Download Read 27th May, 1929.

GEOLOGY OF NORTH-WESTERN DAMARALAND, IN S.-W. AFRICA.
THE
G:EOLOGY
OF
N ORTH- \VESTERN
31
DAMARALAND,
IN SOUTH-WEST AFRICA.
(Read
27th
May,
1929.)
By T. W. Gevers, :M.A., Dr.PhiL, and H. F. Fromnlurze, B.Se.
(Published by authority of the South-"West African Administration.)
[PLATE
II.]
CONTENTS.
INTRODUCTION.
FJXTENT OF AREA.
PHYSIOGRAPHY.
STRATIGRAPHY.
I.-ROCKS OF THE
FUNDAMENTAL COMPLEX.
A.-1'he Schist Formation.
(a) Quartzite Series.
(li) Marble Series.
(c) Mica-Schist Series.
(d) Mode of Deposition.
(e) Age of the 8chist Formation.
B.-The Old Granites.
(a) Gneiss-Granite.
(b) Salem Granite.
Red Granite.
Non-Porphyritic Gmnite.
Aplitic Granites.
Pegmatitic Granites.
(g) Progressive Differentiation of Magma.
(h) Pegmatites.
(i) "Dome-Granites."
(c)
(d)
(e)
(j)
I/.-YOUNGER ROCKS.
A.-Sediments.
(a) The Erongo Sediments.
(b) The Otjongundu Plateau.
(c) The Sediments of the Brandberg.
(d) The Sediments of the Etjo and Great Omatako.
(e) Age of the Sediments.
B.-Igneous Rocks oj the ETongo S1£ccession.
(a) Stocks of Diorite and Granodiorite.
(b) Erongo Granite.
(c) Sequence of Intrusion.
'J'ECTONICS.
32
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
INTRODUCTION.
IntToduction.-In the course of the geological investigation of the
Erongo Tinfields, the writers jointly mapped a large area embracing
practically the whole of north-western Damaralancl (Plate II). By far
the largest portion of this area is occupied by sediments and igneous
rocks belonging to the F'undamental Complex. The unravelling of the
stratigraphical succession and very complicated structure of this group
of ancient rocks wa,s one of the main aims of the geological survey of
the area.
The results of these investigations are herewith presented in the
form of a short paper, which is intended mainly as a preliminary
publication.
Extent of the Area.-The area, investigated may be said, roughly,
to comprise the whole country extending from a line joining the
Brandberg, the Otjongundu Plateau and Etjo in the north to Ebony,
Usakos, Karibib and Wilhelmstal in the south.
Physiography.-The area thus delineated embraces two very
distinct types of country, there being a very marked difference between
its western and eastern portions. While the latter is densely clad with
bush and trees, the former is an arid desert, and forms part of the
Namib, the extensive desert coastal tract of South-West Africa. A
stretch of country exhibiting intermediate characters separates the
west from the east. The average rainfall rises from less than two
inches in the west to 14 inches in the east.
The area is drained by the. Ugab River in the north, the Omaruru
River in the centre and the Khan River in the south. None of these
rivers are perennial. The underground flow, however, in the case of
each river is considerable, and open water occurs in the Ugab and
Omaruru rivers at se.veral localities throughout the whole year.
Large portions of the whole area are sand-covered, and the
characteristic morphological feature are prominent hills and mountains
of the " Inselberg " type rising above the general plain of the country,
which, from east to west, becomes considerably lower in altitude. In
the southern portion of the area, marble and quartzite ridges form a
very rugged mountain-land.
The highest elevation, not only of this area, but also of the whole
of South-West Africa, is the Brandberg, which rises to a height of
8,550 feet. The Erongo Mountains attain their greatest height along
their western escarpment, where the Hohensteine tower 7,670 feet
above sea-level.
Another prominent range are the Kompaneno
l\1ountains, north of Omaruru, which, in the Okongue Peak, attain an
elevation of 5,800 feet.
Stratigraphy.-Disregarding the superficial deposits, both igneous
and sedimentary rocks belong to two widely separated geological
GEOLOGY OF NORTH-WESTERN DAMARALAND, IN S.~W. AFRICA.
33
periods, for while the majority represent some of the most ancient
rocks known in South Africa, the others are, in comparison with them,
very youthful.
The rocks represented in descending order of age are:(1) Ancient sediments of the Schist Formation forming part of
the Fundamental Complex.
(2) Old granites intrusive into the fOrTIler.
(3) Sediments of the Erongo, Brandberg and Otjongundu
Plateau, probably identical with the Kaoko-sediments and
more or less contemporaneous with
(4) The Etjo and
Stormberg) .
(5)
vVaterberg
sediments
(Upper
Karreo:
Intrusiv~
and extrusive rocks of the. Erongo succession:
(a) Dykes and sills of diabase and dolerite; flows of
amygdaloidal olivine-diabase and melaphyres.
(b) Dykes and flows of porphyrites, quartz-porphyry, dioriteporphyrite and granite-porphyry.
(c) Stocks of diorite and grano-diorite.
(d) Erongo granite.
(6) Surface Deposits:
terraces, sand.
Surface limestone, gravel. and boulder-
Rocks of the Fundamental Complex.-The Fundamental Complex
is exposed over enormous areas throughout the whole of South-West
Africa. It is constituted of ancient gne.issoid and granitic rocks,
schists, quartzites, crystalline limestones and other more or less highly
metamorphosed sediments. The task of unravelling the stratigraphical
succession of its sedimentary components presents great difficulties,
for not only have they suffered intense metamorphism, but the.y are
usually also so completely interfolded that unconformities, which no
doubt exist, have, to a large extent, been obliterated.
lRimann, in the central portion of South-West Africa, has proved
the existence of tvvo groups, 2Beetz and Kaiser, in the Luderitzbucht
littoral, that of three groups of ancient sediments separated by unconformities. The oldest of these is generally referred to as the " Schist
Formation," and it is this group which covers the largest portion of the
area under discussion.
The Schist Formation (Beetz: Karibib Beds).-lRimann has
divided this group into a lower Mica-Schist series, a, middle Quartzite
lRimann, E.: "Geol. Untersuchungen des Bastardlancles in D. Sud-West
Afrika." Berlin. 1925.
2Beetz, ·W.: "Versllch einer stratigraphischen Gliederung del' praekambrischen Formationen Sud-west Afrikas."
N e1teS J ah-rbuch, Beilage Bd.
LXI, Abt B 1929.
34
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
series and an upper 1\1arble series. The detailed mapping, however,
carTied out in north-western Damaraland has shown that the same
series found by Rimann in the Bastal'dland area are also represented
here, but that the order of succession as given by him does not apply
to the area under review. Here a great thickness of mica,-schists
forms the highest represented group of the Schist Formation, and is
followed in descending order by a marble and a quartzite series. It is
not known whether the latter forms the actual base of this group of
ancient sediments, although the presence of coarse grits and conglomeratic arkoses in its lowest exposed portions supports such an
assumption. It could also not be definitely ascertained whether
the 11ica-Schist series actually forms the top of the entire group and
is not followed by another succession, possibly that given by Rimann.
In view of the highly folded nature of these sediments, however, this
does not appear probable. It must also be mentioned in this connection that overfolding on an extensive scale is a marked feature of the
structure of these ancient sediments.
The various series to be described in the following pages succeed
~ach other quite conformably.
The Quartzite Series .-Well-bedded phylEtic qua,rtzites form the
topmost portion of this group, and are succeeded down the scale. by
phyllitic and coarse conglomeratic arkoses. The latter are usually
very massive, entirely unbedded and rendered markedly gneissose by
regional metamorphism. They are, as a consequence, very easily
mistaken for gneissose granites. Both quartzites and grits are usually
highly felspathic and sericitic. Their constituent quartz-grains are, on
the whole, rather angular, and secondary quartz is much in evidence.
The felspar is mostly orthoclase and microcline. Plagioclase is inconspicuous. Tourmaline is fairly abundant, occasionally also horneblende. Highly micaceous varieties sometimes contain porphyroblasts
of biotite and magnetite. At the Chuosberg, felspathic grits have been
found to contain gold, sometimes in considerable enrichment.
The thickness of tb)s group could nowhere be definitely ascerLained, since the quartzites usually only appear in the core of antidines and domes. At the Great Rooiberg the thickness of the upper
quartzites seems to be well over a thousand feet. Apparently these
arenaceous sediments were laid down very irregularly on an uneven
floor, for they are often found to pinch out very rapidly. The area
around Usakos and Karibib appear'S to be the northern limit of the.ir
deposition, for north of the Khan their thickness is very attenuated
and their distribution most irregular. North of the Kudukuppe they
are entirely unrepresented, their stratigraphical place being taken by
rr:ica-schists and grey-wackes underlying the marble horizon. South
of the Khan River, however, their thickness rapidly increases, and in
the area between this river and the Swakop they build up several
prominent mountain ranges, such as the Chuosberg and Etusis
Mountains. The reddish colour of the quartzitic rocks contrasts very
GEOLOGY OF NORTH-WESTERN DAMARALAND, IN S.-W. AFRICA.
35
strongly with the brilliant white, of the overlying lowermost marble
band.
Other Rocks Underlying the Marble Horizon.-North of the
Kudukuppe, and sometimes south of the Khan River around Karibib,
the place of the quartzites and grits is taken by mica-schists and greywackes similar to those overlying the marble horizon. A few miles
south of Karibib the latter is found to be underlain by a massive
paragneiss formed by the lit-par-lit intrusion of black amphibolitic
schists by an acid gneissose granite.
The Marble Series.-Bands o,f crystalline limestone are very common in the sediments of the Schist Formation, and also occur intercalated in the quartzite, and more abundantly still in the mica-schist
series. The majority, however, comprising a ma-in band frequently
over a thousand feet in thickness, are situated within a definite marble
horizon. The number of individual bands within this horizon varies
greatly. UsuaIly, there is at least one ve,ry massive bed of crystalline
limestone accompanied by a varying number of very much narrower
bands, either above or below the former.
The macroscopic appearance of these rocks greatly differs with
the intensity of metamorphic changes they have undergone., principally
the extent to which they have suffered recryfttallization. While at
numerous localities the limestone is completely recrystallized, and on
weathering disintegrates into large or small rhombohedra of calcite
and possesses a more or less white coloration, the less metamorphosed
fine-grained rock is of a greyish-blue colour. In the latter case, its
weathered surface is very similar to that presented by the Transvaal
and Otavi Dolomite, or the Schwarzkalk of the Nama system. Banded
and brecciated varieties are very common, and exhibit a great wealth
of different patterns. Near Karibib an attempt has been made to
quarry these highly ornamental stones.
Amphibolites derived from the alteration of impure argillaceous
limestones occasionally occur intercalated with or in close proximity to
marble bands.
In the area south of the Khan River, a band consisting of a very
fine-grained sugary limestone, possessing a, brilliant white colour,
occurs in the basal portion of the serie.s, and forms a strong contrast
to the underlying reddish quartzites. Its thickness is about 100 feet.
In the area south of Neineis and Okambahe, the Marble horizon
contains a very useful marker-bed forming the topmost bed of the
main marble band. It consists of a very compact and fine-grained
sugary limestone of a brilliant white colour, and is, in addition,
characterized by an abundance of graphite flakes. Its thickness varies
from a narrow band to over a hundred feet. On account of its compact weather-resisting nature, it forms a number of conspicuous white
hills.
36
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
In: the same area, the basal limestones are also very characteristic
and useful as a marker. To a considerable thickness they are highly
ferruginous and full of chert. As a result, they weather to a brownalmost black colour.
The thickness of the main marble band va,ries considerably. West
of Karibib it averages about 900, and in the Kompaneno Mountains
about 1,400 feet.
Conta-ct Metamorphis1n.-At numerous localities the limestones
have been invaded and·· intensely altered by granites with the production of calc-silicate rock. :Massive garnet and fluorite are developed
in large masses at several localities.
The Mica-Schist Series.-The marble series is conformably overlain by a tremendous thickness of schistose rocks, predominantly
biotite-schists.
:Muscovit8- and muscovite-biotite-schists, and less
commonly biotite-hornblende- and chlorite-schists also occur. By a
progressive increase of quartz, these pass over into arenaceous schists
and, finally, micaceous quartzites. Some of the less micaceous rocks
very closely resemble grey-wackes. On account of the more or less
intense metamorphic changes which these rocks have undergone,
porphyroblastic texture is very common, and spotted and knotted
schists and quartzites are widely represented.
Over wide areas, however, the rocks here grouped together as schists,
are not fully schistose or even phylEtic, but from typically lepidoblastic mica-schists they pass over a variety of phyllites in different
stages of metamorphism into micaceous slates and finely laminated
shales. The presence of a possibly younger group characterized by less
intense a. metamorphism of its constituent rocks, such as the
Phyllite Forma,tion, was nowhere indicated.
Intercalated with the mica-schists occur numerous narrow bands
of altered, usually highly ferruginous limestone, as well as bands of
" marble" -granulite, garnet-granulite and, less commonly, amphibolite.
Ortho-amphibolites derived from ancient basic intrusions,
while generally commOn in the Fundamental Complex, are rather rare
in the area under discussion. Locally, the schists and slates have been
altered to massive hornfels.
.
Mode of Deposition.-It is apparent that in their original state
the sediments described were represented by a lower group of conglomeratic arkoses and sandstones, a middle group of limestones and
calcareous shales, and an upper group of mudstones, sha.1es,
arenaceous shales and interbedded layers of limestone, calcareous
shales and sandstone.
This order of succession is in accordance with the gradual deepening of an ancient sea, the deepest part of which in this area seems to
have been in the region now traversed by the U gab River. The main
area of denudation appears to have been in the south, in which
direction the lower grits, arkoses and quartzites increase in thickness,
while north of the Khan they are hardly represented. The sand and
GEOLOGY OF NORTH-WESTERN DAMARALAND, IN S.-W. AFRICA.
37
granitic detritus was deposited over a very uneven floor, the latter
being probably derived from the immediate neighbourhood before the
inundation of the subsiding area had taken place. Since the quartzites
are replaced by mica-schists north of the Khan, the deposition of sand
and accumulation of shales and mudstones to some extent evidently
proceeded contemporaneously, and the invasion of the sinking area by
the sea, seems to have progressed from the north-west and north.
The Age of the Schist FOl'mation.-No definite information is as
yet available concerning the relation of this group to possibly still
older sediments.
That the rocks of the Schist Formation do not represent the first
veneer of clastic material derived from the weathering of the original
solidified crust of the earth, and that this group is not the oldest of
the sedimentary formations present in the :Fundamental Complex,
is shown by the presence of pebbles of quartzite and marble in the
conglomeratic bands of the, Quartzite series.
An upper limit to its age is given by the Phyllite Formation,
which, in the northern portion of Bastardland, has been found to
overlie the Schist Formation unconforniably (Himann, loco cit.).
In the Luderitzbucht littoral, the researches of Beetz and Kaiser
have. shown the existence of another group, the Chlorite-Schist
Formation, unconformably overlying rocks belonging to the Schist
Formation, which also in this area is heavily intruded by old granites.
The Phyllite Formation is here unconformably overlain by the Konkip
group. The latter is usually regarded as the equivalent of the
vVitwatersrand and Vent,ersdorp systems of the Union, and the
Chlorite-schist and Phyllite formations have been correlated by Beetz
(loc cit.) with the lower and upper Huronian of North America.. The
age of the Schist Formation, therefore, falls well back into the
Archaean.
The Old Granitcs.-A very considerable prO'portion of the area
occupied by the Fundamental Complex is taken up by granites
intrusive into the ancient sediments. Excepting the Erongo granite,
which is very much younger, there are at least three separate, generations of granite 3 • Two of these antedate by a very considerable period
the deposition of the rocks of the Phyllite Formation, while the last is
still intrusive into them.
Gneiss-Gl'anite.-The oldest generation of granite rocks intrusive
into the Schist Formation is represented throughout South-West
Africa by markedly gneissose granites, generally referred to as Gneissgranite. In the area under review, a very acid rock that does not
appear to have penetrated higher than the marble series, and was
responsible for the production of paragneisses underlying this horizon,
3P. A. 'Vagner has already drawn attention to this fact: On some mineral
occurrences in the Namib Desert. Trans. Geol. Society, S.A., Vol. XXIV,
1922, p. 73.
38
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
seems to belong to this early generation. 4 Everywhere there occur
associated with it reddish aplites resembling felspathic quartzites. Its
outcrops, however, are very limited.
In the Luderitzbucht area there is a well-marked interval between
the gneiss-granite and the succeeding generation, the Salem granite.
The former was here apparently intruded while the folding process of
the ancient sediments was still in active progress, and has led to the
formation of large areas of mixed rocks. The, Salem granite, on the
other hand, was intruded after the sediments were already in a highly
folded and consolidated state, with the result that it forms accordant
to discordant bodies within them.
The Main Period of Granitic lntrusion.-In the central and
northern areas of South-West Africa, the bulk 6f the old granites were
intruded while the folding movements, to which the sediments of the
Schist Formation were subjected, were still in active progress.
By far the most important members of the main period of
intrusion are the Salem granite and a somewhat later phase, the Nonporphyritic granite. They have been grouped together by Reuning
under the term " Hauptgranit. ' ,
Salem Granite.--The name of this granite is derived from the
fact that it was first observed by Gurich in the vicinity of Salem on
the lower Swakop River. It is, however, very widely distributed
throughout the whole of South-vVest Africa and in the tin-bearing
area by far the most abundant of all the varie.ties of granite
represented.
As typically developed, the Salem granite is a grey porphyritic
biotite-granite, with tabular phenocrysts of orthoclase and sometimes
microcline. 5 These phenocrysts usually exhibit flow structure, i.e., a
parallel orientation, which is usually strictly conformable to the strike
of the schist and granite-contact. This is particularly noticeable in
the smaller massifs, e.g., that on Ameib and Goabeb.
Usually, there are two varieties of Salem granite: The typical,
coarse-grained rock with large felspar phenocrysts, which is the more
abundant, and a. variety showing a considerably finer grain and very
much smaller felspal' phenocrysts. This latter variety i:::l well represented in the rock of the" bank" at the Sandamab water-hole. The
flow-structure, i.e., parallel orientation of the felspar phenocrysts, is
frequently particularly well developed in the finer-grained rock.
Gradual diminution of the phenocrysts leads to types in which the
porphyritic structure of. the rock is not very distinct. The high content of femic constituents (biotite), however, still distinguishes this
4A description of this type of granite is given by P. A. ·Wagner in the
sarno paper (P. 73), in ,,,hich he also draws attention to the fact "that it
was this rock that brought about the widespread development of composite
gneiss and mixed rocks" (p. 73).
5 An excellent d?scription of this granite is given by P. A. \Vagner in:
The Ge010gy and Mmeral Industry of S.\V.A. Geol. Survey Memoir No.7,
p. 41.
GEOLOGY OF NORTH-WESTERN DAMARALAND, IN S.-W. AFRICA.
ill)
rock from the more acid, non-porphyritic later phases of the granitic
intrusion. As a matter of fact, the high content of fernie minerals is
one of the main characteristics of the Salem granite. A direct result
of this feature is its almost universal grey colour.
Microscopic examination of the Salem granite shows it to be an
alkali-granite. Its essential constituents are quartz, orthoclase and
biotite. Very frequently, however, the orthoclase is entirely replaced
by microcline exhibiting a very marked gridiron structure. Where
this is not the case, the orthoclase is usually intergrown with albite on
an intensive scale. Twinning, according to the Carlsbad law, is very
common. Plagioclase, except albite, is rare, though oligoclase and
andesine are present in some specimens (Paukwab).
J\luscovite is occasionally visible as an accessory constituent. Of
the latter zircon, frequently in large crystals, as well as apatite and
topaz, are invariably present in considerable abundance. Tourmaline
is present in most specimens, and epidote is abundant in a number of
them. Fluorite. is occasionally present, but metallic ore-minerals are
rare, though magnetite, pyrite. and molybdenite were noticed in a few
sections.
Along the contacts of pegmatite dykes cutting through the Salem
granite, the soda-orthoclase phenocrysts frequently assume a beautiful
pink coloration. In the Otjimboyo area, and particularly well-exposed
on Okapaue, there outcrops a coarsely crystalline porphyritic granite
entirely identical with the ordina,ry Salem granite except for the
salmon-redcolouration of its felspar phenocrysts. rrhis rock fringes
the outer edge of the normal Salem granite intruded into a schistsyncline, is of a considerable width, and where contacts are exposed
(e.g., near the homestead of Okapaue), intrusive into the former.
This porphyritic granite with salmon-red felspaJ"s thus appears to belong
to a slightly ya.unger phase of the granite intrusion, which evidently
extended over a long period coincident with the close of the tectonic
movements that affected the ancient sediments.
Particularly the Salem granite, but also the succeeding phases
of the granite intrusion, everywhere show the greatest conformability
to the tectonic structure of the invaded sediments. So close indeed
is its adherence to the tectonic plan of the whole region, that in
nearly every case the Salem granite forms concordant bodies of the
nature of phacoliths and laccoliths, if the la,tter term can be applied
to concordant magmatic bodies intruded into highly folded strata.
Seldom and usually only over small distances does this granite ever
break a.cross the strike of the sediments.
The intrusion of the Salem granite was followed probably at no
great intervals by a number of other granitic rocks belonging to
successive phases of differentiation and solidification. These related
granites were collectively designated by Cloos6 as normal granites.
6Clo08, H.: "Del' E;rongo." Beitraege
Schutsgebeite, Heft. 17, Berlin, 1919.
ZU1'
Erforschung der deutschen
40
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
They present" ho,wever, widely different aspects not really typical of
ordinary normal granite. Although their distribution is very widespread, their outcrops are not nearly so abundant as those of the
SalerYl granite.
For the classification of these various graniti,e rocks, their petrographical characters and modes of occurrence will be used as a basis.
Being derived from the same magma as the Salem granite they are
all alkali granites.
Red Granite.-This is usually a coarse-grained, rather acid granite,
consisting almost entirely of quartz and dark red felspar.
Biotite and muscovite are present, but on the whole femic constituents
are less in proportion than in the Salem granite. It is intrusive into
SaIem granite and its somewhat later variety characterised by
salmon-red felspar phenocrysts. Contacts between this and the following granite have unfortunately not been observed, and it cannot, therefore, be said which is the elder of the two. Judging, however, by its
coarse grain and tendency to be porphyritic, the red granite probably
represents a phase midway between t,he Salem granit.e and nonporphyritic granites.
It is very well developed in the south-eastern portion of the tinbearing area, east of Otjimboyo, where it covers a large area. Considerable outcrops of this granite were also found along the U gab
river around Sorris-Sorris, Anichab and Davib.
Non-Po1'phY1'iti'J Granite.-Of all the granites later than the Salem
granite, this has the widest distribution. It somewhat resembles that
t:Y'pe of Salem granite in which its typicaI porphyritic nature is not
'very distinct, but may be distinguished froIn it by its considerably more
acid nature and the lesser content of femic constituents. Its main
constituents are quartz and soda-orthoclase, the latter frequently
replaced by microcline. Biotite is not nearly as abundant as in the
SaIem granite, and muscovite frequently predominates over biotite.
~ropaz, zircon, and apatite are accessory constituents, while magnetite and fluorspa,r 00cur as rare accessories.
Plagioclase of the more acid types is more common than in the
Salem granite. Epidote is also present, sometimes in considerable
quantity.
As a rule t,his granite is of middle grain, though coarse-grained
varieties aIso occur. ~rourma1ine (Schod) is far more abundant than
in Salem granite, and gradually increases in importance as the differentiation of the magma proceeds. Sometimes it is alreadY' present in
such excess that this type of granite passes into a, Tou,rmaline granite
(Kakausib) .
vVherever this granite occurs in contact with Salem granite, it is
found to be intrusive into the latter.
Representing merely a more acid phase of the magma. which
had already been deprived of the grea,ter portion of its femic constituents, this type of granite was still intruded in great volume and
GEOLOGY OF NORTH-WESTERN DAMARALAND, IN S.- W. AFRICA.
41
covers extensive areas. Since its intrusion took place after most
of the available spa.ce formed by differential movements within the
schists during the orogenic movements had already been taken up
by Salem granite, it is frequently found intruded along the margin
of the latter, thus enlarging the granite area., and also forced into
the more resisting beds, such as the compact limestones of the marble
hOJrizon. The powerful alteration orf the la.tter by such granite
intrusions has already been mentioned as well as the fact that the
granite in the vicinity of marble bands is usually highly garnetiferous
(Grossula.r).
Aplitic Granites.- \Vith progressive differentiation of the parent
magma" the granitic intrusions become more and more acid and at
the same time more scattered, less -coherent and extensive.
The acidity of this .group -of granites varies, but generally they
_consist almost entirely of quartz and white felspa.r. Small flakes
of mus.covite are common. The rock is usually very fine grained,
compact and weather-resisting. Hence it frequently forms eminences
of the Inselberg type. The most conspicious- hill formed by this type
of rock is the Giftkuppe, a few miles east of the Erongo .station.
This intrusion appears to be in the nature of a stock, but, since
the entire surroundings are covered by sand, hardly any -contacts are
to be seen. The highly acid rock is here alm'Jst white in colour and
heavily intruded by coarse pegmatitic granite.
A plitic granite of not quite so acid a type, and also intruded by
pe,gmatitic granites on a large scale, has also been found to make up
the bulk of the mountainous complex known as the Grober Gottlieb I
and II. These. mountains have been put down by Cloos as consisting
of the very much more youthful Erongo granite, but this latter rock
only forms a sma1l portion of the Grober Gottlieb I, and is only found
as a very limited intrusion at the northern foot of Grober Gottlieb II.
Aplitic granite also forms a very prominent ridge within the
northern portion of the Kompaneno J\Iountains, culminating in the
OkongjJe Peak (5,800 feet).
Pegmatitic Granites .-Apart from the innumerable pegmatitic
dykes that have invaded the enveloping schists to an enormous extent,
there occur numerous bodies of granite highly pegmatitic in nature.
These have a very wide distribution and occur as broad dykelike bodies
usually along the edge or in the vicinity of earlier intrusions. Sometimes they also occur as sma1l individual massifs of their own.
Some of these bodies assume eonsiderable proportions and, on
account of their weather-resisting nature, form conspicuous eminences.
The most prominent of these are the Omaruru Kop, which consists of
a very eoarse-tgrained pegmatitic granite, and the Great -and Little
Kainachab on Davib Ost and Davib West.
This type of granite is always of very coarse grain, and presents
many of the typical features of pegmatites. Owing, no doubt, to the
greater volume of residual magma, the enclosed fugitive constituents
42
TRANSACTIONS OF TilE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
did not alter the rock to the same extent as in well-defined pegmatite
dykes or smaller bodies, with t.he result that their pneumatolytic
effects are not as concentrated. The rock is, however, usually
traversed by additional pegmatite dykes or sheets, which exhibit the
usual pneumatolytic phenomena in more concentrated form.
Frequently the volume of such dykes exceeds that of the rock constituting the ordinary pegmatitic granite.
In addition to muscovite, which frequently occurs in large wellformed books, the characteristic mineral of these pegmatitic bodies
is black tourmaline (Schorl). The slopes of hills formed of this type
of rock are usually thickly strewn with fragments and well-formed
crystals of Schorl, some of exceptional size. Frequently such crystals
exhibit a central core of corroded. crystalline quartz, showing that the
crystals were formed by the replacement of quartz. Garnets are also
frequently found in such pegmatitic granites, not only when these
have invaded marble bands (Grossular), but also away from the latter
(almandine, spessartite).
Very frequently (e.g., near Vis, at the Omaruru Kop and below
the Hohensteine) such pegmatitic bodies, which necessarily vary
greatly from place to place, exhibit feathery and stellar aggregates,
consisting of cookeite and quartz.
By the· diminution in size of the various bodies and an increase of
the pneumatolytic alteration of the rock, these pegmatic granites
gradually pass over into typical pegmatite.
Prog1'essive Differentiation of the Magma.-The order of intrusion
of these various granites into the sediments of the Schist Formation
forms an excellent illustration of the progressive differentiation of a
single parent magma. The oldest is the porphyritic phase, the. Salem
granite. Its richness in fernie constituents indicates that it represents
the first fraction of a slowly cooling magma intruded into the upper
portions of the crust.
That processes of assimilation, however, were also concerned in
the production of the higher fernie content and porphyritic nature of
the Salem granite is shown in numerous places by its contacts. A
good illustration of these features is afforded by a narrow elongated
body of Salem granite intruded into schists, south of Aubinhonis. At
the edge of the Salem granite massif it is difficult to distinguish
between contact altered schists and the dark-grey micaceous Salem
granite.
There is a wide zone of gradual transition from the
unaltered schists to the typical porphyritic granite.
Beginning from the schist side, the latter exhibit the usual
phenomena of contact alteration (spotted and knotted schists), and
then become still more highly micaceous (biotite). Then follows a
zone of minute and intimate lit-par-lit intrusion of the schists by the
granite. The granitic veins at first consist predominantly of \-"hite
felspar crystals with their ,axis of elongation arranged parallel to the
micaceous bands. (strike of the schists). Individual felspar crystals
GEOLOGY OF NORTH-WESTERN DAMARALAND, IN S.- W. AFRICA.
43
entirely surrounded by altered, hi,ghly micaceous schist are very
common.
Further towards the actual granite, such felspar crystals become
more numerous, and begin to form coherent bands separated by narrow
strips of a.ltered schist. The latter a.re finally reduced to thin lamellae
of biotite separating long rows of parallel felspar crystals. Finally,
these alternating bands become disrupted, and the individual flakes
of biotite are scattered about within the granite, and the felspar
crystals are no longer in their entirety parallel to ODe another, though
both the latter and "schlieren" of biotite still exhibit a marked
parallelism to the strike of the adjoining schists.
It is highly unlikely that felspar crystals already formed within
the magma were squeezed into the altered schists, and there is little
reasonable. doubt that they were formed in place. Exposures of this
nat·ure also suggest that the fernie constituent minerals (biotite) of the
marginal portions of the Salem granite are at least partly derived from
a process of assimilation of the invaded schists. Further, the porphyritic na,ture of the granite appears to be at least in part due to similar
assimilative processes, during which, with progressive assimilation
and enlargement of the space occupied by the magma, the newlyformed felspar crystals gradually drifted from the sides of the body of
. magma into its interior. Being the first granite intruded on an
extensive scale and in close conformability to the tectonic plan, the
Salem granite had a far better opportunity to effect assimilation, even
if on a comparatively limited scale, than the later granites which were
intruded after the sediments had already been consolidated to a very
much higher degree.
The red granite and non-porphyritic granites represent the
residual fractions of the magma already deprived of the bulk of its
femic content. Particula,rly the latter is very much in the nature of
a highly acid residual magma. The majority of the later phases of
intrusion also possess a finer grain, and evidently cooled considerably
more rapidly than the Salem granite.
There is a very instructive exposure on Goabeb, close to the road
from Usakos to Davib Osi, which clearly demonstrates the order of
intrusion of the various fractions. Here the Salem granite, is found
to be intruded by a more acid non-porphyritic granite, while both are
invaded by a highly acid aplitic granite, and this and the former rocks
in turn by typical dyke aplites.
Everywhere the pegma.tit.es represent the last phase of magmatic
intrusion. vVhere occurring in juxta,position the a,plitic granites are
found to ·be earlier than pegmatitic ,granites and wherever exposed
together aplite dykes were inva,riably seen to be cut and even faulted
by pegmatite dykes. Actual graduations from typical aplite to
typical pegmatites were found to be very rare. The former, therefore,
appear to belong to the stage of solidification closely preceding the final
attainment of the eutectic mixture roughly rr.presented by the pegmatites. Slight changes in the conditions controlling soJidification
44
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
would na,turally result in varying propmtions of qua,rtz and felspar
being separated out to re-establish equilibrium and the aplites hence
show a somewhat varying composition. They are almost invariab.l;v
barren of pneumatolytic minerals and the saturation point of th t
residual magma. with volatile constituents at the time of their intrusion
was evidently not yet nearly reached. The time-interval separating
the intrusion of the two types of rock, however, is probably only
very small.
Pegmatites .-A vast number of pegmatites are associated with
the various granites belonging to the main period of intrusion. They
include tin-bearing pegmatites which exhibit a number of different
modes of occurrence of cassiterite. The latter are scattered over
a. wide area, extending from Ka,ribib to the southern portion of the
Kaokoveld, and from west of Usakos as far east as Omaruru.
A great variety of other minerals have been found to occur in
pegmatites asso~iated with the old granites; as a matter of fact, with
the exception of the diamonds and the copper, lead, zinc and vanadium deposits of the Otavi area, the majority of the more important
mineral occurrences in Sout!h-West Africa are associated with the
liquid-magmatic, pneumatolytic and hydrothermal derivatives of the
old granites.
The "Dome-Granites. "-This collective term has been applied
by Reuning to a number of different granitic rocks, which, on account
of their wea,ther-resisting capacity, usually form prominent eminences
of the" Inselberg " type, and frequently weather into rounded domes
or cupolas. He considers them to be only slightly younger than his
" Hauptgranit" (Salem, red and non-porphyritic granites).
On
his geological map of the central portion of South-vVest Africa,
however, there seems to be a considerable confusion of these
granites with the very much younger Erongo granite. Thus, the
Brandberg, the two Spitzkopjes, the Erongo-granite massif of
Ukarenz, Anibib and Omandumba near the north-western corner of
the Erongo, a portion of the Erongo-granite on Ameib, the Grober
Gottlieb I and II, the Omarurukop and the Giftkuppe are all designated as " dome granites."
The granite of the Brandberg, however, is intrusive into beds of
the Kaolw-formation (Karroo), and is definitely Erongo-granite, as
well as the granite of the two Spitzkopjes. The two Grober Gottliebs,
on the other hand, consist predominantly of aplitic and pegmatitic
granite, into which the Erongo granite is intrusive. The Omarurukop
has already been described as consisting of coa.rse-grained pegmatitic
granite (old granite), while the Giftkuppe consists of a highly aplitic
granite, most likely also belonging to the same period of intrusion, but
older than the pegmatitic phase.
It is quite evident, therefore, that Reuning's group of " dome
granites" cannot be upheld, since he has applied that term to
granites of widely differing ages.
GEOLOGY OF NORTH-WESTERN DAMARALAND, IN S.-W. AFRICA.
45
It has already been stated that in the Bastardland area there
occur pegmatites intrusive into the Phyllite Formation, and which
evidently belong to a granite considerably younger than the granites
of the main period of intrusion. Also, in the Luderitzbucht area,
there are very acid aplitic and pegmatitic bodies intrusive into the
Chlorite-schist and Phyllite Formations. The Salem granite in this
area is nowhere intrusive into rocks younger than the Schist Formation. A considerable period, therefore, separa,tes the intrusion of
these acid aplitic and pegmatitic granites from the main period of
intrusion to which the Salem granite belongs.
To what extent similar aplitic and pegmatitic granites in the area
under review, e.g., that of the Giftkuppe, belong to this later period
it is impossible to say, for up to the present the Phyllite Formation
has not been identified north of the Auas :Mountains. Since, however,
the pegmatites cutting the aplite of the Giftkuppe are entirely
identical with those in the vicinity connected with Salem granite, it is
highly probable that the former also belongs to the main period of
intrusion.
The Sediments of the Erongo, Bmndberg and Otjongundu
Plaieau.-The hi.ghly folded beds of the Schist Formation a.1'e conformably overlain at a number of localities by more or less horizontally
disposed clastic rocks which present a comparatively youthful appearance. They consist essentially of great thicknesses of coarse arkoses,
conglomeratic and felspathic grits and sandstones, with alternating
layers of shales and mudstones. Their petrological characters, however,
vary greatly from place to place, for while at the Erongo and Otjongundu Plateau felspathic arenaceous rocks predominate, at the Brand~
berg shales and mudstones are developed in excess. Their mode of
deposition appears to have been very irregular, and they seem to have
been deposited on a highly une.ven surface. As a result, their thickness varies tremendously, and may decrease from 1,600 feet to nothing
over a distance of only a few miles, as, for instance, is the case at the
Erongo.
In general, they are reminiscent of Karroo sediments, but no
fossils have so far been found in them, at least not at the a.bove
localities.
The Erongo Sedin16nts.-At the Erongo these sediments present
two distinct facies. The. one consists of considerable thicknesses of
massive unbedded arkoses, conglomeratic and felspathic grits and
sandstones with intercalations of argillaceous sandstones and
arenaceous mudstones. It may be referred to as the Normal Arkosic
It is typically developed at the Onguati-Ecke and the
Facies.
southern escarpment of the Erongo. At the former locality, these
sediment,s attain a thickness of at least 1,600 feet.
The sediments found to underlie the diabase-capping of the
" Sargdeckel," south of Karibib, belong to the same facies. They
46
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
consist of arkoses, grits and felspathic sandstones (60-70 feet) overlain by baked shales and mudstones (40 feet) similar to those exposed
at the Onguati Ecke.
The other facies is represented by very coarse and massive
boulder beds, and may be referred to as the Boulder Facies. The
constituent boulders consist predominantly of granites, practically all
the granites from the Fundamental Complex being represented. In
addition, slabs and angular fragments of schists, phyllites, hornfels
and amphibolites are present. The majority of the boulders are well·
rounded, and numerous pebbles exhibit the bun-shaped outline
characteristic of beaeh-pebbles. The matrix consists essentially of a
very coarse granite-detritus.
The boulder-facies evidently represents the littoral deposit of the
basin, within which the Erongo sediments were deposited.
7'he Otjongundu Plateau.-Here the sediments rest on a very
uneven floor of a coarse, reddish granite. Their base represents a
massive and very coarse granite detritus, the bulk of which appears
to have accumulated in place. The major portion of the rocks, therefore, consists of very coarse arkoses and felspathic grits, some of them
containing conglomeratic layers which are clearly of fluviatile origin.
Interbedded with these coarse rocks occur narrow bands of red
laminated shales, which, however, playa very subordinate role.
These sediments are obviously of terrestrial origin. They closely
resemble not only the basal beds of the Erongo sediments, but also
some of the arenaceous beds occurring at the Etjo and Waterberg.
The plate.au is capped by a series of extensive sills and flows of a
light brown porphyrite very similar to the lower porphyritic lavas of
the Erongo. Thickness: 4-500 feet.
7'he Sediments of the Brandberg.-These stretch in the form of
a fringing girdle around the lower slopes of the circular mountain. To
a height of about 400 feet their base is formed of very hard and
intensely baked shales and mudstones, the lowest of which are very
much brecciated. Their colour is dark red, and frequently they con·
tain numerous calcareous nodules and concretions. Higher up the
s(;ale. these argillaceous rocks alternate with conglomeratic and
felspathic grits closely resembling those of the Erongo and Otjongundu
Plateau. These latter rocks gradually increase in bulk, till over a
height of several hundred feet they make up the series almost
exclusively. Still higher up, intercalations of reddish baked shales
appear again, and, finally, the latter again form the bulk of the
sediments.
The total thickness of these sediments at Hungorob is about 1,500
feet. They are overlain by highly amygdaloidal melaphyres (400
feet).
The Erongo granite building up the. Brandberg is intrusive into
these sediments. There is very little doubt that these sediments are
identical with the Kaoko sediments, of which a remnant occurs only
GEOLOGY OF NORTH-WESTERN DAMARALAND, IN S.-W. AFRICA.
47
20 miles to the north (Dorosberg), and in which Reuning has found
Karroo fossils.
1110de of De LJosition.-There is no doubt that both the basal beds
of the Erongo sediments and the sediments of the Otjongundu
Plateau are of terrestrial origin. The bulk of the sediments of the
Brandberg seem to have collected in water, and the same probably
also applies to the upper beds of the Erongo sediments. The boulder
facies appears to represent a narrow shore-deposit marking the edge
of the basin within which the former were deposited. The shore of
this basin appears to have run approximately parallel to the present
coast line. vVhether the basin was marine in nature is diffieult
to say. It appears to have been very shallow.
The TVaterberg Sediments of the Etjo and Great 011wtako.These may be divided into two series: a lower one consisting mainly
of deep red argillaceous sandstones and arenaceous shales, and an
upper series composed of a fine-grained very massive sandstone.
At the Etjo, the lower series is some 1,600 feet thick, and contains numerous beds of conglomeratic and felspathic grits intercalated
with the reddish arenaceous shales and argillaceous sandstones. In
general, these rocks are reminiscent of Red Beds, while the overlying
massive sandstone, except for its somewhat coarser grain, very
closely' resembles Cave Sandstone. Like the latter, it is very uniform
in composition, felspathic and its bulk entirely unbedded. In colour
it is pale reddish, but weathers to a rusty yellow or pale cream. At
the Etjo its thickness is from 200 to 300 feet, while at the Great
Omatako it reaches 450 feet. There is very little doubt that this
massive sandstone is aeolian in origin, and was deposited under conditions very similar to those under which the sedimentation of the
Cave, Bushveld and Forest sandstone took place.
At the great Omatako this sandstone is conformably overlain by
a great thickness (1,400 feet) of a doleritic rock resembling the
doleritic basalts so common at the base or. the Stormberg Lavas. In
addition, the sediments are everywhere cut by numerous dykes and
sills of dolerite.
It is interesting to note in this connection that reptilian tracks,
probably saurischian, have been found in these sediments (lower
series) -at Otjihaenamaparero, thus indicating their probable Stormberg age.
The Age of the Erongo and Related Sediments.-While fossil
finds in the former and the Kaoko sediments (Mesosaurus, Archeotherium Reuningi and rossil tree-trunks) haNe definitely shown them
to be of Karroo age, unfortunately no fossils have up to the present
been found in the Erongo and related sediments. There is very little
doubt, however, that the Brandberg sediments are identical with the
closely neighbouring Kaoko sediments, and that the Erongo sediments
and those of the Otjongundu Plateau most probably also belong to the
same age.
48
TRANSAm'IONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
The chief indirect evidence leading t,o this conclusion is : (1) The
youthful and unfolded nature of the sediments and the highly folded
state of the Otavi beds (Nama) in the vicinity, which, moreover, are
dIrectly and unconformably overlain by Kaoko sediments; (2) the
Erongo lavas ranging from olivine-diabases to quartz-porphyries are
identical with the volcanic rocks of the Kaoko formation, which, in
the southern Kaokoveld, are intrusive into sediments in which Karroo
fossils have been found.
The Igneous Rocks of the Erongo Succession.-These rocks have
already been described in some detail by Cloos (lac. cit.). It will
suffice, therefore, merely to enumerate the different types of volcanic
and plutonic rocks and briefly to describe only their most important
members : (a) Sheets and dykes of ophitic olivine-diabase and dolerite, as
well as flows of the former, forming the base of the Erongo
lavas.
(b) Dykes and flows of dark melaphyres, frequently amygdaloidal, and green and greyish augite-porphyrites (Si0 2 up to
55 per cent.).
(c) Basic tuffs.
(d) Stocks of diorite and granodiorite (Sio 2 -61 per cent.).
A large stock of granodiorite Gc,curs in the Inner
Erongo. It has the following components: Plagioclase (oligo~
clase-albite) , hypersthene, biotite, quartz, orthoclase in
granophyric intergrowth with the former, apatite, zircon,
magnetite and pyrite. There is a gradual transition from
this coarse holocrystalline rock to a, dark quartz-porphyrite, a,
member of the Erongo lavas. It appears, therefore, that the
granodiorite represents the parent magma, from which, at
least, some of the porphyritic lavas were fed.
A smaller stock of diorite on Ameib and a broad dykelike body of the same rock on Davib represent a rnore basic
endomorphic marginal facies of the main body.
A large circular stock of diorite occurs a few miles south
of Karibib, in the surroundings of the " Sargdeckel" and
Virgin Peak. The central portion of this body consists of a
light grey biotite-hornblende-diorite, which is surrounded by
a very dark augite-diorite, representing a more basic
marginal facies of the rock.
(e) Dykes of diorite-porphyrite and granite-porphyry (phenocrysts of orthoclase, and, to a lesser extent, plagioclase,
quartz and biotite embedded in 11 granophyric ground-mass of
quartz, orthoclase and biotite).
(f) Dykes and flows of acid (quartz) porphyrite and quartzporphyry (Si0 2 up to 69 per cent.).
(g) Porphyry tuff.
(h) Erongo granite in stocks, irregular bodies and dykes.
GEOLOGY OF NORTH-WESTERN DAMARALAND, IN S.-W. AFRICA.
49
As typically developed, this is a rather coarse-grained miarolitic
biotite-granite with abundant nests of tourmaline (Schorl) scattered
uniformally throughout the rock. It is an alkali-granite, its composition corresponding very closely with that of the alkali-granite of
Drammen in Norway. Its Si0 2 content is almost 77 per cent. Its
characteristics, in general, show it to be in the nature of a highly acid,
residual magma. Under the microscope, it consists of abundant
quartz and orthoclase, almost invariably poikilitically intergrown with
albite. vVhen fresh, the orthoclase is almost white, but weathered
surfaces in the Erongo area always possess a characteristic creamyellow colouration. At the Great Spitzkopje, however, the felspars
possess a distinctly reddish tint, while the Erongo granite of the
Little Spitzkopje and Brandberg is of a uniform light red or pink
colour.
Biotite, while always present, is never abundant. Muscovite is
rare, occurring occasionally as inclusions in the quartz. Apatite,
zircon, topaz, represent acce~sory constituents, and epidote a product
of alteration. In a section of the Brandberg granite fluorite and
magnetite were found.
The tourmaline nests are only characteristic of the Erongo area.
Their dimensions vary with the grain of the rock (usually 3 in. to 4
in.). In the £ner-grained marginal portions, or in small bodies, they
are. very much smaller, and often reduced to mere specks. The nests
weather out in the form of balls, and the weathered rock surface, as
a result, usually assumes a very pitted appearance.
In the rock of the Great Spitzkopje, the nests are reduced to
occasional black patches of granular schorl, but in that (-'£ the Little.
Spitzkopje and Brandberg they appear to be altogether absent.
The Erongo granite no doubt solidi£ed at a very shallow depth,
less than 2,000 feet, and th~ s probably in part explains the almost complete absence of pegmatites. These have been found only in a narrow
zone along the western margin of the Little Spitzkopje, where they
contain beryl, aquamarine, topaz and fluorite. They are, however,
very irregular and restricted both as regards dimensions and occurrence. Otherwise pegmatites appear to be completely absent, and
even aplitic veins are not numerous.
The Sequence of Intrusion.-The igneous rocks of the Erongo,
both intrusive. and extrusive, represent a very regular succession, and
form a good illustration of the progressive decrease in basicity and
increase in acidity of the various successive fractions derived from
the crustal invasion by a single parent magma. In addition, the
Erongo Mountains afford a very clear example of the progressive
ascent of the. feeding magma within the crust. Thus, the diorite was
intruded to within its own derivatives (porphyrites) already poured
out on the surface, while the Erongo granite, marking the £nal culmination of the whole process, was intruded to still lesser depths than the
former.
Coincident with this progressive ascent of the magma,
the volcanic activity also became more loca.lized, for while the basal
00
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOl:TH AFRICA.
basic lavas appear to have been derived from great depths and poured
out from fissures as indicated by the abundance and wide distribution of
basic dykes, the intermediate and acid lavas are connected with
definite centres of eruption, of which the Erongo was one. Intermediate and acid dykes, moreover, are almost entirely restricted to
the surroundings of such centres of eruption.
'Tectonics.-The ancient sediments of the Schist :Formation everywhere have been intensely folded, and there is evidence to show that
the folding movements were of very long duration, and were at least
once renewed after a considerable period of rest.
The general strike of the sediments is N.E.-S.vV. This strike is
predominant throughout the whole of the. central and northern
portions of South-West Africa. It is, however, by no means constant,
and the sediments deviate from it to an extent that at once suggests
the marked influence of multilateral stresses. Sudden changes into
directions at right angles to the general strike are characteristic
features of the structural plan of these ancient sediments. Excellent
examples are the marble ridges of the Kompaneno Mountains (JYlap,
FIg. 2), and the complicated marble " dome" on Goabeb bet,ween
the Khan River and the Kudukuppe (Map, :Fig. 1). Locally, the
sediments are powerfully ruptured and dislocated in such right-angle
folds, and also heavily invaded by granite, diabase and porphyry
dykes. The irregularity of the folding movements is particularly well
demonstrated by the outcrops of bands of crystalline limestone constituting the marble horizon. The individual bands usually meander
about in a most bewildering manner, frequently producing an
intricate network, a veritable maze of one and the same marble band.
The north-western and northern limbs of syn- and anticlines
nea.rly always possess a very much steeper dip than the south-eastern
and sout,hern. Actual overfolding is very common, and invariably
from the S.E. and S. to the N.W. and N. The dominant stress during
the folding movements, therefore, came from the S.E. and S.
The complex nature of the folding, however, and the features
already described immediately suggest that the stresses were not
entirely unilateral. On the contrary, it is apparent that the sediments
when folded were not free to yield laterally, but experienced a considerable amount of resistance also in a direction at right angles to
the dominant stress, or, in other words, that they were subj eeted to
multilateral stresses. Hence the bewildering irregularity of the folds.
If sediments are compressed in one direction only, and are entirely
free to yield sideways in a direction at right angles to the stress, long
continuous folds of great regularity result, the tendency of a single
stress in one direction only being to reduce the width of the outcrop
in the direction parallel to the stress and extend it in the direction at
right angles to the latter, in which direction an active' strain is
operating. In the case of multilateral stresses, however, particularly
if the sediments are also compressed by stresses operating at right
angles to the dominant stress, the rocks are not free to yield sideways,
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52
TRANSAC'I'IONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
and the resultants of thes~ lateral stresses produce movements that a.re
mainly directed upwards. Hence the large number of dome-like
structures which characterise the tectonic plan of these ancient
sediments.
The convexity, outline and direction of elongation
naturally depend on the relative strength of the operating stresses at
anyone point. The tendency of multilateral stresses, therefore, is
not to reduce the width of the outcrop in one direction only and extend
it in a direction at right angles to the former, but generally to reduce
the circumference and surface extent of the whole area of outcrop,
and to increase the vertical thickness of the folded rocks. This same
circumferential compression and the resultant of multilateral stresses,
the effect of which is to produce movements mainly directed
upwards, would naturally also concentrate the highly mobile
magmatic material at their disposal into such dome-like structures,
and it is not surprising, therefore, to find the latter particularly
heavily intruded by granite (Fig. 1).
One of the largest and most complex dome-like structures of this
description are the Ka,mpaneno or Tjirundu :Mountains north of
Omaruru. The representation of this area by Reuning on his sketchmap of the central portion of South-West Africa 7 is highly inaccurate.
The numerous closely parallel marble bands cut off in the west by
granite intrusions do not exist as such, but the marble horizon has been
strongly arched upwards and bent about in an extraordinary fashion
(1fap, Fig. 2). The whole structure could, perhaps, be referred to as
an irregular compound dome.
Throughout the entire area there is abundant evidence to show
that the intrusion of the old granites and their pegmatitic derivatives
was closely associated with the folding of the ancient sediments.
There is no doubt that even the last phase of intrusion, i. e., the
innumerable pegmatites, were injected while the folding movements
were still in active progress. Not only is this the case, but the
intrusion of the granites and pegmatites appears to have been mainly
controlled by the tectonic forces operating.
Reuning, in his paper on the granite intrusions of Hereroland 7 ,
expresses the opinion that the intrusion of the Salem granite and its
younger phases, i.e., the main granite-intrusion, took place subsequent
to the main period of folding on discordant to accordant lines. In the
area under discussion, howev,er, the representation of the granite
contacts, etc., given by him on the map accompanying the paper are
very inaccurate. Where granites appear to penetrate the sediments
discordantly, these features are very generally merely due to a sudden
change in strike of the latter, and the fact that the granites almost
invariably closely follow the structural plan of the sediments, even
down to the minutest detail.
7Reuning, E.: "Del' Intrnsionsverband del' Granite des mittleren HereroIan des und des angenzenden Kilstengebietes in Sii.d-west Afrika."
Geol.
Bundschau, Vol. 14, 1923 p. 209.
J
GEOLOGY OF NORTH-WESTERN DAMARALAND,
IN S.-W. AFRICA
513
In the area mapped, the granite intrusions, with the exception of
the very much younger Erongo granite, are generally phacolithic in
nature; they form the cores of anticlines and domes, conformably
fill or form part of synclines and basins and occur as numerous lens-GEOLOGICAL MAP-OFTHE---
-KAMPANENO
1~'IG.
2.
shaped and irregular bodies, the outlines of which are determined by
the strike and disposition of the sediments. Discordant contacts
Laturally also occur locally, but are, on the whole, very restricted.
The flow-structure of the Salem granite, micaceous and acid
" schlieren" and all parallel textures in the granite usually -closely
conform to the strike and dip of the sediments, and thus indica,te that at
the time of intrusion the magma was subjected to active stresses.
In addition there is abundant evidence throughout the entire
area to show that the position, outlines and distribution of granitebodies and pegmatite dykes are very frequently the direct result of
54
TRANSACTIO~S OF THE GEOLOGICAL SOCIETY OF SOeTHAFRICA.
tectonic movements. Differential movements along bedding planes
the schists in areas of powerful torsion are the principal factor in
this connection. Disruption of the individual beds and an inten$e
intrusion of granite phacoliths and pegma.tite dykes in the outer arc
of tension and crumpling up of the beds and an absence of igneous
intrusions in the inner arc of compression are features agam and
again met with.
(Fig. 3.)
or
FIG. 3.
Schematic plan of granite phacolith filling space formed by differential
movements in schists during bending; also showing abundance of pegmatite
dykes in the outer arc of tension.
Swarms of pegmatite dykes usually occur at localities where there
is a marked bend in the strike of the schists. Where the latter
straighten out again, dykes are usually very much rarer or altogether
absent.
By far the greatest number of pegmatite dykes are parallel to
the strike and dip of the schists. In the area immediately southwest of the Erongo, of 1,710 pegmatite dykes that were noted and of
which the direction of strike was measured, 79 pel; cent. were oriented
parallel, 13 per cent at right angles and 8 per cent. at diagonal and
oblique angles to the strike of the schists.
As to the number and age of the folding movements that have
affected the rocks of the Fundamental Complex, only a few general
statements can be made at present. It is clear that in the central
and northern portions of South-,\1 est Africa, there were at least two
periods of folding separa,ted by a considerable interval: one tha.t took
place prior to the deposition of the beds of the Phyllite formation
and one that occurred subsequently. The old granites were intruded
during the first period of folding, probably during its closing stages.
GEOLOGY OF NORTH-WESTERN DAMARALAND, IN S.-W. AFRICA.
55
A comparison of the tectonic movements and the nature of the
old granite intrusions in the central and northern portions of South\Vest Africa with those in the south, e.g., in the Luderitzbucht area,
reveals several important differences. In the south, the main folds
in the sediments of the Fundamental Complex are oriented approxi.
mately N .-S. The same axis of folding was then repeated at intervals
and at very much later periods by the tectonic upheavals affecting
the beds referred to by Eeetz and Kaiser as the Folded Nama and
further south by the Malmesbury series and the Cape system. In the
central and northern portions of South \Vest Africa on the other
hand the folding, warping and faulting movements from the first
folding of the Schist Fonnation till after the deposition of the Eanoo
beds have been along axes oriented approximately N.E.-S.\V. According to verbal communication by Dr. Beetz, the turn-in-strike along
the coast takes place a little north of Spencer Bay. It is interesting
to note, however, that faulting along the northern axis has also taken
place in the south (Kharas mountains and \Vitputz trough), while the
approximate N.-S. direction is evidenced in the north-vvest (Kaokoveld)
by post-Kanoo faulting.
A comparative study of the old granite intrusions reveals the
fact that if the intrusion of the Salem granite took place contemporaneously in the two areas, the first folding movements affecting
the Schist Formation came to an earlier close in the South. Probably
they also began earlier. In the north, the Salem granite was intruded
generally on conformable lines, while the folding was still in active
progress. In the south, on the other hand, this granite was intruded
mainly on discordant to accordant lines into sediments and mixed
rocks already highly folded and consolidated. ~rhe earlier generation,
the gneiss-granite, was here intruded contemporaneously with the
folding of the sediments and has led to the production of large areas of
mixed rocks.
\Vith regard to post-Kanoo warping and faulting movements,
the area under discussion yields but little new information, and the
reader is referred to Stahl's paper on the subjects. It is interesting
to note, however, that the position of the foci of eruption in late
Karroo times and the intrusion of the Erongo granite was evidently
influenced by zones of weakness in the earth's crust, which in postKarroo times developed into powerful faults. The foci of intrusion
and centres of eruption exhibit linear arrangements which are certainly
remarkable and in the most pronounced cases continue post-Karroo
faults, e.g., the Great Wa.terberg fault.
8Stahl, A.: "Die Grundzuge del' Schollentektonik Sud-west Afrik·as."
Zeitschr. d. de1dsch Gcol. Ges., Vol. 79, No. 3/4, p. 54, 1927.