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Lecture Notes in Earth Sciences Editors: S. Bhattacharji, Brooklyn G. M. Friedman, Troy H. J. Neugebauer, Bonn A. Seilacher, Tuebingen
40
Sunday W. Petters
Regional Geology of Africa
Springer-Verlag Berlin Heidelberg NewYork London Paris Tokyo Hong Kong Barcelona Budapest
Author Sunday W. Petters Department of Geology University of Calabar Calabar, Nigeria
"For all Lecture Notes in Earth Sciences published till now please see final page of the book"
ISBN 3-540-54528-X Springer-Verlag Berlin Heidelberg New York ISBN 0-387-54528-X Springer-Verlag New York Berlin Heidelberg This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broadcasting, reproduction on microfilms or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. @ Springer-Verlag Berlin Heidelberg 1991 Printed in Germany Typesetting: Camera ready by author Printing and binding: Druckhaus Beltz, Hemsbach/Bergstr. 32/3140-543210 - Printed on acid-free paper
Dedicated to:
Wissenschaftskolleg
zu Berlin
- Institute for Advanced Study -
Preface This book
represents
the
first attempt
in three decades
to m a r s h a l l
available information on the regional geology of Africa dergraduates
and
beginning
African
universities
African
regional
inability
of
maintain
journal
Africa
is
content
is
This
African
so w i d e l y
dispersed
is
to
that
lack
greatly
and
Since geology is a universal
in on
by
the
books
and
information
about
comprehensive
course
is beyond the routine p r e p a r a t i o n
u n i v e r s i t y teachers.
education a textbook
reference
geologic
a balanced
of
exacerbated
purchase
Besides,
for advanced un-
Geologic
by the
situation
universities
subscriptions.
on A f r i c a
students.
severely hampered
geology.
most
graduate
out
of
lecture notes by
subject and A f r i c a
is
one of the largest landmasses on Earth with one of the longest continuous records
of
Earth
other parts
history,
there
of the w o r l d will
is no doubt
benefit
that
geologic
education
from a c o m p r e h e n s i v e
in
presentation
of A f r i c a n geologic case histories. The scope of this text also addresses the need of the professional
geologist,
who may require
some general or
b a c k g r o u n d information about an u n f a m i l i a r A f r i c a n g e o l o g i c region or age interval. Africa occupies a central position in the world's mineral raw materials trade.
Because of its enormous extent and great g e o l o g i c age, the di-
versity and size of Africa's mineral endowment is unparalleled. the leading supply
of
of gold,
strategic
platinum. solely
source
minerals
Consequently,
on
mineral
diamond, such
African
exports
for
uranium,
as
and dominates
chromium,
nations
manganese,
from Algeria
economic
survival.
to
The
Africa is
the world's cobalt,
and
Zimbabwe depend geologic
factors
which govern economic mineral deposits are stressed in this text. The that
geological
is unique
planet
match
history
both
the
of Africa
in duration
plethora
displayed
in
the African
evolution
decipherable
and
spans
of geologic continent.
3.8 billion
continuity. phenomena
From
from the Archean
and
the various
of
years,
Few other
a record
parts
processes stages
southern Africa,
of our
that
of
are
crustal
through the
plate tectonics scenarios in the ubiquitous Late P r o t e r o z o i c - E a r l y Paleozoic P a n - A f r i c a n m o b i l e belts of n o r t h w e s t Africa,
and in the H e r c y n i a n
to the East African
and A l p i n e
Rift Valley,
Africa
orogenies is replete
with e x c e l l e n t examples and problems for a course on regional tectonics. Teachers Africa's
as the Great pluton;
of
igneous
anorogenic
and
magmatism
metamorphic
petrology
(e.g.. layered
Dyke and the Bushveld Complex;
alkaline
complexes;
basaltic
can
ultramafic
hardly
ignore
intrusives
such
the Tete g a b b r o - a n o r t h o s i t e
volcanism),
or
tantalizing
high-
VIII
grade m e t a m o r p h i c
terranes
such as the Limpopo belt,
the Namaqua mobile
belt, and the M o z a m b i q u e belt. From the extensive
Precambrian
supracrustal
sequences
throughout
the
continent with enormous thicknesses of sedimentary rocks that have hardly been
deformed
Africa's
or
metamorphosed,
present-day
spectrum of
passive
facies models
to
the
continental
upon which
stratigraphic margin,
evolution
there
is
of
a
complete
to base a course on basin
analysis
and stratigraphy. To m a i n t a i n the world
its i n t e g r i t y a course on historical
must
address
the
theory
of
Continental
g e o l o g y anywhere in
Drift
beyond
past continuities between West Africa and South America. between West so also
Africa
connections
geography
of
Precambrian
and
between northeast Africa
southern
Gondwana
fossil record,
mammals and dinosaurs, contributions Although
eastern North America must
it
where
Africa
the transitions
and Arabia, occupied
and the paleo-
centre
from reptiles
stage.
The
to the earliest
and the evolution of Man are among Africa's unique
today
in
the
tropics
Earth's m o s t - s p e c t a c u l a r glaciations.
Africa
was
Africa
still
during the Quaternary. cannot
the
theatre
of
the
Even after the scene of continental
glaciation had shifted to the northern continents
pology
e q u a l l y be explored,
to the h i s t o r y of life and the story of organic evolution. lies
Pleistocene,
invoking
Past connections
witnessed
only lately during the
spectacular
climatic
fluctuations
C e r t a i n l y students of a r c h e o l o g y and paleoanthro-
overlook
the
Olduvai Gorge in Tanzania,
Quaternary
paleoenvironmental
the Lake Turkana basin in Kenya,
record
of
the
the Nile val-
ley, the Sahara, and southern Africa. But logic revive after
since A f r i c a n
textbook, the this
idea
examples
I have of
idea was
been
a full-length abandoned
swer, as a l r e a d y stated, mation about Africa
often
have
by
already been asked w h y
textbook
cited
it has
become
on A f r i c a n
the geologic
in standard geonecessary
geology,
community.
My
simple
an-
is that the w e a l t h of a v a i l a b l e geologic infor-
is so enormous and fascinating,
but so diffuse,
an attempt must be made to assemble and pass on this knowledge.
Berlin, May 1991
to
30 years
Sunday W. Petters
that
A c k n o w l edgemen ts
I would German
like
to
acknowledge
institutions
eleven
months
to
of
the
write
unique
this
residence
at
support,
through
which
Africa,
I
excellent
enjoyed
is
the
Problems
Dr.
project
leader
of
and
research
Ethiopia)
has
and
in
to
graduate
Klitzsch.
enormously
During
from d i s c u s s i o n s
Africa
was
for fel-
literature
Secondly,
on
I wish
U n i v e r s i t y Berlin
Project
from
and
(Egypt,
various
through
preparation with
Berlin
assistance
"Geoscientific
(Sonderforschungsbereich
students
the
two
during
zu
institution
geologic
Research
by
69)
funded
(DFG). Special Project 69 is devoted to
northeast
to the W i s s e n s c h a f t s k o l l e g
me
prepared
secretarial
of the Technical
Special
in A r i d and Semiarid Areas"
geoscientific
fessor
I thank this access
Eberhard Klitzsch
by the G e r m a n R e s e a r c h Foundation
visit
was
Wissenschaftskolleg
bibliographic
limitless
afforded
text
and e q u a l l y important, had the m a n u s c r i p t typed.
to thank Prof. who
and
This
the
(Institute for A d v a n c e d Study Berlin). lowship
opportunity
text.
of
Sudan,
parts
of
Somalia,
Africa.
the r e c o m m e n d a t i o n the
manuscript
suggestions
I
My
of Pro-
benefited
from the g e o l o g i s t s
in
Special Project 69. The idea of w r i t i n g a textbook on the regional g e o l o g y of Africa was c o n c e i v e d during my 15 years of teaching various geology courses at five Nigerian
universities.
During
this
period
I sought
to
enrich
contents by v i s i t i n g several European libraries and museums. pect I wish to thank Dr. M.C.
my
course
In this res-
Daly and his wife and Dr. C.S.
Orereke for
their h o s p i t a l i t y during my visit to the U n i v e r s i t y of Leeds in 1984. Dr. M.
Oden was my host
Museum lege
of Natural
library.
in London
History,
I thank
that year during my v i s i t
the Geological
Prof.
P. Bowden
Museum,
and
Dr.
and
to the British
the
Imperial
J.A. K i n n a i r d
for
h o s p i t a l i t y during the colloquium on A f r i c a n g e o l o g y at St. Andrews versity
in
1985.
Professor
H.P. L u t e r b a c h e r
was
very
helpful
Coltheir Uni-
during
my
visit to the U n i v e r s i t y of T~bingen in 1987. I am g e n e r a l l y greatly indebted to all geologists who h a v e w o r k e d in Africa,
from whose publications
I would
also like to thank e s p e c i a l l y all those who
books, include
reprints,
and
Profs.
J.B
L.B. Halstead,
brates.
Wright,
I. Valeton, and
L.L. Jacobs
of
their
J.A. Peterson,
J.R. Vail, supplied
and
B.-D.
Wilde,
of
These
Erdtmann,
C.O. Ofoegbu,
N.J. Jackson.
illustrations
important
on Africa.
R. Caby, P.
for this text.
sent v e r y
publications
S.J. Culver,
V. Jacobshagen,
J.D. Fairhead, E. B u f f e t a u t
pre-prints
I have drawn the m a t e r i a l
Professors
African
verte-
I am g r e a t l y i n d e b t e d to Prof. Rushdi Said who was also in residence at the W i s s e n s c h a f t s k o l l e g
during
the
1989/90
session.
constant advice and e n c o u r a g e m e n t kept up my spirits.
Professor
Said's
I thank Profs. R.K.
Olsson, R.C. M u r r a y and B.W. A n d a h for encouraging me to pursue this project over the years. I am v e r y grateful the
Precambrian
thank Drs. the
and
offered
very
useful
suggestions.
East Africa.
als0
chapters.
Dr.
Muhongo
greatly
improved
my
coverage
special
thanks
and
all
invaluable help.
go
to
the
Ms
R. Plaar
secretarial
for
staff
of
the
preparation
the
Institute
patience
and
Prasser,
who, as
hard
work.
I would
in addition
the was
of
for
the
their
Special a p p r e c i a t i o n goes to Mrs Maria A. Gowans and Ms
Linda O ' R i o r d a n who p r e p a r e d the final c a m e r a - r e a d y manuscript,
assistance
of
P r o f e s s o r N. Rutter kindly reviewed the Q u a t e r n a r y chapter.
manuscript,
moment
I
S. M u h o n g o and H. S c h a n d e l m e i e r for their comments on some of
Precambrian
My
to Drs. M.C. Daly and G. Matheis who read through
chapters
like
to
acknowledge
to his great hospitality,
liaison provided
also
with by
the
Mrs
publisher.
Gesine
Reinhard
served at the final
Excellent
Bottomley
for their
Mr
and
bibliographic
her
staff
at
the
W i s s e n s c h a f t s k o l l e g and by Mrs Evelyn Kubig of the G e o l o g y L i b r a r y of the Technical University,
Berlin.
Messrs
Umo Harrison,
E. Umo,
Joe Sams,
and
Richard Ingwe and his colleagues rendered cartographic assistance. I thank P r o f e s s o r Charles of Calabar
for moral
managing director
Effiong,
and m a t e r i a l
of Mobil
Vice-Chancellor
support.
of the University
Dr. A l f r e d Koch,
P r o d u c i n g Nigeria
and Mr.
Wande
chairman and Sawyerr,
ex-
ploration m a n a g e r of Mobil, also encouraged this project. Finally,
on
behalf
of
my
wife
Janet,
and
Ekanga and Unwana, who were with me in Berlin,
my
children,
and
his
wife
senschaftskolleg grateful.
for
were
their
very
hospitality.
friendly
to
us,
Emem,
I wish to express profound
gratitude to the Rektor of the Wissenschaftskolleg, nies
Mfon,
All and
Prof.
the for
Dr. Wolf Lepe-
staff
of
the
Wis-
this
we
are
very
TABLE OF CONTENTS
CHAPTER
1
INTRODUCTION
i.i
The Physical
1.2
Geological
CHAPTER 2.1
2
Setting of A f r i c a
H i s t o r y and M i n e r a l
THE P R E C A M B R I A N
Tectonic
OF AFRICA:
Deposits
of A f r i c a
AN INTRODUCTION 8
Framework
2.2
The P r e c a m b r i a n
Time-Scale
13
2.3
Orogenic
Cycles
in A f r i c a
16
2.4
Dominant
Rock Types
C~PTER
3
19
THE A R C H E A N
3.1
Introduction
21
3.2
Kalahari
Craton
23
3.2.1
Kaapvaal
Province
3.2.2 3.2.3
3.2.4
3.2.5
25
Ancient Gneiss Complex The B a r b e r t o n G r e e n s t o n e Belt S t r u c t u r e of the B a r b e r t o n G r e e n s t o n e Belt G r a n i t o i d Emplacement and C r a t o n i z a t i o n Other G r e e n s t o n e Belts in the Kaapvaal Province
26 28 38 39 41
Pongola B a s i n
42
.Zimbabwe
44
Province
G w e n o r o D a m Basement G n e i s s e s O l d e r G r e e n s t o n e Belt (Sebakwian Group) Bulawayan Greenstones S t r u c t u r e of the B u l a w a y a n G r e e n s t o n e Igneous Intrusion and C r a t o n i z a t i o n
45 47 48 54 54
Limpopo
Province
56
Northern Central Central Southern Tectonic
M a r g i n a l Zone (N.M.Z.) Zone in the Limpopo V a l l e y Zone in B o t s w a n a Marginal Zone (S.M.Z.) Models
56 57 58 59 60
Archean Mineralization
on the Kalahari
Gold Chrome Massive Base-Metal Sulphides Iron Ore Pegmatite M i n e r a l i z a t i o n Corundum Asbestos
Craton
64 65 68 68 69 69 69 7O
XII
71
Zaire Craton
3.3
72
3.3.1
Kasai-NE A n g o l a
3.3.2
N W Zaire
Craton
Shield
74
3.3.3
NE Zaire C r a t o n
76
Bomu Gneiss C o m p l e x West Nile G n e i s s i c Complex G a n g u a n G r e e n s t o n e and Schist K i b a l i a n G r e e n s t o n e Belts Granitoids Gold Mineralization
77 79 80 81 82 83
Belt
3 4
Tanzania
Craton
84
34.1
Geologic
Framework
84
34.2
D o d o m a Schist
34.3
Nyanzian-Kavirondian
3 4.4
Gold M i n e r a l i z a t i o n
3 5
West African
35.1
G u i n e a Rise
90
G r a n i t i c Gneiss Basement G r e e n s t o n e Belts
91 92
3.5.2
Archean Mineralization
98
3.5.3
Reguibat
86
Belt
86
Schist Belts on the Tanzania
Craton
87 90
Craton
on the Guinea Rise
99
Shield
3.6
Other Archean
3.6.1
East Saharan Craton
102
Jebel Uweinat Tuareg Shield
102 104
3.6.2
Madagascar
3.7
Archean
3.7.1
Classical
Terranes
105
Tectonic
Back-arc-Marginal
3.7.3
Archean
4
105
Models
105
Models
3.7.2
CHAPTER
in A f r i c a
102
107
Basin Models
107
Plate Tectonics
EARLY PROTEROZOIC AND MOBILE
CRATONIC
BASINS
BELTS
113
4.1
Introduction
4.2
Kalahari
4.2.1
Introduction
115
Witwatersrand
119
4.2.2
Cratonic
Basins
Basin
115
Stratigraphy Mineralization
121 124
4.2.3
Ventersdorp
126
4.2.4
Transvaal-Griqualand Stratigraphy
Basin West Basins
127 127
XIII
4.2.5
4.2.6
4.2.7
M i n e r a l i z a t i o n in the TransvaalG r i q u a l a n d West Supergroups
132
Iron and M a n g a n e s e Gold Base Metals Industrial Minerals
132 132 135 137
Waterberg,
Soutpansberg,
and M a t s a p Basins
137 137 139
Umkondo
139
Epeiric
Basin
139 140
Stratigraphy Mineralization 4.3
Anorogenic
4.3.1
The Great Dyke
Magmatism
on the K a l a h a r i
B u s h v e l d Igneous Occurrence
Craton
140 141
and O r i g i n
141 144
C o m p l e x Occurrence
144 144
Occurrence, Composition, Mineralization 4.3.2
137
Waterberg Basin S o u t p a n s b e r g Trough Matsap B a s i n
Igneous S t r a t i g r a p h y G e o c h e m i s t r y and O r i g i n Mineralization
144 148 149
4.3.3
Palabora
151
4.4
Vredefort
4.5
N a m a q u a M o b i l e Belt
153
4.5.1
Eastern Marginal
154
4.5.2
Western
Zone
4.5.3
Central
Zone
4.6
Igneous
4.8
151
Dome
Zone
156 (Namaqua M e t a m o r p h i c
Complex
157
Central Zone in N a m i b i a Namaqualand Bushmanland Igneous Intrusions in the Central Zone Tectonics of the Central Zone M i n e r a l i z a t i o n in the Central Zone
159 159 160 160 162 164
Natal
166
Province
N o r t h e r n Marginal N o r t h e r n Zone Central Zone S o u t h e r n Zone Tectonic Model 4.7
Complex
Magondi
Zone
M o b i l e Belt
168 168 168 169 169 169
S t r a t i g r a p h y and Structure Mineralization
169 172
West A f r i c a n
174
Craton
4.8.1
Introduction
174
4.8.2
Birimian
176
Supergroup
The B i r i m i a n in Ghana The B i r i m i a n in Other Parts of the G u i n e a Rise G r a n i t o i d s and S t r u c t u r e of the B i r i m i a n T e c t o n i c Models for the B i r i m i a n S u p e r g r o u p
179 184 184 186
XIV
4.8.3
4.8.4
Birimian Mineralization
188
Gold Manganese Diamonds Iron Base Metal D e p o s i t s
188 190 191 191 192
The Reguibat
192
Shield
195
4.9
Zaire Craton
4.9.1
Introduction
4.9.2
Kasai
4.9.3
E b u r n e a n Basement
4.9.4
E b u r n e a n Basement in the Internal and Foreland Zones of the West Congolian Orogen
197
4.9.5
Gabon Orogenic
2O0
4.10
195
- NE A n g o l a
Shield
195
of S o u t h e rn A n g o l a
197
Belt
S t r a t i g r a p h y of the Gabon Orogenic Belt Structure and M e t a m o r p h i s m T e c t o n i c Model for the G a b o n Orogenic Belt
20O 203 203
The U b e n d i a n
2O5
Belt of Central A f r i c a
2O5
4.10.1 I n t r o d u c t i o n 4.10.2 U b e n d i a n
4.11
Rock A s s e m b l a g e s
and T e c t o n i s m
207
Malawi and NE Zambia U b e n d i a n Terranes along the S o u t h w e s t e r n M a r g i n of the T a n z a n i a Craton The U b e n d i a n in Burundi, Rwanda and Zaire The Ruwenzori Fold Belt Mineralization
207
The B a n g w e u l u
214
4.11.1 Geological
CHAPTER 5
Block
207 210 210 213
214
Evolution
THE M I D - P R O T E R O Z O I C
K I B A R A N BELTS
Introduction
220
5.2
K i b a r a n M o b i l e Belts
221
5.2.1
The Kibaran Belt
223
Lithostratigraphy S t r u c t u r e and M e t a m o r p h i s m Intrusive A c t i v i t y T e c t o n i c Model Mineralization
223 226 227 229 229
The Irumide Belt
231
Stratigraphy Structure
231 236
5.1
5.2.3
5.2.4
Southern Mozambique
Mobile Belt
241 243 244 246
Central Malawi Province S o u t h e r n Malawi Province Tete Province M o z a m b i q u e Province 5.3
Regional
Tectonic Model
240
for the Kibaran Belts
248
×V
5.4
C~PTER
O t h e r M i d - P r o t e r o z o i c T e r r a n e s in A f r i c a
250
Angola East Saharan Craton Madagascar
25O 251 253
6
LATE PROTEROZOIC-EARLY PALEOZOIC PAN-AFRICAN M O B I L E BELTS
254
6.1
Introduction The W e s t A f r i c a n P o l y o r o g e n i c Belt
257
6.2.1
Geological and Geophysical Framework
257
6.2.2
T e c t o n o - s t r a t i g r a p h i c Units
260
Foreland Units External Units Axial Units Internal Units
262 263 265 266
6.2.3
Tectonic History
267
6.2.4
T r a n s - A t l a n t i c Correlations with S o u t h e r n Appalachian, U . S . A
271
6.3
The M o r o c c a n A n t i - A t l a s
272
6.3.1
Stratigraphy
272
6.3.2
The Bou A z z e r 0phiolite
273
6.3.3
Mineralization
275
6.4
The T r a n s - S a h a r a n M o b i l e Belt
276
6.4.1
Geodynamic Setting
276
6.4.2
The Tuareg Shield
278
P o s t - E b u r n e a n S e d i m e n t a t i o n and A n o r o g e n i c Magmatism M i d - L a t e Proterozoic P l a t f o r m S e d i m e n t a t i o n Mafic and U l t r a m a f i c Rocks Related to Crustal Thinning V o l c a n o - S e d i m e n t a r y Sequences and C a l c - a l k a l i n e Magmatism D e f o r m a t i o n and M e t a m o r p h i s m Syn-orogenic and P o s t - o r o g e n i c M a g m a t i s m Molasse Sequences 6.4.3
6.4.4
6.5
280 280 281 281 285 289 292
The G o u r m a A u l a c o g e n
292
Stratigraphy The A m a l a o u l a o u Mafic Complex Structure
292 294 294
The B e n i n - N i g e r i a Province
296
The V o l t a Basin The B e n i n i a n Fold Belt The Nigeria Province The Cameroon Basement T r a n s - A t l a n t i c Connections Mineral Deposits in the T r a n s - S a h a r a n Belt
298 301 302 311 314 316
South Atlantic Mobile Belts
318
XVI
The West C o n g o l i a n Orogen
319
Lithostratigraphy Tectonism
319 323
The D a m a r a Orogen
322
Structural Framework Rift S e d i m e n t a t i o n and V o l c a n i s m Regional Subsidence and Marine Transgressions Tectonism Mineralization
323 325 327 331 332
The Gariep Belt
336
Stratigraphy336 Tectonism Mineralization
339 340
6.5.4
The S a l d a n h i a Belt
340
6.6.5
P l a t f o r m Cover of the Kalahari Craton
343
The Nama Group
343
6.7
Katanga
346
6.7.1
Regional Setting
346
6.7.2
The L u f i l i a n Arc
349
Stratigraphy Tectonism
349 352
6.7.3
The K u n d e l u n g u A u l a c o g e n
354
6.7.4
The Zambezi Belt
355
Regional Setting Stratigraphy Structure
355 355 356
M i n e r a l i z a t i o n in the Katangan Orogen
356
StratiformMineralization Vein M i n e r a l i z a t i o n
356 362
6.8
Western
363
6.8.1
Regional Setting
363
6.8.2
The S o u t h e r n Sector
364
6.5.1
6.5.2
6.5.3
6.7.5
Orogen
Rift M o b i l e
Belt
365
6.8.3
Itombwe S y n c l i n o r i u m
6.9
Platform
6.9.1
Regional D i s t r i b u t i o n
366
6.9.2
Sequences on the Zaire Craton
368
Mbuyi Mayi Group Lindian Supergroup
368 369
6.9.3
Sequences on the T a n z a n i a Craton: B u k o b a n and M a l a g a r a s i a n Supergroups
370
6.10
The M o z a m b i q u e
372
Cover of Zaire a n d Tanzania
Cratons
Belt of Kenya and T a n z a n i a
366
6.10.1 Regional Framework
372
6.10.2 Tectonic Features of the K e n y a - T a n z a n i a Province
374
6.10.3 F o r e l a n d and External Zones
377
XVII
6.10.4 The Internal Zone G r a n u l i t e Complexes Central Granulite Complexes of T a n z a n i a U l u g u r u Mountains G r a n u l i t e Complex Pare-Usambara M o u n t a i n Granulite Complex Kurase and Kasigau Groups of Kenya N o r t h - C e n t r a l Kenya G r a n u l i t e Complex K a r a s u k - C h e r a n g a n i Group
378 378 378 379 38O 38O 381 385
6.10.5 0phiolitic Rocks
385
Sekerr and Itiso Baragoi Moyale Pare M o u n t a i n s
386 388 388 389
6.10.6 Molasse
389
6.10.7 M a d a g a s c a r
389
6.11.8 G e o d y n a m i c Model
390
6.10.9 M i n e r a l i z a t i o n
391
6.11
The A r a b i a n - N u b i a n S h i e l d
392
6.11.1 Tectonic Framework
392
6.11.2 Gneisses in P r e - P a n - A f r i c a n Terranes
396
6.11.3 M e t a - S e d i m e n t a r y Belts A r o u n d the Red Sea Fold Belt
399
S o u t h e r n Uweinat Belt Jebel Rahib Belt North Kordofan Belt Darfur Belt Eastern Nuba Mountains Belt Bayuda Desert Exotic M e t a s e d i m e n t a r y Terranes Inda Ad Group (Northern Somalia) Tibesti M o u n t a i n s (Chad-Libya) Paleo-Tectonic Setting for the MetaS e d i m e n t a r y Belts 6.11.4 V o l c a n o - s e d i m e n t a r y and Ophiolite A s s e m b l a g e s V o l c a n o - s e d i m e n t a r y Assemblages Ophio!ites Ophiolitic M~lange and O l i s t o s t r o m e s
399 399 400 400 400 4O0 401 403 403 403 404 404 404 407
6.11.5 Syn- and Post-orogenic and A n o r o g e n i c M a g m a t i s m
411
6.11.6 Molasse
411
6.11.7 T e c t o n i s m
412
Tectonic Model Red Sea Hills Central and Southern Eastern Desert Tectonic Evolution 6.11.8 M i n e r a l i z a t i o n Syngenetic S t r a t i f o r m Ores 0 p h i o l i t e - r e l a t e d Deposits V o l c a n o g e n i c Base-metal Sulphides Magmatic Deposits
412 412 413 414 417 418 418 418 418
XVIII
CHAPTER
7
PRECAMBRIAN
GLACIATION
7.1
Precambrian
7.1.1
Late A r c h e a n - E a r l y
7.1.2
Mid-Late
Glaciation
Proterozoic
Era
423 423
and Paleolatitudes
428
Paleomagnetism The P r e c a m b r i a n
7.2.1
The A r c h e a n
7.2.2
The E a r l y - M i d
7.2.3
The Late Proterozoic
7.2.4
The E d i a c a r a n
Glacial
Glacial
Eras
7.1.3
8
421
Proterozoic
7.2
C~PTER
AND FOSSIL RECORD
Fossil
Record
428
Fossil R e c o r d Proterozoic
431 Fossil R e c o r d
Fossil
433
Record
434
Fauna
PALEOZOIC
435
SEDIMENTARY
8.1
Structural C l a s s i f i c a t i o n S e d i m e n t a r y Basins
8.2
Paleogeographic
8.3
The M o r o c c a n H e r c y n i d e s
8.3.1
Structural
8.3.2
Stratigraphy
BASINS
IN A F R I C A
of A f r i c a n 439
Framework
442 446
Domains
446
and Tectonic
Evolution
451
The P r e c a m b r i a n - C a m b r i a n T r a n s i t i o n (Infracambrian) C a m b r i a n subsidence and V o l c a n i s m O r d o v i c i a n P l a t f o r m and the Sehoul Terrane S i l u r i a n Post-glacial T r a n s g r e s s i o n Early Middle D e v o n i a n Platforms and Trough Late D e v o n i a n Basins, Platforms and D e f o r m a t i o n C a r b o n i f e r o u s Basins and H e r c y n i a n D e f o r m a t i o n
452 453 453 454 455 456 458
8.3.3
Correlations
462
8.4
N o r t h Saharan
8.4.1
Tectonic
8.4.2
Tindouf
and Reggane
Central
and Southern A l g e r i a n
8.4.3
with N o r t h America Intracratonic
Control
Bechar-Timimoun Illizi B a s i n
and Europe
Basins
466
of B a s i n D e v e l o p m e n t
466
Basins
469 Basins
473
Basin
in A l g e r i a n
473 476
8.4.4
Petroleum
8.4.5
Ghadames
Paleozoic
8.4.6
Murzuk Basin
8.4.7
Kufra Basin
8.4.8
Correlations
with the Paleozoic
8.5
West A f r i c a n
Intracratonic
Basins
478
Basin
8.5 .I
Taoudeni
8.5.2
Bov~ Basin
8.5.3
Northern
479 483 484 of Saudi A r a b i a
Basins
488 490
Basin
490 494
Iullemmeden Exposures
Basin Along
494
8.5.4
Paleozoic
8.6
The Cape Fold Belt
the West A f r i c a n
497
8.6.1
Aborted
497
Rifts and G l a c i a t i o n s
Coast
496
XlX
8.6.2
The Cape Supergroup
498
Table M o u n t a i n Group Natal Group B o k k e v e l d Group W i t t e b e r g Group
500 500 5O2 505 508
8.7
Karoo Basins
8.7.1
G o n d w a n a Formations
508
8.7.2
Regional Tectonic Settings
509
8.7.3
8.7.4
8.7.5
The Karoo Foreland Basin of South Africa
510
Dwyka Formation Ecca Group Beaufort Group U p p e r Karoo Formations
512 513 515 516
Other Karoo Basins
517
Ruhuhu Basin Morondava Basin M i d - Z a m b e z i Basin Regional Karoo Correlations
517 520 523 523
Aspects of Karoo Life
525
CHAPTER 9
MESOZOIC-CENOZOIC
BASINS
532
Formation
9.2
The A t l a s Belt: A n A l p i n e O r o g e n Northwest Africa
9.2.1
Tectonic Domains
533
9.2.2
Synoptic Tectonic History
534
9.2.3
The M o r o c c a n or High Atlas
537
9.2.4
The Saharan Atlas
54O
9.2.5
T u n i s i a n Atlas
542
9.2.6
The M o r o c c a n Rif
545
Palinspastic R e c o n s t r u c t i o n S t r a t i g r a p h y of the M a i n Structural Units in the Rif Geological History
545
The Tell Atlas
550
Palinspastic R e c o n s t r u c t i o n S t r a t i g r a p h y and Tectonics of Structural Zones
550 550
Stratigraphic Platform
552
9.2.7
9.3
of the A f r i c a n
IN A F R I C A
9.1
Evolution
Plate in
of the E a s t e r n
533
546 548
Saharan
9.3.1
Structural Framework
552
9.3.2
Paleogeographic Development
552
Triassic Jurassic Cretaceous Paleogene Neogene
552 553 556 557 557
9.4
Evolution
9.4.1
Origin and Structure of the A f r i c a n A t l a n t i c Margin
of the A t l a n t i c M a r g i n of A f r i c a
559 559
XX
9.4.2
N o r t h w e s t A f r i c a n Coastal Basins
563
9.4.3
Equatorial A t l a n t i c Basins
567
Liberian Basin Ivory Coast Basin D a h o m e y Basin Niger Delta
567 568 570 570
94.4
A p t i a n Salt Basins
575
94.5
Southwest A f r i c a n Marginal Basins
580
94.6
South A f r i c a n T r a n s l a t i o n M a r g i n
582
9 5
Evolution
584
95.1
Plate Tectonic H i s t o r y
584
9 5.2
Paleogeography
586
95.3
Selous and M a j u n g a Basins
588
95.4
M e s o z o i c Rift Basins in the Horn of Africa
589
9.6
West and Central
594
9.6.1
Origin
594
9.6.2
Benue Trough
596
9.6.3
Chad Basin
601
9.6.4
Cameroon Cretaceous Rifts
602
9.6.5
Sudanese Rift Basins
602
9.7
Interior
606
9.7.1
Iullemmeden Basin
606
9.7.2
Zaire Basin
606
9.8
Tertiary
6O8
9.8.1
The Red Sea and the Gulf of Aden
608
Tectonic History Stratigraphy
6O8 610
The East A f r i c a n Rift System
613
Introduction G e o m o r p h o l o g y and Structure Stratigraphy and Depositional Models Tectonic Model
613 614 618 619
9.8.2
CHAPTER
I0
of the Eastern A f r i c a n M a r g i n
African
Cretaceous
Rifts
Sag Basins
Rifts and Ocean Basins
PHANEROZOIC
INTRAPLATE M A G M A T I S M
IN A F R I C A
I0.i
Introduction
622
10.2
Alkaline
622
Complexes
10.2.1 Types and Structure
622
10.2.2 The West A f r i c a n Younger Granite Ring Complex Province
625
10.2.3 Northeast A f r i c a n Province
627
10.2.4 Southeast A f r i c a n Province
628
10.2.5 Southwest A f r i c a n Province
628
10.2.6 Tectonic Controls of Ring Complex Emplacement
630
10.2.7 M i n e r a l i z a t i o n in A l k a l i n e Complexes
630
XXl
10.3
632
Basaltic M a g m a t i s m
10.3 .I M e s o z o i c
Basic
Intrusives
632
10.3.2 Karoo V o l c a n i s m
635
10.3.3 K i m b e r l i t e s 10.3.4 Cenozoic
636
Continental
Hot Spots
East A f r i c a n Rift S y s t e m O t h e r Continental V o l c a n i c 10.3.5 Oceanic
CHAPTER Ii
639 639 640
Centres
641
Hot Spots
THE Q U A T E R N A R Y IN A F R I C A
11.1
Introduction
643
11.2
The Q u a t e r n a r y Physical G e o g r a p h y of A f r i c a
647
11.3
Q u a t e r n a r y Deposits in A f r i c a
649 650
11.3.1 West A f r i c a Coastal Plain Sequences Sequences O v e r l y i n g Basement Forest and Savanna Zones Savanna-Sahel Sequences Western Saharan Successions 11.3.2 N o r t h A f r i c a n
651 in the Rain 651 653 653 657
Successions
11.3.3 The Nile V a l l e y
660
Fill
11.3.4 East A f r i c a n Rift V a l l e y
Successions
663 663 665 668
E t h i o p i a n Rift Kenya Rift T a n z a n i a Rift W e s t e r n Rift 11.3.5 Q u a t e r n a r y
Deposits
in S o u t h e r n A f r i c a
Kalahari B a s i n V a a l - 0 r a n g e Basin and Continental A u s t r a l o p i t h e c i n e Cave Breccias
11.4
Shelf
Quaternary Paleoclimatic Reconstructions for Africa
11.4.1 The Land R e c o r d Southern and Eastern A f r i c a The Sahara 11.4.2 The Oceanic
662
Record
669 669 670 671 671 672 672 677 677
11.5
Aspects of Human O r i g i n
682
11.6
Reflections on C o n t e m p o r a r y E n v i r o n m e n t a l Problems
683
References
685
Chapter I Introduction
1.1 The Physical Setting of Africa Africa
is the second
largest continent,
occupying
one-fifth
surface of the Earth. Surrounded on all sides by oceans, tinent
is
like
a
huge
island.
The
boundaries
of
of the land
the African con-
the
African
(Fig.l.l), except on the northern side, lie along mid-oceanic
plate
ridges. The
African plate is growing in size as new material is accreting along these spreading centres0
But what Africa gains is lost elsewhere by subduction
in the global system of moving plates.
World-wide estimates of the rates
of plate motion indicate that the African plate is moving slowly towards the northeast at the rate of about 2 cm/yr.
Figure i.i: Major plates of the Earth; spreading directions are shown with arrows. (Redrawn from Braithwaite, 1987.) Africa is the most tropic~l of all the continents, evenly astride the equator,
the African climate and vegetation are quite extreme. tremely
hot
and
arid
in
for it lies almost
and extends from 37°51'N to 37°51'S. However,
the
Sahara
in
the
north
They range from ex-
and
the
Kalahari
and
N a m i b deserts
in the
southwest
(Figol.2),
through
tropical
to tundra on the highest snow-capped m o u n t a i n peaks equator.
A Mediterranean
type of climate
rain
forests
located right on the
and v e g e t a t i o n
prevails
in the
n o r t h e r n and southern extremities of the continent with low shrubs, evergreen bushes,
and forests.
The climate and v e g e t a t i o n of Africa are dis-
cussed in g r e a t e r detail in the final chapter in relation to the environmental changes in Africa over the last 2.5 to 1.8 million years.
Figure 1.2: Basins Pritchard, 1979.) For unusual
a
continent
in that
it
of
its
lacks
cept the Atlas ranges
and
"Swells"
enormous
size
in
(30.3
high and extensive
(2,100 m high)
Africa.
m i l l i o n km2),
folded m o u n t a i n
from
Africa
ranges,
is ex-
in the n o r t h w e s t and the Cape ranges
in South A f r i c a
(1,800 m high). This morphology,
that A f r i c a
the
has
(Redrawn
largest area of basement
however,
belies the fact
terrain with
ancient moun-
tain belts which have been completely bevelled and exposed at their deep roots.
S t e a d y uplift,
face p r o c e s s e s m i l l i o n years.
deep weathering,
that have
and erosion are the d o m i n a n t sur-
shaped the African
continent
over
the last 450
The
topography
(Fig.l.2). interior
of
Basement
basins
Africa
upwarps
is
form
characterized large domes
lie in broad basement
by
or
basins
and
shields w h i l e
downwarps.
The
swells
swells
extensive
are highest
w h e r e capped by v o l c a n i c flows as in East Africa and central West Africa. Generally
the
is h i g h e r
in
north.
continent the
can be d e s c r i b e d
eastern
and
southern
as a large uneven
parts
and
lower
plateau
in
the
Rising the
abruptly
above
sea
Ethiopian
swell
(Fig.l.2)
level
to
a
rolling
is part
upland
2,000-2,400 m
of an eastern
African
which continues through Kenya where it is 3,000-4,000 m high, interruptions
ruptured
through
into
South
these
Africa.
swells
and
The
East A f r i c a n
created
some
of
the
ern arm of the rift v a l l e y system, ment horst
From the R u w e n z o r i
ley drops down a fault scarp to the rift floor, tacular
fault
Ethiopia. in Kenya
scarps
are not unusual
On the basement and Tanzania,
the
south.
the
vast
Prolonged
4,000 m below°
associated
with
the
East
crustal
Africa
Uplifts
have
where
also
continent.
process
and
the rifts
Kenya
(5,199 m
in Tanzania to and part of
African
Rift
fresh and saline, occur in the rifts,
stability regional
created
But
of
Valley.
the deepest
(1,470 m). punctuated
by
uplifts
of scarp retreat and erosion of e x t e n s i v e
eastern
Mt.
(5,895 m)
Such spec-
in Kenya
form the shoulders
and Mt. K i l i m a n j a r o
a base-
the rift val-
are a c t u a l l y c o m p o s i t e volcanoes,
fields
Lakes, great and small,
cycles
which
has
spectacular on the west-
along the rift v a l l e y
stand two snow-clad mountains,
Both m o u n t a i n s
volcanic
being Lake T a n g a n y i k a
the
upwarps
right on the equator,
Valley
most
towers the R u w e n z o r i Mountain,
5,000 m high clad with snow.
swell
and extends
Rift
horst and g r a b e n landscapes on Earth. West of Lake Victoria,
high)
and
The continent thus appears to be tilted to the northwest.
high, with
that
west
by
planation
great
far
the
surfaces
escarpments most
that c h a r a c t e r i z e s Africa
were
in the
profound
have
surfaces,
and
sustained
e s p e c i a l l y in
first
recognized.
southeastern widespread
is the d e v e l o p m e n t
part of
geomorphic
of e r o s i o n
surfaces
and r e s u l t a n t h e a v i l y leached residual soils. Rich in s e c o n d a r y oxides of iron
(laterite),
these
soils
are
aluminium inimical
(bauxite) to
or both,
agriculture.
c o n d u c i v e to the c o n c e n t r a t i o n of mineral ganese,
and d e p r i v e d of nutrients,
They
are,
deposits
however,
sometimes
such as bauxite,
man-
iron ore, and gold.
Another
characteristic
weathering,
scarp retreat,
small
isolated
bergs
break
steep-sided
the m o n o t o n y
African
product
of
prolonged
deep
and stream incision are inselbergs. residual
hills made of r e s i s t a n t
of the A f r i c a n great plains.
tropical These are
rock.
Insel-
T h e y are best de-
v e l o p e d in open w o o d l a n d s and grasslands on the plateau c o u n t r y of Africa
where they have created a distinctive scenery, region of West Africa,
the Masai
for example in the savanna
steppe of Kenya,
and in the Great Karoo
of South Africa. African they
are
drainage
frequently
systems
also
interrupted
bear
the
imprints
by waterfalls
and
of
uplift
rapids.
in
This
has
that en-
m o s t parts of the continent w i t h almost limitless h y d r o - e l e c t r i c i t y
dowed
potential. W h i l e some of the principal rivers such as the Nile, the Niger and
the
their
Orange
mouths,
discharge
their
others s u c h
as
sediment the
load
Zaire
into
River
large
empty
deltas
through
across
submarine
canyons into d e e p - s e a fans on the ocean floor. The m a j o r deltas including the
Cross
fans.
River
For
sandy,
the
but
delta
most
along
in
part
southeastern the
African
the eastern African
Nigeria
also
continental coasts
and
construct
shelf
shelves
is
deep-sea
narrow
and
there are areas
of c a r b o n a t e sedimentation, w h e r e coral reefs thrive. With strewn
huge
reserves
in alluvial
of
petroleum
terraces along
in
the
Niger
the Orange River
delta,
and
diamonds
in South Africa,
and
diamonds on the beaches of Namibia and on the s h a l l o w shelf of the Orange delta,
the economic potentialities of African rivers sometimes sound like
fairy tale.
1.2 Geological History and Mineral Deposits of Africa The geological record of Africa spans at least 3.8 billion years of Earth history.
Few other continents,
n o t a b l y West Greenland,
North America,
and
the USSR m a t c h this a n t i q u i t y and continuum of geological history. Whilst
only North America
exceeds
Africa
in the overall
spatial
tent of the rocks that formed between 3.8 and 2.5 billion years,
ex-
in South
Africa alone the rocks of this age have supplied over half of the world's gold.
Most
babwe
(Fig.l.3)
of the world's
lie in the Great
Dyke of Zim-
which is about 2.5 billion years old. Apart
from mineral
production,
the w e a l t h
diverse
and
very
spired
classical
chrome
of
peculiar
information rocks
geological
W i d e s p r e a d occurrences
reserves that
of this
models
and
has
age
in
accrued
from
the
southern A f r i c a
treatises
about
this
highly has
in-
period.
of these early rocks in the African basement,
ei-
ther as ancient nuclei or as relicts, attest to the c o n s o l i d a t i o n of what is n o w Africa,
so long ago. A l t h o u g h models
tory
a
plates
indicate
hotter
than exist today,
planet
with
of this phase of Earth his-
smaller,
thinner
enough evidence is emerging
and
more
mobile
from A f r i c a to sug-
gest
that
tectonic
processes
3.8-2.5 billion
compatible with plate tectonic processes. els are discussed
"
a' - - ~
~
were
generally
minerals,
and mod-
I '°'~Tunisia
Algeria
{
,J
ago
in Chapter 3.
Morocc Spanish Sahor I
years
These rocks,
I I-
~ .,.a
1
EO.O I
Mauritania enegal .,~. . . . . .
] Mall "~,..,....""
\
] Burkina Faso I
i
Niger
I "-'-~.e~•~.fiuinen .;-~r''''%.-,
~,/
[" Tchad
r-
,l,,o./L.,,.." : ~.2 Ivory,~~-c~' ~ , •.,' .~g.io ~ ~
S terr LeOne
,Coast , . e ~ . ~ ,
r
j~ Liberia
Togat j l
I
~
-
v
Equatorial Guine
n
,
C.A.R.
~.
",ero • _ j
~. n i j"Zaire
~ u
...~
Gold Diamonds
Angola
C
Copper
~-- . . . . . ~,.
O Cobalt
", --~
• "r--
~
.
A
',,
J
,
~_j--1"
I
~.
2"
i
'--,,"c~'Z°n~°°i°c' ^""'- "~t
Malaw
l - - - - - .-'AV _ ' ~ '.[ - - F - . . . " ~ . - - v v " ' .~,.,,,,
~% fA
Manganese
v Chrome ]k Platinum Phosphates U Uranium 41,
--
%_
r - " - -'=~~'~ Zambia C,..
Bauxite
_
Rwanda qu.. ~. . • I~ "--t~urunam~, -~/2 • " x,
A
li-- " -"~"
!
~..,, ~.I Ethiopia " "';. ,
t_j--
/"
>i"
! I
Sudan
/~.~ r..-%.~ ~-~-
~bon ,I
• A
I"
Afar and Issas
~
~'~ ~ ' .
~
"-.(3
~ -~ ..'. / r',¢, 11 ~'.~@
,./Botswana )-~.w~o
°_
.
A ~"JAfrlcn°l;"i ~South%~• •l"11".••u(.w.~./' j ~ -Swazilandkesotho
Petroleum 500 Km
Figure 1.3: deposits. The Earth
period
history.
sufficient continental
Outline
between Large
stability
2.5
parts
map of Africa
and of
1.75 billion
southern
and rigidity
sedimentary
basins.
with algal mats accumulated
showing
Africa
some m a j o r
years
to form the sites
Shallow
water
ago
was
had by that
mineral
crucial
time
of extensive
sandstones
and
in
attained intra-
limestones
profusely for the first time in these prim•r-
dial seas. lies
in
But the global
their
gold,
zinc deposits.
importance of these early South African basins
uranium,
manganese,
iron
ore,
fluorite,
copper
and
Chapter 4 deals w i t h this phase of A f r i c a n geological his-
tory. In central and w e s t e r n Africa m o u n t a i n - b u i l d i n g p r o c e s s e s quite similar
to
those
2.5-1.75
of
later
billion
years
geological ago.
periods
Gold,
created
diamond
rich u r a n i u m and m a n g a n e s e deposits
and
major
mountain
manganese
in
chains
Ghana,
in Gabon
(Fig.l.3)
have
to r e c o n s t r u c t i o n s
and
are among the ma-
jor m i n e r a l deposits of this period. Studies continents
on
p a l e o m a g n e t i s m which
led
of past
show that one supercontinent emerged from the above episode of
mountain-building.
From
1.75 b i l l i o n
and
matching
rocks
has
been
between like
rifting
and
this
supercontinent,
mountain-building
(Chapter 5).
Paleomagnetic
and a s s e m b l i e s distinctive
in
was
the
types
mostly
eastern
reconstructions
d u r i n g the period between
rock
it
dated
that
of
years
the
established
parts
950 m i l l i o n
of
Africa and South A m e r i c a formed one continent this long ago. Africa, other
years
the
reveal
fine
examples
western,
of
ancient
of
950 and
central
450 m i l l i o n
that m o u n t a i n - b u i l d i n g
mountain
Now exposed
part
for
Africa
positions years,
processes
and
operated
Chapter 6 is replete with many
chains
in
Africa
at their deep roots
that
in linear
formed
belts
central and eastern Africa, these m o u n t a i n chains,
and the Himalayas,
except
of past continental
in a c c o r d a n c e with m o d e r n plate tectonics.
this period.
quiescent,
during
throughout
like the Alps
formed by the opening and closing of oceans
involving
the c o l l i s i o n of ancient continents. The rifting and v o l c a n i s m which preceded
the opening
of one of Africa's
oceans
years ago created one of the world's per in the Z a m b i a n - Z a i r e a n copperbelt It is p e r t i n e n t
to m e n t i o n
between
950 and
450 million
largest deposits of cobalt and cop(Fig.l.3).
at this
juncture
a major Africa-inspired
c o n t r i b u t i o n to the geological sciences--the theory of Continental Drift. From
the
through
original the
ideas
theoretical
of A l e x a n d e r formulations
practical demonstrations Africa
and
theory.
South
evidence
similarities, ages
Humboldt
has
been
the
of A l f r e d W e g e n e r
of A l e x du Toit in 1937,
America
in
the
focus
of
19th Century,
in
1912,
and
continent
of
the
Continental
workers
paleomagnetism. to w h i c h
Drift
of the unity
from the m a t c h i n g of the present coastlines,
Permo-Carboniferous
modern
and the
the connection between
W h i l s t the e a r l y workers derived their restorations
between both continents on the
von
Africa
base
glaciations
their
other
reconstructions
Reconstructions and
and
South America
of
Gondwana,
belonged
on
radiometric
the
at the
and
geological
southern
end of the
950-450 year interval
(Late Proterozoic-Early Paleozoic),
have furnished
the framework for understanding the subsequent geological history of the African continent. The history of life in the Precambrian and the record of Africa's early glaciations are reviewed in Chapter 7. African sedimentary basins, the subjects of Chapters 8 and 9, record essentially the history of marine transgressions and regressions,
except
along the Atlas and Cape fold belts where mountain-building processes in other
parts
of
the
world
marginally
affected
transgressions climaxed in the Early Silurian, the
Early
Carboniferous.
Paleomagnetic
Africa.
Paleozoic
marine
the Mid-Devonian,
and in
reconstructions
of
the
shifting
positions of Gondwana reveal that the South Pole was located in northwest Africa in the Late Ordovician. This caused widespread continental glaciation
in Africa,
followed
by the extensive
after the melting of the polar ice caps.
Early Silurian
transgression
During the Late Carboniferous-
Permian southern Gondwana moved near the South Pole, thus triggering another widespread glaciation which affected all of southern Gondwana. This marked the beginning of a distinctive phase of continental sedimentation known as the Karoo cycle.
Referred to as Gondwana
formations
in
India,
South America, Australia, Antarctica, the deposits of the Karoo cycle accumulated mostly in continental rifts. They contain extensive coal measures
and
uranium,
the
distinctive
southern
Glossopteris
flora,
and
unusually abundant reptiles with mammal-like features showing transitions towards the earliest mammals and dinosaurs. The Mesozoic-Cenozoic history of Africa was dominated by the fragmentation of Gondwana and the formation of the present continental margins and marginal basins along the Atlantic, the
Gulf
during
of
the
Aden.
Major
break-up
of
Indian Ocean, and the Red Sea and
intracontinental Gondwana.
The
rift
igneous
basins
in Africa
activities
that
formed
attended
this continent-wide phase of rifting, from the end of Karoo sedimentation to the initiation of the East African Rift Systems, are reviewed in Chapter I0.
Chapter 2 The Precambrian of Africa: An Introduction
2.1 Tectonic Framework A means of a p p r e c i a t i n g the vastness of P r e c a m b r i a n crust in Africa relative to o t h e r continents of the world. (1989)
is to glance at the tectonic
The tectonic map of the world r e c e n t l y
shows that Africa
or geological map compiled by Condie
has the largest area of P r e c a m b r i a n crust,
fol-
lowed by North A m e r i c a and Antarctica.
I'~::',:.-~:~i
J
~Archeon
Figure 2.1: Pre-Mesozoi~ showing a p p r o x i m a t e extent Windley, 1984.) But
in
lieu
of
a global
l~roterozoic
drift r e c o n s t r u c t i o n of the Precambrian.
geological
or tectonic
of the Earth (Redrawn from
map which
cannot
be
c o n v e n i e n t l y r e p r o d u c e d here,
the relative extent of the A f r i c a n Precam-
brian
from a highly
can
still
be a p p r a i s e d
schematic
pre-Mesozoic
drift
reconstruction of the continents which simply depicts the Precambrian and Phanerozoic
regions
of the world
(Fig.2.1).
This map
clearly shows
Africa is almost entirely made up of Precambrian rocks, northwestern
and southern margins of the continent where narrow Phanero-
zoic mountain geology
belts
is therefore
in the many African brian rocks.
abut
the
Precambrian
essentially countries
study
on African
a study on the Precambrian,
especially
that are
landmass.
completely
A
underlain
by Precam-
(Fig.2.2).
• ".'-.-..-. ;-::.'....~.,j-.,.-.
IULLEMEDEN : : : : "
II
MESOZOIC AND YOUNGER VOLCAN1C~ ROCKS C. 2 5 0 - 0 Mu
Z A I R E ..~B A S I N " "-:
MESOZOIC 10 TERTIARY AND RECENT BASINS C. 2 5 0 - 0 Ma PHANEROZOIC C. 3 5 0 - 5 0 M a
~
FOLD BELTS
U
LATE PRECAMBRIAN TO EARLY PHANEROZOIC BAS1N C.1000-350 M= PRECAMBR1AN BASEMENT C.3700- 500 Mu
1 ~'~
RIFT
that
except along the
VALLEY •
IOO0
Km
I
Figure 2.2: Geological outline map of Africa showing basement outcrops and basins. (Redrawn from Wright et al., 1985.)
10
The wealth
unparalleled and
the
diversity
complete
of African
span of
the
Precambrian
Precambrian
rocks
age
and
mineral
represented
on the
continent r e i n f o r c e the p r e e m i n e n c e of the Precambrian in Africa. The term "basement complex" is c o m m o n l y loosely used in A f r i c a n countries into
to
refer
to
the
Early
Paleozoic
phosed These
and
many
Precambrian
undeformed
supracrustal
metamorphosed
rocks
and
Late
sedimentary
and
deformed
even
contain
though
"basement"
significant
amounts
Proterozoic-Early
and
volcanic
crystalline
rocks
basement
rocks
range
of
unmetamor-
Paleozoic
sequences.
which
on
sit
rocks
highly
attest
to
the
e x i s t e n c e of vast s e d i m e n t a r y basins during the Precambrian.
Consequently
Precambrian
most
supracrustal
conventional
methods
petrological,
cratons
cambrian
the
and
basin
and
belts.
in
crust
which
context have
E a r l y to M i d d l e P r o t e r o z o i c
studied
using
addition
cover of
in
(Fig.2.2) been
to
the
of
the
structural,
a
is g r o s s l y
physiographic
divisible sense
are u s u a l l y included
Pre-
in the
will mean the stable parts of
deformed
(Fig.2.3).
complex,
thin and
of Africa
"cratons"
not
times
exposed parts of the basement and o v e r l y i n g
geology Although
"platforms"
in the present
Precambrian
been
analysis
Precambrian
mobile
"shields"
"craton",
of
have
g e o c h e m i c a l and isotopic methods of b a s e m e n t geology.
Structurally into
sequences
or
metamorphosed
Precambrian
while platforms
relatively undeformed
shields
since
are the
refer to basement sedimentary
rocks
(Fig.2.2). B o r d e r i n g the cratons are that
suffered
Early mobile
belts
Proterozoic. which
metamorphism
Paleozoic
and d e f o r m a t i o n
Pan-African
but
"mobile belts" w h i c h are composed
experienced
orogeny.
The
deformation
the Late and
in
Archean
the
Proterozoic-
Ubendian and
are the
also Early
"Cratonic nuclei" refers to the smaller parts of the cratons
are of A r c h e a n
age and have not been affected
d e f o r m a t i o n for the past 2.5 billion years As evident ern A f r i c a
during
Limpopo
of rocks
by m e t a m o r p h i s m and
(Fig.2.3).
from Fig.2.3 African cratons differ w i d e l y in age.
contains m o s t l y Archean cratonic nuclei
(Kaapvaal
South-
, Limpopo,
Zimbabwe provinces)
surrounded by younger parts w h i c h became cratons af-
ter M i d - P r o t e r o z o i c
orogenic activity.
In contrast,
smaller cratonic nu-
clei occur in equatorial Africa. Among these is the Tanzania shield. tonic
nuclei
also
occur
in
the
central,
northeastern
and
Cra-
northwestern
parts of the Zaire craton, the bulk of the craton having stabilized after an Early Proterozoic orogeny,
like the West A f r i c a n craton.
The Bangweulu
block in central Africa is e n t i r e l y of Early Proterozoic age and has only locally been involved in major orogenic a c t i v i t y since then. A poorly ex-
11
posed
and
cance
seems
poorly
where Archean what
defined
to stretch
tectonic
signifi-
n o r t h of the Zaire craton as far as J e b e l
cratonic
area
Uweinat,
and Early P r o t e r o z o i c
is r e g a r d e d
of
rocks
as the East Saharan
considerable
outcrop
in the n o r t h e r n
part of
craton.
I e I
c,,'+ ~" '.C. ' " .: ~i.. . . "" '' .. ' l : E *
% I I
iI I I
aS J
I
J
I
I
I # i % t
I
0ROGEN I C ACTiV[T[ES
/
I
I
• ... ":-.- :..~ .-.."
LATE PROTEROZO|CEARLY PALEOZO|C
"'ZC ;-ij
%:.- . . . .
.,, # j .,,
~:.-- EARLY PROTEROZO,C I::'I IV21 ARCH~AN
j
),f.'. :: ./.-:,,;:,., - ... : .'~..-- (".~) : .~
• ". :
IERATONS S
BANGWEULU
ES
EAST
BLOCK
$AHARAN
CRATON
KC
KALAHAR|
CR ATON
T
TANZANIA
CRATON
WC
WEST
ZC
ZAIRE
C RATON
Figure
2.3:
AFRICAN
based
KIBARAN
BELT
UB
UBENDIAN
BELT
CRATON
Cratons
The b o u n d a r i e s defined
K
on
discontinuities.
between
and m o b i l e
cratons
structural, Thus,
the
belts
and m o b i l e
geophysical,
limits
in Africa.
of
the
belts
are
radiometric, West
African
sometimes and craton
clearly
metamorphic have
been
12
clearly defined by the so-called circum-West African craton belt of gravity highs
(Briden et al.,
the earliest
systematic
1981;
Roussel
and L~corche,
1989).
cambrian of West Africa and South America, Hurley and Rand fied age provinces limit
the
(Fig.2.4). utilized
with
southern The
well-defined
margins
same age
to prove
In one of
regional radiometric age surveys across the Pre-
of
the
provinces
the continuity
belts with those of Venezuela,
boundaries craton
were
which
and
nucleus
in South America
of the West African
Guyana and Brazil
to de-
Archean
its
recognized
(1973) identi-
they used
craton
and
and mobile
(Fig.2.4), in one of the
strongest confirmations of continental drift.
1o" ....''
o°
1o"
' .I FR [ C A 1 t el. @/:/~/, ~ ~ L E u nean Trans-Amazo : :" leeJMe~camorphic Rocks •nd ~',vy[~ e~ _ I Granites ¢.a. 1900my k%,k~%~vv ,ee~eeel//////)/77----~?~ /. Liberian,,mat.can A r c ~ ~ ~ ~ / Z//1 ~"
r~L__
Pan-AfricanCarirDaka%k~ I ~// W M°bile b r Seltsc'ct'6OOmyn ~ {
.
B,:,,,.,,,..,
,
E S
/I
I
T
'A
,0"
26oo
I0'~
O° SOU
T
1o" o~
?o"
Figure 2.4 : of Precambrian South America. Since cratons
6~ 1~
s~
2~
Pre-drift reconstruction showing the continuity ages and structural trends across West Africa and (Redrawn from Hurley and Rand, 1973. ) generally acted as the foreland
to the younger mobile
belts, prominent thrust zones constitute major structural discontinuities and tectonic boundaries
around cratonic margins.
lack of well-defined structural, Limpopo
province
metamorphic
isograds
discontinuities, aries and
have
of the Limpopo
Henderson
(1977)
clearly outlines the
and
mobile
the in
adjoining addition
been showed
the major
belts.
the
Furthermore,
that
and
seismological
as
the gross
cratonic
Fundamental
Kapvaal
to
adopted
province.
However,
because of the
age and stratigraphic breaks between the
areas,
differences
northern
Zimbabwe
provinces,
and
gravity
and
southern
in southern Africa distribution
anomaly bound-
Fairhead
of earthquakes
seismicity being confined to also
exist
in
the
thermal
13
structure latter
between
southern
exhibiting
greater
African heat
cratons
flow
than
and
the
mobile
former.
belts, These
with
the
differences
reflect the cold and stable nature of the cratons which have thick lithosphere
in contrast
to the surrounding
mobile
belts which are often
acterized by thicker crust but thinner lithosphere, ments
that
are
shear
zones
intruded
(Black,
by
abundant
1984).
Further
granitoids
attesting
especially
and
sliced
to the
char-
along segby
numerous
fundamental
differ-
ences between African cratons and mobile belts is the fact that the zones of Mesozoic located
rifting which
along
the
led to the break-up
all-encircling
Late
of Gondwana
Proterozoic-Early
(Fig.2.l)
were
Paleozoic
Pan-
African mobile belts.
2.2 T h e Precambrian Time-Scale Cahen et al.
(1984)
of available
radiometric
terpretation
of the tectonic
has provided
the most cogent and comprehensive
for describing
presented
a benchmark
ages
in Africa evolution
the Precambrian
compilation
upon which
and
interpretation
they based
of the continent.
their
in-
Their
synthesis
geochronological
framework
regional geology of Africa and correlating
it with those of other world regions. Various gions
of
Precambrian the
world,
geochronological
scales
present
to
purpose
broad subdivisions prehensive are those
and
time-scales but
neither
will
simply
be
have a
been
proposed
review
attempted
highlight
nor
here.
the
authoritative et al.
discussions (1982),
of
James
for
various
critique
Let
principal
which are tenable for Africa.
of Harland
a
it
of
suffice
age
re-
these
for
boundaries
our and
Among the available com-
the
Precambrian
(1978),
Salop
time-scale
(1983)
and Sims
(1980). According milestones" billion
to Cahen et al.
years)
Early-Middle (M denotes
for
the
Proterozoic
mega,
zoic boundary. those
meaning
The
logical
the most
recommended et al.
scale
Archean-Proterozoic boundary;
significant
by
the
International
(1982) for
(Sims,
the one used by Tankard et al.
present (1982).
Ga
for
(1984) proposed
used
Union
of
here
950 Ma
Protero-
(Fig.2.5A)
Geological
the
are
Sciences
1980) which were also adopted
for southern Africa. our
1.75
for the M i d d l e - L a t e
of the Archean
on Stratigraphy
adopted
years)
"chronological
for giga, meaning one
boundary;
and also Porada
one million
subdivisions
(IUGS) Subcommission by Tankard
(1984)
for Africa occurred at 2.5 Ga (G stands
purpose
The Precambrian is
almost
geochrono-
identical
with
14 AREA 3
A U1 LLI
IuGs o~.'~. ~1 o~0GE~I¢ CYCLES C O
0-
-~_ Alpine
mo
)
~EN L - - L a t e Hercynian O I ~ ~= , - E a r l y Hercynia,'~
EIC LU
r
Z
0
..,
0"5
1.0
3°L
AREA
2
,
11111, i , i ,
3-0
1.0
t i
, ,
3.5
/,t)
Africa-Arabia
~
J
India
China South America
~,.=~, . . . . . 0-5
z _U
2.5
2.0
1-0
z
~
1
1-5
¢"==.Co,l e d o n i a n
! o0 O
,°L
Australia AntarcticaL
1.5
2.0
2-5
3.0
,~,
,~,
3-5
/.'0
er
I.,- LU
AREA 1 ~E,
North America Baltic Shield
-
4000
~
~Z
z 34
Anor0genic ~ Granites ~iij
10 '-' I--
~E ~
;1500 0-5
(3_
1-0
1-5 A6E
Zz
s L2ooo
2"0
2-5
(6a)
3-0
3.5
~D
B
A
=-
5"
ha
~w
~m
u.l o n .Event
in
Equatorial
Africa
LU< event ( West Afric(~) event (West Africa ], Watian [ Equatorial Africa], Musefu (Kasai), Ntem (Cameroon). Limpopo belt
-Liberian ,~
Z
r//.vzlw •
,30C~
~ B~ bertonian JJllSwazilandian
~--J "
/ /
I
/ ~-~i----~..,,
~,.LlTHOS PHE R E J " - - " ' " , , ~
= = -,', l "~
Figure 2.6: Idealized stages of the Wilson Cycle compared with a Pan-African collision suture in the southern T r a n s - S a h a r a n mobile belt of West Africa. (Redrawn from Burke and Dewey, 1973; Candle, 1989.)
18
Whether
formed
by Wilson
g e o d y n a m i c model, terminal
phases
gardless
of
the of
important point that
an
whether
Cycle processes
orogenic
or
not
cycle
the
or a c c o r d i n g
is stressed
can be
various
dated
stages
to
here
some other is that the
radiometrically,
of
the
Wilson
re-
Cycle
are
c o m p l e t e l y decipherable. Due to metamorphism,
m a g m a t i s m and d e f o r m a t i o n most radiometric ages
record the final stages of the collisional part of the Wilson Cycle. However, this is not e x c l u s i v e l y so--dyke events may r e p r e s e n t the early extension and rifting and c a l c - a l k a l i n e magmas the arc phase pre-collision-hence a blur on the collision age. Cahen
et al.
cambrian Fig.2.5A. or
(1984)
orogenic
have
cycles,
provided
the
most
radiometric widespread
These cycles are often referred to as
simply
as
"events"
and
within
"episodes" of shorter d u r a t i o n
an
ages
of
for African
which
are
Pre-
shown
in
"tectono-thermal events"
event
there
can
be
tectonic
. The earliest orogeny identified produced
some of the h i g h - g r a d e m e t a m o r p h i c rocks in the Limpopo province at about 3.8
Ga.
ages
More
widespread
clustering
2.55 Ga.
The
West Africa
events 3.5
where
The
events
the
2.75
Ga
Eburnean
Ga
Africa
by
the
south
Kibaran of
tectono-thermal Paleozoic) tonic
the
cycle
which
activities
Archean 2.95
greenstone
Ga,
2.75
affected m o s t l y
Ga,
the
known
between
which
equator. was
is
equatorial
as
2.27
belts
2.65
the
Ga
and
Africa
and
and the Leonian
Liberian
and
with
Ga,
2.03
event
Ga
in
affected
and was followed at 1.4 - 1.3 Ga and at about
events the
affected in
event
event,
n e a r l y the w h o l e continent, 1.10
3.2 Ga,
the 2.9 Ga event is termed the W a t i a n
and
Africa.
affected
Ga,
later A r c h e a n
respectively; West
around
appear
Another
interludes
have
extensive
Pan-African
the entire
to
event
continent between
been and
(Late except
the
restricted more
to
prolonged
Proterozoic-Early the
crayons.
tectono-thermal
Tec-
events
were limited to m o s t l y anorogenic m a g m a t i s m and rifting. In
spite
of
the
uncertainties
which
surround
the
available
radio-
metric ages in A f r i c a and the inherent problems of the poor resolution of some
of
pears Condie Late
the
to
dating
roughly
techniques, coincide
with
African that
regional of
other
orogenic
episodity
continents
(1989) stressed two m a j o r w o r l d - w i d e orogenic episodes: Archean,
and
another
in
the
Early
Proterozoic
ap-
(Fig.2.5B). one in the
(Fig.2.5B).
The
Kibaran and the P a n - A f r i c a n events affected m o s t l y the Gondwana continent (Fig.2ol) nents.
and
did
not
seem to have
strong
counterparts
in other
conti-
19
Two First, of
notable outside
crustal
belts. where
features the cratons,
weakness
This
of
orogenic
orogenies
which
is p r o f o u n d l y
African
ofttimes true
of
cycles
deserve
r e p e a t e d l y affected were
the
the
sites
Pan-African
mention.
the same zones
of
earlier
belts
of
mobile
East Africa
the P a n - A f r i c a n orogeny was superposed on the K i b a r a n and Ubendian
(Eburnean)
mobile
reactivation rocks,
of
belts older
(Fig.2.3). terranes
The
is
consequence
the
of
this
preservation
of
reworking
older
or
basement
structures and radiometric ages as relicts in the y o u n g e r rocks, a
problem ondly,
that
has
bedeviled
it is evident
Precambrian
from Fig.2.3
that
tectonic
interpretations.
the d i s t r i b u t i o n
Sec-
of orogenic
cy-
cles indicates the p r o g r e s s i v e growth of the c o n t i n e n t with time. Most of the
continental
the
Late
the
widespread
masses
of
Archean-Early Archean
younger mobile
belts.
the w o r l d
Proterozoic; and
Early
A natural
are
believed
and
in Africa
Proterozoic
consequence
to
have
this
relict
formed
is
ages
found
of the W i l s o n
during
evident
from
in
Cycle
the
or oro-
genic cycle is the a d d i t i o n of n e w c o n t i n e n t - t y p e crust to the volume of the
continents,
This process
due
to
oceanic
subduction
of c r a t o n i z a t i o n resulted
Cratonization
is
indeed
evident
and
calc-alkaline
from plate motions
in the
Precambrian
magmatism.
and collision.
crustal
evolution
of
Africa.
2.4 Dominant Rock Types Before
traversing
the vast
of immense time span,
Precambrian
terranes
of A f r i c a
in an odyssey
it is useful to distil out of the m e d l e y of Precam-
brian rocks a few salient characteristics of their c o m p o s i t i o n and structure. Wright
In this
regard
et al.
the
stressed
cambrian
rocks
complex,
supracrustals,
In
according
to
intrusions, to
the point
can be grouped
addition
granitic
synthesis
their
of W r i g h t
et al.
that regardless
into a basic
(1985)
is germane.
of g e o l o g i c a l
stratigraphy
and granitic intrusions.
grouping
them
Precambrian
ages.
into
rocks
As already
basement,
can
also
pointed
out
be
supracrustals broadly
of m o d e r n (1989)
rocks,
most of which exhibit
orogenic
enumerated
terozoic rocks.
belts the
key
formed and
as
features
a result
contrasting
the A r c h e a n - P r o t e r o z o i c
that are similar
of
and
categorized
b o u n d a r y separates A r c h e a n rocks with different c h a r a c t e r i s t i c s terozoic
age Pre-
of the basement
plate
features
of
from Proto those
tectonics. Archean
Condie
and
Pro-
20
Archean
crustal
provinces
are
dominated
by
h i g h - g r a d e rocks and g r a n i t e - g r e e n s t o n e belts. trast,
are h i g h l y varied.
two
major
rock
P r o t e r o z o i c rocks,
Seven m a j o r rock associations
types: in con-
have been recog-
nized in them. These are: a q u a r t z - p e l i t e - c a r b o n a t e a s s o c i a t i o n which was characteristic
of
and mafic dykes amounts
of
ophiolites genic
platform
basins;
of continental
greenstones
which
are
like those of m o d e r n
granite-anorthosite
bimodal
volcanic-arkose-conglomerate
rift or aulacogen similar
ocean ridges
complexes
which
tectonic
to m o d e r n
setting;
volcanic
or b a c k - a r c
were
rocks;
basins;
anoro-
restricted
to the Middle
Proterozoic; mafic dyke swarms; and layered igneous intrusions. and
cratonic
Early
lithological
Archean,
non-existent
assemblages
continental prior
to
rift
about
and 2.0
have
been
ophiolite Ga.
In
recognized
While arc
back
assemblages
Africa
small
arc
are
to
the
rare
ophiolites
or
became
w i d e s p r e a d during the P a n - A f r i c a n o r o g e n y as a result of the operation of the W i l s o n Cycle in m o s t parts of the continent. Because of p o s t - o r o g e n i c isostatic uplift and c o n s e q u e n t erosion many A f r i c a n P r e c a m b r i a n orogenic belts are exposed at v e r y deep crustal els
(Fig.2.6
assemblages lost
(Burke
belts
such
F). of
convergent
and as
Consequently Dewey,
the
plate
1973).
Limpopo,
most
of
the
margins
and
Collision
Mozambique
characteristic plate
zones and
in
collision deeply
Benin-Nigeria
lev-
stratigraphic sutures
eroded
are
mobile
provinces
are
r e p r e s e n t e d by cryptic sutures and h i g h - g r a d e m e t a m o r p h i c rocks. The above outline of some of the parameters that will be used in subsequent chapters to discuss the Precambrian geology of Africa leans heavily on plate tectonics,
even though this model has h a r d l y been presented
here in any c o m p r e h e n s i v e or systematic manner. our
understanding
of
the
processes
that
m i n e r a l d e p o s i t s during P r e c a m b r i a n times cambrian
metallogeny
in Africa
has
simply
Plate tectonics also aids
controlled (Sawkins, been
the
distribution
1990). Hitherto,
viewed
as
age: older cratons contain important gold, iron, manganese,
a
Pre-
function
chromium,
of
of as-
bestos and diamond deposits; while younger mobile belts are characterized by m a j o r deposits of copper, bium-tantalum
(Clifford,
lead,
1966).
zinc, cobalt, tin, beryllium,
and nio-
Chapter 3 The Archean
3.1 Introduction At the very beginning of geological time the Archean eon is very significant. A complete range of Archean rocks is represented in Africa, which,
for example komatiites
from this perhaps
continent.
Being
some of
and greenstone belts, were first described
largely underlain
stood the best chance of preserving
by
stable
cratons,
Africa
the Archean
geologic
record
either in isolated cratonic nuclei completely removed from later orogenic activities,
or as relicts that had survived in the younger polycyclic mo-
bile
(Fig.2.3).
belts
A
nearly
complete
span
of
Archean
times,
about
3.9 Ga to 2.5 Ga, is represented in Africa, where like in West Greenland, the oldest rocks on Earth are found. In terms of overall second
after
spatial extent,
those of North America.
Africa and Zimbabwe alone, mineral their
wealth
(gold,
world.
Furthermore,
in the Republic
the diversity of Archean rocks,
diamond,
paleontological
the Archean rocks of Africa come
However,
record
chromite,
are
cobalt,
their enormous
uranium,
so far unmatched
of South
etc.),
anywhere
else
and
in the
the oldest well preserved cratonic sedimentary basins
are found in South Africa which have furnished the earliest reliable record of the paleoenvironmental dial
Earth.
been
the
planet,
It is hardly
cornerstone and
to
conditions
surprising
that prevailed
therefore
our understanding
consequently
this
region
has
of
that the
inspired
on the primor-
southern early
Africa
history
classical
geological
models and treatises on the Archean eon (e.g. Condie 1981; Nisbet, In
Africa,
like
elsewhere,
study of the Archean. time
duration,
lasting
half of the remaining
First, for
of which
the Archean
about
1.3
peculiar
problems
years,
time.
1987).
confront
has been assigned
billion
span of geological
tain only algal stromatolites by means
several
which
Since Archean
has
of our
the
a very long is
nearly
a
rocks con-
and doubtful bacteria and no index fossils
stratigraphic
subdivisions
and correlation
can be es-
tablished,
Archean regional stratigraphy is therefore very imprecise and
uncertain,
especially in a continent like Africa where vast geographical
areas and thick rock sequences belong to this interval. ing, field mapping, stratigraphic record.
But
structural analysis, petrology,
analysis are the primary tools further compounding
the problems
Radiometric dat-
and geochemistry,
for unravelling
and
the A r c h e a n
of interpretation,
are
the
22
structural
complexities
found in ~ucchean terranes,
w h i c h are usually the
products of m u l t i p l e episodes of deformation, m e t a m o r p h i s m and magmatism. Some v e r y p e c u l i a r rock types also occur in the A r c h e a n which in the absence their
of m o d e r n
origin.
analogues
These
include
chean g r e e n s t o n e belts. of rocks mental
such as
had
and
banded
from
speculations
iron-formations,
about
and Ar-
the absence or r a r i t y in the Archean
attest to rather unusual
there was
of
and evaporites w h i c h
compositions
since
a wealth
komatiites,
v e r y much unlike m o d e r n
different
exist;
evoked
Conversely,
carbonates
indicators,
least were
have
times.
are good
conditions, Archean
modern
times;
no v e g e t a t i o n
cover,
which to say the
oceans
the
paleoenvironand atmosphere
biosphere
the rates
did
not
of w e a t h e r i n g
and erosion m u s t have been p r o f o u n d l y greater. The A r c h e a n physical surrounding is believed to have suffered greater meteoric
impacts;
amounts
of
A~chean
ocean
down
to
heat
must
the
compatible
there
were
emanated have
more
been
lithosphere.
with
the
volcanic
from the mantle, simmering, A~chean
subduction
of
eruptions; conditions
even
plate
ocean
below
and in
the
tectonic
crust,
since
and
higher
around
nascent
processes
differed
from
the
crust, though
the
later
P r o t e r o z o i c and Phanerozoic ones as evident from the e x t e n s i v e occurrence of komatiites, A
tonalites and trondhjemites.
striking
(Fig.3.1)
is
feature
the
of
Archean
remarkable
rocks
similarity
of
in
all
their
parts
gross
of
the
world
lithologies.
Two
major lithological assemblages today c h a r a c t e r i z e the Archean:
greenstone
belts
consist
and
high-grade
metamorphic
terranes.
Greenstone
thick and d e e p l y infolded compact dark-green
belts
altered basic
of
to ultrabasic
p r e d o m i n a n t l y v o l c a n i c s and associated sediments which have suffered lowgrade m e t a m o r p h i s m and intensive granitic intrusions. are the h i g h - g r a d e bolites
terranes
and m e t a s e d i m e n t s
morphism,
comprising various granitic gneisses,
which
have been
often at the granulite
granite-greenstone
and
S h a r p l y contrasting
facies.
high-grade
subjected
The structural
terranes
are
amphi-
to high-grade meta-
often
relationships
uncertain
so
of
that
their relative ages are often debatable. The Archean province and
regional is one of
3.1 Ga;
pattern
in which
of
South Africa were followed
by
the
those of the Zaire-Tanzania bilized belts
at
the
the
end
of
crustal
evolution
the g r a n i t e - g r e e n s t o n e £he earliest Zimbabwe
in Africa
terranes
to
of
stabilize,
province
at
about
during
the
the Kaapvaal
between 2.5 Ga;
3.2 Ga while
craton and the W e s t A f r i c a n craton also sta-
the A r c h e a n
(Fig.3.2).
Regionally
the
greenstone
show a n o r t h w a r d d e c r e a s e in Africa in their state of preservation
23
and
in
after
their
lithofacies
the Archean
the Kalahari
reduced
development.
Repeated
the preservation
metamorphism
of
greenstone
craton and caused the preponderance
granitoid
terranes
provinces
of Africa are considered
in the northern
cratons
during
belts
of high-grade
of Africa.
Below,
and
outside
gneiss and the Archean
from the south to the north,
beginning
with the Kalahari craton where they are best preserved and better known.
~
Archeon Provinces
Figure 3.1: Archean provinces of the Earth: i, Superior; 2, Slave; 3, Wyoming; 4, North Atlantic; 5, Guyana; 6, Guapore; 7, Sao Francisco; 8, Kola; 9, Ukrainian; 10, Anabar; ii, Aldan; 12, Chinese; 13, Indian; 14, Pilbara; 15, Yilgarn 16, Kaapvaal; 17, Zimbabwe; 19, NE Zaire Craton; 20, Kasai; 21, NW Zaire Craton; 22, Liberian; 23, Mauritanian; 24, Ouzzalian. (Redrawn from Condie, 1981.)
3.2 KalahariCraton The Kalahari Zimbabwe state belt
craton
craton
(Botswana, (Fig.3.3).
comprises
to the north, Zimbabwe,
separated
South Africa)
But to avoid
order to emphasize
the Kaapvaal
border,
the redundancy
the lithologic,
craton
to the south
in the middle,
and the
around
the tri-
by the Limpopo
orogenic
of the term craton,
structural,
metamorphic
and in
and radiomet-
24
ric age similarities terms
tectonic
here.
The L i m p o p o
hari
craton
and d i s t i n c t i v e n e s s
province
(Kr6ner and Blignault,
province
because
of each P r e c a m b r i a n region,
it was
is included stabilized
1976)
or domain
are used
in the A r c h e a n part of the Kaladuring
the
Late
Archean
tectono-
thermal events w h i c h also affected the Zimbabwe p r o v i n c e to the north.
N.W. ZAIRE R
N.E. ANGOLA `'/ SHIELD
~
~ ~
Cratoni¢ since ¢. 2.5 Go.
TANZANIA
~,'_
the
"o. : " ." ~. """ ZIMBABWE
LIMPOP0 :KALAHARI
2-5 Go. crotons under youngercover
~KAAPVAAL
Reworked Archean during later events
Figure 3.2: Distribution of Archaean Africa. (Redrawn from Cahen et al., 1984.)
cratonic
nuclei
in
25
Unlike
the
Kaapvaal
metamorphism ince which
with
and
Zimbabwe
stronger
is believed
provinces,
deformation
to represent
facies
in the Limpopo
zones
prov-
of the Archean
(Coward et al.,
geo-
of the
1976; Burke
1977). The tectonic link between these three provinces has mani-
fested in the progressive of the
Zimbabwe
Limpopo
The
on
below
blages
-
the
belts,
and
all
sides
of
the
intrusive
by
towards
younger
Kaapvaal,
according
high-grade
the
provinces
As shown by Tankard et al.
geology
summarized
increase in metamorphic grade from the borders
and Kaapvaal
province.
surrounded
belts.
granulite
shearing and overthrusting
Kaapvaal province over the Zimbabwe province
is
of
predominate
the root
suture along which there was repeated et al.,
rocks
to
the
gneissic
craton
Pan-African
mobile
and
Limpopo
principal
The
zone of the
Proterozoic
basement,
granitoids.
central
the Kalahari
Zimbabwe
three
the
(1982)
the
domains,
Archean
assem-
or
schist
greenstone
earliest
cratonic
are
rock
sedimentary
basin in the Kaapvaal province is also discussed. 3.2.1 Kaapvaal Province Detailed
investigations
province
(Fig.3.2)
Anhaeusser,
by
of the Archean several
workers
greenstone (e.g.
belts
Viljoen
of the Kaapvaal
and Viljoen,
1969;
1971; Tankard et al., 1982) has rendered these among the best
known Archean greenstone belts in the world. However,
the global signifi-
cance
greenstone
belt of
lies in its excellent geologic exposures,
the lo-
of
the
Barberton
the Kaapvaal province,
Mountain
Land,
the principal
cation of some of the earliest evidence of life, and in the fact that the Barberton
Belt
is
the
type
locality
of
komatiites,
the
unique
Archean
magnesian ultramafic lavas. Together province,
belts.
the
more
the Kaapvaal
posed Archean granulites
with
rocks
northerly
supracrustals
in the Kaapvaal
greenstone
belts
of
the
comprise only about i0 % of the exprovince,
the vast
remainder
and granitoids which engulf the narrow keel-shaped
Although
their structural
Kaapvaal
relationships
being
greenstone
are very complex,
it has
been suggested that the gneissic terranes were the contemporaneous
sialic
basement which existed during the accumulation of the oceanic volcano-sedimentary
sequences
of
the
greenstone
belts
(Paris,
1987).
Since
they
contain the oldest rocks in the Kalahari craton and are also more extensive,
the
high-grade
rocks
are presented
first,
followed
stone belts, and the late-or-post-tectonic granitoids.
by
the green-
26
Figure 3.3: Exposed part of the Kalahari Craton. i, Cover rocks; 2, Igneous complexes; 3, Greenstone belts; 4, Granites and gneisses; 5, Margins of mobile belts. The numbered greenstone belts are: i, Salisbury-Shamva; 2, Makaha; 3, Gwelo; 4, Midlands; 5, Mashaba; 6, Victoria; 7, Belingwe; 8, Buchwa; 9, Shangani; 10, Bulawayo; ii, Gwanda; 12, Antelope; 13, Tati; 14, Matsitama; 15, Sutherland; 16, Pietersburg; 17, Murchison; 18, Barberton; 19, Amalia. (Redrawn from Cahen et al., 1984.)
Ancient Gneiss Complex This is a collective
term for the basement gneisses of the central Swazi-
land
area
the
al.,
1982).
the north
south
of
Barberton
Mountain
greenstone
belt
(Tankard
et
Similar gneissic terranes which are less well known, occur to of the
Barberton
Mountain
Land
(Fig.3.3).
The Ancient
Gneiss
27
Complex,
as summarized
decreasing age,
age),
by Tankard et al.
the Mkhondo
comprises
(in order of
the Bimodal Gneiss Suite, migmatite gneisses
the Dwalile Metamorphic
trusive Suite,
(1982),
Suite,
of unknown
the Mponono
In-
lenses of homogeneous medium-grained quartz monzonite,
and
Valley Metamorphic
the Tsawela Gneiss,
Suite.
The gross
structural
relationship
between these gneisses is one in which the 3.5 Ga Bimodal Gneiss Suite of interlayered
siliceous
low-potassium
leucocratic
tonalites,
and
the
amphibolites of the Dwalile Metamorphic Suite are intruded by the Tsawela biotite-hornblende
tonalite gneiss which has been dated at about 3.3 Ga.
The Mkhondo Valley Metamorphic
Suite of unknown age,
amphibolites,
while
the migmatite
modal
Suite
within
Gneiss
which
gneisses
appear
the Mponono
consists of layered
to grade
Intrusive
into the Bi-
Suite
occurs
as
sheet-like intrusions of hornblende anorthosite. Structurally the Ancient Gneiss Complex shows a very complex superposition
of
isoclinally
several
generations
folded
gneissic
of
strong
layers
and
deformation
quartz
veins
which
and
in
produced which
the
axial-planar schistocity in the Bimodal Gneisses are cross-cut by the intrusive contacts of the Tsawela tonalite gneiss. Petrologically
and
geochemically,
Gneiss Complex of Swaziland, terranes elsewhere, the
high-grade
supracrustal
the
various
like their counterparts
are tonalitic in composition.
metamorphic
parent
end-products
materials,
desitic
magmas
evokes
similar
magmas
are
rocks
the
comparisons
generated
of
a
preponderance modern
(Nisbet,
1987).
the Ancient
in Archean gneissic
Although they represent
variety
with
of
of
of
magmatic
tonalitic
tectonic Since
and
regimes
these
and an-
where
tonalitic
gneisses are so voluminous in Archean terranes and will be encountered in all
the African
provinces,
chemical
characteristics
possible
origin.
low initial suggest
it is important
of the Kaapvaal
The Bimodal
87Sr/86Sr ratios,
the
derivation
(Tankard et al.,
of
to mention
gneisses
which
Suite and the Tsawela low 518 values, their
parent
the
salient
relate
tonalite
geo-
to their
gneiss
show
and low K20 contents which
magmas
from
mantle
sources
1982), possibly from the partial melting of sinking ba-
saltic crust in a manner that evokes analogy with the generation nalitic batholiths
above modern
subduction
zones
(Nisbet,
1987).
of toThe ab-
sence of intermediate rocks in the Bimodal Gneiss Suite rules out its derivation
from the fractionation
of basaltic
parent magmas.
However,
the
high Rb/Sr and K/Na ratios, enrichment in light REEs, slight depletion of heavy REE s and the prominent negative Eu anomalies Metamorphic
Suite
in the Mkhondo Valley
suggest that these could have originated
partial melting of pre-existing trondhjemitic-tonalitic et al., 1982).
later by the
gneisses
(Tankard
28
The Barberton
Of the
Greenstone
Belt
six greenstone
Barberton
belts
in the Kaapvaal
berton belt extends
as a wedge-shaped
tween the Drakensberg escarpment east.
The
greenstones
Supergroup.
This
predominantly by
a
into
minor
group
shale
the
of
interbeds.
belt
for over 140 km beSwaziland
of a thick v o l c a n o - s e d i m e n t a r y
pile with
graywackes, sequence of
are
of
termed
at the base,
shales
and
followed
chert,
conglomerates,
slight
supracrustals
by means
chain
the
The Bar-
the
Because
these
structures
Barberton
to mafic volcanics
cyclical
facies,
mountain
(Fig.3.4),
(Fig.3.5).
in the west and the Lebombo Range in the
consists
sequence
another
greenschist dimentary
of
ultramafic
cyclical
upward
province
belt is the largest and the best preserved
passes
quartzites
metamorphism,
have retained
upward
which only
to
with lower
their original
of which their paleoenvironments
se-
have been
p r e c i s e l y determined. The Swaziland
Supergroup
underwent
several
episodes
formation in which the entire sequence was repeatedly to the
extent
that
cating
enormously
land Supergroup.
Therefore,
belt
gives
an
stone
belts.
The
about
3.2 Ga
(Cahen
rence plex
of
of granitoid in
one
suggests the
those
regional age
that
sialic
posited
of
the
basement
set
structural
of the
basal
similar tectonic
Gneiss
upon which
the
These are,
predominantly
luvial-deltaic northern
thickness of
for
this
granitoids to those slivers
Complex
thus
granitoids and
ranges ages
from
of
its
the
the volcano-sedimentary
3.5 Ga to vol-
The occurGneiss
Swaziland
equivalent
green-
basal
of the Ancient
or its
Swazimerely
other
respectively°
in
compli-
of the
of the Barberton green-
"sea"
the
and thrust,
repeated,
Supergroup on
folded,
de-
Com-
Supergroup
was
probably
sequence
was de-
1987).
(Fig.3.6).
middle
a
setting
based
Three major lithostratigraphic group
true
in
Swaziland 1984)
surrounding
gneisses
the Ancient
(Paris,
keels
et al., the
of the
are
the usual description
synclinorial
overall
and
successions
the determination
stone
canics
as
stratigraphic
of intensive
Moodies
graywackes
Group.
part of the Barberton
sin was deepest,
sequences make up the Swaziland
the lower ultramafic-mafic The
Fig
Tree
entire
belt
Group;
and
supergroup
(Fig.3.7)
where
Onverwacht
is
the
SuperGroup;
upper
thickest
al-
in the
the depositional
ba-
and thinner in the south which apparently was undergoing
uplift and thrusting at the time the northern part of the basin was filling.
The
thickness
South
African
Committee
for
of 24 km to the Swaziland
ble even on stratigraphic cal evidence,
upon which
grounds Darracott
Stratigraphy
(1980)
assigned
a
Supergroup which was highly improba-
(Burke et al., (1975)
1976),
had earlier
and on geophysibased
an estimate
29
of
8 km.
Also,
effects
of
Paris
nappes
stratigraphic
(1987) and
sections,
previous
estimates.
proposed
by Paris
gave
polyphase
revised
stratigraphy
The (1987)
is s h o w n
LIHPOP0
BELT
in
Supergroup
"T
++4"
•
,
e
*
m
A L A PLUTON v ~ ~ ~~. ' ~+ 3 ~I M"~HPAGENI-TYPE | ¢r:=~ • "+ + + .
Op,j + , • ~:~.%.*~Rooiber-
e + ~ + e + . l + ÷ + l . e
.
"~'~:.>~.+ e '~ Vrybur~, +IV
\
+ Ce Klerksdorp ~ u-
! ]
. . . . .
~
HLIBA STOIZ BURG VALLEY~.'~ ~ . ~ / ___ ~ , ~ S I N C E N I PLUTON HBABANE P L U T O N ~ .~HOOISHOEK PLUTON
NGWEMPISIPLUTON~.~ ~KWEITTA PLUTON SICUNUSA PL UTO N ~-~--~
/
PRE-MOZAAN
Non-Granitic
Rocks
.._.._i;~
~
tochiel
Post-Waterberg (? ! Granite
~
Nelspruit
Bushveld
~
Gronodiorite
fira.ite _+1-95 by
Granite .~ 3-0 by Migmatites Suite
Gaborone and Palala Granite z2.3 by ? ~
Tonalitic
Oiapirs
Mooishoek
Tonolitic
Gneisses
Granite
and
3"2 by Gra.ite
Mpageni
Granite z2"65 by
Granites ( undifferentiated )
Kwetta
Granite
Granite Plutons [undifferentia-ted )
Dalmein
Granite ± 2-9 by
Greenstone
F i g u r e 3.4" Outline geologic ( R e d r a w n f r o m Condie, 19°81. )
of
Swaziland
many
account
,
+
the
the
into
~*:'~:.'::'..~"
,
I
repeated
taken
the
~+ +.+'.~t- - =====~, + ~ ~..++;~J , + ffusl"e.ou g 0~ ~'++ SALISBURY K0P '+g~ t ~ ::~ -KAAP VA+ L.E~'+.~+~ PLUTON ~V2.'+%'+'+7 -~re;orla D I A P I R ' ~ * *+.~.~r , -" /.;%... % % % % * +.-.-+( J++2 ~Ventersdor¢~" N E " H . . . . . ' ~ :-~.:'.:i~ + ' ~ P'+V-~ c~, uuu~t. ~ DALHEIN PLUT ON __ " ?~
/ ; •
(P~x
had
removing
3.1.
--'-~---
- "+',- • _ " ~~ MMATHETHE,:~uooorone
j
after
which
of
:
-HOSHANENo
8 km
h a d not b e e n
in T a b l e
O%?.. +
/"
of
deformation which
0%., ' '
""
estimate
a factor
~.%o~
?
an
Table
3.1
lower
three
ultramafics
shows of and
that which
the
0nverwacht
belong
mafics
to
the
(Fig.3.6).
map
of
Group
Kaapvaal
comprises
Tjakastad The
belts
upper
Subgroup three
province.
six --
formations, a
sequence
formations
are
30
mainly calc-alkaline volcanics belonging to the Geluk Subgroup. A regionally persistent unit, the Middle Marker occurs at the base of the Geluk.
• Usushwana lr [ ~ Gran|toids (3.0 Early Potassic Dalmein type Granodiori~.e BOlmonskop r~Tona[itic P|ut! Ancient G~els 1 Satisburykop 2 Daklein Plut 3 Jamestown $ 4 Slolzbury Sy 5 Saddieback S 5 Eureka Sync 7 Ulundi . SWAZILAND SUPEI~ ~ Moodies Gro~ Fig Tree Grol Geluk Subgro Tjakastad Sub! Ultrab~$ic C Figure 3.5: Outline geology of (Redrawn from Tankard et al., 1982.)
the
Swaziland
Supergroup.
The Middle Marker is 10 m thick and comprises microcrystalline and
chert
with
significant prominent
hematite.
The
coarse-grained
minor
rock
type
upper
part
water-worked throughout
of
the
detritus.
the
Swaziland
Middle
chert
Marker
Cherts
are
Supergroup
very
but
they
are predominant in the Onverwacht where in the Swartkoppie Formation, example, their
they are up to 400 m thick
intriguing
they
contain
origin,
the
carbonized
(Tankard et al.,
Barberton
spheres
which
cherts are
are
1982).
Apart
significant
believed
to
be
has
a
for from
because
among
the
earliest microfossils. The Onverwacht hypabyssal exhibit wide
rocks
pillow
range
of
Group contains predominantly volcanics which
erupted
largely
structures.
Although
composition
from
under
their
ultramafic
chemical to
and associated
subaqueous
conditions
analyses
felsic,
by
far
notable are the highly magnesian lavas known as komatiites, Komati
Formation
matiites
of the Onverwacht
are ultramafic
komatiitic
basalts
Supergroup
rocks with an MgO
are those with MgO
is the type
content
in the range
1987). They commonly exhibit spinifex or quench textures.
a
the most
of which the sequence.
of about of
and
indicate
Ko-
18 %, while
10-18 %
(Nisbet,
Chemically, ko-
Figure
3.6:
Stratigraphic
columns
for t h e S w a z i l a n d
Supergroup.
:'--i--_: i~
(Redrawn
Cl.tho~ / F'i~
. . . . .
from Candle,
,,it
1981.)
ONVERWAEHT GROUP FI~ :16 1REE GROUP )nv~r~cht Anticline and Kromberg Syncline HanDlES GROUP 5213 m -PI;.~ -I~1~-"" I ~ ' Stolzbutg Syn£line Ulundi Syncllne Eurekn Syncline Eth. __915m. . . . . . . . L~\ conglommate ~ 31&0m~~ : c - : ~^oo q~tzific_ . . . . . . . . . .sandstone ..... conglomerate__ Cycle Kromberg Fro. ~ neccia #jlomemte lava ?~ ' Bavioaskop ~:'-.:--: sandstone, subgreywocke - - - 3 rr/: / 1920m / /- m \ Fo~'mation i i . - . 9 fit. shale Cycle he-grained tufts gre~a~ \ I '~es® ~ qu_°:_~_it:_c°_n_gL°P_e'Ate ...... Znd rad,ng downwards HAFIC TO 1to coarse-grained \\ Joe's-Lu'¢~c"~k,"/; subgreywacke, grit. shale Cycle uffs ELSIC UNIT 'l ova '~:.~i-~':~';~'RENACEousF'm~m,~v~. . . . . jaspilita bonded i~onstone amygdaloidol |o~ heft breccia quartzite conglomerate -_-_= iEDIHENTARY . . . . orkgreen shale . . . . . . . . . . .. . . . ) Formation ,ft &8&Sin oncly shale shale [ " ~~, banded ironstone C'y~'ie sasp hurt g,eywac~ ........ sub~,eyvacke. onded fm'~uginous chert iddle Mor~rJ~__~.~ chert -----"160 . . . . . shale =---telspothic quartzite hurt .:.-_ ~C.-_. _~[~; i~:\~: calcareous quartzite ~-~.}i 0~ . . . . . , ~ b_~o_, ¢o~,o.e~_o~ . . . . . . . . . . . . i Komoti Fro. chert "---~- 700m reywocke LOWER . . . . . . . . . . hale gre ywacke :.~. c ULTRAHAFIC :l UNll lheespruit shale ...... I~l intrusive tonolitic gneiss ¢ .--:!.:'-"~: l chert with minor shnle and limestone ::,~"-" ¢g felsic Iovos, tufts, agglomerates and porphyries ¢ ~2 'ey~ocke grey~uc ke ! m , E~ mafic pyroclasts, agglomerates, pillow breccios, e t c Formarian te-i ¢ .~ 1213&m hale mofic lawns me~a- tholelites Hiddle Marker:. chert, limestone and shale I ~ felsic tufts ( often siliceous anti aluminous). I-1 motic lavas(primitive metobosatts and pyroclasts). E~I ultramofic lawns ( metoperidotitesL
o
32
matiites
have
high
while komatiitic alkalis, rocks,
Ti,
CaO/A1203,
basalts
Nb,
Apart
later,
and
low
Ti
compared
basalts,
Mg, Ni, Cr and low
from their economic
provide
to
the evidence
importance,
these
for the composition
of the Archean mantle.
Saddleback Syncline "80 km t
Swaziland Border ,,, t
SOUTH
Ni
also exhibit high CaO/AI203,
Zr, Fe/Mg.
as will be shown
and temperature
Cr,
~
Present Outcrop
Limit ~ _ _ . . ~ A ~ ~.__...~-
,,.
..
Eureka - UlunO|- S t o l z b ~ g Syncline f NORTH
, ,~~~-_----~--.
~
4000 m
•
-3000 •
. ,
.
-
-2000
-IOOO ~- o o o b.
==================================== -o [ Onvorwocht I" : :,:.~ Conglomerate
Iron-Formation/Chert
Texturally-immat ure Arenlte Textually-Mature Aronlte
Wacke Volcanic Rock
SIItstone- Mucletone .t
Transgressive Surface
,! Interbeddod Sandstone- Mudstone
Unconformity
Figure 3.7: Stratigraphic cross-section showing relationships in the Swaziland Supergroup. (Redrawn from Eriksson et al. (1988.) The Fig Tree and Moodies nostic lithologies (Fig.3.6) mainly
three
of graywackes, begins
graywackes, The
and primary sedimentary
contains
Formation
with
shales,
overlying
breccias
Groups are sedimentary
formations.
shales a
minor
and minor
massive felsic
Schoongezicht
chert
structures. lower
chert;
The Fig Tree Group
Sheba
Formation
whereas
unit,
but
Formation
is
composed
consists
the middle
consists
tuff and some ferruginous of
Belvue
mostly
chert felsic
of
bands. tuffs,
along the
northern part of the Barberton belt in the Eureka and Stolzburg
synclines
which
were
the
These formations
with diag-
are best developed
(Fig.3.5)
and agglomerates.
The
sequences
deepest
which the clastic lithofacies
(geosynclinal)
parts
of
(Fig Tree and Moodies Groups)
the
basin
in
are thickest.
33
Table 3.1: S t r a t i g r a p h y of the Barberton g r e e n s t o n e belt based on the South A f r i c a n Committee for S t r a t i g r a p h y (A), and as revised (B) by eg. Paris (1987).
o
3 sedimentary cycles (conglomerates, q u a r t z i t e s , shales, greywackes, jaspiLites, magnetic shales )
E
Sch oongezicht Formation Belvue Road Formation Sheba Formation
I-- D
~ .0 ~
~
cherts, shales. -- greywackes banded I ferrugin0us c h e r t s
o. Swart.koppie F o r m a t i o n i Kromberg F o r m a t i o n
o'~
mafic to felsic volcanic cycles, c h e r t s
Nooggenoeg Formation i
.¢ ~.
Middle Marker (chert)
-~
~ ~ o Komati Formation c, Theespruit Formation ~'~ Sandspruit Formation i~.tn
o o
"r
! F5 q u a r t z - arenite, siltstone F4 conglomerate in matrix of both I chert and single crystal quartz I grains S c h e r t - q u a r t z arenite , I i conformable to unconformable I
MALOLOTSHA GROUP
-2km
A.
~_ uttramafic to mafic volcanic c y c l e s , cherts
CONTINENTAL ALLUVIAL FAN
I I I
DIEPGEZET GROUP ~o ~v ~2km L~ u
F3 c h e r t - arenite, conglomerate in matrix of chert grains I It F2 ferruginous and t u f f a c e o u s l sittstone, ferruginous c h e r t I = arenite
"~ FI jaspilites,
ferruginous chert . ferruginous t u f f , shale and sittstone , conformable (?)
.= E'
ONVERWACHT GROUP -3kin
o "D
OCEANIC PROGRADIN G SUBMARINE FAN
! ! 1 i
I
votcanictastic unit (distal and i proximal turbidites facies and I subaeriat facies), mafic iI and uLtromofic unit I !
OPHIOLITE ARCHEAN OCEANIC
B.
CRUST
Unconformity or tectonic contact
5RANIT01D
I I I
SIALIC
CRUST
34
The
deepest
part
of
where
the graywackes
Also,
in
clastic which
the
this and
Eriksson
is
Belvue
with
represented
units
there
intercalated
et al.
by
shales display typical
overlying
deposits
basin
(1988)
Sheba
Formation
Bouma turbidite
are
banded
the
prograding
facies.
fine-grained
iron-formations
and
interpreted as the lower submarine
basin floor, and basin slope environments
chert, fan and
(Fig.3.8). The presence of soft
sediment folding in the iron-formations suggest gravity displacement in a slope environment. Fig
Tree
Group
Schoongezicht northward south.
An overall
and
the
Formation
suggest
progradation
The
of
conformably
glomeratic
lithofacies
upward
presence
coarsening of
basin
proximal
overlying
filling
more
and
Group
during
top part
in
the
shoaling
landward
Moodies
was deposited
when deltaic and alluvial
of the
conglomerates
due
sediments
with
its
to
the
from
strongly
this phase of basin
conditions were established
of the
overlying the con-
filling
in what had been a
deep turbidite basin (Fig.3.8). Sedimentary cessfully mental
structures
utilized
and
(Eriksson
interpretations
textural
et
al.,
characteristics
1988)
for
have
detailed
of the Moodies Group lithofacies.
been
suc-
paleoenviron-
In the northern
Eureka syncline the contact of the Moodies Group with the Fig Tree Group is
gradational,
poorly
sorted,
with
conglomeratic
beds
these
conglomerates
are well
ternal grading and weak imbrication, of
plane
or
cross-stratified
environments glomeratic modern
which
sandstones
which
longitudinal
bar
facies
conglomerates
are more
Formation,
abundant
northward
dominant
in
the
are
while
with
north.
thinly
prominent subarkose
shale
and
Overlying
deposition
which
in
The lower con-
is similar to the contains
sandstone beds in the southern
and quartz
banded
the
interbedded
in-
pebbles,
In the Clutha Formation these
are represented by cross-bedded
of the Clutha
Formation
suggest
(lower part of the Clutha Formation)
conglomeratic
Although
displaying
channelization and the intercalation
cobbles and boulders separated by channels. general,
upward.
similar to modern-day upper alluvial plains.
facies
lithofacies
thicken
stratified,
beds
cross-bedded
In
source region
arenite
become more
iron-formations
conglomeratic
plane-to
(Fig.3.9).
being
of
the
preClutha
sandstones
and
shales which show bimodal-bipolar paleocurrent patterns,
indicating tidal
current-induced
on
The planar, while
the
herringbone
reversals
cross-bedded channelized cross-beds
of
flow directions
sandstones sandstones
and
indicate with
superimposed
formed
as washover
sand
sheets
thin sandstones represent tidal flats.
tidal
flats. facies,
small-scale trough , planar ripple
tide sand flats with shallow tidal channels. probably
(Fig.3.9)
flood tidal deltaic structures
reflect
The plane-bedded
while
the mudstones
and low-
sandstones within
the
35
Ancient GneissComplex
3ches
Figure 3.8: Depositional models for the Fig Tree Group and Moodies Group (B). (Redrawn from Eriksson et al., 1988.)
(A),
36
Overlying the Clutha Formation is the Joe's Luck Formation which contains tuffs,
agglomerates and a thick upward-coarsening
quence which
displays
tide-dominated
features
lands, deltaic, and shallow shelf deposits
depositional
of prograding
barrier
seis-
(Fig.3.9).
INTERPRETATION 5-30m
-
Bar-top deposition during falling water stage
~[;.~..:
ft
0
"h°
"~J"
.°°
.
°..*
-...o
~/ -.;:
Midchonnel bar and channel floor dune migration at high water stage
o.. :'~
u)
. .'." "
" . " 4:""
b4
0 0
.~-..~ %...:.
••
..,. "'... °.
Log on channel floor
Plane-bedding Ripple-drift cr ass-lamination Shale drapes and shale clost Trough cross-bedding and ripple X-Ion,Motion
Figure 3.9: Moodies Group 1982.)
Interpretation of sedimentary structures in the (Clutha Formation). (Redrawn from Tankard et al.,
The banded iron-formations
at the base of the sequence formed in the
deeper part of the shelf under quiet conditions
which
favoured
far away from clastic influx.
chemical
and suspension
sedimentation,
of the Moodies
Group was deposited during a regression when there was a
return to tidal flat and alluvial plain environments.
The top part
37
According berton
belt
sediments chert
to
were
terrain
(Fig.3.8).
plain
shallow
marine
ments with
with
et al.
Tree
and Moodies
derived
from
a
deposited
abrupt in the facies
a narrow
southern
along
transition Fig Tree reflect shelf.
reworking
alluvial
formed
plain
barrier
the p a l e o g e o g r a p h y sedimentation uplifted
from
submarine
Shallow marine
along
and
deltaic
complexes
and a
the
steep
extensive
extensive
margin
sedimentation
sedimentation
with
the Barin which
continental
suggest the d e v e l o p m e n t
and
of one
sialic-volcanic-
fan
Moodies
sedimentation
was
mixed
a northward-facing
and the basal
h i g h e r up in the Moodies braided
(1988),
Fig
and
The
braided margin
Eriksson
during
continental
coastal
sedi-
of a w i d e r
shelf
in w h i c h
back-barrier
coastal tidal
flats.
. Fig Tree Group Sedimentation
,\J 0 .I\7_ Swartkoppie Formation
Calc- al koline Volcanism ;'/{ Hoog,enoeo, Kromberg)%J
;~<J
illllllllllllllllllllllllllllllll/ N
to
absence of
Initial phase of Submarine ultramafic-mafic Volcanism ~ (Tjakastad SubQroup) S
Figure 3.10: Model for the s t r a t i g r a p h i c and tectonic evolution of the B a r b e r t o n g r e e n s t o n e belt. (Redrawn from Lowe and Knauth, 1977.)
38
As
postulated
mentological initial
phase
Tjakastad kaline
by
Lowe
and
Knauth
of
the
Barberton
evolution of
submarine
Subgroup
volcanism
was
(1977), belt
mafic-ultramafic
formed
represented
the
(Fig.3.11).
(Fig.3.10)
volcanism
This was
by the Hooggenoeg
during
the
of
which
an the
Formations;
the
uplift
involved
during
and Kromberg
Formation;
Group,
sedi-
by calc-al-
of the Swartkoppie
Tree
and
followed
and by the deposition Fig
tectonic
and the deposition of
granitic
southern
sediment
source areas.
ONVERWACHT GROUP
FIG TREE
MOODIES
GROUP
GROUP
. . . . . .
Ab
%\
\
I GELUK< SUBGROUP
.......
-
\\
~_-----
% %
,
_--_--_-_ ___ --_------
\
\
0='
",,',
TJAKASTA~ SUBGROUP
I
AD
pe#
~CI ~ m
_ ~
M
j,
u''~A
JL
\ %
'w I&~
Bo ;...°,
sc
•
a •
-
,m,m
41'
,n
,F'2 i,o
Figure 3.11: Simplified stratigraphy of the Swaziland Supergroup. Sa, Sandspruit Fm.; Th, Theespruit Fm.; Ko, Komati Fm.; MM, Middle Marker; Ho, Hooggenoeg. Fm.; Kr, Kromberg Fm.; Sw, Swartkoppi Fm.; Sh, Sheba Fm.; BR, Belvue Road Fm; Sc, Schoongezicht Fm.; CI, Clutha Fm.; JL, Joe's Luck Fm.; Ba, Baviaanskop Fm.; i, ultramafic lavas; 2, mafic lavas; 3, siliceous and aluminous felsic tuffs; 4, chert with minor shale and limestone; 5, metatholeiites; 6, felsic lavas, turfs, agglomerates, porphyries; 7, mafic pyroclastics, agglomerates, pillow brecias; 8, graywackes and shale; 9, shales; 10, tuffs; ii, conglomerate and quartzite; 12, amygdaloidal lava. (Redrawn from Frazier and Schwimmer, 1987.)
Structure
In his
o f the B a r b e r t o n
structural
mapping
part of the Barberton of deformation repeated, signed
and
belt,
sedimentological
Paris
(1987)
during which stratigraphic
leading
to the
Greenstone Belt
to the
Swaziland
24 km thickness Supergroup.
studies
identified
the
southern
four major
on
episodes
units had been dismembered
and
which
as-
Although
had
Paris
been
previously
applied
new
strati-
39
graphic
terminologies
marine
facies
doubtedly
overlain
these
respectively.
(Table 3.1),
are
by
a
the
part
identified
continental
equivalents
The structural
for the northern
he also
picture
alluvial
fan
the
Tree
of
presented
of the Barberton
a prograding
belt
Fig
sub-
sequence. and
by Tankard et al.
also reveals
Un-
Moodies (1982)
polyphase
de-
trending
NE-
formation like in the south. Regionally,
the Barberton
belt
is a broad
NNE and comprising tight to isoclinal turned to the west and separated deformation clines. sodes
episodes
Both
about
followed
transverse facing
NE-
by
or
slaty
radically
were
along
along
cleavage with the
and
produced
There
is
crescentic
no
formations
doubt
that
(Paris,
evident
in
which
schistosity,
axial
and
folds.
to
a
folds
synform
which
buckled
interference
minor
conjugate
forces
played
was
downward-
major
spo-
in
a
synclines
and
role
of
produced
folds
a
This
formed
deformations the
synepi-
development
concomitant
The
compressive
folding
planes.
the
These
next
and Ulundi
and minor
led
Several
faults.
these
de-
1987).
GranitoidEmplacement As
in the Eureka
SE-dipping
axes
(Fig.3.12).
syncline.
large n o r t h w e s t w a r d - t r e n d i n g and
and
NNW
steeply-plunging
Eureka
slices
formed during major
NNE-striking
shortening
structure
by tectonic
have been recognized
synclines
synclinorium
synforms which are generally over-
and Cratonization
Fig.3.4
granitoids
are
eastern part of the Kaapvaal province.
the
most
pervasive
rocks
in
the
They were emplaced into the green-
stone belts and adjacent gneisses from about 3.35 Ga to 2.6 Ga, after the volcanism (1982)
and
gave
deposition
the
the major granitoids. foliated and
rock
of
following
ranging
predominantly
the
Swaziland
sequence
The Granodiorite in
composition
granodiorite
and
Supergroup.
of events Suite, from
during
Tankard
a coarse-grained,
hornblendite
tonalite,
was
et
the emplacement to
al. of
slightly
granodiorite
emplaced
into
the
southern gneissic terrane of the Barberton belt at about 3.35 Ga. Between 3.4 Ga
and
plutons
3.1 Ga,
intruded
several
the
lower
leucotonalitic,
trondhjemitic
members
Onverwacht
of
the
and
Group
southwestern and northwestern margins of the Barberton belt,
tonalitic along
the
and thereby
marked the end of the deposition of the Swaziland Supergroup and the approximate
period
for
the
stabilization
of
this
part
of
the
Kalahari
craton. One of the largest granitoid plutons rite.
is the Kaap Valley quartz dio-
Even more extensive are the Nelspruit porphyritic granite and mig-
matites
which
underlie
a vast
area
north
of
the
Swaziland
Supergroup.
Figure 3.12:
Xecocatu M
TRAv£Rs~C-O OIoIotshoGroup(M) Diepgezet Group (D)
~
Thrust
-e-Younqinq Direction
D2 tectonic brecclo DI tectonite
Structural relationships in the Swaziland Supergroup. (Redrawn from Paris, 1987.)
0
B
SE
/ T - Junction thrust
TRAVERSE A-8
0
,,
~ I C n e r t and love n _ .. • bearing chert [-onverwocnt SloheroJd . . . . Lopilli beorJnq chert J ~ r ° u p [ ° ~
~
lOOm
h
lOOm
T~
waterfall $¥nclDnorium
NW
,,,
4~ O
41
Within
the
which
Nelspruit
t o g e t h e r with
most
extensive
like
quartz
Swaziland
porphyritic
granite
the Nelspruit
intrusive
monzonite
Supergroup.
suite,
which
is
Suite was
however,
surrounds
the
Hebron
emplaced
is
the
the
granodiorite,
around
younger
3.2 Ga.
Lochiel
southeastern
margin
M i n e r a l i z e d pegmatites w i t h cassiterite,
m o n a z i t e are common in the Lochiel complex.
The
sheetof
the
beryl and
The Lochiel quartz monzonite
was e m p l a c e d s y n t e c t o n i c a l l y at about 3.0 Ga. The g e o c h e m i c a l models for the origin of these tonalites are based on their
characteristically
low K/Na
limited range of Rb/Sr ratios low 8180 values;
ratios
(< 0.5);
variable
(close to 0.1); e n r i c h m e n t in light REE and
all of these suggest mantle d e r i v a t i o n p r o b a b l y by par-
tial m e l t i n g of the 0nverwacht Group basalts. However, monzonite
with
K / R b ratios;
its
high
initial
87Sr/86Sr
ratios
the Lochiel quartz
and
high
slSo
values
could have been g e n e r a t e d from crustal sources.
Other Greenstone Belts in the Kaapvaal Province The smaller g r e e n s t o n e belts in the Kaapvaal p r o v i n c e are less well known than
the
Barberton
berton belt eastern
belt.
These
outcrop m a i n l y
(Fig.3.3) while two belts,
inliers.
B a r b e r t o n belt
The
occurrence
to
the north
of
the Bar-
the M u l d e r s d r i f and the Amalia are
of these
other
belts
far a w a y
from the
suggests that the A r c h e a n g r e e n s t o n e belt of the Kaapvaal
province was o r i g i n a l l y much more e x t e n s i v e than what now remains.
l. Pietersburg from
beneath
the
Belt.
This
cover
of
belt
the
extends
Transvaal
in
a
northeasterly
Supergroup,
for
direction
about
100 km,
with an average w i d t h of about 10 km, tapering out between two granitoids at its n o r t h e a s t e r n lavas w h i c h
are
berton belt.
end.
It has a lower sequence of u l t r a m a f i c
chemically
similar
an upper sequence of conglomerates, the
Moodies
belt.
Group.
Three
There
periods
of
the
komatiitic
is
pile
facies.
facies
Amphibolite due
to
contact
no
quartzites
calc-alkaline
deformation
volcano-sedimentary
probably
to
There are m i n o r chert bands and banded
was
affected
weakly
rocks
suite the
adjacent
as well
of
the
in
belt
the
to
the
a regional
and like
Pietersburg
during
to g r a n i t o i d
as
Bar-
iron-formations,
and shale interbeds,
metamorphosed
occur
metamorphism
lavas
to mafic
which
the
greenschist intrusions increase
in
m e t a m o r p h i s m n o r t h e a s t w a r d towards the h i g h - g r a d e L i m p o p o province.
2.Murchison and
up
to
Belt.
15 km
ultramafic-mafic
This
wide.
is a n a r r o w g r e e n s t o n e
The
succession
sequence and passes
here into
starts
belt
some
with
quartzite
140 km
long
Onverwacht-type
metasediments
and
acid p y r o c l a s t i c rocks, all of w h i c h have been subdivided into six litho-
42
stratigraphic
units
relationship structural
(Tankard
between
et
al.,
these units,
interpretations.
1982).
However,
being uncertain,
the
stratigraphic
is based
The M u r c h i s o n belt was
entirely on
intruded by the Rooi-
w a t e r layered complex and granitoidso 3.Sutherland
the
Kaapvaal
posures rocks
here
with
This
Belt.
province. are
It
poor
minor
occurs is
at
about
comprising
the
extreme
60 km
long
schistose
intercalations
of
northeastern
and
15 km
and m a s s i v e
banded
wide.
part The
of ex-
mafic-ultramafic
iron-formations,
chert
and
q u a r t z - s e r i c i t e schist. 4.Muldersdrif
Barberton
belt.
and
Amalia
Belts.
The M u l d e r s d r i f
Both
belts
underlies
lie
a small
to
the
area
west
of about
of
the
150 km 2
on the edge of the J o h a n n e s b u r g Dome and contains p r e d o m i n a n t l y basic and ultrabasic
massive
and
u l t r a b a s i c intrusions The A m a l i a
schistose m e t a v o l c a n i c s
belt of w e s t e r n
Transvaal
rounded by the V e n t e r s d o r p Supergroup. grits,
with
remnants
of
layered
(Tankard et al., 1982).
shales, ironstones,
is p r e s e r v e d
as an inlier sur-
It consists m a i n l y of graywackes,
lavas and tuffs.
3.2.2 Pongola B a s i n In this basin the Pongola Supergroup u n c o n f o r m a b l y overlies granitoids in the southern part of the Kaapvaal province b a b l y the e a r l i e s t and 2.9 Ga.
stabilized
The Pongola
cratonic
up
salts,
basaltic
ty.
30-m
A
to
8 km
thick,
thick
recrystallized
(Grotinger, platform ramp,
1989).
deposit
based
on
siliciclastic
between
3.1 Ga
7.5 km
with
stromatolitic
of
into a lower Nsuze interfingering
w i t h tholeiitic structures
occurs
ba-
affiniwithin
and
clastic-textured
cross-bedding
and
dolomites contain
which
large
display
stomatolite-
all of which suggest t i d a l l y i n f l u e n c e d environments This
which the
in what was pro-
This carbonate body is laterally d i s c o n t i n u o u s with
herringbone
derived intraclasts,
It is d i v i s i b l e about
dacite and rhyolites,
carbonate
dolomite
well-developed
comprising
andesites,
the Nsuze volcanics.
3.4),
Supergroup is a sequence of tholeiitic volcanics
and tidal flat sandstones and shales. Group,
(Figs.
shelf environment,
is
probably
evidence
sediments,
believed
and
for on
to
be
accumulated tidal the
the on
activity
oldest a
known
carbonate
high-energy
carbonate
in
comparatively
both sparse
carbonates occurrence
and of
stromatolites. The u p p e r part of the Pongola Supergroup consists of an unconformable sequence ture
(Fig.3.13),
quartz
up to 1,800 m thick,
arenites,
shale
and
banded
in which
there
iron-formations.
are m o s t l y maThis
is
the
43
Mozaan Group which contains sedimentary structures that are suggestive of sedimentation
in
tidal
flats
and
tide-dominated
shelf
environments
(Tankard et al., 1982).
VRYHEID-PIEDRETIEF AREA
I~,m '11
-10 -9
AMSTERDAM AREA WIT-MFOLOZlINLIER GROUP ~ I , S
NSUZE ~
NKANDLAAREA
GROUP •/
.,"
2
r~
Ushushwar~ Complex
F~
Iron-rich Sediments (where thin Indicated by o
1
0 %.
Argillaceous Sediments Arenaceous Sediments Volcanic Rocks Granite Basement Unconformity Figure 3.13: Stratigraphic (Redrawn from Nisbet, 1987.) The
shelf
sediments
columns
in the Mozaan
for the Pongola
Group
consist
of
Supergroup.
ironstones
with
shales and siltstones which are arranged in upward-coarsening motifs that suggest
beach
sedimentation.
The banded
iron-formations
at
the
base
of
44
the
Mozaan
believed
contain
to
be
interlaminated
chemical
chert-jasper
precipitates
in
and
distal
magnetite
shelf
and
are
environments
far
away from detrital influx. Pongola Supergroup volcanism and sedimentation ended with
the emplacement
of the Usushwana mafic-ultramafic
intrusives
at about 2.87 Ga. 3.2.3
The
Zimbabwe
Zimbabwe
Province
province
is
an
oval-shaped
300,000 km 2 granite-greenstone
province which underlies eastern Zimbabwe and extends southwestwards into eastern
Botswana
formed
and
rocks,
(Fig.3.14).
metamorphosed
gneisses,
older
In it are exposed
rocks
which
granitoids,
suites
include
various
of
complexly de-
high-grade
distinct
sets
metamorphic
of greenstone
belts, intrusive complexes, younger granites and the Great Dyke. The ages of these rocks range from about 3.8 GB to 2.5 Ga, the last age being that of the Great Dyke. Among the points of interest in the Zimbabwe province are the huge deposits
of chrome,
bestos,
and other minerals;
some of
the earliest
gold,
nickel,
platinum,
iron
ore,
and the fact that the metasediments
unequivocal
stromatolitic
carbonates.
The
as-
contain Zimbabwe
province is also one of the few regions in Africa where old supracrustals appear to have been laid down on a continental basement The
oldest
rocks
in
the
Zimbabwe
province
(Nisbet,
include
a
1987).
variety
of
gneisses and tonalites such as the Chingezi gneiss, the Mashaba tonalite, and the Shabani gneiss. granulites
and
supracrustals vince.
These
In the southern part of Zimbabwe are the various
amphibolites constitute
form
between
the
exhibit
concordant
the
which
belong
the schist intervening
"gregarious"
tonalitic
contacts
to
or gold elongate
the belts or
batholiths
Limpopo of
arcuate belts
The
Zimbabwe
synformal
(Fig.3.15)
with the greenstone
province.
the
which
probelts
commonly
and show gneissic
foliation and a relative abundance of greenstone xenoliths.
Three gener-
ations of greenstones are discernible in the Zimbabwe province. As summarized
by Foster
older
greenstones
stones, wayan
the
or
and Gilligan which
"Belingwean"
upper
and by Nisbet
are
collectively
or
lower Bulawayan
greenstones
youngest greenstones
(1987)
(Fig.3.14).
or the Shamvaian
termed
(1987) the
Sebakwian
greenstones,
What
was
(Macgregor,
they are: and
previously 1947),
the
the
greenBula-
termed
the
is now regarded
as the upper part of the Bulawayan Group, representing the terminal phase of greenstone volcanism and the accumulation of granitic detritus in probably
local
isolated
basins
(Wilson,
1972).
Representative
gneissic,
greenstone and granitoid domains from the Zimbabwe province are described below in order to illustrate the stratigraphic, characteristics of this province.
lithologic and structural
45
~
-~SEBAKWIAN GROUP
o z
l0
BIMODAL UNIT (WEST). MIXED UNIT (EAST)
z m ~ ~ :;o
BASALTIC UNIT INCLUDING KOMATIITIC AND BASAL SEDIMENTARY UNITS WHERE DEVELOPED
z°
LOWER GREENSTONES
Figure 3.14: Archean greenstone from Foster and Gilligan, 1987.)
Gwenoro
Dam Basement
LATER COVER ROCKS (PROTEROZOIC TO RECENT) 5HAMVAIAN GROUP
belts
of
Zimbabwe.
(Redrawn
Gneisses
This is a structurally complex basement region south of the Gwelo schist belt,
between
the towns of Gweru and Shurugwi.
It illustrates
the field
46
~:- *.~ ~ ..
~
",:S. ~ ~
.
~:? ;
.
,.
•
~'.,,~ ~.i~ r,~
~
:
,
:
~
z%,~ ,., %
~
~
,
1
I
A.
10
YOUNGER GRANITE
Km
30°00
' E
GREAT DYKE
TONALITE AND GRANODIORtTE FOLIAIED HOMOGENEOUS C¢4EISS ~"~IvEINED PEGNATOID GNEISS BANDED 1,4IGMATtlE AND NEBUUIE ~m~ SCHIST BELT ROCKS
CORDIERITE-AMPHIBOLITE & ASSOCIATED ROCKS --
MYLONtTtC GRANODIORITE FAULT
~"
INFERRED TECTONIC SLIDE WITH DIRECTION OF DIP e
•
,
...~. ~ % ~~.~
_
b
i
~
~.~ ~ ~'.F~?...~* _~ ~ ~ ~
[[[] YOUNGER ROCKS ~[I GRANITES
+
B
"~/~JSTRUCTURAL TRENDS
EH,STS
Figure 3.15: A, Outline geological map of the Gwenoro Dam a r e a ; B, S t r u c t u r a l relationships in Zimbabwe greenstone belts. (Redrawn from Condie, 1981; Frazier and Schwimmer, 1987.)
47
relations belts
between
gneisses,
(Fig.3.15,
gneisses with layers,
to
abundant
b).
The
alternations
faintly
associated gneisses
granitic
range
in
plutons
composition
of q u a r t z o - f e l d s p a t h i c
foliated,
homogeneous
migmatite-agmatite-nebulite
and
bands
gneisses.
terranes.
from
banded
and biotite-rich
There
Since
greenstone
are
the
locally
gneisses
are
m a i n l y of tonalite or trondhjemite composition within mafic enclaves,
and
rocks
re-
of
garded
intermediate
as
parallel
bimodal.
between
Stronger
inclusions grees into
the
adjacent
entire
Fig.3.15A,
terrane
foliation
the g r e e n s t o n e
belts
is
is
generally
especially
close
show
supracrustal
fragmentation
gneisses.
a
transition comprising of
greenstone
pluton
there are a b u n d a n t
show p r o g r e s s i v e
transitions
between
plex-granite dational.
the
Some of the inclusions
and
foldbelt
and
connections
in
and
rare,
foliation develops where
inclusions
trends
are
but foliation becomes v a r i a b l e in the intervening re-
assimilation
of
These
depicted
in the gneisses.
of
trains
As
the gneisses
to their contacts; gions.
composition
contacts
by
from
belts
the
banded
inclusions
the
from
greenstone
migmatites
are
(Condie,
range
Ghoko useful
1981).
sharp
and
and for
The
de-
Sometimes, belt
nebulites. tracing
gneissic
discordant
to
the comgra-
Sheared contacts render it d i f f i c u l t to a s c e r t a i n w h e t h e r such
contacts are u n c o n f o r m i t i e s or intrusive.
Older Greenstone Belt (Sebakwian Group) The Sebakwian accumulated
greenstone
around
succession
3.4 Ga,
of Zimbabwe,
in the Shurugwi,
are
to be
believed
morphosed rences, within
to
the
they are gneisses.
the oldest
amphibolite found
comprising
are best d e v e l o p e d
lavas and sediments in the
Lower Gwelo and the Mashava greenstones facies.
scattered
which
Outside
elsewhere,
Near the town of Shurugwi
have
the
the
regions.
been
as
infolded
lower part
is
intruded
by
a major
suite
of
occur-
remnants
of the Sebak-
w i a n sequence includes m a g n e s i a n basalts and possible komatiites, nor m e t a p e l i t e s and banded ironstones.
part These
m o s t l y meta-
south-central
mostly
which
south-central
and mi-
The lower S e b a k w i a n in this region
ultramafic
bodies,
some
of
which
bear
chromite. Resting
unconformably
on the
lower
sequence
is the
s e d i m e n t a r y Wan-
derer F o r m a t i o n w h i c h has a very diverse a s s o r t m e n t of sediments rapid
lateral
facies
variation
iron-formations.
Clasts
Jaspilite
granite
eroded
from
(Fig.3.16) into
a
chert, a
highly
through
large
nappe
of
from conglomerates
talc-carbonate
and gneiss
varied
Shurugwi, structure
suggest
terrain. the
As
Sebakwian
before
the
rocks,
to pelites chromite,
that
the
shown
in
the
sequence
has
intrusion
showing
and banded metabasalt,
conglomerates
of
cross been the
were
section deformed
Mont
d'Or
48
g r a n i t e about 3.35 Ga ago.
In this nappe the g r e e n s t o n e s u c c e s s i o n is in-
verted.
is
The
nappe,
which
about
i0 km
wide
and
can
be
traced
in
a
n o r t h w e s t e r l y d i r e c t i o n for about 60 km, appeared to have transported the s u p r a c r u s t a l sequence over a distance of about 50 km (Stowe, 1984).
SELUKWE NAPPE LONGITUDINAL SECTION ° SOUTHERN WEDGE COIMPLEX I
S3(~W ~n~J-
"~
/
7G
/
~
Tibillkw¢
-.
~ -
su
~t~m,~
Complex
SG
Selukwe
Formation
tn
ton=~ti¢
c,-~,
MC
Mont
Figure 3.16: from Nisbet,
-
Greenstone
/
m,
SELUKWE PEAK - - ' ~ " WOLFSHALL
MONTDIDR
~---'-
s,
--~
,
I
~'-¢~""
51qvL,..~7 .-
~
30=E HIGHLANDS w,
•.-,,
lO00ml~q~ll~~~:~,.~.-'~ 0 m 1 ~ --_'~_-~",'I I~..'..~_'--_'--_'--_'--_'~"" t N . ? ' - - - ' " I
dbr Complex
I ~--_--~ /
C r o s s - s e c t i o n through the Selukwe nappe. 1987.)
(Redrawn
Bulawayan Greenstones These
greenstones
widely vince
of w h i c h
recognized
and
(Fig.3.14).
The
unconformable
the Mberengwa
correlated lower
belt
is a microcosm,
across vast parts
part
of
the
of the
succession
is
has
been
Zimbabwe pro-
almost
entirely
upon an older basement comprising the Chingezi gneiss,
the
Shabani gneiss and the Mashaba tonalite which range in age from 3.5 Ga to 2.9 Ga.
Two s u p r a c r u s t a l
the
Bulawayan
are
the
greenstones
"Belingwean"
or
B u l a w a y a n greenstones.
sequences which the
separated by an unconformity,
formed between
Lower
Bulawayan
In the M b e r e n g w a belt,
p a r a t i o n between both greenstone
successions
2.7 Ga and
greenstones
make up
2.6 Ga. and
These
the
Upper
the l i t h o s t r a t i g r a p h i c
se-
is h i g h l i g h t e d by the pres-
ence in the Upper B u l a w a y a n sequence of a d i s t i n c t basal marker bed, the Manjeri F o r m a t i o n The M a n j e r i
(Fig.3.17).
Formation
ranges
in thickness
from
0 to
i00 m
and
com-
prises a coarse and p e r s i s t e n t conglomerate bed w i t h clasts of the underlying tonalite, thin
dolomite
graywacke, contain
o v e r l a i n by a sequence of s h a l l o w - w a t e r which
passes
upward
into
chert
followed by banded iron-formations.
a Manjeri-type
marker
unit w h i c h
can
and Many
a
siltstones and a
thick
Zimbabwe
be used
sequence
of
greenstones
regionally
to se-
49
parate lower
the
lower
from
greenstones
canics
are
comprising
the
upper
Bulawayan
characterized
pillowed
mafic
by
and
greenstones.
lower
sequences
ultramafic
and
The of
upper
bimodal
felsic
and
vol-
volcanics.
However, major regional lithofacies developments occur in the Upper Bulawayan succession.
I
30m J ' " ' ' ' - ~ k . r''''-'.~ ~ : J ~ P I;:~-:!~.:.~ --
\~
~
- . J
I..~..;~,1
\.
I-'.'.'.',';
, .
-
++i
_ =
On*
:.-.:.+
+
....
lore
Graded bed ~ ' d y k e
~ / ~
......
~andstone
I -
-
~ ~ 1 0 -l'n~j~/~
~+'~Om
~ Ripple marks a," Floser bedding
1/
•
.
Jospilite and Chert Argilite Arenite
Figure 3.17: near Shabani.
,roe
ross-
bed,,in,
'~ Festoon cross bedding ~" Slump structures nnlconglomerate ~ Sandy Dolomite ~ Folioted Tonalite
-
Stratigraphic column for the Bulawayan greenstone (Redrawn from Candle, 1981.)
the upper part of the western
appreciable
calc-alkaline
and
Formations,
Felsic
w.~'2~J-lOOm
uroaea arenacaous
"-, beds ,,,ith ~ o+,,,o~eous partin,,s ~
I+:: :::~
First,
RELIANCE Fro.( 1Kin, thick, Komotiites and Komotiitlc basalts)
High Hagnesla Basalt ~L \ ( Kro matilte) ........ Sulphite iron Formation /V~t-w;/, ~
:+OreI++::::::~
[~
CHESHIRE Fro,( basal congl., passing into ~hales, siltstones and stromafolltlc limestone~) ZEEDERBERG Fm.(SKmthick) (Pillow basalt and flows, no sediments)
Calc-alkaline
volcanic
volcanics
greenstone
(Fig.3.14)
while
the
eastern
suites
are
not
successions
as found
greenstones
common
in
Cheshire studies
Formation, of
some
of
from which the
sedimentological
earliest
stromatolites,
light on Archean shallow marine environments
remained
the Archean.
there is a major carbonate body in the Upper Bulawayan and
bimodal. Secondly,
sequence,
isotope
have
contain
in the Maliyami
shed
in the
geochemical considerable
(Abell and McClory,
1987).
l.Mberengwa Belt. In the Mberengwa greenstone belt the upper greenstones
rest on the
lower greenstones
in a synform
(Fig.3°18).
The lower
greenstones belong to the Mtshingwe Group which has been subdivided four formations.
The lower Hokonui Formation
comprises
into
2-3 km of dacitic
50
pyroclastics
and
andesitic
flows
with
a
spectacular
vent
agglomerate
which includes huge blocks of the underlying tonalitic country rock.
.~3ds
9
0
Figure 3.18: 1987.) This southern
tonalite part
of
Hokonui
9
0
36 od~
Belingwe
greenstone
intrudes
into
the
pile of komatiites,
1
Mberengwa
the belt,
belt.
i
(Redrawn
Hokonui the
in
Bend
from
some
places.
Formation,
komatiitic basalt and banded ironstone,
unconformably,
and
is
2-5 km
thick.
The
Nisbet,
absence
In
the
a remarkable overlies the of
clastic
sediments in the Bend Formation suggests that volcanism possibly occurred well away from clastic detritus.
The eastern part of the Mberengwa belt,
in
sequence
contrast,
contains
a
thick
of
coarse
conglomerates
and
51
breccias
which
basalts,
shales
Formation
which
ultramafics, edge
passes and is
thus
proximal
formations,
the
suggesting
a
lithofacies
shales
volcanics.
a
The
belong
basinal of
Brooklands
the
transition
the
1,000 m
with
Bend
of
Bend
mafic-
from
trough
banded
coarse
is
the
conglomerates
warped
Bulawayan
iron-
Formation
Group was
of the upper
komatiitic
the
of
facies
entire Mtshingwe
and
to
equivalent
Overlying
consists
the deposition
komatiite
paleogeographic
into
which
of
These
lateral
and komatiites.
and
before
assortment
iron-formations.
Formation
felsic
an
probably
Koodoovale eroded
into
and
partly
greenstones
in the
synform. In the Mberengwa belt the Ngezi Group constitutes the youngest greenstones
which
overlie
(Fig.3.18). gneisses,
Since
this means
material
as
Mberengwa
were
the
limestones
laid
of
about tiites,
believed,
down
the
on
and
Manjeri
thick.
erupted
strata
with
a
oversteps
rather
marked
older
unconformity
greenstones
represent Archean
some
greenstones
pre-existing
continental
shallow-water
sandstones
like
crust
and
oceanic those
in
(Bickle
et
1987).
deposits,
1,000 m
older clearly
that not all greenstones
intertidal
deeper water
the
Ngezi
previously
al., 1975; Nisbet, From
all
the
Formation,
overlain These
as
is
an
stromatolitic
upward
change
by the lavas of the Reliance
lavas
flows,
there
and
include
pillow
lavas
komatiitic and
basalts
tuffs.
The
into
Formation, and
koma-
5.5-km thick
Zeederbergs Formation overlies the Reliance volcanics and comprise mostly pillowed and massive tholeiitic basalts with little or no interbedded sedimentary material. the top from
The Cheshire Formation,
of the Ngezi
the
mations,
Group.
Zeederbergs and
upward
It comprises
Formation) into
which
up to 2.5 km thick, occurs at a basal
passes
an assortment
of
conglomerate
laterally
into
shallow-water
(derived iron-for-
sediments,
in-
cluding very extensive limestones which, in places, are profusely stromatolitic.
Figure 3.19
is
the
inferred
upper part of the Ngezi Group (Nisbet, 2.Midlands
the
Greenstone
Kwekwe-Gweru
Gilligan, wayan
1987),
and
contains
greenstones
(Fig.3.14)o
The
Belt.
the
in
western
paleogeographic
The Midlands
greenstone
greenstone
some of the best exposures western
greenstones
for
the
1987).
Chegutu-Kadoma
the
setting
part as
of
already
the
belt,
belts
comprising (Foster
and
of the Upper BulaZimbabwe
noted
are
province
peculiar
in
their calc-alkaline volcanic suites and in the significant development of the
Shamvaian
Group.
Mafic Formation,
At
Kwekwe
the
lowest
lithostratigraphic
is a sequence of pillowed mafic
unit,
the
flows interlayered with
chert and minor felsic ruffs and conglomerate with granitic
cla~ts which
52
indicate
the
Formation
presence
of earlier
is intruded
pre-greenstone
by the Rhodesdale
and is overlain by the Maliyami
sialic
batholiths
Formation
(Condie,
are augite
in which
altered
andesites
groundmass.
lavas
imply
lavas
contain
pyroxene. crysts,
that
Porphyritic
likened
the
trusion, above
North
rounded
by
overlain flows, the
volcanics Balholith,
zones
America,
the
of
similar
and
and
pseudomorphs
and
phyllites
its
in
age.
belt
The Maliyami lesser
uplift
and
unconformable
and conglomerates
is present
and of
(Harrison,
Nisbet
the
assemblages
is
stripping,
contact
with
of
is sur-
conformably
andesitic mafic
in-
Cascades
intrusives
Formation of
(1987)
tonalitic
andesitic
mainly
amounts
groundmass
orthopyroxene.
composition,
Sierras
some
feldspar pheno-
after
basaltic
the
in
amygdaloidal
rare
contemporaneous
comprising
folding, its
chemical
calc-alkaline
with
implying pile
as
Formation
tuffs
to
fresh within an
serpentine
fine-grained
mineralogy
to modern
where, a young
Felsic
surface
Some
of
rarely found in Archean terrane, and
volcanic
graywackes,
facies
Maliyami
breccias,
erosional of
present.
and
lavas
by
is typically
aggregates
and some chlorite
the Sesombi
western
was
of
on its p e t r o g r a p h y
subduction
flows and andesitic
lavas include examples with altered
long,
Devitrified glass, Based
clinopyroxene
presence
olivine
bodies,
which consists of a
1981). The lavas which are dated at 2.7 Ga,
low-greenschist
1-3 m m
1970).
The
The Mafic
or ultramafic
(Fig.3.20),
thick sequence of intercalated mafic and andesitic dacitic pyroclastics
crust.
to dacitic
volcanics. marks the
An
the top
overlying
of the Shamvaian Group.
stromatolites
a~cte r ia
~
~,~/'.:. "#f~.-
.~
vents
Fm
Figure 3.19: Paleogeographic setting tion. (Redrawn from Nisbet, 1987.) 3.Tati
thick
deformed
of
volcanics
meta-arkoses.
prises a bimodal
The
sequence,
mafic and ultramafic
Fm
for
the
Cheshire
In the Tati belt of northeastern
G r e e n s t o n e Belt.
sequence
~o
and
sediments
unconformably
lower
one-third
of
the
Tati
Forma-
Botswana,
overlie
a
highly
succession
com-
the Lady Mary Formation which is a sequence of
flows and sills with minor felsic tuffs,
arkose, and
-ueexB
e~qgq~TZ
emos
('T86I 'eTpUOD mox~ u-~xpe~) xo~ su~nToo aTqdgxBT~gX~ S
"s~Te q e u o ~ s :0Z'Z eanBT~
.+' ..,~v., I tl,,
,,+-'+"vl
11 ^-...~^.;
SOlUDOIOA O | t O t U O l l l r l
,+,,,.,+ °,+°,,
.....
• l+:+,.,:l
~"+++']< +
i~
P,,i
•,,..°.°+,o~ i i ~ .,o.+o,,.,.o+,+,.+..,+j.+~+ +~ p u o 1/401-1
'""
.o,, ~°'09:~',°~ I._1 ..~
81.1I1+10.IO
-.,z+,.,o,,o II • #..BJD
- o~,.,..(Ol.,O
~lrlb
+
'1. A ~1
+,i,.+,,.,..I .
l+'V:l
N,,,v^+,vHS
-NVAVMVINB
",,,'+,," 3,~3non$ 1J.~'.L
£5
54
chert with m i n o r carbonate graywackes,
(Fig.3.20).
The Penhalonga M i x e d F o r m a t i o n of
g r a p h i t i c phyllite, mafic flows and some a n d e s i t i c and felsic
volcanics overlies formations,
the L a d y M a r y Formation.
carbonates
and conglomerate
breccias and tuffs in the upper part.
There are m i n o r banded iron-
in the P e n h a l o n g a with andesitic
The Selkirk Formation,
composed of
d o m i n a n t l y a n d e s i t i c to felsic pyroclastics with m i n o r basalts and chert, constitute the upper unit.
The Penhalonga
and the Selkirk
Formations
present a c a l c - a l k a l i n e sequence like the M a l i y a m i - F e l s i c
re-
sequence of the
M i d l a n d s belt.
Structure of the Bulawayan Greenstone As
already m e n t i o n e d
for the older
Shurugwi
greenstones
belt,
there are
w e l l - d o c u m e n t e d nappe structures in the schist belts of the Zimbabwe province. the
This
implies
deformation
(strain)
studies
revealed
that
that horizontal
of
greenstones
of
in
the
the
greenstone
western
probably
as
and lineations a
result
of
belts
of
Four
deformation
Botswana,
stone belt, nalitic
belts
structural
Botswana
there
are
which
the
movement
phases
thonous.
A
gneiss
Victoria
second
granitoids, foliation
probably
greenstone and
steep
plunge to the northeast or south-southwest, of
the
exist
in
Zimbabwe
province
(Coward et al.,
the
to
the
1976).
granite-greenstone
initially belonged
are overturned to the northeast
(Antelope)
have
belts
of
of dia-
the Tati, Vumba and part of the M a t s i t a m a greenstone belts
basement
Mberengwa
phase
and
In a p r e - c l e a v a g e d e f o r m a t i o n prior to the e m p l a c e m e n t
piric plutons, of
Detailed
Zimbabwe
while in the south and east the foli-
southwest relative to the Limpopo province
Zimbabwe.
forces w e r e important in
1981).
granite-greenstone
foliation and d o w n - d i p lineations, ation curves,
compressive (Condie,
appears
to
succession. of
thrust
However,
greenstones,
phase
be
to one
though
deformation
continuous
(Fig.3.16). over
the
folded, was
In fact, the
Lower
Bulawayo, are
caused
the
Gwanda
Shangani,
probably by
greenthe to-
autoch-
intrusive
e s p e c i a l l y the syntectonic plutons which p r o d u c e d local steep
and
lineation
of regional
in the contact
deformation
zones with
produced a few m a j o r
greenstones. structures
spread cleavage in both granite and greenstone terranes. the greenstones
were
shortened
by up to
65 %. Late
The main but wide-
During this time
deformation
produced
c r e n u l a t i o n s and tight folds which deform the earlier fabrics.
Igneous Intrusion and Cratonization The
terminal
vince
was
phase
of d e f o r m a t i o n
accompanied
by
the
and m e t a m o r p h i s m
emplacement
of
in the
mafic
and
Zimbabwe progranitoid
in-
55
trusives.
The
cratonic some
parts
which
of
large
veloped
cratons,
basins
in the
which
some
Kalahari
intrusive
in
the
newly
had
stabilized,
regions
such
craton,
as
there
bodies
rose to
formed
continental
became
the was
fill m a j o r
the G r e a t isting
Dyke,
intrusives,
respectively,
granite-greenstone
sites
magmatism
during
It was
which
during
were emplaced, of
cutting
Zimbabwe.
in
had dethis
re-
2.5 Ga that large min-
such as the M a s h a b a u l t r a m a f i c
terranes
large But
fractures
masses.
of
province.
renewed
newed phase of m a g m a t i s m between 2.7 Ga and about eralized ultramafic
the
Kaapvaal
The
across Great
suite and
the pre-ex-
Dyke
ever, c o n s i d e r e d as m a r k i n g the inception of the Proterozoic,
is,
how-
hence it is
d i s c u s s e d under the Early Proterozoic° l.Mashaba Mberengwa trusions
Ultramafic Suite.
greenstone which
have
belt been
Scattered
are dated
several at
about
termed the M a s h a b a U l t r a m a f i c Suite is
the
Shabani
related
to
the
c o u n t r y rocks. rock w h i c h in
main The
is
or
intrusive
Shabani
It
which
consists layers
cut
Complex
about
1,500 m
of
of
p y r o x e n i t e into gabbro. and
Mafic
the
perimeters
mafic
2.7 Ga.
to
These
of
the
ultramafic
in-
are
collectively
(Fig.3.14), the m o s t n o t a b l e of which komatiitic across
dykes
most
of
which the
may
not
be
granite-gneiss
is a large slab of m a i n l y ultramafic
is exposed along the n o r t h e a s t e r n edge of the Upper Bulawayan
greenstones. body
Complex.
around
major
a
simply
dunite
differentiated
pass
upward
sill-like
through
igneous
peridotite
and
It outcrops over an area of about 15 km by 2.5 km thick,
the sill
with
60 ° dip
S e v e n t y per
cent of
is dunite;
pyroxenite,
w h i l e gabbro constitutes
in
the
southward
20 % consists
direction.
of p e r i d o t i t e
up to i0 %. The Shabani
and
Complex has
one of the w o r l d ' s largest deposits of asbestos. The
Shabani
Complex
probably
represents
a
which was fed from b e l o w by ultramafic liquids,
stratified
magma
chamber
in w h i c h d o u b l y d i f f u s i v e
processes could have operated to produce the v o l u m i n o u s
l o w - d e n s i t y erup-
tive basalt lavas of the Zeederbergs F o r m a t i o n
1987).
(Nisbet,
2.Younger Granitoids. Granitoids such as the Sesombi tonatite and the more
potassic
stone
belts
ration
Suite
2.6 Ga.
The
(Fig.3.14) Sesombi,
were with
intruded its
low
into
the green-
initial
87Sr/86Sr
(0.701) m a y have been derived from the m a n t l e or from deep crustal
granuliteso stones, With
Chilimanzi
around
the
This
hence
renders the tonalites
they
initial
probably
87Sr/86Sr
originated
ratio of
indistinguishable from
the
same
0.7025-0.7045,
m a y have had a m o r e crustal component at its source.
the
from the greenmelting
process.
Chilimanzi
Suite
56
3.2.4 Limpopo Province The Limpopo
belt,
as
it is commonly
known,
is a major
zone of Archean
high-grade metamorphic and igneous rocks located between the Kaapvaal and the
Zimbabwe
cratons
(Fig.3.21).
vince into three subdivisions. culled mostly
from Tankard
Mason
(1973)
divided
the
Limpopo
pro-
The following account on this province is are
two marginal
zones
each adjacent to the Zimbabwe and Kaapvaal provinces.
The marginal
zones
are characterized
et al.
(1982).
There
by highly sheared rocks
striking parallel
to the Lim-
popo belt and composed chiefly of deformed granitoids and subordinate sequences
of greenstone
the granulite 3.8 Ga
old)
granulite
facies. and
There
all of which have been metamorphosed
is a central
supracrustal
and amphibolite
the central of
affinity,
rocks
facies.
metamorphism
in
have
The marginal
zone by shear belts.
regional
zone comprising
which
been
zones
basement
to
(ca.
metamorphosed are separated
to from
Although the timing of all the periods
the
adjoining
terranes
is
not
completely
known, a major tectono-thermal event occurred at about 2.7 Ga (Van Reenen et
al.,
1987).
The
overall
progressive
increase
in
metamorphic
grade
towards the Limpopo province from both adjoining provinces suggests a relationship grade
between
outwards
these
from the
three centres
suggests either differential deeper
crustal
levels
provinces.
of
of the
The
increase
Zimbabwe
and
in metamorphic
Kaapvaal
provinces
uplift in which the Limpopo belt represents
granite-greenstone
terranes
or
there
was
in-
crease in geothermal gradient towards the Limpopo belt.
Northern Marginal Zone (N.M.Z.) This
extends
like
a
wedge
from
southern
dying out south of the Great Dyke by the Tuli-Sabi
Zimbabwe
(Fig.3.21).
and
tapers
westward
It is bounded to the south
shear zone, while to the west in Botswana,
the boundary
between the N.M.Z. and the central zone of the Limpopo province is not so well defined. wards
In Botswana
directly
into the
the Central
Zimbabwe
Zone is believed
province
north
of
to grade north-
the Tuli-Sabi
shear
belt. The cover
rocks
of the N.M.Z.
granulite-grade
greenstone
positions
as
careous
such
belts
ferruginous
are represented
(quartz-bytownite-diopside rocks).
Low-pressure
2.9 Ga producing as
Botswana.
two pyroxene
Between
2.7 Ga
linear relicts
rocks),
granulite
possibly and
metamorphism
assemblages and
and rocks
2.6 Ga
throughout the
of
occur with com-
(quartz-magnetite-pyroxene
(cordierite-sillimanite-biotite-sapphirine thene
by
in which metasediments
rocks), pelitic
calrocks
sapphirine-hypertook
place
the N.M.Z.,
metamorphic
rocks
before as far of
the
57
N.M.Z.
were strongly deformed into major upright
enderbites gneisses.
were
produced
This
was
quartzo-feldspathic ditions
as
after a
granulite
result
eastern
displacement
areas,
the
Charnockites
metamorphism
of
segregation
and
the
while
Deformation during this phase
of the Zimbabwe province in the western
trusion of the Chilimanzi
region
for about
there was
of
from the
caused the
200 km in the
a displacement
partial
melting
between
of the
of
before the in-
Intrusive Suite of granitoid batholiths.
were intruded north of the N.M.Z.
They resulted
granulite
injection
about 50 km. This was followed by another shearing event, batholiths
and
and granite veins and magmas crystallizing under con-
of low water pressure.
sinistral
of
folds.
These
2.7 Ga and 2.6 Ga.
granulite
gneisses
which
formed at about 2.9 Ga. Central Zone in the Limpopo Valley The Limpopo valley believed
deformation between pre-cratonic are
among
Gneiss
falls within
3.8 Ga and 2.6 Ga.
the
oldest
comprises
rocks
nebulitic
known
area
(Fig.3.22).
quartzo-feldspathic
which
has
the gneisses
quartzite,
all
Beitbridge
Sequence,
comprise
The basement
to the Central
are
of which are
metaquartzites
(Tankard et al.,
1982).
Africa
gray
(3.8 Ga).
gneisses
of
The
of
banded
suggest
the
of
Diti-Shanzi
sedimentary
probably
and
parent
Nuli
The pre-cratonic
facies
in the include Suite
marble
meta-
rocks.
and
Also
in
Suites
eugeosynclinal
intrusions
to-
These are
Sequence
Metamorphic
deep-water
River
Metamorphic
iron-formations,
the Messina
Sand
dykes.
rocks of the Beitbridge of
which
granodioritic,
Sequence which has a carbonate
Other
Zone
(Fig.3.22)
composition with metabasite
gneisses
intercalations
in
layered
overlain by the Beitbridge the
Zone where
cover is made up of the Sand River Gneiss
nalitic and quartz dioritic Messina
the Central
(Tankard et al., 1982) to have undergone at least six periods of
the
which origin
of the Central
Zone
include the Messina Intrusive Suite (metamorphosed anorthosite and leucogabbro)
which
was
emplaced
between
3.2 and
3.1 Ga,
and
the
Bulai
Granitoid Gneisses emplaced at about 2.7 Ga. The emplacement of the Bulai granitoid was
followed by a widespread
and intensive
episode
shortening which was coeval with similar tectonic events province.
Throughout
of crustal
in the Zimbabwe
the Limpopo valley the flat-lying gneisses were de-
formed by this thermo-tectonic event into tight isoclinally upright folds with NE axial tocity.
trends,
caused by
shearing
and
strong
axial
plane
schis-
58
i, ~ v / . f
KAA PVA AL PROVINCE ~_---'~L
0I,, ~
t
km t
=
.'.L '., ; "A
tOO U
Crotonic cover Northern Morgincl,Zone1
~ C e n t r o l Zone i> LIMPOPO ~ 1 ~ SouthernMorginolZone ) ~ ~
Greenstonebelt Granite-Gneiss
Figure 3.21: Tectonic Zones from Tankard et al., 1982.)
Central
The
PROVINCE
ZIMBABWE PROVINCE KAAPVAAL
in the Limpopo
PROVINCE
province.
(Redrawn
Z o n e in B o t s w a n a
northern
structural
margin
of the
Tuli-Sabi
boundary
between
the Limpopo
shear
belt
province
is believed and
the
to be the
Zimbabwe
pro-
59
vince (Fig.3.23); otherwise there is no precise boundary. there
are
gray
representing
layered
basement.
tonalitic
The
age
gneisses
of
the
of
At Baines Drift
unknown
overlying
age
probably
metasediments
of
the
Baines Drift Formation is also uncertain although the entire Baines Drift Metamorphic
Suite
equivalents
of
is
the
generally
believed
shallow-water
facies
to of
represent
the
such as the nearby Matsitama belt in Zimbabwe. complex
Matsitama
sequence
and
and
there
are
folded
shales
large
sheets
into nappes
appears
of
during
everywhere
in
consists
basalts
and
layered
2.7 Ga-greenstone Zone;
current-bedded
dolerite
sills.
deformation
Limpopo
belts
the structurally quartzites,
Near
Baines
metagabbro-anorthosites
the major
the
of
high-grade
Metamorphism is much less
intense in the Matsitama belt than in the Central marbles,
the
phase
province.
The
of
Drift
which
2.6 Ga,
strata-bound
were which Ni-Cu
sulphide deposit at Pikwe was formed during the intrusion of the layered metagabbro-anorthosite.
Southern Marginal Zone (S.M.Z.) This
zone
from
a
gneiss shown
which
displays
typical grade by
the
low-grade
was
these
described
authors
mafic,
typical
ultramafic,
Pietersburg, Africa
and
detail
Southern crust.
felsic,
and
deformational to
the
high-grade
by Van
Reenen
et
al.
(1987).
Marginal
Zone
In this
zone
represents
(Fig.3.24)
lithologies,
volcano-sedimentary
and Rhenosterkoppies
are tectonically
transition
terrane
granite-greenstone
Sutherland,
(Fig.3.24),
in
the
section through the Archean ward-dipping,
metamorphic
granite-greenstone
a
cross
steep north-
comprising
assemblages
greenstone
As
belts
the
of of
the
South
juxtaposed with and overlain by pro-
gressively higher lithologies from south to north. The Pietersburg greenstones,
at least 3.45 Ga old, is at greenschist-
grade in the central and southwestern parts and is succeeded along shear zones
by amphibolite-grade
rocks
and Sutherland greenstone belts grades
relative
trondhjemitic
to shear
in the northeast.
zones and are surrounded
Baviaanskloof
The Rhenosterkoppies
show similar arrangements
Gneiss
which
is
of metamorphic
by the tonalitic
about
3.5 Ga
old.
and The
Baviaanskloof Gneiss and the greenstone assemblages can be followed uninterrupted
across
the
transition
from
amphibolite
grade
to
granulite
grade. At this transition
there is a significant change in deformational
style in which high-grade
greenstones are highly reduced compared to the
more
extensive
(Fig.3.24). semblages
In and
outcrop the their
of
the
granulite intrusive
lower-grade terrane,
lithologies
metamorphosed
granodioritic
plutons
to
the
greenstone
have
yielded
south asages
60
around
2.65 Ga w h i c h
reflects a w i d e s p r e a d
tectono-thermal
event of this
age.
CRATONIC COVER ( SOUTPANSBERG,KARO0) 8ULA! GNEISS MESSINA INTRUSIVE SUITE
SINGELELE GNEISS
|OR]
s~Nz, M~AMORPHJCSU,TE J
,,~
LoJr!
.ETAMORPH,C SU,TEJ
SAND RIVER ONEISS .
m
FAULT ~.,/~/~
TRACE OF LAYERING
Figure 3.22: Type area Tankard et al., 1982.)
of
the
Central
Zone.
(Redrawn
from
Tectonic Models
V a r i o u s models have been proposed for the origin of the Limpopo belt. The only points of agreement,
as summed up by Shackleton
(1986), are that the
L i m p o p o belt shows evidence of drastic tectonic crustal thickening,
and a
61
complex
deformation
sequence
continent
plate
movements
of the Kaapvaal
on
the
both
collision.
Tuli-Sabi
cratons
1983; Light,
APPROXIMATE
shear
rotated
of
great
The various and
zone
towards
Zimbabwe (Coward, one
collision cratons 1976),
another
1982; Van Reenen et al.,
LIMITOF --.,~,. --.,
intensity,
suggesting models
either
or
(Barton
involve
relative
as dextral
compressional and
continent-
Key,
motion
motion
1981;
Fripp,
1987).
-.2700Mo]
,,•v
10
Retrograde Isograd j
~6 .Q Y
~
And
150
'3~o
Figure 3.25: Marginal Zone. this
'
i ¢~..o :P,o,o, ...,/(~)PHz0:02Ptotal
~ ~0
Vm [ n o o Ma]
During
/
: - / / \ , /, , Sill /Anth / =En.Qt,*H=O ~ ,~"~,,,~
."/
produced
i
Ky . . ' " t ~ , '
¢;..~,e~,,," 0
/,~W~
[ >2/.,50 Ma] ~ ' - -
'
-
' ~ 0 . . .600 . . . . 750 .
Temperoture(°c)
A.
9~0
Pressure-temperature-time path for the (Redrawn from Van Reenen et al., 1987.) decompression
vast
in virtually all rock types;
The granulite-grade
rocks
volumes
of
granitic
Southern
melts
were
and the Matok pluton was emplaced.
of the S.M.Z.
were uplifted
during
this event
64
and the g r a n u l i t e terrane was established in this zone at that time. The southern m a r g i n regional the
of this
encroachment
retrograde
d e h y d r a t e d granulite
of
C02-rich
orthoamphibole
fluids
isograd
terrane was
which,
in
by
Fig.3.24
subjected
rehydration, which
can
to a
caused
be
traced
over a d i s t a n c e of 150 km. Van Reenen et al. s u g g e s t e d that the behaviour of the entire n o r t h e r n part of the Kaapvaal p r o v i n c e was consistent with their o b s e r v a t i o n that the high-grade a s s e m b l a g e s of the S.M.Z.
had been
buried down to 27 km before being uplifted. The Central the rocks they
Zone, however,
of the M e s s i n a
underwent
had experienced
a unique history in which
area had been buried down to about
high P/high T
granulite-facies
35 km where
metamorphism
at
about
0
10 Kbar
and
800
ditions
existed
(Shackleton, perienced S.M.Z.
C
before
about
1986);
a
approximately
2.7 Ga
and
ago
when
thereafter
the
the
pressure-temperature-time
3.12 Ga
ago;
Bulai
rocks
of
evolution
amphibolite
Gneiss the
was
Central
similar
to
con-
emplaced Zone
that
of
exthe
(Van Reenen et al., 1987). A c c o r d i n g to the latter authors the de-
c o m p r e s s i o n event in the Limpopo belt, during w h i c h h i g h - g r a d e rocks were brought to the surface, was accompanied by a coherent and coeval regional deformation,
in
which
in
t r a n s p o r t e d to the west; to
the
north;
ported
to
and
the
et
lated
(Fig.3.25,
(1987) B)
thickening
Himalayas. rapid
rebound
tectic
the
rocks
of
the
tectono-thermal the
Limpopo
initial
phase
to what
the
its
Central
is
event
orogeny, of
lateral
spreading
for which
they postu-
of
by igneous at
trans-
2.7 Ga, Van
collision
taking
on m i x e d
were
about
continental
currently
isograds
Zone
at
Zone were
Zone were thrust
Marginal
followed by the r e - a d j u s t m e n t
of high-grade
and
crustal
of
Southern
of the crust accompanied
melting
Finally,
termed
similar
This was
the e s t a b l i s h m e n t
This
an
west
rocks of the Northern M a r g i n a l
rocks
south.
Reenen
crustal
al.
the
the
place
the
with
under
the
isotherms,
and
lithologies
at depth;
d i a p i r i s m due to ana-
depth
(Fig.3.25,
e q u i l i b r i u m was attained in w h i c h the crustal
B,
4).
thickness
of the u p l i f t e d areas approximated those of the surrounding cratons. 3.2.5 A r c h e a n M i n e r a l i z a t i o n on the Kalahari C r a t o n Archean
high-grade
mineral
deposits
formation.
amphibolite-granulite
because
However,
regions
have
of their high m e t a m o r p h i c
not
supplied many
grade and
strong de-
as a l r e a d y pointed out, an economic deposit of Ni-Cu
occurs in a 50 m wide amphibolite layer in the S e l e b i - P i k w e area of Botswana in the central zone of the Limpopo province By nomic
far,
the
mineral
granite-greenstone potential
which
are
belts
have
ranked
(Fig.3.23). yielded
among
the
the
highest
world's
eco-
largest
85
sources of Au, Ag, Cr, Ni, Cu, and Zn. Before r e v i e w i n g southern Africa's enormous m i n e r a l work will
deposits
(Fig.3.26),
first be considered
the g r e e n s t o n e m e t a l l o g e n i c
in general
terms.
A unique
of A r c h e a n
terranes all over the world is their r e m a r k a b l e
the
and
types
modes
of
mineral
occurrences,
hence
frame-
characteristic similarity in
A_rchean
greenstone
belts c o n s t i t u t e a d i s t i n c t m e t a l l o g e n i c p r o v i n c e in the d i f f e r e n t shield regions
of the world.
deposits
and
asbestos, gold,
There
the m a j o r
magnesite
silver,
granite-greenstone
and
copper
is a close r e l a t i o n s h i p
talc
and
occur
zinc
are
in
rock
types.
ultramafic
found
in
between
the mineral
Chromite,
flows
and
the m a f i c - f e l s i c
iron ore, m a n g a n e s e and barytes occur in s e d i m e n t a r y rocks; and
pegmatites
molybdenum
and
are
the
sources
bismuth.
The
of
lithium,
primary
source
tantalum, of
most
occurs
in
sulphide,
quartz
lode
quartz
lodes
occur
(Anhaeusser, formations carbonate
modes
and
within
1976).
where
four
as
beryllium,
of
a
of
the
gold
of
(Fig.3.27).
deposits
are
leaching
and
or
massive
Most
of
granitoid
found
facies
tin, miner-
Gold miner-
deposits,
surrounding
in the oxide
result
stratiform
disseminations
Stratiform-type
as
as
the margins
they occur
facies
viz.,
volcanics;
and granites
a l i z a t i o n in southern Africa were the m a f i c - f e l s i c volcanics. alization
nickel,
intrusions;
in
banded
in the
the
plutons iron-
sulphide
precipitation
of
gold
and by
o
circulating
volcanic
thermal brines
at temperatures
below
400
C
(Fripp,
1976).
Gold Gold occurs in the g r a n i t e - g r e e n s t o n e belts of Zimbabwe and South Africa, but the largest deposits are concentrated in the A r c h e a n - P r o t e r o z o i c Witw a t e r s r a n d s u c c e s s i o n which will be treated later. mining
dates
back
to the Middle
deposits.
As
summarized
Zimbabwe,
30
are
Sebakwian sulphide mostly
deposits
in
the
by Hutchison
stratiform
greenstones,
without
there (1983),
mineralizations
seven
Bulawayan
ages,
occur
in
stratigraphic and
Shamvaian
In Zimbabwe where gold
are m a n y of the
varieties
100
associated
more
massive
control,
56
successions
are but
of gold
larger mines mainly but
with
stratiform
in quartz also
in the
in
lodes
the
Se-
bakwian, w h i l e seven are strata-bound d i s s e m i n a t e d deposits p r e d o m i n a n t l y in the B u l a w a y a n and Shamvaian successions. In the S e b a k w i a n where most of the s t r a t i f o r m gold d e p o s i t s Zimbabwe,
mineralization
formations
that
tuffs.
individual
The
are
is
found
interlayered gold-bearing
in
several
with
mafic
beds,
are c o n f i n e d to sulphide beds and mixed iron-formations
(Foster
and
Gilligan,
thin and
beds felsic
generally
less
of
Gold
banded
iron-
water-deposited than
sulphide-carbonate 1987).
occur in
5 m
thick,
facies
in the
occurs
as
minute
66
K/ •
.
.
.
•
'
KARI.A 2, .
°
°.
. I
I
:\'.
. " CRAyON:~
,.k::
• :..
•
•
•
.
°
.
°'
.~
,~
f.°"
LEGEND , e ~ 4~ . , ~ Younger cover ' " "
~P.../... . °/•
4"P, Felsic phaseI Bushveld ] Igneous %(IOTSWt + + I~////~ Mofic phase Complex ~- + / '~"~ rchean granites gneieses Archean Greenstonebest(Gold belt~ +
~ ~
~UE
,~:: ~.~.'..
Sedimentary unit ,a,io-,.l.,c un, .,< Ulh'amafia- maficunit
I
! ''p°"on, pegmatite fields
N
"
• •
+ MICA 3USHVELD
":r~ "".~ :" "
' : •
.
";bur~'" " " ~ ~; . ~.SWAZILAND .; o h a n n e s • • KAAPVAAI J
~' ~I~7-:/ •
-~ ~ -~
•
•
•
W I T W A T E R S R A N D
e •
" "
" "
~OLDFIELD
Known entry pointsofsedimente and gold
IO O k m
////
Source area for Witwatererand gold
F i g u r e 3.26: Some mineralizations ( R e d r a w n f r o m H u t c h i s o n , 1983.)
on
the
Kaapvaal
craton.
67
50-micron grains of native gold with arsenopyrite,
in an ore grade which
averages ii ppm (at-l).
'
I
f~
C5o752 -.
•
I
'
_
volcamc
centre
~.lj;,.~---"
~k'~
O
~.jJ) ']-) _
or jumorole~..~-q~'~f
...... ~
'
~.
~Sso
,P~u="SU/':~! A V ' - - ~ ' =
•
"
Level
-
b a s in
s E DIME N T A R Y - "
.,
"=/:/A'>>/ i >/:> >>>>>>:
R
::-_-:::::Sediments incf uding shales
3yroclastic rocks :heroical sediments ~yroclastic rocks Ore Ore reLsic volcanics :hemico( sediments
Z W -r ,
/- , ' / ~
-
2;/
WEST POST ROKELITE COVER
MYLONITE ZONE RRG
~z O
ROKELITE TECTONIC AND 0,. REACTI YAT~ON tu
--
.~ LIMITED RIFTING
~
¢
., ~.~"
NQtal mobile bc[t
A. Kasai - NE Angola shield mobile belt B. Southern Angola [. Gabon Orogenic belt D. Ubendian belt
~
Cratons older than 2-0 5a
~
Inferred pad of older crafon |>2"0 6a) TerrQnes between2'5 and t-TGa reacfivafed by later events
Figure 4.1: Distribution of Early Proterozoic rocks in Africa. i, Pongola basin; 2, Witwatersrand basin; 3, Ventersdorp basin; 4, Transvaal-Griqualand West basin; 5, Waterberg-Soutpansberg-Umkondo-Matsap basins. (Redrawn from Cahen et al., 1984.)
115
4.2 Kalahari Cratonic Basins
4.2.1
Introduction
The
Late
the
formation
tained
Archean-Early of
crustal
Proterozoic
extensive
stability
in
(Fig.4.2,A). ,,~,~ I } ~
r,.'-.>,.".>~
~
"-.~-.J-.-~'. ' ". : -
//. . ~ , . - ~ ~ e r , .
Vermoas \~.111 /.Kle k d o r p . : . / . : - - ~ : Dome .. - - ~ .- "/~.~red~,a. P ~ h ,~/I.~.~.: Wesselbron
'.: ~. -'3 I ~ , ,
"./, ~
: . . " • . -:/f'~
....
../
:." " " " : ~k~k-C~ ~
: : L. _~_" _ _ "."
Oo~e~ ~'~'-y/ "\ ~"-
" ~- ~
/
- ")" \ \ \ = ~ . ~ ' / /
V -Ira'
) .///",~ "
)
". ".~I CJI,~-/-'~
--
-.~
./ Steynsrus
"
Cedarmont
Dome
heunissen
r== "~m'ru.= r ~,u" IJ ~
L
200kin ,
Elongationdirectionsfor platform- subtidal mounds Outcrop and subcrop of Transvaal and Griquoland West Supergroups Iron-formation Carbonate - chert
A NW
Basinal cherts and iron-rich dolomites upward into iron formations/
~_~o~/~ "~ Volca~c ~ " rocks B
...High-water level , . / , / L o w - water level
SE
"-~" -'---Z--------------~--Platform tidalflat ( recrystallized dolomites and cherts) Platform subtidal ~ (iron-poor doldmites) ~
Platform- edge (limestones and iron-rich dolomites)
Figure 4.10: Tectonic setting for the Ghaap and Chuniespoort carbonates. (Redrawn from Tankard et al., 1982.) Basinal iron-formations, the Asbeshuewels Subgroup, and its metamorphosed equivalent, the Penge iron-formation of the Transvaal area (Fig.4.9), contain lithologic units which are laterally persistent over
131
v e r y long distance.
These are chemical deposits with stacked vertical cy-
cles of v o l c a n i c material,
chert,
siderite, m a g n e t i t e and m i c r o b a n d e d he-
matite. The u n c o n f o r m a b l y o v e r l y i n g Pretoria and P o s t m a s b u r g Groups are
primarily
shales,
regressive,
with
fluvial
arkoses,
tidal-flat
(Fig.4.9)
arenites
and
shoal w a t e r quartz arenites and oolitic ironstones and volcanics,
all of w h i c h signal a dramatic change in the pattern and source of sedimentation.
The e m e r g e n c e of most parts
of the T r a n s v a a l
land surface with little relief and little erosion, sion
of
ering
the
and
Chuniespoort-Ghaap
paleosol
epeiric
development
sea,
(Reimer,
basin
as a vast
f o l l o w i n g the regres-
favoured
1987).
pronounced
Products
w e a t h e r i n g o c c u r in the Pretoria Group as a l u m i n a - e n r i c h e d
of
weath-
intensive
sediments,
and
as "minette"-type ironstones, which are p r o b a b l y the oldest ironstones of this
type
in the
however,
geologic
contains
manganese,
formation
of
complexes w h e r e stones
The more
sediments
chert
of
silica
whereas
basinward
such
in the G r i q u a l a n d West basin.
large-scale m o b i l i z a t i o n the
record.
chemical
the
was
group.
Group,
iron-formation
sea w h e r e
mobilized
and
The paleosols with
it c o n t r i b u t e d
into
it formed the oolitic and p i s o l i t i c
of the Pretoria
Postmasburg
banded
P r o n o u n c e d w e a t h e r i n g caused the
into
iron
as
deltaic
"minette"-type
over
20% AI203
to
coastal iron-
was the
source of alumina enrichment in the Pretoria Group shales. Another Griqualand
interesting West
aspect
Supergroup
of
the
relates
stratigraphy
to
the
of
the
implications
Transvaal of
c a r b o n a t e s e d i m e n t a t i o n to the e v o l u t i o n of the p r i m i t i v e atmosphere. carbonate being
platforms
among
of
the
the oldest
tually throughout
Chuniespoort
in the geological
their
1,500-m thickness.
and
Ghaap
record,
Groups,
are
and
large-scale The
apart
from
stromatolitic
vir-
Since there w e r e no metazoans
in the Early P r o t e r o z o i c seas to graze upon them, the s t r o m a t o l i t e communities
were
able
green algal ing that some the
to
colonize
several
large amounts
2.2 Ga
ago.
regional
there
were
limited
unconformity: separating
that
the
Well-preserved
blue-
stromatolites
prov-
from these
of oxygen were being
Whereas
from the P r e t o r i a - P o s t m a s b u r g Groups, formity,
environments.
filaments have been recovered
generated
by photosynthesis
oxidizing
conditions
Chuniespoort-Ghaap
below
carbonates
there is evidence above this uncon-
following the deposition
of the carbonates,
o x i d i z i n g con-
ditions d e v e l o p e d
in the hydrosphere and p r o b a b l y also in the atmosphere
(Tankard
1982).
breccia taken
et al., which
into
rather was
defines
solution
The
the
during
precipitated
presence
unconformity
upon
of
manganese
suggests
oxide
that
subaerial
weathering
of
oxidation
to its h i g h e r
in
the
manganese the
chert
was
not
dolomite
but
valency.
With more
oxygen a v a i l a b l e the ironstones in the Pretoria Group and the iron-forma-
132
tions
in the upper Postmasburg
presence
of
significant
quantities
m i n e r a l s in the calcareous formation
are
further
Group are deficient of
manganese
in ferrous in
iron.
The
quadrivalent-state
shales which are i n t e r l a y e r e d w i t h i n the iron-
evidence
suggesting
the
prevalence
of
oxidizing
conditions. 4.2.5 M i n e r a l i z a t i o n in the T r a n s v a a l - G r i q u a l a n d W e s t S u p e r g r o u p s Iron and l~nganese As
aforementioned
are
present
"Minette"
in
type
"minette"-type the
is
Transvaal
mined
around
ironstones and
and
banded
Griqualand
Pretoria,
with
West
iron-formations Supergroups.
reserves
of
about
The 6x109
tons at 45% Fe. M a n g a n e s e occurs in calcareous sediments interbedded in the iron-formation group
immediately (Fig.4.11).
the world's
above Known
the Ongeluk as
lava
the Kalahari
in the G r i q u a l a n d West Manganese
Field,
this
is
Superamong
largest reserves of manganese, with over 800 m i l l i o n tons. Up
to three layers of conformable m a n g a n e s e ore, with strike lengths of over 50 km, are d e v e l o p e d w i t h i n the basal 100 m of the banded iron-formation. The lowest m a n g a n e s e band is up to 25 m thick. N e a r - s u r f a c e supergene-enriched ores and the p r i m a r y ore are mined. The p r i m a r y control of manganese m i n e r a l i z a t i o n was the original sedimentary are
environment.
interbedded
Discrete
within
the
bands
banded
of
chemically
iron-formation
deposited (Fig.4.12).
manganese Iron
and
m a n g a n e s e oxide facies occurred in the nearshore part of the depositional basin,
while
limestones,
sent the distal basinal seawater
with
Hutchison,
small
1983).
iron-formation
carbonates
repre-
facies. M a n g a n e s e was m o s t l y held in solution by
addition
With
and m a n g a n e s e
from
hydrothermal
the onset of more
sources
oxidizing
(Beukes,
conditions,
p r e c i p i t a t i o n of m a n g a n e s e took place in deep marine water
1989;
chemical
far away from
clastic d e p o s i t i o n a l sites. Cold Gold was
first m i n e d
Rest-Sabie
gold
on a large-scale
deposits
in South Africa
(Figs.4.8;4.11)
in
eastern
from the Pilgrims Transvaal
in
1872.
M i n i n g was by the w a s h i n g of elluvial and alluvial sediments and by working o x i d i z e d and sulphide ores. After nearly a century of mining production
ceased
centimetres
(Anhaeusser to
tens
of
and
Button,
centimetres
1976). thick,
Quartz-pyrite
cross-cutting
reefs,
veins,
a
few
sausage-
-
':::::':.:'
.
:
.
/
Centr~z~ zone ~i . Namaquo Province < 30 Eastern Marglno[ z o n e ] -~ 3-0 Kaopvaa! Province TVMB Tan~olffe Valley rnytonite beIt
zones of the Namaqua p r o v i n c e and geothe Central Zone. (Redrawn from T a n k a r d
154
the
Early-mid
Proterozoic
Namaqua
along the southern margins belt
is situated
and
Natal
of the Kaapvaal
on the northwestern
mobile
province,
part
of the
belts
are
whilst
the Magondi
Zimbabwe
located
province.
At
the end of prolonged and intermittent tectonic activities the Namaqua and Magondi
belts
(Clifford,
stabilized
and
became
parts
of
the
Kalahari
craton
1966).
The Namaqua belt is a highly complex high-grade metamorphic mobile belt comprising several terranes of varying ages
polyorogenic
(Fig.4.20), rang-
ing from about 2.0 Ga to 1.0 Ga. Prolonged tectonism involving high-grade metamorphism,
crustal
tectonically
comparable
reworking with
most of the crystalline
and
the
basement
shearing
Limpopo
renders
belt.
complex
The
the
Namaqua
Namaqua
of southwest
belt
Africa
stretching
from southern Namibia to the southwestern part of South Africa. qua
basement
volcanics
is
to
the
concealed
beneath
north
south
and
the
and
Nama
domain
(with
Zone (Fig.4.20),
supracrustals
the Richtersveld
domain),
about with
Zone or Namaqua Metamorphic orogenic
heterogeneous
mineralized
1.3-Ga
metasediments
The Namaqua province
about
Complex which
collage of
The Nama-
sediments
and
of
the
consists
formerly referred to as the Kheis
3.0 Ga old);
rocks
Karoo
by deformed
Late Proterozoic Gariep Group in the west. of an Eastern Marginal
and
belt forms
a Western
2.0 Ga old;
and
(formerly
the
Central
is a complexly deformed
low-to high-grade
volcano-sedimentary
Zone
gneisses
supracrustals
last
poly-
and highly affected
by
tectonism at about 1.0 Ga (Tankard et al., 1982). 4.5.1 Eastern Marginal Zone This is a narrow (15-30 km wide) precratonic the
Doornberg
Brakbos
zone of low-grade but complexly deformed
cover rocks which is separated
fault
fault,
and
from
(Fig.4.20).
The
the
from the Kaapvaal
Namaqua
Eastern
Zone
Metamorphic is
province by
Complex
metamorphically
by
the
transi-
tional between the Namaqua gneisses in the west and the Kaapvaal basement and cratonic cover sequences to the east (Tankard et al. 1982). The rock assemblages in the Eastern Zone comprise the Marydale Formation
and
the
Matsap
Group
(Fig.4.21)
which
are
similar
to
the
Archean to Early Proterozoic platform cover of the Kaapvaal province. unconformably included
in
greenstone
overlying this
belt,
Late
chapter. 3,0 Ga
Proterozoic
The Marydale old.
It
was
clastics Formation
intruded the Marydale
represents
metamorphosed
facies at about 1.9 Ga. Late Archean granitoids the only Kaapvaal
and volcanics to
the
Late The
are not
an Archean greenschist
of the Kaapvaal province
Formation at about 2.9-2.5 Ga. The Matsap Group is
cratonic
sequences
that has been directly
traced into
155
the Eastern Zone where it is represented by m e t a s e d i m e n t s sericite
schists,
hematite
quartzites,
such as quartz-
metaconglomerates
and
schistose
basic lavas.
I CE N M A NTA RAL ZON Ei ~
K AE AL I PA RA OP VN IVC Axial trace of F2
Upington \x x ~
,!;!;!i!i i ;
........ Bound~/between Namaqua ~ ~" and Kaapvaal Provinces -- -- -- Boundor y between Eastern i~argin~ Zone a n d Centra! Zone Unconformity or intrusive contact in stratigraphic column ~ - - - - Major fou(t or shear zone ~T'Thrust fault xxx~ Kaa)enHi( AGE (Go)
)-2
~
KorasGroup
>1.2-~'0 Namaqua granitoids}
~ :
NomoquaGneisses
2.0
Complex
~ m
2-6
x
Wilgenhout Drift F* "l "] Koa)en Formation LMatsapl Groblershoop.D obep F. etc]Group |
Gr~.oo,aodwest.ope.~oop ~
.:?:2'.)i?:'
Ventersdorp S;Jpergroup
x x'~ X X
|KoGpvaa|
roA%?,c .~
2,9- 2.6 >~30
greenst one "•Archeon be(t
Mar ydale Formation
l
,
CENTRAL ZONE )EASTERNMARGIi NAMAQUA PROVINCE
Figure 4.21: Eastern M a r g i n a l Zone and adjacent areas of the Central Zone and the Kaapvaal province. (Redrawn from Tankard et al., 1982.) Structurally,
the
most
characteristic
features
include A r c h e a n isoclinal folds of v a r i a b l e sizes; r e f o l d i n g which resulted
of
the
Eastern
Zone
E a r l y - L a t e Proterozoic
in complex fold interference patterns;
and mid-
P r o t e r o z o i c NNW faults and shear belts. A late m e t a m o r p h i c e v e n t at about 1.35 Ga p r o d u c e d greenschists to g r a n u l i t e facies rocks.
156
4.5.2 Western Zone The vast Central west-central
Zone or Namaqua Metamorphic
rocks and high-level the Western age,
Complex
is occupied
part by a small wedge-shaped belt of low-grade intrusions,
in its
supracrustal
known as the Western Zone. The rocks of
Zone consist of the Orange River Group of Early Proterozoic
and a composite
granite batholith,
the Vioolsdrif
Intrusive
Suite,
of slightly younger age (Fig.4.22). The Western
Zone is,
morphic Complex,
however,
an integral
part of the Namaqua Meta-
in spite of the fact that the rocks of the Western Zone
are weakly metamorphosed and deformed, and in spite of the rarity of pre-
AGEIMa) 50kin
< 550 Nomo , K o r o o
!
covet
550 Kuboos - B r e m e n !! ! ~J Suite ~ 900 G o r i e p
Intrusive
Group
920 Richtersveld Ánirusive S u i t e
C E N T R A L ZONE ~
Rosh Pinah=
No2noqus Metamorphic Complex
"".%, .x, ."..
" .~
.....
..
' .....
1900~ 1730 Vioolsclrif Intrusive Suite f~I~/;[:~i~ RosyntjiebergF m I Orange ~
~" - ~ " /
ZONE
Goodhouse ond Helskloof gronitoids
r///xR,l
< ........ . ,,
~ . . -
WESTERN ~
2000
~,~ N ~
~
NAMIBIA
/ DeHoop/
y.~b
~- River = Grou
FmJ
P
T a n t o l i t e Volley m y l o n i t e belt T h r u s t nappe
H b - H o i b mine S b - Subruins Sw- Swartkop He- Klein Helsktoof H - Henkries
q.-
)ranj
ATLANTICocEAN L SOUTH AFRICA ! ~ L
Figure 4.22: Western Zone from Tankard e t a l . , 1982.)
:.'.c%-..
of
w pe g matiter, ......................
the
Namaqua
, .....
province.
....... . .........
.... ..
(Redrawn
157
served basement rocks. The Orange River Group comprises the De Hoop Subgroup of intermediate and acid volcanics;
the coeval predominantly acid
and basic volcanic Haib Subgroup which is dated at 2.0 Ga;
and the con-
formably overlying Rosyntjieberg Formation which comprises metaquartzites with ripple marks and cross-bedding and intercalations of magnetite ironformations, foliation
chlorite schist and metapelite.
accompanied
by
the
low-to
Largely cataclastic regional
medium-grade
Orange River Group and the Vioolsdrif
metamorphism
Intrusive Suite,
of
the
trends east-west
along the nose of the Western Zone, but swings from a NW-SE to N-S direction in the northwestern area. The
vioolsdrif
Intrusive
Suite
was
emplaced
1.87 Ga, structurally below the Orange River Group. basal
basic--ultrabasic
diorites
with
minor
layered
diorite.
suite generally exhibit aging 0.7031,
suite,
The
extensive
intermediate
between
tonalites,
rocks
low to moderate initial
2.0 Ga
and
It comprises a small in
and
the
grano-
Vioolsdrif
87Sr/86Sr ratios,
aver-
suggesting major additions of mantle-derived calc-alkaline
volcanics to the crust during the emplacement of the Vioolsdrif Suite in the Early Proterozoic.
Porphyry-type copper and molybdenum sulphides are
found in the porphyritic granites of the Vioolsdrif Suite; these are the orebodies in the Haib mine. 4.5.3 Central Zone (Namaqua Metamorphic Complex) This vast medium-to high-grade metamorphic terrane consists of a heterogeneous basement and an overlapping sequence of supracrustal volcano-sedimentary rocks which witnessed tectono-thermal events between 1.9 Ga and 1.75 Ga in the Early Proterozoic and again at i.i Ga in the mid-Proterozoic
(Moore
et al.,
1989).
The
divided into the Namibian part, land
section
order.
Since
(Tankard
et al.,
primary mineral
Central
Zone
(Fig.4.20)
has
been
sub-
the Namaqualand sector and the Bushman1982),
which
assemblages
are
and
here
described
sedimentary
in
that
features
have
been considerably obliterated in the Namaqua Metamorphic Complex by metamorphism and deformation, the parent
the
following account
stresses
the
nature of
rocks which were referred to by their bulk composition,
assigned
to
(Tankard
et al.,
tectonic
environments
1982).
Because
large-scale discontinuities
and
on of
the
basis
structural
thrusting,
the
of
their
geochemistry
complexities rock
types
and
are
involving not pre-
sented in their inferred order of superposition, but rather in a structural sequence.
~1]
~
Aus
\\
Luderltz vvv
:,.... District
20S7±65 Mo
2022±50Mo
1918+-1& Ha 19/-6+-33 Mo
? 1600 Ha
Chegga Assemblage . . . .
deformation, greenschist facies Aguelt Nebkho group
Yetti granite Imourene group Aioun Abd el Molek group
2039.~/.9 M= 2057.+66Ma 2039.+&gMa Polyphase
Folding with thrusting n a ppes
2022~_50M0
rupakivi
Tiguesmot granite Ain Ben Till granite
El Archeouat granite
Bir Moghrein granite (and closure of its biotites)
Tobatanat
Bose at>10S0 Mo
1912 +. ~.7 Me 1970+-&6 Ha
1877".35Mo
1755.'65 Ha
1563 ± 28Ha
3270Z 3&7 Me Hassi el Ghollaman gneisses ( granulite to amphibolite facies)
group
c.2710Ma migmatitic complex
WESTERN EASTERN
(Redrawn from Cahen et al.,
2 EASTERN PROVINCE
of the Taoudennl basin
~E 2539_*S&Mo 6hallaman granites
Polyphase deformation
erasure ages
biotite
closure ages
cover
? 1600 Ha do|erltes
Supergroup 1 of the
PROVINCE
biotite
SW~- . . . . . . . . .
2050.~119 Mo
1811.'56 Ma 1872.*$2 Ma
15&6 ~ 32 Mo
1 SOUTH-WESTERN
Table 4.3: Tectonic events in the Reguibat Rise.
C
BASEMENT
A
B YETTI CYCLE
CYCLE
EOLAB
195
4.9 Zaire Craton
4.9.1 I n t r o d u c t i o n Unlike the K a a p v a a l craton which had m o s t l y a t t a i n e d crustal stability by the
end
of
the
dominantly
the
(Clifford, the
the
Zaire
of
the
1970).
Eburnean
Angola;
Archean, products
and
In the
orogenic
Zaire craton
cycle
and
belt.
foreland
in the West African
quite p r o l o n g e d about
2.4 Ga
2.15 Ga
in
foreland
in
West
there
the
African
cratons
Proterozoic
is u n a m b i g u o u s
Kasai-NE
are
Eburnean
Angola
pre-
orogeny
evidence
shield;
for
southern
in the Gabon orogenic belt, where Eburnean rocks provide the
basement As
and Early
to
the younger
West
the Eburnean
Congolian
o r o g e n y was
in the Zaire craton where orogenic episodes
and
2.2-2.0 Ga
southern Angola;
of
craton
Pan-African
the
West
in
the
and
Kasai-NE
at about
Congolian
belt,
2.0
and
apparently
are known at
Angola
shield;
in the
internal
in
the
Gabon
mobile
at
about
zone
orogenic
and belt
(Fig.4.1). 4.9.2 Kasai - NE A n g o l a Shield On the u p l i f t e d Proterozoic A
basement
2.2-2.0-Ga
this
southern part of the Zaire
part
gneisses,
tectono-thermal
of
the
Eburnean event,
craton
migmatites event
(Cahen
et
was
craton patches
and m e t a s e d i m e n t s
the
al.,
last
orogeny
1984)o
locally termed the Mubinji
orogeny
localized A r c h e a n
and
and m e t a m o r p h o s e d
the
about
deformed
2.42 Ga
(Table 4.4).
The
Luiza
Luiza
are exposed. affected
the
earliest
(Cahen et al.,
and rehomogenized
basement
metasedimentary
Supergroup
is
a
to
which
However,
had r e c r y s t a l l i z e d also
of Archean
1984),
gneisses cover
at
metasedimentary
sequence of quartzites, mica-schists and banded i r o n - f o r m a t i o n s
lying un-
c o n f o r m a b l y on the Archean Kanda-Kanda tonalitic and g r a n o d i o r i t i c gneisses
(Fig.3.31).
Angola
as
Similar
outliers
of
metasedimentary
the
Luiza
rocks
Supergroup.
A
occur
near
later
Eburnean
Mufo
in
NE
tectono-
thermal event at 2.2-2.0 Ga caused more w i d e s p r e a d m e t a m o r p h i s m of basement rocks and the emplacement of anorogenic granites and pegmatites
some
of which cut the younger Lukoshi m e t a s e d i m e n t a r y formations. The the
Lulua
lateral
volcanic
Group which foreland
assemblage
Supergroup
which
are
about
(Fig.3.31)
metasedimentary
either
equivalent in
components
interstratified
6 km a belt
post-dates of
it,
thick, about
the
is
Luiza
Supergroup
a metasedimentary
lying 170 km
to
the
long
north and
20 km
of the Lulua G r o u p are slates with
greenstones
comprising
of
or
is
and
meta-
the
Luiza
wide.
The
and quartzites,
spilitic
basalts,
196
lavas
(including u n m o d i f i e d
(Cahen et al.,
pillow lavas)
and p o s t - t e c t o n i c
1984).
Table 4.4: M a j o r tectonic events in Kasai and adjacent the Zaire craton. (Redrawn from Cahen et al, 1984).
6, LOHAMIAN
granodiorite
OROGENY
5, POST-LULUA
of
c. 975- 9/.8± 20 or 937*- 20 Ma. - - M b u j i Mayi supergroup
FOLDING
&. c.2200 - 2 0 0 0 Ha
parts
Pre-llSG±lGHa (age of a pos|-tectonic syenodiorite silt ) --?Lulua group (interstratified spilitic lavas 1468± 30 Ma.
orogeny
f
pegmatites: syntectonic
c. 1920 Ha 2037 ± 30 Ha
granites:
post-tectonic
2200 - 2050 Ha
events:
__ perhaps Lukoshi formations 3. H U B I N D J I
OROGENY
metamorphism af
2423',"&8 Ha
Luiza metaseclimentary group 2. M O Y O - MUSEFU
EVENT
f
b. Mayo episode:
a. Musefu
erasure of biotite:
Haiafudi granite: migmatization and cataclasis: Oibaya migmatite and granite
episode:
2560 Ma 2593_+92Ma 2680±S Mr assemblage
charnockitiz-tion and granulite 2820 Ha
facies metamorphism:
Kasai-Lomami gabbro-norite and charnockite
assemblage
1. PRE- MAYO- MUSEFU 'CYCLE' m
Kanda Kanda tonalite and granodiorite gneiss: undated
Upper Luanyi granite
gneiss:
c. 3&00 Ha
In outcrop the Luiza is mostly bounded by longitudinal faults and was fragmented ments.
by
Lulua
late beds
NNE-SSE are
faults
folded
which
along
a
produced
WSW-ENE
horizontal
trend
with
a
displacenortherly
v e r g e n c e and a n o r t h e r l y attenuation of folding and metamorphism. w e s t e r n outcrop
some w e a k l y
In the
folded and u n m e t a m o r p h o s e d beds of the Lulua
Group
rest
u n c o n f o r m a b l y with
a basal
c o n g l o m e r a t e on
ment.
Based on the age of an i n t e r s t r a t i f i e d lava, the Lulua Group is not
y o u n g e r than 1.46 Ga and m a y be older than 2.0 Ga
crystalline
base-
(Cahen et al., 1984).
197
4.9.3 E b u r n e a n B a s e m e n t of Southern A n g o l a After
consolidation
during
s o u t h w e s t e r n Angola Proterozoic
orogens,
Damara-Kaokoveld addition
the
remained the
orogen
to A r c h e a n
Eburnean
orogeny
as a stable
West of
Congolian
Namibia
gneisses
to
the b a s e m e n t
crustal
block
orogen
to
the
the
south
the
basement
(Fig.3.32),
complex
between north
w e s t e r n A n g o l a contains a large terrane of E b u r n e a n g n e i s s e s ites
formed
of
older
Eburnean-deformed
Archean
low-grade
protoliths,
metasediments.
separating These
and of
syntectonic
(Unrug,
1989).
glomerates; lenses;
and
post-tectonic
volcano-sedimentary
meta-arenites;
schist
metasediments
and hypabyssal
and Cahen et al.
talc
schists,
and volcanic (1984),
porphyroblastic
lithologies
metagraywackes
ortho-amphibolites;
gneisses (1987)
granitoids
Their
with As
include
belts
of
occur
as
and regranites
basal
crystalline
chlorite
rocks.
schists,
con-
limestone migmatitic
shown by C a r v a l h o
lithostratigraphic
In
south-
and migmat-
rafts and synclinal keels among rhyolitic to a n d e s i t i c v o l c a n i c s lated
the
(Figs.4.1;3.32). complex
in
two Late
subdivisions
et al.
and cor-
relations of the Eburnean assemblages of Angola are still v e r y tentative. Summing region,
up
Cahen
the
array
et al.
of
(1984)
available concluded
geochronological that
an
orogeny
data
from
affected
this r e g i o n at about 2.15 Ga, during which the m a i n m e t a m o r p h i s m ,
most
the of
granit-
ization and d e f o r m a t i o n took place, followed by e x t e n s i v e late- and posttectonic, tween the
and
anorogenic
2.05 Ga
and
"homogeneous
granitic
1.75 Ga
or
intrusions
1.65 Ga.
regional granites"
tion of Juvenile crustal material
Low
and
volcanic
initial
Sr
activity
isotope
of southern A n g o l a
ratios
bein
suggest the addi-
(Unrug, 1989).
4.9.4 E b u r n e a n B a s e m e n t in the Internal and F o r e l a n d Zones of the W e s t Congolian Orogen This p o l y o r o g e n i c along
the
domain
equatorial
this
basement
from
Archean
rise
is part of the A t l a n t i c
Atlantic
are
granitoid
exposed and
margin
of
a wide
central
range
charnockitic
Rise
of
or b a s e m e n t
Africa
(Fig.4.36).
basement
massifs
of
swell
rocks
southern
On
ranging
Cameroon,
Gabon and Congo Republic to Eburnean rocks d e f o r m e d d u r i n g the Late Proterozoic West C o n g o l i a n deformation.
Eburnean rocks o u t c r o p e x t e n s i v e l y as
the b a s e m e n t to the W e s t C o n g o l i a n m o b i l e belt and on the cratonic land
of
this
orogen.
lithostratigraphic northern Angola
Table
sequences
4.5 in
shows this
(Cahen et al., 1984).
the
correlations
region,
from
of
southern
the
fore-
Eburnean
Cameroon
to
198
I Libreville
~.>'~;-.".~'r: ::-',7.
x I ~, I-/
'l ~
i
(
/
t
~
.'"" :
---
~
/#//
"'.
f
Figure 6.2: Africa showing Pan-African mobile belts and stable areas with cratonic cover. A, Cratonic areas where Pan-African supracrustals are covered by the Phanerozoic; B, cratonic areas stripped of P a n - A f r i c a n cover. (Redrawn from Cahen et al., 1984.) quiescence
after
the
Eburnean
orogeny
blocks
into the s u p e r c o n t i n e n t
during
the
Pan-African
prate-North of
which
Southward,
of
continental
orogenic
belts
known and
and
as
(Fig.6.1).
West
Iapetus,
of
Shield
to
along
with
which are now preserved
opening
Damara,
and
closing
in the vast Pan-African
in eastern Africa
the opening
evolved.
resulted 3000-km
in chain
the of
in a series of re-
Gariep, of
of a
eastern margin
(Fig.6.1) I
a
of continental I was fragmented
the
orogens Pangea
Ocean
African craton and the East Saharan craton, the Arabian-Nubian
Pangea
leading
Rokelides
Congolian,
The
are also now well d o c u m e n t e d
as the the
Atlantic
geosynclines the
event
separation
prate-South
margin
known
Ocean
rifting a
saw the amalgamation I (Fig.5.1).
tectono-thermal
Mauritanides
the
formation
entrants
Atlantic
the
Pangea
and
Saldanhia
Pan-African
oceans
belt between
the West
and in the Mozambique
belt and
(Fig.6.1).
Plate
collision
at
257
the very of
the
Fig.
end
of
Pangea
the
II
from
the
mobile
initiation
orogeny
the
cycle
resulted
Gondwana
the
part
the
mineralization
in the
of which
the
The
Pan-African
same
time.
which
by
close
Pb,
was
Organic
emergence
is
shown
in
of
the
and body
an
era
evolution
times,
had
Precambrian are
which
had
cratonic.
accumulated
parts
characterized
of
in
belts the
of algal
had
Africa
almost
at
course
of
stromatolites,
agents
evolved.
of
carbonate
seas.
By the
Their
traces
and in the Nama
Damara
by
and diamond.
epicontinental
orogen
the
Africa.
terms
glaciation
dominant
metazoans
between
in
In
throughout
appearance
found in the Katanga
Pan-
the older cratons
Pan-African
Pan-African
soft-bodied
are
whereas
widespread many
the
along
the
two tectonic-metallogenic
gradually,
become
early
chains
Cr, asbestos
of in
located
and
and
and southernmost
and Pan-African)
appearing
Apart
cratons
Gondwana,
were
remaining
of Au, Fe, Mn,
also
stable
of
mountain
Zn, Co, Sn, Be, Nb-Ta,
in widespread
fossils
sediments
Africa
respects.
the last period of w i d e s p r e a d
in the northwest
led to the widespread
Pan-African
sedimentation
extensive
(Kibaran
deposits
the Precambrian,
of
into
break-up
(1966) had distinguished
deposits
glaciogenic
of
rest
orogens
of Cu,
contain important
Africa
and
in many
in the Mesozoic
only occurred
Clifford
major deposits
Cycle
formation
The y o u n g e r
of
rifting
The Pan-African m a r k e d
orogenies with
significance
differentiation
subsequent
belts.
and
Africa,
with
of great
of a new Wilson
Subsequent
units.
era was
structural
belts,
African mobile
the
orogenic
6.1. The P a n - A f r i c a n
of
Pan-African
supercontinent,
and G a r i e p
cratonic belts
in
southwest Africa.
6.2 The West African Polyorogenic Belt
6.2.1. The
West
along the
Geological African
the
Senegal,
and
as
Sahara,
craton
margin
in
or
Liberia
of
Framework
mobile
the
and
West
Sierra
the Mauritanides this
Cenozoic coastal the
polyorogenic
western
Rokelides
Western
and Geophysical
chain
basins
tabular
Repeated
Late
complexly
deformed
of
belt
Leone,
in n o r th e r n
mobile
belts
on its w e s t e r n part, cratonic
cover
Proterozoic-Early and metamorphosed
(Cahen
African
along
Paleozoic rock
et al.,
craton
the
Bassarides
Senegal, is
1984)
extends
(Fig.6.3). in
Known Guinea
Mauritania
covered
by
the
and
eastern orogenies
assemblages
flank have
the
Mesozoic-
and is in turn t h r u s t its
as and
against
(Fig.6.3). produced
in the M a u r i t a n i d e s
258
and Bassarides the
belts
which
lithostratigraphic
treated
as
imposed
defy
units
in
tectono-stratigraphic
upon
them
by
the
simple the
stratigraphic
mobile
units
zones
in order
tectonic
of
to
processes
correlations. these
orogens
reflect they
Thus,
the
are
features
have
undergone
(Dallmeyer, 1989; L ~ c o r c h e et al., 1989; Sougy, 1962).
CIRCUM-WEST AFRICAN
GRAVITY
HIGH
OUTLINING PAN-AFRICAN SUTURES AND CRATON BOUNDARIES PALEOZOIC FOLD BELT
PALEOZOIC PLATFORM COVER
PAN-AFRICAN MOBILE BELT
EBURNEAN
ARCHEAN NUCLEUS
MADINA
- KOUTA
BASIN
Figure 6.3: West African craton delimited by a belt of gravity highs (black), showing sutures and mobile belts. (Redrawn from Roussel and L~corch~, 1989.) Regional crustal structure and terrane boundaries have been delineated around
the
entire
geophysical prominent portion
West
methods regional
and
African
(Roussel belt
suture
of
of the
and
craton
using
L~corche,
gravity
highs
Pan-African
gravity
1989;
and
Ritz
(Fig.6.3)
orogenic
geoelectrical
et al.,
1989).
defines
belts
the
A
axial
surrounding
the
craton. In African rides,
the
West
craton and
(Fig.6.4,A).
African gravity
demarcate The
polyorogenic
high two
eastern
lie west
belt,
segments
of
the
of the Mauritanides
gravimetrically
contrasting
terrane corresponding
circum-West
and the Bassa-
crustal
to the craton,
terranes
is defined
by a broad regional negative anomaly (Fig.6.4,B) which is characterized by
259
NE-SW gravity the
positive western to
trends.
Bouguer coastal
the
east
terranes
is
positive
anomaly
It
westwards
a
runs
wavelength
of
a
gravity
Pan-African
suture
is
is
nearly
basement
the
orogen, denser
to
the
the
but
underneath
a
generally
existence
of
a
axis
of
block,
and
western
crustal
NNW-SSE-trending
belt
of
is known as the Mauritanian the
is displaced
western
reflect
the
eastern
continuous,
westward-dipping highs
with
by
is denser and thicker than the craton
This density d i s c o n t i n u i t y parallel
terrane
characterized
consistent
Separating
prominent,
beneath
remnant
that
the western
basin
block which
(Fig.6.4,B).
anomalies.
anomaly.
coastal
basement
Mauritanide-Bassaride
the
In contrast,
Mauritania-Senegal
exposed
parts
of
slightly westward.
and
suture
zone.
unrooted
dense
is b e l i e v e d In
the
bodies
the
It dips
to represent
Bassarides trapped
short
along
the
zone.
wsw
mgczI
B
6O &O ~ 20 0
- 2O
.
20
O
0
80
100
~
t ...... k
~
O
-40 60
t- Ma u r i t o n ia n -~.~--- o u t c r o p p i n g b e l t basin Songarafa
w i sw
o ~'5
~i \/-'\ ~ " ~ -
--~
:IF
F'orelond --
I 1
TAGANT ENE
~"
r;>_/,/~;/Z//A.//~. J L
J I \:
"~/2/////.,,~//////
;~ "P '4- -Ik-J"
,l~z/,,s
has
-
GRAVITY
HIGH
GRAVITY
LOW
PRESUMED
FAULT
ZONE
Figure 6.4: Bouguer anomaly map of the M a u r i t a n i d e s (A); g r a v i t y p r o f i l e across the Mauritanides. (Redrawn from L ~ c o r c h ~ al., 1983.)
B, et
The
orogen
long
been
ridge with
wavelength
interpreted its
crest
as
Mauritanian an
anomaly
asymmetric
at a depth
along
the
mantle-rooted
of about
Mauritanide mafic
or
ultramafic
15 km and a s i g n i f i c a n t
westerly
260
dip
(Fig.6.4,B).
with
the
Since
segment
Paleozoic
related
to
(Senegal
a
and
anomaly
eastern this
is
(1989)
is associated eastward
during
the
North America
collisional
translation
collision
offers
a
of the West African mobile belts.
A
survey
magnetotelluric
tectono-stratigraphic revealed above
a
crustal
Bouguer
units
by
a
(Fig.6.5,B)
ohm-m)
at
depths
depth.
The
highly
(5,000
ohm-m)
sequence
In
of
uppermost
zones
resistive crust.
about
7 km.
of
the
tectono-stratigraphic
9 km,
can
correlated
zone.
In
from
volcano-sedimentary
craton
the
of
around
high
in
part
the of
have
with
upper 80
been belt.
the east
formations.
The
axial has
15
being due to West
less
African
resistive
and
a
maximum
resistivities with
western in
the
the
in
300
the two
of
5 to
the
internal
ohm-m
material
as
volcanic
or
with
resistivity
values of 3,000 ohm-m at depths of 12 to 16 km was interpreted
as a basic-
ultrabasic
body
separating
Senegal microplate)
(1989)
stratigraphic African overlain
craton by
and
units is
crustal
blocks
(West
with different geoelectrical
6.2.2 T e c t o n o - s t r a t i g r a p h i c Dallmeyer
two
the
horizontal
African
craton
and
structures.
Units
L~corche
across
wedge
in the
Mauritanides
range
interpreted
west-dipping
(2 -
at greater
Proterozoic-Paleozoic
complex
zone
been
a
ohm-m
The
thickness
crust
in resistivity
correlated
calc-alkaline
to
layer
the
overlies
moderate
fold
body
with
west
the
to
is
upper
basin vary
and
the
resistive
eastern
rapidly
ohm-m)
the
microplate
resistive
of
1989)
with
conducting
crust
the
various
et al.,
lower resistivity
upper
values The
units
by
the
(30,000 On
the
shown
for
the
ohm-m)
again
craton
As
accounts
Senegal
highly is
1988).
compatible
(I,000
crust
the
Mauritanides
characterized
thinning
a
thick The
contrast,
resistivity
thickness
be
6 km
from west to east;
lower
shows
2
basement
African
(Ritz
is
The
in the M a u r i t a n i a n - S e n e g a l
invasion.
is
which
of
western
across
Mauritanides
resistive
12 - 18 km.
sediments
craton
southern
(Fig.6.5B,C)
geometry
that was
and Culver,
resistivity
interpretations.
a
of
from 3 to 30 ohm-m, water
crustal
moderately
overlying
sea
in
structure
anomaly
characterized
of
Late
stress
West
that
the
Pan-African
the
a
the
mechanism
evolution
earlier that
of
of
coincides
where
compressional
(Venkatakrishna
model
on
inferred
with
anomaly
orogen
superimposed
L~corche
considerable
microplate)
of the Mauritanian
Mauritanide-Bassaride
orogeny
Roussel
the M a u r i t a n i a n
below
the
Hercynian
deformations,
against
the extension
of
the
basement mid-Late
et
al.
West or
(1989)
African
defined
polyorogenic
foreland
zone
Proterozoic
and
which
several belt. is
tectonoThe
West
unconformably
Early Paleozoic
cover
261
BOVE
~
*
÷ + + +
.AS,. ~:++++F;+++. h\+l ,
,oo
..
4 wEsT "~, ÷
•
÷ AFRICAN ~
I•*I,
indic,~.
,
I.'!
island
events
-'""
" . , 9~ , ~~-
vo|canism
Correlation of principal tectonic Schandelmeier et al., 1990)
arc e v o l u t i o n
-
"k#~ . . . .
~
,
-%~j
pia)
S Somct[ia
I
-
÷++
'¢
in the basement
u
+
of the ANS.
÷++÷4 ÷,,++ +
+
thrusting and I or strike slip fau[tlng
~
++4
~.~ °
+÷
+÷+÷+-, +++ ÷ ÷÷÷++÷ +++. ++ +++÷ +++ + ++ ++~ ~ ++÷ + ++ ÷+++ + ++ +
@++++L
N Somalia
Horar CE Ethio-
d y k e emplacement
evolutll
arc .
_+_+_+.+.
;.i
,
S E
UcJan da
NW Kenya S Sudan
episodes
ext e n s l o n a l
bosins
9
%'11 t ,
~
~
rifting
-l.
,-,~,,77 .~
',1 . . . .
-~.. ;;.
;;~#~ s e d i m e n t a r y
+ + + + +
metamorphic
rifting
,",~,t~)
K
~¢,
of
facies
Blue Nile
-,-+-.-,- - m -- +..~, ÷ .
Nuba ~ountalns
+oc~. . . . . . .
--=
~~-~.-~-
;:::::
r-.t~,i>, i e v o l u t i o n
+ ÷ ÷ +
Sab~ioka
within p l a t e g r a n i t o i d emp{acement
rifting
t';'t/'~'~D
"; ".
,~ - ~'.
,
:7_~
?
NE A f r i c a
Pan-African of ~ cant,
•
"
~-~
+
Tibesti
;o ~,,., -" ~" ..~ ,..,-:','-"'1 . ,
C/
SED
Bayuda Desert
Lale tectonic granJtoid emplacement
rifting
- i" ~ " / /3i
.,,.( "~; i ~ t V"V -
.
t~
~£D '"
Eastern Desert
~-',L" ~
'-' ",">. ) ;'L "¢J .'
",,
Figure 6.71: (Redrawn from
~
r,'"
I d" l~. . ., t
.1÷÷
I+ + ++
i+ ÷ l+ +
Red Se~ Hills
-~_~J.,,
....
.'.'.'.
,T.r
Arabian Shield
- ~7",:',
900,,
8oo
700
500"
Time (Ma)
o~
398
the
extreme
Uweinat
eastern
complex
part
and
of
as
Jebel
the
Kamil
in
high-grade
the
Egyptian
granitoid
part
basement
of
with
the
minor
intercalations of metasediments in the Bir Safsaf-Aswan uplift (Fig.6.70).
OF
4 0 E N
A Norgeiso /
GuLF
ADEN
OF
t
80Km
i
oF Molt~..
~
Wagdefia
ADEN
LO'~SKhoreh ~ -~ . . . .
~
=.
iTeser,os {
.%,.. Anticline
~
Phanerozoic
Synci|ne
~
¢o~ r foc k s
--/- Thrust
.
NE
-Ros Hontccro
~Ras Hantoro granite Gneiss " Diorcte ~(E bQsement o~ly) Syenite n-Ne synite (? Pre-teclonic) Gobbro(mainly interlectonic) "
BRANCH OF
MOZAMBIQUE
= 3E ~ ~ ~
~ ~ ~ ~
BELT
Harlro .I,40 Calcareous series . . . .senes BotomQ- Ubah. pehtic (omphibolites predominote toca~ty~ Oebile psommitlc" series -extensively migmatised Granitic gneiss and migmatite including remnonts of ? Pre Mozambique gneiss
"
E
W E, MARGINOF ETHIOPIA:IS. ARC BASIN Abdul Qodr.
Maydh Greenstone T T T *Diorite
Be, Nb,
-or~d post- tecgronites
INDA AD BASIN ? E.GONDWADA Possible Sn Sn PLATE suture .
]NDA AD SERIES:Folded
F~-~lOtder(Pre.Mozombique ) gneisses: L~ x x J Continental cr us'( and marble ABDUL OADR VOCANIC SERIES: Intermediate- Qcid volcanics
tonic ~Syn
L:'...~:./f,).'t mudstone, wackes quartzite
~Dior~te
~
~Gabbro
~-~'~
MAIT GREENSTONE:.e'a-piIIow bas.lt, ac,ino[ite schist
~
LAYERED SEGUENCE: ~ Mozambique Belt gneisses: qz~- fetdspathic ctostic, pelite gneiss and metacol¢areous rocks
~Phyilite
peiit~c schist
Metabasalt greenschist
Figure 6.72: A, geological sketch map of N. Somalia, B, PanAfrican p l a t e t e c t o n i c s f o r t h i s r e g i o n . ( ( R e d r a w n from Warden and Horkel, 1984.)
399
Also, as
small
inliers
exotic
of amphibolite-facies
terranes
assemblages
in
the
within
Southern
the
gneisses
and metasediments
lower-grade
Eastern
Desert
of
volcanogenic
Egypt,
in the
Sea Hills of the Sudan,
and in Ethiopia and Saudi Arabia.
6.11.3 M e t a - S e d i m e n t a r y
Belts Around the Red Sea Fold Belt
Schandelmeier
et
semblages
tectonic
between
and the
East
(Fig.6.70).
presented
evolution
Saharan
the
an
the
and
interpretation
the
Red
Sea
these
along the eastern of
presented
a Late
ting for the region are largely culled
and
rock
thrust
of the
belt
East
Saharan ocean
the paleotectonic
from Schandelmeier
as-
a zone of
Pan-African
with
Red
scattered
represent
margin
Proterozoic
below together
the
belts
fold
belts
ophiolite
adjacent
of
meta-sedimentary
the fact that
initiation
The outline
of
craton
that developed
during
the ANS.
(1990)
They emphasized
early rifting craton
al.
occur
et al.
in
set-
(1990).
Southern U w e i n a t B e l t
South
of
exposed bolite
the
Uweinat
a belt and
of
block
psammitic
banded
metasediments
extension. tectonic this
Although
is
lithologic, belt NE-SW
a
believed
and
characteristic
that
are
sharply to
and
(Fig.6.70).
axes
rocks
of w h ic h
be
suggests
in to
outline
even
from the
on
syn-
with
The
to
wrench
under within
entire
basin
to post-
basis
the
belt
is
amphi-
in this
the
Uweinat
isoclinal. dextral
marble,
metamorphosed
structures age
southern
Sudan
synsedimentary
similarities
the
the
are intercalated
early
older
of
minor
been
Pan-African
structural
open
sygmoidal
truncate
of
Folds
with
have
no age data are available which
extremity
Bimodal v o l c a n i c s
manner
metamorphic
nearby
pelitic
all
conditions.
in
granitoids
basin
and
ironstones,
low- to m e d i u m - g r a d e the
in the n o r t h e r n m o s t
belt, of
the
Jebel
Rahib
strike
along
belt
owes
faulting
late
basic
igneous
its
in
the
Pan-African.
Jebel R a h i b B e l t
This
belt
and
a
contains thick
metasediments Pan-African
which rift
volcaniclastic lithosphere
complexly
sequence
was
have
basin.
been Since
derivatives probably
deformed
of
not
ultrabasic
arenaeeous
and
interpreted
as
no
have
a r c - ty p e been
involved
deposits magmatic
found, during
Rahib basin.
An age of 570 Ma from p o s t - o r o g e n i c
affected
the
by
penetrative
NNE-SSW
of
the
a
Red
rocks
closing
granitoids shearing
rocks
carbonaceous Sea-type
and
related
of
oceanic
subduction
s t ri k e - s l i p
sets the m i n i m u m age for its deformation
and
subordinate
of
the
Jebel
w h i c h were not in
this
and low-grade metamorphism.
belt,
400
An
ophiolite
chromites, oceanic
assemblage
with
ultramafic
massive and layered gabbros,
ridge
affinity,
and
rocks,
dykes,
chert
pyroxenite,
pillow-lavas
deposits,
podiform
of clear mid-
furnish
the
evidence
supporting the appearance of oceanic crust in the Jebel Rahib rift. ophiolitic
rocks
impose
some
constraint
on
the
geodynamic
These
evolution
of
this area, and imply that juvenile Pan-African rocks were generated in the Nubian Shield outside the Red Sea fold and thrust belt. North K o r d o f a n
Belt
In its depositional
setting and structural
is similar to the Jebel Rahib belt, found.
Although
the ages
style the North
Kordofan belt
except that o p h i o l i t e s have not been
of the deposition,
metamorphism and deformation
of the m e t a - s e d i m e n t a r y pile in the North Kordofan belt have not yet been ascertained, has
among
been dated
the
at
intrusive
about
granitoids
590 Ma.
Also
a tourmaline-bearing
late Pan-African
shear
granite
zones
which
are sealed by mica-bearing pegmatites have yielded an age around 560 Ma. D a r f u r Belt
The
low-grade
gneisses North
meta-sedimentary
in the
Kordofan
southeastern and
Jebel
unit
Darfur
Rahib
structurally
block may also
metasediments.
overlying be
basement
equivalent
Intrusive
to the
granitoids
have
yielded ages of about 590 Ma and 570 Ma in the Dafur belt. Eastern Nuba M o u n t a i n s Belt
In
the
eastern
Nuba
Mountains
a NE-
to
NNE-striking
belt
(Fig.6.70)
of
low-grade volcano-sedimentary rocks is exposed which contains fragments of highly
dismembered
ophiolites
and
basic
to
acidic
plutons.
These
arc
ophiolite assemblages were metamorphosed around 700 Ma, with post-tectonic m a g m a t i s m ceasing around
550 Ma.
Since the eastern Nuba Mountains
do not
represent the boundary with the volcano-sedimentary and o p h i o l i t e belt of the
Red
Nuba
Sea
fold
Mountains
distance
belt,
the
represents
from the east,
Pan-African either
or more
a
juvenile
klippe
terrane
thrust
over
of a
the
eastern
considerable
likely it represents a m i n o r ocean basin
behind a large probably rifted-off continental fragment. Bayuda D e s e r t
Here Pan-African rocks occur as two different tectono-stratigraphic units. First,
on the eastern part along the Nile,
metasediments,
meta-volcanics
and
is a n a r r o w strip of low-grade
granitoids
which
range
compositionally
401
from early tonalites through granodiorites
to large peralkaline granites.
The tectonic evolution of the area involved a main metamorphic event which followed
plate
emplaced
collision
above
a
at
about
subduction
761 Ma.
zone
at
Before
about
then,
898 Ma,
granitoids followed
were
by
the
emplacement of other subduction-related granitoids at about 678 Ma; and by anorogenic within-plate magmatism at about 549 Ma. An
extensive
meta-quartzites Gabgada the
(Fig.6.70),
only
position
imply
(Fig.6.70) oceanic
that
of
which
former
marbles
along
located
the
basin,
in
and
intercalated
Nile
south
separated beyond
reflects
the
(Fig.6.69).
the
of
This
Abu
Hamed
from
the
major
Red
Sea
Hills
1990). Terranes
et
al.
(1987)
rifted
canogenic-ophiolite-granitoid assemblages the
that
margin
fragments
craton occur as high-grade meta-sedimentary exotic Desert,
and
(1987) may represent
deposit
continental
independent
was
Kr6ner
margin
assemblage
an
Exotic M e t a - S e d i m e n t a r y
to
the
arc
to
(Schandeimeier et al.,
According
continental
of
the
belongs
basin
sequence
which according to Kr6ner et al.
autochthonous
approximate would
meta-sedimentary
is exposed between the Nile and the Red Sea Hills west of
Southern
Eastern
Desert,
of
the
in the Egyptian
and
the
East
Saharan
terrane among the volCentral Eastern
Sudanese
Red
Sea
Hills,
notably at Meatiq, Hafafit and in the Sasa Plain of Gebeit, and near Haya, southwest of Port Sudan the eastern Arabian gneisses
(Stoesser
African
(Fig.6.73).
Shield
was
et al.,
tectono-thermal
Also,
the Afif
identified
1984)
which
events,
as
though
bears
terrane
an exotic
(Fig.6.70)
block
remobilized
resemblance
to
in
of ancient
by
the
African
Pan-
cratonic
gneisses. The exotic m e t a - s e d i m e n t a r y terranes, ces"
and regarded as
the
oldest
rocks
fully discussed by Kr6ner et al. nic
setting.
(Fig.6.73), schists
which
sedimentary rocks.
As
exposed
in
are m o s t l y
structures
Southern old,
composite
dome
survived
and
locally
the
intense
Some of those metasediments were aluminous
sence locally of sillimanite. Eastern
which
Desert
suggests
that
Eastern
Desert,
structure
consist of m e t a - q u a r t z i t e s
feldspathic
had
"older shelf sequen-
in the
were
(1987) within a Pan-African paleo-tecto-
the
these metasediments
termed the
found
of
Hafafit
and quartzitic
cross-bedded
where
metamorphism
in
the
these
as attested by the pre-
The clastic m e t a s e d i m e n t s of the central and
have yielded their
U-Pb
provenance
zircon lay
in
ages an
as old as ancient
2.06 Ga
continental
crust exposed probably along the margin of the East Saharan craton.
402
Figure 6.73: Precambrian rocks in the Egyptian Eastern Desert. NED, North Eastern Desert; CED, Central Eastern Desert; SED, South Eastern Desert. (Redrawn from Greiling et al., 1988.) Local bably
volcanic
derived
rifting
and
800 Ma
ago.
lowest
positions
quartzites Plain
south
components
from
a
formation
in
of Gebeit
as the h i g h - g r a d e of Port Sudan.
the
margin Red
associated and
the
magma
of a passive
Continental
with
among
primitive
Sea
Hills
of
found
in extensive aluminous
metasediments
perhaps
continental
deposits
marbles
and partly
Hafafit
source,
during
margin
also the
occupy Sudan.
at about the
small
outcrops
areas
south
of Wadi near
pro-
initial
900 Ma to
tectonically
These
in
metasediments
were the
include in
Amur;
Haya,
the
the Sasa
as well southwest
403
Inda A d Group (Northern Somalia) In no r t h e r n
Somalia
the e a s t e r n
border of the local volcano-sedimentary
of
the
"Maydh
the
Inda Ad Group
Greenstone
Belt"
of metasediments
(Fig.6.72,B).
(Fig.6.72,A)
and o p h i o l i t e
The
pelitic
and
rocks of the Inda Ad Group which are intercalated with marbles, along N-S-trending facies. is
The
Inda Ad Group
equivalent
granitoids
regional
to
the
-
extends
Ghabar
have yielded
Inda Ad Group
fold axes and metamorphosed northward
Group.
A
ages.
Belt"
southern
are folded
Yemen
and
that
of
where
it
post-tectonic
The tectonic
resembles
psammitic
in the greenschist
granodiorite
late Pan-African
"Maydh Greenstone
into
forms
sequence
setting of the the Jebel
Rahib
belt in the Sudan.
Tibesti Mountains Although
located
the C h a d - L i b y a
far out on the western
frontier
and m e t a m o r p h i c and Rodgers, during
the
(Chad-Libya)
(Fig.6.70),
rocks which
1978)
which
Pan-African,
divided
micaceous
slates
quartzites,
and
rhyolitic
Subduction basin
granitoids,
lavas)°
and
and
geochemically
late akin
late
age
coeval
The
Tibesti basin are believed
(1988b)
sedimentary narrow
Pan-African
summarized belts
1980),
supracrustal
Jebel has
and
schists,
pyroxenites),
alternating are
with
also
like
in the Tibesti
but
to
eastern
only
520 Ma
basaltic
dykes
from
granitoids
the
the are
are
southern
from
Jebel
to have been induced
side
late well
of
the
Pan-African dated.
These
petrologically Egypt Ben
and
and
northern
Ghemah
on
the
by subduction.
Setting for the Meta-SedimentaryBelts
of
continental
(Jackson,
750 Ma, 590 Ma
intrusives
Sudan.
Vail
such as mica
are more a b u n d a n t
on
to
western
Paleo-Tectonic
fully
the
(medium-grade
metasediments
occurred
and
from
rhyolitic
to
(Ghuma
of the Tibesti
and arkoses
these
to
Tibestian
rocks
on
are less prominent.
1.0 Ga
in
similar
amphibolites
quartzites
volcaniclastics
metamorphism
between
schists,
Although
deposits
ranging
granitoids
with basic volcanic
(low-grade
those of the Rahib basin,
Lower
basin
and d e v e l o p e d
the Precambrian a
craton,
contain magmatic
of an ocean
at a period
into
hornblende
Tibestian
area and calcareous
Tibesti
1966)
intercalated
and an Upper
and
Lithologically,
(Klitzsch,
metasediments
the relics
in the mid-Proterozoic
in a style
Rahib rift to the east. been
the Tibesti Mountains
represent
began
part of the East Sahara
the
the
ANS.
margin
which
infillings
He
with
rested of
paleogeographic regarded a
upon
early
implications
them
as
shallow-water a
gneissic
Pan-African
of
representing
the
miogeosynclinal
cratonic
continental
foreland, margin
meta-
either
a
wedge or
rifts,
as a
404
view shared by Schandelmeier et al. into
small
Huba
Mountains,
Uweinat,
ocean basins
Darfur
attained.
Thus,
that
and
Inda
Ad
and
North
(1990).
Some of these rifts developed
later closed, basins
Kordofan
for example
(Fig.6.72,B), basins,
or
the Jebel Rahib,
as
in
the mini-ocean
the
Southern
stage was
not
prior to or contemporaneously with extensive oceanization
in the Red Sea fold belt and in Saudi Arabian parts of the ANS, processes of
crustal
extension,
lithospheric
thinning
and
the
development
abortive rifts transpired extensively in other parts of the ANS
of
(Jackson,
1987). 6.11.4 V o l c a n o - s e d i m e n t a r y and Ophiolite Assemblages
Volcano-sedimentaryAssemblages These
are
heterogeneous
Andean-type volcanic water
shales,
Fig.6.70
piles
oceanic
island-arc,
and
plate
and associated pyroclastic volcanogenic
siltstones
they
of
occupy
and
the
limestones
Sinai
(Vail,
peninsular,
1988b).
most
of
margin
and shallowAs
the
shown
Central
in and
Southern Eastern Deserts of Egypt, the Red Sea Hills, most of the basement of
Ethiopia,
forming
the
and core
calc-alkaline, rhyolitic
the
western
area
of
ranging
types.
Arabian
the
ANS.
Shield
compositionally
Because
amphibolite facies, Vail
of
their
(1976,
and
The volcanic from
Yemen
rocks
basaltic
characteristic
basement,
thus
are
predominantly
and
andesitic
greenschist
to
to
lower
1979) grouped those in the Sudan into what
he termed the Greenschist Assemblage (Table 6.4). Jackson
(1980)
summarized
the
stratigraphic
terms
that
have
been
assigned to the v o l c a n o - s e d i m e n t a r y units which he collectively termed the "younger m e t a - v o l c a n o - s e d i m e n t a r y units"
(Table 6.4).
In Egypt,
those are
found in the upper formations of the Abu Ziran Group; they are referred to as the Jiddah,
Samran,
Halaban
Thalab and older volcanic in northeast Ethiopia;
rocks
and Hulayfah Groups in Yemen;
in Saudi Arabia;
the
the Tambian and Tsaliet Groups
and are included in parts of the
"Older Series" of
northeast Somalia.
Ophiolites Closely
associated
masses
of
which
from
with
tectonized base
the
volcano-sedimentary
mafic-ultramafic
upward
typically
complexes,
contain
(Kr6ner
1988b) serpentinized pyroxenites and peridotites, dyke complexes, all
pointing
to
p i l l o w lavas, an
ophiolite
assemblages comprising et al.,
are
linear
a succession 1987;
layered gabbros,
Vail, sheeted
and rare siliceous bands and plagiogranite, suite
(Fig.6.74).
In Egypt
the dismembered
7
UNNAMEO Ajal Bohoh Boish Nali Jiddoh
Kisll "Series"
Older 'Serle s °
I 1000
bSystem°
900
i
CRUDE
UNITS
Uhu
z
700
Group
Bukobu n
Older "Series"
RADIOHETRIC SCALE (Ha)
000
I
Bukoban System Busondo Group Ikorongo Group Kuvimbo Group
Hozambiquian
Group Group Group Group Group
HETAHORPHJC
Correlation of the Late Proterozoic of the ANS.
Saramuj 'Series"
.......I 600
TANZANIA
UGANDA
KENYA
AbLun "Series" Embu "Series' Milyana "Series' Bunyaro 'Series' 'System'
SOMALIA
Inda Ad "Series"
YEMEN ARAB REPUBLIC
PEOPLES DEMOCRATIC REPUBLIC OF YEMEN
ETHIOPIA
SUDAN
EGYPT
SAuDI ARABIA
JORDAN
(Redrawn from N. J. Jackson, 1980. )
Samron Group Fatima Group Holobon Group Shammer Group HALl GROUP Huloyfoh Group Murdomo Group Urd Group Jibaloh Ablnh Group Group Ziron Group Rubshi O o k h o n Hommamot Group Abu Group Avat, As~ribu, MITIQ 6NEISSES Oeosynclinol Metasediments Ceosynclinal Metnvotcanics HaNgar Shod[i and other vol¢on;cs Greenschist Hetosedimentr y Group Assemblage or Haflrdelb Or. e~. ( Kushebib Group ) Tsoliet Group Oldykama Formation Hormora Group Tombiun Group Shiraro Formation Adolo Group Motheos Formation Older volconics Ghober Group Aden Metamorphic Group Tholob Group Thaniyo Group unamed units ?7 ? unnamed units
Table 6.4:
0
406
ophiolites
belong
Saudi Arabia. the
Sudan,
the
Rubshi
The ophiolite
namely:
Nakasib-Oshib the Tullu
to
belts
the Sol
Complex,
Group;
the
Dimtu-Akabo-Birbir
belong
of the Nubian
Hamed-Wadi
and
they
Wad
to
the
Shield
Onib Complex
Wadela-Ingessana
Urd
Group
in
include
those of
(Fig.6.70),
the Khor
Complex.
belt of western Ethiopia,
Others
are
the Adola belt in
eastern Ethiopia and the "Maydh greenstones" in northeastern Somalia. As already pointed out the volcano-sedimentary and ophiolite the ANS extend southward in two main prongs
,1
E
(Fig.6.68)
PiUow b~alf =o~.,~q-Cher~ C~Icoreous sediment .... ~--Sheeted Dikes Isotropic gobbro,Injected by Oi kes and grading downvords into
~
Ptagiogr~nite
7
zones of
into the Mozambi-
dikes ond plagiogranite layered gabbro
Cumulate layered gobbro showing tiqht lsoclinal folding locally and with rare serpentinitic lenses -
_
~
l
-llmmm i o
Gabbro to marie gabbro containing serpenfinite ~nd
N
P~oxenite rods
c5 c
E
~Serpentini~
~Hointy pyroxenite with occasional serpentinife {at places
Jcarbonated) and rare m~fic gabbro
~ , ~ , ~ Disseminated Cr lPyroxenite with disseminated chromffe and layered, mass,re ~'2-~T'--~--Hassive Cr Jchromite lenses Basal ultmmafic unif[serpenfinite)with lenses of peridofite
4--
~,~,~DisseminatedCr
C
Figure 6.74: Schematic section through Wadis ophiolites of the northern Red Sea Hills of the from Kr6ner et al., 1987.) que
belt
of
East
Africa
where
sutures in the latter region
they
define
the Blue Nile region of Sudan and Ethiopia north-south-trending granitoid
assemblage
ophiolite
belt
of
bordered
to
Sekerr
ophiolites are
the
(Fig.6.70) west
collision
as an approximately and
by
the
ophiolite
and
Ingessana-Kurmuk
zone of eastern Sudan and to the east by the Tullu Dimtu-Akabo-
the
dismembered
Both ophiolite zones have been correlated
ophiolites
of
Uganda-Kenya.
of the Blue Nile region have so far not been
probably
magmatic
Pan-African
1988b). One prong lies in
volcano-sedimentary
Birbir zone of western Ethiopia. with
the
(Behre, 1990; Vail,
Onib and Sudi Sudan. (Redrawn
rocks
orthogneisses
older
than
850 Ma,
and metamorphism (Selak
Formation)
the
age
of
some
of
However,
dated; the
syn-tectonic
in the surrounding h i g h - g r a d e and
paragneisses
(Tin
the
but they
Group).
migmatitic Late-
to
407
post-orogenic 500 Ma.
The
granitoids
in the area
volcano-sedimentary
have
rocks
been
and
dated
between
ophiolites
of
520 Ma and
the
Ingessana-
Kurmuk area are in thrust contact with the Selak and Tin basement rocks. Another eastern
southward-extending
Ethiopia
northern
Kenya
(Fig.6.75,A)
(Fig.6.62).
mafic-ultramafic and
onto
rocks
basement
transport
The
which
gneisses
towards
the
ophiolite which
east
Adola
have
migmatites a
in
the
Adola
area
of
Moyale
belt
of
into
the
belt
contains
intensely
involving
considerable crustal shortening
is
ophiolite
been
and
belt
continues
thrust
(Fig.6.75,B)
minimum
(Baraki et al.,
of
imbricated
over
each
with
30
to
other
tectonic
40 km,
and
1989).
Ophiolitic M~lange and Olistostromes Two types of subduction-related
lithologies occur among the ophiolites of
the Eastern Desert and the Red Sea Hills. Both represent a chaotic mixture of
heterogeneous
mappable
body
rock
of
pervasively sheared, with
diverse
and
also a mappable
material
deformed
in
a
pelitic
heterogeneous
matrix.
rock
A
m~lange
material
is
consisting
a of
fine-grained commonly pelitic matrix thoroughly mixed
angular, lens-like
poorly
sorted
inclusions.
chaotic unit of
An
olistostrome
intimately mixed
is
heterogeneous
material that lacks true bedding but is intercalated among normally bedded sequences The
(AGI, 1972). ophiolites
of
the
Eastern
Desert
form
part
of
an
extensive
tectonic m~lange which resulted from the complete dismemberment and total disruption of their original stated by Hassan and Hashad
stratigraphic (1990)
character and distribution.
the m~lange
As
of the Eastern Desert are
characterized by the presence of a significant proportion of serpentinites either
as
matrix
ophiolitic
or
as
fragments,
Other components
variably
deep-sea
sized
blocks,
sediments
such as granitic rocks,
and
in
addition
calc-alkaline
carbonate
rocks,
to
other
volcanics.
quartzites
and
mudstones attest to the characteristic h e t e r o g e n e i t y of the m~lange which nevertheless, m~lange
are
still
constitute
commonly
thrust
mappable sheets
or
lithostratigraphic slices
which
entities.
were
The
incorporated
within allochthonous belts of metasediments. At
Wadi
Ghadir
(Figs.6.73;6.76,A) distal
facies
rolled
and
in
the
there
Central
(Hassan and Hashad,
fragmented
Eastern
Desert
is a large ophiolitic
rock-debris
1990).
The proximal
of highly
near
Jebel
Hafafit
m~lange with proximal
variable
facies sizes
consists
and of
in a sheared
matrix of scaly and schistose mudstones; abundant serpentinized peridotite blocks,
some of w h i c h are surrounded by sheaths of schistose talc-carbon-
408
4TARY BELT
6MATITES ;OMPLEX)
JSIVES
|
lS wm
~
A
E est
West
A
A'
/w ~
-- . . . .
"
-~,~{~'~J
A I []
METAVOLCANOSEDIMEN~AR¥ BELT
A 2 ~I~MAFIC-ULTRAMAFIC
BELT
B I ~CENTRAL BASEMENT] HIGH B2
;5 A1
--
B 2 []WESTERN BASEMENT~ GRADE B 3 []EASTERN BASEMENT] GNEISS
E)
BI
[]
DEFORMED GRANITES
~ YOUNG INTRUSIVES [ ~ THRUST
i
V--IDE'OR"EO
AI B
Figure 6.75: Tectonic units in the Adola fold and thrust belt of southern Ethiopia (A); B, schematic sections showing structural relationships. (Redrawn from Baraki et al., 1989.) ate rock produced by squeezing and rolling of the blocks; debris
including
volcanic
granite, and amphibolites.
material,
graywackes,
and other rock
quartzites,
chert,
The distal facies is a low-grade pelitic schist
with pockets and lenses of highly schistose talc carbonate rock. A genetic
409
model
(El Bayoumi,
westward disrupted where
1984)
subduction
of
ophiolites
for the Wadi Ghadir ophiolitic oceanic
and
crust
continental
resulting margin
they mixed and formed a chaotic mass
in
m~lange
gravity
sediments
(Fig.6.77).
involved
sliding
into
the
of
trench
The area was later
intruded extensively by dykes and calc-alkaline granites and leucogabbros.
%
~jq
. + + +
s
--J_\~...~Marso A~om
~-
Phanerozoic COver "i'.~ Post-tectCnlc granites Deformed granitoids(->682 t, 11Ha ) Gneisses(dominantly grctnitoids),shelf fackes metasediments, n~nor igneous rocks
regional Qntiform overgrinted by gravitative doming
W
[~ ~ ~ ----'-"
}i Ophiol+t'¢ m41ange and caic-Qllcaline igneous rocks / with uitramafic and mofic fragments / Thrust at the bo.se of the ophiolitic m61onge complex / Hinor thrust
A J
" ~ EN£
M_-'C..., . . . . . . " ramp aria/or H~glf- HafofTt thrust antiformal stack giving rise tO regional ~nt form "'-=
I,
20 Km
I
,
horiz~ontal s c a l e : v e r t i c a l
scale
. . . . .
I
'
"
~
'
\
t. - t h r u s t
Figure 6.76: Schematic map (A) and section (B) through Wadi Hafafit Culmination. i, volcanic rocks near Marsa Alam; 2, Wadi Ghadir ophiolite; 3, Hafafit igneous suite. (Redrawn from Greiling e t a l . , 1988.) In
the
olistostrome
Wadi
Mubarak
(Shackleton,
area
the
1986).
m~lange Attesting
developed to
this
initially origin
as
are
an the
410 unstratified,
mainly pelitic matrix with little sign of deformation other
than late cleavage; enclosed angular blocks and large masses of ophiolites and
sediments;
sediments;
the
and
an
sharp
contrast
extensive
mass
between
of
the
ophiolitic
m~lange m~lange
and which
normal in
one
locality rests with normal
sedimentary contact on turbidites and pelites.
Much
vicinity
further
south
serpentinite, complex either
in
the
meta-gabbro
and
(graphitic pelites, part
of
a
of Wadi
graywackes,
tectonic
Haimur
amphibolite
m~lange
within
psammitic
rather
ophiolitic a
sediments,
than
an
lenses
of
meta-sedimentary marbles)
olistostrome,
or
are an
original olistostrome that has been so highly deformed that angular blocks have become lenticular,
and marbles flattened and stretched to the extent
that they extend for several km along strike (Shackleton,
Continental
Crust
Trench
÷ : "i'+ ÷÷ ÷++ ++÷ "I'÷÷ ÷'¢" .¢,
"4:: +
÷
+
4.
, ~..+~ - ~ -
4-
+ '""+~"" Trench~---3-"-~-'v ÷......+ ~ -- ÷
+~~" ,~~
~"~ ,,"~
V•
v
v
V
v
v
v
V
v v
V
v
v
V
v v -
Y
S:J.> f d:y
Dis~z[ M41anse serpentine'
r.* T " : " ~ ~
%,
Vv
w r,.
¥
÷
~÷÷;'+÷+
Crust
Oceanic
+
1986).
E
Proxlmul Helan~le '
-d
Ophiolife
k
Figure 6.77: Model for the origin from Hassan and Hashad, 1990.)
"
of
Ghadir
Granite
'
m~lange.
(Redrawn
411
6.11.5 Syn- and Post-orogenic and Anorogenic M a g m a t i s m Intense
plutonic
associated with ANS.
Igneous
activity
rocks
occur
complexes
of
granites,
previously
large
of
Granites" late-
are
in
heterogeneous
the
the
margin
ophiolite
"older
Sudan
batholiths
tonalites
granitoids"
(Vail,
1987;
post-tectonic
plutonic
developed which
is
a
Red
Sea
complexes, Hills
of
and
They
with
ring
complex
well
granites
the
Sudan
is
alkaline
in the Northern the
suite
of
alkaline
southern Egypt
province
which
extends
from
calcand
Eastern
anorogenic
syenite and rare foid syenite ring complexes and plutons,
major
most
(Fig.6.73).
as
as
the
are
bimodal
An important later magmatic development the
adamellites-
Egypt
1988b)o
bodies
syenites.
and
and in
plutonic
characteristically
gabbro-granite
Desert
and
is
of the
are
alkaline
granite,
environments
assemblages
in the Northern Eastern Desert of Egypt
and
also
plate
and
granodiorites,
termed
extensively developed structures
as
diorite-gabbros,
"Batholithic High-level
suggestive
the volcano-sedimentary
northern
in what
Uganda
to
(Vail, 1989a).
6.11.6 M o l a s s e
Jackson
(1980)
depositional
referred
sequences
to
in
the
the
metamorphosed
"Infracambrian
and are generally of subaerial or very shallow-marine origin. the
sedimentary
units
characteristic features of molasse. unconformably
overlain
by
the
sequences of purple-coloured, and
equivalent
of
rest
volcano-
successions,
below,
which
slightly
the
and
shown
units",
as
sedimentary As
sedimentary
uppermost,
ANS
unconformably
this
assemblage
on
older
exhibit
the
In Egypt older m e t a m o r p h o s e d units are
Dokhan
volcanics
(Table 6.4),
which
are
porphyritic acid and intermediate volcanics
pyroclastics,
with
minor
components
of
volcaniclastic
sediments. The youngest Pan-African sequence exposed mainly in the Central and Northern the
type
graywacke, typical
Eastern Desert is the Hammamat Group,
locality,
comprising
limestone,
molasse
thick
sequences
slate and minor volcanics.
sequence,
deposited
in
about 4,000 m thick at
of
conglomerate,
The Hammamat
alluvial
arkose,
Group
fan-braided
is a
stream
complexes and playa lakes in disconnected intermontane basins as a result of rapid uplift and erosion (Hassan and Hashad,
1990).
In the Sudan the equivalent to the Hammamat Group are termed the Abu Habil
Series
and
Didykama,
Shiraro
composed
of
limestones.
the and
Amaki
conglomerates,
Lithologic
Series
Matheos units
(Vail,
sedimentary sandstones,
equivalent
to
1988a);
and
formations slates the
in
Ethiopia
the
(Table 6.4)
are
and
Hammamat
stromatolitic Group
developed in Saudi Arabia and have been variously designated
are
well
(Table 6.4).
412
6.11.7 T e c t o n i s m T e c t o n i c Model
Before examining a few examples of the deformational Red
Sea
fold
tectonic
and
setting
thrust that
belt,
has
it
been
is
illuminating
postulated
for
styles
first
this
found in the
to
consider
the
structural
province.
This approach of going from the tectonic model to the resulting
structure
is preferred here partly because there is considerable unanimity regarding the
plate
(e.g.
tectonics
Burke
Kr6ner
regime
and Seng6r,
et al.,
1987;
Stoesser and Camp,
that
1986;
operated
Schandelmeier
1985);
in
the
Pan-African
Ei-Gaby and Greiling, et al.,
1988;
1988;
of
the ANS
Jackson,
1987;
Shackleton,
1986;
and also because the deformation mechanisms are
more readily understandable within the plate tectonics framework. The analogy between the island-arc and
ophiolite
plate
assemblages
tectonic
setting
of
the
setting of the volcano-sedimentary
ANS
and
the
Recent
is now widely accepted.
southwest
A microplate
Pacific
arc-back-arc
ocean basin existed between 900 Ma and 600 Ma in the Red Sea fold belt and in
Saudi
et al.,
Arabia
1987).
similar
to
The modern
the
situation
in
Indonesia
today
island-arc setting is characterized
arcs and associated volcanic
flows,
pyroclastic deposits,
(Kr6ner
by volcanic
tuffs;
volcanic
fronts which occur some 80-150 km inland from the trench where tholeiitic and
calc-alkaline
andesites; range
active
from
turbidites
are
found
basins
volcaniclastics
to
with
over
mostly
fragments
of
back-arc
directions
subduction
oceanic have
pelagic,
(e.g.
Riess
most workers
favour westward
ling et al.,
1988),
there
crust
been
with
et al.,
1989).
1983;
is a consensus
and
where
hemipelagic
for
(e.g.
zones
the
favouring
1986),
Ei-Gaby et al.,
that
disparate
some
Shackleton,
and
representing
Although ANS,
basaltic sediments
sediments
and ophiolites
(Condie,
proposed
subduction
andesites
subduction
in the distal parts of the basin;
subduction eastward
magmas back-arc
while
1988; Grei-
the ophiolites
of the Red
Sea fold and thrust belt represent sutures which resulted from arc-arc and arc-continent
collisions
at various
times.
The
structures
which
resulted
from these collisions are considered below before examining the timing of the collision events. R e d Sea H i l l s
Unlike
the
low-angle Hills
of
Egyptian thrust
Eastern
regimes,
Desert
the
the Sudan are often
major
which
is
tectonic
steep with
large
characterized boundaries shear
in
by
extensive
the
zones which
Red
Sea
contain
highly sheared lensoid mafic-ultramafic bodies which represent dismembered
413 ophiolites
(Kr6ner
separating
et al.,
successively
correlated
the
major
Hills with
those
in
1987)0
accreted
The
ophiolites
island
northeast-trending Saudi Arabia
define common tectonic
terranes
arcs. and
in the region.
the
KrOner
et
belts
in
ophiolite
(Fig.6.70)
define
sutures
al. the
(1987) Red
Sea
used them as sutures
to
The Onib-Sol Hamed suture
zone which separates the Midyan and Hijaz terranes shows steep to vertical dips
and
faces
the
southeast
southeast to the southwest the northwestern NW-
and
part
SE-verging
regional
shear
placements
of
also
the Red
folds
zones
I
occur
in
R, Nile :
~2°E!
I
Sea level~
and
with
characterized by mylonites
6ranife
(Fig.6.78,A)
suggesting
(Kr6ner et al., Sea Hills
minor large
the
1987).
trends
thrusts. sinistral
Red
Sea
obduction
dominantly
Prominent and
Hills.
the
locally These
SW-NE with
late
north-south
dextral
shear
dis-
zones
are
(Almond, 1987).
W. Haimur ~ . . . . --~
Sol Hamed Opbiolite
3"¢,~
T ~ ....
~°'~eCt.~ts,,~.
. . . . . . . . .
~___
from
The structural grain in
Halaib
Unconfor~ ty
~i,~,'.: !~!!!~!:~
A
Approx, 600 Km W to Arhaean of J Uweinat 100 Km
WSW
.
Heatiq . Dome
r~77~ ~
~. , , . ~ , / , . G'~';,~ .~',~,~S~>,~_~.~jZ,,.~o. o~-~"~2~9/..~.~Z~/.,
[ •Cretoceous ~
,~z/~.~..~ , , _ ,~ "~ ~ ' ~ ' ' ~ ~ . . ~ , , b ~ "
•
Smnite
~Para
This
mylonifes
the
Eastern
Desert
of
Egypt.
Eastern Desert
is the fold and thrust
composite
~ooK•
gneisses
Figure 6.78: Sections across (Redrawn from Shackleton, 1986.) Central and Southern
B
--
Ophioliticmelange 0phiolite '] Sch is ts, amphibolifes,
DCalcaikcdine
ENE
allochthonous
belt
thrust
sensu
sheet
stricto,
which
(Fig.6.78,A),
the
appears
to be one
leading
edge
of
414
which
is located
tains
huge
gional zone
along
fragments
ophiolite
shows
African
recumbent
high-grade island-arc
entire
complex
mylonites
windows
Migif-Hafafit
as
Dome.
thrust.
of the basal
footwall
gneisses,
whereas
ophiolite
assemblage
the
the
at
Dome
shows
North
and associated
with
transi-
is
an
allochthonous
this
1988).
(Fig.6.79)
As
thrust
Eastern Group
Desert
and
sediments
by
the arc
Sinai,
which
composite
basal
direction
stack
and
shows SE
of tectonic
et
assemblages
al.
of
(1984),
the Migif-
craton.
late
are widespread,
is
high-grade
stack w h i c h
Ei-Ramly
at
Migif-Hafafit
volcano-sedimentary
dominant
shown
and
(Greiling
of the older
greenschist
and are
Dome
feature,
antiformal a major
consists
the regionally
and the Hammamat
and high-grade meta-
the Meatiq
that
above
thrust
younger
et alo,
processes
the
ductile pre-Pan-
investigations
Hafafit area onto the margin of the East Saharan In
older
molasse-facies
structural
(Fig.6.76)
lineations,
and the Hammamat
the
in a re-
This
volcanics
forms the roof of the antiformal
(Greiling
collisional
by
Culmination,
thrusts
transport
Calc-alkaline
terranes)
Detailed
Migif-Hafafit
to NW stretching
zone.
by
rocks are themselves
(exotic
of w e s t w a r d - d i r e c t e d The
underlain
sheet con-
enclosed
thrust
older shelf-facies
of the Migif-Hafafit
the
are
overlain
The underlying
group and granitoid
known
and upper mantle,
which
unconformably
tectonic
1988)
The thrust
chemistry are thrust over the ophiolite m~lange;
is
sedimentary
et al.,
1986).
by a ductile
basement.
exposed the
crust
underlain
gneissic
(Fig.6.78,B).
in
(Shackleton,
of oceanic
m~lange
tional
Group
the Nile
orogenic whereas
acidic
plutons
o p h i o l i t e m~lange
rocks occur as minor remnants.
Tectonic Evolution The
Early
margins Desert early
to
Middle
of the ANS, and
continent (1985)
at
which
centered
700 Ma
there
margin
a period
and
950 Ma,
in was
between
the an
accretion
until and
tween
about
But
680 Ma
and and
case
development
ago
conditions
took
place
an
ensimatic
arcs
between
of
which
about
the
of
arc
ANS
were
the Afif
microplate
700 Ma and
plutonism
and
continental
east
main
basin
950 Ma
the African
persisted
Camp
between
ocean
about
The
and
in NE Africa of
of
the African
Between
granitoid
Cratonization
to rifting of
Stoeser
creation
ANS.
(Fig.6.80,B).
post-collisional 620 Ma.
thinning
of
parts
southern
of the Eastern
attest
cycle.
in the east and
ensimatic
events
the
and
terranes all
Pan-African
of
western
in many
subsequent
part
the
Arabia,
lithospheric the
terrane
640 Ma
collision
the
the
extensive
about
(Figs.6.79;6.80,B)
of and
eastern
the Afif
(Fig.6.80,A).
terrane
of
on
in the exotic
in Saudi
as was
beginning
postulated 1.2 Ga
gneisses
those
microplate
terrane,
the
about
situated
including
the Afif
Precambrian
Proterozoic
640 Ma
occurred was
then
becom-
415
pleted.
From
about
630 Ma
truded the cratonized
to
about
540 Ma
E a s t e r n m a r g i n of t h e NE A f r i c a n p l a t e - - ~ : passive continental .
~ //.,~.,
/cq ~
.
.
.
~
,
-Nubion
margin
e v o l v i n g arc
-~"
basement
\ -a/oceaniccrust.-.-: ,/ , ~ ~ ,,:,, , -
"/
~ "
,
~
,
;~--:r-~-x~--,~ ~ ; ~
- I ~ ~
However,
Rather,
~
t
i
c aec ompression
available
at
faulting
,~,
terrane
in
the
continuous 1987)
terrane
to
between
800 Ma
Following
correlations
between
Red
Sea
developed
the
900 Ma
north and
resulted
which
700 Ma. from
welding
and
Red
Sea
670 Ma
and
620 Ma
Haya
as
development
that
the
Shield
terrane
Shield
at different
times.
which
is
(Fig.6.80,A),
of
the
between
both
believed
about
700 Ma
microplates
(Haya
an
extensional
indicated
by
the
the
terrane
suture
underwent
to
be
(Kr6ner et al.,
while
Hijaz
Umq-Nakasib-Amur at
ter-
(Fig.6.80)o
to the Nubian
800 Ma
with
the
island-arc
in Saudi A r a b i a
Bir
of
simultaneously
terrane
collision
Hills
I
and accreted
correlates The
together
the
reveal
develop
from the Arabian
Hills
I
550Ma-
ages
all
island-arcs
between
the
the
terranes),
not
with the At Taif-Jiddah
evolved
microplates
did
I
granites
around
radiometric
microplates
show that different Thus,
/
granitoids
Figure 6.79: Plate Tectonic model for the ANS. (Redrawn from Schandelmeier et al; 1988.)
or
:~.
/ _ ,
" ~ ","
"q ~
myl o n i t e
~
. I ' - , ~, .~ '
of S - t y p e
e r o s i o n / u p l i f t, b l o c k
1686':'52oMol
ranes
-~ ~
Mo ] B]R-SAFSAE- ASWAN UPLIFT
~
520 M
Shield
back-arc b a s i n
.
generation
~
in-
,. ~ * \t
/ : ~'P~e-Pan-African
.
granitoids
4,,~
/
[720--680
intracratonic
shield.
Gebeit evolved
between and
both
680 Ma.
and
Gebeit
tectonic
regime
occurrence
of
volcanics
416
which do not
show the penetrative NE-SW structural
grain produced during
the microplate collision along the Bir Umq-Nakasib-Amur belt.
S'" A' ARC ~ ul ~
9oo-~oo 4 ;
-
~
and merQinal
ALAy,A-arc" ~
arc terronll continent
~.i°.
•
Imar~)'hal . j b o , m ,. • •
PlY |Jt' ~
//o / "~ ~4~/ Asm o. ARc
I:~
"
O,ATO.." 1 •
.
.
.
ii(g' ]J
A .~
NAJD FAULT SYSTEM
AOO.TEO
AF.,~A. '-, ' . ' , ' '." ' " ' " AL AM,~
t T~""A"E
intraplate strike-slip
faults
~ . : .,,,.../.. ~-:\ \ , ." "I"-":,:.::I:.X \ \n • ./-~.."L' . " t :::~A U
"
.
i ,AYA I
• '
60OMa
/
I,LX.;\ (
ARC COMPLEXES,"
'
":~'4
, \\~p
• l',.-
"If,": :. • ~Li • ". ,"
PLATE
.?..: ~ ' ~
c
c01,i,~0;oiEo,, Ar°b,on • tt " c o n t i n e n t a l p l a t e with s e c r e t e d .~ i=land-arcterrone 1
Figure 6.80: Progressive development figure supplied by N. J. Jackson.) In the Tokar terrane of
northern
Ethiopia
predominates,
which
volcaniclastics et al.,
°goinst Afric~.
EAST CEN. EGYPT ,. . , )/ ::::. AFIF \ • ' " ' ' I'" ' " CONTINENTAL ~ ".'.' " : .' : . : . - - ~ 'tMICROPLATE • " '. : ' " " :1 k .:. " . l ' . ' , ( " X
.,;.,I., E,¢ //// \ "
"
7006 4 0 Me tectonic juxtapoIJtioning of i l l ° h a l -
Island.ore fesfoonl
~
1978).
that
the
ANS
(Redrawn
from
further south in the Tigre and Eritrea provinces
low-grade
meta-volcanics
consists
mainly
accumulated
These
of
island-arc
in
of a
rock
of
island-arc
and°sites
shallow-water
suites
contain
and
character associated
setting strongly
(Kazmin deformed
bodies of syn-tectonic diorites and granodiorites, and intrusions of lateto-post-tectonic granites and granodiorites with cooling ages ranging from 700 Ma to 450 Ma. In the
Eastern
Desert
installed in the central emplacement of and
710 Ma
initial
suggests
a
of Egypt a passive and southern parts
arc volcanics change
to
margin
until
about
and granitoids
subduction,
hence
seemed
to have
800 Ma.
Here the
between about conversion
been
to
770 Ma an
en-
417
simatic tectonic regime with ophiolite subduction leading to the development
of
arc
systems
which
lasted
until
about
680 Ma
ago
when
collided and were accreted onto the margin of the Nile craton Lastly,
following molasse-type deposition,
there was
the
arcs
(Fig.6.79).
low-angle
thrusting,
strike-slip faulting and the emplacement of late-tectonic plutons at about 600 Ma to 570 Ma
(Stern, 1985).
In contrast, compressional oldest rocks
the Northern Eastern Desert evolved mainly in the strong
regime
which
610 Ma old;
the bimodal
arc
accretion.
Dokhan volcanics
clastic Hammamat
molasse
tensive
rifting.
about
followed
Excluding
Sinai,
in the Northern Eastern Desert are granodiorites,
phase 600 Ma
of and
formed between Late-tectonic
570 Ma;
and
and their intimately 600 Ma and 575 Ma, granitoids
bimodal
dyke
were
swarms
the
680 Ma to associated
during an ex-
emplaced
intruded
between
from
about
590 Ma to 5~0 Ma. The m i c r o p l a t e collision and accretion events of the Red Sea fold and thrust
belt
were
felt
in
the
Mozambique
belt
as
well.
The
island-arc
systems in the southern terminations of the ANS, though poorly dated, also were
folded
along
the
and
thrust
onto the
Adola-Moyale
belt,
Dimtu and Sekerr sutures. Tanzania
was
also
surrounding
and
along
Further south,
involved
in
severe
basement
the
areas,
Ingessana-Kurmuk
for example and
Tullu
the Mozambique belt of Kenya and continent-continent
collision
and
suturing at about the same time, between 900 Ma and 600 Ma ago. 6.11.8 M i n e r a l i z a t i o n Syntheses (1984,
on
the
1988)
widely d i s p e r s e d various
mineralization
and by Vail
(1979,
publications,
countries
in
the
1985, and
ANS
an
elaborate
following
synopsis
is
account
of
largely based
the on
been
technical
For example, mineral the
presented
by
Pohl
in addition to numerous and
inaccessible
that make up the ANS.
furnished
have
1987),
Hussein
deposits
genetic
reports in
in the
(1990) Egypt.
descriptions
of
has The Pohl
(1988) as shown on Fig.6.68. The ANS
is generally not considered
genic province,
to be a very productive metallo-
although gold mining dates
from antiquity,
e s p e c i a l l y the
Pharaonic times; and a wide range of metallic and industrial minerals have been exported in small quantities from the Sudan and Ethiopia, platinum, mineral
chromite and mica. However, prospects
in
international metal 1988). A genetic
the
ANS
will
commodities market
for example
the development of a large number of depend
on
the
recovery
of
from its present d e p r e s s i o n
classification of the mineral deposits
of the ANS
the
(Pohl, shows
418
that
syngenetic
base-metal
stratiform
sulphides,
mineralization,
ores,
ophiolite-related
and magmatic
deposits
deposits,
including
volcanogenic
extensive
pegmatite
are quite promising.
Syngenetic Stratiform Ores In Egypt
and
Saudi
Arabia
tion characteristics origin.
Some
deposits
of
host
carbonates
magnetite
these
gold
hematite
ferruginous-banded
as
well.
in Saudi Arabia
in the terrigenous
and
with
banded
iron-forma-
occur which are probably of volcanogenic-hydrothermal
There
where
are
cherts
associated
magnesite
deposits
with
in
there are M n - Z n - C u - b a r i t e
these
sedimentary
lenses
as well
metasediments.
Ophiolite-related Deposits In
Egypt,
Sudan
ultramafics
Ethiopia
; magnesite
serpentinites. zones,
and
In
veinlets
Egypt
in addition
there and
high-grade
are
stockwork
talc
to the occurrence
chromium
and
bodies
deposits
are
platinum
ores
occur
in dunite
found
in
some
of low-grade t a l c - c a r b o n a t e
in and
shear
rocks.
Volcanogenic Base Metal Sulphides The
most
prominent
and A n d e a n - t y p e ment
Cu,
arcs.
magnetite
granodiorites. Zn,
Pb,
breccias. beds
a
exhalites, Saudi
Au
and
at
in the
associated
contacts
and proximal
with
massive
Ag
are
associated
are
the
more
with
and
There
are
in
around
or
graphitic
quartz
veins
acidic
ANS
are
acidic
with
them are replace-
tuffs,
for
domes
Zn-Pb-Cu and
with
example
at
bodies
intrusions.
and
containing
subvolcanic
stockwork
subvolcanic
ensimatic
diorites
deposits
lenticular character,
and
the
gabbros,
sulphide
distal
hydrothermal-sedimentary
calc-dolomite
mineralization
the
present
more
Arabia.
environment
Among the deposits
ores Stockwork
Also
with
metallogenic
bands
of
Nuqrah
in
with The
and
sulphide
Au-Ag
numerous
small gold fields of Egypt are of this type.
Magmatic Deposits As
already
while mation about
post-orogenic
the
640 Ma island-arc rocks
late
granites
to
evolution was
of
obtaining
the
ANS
in one
accretion
are
alkali
at
was
so
part,
plutonism were still active
intrude
intrusives
including
tectonic
magmatism
and syn-tectonic
magma t i c these
shown,
orogenic
elsewhere.
and suturing had l a r g e l y
shallow
levels
granodiorites, granites
and
until
often
that
defor-
However,
by
ceased allowing
about
monzogranites, syenites,
complex
540 Ma.
Among
alkali-feldspar with
equivalent
419
volcanic rocks and layered gabbroic rocks, especially in the southern part of
the
ANS.
The
intermittently
Najd
during
fault
this
system
period
in
Saudi
resulting
Arabia
in
remained
greenschist
active
metamorphism
and coarse molasse deposits. Among
the
important
associated with involved
types
those at Abu
Dabbab
of highly
phases
and
their
are Ta-Nb,
evolved
and greisenisation.
in Egypt,
pegmatitic
granites
of mineralization
copolas
in albitization
marginal Alkali
small
granites
These
deposits,
include disseminations
and
external
quartz
Sn, Be
which
pegmatitic-hydrothermal
been
for example
within
veins
and W
had
the copolas,
and
stockworks.
are
mineralized
suite
with Nb, Zr, Y, REE , U and Th; and ilmenite and magnetite occur in layered mafic complexes.
In the Baish Group of Saudi Arabia
(Table 6.4) scheelite
with
calc-silicates
amphibolite
the
quartz
and
immediate
vicinity
of
in hornblendite
a post-tectonic
and
muscovite-biotite
occur
granite
in
thus
indicating a genetic link with acidic magmatism. The
coarse-grained
northern Sudan, of muscovite: 644 Ma;
pegmatites during
are
the
pegmatite Berbera
Mg-Ti-Li-rich event
metamorphism
of
carry
the
Bayuda
desert
two different
muscovites,
at
552-526 Ma
believed
to
be
which
formed
main
in
generations
in
mica),
in the
the
phase
the
of
syntectonic
(K~ster
(former
mining
amphibolite-grade
pegmatites of northern
metamorphic
basement
units
These
regional important
1990),
district
district for tin and tantalum). According to KHster et al.
lower-
granitoids
Other
et al.,
and in the Bosaso area to the northeast
metal-bearing vein-type
a
1990).
products
with
Pan-African.
Somalia
northwest
et al.,
anatectic
of
northern
reflecting
(K~ster
contemporaneously
tectonic lie
region
phengite
dated
fields
columbite,
pegmatites
for mica,
Rb-Cs-Sn-Nb-rich varieties which were emplaced at about 698-
and
temperature
muscovite
formerly mined
in
for
the
beryl,
(former mining (1990) the rare
Somalia were emplaced into and
into
the
greenschist-
grade m e t a - s e d i m e n t a r y Inda Ad Group between 497 Ma and 392 Ma, after PanAfrican
granites
had
triggered
the
circulation
of
fluid
phases
in
a
tectonically reactivated terrane. Gold-quartz be Ag,
Cu, As,
and
gold-carbonate veins,
Pb and Zn, are widespread
with
pyrite
in the ANS.
in which
there may
These are hosted by
intrusive volcanic and ophiolitic rocks, including post-tectonic granites, and quite often, evident. hundreds showing sometimes
a direct relationship with cooling intrusives may not be
The
gold
of
m
strong caused
veins
long.
are
These
tectonic
usually veins
control
boudinage
of
thin,
often by
less form
ductile
the veins.
than
one mm
systems or
Almond
several
brittle et
al.
and
several
km
long,
shearing
which
(1984)
explained
420
these
veins
as
originating
from
large
hydrothermal
systems
which
were
either induced by metamorphism or by the cooling of unexposed intrusives.
Chapter 7 Precambrian Glaciation and Fossil Record
7.1 Precambrian Glaciation A major
aspect of the Precambrian
lier in passing, cially
in
abounds
the
in
(Fig.7.1). deposits
s t r a t i g r a p h y of Africa,
is the w i d e s p r e a d Late
the
Precambrian
From
a
Hambrey
occurrence
Proterozoic. of
compilation
(1983)
Evidence other
of
of glacial
for
deposits
continental
continents,
the
and Harland
mentioned
Earth's
espe-
glaciation
except
Antarctica
pre-Pleistocene
(1983) d e t e r m i n e d
ear-
glacial
that the intervals
of w o r l d - w i d e expansions of continental ice sheets can r o u g h l y be grouped into glacial eras, periods and epochs as shown below: (I) Late P r o t e r o z o i c Glacial Era: (i) Late Sinian Glacial Epoch: (ii) V a r a n g i a n Glacial Period (with 2 main epochs):
610-580 Ma 650-610 Ma 720-660 Ma
(iii) S t u r t i a n Glacial Period (with 2 main epochs):
790 Ma 800 Ma
(iv) Lower Congo Glacial Period (with 2 main epochs):
820 Ma 950 or 865 Ma 2.0 - 1.0 Ga
II) M i d d l e Proterozoic Glacial Era:
(III) Late A r c h e a n - E a r l y Proterozoic Glacial Era: H u r o n i a n Glacial Period (with 3 or more epochs): 2.3 Ga W i t w a t e r s r a n d Glacial Period (with 4 or more epochs): 2.65 Ga Though direct
sometimes
evidence
mixtite),
such as
striated
friction cracks, indirect rise
sorted
rock
and
the case
glacial
such
as
in
glaciation tilloid,
surface
which
and other geomorphic
post-glacial
ranging
(tillite,
basement
1983). Till and tillite
debris,
for ancient
deposits
polished
roches m o u t o n n ~ s
evidence
(Crowell,
ambiguous,
rapid
and
grain-size
form
diamictite,
commonly
forms;
clay
to
shows
as well as
pronounced
(consolidated till)
involves
sea-level
consist of unboulders,
with
some of the larger stones having been t r a n s p o r t e d by ice over great distances
in w h i c h
traced
to
doubtful with
their origin.
boulder
case they are source
areas.
Diamictite
beds,
clays
is
and
sometimes Tilloid a
faceted refers
general
sand,
term
pebbly
and
to
striated
and can be
tillite-like
for
an
sandstones,
rocks
unsorted and
of
deposit
mudstones.
422
Tillites
and
tilloids
African mixtites
are
sometimes
termed
mixtite.
The
origin
of
some
is controversial.
~c
A
(7 .-.
300Kin
!
el
b
J
d
Geological
eJ ¢;n
R3
R2.~ R1
1000 680
16S0 1000
V.R&
680-' 560
Ha
PR1 -~ A
? ?
>1650
I
Tillite
4~x
Other gl. roc ks Mixtites
A
Non gl. mixtites
t>
Figure 7.1: Global (Redrawn from Windley, For example, been
beginning 1989;
whereas
attributed
believed Salop, however,
distribution 1984.) the mixtites
debris
flows
of new sedimentary
Stanton
Formation
to
in
et al., the
to be
1983; to
1963),
Damara
of glacial
Tankard correlate
V
cycles
by
by
1982).
1983),
some workers There
Precambrian
has
these
(e. g.
been
mixtites
orogen
subsidence
(Cahen and Lepersonne, (Porada,
tillites.
Congolian
strong
a mode of origin also invoked
origin
African
Precambrian
of the West
triggered
Supergroup
et al.,
of
have
at
1976;
the
Porada,
for the Chous deposits Harland,
a general regionally,
are 1983;
tendency, and with
423
glacial
deposits
inferred
ages
1978). their
other
so permit
Since
directly,
in
their
parts
of
(Chumakov,
precise
the
world,
1981;
ages
especially
Deynoux,
are
often
1983;
when
Deynoux
difficult
to
their
et al.,
ascertain
glacial deposits are usually assigned approximate ages based on
stratigraphic
position
above
and
below
radiometrically
dated
intervals. 7.1.1 Late Archean-Early Proterozoic Glacial Era The
Witwatersrand
Supergroup
(Harland,
1983;
Tankard
overlying
Ventersdorp
contains
et al.,
lavas
the
1982),
dated
earliest
estimated
at about
known
to
2.64 Ga
be
glaciation
older
than
and younger
the
than an
underlying granite which is about 2.66 Ga old. These glacial deposits belong
to
the
witwatersrand
with
striated
pebbles
shelf
deposits
at
Group
(Fig°4.5A).
Glacial
associated
two
or
three
Tankard
Period. with
They
alluvial
stratigraphic
et al.
consist
(1982)
fan
diamictites
deltaic
levels
postulated
of
in
and
the
distal
West
that the most
Rand
likely
agent of deposition for the West Rand Group diamictites was submarine debris flow triggered from accumulations of ice-rafted moraines. Named after the Huronian tillites of Ontario, terozoic
Huronian
diamictites within
the
(Fig.7.2). pavement
which
occur
Postmasburg These
and
stones,
Glacial and
associated
glacio-fluvial
and
and
is
sporadically
glacial
mudstones
Period Pretoria
beneath Groups
diamictites shales
glacio-marine
the
of
the
contain
conglomerates,
varved
Canada,
represented
regional
have
et
Brazil,
and Wyoming
(U.S.A.)
of
by
Supergroup a
striated
sandstones,
been
(Tankard
Africa
unconformity
Transvaal
remnants
silt-
interpreted
as
al.,
Salop
(1983) considered the glaciogenic deposits in Kimberley Africa
the Early Pro-
South
cross-bedded
which
origin
in
to be roughly equivalent
1982).
of
(N.W. Australia), to those of South
(Fig.7.2).
7.1.2 Mid-Late Proterozoic Glacial Eras Mid-Proterozoic Tuareg Shield Earth,
compared
Silurian, the Late count
even
mid-Late the
are known below the Stromatolitic
bulk
with Glacial of
Series
in the
But by far the most extensive glacial period on
Devonian,
Proterozoic
for
Fig.7.1.
tillites
(Fig.6.15).
the
later
glaciations
Late
Paleozoic,
Era.
The glacial
Precambrian
(during and
the
deposits
glacial
the
Ordovician-
Pleistocene), of
deposits
this
was
era ac-
plotted
in
424
African
Late
Precambrian
platforms
and mobile
belts
were
awash with
tillites.
USA
BRAZIL
S.AFRICA NW. AUSTRALIA N. AUSTRALIA - KIMBERLY
PINE
CREEK
IZ!
o
)-
m
,...i
v
,¢
0
-'-~
0
iff. !z 'n-]
N xz
.......
Figure 7.2: Geologic columns showing correlations of Precambrian diamictite-bearing supracrustals. (Redrawn from Salop, 1983.) In the West Congolian Glacial
Period,
sup~rieure
du
the Bas
Groups respectively.
mobile belt,
"Tillite
Congo"
the type area for the Lower Congo
inf6rieure
underlie
the
du
Bas
Louila
Congo" and
the
and
the
"Tillite
Schisto
Calcaire
The age of the lower tillite is believed
to be about
425
950 Ma,
while
that
of
the
upper
tillite
is
probably
820 Ma
(Harland,
1983). The equivalents of both tillites are the Grand C o n g l o m ~ r a t and the Petit
Conglom~rat
mixtite basin
of
the
of
the
Lindian
correlates
with
Katangan
Supergroup
Supergroup the Grand
in
the
NE
(Table Zaire
Conglom~rat
and
6.3).
The
Akwokwo
Precambrian
the
"Tillite
platform
inf~rieure
du Bas Congo". In the
Damara
Supergroup
of
Namibia
the
diamictites
of
the
earlier
Sturtian epoch occur in the Nosib Group at the base, whereas those of the later epoch include the w i d e s p r e a d Chous mixtite, Numees
mixtite
in the Gariep
in the Tuareg Shield
(Fig.6.15)
and of the T a f e l i a n t Group Deynoux deposits
Group
(Figs.6.42,
Africa.
Sturtian
include the tillites of the
the
tillites
"Siere Verte"
(Fig.6.17A).
(1983) p r e s e n t e d a synthesis
in West
and its equivalent
6.46).
These
are
on the Late P r e c a m b r i a n glacial
exposed
as
a
thin
ribbon
along
the
n o r t h e r n and w e s t e r n parts of the Taoudeni basin
(Fig.7.3), and belong to
the V a r a n g i a n
deposits
Glacial
Period.
Varangian
glacial
in West Africa
include the tillites of the Tabe Formation at the base of the Rokel River Group
(Culver et al.,
their
equivalents
1978); the Kodjari tillites in the Volta basin; and
dated
at
about
n e a r b y B e n i n i a n m o b i l e belt. fall
between
Group)
in
age
the A d r a r
green shales The
the
of
675 Ma
in
the
Buem
Formation
of
the
In the Taoudeni basin the V a r a n g i a n tillites
the
region,
upper dated
middle at
part
about
of
Supergroup
775 Ma,
and
the
I
age
(Atar of
the
(595 Ma) in the overlying S u p e r g r o u p II (Fig.7.4A).
"Jbeliat
Group"
is
the
collective
lithostratigraphic
term
pro-
posed for the A d r a r tillites and other V a r a n g i a n tillites in the Taoudeni basin liat
(Deynoux and Trompette, Group
was
presented
by
1981).
A detailed description
Deynoux
(1983).
This
deposit,
of
the Jbe-
up
to
50 m
thick in its type area in the Adrar,
consists of two u n c o n f o r m a b l e phases
of terrestrial tillite accumulation,
each o v e r l y i n g an erosional
The erosional
surface represents
the pre-glacial
tillite, and an irregular surface with tillite
(Fig.7.4B).
lacustrine glacial rarely exhibit
The tillites
or marine
retreats.
slump
for the second
are succeeded by fluvial
sandstones and
dropstones
interglacial argillaceous
structures
related
for the lower
"roches moutonn~es"
shales with
The
conglomeratic,
substrate
surface.
which were
deposits
siltstones) to
friction
deposited
(fine-medium between or
the
the
during
sandstones, two
ploughing
tillites of
ice-
blocks on m u d d y tidal flats. The Jbeliat glacial deposits are capped by a thin and e x t e n s i v e d i s c o n f o r m a b l e structures w i t h i n
sandstone h o r i z o n
sandstone wedges.
This
containing
polygonal
is o v e r l a i n by p o s t - g l a c i a l ma-
rine t r a n s g r e s s i v e deposits belonging to the T e n i a g o u r i Group
(Fig.7.4A).
426
Two
regionally
occur rich
persistent
immediately calcareous
bedded
chert
above
and
characteristic
the polygonal
dolomite
horizon,
(Fig.7.4B).
The
post-glacial
sandstone
3 - 5 m
mixtite,
thick,
dolomite
horizon. is
A
lithologies thin baryte-
overlain
with
baryte,
by marine and
chert
c o n s t i t u t e the triad, a regional marker for the V a r a n g i a n tillite in West Africa.
I
J O,uaternary and Heso-Cenozoic Late
/,~
cover
Catedono-Hercynian Pan-African
fold
fold
Precambrian
basement
Outcrops of Late P~ecambr(an glacial deposits
Precambrian and Paleozoic cover belt
Aree
belt
shown in
Fig. 7-~
Figure 7.3: Distribution of late Precambrian tillites Africa. (Redrawn from Deynoux, 1983.)
The glacial in
South
Africa
deposits and
of
the Late Sinian
Namibia
in
(Table 6.2), a c c o r d i n g to Harland
the
lower
in West
Epoch are b e l i e v e d parts
of
the
(1983) and Tankard et al.
to occur
Nama
(1982).
Group
427
Upper Ordovician
gla=~o,d~pos~ts
~-_~-.~-----_--'..~_-OLTED-CH.IG'~GR0~
~ ~
t ~ o ~ o oNJAKA.E-A%mW GR0U-~'~
O
V
V
.~oundary ,?~_:,, II, , , .l]. ,' ; , * ' 1 l ,', -
O
O
0
'
u
- . p FO ,, , , ' ".-V'.'OUJEFT ' , "
"II-
3
O
.',;.-', "
.
.
.
.
.
~•' ' ~ ' " ' . # - . " z"" ". " . , . . ~ .- " . ~ - . .7:. .. . . 5ROUP . . ,,.
500m
".'." .y_7". : ~.',"T.'.'.Tr.'.." ".Tr.i,i,"~'." .'.Tr'." ." ~. /'SUPERGROUP
.~.~--~T~-:'.-~-'..:--7-.-:-:--~:~-GROUP..=t'~;: Late Precambnan ~/~__/-~ glacial deposits ~ _ _ _ ~Xx"o-- ~.b+ ' 6 - '
~.o- -6,~'~o-,-~
LOWER PROTEROZOIC~
FENIAGOURI GROUP ' ~ ' ~ ~- . . - ~ - . ; -
-- , ;
~
"
~
~
" ~ "':'. SUPERGROUP •
+.
-,~..~.:., A
~
Coarse sandstones and carbonates
~
Glacial deposits
Shales and siltstones, bedded cherts in the Teningouri Group
~
Very fine sandstones, siltstones and Shales
Fine sandstones with Scolithus
~
Cross-bedded Argilaceous
fine sandstones
tromatolitic carbonate rocks, Siltstones and Shales
~
Sandstones, conglomerates and Sittstones Polygonal structures and sand-wedges copped by calcareous dolomite
sandstones
NE
Oued Jbiliat O u e s t
.
.
.
.
::. .............. . . ~ - . . . : . ~
~...........
20m] ,2Kin
]
B
Teniagouri Group (silexite)
]
Giouconitic sandstone
[~
Calcereous dolomite with barytes
Conglomeratic sandstone
[]
Shaly sandstone with conglomerate
Bose of glacial units (Assobet-Hassiane Gr )
Figure 7.4 :
the Taoudeni
Late
basin.
Precambrian
and
Early
(Redrawn from Deynoux,
Paleozoic
1983.)
sequence
of
428
7.1.3 P a l e o m a g n e t i s m Polar wandering Precambrian
and Paleolatitudes
and global
glaciations;
climatic changes have been
but
as
yet no generally
has been found. N o r have the paleolatitudinal been
established
pletely al.,
with
certainty.
contradictory.
1973)
believed
One
that Africa,
ing the Late Proterozoic al.,
1974;
cated
Veevers
near
the
of
Pole
(1983)
pointed
(Fig.7.SB,C),
we have reviewed. Precambrian tudes
changes Global
tillites
(Windley,
could
hence
that
com-
Piper
et
the
(McElhinny et
Africa
great
have triggered
was
remain uncertain,
the profound
Australia
Har ~
and worldglaciations
for the occurrence
North America,
lo-
glaciations.
of polar w a n d e r i n g
cooling would account
in Europe,
are
g.
lay along the equator dur-
postulated
out that a combination
wide paleoclimatic
(e.
whereas another school
While the causes for global Precambian glaciations land
of the continents
plaeomagneticists
1976)
to explain explanation
interpretations
for example,
(Fig.7.5A),
and McElhinny,
South
positions
Paleomagnetic
school
invoked
acceptable
of Late
and at low lati-
1984).
7.2 T h e Precambrian Fossil Record Because
fossils
a plant
or
Earth's
crust
life
by definition
animal
that
since
.... " (AGI,
has
some
1972),
taphonomic
features,
of
paleontology.
starts
from
sediments;
been past
preserved geological
and geochemical
3.5
"any remains, by
trace,
natural
time;
or imprint of
processes
any
in
evidence
of
Seen
Ga,
and consists
in
the
markers
this
age
have all been placed
light,
of
the
of only indirect
the
Archean
oldest
evidence
known
the past
the remains of Precambrian micro-organisms,
domains
about
embrace
their in the
fossil
record
unmetamorphosed
of life in the form of
inorganic structures
and organic chemical compounds which are believed to
represent
remains.
microbial
evolutionary verse first
pathways
soft-bodied time
Precambrian
paleontology
tal indicators
than
trace
and geochemical
Frazier
(Knoll,
and algae,
such
record
fossil
analyses
cryptic
evidence
been hypothesized
(Ediacaran
therefore
fauna)
towards entails
studies
of ancient
as
leading
which
the
for
the
the Proterozoic.
morphological
metabolic
all
to the di-
appeared
the end of
of preserved
hangs
investigations
microbial
communi-
and paleoenvironmen-
1990).
Schwimmer
1.0 Ga into:
blue-green
that have
metazoans
in the geological
of micro-organisms; ties;
Upon
spheres
(1987)
grouped
and bacilliform
bacterial
or fungal
most
Precambrian
structures
spores,
fungi);
fossils
older
(possible bacteria, filaments
possibly
50*W
0
A
Figure dering
.,," ~ . . ' - -
/ ~ / ~ . i ; ":: ~ :,"
B
~
\
7.5: Late P r e c a m b r i a n - E a r l y P a l e o z o i c a p p a r e n t of the S o u t h Pole. (Redrawn from Deynoux, 1983.)
::~'i
~;!:~..\
wan-
C
430
of
algae;
spheres
stromatolites
(colonial
(bacteria,
algae,
(algal
bacteria fungi,
shaped single-celled
and
bacterial
or algae);
structures);
spheres
undergoing
or other single-celled
structures;
clusters cell
eukaryotes);
of
division
irregularly-
and fossil carbon compounds. 613Cpd b
o
-1'o
-2o
-;o
-go
I MOODIES FOSSIL S ( Archaeosphaeroid~ Eobacterium
FOSSILS{ 20Jam spheres filaments) FOSSILS( 20sWm spheres filaments)
FI5
_
TREE
p
IL
S S
0
D sS
e
S
SWART
KOPP!E.... ~_ KROM= BERG ~.j
D D
0 e
e
e e
D (Z
w -r
MIDDLE MARKER 3280± 70 Ha)
P
(age
"r
~:
o
z
FOSSILS( lO~m
I
THEESPRUIT
o
spheres filaments )
tsA.oj SPRUIT
I
|= organics 1 D= dolomites 1 I ~,,S=siderites J
Figure 7.6: Distribution of microfossils and carbon data for the Swaziland Supergroup. (Redrawn from Windley, A
chronological
comprising
some
account
of these
of
types
the African of organic
Precambrian
remains,
fossil
is presented
Mention is made in passing of those in other Frecambrian to fill the missing gaps in the African record.
isotope 1984.) record, below.
regions in order
43t
7.2.1 The A r c h e a n Fossil Record Windley
(1984) p r e s e n t e d a comprehensive survey of the known A r c h e a n - P r o -
terozoic m i c r o f o s s i l s of Africa. land
Supergroup
greenstones
yielded microfossils
Three s t r a t i g r a p h i c levels in the Swazi-
(Fig.7.6)
at least
in
sils are carbonaceous cell-like spheroids, ies, and
filamentous t h r e a d - l i k e
the L o w e r Onverwacht G r o u p in the Upper O n v e r w a c h t 55
microns,
of the the
these
section, forms
Group,
up in the
contain
carbonaceous
spheroids
flagellates.
Also,
at
about
represent
Some
the
the
evidence
compounds
of of
metabolic
cherts
increase
in
size
upward
contains
spherical
in
the
stratithe size
Onverwacht
cell division,
organic
Fe,
Ni,
processes;
to
and
some of
some of the
and
and
of
microfossils
in
the
bodies Ca
with
which
plants.
however,
of
probably
evidence
been
in favour
Supergroup
coatings
were
isotopic
which
columnar
have,
Swaziland
dated
aggregates
primitive
(1983). But among the arguments
matter
algal
carbonaceous
diaspores
ascribed
possible Cu,
In the Upper
of binary
vegetative
of
and
in the Fig Tree Group resemble algae and cysts of
materials
presence
occurrence
black
Ranging in size from 1 micron to
section.
q u e s t i o n e d by Schopf and Walter of
in chert and argillite
the Pieterburg greenstone belt of South Africa,
2.6 Ga,
probably of
They are found in cherts in
and in the o r g a n i c - r i c h
microfossils
have
r o d - s h a p e d b a c t e r i u m - l i k e bod-
structures.
(Fig.7.6).
province
so that those in the Lower O n v e r w a c h t are half
higher
spheroids
Kaapvaal
These p r o b a b l e microfos-
(Theespruit Formation),
shales of the Fig Tree Group graphic
the
3.5 - 3.4 Ga old.
are
the
sulphur
and
precipitated
suggesting
by
carbon
f r a c t i o n a t i o n through photosynthesis. The S w a z i l a n d m i c r o f o s s i l s are believed to have carried out photosynthesis,
a vital process which could even have started e a r l i e r and liber-
ated oxygen
The
above geological
e v i d e n c e and findings in other A r c h e a n g r e e n s t o n e belts
into the
anoxic
primordial
evironment.
such as the War-
rawoona G r o u p in w e s t e r n A u s t r a l i a suggest that the e a r l i e s t A r c h e a n life consisted
of
groups
procaryotic
which
of
include
single-celled cyanobacteria
species of bacteria; contains
bacteria
procaryotes
organisms
(blue-green
the a r c h a e b a c t e r i a
that
can
thrive
and a third group of organisms modern
eucaryotes
cyanobacteria
were
(cells
probably
algae)
nuclei).
comprising
and m o s t
of
the
commoner
acid
or
salty
environments);
which were p r o b a b l y the ancestors with
the
the
of
Three
eubacteria
(a d i s t i n c t p r i m a r y k i n g d o m which
in hot,
(micro-organisms builders
without
existed,
nuclei). earliest
the Late A r c h e a n had appeared in great abundance.
It
is
to the
believed
stromatolites
that
which
by
The c y a n o b a c t e r i a could
i n i t i a l l y have utilized H2S for photosynthesis without g e n e r a t i n g oxygen,
432
but
later they were
tive
sources
able to exploit both
of energy
The e a r l y b i o c h e m i c a l cussed
in detail
record,
for
sunlight and w a t e r
food manufacture,
pathways
by Nisbet
thereby
in these primitive systems
(1987).
Our main
concern
of w h i c h c y a n o b a c t e r i a made their impressive
as alterna-
liberating
oxygen.
have been disthe
fossil
contribution
here
is
in the
form of stromatolites. Since Cheshire
they
are among
Formation
in
the b e s t - p r e s e r v e d
the
Upper
Bulawayan
stromatolites, greenstones
those
of
in the
Zimbabwe
are
d i s c u s s e d here in detail. While the occurrence of true stromatolites have been
doubted
in older Archean
strata
such
as
the Fig
Tree Group
and in
the M i d d l e A r c h e a n Nsuze Group of the Pongola basin in South Africa, morphological al.,
studies
1980)
modern
and
of the Late Archean Cheshire
geochemical
stromatolites,
stromatolites
are
studies
built by blue-green
carbonate
or
mats of these micro-organisms; with
organic
domes
(Fig.7.7) algal and
filaments
(with
radii
suggest
of
chert
these
the mats growth.
up
400
mm)
which
intertidal
Like
modern
origin,
stromatolites
the
Cheshire
(Martin et similar
are
Modern
produced
The
wavy
in
the
laminations Cheshire
which
forms
are
are
and
large
attest to their truly commonly
enclosed
in
of
lagoonal
shales
'
33 1 .. ,;...., J
..... I
EXPLANATION SHOWING TYPICAL CYCLE UNIT
321 -'
22
311
30] ..... 21
Horizon No.
Cycle No.
~Well-laminated brown-weathering dolomitic Clotty lamination ~ ~ l i m e s t o n e . 30 I~-L~.--_'_~116 Blue-weathering limestone with radiating Smooth lamination ~ ~ crystal structure. Crinkle lamination __._/'-~" Well-laminated dolomitic limestone with rare chert,
...._......-UPPER ZONE
2912~=_~_ %. 20
-,~1:19
281~'--~-=r..~_.1B
by
stromatolites
siltstones of intertidal origin.
~_".:
'"
-~
_ ~ > o -
[ LOWER
~
CYCLES
~.~
,-
Z LU LU Z 0 LL
~AN
TYPEI It'E -- DisconformitySarah
Era.
90.300m' ~ Glacial U n c o n f . ~
Z a r q e Fro.
a]
0-115m
261m
I Unconf. ~ ASNILLIAN ? C ARADOCIAN
~
L LANDEILIAN L LAN VIR NIAbI.:, ARENIGIAN
--z-4
o 663m
CAMBRIAN ? TO ARENIGIAN o < (.o
Regional -~. d i s c o n f o r m i t y "h.. CA M BRIA N?
Major Unconf.~ PROTEROZOIC
BASEMEN T(Shield
Figure 8.35: Schematic Paleozoic succession Arabia. (Redrawn from Vaslet, 1989.)
for
Stratigraphically, the
of
tics and carbonates,
Taoudeni
basin
2,000-3,000 m thick,
consists
central
Saudi
fine-grained
clas-
the type sections of w h i c h are
492
i
~ . + ~)
/ 0
/ ~
4
+ + +. + + + + + +
ZEMMOUR / ~ / ~ II Bir M°ghrein
~
b'~
"t"
"~
~
I~/ ~ _: : ~
,~
u..-----J
.., . . . . . ...:::;/ / O m e n d a
.:. " . : : . : " -
•
.:..,
"/'Tok oJ'Z-,
..-:;__._.:.w
.,"
• :12.~ '~i:"
....
Oumpum
c~*
Asemkaw
Figure 8.39: A f r i c a n coast.
Paleozoic exposures and sequences (Redrawn from Talbot, 1981.)
along
the
West
497
Preserved Takoradi,
along
the coastal
strip in Ghana,
are small discontinuous Paleozoic sections
to as the Sekondi Series. The Sekondi Series, in
faulted
shales
blocks
resting
and
or
is
predominantly
unconformably
in these sections ine
to the east and west of
upon
a
lacustrine
deposit
that
glacial
conditions
probably
(Talbot,
1981).
oldest
The
sequence
the Birimian
is the basal Ajua Group, the
have yielded Late Devonian microflora
referred
sandstones
(Fig.8.39).
under Late
biostratigraphically
ever, at the base of the Takoradi Sandstone Ajua glaciogenic group.
of
an intertidal
accumulated
during
(Fig.8.39),
1,245-1,325 m thick, occurs Most
notable
to shallow mar-
locally
freezing
Ordovician dated
and
or
glaciation
horizon
is,
how-
(Fig.8.39) where basal shales
from a horizon
300-400 m above the
Poorly preserved brachiopods,
pelecypods and fish
remains also occur at this level. Further
east
area of about West
near
Accra,
African
coast.
Believed
faunal and p a l y n o l o g i c a l base:
coarse
and shales; massive
evidence,
sandstones
sandstones
assemblage
(Kesse,
the A p p a l a c h i a n
was
assigned
(Johnson and Boucot, strata
Group
in
the
to
is
exposed
to M i d d l e
in
Devonian
comprises
alternating
a
small
section on the age
on
from its
fine sandstones
shales with trilobites and brachiopods; and
alternating Based
brachiopod
on
fauna,
Appalachian
shales its
and
thin-bedded
paleobiogeographic
the A c c r a i a n
brachiopod
paleobiogeographic
province
thus placing the West A f r i c a n coastal Paleo-
northern
North and South America
Early
the A c c r a i a n Group
1985).
the
1973),
of
sandstones;
thicker fossiliferous
a f f i n i t y with
zoic
to be
p e b b l y cross-bedded
cross-bedded
micaceous
the A c c r a i a n
11.7 km 2. This is the best dated Paleozoic
part
of
a
Devonian
seaway
that
came
from
(Fig.8.40).
8.6 The Cape Fold Belt
8.6.1 A b o r t e d Rifts and Glaciations Two d o m i n a n t
factors d e t e r m i n e d basin development
the Paleozoic.
First,
in South Africa during
the Lower Paleozoic Cape Supergroup,
thick sequence of n e a r s h o r e and shallow shelf sandstones, the initial
the Early Paleozoic. and Antarctic km
in
rifts along which southern Gondwana a t t e m p t e d to break up in Figure 8.41
aries which formed a triple 1,000
a phenomenally accumulated
plates.
further
However,
south
(inset) shows the incipient plate bound-
junction between the African, the d e v e l o p m e n t
(Fig.8.41)
the Cape region of South Africa.
aborted
South American,
of a subduction
further
crustal
zone some
extension
in
The Cape region instead r e m a i n e d as the
498
passive
continental
geosyncline.
margin
of
what
Northward-directed
Du
Toit
flat-plate
(1937)
termed
subduction
the
(Lock,
Samfrau
1980)
sub-
duction generated compressional forces that deformed the Cape Supergroup clastic wedge which then became the Cape belongs
to the Gondwana
orogenic belt.
fold belt.
Other
The Cape
segments
of
this
fold belt orogenic
belt are now widely dispersed in remote regions such as Bolivia, Peru and Argentina
in
South
America,
and
in
Antarctica,
and
eastern
Australia
(Tankard et al., 1982).
Figure 8.40: Early Devonian paleogeography (Redrawn from Tankard et al., 1982.)
of
Gondwana.
As already mentioned in the introduction to this chaper, South Africa witnessed spectacular environmental changes during the Paleozoic.
It ex-
perienced the Late Ordovician glaciation, and later lay at the centre of the great Permo-Carboniferous glaciation of southern Gondwana. 8.6.2 The Cape Supergroup This is an 8-km thick Early Ordovician to Early Carboniferous clastic sequence which forms folded mountain ranges along the coast of South Africa (Fig.8.42A).
Its equivalent, the Natal Group,
is exposed along the east-
499
PERMO- TRIASSIC MADAGASCAR
AFRICA
+
SOUTH AMERICA
ANTARCTICA
" ~'"-'-Co FO
%
t
e~['~'--
M.$ . ACTtVE
\, t\
// Natal Group
African Plate
S o ~ p'~
I 0 0 Km L ---
I
Incipient plate boundary
Figure 8.41: Tectonic model for the Karoo basins and the Cape fold belt; paleogeographic setting for the Cape Supergroup on a pre-drift reconstruction of Gondwana. (Redrawn from Daly et al., 1989; Tankard et al., 1982.)
500
ern
seaboard
of South Africa.
Pan-African and
metasedimentary
Klipheuwel
Fig.8.42A,
and
the
Cape
Supergroup
and the W i t t e b e r g
up
Table
the
tailed
Mountain
synthesis
Supergroup
granitic
post-Pan-African
Bokkeveld, of
The Cape
basement
molasse
is
divided
Groups.
Group.
rests and
unconformably
on the
formations. into
the
Franschhoek
As
Table
shown
in
Mountain,
the
A b o u t half of the s u p e r g r o u p
Tankard
et
for the Cape Supergroup,
al.
which
(1982)
on
is made
presented
is summarized
a
de-
below.
Table Mountain Group Of
Early
sists in
Ordovician
of quartz
an
to Early Devonian
arenites,
elongate
depositional
coast of South Africa. dence
and
faulting within
tain
correlates
the
Group
water
During
with
the Late
which
are
referred
tains
well-developed
and
and
roches
contains
Cedarberg
the
hence
basal
tillites
the
lagoonal
following
superjacent
shoreline
8.4
comprises
lower sequence, is o v e r l a i n Formation,
Mounshows
the Pieke-
by tidal
which
and
basin
lay along
flat
interfin-
shallow
shelf
Whereas
assemblages.
Formation
of an
accumulated
the Pakhuis
proglacially
associated
retreat.
the m a r g i n
sediments
transgressive
Nardouw
Table
(Fig.8.43A).
with
glacial
thickness
which
laminites,
brachiopod
in
Table
Formation.
overlying
and
subsi-
Group
glaciogenic
glacio-lacustrine
prevailed and
the Cape
present
east.
the
barrier-beach
Formation
the
in
Mountain
In the
high-energy
to
differential
facies
con-
accumulated
The
Graafwater
to as the Pakhuis
and
clastic
Group
Group
which
(Fig.8.42B).
fan sequence,
the
sheet,
moutonn~es,
conditions
glacial
ice
massive
tidal
of
Mountain
parallel
of
units
sequence.
Ordovician
Gondwana
tillites
stacking
Natal
the Peninsular
extensive
trended
of the Table
an alluvial
deposits
southeastward
quart z - a r e n i t e s ,
that
the
the
and an upper
Formation,
shallow
gers
with
subdivions
sequence
nierskloof and
in
the Table
and mudstones
basement-controlled
lithostratigraphic
stratigraphic
a lower
axes
Pronounced
resulted
variations
age,
conglomerates,
con-
reworked
striated
pavements
Cedarberg
Formatiom
These
The
environmental
upper
mark
a
part
of
return
to
the pre-
sedimentation.
Natal Group In the Natal under
greater
Group.
Since
stron g e r shoreline in the
embayment
tidal
wave the
the Natal
and Natal
currents
resulting
stratigraphic
tidal
Group,
current
embayment were
generated
in lenticular sequence
was
tidal
about
I000 m thick,
intensity
than
the
funnel-shaped and d i r e c t e d sand bars
as truncated,
was deposited Table
inset)
perpendicular
to the
(Fig.8.43B)
stacked
Mountain
(Fig.8.41,
and
which occur
en ~chelon
sand-
501
stone
units.
Otherwise,
as shown
in Fig.8.43,
the Natal
and Table Moun-
tain Groups have similar stratigraphic characteristics.
Early Carboniferous (WITTEBERG GROUP)
N A T A L GROUP
BOKKEVELD GROUP
Ordovician {TABLE MOUNTAIN GROUP}
Port Alfred
Cape
P o r t Elizabeth
(A) 200 Km
North
South
[ ~ ~ ! ~ } ~ i } "":"' ~~":%'.-':;;"-:' . -::'~.:":-:'.'-'.:'".:: ).ii~ili'~........................... i i ~i~i .!:~i:i!i i~ili~i 'i~!i:.:-~ii!i i ili!i i i:~ii ].
, 5
~ ~
oreoit. ~Con.gtomerate.sandstone= ~ subordinate mudstone
~ ~
4 Pa~o~ber~
~
3
Formations Peninsular Formations
2
Groatwater- Formation s
[
PiekenierskIoof Formations
~
~ ~
~
I---1 Pre-Cape basement ~
" (B)
Figure 8.42: Occurrence of the Devonian in South A f r i c a (A); and N-S section of the Table M o u n t a i n Group. (Redrawn from Hiller and Theron 1988; Tankard et al., 1982.)
502
Table 8.4: L i t h o s t r a t i g r a p h y (Redrawn from Tankard et al.,
WESTERN CAP,E FORMATION
of the 1982.)
Table
NARDOUW
w -'
1100 Coarse- grained quartz arenlte, trace fossils
CEDARBERG
1/,0 Fine-grained sandstone, siltstone,and mudstone, marine invertebmtes
PAKHUIS
120 Sandstone, conglomerate, d/arnlctite
=c,.
THICKNESS (m)
FORMATION
.-~
LITHOLOGY
AGE
BAVIAANSKLOOF
150 Shale, mudstone, quartz arenite, marine invertebrate s
KOUGA TCHAND0 CEDARBER5
)~0 Quartz arenite 200 Sandstone SO Shate,mudLATE ASHGILLIAN stone, fine( END ORDOVICIAN) grained sandstorm
PENINSULA
o
Group.
20°E .......... EASTERN CAPE
THICKNESS (m) LITHOLOOY
o
g
Mountain
1800 Medium- to PENINSULA coarse- grained quartz a renite with quartz pebbles, trace fossils GRAAFWATER &&0 Interbedded quartz arenite, siltstone, and mudstone, trace fossils PIEKENIERSKLOOF BOO Conglomerate and coarsegrained sandstone
SILURIANSIEGENIAN (EARLY DEVONIAN)
2150 Medium-to coarse-grained quartz orenite with quartz pebbles, trace fossils
EARLY- LATE ORDOVIEtAN
EARLY ORDOVICIAN
EARLY ORDOVICIAN
Bokkeveld Group This
is
a
deltaic
sequence,
about
(where subsidence was greatest) Cape.
According
essentially units,
each
represent cycles and
of of
to
Hiller
and
3,200
Theron
argillaceous
horizons
which
formation
the vertical
(Fig.44B)
caused
regressions.
Hiller
is
a
m
thick
in
the
eastern
Cape
and at least 1,500 m thick in the western (1988) which
the
Bokkeveld
alternate
(Fig.8.44A).
with
These
consists arenaceous
alternations
stacking of five or six u p w a r d - c o a r s e n i n g deltaic by t e c t o n i c a l l y - c o n t r o l l e d and
Theron
(1988)
adopted
marine the
transgressions
sedimentological
503
Gr~
N
uvial ~te ~|aJ erkose lde dominated f quartz arenite tn basement
Figure 8.43: Depositional environments of the Table mountain Group (A); and the Natal Group (B). (Redrawn from Tankard et al., 1982.)
504
criteria
established
interpretation Early
of
Devonian
(Fig.8.45)
for
the
Recent
Niger
sub-environments
Bokkeveld
the
the
Group.
Bokkeveld
was
As
delta
(Fig.9.27)
(Fig.8.44B)
shown
deposited
in in
the
that
for
existed
their in
the
paleogeographic
model
southward-prograding
wave-
d o m i n a t e d lobate deltaic systems.
GROUP
FORMATION
FORMATION
:~[~'Wifpoor t'."-
Famenn[an
W t p o o r t "-','~"
Swart ruggens Witteberg
Mar ine reworked
Frasnion
':'Biinkb'erg"~:;':' W e t t e v r ~ e
,
sands
deltaic Wogen D r i f t
Q_
n~
Karoopoort
41
.,0-
SandPoort
Givetion
:: o,ber~ :::::
Tidal
~= Klipbo.kop'~ Adolph,poort m O
,'. ','.','."
m
Dirkskroal
e
oE
~>
,>... ,,,,,; Pro-delta Shelf
Miller Diomictite
I
nr IJJ LL_
11
=
i
Wooipoort Shale
Z o m n-
Floriskraal Sdst.
~ L e w n ~ i n ~ ' B o s l n
÷
.÷
*
+
÷
÷
+
-#,
+
+
Mediterranean
4.
+
+
÷
+
+
÷
region.
During the Middle and Late Jurassic the external Betics in Spain, the Rif, the Tell, and the Alboran continental margins subsided rapidly while
535
Ii .... ~ ~ _ _ _ . : _ .
.
CO n o d
÷"
(]
.
÷
÷
PAL,ozo
--M e.s e t Q.
+
o
+/i--X.
------
--~J. ~0~/ ~c'°
,tlZ/'. . . .
- ~tk~ \ "
--
.
Iberlco.. . .
--
-
/~'~'?-~.
oRoG,NEs,5
c
( A LRINE OROGENES I S )
A TClssili
+
T
/ ~
+
CRYST.BASEMENT
T RIASSIC- JURASSIC SINISTRAL TRANSTENSION
/ J
+
7.,~d"
GIBRALTAR
~..,^~L,~
N.
,100km
,
c
D
~
CRETACEOUS-PALEOGENE DEXTRAL TRANSPRESSION
3-/~
~
NEOGENE OPENING OF z/~: - THE ALBORAN SEA
Z~1 /
~. ~
.
.
"-
.
.
.
.
.-
~" ""
.
RIF
ALBORAN
/_.,/7"~/.j~_ x
Figure 9.2: Atlantic and Atlas rift systems and their structural development. (Redrawn partly from Stets and Wurster, 1982.)
536
strike-slip m o v e m e n t affected the clastic-filled Atlas basins. of the A f r i c a n plate Late J u r a s s i c which
from the southern European plates in the Middle and
reached the point when ophiolites
lay in the central part of the Tethys;
tend into the w e s t e r n m o s t from
formed
in the Alps
but ophiolites
were
did not ex-
Tethys, where only few submarine volcanics
known between Africa and Iberia. siliciclastics
Separation
Africa,
are
In the Early Cretaceous large amounts of
Iberia
and
the
Alboran
continental
blocks
were d e p o s i t e d in the Tethyan basin as deep sea fans during eustatic lowstands of sea level.
Actual
~ -
C o a s t Line
Jurassic S u t u r e
Zones
betweenthe
Continental
Blocks
Emerged Land •
marine
^ m
~,'~2~
Ep,continento{ and C o n t i n e n t e ( deposits
Open
^
,c~,~%^
deposits
~
l-~i~
Ge r m a ~1~C'~9~
-~__'~C~_~ basin-~-~'_
Bisc°y~
A ~ "
"~'~
":'." A ATLANTIC _ . ~ , .Atlas Gulf IMARGI ~ 2 , A ~ I = ~ A
^ -
A
"
-'.
A
^
^ A
~''.~S~nic
"
^
k
"~,,~'='~-
~__~'-
=-- ~ = = Future R i f - B e t i c
,
I
Figure 9.3: Late Triassic p a l e o g e o g r a p h y and lithofacies the Western Tethys. (Redrawn from Waldi and Favre, 1989.) However, ably
so
that
basaltic sional
as
from the Late Jurassic onward rifting d e c l i n e d consider-
by
Eocene
magmatism
to
of
to
compressional
times
it
alkaline
had
ceased.
intrusions
tectonic
regimes.
An
attendant
suggests The
a
opening
change
from
change
from
of
North
the
tenAt-
lantic Ocean and the Gulf of Biscay during the Cretaceous and the Paleogene
had
caused
introduced
uplift
deformation Miocene
in
times,
and the
a compressional
folding central
geodynamic
in the Atlas High
Atlas
c o n c o m i t t a n t l y with
setting
basins.
Uplift
occurred
from
the main phase
(Fig.9.2C) and
which
compressional
Oligocene
to
of d e f o r m a t i o n
Early in the
internal zones of the Rif and the Betic chains. The last thrusting in the Rif d u r i n g the Pliocene coincided with uplift in the central High Atlas.
537
Here
uplift
thrusting Sea,
persisted
down
to
the
Quaternary.
The
terminal
phase
of
in the Rif has been linked to crustal e x t e n s i o n in the Alboran
caused
by Neogene
upwelling
of
the
asthenosphere
(Fig.9.2D)
along
the G i b r a l t a r fracture zone. Neogene uplift in the central High Atlas has been
accompanied
by
which are parallel
subsidence
of
to the Atlas
narrow
chain
and
elongate
(Fig.9.4A)
foredeep
basins
and contain
continental
Moroccan
High
strata. 9.2.3 The M o r o c c a n or High Atlas About
800 km
(Fig.9.4) Western ranes
long
and
consists
strata;
and
Central
by Carboniferous
Eastern
stratigraphic development for about
zoic
evolution
Atlas
very
the W e s t e r n
in
of Paleozoic
covered
slightly
Atlas
In
was
strata;
its
more
of northwest Africa
the ter-
by Mesozoic
deformed
Mesozoic
1987).
High
Atlas
Infraand
tectonic related
the and
to
the
than to the rest
From the Atlantic coast of M o r o c c o it extends eastward
the W e s t e r n
Cretaceous
continental
High
basin
(Fig.9.4B)
where
e s s e n t i a l l y detrital
facies with evaporitic
and
thinly of
or
from west to east,
a horst
thin
(Schaer,
50 to 70 km to the Argana
strata
whereas
High
and
a horst
with
5 km thick and comprises
glomeratic Jurassic
rocks
the
There are,
granites
of the A t l a n t i c m a r g i n
of the High Atlas. is up to
wide,
High Atlas,
High Atlas,
Precambrian
and
i00 km
the Paleozoic
the Precambrian
Cambrian
to
of four main parts.
High Atlas;
intruded
40
sequences a p p e a r i n g Atlas
calcareous
thicken
marly
and
strata are predominant
to
the Triassic
and
locally con-
in the west.
the w e s t
Meso-
and
contain
locally e v a p o r i t i c
facies,
in the east.
The W e s t e r n High
Atlas was only affected by differential subsidence. The Central and Eastern High Atlas, Atlas because rift
trough
of thick Mesozoic all
the
way
structural evolution, (1988),
the
carbonates Algerian
created
prograded
towards
1982; Mattis, and
the Atlas the
1977)
(Fig.9.4B), border.
rift
centre
of
Continental
in which broad the
grabens
and deposited brick-red
mudstones.
horizons of dolomite,
Its
summarized by Stets and W u r s t e r
show the following m a j o r phases.
Triassic
erates
to
also known as the Calcareous High
These
mudstones
are
extends
and
(1982) and by Warme rifting
alluvial
(Lorenz, fluvial
as a deep
stratigraphic
in the Late
fans
1988;
(Fig.9°5A) Manspeizer,
sandstones,
intercalated
with
conglom-
evaporitic
gypsum and halite, and with tholeiitic dolerites at
the top of the Triassic sequence. W h i l e the supply of terrigenous clastics continued into the Jurassic, marine
invasions
coming
along
the
Atlas
gulf,
flooded
the
meseta. A n o t h e r t r a n s g r e s s i o n proceeded eastward from the nascent
Moroccan
538
lOOkm
t
L:.:'< :?.-~:L
R i f
RQbot
i!i:i!:!iiiiii!!i!i:i!ii:i:
A
PR
"
"
~
.
A,-... . ,",- - ': ! i . i.:,..-: . -~~.:.';nti-A,,o° ... ..
Agadir
~ . .. - . . , ,,
•
." .
..,
.
,o,~ed ,omo,ns
Post-tectonic subsidence
. . . ,
Alpine ~
Post-tectonic basins
unfolded ~ domains
Mesozoiccover Paleozoicbasement Anti-At las
ATLANTIC GULF Western Agodir
I
L~o,m "
Rif and Atlas cover material Mesozoic }ntrusions(}nCentral Paleozoic material AtlQs
GULF OF TETHYS Paleozoic Precambr}an Argana
1
T,ol,
Central
Toubkal
Imilchil
I T,oi,
B
Eastern Rich
f
"
~
I
~
"
Figure 9.4: A, structural divisions of the schematic cross-section through the High Atlas Tamlet in Morocco. (Redrawn from Schaer, 1987.)
~'a,emeo, High from
Atlas; Agadir
B, to
539
C e n o c z o i
ATLAS
L/ Cr e t a c
eous
C.~
i
Jurassic B.
Triassic
~2~-:~
MESETA L . ~
ANTI-ATLAS
Figure 9.5: Tectonic evolution from Stets and Wurster, 1982.)
Atlantic carbonate (Fig.9.6) whereas
Ocean
to
the
build-ups were
west.
From
including
established
gravity-generated
on
of
the
the
Early
High
to
epicontinental fault
limestones
blocks and
Atlas.
Middle
Jurassic
limestones which
were
olistostromes
(Redrawn
and shoal
major reefs areas,
accumulated
in
540 adjacent the
deeps
Atlas
which
(Warme,
rift
extended
and
1988).
global
Early
to Middle
sea-level
over adjacent
rise
platform
Cretaceous
caused
areas.
subsidence
maximum
During
in
transgression
the regression
that
followed fluvial and deltaic fans prograded into the Atlas gulf from east and west
(Figo9.5C).
Subsidence
ended after
the Turonian
and
from later
Cretaceous the Atlas rift began to rise; and border faults d e v e l o p e d into thrust
faults
thrust
onto
eroded
into
along
the a d j o i n i n g new
(Fig.9.5D). trending
(Fig.9.5D)
folds
platforms.
alluvial
Among
the
with
bottomed synclines
which
fan or
of
The trough
systems
structures tight
slices which
in
the
faulted
Mesozoic
fill,
marginal
High
isoclinal
Atlas
are
anticlines
flat-
(Schaer,
1988).
EAST
WESTERN HIGH ATLAS HAHA BASIN
CENTRAL AND EASTERN H}GN ATLAS
ARGANA BASIN
COURIKA VALLEY
MASSIF
ANCIEN
ATLAS
HEAD OF BASIN
MARGINAL MARINEBASJN
UNLIFTED MASSIF
PRESENT
UMESTONE
I UPU~TED
STRUCTURE
PLATEAU
JURASSIC PALEOGEOGRAP. HY
ENE-
and
WEST
GEOGRAPHIC PROVINCE
was
foredeeps
(Fig.9.7); and in the western part of the Central High
Atlas folding was caused by basement faulting
GEOLOGIC PROVINCE
were
now uplifted,
filled
Central
strata
'i Ii
TAHLELT
TETHYAN MARINE BASIN (RIFTOR AULOCOGEN)
!i
EAULTED
MASSIF
RANGES
MARGIN I
i
FOLD AND UPTHRUST BELT
I PLATEAU
,
I
PERIOD
CRETACEOUS
MALM (UPPER)
NONMARINE OR
~ i
C ~ ~ . . . . . ,i' O SHALLOW MARINE {r ~"X UES ~ / ~'I~X
DOGGER --~
R
(M,DOLE}
~U>f
LIASSIC (LOWER)
~'w/~ II i
~
SHALLOW EPIERIC SEA5
CONTINENTAL
X C ~,~ . . . . . . . . . . . . . X O SHALLOW MARINE DOLOM TEl & LIMESTONE(THIN) ' " ~ ×'" × "~, , , , ,
,
L',~Y~ojSY< "
/
'
S
'
.#-&Qjj>~y.~.../_
.1.1 I /
--_ -- ~
%.-/
-"--"
/
~
."
/
J
.-.>-..
/
I
~
/
l_'Z _ -
.
.
-
~---,.._~
fl-2;- ~
NAKNABSY
~
~
\
~
"/
~,---7
~
~
I
Figure 9.8: Tectonic sketch map of Tunisia. i, Numidian nappes, Tellian units, and para-autochthons of Heldi; 2, M e d j e r d a p a r a - a u t o c h t o n and autochthon; 3, thrust zone of Teboursouk; 4, M i o c e n e foredeep; 5, diapiric zone of T u n i s i a n Atlas; 6, central and southern zone of Tunisian Atlas; 8, eastern p l a t f o r m (Western part of Pelagian block) 9, Saharan platform; 10, thrusts. (Redrawn from Salaj, 1978.) A
complete
and
The Lower Cretaceous
well-exposed
Cretaceous
sequence
occurs
in
Tunisia.
is represented along the margin of the Saharan plat-
544
,,L -~
7
//
~ -~% - -- - A~- ~ ~o ~ _.
/
.
..
-
A
I
'Y
,
lOOkm
/
/x
~
z~r
,
1~:=1 2 ~ 3 ( ~ 4E:~ Sf~:~ o E ~ ~ I ' ~ s rl~l ~ Pk'-;hoE23 ~d--1
A
I i-~'I 2 I-~--I 3,[~3 4 r=~l 5 ~
/
\.
Jurassic
Albion-Turonion
N
•
6 i--~ ? IZ2]
B
N
' '" . :."-':i.
_
;>- .---~. • . . . \
-
:~~ :.. v . - ~ , :
7[UTq [I--'I]9 []Z]I0[~11 [[~]121~--I
Paleocene-Middle Eocene
"60 km IE~] 2 l~E33 IZ~4122ZI s E ~ 6 I ~ 7 FK-Ie r-:~9 I~IoF'UD
Late Eocene Oligocene -
D
Figure 9.9: Paleogeographic maps of Tunisia. A: 1-2, 5, pelagic facies; 3, 6, 9, littoral facies. B: i, rudistid reef; 3, 4, 9, pelagic; 7, evaporitic laguno-neritic. C: 1-4, E1 Haria Fm; 5-6 Metlaoui fm. D: 1,2 Souar Fm., 4, marly limestone; 5, gypsiferous strata; 7, Numidian Sst; 9, Nummulitic limestone; i0, limestone with Lepidocyclina.
545
form by neritic sandy oolitic limestones overlain by lagoonal gypsiferous shales,
and g y p s u m with
tracodes. graphic
Coeval
sandstone
intercalations
containing
lagoonal os-
strata in northern Tunisia consist of pelagic
limestone w i t h marl.
The Pelagian
sublitho-
p l a t f o r m in Tunisia
was
emer-
gent in the Early Cretaceous since it consists of n o n m a r i n e deposits. Late
Aptian
in
Tunisia
was
marked
by
a marine
transgression
c o r a l - b e a r i n g and orbitolinoid limestones accumulated. climaxed
in the
Late
widespread rudistid ran p l a t f o r m trough.
Cenomanian-Turonian
Carbonate
and pelagic marls
sedimentation
in Tunisia.
in
which
(Fig.9.9C). and marls gests
A
there
change
to Late
prevailed
1987)
throughout
and
deposited
was
from
continuous
and
classic exposures
lies within a unit deposition
Paleocene-Early
shoaling
in the Tunisian
the Late Cretaceous
contains
Eocene
Eocene-Oligocene n u m m u l i t i c
progressive
This transgression
and limestone
Northwestern Tunisia
where the C r e t a c e o u s - T e r t i a r y b o u n d a r y mation)
The
which
limestone and d o l o m i t e along the m a r g i n of the Saha-
(Fig.9.9B),
to P a l e o g e n e
(Wiedmann,
in
emergence
(El Haria For-
across
this
globigerine
limestones
during
the
boundary limestone
(Fig.9.9C)
Late
sug-
Tertiary
in
which there was a significant t r a n s g r e s s i o n in the early M i d d l e Miocene. As in other parts of the Atlas of Tunisia tectonic the
Pelagian
Tunisia, Western
which banks,
basin,
of depocentres region.
the main
part
from a marginal
the s t r a t i g r a p h i c were d i r e c t l y
related
to the
by C l i f f o r d
(1986)
However,
as
of which
lay offshore
sag basin
shown
into
evolution
of
present-day
a wrench-modified
on account of transcurrent m o t i o n between North A f r i c a Late effect
reefal
was
basin
their
Pelagian
movement
throws;
inversion
developed.
mentioned,
and The
flourished
limestone had shoaled.
reservoirs in Tunisia;
the
Transcurrent
it reversed
build-ups
already
globigerinid
Consequently,
Cretaceous.
faults;
net
of this
Mediterranean.
from the of old
the creation
changed
(Fig.8.2)
The
and
evolution
foreland,
and the
basin
caused
Late
the
triggered creation
was
uplifted
intrusions.
paleo-highs
Eocene-Oligocene
after
the
Lower
These nummulitic
as
reactivation
salt of
basin
and the
Eocene
on
nummulitic deep-water
banks are petroleum
they are sourced by the g l o b i g e r i n i d facies.
9.2.6 The M o r o c c a n Rif
Palinspastic Reconstruction The Rif M o u n t a i n s
of n o r t h e r n Morocco,
Atlas of Algeria,
constitute the southernmost segments of the A l p i n e oro-
genic
belt.
T o g e t h e ~ they
up to
form the A f r i c a n
1,500 m high,
part
of
and the Tell
the Maghrebides,
an
Alpine o r o g e n i c chain which a c t u a l l y extends from the Betic c o r d i l l e r a of southeastern
Spain
and
continues
beyond
Algeria
into
southern
Italy
546
(Fig.9.1). The Betic and the Rif constitute the A l b o r a n m a r g i n or the Arc of
Gibraltar
which
rims
the
Alboran
Sea
in
the
Western
Mediteranean
(Fig.9.10A). The s e d i m e n t a r y sequence in the A l b o r a n
(Betic-Rifian) m a r g i n are be-
lieved to have initially accumulated from Triassic times at a more easterly location n o r t h e a s t of p r e s e n t - d a y Tunisia.
This was along the conti-
nental m a r g i n
of an ancient m i c r o p l a t e
Alboran block
(Fig.9.3). The sequence was t e c t o n i c a l l y transported to its
present w e s t e r n (Durand-Delga
in southern
Europe,
probably
the
location by progressive WSW movement of the A l b o r a n block
and
Olivier,
1988)o
The
microcontinent
collided
with
the
A f r i c a n plate in O l i g o c e n e - M i o c e n e times and produced the complicated Rif overthrust
(Fig.9.10B).
Because
of
the
large s e p a r a t i o n
between
the Rif
and the High Atlas Durand-Delga and Olivier concluded that it is impossible to trace direct p a l e o g e o g r a p h i c can
Atlas
adjacent
to
it,
links between the Rif and the Moroc-
moreso
as
their
contacts
are
tectonic
(Fig.9.4A)° These authors presented stratigraphic and p a l e o g e o g r a p h i c interpretations
for the various
structural
units
in the Rif based
on this
p a l i n s p a t i c reconstruction. S t r a t i g r a p h y of the M a i n Structural Units in the R i f The
internal
Ghomarides Choubert
zone
and and
of
the
the Rif
"Dorsale
Faure-Muret
(Fig.9.10B) calcaire"
(1973)
consists
of
(Durand-Delga
referred
to
the
Ghomarides as the Rifides and the "Dorsale calcaire" In
the
Sebtides
mantle
peridotites
of
uncertain
the
Sebtides,
and Olivier, Sebtides
the
1988).
and
the
as the Ultrarifaine.
age
are
overlain
by a
thick p r o b a b l y P r e c a m b r i a n to Paleozoic sequence w h i c h passes upward into Permo-Triassic
strata
alpine nappes.
G e n e r a l l y thrust over the Sebtides,
at
the
greenschist
facies.
The
latter
occur
the Ghomarides,
as
which
are s t r u c t u r a l l y more complex towards the southwestern and southern borders
of
These
the
slates
internal are
Rifian
overlain
zones,
are
disconformably
essentially by Triassic
Paleozoic red
slates.
sandstones
and
by a thin and d i s c o n t i n u o u s Late T r i a s s i c - E a r l y J u r a s s i c carbonate cover. The
youngest
formation
"Dorsale calcaire",
in
the
comprising
Ghomarides
are
largely Mesozoic
of
Eocene
carbonates
in
age.
The
locally over-
lain by P a l e o g e n e detrital formations, occurs as folded and thrust sheets (Fig.9.10B). caire"
From the more internal to external parts,
the
"Dorsale cal-
shows m a r k e d facies changes from shallow d e p o s i t i o n a l environments
('Chaine c a l c a i r e
interne")
and more pelagic conditions
through intermediate depth ("Chaine calcaire externe").
facies,
to deeper
547
0
~O"a
~J . ~ dJ
0"~
~0
I
~n
< m~
o N
~
iz
•
X ,C'~ gH,.~ •,4 ~
~.~'~
E ~ 0 ,~ ',~ ~
0~,.~ ,' ,.~
~ ~
.,4
o o
2-O~m
gl
~
.,--t ~
~
,1~,_ t o N
~
z
•,-l,Z~ I .i~oo
_1
" , ~
z
I
•~
ul
I~i~ I,,,I,,~
~
~
~-~
2 -r 0~
~ 0 ~,~ .~ ~ ~.~ ~: ~0
548
Occupying highly
thrust
deformed
which
enclose
flysch
pelites
between
tectonically
slices
the P r e d o r s a l i a n eral
contacts
mixed
limestones
units.
nappes
locally
of
Southeast
which
and and
quartzitic-pelitic
careous
flysch
flysch.
The
careous
complex
and m a r l y
(Numidian
nappes
flysch;
are
flysch
flysch
known
(Fig.9.10A)
organic-rich
to the
sandstones siliceous
northwest
to Lower
of
to M i d d l e
Eocene
and
unit,
the
calJebha
Eocene
flysch
as
are sev-
and Upper Cretaceous
Upper Cretaceous
and Paleocene
zones are
marly
These
fault
flysch,
located
and include
and
flysch.
Barremian-Albian
by a M i d - C r e t a c e o u s
and by A p t i a n - A l b i a n
fault are more
and external
argillaceous sandy
of the Jebha
include
overlain
the internal
cal-
sandstones
nappes).
Geological History As a l r e a d y
mentioned
initially
been
tal m a r g i n and
in southern
Dorsalian
Predorsalian deep
ocean
thick
rift facies
Atlas
flysch
somewhere
represent
deposits.
the
slope,
the
subsident
in the Late Triassic
slope
Early
had
transcurrent end
of
the
African
vergence
plate.
at
producing
the
Stretching area
of
crust
the
and
ranean
triggered
comprising the
Early
The
of
present
Collision,
the
very
Late
over
mature
quartzose
Miocene.
the
After
coarse
in the
late
the
a
had
thrust
internal
Jurassic-
left-lateral
(Fig.9.3).
detached
sheets and
along this
Late
this domain
the
floor
a continen-
large
Africa
as
in the
being
ocean
times
in the
along
domain
crust
Sea
marls
subsidence
the
occurred
leading
marked
of
difference on
the
especially
sedimentation
active
from
margin,
originated
By the
from
Europe;
collided
with
external
an
with
external
zones,
thus
of the Arc of Gibraltar.
thrusting,
deposition
and
Shelf
Europe
formed
continental Alboran
The Ghomarides
this
facies,
accumulated
was
to have
continen-
had counterparts
the m a i n
and Early Miocene
between
which
facies
located
Betic-Rifian
curvature
the
basin
was
collision
boundary
subsidence
Sea.
sandstones,
reduced
the
the present
sedimentary
margin.
separated
the Late Oligocene
along
and by Early Jurassic
flysch
margin
which
Cretaceous
and during the
The
This
fault
the
which
continental
formed.
Cretaceous.
passive
while the flysch nappes
Rif
along a more
tal
facies
sequence,
margin
are b e l i e v e d
north of Tunisia.
and the carbonate
in
units
to O l i g o c e n e
shelf
Presumably
Mesozoic-Cenozoic
succession
began
structural
a Triassic
Europe,
terranes
basin
Rif
along
the continental
the Triassic Tunisian
the above
deposited
the
to
the
clastic
of
the
Miocene
Ghomarides. was
also
The
in
of
first
siliceous
the
oceanic MediterSea
conglomerates, Numidian
deposited
sedimentation
Early Miocene,
Neogene
of the M e d i t e r r a n e a n
Oligocene-Early
clastics
the
appearance
initiation
and subsidence
Rifian
in
ceased clays,
flysch
throughout because marls
of and
549
ON
~
•~
m ~ ,...~ m "el v
v
e~
!°
..~ E L o
u~
'.,...~.~-!~#.:.~
::::'..i':.'....~.-7:.'~i.!~.::".:b .
""
rY
.'.':': -." ".. "." ".,I-,'[. ;" ;;" .......
i
:':'..-:;::..£..,---::.'.:-'. ...... '".::"::!:::.i
,
MeSozoic and £enozoicmarineemb~,yme""'"'-=:" \.
N3k,~
(: ~'~
"~
Mid-Mesozoic continental facies
t
A
f ' . ' . ' .'." -" .,'-[ :l : =
.
k,
Stream flow directions from current beddingmeasurements_~.~_.~__
.-f~'~
ZAIRE j(/"'~ \" b 'i" ..~~. ~.
:"1>
~'~
~:i~..~).~:.:j'~"'i~,ij:ii'j~...'~j"j:!..'.~'.,.!ii "
~.':I
A I-
• ,~ I
,,
[I'~'~.
s
~ .,. 6
i
ii
Figure 9.24: A, structural sketch map of the eastern Ivory Coast basin; i, sub-cropping basement; 2, oceanic basement ridges; 3, oceanic basement; 4, Ivory Coast ridge (deformed sedimentary and basement wedge); 5, main distensive faults; 6, main anticline axis; 7, main synclines and basin axis; 8, isochrons of the Lower Cretaceous-Upper Cretaceous unconformity (Albian?). B, cross-section across Ivory Coast basin. (Redrawn from Mascle et al., 1987; Clifford, 1986.)
570
when
the
continents
formity represents.
separated, As
the
an event w h i c h
lithosphere
cooled
the M i d - C r e t a c e o u s normal
thermal
uncon-
subsidence
affected the w h o l e basin.
Dahomey Basin An arcuate coastal basin
(Fig.9.20) underlying the onshore parts of Togo,
Benin, and s o u t h w e s t e r n Nigeria, Benue
trough
the western Adegoke,
by a basement
ridge,
flank of which
1981).
De Klasz
the Dahomey basin was separated from the the
consists
Okitipupa
of horsts
ridge
(Adegoke,
and grabens
1969),
(Omatsola and
(1978) presented a stratigraphic
summary of the
Dahomey basin w h i c h he termed the Benin basin. From
Togo
to
sandstone
(Abeokuta
Cretaceous posed
southwestern
Nigeria
Formation),
in the subsurface
part.
Large
reserves
thickens
to about
the
basal
age
(Okosun, of bitumen
posits in the A b e o k u t a Formation quence
the
of
sequence
which
1990),
is
ranges
a
fluviatile
from
to M a a s t r i c h t i a n
have been
proven
(Adegoke et al.,
3 km near the present
the
in tar
sand de-
1980). Downdip, shoreline,
Early
in the ex-
where
the sea thick
p r e d o m i n e n t l y m a r i n e Upper C r e t a c e o u s - T e r t i a r y section succeeds the basal Abeokuta Eocene
Formation.
phosphatic
phate quarries. subsurface
The
Oshosun
Lower
Tertiary
Formation)
is
(Paleocene exposed
in
A highly fossiliferous black shale
contains
the M a a s t r i c h t i a n - P a l e o c e n e
Ewekoro
Limestone,
limestone
and
(Araromi Shale)
transition.
phosin the
Although
the
exposed y o u n g e r T e r t i a r y deposits are nonmarine, marine latest OligoceneMiocene deposits are known in the subsurface
(Fayose,
As in the n o r t h w e s t African coastal basins,
e s p e c i a l l y the Senegal basin
and
in the Dahomey basin,
In the western in
Togo.
favoured
upwelling
formation
basin, whereas
deposits
of
Dahomey basin phosphate mining
Coastal the
economic
of
and
thick
Eocene
beds
in southwestern Nigeria nearness
phosphates
occur.
is carried out at Hahatoe
remoteness phosphate
1970).
from in
clastic the
deposition
western
Dahomey
to the Niger delta clas-
tic province inhibited phosphate sedimentation.
Niger Delta The Niger Delta is among the world's largest p e t r o l e u m provinces,
and has
been rated as the sixth largest oil p r o d u c e r and twelfth giant hydrocarbon province
(Ivanhoe,
The southernmost in the Benue trough, contains
over
1980).
and last of the major deltaic complexes
constructed
the Niger delta began to form in the Eocene and now
12 km of
clastics
which
fill
the entrance
into
the Benue
571
trough
(Fig.9.18).
quence which
The
prograded
Niger
delta
across
complex
is
the southern
a
regressive
Benue trough
offlap
se-
9.25)
and
(Fig.
spread out onto cooling and subsiding oceanic crus%, which had formed as Africa
and
detailed Onuoha, 1982;
South America
information 1981),
Short
Recent
separated.
is available
petroleum geology
and Stauble,
sedimentary
for high energy,
1967;
environments
wave-dominated,
Because
of
its
petroleum
on the geophysics (Evamy et al.,
Whiteman, (Allen,
resources
Hospers,
1965,
1978; Orife and Avbovbo,
1982) 1965)
(eg.
of the Niger serves
constructional,
delta.
The
as a classic model
arcuate-lobate
tropical
deltas. The oldest arcuate
formations
exposure
Paleocene Imo Shale
the delta
in the Niger delta
form an
(Fig.9.25A).
are
frame
These
the
(fossiliferous blue-gray shales with thin sandstones,
marls and limestones, Ameke Formation
(Paleocene-Eocene)
belt along
and locally thick nearshore sandstones);
the Eocene
(fossiliferous calcareous clays, coastal sandstones);
the
Late Eocene-Early Oligocene lignitic clays and sandstones of the OgwashiAsaba
Formation,
Sands).
These
subsurface
and
where
(Fig.9.26).
the Miocene-Recent
formations
The
they Akata,
facies equivalents
have
Daukoru,
highly been
Agbada,
representing
vironments respectively vironments
are
and
Benin
assigned Benin
pro-delta,
(Fig.9.27).
Formation
diachronous
Unconformities,
(Coastal
extended
different
formations
Orife
and
Avbovbo,
names
and delta-top en-
Figure 9.27 depicts the delta sub-en-
large
1982).
the
interfingering
clay
(Allen,
fills
of
1965; Weber and
ancient
submarine
canyons, and deep-sea fans occur in the eastern and western delta 1972;
Plain
into
formation
are
delta-front
and their sedimentary characteristics 1975).
and
These
formed
mainly
(Burke,
during
Early
Oligocene and Tertiary lowstands of sea-level. Whiteman
(1982) gave the following outline of the geological
of the Niger delta. in
the
trough)
Paleocene-Eocene were
the Cross the
By the Middle Eocene the major depocentres
the
sites
River drainage
sediment
supply.
(in
the
Anambra
of deltaic systems
Both
basin,
outbuilding (Fig.9.25B)
drainage
systems
Oligocene
and formed the present Niger delta.
initiated
in
Mountains
provided
River,
the
Oligocene.
During
the
a new and dominant
Afikpo
with
the
history
initiated
syncline,
Ikang
Niger-Benue
and
accounting
for the bulk of
merged
the
Simple growth
Miocene
sediment
at
uplift
of
end
of
the
faults were the
supply through
Cameroon the Cross
thus constructing of the Cross River delta. As shown in Fig.9.26C
the shoreline progressively migrated seaward during deltaic progradation. This was greatly accelerated in Miocene-Pliocene times with attendant increase in growth faulting and large-scale diapiric movement Shale.
This
involved deep mass movement
of the Akata
of the undercompacted
and over-
572
N
"If "" I
I
G
E
R
"'
/ ./ (.Jt'il f'--.. )
Nupe BaSin
--j /"
I ~AIbian
~Eocene
]_~
'Mamle~""J'" l,'.~AIbian, l';7-~lMioo~ne
,Emboy. $
Lo gas
~L.Tor. ,=,,Ce°om
-"
~
Qmb
Meander belt
Qd Qfs
Deltaic Plain Fresh water (
~]
Hilly or dis Dry and fiat Mangrove swat Dry land with Fresh water .¢ Estuaries Bec~:hes and Marine
1
200Km
F i g u r e 9.25: Outline geological e n v i r o n m e n t s of the Niger delta. Allen, 1965.)
Tur.
~
Pleistocene(ChadFm~a
^ ., -m-~TtCon t inental ~ ~'amp.r ,_~
"
'..'.;-~
LAVAS
3, U. CRET. SEQUENCE ~CONTINENTAL 4, L. CRET. SEQUENCE 5 U. JURASSIC SEQUENCE I~:'.
Main riff valley of spreading ridge(0ceaniccrusf) and/or isolated deeps
~/~
Yombo
o
_.;V',
.... ~
0
%
"
'l\f"
Transform
faults
~1 Southern limit of marine Cretaceous
~,~° ~/-,
~I/\i." "t~'~b
o
Exploration w e | i s
j)
Hydrocarbon field ~ll:~
,
\,/~ A
Condensate/gas discovery Oil seeps
5 ~ km
--~
' , ~' ~I, 1] ~ i J ,k~4a n d a b " = ~ - ~
Afar ~rl '.(%~l~I~,~,~."---'--"~a f ~-" ,, L I ~ J ~---Aden
0
F i g u r e 9.46: Aden. ( R e d r a w n
Outline geologic map f r o m B e y d o u n , 1989.)
Sharmah field /.
, , I /
M
of
A
LI
the
Red
Sea
and
Gulf
of
610
A notable hot
brine
feature of the Red Sea median v a l l e y
pools
below
which
metalliferous
is the o c c u r r e n c e of
sediments
are
found.
These
m e t a l l i f e r o u s deposits contain i00 to 200 million tonnes of ore deposits, with an ore grade of silver
(Sawkins,
3.5%
1990).
zinc,
0.8% copper,
It is believed
and
that
significant
the metals
and
amounts of
sulphides
in
the brines w e r e transported by seawater which a c h i e v e d increased salinity from c i r c u l a t i o n through Miocene evaporites,
convection circulation being
g e n e r a t e d by heat e m a n a t i n g from hot basaltic rocks in the slowly spreading axial zone. S e a - f l o o r spreading in the Red Sea and the Gulf of Aden was accompanied by vertical
faulting w i t h displacements
This o c c u r r e d along both sides of the rift Sea
margins
Miocene, tinued Aden
had
essentially
attained
their
sea-floor spreading, volcanism,
till
present-day.
The
of several thousand meters.
(Fig.9.47A). A l t h o u g h the Red
structure
present
of
the
Red
is e s s e n t i a l l y c h a r a c t e r i z e d by deep rifts
and by various salt structures
A related (Fig.9.46).
feature
This
is
to the Red a
Sea
by
the
Early
and
the
Gulf
of
sediment draping over
(Fig.9.47).
is the Gulf
northwest-trending
Sea
complicated by extensive
faulting of the floor and the overlying sediments, fault blocks,
form
and sediment infilling have con-
of
Suez,
intracratonic
to the north
basin
separated
from the Red Sea by the Gulf of Aqaba transform fault, and bounded to the east by the Sinai massif, and on the west by the Red Sea Hills. of
Suez
is an
extensional
basin
that
began
in the
Early
The Gulf
Oligocene with
normal
faulting and dyke injection which produced tilted blocks
that re-
semble
half
zones
grabens.
Horizontal
extension
along
shear
fracture
in
the Gulf of Aqaba and within the Gulf of Suez itself is believed to have caused rifting in the Gulf of Suez, with the driving m e c h a n i s m being the counter-clockwise (eg., Meshef,
rotation
1990; Morgan,
of
Arabia
from
Africa
as
from
Eocene
times
the
Gulf
of Aden
1990).
Stratigraphy The
stratigraphic
prises
a
and
upper
an
post-rift
lower
succession
series
sequence
mainly
of
in
pre-drift
of Late
clastic
the
Red
deposits
Oligocene
fill
and
Sea
and
associated
to Middle
related
with
Miocene
volcanics
com-
volcanics,
syn-rift
(Fig.9.48).
and
In the
Red Sea syn-rift clastic deposition was interrupted by w i d e s p r e a d evaporire
sedimentation
(Fig.9.47A).
basins
contain a Late M i o c e n e
marine
Late
Pliocene
to
Both
the
Red
Sea
to Pliocene clastic
Recent
post-rift
and
the
Gulf
of
Aden
fill and a more open-
sea-floor
spreading
with coarser clastics which are restricted to the margins.
sequence,
611
SUDAN
SAUDI ARABIA Axial
L
. -; Loasn ne Post Miocene " ~'X marine oozes
avas
Trough . I Miocene . I Eva.p.or,ires
Reflector-S .
/.
, , Jurass,c
_ . . . ~ ~
A ~,lOOKms 4
D
Presumed M a n t l e
SUDAN
RED
SEA
SAUDI ARABIA
D
Figure 9.47: East-West structural (A) and s t r a t i g r a p h i c (B) sections across the Red Sea. (Redrawn from B r a i t h w a i t e 1987., Peterson, 1985.) Regarding
their
paleogeography,
O l i g o c e n e - E a r l y Miocene extensive sediments.
continental Marine
it
which
was
occupied
although
Subsequently,
also
became
evaporites
triangle.
restricted, throughout
by
the
Late
by
a
lake
that
received
coming
from
(Fig.9.17C), but was not connected with the Gulf of
through
Afar
that
in the Early M i o c e n e
Aden by the M i d d l e Miocene, the
believed
the Red Sea had become a m a j o r d e p r e s s i o n and an rift
incursion took place
the M e d i t e r r a n e a n Sea
is
the
giving Red
rise Sea
some tenuous to
basin.
links thick
link m a y have existed with
the
Middle
Evaporite
to
Mediterranean Late
formation
Miocene climaxed
612
during
the
Late
Miocene
Mediterranean
Sea
later between
the Red
during
-
dried
(Messinian)
crisis,
up
al.,
Indian
NORTHERN REDSEA SUDAN SAUOI
EGYPT
! , = , ,..) ~ I I =
RECENTPLIOCENE
-
~
~
~
'',%,', , •
1973). Ocean,
MIDDLE
:Z--JCZZZZE~--,
~ ~ lJ .
~,
JURASSIC
In
the
established
the Gulf
of Aden,
.:_-.
_
contrast, its
Although
from In
the
lacustrine
major
Gulf
.¢:z4"~- ) 2 ;
oil
seeps
petroleum
this
field
is
listric
Marl
serving
are
sealed
petroleum,
as t h e by
but
Y
of
an from
of
Aden
and
occur
the
related
upper
salt
connected
Sea
There
layer.
sandstones
margin from
tectonics
Gulf
.
of
of t h e N u b i a n
from
the
were
Indian Conse-
limited.
(Fig.9.46), Red
Sea
in w h i c h
Su-
growth
Globigerine
sand reservoirs Suez
the basin
in o f f s h o r e
the Miocene
are Miocene The
z •
to
the
field/located salt
"
~
history.
environments
evaperites t with
rocks.
faulted
to
c¢
~,
~.~--~_
(Redrawn
Tertiary
directly
Suakin
-
basin.
its
Red
is
~-l~=
~"
-"
~~. A~onfain" t W ,~nran ° =
openly
the
0e'""
,-~:¢:;~'.',,~
Sea
evaporitic
that
into Miocene source
was
along
field, is
Red
'_'+"+" w.'.+.'~*.~.v
".Hob~b.:;"--" • ~ . ~ . A
..
throughout
production
traping
the
,!'
6~------
= ~L~.*-.. ~
_ . . . .
:~:~-.,,,
i,,,
AA ~ ÷ --
and r e s t r i c t e d
a gas/condensate
faulting
v,v.v.
~
inception
N
; ' ' ' ~C. ".
°."~'-'l."...-=:;:.:.:.:.:.:.:.:~...:.. I~:,-
"
/
from
quently,
dan.
was
which
:.Io,':...o ,'*'z,~-y,-T -';-.., ueseI~;~1,[''~r"-,.:.----,- % ~
~ A _ A
F i g u r e 9.48: Stratigraphy B e y d o u n , 1989.)
is
of
SOUTHERN'"RED SEA ETHIOPIA S/ARABIA/YEME
;-,~,T,,--°,,~, *
'.
:-q_--~_~--/~-~%-----~--~_~_'--~-~A, "
only
link
. . . " .~ ' ' '
^^^&~l"=.-=_-----=~--=-"\
~------
Ocean
A
course
through
ARABIA
:~,"~'~'~~--T.T:Tf%.'A
Z[ o
et
the
the
the Pliocene.
AGE
-
(Hsu
Sea a n d
in
also
depositional
which
produces cycle.
613
9.8.2 The East A f r i c a n Rift System
In troducti on
Our survey of the d e v e l o p m e n t of M e s o z o i c - C e n o z o i c basins in Africa began with Triassic
rifting
in n o r t h w e s t e r n Africa.
Having e x a m i n e d
gation of rift systems all around the continent, of
the
Atlantic
and
Indian
margins
from J u r a s s i c
oceans,
the Red
story with East
Sea
Rift
basins
and
to Early Cretaceous
and the Gulf
a glimpse
African
Ocean
at
the
System
grand
we
theatre
(Fig.9.49).
their
times,
of Aden, The
African
continental
up to the birth of new
can now
of
the propa-
leading to the formation
conclude
contemporary
tectonic
the rift
rifting--the
significance
of
the
East African Rift System lies in the enormous scale of continental breakup, involving the generation of oceanic crust--the i n i t i a t i o n of the Wilson Cycle par excellence. But before Rift
Valley
c o n s i d e r i n g the global tectonic
as
it
is
sometimes
called,
it
s i g n i f i c a n c e of the Great
is
pertinent
universally acknowledged
aesthetic quality of its great
mountains,
Apart
and wildlife.
travellers, was best tem
of
nomic
the
impact of the Rift V a l l e y on the lives
affairs
has
profoundly
the
abundance
of
rift terrane,
the political
the rift,
the agricultural
and
Rogers
affected
of East Africans,
day--witness
the
snow-clad
from its lure to both colonial and modern
summed up as follows by
valleys
to m e n t i o n lakes,
the
social,
that
finds
(1989):
"The sys-
political,
from the time of H o m o archaeological
boundaries
of East Africans
and Rosendahl
and
to modern
erectus
associated
follow the W e s t e r n
import of rift valleys.
rift will continue to p l a y an economic role in Africa,
The
eco-
with
the
Branch of
East African
perhaps
a crucial
one if the boom in p e t r o l e u m exploration in East Africa proves fruitful". Since
Chapter
i0
is
devoted
to
Phanerozoic
magmatism,
of
which
volcanics of the East African Rift System form a major component,
the
the ig-
neous a c t i v i t y that was associated with this and e a r l i e r rifting will not be
considered
stratigraphy
in
of
detail
the
rift
in
this
section.
system will
be
Similarly,
treated
the
in Chapter
Quaternary ii.
This
is
because the Q u a t e r n a r y of the Rift V a l l e y is the b a c k b o n e of the African Quaternary.
As
aforementioned,
unique record of vertebrates, and
one
records
or
the
for the
most last
the
Rift
Valley
Quaternary
contains
including the stages of homonid
comprehensive 2.5 million
and
years.
best
documented
However,
in v i e w
a
evolution,
paleoclimatic of
its
impor-
tance to the u n d e r s t a n d i n g of the stratigraphy of the rift system in general, d e p o s i t i o n a l models will be d i s c u s s e d in this section. is m o s t l y d e v o t e d to the geemorphology,
structure,
general
What follows stratigraphy,
614
and
the
origin
of
the
Rift
Valley,
a
term
commonly
used
for
the
East
A f r i c a n Rift System.
Figure 9.49: The Red Sea and the (Redrawn from Braithwaite, 1989.)
East
African
Rift
system.
the
world's
most
spectacular
rift
system.
Geomorphologyand Structure The
East
faulted
African
Rift
land-scapes.
gionally,
it belongs
System
is
one
of
It is part of the world's
oceanic
to a system of rifts that cuts through northeastern
Africa and the M i d d l e
East before entering East Africa
its
system
course
graben
this
in Jordan,
of Aden.
At A f a r
rift
Re-
includes
major
rifts,
the Gulfs of Aqaba and Suez, the m a i n
rift systems
veers
(Fig.9.49).
such as
the
Along
Dead
Sea
the Red Sea, and the Gulf from
its NW-SE
course and
615
enters
East
Africa
where
it
runs
through
Ethiopia
to
Mozambique
(Fig.9.49). The East A f r i c a n Rift System has been divided into the Eastern branch (Gregory about
Rift)
Ethiopia faults
the
wide
and Kenya
in
and
Western
runs
from
before
northern
the Ethiopia the
and
50-80 km
The
Afar
Eastern
triangle
entering a diffuse
Tanzania.
the
branch. the
Kenya
It crosses
domes.
lithosphere-asthenosphere
domes
boundary,
generally
region of graben
two areas
These
branch,
southwestward of
basement
reflect
and
broad
topographically
through
and
splay
uplifts,
uplifts they
of
form
plateaus w h i c h sometimes rise to over 3,000 m0 Prichard Rift
(1979)
Valley.
presented
From
the
(Fig.9.50A)
the
Eastern
(Fig.9.50B)
which
Ethiopia
wide
fault-bounded branch
record
throws
A
major
Ethiopia
and
rift
is
in Kenya
of
the g e o m o r p h o l o g y
triangular
displays
of
up
the n o r m a l l y w e l l - d e f i n e d
b a s i n - a n d - r a n g e province, lies.
a description
to
Danakil
prominent
3,000 m
Rift V a l l e y
of
depression
fault
or more.
is replaced
the
In
scarps southern
by a diffuse
up to 150 km wide, where the Turkana d e p r e s s i o n believed
to
lie
the Rift V a l l e y
beneath
floor
Lake
Turkana.
is occupied
Both
in
by a series
of
small lakes which in Kenya are not deeper than 16 m, except Lake Turkana (116 m). The
Western
through
Lakes
branch
extends
Malawi,
from
Tanganyika,
the
to
against the Aswa shear zone (Fig.9.50A). around
Lake
partly
drowned
Kivu
valley which trough. in
a
and
to
divides
Towards
the
form
Ruzizi
rias.
the northern
fault-bounded
trough,
plain
of
Mozambique
Mobutu
where
it
terminates
In the n o r t h e r n part the terrane
Mountains
Lake
northward
coastal
Lake
Edward
is
rugged
occupies
and
faulted
a well-defined
into a lowland and a trough,
end of the Western the
scarps
of
branch
which
and rift
the Semliki
Lake M o b u t u
gradually
lies
decrease
in
e l e v a t i o n until they are replaced by the zig-zag fault zones in the Nile region.
The southern segment of the Western branch
landscape
with
and Malawi.
sub-parallel
Lake Malawi
faults,
occupies
within
which
a deep trough
is a h o r s t - a n d - g r a b e n lies
Lakes
(Fig.9.50C),
Tanganyika about
80 km
wide and 650 km long. The
structure
Chorowicz Rosendahl
et
al.
(1989).
Valley
which
faults
located
of
the Western branch has been d e s c r i b e d
(1987),
Rosendahl
et
al.
G e n e r a l l y there are arcuate
define within
half-grabens the
grabens,
them into r h o m b - s h a p e d outlines
(Fig.9.50). link
the
(1986), border
and
faults
Smaller border
in detail by
NW-SE
faults
Specht
and
in the Rift transverse and
dissect
(Fig.9.50A). These t r a n s v e r s e or transfer
616
~ Otorgesailiel Kirik ;fi ~Basaffs 1Baser
•Sasement System ~
!~,';,:;
~
I
:
~O~er ~ Sed;mentary ~votcan;cs deposits Otor-esaiSevo|cano
'
: approx
:
~ m
~
600 300
lOOiCrn
B
.
Lake Halaw;
C
~_~Km
~
Recent sediments
~
Basement
Figure 9.50: A, structural map of the East African Rift: I, major fault zone; 2, other faults; 3, inactive fault; 4, major volcano; 5, major dip; B, Buhoro Flats; D, Dombe trough; E, Elgon volcano; G, Gilgil fault; K, Kilimanjaro; L, Livingstone fault; M. Mahali Mounts; Ma, Mau Scarp; N. Nandi fault; Ny, Nyanza rift; R, Rungwe volcano; S, Sattima scarp; U, Urema trough; V, Virunga; Y, Yatta plateau. B, cross-section of the Eastern Branch; C, cross-section of Lake Malawi basin. (Redrawn from Chorowicz, 1990; Pritchard, 1979.)
617
zones are characteristically defined by fault-bounded upstanding basement blocks which subdivide the half-grabens.
Mt. Elgon
km
Ba ringo-Bogor jcl Sub-Basin
Elgeyo Escarpment
.5" SeaLevel 204060:
•
°"°'°n"°"n0
80. tO0-
. .. .- ,
..-...
...
~o~,e,
. . . . . . . .
:.- .\-1.
:.---
.....-.
". ", ". " , ' : . : . !1 KenyG Dome hot spot
."
20kin,
i.:., 111 ff
Figure 9.51: Sections showing the deep structure underneath the Eastern Branch (Gregory Rift). (Redrawn from Karson and Curtis, 1989.) While the Eastern branch shows pronounced magmatism which varies with crustal depth
(volcanic eruptions,
fissuring,
faulting and block rotation
in the uppermost crust, with magmatic intrusions dominating in the middle and lower crustal depths), less advanced rifting. than the Eastern branch derneath
the
and Curtis,
the Western branch has much less volcanism and
The latter branch is therefore considered younger (Chrowicz et al.,
Eastern branch 1989),
revealed
1987).
by geophysical
is related to the magmatic
the origin of the Rift Valley.
The deep structure unsurveys
processes
(eg. Karson
associated
The axial positive gravity anomalies
with and
618
high c o m p r e s s i o n a l w a v e velocities basic
and
ultrabasic
igneous
underneath the Rift V a l l e y are due to
bodies
in the
crust
and
upper
mantle.
The
rift crust and upper mantle have been intruded in the Q u a t e r n a r y by individual m a g m a t i c diapirs which are probably body of along
asthenosphere
its
apex.
(Fig.9.51),
Seismic
area mantle diapirs
with a c o n c e n t r a t i o n
reflection
correspond
fed by a vertical,
data
show
that
units
that
was
since
the
insufficient
amount
to
of partial
in
the
Lake
to Quaternary volcanoes w h i c h
beneath the centres of individual half-grabens. concluded
wedge-like
of
have
crustal
caused
melts
Turkana
are located
Karson and Curtis
extension
mantle
across
upwelling
and
partial melting, m a n t l e d i a p i r i s m and associated magmatism,
(1989)
the
rift
extensive
instead, con-
trolled the g e o m e t r y of rifting at the surface.
Stratigraphy and Depositional Models The
East
African
plateaus
of
Rift
Cenozoic volcanics custrine,
rifted
Ethiopia ments,
are
characterized
rocks
that
by
are
broad
mostly
(Figs. 9.50B;
of
most
Ethiopian
plateau.
likely
underlain
by
fluvial,
by la-
51A). Cenozoic basaltic and
2,000 m thick,
the
uplifted
covered
and by rift basins which are filled with
The
floors
flat-lying
cover the cenof
the
Mesozoic
rift
marine
in
sedi-
above which are Cenozoic nonmarine strata.
Because the Rift Valley earlier
posits
notably
the m i d d l e et
al.,
and
Karoo of
rift.
Jurassic
Petroleum
the Western
sedimentary
Late M i o c e n e
custrine deposits and expands
the a
zones of crustal
certain parts of the rift are superposed on
segment
1989)
(with coal),
cent deposits.
Karoo
is located along persistent
in East Africa,
riftS,
in
(Morley
The
is
flows and turfs as much as
part
reactivation Rukwa
System
crystalline
and volcanic material
other v o l c a n i c tral
Valley
Precambrian
sequence
or Cretaceous
exploration
branch
(Fig.9.50A)
which
red beds
includes
in
Lake
revealed Karoo
and T e r t i a r y
de-
to Re-
fluvial red beds and Late M i o c e n e - R e c e n t
have been dated p a l y n o l o g i c a l l y u n d e r n e a t h
Tertiary-Recent
sequence
fills
a half-graben
g r e a t l y towards the eastern border fault where
la-
Lake Rukwa. in the
lake
the thickness
of the Late M i o c e n e - R e c e n t section is estimated at 6-7 km. The
enormous
sedimentary
atypical and quite localized, in
the
Rift
diversion
Valley.
of
Frostick
drainage
and
thickness
in
Lake
Rukwa
and
Reid
sedimentation
(1989) away
stressed from
the
d o m i n g and b a c k - t i l t i n g of footwall fault blocks. Also, chitecture
of
the
rift
does
may
however,
be
considering the Recent depositional pattern
not
permit
through
the
considerable
rifts
because
of
the segmented ar-
drainage.
Consequently,
619
parts of the Rift Valley are actually starved of sediments,
while other
areas were active depocentres.
was
Pickford
(1982) suggested that in the Kenya Rift Valley sedimen£ation
cyclical
in these depocentres
and reflected
initial
downwarping
and
rapid scarp erosion and deposition of coarse alluvial fans. As the scarps are levelled by erosion finer deposits accumulate in lacustrine environments together with biogenic sediments lites tion
(Casanova, of
lava
that often prints
flows
contain
and
pyroclastics
which
rich
terrestrial
fossil
(Behrensmeyer and Laporte,
Cohen
et
(Fig.9.52)
al.
for
(1989)
the
deep
basin
(1,470 m),
clastics,
of
branch
without
bury
and
assemblages,
preserve
soils
including
foot-
contrasting
depositional
the Western
branch.
of
in a semi-humid
significant
Lake
Turkana
in a volcanic
belt,
volcanism.
models
and is a very
Both
basins
are,
Lake Turkana sedimentation is dominated by the
oxygen-poor,
because
two and
is situated
sediment-starved.
accumulation
may
is located within a semi-arid region,
Lake Tanganyika
however,
and stromato-
1981).
presented
Eastern
which is 116 m deep, basin.
such as diatomites,
1986). Sedimentation is often interrupted by the deposi-
terrigenous
muds;
in marginal
depocentres
ponding
rare
deep-water by
coarse
volcanic
bar-
riers. There are low biogenic components and low carbonate content in the sediments. biogenic
Lake
muds.
Tanganyika, Anoxic
in
contrast,
conditions
is dominated
prevail
posited at great depths by subaqueous
below
150-200 m.
gravity flows;
very rare along the margin and littoral carbonates morphology ganyika
allows
has
and prior north ity,
limited
sedimentation
(Fig.9.50A). and
only
steep relief
climate
clastic
upstream
Thus,
input
with backsloping, in Lake
by organic-rich,
are common.
into
the
lake.
limited drainage Kivu,
which
is
contrasting basin morphologies,
have
produced
different
Rift
System has
Sand
is
de-
sediment ponding is
lithofacies
The basin Lake
Tan-
basin area,
located
to the
volcanic activ-
in the
rift
lakes
(Fig.9.52). Tectonic Model
The
East African
including
doming
activity,
and
by
asthenolith
variations
in the
generated injection, theme
models were reviewed by Chorowicz et al. Rift Valley developed in stages
of
a wealth mantle
of genetic
convection,
lithosperic
models, hot
stretching.
spot These
(1987), who also argued that the
(Fig.9.53), dominated by strike-slip tec-
tonics, a model that is gaining widespread acceptance.
620
Large ":-" .::
~
(~)
.
-,- Coarse
/
/
intumescence
/A //
l
a
s
I
t
i
\ h c o o r s e clostic Aleccumulotion
I ~ j" .~Z.~/ V
....... ~
h r ~ ' . A \ ~Minimall
/ K ~
~ c
. , , ~ Ma r gin al coarse
s
/l..'/~.
t[ops /
~ ~_, t _, ~ t /l c ~s : ~t:/-62.k~-.r-:.l /
/
~""
O~onlc~aoorctosl~.
-
]
;~%°
Turkano Type Basin- Process
Sionl fi¢o~f
I
.
.
:g,;72"
Turkana Type Basin- Product
Surface flow ~ Distribution : U~IrCU l l ~ ] S [
Dra n a ~
,
Tanganyika Type Basin- Process
~
Tonganyika Type Basin- Product
Figure 9.52: Depositional processes and resultant lithofacies in lakes Tanganyika and Turkana. (Redrawn from Cohen et al., 1989.) At the pre-rift stage, turing,
characterized
topographic d e p r e s s i o n for t h o l e i i t i c place
since
by
formed,
volcanism
Late
initially horizontal motion led to dense fracstrike-slip
faulting,
and open gashes
(Fig.9.53A).
Oligocene-Early
while
in the crust created room
Horizontal
Miocene
a shallow but wide
slip m o v e m e n t
times
in
a
NW-SE
along fractures and lineaments such as the Aswa lineament
has taken direction,
(Fig.9.50A) and
shallow lakes similar to Lake Mweru, may represent the Recent topographic manifestation
of
this
pre-rift
phase.
Initial
tholeiitic
volcanism
may
621 have been similar to the present-day Virunga volcanic
chain
(Kampunzu et
al., 1983).
)
.
~-~Crust [ - ~ Upper Mantle ~
Tension Gashes
o Km
Figure 9.53: ent stages of 1987.) After normal
being
faults
Tectonic model showing the lithosphere at differrift evolution. (Redrawn from Chorowicz et al.,
initiated,
which
border
typical
rifting
the main
(Fig.9.53B,
tilted
blocks,
the rift floor and uplift along the shoulders ensued. ing
stage
manifested
in
significant
magmatic
C)
while
occured
along
subsidence
of
The advanced rift-
intrusions
axis at which stage the initial oceanic crust had formed.
along
the
rift
In the Eastern
branch the typical initial rifting stage characterized by major uplift of the rift shoulders began in the Late Miocene, stage was
attained
the oceanic stage.
in the
Pliocene.
The Afar
while the advanced rifting depression
corresponds
to
Chapter 10 Phanerozoic Intraplate Magmatism in Africa
10.1 Introduction A f t e r P a n - A f r i c a n orogenic activities, variety
of
intraplate
the Atlas M o u n t a i n South
Africa,
times d u r i n g
or anorogenic
belts
where
Africa became the scene of a wide
magmatism.
of northwest Africa,
subduction-related
the Phanerozoic. the
The only
magmatism
involved
emplacement
intrusions,
b a s a l t i c volcanism,
of
fold belt of
occurred
M a g m a t i s m in the A f r i c a n
Phanerozoic
exceptions were
and the Cape
alkaline
at
various
plate during
ring
complexes,
the
basic
and other e c o n o m i c a l l y important alkaline
rocks such as kimberlites and carbonatites. The climax of Phanerozoic alkaline m a g m a t i s m in A f r i c a was related to widespread
Early
wana.
Extrusion
Early
Jurassic
lineaments.
The
Mesozoic of
as
rifting
Karoo a
result
of
emplacement
peak
in the Jurassic.
most
intensive
phase
which
flood basalts the
of
preceded
reactivation
alkaline
Following
ring
in the wake
of kimberlite
the
climaxed
in of
break-up
the
Late
of
deep-seated
complexes
also
in southern
-
basement reached
of Karoo volcanism,
intrusion
Gond-
Triassic
Africa.
was
a
the
Resur-
gence of basaltic v o l c a n i s m occurred in the Late C e n o z o i c during the form a t i o n of the East A f r i c a n Rift System. This was i n i t i a t e d by a new phase of continental b r e a k - u p in eastern Africa.
10.2 Alkaline Complexes
10.2.1 Types and S t r u c t u r e A c c o r d i n g to K i n n a i r d and Bowden m a t i s m was ture, are
often
of
magma,
two and
(Fig.10.1). plexes
are
Ethiopia,
(1987) African P h a n e r o z o i c alkaline mag-
c h a r a c t e r i z e d by small centres of subvolcanic in
the
types,
form of viz.
those Prominent well
with
complexes.
dominated
Alkaline
by
undersaturated
magmatic
are
(Almond,
Nigeria,
1979;
with
oversaturated
complexes
and
carbonatites
Niger,
Turner,
Provinces
centres
among the oversaturated provinces,
developed,
and A r a b i a
ring
those
to plutonic na-
where ring com-
Cameroon,
1976;
Vail,
Sudan,
1989b).
Egypt, The un-
d e r s a t u r a t e d p r o v i n c e s include Namibia, Angola, and the East A f r i c a n Rift System.
623
¢=~
I' ~P"
Uweinot %t~90 .. '.. Nubio ~L
iA'i, " 480- 400 Domoqorom • ":320-290
650-25J1~
*
%,.
Afclr 25
LOS
96
5 '. ~omeroon
"(
= b ~ 60-50
%
~% it
f ~
TERTIARY- RECENT ALKALINE VULCAN|SM *
•
OVERSATURATED COMPLEXES
J
ALKALINE
MIXED ALKALINE COMPLEXES i UNDERSATURATEDALKALINE COMPLEXES CRATONS STABLE DURING THE PAN- AFRICAN
&A
~]
(..~:
Nornibic 190-120
Luderit 130
Figure i0.i: Distribution of alkaline Africa. (Redrawn from Cahen et al°, 1984.) Anorogenic
alkaline
display similarities
centres,
root
zones
neous, den,
where
the
unfoliated,
1987).
whether undersaturated
in their structural
are usually exposed at various plutons
and
levels, have
commonly
The
sharp
classic
locality Rift
for
System.
volcanoes
alkaline
contacts
textures
provinces
and
(Kinnaird
to the homoge-
and
the sub-volcanic
in composition,
(Fig.10.1).
undersaturated Here all
(01doinyo
(e.g. Napak in Uganda), Malawi
The centres
Bowin-
with basic rocks
for 5% or less of the total surface area.
from alkaline in
in
or oversaturated,
(Fig.10.2).
intrusive
porphyritic
In the oversaturated
the East A f r i c a n tres
features
rocks
ranging from the volcanoes,
trusions are m o s t l y syenitic to granitic accounting
magmatic
(carbonatite)
levels
Lengai),
of erosion
complexes
to the partially
eroded
and well exposed root zones at Chilwa
Combining
all
three
is
are displayed,
different
cen-
Island
erosional
624 (chr0nostratigraphic) den
(1987)
deduced
horizons
the
compositional
tion of an ideal carbonatite cent
volcanic
products
agpatitic phonoliteo cores
and
alvikitic
roots
of
complex
include
variation volcanism
nephline
carbonatite
and plug-like bodies,
Kinnaird
in a vertical
sub-volcanic
ijolite,
Other
rocks,
(Fig.10.2B).
carbonatite
The equivalent
carbonatite.
igneous bosses,
in these magmatic
cross-sec-
In this complex the Reinclude
nephelinite
(plutonic) syenite,
occurrences
ring-dykes,
and Bow-
and
sovitic
include
veins,
and
bodies in the or
xenolithic
and cone-sheets.
A B C
C
(B) OVERSATURATED ALKALINE ZOHPLEX
CARBONATITE COMPLEX
Volcanic pile (basalt,rhyotites trachyte) (Quartz-porphyry volcanic feeder
~
F'g-] AIbite zone [ ~ - ] Micro¢linezone F ~ ] Ring dykes F~ I
~ ~
Alkalic s~rotovohQno with phonolite, nephitenite Notrocarbonatite
[ ~ Breccia zone F - ~ Carbonatite ring dykes F ' ~ Carbonotite cone sheets J ] Country rocks
Cone sheets I Country rock
~
m Syenite [ ~ Foyalite granite [-@---] Arf vedsonite granite FD-'~ Arfvedsonite olbi~e apogranite '~TI Blotite granite
Fenitized country rock
~ ' ~ Syenite fenite ~
Nepheline syenite
I;olite [',~"1 Carbonafile core
Figure 10.2: Schematic sections across oversaturated alkaline complex (A); and carbonatite complex (B) where (A) represents Oldoinyo Lengai; (B) Napak; (C) Chilwa Island. (Redrawn from Kinnaird and Bowden, 1987o) Vail African
(1989b) ring
furnished
complexes,
a
regional
summarized
account
below.
He
of
the
observed
distribution that
continent probably has the largest number of ring complexes Over 625 ring complexes from m i d - P r o t e r o z o i c
were recognized
to Tertiary
on the continent, The
Pan-African
the
of
African
in the world. ranging in age
late
or post-oro-
625
genic ring complexes, d a t e d 720-490 Ma, w e r e m e n t i o n e d e a r l i e r in connection w i t h (Ch.
the
Tuareg
6.11.5).
shield
In this
(Ch.
6.4.2),
and
the A r a b i a n - N u b i a n
chapter we shall be c o n c e r n e d with
shield
ring complexes
of O r d o v i c i a n to T e r t i a r y age. 10.2.2 The W e s t A f r i c a n Younger Granite Ring Complex Province This
is
Africa
one
of
the
(Fig.10.3).
best The
known
and
finest
Early-Middle
alkaline
Paleozoic
was
a
ring
provinces
time
of
in
important
a n o r o g e n i c ring complex emplacement in the Sahara,
from the Tuareg shield
to
Early-Middle
the
Nubian
complexes
shield.
occur
Younger G r a n i t e Republic
in
On
the
the
the
shield
northernmost
ring complex
through
Tuareg
and
oldest
province w h i c h
Jos
plateau
in
the
part
extends
of
the
Paleozoic meridional
from n o r t h e r n
Nigeria s to
the
Niger
Benue
valley
(Fig.10.3). The northern Niger complexes consist of well d e v e l o p e d structures w i t h ring-dykes from 2.5,
and cone-sheets of granite.
to 65 km in a gabbroic complex.
They range in diameter
The complexes o v e r l a p and dis-
play shifting intrusive centres. South of Air,
in the Damagaram region of
Carboniferous-Permian
alkaline
ring p r o v i n c e
southern which
Niger,
extends
there
into
is a
Nigeria
(Fig.10.3). These are the best developed Late Paleozoic ring complexes in Africa. About a dozen granitic ring-dykes, diameter,
intrude
ages
range
side
(Rahaman
from
a
highly
323 Ma
et al.,
deformed
in southernmost 1984),
matic
activity, 1974).
alkaline
characterizes
complexes
in Tadhak and at Timetrine nearby. type
of
alkaline of
magmatism the
258 Ma
inlier.
on
the
a southward province
in the West A f r i c a n
province
here.
in Africa,
craton,
Timetrine,
structures, biotite, bros,
and
long
(N-S)
and
150 km
2 to 15 km across. fayalite
dolerites.
lie
no
ring
on
the
structures.
the Jurassic was the most intense phase of
in Africa.
among the Younger granites of Nigeria 400 km
contains
located
edge
zone
in
(Turner and
there are u n d e r s a t u r a t e d foid syenites and carbonatites.
development
Nigerian
These are of Permian age. A different
prevailed
African
this
eastern
Like e l s e w h e r e
Their
decrease
Rather,
ring complex
West
to
basement
This suggest a southward m i g r a t i o n of mag-
a feature which
The w e s t e r n m o s t
Niger
thus d e m o n s t r a t i n g
the age of the ring complexes. Webb,
a p p r o x i m a t e l y 2 km and more in
Proterozoic
granites,
There
are over
(Turner, wide
40 granite
complexes
1976). T h e y lie in a broad
(Fig.10o3),
and
display
ring
They include soda p y r o x e n e and amphibole syenites,
and
There are rare volcanics
trachytes such as
with
m i n o r gab-
rhyolites,
tuffs,
626
and
ignimbrites
Turner,
characteristic
evident trend
1976;
Ike,
1983;
Jacobson
et
al.,
1958;
1976).
The of the
(Badejoko,
southward
in the Paleozoic, ring
complexes
migration
of magmatic
was quite pronounced
of the Nigerian
Younger
activity
in the Mesozoic. Granites
already The ages
illustrate
this
(Fig.10.3).
Figure 10.3: West African. A broad Younger anomalies
Sketch map of the Younger Granite (Redrawn from Cahen et al., 1984.)
regional
Granites
on
complexes
negative
gravity anomaly characterizes
the
plateau
over individual
Jos
complexes
(Ajakaiye,
1976).
of
the Nigerian
Large
suggest that the complexes
negative extend to
627
depths
of
bodies. been
10-12 km
The
that
they
are not
north-south alignment
attributed
zone.
and
to
their
of
probable
underlain
the
by
deeper magmatic
Younger Granites
location
along
a
province has
deep-seated
shear
Shear motion is believed to have generated frictional heating, and
to have released pressure for magma to ascend through fractures. The timing and
localization of
shearing was
probably responsible
for the pro-
gressive southward age decrease. Outside small
the
Tertiary
Younger alkaline
Granites plutons
province which
through Cameroon into Chad Republic. Cameroon, ultimes"
they
coincide with
lies
extend
a
NE-trending
from
Known as the
the
Gulf
band
of
"Granites ultimes"
the Cameroon volcanic
line.
consist of plugs and ring complexes of granites,
The
of
Guinea in
"Granites
syenites,
and
microgranites, numbering up to 38. They are obscured by younger volcanics in some places. the
Younger
Younger Granite age
from
Being petrographically and geochemically very similar to
Granites,
67 Ma
ring in
they
are
complex
the north
considered
province. to
38 Ma
as part
The
of
"Granites
in Poli
the
West African
ultimes"
range
in
central
part,
to
in the
46 Ma near Douala on the coast. This, however does not reflect an age migration of magmatic activity. 10.2.3 Northeast African Province This province includes the ring complexes of the Sudan, and
Uganda
syenitic,
(Fig.10.1).
and
In
gabbroic
this
plutons
province intrude
various
the
Egypt,
anorogenic
basement
complex.
Ethiopia, granitic, Ring
com-
plexes of Early-Middle Paleozoic age occur in the Nuba Mountains of the Sudan, in the Bayuda desert, the Nile valley, and in the Red Sea Hills of northeastern Sudan.
Sabaloka has the largest complex.
The Paleozoic com-
plexes are characterized by alkaline microgranites, quartz-soda pyroxeneamphibole chytes,
syenite,
alkali
with
lavas,
the
and
extrusive
phase
pyroclastics.
being
These
are
trachybasalts,
tra-
preserved
in
down-
sometimes
indistin-
faulted cauldrons which are enclosed by ring-dykes. In
the
Bayuda
desert,
Mesozoic
guishable from the earlier ones,
ring
complexes,
have intrusive cores.
These cores con-
sist of soda pyroxene and amphibole granites, quartz syenite, and associated volcanics
such as rhyolites and tuffs. Mesozoic alkaline ring com-
plexes with granites and syenites also occur in the Kordofan province of central Sudan.
In the Red Sea Hills of Egypt and the Sudanese ring com-
plexes alkali granites,
quartz and fold syenites,
trachytes and gabbros
pierce
through Pan-African volcano-sedimentary-ophiolitic supracrustals.
In the
Sudan these
complexes are meridionally aligned
for over
300 km,
628
whereas
in
southeastern
230 km long.
Egypt
The distribution
ment of brittle fractures Another south
group
trend,
850 km
Uganda.
and foyaites,
lie
in
a
broad
NNE-trending
band
pattern is believed to reflect the arrange-
in the basement.
of Mesozoic
about
to northern
they
alkali
long,
ring
along
A prominent
complexes
the Sudan-Ethiopia
group of Tertiary
of which Jebel Uweinat
the Uweinat Archean basement inlier
occurs
in
a north-
frontier,
alkaline
down
ring granites
is the best known example,
intrudes
(Fig.10.1).
10.2.4 Southeast African Province Southern Mesozoic. 300 km
along
province band
Africa
High
was
level the
centre
(Fig.10.4).
extending
consist syenites,
exempt
of
The
from
of
not
ring-dykes the
and
a nepheline
alkaline
cone
Limpopo
Nuanetsi
Transvaal
microgabbros,
from
and
sheets belt,
SE
lie
microgranites, syenite.
along
Mozambique.
The
The
and
known
during
extend
constitute
intrusives
to
intrusion which
for
the a
Nuanetsi
NNE-trending
intrusive
phases
granophyres,
ages
of
the over
these
quartz
intrusives
range from 186 to 173 Ma. Another major occurrence in southern the
Malawi.
high-grade
gneisses
preserved
granite,
well dykes, tween
cone sheets, 139 Ma
and
Mlanje Mountain,
of alkaline
In this of
region the
Southern
syenite, form
dozen
is in the Chilwa area ring
Mozambique
foid
and radial dykes.
108 Ma,
complexes
several
syenite,
complexes
belt.
and
mountain
contain
carbonatite
This group of intrusives,
prominent
intrude
These
ranges,
ring-
dated be-
including
the
over 3,000 m high.
10.2.5 Southwest A f r i c a n Province In southern comprising extensive truded
Namibia
there
ring complexes dyke
at
swarm,
about
is the Luderitz (nepheline
which
133 Ma,
lie
in
probably
alkaline
syenite,
a NE-trending
along
the
province
foyaites,
(Fig.10.4),
syenites),
belt.
These
continental
and an
were
extension
inof
a
South Atlantic oceanic fracture zone. Northward
along
complexes
lies
from
coast
the
ite,
and
radical
Africa,
ENE-trending
landward
(1984) the plutons sheets,
southwestern
in two
for
linear
about
in the Damaraland dyke
swarms.
The
the
Damaraland
belts
370 km.
According
show well developed intrusions
syenite,
or gabbro;
with
more alkaline
complexes.
The Damaraland
carbonatite
being
group
(Fig.10.4) to
of
which Cahen
present
ring complexes
et
ring-dykes,
are p r e d o m i n a n t l y in a
ring
extend al. cone gran-
few of the
are located along
629 the axis of the Pan-African 126 Ma.
Thus,
they were
Damara belt.
emplaced
Their ages range
at about
the
same
time
from 194 Ma to as the Nuanetsi
and Chilwa ring complexes.
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