower Matsap stage. This
slip amounts to at least one mile. The plane of the thrust lies at the base of the
Lower Matsap ridge (Fig. 8, Section F-F', western part).
8. Further Cases of Possible Overthrusting.-In the west-central portion of
the area the Hartley Hilllav3:s are seldom exposed or are conspicuously absent,
and the westernmost outcrops of the Lower Matsap strata. and the easternmost
outcrops of the Upper Matsap beds are in some places less than! mile apart.
On the farm Mamatlun, five miles north-east of the Magajkwa beacon, the Hartley
Hill basal conglomerate is exposed barely 400 yards east of the nearest outcrops
of Upper Matsap quartzites, which dip at 35° to the west._ In this locality and
farther to the south the thickness of lavas present must be less than 1,000 feet,
whereas in the type locality near Olifantshoek, it is about 2,000 f€et.
This rapid decrease in thickness within such a short distance from the type
locality, where the s~ccession is normal, suggests that a certain amount of sliding
of the- Upper Matsap quartzites over the Hartley Hill lavas did take plac,:.
This view finds support in the fact that the Hartley Hill lavas and the underlying Lower Matsap beds dip very steeply to the west, in some places at 90°,
whereas the Upper Matsap quartzites dip in the same direction at only 35-40°.
The evidence is, however, not conclusive, as some natural thinning-out of the
lava flows may have occurred.
The possibility of small overthrusts, developed from overfolds, in the upper
Matsap quartzites has also been alluded to (p. 218), but again positive proof is
lacking.
9. The Associated Normal Faulting.-In the neighbourhood of Hartebeeshoek a normal fault with a downthrow to the west has been traced for roughly
six miles in a north-south direction. North-east of Hartebeeshoek the banded
jaspers forming the overriding block of the Matsap Hills thrust have been let
down against Gamagara quartzites which form the floor 'of the thrust. The
fault, therefore, post-dates the Matsap Hills ~hrust. On Hartebeeshoek the
banded jaspers overriding the Gamagara quartzites along the elongated dome,
dip south-eastwards at 25°, while against the slope of the hill It miles farther
east, i.e., below the Langkloofheacon, the Gamagara basal conglomerate crops
out 250 feet above the base of the hill. Here the fault has its maximum throw
of at least 300 feet (Fig. 10, Section Q-Q'). Southwards, the anlOunt of throw
decreases, and in places is so small that the fault cannot be traced in the Gamagara quartzites. N ear its southern end the Gamagara basal conglomerate and
some underlying banded jaspers are exposed on the upthrow side of the fault,
the conglomerate lying some 50 feet above the top of the quartzites on the downthrow side.
STRATIGRAPHIC FEATURES AND TECTONICS
247
From the above brief description it may be inferred that the fault has
resulted from irregular sagging of the earth's crust subsequent to the overthrusting. This may be due, as Truter has pointed out in the case of the normal
faults following on the Potchefstroom overthrust, to a readjustment of localised
pressure resulting from a piling-up of rock masses during overthrusting. Such
localised pressure will give rise to tensional stresses and readjustment will be
effected by normal displacements (20, p. 453).
D.
THE GENTLY FOLDED EASTERN BELT.
This belt occupies the eastern half of the area shown on the map, and south
of Postmasburg extends well into the western half. vVithin this area the rocks
belonging to the Dolomite series and the lower and middle divisions of the Pretoria series have been thrown into a series of geptle asymmetrical anticlinal and
synclinal flexures.
1., The Folding.-In describing the Dolomite east of the Gamagara rand,
the fact that the beds dip persistently eastwards (south-eastwards in the neighbourhood of Postmasburg and north-eastwards in the vicinity of the Gamagara
valley in the north) has been mentioned. Only along the Gamagara rand the
beds are horizontal or show local westward dips. Rogers regarded th~dolomite
flats' as forming part of a broad low anticline, which he named the Maremane
anticline. Clearly only the eastern limb is exposed to view to-day, the Gawagara
rand being situated more or less along the axis. It has been stated.on page 209
that the Gamagara series had been laid down unconformably on an erosion
surface of gently tilted rocks belonging to the Transvaal system, and the Maremane anticline is therefore a pre-Matsap structural feature.
To the north"'east and south-east of the Maremane anticline, two great
asymmetrical synclines, the Dimotensyncline and the Ongeluk-Witwater
syncline, are developed, connected by a saddle-anticline lying to the north of
Groenwater. As features complementary to the Maremane anticline, they have
probably originf1ted at the same time. A probable relationship between these
features and the post-Matsap tectonic disturbances has already been hinted at
and will be stressed below.
The eastern limb of the Dimoten and Ongeluk-Witwater synclines is formed
by the arc-shaped Kuruman hills and the Asbestos mountains; the western
limb is formed by a slightly less conspicuous range of banded ironstone hills
which die out in the north on Kathu and in the south on Kameelfontein on the
Postmasburg loop. The axes of the two synclines pitch to the north and south
respectively and curve slightly to the west.' The Ongeluk-Witwater syncline
measures fully 40 miles along the axis and 20 miles across at its widest point.
West of the Ongeluk-Witwatel' syncline the Matsap hills 'form a broad, flat
anticline, the axis of which runs in a N.N.E. direction and pitches to the north.
Ganlagara beds are found along the crest of this anticline in the neighbourhood
of the Langkloof beacon, and display the same anticlinal structure: The anticline in its present shape is therefore a post-Matsap feature. That this anticline
is probably connected with the Maremane anticline is indicated by the exposures
of banded jaspers· on Witboom, in which an anticlinal structure may still be
observed. The writer's opinion is,' however, that the pre-Matsap Maremane
248
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
anticline had a southward extension into, and was connected with the MatRap
Rills anticline by a shallow intervening saddle-syncline. The Matsap Hills
anticline was then probably much flatter than now, and the Gamagara bed~.
which to-day rest on the banded jaspers with no angular unconformity, were
most likely deposited on dipslopes of very gently tilted beds along the flanks ot
this pre-existing anticline. Crnstal deformation in post-Matsap times had an
accentuating influence on the pre-Matsap structural features.
In the south-western portion of the area the gently folded belt approaches
very close to the intensely folded western belt and grades into the central thrustfaulted belt, which here extends along the western limb of the Matsap flills anticline and is accompanied locally by overfolding. This has been described in
connection with the overthrusting on pages 242 and 244.
2. The A880ciated Marginal Faulting.-Several strike faults, accompanied
by transverse and oblique faults, occur in the eastern limb of the Dimoten and
the Ongeluk-'Vitwater synclines. It is rather characteristic that they are
located in the terminal portions of this great arc, and have not been found in
the central portion of the latter.
The faults situated in the eastern limb of the Dimoten syncline have already
been described (22, pp. 59-60). East of the Chee beacon is a major strike fault
running in a N.N.W.-S.S.E. direction, with a downthrow to the east. It is
accompanied by several transverse faults, most of which are terminated by the
strike fault, and a number of oblique faults, most of which cut the strike fault
but have caused practically no displacement in the latter. They are mostly
orientated in a N.N.E.-S.S.W. direction, but one or two run in an E.S.E.W.N.W. direction. The strike fault runs along the foot of an eastward-facing
escarpment formed of dolomite and capped by w~stward-dipping banded ironstones. The banded ironstones east of the fault dip in nearly every conceivable
direction and are seen to pass from locally contorted, through sharply pl1cated
into intensely brecciated strata near the fault-plane. The brecciated banded
ironstones are practically indistinguishable from the non-ferruginised Blinkklip
breccIa.
Duplication of the Lower Griquatown beds and the Ongeluk lavas to the
south of the Dimoting beacon and farther south on Schietfontein has led to the
recognition of two strike faults, trending roughly N.N.W.-S.S.E., with downthrow
to the east. In the case of the Schietfontein fault, the asbestos zone on the
upthrow side has been raised to the level of the Ongeluk volcanics, and the throw
must therefore be of the order of at least 2,500 feet. The strike fault south of
Dimoting is accompanied by one transverse fault, striking E.N.E.-W.S.W.,
and one oblique fault striking N.N.E.-S.S.W. In both cases the strata on the
southern sides of the faults have suffered a relative eastward displacement.
In the vicinity of Griquatown, Boardman has mapped a number of faults
which bear a striking similarity to those described above as regards their position
and the effect on the beds involved. The writer's knowledge of these faults has
been gained from rather cursory inspections, while mapping in the areas farther
west. As detailed descriptions of the faults are still being awaited, the views
expressed below should not be regarded as authoritative.
STRATIGRAPHIC FEATURES AND TECTONICS
249
On Hopefield, north of Griquatown, a strike fault, trending roughly N.E.S.W. with the downthrow to the south-east, has been mapped. Thereby the
asbestos horizon on the downthrow side of the fault has been let down to below
the dolomite-banded ironstone contact on the upthrow side. The throw, therefore, amounts to more than 100 feet.
West of Griquatown, on the eastern portion of Taaiboschfontein, rocks
belonging to the tillite zone have been let down on the eastern side of a N.N.E.S.S.W.-trending strike fault against banded jaspers lying several scores of feet
below the tillite zone on the western (upthrow) side. The N.N.W.-S.S.E.-trending oblique fault on Taaiboschfontein has caused a duplication of rocks belonging to the banded jasper and tillite zones and the Ongeluk volcanics. This fact
was noticed by Rogers in 1905 (14, p 167), and he regarded the fault as of the
normal type with downthrow to the east. The amount of relative displacement
along the fault-plane is seen to decrease in a S.S.E. direction, and it may be
regarded as of the hinge fault type. Boardman, however, marks the fault as a
reverse fault. The writer agrees with him" regarding the fault as an obliqueslip reverse fault, the horizontal component of which has caused a relative
northward displacement of the strata on the upthrow side. The smaller oblique
fault shown to the south of the former, whereby Ongeluk lavas on the upthrow
side have come to occupy a position adj-acent to feldspathic quartzites, lying
several tens of feet below the tillite on the downthrow side, is apparently similar
to the one described above, except that in this case the horizontal (tear) component acted in a southward direction. In this neighbourhood, Boardman has
also mapped a small thrust-fault in the banded jaspers, which indicates that the
faulting in general is due to compression rather than tension.
Truter and Wasserstein (22, p. 60) have already focussed attention on the
fact that the displacement along the faults in the eastern limb of the Dimoten
syncline is out of proportion to the gentle pre-Matsap flexuring and that the
movements appear to be of such a violent nature as to suggest that they are
related to the post-Matsap tectonic disturbances. This seems to hold equally
well for the faulting in the neighbourhood of Griquatown, and the writer agrees
with the authors quoted above in interpreting the strike faults as reverse faults
and the transverse and oblique faults as tear faults, due to a differential transmission and local resolution of the stresses from the west into components acting
in north-easterly and south-easterly directions. The evidep.ce is not conclusive,
but rather suggestive.
E.
THE ORIENTATION OF DYKES AND FRACTURE ZONES.
The conspicuous emplacement of post-Karroo dolerite dykes in the foreland
of the central thrust-faulted belt is at once apparent from a glance at the map.
Longitudinal cracks filled with vein quartz, and transverse and oblique fractures
along which crushing and brecciation have taken place, are also found only in
front of the great thrusts.
Along the eastern flank of the Matsap hills, a few north-south cracks, filled
with vein quartz, have been observed in the banded jaspers. South and southeast of the Langkloof beacon three fracture zones have been found, two of which
strike N.E.-S.W. and one N.W.-S.E. A fourth in the Gamagara quartzites near
250
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
the beacon strikes E.S.E.-W.N.W. Significant is the fact that most of the larger
dykes in the area to the east give preference to the directions mentioned above
(p. 216). The small transverse and oblique faults characterising the eastern margin of the gently folded belt, strike in directions corresponding to the above.
These features therefore seem to be genetically related to one another as well
as to the post-Matsap tectonic disturbance.
The north-south fractures are therefore regarded as tension cracks, along
which crustal settling took place, subsequent to the thrust-faulting, by gravity
faulting on a small scale. Boardman has suggested that the composite dyke
east of the Gamagara rand and the Klipfontein dyke follow the sites of
relaxational faults of post-Matsap age, and that small-scale trough-faulting
might have taken place between the parallel dykes. No conclusive evidence for
this statement could, however, be obtaineq. (1, p. 48).
The transverse and oblique fractures are regarded as tear fractures or planes
of shear, which have arisen through the local resolution of the compressive
forces into components acting roughly in N.E. and S.E. directions. Such
fractures were favourable for the emplacement of the post-Karroo dykes.
F.
THE NATURE AND AGE OF
THE
DIASTROPHIC MOVEMENTS.
Conditions similar to those determined by Truter in the Potchefstroom
area (20), but on a more extensive scale, seem to have prevailed in this area
during the period of post-Matsap orogeny. That the dominant stresses acted
from the west is indicated by the orientation and inclination of the thrustplanes and the inclination of the axial planes of the overfolds. The curvature
of the fold axes and the arcuate shape of the structural belts and the folded
mountain ranges indicate that the focal point of the tangential stresses lies to
the west of the folded and faulted belts, more or less along an east-west line
passing through the Magaikwa beacon.
The existence of two sets of fracture zones in the thrust foreland, roughly
at right angles to each other, and the presence of local cross-warps in the Inain
folds indicate a local resolution of the west-east stresses into centripetal components directed roughly from the north-west and the south-west.
Th(j post-Matsap age of the diastrophic movements has become evident
from the descriptions of the thrust-faults on preceding pages. Now the horizontally disposed Dwyka beds, of Karroo age, fill erosion-hollows in the folded
and faulted Transvaal and Waterberg rocks. In addition, dolerite dykes, believed
on lithological grounds to be of Karroo age, cut through the folded Matsap beds
in a number of localities. This therefore fixes the time of the earth movements
as post-l\Iatsap and pre-Karroo.
IV.-THE RELATION OF THE MINERAL DEPOSITS OF THE AREA TO THE
EARTH MOVEMENTS.
1. The Asbestos Deposits.-Despite the diverse opinions on the time and
mode of origin of the crocidolite asbestos, there are indications in many of the
larger workings that the deposits are genetically related to the widespread
post-Matsap tectonic disturbances. Fairly strong folding and overfolding
STRATIGRAPHIC FEATURES AND TECTONICS
251
from the west have taken place locally, the axes of these small folds being
parallel to the major fold axes. The Blackridge asbestos mine is situated in the
western limb of an overfold, and this limb has been thrown into a number of
smaller wrinkles. Asbestos of longest fibre and best quality seems to be confined
to the crests of the small anticlines and overfolds and the direction of growth
of the fibre is parallel to the axial planes of the folds. Whether the constituent
chemical components were originally occluded in the parent rock or filtered in
at a later date is not yet clear. It would, however, appear that during the period
of post-Matsap mountain-building, conditions favouring the crystallisation of
crocidolite were created. The bearing of the post-Matsap earth movements on
the genesis of the asbestos should therefore be given closer consideration in
future.
2. The Barite Occurrence.-Twenty-four miles south of Postmasburg a
barite deposit has been found in a crush zone in the Ongeluk volcanics, striking
N.W.-S.E. The mineral occurs as a metasomatic replacement within the zone
of brecciation and is accompanied by stringers of vein quartz.
3. The Iron Ores.-They are related to the post-Matsap earth movements
mainly in point of time. WagI!er (23) states that there were several periods of
haematitization in this area, the earliest- and most important of which is of preMatsap age. Boardman (1) has established three periods of iron ore formation,
all of which fall within cycles of erosion. The first took place in pre-Matsap times
when the banded ironstones were enriched in iron. The second, for which
more direct evidence is available, took place in post-Matsap tinles, when the
Ganiagara basal conglomerate and lower shales, as well as the Blinkklip breccia,
were extensively ferruginised. Glacial pavements of Karroo age on the ferruginous conglomerates, help to assign to this period of haematitization a preK.arroo age. The third period of iron ore formation covers the post-Karroo
cycle of erosion.
4. The Manganese Ores.-It can be stated definitely that these ores are of
post-Matsap age, since the basal Gamagara beds (the conglomerates and shales),
as well as the Siliceous and Blinkklip breccias, which have also been shown to be
post-Matsap in age, .are replaced by manganese. The relation of the breccias
to the post-Matsap earth movements has already been described, so that
manganization must have taken place subsequent to the post-Matsap folding
and thrus~-faulting. This is borne out further when the position of the ore
deposits is considered. They are practically confined to the central portion of
the area and are found on the Dolomite, where the latter is ov:erlain by rocks
belonging to various subdivisions of the Transvaal and Waterberg systems,
and suggestively along a thrust contact. The post-Waterberg tectonic disturbances were apparently instrumental in creating suitable zones of weakness
along which replacement by circulating aqueous solutions could most easily be
effected~
252
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
V.-THE GEOLOGICAL HISTORY OF THE AREA.
The geological history of the area is characterised by the following chronological sequence of events : 1. The Transvaal Sedimentation and Volcanic Period.-The succession is
conformable throughout the area. Fine-grained chemical deposits bulk largely
and two periods of contemporaneous volca.nicity are represented. The first was.
accompanied by a sFghtly younger intrusive phase.
2. The Post-Transvaal Deformation.-The important event was the gentle
folding of the rocks belonging to the Transvaal system, the forlnation of the
Maremane anticline and, probably, the two synclines lying to the north-east·
and south-east. To the west of the Maremane anticline a large geosyncline was.
probably created simultaneously""
3. The Waterberg Sedimentation and Volcanicity.-This was accompanied
by erosion of the Transvaal rocks to the east of the basin of sedimentation, in
particular along the Maremane anticline, and ended in the laying bare of the
Dolomite along the crest of that anticline. The succession comprises coarse,.
detrital, off-shore deposits, while finer-grained sediments were deposited in the
deeper portion of the geosyncline. The coarser, cross-bedded sediments, forming
the upper division indicate a filling up of the geosyncline, while basic lavas were
poured out as well.
4. The Post-Waterberg Orogeny.-With the setting-in of tangential stresses
from the west, the Matsap beds within the geosyncline were the first to suffer
deformation, and were intensely folded and overfolded. The strata along the
western limb of the Maremane anticline, underlain by perhaps 5,000 feet of
massive dolomitic limestones, yielded to extensive and repeated low-angle compressional faulting (thrusting) accompanied by brecciation and mylonitization.
East of the Maremane anticline the pre-Mat sap folds were accentuated and, as
a result of the differential propulsion of the thrust blocks from farther west,
were accompanied by reverse and tear faulting, and crushing and shear fracturing in the thrust foreland. The period of compressional deformation was.
terminated by crustal settling, which resulted in normal faulting and tensional
fracturing.
5. The Post-Waterberg Intrusions.-They are really of minor importance
and their exact age relationships cannot be deternlined. In addition to being
post-Matsap in age, they also seem to post-date the iolding and faulting, but
are regarded as pre-Karroo in age on lithological grounds.
6. The Karroo Glaciation and Sedimentation.-Th~ traces of Dwyka.
glaciation are few but fairly widespread, their importance in the closer dating
of the post-Matsap tectonic disturbances, the period of slumping and the time
of manganese formation being considerable.
7. The Post-Karroo Intrusions.-On account of a lack of more direct
evidence for dating these intrusions, a post-Karroo age is tentatively assigned to
them on lithological grounds.
8. The Post-Karroo Erosion.-The present topography has been largely
determined during this cycle of subaerial denudation, during which the climate
was presumably much the same as it is at present. Subsurface solution of the
TRANS. GEOL. SOC. S.A., VOL. XLVII.
PLATE XXXlJl.
FIG. I.-Eastern Buttress, National Park, view from tho top of Tugela Fall, showing the
westward slope of tho summit plateau here. The pattern of the hill on tho right is
often repeated arlong the crest of the escarpment. 'The basalts are well-stratified and
more than 4,000 feet thick.
FIG. 2.-The scene of the Tugela Capture. The former course ran from right to len,
then round the line of dots to the southward. N ow the stream flows past the two lower
arrows and over the Amphitheatre wall on the loft as tho Bibbon Fall. Tho Pool above
" D" marks tho continental divide! The lower arrows denote the Indian Ocean drainage,
the upper Atlantic Ocean drainage.
STRATIGRAPHIC FEATURES AND TECTONICS
253
dolomite and the collapse of the overlying beds under gravity into cavities
created in this way, took place during this period, and the manganese deposits
were formed during its later stages.
9. The Post-Cretaceous Eruptions.-The kimberlite pipes are representa ti ve of this period.
10. The Recent Accumulations.-In comparatively recent times most of
the irregularities in the post-Karroo topography have become smoothed out
by the accumulation of deposits of surface limestone and sand. Much of the
latter can be classed as Kalahari sand and has probably drifted in from the
west.
VI.-ACKNOWLEDGMENTS.
The writer gratefully acknowledges the help of the members of the
Geological Survey Drawing Office staff in re-drawing the map and the text
figures. To those of his colleagues, particularly his co-mappers in the area,
with whom the structural problems have been discussed and others who have
made suggestions for improving, or have critically reviewed, the manuscript, he
remains greatly indebted. His indebtedness to his predecessors in the area is
a,t once apparent from the frequent references to their contributions to our
.geological knowledge of the region.
VII.-LIST OF REFERENCES.
<1) BOARDMAN, L. G.
"The Geology of the Manganese and Iron Deposits North of
Postmasburg, C.P." Diss. Prete Univ., 1941, 169 pp.
(2) BOARDMAN, L. G. "The Geology of the Manganese Depoits on Aucampsrust, Postmasburg." Trans. Grol. Soc. S. Afr., XLIII, 1941, pp. 27-36.
(3) BOARDMAN, L. G. "The Black Rock Manganese Deposits in the South-Eastern
Kalahari." Trans. Geol. Soc. S. Afr., XLIV, 1942, pp. 51-60.
(4) CADELL, H. M. "Experimental Researches in Mountain Building." Trans. Roy.
Soc. Edinburgh, :xXXV, 1890, pp. 337-357.
(5) CHAMBERLIN, R. T., AND MILLER, W. Z. "Low-angle Faulting." J. Geol., XXVI,
1918, pp. 3-44.
(6) COOPER, J. R. "Geology of the Southern Half of the Bay of Islands Igneous Complex." Newfoundland, Dept. Nat. Res., Geol. Sect., Bull. No.4, 1936, 62 pp.
(7) DAKE, C. L. "The Hart Mountain Overthrust and Associated Structures in Park
County, Wyoming." J. Geol., XXVI, 1918, pp. 45-55.
(8) DEISS, C. "Structure of Central Part of Sawtooth Range, Montana." Bull. Geol.
Soc. Amer., 54, 1943, pp. 1123-1167.
(9) Du TOIT, A. L. "Some Reflections upon a Geological Comparison of South Africa
with South Ameriea." Proc. Geol. Soc. S. Afr., XXXI, 1928, pp. xxxi-xxxvii.
(10) Du TOIT, A. L. "The Manganese Deposits of Postmasburg, South Africa." Econ.
Geol., XXVIII, 1933, pp. 95-122.
(11) HALL, A. L. "The Manganese Deposits near Postmasburg, West of Kimberley."
Trans. Geol. Soc. S. Afr., XXIX, 1926, pp. 17-26.
{12) LINK, T. A. "En Echelon Folds and Arcuate Mountains." J. Geol., XXXVI,
1928, pp.
(13) NEL, L. T. "The Geology of the Postmasburg Manganese Deposits and the
Surrounding Country." Special pUblication; The Government Printer, Pretoria,
1929, 104 pp.
(14) ROGERS, A. W. "Geological Survey of Parts of Hay and Prieska, with some Not.es
on Herbert and Barkly West." Rep. Geol. Comm. C.G.H., 1905, pp. 141-204.
254
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
(15) ROGERS, A. W. "Geological Survey of Parts of Bechuanaland and Griqualand
West." Rep. Geol. Comm. C.G.H., 1906, pp. 7-86.
(16) ROGERS, A. W. "The Campbell Rand and Griquatown Series in Hay." Trans.
Geol. Soc. S. Afr., IX, 1906, pp. 1-9.
(Z7) ROGERS, A. W. "Geological Survey of Parts of Vryburg, Kuruman, Hay and
Gordonia." Rep. Geol. Comm. C.G.H., 1907, pp. 11-122.
(18) ROGERS, A. W. "The Pioneers in South African Geology and Their Work." Trans.
Geol. Soc. S. Afr., XXXIX, 1937, Annexure, 130 pp.
(19) STOW, G. W. "Geological Notes upon Griqualand West." Quart . .1. Geol. Soc.,
London, XXX, 1874, pp. 581-680.
(20) TRUTER, F. C. "Observations on the Geology and Tectonics of a Portion of the
Potchefstroom District." Trans. Geol. Soc. S. Afr., XXXIX, 1936, pp. 441-455.
(21) TRUTER, F. C. "Discussion on L. G. Boardman's paper: 'The Geology of the
Manganese Deposits on Aucampsrust, Postmasburg'." Proc. Geol. Soc. S. Afr.,
XLIII, 1941, pp. lvii-lix.
(22) TRUTER, F. C., WASSERSTEIN, B., BOTHA, P. R., VISSER, D. J. L., BOARDMAN,
L. G., AND PAVER, G.L. "The Geology and Mineral Deposits of the Oliphants
Hoek Area, Cape Province." An Explanation to Sheet 173. (Oliphants Hoek~)
The Government Printer, Pretoria, 1939.
(23) WAGNER, P. A.' "The Iron Deposits of the Union of South Africa." Mem. Geol.
Surv. S. Afr., No. 26; The Government Printer, Pretoria, 1928, 264 pp.
TRANS. GEOL. SOC. S.A., VOL. XLVII.
PLATE XXVlI.
THE GEOLOGY
OF
PORTIONS OF BECHUANALAND
AND
z
GRIQUALAND WEST
Compiled by D.J.L. Visser D.Se.
-
r
-<
c=J
REFERENCE -
SuperfiCial deposits
Cotoclostic rocks
~ ~Gomogoro
«
~ Upper
o
0:::
o
}
WATERBERG
SYSTEM
Griquotown Beds
z
o
and Motsop Sen'es.
volcanics
I: .,'. '.'..:."Lower
I: ': : : : : : IDolomite
Griquotcwn Beds
TRANSVAAL
SYSTEM
Series
- - - lPo/erite dykes
- _/_ Normal faults and tear {aults
_~_
Reverse {au Its
- L!._ Thrust {au Its
25J1'
Dip of strata wi.th amount in degrees
_._.- Divisional boundaries
>-
«
I
-
Miles 10
SCALE -
5
1 : 500,000 -
0
10 Miles.
~1~E3~~E3~==b3~EE33C~E4=t======~======:a1
TRANS. GEOL. SOC. S.A., VOL. XLVII.
PLATE XXVIII.
FIG. 1.
The Matsap series on Hartley, looking south-west. On the right the Hartley Hm
lavas dip southwards under Upper Matsap quartzites; on the left the quartzites dip
steeply to the west.
FIG. 2.
Bergenaars Pad, in the Langeberg, seen from the east.
TRANS. GEOL. SOC. S.A., VOL. XLVII.
PLATE XXIX.
FIG. 1.
Hipple marking in the Upper Matsap quartzites.
FIG. 2.
Small canoe-fold, pit,ching to the south, in the Upper Matsap beds, north-west of
olifantshoek.
- - - --.---
TRANS. GEOL. SOC. SA., VOL. XLVII.
PLATE XXX.
FIG. 1.
Asymmetrical, open folning in the Upper Matsap beds north of Olifantshoek, seen
from the north.
FIG. 2.
'1'he Matsap I-Hlls thrust on Oamelthorns, seen from the south.