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 <~m O
~-
p ~Belin~ean
! 3500
Z
.,n;~,,
Gneiss complex (Kaapvaal), Tonalite gneiss (Zimbabwe), Hoggar. Madagascar. Kbsai (Zaire), NorthernZeire
Sand River Gneiss (Limpopo be{t)
~,~ ~ .~ooo LUr¢
,,:[
Figure 2.5: Distribution of African orogenies (A); B: Frequency distribution of U-Pb zircon ages from different Precambrian regions. (Partly redrawn from Condie, 1989.)
15
It is c o n s i d e r e d relevant here to outline the various
arguments that
have been advanced in support of some of the above boundaries, for the
Proterozoic.
Windley
(1984)
favoured
the w o r l d - w i d e A r c h e a n - P r o t e r o z o i c boundary. ological earth
features
history
boundary
for
that
are
to
Africa
are be
associated
could
Archean-Proterozoic
perhaps
boundary
crustal evolution. i)
rocks
terozoic are
formed
lated
by
platform
terozoic stable
plate
and
other
boundary
to
continental
about
turning
point
3.0
Ga.
if the
In
essence
criterion
ity had
and
structurally
tectonics;
separating
and
very 3)
attained
similar is
of
across
to
the
earth
and
earlier
in
Early Pro-
the
of
(1989) re-
Archean-Early rapid
Pro-
growth
the oldest
of
cra-
is dated
s t a b i l i t y and rigid-
Africa.
But
as
Cahen
et
al.
2.5 Ga that larger areas of the
A f r i c a n continental crust became sufficiently stable and rigid to form the sites of p l a t f o r m basins. Also, structure
belts
orogenic
abundance
Condie
Since
that crustal
southern
crust
mobile
modern
greater
the
the Late Archean.
(1984) e m p h a s i z e d it was not until about
dated at about
the
h i g h l y deformed
Proterozoic
there
changes
cooling
crust during
Early
belts in the Archean.
geodynamic the
2)
3.1 Ga and 2.9 Ga, it means
been
in
for it is
tonic basin is the Pongola basin of the Kaapvaal p r o v i n c e which at between
Ga as
in the e v o l u t i o n of the earth's
sequences;
and granulite-gneiss
these
2.5
Archean-Proterozoic
from slightly or u n d e f o r m e d and u n m e t a m o r p h o s e d
cratonic
greenstones
major
better
of a major u n c o n f o r m i t y
stratigraphically
belts
at
especially
of
The d i s t i n g u i s h i n g features of the A r c h e a n - P r o t e r o z o i c
the p r e s e n c e
Archean
be
this a
is diachronous,
b o u n d a r y which mark a m a j o r phase are:
then
adoption
But he added that if the ge-
with
considered
the
(cratonic)
the Great Dyke of Zimbabwe is
2.5 Ga. This dyke which is intruded into an abortive rift
suggests,
like
other
cratonic basins of the Kapvaal
Archean-Early province,
Proterozoic
rifts
that w i d e s p r e a d
in
the
stable conti-
nental crust had existed in southern Africa by 2.5 Ga. Kr6ner Africa
(1984)
argued
that
the
Middle-Late
going o r o g e n i c phase, the Pan-African orogeny. which
Proterozoic
started
about
950
Ma
ago,
Africa
a c t i v i t y was
Paleozoic.
differentiated
However,
episodic
and
lasted
until
about
450 Ma
(1983). As emphasized by W i n d l e y
Early
the age of
(1982), and even at
(1984) the p r o b l e m cen-
tres around where to assign the earliest known a s s e m b l a g e organisms
in the
the oro-
This b o u n d a r y is debat-
It has been placed at 590 Ma by Harland et al.
650 Ma by Salop
into
Pan-African
for the upper b o u n d a r y of the Precambrian,
570 Ma r e c o m m e n d e d by the IUGS, is adopted here. able.
in
During this o r o g e n i c phase
was
p r e s e n t l y observed pattern of cratons and mobile belts. genic
boundary
should be placed at the beginning of the last m a j o r and thorough-
(Ediacaran fauna) which appeared towards
of soft-bodied
the close of the Pre-
16
cambrian
and h e r a l d e d
geochronological taining
Ediacaran
"Vendian",
the beginning
of the Phanerozoic
terms that have been proposed faunas
"Ediacarian",
and and
rocks
have
varied
"Infracambrian"
eon.
Just as the
for the age interval con(e.g.
"Eocambrian",
in Africa),
opinion
is di-
vided on w h e t h e r to assign the Ediacarian interval to the latest Precambrian,
the
earliest
Phanerozoic,
or even
to
a separate
"Ediacarian"
or
t r a n s i t i o n a l age interval.
2.3 Orogenic Cycles in Africa Orogenic which
cycles
produced
tain belts.
or o r o g e n y
crustal
can be d e f i n e d
deformation,
An orogenic
as the tectonic
metamorphism
and m a g m a t i s m
or m o b i l e belt is the crustal
genic a c t i v i t y has taken place.
processes in moun-
region where oro-
G e n e r a l l y an o r o g e n y involves a progres-
sive sequence of tectonic events which in plate tectonics terms often implies
the opening
and closing of an ocean.
Since o r o g e n i c
processes
are
now b e t t e r u n d e r s t o o d within the framework of plate tectonics it is better to c o n s i d e r African orogenic cycles in the context of plate tectonics even
though
classical
some
of
concept
the
for ease of expression. tectonics
to
familiar
of the
the
Furthermore,
interpretations
Burke and Dewey,
1973;
L ~ c o r c h e et al.,
1989; Kr6ner,
al.,
1989;
have
greatly
tectonic
processes.
1986; our
The
1984,
entrenched cycle"
African
be used
Precambrian
1976; Caby,
1987,
1989; Porada,
Tankard et al.,
understanding
Late
terminologies
will
of
the
in this text
the successful applications of plate
of
Burke et al.,
Shackleton, enhanced
and
"geosynclinal
orogenic
(e.g. 1988;
1989; Schandelmeier et
1982;
Wright
of A f r i c a n
Proterozoic
terranes
1989; Daly,
et al.,
orogenic cycle
in
1985)
cycles
and
Africa
has
been most s u c c e s s f u l l y interpreted within the framework of the Wilson Cycle
(Burke et al.,
1977) which attributes orogenic cycles and the forma-
tion of m o u n t a i n belts to the opening and closing of oceans. The
Wilson
clinal cycle. marine
rift
Cycle
incorporates
the
It starts with doming, sediments
and
stages
of
the
classical
geosyn-
rifting and the a c c u m u l a t i o n of non-
volcanics
(Fig.2.6A),
followed
by
crustal
s u b s i d e n c e and the opening of an ocean basin during w h i c h thick piles of geosynclinal deposits gins the
and ocean
on the begins
a c c u m u l a t e along both the passive continental mar-
adjacent to
seafloor.
close;
and
Subduction
concomitantly,
starts
at
some point
syntectonic
magmatism
and is
g e n e r a t e d above the subduction zone. The ultimate result of ocean closure is
continent-continent
collision
which
causes
intense
deformation
and
17
plutonism;
and finally differential
uplift,
v o l c a n i s m and molasse deposi-
tion.
VOLTA
I
BUEM
BUEM TOGO
ooI
s~,
,
~
,,
l
-o Iooo'
G
VOLTA
BUEM
TOGO ,
high p l a t e a u volcanoes?
granodiorite
e xogeosync line THICKENED
F
- .*.~"
-
~ N O C
WEST WEST
;-'."-.5 '1: -';~;;d CONT,NENTAL ' -~ ~T+ ".'t-'~:.-,t~,'i~q cRusT
KITICR OCK'$C Roc,' r a'Z-'~'~--~'T'=C II
r e a c t i v a t e d Dshomeysn with remobilised granite and dragged up charnockitic rocks and
Collisional Orogen
residual
anorthosite
E
D
~ ~ ] ~ , ( I ,
OCEANIC% ~---, ,,- .,,..-,.~. . . . . -,,-h,,,,
CRUST % '
\ ^..
\
Pn~civ~ C n n t i n e n t a l
\
C
B I
"\ \
A
']-" '-'~'Ir=~-'-',"'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).
-.<~r,v" ~,,~, ~ . . . . ~ ~ _ _ _ _ ~ . ~ _ _ ~ _ _ .
~ ~ . ~ - / / / ' ~ - - - - - - ~ A N D CATACLASTtC FABRICS SPATIALLY ~ ¢ , ~ / ~ A S S O C I $A T - E O ~ --ASSOCIATED - - - - WITIW ' ITHTULI-SABI SHEAR BELT
~:SOI~'R-AFRICA'~,I CRATONIC COVER (PALAPYE, KARO0, KALAHARI }
P0 STTECTIONIC GRANIT01D PLUTONS SYNTECTONIC GNEISSEC INTRUSIONS GREENSTONE BELT INVERTED LIMB OF D| NAPPE BAINES DRIFT METAMORPHIC SUITE AND LAYERED BASIC tl,rrRU~ONS LAYERED GNEISSES AND MIGMATITES, INCLUDING POSSIBLE BASEMENT .....
INTERNATIONAL BOUNDARY
f /.6~//I.
TRACE OF LITHOLOGtc LAYERING
"
Figure 3.23: et al., 1982.)
Central
FAULT
Zone
in
Botswana.
(Redrawn
from
as
Tankard
62
The
early
are,
tectonic
however,
crust
quite uncertain
(ophiolites)
associated
settings or
Zimbabwe
and
Kaapvaal
al.
(1977)
et
magmatism
levels
in the adjoining
of
the
Limpopo
which
prior
to
sheared
of
collision.
But
geosutures
and uplifted
granite-greenstone
belt
of oceanic
could
of an ocean that was subducted
indicators
belt has been
evolution
in the absence of true remnants
cratons
these
the Limpopo than
the
calc-alkaline
with the existence
Burke
during
have
been
between the
as
suggested
are missing
by
because
to expose
deeper
crustal
terranes.
Thus,
even at
2.7 Ga the tectonic processes
in the Limpopo belt are entirely compatible
with plate tectonic processes
seen today.
-
-
-
--I
-
-'N-----:
-
~
~
_-_
~
.
_ _
_-_ ~
~
-
:
~
---i --__
2
~
-
"..
~
~~
/ , r i : ; -~
?~-
._~,,~'<....
_
~----
"
Greenschist Facies} ~ 2650 Ma Granite ~ ] Shear Zones Amphlbollte Facie.s|Gre.en.stone ~ 2 4 5 0 M a Granite ~ Thrust Zones Granullte Facies J .w.i ~Schleland Phalaborwa Charnocklte AlkalineComplexes2050 Ma ~-~ Cover Bavlaonskloof Gneiss ~ Retrograde Orthopyroxene Isograd
~
Figure 3.24: Southern Marginal Zone and northern part of the Kaapvaal craton. P, Pietersburg belt; R, Rhenosterkoppies belt; S, Sutherland belt; M, Murchison belt. (Redrawn from Van Reenen et al., 1987.) Van
Reenen
et
temperature-time Marginal these
Zone
authors
followed
by
al.
(1987)
evolution
(S.M.Z.)
of
traced
in the
isothermal
interpretation
metapelitic
of
gneisses
the
of
the path shown on Fig.3.25,A.
m a x i m u m metamorphic rapid
their
conditions
decompression
(P>9.5 Kbar; of
about
the
pressureSouthern
According
to
T>800 ° C) were
2.5 Kbar
between
63
about
2.7 Ga
cordierite
and
2.65 Ga,
and hypersthene
as
evident
in
the
decompression
textures
of
after garnet.
STAGEI : CONTINENTAL COLLISSION(ISOGRAD DISRUPTED)
b-.'.q
"--'4 I5OGRAO
STAGE 2 : THERMAL READJUSTMENT
F
2
II
I--....
I
STAGE 3: ISOSTATIC READJUSTMENT •" - . ~ - L - - - L - ~ - - E R OOED 4 ~I F
lit
B. MECHANISMS
OF UPLIFT
UPLIFT
Z+
///I////,,'~\
f ,/,,,,////// ~-ANGLE THRUSTAND
ANATECT,C MELTS
REVERSE FAULTS
[>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 - "
.,
"=<" , • • • --"
....
*
~
• I"
" "
'
/
.
~
.{.'-:--.. - . . . . • {"
."
ptuton.
'
.
•
".
'l. [ • . .'~"*." " . . "
.~T'%|
" "
~ ',
" • ".'..".
• . .
~.\II
"
.
.~,3~1~'.-..
"
.'l,~r~41.J " tII'~FIiV : .'"~/t~.y
.
.
.
.
.
.
.
.
.
"
• • "
"
" .
.
.
.
.
.
volcanic
.
.
.
.
•
.
.
.
:'d
• ~,,, .
.
•
Vl
v~.\ /
'
~".~
•
.
.
.
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•
"..--._.._~---:~st~ck\U--~. ~'""rr~--=="-""~r:~l,,,,,/ .
.
.
.
.
.
UNIT
activity-.."
~, P l a c e r s
.
I. 2. 3. 4, 5.
.
"
.
.
.
•
•
"
• •
OF
rz
"
--
.
" •
ARCHAEAN
/
"
"
•
/
I..'
.,-', I / 1 "
" -
".
""
"\.~/\/ SOLD
'._~.-
V" ,Iv: . . . ."
-/|
: • . ' I.%..,:
• • " • " " "
TYPES
Q u a r t z l o d e s , v e i n % s t o c k w o r k s and s h e a r s S t r o t i f o r m deposits in bonded iron-formation 2a-proximal or o x i d e f a c i e s 2 b - d i s t a l or sulphide and carbonate f a c i e s
. ,~,~
.
"
~ "
.
"
"
.
ULTRAMAFIC-MAFiC
Site of continued .....
. .
~ ' J ~ " 7 ~ "
:.
:
"
• • • • "" . •
". . •
.
....
DEPOSIT
I
V o l o o n o g e n f o s t r o t o b o u n d massive sutphides { V o l c a n o g e n i c - s t r o t i f o r m suboqueous e x h o | o t i v e { Epithermal veins I Plutonic s t o c k s o r silts / Placers /
B
Fumarolic discharge of Au~ A s , Fe~ S= Si~ CO 2 ( Au- thio - species)
Volcanic centre [~e~, . . . . . OxIoe
0
4
SEBAKWtAN TIMES
C
Sulphide and corbonafe facies
Toclee
Sea Level
0
.';'.
'
.
Maflc
"
"
•
•
•
•
"."a.s.s b~m j ~ c auO ~"
- ulframof|c
C "
BULAWAYAN
TIMES
d i s c h a r g e o f F e , Si
..
4
Sea
Level
3
0
I
I
Z
2
3
3
4
4
5
5 km
km
Figure 3.27: Relationship between Archean greenstone mineralization and volcanic-exhalative activity (A); B, model explaining the association of stratiform gold and sulphides with iron-formation; C, later lode or vein deposition in tectonic fractures. (Redrawn from Hutchison, 1983.)
of
Lode deposits
of gold occur in the thick mafic-felsic
volcanic rocks
the
greenstones
the
Bulawayan
structurally pyrite,
controlled.
chalcopyrite,
Lode
stibnite
where
the
distribution in
of
gold
usually
occurs
and
galena.
Hydrothermal
veins
association solutions
are with were
responsible for transporting the gold and sulphides form the mafic-felsic
68
source rocks into the veins.
Silver is r e c o v e r e d as a b y - p r o d u c t of gold
m i n i n g in Zimbabwe.
Chrome Most
of
the
world's
chrome
reserves
are
in
Zimbabwe
where
they
occur
m a i n l y in the Great Dyke and in the Shurugwi Complex.
A r o u n d the town of
Shurugwi
chrome
ultramafics
Shurugwi
Ultramafic
have
been
that
of
seams,
the the
in
Great
a
Early A r c h e a n
from which
Whereas
deposits
lenticular up
in the
over
mineralization
Dyke.
Shurugwi
range
occur
Formation
produced
ribbon-shaped, bodies
deposits
in
are
pods.
to about
15 m
the in
12 m i l l i o n
setting Great
the
In
the
thick
and
Dyke
form
traced
from
occurs
extremely deposit
the
of chrome
different
chromite
of
Shurugwi can be
tons
totally
of
elongate
typical
along
in ore
strike
for
over 300 m. The chromite horizons are restricted to rocks which were originally
olivine
cumulates.
The
cumulates, Cr 2
03
at
or
contents
near
of
their
the
ore
contact
are
with
about
pyroxenitic
60 %
with
Cr/Fe
ratios of 3.5 or 4.0 to 1.0. Chromite formed in a magma chamber with conv e c t i o n currents w h i c h produced the lenticular shape of the ore bodies as well
as
features
which
resemble
as slump and flame structures, ing.
However,
subsequent
primary
"sedimentation"
structures
such
load casts, m i n o r u n c o n f o r m i t i e s and grad-
deformation
has
complicated
the
original
shape
of the chromite ore bodies.
Massive Base.Metal Sulphides The
ultramafic-mafic
belts
of
pyrite,
Zimbabwe
sills
and
pyrrhotite,
and
felsic
South Africa
volcanic
rocks
have yielded major
intrusion
matic, late.
with Apart
trusive
(Fig.3.14),
pyrite
and
the
sulphide
pyrrhotite
from the Shangani
ultramafics
the
greenstone
sulphides
such as
pentlandite and chalcopyrite in w h i c h there are eco-
nomic c o n c e n t r a t i o n s of Cu, Ni, Fe, Co, S, and As. mafic
of
which
are
In the Shangani ultra-
mineralization
crystallizing
nickel deposit,
early
nickel
stratigraphically
was
and
late
also occurs
equivalent
mag-
chalcopyrite to
in ex-
the
Bula-
w a y a n komatiites of the M b e r e n g w a greenstone belt. This is because of the characteristic
association
of
nickel
with
komatiitic
fluids
especially
where such fluids are saturated with sulphur. In the M u r c h i s o n greenstone belt of South Africa m e r c u r y and stibnite are m i n e d at this strike canics,
from the v o l c a n o - s e d i m e n t a r y sequence.
l o c a l i t y occur as concordant distance
of
dolomites
about
50 km.
and ironstones.
The
The stibnite ore bodies
lenses which are strung out along a host
rock
consists
of
M e r c u r y and a n t i m o n y were
altered
vol-
concentrated
69
in these sediments as a result of submarine hydrothermal related
igneous
activity
which
is represented
prophyry or basic lavas (Hutchison,
by the
and genetically
associated
quartz
1983).
Iron Ore
Although
the best development of banded iron-formation
occurs around the
Archean-Proterozoic
boundary,
the Buchwa
iron ore body in Zimbabwe
good
economic
deposit
a mid-Archean
example
mation.
This
(Fig.3.26). large
of
an
deposit It
lies
occurs
in the
in a host
Buchwa rock
syncline
of
and
formed
thereby
through
facilitates
southern
which
granitic
is a
iron-forZimbabwe
outcrops
terrain
as a
on account
The ore body occurs near the top of the
mining
the replacement
banded
in
jaspilite
900 m high hill above the surrounding
of its resistance to weathering. hill
of
operations.
of silica
culating solutions during metamorphism,
This
iron
by iron oxide,
ore
probably
either
by cir-
or through a process of supergene
enrichment involving the fluctuation of the ground water table. Pegma ti te M i n e r a l i z a t i o n
The best
known
Archean
example
in Southern
Africa
eastern extremity of the Victoria greenstone belt economic
deposits
of petalite,
lepidolite,
mineralization
geologically
at
oldest
Bikita
tin
which
deposit.
tin
Bikita
now
at
beryl,
there was also
exhausted,
occurred
the
Apart from
pollucite,
(Fig.3.28),
although
The
in
spodumene,
eucryptite and amblygonite in zoned pegmatites tin
is
(Fig.3.26).
was
disseminated
the with
tantalite and microlite in marginal pockets in quartz-rich zones in large masses
of
lepidolite
greenstone
belt
there are important lumbium,
corundum,
pegmatites widely
in
which the
greisen.
deposits feldspar,
already
of mica, emerald,
Bulawayan
amphibolite
unrelated
later
greenstones
pointed region
beryllium,
the
Murchison
lithium,
of
Zimbabwe.
granitoids.
The
terranes
They are
where
tantalum,
above
co-
from the
Tungsten mineralization
facies metamorphic
intrusive
out,
in South Africa,
and beryl which are mined
intrude biotite schists.
occur within to
As
is an important pegmatite
occurs
pegmatites
and are apparently
the products
of
ana-
tectic melts which were produced in situ or nearby during regional metamorphism. Corundum
This Zimbabwe.
is
economically
Boulder
corundum
important deposits
in
the
Archean
are most
greenstone
important
and
terized by features which reflect the metamorphism of Al-rich
belts
are
of
charac-
sediments.
70
Most deposits contacts.
occur near the granitoid-greenstone
They
appear
the corundum occurs of
andalusite,
ultramafic sediments
conformable
as lenses
sillimanite,
rocks
(talc,
with
the
in Al-rich or
schists,
or mafic dyke-granitoid
greenstone
stratigraphy
schist and contains
kyanite.
The
associated
serpentinites),
rocks
ironstones,
and
one or more include
argillaceous
or gneisses.
BIKtTA
~ . . ' ~.--" ~ \
;, --> . - . . . ~ Polluclt/e .i..eREENSTONE\ ~%-__~.~><'
~cO~LE\ s % - ' ~ . o)"~L~/-. ROCKSl \
MAIN ZONE OF ~' MINERALS PREDOMINANCE ~Quortz } Lepidolite Core zone Amblygonite AII mix } Spodumene Intermediate
~
Pollucite
t~~
LEPIDOLITE
zones
Petolite
~Muscovite Feldspar
} Wall zones
lOOm
I
Greenstone
Figure 3.28: in Zimbabwe.
J
Countryrock
Cross-section through the main (Redrawn from Hutchison, 1983.)
Bikita
pegmatite
Asbestos
Large
asbestos
Zimbabwe.
deposit
M a n y Archean
chrisotile
occurs
Most
Occurrence
in volcanic
in southern the
are
rocks.
Africa,
ultramafic
bodies
greenstones; volcanism million
but
tons
and
in
(i)
portions associated
(iii),
pre-date
in Zimbabwe
fibre
layered of
ultramafic and
(Anhaeusser,
South
complex
Africa
in
contain
of
ultramafic
greenstones
with
intrusive
the
1985).
ultramafic
are three modes
granitoid
of asbestos
Shabani
and Southern Africa ranks third in the
serpentinized
There
viz.,
mafic-ultramafic
the
ultramafics
fibres of good quality,
world production of chrisotile
are
in
bodies
although
ultramafic (e.g.
to
associated
felsic
bodies
(ii)
with
layered
portions
which
Shabani).
fibre have been mined m o s t l y
some
mineralization
successions;
mafic
intrusion
sills
asbestos
of
post-date
More
than
3
from the Shabanie
71
mine
in
Zimbabwe.
ultramafic
suite
Other
footwall
concentrated hydrothermal
occur
in
other
parts
of
the
Mashaba
from Filabusi and Norma to Zvishavane and Mashava along
a 150 km arc (de Kun, the central
deposits
in
1987).
In the Shabanie mine the ore bodies occur in
dunite of the Shabani
zones
where
alteration
of
extensive
dunite
Complex w i t h
fractures
(Fig.3.29).
the main
fibres
faulting
aided
and
Other
mineralizations
in
the g r e e n s t o n e belts of southern Africa include m a g n e s i t e and talc in the ultramafics and beryl in the volcanogenic deposits.
v
ooo.=.oo'h I'- "
u ~Z
-
o
-
"IDY kes
-
W / / / J B i r b i r i t e w e a t h e r e d zone
z=---_V-ITatc zones
"~
-/-
-- ~¢t
- -
--
-/'--
' ~ . ,-V'X
-
--
°
.
/,:#.
,,
--~\~''.'-_~_'_\
- ~ ~ k v
u 1PQrtialty serpentinized zone - - / " -- -- -- ~:.~-..'~ {\',-:,~'.'.-.tSi~y f~ore orebody with fibre s e o m s ~ . . . . ~.~--k~_~ ~ / I':':-::';I Brittle fibre zone ~__ _ _ _ ~ ' ~ z'_-~,. ~" [ - - - - - I TOIC c a r b o n a t e r o c k s 2:1 Value r a f i o ' ~ -- -- ~ ~ ~ : . . ~ / / /
Figure 3.29: bestos mine.
Composite c r o s s - s e c t i o n through (Redrawn from Nisbet, 1987.)
I
/11~
-
I
~ l/It!
I
,,
~'lJ/
I
//~l ~ I/;7
the
I ~1
Shabanie
I
as-
3.3 Zaire Craton
A r c h e a n terranes are exposed in three p r i n c i p a l parts of the Zaire craton in
equatorial
Kasai-NE
Africa
(Fig.3.2).
In
Angola
shield
which
and migmatites.
In the
northwestern
Chaillu In
the
of
the
part
craton
stretching
there
charnockitic lie
the
from southern
is
the
gneisses Ntem-du Cameroon
as the foreland to a y o u n g e r P r e c a m b r i a n m o b i l e belt.
northeastern Archean
southwestern high-grade
part
c h a r n o c k i t i c - g r a n i t o i d massifs
to Congo Republic,
varied
the
comprises
part
gneissic
of and
the
Zaire
craton,
granite-greenstone
however, terrane
is which
a
vast
and
spans
the
72
s o u t h e r n part and w e s t e r n the
western
of
the Central African
Uganda
(Fig.3.30).
parts
of
the
Republic,
NE
Zaire,
southern
The p r e d o m i n a n t l y h i g h - g r a d e
craton
will
be
described
Sudan
terranes
first
before
in the
n o r t h e a s t e r n parts where there is m o r e information and a clearer picture of the A r c h e a n g r a n i t e - g r e e n s t o n e assemblages and t e c t o n o - t h e r m a l events. Remnants
of
Archean
rocks
in
the
later
Proterozoic
mobile
belts
which
c o m p l e t e l y surround the Zaire craton are indications of an initially more e x t e n s i v e A r c h e a n terrane in this region.
ARCHAEAN
~
Granites ~--~ Greens~on.beff ~Arnph;bo[~lefacles gneisses PROT£ROZ01C baslcg n e b ~
~ - ~ GrQnuff,e,ac~es gnelsses
~Archaean
Figure 3.30: A r c h e a n cratonic (Redrawn from Condie, 1981.)
nuclei
on
the
Zaire
craton.
3.3.1 K a s a i - N E A n g o l a Shield This region includes the basement of western Kasai and southwestern Shaba provinces ancient
of
Zaire
and
metamorphic
basement
is
exposed
cover
about
the
4°S
here it
on
southern
is
a
(Fig.3.31).
shield
bounded
and
by
which
western
an
An dips
In the east the Archean terrane is bounded by the K a t a n g a n system rocks
at
Angola
fault.
Phanerozoic
where
in
to
shield.
north
Luanda
gently
while
the
northeastern
important
ends
of
the
73
21'
22"
23'
24"
23"
24"
/ 2|"
22"
Figure 3.31: Central Kasai province, Zaire showing: i, Karoo to Recent cover; 2, Mbuyi Mayi Supergroup; 3, Lulua Group; 4, Luiza Supergroup; 5, Gneisses, Dibaya Granite, and migmatite assemblage; 6, Amphibolites; 7, Gneisses; 8, Migmatites; 9, Granites; 10, Gabbro-norite; ii, Charnockites of the Kasai-Lomami gabbro-norite and charnockite assemblage; 12, Kanda Kanda tonalitic and granodioritic gneisses and Upper Luanyi gneisses. (Redrawn from Cahen et al., 1984.) As shown by Cahen and Lepersonne
(1967), most of the Kasai and NE An-
o
gola
shield
inclusive
between
of
the
mainly by poorly rocks,
7 S
o
and
ii S
Dibaya-Luiza-Kanda exposed
dated at about
pegmatites
lats.
gneisses
o
and
Kanda
longs. type
areas,
and migmatites
facies.
These are separated
and granodiorite
gneisses
The
contain
lenses
gneisses
which are hololeucocratic of granulite the nearby
are
24 30'E, underlain
The oldest
3.4 Ga, are the Upper Luanyi granite gneisses with
of amphibolite Kanda
and
(Fig.3.31).
Kanda Kanda gray tonalites Kanda
o
22 E
pink rocks that probably
facies metamorphism
charnockitic
diffuse
rocks,
or as intrusions the Kasai-Lomami
of
from the adjacent
by inferred alaskite
faults. gneisses
formed near the limit which
originated
gabbro-norite
and
from char-
74
nockitic
assemblage.
Although
their
exact
ages
are
unknown,
the
Kanda
Kanda gneisses have been placed between 3.4 Ga and 2.82 Ga. The Kasai-Lomami gabbro-norite and charnockitic assemblage comprises
two
amphibolites
rock
suites.
There
and anorthosite)
is
the
mafic
part
(Fig.3.31)
(gabbro,
which originally comprised
norite,
a heterogeneous
suite of hypabyssal intrusives or effusive magmatic rocks and deep-seated intrusives which have undergone granulite facies metamorphism. part
is
the
acidic
component
enderbitic
composition
granulites,
both
before
their
of which
suite
textures
and
of
the
(or
respectively.
are
gneisses rare)
reworking.
metadolerite Kasai-Lomami
cataclastic
metamorphism
dark
of crustal
contains
components
comprising charnockites
probably had partially
long history
Kasai-Lomami acidic
(true
which
charnockitization)
sedimentary
precursors
acidic
Both
part
the mafic
contain
reflect
and
charnoaluminous
assemblage
deformation
of
and
The
dykes.
The second
of
the
and
the
granoclastic
granulite
regional
facies
deformation
These events are dated at about 2.8 Ga.
The youngest
Archean
rocks
in this
region
are
the extensive
Dibaya
granite and migmatite assemblage which covers a large part of Kasai proO
vince between Alto
O
5
and
7 S and probably continues
Zambeze province
comprises
of Angola
granite-to-tonalite
(Carvalho,
migmatic
southwestwards
1983).
gneisses
The Dibaya assemblage
and
calc-alkaline
ites. These rocks have prophyroclastic to heteroclastic locally mylonitic. transformation migmatite
assemblage
formation appear.
They have yielded
of pre-existing between
radiometric
Archean 2.82 Ga
into the gran-
structure and are
ages which
suggest
the
rock into the Dibaya
granite
and
Dibaya,
de-
and
2.56 Ga.
South
of
(oataclasis) decreases so that structurally undeformed granites The
Malafundi
anatectic
granites
occur
in
the
southern
area
linked to the migmatites. Archean western
rocks
Angola
are also exposed
(Fig.3.32),
in several
for example
the River Cuanza
near Cariango where Archean
and
and
NE-SW
trends
comprise
places
at Malanje,
enderbites,
in the basement of Dondo,
assemblages
charnockites,
and
south of
show east-west kinzigites
and
O
granulite
gneisses.
o
South
O
of
Nova
Lisboa
in
the
area
between
13
and
o
15 40'S and 14 30' and 17 30'E, the volcano-sedimentary Jamba Group is of Archean age. 3.3.2 NW Zaire Craton This
part
stitutes
of the
the
Zaire
foreland
of
craton the
is
a
younger
broad
basement
upwarp
which
con-
West
Congolian
mobile
belt
(Pan-
76
iin& ~l 'l'l[I
,~ .
"'
"
X>:H~ ~;,,!~'y,,,,,..~" .. • ,
I iI I Ill i
~:
--
'
~.:----~
I
.'.
iIIl,I
"
*"
~-.
'
t.-©
~
o,,oo~o~ ~ ~ !
'-
"
~~I.
-~, " i"
'
~--=~['di',~ , ; . .... ." v :-...' ------= I "" . "". " llli~-~,., I.~~..."~..... ~ ......
..;.~--~,~ ';. ::',';. ,',i~~
\ ~gunzc~ ;abo,o
......
I "
,
4~ I
# "
~. ' ~ " ~ "
" ~ " ~.~..°..~ I
'." " .
. "~
"i"~
'.~ "
:..,--:I;;::-.~.~......._
'~' - - ~ ~ ~ ~ I
' .....: '. ......~,'.: " ." ."."" -..::Y,:' ~ ~ - . ' "' Luen~,~..[ ;,.....:.;~LTA~ ~'~ -~-~-~--~-z~ ~ " ~ " ~ = ~ ll~'~J" '" " " " ~ " " "" " :':"""~-~"~ Lobito .... ~ ~"~_~.~_--~--%~,--47.Z&~i-~lIIII~ ~ ~ - ' ~ . ~i[ %:..! ~';1%°. . " " """" ""'I:;.. "-;~ ^,',n ;-~;:-.~///'~,~:( ." • .I ~_ L I 4 1 v l D r " Lr_.: @ / I I I , E#]~ =
~--~
~_~-e_
~-~__-
..=- ~,~_--._- -e~z_-~ - w .:_:
--
,.-- - ~ -
i ) I '., " : .-
: .
","
• :
. ,'...[
: " ": : "
. : .... . - • . . . ;-,.,: I.. ...... •".-" .... ,: :~;'.,
F~:Y_-J"~'~.~--~-=P---:~
"r~--~=-~ " '.: ' ' ,:: " ~ ...... _-,_(:~..: .-....." .~.-.....-..
.
~:"
.
:
I
.
s
~
,.. ..... v " : : ':I" "."-:": '(~"i-:;":--".'I'\
IBo
ZO °
22 ~
24
Figure 3.32: Tectonic map of Angola. i, P h a n e r o z o i c cover; 2, Pan-African; 3, Kibaran; 4, Kibaran-Eburnean; 5, Eburnean; ~, Eburnean-Archean; 7, reactivated Archean;8jArchean; 9, older Archean reactivated; 10, older Archean. (Redrawn from Carvalho, 1983.)
76
African). massif,
It extends as a vast granitoid massif known as the du Chaillu
from about
is mostly
3°45'S in the Congo Republic,
concealed
Cameroon
where
beneath
the
Late Proterozoic
granitized
Ntem
through Gabon where it
supracrustals,
charnockitic
to southern
massif
is
exposed
(Fig.3.30). The Ntem complex, du Sud Cameroun", Equatorial granulite the
otherwise
outcrops
Guinea,
across the borders
charnockitization These
of
rocks which
highly
"Complexe
formed at about
evolved
Cameroon with
It comprises
precursor
later suffered cataclasis,
cally retrograde metamorphism,
calco-magn~sien
of Southern
Gabon and Congo Republics.
facies metamorphic
lerite dykes.
known as the
a variety of 2.9 Ga through
rocks
including
recrystallization
do-
and lo-
as well as a granitization event at 2.7 Ga
which caused the emplacement of the du Chaillu granitoids in the south. The du Chaillu massif generations biotite
shows a north-south
of granitoids,
viz.,
or biotite-amphibole
which occur as veins schists
and
types,
greenstones
exist
as
to quartz dioritic
and pink mostly potassic
cutting the gray granitoids.
transformed by granitization (2°20'S,
foliation and contains two
a gray granodioritic
septa
which
Within
have
not
migmatites
the granitoid, been
completely
(Cahen et. al; 1984). For example,
at Mayoko
12°50'E) one of the relict greenstone belts occurs in an area 20
km long and 5 km wide and consists of sub-vertical banded iron-formation, amphibolites, trend.
pyroxeno-
amphibolites
and
biotite-gneiss
Another relict greenstone belt occurs at Zanago
with
(2°45'S,
a
N60°E
13°33'E)
and is 30 km long and 25 km wide. This belt consists of north-south-tending,
steeply
quartzites,
dipping
banded
amphibolites
dunite.
Since
schists
and
the du Chaillu greenstones
iron-formations,
with residual which
granitoids are
amphibolite-bearing
pyroxenites, have
and a small mass of
been dated
engulfed
by
the
at
2.7 Ga,
granitoids
are
the ob-
viously older (Cahen et al., 1984). 3.3.3 NE Zaire Craton In the northwestern part of the Zaire craton and granite-greenstone
(Fig.3.33)
Archean gneisses
terrane underlie most of the Haut
Zaire adminis-
trative province and extend into the neighbouring territories African
Republic,
geologically semblages:
western
complex
Uganda
terrane,
and southern
Archean
rocks
Sudan.
comprise
of Central
In this vast
and
the
as-
following
(i) old basement gneisses which date from about 3.5 Ga and are
known as the Bomu and West Nile Gneissic Complexes;
(ii) scattered green-
stone belts known as the Ganguan greenstone belts in the west and as the Kibalian
greenstone
belts
in
the
east,
both
of
which
represent
two
77
periods
of greenstone
main generations at about Massif
emplacement
of granitoid
broke
through
the
240
I
'CE.T' . ~
[
I
I
~
: ~
at between belts
~o
,,o
and
(iii), two
2.9 Ga and
events the Upper
greenstones
.FR,'CA,NE'
_
.'x.N..... ; . moo
the
2.7 Ga and
Zaire Granitoid
basement
granulite
~
~
,
'
-
.
~
"~
.'* Y
'
*
~
+
+
~
,
~+
"
:'" Q "
~
%,, --
"
_ _
'
3~ i
'
I
I 1
'
,,
__
n". ~ 7 " . * , , . 9 ~ --
,::io/..~i!~: < - ~
"
30.
-:
..~..:~:'~.
~~VJ
/
3.2 and 2.6 Ga; and
(Table 3.2).
,2.
,
emplaced
2.5 Ga. During these intrusive
gneisses
~
between
--
"VI~
.
, ~..~._~..
/'~
--
-
%~ ~'6~.-.~--~,.~i - - ¢ , , _ -
~"
~lt./_*
~
"
0
-.
s0
.
u,,-.
'els-.
~"P\
.
--
--
,.,.,--
--
--
__Co,-on%"
-
-
",
4
,
P
t
~-,./
i
--_j
1
-,,¢
9~~(~ ~-_
Figure 3.33: Granite-greenstone belts of NE Zaire: i, Mesozoic cover; 2, Lindian Supergroup; 3, Upper Zaire granitic massif; 4, Bomu gneissic complex; 5, West Nile gneissic complex; 6, Greenstone belts. (Redrawn from Cahen et al, 1984.)
Bomu Gneiss Complex This
is
exposed
rivers where tral
African
B~r~m~,
around
Nzangi
Republic
and
gneisses
(Cahen et al.,
gneisses
and occupy a synformal schistose
confluence
northern
(Fig.3.33).
and Monga
plex in this region bole-pyroxene
the
it underlies
which
of
Several
Bomu
gneiss
actually
(Mbomou)
1984).
gneisses,
border
outcrop
gneisses.
complex
quartzites,
Bomu,
of
about
50,000
Zaire. The Bomu gneisses
and have undergone
a suite of micaceous
and amphibole
the
com-
The largest is the Bomu amphi-
form a large
of the Bomu
Uele
of Cen-
the basement
retrograde
are
the
mica schists,
km 2, are
metamorphism.
They also contain massive and banded tonalitic and monzonitic At the eastern
and
parts
complexes,
constitute
structure in northern
garnetiferous
the
Zaire and the southern
granitoids.
synformal
B~r~m~
biotite gneisses
Both the Bomu and B~r~m~ gneisses have ENE and NW
3.&Go tonalite precursors Nzangi gneisses high grade tectonothermal event marie precursors Bom~ gneisses
tectono -thor mol event ? ? Bereme g neisses
fionguon
2.91~0.05 Go tee+one°thermal event
B
A
I.
I
A
2J.6_~0.03 Oa granite intrusions
lturi
event
pare gneisses
+octane-thermal
northern
6,West Nile and central Uganda extreme (N. E. Zaire )
Upper Kibalian ( I(/boli beds)
2.68Z0.06 Go tectono-thermol event [ early-Aruan. Up to amphibo~He facies )
2.6L,~.O.0g Go late-to posttectonic monzonite
KibaHan of West Nile (Adido belt )
2.55 Go tee+one-thermal event redone-+herren| eve~ [late-Aruon, up to [ greens chist amphibolife facies J facies )
e.2.0SGa teetono-fhermczl event
S. Aru-Zanl (N.E. Zaire)
in
Kovirondian
grQnite intrusions 2,g&±o,o& Ga tectono-thermal event (greenschist to amphibolite facies)
2J.7± 0.06 Ga
c.2.OgGQtectono-thermal event
7. Northern part of Tanzania shield
Zaire
2.72-~0.08 Go [2.8g Ga} granite, etc 2.85-* O.0g Ga tonolite, etc. intrusions granite, etc. intrusions intrusions tectono-t~rmoI event ~.,~tano-thermal event 2.91 ~ Wotion event tectono-the~mol event {granulite facles) { greenschist |greenschist | §reenschlst facies) facies) facies N'tcmzion lower Kibolian Lower Kibaiian |Aru beds) pre-Wofion precursor rocks
2.~1±0.13 Ga granite intrusions
N
&. Kilo (N.E. Zaire}
C. 2.05 Ga redone-thermal event
I
3. Southern Ituri CH.E. Zoire)
2.h9.'0,07 rao 2.79_~0.07 Ga tanatite, etc. tonalite, eft, intrusions intrusions tee+one-thermal eYent te~tono-~etmai event ( greenschlst ( gteenschist facies) facies) Lower Ki~)alian Lowe~ Kibaiion
Upper XibaHon
redone-thermal event [greenschisf facies )
Z60.~0.03 Ga tectono-ther mol • vent ( amphi halite facies )
K
2.g1~0.06 Ga granite intrusions
2, Hate (N.E. Zaire)
Z/*5± O.O3 Go gr~nffe intrusions
6AGUAN
1. Bomu region (N. Zaire}
Table 3.2: Early Precambrian correlations craton. (Redrawn from Cahen et al., 1984.)
A
U
fonolite precursors Nzangi gneisses high grade tee+another mol event mofi¢ precursor~ of Bomu gneisses
? Bereme gneisses and Ifuri poragneisses
2.a&±O.Og Oa fonatite, etc. intrusions 2 . ~ O,O~, G~ WATIAH +octane-thermal event (Watiaa) Deposition of levee Kibeiion end Nyanzian Depositioo of Gonguan pro-WaHoo precursor rocks [oge unknown) tectono- thermal event
N
2.65±9.06 Ga A eotiy-Aruan tect onethermal event
2.6~,.'0,09 Ga late-to post-tectonic monzonite
deposition ef Upper Kibalion, Xiba[ion of W, Nile and Kovir~ndion g
granite intrusions 2.57~ 0.0/. Ga |late-Azuan tecta nothermal event
¢.ZOSGa ted'ano-therma! event 2.I.6±0.05Ga
8, 5eguence of ~ssemblages and events
79
structural
trends,
the
of
south
the
around Bondo. which
are
gneisses
the later structures B~r~m~
complex
are
being older than the former.
the
Nzangi
gneisses
which
These are massive or banded basic-to-intermediate
associated medium-to
with
quartzites
high-grade
and
mica
metamorphism
schists.
In
followed
by
was
metamorphism and by the intrusion of the Bondo granite.
To
outcrop gneisses
the
Nzangi
retrograde
The Monga gneiss-
es occur to the south of the Bomu gneisses and include mica schists with the gneisses. The geological history of these gneisses began with the deposition of the probably oceanic precursors cursors
of the Nzangi
of the Bomu basic gneisses
gray gneisses
at about
3.5
This was followed by high-grade tectono-thermal the
intrusion
of
B~r~m~ gneisses
tonalites
at
about
followed before
and the pre-
(Cahen et al.,
1984).
activity which ended with
3.41 Ga.
The
the deposition
emplacement
of
the
of the Ganguan greenstone
and schist belts about 3.2 Ga to 3.1 Ga ago. West N i l e Gneissic Complex
Included in this collective term are several basement gneisses which are poorly
exposed
through
from
the
Nile
Province
of
and
NE
Zaire,
presented below mainly apply to the better known parts
in West Nile and
(Hepworth,
into Central African
Uganda
The descriptions
Zaire
Sudan,
West
Republic.
NE
southern
1964;
Lavreau,
1980).
This
region
represents
what
were probably coeval continental areas during the deposition of the nearby
Kibalian
greenstones.
This
terrane
attained
granulite
facies
during
the tectono-~hermal episodes which subjected the greenstones to low-grade metamorphism.
However,
no
continental
basement
is
known
which
directly
underlies the Kibalian greenstones. Granulite
rocks
belonging
to the so-called
pre-Watian
the West Nile Province of Uganda and northeasternmost nockitic
dolerite
vertical
axial
intruded
by
dykes
planes
dolerite
and
are
trending dykes,
the
probably of volcano-sedimentary rocks
were
metamorphosed
characterized
E or ENE. parent
origin
at greater
by
Before rocks
isoclinal
they were
to
these
(Cahen et al.
crustal
assemblage
depths
of
Zaire contain charfolds deformed
granulites
with and were
1984). These parent to
granulite
facies
during the Watian tectono-thermal event at about 2.9 Ga. Charnockites developed at this stage. The Watian event was probably followed by the formation
of volcano-sedimentary
rocks
which were
later metamorphosed
into
the so-called western grey gneisses group comprising well-layered gneissic rocks which
are predominantly
blende gneiss with microcline.
upper amphibolite
The metasedimentary
facies
biotite-horn-
origin of the western
80
gray gneisses bearing
is suggested by the conformable
quartzites.
These
gneisses
extend
from
the northern part of the West Nile Province. pre-Watian
granulite
gneisses
exhibit
morphism
which
gneisses
NE-plunging are
dated
is, folds
with and
and sillimanite-
northeastern
Zaire
into
Their relationship with the
however,
2.68 Ga
fuchsite
uncertain.
steep
axial
assigned
to
The western planes,
the
and
Aruan
gray meta-
tectono-
thermal event. In the southern part of the West Nile Province and in the adjoining NE Zaire there are low met@morphic the eastern
gray gneisses.
gray gneisses.
grade rocks which are referred to as
These are probably equivalent
to the western
The remaining parts of the West Nile Gneissic Complex are
made up of other gneisses
which
are probably
equivalent
to the eastern
and western gray gneisses. Ganguan Greenstone and Schist Belt This occurs in several exposures mainly east of the Bomu-Uele confluence completely
detached
from
the
eastern
Kibalian
belt
(Fig.3.33).
In
the
northern part, the Ganguan rests unconformably on the Bomu mafic gneisses and
on
a
part
of
B~r~m~
gneisses,
with
which
they
share
NE-trending
folds. To the south, the Ganguan is unconformable on the Nzangi mafic and intermediate
gneisses
Lithologically quartzites
the
and
and
have
Ganguan
quartz
been
folded
comprises
phyllites,
(in
with
its
Nzangi
ascending
quartz-poor
talc
schist, chlorite schists, black schists, and phyllites. cut by gold-bearing quartz reefs. the
2.98 Ga
tectono-thermal
order):
sericite
schist,
sericite
The phyllites are
The Ganguan greenstones
event
which
affected
basement.
are older than
them.
However,
at
Matundu near the Central African Republic border, the Ganguan greenstones are
older,
having
formed
at
about
3.2 Ga
and
suffered
two
deformation
events, whereas only the 2.9 Ga-event affected other Ganguan belts. Northwest
of
the
Bomu
gneisses
Bandas and the Dekoa belts, have 1981).
been
correlated
with
the
The Bandas greenstone
predominantly (Fig.3.34). schists;
volcanic
are
two other
greenstone
lying in Central African Republic. Ganguan
belt,
unit,
about
and
an
greenstones
the
Both belts
(Poldevin
250 km long, upper
belts,
comprises
et
al.,
a lower
metasedimentary
unit
The lower unit consists of about 500 m of quartz-feldspathic
a middle unit of alternating tholeiitic
basalts and itabirites,
about 2,600 m thick; and an upper suite 1,000 m thick, composed of itabirites,
basalts and andesites.
The metasedimentary
sequence in the Bandas
belt consists of graywackes intercalated with acid volcanic tuffs, and is about
500
m
thick.
These
rocks
are
intruded
by
granitic
rocks.
De-
81
formation was
polyphase
tural
trends
and
about
150 km and
mostly gneissic
resulting
steep lies
plunges.
in isoclinal The
Dekoa
to the south of the
granitoids
with associated
folds and NWW-WNW
greenstone Bandas basic
belt
belt.
struc-
extends
It consists
rocks and banded
for of
iron-
formation which exhibit NW foliation.
A
GOUBADJIA
GOUMBROU
AOU ???
~-.~
~,,,~TA SEDIMENTARY
~q3
IAAA :a^^ AAA
D~
~s ~6
IAAAA a~AA A IAA A
B
~7
UNIT
i%% ^ ...... h,h,
I
:A A A A'~
!
:..÷.
l,,,7/.,,,,,k
;~'X'",<'£'~ ~ , m ÷,, \
0
,
0,5 km
\ I
Figure 3.34: A, Bandas greenstone belt. B, Stratigraphic columns for Bandas greenstones; i, granitoids; 2, tuffs; 3, basalt; 4, rhyolite sills; 5, andesites; 6, graywackes; 7, acid volcanics. (Redrawn from Poidevin et al., 1981.)
Kibalian
Greenstone
Although
exposed
in
Belts
several
Upper Zaire granitoid massif,
isolated
belts
(Fig.3.33)
separated
by
the
the Kibalian greenstone belts probably re-
present one previously continuous
greenstone terrane,
which has now been
82
separated into an eastern facies and a w e s t e r n facies. The eastern facies w h i c h o u t c r o p at Moto, of m a f i c
Ngayu and M a m b a s a
show a predominance
to i n t e r m e d i a t e volcanics while the w e s t e r n
facies which is ex-
posed
at
Sili-Isiro
less
mafic
rocks.
Zani, Kilo,
and
The
Tina
contain
Kibalian
is
mostly
further
banded
subdivided
more e x t e n s i v e lower Kibalian and an upper K i b a l i a n At
Moto
the
metavolcanics E to
ESE
lower
with
which
consists
of
comprises
trend.
have
mainly
been
Its upper age dated
volcanic
at
which
is
2.89 Ga.
superposed
on
into
a
regionally
(Lavreau, mafic
and
1984).
to
intermediate
and shows a p r e d o m i n a n t l y
limit
agglomerates
quartzites and banded ironstones. synform
mainly
some banded iron-formations
structural
tonalites
Kibalian
iron-formations
is
The
and
set by the
upper
intruding
Kibalian
meta-andesites
at Moto and
some
The upper Kibalian forms a N-S oriented the
structures
in
the
lower
Kibalian.
Granitoids which are dated at 2.5 Ga intrude the upper K i b a l i a n at Moto. At
Zani
the
sequence
lower
Kibalian
consists
lite
and
more
to the
of
chlorite
is
sericite
schists.
lower
than
A
similar
desires,
and amphibolites.
dacites,
Ngayu
mafic
river
to
is
talc-,
composition
Kibalian.
At Mambasa
comprises
metavolcanics
where
probably
at Kilo
The g r e e n s t o n e and
which
belonging
and
comprises
sequence to the south
consists
are
chlorite-,
of
probably
the lower Kibalian
(greenish
the upper
quartz-phyl-
and a m p h i b o l e - s c h i s t s as well as an-
probably monocyclic
intermediate
occurs
while
banded
assemblage,
Kibalian
chlorite-,
at Moto,
iron-bearing
greenstone
the upper
m o s t l y sericite-, at
to that
schists,
metavolcanics
part
of
is p r e s e r v e d
the
of
lower
the sequence
sericite-schists;
volcanic
tuffs) and banded iron-formations. The
Kibalian
Group
of
the
g r e e n s t o n e belt which consists amphibolites
resting
West
Nile
Province
belongs
to
of an a s s e m b l a g e of low-grade
unconformably
on
the
western
gray
the Adida
schists and
gneisses.
The
Adida g r e e n s t o n e sequence has been correlated w i t h the upper Kibalian. An balian
island
arc
tectonic
greenstones,
(Cahen et al.,
some
setting
of which
has are
been
invoked
believed
to be
to
explain
the Ki-
of oceanic
origin
1984).
Granitoids These are the m o s t e x t e n s i v e rock types on the NE Zaire craton,
covering
about five or six times more surface area than the schist and greenstone belts.
The
granitoids
the r e w o r k i n g ations
are
of m o n z o n i t e
of granitoids
mostly
orthogneisses
granites
(Table 3.2).
which
and tonalites.
were
There
The first g e n e r a t i o n
derived
are
from
two gener-
is dated at about
83
2.84 Ga and belongs to the Upper Zaire G r a n i t o i d Massif w h i c h consists of tonalites dated
at
with
diorites
about
2.46 Ga
and is
granodiorites.
the
most
The
abundant
second
and
group
consists
which
of
is
medium-to
c o a r s e - g r a i n e d quartz monzonites w h i c h intrude the first group. Gold Mineralization
The
granite-greenstone
gold
in
Zaire
since
terrane
of
production
NE
Zaire
first
began
has
been
a major
in
1904,
and
source
by
the
1980's the total output from this region had reached 350 tonnes 1984).
Half of the p r o d u c t i o n has come from placer deposits,
of
early
(Lavreau,
the extents
of which g e n e r a l l y do not overstep the limits of the K i b a l i a n greenstone belts. L a v r e a u
(1984) p r e s e n t e d an i n t e r p r e t a t i o n of the controls of gold
mineralization
in the NE
posits
also
occur
the g r e e n s t o n e s centration.
as
Zaire greenstones
a result
in w h i c h
Other
Such deposits
geologic
from
the d e v e l o p m e n t veins
were
which
had
nations units.
of
later
shear
across along
Quartz
veins
account
are
located
cated
on
a
the
supergene
Proterozoic
for more
zones,
three
concentration
or
planes
than half
belts.
within of
which
lineament
which
the
are
is
several
bearing,
mylonitic
zones
along
gold
Gold
to
quartz zones
impreg-
greenstone
primary
gold ex-
g o l d - b e a r i n g quartz subparallel
with
an
some of these are lo-
observable
which
of
shear
some
on
Landsat
(Fig.3.35). The n o r t h - s o u t h structure is a m a j o r shear zone with
of
north
linked
Au-bearing
tonalites.
for example,
of
are
Pan-African
w h i l e a fourth trends north-south;
major
weathering
and at Subani
enrichment,
and
greenstones foliation
In the Kilo belt,
along
the
in the K a b a l i a n
the
occur
ENE-WSW strike,
near Kilo
influenced
and
Early
also
tracted from NE Zaire. veins
which
zones
into
cut
tropical
gold de-
(Fig.3.33).
regoliths,
emplaced
pronounced
have been mined
features
placers,
the
Eluvial
supergene processes has also e n r i c h e d gold con-
of the M o t o g r e e n s t o n e belt
apart
of
(Fig.3.33).
quartz-veins,
images
(Fig.3.35, b)
sometimes
gold-
are a r r a n g e d in an "en ~chelon" pattern.
Gold also occurs
as impregnations
near the b a n d e d i r o n - f o r m a t i o n ho-
rizons in the v o l c a n o - s e d i m e n t a r y parts of the greenstones,
w i t h o u t being
related to quartz veins.
In such deposits the m i n e r a l i z a t i o n was probably
deposited
enrichment
by
syngenetic
by
the
co-precipitation
of
gold
to-
gether w i t h iron and silica, or as a result of the supply of gold through fumaroles.
84
A
\
7
B 4
Figure 3.35: A, Kibalian of Kilo. I, mafic; 2, andesitic; 3, amphibolites; 4, granitoids; 5, itabirites and gossans; 6, mineralized areas. B, N-S cross-section through Isuru-Kanga shear zone; I, amphibolites; 2, granitoids; 3, mylonitized rocks; 4, cataclazed rocks; 5, lamprophyre; 6, dolerite. (Redrawn from Lavreau, 1984.)
3.4 Tanzania Craton
3.4.1 Geologic Framework The
Tanzanian
craton
or
shield
Central Plateau of Tanzania,
(Fig.3.36)
largely
coincides
and consists of Archean granitoids,
with
the
gneiss-
85
es,
migmatites
schist
belts
Victoria
and
which
(Nyanza)
southeastern largely
extend and
Uganda.
cratonic
Proterozoic
irregularly
northwards
the
the
Late
subsequent
Pan-African
and
into
surrounding
Since
with
shaped
the
region Archean
tectonism
Mozambique
scattered
belt
eastern
of
greenstones borders
southwestern
this
being
region
has
confined
to
to the east;
the
Kibaran
and
southwest;
and
the
mid-Proterozoic
and
remained the
Late
the Early Protero-
the Ubendian
west
Lake
Kenya
zoic Usagaran mobile belt to the south-south-east, northwest
of
and
belt to
belt
to
the
(Fig.3.36).
m13 ~4 ~5
W, M. MIGOR|
GOLDFI£LD$
Mu,MUSOMA-
,o
g,,,
MARA
GOLDFIELDS
!
Figure 3.36: Geological sketch map of the Tanzania craton, i, Mesozoic-Cenozoic cover; 2, Bukoban and equivalents; 3, KaragweAnkolean (Kibaran) and Ukingan (in the south); 4, Buganda-Toro; 5, Mubende granite; 6, Granites, migmatites and gneisses; 7, Kavirondian and Nyanzian; 8, Dodoman; 9, Gneisses, etc, (Redrawn from Cahen et al., 1984.) The
Archean
terrane
of
the
Tanzania
shield,
African cratons cannot be readily delimited stone
and
granitoid
terranes.
Rather,
unlike
those
on
into separate gneiss,
there
are
distinct
schist
most
greenbelts
86
surrounded by a vast granitoid-migmatite-gneiss Bell
and
Dodson
Nyanzian
and
constitute
(1980),
the
there
are
Kavirondian,
three
schist
separated
the bulk of the Archean
terrane.
by
As summarized by
belts,
series
the
of
Dodoma,
granitoids
terrane of the Tanzania
the
which
craton.
Wide-
spread granitoid m a g m a t i s m occurred at about 2.54 Ga on the Tanzania craton, and assimilated material,
a lot of pre-existing
and granitic rocks
high-grade
(Cahen et al.,
rocks,
supracrustal
1984).
3.4.2 Dodoma Schist Belt In its type area around Dodoma the Dododma Dodoman
System
high-grade as
consists
metamorphic
supracrustals.
banded;
sericite
bearing rocks; tites
have
The
belt generally
schists;
strike
of the Tanzania
comprise
quartzites,
for
by the widespread (Gabert,
7,500 m thick, group
the
and pegma-
Dodoma
or steeply.
event
part of the Tanzania
craton and
The Dodoman System provided the
(Gabert,
1990).
The
Nyanzian,
of pillow lavas with
local banded
iron-formations,
agglomerates.
intercalated
ruffs
with andesitic
group
ruginous
overlies
slates
occur
iron-formation
and
tuffs near the top
the
whole
at the base of the
Resting base
of
the
phyllites Arkoses
Kavirondian
Kavirondian
with
some
with pebble beds
Kavirondiano
about
is a pelitic
fine-grained rocks
is another 1,500
to
sequence
sandstones
and agglomerates
The Kavirondian
upward
into
A slaty and an-
Tuffaceous
The Nyanzian
sub-acid
silty has
and
fer-
unit while
simple
folds
folding and thrusts.
on ~he Nyanzian System,
pass
slaty and andesitic
occurs near the top.
unconformably
the
These
(Fig.3.37).
succession.
and shear belts with local isoclinal
sequence,
about
occurs at the base, and consists of a basal mafic volcanic
lavas
banded
the rest
Schist Belts
volcanic group of rhyolites,
desitic
The Do-
(Table 3.2) and
followed by an intermediate-to-acid with
schist
1990).
greenstones
composed mostly
graywackes
and
and corundum-
Granites
of
of
as well
hematitic
2.5-Ga event which affected
occur in the northern
these
rocks
tectono-thermal
in western Kenya and southeastern Uganda. basement
locally
and talc-chlorite The
or the
exposures
and migmatites,
and hornblende gneisses.
the Watian
(Fig.3.36)
ESE-trending
ESE or ENE and dip v e r t i c a l l y
craton
schist belts
elongate
as granitoids
metasediments.
3.4.3 N y a n z i a n - K a v i r o n d i a n These
such
quartz-schists
the
formed during
was also affected
several
latter
amphibolites;
intruded
doma belt
of rocks
schist belt
of and
occur
volcano-sedimentary
3,000 m
thick.
At
the
slates,
mudstones
and
volcanics
(Fig.3.37).
in the upper part of the
have structures
which are
subparallel
87
to
the
Nyanzian
within
virondian
is
virondian,
ranging
cession,
which
simpler
than
they
the
are
infolded.
Nyanzian.
Structurally,
Granitoids,
in age from 2.7 Ga to 2.5 Ga,
for example
at Buteba and Masaba
mostly
intrude
in Uganda,
the
Ka-
post-Ka-
the above suc-
and at Mumias
and
Migori in Kenya.
Volconics
u
o
z~
IX D.
Gronites
I" \'I -I °
Felsic v olcanics
5
:">/:/A'>>/ i >/:> >>>>>>:
R
::-_-:::::Sediments incf uding shales
3yroclastic rocks :heroical sediments ~yroclastic rocks Ore Ore reLsic volcanics :hemico( sediments
Z W -r ,<
nterrnidiate to elsic volconics
L
U
c
vlafic volcanics
o
r
I":'::':~""::I
Figure 3.37: Schematic stratigraphy of the Geita area. (Redrawn from Kuehn et al., 1990.)
3.4.4
Gold
Mineralization
The greenstone for
syngenetic
(1990)
belts and
of
on
the
the Nyanzian
epigenetic
and Kuehn et al.
Tanzania
(1990)
gold
Precambrian
of
Craton
System are an
important
mineralization
provided an evaluation
host
rock
(Fig.3.36). Gabert of the geological
88
factors
controlling
(Table 3.3)
has
these
been
mineralizations.
made
from
the
Small
Nyanzian
but
steady
greenstones
production
in
Tanzania,
Kenya and Uganda over the years. Table 3.3: Summary of statistics on gold production in East Africa. (From Kuehn et al., 1990.)
Goldfield mine/prospect
Productive y e a r s
Production ( kg Au )
Mineralization type
Host rock
Migori
1933-1966
950
Buhemba
1913-1970
12170
Quartz reef
Mafic schist
Z
Kiobakari
1933-1966
8810
Quartz reef
Andinol rock
N Z .~ >. Z
Geifc, mines
193B-~966
27440
Sfratabound- stratiform
BIF, tufts
Buck Reef
1982- present
100
Quartz reef
Basalts
Canuck
1945-1953
230
Quartz reef
BIF
Mahene
1946-$956
15
Strata bourld -stratiform
BIF
Sekenke
1909-1956
4300
Quartz reef
Diorite
1935-1960
25000
Quartz reef
Basalt
1950-~960
2170
Quartz reef
Gnelsses, schist
UBENDIAN
t
Lupa Mpanda
Stmtabound - stratiform
~L~aI~, BIF
The Dodoma System has also yielded eluvial gold from its supracrustal rocks at the type locality where gold was derived from the weathering of gold-bearing quartz veins which may be related to late-tectonic
granitic
activities. In
the
Nyanzan
greenstones
most
of
the
known
gold-quartz
vein
de-
posits occur in relation to mineralized shear zones, but there is a rare case where significant gold mineralization occurs in a granite. three main types belts
(Gabert,
of primary gold occurrences
1990).
Strata-bound
in the Nyanzian
syngenetic gold deposits occur in the
sulphidic and carbonate facies of the banded iron-formations, companying
tuffs.
auriferous
pyrite,
pyrite
bodies,
veinlets.
Such deposits
as well
Another
are characterized
arsenopyrite,
There are greenstone
pyrrhotite,
locally
as gold- and sulphide-bearing
type of mineralization
and the ac-
by disseminated developed quartz
gold,
massive
and calcite
is the epigenetic-hydrothermal-
type in the form of tectonically controlled quartz veins or reefs, which occur preferentially belt.
This
gold,
pyrite,
of
gold
is
in carbonized mafic metavolcanics characterized
pyrrhotite,
mineralization
pregnations reef
type
wall
is
chalcopyrite the
of both the banded rocks,
and
by
following
of the greenstone
paragenesis:
and arsenopyrite.
epigenetic-metasomatic iron-formation
quartz-sulphide
rence is limited to tectonic
the
gold-sulphide
host rocks
replacements.
native
The third mode
This
im-
and the quartz type
of
occur-
zones and occurs mainly in the banded iron-
89
formation.
The
pyrrhotite,
quartz-sulphide
chalcopyrite,
replacement
arsenopyrite,
is
characterized
galena
and
by
pyrite,
sphalerite
para-
genesis. Ore banded
bodies
often
occur
iron-formations
formation.
They
preferentially
and tuffs,
also exhibit
along
or within
structural
the
contact
between
ruffs near the banded
control
in which
they
iron-
are con-
centrated in the fold hinges of small-scale folds which occur in mediumand
large-scale
folds;
and
the
positions
of
the
reef
orebodies,
which
often extend to considerable depths, are often controlled by zones of intense folding, at
their
fracturing and shearing in the greenstone belts especially
contacts
with
granites.
there was
lithological
laminated
sulphide-facies
the best places. formation
and
In
strata-bound
and stratigraphical of
the
banded
gold
control
iron-formation
tuffs
and
the
sulphide
the medium-
seemed
Gold-sulphide mineralization preferred
associated
mineralization
in which
to
offer
the banded iron-
ores
show
a
distinct
preference for the iron-rich bands. In the Migori goldfield in southwestern Kenya host
rocks
graywackes,
consist
of
mafic
conglomerates,
the form of auriferous
(1987)
quartz reefs and impregnations
pyrrhotite,
zones
the Maccalder
and mafic volcanics.
Gold occurs in
of host rocks such As shown by Ogola
up to 300 m long and 2 m wide,
and
faults
mine,
have
(Figo3.38, produced
B).
gold
Several
the Nyanzian
iron-formations,
The main sulphide ores
arsenopyrite and chalcopyrite.
the quartz reefs,
by shear
(Fig.3.36)
banded
shales and andesitic volcanics.
as the banded iron-formations are pyrite,
volcanics,
were controlled
small mines
from the Migori
including
greenstone
belt
(Fig.3.36)
pro-
(Table 3.3). In the Musoma-Mara duction
came
from
quartz
stratiform orebodies. canics,
and
also
even granites; ducer,
stratiform
hosted
with
1990).
gold
felsic
mineralized
shear
zones,
volcanics
and
banded
and
planar
iron-formation,
deposits
reef
quartz
reefs
constitute
most
of
the
In the nearby Geita-Kahama goldfields in
arsenopyrite,
quartz
Tanzania
and
and in the Buhemba mine, which used to be the largest pro-
iron-formations
pyrrhotite,
reefs,
in northern
The deposits are mostly associated with mafic vol-
shear-zone-hosted
(Kuehn et al., banded
goldfields
occur the
in the oxide form of
with or without
mineralization
also
and
sulphide
Fe-sulphide-rich chalcopyrite).
occurs
near
orebodies
strata-bound facies
bodies
of the (pyrite,
Mafic volcanic-
granite
intrusions.
The Nzega-Sekende
goldfields
have produced gold mainly from structurally
controlled quartz
reefs with strong associated wallrock alteration;
is hosted here by dolerite or mafic volcanics.
gold
90 4
'~P<."
,~.,.,.
~
'. I
i,
k
t'
' \"
^
A
~p
z /
:
" /%
~o
r
p
h~
.C
EXPLANATION '
NYANZIAN
SYSTEM
Porphyriticandesite i "]~ A Phy[lite tndescti,anedescti-dacctis . .
/ /
,~
~y~ r
'-.-, [-~2-?.~-k-'C..~..-...~ [] Banded ironstone " ~'>."2-'_~ ~ Sandstone ~ .~ I~ Metabasalt.
,, A
~
~
ROCKS
^' ~ Ouartz porphyry J;~ Younger dolerite [] Older amphibolized melanocratic doterite
^ +
INTRUSIVE
:::
+ "%,. +
~
x~...~.~~ [ ] Faurs
,, 3kO M
t
• Macalder mine
i NNE
SSW
I Coarse-mediumand fine grained sandstone Banded iron-formation Metabosaltin places stronglychtoritized Quartz porphyry Sulphide ore Faults
B
r
r
~ ~ON
rI%,r
J
r-
~r-k
Figure 3.38: G e o l o g i c a l sketch m a p (A) and section (B) area a r o u n d the M a c a l d e r Mine. (Redrawn from Ogola, 1987.)
of
the
3.5 W e s t A f r i c a n C r a t o n
The West A f r i c a n Rise
to
downsag,
the
craton
south
and
the T a o u d e n i
the north,
(Fig.3.39) the
basin
Reguibat
is exposed Shield
to
in two segments, the
lying in the centre.
east and west b y P a n - A f r i c a n
north,
with
The craton
the Guinea a
shallow
is bounded
to
m o b i l e belts.
Guinea Rise
3.5.1
An A r c h e a n known
nucleus
as the K a n e m a - M a n
of the West A f r i c a n Domain,
on the
craton
lies
southwestern
in a tectonic
corner
unit
of the Guinea
91
Rise,
otherwise
Kanema-Man
known
tectonic
as
the
domain
Dorsale
de
underlies
Man
an
Shield
area
of
c o v e r i n g n e a r l y all of Sierra Leone and Liberia, a small part of the western
Ivory Coast
about
150,000 km 2 ,
(Fig.3.40).
supracrustals,
to the east by the
fault zone which separates the K a n e m a - M a n domain from
grades
along
al.
(1985),
into u n d i f f e r e n t i a t e d basement
the G u i n e a - M a l i
and field descriptions, at about
and
It is bounded to the
the Early P r o t e r o z o i c Eburnean mobile belt; and in the north, gneissic b a s e m e n t
The
s o u t h e a s t e r n Guinea,
southwest by the Pan-African reactivated Kasila belt, Mt° T r o u - S a n s s a n d r a
(Fig.3.39).
frontiers.
and the
While
as presented in Cahen et al.
the Archean Eburnean
radiometric
have enabled the recognition of m a j o r t e c t o n o - t h e r m a l
2.96 Ga and 2.75 Ga in most of the granitic gneiss
in the g r a n i t e - g r e e n s t o n e terranes,
dates
(1984) and W r i g h t et events
basement and
the lack of such data in the northern
parts of the K a n e m a - M a n domain has h i n d e r e d the d e l i m i t a t i o n
of the pre-
cise n o r t h e r n extent of the Archean nucleus of the West A f r i c a n craton.
Granitic
The
Gneiss
basement
local names. the M a h a n a Gneiss
Basement
complex
in
Sierra
Leone
and
Liberia
has
not
But the adjoining parts in Guinea have been Gneiss,
east
to the west,
of
the
Simandou
supracrustal
and the Guinea Gneiss
been
assigned
subdivided into
belt,
the Macenta
to the n o r t h w e s t
(Table 3.4).
In Ivory Coast the basement complex, w h i c h is known as the M i g m a t i t e and Gneiss
Formation
has
two
higher-grade
(granulite
facies)
areas
referred
to as the Mt. Douan Formation and the Man c h a r n o c k i t e complex. In general gneisses (1985) facies.
formed
85 % of the K a n e m a - M a n domain comprises According
which vary
The from
in m e t a m o r p h i c
composition diorite
relicts
of
is
through
tonalite
basaltic
dykes
Coast
includes
noritic series,
a granite
occur
from a m p h i b o l i t e granodiorite,
granite. as
of
et al.
small
to granulite
but
there
Metamorphosed lenses
and
The Man c h a r n o c k i t e massif series
granitic
and W r i g h t
charnockites
is
and
fied in the Man charnockites. facies,
while
the
a
de-
sheets
of
of southwest
sensu
stricto,
an a m p h i b o l e - p y r o x e n i t e series, m a g n e t i t e quartzites,
lit-par-lit injection gneisses. granulite
to
(1977)
b i o t i t e and h o r n b l e n d e - b e a r -
grade
predominantly
a m p h i b o l i t e w i t h i n the gneisses. Ivory
to Bessoles
these are m o s t l y q u a r t z o - f e l d s p a t h i c
ing rocks range
at least
and migmatites.
a
and
Two phases of t e c t o n i s m have been identi-
During the first phase m e t a m o r p h i s m reached second phase
involved
cataclasis
and retro-
g r e s s i v e m e t a m o r p h i s m of the various units w h i c h contain the hypersthene gneisses
and granites
of the first phase.
in the h o r n b l e n d e - g r a n u l i t e
R e t r o g r e s s i v e m e t a m o r p h i s m was
and a m p h i b o l i t e
facies.
The Man
contain rocks w h i c h have been dated at 3.1 Ga. The main
charnockites
structural
trend
92
in the basement of the Kanema-Man domain is north to northeast, wards the north it swings to the northwest
but to-
(Fig.3.40).
SOOKm....
/
~
~
.~.,Ph:rLCREG SHIELD belt=sion Polycy¢lic central Hoggar Ai'r
DENt
'~ ~
Roket
ern
5,
'~~~
"';'"
~~"~_J~'PAN~r
AFRICAN TUAREG AND BENIN NIGERIA SHIELD
~
~
Precambrian undifferentiated (stabilized ~ "/30 Ha ) .] Upper Proterozoic Volcano- defrlfic formations (Phorusian } HERCYNIAN HAURITANIEIES WEST AFRICAN CRATON ~Polycyclic basement I I(mainly Lower Proterozoic) ~ 7 " ~ Mef°m°rph°sed Precombrian e ~ Cambrian ( Obosom Group) Lover Poteozoic m
1
~
Lower Proterozoic( Birimion}
]~Archeon
~ Vendion- Cambrian ( Supergroup 2 ) *..... z(Tiririne Group in Tuareg Shield) I Upper Proterozoic ( Supergroupl )
[]
Middle Proterozoic (Tarkwaion, Guelb el Hadid Group}
Figure 3.39: Schematic Precambrian (Redrawn from Black and Fabre, 1983.)
geology
of
West
Africa.
Greenstone Belts
In
the
Kanema-Man
domain
relicts
of
greens)one
belts
occur
mainly
in
north to northeasterly trending synformal structures which are surrounded by
basement
granitoids. and
granitic
gneisses
and
Based on their sizes,
metamorphic
domain
are
Sierra
Leone,
grade,
divisible and
Leone,
Liberia
western
part
an
and
the
into
part
southwestern
is characterized
stratigraphic
supracrustal
regionally eastern
autochthonous
a
which Ivory
and
thicknesses,
sequences western occurs Coast
by larger typical
paratochthonous
of
part in
lithologies
the
Kanema-Man
covering
most
southeastern
(Rollinson, greenstone
Sierra
1978). belts,
of The
up to
Ultramofic Schist
Bgnded Iron~torrn~tion Amphibolite
)Sula
IountOinS
~
Pan African Rokelide Belt
~
I
Hill
(5)Bctgb 0lo
deposits
Gneiss, Migmot ite, Gronulite B~sement (with fohotion trends; Man Chornockite Complex
",,<<.
~omDul HIllS
\~
.............
__ /rn:moro.iOo bell (~ Suprocrustol ~[ts(k= kasilo belt /
Liberion Granites
M~nly Tertiary Gnd Younger
3)Gori Hills
~
))South ~imini Hiffs
"-----2
"------2 -__---
-_---
-_---
~igure 3.40: Schematic geologic and mineral Redrawn from Wright et al., 1985.)
~
Peltic Schist
Ouortzite
EG E NO
ooooooo
o0 0%°0%%% °o %0 ~ .... o%OoOoOo, ~
t. - ' ~ l ~
200kin J
"J
/
Hills
\
', "
Ronge
(9) GOe
.o~.toi°s
of West Africa.
Ronge
(8) Bong
I
nucleus
(7)8omi
~
map of the Archean
,,,Mooo River Mine
'.
I ~,~
~..., / / /
".: /
3
94
130 km long with successions part
are ultramafic-mafic
and vesicular schists,
textures,
with
as quartzites
amphibolites
occasional
hibit
features
occasional
mica
and rhyolites.
greenschist
to
folding
and
faulting
are metasediments
schists,
metacherts,
quartzo-feldspathic
such meta-
schists,
showing the
and subordinate amounts of metavolcanics
The western greenstone
epidote-amphibolite
facies
and chloritic
fuchsite-bearing
weakly metamorphosed graywackes
of turbidites,
almandine-amphibolite intense
fuchsite,
cordierite-garnet-schists,
minor banded iron-formations, with dacites
and
In the lower
and amygdaloidal
to serpentinites
In the upper part of the sequence with
conglomerates, typical
(Fig.3.40).
lavas and sills with pillows
now metamorphosed
interbedded
metasediments.
up to 6.5 km thick
developed there
near
are
belts
(Fig.3.40)
metamorphic intrusive
rapid
and
facies
contacts.
irregular
exwith
Due
to
changes
in
lithology and thickness within the western greenstone belts.
Table 3.4: Correlation of the Archean of West Africa. (From Wright et al., 1985.) Event and approximate age
Liberian, 2.75 Ga
Sierra Leone
Guinea
Liberia
Ivory Coast
deformation and metamorphism of supracrustals, basement reactivation and granite intrusion Kasita Group Rotokolon Fro.
(U,) Broadly
(metased.)
contemporaneous deposition of supracrustal sequences
Morompg Group (L.) (U.)
Loko Group
Simandou and Beyla
Nimba Group
Tonkolili Fm. "1 x (metased.)
Sula Group (L,) unconformity Leonian, 2,95 Ga
Mototo Fro, (metavolc.)
M'L Gao Formation
Groups
t
Sonfon Fro. (metavo~c.) Macenta Gneiss, Migmatite and Mahana Gneiss Gneiss
gnelss-mlgmatite-granulite basement
pre- Leonian, 3.1 Ga and older
and Guinea Gneiss Formation Pit Douan
not identified but probably present
Formation and Mancharnockite
complex
The eastern greenstone belts are generally smaller, long, banded grades.
with
thinner
stratigraphic
iron-formation, The
with
paleogeographic
less
successions greenstones,
implications
of
only up to 40 km
(Fig.3.40) the
and
higher
above
dominated
by
metamorphic
regional
facies
95
distribution
is
shallow ensialic in a deeper, al.,
that
the
shelf
basins
less
eastern
stable
greenstones
whereas
and
probably
the w e s t e r n
developed
facies
ensimatic
geosynclinal
This
in
were
setting
in
deposited (Wright
et
1985). l.Kanema
Greenstone
Belt.
belt
type area of the K a n e m a - M a n domain; granites,
acid
schistose
sediments
gneisses, and
have been r e c o g n i z e d amphibolites
with
formations
and
comprising
a
granulite
facies
volcanics.
Two
in the area,
subordinate
a
younger
lower
The Loko G r o u p
rocks
namely,
greenstone
metavolcanic
Sierra
Leone
and g r e e n s t o n e
separate
is
the
suites
of
belts
of
greenstones
an older Loko Group comprising
serpentinites,
quartzites
succession,
formation,
upper sequence of metasediments,
tectono-thermal
central
it consists of a b a s e m e n t complex of
the
the
and banded Kambui
Sonfon
iron-
Supergroup
Formation
and
an
the Tonkolili Formation.
supracrustal
greenstone
event dated 2.96 Ga.
belt was
affected
by a major
During this event w h i c h
is known as
the Leonean event, the g r a n i t o i d basement on w h i c h the Loko G r o u p was dep o s i t e d was
metamorphosed
to the granulite
stones were m e t a m o r p h o s e d structures tight
and
minor
fabrics which
folds
with
facies while
into amphibolites
an
formed during
axial
planar
the
Loko green-
and serpentinites. the Leonean
schistocity.
East-west
event The
consist
Leonean
of
event
t e r m i n a t e d with the formation of autochthonous g r a n i t o i d s w i t h K - f e l d s p a r porphyroblasts. The
Kambui
tectono-thermal dominant
unit
Supergroup event. in
serpentinitic amphibolites
with occur
iron-formation morphosed
to
The
near
horizons
nounced
in
resulted formation
and
low-pressure
a
thick
the
underlying
It
6 km
thick,
comprises
sequence
of
this
top.
The
unit
banded
Group.
ovoid-to-ellipsoid
Northerly
the
pre-
ultramafic and
massive
facies
was
meta-
during
the Kambui
the
Supergroup
however,
structural
For-
clastic with
Supergroup
and synclines w h i c h are,
Loko
is
Leonean
Tonkolili
is m o s t l y
Kambui
the
basal
In the o v e r l y i n g
but
During this event
from the L i b e r i a n event. of
about
after
greenschist-to-amphibolite
2.75 Ga.
into anticlines
the
Group,
the base, near
accumulated
Supergroup.
some p i l l o w horizons.
L i b e r i a n event at about was d e f o r m e d
Sula
Kambui
extrusives
mation
tuffs
the
supracrustals
more pro-
orientations
The L i b e r i a n event t e r m i n a t e d with the
granitoids
which
are
elongated
parallel
to the L i b e r i a n foliation trend. 2. Kasila Domain. A belt of granulite facies rocks, extends along the coast from Liberia to Guinea
the Kasila Group,
(Fig.3.40).
F u r t h e r inland
are related d i s c o n t i n u o u s greenstone belts b e l o n g i n g to the M a r a m p a Group
98
(Fig.3.41,
inset).
The
lower
part
of
the
Marampa
metamorphosed basaltic and andesitic volcanics phyritic
and
ultramafic
flow
structures
intercalations
are
preserved
(Wright et al.,
gneisses grade
and
quartzite,
1985).
banded
metabasic
quartz-schists,
iron-formation.
igneous
rocks
mica
of
largely
serpentinized
The upper part is metafuschsite-quartzite,
schists,
The Kasila
(now
consists
in which pillows and por-
with
sedimentary with pebbly and cross-bedded quartzites, manganiferous
Group
Group
pyroxene-bearing
garnet
schists,
consists
of high-
basic
granulites)
with metamorphosed and dismembered remnants of large layered anorthositic and gabbroic long. and
The
rocks,
Kasila
now occurring as lenses up to 200 m thick and 50 km
also
contains
aluminosilicate-bearing
equivalents
of banded
minor
rocks
quartz-magnetite,
which
iron-formations,
represent
marbles,
quartz-diopside
highly
metamorphosed
and pelites
respectively
(Wright et al., 1985). Although similar
the
Kasila
to greenstones,
Group
is
lithologically
Wright et al.
(1985)
and
stratigraphically
interpreted
it as the root
zones of nappes which formed as a result of Alpine-Himalayan-type
conti-
nental
a West
collision
African
and suturing of two Archean continental
cratonic
plate,
and a South American
(Guyana)
plates,
plate
(Fig.3.41,
B). The higher metamorphic grade of the Kasila Group, and the occurrences of
layered
anorthositic
rocks
(suggesting
deeper
crustal
levels),
are
suggestive of deep crustal processes which perhaps took place during subduction and plate collision. (Fig.3.41)
Collision and thrusting of the Kasila Group
are also suggested by the predominance of NW-SE-trending foli-
ation with consistently moderate dips to the southwest; of a northeastern dip
at
boundary
moderate
thrusting
angles
zone of sheared gneisses to
onto the granitic
the Marampa
the
southwest,
basement°
Group probably represents
Kasila
belt
(Fig.3.41).
tures,
low-angle
lying granitic
This
nappes
is suggested
thrust-faulted
basement
According
contacts
and the presence
and mylonites which
suggesting to Wright
medium-angle et al.
that were derived
by the
recumbent
of the Marampa
and the lower metamorphic
from the
fold
with
(1985) struc-
the under-
grade which
increases
from greenschist in the east to amphibolite in the west. After mylonite was
they were suture
reactivated
amalgamated
with
zone in Late Archean as the
internal
the West times
African
craton
(Figs. 3.41, B),
zone of thrusting
along
the
this suture
and ductile
shearing
during the Late Proterozoic - Early Paleozoic Pan-African tectono-thermal event
(L~corche et al., 1989; Williams and Culver,
1988).
97
~.Grou p " ~ \ ~ . : . ~
/
....
2
"
~o.v.,,,
-
;'::
....
/
'
/ /
- .......
'
" ~ , - \ / . t
"
KEY ~";'IPOST ARCHAEAN ~ THRUSTS AND MYLONITES ROCKS [771ARCHAEAN BASEMENT M MARAMPA GROUP B
(I) GUYANA FA'UELO
WEST AFRIfAN CRATON
A
($1
KASILA
7,
GROUP MARAklPAG~OUP
> /- , ' / ~
-
2;/
WEST POST ROKELITE COVER
MYLONITE ZONE RRG
~z O
ROKELITE TECTONIC AND 0,. REACTI YAT~ON tu
--
.~ LIMITED RIFTING
~
¢<
EAST ~gXl ONV'A". ~
G I~,l'f,
NG
III I :ROKEL R I y E R GROUp ( folio
----
AMALGAMATION OF ARCH.AEAN
TERR,NES I
Figure 3.41: A, tectonic model for the Kasila and Marampa Groups; B, time-terrane evolution of Precambrian of Sierra Leone. (Redrawn from Wright et al., 1985; Williams and Culver, 1988.) Under Marampa
the
Groups
mylonitization.
Pan-African suffered
tectono-thermal retrograde
regime
metamorphism
the
Kasila and
and
the
penetrative
98
3.Other
Greenstone
supracrustal
succession
migmatite-gneiss emplacement
of
In southwestern
Belts.
was
basement
eruptive
described with
by Papon
arkosic
volcanics,
Ivory
Coast
(1973)
who
precursors,
basic
a
dual-cycle
identified
followed
laccoliths,
sills
by
and
a
the
flows,
which were transformed to various amphibolites and pyroxenites of the Mt0 Douan charnockitic gneisses and leptinites
(Table 3.4). Then followed the
deposition of a volcano-sedimentary sequence, the Mt. Gao Formation which were
metamorphosed
into
amphibolites,
amphibole-pyroxenites
pyroxenites and folded into tight synclines. was
the
syntectonic
emplacement
of
and
garnet-
During the later event there
porphyroid
and
migmatitic
granites
which were succeeded by post-tectonic gneissic granodiorites. The metavolcanics of southeastern and
banded
and metasediments
of the Simandou
Guinea contain amphibolites,
ironstones.
On
the
nearby
biotite
Nimba
greenstone
schists,
mountains
in
Liberia
greenstone belt includes an important sedimentary iron deposit. sedimentary
rocks
of the Nimba Mountains
in the
belt
quartzites
Ivory Coast
the
The metacontains
a
lower group of basal conglomeratic quartzites and intercalated calcareous and
pelitic
group
sediments
comprising
iron-formations. to
the
(Black
phyllites
and and
Fabre,
itabirites
The Nimba greenstones
surrounding
basement
1983).
complex
There
with
is also
an upper
intercalated
silicate
are weakly metamorphosed upon
which
the
compared
greenstones
are
probably unconformable. 3.5.2 Archean Mineralization on the Guinea Rise According to Wright et al.
(1985) several factors account for the limited
range of Archean mineralization in the Kanema-Man province, the Zimbabwe province, andesite-rhyolitic limits
volcanics
the prospects
the generally the polycyclic
compared with
for example. These include the relative paucity of among
the
supracrustals
for base metal mineralization
higher metamorphic
which
(Cu,
Pb,
grade of the surrounding
nature of this province which probably
therefore Zn, As-Bi);
basement,
accounted
and
for the
loss of potential ore-forming elements during the passage of volatiles. Gold has been mined
from placer deposits around the greenstone belts
and from gold-quartz
veins
dwindling
from
production
interest
has
ruginous
schists
shifted
to
in Sierra Leone and Liberia alluvial the
mining
primary
(metamorphosed
of
source
banded
river in
(Fig.3.40).
gravels,
amphibolites
iron-formations),
Iron deposits
are the
principal
which
Archean
lie mainly
mineral
in the
resource
eastern
of West
and
where
quartz veins occur sometimes containing pyrite and arsenopyrite, maline.
fergold-
or tour-
greenstone
Africa,
With
commercial
coming
belts from
99
Liberia,
Guinea
reserve
and C6te d'Ivoireo
of
over
4.4
Riverhills,
Bomi
Hills
billion and
Liberia,
tonnes
at
a leading
Nimba,
Bong-Zaweah
Wologisi-Bea
(de Kun,
1987).
grade is about 65 % Fe and ranges from 35 to 70 % Fe. at
Nimba,
the
banded
iron-formation
is
up
to
formation
Cretaceous grading
and
of
weathered reserve
of
earlier
the
ore is
peneplains
zones
also
Simandou-Nimba
region is
80 km.
Other deposits and
1,600 m
itabirite
vast,
ironstone
quality
about
over
where 350
-
1,300 m
above
from
(Fig.3.42, B).
banded the
at
Nimba
at
billion
grade
1,000 m
is
thick
thick sea
to
40
-
tonnes, also
ore
(Fig.3.42).
level
level, caused
with
Mano
average
Deep w e a t h e r i n g during
above
Fe
Mts.,
The
the
supergene
up-
Fe
in
the
of Guinea
the
especially
about a
in
68 %
In the n e a r b y R e p u b l i c
2.5
the
sea
38 %
holds a
In the largest mine
500 m
M a g n e t i t e and h e m a t i t e are the main ore minerals. the
producer,
65 % Fe.
strike
in
the
Here
length
of
the
about
occur in C6te d ' I v o i r e and Sierra Leone where both
reserves
are
economic.
Archean
itabirite-type
posits also occur in the Imataca complex in V e n e z u e l a
iron
de-
(Fig.3.42, A), thus
s u p p o r t i n g the link between the Guyana and W e s t A f r i c a n c r a t o n s i n Archean times.
3.5.3 R e g u i b a t S h i e l d The R e g u i b a t
shield is the n o r t h e r n
part of the West A f r i c a n
is bounded to the south by the Taoudeni i n t r a c r a t o n i c basin,
craton.
It
to the north
by the Tindouf basin and to the west by the West A f r i c a n p o l y o r o g e n i c mobile
belt
(Fig.3.43).
Like
the
Kanema-Man
shield
to
the
south,
the
R e g u i b a t shield is composed of A r c h e a n rocks in the west and centre, with Lower of
Proterozoic
the
Reguibat
Ghallaman
Group
metamorphic morphic
es, and
east, of
granulites;
granulites,
quartzites.
known
comprising
consists
amphibolites;
predominating
to the
unit
unit
rocks shield,
beryl-tourmaline
has at
are
east.
The A r c h e a n
Group
in
the
(from
cut by
pegmatites.
biotite
quartzites.
The
part
of
Reguibat bolites,
the
marbles,
of biotite
quartzites
absent
Archean
rocks
from are
which the
are
central
the
and a
The
meta-
pyroxeno-
ferruginous anorthosites
granite
and by
10 km thick.
includes abundant
amphibolites,
represented
and migmatites.
and
is about
shield the A r c h e a n gneisses,
and
sillimanite-gneiss-
by metagabbros,
The Amsaga a s s e m b l a g e
are
garnet gneisses,
charnockitic
small massifs
muscovite
ferruginous
shield
shield
and
granulites.
top):
intruded
basement
west
typical
to
amphibolites,
are
In the central part of the Reguibat leptynites,
base
bottom
the
into a migmatitic,
p y r o x e n e - a m p h i b o l i t e rocks,
assemblages
and
in the
the Amsaga
been d i v i d e d
biotite gneisses;
These
serpentines
as
marbles
and
typical
of
the western
part.
In
the
by
leptynites,
eastern amphi-
I00
LEGEND
°'°
-~as///
Monrov~
~/~ J
[~/~O
/7
~
.
.
/7
.
//"
BERIA
•
//(
///
ly,o;¢bo%C- --"~, S ~
NIMBAaL, BERIA
/ / 7/
~',LI
4 ~21~ I
- NW-
Y////~Precam briQn shield t"%--'~Reg;ohal strike 0 Itabir;tes ( h e m a t i t e -
E-
• I 2 3 /, S E
Semi Hills Mane river Bong range Nimba Simctndou Maramp a
Cementation arc Alumina-rich
~
quartz schist )
High- grsde ores of metamorphic differentiation High-~llrQde ores of ait~ratJon
!
A
Zone A high- grcldz C, usta! art 63"6S°1. F'e
446
a
A6
~':':..":'- "
a, 4
"
~"
~ I
*as~**
.v..-
Octrita| i t a ~ ~ o f t / //~r~/ ,,Jurnina~rich.///,E~c.,a ,cy/,/p'*~/!_~
• '~, 6 *
':;"
'2;
~
jlO0 Km I
Hard ore
~
l|ahil~te
[----] Soft ore
~
Schist
Itabiritc mica+ ;mphibnlc
mztamorphlc dcffercnti0~ion 63-68%Fc
Figure 3.42: Archean iron mineralization (itabirite) on the West African craton and the Imataca Complex in Venezuela (A). B, supergene enrichment of Nimba itabirite (Liberia). (Redrawn from Hutchison, 1983.) The
metamorphic
amphibolite are
abundant
phyritic
facies in
biotite
yielded
ages
whereas
the
grade and
the
eastern
granites,
indicating granites
Eburnean reactivation 1.5 Ga.
varies
decreases
are
from
low
towards
zone
and
granodiorites a
major about
pressure
the
include
old
facies
Syntectonic
two-mica
and diorites.
tectono-thermal 2.4 Ga
granulite
east.
(Black
granites,
por-
The gneisses
event and
around Fabre,
to
granites have
2.7 Ga, 1983).
produced biotite ages falling in the range of 2.0 -
101
~LnJ~
HAURITANIDES HERCYNIENNES ~
Noppes post-Frosniennes comprenonf ~ du sode remis en mouvement PALEOZOI(1UE
Si|urien, Devonien,Csrbonitere ~]~ tillite de I'0rdovicien terminal
~Group
~ ~
s~-~lTremadoc -Arenig ~ ~
UU
n~NK
Group de (Assabet el Hossione (=gres de BirAmrane etde Cheikhio) Group ci'Atar a recifs stramatoliflques (Adrar Khar Honk el Thiethyale) de Char DORSAL/: REGUIBAT
Granites intrusifs et microgranifes rhyolifes associes Series volcano-sedimentaires d'lmourene. Aguelt tekhneig et Aroun Abd el Halek erie d'Aguelt Nebkhn ef granite du Yefti. Series onciennes(Amsaga Tosiasf, Ghallaman, Chegga) ef gronife.s associes
Figure 3.43: Geologic sketch (Redrawn from Bessoles, 1983.)
map
of
the
Reguibat
shield.
102
3.6 Other Archean Terranes in Africa
3.6.1 East S a h a r a n Craton Sometimes craton
r e f e r r e d to as the Nile craton
(Cahen
et
al.,
1984)
is
(Rocci,
a poorly
1965),
exposed
e a s t e r n Sahara, w i t h the Uweinat basement inlier Archean
nucleus.
Vail
(1988a)
furnished
a
A)
which
G n e i s s i c Complex
may
be
the
may
include
Dafur p r o v i n c e are exposures
in
the
synthesis
on
the
largely
Sudan and Chad Republic
extension
of
the
West
Nile
(Fig.3.33) w h i c h stretches from eastern C a m e r o o n through
the Central A f r i c a n Republic, which
northward
block
(Fig.3.2) as the exposed
undated but suspected Archean terrane in western (Fig.3.44,
the East Saharan
cratonic
Archean
of w e s t e r n
northern
age
are
Sudan.
Zaire to Uganda.
present
Around
of biotite gneisses with
in
Ancient
eastern
Jebel Marra
Chad
gneisses
and
(Fig.3.44,
flaggy quartzites,
in
A)
there
migmatites
m y l o n i t e s of unknown age which are referred to as the Gneiss Group; are p r o b a b l y older than the nearby M i d - P r o t e r o z o i c metasediments,
the and
these
and may
be c o m p a r a b l e to the Archean rocks in the Uweinat inlier. Archean
rocks which
have
been
suspected
in other parts
of the Sudan
include the Older Plains Group in the northern part of the Sudan basement comprising into
gneisses,
the West
among
the
Nile
oldest
schists Gneissic
rocks
in
and
quartzites;
Complex the
of
these
Zaire
Sudan
are
and the
extend Uganda.
along
Sabaloka
basement
Nile and
Blue Nile valleys.
basement Archean
the
River
inlier;
Nile;
rocks in the Sudan, basement
complex
the
and most
of
These
Also
charnockitic
gneisses w h i c h occur in the Imatong and Acholi Mountains; Gneisses
uninterrupted
granulite
facies
of the basement
the Fundamental gneisses
gneisses
suspected Archean
included granulite
in
the
west of the
- Early
Proterozoic
like the West Nile G n e i s s i c Complex and the southwestern
Nigeria
and
the
Bur
region
of
Somalia, were t h o r o u g h l y reworked during the Pan-African orogeny. Jebel
Uweinat
Archean
charnockitic
(Oweynat)
basement
granulites
are
exposed
inlier in northeast Africa
of the Libyan,
Egyptian and Sudanese frontiers.
prises
and a m p h i b o l i t e gneisses,
biotite
charnockitic
gneisses
(Cahen
et
al.,
the
Jebel
Uweinat
(Fig.3.44)
on
at the
junction
The Uweinat basement com-
m i g m a t i t i c biotite
1984).
Paleozoic
m e n t a r y rocks u n c o n f o r m a b l y overlie the Uweinat basement, of
syenites,
granites
and other
igneous
rocks
intrude
gneisses and
- Mesozoic
sedi-
while a variety
both
basement
and
sediments. A m o n g the basement rocks is the Karkur M u r r series, a group of pyroxene
granulites
containing
charnockitic,
noritic,
and
diopsidic
103
.... ....../:,.~-.--
( k ZAmE
~
~:~/
X'%cR-o.<\ )~_:(
A I
~o
u
F
\
/
4F
o1
16°
soo km
~
F
o ,~Oualian ~'1~ l~,Edlll_ I \
/
/IES(\-'q
1
$/
:
I~G o.
r
.
B
~7
Figure 3.44: Archean components of the so-called East Saharan craton. A, Sudan; B, Tuareg Shield. (Redrawn from Cahen et al., 1984; Vail, 1988a.)
104
g n e i s s e s and metaquartzites. series Murr
retains
gneisses
facies.
remnants have
Although folded on E-W axes,
of recumbent
undergone
A mylonite
zone
folds w h i c h
retrograde
separates
the Karkur Murr
strike
metamorphism
the Karkur Murr
N-S.
in
The Karkur
the amphibolite
charnockites
from the
s u r r o u n d i n g migmatites. The g r a n u l i t e at
2.9 Ga,
and
nockitization
facies m e t a m o r p h i s m is
in
therefore
Africa,
nockites in Cameroon; NE A n g o l a
shield;
of
within
for
example
the Karkur M u r r the
phase
the
of
formation
has been dated
widespread of
the
char-
Ntem
char-
those of the Kasai-Lomami a s s e m b l a g e in the Kasai -
and the pre-Watian granulites
on the Zaire craton.
of the West Nile Complex
The timing of retrograde m e t a m o r p h i s m around 2.63 Ga
in the K a r k u r M u r r charnockites agrees with the same event in these other g r a n u l i t e rocks of the Zaire craton and reinforces the v i e w that the East Saharan
craton
is a c t u a l l y
a northern
continuation
of
the
Zaire
craton
(Fig.3.44 inset).
Tuareg Shield High-grade
Archean
(Ahaggar) M o u n t a i n s of the w e s t e r n called
East
Tuareg
Saharan
(Cahen et al., The
occur
shield, craton
In O u z z a l - I f o r a s
(Fig.3.44,
B)
thinning
des
the
Tuareg
shield
(Fig°3.44,
or
Hoggar
in the In O u z z a l - I f o r a s
and in the O u m e l a l e n - T e m a s e n t
occupying
domain
B)
of
the
eastern
the
is a n a r r o w elongate
full width
towards
Iforas.
of
the
the M a l i - A l g e r i a n
in a w e s t e r l y d i s p l a c e d
the A d r a r
in
part
and the so-
Tuareg
shield
1984).
the north, pearing
terranes
of central West Africa,
Archean
Tanezrouft-Adrar frontier,
block which widens
granulites
in
submeridional
this
and
to the region
block
zone
in
then re-ap-
south through were
later af-
fected by the Early Proterozoic Eburnean o r o g e n y during w h i c h they underwent lower grade metamorphism. O r i g i n a l l y included among the surrounding P a n - A f r i c a n amphibolite and greenschist
Suggarian
rocks,
the
In Ouzzal
granulite
facies
rocks
were
later found to have N-S shear zone tectonic contacts w i t h the Pan-African rocks. These A r c h e a n granulites occur as high level nappes which were emplaced
from
Proterozoic complex
of
potassium
SSE
to
sediments
consists
quartzites, lenses
the
of
a
leptynites
NNW
direction
before
the
deposition
1978).
The
In Ouzzal
(Boullier
et
range
aluminous
and
pyrigarnite augen-gneisses
of
marbles
and are
al.,
commonly
lherzolite. the
metapelites,
oldest
banded
associated
Gray
gneisses
rocks
in
the
with and
of
Late
granulite magnetite norite
and
associated
region,
dated
105
3.48 Ga. Interstratified within the metasediments of the In Ouzzal granulite
complex
are
lenticular
bodies
of
orthypyroxene-sillimanite
granu-
lites varying in thickness from 100 cm to 150 cm. These Ai-Mg-rich rocks, according to Keinast and Ouzegane
(1987), have undergone widespread high O
pressure
(10 Kbar),
crustal were
depths
later
orogeny
of
high temperature up
affected
to by
35 km,
O
(750
between
lower pressure
C - 805
3.3 Ga
C), metamorphism
and
2.9 Ga.
metamorphism
(2.1 Ga) during which other granulites
These
during
the
at
rocks
Eburnean
formed in the region.
3.6.2 Madagascar The Antongilian which
are older
of Madagascar
than
3.0 Ga.
Archean
exposed
the
gneisses
the Masora Group to the south, the granite-migmatitic in
gneisses
were metamorphosed
these
younger
rocks
migmatites
belts
comprise
(Ambodiriana
granodiorites
Antongil
in the interior of Madagascar
outcrop ever,
the
on
(Cahen
et
3.48 Ga
mostly
granitoids
migmatites).
and quartz
al.,
at about
The
oligoclasolites
terranes
and the more
potassic
ridges
are
comprises
of gray gneisses
Tamatave,
the anticlinal
which
(Fig.3.45)
It consists
massif
north
of
facies on
and the cores which
1984).
The
(Cahen et al., (Antongil
granitoids
Antongilian 1984).
granites) range
Howand
between
on the one hand and calc-alka-
line, sometimes potassic leucocratic granites on the other. Granodiorites predominate followed by tonalites, diorites and amphibolites.
3.7 Archean Tectonic Models
3.7.1 Classical Models Since
some
greenstone
of
the
earliest
and
models
belts were actually developed
chean of southern Africa, models
tectonic
thence
for
the
origin
from investigations
of
Archean
of the Ar-
it is pertinent to briefly review some of these
cursorily
examine
some of the prevailing
views
about
the origin and development of Archean terranes especially in the light of plate
tectonics.
Condie
(1981),
Discussions Nisbet
on Archean tectonic models
(1987),
Kr6ner
(1989),
and
in
can be found in many
other
Pre-
cambrian research papers which are devoted to this controversial topic. According to the classical or downsagging basin model al.,
1969; Viljoen and Viljoen,
1969; Glikson,
(Anhaeusser et
1972), Archean greenstone
belts were initiated along fractures in downwarps at the boundary between thin
continental
crust
and
oceanic
crust
or
in
parallel
fault-bounded
106
troughs with
or rifts
the
the basin into
in the primitive
accumulation to downsag
the
crust
stones
in
caused
the
and
tonalitic
its
compressional
characteristic
synformal
major d e f i c i e n c i e s
crust.
Basin d e v e l o p m e n t
which
beneath the heavy volcanic
diapiric around
sialic
of volcanic m a t e r i a l granites
margin.
The
deformation shape
of
as
pile.
rose
and
intrusion
of
the
Archean
it thickened As
the basin
invaded
of
the
tonalitic
greenstone
greenstone
basin
belts.
of the classical or d o w n s a g g i n g model
began caused sank
greendiapirs
into
the
Among
the
are its failure
to explain the origin of the extensive high-grade g n e i s s i c terranes which surround
the
granite-greenstone
belts
and
the
fact
that
the
high-grade
terranes w e r e not considered as the p o s s i b l e b a s e m e n t to the greenstones. As shown in the southern part of the Barberton belt the gneissic terrane was
the
Viljoen
sialic
basement
and Viljoen,
Zimbabwe province,
to
1969;
the
greenstones
Glikson,
1972),
(Anhaeusser
as was
the
et
case
al., also
1969; in the
and on the Tanzania shield.
gene volcanlcs '.oceous-Tertiary Intrutions MORON
taceous volconics erior Basins {Neogene} roo- Quaternary
ECAMBRIAN drarona (Devonian?) ver- middle Proterozoic :hean (Antongilian)
Figure 3.45: Hottin, 1976.)
Geologic
sketch map
of Madagascar.
(Redrawn
from
107
3.7.2 Back-arc-Marginal Basin Models Of the many subduction-related posed
to explain Archean
the island-arc
1987; Windley,
lar
modern
calc-alkaline Archean
1984).
island-arcs volcanics
greenstone
(Fig.3.46)
and
granite-greenstone
subduction model has been the most popular
Nisbet, to
plate tectonic models that have been pro-
granulite-gneiss
where
(Condie,
Archean granite-greenstone
in
their
basal
low-K
belts, 1989;
belts are simi-
tholeiitic
volcanics,
and in the chemistry of their tonalitic plutons.
belts
are
similar
to modern marginal
the volcano-sedimentary
sequences
back-arc
basins
are usually best pre-
served, unlike other subduction settings where the oceanic lithosphere is often
destroyed
by
subduction
and
later
by
erosion
after
back-arc basin setting has therefore been the preferred for
Archean
southern
greenstone
Chile
has
similar
to Archean
include
the
been
facies
intrusive
Rocas
features
(Tarney et al.,
of
metamorphism, pile,
Tertiary
showing
belts
structure
the volcano-sedimentary younger
The
cited as
greenstone
synformal
greenschist
belts.
the
Rocas
structural
Verdes
which
are
remarkably its
However,
size,
stratigraphy
the geochemistry of the volcanics,
tonalite-granodiorite.
of
Similarities
basin,
the
The
setting
complex
1976).
Verdes
style,
uplift.
tectonic
the
main
of
and the
difference
lies in the absence of undisputed ophiolites in Archean greenstone. The
marginal
positional
and marine deposits
from the deep
graywackes
in
offers
a
broad
spectrum
(e.g.
and
greenstone
basin environments
the Tonkolili
and intertidal
Moodies
Group,
Formation
environments Cheshire
of
de-
the variety of volcanogenic,
found in African
flysch turbidite
Zimbabwe,
deltaic
craton
model
belts.
This
represented
ssch as the Fig Tree Group in the Barberton belt,
Formation proximal
basin
settings which would accommodate
continental ranges
back-arc
in
the Manjeri
in Sierra
Leone,
to
like those of the Kalahari
Formation,
Shamvaian
Group).
Chemical sediments such as chert and banded iron-formation accumulated in parts
of
the
shelf
sedimentation,
while
and
deep
basin
stromatolites
which
were
far
away
from
like those of the Cheshire
detrital Formation
flourished on intertidal flats, lagoons and bays. The volcanic centres in the back-arc basins were not only sediment
sourcelands,
but also sources
of mineralizing fluids rich in gold and sulphides. 3.7.3 Archean Plate Tectonics. A
full
consideration
geothermal lithosphere, theses
are
gradient
of
the
and
the
tectonic
implications
properties
of
the
of
the
Archean
high
Archean
asthenosphere,
and crust is not intended here since very authoritative available
in
Condie
(1981),
Nisbet
(1987),
and
Frazier
synand
108
Schwimmer of
the
(1987),
African
metallogeny,
to mention a few. However, continent
is
it is pertinent
and
Dewey
although writing
(1973);
since the crustal evolution
to
to consider
on Archean crustal evolution. Burke
fundamental
its
regional
geology
cursorily the prevailing
and
views
The overview presented below is culled from
and
from
Frazier
about North America,
and
Schwimmer
found the Archean
(1987),
who
of South Africa
and its global tectonic implications worthy of a full-blown account. Burke
and
Dewey
(1973)
Archean
tectonic
namely:
the primitive
the permobile
Frazier
to
have
and
transitional phase.
for the primitive which
convection
of
phases, regime;
development
spreading
considered
main
and the
mantle-derived magmas the
(1987) three
tectonic
crust
Small-scale
through
phase being the earliest Archean
phase
(Frazier and Schwimmer,
sumes a thick convecting asthenosphere, mafic-mafic
Schwimmer
evolved
stage which characterized most of Archean history;
Archean-Proterozoic The model
and
processes
the
of
initially
dominated
contained
little
resulting
sialic
in the asthenosphere
ascended at randomly oriented
three-armed
rifts
three
1987)
as-
a thin lithosphere and an ultra-
in
the
plates
so that
heat
"hot spots",
thin
radially
material.
lithosphere away
from
and
causing and
the
the
three-
armed rifts.
The plates probably moved across the surface until they met
plates
opposing
with
under
the
other
motion
could
have
in which
case
underthrusting
resulted
(Fig.3.47).
Since
of
one
the
plate
geothermal
gradient was too high in the Archean for eclogite to form from basalt as is the case plate
in the Phanerozoic
and pulls
Archean
the plate
plate probably
where
eclogite
steeply down
along
into the mantle,
simply slipped beneath
very low angle as shown in Fig.3.47.
forms
a down-going
the down-going
the overriding
Instead of eclogite,
plate at a
garnet granu-
lit e, a lower density material, could have formed, which did not have sufficient
density
Calc-alkaline chean,
was
going
slab
to
pull
material
probably (Fig.3.47
the
down-going
which increased
generated A);
material at hot spots
due
slab
steeply
into
the
in quantity with time
to the
or as a result
partial of
the
(Fig.3.47 B); or still,
melting
mantle.
in the Ar-
of
thickening
the down-
of volcanic
the lithosphere
could have
buckled at zones of opposing plate motion (Fig.3.47 C). Burke
and
mobile phase cratonic bounded included outcrops
Dewey
highlighted
(Table 3.5) to include:
areas; by
(1973)
and
individual generally
calc-alkaline
intruded but
characteristics
of the
the absence of continents
dominantly
volcanics
which were wider
the
granodioritic
the
and
shorter
curved
areas
greenstone
volcaniclastics; than modern
plate
per-
or large
which belts
greenstone sutures
were which belt
on ac-
109
count of the smaller sizes of Archean plates;
and the general absence of
large volumes of sedimentary units in the Archean because of the absence of stable cratonic areas where they could have been deposited.
•=.---
~
~
volcanic arc
~
.......
" ~--~.
"
'
~
Basalt
I-" ~ f 1 _
t ':~
Cr/,Jstal .
:
Volcanic
:::'~ ".*, ~
~ ~f::/,
~
~,. ,,
Extension
Phase
. . . . . .
B
W
Sediment
Continental Sediments
Lavas
Sedimentary
Phase
Deformation
~
~
lite
-
Basin
Closure
ronites
Figure 3.46: Back-arc marginal basin model for the development of Archean greenstone belts. (Redrawn from Windley, 1984.)
110
Subduction
during
the permobile
curred
at the boundary
result
of
the
between
buckling
of
the
regime
simatic
(Fig.3.48,
and
lithosphere.
sialic
B)
crustal
Low-angle
calc-alkaline
volcanic Felsic
western
and
material
over
Formations
in
extensive the
upper
have
areas
oc-
elements
subduction
down-going plate and partial melting could have produced Maliyami
could
as a
of
the
large volumes of
as
evident
Bulawayan
in the
greenstones
of
Zimbabwe.
A primitive
crust
calc - alkaline
B mantle
diapir
C partial
melting
Figure 3.47: Suggested mechanisms for the generation Archean calc-alkaline magmatism. (Redrawn from Frazier Schwimmer, 1987.) The transitional by
the
development
relatively
a rigid
lithosphere
probably
boundary was marked as
rapid decline in radiogenic heat production;
associations
and structures
progressively sociations
phase at the Archean-Proterozoic of
less
which
characteristic (Burke
and
developed
during
the transition
of the availability
of a rigid
areas were dyke swarms,
Dewey,
lithosphere
the
1973).
The
phase
and extensive
the Great Dyke of Zimbabwe,
result
consequently
of t h e permobile
common
of and
a
rock
regime became new
as
of
rock
as-
a consequence
stable
cratonic
and the Witwatersrand
Triad,
the oldest thick extensive group of rocks deposited on continental
crust.
The Great Dyke was emplaced around 2.5 Ga, at the Archean-Protero-
zoic boundary.
PHASE
PHASE
e
c
SWAZI.
high heutflov-thin lithosphere growth of permobile crust by convective scum mechanism or vertical grovth of gmnodiorite-
~
z~oo I
SUPERIOR
•c- ~ =~"~
~
=~ ,~
HURONIAN
belt
crotons
2o'i o........o
6T
,zoo
progressive thinning of asthenosphere by bnsel accumulation of refractory lithosphere slabs continental accretion in volcanic arcs KIOARA DAHOHEY. APPAL. URALS ALPS
lithosphere thickens-plate margins become more narrowly-defined increasing stresses of sabduction zones and higher pressure assemblages ncreas ng y-narrow and buffer-defined accreting plate margins leads by 600 my to generation of ophiolite sequences |= oceanic crust and mantle ] increasing proportion of alkalic to tholelitic basalt in dike swarms progressive sharpening of facies changes in orogenic belts
exogeasyncllnes-marginul thrust zones basement reactivation by Tibetan Plateau-type mechanism-cryptic sutures
1750 ' !
i
ophiolite sequences b|ueschists low rnnk I high rank
sedimentary polarity reversals from cratonic to tectonic land supply
,, kyanite
ophiolite lithologies - pseudo- ophiotite sequences
LABRADOR
~o.~-- ~'
-~= , ~ G o~-o
~.
.-~
. . . .~ sedm i entsedm i ensupply t sediment supply from rising g r n n ~ terrunes and volcanic c o m p l e x e s ~ f r o m pervasive reworking of basement in permobile regions
n~rrow wag-defied linear/arcuate orogenic
stable crotons - major marine transgressions flood continents
continents oceans orogenic belts
extensive cratnnic instability-elastic basins and troughs- basaltic dike s~arms- vulcanicity
quantitative
stages.
npproach from qualitative npproach mngnntic from. faunal anomnlies provinces
TECTONICS RE61HE
and plate tectonic
miogeaclines [continental shelves} sharp boundaries vith eugeosynclinul fiacies strongly zonal sedimentary igneous and metnmorphic facies relationships
aulctco~len first major dike swarms ~
PLATE
transitional,
semiquantitnflve approach from paleomagnetic data qualitative upproach from paleoclimatic indicators
? andatusite siiim~ite "'~oTdie-rit~". . . . cnlc- alkaline assemblages
non-linear orogenic pQtterns-greenstone buff t
~ f
TRANSITIONAL
through the permobile,
crgressivestabitisafion atons-toco,[ semi pervasive permobite conditions ~,~table platforms no croton or plcltforms locatisation o?~,,~nd troughs permobi|e r e g i o n ~
PERHOBILE
Table 3.5: Lithospheric evolution (From Burke and Dewey, 1973.)
-.L
112
gravity ÷
basic
ultra bas'c c r us
t h i n ' l i t h o s p h e r e ° •
-
-
.
. -
__-_-~...:._z
"" ....
.
<'-.~
anomalies
--
÷
--
b a s i c -
- ~ , - .C['~.'.~ c~.k
. , -= ' , - ¢ ~ , ~ z -
.
""
astbenosphere
~
-
-
"
.
-i----
f, :.~
"
~"~ ' ' 7 ~
" tholeiitic49. - magmas~:'-
i-~---'--
"-- - ' " " ~ ' ' ' ~ '
/avas
b a s i c
"-' ~3-' •
." " . " , ".-/ " • " • "'~';~'" " " "
i m b r i c o
older • "supracrustals
ultra,
~,
" " • .'. ." " - •
A
""--'-----'['I~'~"%"~L-L-'--
k o m a t i i t i c
mantle d i a p i r s
intense deformation metamorphism,== . . . . . . . . . . .
calcand
alkaline
ond
~-r,~-,
- ~.'~
25km
lavas
sediments
. . . . . .
"~-
~
-
--
~
. . . . . . . .
q ~:iI,-~<4 , ~ . . ~ - ~ ~ . . . . ; ~ ' : ~ , ' ~ - - ~ p < q ; ~ v . ' ~ : , i ~ - .
•
%%%.
.
°
'
.
.
.
•
ogmas
.
°
.
.
.
°
•
.
.
,
,-~,"
.
,
•
.
".
.
-
.
8 Figure 3.48: Actualistic model for permobile ics. (Redrawn from Frazier and Schwimmer, 1987.)
Archean
tecton-
o
After the falling near-surface and
eclogite
which
ever
probably change
formed since
initiated
instead
has
of
geothermal granulite,
characterized
(Frazier
and
gradient steeply
convergent
Schwimmer,
Schwimmer,
was
geothermal
gradient
geologically would
from granulite to eclogite.
"sudden"
have
dipping This
according
because only a small
been needed
15
C/km,
subduction
plate-boundaries,
1987).
in the tectonic behaviour of the Earth,
reached
to cross
final
was
dramatic
to Frazier and change
the phase
in the
boundary
Chapter 4 Early Proterozoic Cratonic Basins and Mobile Belts
4.1 Introduction Although
the Early Proterozoic,
by most conventional definitions
started
from about 2.5 Ga and lasted in Africa until about 1.75 Ga (Cahen et al., 1984), the crustal evolution of Africa precludes strict adherence to this time framework in several respects.
First,
the assembly of large Archean
continental blocks out of the m@l~e of colliding island-arcs
established
stable crustal areas in the Kaapvaal province as early as around 3.0 Ga. Between
2.8 Ga and 1.8 Ga, vast portions stable
(Kr6ner,
1989) successively, as sites of Pongola, Witwatersrand,
dorp,
and
(Fig.4.1)
in
southern
West
and
the
basins
from
prohibits
any
belts
distinguishable
River orogeny being
in
Proterozoic,
(Fig.4.1). the
Although
strict
Namaqua
belt,
giving
mid-Proterozoic
age,
rise
two distinct the
Early
(2.0-1.7 Ga) and the Namaqua orogeny
of
et al.,
from the Late adherence
to
limits of the Early Proterozoic in this region. Sec-
Early to Middle
and Natal mobile
Venters-
(Tankard
sedimentation dating
therefore
developed
Waterberg-Soutpansberg-Um-
sedimentation
outside the cratonic areas in southern Africa crustal
prevailed
ter
intracontinental
epicontinental
Africa
the geochronological
are
which
The long continuum of cratonic
Archean ondly,
on
Transvaal-Griqualand
kondo-Matsap 1982).
cratons
of this province had become ex-
tensive
recurrent
instability
to the Namaqua orogenic
cycles
Proterozoic
Orange
(1.2-1.0 Ga), the lat-
and
protracted
orogenic
activity in the same region renders it again impracticable to enforce the arbitrary
Early Proterozoic
age
that after the Early to Middle
limits.
Equally
Proterozoic
significant
tectogenesis
is the fact
the Namaqua mo-
bile belt stabilized and became part of the Kalahari craton. Thus, zoic
in
geologic
although this chapter dwells principally on the Early ProteroAfrica,
the
processes
continuum
of
Late
in southern Africa
these other age intervals
Archean
and
Middle
has necessitated
in this chapter.
Proterozoic
the inclusion of
Also considered
here are the
Eburnean orogenic belts of the West African and Zaire cratons, dian
mobile
belts
of
central
Africa,
Proterozoic anorogenic magmatism.
and
the
areas
affected
the Ubenby
Early
114
1 • ~
." ." .'.''
"
o
::
IBcni~Nigcria shidd
.... •
.2...,' -
.}!
Namaq u a
.
.
.
~
.
, D ~,~? ,',>., ~.~"
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). <¢11 K u n o n e l n n l i s t V e l l f e
~.--'_--.~.~
:-'-
-½
v, z c*
,-, "7o
~ ~
~
P
....
.r___~
'
.7::~, K/'~
Bosh
4I ~
was ~- ~.
. %--I
-~':':a" :
"%(
~ G r l m l O " k ~'ucur~l
I
/
.~.%4. S o u ~ n s b e l g
...:.'i-~P'..*,l'~"/,,.~ ,~l'~l~ ,~ /
Folded crofonlc cover
r
O
V
t
-
".Z,
NCE
..z~...' ..'"":Xk /
'
'~'.~.,'~:: :. 4,-' : "::, C
~Igneous / " .~ {corn" ~ ' ~ "
~
/
/
Cra.tonk coyer exposed or conceoled
~
(outcrop/in,red
, 500 Km
~
subcrop ) i
F~Low
grade
[
t\
t~--
<
rcmsifionc~l
| ~, l i
~'~" \-~l/l
o
.......
~
MA~ONB, '..
........
_..
~++-t+
'
.,~
~
i'l'./'~
......i
,'" ~ ,
, . ' ; ~ " . . . . . SQl,sbury -"
:-.. :-'--
'~"
i
""
"%'
\I~" ~
i
"
I "l
I
•
~%
! / ," { % i /"
"
ZIMBABWE PROVINCE) %, I / I',./\ / \." ,, I ~ , / t
['~
High grode
* *
Mc~jo~fhrust f ~ l t
++++ ~gL\ ~
~
Major sheet bell
% i~x" 1 i I "t I t/ t~Rehoboth ~c i \ ~ NAMIBIA',. \ \ _ " \ % ['. " I |~ i | ~,'~9~:"" PROVINCE% / [ . , ' - . ~ J | KAAPVAAL i / / ~ . '~:~'W ~. Sincloir I \ /'l i i i " ~ ,.~ '" " i ~ A 6~_ - ,~,.J0honnesburl # ~ " 7
o
"
~AA"2~.17"~' ~"i b=,4. ~ " ' ' 6
'<:;F,,'IT, ,
%~q;'~:-:t lt+~"~ ,~0~\.~.~ +
, t
, % i
',
T -~-i.+k-,~' ,., \
i
ProviNce k:~-I .~ •
'..,.-',
--~J
by atre-
Bc~sic- ultr~b.slc layered intrusions ~nd ossoci~fecl gr~aitoicls
A
-~z
had
activity
"<-~ '~ '~ I x . . . i ~.-."~' : :.':' ".'~" ~: ' . ! , : - ;.~'f:~,-,M~-%h~ ~,~ ~
~.~.~
marked
that
,-., ,.:, %,,.',',-',,,@
, t
was
areas
~ ....0 ~ J~\'~ J ~ - " ,,,ZIHB~ tWE ~rPl0VlN~. E .".~ ~ ~ - # ~" ~-~ ~
i
.
~
~"'~
'J
~i
\ •
craton in
~.-Piriwirt-Lomiigunl
.....
\ ~@ \ '? \'
'
Kalahari basins
Tectono-thermal
p.q!t~.: .... ,-:-'---',:',
/
the
intracratonic
$
.,'tPRO.VINCE-",~/
F i g u r e 4.2: Tectonic outline map of the Kalahari craton showing A, s u p r a c r u s t a l development from 2.9 G a t o 1 , 8 Ga; B, Early to Middle Proterozoic orogenic b e l t s f r o m 2.1 t o 1.0 Ga. ( R e d r a w n f r o m T a n k a r d et al., 1 9 8 2 . )
116
stricted
to the Namaqua
of the craton, (Fig.4.2,B). the
stable
nuclei.
and Natal mobile belts
along the southern parts
and to the Magondi mobile belt on the northwestern margin
Elsewhere,
igneous
activity
continental
crusts
of
These
Early Proterozoic
periodically
the
Zimbabwe
magmatic
suites
broke
and
out
Kaapvaal
include
through cratonic
layered
igneous
intrusions such as the Great Dyke of Zimbabwe and the Bushveld Complex of the Kaapvaal province.
The major magmatic events were followed closely by
the emplacement of granophyres, Cratonic
granites and basic dykes and sills.
sedimentation began quite early in the Kaapvaal
province where,
around 3.0 Ga, the Pongola basin had subsided along the eastern margin of the
craton
(Fig.4.3).
Witwatersrand
basin
there cratonic
Waterberg-Matsap 2.8 and 1.8 Ga
axes.
cratonic
centre
of
subsidence
the
Kaapvaal
Traansvaal-Griqualand
alluvial lacustrine
lated. lent,
The the
and
thick
overlying
shelf.
in the Witwatersrand
and the asymmetric
basin disrupted
bonate-pelite
Supergroup
Supergroup,
deposition
lithofacies
continental
reflect
immature
Following sea,
the regression
rifting
Within
these
resumed
and
its
platform
southwestern
equiva-
cratonic
rifts
accumulated
Soutpansberg and Matsap Groups. crustal
stability much
part
the earliest
of
subsidence
cratonic quartz-carto platform-edge
set-
transgression over
in the Late Archean.
of the Transvaal-Griqualand the western
fan-la-
terrain where accumu-
extensive
that had been assembled
along
to-
clastics
of the characteristic
in a carbonate
mass
and
basin was
the alluvial
ting, during what was probably the first major marine the vast
West,
Faulting and volcanism along the
fault-controlled
Transvaal
Griqualand
from
fans and fan deltas which prograded
flank of the Witwatersrand volcanics
the
(Fig.4.3), between
custrine basin and created the Ventersdorp basin-and-range bimodal
to and
proceeded along progressively westward-migrat-
The Ventersdorp,
(Table 4.1). Sedimentation
by extensive
shifted
province,
cratonic basins followed successively
wards a northwesterly western
the
subsidence
ing depositional
dominated
Subsequent
in
West epicontinental
the
Kaapvaal
red beds
of
province.
the Waterberg,
The Zimbabwe province which had attained
later in the Late Archean
had deposition
centred i
along its western and eastern margins, respectively.
This
suggests
in the Magondi and Umkondo basins
continental
margin
sedimentation
in
these
areas. The overwhelming global importance of the Late Archean-Early Proterozoic
of
South
watersrand
Africa
basin
lies
dominates
in
its
economic
the world's
Transvaal basins and the Namaqua Metamorphic vast strata-bound mineralizations
significance,
gold and uranium
for
the
Wit-
production.
Complex are repositories
such as manganese,
iron
ore,
The of
fluorite,
117
copper
and
Complex,
zinc.
Furthermore,
layered
intrusives
such
as
and the Great Dyke of Zimbabwe host some of the
i
~..
.i
~SedimentsX"
,,. ~ l ) '
• !......
the
Bushveld
m o s t spectacu-
~
:>-:../
~ :(',v.~,K
..___...<,,°°°
Supergroup > 3000 ma 4) Archoean greens~one belt ~21300 m Exposed granitesjgneisses ~Basin Estimated original and migmati~es IAx~s.s~.~Arecd exlentof I' ' Je ~ ~ Deposilional basin "-: J Area, of outcrop j
F.
.-,.
h
f
i
h
- ~
,, .........,_i.:::/
../' omln,on
ed,m:n,s
~Votconics ~
,ed,..ootsX
Reef-
/
Supergroup 3000 rna ~10~700 m
"--;
,_.
.
f ~ , ..... L.<% /
L~ Volcanics (
Witwat ersrand Supergroup ~1199m2700 - 2500
r
:.
~
/ "
JVentersdorp Supergroup 2500 - 2 3 0 0 m a
~5000m Basin axes migrate I progressively northwards J Areal extent of basin "--\.|increseswdh time
::
- )t '')`
,A~/ ~
~
)
~ j
~SedimentsN Tronsvoa|
Volcanics ~
Supergroup b 8ushveld igneous complex 15200m
J
,.r"~ /
[ ] Sediments \ ~$1 Volcanic s
~
]
23o0-2100mo ~ IO,700 m
/:'Woterberg
Motsap S upergroups 2000 1700 m o ~6500m
Figure 4.3: Progressive d e v e l o p m e n t of A r c h e a n to Early Proterozoic cratonic basins on the k a l a h a r i craton. (Redrawn from Anderson and Biljon, 1979.) lar
magmatic
ores
known,
viz.,
chromium,
diferous and titaniferous magnetite.
platinum,
nickel,
and
vana-
sequences.
Guartzite, conglomerate, marie and Feisic vo[canics,
Shale, quartzite, bonded iron-formation, 2700.* conglomerate,
Hafic and felsic volcanics, quartzite conglomerate, shale, carbonate rock,
Dolomite. bonded ironstone, shale, quartzite, mafic volconlcs
Red and white sandstone, conglomerate, shale, greyvacke, amygdaloidal lava
Amphibolite. amphibole schist, quartzite 1100-1 3 0 0 ? sericffe schist, biotite schist quartzite, conglomerate, dolomite,
Shale. orthoquartziteosondstone, conglomerate, tiHite,
Tillite. shale, mudstone, sandstone, siitstone, conglomerate, basalt, rhyolite, carbonaceous rock.
Shales, siltsfone, sandstone, c o n giomerote, limestone,
DOHINi0N
WITWATERSRAND SUPERGROUP
VENTERSDORP SUPERGROUP
TRANSVAAL SEQUENCE GRIO.UALAND WEST SEO.UENCE
WATERBER6 GROUP (GLIFANTSHOEK SEQUENCE)
KH£1S G R O U P
CAPE SUPERGROUP
K A R0 O SEQUENCE
HESOZOIC AND YOUNGER
160
1 60-300
/*$0
1750-2100
2200
2300-2650
2B00
2900-3100
Hoflc and felsic volcanics, shale, phyllite, limestone, quartzite cong|omerate, bunded ironstone,
3200-3/*00
15000m (thickness muy be too high as result of duplication)
Low grade metamor-
2650m
Underformed and unmefomorphosed.
Underformedand un metamorphosed in central and nodhern port of basin; Slightly folded in south.
Strongly folded; Low grade metamorphism.
Isolated remnant in granite-gneiss terrane, Strongly folded; Amphibolite to granulite grade of metamorphism.
--
Copper, lead. zinc. silver.
(Copper~uran~um?)
Hangonese. iron, fluorite. crocidolite and omos~te a.sbestos, gold, andolusite. limestone (copper, lead, zinc),
Go}d,
Gold,urclnlum, silver, pyrite.
Gold, t~anium, pyrophyllite.
Gold, antimony, copper, lead, zinc. mercury, iron. chrysotile asbestos mognesite, talc ( n i c k e ) Gold.
5700m-*.
(Oil),gos(coal). alluvial diamonds,ilmenite, futile zircon, monclzite~ salt,
Sediments attain thick- Coal, uranium, cloys, o i l ness of more than shale (titanium-sands). 10,000m capped by 1/.00m of volcunJcs.
8S00m
Unknown.
Faulted and tilted blocks in the Transvoa{- 7700m unmetomorphosed; in northern Cape succession tilted to west and intensely foldedlow grade metamorphism.
In the Transvaal, succession tilted towards 8500m centre of basin - in northern Cape succession tilted westwards; Local folding into open anticlines and synclines; Low to high grade contact metamorphism
Succession tilted toward centre of basin, 5000 m faulted; Overturned around Vredefart Dome.
Succession tilted towards centre of basin. 7500m faulted; Overturned around Vredeforf Dome; LoW grade metamorphism.
Slightly deformed, phism.
Relatively underformed, Lower sequence low Lower Hsuzi Group up grade metamorphism, Upper sequence fa 5Sg0m in South. unmetamarphosed. Upper Hozaon Group up to 50001n in north.
isolated remnants in Basement granite-gneiss, fsoclinafly folded, 6reenschlst metamorphism,
Economic Minerals a n d Metals
(From Anderson and Biuljon,
Age in Deformation and Metamorphism Maximum Million Ye~urs Thic k n e s s
PONGOLA GROUP
Rock Types
Uitrumafic, mafic and Feisic volcanics, shale, schist, greywackeobonded ironstone, chert, quartzite, conglomerate,
Main
SWAZILAND SUPERGROUP
Sequences
Table 4.1: Main features of South African supracrustal 1979.)
Oo
119
4.2.2 W i t w a t e r s r a n d The W i t w a t e r s r a n d
Basin
basin,
in its tectonic
intracratonic
trough or yoked basin
the
of the foreland
rifting
of the Kaapvaal
~Johonneyborgy.(~_~
~t /
::~-....
?
---.~.-:::~,~'~.~'~ ,
r" :eo'swano
I
I'. !/ i
I
.~//
.
~
~
~-.,.._;
~ ' ~
\
( ~ ;~27r , "Piil~ala'/" "---..~.-" ~ _, ~ u/r b s n
....
~
.... , / "
S.or,
I
Side
deposltional a:liy
4,-- offlap
I
I
',
A
/
-Cover
~ e n , r o , ~ono ~---"-'1 Wey! Rand ~ Granitic BeYement ~ Foully km
S ~ I10I .-...a
0
I
10 a
S Long side of ....
b¢/sin
onlap
~/ Proterozoic Expo yures
Basement E xpoyure y
Phanero zoic Cover
"~-. ~v v v vvv v v vv v v v vv v v v v.~_~=- .'.-:..=.
i ::::;i !:~!!i ! ;!;!~!i;;;!:.?si..~i~!i~::!:~-~-,pp er
================================
%
.~"'~~ coorye
and fine clestics
Z , I ~ I, l:-"l - - - p i v o t o fine classics and non ~ -~-.L I I ~o o elastics o - oo -o.o. .o. o o% ~ b- / /
B
-
Oo oOO o o o .~-.~ %'~;~'~o o o o_"~.~ Lowercoarye clastics and fine clostics ",,~v q - v - o - ~ 6 / "
~%
"X'9"L~'basol votconics and coarse clasticy
inlensity of folding
Figure 4.4: Geologic outline map the W i t w a t e r s r a n d basin. (Redrawn 1989.)
an
from
(Burke et
(~" "
~
"
Basin
represents
cratonic nucleus
-',
Nige
izzq Ventersdorp Basin ~
Wits.
Bosemen% Exposures
0
,/
/ " ~
(Fig.4.4),
1975) that developed
o
I
T.V.L ~loh~nneyb~)
Cope
N
setting
(Pretorius,
(A) and c r o s s - s e c t i o n (B) of partly from M a r t i n et al.,
120
al.,
1986).
volcanics
Up
to
respectively and
clastic
sediments,
basin was
initially
clastics, by
felsic
Dominion
Group;
belonging
F ~ %-.':
Turff ontein
k
mafic
(Fig.4.4,B).
2.7 km of mixed
followed
by about
to the West Rand Group
the 2.4 km thick arenaceous
Central
and
Supergroups
rift basin
filled with about
the
and quartz arenites
overlain
and
and the Ventersdorp
in what was a one-sided
immature
4.5 km of mudstones (Fig.4.5,A),
of
the Witwatersrand
accumulated
The Witwatersrand volcanics
16 km
representing
Central Rand Group
SR
Rand
Johannesburg
:".::
~
Main
2m
] ---- M R L
Conglome- i rate
Jeppest o w n
"-:..?'~
Government
.----: -_-j |3U
Hospita! Hill
~ "--~.
~loraisburg Duartzile
::"'
Roodepor
West Rand
Slack Bar MR
900
~Conglomerote I~
DQuortzite
~
AL
Shale
A
Ti II oid 28 ° E
Pr e t oHo
Dome / / /~- - -\ . ," rp'/~..~ \ g t I ~%/~'~ /) |) JiDom e .~ [ ! /"~'T~N'J°hannesb'uig~"/ /, ^e i
Westerdom Dome -
," ~ c ~ , . . .
,'~',
\Cor*e,on",,~,:?' ;
~ (~,>,,~,~ 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==<.t .r~)(~oj a.)-n:st,~pnuiodr t ¢en,ra,, add Groao ~/-'~'-J B a s e m e n t
Dome
" '
~ Eva°dot
/D.~.~
" . . .
t" "*.
~)e,,ooDome
.Xj- \ ~ ,
.~-~.. " ~ ' . . . - - . ~DOme~'-'l~]// J/' ",.- :: ", r - " • <, , ~ c ~ , ' , ~V'w~lka~ /~ \ k-5,~5~) ] / _
TheunlsSen/,'/,~l'.-~
26"S
B
Figure 4.5: A, Stratigraphic succession Supergroup; B, Distribution of the Central from Tankard et al., 1982
0
km
100
~
'
'
in the Witwatersrand Rand Group. (Redrawn
12t
(Tankard
et al.,
1982).
The
conglomerates
of
the
West
Rand
Group
host
placer gold and uranium mineralization. Since the Witwatersrand Supergroup has been metamorphosed only to the lower greenschist and
primary
facies, the unusually well preserved sedimentary fabric
sedimentary
mental
reconstructions
et al.
the
structures (Tankard
depositional
essentially
one
model
in which
have
allowed
et al., of
alluvial
the
fans
detailed
1982).
Witwatersrand and
paleoenviron-
According
to
Tankard
Supergroup
fan deltas
prograded
was
across
finer-grained lacustrine deposits on a humid and semi-arid landscape that was devoid of vegetation.
Along the fan-delta
shorelines
there was tidal
and wave reworking of sediments while finer grained clastics and chemical sediments
accumulated
offshore.
Epeirogenic
tilting
and
warping
controlled the geometry of the sedimentary units, unconformities, paleocurrent
patterns
in the basin.
fluvial
sedimentation
Central
Rand
furnished
Group
the
and
the
(Fig.4.5,B).
detailed
Diapiric
formation
geologic
granite
of
rich
Exploration
for
information
that
doming
gold gold is
and the
accelerated
placers and
in
the
uranium
has
available
on
this
ancient basin.
Stratigraphy The Dominion Group is preserved in an area of about 15,000 km 2, in western Orange Free State and southwestern Dominion
strata
rest
non-conformably
Transvaal, on
where up to 2.7 km of
Archean
basement
granites
3.0
Ga to 2.7 Ga old. Intrusive and extrusive volcanics in the Dominion Group have been dated at 2.8 Ga, which implies the initiation of the Witwatersrand basin around that time. An immature, with placer concentrations nite occurs
cross-bedded
of detrital pyrite,
fluvial
monazite,
at the base of the Dominion Group.
sandstone
gold and urani-
A mixed volcano-sedimen-
tary interval follows comprising basaltic andesite and tuff, acid and andesitic lavas, volcanic breccia and quartzo-feldspathic Comformably
overlying
the
Dominion
Group
(Fig.4.5)
which occurs over an area of about
thickness
of 4,650 m,
northwest.
A
basal
quartz pebble
offshore group)
sandstone
thickness
of tidal
inlet
the
West
of about
and beach
7,500 origin
mature quartz arenite with bimodal-bipolar
Beach and tidal
shales
sandstones
and a thin contorted
and
siltstones
iron-formation
grade upward into the fine-medium-grained,
and trough-bedded
subarkosic
fan-deltaic
Rand
Group
42,000 km 2 with an average
lag deposits overlain by trough cross-bedded,
and plane-bedded directions.
and a maximum
is
lava.
with
m
comprises
fine grained paleocurrent intercalated
(Hospital
Hill
upward-fining,
arenaceous
deposits
in the
Sub-
planar-
of the Gov-
122
ernment
Subgroup.
The
occurrence
Subgroup has been interpreted
of
diamictites
within
the
as the evidence of the oldest known glacia-
tion,
representing
ice-rafted moraines which were displaced
shelf
environment
by
part
of
planar
the West
proximal
shelf
submarine
Rand Group
cross-bedded
flow
(Tankard
comprises
subarkose
siltstones
with
and
overlying Central Rand Group
largest
shales
source of gold.
pass
detail
in
course
of
gold
on
paleogeography
of
these
Central
maximum cally
Rand
Group
thickness
this
is
group
subgraywackes
is exposed
about
consists
mining;
4.5,B;4.6)
western
this
an area
2,880 m near of
the
has
and
shed
of about
time the centre
fields developed surrounding over
the
Dome.
coarse-grained,
in alluvial
fan fluvial
show
a
while
down-slope the
other
(Fig.4.6)
plain.
Lithologicross-bedded
packages with
of
the
basin
(Figs.4.4,B;
short braided
Bimodal
component
component
which
is
Two
horizons These
ceous
matrix;
which
accumulated
an environment
they
was
related
diamictites
are
characterized
downslope
with
Several
were
in
in
the
of d y k e s
and
during
cover,
intruded.
and during
channels
longshore
Central
cur-
Rand
fans.
snow.
interlace
the
overlying
the Cretaceous
suspension In such
rock debris
or melting
sills
Group
in an argilla-
muddy
alluvial
weathered
ism; during the intrusion of the Bushveld Complex; of Karoo dolerites;
by
as high-density
the
sandstones
stream
supported
in arid or semi-arid
emplaced
in the
the
(Fig.4.6).
by clasts
originated
gold-
From the
emerged and flowed
to distribution
occur
sparse vegetation
generations which
streams
produced
have been wetted and displaced by rainstorms
Supergroup
inland sea.
and the various
cross-stratification
of
probably
unstable
between the basement uplifts.
rents in fan deltas along the lake margin
(Fig.4.5,A).
its
fans formed m o s t l y along the
margins
of the basin became
highlands
in
more gentle southeastern margin of the basin.
in the down-warps
piedmont
the
sediments.
9,750 km2;
At the centre of the basin was a shallow lake or an enclosed With
into
considerable
ancient
Vredefort
predominantly
fault-bounded
and not on the
and
has been unravelled
unmetamorphosed
over
which were deposited
and
trough
economic importance being the
an average thickness of 250 m. The alluvials northern
upper
directions
gradationally
Its sedimentology
light The
the
The
fine-grained,
paleocurrent
which
into a distal
1982).
(Fig.4.5,A).
great
the
et al.,
fluvial,
bipolar
The Central Rand Group is of tremendous world's
Government
could
Witwatersrand
Ventersdorp
volcan-
during the emplacement
when k i m b e r l i t e
dykes were
{
M,ne,ot
Heavv
~
F i g u r e 4.6: from Hutchison,
,
~
Geologic 1983.)
r__£ a of
~
'
r~
~ .
. ~ t 6
_-.,~
,
~
section
and
; ! i .UO:l I1 i I Pyrite" y t Chromite Zircon
I l, ,dl/
J
/
I
facies
I
" - ; ' . ; , "',~'~I
,," .h..,!'~,'~ ,~-;'77;','41 I
~
,~osP oRANG~"
model
""
I /
L
\-
~r~r--~eitaic~epos,<
/
, ~ , .
/~
~
for
the
~
1- ~
J
\
.......
Witwatersrand
alluvial
\
,ran,um
"
goldfield.
--
into succeeding fansJ
Zo.e of sub.q--t
Lacustrine Longshore Currents
~
(Au)
~.~
~'S£mMENT$
~
- z o . e of w i n n o w i n g ~ " . . ~
~
-----/--~PT~----~
<_
ap
Profile of larg_erdVer--s t~e:kn~'~
i i I IFluvial S,~,,stem
/,~7/-'//.(,
" ~
JKPP ESTOWN ~ ~ ~ -- -- ~
s
as
~
- ~ ~-
_
.s ~ . , ~ ' ; "
:
~7,~!'.-'~--.,~7~
k
Accumulation -'"
/
~,~ ~
/e
/ , ~
/
Depositionol floor of banket
~
THE BANKET,on a smooth depositional floor is fed by ellipsoidal Pebbles clccumuloted on the littoral pebble mill released under exceptional but not cataclysmic conditions. The pebbles were moved by stream currents assisted by gravity
(Redrawn
~0
124
Mineralization
The W i t w a t e r s r a n d area
of
about
South Africa, ginning
Supergroup,
has
supplied
the
of South Africa's financial
fields along
nearly
(De Kun,
mining
industry
earnings.
and western
osmiridium
and
Supergroup.
pyrite
Gold mining
over half-a-million; to over
2,000
frigerated
are
the
On a regional
fluvial
is
channels
the elevated delta
was
sites
generally
unimodal, low
gold or
channels;
localized
the
and
ticles
fill
formities which
force of
shafts,
are sunk
dewatered,
and re-
Supergroup
sandstones
the
planar
channels
interface
which
reworked
and
redistributed
packages
These
deposits;
cross-bedded
two
the
are
the
overlying
dune migrations
units
of
litho-
were
po-
scourtrough,
in shal-
representing
distal
1982). in the strongly pyritic
(Fig.4.7).
Gold,
which cover intraformational cycles
the from
shoreline
The various
depositional
suggest
fans.
between
uranium
sands that
and pyrite
sands;
in the
discontinuities
sedimentation;
u n c o n f o r m i t y that separate short depositional
in mud cycles;
along
the
par-
stratior uncon-
(gold was derived by reworking the immediately-underlying
separate
is
in the Wit-
of alluvial
(Fig.4.6).
lie on the foreset beds of cross-stratified
form conglomerates
Witwatersrand
and heavy minerals
concentration.
gravel-bar
(Tankard et al.,
erosion
where
labour
and the lacustrine
were
1979)
fluvial
uranium
the
sediments
sediments
Gold and uranium are concentrated usually
a large
of gold and uranium along
the
(Pretorius,
braided
and
7 gold-
such as uranium, the
The W i t w a t e r s r a n d
to the northwest,
Rand Group
cross-bedded
braided channels
of
ventilated,
from
(Fig.4.5,B)
by the development
transported
where
of lag
for 70% of the indus-
part
engages
technology.
controlled
source areas
pebble
upper
tons of
is the hub
leading sources of uranium and its byproducts.
in the Central
tential based
in the
of
since the be40,000
principally
economic mineralizations
found
latest
that
systems
been
over an
Republic
basin
rim of the basin
in South Africa
along the paleostrandline facies
has
scale the distribution
sequence
Mineralization
gold Over
The W i t w a t e r s r a n d
and are excavated,
also among the world's
watersrand
the
the mines which are 8 to 10 m-wide
m depth
using
2.3 Ga,
of
in 1886.
for it accounts
Production
the northern
2.8 and part
55% of the world's
1987).
most of the gold and associated
fan
between
north-central
of the gold rush near Johannesburg
gold have been mined try's
deposited
42,000 km 2 in
beds)
planes
of
and in carbon seams
which are d e v e l o p e d on or immediately adjacent to planes of unconformity. However, mediately These
the greatest adjacent
concentration
to bands
are p r e f e r e n t i a l l y
of gold and uranium occurs
of conglomerate
developed
(locally
known
in or im-
as bankets).
at or near the base of each cycle of
125
sedimentation.
Within
the J o h a n n e s b u r g thickness
whereas
on
only 1% of these or
conglomerate
the
area,
Dominion
these the
southern
50 mm
They jasper,
sisting
of
muscovite,
E
and
(Fig.4.7)
are m o s t l y secondary
of
the
Central
Group
to
subangular
and other minerals.
the total
basin
they
constitute
1983).
well-rounded
quartz,
vein
The bankets
pebbles
quartz,
which
accompanied
The m a t r i x
phyllosilicates,
in
8% of
(Hutchison, of
Rand
about
the pebbles range in d i a m e t e r from 20 mm
oval
quartzite,
consist
the
occupy
margin
l i t h o s t r a t i g r a p h i c units reefs
form 70% of the rock volume; chert,
Group
conglomerates
to by
is compact con-
sericite,
pyrophyllite,
chlorite, and chloritoid.
"'.'.,
c
'
u m
'
. .'...'
.
%; .'.',
""
"
'"
".
•
"
o
o
2
l a HorlzaatQ{
60 ~C3.~£0- - -
l
"- -4[i
I Scale
in
3
4
I
J
m
_ ___ _
:~
~
4
0
c
0
T
I0
"27
ppm GoldIppm Uranium
2 o 1
0
L
2
I
3
i
Horizontal
4
t
scale
in
J
m
Figure 4.7: C o n c e n t r a t i o n of gold and u r a n i u m in pyritic fluviatile erosion channels of the Steyn placer. (Redrawn from Tankard et al., 1982.) In general
gold
particles
may
be d i s t r i b u t e d
in
a dispersed
manner
throughout the banket matrix; as isolated clusters; or as thin streaks on either
the h a n g i n g wall
or the footwall
contact.
Since
gold and uranium
126
were
concentrated
hydraulically,
good
correlation
scale,
between their distribution both vertically
tally
(Fig.4.6).
Group,
gold was
uraninite,
In
the
Steyn
concentrated
zircon,
and
(Fig.4.6).
Kerogen
occur
seams
were
with
(Tankard et al.,
of
lower
transported
in association
Witwatersrand Supergroup
and
on
(Fig.4.7)
placers
in the upper
and chromite
the distal fans
Basal
exists
a
and horizon-
the
Central
fan plains
further
the placer
local Rand
whereas
basinward
deposits
into
the
of
1982). The seams, consisting of
hydrocarbons with organic sulfur and oxygen, originated by polymerization of biochemical At
least
These
two
are
0.2 mm
compounds types
of
Thuchomyces
produced by decaying primitive micro-organisms. organisms
have
lichenoides,
in diameter with
been
identified
a fibrous
spheroidal
form
vegetative
in
the
kerogen.
0.5-5.0 mm
diaspores;
long and
Witwater-
and
omyces conidiophorus, the hyphae of saprophytic fungi. The association of kerogen with mineralization fortuitous
since
algae
in the Central Rand Group
could
have
grown
on
is believed to be
channel
scour
surfaces
on
which detrital heavy mineral lags were also concentrated. The
provenance
surrounding
of
greenstones
the
Witwatersrand
and granites.
gold
Gold was
and
uranium
eroded
from
was
the
the Archean
gold-rich greenstone belts while uraninite came from the younger granites that envelope the greenstones.
The uraniferous conglomerate ores occur in
the Late Archean-Early Proterozoic interval in environments which reflect sedimentation
throughout a period when the Earth's atmosphere was anoxy-
genic.
this
Under
condition
transported and deposited dizing
conditions
detrital
in placers.
Soutpansberg,
4.2.3 Ventersdorp
Basin
is
an
elliptical
basin
pyrite
and
gold
were
This ceased with the onset of oxi-
late in the Early Proterzoic
of the Waterberg,
This
uraninite,
when the oldest red beds
and Matsap Groups were deposited.
of
over
200,000 km 2 in
area,
encompassing
most of the surface and subsurface extent of the underlying Witwatersrand Supergroup group, lavas
(Fig.4.4,A).
which
is over
at the base,
The basin
7,860
m
is filled with the Ventersdorp
thick,
comprising
overlain by conglomeratic
basaltic
and
subgraywackes
Super-
rhyolitic
and subordi-
nate shale and limestone at the top. The stratigraphy of the Ventersdorp Supergroup
is complicated by local unconformities,
like depositional geometries,
repeated lithologies,
and faulting which occured during deposition.
lenticular and wedgeoverlaps,
and folding
127
At the base of the succession is the Klipriviersberg Group which was deposited during the phase of stress and faulting that
followed subsid-
ence in the Witwatersrand basin. Faults acted as conduits for voluminous (1,830
m
thick)
amygdaloidal,
continental
tuffaceous,
conformable upon
tholeiitic
and
with
basalts
breccia.
which
This
are
porphyritic,
volcanic
sequence
is
the Witwatersrand Supergroup in the northeastern part,
but on the northwestern margin of the basin the contact is unconformable with a basal conglomerate that is mineralized with gold and uranium that were reworked from the underlying truncated Central Rand Group. The middle Platberg Group,
about 5,000
m thick,
succeeds unconform-
ably. This is a sequence of immature clastics with coarse scree deposits, sandy graywackes which were alluvial
and
shed
fan
from
systems
debris
flows.
Graben margin
surrounding horsts and
graben
lake
change
boulder
conglomerates
basinward
into multiple
stromatolitic
calcareous
shales.
Basin-and-range topography prevailed during Platberg times and persisted throughout
the
deposition
of
the
overlying
Pinel
Group
alluvial
fan
deposits.
4.2.4 Transvaal-Griqualand West Basins The
Transvaal-Griqualand
West
basin
system
(Figs.4.2.A;4.8)
are
the
remnants of a once extensive epeiric basin which covered a large part of the
Kaapvaal
cratonic nucleus, up to an area of about 500,000 km 2, from
about 2.3 Ga to 2.1 Ga. The Lobatse basement arch (Fig.4.8) separated the Transvaal basin to the northeast from the Griqualand West southwest. ments,
The thickest parts of both basins, with about
lay in the Transvaal basin.
basins are, Lobatse which
however,
arch
occupy
The
Transvaal
northeastern
and
and
to the
12 km of sedi-
The lithostratigraphic units
correlatable across the
(Fig.4.9). the
basin
in both
130-km gap occupied by the Griqualand
southwestern
West
basins
Supergroups
respectively are
lithologically varied epicontinental deposits which host a great variety of
strata-bound
fluorite,
lead,
mineral zinc,
deposits
vanadium,
such
as
asbestos,
iron
ore,
manganese,
aluminous minerals,
gold,
and
lime-
stone deposits.
Stratigraphy At the base of the Transvaal Supergroup are the cross-bedded, atic
and
arkosic
This
group
sits
deltaic
sandstones
unconformably
equivalent to the much thinner
upon
and
shales
of
the
Ventersdorp
the
conglomer-
Wolkberg
Supergroup
Group. and
Vryburg Formation in the Griqualand
is
West
128
rims Rest e Goldfield ms Rest
X
[ • - ] Basement ~
--
Witwatersrand Goldfietds
1. OrangeFree State 2. Klerksdrop 3- West Wits line &. West Rand 5. Centra( Rand
6. 7, 8 9-
X~
East Rand Evander So,JthRand Yredetrort
UUtCrOpof Transvaal I1~ Supen~rou p
Crocidolite
>4~A Amos~te >~Fe Iron ~Ls Limestone
Figure 4.8: Distribution of the Witwatersrand, and TransvaalGriqualand West Supergroups and their mineralization. (Redrawn from Anhaeusser and Button, 1976.) basin.
The
overlying
Chuniespoort
and
Ghaap
Groups
(Fig.4.9,B)
are
a
thick succession of mostly dolomites and dolomitic limestones with extensive banded iron-formations. tralia
(Hamersley Group),
Like
similar Precambrian sequences in Aus-
these carbonates represent the oldest extensive
carbonate platforms in the geological record. Tidal-flats, ronments,
platform
edge and basinal
carbonate
in the Chuniesport and Ghaap sequences Tidal
flat sedimentary
and domical
structures
algal stromatolites;
morph after supratidal gypsum; tidal
flats;
indicators
and
include dark,
gate stromatolitic matolites
quartz
subtidal envi-
facies are well displayed
(Tankard et al., 1982). include
fibrous,
flat-laminated,
low-relief
bladed dolomites which pseudo-
tepee structures which suggest evaporitive
oolitic
dolomites.
Fe-poor dolomites
Subtidal
paleoenvironmental
characterized
mounds with no exposure-related
by large,
features.
elon-
These stro-
grew below wave base. As shown in Fig.4.10 the carbonate plat-
129
P~t~~espoort ! 100 Km ~ ~ . _ -
~y,.%
1
~"~Thobazimb
t~J ~
!2:2
Svoz~ond
~ s \~
~ / ~ ), ~/~.~-~ N~mHERNc~PE ~ fi~'.~'#~ ~
~
~:;.~H
~
/S/
~/~/" ~"('~
r
TRANSVAAL PROVINCE TRANSVAAL ~shveid Igneous Complex •I Iron formetion of run fl 'm-rio _" . ! _ firou P I• Grfqud~nd . ,~ pPre2ojja~ rli'oria~ Group_ firo • Eiri~ugl n nd ~ G~ b;'./'/',~ .~ Griqu~tovn Jasper -nd ~ 4 Duitschland Formation t , o )e e.s " " UUii scnlo~O Koega~s Formation m i n i Pengeiron Formation t ' - ~ Kuruman Iron Form,fion~'mz,~ Halm~ni Dolomite /~,~ ~pbell Rand ~Qnd J ~ite ~; ' ~ nl n~nt Dolos Campbei Dolomite cover rocks r'--'] ~ Ba.semenf
:~,, ~;~-~,
A
I j I NORTHERN CAPE PROVINCE J
j
TRANSVAAL
.
PROVINCE 1 CHUNIESPOORTso00,
/._
=
o. , ~
~ ~ o
m
~
~
~
;
&O00]
~
,~:~-1\ 3.o0ot
SABLE
KOEGAS
BIC=Bushvdd Igneous C o m p ~ x
\ CENTRAL ~ TRANSVAAL °
~
/
~ . / B
Figure 4.9: Geologic sketch map (A) and stratigraphic columns (B) for the Transvaal-Griqualand West Supergroups. (Redrawn from Reimer, 1987.)
130
form edge lay to the southwest. The platform edge lithofacies are represented by oolites with megaripples, oncolites, edgewise rip-up breccia and current ripples; while slump breccias, graded carbonate turbidites with alternating algal debris and pelagic shales and basic ruffs suggest basinal environments.
LIMPOPO
/°"
e9~°
/
o
i
. ~
~mt~t
•
~J
~-" .._pj
j
l,
~
lift--,,
\ ,,.'~k
:......,...'.... :.:. :. :. :, :, ,j ::::::::::::::::::::::::::::::: .':':".".~-'.'/','.".':':':':;:'i ....
~ -~ ~=~--~
.l~grRmm~/PLATFORM._
~
%"
1
BASIN
,~%',~...~
/
J
~
/
PROVINCE
KAAPVAAL PROVINCE
~ " "~J~p~-,~n'~~'> "~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-
-
':::::':.:'
.
:
.
/<
'
|
I ~,
, ~,~
~
I --I
~
~
.
~
.
~Archaean
gra6ites
r~
r-7
,, ""
"
/
..
""
_~
K o o ( i n ~BlackReef __'.,.J'~, Ouor tzite
Archoeon qreenstone bell
Arenaceous-Rudsceous sediments Volcanic rocks
A r g i l l a c e o u s sediments Arenaceous sediments
""
/
/
I" %. /~ I
~ , ' ~ - ~
[Amosite~s ,estos~l.Crocidolite Asbestosl / / [Highgrade Iron o~I
~
Mon~
~
", \ I /
~.~
'
Mafc ntrusives(Bushvedage)~
~%~
Aureole of C o n t a c t .. I _ M e t a m o r p h i s m
,,,..... U n c o n f o r m i t i e s
~,~
o
t . "% ,
Figure 4.11: Schematic section depicting the stratigraphic setting for stratiform ore deposits in the Transvaal basin. (Redrawn from Anhaeusser and Button, 1976.)
ooo
,0®m
. o~o~ v~e4°~e
to
134
rock
.,"\~|'~--/~
A ~H/:,
~y
//\
i:iiTo
~--'CI:[ZZ2~__~
v
-,
v~-----~_=h/::
Bluck
-
. , . . . . . . , , . : ~ ~ 1 ~ :
'
• ;.: ":-:,: -: .: .: /
:
Hotazel
q
:L=;v"
.-27 3 o ~ : . ~ - - - ' - _ - - ~ - - ~ 5 ~
": ":': "
Rock
-
'
:
'
:
'
,
~
V"
~-------'
Matsa p
~
-_-_ ~ E ~
Group
. . . . . .
Uncon'~ormi ,~y
--- -~ -'L'-_
Manganese
Ore
Banded
Iron - F o r m a t i o n
VvV V V
V V
S OUl h
V V
V V
V
"v" V
V
V
V
V V
V V
V
Manganese
Ore
~
Ongetuk 0
V
Lava
V
V
V
V 100 metres
I
.....l
V V
0 V ~,,
20 I
V V
V V
V
V
V V
V North
Ferruginised Manganese
,"
V V V V Wesl V
V V
V
Surface- Enriched Mangsrlese Ore
F.:., . ~ . . . : . ' .
V
V
40 metres J V
V V
V V
Ongetuk V
V
Lava V
V
V V
V
V V
V
V
V East
Figure 4.12: Outline geology and cross-sections of the Kalahari manganese ores and banded iron-formation. (Redrawn from Hutchison, 1983.) like swellings in stratiform quartz veins and gold impregnations were the primary types of mineralization.
135
Stratified
gold
Wolkberg
Group,
Pretoria
Group
veins
the
are
Black
hosted
Reef
(Fig.4.11).
by
the Malami
Quartzite
and
Mineralizing
the
fluids
Dolomite,
the
1,700
basal
concentrated
m
upper of
the
the
aurif-
erous ores by m i g r a t i n g through c o n f o r m a b l e p a s s a g e w a y s created by intrastratal around
tectonic the
the shaly rocks. extend
up
to
early-phase pyrite,
movements
reefs,
offset
which
dykes,
10 km
along
strike.
quartz;
carbonates
pyrrhotite,
sphalerite
copper,
Pilgrims fluids
sourced
gold
by the
However,
fluids
could
cially
the
as
pyrite
with
and galena;
Bushveld
with
have
been
shales
in
Igneous
generated the
(Anhaeusser and Button,
been
dolomite
of
arseno-
byproducts
such as sil-
mined.
The o r i g i n of the
some
contain
to
hydrothermal
80 km west
that
Transvaal
which
consist
chalcopyrite
attributed
the
in
and
possibility
from
reefs
scheelite,
gold
Complex,
is a strong
the
some
In addition to gold, has
surfaces
cleavage
dip g e n t l y to the west and
Mineralogically,
and
deposits
there
slickensided
non-penetrative
arsenic, bismuth and pyrite w e r e
Rest-Sabie
goldfield.
produced
The reefs, which are banded,
among later m i n e r a l i z a t i o n ° ver,
also
as well
of
the
the m i n e r a l i z i n g Supergroup,
about
0.i
espeppm
Au
1976).
Base Metals
Lead,
zinc,
near
vanadium
Zeerust
oldest
in
and
the
fluorite m i n e r a l i z a t i o n
western
known Mississippi
production
of
lead and
Bushveld
Valley-type
in the Malami
(Figs.4.8;4.11)
mineral
zinc has ceased,
province
but
fluorite,
is
the
in the world.
The
with
100-150 mil-
lion tons of total reserve and a tenor of 15% c a l c i u m fluorite, the largest deposit in the world
(Hutchison,
Dolomite
perhaps
is one of
1983).
The Malami D o l o m i t e fluorite deposit has m a n y features in common with the
Mississippi
telethermal an o v e r l y i n g Dolomite high
Valley-type
stratiform impervious
(Fig.4.11).
where
ore,
desposits
including
the
in carbonate
layer which
is
fact
rocks
chert
in the
The Zeerust fluorite d e p o s i t
mineralization
was
controlled
by
are
case
of
Chuniespoort
overlying
Group,
Pretoria
group.
veins,
disseminations
length
of
about
beneath
and
60 km.
The
the
ore
stratiform
The
ore
unconformity
bodies
which
several
which
include
replacements,
is of
are
the Malami
paleo-porosity
(Fig.4.13).
occur
varieties
of
the e
The ore bod-
ies o c c u r at and n e a r the top of the s h a l l o w - d i p p i n g M a l a m i the
both
capped by
is located on a paleo-
the
d o l o m i t e such as v u g g y horizons and p a l e o - k a r s t s
that
which
Dolomite
separates breccia over
of the
pipes,
a strike-
including
algal
ore, w h i c h occurs as irregular nodular masses and less c o m m o n l y as laminae of
fluorite
tions was weak.
in stromatolites w h e r e Black spar
the effects
occurs as finely
of t e l e t h e r m a l
solu-
c r y s t a l l i z e d b l a c k fluorite
136
GUBBINS
ALANKOP HILL
:7 I
'
~
.
,
'
-
b~..
-....%" ',
',
z
//
MINE
/i /1 ' ~ / - : ~ : ~
,
/ 2 d - 7 - - I~ , ~-":. . ... .. ... ... ... . . . " " ~'d -'-~'-" / / .... ,~ . . * * * - . . • "~, " / / / / / / J 50m
"-" -" .....
j
o - Bonded chert
o-
b - Blockchert siliceous shale with white chert frooment$
dolomite
c - massive fluorite d - breccia cemented with black shale and fluorite
c - Siliclftecl and mlnerolised dolomite d - Highly weothered dolomite
'
$TAVOREN i
~ ~ . ~ W7 I N T E'" R S H O VVVVVVVt/ VV V v VV v VVV VvVVVVVVVvyvvvvvwv~v~vV I
Assumed Fractures
~:~ Poleocave In central p a r t of Wlntershoek I v e r t l c a l s e c t i o n l a : dolomite with algal ore, b = tremolite-rich, c : stlicified dolomite very rich in fluorlte~d = siliceous shale { k o r s t channel filling)
replacing dolomite
(the black
~
Feisite
Bushveld Granite
E ~
Union Shale
Stanniferous stacks of Bobbejoankop Granite Upper members of Transvaal Supergroup
Figure 4.13: Zeerust fluorite (Redrawn from Hutchison, 1983.)
ited from the dolomite).
Granaphyre
colour
mineralization
is due
and
to carbon
tin
lodes.
inclusions
inher-
There is i r r e g u l a r l y banded spar p o s s i b l y depos-
ited by v o i d - f i l l i n g along the main p a t h w a y of fluid m o v e m e n t in cavernous dolomite;
and also breccia spar, which forms a cement of breccia bod-
ies in the dolomite,
creating pipe-like bodies w h i c h are
ated
with
mineralization.
sink
holes
Major
lead-zinc in
association
east-northeast
and
with
Fluorite
impure
north-northwest
black
also
chert
tectonic
locally associ-
occurs
around
paled-
(Hutchison,
1983).
zones
in the Kaapvaal
p r o v i n c e w h i c h controlled the emplacement of the B u s h v e l d Igneous Complex and numerous y o u n g e r alkaline intrusions such as the P i l a n e s b e r g Complex, c o n t r o l l e d the i n t r o d u c t i o n of fluorine from the alkaline intrusives into the Malami D o l o m i t e
(Hutchison,
1983).
137
Industrial Minerals The
Transvaal
minerals basin
and
Supergroup limestone.
(Fig.4.11)
mally altered
is
also
a
Chrysotile
where
rich
Bushveld-age
the Malami
source
asbestos sills
Dolomite.
have
During
of
asbestos,
occurs
the
around
intruded
aluminous
the
Transvaal
into
formation
and
ther-
of the deposits
Mg and Si were supplied by dolomite and chert r e s p e c t i v e l y w h i l e the sill was
the
sources
alteration sills, after well
shows chert as
of
zone,
the water
and the
heat needed
dedolomitization in w h i c h
ripple
into
the nodular
marks
including andalusite,
are
calcite,
is
(Fig.4.11).
inherited.
A
reactions.
contacts
serpentine
laminations
variety
of
An
of the
pseudomorphs
in the
chert as
aluminous
minerals
staurolite and kyanite occur in the thermal aureole
recovered
Primary
and
and algal
of the B u s h v e l d Complex w i t h the aluminous Andalusite
for the
a metre or two above the u p p e r chilled
from
the
limestone
is
Marico
shales of the Pretoria Group. district
quarried
at
in
several
western
Transvaal
locations
in
the
C h u n i e s p o o r t Group.
4.2.6 Waterberg, Soutpansberg, and Matsap Basins These are coeval Early- to m i d - P r o t e r o z o i c i n t r a c r a t o n i c basins which are strung out along the w e s t e r n margin of the Kalahari craton, popo p r o v i n c e to the Kaapvaal p r o v i n c e by
ubiquitous
These that
are
red
the
beds
earliest
oxidizing
which red
conditions
were
beds must
deposited in
have
from the Lim-
(Fig.4.14). They are c h a r a c t e r i z e d
the
between
geological
existed
in
the
either d u r i n g s e d i m e n t a t i o n or shortly afterwards
2.1 Ga record,
and
i.i Ga.
which
Earth's
imply
atmosphere
(Tankard et al.,
1982).
Waterberg Basin This
basin
is filled w i t h
about
5 km of continental
beds,
the Waterberg
Group. The W a t e r b e r g G r o u p rests u n c o n f o r m a b l y on the Kaapvaal craton and comprises stones,
red c o n g l o m e r a t i c
planar
calations
of
and
trough
trachyte
and
and m e d i u m - g r a i n e d cross-bedded quartz
fluvial
porphyritic
to g r a n u l a r arkosic sandstones, lavas
with
(Fig.4.14)
sandinter-
in
the
southern part of the basin.
Soutpansberg Trough This
is
a yoked
basin
or
faulted
trough
located
within
the
previously
u p l i f t e d and eroded high-grade gneiss terrane of the Limpopo province. is
filled
by
the
Soutpansberg
Group which,
in
the
lower
3,600 m of lavas w i t h subordinate sandstone i n t e r c a l a t i o n s the
upper
part
of
the
group
are
800 m
of
litharenites,
part
It
comprises
(Fig.4.14). ~ In lithic
wackes,
138
siltstones, mudstones, pyroclastics, and thin conglomerates. Pl~nar cross-beds and upward-fining depositional cycles suggest fluvial sedimentation. UMKONDO GROUP
ArkoiM Untf
~Arkou ~ renlte In flnlng-up point bar cy~e~
UDoer Ar gllloceous Unit
:::22 T.e
MOCked¢,onnell
0
Southern warreD°ms bo.ln
Ouartz arenlt,
E~
rs,.,.
~o..r,*o.,o.d ,..,,cu~r. ~ o . l ~ - , ,~nds~, and ,.at°
~
r ~
,,
NAMIeIA
235m ~
[:~,~.-~.~o
~.~.,
Argillaceous
....
oJ4!I I
.oTswA.,
~
Snail and llltlltOml Re~slit°tone ~mestone bl0herml Calcareoushornfels ~metomorpholled mOt|l} LIF~estone
t ""5":'~':
I~:'.i::.l
m'~
I I I |
ZIMBASWf~ ChiDing J~.o~ U.KONOOG,OU~" tvvv.vl % SOUTPANSBE~' t;::"~'] " ~ " k GROUP, /
I | |
~
"
~.(--~,~
~
[, 1
..
h • I \
.~/.' .__~_."~.~k.WATERB[RG ~.." WoYmbothS/GROUP ; " ""'~"(,,,,,v,,,"~ ~
•'..F
~' \ ~=
I "::Y""
t ::"::i~;:~ | :' 'i'~<:':"
Sandstone
o.ou
Ouart=tIcUnlt
~ I::::::] ~'1:3
WATER BERG
700m : U,'~-< ! ..,!,~:.iii¼ "
~):/.'..:..~
Interl~clcle~J~=an~tone,llttltone
...
E~ ~ ~'l ~J
L .... f ~ MATSAPGROUP-~' - •
/ / / ~
•
~ T
, ' '~SWAZ LAND
~-~
STATE./.([" NATAL / ~ f/" % / ~ LESOTHO / ...... " - J - /
_ ,'---'~
~
~
~ 4- I I~ ' I I% I + [~+
,(.
.%,~ .{. 4-'" +
. . . .
UpD~fS0utpa.iberg(arenlfe-argllllte-10vo-pyr0clolticr0¢k)
Mlddll Soutponlberg ( or°nit°- lovos to east ) ~ L o w a r S o u f onsber (voicon~crocl() ~:1 Umpopo ba.ement
SOUTPAN o
~ VJ....~'-j.~f. T ~ ~ -r %4\ t ' + ~,~" +
~
" LCAPE "tOWN
II ' , II I" .11 ..t.
~o¢
SBERG
GROUP
Figure 4.14: Distribution and representative geologic columns for the Waterberg, Soutpansberg, and Matsap basins. (Redrawn from Tankard et al., 1982.)
139
Matsap Basin This basin lies on the southwestern margin of the Kaapvaal province where a thick
sequence
unconformably
of continental
on
the
Lucknow Formation, subordinate
beds
Griqualand
and
West
lavas,
the Matsap
Supergroup.
The
Group,
lower
unit,
about 1,000 m thick, consists of sandstone,
conglomerates,
dolomitic
limestones
sits the
shale, and
and volcanics.
The over-
lying Hartley formation has an average thickness of 1,300 m and comprises amyqdaloidal quartzites. stones
lavas
with
intercalated
ruffs,
quartzites
In the Volop Formation at the top arkosic
predominate
with
minor
chert
and
jasper;
and
breccia
cross-bedded
this
unit
sand-
is
about
1,500 m thick. While
the
Griqualand
older
West
Supergroups
Kaapvaal province, margin
of
the
stratigraphic
sequences,
are abruptly
such
as
terminated
the
Transvaal
on the
edge
and
of the
the Matsap Group which accumulated on the southwestern
craton
at
about
2.1-1.8 Ga,
shows
lithologic
continuity
with the adjoining Namaqua Mobile belt. 4.2.7 Umkondo Epeiric Basin
Stratigraphy The basement complex of the Zimbabwe craton is nonconformably overlain in the
northeast
dominantly deformation the
by
relatively
clastic
unmetamorphosed
sediments which
away from the craton.
Umkondo
Basin
is
termed
continuity.
The Umkondo
Umkondo
Group
limestones overlain by sandstones, The calcareous underlain
unit
was
inferred
laminations, cross-bedding
from
diagnostic
flat or sabkhha
is about
intraclast
which
and
comprising
Group
(Fig.4.14)
are succeeded
such
environment
which
1982).
is
by supra-
as
for the dolomite
delicate,
crinkly
ghost bird's eye structures,
breccia
origin.
south
siltstones with greenish gray
Supratidal
features
(Tankard et al.
unit is of shallow shelf
sequence in
the
1,250 m thick
of the Umkondo
conglomerates
tepee-like structures, and
in
these sequences are almost in
and lacustrine or lagoonal
shales and feldspathic arenites.
Group
of pre-
shales, and altered basic lavas.
at the base
by reworked basal
tidal dolomites
sequences
in metamorphism and
Although the stratigraphic the
the Gairezi Group in the north (Fig.4.2,B), outcrop
cratonic
show an increase
are
indicative
The overlying
This is succeeded
algal
ripples,
of supratidal
lower argillaceous by a thick sequence
of deltaic cross-bedded arkosic subgraywackes and wackes.
Near the top of
the Umkondo Group is the upper argillaceous unit which contains point-bar sequences,
overlain by arkosic sandstones
and lavas.
The facies progres-
140
sion in the Umkondo G r o u p
is one in which continental
lakes and
w e r e d r o w n e d by a m a r i n e t r a n s g r e s s i o n coming from the east, fan deltas
prograded
flux m e a n d e r i n g doned
deltaic
braided
across
and with
reduced
after which
terrigenous
in-
fluvial systems d e p o s i t e d alluvial plain clays over abanlobes.
river
the basin,
sabkhas
In the
Chipinga
area
(Fig.4.14)
system p r o g r a d e d eastward across
a higher
the m e a n d e r i n g
gradient
river belt
and d e p o s i t e d the upper arkosic unit. From west
to east
sive increase facies.
and Gairezi
grade,
sequences
from greenschist
show a progres-
to high amphibolite
There is a c o r r e s p o n d i n g west to east increase in structural com-
plexity
with
Lithologic belt.
the Umkondo
in m e t a m o r p h i c folding
units
The
age of
lying b a s e m e n t
and
lose
thrusting
their
the Umkondo
(2.5 Ga)
and
along
identity Group
falls
that of
north-south
within
the
between
to
NE-SW
adjoining the
the intruding
age of
axes.
Mozambique the under-
Umkondo dolerites
and
lavas which were emplaced about 1.78 Ga, after the d e p o s i t i o n of the sediments
(Cahen et al.,
1984).
Mineralization
The Umkondo ing.
lavas which
overlie the upper arkosic
unit
m i n e r a l i z a t i o n in several parts of the Umkondo G r o u p ton,
1976).
ites
and
and
are copper-bear-
They were p r o b a b l y the source of the c o p p e r - i n - s a n d s t o n e
as
Copper-bearing
shales.
solutions
formed
ores
At the Umkondo Mine bornite
disseminations
in
quartzite.
There
in the Umkondo
occurs
are
epigenetic
(Anhaeusser and Butas nodules
sporadic
quartzin shale
occurrences
kyanite in p e l i t i c gneisses which correlate with the Umkondo Group. ite, w h i c h sometimes occurs with sillimanite,
of
Kyan-
is found here because these
rocks have been involved in the h i g h - g r a d e regional m e t a m o r p h i s m and deformation
of
prospects
where
the
Zambezi
mobile
belt.
Kyanite
is
mined,
the host s e d i m e n t a r y succession consits
and
there
are
of interdigitat-
ing pelitic and semi-pelitic Umkondo gneisses.
4.3 Anorogenic Magmatism on the Kalahari Craton A p a r t from the d e p o s i t i o n of thick cratonic sequences another consequence of
cratonization
A r c h e a n was running Dyke and or
in
the
Kaapvaal
and
Zimbabwe
provinces
in
the
Late
the initiation of a m e g a - f r a c t u r e system along a N E - S W trend
from
the
Zimbabwe
the B u s h v e l d
incipient
province
to
the
Complex were emplaced
intracontinental
rift
Kaapvaal along
lineaments
province.
these m a j o r during
the
The
Great
fractures Early-mid
141
Proterozoic
(McConnell,
as the M o s h o n a l a n d as well
as
the
structural
1972).
Swarms
at
lineament
about
Kaapvaal of
the
layered
granites),
carbonatite
truded
the
the
the high-grade
same
into the Zimbabwe g r a n i t e - g r e e n s t o n e
ter-
though
phase)
to
not
(the
and
Umkondo
gneisses
(Cahen et al., classic
contain
and
along
as already noted,
of
age
intruded
into
from 2.05 Ga
of
the
dolerite
The Great magmatic
last dyke
greenstone
1984).
examples
was
sediments
1.4 Ga
Mashonaland Archean
Complex
cratonic
at 1.9 Ga and 1.78 Ga the mafic bodies,
such
the
veld
Complex
complex,
Bushveld
and overlying
mafic
while
and extrusives
(Fig.4.2,A).
2.46 Ga,
basement
dykes
and Umkondo dolerites were emplaced during this phase,
Palabora
The Great Dyke was emplaced rane
of mafic
the
(the age
associated swarms
belts
of
in-
Zimbabwe
Dyke and the Bush-
mineralization
while
are associated with stratiform
copper
deposits. The wide.
Early-mid-Proterozoic
Other intrusives
Sudbury
Irruptive
phase
of ultramafic
was
are the Stillwater Complex in Montana,
in Canada,
the giant dyke suites
the dolerite dyke swarms in West Greenland. with regional
intrusions
world-
U.S.A.,
the
in West A u s t r a l i a and
Dyke intrusion was associated
stress systems that developed during early a b o r t i v e a t t e m p t s
to break up the newly assembled continental
plates
(Windley,
1984).
4.3.1 The Great Dyke
Occurrence, Composition, and Origin The
Great
Dyke
is
a
spectacular
layered
ultramafic
long; and with an average width of 8 km. It consists continuous
elongate,
gently
inward-dipping
plexes known as the Musengezi,
Hartley,
most of its
Dyke,
length,
the Great
layered
its
plex
fracture
greenstone
two parallel
belts
Which
cut
northeast
splits the
and batholitic
granite.
satellite
high-grade have
metamorphic
disrupted
and
rocks
of
displaced
of
on
the
com-
the Mashona-
At its southern end
the Southern
the
For
dykes
and the gabbro-filled
Dolerites
into a number of smaller bodies,
faults
subcom-
(Fig.4.15).
zone further east, were intruded into the basement
land suite and granitic veins cut the dyke in places. the Dyke
480 km
lopolithic
Selukwe and Wedza
either side, known as the Umvimeela and East Dykes, Popoteke
intrusion,
of four distinct but
Limpopo dyke
at
Satellites belt.
its
East-
northern
end. Each subcomplex has a broadly gentle synclinal layering
dips
inwards
towards
the dyke
centre
structure
at angles
as
in which the steep
as
25 °
W
sotellite complexes and dykes
WEOZA COMPLEX
SELUKWE COMPLEX
HARTLEy COMPLEx
:sO0
ZOO-
JOo-
J~
Basal norite
~ ' ~ r Cbromitite
E"~ HorZburqite/dunite
Bronzirile ;~ ~ 1 ~ Olivine bronzitite
Clf
[]
I Cla
~Gabb
,~
" ' SI $2 $3
Cle . . . . . it~ L
__,
" "
. _.
O- '~ ~
:~ -~ -~'"
eoo0
i Jeoo
i ~000
i
i
CYCUC oUNIT.. ,,
~Gabbronorite [ ] Olivine gObbro ~Websterite ~ J Bronzitite ITI11]JOlivine bronzitite ~7,7~Granular harzburgite
S
Sequence.
Sulphide Zones
(Redrawn
[ ] POikillflc barzburqite C I C [ ] Chromitite With number
The Great Dyke of Zimbabwe and the cyclic units of its Ultramafic
[rom Wilson, 1989.)
~igure 4.15:
I
T]s
~
MUSENGEZI COMPLEX
143
near
the
outer
contacts
Gravity studies
across
is V - s h a p e d with tact b e t w e e n
and
as
shallow
as
5°
in
the
central
areas.
the dyke indicate that at depth the cross-section
the p o s s i b i l i t y of deep central
feeder dykes.
the dyke and the surroundig country rocks
The con-
is steep,
sheared
and d e s i l i c i f i e d but not chilled. Xenoliths of country rock have been incorporated into the gabbroic parts of the dyke. In
his
description
Dyke W i l s o n consists lain
(1987,
of
1989)
of two parts,
by
the
(Fig.4.15).
Mafic
The
the
geology
and
showed that the
mineralization
layered
a lower Ultramafic Sequence, Sequence
Ultramafic
of gabbroic
Sequence
was
rocks, further
ing cyclic units. which
the
Great
of the dyke
2,000 m thick;
over-
about
1000 m
thick
into
lower
divided
Dunite S u c c e s s i o n and an upper Bronzitite Succession, a dunite
of
succession
a
the latter compris-
Each cyclic unit comprises a lower chromite overlain by
in
turn
harzburgite-olivine
passes
bronzitite
gradationally
through
to a b r o n z i t i t e
layer.
a
complex
dunite-
The olivine bronz-
itites or h a r z b u r g i t e s exhibit small-scale layering on the scale of a few centimeters.
This
resulted
from the rhythmic
tions of olivine and orthopyroxene. has a v e r y
characteristic
fluctuation
On the w e a t h e r e d
appearance
with
the pyroxenes
to w e a t h e r i n g and p r o d u c i n g sharp protuberances. of the u l t r a m a f i c
succession
in the propor-
surface harzburgite being
resistant
W e b s t e r i t e marks the top
in all four subcomplexes and is immediately
overlain by the gabbroic rocks. Wilson's
(1987) study of the mineral c h e m i s t r y and g e o c h e m i c a l model-
ling suggests was
highly
parent about
that the Great Dyke originated
magnesian,
liquid
was
1 km high
with
15%
repeatedly
and with
MgO
a doubly
origin of Cyclic Unit I (Fig.4.15), series
of pulses
(a
injected
from tholeiitic magma which
komatiitic into
a
diffusive
chamber.
for example,
volume
and
turbulence
to drive
the p r i m a r y field of chromite, of Subunits
tion d u r i n g
unit.
the
with
dominant
The
the U l t r a m a f i c
bronzitite
last
phase
Sequence,
The
chamber the
invoked a
into
cooler
The pulses were of suffihybrid
mixture
into
at the base
Subunit Ib w h i c h was i n i t i a t e d by a smal-
cumulus
Subunit Ib caused olivine
orthopyroxene clic
as
injected
resulting
failed to produce chromite,
orthopyroxene
(1989)
to give the thick c h r o m i t i t e s
Ic and Id. However,
ler p u l s e of m a g m a place
the
liquid). magma
In e x p l a i n i n g
Wilson
of hot primitive magma w h i c h were
and more evolved resident magma in the chamber. cient
basalt
stratified
forming
of magma
caused
but caused o l i v i n e to re-
phase.
Continued
to once again the d o m i n a n t
injection
olivine
give w a y
to cumulus
lithology
of the cy-
during
to appear
fractiona-
the
only
emplacement
temporarily
of
as a
n a r r o w olivine b r o n z i t i t e in Subunit Ia. It has been suggested that magma was injected
as fountains.
The hybrid
liquid p r o d u c e d by the entrainment
144
of
the
would
more
evolved
have
created
broken
into
a compositionally
convection magma.
resident magma
down
system
a
by
the
series
stratified
could have been
turbulence
of
double
magma
in
the
diffusive
column.
The
fountains, layers
double
sustained by successive
that
diffusive
pulses
of new
Since Cyclic Unit I maintains similar s t r a t i g r a p h y and lithologies
t h r o u g h o u t the Great Dyke,
the above genetic m e c h a n i s m g e n e r a l l y applies
to the lower U l t r a m a f i c Sequence in the dyke.
Mineralization The Great world. 190
Dyke
contains
one of
the
largest deposits
of
chromium
in the
C h r o m i u m reserves in the dyke, computed to a depth of 150 m, holds
million
mium/iron
tons
ratio
of of
metallurgical 2.8;
49% Cr203
and a c h r o m i u m / i r o n
vial
(Hutchinson
ore
Ultramafic
Sequence
chromium
350 million
1983).
tons
ratio of Cyclic
is important
with
of
2.3; and
Unit I,
because
48%
Cr203
and
a
chemical/refractory 60 m i l l i o n
the
uppermost
it is host
chroore
at
tons of eluunit
to several
of
the
sulphide
layers bearing p l a t i n u m group elements.
4.3.2 Bushveld Igneous Complex Occurrence This
is the world's most extensive
cupying
an
area
cratonic basin
of
about
into which
layered u l t r a b a s i c - b a s i c
65.000 km 2 it was
in
the
intruded.
centre
of
It consists
complex, the
of
oc-
Transvaal
four lobes,
a
w e s t e r n "lobe, southeastern and eastern lobes which are largely covered b y M e s o z o i c rocks,
and a northern lobe (Fig.4.16).
s e d i m e n t a r y rocks represent
of the Transvaal
upfolded
portions
of the country
of the w e s t e r n and eastern lobes plex
comprises
picted
granite,
in Fig.4.16,
complex.
The
layered
Domical areas of deformed
Supergroup and older formations which rock,
occur within
the cores
(Fig.4.16). The central part of the com-
microgranite,
felsite,
and
granophyre.
As
de-
the Transvaal S u p e r g r o u p also forms the floor of the sequence
of the u l t r a b a s i c
and basic
rocks
in each
lobe dips, u s u a l l y at low angles of between 10 ° and 25 ° , towards the centre. G r a v i t y studies reveal that each lobe could be r e g a r d e d as sill-like in form, t h i n n i n g laterally from a feeder dyke near the centre.
Igneous Stratigraphy The
Bushveld
about been
Complex
consists
of
7.6 km thick and consists correlated
regionally
based
the
Rustenburg
of mafic on
Layered
and ultramafic
persistent
marker
Suite rocks
which
is
that have
horizons
such
as
145
the m a i n all
of
Chromite
which
granophyres cated near into
Seam;
are
the M e r e n s k y
easily
and felsites, the V r e d e f o r t
five
zones
mappable.
The
and various Dome.
(Fig.4.16),
Reef,
and
complex
also
Magnetic
contains
from
the
Suite has
base
been
upwards
are:
Seam,
granites,
satellite intrusions which
The Rustenburg
which
the m a i n
are lo-
subdivided the
Chill
Zone, the Basal Zone, the Critical Zone, the M a i n Zone and the Upper Zone (Tankard et al., A
zone
rocks,
of
notably
1982; Hutchinson, norites
of the Transvaal in some places.
which
quartzite,
are
intrudes
Supergroup.
1983; Sawkins, highly
1990).
contaminated
transversely
This is the Chill
into
with the
sedimentary
Pretoria
Group
Zone w h i c h may be missing
The lowest unit of the complex is often the Basal Zone in
ROCK TYPES
N ~
Ferrodlorite, troctollie, gabbro, gnat thostfe m a g n e t i t e seams
~I//, Main magnetite
seam
i~ ,7/:1
,,
'
J
r o c k st,, / d O e r"l '
I
I
[;
o
~ "
..',~I , Gabbro,onorfhoslte,
#
!.
~" "'--"
.~
< k/N:!::?I~L
i >/;.~ AI~'%
~"~
Merensky R e e f
o
-u
~
Norlte chromlfile seams
e o , o l zo.,
pyroxenlte
o,o°,,.
Main chrome
":,,-
c / O l d e r rocks
~"
seam
v"f' v~ =l~'7 • V r~v Pyroxenlte, harzburgtte ¢hromiflte seams o ¢v ~~v *~ norite (suborclinate) o
"~
I 1 LI
I'tlJIIIIIII!lllllllllTm,¢'~ I
i / T..ltllllllllnllTIIIIIIIIIllr t'OCkl I
bv~
Figure 4.16: through the 1990.)
Geologic sketch map and schematic Bushveld Igneous Complex. (Redrawn from
sections Sawkins,
146
which
the rocks are p y r o x e n i t e and harzburgite
containing
chromite seams
which
are
iron
in
richer
stratiform layers.
aluminium
The
Basal
and
poorer
Zone
in
displays
than
marked
the
layering
higher
with
thick
In the o v e r l y i n g Critical Zone layering is even on a larger scale
and ranges ite,
in
series.
from banded norite to variable successions of pyroxenite,
anorthosite
chromitite
seams;
suite of this formation
chromite.
chromite
zone.
the two most phases.
and
a
Critical
cumulus
Zone
phase
contains
within
most
the
nor-
of
the
anorthosite
The M e r e n s k y Reef and the o v e r l y i n g Bastard Reef are
complete
of
is
The
both
cyclic
units,
units which
chromite
terminate
and
this
olivine
zone.
disappear
After the as
cumulus
The M e r e n s k y Reef is unique because it is p r o b a b l y the most per-
sistent m a g m a t i c - s e d i m e n t a r y layer of all layered complexes in the world, and
because
it
represents
n u m b e r of cumulus phases. ring olivine, Merensky which
complete
mineral
grading
with
The cumulus phases are chromite,
the
maximum
locally occur-
o r t h o p y r o x e n e with some c l i n o p y r o x e n e and plagioclase.
Reef
was
a
apparently
close
boulders within
to
differentiated
a
basaltic
an underlying
from
an
composition.
isolated There
anorthosite m a r k e r
magma
are
horizon;
The
segment
melanocratic these boulders
p r o b a b l y sank through the magma and disrupted the earlier chromite cumulus layers to form a basal pegmatoid reef. The
Critical
layered gabbroic
Zone rock
basal part of the Main is gabbroic. progressive the top
is
which
succeeded constitutes
Main
relatively Zone
homogeneous
(Fig.4.16).
The
zone is p r e d o m i n a n t l y norite while the upper part
change
in o r t h o p y r o x e n e
from En75
near
the
base
to
En40
at
The overlying Upper Zone is d e m a r c a t e d at the base by
the a p p e a r a n c e of cumulus magnetite; proportions
gioclase.
a
the
In the Main Zone cryptic layering is evident from the upward
(Fig.4.17).
of v a r y i n g
by
of
it is d i s t i n c t l y layered as a result
cumulus magnetite,
olivine,
pyroxene
and p l a -
There are 20 seams of m a g n e t i t e ore as well as anorthosite and
tractolite
layers.
Hutchison
(1983,
Fig.6.4)
illustrated
the
spectacular
cryptic layering in the Upper Zone where olivine grades upwards from Fa54 to Fa100, the
base
Upper
while
plagioclase
to An34
Zone
is
at
shown
w h i c h A = Na20+K20; Basic prominent
by
sulphides;
and u l t r a b a s i c throughout
dunite which
top the
becomes
sodic upwards,
(Fig.4.17). diorites
in
Strong the
ranging
iron
AFM
from An60
enrichment
diagram
in
at the
(Fig°4.17)
in
F = total iron as FeO; and M = MgO.
bronzitite pegmatoids
pegmatoids
the
the
pegmatoids Rustenburg
in the
form of p i p e - l i k e
sequence.
The
various
bodies types
are are:
in the Basal Zone with p h l o g o p i t e and nickeliferous
pipes
in
the
Basal
form irregular masses,
and
Critical
Zones;
a n a s t o m o s i n g veins
diallagite
and pipe-like
bodies in Critical, M a i n and Upper Zones; pegmatoids in the Critical Zone
147
Fo> FsTAn ~ MOI % 10 20 30 40 50 60 70 80 90 I00
ROCK TYPES 0 Fe-dio
J PLAGIOCLAS1
UPPER MGTE SEAM
I
I
I
OLIVINE
O R T H O P Y R O X E N E -
Fe- did, fro an% mote pyr (rare)
Go~tro, and, mote
MAIN
MAGNETITE
GO, and, m te Mottled ano~ ga Go, nor, porphyritic and mottled in Noces
TOTAL Fe
Go/nor •~m- scale layering
OF /
Norite and Gabbro
/
Y
)hyritic marker Nor Needle nor Spotted Nor, some mottled and .
J
4(
--
~of
Basal Zone,
O Chill zone Hyperite MO
I
O Main zone Anorthosite " Critical zone Anorthosite
I
It\LIII
:~-Or thopyroxene !~ norite and porphy• ritic pyroxenite
Per Pyr Nor
' | r ~ Or thopyroxene Oliv i n e ~
Pyr
~_j
Hyperite (thickness I variable quartzite J :~J;on;-,
~
i | Or thopyroxene in y pyroxenite MAI~4 CHROMITE SEAM
Pyr
--
I
',~ob~ro, at mo,° zone
MERENSKY REEF
Norite interloyered with Pyroxene
--
< ~ f.~
~pper mottled marker Nor/go banded porphyritic in places
\ I HORTONOLITE
PLAGIOCLASE Ortr
.......
u
Bushveld Complex showing mineral trends in the Figure 4.17: cryptic layering and differentiation trends (AFM diagram). An, anorthite; ano, anorthosite; dio, diorite; fe-dio, ferrodiorite; fa, fayalite; fs, ferrosilite; ga, gabbro; mgte, magnetite; nor, norite; per, peridotite; pyr, pyroxenite; tro, troctolite. (Redrawn from Hutchison, 1983.)
148
with
inclusions
anorthosite;
of
leucoamphibolite,
magnetite
pegmatoids
vermiculite pegmatoids
amphibolite,
in
the
Main
chromite and
and
Upper
in the Upper Zone of the eastern
mottled
zones;
lobe.
and
These peg-
matoids o r i g i n a t e d from the filling of dilation fractures and by forceful e m p l a c e m e n t during the aggregation of volatite fluids. various berg
acid intrusives
layered
Layered
felsite
Suite.
porphyritic
The
acid
metasediments
group,
about
Rashoop
rocks
and
form part of the B u s h v e l d Complex.
is
2.22 Ga
granophyre
underlies
also
in
old,
suite,
the
predates
contact
with
the
comprising
Rooiberg the
The RooiRustenburg
fine
felsites
to
and
Rustenburg
Suite.
c o a r s e - g r a i n e d intrusive rocks such as the Nebo layered granite 1.92 Ga)
and the M a k h u t s o
The age of the M a k h u t s o
coarse
Transvaal Other
(dated at
granite form part of the Lebowa Granite Suite.
granite,
the age of the Bushveld Complex
about
1.67 Ga,
(Cahen et al.
sets
the upper
limit of
1984).
A number of satellite bodies which are c o m p o s i t i o n a l l y similar to the Bushveld the
Complex
cratonic
were
emplaced
sequences
mostly
contemporaneously with around
bodies include basic intrusions; g r a n o p h y r e dykes; and dolerite,
the
syenite,
Vredefort
the
complex,
Dome.
The
nepheline syenite,
into
satellite
bronzite and
gabbronorite and p y r o x e n i t e sills.
Geochemistry and Origin Major-and revealed silica
trace-element that
the
saturated
average
geochemical
Bushveld parent
tholeiitic
studies
Complex
magmas
basalts.
which
The
(Tankard
crystallized were
average
from
et al; highly
considerably
Mg0
content
more
in
the
1982)
have
magnesian, basic
than
Rustenburg
Suite is between 13.0% and 13.4%, with Cr over 1000 ppm. Above the Critical
Zone
the
major-and
M a g n e s i a decreases and
Ni
about
contents 1500
tively. local the
decrease
ppm and
At
the
(Fig.4.17). erishment
in
geochemistry
correspondingly
200 ppm
level
enrichment increase
trace-element
changes
markedly.
to a m e a n of about 6% in the Main and Upper Zones; Cr
of
in both the
Fe
The m a g n e t i t e
in the V205
respectively,
the
main
contents
content
to
the
layer,
The decrease of
both
and
in Mg
Zone
Zone,
from
70 ppm respec-
there
olivine
in the Upper
from about
Critical
200 ppm
magnetite
Cr and Ni. layers
above
is,
however
is reflected
and
a in
orthopyroxene
show a steady impov-
2% in the m a i n m a g n e t i t e
layer
to b e l o w 0.3% in the topmost layer in the eastern lobe. A
mantle-derived
established igneous
although
layering,
magmatic the
source
mechanism
for for
the the
up to 100 km along strike,
Rustenburg repetitive
Suite and
is
well
persistent
has r e m a i n e d uncertain.
The
149
two p r e v a i l i n g h y p o t h e s e s for their origin are that magma d i f f e r e n t i a t i o n took place at depth, with
or
without
occurred ating
in
w i t h subsequent intrusion of the separate
further
situ with
body.
The
differentiation;
or without
latter
element geochemistry.
additions
hypothesis
However,
or
is
that
magma
of m a g m a
supported
by
fractions
differentiation
to
the differenti-
the
trends
in major-
it is g e n e r a l l y a c c e p t e d that the Bushveld
Complex r e p r e s e n t s the emplacement of olivine t h o l e i i t e m a g m a through five main
vents
(western,
melting
at
a mantle
partial
melting
of
eastern, magma
the
central,
source
sialic
crust,
Nebo granite and its variants derived
from
a crustal
northern,
generated
to p r o d u c e
crystallized.
source.
As
southeastern).
the magma
which
liquids
The felsites
already
noted
the
Shallow
induced
the
from which
the
could have been
emplacement
of
the
B u s h v e l d Complex caused extensive contact m e t a m o r p h i s m and d e f o r m a t i o n in the s u r r o u n d i n g c o u n t r y rocks. Mineralization The
Bushveld
Complex
is very
rich
in
sulphide
ores
with
d e p o s i t s in the M e r e n s k y Reef, the UG 2 C h r o m i t e Layer, Disseminated Layered
sulphides
Suite
sulphide,
are
up
to
2
volume
and in the M e r e n s k y Reef.
followed
by
pentlandite,
percent
Pyrrhotite
chalcopyrite,
very
extensive
and the Platreef. in
the
Rustenburg
is the m o s t pyrite,
abundant
cubanite
and
mackinawite. Chromite chrome-ore depth
of
occurs
in
reserves
of
only
(Hutchison,
300
commercial about
m,
quantity
2,300x106
with
ten
in
tons
times
the
down
this
Critical
to
a
Zone
vertical
reserve
estimate
1983). The chromite layers extend for distances,
with
mining below
up to 65 km;
and are b e l i e v e d to be the products of cumulate m a g m a t i c s e d i m e n t a t i o n in which an increase in oxygen fugacity was r e s p o n s i b l e for the formation of the m a s s i v e chromite layers. Vanadiferous cumulate there
layers;
is about
the M a i n
2% v a n a d i u m
The
1.8 m thick,
as
plug-like
at the Kennedy's
pentoxide.
pegmatite
The m a i n
Zone w h e r e
bodies
and
as
Vale p l u g - l i k e mass where magnetite
layers
are
in
the content of v a n a d i u m pent-
from 2% in the lowest layer to 0.3% in the u p p e r m o s t layer.
The ore m i n e r a l s
1983).
occurs
it is mined
Zone and in the Upper
oxide varies
dioxide.
magnetite
include m a g n e t i t e and ilmenite,
vanadium
pentoxide
resources
of
with up to 14% titanium
the
are e s t i m a t e d at 17xl09kg in 2x109
main
tons
magnetite
of ore
layer,
(Hutchison,
150
C a s s i t e r i t e occurs in the late-stage granite intrusions of the Lebowa Granite Suite. ment
rocks
The Lebowa granites were derived by anatexis
of
the
highly enriched mas
were
metals
by
highly
volatile
during
which
acted as the flux.
they
stripping
in f l u o r i n e - r i c h
sistent tectonic
Complex,
in these rocks,
volatile
crustal rocks
Bushveld
became
or
enriched
scavenging.
in
of the base-
fluorine,
which
is
Since the fluxed magtin
and
other
Fluorine-fluxing
of
trace sialic
zones w i t h i n an old, d e e p - s e a t e d and per-
trend in the Kaapvaal province,
accounts
for the occur-
rence of late-stage tin-bearing granites in the m e g a - f r a c t u r e
zone, along
which the B u s h v e l d Complex and the Great Dyke w e r e emplaced.
A
orite
J~
rnatolO ~otoid ~xenlte
2000 m I
~xenite
[o~'aIiy
Figure 4.18: A, outline geologic m a p of the P a l a b o r a Igneous Complex w h e r e d i s s e m i n a t e d copper ores are m i n e d by open-pit t e c h n i q u e s from the L o o l e k o p carbonatite. B, d i s t r i b u t i o n of p h o s p h a t e in the complex. (Redrawn from De Jager, 1988; Sawkins, 1990.)
151
4.3.3 P a l a b o r a Igneous Complex The P a l a b o r a Igneous Complex,
comprising a l k a l i n e rocks and carbonatites,
is one of the oldest ring complexes and perhaps the m o s t atypical complex of this type cated
in A f r i c a
in the A r c h e a n
inset),
gneiss
the Palabora
intrusion
(vail,
1989b), w i t h an age of about
terrane
(Phalaborwa)
(Northern
Pyroxenite,
around w h i c h
various
north-south
alignment
rock
types
of the eastern
2.05 Ga° Lo-
Transvaal
(Fig.4°18,
complex consists of three centres of Loolekop
are
Lobe,
arranged
(Fig.4.18,A).
The
Southern
in a
zonal
Pyroxenite)
pattern
quartzo-feldspathic
with
a
gneissic
basement is intruded by micaceous pyroxenites and syenites with an intervening
zone
of
fenetization,
the youngest
member,
a s~vite
carbonatite,
lying in the centre in the form of arcuate concentric bands. The p r e s e n c e of copper sulphides,
vermiculite,
apatite,
and magnetite
renders Palabora one of most highly m i n e r a l i z e d complexes of this type in the world
(Vail,
1989b).
of copper reserve
Sawkins
(1990)
attributed
the
300 m i l l i o n
tons
(at 0.69% Cu) to d i s s e m i n a t e d copper m i n e r a l i z a t i o n as-
sociated w i t h the final phase of the irregular d i k e - l i k e carbonate intrusion,
and to the presence of numerous
c o p p e r - b e a r i n g veinlets w i t h i n the
surrounding carbonatites and pyroxentites. the
orebody
cubanite, sulphides.
include
pyrrhotite
chalcopyrite and
various
Figure 4.18,B
shows
and
bornite,
nickel, the
The m a j o r sulphide minerals in with
cobalt,
distribution
apatite) w i t h reserves of about 13x109 tons
small
copper, of
amounts
lead
and
phosphate
of zinc
(fluor-
(de Jager 1988).
4.4 Vredefort D o m e Located on the southern part of the regional lineament on w h i c h the Great Dyke and the B u s h v e l d Igneous Complex are situated, (Fig.4°19,A). collar
of
The V r e d e f o r t dome
steeply
dipping
V e n t e r s d o r p and Transvaal
and
is an almost
overturned
Supergroups
strata
(Fig.4.19,B)
A r c h e a n d e e p - l e v e l granulites and granite gneiss, section
through
the
crust
of
the
Kaapvaal
is the V r e d e f o r t dome
circular of
structure with a
the
Witwatersrand,
surrounding
a core of
thus r e v e a l i n g a cross-
province.
The
radius
of
the
b a s e m e n t core is 18 km and the widths
of the collar and rim synclinorium
are
(Tankard
17 km and
20-30 km,
respectively
et al.
1982).
The
core of
the V r e d e f o r t Dome comprises a ring of A r c h e a n granite gneiss surrounding a central part of granulite four m e t a m o r p h i c events.
facies rocks.
The V r e d e f o r t
Dome experienced
152
/
Craton
~" <J i ~ / VC t" LJL)
'.-' Dome ps/~]!}:, /
k~ ~ /
/Durbon I Indian Ocean
/ J~Por 20kin
~
B_~~,-¢~
500kin l
i
~ t Elizobeth
A.
........... Mafic "si(~'ellIlc rR~Sl~s I n c l u s i ~
J
&
~. ":',,'',
Outer Granite Gneiss
~--Witwotersron~
B. MAFIKENG
~
VREDEFORT
HARRISMITH
x,,&
I
,;~~-~:
t
Figure 4.19: Regional tectonic setting for the Vredefort dome (A) and schematic cross profiles (B,C). (Redrawn from de la Winter, 1989; Marsh et al., 1989; Nisbet, 1987.) The about
latest
2.0 Ga,
event
that
represented
has
been
identified
by shatter
cones
is
which were caused by fracture-filling
with mobile
duced
during
at high
temperature
and
strain
This was preceded by thermal metamorphism which rocks.
produced An
albite-epidote-hornfels
earlier
metamorphic
morphism of the Witwatersrand
event
shock metamorphism
and pseudotachylite the
silicate material formation
resulted
Supergroup
- hornfels in
pro-
of the dome.
of the Witwatersrand
and hornblende had
at
veinlets
low-grade
facies meta-
rocks, and this in turn was preceded by the
153
earliest
metamorphic
event
the
that the dome p r o b a b l y originated another
probably
took
(Nisbet,
1987),
models
for
where
the
Fig.4.19,
view
place
the
at
coesite
explosive
that
during
the
origin
is
an
time of of
and
produced
suggests
event,
of
had
Although pact
presence
which
the
stishovite
explosive the
and
uplift
Complex
dome
is
the V r e d e f o r t
intrusion
Bushveld
model
in
rocks. rocks
from an e x t r a - t e r r e s t r i a l
metamorphism
Vredefort
intrusion
granulite-facies
are
shown
of
at was
about in
Fig.4.19,
imdome
2.0 Ga
emplaced.
depicted
in
the
Both
Fig.4.19, B
whereas
C implies the e x t r a - t e r r e s t r i a l impact origin of this enigmatic
feature.
4.5 Namaqua
Mobile Belt
W r a p p e d round the K a a p v a a l - Z i m b a b w e A r c h e a n linear n e t w o r k of
[./\
Proterozoic mobile belts
cratonic nucleus (Fig.4.20,
is a curvi-
inset), of
which
N amaq
4 ; }0!~ ~
.•i
D. fault . . . berg
1Gr~nau
J r
+ + + +
Oranjemun~ /
Fig.4.22"
I
ATLANTIC
A ~
iIRatelpoor t • \ lOkiep | lsprinbok ~
~
OCEAN
-IZl
lOOkm ,
l /
Figure 4.20: Tectonic graphic subdivisions of et al., 1982.)
/
=~
6o J i,~0.6 Ph. . . . . . ic ~ O~JGariep Province
Brakbos fault 1-2 Koras Group 1.3 Sinctair Group(Nomibia Province)
l
tter fontein
I
~.
8 1
Swartiintiies River mouth
I
~ .
/ Gomsberg I
~20Western zone < 21>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
<,J l
i~"
District
"~'~'.-
-
•r/~/,
~
~
\ ! J qamaquatand
lll~
1t I r/~°" u~
\X\
\
.
n .~. u=.~-- | "l Suite -1
~ar...........
~ ~
j°'
----2----=_-
beachbarrier complex ~carbonote ti%\Olatform
-__~/~" - --r'oTuuaer Gamsbergmine Western Namiesberghil ~ _ ' _ ~ r - - - - ' : ' B u shman land
~m
,!KleinK e r a s - - - ~
",%
l
Figure 4.23: S u g g e s t e d l i t h o f a c i e s map of the p r e - t e c t o n i c cover sequence in the Namibia Central Zone (Namaqua province). Inset d e p i c t s t e c t o n i c setting for the N a m i b i a n supracrustals in a backarc b a s i n and i n t r a c o n t i n e n t a l rift. (Redrawn from T a n k a r d et al., 1982.)
ATLANTIC OCEAN
~
~-.~.
~"k~..
\L_
~.as~ernvvvv
~ 0 INCE
I~
"
~
.~.H..ottentot Bay....
"~
CENTRAL ZONE '1 Rock Suite Znferred Parent Rock Bushmanland i~letam,suite Shal(ow to marginal marine Amphibolite and GrayGneiss Basic intermediate volcanicsJ Metapelite Distal marine and silt Marble, col- granofels Carbonate platform Metaquartzite Marginal marine Quortzofeldspathic pink Ftu via( ar kose Gneiss
G0
159
C e n t r a l Z o n e in N a m i b i a
The
Namaqua
Metamorphic
igneous and m e t a m o r p h i c gneisses
which
western part Suite ite,
Complex
in
represents
the
graphite,
(Fig.4.23)
pretectonic
(Luderitz district)
comprising
Namibia
basement and a h e t e r o g e n e o u s
metapelite
carbonate
rocks
in
carbonate
shelf
association
the
of
an
cover.
This
cover
in
the
is referred to as the Garub Metamorphic
gneisses
and dolomite.
consists
sequence of layered
with
intercalations
The dolomite
Namaqua
province
(Fig.4.23)
represents and
has
which
of
metaquartz-
the most
been
extensive
interpreted
broadens
westward.
as a
A wide
tract of gray b i o t i t e - h o r n b l e n d e gneisses of volcanic and p l u t o n i c origin separates
the
igneous
rocks
from the broad m e t a p e l i t e of the N a m i b i a n sector, gneisses which
with
point
(1982)
the
of the Vioolsdrif
Suite
region to the north.
relict
cross-bedding
to an ancient
and
continental
Early-mid-Proterozoic
arkosic
rift.
paleo-tectonic through a delta,
situated
in
the
terrigenous northwest.
muds Along
and the
signatures
to Tankard
setting
(Fig.4.23,
with
geochemical
According
the Namaqua M e t a m o r p h i c Complex filled
Zone
s o u t h e a s t e r n part
there is a broad zone of pink q u a r t z o - f e l d s p a t h i c
tinental rift trended northwest was
in the W e s t e r n
In the
of
this
et al. part of
inset) was one in w h i c h a con-
silt,
into a p a l e o - g u l f ~ w h i c h
with
southern
a
carbonate
margin
of
lay the c a l c - a l k a l i n e m a g m a t i c chain of the V i o o l s d r i f
the
platform
paleo-gulf
Suite and the vol-
canic and p l u t o n i c rocks now r e p r e s e n t e d by the b i o t i t e - h o r n b l e n d e gneisses. in
A back-arc
Fig.4.23
basin
(inset)
carbonate-quartz 2.2-2.1 Ga). are West
shelf
Tankard
et al.
association
This
implies
rift model
(1982)
who
was
also
postulated
argued
of the Garub M e t a m o r p h i c
p r o b a b l y equivalent
Supergroup.
western
and intracontinental
by
that
to the
carbonates
the Garub
of
dolomites
that
Suite
as the
(dated
the Griqualand
accumulated
on a
shelf at the same time with those of the epeiric G r i q u a l a n d West
S u p e r g r o u p in the Kaapvaal cratonic basin
(Figs.4.23,
inset;
4.21).
Namaqualand In N a m a q u a l a n d tonic
rock
(Fig.4.20)
assemblages
basement has not been identified. belong
(Tankard et al.,
1982),
the
Namaqualand
base
of
the
quartzo-feldspathic origin.
This
of
the
Namaqualand
Suite
there
gneiss
is
of
minor metaconglomerates,
Namaqualand
Suite
is
The pre-tec-
Metamorphic
Suite
also known as the Okiep or N a b a b e e p sequence. At
biotite
istics of tholeiitic Namaqualand
the
Metamorphic
lavas, famous
a wedge
probable
is o v e r l a i n by ~ e t a s e d i m e n t s
metaquartzites, Part
to
of
pink-weathering
continental
such as metapelites, calc-silicate
Suite
shows
rocks
geochemical
arkosic gneisses,
and marble. character-
suggesting the latter as the protoliths. for
copper
mineralization
near
The
Springbok,
160
where
there
are garnetiferous
quartzo-feldspathic
gneisses
and
metaquartzites as well as syntectonic granitoids and granitoid gneisses. Bushmanland
In the Central
Zone of Bushmanland
comprising
layered
by
but
a
thin
rocks
in this
regions
biotite
extensive terrane
nearby,
and
(Fig.4.20)
gneisses
with
metamorphic
merge with
comprise
cover
those
a poorly exposed basement
folded
schistocity
sequence.
The
of the Namibian
a variety
of
gneisses,
is overlain supracrustal
and Namaqualand
schists
and meta-
quartzite which are metamorphosed mainly at the lower amphibolite facies. In eastern
Bushmanland
pink gneisses
of pelitic
origin
are overlain
by metamorphosed basic and calcareous rocks or by a metamorphosed clastic sequence
in which
trough
cross-bedding
and pebble
sheets
are well
pre-
served. Based on its major element geochemistry the pink gneisses are believed to be derived posited and
possibly
its
trench
or
on
1982).
Quartz
inated
from sediment
was
followed
clastic
from shallow-water
in a fore-arc the
arenites by
the
sediments
pelitic
sediments
or back-arc basin between
continental
overlie
side
these
reworking.
of
pelitic
an
In some parts
accumulation
of
of
low-energy
and by Kuroko-type
arc
rocks
that were dea volcanic
(Tankard
and
of Fe,
orig-
this
predominantly
mineralization
et al.,
probably
Bushmanland
arc
unit
siliceous
Mn,
Zn,
Pb,
Cu, and S, which are capped by predominantly calcareous metasediments.
At
the top
of
of the Bushmanland
structural
sequence
are
layered
gneisses
granitoid or basic composition which may represent
intermediate to basic
metalavas.
seem
Since
the
Bushmanland
supracrustals
to
be
younger
(1.7 Ga), correlations with the Eastern province and the Matsap Group are unlikely,
because the latter is about 1.8 Ga old (Tankard et al., 1982).
I g n e o u s I n t r u s i o n s in the Central Zone
The Namaqua granitoid
Metamorphic
gneisses,
Complex was intruded
charnockites,
toids in the Namibian sector tectonic basic intrusions metalavas, sions.
highly
Syntectonic
and
syntectonic
mid-syntectonic
augen grani-
(Fig.4.24), while in Namaqualand early syn-
are represented by metabasites with tholeiitic
deformed
basic
intrusive
sives occur in Bushmanland.
gabbroids
by early
dykes
augen
and
granitoid
sheets,
and
gneisses
layered
and
basic
intruintru-
~
":'i:-.
~
.:,
8¢~y
.
.
.
krn l
50
~
~
s
s
l
c:~
INC
f
"
,,-o,S
',.)-9 :~#$L-'~-{~, ~ f)
';'e
District
,t
A FR IC A
Karasburg
N A MIBIA
suite
intrusions and
"-..'~.
Eastern
* . ~ ~.~.,,~.~.,,~'--'T'~m•Western
,f Karas • ' -
,,eio
.; ,,,
(mainly augen g. . . . diorite gneiss with even°rained toncdite at coast)
Charnoekile and enderbite
1
%ntrusloP~ ~ Lc=le-syntec,onic votccmic and cI;sti . . . . . . 1 Mid-syntectonic alaskilic megaw:rysticond biotite Syntect ethic gebbroid grsnite. LGte-synteclonic acid to inlerrnediQte
dp,,tecton~. . . . . .
CENTRAL ZONE ]U.e*po~d are. . . .
D1-D2 ~ E a r l y - s y n t e c t o n l c
~" to pre D2
I
I D2-D3 ~ D1 -?D3 /
": post D3 [ - ~
",~, intrusive \~uite :,¢,~
~ " Distrit z
LiJderitz
"~.~xcelsox rny|onite bell ~ ' - E a st e r n
-~x_;~_X
,~ ' %~ ' -.~ •
Aus
~
I
Figure 4.24: Central Zone (Namaqua province) syntectonic cover sequence. (Redrawn from Tankard et al., 1982.)
.
OCE A N
ALANT,
I ~
|~Hottentot
~ =
N A M I B I A --114elmerin gh° usen
i ~ w,ste,~ LUdemz DiSi,ict ~
_i
162
Tectonics
Two
of
the Central
orogenic
events,
Namaqua o r o g e n y Thus
the
the
Orange
River
orogeny
(2.0
into
history
early
of
the
low-angled
Namaqua
and by the intrusion
of sheet-like
an event involving upright folding, Major which
thrusts
separate
Bushmanland
1.7 Ga)
Metamorphic
thrust-dominated
and
and
it
the
bodies.
which
can
were
be ac-
up to the granulite This
was
followed by
late b u c k l i n g and ductile shearing.
shear
zones
the
Kaapvaal
from
Complex
events
c o m p a n i e d by the most intense degree of metamorphism, facies,
-
(1.2 - 1.0 Ga) have been r e c o g n i z e d in the Central Zone.
structural
simplified
Zone
occur
in
the
province
s u b p r o v i n c e to the west.
Eastern
to
the
Marginal
east
and
Zone
from
the
In the latter region up to four de-
formation episodes have been recognized involving the formation of thrust sheets,
imbricate
thrusting
structures,
episodes
were
ramps and large scale r e c u m b e n t
followed
by open
folding
and
folds;
subvertical
the
shear-
ing. Early d e f o r m a t i o n in Namqualand resulted in a regional finite fabric and
folding
clinal
ranging
folds
periods
of
in
from
shallow
southern
steeply
and
inclined
and north-northwest,
folds
eastern
in
the
north
Namaqualand.
folding with
axial
to
trends
during which major periclinal
tight
This
was of
folds
and
iso-
followed
by
east-northeast formed through-
out eastern N a m a q u a l a n d and re-folded the older flat-lying folds. Major terranes
shear and
Groothoek Western result
zones define most
zones
thrust
Zone of
in
and
1989). toids
at
Thrusting with
of
(Fig.4.25,B)
was the
Metamorphic in
western
the Namaqua
tectogenesis,
about
of the W e s t e r n
province;
rocks
Namaqua
dips
Namaqua
(Fig.4.22)
from the
the
westerly direction west
the
belt
of the boundaries
between
Complex.
For
Namaqualand
province
the W e s t e r n
accompanied
the north
the
south.
generally
As a
in a southstrikes east-
(van der Merwe
by prograde m e t a m o r p h i s m grade
the
in the
Zone and by retrograde m e t a m o r p h i s m metamorphic
example
separates
Zone m o v e d
along a thrust plane w h i c h
30 ° towards
the various
and
Botha,
in the graniin the Namaqua
increasing
towards
the
south. The Namibia
Tantalite
Valley
from
Western
the
mylonite Zone
belt
separating
(Fig.4.23)
trend
the
Central
roughly
northwest
about 510 km and marks a zone of m o s t l y dextral d i s p l a c e m e n t 85 km al.,
under
to
upper
amphibolite
in for
for at least
metamorphism
(Tankard
et
1982). Watkeys
Proterozoic age.
greenschist
Zone
(1989) age,
observed
it shows
Both h i g h - g r a d e
that although
similarities
terranes
the Namaqua m o b i l e
with
the Limpopo
record an initial
belt is of
belt
of Archean
crust-forming
event fol-
163
~ •~
Nama and Koroo sedimentary overburden ~ Lote Precclmbrian Gariep Bell ~-., Nomdqua P. . . . . c e Nclmoqua. "~' i~/
t I
I
~S,udy ....
%~
Teclonic
o o o X;:'\
Direction /
/A ] /!gl ~,'6'~.i..\'~
o o
Stretching
"~-o ~
:::::-
.
I
:/..'.:" . . . .
Oranjemun-~5 o ° ~ . . ' " ~ . ~ ' k .
~rm?.qo,.~/
OoOOo ~,~
I
~--~uffels
T Q
17" & s ' T B
1
. > : . . / "
A
River Shear
J
'T
I
18° '00'
Eo~o o--~Or~,ge ~,er
'ioo,,dr.
j
'
-29"
~00 ~
I
~',,'~':$'~ -~qiver T h E u s ~ : ~ [ ' : L _ _
B
Y////2~.."~ : : ,,,,?o~-%;;';~,o Vyo% ;;o o\ ,~-{_,''L,~
oo ~'°PO°"
" ' " ""
~'~",~,;,'Z",] Kinderl~
~~.._..!~.i!r,,v.,o
(1200--1900 M o )
~
~o,~ so00rooo ( 2000
• , • :'~-~--~.~ -" Province
)" I~ ~
~
-
~,',"~X\\\N 9 m a q u a Province
Tomibosmond
Thrust
Namoquo
M a) F . . . . tion
~
Groothoek
k\\\l
Area of influence of the Groot hoek Thrust
I
province --'P I ~
Groolhoek
Gneiss
Richterseld
Thrusl NOUS
10
0 L
,
I
Province
Sheors
20 km -----J
Figure 4.25: Geology and tectonics of western (Redrawn from van der Merve and Both°, 1989.)
Namaqualand.
164
lowed
by
the
deposition
miogeosynclinal forming
mechanism perhaps
which
as
causing
sequences
in both belts,
opposed
to
of
Whereas
quartz,
and
pressure was the varying
dis-
However,
the
domains
between
or
horizontal
movements.
in both
processes
carbonate
in a crust-de-
large-scale
tectonic
metamorphism
contrasting
pelite
prior to reworking
involved
vertical
granulite
on account
Proterozoic.
dominantly
supracrustal
event,
placements,
of
was
different,
the Archean
and the
factor in the Limpopo belt
where a high pressure granulite event was followed by rapid depressurisation to low pressure bolite with
facies,
steep
paleo-geothermal
the crust. 1.0 Ga rare
The syn-to
in the the
conditions
two
belt represents
gradients
belt which
belt.
regions
were
due
was to
probably
due
to the
belt with a colder
where the continental margin was subducted, much crustal
reworking.
The Namaqua belt,
genic belt that involved crustal
the amalgamation
transfer
in the
that fact
in
1.2 -
crust
are
the differences that
thicker
5 kb,
at about
transfer
concluded
to amphi-
at about
heat
intrusives
heat
(1989)
constant
magmatic
granitoid
caused
Watkeys
an orogenic
and then rehydration
belt pressure
late-tectonic
Namaqua
in the Limpopo
between
granulite
in the Namaqua
the
Limpopo
lithospheric
root
deformed and uplifted without
in contrast,
represents
of various
terranes,
an oro-
extensive
reworking and the addition of juvenile material.
M i n e r a l i z a t i o n in the C e n t r a l Z o n e
In the area north of Springbok and Okiep in Namaqualand bearing
late-syntectonic
bodies
such as gabbronorites,
ites intruded mostly granitoids, the emplacement copper
mining
are g e n e r a l l y
small,
irregular
shape
pipe-like
shapes
an average
in the horizontal in
the
emplacement
was
from vertical
structurally
and
within
gneisses°
controlled by the deposition in the original At Copperton ization siliceous
occurs
The
lavaso
Tungsten
deposit
of wolframite
in southeastern
to pelitic
is of magmatic
in the Springbok
sedimentary rock
between
was
The
They exhibit
et al.,
Their
Their
major
mineraliza-
in a quartz-biotite-
probably
stratigraphically
as a heavy mineral
Bushmanland,
concentrate
1976).
strata-bound gneisses
large ore reserves
pinching
being derived
(wolframite) area
or
1982).
bodies
origin,
(Anhaeusser and Button,
quartzo-feldspathic
gneiss.
with
bodies
sheet-like
most
cupriferous
rock
(Tankard
basic
1 km.
controlled.
from parental garnet
of about
horizontal
Copper m i n e r a l i z a t i o n basic
This is the centre of the
copper-bearing
lithologically
to near
sporadically
length
section
out at depth.
tion also occurs
The
section and anastomosing
vertical
axes
plunge
granitoids.
in South Africa.
with
and dior-
around 1.07 Ga, after the major phase of
of late-syntectonic industry
over 700 coppernorites
Cu-Zn mineral-
and in this
an
overlying
deposit
are
165
the second largest source of zinc in South Africa,
after those of western
B u s h m a n l a n d w h i c h are d e s c r i b e d below. The
Aggenys-Gambsberg-Swartberg
(Fig.4.23)
has
the
largest
area
zinc deposit
in
in Africa
tones of ore at 7% Zn and 0.5% Pb (Goossens, This d e p o s i t zation.
with
in
the
Aggenys-Gamberg
Bushmanland
over
in
intimate
stratigraphic
association
b a n d e d i r o n - f o r m a t i o n and quartzites c o n t a i n i n g Mn and sulphides. minerals barite
include
sphalerite,
horizons
occurring
argentiferous
as
lateral
The f o l l o w i n g are the evidence
galena
and
equivalents
of
for the d e p o s i t i o n a l
1982).
iron minerali-
occurs at three d i s t i n c t sequence,
150 million
1983; T a n k a r d et al.,
is part of a huge base metal and A l g o m a - t y p e
Base m e t a l m i n e r a l i z a t i o n
horizons
western
with
The ore
chalcopyrite the
sulphide
environments
with ores.
and the
source of the metals. Fine b a n d i n g
in the ores
tary conditions.
suggest
precipitation
the metals w e r e derived from upper crustal sources. with barite
and sulphides
cipitation distal
under
quiet
sedimen-
Lead, s t r o n t i u m and sulfur isotopic studies suggest that
from
suggest a submarine e x h a l a t i v e o r i g i n and pre-
seawater
position
from
the
The iron oxide facies
under
reducing
volcanic
vent
conditions
(Strydom
in
a
et al.,
proximal
1987).
or
Erosion
and r e n e w e d supply of clastics after the chemogenic phase is suggested by the
overlying
mainly mafic extrusive
metamorphosed metavolcanic
tholeiitic
lava
arenite
rocks. flows
and
conglomerate,
The mafic
volcanics
in
a
sequence
probably
in a shallow basin w h i c h was
of
represent part of an
arc to b a c k - a r c tectonic setting. The
genetic
iron-formation is v e r y
model
for the base metal m i n e r a l i z a t i o n
in w e s t e r n Bushmanland
instructive
regional tectonic
in that
framework
it places
and A l g o m a - t y p e
p o s t u l a t e d by W a t s o n et al.
(1989)
the m i n e r a l i z a t i o n w i t h i n a lucid
(Fig.4.23). An Early P r o t e r o z o i c plate colli-
sion event b e t w e e n 1.9. Ga and 1.75 Ga, involving the b a s e m e n t components in
the
area,
was
followed
by
the
extrusion
rocks to the n o r t h in the V i o o l s d r i f area Immature posited
and in
reworked
a
closed
a c c u m u l a t i n g deposits metal-rich creased
brines
metal-rich sedimentary
of
volcanic
near
the
expelled.
in i n t e n s i t y with
time,
These
detritus
original
were affected by c o m p r e s s i o n a l
were
acid
volcanic
(Fig.4.23) of the W e s t e r n Zone.
subaerial basin
subaerial
events
compressional
finally ended
were
de-
suture.
The
during which
events
at the onset
of
which
in-
intensive
t e c t o n i c a c t i v i t y as e v i d e n c e d by the r e d u c t i o n of the size of the basin and
by
the
presence
of
intraformational
conglomerates
and
eruptive
tholeiitic v o l c a n i c rocks at about 1.65 Ga. In the anoxic portions of the basin metal
sulphide deposits
accumulated
from brines
e x p e l l e d along in-
cipient
thrust
followed
by
faults near the southern boundary of the basin.
low-angle
thrust
faulting
which
rocks in thrust slices and associated tectonic activity,
preserved
synformal
the
This was
supracrustal
folds. A long history of
granite intrusion and regional metamorphism then fol-
lowed which apparently lasted over the ensuing 500 million years
(Watson,
1989). According to von Knorring and Condliffe tantalum
pegmatites
in Namibia
are those
(1987) some of the best known of the Tantalite
are located about 30 km south of Warmbad (Fig.4.24).
Valley which
These pegmatites be-
long to the much larger pegmatite region in the Orange River belt within the Namaqua Metamorphic (Homestead, abundant
White
Complex.
City,
tantalum
Three of the larger
Lepidolite)
mineralization
Tantalite-microlite,
in
the
which
montebrasite,
lithium pegmatites
Tantalite
occur
spodumene,
in
Valley,
contain
gabbroic
rocks.
beryl,
common
caesian
beryl, and bismuth minerals have been mined from these pegmatites.
4.6 Natal Province This Early to Middle Proterozoic orogeny,
located along the eastern coast
of South Africa covering a distance of about 280 km in the Natal Province (Fig.4.26,A), underneath Kaapvaal inset). Africa Late
is
the
a
province, Tankard
into
gneissic
cratonic to
et al. to
Saldanian origins). according
cognizable
link
terrane along
up with
(1982)
which
the
Early
grouped
basement
Paleozoic
the
Tankard beneath
et.
of
westward
the Archean
belt
belts
and Natal
Pan-African
al.,
extends
mobile
orogenic
Namaqua
The main differences
to
probably
southern margin
the Namaqua
the Early-mid-Proterozoic
Proterozoic
belts,
cover
belts
(Fig.4.26, of
southern
belts,
and the
(Damara,
Gariep,
between both groups of orogenic are
the
the o l d e r orogens.
extreme
rarity
The supracrustal
of
re-
succes-
sions in the Natal and Namaqua belts have been correlated with the Early Proterozoic
sequences
in the Kaapvaal
West and Matsap sequences;
province,
against the orogens with a tectonic contact Pan-African orogens, ments,
such
as
the
Griqualand
and these older cratonic cover often terminate (Figs.4.21;
4.26,A).
All the
in contrast are underlain by rigid crystalline base-
and contain thick miogeosynclinal and eugeosynclinal sequences.
Only the northern part of the Natal mobile belt is exposed, where allochthonous greenstones,
rocks have
(Fig.4.26,B,C), been
some of Which resemble typical Archean
extensively
foreland of the Kaapvaal province
thrust
onto
(Tankard et al.,
the
Archean
1982).
cratonic
The Natal belt
?igure 4.26: Geologic map ~ankard et al., 1982.)
~
~
~
NATAL PROVINCE
.
~
~
. " : .'.'.:.::.:,:i:'.~.~-'. : : ~ : ~ , " ' C / ' ~ ' :
~--~.iC suite ~ Granitoid gne [sses
~,.~IUlra basic inlrusions E'I-L'~ Mapumulo metQmorph[{;.~]{~'.l
.j
Melange
Structurat sequence ~ M f o n g o s ~ meiamofphic ~ Siide Zone • Tugeta metam. Suite ~ !
/ f
.
;=~::~:~: :
~-~ ~ +. f j ~
A ~ ~':
C
~
Group
Gneiss Gneisses
Redrawn from
Nondweni greenstones Nondw
Pre-Nsuze gr :~nitoids
Nsuze
Ntlngw
N11ngwe FormQtion
Po,,-N Post-Nsuze
~
~
IA APVAAL PROVINCE M ,AAF' VAAL ,"I'
~
t
KAAPVAALPROVINCE
: ":'.". C.~.7_,': . : "
(A) and c r o s s - s e c t i o n s through the Natal province.
ZN
Zone
U
168
has been the
subdivided
Central
Zone,
into the Northern Marginal and
the
Southern
Zone.
these zones is drawn from Tankard et al. Northern Marginal
This tectonic row,
thrust
following
Zone,
description
of
(1982).
large thrust nappes which
onto the stable Kaapvaal
southward-dipping
Marginal
the Northern
Zone
zone comprises
ported northward
The
Zone,
belt
Natal
thrust
comprises
foreland,
belt
have been trans-
separated
(Fig.4.26,B).
imbricated
slices
of
by a nar-
The
Northern
two
unconformable
supracrustal sequences metamorphosed to greenschist facies.
These are the
younger Ntingwe formation representing a transgressive sequence deposited unconformably on a stable shelf which was the crystalline Kaapvaal land;
and
older
Mfongosi
Metamorphic
al-
lochthonous thrust nappes that have overridden the Ntingwe Formation.
The
comprises mostly argillites
Suite
which
and basic lavas.
consists
foreof
Mfongosi
the
South of the Natal
thrust belt is a wide westward-plunging nappe complex comprising four extensive
thrust
morphic
Suite
lavas
and
nappes
which
collectively
(Fig.4.26,B,C),
subordinate
clastic
constitute
an
ophiolitic
and
chemical
the
sequence
Tugela
with
sediments,
Meta-
metabas~te
which
have
been
metamorphosed to the amphibolite facies. Northern
This
Zone
zone
cover
contains
rocks,
heterogeneous
grade
steeply dipping
to
synformal
as the Mapumulo
belts
Metamorphic
of
precratonic
Suite.
This
is a
suite consisting of high-grade migmatized psammitic gneiss,
metagraywacke} continental
four
referred
and metabasalt,
provenance
"basement"
all of which
deposited
in
is present which,
represent
fault-bounded
however,
clastic wedges of
grabens.
A granulite-
may represent younger grani-
toids that were intruded syntectonically beneath the supracrustals, the latter had formed.
Metamorphism at high-temperature
after
and low-pressure
caused partial melting in both "basement" and the supracrustals resulting in the formation of migmatites and elongated granite plutons of anatectic origin. After folding, Central
This
is
massive
shear belts and pseudotachylites developed.
Zone
a
large
or
zone
foliated
characterized biotite
or
by extensive
hornblende
masses are separated by septa of augen gneisses, and
metamorphic
metamorphism
suites
which
that
locally
formed
under
attained
the
batholithic
granitoids.
The
bodies
of
batholithic
charnockitic intrusions,
high-temperature granulite
facies.
low-pressure These
rocks
169
probably
represent
relicts
of
a
formerly
much
more
extensive
granulite
terrane w h i c h has largely been retrogressed to upper a m p h i b o l i t e facies. Southern
Zone
This n a r r o w zone is characterized by a more e x t e n s i v e d e v e l o p m e n t of the charnockite-granolite
assemblage
Here the p r e c r a t o n i c isoclinally,
northern
parts.
cover sequence has been intruded by granite,
folded
metamorphosed
to
than
the
is
found
granulite
in
the
facies,
and
intruded
by
an
e x t e n s i v e charnockitic suite. Tectonic Model The N o r t h e r n M a r g i n a l
Zone of the Natal m o b i l e belt contains the tectonic
imprints
collision
of
thrusts
a
and
foreland, Tugela
major
fold
its
nappes
supracrustals,
basic-ultrabasic
believed
to
represent
of
lavas
with
northerly
and
on
suite
and
sediments,
that
the are
was
obducted
of
pillowed
metalavas,
subordinate metachert and carbonate rocks, with layered basic-
ultrabasic
intrusions
of the Tugela
lavas
of the Natal
province
tectonism
structions
may
Suite,
the
verging Kaapvaal
The
the M f o n g o s i
by
the
argillites
subordinate
ophiolite
supported
its sheets
northward.
suggest
is
an
from
thrust
Mfongosi
extrusives
view
Apart
override
the
parts
latter
zone.
which
association
in which
case the
represent m e t a m o r p h o s e d
oceanic
have not been e x t e n s i v e l y
dated,
around
1.0
Ga.
With
Proterozoic
of past continental positions
suggesting
latter and
crust. and the
The rocks few ages
paleomagnetic
recon-
that the eastern ex-
tension of the Natal province lies in the h i g h - g r a d e terrane of Dronning Maud Land lision
in Antarctica,
probably
it is conceivable
involved
an
eastern
that plate m o v e m e n t s
cratonic
nucleus
in
and col-
Antarctica
and
the Kaapvaal p r o v i n c e being driven against each other.
4.7 Magondi Mobile Belt
Stratigraphy and Structure The Magondi cratonic
nucleus,
over about mobile
belt
belt
is
located
where
on the
it extends
northwestern
a thick
of
the
in the n o r t h - n o r t h e a s t e r l y
250 km, with a width of about contains
margin
supracrustal
50 km
(Fig.4.27,A).
sequence
that
Zimbabwe direction
The Magondi
underwent
phase d e f o r m a t i o n and m e t a m o r p h i s m from g r e e n s c h i s t to a m p h i b o l i t e during the Magondi orogeny between 2.15 Ga and 2.0 Ga;
and was
polyfacies
later in-
.
25
r
30" East 0"
East
•
÷
~
+
+
~
[~
Shales Archean granitic crust
Conglomerate
~ [~
3
÷
~
a
-
+
"~ ÷
~---0-o8o
,
+
~
l
+
+
+
3km
I0kin01
+
+
"
' St d bI e__..,.. j Shelf
.i
oU--.~'~."rO~.-.,,
.=~fTransitional
i
,,~ LOMAGUND~ GROUP--~
• , _o~O ~ > ~
-~"'~ °~°~--~'--_=
Volcanlcs Chert
+
.~ 0 ~
--Z'~
, ~ Quartzite Dolomite
÷
~/,
G.,n,~..-'-c"_ -.-'~
) Cover
~
Oro~wacke arenite
÷
_
GROUP
FLYSCH
PIRIWlRI
Archaec~n Basement Fault
D eweras
=.omogoo~i
ii~Graphitic shales [ ~
÷
~
OJ~"I''-4~ •
• °~ / ~ / C ~
I
I
--
Se.ime.,s Sedimenls Voicanics
piriwiri
~igure 4.27: Geologic map (A) and stratigraphic sections (B, C) across the Magondi mobile belt. [Redrawn from Leyshon et al., 1988.)
)
(-Copper King ~- Copper Quee
,.-4 Northern [i( Green schist Met amorphi:
X~ Archaeon (
*---1Intrusive gr,
Deweras
Lomogundi
YI Phanerozoi¢ i Piriwiri Diatreme G
~agondi *'~Limp opo ;//Trend /
~ Ph(~nerzoic (1 .~ Zambez~ ) r t
~
~C~
otswanc= x
L,
Zambia
171
truded with veins during a late stage of the Pan-African along its northern margin. The metasediments
and metavolcanics which con-
stitute the Magondi
Supergroup are divisible
and Piriwiri Groups
(Fig.4.27,A).
The Deweras Group comprises of the alluvial arenites
(Fig.4.27,B);
into the Deweras,
unconformably
intercalated
on
the
volcanics
vesicular and amygdaloidal
Lomagundi
a lower sequence of continental
fan facies with conglomerates,
resting
Zambezi orogeny
cratonic
with
red beds
lithic wackes and arkosic basement
alkali
of
Zimbabwe
metabasalts,
massive,
lavas; and an upper arenaceous
formation, with
argillites and subordinate arenites which represent the shallow water deposits
of
an
extensional
basin.
mentology of the copper-bearing lower
sequence
consists
Masters
(1989a,b)
discussed
of the initial
rift alluvial
Mangula Formation and rocks of a playa environment, both of which have been studied in detail alluvial sists
fan
of
arenites
and braided
trough of
interfingered argillites sequence bedded
lithic
the
distal
or
a
rapidly
massive
anhydrite-bearing
wackes, fan
which
dolomites,
Formation
conglomerates
and
and and
Norah
of
arkosic
interbedded
with
argillites
and
meta-
formation
shallow
The con-
laterally
carbonate-bearing
The
succession
dolomitic
the
the Norah formation,
are vertically
environment.
arenites,
The
fan deposits,
of the Mangula
siltites
alternating
arkosic
sedi-
in the Norah copper mine.
braided-stream
environment
with
of of
stream deposits
cross-bedded,
the mid-fan
the
lower sequence of the Dewaras Group.
is
a
water
plane-
thinly
bedded
argillites.
The
dolomitic units, which are very thinly bedded and with flaser and lenticular
bedding,
represent
arid
evaporitic
playa
flat
deposits
on
the
shores of an ephemeral playa lake which were subjected to rapidly alternating
wet
and
dry
spells.
Since
environment,
the Norah Formation
lake
or
shrank
margins,
disappeared
the alluvial
it
represents
is impersistent
during
renewed
fan lithologies
an
ephemeral
playa
so that when the playa
tectonism
along
the
rift
of the Mangula Formation prograded
over the Norah playa facies. The Lomagundi Group which succeeds the Dewaras Group is predominantly a
clastic
carbonates, slates and
sequence while
with
and graywackes,
Tennick
(1988)
miogeosynclinal
pyritiferous
the Piriwiri
flat,
the
and
essentially
stromatolitic of phyllites,
of chert and quartzite.
Lomagundi
subtidal,
with carbonate platform development,
argillites
consists
with minor bands
interpreted
tidal
Group
Group
marginal
as
stable
orthoquartzite
Leyshon shelf
or
deposits
and the Piriwiri Group as the coeval
flysch deposits of the eugeosynclinal basin
(Fig.27,C).
t72
The The
Magondi
bulk
Magondi in
of
the
belt
the
belt
(Fig.4.28,B).
deformation south
The
form
of
in
the
by
SE-directed
central
and
thin-and
and
thrust
southern
by a thin-skinned
both
central
facies,
tectonic
parts models
attained
tectonic
granulite
processes
the
apply
increased
horizontal
of
the
whereas
belt
and in some places
parts
of
model,
thick-skinned
southern
tectonics.
but towards the north regional m e t a m o r p h i s m
to the amphiobolite between
characterized
can be explained
extreme
greenschist
is
facies.
operated
in
Apparently
the
some
Magondi
belt
2.15 Ga and 2.0 Ga, although the details are not yet clear.
Mineralization
Strata-bound per mines
copper mineralization
are
situated
(Fig.4.27,A).
In these mines
200 m thick, quartzites, bornite which
with
and
mation.
The
oxidizing the
(Leyshon
include
copper-silver
red beds,
ore
basin
absorbed
moved
copper
2 km
and
gold,
bodies
permeable
of argillites,
1988).
The
platinum,
were
which
Norah
in a zone up to
rocks
to the evaporites
brines
and
The major ore minerals
Tennick,
onto alteration
along
at Molly
occurs
in host
silver,
was related
chlorine-rich
picked up metals
of
sediments
and conglomerates.
chalcopyrite
Mineralization
in the Deweras Group where cop-
clastic
length
grits
produced
occurs
the
disseminated
a strike
arkoses,
are
selenium.
within
formed
reacted
horizons
by-products
palladium
and
of the Norah
for-
diagenetically
with
products
are
of
and the
the
labile minerals basal
by
evaporites, in
unconformity
of the Deweras Group, and precipitated
sulphides under reducing conditions
in
(Masters,
the
Mangula
orogenic
and
events,
microcline
Norah
the ores were recrystallized
veins
injected
and cleavage-parallel A
variety
Groups the
of
(Leyshon
Piriwiri
Formations
into surrounding
the
adjacent
mineralizations where
occur
1988).
gold
northeast-trending to
small
diatremes
Copper
King
galena,
arsenopyrite,
domes
are
in
Over
Group.
sidered
the Cu-Pb-Zn and
Although
as sources
quartz
the
Piriwiri
into quartz-
with
intrusive
the
and
deposits
occurs
in
in dark brown vein quartz;
and
Belt
Around
deposits
an
known
red
Mineral
in
Lomagundi
are
usually
the
Copper
(pyrrhotite,
magnetite,
cubanite,
within Queen
or and
chalcopyrite,
marcasite,
etc.)
lenses in a skarn which is part of the Piri-
of the metals, are
Piriwiri
unusual
(Fig.4.27,A).
ores as syngenetic veins
the
50 gold
mineralization
sulphide
pyrite
which occur in discontinuous wiri
and mobilized
subsequent
rocks and into local cleavages
earth with boulders of Lomagundi dolomite; along
During
veinlets.
and Tennick,
Group
1989).
granites Leyshon
strata-bound
sources
of
of the
domes
and Tennick
mica,
could
be con-
(1988)
considered
mineralizations.
Pegmatites
beryl,
tantalo-columbite,
lunon( I Vond
od
km
3
~
,
,~,,
~-q Archraean
Argitliles guartzites Cherts on, d/or Dolomites [ ] S e dBasalt iments
Th in-Skinned
> DEWERAS
L OMAGUNDI
~-BASEMENT
COVER
Rhno 1
Thick-Ski~ned
Rhino
, Thln-Skinned
\
W~
I
Thrus~
I ~I
O~
I,
2.
I
Murison I
0
:~3 km
~-C
km
(~
Murison @
. - ..... "FC~ult
..i~- ~
I
Chig. . . . I
Chigwena
?igure 4.28: Tectonic map (A) and structural sections (B) showing suggested thin- and thick3kinned tectonic models for the Magondi belt. (Redrawn from Leyshon et al., 1988.)
A
te mite
•
~')Q
0
) ~
4
174
topaz,
tourmaline,
mineralization
tin
may
and
relate
tungsten to
the
at Mwami
Late
near
Karoi,
Proterozoic
although
event
in
the
this
nearby
Zambezi belt. Lenses of calcareous m e t a - a r k o s e in the L o m a g u n d i graphitic schist
host
extreme
a stratiform
north
evidence thermal
of
the Magondi
suggest fluids
copper-iron
that
near
these the
belt.
sulphide Sulphur,
sulphides
sea
bed,
near
Shamrocke
oxygen
could
or
at
have hot
and
Mine
carbon
originated
spring
in
the
isotopic
from hydro-
vents
during
the
formation of m a r i n e limestones in an evaporitic environment.
4.8 West African Craton
4.8.1 I n t r o d u c t i o n Outside
southern
Africa
the
West
African
craton
is
the
next
region
A f r i c a w h e r e L o w e r Proterozoic rocks are e x t e n s i v e l y preserved. mian
supracrustals
stones except the
amount
wackes
of the Guinea Rise
(Fig.4.29)
r e s e m b l e Archean green-
for the absence of komatiitic volcanic
of
chert
(Condie,
and
1989).
the
preponderance
The
West
African
in
The Biri-
rocks,
reduction in
of v o l c a n i c l a s t i c s craton
stabilized
and
gray-
during
the
Early P r o t e r o z o i c Eburnean orogeny which also s t a b i l i z e d the Zaire craton (Clifford,
1970),
neighbouring
and
regions
affected
in
South
vast
America
Eburnean tectono-thermal province The eastern in
the
south
Eburnean
parts that
and
the
Reguibat
where
in
coterminous
in both
the
Archean
Africa
and
with
the
1971).
craton
shield
reactivated
western
were
(Hurley et al.,
part of the West African
domains,
of
the Guinea
north,
Rise
constitute
basement
rocks
and
the Lower
P r o t e r o z o i c schist belts and their engulfing large concordant syntectonic batholithic orogeny,
granitoids,
and
(Fig.4.29,
became
inset).
attained
welded
East
and
cratonic
onto
the
north
of
conditions d u r i n g
western the
Archean
Eburnean
A r c h e a n and Eburnean b a s e m e n t rocks dated between
the Eburnean
cratonic
craton,
nuclei
remnants
2.2 Ga and
1.8 Ga,
of are
known, w h i c h had survived the Late Proterozoic P a n - A f r i c a n events. Relict Eburnean
basement
ortho-
and
Ibadan
granite
shield
(Fig.3.39).
inliers augen show
para-gneisses
occur
gneiss, an
occurs
gneisses
high-grade
in
the West
and
the
Further north,
as
the
and
as
important
as
Kerdous the
granitization
and
Kaduna
Central
schists,
migmatites event
at
or
granulite
Hoggar,
migmatites
in
in the A n t i - A t l a s
mica
Zenaga
amphibolite
1.95 Ga
the
Eburnean
migmatites
granites, (Cahen
among
Benin-Nigeria
(Fig.3.39),
amphibolite and
and
the
facies
all et
of
al.,
and
which 1984).
LIBERIAN CRATONIC NUCLEUS (KANE;MA -;MAN )
EBURNEAN- LIBERIAN TRANSITION ZONE
EBURNEAN PROVINCE WITH VOLCANIC BELTS ( BIRIt41A;M )
Y COVE,
~ohrovia
," -41
,
KENEMA ~-MAN DOMAIN t
~OU
./~'J.,,~'~>)"k
~:..
I
,'r~,V..ixA.l,,/I;,en~'+ T /;97/P"
~td|on~
.......
/i'~'~(
300km
ACCrO
m
'igure 4.29: Sketch map of the Birimian of West Africa. I, Tarkwaian facies; 2, Sedimentary [lysch; 3. Volcano-clastic facies; 4, Greenstone; 5, Eburnean granitoids; 6, ? pre-Birimian; 7, Un[ifferentiated "basement" of the Baoule-Mossi domain; 8, Voltaian Supergroup; 9, Limit of Phanero;oic cover; 10, Recent. (Redrawn from Cahen et al., 1984).
/,~
SEOI,,N,,
/,
PAN - AFRICAN PROVINCE
,o,T-,,o,,,,N
I ; /
/ ._%": ~T~.'/. \ \ t4 :, I
I' ~
"" L/-..~)~LEKenieboand
176
The
Uweinat
inlier
in
the
eastern
Sahara
(Fig°3.44, inset)
also
c r a t o n i z e d d u r i n g Eburnean tectono-thermal events at about 1.8. Ga. 4.8.2 B i r i m i a n S u p e r g r o u p The B a o u l ~ - M o s s i domain in the eastern part of the Guinea Rise was where the
Birimian
Coast,
Supergroup
Ghana,
(Fig.4.29).
Burkina
The
accumulated Faso,
Mali,
Eburnean province
w i t h the B i r i m i a n terrane
in parts Niger,
of
the
Guinea,
is a regional
(Fig.4.29,
inset),
republics Senegal
t e r m that
of
and
Ivory
Liberia
is synonymous
and used m o r e c o m m o n l y than
the B a o u l ~ - M o s s i domain. The
Birimian
Supergroup,
following
the
definition
(1984), includes the T a r k w a i a n Group as well. Ghana, and
the B i r i m i a n is p r e d o m i n a n t l y a tightly folded,
volcanic
during
the
teristics
of
Birimian Archean
sequence, Eburnean this
is
extensive
believed
crust,
to
which
Liberian
basement
However,
no
which
in
deformed it
case the
has
in
was
been
be
that
nuclei
between
the
evidence cludes
for
the
Birimian nature
the
sometimes in a r e a c t i v a t e d state
presence
contrasting
the
Ivory Coast
clasts
and
and
migmatites.
Archean
axial
general
NNE
in
the
basement
and
surrounding
Faso,
which
seems
suggest
on the
west. Super-
Indirect
Birimian
in-
between
the
styles
rocks,
conglomerates
to
the
the
is exposed all
the
crystalline
basal
to
(Fig.3.39).
beneath
metamorphic
Birimian
in
that
and
the
parts
of
the
Birimian
from and deposited on L i b e r i a n - a g e b a s e m e n t gneisBirimian
directions
Birimian
trending
and
within
Burkina
was p a r t i a l l y derived ses
deformation
supracrustals
of
of Archean
with
Birimian
group and its p u t a t i v e Archean crystalline basement w h i c h around this domain,
characthat
developed
continuous
cratonic
traced
et al.
2.13-1.8 Ga
regional observed
basins
probably
Archean
about
the
should
accumulated
Cahen
low-grade clastic
at
considering
supergroup
this
contact
last
Before
have
in
exposed
direct
was
orogeny.
of
In its type area in western
schist
of
structural
belts
Ivory Coast
trends
the K a n e m a - M a n occur
and Ghana
are
domain
superimposed (Fig.4.29,
as
parallel,
but
turning
elongated ENE
on
inset).
the In
troughs,
in Burkina
Faso,
and to the NW in Mali. A c c o r d i n g to Black and Fabre occurs either
in troughs
(1983),
containing
and Tagini
(1971),
flysch deposits
s h a l l o w basins w i t h v o l c a n o - s e d i m e n t a r y sequences
the Birimian
(Type I) or as broad
(Type II). The troughs
and basins are s u r r o u n d e d by vast antiformal areas of A r c h e a n reactivated and
undifferentiated
basement
gneisses,
migmatites
and
granites.
The
superficial s i m i l a r i t y between the Birimian and the n e a r b y A r c h e a n greenstones
lies
in this
structural
setting
and in the
Birmian volcano-sedi-
177
mentary
lithology
which
includes
phyllites and graywackes. Birimian
schist
belts
Burkina
Faso,
while
Burkina
Faso
and
Birimian
belts
greenstones, by
rhyolitic
and
posits acid lavas
Niger.
dacitic
predominant In
southern
similar,
occurring
were
Type
deposits
volcanism
tuffs,
which
(Fig.4.30,A).
in
Ivory
intrusives,
Coast
and
central
and
(1983) Type I in
western
in Ghana,
and
eastern
Ivory
and were believed
and
unconformably
II successions
composed minor
with
rocks
Coast
Type
of
and
calc-chlorite
succeeded
bodies
intrusions.
and Type
metarhyolites,
and by
are mostly
volcano-detrital
ultrabasic
subvolcanic
of
phyllites
a
persistent
flysch-type
shallow water sequences
basic
to
sub-
with
much
intermediate
II sedimentary
graywackes,
de-
quartzites,
formameta-
schists. On its westernmost outcrop in Miogeosynclinot Belt
VoIic~e°n'-tary /')'¢" ~/2"~./"'r~ )"
A
rocks
S.E~ N-WTarkwa
,
Kawere "~roup*
Torkwa Phyllit e Bonket *Series" ~
I
to start with basal
composed
quartzites,
tions are represented by sericite-schists, conglomerates
and
II is extensively developed
volcano-sedimentary
horizon;
continental
lavas
and metabasalts including pillow lavas; to be disconformably
overlain gondite
are
Type
are very
basic
As summarized by Black and Fabre
~
B
Birimian
Banket Conglomerates
Figure 4.30: A, possible tectonic model for Tarkwaian Group. (Redrawn from Bessoles, 1977.)
the
Birimian;
B,
I Huni formation (qu~rtzites and ph~llites ] Torkv= formation { phylLites ) Banker formation (quartzitos and cangtamerates) Kevese formation [conglomerates)
2.65a
f Upper arenoceous formation (sandy ftysch) Upper argil{aceaus formation (politic fiysch ) LOWER BIRRtH1AN Hiddle arennceous formation {sandy- politic ftysch) Lower argillaceous formation Lover arenaceous formation
I Syntectonic and intrusive granffoids ~sic volcanic formation UPPERBIRRIHIAN Acid volcanic formation • Volcanic arenaceous formation vw~lOcal u n c o n f o r m i t y , ~ , w , ~ . ~ . ~ v 2.27Ga
2.136a
TARKWAIAN
2.03 6a
Ghana
~ ? ~ Niega- PauU Plage gneissified granite Honagaga paragneiss
Eburnian I 0rthogneissified granitoid Flyschoids Kaunoukou and Douloyeko paragneiss
Eburnian II ~ Bo.aule type granites Volcano-elastic formation de Lauga-- S~ries de ]nahiri
Kink~ne" series
Wind,no' granite and Bondoukoutype granites
COte d ' l v o i r e
?
Pre- Birrimian crystalline basement
Hesozana| metamarphites of the Oibirshi and Tambao formations
6rctnitoids
Epizona| sediments of the Amarasinde and Bettekoine' tormatlons ~the Liptakoian
Volta,
(From Cahen et al.,
Liptako. NEHaute and W Niger
Table 4.2: Correlations of the B i r i m i a n Supergroup and Eburnean events.
1984)
co
179
the K e n i e b a
and Kayes
inliers
(Fig.4.29)
in eastern
Senegal
and Guinea,
the B i r i m i a n consists e s s e n t i a l l y of low-grade v o l c a n o - d e t r i t i c sequences w i t h flysch, i n t r u d e d by a v a r i e t y of Eburnean granites. A major succession
discrepancy erected
(e.g. W r i g h t et al., clastic
sequence
believed Ledru
to
be
et al.
mentary
existed
underlies case
(1989)
the
in
at
the
base
relations of Cahen et al.
1985).
Whereas
francophone
the of
regional
the
group,
region
the
opposite
(Fig.4.30,A),
occurrence
Birimian,
of West Africa
in Ghana a thick flysch
thus
of
the
was
until
metasedi-
supporting
the
cor-
(1990).
in Ghana
As shown by Kesse subdivided
lithostratigraphic
parts
(1982), shown in Table 4.2, a c o r r e l a t i o n which
has r e c e n t l y been m o d i f i e d by Leube et al. The B i r i m i a n
Birimian
volcano-clastic
the
confirmed
the
in the francophone
1985; Kesse,
the
sequence
between
in Ghana and
into
(1985) the Birimian Supergroup a
lower
predominantly volcanic
metasedimentary
upper Birimian,
in Ghana was previously
Birimian
and
an
unconformable
which is also u n c o n f o r m a b l y - o v e r -
lain by a g o l d - b e a r i n g molasse sequence,
the Trakwaian Group
(Table 4.2).
The Lower B i r i m i a n is w e a k l y m e t a m o r p h o s e d and changes upward from mainly phyllites ruffs,
and
subgraywackes
graywackes
and
to
phyllites
feldspathic
and
sandstones.
slightly
The
metamorphosed
conglomerate
horizons
in the a r e n a c e o u s units were believed to have been derived by the erosion of basement.
Some of the phyllites
also contain pyrite and assigned
mostly
basaltic
interstratified added
with
overlying
and
andesitic
graywackes
manganiferous
and
(Bessoles,
Leube et al. interpretation stressed
lavas
in the upper part and
and
(Fig.4.30,A)
The Upper Birimian was
and
graphitic
phyllites
p o s i t i o n a l model for the Birimian clinal flysch
are silicified
fine carbonaceous matter.
tuffs
often
phyllites; gondites.
spilitic,
to
A
which
was
previous
de-
c o n s i d e r e d it as eugeosyn-
1977), a v i e w w h i c h has been m o d i f i e d in Ghana.
(1990) p r e s e n t e d a s i g n i f i c a n t l y d i f f e r e n t stratigraphic for
lateral
the
Birimian
lithofacies
Supergroup
relationships
in
Ghana,
within
this
in
which
group.
they
The
most
notable aspect of their r e - i n t e r p r e t a t i o n was the r e c o g n i t i o n of the fact that the so-called Upper Birimian, the
lithofacies
Leube
et al.
equivalent
(1990)
spaced belts
terranes
consisting
Fig.4.31.
The
the
Birimian
metasedimentary
to the parallel
regional
of folded Birimian m e t a l a v a s w h i c h
of
isoclinally
latter
volcanic
c o m p r i s i n g e s s e n t i a l l y metalavas,
the Lower
d r e w attention
of evenly
granitoids;
of
constituting
belts,
folded the
15-40 km wide
and
about
disposition
a l t e r n a t e with
metasedimentary Birimian
was
rocks.
basins 90 km
rocks shown apart
and in from
180
°
-~ ~ k. \ ,.
-i I--U3
"1 .,.,.
0
' /~ BIRIMIAN A BoleS.7 o ~..~,
" , .. .' /. . o~ o 9
-~-7 :y [ ~
IN
CENTRALANDWESTERN
' /
i (,"
<
- 9 >A u A N D M n O C C U R R E N C E S
~ •
GHANA ~
Volcanic facies portly
~ /
covered by Tarkw~an sediments
~' .. ~ .
. n Occur . . . . . .
r m.no
v',: , ~ . .a ,~.
'4"~' / s~. ,%
0
AU Occurrence or Mine (numbers refer to names
Figure 4.31: Simplified geology of the Birimian in western Ghana showing gold and manganese mineralization. (Redrawn from Leube et al., 1990.)
181
one
another,
flat-lying southern
have
been
Late
Ghana
traced
by
gravity
Proterozoic-Early
the
Birimian
belts
surveys
Paleozoic are
less
eastward
Voltaian
eroded,
beneath
the
sediments.
In
whereas
in northern
Ghana and in Burkina Faso where the level of erosion is d e e p e r due to the uplift
of
the West
African
shield,
the v o l c a n i c
belts
are
n a r r o w e r ~ and
g r a n i t o i d s d o m i n a t e on account of the exposure of d e e p e r levels. Most of the
Birimian
metalavas
are
tholeiitic
similar to m i d - o c e a n i c ridge basalts
with
chemical
characteristics
(MORB), and w i t h g r e a t e r resemblance
to A r c h e a n tholeiites than to Phanerozoic varieties
(Leube et al.,
They
settings
show
chemical
continental
rifts);
affinity
to
rift-related
and v i r t u a l l y no r e l a t i o n s h i p w i t h
tectonic environments, w h e t h e r arc or calc-alkaline. (1990)
pointed
setting
out
identical
basin ridge;
that with
rather,
tension-related
the or
Birimian
similar
to
interbedded pyroclastics
domain.
not
Leube et al.
metalavas
did
a modern
mid-oceanic
Lithologically
and
collision-related
However,
originate or
it seemed as if the Birimian t h o l e i i t e s
tectonic
1990).
(MORB
in a
back-arc
formed~n
the m e t a l a v a s
are now between p u m p e l l y i t e - p r e h n i t e
and
a
their
facies and
a l m a n d i n e - a m p h i b o l i t e subfacies. The
Birimian
metasediments,
according
to
Leube
et
al.
(1990),
d i v i s i b l e into v o l c a n i c l a s t i c rocks; t u r b i d i t e - r e l a t e d wackes; rocks;
and
boundaries logic
chemical
sediments;
between them.
characteristics
Figure
these
lithofacies
argillitic
show
gradational
4.32,A shows the facies model
of the various
lithofacies
are
recognized
and lithoby Leube et
al. The v o l c a n i c l a s t i c
facies represents v o l c a n i c islands or chains where
tholeiitic
pyroclastics
lavas
wackes
were
slopes
at
silts, some
the
pyroclastic
basin
lithology
in
while
between
and
the
form
edges
and
Birimian
the
carbonaceous
of
manganese
basins
chains
plant-like
epiclastic
turbidity
the
volcanic
carbonates,
by
erupted;
of
Ghana,
organism.
and sulphides
turbidite-related transported
Argillites,
represent
occurring
basin
the
sediments
currents.
schist
and
and
in
sediments
Chemical
a
basinal
the
muds
transition
suggests
sediments
the
as
and
zones
growth
such
occurred in the basin
down
widespread
of
chert,
in relation to
v o l c a n i c and h y d r o t h e r m a l emanations w h i c h were also the p r i m a r y centres of gold formation. Leube
et
Supergroup flows
al.
of
(1990),
Ghana,
(metalavas)
(Fig.4.32,A)
and
in
occurred that
their
postu!ated
this
depositional
that
the
the
two
major
rock
for of
the
the
Birimian
tholeiitic
s i m u l t a n e o u s l y w i t h that of the m e t a s e d i m e n t s is
evident
in
rocks into s e d i m e n t a r y rocks along strike, tween
model
deposition
types
the
transition
from
volcanic
and in the i n t e r f i n g e r i n g be-
(Fig.4.32,B).
Also,
Sm-Nd
isotopic
analyses of samples from the Birimian s e d i m e n t a r y basins and from volcan-
182
,,
f
~
s
f
leve|
ff
A
facies
~
argillite -volconictastic facies
~
voiconiclastic- argiliite facies-.... wacke facies {turbidite related } volcanic -
f
a
f
argilllte
~magmo
e
~
chemical facies
voicaniclostic facies
chamber and feeder.channel
fault
BASALTIC
v v
B
I Iv
(P ROOLA T,C . EPICLASTICS, WACKES, ~
A"G,LL,TE
%
~
TARKWAIAN CONGLOMERATES AND QUARTZITES LATE K-RICH GRANITOIDS (BONGO TYPE )
DIXCOVE GRA NITOIDS
IRIMTAN LAVA$
UNDIFFERENTIATED ~ J PRE- BIRtMIAN ~'1 PROTOCRUST
V nI AVn 'l
Figure 4.32: A, suggested facies relationship in the Birimian of Ghana; B, schematic relationship of major rock units, excluding the Tarkwaian. (Redrawn from Leube et al., 1990.)
183
ic
flows
in
the
accumulation were
volcanic
between
therefore
belts
2.31 Ga
not
of
confirm
and
their
2.01
Archean
Ga.
contemporaneity
Birimian
provenance,
but
and
their
in
Ghana
derived
~from
sediments were
c o n t e m p o r a n e o u s v o l c a n i s m along adjoining v o l c a n i c belts. About
the
unconformably
v i e w (e.g. Kesse, quence
(Fig.4o30,B)
tectonism.
overlying
Tarkwaian
Group,
the
traditional
1985) has been that this is a gently folded molasse sethat was d e f o r m e d d u r i n g the w a n i n g phase of Eburnean
The T a r k w a i a n is believed to be 2,500 m thick in the type area
and up to 6.7 km thick in other parts of Ghana. At the base of the Tarkwaian Kawere
"group";
of the Banket lite
(Table
Tarkwaian were
this
intermontane
In
of
as
basins
et
account al.
which
poorly
is
fans
by
lateral
variability
and
the
economically
emphasized
Tarkwaian
minor
sandstone. sediments
The which
intracratonic
granitoids
important
the
consists
phyllites;
representing
patterns.
poorly
sorted
transportation. conglomerates
Pebbles
and
supra-
following of
and
and
Tarkwaian
alluvial
sedimentological
conglomerates,
shows
as
a
vertical ~ and
deposits
in the s t r a t i g r a p h i c a l l y
polymictic
quartzites,
significant
fan
with
of
braided
lower conglom-
size of boulders;
result
Sequence,
short
they are
distances
of
H i g h e r up in the s t r a t i g r a p h i c sequence the q u a r t z - p e b b l e
are
"clean"
quartzites
c o m p o s i t i o n includes granitoids, carbonaceous
schist,
h e m a t i t e - b e a r i n g hornfels. channel
Huni
elongated
Birimian
are p o o r l y rounded and locally of the
chert,
and conglomerates
irmnature
in
as the
into the Tarkwa phyl-
the
sorted,
alluvial
bounded
(1990)
rocks
stream channel
of
The
arkosic
also
over
grits
upward
known
from w h i c h the Tarkwaian sediments were derived.
characteristics.
erates
in turn pass
coarse,
piedmont
rift
their
Leube
which
Fig.4.30,B),
consists
deposited
crustals,
is succeeded by quartzites,
"series"; 4.2;
is a c o n g l o m e r a t i c m e m b e r
scouring,
which
having
basalts,
quartz,
been
more
rhyolites,
Mn-rich
rock,
reworked.
Pebble
tuffs, black pyritic and
siliceous
and
The sandstone facies reveals c r o s s - b e d d i n g and
are
indicative
of
shallow water
conditions J The
T a r k w a i a n sediments are only w e a k l y metamorphosed. Finally,
of
tectonic models al. of
(1990) a
relevance
any
belt
structural unit.
paleogeographic
for the Birimian of Ghana,
that the v o l c a n i c l a s t i c
previously
volcanic
to
more
has
to
widespread be
belts
and
is the o b s e r v a t i o n of Leube et cannot be
stratigraphic
considered
reconstructions
as
a
regarded sequence,
well-defined
as remnants hence
each
independent
184
The B i r i m i a n in O t h e r P a r t s o f the G u i n e a R i s e
No
attempt
Birimian
was,
the efforts the
however,
stratigraphic of Ledru
conflicting
neighbours, the
to refer
et al.
(1989)
stratigraphic
to resolve
francophone
craton.
volcanogenic
these
conflicts.
have
However,
also
retained
tholeiitic
their
spite of
to redress and
no attempt
her
is made
the case in
the term Lower Birimian (Table 4.2)
lie
the
in
Ghana
previously
succession to
extend
(1989)
between
As was
assumed
to
Thus,
et al.
have retained
are
comprising
(1990)
Ghana.
and Milesi
still persist.
which
They
unit
al.
interpretations
geologists
sediments
et
outside
to the basal metasedimentary
flysch-type Archean
by Leube
to areas
major differences
here either, Ghana,
made
model
comprising
unconformably
Upper
submarine
Birimian
flows
on
the
for
the
at the base and
calc-alkaline pyroclastics at the top, separated by a sequence of cherts, black sediments and acid pyroclastics. Milesi et al. evolved
through
accumulated
(1989) considered the Lower Birimian succession to have three
main
stages.
Flysch-type
in a steady subsiding basin,
instability leading to the deposition of coarser, sediments.
Sedimentation
resulted
in
the
was
accompanied
appearance
of
deposits
initially-
followed by strong
sedimentary
turbidite-type
by hydrothermal
cryptocrystalline
clastic
activity which
tourmaline
with
dis-
seminated sulphides in breccia pipes and in the matrix of the immediately surrounding input
sandstones
clay-carbonate
volcanic
activity
and conglomerates. sedimentation
represented
by
With a decrease
in ferrigenous
followed,
with
intervals
pyroclastic
and
epiclastic
of
acidic
deposits,
dykes, and sills. The tourmaline-bearing turbidites also contain gold deposits
which
are
associated
with
volcanism
and
hydrothermal
activity ~
which appeared late during the deposition of the Lower Birimian. The stratigraphic position of the Lower Birimian metasediments at the base of the Supergroup has been upheld to be valid for Ivory Coast, Burkina Faso, Mali, and Senegal, by Ledru et al. (1989). al.,
These
1984)
Birimian Ghana,
authors
also
retained
the previously
that a major unconformity
(Table 4.2),
where
Wright
an opinion et al.
(1989) and by Milesi et al.
separates
held view
the Lower
not shared by Leube
(1985),
had also reported
(Cahen et
from the Upper
et al.
(1990)
for
a conformable
re-
lationship between the Lower and Upper Birimian. G r a n i t o i d s a n d S t r u c t u r e o f the B i r i m i a n
Abundant granitoids are closely associated with the Birimian schist belts (Fig.4.32,B). basal
pebbles
These comprise pre-tectonic granites from which some of the of
the
Birimian
were
derived;
and
large
concordant,
185
foliated
syntectonic
b a t h o l i t h i c granitoids
Cape
Coast
type
in Ghana),
which
are
either
emplaced
ment
or
produced
formations.
and
biotite-hornblende
and
in p r e - B i r i m i a n m i g m a t i t e s
granitization
of
the
in Ivory Coast,
folded
2-mica varieties
and g n e i s s i c
Birimian
domain
of
the
where
Type
syntectonic
hornblende.
The
and
II
Birimian
belts
post-tectonic
post-tectonic
intrusives
predominate;
granites are
contain
small
and
d i s c o r d a n t g r a n o d i o r i t e s with localized thermal aureoles. red to as Ghana.
the B o n d o u k o u
Basic
Tarkwaian
base-
geosynclinal
T w o - m i c a granites are subordinate in the eastern part of the
Baoul~-Mossi majority
by
with
(Baoul~ type
to
type
in Ivory Coast
intermediate
Group,
including
intrusives
a norite
and
are
body
about
Dixcove
in
20 km
subcircular
They are refer-
as the
known
the
biotite
Ghana long
type
among
near
in the
Tumu
in
characterized
by
n o r t h e r n Ghana. In
Ghana
isoclinal
symmetrical southeast; out
the folds
steeply
striking
oblique
Cleavage
intrusions. Ashanti
of
the
near vertical
in
the
Birimian axial
volcanic
rocks
planes;
belts
with
is
locally d e v e l o p e d slight
vergence
open
to
the
an axial plane cleavage d e v e l o p e d parallel to b e d d i n g through-
the
1990).
structure
folds w i t h
inclined
to
and the
by first
a
weak
second
cleavage
cleavage
(Leube
et
al.,
is more pronounced and more regular around the granitoid
Three
belt
sediments;
or p e r p e n d i c u l a r phases
(Fig.4.31)
of
fold
deformation
high-angle
occur
in
Ghana.
reverse faults or u p t h r u s t s
In
the
are found
in mines. The shows
Tarkwaian
different
(Leube
et
al.,
forms
a large
wide.
In
are
Group
1990).
In
symmetrical
addition
to
folding
of
the
turns
are
repeated
al.
(1990)
gravity
Bui
post-Tarkwaian
the
belt
of
on
the
(Fig.4.31)
trough,
110 km
structures
in
Birimian volcanic
belts
the
Tarkwaian
Group
long and
up to
15 km
Ashanti
belt,
there
the
and
isoclinal
folds;
west
the
directed is
deposition
by
the
and
east,
Tarkwaian.
sediments
style and
from the the
in w h i c h
to t e n s i o n - r e l a t e d
Tarkwaian
Tarkwaian
and
the centre,
overthrust
structures of
event.
towards
strata
five
the
Birimian
tectonic
each
towards
attributed Tarkwaian
during
unconformably
vertical
belt
into thrusts the
the
in
synclinal
tight,
Ashanti
and
rests
styles
symmetrical
locally d e v e l o p e d
centre
which
structural
rather
open
strata
Leube
et
folding by than
to
a
i n t e n s i t y of deforma-
tion are s i g n i f i c a n t l y different from those in the B i r i m i a n strata. O u t s i d e Ghana, deformation
the Birimian
and metamorphism,
g e n e r a l l y penetrative, (Milesi et al.,
1989).
coaxial,
S u p e r g r o u p also e x h i b i t s
DI,
D 2 and
D3
(Table
4.2)
three phases in w h i c h
of
D 1 is
and affects o n l y the L o w e r B i r i m i a n Group
During D 2 and D 3 the tectonic
setting was
one of
186
transcurrent
movement,
verse d i s l o c a t i o n s granitoids.
The
in which
caused
large
D2
(sinistral)
D3
folding and also controlled
D 3 transcurrent
Ghana are best d e v e l o p e d
and
faults
which
(dextral)
trans-
the emplacement of are
hardly
seen
in
in the northern Birimian belts and have imposed
N E - S W structural trends on the n o r t h e r n terranes. Tectonic Models for the Birlmlan Supergroup According
to Milesi
of Cahen
et al.,
et ai.(1984)),
(1989),
which
an early orogenic
they termed
phase
the B u r k i n i a n
(Eburnean I
orogeny
(Table,
4.2), had a f f e c t e d the Lower Birimian at deep crustal levels in the Ivory Coast.
Judging
from the extensive
L e d r u et ai.(1989) from
several
tinental
plate
al.
in
eastern
them as
and
consequently
rather to a d i f f e r e n t
in-situ
Senegal,
Yaour~ belt in Ivory Coast. Ghana,
at different
times d u r i n g
The Upper Birimian m e t a v o l c a n i c s
considered
belt
schistocity,
a phase of
(Table 4.2),
con-
on the
a c c u m u l a t e d in d i s c o n t i n u o u s and independent basins. Ledru et
(1989)
Mako
collisions
accretion.
other hand,
area a f f e c t e d by a first
c o n c l u d e d that the B u r k i n i a n o r o g e n y p r o b a b l y resulted
the
zones of rifting,
Gaoua
belt
in
for example
Burkina
Faso,
the
and
the
One could, perhaps add the B i r i m i a n belts of
relate
their origin not to plate
geodynamic process
-- as was p o s t u l a t e d by Leube et al.
-- continental
collision,
but
rift development
(1990) in their g e o d y n a m i c model for
the Birimian. Leube Ghana
et
which
al.
(1990)
involved
temporaneously
proposed
a
tectonic
small-scale,
operating
model
for
equidimensional,
convection
cells
in
the
the
Birimian
parallel upper
and
mantle
of
conwhich
caused the rifting of a highly thinned protocrust as well as linear eruptions
of t h o l e i i t i c magmas
above
sea
subsiding
level,
crustal the
volcanic-derived
parallel
compression
was type
of
intermontane
were
and
After by
two
folding
granitoids.
B i r i m i a n belt was presents
basins.
generated
shortening
Baoul~
(Fig.4.33).
With
sediments volcanic
final
leading stage
in the volcanic
belts.
in
to
the
the
from
Birimian
rocks
eroded
g r a v i t y d e f o r m a t i o n of T a r k w a i a n sediments,
in
of rifts
the
slowly
and
lateral
sialic
blocks,
emplacement
evolution and the
of
nearby.
This
formation
sediments
was
of the
The T a r k w a i a n molasse
the i n f i l l i n g of these later basins with clastic
derived
the volcanoes
ceased
contracting
caused by the reactivation
basins
of
accumulated
activity
adjacent
resulted,
The
the growth
rethat
followed
by
and by the i n t r u s i o n of mafic
sills and p o s t - E b u r n e a n Bondoukou or Dixcove type K - r i c h granitoids.
187
MODEL
FOR THE
EVOLUTION
OF THE BIR|MIAh
~SIAL1C CRUST ~ S E D I M E N T 5 [ + ~ ] 6 2 - GRANITOID ImNTHOLEI ]TIC BASALT
I
STAGE
,
~
[ ~ G l - GRA- ~ C O N G L O M E R A T E 5 l SANDN ITOID STONES(TARKWAIAN)
f
f
~
,
~ " ~ , ' ~
'q/"
STAGE
f
f "
qC'
il
• I
"/'~
~ '
"hP
l
f
STAGE III
ash f o i l - o u t
f STAGE
f
f
f
IV
. . . . . . . .f . . . .
STAGE V
f
f
f
f
f
~2 ~ j - -'_-t,.
Figure 4.33= Tectonic Leube et al., 1990.)
f
model
.+_t_ . . . .
for
the
_,.
Birimian.
(Redrawn
from
188
4.8.3 B i r i m i a n M i n e r a l i z a t i o n The Eburnean part of the Guinea Rise is by far more m i n e r a l i z e d Archean
cratonic
nucleus
to the west
Archean
granite-greenstone
belts
(Wright et al.,
in
Sierra
Leone,
Ivory Coast have y i e l d e d m o s t l y gold and iron ores, mineralization manganese,
occurs
diamond,
mineralizations cobalt, the
and
Birimian
iron
ore,
including
lithium,
mineralization
m e n t a r y sequence weathering
in
and
lead, and
occurs in
also
in
the
soils
the
Liberia,
Guinea
and
a greater v a r i e t y of
antimony,
Birimian
especially
but
economic silver,
in pegmatites volcanics
and
bauxite)
and
nickel
in
Mali
and
Most
of
volcano-sedi-
and as residual
gravels
seems to be p r e f e r e n t i a l l y
above
the
concentrated in
an e a s t e r n belt along an arc following the main n o r t h e a s t e r l y trend in Ghana,
gold,
sulphide
(Fig.4.34).
in Tarkwaian clastics,
(including
Mineralization
Whereas
belts,
small
copper,
niobium
(Upper Birimian),
products
supracrustals.
tin
greenstone
than the
1985).
structural
Ivory Coast and Burkina Faso, and in a n o r t h w e s t e r l y belt
parallel
to
the
main
structural
trend
(Fig.4.34).
Gold
and
m a n g a n e s e m i n e r a l i z a t i o n appear to be inherent in the Birimian which also has good prospects
for massive sulphides.
Gold
Gold in Africa was produced in colonial times m o s t l y from Ghana Gold Coast) tween the
which
supplied two-thirds of the continent's
(formerly
gold output be-
late 15th century and the m i d - 1 9 t h century w h e n the Witwaters-
rand
goldfields
gold
reserves
in
are
South about
Africa 5,000
started
tons
to
(Wright
Fig.4.31 most G h a n a i a n gold occurrences
produce. et
Ghana's
al.,
and mines
1985).
As
concentrated
estimated shown
in
in narrow
"corridors" of 10-15 km w i d t h in the transition zone b e t w e e n the volcanic belts and the s e d i m e n t a r y basins where the chemical facies and regionally extensive
shear
found
(Leube
quartz
veins
et
zones
at
al.,
1990).
and
the
lenticular
volcanico-sedimentary The
primary
reefs
and
gold
in
interface
deposits
some
of
which
the
are
also
occur
tuffaceous
in and
a r g i l l a c e o u s rocks, are accompanied by sulphides e s p e c i a l l y arsenopyrite, pyrite, 10%
pyrrhotite,
silver
is
chalcopyrite,
associated
with
bornite,
gold,
galena
which
and
renders
sphalerite.
gold
refining
Up to more
profitable. Gold lodes are a s s o c i a t e d with persistent and d e e p - s e a t e d shear zones and
are
also
phyllites comprising turfs,
and
located the
along
metamorphosed
graywackes
the
transition
volcaniclastics,
and
and
carbonaceous
basic
and
zones
between
generally
and
intermediate
in
the
manganiferous magmatic
Birimian
country
rocks
argill~tes,
rocks.
In
Ghana
. . . . .
~'" :.
.Sigutrt.
~%'~:_-
_
.
OI
-
+~
.~ ++~'+~
.
" •' .
_
Torkwoion
International borders
Figure 4.34: Mineralization from Wright et al., 1985).
--'--
Birimian with subordinate volcanocJenic component ~ Birimian with dominant volconogeni¢ component ~ ] Undifferentiated granitic rock including Archaeon basemen and Ebumian intrusions I'+-F-~+ Archoe o nj Di=diomonde
~
~
- ~ ' ~ Late Proterozoic to Tertiary
key
+ + + + + + + + + + + + + ++ + + . + ~ $ ~
~+ + + + + ~ " +++++++~ + :F"~--1 ++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Jr++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++ ++++ ++ ++ +++++++++++ ++++++++++++++++++++~/~ +++++++++++++++++++
~+++~_\
~ . .
~l..~f,,~ "-..:~, ~
~--~-
+++
-
of
Lower
on(Ira
•
•
. • ", .
Proterozoic
Lahou
('
(
rocks
-- )-
in W e s t
Tov, orodt
.~$uto
__
(Redrawn
2 0 o km
Africa.
,
~¢+ro
(onongo ~_ iI~Bidm) __~
/-/- V-OL TA
~-BAsI._
..~. ~
IAu
| Pouro
~(Ni,Co3"
Tomboo~
__
190
active
mines
are
located
at
Presta,
Obuasi
and
Konongo.
Wright
et
al.
(1985) c o n s i d e r e d gold m i n e r a l i z a t i o n in this setting as p r o b a b l y of syngenetic
volcano-exhalative
associated
origin,
sedimentation.
morphism
and
Gold
concentrated
related
was
along
to
greenstone
remobilized major
shear
volcanism
during
Eburnean
zones.
Emplacement
and
metaof
Eburnean granite intrusions could have provided an a d d i t i o n a l heat source for m o b i l i z i n g Mali
granodiorite At
gold
(Fig.4.34)
Kalana
quartz
into
where
intrusions
where
gold
veinlets
with
veins.
This
gold-bearing
process quartz
within volcanic
is
apparent
veinlets
Birimian
reserves
are
estimated
sulphides
are
a
rocks
at
at
occur
Kalana near
(Goossens,
100
tons,
few m i l l i m e t e r s
to
Poura with bearing over
23 tons
quartz
of
veins
recognized reach
300 g o l d - b e a r i n g
down
quartz
reserves; to
veins
several
a s s o c i a t e d w i t h d i s s e m i n a t e d sulphides Gold
mineralization
in
the
paleo-placers
in South Africa.
western
Ghana,
sedimentary
rutile,
zircon
and abundant
erates
(Fig.4.30,B)
eastern
margin
of
and heavy m i n e r a l s matrix-poor
near
the
are
another,
they
not
base
layers
secondary
Boura-Gangaol
where
and rhyolitic
tuff
1983).
deposits
of
the to
type
with
hematite,
close
within
quite
the
of
the
locality
heavy occur
Takwaian in south-
minerals
in banket
Tarkwaian,
the
mostly
sediment
system
bankets.
persistent
extensive
h u n d r e d s of meters of distances. and
associated
detrital
syncline
gold-
is v e r y similar to the W i t w a t e r s r a n d
stratigraphically
are
the Ofin d r a i n a g e
fan
In Bur-
including
source
such
as
conglomalong
area.
the Gold
are concentrated in the more m a t u r e and better sorted
gravel
horizons
(Goossens,
At the Tarkwaian
gold
the
and
centi-
where
in rhyolite
alluvial
G r o u p i n t r a c r a t o n i c p i e d m o n t basins
locations
Guiro-Bayildjaga
50-m depth; occur
1983).
gold-bearing
meters wide along p r e f e r r e d structures up to several meters wide. kina Faso g o l d - b e a r i n g quartz veins occur at several
in
small
with
Although
the
gravel
from
one
location
consistent
ore
grades
to over
In Ghana m o d e r n gold places occur along
supplied by the Birimian
gold mineralizations;
such
and
a deposit
Tarkwaian
is mined
at
primary Dunkwa.
Gold of Lower Proterozoic provenance also occurs in m o d e r n beach sand and in P l e i s t o c e n e
alluvial
deposits
in Ghana,
Liberia,
northeastern
Guinea
and Ivory Coast.
Manganese Manganese
oxides
in
w e a t h e r i n g of protores
the
Birimian
Supergroup
such as m a n g a n i f e r o u s
are
concentrated
phyllites,
gondites
by
the
(highly
m e t a m o r p h o s e d m a n g a n e s e sediments comprising spessartite and quartz), Mncarbonates and silicate horizons in the Birimian v o l c a n i c l a s t i c s or sometimes
near
the
transition
between
the
metalavas
and
the metasediments.
191
Manganese and
in
tion
ores
occur mostly
southwestern
is
where,
in in
the
amounts
of name
formed
Niger.
Birimian
addition
derived
as
an
to
ses.
Nsutite
ore
and g o n d i t e s the u p p e r
part
in the
manganese
deep
occur
in
origin,
at
on
the
to
Wright
have
Birimian,
manganese
exploration
could
a
proces-
phyllites in
believed
that
submarine
vol-
association as
during
Supergroup
(1985) had
be used
locally-
enrichment
Birimian
et al.
could
large
depleted,
manganiferous
top of the
gondites
the
almost
Ghana
be
peneplain
by supergene of
mineraliza-
to
which
now
Faso,
southern
used
Tertiary
and that with the common
in the g e o c h e m i c a l
in
there
ore,
a
Burkina
manganese
Nsuta
Mn02,
sequence
group.
and
largest
nsutite
a
below
Ivory Coast,
ore,
pure
The
oxides w e r e p r o d u c e d
bodies
phyllites
in the
the
sequence
weathering
of the p h y l l i t e
cano-exhalative
ment
given.
450-600 m thick
manganese
far
battery-grade
of
which pure m a n g a n e s e
by
metalava
was
result
But
southern
metallurgical-grade
unusual nsutite
a
in Ghana,
of gold with
a pathfinder
ele-
for gold.
Diamonds
Diamonds Coast.
have
been
In the
diamond-producing of
Birimian
diamond
area
are
from
diamond
B i r i m i an
field
in
in
about
the
later West A f r i c a n M e s o z o i c
Ghana the
diamonds
and
Proterozoic
in
Ghana,
alluvial
200 m wide
Lower
rocks
southern
in West Africa,
conglomerates
sources
the m u c h
produced
Birim
50 km
rocks
kimberlites
and
come
long.
and
which
the
Ivory
largest
from a band The
are
single
Birimian
unrelated
are also
to
an impor-
tant source of diamond.
Iron
Iron
ores
Whereas in
in
the
other
the
Early
parts
of
Lower
Proterozoic the
climax
was
attained
(1985)
manganiferous
the Bi r i m i a n are
unlike
generally
the
was
thicker
Late
deposit
at
ore at 45-50%
deposits,
of
probably
more
Senegal
with
magnetite
the
magmatic
Faso and near Takoradi
South
for
are
banded
Africa,
As
to have
observed
superseded
than
those
is a m a j o r reserves
100 m i l l i o n origin, in Ghana.
of
in
Africa
Wright
the
et
al.
iron-formation
of
Archean
times.
Proterozoic million
norites
in
iron-formations
tons at 62-65%
occur
extensive.
iron-formations
by
Early 400
not
in West
Early P r o t e r o z o i c
extensive there
Africa
climax
where
estimated
Fe, and
West
Archean.
seem
regions
and
of
including
sediments other
in e a s t e r n m o s t
Burkina
world
in
However,
Fal~m~
Proterozoic
Fe. and
iron
tons
of
O t h e r iron gabbros
in
192
Base Metal Deposits A c c o r d i n g to Goossens occurs
(1983) a massive sulphide deposit rich in Zn and Ag
in the Birimian belt of Burkina Faso at Perkoa
c o n t a i n i n g pyrrhotite,
of zinc is over 4% for a m i n i m u m of 30-m width, intersections
ranging
from 3 to
15 m.
and exceeds
Perkoa was
the
m a s s i v e sulphide deposit d i s c o v e r e d in the Birimian. reported
deposits
Birimian Kourki
belt
in
(Fig.4.34)
pyrite and sphalerite, and 20 oz Ag/ton.
with
in Niger
Niger w h e r e
porphyry
copper
and Burkina 0.06%
Mo
Faso.
and
20% in drill
first volcanogenic
Goossens
affinities
in ores
The grade
the
northern
These m i n e r a l i z a t i o n s
occur at
0.07%
Cu
were
in
(1983) also
found;
and
Gorem
and
Gaoua in Burkina Faso with 40 m i l l i o n tons at 0.15% Cu and 0.05% Mo, and 22 m i l l i o n
tons
considered
sub-economic
at
0.08%
Cu
respectively.
they
contain
Although
rich
copper
these
veins
deposits that
have
were been
mined from time to time. 4.8.4 The R e g u i b a t Shield As in the Guinea Rise to the south, Archean b a s e m e n t rocks in the western part and
of the R e g u i b a t
shield give way to Lower
volcano-sedimentary
assemblages
the eastern domain of the Reguibat retrogressive the
eastern
(Fig.4.35).
they
later than in the Guinea 2.0 Ga and
occurred
disturbed
had
survived
the
between
volcano-clastic
Rise,
1.87 Ga, 2.4 Ga
since
unlike the and
sequence
and
orogeny
(Cahen et al.,
the Eburnean
(Table 4.2).
extensive
1984) it
shield was
in the former
latter w h e r e
1.8 Ga
occur in
Eburnean
it o c c u r r e d
in
facies and
(Chegga assemblage)
that the Eburnean orogeny in the eastern R e g u i b a t
region b e t w e e n events
where
rocks
However,
shield Archean a m p h i b o l i t e
facies migmatites
extremities
igneous
(Fig.3.39).
From a v a i l a b l e geochronological data
is apparent slightly
greenschist
Proterozoic
in the east
rhyolitic
The
little
volcanism
of
the eastern R e q u i b a t shield also contrasts with the B i r i m i a n Supergroup. The Yetti Group and its equivalent the Aguelt Abd el Malek Group, are the
oldest
These
are
masses
of
volcanic facies
Proterozoic preserved detrital
flows,
and
in
assemblages
and
all
of
intruded
by
in
NW-SE-trending pyroclastic which the
are
Yetti
the
AY
shield
where
material
weakly and
Reguibat
belts,
they
with Tili
4.3). thick
sills,
metamorphosed Ben
(Table
comprise at
granites.
dykes
and
greenschist The
Yetti
Group suffered two phases of d e f o r m a t i o n before the i n t r u s i o n of granites at about
2.03 Ga;
isoclinal
folds,
the first deformation being c h a r a c t e r i z e d by recumbent and
the
second
v e r t i c a l or inclined axial planes.
by
open
folds
of
great
amplitude
with
193
/' ~)"
0
~ j o u n +~
ATLANTIC
J~
/
o
0
0
o
or)
0
°Ti
~o ' nu u,
0
BOU ernous
0
0
0
.
OL-
~,~
~
.
.
=el I
~
t
~
.
~
'
/
~
/
.
:
~
~
Z . o
.
,~ ~
~
~
o ~ = o go
~
~
~
~
L
\'
~
r~,
.
~
.
.
~
°
1,52"
/~
"
-
o
o
o
o
o
9>-- ........
°1
-
I
~'~"
~
l;'b;I
F/"/+7,1
Figure 4.35: Geologic sketch map of the Reguibat Rise showing: i, Late Proterozoic - Paleozoic; 2, Mauritanide (Hercynian) nappes; 3, Late Proterozoic, 4, Guelb E1 Habib Group; 5, Eglab volcanics; 6, Intrusive granites and associated rhyolites; 7, Oued Souss Group and Yetti Group; 8, Basement groups (Amsaga, Tasiast, Ghallaman, Chegga Groups and associated granites) (Redrawn from Cahen et al, 1984). Early phases of Eburnean tectonism were followed by the deposition of volcano-detrital
groups
(e.g.
The Oued Souss
Group)
which
are preserved
in NNW-SSE structures and were folded and slightly metamorphosed at about 2.02 Ga before to
1.87
domain
Ga. are
the onset of extensive
Among abundant
The
Eburnean
high-level
anorogenic
magmatic granitoids
adamellites,
coarse pink biotite granites,
and granite
porphyries,
which
rocks
have
magmatism in
the
including
from 1.97 Ga
eastern
sub-volcanic gabbros,
invaded
the
Reguibat
porphyritic
thick
and vast
biotite diorites sequence
belonging to the Aftout rhyolites and ignimbrites with subordinate dacite and
andesites
granites
near
the
and a nepheline
volcano-detrital comprising
base. syenite
There
are
complex.
also Resting
and intrusive assemblages,
unfolded
and
unmetamorphosed
sub-volcanic
alkaline
unconformably
on this
is the Guelb E1 Habib Group, coarse-grained
which are considered as the Eburnean molasse
clastic
rocks
(Table 4.3). Dolerite intru-
sions into the Guelb E1 Habib, dated around 1.6 Ga, probably also belong to the regional post-Eburnean anorogenic magmatic phase.
Amsago
assemblage:
Sooudo
PART
syenite
?Hassi el
Fogra ~Series'~. . . . . .
Fogra syenite .~Chenachane- Erg Cheich groups
lsoclinal recumbent folding {P1} Yetti group
Open folding {P2)
Yettl-£glab junction granite Folding with thrusting noppes (P3) Akilet DeileL group Oued Souss group Yetti granite
Aftout (pink} granite
Eglob volcanics
Guelb el Habib group
dykes
1984).
doierite
PART
2039~ 69 Ma 2008± ~1Mo >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
~
.'"" :
---
~
/#//
<
]':.;"
.':-'."
• ....
" • .
,<,/i~
• ,'f°
Cabindl
~/: .'.'. 1
Som£
i~s
,
r .".
~
/.
- . "
:'k~' ~'~"
-.
,
,...., .
~
•
"
,
: ...
•
~ \
• . : . ". "
:
"
•
. •
o • ."
.
" .
•
• o
•
." •
- . ' o .
-o •
"
:."
...
,
• o
•
• .. .
' - :'.
7
:
. .
' ,
$OOKm
:.
,
Figure 4.36: Geologic sketch map of the Atlantic Rise showing: i, P h a e r o z o i c cover; 2, Tabular West C o n g o l i a n sequence; 3, Folded West Congolian; 4, Sembe-Ouesso Group; 5, pre-West Congolian formations in the West Congolian orogen; 6, Francevillian; 7, Ogoou~-schists; 8, Archean basement (du Chaillu). (Redrawn from C a h e n et al., 1984.) In
the
Eburnean Gabon
internal
imprints
and
southern
activation of
cover
Doussa"
the
West
as well
Congolian
the
as
as in n o r t h e r n Angola.
the K i m e z i a n
Supergroup
"S~rie de la Lo~m~"
the
basement
which
are
now
Basement re-
and m e t a m o r p h i s m
in n o r t h e r n
in Congo and the
migmatites
and
o r i g i n a l l y been m e t a m o r p h o s e d
in the a m p h i b o l i t e
along
trends.
mainly
NE,
mainly
NNE
and
of mica
N-S
schists
The
"S~rie
and two-mica
Angola
facies. de
and
"S~rie de la
gneisses.
gneisses
earliest
in northern
in Gabon. The K i m e z i a n Supergroup c o n s i s t e d of schists,
limestones
consists
orogen
charnockitic
s i m u l t a n e o u s l y w i t h the d e f o r m a t i o n
known
its correlatives, and
of
Cameroon
occurred
rocks
zone
occur as reactivated
quartzite These
had
They are folded
la Lo~m~" with
in Congo
garnets,
and
para-amphibolites.
The d e f o r m a t i o n and m e t a m o r p h i s m of these cover rocks
occurred
2.08 Ga during what
at
about
orogenic event
(Cahen et al.,
1984).
is
locally
known
as
the
Tadilian
[. 2.9-3.0Go
11.2.16a
TADIL1AN OROGENY | 2.08 GO,)
HI
IV. t0Ga
Congolian supergroup
supergroup
unconformity
Kimezlan
supergroup
unconformJ~'y
Lower Zodinian or Mutadi-Palaba group
disconformity at least
lshe[a-VQngu group
Upper Zndinion or
Zctdinian
Hayumbian supergroup
CONGO
Syst~me du Congo occidental Syst~me des Hunts Bamba
3.
de In Bikossi
inf~rieur
S~rie de I0 LuPine"
7 unconformity
Groupe
Groupe sup~'rieur
S~rie
S~ries de in Loukoulo
disconformity fo unconformity
West
V. 0.73-0'65,,
BAS-ZAIRE
2.
I. Ar, E
Oouigni
S~rie de lu Ooussn
S~rie des Hunts Kouboulu
S~rie de la
unconformity
S~ries de ¢a Loukoulu
unformity
Syst~me du Congo occidental Syst~medes Hunts Bamba
la
Noy-
Groupe des gneiss plagioclase bosiques
unconformity
Groupe des gneiss ~z plagioclase acides
Groupe des micuschisfes
unconformity
firoupe des chlorito-schisfes
presumed unconformity
Syst~me de
NORTH
zone of the W e s t C o n g o -
SOUTH S. 6ABON-MAYOHBE
in the i n t e r n a l
&. 6ABON-MAYOMBE
T a b l e 4.5: S t r a t i g r a p h i c s u b d i v i s i o n s of the s u p r a c r u s t a l s l ian orogen. (Re drawn from C a h e n et alo, 1984.)
200
In
the
overlain
Bas-Zaire
by
the
and
northern
Zadinian
Angola
Supergroup,
the
Kimezian
comprising
two
is
unconformably
distinct
groups:
a
lower group w i t h m i c a c e o u s quartzites, mica schists and acidic lavas; and a disconformable
upper
group which
begins w i t h abundant
is followed by chlorite and sericite-schist, pyroclastics
in Bas-Zaire.
mafic
lavas
and
quartzites,
and some acidic
In the latter area tholeiitic
lavas which are
andesitic towards the top, and over 500 m thick, occur at the base of the upper
group.
Zadinian
fall
between
2.08 Ga
the
age
the
of
(Table 4.5). was
(the age
upper
the
along
ENE
southern
of
part
Deformation,
initially
towards
sedimentation, the
of
to NNE
underlying
the
which was
part
metamorphism
trending
intense axes
deformation
Kimezian)
overlying
not
in Angola.
and
and
Mayumbian
with
1.02 Ga,
Supergroup
in B a s - Z a i r e
and Congo,
increasing
Metamorphism
was
at
all
a
intensity
lower
grade
than during the Late Proterozoic West C o n g o l i a n orogeny. 4.9.5 G a b o n O r o g e n i c Belt The
Tadilian
orogeny
(Cahen
et
al.,
1984)
b a s e m e n t in the internal and foreland (Fig.4.36)
is
better
Gabon o r o g e n i c belt
known
further
(Fig.4.37)
that
produced
the
Eburnean
zones of the West C o n g o l i a n orogen north
in
central
Gabon,
where
the
comprises a m a j o r Eburnean tectono-thermal
event i n t e r p r e t e d as a collision zone demonstrated
which
the m e t a m o r p h i c
(Ledru et al.,
rocks
of the Gabon
1989).
Ledru et al.
orogenic
belt dis-
play h o r i z o n t a l d e f o r m a t i o n and petrological c h a r a c t e r i s t i c s which attest to E b u r n e a n plate collision that involved m a j o r crustal shortening.
Stratigraphy of the Gabon Orogenic Belt On
the
eastern
medium-to
slopes
high-grade
of
the
Atlantic
granite-gneiss
Rise,
terrane
the
Archean
(3.15-2.6 Ga
old)
du
Chaillu
is
uncon-
formably o v e r l a i n in eastern Gabon by the Early Proterozoic Francevillian S u p e r g r o u p in the F r a n c e v i l l e and Boou~ basins. lian, dated between
2.3 Ga and 2.0 Ga,
by the Ogoou~ M e t a m o r p h i c s Early
Proterozoic
s y n c l i n o r i u m and occurring
age.
The
consist
The u n d e f o r m e d Francevil-
is o v e r t h r u s t on its w e s t e r n side
(Fig.4.37) which are also considered to be of Ogoou~
Metamorphics
are
essentially of paragneisses,
locally at the base
(Ledru et al.,
lian S u p e r g r o u p and the Ogoou~ Metamorphics
1989).
exposed with Both
in
a
broad
orthogneisses the Francevil-
constitute the Gabon orogenic
belt. AS summarized by Bonhomme et al. comprises
five
feldspathic
subdivisions
conglomeratic
or
(1982), the F r a n c e v i l l i a n Supergroup
formations.
Mabinga
At
sandstone
the
base
formation,
is
the
coarse
about 1,000 m
201
Figure 4.37: Geology of the Gabon orogenic belt. i, Mesozoic cover; 2, Upper Proterozoic of the Nyanga Syncline; 3, Lower Proterozoic (Francevillian); 4, N'Djole Basin; 5, Ogoou~-Metamorphics; 6, Migmatitic domes; 7, Iron formation; 8, Chaillu and North Gabon granitic domains; 9a, medium- to high-grade gneisses of the Monts de Cristal; 9b, Lambar~n~ Migmatities 3.09 Ga old. (Redrawn from Ledru et al., 1989.) thick.
The
anhydrite contains
sandstones
and gypsum, pyrite
and
comprise while
asphaltic
from the top of the Mabinga Mounana about
(Fig.4.38).
600 m of
channel
calcareous
manganese
deposits
the
at Moanda carbonate and
siderite
1.6 million
tons
cesses
enormously
had
discordant
black
rich in m a n g a n e s e
facies
material.
shales
Poubara
taining
red
conglomerate
Slightly
sandstones,
a
a discordantly
Rich
hematite,
over
shales)
The
rich
from the weathering
which occurs with banded
greenalite.
and reserves upgraded
With
an
annual
of up to i00 million the
deposits
formation
the Mabinga
(Bangomb~
manganese
black shales to about 45 % in the ores
facies
are mined
at Oklo and
sandstones and and
are
lenticular productive
of a top horizon
iron-formation production tons,
content
(Hutchison,
dolomite,
non-oxidized
uranium
sandstone
sandstones.
formed
with
overlying
conabout
supergene
from
1983).
of
15 %
in
prothe
202
X
.
.
.
•
.".."
. ....'.'."
x
,
X~.o.
~
.....
t
.
.
'
.
~
/
~
/
/
/
~
/
/
/
/
/
X
,",',;:" 'i:'~'L'
." " ' ~ x
/
.
X
I
/
--
.
~Djoumou
'm
sholes
Mong~nese ore deposi,s o~'Moand¢
,',:-nPouboro s a n d s t o n e s 'J-:-UBengornbe'shales I ~ M o b ~ n g a coorse s a n d s t o n e s
. . . . ~
' " . m ~ . IX] Du Cheillu differentiated -
"'" " " "
/
" o
k^ k m
~
.
bosement
h PX
•
X
.~
X
• +~i :..:::.
" "." ." ." ." .'"~'k..~3,.,_~ " "[/. "~7"~, ~ " .
Y
MOANDA
w, + , "
//
~IV,~ C ~/J~//'/////J,'~/~ / ' ~ / ' / ~ "
/
X
/
~
:
~X X~ - O k e l o b e n d o ~ , ~ "," ~ I . .""~-<'~4kk~'Vllll~,'. ' "" IX ~-~ - " ~ ~ "
~
.
/~.}.:~.~:../~! Bamboi
/
/
"." . ' . "
.
[
•
°.
,
X
'
X
- " - "
" " -
. "
.,
." . " -
:,,:',':, :" .:,:':'.
°
X
Figure 4.38: Geologic map and stratigraphy of the Francevillian Supergroup showing manganese deposits. (Redrawn from Bonhomme et al., 1982.) The
overlying
Djoumou
shales
constitute
thickly banded chert and massive dolomite, steps onto basement.
a
marker
unit
of
mostly
about 50 m thick, which over-
The next unit comprises the Bambai black shales with
associated ignimbritic tuffs at the top. The Lepaka sandstones
constitute
the uppermost unit and comprise alternations of sandstones and shales. In
the
morphosed
Franceville
basin
and undeformed
had favoured the concentration basin
the
except
Francevillian
in the
zones
of uranium.
Supergroup
of early
is
unmeta-
fracturing
which
But further west in the Boou~
(Fig.4.37) where the Francevillian rests on the Archean basement of
the Lop~ horst with a basal
conglomerate
and sandstone unit,
the Super-
group had undergone a phase of deformation accompanied by low-grade metamorphism clastic morphics
(Ledru rocks
et
alo,
1989).
terminate,
(metasandstones
Further
having and
been
to
the west,
overthrust
metapelites)
and
by
by
granitic rocks of the Abami~ and Diany Miyol~ domes.
the
Francevillian
the
Ogoou~
the
Meta-
migmatitic
Ledru et al.
and
(1989)
discussed the structure and petrology of the Francevillian Supergroup a n d the Ogoou~ Metamorphics
in detail, thus furnishing the basis for the pre-
203
sent account. volcanics
Ogoou~ rocks at the base contain m a i n l y basic to acid meta-
which
wrap
around
the m i g m a t i t i c
domes.
The
Okanda
or
m e d i a t e series occurs at the w e s t e r n contact of the Lop~ horst
Inter-
(Fig.4.37)
and has a l i t h o l o g y that is identical with the basal part of the Francevillian,
but
shows
structural
and m e t a m o r p h i c
characteristics
which
ap-
pear to be continuous w i t h the Ogoou~ series. Structure and Metamorphism The F r a n c e v i l l i a n
Supergroup in the Boou~ basin s u f f e r e d two m a i n super-
posed
deformation.
phases
of
An
earlier
phase
of
folding
produced
r e c u m b e n t isoclinal folds w i t h an axial plane s c h i s t o c i t y w h i c h is parallel
to
bedding
sistant
in
pelitic
sandstone
beds
intervals.
and
resulted
in
the
Deformation resulted
repetition
of
re-
in e a s t w a r d overturned
folds w h i c h w e r e a c c o m p a n i e d by g r e e n s c h i s t - f a c i e s
metamorphism.
A later
phase of folding p r o d u c e d some tight folds w i t h NI40°E axes. However, tabular tion,
Francevillian
whereas
phases
of
to
to
the
greater
the
west,
east the
intensity,
which kyanite appeared.
did
not
Okanda with
show
series
such
polyphase
exhibits
intermediate-grade
Metamorphism
in the Okanda
two
the
deforma-
deformation
metamorphism
series
in
suggest burial
to a d e p t h of about 15 km. But since the m a x i m u m thickness of the Francevillian did not exceed about
2,000 m,
this depth
of burial
has been at-
t r i b u t e d to tectonic thickening of the crust. The e a s t w a r d o v e r t u r n e d recumbent Archean
folds,
flat-lying
contact,
and
schistocity,
the
presence
d~collment of
late
at
the
Francevillian-
metamorphic
shear
zones
(Fig.4.39) ~ii suggest that the western part of the F r a n c e v i l l i a n was involved in horizontal d e f o r m a t i o n and crustal thickening. In contrast
to the Francevillian,
phases of deformation.
the Oguoo~ M e t a m o r p h i c s
show three
An early phase, w h i c h was o v e r p r i n t e d by the main
phase during w h i c h folds were produced w h i c h are o v e r t u r n e d away from the axis
of
phase they
the A b a m i ~
diverge are
eastern
migmatitic
away
from the
overturned margin.
here
The
dome
core
of
westward,
third
phase
of
(Fig.4.39).
The
the A b a m i ~ they
are
folds
nearly
deformation
of
the
anticlinorium is
second
and while
horizontal
represented
on by
the open
folds w i t h subvertical or n e a r - h o r i z o n t a l axial planes. Tectonic Model for the Gabon Orogenic Belt The
salient
al.
(1989)
directed and
the
structural in
their
thrusts, inverted
the
features of the Gabon m o b i l e belt w h i c h Ledru et model
have
sought
superposition
of
m e t a m o r p h i c isograds.
to
explain
progressively
are
the
deeper
eastward-
structures,
An east to w e s t e x a m i n a t i o n of
204
this mobile et
al.,
belt revealed the following
1989):
(i)
clinorium where (2)
the
eastern
flank
eastward-directed grade
Ogoou~
of
on
either
whereas
slices
being
been interpreted
LopQ
the Francevillian
as a mature,
anti-
they diverge;
been
affected
is
and affected
the
by the
basin,
with Archean
The Abami~
mushroom-shaped
(3)
parautochthonous,
is truly autochthonous;
in places.
by
of the medium-
Francevillian;
Lastoursville
to varying degrees,
incorporated
Abami~
the
has
horst
basement
the
(Ledru
the thrusting
low-grade
the
in
in
features
and from which
synclinorium
the
of
tectonic
reached
tectonics with onto
side
Archean has been involved tonic
Ogoou~
from its Archean
tectonics,
100 km to the east,
was
are centered
the
horizontal
been detached
tangential
metamorphism
Metamorphics
Francevillian having
peak
the structures
general
more
same than
and (4) the
basement
gneissic
tec-
dome has
diapir.
E
W
.-
I i YC: I; Western
Gnetss
•,
u ....
HtIC
Ogooue
EJ/~.IC//j//'~_~. */'~ x 4 ) ~
"/-I.
~
"l.~/
'
/
F
~
~
,
~
~~ '
~
,
-
~..,"
~
n
~ ~" -/,
"
O k a n d d s e t es
n %A~hseon bDoQs~e m ent ......
/
/, ' ~
I
"
_
sync |inorlum
Ab0mte "'Y'"~"
2
3
/
A
Figure 4.39: Structural section across the Gabon orogenic belt. i, SI; 2, S2 in the sillimanite zone correlative with the foliation in the Abami~ migmatites; 3, $2 in the staurolitem u s c o v i t e - b i o t i t e - g a r n e t zone correlative with the first schistscity in the Francevillian; 4, stratification in the Francevillian. (Redrawn from Ledru et al., 1989.) Ledru et al.
(1989) likened the tectonic features
in the western part
of the Gabon mobile belt to those in continent-continent Such
belts
contain
broad
areas
of
medium-to
collision belts.
high-grade
metamorphism
;
nappes emplaced during Barrovian m e t a m o r p h i s m upon a prograde metamorphic foreland; al.
and migmatitic
placed
the
tectonic
domes
with granitic
events
recorded
orogenic belt in the Eburnean orogeny
in
intrusive the
suites.
rocks
of
Ledru the
et
Gabon
(about 2.0 Ga). This suggest an im-
205
portant regional
convergence during the Early P r o t e r o z o i c
leading to the
crustal s h o r t e n i n g seen in the Ogoou~ series and the Francevillian.
These
features were p r o b a b l y generated in a collisional setting. Ledru
et
al.
(1989)
drew
attention
to
the
flows in a v o l c a n i c sequence in the Eteke area basal Okanda
series,
Francevillian
Supergroup
Ogoou~
contains
point
to
an
Proterozoic of a full orogenic
in the acid
extensional
rocks
cycle.
southern
and
basic
regime
Cycle
operating
Needless
to
say,
region.
during
the
continued
Eburnean
southward
basement
Also,
the
belt,
and
suggest belt
of
Congolian orogen
the
and
internal
of
of
the
suites
the
basal
the possibility
during
the
Eburnean
Gabon
belt
did not
the
considered
base
magmatic
deposition
in the G a b o n the o r o g e n y
the
These
into what we
of
belt with
located near the base of the
terminate at the southern frontier of Gabon Republic, which
komatiitic
(Fig.4.37) p r o b a b l y in the
volcanics.
in the Gabon mobile
Wilson
of
and to a substantial v o l c a n i c - s u b v o l c a n i c
u l t r a b a s i c rocks and tholeiitic metabasalts, series
presence
but a f f e c t e d a belt
earlier
foreland
(Ch.4.9.4)
zones
of
the
as
West
(Fig.4.37).
4.10 The Ubendlan Belt of Central Africa
4.10.1 I n t r o d u c t i o n The
Ubendian
orogenic
belt
the Early P r o t e r o z o i c geny
(McConnell,
al.,
1984).
1972),
Western
highland
basin
Rift
regional prints of
central
region
from
Rise
uplift,
is
which
Africa
2.05 Ga and
encompasses
Tanzania
topographically
affected
1.85 Ga
the W e s t e r n the
plateau) the
g e o l o g i c a l l y it is a p o l y o r o g e n i c
within
the Ubendian
belt
are
of
Whereas
the
Mesozoic-Cenozoic
belt
scattered
a
Valley,
(upland separating
(Fig.4.40).
product
(Cahen et
Rift
bearing
Early, M i d d l e and Late P r o t e r o z o i c t e c t o n o - t h e r m a l
Furthermore,
by
Rift Rise,
Western
and the Zaire-Nile w a t e r s h e d the
was
locally termed the U b e n d i a n oro-
and dated between
the R u w e n z o r i Mountains, Zaire
of
The U b e n d i a n belt is exposed along
north-south
the
(Fig.4.1)
Eburnean orogeny,
remnants
the im-
activities. of Archean
rocks w h i c h w e r e once continuous with the T a n z a n i a craton to the east and with the Zaire craton to the west. Between the Late A r c h e a n and about thick
supracrustal
morphosed northern
during
assemblages the
1.85 Ga in the Early Proterozoic,
accumulated
Ubendian
M a l a w i and n o r t h e a s t e r n
event.
The
and
were
Ubendian
deformed belt
and
extends
metafrom
Zambia, w e s t e r n and s o u t h e r n Tanzania,
206
We~
÷
-~'i-Ni
Ie
"1-
ib Ii 4
nucleus
.,a'=,"
~/no'/o~" " ~
SHIELD
+~ JPalaeozoic to Recent
~
Rift System volcanics :TertiQry to ~ecent
LIndl~
Tabular PrecambHon * Lower Palaeozoic
~Granlte
KATANGA -C ='~
ivris uku Belt
L2 f111 ~,.,',~::'/.
\ Co./, ,. C=
I
MAJOR PRECAMBR|AN ASSEMBLAGES Each ~ncludin 9 earller units reactivated
~
Kotangan and Mozambique belt
~
'1 4, 4"
/,¢
eG 0
: oH ages
~
Earlier structures reactivated by the Pat,African K~bartdes, |r umides, Kotagwe - Ankolean
P ~
Ubendian and
~
Dodoman,N yanzian, Kar ira ndian,West Nile
(KA)
equiva(ents
['~-~+-']Early Shield complexes Vd'/S c~lisbury
.~
.¢~
/
Major Rift System fault Other faults : including post-~rroo
e ~ - . Precambr~an t,~heor zone rocks & intrusives |EIRA
L.,+
+ Inhambane Basin
+
+
+
AAP VAA5
SHIE+LD'F +
Figure 4.40: Outline Precambrian (Redrawn from McConnell, 1972.)
....... Dislocation zones :imprir~c with Mozambiquian dates ..---e. Vergence of fo|d be|is D Dodomo K Kitimanjoro L Lusako Ko Kavirondo K HitUi MP Marungu P|ateau
tectonic
map
of
N Namanga P Pare Mts U UsambaraMts Pa Pangan) R. N Nuanetsl
East
Africa.
207
Burundi,
Rwanda
Mountains
and
northeastern
and
the
neighbouring
adjoining
parts
parts
of
of
Zaire,
western
to
and
the
central
Ruwenzori Uganda
and
Zaire.
4.10.2 Ubendian Rock Assemblages and Tectonism Malawi
Zambia
and NE
In its southernmost parts Ubendian Misuku
belt
belt
subdivided
into
to
Songwe or
group
quartzite
pelitic
Chambo
the
the
Chambo
Songwe
belt.
which et
form
gneisses,
garnet-sillimanite-mica
gneisses
and
al., a
and some augen-gneisses
southernmost
gneisses group
of
with
and
gneissic
rocks;
schists,
with
quartzite.
foliation
these
fer-
and semi-
and ferruginous
east-southeast
the
1984).
southeast-
schists
and
The
has been
gneisses
(Cahen
gneisses
amphibolite
Zambia the
Misuku
unit of gneisses
granulites
the
and
name,
separated by a shear zone from the pelitic
cordierite-sillimanite the
local
River
border
of micaceous
widespread migmatites stitute
the
gneisses,
Jembia
the Malawi-Tanzania
ruginous
by
is a structural
the
gneisses
trending
The
in northern Malawi and northeastern
referred
(Fig.4.40)
cordierite Along
is
have
con-
tectonic
contacts with the Chambo gneisses within which they are sometimes
inter-
layered. In
northern
thermal in the
events
Malawi
south,
through
upper amphibolite stroyed. which
northeastern
lower-to-middle
Zambia grades,
large-scale
Ubendian
facies,
facies in the north where cordierite
in a regional
folds with
lowed by boudinage and
the
foliation
associated
migmatites
and
is completely de-
fold pattern
axial planar
with
and
isoclinal
schistocity,
and irregular folds in the Chambo gneisses.
their
facies
to middle
affected the Misuku gneisses
and
southeast-trending
tectono-
from greenschist
amphibolite
Two episodes of tectonic movements
resulted
granites
and
show varying metamorphic
satellite
fol-
The Nyika
granite
bodies
were emplaced at 2.05 Ga during the peak of the Ubendian orogeny. U b e n d i a n T e r r a n e s a l o n g the S o u t h w e s t e r n M a r g i n of the Tanzania Craton
This
region
high-grade in
several
terranes margins
is
the
type
area
metamorphic
rocks
discrete
blocks
extend
from
of Lake Rukwa
Lake
of
the
of both or
Ubendian
terranes
Tanganyika
and
where
the
a variety
igneous
(Fig.4.41,A).
through
to the northern margins
gneiss complex corresponds
belt
sedimentary
Ufipa
origin
The
to the Chambo gneisses
Ubendian
plateau
of Lake Malawi. in the south
of lie and
The Ufipa (northern
208
Malawi
and
NE
Zambia),
while
the
Ubende
"Series"
corresponds
to
the
Songwe gneisses. LAKE ~ T A N- - G A N YIKA
--
...
.... -
N T
•
&
.
•o _ _1..
100kin i
+
+ +
• ..:.
--
,4"~-~
+
-~,
-
TANZANIAN CRATON +
+
..I-
+
---_ ~Circo
1800~,:a
4-
/
/ +
•
8elf
"~C/~
Granite
~,,
,,"-
'-,
/ /errane Boundary Shear Zone ...... I n f e r r e d Fronlal Thrust Belt
~/l
/':',~i
"::'"~---:'~'~" A
Lake MQlawi TANZANIA
CR ATON
/'~--
---~--~ " --
B Figure 4.41: A, Ubendian terranes, each bounded zones; B, tectonic model for the evolution of Ubendian ranes. (Redrawn from Daly, 1988.) Figure
4.41 A shows the terranes
stratigraphic area.
These
anorthosite) basites
comprise,
(gneissic
lineation
(cordierite
(meta-basites) comprising
from
south
intermediate
granite)
granulites)
with
to
ENE-WSW
with
(i)
trend;
with
east-west and
the
Upangwa
(2) the Mbozi
quartzites) trending lineation; (7)
on litho-
in the Ubendian
(meta-volcanics);
lineations
lineation;
schists.
1988)
north:
lineation
(3) the Lupa
stretching
alumina-silicate
(Daly,
granulites
trend; with
that have been recognized
criteria
with NW-SE stretching
and
stretching Nyika
and structural
by shear belt ter-
the
with
type (meta-
(metaNE-SW
(4) the Ufipa NW-SE; (6) the Wakole
(5) the Ubende terrane
Together these terranes occupy a NW-
209
SE-trending
belt
which
extends
over
a
distance
of
about
400 km,
with
major shear zones defining the individual terrane boundaries. Daly the
(1988)
Ubendian
presented
a structural
terranes.
The
analysis
terranes
are
and
tectonic
internally
model
for
heterogeneously
deformed with extensive tracts of foliated mylonitic and ultra-mylonitic gneisses in which the main foliation is folded generally about axes which are sub-parallel strain
to the elongation of the terranes.
orientation
noted above,
and
the
lithological
Variations
differences
among
in finite
the
emphasize the discrete nature of each terrane.
terranes
The terranes
are bounded by major steep ductile and brittle shear zones which persist for
up
to
(1988) that
600 km
tectonic
the
and
terranes event
overthrust
and
accreted
complex for
structural
the
adjacent
Ubendian
to the
history.
terranes
Tanzania
a
the
series
of
Tanzania
NW-directed
craton
as
thrust
shown
Daly's suggests
craton
orogeny which he termed the Usagaran
caused
onto
long
(Fig.4.41,B)
were
the Early Proterozoic Usagaran
had
model
during
orogeny. sheets
The
to
be
diagrammatically
in
Fig.4.41 B. During the late stage of the Usagaran thrusting the Ubendian terranes
probably
developed
as
a
series
of
tectonic
slivers
which
accreted laterally onto the Tanzania craton. Along
the
southeastern
margin
of the Tanzania
craton
is
the
ENE to
NNE-striking Usagaran belt (Fig.4.41,A), wherein lies the Usagaran Supergroup which was
folded during the Early Proterozoic
Tanzania craton,
its foreland.
Later,
orogeny against
the
the Usagaran belt in turn acted as
the foreland to the Late Proterozoic Mozambique belt. The Usagaran Supergroup unconformably overlies the Archean basement of the Tanzania craton. It is
a sequence
basal
Konse
marbles, blende
of psammitic a n d
Series
of quartzites,
conformably
gneisses
pelitic
metasediments
amphibolites,
overlain by the middle
of psammitic
and pelitic
comprising
hornblende
and upper biotite
origin.
Thus,
the
nockites, and
metamorphism
migmatites,
eclogite.
The
to
the
amphibolite
interlayered
quartzites,
Usagaran
is
unconformably
facies), various
Usagaran
and by
some
the
is
(showing char-
intrusive
overlain
and
and horn-
predominantly of the amphibolite facies with pyroxene granulites retrogressive
the
gneisses
rocks,
Ndembara
Series which is up to 3,500 m thick consisting of intermediate-to-acidic volcanic
rocks
and
subordinate
phyllites
and
quartzites
(Cahen
et
al.,
1984). The occurrence garnet-bearing precursor
volcanic
high-pressure
of eclogites
gneisses plug,
showing
within the
suggests
metamorphism
of
the Usagaran
oval-shaped
that
probably
the
amphibolites
foliation
eclogites
mantle-derived
and in
pattern
originated precursor
of
a
during rocks
210
(Muhongo, gneissic the
1989).
with
craton,
easterly
dips
and
facies
in
emplaced when
the Usagaran
the
charnockites
may
represent
a
cryptic
1973). The Usagaran rocks are i s o c l i n a l l y folded are
and m e t a m o r p h o s e d
amphibolite
probably
in the s o u t h e a s t - n o r t h w e s t d i r e c t i o n over
whereas
(Dewey and Burke,
folded
were
complexes were thrust
Tanzania
suture
The eclogites
thrust
over
the
at greenschist
the
south,
and
craton.
facies
was
The
in the
also
Ndembara
north
intruded
and
by
was
at the
granites
at
about 1.86 Ga and 1.77 Ga. To the north and n o r t h w e s t of Kalemie of Lake T a n g a n y i k a
(Zaire) along the western part
are poorly-known gneisses,
and w h i t i s h - t o
greenish
various mafic
rocks
g a r n e t - b e a r i n g mica
schist
i n t e r s t r a t i f i e d q u a r t z i t e s w h i c h are intruded by
and granites
dated
at
1.8 Ga.
These
gneisses were
also d e f o r m e d by the Ubendian events and comprise the n o r t h e r n extension of
the
Irumide
low-grade
to
belt.
In
this
region
amphibolite
southeastern
tip
of
sequence
greenschist-to-amphibolite
of
volcanics
which
Lake
rest
and
the
the
metamorphic
hornblende
Tanganyika
and
unconformably
Zambia facies
on
grade
granulite
the
is
ranges
facies.
the
Chocha
rocks.
the
Group,
metasediments
Ubendian
from
Near and
a
meta-
The
Chocha
Group is y o u n g e r than 2.05 Ga but is older than its a c c o m p a n y i n g granites such as the Kate and Luongo granites which are dated at about 1.8 Ga.
The Ubendian in Burundi,
R w a n d a and Zaire
A r c h e a n basement rocks in this region gressive the
metamorphism
in
Ubendian' orogeny.
from
the
south Kivu, tightly south
Kazigwe
and
(Fig.4.42) were subjected to retro-
greenschist
facies
amphibolite
retrogressive
Zaire
schists,
These
orogeny
deformation
facies
complex of
during
resulted
the
Archean
On the Itombwe plateau in
(Fig.4.42), the Archean basement has superposed upon it
mica
strikes.
and
metamorphism
complex in southwestern Burundi.
folded
Ubendian
The
mylonitization
Kikuka gneiss
the
in
southerly Ubendian
mica-quartzites
and
gneisses
supracrustals
are
the
deformed view
of
their
type area.
structural
Around
Uvira
in
continuity Zaire,
with
products with
northof
the
the more
similarly deformed
g n e i s s e s and m e t a s e d i m e n t s with north-south trends a p p e a r to be the products
of
the
Ubendian
deformation,
especially
as
they
are
cut
by
pegmatites w h i c h are 2.03 Ga old.
The R u w e n z o r i F o l d B e l t The
Ubendian
(Fig.4.40).
belt
The
terminates
Ruwenzori
belt
northward
as
the
is an east-trending,
Ruwenzori strongly
fold
belt
tectonized
o r o g e n i c belt w h i c h is situated south of the A r c h e a n b a s e m e n t complex of
211
Z A IRE
"''~ I
i
;ement
of.
.Ug,=,~d(,,.
VICTORIA
~1
~ ~ _ _ U G ' '
_ANDA .
~
.: :;:
\3.
~s
"":~ : "'.
.
.
.
.
.
.
.
.
T A NZ A N I A
~\\.
lookm
G:::lj.
B
"'."( * 44**
~I :.Ai F;-;--14
A~
;~ 5 L lOOkm
I!; KQ|err
Figure 4.42 : A, Ruwenzori fold belt showing: i, Phanerozoic cover; 2, Karagwe-Ankolean (Kibaran) ; 3, Main granites of the Ruwenzori belt; 4, Buganda-Toro Supergroups; 5, Basement of Uganda, north of the Ruwenzori belt. B, Western Rift rise showing: i, Cenozoic cover; 2, Bukoban and Malagarasian tabular sequences, folded formations with trends in the Itombwe Supergroup, and Upper Rumbu alkaline complex; 3, Burundian and Karagwe-Ankolean Supergroups (Kibaran belt); 4, Basement affected by Ubendian orogeny (C° 2.05 Ga). (Redrawn from Cahen et al., 1984. ) Uganda
and
northeastern long
to
the
east
of
Zaire.
the
Archean
Kibalian
The rock assemblages
Buganda-Toro
Supergroup
granite-greenstone
of the Ruwenzori (Fig.4.42)
which
belt
of
fold belt beare
the
Early
212
Proterozoic
metasedimentary
in central
and w e s t e r n
and m e t a v o l c a n i c
Uganda.
The
sequences
Buganda-Toro
that
are
Supergroup
exposed
rests
uncon-
f o r m a b l y upon the A r c h e a n basement of Uganda and extends w e s t w a r d s the n o r t h e r n
shores
of Lake Victoria
across
the Ruwenzori
the W e s t e r n Rift V a l l e y into n o r t h e a s t e r n Zaire.
along
Mountains
and
In the southern part the
R u w e n z o r i , s u p r a c r u s t a l s are u n c o n f o r m a b l y o v e r l a i n by the m i d - P r o t e r o z o i c Karagwe-Ankolean
Supergroup.
Cahen
et al.
(1984)
placed
the
deformation
of the R u w e n z o r i fold belt at between 2.5 Ga and 1.84 Ga ago. The Buganda Supergroup of central Uganda sequence of quartzites, pass
upward
ultrabasic flows
conglomerates,
into
basic
volcanics,
rocks.
Among
the
with
tuffs
equivalents
such
and
as
basic
volcanic
dolerites
(Fig.3.36)
phyllites,
amphibolites suite
are
and
quartz
tuffs,
massive
agglomerates,
and
comprises a basal
slates or shales, which and
and
dolerites.
with
some
pillowed
lava
their The
hypabyssal
Buganda
Super-
group g e n e r a l l y strikes east-west. The
Toro
Supergroup.
(Tanner,
upwards: (sometimes
and
pillowed) pelites;
and
bands
of
structures
basement
lithologically
to
the
Supergroup
This
comprises,
metamorphosed
Buganda
andalusite-cordierite schists;
sequence
and
sill-like
less-common
is
and
sillimanite-
conformably
units
of
of
marble
bands
manifested Mountains,
gneisses
These m a j o r
in and
which
major
ENE
synclinal
well-developed
are
parallel
to
base lavas
banded e p i d o t e - a m p h i b o l e succeeded
Formation which is composed of m a s s i v e with
from
tholeiitic
and
folds
rocks,
foliations the axial
with
quartzite. in
in
of
The
The first
parts the
planes
by
and pillowed
doleritic
in the Toro Supergroup reflect two tectonic events.
deformation Ruwenzori
Toro
sills;
marbles.
interbedded tuffs
the
conglomerate;
and
the S t a n l e y V o l c a n i c flows
similar
less common biotite
dolomitic
lava
very
1973),
quartzite
muscovite
is
Based on the sequences which were r e c o g n i z e d in the Ruwenzori
Mountains
rocks
Supergroup
of
the
underlying the
folds.
folds were affected by a younger event with axial traces ex-
tending north-south. The
Buganda
and
Toro
Supergroups
are
equivalent
lithostratigraphic
units, not o n l y because of their lithologic similarities but also because the
base
tinuously Uganda folding,
of
the
from
(Tanner,
combined the
1973).
schistocity
schistosities.
Buganda-Toro
Ruwenzori There and
Supergroups
Mountains are
also
cleavage,
to
the
structural
with
two
extends
Jinja
almost
area
in
similarities
folding
con-
central in
episodes
the and
213
The L u h u l e - M o b i s i o Group in the a d j o i n i n g parts of n o r t h e a s t lithologically Mobisio slates and
similar
Group and
trends
to
the
east-west
phyllites
with
Buganda-Toro
Supergroups.
and
of
small
pebbles;
locally p r e s e r v e d ripple marks;
basic this
complex
comprising
sequence
western
appears
outcrops
form
a
and
(from
quartzites
shales,
intrusions
to
with
phyllites;
lavas.
syncline
In
Luhuleto
top):
conglomerates
quartzites;
the
eastern
the
of the L u h u l e - M i b i s i o Group d u r i n g the Ubendian
tec-
with
steep
flanks,
outcrops The
folded
steep
and a
planes.
isoclinally
with
The
bottom
while
are
intense d e f o r m a t i o n
consists
Zaire is
axial
t o n o - t h e r m a l event at about 2.0 Ga also a f f e c t e d the s u r r o u n d i n g graniteg r e e n s t o n e belts of northeast Zaire. However, m e t a m o r p h i s m in the LuhuleM o b i s i o Group was weak. When traced the southern
southward,
outcrops
the north-south
Ubendian
structural
of the Toro Supergroup appear
trends
to link up with
in the
U b e n d i a n belt of Rwanda through an U b e n d i a n terrane of a n a t e c t i c granite gneisses and migmatites. semblage w i t h i n
the
one t e c t o n o - t h e r m a l have
metamorphosed
Cahen et al.
2.05-1.85 Ga
(1984) p l a c e d the B u g a n d a - T o r o as-
interval
during
which
there was
event or two. An earlier event at about the
lower
part
of
the
Toro
Supergroup
either
2.1 Ga could into
gneisses
and schists before the deposition and t e c t o n i s m of the upper part of the Toro
Supergroup.
Buganda-Toro northeastern
Regionally,
Supergroup,
and
the
basic-to-ultrabasic
possible
intrusives
lavas
which
of
occur
Zaire to the area east of Jinja in central Uganda,
the from
have been
c o n s i d e r e d as ophiolites which occur over a d i s t a n c e of about 550 km. The lavas
and
their
temperature-low
related pressure
intrusives
and range from an u n m e t a m o r p h o s e d to
the
amphibolite
Ruwenzori the
facies,
Mountains.
imprints
of
the
exhibit
metamorphism
Except
state,
with on
as
the
the
mid-Proterozoic
the
the
same
degree
surrounding
of
through the g r e e n s c h i s t latter
Ruwenzori Kibaran
mainly
developed
Mountains event,
where
other
high
metasediments facies, on
the
there
are
parts
of
the
B u g a n d a - T o r o terrane show no effects of later t e c t o n o - t h e r m a l events. Mineralization The
Ubendian
belt
is
generally
poorly
mineralized.
A
stratiform
syn-
genetic c o p p e r - c o b a l t m i n e r a l i z a t i o n occurs at Kilembe in the Toro Supergroup
(Fig.4.42).
The s t r a t i f o r m s u l p h i d e deposits at K i l e m b e occurs in a
b a n d e d a m p h i b o l i t e h o r i z o n within a sequence of schists and amphibolites which are b e l i e v e d to be of sedimentary origin
(Tanner and Bailey,
1971).
D i s s e m i n a t e d pyrite and chalcopyrite m i n e r a l i z a t i o n c h a r a c t e r i z e s all the basic rocks of the Buganda Supergroup and the S t a n l e y V o l c a n i c Formation of the Toro Supergroup.
214 In T a n z a n i a vaded
by
and fissures
hydrothermal
r e s u l t i n g in epigenetic
(Fig.3.36),
(Harris,
zones
mineralizing
activity, Mpanda
shear
1981).
in the U b e n d i a n
fluids
during
rocks
were
late-orogenic
in-
igneous
lead, copper and gold m i n e r a l i z a t i o n at
and gold m i n e r a l i z a t i o n at Lupa near the Rukwa trough
The Mpanda m i n e r a l i z a t i o n occurs
as fissure veins and as
d i s s e m i n a t i o n s and replacements in shear zones w h e r e there are galena and chalcopyrite, w i t h some silver and gold in a q u a r t z - s i d e r i t e gangue. Gold mineralization which,
with
in
the
related
Lupa
goldfield
diorites,
intrude U b e n d i a n m i g m a t i t i c gneisses. fined veins
in
fissures
and
shear
occasional m i n o r chalcopyrite; pyrite,
chalcopyrite
gangue,
and
is
albitic
associated
rocks,
and
with
granodiorites
potash-rich
alaskites,
The gold occurs at Lupa in well-de-
zones
in a s s o c i a t i o n
with
pyrite
and
in association w i t h varying proportions of
galena;
or
without
sulphides
in
a
siliceous
sometimes in a s s o c i a t i o n with chloritic material.
4.11 The Bangweulu Block P r e v i o u s l y referred to as the Zambia craton nucleus
(Clifford,
a
Eburnean
small
set).
the Bangweulu Block situated
in
east,
Zaire the
foldbelt
to
Proterozoic (1984)
and
Tanzania
Middle the
schists,
northwest
Lufilian
showed
cratons
Proterozoic
that
by
Kibaran
and
the
granitoids,
Block
metavolcanics
Ubendian
foldbelt
southeast,
Bangweulu
Zambia
1972)
(Fig.4.40,
surrounded and isolated
arc in the southwest
the
1977), or the Zambia
(Drysdall et al.,
northeastern
The B a n g w e u l u Block is completely
nearby
of
1970), craton
(Kr6ner,
mobile
and
the
respectively, (Fig.4.40).
comprises
cover w h i c h formed during the Eburnean event,
belt
by
Anderson
a crystalline
and a w e a k l y
from the to
coeval and
deformed
is in-
the
Irumide the
Late
and Unrug basement
sedimentary
between 2.0 Ga and 1.8 Ga,
as a c o n t i n u a t i o n of the Ubendian mobile belt.
4,11.1. Geological Evolution A n d e r s o n and Unrug
(1984) did not identify A r c h e a n rocks in the Bangweulu
Block, hence they a t t r i b u t e d it solely to the Eburnean. plex
of
the
basement in the
of
Bangweulu
Block
n o r t h e r n Malawi
Bangweulu
basement
is and
actually
an
southwestern
occur
as
small
The basement com-
extension
of
Tanzania.
The
schist
belts
the
Ubendian
oldest
rocks
(Fig.4.43)
where
q u a r t z o - f e l d s p a t h i c rocks derived from semi-pelitic to p s a m m i t i c and acid volcanics
are
believed
to
be the
stratigraphic
equivalents
of
the
pre-
215
N
• •
.
~Mpo rok o s o .::
~
~
~'-:'T.
~
"P
"-
~:-)<
.- - (/<
Mporokoso Group
~----~or ogenlc belt
younger sediments basement 0
, km
g l O0
graphite
Figure 4.43: Geologic map and tectonic setting of the Mporokoso Basin. (Redrawn from Andrews-Speed, 1989.)
216
Ubendian Misuku sidered. large
and Jembia
However,
the
concordant,
(Fig.4.43). rhyolite.
Bangweulu
composite,
The
hypabyssal
River migmatites
and
is
batholithic
metavolcanics
intrusions
basement are
flows
and granulites predominantly
granitoids
mostly
of
Andean-type
modern
composed
and
porphyritic
with
andesite,
zones
(Kabengele
small
dacite
and
in the northwestern
Block are chemically and petrographically
subduction
of
metavolcanics
pyroclastics
These extensive high-K calc-alkaline rocks
part of the Bangweulu
already con-
and
Lubala,
akin to
1987).
They
suggest a similar type of magmatism in the northwestern Bangweulu region between
2.0 Ga
(Anderson and Unrug,
of the block,
the Kate
Eburnean discordant and metavolcanics The
Supergroup 1982).
and
uranium
marine four
Block
The Mporokoso
unconformably sequence,
one of the late
the Bangweulu
and
The Mbala
unconformity-bounded
overlain
about was
5 km thick, deposited
Formation,
depositional
by
plain,
area
while
the
1989).
for these
acid tuffaceous
upper
two
about
Daly and Un-
contains
placer gold
fluvial
and
in
2 km thick,
sequences sequences
unit,
Paleocurrent
clastic
deposits.
shallow
consists
(Fig.4.45),
the
of
lowest
(C,D)
each
which
closely
resemble
reflect
the
shallow tidal
data suggest a southern sediment An unconformable
the Nsama Formation,
succeeded by the Kabweluma Formation arenites
cratonic
(A,B) representing the deposits of braided rivers on a wide
(Andrews-Speed,
source
a
1989;
flooding and reworking of fluvial sediments by an invading sea
granitoids
the Mporokoso Group of the Muva
1984; Andrews-Speed,
Group,
mineralization,
environments.
fluvial
is
and volcanogenic
two sequences
(Fig.4.44,A),
separates
from the migmatites and gneisses of the Ubendian belt.
(Anderson and Unrug,
rug,
granite
intrusive bodies,
Bangweulu
siliciclastic
intrusive
1984). Along the northeastern margin
and
extensive
is up to 600 m thick,
and is
(Fig.4.45) which consists of quartz-
the tidal
part of the underlying Mbala Formation.
marine
sandstones
at the
top
The Nsama Formation was probably
derived from a magmatic arc which lay to the north
(Fig.4.44,A).
The Mporokoso Group was deposited between 1.8 Ga and ioi Ga on an extensive (1989)
post-Ubendian
silicic
magmatic
showed that the sandstones
arc
(Fig.4.44,A).
and conglomerates
Andrews-Speed
are locally enriched
in placer gold deposits which were derived from gold-bearing quartz veins in
the
Lupa
(Fig.3.36). in uranium
goldfield
Also,
in
the
the magmatic
Ubendian
belt
of
rocks of the Bangweulu
(10-60 ppm) which according to Andrews-Speed
been redistributed
into the Mporokoso
nearby Irumide belt (1.4-1.0 Ga).
southern
Tanzania
basement are rich (1989) could have
Group during the evolution of the
217
,C
~ ÷" ~ K~ALI-TORO-BUGANDA
/~'e\ II "q..¢
.,.,
;i(A
I SH,ELD
\
._.
/
.."+-,Ib+t,
1
~3..-¢+ ++ ~'.%~: 'i
%:..-; ."9 + (9
,+
,,
*
..:" .......
.."."::: .......
""
/+'>;" ':ll
I
j
... . . . . . . v : : '.':. :V.":.'I
A EBURNIAN
....
÷
8 KIBARAN EVENTS
EVENTS
LEGEND
,./(~"
Plutonism +
Volcanism
Kate Granite X, Syenite
Mafic and
V
Lacco|i'lhe
ultramafic roclcs A
Quartz vein2 ~
Schist b e l t ~ ~
Platform sedlmentsl---'-::Pcdeocurrent t MsuJI "::!
~ I , '%=-".k x
_xf--T'~k %.%,
,-~
)
Faults#'
Shear zones ~z
Structural t r e n d s , /
"'/
Rotation
Soundary of Sangweulu Slack
previously formed
Front of Lufllian A r c / / D i
.,-
:~<:Z')'.:!'~.
Subduct[on
zone .#~
Rejuvenated shear zones,," Uplifted
source
area
of ciastics ~;~,~
C PAN-AFRICAN EVENTS
Figure 4.44: Tectonic evolution of the Bangweulu block during the Eburnean, Kibaran and Pan-African. (Redrawn from Anderson and Unrug, 1984.) Figure dynamic
4.44
(Anderson
evolution
of
the
and
Unrug,
Bangweulu
1984)
shows
terrane.
The
a model
for
the geo-
crystalline
basement
formed during the late Eburnedn by the diapiric emplacement of granitoid batholiths
in
the
southeastern
part
of
the
Bangweulu
Block,
whereas
slightly later, a high-K calc-alkaline volcanic arc and associated intermediate and acid batholithic intrusives and basic plutons are believed to
218
have
occupied
the northwestern
also corroborated the
Mporokoso
event,
and
orogenies
continued
during
:'-'
."." i ." ".:. :.:':D' " . ' " . ' : :--;=-
later
during
Kibaran
situation
took place in the
and
Eburnean
Pan-African
/ I-"
"
""
"
'
'
" ,
. '
, "
.
L,t.l__: . :
'
.'
"
•
'.
. +
fan
=• f V
.
•
!
.
• •
J
'
:
" " ' "" . . . .
" *'
'
NSAMA
F~
'
C H E R T , ~'UFF & SAND
~
MARINE
SAND
~
DEBRIS
FLOW
.
;'-'":''
;'] FLUVIAL
Mbala
Formation
~
....
~ , , . . "..'
3
]
~
' KASWELUMA F~ 5
'. entrenchmerrt ]
Lithostratigraphy
'-'.'" " " ' !
"=-:'---:--'--:-::-
. . . .
A ' • : " • .' " . '... .. I~ . . " . ". ~ . ' .
Scarp
Scarp retreat
much
succeeding
N aRT I-
""'"
Scarp retreat
Change
a tectonic
(1989). Sedimentation
basin the
the block,
(Fig.4.44,C).
Volcanism
Fan-head
of
by Kabengele et al.
intracontinental
Geomorphic SOUTH events
'
part
~
FLUV]AL
~
VOLCANIC$ 7 G R A NITOI D$
2 ~
:! "'/:AU
'"
.........
"'~
....
"'"::-""
SAND
::'"
:i:':':":'::.'.':::.::':":'."','.:.':'",:.'::'.','",',',:.':.".':',:.':" ' ~ " : : :
GRAVEL
METASEDIMENT
~" ""
/.~ ",':'.:":" "," '-".:.::'.':, 'A', ',',":',::':. ".- " "':',', i'"..".':" ".:::':'~',':' :::'".' :.':":'.:.:" :'"'"::. "': 1
Au
.
....
FACIES DESCRIPTION ASSEHB LA(~E
INTERPRETATION
Planar cross-bedded sandstone, texturally immature;
Sandy braided river dominated by sand waves and transverse bars,
Trough cross- bedded sandstone, tex~uratly and mineralogically immature, with wellsorted trough cross-bedded and massive conglomerate.
Sandy to pebbly sheet-braided rivers with dunes and |ongitudinaL bars
Poorly sorted
Grain flows forming a fraction carpet or sandy debris flows
conglomerates.
Texturally and mineraloglcally mature planar and trough cross-bedded sandstones
Tido[-dominated, shallow shelf sea
Hudsfone, tuff and chert, with minor siltstone, sandstone and carbonaceous mudstone
Voicanogenic
Figure 4.45: Stratigraphy and gold and uranium mineralization in the Mporokoso Group. (Redrawn from Andrews-Speed, 1988.)
219
Deformation fold
zone
lated
to
(Daly,
known
of
the
as
the
NW-directed
Mporokoso Luongo
basin
fold
thrust
was
belt
largely
(Fig.4.46).
tectonics
of
the
confined This
Kibaran
to
folding age
a
narrow was
Irumide
1986a).
J
+
+
÷
÷
÷
t,41:
÷ ÷
+
+
Makasa
-
/"
/
÷
÷
÷
+
+
+
~gu
÷
/
+
÷
÷
/-
N
~cm~ipil!.. + •6 ÷
+
+
÷
Figure 4.46: (Redrawn from
+
t00 km '
Structural sketch figure supplied b y M.
[71Luapulo
I
map of C0 D a l y . )
beds
F'--I Mporokoso Group
the
~
Bongweulu granite
~
Luopulc& volanics
Luongo
zone.
rebelt
Chapter 5 The MId-Proterozolc Kibaran Belts
5.1 Introduction Widespread Early
crustal
stability
Proterozoic
prevailed
Eburnean
orogeny ages,
reconstructions,
radiometric
Lower Proterozoic
rock assemblages
Africa
America
and
South
continents mass;
(Fig.5.1).
while
continental
mass.
1980),
suffered
which
mostly
development, Proterozoic including
to
and
matching
crustal
and
western
Antarctica
the
ensuing
the
end
of
Europe
also
of
the
continental
the
widespread
land masses of
continuity
rifts
intrusion
and
of
emerged
among
formed
formed
a
both
one
land
continuous
(Morel and Irving,
mid-Proterozoic
magmatism,
and only very localized
the
the
one supercontinent in
anorogenic
abortive
m o s t l y restricted
the
from
Paleomagnetic
It is believed that at the end of the Early Proterozoic
w o r l d - w i d e orogenies, Piper,
and
America
India
Africa
across the consolidated
attest
North
Australia,
in
(1.8 Ga).
rifting
orogenies.
anorogenic
and
all
1978;
950 Ma),
intracratonic
The predominance
magmatism
andesine-labradorite
(1.77 Ga -
basin of mid-
over the world,
anorthosites
which
are
to this age, was possible because the Earth had evolved
a stable crust by mid-Proterozoic
times
(Windley,
1984).
Greenland 30" N
"''-..f
Hadagacar i
Figure 5.1: Paleomagnetic nent in the Late Proterozoic.
reconstruction of the superconti(Redrawn from Condie, 1989.)
221
In Africa the mid-Proterozoic crustal quiescence was interrupted only in limited parts of the Eburnean terranes south of the Sahara,
including
the Namaqua and Natal mobile belts which, as already noted, stabilized as parts of the Kalahari craton after mid-Proterozoic orogenesis. of
mid-Proterozoic
deformation, the
was
terranes
rifting,
sedimentation,
place
between the in
the Kibaran, rifting,
Zaire,
Tanzania and Kalahari
Africa
event,
known
northeast-southwest-trending
as
cratons.
the
between
and
did
1989a);
it
Africa,
(Black,
is
and
about
not also
magmatism,
extend doubtful
1984;
Caby,
950 Ma,
northward whether 1989).
into the
in
and
central
the
southern
Africa
orogeny
outside
belts,
deformation and
northeastern
Kibaran
However,
orogeny,
mobile
(Fig.5.2). Kibaran
metamorphism
1.45 Ga and
in
In this
Kibaran
intracratonic
Irumide, and the Southern Mozambique belt
sedimentation,
transpired
metamorphism
concentrated in the eastern part of central Africa,
region a mid-Proterozoic orogenic took
magmatism,
The scene
(Vail,
affected
West
Kibaran mobile
belts Kibaran-age sedimentary and magmatic rocks are known in Angola, the Sudan,
the
(Fig.5.2).
Tuareg
Shield,
Since
these
the Benin-Nigerian Shield, include
and
volcano-sedimentary
in Madagascar
assemblages
actually belong to the initial phase of the Pan-African orogeny,
that
some of
these are considered in the next chapter. From
the
mid-Proterozoic
mineralization
in
terranes
the
of
mineralizations diamond,
the
what Clifford is
Burundi,
the
tin,
are
and
dominant
what
gold,
younger
it
was
Whereas iron,
type in
the
ore
cratonic
predominant
chromium,
terranes
of
the
asbestos
in Africa
and
constitute
considered the structural metallogenic domain that
by
major
tungsten
Rwanda,
the
from
Precambrian.
cratons
mid-Proterozoic (1966)
onward,
changed
earlier
in
characterized
beryllium,
Africa
western
deposits
and
of
copper,
niobium-tantalum.
Tanzania,
and
lead,
In
the
southwestern
zinc,
cobalt,
Kibaran
Uganda,
belt
there
of are
economic deposits of tin, beryllium, tungsten, niobium-tantalum, gold and lithium.
5.2 Kibaran Mobile Belts The
Kibaran
tectono-thermal
contemporaneous orogenic belts northerly Kibaran
belt
and the eastern belt, these
basins
event
occurred
three
in central eastern Africa
consists of two
segments,
and together with the
constitute
in
intracratonic
(Fig.5.3).
the western
Irumide belt
extensional
parallel
type
to the
basins
The area
south,
which
are
transverse to the northwest-southeast-trending Ubendian belt. The Kibaran
222
N
I
° ~o
1500 km
I. The Tonzanion croton, Ubendion-Usogaran belt, Bangweulu block ~. The Kapvaol croton 3. The Z a i r e croton 4. Mwembeshl shear zone 5- B r a k b o s - Doornberg fault zone 6. Pofadder shear zone T. Mugeese shear zone 8- Ghonzi- Chobe belt 9. Rehobothion
Figure 5.2: Mid- to Late-Proterozoic tectonic features Southern Africa. (Redrawn from figure supplied by M. C. Daly.) basins
(Fig.5.3)
during
sinistral
(Klerkx origin
et al.,
are believed shearing 1987).
of the Kibaran
southern
California,
to have originated
along
Klerkx
wrench et al.,
belts
to that
U.S.A.,
where
faults (1987)
of the
between in
1.4 and 1.35 Ga
the
likened
Ubendian the
Basin-and-Range
strike-slip
faults have produced oblique normal fault-bounded
of
movements
belt
extensional province along
of
wrench
sedimentary basins.
223
N
L.Edvard
Buru
linn
L.
Figure 5.3: Klerkx, 1988.)
Kibaran
basins
In contrast with the Kibaran Southern
Mozambique
belt
of
central
Africa.
intracratonic
of
eastern
(Redrawn
basins
Zambia,
to the north,
Malawi
and
Mozambique appear to have resulted from plate margin processes 1984; Piper et al., 1989; Sacchi et al., region
of
high-grade
ultramafic
rocks
gneisses
suggests
that
with the
older Limpopo
belt
to the south,
and collision
belt
(Burke
the
northern (Andreoli,
1984). The preponderance in this
slices
Southern
of
1977),
ophiolitic
Mozambique
is a deeply
et al.,
from
exposed
where
island
mafic
belt,
convergent arcs
and
like
and
the
margin oceanic
crust accreted onto the African continent. 5.2.1 The Kibaran Belt Li thostra ti g r a p h y
The
Kibaran
Zaire,
mobile
through
Tanzania.
It
belt
Burundi is
poorly
extends
and
for
Rwanda,
developed
to in
about
1,500 km
southwestern the
middle
from
southeastern
Uganda
part
where
and
western
the
Lower
224
Proterozoic
Rusizian
basement
of the Ubendian
belt separates
belt into a western basin and an eastern basin. the
Kibaran
quartzites similarly Klerkx the
belt
and pelitic deformed
et al.,
Shaba
Supergroup, Supergroup
contain
and
Kivu
whereas
The
provinces in
the
,
of
in both basins sequence
Zaire,
eastern and
However,
sequences
both segments of
(Fig.5.3),
and intrusive granitic
sedimentary
in Rwanda-Burundi,
"..:::
A
sediments,
rock
and metamorphosed
1987).
Uganda and Tanzania
"
similar
basin as the
is it
the Kibaran
(Cahen et al.,
in the western
referred is
mainly
rocks, which are
known
to as
Karagwe-Ankolean
as
the
the
1984;
basin,
in
Kibaran
Burundian
Supergroup
(Fig.5.4).
'~ '.i
100 KI~
~3 IINI t-ll Figure 5.4: A, Kibaran belt in Central Shaba, Zaire; i, Kibaran Supergroup; 2, Granites; 3, Katangan Supergroup; 4, Phanerozoic; B, the Kibaran belt in Burundi showing: i, Archean; 2, Burundian metasediments; 3, Kibaran granitoids; 4, Mafic and ultramafic intrusions; 5, Late Kibaran alkaline intrusions; 6, M a l a g a r a s i a n (post-Kibaran) sediments; 7, post-Kibaran alkaline complex; 8, Cenozoic; 9, major axes of upright folding (D2); 10, major late-Kibaran shear zones (D21); ii, stratigraphic boundaries and structural trends. (Redrawn from Cahen et al., 1984; Klerkx et al., 1987.)
in
225
In the w e s t e r n Kibaran basin, the basal sequence, which
was
monotonous of
a
deposited
soon
phyllites,
greenstones
higher
Group.
The
metamorphic Lufira
after grade
Group
the
Ubendian
orogeny,
and carbonates.
than
the
comprises
the Mt. Kiora Group comprises
The Mt.
unconformably
basal
dark
Kiora Group is
overlying
conglomerates,
Lufira
quartzites,
p h y l l i t e s w i t h occasional doleritic lavas at the top. This is overlain by the
dark-coloured
Hakansson
Group,
comprising
slates
which
basal
and
conglomeratic
is d i s c o n f o r m a b l y
conglomerates
and
quartzites
succeeded
arkosic
by
of
the
the
Lubudi
graphitic
Mr. Group
shales
and
s t r o m a t o l i t i c limestones and d o l o m i t e at the top.
A
i / -
"-'''7"':'~:'~"
B.......:.........~.- .....: ....-..-..,-• .~ ... :
:
~-.:...o-.-...• -.-, -......, ./"2
-
,
"1\/\
~i\° :
.
::'~
_
\ z \l \ t
ml Figure 5.5: C r o s s - s e c t i o n through the Kibaran belt in Burundi d e p i c t i n g D1 phase deposition, d e f o r m a t i o n and magmatism. A, at lithospheric scale. B, across the crust of Burundi; I, U p p e r Burundian sediments; 2, L o w e r and M i d d l e B u r u n d i a n sediments; 3, Quartzites in 2; 4, p r e - K i b a r a n basement; 5, 6, g r a n i t o i d s intruded in D1 phase; 7, mafics intruded in DI. (Redrawn from K l e r k x et al., 1987.) In the eastern
Kibaran
Supergroups
are
also
(Fig.5.5).
Here
the
turbiditic
pelites
basin
amenable lower with
carbonates
and
phyllites,
occasionally
the
Burundian
to a t h r e e - f o l d
groups
are
a
more
of
arenaceous,
interbedded
the
characterized
intercalations
volcanics;
and
with
Karagwe-Ankolean
stratigraphic by
mature
subdivision
dark
laminated
quartzites,
reddish
conglomeratic
middle
rare
group
quartzites
of and
226
minor
basaltic
group
of
and
dacitic
immature
arenaceous
ferruginous quartzites. south,
the
correlates
with
Ubendian
the
complex
cumulates
and
and
(Fig.5.3)
sulphide
of
basin
the
shortly
conglomerates
containing
of
a
the
basal
and a main
series
Itiaso
by
chrome-magnetite
zone;
further
Itiaso
The
intruded
comprising
and
the
and
the
Kapalagulu
zone
of
olivine
concentrations;
a
zone w i t h anorthosite
rhyodacitic volcanic
Supergroup,
sedimentation
after
Ga
1.40
at the base of the
in
(Klerkx
the
rock at the
eastern
Kibaran
1987).
Fluvial
et al.,
lower group and the presence
of graded
ripple marks and slump features higher up in the pelites of the
lower g r o u p of
are
of an interbedded
Burundian
began
bedding,
disconformable
1963). The K a p a l a g u l u complex is over 1,500 m thick.
Based on the age base
upper
deposits
Supergroup.
gneisses
r h y t h m i c a l l y layered intermediate (Wadsworth,
an
shales
Karagwe-Ankolean
local
and
conglomeratic
phyllitic
basement
(1.23 Ga)
with
rocks;
On the eastern m a r g i n of Lake Tanganyika,
quartzites
underlying layered
volcanic
and
clastics
rare
into
stromatolitic
a
rapidly
carbonates,
subsiding
suggest
trough.
About
the
rapid
ii
to
influx
14 km
of
sediments a c c u m u l a t e d in the eastern Kibaran basin; and the w e s t e r n basin was
probably
even
conglomeratic
upper
of
tectonic
thicker part
movements
(Cahen
et al.,
of the Burundian
with
uplifted
1984).
The
Supergroup
local
sediment
arkosic
suggests
and
the onset
source
areas
Kibaran
belt
within
the Kibaran belt. Post-Kibaran
molasse
deposits
occur
within
its w e s t e r n and eastern cratonic forelands. the Mbuyi
Mayi
and
Malagarasian
the
Supergroup
on the western
Supergroup
S u p e r g r o u p on the Tanzania craton the
Kibaran
belt.
The
Kibaran
and
foreland its
on
the
and
on
Zaire
craton,
the
Bukoban
equivalents
(Fig.4.42),
internal
the
The basal c!astic sequence of
are the foreland molasse of
molasse
lie
in
the
north-south-
t r e n d i n g Itombwe synclinorium.
Structure and Metamorphism Detailed
structural
et al., phase
(1987) of
the
revealed
horizontal
metamorphosed in
has
analysis
D!
thrusting
the
four main
deformation
structural
basement.
thin-skinned
of
(DI)
deformation along
the
s e d i m e n t a r y cover.
Kibaran
deformational occurs
levels near granite produced western
regions of intense granitic intrusions
Rwanda,
eastern
at
belt
episodes.
deeper,
intrusions of
the
Klerkx
The
more
first
strongly
in anticlines or
bedding-parallel part
by
foliation,
Kibaran
belt
in
(Fig.5o5,B) and d ~ c o l l e m e n t of the
In the eastern less m e t a m o r p h o s e d parts of Burundi and
D1 d e f o r m a t i o n caused basement mylonization.
M a j o r granite-gneiss
227
domes were e m p l a c e d during the D1 event. (D2)
affected
all
Kibaran basins. NE-SW,
but
Kibaran
NW-SE
the r e g u l a r structural was
to o b l i t e r a t e folding
and
producing
in
both
the
eastern
and
the
western
It produced open, upright folds w h i c h are m a i n l y oriented
swing
D2 d e f o r m a t i o n
rocks
The second phase of deformation
in the northern
of
the
eastern
belt,
D1 structures. cataclasis
but
it was
not
sufficiently
A late phase of D2 is a s s o c i a t e d with shear
which
NE-SW-
were
and
superimposed
NW-SE-trending
on
previous
shear
zones
is located in a zone of persistent crustal instability, also a f f e c t e d
domes.
penetrative structures which
a f f e c t e d the alkaline intrusives emplaced during D2 deformation. was
where
trends are d e f l e c t e d around g r a n i t e - g n e i s s
compressive,
vertical
part
by later tectonic
events
ranging
also
Since it
the K i b a r a n belt
from the Pan-African
o r o g e n y to Cenozoic rifting. Regional
metamorphism
greenschist
facies,
facies and m i g m a t i t e s highs.
There
are
in
the
especially
in
two
main
phases
and
D2
with
intrusives.
Variable
belt
is
synclinoria,
generally whereas
occur near large b a t h o l i t h i c granites
D1
associated
Kibaran the
contact
of
regional
deformation
metamorphic
which
syn-orogenic
aureoles
were
the
and basement
metamorphism
and
of
amphibolite
are
granite
produced
during
these intrusions.
Intrusive Activity The
Kibaran
belt
distinguished Cahen
by
et al.,
from Kivu and ubiquitous
(1984)
and
granitic
rocks
in
the
referred
to
the
G1
intrusives
as
in
Klerkx
et al.,
Kibaran and
G2
Zaire
and
belt.
to
granite (1987)
The
granitoids,
G1
Burundi
and Rwanda
intrusions
discussed
first are
found m o s t l y within the lower Kibaran
sets of intrusives, The
Shaba
granitoid
two
four
types
types,
syn-orogenic sequences.
is
(Fig.5.4). of
commonly concordant
The
last two
G3 and G4 are p o s t - o r o g e n i c a l k a l i n e granites.
batholithic
granitoids
are
homogeneous,
porphyritic
gneissic
biotite a d a m e l l i t e s with low initial 87Sr/86Sr ratios w h i c h were emplaced around
1.35 Ga
porphyritic chemistry.
(Klerkx et al.,
peraluminous Such granites
1987).
two-mica
The G2 g r a n i t o i d s
adamellitic
are t y p i c a l l y syn-tectonic
depth where m i g m a t i t e s
form as a result
G2
common
granitoids
during both G1
D1 d e f o r m a t i o n and
cataclased more
are more
G2
porphyritic
central
around
granitoids
portions
1.28 - 1.26 Ga
adamellites
(Condie,
(Klerkx et al., Kibaran
which
gneissic,
of the granitic massifs,
not
S-type
1989).
they were
in the w e s t e r n are
with
and were e m p l a c e d at
of anatexes
in the Shaba province;
co-exist
are u s u a l l y non-
orthogneisses
and pass
1987). belt,
into
The
emplaced Where the G1
occupy
the
completely
228
gneissified G2 granitoids which are usually situated in the outward parts of the massifs. deformation. ratios,
The G2 granitoids
On
the
basis
of
recrystallized
their
in spite of strong crustal
under
generally
low
contamination,
continental
crust
and
became
during D1 87Sr/86Sr
Klerkx
suggested that the primary magmas for these granites lower
stress initial
et al.,
originated
progressively
(1987)
from the
contaminated
with
crustal material during their emplacement. Although 1.19 Ga
classified
and
subdivided
1.0 Ga
as
by
this generation
the late intrusives
post-orogenic
Cahen
et al.,
of granites
G3
granites
(1984),
and
Klerkx
in the eastern
typically
unlike
G2
first ages
intrusive,
group
and
around
compositionally
they
consist
also exhibit
of G3
(1987)
which are associated with D2 compressive deformation
granites;
compositions
between
Kibaran belt into
and those intrusives which are associated with shearing. are
dated et al.,
granites
1.18 Ga,
which
of
crustal
occupy
the
homogeneous
unfoliated,
two-mica
granites
strontium
isotope
signatures.
the
anticlines
cores
also dates
The first group and
the
of
D2
formation
with
of the
different The with
anticlines.
The second group of G3 granites comprises intrusions of alkaline granites which
are
spatially
about
1.10 Ga,
associated
the
age
of
with NE-SW D2
D2
shearing.
shearing
The
and
relatively
are dated low
at
87Sr/86Sr
initial ratio suggests a deep crustal origin for these alkaline plutons, and thereby also imply a deep crustal origin for D2 shearing. In the eastern Kibaran belt there is a string of mafic and ultramafic intrusions shearing to
aligned
northeast,
direction
Lake
parallel
(Fig.5.4,B).
Victoria
in
to both the major
They extend
northwestern
from
D2 folding and
Burundi
northeastwards
These
are
Tanzania.
intrusive
peridotites with or without associated gabbro, norite and leuconorite, which the gabbro-noritic et al.,
1987).
ultramafics crystals form
related
layers
of
peridotite
and
norite
(Klerkx in
the
suggest that they were emplaced as a crystal mush of olivine
in a noritic
the
rocks exhibit some evidence of layering
Alternating
in
liquid matrix,
ultramafic
these
mafic
while the mafics
crystal-liquid and ultramafic
mixtures.
bodies
were also derived
Klerkx
to mafic
et al.,
magmatism
(1987)
which was
generated during an early phase of crustal extension. The last major group of intrusives are the post-tectonic equiangular alkaline These
granites
are
"tin" granites or G4 granites of
economic
importance
since
(Cahen et al., they
mineralized pegmatites.
The G4 granites are cataclastic,
and cut across
rocks of older granitoids
They
consist
of
country quartz,
microcline,
albite,
leucocratic
are
1984).
invaded
by
locally sheared
and Kibaran
and muscovite
sediments.
(or biotite)
229
with accessory apatite,
zircon and tourmaline.
Their roofs are invaded by
mineralized pegmatites and quartz veins. The G4 granites were emplaced at about 976 Ma, probably from a deep-seated magma or from the fusion of G1 and
G2
granitoids
or
pre-Burundian
basement
gneisses
(Klerkx
et al.,
1987). Tectonic
Model
In their discussion of the tectonic evolution of the Kibaran belt Klerkx et al.,
(1987) considered this belt as a linear intracratonic trough, the
origin
and
early
development
of
which
was
controlled
by
crustal
extension. Like geologically younger regions with lithospheric as
the
Late
Paleozoic
States,
where
rifting,
the
which
are
generated
bimodal large
Morocco
mafic
number
associated by
of
with
crustal
and
of
and
the
acid
granites
the
The
magmatism and
first
extension.
Cenozoic
are
gabbros
western
such
United
associated
in
deformation granitic
stretching,
of
the
(DI)
magmas
with
Kibaran
belt
may
have
been
could
have
been
produced by the partial fusion of the lower crust by the heat supplied by mafic magmas generated by the intrusion of hot asthenosphere A
slow
and
protracted
1.26 Ga probably
extension
triggered
very
of
the
little
Kibaran
fracturing
belt
(Fig.5.5,A).
between
and thereby
1.35
and
inhibited
volcanism in the belt. Rather, heat from the mafic magmas accumulated and caused
the
partial
fusion
of
magmas which were emplaced extension
(D2).
peraluminous,
The
crustal
granitic
S-type
rocks
thereby
producing
during the climax of deformation magmas
character
by
could
crustal
have
acquired
contamination
granitic
and crustal their
at
strong
the
upper
the Kibaran
belts
crustal levels while ascending. Mineralization
Although
production
constitute with
is not
a metallogenic
them
economic
high by global province where
deposits
tantalite and lithium-ores, of
bastnaesite,
uranium
of
tin,
standards, "tin"
granites
tungsten,
have associated
beryllium,
colombo-
in addition to minor but exploitable deposits ores,
mica
and
semi-precious
stones.
Small
amounts of gold are won from Tertiary and Recent alluvial placers, which are
derived
from
the
Kibaran Supergroup Mineralizations (Fig.5.6)
have
basal
(Radulescu,
conglomerates
and
quartzites
of
the
lower
1982; Tissot et al., 1982).
associated with the G4 post-tectonic Kibaran granites
been
interpreted
by
Bugrov
et al.,
(1982), Radulescu
230
f..~.~l~b]
Ultromafic
/ mofic
belt
Kibaran metasedfments,
and
oeromagnetic
granitoids
and
~nomalies
basement
domes
Bugondo - Taro
Figure 5.6: Geologic sketch map and mineral deposits in NE Kibaran belt. Mining districts in Uganda are: Bj-Buhewju plateau; Ka-Kamena Fe deposit; Ki-Kirwa W; Ke-Karenge G4 granite; MMashonga Au; No-Nyamulilo W; Rh-Ruhiza W; in Rwanda to the north are B-Bugarama W; Bu-Burange pegmatite; Mi-Miyoye Au; Bi-Bisesero Au; G-Gifurwe W; N-Nyamulilo W; Ny-Nyungwe Au; R-Rutongo Sn; M-TMusha-Ntunga Sn; in Burundi they are Ci-Cibitoke Au; Ca-Cankuso Au; Bo-Buhoro Gabbro; Mu-Musongati Ni; Re-Kayonde Ree; in Tanzania they are Kb-Kabanga Ni, Cu, Co. Dotted lines enclose auriferous zones. (Redrawn from Pohl, 1987.) (1982),
Tissot
with mostly metallics
et al.,
Sn and Nb/Ta
(1982)
and
Pohl
(and subordinate
such as muscovite
(1987) Li,
and kaolinite);
as
comprising
pegmatites
Be, W, Bi, U/Th,
quartz
veins
and non-
with mainly
Sn
231
and W and
(and pyrite,
quartz
veins
pegmatites
and
with
the
ores.
In
had mostly their
Bi, Au,
gold.
tungsten
tectonic granites mineralization,
siderite,
(Fig.5.7) view
Pohl
quartz
of
metals
from
limonitic
(1987) veins
silicification
demonstrated
are
that
concentrated
the
apparent fluids,
H20 , and boron the
granites
rarity
of
flourine
than
post-
from
of
during
sodium-rich,
the volatiles,
rather
the
the sources
some of which were
among
zones
tin-bearing
in
and that these granites were
the mineralizing
phosphorus,
U);
and derived the
Kibaran
metasediments.
Ferberite quartz veins Sn quartz veins Huscovite (Au.$n) Huscovite( A u ) , , , ~ BU ,
HETASEDIMENTS \ . ~
" '_
/ \
~
~ //\"<
/
_
\-
\
I
~"
Au quartz veins Pegmotite fields \l
MI
~ 8Sn(W) HT
~ -, ~
~
\ - -~ ;
-\
/
_.,- ~ /~/6(,~
~
/A -
V Lt. ee.(Sn, rb/ta,th)
- j ~- /
\/ -~
i
Figure 5.7: Kibaran mineralization related to G4 "tin" granites. Figures denote types of pegmatite. Bu-Bugarama; K-Karenge granite; Mi-Miyoye; M-T-Musha-Ntunga; R-Rutongo. (Redrawn from Pohl, 1987.) In northern (1.30 Ga 1985),
old)
Burundi, which
contains
million tons
is
northeast of Bujumbura, a part
of an
alkaline
the Matongo massif
an igneous phosphate deposit with a reserve
(Kurtanjek and Tandy,
carbonatite
(Kampunzu
et al.,
of about 40
1988). Apatite is the primary mineral.
5.2.3 The Irumide Belt Stra ti graphy South
of
the
Kibaran
intracratonic orogen, to the
south by
Bangweulu grade belt
block,
gneisses occupies
belt
is
a
parallel
the Irumide fold belt.
the Late
Proterozoic
coeval
mid-Proterozoic
The Irumide belt is bounded
Lufilian
arc,
to
the west
and to the east the Irumide belt passes of
most
the
Southern
of eastern
Mozambique
Zambia where
northeastward into northern Malawi
(Fig.5.8).
belt
into the high-
(Fig.5.8).
it extends
by the
The
for about
Irumide 700 km
232
IQnyi ka
~j
L. Nweru
I]T 2
TANZANIA
ZAIRE Hills ,14
•,r.-.. ". ",b --'"
4
~
• 1 nilon9o~" • "~: • ¢ " - " : ~ - " - ---.6":-'.-" o " . $',,,>.'. : "-: -. • . : • So wez -. .. ~ " ~ , , . -. ~,.. ~'.." . : ' . . . . ~..=.,~.'. , .
-....
: . ....~ .......
-
/(11111Vl£~un~zL,"
.~ :.
/IIIIWY
IolQ
%
/:-
I
tgJLv
o.g \ ÷ ,, % ~ 4~r
o~-;-u'xChip=ta S=sore
OZAM B! QU E L~"".Z ,,, m ~ e ' Z
i
b et~
%
ZIMBABWE
\ .<
£1~?'" " ~
"-
'~
BOTSWANA
200Kin
\
Figure 5.8: Outline geology of Zambia. i, Cretaceous cover; 2, Karoo; 3, Katangan; 4, Zrumide; 5, Katangan basement; 6, Mporokoso Group; 7, granites, gneisses, volcanics of the Bangweulu block. (Redrawn from Cahen et al., 1984.) The basement Proterozoic belt,
in
in the
Mkushi
the
Musensenshi
Irumide belt consists
Gneiss
Rufunsa
Group.
complex.
area
The
Irumide
Lufilian arc, the Lufubu schist granitoids
of
the
In the
(Fig.5.8),
Bangweulu
of the circa
southern
the
basement
part
basement and
that
1.8 Ga Early
of
is
the
known
of
the
Irumide as
the
adjoining
(Table 5.1), and the basement schists and block,
all
formed
during
the
Early
Proterozoic Ubendian orogeny. The Mkushi Gneiss and the Musensenshi Group consist
of
amphibolite-facies
gneisses
and
metasedimentary
schists
and
migmatites with white, granular bands of quartzite. Daly sequences
and
Unrug
(e.g.,
the
(1983)
attributed
Mpanshya,
Groups),
which
had been
Irumide
belt
(Table 5.1), to
Nwami,
previously the
most Sasare,
described
Irumide
metasedimentary
Musofu,
Fombwe,
in different
Muva Supergroup
Mafingi
parts
(Fig.5.9).
The
of
the Muva
233
Table 5.1: Suggested stratigraphic correlations bia. (Redrawn from Johns et al., 1989.) RUFUNSAAREA
NYIMBA AREA
PETAUKE AREA
Ka~GALUW£ FOMATION
UPPER SASARE GROUP
HWAHi FORMATION
MVUWEGNEISS ~" FORMATION 0
o
MVUVYE5ROUP I !' LUSANDWA GROUP SASAREGNEISS~
m PRE-MCHINJI
SINDA FJROUP
AND6NEISSES BASEMENTCOHPLEX
NYAN)I ONEISS FORMATION
is essentially
lithologies
a thick sequence
which
have
been
of alternating
strongly
subjected to varying degrees of metamorphism. expanded
usage
geological
(Daly and Unrug,
time,
g
~:
GRANULITES
(~ROUP
MAMBOONEISS FORMATION
pelitic
~ LIFUZHERE SCHIST FORMATION
FORMATIONS
~ FORMATION
MUVA
~
~UARTZITE
MVUVYEMARBU ~ FORMATION
Supergroup
(UPPER PARTi KACHEBERE FORMATI0 N LOWERPART -~ MCHINJ) RIDGE FORMATION x PATE H,LL ~c ~ FORMATION
VARIOUS
~= CHIPIRINYUMA o <: GNEISS ,,.o MOFMEYER SEIIUENCE (And Mkokomo sequence7)
PROBABLE SUPEROROUP E~,UIVALENTS
~¢HEBERE FORMATION KATANGAN
LOWER SASAR£ GROUP
CHITUNDULA SCHIST AND QUARTZITE FORMATION
Zam-
MCHINJI A R E A ( HALAWI)
SASAR£GROUP
MULAHBA FORMATION
MUSENSENSHI FORMATION
eastern
SASAREAREA .USANDWAR, AREA CHIPATAAREA
~ RUFUNSA
IETAVOLCANI[ FORMATION ,,, CHAKWENGA ~- RIVERSCHIST EHIHPl FORMATION ! z KAULASHISHI SEO,UENEE O,UARTZITE ; ~: FORMATION
in
from the
1983),
folded
and
over a
orogenesis
and
thrust
The Muva Supergroup,
accumulated
end of Ubendian
quartzite
long
(at the
and
in its span of
same
time
the Ndembara and Chocha Groups were deposited on the Tanzania craton) the
beginning
of
Sedimentological
Irumide
deformation,
analysis
late
in
in the northwestern
the part
Middle of
the
Irumide
belt, where the Muva Supergroup oversteps the Bangweulu block shows
a
trough
rapid
thickening
of
(Anderson and Unrug,
the
1984;
Muva
Supergroup
Daly and Unrug,
fold
(Fig.5.10),
towards 1983).
to
Proterozoic.
the
Irumide
In this region
the thickness of the Muva ranges from 100-300 m on the Bangweulu block to about the
8 kln in the Irumide
Unrug,
basin
1984;
Daly
Irumide
trough
margin and
by
(Figo5.10),
extensional
Unrug,
1983).
In
suggesting
normal the
the
faults
(Anderson
northwestern
part
Irumide belt, where the Muva Supergroup has been subdivided lithostratigraphic lateral
equivalent
(Fig.5.10),
were
the
basal
Kasama
Mitoba
River
Formation
on
Group, the
River Formation and the Manshya River Group
deposits
derived
northwest
of
the
of
of and the
into several which
is
Bangweulu
the
block
is overlain in the Irumide trough by the marine sediments of
the Manganga fluviatile
units,
control
from
on the
of the Kasama
Formation
the
and
uplifted
eroded
Bangweulu block, whereas
(Fig.5.10).
and the Mitoba Mporokoso
River
Group
to
The
Group the
the overlying Manganga River
234
MWAMI GROUPAND UPPERSASAREGROUP(-KA'IANGAN?)
J~
DOMINANTLYCHITUNDULA SCHIST~, QUARTZITEFORMATIONAND ( -MUVA? E(IUIVALENTS O,UARTZITES STIPPLED DOMINANTLY SASARE GNEISSES GROUPAND EQUIVALENTS , I-BASEMENTCOMPLEX)
{GRANULIT£S AND MARBLESORNAMENTED)
Figure 5.9: Outline Johns et al., 1989.) Formation
and
transgression be
shown
tectonic
the
geology
Manshya
of eastern
River
Group
Zambia.
were
(Redrawn
deposited
by
later,
this
paleogeographic
implications.
pelites
basement
of
marine
Further
east
setting beyond
has
the
considerable
Irumide
the
Mafingi
Group,
rest
directly
on
regional
trough,
on
the
the psammites
the
crystalline
(Fig.4.43).
Southward, occurs in
a
coming from the eastern part of the Irumide basin. As will
Malawi border other deposits of the Irumide transgression, and
from
the
in the region east of Lusaka, Rufunsa area.
an atypical
rock assemblage
These rocks which belong to the
Mpanshya
235
~ KASAMAFORMATO IN Itake , FLUVIALPALAEOCU~ENT~ ;z ika, PALAEOCURRENTS/~ ~/..~PT",'~,' ~g~i~k~ mm~ / ,~ / /,,Z.~HevU~. EH,':-"~(z,;-~"
OROKOSOGROUP ~LUVIAL
\
~
t ~ .~,'~O~'J "~."~j~-~',)~Ik'/'J'.'.'.'.'.~__ ~
IVPPJ~'TRANS-
-"~I,//,K, BANU'WEULUBLOC~ ~ z - - - _ ~ L U A N ~ V ~ / ~Ho~shy~ River {rod H~fingi Group
9~I.
~ Hitoba River (3roup ~Kasama Formc~tion
~" ~
Pebblesize gradient Poleocurrents vec tots Fluvial sediments Aeolian sediments
Hporokoso Group
HABULA VALLEY
MBESUMA CHAPALAPATA CHIMBWE HILLS SYNCLINE
OM 1000
ya
2000 3000 mgcx
z.O00
fion 6000
•
~000
Other symb
.I0000
CHISHIMBA
MABULA
FALLS
I Km MIBANGA 0
Km
KAKUMBU
VALLEY
HILLS
CHAPALAPATACHM I BWE CYNCUNE MBESUHA
B~ngweulu Block
HILLS ~.,~,h~ow m~rine Beachc~,nd~
T~ns fional zOnerumlde fold Belt
200Km Figure 5.10: Geologic sketch map and s t r a t i g r a p h i c columns for the Muva Supergroup. (Redrawn from A n d e r s o n and Unrug, 1984; Daly and Unrug, 1982.) G r o u p are n e i t h e r similar to the Irumide b a s e m e n t Muva
Supergroup
Mpanshya
Group
as
defined
consists
of
above.
Rather,
a highly
as
deformed
lithologies
shown and
by
Barr
extensive
nor to the (1976), sequence
the of
236 tholeiitic
and
andesitic
locally developed. of
the
Muva
(1983);
amphibolitic
The Mpanshya
Supergroup
by
volcanics,
with
pillow
lavas
Group was correlated with the lower part
Cahen
et
al.,
it extends c o n t i n u o u s l y for about
(1984)
and
by
Daly
and
Unrug
200 km, and is a s s o c i a t e d with
a wide v a r i e t y of intermediate and acidic volcanics
and minor ultrabasic
intrusives of p a r t l y submarine origin. Whether the M p a n s h y a volcanics are ophiolites
has,
likelihood
that
however,
not
the M p a n s h y a
yet
been
represents
ascertained.
a plate
Although
collision
suture
the
cannot
be ruled out. Da!y (1986b) argued that the M p a n s h y a volcanics are not the relicts
of
an extensive
oceanic
crust,
rather
they
are
the
products
of
p r o n o u n c e d crustal extension prior to the Irumide orogeny.
Structure
After
a
Irumide
detailed orogen
shortening. zone
and
similar
The
an to
is
structural a
fold
Irumide
internal a
typical
analysis and
belt zone.
is
Daly
thrust
belt
structurally
The
Phanerozoic
foreland foreland
comprises three p r i n c i p a l structural domains
(1986a)
concluded
with
considerable
divisible
zone,
into
that a
foreland
some
350 km
wide,
is
and
thrust
zone.
It
fold
(Fig.5.11).
-:.:-':'::"i~:
~Y,~FTcmzo,ninn~ ~ / N ::•:.:;:.
\
:.:..:.':..':"
,:~'SM B ,': :.', b~zi--,~,, belt\ "~".".': ,,.'~: ' : y SOOKm
~ Rm-African orogenic belt Foreland zones "~ Internal zones J-,,1lOOmy orogenic belt ,,-11100my granites ~2100my orogenic belt SH B Southern Mozambique Belt N Niassa polycyclic pre-kibaron Figure 5.Ii: Distribution of Kibaran belts Africa. (Redrawn from Daly, 1986a.)
the
crustal
basement in central and East
237 The w e s t e r n m o s t (Fig.4.46). folded and arcuate
In
this
deformation
thrust M p o r o k o s o
thrust
(Fig.5.12,A). are
structural domain is the L u o n g o
located
and The
shear thrust
along
detachment
surface
Supergroup
have
the
zones and
in
which
the
shear
is
which
the
displaced.
climb
of
of
This
the
wide,
the
major
Bangweulu
block
the
basement
folds
Luongo
zone
overlies
contact
asymmetric
Throughout
50 km
the
out
contact. large
about
Supergroup
basement
zones
basement-Muva
along
been
belt,
Group of the Muva
fold and thrust
acted in
fold
and as
the
and
a
Muva
thrust
zone fold v e r g e n c e swings from WNW to NNW and the s t r e t c h i n g lineation in the
underlying
suggesting
basement
NW-directed
shear
zone
tectonic
always
shows
transport.
a
southeasterly
About
shortening was e s t i m a t e d across the Luongo zone
10 km
of
(Daly, 1986a).
The Irumide belt
Foreland fold belt Infernal zone of sfructun21 divergence divergence NW facing SE facing
Foreland she~r zone
.,structure
NW
sfruch~re
7binned Irumide Ix~in crust
50 ,50 Km ,
~
Bosemef
Northern imbricates NWfacing folds and thrusts
NW
plunge crustal
A
~ f - ~ Muva Sediment
Steep zone
Southern zone
Upright folds and fubric Horizontul flutfening giving verfirnl fabric in the "pop up'core
KI
Fist lying SE verging major sheath folds of basement ~ cover below 0.H.T. SE
Km 15 PosfuNtedlinked fuult-shem" zone system gzvmg pod up geometry
K~rroo
[ ~ Muvasediments [ ~ Basement
B
10Kml
i
Figure 5.12: Structural sections through the Irumide belt; B crosses the Southern Irumide belt in the Mkushi area. (Redrawn from figures supplied by M. C. Daly.) The C h a m b e s h i fold and thrust zone, east of the Luongo zone, consists of
open
to
tightly
folded
quartzitic
sediments
in
thrusts have locally caused the r e p e t i t i o n of beds.
which
bedding
plane
The folds and thrusts
238
which
are
which
directed
developed
Ng'andu
fold
concentric developed
northwest
in
zone,
upright above
(Fig.5.12,A).
a
the
basement I00 km
basal
resemble
for
the
Shiwa
near
a
fold
zone
Shiwa
of
major
apparently
basement-sediment
above
the
basal
About 40 km of crustal
Ng'andu
which
thrusts
the
sequence
folds
the
those
SE-directed
Eastwards,
comprises
disharmonic
d~collment
folds
by m a j o r
(Fig.5.12,A). wide,
locally
the Jura M o u n t a i n s of Europe. estimated
refolded
about and
These
are
where
contact
d~collment
in
shortening has been the
d~collment
dips
eastwards towards the internal zone of the Irumide orogen. The
internal
divergence
which
(Fig.5.13,A). much
beyond
there
was
show of
can
the
be
the
Irumide
traced
general
orogen
throughout
greenschist
granite magmatism.
thrust
while
of
is
the
a
zone
Irumide
of
structural
belt
in
Zambia
Here the metamorphic grade attained the amphibolite facies,
also
NW-verging dome,
zone
to
structures
the
southeast
southeasterly
NE-striking
lies
vergence
shear
zones
facies
the
Muva
In the
internal
zone
to
the
northwest
this
basement
across
of
of
a belt a
(Fig.5.12A,B).
Northward northern
of
large
dome,
represent
the
Supergroup;
the
and
steep
basement
structures
in Malawi
a series
continuation
of
the
internal zone of the Irumide fold belt. However, major
in
the
d~collment
foreland
southern
and
fold and
Irumide
arcuate
thrust
strike to form a major
belt
basement
Daly
zone of the n o r t h e r n
imbricate
zone
(1986a)
thrusts
showed
which
Irumide belt,
(Fig.5.12,B).
that
constitute
the ' the
merge along
This has resulted in
local crustal t h i c k e n i n g and in the inversion of the m e t a m o r p h i c isograds with the m e t a m o r p h i c grade increasing towards the foreland. M o v e m e n t here was also d i r e c t e d this
divergent
zone
changes
metamorphic
towards
structure through
grade,
the northwest. in
a
into
the
belt
Figure
southeasterly of
5.12 B shows direction,
upward-facing
southeast-facing,
that across
the
structures
mylonitic,
imbricate of
recumbent
lower
fold
and
thrust structures. In the C o p p e r b e l t area of the Lufilian arc to the west of the Irumide belt, marked
the Irumide and the younger Lufilian structures are separated by a structural
Copperbelt
discordance.
(Fig.5.8)
where
the Irumide structures, using
stretching
the
In
contrast
Lufilian
to
the
structures
southeast
are
of
superimposed
the on
separation of these d e f o r m a t i o n has been possible
lineations
(Daly
et.
al.,
1984).
Here
the
Irumide
fabrics contain a downdip extension lineation indicating southeasterly to northwesterly strike
thrust
parallel
to
direction, the
Irumide
whereas
the
structures
Lufilian reveal
structures horizontal
which linear
239
fabrics
showing
the
characteristic
Lufilian
ENE
to
NE
direction
tectonic movement.
~
~1100my ~e, gneissic granite ~ . B j ContractionaJ ~',-~'$~, W , faults & shears ~',.~ ,,.. ~/,~ .. .--,~-'Irumi de foliation~, .~.~;~'-~." /k Extensionhneahon . . . . Is, .. ,-. ,) i . ~ .
, SOKm
,
,, ~
,~I..
q'
_
:,, :.l'~'h'h
.,
,s
I
,.,'
: i':.~tt 117' .
Figure 5.13: Irumide belt.
~" \
,
"//
Sketch maps showing regional structures in (Redrawn from figures supplied by M. C. Daly.)
the
of
240
Irumide structures Tanzania
(Fig.5.13).
Irumide
displacement
terminate against the Ubendian belt of Malawi and Syn-tectonic in
of
of
Sacchi
Mozambique
et
constitute
Andreoli
al.,
(1984),
(1984)
the
Daly
the
testify
belt.
Southern
(1986),
represents
gneisses
the
of
to
Irumide
These basement gneisses which extend from eastern zambia into to
in
age
Irumide
Mozambique. according
southeast
Irumide
the
continue
and
the
granites
portion
structures Malawi
to
this
Malawi
Mozambique
Piper
internal
et
al.,
zone
of
belt
and which
(1989), the
and
Kibaran
collisional orogen. 5.2.4 Southern Mozambique Mobile Belt The Southern Mozambique deeply
exposed
granulite
zones
ophiolitic Sacchi
with
rocks
(Andreoli,
belt
(Fig.5.11)
mid-Proterozoic
1984;
et al.,
interleaved
represent Daly,
the
1986a;
1984).
belt
slices
the
of
deeply
in
of
remnants
Cadoppi
Although
is a complex,
orogenic
basic plate
et al.,
of the Mozambique belt by Holmes 1986a; older
Sacchi, orogenic
suggesting
have
Mozambique
the
Mozambique
This in
Mozambique
is
Southern
During belt
the
Mozambique belt
Mozambique
belt
and
justifiable
this
belt
of
has
often
should
be
Late
the
northern
was
the
of
in which an belt,
treated
by
as
a
Proterozoic-Early parts
of
last major
Kibaran
tectono-thermal
only mild deformation,
(e.g., Daly,
a younger
the
because
belt
1989;
belt to the north,
situation
area
from
Tanzania
Pan-African
suffered
thermal rejuvenation
in
sutures
et al.,
(1951). Subsequent workers as the type
orogenic belt
separation
the
i.I0 Ga.
Southern
ultrabasic
and described as the type area
sought to redress
is adopted
separate
Paleozoic Africa.
belt
that
completely
event
1984)
and
Piper
been regarded as a part of the Pan-African Mozambique it had been considered as mid-Proterozoic,
high-strain
collision
1987;
Southern
eroded and
which
age,
event
at
the
granite
East
orogenic about
Southern
emplacement
and
(Sacchi et al., 1984).
The Southern Mozambique belt consists of the Malawi tectonic province and the Mozambique province of Cahen et al., includes Malawi
the Tete region of Mozambique. was
southern
previously
subprovince,
subdivision
is
petrological
basis.
subdivided as
merely The
shown
into by
geographical basement
(1984).
The Malawi province
Although the basement complex of a
northern
Carter as
and it
complex
in
subprovince
Bennett
has
no
both
and
(1973),
structural
geographical
a
this and areas
comprises mostly amphibolite facies gneisses and granulites, with patches of Ubendian the
rocks
available
interpretations
occurring detailed (Andreoli,
in the northern geochemical, 1984;
Piper
basement. structural
et al.,
However, and
1989),
based on geodynamic
the
basement
241 complex
in central
complex
(Barr and Brown,
Central M a l a w i
and southern
and the Tete gabbro-anorthosite
1987) are highlighted
below.
Province
A recent geological
synthesis
shows that the basement orthogneisses.
Malawi,
for central Malawi
The paragneisses
gneisses interbanded
by Piper et al.,
complex in this region comprises
with
(Fig.5.14)
granulites
(of
consist
(1989)
paragneisses
probable volcanic or volcani-
a3
LEfiEND INTRUSIVE ROCKS
~Syenite ~-]Granite, Syenite ( Basement Complex ) POST- BASEMENT COMPLEX
~---~ Superficial deposits ~ Sediments and votcanics(Permo-Triassic-Quat.) BASEMENT COMPLEX
Mafingi 6roup and post-Mcffingi cataclastics( greensl
Amphibolite gneisses 5nanulite facies ,
Figure 5.14: and Bennett,
Outline 1973.)
2,oo Km
geology
,
of
and
of biotite-hornblende
Malawi.
(Redrawn
from
Carter
242 clastic
origin);
biotite-hornblende
muscovite-graphite metasedimentary tholeiitic inferred
gabbros.
that
continental sequence,
pelitic
units
intruded
Based
the
which
schists which are
are
on
by
their
protoliths
shelf-rise
gneisses
of
a
the
not
banded;
of
central
whereas
low
Piper
Malawi a
TiO 2
et
al.,
semi-pelites
and
pelites,
of
the
equivalent
of
the
were
central Muva
interpreted
Malawi
as
the
paragneisses
Supergroup
of
the
proximal
to
(1989)
the
Irumide
were
metasedimentary
the Mchinji Group which comprises m e t a c o n g l o m e r a t e s ,
equivalent
olivine
paragneisses
fourth
and
These
locally pyritiferous. suite
geochemistry
deposits,
are
psammites, lithofacies
east
belt
and in
the
Zambia
(Table 5.1). Piper
et
geochemical Kibaran
al.,
(1989)
also
grounds
and
assigned
granitoid
(Fig.5.15)
and
alkaline
identified them
granite
to
granitoid the
orthogneisses
widespread
plutonism.
episode
Southwest
of
on of
Salima
there is an e a r l y - k i n e m a t i c anorthosite b o d y which is exposed
~
~':'~:~ :':;~':;.:i L %\ OOM~i ~'.~'!'i'['-~ ~
~l Huscovite-fr~phitePelites ~Sio'ti~-Hornblende6neissas
KASUNG/U ~-'.M_~ / "' / @~~" [H'l'~:~-:!)Line of ~,j)
DO.WAZ_._~.~f~/\E~ ,~ C'.SLL ,H,
"Q~z~"-.~-A
I
NTCHISI I
~.'
~<~] Hchinji grouppso.mmites I ~ Gronifoidorthogneisses / ~ Metn-Anorthosite ] {_~., ".. Limitof superficiQlco~r
\ , soK~ ,
A
B DOWALILONfiWER. LINTHIPE LAKE I I I .AU W,
Figure 5.15: G e n e r a l i z e d basement map of central B, cross-section. (Redrawn from Piper et al., 1989.)
Malawi
(A).
243
over
an
area
associated al.,
of
with
(1989)
this
belts
Malawi
in
other
genetically of
unlike
probably
the
intruded
to
which
the
of
is
the
are
world
those
body
spatially
granitoids,
similar
suites
parts
anorthosite,
Malawi,
anorthosite
adamellite-dominant
them
granite-anorthosite
Proterozoic southern
Because
perthite-rich
considered
charnockitic central
250 km 2.
found
in
(Windley,
the
Piper
Tete
et
adamellitemid-
1984).
The
province
and
metasedimentary
pile
during
anorogenic plutonism along a continental margin in central Malawi. Two phases of deformation have been recognized in central Malawi; early
phase
asymmetric locally
flat,
folding
to
transport The
with the
which
west,
directions
paragneisses
shearing.
isoclinal
structures;
shows
vergence
suggesting
compatible with
and
As depicted
gabbroic in Fig.
to
north
and
a
the
north,
and
second
phase
northwest
northwesterly
an
with or
tectonic
the Irumide belt in the northwest.
suites
5.15,B,
preferentially
the asymmetric
show
synclinal
ductile folds
in
the northern part change southwards into steep, upwardly- converging dips in the southeast with granulite-facies assemblages the steeply dipping zone.
Since these structures
crustal
shear
levels,
evident.
The
ductile
structures
and
zones
rather
tectonic
in the central part of probably formed at deep
than
transport
thrust
planes
directions
in
are
central
Malawi are shown in Fig. 5.13,B. Southern Malawi Province
As
in
central
constitute (1984)
and
the
northern
bulk
demonstrated
Lilongwe
and
in the
that
migmatitic
alkali-olivine
to
Malawi
silicate (1984)
rocks
the
rocks
granulites
which
region
rocks,
high-alumina
basaltso
The
and marbles. that
a
interbanded
Based
on
and
with
part
of
granulites
mostly
were also
supracrustal
their major
substantial
and
Malawi.
occur
(Fig.5.14)
plutonic
generally
gneisses
in southern
and
are
concluded
amphibolite
Blantyre-Zomba
supracrustals, southern
Malawi,
of the basement
Andreoli east
from
probably
form
granulites
meta-pelites, element the
of
derived
data
southern
of
calc-
Andreoli Malawi
granulites were derived form island-arc volcanics and associated plutonic rocks
and graywackes.
migmatites,
The amphibolite-facies
paragneisses
and occasional
gneisses
relicts
consist mostly of
of gabbro,
dolerite
and
are
diabase. On
the
preserved assemblage
Kirk in
the
Range
relicts
Lukudi
comprises
River
granulite-
of and
serpentinized
Alpine
peridotites
Chimwadzulu
areas.
This
and amphibolite-facies
basic
ophiolitic rocks which
244
are
believed
terrane
Tete
to
have
(Andreoli,
accreted
onto
an
older
granodioritic
basement
1984).
Province
In southern
Malawi
there
are also
gabbro-anorthosite
plutons
especially
in the L i n t h i p e area, but by far the largest c o n c e n t r a t i o n of anorthosite complexes
is
in
the n e a r b y
gabbro-anorthosite world,
occurs
Tete
complex,
province
one
of
(Figso5.3;5.16).
Barr
the and
of Mozambique, largest Brown
of
such
(1987)
where bodies
presented
the Tete in a
p r e h e n s i v e account of the geology of the Tete complex.
~A,B,, .......
...,OAL~h.
,
'
i
s'
.,~;/,/A • • ":.,;.,"/.,(-,:::;I:XL~'/.,"2"~"
~ZAMBUE."
.......
W "' > " GRANITE
R-"P. " "F / / ~ / / / / / / / / / ~ : . .
÷*
<* " c , . V~7~ANOONiOn,
• .~ ' ~ . ' . ' ~ G UP/ ~ . "" .oTY . .
\ .~2~./_~_•~ ~ ~ / ~ / / / ~ / / / ~ " I COMPLEX~
~
L
~, ~ , ~
~s>/72
~
ALAW,
.... ,.,.,,,.
~~),
NORTH O F " ~ ' N ~ U ' ~ ZONE . .~.~... ;' 0 " ~';~' LAYERED MAFIC- ULTRAMAFIC ~ : : ~ S O U T H OF S A N A N G O E COMPLEX ~'--~~" THRUST ZONE POST-FINGOE AND LATE GRANITE "~ " ~ " ~ ~ ~ ~'k :,,,,, TETE COMPLEX
1
r~--~ PRE-PINGOE~FINGOG EROUPGRANITE AND GRANODIORITE ~ ' ~ % ~ \ ~ ' ~ ' ~
~
[~i-~ RUSHINGA GROUP ~--~<~ BARUE GROUP
O~UARTZITE AND SCHIST OF ZAMBUE GROUP
~ GABBRO- ANORTHOSITE ~7//'~PRE-FINGOE GNEISS AND GRANULITE WITH CHARNOCKITE AND BIOTITE GRANITE
A
SO Krn
:
-
xx
, "'~ILINTHIPE~{k
,
L,LONG E
E~ISEDIMENTARy
~
SYENITE x x x
~ "-~ \
LAKE
\4\
\
~k
I
"
I
"~.NCHEU(~,._I
"~ \
~
~
~ANORTHOSITE Xx%/x x x Z I ~ / ' / , xx .L-:JRocKs WiTH TREND U~I~;~J/~
~
-J ~ I
I
B
-
~I
Figure 5.16: Geologic sketch maps and c r o s s - s e c t i o n the Tete Complex. (Redrawn from Barr and Brown, 1987.)
through
the com-
245
In
the
country
Tete
rocks
province
gabbro-anorthosite
comprising
gneisses,
bodies
granulites,
are
and
emplaced
within
charnockites
and
b i o t i t e granites,
all of which are surrounded by large bodies of granites
and g r a n o d i o r i t e s
(Fig.5.16,A).
the
gneisses
and
metasedimentary suites
of
granulites followed 1984). Kirk
and by
the
was
in
turn
anorthosite the
in
overlain
Fingoe
ultramafic
mylonitization
and
are
norite-gabbros
area and
Range
of
and
and
Other
Linthipe
rocks
granite
anorthosites
In the w e s t e r n part of the Tete province
granulites
bodies
and The
intruded
cut and
Group rocks.
by
Nsanje
are
intruded
Tete
the
eastern
in
region
this
area
of
gneisses
younger
basic
(Cahen
and dykes
et al.,
those
of
occur
in the
which
Malawi
of
gneisses
including
southern
the
complex
north-south-trending mineralization
by
by
basic
uranium
layers of anorthosite
the
unconformably
the
(Fig.5.14)
are
coeval with the Tete complex. The Tete complex is a large sheet or lopolith thick,
extending
over an area
of about
(Fig.5.16,B),
6,000 km 2.
It consists
l i g h t - c o l o u r e d gabbro and norite with s u b o r d i n a t e a n o r t h o s i t e numerous
dolerite
dykes.
p e g m a t i t i c textures.
It
has
known as the Chidue Group, as
well
metasediments
of
Tete
was
complex
metamorphism, high-grade level.
the
as
A l o n g its western
intruded into f l a t - l y i n g m e t a s e d i m e n t s
Chidue
along
Group. at
the eastern
western
the
contact
Apparently
shallower
the
depth
part of
rocks and charnockites its
or
there are zones of c o u n t r y rock inclusions skarns
emplaced
whereas
Along
layers and
coarse-grained
to the border w i t h Malawi.
m a r g i n w h e r e the Tete complex was complex
very
The Tete complex extends from the big bend along the
Zambezi River in M o z a m b i q u e
the
medium-to
10-20 km of mostly
with western
under
the
the complex was
complex
in
carbonate
part
of
the
amphibolite-facies
implying e m p l a c e m e n t
margin
the
intruded
into
at a deep crustal
exhibits
pneumatolytic
alteration. The
Tete
complex
a n o r t h o s i t e plutons
is
similar
to
G r e e n v i l l e P r o v i n c e of North America. older
large
complex.
basic
Unlike
p o o r l y banded in
the
albite
complexes
these
and
contains
lacks
but
poor
mid-Proterozoic
opaque in
It is, however,
as
layered
rhythmic
end-members
abundant
such
African
olivine and p y r o x e n e which
vanadium,
other
gabbro-
such as the A d i r o n d a c k and M o r i n a n o r t h o s i t e s the
Great
intrusives,
layering;
very distinct
Dyke
or
the
it contains
(andesine-sodic
the
Tete
complex
plagioclase
labradorite);
it
minerals and
which
are
cobalt
enriched
(Barr
g e o c h e m i c a l contrast between the Tete complex,
and
is
in
is
rich
poor
in
and it
titanium
Brown,
from
Bushveld
contains more of the iron end-members;
chromium
of the
1987).
and The
and the Great Dyke and the
B u s h v e l d complex includes the fact that the former is high in c a l c i u m and
246
low in m a g n e s i u m and also exhibits a d i f f e r e n t alkali enrichment and rare element pattern. Throughout anorogenic (Windley, 1.0 Ga
the
world
magmatism 1984).
anorthosite
is
in
high-grade
intermediate
crustal
the Tete complex
were
to
mostly
the
1984).
The
southern
Malawi
belong
complexes.
and
depths
of
a
unique
Middle
emplaced
Windley,
gabbro-anorthosite
associated
represent
peculiar
bodies
1989;
bodies
anorogenic
which
These
(Condie,
anorthosites
Tete
The
the
coarse
1.70
and
mid-Proterozoic
igneous
texture
country
rocks
origin
least
eastern
(Barr and Brown,
at
and
related
charnockitic for
of
Proterozoic
between
complex
to
type
the
and
suggests part
of
1987).
Mozambique Province The M o z a m b i q u e province contains two distinct collision suture zones, the Lurio
belt
conjuction
and
the
Namama
with
the
Chimwadzulu
belt
(Fig.5.17), zone
in
the
which Malawi
the Southern M o z a m b i q u e belt as the collisional compressive orogeny The extends
were
generated
Lurio
belt
It
is
crosses
Malawi South
displays
shows
southeasterly
products
of
to
the
the
easterly
the
collisional
border, of
belt,
a
Mozambique
northeastward
eastern
(Fig.5.17,A).
In the An
in ~
qualifies
zone from which regional
(Fig.5.13)
Kibaran
Indian from
Lurio
thrust
vergence.
orogenic
from
during
ocean;
which belt,
the
Kibaran
orogeny
which
was
course
Be, Nb-Ta), detail
by
Sacchi et al.,
of
developing
the
the
the
where
the
Lurio
belt
belts
referred
(Sacchi et al., prospects
it near
originates
belt,
Namama
locally
mineral
Mozambique
from
branch
thrust
unlike and
the
is a b i f u r c a t i o n
northern
a major
Lurio
in
Malawi
there
a
vergence, The
belt
southern
Lurian event and a s s i g n e d an age of i.i0 Ga
Li,
considered
(Daly, 1986b).
province.
the
displacements
when
province,
Namama which
are
the
as
the
to
1984).
(pegmatites
with
the M o z a m b i q u e p r o v i n c e has been m a p p e d and investigated
several
mineral
exploration
teams
(Cadoppi
et al.,
1987;
1984) who have furnished a clearer regional structural re-
i n t e r p r e t a t i o n of this crucial part of the Southern M o z a m b i q u e belt. The
pre-Kibaran
basement
rocks
in
the
Mozambique
province
are
the
biotite- and b i o t i t e - h o r n b l e n d e - g n e i s s e s which exhibit diffuse pre-Lurian migmatitic
textures.
These are v a r i o u s l y termed the Nampula and Namarroi
Series or the Mocuba Complex the
protoliths
to
these
(Table 5.2). G e o c h e m i c a l data
gneisses
were
volcanic
rocks
suggests
and
that
subordinate
d i o r i t e - g r a n o d i o r i t e intrusives. Gneissic supracrustal cover rocks in the
247
u, ,,
71 ,,
I\lkl/x
....q
I"
,
.- . I _ .
.
.
.
.
E
"
2 5 km
Figure 5.17: Tectonic map of the Southern Mozambique belt (A), and schematic cross-section across the Namama belt and Mugeba Klippe. i, late- to post-kinematic granite; 2, pre- to syn-kinematic granite; 3, basement (Mocuba Formation); 4, various supracrustal covers - Mamala, Cavarro, Rio Molocue units; 5, ophiolitic subunit of the Rio Molocue Group (Morrua Formation); 6, Granulites of the Mugeba Klippe. (Redrawn from Sacchi et al., 1984.) Mozambique province include the "Serie de Metil" comprising of
mostly
rhyolitic
volcano-detrital
metasedimentary
rocks
quartzites
ferruginous
and
supracrustals ultramafic intrude (about
the
500 Ma)
granitoids.
as
origin
mica
quartizites
Syn-tectonic basement
Pan-African
and
(Rio
granitoids, the
(Mamala
schists,
also contain local occurrences
rocks.
both
such
formation);
fine-grained Molocue
dated
granites
at
intrude
These
ophiolitic
about
cover,
and
gneisses,
Complex).
of metamorphosed
supracrustal
prophyritic
leucogneisses
1.10 Ga,
while
younger
the
earlier
248 Table 5.2: Sequence of Kibaran tectonic events M o z a m b i q u e belt (from Cadoppi et al., 1987).
$y n- to - post - tecto nic granite and pegmatite
~) Ht.lnrepele ~ cl) teucocratic gneiss;
~(Sranite and pegmatite ~Hocuba Complex ~ (Higmatite
of
(Fig.5.17,B). the
Malawi
distance (Sacchi
of
tectonic
erosion
intrusive
The Mugeba
border about
et al.,
to
is
west
The
are
_
-
~
it
position
of
from
was
present
Mugeba basal
and
root
Monapo
comprising ultramafics
the Lurio
thrust
position
the
>1100
allochthon, and
originated
before
its
.,.--,.
the
largest
protoliths
allochthon
200 kin, to
1984).
the
basic
the
1000-1100
Granite and pegmatffe emplacement Heto~morphism and deformation ( pre-Lurian}
significance
klippen
1000-1100
deposition and volcanic activity
\gnelss
Mugeba
? 1000
deposition
/Biotite (-hornblende)
considerable
~S0~500
deposition cmd magmotic activity
~
granulites
AGE(M~
deposition
quartzite b) amphib°lite; Rio Holocue ~ 1 Ri) ~CHo°~r~l~exl quartziteulframafite; impure Group Nipiodi ~ I c) biotite gneiss; o Formation "~ ) micaschist Hagnetite- Leucocratic Hamala Formation gneiss ~ Lepfinite)
The
Southern
5r~nffe and pegmatite e replacement: rudiometric rejuvenation of the minerals (Pan-African event); stow uplift Granite and pegmatite emptQcemenf Orogeny and metamophism (Lur ion); uplift
5ranite and pegmaflte
Of
the
EVENT
LITHOLOGY
klippe.
in
belt
eastward
in
the
zone
of
near
over
Namama
a
belt
the granulite
Mugeba nappe in the central part of the Lurio belt near the Malawi border is structurally Malawi
in which
compatible with Andreoli's ultramafics
suggest
(1984) thrust belt in southern
the existence
of a collision
suture
in the region.
5.3 Regional Tectonic Model for the Kibaran Belts A
regional
Africa
must
features
tectonic
in this
Irumide
Mozambique
belt
of
and
region.
fold belt. slices
synthesis
accommodate
for
the
account
There
lithofacies
where
higher
ophiolitic
basic
Kibaran
for
is crustal
the
shortening
and structures grade and
belts
of
following
across
continue
granulite-facies ultrabasic
eastern salient
rocks
the
central geologic Irumide
into the Southern paragneisses and
with
island-arc
249
volcaniclastic deposits Mozambique from
the
cover
favourable
In the Southern
belt a granulitic klippen w h i c h has been thrust Lurio
rocks
Malawi
are known in a number of places.
belt
in
for
overrides
the
the
province.
older
Namama
belt.
emplacement
The
Kibaran
southeastward
pre-Kibaran
basement
gneisses
Conditions
appeared
particularly
of plutons orogeny
of
in
gabbro-anorthosite
the
Southern
in
Mozambique
and the belt
w h i c h took place at about 1.10 Ga coincided with folding and thrusting in the Irumide belt, compressional d e f o r m a t i o n and a l k a l i n e m a g m a t i s m in the K i b a r a n b e l t further north, and with n o r t h w e s t - t r e n d i n g sinistral strikeslip m o v e m e n t along the U b e n d i a n belt. Andreoli's offered
a
features
(Daly,
et al.,
(1984)
widely
1984).
plate
accepted
1986a,b; Andreoli
tectonic framework
model for
Klerkx et al., postulated
for
1987;
that
the
southern
explaining the
the
Piper et al., orogeny
in
Malawi
above
has
tectonic
1989;
Sacchi
southern
Malawi
was initiated by collision between the eastern passive m a r g i n of a Niassa craton
(Fig.5.11)
with
an
island
w e s t e r n m a r g i n of a Lurio craton
I RUMIDE BELT
arc
which
(Fig.5.18).
, S. MALAWI r-IMBRICATES
developed
the
The s t r o n g l y recrystallized
...=.
G.Z. ,
~ , . . . ~
IRUMIDE INTRACONTINENTAL BASIN
/
1"
v.- 7-
~
X
..
--
PLATE
to
LURIO ZONE r'- NAMAMA t ZONE
~.,~,%
PROTO- AFRICA
adjacent
M-P.
MARGINAL BASIN
....
ISLAND ARC
NIASSA CRATON
MICRO- PLATE
LURIO
INDO-MALAGASH PLATE
Figure 5.18: Schematic structural section across the IrumideSouthern Mozambique belt showing thrust zones, lithospheric plates and a s s o c i a t e d ophiolites. N.C., Niassa craton; L.P. Lurio plate; I-M-P, Indo-Malagash plate; C.Z. Chimwadzulu Zone. (Redrawn from figures supplied by M. C. Daly.) polycyclic
pre-Kibaran
(Fig.5.17)
are
mafics
ultramafic
and
basement
regarded
as
slices
the
at relict
in
the
Niassa
and
cratonic Mpanshya,
in
the
nuclei.
Namama
The
Chimwadzulu
belt
ophiolitic and
Namama
250
zones possibly represent relict ocean floor volcanics and
and
gneisses
associated of
high-grade equivalents, the
continental
margin the
of
the
are The
represented Muva
in
the
Supergroup
and
amphibolites its
the paragneisses of central Malawi, assemblages
craton.
Malawi
and
Malawi.
shelf-slope
Niassa
southern
granulites
sediments
southern
(Fig.5.18), while island-arc
Eastward
island
amphibolites
arc
in
which
probably
the
accumulated
subduction
and
along
the
underthrusting of
generated
region while
eastern
are probably
the
syn-tectonic
subsequent
island-arc-
continent and continent-continent collisions produced the fold and thrust belt
tectonics
in
the
Southern Mozambique
belt
(Fig.5.18).
Anorthosite
plutons which crystallized from mantle partial melts later recrystallized at the granulites facies and were isoclinally infolded within granulites and amphibolites. As
suggested by
relationships several
in
Daly
the
collision
(1986a,b)
Southern
sutures
the
complex
Mozambique
existed
in
the
region.
suggests
The
existence
pre-orogenic
northwest-verging directed
of
structures
directions
implies
the
and
implies
the
the
dominance
that
changes
directions
predominance of
in
fragments in
transport
while
geologic
likelihood
several crustal
northwest-southwest
subduction
subduction zone.
and
region
subduction directions and the accretion of the
structural
belt
of
southeasterly
During the culmination of the Kibaran orogeny
at about i.i0 Ga structural reactivation occurred along the Ubendian belt which underwent crustal
large-scale strike-slip movement which
shortening along
trending
strike-slip
the
Irumide belt.
deformation
in
the
This
Ubendian
accommodated the
resulted belt
in northwest-
parallel
to
the
Irumide thrust direction, and caused the emplacement of the syn-tectonic granites in the Ubendian belt which are dated at about 1.13 Ga.
5.4 Other Mid-Proterozoic Terranes in Africa
Angola In Angola stable
sub-horizontal beds
cratonic
central Africa
areas
of Kibaran metasedimentary rocks occur in
(Fig.3.32).
Like
the
Kibaran
the mid-Proterozoic cratonic platform
belts
of
eastern
cover sequences of
Angola trend northeast-southwest. These sequences include the Chela Group and the Leba-Tchamalindi Formation of southern Angola, which extends into neighbouring Namibia; Angola, Chela
and Group
the
part of the Oendolongo Supergroup in west central
Malombe
consists
and of
Luana
Groups
of
conglomerates,
northeastern quartzites,
Angola.
The
sandstones,
251
siltstones,
shales,
(Carvalho et al., and
the
c a l c - a l k a l i n e volcanics and v o l c a n o - s e d i m e n t a r y units
1987). Clasts of the Cuenene g a b b r o - a n o r t h o s i t e
surrounding
conglomerates (Carvalho
et al.,
overlies
the
quartzites
intrusive
suggesting
an
1987).
Chela
by
the s t r o m a t o l i t i c the
Leba-Tchamalindi
and
is
craton
Among
the
intrusives
Angola
is
of the Mbuyi
Chela
Group
the
Chela
Group
of
unconformably
basal
conglomerates
such
as
and
stromatolitic
vast
The
Mayi
Supergroup
fact
that
on the
eastern
the L e b a - T c h a m a l i n d i
I.i0 Ga old, places it in the Kibaran.
emplaced
the
extreme south of Angola.
the
Formation
sediments
(Fig.6.59).
is cut by noritic dolerite dykes,
southern
composed
in for
The L e b a - T c h a m a l i n d i a p p a r e n t l y correlates with
dolomites
Zaire
occur
1.40 - 1.30 Ga
chemical
limestones and dolomites. part of
grantites
of
The
Group
overlain
red
age
complex
into
Cunene
the
Eburnean
basement
gabbro-anorthosite
Emplacement was at about 1.5 Ga
complex
complex
in
(Vermaak,
of the
1981).
This complex hosts an i r o n - t i t a n i u m ore deposit. A l t h o u g h p o o r l y exposed, it
is
Angola
believed and
(Simpson,
to
occupy
northernmost 1970;
an
area
Namibia.
Vermaak,
of
about
17,000 km 2
Its e s t i m a t e d
1981).
The
complex
in
thickness which
southwestern
is up to
comprises
14 km
over
70 %
a n o r t h o s i t e with granitic rocks and minor u l t r a m a f i c b o r d e r facies making up
the
remainder
bodies
(with
an
iron-titanium
of
its
average
oxide
composition, of
49.5 % Fe,
segregations
central parts of the complex
East
Saharan
contains
titaniferous
18.7 % TiO2)
that
are
which
scattered
(Sawkins,
1990).
and Vail
(1988a)
magnetite
are
through
probably
the
north-
Craton
S c h a n d e l m e i e r et al.,
(1990)
furnished a summary of the
suspected Early-Middle Proterozoic assemblages w i t h i n the b a s e m e n t of the Sudanese
part
(Fig.5.19)
Zalingei
area
in the northern
Desert
in
southern
Blue
of
the
Nile
East
Saharan
craton.
part of South Darfur Province,
and
the
These
include
Province,
Equatoria
the
the Bayuda
Province,
and
the Red Sea Hills. In the North and South Darfur Provinces p r o b a b l e m i d - L a t e Proterozoic rocks,
which
are
infolded
ridges
schists.
Similar
referred of
to
as
quartzite,
but h i g h l y
base
(Kongyo Hills
intercalations, Zalingei Sudan
which
Semipelites,
and
pass
Quartzite
flaggy
occur near
Sandstones) upward
Group,
biotite
folded a m p h i b o l i t e
M i d d l e P r o t e r o z o i c age or older, at the
the
gneisses facies
the
quartzites Golba
into the Tari G r a p h i t e - Q u a r t z
neighbouring
Uganda
and
western
with
a
sericite
These
Schists.
long
of probable
Siltstones
Kenya
of
and
rocks
Zalingei town.
massive
through
consist
contain
psammitic and
the
In southern supracrustal
252
sedimentary unit unconformably Middle
to
upon
Late
Metasediments African
comprising massive basement
Series
gneisses,
Proterozoic
age.
and
The
is
and quartz regarded
Madi
Group
basement
Proterozoic
or Gray Gneiss
age. Group)
Group (Bayuda Formation), of quartzites, middle unit
in
the
This
Bayuda
Desert
comprises
which
are overlain
as
of
rests
probable
the
Kinyeti
sequence.
The Pan-
is
granitic
schists
and
in the Sudan belong to this supracrustal
reactivated
pre-Middle
quartzites
probably
gneisses
even
of
(Abu Harik
by the Metasedimentary
a geosynclinal succession with a basal sequence
quartzo-feldspathic
gneisses,
mica
schists
and marble;
a
comprising acidic gneisses, biotite- and hornblende-gneisses
,..~ • •
Kasheblb. 0 Red Sea \
Sea
b
J
I
200Kin
I
Figure 5o19: Lower-Middle Proterozoic Sudan. (Redrawn from Vail, 1988ao) and
amphibolites;
and
consists
of mica
believed
to represent
an
schists,
upper
basement
volcano-sedimentary
ferruginous
an island-arc
quartzites
depositional
rocks
in
suite.
The
and marbles setting with
the
latter
which
are
sediment-
253
filled
back-arc
basins
and
shelf
facies
which
existed
prior
to
1.0 Ga
(Vail, 1988a). In
the
basement
southern
rocks
migmatitic
Blue
(Tin
gray
Nile
Group)
gneisses,
Province
comprise
a
enclosing
Early
lower
to
Middle
Proterozoic
unit
(Selak
Formation)
amphibolitic
bands,
and
an
of
upper
supracrustal m e t a s e d i m e n t a r y cover, the Gonak F o r m a t i o n w h i c h consists of paragneisses,
pelites
and
calc-silicate
rocks.
In
the
Red
Sea
Hills
M i d d l e P r o t e r o z o i c or older rocks are s u s p e c t e d to be the exotic basement terranes among Late Proterozoic rocks. gneisses,
hornblende
schists,
The exotic b a s e m e n t
chloritic
includes
slates and marbles,
which
acid
are of
the a m p h i b o l i t e grade.
Madagascar Hottin
(1972,
1976)
and
g e o c h r o n o l o g i c work, and
southern
Madagascar
parts
are
schists,
of
Madagascar
predominantly
crystalline basement
(1979)
have
supracrustals
facies.
demonstrated,
based
on
rocks occur in the northern
(Fig.3.45).
dolomites,
apparently deeper marine Archean
Vachette
that Middle Proterozoic
Mid-Proterozoic such
volcanics,
as
and
rocks
quartzites,
in
mica
metasediments
of
These rocks rest u n c o n f o r m a b l y upon the
of Madagascar.
The
1.10 Ga
orogeny
which
affected
rocks
in
the
Kibaran belt has also been recognized in Madagascar. In
his
Proterozoic
review of
ophiolitic
suture
ultramafic
rocks
distance
of about
dunites, gabbros
of
the
eastern zone
and
spreading, imbricated collision,
in
800 km,
a
These
after
in an active in
occupies
suture
ophiolitic (1990)
referred
Madagascar.
a
narrow
with
rocks
nickel
represent
mid-Proterozoic
subduction zone
5-20 km
and
the
of
wide
an o p h i o l i t e
Kibaran
Late
probable maficover
It consists
chromite
rifting,
the
a
zone
trend.
zone environment.
during
to
This
area
following a n o r t h - s o u t h
associated
amphibolites.
originated
of
Berhe
northeastern
(1.4 Ga)
harzburgites
probably
occurrence
Africa,
during
a of
deposits, belt which back-arc
The ophiolites were continent-continent
thus c o r r o b o r a t i n g the abundant evidence for K i b a r a n collision
tectonics w h i c h we have seen in the Southern M o z a m b i q u e belt.
Chapter 6 Late Proterozoic-Early Paleozoic Pan-African Mobile Belts
6.1 Introduction Kennedy
(1964)
originally
defined
the
Pan-African
as
a major
and
wide-
spread tectono-thermal event that led to the structural differentiation of Africa
into
Kennedy's
cratons
time,
and
orogenic
refinements
inter-continental
in
correlations,
led to the g e n e r a l l y
areas
about
500 ± 100 Ma
geochronology, and
the
extensive
concept
of
ago.
field
plate
Since
mapping,
tectonics
accepted view that Kennedy's definition
have
of the Pan-
African o r o g e n y referred only to the final thermal episode of an orogenic cycle which almost
the
only of
spanned
from at least
duration
of
the
such magnitude
belts,
involving
Precambrian
and
the
The
several
but the regionally extensive
world-wide system of mobile belts the
950 Ma to about 450 Ma
Phanerozoic!
(Kr6ner,
Pan-African
1984),
orogeny was
orogenic episodes
in
not
individual
Pan-African belts are also part of a
(Fig.6.1)
Phanerozoic
which mark the limit between
(Black,
1984).
It will
be
shown
this chapter that the Pan-African orogeny in the individual belts,
in
start-
ing from the initial rifting phase with related sedimentation and magmatism,
through
ocean
(geosynclinal) collision
opening
sedimentation,
magmatism,
and
concomitant
continental
margin
to subduction and plate collision, and post-
spanned
the
entire
Late
Proterozoic
to
Early
Paleozoic. The term Pan-African will be used here with a dual meaning. used
as
Early
a
collective
Paleozoic
term
age,
as
for
the
well
as
orogenic for
cycles
this
of
age
Late
span,
It will be
Proterozoichence
in
a
geochronological sense, equivalent to an era. The Pan-African belts and structural
(Fig.6.1) display all the sedimentary,
magmatic
facets of modern orogenic belts that are related
to plate
tectonics, and provide conclusive evidence for the operation of the Wilson Cycle and
in the Precambrian.
island-arc
(Caby,
1970),
continental
It was after the identification margin
volcaniclastic
and dismembered ophiolites in Morocco
the A r a b i a n - N u b i a n clear that m o d e r n the i d e n t i f i c a t i o n
Shield plate
(Garson a n d Shalaby,
tectonics
had operated
of
Hoggar
1981) and in
1976) that it first became in the Precambrian.
of Pan-African cryptic collision sutures
their similarities with
in the
(Leblanc,
metamorphic terranes along the margins of the West-African recognition
of Andean-type
sequences
Himalayan-type
Also,
in high-grade craton and the
collision
belts
255
WEST GONDWANA
J
~
,
t
j
,.~
---'U-------------~z
".--3~
~ ~
=~ EL,,~-~.
,)~~_----i~
( I
"~
~
Upper Proferozoic aulocogen deposits
1
Upper Prolerozoic geosynclln•l
,~ ,,
~--/-----_'.~
I;
//wc
Pan-African belts
~--'~
1
~ ~-_--~-~'k~'.J
S
Proto-South Atlantic
"~-fff~-T.5' /
A
Adomo, te, Oc,o,
h = - - c A . - . <,
deposits
Ocean
l " 17t.
~- - -
EAST GONDWANA
d
Figure 6.1: Pre-drift reconstruction of Gondwana showing PanAfrican belts, i, Pharusian belt; 2, Gourma aulacogen; 3, Dahomeyan belt; 4, Rokelide belt; 5, Maritanide belt; 6, Northeastern fold belt (Borborema province); 7, Araguaia belt; 8, Paraguay belt; 9, Sierras Pampeanas; i0, Ribeira belt; ii, Mantiqueira belt; 12, West Congolian belt; 13, Kaoko belt; 14, Damara belt; 15, Gariep belt; 16, Saldanhia belt; 17, Lufilian arc; 18, Shaba aulacogen; 19, Zambezi belt; 20, Mozambique belt; 21, Red Sea fold belt; 22, Transantarctic belt; 23, Adelaide belt. (Redrawn from Porada, 1989.) (Burke and Dewey, grade
terranes,
Actually
Clifford
orogenic
belts
termed
as
the
(1970)
which
upper Precambrian Congolian,
1972) ushered such
had earlier
he
belts),
vestigeosynclinal
Cameroon
province).
fitting
of
both
and
of
zones in
two
zones
of similar high-
East of
Africa
types
of the
Pan-African
arc,
rejuvenated belt,
into
deformed
Damara,
basement
Zambezi
ensuing
belts
(Fig.6.1).
of Pan-African
orogenically
(e.g. Lufilian
(Mozambique
happened of
as
sediments
belts
What
types
belt
recognized
characterized
geosynclinal
Pharusian
in the re-interpretation
Mozambique
belt,
two
decades
the
plate
West
which
he
Nigeriawas
the
tectonics
paradigm. Cahen et al. in
the
crustal
(1984) best summed up the significance evolution
of
stability which had prevailed
Africa,
when
they
stated
of the Pan-African that
the
crustal
since the end of the Early Proterozoic
oro-
geny (Eburnean) was only locally interrupted during the Middle Proterozoic (Kibaran),
but was followed by widespread
950 Ma to
after about
600 Ma
tectonism
(Pan-African).
(Fig.6.2)
The long
after about
phase of crustal
256
":'
" ~ * 2,:::
:'"::~:::'~"
':-'2%'"
.--,::
• "-
:':." "
.:j'/" mobile belts tabular cratonic
/:i!.
"f
___i
cover
""
-/
: .....
~
~ m
.'.:j. ;.':" ::: .':. ;;•..
A
Pon- African Pan_ African
2,>"'.
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 ~
<~;~tc.~o.,,
'-.,'t..1+%%%÷~ "hA+÷÷++++!
Ultml~,ic Hw
,
.,
.~. . . . . . .
I
,0oo
Volconics and volcano-elastics
' ..........
Moho
I
I
+~+.
. .o" t
..,,~; ;;;
'
r:;o,''~~m,°oooo,
,; .......
J
\Allianne complex
SW
,o-.,I\ . X
I
°7 ~
,4
body 7 SE
.................
lO
I . ~
L--t"t
# I
I
nll'i, ll,
,=
I
r n .s ..,
upplr manila
"
(20001
NW
SM
( 50000
$iE
Basin
SW
NE
Maurttonldes
I
.
,,"
40
)
Senelal
Mlcroplate
,'=7
,,"
."
"
~"~L!
#
•
" West A f r i c a n
#
•
* i i
i
!
i /
Cratorl
Figure 6.5: Crustal electrical resistivity interpretation across Southern Mauritanides based on the magnetotelluric method. (Redrawn partly from Ritz et al., 1989.) sediments
(Fig.6.6).
geosynclinal metamorphosed
The
clastic and
external
sequences;
deformed
rocks with d i s m e m b e r e d
zone the
to
axial
allochthonous
ophiolites;
whereas
the
immediate
zone rift the
west
contains
of
variably
consists
volcanic-volcaniclastic
internal
zone
the west comprises v a r i a b l y retrogressed gneissic terranes.
furthest
to
262
SE 0uled Dhlim ReguibQt Shield
NW
-',2 :';, (~)
ESE Akjouj+
WNW
Ad~r
W HogtoLahjorGo~ka;IOAouei~:ng~ *rotel
"--]17
/
logonet
k-k-lI5 6uidimako Oiolo-Bouonz~
W
lllll13 ll
[~
==~I0 i21I~9
W
ll
E Assoba
I Harndallaye
I
SSE
Simenti Group
Hoii Gp"
""A,ooo,o°IYoukou° ou.'°u/'" ."
NB
~7 521116
6
WSW
5
Kenema
Rokel River 5p,
Kosilo H~ompa s~,hl
£H£
Leo Shield
~3 NE
[~
1
(~ 20Kin
Leo Shield
? 6ibi Mountain
SW 0teon
,." .........
~
.v-r,
Figure 6.6: S k e t c h map showing orogens of the West African p o l y o r o g e n i c belt. 5, u n d i f f e r e n t i a t e d b a s e m e n t ; 6, S u p e r g r o u p i; 7, Unit A; 8, U n i t B; 9, Unit C; 10, S u p e r g r o u p 2; II, 2a; 12, S2b; 13, u n d i f f e r e n t i a t e d Supergroups 2, 3; 14, S u p e r g r o u p 3; 15, internal nappes; 16, W e s t e r n m o s t H e r c y n i a n (?) formations; 17, M e s o z o i c and younger. Lines of section shown and legend for crosssections are: I, u n d i f f e r e n t i a t e d basement; 2, S u p e r g r o u p i; 3, Unit A; 4, Unit B; 5, Unit C; 6, e x o t i c basement; 7, S2a; 8, u n d i f f e r e n t i a t e d S2b and $3; 9, S u p e r g r o u p 3; 10, L a t e P a l e o z o i c allochthons; ii, Mesozoic and younger cover. (Redrawn from L~corch~, 1989.) Foreland Units
The
foreland
regionally
are
extensive
unconformable of
units
sequence
quartzitic
part
tabular termed
sandstones,,
s e q u e n c e is 1,400 m thick. is u n c o n f o r m a b l y
of
the
T a oudeni
epicontinental Supergroup shales
and
I by
cratonic sequences. Trompette
stromatolitic
It is of m i d - L a t e P r o t e r o z o i c
o v e r l a i n by S u p e r g r o u p
2, a s e q u e n c e
basin. At
the
(1973),
They base
an
consists
limestones; age.
are
this
Supergroup
I
of Late Proterozoic-
263
Cambrian the
epicontinental
orogenic
belt,
Supergroup
tectono-stratigraphic sequence
of
red
which
Cambr i a n - O r d o v i c i a n are
conformable
with by
the
trace
Ordovician
been recognized Bov4
relatively basin
and
basin
in
by
Guinea
overlies
the
and
northern
sandstones
The
Middle
of
is
best
of the
at
marine
the top
sandstones succeeded
begins
beds,
The
with
and
by
Late
Devonian
Carboniferous
has
not
belt.
Paleozoic
correlate
part
and
unconformably
c!astic
a
and
feldspathic
whereas
latter
polyorogenic a
2b,
2 is
(Fig.6.7,A).
is
(Fig.6.3)
of
2a,
chert
suggestive
fine-bedded
Silurian
which
into
disconformably faunas
in
metamorphosed
subdivided
cross-bedded,
base
Mauritania.
deposits
of
2a
the
and
Westward
baryte-limestone,
Supergroup
in the West African
upward into Devonian
with
the
basin
containing
SupergrouP
Rokelide
and
is filled with Silurian
3.
the
This
southern
shales
which pass
units
containing
and shale.
Units
In the external deformed
and
Supergroup
parts of the orogens
weakly 2
on
metamorphosed the
craton.
stratigraphic
units
the Rokelides
in Liberia,
are
Bakel
Group,
external
zone
locally
thrust
sequences.
progressively The
is
within
the
Group
about the
Archean
sandstones
clays which
and
tillite
of
5 km
The
sequence
the
the
zone
Gibi
River Group
of
the
these
equivalents these
Mountain
in Sierra
deepening
of
axial
the
change
littoral
and
but
zone
and
basement begins
Formation
Leone,
in
the Mali
names
such as
(Fig.6.7).
In the
of
Supergroup
west,
2
are
foreland
they
become
units of that zone. consists
occupies which with
it
of Supergroup basin
a
of
sediments
broadly
overlies
with
as tillites
2 (Culver is
sandstones
through
a
marked
feldspathic belonging
et al.,
suggested
and
synclinorial
conglomerates,
interpreted
Rokelide
sublittoral
the
of
tectono-
unmetamorphosed
to
Rokelides
thick
have been
the
Mauritanides
equivalents
deformed
at the base
progressive from
belt over
unconformity.
the regional
termed
parts
fold
towards
River
it
are
external
imbricated with the tectonic
Rokel
structure
the
which
and have also been assigned various
various
the
In
the Rokel
eastward
But
volcanics;
in
of
there are structural
sequences
variously
Group in the Bassarides, the
at
shale
been
brachiopod
sandstones
followed
part of the Bassarides
External
occur
throughout
undeformed
which
Supergroup
Scolithos.
siltstones
pre-deformed
sequence
Inarticulate
quartzitic
tillites,
limestones,
a
1,200 m thick.
2 has
tillite,
2b,
boundary
3
2 overlies
succeed
fossil
Supergroup
The
and
in Mauritania.
about
Supergroup
Proterozoic
dolostone;
sandstones
developed
units.
upper
stromatolitic
clastic deposits,
by
to
1978). A an
neritic
upward shales,
264
subarkoses
and
Overlying basin,
orthoquartzites
prodelta
which
structures.
was
turbiditic
and mudstones
followed
by andesites,
indicate basalts
At the top are deltaic shales,
and orthoquartzites
Orogen$
into
shales
Westermce=t Formc~Hons Uppermost ~'~
A~ol
subarkoses
Formations
S.3
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El=~erna!
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(at c~ ~elole~ ba~et,et~
Cintra[
of
with pillow
arkoses,
.
N-Western
sandstones.
and spilites
Formations
U nkno%vn
\
provinces
shoaling
1982).
Inferno[ Formations
O~l~td O h l ~
Southern
o
and
upward
siltstones,
(Williams and Culver,
Reg|ons
Country
shales
5.2b basemen~
Fa, k~ka { U I } )
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Kelbe(Colc-alk.) Ei Aouei~a (U.A.)
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S.3
< X
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Oua-Oua
S. 3
Oiala. Boua nze~(U.8.)
Mbout ( C-=tk; Guidimakha
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MomdaHCtye (U.A)
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5.~(Mod~a- Xout~ basement
Xolent4 ( S.:~a )
basement ) Kenema(b~sement) M¢ltompa( baseme ,tRoke~ ~ / ~U. Sr - basement ) U kno n _~ver .2a n ~ GIbi Mou"min( S. 2a ) basement
KOSHO basement Coostal xone base--hi)
Liberia
Faleme (S 2a )
Mall ( 5.2a ) <
Un~n~n o
S.2a basement
Kidlra( 5, 2o ) Sakel
Damon{an Niokilo-Koba
....... ~o~~.~. " mY o ~ o ~ k o u n
m
Gabou (U. A.)
Debi(Calc- olk.) %
5E
B
[~
s~lesand si~stones
~gt~o~afions
~dolostones
Bred
~limes~o.,s
FT~q~.,~mc
[~
basement
E~¢ross
beds
be~
~sand$1or.es san~to~
M L :m~ddte:i°we~
Ondq.$a~stoaes
u : u.c©.~o~ 4 : 4~confacm~ty mu : mQin ua
Figure 6.7: Tectonostratigraphic correlations in the Mauritanide, Bassaride, and Rokelide orogens (A); and (B) generalized tectonostratigraphic cross-section of the Mauritanian Adrar. (Redrawn from L~corch~ et al., 1989.) In the Bassarides Mali
Group,
undeformed
an and
of northwestern Guinea and southeastern
equivalent
of
unmetamorphosed
the and
Rokel
River
comprises
Group, basal
is
Senegal, also
the
largely
conglomerates
and
265
sandstones
of
glacial
origin.
These
basal
tillites
are
succeeded
sequence of shales, dolomites and quartzites which predominate and
graywackes
where
more
Group
to
and
open
the
indicate
marine
south.
deposition
environments
The
Mali
on
a
prevailed
Group,
unlike
underlain by older eugeosynclinal graywackes, alkaline
intrusives which
represent
passive
an
the
in
Rokel
the
a
over lavas
continental
than
by
margin
Rokel
River
River
Group,
is
abundant volcanics and calc-
earlier
active
continental
margin
that was m i l d l y deformed and metamorphosed prior to the deposition of the Mali-Rokel River sequences.
This older succession belongs to Supergroup I
of mid-Late Proterozoic age. Axial
Units
Allochthonous (Dallmeyer, 1962).
units
1989;
occur
Dillon
Lithologically
volcanic
and
in and
parts
these
are
metasedimentary
rifted-margin
prism
that
of
Sougy,
the
1974;
Mauritanide-Bassaride L~corche
variably
rocks
which
probably
et al.,
deformed represent
correlates
with
and a
Sougy,
metamorphosed
Late
the
orogen
1989;
Proterozoic
upper
parts
of
Supergroup I. Among the allochthonous units there are intercalated marine sequences
in
the
form
of
graywacke,
tholeiitic and alkaline basalts. (Fig.6.7,B)
have been grouped
to alkaline volcanic sequence grade
(amphibolite)
(U.B).
In
the
continental basaltso a
into
the and
Mauritanides,
supracrustal
and
jasper,
serpentinite
slightly metamorphosed metasedimentary
the
components
Farkaka
and
sequence
intraplate
continental
margin
block
(L~corche et al., deposits
that
that 1989).
probably
setting
along
the
probably
existed
eastern
west
of
and the higher
along
the western
sequence
(U.B)
includes
rift-related side
the West
Similarly Unit A represents accumulated
tholeiitic
western
The paleogeographic setting for the U.B assemblages
continental
and
formations of the axial zone
(Unit A or U.A in Fig.6.7,B),
metavolcanic
central
chert
The various
was probably of
a
rifted
African
coeval margin
meta-
craton
rifted-margin of
the West
African craton during the same phase of ocean spreading. There
are
uncertainties
metasedimentary
units
Mauritanide-Bassaride known
(L~corche
grade,
deformed
in
regarding the
orogen,
et al.,
the
correlations
allochthons because
1989).
For
their
of
the
foreland
example,
the
sandstones which contain Devonian
of
axial
some zone
equivalents
sequence
of
of
the
of
the
are
very
not low-
faunas and structurally
overlie pre-deformed high-grade axial units at Sangarata
(Fig.6.6,
5), may
be equivalent to Supergroup 2 or 3. Also uncertain are the correlatives of the extensive mylonitic quartzite nappes region
of
northwestern
Mauritania
which
(Fig.6.6,
are exposed
6).
An
in the Akjout
extensive
klippe
of
unknown origin rests on the western margin of the Reguibat Rise. A variety
266
of variably metamorphosed and deformed rocks including anorthosite plutons are
part of this internally imbricated nappe complex which is known as the
Ouled
Dhlim
complex.
The
structurally equivalent
Ouled
Dhlim
to the Akjout
complex
may
internal
at
nappes.
least The
in
part
be
latter nappes
contain assorted rock assemblages including metasediments, meta-volcanics, and retrogressed gneissic rocks. Internal Units
Variably
retrogressed
mylonitic
gneissic
rocks
internal structural units in the West African Rokelides
these
are
considered as exotic
constitute
fold belt
basement
terranes
represent fragments of the Guyana craton of South America Pan-African Kasila orogen.
and In
reworked Marampa the
calc-alkaline,
Archean
Groups,
granulite-greenstone
constitute
Mauritanide-Bassaride variably
deformed
and
the
the
In the
(Fig.6.8). These
zone of
gneissic
metamorphosed
include felsic volcanic units and associated
western
that probably
basement
internal
orogen
the
(Fig.6.6).
igneous
rocks, the
rocks
the
Rokelide contain
rocks
which
co-magmatic granitic plutons
that are dated at about 685 - 675 Ma.
0 0 : Osceola Granite CG: Coyah Gronite w v v-
Late Poleozoic Lore Proterozoic- Combrion
Approximote edge of The Goodwona-
Wiggins
Arch~
- 7 Po,.o=oi¢ so~,n,
..
~" '
~/.':'.":'. ' ~ ~ ".""""'t
e"-- "/ .--X- . ~ ' -
Figure 6.8: Pre-drift reconstruction of NW Gondwana showing the relationship between pre-Cretaceous lithotectonic units in the subsurface of SE U.S.A.; and correlative sequence in West Africa and NE South America. (Redrawn from L~corch~ et al., 1989.)
267
6.2.3 Tectonic H i s t o r y The polyorogenic West African fold belt was initiated during the mid-Late Proterozoic, the Pangea
between
about
i.i0 Ga and
I supercontinent
(Fig.6.1)
770 Ma, along
by continental
rifting of
the western part of Gondwana
(Fig.6.1). Continental margin depositional wedges accumulated as sea-floor spreading Iapetus
opened
a
vast
(Fig.6.9,A)
paleogeographic
Proterozoic-Paleozoic Laurentia
reconstruction
southeast-trending orogenic belt
Late
between orogen
and
(Condie,
which
in South America
1989)
probably
as In
Rokelides
up
Recurring
known
Gondwana.
the
linked
(Fig.6.1).
ocean,
western
with
formed
the
orogenic
the this a
Araguaia
activity
in
the West African polyorogenic mobile belt resulted from the convergence of the
plates
within
and
around
the
Iapetus
(Fig.6.9A,B);
and
from
several
collisions of these plates at different times. In (e.g.
the Mauritanide-Bassaride Term~s~,
belong with
to
rift
tholeiitic
components
alkaline
to
the
Limited
Farkaka
phase
complex (ca.
west
which
was
Groups) as
narrow
the
Mauritanides.
in
the
internal
Based
zone,
supracrustal
with
the
(Dallmeyer,
during
the
Late
1989;
L~corche
Proterozoic
deep
water
ocean
Bassarides
the
ocean
an
of
but
the much
age
of
the
calc-
closed
in
the
Late-
subduction
zone.
ensialic volcanic arc over a
western tectonic block of continental crust model
zone
continental
765 - 700 Ma) along a w e s t w a r d - d i r e c t e d
This resulted in the formation of a western,
axial
rift-facies,
on
the
equivalents
in the
produced
in
its
These
interfingered
spreading
in
I and
(Fig.6.10,A).
and
sea-floor
(Fig.6.10,A)
northwards
Proterozoic
Supergroup
and alkaline basalts as well
units.
Sea-type
wider
Diala-Bouanz~,
sedimentation
accumulated
sedimentary Red
Guigan,
the
belt,
(Fig°6.10,B).
et al.,
(675 - 650 Ma)
In this tectonic
1989)
continental
caused
the
first
collision Pan-African
episode (Pan-African I) which produced m e t a m o r p h i s m and deformation in the Bassarides and Mauritanides By
the
latest
southward towards the
glacial
River
and
Group,
(Fig.6.10,C).
Proterozoic,
western
Gondwana
(Fig.6.9)
the South Pole so that between about locally
Mali
flyschoid
Group)
were
sequences
deposited
of
Supergroup
(Fig.6.10,D).
had
drifted
650 Ma and A
2
575 Ma
(e.g.
second
Rokel
orogenic
episode hook place in the Early Cambrian between about 575 Ma and 550 Ma, which
had
greater
intensity
in
the
south
where
there
was
continent-
continent collision between the West-African craton and the Guyana craton. In
the
Rokelides
this
compression
produced
very low-grade m e t a m o r p h i s m in the western whereas
in
the
eastern
part
this
sequence
only
slight
part of is
deformation
the Rokel
unmetamorphosed
sporadically m i l d l y deformed (Williams and Culver,
and
River Group and
only
1988). The Kasila Group
268
&NTARCTK:& ~
AUSTRALIA SOUTH CHINA IIROOK$ - ¢HUKOTSX
YUCOtO
I
~
•
W. Newfou=d~o~d
~ ( O 0#
CHINA EUROPE
TH ~K)U
S1B£R~A
;:1;:;:!
Suture
CoIIisional Thrust|nq
]
(C) IFEROUS
Figure 6.9: Phanerozoic from Frazier and Schwimmer,
continental 1987.)
reconstructions.
(Redrawn
269
A
D 650-575Ma 1100-700 Mcl
DEPOSITION OF ROKEL
E
B
575-550 MQ
7 0 0 - 6 7 5 Ma
x
R. Gr,
x
w. •
+
,
,
÷
PAN- AFRICAN I[ OROGENY ( E CAMBR|AN)
C
675-650MG
[]
YOUKO UNKOUN
[]
MAL!
[]
GRANITE SIMENT]
[~
GROUP
NIOKOLO-KOBA
~1~ GUINGUAN PAN-AFRICAN [ OROGENY
GROUP
GROUP
GROUP
GROUP
[]
TERMESSE
]
MADINA - KOUTA
GROUP
[]
WESTERN
[~
BASEMENT OF WEST
SUPERGROUP
BASEMENT
BOVE BASIN
CRATON
TAOUDENI BASIN
I
.....................................................................................
F
AFRICAN
SG.3
}. . . . .
550-360 Ma
DEPOSITION
OF
SEQUENCE IN
BOVE AND TAOUDENI
- ----~ .~/P
7
BASINS
TAOUDENI
G 320-275 Ma •
Figure 6.10: Geodynamic evolution genic belt. (Redrawn from Dallmeyer, to
the
et al.,
west
was
1985;
reactivated
Williams
and
during
Culver,
of the West A f r i c a n polyoro1989; L~corch~ et al., 1989.) the
Pan-African
1988).
The
II
event
5 km-thick
sheared gneisses and mylonites which dip at moderate angles west
with
medium-angle
(Figs.6.1,6.2), Kasila which
Group abuts
as
support a
thrusting
Burke
and
cryptic s u t u r e
against
unreactivated
onto
Dewey's in
a
the
(1973)
Pan-African
basement.
The
(Wright
zone
of
granitic
basement
interpretation reactivated mylonite
the
to the southof
the
basement
zone,
about
400 km long, continues southeastwards into Liberia as the Todi shear zone, although it is doubtful whether the mylonite zone in Liberia
is part of a
270
collision
suture
deformation basement
the
was
Mountain
(Williams intensity
thrust
Formation
Pan-African
and of
Culver, tectonic
eastward
over
II produced all
event
in the Mauritanide-Bassaride event
molasse
in Guinea.
The
deposition
its equivalents
local
Permian, was
of
the
orogen
of
at all
African
craton
such
that
II
Archean
the
Gibi
sequences the
in
western
The Hercynian southward
part
where
and of
Senegal
the
final
Carboniferous-
Senegal
microplate
thrusting
nappes,
the
and rift
of the
the more
sequences
Mauritanide-Bassaride
terrane
Bassarides.
Pan-African
Late
Proterozoic of
basin
the
With
the
orogeny was pronounced
in the
between
extensive
Late
western
Taban
to the Late Devonian,
the
foreland
the
(Fig.6.10,F)
(Fig.6.9,B).
in as
and
Taoudeni
movements
(Fig.6.9,C),
resulted
after the Pan-
Youkounkoun
in the
effects
tectono-thermal
II fold belt
arc and host gneissic
in the Rokelides
and m e t a m o r p h i c
Pan-African
Gondwana
orogeny
In the
the calc-alkaline nappes.
Pan-African
Uplift and erosion of
Pan-African
western
foreland
it attenuated
belt.
relative
and
ophiolites).
structural
to the
This
imbrication
internal but
West
eastward.
(including
was
from the Late Ordovician
due
the Hercynian
intracontinental
the
Proterozoic-Cambrian
3 successions
submerged
Laurentia
driven
Late
earlier
deposition
took place
and
the
Supergroup
on the
during
ductile
of
of
the
deformation
microplate collision
traces
caused
the Mauritanides with
transport
the
penetrative
obliterated
II
During
in Liberia.
which
African
1988).
Its
were
thrust as the
in the Mauritanides, effects
II was
the
last
of Florida
and
of
were
not
felt
tectono-thermal
episode. Similarities mobile
belts
lithotectonic
units
allochthonous West
between
Africa
east
rather
(Fig.6.9A,B).
than
Also,
Appalachian,
sequences which were African
of
the
when
to
Hayesville
(1989) the
traced
could
located
of
exotic have
America,
orogeny,
fault
point
the
African
(Fig.6.8) affinities
to
fact fold
upward,
miogeosynclinal
their
collision
along
that belt
earlier
the and
become
various southern
increasingly
terranes
microplates
Dallmeyer represent
(Fig.6.9,A,B)
away from both the Laurentian of Laurentia
the Ordovician-Devonian
and
lithologic
are with
Iapetus Ocean
successions.
lithostratigraphic
accumulated
the
all
stressed West
in the Iapetus Ocean, With
thrust
paleontological
stratigraphically
underlying
these
West African
in the eastern part of the
units
land masses.
the Hercynian
North
Dallmeyer
their
that which
with
the
and the fact that all southern Appalachian
to West Africa,
tecto n o - s t r a t i g r a p h i c
postulated
basement
1989),
and show Ordovician-Devonian
location adjacent
unrelated
the
(Dallmeyer,
Gondwana
and
during
units which were
271
part
of
these
exotic
American margin, the
West
terranes,
were
thrust
westward
onto
the
North
while the Senegal microplate was driven eastward against
African
craton.
The
latter movement
generated
the
Ouled
Dhlim
klippe, and the Akjout and Magta Lahjar nappes in the northern part of the Mauritanides. 6.2.4 Trans-Atlantic Correlations with Southern Appalachian, U.S.A The
tectono-stratigraphic
pre-Cretaceous Atlantic
basement
Coastal
(Dallmeyer,
Plain
1989).
calc-alkaline,
units in
and
deformed
Coast
granites
are
the
of
counterparts southern
southeastern
volcanic-volcaniclastic
which
have
of
and m e t a m o r p h o s e d
Florida, dated at about 690 - 675 Ma, alkaline
above
subsurface
the Gulf
Variably
felsic
outlined
the
rocks
part
in
the
of
the
United
granite
in the
States
and
host
subsurface
of
correlate with similar coeval calc-
exposed
in
the
internal
parts
of
the
Bassarides in West Africa. The
Osceola
granite
in
the
subsurface
of
the
central
peninsula
Florida is similar to the Coyah granite of the northern Rokelides Republic of Guinea
(Fig.6.8).
Dallmeyer
(1989)
of
in the
suggested that the Osceola
and Coyah granites were initially part of the same suite of post-kinematic shallow-level plutons which were emplaced along the northwestern margin of Gondwana. Lower
In
the
Suwanee
Ordovician-Middle
basin
in the
Devonian
subsurface
sandstone-shale
of
northern
sequence
Florida
with
a
Gondwana
paleontological affinities is similar to the sequence in the Bove basin in Senegal small
and
area
Guinea. in
the
Osceola granite, cooling history and
Liberia.
amphibolites and
a
Arch
Also,
high-grade
subsurface
of
east-central
exhibit petrological similar
to
Finally, in
the
low-grade
subsurface show
contradistinction
to
Atlantic
Coastal
the
rocks
in the Rokelide
Wiggins
reactivation coeval basement Plains, which
in
also
southeast
in
of
a
the
and a post-metamorphic orogen
assemblage
Arch
sequence
penetrated
Florida,
similarities
interlayered
metasedimentary
(Fig.6.8), and Gulf
those
an
metamorphic
in Sierra
of
southwestern
penetrated
dated
310
rocks
in the
Leone
gneisses
Mississippi
in the
to
and
Wiggins
300 Ma,
subsurface
of
in the
show no apparent evidence of any
tectono-thermal activity during that time span. The
above
similarities
southeastern United
States
with
the
suggest that
pre-Cretaceous
the basement
Gulf Coastal Plains were part of the Mauritanide, orogens Gondwana,
of
West
Africa
(Dallmeyer,
1989).
They
which after the amalgamation of Pangea
basement
of the Atlantic
of and
Bassaride,
and Rokelide
represent
fragments
of
II, were stranded during
272
Mesozoic
rifting
that
was
associated
with
the
creation
of
the
present
North Atlantic Ocean.
6.3 The Moroccan Anti-Atlas The
West
beneath and
African
coastal
basins,
re-appears
in
("boutonniers") anticlinoria the
swings
the
which
mobile
though
northeastwards
Moroccan
lie
belt,
in
around
Anti-Atlas
the
cores
concealed
the Reguibat
as
of
largely
Shield
Precambrian
the
inliers
Anti-Atlas
Hercynian
(Fig.6.11). Those inliers including those at Bas Dra, Zenaga,
Ifni, Kerdous, expose
polyorogenic
Tagragras, Bou-Azzer,
Pan-African
Siroua, Saghro and 0ugnat
basement which
underlies
(Fig.6.11),
the Paleozoic
which although mildly deformed in the Anti-Atlas,
sequences,
are severely tectonized
north of the South Atlas Fault. Since it has been possible in the Moroccan Anti-Atlas
to
stratigraphic
separate
the
Pan-African
sequences
and
orogenic
basement
events,
from
it
is
younger
Paleozoic
appropriate
here
to
according
to
consider only the Pan-African tectono-thermal events. The
Late
Leblanc began
and with
followed formed
Proterozoic Lancelot platform
by
an
at
about were
followed
by
787 Ma,
615 Ma
phase at
and
by
-
and
which
and in
The
et Azzer
which
and
marks
the
(Cahen et al.,
initiation
of
the
(1984),
an
latter
the
Bou
unknown dates
Caledono-Hercynian
Group),
ophiolites Azzer
subduction
obtained from an unconformable volcano-sedimentary sequence, Group,
al.
Quartzite
Bou
Subsequent
plate
563 Ma.
Cahen
the
tectonism 685 Ma.
the African
578
the Anti-Atlas,
(Limestone
about
between
of
(1981),
during
followed
obducted
collision
between
cycle
Leblanc
sedimentation
extensional
ophiolites plate,
orogenic
(1980),
was
northern
which
were
the Ouzarzate
orogenic
cycle
1984) which is not considered here.
6.3.1 S t r a t i g r a p h y The basal Limestone and Quartzite group rests unconformably upon Eburnean basement
in the
southwestern Anti-Atlas
Kerdous and Zenaga Plain inliers. sequence
consists
of
several
conglomerate
and
stromatolitic
(Cahen et al.,
the succession. Graara, comprises
similar black
and are exposed
in the
Bas Dra,
In these inliers this basal Pan-African thousand
interstratified 1984).
m
of
limestones
quartzite which
are
with
basal
occasionally
Mafic sills have been injected within
To the northeast in the Siroua massif and at Bou Azzer-El sediments shales
with
belong
to
the
interstratified
Tachdamt-BleYda siltstones
and
Group mafic
which pillow
273 !avas
accompanied
limestone reflecting the
Bou
by
horizons. platform
Azzer
jaspers,
graywackes
The southwestern conditions
ophiolite,
is
and
ruffs,
lithofacies
whilst
the
believed
to
has
northeastern represent
quartzite
been
and
interpreted
as
facies,
including
formations
deposited
along a continental margin.
~0~0 CCO Southl
~ 0 ~ _ ~ ~ _
N
•
Maider~-~- 8echar basin
_~ . . , ~
,4~adir
]fni
,,,,5ok?
Post- Pateozoic strata
I I
Poleozoic strata
Precombrian rocks Anticline
Fault
~+/+
. ,o,;o:,
d>"
Thrust
Figure 6.11: Tectonic sketch (Redrawn from Pique, 1989.)
map
of
the
Moroccan
Anti-Atlas.
6.3.2 The Bou Azzer Ophiolite This has been considered the
earliest
tectonics
undoubted
to be among the oldest true ophiolites evidence
in the geological
record
for
the
(Leblanc,
This ophiolitic complex, about 5 km thick, southward
thickening
dismembered
basic
sedimentary
and
(Bloomer et al.,
sequence
alkaline
mafic 1989).
of
Leblanc
blocks (1981)
top, about 2,000 m of serpentinized
1981;
modern
plate
Bloomer
et al.,
1989).
among
from north to south, a eruptive
complex a
reported
peridotites;
and among
of
includes
calc-alkaline
volcanic-pluton
igneous
operation
and
sheared
a
material, m~lange
argillite
(Fig.6.12)
a
with matrix
from bottom to
about 500 m of discordant
274
layered
gabbros,
discordant
large
stocks
of
quartz-diorites;
about
500 m
of
layered gabbros; about 500 m of basic lavas with pillow lavas;
and approximately
1,500 m of volcano-sedimentary rocks.
The
serpentinites
constitute up to 40 % of the entire complex and were derived from dunites and
harzburgites.
interlayered subordinate to dacite.
In
the
sequence
of
volcano-sedimentary
graywackes
spilitic pillow
and
lavas which
series
there
calc-alkaline
range
is
volcanics
in composition
an with
from basalt
Jaspillites and calcareous tufts occur among the sediments. The
graywackes
are
of gabbros,
locally
conglomeratic
microgabbros,
basic
near
lavas
their
and
base
with
fragments
quartz-diorite.
Arkosic
sandstones occur at the top of the volcano-sedimentary series.
A Continentol mor~in deposits with poIt.tectonic granodiorites
" ~ - ,
o
I
I
5Km i
i
B
Sheet of ophiolitic materiat
SW
'k ~°¢:~o~
oov.~
',
sou Azz,r
~
}bel Oumarou
~'~,
m,,o°,,,
,.:.: . . . . . . . . - , ~ k ..:.
o_o,o,,,.,/
-%
,~ -,
NE
.,d~;,.,_
oo,,,,.
gobbro$ / ".
~
Continental
Figure 6.12: Sketch map and cross-section ophiolite. (Redrawn from Leblanc, 1981.) Whilst
Leblanc
(1981)
considered
representing oceanic crust, Bloomer et al. and geochemical not
by
data,
Precambrian
representing
Bou
Azzer
Bou
Azzer
ophiolite
as
(1989), based on new structural
re-interpreted the ophiolite complex as originating
ocean-floor
a m~lange
the
showing
of
spreading,
a rifted
but
continental
rather, margin,
at arc
a
suture and
zone
fore-arc
275
terranes
which
were
stacked
against
the
foreland
of
the
West
African
craton during Pan-African plate collision. During the Pan-African deformation at about
685 Ma,
the ophiolite was
obducted and dismembered into several sheets which were thrust against the continental type
margin
granodiorite
vergence,
low-grade
Kerdous
Deformation was
metamorphism,
and
intrusion at about
by a northern and
(Fig.6.12).
greenschist
615 Ma.
north-south
metamorphism,
was
accompanied
followed
by Barrovian-
by
post-kinematic
Similar d e f o r m a t i o n
crustal
occurred
in
shortening, the
characterized
ductile
western
shearing
Anti-Atlas
near
(Fig.6.11).
After
this
granodiorite,
deformation,
the
conglomerates,
graywackes
with dropstones,
and
Tidilline
the
Series
emplacement
was
(turbidites),
feldspathic sandstones,
of
deposited, laminated
the
BleYda
comprising
siltstones,
and volcanic rocks.
basal
mixtites
This sequence
rests unconformably on the Tachdamt-Bleida Group and on the post-tectonic Bleida granodiorite, African
event.
In
and was deformed without m e t a m o r p h i s m by a late Pan-
the
Bou
Azzer
inlier
this
deformation
with WNW-ESE trends and vertical axial planes,
produced
folds
fracture cleavage and WSW-
ENE faults. 6.3.3 Mineralization In their Bensaid
analysis et
deposits
al.
formed
important
of
the
(1985)
tectonic
pointed
throughout
mineralizations
out
the are
control that
of m i n e r a l i z a t i o n
although
geological
major
history
associated
with
of
in Morocco,
metallic this
certain
mineral
region,
terranes.
some Thus,
while the main copper and cobalt deposits are concentrated to the west of a
deep
crustal
north-south
concentrate to the east
lineament,
(Fig.6.11).
v o l c a n o - s e d i m e n t a r y formation,
all
have
Zn-,
Ag-,
pegmatites
Au-,
Pb-Zn
deposits
Based on copper m i n e r a l i z a t i o n
The Bou Azzer ophiolite has deposits of cobalt, inliers
major
in the
BleYda is the centre for copper production.
Middle Precambrian granodiorites Pan-African
the
in the Siroua,
Cu-
bearing
and W-bearing
Be,
alkaline granites in the Tazenakht,
Li
and
Nb
chromium and asbestos. The eastern Saghro and Ougnat veins
and
traverse
disseminations.
lower
Precambrian
Iguerda Agadir, T a z e r o u a l t and Kerdous
areas. Also, the Precambrian mantle-derived Beni-Boucera peridotite in the Rif domain and
of northern Morocco has economic deposits
vermiculite.
Among
Morocco's
silver, iron ore and antimony.
metallic
mineral
of nickel, exports
are
graphite copper,
276
6.4 The Trans-Saharan Mobile Belt
6.4.1 Geodynamic Setting As defined
by
Cahen
et
al.
north-south Pan-African east
(Fig.6.3).
Its
(1984)
the
Trans-Saharan
mobile
belt
boundary
with
the
craton
is
defined
by
the
segment of the circum-West African belt of gravity highs which to
Roussel
and
L~corche
comprising unrooted, can be
followed
length
of
(e.g.
the
1984;
craton and an active remnants eastern
of
which
Hoggar
of
1987)
between
an
2,000 km
mobile
Caby,
collision
marks
the
belt.
bodies.
this
passive
This
(Fig.6.3)
Prevailing
relate
the
eastern
according
eastward-dipping
and ultramafic
a distance
Trans-Saharan
Black,
continental
(1989),
dense mafic
over
is
belt which borders the West African craton to the
along
zone
the
entire
geodynamic
models
suture
margin
suture
suture
to
of
the
Pan-African West
African
codilleran-type margin of an eastern continent, are
and
the
the
Archean-Early
Benin-Nigeria
Proterozoic
Shield
terranes
(Fig.6.13),
the
of
the
which
were
reworked during Pan-African tectono-thermal events. Although across Mali, it
is
the
exposed
outcrops Nigeria
Trans-Saharan
mobile
belt
in
eastern
extensively Shield.
In
Ghana,
only the
in
Togo,
the
Sahel
Benin,
Hoggar
the
from
southern
belt
downwarp,
the
Iullemmeden
entrant
the
middle
the
West
aulacogen
along
into
of
15°N
and
Algeria
Cameroon, in
In
Mali
is
Benin-
a
shallow
a
major
occupied
Trans-Saharan
it
the
beneath
craton
The
and
lies
basin.
African
latitude.
Nigeria
Mountains,
mobile
Mesozoic-Cenozoic Gourma
runs
Niger and Chad in the Sahel, to the West African coast where
by
mobile
rethe belt
extends for about 1,000 km across from O ° longitude to 15°E. The Trans-Saharan mobile belt will be described in its three principal outcrop
areas,
Pharusian, domains;
the
Tuareg
Shield
(Fig.6.14)
the
Gourma
aulacogen;
and
the
the north
Benin-Nigeria
belt is characterized by roughly meridional through
in
all
its
tectonic
domains.
rift-related
suites
calc~alkaline
and
volcaniclastics,
subduction-related
paired
metamorphic
Wilson
Cycle
1989).
the
in
the
belts,
sediments,
together
geochemical strongly
with
Trans-Saharan mobile
vast
the
belt
(Fig.6.15),
platform
extensive
zonation,
suggest
The mobile
shear zones which have sliced
of pre-rift carbonate-quartzite
magmatic
Shield.
Stratigraphically
widespread occurrence
showing
comprising
the polycyclic central Hoggar-AYr and the eastern Hoggar-Ten~r~
continental batholithic
and
the
operation (Black,
the
sequences, margin plutons
presence of
1984;
a
of
complete
Caby,
1987,
277
\
E~'~'C;J""
%
\\
/ [
SAHARA
,,ff'~ Ix C'%/In Ouzzal+. ~+ ",~ fironutites \
1..J
I~. %
j
Fig. 6.19A ;
/
/
',
; A
I
/
t
Timbuch
t
-k;
t
,/ "'~-""', L E 0 SHIELD'
",. "¢/.,
.
.,./,./,
,
I
/
,
F-A+
.~13
~s ~6 7
s
//is
g--rl 8
I
'I/ I
J s •
/
•
t
I1"*1
NIGERIA
-,j,,, I|
/""
ccro
Figure 6.13: Geologic sketch map of the Trans-Saharan mobile belt. i, molasse; 2, Upper Proterozoic passive margin sediments; 3, Atakora and Gourma metamorphic nappes (mainly quartzites and phyllites); 4, same as 3 with high pressure - low temperature metamorphic assemblages in Gourma; 4, major mafic-ultramafic massifs along the Pan-African suture; 6, metasediments and gneisses affected by the Pan-African; 7, high temperature - low pressure equivalent of 6; 8, undifferentiated gneisses; 9, rocks stabilized before 725 Ma in Djanet - Tafassasset domain; 10, undifferentiated rocks of central Hoggar, partly stabilized at about 840 Ma; ii, pre-Pan-African granulite basement 2.0 Ga old; 12, West African craton; 13, main frontal thrust of Gourma and Atakora nappes; 14, main direction of movement; 15, vertical shear zones and strikeslip faults. (Redrawn from Caby, 1987.)
278
6.4.2 The Tuareg Shield The major north-south province
shear zones in the Tuareg Shield have divided this
into three domains.
western
branch
The Pharusian belt to the west,
and an eastern
branch,
Proterozoic volcano-sedimentary rocks
both
containing
(Black,
1984).
comprises a
predominantly Late
In the centre is the
polycyclic Hoggar-Air domain which is largely composed of Archean-Eburnean gneisses
reworked
African.
The eastern Hoggar-Ten~r~ domain suffered an early phase of Pan-
and
intruded
by
abundant
granitoids
during
the
Pan-
African tectonism around 730 Ma.
%
%
o
/
,%
~2
"
17~ 3
J MI7 Figure 6.14: Principal tectonic units of the Tuareg shield, i, Recent lavas; 2, Nigritian and Ahnet Purple Series; 3, Tiririne Series; 4, Tiririne fold felt; 5, In Ouzzal granulites; 6, East Saharan Craton (?); 7, Pharusian I. (Redrawn from Cahen et al., 1984.) The polycyclic Archean-Early Proterozoic high-grade assemblages
which are
preserved as the In Ouzzal and Iforas granulites were last affected by the Eburnean in
the
orogeny Tuareg
(Fig.6.14),
Shield
were
whereas
profoundly
other parts
of the
reactivated
Eburnean
during
the
terrane
Pan-African
events. These granulites and ~he Pan-African reactivated rocks constituted the basement basement
for the Pan-African orogenic cycle. The reactivated
rocks
include
the
large
orthogneissic
masses
of
Eburnean
the Arechchoum
279
Series in east-central Hoggar; Series,
the
Kidalian
polycyclic
assemblage
the metasediments of the Eg~r~ and Aleksod
granulite
in Adrar
and amphibiolite-facies
des
Iforas;
and
in
gneisses
Tamanrasset
and
of the Arefsa
there are granulites and the Gour Oumelalen gneisses of central Hoggar.
•
• M
530 560
600
M•
-
O L
A
•
S S E
~ ~ m ,~
•
•
.
•
•
-.-,===~tillite
-7"
•
~s~/9,q
•
•
//~
• ee •
_
.~------~'~!9ni.m. b r i t e s
//~%{,~ alkaline
--,
(, f ~ i 1 ~ronlr,
~m+-./~,">" !l[i,d ,l~ ir{trusive +~ tl[i I orani~'e
Ma
Ma
dykes I ' GRANITES
SYNOROGENIC
Cordilleran
to ~-~" volcanic qroywockes "."-~enous tlysch ~2J'.5.~rhyolites ~.'~- . ' ~ ~ ~ docites Island-arc type ~ ' - ~ . " . . ~ ~ ~ , ~ , , environment ~ ~ " • . °• ~ 2 7 . . ~ - P ~ : - P [I II ~ ~P'PZ.,'~., •~ " " ~ .. ~'./~qlmm,'~aRRll& ¢-~ " ~ ., - ,. , j., • I ~ /" -f" .~:.~'% ondesltes "nm~r'~". ..... . - . ~ . . ~ " u-oJoritestj.4i- P ,,] / . - --..~.,/ %~ • • • • IIIIIT ~.~-.I-I- ~P---.T.JI,, b-- ~ ~7 • • • • • • u ~ --~-mit~ = ~ ~JIIf ~'~-" 8 O 0 M a . o ~ (: ~ ?)~ - ~ -+ . tt catc o,k. , ,Jill111 ,/~1-0 m ~"T--'. ~ . "~ --~ :~, Q-diorlfes ~ .,.J lilll i v
(~ - ' ~ ,..
~-'q"P'~q"=~ "
~, ~
~
o
v
\o'l
-res
|~ljm~AH~l~d ykes
"~ .
~: fluviatile "quartzites.':- : . ' . . " " ". - " .O 4 W • 6 • ' • ' % ~ • • • o f i l l i t e
,. •
n
......
,.....
~',,~,
•• .# .
. . . .
• • .. •
.n~,tusiOn s f?'
. ~~ ' pe~'mc f .... ~ . ] . . / . l o w e r q.uortzi'te'unit "': " . .
.~.~*
,.."
:.-:--
: 2.0
GO
EBURNE
:.-P~',. •
;-:
• ".-~"
:: :
A N
GR'ANITES
Tassenc~janet o series
I
ARCHEAN 3-0
Go
IN
OUZZAL
suprocrustols
Figure 6.15: Summary of stratigraphic and the Tuareg shield. (Redrawn from Caby, 1987.)
tectonic
events
in
280
Post-Eburnean
In
the
sedimentation
Pharusian
deposited
after
Proterozoic
belt
the
the
tectono-thermal
event,
orogenic
in
(Caby,
1987).
Proterozoic
At
there
Eburnean
rocks,
cycle
and AnorogenicMagmatism
piles
hence in
metasediments
of
the
the
included
3,000 m
were
only
a
single
in the Pan-African in
belt
of
which
overlying Upper
orogeny
Pharusian
over
the
suffered
Kibaran
northern
comprising
with
sequences
they have been
absence
Ahnet
of
Together
post-Eburnean
the
sequence
are
orogeny.
this
there
weakly
is
region a mid-
metamorphosed
deltaic and m a r i n e quartzites and pelitic schists c o n f o r m a b l y overlain by carbonates. there
Elsewhere,
are
aluminous
peraluminous important
phase
of
alkaline
sequences
metaquartzites
schists.
Basinial
are
with
highly
kyanite
sedimentation
rift-related
been d a t e d b e t w e e n banded
similar
anorogenic
metamorphosed,
and
and
sillimanite
was
succeeded
alkaline m a g m a t i s m
and
by
an
which
has
1.82 Ga and 1.75 Ga, during w h i c h a l k a l i n e rhyolites,
intrusives
(now
gneissic)
and
some
hyperalkaline
and
syenite rocks, were emplaced. Mid-Late
Proterozoic
Platform et al., the
sediments
Platform
once
Sedimentation
ascribed
to the
lower P h a r u s i a n
group
(Bertrand
1966), occur e x t e n s i v e l y in the w e s t e r n P h a r u s i a n belt.
remnants
of
a platform
sequence
of quartzites
and
of
They are
stromatolitic
marbles w h i c h are comparable with Supergroup I on the West African craton (Black,
1985).
uniformity; basement,
These
they in
the
metamorphosed
deposits
rest
Tassendjanet
Stromatolitic
sandstone of beach origin, followed latter gray
gradationally
unit
has
pelite
stromatolitic
Series
sequence
by
on
the
of
northwest
region
by
uniform
intraformational
gradationally
characterized
has
a
great
granulitic
basal
lithologic
and
Hoggar, unit
of
granitic
the
weakly
well-bedded
50 to 100 m thick, with rounded quartz gravel,
intercalations
siliciclastic upward
are
unconformably
and
quartzite,
quartz
up
pebble
potassic
to
600 m
conglomerates,
rhyolites
near
the
thick.
The
aluminous base.
This
is succeeded by dolomitic passage beds which pass in
the
Tassendjanet
calcareous-dolomitic
area,
deposits
into
with
over
shaly
and
4,000 m
of
quartzite
intervals. The age of the Stromatolitic
Series has been a s s e s s e d using indirect
evidence such as the age of the pre-tectonic gabbro sills the
series
at
Tassendjanet,
and
the
age
of
the
(793 Ma) within
quartz-diorite
plutons
(868 Ma) w h i c h cut the equivalents of the S t r o m a t o l i t i c Series in central Hoggar
(Caby,
carbonate
1987).
sequence
was
Thus,
the
deposited
platform prior
to
siliciclastic-stromatolitic these
dates,
probably
in
the
281
Middle
to
early
stromatolitic on
the
Late
carbonates
presence
of
Conophyton
occurring
correlates
with
Mauritanian Pouchkine
Proterozoic.
(1988),
stromatolite
Supergroup
similar in
the
Adrar,
with
Atar
Group,
deposited
biostromes
sequences.
The
(middle
according
correlation
I of the Taoudeni
stromato!ite
both
which was
Paleontological
to
basin
the
form
Stromatolitic
of
Supergroup
1.0 Ga
these
is based
with
Bertrand-Sarfati
between
of
and
Series
I) and
700 Ma,
in
the
Moussine-
based
on
the
meta-mafic
and
assemblages.
Mafic and Ultramafic Rocks Related to Crustal Thinning Throughout
the
Trans-Saharan
meta-ultramafic grade
masses
metamorphic
amphibolites gabbros gabbro
at
or
However, Iforas
form
these
and
occur
mafic
Silet tectonic calc-alkaline
belt.
emplaced
into
In
diapiric sills,
the
highand
Granulite
meta-
the
meta-
to mark
Kabr~
the
suture
the
continental
layered
or
of
and
which
laccoliths
Adrar
peridotites,
Ougueda
the
mafic
basement
with in
within and
either
between
in
the
ultramafic about
the
pelitic contact
layering,
Tassendjanet
tholeiitic
complex and
in
Examples of
magmatic
the
they were
intrusions
carbonates exhibit
sills occur
and Adrar
Rather,
multiple
intrusives
serpentinite
areas
in the Hoggar
bodies.
quartzites,
and the cumulate in
in
in
gneisses
with the host metasediments.
wehrlite
these
masses
lapoliths
ultramafic contacts
and
rocks
Pharusian
and
ophiolitic
comprising
include
zones;
lenses
the
sizes.
believed
and ultramafic
as
and
pyroxenites
various
are
and
within
aulacogen,
belt
of the above
or tectonic
intrusives
Gourma
small
layers
occur of
are
(Fig.6.13).
asthenoliths,
The
there
concordant
boudins
the
metasediments
metamorphism
dunites,
in
belt
They
Togo-Benin
the other mafic
of
schists.
the
are not part
surrounding
form
isolated
collision
mantle-derived; the
as
in
the Pan-African
des
which
terranes.
Amalaoulaou
massif
mobile
and
gabbros and northwestern
intrusions
1.0 Ga
and
were 800 Ma
(Fig.6.15).
Volcano-Sedimentary Sequences and Calc-alkalineMagmatism These rock assemblages they characterize. (Fig.6.16) which, the
representing
during
western
Li~geois
are thick and extensive
They were
collision,
margin
et al.,
of
1987).
the island-arc
the
newly
were
thrust onto the West African
the
East
formed
in the Pharusian
Saharan
belt which
and c o r d i l l e r a n v o l c a n i c l a s t i c s Pan-African
craton
(Black,
continental
crust
craton and onto
1984;
Caby,
1987;
282
,
CRATON
ADRAS 0ES IFORAS
(B) ]~r WEAK DISTENTION TRANS-CURRENT MOVEMENTS
(A) El UPLIFT (South 580-570MQ ( N o r t h 560-545Ma WEST AFRICAN W CRATON 6.50' IFORAS
(South 580 ?(Min 54,0) (North 550 WEST AFRICAN . 5 . iF,~O^= vn~ W --CRATON~ 0430 . .. "P're'sent Erosion E ~ C , ' & ~ . I J!,F~ I surfoce •
E
:!:!:~:!:!:~!:i:i~i:~i:~:i:::.:+:.:.:...
(C)
,'I,';
(o)
Figure 6.16: Tectonic model for the evolution of the Iforas batholith during the Pan-African orogeny. (Redrawn from Li~geois et al., 1987.) In
the
Pharusian
stromatolitic
marbles
(Fig.6.15).
The latter
continental
margin,
which
suggest
craton
from
associated
the the
belt are
the
sequences
with
of
calc-alkaline
The
volcaniclastic
increasing
deepening,
to the deep ocean which Resting
batholiths
thousand
of
Upper
belt
(Caby,
Pharusian
m
ocean
(Caby,
The
rocks
an
are
on
eastern
deeply
The
the
and
sequences
of an active
lithologies
volcaniclastic
(Fig.6.16)
West
African
sequence
and
formed in the Late Proterozoic
below
Pharusian
marginal
from
belt,
sea
east
to west,
in a direction
across
an island
of
arc,
(Fig.6.16,A).
eroded
between
Proterozoic 1987).
arc
features
separating
described
lay to the west
dated
quartzite
1987).
to the western
from
of
volcano-sedimentary
island
open
craton.
sequences
unconformably
subalkaline
an
and magmatic
from the eastern
sequence
thick
display the typical
Saharan
between about 850 and 700 Ma
that is,
by
characteristic
presence East
platform
overlain
and
880 Ma
altered and
volcaniclastic sequence
begins
calc-alkaline
839 Ma, rocks with
in
and
are
several
the
eastern
clastic
units
and
283
lenticular
dolomites
volcaniclastics. dacites which
of
probable
Upward, the
marine
succession
"Serie
verte"
of
the
alkaline
batholiths
granodiorites,
and
southwestern
Iforas
a
sequence
Pharusian
plagiogranites. of
Upper
basal
the
andesite-
upward into
belt.
are similar
These
to the
graywackes
and are intruded by m a n y dolerite
comprising
part
thick with
sediments
and
that pass
with
In the eastern and northern parts of
northwestern
terrigenous
interfinger
rhyolites
pelites,
Pharusian belt there are graywackes which
mixed with
the
which
includes
are overlain by terrigenous
p r e d o m i n a n t l y rhyo-dacite volcanics. the eastern
origin
quartz-diorites, A uniform
eastern
Proterozoic
pelitic
conglomerates
rests
micro-diorites,
terrigenous
branch,
and
in
detrital
upon
are
calc-
sequence
eastern and
covers
Adrar
des
volcaniclastic
mid-Proterozoic
quartzites
and alkaline orthogneisses. In the northwestern part of the Pharusian belt the thick
monotonous
flysch-like
sequence
of
green
"Serie verte" volcanic
is a
immature
graywackes which passes northwards into a subaerial calc-alkaline volcanic complex, about 6,000 m thick, probably representing part of the island arc itself.
This
complex
consists
of
basic
dacites with rhyolites near the top. tillite
occurs
of
continental
subduction
zone,
the
"Serie
margin
by
the
verte"
and
the
partial
In
the
central
part
of
volcaniclastic
derivation
andesites,
of
of
The geochemical
complex the
suggests
andesites
the mantle,
an
from
followed
by
a
low
1979).
Adrar
sequence
complex.
calc-alkaline
melting
pressure fractionation (Black et al.,
Proterozoic
normal
Polygenetic conglomerates with basal
at the base of the calc-alkaline
characteristics active
andesites,
des
Iforas
belongs
(Fig.6.14)
to the Tafeliant
the
Upper
Group which
rests unconformably on pre-Pan-African or older basement
(Fabre,
1982) and
comprises
blocks,
littoral
basal
calcareous tillites
unstratified
sandstones
and a unit
(Fig.6.17,A).
graywackes
with
abundant
pelitic
siltstones,
bimodal
suite of basalts
Tafeliant
Group
north-south
was
faults
tillite
The
with
of black
succession
andesitic
large pelites passes
material
deposited (Fabre,
in
1982).
a
clasts
from the
into
mixed sialic
may
marginal
correlate
of marine volcanic with
semi-
basement;
occurs at the topmost
shallow
It
with upward
partially
and with arkoses derived and rhyodacites
local
part.
a
The
basin
bounded
by
westward
with
the
Tessalit-Tilemsi volcanic graywackes which contain deep trough sedimentary features basalt
including a marine diamictite.
complex,
located
at
about
correlate with the Tafeliant Group.
The Oumassene Group,
i00 km
to
the
north
of
an andesite-
Tafeliant,
may
284
TAFELIANT GROUP
S H E L F - T Y P E COVER
S~ ( B+ot+}
$2
S~
._,-.~...,.~-.---;,--- ~ --.+:+...+....~.,..,>:;..:~+.,.~~-x+× .<-- .~+.- . 7---~ ..-.-.~. ?]>.~; ~ +
-.-~+:v
i
.-,. i._.+. i
..
.'+"++" : ~
km
""
:
~
"+'-+""+;-::.:+<"~C-.:5-o~.
~
"~
~
~
.~
"
~
~
.......
. . . . .
~
++~..+"es~
".<
BASEMENT
m
o
~ P[aglogranite
,
,v,e I~+,---'-+ . . . . . . ~ ~ . ~++~...
"' ~ ,'::,E, t+'+' , ' +
'+/-" ~++",-S .+" ,.. + ~ - +?', , . ~ ~ "+
oc ~'e '~C+'~
++
o o~ u ~" c &" e 0 ~0 ~ ~
DiWe complex (Di~ba~e microdioflte)
.
.
.~++'+;-"-'--~':.~"+
.
.
.+
.
.
o.+ ,-,~'C".','-..~ -..- -
"
~.P.."~.+JF~.~'~//J//~~"
+m+Y+O+AL,+;C
.
.
. Q- D i o r i t e
++
~+,~t '+4+':'++. . . . +. .+ . +.- - . <.. . .~ ;.~ I. ~ % . ,. ~ V . ~. +." +. . >. . . " .. " - - L
~//j"
.
~
~-~
~" +:+ ++ + -.,,,+
(.. +.,re~.~o~ Z+~2.;'
." •
,+
B
GN+,SS+S
Figure 6.17: A, cross-section through the Tafeliant Group. i, Tafeliant Group with basal glaciogenic sediments; 2, intrusive foliated metadiorite, 793 Ma old; 3, shelf-type cover of Upper Proterozoic age; 4, pre-Pan-African gneisses; 5, direction of younging strata. B, schematic section showing relationship of layered sequences and plutonic rocks in the Tessalit-Tilemsi island arc. (Redrawn from Caby, 1987.) In
the
northwestern
part
of
Adrar
des
Iforas
the
Tessalit-Tilemsi
volcanic and volcaniclastic series
(Fig.6.17,B) is of island-arc affinity.
This
belt
sequence
characterized from
the
rest
defines by
a
broad
metamorphic of
the
and
Tuareg
Shield.
Tessalit-Tilemsi Series consists, sequence with meta-basaltic followed
by a thick
about
As
lavas,
of volcanic
dacites
sedimentary
structures,
and
a
of
rocks
make
up
Among
cumulates;
the
plutonic
in
that
are
is
distinct
Fig. 6.17,B
which rocks
are
and younger
graywackes
turbiditic series.
wide
which
depicted
generations
plutonic
100 km
features
the
in the lower part, of a bimodal volcanic
pillow
sequence
of
structural
large about
with
volume 70 %
meta-gabbros
microdiorite and dolerite bodies,
andesites,
deep water of
of the
and
several entire
and
ultramafic
stocks and dykes;
and banded
285
and
differentiated
and lopoliths. was
early
gabbro-norite,
tonalite,
and
granodiorite
laccoliths
During the emplacement of these mantle-derived
anatexis
among
the
surrounding
metasediments,
rocks
from which
there were
generated tonalitic gray gneisses and migmatites of low-pressure granulite facies, at about 720 Ma. Since the geochemistry of the plutonic magma
evolution
volcanism,
Caby
magma chambers
beginning (1987)
with
rocks
oceanic
concluded
that
attest
tholeiites
the
plutons
to a sequence of to
calc-alkaline
represent
deep-level
that are related to the calc-alkaline volcanism;
and that
they define the suture zone which has a positive gravity anomaly signature (Fig.6.3).
The calc-alkaline batholiths were emplaced on the eastern side
above
eastward-dipping
the
prior to collision. the
pre-orogenic
eastward
is
subduction
zone
(Fig.6.16)
at
about
633 Ma,
The distribution of the plutonic complexes, especially
suite,
similar
to
and
their
progressive
the
magmatic
increase
zonation
across
in
K20
modern
and
SiO 2
subduction
zones. The Tessalit-Tilemsi complex was p r o b a b l y an island-arc environment that was active from about 800 Ma to 630 Ma. of
gabbroic
lopoliths
and
basic
Since it had a magmatic root
gray gneisses
(Fig.6.17,B),
it
probably
had no continental crust (Caby, 1987). The Tessalit-Tilemsi accretion zone follows the sheared western margin of the Iforas Pan-African their
(Fig.6.14).
suture
kindred
developed spatial
east
zone,
Notable also
calc-alkaline of the
distribution
suture. of
is the total absence,
of the Tessalit-Tilemsi
the
magmatic
rocks,
As postulated rock
graywacke which
by
assemblages
Black can
west of the
formations and
are et al°
be
extensively (1979)
accounted
this
for
by
positioning an ocean basin between two continents in this region; the West African craton on the western side, and an eastern continent on the other side with an island-arc complex in between
(Fig.6.16A,B).
Deformation and Metamorphism Caby
(1987)
divided
the post-Eburnian
structural
and metamorphic
history
of the Tuareg Shield into two phases--localized early Pan-African tectonothermal events at 870 Ma,
730 Ma and 720 Ma; and the regional Pan-African
event which affected most parts of the T r a n s - S a h a r a n mobile belt at about 600 Ma. Uncertainty
surrounds
the existence
of a Kibaran or Aleksod event
the central Hoggar which was postulated by B e r t r a n d and Lassere by Cahen et al. produced
(1984).
Kibaran or Aleksod
kyanite-sillimanite
and
granulite
o r o g e n y was facies
(1976) and
believed
metamorphism
in
to have and
sub-
286
horizontal absence
foliation
and
of reliable
recumbent
radiometric
folds
dates
in
central
and in v i e w
Hoggar.
of
the
But
in
the
lack of marked
structural d i s c o n t i n u i t y between these probably older polycyclic basement rocks
(at Aleksod, Arefsa and Tamanrasset) and the surrounding terranes in
which
a
single
consigned
the
Pan-African
Aleksod
tectono-thermal
event.
conclusively
unravel
east-central
Hoggar
amphibolite thus
and
lenses
indicating
However,
the
that
U-Pb
well
established,
ages
history
sometimes
are enclosed
attainment
of
of
rocks
to
are
required
the
contain
(1987)
Pan-African order
to
polycyclic
gneisses
of
eclogites
in
in orthogneisses
high-pressure
Caby
the in
garnet-
and metasediments,
conditions
estimated
at
Such extreme metamorphic conditions were reached prior to
the
low-pressure anatexis
the
central
African
is
metamorphic
thermal
which
the
15 kb at 700°C.
event
related
Hoggar,
which affected the surrounding
southwest
retrogression
and
of
Tamanrasset,
overthrusting
of
U-Pb
rocks.
ages
Eburnean
Also,
suggest
in
Pan-
granulite-facies
rocks. In the eastern part
of the central Hoggar a tectono-thermal
event at
870 Ma affected a platform quartz-carbonate sequence which correlates with the
Stromatolite
around
868 Ma
batholiths, 1987).
Series. from
This
event w h i c h
late-tectonic
is
supported
quartz-diorite
The
Djanet-Tafassasset
domain
meta-pelites
and
with
deeper
graywackes
syn-tectonic
late-kinematic
granites
at
metamorphism,
porphyritic deformation,
(or Tiririne the
fold belt
Tiririne which
and
molasse
dated
(Fig.6.18,A)) was
overthrusts
730 Ma.
stabilization
belt
in a Himalayan-style
This
The
Tiririne tectonics.
and by
suggests
that
Djanet-Tafassasset 730 Ma,
high-grade
fold
and
vertical
granitoids
the
belt
represents early Pan-African metamorphic rocks which Tiririne
in
occurred before
deposited.
the
of
of
(Caby,
greenschist-
is cut by many
Issalane
ages
granodiorite
occurring
north-south-trending belts
which
U-Pb
has not been recognized elsewhere in the Tuareg Shield
amphibolite-facies
domain
by and
after
gneisses
of
(Fig.6.18,C,D)
later overthrust the
High-level
granitoids were
emplaced by anatexis during this early Pan-African event. The Tessalit-Tilemsi
domain
in the western A d r a r
des
Iforas contains
the vast island-arc calc-alkaline volcaniclastic complex which appears to have
been
subducted
at
about
720 Ma
basic plutonic rocks, metamorphism,
giving
rise
intensely deformed plagioclase gray gneisses the
hornblende-granulite
subduction folding
producing
(Fig.6.17,B).
facies.
migmatites The
late
to
the
emplacement
of
and deformation which resulted in the
and
(Fig.6.17,B) metamorphosed at
Anatexis foliated
Pan-African
also
resulted
dioritic
event
at
rocks
about
from
the
with
flow
600 Ma
caused
the partial retrogression of these rocks to the g r e e n s c h i s t facies and the
287
formation of open folds with sub-vertical flow schistocity,
especially in
the low-grade metasediments. eo a.c ,.:,.~ ; ... (,~e ~ ~ , ~ . ~ _ 2 Tnnrlne ...., ~o o ~ -'-$I syncline
/ l / i l l
....
iSSALANE
51
Ghoras sync|ine
>" O \ t y y
."
:, ,--,~.-~o,°~/~'/"
"#,.':'t-
8* 30" Shear
",. . . .
Tln
,
4
]~F/"
//~/.~[
:one
~krn
Tiririne trough •
- - - t - . . - - - ~:'- ~
. ,C.~.:" t
lj L ,
~.r
~151~.5T5AHARAN CRATOIN
~
B
725Ma
650Mo
C 610 Ma
Honog gronitic axis
~
Aghe~er syncline
D 590 Mc~
Anotexis
Figure 6.18: A, cross-section of the late Pan-African Tiririne fold belt (Eastern Hoggar); B, C, D and stages in the origin of this fold belt. (Redrawn from Caby, 1987.)
of
The late Pan-African
event
at about
the
where
early
Tuareg
practically movements.
Shield
all
domains
Metamorphism
resulted
involved
600 Ma a f f e c t e d gently from
an initial
inclined
nearly
all parts
schistocity
large-amplitude
in
horizontal
intermediate-to-high-pressure
phase followed by lower pressure and high temperature conditions. In Adrar central
des
Iforas
Iforas
("Kidalian
(Fig.6.19,A)
the
assemblage")
middle-to-high-grade
belongs
to
the
middle
zones of the Pan-African and includes pre-Pan-African basement,
gneiss and
of
deep
cover and
288
pre-metamorphic
intrusives which were
involved in Pan-African
large-scale
horizontal movements before 610 Ma. Limited northwestward thrusting of the frontal part of the Eburnean granulites over the "Kidalian assemblage" evident
from
However,
the
gently
dipping
foliation
in
the
"Kidalian
is
assemblage".
since the Eburnean granulites are bounded to the west and to the
east by vertical shear zones in which sub-horizontal stretching lineations suggest
strike-slip
diorites (1987)
fault movements,
and elongate masses
of
granitoides,
and gabbros were emplaced along these vertical shear zones, Caby concluded
that
the
large
masses
of
Eburnean
charnockites
and
granulites are not completely allochthonous but are partly in place, being rooted in the upper mantle.
TILEMSI ISLAND-ARCASSEMBLAGE TAOUNANT Na.ppes CENTRAL BATHOLITH MOI asse Lote Gobbros /
IM,ylonites
~ Cretaceous./
GRANULITE UNiT
\
\volcaniclastiCs /
wE~'A~'~,--~?~;.:',~:I:-~:gI'~:!V',.
¢over*"~ cover ~
ADRAR
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A GQbbros and
eCIfIy anotexts at 720 MS
10kin
AGUELHOC
REWORKED
Metomorph|sm
BASEMENT SUPRACRUSTAL
L--J
"~"":'.F~:,':
INTRUSIVES
( Kidcdiclr clssembiage)
N N W
TASSENDJANET NAPPE ,
,
.
,
• ~;' ',::. "2100 My. . . . / s 4 Granite * + ,. . 10
I
4"
j
°
km
NNW
C
SSE $
S
0
B
% % ~ %~7 %
* .
.
, ° °\ ~"'-:., .,.-.~..~. " . ~ / ' ~ ~ ' ~
~
~
~
L
,
~
,
~
/ ' / ~ ~ ~ ' ~ ' ~ , ~ _ ' . . . . ' , ~+ " " ~ . . . . . ~2 ?------~-'. "S~" "K"' Ol --'''-~---'", " S e r i e ' v e r t e " v olcanic graywackes
TASSENDJANET NAPPE
"
.~
.
.
~
SERIE VERTE GRAYWACKES . . , .
.'0 Volcanics and q graywack es
TIDERIDJAOUINE ky IN j" - , . OUZZAL
SSE
20kin,
Figure 6.19: A, schematic East-West section through central and Western Adrar des Iforas showing nappes, central b a t h o l i t h bounded by vertical shears. B, C Tassendjanet nappe representing a Northern continent; black represents ultra-mafics. (Redrawn from Caby, 1987.) A l t h o u g h polyphase deformation appears to be the case in several parts of n o r t h w e s t e r n Hoggar and in Adrar des Iforas, Caby
(1987) was in favour
289
of one continuous
Pan-African deformation episode.
in northwestern Hoggar at
shallow
Tafeliant shows
depth
(Fig.6.19,B,D)
which
change
synclinorium
only
northwest,
a
single
and
with
southeastern margin
to
fold up
shows large horizontal displacements
recumbent
(Fig.6.17,A),
the
generation
to
of this
30%
The Tassendjanet nappe
folds
youngest
with
east-west
synclinorium,
open
at
deeper
levels.
Pan-African folds
crustal
however,
The
structure,
trending
north-
shortening. there are
In
the
transitions
from open folds with vertical axial planes to vertical recumbent folds at greater
depth,
assemblage".
which
Caby
are
similar
to
those
in
the
older
"Kidalian
(1987) pointed out that these rocks were m o s t l y affected
by the regional
Pan-African
event between
620 Ma and
600 Ma during which
gently inclined
foliation and recumbent and sometimes re-folded folds and
crustal thickening were produced under Barrovian metamorphic conditions. East of the regional Pan-African suture (Fig.6.14),
the
subsequently
syn-kinematic
altered
the
emplacement
Barrovian
zone at Egatalis and Aguelhoc of
gabbro-norite
metamorphic
bodies
grade
and
have caused
sillimanite to pseudomorph after kyanite. Post-orogenic
strike-slip
movements
occurred
along
N 20 ° shear zones and faults, between 566 Ma and 535 Ma 1983);
and a
brittle
final
conditions,
(Li~geois et al.,
post-collision deformation produced
a
conjugate
north-south
phase which set
of
to
(Lancelot et al., occurred
strike-slip
under faults
1987).
Syn-orogenic and Post-orogenic Magmatism Pan-African granitoids make up to 50 % of the Pan-African outcrops Tuareg Shield,
in the
especially in the west-central part of the Pharusian belt.
Syn-orogenic granitoids with migmatitic margins predominate in the amphibolite-facies terranes. suffered
only
graywackes
These are often calc-alkaline in domains that have
Pan-African
tectonism
and meta-volcanics.
from q u a r t z - d i o r i t e
Some
to granodiorite,
terranes
with
foliated masses
or
in
range
but
abundant in east-central Hoggaro However,
the more
potassic
abundant in
meta-
composition
varieties
are
the distinction between foliated
truly syn-tectonic masses with diffuse margins
and unfoliated masses with
sharp contacts depends mostly on the level of intrusion. A unique syn-orogenic granitoid complex is the composite central calcalkaline b a t h o l i t h of Adrar des Iforas for over 300 km and is over 50 km wide; Tessalit-Tilemsi
(Fig.6.20). This batholith
extends
it is bounded to the west by the
island-arc accretionary terrane,
and to the east
290
by
the
Eburnean
(Figs.6.20,6.21).
Iforas
polycyclic
granulitic
According to Bertrand and Davison
basement
nappes
(1981) and Liegeois et
Figure 6.20: Geologic map of the region intruded by the Iforas batholith. I, Eburnean granulites; 2, undifferentiated schists and gneisses; 3, volcanic and associated plutons of the Tilemsi island arc; 4, Aguelhoc gneisses; 5, Pan-African plutons; 7, rhyolite flows; 8, post-tectonic plutons of the Kidal assemblage; 9, alkaline ring complexes. (Redrawn from Li~geois et al., 1987.) al. as
(1987) pendants the
Tafeliant
batholith lavas
Oumassene
Groups occur
(Fig.6.21).
The
batholith
is
of
dyke
comprising
spectacular swarms
essentially
tes , micro-adamellites and perthite
capped
in by
the
central
flat-lying
such
Iforas
rhyolitic
and cut by sub-alkaline and alkaline ring-complexes.
in Fig.6.21
generations swarms
and belts of low-grade metasediments
the
(Nigiritian)
depicted
of gneisses
and
granite.
dyke
exist.
swarms
First,
subordinate
and felsites;
occur there
basic
in the batholith.
As Two
are
east-west-striking
dykes,
quartz-microdiori-
and subsequent high-level adamellite
The second generation
consists
of north-south
basic
291
dykes and several
sets of quartz micro-syenite,
which can be traced within
the Nigiritian
granophyres
rhyolite
and rhyolites
field for 250 km along
the axis of the batholith and along the alignment of ring complexes. Based on the petrology, lith,
Liegeois
et
al.
geochronology
(1987)
concluded
and geochemistry
that
the
magmas
of the batho-
from which
this
composite cordillera batholith was derived all share low initial 87Sr/86Sr ratios,
suggesting
derivation
from the subduction
island-arc calc-alkaline volcano-sedimentary shown in the model of
collision,
(Fig.6.16,D),
alkaline
magmas
asthenosphere,
thus producing
ring complexes
(Fig.6.20).
Late- and These
post-tectonic
include
comprising chemist~
the
a homogeneous and
concentric
probably
anorogenic
granites
suite
generated
alkaline
Also, as
direct
from
the
magmatism with alkaline
are common
in west-central
Hoggar.
in
dated
560 Ma,
Hoggar,
of granites
structure.
tungsten-tin mineralization
(Fig.6.16).
some 50 million years after the beginning were
granitoids
Taourirt
of the Tessalit-Tilemsi
sequence
A
with
suite
of
at
about
restricted
calc-alkaline
high-level
granites
with
cut the gneisses east of the 4°50 shear zone.
W
E
ACCRETION DOMAIN
MAIN
IFORA $
8ATHOLITH
AOUKENEK G R A N I T E
t
13
12
,~Lz
1
11
~
~
CENTRAL
TAOJOUOJEMET 10
3
.~1
9
GRANODIOR~TE
8
G
t
I
~=1
v
,,o,,,,,,
7
3,
DOMAIN
IN BEZZEG &
3
I
WE
,
METADIORITE 2
1
s,.s=,~,>l?:,.2'Z S • ",
E
Figure 6.21: Cross-section through the Northern part of the Iforas batholith, i, main thrust contact of the Iforas granulite unit; 2, Kidal assemblage; 3, mylonite belt; 4, low-grade upper Proterozoic supracrustals; 5, Kidal assemblage structurally below granulites; 6, deformation with folds and foliation; 7, Tedreq acid intrusives; 8, remnants of Kidal assemblage; 9, late prophyritic granite; i0, andesites; ii, central part of batholith with flat-lying rhyolites of the Nigiritian and alkaline ring complexes (solid black); 12, migmatites; 13, Tessalit Formation (clastic and volcanoclastic). (Redrawn from Betrand and Davison, 1981.)
292
Molasse Sequences The oldest
molasse
fold
belt
(Fig.6.14),
6 km
of
greenish
derivation east, and
a
sites,
(Caby,
early
molasse
about
400 km.
genic
conglomerates
glacial
arkoses,
purple sequence
deformed The
Pharusian graben of
S~rie
of
green
Pourpr~e
(Fig.6.14) age,
of
similar
ignimbrites and syenites
tillites, facies
the
the
suture
the
molasse
1 ° shear
zone
rhyolites)
ring complexes
for
300 km long,
arkosic
along the
sandstones,
conglomerate event
at
of the Iforas
fine
lenses,
and
this
is intensely
intrusives.
pelites,
belt Its
is a late molasse
lithologies
green-khaki
and thick g l a c i o g e n i c
and
of
Pourpr~e.
into a
argillites.
Taoudenni
zone
sequence
S~rie
andesites,
up to 6 km thick.
the
vertical mylonite
clasts which pass upward
(basalt,
continental
argillites
in
north-south-
poly-
of the Pharusian
varved
facies
at
ande-
siltstones,
by basic and acidic
glauconite,
with
the
intruded
Tessalit-Anefis
600 Ma Pan-African
in illite and occasionally
"triad"
in
uplifted
dacites,
in
of
and khaki
polygenic
tillites,
lates
the along
ignimbrites,
and
the
and
east
fills a narrow graben
and reddish
argillites
is
runs
volcanic
sequence
shales
of Early Cambrian
either
molasse,
sandstones
rhyolitic
post-dates
sandstones
belt
which
fine
sequence
and metamorphosed
sequence arkosic
or
was
belt an early molasse
is cut by the alkaline
Composed
being
rhyolites,
preserved
the younger
with abundant
A molasse
fault.
Series
up to suggest
domain
and was
Tiririne
immediately
Pharusian
a volcanic
The sequence
batholith.
molasse
the
a narrow
and
indicators
by major north-south
situated
underlies
It consists
facies
the top.
Adrar
in
in
occurs
730 Ma
comprising
are
bounded
In the northern
which
comprises
Djanet-Tafassasset
the
molasse
basins
and graywackes
Paleocurrent
around
sills
in the Tiririne
Series
and granodiorites.
(Fig.6.14)
in residual
1987).
arkoses
of
Pan-African
grabens
or
the
deformation
diorites
later
trending
its
from
Hoggar,
Tiririne
graywacke.
stabilized
a variety
gabbros,
The
zones
by
and
had
in the eastern
the molassic
sediments
that
Before
660 Ma
occurs
where
arkose
the
terrane
eroded.
about
An
of
sequence
The
cherts, basin.
basal and
units part
such as
with
limestones
Effusive
include
rocks rich
corre-
rhyolites
occur at the base of the S~rie Pourpr~e w h i l e alkaline
the
and
basalts
are present at the top.
6.4.3 The Gourma Aulacogen
Stratigraphy Mid-way lies
the
along the Pan-African Gourma
belt
of
Mali
suture
zone of the T r a n s - S a h a r a n
(Fig.6.13).
The
Gouruma
fold
mobile belt
belt
has
the
293
characteristics bifurcation
of
of
a
the
triple roughly
junction
in
that
it
north-south-trending
marks
the
rift
westward
system
which
developed along the eastern margin of the West African craton around 850 800 Ma ago. The Gourma trough evolved as an aulacogen or failed-arm of the Pan-African ments
rift
accumulated
Fig.6.22,B,
system. in
Over
this
12 km rapidly
the sedimentary succession
2"
Toko,mbol Q ~
of
presumed
subsiding
Upper basin.
Proterozoic As
sedi-
depicted
~
. O"
N
~ I
~ 7
A 15'
0
50
100km
BAND|AGARA
I
SANDSTONE
in
(Black et al., 1979) begins with an
/ i
N NE
II~"
,~
ForrnQt ions
V
I
III
I
sO krn
B
Figure 6.22: A, outline geologic map of the Gourma aulacogen. i, Quaternary; 2, Bandiagara Sandstones (Cambrian ?); 3, und i f f e r e n t i a t e d possibly Upper Proterozoic formations; 4, same as 3 but parautochthonous; 5, Fafa parautochthonous unit; 6, exter-nal nappes with shallow greenschist metamorphism; 7, internal nappes; 8, internal nappes with high pressure low temperature m e t a m o r p h i s m including eclogite; 9, shales and quartzites of the Guemri window; i0, Amalaoulaou mafic-ultramafic massif marking suture zone; ii, Cenozoic of Gao trough; 12, u n d i f f e r e n t i a t e d basement, 2.0 Ga old of the West African craton and the Bourr~ inlier; 13, fold axis; 14, thrusts. B, section showing facies and thickness. (Redrawn from Caby, 1987; Black et al., 1979.)
294
early terrigenous phase
(Units I and II), followed by a carbonate sequence
which
west-east
shows
a
lateral
facies
platform through basin slope to trough passive Slope
continental
facies
such
margin as
carbonate
sequence.
Sandstone
Group
non-marine
breccia
and
a
clastic
analysis
converging
from
are
in
(Units
well
the
drainage
the
cratonic
ending with a typical
sequence
turbidites
Paleocurrent
reveals
progression
(Unit III),
IV and V).
displayed
overlying
pattern
in
the
Bandiagara
centered
on
the
Gourma trough with transport from the southwest and west.
The Amalaoulaou Mafic Complex Two important igneous complexes are exposed in the Gourma aulacogen. These are
the
facies
Amalaoulaou and
the
basic
Eburnean
intrusion,
Bourr~
metamorphosed
basement
horst
pegmatites and banded migmatised amphibolites mentioned 800 Ma,
the
Amalaoulaou
marks
collision. contact
the
This
with
original
complex the
(Fig.6.23,A)
basic
complex
eastward-dbpping
at
direction
(Davison,
which
suture
about
40°E
the
suture
of
of
the
which
granulite
emplaced
of
at
about
Pan-African
plate
cumulate body.
believed
(Caby,
the
orthogneisses,
1980). As already
underlying
is
zone
was
zone
is an unrooted gabbroic
meta-quartzites
dips
under
comprising
The lower
internal
to
nappes
approximate
1987).
Along
contact are complexly and intensely deformed meta-mafic
rocks,
the
the lower
comprising
chlorite schists at the base with banded metasomatic and hematitic jaspers which
contain
magnetite,
altered
chromite
and
a
blue
amphibole.
Amphibolites with blue-green hornblende or actinolite occur in the middle; and above
the
cumulates
show
contact
are eclogite assemblages.
tholeiitic
chemistry
and
had
The Amalaoulaou
re-crystallized
gabbroic
under
high-
pressure granulite facies conditions.
Structure The
Gourma
aulacogen
displays
large-scale
nappes
which
were
emplaced
during the collision of the West African craton against the East Saharan craton zone
at
about
300 km
mostly
600 Ma
long
mica
and
schists
(Black et al.,
50 and
- 80 km wide quartzites
1979).
The
show
(blueschist) metamorphic
the
attains
pyroxene
is
conditions for the jadeitic
where
it
present.
eclogite
The
flat-lying
pressure/low-temperature southwest
nappes
(Fig.6.22,A).
grade
(P = 18 Kb; T = 800°C) has been suggested
internal pyroxene
nappes +
based
on mineral
pyrope-rich
garnet
foliation
conditions
High-pressure/low-temperature
aluminous mica schist at Takamba (Fig.6.23,B).
rutile)
over
and
which
initial
high-
in
the
to
jadeitic
metamorphic
(de la Boisse, (e. g.
a
nappes,
that increases in
associations +
outcrop internal
1981)
phengite
+
eclogitic
295
Earlier bearing minor
sharp
folds
phengite
folds
internal
in the Gourma
muscovite.
Stretching
that are perpendicular
nappes.
(Fig.6.22,A)
In northern
the
autochthonous
internal
folded
towards
the
southwest
against
the
Eburnean
trough
are
cut
lineations
to the general
Gourma,
nappes
by
associated strike,
along the banks
are
in
direct
greenschist
facies
formations
whereas
to the
(Fig.6.23,A).
planes
with
NE-SW
occur within
of the
contact
(Fig.6.23,B); Bourr~ massif
foliation
with
by
south
the
River Niger the
para-
underthrusting they are
The Bourr~
faulted
horst was up-
thrown after the passage of the external nappes.
SSW
Basal
External
sandstone nappes -- ~----~~,...
Faro window Cover . ~'3.'-~"~-~ Internaitnappes'~
_. ,=
Paroulochthonous
Eburnean basement
External nappes ~/ .., ;~-
~
'
:
Autochthonoas Gourrna basin
)~¢k
"U w
~'¢/~
"
r~ll~ISl I t S
Guemri
,
A
"
window
TAKAMBA C HABARIA /#lmernal . . . . ..appes It -~.....,,,,:--~.,_--~..~;:.~-.....~ ,...- .
...~ - -. .-. . . .
'
P { e (~
horst 10kin L,,
W
/
I:~O U r f e
untl
NNE Amaluoulaou
Albilic
mlcaschists
"'~
micaschists
10 km I
,
TAOUSSA Sericite-quor tzit es ano- . ;a spers
l
B SW
NE
F, . . . .
--
~. % .
~'Z
C
uni,
sw
"--. .~
s w
50m env.[
. - ' f ~ .
[
Mylonltes
~
Mylonit~c jQspers
~
:..~'..
.
52 Figure 6.23: Caby, 1987.) The
external
greenschist
Sections
nappes
basinal
are
through
Faro
the
composed
of
.
.
D
unit
Gourma
subhorizontal;
formations
NE
they
region.
comprise
quartzites,
(Redrawn
from
mostly
schistose
marble,
siliceous
296
dolomites
and turbiditic
morphism.
The
have
been
Gourma
formations
intensely
(Fig.6.23,C,D).
sediments;
deformed
As evident
and are devoid of high-pressure meta-
(Fig.6.22,B)
to
form
the
which
lie
in
the
aulacogen
para-authochthonous
from these diagrams
(Caby,
1987),
foreland
major
folds
in the external nappe are overturned to the southwest and are superimposed on early stone
pre-nappe
the Gourma al.,
east-west
(Fig.6.22,B)
folds.
The non-metamorphic
Bandiagara
Sand-
of probable Cambrian age, which u n c o n f o r m a b l y o~erlies
sequence,
was
affected
by a later phase of
folding
(Black et
1979).
6.4.4 The Benin-Nigeria Province The Benin-Nigeria province
(Black,
1984; Caby,
tinuation of the Trans-Saharan mobile belt Faso,
southeastern
(Fig.6.24).
Ghana,
Togo,
Benin,
1987)
is the southern con-
(Fig.6.13) Nigeria
into Niger,
and
western
Burkina Cameroon
From the southeastern margin of the West African craton east-
J 8enin- Nigeria
81r,mia.and
~
Taro Formation ~
gtat~itel
Supra©rust.l belts In Oohorrloyon
(p0n~fri©a.)
~'~
Metozoic and younQet cove~ (younger Igneoum r a c k s not Bhawn)
~
Ser.atinebodhl'
Figure 6.24: Geological outline map of the southern part of the Trans-Saharan mobile belt. (Redrawn from Wright et al., 1985.) ward,
three
major
tectonic
These are the Voltain
domains
occupy
the
Benin-Nigeria
province.
foreland basin with flat-lying foreland nappes,
Beninian fold and thrust belt,
the
and the Nigerian high-grade gneiss terrane
297
(Fig.6.25).
In view of their structural
these domains
is discussed
and lithologic
complexity,
each of
separately below. -~
Phanerozoic
~
olfoian
~
A
cover
basin
Parsutochthons( Buem etc.)
~
Atakora
~
Undifferet;ated
noppe
J ~
Charno ck ires ( Pan'~A f riccin )
allocbthonous
1£burnian granulite block ILTTTITn l l l l l H M e t a b a s J c m a s s i f s of t h e ~
Gfanulite
gneisses
Suture
facies
P~:n- A f r i c a n
U n d i f t e r e n t [ a t ed p r o t e r o z oic SChist belts /
9neisses Maior s h e a r ~zone
\<..z:
÷ +
•
+
÷
4.
Ifeware
4./
- - ~
÷
4--
@ 4'
4. 4"
4.
"b
.
.~
4. 4.
~.
4. 4-
÷
4.
4
•
4.
+
/ Cotonou
--
Lag°s
t
GULF
G U
OF
I
t 0 0 km
P
Lomc~
I
N E A
Accra
(P~rautochthons) F
I
0 RELA
ND
NAPPES
N
I ~l~-~
30kin
~
,b
I
G E
\%~ BeniniGn
,F
( M o
H
-~
0 )
4,
4.
+
@
R
I
\%
~'
A
N
Nigerian-
P
R
O
V
I
N C
Cameroon Polycycllc
"I~'Kon di
E
E
baseme.t
Fault
Figure 6.25: A, geological sketch map of the Benin|an belt. B, cross-section; VB, Voltain basin; B, Buem parautochthon; Kp Kand~ phyllites; A, Atakora metaquartzites; K, Lama-Kara leucocratic granite-gneisses; Ka, Kabi~ Klippe; i, Benin|an gneisses; 2, granulite belt; 3, garnet amphibolites; 4, polycyclic gray migmatitic gneisses. (Redrawn from Caby, 1989.)
298 The Volta Basin Stratigraphy.
The Volta
basin
is
a gentle
synclinal
basin
in which
the
oldest deposits are exposed around the margin, with the youngest occupying a roughly central position. Geophysical data suggests that the Volta basin deepens eastward where it is probably up to 10 km thick 1985).
Two
unconformities
each
fill into a lower massive, pango-Bombouaka with
a
basal
Group;
tillite;
(Fig.6.26).
These
across
craton
the
marked
by
a
tillite,
and
an
flyschoid
upper
sequence,
molasse
and
is
laterally
led
quence
to
the
concept
(Fig.6.26,A),
of
a major
BASIN
the
basin
Pendjari the
Supergroup which
equivalent
to
Group,
Obosum
the
Group
extends
folded
Voltain-Buem-Atakora
lithologic
1973).
BENINIAN
"~,~0
BELT
IUIrM FORMATION
1~3
~7
(AI O,%~TIE.AN/ObosumGroup-M01asse F Localunconformity: 600 re,y,
o1.~
and
(Fig.6.25,B). This correlase-
whose depositional history spans the duration of the
Pan-African orogenic cycle (Grant,
VOLTA
the
sequence,
sequences make up the Voltain
thrust eugeosynclinal Buem and Toga Formations tion
separate
cross-bedded arkosic sequence known as the Da-
a middle
margin
(Ako and Wellman,
/ °tIGr°up
~ ~.-.___],._._ Angular unconformity with I-. _ . I glaciation
~
im~,a
,°C°z°, Figure 6.26: Stratigraphy of ton et al., 1980; Grant, 1973.)
the Voltaian.
(Redrawn
from Affa-
299
The the
Dapango-Bombouaka
Eburnean
Taoudeni Group ty,
basement
basin
(Oti
Voltain been
basal
partly
or
phosphate
Supergroup Among
the
siltstones
and
Group,
to
up
shelf.
the
2,000 m
increasingly
with
in Burkina
Obosum molasse
which
accumulated
flooded
Niger
of
the
the
planes.
are
part
margin
craton,
the
sea
eugeosyncline
interval
Group
At Kodjari
(Lucas
and
with a
phosphate which
well
bedded
of of
18
to
the
35 %,
northern
phosphate
and
a
Volta
Of
is
of
the
by the
in
Burkina
(Fig.6.27,A)
epeiric into
sea which the deeper
1986;
Slansky,
of
phosphorite
reserve basin
of
were
material
under
pyrite
slightly
(Trompette,
reducing 1988).
the stratigraphic is barren.
shows in the epizonal
equivalent
But eastwards
units
lower
Tapoa
to
the
an
about
100 million
probably
deposited
with as
southern
of the Buem Formation.
At
Formaaverage t.
The
between
a low influx of indicated
part
of
by
the
of the p h o s p h a t e - b e a r i n g
and basinwards
cont.
Kodjari with
in
massive
60 million
occurs
conditions
In the
at
30 m and a P205
about
is equivalent
in the
and
fine-grained,
933 Ma and 660 Ma on a shallow marine p l a t f o r m
in Ghana,
be-
folds
tillite.
basin
Faso
0 to
reserve
unit
to very
jari Formation
marine
part
Formation
in Burkina
a thickness
a
of
shales,
overlain
et al.,
of
Pendjari
the Pendjari
descended
the Kodjari
the primary
sorted,
25 %;
is
(Fig.6.26,B)
in
presence
are The
deposits
Pan-African
base
asymmetric
Volta
the
fine,
basin
of
phosphate
the
(Fig.6.27,B),
clastic
the
northern
of
fine
about
the
where
tion,
content
In
economic
subsiding
is u n c o n f o r m a b l y
economic
Tapoa
P205
rapidly
and
polyorogenic
Group
places.
NNE-SSW-trending
Group
northern
the
phosphate
and well
phosphorites
at
African
Pendjari a
have which
limestone , and
which begins with a Late Ordovician
Beninian
Pendjari
about
on
Pendjari
bearing
occurs
in
Lower
baryte-bearing
West
the
unconformi-
sequence
tillite,
also
the
Pendjari
1988).
Republic.
of
of
glauconitic
gently
axial
there
along
the
richest
or bedded tent
of
Trompette,
The part
In
and Benin
basin
of
a
argillites
the
accumulated
the Pendjari
the West African
marine
in
the
(locally
which
Group
are
into
(Fig.6.26,B)
Niger,
triad,
lithologies
inclined
Phosphate Deposits.
by
on
I of
the
underlying
fold and thrust belt is approached
Faso
Faso,
of
association
which
deformed
southeasterly
basin
beds
silicified
or
angular
succeeded
are
carbonates
the
slight
conglomeratic
and
thick,
a
Supergroup
the
River
sandstones
As the Beninian
comes
as
principal
with
unconformably
Voltain
into
The
Rokel
with
incised
which
chert,
is known
2 and
follows
slumped
deposits.
rests
The Middle
channels
basal
Group
correlated
1985).
tillites
brecciated
baryte-silexite
1986;
The
as
Voltain
been
in Ghana)
stromatolitic),
calcium
has
et al.,
(Fig.6.26,B).
includes
Lower
erosional
interpreted
belt.
and
(Wright
Formation
showing
or
the
Volta Kod-
there are phosphate
300
D2
TAPOA
D5
N
B "~4o 200
~'160 8D O W,
-40 ~80
I
tS0~ 110J
D?
'
II
-J
3O
Drill holes with those portions of the seq . . . . . . . th P205 content> I•*I,
indic
- - --
~
QUATERNARY
-- F a u l t
Alluvium Weathere d zone : brecclas with kaolinite phosphates
PLIOCENE POST-EOCENE Moussa
b|ack
ver tlcaI Iine.
:-_-----~-
Kwara
Formation
f'[]
an d alum~n~um
Clays;tone and siltstones
~
Basal
I IB
Quartz sandstones and sondstones with phosphorites
UPPERVOLTAIAN SER|£S PhosFhorlte-becu'ing ( Upper Precambr{Qn ) Formation ]~ [ ~ C~yney-silt y F o r m c P ~ i
conglomerate
Grainstone(portiy oollt~c ) phosphor~tes Siltstone snd sandstones phosphorites limestones SHtstones and
alternating with siltstone
block p y r i t i c c|ays~ones
Figure 6.27: Phosphate deposits of the from Lucas et al., 1986; Trompette, 1988.)
Volta
basin.
(Redrawn
301
The Beninian
Fold
Belt
This is the external
zone of the mobile
Dahomeyides,
or the Togo belt
of
belt
the
fold
whereas thrust vince
to
the
contact
the
with
against
deformed
the
1985).
Volta
granulite
to as the
To the west
the rocks
basin
metasediments
the basement
referred
of
or
onto
the
gneisses
the
fold
of
the
craton,
belt
are
Nigerian
in
pro-
(Fig.6.25,B).
and
The Buem Formation outcrops
marks
ward-dipping
the
eastern
largely
and subordinate
similar
the
to
margin
Pendjari
Formation,
ophiolitic
alkaline
calc-alkaline
schistose
and
massive
These
folding
the tectonic
and
thrust
Formations.
(now
In the
of
the
Kirtachi
Quartzite
and
It
some
in
where
lenses
of
P = 16 Kb,
during
the
nappes
of
of the Buem pillowed,
chromite
and
in
has
very
slices include
and
with
cross-cutting
Ghana;
caused
(Fig.6.25,B),
but
by the masked
and
being thrust westward
Pan-African
the
towards
meta-basic
relict
suture
the
of
there, stacking
the
true
the
basaltic
more
Togo nappe
volcanics
about
(or
conglomeratic
internal grade at
units in
(Caby,
and
the
the from
kyanite-staurolite
are now in the of
and
1989).
orthogneisses;
to have been derived
meta-quartzites
of
south
T = 700°C
600 Ma
leucocratic
zone
and
meta-quartzites
metamorphic
at
the Lower
Atacoran
contains
The
rocks w h i c h
internal
Akwapim
crystallized
includes
quartzites
and
belt
which
were
orogeny
foreland were
Beninian
belt
before
the craton.
oceanic
Beninian
of granulitic
of
with
Atakora
Niger)
mas@if,
represent
masses
the
which are considered
These
terms
Beninian
high-pressure
Atakoran
in
the
muscovites
and
Pan-African
Benin.
essentially
with
the
and
of the Atakora
metamorphosed
to
of
kyanite-eclogite
granites; in northern
consists
meta-quartzites.
attained
Eburnean
detached
presence
to the Buem Formation
southwestern
granite-gneisses,
Kabi6
part
marble;
Lama-Kara
The
units
developed
synonymous
ferruginous
Togo
believed
sandstones,
It is lithologically
successions
other
phyllites
Formation
The foreland
is
and
Atakora
severely
of
sequence
external
the
schists
well
are equivalent
Kand~
greenschists)
layers
the
This
which
(Fig.6.26,A).
consists
upon
Formation.
schists
Voltain
are
repetition
mainly
occasionally with
15 km
It is a southwest-
of
in the
The ophiolitic
is about
of the Buem Formation.
Atakora
and mica
one
differs
basalts,
lithologies
sheets
thickness
rocks.
but
serpentinites,
dolerities.
basin.
sequence
shales and mudstones.
of dismembered and
in an area that
of the Volta
unmetamorphosed
siltstones
of
sometimes
(Wright et al.,
thrust
east
Buem Formation.
wide
are
belt,
a
part crust,
fold
of
belt
meta-cumulates
the
Atakora
now found
caught
(Fig.6.25,B). dip
gently
at
in
Togo,
is
up along
the
In the
massif
15 ° to
30 ° E.
302
The base of the massif is marked by garnet-amphibolites which separate the meta-mafic
bodies
probably intruded During
the
subjected partial
from
orogeny to
the
orthogneisses
below.
The
mafic
the base of the crust in an arc-type meta-noritic
deep
gabbros
amphibolite-to
crystallization
into
of
calc-alkaline
granulite-facies
kyanite
eclogite.
bodies
had
tectonic setting. character
conditions
were
before
Blastomylonitic
garnet-
amphibolites with eclogites occur along the sole of the Kabi~ klippe. The klippe was probably thrust westward from the actual suture (Fig.6.25,B). The Nigeria Province
Previously
known
as the
the Nigerian province
Dahomeyan basement
(Caby,
1989)
(e. g. Wright
includes
et al.,
1985),
the Beninian gneisses
in the
internal zone of the Pan-African Benin-Nigeria orogen, (Fig.6.25,B), Archean
and
the
basement
presented a regional Nigerian
province
expanse
allow
and
of
reactivated
Proterozoic
high-grade,
supracrustals.
synthesis and re-interpretation
is
the
reactivated basement. (Fig.6.25,A)
vast
gneisses
southern
east of the suture
continuation
probably
Caby
(1989)
of this region.
of
the
central
The
Hoggar
Thrust and shear zones within the Nigerian province the
subdivision
of
this
region
into
the
Beninian
gneisses and the Nigerian-Cameroon polycyclic basement.
Beninian Gneisses. These are high-grade amphibolite quartzo-feldspathic and magmatitic rocks, often migmatitic
granitoids.
overthrust (Black,
the
Atacora
1984).
elongated
The
These
granulite
Beninian
unit,
which
gneisses belt
mafic
are
composed
facies,
anatectic,
gneisses
is
believed
in
turn
mainly
amphibolites
to
be
their by
kinzigites,
with
and
(Fig.6.25,B)
overthrust of
predominantly
relict
with flatly
equivalent an
eastward
two-pyroxene
granulites
and
assemblages.
These rocks could have attained high-grade conditions in the
kyanite-eclogite
internal zone of the orogen simultaneously with the eclogites of the Kabi~ meta-basic massif. gneisses
which
(Fig.6.25,B) Archean marble overlain
are
basement sequence, by
Whereas the granodioritic to tonalitic hornblende gray
outcrop
in
Benin
lithologically in Nigeria, and
pelitic
similar
east
to
the Badagba
associated gneisses)
Republic
the
are
to
exposed
the
adjoining
quartzites
sub-alkaline
which
of
Kandi
fault
reactivated
(orthoquartzite
syenite along
the
and
orthogneisses Kandi
fault
probably represent a metamorphosed monocyclic Proterozoic cover.
The Migmatite-Gneiss consists
of
granodioritic Proterozoic
Complex of Nigeria.
The basement compplex of Nigeria
predominantly
Archean
to
composition;
cover
tonalitic now
represented
by
polycyclic remnants
variably
gray of
migmatized
gneisses
of
unconformable metasediments
303
which are preserved in synclinorial schist belts; and many syn-tectonic to late-tectonic 1972).
intrusions
(Ajibade
et al.,
The Proterozoic metasediments
Metasediments
of Early Proterozoic
1987;
McCurry,
1976;
have been classified
age,
Oyawoye,
into the Older
and the Younger Metasediments
of
Pan-African age. Reactivated
Archean
gneiss complex,
basement,
includes the migmatite-gneisses the
northeast
Schlag,
(Fig.6.24);
1989)
in
southeastern
quartzo-feldspathic
and migmatites, staurolite rocks
are
referred
to
as
the
migmatite-
of
Obudu,
Nigeria;
and
and the
Oban
(Ekwueme
and
migmatite-gneisses
in
The migmatite gneiss complex is dominated
biotite-hornblende-bearing
gneisses;
and
in which minerals such as garnet, sillimanite,
suggest high-amphibolite confined
It
of the Zinder inlier in Niger Republic in
those
neighbouring Cameroon Republic. by
often
occupies nearly a half of the surface area of Nigeria.
facies metamorphism.
to charnockite
bodies
schists
kyanite and
Granulite-facies
which are generally
associated
with granites of probably igneous origin. In the Ibadan area of southwestern Nigeria migmatite-gneiss the
complex has been
predominant
(Fig.6.28)
studied
(Figs.6.25,6.28)
in detail
rock types are banded gneisses,
representing metamorphosed
where the
(Burke et al.,
1976)
schists and quartzites
shales and
graywackes with inter-
bedded sandstones, whereas the interleaved amphibolite layers are probably metamorphosed
tholeiitic
intercalated
within
supracrustals
which
basalts.
the are
Although
Ibadan
banded
considered
the
schists
gneisses
are
separately
below,
here in order to demonstrate the complex geological of
the
Nigerian
polycyclic
basement
history
complex.
(Burke et al.,
The
Ibadan
1976) which
and part
they
quartzites of
relict
are mentioned
history of this part
banded
gneisses
had
a
in spite of the paucity of
reliable isotopic ages, is also true of most of the Nigerian basement. The with
geological
the
basalts.
history
deposition An
metamorphism
early which
of
of
shale
phase formed
the
the
Ibadan
graywackes of
folding
banded
migmatite-gneiss and
sandstones
and
gneiss,
complex
with
high-amphibolite quartzite,
and
began
interbedded facies
amphibolite
was succeeded by the emplacement of semi-concordant aplite schists in the banded gneiss
at about
2.75 Ga,
during
the Liberian orogeny;
and by the
emplacement of microgranodiorite dykes. A second phase of intense folding followed,
in
which
the
Ibadan
granite
gneiss
was
emplaced
2.20 Ga, during the Eburnean orogeny.
The Ibadan granite gneiss,
granitic
probably originated
body of uniform
composition
fusion of the banded gneiss country rock (Freeth, 1984). Archean gray gneisses are also intruded by
at
through
about
a coarse partial
In Ife (Fig.6.28)
Eburnean orthogneisses similar
304
:::t:::::
J~.
,
:~./,
::
:: ::~.i
:
'
OYO
~ :
""
~ ~
:
"
. . . .
"
i..
'
[
•
•
.
.
.
.
~
!:.".
":~::."
.
2
3
4
!;
::
X beokula
'k~
~
c
x :~
,i i
C,b o n g a n ~
~--/~
(p~u
,~
•
•
i
z
x
x
x
20km
1
x
/ /-
Ifewara
5
6
7
8
9
10
11
I~) Iseyin Oyon bett ( ~ l b o d a n - G n e i s s - Q u a r t z - $ c hist Complex ( ~ ILesho bert
12
Figure 6.28: Sketch map of the basement of SW Nigeria. i, migmatites and gneisses with amphibolite intercalations; 2, gneiss and schist complex; 3, quartzite and schist; 4, pegmatized schist; 5, schist and epidiorite complex; 6, epidiorite (Archean and Eburnean); 7, granite gneiss (Late Early Proterozoic); 8, quartzite; 9, feldspathic quartzite; 10, granites; ii, Late Proterozoic quartz syenite and charnockites; 12, Cretaceous-Recent cover. (Redrawn from Ajibade et al., 1987.) to
the
Ibadan
granite
gneiss
(Fig.6.29,B),
which
have
yielded
ages
of
about 1.82 Ga. The last tectonic event in the Ibadan-Ife area was the PanAfrican event. Older
Supracrustal
Metasediments.
relicts
in
the
Nigerian
basement
are
either intercalated within reworked Archean gneisses as afore-mentioned or are
unconformably
disposed
in
synclinorial
schist
belts.
The
former
category belongs to the so-called Older Metasediments which are again best known in the Ibadan-Ife region of southwestern Nigeria.
In the Ibadan area
these occur as schists and meta-quartzites within the banded gneisses and extend
for about
the Ife area to be older
bounded
by
the
Ifewara
fault schist
Igara-Kabba-Lokoja
1989) to the east.
into the Iseyin
area and eastward into
In the latter area the Older Metasediments appear
Iseyin-lbadan-Ilesha
Proterozoic Caby,
75 km northward,
(Fig.6.28).
system which belt
schist
from belt
seems the
(Ajibade
to
separate
presumably et al.,
this Late 1987;
305
Caby
(1989)
observed
that
the
Ibadan
meta-quartzites
overlain
pelitic schists which were intruded by Mg-rich mafic sills, a monocyclic pebbly
Early
Proterozoic
meta-quartzites
quartzite-schist aluminous
mica
mark
the
assemblage schists
(Eburnean) basal
rests
with
two
sequence.
unconformity
upon Archean micas
and
In
gray
may represent
the
with
Iseyin
which
and
these
aluminous
probable schist, of
metasediments
rhyodacitic
origin.
talc-tremolite
the
Ilesha
geochemistry basalts
and
schist
belt
which
these cut
carries suggests
(Olade the
and
Elueze,
1.8 Ga
schist,
gold
gneisses
talc
in the
and
magmatic
rocks
of
1989; Hubbard,
the
Ife-Ilesha
(Fig.6.29,A),
prominent and
horizons
muscovite-schists
and
marbles in
(Ajibade
et al.,
and biotite-schists) several
belts.
calc-silicates. include
quartzites
Some belts
spessartite-bearing
belts,
with
(represented by
contain
quartzite,
Igneous
rocks,
amphibolites
generally
(originally
fractures
belts.
There
during are
the deformation
also
small
of
the
occurrences
occur
in
in
the
northwestern
synclinorial
schist
gradational,
migmatite-gneiss
faulted complexes.
or
part
belts
characterized by tight to isoclinal with
minor
lavas
or
supracrustal
of
of
in
the
which
country the
acid
but they are
(Fig.6.24).
low-grade
sheared
boundaries
with
a cataclastic
the
They
rocks
folding and steeply dipping
For example,
meta-
1985).
The Younger Metasediments occur in southwestern Nigeria, found
forming
ferruginous
conglomeratic
volcanics of dacite to rhyolitic composition (Wright et al.,
mostly
1987;
serpentinites and other ultramafics which were probably
intruded along deep 1982)
and
these
minor intrusions), (Bafor,
in
quartzites,
constituents
age
These are Late Proterozoic pelites
strike ridges
banded
Eburnean
these
sediments
1975).
Younger Metasediments° phyllites,
probable
The
subcrustal
supracrustals may therefore represent pre-Pan-African m o n o c y c l i c Caby,
chlorite
from
Since
gneisses
of
schist belt
mineralization.
derivation 1978).
granite
Ife are
Southeast of
layered
schists occur
also
by
by
amphibolite
amphibolites
peridotites is
Massive
overlain
rock and pelitic
area
of
are
~uround
sillimanite,
exposed which often show well-preserved sedimentary bedding. Ife
area
the meta-
gneisses.
garnet
by
are
foliation
surrounding
belt,
the
Zungeru
mylonites extends on both sides of the Birnin Gwari schist belt
(Fig.6.30)
and p r o b a b l y originated as a major thrust between the b a s e m e n t
(Kusheriki
Formation) Formations).
and The
the
supracrustals
Birnin Gwari
(Birnin
Formation
schistose mudstone conglomerates.
Gwari,
includes
Kushaka, phyllites,
Ushama
Schist
schists,
and
306
I IFEWARA
/ .~- / ~
I-" _~ ~
~--
/
ssw
---
-~
./
/
)o If
.,-i .2~/_.-~
~"~--
N HE
FAULT
~..~.._--_ _ . I
~,.,~
L
, ~
~, .V~)%xr.~
x
17"xw'
A
IBADAN SHEAR ZONE
E
~,bl/q]
!I
PRESENT DAY
TOPOGRAPHY
B
EGBE pROFILE (AT 8" 13' N)
WEST WR RblSr OG 63~Z16Ma b:O7229 ~
EAST WR RblSr pegm534Z 9Ma b=07678
MarginG~
t
5km
J
F i g u r e 6.29 : Geologic sections through SW Nigerian schist belts. A, Ife region: i, 1.8 Ga orthogneisses; 2, P r o t e r o z o i c sil l i m a n i t e - schists and q u a r t z i t e ; 3, mafic a m p h i b o l i t e s ; 4, A r c h e a n g r a y gneisses. B, Ibadan region: i, P a n - A f r i c a n syenite; 2, P r o t e r o z o i c cover; 3, schists w i th lenses of m a f i c to ultram a f i c s and basal, l o c a l l y d e t a c h e d quartzites; 4, A r c h e a n gray gneisses. C, Egbe area. (Redrawn from Caby, 1989; Matheis, 1987.) A
major
belt
western Nigeria
of
Younger
Metasediments
through K a b b a and Okene
extends
(Annor,1983)
from
Igarra
to L o k o j a
in
south-
(Fig.6.24).
307
The Igarra sequence
includes
semi-pelitic
and
across
matrix
(Caby,
1989; gneiss,
marbles
the
from
relict
features
attesting
grades
underlying schists
sedimentary
in arenites, to
laterally
crystalline 1976).
pegmatite,
biotite
cross-bedding all
abundant
Odeyemi,
granodiorite, hornfelsic
basal polygenetic The
units.
The
such
as
and
origin.
which originated
This
1 m from
is
and
overlain
concretions,
parallel-laminated
turbiditic
up to
derived
calc-silicates
meta-mixtite
channels
layers with a
cobbles, were
calc-silicate
and cross-cutting
a probable
angular cobbles
amphibolite,
containing
into migmatites
meta-mixtite
by and
meta-pellites,
with pebbly infill, turbiditic
sequence
from the emplacement
of
voluminous sheets of late tectonic granitoids.
[]I []3 []s []7 []9 [], [], []8 6,° Figure 6.30: Geological map of Zungeru area. i, migmatites and gneisses; 2, kyanite and sillimanite-bearing quartzites; 3, Zungeru mylonites; 4, Kushaka Formation; 4, Birnin Gwari Formation; 6, Ushama Schist Formation; 7, foliated tonalites and granodiorites; 8, Late Pan-African granites; 9, Cretaceous-Recent. (Redrawn from Ajibade et al., 1987.) Southeast rian basement,
of Kabba
in the northeastern
there are metamorphosed
part of the southwestern
banded
ironstones,
Nige-
rich in magnet-
308
ire and hematite. quarried.
Near Kabba and Jakura dolomitic marble occurs
Numerous
thin sheets
and
lenses
of calc-silicates
which
is
probably re-
present m e t a m o r p h o s e d impure limestones. Among
the
northwestern
best part
canic-sedimentary clinally
folded
examples
of
preserved belts
into
these
250 m
thick,
greenschist (Utke,
basement
to
and
with
rounded
quartz,
fine-grained
The Anka belts.
grabens
pre-tectonic with
They
are
to angular volcanics
(Black,
and
in
the
are
iso-
belts
comprise cover
intruded
are
poly-
of
low-
by a suite
granodiorites
interbedded with are
(Holt
and
psammites.
laterally
clasts
1984).
of
and
granite,
pelitic
At
volcano-sedimentary
volcanic
facies
and Maru
an infolded
cover and
those
eleven meta-vol-
amphibolite
of
et al.,
material
sedimentary
Bunkasau
pile
of
clasts;
as
set
in
a
belt
volcanics,
well
150 imper-
as
1978),
there
coarse
high-K
vein
psammitic
(Holt et al.,
in the Anka
acid
Up to
vertically
orthoquartzite,
matrix. They were deposited rapidly in shallow basins probably
are
(Fig.6.31,A) pelites and semi-pelites predominate,
meta-conglomerates
sistent
Nigeria
Here about
1987).
granites
meta-conglomerates the
in
sequence with
both basement
late-tectonic
1978). In the Anka belt with polymict
belts
(Fig.6.24).
meta-volcanic-sedimentary
grade metasediments; to
in
basement
cyclic migmatite-gneiss syn-tectonic
schist
of the country
is a
clastics
calc-alkaline
volcanics. Holt et al. metasediments locally
are
predominantly
interbedded
ironstones near
(1978) have shown that in the Maru belt
are
Maru
micaceous
intercalated
township
where
pelitic
with
they
and
psammites pelites
are
up
to
(Fig.6.31,B),
semi-pelitic
and
rocks
orthoquartzites.
and
semi-pelites
2 m
thick
with
the with
Banded
(Fig.6.31,B) characteristic
alternations of quartz-rich and iron oxide-rich laminae similar to typical banded
iron-formations.
These
lithologic
features
point
to
a
basin
of
quiet water sedimentation in the Maru belt where the presence of pyrite in some pelites
suggests deposition under anoxic
predominate
throughout
the
sequence
conditions;
implying
free
but iron oxides circulation
of
oxygenated waters in most places. Basalts of ocean floor affinity occur in the Maru belt the Anka
Kanoma belts
African
(Ogeze,
pluton.
age,
(Holt
1977), as well as a sodic hornblende syenite pluton,
Although et al.,
like
similar
Kibaran
1978),
age
both
widely
was
belts
developed
suggested have
been
for
the
assigned
supracrustals
Maru to
(Black,
and Pan-
1984)
considered here. Utke
(1987)
regarded
volcano-sedimentary
the
assemblages
schist
belts
of
with
tensional
NW and
Nigeria
as
typical
compressional-related
rocks. These rocks display the influence of paleo-rift systems.
309
~
:
_ __
D;!~''
lI+1ig rnoilt ies, G neiss_.~ ~
F'hyHite,Metosiltst one Schist
~.~o,,..,..m,
l
d feld.thic
......
SChlS,
Bonded Ironstone
17~J
--
~Acid-BoslcPegmoliteComplex ~lUItra-
M(dics ondMofics
~o,,,,
30 km
-
-
I I
~
'
.
"
;
7
'
T
f
~
.+ .+ .{.+U.,~ • :,..''/ ;""";~'~'81 :'f.-L J +.:~...
: ~ +
~ ',~_:~=-_Y+
..::.,. ~; .:
.
~
~ ~ :
+,
:4/. ~ ' ~ + +,,/~l ~ n ~ o i ~ . e~, ~.~,
.<"-~ +
£
'
-- - . . . .
~%l~':)).7::jl!:#l\"*/i
:-;,::;.:,:.:¢:::: P' + .,'::~,
~
-
X
.~ ~ "~
,..!:~i.i::iL~.,~~,7"~j)
"- - - - -
. . . . . . . . . . . . . . :
~!~:" --
. . . .
- _ _ _
I ~ ~ ~ : I~ ~ . . . . . .
j
. . . . .
y
+ > ~: .. -,.
+ 'J/f:~';2~?" + &Y..";,'.£L<:~~, ~"~.."¢i;:'.~ ;;
\
+ + )
"~l~ii!~'..(~.
+
+
+/"I
'...... F : ? + I:~/~
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GEOLOGICAL MAP OFKANOHA DISTRICT
,; ,'
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.o,n..
,
~
Granite
i/I t
I ~
/#
/' / /.
t
-",'~t/
/ll
o
t/,
o , I
/
~I!I
' /
/
I
;
-,--
Soundory(opprox,)
1,76 ~
J
Ill
I
II
,I 72k
l/
Sye.,,e
~.e,o.oo,..n,s
~
t l,
!1 Z~ ,,,. /,I I z
i
,i
tl
"~ t I 'DIS' S/? // " .< S/~ /ti 'lit'~o ....L ' " I ?'~"J'l.~'/7+~+
t
Foult ,
Crenuicition cleovoge and Kink - bonds.
-
-
~
\
~'\
:
"ig! i
t
~0 t0
'tX,~.~
I /
, !901 I : i 7 \ \ ~
.~.:~t;~.:, 17 ~ ~
,,'~ ,,, ,.,,,1\~.. I I
~
l,,, I '1
',
"*/,
i , v~ ;I '1 Ill
,l I
,
;,
I
I
.,;I
/
I
+
ii
/
/ "+ill
/:
7
/
/~,,
i
XH'II
I
~!k=
B
Figure 6.31: Sketch maps of Anka and Maru Nigeria. (Redrawn from Holt e t a ! . , 1978.)
schist
belts
in
NW
310
Structurally, isoclinal
the
folds,
closures
are
Younger
often
defined
obscured
by
foliation and by abundant trend
in
places
the
schist
earlier
noticeable Grant
Metasediments by
minor is
structures.
et al.,
(1973) a single
Archean-Lower
Proterozoic
and
ridges.
to
NNE-SSW, with
axial
dips
to
to Fold
plane
structural
although
shallow
according
tectono-thermal
their
tight
The characteristic
However,
Pan-African
parallel
or
structures
1985).
major
quartzite
shearing
North-South
east-west-trending
(Wright
prominent
intense
belts
include
in are
Caby
some still
(1989)
and
event affected both the
supracrustal
cover
in
the
Nigerian
province. During
the
Pan-African
amphibolite-facies in the Ife
basement
huge
gneisses
gneisses
nappes
metasediments
orogeny
into
of
whereas
steep,
syn-metamorphic
in other
areas
Proterozoic
cover
structural
observed
that
the
(Caby,
The
Nigerian
penecontemporaneous
zones was
In some
formed at deformed
separated latter
schist
wrenching
near
Proterozoic
basement
1989).
deep
foliation
(Fig.6.29,A).
(Fig.6.29,B)
of
the
example,
and
shear
Archean
domes
disposition
For
basement
towards the northeast
in
recumbent
metasediments.
Archean
north-south-trending
characteristic (1989)
the
were transported
elongated
synforms
overlying
involving
temperatures,
tectonics
generated high-temperature and
places north-south-trending, high
horizontal
and
by
is
the
belts.
Caby
northeasterly
thrusting near Ife is similar to that of the central Hoggar which occurred between
629 Ma
lower
temperatures
and
ultramylonites,
614 Ma.
A later phase
produced
for example
green
along
the
of
renewed
dextral
biotite-bearing Ifewara
fault
shearing
at
retrogressive
system
(Fig.6.29,A).
There was brittle reactivation during the Phanerozoic. Granitoids.
One
of
the
implications
tectonic model of eastward
of
the
widely
the southern part of the Trans-Saharan mobile belt 1973; Wright et al., Nigerian
province
diorites
and
in
(Fig.6.32).
syenites
southwestern
Nigeria
facies these
distinguish Shield
is
500 Ma. including
In
Syn-tectonic
were
intruded
supracrustals
Nigeria cuts
(e. g. Burke and Dewey,
a
Pan-African
into
the
late-tectonic
both
syenite are
by
abundant
northwestern
dacites
and
Pan-African
Nigeria
shonshonites
there
migmatite-gneiss locally
cover
termed
the
are
Older
Granites
In to
The central Nigerian dated
volcano-detrital age
developed
(Fig.6.29,B).
granitoids
of Cambrian
granites,
For example at Shaki
with
Proterozoic
intrusives
the
(Fig.6.29,A,B).
potassic
through
to
them from the Jurassic Younger Granites. occupied
Pan-African collision in
1985) is that it generated abundant granitoids in the
complexes and overlying granulite
accepted
subduction and continent-continent
in the Anka
at
700
-
materials and Maru
311
belts which
Black
(1984) interpreted as probably representing Pan-African
molassic grabens.
W
E
KEY BuemFormation and
~ ~
~
Dahomeyan basement
Middle Volfczian Togo Formation and ~ Lower Voltaian Older Granites(Including charnockifes etc.) ~
~
West African craton upper mantle
suprncrustctl belts)
Figure 6.32: Plate tectonic explanation of the basement geology of the Beninian belt. (Redrawn from Wright et al., 1985.) The Older Granites range in size from small stocks
to
large
elongate
batholiths.
Some
porphyritic
textures.
intrusives,
for
Jurassic
batholiths
have
Some
example
Younger
concordant
of
at
the
of
bearing
the
country
variety
post-tectonic
where
known
basalt
as and
exhibit
at
Bauchi
also
there
occur
is
(Oyawoye,
dolerite
granitic
ring-structures
east of Ibadan,
bauchite
granodioritic
composition
post-tectonic
Charnockites
Granites in southwestern Nigeria, part
adamellitic
Toro,
Granites.
subcircular cross-cutting
predominantly
dykes
coarsely
and
dioritic
similar
among
to
the
the
Older
and in the northeastern a
distinctive
1972).
also
and
fayalite-
Pan-African
occur
in
late
the
to
Nigerian
basement. The Cameroon
Basement
The Pan-African belt in Cameroon , although belonging to the Trans-Saharan belt, marks Ntem
the transition to the Zaire craton.
complex
into the
Zaire
Pan-African Archean,
of
southern craton.
mobile
the
Nyong
(Fig.6.33)
The rest of Cameroon
belt
(Cahen
gneisses
"intermediate"
Precambrian
complex;
syntectonic
and
Cameroon
of
groups
et al., pyroxene
granites
lower Paleozoic rocks as well.
and
southward
is considered
comprising
gneisses the
part is the
continues
Republic
1984),
including
generations of post-tectonic granites;
The cratonic which
and
Dja
migmatites.
reactivated
migmatites;
Group
as a
and
There
the
mixtite are
two
and in northern Cameroon there are
312
200 km
[--]I
'
~'
~-~ %.~
'
~4
~i~o~d
rmrm s
Fc'='l 7
Garoup ~ + ~
l'X-;1 8
L
÷-
,'..
C
-
•
- ~.o+P + .-.,+ - x ~
~ 7+_+ + ~7++ + - -÷~ OoUsl_ak-A. Z
+ ÷+
•
Figure 6.33: Geological map of Cameroon. i, Phanerozoic cover and "granite ultimes"; 2, Lower Dja Group; 3, "Intermediate" groups; 4, formations of doubtful age; 5, Ntem Complex; 6, part of 5 reworked in the Pan-African; 7, migmatites; 8, granites. (Redrawn from Cahen et al., 1984.) Although
the so-called
been
in
have
weakly rocks
placed
metamorphosed of
sedimentary
generation Bengbis
of
Pan-African
Poli-Matoua
metamorphosed,
and
the
the
former
by
was
less
the
Sembe-Ouesso
involved
being
abundant
in
the
belt
Nola Group
in the Central African
1984).
lower Dja Group
The
about are
overlain
Congo
and
In spite
Preca m b r i a n
of age uncertainties
with
the
terrane
Nigerian in
are
Group
which
comprise
intruded The
correlates to
by the first
sedimentary lower
Dja The
Group
a
south,
to the n o r t h e a s t complex
and
lower
with
the
extends
the
Ayos-Mbalmayo-
which
weakly
which
is
Dja
Group
similar
unit,
and with
the
(Cahen et al., is equivalent
belt in the south.
and the lack of a d e l i b e r a t e
province,
Cameroon
these
schists.
a mixtite
to the lower mixtite of the West Congolian
correlate
by the
they
Among
520 Ma.
also
Republic
Republic
includes
Lom
that were
at
Group
have not been dated,
belt.
the
sericite-chlorite
in the mobile Group
and
origin,
granites
Yokadouma
groups"
mobile
Group
and volcanic
post-tectonic
Group
characterized
"intermediate
the
there across
is
no
the
doubt Nigerian
attempt that
to the
frontier
313
(Fig.6.34) occur.
into
The
southeastern
Pan-African
the
low-grade
at
600 - 565 Ma;
Pa!eozoic
history
metasediments; the
granites
of the M a n g b a i
[~
Nigeria where
562 Ma
before
•
~/
Najor shear zone
~/+{/
I~'
LateProterozoic suture
/
Major thrust
+
+
+
+
+
+
•
÷
+;
.
÷
..
• ,i,
+
•
+
+
÷
.
÷
.
÷
÷ ÷
+ +
÷
°
<
+ +
+
÷
. Z4!
". "
+ ,i,
÷
+ ,+ / / ,
÷
+
+
+
.,
+
,,,.
+ ÷,~ l'
+S
,i
./
÷h ÷ +
/ +
÷
'
+ +
÷
+ ÷
+ +
+
÷
+
,
.
÷
+
1 +I "
.._.:..
l
.I+
EASTERNHOGGAR
+
• ~i
+
1984).
~/+
+
+
•
.....
+,
deposition
. ÷
+
+
+
°
+I
. . . .
++ %
and
•
;+,++.-
•
+
+
+
+
the
+ +!
f •
+
early
and
.
+,
,~+.s ~
+
Precambrian
respectively;
CENTRAL .HOGGAR
",4~
I
of
event
÷ ÷
Reworked Archean and Proterozoic c o v e r
.
accumulation
a tectono-thermal
+ + '*~
' ~
÷
late
the
and m i g m a t i t e s
+ *
Western Hoggar Upper Proterozoic
•
gneisses
involved during
(Cahen et al.,
÷ \
noppes
+
very
516 Ma
400 Ma
÷ PIUc~eJ ~
of
and
Phenerozoic
~Foreland
Cameroon
migmatization
intrusion
at
Group
of
similar
\ IOOOKm
F i g u r e 6.34: C o n t i n u a t i o n of the T r a n s - S a h a r a n NE Brazil. ( R e d r a w n f r o m Caby, 1989.)
mobile
belt
into
314
Trans-Atlantic
Connections
Trans-Atlantic Precambrian geochronological correlations 1968), and geological,
and structural
(e. g. Caby,
(Hurley and Rand,
1989; Shackleton,
1976)
correlations
between West Africa and Brazil have shown that prior to the
break-up
Pangea
of
II
(Fig.6.35)
and
the
opening
of
the
South Atlantic
Ocean, the West African craton and the Trans-Saharan mobile belt continued southward into Brazil. Luis
craton whereas
province
The cratonic remnant in Brazil is known as the S~o
the Pan-African belt
is referred to as the Borborema
(Figs.6.34,6.35).
~iolites
River Zone a
~o
'% 5
Z~ire Kalahad
francisco
AD
Acto]~
A W
Al Amar Jab~! al Wask
Figure 6.35: Probable Pan-African ophiolites sutures on pre-Mesozoic drift reconstruction. Shackleton, 1976.) The
Trans-Saharan
continuity with
the
mobile
Brazilian
belt
shows
Borborema
close province,
"
.Kenya.
and collision (Redrawn from
geologic
and
suggesting
structural that
during
315
the
Late
Proterozoic
orogenic
belt
Pan-A f r i c a n Borborema Caby
tectono-thermal
(Caby,
1989).
orogeny
in
and Nigerian
(1989)
Borborema
provinces
which
include
with
minor
the are
of
aluminous
meta-quartzites
Jucurutu
intruded
by
2.0
disconformable, turbiditic
sequence
belt of southwestern Like for
in
the
both
regions
pervasive, of
the
Archean
in
green s c h i s t
continues
system
Brasiliano
However, Pharusian suture
rocks
similar
are
supracrustals as
the
to the
with
Serido-
Igarra
schist
evidence
Rather, orogeny
in are
with m a j o r nappes
under
high
temperature
muscovite-free
granulite
and
was
charnockite
at
similarities
the
with
lower
areas
that
of
where
they
of
Pan-African are
shear
nearly
all
zones
also
with horizontal
The Sobral
underlain
lineations
by that
fault or the Trans-Brasiliano
into the Kandi fault and the 4050 ' fault of thus
dextral
generated
in
representing
lithospheric
the
final
(Caby, suture
aulacogen
of
continuous
the
movement.
Pan-African-
collision,
and
was
1989). zone
is evident
and the Beninian
in Borborema
a
strike-slip
stages
continent-continent
a Pan-African
seen
synchronous
as
pattern
belt,
during
been
convincing orogeny.
metamorphism
and ultramylonites
mobile
whereas
no
grade.
northward
was
well
lithological
branched
lineament
has
kyanite-bearing,
as
later in the Phanerozoic
have
of
synformal,
Pan-African-Brasiliano
belts
of
belt and the Gourma
zones
Borborema
are
supracrustals
known
supracrustals,
schist
(Fig.6.34)
orogeny
react i v a t e d
there
consisting
Younger
(Kibaran)
foliation,
displacements.
Trans-Saharan
fault
the
contain
spite
mylonites
transcontinental This
in
and
horizontal
lineament, the
linear
Borborema
retrogressive
these
flysch-type
Borborema
Proterozoic
Proterozoic
sinuous in
suggest
of
regions
facies
the
migmatites
regions
granites.
Proterozoic
recumbent
higher metamorphic
The occurs
and
some
and
and
known as the Cera Group and
are very
province,
a Middle
effects
Both
In
attained
the
rocks
assemblages belts.
of
and produced
conditions.
evolution
Nigeria.
Nigerian
existence
of the
and granodiorites,
both
concretions,
in Borborema,
one
high-grade
Nigerian
gneisses
in Nigeria,
Proterozoic
Ca-Mg-Fe
the
metasediments
anorogenic
Late
The
In
as
equivalent
of their crustal
gray
schists,
As
behaved
of
trondjhemites
monocyclic
and pelitic
and
is the
similarities
greenstones.
1.80 Ga
probably
event
by mostly
in Brazil.
-
facies
Cachoeirinha
and
regions
correlations
tonalites,
Proterozoic
Formation
his
both
and comparison
underlain
supracrustals Early
In
following
reworked Archean
remnants
the
The Brasiliano
Brazil.
provinces
stressed
events
(Caby,
are foreland basins which are equivalent
1989).
in
the western
belt,
no possible
Also
missing
to the V o l t a
basin.
from
316
Mineral Deposits in the Trans-Saharan Belt In contrast
to
the
West
not a rich mineral which
could have d e p l e t e d
reactivation,
could
(Wright et al.,
abundance
minerals
produced
Also,
relatively
some scope
of the rural d e v e l o p m e n t
Gold.
Gold
about
12 t. produced
has
mechanized
been
mining
of
primary
Woakes
(1989)
where
is found
of
gold
quartz,
the
ranging
in
several steep
in Nigeria,
hundred dips
and
veins
(1985)
Anka,
Ilesha
in veins,
from The
largest
and
deposits
elements rocks
the variety
and
small-scale
industrial
mining as part
belts
cm
en
with
rise and
in the
schist
lenses,
reefs
fractures Placer
include
basement
belts and
few m,
in
pinch-and-swell
and
shear
gold
was
has been
both
of
are
to
often
(Elueze,
probably
up
structures,
They
zones
the
are small
lengths
arrangements.
rocks.
(Fig.6.24)
gold veins
with
in the
similar bodies
rocks,
Primary
have
echelon
1913 with
investment
that gold mining
Egbe
to a
since
renewed
should veins
and
sometimes
or
cross-cutting.
1986);
con-centrated
pods and veinlets with low gold content.
belt gold is associated
deposit
certain
However,
quartz-tourmaline
several
associated
In the Beninian
crustal
belt.
observed
stringers,
veins
mostly from d i s s e m i n a t e d
The
in quartz
et al.
parallel
are
in
schist
in the migmatite-gneisses.
m.
and
although
is
nature
and ultramafic
metallic
production
occurring
Maru,
thickness
concordant some
and
in the mobile
belt
mafic
1989). With the recent
quartz-feldspar
supracrustals
enrichment
from the Nigerian
(Woakes,
and Wright
to
mineralization,
local
minor
mobile
of its polycyclic
drive in debt-ridden West African nations.
gold
restricted
Trans-Saharan
for labour-intensive,
mined
gold-mining
the
on account
unlike the craton,
insignificant
of
offer
craton
probably
it of economic
have
1985).
are v o l u m e t r i c a l l y and
African
province,
is
at
Pemba
with Pan-African
in northern
Benin
where
thrust zones.
mining
has
been
Beninian
fold
carried out.
Chromite. belt.
This
Other
northwestern along sources
a
occurs
in
Nigeria
major
a
shear
of asbestos,
are
linear zone
through schist
defined
ENE-WSW
the W a m b a - J e m a a belt,
to
pegmatites
were
conformably
into
serpentinite in
the
belt
the
with
to
serpentinite These
in
area
Ilesha
host
400 km long,
are
(Elueze,
stretching
(just south of Jos area
in
between rocks.
southwest 580 Ma
east,
bodies
bodies
and nickel
the
the
Tin-tantalum-niobium-bearing
belt
emplaced their
bodies
basement
(Fig.6.24).
talc, magnesite
Pegmatite Mineralization. a well
small
occurrences
NE-SW
potential
pegmatites from
Nigeria
M i n e r al i z a t i o n
trends
also
in
1982).
Plateau)
and
where
the
occur in
Jos
through
Plateau the Egbe
(Fig.6.24).
These
530 Ma,
more
or
in these
pegmatites
less was
317
apparently
influenced
pegmatites
in
belts,
are
emplaced 1987).
enriched
into in
Nigeria, annual
rich
which
about
lithology, into
relative Plateau
Sn,
Nb,
tin
provide
Small
are from pegmatites,
Li,
with
averages
rock
emplaced
tantalum in
pegmatites
production.
host
on the Jos
association
Mineralized
the
Nigeria
in
gneisses
Pegmatites
massif,
by
southwest
to
are
Ta,
which
nearly
niobium,
Cs
also
5
t.
per
of
Nigerian
case
in
annum,
but
tin
in
the
Oban
times.
production
not
and
those
(Matheis,
colonial
tantalum
the
schist
whereas
in tin
occur
mined the
amounts
which
enriched
was all
in
meta-sedimentary
exceeding
niobium
in
20 t.
production
the bulk of these coming from the Younger Granites of
the Jos Plateau. Nigerian pronounced rich
mineralized
mineralization
pegmatites varying
pegmatites
pinch-and-swell are
in
the
dominated
amounts
Mineralization
of
quartz
generally
hydrothermal
solutions
feldspars
quartz-mica
or
by
that
commonly
showing
swelling.
oligoclase,
is
are
structures,
The
and
microline, with
secondary
Apart
from
carry
apatite,
tourmaline pointed emplaced the
a
host
monazite, and
out
accessory Li-rich
minerals, mica
although
Nigerian
pegmatites
and
reactivation
there
is no
the Older Granites
including
(Zn-rich
probably
caused
greater
Na-rich of
the
columbite
scheelite,
black
and
spinel).
and
Matheis
between Rather,
re-cycling
and
from which
beryl,
pink-green (1987)
pegmatites
during the
link
(Fig.6.29,C).
in the basement and supracrustals
by
tourmaline.
late-stage
rare-metal-bearing genetic
and
barren
the mineralized pegmatites
simultaneously with the Older Granites event,
or
albitization
lepidolite,
gem-quality blue gahnite
that
Pan-African
elements
of the
with
accompanied
and
cassiterite
tantalite which are the main economic minerals, also
often
muscovite
associated produced
bodies
albitization
non-mineralized
biotite,
greisens.
massive
intense
were
last phase of
the mineralized repeated
crustal
concentration the
of
late-stage Na-
rich hydrothermal fluids derived their enrichment. Uranium.
This
occurs
northwestern Cameroon
in the
Pan-African
granitoids
of
the
Poli
area
(Fig.6.33). There are also uranium prospects
in
in the
Tessalit region of western Adrar des Iforas in Mali. Iron
Ore.
Iron
ore
is of economic
importance
in some
Pan-African
schist
belts in w e s t e r n Nigeria and in the Kribi region of southern Cameroon. the latter region the grades are about there the
is
Buem
secondary and
hematite
Atakora
40 % Fe, with
enrichment.
Formations
in Togo;
Banded near
In
70 % attained where
ironstones Bandjeli
occur
in
90 m i l l i o n t.
also
of
ore are known, whereas around Dako m a g n e t i t e - h e m a t i t e ores are associated with quartzite and mica schist with about 40 - 45 % Fe.
318
In with
the
Maru
schist
magnetite,
phyllites
and
Itakpe
ores
occur
Hill,
an
where
with
iron
ridges
ore
southerly
dip
and
are
steel complex nearby.
least
Minerals.
favourable
40 % Fe.
the
deposit
prospects
1978).
southeast
of Kabba
The
Itakpe
The
ores
at
35 - 50 % Fe.
commercial
main
products
kyanite
from
and
has
the
massive
minerals. It
in southwestern
kaolin,
and at
south-east,
are
ore
purer
basement
Hill
to the
the
t.
and
among
as
at Jakura
for
Richer
and closing
rocks
amphibolites,
interbedded
started.
(Olade,
150 million
Among
are the marble
also
has
WNW-ESE
hematite
reserves
belts
to
near Okene,
mining
Ajeok u t a
Industrial
up
fold oriented
magnetite at
Nigeria quartz-hematite
garnet-grunerite-schist,
contain
as prominent
overall
banded,
with
in the Older Metasediments
form of an isoclinal with
in northwestern
quartzite,
itabirite-type gneisses,
belt
associated
will
the
and
Proven feed
the
Pan-African
Nigeria.
There are
sillimanite
in
this
region.
6.5 South Atlantic Mobile Belts The
West
out
along
of
Congolian, the
3,500 km.
the
West
and
and
belts
Brazil
opening
(Porada,
attests
and
1989;
of
the
strata
suggests shield,
Porada
(1989)
Gondwana. South sent
The
Atlantic
closing
of
a Late
Ocean
South
Atlantic crustal
Atlantic
opened
tic
have
northern
of
which
is
weakness.
progressively followed
a
cratonic
from
Brazilian
continuity Africa
Atlantic
in
this
the
in
and
by
of and
Ocean
case the
collision. was
along
same
the
path,
for
it
(Fig.6.1).
the
of
which
proto-
the
pre-
zone the
proto-South
remained
The
western
the
persistent
during
south to north,
shown by
in
Of
development
Mesozoic
of a contiAs
located
evolution
subsequent
the
in the lower fully marine
from the rifting
and South America remained bridge
the
rocks
succeeded
spreading, and
aligned
similar
Gondwana,
h i s t o r y which relates to
proto-South
are
originated
Gondwana
As
western
The
strung
a distance
southwestern
and m a g m a t i c
orogens
shield
west
foreshadowed
part where Africa
Francisco-Zaire
of
all
1981).
by sea-floor
original
rifting
are over
with
(Fig.6.1).
belts
sedimentary
orogens
of
continuous
Proterozoic
and Cordani,
that these followed the
are
to their common geodynamic
Torquato
Pan-A f r i c a n
may
belts
respectively
Pan-African-Brasiliano
nental
orogens
(Fig.6.1),
reconstruction
orogenic
The fact that rift-related part
Saldanhia
of Africa
drift
Damara
Ribeira
and
margin
a pre-Mesozoic
Pan-African-Brasiliano
eastern the
On
Gariep
Atlantic
Congolian
Manti q u e i r a the
Damara,
South
closed
of
South Atlanin
the
u n i t e d along the S~o proto-South
Atlantic
319
evolved through a complete Wilson Cycle leaving behind a chain of orogens the West Congolian, however,
belonged
Damara, more
to
Gariep, and Saldanhia belts. a
southern
ocean,
stretched eastward from Antarctica to Australia
the
The latter belt,
Admaster
Ocean
(Hartnady et al.,
which
1985).
6.5.1 The West Congolian Orogen Lithostratigraphy The West Congolian mobile belt extends for over 1,300 km from Gabon southwards through
Congo,
Zaire, to northern Angola
(Fig.6.36).
It contains
GABON
CONGO
f
BRAZZAV~LLE
ESQU]SSE GEOLOGIGUEDE L'OUEST CONGOLIEN AU BAS-ZAIRE ET DANS LES REGIONS VOISINES L~gende AHUViOnS COtJVERTURE MESOZO'~OUEET ¢ENOZOIQUE
Groupe du
4~
\Iz
HGu|-$httogngo
Lave= [nterittatlfide dan$ [o
~,
Inf4tieure
sills dons
rnixtite te S o n s i k w a
Groupe de Icz Sanlikwo(so~ groupe'supe'rieur) Orouge de la Sanlikwo~ou$ groupe infc(rieur) .~I~__~ Mayurnbien G r a n i t e s post - Mayumbian ~o[~ Terrcsinsonte'-Mayurnbien de la zone mz~clictne de ['oroge'ne
ANGO
8(}kin
Figure 6.36: Geological map (Redrawn from Cahen, 1978.) three structural zones east
to
west,
the
of
the
West
(Cahen et al., 1984; Porada,
external
zone
with
Congolian
orogen.
1989), comprising from
subhorizontal
strata;
the
median
320
folded zone; and the internal zone to the west which, except for intrusive rocks,
consists
external
and
sequence
of
of
pre-West
median
Congolian
zones
low-grade
contain
basement
the
metasediments.
rocks
West
Since
(Fig.6.37,A).
Congolian
older
The
Supergroup,
rocks
considered
a are
exposed in the internal zone, it is therefore appropriate to start with a description of that zone. The
older
Zadinian
rocks
in
Supergroups
orogen
the
Middle
proterozoic and
internal
(Fig.6.38).
Mayumbian
Supergroup
the
Throughout
Supergroup
equivalents
and
in some places The
volcanic
and
most
et al.,
(Vellutini
1983).
the of
Mayumbian
volcano-sedimentary
Mayumbian the
1984).
and
West
overlies
meta-volcanics,
(Cahen
the Zadinian are, however, et al.,
are
unconformably
metasediments
its
zone
the
Congolian
post-Eburnean-
and The
the
Zadinian
upper
parts
of
transitional with the Mayumbian comprises
sequence.
In
a
Bas
lower
predominantly
Zaire
the
volcanic
facies of the Mayumbian consists of a 3,000-m thick sequence of rhyolites with dacites, hypabyssal granites and microgranite. The age of the Zadinian-Mayumbian succession falls between that of the underlying Kimezian Supergroup which was affected by the Eburnean orogeny at about old.
2.0 Ga, and the age of the Mativa granite which is about 1.02 Ga
Various
origin
and
et al.,
plate collision scenarios deformation
1983;
Franssen
the Zadinian-Mayumbian continental
rift
of
the
have been advanced
Zadinian-Mayumbian
and Andre,
1988).
But
sequence represents
which was
deformed
and
African orogeny in the West Congolian
according
thrust
now
schists
or mixtites,
interpreted
from the
Haut
as mud Shiloango
eastward
during
the
Pan-
from base up, into
the Schisto-Calcaire Group,
considered
(Porada,
Group,
(1989)
(Schisto-Gresseux Group). Two horizons of
formerly
flows
Porada
(Fig.6.37,B).
the Haut Shiloango Group,
and the Mpioka and Inkisi Groups pebbly
to
(Vellutini
the infilling of a Kibaran-age
The West Congolian Supergroup has been subdivided, the Sansikwa Group,
to explain the
sequence
and
1989)
the
as glacial
separate
latter
the
from the
deposits, Sansikwa
but
Group
Schisto-Calcaire
Group (Fig.6.38). Because energy the that
of
its elongate
debris
lower
flow deposits
part
of
in
Congolian
rift.
rift
a
in
the
Congolian external
fault-bounded The
West
structure
of
geometry,
(mixtites),
the West
sedimentation
initiated earlier
basin
Kibaran
the occurrence
beds
Supergroup,
and
continental
Congolian
red
and
rift age
and basic Porada
median
zones
rift,
which
he
bounded
to
was
within
of
which
volcanics
(1989) the the
in
surmised
orogen
termed the
of high-
was
the West
west
by
volcanic
an and
321
sedimentary
rocks
of
the
Zadinian
and
Mayumbian
Supergroups
accumulated
before being intruded by anorogenic end-Mayumbian granites of Kibaran age. By the
onset of West
Congolian
rifting and deposition,
the
Kibaran
rift
was already a positive feature supplying the basal coarse clastics of the Sansikwa Group.
I~
~"<'!' ~ "" :
i
: :'
A
:. ":?,' :•
7~
%"" /
"" /
~
10 k__m INTERNAL ZONE
MEDIAN ZONE
EXTERNAL ZONE
B 50km EBURN[AN
ZAD1N|AN MAYUMBIAN
Z. MAYUMB,
20kin. Figure 6.37: A, Tectonic map of the West Congolian orogen, i, Paleozoic - Recent; 2, "Schisto-Greseux" Group; 3, "Schisto-Calcaire" Group; 4, "Schisto-Calcaire", Haul Shiloango, "Serie de la Louilla" and Sansikwa Groups of the median zone; 5, Mayumbian Supergroup; 6, Kimezian Supergroup; 7, u n d i f f e r e n t i a t e d basement; 8, transcurrent fault; 9, thrust zone. B, cross-sections; I, Mayumbian and basement; 2, Sansikwa and Haul S h i l o a n g o Groups; 3, "Schisto-Calcaire"; SG, Schisto-Greuseux"; SCH, schists. (Redrawn from Porada, 1989.) The
Sansikwa
carbonates;
the
Group Haul
is
mostly
Shiloango
a
clastic
contains
sequence
mainly
with
argillites;
subordinate whereas
the
322
Schisto-Calcaire which was
Group
Bouenza
(Cahen, and
1978),
of
eastern
has
an age of
the
West
carbonate
of
i. Ii Ga,
Congolian
which
fill
units
sediments of the volcanic
Haut
rocks
Shiloango
in
Group
it is believed that rifting
Supergroup
began
and
around
this
time
the Schisto-Calcaire Group
comprising
depressions
Cahen
the
deposition
angular
shallow
limestone
synclines
in
and the
(1978) related Mpioka sedimentation to uplift
Ngungu
ridge
in
the
southern
part
of
the
orogen
from which limestone conglomerates were derived. The Mpioka is
4,000 m
(Fig.6.38).
equivalent
conglomeratic
the M ' b r i d g e - M b a n z a
(Fig.6.36), about
of
the basic
The Mpioka Group overlies
with
clasts
Based on the age of
underlying carbonates. of
predominance
the
which
the Kibaran.
unconformably chert
Group.
Series,
deposition
during
the
followed by the in-pouring of the terrigenous
Schisto-Gresseux the
marks
thick
in
Angola
and
decreases
in
thickness
northward
It is unconformably overlain by the Inkisi Group which is over
1,200 m thick and comprises conglomeratic arenaceous and pelitic red beds. The Mpioka
and
Inkisi
Groups
are considered
to be molasse
deposits
down after the first orogenic episode of the West Congolian belt.
ANGOLA
I F-..--.J ~o,"'"~ "1 CO.GO
~~, ~
o ooP
\~4 I r i ~
; :
MAYUM-
Ls~L
" ~4~'l~J" ~ ' -
"~
MVOUTI
"~ "~ ;*
LA
;%
+:ttJ
¢ ~ / '
GROUp
I,%?L] co0~
E-------J --_----
~
Gtes
T,,,o,oso, o,,t,tss ~
$tromat0titic
~
Arkoses
limestOneS
UPPER TILLOID O ~ C A L C A I R E [ : ~ 2 : ~ 5 ::~"
~
.',m
.,_', I=_-.M <~< 9
/
.
. GROUP
<~:=,
~2:4;I
~ ° rAg~lc°"'L"
,.,
+ ,
Figure 6.38: Stratigraphic correlations in orogen. (Redrawn from Vellutini et al., 19830)
the
West
Congolian
laid
323
Tectonism Deformation west
and
and metamorphism
was
in
two
produced
the
thrusts,
isoclinal
The
first
internal
west were zone
cleavage folds
zone
thrust
and
caused
the
crenulation
1981).
The
in
shear
at
westward
low-pressure temperature
of
older
and Andre, magmatism
phase
facies
have been 1988).
basaltic
pillow
lavas which accompany
similar,
but
altered
lower
foreland massif
1978).
part
West
Congolian
and collision occurrence still
a
the
southwest
the
zone
external
and
dykes are
well
as
pressure2 - 3 Kbar
of
the West
intruded
feeders
and
near
du
the
stated
fresher
the
the
for
As a l r e a d y
younger
Series,
at
zones
the
but
zone and to the
which
lower mixtite.
Bouenza
to 536 Ma,
where
550 ° - 600°C
and
folds
(Coward,
low-grade,
in the median
internal
intrusives
as
verging
very
also
structures
back-folding
low-to
at
D1
sills
Chaillu
in Congo.
Based on the asymmetrical the
These
sills, of
at
sills
(Cahen,
highly
is
to the
pronounced
625 Ma
In the median
Group
D1
the
locally
1984).
the
to
D2,
dated
zone; zone.
in the median
from
estimated
limited
gneisses
1984),
of
event
internal
(Cahen et al°,
facies
in
and
and
result
D1
et al.,
During zone
has been
Sansikwa
the
a
metamorphism
was
in the
sequences
small-scale as
to the greenschist
(Franssen
in
rocks.
from east to The
in the median
granites
(Cahen
and migmatites
zone
conditions
developed
internal
zone
1989).
structures
basement
medium-to
amphibolite
Congolian
occur
fold
734 Ma
the
deformation
external
increases
(Porada,
zones
about
median
granites
Congolian
D2
over the overturned
in
and
the
second
the
increases
Dated
refolded
from the affected In
and
the Eburnean
cleavage
produced
and
and m a j o r
rehomogenization
coaxially
were
D1
northeasterly
(Fig.6.37,B).
were
in the West
stages,
belt,
probably of
along
debate
(Fig.6.37)
(1989)
took place west
of ophiolites
subject
structure
Porada
that
of the internal
the w e s t e r n
(Vellutini
and the m e t a m o r p h i s m
suggested
margin
et al.,
plate zone.
of the
1983;
in
convergence However,
internal
Franssen
the
zone is
and
Andre,
1988).
6.5.2 The Damara Orogen
Structural
Framework
The
orogen
Damara
Namibia. 500 km Kaoko
is a major
It comprises wide,
branch
between which
Pan-African
belt
a northeast-trending the
extends
Zaire
and
Kalahari
northwards
into
located
along
intracontinental cratons; Angola
and
and
the
coast
branch, a
coastal
towards
the
of
400 or
West
324
Congolian with
the
Gariep
belt
belt
view
(Fig.6.39).
Lufilian
of
arc
align the
While
parallel
branch
intracontinental
the K a o k o b r a n c h
to t h e
stratigraphic
intracontinental
the
(Fig.6.1),
Ribeira
and
belt
of
structural
a n d the K a o k o belt,
branch
is
the
coast
along
Brazil
and
continuity
on
trend
and the
Uruguay. between
b o t h are c o n s i d e r e d
In the
as part of
t h e s a m e D a m a r a orogen. The
Damara
1.0 Ga a n d of
the
1983) the
orogen
700 M a
orogen based
was
lithofacies;
of p r e - D a m a r a
÷
through
1989).
(Fig.6.39,A)
on
location
originated
(Porada,
The
postulated
the
general
basement
+
(e.
g.
at
Martin,
the location
Kaoko Rift
.?:
CrQton Northern
.-.,
Porada,
distribution; lines;
.
~_6~LI~ ~;..:t:"
o
between beginning
of h i n g e
+ Zc*ire
the
1983;
thickness
\ ÷ ~ \
rifting
of r i f t s
sediment
inliers;
+
zo~re c~aton
continental
existence
-
,
Rift
-
Central Rift M0in R~t~.'::'i{i;:::!:~i~Souther n Rift
Belt
-
B
t - "I ~C P " "
~
Ka|oharJ Croton
÷
/~
I Khomas Rift
KQIahor{ Croton
Northern
Platform
Southern Platform Northern zone MA
Centrol zone
-Oomoro
8elf
W
Southern zone Eoster n
Western zone
=
N NC
=
Windhoek Bay,
Noukluft Nappe Complex
t Kaoko Belt
F i g u r e 6.39: Tectonic subdivisions ( R e d r a w n f r o m P o r a d a , 1983.) and
the
distribution
intracontinental
belt
OkQhandio LineQmen~
WB -- Wo~vls
zone
CentrQi zone
= Matchless AmDhibollte
0 L --
of
branch
rift-related has
been
of
acid
subdivided
to
the
Damara
alkaline into
three
orogen.
volcanics.
The
separate
rift
325
zones,
each 50 - 70 km wide and over 200 km long.
(graben), half
and
Central
grabens.
The
zone,
rift
known
as
basement
has
been
The above
by
zones
are
the
similarly,
Sesfontein
A Western
inferred
based
rift d i s t r i b u t i o n
on
on
is a Northern
been
both
there
zone
interpreted
sides
of
what
as
looks
also
inlier
zone,
about
50 km
wide,
and to the west
otherwise
pronounced imposed
is a north-south-trending
graben,
basement
rift
has
have
(Fig.6.39,B).
the Kamanjab
block.
There
which
located
trough
Kaoko belt,
to the east
buried
zones
the Khomas
In the northern Eastern
Southern
latter
like a fourth rift,
bounded
and
westward
known
as
sediment
a generally
by a
the Kaoko
thickening.
asymmetric
morphology
on the Damara orogen.
Rift Sedimentation The Damara and
rifts were
locally
volcanics
1983).
Supergroup.
sediment
the
erosion
graben
rhyodacites, Naauwport
southern were Summas
Here
mittent
influence
with
which a
large
during
of
of
the
rift
playa
sediments Formation.
on sedimentary cracks,
evaporites
(e.g.
and
and
Damara
Formation,
on
borax,
The
Summas
Formation
sediments
and
blocks
margin
of
ignimbrites belong
Naauwport
fault,
to
the
Formation
which
marks
The Naauwport missing
the and
the
volcanics
north
of
the
is exposed.
involving is
thrusting,
still
a sequence
of
of
in
fluviatile
of
the
of the Duruchaus
gypsum);
the depositional
evident
large
crystal
the
Southern
fan and inter-
and conglomerates,
siltstones)
origin
presence
trona,
Etusis
1982)
fault-bounded
such as cyclic sedimentation, the
and
agglomerates,
completely
rifts
and
the basement
alkaline
(Fig.6.39,A).
such as quartzites (pelites
and
northern
thick.
the
are
basement
The playa
evidence
fault,
inlier
to
and
6,000 m
of
the
of
Eburnean
first-cycle
Along acid
intrusives
deformation
early
the Kamtsas
mostly
is over
rifting
intense
in
the
alkaline
(Tankard et al.,
in the basal
reflect
sedimentation.
variety
stream d e p o s i t s
Duruchaus
cation
a
rift
to
(Table 6.1)
orogen,
furnished
lenses,
fault w h e r e pre-Damara
In spite
with
of
acid
Nosib Group
and granitoids
variations
m a i n l y fluviatile
and
Damara
conglomerate
edge of the Kamanjab
environment zone.
of
consanguineous
extruded
the
much of the
zone thickness
Formation
is associated
sediments,
constitute
gneisses
channels
the phase
Northern
These
Throughout
distribution
fluvial
filled with terrestrial,
playa-like
source areas to the Nosib G r o u p
In the Central
during
or
metasediments,
elevated
and
initially
lacustrine
(Porada,
the Damara Kibaran
and V o l c a n i s m
interfingers
partly
calcareous
Formation
is based
fine bedding, pseudomorphs
and on geochemical
evidence
desicafter in the
Z
CO (~
111
O
"~ k-
I ...............
::E
-J
Z
GROUP
Mixtit e, data mite, limestone ; sctndstone)itobirite oolite •
CHUOS
' Dolomite limestone ooli~ ' tiC chert sandstone
GAUSS 750 m
BERGAUKAS Do~om re, | m e s t o n e s t r o m S2 5 m atolltes ~ar kose~ gre3~vcck e
OoIomite~ l l m estate' marI~ S h a l e • stromQloiites
AUROS 450 m
DISCORDANCE
700m
D o l o m l t e, I [ mestone~ s l u mp b r e c c i a
MAIEBERG 950m
ELANDSHOE~ D o l o m i t e w i t h c h e r t s i r 1.1 K m omotolites
ASKEVOLD Rhyohte t uH,Qgglorne rat eo NAAUWPOORT Qndesite , epiclosite 6Kin bostonile Q u a r t z i t e , a r k o s e , c o n 9{oN ABI S merate
VARIANTO Mixtil~' t uff ,it a b i r l t e
I L.OC.A,, 'SCOR!Aki
ABENAB
LOCAL
TSUMEB
I'I
H O T T E N B E - , D o t o m l t e w i t h chectjshole RG l i m e s t o n e ,st romcttolltes, 9DOra oolites
in the Damara orogen.
CEN TRE
correlations
Quartzite .conglomerate a r k o s e , arcJillit e
Sho.led o l o m i t e lenses
KOMBAT
TSCHUD[ 3Kin
ShcHe. m a r t . siltstone. sandstone
OWAMBO 100Ore
L ITHOLOGY ( MAX, THIC K NES5)
Stratigraphic
SUBGROUPFORMATIOi'
NORTH
Table 6.1:
SOUTH
(Redrawn from Porada,
1983.)
327
form
of
high
K,
Na,
Li
and
B which
suggests
extreme
chemical
concentra-
tions. and
felsic
volcanics
southern
Mafic
margin
of
Damara
relevant.
The
Sinclair
with
interbedded
rare
limestones
by
large
mediate
the
Group
et al.,
are
of
1984).
of
high-level from being
Sinclair
thick
The
emplaced
Group
are
also
acid
and
Sinclair
on and
the and
and
among
rift
sediments
in the basement
in a rift
in the
are
Southern
and
intruded
basic-to-interalong the
setting,
of the Sinclair Group which have been dated between
intercalated
lavas
sandstones,
deposits
rhyolites
probably
along
significant andesitic
argillaceous
granites,
southern margin of the Damara orogen, canics
Kibaran
(Fig.6.40,A)
quartzites,
(Cahen
Apart
the
consists
conglomerates,
amounts rocks.
in belt
the vol-
1.25 and 1.0 Ga
zone
of
the Damara
belt. In the Kaoko belt, in
thickness
graben lier
to
along
the
missing
Sesfontein
rift,
the presence
plutons
rift
on
southern
volcanics
reduced
the
side,
the
its
part
equivalent
the in-
Group
thin;
Along
either
outside
thus
the
suggesting flank
meta-volcanics
Further acid
is
the western
acid and basaltic
abundant
of
basement
Westward,
or very
with early rifting.
with
western
Nosib
in thickness.
high to the west.
are at least partially
Correia,
on
the Nosib Group varying
But on the K a m a n j a b
missing
are bimodal
Angola
shows
1,000 m
eastern
of another basement graben
graben
part.
is also
which were associated of
about
the
or greatly
the Nosib
Sesfontein
Group
from
in the northern
completely
of the
strike
5,000 m
bordering
the Sesfontein
north,
the Chela
volcaniclastics
to the Nosib G r o u p
and
and
(Kr6ner and
1980).
Regional Subsidence and Marine Transgressions The
rifts
of
accompanied clastic
the by
sediment
transgression Sesfontein
tial
orogen
more
graben,
show
two
encroachments
transport
was
(Fig.6.39,B), group
Damara marine
from the east
extensive
and
in
the
the dolomites
subsidence to
equivalent sidence
500
the -
Ugab
in
came earlier
this
in
Subgroup region.
thickens the
in
areas
light-coloured
from
However,
in the d i s c o n f o r m a b l y
Northern in
the
0 -
rift of
the
and
The
first marine where
Northern of
the
Because
30 m
on
The also
Northern
underlying
were
zones
shales
graben. zone
which
(Fig.6.40,B),
1983).
(Table 6.1).
Sesfontein the
subsidence
west
(Porada,
adjoining
and
Abenab
700 m
of
the
in the northern
of the Otavi Group were deposited
graben
phases
from
in the
platform
Abenab
the
flanks
of
wide
extent
of
suggests
rift
marine
Khan F o r m a t i o n
Sub-
of differen-
rapid
the the sub-
influence
at the top of
328
the N o s i b
Group
ence a c a r b o n a t e
(Table
6.1;Fig.6.40,B).
shelf-to-basin
setting
During
this
(Fig.6.41)
existed
PO
1 Sesf
1 Sesfontein
3
3 Karlb~b
6
o:.O.
Sum aS mrs. lO Mi~tenfold II Ts~un 12 Hokos rnts,
....
%
Bosement
of subsid-
in the N o r t h e r n
%, 4}-
~7~
Ba¥(~.~
,ho,,.o,
-
e a r l y phase
,
~ \
~========= KhomosTrough]
C
continuing it~o| belt J
J Goriep
/i
waw,s ~
)
Pre- Domoran '1~ basement. ~'~ ~"i ~-~ Sy . . . . ~ , ¢ Flysch~
.~
.......... i,~/-,..,.,, , ~.
"
',',~~#,~ .~======~
r~ 13
Late to Posl-Orogenic rnolasse ( Muldln group)
~.~
Sin¢lQir Group
Figure 6.40 : S k e t c h maps showing the d e p o s i t i o n a l the Damara belt. ( R e d r a w n from Porada, 1983.)
framework
of
329
N
SlW
I.
2.
E
3.
t,,.
+".+
S+
+Kuiseb Fm+ Karibib Fm. book reef
+
+
ii:+i
i!!;iiiiHt
° +Dolomffic schist Lm~inafed Dolomite
[
Dolomitic qu~tzffe
500
Dolomffe Sigstone
basin:
Figure 6.41: Facies from Porada, 1983.)
relations
in
zone in which well developed back-reef, facies tion
the
contains
si!tstones and
and
rudites
Subgroup.
(Redrawn
and turbidite
The Okunguarri turbidite forma-
quartzite
which
Ugab
~- O(m)
fore-reef, off-reef,
(Okonguarri Formation) accumulated.
calcarenites
1000
turbidites
originated
from
the
intercalated
with
reefs
were
which
situated northward along the shelf edge of the Northern rift. The
next
phase
of
subsidence
found than the earlier one. basin
configuration
Kalahari cratons.
and
and marine
transgression
was
more
pro-
It altered the pre-existing p a l e o g e o g r a p h y and
created
a wide
basin
between
the
Zaire
and
the
The shelf-to-turbidite basin in the N o r t h e r n zone filled
up and together with the Central
zone became
the Karibib
carbonate plat-
form (Fig.6040,C) which continued southward towards a new and rapidly subsiding
turbidite
carbonate
basin known as the Khomas
platform
was
Sesfontein trough. Also, the southern of
the
regional
orogen.
trough
mixtite,
equivalent,
by
the
Northward
turbidite
A the
the
Chous
western more
Chous Formation was reinterpreted
in
westerly
underlies
Originally (Porada,
platform
Kaoko
trough the
regarded
as
the
situated along
throughout the d e p o s i t i o n a l
carbonate
Formation,
Subgroup.
the Karibib
basin
a northern carbonate platform was
from
Tsumeb
trough.
another
flank of the Zaire craton
Damara
Sesfontein
bordered
phase
separated (Fig.6.40,C). Karibib a
the A
and
its
tillite,
the
1983) as s l u m p - d e r i v e d mass mud
flow deposits which were triggered by rapid subsidence and syn-sedimentary
330
faulting areas
at
the
derived
the beginning Chous
clasts,
Figure
towards
into
a
the
narrow
(Miller,
1983).
to 700 m thick, Tinkas
zone,
clastic
However,
Hakes
second
and
marine
accumulated zone.
(Porada,
The
belonging
to
in the
ZONE
the
Aus
Ugob
Formation
margin
towards
sequence
to correlate (Table
opened
Damara
orogen
carbonates,
zone
the
pelites)
and
were
the
prograding
Khomas
trough.
in the Southern zone
6.1)
about
Southern
up
sequence,
in the Northern
Corona
in the eastern
carbonates
with
the
2,000 m part
are
of
the
and
first
thick, the
or had
Southern
equivalents
of
zone.
KHOHAS TROUGH
Summas Fault
SOUTHERN
Kudis
C.huos
ZONE
Kamtsas
......".. ~"v~:~:'" " Chuos stage
..,~.,.._._.j~%~.,..:::d:,~ r ... K
the
which
(originally
Quartzite,
delta
in the Central
NORTHERN
in
were accumulating
sequences
Hakes
turbidites
....~...,:,_.
schists
basin
clastic The
1Nsuupooct
subsidence
locally
from the Northern
trough,
the Karibib p l a t f o r m
- whether
in a large
turbidites
In proximal with
areas.
settings
Khomas
of the lower marine
1983)
cycle.
Chausib
the Tinkas
the
carbonates
southern
Blaukrans
earlier
maximum
biotite
the Karibib
the
subsidence.
with a 3 - 4-km thick turbidite
comprising
the correlation
is uncertain
across
interpretation
interfingers
from
of
conglomerates
in the distal basinal
zone,
during
sequences
simul t a n e o u s l y
the
ocean In this
While
phase
unsorted
the inferred depositional
Southern
Member,
siltstones.
second
contains
and pebbly schists
6.42 shows
zone
of the
mixtite
÷
~
Tinkas
Aua,,,s~
~
J
L
Mo,chiess Amphibolite ~ . ~ l ~ - ~ k " ."~= ':"~'~" -
Kuiseb
~'L.= Formation
Oceanic Crust
4---
~"'~:~"~~/.'L:~
C
Matchless stage
Figure 6.42: Cross-sections suggesting depositional for the Damara Supergroup. (Redrawn from Porada, 1983.)
settings
With
into
basin
in
pronounced which
accumulated
subsidence
about
i0 km
(Fig.6.40,C).
of
Near
the
Khomas
pelites the
top
trough
and of
extended
quartzites the
Kuiseb
(Kuiseb are
a
deep
schist)
tholeiitic
331
basaltic
rocks,
the
characteristics also,
an
structures
and talc
ocean
arm
(Miller,
Amphibolites
of the Matchless
pillow
chlorite
Matchless
1983)),
Amphibolites
and
schist
opened as
was
belt
the
suggest
associated
affirm oceanic
along
(Fig.6.42,C).
the
axis
postulated
the
and
and support
Khomas
earlier
chemical
ocean-floor
meta-gabbros
origin;
of
The
the view that
trough
(Fig.6.35)
basalts;
ultramafic
(Fig.6.42,C) by
Shackleton
(1976). In the which
Kaoko
separated
semi-pelitic rhyolites
for
Sesfontein
schistose
and
Eventually whereas
the
the
regional
of
distribution submergence
of
and
platform
throughout
of
clastic
4 km
thick
Formation
the
sediments
to the
in
the
Tsumeb
of
the
of c a l c - s i l i c a t e
by the
absence
fill
of
Mulden
a
in the
Group,
4
conglomerates
and
molasse
whereas
basin,
filled with
the
on
by
the
It also
Kaoko the
graywackes,
orogen.
trough
in the
Syn-orogenic upper
part
deformation the
5 km
Group
sequence
(Porada,
1983).
account
of
flysch
of
and
supply 3 to
the
Kuiseb
The Kuiseb
flysch
of beds
and
of the Kuiseb
schist;
and
staurolite
the
and
Northern
Ovamboland
thick,
was
sandstones
basin
andalusite
platform
which
Southern
the
and deeper
platform
is
became
the
where
the
(Fig.6.40,D)
deposited.
along
the more distal
The Nama
turbidite shelf
by the absence
are typical
of kyanite,
The
implies
a tremendous
(Fig.6.40,D).
schists
the site
of
erstwhile
implies
and
trough.
(Table 6.1).
cessation
onto
entire
4 km of
(Fig.6.40,A)
The
Mulden
proximal parts
and shale with a few stromatolitic
the
over
Kuiseb.
arkosic
siltstone
a geanticline
Kuiseb),
the
carbonates
Kuiseb
Amphibolites
flysch
basin,
about
in
Formation
the
rocks which
Pan-African
molasse
the
eastward
platforms, of
Khomas
are found in the underlying Following
on
while
to
deposited
Kuiseb
from the underlying
lenses
site
troughs,
platform
the orogen.
the Matchless
is distinguishable
were
overstepped
overstep
occurs above
was
Kaoko
(equivalent
carbonate
sedimentation, areas
the
accumulated
trough which by then had filled up, became
accumulation
widespread
and
porphyry
geanticline
the Sesfontein
carbonates
graywackes
feldspar
the
Tsumeb
considered
comprises
part
of
the
of the basin were carbonate as
another
layers. molasse
Tectonism
A
recent
the
Pan-African
orogeny
1989)
shows that the orogeny was polyphase
about
480 Ma.
recognized, (between
In
the
Kaoko
the first,
Africa
and
belt
three
in
the
main
America)
west
of the
belt
from about
deformation
(DI) which probably resulted
South
Damara
and lasted
events
(Porada, 650 Ma to have
been
from plate convergence
present
coast
of Namibia,
332
produced
a
followed
by
erosion
bedding-parallel the
and
proba b l y
by
the
Subgroup
deposition
large-scale
were
molasse
whereas
the
which
are
granitic
nappes
in
D2
the
folding
Central
by
which
(Porada,
syn-tectonic
and
event
was
D2
formed
Group
plates;
by
the
Kuiseb
carbonates
and
Mulden
following
back-folding
zone was also polyphase
and collision
trough
rocks
widespread
the
before
producing
bedding-parallel
appear
to
be
zone
Post-Dl
have
granitic
been
suites,
foliation,
related
in the Kaoko belt,
1989).
D1 and D2
intrusive
dated
the
both
to
and to subduction
at
dioritic
650 Ma,
Salem
to
whereas
granites
of
the
zone are dated at 570 - 540 Ma.
In
the
Khomas
trough
progressively
This the
southern
large-scale
doming
partial
five
the
and
meta-pelites
imbricate the
northern
margin
intruded
feature
the
evidence
been
sediment parts
greatly
of the
The
Southern
of
the
orogen,
is at the greenschist
but increases
conditions towards
in
zone along w h i c h
the
(Tankard et al.,
northeastern
for orogen parallel displacements
arm
facies
trough.
of
as outlined
The
the
and The
where
the
folding
and
(Fig.6.43,C) the
Donkerhoek
1982). Damaran
by Coward
in the n o r t h e r n
to high-temperature,
Khomas
lineament
D3
folding,
1989).
trough
by recumbent
Okahandja
caused
intrusions
(Porada,
Khomas
480
thrusts
upright
granitic
cover
540 Ma
about
The collision
of the
thickened
around until
including
characteristic
syn-tectonic
and
deepest
trough
phases
(Fig.6.43,B). with
at about 520 - 500 Ma
evolution
Metamorphism
creases
basement the
(Fig.6.43,B).
granite was in
of
have
thrusting
zone,
occurred
in this
deformation
concomitant
preserves
collision
sequence
foreland
Central
melting
zone
the
successive
cratonic
in
Southern
continental
deformed
produced
defor m a t i o n
ture
and Otavi
characterized
recumbent
Khomas
the
was
at about 570 Ma.
in the Central
convergence
Damara
folds;
This uplift
and South A m e r i c a n
a final D3 event caused west-vergent
of other granites
continental
marks
recumbent
The
dated at 590 Ma were emplaced
and
Kuiseb
molasse.
of the African
rift.
rocks,
Granites
structures
the
Mulden
Kaoko
(Fig.6.43,A).
thrusting,
onto
the
of
the
granitic
over
Deformation
Ma.
old
eastward
the intrusion
Central
in
650 Ma
thrust
D2 event,
which
foliation
of
caused by the collision
it pr o d u c e d
in
S1
emplacement
A major is
the
(1983).
part
of the
m e d i u m pressure-tempera-
metamorphic
grade
again
de-
the Southern platform.
Mineralization
Base
metals,
types of deposits
mineralized
pegmatites,
and
in the Damara belt of Namibia
uranium
are
the
(Fig.6.44,A).
principal Stratiform
333
WEST EUGEOSYNCLINE A KuisebSubgrOUpnaPpes(flysh) ChuosFm, (d~omctites)
.
.
.
EAST
PF %oo~!i~ ,~ . 0 ~ . ZA|RE ~O'~" . . . . ::~ PROVINCE Mulden Group (Exogeosyciine) ~
.
A
.
~<'
~o
Donkerhoet granite
~ ,
Oamara Supergroup
.g"
*'*'..~" Boundary b e t w e e n e u g e o s y c l i n e and miogeosycllne High-angle ~autt KI Kr~maniab inlier l -.,k_ Thrust fault NNC NCcukluft nappe comp{ex Vergence 0L OkahandJo linec~ment AS 8 Areb shear belt STF Sesfontein thrust fault GI Grootfontein inlier PF Purros fault NORTH WTF Wc~terberg thrust fault AHocht:honou s Swakop Group B
Inclined Dn .1 )) ~ ~-~
~ • eL' ee o ~ ~
~o
.o
• ,, SOUTH
Dn sole thrust
....
J
Nauklutt n a p p e s (largely Swakop Group)
~
~
G a u b V Q I l e y GrQuwaterMetoillorphic Metctmorphi¢; Suite 10 km Pre-Damara Basement
Weerler
=
C
~OOkrn
On thrust folded by Dn.1
~///
. .::.~'.'::...
,'~;~-~,B ~ Kaiaharl
~--~ Bssement
/"3 ~..")/.
Suite
of the
(Fig.6.44,A),
the Nosib Group marine
the Mississippi
Nama Group
Lower
in
the
Damara
orogen.
copper and iron sulphides and native silver are produced mine and elsewhere part
~
j
Figure 6.43: Structural styles from Tankard et al., 1982.)
and
--
beds
at
the
(Tankard
Valley-type
where mineralization contact between
et al.,
1982).
the
(Redrawn
from the Oamites
occurs
in the upper
continental
Lead-zinc
red beds
mineralization
is hosted by the do!omitic
of
limestones of the
Otavi Group in the north, near Tsumeb and Aukas. In Namibia pegmatite mineralization is e x t e n s i v e l y a s s o c i a t e d with the metasediments
and
Pan-African
granitoid
rocks
Damara orogen
(von K n o r r i n g and Condliffe,
(550
- 470 Ma
old)
of the
1987). Figure 6.44,A shows that
pegmatitic tin and t a n t a l u m deposits are restricted to three well defined, N E - S W - t r e n d i n g belts, up
to
1,000 m
long
each over 100 km long. At Uis a gigantic pegmatite, and
tantalite as by-product. coarse-grained
bodies
about
i00 m
wide
is
exploited,
with
The pegmatites appear to be zoned,
with
microline,
albite,
muscovite
columbite-
coarse to very
and
quartz.
The
a c c e s s o r y minerals in the Namibian tin belts include beryl and tourmaline.
.
.
.
n
~,,l
Sawkins,
Figure
5Km
1990.)
6.44:
~
[r~'lOrunitegneiss& I gronite~
B.~']Abbab[s Fm [~] Pegmafitic Fm
c~ :::, ~ : " ~ Bi O | l t e
l~
an~lsS, -w =,-...lr'z'~Felds 'Ji po~hic : _ ~~ t.=..~quartzite J ~j
< Chous . a:~cl:--~Khon Fm
~Khomos Fm c,.~Wetwi~ch Fm
Salt
Tin C°~umbium
Mica
~. m
Mineralization
Cesium Wollostonite
D
,,
pyro . . . . . horn6[ene gneiss
in
the
Damara
/
," > s00mmrol.
ONOANJA
orogen.
!
"
i
]
)
•
from
de
Kun
C
ZERO REFERENCE LINE
(Redrawn.
':~
.
...... ©
/ SWARTMOODER
, SO0 Ft
C~Lower pyroxene-hornblende gneiss
![~'] Pyroxene - grarnet
:~Upper
OneissJ gneiss , ~ ~ T 4 ~
,=o,Looete
~
-"
/
m mgranite U r o n i f e r opegmotitic us
mun
.
/. /~ W)ndh6(w /
KARIBIB
"
: •. • . F
DKARUSU
.~k- U U ,',ii~",~) )
W+
UlS
SPITSKOPJE
RANOBER6
[ ] U p p e r marble
~
Tungsten V Vanc~dium : ~ L e a d . Zinc, Silver
W
U Uranium i Lithium Z Sulphur ~ Z i n c with Lead
X
k
1987;
'- i
335
The southern tin belt is characterized by w a l l - l i k e p e g m a t i t e dykes which intrude
the
desert
Damara
plain
metasediments
from
as prominent wall-like
which
they
features w h i c h
weather
out
can be
in
the
followed over
great distances. Mineralized
pegmatites
in
the
Karibib
area
have
for
over
50
years
yielded a host of rare-element minerals such as a m b l y g o n i t e - m o n t e b r a s i t e , lepidolite, minerals great
petalite,
in a d d i t i o n
variety
productive quality
pollucite,
beryllium-niobium-tantalum
to industrial mica,
of
attractive
lithium
pegmatites
are
occurs
a
tourmaline
ceramic
gemstones.
in
known
zoned
feldspar,
Here as
and
the
pegmatite
quartz
larger
Rubicon
and
in
the
bismuth and a
and
most
Helicon.
Gem-
Karibib-Usakos
area. Sawkins A)
(1990)
as an example
collisional
discussed
the u r a n i u m deposit
of vein-type
uranium deposits
(S-type)
granites.
The Rossing
at Rossing
(Fig.
6.44,
that are a s s o c i a t e d with
u r a n i u m d e p o s i t was
believed
to be the largest u r a n i u m producer outside the former communist block. As shown
by
Sawkins
metamorphic intruded
in
a
early
zone
alaskites,
of
the
the granitic
range of textures, intrusions
vary
and
(6.44,
possibly
pegmatitic and
in
rocks which
central,
anatectic (Abbabis
volcanics
largely
granites
belonging
concordant
and
Formations
the
large
to
6.44,
intrusions
to
the
occurs
schists,
C).
contain the u r a n i u m d i s p l a y
discordant
and
relationships
gneisses,
(Fig.
have
Formation)
from aplitic to granitic and pegmatitic.
from
high-grade
granites
The u r a n i u m m i n e r a l i z a t i o n
by
Rossing
the
where basement
B).
characterized Khan
lies
belt,
Precambrian
rocks
uranium-bearing
marbles
deposit
Damara
Formations
migmatite
dykes,
this
the
sedimentary
and Khan
between and
of
remobilized
granitized Etusis
(1990)
zone
Termed a broad
The alaskite
thin
conformable
t y p i c a l l y d i s p o s e d of in closely spaced arrays.
Most of the a l a s k i t e in the Rossing area are u n m i n e r a l i z e d or w e a k l y mineralized,
and
uranium
of
economic
grade
is
concentrated
alaskite was emplaced into a garnet g n e i s s - a m p h i b o l i t e unit zone)
or
into
the
cordierite gneiss type
of
ore
amphibole-biotite
lower
sequence in the central ore zone.
localization
uraninite w h i c h
schist,
are
is confined
not
known.
The
biotite,
or w i t h i n
exhibits
amounts
of
ore
sulphides oxides
are
association with
associated
(pyrite, chalcopyrite,
(magnetite, hematite,
with
the
lower
mineral
small grains
biotite
and
uraninite,
bornite, molybdenite,
of
and
Uraninite
zircon.
while
is
several
feldspar,
cracks or i n t e r s t i t i a l l y to these minerals.
a preferential
betafite
and
The controls of this
microns to 0.3 mm, o c c u r r i n g either occluded w i t h i n quartz, also
the
(northern ore
marble,
primary
to alaskite as v e r y
where
Small
flourite,
arsenopyrite),
and
ilmenite) occur somewhat s p o r a d i c a l l y in the
336
ore. Beta-uranophane and other secondary uranium mineral represent 40 % of the
uranium
in the
orebody.
Whilst
a precise
genetic
model
has
not yet
been advanced Sawkins
(1990) suggested that the formation of the alaskite
must
concentration
have
involved
alaskite or the
a
metasomatizing
of
uranium
in
those
fluids were generated,
areas
where
the
especially as the
uranium levels in the basement rocks are generally high. 6.5.3 The Gariep Belt
Stra ti graphy The
Gariep
belt
lies
along
the
Namibian-South African boundary Sperrgebiet diamond)
(or forbidden
the
Namibian
coast
of
southwest
(Fig.6.45).
Africa
has
the
Previously referred to as the
territory because its beaches
sector
across
consequently
were
yielded
strewn with
less
geological
information than the South African or Richtersveld sector. Like
other
South Atlantic
Pan-African mobile
belts
the
stratigraphic
succession in the Gariep belt begins with a rift sedimentary-volcanic sequence,
followed by a continental margin
sequence. A unique feature, however,
(miogeosynclinal-eugeosynclinal)
is the presence of widespread indica-
tors of a collision suture in the form of ophiolitic basic volcanic rocks (Fig.6.35), as well as some blueschist metamorphic rocks The
basin
fill
which
is
known
as
the
Gariep
conformably on the Namaqua Metamorphic Complex, escaped
reworking
during
Group
rests
non-
in areas where the latter
the Pan-African orogeny
Where the~e was reworking,
(Kr6ner, 1974).
(Tankard
et al.,
1982).
the Namaqua-Gariep contact is a paraconformity.
At such contacts the sheared and truncated basal greenschist
facies rocks
of the Gariep Group grade downward imperceptibly into Namaqua greenschists (schists that
and
had
However,
phyllites)
been
altered
whereas
representing
retrograde
amphibolite-facies
during
Pan-African
tectono-thermal
structural
roughly north-northwest
the
trends
in
the
Gariep
Group
those of the Kibaran Namaqua
are
rocks event.
oriented
Metamorphic
Complex
trend mostly westward. The eastern
lithofacies rift
carbonates, and
a
of
and shelf shelf
western
the
Gariep
Group
(miogeosynclinal)
clastics, continental
diamictites
is
broadly
divisible
sequence w i t h and
slope-ocean
continental
subordinate
floor
into
volcanic
(eugeosynclinal)
an
beds, rocks; facies
(Figs.6.45,6.46). As shown in Fig.6.46 both lithofacies either interfinger or
are
et al.,
tectonically 1982).
juxtaposed
Sedimentation
in
(KrSner, the
Gariep
1974; orogen
Porada, was
1989;
controlled
Tankard by
an
337 oscillating
shoreline,
hence
the
pronounced
interfingering
of
the
lithofacies.
q> ( Diamond Area ) Sperrgebiet
J'-~Sperrgebiel
"
boundary
~Boundary of Gariep Province
" ~
..Boundary
between
eugeosyn_
..'" cline an d miogeosyncline
• ~ A " " Trace of thurst
Aurus
Witputs
Mrs
Cha meis Bay
[ POSt• Nama ] cover ~x~-'~jKu b o o s - Br e men Intrusive KI Sw art bartk Suite K2 Kaboos K3 Tatasberg- K aria beam K4 Younger Bremen KS Garub Group Nama Group Eugeosyncline
Aussenkier
Oranjemun~ Miogeosycllne N. . . . .
Or angeRiver maul h~l ~ Fm.]
At . . . . der Bay
I
Bogenfels rrr~ ITg:,';lHeioab Fro. F Gariep Group ~Hilda Fro. | ~0ro~jemund Fn~~-~ Kapok Fro,| ~]]~ Groat der m Fro. ~ Stin kfon~ein.J Fm, ~Gonnskouriep dikes ~Holgat
/
Fro,
ATLAN TIC OCEAN
Richterveld Intrusive Suite RI Main bathotith B1 Older Bremen R2Rooiberg R3 Lekkersing Refotialed basement Namaqua basement
~
~
/ SOUTH t / ! i / AFRICA
Port NolIoAh
It/
Kleinz ee
•Springb°k
lO0km
Figure 6.45: Geological Tankard et al., 1982.) The
initial
Formation, thick,
a
in
which
conglomerates top.
In the
rift
coarse and South
map
deposits
arkosic
and
upward-fining
of
the
are
fluvial
sector the
belt.
represented
subarkosic
shelf dolomites w i t h African
Gariep
fan
by
the
sequence,
sequences
algal
(Redrawn
Stinkfontein 150
contain
structures
Stinkfontein
rests
from
-
3,000
m
lenticular
occurring
at the
non-conformably
338
upon
the
in
Namaqua
Namibia
by
(Fig.6.46,A). and
an
pyroclastics
The
and
containing
arkoses,
1982).
In the
laterally thick),
and a
arkose,
complexes
or
where
Rosh
it is replaced
Pinah
Formation
lower
sedimentary-volcanic
member,
with
rhyolite,
various
of g r a n i t e
known
and
Kapok
(700
-
trachyte,
to s y e n i t i c
as the R i c h t e r s v e l d
Formations
3,000
m
stromatolitic
conglomerates, western
part
downward
shelf
Kapok a
member
member,
northward
composition.
Suite,
the
South
schists
very
and
overlain
the
the
cross-bedded
and thin dolomites.
\
~
~
v
V V V
\ , ; -. . . .
Na mQqua basement
V
v
v
et al. ,
Hilda
Formation
grades
(5and
6 km graded
In the southwestern,
Numees Fro.
v
_.<--.,.~':
Kapok Fro.
-.L.~_~ I ~k%~'~kLOCATION 'r~k'IH-~"MA P"
A
Namaqua basement
yncline
Eugeos
l "N
...
a
"b,,,
F<-:c"-.
°:%h b v v v v v v v v v ~--::_.o,~o,
~" V v
v
"-.
°m.es
~'m-~~:;.:"'..'.':'.W-~,'.-~oigo,
V V V V V V V ttl'-- C''~'~; - -: = ' - - - - ~ ~ ' : ~ : ~ : , ~ , ' . ~ ' ~ t ' , - ' ~ j I
v v v v v v v ~ - - - = = = : ~ = ~ ~ % : ; . : . > : ~ ' , ~
~m. l
St}nkfontein
Fro.
v v v v v v vl~?.T.~':.~->,.>~f~'~,:::'."::'..,:;,.'?.¢,.;.v:..:'.!.::..;.:.~l~
\~v v v v v v v t ~ - ; " v v v v v vP" . . . . . . .
~ D
F i g u r e 6.46 • Geological sections (Redrawn from Tankard et al., 1982.)
k \
by
shelf
quartz-
(Tankard
sector,
Holgat
phyllites,
marine
coarse-grained
schists
African
into
containing
unconformably
a heterogeneous
dolomites,
phyllites
(Fig.6.46,B)
sequence
quartz-sericite
of
are
thick),
--.,.~/~,--:-,"I:'~ . " genfiliFro. ' ' A~i0a'~#_m~"_.~HilCg~"I
I
is about
1989).
Formation
ites,
comprises
subvolcanic
Stinkfontein
sequence
out
Pb-Zn-bearing
volcaniclastic
(Porada,
Hilda
but p i n c h e s
latter
volcaniclastic
920 Ma old
the
the
The
upper
The u p p e r
basement,
I'/,J\~l:l I,';.TC/-b~am~q,-, \l~Z',;'Y,,J b0~eme.,
through
the
Gariep
belt.
339
more
basinal
Formation. probable
area,
A
the
Holgat
diamictite,
glacio-marine
rests
the
unconformably
Numees
origin,
Formation
rests
upon
on
the
Stinkfontein
(about
500
m
Hilda
and
Stinkfontein
the
thick)
of
Formations. The
eugeosynclinal
volcanic
Grootderm
equivalents
Formation
of
and
the
the
Holgat
Formation
overlying
are
the
volcano-sedimentary
Oranjemund Formation. The Grootderm is a 4 - 5-km sequence of basaltic and andesitic (1,000 The
lava,
- 1,500 m thick)
Grootderm
mass.
volcanic
agglomerates
and
tuff;
is predominantly graywacke with
Formation
Regional
breccia, is
metamorphism
believed has
to
represent
greatly
an
the
Oranjemund
rare carbonates.
obducted
obliterated
the
ophiolite
sedimentary
and
volcanic features of both the Grootderm and the Oranjemund. In basin
his
interesting
Kr6ner
biogenic
(1976)
dolomites
quiescent
to
submarine
started
with
crushed
and
the in
the
with
which
of
of
the reef
Deposition
organic
the
clasts
graywacke.
or
Namibia
(Fig.6.45)
to the north,
a
character
with
rich
Bogenfels
Formation
Grootderm Formations
later
supported In
and arenites
marked
it
overlaps
the
the
by
on
probably displaced, to
a
form
matrix
of
area
of
Bogenfels
(Martin,
and
or
is thin and has 1965)
which
pass
(Fig.6.46,A). The carbonate-
end
the
rocks
guyots
Oranjemund were
the Oranjemund Formation
dolomites
because
on
volcaniclastics
are
upward into the onlapping Bogenfels Formation differentiation
eugeosynclinal
volcanic
the
which
surrounding
carbonate
of
growth
of
reefs
rocks
volcaniclastic
Oranjemund
association
carbonate
volcanoes.
sheared shelf
close
possible
building
mixed
olistostromes
reconstruction
linked
of
basin
eugeosynclinal
filling
and
0ranjemund
and the northward rising Namaqua basement.
and
It grades
laterally eastward into clastic lithofacies, the Heioab Formation which is probably the northern equivalent to the Hilda Formation,
or it represents
a non-volcanic facies of the Kapok Formation (Tankard et al.,
1982).
Tectonism
Deformation
in
southeastward ocean-floor units
are
between
the
Gariep
thrusting
ophiolitic thrust
700 Ma and
as
of
belt
the
is
Gariep
characterized
Group
(Fig.6.46,A)
Grootderm
Formation,
and
flat-lying
sheets. A
major
530 Ma
by
is believed to have
basement plate
caused
large-scale
especially
the
gneisses.
These
collision
event
the orogeny in the
Gariep belt. The evidence for the existence of a collision suture includes the
occurrence
nappes,
of
basic
volcanic
rocks
of
ophiolitic
affinities
and the presence of glaucophane and other sodic amphiboles
G r o o t d e r m meta-lavas suggesting high-pressure m e t a m o r p h i s m
(Kr6ner,
in
the
in the 1974).
340
Elsewhere,
metamorphism
greenschist
facies
greenschist was,
in
facies
however,
in
in
the
the the
attained
Gariep
belt
Stinkfontein
commonly
Formation
Hilda
Formation.
the
Stinkfontein
in
was
The
at
the
lower
the
upper
amphibolite
grade
and
lower
Formation
at
in
the
Namibian
sector. The
Richtersveld
batholiths),
and
emplaced
before
911 Ma.
A major
of which
cut
or
chain
of
onset
swarm with
Richtersveld
continental
Kuboos-Bremen
the
Suites,
and carbonatite
of
Gariep
alkaline
to
batholith,
break-up were
varying
and
tholeiitic
in
for
leucogranite
920
composition,
some
of
the
Gariep
(Tankard
composition
from
intruded
between
of
phase basin.
of a linear
et al.,
granitoid
-
some
100 km east
by the emplacement
were
Suite were
around
to the dyke injection
initiation
line
and sheets,
and
Intrusive
sedimentation
extends
followed
Kuboos-Bremen
diatremes
(granite Old Bremen
These dykes belong
and m e t a m o r p h i s m
plutons,
Suite
to syenitic
the
(Fig.6.45).
heralded
Deformation
after
dyke
the
the Gariep belt which
Intrusive
the granitic
1982). to
The
syenitoid
550 Ma and 500
Ma.
Mineralization
In
the
Namibian
strata-bound the
Kapok
recovered chert
and
sector
stratiform
Formation. from
and
this
in
In
dolomite
6.5.4 The S a l d a n h i a on
Saldanhia
the
Nolloth The
and
Malmesbury Kaaimans
deposits
The
with
belt
Groups
to
lead
and
mineralization
tip
is
is
zinc,
exploits
Pb-Zn
member of
silver
hosted
intercalations.
rocks
the
is
also
in
carbonaceous
is
probably
It
of
1982).
eastern
a
western
These
of
South
sequence
of
Africa,
the
deformed
and
between
Port
(Cape granites)
southwestern a
outliers,
(Fig.6.47,A).
Republic
as
and granites
in
comprises
and
of
exposed
Elizabeth
belt
Group
(Fig.6.45)
(Tankard et al.,
southwestern
Porth
mine
in the upper volcaniclastic
clastic
origin
sedimentary
Saldanhia
Pinah
Belt
orogenic
metamorphosed
Rosh
addition
mine.
sedimentary e x h a l a t i v e
Located
the
Cape
Province
meta-sedimentary known
as
low-grade
the
(Fig.6.47,A). sequence,
Kango,
the
Gamtoos
metasediments
and
accumulated
between the end of the 1.0 Ga Kibaran event and about 500 Ma. The
Malmesbury
lithologic
zones
Group
consists
(Fig.6.47,B),
turbidite
deposits
(phyllites,
Formation;
the central
namely;
of the
graywackes)
zone with formations
three
distinct
southwestern belonging
fault-bounded zone
to
the
with
thick
Tygerberg
that are characterized
by mica
341
schists,
fine-grained
dolomite
lenses
northeastern
zone
quartzites, bands
(Dunlevy,
suggest
schists
to
those
quartz
marine
contains
quartz
similar
quartzites,
which
a
complex
and psammites
of
schists
shelf
modern
with
limestones
environments;
sequence with
of
and
whereas
the
coarse-grained
conglomerates
and phyllite
depositional
environments
near-shore
1988).
Although
poorly
exposed,
from south to north, (Tankard et al., flat-shallow unconformably
somewhat
similar
lithofacies
progression
occurs in the eastern outliers of the Saldanhia belt
1982).
shelf
a
Here the Kango Group shows a succession
deposits
overlain
by
overlain
by
post-orogenic
turbidites
which
continental
of tidal
are
in
deposits.
turn
In
the
Gamtoos Group, a sequence of shelf clastics and carbonates are overlain by alluvial the
fan and fan delta deposits,
Kaaiman
Group,
sandstones, The
Group
continental
consisted
of
clastics;
whereas
sequence
of
the
in the southernmost
is
sediments
These of
the
eastern
sequence,
shallow-water
margin
location
and
margin
pelagic
and
conglomerates. passive
flysch
succeeded
by
inlier,
shallow-water
shales with occasional thin carbonate beds.
Malmesbury
classic
a
and
belt
coastal Ocean
on
therefore
ocean
shelf-sea lay
a passive
and
deposits
and
the
et al.,
margin
is
a
landwards
carbonates
along
(Hartnady
display
basin
arenaceous
environments
Adamaster
Saldanhia
from
turbidites;
and
depositional the
out!iers
which
northern
1985).
suggested
The
by the
sparseness of basic and intermediate pyroclastic and extrusive rocks among the
Malmesbury
Group.
Dunlevy
(1988)
suggested
that
the
subduction
of
oceanic crust beneath South America on the opposite side of the Adamaster Ocean
(Fig.6.1)
eventually led to the collision of the Kalahari
and South
American cratons, and thus, the Saldanhia orogeny. The
Saldanhia
During the
orogeny was
polyphase
spanning
from
610 Ma
first phase, prior to the impact of the opposing
deep ocean and continental margin turbidite sequence
500 Ma. the
(Tygerberg Formation)
was deformed
into a series of vertical
the tectonic
front to migrate northward into the shelf-sea and eventually
into
the
nearshore
region,
p r o g r e s s i v e l y decreasing
isoclinal
to
cratons,
producing
intensity away
a
folds.
deformational
from the
suture
The occurrence of ophiolitic greenstone and chert units Formation
of
the
central
zone
is
indicative
Collision
of
caused
pattern
(Dunlevy,
of
1988).
in the Bridgetown
thrusting
from
a
suture
which was located somewhere to the southwest. Post-collision
isostatic
uplift
generated
faulting
and
small
post-
tectonic basic and intermediate intrusives. The mountain ranges and fault-
342
bounded
intermontaine
became the
basins
which
formed during
depocentres for the molasse-type
Q
St. Helena
.
Saldanhia
orogeny,
Klipheuwel Formation
,. --. ~-. "~ ." .
\
the
..
of the
.
/
.. \
\
\ •
.
.
\ ~
\
.I
(, .
$8
V
•
,
\o
So,~o~h~'C~'~:
"
" \"
" ~
"k,~.pil, , , b e f g ' - (
".
"...
,
20kin
CPB
Cape:Peninsula
batholith
DB
Darling
SB
Sol danho batholith
~"]
botholith
C
Cope Supergroup
~-----7 Klipheuvel Formation [~"1
Cape Granite Northeast Domain Central Domain Southwest Domain
....... Boundary
between tecton
Fault Zone
Zone 2
~ "-
Zone
A
3
B
I
\ _~
~',~ Northern ~ \ Plutons Plutons Port\ •o o i Nollo~h'~ ~ / ' Mclrgln of Molmes bur y Geosyncl;ne "Q Granite Pluton
WESTERb/~ SALDANH[A~
PROVINCE
So=h
"~ " " ~ \ ~"1 - J ~
EASTERN 700kin SALDANHIAN ' PROVINCE Gam'toos Kango Iniier InUer /
p,otoo,
/ KclQimons Inlier
Figure 6.47: Geological sketch maps of the Saldanhia (Redrawn from Tankard et al., 1982; Dunlevey, 1988.)
belt.
343
Malmesbury
Group,
Cape Supergroup
which
heralded
the
initiation
of
the
Early
Paleozoic
deposition.
6.6.5 P l a t f o r m C o v e r of the Kalahari Craton The Nama Group
The
Nama
Group,
accumulated type
faunas
parts upper
of
a
platform
between (Tankard
the
Damara
parts
of and
folding The
et al.,
the
Pan-African (Fig.6.48).
and
Nama
Naukluff
represent
margins
complex
/7
of in
the the
craton
age
of
roughly
with
and Malmesbury Pan-African Nama
basin
Damara
on the northern
Ediacarathe
upper
Groups.
molasse.
were
and
(Fig.6.48),
its
affected
Gariep
margin
of the Nama
~! "".,
eth~,ieKeetma,ns'.:""..";:, .j_ :c AUS ~ ;-:'-':'~l''"
'
"":-
~ , , o p
\
" ::', "ho.op'-~
i r~,l~ISn rlnv~/river
I~
":: ::'1,,.
IVY^ .....~.-our~.-.~::.;,'~
N0ma
Group
Folded Nama Thrust
.,ooK=.
.....
MoJmesburesbur~ Vanrhysdorp Figure 6.48: et al., 1982.)
The
Nama
Group
foult
Contact with younger cover Contact with K a l ~ a r i bQsement
Lines of section in Figure 6./, 8 in
Namibia.
(Redrawn
from
by
belts
i~,
~:~/':;i:::
The The
!
~'~ ~ .,~.Schip
't Luderlfz
the
from the Damara orogen onto the Nama Group. ~.doek
I
Kalahari on
the Gariep
thrusting nappe
the
based
It correlates
and
Group
southwestern and
on
550 Ma
1982).
Supergroup
northwestern
basin was thrust
sequence
650 Ma
Tankard
344
The Nama Group
is of great
paleontological
the earliest
metazoan
are well
preserved
pearance
of
metazoan
fauna which
ran fauna,
soft-bodied
Precambrian Nama
Subgroups lithofacies
Group
pattern
later
organisms the
which attest
latest
belongs
and later
in it
to the ap-
Precambrian.
in detail,
in the Nama Group,
because
The Nama
to the Ediaca-
found
in similar
strata in other parts of the world. comprises
(Table 6.2).
progressively
during
is discussed
first discovered
uppermost The
organisms
significance
It
with
shallow
intertonguing
Table 6.2: Stratigraphy et al., 1982.) Formation Member
the
Kubis,
thickens
with
of
marine
Nama
and
platforms
argillaceous
Group.
and
displays
carbonate
intertidal
the
Schwarzrand
westward
(Redrawn
Fish an in
River overall
the
west
fan deltaic beds
from
Tankard
Interpretation
Description i,,,
Shale and mudstone.
~r
Tidal flat Gad fan delta
Phycodes pedum
Braided a l l u v i a l plain
Sandstone ---
- _--
•
.--'7 . . . . . .
OI
.......~ ? >
~ Horibe,
•":)C~.~:'~.~g..
.~. . . . . . . . i......-..i •
cc NABABI5 ; > Zarnnarib ¢¢
;
-
I
!T,o .
.
.
.
Braided a l l u v i a l plain and d)stal fan delia Braided alluvial plain and fan d e l t a
U-shaped v a l l e y s , striated floors t shale r diamictite
(~Itn ~n S Glacial~
spitskop
Blue limestone(oolites,cross-bedded grainstones columnar stromatolites, Cyclomedusa
C l e a r - w a t e r shoaling platfof m. stromatolites biostromes, t i d a l flats
eldshuhho.
Green and red shale and siltstone with i n t e r bedw~ed sandstone. Trace fossils
subtidal followed by fan
m N
I .
.
.
(3
~ ,n
Tidal flat
! URUSIS
g ]o I ~o I _e
Ho.s
Car bangle platform, stromatolites biostromes, fida| fiat, Fan d e l t a i n N, intertidal a n d subtidal with carbonate shoals in 5
Blue and yedow limestone(Gables, first stomatolites C~o udina, Cyctomedusa Arkosic sandstone; interbedded mestone quadz arenite and diamictfte. NasepiCt, Ptertdinium Parameduslum
' ~ ' .". "~.":?;, "B
NUDAUS
Distal f l u v i a l and intertidal inN; b a r r i e r islands a n d carbonate shoals in S Reef-Lagoon complex, carbonate p l a t f o r m
Quartz arerlite and arkose Diamlctite, Rangea, Pteridlnium. Planolites Sko[ithos Blue limestone, shale. Clouding
:;..:?.V]F.7:..,,:~ i=//:e:,:'~';~ ~ to °l ~
plain
Tidal flat and fan delta tallowed by braided alluvial plain in N;
~o:~;~'2~c g g lllll
alluvial
Red mudStone and sandstone, blue-green mudstone and limestone overlain by sandstone and shale in S. Phycodes an d Planolites
VEROESIGNlep NO~4TSAS Kreyriver
.
Braided
Tida( flat and fan delta
Red mudstone
STOCKDALE Red sandstone and mudstone at ~iop
=u" ~
~ -
sandstone
-/J
£..:~.C.'..:-.L./{,
1
Red orkosic cross-bedded
~ BRECKHORN Sandstone(trough cross-bedd ng, mudstone partings)
.-,.., • • ,/
Tidal flat and fan d e l t a
Mudstone with thin sandstone i n t e r b e d s Phycode s pedum, Skollthos. Enigmo~ichnus african[
I
.,phoe,
DABIS ~ MoralKani-
Arkosic sandstone evaporite diamictile Rangea I Pteridinium Namalia Er nietta, Or thogonium, Skollthos J
AIluviQI fan. t l d a I fiat, Lagoon in W
Conglomerate arkose, quarlZ arenite, organic-richJ dolomite limestone, Clouding J
Braided f l u v i a l plain, debris flows, bioherms and tidal flats in W
I
345
in the east
(Fig.6.49).
and less mature graphy during marine
alluvial fan deposits further to the east.
the deposition
transgressions
oscillations, carbonate
The fan deltas in turn interdigitate with coarser of the Nama Group was
from
the
west
which
because
units.
The
siliciclastics
show
by shallow
of
created an alternation of clastic shoreline
platform
The paleogeo-
dominated
shoreline
lithofacies and
upward
decrease
maturity.
E
_
~
_ -
.
~ : o
:1:: ~:
Nabc~b;s
S~ockd~teFm
u~
~
_ - _ T ~
.
.
Fro. " •
"
- . L
. "
•
_
-
-
.
-
" . .--'."
,
-
.
.
.
-~... •
E " , ~ - . "
groine~
~
_
. "
r se-groined
":'-::-t
" . . . .
L :~'-
-."
.
,,G
archive orenile
Conglomerate
D
N
/
~
.
T
r
N
Gross
°ugh
bedding
......
_ % . , . ...... ,
/ AubFm
S E
NabrabsFm" L u ~
"
*,' ' . . . . •
"'
-
•
.-". • ".' -'-
~ . . . . .
"
. . . . . . .
"
" "
"
....
:
~"
>_o
in
lEE
E ~
StockdaJeFrr
~
Norn,$QsF
N O
U
- - ~:
E~
'
B "~2,
"
'
4.
,I.
Zoris
//
+
Figure 6.49: Stratigraphic (Redrawn from Tankard et al.,
sections 1982.)
through
the
Nama
Group.
in
346
A westward ted the
basin
of
basal
sequence quartz
projecting
Nama
arenite
into
deposited
intertidal
in
the
with
and
stromatolitic
reefs
serpulid-like
polychaete
are
two
The on older origin,
River
Nama with
arkosic
of
micritic
the
and oolitic
the
of
diamictites
Subgroup,
deposits.
a late
trace
limestones
huge
domical
in association
In the Schwarzrand
origin
which
and
ridge,
are
with there
probably
of the Gariep Group. Damaran
It comprises
the marine
platform
shelf margin
glacial
Fig.6.49). fluvial
basement
contain
and
fluvial
limestones
distal
Osis
carbonates
grew along
sandstone
(Table 6.2,
Subgroup,
separaa varied
braided
stromatolitic
environments
The
diamictites
to the Numees
Fish
and
north
initially comprises
eastern
worms known as Cloudina.
disconformable
equivalent
an
Schwarzrand
bedded,
which
in
shales
deposits
(Fig.6.49,B).
Ridge
Subgroup
coarse-grained
subtidal
overlying
thickly
south
the Osis Kubis
deposited
westerly
argillaceous
interfinger in
more
unconformably
intertidal
The
were
and
the
high,
conglomerate,
which
environment;
In
basement two parts.
molasse,
red-beds
of
rests
alluvial
unconformably and
tidal
flat
Phycodes occurring abundantly at the
fossil
top.
6.7 Katanga Orogen 6.7.1 Regional
Setting
Porada
defined
(1989)
interior
central
Zambezi
belt
between
the
arc, by
(Fig.6.1). Zaire,
an arcuate
respectively.
It lies
intensely
deformed
strat i g r a p h i c a l l y
system belt;
an
Zambezi
(Porada, the central
the
the
belt
1986).
the
Lufilian
and
1989). arms
The
rifts
than
latter,
arm
The Lufilian and
the
southeast
Irumide
frontier
the
situated
belts
southwestwards
The Zambezi belt is also arcuate
arc
of
the nuclei
arc,
the
being
are
(Fig.6.51),
southern
constituted
and
known
and
provinces
to the northeast
of the Lufilian
the
well
arc
(Fig.6.1).
Zambia-Zaire
(Daly,
continental
tectonic
Kibaran
highly metamorphosed of
two
Lufilian
cratons
is bordered
belts,
extension
comprising the
contiguous
to the southeast and
as
belts,
(Fig.6.50,inset).
shear zone
from b i f u r c a t i n g
orogen
and Kalahari
astride
Angola
in shape.
are
belt,
mobile runs
into e a s t e r n m o s t
the Mwembeshi
Both
orogenic
It
Katanga
Pan-African
Bangweulu,
mid-Proterozoic
The
the
African
and although arc,
it
separated
from
it by
believed known
this
more
Lufilian
rift
as
to the
lay
have
evolved
Katanga
in the
of the Lufilian
is
rift
Zambezi
arc; whereas
347
the northernmost
arm of
angle
re-entrant
from
the
Kundelungu aulacogen
the
(Unrug,
rift which of
the
extended
Lufilian
northeastward
arc,
developed
at a high into
1987).
m
) L
KUN LUF]
OTHER PAN
Figure 6.50: Geological map of the Lufilian arc. i, PaleozoicRecent; 2, granitoids; 3, Katanga Supergroup; 4, dolerite; 5, basement inlier; 6, metasediments and sheared basement in the Zambezi belt; 7, Kibaran belt; 8, Bangweulu block; 9, Archean-Lower Proterozoic basement; i0, thrust; ii, strike-slip faults; 12, lakes. H, Harare; L, Lusaka; N, Ndola; U, Urungwe klippe; MSZ, Mwembeshi Shear Zone; I, external fold thrust belt, II, Domes regions; III, synclinorial belt; IV, Katanga high; Y, Shaba aulacogen. (Redrawn from Porada, 1989.)
the
348
The
Lufilian
Drysdall, region;
1964): the
arc
consists
of
the
external
arcuate
synclinorial
belt;
four and
structural fold
the
and
zones
thrust
Katanga
(de
Swardt
belt;
the
high
and
domes
(Fig.6.50).
The
Kundelungu aulacogen is underlain by the Eburnean Bangweulu block; whereas the
Lufilian
basement
arc
has
folded
and
metamorphosed
in the Shaba Province of Zaire,
Kibaran
and the Ubendian
rocks
as
its
Lufubu schists
and the Kibaran Muva Supergroup as basement in the Zambian Copperbelt.
i.o.'.'."
\ \
'\ \ ~
.
~i~!
o~
/ "*
/
,D 2~ 3~ 5~ 6--
1000 Km
Figure 6.51: Inferred Upper Proterozoic rift systems in Southern Africa and Eastern Brazil. MSW, Mwembeshi Shear Zone; i, rift fillings; 2, aulacogen deposits; 3, rocks deposited and/or affected by the Kibaran event; 4, Sao Francisco-Congo cratonic bridge; 5, transcurrent fault; 6, assumed rift margin. (Redrawn from Porada, 1989.) The
thick
in the Katanga
and v a r i a b l y deformed orogen
and m e t a m o r p h o s e d
is known as the Katangan
stratigraphic
Supergroup.
pile
The Katangan
Supergroup has attracted world-wide attention because it holds half of the world's areas;
cobalt
reserves;
it
is
among
the
world's
largest
copper mining
and with a host of other mineral deposits such as Zn-Pb su!phides,
U oxides
and noble metals,
the entire Lufilian
greatest s t r a t i f o r m m e t a l l o g e n i c provinces.
arc
ranks
as
one of
the
349
6.7.2 The L u f i l i a n A r c
Stratigraphy In the
Lufilian
arc
the
Katangan
mineralized
Roan
Group,
ferences
the
original
the
in
Lufilian
different sequence
arc
Fig.6.52
arc;
it
an
(Unrug,
Supergroup Lower
the
and
in
most
and
aggregate
comprises
Upper
and
the
in
stratigraphic
thickness
of
heavily
structural
evolution
variations
between
arc.
the
lower
Groups.
the
Lufilian
the
Kundulungu
stratigraphic
complete
is a generalized
shows
Supergroup
for
domains
thickest
belt.
the
rift morphology
account
structural is
and
The
external
section
about
of the
stratigraphic
fold
across
9 km
Dif-
for
and
the the
thrust
Lufilian Katangan
1988). KUNDEL U N G U LUPILIAN
--
FOLOBELT
AULACOGEN
~YT
SYNCLtNORIAL
F000M
BELT
DOMES
INNER UNIT
';'n. r~
REGION
MIDDLE
UNIT
O
2000
.... 4000
~
PETIT
C O N G L O M E RAT_
-~ -~ •: ' - - ' ~ ---
000
", ~ MWASHIA' UPPER ROAN
LOWER KUNOELUNGU VOLCANICLASTIC PELITES
" ~
KAKONTWE LIMESTONE GRAND CONGLOMERAT
P• • • i • • • • '" ,.,---r~_ .
ROAN ".
~--
SHALE
""* ÷I
:RAT ROUGES ~•
Anhydrite
~ ~
SJltstone~ Mudstone
Carbonaceous Siltstone
M i x tite~ L o m i n i t e ~'~
Sandstone
' ' ' " "
•
and
~
e • •
MINDOLA C LASTICS F'M, B O U L D E R C O N G L O M E R A T E ~°o~/7*. " . ' l S E R I E DES MINE ¢-
I0000
// /
' ~ ~ ""
t/
Dolomite, Limest one
,/
~=~,~_~
GROUP " . ORE
/
/
/
~,,
DOLOMITE ~
8000
--
~,.___~--'--~
ii tJ
Arkose
•
Conglomerate Basement
Figure 6.52: Stratigraphic (Redrawn from Unrug, 1988.) In either
the
gneisses. 1.2-1.1 Ga Group
external
Kibaran Since old,
represents
fold
metamorphic
belt
columns
the
rocks
Roan
or
upon
for
the
Group
the
first
presumably phase
of
rests
Ubendian
the Roan rests upon the Nchanga deposition
Katanga
started
Supergroup.
unconformably
basement
granites
upon and
Red Granite w h i c h
is about
after
The
sedimentation
this
time.
(Fig.6.53,A),
Roan
in w h i c h
350
the
initial
uplift
and arkosic
unit,
the accumulation
and rifting
the Mine Series, lagoon
the deposition
of a conglomeratic
followed by a marine transgression
of shallow marine
platform-hypersaline (Fig.6.52).
caused
clastic
evaporitic
and
rocks and the mixed carbonate
sequence,
known
as the Upper Roan
The Upper Roan is conformably overlain by the Mwashia Group, a
sequence of carbonaceous
shales and siltstones;
locally developed
pyroclastic horizons.
gabbro Group.
the
occur
in
UPPER
Upper
Roan
and dolomitic
Many irregular
and
in
the
shales with
sill-like bodies of
overlying
Lower
Kundelungu
KUNDELUNGU LUFILIAN FOLDBELT AULACOGEN INNER ZONE MIDDLE ZONE OUTER ZONE N/ O M 5 ....... :i::~'!?::'.. •':"."::<;':'~~':';~C:;:~I;::.:~ ......'-
GROUP
~
oo o o
°o , ~ o ~ o / ~ ' ~ . _ : ' ' ' " : ' t " ; "
,ooo
:.:... OOoo
.:.:::
--"=
LATE KUNOELUNGU TiME
~000 M
C
9M LOWER
GROUP
-
,.1000
~____~~ ', 3000
i"
:::::..
000
7000M EARLY KUNOELUNGU T~E
* " " "
B
~M
ROAN GROUP
000 ~0OO M
ROAN TIME
A Figure 6.53: Basin evolution for the Katanga Supergroup. See Fig. 6.52 for explanation of symbols. (Redrawn from Unrug, 1988.) According to the second stage
Unrug (1988) the overlying Lower Kundelungu represents (Fig.6.53,B) in the evolution of the Lufilian basin, in
351
which the northwestern and northeastern margins of the basin were uplifted and
glacial
conditions
et al.,
1984).
phase;
and
it
glaciomarine granite
and
basalts
at
about
developed
Consequently, consists
laminites
of
with
pegmatite the
the
of
the
which
the
surrounding
conglomerates,
dropstones.
clasts
base
in
highlands
"Grand Conglomerate"
"Grand
glacial
Since
have
formed
the
been
this
diamicites,
and
conglomerates
dated
Conglomerat"
(Cahen
during
at
have
976 Ma,
yielded
contain and
an
the
age
of
948 Ma, the latter date is probably close to the age of the glacial
deposit.
The
overlying
Kakontwe
Limestone
(Figs.6.52,6.53,B)
the development of carbonate platforms on basement horsts part of the Katanga rifts. Copperbelt, there
is
but
a
change
towards the northeast. belt during
This limestone is up to 350-500 m thick in the
thins out northward towards
facies
represents
in the southern
into
the external
subgraywackes
fold belt where
(Fig.6.52)
which
coarsen
Rifting and block faulting occurred in the external
the deposition
of
the Lower Kundelungu
(Fig.6.53,B),
leading
to the accumulation of up to 5000 m of pelitic sediments; whereas north of this
subsiding
zone,
only 400 m thick The
third
Kundelungu developed
phase
Group. on
conglomerate
in the Kundelungu aulacogen,
the equivalent
of
basin
Because
the
Kakontwe
tongue--
evolution
of
uplift
Limestone,
spread northward
is
represented
in
the
south,
and
the
"Petit
into
the basin.
of the basin margins and subsidence of the basin aulacogen sequence
(Fig.6.53,C) of
the
Kundelungu
unit is
(Unrug, 1988).
Hpper
was
resulted
in
Kundelungu
syn-orogenic,
the
but
Upper
Conglomerat" Pronounced
of
Deposition
the
the
unconformity --a
uplift
floor in the Kundelungu
deposition
Group.
by
an
the
thick
of most
sequence
of
clastic
the Upper
terminates
with
a
distinctive purple arkose which may represent the m o l a s s e of the Lufilian orogeny. This molassic part has been traced far into Zaire and Angola. Rift-related m a g m a t i s m accompanied the e v o l u t i o n of the Lufilian basin especially
during
the
deposition
Groups. As outlined by Unrug and was
of
the
Roan
and
the
localized along rift faults which delineated
the Lufilian aulacogen.
basin
and
Lower
Kundelungu
(1988) volcanism was bimodal, acid, and basic
in the north along
the western part of
the margins
of
Trachyandesites are exposed in the Roan Group
the Kundelungu
in a belt 180 km
long in Alto Zambezi in Angola; rhyolite intrusions and tuffs occur in the Roan
in the
rhyolites
north-central
and
dolerites
comminuted
volcanic
rock
Kundelungu
Group.
These
part
of
occur
in
fragment
the
Lufilian
the
"Grand
makes
volcaniclastic
up
arc;
the
pelites
material
regionally widespread hydrothermal alteration.
and
clasts
Conglomerat'o
Basalt
show
of
Up
of
the
both
to
50%
Lower
evidence
of
lava flows occur at
the surface along both margins of the Kundelungu aulacogen.
352 Tectonism
Based
on
structural
interpretations
Daly (1984), Daly (1986c) and Unrug
by
Cahen
et
al.
(1988) two major
(1984),
Coward
and
(DI and D2) phases of
the Pan-African orogeny affected the Lufilian arc. These have been locally termed
the
Lusakan
dated at about nappes where
the
areas.
The
initial
slices
phase
of
the
Lusakan
or
of
the
Kundelungu
also
occurred
nappes
could
in
the
have
aulacogen
and
onto
synclinorial
belt
from
been
thrust
onto
(Fig.6.54) which acted as a ramp during this orogeny. and
thrust
compression
or
belt
crustal
folding,
caused the northeastward thrusting of Lufilian arc
re-entrant
Thrusting
basement
region fold
850 Ma,
towards
cratonic
orogeny.
resulted
shortening
from
very
in which
intensive
the domes
the
domes
The external northeastward
region was
probably
detached and forced northeastwards.
S
N INNER UNIT MIDDLEUNIT OUTERUNIT { SYNCLINORIAL) { DOMES) ( EXT.FOLD AND THRUST) Zombezi belt M m" shi High grade Metamorphism U.Kundelungu Kibamn
~Lwe De', '-ravit" minimum ,I U|$|o{a.I|ON | ~ ,¥ y ,.HookGroni~ ~ 1 / post-orogenic syenite i ,, ,' ,' ,, .. / r,' ![.2 .-;:'")" 1""
A
later
,Foldbelt
II'K°°n
III
I
,-.
:.:.-..:
"":<";'""
:';':';;'!i!ii
Figure 6.54: Schematic section (Redrawn from Porada, 1989. )
(Porada,
iL.Kundelungu
h
Lufilian
1989 )
was
orogeny
(Cahen
probably
across
the
et al.,
responsible
Lufilian
1984)
at
the
D2
for
fold
about
belt.
750-700 Ma
deformation.
It
produced the present arcuate shape of the Lufilian belt.
It also affected
the Upper
phases
Kundelungu
recognized
in which up to
Fold
characterized recumbent
D1
Roan
the
over
and
by
an
folds
Thrust
Belt.
arcuate
(Fig.6.54),
Kundelungu.
Over
belt and
Angola
Copperbelt.
through
The
external
of
east-west
fold
and thrust
thin-skinned
local
a distance
southwest to the zambia Copperbelt, in
have
been
(Cahen et al., 1984) at the re-entrant of the aulacogen.
External
NE-SW
five deformational
nappes of
belt
outward-facing
is to
and
overthrusts
of
the
800 km
from Angola
in
the
the southeast axial trend swings round in
southern
Zaire
During D1 thrusting the Kafue anticline
to
SE-NW
(Fig.6.50)
in
the
controlled
353
the
external
fold and
decoupling contact cover
horizons
or within
and
movement
During
the
direction.
The
thrust
belt
resulted
D1
the
inliers
affected
the
Region. with
in
to
at
The
the
domes
thrust
southward
sheets.
culminations In the
domes
vicinity
exposed
to
of
the
suggest
is
was
grade
fold have
movement;
whereas
belt
and been D2
Zambia and Zaire
there
This
is
low-grade
increases
characterized
slices
also
The
of
the
slightly
thick-skinned rocks
forming
gravity
anomaly
basement
slices
northeast
and biotite east-west region
schists
been
D1
which were
trending
has
during
folds
with
interpreted
as
1986c). sediments mineral
of
the
were
metamorphosed
assemblages, Kabompo
intervening
metamorphism
decreases
by
basement negative
towards
domes
(Daly,
the
of
shows
recumbent
zones in
the
Kundelungu,
superposition
Katangan
domes,
in a northerly
in
for
dome,
indicate
Kundelungu
which
westwards,
to the
example
Group,
hardly
mainly
attained
southwards,
in
high-
and
the
towards
fold and thrust belt.
completely
scanty
structural
region
was
with
But
the
Region
basin which
granitic
sheared
structures
in northwest
facies.
region
kyanite-rich shear
conditions.
Synclinorial also
the
and
almandine-amphibolite
marginal
basement-
the
re-folded
thrust
and
the
planes.
region
between
the external
in
developed
Upper
quartz-muscovite
large
axial
grade;
and
This
above thrust ramps
amphibolite
to producing
in the south and west.
by
transport
folded
dipping
the
strata
region
thickening
event produced
subsequently
It generated cover-basement
tectonic movement.
shearing
event.
were
best
greenschist
facies
Tectonic
pressure
below
fold
the
basement
crustal
This
the
lineations
east-northeast-directed
external
metamorphism
are
the younger
(Fig.6.54).
the
which
Copperbelt
earlier
to the epidote-amphibolite
due
in addition
D1 structures
structures
an
Throughout
Domes
at
the northeast.
which
tectonics
Copperbelt.
either
northeasterly-facing
from a northerly directed
regional
Zambia
Stretching
of
D2 deformation
to
structures
bedding
prevalence
of
attributed
in the
to
structures.
the
towards
belt
lower Roan sediments,
imbrication
basement
thrust parallel
also
folded
during
information affected
intrusions
contact
and Katanga
in
metamorphic
grade metasediments
High.
The
synclinorial
lay to the south and west of the domes
by
the
at is the
least
two
Lufilian
aureoles
which surround
two
available
deformation about
deformation arc
are
developing
the
in
the intrusives.
events.
Katanga
events.
found the
belt
in
the
was
region.
Although
high,
Almost
this
all
Katanga
otherwise
the
It was
very
the
high, low-
354
6.7.3 The Kundelungl/ A u l a c o g e n This
is
which
a
wedge-shaped
branches
from
and
the
northeastward
northern
tapering
re-entrant
to
fault-bounded
the
Lufilian
trough
arc.
The
Kundelungu aulacogen runs northeastward for 600-700 km between the Kibaran belt and the Bangweulu block.
Nearly 7 km of u n f o l d e d Katangan Supergroup
(Fig.6.53,C) lies in the Kundelungu aulacogen. In
the
aulacogen
continental-marine
the
cycles
Roan
and
Group
is
consists
disconformably
of
four
overlain
alternating
by
the
"Grand
Conglomerat" which along the southeastern margin of the Kibaran belt is up to 1,200 m thick probably reflecting active
faulting,
existence
region.
thickens
of
active
from about
in the centre. and
glacial
along
is a corresponding the
centre. The overlying Kundelungu
and
Kundelungu
Group
transcurrent basin
on
margin
downwarping and the The
Lower
to
facies
more
change
pelitic
Kundelungu
to about 400 m
from conglomerates
deposits
in
the
basin
"Petit Conglomerat" rests u n c o n f o r m a b l y on the Lower the
basement
which
faulting,
margin
in that
250 m along the edge of the aulacogen
There
psammites
fronts
along
accumulated consists
(Fig.6.53,C)
the
during
a
basin
margin.
phase
of
of conglomerates
and
siltstones
and
The
Upper
compression
and
sandstones
shales
towards
near the
and the
basin
centre, with over 1,000 m of arkoses at the top. Like
the
Athapuscow evolved
Gourma
from
compressional
a
which
clastic
sequence
downwarping stage. et
al.
Africa
stage;
whereas
and the
and
a
the
the
which
overstep (1989)
Middle
downwarping
volcanics
of
the
followed by a phase calcareous the
upper
trough
Proterozoic
Kundelungu Roan
the
Lomamian
to
a
Group were
of graben filling of
and
margin
the Mwashia
the
orogeny
succeeding
represents
suggested that the downwarping
termed
aulacogen
stage,
part
"Grand Conglomerat"
Porada (1984)
or
Canada,
through
clastics
argillaceous
accumulated;
Cahen
phase,
the graben
the
in West
northwestern
The
Subgroup
what
of
graben
phase.
deposited during during
aulacogen
aulacogen
of
the
stage was
about
976 Ma,
which affected Katangan deposits along the western flanks of the Kibarides where
it
accumulated
produced after
SW-NE-striking the
beginning
faults. of
the
The
Upper
compressional
Kundelungu stage
Group
which
as
already noted was caused by the first Lusakan d e f o r m a t i o n at about 850 Ma.
355
6.7.4 The Z~mbezi Belt
Regional Setting The Zambezi belt occupies the northern part of Zimbabwe, and a small portion of the western Mozambiquian pedicle The
Zambezi
belt
is
an
arcuate
deformed
metasediments
southern
part
truncate
the
orogenic
belt
and
Umkondo
separated
with
the
from the
thrust
belt
intercalated Zimbabwe
of
basin
high-grade
basement
craton
southern and
gneisses
and
the
(Fig.6.50).
Zambia,
(Fig.6.50, inset). intensely
which
in
adjoining
The
Zimbabwe craton by an arcuate thrust
Zambezi zone.
the
Magondi belt
Along
is this
thrust zone m e t a s e d i m e n t s and basement gneisses have been thrust southward to the extent Snelling,
of detaching and transporting
1971),
from the
Zambezi belt,
the Urungwe k l i p p e
and thrusting
(Vail and
it over a distance
of 40 km onto the craton. To the east of the Zambezi belt lies the Southern Mozambique belt; and to the north the Mwembeshi Lufilian
arc.
transcurrent
The
shear zone separates the Zambezi belt from the
Mwembeshi
boundary
which
shear
sharply
zone
marks
separates
a
the
major
low-grade
sinistral metamorphic
rocks of the Lufilian arc with east-northeast tectonic transport, deep
crustal
medium-to-high
(de
Swardt
and
grade
Drysdall,
metamorphic
1965)
southwesterly m o v e m e n t direction
which
rocks
are
of
the
from the
Zambezi
characterized
by
belt west-
(Daly, 1988).
Stratigraphy In spite of the difference in metamorphic grade Katanga metasediments have been traced across the Mwembeshi shear zone and are believed to have been metamorphosed to a higher grade in the Zambezi belt.
The m e t a - s e d i m e n t a r y
sequence
its
of
Formation
the
Zambezi
comprising
dolomitic
belt
(Porada,
feldspathic
limestone.
There
1989)
has
quartzite,
are
also
at
base
calcareous
epidosites,
the
Chunga
schists
and
garnet-bearing
amphibolites,
and rhyolites which are probably the Kafue rhyolites. At the
base
Chunga
of
the
basement gneisses. limestone
which
is
quartzite.
The
next
carbonate
rocks
at the top of its upper coarse
overlain unit,
ranging the
part,
(Porada,
are
schists
is
by
probably
the
quartz-muscovite Lusaka
from dolomite
sequence
and
conglomerate
Supergroup
sequence
representing
altered
The overlying Cheta Formation consists of a thick basal
which
includes
comprises
limestone.
is argillaceous
unconformably
1989).
dolomite, to
(chlorite)
clasts
at
from
a the
few
and
variety
The Kawena
at the base
overlain
a
schist
and psammitic localities
underlying
of
Formation in
by a
Katanga
356
The metamorphic
grade
in these
rocks
range
with relict two-pyroxene granulite-facies rocks
in southern Zambia,
(Vail and Snelling,
from medium-to-high-grade
assemblages
in sheared basement
and sillimanite-bearing assemblages
in Zimbabwe
1971).
Structure
The
structure
of
basement-cover thrust
(Porada,
which
at
in
belt
with
Zambia
consists
high-grade
consist
the epidote-amphibolite
1989).
tectonic
Zambezi
imbrications
nappes
limestones
the
Daly
transport
(1988)
direction
large-scale
metamorphism
of Katanga and
inferred along
of
a
high-level schists
metamorphic
dominantly
the entire
and
Supergroup
greenschist
deep-level and
facies
west-southwesterly
Zambezi
thrust
belt
and a
general structural vergence towards the SW and WSW (Fig.6.55). Two deformation phases similar to those of the Lufilian arc have been recognized
in
deformation
the
contemporaneous between
Zambezi
caused
950
the
with Ma
the
and
belt
(Daly,
1986
c,
1988).
ENE-WSW-directed
movement
which
E-NE-directed
thrusting
in
850 Ma.
The
D1
D2
event
was
The
early
was
the
D1
probably
Lufilian
north-south
arc
directed
(Fig.6.55) and caused emplacement of the Urungwe klippe of Zimbabwe around 900-800 Ma
(Porada, 1989).
6.7.5 Mineralization in the Katangan Orogen The Zambian-Zairean Copperbelt of the Lufilian arc holds more than a half of the world's reserve of land-based cobalt deposits and about world's in
the
copper reserves. Copperbelt,
Zaire
has
the
largest
tons, while Zambia holds 1.7 million tons is a polymetallic metallogenic province, vein,
and
sulphide, stratiform the
most
in
oxides
and
which and
of
the
noble
share,
in
(Goossens,
The
deposits
Unrug's the
explanation for the
concentration.
with
about
1983).
3.1 million
The Copperbelt
the major types being stratiform,
dominant metals.
vein mineralization
comprehensive
processes Unrug's
skarn, Cl
12% of the
Of the 4.8 million tons of cobalt metal reserve
following
are
(1988)
Copperbelt sources outline
Cu-Co genetic
and
(Fig°6.56)
of is
metals based
Zn-Pb
model
for
offered
and
their
mainly
on
(1988) model.
Stratiform Mineralization
This occurs mainly in the external fold belt and in the domes region where the
major
associated
deposits
are
copper,
copper-cobalt
and
uranium.
include nickel, gold, platinum group metals,
Other
metals
selenium, cerium,
357
4t
Lufilion
a
o,emoo
~
Mwembesh~
~--]
zone
~
-
Zombezi
Pre-Pon- African
Zimbabwe Croton
~it
KI extention lineotion
~'~
K2 extentlon [ineation
lOOkm
,'~1 ~ ,.'Chiman~moni
B
/" Lufillan
Ndoio
~
.~
/
,I
," ,"1"" / ~
~-~f~
~ ~,_~
deformo,ion
~-~_~
Zombezi
~
belt
~ . ~
(~> Urungwe KHppe
100kin
/
Locolised Z o m b e z i
,....... [..~--/J/._
o~'~-*- ~ \ / / "¢ inter - cant I nentol Shear zone
,/ : /
,,,"vo,,e~'/" sheo. ~o..
/
.
;
Figure 6.55: Mwembeshi Shear (Redrawn from Daly, 1988.)
Zone
and
.
\~ Zombezi '~" ~ Defor mat Jan '~ "
associated
thrusts.
molybdenum, vanadium and tungsten. Uranium occurs as uraninite and in many secondary
minerals.
Copper
chalcopyrite are present,
sulphide
such
as
and in the domes region
chalcocite, there
bornite,
is a zonation of
these minerals in the above sequence away from b a s e m e n t highs and from the base
of
the m i n e r a l i z e d
into
pyrite.
The
copper
panied by carrolite, form
zinc
and
lead
zone
upwards
sulphides
in
where
copper
mineralization
copper-cobalt
deposits
are
grades accom-
cobalt pentlandite, and cobaltiferous pyrite. Stratisulphides
occur along
posits, especially at Mufulira.
the
fringes
of
some
copper de-
.I
Q
•
,l.-o'oJ Kale n g wa'~'"
• ZAMBIA
Kolwezl District Tilwezembe
•
Figure 6.56: (Redrawn from
ZnlPb
Au
culcoIu U
CulCo CulColNi
Cu
Mineralization
Vein
~1~
J.~
•
~
II
showing
Skarn ~"
Lufilian arc Unrug, 1988.)
,~
~
0
Stratiform
• ~
ipo+ +
+ubungu-
~
Kalabi
'?n,e+
e, b w ~
.v~oon
Antelo e
the
Mwembeshi
distribution
÷ ,.,,,,.,,,
of
Zone
mineral
Dislocation
deposits.
.. , ~
Bwana Mkubwa
il ira
. . . . ~ ]MotQIQ~k~ . . mm a ....... ++ + + + ÷ 4 + *IIOlaQ# m w # m e mmmmBmm,eWm.m~
I
,'L~
•
~
h~fumpl
Shlturu
~ "go
Malundwe
Mufumbwe Dongwe M U F A L I R A - P r o d u c e r Presentor Past .='~'J~$""v Mokombo - U n d e v e l o p e d Deposits ., + ~ Mineral Occurrence + +, " " L o l o f u t o
ANGOLA
.,,,z*--.
Katongan Andeshes Basement Inllners Gr an;told Syenite Kiboron Ter rone , MoiorThrust Fault , Minor Thrust Fault ......... Fault m i m Slnlstral Tronscurrent Fault ...... International Boundary
~o
359
Stratiform (Fig.6.52) pebbly
in
sulphide the
arenite,
siltstone,
dolomitic
silty
arenite,
dolomitic
hanging wall
occur.
entire
quartzite,
stromatolitic
stratigraphic
cobalt,
"Grand
impermeable
Conglomerat".
dolomitic
Where
the
is found
gold,
copper
lead, chromium and molybdenum also
layers
of mixtites
mineralization
karstified Kakontwe Limestone,
and
of
the position of
region where minor
The upper boundary of stratiform mineralization by the
Group
arkose,
dolomite,
control
in most mines whereas
the basement inliers of the domes
provided
Roan
conglomerate,
Stratiform uranium m i n e r a l i z a t i o n
sulphates and traces of nickel, was
the
carbonaceous
Good
occurs
in
including
siltstone,
argillite.
the footwall is unpredictable. around
occurs
lithologies
feldspathic
sandstone,
mineralization
mineralization
permeable
in the Roan Group
and
extended
laminites upward
in the
into
the
the thick Lower Kundelungu pelites provided
the impermeable layer. Stratiform mineralization which
determine
stratiform
the
was
also
of
the
position
deposits
at
the
surface
structurally Roan
at
a
Group depth
controlled, rocks
that
by
that
is
folds
host
the
accessible
to
mining. Also faults determine the position of the ore body. The early hypothesis which
were
advanced
(e.g. Clemmey,
to
explain
the
1978; Garlick and Fleischer,
origin
of
the
Copperbelt
1972)
stratiform
mineralization emphasized the sedimentary origin of the ores. These models were
based
on
the
geochemical
and
diagenetic
processes
that
during the transgressive - regressive cycles in the Roan Group According
to these models
the
basin
Roan
where
during
high
the regressions
concentrations
of
saline
sulphides
lakes and
prevailed (Fig.6.57). existed
borates
in
took
place and iron and cobalt sulphates formed in the lake mudflats or coastal sabkhas°
During
established indurated
the
the were
concentrated
transgressive
sulphides swept
on
and
foreset
phases
when
the
in the sediments which had re-deposited
beds
and
in
as
detrital
truncation
lakes
become grains
planes
along
other
Since terrestrial
can mobilize and transport large amounts of elements
copper,
lead,
silver
and
zinc
as
they
migrate
with their
were
with
heavy minerals.
sulphide-charged algal mats of former lakes,
through
re-
which
high E h values, as
formation waters,
were
sufficiently
low pH and
the
such
hydrogen
such brines were believed to
provide the mechanism for metal concentration in the stratiform deposits. Although
the above salient sedimentological
features
and perhaps some
of the geochemical processes contributed to the Copperbelt stratiform mineralization, circulation
Unrug of
(1988) emphasized the importance of the convective re-
basinal
brines
that were
dients in the Lufilian rifts. Unrug
driven
by the
(1988) and Annels
high
thermal
gra-
(1984) also stressed
360
ENVIRONMENT OF DEPOSITION ,, , 'i------. . .
Argh~te- ---------' . . . . .
I .... I i,, I I t [ t
IIID [ I I
SURRAT'DAL
I
t
~ -
. . . . . . . . . . . -- --
,,,
I
CholcociteBornileond Minor Chalcopyrite Cuprlte N a t i v e Mqtachile
~ E O= "~ Grey
CONTINENTAL
Quartzite
O=
Gfi~ ! Bounded S h Q l e - O u a r t z i t e .. ... . . . . . . . . . . . . :.: .
-
m
-
•
I
Dotom}~e Doto mite Gypsum _ _ I I t I
P
ShGie
7--[ I
I I I , I .I ii
INTERTIDAL
I
Minor Bornile
I [1=1
Bornite
Catbonace
>-
. -
":
'"' ond
ChaI¢ocite
Grey QuOrtzlt e '~ •
- - L FLATS
Minor C h ~ I c o c I l e Qnd N a t i v e Copper
O(~rk Shales, •
-
SABKNA
SUB 1 0 A ~ L
......
P~nk Ou(~rtzi~e Arkose
~
5UPRATIDAL
~5
, Minor B o r n i t e and ChalcopyrJte
~ ~ X ~
Gypsum
I FLATS
10
Spe¢~IorI,e
o X
~
S *o
Minol" p y r i t e , Chalcopyrile and bornite
3 I
I I I
_~--- A~gilllte Quartzite
SABXHA
I
ORE MINERALS
LITHOLOGY
m~O
AGOON
: O~
MIr~orCt$ol¢o¢ite ond C h o ~ o p y r i t e Malachite
"~
CONTINENTAL SUPRATIDAL
Pink Q u a r t z i t e Arkose ~ - Bo,nded IShale
& ~
.
~
- 0.~artzi~, _LShale.
SABK~4A
Bornite
u ~
Ch:41cop)'r!te
,=,
Dolomite ~
z
Pryrite
--~ D o l o m l t i c SHtstone-"~'[ -u/
- SUBTIDAL FLATS ~
Carb~on r
CONTINENTAL
Ounes
Grey ~ • Quartzite
~ ¢ O : O~
Bo~nite M~o{coci~e c~nd P y ' i t e
•
q
_~ ~
Cha~copy ri~,e
~
Ch~ysocol~o
O ~
M(~QCh~t e
o o
Bioherm Oxidation Potential ( E h )
%
Cu
Dunes Cu Fe Co
An"" . e . o
e
"~.
Figure 6.57: Depositional control model of copper mineralization in the Copperbelt. (Redrawn from Hutchinson, 1983.) the
role
of
contributions
and
hydrothermal
in sourcing the polymetallic mineral deposits.
igneous
and
volcaniclastic
rocks,
The following
361
aspects
of
the
widespread wide source in
the
into
replaced brines
the
took
basin
pyrite;
the
and
brine
and
was
interaction why
the
at
in
a
fluid of
replaced
in the
late
copper
hypothesis
exploration.
It
also
by Garlick
of primary metal
sulphides
metals
were
sources
of
Lufilian
the
arc,
count
for
which
have
the
ted,
in
alteration
which
case
the
the
those
in
Tanzania,
ganyika.
In
deposits, sites
the
Recent
precipitate and the
(Fig.6.58,A,B). induced
depth)
Here
high
hydrothermal
sediments
which
area
have
heat
the
Rift
in
of
major
et
al.
that with diagenetic
rocks
sills,
(1989)
northwestern
part
Pb,
under
of
intense
a
thick
pile
show
opera-
pulse
and
Tan-
sulphide
lake
bottom
Tanganyika
seismicity (up
Zn
Kenya
Lake
Lake
and other metallic
of
analogous Cu
recovered
and
metals,
Ethiopia,
shallow
deep
through
ac-
times.
probably
faults
the
sulphides
the
could
rocks
first
where
The
of
processes
Djibouti,
from
and
famous
meaning
the
was
System
areas
suc-
described
of volcanic
hydrocarbons
flow
of
platinum-group
provided
rift
copper
mineralization.
and
circulation entrapped
and
the
explains
of
structures,
the
metal
planes
hydrothermal
Lufilian
African
it
dense
proved
during early Kundelungu
Tiercelin
on
This
volcaniclastic
fluids
intersection
fluids
(1972),
clasts
that
the
latter
and
nickel,
geothermal
at
water
East
the
origin
truncation
for
Since
from
hydrothermal
(20 m
in
of
during
times.
of gabbro-dolerite
hydrothermal
activities
and
igneous
it is believed
were
which
sulphides
Copperbelt
sedimentary
cobalt,
cobalt
deposition
explains
input to the Roan Group aquifers
sulphides and
of
and
during
the
metal
Zambia
affinities.
the
basin-
constituted
Kundelungu
of the
the presence
volcanic
Hydrothermal with
with
occurrence
mafic
metasomatic
metal
which
other
with
a
circulation
pyrite
accounted
the
the
and Fleischer
pyrite
Unrug:
during the mineralization.
foresets
replacement
copper
diagenesis
synsedimentary
deposits
the a s s o c i a t i o n by
and
by
requires
stratiform mineralization
indicate
in early
cross-stratification mine
of
primary
of
belt
folding;
stage
model
brines
stressed
mechanism;
(200 ° to 250°C)
by
were
Lufilian
before
inclusions
stratiform
from the M u f u l i r a
entire
place
of metal-rich
mineralized
the
circulation
earlier
sulphides cessful
mineralization in
and emplacement
basin
at temperatures
In trap
of metals
entire
brought
Copperbelt
mineralization
to and
have
6 km)
of
nonmetal-
lic deposits. During Group
pressured dewatering another
the
subsided
second by
pelites of
8 km
and
(Lower Kundelungu
these
basin-wide
depositional
some
pelites pulse
of
which
phase was
Group) are
metal-rich
in
the
overlain
Lufilian by
at least
rich brines
in
a
arc
sequence
5 km thick
volcaniclastic
could
have
been
the of
Roan over-
. With
the
material generated
362
which,
under
a
high
regional
heat
flow,
could
have
deposited metal sulphides in the permeable Roan Group. LUFILtAN FOLDBELT
S
AULACOGEN
(Fig.6.58,C).
oM
. .. ~9
~
ER
and
Meteoric• (and/or q l l ......... Hydr0thermal F4Uld L] Lacustrine Wotersl (Pembo.CapeBc~nza)f ,, 27" C l/I" 65-80°C "1
KUNDELUNGU
Inner Zone Middte Zone Outer Zone
circulated
ooo
Ill" ""~'8-8'Z
}
. t , SULFIDES
J
..
-
~.~jfiN2S-CH4_RICHJ
I
~__/LyA.,,O,,,,TE~ |
KAKOTWE llMI _~,,~----h E LTrt~--C
4~"
~
3000
~ ~ - - ~
~
'
N
.,..~v ,b
-
~otecln
...
~~LT/i;:!?: ...
,j
.I .
.
5000
7000
DOLERITE C
B
IRUNGA N
UVIRA
h,.EAULT
0v,.A i BARAKAI'~~ ~I~CAPE
.~,~u% r,,,,<91,,) KALAMBA
~
~
r
.
RUNGWE
- 7 !Jndfffer entioted Sediments
'~CenozoicVolcaniccentres iiTRM TranscurrentFau[t Zone vA- kOcQtion Of High Heot flow
v13|ues
A
Figure 6.58: Tectonic models for the origin of mineralization in the Copperbelt. (Redrawn from Unrug, 1988; T i e r c e l i n et al., 1989.) Vein Mineralization
This occurs mainly in the external
fold belt and in the synclinorial belt
363
and in the Katanga the
domes
copper
gold at
granitoids The date
quartz
where
present Kabwe,
veins
the
in
are
latter
association and
of
associated cut
vein
thrusting
complex
is the only one located of zinc,
metal
Shinkolobwe
and with
thermal
which were rejuvenated
folding
deposit
Major and minor deposits
are
Kipushi,
Gold-bearing located
The Kansanshi
region.
and
exploited
high.
Zn/Pb
and
(Fig.6.52),
Cu
at
Kansanshi
major
above
Conglomerat",
the
mineralization the
deposits
impermeable
suggests
that
shear
aureoles
and
are
(Fig.6.56).
zones
around
and
are
pre-Katangan
during the Pan-African.
suggests
vein
uranium,
associations,
with
late
presence
of
of fluids in the basin after the major d e f o r m a t i o n large
lead,
in
late
high
mixitites faults
ore fluids to cross permeability
in
phases.
the and
created
faults
which
considerable
post-
amounts
The location of
stratigraphic
laminites pathways
of
that
section
the
"Grand
enabled
the
barriers.
6.8 Western Rift Mobile Belt
6.8.1 Regional The Western
Setting
Rift
Rise,
Tanzania
cratons
orogenic
belt which
orogenies Rift
Rise
Southern
where
the
of
the
mostly
Cameroon
Pan-African
The Western
supracrustal
is
basement
Rift mobile and
It also cover;
of
pre-existing
Malawi
northward Lake
which
runs
across
(Fig.6.59).
where
along
the
the
all
the
it
northern
latter
it merges
of
of the
Kivu w h e r e
This
spanning
Uganda
extremity
province
of the
reactivation
and Kibaran
northern
extends
and
poly-
orogeny the Western
end
cratons
to eastern
presented
belt
is s t r u c t u r a l l y
Kibaran
includes
as
geology of the W e s t e r n (1984)
the
a
way
belt from
eastward
belt.
Ubendian
supracrustals.
It
terrane
Tanzania
in the west,
with the Mozambique
from
Zaire as
by the Ubendian
from the K i b a r a n
(Fig.6.59).
the
4.10.2)
reactivation
starts
far as the northern
and
separating
(Chapter
affected
foreland belt
Pan-African
Zaire
belt
During the P a n - A f r i c a n
the Rift
belt as
vast
Republic
reactivated
1984).
fold
mentioned
it branches
Mozambique
with
and
been
successively
Western
Rift V a l l e y
margins shows
was
constituted The
the Lake Malawi
merges
a highland already
(Cahen et al.,
structures.
Western
has
well
as
basement
domains
with
abundant
in
two
parts,
and
contain
deformed
but
post-tectonic
Rift belt which was well below
complex
which
summarized
the
southern
no
incorporates Pan-African
well
preserved
intrusives.
The
by Cahen et al. part
covering
364
northwestern ganyika;
Malawi
and
a
and
northern
both
sides
segment
of
southern
wherein
lies
and
the
central
Itombwe
Lake
Tan-
synclinorium
(Fig.4.42).
Figure 6.59: Platform sequences on the Zaire-Tanzania craton. i, cratonic areas since Early Proterozoic; 2, Early Precambrian partly reactivated by the Pan-African; 3, Kibaran belt; 4, PanAfrican m o b i l e belt; 5, Pan-African stable zones; 6 as (5) but under Phanerozoic cover. (Redrawn from Cahen, 1982.) 6.8.2 The Southern Sector The southern part of the Western Rift mobile belt starts from northwestern Malawi where of
East
Pan-African
Africa,
was
tectonism
caused
alignment
of most
as
along
the
the
controlled
Irumide
Mugesse
deformation
formations
were
regarded
by
tightening
greenschist-to-amphibolite various
tectonism,
pre-existing of
structures; shear
zone.
facies.
episodes
accompanied
No
have by
as part
the
of the Mozambiquian
structures.
Pan-African
pre-existing
Ubendian
folds;
re-
and important
shear movements
such
Diastrophism metasediments
been
reported.
emplacement
of
took
place
associated However, syenites,
under
with
the
these
de-
nepheline
365
seynites and pegmatites.
These intrusions probably took place at the same
time with those in the neighbouring Katanga orogen. Along Zaire)
both
sides
of
metasediments
Lake
Tanganyika
are preserved
(western
in symmetrical
Tanzania external
and
eastern
zones in Tan-
zania and northeast Shaba, on both sides of an internal zone where closely folded metasediments are found.
The metasediments
zone in Tanzania belong to the Buanji Group which towards
the
Supergroup.
tabular
However,
cratonic
cover,
the
in the eastern external extends
Bukoban
northwestwards
or
Malagarasian
the Buanji and the Bukoban-Malagarasian are not con-
tinuous. The
Buanji
Group,
about
Kibaran Ukinga Group. mudstones, shales,
si!tstones,
and
siltstones,
graywackes, containing dolomitic
1,700 m
thick,
rests
unconformably
upon
arkosic
quartzitic
sandstones; and
a
middle
conglomeratic
sequence
sandstones
with palynomorphs of Vendian age; and an overlapping amygdaloidal limestones.
vergence with
the
It consists of a lower alternating series of shales,
lavas,
The
pelites,
Buanji
folds dying
out
is
sandstones,
strongly
towards
the
folded
foreland.
sequence
conglomerates, with The
of and and
a northeastern sequence on the
western foreland belongs to the Marungu Supergroup. The lower part of this Supergroup is a sequence of coarse-grained detrital rocks which are folded along NNW axes, with folds which disappear towards the southwest.
The rest
of
Katangan
the
Marungu
Supergroup.
Supergroup
are
probably
equivalent
The internal zone is poorly known,
folded metasediments
which
have been
structurally
on the western shore of Lake Tanganyika,
to
the
but is believed to contain followed
where it appears
up to Kalemie
to connect with
the Itombwe synclinorium of northeastern Zaire (Fig.4.42). 6.8.3 Itombwe Synclinorium This
represents
Itombwe
the
northern
synclinorium
part
extends
of
the Western
approximately
Rift
mobile
belt.
from
Kalemie
north-south
The to
Lake Kivu and is occupied by a deformed m e t a - s e d i m e n t a r y sequence belonging
to
Kasiba
the
Itombwe
Group
Tshibangu
Supergroup.
(1,000-1,500 m
Group
Supergroup,
the
quartzites
which
conglomerates,
(2000 m Nya pass
The
Itombwe
thick)
and
thick).
Kasiba upward
Resting
Group into
Supergroup the
unconformably
comprises graphitic
comprises
disconformably basal
on
the
the Nya overlying Burundian
conglomerates
phyllites,
quartzites
and and
and black or gray slates and phyllites. The Tshibangu Group
is underlain by a mixtite, and consists p r e d o m i n a n t l y of pelites.
366
The
Bukoban
Tanzania
or Malagarasian
craton
correlatives
is the
of the
foreland
Itombwe
cratonic
tabular
formations
outcrops
of
Lindian
craton
Nya
Itombwe
equivalent
Supergroup
the
of the
on the
Supergroup
on
the
western
margin
Itombwe
Zaire
which are generally
Supergroup
was
The first deformation
Kasiba
Group;
Tshibangu
Group
schistocity south
of
the
Supergroup.
The
craton
are the patchy
classified
northern
under
shear
as the southern
parts
of
the
Zaire
was
symmetrical
open
last deformation
east
to
times
along
north-south
took place at the end of the deposition of the deformation
The
west
at
which
after
and
shear
compressive
660 Ma
by
and
axial
of the
plane
flow
folds along north-
stress.
alkaline
on both sides
cataclased
the deposition
folds
produced
regional
about
and granites,
folding
three
which
intruded
gabbros
deformed
second
during
was
syenites,
the
formed.
axes
Supergroup
the
on
(Fig.6.59).
The axes.
the
Supergroup
The
massifs
of the Western
sometimes
Itombwe
comprising
Rift,
before
recrystallized
the
intrusions.
6.9 Platform Cover of Zaire and Tanzania Cratons
6.9.1 Regional Tabular
cratonic
margins lying
Distribution
Pan-African
successions
in
(1982)
observed
stable
craton
in
sedimentary
(Fig.6.59),
cover
the
sequences
that
the
generally
areas
by
cover
therefore
were
located
mildly
felsic
directly during
ages are available
after
volcanism. contain
Pan-African
deformed
on
molasse. the
underlie.
Supergroup
the the
of
these
The
parts
the
of
northern
(Table 6.3).
part
tabular
Between
the
of the
only
Zaire
Zaire
and
is
Cahen on
the
preceded
Pan-African
while
the upper
cover which orogens
few
stratigraphic
were
radiometric correlations
in these sediments.
Zaire
sequence
the
cratonic
platforrm cover of the Zaire craton
southeastern
deformed
and was
Pan-African Since
and acritarchs
the
the
flat-
belts.
sequences
as well,
the
sequences,
in
to
orogeny
of
tectonism.
sequences
extensive
mobile
tabular
basal
of
once
equivalent
the Kibaran
foreland
Pan-African
intracratonic
On
Supergroup;
are
a
Pan-African
The parts
from the cratonic
The Pan-African
of
Kibaran molasse
have depended on the stromatolites
Phanerozoic
rest on the Zaire craton along its
remnants
which
deposition
began
some
includes
the
craton-encircling
cratonic part
formations
representing
crop out around the
basin
craton known
Tanzania
which is as
they
also
the Mbuyi
Mayi
the
cratons
Lindian southerly
367
Table 6.3: Correlation of platform cover sequences on and Tanzania cratons. (Redrawn from Cahen et al., 1984.) Welt Congo (Zalre, Ango~o Kasoi and Western Shoba Congo, West Congolion (Zoire) MbuJi Moyf Supergroup ) Superg roup >.62:5 MO
Bhoboand Zambian Copper Belt Kotongan
>590
Aruwimi Group Banolio orkoses Lowest port of "Sonalio orkoses" AIOIOshales Galombogo quartz
| ~
734
Upper KundeLungu
Mpioka Group
AAAAAA
Lovoe and lower mixtite
788
"Petit conglomerate" mixtite Kotongan
Lokomo Group
Lower Kundelungu aaa~
948
Sonsikwo Group
f
81i Group co 1050 BI Group Mayumbion BO Group volcano sediments
LOVOS
Bll e 811 BI7 BII BII
d C
"Grand conglomerate" and Lovos AA~&~ 976 Mwashya Roan
950
Akwokwo mixtffe
Ituri Group
UPperRoan
b O
LOWer ROan 83 ca I100- 7200 Nchongo Red Granite
1310
Central African Republic
][
Z
fclv Schisto aolcoire,~ cllI;84 Group lCl Upper mixtite Hour Shiloongo Group
Zaire
N.E. Zoire Lindlon Supergroup
602
lnktst Group
the
1310
Eastern Kivu (Zaire) Itombwe S u p e r g r o u p
SE Burundi and W. Tanzania Mologorosion [Bu] and Bokobon (T) Supergroup
660 700 D iolingo Formation Bakoumo Formation Bondo 'fluvio-glaciol conglomerate' Nakondo quortztte
Kibago Group (Bu]
Monyovu Red beds (T) [ Bugoago(Bu)=Ilogolo(T)
774
Bougboulou Group
MOSeOGroup l c13rbonole rooks (Bu) Kob,Jye(Bu)=Gogwe(T] omygdoloidol IOVOS A A A A A 813 Tohibongu Group wi)h basal mixflte
Mixtite
Uha Group (T)
Nkoma Group (Bu ] ~J900± 24 4MusindoziGroup (Bu) = Kigonero Flogs(T] a itlc|uding carbonate Aroake
978 Kovumbwe
MOIOntWo Ifiaso (K~boran)
patchy outcrops as already the Bukoban of the
of the Lindian correspond
noted.
or Malagarasian
Itombwe
Supergroup.
foreland tabular described.
Eastward
sequences
on the western Supergroup The Mpioka
is
to the folded margin the
and the
Itombwe
of the
eastern
tabular
Inkisi Groups
of the West Congolian
orogen,
Supergroup
Tanzania
craton,
equivalent
which
are the
have a l r e a d y
been
368
6.9.2 Sequences on the Zaire Craton
Mbuyi Mayi Group Formerly referred to as the Bushimay Supergroup,
the Mbuyi Mayi is exposed
in
Kasai
a
narrow
belt
which
southeastwards
into
nonconformable
upon
extends
western the
Group.
Shaba
As
shown
comprises
conglomerates The middle
a
rocks
group
and quartzites
group
(BI)
Province
the
termed
that
comprising
base B0
Province
in
(Fig.3.31).
basement
of
in Shaba where
in Table 6.3
basal
eastern
Shaba
Eburnean
u n c o n f o r m a b l y upon Kibaran Roan
from
Kasai,
it is
of Mbuyi
(Cahen,
Zaire
It
is
and
rests
equivalent
to the
Mayi
1982)
Supergroup
which
has
in
mostly
are considered as the Kibaran molasse. siltstones,
shales
and dolomitic
shales
with some dolomitic beds at the top, constitute the base of the Mbuyi Mayi in Kasai where it is about 550 m thick, whereas the combined thickness of B0 and BI in the Shaba Province to the east is nearly 3,000 m. The upper group
BII
is
essentially
composed
of
dolomitic
and
stromatolitic
Baicalia, Tungussia, and Gymnoselen. These forms have been reported from the lower
carbonates part
of
(1972) the
species
Supergroup
I in
and
the
(1987)
getting
recovered
the
western
the
Taoudeni
Upper
from
younger
towards
sequence
Riphean
boreholes
the
(BII)
epicontinental
sea
east
was
which
Conophyton,
stromatolite
basin
by
Bertrand-Sarfati
(1.0-700 Ma
which
being
(Fig.6.60,C).
deposited
old).
penetrated
concluded that BII is diachronous,
stromatolitic Riphean
of
correlated with
acritarchs
Baudet and
with
in
Based on and
BII,
older in the west
This
a
BI
was
because
transgressive
progressively
encroached
during
Mayi
the
Upper
from
the west
deposition
(Baudet,
towards the east. The 1987)
paleogeographic was
deposited
one as
in
setting
which
molasse
uplifted
(B0)
in
Mbuyi
Kibaran
bordering
mountains
troughs
were
(Fig.6.60,A);
followed by the filling of the trough with finer clastics cessation of terrigenous colonization
by
sedimentation an epicontinental
stromatolites
invaded
the
region
eroded
as
this
and was
(BI). With the
sea suitable for
from
about
1.0 Ga.
Deposition of the Mbuyi Mayi Supergroup terminated in Kasai Province with volcanism at about
948 Ma,
when a widespread amygdaloidal
prominent regional marker horizon The Mbuyi Mayi
(Fig.6.60,C),
Supergroup was deformed during the Lusaka
in the nearby Lufilian arc at about 850 Ma.
basalt
accumulated
unit,
a
in the basin.
folding episode
369
NE
~
SE
®
KAM w
~-~, 9~,
....
-~'-~-
SHe
L,-
BIIo-c
l~.~ I - I ~
~
J. . . . . . . . ~ w I ~ I w t/-J..',
,.. . . . .
~
~ ~ - ~-Z ' 2- ? D ' - i ' ~ J - -
I
~ oi ~ / v
".'."~"~"T~:o~'d"
"~:~'~"c~O°'+'°::~'+
l~~.~.,i;. ""'
1125B~ I. . ~ '- . ' ; ; , LUB] BUSHIMAY L UILU
Bo)" ,.'.'.'. .£'£>-~,; ca" ,"
i"/';"~i°~2~';~c;},
' I- ' , , , . _ ~ - - z ~ : ~ . , . . V~
.',:.'..,'.~::+S~
......:.'..~o~.
........~,:
LUEMBE
MAKUKULU RIANKODI
EASTKASAI
BUKAMA
SHABA
Figure 6.60: Tectonic model for the d e p o s i t i o n of the M b u y i Mayi Group. A r r o w s to the left d e p i c t d i r e c t i o n of m a r i n e transgression; right arrows depict dominant sediment source area. (Redrawn f r o m Baudet, 1987.)
Lindian Supergroup This the
is a s u b - h o r i z o n t a l type
Zaire and
area
(Fig.6.59).
westward
western
of
part
which
non-metamorphosed lies
Outliers
in
Cameroon
of
the
Zaire
north occur
and
and
sequence,
northeast
northward
Congo
in
Republics.
intracratonic
basin,
of
up
to
Central In
2,500 m thick,
Kisangani
the
in
African
northern Republic,
subsurface
beneath
the
of
the
Phanerozoic
370
cover,
the
Lindian
Inkisi Group
northern areas, Pan-African
appears
(Cahen,
to
1982).
pass
westward
is considered as the
mobile
probably
extended
northern
Zaire
observed
that
belt from
into as
which,
the
evident
through
from
part
of
the
and in its
basement
Central
(Cahen
et al.,
from
northwest
moves
upper
locality,
foreland sequence of a poorly known
as
Cameroon
Uganda
one
into
The Lindian at its type
Africa
1984). to
re-working, Republic
Cahen
et
northeast
al.
or
and
(1984)
east,
the
Lindian Supergroup becomes deformed and metamorphosed in mobile belts, which
refoliated
lower
Precambrian
gneisses
are
the
typical
of
tectono-
thermal imprints of the Pan-African orogeny. Verbeek's identified base
is
(1970)
three a
deposited
description
groups
typical under
of
stable
mixtite,
separating
the
The
epicontinental shallow
the
Akwokwo
Ituri
form
the
Lindian Ituri
in
the
Group
type
marine
conditions
flourished
mixtite
is
overlying
where
Lokoma
and greenish cross-bedded arkose, quartzite, deposited
succeeded
by
influence. and or
intracontinental
dolomitic
This
silty
comprises
in
mudstones from
lagoonal
and
passes
to
the
Group.
The
of
the base,
limestones,
into
lagoonal and
limestones reddish,
origin.
quartzites
of
1972). A
unconformity latter
has
pink,
The
nonmarine
suggesting
beds
marked
finely stratified,
The
Aruwumi
Group
and
euxinic
at
shales;
are
marine
micaceous the
fluvial and eolian origin;
dolomites,
a
lilac
subgraywacke and shale, which
depressions.
oolitic
upward
the
stromatolites
(Bertrand-Sarfati,
linked
at
association
piedmont and fanglomerate lithology comprising coarse-grained, were
area
(Table 6.3)
orthoquartzite-carbonate
similar to those of the Mbuyi Mayi, middle
the
(Fig.6.61).
and
top
marine a
thick
deltaic arkosic sequence which extends westward into the Inkisi Group. 6.9.3 Sequences on the Tanzania Craton: Bukoban and Malagarasian Supergroups On
the
western
Tanzania and in
an
part
its
arcuate
of
the
equivalent
exposure
Tanzania
craton
the
the Malagarasian
belt
where
they
Bukoban
Supergroup
Supergroup of
are
slightly
Burundi
folded
of
occur
along
the
Western Rift mobile belt. The folds attenuate towards the Tanzania craton. The Bukoban is over 3,000 m thick. Group
sits
unconformably
unit the M a s o n t w a Group, beds
belonging
Bukoban its
to
Sandstone
gabbro
overlying
the
on
In Burundi the oldest unit, the Kavumwe
the Kibaran
Kibaran
Itiaso
Group
(=Kavumwe of Burundi)
intrusive
whereas
in Tanzania,
the
unconformably overlies older pre-Bukoban
which
is dated
(Table 6°3).
at about
M u s i n d o z i dolomitic limestone
The
is slightly greater 1.02 Ga.
and quartzite
The
age than
oldest clastic of
the
that of
unconformably
group (= Kigonero
371
~
~'~5
~
B
ine ~f Section
~
MESOZOIC-CENOZOICCOVER
1
ARUWUMI SEQUENCE LOKOMA SEQUENCE
.,~
I ~ ,TUR, SEaUENCE
_=~. . a u,~
~
p
300Kin
A
Bafwabalinga
Bafwasencle
Figure 6.61: Sketch map and geologic section Supergroup. (Redrawn from Verbeek, 1970.)
/
Aruwumi
for
Ituri .J.k.m.
\
~
the Lindian
Flags of Tanzania) has columnar stromatolites which correlate with BII of the Mbuyi Mayi Supergroup, thus suggesting that the Musindozi is older than 948 Ma. The upper Bukoban comprises conglomeratic sandstones disconformably overlain by dolomitic and silicified limestones, with a disconformable clastic sequence at the top. In Uganda the correlatives of the Bukoban include the Mityana and the Bunyoro "Series" (Jackson, 1980). The former is very thin and consists of
372
conglomerates, in
Late
Proterozoic
consists On
sandstones
valleys,
of clastic deposits
the
Bukoban
eastern
Supergroup
volcano-sedimentary fully
and siltstones
in
the
Mozambique
next
the
much
of fluvio-glacial
side
of
are
the
group
while
which where deposited
the
Group
western
Kenya.
since
they
the
of
lie
Bunyoro
"Series"
origin.
craton
Ikorongo
of
section
Tanzania
thicker
subaerially
correlatives
Tanzania
Both
in
and
groups
the
the
are
foreland
of
the
Kisii
discussed
zone
of
the
trends
and
belt of Kenya and Tanzania.
6.10 The Mozambique Belt of Kenya and Tanzania
6.10.1 Regional The
Framework
Mozambique
extends
belt
exhibits
from northern
and Kenya,
into
(Fig.6.62), gneisses
Mozambique southern
Ethiopia
and northern
distance
of
orthogneisses
belt,
part,
examine
a
as
and
here
the
no r t h - s o u t h
and Malawi
Sudan,
over
and
generally
Mozambique
we of
6,000 km.
at the
have
part
south
Somalia
Comprising
(Chapter age
of
5)
from
this
predominantly
para-
granulite-facies,
is
of
Tanzania
belt
through Tanzania
on the Red Sea coast
amphibolite-to
seen
Pan-African
central
structural
in the
Kibaran
age
northwards.
which
lies
We
in
the
in
the
shall
Kenya
and
Tanzania. The
term
orogenic cepts
that
regional
cutting
earliest
historical that
intervening
belt,
Of
Mozambique orogenic
of
interpretations
can (Cahen
be
used
which
the
was
younger with
than
the
east-west
belt which has NW-SE structural belt
was
In
originally
perceived
(1984)
subdivided
the
African
conto his
cross-
relative was
the
continent,
the
high-grade
by a north-south
adjoining
low-grade
structures; and
the
trends. as
belt along the eastern m a r g i n of southern Cahen et al.
was
that
what
that
1.3 Ga, and c h a r a c t e r i z e d
Shield
time
determine
proved
youngest
significance
Holmes'
1984). to
the
geologic
the concept
to
et al., dating
ages
for
classic
in
belt d e m o n s t r a t e d
radiometric
Tanzanian
(1951) the
considerable
actual
(Fig.6.62),
the
Ubendian
However,
Africa.
geologic
dated at about
trends
Holmes
one
relationships
furnished
by
embodies
the Mozambique
of
of
coined
in
application (1951)
structural
Africa.
inspired
terranes
greenstones
The
belt,
Africa,
Precambrian
Mozambique
trending
East
structural
of
Holmes
in
was
and
observation
ages
Mozambique
belt
a
meridionally and equatorial
the M o z a m b i q u e
belt into a
373
BAUD|
ARABIA IdsQs
$uture
£G Y P T ~itan ture
600km
Khartoun~,
E
T HI
Aden
O~P ibou t'i
Gu~I
AbClba ~ /
My ~,
Ba J
*
°°-
~'/ u GAN'D'A" L,Vict oria
TANZANIA
I~.
L=ke Nyasa
~
STRUCTURAL TREND LINES OPHIOLIT ES MAJOR F A U L T S
• I j MOZAM BtQUE i ]
. \
¢.y
RIFT VALLEY TANZANIAN CRATON
MO' . ,,
Figure 6.62: Structural trends in the Mozambique belt. (Redrawn from Behre, 1990.)
374
Mozambique Zambezi
province
province;
which
and
is
a
the
type
Kenya-Tanzania
Malawi and Mozambique provinces,
(1951) and subsequent workers
belong
to
the Kibaran the
Early
Zimbabwe
craton,
Tanzania province,
orogenic well
as
Malawi
province.
As
province;
a Mid-
aforementioned
(e. g. Andreoli,
cycle.
Proterozoic
as
a
the
1984;
The Mid-Zambezi
Umkondo
belt
Pan-African
Daly,
province
which
lies
Zambezi
the subject of this section,
1986a), actually
east
belt.
of
The
and the northern segments
are the products of the Pan-African orogeny.
the
is
polyorogenic. granite
The
and
Pan-African
of
domains
of
that
the
of
the
Archean
to
Mozambique
experienced
emplacements
Pan-African
domains
rocks
events
older
evident
pegmatite
manifestations older
it
at
and
Mozambique
(Almond, 1984; Shackleton,
belt
age
least
belt
mild
thermal
tectono-thermal
Kibaran
it
has
is
From
indeed
deformation,
rejuvenation
episodes,
were
the
Kenya-
of the Mozambique belt, aforementioned
the
being of mid-Proterozoic age according to
Holmes
includes
area;
whereas
been
as
in
the
suggested
overprinted
by
that
Pan-African
1986; Key et al., 1989).
6.10.2 Tectonic Features of the Kenya-Tanzania Province Having singled out the Kenya-Tanzania province as the Pan-African segment of
the
define
Mozambique
belt
in
East
its salient geologic
distinctive rock assemblages, (1979) and Pohl
Africa,
features
it
is
now
pertinent
and its limits before
structures and tectonic
to
closely
outlining its
history.
Shackleton
(1988) summarized the main features of the Mozambique belt
as: the ubiquitous presence of Pan-African ages of about 600 Ma; the highgrade and polyphase m e t a m o r p h i s m which facies
with
granulites,
incipient
charnockites
environments; the
ubiquitous
western
polyphase orogenic
and
anorthosites
deformation; front
is generally of upper
migmatization; the
adjacent
the
deep
dipping
Tanzania
amphibolite
occurrence
representing
easterly
to
the
thrusts
craton;
of
crustal along
and
the
general rarity of granitic intrusives in this belt. These
characteristics
are
believed
to
represent
the
tectonic
signatures in the exposed deep crustal levels of a Precambrian orogen that had
suffered
Tanzania
continent-continent
province
from
the
collision.
cratonic
An
examination
foreland
in
the
of
the Kenya-
west
across
the
internal or mobile zone shows that the rock assemblages and the thrust and fold
structures
collisional Shackleton, Except craton
display
suture
the
(Berhe,
characteristics 1990;
Key
of
a
et al.,
gigantic
1989;
east-west
Muhongo,
1989;
1986). along
where
the
small
foreland,
tabular
on
cratonic
the
eastern
margin
supracrustals
have
of been
the
Tanzania
assigned
to
375
the
Pan-African
sequences
in
high-grade, have
eroded
out,
Rather,
the
and
the
belt
of
of
orogenic
Tanzania
craton
fronts
near
occur
Iringa
nature of the M o z a m b i q u e of successive the margin front
orogenic
of
or thrust
geosynclinal
orogeny
at about
the Ndembera strongly
first
two
where
K/Ar
orogenic
the
the
remain or
thus
original
the
are
were
they
belt
is
several
sequences
may
However,
are
those
As
stated
different
but
represented
spite
the
metasediments
summarized
the
lithofacies
of in in
their gross p a l e o g e o g r a p h i c
and tectonic
quartzite
lithofacies
shelf
Ubendian
unconformably
on the western
the
by a
by
part
(Fig.6.63).
The
third
or
western
limit
of
lies
the
in
eastwards about
from
500 Ma
zone about
which
and pitfall
the the the
in
based
are in
Pan-African
(e. g. by
dated Rather,
metamorphism,
Shackleton,
Shackleton
the
in
the
surrounding
the
belt,
(Fig.6.63)
exposed
Shackleton
and
Fig.6.63
mainly
"it
similar
metasediments
Mozambique
1986)
(1986)
lithologically
uncertainties
belt
well age.
ages of the rocks affected
implications. are
on
Pan-African
of
The original
1989).
be in
the
principle
not of
could have been A r c h e a n
et al.,
Usagaran
during
front
of
the Konse
the
front
along
values
from
westerly
of
succeeded
decline
exclusively
ages
the west
sequence
Pan-African
Mozambique
that
a most
belt.
and uplift.
Key
marble
to
located
attain
of
and
zones
are overlain
generally
margin,
that
Belt". ages
which
radiometric
(e. g.
transgressive Mozambique
(1986)
of
magmatism
appears
is
the
along
undeformed
age
of
the polyorogenic
fronts
post-Ndembera
The
of the Mozambique
conjectural;
Kibaran
which
margin
have been telescoped
Here then lies the cardinal
sequences
available
deformation,
ages,
cratonic
stratigraphy
stratigraphic
the
granites.
ages.
the stratigraphy
of
The
scanty.
metamorphosed
of Ubendian
front
mineral
2.0 Ga near
are
rocks
foreland
or external
lower
although
known.
et al., 1 9 8 4 ) .
identified
rocks
the
and
which
east
which
1979)
not
in the
illustrates
The Konse gneisses
tectonized
the Pan-African
The
deformed
volcanics
post-Ubendian
This
Archean
are
southeastern
Shackleton
(actually
1.9 Ga.
fronts
Pan-African deformed
craton.
separating
were
acid
the
(Shackleton,
Tanzania
which
(Cahen
he rightly
Proterozoic
included
belt is indeed
(Fig.6.63).
belts
zone
been
Lower
they
interesting
as
sediments
and
belt
along
the
because,
belt and how the w e s t e r n
paragneisses
Supergroup)
are
the
has
Mozambique
record of the Mozambique
Several
such
to very
belt model,
raised
sediments
Archean
stratigraphic
metamorphosed
collision
however,
for example,
the
been
eroded
records of
reworked
Pan-African
either
to the eroded
craton,
zones
belt,
have
(1984),
stratigraphic
the Tanzania
stratigraphic
belt
Almond
happened
narrow
external
Mozambique
in the deeply
away.
of what
pointed
along
the
Mozambique
or as implied
been
question
of
the
stressed
shows that
in the
western
376
part
of
the
Mozambique
belt,
while
marbles
occur
in
the
central
eastern supposedly more basinal parts of the belt. ~%~ l
",-,,.,
",,,%"..""~ ] ETHIOPIA :L /+
".
I
'~,"i
....
I..;.:. i
,!
,, .... .,.,.°
I
"~
~
:
~:~o,-ogo~..,
so,.,,
, I
t
' ~ ',oCh,r°~oo, '
MOZAMBIQUE B~LT
~',, ,:" ~
~/
~,~=, o , , ..... ~r~. . . .
~ot.,
l
"
I
..... ,,,oooron,,..
Ul~romoflcI, probably ophtolltes Mofic ond schist p r O b ~ l y ophiolites
gneiIllI
(½
~lil ~
~
QuortzJtes
V ~ ' ~ Bonded Jronstonel Marbles
Gronuiifes
'...'I ~, "..,.s,-o~i2.
"..
~
Arohoeon
~
Mylonttizid Archoeon Undlfferenttoted mainly metosedimentI I " I With strike of bedding or fotiotion (Tonzo nio Croton ) L-~ Kisii ond Ikorongo
t I // TA
,
Succelsive Orogenic Fronts : i" 1-85-1,9 GO Tr 1.85 Ga ~ 550 Ma
"-
NZANIA
,'
~ c°
Y
.: < D
,r,k ; 7 -
•"
..
' P-U
, ,%
l~-:-~a/ TANZANIA CRATON ~
J
/
/
,o,op
Ubendlon 1- 1850 MO) granites
~
I"
__
\%~%\KENYA Is, ' iI
-: ..
,
~"',. . . .
oINon~yukl
k"~
• ÷"" [:
I .....................
O Mareoblt
~I,,
/÷' ; ~ff
,
< [-
/
•
ttis jt o . ~
-
W
WAMI RIVER
UL
ULUGURU MTS.
. .
."
Oodomoo .
X~2.~% I
PARE-USAMBARA MT5.
II,/v - - . ....... I
INDIAN OCEAN
;/1' Da ces..Soloor
--:
~.;
f I
Figure 6.63: Structural and stratigraphic sketch M o z a m b i q u e belt. (Redrawn from Shackleton, 1986.)
map
of
the
and
377
6.10.3 Foreland and External Zones Resting
unconformably
between about which
have
along
the
been
correlated
(Cahen et al.,
Soit Ayai Groups of Tanzania, Kenya,
which
formations central
margin
of
the
Tanzania
craton
3°S and the equator are low-grade and tabular metasediments
traditionally
northwestern Tanzania of
eastern
are
that were
Serengeti
the
deposited
Soit
foreland
Bukoban
Supergroup of
representatives
in the Mozambique
in northwestern
The
the
and the Kisii Group of the adjoining parts putative
is only slightly metamorphosed, metasediments.
with
1984). These are the Ikorongo and the
Tanzania
orogen
(Fig.6.63)
and comprises about
Ayai
Group,
equivalent of the Ikorongo Group,
the
to
the
of
the
the east.
In
Ikorongo Group
1,500 m of quartzitic
more
highly
metamorphosed
lies some 60 km eastward.
The lower formation of the Ikorongo Group is a purple quartzite with ripple marks and cross-bedding and containing
boulders
mostly
thick sequence of siltstones, formation There
which
correlates
is a progressive
Ikorongo
Group
to
greenschist
facies
metamorphic
grade,
paleomagnetic (c. 1.0 Ga) the
Soit
while
pole
Ikorongo
Group
is
The Kisii (Fig.6.63) Nyanzian basins The
a
falls
some
of
1986),
Kavirondian
comprises
soapstone,
the
Group
between to
have
15 - 20 m
sometimes
are
thick;
attained
of
the
occurred
in
An upper rhyolitic
siltstone and chert; intercalated
interval
southwestern Kenya
The
Kisii
Group
slightly tilted the
about
upon
forms
towards
oldest
being
basalts
shallow
the west. the
Kisii
(170 -
500 m
The middle
blanket-type
1.3 -
the Archean
subdivision
conglomerate
on
the
is a sequence
and quartzites with thin pebble beds.
subdivision of considerable tuffs;
Sandstone
this
rocks,
eroded basement in places where the lower group is missing, of ferruginous
higher
the deposition of
within
flat-lying
(0 - 200 m thick). a
a
the
Based on its
Bukoban
(c. 900 Ma)
Group.
from the
attaining
staurolite.
non-porphyritic
with
the upper
Ayai
Ikorongo
resting n o n - c o n f o r m a b l y
subdivisions,
resting
Soit
A
1975).
relatively
axes which
three
rocks.
constitute the
having
that
Flags
Archean
and m e t a m o r p h i s m
garnet and
metasediments.
and porphyritic basalts
(15 - 150 m)
with
Ayai
1973; Piper,
succession
with north-south
thick);
Soit
believed
(Shackleton,
and
group
in
Group is exposed near Kisii township
as
1.0 Ga old
and sandstones
Group,
and that of the Kigonero
(McElhinny and McWilliams,
underlying
in deformation
kyanite,
which
the
meta-gneisses
Ayai
the
contains
from
slates,
with
increase
the
locally developed coarse conglomerate
derived
thickness
feldspathic
consists
sandstones
of
and
rhyolites
and
conglomerates;
andesites and dacites; and porphyritic and non-porphyritic felsites.
378
6.10.4 The Internal Zone
Granulite Complexes Granulites
and
region
east
domains
in
associated
of
the
the
amphibolite-facies
Tanzania
granulite
craton.
terrane
rocks
Hepworth
of
occur
(1972)
central
and
in
the
delineated
eastern
vast
several
Tanzania
using
aerial photos, and also adopted for this region the blanket term Usagaran, a
stratigraphic
term
which
Proterozoic age. Similarly, system
to
the
in
its
Sanders
Mozambiquian
type
area
denotes
rocks
of
western
Kenya,
which
marked unconformity on the cratonic foreland. However, rightly insisted that the term to
its
type
area
near
rocks
of
Early
(1965) applied the common term Turokan
"Usagaran Supergroup"
Iringa
where,
as
rest
with
Cahen et al.
a
(1984)
should be restricted
aforementioned,
the
Early
Proterozoic Usagaran
Supergroup rests on the craton and forms a distinct
orogenic belt which,
however,
served as the tectonic
front for subsequent
Precambrian orogenies. The
internal
Tanzania
or
province
assemblages
mobile
are discussed Recent
of
of
the
Mozambique
predominantly
(granulite- and amphibolite-facies)
significant ophiolitic granulite
zone
consists
below
form
ages
and Kenya
(Key et al.,
reworking
of
Archean
they are best
the
granulites
of
known.
Kenya-
In Tanzania
discontinuous Tanzania
Kibaran
rocks
during
the
the
domains.
(Muhongo,
1989) range from 2.75 Ga to 538 Ma, to
the
metamorphic
and minor but tectonically
north-south-trending
from
in
Both the granulite and ophiolitic complexes
in areas where
complexes
radiometric
rocks.
belt
high-grade
1989)
suggesting the
Pan-African
tectono-
thermal events. Hepworth
(1972) had grouped the granulites of Tanzania
(Fig.6.63) into
western or foreland granulite complexes which lie on the craton and do not exhibit which
Pan-African
are
ages
characterized
eastern granulite
by
complexes
(Muhongo, the
1989);
abundance
which
central
of
granulite
quartzitic
complexes
lithofacies;
and
lack or seldom contain quartzites,
but
contain mostly marbles.
Central Granulite Complexes of Tanzania According to Malisa and Muhongo Tanzania, complexes,
represented have
(1990), the central granulite complexes of
by the Loliondo,
fault-bounded
Longido,
structures,
and
Ifakara, consist
Fura
and Songea
lithologically
of
pelitic and psammitic metasediments and their m i g m a t i z e d equivalents. They ~re
predominantly
biotite-hornblende
and
garnet-pyroxene
granulites;
379
feldspathic
micaceous
quartzites;
and
chlorite
schists.
Metamorphosed
equivalents of igneous rocks include meta-gabbros, meta-pyroxenites, dolerites, occurring
and
amphibolite
quartzites
lenses.
suggest
Pegmatites
sedimentation
are
of
the
abundant. granulite
meta-
Commonly protoliths
under the influence of granitic and gneissic source areas which lay on the Tanzania craton. The
central
amphibolite complexes
granulites
grade,
occur
with
as
axes
lineations
been
metamorphosed
granulite-facies
synformal
east-west-trending stretching
have and
swing
reached
antiformal
towards
dominantly
to
the
places.
southeast
general
and
mineral
and
northeast,
suggesting the refolding of earlier folds in the Mozambique belt. was p r e d o m i n a n t l y of
the
central
in the NE-SW direction.
granulite
complexes
The original
are not
only Pan-African ages (Malisa and Muhongo,
known,
but
The
Their
southeast;
the
almandine-
certain
structures.
the
plunge
in
at
Faulting
ages
of
the rocks
they
have
yielded
the
eastern
1990).
Uluguru Mountains Granulite Complex The
Uluguru
granulite
Mountains
complexes
of
Tanzania
which
are
(Figo6.63)
large
concealed beneath Cenozoic volcanics
belong
fault-bounded
and sediments.
to
structures
The eastern
partly
granulite
complexes include also the Pare Mountains, Usambara Mountains, Wami River, and
Nachingwea
complexes.
These
polymetamorphic
Archean to Pan-African ages (Muhongo,
complexes
have
yielded
1989).
The striking peculiarities of the eastern granulite complexes include: the p r e p o n d e r a n c e of marbles; the probable occurrence of charnockites; the
occurrence
of
eclogites
labradorite anorthosites, dykes
and
and
metamorphosed
ophiolite
pyroxenites, dunites,
magnesitiferous
peridotites
and
rocks
serpentinites,
dolerites
in
and
including
amphibolite
shear
zones,
suggesting the preservation of cryptic sutures in this region. The Uluguru Mountains have a long and complex Precambrian history,
and
comprise three groups, a granulite group, an acid group, and a crystalline limestone
group
complex history, than
the
granulites which
on
(Cahen
limestone showing a
wider
granulite
group
qranulite
facies
et al.,
the granulite group.
1984).
Since
and acid
they
groups
A meta-anorthosite
probably
body
occurs
conformable contacts with the granulite scale,
including
appears the
metamorphism,
to
be
intrusive.
The
a
more
be older in
the
foliation,
mostly
but
of
the
initially
at
the
were
but
underwent
later
to
rocks
meta-anorthosite they
have
are believed
retrogressive
380
metamorphism
in
the
amphibolite
gneiss group which contain The 1984)
complex
the regional same WNW
tectonic
involved:
gentle
history
isoclinal
of
folding
smaller-scale
involving
folding
north-south
all
giving
resulting
in
units
of
the
acid
relicts of granulites. the
rise
Mountains
or northeast
on major
groups
of
to broad
NNE-SSW
rocks
in
anticlines
axes; and lastly the intrusion
et al.,
resulting
structures
axes
the
and
(Cahen
axes
with smaller-scale
folding
three
Uluguru
on north
strike of the foliation,
pattern; axes
facies
on the
and minor
complex;
in
ESE-
subsequent
synclines
with
mainly
of mica pegmatites.
Pare-Us~mbara Mountain Granulite Complex These
form
consist
horst-like
of high-grade
mountain rocks
assemblage
with
1984).
The
Pare-Usambara
Steppe
Group
enderbites,
and
to
amphibolite-facies Pare-Usambara
Mountain
well-defined structures south, The
NNE
Masai
are
in
and
the and
irregularly
flanked the
dipping
to
Umba
gently
the
Steppe
about
of pre-granulite
the
and the
to
Group. of
younger
latter
the
Groups
by
Both
the Masai
age
than
consists
northeast.
are the of
Pre-existing
from northeast
contain
an
et al.,
groups
group
structural
and
Group,
(Cahen
northwest
Steppe
youngest
structures
Tanzania,
Mountain
anorthosites
folds seem to swing
and
Umba
folded
and
Structurally
north,
northeastern
migmatites,
Large-scale
the granulite
contain relicts
is by
Group.
layers
Steppe
granulites; but
flat
are rare.
to
east
gneisses
in
as the Pare-Usambara
granulites
horst
the
ranges
known
trends
relicts
in the
are NNW.
of
pyroxene
are not flat as in the Pare Mountains northeast
axes.
These
younger
groups
structures.
Kurase and Kasigau Groups of Kenya Key
et
Kenya. has
a!.
been
(1965))
(1989)
Throughout
outlined this
recognized. in western
craton,
which
Mozambique
Part
Kenya
has
belt
been
the
overlying
The
uppermost
shelf
component
main
features
migmatite
this
traced
the
for
group
lithofacies
metasediments
i00
Elsewhere
of
to
km in
of
of
east
of
into
the
mid-Proterozoic
facies
into deeper water
in the form of scapolite-bearing
Sanders
In southeastern shows
in
the Tanzania
eastwards Kenya
zone
metasediments
migmatites
metasediments
the
internal the
Archean
basement.
eastward to
the
(Turbo
reworked
about
1983).
of
basement
basement
from the migmatite
Kurase-Kasigau
from shallow water shelf
evaporite
of
a
represents
(Vearncombe,
ages have been obtained
the
region
Nairobi gneisses.
Kenya change
sediments. contain
an
381
The Kurase-Kasigau Group of metasediments oGcurs in the Voi-Tsavo area of
southeastern
Mountain
Kenya
terrane.
(Fig.6.63)
The
as
a continuation
Kurase Group comprises
such as marble and quartzite; and graphite, and
schists;
The
overlying
consists
biotite-hornblende Kasigau
mostly
of
Group
Facies
which
amphibolites
lithologies
(Gabert,
eugeosynclinal
been
with
occur
Pare-Usambara
miogeosynclinal
the
have
gneiss,
transitions
the
sillimanite and kyanite-gneiss
and
represents
graywackes
feldspar-biotite-hornblende amphibolites.
gneiss;
of
facies,
metamorphosed
to
intercalations
between
the
1984).
Kurase
and
quartz-
of
ortho-
and
Kasigau
Groups; and basic and ultrabasic rocks were emplaced along regional faults or thrusts zones. Three Kasigau
phases
Groups,
of
deformation
the
pegmatites and the
last
of
have
which
been
recognized
controlled
the
joint pattern in this region
in
the
emplacement
(Gabert,
1984).
and open folds with NNE and NNW-striking axes are prevalent and
Kasigau
Groups.
In
the
area
southwest
show predominantly
northerly dips
minor
are
folds;
there
the stretching
which
also NNE-dipping
lineations
of
are
Voi
foliations
of
late
Isoclinal lineations
to the axes
in the area
to the
and
in the Karuse
stretching
parallel
are mostly parallel
Kurase
of
the
in which
roughly north-south
trend of the M o z a m b i q u e belt. Although Gabert
the
(1984)
outlined
Kurase-Kasigau metamorphism;
succession the
area
geologic
late phases
as
activity
pegmatite
events of
comprising:
hydrothermal
mineralizat±on;
of
was
the
not
geological
Barrovian leading
emplacement;
to
and
clearly
history of
medium-to syngenetic
widespread
defined, the
high-grade base
metal
re-setting
of
radiometric ages during Pan-African tectonism at about 550 Ma.
North-Central Kenya Granulite Complex This
is a vast
the
Samburu-Marsabit
(1989)
exposure of the Mozambique
unravelled
(Fig.6.64)
that
belt
area of north-central
the
complex
it warrants
geology
of
south of Lake Turkana
Kenya this
a close examination
(Fig.6.63). region
in
in
Key et al. such
of its tectonic
detail features
and scenario in order to gain more insight into the polyphase evolution of the Mozambique belt as a whole. The lithostratigraphy of the Samburu-Marsabit area of
the
basal
Mukogodo
Migmatites
which
metasediments such as banded gneisses, thrust Loroki,
as
subconcordant
Kotim
gneisses)
sheets.
are
overlain
by
into w h i c h the migmatites have been
Continental
comprising
(Fig.6.65) consists
unconformably
clastic
meta-arkoses,
units
(e.
g.
Ndura,
meta-quartzites
and
382
manganiferous
sandstones,
with
are among the banded gneisses. into more
pelitic
locally
sedimentary
structures,
These continental units show facies change
metasediments,
the Samburu-Marsabit area.
preserved
the Don
Dol Gneisses,
The basal metasediments
in
the
centre of
are overlain by
IL BUSI GNEISSES
~
LOLKOITO| GNEISSES OL o o , . Y o GNEISSE5
~
DON
~
KOTIM
~
LOROK1
~
.C, RO
DOL GNEISSES GNE1SSES GNEISSE
BASE OF SiAMBU COMPLEX
~
LOLDAIKA MtGMATITES NOURA
COMPLEX
MUKOGODO COMPLEX
~
CHAPARKROM COMPLEX SIAMBU COMPLEX
•]GRANITES *
• *
~ i .
Figure 6.64: Geological map of Northern Kenya area). (Redrawn from Key et al., 1989.)
MAJOR THRUST FAULT OR SHEAR ZONE
(Samburu-Marsabit
a
383
W O--~D°in + Y~)+NLg* q+r°+* Gn+eiss~e + + ~"++%+
o,o, On,,,,,, \
I
J~
.... .:
Don Dol Gneisses
\
]Shelf
Matoni Gnei:lse?Si~+Gneisses +.F'+'~÷++ ++-r~,E
l nllillIll
-,0o, /
"%*++÷ %
sediments
.
~
~. ~ ~ + + e_
/ t / ~ U . kOgOdO Migmatite~ / I , ,~ ~ \~/|l\\I/~jl/ ~, / " [ ] Coarse clastic sediments
Mixed arenaceous/argilloceous basinal deposits
[ ] ' C o l d ' sialic platform
~lO~ The llthostrotiqrophic groups Lithostratigraphic group
~lot I~own
Litholoqies
Maximum thickness ( m)
Chaparkrom Melanocratic hornblende ~-quartz gneisses Complex and fissile leucocro~'ic qneisses, disrupted Korr Complex marbles, metoquartzites, omphibolites ultramafics
Remarks
Unknown
Slice of ophiolitescomplex ( ? ) tectonically emploced over Kotim and Lolkoitoi Gneisses
+4000
Defines the Moritem Synform: thrust over the OI Ooinyo Ngiro and Loroki Gneisses: local miqmatisation along contact.
II Busi Gneisses Fine-grained, dark-grey grophitlc ~neisses ,~ I 0 0 0 marbles, metoquortzites
Forms the highest synform in a listric fault complex in the east.
Siombu Complex
Assorted melanocratic hornblende-rich gneisses, and fissile biotite gneisses marbles, metoquartzites and sheeted d y k e s : miqmatites and allochthonous plutons derived from the host complex during subsequent metamorphisms
Matoni Gneisses Fissile garnet + siltimanite qneisses and schist and minor biotite gneisses muscovite qneisses, marbles and ealc- silicate Lolkoitoi
Banded fine-grained and biotite gneisses
IO00
Local unit above the Kotim Gneisses
~1000
Overlies the Kotim Gneisses
~4000
Overlies Ndura Complex and Don Dol Gneisses, tectonically interleaved with Ndura and $iambu Complexes
Gneisses
Grey biotite gneisses with major marbles Ol Doinyo Ngiro Gneisses metacherts, metoquortzites graphite gneisses, corundum -~garnet gneisses, quartzo-feldspathic qneisses and rare meta-ironstones Don Dol Gneisses
Grey biotite gneisses and darker hornblende gneisses, quortzo-feldspathie gneisses, metoquartzites, graphite gneisses
Overlies Mukogodo migmatite in centre of mop area
=,4000 Kotim Gneisses Massive cluortzo-feldspothic qneisses, metaquortzites, charnockitic qneisses (NW) mangoniferous metopsommites, biotite qneisses ~2000 Loroki Gneisses Quartzo-feldspothic gneisses. Ndura Complex Leucocratic, banded grey gneisses with from 15 to 5 0 % concordant quartzo~ (Cover) fetdspathic sheets (stromofic migmotltes) with mafic orthogneisses and qarnetiferous migmatltic gneisses Mukogodo Migmatites ( Basement I
Aqmotites and nebulltic miqmatites with quortzo-feldspathic palaeosomes as well as K-feldspar and garnet porphyroblasts, mafic dyke swarms and numerous felsievein phases
Figure 6.65: Suggested stratigraphy area. (Redrawn from Key et al., 1989.)
Major thrusts cut the area
Litholoqically similar to the Kotim Gneisses
-I- 1500
Underlies the Loroki and OI Doinyo Ngiro Gneisses
Unknown
Basal unit
of
the
Samburu-Marsabit
384
sequence
of marbles,
Doinyo,
Ng'iro,
slices,
Lolkoitoi,
along
meta-sedimentary
part
Ii
subhorizontal
thrusts
Gneisses). Korr,
belt
was
probably
symmetry
which
the nappes
of western
area
Meta-volcanic
levels
extend westward
of
the
overthrust
western
face to the
zone
in the
underneath
located
backthrust
and those
southeast.
on the
units
external
zone of
corresponds
had postulated
zone of the M o z a m b i q u e
southeastwards units
This
(1965)
face northwest
area
this was the root
separation
located. Sanders
Kenya
of the Samburu-Marsabit (1989)
(01
Siambu Complexes),
into high tectonic
of the Samburu-Marsabit
of bilateral
et al.
Busi
(Chaparkrom,
gneisses
strata of the EaSt African Rift V a l l e y where the root
Mozambique
where
Makoni,
graphitic
pile.
The granulites the Cenozoic the
and vanadiferous
some of w h i c h are ophiolitic
were emplaced
axis
meta-pelites
to
the
as the one
in the eastern
According
to Key
orogen marking
in
the
and
east
foreland
the
from zones
the near
the craton. Several the
syn-tectonic
Samburu-Marsabit
which
are either
vertical
the
foliation
migmatite
and
dykes and veins
event major
First in
the
was
folds
The
post-collisional
produced
regional
zones which
strike
the intrusion shears mark
gneisses
of tectonic plate
with
and
are
with
occur
There are
concordant
narrow
in
sheets
they intrude;
plutons.
partial
and common minor
events was recognized
collision facies
ductile
cover,
which
and
at
folds
subparallel
and
vertical
High
orogenic events.
500 - 480 Ma.
about
of
a
with melt
felsic
which
by Key et al.
tectono-thermal
820 Ma,
and
produced
interleaved
ophiolitic
involved
Between
to the orogenic
granites.
caused
thrusting slices
greenschist-amphibolite
uptight
the terminal
which
stocks;
and metabasic dykes.
of syntectonic
been dated at about
the gneisses
plutons
first phase of this deformation
crustal melt granites was
trondhjemite
small
ages.
meta-sedimentary
complexes.
intrusives
of with
some of the larger
amphibolite-granulite
recumbent
basement,
adjacent
sequence
there
granitoid
and high level discordant
micro-granite
of various
The following (1989).
the
rimming
consist
or discordant
plutons;
narrow
of
zones
post-tectonic These
concordant
low level concordant also
and
area°
the
meta-volcanic
the emplacement
of
620 Ma and 570 Ma there
facies ductile strike.
level open
deformation strike-slip This
which shear
culminated
in
folding and brittle
The final uplift
and cooling has
385
Karasuk-Cherangani Group The
basement
of
northern
migmatitic gneisses age which
have
Uganda
comprises
mostly
granulite
grade
and hornblende schist of Archean or Early Proterozoic
been
1969; Cahen et al.,
strongly
overprinted
with
Pan-African
ages
(Almond,
1984). An extensive meta-sedimentary unit known as the
Karasuk G r o u p in eastern Uganda, and the Cherangani m e t a - s e d i m e n t a r y group in western
Kenya,
overlies
this
basement
sometimes with
Although known by different names in each country, metasediments into
Sudan
(Fig.6.66).
quartzo-feldspathic
calc-silicate interpreted
gneisses
as
The
metasediments
psammites,
and minor
miogeosynclinal
crystalline
deposit
are
made
mica and graphitic limestones.
along a stable
and
is
represented
sedimentary assemblage The
by
that
(Shackleton,
dipping m y l o n i t e imbricate
area
has
relate to the
1986).
Sekerr
They
have
continental
been
margin
lies to the
ophiolite-island-arc
yielded
tectonics
significant
volcano-
structural
of the western M o z a m b i q u e
belt
The western half of the area consists of a northeast-
zone which
lies along
the margin
of the
craton;
and an
stack of thrust slices of Archean basement followed eastward by
another imbricate zone of Proterozoic metasediments ophiolites
are
complicated structures
of
(Fig.6.66).
Karasuk-Cherangi
information
the
up
schists and
to the west. An active outer eugeosynclinal volcanic-arc belt east
contacts.
extend across the Uganda-Kenya boundary and even northwards
southern
quartzites,
thrust
the Karasuk-Cherangani
thrust
imbricate facing
westward zone
at
alternately
at
a
Sekerr upwards
higher dips and
(Fig.6.67).
The Sekerr
structural
gently
to
downwards,
level.
the and
The
east
with
includes
an
extensive zone with repetitions of dismembered ophiolites now occurring as lenses. been
These
are
features
of
a major
crustal
shear
suture
on
account
of
well-defined
(Shackleton, 1986; Vail,
1988b).
interpreted
ophiolites
6.10.50phiolitic The
internal
the as
zone
of
grades m e t a m o r p h i s m these
ophiolitic
1986;
the
Mozambique
data Behre
belt
contains
and intense deformation
rocks
Vearncombe,
significance.
zone
which
has
zone
of
imbricate
slices
of
rocks whose ophiolitic nature has survived the highhave
(e. g. Kazmin et al.,
structural
the
Rocks
mafic and ultramafic
authors
a
1983), which also
been
widely
reported
by
1978; Prochaska and Pohl,
Behre
(1990)
have
further
traced
define cryptic sutures in the
in the mobile
has
northward
added these
Mozambique belt,
Although
several
previous
1984;
more
elucidated
zone.
Shackleton,
geochemical
their
ophiolitic
and
geodynamic rocks
and linked them
which
with the
386
ii.;3:.-.:
" ': "
i~Kv~
~tate
•" vx v <\
k.
",, \
\~,
\\
. ~ ~'~<"
\"
~ \ ',
\
\~"-
X~¢'~
\\
cover
~
Ophiolite suite
~
Netasedimentar y supr acrus~al group Gneisses
......... .. .~'7":':.[~
:\ 9t !
, . .b~...~ / \
intcusives
~'I:5UDAN ~ P h a n e r o z o i c
I:I
Fractures Tnternationol borders
.
./. \...
~
. ....." KENYA
k \\ \ \ \
~ v :.,:V.' v :: '. .'. '-vtSt.v~,v. v
/ \~
i'..
.opo.~ ~,>[\ \
{
!-:i'
i
'
I
'..
UGANDA
~,,-,~',..'
"
o
W F i g u r e 6.66: G e o l o g i c a l s k e t c h m a p of k e r r area. ( R e d r a w n f r o m Vail, 1988b.) extensive
ophiolites
demonstrating by
the
that
same
ophiolites
the
Arabian-Nubian
the Mozambique
regional
of
Mozambique
of
collision
northern
Kenya
the
belt
Shield
and n o r t h e a s t
events.
The
(Fig.6.63)
b e l t and t h e A r a b i a n - N u b i a n
Kapoeta-Karamoja-Se-
Sekerr,
provide
(Fig.6.62),
Africa
were
Baragoi
the
links
thus
affected
and
Moyale
between
the
Shield ophiolites.
Sekerr and Itiso As
aforementioned
which
rests
which
has
upon
been
there the
is
a
complicated
metasediments
followed
northward
of into
the the
imbricate Sekerr
zone
area
Karamoja
of
ophiolites
(Vearncombe,
district
of
1983)
southern
o~
~,
Figure
~'- ~ ,~:...~:[~ " ~-'~.-..=. ~. .
Garnet
Cross-sections
÷ •
Belt
belt.
~ ~.-,~,,,, . ÷ •"'. :~ \~. -
.
~q',
/t~ ,
Baragoi ophiolite Zone
B
A
(Redrawn f r o m S h a c k l e t o n ,
\ ~
Belt
O p h i o l i t e s of Sekerr suture
Mozambique
"-,'\ . .
I m b r i c a t e z o n e of shelf sediments
1
B
36~30'E
t h r o u g h the N o r t h e r n M o z a m b i q u e
50 km
,\. . ...- + + + + . + ." ~+-.
....
Mozambique
M ozamb~que Belt
T h r u s t shelf s e d i m e n t s and Archaean
GraGite,granodiorite (Archaean)
6.67:
~'~
~I<
Ophiolite ~l~ ZOR w,I
<~I
UM~tfr~Cmaf fc ] - Prate rozoic O phioliie
~+- ~+- ; "+v ~.- ; -+~ - *+- ' ~+" ~. - , + ;<~ ' ~.,\ , ~ ~
Mylonites = = \ %
A rchoean ~,~ [ T a n z a n i a Croton)
,-,/,u,,,¢==
Boundary 9o obscure e/ u ........ o
A r c h a e a n ~, ,~ (Croton)
1986.)
388
~udan
(Vail,
1988b) where there is strong thrusting of the ophiolites onto
the m e t a - s e d i m e n t a r y units. According to Vail ophiolites pillow
(Fig.6.66)
lavas;
are
gabbros
a
sequence
with
of
preserved
(1988b) the Sekerr-Karamoja
andesitic
meta-volcanic
layering;
hornblende
rocks; schists;
serpentinites with podiform chromite; basic dykes which probably represent a
sheeted
dyke
complex;
marble
lenses;
and
narrow
bands
which are believed to have been original chert layers; schists
which
sediments.
were
All
of
probably
the
tuffs;
above
lithofacies
island-arc ophiolite sequence. fragments
are
tightly
gneissic metasediments,
and
pyroclastic
are
of
quartzites
psammites and
believed
to
and mica
turbiditic
belong
to
an
In southeastern Sudan gabbro and pyroxenite
infolded
with
marbles,
hornblende
schists
and probably also represent a d i s m e m b e r e d
and
island-
arc and ophiolite suite (Vail, 1988b). The Sekerr ophiolite represents a regional ophiolite belt that extends all
the
way
region.
through
western
Ethiopia
where
it
This ophiolite belt probably continues
along
the
eastern
Itiso
mafic-ultramafic
ultramafic
rocks
Mozambique
front
complex,
(Behre,
lavas
Shackleton,
exposed
in
the
Akabo
south of Sekerr into Itiso
in Tanzania
pillow
1990;
is
(Fig.6.62) are
where
associated
1986).
Trace
in
the
with
element
the data
suggest the origin of the Sekerr ophiolite in a back-arc basin between 1.0 Ga and 663 Ma ago (Behre, 1990).
Baragoi Several ophiolitic complexes occur in the high-grade rocks of the SamburuMarsabit
area
Baragoi.
The
in
north-central
Baragoi
sheeted dykes with
ophiolite
Kenya, includes
trace elements
including
the
Siambu
metamorphosed
mantle
showing a transition
complex dunites
at and
between mid-ocean
ridge basalts and island-arc tholeiites. Two separate suites of ophiolitic gabbroic rocks in the Baragoi area have yielded ages of 796 Ma and 609 Ma. The Samburu-Marsabit ophiolites occur as thrust slices, w h i c h at Marsabit have been
transported up to a distance
of about
100 km,
thus
reflecting
severe crustal shortening (Key et al., 1989)
Moyale At Moyale in northernmost Kenya extend
into
the
Moyale
ophiolite
Adola
fold
includes
(Fig.6.63) low-grade ophiolite occur which
and
thrust
serpentinized
occur as thrust slices among continental
belt
of
southern
harzburgite
and
shelf meta-pelites.
assigned this ophiolite to a back-arc tectonic setting.
Ethiopia.
The
gabbros
which
Behre
(1990)
389
Pare Mountains
A
highly
dismembered
serpentinites, Ophiolites
and
scattered
meta-pyroxenites,
ophiolite
occurs
meta-gabbros,
here
comprising
and
amphibolites.
of
southeast Kenya
also occur in the neighbouring Taita Hills
in what probably represents the continuation of a suture zone which Behre extrapolated northward into the Adola-Moyale belt
(Fig.6.62).
6.10.6 Molasse Low-grade
metasediments
probably Kenya
unconformably
(Pohl,
1988).
which upon
These
may
represent
high-grade
rocks
Pan-African in
the
are known as the Ablun
molasse
Mozambique
Series
rest
belt
of
in northeastern
Kenya and as the Embu Series
in the central part of the country
(Pulfrey
and Walsh,
Series
slightly
1969)o
metamorphosed
The Ablun
rocks,
including
Embu
Series
consists
of
conglomerates,
limestones.
The
consists
limestones,
sandstones and conglomerates.
tightly
folded,
sandstones,
of mostly
pelitic
Tourmaline
phyllites,
rocks,
with
and some
is predominant among
the accessory minerals in both groups of metasediments. 6.10.7 Madagascar Pan-African
ages
are w i d e s p r e a d
in the
island
of M a d a g a s c a r
the reworking of the extensive Archean to Kibaran the were
characteristic imposed
on
Madagascar
as
reinforced
by
alongside
an
north-south
the
integral
plate
the
island.
foliation Cahen
part
of
reconstructions
coast
of
East
et the of
Africa
trends al.
terranes, of
the
(1984)
belt.
paleo-position
(Fig.6.62)
based
suggest
Mozambique
therefore
Mozambique the
and
during which
on
This of
belt
considered link
is
Madagascar
the
evidence
provided by magnetic anomalies and other geophysical and regional geologic correlations
(e. g. Rabinowitz et al., 1983; Reeves et al.,
1987).
Most of what is known about the Pan-African of M a d a g a s c a r is based on its
abundant
regional
radiometric
field geologic
(Cahen et al.,
1984)
dating,
rather
investigations.
than
on
detailed
The scheme
structural
and
for Pan-African events
involved metamorphism at about 845 Ma which attained
the granulite facies in some places but was only at the greenschist facies in the
Early Proterozoic
of Madagascar processes intrusions
took
(Fig.3.45). place
between
at
750 Ma
Vohibory metasediments Intrusions about and
of granites
740 Ma. 600 Ma
in the
There
during
and
were an
southernmost
part
another metamorphic
granite
episode
and
of mild
pegmatite folding.
Granites and pegmatites were again emplaced as from 600 Ma to 480 Ma.
390
6.11.8 Geodynam~c Model The fact that the Mozambique belt represents the lower crustal huge
continent-continent
1990;
Burke
and
Shackleton, evidence below.
1986).
that
The
have
Whether
or
complete Wilson
collision
Senghor, been not
adduced
in
is
is
Key
now widely
et al.,
multiplicity
there
Cycle,
orogen
1986;
of
support
enough
of
this
evidence
and what plates
accepted
1989;
structural
collided,
(Behre,
Muhongo, and
the
1989;
petrological
model
for
level of a
is
summarized
operation
are pertinent
of a
questions
raised by this model. The latter issues are also addressed in what follows below. Shackleton structures
(1986)
argued
convincingly
that
the
fold
so glaring in the Kurasuk and Cherangani Groups
the products
of plate
collision,
a view
shared
and
thrust
(Fig.6.67) are
by Key et al.
(1989) who
placed the granulite internal nappes which were generated at the root zone of the collision somewhere beneath the G r e g o r y Rift of Kenya. granulite slices
complexes
(Maboko
of Tanzania
et al.,
1985)
have been
interpreted
on the evidence
as
(Muhongo,
The eastern
tectonic
thrust
in press)
of
the
hornblende-pyroxene granulites and amphibolite gneisses which are commonly thrust
over
lithologic
graphitic boundaries
pseudotachylites for
example
local
marbles.
in
which
are
also
strong
major
or
meta-anorthosites
Uluguru
and
shear-zones are
Pare
within
thrust-zones
schistocity,
have developed between granulites the
thrusts
in
There
Mountains. schists,
mylonites
at and
and meta-anorthosites, The
occurrence
gneisses,
of
many
granulites
and
further evidence of intensive deformation among the
eastern granulite complexes. Although the actual number of sutures and the age of suturing have not been
precisely
zones),
determined,
the position
of
(e. g. Behre
some of these
(1990)
sutures
suggested
two
suture
is roughly known based on
the dismembered ophiolites which decorate them. The stretching commonly plunge zones
and
NW-SE
plate
zones
show
resulted
the
lineations
Aswa
wrench
motion. a
in the western
part of the Mozambique
to the southeast and trend NW-SE,
rather
from a
plate boundaries
late
fault
However, persistent orogenic
(Fig.6.62),
the
stretching
roughly
like the brittle
thus
implying
lineations
north-south
relative motion
trend
of plates
belt shear
approximately
in
the
which
central probably
parallel
to
the
(Shackleton, 1986).
A possible Wilson Cycle scenario for the Mozambique belt was proposed by Behre
(1990)
who
suggested
an early
stage
of
rifting,
followed
by a
391
phase
of
continent
subduction
and
collision
Although
in the M o z a m b i q u e
island-arc the
belt is quite
accretion,
ending
stratigraphic
tenuous
with
record
continent-
of early rifting
and circumstantial
compared with
coeval Pan-African mobile belts already reviewed, the rifting of a Kibaran continent Kibaran zones;
along
rocks and
sediments
East Africa would account in
also (Kisii,
Mozambique belt.
central
Kenya
explain
the
Ikorongo,
and
in
Madagascar,
existence
Soit
Ayai
for the presence of Archean to of
east
passive
Groups)
along
the
suture
continental
margin
the
of
foreland
Subduction and island-arc accretion are evident
of
the
from the
ophiolites of the Mozambique belt, while continent-continent collision, already
mentioned,
generated
nappe-type
folds
and
(McWilliams,
1981)
in
thrusts
on
as
a regional
scale. Paleomagnetic
evidence
the
form
of
a
large
misfit between East and West Gondwana suggest that a large ocean separated both
regions
Ethiopia craton
and
and lay
that
Somalia
in
West
northern segments
Madagascar belonged
Gondwana
and
to
parts
East
of
eastern
Gondwana,
(Fig.6.1).
The
Tanzania,
whereas
tectonic
the
Kenya,
Tanzania
evolution
of
the
of this eastern Pan-African ocean will be examined next
in the A r a b i a n - N u b i a n Shield. 6.10.9 M i n e r a l i z a t i o n
Metamorphic processes strongly controlled mineralization in the Mozambique belt.
In
general
economically
region. As pointed out by Pohl of
this
belt,
minerals kyanite
which in
the
gem-quality
the
mineral
are
deposits
economically
metasediments;
vanadium
important
mineralization
is
rare
in
this
(1988), because of the h e t e r o g e n e o u s nature vary
amphibole
grossularite
widely
important in
(Fig.6.68).
include
asbestos
flake
in
calc-silicate
Metamorphic graphite
ultra-mafics, graphite
and green
schist;
and
of Tanzania have a high potential
for
blue zoisite in impure marbles. The eastern granulite complexes gemstones scapolite, quartz,
such
as ruby,
hornblende,
zircon,
varieties
of garnet,
tremolite,
apatite
and
emerald,
enstatite
green
tourmaline,
alexandrite,
(Malisa
varieties
and Muhongo,
gemstones are commercially exploited by small-scale mining.
sapphire,
1990).
of
These
T h e y are found
in pegmatites either as conformable folded bands and layers of metamorphic mobilisates or as undeformed cross-cutting veins Of
lesser
stratiform magnetite
ores
economic such
as
associated with
importance the
Fe-
so and
far
(Pohl, 1988).
are
the
Mn-quartzites
the basic meta-volcanics
of
small in
syngenetic
Kenya;
and
the
the Kasigau Group.
392
The
ophiolites
copper.
There
contain are
small
also
ore
deposits deposits
of
chromium,
which
are
nickel,
related
to
magmatic rocks.
These include copper in the diorite-rhyolitic
the
in
Voi
gneisses,
area
Kenya;
anorthosites,
and
magnetite
and
ilmenite
in
platinum and intermediate gneisses of charnockitic
and associated pegmatites in the Pare Mountains of
Tanzania.
"atiform
)|os c: tted intrusives ~its
Figure 6.68: Metallogenic map of the northern Mozambique and the Arabian-Nubian Shield. (Redrawn from Pohl, 1988)
belt
6.11 The Arabian-Nubian Shield 6.11.1 Tectonic Framework Beyond
the
segments
Kenya-Tanzania
(Fig.6.68).
province
the
Mozambique
One segment continues northeast
belt
splits
through
into
two
southeastern
393
Ethiopia and Somalia segment runs
into Yemen across
northwestwards
into
the Gulf
the Sudan.
by high-grade gneisses and metasediments, low-grade
volcano-sedimentary
rocks
of Aden,
Both
while
segments,
the other
characterized
are separated by large wedges of
containing
ophiolites
(Fig.6.68).
In
this northern region the Mozambique belt becomes part of another tectonic province, the ANS
the Arabian-Nubian includes
Egypt,
Shield
Sudan,
(ANS).
The Nubian or African part of
Ethiopia and Somalia,
whilst
Saudi Arabia
and Yemen constitute the Arabian part of the shield. A Late
Proterozoic
paleo-tectonic
sketch map
of NE Africa
(Fig.6.69)
in which the Red Sea, a much later geological feature is removed, displays the tectonic extending
continuities
from
(Fig.6.70), referred
about
thus
to
as
the
Kenya,
Somalia,
includes which
extends
developed.
and
into
Archean in
polymetamorphic
rocks
Pan-African
assemblages The ANS
(1987)
from
Arabia
and Sea
fold
which
it
the
is
terranes.
As
and
stated
Saudi
more
the
1978)
extensively
the
et
al.
continental
in sharp thrust
volcano-sedimentary
or
Pan-African
Schandelmeier
Arabia,
ANS
ophiolite)
Pan-African
overlying
of the Mozambique belt are oceanic
and
in
northern
( K a z m i n e t al.,
even
by
exposed
defined,
surrounding
metasediments
are
30 ° N
previously
Uganda,
Thus
belt
to
rocks
northern
Yemen.
where
includes
juvenile
equator
(volcano-sedimentary
Red
Ethiopia
defined the ANS as
the
Complex,
Sudan,
Arabia
shelf
gneissic
Sudan,
and
Basement
the
also
Vail
pre-Phanerozoic
Desert,
ensimatic
continental
(1990),
50 ° E those
Saudi
Saudi
ANS
reworked
with
in
constitute
The
Mozambiquian
to
Eastern
low-grade
that
the ANS.
Precambrian
Egyptian
the
assemblages
28 ° E
encompassing
the
Sinai,
across
and
contact
ophiolite
(Fig.6.69). is characterized by two principal
lithological-tectonic units
reflecting two contrasting tectonic environments the gneisses
and meta-sedimentary
African rocks,
rocks
(Vail,
including
1988b). First, are
both
reworked
as well as Pan-African continental margin
pre-Pan-
sequences.
These
occur as scattered basement inliers east of the East Saharan craton and as exotic basement and
ophiolite
terranes
further eastwards
assemblages.
The
latter
within
constitute
thrust belt,
and are by far the most extensive
in
They
the
ANS.
represent
oceanic
margin
the volcano-sedimentary the
Red
Sea
fold
and
lithological-tectonic unit
and
island-arc
settings
and
their associated syn- to post-tectonic intrusions.
The v o l c a n o - s e d i m e n t a r y
and
greenschist
ophiolite
units
are
low-grade
rocks
at
the
contrast to the amphibolite-facies q u a r t z o - f e l d s p a t h i c in the gneisses and meta-sedimentary units.
facies,
in
and foliated rocks
394
CAIRO
1
l
1
•
,
/ I
EAST
%
I I
S A H A R A NI I
/
,,
I
lI
CRATON/
ii '
,"
/
II " 9
AFRO-ARABIAN
DOMAIN {:
EAST
~" o " :.i~.-~,~':::
•
ARABIAN PLATE
.:. :
I
t
I
t
0 ~
~.
:,k
RTOUM
/ t " l -- t
"
k " ~l.'
~I
I
fif
"
X
--I /
"
~
~.)
L.Turkana
/
8o,~, U
/
/
/ volcanosedit.- ophiolite assemblage and position of Pan-African Continen-
,4 morgan
/
,.p-\ if I'~., I ~
Phanerozoic
cover
Proterozoic volcanosedit-ophiolRe assemblage / ; ~\Tanganika ,;
:-' •
MOF
Mozambique Orogenic Front
ASZ
Aswa
' r ,'-'-" L~ /
Halawi
800kin
F i g u r e 6.69: the p r e - d r i f t N. J. Jackson)
Rocks of C . 2 0 0 0 M a a f f e c t e d b y ].Gneiss m e t a late Proterozolc metamorphismJsedit, group Rocks of C . 2 0 0 0 M a u n a f f e c t e d by l a t e Proterozoic m e t a m o r p h i s m
~,-
L.':
L
--'--- _
I
;'/,
shear z o n e
;~
T e c t o n i c s e t t i n g of the reconstruction. (Redrawn
Arabian-Nubian Shield from f i g u r e s u p p l i e d
on by
395
Sutures / faults
Ben Ghnemoh
S u p r a c r u s t a l tow to m e d i u m metosediments
MEDITERRANEAN SEA
i
~
Ma f i c / u l t r o m o f i c
ophiolitic
grade assembloge
P o n - A f r i c a n j u v e n i l e i s l a n d arc and conUnentQt margin associotions Pre-Pan-Africon portly reworked in the L a t e P r o t e r o z o i c 150kin
SAUDI EGYPT
.~
LIBYA
ARABIA
Bif ~o,=~" rail
°*'°°'r
v -f Wadi Hatfa
Nub[
(~ Oeser t ~ . ) J SUDAN
.~.
J
ETHIOPIA
Hater
SOMALIA
+ ÷ • • ÷ + • ÷ ÷ • • • • • • • • ~ + ÷ + • • ÷ ÷
500kin I
+ +÷ • • • ÷ ÷ ÷ ÷ • ÷ + +
• I
Bur region
e- ÷ ÷ ÷ + ÷~. odishu ÷ ,
Figure 6.70: Major from Schandelmeier et
The rifting,
ANS
evolved
oceanization
÷ + • ~, ÷ + ÷ ÷ +
KENYA
+ -o ÷ ÷
basement tectonic al., 1990)
through
the
with
multiple
Wilson
units
Cycle
island-arc
and
of
the
displays
complexes
ANS.
(Redrawn
the (Fig.6.71)
stages
of
of
the
396
Indonesian-type,
which
the
had
collided
surrounding
several
continental
times
margins
and of
were
the
ultimately
thrust
onto
craton.
The cratonization of the ANS through the accretion of island-arcs
East
Saharan
is a notable departure from the continent-continent collision regime which produced
the
granulite
facies
metamorphism
in
the
central
and
southern
provinces of the Mozambique belt. 6.11.2 Gneisses in Pre-Pan-African Terranes Between which
the
East
Saharan
lay approximately
there
is
a
belt
of
craton east
southwards
through
southern
Somalia
(Fig.6.69).
the
the
scattered
Kenya
and
and
of
Late
River
exposures
Ethiopia, The
Proterozoic
Nile
in
of
Egypt
gneisses
northwestern
precursors
oceanic and
these
realm Sudan,
which
extends
into
northern
Uganda,
of
the
gneisses
are
of
Early to Middle Proterozoic age, with relicts of the Archean also involved (Schandelmeier para-
and
mineralogy
et al., 1990;
orthogneisses
which
and
at
generally
granulite facies and
Vail,
1988b).
are
the
are
quartzo-feldspathic
banded
grade,
with
contrasting
sometimes
rising
to
(Vail, 1987). They contain marble bands, rare quartzites,
amphibolites are
variously
amphibolite
and
also
show
migmatization
Because of their intense foliation, contacts
These
not
always
and
granite
emplacement.
the original lithologies and internal
decipherable
in
these
reworked
pre-Pan-African
rocks. From
Somalia
inliers
right
up to Egypt
(Fig.6.70)
which
thermal
activities
(Schandelmeier,
Somalia
to
the
southeast
granitic orthogneisses to
post-tectonic
uncertain.
were
the
gneisses
reactivated 1990).
(Fig.6.69)
In
although
the
high-grade
and amphibolites were
granites
the
occur
during
as older
Pan-African Bur
area
paragneisses,
of
basement tectonoSouthern
migmatites,
intruded by Pan-African
age
of
the
metamorphism
synis
In northwestern Somalia and extending into eastern Ethiopia the
reactivated Early to mid-Proterozoic basement consists of paragneisses and migmatitites w i t h carbonate
intercalations of amphibolites and local occurrences of
rocks,
(Fig°6.72,A).
quartzite,
There
are
and
several
acidic
syn-
to
to
basic
post-tectonic
meta-volcanics intrusions
of
granites, diorites and gabbros. This basement can be correlated with that of Yemen on the opposite side of the Gulf of A d e n Pan-African reworked basement in the Sudan Mountains,
the
Darfur
block,
and
the
Bayuda
(Fig.6.70).
includes those of the Nuba and
Nubian
deserts.
In the
latter area high-grade granitoid gneisses with minor inliers of high-grade metasediments
were migmatized
in the Pan-African.
and intruded
by voluminous
granitoids
late
Small patches of reworked migmatitic gneisses occur on
~
10001
.z;
~/-"
/.-
>,~.
,
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.
~_".:<,J"3~
to
trap and bind detrital materials
and their w e l l - p r e s e r v e d m i c r o s t r u c t u r e s
origin.
were
algae or cyanobacteria.
structures
during to
stromatoliles
that
::":?~'.~-MIDDLE ZONE OWER ZONE
~ " t
Vertical scale of metres
],m
Figure 7.7: S t r a t i g r a p h i c columns for the main stromatolite o u t c r o p in the Cheshire Formation, Belingwe g r e e n s t o n e beltj Zimbabwe. (Redrawn from Nisbet, 1987.)
and
433
The C h e s h i r e stromatolites show cyclical laminations r e f l e c t i n g local fluctuations
of
supported
carbon
and
forms
of
by
1987).Various
Baicala,
energy
Conophyton,
conditions, oxygen
and
temperature
isotopic
stromatolites
Irregularia,
variations
similar
to
and
modern
Stratifera have
and
salinity,
(Abell
and
forms been
as
McClory, such
as
identified
from the C h e s h i r e Formation. 7.2.2 The E a r l y - M i d Proterozoic Fossil Record By
far
the
most
extensive
Early Proterozoic, province,
thick
These
and
of
low
Groups are
ranged
stromatolites
epeiric
relief
to shallow subtidal
Ghaap
stromatolites
mounds w h i c h
of
when, as in the Transvaal
accumulations
built the intertidal niespoort
profliferation
and
characterized from 5 to
by
large
200 m
Also,
as
previously
noted
in connection
elongate
the
the Kaapvaal in the Chu-
West
and were
in
stromatolites
carbonate platforms
with laminae relief of over 3 m (Tankard et al.,
reefs
sea of
domical
of the T r a n s v a a l - G r i q u a l a n d
in width
was
Supergroup.
stromatolitic
up to
40 m
long,
1982).
with
the W i t w a t e r s r a n d
gold
(Chapter 4.2.2), kerogen seams with remarkable remains of micro-or-
ganisms
have
lichen-like
been discovered, plants.
These
which
include
bacteria,
forms are believed
algae,
to have e x i s t e d
fungi,
and
as carpet-
like e n c r u s t a t i o n s w h i c h extracted gold and u r a n i u m from the surrounding water,
in a m a n n e r similar to modern fungi and lichen.
From the coeval Gunflint about 2.0 Ga), nia
and the younger Beck Springs Dolomite
(1.4 - 1.2 Ga old)
caryotic
cells.
history
at the
also
the
the
enhanced
Proterozoic old)
has been u n c o v e r e d
biota
includes
widespread oxygen Springs
contains
diverse
of mitotic
appearance
of
terozoic.
thus
in eastern Califorfor the e a r l i e s t
eu-
r e s e m b l e modern
levels chert
sexual
enhancing
of
through in
division.
reproductive
evolutionary
banded
in geological
iron-formations
photosynthesis.
central
algae including
cell
at an interval
accumulation
Australia
The
Mid-
(1.0 Ga - 900 Ma
green algae w h i c h w e r e
This
finding
processes,
tionary step w h i c h established the m e c h a n i s m material,
(dated at
and also contain some of the strongest geochemical
Bitter
stage
Canada
forms w h i c h
for the presence of chlorophyll
when
suggests
evidence
The G u n f l i n t
soil m i c r o - o r g a n i s m s , indications
Iron Formation in Ontario,
an
further
found
attests
improtant
to
evolu-
for the t r a n s f e r of genetic
diversification
in
the
Late
Pro-
434 7.2.3 The Late P r o t e r o z o i c Fossil Record By the Late Proterozoic, s t r o m a t o l i t e s , w h i c h had w i t n e s s e d u n r i v a l l e d proliferation
earlier
in
the
Proterozoic,
attained
the
acme
d o m i n a n c e and e v o l u t i o n a r y radiation throughout the world.
of
their
This situation
r e n d e r e d it feasible to subdivide and correlate Late P r e c a m b r i a n sedimentary
supracrustals
using
stromatolite
assemblages
- an
undertaking
which Russian geologists blazed the trail. Russian b i o s t r a t i g r a p h e r s divided
the
(Fig.7.8)
Riphean based
period
on
(1.65 Ga
distinctive
-
680 Ma)
into
four
biostratigraphic
major
for sub-
units
assemblages
of
stromatolites w h i c h are now used routinely all over the world.
/ x
..... ,
,'",, •
,-'\,'\
,-"---" . . . . . . . . . .
t
I
:',,1
t
I
t
J
I
I
I
/
rl
~ t
/
'
/
I (t
4
diversification
~'--"--
//
,
~
Sfromatolifes
~
o
(families) g °
?o - o o o o o o o ~ _ / Riphean
9;0 0'00 95o '
8~o
I Ve,ndian ,I Cambria'n"
8;0
7;0
7~o
6;0
600
sso
soo
( 106 yrs}
Figure 7.8: D i v e r s i t y of stromatolites and m e t a z o a n s during the late P r e c a m b r i a n and global changes in ocean c h e m i s t r y and sea-level. (Redrawn from Morris, 1990; Windley, 1984.)
For
example,
the
Late
Riphean
(Fig.7.8)
has
been
recognized
t h r o u g h o u t the A f r i c a n Late Proterozoic s t r o m a t o l i t i c sequences and
utilized
craton,
to
to the
correlate lower
from
part
of
the the
Taoudeni Mbuyi
basin
Mayi
on
the
Supergroup
(Fig.7.9)
West on
African
the
Zaire
craton, and to the Bukoban Supergroup on the western part of the Tanzania craton
(Bertrand-Sarfati,
1972).
pointed
out
of
that
stromatolites forms.
Late
in
are
spite
paleoenvironmental
taxonomic
morphologically
Proterozoic value
Bertrand-Sarfati
uncertainties
different
stromatolites since
and
biological
from are
the
Walters
(1981)
Early
Proterozoic
Late
Proterozoic
also
(Trompette,
of 1982)
great and
435
paleoecological
parameters
their
growth
maximum
Pouchkine bioherms
(1988) in
such
as
(Cloud,
sunlight
1968).
used the extensive
the
northwestern
and
range
influenced
and
Moussine-
Bertrand-Sarfati
columnar
part
tidal
of
the
stromatolite biostromes
Taoudeni
basin
to
and
interprete
subtidal epeiric s e d i m e n t a r y environments. O t h e r m i c r o b i o t a s underwent e v o l u t i o n a r y changes d u r i n g the Late Proterozoico
A c o m p i l a t i o n of available e v i d e n c e by T r o m p e t t e
that up to about bacteria)
1.45 Ga ago p r o c a r y o t i c cells,
and filaments
that time there was a marked increase in size, sification are
of
taxa.
Acritarchs
organic-walled
sheaths
(bacteria and oscillatoraceans) appeared
microfossils
that
(1982)
showed
(cyanophytes and w e r e small; after
a c c o m p a n i e d by the diver-
in
the
Late
Proterozoic.
occur
in
fine
clastic
These
sediments.
They range in size from a few microns to several tens of microns in diameter.
Generally
spheroidal
in
shape
affinities with m o d e r n taxonomic groups protistans,
annelid
Proterozoic
sediments
initiated Riphean,
all
by the Russians, Vendian,
stromatolites. Africa
eggs)
is
and
uncertain.
over
the
often
unicellular,
(e.g. algal cells, But
world,
being
their
example,
acritarchs
have
been
cysts,
in
Late
biostratigraphy,
also
has enabled more precise
abundant correlation
and Early Cambrian strata than has been
For
their
spores,
feasible using
utilized
to e s t a b l i s h the age of the Mbuyi Mayi S u p e r g r o u p
and the age of the Roan Group in the K a t a n g a n orogen
of Late
in equatorial (Baudet,
(Binda,
1987),
1972).
7.2.4 The E d i a c a r a n Fauna The
Ediacaran
organisms strata,
fauna,
which
were
comprising
are
the
mainly
first
the
imprints
preserved
multicellular
in
(metazoan)
the g e o l o g i c a l record, during the Vendian 2.2
the
ranges
age from
of
the
650
to
Ediacaran 570 Ma,
organisms
in
the
fossil
in
Morris
However,
record
is not
moulds
of
soft-bodied
marine
organisms
terrigenous to
appear
in
(Fig.7°8). As stated in Chapter
assemblage,
although
about 620 - 570 Ma preferable.
and
shallow
its
(1990)
broadest
definition,
considers
a
span
of
the first a p p e a r a n c e of Ediacaran precisely
known.
The
presence
of
animal burrows at an interval dated at about 1.05 Ga in the basal beds of the Roan
Group
in
the
Zaire-Zambian
p r o b a b l y a p p e a r e d this early
First d i s c o v e r e d in Namibia, Namibia
was
a German
Copperbelt,
(Clemmey,
suggests
that metazoans
1976).
prior to the Second W o r l d War,
colony known
as
Deutsch
Sud-West
Africa,
at a time the
Edi-
acaran fauna was announced and christened in South A u s t r a l i a where faunas similar to those of Namibia were later uncovered. truly
cosmopolitan;
for it appeared
at the
same
The E d i a c a r a n fauna was geological
age
in Amer-
436 ica, Asia,
and Europe,
without many provincial
characteristics
in any of
these regions.
Anti
Atlas•
~ Abadla
~ / / ~ ~ ~--Chenochclne ~4~:~'e'~\ . . . . .
a Tiguentourine
Zemmour/(Jr /
86me {t Collenia
"~\
6our Roouo
/
\ \
Cocobeoch . . . . . . ~ ~ Koumiki('a ~p..~Fr, nncevilhen v/ N~..kaQ~-~ ZAIRE u\
Thy..?l, / 1500Km
......
~,Ph~tform Series post-2"06, c] Post-Cembrien S'rrometotifes
I
\
~,~ , i, /~8uk!ban
Hb y, Zy,/ C RATON//
c~Mufu|ira |
1 ~
tQ~'i
[~AHA"|f
~
BuI"waY;/c~RATON/ ( kVenfers~r,p .e
(
the
• J/
bilaterally worms;
the unknown organisms whose imprints
Ediacaran
(sometimes
fauna
shaped
like
symmetrical
segmented
forms
that bore no resemblance
were
morphologically
feathers, but
combs,
annelids;
to living creatures
stromatolites
and moulds
closest
fans,
non-segmented
resembled
/
AmbcttoH~"/an"
Figure 7.9: Known distribution of Precambrian in Africa. (Redrawn from Bertrand-Sarfati, 1972.) Generally,
/
/ //
coelentrates
and brushes);
organisms and
to like
still,
(Seilacher,
constitute others were
platyhelminth
forms
were
there
1984). Two broad
437
Ediacaran such
as
faunal those
assemblages
in
Namibia
are
and
known,
the
p r o b a b l e d e e p - w a t e r assemblage,
namely:
Ediacaran
shallow-water
Hills
in
dwellers
Australia;
also found in Australia,
and
a
and in Charnwood
Forest, U.K. The
Nama
Ediacaran
platform
assemblage
group
which
in
Namibia
inhabited
lived among s t r o m a t o l i t i c biostromes
Namalia,
(Table 6.2)
tidal
flats,
contains
shallow
(Tankard et al.,
(Rangea,
Pteridinium,
Ernietta)
on tidal
flats in front of braided alluvial plains,
1982).
and w o r m - l i k e
a
diverse
seas
and also
Coelentrates
forms
flourished
w h i l e the polychaete
Cloudina scavenged the stromatolites for food. In a d d i t i o n to these body traces known as Phycodes and Skolithos r e p r e s e n t other soft-bod-
fossils,
ied organisms that b u r r o w e d through the sediments. The
Ediacaran
faunas
are
of
great
scientific
interest
because
they
mark a m a j o r step in the evolution of life -the a p p e a r a n c e of the earliest
metazoans.
This
cursory
review
of
Precambrian
life
shows
that
Archean organisms consisted of procaryotic m i c r o - o r g a n i s m s with stromatolites
dominating
flourished caryotic nity.
so
algae,
The
the which
arrival
communities.
scene
throughout
extensively, perhaps later
Ediacaran
metazoans
the i m p o r t a n t
Proterozoic. of
Stromatolites
contributions
stromatolite
from
mat-building
suggests
the e x i s t e n c e
of
other
speculated that
the ancestors
of
the
faunas was found among acritarchs the
the
account
joined the
of acritarchs
It has been
on
(Trompette,
are not
invertebrate
believed
groups
Echinodermata)
w h i c h appeared
as
the
to
1982). However, have
(Archaeocyatha,
been
the
commuorganic
Ediacaran
ironically, ancestors
Mollusca,
in the Early Cambrian,
eu-
of
Brachiop0da,
already
so advanced !
to
suggest
Precambrian that
these
(Fedonkin,
of
well
established
phyletic
had
existed
in
tion near the dawn of the Phanerozoic.
brief duration.
in the
the
latest
Proterozoic,
but
as
they had no m i n e r a l i z e d or p r e s e r v a b l e skeletons.
(1990) advanced an interesting h y p o t h e s i s
acaran assemblages
lines
1990). Their advanced e v o l u t i o n a r y stage indicates
invertebrates
g e n e r a l l y believed,
Fedonkin
existence
for biotic evolu-
Low species d i v e r s i t y among Edi-
suggests that this group lived for a g e o l o g i c a l l y
In m a n y parts of the world Ediacaran faunas are found af-
ter the last P r e c a m b r i a n glaciation.
The w o r l d - w i d e p o s t - g l a c i a l marine
t r a n s g r e s s i o n p r o b a b l y caused the Ediacaran faunas to r a p i d l y d i v e r s i f y and attain their peak of e v o l u t i o n a r y radiation, clined.
after w h i c h t h e y de-
Their d e c l i n e is evident in their general lack of endemism, mean-
ing that Ediacarans did not evolve into new niches. A n o t h e r indication of
438
t h e i r d e c l i n e is t h e i r i n c r e a s e in i n d i v i d u a l b o d y size c o m p a r e d to the first v e r y small s h e l l y fossils w h i c h a p p e a r e d at the b a s e of the Cambrian.
Mass e x t i n c t i o n s among E d i a c a r a n c o m m u n i t i e s
from the M i d d l e Vendian,
could h a v e set in as
p r o b a b l y b e c a u s e of c o m p e t i t i o n f r o m the m a n y
small and m o r e e f f i c i e n t feeders w h i c h w e r e the a n c e s t o r s to the C a m b r i a n invertebrates.
Chapter 8 Paleozoic Sedimentary Basins in Africa
8.1 Structural Classification of African Sedimentary Basins Africa lay at the centre of Gondwana at the close of the Precambrian. P a n - A f r i c a n o r o g e n y had ern margins.
joined other continents
Throughout most
of the Paleozoic
southern seaboard of the Iapetus Ocean
The
to its e a s t e r n and westNorth Africa
(Fig.8.1),
occupied
and South Africa,
the too,
was b o r d e r e d by a shelf sea. A f t e r the Iapetus closed in the m i d - D e v o n i a n and the H e r c y n i a n maining
northern
orogeny had in the Late C a r b o n i f e r o u s continental
more
interior
location
this
position
until
each
continent
in
blocks
this
into
new
Mesozoic-Cenozoic
went
its
separate
Pangea,
Africa
supercontinent. times
way,
when
leaving
brought
the re-
assumed
an even
Africa
Pangea Africa
remained
fragmented surrounded
in and
once
again by the ocean, as was the case before G o n d w a n a began. Seen
against
this
global
Phanerozoic
paleogeographic
c l a s s i f i c a t i o n of A f r i c a n sedimentary basins readily
appreciated.
Clifford
(1986)
and
Picha
basins into: d i v e r g e n t passive marginal basins; intracratonic
fracture
basins;
and
(1989)
grouped
intracratonic
African
sag basins; Figure
8.2 and
the s e c o n d a r y or m o d i f i e d basins,
foreland
the
can be
shows the d i s t r i b u t i o n of these p r i m a r y types of s e d i m e n t a r y basins, continent.
cratonic
backdrop,
into four m a i n types
basins.
all of which cover about a half of the
D e p e n d i n g on their structural location and on the d o m i n a n t de-
positional process,
the passive or m a r g i n a l
c l a s s i f i e d as wrench,
deltaic
sag basins have been
sag, and fold belts.
Although
this
further chapter
deals p r i n c i p a l l y with the Paleozoic it is considered necessary,
however,
to
in
place
all
basins
including
structural settings, polycyclic tions
their
of
Mesozoic-Cenozoic
age
their
e s p e c i a l l y considering the fact that some basins had
structural
since
those
settings
inception.
and
histories,
Some
Paleozoic
having
suffered
basins
also
modifica-
assumed
new
structural settings in the Mesozoic-Cenozoic. The above termined
by
drifting
and
1989).
Only
collision
stage
sagging, two parts
tectonics.
foreland and orogenic
classification
their
of African
in the
and
plate
subduction
Phanerozoic
tectonic and
continental
of Africa were m a r g i n a l l y Northwest
thrust belt
Africa
basins
(Wilson)
comprising
and the M e s o z o i c - C e n o z o i c
systems with thin-skinned thrust belts,
been derifting,
collision
involved the
has
Cycle:
Moroccan
Atlas
(Picha,
in Phanerozoic
and
Hercynide Rif Alpine
and the Early Paleozoic
440
Cape
fold belt
The basins
in South Africa,
in the
rest
of the
the plate tectonic process. the
Saharan
platform
which
were
affected
continent
by P h a n e r o z o i c
are a t
the
first
two
orogenies. stages
in
Paleozoic intracratonic sag basins constitute extends
from
Mauritania
and
Morocco
in
the
west to Egypt and the Sudan in the east.
gJll) Figure 8.1: Global p a l e o g e o g r a p h y of the Ordovician-Silurian. (Redrawn from Scotese, 1986.)
In sag
sub-Saharan
basins,
Africa
although
the
Zaire,
containing
Okawango
Precambrian
and to
Etosha
intracratonic
Phanerozoic
sediments,
441
Rif-Beitics
Essaouira
S,a
Me diterran
/
Bl== Nile Delta of Suez
E. Desert
Red Sea
,,A,African ^~.;~.~'~2" luIIemeden N.W. 1
-~°"°' )
Mad,no " Kauto
t
b OongOtO~
5
Ivory
o.~ of
.,.,.,f /Gulf
Gulf of Aden
-- \
Oelra
Guinea
Zoire
amu Emboyment
BASIN T Y P E Intracrotonic Inferior Sag Crotonic Foreland
;to!
Aptia ~)kawongo
Intracrotonic Interior Fracture Marginal Sag Etosho
Deltaic Sag Wrench Foldbelt
Indian
Ocean
Major Transforms Coastal I 6 0 0 km I
D,F.Z.
Oui'eniquo Basin DAVIES FRACTURE ZONE
F-A.F.Z. FAULKLAND-AGULHAS FRACTURE ZONE
Figure 8.2: Types of African Clifford, 1986.)
sedimentary
basins, (Redrawn
from
442
subsided The
mainly
main
Early
in
Karoo
belt.
(Late
basin
Gondwanide
Three
Africa, viz:
times
foreland
Triassic
fold
Karoo
Carboniferous
formed
orogeny
generations
in
which
of
response caused
to to
Early the
Late
deformation
intracratonic
rift
Jurassic). Permian-
in
basins
the
Cape
occur
in
the Late Paleozoic to Early Mesozoic Karoo rifts in southern
and eastern Africa; tral Africa;
the Early to Late Cretaceous
rifts
and the Cenozoic rifts in East Africa,
in west and cen-
the Red Sea, and the
Gulf of Aden.
8.2 Paleogeographic Framework Scotese's of
the
(1986)
paleomagnetic
continents
during
global p a l e o g e o g r a p h i c
reconstructions
the
Paleozoic
framework
graphic data. for Africa
Paleogeographic
were
also
At
the
regional
(Figs.
a
well
8.1;
constrained
8.3),
paleoclimatic
positions
especially
and paleobiogeo-
implications of Paleozoic continental drift
discussed
Hargraves and Onstott
furnishes
for Africa
since it took into consideration
of the changing
by Van
Houten
and
Hargraves
(1987),
and
(1987).
beginning
of
the
Paleozoic
the
South
Pole
was
located
just
north of A f r i c a in the Iapetus Ocean. Q u a r t z - r i c h sandstones began to accumulate marine brian
in the C a m b r i a n along north Africa when there was a progressive
t r a n s g r e s s i o n which sandstones
form
progressed
northward
into tidal
and
basin
in
1990).
the
as
from braided
the
High
into the O r d o v i c i a n sheet
fluvial
Atlas
of
Tibesti,
Taoudeni,
Ougarta,
(Fig.8.4).
Mauritania
streams
Morocco
did not exist at that time. Tindouf,
from
facies northward,
such as the Ahaggar,
bian M o u n t a i n s cian
continued
continuous
shallow-marine Western
Uplifts
a
in the
to
south,
and passing
(Burollet,
Sinai
Camand
grading
into a deep
1989;
Klitzsch,
Gargaf and the Egyptian NuThese rose after the Ordovi-
Ghadames,
Murzuk,
Kufra,
and
Dakhla i n t r a c r a t o n i c basins subsided. Gondwana ~Neugebauer,
drifted 1989)
over
the
glaciation
Early S i l u r i a n back
into
caps,
and
entire
during
the
Ordovician
in Africa
(Fig.8.1A),
(Fig.8.5A).
leading to w i d e s p r e a d
Rapid
northward
drift
in
conthe
(Fig.8.!B) brought the northern margin of w e s t e r n Gondwana
a warm
climatic
a profound
Saharan
Pole
and by the Late Ordovician the South Pole was located
far inland in n o r t h w e s t e r n Africa tinental
South
marine
realm,
causing
transgression
the m e l t i n g (Fig.8.4)
p l a t f o r m with m a r i n e embayments
the Cape region of South Africa
(Fig.8.5B).
of
which
r e a c h i n g as
the
polar
ice
inundated
the
far
south as
443
Late Carboniferous (Westphalian)
\,\
t
Early Carboniferous (Visean) Figure 8.3: Global (Redrawn from Scotese, Major
marine
paleogeography 1986.)
transgressions
also
of
took
the
place
Carboniferous.
in
the
Mid-Devonian
(Fig.8.5C) and Early Carboniferous.
Basin subsidence a c c e n t u a t e d the Sil-
urian and D e v o n i a n
and caused the n o r t h w a r d
transgressions,
progradation
of sand bodies over marine shales. Hercynian d e f o r m a t i o n a f f e c t e d Morocco in the Middle form
including
to Late Carboniferous. the
Ougarta
ranges
east-west uplift d e v e l o p e d across
It also a f f e c t e d
which
were
faulted
the Saharan platand
uplifted.
An
the eastern m a r g i n of the Kufra basin.
444
R e j u v e n a t i o n of basement Mountains strata. the
created
faults along the northern border of the Ahaggar
horsts
In the n o r t h e r n
Early
Carboniferous,
post-Hercynian
and
grabens
Ghadames
Paleozoic
transgression
with
drape
basin which was strata
(Burollet,
were
1989).
folds
in
Paleozoic
uplifted
at the end of
stripped
off
In
Tunisia
before
and
a
northern
Libya a set of faults created tilted blocks and steps along the southern margin
of
a proto-Tethys
of C a r b o n i f e r o u s
and
Ocean
Permian
(Klitzsch,
1971),
shallow marine
causing
lithofacies
the deposition with
corals
and
fusulinids.
Sea
Level
Cycles
GLOBAL
EGYPT
÷
--
÷
- -
J
¢
P
~
D
o
Figure 8.4: from Morgan,
Global 1990.)
and
Egyptian
sea-level
curves.
(Redrawn
S o u t h e r n A f r i c a m o v e d over the South Pole during the Late-Carboniferous
(Fig.8.3B),
geological accumulated
thus
history, during
the this
witnessing
the
most
Permo-Carboniferous glaciation
covered
pronounced glaciation. all
the
glaciation Tillites
southern
in
which
continents
445
from
South
America,
peninsular the
to
Antarctica.
predominantly
Africa,
with
fluviatile
times.
and
lacustrine
the
Gondwana
Karoo
in this
and
The
eastern
Africa
through
Permo-Carboniferous
Karoo
rifting,
Gondawanan
southern
Since
is i n c l u d e d
nonmarine
widespread
characteristic throughout
southern
depositional basin
ciastics.
flora
during
depositional
Late cycle
deposits (Fig.8.6)
Carboniferous began
in the
to
Arabian initiated
(Klitzsch,
and d e p o s i t i o n
Karoo-type
Glossopteris
glaciation
cycle
subsidence
the
1986)
with
their
accumulated Early
Late
Jurassic
Paleozoic
chapter.
:')::"'G::....~""~6~:J:~-cF.".
~":'/ ""6 " .G,:G:.,..:..~i[~ ~.,:::.%,
M(d - Ordovicion shoreline G= Lale Ordovician-earliest Silurian glacial deposils
in
of thick
Eoriy Silurian shoreline G=Eorly Silurian gtociol deposifs
Mid-Devonian shoreline G=LaIe Devonian glacial deposits 6 7 Probable Early [obonifereous glacial deposits
[oLd-water (Molvinokofric) realm Warm- water realm F i g u r e 8.5: Ordovician Devonian paleogeography of w e s t e r n Gondwana showing glacial deposits. ( ( R e d r a w n f r o m H a r g r a v e s and V a n Houten, 1987.)
it
446
8.3 T h e M o r o c c a n H e r c y n i d e s
8.3.1 S t r u c t u r a l Because
of its structural
consi d e r
first
tectonic
and
tural
the
and
between
to
and stratigraphic
structural
stratigraphic
subdivisions
attempt nent
Domains
and
framework
evolution
Morocco
to
southern
Europe,
since
both regions
during
terminologies
relate
the
have
tectonic
Morocco
complexity
of M o r o c c o
it is n e c e s s a r y
before
the Paleozoic. been
applied
domains
occupies
of
examining Various
struc-
to M o r o c c o the A f r i c a n
a transitional
to its
in an conti-
position
(Fig.8.7).
60ON
30ON PALAEOEQUAT0 R
30os
60oS
r-IAN6ARAN l~'~M ERICAN" ~[['~60NDWANAN Figure 8.6: from Chalnor
According tonic
consists African also
of
belongs.
of
et al.
Mediterranean
the
Rif
Maghrebide
fundamentally rest
Global p a l e o g e o g r a p h y and Creber, 1988.)
to Bensaid
domains,
EURAMERICAN CATHAYSIAN
[]
The from
Morocco
(1985)
and
Mountains
Mountain
middle
(African
and
to which belt
of
southern
Morocco)
Permian.
consists
Morocco.
(Fig.8.7)
a
the
Morocco
African
chain
Maghrebide,
of
which
of two main
Mediterranean are
part
of
Morocco. to
the
nappe South
of
North
of Algeria
tectonics
relatively
tec-
Morocco
the
the Tell M o u n t a i n s alpine
belongs
(Redrawn
the
differs Rif,
stable
the West
447
© /J
~
~
.
~
j
~
[srbiscen events AIleghen~o.n 2 4 0 ~ 7 0
/
.....
~_.>~ Rif
y~ 2c jr
_..
,
Me
~sodo,,cAs,o,,oo3oo,4o~o
-~L , - ~
~Y
,- \ \
",,~,,,
~.~
~
500
Figure 8.7: Hercynian orogenic belts Africa. (Redrawn from Windley, 1984.)
of
Europe
and
North
448
A f r i c a n cratonic margin which behaved as a rigid block during Phanerozoic orogenies. prising middle
African
Morocco
the Anti-Atlas
and
segment w h e r e i n
Mountains.
consists
of
the Tindouf
a
stable
basin
pericratonic
(Fig.8.8),
part
com-
and an unstable
lies the M o r o c c a n plateau or meseta and the Atlas
The middle part of Morocco
is a q u a s i - c r a t o n i c
spite of similarities in Paleozoic sedimentation,
zone, w h i c h in
is d i s t i n g u i s h a b l e
from
the A n t i - A t l a s by its lesser rigidity during the P a l e o z o i c and later orogenic cycles.
( A i bO r o n/~'% S
e G
/
/(
~tl Atlos
GeC
oXe 0~
0 .(, \\ ~ t r . Moroc. Me seta
.0 ~9 ~
O~ ~
W~A
~~
/ I
iC
Croton
~r
Figure 8.8: Major structural from F r o i t z h e i m et al., 1988o)
Piqu~
and M i c h a r d
(1989)
whose
provinces
structural
of
Morocco.
subdivisions
(Redrawn
are
followed
here, d e s c r i b e d the Paleozoic sequence of Morocco as o c c u p y i n g five major structural Atlas, feature
domains,
viz.,
the
Saharan,
the Anti-Atlas,
the
Meseta,
the
and the Rif domains. Whilst the Tindouf basin, the main structural of
filled with
the
Saharan
Paleozoic
domain
(Figs. 8.9,
epicontinental
the n o r t h e r n cratonic border,
8.10)
sequences,
is
a
large
syneclise
the A n t i - A t l a s
occupying
is an anticlise or anticlinorium.
The Anti-
449
Atlas
is
characterized
by
thinner
Late
Proterozoic
and by domal uplifts
where
the underlying
(Fig.8.10).
major
pericratonic
Another
the southeast
(Fig.8.8),
cambrian
flows and v o l c a n i c l a s t i c
lava
WesternMesefa ~ . ~ L
is
sediments
Paleozoic
basement
the
strata
is exposed
Ougarta
block
and e x t e n s i v e
to
Late Pre-
sequences.
E-stern
,
Pan-African feature
with Paleozoic
to
SPAIN
"HighAtlas
Tonglib" K Rift RIF
Robot
"~/~\.Hekkam Debdou
Casablanca El J o d i d ~ ,200K~
Safi /
~Rehom.mnaj9 Ben Zireg ".-..~,
Morrokect
t
~
.. :..-" Holder "o,
Ag~ir • "" " ~ ' ". '...~ ! ~ . •.W..'~
Kerd~t :.: q,
~'Bechor bosin •
,
:.. ...
,. ~
.-.
-...-
100Km
,'.1.'.
:,
-"
.
Post Paleozoic rocks Pa[eozoic rocks
"~. :'., ~ , ~ ° :
Precombrion
rocks
£ :::---.:-:
Figure 8.9: P a l e o z o i c massifs and structural (Redrawn from Piqu~ and Michard, 1989.)
The meseta
Meseta and
(Fig.8.9),
central
formably
Oran
which or
is a d e n u d e d
accentuated the
domain
the
by
segment
late-orogenic
anticlinoria.
overlie
is
Eastern
Paleozoic
separated meseta
the
of a H e r c y n i a n
granite
intrusions
Undeformed rocks
into by
domains
the
of Morocco.
Moroccan
Middle
Atlas
(Variscan) that
are
Mesozoic-Cenozoic
in the Meseta,
and w h e r e
or
Western
fold
belt
belt w h i c h was concentrated
in
strata
uncon-
eroded,
Paleo-
450
~
~
~ .4,
~
I
o
D 0 0 0
O
~u E Ig 0
o °
N
o
E
®
U~
0 .,-t
o
E
D
a; o
w~
I
0
0
0
°
o
c
E
v
c
•
0
~
o
u
:
0
o
.B
D .,4 .,-I
g N
F!a,'
Ib
@
o
t//I
g
~
/5
*.
._~
o
~
~
~
~
o
u
Q
N
O
m
0
451
zoic m a s s i f s - - C e n t r a l can
meseta.
hence
In
the
Paleozoic
Midbelt,
massif, Oran
meseta
massifs
Running
from
over
the Atlantic
a distance
folded
over
domain
is known
las.
Paleozoic
and
of the central and
the
formation exposed
High
phases, at
Atlas
Since orogeny
folded
the
AYt-Tamlil,
the
lectively
termed
tectonic
Michard
Moroccan
below.
Anti-Atlas
and
to
the
initially
Moroccan Late
coming
region
during
cupied
the
Early
platform
progressively
boniferous,
part
deformation at
a
by
Piqu~
are
Atlas
the
Hercynian
they were
and
Michard
Ordovician
synthesis
age
col-
(1989).
(Caledonian
Morocco.
on
the
an
outline
the
Middle
Sehoul
the
setting.
zone)
phase
the
Paleozoic shelf was
of Morocco,
when
the into
the main phase of Hercynian
characterized and
with
Sehoul
part
fault-bounded during
movements
of
deep
basins.
in this
basin
oc-
block.
The
basin
Morocco
the M i d d l e
one
terrigenous
of
the
by post-
constituted
turbidite
western
closed
be-
installed
in this n o r t h e r n
is
generally
a deep
south
which
the
was
Early
while
of
and
Devonian
of redbeds
A carbonate
place
stratigraphy
and middle M o r o c c o
Devonian,
and
This
deposition
southern
disintegrated deformed
by
Hercynides,
molasse
took
time
affected
(except
during
to Middle
Middle
Carboniferous
to
from the Sahara.
Devonian,
Anti-Atlas
shelf
de-
inliers
or Ghomarides.
P r o t e r o z oi c
depositional
northwestern
Hercynian Late
Morocco
Later,
all
Late
a
the
volcanism.
the Anti-At-
and T e r t i a r y
Evolution
the
Western
from
the
block of n o r t h w e s t e r n
of
epicontinental
sediments
of
published
by p o s t - P a n - A f r i c a n
collis i o n a l
Hercynides
From
same
were to Late
deformation
(1989)
in
the
of the Middle
Paleozoic
pal~ozoYque
domains
the Middle
and Tectonic
evolution
presented
and
The Atlas
controlled
consists
in ap-
strata are
1987).
which
Mesetas
Tamlet,
which
also occur in the inner part of the Rif do-
in the Sehoul
Stratigraphy
the
Large
the Nappes
tectonic
the
is known
and
basins
and
Mekkam,
to Tunisia
Mesozoic
its Late Jurassic
domain.
Mougueur,
during
pre-Hercynian
orogeny)
the
eroded
Debdou,
(Schaer,
This d o m a i n
separates
Atlas
terranes
above
(Fig.8.7)
However,
basement
orogeny with
they constitute
main
less
Mountains
rift t e c t o n i s m
Ocean.
which
(Alpine)
main w h e r e
vast
in the Moroc-
in the west
terrane w h e r e e s s e n t i a l l y
Atlantic
Paleozoic
longed
are
Jerada,
are the Atlas
Precambrian
(Fig.8.9).
Piqu~
exposed
strata (eg.
of M o r o c c o
2,000 km,
for its p o s t - T r i a s s i c
The A t l a s i c
8.3.2.
younger
restricted
coast
of
pear to be an a u t o c h t h o n o u s
Atlas,
the
are more
Jbilete--are
etc.).
the east,
opening
Rehamna,
during and
These to
Late
in n o r t h Africa.
the
later Car-
452
The P r e c a m b r i a n - C a m b r i a n Transition In
the
A~ti-Atlas
supracrustals stones)
a
major
(shallow
affected
by
angular
shelf
unconformity
quartz
Pan-African
zoic-Cambrian sequence
(Infracambrian)
arenites
II orogeny,
(Fig.8.11).
has been used in the Anti-Atlas
from
Also
tratigraphic
termed
Anezi-Siroua-Saghro
III
belonging
Group
and
a thick
Late
lime-
Protero-
(Pruvost,
1951)
for the p o s t - P a n - A f r i c a n conformable non-
Precambrian
successions
Precambrian
stromatolitic
The term Infracambrian
m a r i n e sequence b e l o w the lowest fossiliferous trilobites.
separates
and
to
level w h i c h bears Cambrian
(Choubert, this
age
1963),
interval
the A n e l a - O u a r z a z a t e
Group,
the
lithos-
include both
the
of which
g e n e r a l l y contain thick sequences of p r e d o m i n e n t l y redbeds w i t h major intercalations carbonates
of v o l c a n i c materials.
mark
the
initiation
Upward-finning
of epicontinental
clastic
sediments
sedimentation
r e p r e s e n t e d by the c o n f o r m a b l y overlying A d o u d o u n i a n Group shown
in Fig.8.11
this
group
stone and slate sequence
comprises
Lower Limestones,
and
which
is
(Fig.8.11). As a middle
sand-
("S~rie Lie de vin"), and Upper Limestones which
carry the earliest Cambrian trilobites in Morocco.
N
S i
J.Ouourhis
1H,m
/ 1500m
• 0°'..
J. Bani
TQbanit
1035 m ,,, ;,~,,.'/
Feija. t a h t o n i a
9 30m
Shales and sandstones Q uQrtz ires
~I
T~,ta
• , • Go
Irn~tek track
:.'Oo°. o o o ol°°° °oo ~ °~*= I . o %*o~ o olo~oo
Sondst,. Qnd ~ Petitic
I :Sand
ShQLeS t
IS a t e
Tobant Group
'
~k U p p e r ° ° 1 Ll~tles o n e s
r d
o v
i
c
~ a
n
Middle
ISandstones' Lower land s[Gtes '~Limestones
._ o
~-
~,
c
i Adoudounian
Lower
Cambrian
Group is about
Group
~
C o m b r Jan
Figure 8.11: The lower Paleozoic section near Tafa. Legrand, 1985). The A d o u d o u n i a n
z -
I
~-
SHur. IO
•
0°o° o~
E fougania~,
sho, l e s
Go o
z
(Redrawn form
1,400 to 2,100 m thick in the Western
High Atlas w h e r e the entire Infracambrian to Carboniferous
sequence is up
to i0 km thick
in the Western
(Froitzheim et al.,
1988). The A d o u d o u n i a n
High Atlas is well exposed in a succession that reflects Late Precambrian rifting, Killick tion);
differential (1988)
coastal
subsidence
and
marine
the sequence records a marine stillstand
(Tighricht
transgression. transgression
Formation);
According
to
(Ighir Forma-
rift-related
prograda-
453
tional
regression
with
interfinger
with
Formation);
another
transgression
ern Morocco, quences
thus
position
of the
troversial.
Sdzuy the
(1988)
at
reported the
gion was
of
agrees
so that
of
similar
to
plexes
like
into
an
renewed
marine
Adoudounian
of w e s t e r n
Group
and east-
of I n f r a c a m b r i a n
se-
is however,
example,
But algae
to
the
Anti-Atlas
aulacogen
that
A),
gulf
was
that
linked
composita
of
below
Early
Cam-
from the reoccupied
to
a reconstruction stratigraphy
this
Fl~gel
Limestone)
that nhe sea e n c r o a c h e d
an
and
Kundatia
Group
con-
placed
Buggish
(Lower
Adoudounian
Cambrian
stratigraphic
strata of
Most
Cambrian
of
age
the
the
that
what
Iapetus
generally
the
Western
(Destombes
strata d i s a p e a r Anti-Atlas
layers occur the
everywhere
or
is
are
found
the Lower
absent.
and basaltic
flows,
and
High
strata
the
exposed
strata,
they
at
(Fig.8.9, occur
fossiliferous
1985)
and
of
of
the
as thin
Lower
with
Middle by
and
facies
Cambrian
thick
com-
and tuffites.
the
Anti-Atlas
Morocco
anticlinoria. sandstones
while
occurs
and in n o r t h e r n
in H e r c y n i a n
argillites
of
breccias
flank
in
in the A n t i - A t -
represented
southern
8.10),
the
to Late Cambrian
domains,
Traces
are
in
sequences
outcrops
and Meseta
volcanic
Terrane
are
al.,
Cambrian
Local
Anti-Atlas.
in M o r o c c o
et
is of M i d d l e
or thin e a s t w a r d s
in the Atlas
Western
sections
shale-graywacke
and shale group
P l a t f o r m a n d the Sehoul
of
for
vin".
calcareous
(Fig.6.9,
sandstone
detrital
Cambrian
which
(AI Makhzen
and Volcanism
Bani to Ougarta
essentially
lower
(Fig.8.11).
of a n d e s i t i c
from Jbel
or
Middle
those
Ordovician
the
boundary
inf~rieurs"
postulated
in the eastern
occur
Ordovician
the
west
Cambrian
Cambrian
volcanism
to
extension
de
Infracambrian-Cambrian
Tabanit
conglomeratic
and
in parts
(1979),
lie
Cambrian
west,
Moroccan
are
deltas
1988).
age. G e n e r a l l y
Middle
the
Anti-Atlas
Fig.8.11
similar
Schmitt "S~rie
calcaire
the
subsidence
overlying
las,
Early
des
graben
the
classical
Western
the
to
(Killick,
Cambri a n
the
assigned
a
with
strata
the northward
and
of
in
further
Atlas
(1978)
and Fl~gel
Agadir
probably
Ocean
The
and
Buggish
fan
deposits
Formation);
Infracambrian-Cambrian
top
"S~rie
(Fig.8.11), brian.
and
the Lower Paleozoic.
boundary
from
(Taghbar
of some anticlines
suggesting
fans
and peritidal
Formation).
unfossiliferous
beneath
The
stillstand
in the cores
alluvial
fluvial
(Talat-N-Ou-Lawn
Elsewhere, are exposed
basin-margin
basin-centre
They
where, consist
(Fig.8.11)
de-
454
posited rocco
in a m a r i n e
clastic
to the Tindouf
clastics
basin
accumulated
in
trilobites,
brachiopods,
ostracodes,
gastropods),
tion occured
its
overwhelming
feature
clastic
Late O r d o v i c i a n
Unweathered
diamictites
attest
and
deposits
detrital
Volcanic
are
flows
part
separated
zone
the
from
the
low-grade
zone
local un-
sedimentation
was
Turbidites
(Fig.8.9).
sandstones are
and micro-con-
common
in i c e - c o v e r e d
fine
zone
the
Ordovician
by a tectonic by
Sehoul
before
and storm deposi-
of carbonates.
in
Moroccan
polar
strata,
with v o l c a n i c
graptolites.
in the
metamorphism
among
overlain
Lower Ordovician
nautiloids,
or sub-
Sahara.
sequences
Sehoul
is
Ordovician
at ab o u t
the
rates
(graptolites,
numerous
and Tazekka
which
from Mo-
to 2,500 m of
orthoconic
causing
are m o s t l y
intercalated
the
Cambrian
containing that
are
of M o r o c c o
2,000 faunas
Ordovician
biotites
to slow w e a t h e r i n g
extended
conditions,
the r a r i t y
Jbilete
polar areas of the Late O r d o v i c i a n
northern
bottom
oscillations
in eastern
glacial
glomerates.
shelf
neritic
of M o r o c c a n
nature
deposited
and
pelecypods,
bioturbated sea level
shelf w h i c h
On this
Anti-Atlas
echinoderms,
A notable
were however,
in Algeria.
the
with m i n o r
conformities.
epicontinental
flows
contact.
sandstones
But more had
of
folded the
in the
In the and
and
Sehoul
siltstones
significant
been
intrusion
but
in the Meseta
is the fact subjected
Rabat-Tiflet
to
granite
430 Ma.
Silurian Post-glacial Transgression Following Moroccan
the m e l t i n g shelf
form g r a p t o l i t i c ing event. in
the
followed
to
shows by
Late
are
overlain
bears
carbonate
facies
the peak
Llandoverian.
a typical
uniform
which
Ordovician
shale and siltstone
In the Anti-Atlas
Middle
Anti-Atlas
of the Late
Saharan
facies
of
by an upper
concretions
of this The
succession
attests
of Lower
dark
graptolitic sequence.
Scyphocrinites
limestones
shales were p r o b a b l y d e p o s i t e d
relatively world
shallow
(Berry and Wilde,
In w e s t e r n tercalated hibit
water,
Morocco
within
striking
representing
like
lowlands
shale
and the nautiloid
coeval
the
to this
Silurian
was
in
attained
the
western
sandstones,
and
siltstones,
upper
shale
sequence
Orthoceras (Fig.8.12),
uppermost
black
uni-
unique drown-
platy
shales The
the entire
A widespread
transgression
Draa
Silurian
black
ice caps,
and the Saharan p l a t f o r m were inundated.
transgressive
under euxinic shales
with unit.
conditions
i n other
parts
in
of the
1978). (Ouled
Lower
thickness
Abbou
Silurian
region)
strata.
variations,
thin
Silurian
being
basaltic strata
thickest
flows
are
in-
in M o r o c c o
ex-
(1,300 m)
in
the
455
northern where.
half
Thickness
tural
variation
differentiation
northern
Series DEVON
of t h e T i n d o u f
e
Hercynian
basin, has
been
between
domain which
-J C3
D
_l
Draa Eastern Western plain IAnti-Atlas;High Atlas I I ~ or Gedinnian 3ondstones Slack shales Conglomerates :3ridlirnettones S c y p h o c r i n i t e s 3nd lirnestones (~ . . . . . JlUppershaler,li~Cr nodc e ~2 ,-,H~, lime~n~o F J/andlimestones/ . . . . . ~Form(]t[on r. Formation I ~ e ~ I / e [52 ~ ~ (20Oto70O 'f ml.Orth°ceraslf Shales fAmsailikh It,~sto~ I[ and e [51 f plane ~ I
LowerWenloek
0 C) Z
i F°rmatikon
.........
Idwian upper lower
Format
. . . .
.
...... ?
Ii~rtom2Oot~ll
? '___
IF
Grey black
~
iJ~~AF~d/~~r
IIl~'Tizill~/ Ijl/ Shales
~
north-south domain
strucand
the
Cambrian.
Some index fossils ( present in Morocco) t,
.
.
.
Nlonograptus umformls Pristiograptus transgrediens
.
?
--
-
MonocL ultimu% Formosogr.formosu.~ Monocl. tomczycki "Monogroptus" e 9 r Ioehkovensis Saetogroptus leintwardinensis Pristiograptus tumescens Saetogroptus chirnaera~N nilsson Colonograptus colonus, S.fritschi
?
F Rod : and
G~y~rrf Y.ellgwish sna~es and •EWhite yellowish shales i R--ed~ / shalesI # of I~ -'Az~
h~
Monoqraptus flemingi Monoctimacis egr v o m e r i n u s Spiragraptus Monoclimacis spiralis crenulata Monociimacis griestoniensis Monograptus [Globosogr.) crispus Spirograptus turriculatus Monocjraptus halli, Rastrites linnaei Monograptus sedgwicki
/ Phtanites Demirastrites convolutus [ T ~ / Pristiograptus gregarius
IIIIIIlllllllNIIl/ ' "°° °°'
IlJJJll[lllJll/I
Orthograptus vesicutosus Akidograptus acuminatus Glyptograptus persculptus
~'D*~o~i"'fllllllllllll[V AIIIIIII lower
the M i d d l e
_
Rhuddanian
"J
a
Anti-Atlas
Eastern High Atlas
•
upper ,=OO,o IIIIIIIIIIIIl' /11111V .o~°" .o ,O 2semi IIIIIIIlllll~zsoq~l/llll Ain o~-,lllllllllllll~.=~lll[IH ~,~" ~.~
-1
to
200 m e l s e -
i limest°nes ~greyshales N l ' v u l g a r i sG°th°graptus Monograptus E t A t c h a n a ° f nassa' flemingi
(25to700m) I'~'"~"J'"vu' J
upper Telychian [ lower ~, Ain upper ~ Free,on lower / Chebi
<
since
I I Stages
O~ Upper Wenlock
>cr uJ >
southern
began
•
not exceeding
attributed
the
m e s t o n e s
O~
but usually
~ " /
Microconglomeratic green].". . . . II slates or Bent quarfzitesJ uuorrzn'es I Slates
0 R D0 V I C I A N
F i g u r e 8.12: Graptolite biozonation and in M o r o c c o . ( R e d r a w n f r o m L e g r a n d , 1989.)
Silurian
correlations
Early Middle Devonian Platforms and Trough Lower both
and
Middle
sides
eastern
of
Devonian
the
Devonian
carbonate
Silurian
sea,
platform,
Tindouf
Anti-Atlas.
Here
of
basin Wendt
platforms
fluvial
south
strata
the
and
are
well
and
in
(1985,
and
Tindouf
the 1988)
basins.
regressive basin.
exposed
in
Tafilalt-Maider identified
Following
conditions In t h e
southern
area
Middle
the
prevailed
Draa
Morocco
lowlands
retreat on on
the the
in to of
on the
Late the
Saharan north-
456
ern
margin
about
of
the
1,300 m
sequence.
alternating more
of
Each
Tindouf mostly
"Rich"
and
beds
and
lenticular
Middle
Devonian
are p r o g r e s s i v e l y
northwest
where
represented
(Hercynian-Bohemian) As
depicted
in
by
in Fig.8.13
limestone
deposition
1000 m
of
Devonian
NNE-SSW basin
lay
along
where
Early-Middle zone
the
shows
shelf
Middle
had a c c r e t e d
to the M o r o c c a n
According
to Piqu~
separate
ian.
Epeirogenic
imentation the
in
persisted
took
place
and Michard
upwarp
until
Here
the
towards
intervals
are
showing
the com-
European
Early-Middle
which The
occurs reefs
which
accumulated
extreme
limestones
above
about
trended
turbidite during
northwest
implying
Devonian
above
Marrakech-Oujda
granite,
the
Cambrian
the
Sehoul to
Sil-
that the Sehoul
zone
by Early Devonian
the
part
Early
almost
eastern
earlier
(Fig.8.14).
previously
basin.
and
The Lower and
and pelagic
these
by
coarse
top.
a deep
the
(1989)
Morocco,
occurred
Side-Bettache
with
the
"Rich"
followed
sediments
and structural
followed
basin
in what
deformation
the
by
times.
Platforms and Deformation
western
tion
of
To
Meseta
stratigraphical
Sidi-Bettache
at
limestones
shales.
Devonian
and a Late Ordovician
L a t e D e v o n i a n Basins,
area
Morocco
turbiditic
(Fig.8.13).
and
as
limestones.
an extensive
pyritic
urian strata
went
was
in w e s t e r n
dark
and
Devonian Lower
there
western
pelagic
Tafilalt
known
sandstones
occurring
goniatite
represented
limestones
more calcareous
the
is
affinity.
reefal
Lower
platy
by black pyritic
thin
faunal
deposits
of
sandstones
is r e p r e s e n t e d
and
cyclical
Devonian
fine-grained
Devonian
east
Lower
is composed
Middle
pletely
the
detrital
cycle
argillite
massive
basin,
Devonian
of
which
The main
platform
when
marine
Eovariscan
phase
with
of Eovariscan
under-
carbonate
subsided
Morocco
the M a r r a k e c h - O u j d a
contemporaneously
Morocco
in the Late Devon-
strongly
In eastern
Carboniferous was
and w e s t e r n
evolution
form
sedimentadeformation
trough.
the
to
sed-
Eovariscan
subsidence
deformation
was
of the in the
Late Devonian. In the w e s t e r n of about
by Wendt
whilst
1988),
i00 m
covered
uniformly
of
pelitic
in the eastern about
basin margin
about
lowlands)
On the contrary,
developed
(1985, and
limestones were
concretions.
setting
terbeds
(Draa
1,300 m of an e s s e n t i a l l y
calcareous basin
Anti-Atlas
the
Tafilalt in
sequence with
a carbonate Anti-Atlas.
800 m of clays with
slump deposits
siliciclastic
deposited
the Upper D e v o n i a n
the
Here,
platform
and
(Fig.8.14).
Tafilalt-Maider
layers and and clastic
as demonstrated
calcareous
accumulated
limestones
silty
platform
consists
and sandy in-
in the M a i d e r
basin,
condensed
cephalopod
Deltaic
sandstones
region
at
the
Devonian-
457
Carboniferous transition, with some olistostromes generated in the northwestern
margin
of
the
Tafilalt
basin,
east
of
the
Tafilalt
carbonate
platform.
Early
Devonian
Oujda . ~
,,
/
,'
Zekkorg~ ~_%~]e rada ~'~ Debdo'~CMe'kkam
RABAT TIFLETZ. ~..Pq'/"r ) I ' - I ,~,
J
r i _ ~ . .-- - - - ~ .
o
REHAHNAi ~--
T~--
i-I "
,
~.... • •
"~-~z}o-u " "
.'_------
" ': ......
"~
".,
.~
%~
I I
~
tammut'" ,,Tafil~t L~ghdod ,
.M~ider~
t_.~r~_/,
E~]
Undocumented
[]
Turbidites and
•
Shelf limestones
[]
Shales
pelagic
[ ] C o n t i n e n t a l redbeds. [ ]
Figure 8.13: Early Devonian lithofacies (Redrawn from Piqu~ and Michard, 1989o)
North of the Anti-Atlas,
Hauts plateoux
facies
Sandstones
of
northern
Morocco.
basins opened in western Morocco by exten-
sional processes which involved normal faulting. Syn-sedimentary faulting and graben subsidence took place generating gravity sliding of shelf deposits down into the grabens. Thus, while the Moroccan Coastal block, the Za~r rise and the Sehoul zone went up yielding proximal chaotic lithofacies
(olistostromes,
down
into
the
mudflows)
Sidi-Bettache
and
trough,
Middle the
Devonian latter
limestone
subsided
and
breccias received
fine-grained graywackes and pelites into its distal facies (Fig.8.14). Repeated the
Late
regional
Devonian
and
deformation the
Late
in
the
Marrakech-Oujda
Carboniferous
removed
basin Upper
between Devonian
strata from most of this region except in places such as the Debdou inlier
(Fig.8.9). Here Eovariscan deformation caused regional metamorphism
458 dated at about which
according
France sive
366 Ma in the Midelt to
(Fig.8.7).
folding
westwards
Piqu6
episodes
in
the
eastern
Jbilete
contain
Ordovician
and
In M o r o c c o
Michard
produced
Debdou-Mekkam which
and
only the e a s t e r n m o s t
but
rooted
strata
meaning
parts
is
was
also
known
recumbent
verge
west
which
(Fig.8.14),
accompanied
NNW-SSE
area,
were
Devonian
the Early Carboniferous,
(1989),
metamorphism
which
nappes
and Debdou areas
were
western succes-
folds
that
in
verge
ZaYan.
Debdou-Midbelt
thrust
that the Eovariscan
of the M a r r a k e c h - O u j d a
in
by two
eastward
of the
an event
westward
deformation
In axis
during
affected
trough.
Late Devonian T
. . oUrlTOISlOQ
.4/ I I i ~ , . /
.
~
~
~;;;7~
I I I
I I
Oujda...
I II
. ~/~/x~'~<."
FFFIPhysiographicollyhigher LLJJzones, mostly terrestrial I ~ Shales -~Olistostromes,mudflows, ~Pelagic limestones proximal furbidites [ ] S~nds ~ £ ovoriscon belt,,,,,,,,,,,, l
!
Figure 8.14: Late Devonian lithofacies (Redrawn from Piqu6 and Michard, 1989.)
of
northern
Morocco.
Carboniferous Basins and Hercynian Deformation Earliest which
Carboniferous
covered
(Gattendorfia
most beds)
the Draa and Ougarta
time was marked of
were
the
deposited,
areas.
by a shallow
Anti-Atlas
during
followed
The western
marine
which
by r e g r e s s i v e
and southern
parts
transgression
pelagic
facies
sandstones
in
of the Tindouf
459
basin
and
probably
the
northern depocentres. western
Morocco
Tafilalt
uninterrupted
(Korifla-Bou-Rzim
Bettache
basin
tostromes, rocks
(Table
mudflows,
fault
Upper
sequence)
8.1).
and
scarps.
intercalated
emerged
and
Late Devonian paleogeography,
with
clastics
active
also
proximal
the
within
however,
of
p e r s i s t e d in Carboniferous
this
suggest
tholeiitic
the K o r i f l a - B o u - R z i m
into
in the subsiding
margins
turbidites
Contemporaneous
clastics
Devonian-Early
accumulating
Along
shed
Sidi-
basin
olis-
re-deposition
to
alkaline
sequence
down
magmatic
indicate
crustal
e x t e n s i o n which created rifts or back-arc basins.
Table 8.1: Upper Paleozoic correlations and s t r a t i g r a p h i c termino l o g y for Morocco. (Redrawn from Piqu6 and Michard, 1989.)
Southwestern Anti-Atlas
t .......
Sidi-
Central Jbilete
Bettache basin
Northeasterf western limit
western margin
western part
center
eastern part
Morocco
Betana
Jerada
beds Ouarkziz
Bou -Rzim
Betaina
SarhJef
!Kharrouba
Korifla
Tazoui Daraa Benlowland sequences 5limane
Fouizir
Debdou slates
Renewed marine t r a n s g r e s s i o n occurred in the Draa lowlands during the late Early C a r b o n i f e r o u s silts
ditions
prevailed
sion
located
8.1)
comprising
gest
euxinic
tral
sequence central
Jbel
fine
Ouarkziz
clastics
massif,
fold
flows
and and
a
Middle
strandline
(Fig.8.15).
basin
argillites
series)
the
of
that the
were
The
extended
deposited,
Carboniferous
and
series
the
eastward
including region.
eastern
(Hercynian)
part
Here
con-
transgres(Table
chalcopyrite
massif.
in the A z r o u - K h e n i f r a in
this
in the w e s t e r n
Central
accumulated
of
Sarhlef
pyrrhotite
accumulated
belt
and breccias
(Kharrouba Jbilete.
in
thrust
with
in which
paleoenvironments,
limestones
andesitic
southwestwards
near
Jbilete
Eovariscan ates,
(Visean) producing g o n i a t i t e - b e a r i n g pelites and
(Betaina beds) and reefs in the Tafilalt area. But continental
sug-
of cen-
over
the
conglomer-
calc-alkaline A turbiditic
deeper
deformation
part
of
triggered
460 olistostromes limestones nappes.
followed
and
allochthonous
turbidites
At K h e n i f r a
emplaced
by
which
were
the Late V i s e a n
by g r a v i t y
Ordovician
emplaced
is also
and
from
Devonian
the
overlain
east
as
by nappes
pelagic gravity
that were
sliding at the end of the Visean.
Late Visean
~
Surelevate areas, mostly terrestrial
[]Sandy
]Sandstones.& pelites; [ ] S h e l f marine series
[ ] Turbidites & synsedimentary nappes
limestones limestones
[~7 01istastromes
Figure 8.15: Late Visean lithofacies (Redrawn from Piqu~ and Michard, 1989.)
In
the
peared folds
completely
the
in
the
in the A n t i - A t l a s
sociated
en-~chelon
strike-slip acted
Anti-Atlas
as
faults
the
Tindouf
with
folds
occured
southern
Hercynian
NE-SW in
Carboniferous
deformation,
zone
served
northwestern
as
meseta
the
shear
zone,
and
to
basin
Morocco.
attenuated
(Fig.8.10).
It
and
disap-
generated
open
sinistral
strike-slip
faults,
and as-
western
Anti-Atlas.
NW-SE
dextral
the
while
belt in Algeria. Hercynides
the
foreland. the
northern
deformation
in the Ougarta
foreland
Late
Western
the
in
Tizi
Coastal Major
n'Test
during block
shear fault
The Anti-Atlas the and
zones zone
Middle the
such
and
to
Sehoul as
the
Tineghir-
461
B~char thrust w h i c h constitute the so-called South Atlas arate the central part of the Hercynides These or
fault zone, sep-
from the forelands
(Fig.8.16 A).
shear zones are c h a r a c t e r i z e d by intense d e f o r m a t i o n w i t h reclined
recumbent
stratigraphic
folds
(Fig.8.16B),
changes.
To
and
the
by
granite
northeast
the
intrusions
and
Tazekka-Bsabis-Khenifra
thrust zone corresponds to the limit between the Eastern meseta during
Eovariscan
and
Middle
Carboniferous)
abrupt
and
the
(deformed
Western
meseta
(deformed in the Late Carboniferous).
/
~ ~ 4
Tazekka _~;#/ Precam bri(zn
series
5ranites
Fold axial plane trace
//~/F Strike-slip fault
,A"~" Thrust Carboniterous regional I shortening
100Km
t
A
NW
SE WMSZ
^ Coastal , B(ock
u
20
$ t
.
Central Mossvf ~ .,~
APIZ
Eastern Mesela
A-~
A÷,
.,..-~.,as
'
Moho
B Figure 8.16: Structural map (A) and section (B) of the Moroccan Hercynides. For (A); Z, Zaier; O, Oulmes; M, Ment; JT, Jebe! Tichka; A, Azigour. ((Redrawn from Piqu~ and Michard, 1989.)
462
Whereas east
and
the v e r g e n c e
west,
northwest
those
and
to
shortening
Coastal
block
placement
of
of
the
crustal
to
of
Eovariscan
the Carboniferous
southeast.
(Fig.8.16 50%
folds
The
block
increased
for about
event
deformation
in the Central massif,
the Coastal
Devonian)
Hercynian
latter
B) which
(Late
from
and
to
verge
NW-SE
in the western
entailed
I00 km towards
the
to the
involved
15%
also
is
the dis-
the Anti-Atlas.
Since most of the Carboniferous d e f o r m a t i o n was by ductile processes gional m e t a m o r p h i s m was also involved, central place in
and
western
including
the
Central
the
Rehamnao
crystallization
massif
at
reaching the amphibolite
Syn-tectonic
300
to
of
granite
290 Ma,
within
facies in
emplacement
the Oulm~s
also
calc-alkaline
a
sinistral
retook
granite
shear
zone
w h i c h c o n t r o l l e d the deformation and m e t a m o r p h i s m of Paleozoic strata. Late C a r b o n i f e r o u s
(Westphalian)
sedimentation
a paralic basin which extended from the Fourhal basin
eastward
lasted
from
arc-type
(Fig.8.17).
latest
andesitic
At
Visean
to
Tazekka Early
and
centred in
"syncline" and the B6char
Jerada
Westphalian
calc-alkaline volcanism.
in Morocco
turbidite
associated
This
suggests
deposition
with
island-
either subduc-
tion or large-scale intracontinental transcurrent faulting of a thickened continental posits coal.
crust.
Alternating
occur near the top, These
are
overlain
fine-grained
marine
and
continental
and at Jerada
these paralic deposits
unconformably
by
the
Sidi-Kacem
de-
contain
continental
redbeds and by lacustrine limestone, the u n c o n f o r m i t y being due to Hercynian deformation. 8.3.3 C o r r e l a t i o n s w i t h North America and Europe The M o r o c c a n
Hercynides,
continuation
of the western
name Hercynides.
its
tectonics European
and
faunas,
Hercynian
belt
are
often
seen
(Fig.8.7)
hence
as a the
Comparisons of the Lower Paleozoic of M o r o c c o have also
been e s t a b l i s h e d with the northeastern part of the Avalon zone in New England in North America
(Rodgers,
Based on p a l e o m a g n e t i c reconstructions
(Fig.8.18)
1970; Skehan and Pique,
(eg. Scotese,
1989).
1986), these regions
are g e n e r a l l y b e l i e v e d to have been situated close to northwestern Gondwana
(Morocco) as depicted in Figs.
b e l o w are from Skehan and Piqu~ Along
the present western
6.9 A, 8.2. The similarities reviewed
(1989) and Piqu~ and Michard Atlantic
margin
or eastern
(1989).
North
America,
the A v a l o n zone which occupies part of eastern A p p a l a c h i a n Mountains Newfoundland
to
stratigraphic
evolution
Early Paleozoic,
Florida
(Fig.8.18), with
Morocco
shared from
the
suggesting African connections.
comparable Late
tectonic
Proterozoic
to
from and the
Parts of the Avalon zone
463
are b e l i e v e d
to have
arrived
in North America
at d i f f e r e n t
times
during
the Paleozoic.
Westphalian
al c~
o
[ ] L~runic basins
~;..".-
[]
Paralic basin
.w*."
Figure 8.17: Westphalian lithofacies (Redrawn from Piqu~ and Michard, 1989.) Similarities
shown
land and Morocco,
in Fig.8.19,
of
between
the Avalon
Proterozoic
Morocco.
zone
of New Eng-
stromatolitic
carbonates
and q u a r t z i t e s which overlie the Eburnean basement in Morocco.
The equiv-
alents
affect the Middle
northern
in New England belong to the Blackstone Group.
Other
similarities
are the Bou Azzer ophiolites of Morocco
(Middlesex v o l c a n i c s
the
orogeny);
Group
Pan-African and
the
I
"Lie-de-vin"
Group in Avalon);
(Avalonian unit
(Mattapan
olistostrome
Adoudounian
unit
volcanics
in Avalon);
Ouarzazate-Tanalt of
the
Group
in the Newport Group
zone.
The
ments
have been equated with a sequence
basal
conglomeratic
quartzite
and
(Hoppin
the
overlying
Cambrian
graywacke
c o n t a i n i n g the trilobite in Boston and Rhode
and
siltstone
Paradoxides
fine
in New England
Quartzite)
maroon slates and thin t r i l o b i t e - b e a r i n g limestones The M i d d l e
the
Boston
Bay
and the calcareous A d o u d o u n i a n w h i c h is p r o b a b l y equiv-
alent to the upper Upper
orogeny
of the Avalon clastic
that consists
overlain
by
green
of and
(Weymouth Formation).
sequence
in
the
Anti-Atlas
is e q u i v a l e n t to a similar
Island where the P a r d o x i d e s
sedi-
sequence
fauna has close affinity
464
Eo b~
fl)
{.) cO
(P -,,4
f-~
0 4-i
"0
d 0 0
u
I,,..I f~
0
°o
465
with
Morocco
unlike
and
Morocco
whereas
the
of distal
Spain.
where
it
Ordovician
turbidites
Island,
equivalent
clastic
sequences
However, is
the
up
in New
to
sequence 8 km
England
units which
in Morocco
and
has
known as the Dutch
is
thinner
contains
been
shallow
England
volcanics.
recognized
Island H a r b o u r
are
in New
Formation
water
Also,
in a sequence
upward
in Rhode
coarsening
contain diamictites.
BosQIt
R P
Alleghany
Westphol~on AStephonion 8 or C Ftoro
'en ~hode is,F M ]
S;di- Be t toch e Basin seq.
D S
Dutch Is. iarbour FI
Quincy p|ut. ocr~ tGrchs
~Z
3mestown
Por adoxides
£~ .J
Neymouth Hoppin Qt: Boston So Newport ( 5quantum Roxbury C
O~
R~ch Ban~ III 9oni !
J-C i
Jebel 8ehq 534 Mo
Brighton voles. Wottopon voles.
Trochytes 618 MQ
M i l f o r d Gr. 630Mo
Avolonlon
t
Metomorphism 685
Blackston(
Bou A z z e r Ophio. 798 Ma TQhGIQ Gr. 1989 TGzerouQ{t Gr, 1973
Figure 8.19: Comparisons of the Late P r o t e r o z o i c - P a l e o z o i c of the A v a l o n Zone of N e w England with that of Morocco. (Redrawn from Skehan and Pique, 1989o)
The M e g u m a tia with
(Fig.8.18) western
Ordovician
terrane has
on the s o u t h e a s t e r n
been
correlated
Mediterranean
Meguma
G r o u p comprises
quence that was d e p o s i t e d of the
clastics
bro-Ordovician (1989) Meguma African
lying deep-sea
suggested terrane
that
and
continental
with
Paleozoic
the
blocks.
fans
margin,
occur
zone
In Nova
environment
in the
Sehoul
in
the
Portugal
had
the
Sehoul
on the West A f r i c a n
somewhere
southern
of A v a l o n i a
Meguma
in Nova
in
Scotia
a very thick clastic
in the deep-sea
somewhere
part
with
terrane
of
began
Similar
Piqu~
the to
and
Cambrose-
the p r o v e n a n c e
craton.
zone.
part
the
progradational
Cambro-Ordovician, been
Morocco
Sco-
Cam-
and Michard after
the
northwestern separate
from
466
Africa
(Fig.6.9A).
From
then
on~ the
similarity
between
the A v a l o n
zone
and northwest A f r i c a largely ceased. The S i l u r i a n through Devonian stratigraphy and the tectonic evolution of M o r o c c o and New England are disparate with rifting and the production of alkaline p l u t o n i c Silurian,
Devonian)
larities were, tation
and
rocks occurring earlier in New England and during the Early Carboniferous
however,
(Ordovician-
in Morocco.
restored in the Late Carboniferous with
deformation
followed
by
coal-bearing
strata
Simi-
sedimen-
that
were
de-
posited in i n t e r m o n t a n e basins. Although
the strong
similarities
M o r o c c o have long been recognized
between the Paleozoic of Europe and
(Fig.8.7),
the precise Paleozoic
tion of the eastern
part of Morocco
southwestern
(western M e d i t e r r a n e a n - I n n e r
Calabria and
Europe
and Sicilia,
Michard
(1989)
Corsica, traced
an
relative
to the Paleozoic Rif and Betic,
of
Kabylia,
Sardinia)
have remained unresolved.
elongate
and
relatively
loca-
blocks
narrow
Piqu~
zone
of
crustal w e a k n e s s which they termed the N o r t h w e s t e r n Gondwana mobile zone, extending
from
east.
Northwestern
The
during and
by
the
Marrakech
through
Gondwana
mobile
Devonian-Carboniferous
orogenic
events,
most
Oujda, by
Kabylia,
zone
was
repeated
especially
the
and
Calabria
characterized
deep
basin
Late
Devonian
in at
the
least
sedimentation Eovariscan
phase. This m o b i l e zone separated the A f r i c a n cratonic area from the Hercynian orogenic belt of central and western Morocco,
thereby constituting
the southern limit of the European-North African H e r c y n i a n orogenic belt.
8.4 North Saharan Intracratonic Basins Giraud et al.
(1987) pointed out that "from southern M o r o c c o to the Egyp-
tian-Libyan border one can observe a sequence of basins, blocks
or
uplifted
termed
the
North
cluded
the
"Tindouf,
Goussa,
areas
Sahara
of
various
basins
Timimoun,
Syrte and Kufra".
orientations".
by Giraud
et
Oued Mya,
al.,
Ghadames,
troughs,
These
among
basins
which
Mursuk,
This suite of basins constitutes
horsts, were
they
Hun,
Dot
inel
the North Sa-
haran p l a t f o r m basins which contain extensive Paleozoic sequences. 8.4.1 T e c t o n i c Control of Basin Development The
post-Pan-African
(Fig.8.20)
structural
picture
of
the
North
Saharan
platform
i n c l u d i n g the structural effects of the C a l e d o n i a n and Hercy-
nian orogenies can be gained from the Phanerozoic structural evolution of
467
northeast Wycisk
Africa
presented
by Klitzsch
(1987), and Schandelmeier et al.
.;.?M '°<:. ,
~
~
h
~ x
~
1981,
1986),
Klitzsch
and
(1987).
I~ :~%. -_..------_=~L_~. -"--'I 8,
n
~---'k%"-.
,',\%3."~ \
"v4",'.',, I Y ~
i~q~A
I
(1971,
r-$'. ,~
........a
~,'~_~,'%,,,z~ ~V's'~' ~"
'~'~
Figure 8.20: Structural sketch map of northern Africa. (Compiled from figure supplied by J.A. Peterson; Klitzsch, 1981; S c h a d e l m e i e r et al., 1987.)
Regional phases
peneplanation
of the Pan-African
prevailed orogeny.
in
North
Africa
after
the
final
But unlike northwest Africa where,
as
we have seen, continuous c o n t i n e n t a l - s h a l l o w marine d e p o s i t i o n took place across
the
base of
latest
Proterezoic-Cambrian,
the Cambrian
all
the way
a
marked
hiatus
from the Tindouf
basin
Egypt and Sudan in the east,
implying regional arching.
tribution
strata
of
Lower
Cambrian
suggests
occurs
at
in the west
the to
In Egypt the dis-
considerable
structural
re-
lief. The s t r a t i g r a p h i c thicknesses in d i f f e r e n t parts of the North Saharan p l a t f o r m reveal broad graben-type structures and smaller horst blocks which
trend
NNW-SSE
(Figs. 8.20,
WSW-oriented
post-Pan-African
the
shield.
Saharan
Libya, defined
By
8.21).
These were the p r o d u c t s
intraplate
Middle
to
Late
tensional Cambrian
of ENE-
forces w h i c h broke up times
sedimentation
n o r t h e r n Chad and northern Niger was a l r e a d y taking place NNW-trending
(Fig.8.20).
These
(Ordovician-Silurian) depocentres,
grabens
structures after
or were which
troughs
which
accentuated they
were
by the
continued
to
bounded
by
Caledonian determine
in
in well horsts orogeny sediment
and the centres of m a g m a t i s m e s p e c i a l l y along the borders of
the M u r z u k basin,
the southwestern m a r g i n of the Kufra basin,
eastern part of the Ennedi Mountains.
and in the
468
In the eastern 1,000 m
thick
Murzuk
but
very
(Fig.8.20)
where
units
rest
directly
horst
divides
eastern basin
displaced
the
upon
the
of Ca m b r i a n
the
and
tilted
Cambrian
and O r d o v i c i a n
on
the
neighbouring
out
and
Lower
basement.
(Homra)
Faulting
horizontally
Kufra
towards
thin
wedge
the Ghadames
sub-basin.
are n e a r l y of
they
basin
in
basin
the
over
into
Dor
el
truncated
north
a
deep
eastern
Ghadames
basin,
to Early Carboniferous
trough
sub-basin
trough
and
in
The
about
and an
the
Murzuk
deposits
strikes
contains
which
northern
part
northwestwards
2,000
to
3,000 m
strata.
4"
,
units.
which
uplift
transgressive
the T r i p o l i - T i b e s t i
a western
Gussa
by Ordovician
contains
__2,: u L_. ?
,,
'," v
Figure 8.21: A p p r o x i m a t e thickness n o r t h e r n Africa. (Redrawn from figure
The
southwestern
(Fig.8.20). basin,
The
where
quences
were
of
which
the
part
Early
eroded
away.
prolongation Gardeba
experienced
high
the
Darfur-Uweinat-Gardeba generally
trough
basin
runs
Carboniferous Along
of Paleozoic sequences in supplied by J.A. Peterson.)
the
Egypt
by
lies
uplift
the
eastern
Dakhla
subsidence Cambrian
and
the m o r e
extensive
uplift
as the
northwards
towards
of
border
of the D a r f u r -U w e i n a t
greatest
the Uweinat
is known
times t most
thin on this hor~t I p r o g r e s s i v e l y
overstepped
northern
of the Kufra
Erdis-Kufra
after
lies n o r t h e r n East
are
marine
1,700 m of p r e - O r d o v i c i a n
basin
are up to
Tripoli-Tibesti
Silurian
Further
strata
the of
uplift
basin, during
the the
Ordovician disappear
Silurian
effectively
prevented
basin
the
Sirte
Paleozoic
the
Sirte
(Gardeba
se-
basin
uplift).
northern
part
Paleozoic.
On
units.
where
are they
Except
Ordovician,
of the
strata I which
southward
marine
Erdis
in
Sil-
469
urian,
Devonian,
reaching ing
and
eastwards
Saharan
encroach
Early
Carboniferous
into north-central
platform
troughs
southwards
upon
marine
Sudan.
allowed
transgressions
from
N e v e r t h e l e s ~ the NNW-trend-
Paleozoic
marine
the n o r t h e r l y tilted A f r i c a n
transgressions
to
plate,
to
leading
the d e p o s i t i o n of thick sequences in the troughs and a thin sheet of Paleozoic strata During with
the
Europe,
form.
(Ordovician to Carboniferous) post-Visean
ENE-trending
Hercynian
structures
on the structural highs.
tectonism
when
NE-Africa
resulted on the North
This was on account of the reactivation of d e e p - s e a t e d
dextral North
wrench of
the
prominent
faults
include
the
Algeria,
in
became
Nefusa
the
uplift
and
Gulf
in
Suez
this
uplift
sinistral
Libya,
anticlinorium,
graben.
Because
of
Precambrian
shear
reactivation
(Fig.8.20).
northwestern
the A l g e r i a n
of
conjugate
border
Uweinat-Bir-Safsaf-Aswan
eastern oped
which
Egyptian-Sudanese
collided
Saharan plat-
Similar
southern whereas
faults.
produced Tunisia
sagging
widespread
the
structures and
devel-
post-Hercynian
erosion on the North Saharan p l a t f o r m Permian rocks are g e n e r a l l y missing or
very
thin
thicker north ment
of
the
on
the
platform
of the Nefusa Pelagian
especially
uplift
platform
on
the
uplifts,
in the Jefara
(Peterson,
trough
1985).
A
major
continental basin developed across southeastern Libya,
but
markedly
(Fig.8.20)
seg-
Permo-Triassic
n o r t h e r n Sudan and
southern Egypt in response to the U w e i n a t - B i r - S a f s a f - A s w a n uplift. Rocks
of
all
the
Paleozoic
M a x i m u m thickness of Paleozoic where
the
Silurian-Devonian
1989).
Paleozoic
Dakhla
basin
are
the
vast
Algeria
(Peterson,
reserves and
Libya. strata
hydrocarbon
occur
in
the
1985)
is
about
5 km thick
natural
of
and
over
The
3 km
in
ironstones,
widespread
provided
reservoirs
resources
oolitic
Saharan
platform.
basin
in Algeria
14 km
very
thick
in
of v a r i a b l e
trough
Paleozoic
and
oil
source
abound
of
gas
graptolitic
petroleum
quality
and
(Guerrak,
part of the
northeastern
the
Silurian
rich
thick
in the n o r t h e r n
eastern Algeria and in the Jefara
important
boniferous
succession
rocks are about
n o r t h w e s t e r n Libya, Among
systems
strata is in the Tindouf
and
(Fig.8.21). North
Africa
especially
shales rocks.
throughout
and
in
Car-
Sandstone the
Paleo-
zoic of North Africa.
8.4.2 T i n d o u f a n d R e g g a n e B a s i n s
The Tindouf
basin
synclinorium,
is a large p o s t - S i l u r i a n
ENE-WSW-trending
800 km long and 200 to 400 km wide
asymmetrical
(Deynoux et al.,
e x t e n d i n g from southeastern Morocco into southwestern Algeria, of
the
basin
basin
lies.
(Fig.8.22).
Eastward, Cambrian
the
Tindouf
basin
to Carboniferous
merges
strata
fill
with
1985),
where most the
Reggane
the Tindouf
and
470
Reggane
basins
northern the
part
Cambrian
Glacial rect
Late
on
basin.
the
is
the the
shales,
and
the
(Fig.8.24)
sequence Tindouf
absent
and
Ordovician basement
Elsewhere,
age
is much
basin
the
the
bulk
marine
(Fig.8.23A).
Carboniferous
the
in
than in the Tindouf
along
sometimes the
carbonates
Reggane
also
Silurian
and
basin
of
from
was
di-
Tindouf Silurian
Devonian
to
influ-
continental
basin.
]~T~ CARBONIFEROUS
[ IVOLTAIAN I • (GHANA)
I
DEVONIAN-SILURIAN
~LATE
CAMBRIAN AND ORDOVICIAN
~
~
rest
marine
more
where
missing.
the
marine
clastics
removed
often
of
in the
flank
shales
flank
comprises
further
complete
southern
is
southern
sequence
Being the
and more its
Ordovician
and
along
of
thicker
than
Lower
deposits complex
alternating
Carboniferous ence
and of
PROTEROZOIC-EARLY PALEOZOIC
BASEMENT
METAMORPHOSED P~ - EARLY PALEOZOIC
Figure 8.22: D i s t r i b u t i o n of some m a j o r Paleozoic exposures in n o r t h e r n Africa. (Redrawn from Whiteman, 1972; Bellini and Massa, 1980.)
In
the
the L o w e r
Reggane
basin
Carboniferous,
littoral followed
and
shallow
by continental
marine
clastics
occur
beds w h i c h m a r k e d
in
tempo-
471
NAHURO-
IGUID] SUB- BASIN DJEBILET SUB- BASIN GENERALIZED LITHOLOGY FORMATIONS Thick.(m) Thick.Ira) ....... FORMATIONS i!~{i!;.~~-." ~i Sandy shales, ' Fine sandstones ~ 350 HASS~ AOULEOUEL g:::::'.:::::::. ££-_---_--~L9 '_".............
WESTPHALIAN
Sh OI es
~
(Z
AIN
--'--'-'"-'" : Shales ~^~.~.~ Limestones -I~--~--tttZ~J clnd Dolomites
EL
CONCEALED
uJ
600
u-
~!i,//~/~/~],Anhy drite
BARKA o
F_--_-~_-~z~ Shales and . . . . . . . . . . . Limestones
VISEAN
KERB
Shales and
ES
310
SEFIAT KERB ES SLOUGUIA
TOURNAISIAN
FAMENNIAN
......
~0-160
KERB EN NAGA
--~---_'-'~ ;"'--'---' l~mestones Shales and ,-'-Z--'Z-~A~ Li mestones
Silstones and Shales Argillaceous I00-150 Silstones ............. Silsto nes and 80-160 Limestones /.0-100 - ~. . . . Limestones and Sh~ie~ Limestones and 50-100 Sandstones 100-1/.0
0UED GHAZAL FRASNIAN
AREA
OUED TSABIA
. . . . . . . . . . . . . . . .
MECHERI
BOU BERNOUS
. .. .. . . . . . . . .. .. . .. .. . . . . . . . .. .. .
= o
>
OUED TALHA
L.DEV.
DJEBILZT SILURIAN C -
GHEZZIAN.E
UPPER DEVONIAN
~oDJEBILET
MIDDLE DEVONIAN LOWER DEVONIAN
~'~__
SUB-BASIN
70
Upper FE"Dj 70 MLEHAS
.... B0-200 ~ ------z--_-~ Shales 0 70 :i:;:{ ?i is:!! Sandstones
SEBKHAMABBES
0
200-250
Lower FEDJ MLEHAS GARA SAYAOA
90 70-1000
)GUIDI SUB- BASIN
~I~' Lar oussi up|ift
P
]
uinet uplift
Bou Bernous uplift
SILURIAN
°'" ÷
÷
~
÷
÷ ÷
÷ ÷
÷
4,
. . -..'° "
÷
÷ ÷
+
0RDOVICIAN PRECAMBRIAN
Figure 8.23: (Redrawn from
W
.
•
.
Approx,mate honzontal scaler
Stratigraphic Guerrak, 1989.)
sections
100Kin
of
+
I
the
+
÷
+
÷
+
Tindouf
+
+
E
basin.
472
rary is
emergence
(Conrad,
represented
odonts
and
by
by
1985).
terrigenous
extensive
Upper
stones,
micro-conglomerates,
and
Cretaceous zoic
to
sequence
Visean
deposits
Carboniferous
sometimes
Late
carbonates
facies.
rine
A
carbonate
algae
Recent
(Hamada
Formation)
in
Tindouf
and
the
STAGES
W
REGGAN
J/
/
I
>
>
>G y p s u m>
i
>
[
i
i
i
.>
>
::
>
i
.
containing thin
ma-
continental
overlies
the
MOUYDIR
Paleo-
BASIN
E
: ~ , ' 'Djebet ~
,
w
, " • " • -. • • . . . . . . "~ . "'" " ; { . .. • . . .
.
. :
. • ". ,'.~
~
~
.
>
. ,
sand-
,................,
>
>
, ,
shales,
basins.
. ,'-" Hossi-Bochir
of Noss~-To~b~ne
,
A
AI~NET BASIN
'. " : • Mott~ F o r m o t i o n , : '
Serpukhovian
red
con-
regressive
JPost-Hercynion ,o' p r e s e n , - d a y
/ •
BashkiriQn
include
unconformably
:,~*.'.;'. Red b e ' d s ' o f . ' . ' : • : ". " " • - " " . " , ' , : . : ". : ., . L . . , , "'. " " Red bed .... Ain echChebbi':'.' "." "" nf • n the North and Azze - " '' - " . . v
to
and
gypsiferous
ostracodes.
Reggane
transgression
foraminifera
upper
intercalations
and
BASIN
/
Lower MoscviGn
an
strata
lacustrine
Namurian
containing
with
regressive and
and
,
.
.
i
r
~ .
,
, LI
,
" .
. r
,
~
-.
.
o m
i
,Berg'e, ' , Limesl06e ' . ' . : 2
Vii-ill V3
Ttrechoumine
Upper
Shales
ViseQn V2b
Vl Lower Viseon
N2c
-
-
~
Vl
TII
Upper
!Tn
3 S h Q I e s
TOU
r r;o ~slo.n T,
Lower Tournc6sian Strunion
(Tn2 1:. -- - _ - - - ---.-_--_--_--'_-~-._-- I---= r_ -._- _--_" Upper Khenig Sand-stones ._:--'-T~-[_--_-- .=-. . . . . . . . . . . I- . . . . . . . . . . . . . . . . . . . Tnlb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,-~ -_
Kh~nig-- Sd6dsto~es-- . . Z.'.. .
Tnla.'------ --- . . . . . . . . .
---- ----Lower Khenig SQ,dstones
FQmennion
~
-.
Khenig
M a r i n e SQndstones.
DeltQic Deposits
~
.
.
.
.
.
.
Shsles
FiuviGtile FQcies
~
Redbeds
Shales Carbonates Evaporiles
Figure of t h e
Along Iguidi within
8.24: Ahaggar.
the
Stratigraphic (Redrawn from
southern
sub-basins, argillaceous
part
numerous and
of
succession of Conrad, 1985.)
the
oolitic arenaceous
Tindouf
the
basin,
ironstones deposits
Carboniferous
in
lenses
the are
(Fig.8.23B)
NW
Djebilet
and
interbedded of
mostly
473
Devonian
age.
Paleozoic
These
oolitic
ironstone
M o r o c c o to Libya. ironstone
lies
Silurian
and
in
these
9.2
belt
formed
shallow
of
the
tons
Local
characterized
by:
(i)
accumulated
the
flanks
the
top
towards rich
of
the
mud
in
with
in
southern
relatively
location
in
cold
a
major
for
(Guerrak,
near
iron
billion
tons
(LOID) in
from
in
the
1989).
The
type
settings
control
of
having such
sediments
(ii) the o c c u r r e n c e
sedimentary cycles;
such
as
source
temperate
3,000 km
i0 billion tons of
(Guerrak,
seas
African
as
1989)o LOID in the Tindouf basin is
of uplifts; regressive
North
over
Deposition
epicontinental
environments
and
1.5
Devonian
paleogeographic
of major
quiet
about
the
coarsening-upward
end
of
extends
Ironstone
Paleozoic
coastal barriers and deltas
at
part
that
deposits
billion
are
on
are
An estimated total reserve of about
ironstones in
ironstones
or
and
regimes
(v)
which
ironstones
mostly
ooid
lagoons
areas;
climatic
cycles,
(iii)
of
located
growth
in
iron-
embayments;
(iv)
paleolatitudinal
after
North
Africa
had
recovered from the Late Ordovician continental glaciation. 8.4.3 Central and Southern A l g e r i a n Basins Paleozoic uplifted
rocks zone
Bled-el-Mass basin
to
underlie
that
most
runs
along
anticlinorium
the
northeast;
(Reguibat Shield)
of Algeria.
They
the
ranges
0ugarta
(Figs. along
8.22; the
northern
which also constitutes
ouf and Reggane basins;
and also
8.25A);
outcrop
along
through
in
the
margins
the southern
the
a NNW-SSE Touat
adjoining of
and
Bechar
Yetti
Eglab
flank of the Tind-
form a girdle around
the Tuareg
Shield
with the Ahnet and Tassili N'Ajjer among the largest Paleozoic exposures (Deynoux,
1983).
Paleozoic
sequences
Bechar-Timimoun Oriental)
basin;
Between the uplifted (Erg
Occidental)
the
Illizi
tailed
stratigraphic
Legrand
(1985) and W h i t e m a n
Bechar-Timimoun
reference
(Polignac)
descriptions
Ghadames
basin; these
the
basins
in Algeria, therefore has
been
Ahnet
were
or
Erg
basin.
De-
presented
by
Sandstone Group)
c o n s t i t u t i n g a stratigraphic
correlated with
sequence
succeeds
the Ougarta
chain
of up to 1,500 m of arkosic
which is glauconitic
cause of the onset of a marine transgression. 0rdovician
and
(Rhadames
(1972).
The Cambrian consists
(the Ougarta
of
the
(1985) the Bechar basin contains one of the most com-
sequences
point which
(Fig.8.26).
basin;
extensive
the largest of w h i c h are the
Basin
A c c o r d i n g to Conrad plete marine
zones of central Algeria
fill great depressions,
unconformably
nearby
sandstones
in the upper part be-
A graptolitic
(Fig.8.26)
into a sandstone complex with numerous m i d - O r d o v i c i a n
and
argillaceous passes
fossiliferous
upward shale
474
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z U.I 123
5~ "r ~
o LLI 123< I--
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E
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476
intercalations. Ougarta
The
chain
the Ougarta
(Fig.8.25)
Sandstone
dovician-Silurian Ali
Clay
boundary
The D e v o n i a n
stones,
succeeds
and are
Early
Carboniferous basin.
(1985)
into up
to
matactis, the main gence
reef
during
builders.
by
that was prone
predominantly
deltaic
trine
deposits.
occur
in the
mark regional
part
(Oued subsand-
the
Sand-
which
which
and
grades
horizons
that
is lo-
on
stable
plat-
contain
stro-
crinoids,
of emer-
platform
group exhibits
into
An
upper
fluviatile
while
red
were
lower group.
unstable
are a s s o c i a t e d
group,
which
to a period
channelling.
the
group w h i c h
an
the m i d d l e
upward
of the upper
on
in
Pareyn
the top of the
accumulated
Lower to
and
The reefs
attests
marks
influence,
Lemosquet
developed
areas.
Devonian
sequence.
lower
fenestellids
intraformational
Some m a r i n e
lower
of
by regressive
accumulated
by
The
which
shelf
Carboniferous,
deposits
strata
groups.
A paleo-karst
and
overlie
subdivided
reefs
corals,
to continental
intercalations
shales Gourara
represented
the
the Or-
neighbouring
clastic
been
detrital
carbonates
marks
by g r a p t o l i t i c
shales
and
has
contains
rugose
the M i d d l e
Characterized
tal
thick
in
the d e p o s i t i o n
unconformity
the
basin,
and upper
by s u b s i d i n g
sponges,
thickest
conformably.
carbonate
middle
4.4 km
forms b o r d e r e d
is
by a very thick C a r b o n i f e r o u s
sequence
lower,
in
Carboniferous
followed
This
is o v e r la i n Shale
in the Bechar
Bechar
cally
which
Fegaguira
Middle
Algeria
A Late O r d o v i c i a n
the Silurian
Transgressive
in
which was a deep trough d u r i n g
Group.
Formation;
basin).
stones,
Cambro-Ordovician
detri-
group and
with
coal
shales
of
lacusseams
at the
top
emergence.
Illizi B a s i n A
regional
cross
Timimoun, brian The
to
Upper
lower
in whi c h
section
the Ghadames
part
(Fig.8.27)
Carboniferous of this
fill
sequence
marine
(Clifford,
1986).
a very
thick
stones
and
carbonates
and
shales
vary
reveals
Illizi
in
Carboniferous
carbonates
in
middle;
and
at
the
the
sandstones
base;
from
with
shales,
overlain to
are overlain
marine
fluviatile
by
305 m
continental
restricted of
1985).
and
244
sandstones
sequence,
Bechar-
(Peterson,
are u n c o n f o r m a b l y thickness
lacustrine
across
up to 3.5 km of Cam-
basin
of m a r l y
Illizi basin Devonian
varied
marine the
the
sandstones
which
In the
in
NNW-SSE
basins
consists
thick C a m b r o - O r d o v i c i a n
Siluri a n
running
and the Illizi
by
sand-
evaporitic
clastics
at
the
top. Since
the
(Fig.8.27) place
at
Ahaggar
Mountains
a discussion
this
point.
border
of the northern
Extensive
Cambrian
the
Illizi
basin
rim of the A h a g g a r to O r d o v i c i a n
to
the
south
is not out of
strata
are
exposed
477
I-j
.,-1 ,-4
0 5
I:1 14 "l:J I1)
I
%
.,-I 1.4
,4-1 0
0
I8 I
18
E
0
%%
L) c) .~1
.,-I 4.J 1.4 .la ra3
o, oZ, 0 1.4 I~.la ~1~
478
around
the
the east
northern
(Figs.
Ahaggar
8.22;
basement
overlain
sandstones.
The
Mountains
sandstones
to
Djado
in
and c o n g l o m e r a t e s are un-
shales which
form
in A l g e r i a n
(1985)
appraised
n o r t h e a s t e r n Africa. saoud
sandy mudstones,
by Silurian
spectacular
pass
upward
cuestas
Paleozoic
into
around
Devonian
the Ahaggar
Basins
is among the world's m a j o r Paleozoic
terson
part
(Fig.8°25).
8.4.4 P e t r o l e u m
Algeria
the western
8.25). Along northern A h a g g a r the C a m b r o - O r d o v i c i a n
c o m p r i s i n g m o s t l y quartzites, conformably
from
oil
the
hydrocarbon
oil and gas provinces.
resources
Two well known supergiant
field w i t h
9 billion
barrels,
and
of
north-central
Peand
fields are the Hassi Mesthe
Hassi
R'Mel
gas
field
(Fig.8.27) with 70 trillion cubic feet of gas and 2.6 billion barrels of crude.
The bulk
of A l g e r i a n
petroleum
lies
in the B e c h a r - T i m i m o u n
basin
and in the Illizi and Ghadames basins. Most
of
structural
the
Paleozoic
traps
(Fig.8.27)
which
oil
are
and
gas
linked
to
fields
in
major
these
basins
are
in
paleo-structural
features
that o r i g i n a t e d during Caledonian tectonic m o v e m e n t s
and were
later strongly a c c e n t u a t e d by Hercynian tectonism. latter m o v e m e n t s
d e t e r m i n e d the present
Two large domal structures
To a large extent the
structure of the w e s t e r n Sahara.
controlled the location of the Hassi Messaoud
oil field and the Hassi R'Mel gas field, which hold the substantial part of
the
oil
and
show e v i d e n c e
gas
of
reserves
the
in
influence
the
region.
Many
of e s p e c i a l l y
of
the
Hercynian
smaller
fields
paleo-structural
growth. In their order of importance the productive reservoirs in Algeria are in
Cambro-Ordovician,
stones.
basal
Triassic,
Devonian
and
Carboniferous
sand-
Fractured Cambrian quartzitic sandstones capped by impervious Or-
dovician
clastics
trapped
the
oil
in
the
Hassi
Messaoud
field.
Basal
T r i a s s i c sandstone reservoirs account for a m a j o r part of the gas reserve in the Hassi R'Mel
field.
In the
Devonian
and Carboniferous
Devonian
sandstone reservoirs
gas
fields
are
located
Illizi basin
sandstone
on
gas and some oil occur in
reservoirs,
are also productive.
anticlines,
faulted
and
in the Ahnet
basin
G e n e r a l l y the oil and
anticlines,
or
on domes.
Important aspects of the major accumulations include early stratigraphicpaleo-structural The
sealing
sequence, rock was
beds
trapping include
Carboniferous the w i d e s p r e a d
and u n c o n f o r m i t y or r e s e r v o i r p i n c h - o u t traps. the T r i a s s i c - L o w e r
and
probably
Jurassic
Devonian
shales.
regional The
organic-rich Silurian shale. Also,
evaporite
major
source
marine shales
479
of
Devonian
and
Carboniferous
age
sourced
hydrocarbon
accumulations
in
the Illizi and Ahnet basins. 8.4.5 G h a d a m e s Basin This basin occupies northwestern Libya and extends into Tunisia and Algeria
(Figs. 8.28A;
Paleozoic the
south
Ghadames
in the
(Table
8.2).
basin
Cambrian
to
extend
part of
covered
and
the M u r z u k
Carboniferous
sequence
fills
is culled
the Ghadames
basin
basin presented from
Bellini
(Fig.8.28 A),
strata
b e l o w applies
and Massa
otherwise
(Fig.8.29).
(1980),
The
Formation) by
the
they
descrip-
Goudarzi
(1970),
and
(1981).
Cambro-Ordovician
8.2).
in
m a i n l y to the Libyan
strata,
known
as
the
Gargaf
Group,
rest
formably upon basement with basal conglomerates and sandstones
lain
basin
caused by C a l e d o n i a n and Hercy-
by M e s o z o i c - C e n o z o i c
tion of the Ghadames Klitzsch
into
Lower Paleozoic strata are well exposed n e a r Jebel Gargaf
southern
are m o s t l y part
Ghadames
b a s i n with major unconformities
nian movements. in the
8.29) where it is also known as the Erg Oriental basin.
strata
containing
Tigillites at the top. This is u n c o n f o r m a b l y over-
Middle
Ordovician
The Late
Ordovician
Melez
siltstones
and
terbedded
with
noncon-
(Hassaouna
sandstones
in a matrix of green shale,
and
Late
Chograne
Ordovician
Shale
fine-grained
is
sandstones
the tillite contains
gneiss and r e - d e p o s i t e d boulders
and pebbles
shales
fossiliferous with
clasts
(Table and
in-
tillite.
Set
such as granite
of shale and sandstone.
The
Silurian Tanezzuft Shale succeeds and passes upward into the Acacus Sandstone.
The upper part of the latter formation,
the Silurian shale below,
sult of C a l e d o n i a n movements, tal
sandstones
(Tadrart
though
fossiliferous
like
shows increasing continental influence as a reand is u n c o n f o r m a b l y
Sandstone)
of
Early
overlain by continen-
Devonian
age
(Fig.8.30).
Devonian facies in the Ghadames basin belongs m o s t l y to the Aouinet Group which consists of ferruginous sandstones,
oolitic ironstones,
iferous
sandstones
and
carbonates
trilobites,
bry-
ozoans,
and corals.
This
sequence grades upward into C a r b o n i f e r o u s
sandy
and
shaly
mains,
feldspars
deltaic tains eous
facies
in which and
environments.
limestones red-brown
with
fine
frequent
with
to very
fine
ferruginous
The Carboniferous fusulinids
dolomitic
brachiopods,
and fossil-
shale
and
sandstones hematitic
in the G h a d a m e s
and brachiopods, of
Late
and
Carboniferous
with
plant
levels basin
age
suggest
also
an u p p e r
recon-
homogen-
with
shaly
dolomites and a n h y d r i t e in the lower part. P e t r o l e u m occurs in the Devonian of the Ghadames basin
(Fig.9.29).
480
A o SW 0m
i ..--~, ~ 100 200[
~" " " ' " " " $ / -.-----:'-'~--:'" ""
o
~
u "~
~
~
Upper Part of
N
O rl
~
~E =~ e® -= c° . ~. t n ~ ~
t--
N
=
NE Alluvia(s(Wadi
I MessakSand- = |
Stone(Continental Mesozoic) ,
Gusso)
Baso Port of Cambr on Hassoouna S~ndstone /
B
! Figure 8.28: Geologic sketch map of Libya (A) and schematic section across the northern edge of Dor el Gussa, eastern Murzuk basin. (Redrawn from Bellini and Massa, 1980; Klitzsch, 1981.)
481
.I0
~o Co.
=EZ T~
0
N
f
0 ,
m
o~ U o
ON
o
z '~"
.~
C,
o
o
z
44 v 0 t~ 0-,4
m
~"
,_
~- :,~ ~-
o
o
~ !
~
o
z
u
~
'
o
~
z
z z
..-.4 N ..el ~-I
482
•~d A'J3 ~4 ¢) v
8 "D <
~.
÷
o,~. m~ 14 ~J ¢) E) !
~J ~Q
o
0 ~4
Q:: 0
O~ t~ (J w
z~
cd ~ •,4 (1)
4J
Z U~
m u}04
I.U
m r~ "r
~4
VI~ 3"J1¥
~
483
8.4.6 M u r z u k The M u r z u k gar
and
Basin
basin
the
is a vast
Tibesti
brian to C a r b o n i f e r o u s ern,
and
southern
stratotypes Wadi
in
the
Tripoli-Tibesti
eastern more
Restricted in wh i c h which
western marine
there
originated
clastic
from
located lower
were
Libyan
at
surrounding
landmasses
Cam-
Dor
and at
sub-basins
el
by
of
Gussa)
an
and
a
Takarkhouri.
eastern
detrital from
east-
Paleozoic
consists
at Wadi
in the
continuous
two
basin
exposed
established and
Lower
(Fig.8.28B),
into
Murzuk
(exposed
the Ahag-
comprises
rim the western,
the
Split
is best
between
fill
sub-basin
the
sub-basin which
oxidation
the
of
sub-basin.
(Fig.8.20),
environments
Lycophytes
bris with
western
heavy
some
in the eastern
sub-basin
was
Its
A system of cuestas
exposing
uplift
predominantly
marine
basin
(Fig.8.20).
strata.
margins
at Dor el Gussa
Takarkhouri
the
intracratonic
Mountains
sub-basin
sedimentation
where
vegetal
de-
were derived.
RELATIONSHIP OF THE DEVONIAN CYCLES OVERLONG SILURIAN CYCLES AFFECTED BY THE CALEDONIAN OROGENY
NNW
SSE NORTH SOUTH TRIPOLITANIA TRIPOLITANIA AWAYNAT WANIN CARBONIFEROUS M'RAR FM. SECTION
-$.
(z:
- ~ - ~TAHARA FMA O v, FM. A0 Ill FM'I'
"
>~ r,
OUANK --YAO
'.
;
~
~
~
.
~
i
I. ; i
.
.
~
f / j/
/
,,'~'"
....
. ~j "/ ' -
SHALES FM-
(TANEZ~T~ID~-FFARA FM.
6
.
/
~
"J
.
-
"
O
Figure 8.30: Schematic section d e p i c t i n g the S i l u r i a n - D e v o n i a n of n o r t h e r n Libya. (Redrawn from Bellini and Massa, 1980.)
As a f o r e m e n t i o n e d is e s s e n t i a l l y 8.2).
The
marine
Devonian
units
with
tains
extensive
Early
Carboniferous
ies,
while
lated.
in
the Lower
Paleozoic
succession
the same as that of the G h a d a m e s is an
also Upper
oolitic
the
continental Devonian
ironstones
the
regressive (Van
is t r a n s g r e s s i v e Late
at
Carboniferous
with red
Houten mixed
basin base
in the M u r z u k
basin
to the north
(Table
passing
paralic and
cycle
Karasek,
marine
continental
and
upward
into
Which
con-
1981).
lagoonal
sediments
The fac-
accumu-
484
8.4.7 Kufra Basin This
is
into
Chad
a
large
Paleozoic
where
it
southwestern
is
basin
known
which
as
the
Egypt and northwestern
extends Erdis
Sudan
from
basin,
southeastern and
(Fig.8o28A).
its
eastern
(Fig.8.31), cian
margin
the Kufra
strata
where
the
Silurian
oversteps
onto
basin has a thick sequence of C a m b r i a n
(Table 8.2).
The
Silurian
marine
into
It is bordered to
the n o r t h e a s t by the Dakhla basin of Egypt and n o r t h e a s t e r n Libya. along
Libya
marginally
Except
basement
to Ordovi-
transgression
which
was
w i d e s p r e a d in this basin, extended as far as eastern Egypt and northwestern
Sudan
(Fig.8.32)
Sandstone Devonian
under
and
shoal
terminated
water
sedimentation
with
conditions.
began
under
the
deposition
of
the Acacus
Caledonian
movements,
continental
conditions
Following
predominantly
and passed upward into open but shallow marine environments with locally developed
lagoonal
the C a r b o n i f e r o u s bly
Visean
conditions. (Schrank,
age.
Mostly
nonmarine
beds
were
deposited
in
1987) with a brief marine incursion of possi-
Hercynian
uplift
altered
the
basin
configuration
and
created a new d e p o c e n t r e transverse to the earlier basin. Table 8.3 shows the subdivisions of the Paleozoic
sequence along the
eastern m a r g i n of the Kufra basin. P r e v i o u s l y termed the Nubian Sandstone and
believed
strata
to
which
Klitzsch
(Fig.8.33). the
Cretaceous
along
and
Lower There
Early
Ordovician
of
outcrop
(1986),
progressive
from
be
the
a
and to
to
Vail,
basin
Lower
and shallow marine
sandstones
shown
by
represent
a
onlap
encroachment
Middle
Paleozoic
been
to
Carboniferous
and
the
have
(1990)
southward
Early
1988A),
frame
Squyres
progressive
Ordovician
fluvial
(eg.
eastern
Klitzsch
0rdovician was
age
Silurian
of
sequence the
times.
sea
Thus,
(Karkur Talh Formation)
which are p r e s e r v e d at Jebel Uweinat are overlain by the Late OrdovicianEarly and
Silurian
Um
intensely
deposits,
Formation
which
pass
into
upward
Devonian
subtidal
clastics. which
(Tadrat
basal and
braided
Formation)
conglomerates, shallow
stream are
one
glacial
of
marine
sandstones.
succeeded
also
fluvial to coastal plain and tidal-
These are overlain by Late C a r b o n i f e r o u s
represent
Permo-Carboniferous
of
coastal
sheet-like
sandstones
u n c o n f o r m a b l y by Lower Carboniferous deposits,
consists
cross-bedded
which
Unconformable
Ras
bioturbated
the
deposits
rare which
northernmost are
more
glacial
lake
occurrences
widespread
in
of the
Karoo S u p e r g r o u p of southern Africa. Post-Hercynian parts
of
Egypt
Uweinat-Bir conditions.
and
sedimentation Libya
Safsaf-Aswan This
took uplift
sequence
in
northwestern
place
in
a
(Fig.8.34)
begins
with
new
Sudan
depocentre
under Late
and
Karoo-type
neighbouring south
of
the
continental
Carboniferous
tillites
485
++ ~ _ 0
-~
• ..:,~ : ' : ~ . ~ + + •..~ ,? .?.:,,. + ,,. :..., "4;;.!.:.,
+ O~
O) .,,-t
tO
"*
'7
T
G)
'3
"" .÷ ++
-,-.I 4..i
u_ o ÷
~
0 4.-I
M
u_
Q)
t~ ...
G)
~
{fl 0 0
0 -:.:. U (1)
t.a
t~ I
io
¢ I-I .,..t
486
(Northern
Wadi
Malik
Formation)
and
passes
upward
into
the
Permian
to
Early Jurassic Lakia Formation which comprises sandstones with paleosols, and m u d s t e n e intercalations with silicified trees and a rich Lower Jurassic flora
(Klitzsch and Lejal-Nichol,
1984).
MEDITERRANEAN
SEA
I 200Km ,
EL Kharga e
-z- £------z-R ; , ; g
...........
.'['l"
Setim{*
Port Sudal Dongola -.. ~,-~---@~
.-....-,..
.'..~;:-.~ )
""
N"..,'.'.'.'.'..':
Khartoum
.'.."2:.":
~Open B
marine
Recentty discovered strata
~
I
Tronsiffonal coastal to fluvlal environment /
Aprox. Limit of Earty Cambrian transgression
/"
Figure 8.32: Ordovician-Silurian paleogeographic NE Africa. (Redrawn from Klitzsch et al., 1990.)
sketch map of
487
UPPER
z
CYCLE
j
z'.~
IMIDDLECYCL~
>
'"
-~
_ ~ > o -
[ LOWER
~
CYCLES
~.~
,-
Z LU LU Z 0 LL
~AN
TYPEI It
STRATA
STRATA
o
PALEOZOIC
Z
STRATA
~c ~o c
c
b. ~ ""~
aC
~_
~o._
~ O -,-I I-I
--
Z
o~
--~k
v,.4v
(I)
u:.J
°
r'r 0
!~1~1 ~~b
0 O~ .,4
~
0
<(
O 0 O
O -H
I..LI - - Z Z ~.-~-0
U
1-..- ~
LLI
Z ~:E I:1:: ~ •~ : o e O
rr"
O~ 0 "~ 14
~
•
N
I--zf . -, -_ e~..
:~: t/3
n
" ~ _~ _- n " ~ _D_
--~'--
I'-
z o-~ 0 0 I1~ Ul
N
if)
:,,~o 1.-zLU i.U ¢.~ (..)
zO >
o-
Q
,,,V
.
'il E
I
C
488
8.4.8 C o r r e l a t i o n s Figure
8.32
Saudi
w i t h the P a l e o z o i c
shows
Arabia
the
through
paleogeographical northwest
of
Klitzs c h
et
comprising sandstone, Siluri a n to
embayment
within
Cambrian.
which
along
the
to
view
Red
Sea
1,500-2,000 of
fluvial
environment
placed (Klitzsch
represents
that centred
in
the
also
1990), Early
et al.,
the w e s t e r n
in Saudi Arabia
and the
outlier
(Fig.8.32).
Early to
Egypt offers
Paleozoic
Paleozoic
coastal
North A f r i c a n
(Seilacher,
were
between It
Coast
m of
of c h a r a c t e r i s t i c
assemblages
therefore
connection
Early
sandstones
Harlania,
to fluvial
Sudan outlier marine
fossil
and
the
reported
conglomeratic
trace
in
Sudan,
(1990)
on the evidence
Cruziana
coastal
paleogeographical
Sinai
framework
Port
al.
of Saudi A r a b i a
with
origin.
Ordovician
The and
types belonging
Paleozoic 1990).
Here strata
The
transitional northeastern
edge of an Early Paleozoic to the east.
NNW
"~ S S E N W Gift Kebir
Gebel Uweinat
S E of G e b e l Kissu
LQkia A r b i a n
Om
SO0
K• or c
~
tillite
~
Latest Carboniferous fluvio-glacial fgss, glacial worves ss, slltst., shale, marine Early Carboniferous becomes fiuvia| southward ss. f|uviat, northward transport Devonian Silurian ~oca|l¥ inch ~s, shallow marine Ordovlclan f|uviQ~, influence from south ss,
o el l s i ~ t s . ,
PC
[~ ~]
i e ~
Figure 8.33: from Klitzsch,
Vaslet more
in
(1989) common
stratigraphic lowing central
Schematic 1986.)
section
across
showed that northeast during
sequence
similarities Saudi
o
Paleozoic for central
with
Arabia
Egypt
were
and
southern
Africa
times.
the
deposited
on
Sudan. a
(Redrawn
and Saudi A r a b i a
Vaslet's
Saudi Arabia
Egypt.
composite
(Fig.8.35) Early
major
shows
Paleozoic
unconformity
had a lot Paleozoic the folstrata over
in the
489
H ED1TERRANEAN
SEA
___~f~ L:-:_:~-
"
, 200Kmj sea in
--.~'~_-'[rlassi c time :'.. "5 ? . : . : " ; ~ C ~ i F 6
\
(i~ " i ::4!:::< in ly contlnenta!
\
..)~ shallow marine transgresslon In k'~ rmlanin Nwtime
I. \\ /~Lo;e Jurassic
iftlll
ELKhar! ~
•
:'::
::.: .A
Permotriassic to Lower :'-. ".~ :.', • conhnental. •".-'.:. -" . ' : . . . , I . . . . . . .
"::-'-: : ; '":" :':: i:": :::" ~:_.: ...:. :.!. :;:
\ Selima
\ \
urassic ) ": ':!
\Port Sudan
.~-/" \
x\j
\\:artoum/\
\
¢
Faults and Intrusions of Permian to Triassic age ]. I1.
Area of erosion and removal of sedimentary cover, exept of some Cambrian remains Area where most Paleozoic sediments were preserved
III. iV.
Area lacking Pre-Hesozoic sediments Recently discovered Early Paleozoic strata
Figure 8.34: Post-Hercynian structural re-organization Africa. (Redrawn from Schandelmeier et al., 1987.)
of
NE
490
peneplained evolved
surface
from
an
environment
to
took place and
shallow
Middle
periglacial
Early Paleozoic
erosional
to
ice caps
of the ice caps There The
in
the
experienced was e f f e c t e d major
in
as
continental
Middle
a precise
several
glacial
link with
by C a l e d o n i a n
with to
Also,
crustal rocks
in Saudi Arabia during
like
upheavals to
Permian)
the
glaciation
was
Africa
the A r a b i a n
under
which Shield
sometimes
basement.
the Late Permian
con-
Like
North
which
the
move-
paleoenvironmental
continental.
movements,
down
of
the melting
in the M i d d l e Llandover-
the P e r m o - C a r b o n i f e r o u s
older
ice caps
Pronounced
and retreats
Following
caused
again
and
in Arabian
Gondwana.
(Devonian-Early
Hercynian
Permian
the
and
the Llandeilian
advances
Africa
peninsular.
Carboniferous
all
Arabia
northeast
marine
Arabian
Silurian
incursions
widespread
in northern
transgression
Saudi
in
to
by p r e - L a t e of
are
setting
to deltaic
marine
Ordovician
deposits
mark
Late
of the Kufra basin,
southern
erosions
was restored
(Fig.8.35)
Paleozoic
megacycle
eastern m a r g i n
Late
establishing
in the
alluvial
brief
in central and western Arabia.
a hiatus
Late
one
changes from
felt
thus,
depositional
the Late Llanvirnian,
the Late 0rdovician
there was a marine
was
Three
Caradocian.
to subaqueous
outcrops,
during
The
continental
conditions.
during
Late
belt.
Paleozoic
marine
unconformities
contin e n t a l
stitutes
Pan-African
Early
continental
which existed
ments.
the
in the O r d i v i c i a n
the
ien.
of
initial
caused
Deposition
epicontinental
conditions.
8.5 West African Intracratonic Basins
8.5.1 T a o u d e n i Centred (Figs.
on
to
east
The
geologic
strata
underlies
most
into
basin
the
two
simple
shallow
interior
shallow
the
Late
mobile
cratonic Thus,
seas
most
(Deynoux,
and extends and
basin
Deynoux
the
Structurally with
very
belts
low
this
of
et al.,
Lower and
and con-
where
one
the
1985)
Faso,
the T a o u d e n i of
Late
had ended.
outcrops
southwest,
of
extremely
flooded
events
Burkina
dips
basin
to the west
sequences
during
extensive
1983;
in
Taoudeni
Proterozoic-Early
periodically
into Algeria,
Senegal
(Fig.8.25C). sag
vast
the
long after the P a n - A f r i c a n
contains
Guineas
known as the Bove Basin
tabular
(Fig.8.37).
time
in West Africa
of Mali,
the
as
Pan-African
contains age
is
served
to the encircling
and p e r s i s t e d
Taoudeni
craton
having
(Fig.6.3),
of
basin
Paleozoic
tinues
African
which
to C a r b o n i f e r o u s
period
Taoudeni
West
8.25B),
foreland
the
Proterozoic long
the
8.36;
Paleozoic and
Basin
basin
it
is
is a
degree
or
491
less
(Fig.8.37).
is exposed
It lies
unconformably
to the n o r t h w e s t
and
to the west by the Mauritanides gin w i t h
folds
and
fractures
southeast.
which
to the northeast
Shield.
Cyclical
1979)
resulted
(Fig.8.37).
epeirogenic in
There
are
d o l e r i t e injections
Eburnean
b a s e m e n t which
flanked
and overthrust
It is
resulted
upon the Rokelides and the Bassarides Iforas
the
along a narrow
whereas to the southwest its extension, des
upon
(10-50 km) t e c t o n i z e d mar-
from H e r c y n i a n
the B o v ~ basin,
(Fig.8.25C).
the Taoudeni movements
basin
in
the
unconformity-bounded synsedimentary
faults
deformation,
sits d i s c o r d a n t l y
In the region of Adrar
is b o u n d e d Taoudeni
by the Tuareg
basin
stratigraphic with
low
throws
(Petters, sequences as
well
as
(Fig.8.37).
" KHUFF FORMATION
LATE
PERMIAN
c ~ ~ '~-~-m,-~ Early P e r m i a n 9ieldF . . . . . -m-idJ :fat I Emsian---letl
I ]~DIBer w a l h Fro. 163 m Taw i l Fro.
]
~
| ] |
SIEGENIAN
B6m GEDINNIAN D i s c o n f o r m i t y .........
-
-
187m >'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~/ ~ _: : ~
,~<~
// I~/
'~
+
+
+
,K ~ ,.~\~, D" ~- u n e
.
,+ ~ J ' / ~ C h i n g u e t t
.:..
i
k
,c,,
•~.
I~ N o u o k c h o t t
~Arotone
Moudjerio~~
TJmbouctou
'Ouolota Nemo
..... ~/
~oo~
1
2" [~ okor
k .,4
Doleritel Hercyniennes
" Unite de Fale'me'
Mouritontdes {the{fie caledono-hercynienne) ~
de -
eov~ 8 ~ s , N
TerrainsPost-Poleozoiques
co
L,"J~IJ~iI~?:LE:~
-
+
÷
Figure 8.36: Sketch (Redrawn from Deynoux,
÷ + 1 +/~o4~ +
map of 1983.)
the
-b
+
4
-p
+ Conokr
Rakel~d¢l (cholne pan-ofglcalne)
8amako
+
+
~
Ordovlcien superieuT proboble
1 ~Jebutant por la fillite.dite superieure
|
Combrlon Drobob~e aveo peut-efret
~"~
un pe d>ordov~e~e~ | Somme! probable@u Precambfi~tn | ......... 0.b . . . . . . . . . . . . . . . f . . % . . . . . . .
I
Fcc=~ Precambrlen super[eur sedJmenfaire debufonf OUX environs de I000 MA J | Socie precombrien moyen au/e~ inferieur et intrusions VOt~onlques dioqe vorJe
Taoudeni
I
and
Bov6
basins.
493
5 S e n e g a l Oriental Guinee
2 Adrar
1 Toodeni
Nonk
-
O u e d Chig
m"
-
1
..
/Grou;
~
/
uma
NNjj,o k c l n e .
~
Abteilli
Group
MALl
,-:. 7i
"~ "_--_
Group Ator
-_=~
Cliff
-£Z
Group
R Richo1~~ ,
~
End I ~ Glaciation
::"2;
't:: Assobet ".':'? a n d
~
!::'2' i:-2:
i'
7
~E
Hombori Goundem
8
MALl
Gr, ~
Ator-
gl
Silurian Shale 5 i | e x i t e ¢{~d J a s p e r
m
Sandy
m
and Shal e ~mSondslone. SkolilhasoCongo l me~oe l Carbonate
DOlomite
~Ioun
N
=,=!
~
~
5hole
~-
Bobo Diou(Qsso
•
.
(X~ 8 o s e m e n t
W
25 Km
.
" "
~
D
o
|
e
r
l
,
e
-
. "
•
. "
•
F~. %~;
E
•
• C~rbonJf
.......
d
. •
S.G.3 so o
1
0
~ °
~"
•
" ' . " . - ' '
*
"1
..
°
"'!
.
"
°
....
.
--.
-.I."
o~d . . . . .
•
.
"..
15
20
S-G.2
i :
~, L
, '
~
, /!
',
I
I
,
',
,
~
I
,
Figure 8.37: Geologic sections across the (Redrawn from Deynoux, 1983; Clifford, 1986.)
, ,
'
, u
'
!
~
S.G.~
r"
Taoudeni
basin.
494
in the A d r a r sion
in
of Mauritania
the
(Mid-Late
Taoudeni
(Fig.7.4A).
basin,
Proterozoic
termed
sandstones
As
seen
in Ch.
supergroups,
and
6.2.2 the
comprises
stromatolitic
succes-
Supergroup
carbonates);
group 2 (Late Proterozoic basal tillites, b a r y t e - b e a r i n g dolomite, cherts and shaly siltsones, stones with tillites,
shales with
reefal
Silurian
and Supergroup
shales
limestones).
and
In Mall
fine
3 (Late 0rdovician
sandstones,
Carboniferous
and Devonian
clastics
and
bonates are exposed which sit u n c o n f o r m a b l y on Devonian shales. siliferous
marine
Lower
Carboniferous
clastics
chiopods are overlain by evaporitic carbonates
marine
Skolithus-bearing sand-
and C a m b r o - O r d o v i c i a n
inarticulate brachiopods);
graptolitic
1
Super-
with
conodonts
(Legrand-Blain,
car-
Here fosand
bra-
1985).
8.5.2 Boy4 Basin This
is
a
gentle
synclinal
feature
strata. As depicted in (Figs. 8.36;
filled
with
Ordovician
to
Devonian
8.25C) the sequence in the Bove basin
rests u n c o n f o r m a b l y on the Archean and on the Rokelides along the GuineaSierra Leone border; K~dougou; Groups
and
which
equivalents (tillite,
is are
of
it overlies Birimian basement in the northeast near
also
unconformable
equivalent
Supergroup
baryte-bearing
to
upon
Supergroup
2 are,
from
calcareous
groups
which
correlate
Saionia Scarp Group (Villeneuve,
sandstones
dolomite,
Taoudeni the
chert);
and
S~gou
basin.
The
Walidiata
Group
the
Group
Mali
and the Sala and L~louma glacio-
other glaciogenic
strata
such as the in S e n e g a l
1989). of Supergroup
(Fig.8.37).
consists
graptolites, Devonian
oldest,
in Sierra Leone and the Youkounkoun Group
The equivalents Group
with
Madina-Kouta
I in the
the
(monotonous siltstones with radiolaria); marine
the
of
Starting
black
followed
sandstones,
paper
by
3 are the T~limele
with
a
shales
shales
basal with
with
conglomerate
abundant
fine
Group
and Devonian the
T~limele
pyritized
Silurian
sandstone
intercalations.
the top of which contain probable L o w e r Carbonifer-
ous b r a c h i o p o d faunas, overlie the T~limele Group gradationally. 8.5.3 N o r t h e r n Iullemmeden Basin Although
largely
a
Mesozoic-Cenozoic
Iullemmeden basin at Tamesna zoic
basin
that
post-Hercynian Tamesna wards
centred
uplift
(Figs.
in the
present
of the Ahaggar.
sub-basin thicken towards
where
they disappear.
Early Paleozoic.
basin,
8.2,
the
8.38) Ahaggar
Generally
the A h a g g a r
The Ahaggar
northern
part
of
the
belongs to a Lower PaleoMountains,
prior
to the
Paleozoic
strata
in the
(Fig.8.38)
Mountains
did
and thin south-
not
exist
in the
C a m b r o - O r d o v i c i a n strata are up to 500 m thick in east-
495
ern
Tamesna
Cruiziana posits.
where
These
100 m thick,
they
are
followed
Late Tournasian Visean
are by
basal
conglomerates,
unconformably
Early
Silurian
Carboniferous
coal-bearing
shales
and
paralic
limestones
deltaic
by
graptolitic
corals
overlain and
2
~
50 Km
3
z~
,,,< n
Z
D
~uezouman
I
y'.:~ ...~h ~g
~|
Terngh
to
and shales,
odonts.
I
de-
up
A transgressive
is d i s c o n f o r m a b l y containing
with
glacial
shales,
sandstones
sandstones.
sequence
(Fig.8.38)
sandstones
overlain
and discordantly overlain by Devonian
and Late D e v o n i a n - E a r l y by
comprise
Skolithu~ and
and
100m
B A Figure 8.38: The Paleozoic of northern Iullemmeden basin, i, Precambrian; 2, Lower Paleozoic-Middle Devonian; 3, Upper Devonian-Lower Carboniferous; 4, Carboniferous. a-k are fossiliferous horizons. (Redrawn from Legrand-Blain, 1985.)
con-
496
8.5.4 P a l e o z o i c Few exposures Takoradi more
A l o n g the West A f r i c a n Coast
of Paleozoic Accra
extensive
Monrovia able
and
Exposures
strata
in Ghana
Paleozoic
occur
sequences
the P a y n e s v i l l e
lie
are
Liberia,
believed
.
to
in the coastal
up to 1,000 m thick,
continental
TAKORAD¢ SANDSTONE cASO m
These
which
Sandstone,
Devonian-Carboniferous
near Monrovia,
(Fig.8.39).
and near represent
basins.
represents
Near prob-
deposits.
..
JUA
FORMATION
'''~
ELMINA SANDSTONE
A.OA o.oo
c
i:l
BIRR|MIAN
ASE MKAW
FORMATION
:,',: .':"
5 Km
.',
9 , , . , ~ ,:
:
" :::":"';
:'"
,
"5"t :-: ~
~,..
7-'-
.:'..'¢
:
•
>
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
,'. ','.','."
<
-
-:z
Tro-Tro
4
¢n V o o r s f e h o e k
Tra-Tra
4
Voorstehoek •
I(
mouth
bar
Eifelion
-
• Delta
.
s l o p e
';:'G~Ak~"S:::'G'~;~kO :: ::: :~' i . . . . - , . . . . . . . Gyoo
Table
bay
Distributary
." C : " - , "-'-'-;-1:'-'-'-'-", "-'-'-'. HexRiver.4 " Hex R i v e r . ' . l
Mountnln
-
Karies
::.t', "'"'.".".1";'>;"" ".'-".";'L 8 o p l o a s - ; . ~ -~ B o p l a o s [-[- .~
Oi
-
W u p p e r t a l "[ ",'.'.'
Waboomberq
8okkeveid
flat
interdistributary
~
Gyoo
•
Erosion
I : ": "['R iet vl ei'":: ': B o v i a o n s k l o o i . " l " "-' . . . . .......l.. ...... . ..
Shelf
Arenaceous ~--~ ArgJllace0us (A)
(B)
Figure 8.44: A, stratigraphic table for the D e v o n i a n of South Africa with fossiliferous formations shown with black triangles; B, schematic celtaic genetic sequence in the Bokkeveld. (Redrawn from Hiller and Theron, 1988.) Analysis
of
Bokkeveld Group on
the
rich
Lower
Devonian
benthic
communities
c o n s i d e r a b l y refined p a l e o e n v i r o n m e n t a l
sedimentological
criteria
(Hiller
and
Theron,
in
deductions
1988).
The
the
based
Bokkeveld
505
benthic like
communities
coeval
communities
p a l a c h i a n fauna nity
by
gastropods, graphic
sequence,
brachiopods,
the
southern
delta
faunal
which
were
and
brachiopods,
and
slope
hyoliths.
province,
part
of
the
unAp-
fewer trilobites
fossil assemblage
infaunal
Higher
paleoenvironment
siltstones
contain
in
with
mostly
(Fig.8.46B), whereas
pelecypods, the
communities
(Fig.8.46C).
the d i s t r i b u t a r y mouth
strate
functional
pedicles;
Large
bars where
while
thicker-shelled
the upper
thick-shelled
they were
infaunal
strati-
coarse-grained
such as d i s t r i b u t a r y mouth bars and tidal
dominated by
Ghana
free-lying
crinoids,
interbedded
deltaic e n v i r o n m e n t s lower d i v e r s i t y
the M a l v i n o k a f f r i c
In the Bokkeveld the pro-delta benthic commu-
thin-shelled,
and
to
was c h a r a c t e r i z e d by the most diverse
trilobites,
sandstones
in
(Fig.8.40).
(Fig.8.46A)
dominated
belonged
shallow
flats had
brachiopods
fixed
to the sub-
and
inarticulate
bivalves
brachiopods d o m i n a t e d the tidal flats.
'.":;".'.:.'. "i.'.:.'..:': , ~'~'.'":=' .":.>~,--, :.'~.
.... .:.. •...
.....-...~_, .,~ , _ . ~ < ~......'.":~-. :.~ , ~ ,:..... ',,"".'......'~.~'........".".... -" :. : ."."...:.'.....'-..~,,,.......~., ,'c ,~.. .. '
.' .:..
•
,.~.
,...
, ,
."
..
.:
.,,
.
.':.'....~;,...
...........;E .......... ,, .,..."-. -......... .
$ ~e~
":i~
~
,
-...... ;
.....;." .... "
Deep
:...,.
,,."
Figure 8.45: Paleogeography from Hiller and Theron, 1988.)
of
the
CT
-
PA
- Port Alfred
Cape
P£
-
Port
Town
£1izabeth
Bokkeveld
Group.
(Redrawn
Wi tteberg Group Named
after
arenites
are
prominent well
over 2,000 m thick.
mountain
exposed,
the
ranges
in
Witteberg
the
Cape
Group
is
region a
where
clastic
quartz
sequence,
It occupies a transitional s t r a t i g r a p h i c p o s i t i o n be-
tween the Bokkeveld Group below, and the basal Dwyka F o r m a t i o n of the Karoe S u p e r g r o u p
above.
Alternating
lished since Bokkeveld tion.
Thick
shelf,
transgressions
and regressions,
estab-
times, also controlled W i t t e b e r g deltaic prograda-
delta
slope,
delta
platform
with
barrier
beach,
506
g,
z2,N:;;C...~~o
Figure 8.46: Devonian faunal (Redrawn from Hiller and Theron,
_
communities 1988.)
of
South
Africa.
507
lagoonal,
and tidal flat deposits,
are found in the W i t t e b e r g
succession
(Fig.8.47) w h i c h represents the vertical stacking of these facies.
STRATIGRAPHY
LITHOLOGY
FOSS ILS
DEPOS ITI O NAL
,_ Formation
Member
== O>
m
Dirkskroal
e
oE
~>
,>... ,,,,,; Pro-delta Shelf
Miller Diomictite
I
nr IJJ LL_
11
=
i
Wooipoort Shale
Z o m n-
Floriskraal Sdst.
Kweekvlei Shote
_ _
Offshore shelf
L I
i i
~ - z
Lagoon
i,
I Skitterykloof Perdepoort
Witpoort Sdst.
ENVIRONMENT
2200"t
Sdst. Southloof Sdst ¢n
~
i
Rooirand
Barrier beech Offshore shelf Open beach
Shoreface
|
Offshore shelf
Z O
Weltevred Shale
Barrier beech
> ~J a Offshore shelf
O_
.
|
B a r r i e r beech
Dark-grey to block mudstone/shole Fine to medium-groined sand stone I//i-
Clost-rich messive diomictite
Figure 8.47: S t r a t i g r a p h y of from Loock and Visser, 1985.)
the
Witteberg
Group.
(Redrawn
Because of the p a u c i t y of fossils in the W i t t e b e r g Group, been somewhat uncertain.
But Loock and V i s s e r
(1985) a s s i g n e d
its age has the Witte-
508
berg to the Late D e v o n i a n - L a t e Carboniferous, ate
terminal
Malvinokaffric
The upper W i t t e b e r g dant
flora
Primitive berg.
which
are
more
acanthodian
advanced
and
the
onset
of
which
and
occur
in
freshwater
than
paleoniscid
The M i l l e r Diamictite
nounces
assemblages
is of terrestrial
on account of its depauper-
those
fishes
in
the
the
occur
basal
origin,
in
part.
with abun-
Bokkeveld
below.
the
Witte-
upper
in the upper part of the W i t t e b e r g Group an-
the
Permo-Carboniferous
glaciation
of
the
Karoo
Supergroup.
8.7 Karoo Basins
8.7.1 G o n d w a n a F o r m a t i o n s The Late
Carboniferous
to
Early Jurassic
is r e p r e s e n t e d by w i d e s p r e a d nonmarine continental
nature
Karoo
the
with
scale.
has
rendered
standard
This is because
it difficult
European
Late
the European
marine
faunas
of the marine
became
rather
more
interval
strata,
Its
to p r e c i s e l y correlate
the
stratigraphic
to use
sub-Saharan Africa
Paleozoic-Early
transgressions
practicable
in
the Karoe Supergroup.
shown
in Fig.8.4. term
strata,
the Karoo
used for the Late C a r b o n i f e r o u s - E a r l y Jurassic sequence
depositional
continental
phase
(Karoo)
(Cape
phase,
Supergroup)
with
greater
and
a
later
climatic
Just
the Cape
as it
"System"
"System" was
(Haughton,
This allowed a d i s t i n c t i o n between an earlier rift-related tic
time-
scale was based on the
the broad
when referring to O r d o v i c i a n - C a r b o n i f e r o u s
Mesozoic
1969).
coastal
similar
extremes.
clas-
but
more
Karoo-type
s e d i m e n t a t i o n was not limited to sub-Saharan Africa, n o r t h e a s t Africa, and Saudi
Arabia,
as
we
southern Gondwana, and the
Santha
distinctive
saw
previously.
yielding,
Catharina
faunas
and
But
for example,
"System" flora
rather,
it
the Gondwana
in South America
(Fig.8.6)
that
was
widespread
"System"
(Kummel,
constituted
in
in India,
1970), the
with
Gondwana
p a l e o b i o g e o g r a p h i c realm. Thus,
K a r o o - t y p e deposits,
mations,
are
sopteris
flora
terrestrial had
the
(Fig.8.6)
vertebrates.
greatest
established habitats tional
world-famous
for
paleontologically. and
rich
reptilian
South Africa,
vertebrate the
being part of the so-called Gondwana
diversity
correlation
of
lying with
Karoo
They faunas in
settings w h i c h
regional
favoured coal formation,
tectonic framework of these basins.
with
the
heartland,
reptilian
Before
Glos-
pre-mammalian
the Gondwana
strata.
of these unique vertebrate assemblages,
contain
for-
biozones
examining
the
and other Karoo deposi-
let us
first consider the
509
8.7.2 Regional T e c t o n i c Settings The term "Karoo" is entrenched in African geologic literature, in a generic sense and
climatically
cession clastic
wedges beds;
Markwort,
from
which and
1989;
Karoo
tends
controlled
comprises,
eolian 1988).
for Late Carboniferous continental
the
base:
interfinger
extensive
Nichols
basins
and
in an east-west
with
Daly,
Tankard
types. across
the
Cape
basin
are
1975).
fold
shallow,
These
Angola,
sag
(Daly
broad
occur
et
et
are
known
1969; al.,
Karoo
South Africa 1989).
in
and and
1982; basin
Wescott, which
ex-
subsided
c o m p r e s s i o n and uplift the
basins
South
fluvial Kreuser
(Fig.8.48)
Outside
sag
suc-
to
Africa,
Karoo
the
foreland
west
Botswana,
(Rust,
Namibia,
The third and eastern group of Karoo basins ac-
(1975) are the narrow grabens and h a l f - g r a b e n s or troughs
in eastern
Africa,
for example
Zambia,
Zimbabwe and M a d a g a s c a r
from
long
a
al.,
Karoo
fan-deltaic
deposits;
The main
intracratonic
basins
Zaire and Gabon.
cording to Rust which
belt
typical
(Haughton,
as a foreland basin because of prolonged regional in
A
being used
tectonically
coal-measures;
lacustrine
flows 1989;
three
direction
sequences.
tillites;
basalt
are of
to Early Jurassic
period
of
in
(Fig.8.48).
regional
crustal
Tanzania,
Kenya,
Uganda,
These Karoo troughs
extension
which
resulted
preceeded
the
f r a g m e n t a t i o n of G o n d w a n a in the Late J u r a s s i c - E a r l y Cretaceous. The most e x t e n s i v e Karoo rifts and the Lower trending the
Ruhuhu
Daly beshi
rifts which
Lukusashi
trough
Karoo basins
crustal
Zambezi
Luano,
of
and
et
al.
(1989)
extension
shear
zone
trend
Luangwa
Tanzania,
(Kent et al.,
in eastern Africa are the M i d - Z a m b e z i
with
roughly
rifts; the
east-west;
and
latter
the
the NNE-SSW-
Maniamba
connecting
rift
with
and
coastal
1971).
attributed
the Karoo
caused by sinistral (Fig.8.41)
during
rifts
in eastern
strike-slip m o t i o n
the P e r m o - T r i a s s i c
Africa
to
along the Mwem-
Gondwana
orogeny.
S t r i k e - s l i p motion caused the complex fault pattern found in Karoo rifts. For example, the ENE-trending fault-bounded Kafue, Luano, Ruhuhu and Maniamba
rift
They
also
basins
pull-apart along
the
movement angwa
are
contain
asymmetric
smaller
structures Nwembeshi
caused
shear
half-grabens
internal by
zone.
faults
ENE-WSW Also
Geophysical
estimates
of
sinistral found
1989).
and
Lower
Zambezi
basins
(Daly
et
border
this
thickness
Luano,
Ruhuhu,
1989;
are
motion
transcurrent
in the L u k u s a s h i
al.,
faults.
which
strike-slip
with
stratigraphic
rifts suggest a m i n i m u m of 5 km for the Kafue, Luangwa,
major
half-grabens
compatible
are the en ~chelon half-grabens
rifts.
with and
and Luin
Karoo
Lukusashi,
Orpen
et
al.,
510
KAR00 BASIN 0-1000 M
N
t
o
1000M- &000M ~
RUFUI
>&.000M LUANBWA
BAROTSE t
Y~ %
NDAVA~
I
I.
500Km
I
7. RUHUHU
KAFUE
2. LUANO
8. HANABA
3. LUKUSASHI
9. LOWER ZAMBEZ
4. SOUTH LUANGWA
10. M]D-ZAMBEZI
5. MID- LUANGWA
11. RUFIJ! 12. KAR00 FORELAND
6, NORTH LUANGWA
Figure 8.48: Distribution of major Karoo Africa. (Redrawn from Daly et al., 1989.)
basins
of
southern
8.7.3 The Karoo F o r e l a n d Basin of South Africa The type area
for the Karoo Supergroup
Africa
(Fig.8.49A)
shales
are exposed
referred
to as the
region which
lies
where
nearly
intersected
is in the Cape Province
horizontal
by dolerite
continental sheets
"Karoo" by early explorers. in the Karoo
foreland basin,
of South
sandstones
and dykes
Being the type area, will
and
in a region
be described
this
first.
511
ll.r "~'-'~
I
........................
~ .....
~% m5esf°nteln
V-
~"
:
..
.
:[,~':'::"
I i
. ~:;:"
N A M IB IA
px_..':<~:".':'::'] ""
.
. .': , : ~',;".',-.
Possarge
!
~,:--.
"x It,~o~1 .... .ooo
x.
B OTSWAN
~:'{V~.,', ".l V : " . ' : ","
Motientha vv,--
;
.;/
Wonkie
A
V :-2,[ ~ v v ' ~
J~"::"~/~-"~!y'.'AZ M B A B W E
T : '.":)
k.
Nuclnetsi V 1 Tu~i V '
. - / ~ °---
V V ~ a t e r b e r g ". " " , ' . : " "" bGs n .~
/
rim
Flats
Job. . . . b ~ , 9 ~
.-_
v
'~v~ ~i ~, %
..... ,o~,o~_ - ,_,:-~
.v
,~v
KARO0 BASIN
~-~ Karoo lavas ~ P o s t BeoufortSec ~ eauf0r~ £cco "
300Kin
Cape
G~afRein.et
~ Dwyka ELSEWHERE
~
Karoo l o v e s Koroosed,undiH-
entiate~ x - - x ' L i n e of Section
A
CAPE FOLDBELT NE
m 2000
1000
~o
B
,00%
1000
Figure 8.49: tions showing al., 1982.)
Karoo basins of South Africa (A) and cross-secmajor geologic units (B). (Redrawn from Tankard et
512
Tankard et al. Karoo
basin.
(1982) presented a regional stratigraphic synthesis on the Here,
the
Karoo
Supergroup
coarse bedload and braided streams,
ranges
deltaic,
from
glacial
through
distal flysch to eolian de-
posits. Four major lithostratigraphic units record this broad spectrum of depositional by
the
environments.
coal-bearing
which
is
that
pass
Clarens
in turn upward
The glaciogenic
alluvial
overlain into
Formation).
by
to
the deltaic
fluviatile
Voluminous
Dwyka
flyschoid and
wedges
eolian
outpourings
Formation
clastics of
Table 8.5: Stratigraphy of the Karoo Supergroup (Redrawn from Tankard et al., 1982.)
FORMATION
DRAKENSBERG (volcanics) ( Previously Stormberg )
CLARENS ELLIOT MOLTENO SOUTHWEST
BEAUFORT
TARKASTAD SUBGROUP ADELAIDE SUBGROUP
succeeded
Ecca-Group,
Beaufort
(Molteno,
Group Elliot,
basaltic
lava
(Table 8.5)
in South Africa.
BIOZ ONE
(Dinosaurs) ( Dicroidlum ) SOUTHEAST BURGERSDORP
NORTHEAST OTTERBURN
KATBERG
BELMONT
BALFOUR
E~COURT
TEEKLOOF
ABRAHAMSKRAAL KOONAP WATERFORD FORT BROWN LA~NGSBURfi RIPON VtSCHKUIL COLLINGHAM WHITEHILL PRINCE ALBERT DWYKA
Ka nnemeyeriaDiademodon LystrosaurusThrin~xodon Dicyno don lacertlcepsWhaitsia AulacephalodonCistecephatus TropiclostomaEndothiodon
MIDDLETON
ECEA
the
deposits
is
the
of Drakensberg
in the Jurassic ended the Karoo depositional cycle
GROUP
of
PristerognathusDi~ctocton D~nocephalian VOLKSRUST VRY HElD PIETERHARITZBURG (61ossopteris)
Dwyka Formation This is a sequence of Late Carboniferous-Early-Permian 700 m thick in the Cape depocentre over Precambrian basement. clude pebbleported
by
silty
and
clay
sometimes up to six units, in-
clasts of local and distant
matrix;
up to
(Fig.8.49B), but wedging out northward
The diamictite,
and boulder-size
diamictites,
fine-grained
origin,
cross-laminated
sup-
glacial
513
outwash and g l a c i o - l a c u s t r i n e These
accumulated
aqueous
mostly
terminal diamictite
occurrences
of
recording
diamictites
especially
that the Karoo basin
by
and
and as t e r r e s t r i a l
several
(Fig.8.50,B).
overlain plant
and varved c a r b o n a c e o u s
as ground moraines
moraines,
glacial advance and retreat fining
siltstones,
cycles
(up
shales. and
to
sub-
nine)
of
Each cycle comprises an upward-
glacio-lacustrine
palynomorph-bearing
in the northern part
and the surrounding
shales. shales
of the Karoo
areas
locally
Sporadic
within basin
the
suggests
supported
Tundra
conditions with plant cover. Based
on
ice m o v e m e n t
directions
inferred
from g l a c i a t e d
the o r i e n t a t i o n of t i l l i t e - f i l l e d glacial valleys, n~es
with
smoothly
glacially from
plucked
which
ice
polished
and
downstream
moved
are
striated
sides,
upstream
the
believed
centres
to
have
pavements,
exhumed roches moutonsurfaces
of
been
and
maximum located
Transvaal, Natal, and p o s s i b l y in the A t l a n t i c area also
jagged
glaciation in
Namibia,
(Fig.8.50,B).
Ecca Group Major
changes
setting
of
in
the
the
Karoo
paleoclimatic, basin
took
paleo-tectonic
place
in
the
This was during the deposition of the Ecca Group. in a w i d e range of paleoenvironments: deltaic
to a deep
trough
and
Early
paleogeographic
to M i d d l e
from alluvial plain through fluvio-
(Fig.8.50,C,D).
First,
from the South Pole and ice sheets melted,
South Africa
drifted
a broad e p i c o n t i n e n t a l
as
sea en-
croached into the Karoo basin from the west. This t r a n s g r e s s i o n sented
by
shales,
the W h i t e h i l l
an
extensive
the Dwyka Formation, cypod Eurydesma, crinoids, While itzburg about
flyschoid
margin,
fossiliferous which
and
rests
phosphatic
conformably
on
the pele-
and foraminifera. deposits
with
500 m m a x i m u m of
of
is repre-
Orthoceras, and Eosianites, brachiopods, echinod spines,
Formations)
complexes
unit
(White Band)
and contains paleoniscid fish, gastropods,
radiolaria,
(Collingham,
turbidites
bathymetric
the Vryheid
the Karoo trough
marker
Formation
Permian.
This group accumulated
depth),
Formation
(Fig.8.50C).
Ripon,
accumulated were
Further
stable
Laingsburg, in
shelf
deposited
landward,
the Karoo
Pietermartrough
prograding
along
(at
deltaic
the margins
of
along the n o r t h e r n basin
lay e x t e n s i v e alluvial coal swamps.
Of the n i n e t e e n Karoo coalfields that are m i n e d in South Africa, with 81 b i l l i o n tons of recoverable bituminous the Ecca Group; 1982).
The
Ecca
coal, e i g h t e e n of these lie in
one lies in the Molteno Formation above coal
basins
in
South
Africa
include
(Tankard et al.,
the
Limpopo,
Sour-
514
DWYKA FORMATION glacial deposits
W I T T E O E R G GROUP ( p r e - K o r o o } shallow s e a - b e e c h - deltaic deposits
::~:
!: ::[
:
::::::!:i:~,
:.:.~ii :: :i: ~ ([i~," ~ ~
....."% 7 Y ::,:(,. ,4,
~::" : i::: ~
::~..'"
{
~':
~:::"
":'~-.~ : :
laclerl
T' 'V ~.!*.":" I
- 2~2g~-
~
shallaw see
. . . . .
:. ::
o o0el,o..
\:!:j,~:J...,'
~ ~:-:~-<~
~
""
_
Er..!.': ,,~
. ,,u~.~- ~.:'{"
t ' , J ",&.~ ~
'.-.:!:
: :::'"i;.~,~
.... " " ' ' ' : ,!
.cu,hw.te,
~ J , ~ : Ecco deltas with ~,coal facies /~::
u..
Ecco deltas
de,to.
t£-;~' ,,.~-"~. eastern X'~.-"-~.:~ ~ E~-o deltas
".'~t'..
MOLTENO FORMATION coarse groined fluvtol deposits
'/;:~ i : :. ] ! ! i i :: :: :: :: i i i! i i ! i i ~ . ~ :
i !~i:!
:;i~"~ J ~
*' ' , ~ ' 4 ~ { : ~ b r a ~ , ~
,ivy,
~ ~"~/. :~ lOW to high ~ C~ ~!~"~sinuo$ity rivers
ELLIOT FORMATION distal fluvio-locustrine deposits
CLARENS FORMATION ephemeral stream/ployo/dunecomplex :-
.'4
lake
~
B E A U F O R T GROUP fine-groined alluvial plain
~.....J'~ '~'~
~:.-..-~:,:-"
.ha,,ow
Ecco deltas ~ : : ~ ~ "~>.:,; ~
. X
ice sheet
ECC.6 GROUP (late Permian) deltaic-lacustrine depceits
~.;~::::"
:: :
,ca ,,ow.
~
~ . ~ /
ECCA GROUP (early Nrmton} continental slope-deltaic-lacustrine deposits
,~L..:
;.......;....
•.
, ,,~~-~',r.-~
western
")1' .... :-: I:.!
.:;.~ ! ,.......
i~i~
~-~
.~",
~:~ ....
-- ,-. ....
;:i':~
Figure 8.50: 1990.)
Karoo
depositional
- .--
- .... r
" "
I
•:.~]] ! ] ]i~]iii!i;:!i'q,r¢¢ ('-Ji~rly phose of ::: : . ' , , :. , ',:q l ..~." ~":'~; ~ . "~ •"~::: _ .:~. ~ .Drake n sberg --'% • : ' % ,~ 'I '!~...:::. :~. peleo :.1[. I volcanoes
models.
Z,i, ~:
(Redrawn
from
Smith,
515
pansberg-Lebombo,
Waterberg,
basins
eastern
(Witbank,
these basins facies.
Springbok
Transvaal,
the distribution
Flats,
Natal)
and
the
northern
coalfields
of coal was d e t e r m i n e d
Coal was m o s t l y restricted to paraglacial,
(Fig.8.49A).
by the
u p p e r d e l t a i c plains,
(Caincross,
factor was
with economic coal m e a s u r e s
in stable p l a t f o r m areas around Witbank, less stable areas in Natal to the east,
In
sedimentary
s t r a n d - p l a i n s and alluvial valleys the rate of subsidence,
Karoo
1989). A n o t h e r controlling
northwest of Natal,
occurring
whereas
the
contain numerous but impersistent
and thin coal seams within a c o n s i d e r a b l y thickenend deltaic sequence. Beaufort Group This is the most e x t e n s i v e l y exposed of all Karoo strata in South Africa, covering about
200,000 km 2 (Fig.8.49A)
3 km in southwestern late
Early
clastic wedges shifted Cape
fold belt
thins
from
to the
foredeep
quartzitic which
age,
basin. is
the
sequence
depocentre
south supplied from
locally
while
The
Permian
to
alluvial
Beaufort
Group
famous
of
the
into the
amounts
Precambrian
Consequently, is
Uplift
clastic wedges subordinate
elevated
lay to the north and to the east. northward.
of Late
fossiliferous
(Fig.8.50,E).
regressive
(Fig.8.49B),
came
of
of about
in an e a s t - t r e n d i n g d e p o c e n t r e which had
Ecca
basin
clastics
The Beaufort Group
a thick
that accumulated
northward
Beaufort
Karoo
Triassic
with a m a x i m u m thickness
of
sourcelands
the Beaufort Group
for
its
distinctive
reptilian fossils upon which its Late P e r m i a n - E a r l y T r i s a s s i c biozonation is based
(Table 8.5).
The B e a u f o r t
is made up of two subgroups.
The A d e l a i d e
prises m o s t l y mudstones which accumulated in distal vial ral Its
sediment levees
source areas)
and
lateral
in
extensive
equivalent,
lithofacies,
inter-channel freshwater
the
represents
floodplains,
lakes
Tarkastad
predominantly
(Yemane
Subgroup, fluvial
S u b g r o u p com-
(in r e l a t i o n to allubordered and
by natu-
Kelts,
1990).
more
sandy
with
channel
environments
showing u p w a r d increase in bedload lithologies that resulted from the migration of h i g h - e n e r g y proximal fluvial environments.
The d i f f e r e n c e s be-
tween B e a u f o r t
parts
formations
foredeep m e r e l y
reflects
in the w e s t e r n varying
and eastern
intensities
of
of the Karoo
terrigenous
influx
into
s h r i n k i n g lake depositories. Uranium Mineralization.
In the highly arkosic A b r a h a m s k r a a l
Formation
at the base of the Beaufort Group in the southwestern part of Karoo basin is a sequence
of
upward-fining
meandering
channel
sandstones
channel flood basin i n t e r l a m i n a t e d siltstones and mudstones. eralization
occurs
in the
channel
sandstones,
especially
and
inter-
U r a n i u m min-
in elongate
or
516
tabular cially
calcareous along
complex num,
the
sandstone base
silicates
phosphorus
of
and
volcaniclastics
and
the
pods which lateral
margins
coffinite
urano-organic
are rich
type,
of
flushing
transported the
more
stones. ducing
into
groundwater
the resulting uranyl
permeable
Local
controls
channels.
occur
together.
espe-
Uraninite,
arsenopyrite,
soil formation,
system.
Mildly
molybde-
Interbedded
basal
portions
and by subse-
reducing
carbonate complexes
coarse-grained
conditions
pyrite,
the
remains
are believed to have supplied ura-
nium which was released through weathering, quent
the
pyrite,
compounds
and basement granites
in plant
groundwater
preferentially
of
the
channel
into sand-
over uranium m i n e r a l i z a t i o n were exercised by re-
created
by
concentrations
of
carbonaceous
debris
and
and by p e r m e a b i l i t y barriers provided by adjacent or interlayered
argillaceous units. Uranium
mineralizations
pelite-hosted, 1987).
as
well
as
in
the
Karoo
coal-hosted,
are
and
These are also found in Karoo deposits
gascar, Angola,
of
vein
the
sandstone-hosted,
types
(Roux
in Zimbabwe,
and
Toens,
Zambia, Mada-
and in Gabon where uraniferous black shale occurs.
Upper Karoo Formations
Previously
termed
the
Stormberg
Group,
the
upper
prise the basal Late Triassic Molteno Formation,
Karoo
w e d g e that was derived from the uplifted Cape fold belt. mation
contains
the
richest
and
best-known
fossils being represented by abundant
Triassic
fossil
formations
com-
a coarse fluvial molasse
insects.
The Molteno For-
floras,
the
animal
The Molteno repre-
sents deposits which are typical of a braided river system of higher energy than the underlying upper Beaufort, of alluvial
that were laid down
in a series
fans under cool, braided conditions with p e r m a n e n t ice in the
mountain heartland
(Cruickshank,
The c o n f o r m a b l y
overlying
1978).
Elliot
Formation
is a sequence
of redbeds
which contain d i a m o n d placers in Swaziland. At the top of the Karoo sedimentary
sequence
is the
Early Jurassic
Clarens
F o r m a t i o n which
consists
of eolian sandstones and associated fossiliferous playa lake sheet flood, and ephemeral stream deposits. m i d d l e of the Clarens verse dune which
accumulated
the west
with
under
barchan dunes desert
represented
conditions
with
by
large
paleowinds
trough
sets
blowing
from
(Fig.8.50,H).
However, part
deposits
Large-scale cross-bedded sandstones in the
Formation has been interpreted as m i g r a t i n g trans-
less severe climatic conditions are r e p r e s e n t e d in the basal
of the Clarens
Formation which
contains
numerous
lake deposits
io-
517
cally c a r r y i n g
freshwater
fish,
crustaceans,
dinosaurs
and d i n o s a u r
foot-
prints. Karoo of the
sedimentation
Drakensberg
Lebombo-Nuanetsi consistent break-up
ended
plateau,
volcanic
magmatic
with
the o u t p o u r i n g
the Stormberg provinces.
event
in
the
range,
These
of vast and
lavas
Gondwana
basaltic
of the
Zimbabwe
represent
continents
lavas
a
that
and
remarkably
heralded
the
of the supercontinent.
8.7.4 O t h e r K a r o o Basins Outside
South
widespread
Africa
few of these basins, their regional
Ruhuhu
continental
in i n t r a c r a t o n i c
basins
especially
cal Karoo
rift basin succession
of southwestern
glacial
and
followed
by a succession
periglacial
fluvio-lacustrine
stratigraphy
of
8.7.2),
result
the
Deposition
basin
cycle
are
Descriptions
of a
b e l o w as well as
began
in
to
attributed
to
mountain
lakes.
The
fluvial
overlying
sequences.
strongly
Each
The cycle
a mudstone/coal
facies.
succession
of braided with
swampy v e g e t a t e d
of
clastic
basin
(Kreuser
Early
Permian
tillite
horizons
coarse
into
an e r o s i v e
with
sequence
channels
splay,
which
levee,
coal
lakes and floodplains
during
lacustrine
has
been
(Fig.8.52B). mudstones
have
glacial cyclical
followed
silicified
by
wood
and ends with
interpreted
to m e a n d e r i n g
deposits,
as
a
river
ending
with
(Fig.8.52A).
cycle w h i c h which
diamictite These
of
base
and
switched
overbank
1989).
lowland
consist
as a
(Fig.8.41).
basal
into s a n d s t o n e - s i l t s t o n e ,
facies
margins
a
issued
with
zone
the
stated
opened
deposited.
measures
clastics
upward
when
coal
begins
already
and Markwort,
was
fa-
redbeds,
described
which
shear
which
upper
accumulated and
As
fluvial
playa
(1989)
along the M w e m b e s h i
a tectono-sedimentary
redbeds
sandstones
detail.
glaciers
fluvial
ox-bow
the
in
and
This
crevasse
followed
uplift
basin
the typi-
Permo-Carbonif-
coal-bearing
is one of the rift basins
sequence
fines
point-bars
fluvial
lower
cross-stratified
fragments.
Then
three
shows
with
stream deposits, Markwort
the
up
thick
and
three phases
with
(Fig.8.48)
Kreuser
movement
through
Tanzania
deposits,
Ruhuhu
the Ruhuhu
evolved
(Fig.8.51)
fine
Karoo
are p r e s e n t e d
It is complete,
of braided
strata.
of s t r i k e - s l i p
The basin
ate
the rifts,
in East Africa.
cies,
been
the
correlations.
erous
(Ch.
of
Basin
The Ruhuhu
and
deposits
south of the Sahara.
was
arkosic
caused
clastic
Monotonous and
oolitic
by moderwedges
alternations limestone
and of and
518
marl
lenses
resulted
lakes
(Fig.8.52C).
phase
of
mainly bouring
from
Up
to
fluctuating
shallow
K6
lake
fluctuations
Zimbabwe
where
in Zambia,
gesting
existence
beds
(Fig 8.51) Similar
These
they
Kenya
climate.
beds
in the Ruhuhu basin
and
fine
are
the
were and
shorelines
deposited
very
thick
siliciclastics
known
as
the
inter-connected
lacustrine
deposits
are
(Kreuser et al.,
giant
during
this
sequences
of
in neigh-
Madumabisa
Mudstone
and in Malawi,
shallow
lakes
potential
1988).
of
occur
("Maji ya Chumvi Beds")
of vast
warm
of
levels.
lacustrine marls
(Table 8.6) the
the
sug-
under humid
petroleum
source
They also p r o v i d e d rep-
tilian habitats.
SW- TANZANIA
S. AFRICA
JURASSIC
I
n 0 t~ STORMBER5
TRIASSIC
NtG_~
_ ~-~-----MA"NDA" B E ' 6 " S ~ - - ~
-K_8. ~-UPPER_ BONE_ BE DS_--[-~ B ~E
P E R M I A N
•
DEP OSITIONAL ENERGY
_ ~
MIDDLE
F
'
LOWER i-" ""
0
LATE
:---" ~----~• ]FrG-~RF-------Z..4
UPPER
..... : .... C-'~: -'.":..,.','~':'~.. -
BEDS
.
!:..c.o..~L.M.e~s..u r. ~s!.::,r..,x ECCA
"iiiii "]]lill]l~-'~:w?iVilliliiII'~
EARLY ?--
[ARB,
DWYKA
K2
COAL MEASURES lllfft
,l,,
PERI~Y.'Xc'i'A'C'S-"i' IIII1
KI illll
BAS A,L,,,,T,,!LL !T,,E,S,, II~l
'~ 14P
Figure 8.51: The Karoo of the Ruhuhu basin in Tanzania. Black squares, intervals dated with palynomorphs and vertebrates; black circle, red beds, half moon, gray/black colours; open circle, light colours. (Redrawn from Kreuser and Markwort, 1989.)
The Lower Bone Beds tional pattern.
(K6) in the Ruhuhu basin mark a change in deposi-
In this unit nodular and thin limestone horizons and bone
beds are found in a p r e d o m i n a n t l y m u d s t o n e lithology of playa origin with increasing d e s i c c a t i o n tic
lenses
skulls
and occasional
(Fig.8.52C).
including
those
The of
been c o r r e l a t e d with the
the
bone
floods w h i c h
beds
sauropod
contain
supplied coarse clas-
in
situ
skeletons
and
Dicynodon lacerticeps which has
zone that bears this
taxonomic name
in the up-
permost Permian of the Beaufort Group in South Africa. The sented
terminal
Karoo
by the m a r k e d l y
depositional disconformable
cycle
in
Kingori
the
Ruhuhu
Sandstone
basin,
repre-
and Manda
Beds,
519
0
.¢,
m~
0
o~ m~
O~ G)
~
0
0 ~r..) .,-4 m t~
8~ CO • ~o'~
520 was
characterized
climatic
setting.
cross-bedded
The
quartz
was d e p o s i t e d Numerous
by fluvio-deltaic Kingori
arenite,
extensive
Sandstone,
is
in a m e a n d e r i n g cyclothems
conditions
regarded
a as
reflecting thick
Early
river
environment
occur
in
the
a new tectono-
coarse-grained
Triassic
under
humid
overlying
Manda
in
and
age,
and
conditions. Beds
which
w e r e d e p o s i t e d during uplift of the margins of the Ruhuhu trough and subsidence and e n l a r g e m e n t of the depocentre. later eroded when the half grabens the
southeast
and
faulted,
The top of the Manda Beds were
in the Ruhuhu trough were uplifted to
probably during
the Late
Jurassic.
The
upper
part of the Manda Beds contains Early Triassic vertebrates. Table 8.6: S t r a t i g r a p h y of the Karoo (Redrawn from Orpen et al., 1989.)
EUROPEAN TIME EQUIVALENTS
MID-ZAMBEZI
LOWER JURASSIC
of
Basalts
I STORMBER6 UPPER
FineRed
KARO0
Morley Sondsto ne
Escarpment
SERIES
Flags
Grit
BEAUFORT
UNCONFORMITY
SERIES
ModumabisoMudstone Upper WankieS~ndstone Black Shale and CoolOroup Lower WankieSandstone
PERMIAN
61acial
basin.
SOUTH AFRICAN EQUIVA LENTS
Pebbly Arkose
Ripple Marked
Mid-Zambezi
BASIN
MINOR Bato ka UNCONFORMITY Forest Sandstone
TRIASSIC
the
LOWER KARO0
ECCA SERIES
Beds DWYKASERIES
CARBONIFEROUS
Morondava Basin
Before
it
Madagascar 1987).
The
drifted
away
developed Morondava
from
Karoo basin
Somalia, rifts
on
Kenya its
(Fig 8.53)
and
western
resulted
Tanzania side from
the
island
(Reeves
et
crustal
extension
which e v e n t u a l l y led to the separation of M a d a g a s c a r from Africa. (1972) basin
subdivided the nearly into
the
basal
Sakoa
12-km thick Karoo sequence Group,
the
Sakamena
Group,
of
al.,
Basaire
in the Morondava with
the
Isalo
Group at the top. Further details of the tectonic and stratigraphic evol-
521 ution and
of
the
above
by W e s c o t t
regional which
sequence
(1988).
paleoslope
lay to the
was
were
During
furnished
sedimentation
to the west
southwest
and was
and
by
Nichols
in the
southwest
and
Morondava towards
intermittently
Daly
(1989),
basin
a
lake
inundated
by
the
basin
the
sea
which lay further south.
4KEY MAP
DETAILEDMAP(BELOW}
JURASSIC-
TERTIARY ~
CRETACEOUSjuRASSI~ C
TERTIARY ISALO
KARO0 ~
SAKAM[NA SAKOA PRECAMBRIAN METAMORPHC IS
BASEMENT ~
/~
MAJOR FAULTS i
lOOKm
I
l
rver
Figure 8.53: in Madagascar.
G e o l o g y and s t r a t i g r a p h y of the M o r o n d a v a (Redrawn from Nichols and Daly, 1989.)
basin
522
Although
tillites,
coals
and
limestones
the southern part of the Morondava basin, tile
sequence
(Fig.8.54)
internal d r a i n a g e
broken
occur
in the
Sakoa
Group
in
this is a p r e d o m i n a n t l y fluvia-
deposited
when
by series
of horsts
the
basin into
was
primarily
sub-basins.
one
of
The Sakoa
G r o u p w h i c h is of Late Carboniferous to Early Permian age is about 2,000 m
thick
and
is
progressively
and
unconformably
overstepped
n o r t h e r n part of the basin by the overlying Late P e r m i a n - M i d d l e Sakamena place
Group,
in
Permian
about
an enlarged marine
4,000
m
thick.
Morondava
transgression
u n d e r l y i n g Sakoa Group,
Deposition
depocentre
which
was
and
was
followed
uplift and erosion,
of
the
Sakamena
preceded
by
the
in
the
Triassic took
by a Middle
tilting
of
the
before the d e p o s i t i o n of the
Sakamena.
AGE
GROUP
LITHOLOGY [
-IT --
o
[
I
I
I
I
I
I
I
I
I
r~-", ~
[
I
[
I
~,1
<.-t
I
I
I
I
I
] ]
J
I
I
1
I
I
I
I
',.9
:-.'v .'." .'.'.:.': : -..: .':.." ' .' ." -'.;.::
o
-:!:.:.:.:.::i-.:'_"i~'~::'..:.:i{'.:?:::::
"' " : '."--~' ":..~.,:.,:', : " " T " ' :~.~,: : , ' "::"~.:..::: ~ :[ "c:" :.~:'~..'.: ,<
.':.t..'~i '..'..,.,.'..:-.',:~"
<
~.~::.,.~=.v.::::_.:.__:....,/~
n
I.:.:'.'< = ".'" ">.' .'-: =':::J
o
-::.:: :.: .:..:-..-.:-~/:,,';,;,,
~_
~ ,"7~,~-T
.-'.:.':.:.
,~'?"~ ~, ~ ,~,~ ';2,
,J-'-.
-
LATE
Figure 8.54: G e n e r a l i z e d Karoo succession basin. (Redrawn from Wescott, 1988.)
in
the
The Sakamena has been subdivided into lower, middle,
Morondava
and upper units.
A c c o r d i n g to Wescott
(1988) the Lower Sakamena Group is c h a r a c t e r i z e d by
very
facies
coarse-grained
border
faults.
reflecting
Braided-stream
renewed
sandstones,
tectonic
littoral
activity
clastics
and
along
stroma-
523
tolitic limestones of lagoonal, in the
Lower
mudstones, part
as
Askin,
Sakamena.
1987).
sea. Wright
of
Sakamena,
an
Fluvial
Early
systems
Triassic
predominantly
marine
discharged
ial s e d i m e n t a t i o n of
Sakanema transition.
transgression
from the east
Triassic
sandy mudstones,
Mid-Zambezi
gravelly
to Middle
Jurassic
Isalo
and
of cross-bedded,
contains
sandy
Group,
fluvalter-
mudstones.
about
5-6
km
The
thick,
g r a v e l l y sandstones
and
Basin
(Fig.8.48)
cession e s t a b i s h e d by Bond
another r e p r e s e n t a t i v e Karoo basin is
of Zimbabwe.
Here the s t r a t i g r a p h i c suc-
(1967), also reflects
the d o m i n a n t
c h a n g i n g climate and vertical tectonic movements. successon
varved mudrocks; grade upward
starting
and
Lower
Karoo
with
diamictites
Coal
Group.
containing thick economic
Glossopteris-bearing
The
Upper Permian M a d u m a b i s a Mudstone.
After a hiatus,
with
the
conglomerate
beds
s e d i m e n t a t i o n appears sandstones,
coal
known
and mudstones
the o v e r l y i n g clastic beds,
contains
to
as
Grit
following
which
d e p o s i t i o n of
the R i p p l e - M a r k e d
Flags,
and
including the eolian Forest Standstones.
Glossopteris
ostracodes,
Sand-
followed by the
the U p p e r Karoo begins
Escarpment of
seams of the
Upper W a n k i e
to have proceeded with u n i n t e r r u p t e d
conglomerates
In a d d i t i o n
and
followed by the pereglacial Lower W a n k i e S a n d s t o n e which
into argillites
Shale
from the
control of
Table 8.6 shows a typi-
stone was d e p o s i t e d during renewed coarse clastic influx,
stone
shallow
all of which are of fluvial origin.
the M i d - Z a m b e z i basin
Black
the
A return to p r e d o m i n a n t l y
sandstones
R e t u r n i n g to the A f r i c a n mainland,
Karoo
(Wright and
into
took place in the Upper Sakamena w h i c h
cross-bedded
consists m o s t l y of alternations
cal
fossiliferous
is shallow m a r i n e in the basal
and A s k i n placed the P e r m i a n - T r i a s s i c b o u n d a r y a p p r o x i m a t e l y
at the L o w e r - M i d d l e
overlying
The Middle
siltstones and fine sandstones,
a result
nations
algal reef and shallow shelf origin occur
and
related
freshwater
flora,
pelecypods,
the
Madumabisa
and
reptilian
Mudbones
similar to those of the A d e l a i d e Subgroup of the Beaufort Group.
Regional Karoo Correlations Karoo-type
nonmarine
(Kent et al.,
beds
lie along the coast of East A f r i c a
1971) where Permian marine sequences also appear,
ple in the Kidodi basin of Tanzania, the
Mandawa
stricted
basin
marine
in
Tanzania
incursions
the Lamu e m b a y m e n t
Permo-Triassic
(Nairn,
1978).
In
6.8 km of Karoo deposits lie in the subsurface
coast
(Mbede,
for exam-
in Kenya;
evaporites the
in grabens
and in
indicate of
1984).
Kenya
reover
524
In
southern
Botswana
Africa
(Fig.8.48)
the
and
Karoo
those
in Namibia
tensions of the m a i n Karoo basin paleontologically
(Haughton,
Strata
to those
equivalent
succession
in
the
are direct
Passarge
basin
stratigraphical
of ex-
in South Africa, both l i t h o l o g i c a l l y and
1963;
Nairn,
described
1978;
above
in
Tankard
the
et
Ruhuhu
al.,
basin
1982). are be-
lieved to occur e x t e n s i v e l y in eastern and central Africa at least in the basal
part
Uganda,
of
other
Mozambique,
basin w h e r e
Karoo
Kenya
rift
and
in
basins
in
Somalia.
Zambia,
They
Malawi,
also
underlie
they are referred to as the Lukuga Series.
izes p a l y n o l o g i c a l Saharan Africa
the
parts of sub-
1984).
Table 8.7: P a l y n o l o g i c a l correlation of Karoo sequences in southern and central Africa. (Redrawn from Dingle, 1978.)
TANZANIA
SOUTH
C__OASTAL KARO0
NORTHERN
AFRICA
-r-
HADAGASKAR I J~Sakornenc
KAROO
ZIMBABWE SE-
Mikumi
7 ! ~-
Ro,,i,I
. NEZAMBIA
i i
s
TANZANIA
iL~
i
i ~
I
i
UPPERJ 4 "~ I
K3
¢ MIDDLE~
K4 Jppe¢ Wo ~kie
Scl ~d~L_
a
~
t E cca I
Az~ illite QI
Co 'K1 No (2 el andst,
- LOWER~ c
"~13~ leosurej ~0
Q"
I
~adu- IShale nclbisa[
tohitoIClKalk
Z,AIRE
~E
c
o
m a b;s~
< ~ c~
Ecca l
(
~2 ez
Madu-
--
--
r i
imbia
L~;il
Couhes de hou~Ue
.owe~ Bec~
I :ouch~
Schlste.¢ nolrsde .Lukuga
Q
_ '.houillc
Lo vet
~Va ~kiE S a ~ds~- 1
)wykQ J
W
K ]wkc
Schistes noirsde WQIikal Assises ~erig~:~Qfes
• I
Zaire
Table 8.7 summar-
correlations of Karoo strata in various
(Kreuser,
Tanzania,
heit
GABON
525
8.7.5 Aspects of Karoo Life and
animal
life
impressive
Plant
record
in South Africa
best
known
enriched
of
our
evolution, record
the
flourished
Gondwana
knowledge
the
during that
flora
of
and
early
highlights
of
Karoo
faunas.
terrestrial
the
times
constitutes
Karoo
and
left
the m o s t
Since plant
these life
paleobotanical
a
very
diverse have
and
greatly
and
vertebrate
and
vertebrate
(Fig.8.55) deserve at least passing m e n t i o n at this point.
Subclass
Upper Permian
Orders and suborders
Scythian Lystrosaurus ZOne
Midd|e and Upper Triassic
Cynognathus Zone
ANAPSIDA SYNAPSIDA
Therapsida Oinocephatia Anomodontia Theriodonta ( mammal-like reptiles) Gorgonopsio Therocephalla Cynodontia
dominant herbivors Lystrosaurus common, least mammal-like advanced types
dominant mammal-tike reptiles Cynognathus
OlAPSIOA
earliest dinosaurs
(gave rlse to dinosaurs, lizards, birds, crocodiles)
Figure 8.55: vertebrates.
ORN~THISCH1A
Stratigraphic ranges of some (Redrawn from Dingle, 1978.)
Just as the m a j o r Carboniferous marine ted w i d e s p r e a d
epicontinental
the formation of coals
transgression
flora
was
the
later
of
coal m e a s u r e s v e g e t a t i o n Triassic.
Karoo
the
basins
crea-
a high latitude coal mea(Fig.8.56)
and
throughout
Glossopteris plants w h i c h appeared dur-
Permo-Carboniferous
(Plumstead,
glaciation,
formed
(Fig.8.6),
viz;
the
the
1969) which c o n t i n u e d right into the
Global v e g e t a t i o n at that time consisted of four m a j o r
tic provinces (or
in
The w e l l - k n o w n
phase
(Fig.8.4)
swamps and coals measures on the Saharan p l a t f o r m
flourishing
southern Gondwana. ing
Karoo
seas in most parts of the w o r l d and led to
and in other low latitude regions of the world, sures
South A f r i c a n
Euramerican
flora,
and
floris-
the Cathaysian
Gigantopteris) flora, both of which existed at the p a l e o - e q u a t o r i a l
latitudes; wana tions
or
the northern Angara or Kuznetsk
Glossopteris flora seemed
(Plumstead,
to
favour
(Chaloner the
and
evolution
flora;
and the southern Gond-
Creber, of
the
1988).
Glacial
condi-
Glossopteris
flora
1973), which expanded with the r e t r e a t i n g g l a c i e r so that the
Early to M i d d l e
Permian
Ecca
coal
swamps
were
thickly
vegetated
with
a
526
Glossopteris
mixed genera
of
lycopods,
leaves
flora.
and
This
mixed
fructifications
flora of
a few ferns and pteridosperms,
contained
Gangamopteris and ginkos.
and the early Triassic when the Beaufort Group was
many and
Early Triassic.
Dicroidium
flora
took
over
and
By the late Permian deposited,
signs of a Dicroidium flora, a lowland temperate vegetation, in the
species
Glossopteris,
in
the
the
first
had appeared Late
Triassic
Molteno F o r m a t i o n w h e r e it beceame the dominant and most diverse element. Interestingly
the
animal
fossils
in
the
Molteno
Formation
r e p r e s e n t e d m a i n l y by insects with 25 genera and 32 species Anderson,
(Anderson and
1984).
Figure 8.56: Glossopteris - Gangamopteris forest times, South Africa (Redrawn from Plumstead, 1969.)
of
The
up
Gondwana
paleontological
(1970) as follows: in South Africa, tile
are
Mesosaurus.
record
was
best
summed
"The most abundant and d i v e r s i f i e d
where The
the Upper Dwyka Ecca
yielded no reptilian remains.
Group
bears
shales a
contain
reptilian the
Glossopteris
Karoo
by
Kummel
fauna is
swimming flora
but
The overlying Beaufort Group, however,
rephas is a
v e r i t a b l e g r a v e y a r d of ancient reptiles in which no less than 600 species have
so far been discovered.
Within the Beaufort,
in fact,
six distinct
527
reptilian
zones
have
been
recognized,
and
they
greatly
aid
in
correla-
tion. The o v e r l y i n g Stormberg Group also contains a rich r e p t i l i a n fauna, sharply d i f f e r e n t i a t e d not common
from that of the Beaufort.
in the Gondwana
lower group
Reptiles are a p p a r e n t l y
strata of Madagascar.
(corresponding to Ecca) has
The m i d d l e
part of the
some genera also present
in Tan-
zania. The fauna of the upper part of the series has affinities with that of the Upper Gondwana is
even
more
valid
evolutionary
of
India".
today
as
The above appraisal
more
reptilian
fossils
link between these early reptiles
are better understood
(Halstead and Halstead,
We also k n o w that Mesosaurus,
of the Karoo are
lived
Fig.8.56;
in
the
and
that
Ecca
coal
another
the most
common v e r t e b r a t e
(Currie,
1981).
no v e r t e b r a t e ated
from
the
a Late C a r b o n i f e r o u s - P e r m i a n amphibious swamps
Hovasaurus
reptile, that
(Plumstead,
lived
in South America,
1969),
boulei
in the Late
as
shown
(Fig.8.57A),
Permian
in was
of Madagascar
During the Late Permian and Early Triassic, M a d a g a s c a r had
faunal
the
and
and mammals
1981).
reptile that inhabited Karoo basins and similar basins also
known
and dinosaurs
fauna
similarities
African
with South Africa
continent
by
a
marine
since
barrier
it was
(Battail
separet
al.,
1987). But by far the most fascinating aspects of the Karoo r e p t i l i a n faunas are
their
evolutionary
(Fig.8.55) primitive
trends.
The
reptiles
of
the
Beaufort
r e p r e s e n t one of the b e s t - p r e s e r v e d e c o l o g i c a l mammal-like
evolutionary
reptiles
transitions
or
paramammals
to dinosaurs
and
in
the
assemblages
world,
to mammals.
Group
which
of
show
Paleoecologicall~
the Beaufort accumulated in large inland basins with terrestrial habitats comprising
extensive m e a n d e r i n g river
floodplains
and
lakes,
and heavily
d i s s e c t e d lowlands where plant and animal life were p r o b a b l y concentrated around
water
courses
(Smith,
1990).
It
is believed
that
heavy
rainfall
o c c a s i o n a l l y led to the overflowing of rivers into their floodplains that
the
(King,
climate 1990;
(dicynodonts),
became
hot
Tankard
et
other
or
semi-arid al.,
reptiles,
with
1982).
highly
seasonal
Plant-eating
amphibians,
freshwater
and
rainfall
parammamals fish
and
the
Glossopteris flora were part of the complex Beaufort terrestrial, aquatic and semi-aquatic ecosystems. Using biozones western
the Karoo
carnivorous tained Late
paramammals, the
(eg. Tankard et al.,
large
Permian
basin
(Table 8.5)
Dinocephalia. numbers
Beaufort 1982).
of
The Early
Cistecephalus
Group
has
been
contained
the
large
Pristerognathus-Diictodon Permian
Zone
had
subdivided
The basal D i n o c e p h a l i a n
dicynodonts,
large
herbivorous Zone
while
herbivores
such
into
Zone in the above
and con-
the
overlying
as
Diictodon
528
~
'10era'
A
Figure 8.57: Some Karoo vertebrates from southern Africa. A, Hovasaurus boulei; B, Mesosaurus; C, Euparkeria capensis; D, Lesothosaurus austrialis; E, Heterodontosaurus; F, Syntarsus; G, Brachiosaurus branchi; H, Spinosaurus; I, Allosaurus; J, Kentrosaurus. (Redrawn mostly from Halstead and Halstead 1981; Curie, 1981.)
529
(Fig.8.58) lying
and m a n y other dicynodonts.
Early
declined
However,
Lystrosaurus-Thrinaxodon
Triassic
Zone,
from about 24 genera and 200 species
genera and
six to nine
Cruickshank,
species
by the time of the overthe
dicynodonts
had
in the Late Permian to two
in the Lystrosaurus
Zone
(Colbert,
1965;
1978).
R e c o n s t r u c t i o n of Diictodon from the Late Permian (Cistecephalus zone) of South Africa. (Redrawn from King, 1990.)
Figure 8.58:
There
was
a
corresponding
floral
change
from
a
Glossopteris-dominated
flora to a Dicroidium-dominated one, which allowed m a n y vacant be
filled
by
herbivorous
(Cruickshank, impoverished
1978). Early
Thus,
an
aquatic
way
of
that
life was
plains
community
genus Lystrosaurus and
of
other
niches
reptilian
is believed
completely
at
dominated
(Fig°8.59). to
home
have in
been
rivers
by
the
to
groups
the Lystrosaurus Zone was c h a r a c t e r i z e d
Triassic
herbivorous dicynodont
hippopotamus
representatives
by an single
This genus took to a and
small
reptilian
lakes,
perhaps
530
feeding
on
aquatic
vegetation
(Colbert,
1965;
Halstead,
1975).
Thrinaxodon, a smaller fierce flesh eater, and Prolacerta, the ancestor of the lizards,
both lived on land nearby (Fig.8.60).
Figure 8.59: Lystrosaurus from the Africa. (Redrawn from Halstead, 1975.) Colbert and
(1965)
amphibian
pointed
evolution
out
that
that had
the
been
Early
Triassic
essential
features
established
at
the
of
South
of reptilian beginning
of
Triassic times continued throughout this period with the dominance in the
Kannemeyeria-Diademodon Zone, of paramammals typical of the Lystrosaurus Zone. The Kannemeyaria-Diademodon Zone represents the landoverlying
d w e l l i n g vertebrates zone
contained
that lived at the close of the early Triassic.
theriodonts
or
mammal-like
reptiles
(Fig.8.55)
that
This had
a p p r o a c h e d the threshold of m a m m a l i a n anatomy and p h y s i o l o g y by the Late An
to large,
w o l f - l i k e paramammal with dagger-like
tive predator.
example
was
Cynognathus
Triassic.
(Fig.8.60),
a flesh-eating,
canines,
medium
that was an ac-
Living at the same time and place with Cynognathus was the
small t w o - l e g g e d
reptile named
Euparkeria (Fig.8.57C), an archeosaur and
531
the
immediate
which
first
ancestor
appeared
of
in
the
dinosaurs
the Late
(Halstead
Triassic
and
(Fig.8.55).
Halstead, Some
of
1981)
the most
primitive dinosaurs ever known are a c t u a l l y from Karoo beds in Lesotho in southern Africa.
(Fig.8.55).
nithischia
herbivorous and
These
dinosaurs,
Heterodontosaurus
Halstead,
1981).
belong
to the
early
The ornithischians the
oldest
consors
Another
Karoo
of
group
known
as
Or-
were lightly built bipedal and
which
(Fig.8.57D,E) dinosaur
dinosaur
was
are
Lesothosaurus
from
Lesotho
Syntarsus
australis
(Halstead
(Fig.8.57F)
and from
Zimbabwe.
Figure 8.60: South Africa.
C y n o g n a t h u s and Prolacerta from (Redrawn from Halstead, 1975.)
The a p p e a r a n c e of the better adapted archeosaurs brought about the demise of the paramammals, not
recovered
from
massive
end-Permian
vaded the niches of the parammamals. the first true mammals life, while 1975).
the
rest
the
Triassic
of
in the Late Triassic
a d w i n d l i n g stock w h i c h had
extinction.
The
archeosaurs
in-
The smallest paramammal e v o l v e d into
that survived
of the paramamals
by r e s o r t i n g disappeared
to a n o c t u r n a l completely
w a y of
(Halstead,
Chapter 9 Mesozoic-Cenozoic Basins in Africa
9.1 Formation of the African Plate Rifting and b r e a k - u p of Gondwana dominated of Africa.
heralded
occupied rifts,
the
In northwest Africa rifting in the Triasof
Pangea.
the eastern part of Pangea began
thus
sic onward,
Late
(Fig.8.6).
fragmentation
creating
gulf which e x t e n d e d which
history
The H e r c y n i a n orogeny had by Permian times welded Gondwana and
Laurasia into Pangea sic
the M e s o z o i c - C e n o z o i c
formed
the
along
flysch
boundary to Late
basin
Ocean
to encroach w e s t w a r d
which
into the
F r o m the M i d d l e - L a t e Juras-
lay along
between
Paleo-Tethys
the Saharan p l a t f o r m and an Atlas
from Tunisia to Morocco.
a large
Cretaceous
embayments
The
the
the African
T e r t i a r y Alpine
transcurrent and
orogenesis
the
fault
European
caused
zone
plates.
thrusting and
folding across this b o u n d a r y zone in Africa and southern Europe. Triassic
rifting in northwest Africa
(Fig.8.2) was p r o p a g a t e d
south-
ward, and from southern G o n d w a n a another rift system w h i c h began in Early Jurassic
times
was
extending
Jurassic,
both
rift
systems
region.
Sea-floor
started
during
Atlantic
began
northward. had
reached
Towards
the
and
through
cut
end
the
the
Middle
to open
North
the
equatorial
Middle
spreading and the opening of the North A t l a n t i c Jurassic
and
progressed
southward.
later in the Early Cretaceous
and
South
Atlantic
merged
as
Ocean
The
South
(Late Aptian).
o p e n i n g of the Equatorial A t l a n t i c started in the Albian. onian
of the
Africa
The
During the Tur-
and
South
America
finally separated. Similarly,
the
eastern margin
of Africa
formed by the
fragmentation
and d r o w n i n g of eastern Gondwana, but this initially involved large-scale transcurrent m o v e m e n t s lier
and
created
along transform faults,
aborted
rifts
during
Karoo
a process times.
that began ear-
Transcurrent
motion
which o c c u r r e d between the Middle Jurassic and the Early Cretaceous along the F a l k l a n d - A g u l h a s
fracture zone
o p e n i n g of the South Atlantic, dian
Ocean
(Fig.8.2)
to
margin the
of
north
Africa. caused
(Fig.8.2)
in the south leading to the
c o n c o m i t t a n t l y initiated Motion the
along
southward
the drift
of
opening the Somali basin in northwestern Indian Ocean. ics s u b s e q u e n t l y widened this ocean.
the southern
Davie
In-
fracture
zone
Madagascar,
thus
Distensive tecton-
533
Another ture
tectonic
zones
along
was
that
pre-existing
or interior
imprint where
of
the
basement
fracture
transcurrent latter
are
lineaments,
basins
(Fig.8.2).
i n t e r i o r basins,
West
into
the
motion
African
interior
and the Chad b a s i n rift
shear m o t i o n
along
ment,
w h i c h was the l a n d w a r d
continuation
of the Gulf of G u i n e a
fault
system
Red
such
system, are bel-
African
linea-
transform
(Fig.8.2).
the
classical
processes
led to the b r e a k - u p Sea
and
the
Gulf
to d e t e r m i n e
the origin and alignment
the e v o l u t i o n
scenarios
major
during
movement
rift
doming,
rifting
of the A r a b i a n - N u b i a n
of Aden
along
of
transcurrent
Various
rifts
basins
of
spreading
continent
and even the Sirte b a s i n in Libya, a Central
frac-
created
to be
the
products
propagated
oceanic
ieved
While
the
along
strike-slip
as the Benue trough and its bifurcations, the S u d a n e s e
motion
the
thus
creating Miocene,
basement
in this
lineaments Rift
chapter
and s t r a t i g r a p h i c
sea-floor
Eocene-Early
of the East A f r i c a n
will be examined
of their structures
Shield,
Late
Precambrian
and
seemed
System.
together
with
fill.
9.2 The Atlas Belt: An Alpine Orogen in Northwest Africa
9.2.1 T e c t o n i c The Atlas
Domains
fold
and
thrust
Maghrebide
internal
(Fig.9.1)
constitute
mostly
in southern
and
geria,
foreland
ternal
zones
ranean
Sea,
(Fig.9.2D), in
North
which It
was
of
times.
Atlas,
includes
the
Africa
against
during
(Caire,
feature
1978).
Atlas,
orogen
loop
of Algeria.
Alpine
orogeny
part
that
of the
the
was
Atlas
Africa
the
the
and
African platform the
in Alconstithe
in-
Mediter-
Alboran
paleo-tectonic
Saharan
Apennines
lies dur-
western
bordering
A major
the
which
of Europe,
the
and
Magheribes
and evolved
In North
from
belt
the
the Saharan
chain.
starting
Betic-Rifian
the n o r t h e r n m o s t this
and
like the Jura M o u n t a i n s
orogen,
the
foreland
Atlas
of the A l p i n e
of the A l p i n e
Alpine
The
the Mediterranean,
The M o r o c c a n
and the Tell Atlas
occupied
overthrust
chains
and beneath
fold belt the
the Atlas
southern
the
and the T u n i s i a n
tute the
includes
domains.
Europe
ing M e s o z o i c - C e n o z o i c
belt
external
Sea
element
promontory in Tunisia.
Dinarides
were
534
9.2.2 Synoptic Tectonic H i s t o r y The tectonic
and
stratigraphic
evolution of the Alpine
orogen
west Africa d i s p l a y the various stages of the Wilson Cycle. Jacobshagen tory
of
(1988)
this
and Wildi
region.
and Favre
During
the
(1989) outlined
Triassic,
rifts
were
northeast-trending
lavas were rift
extruded
system
seaways ways.
failed facies
basins
formed
(Fig.9.2B);
in the Atlas
because
there
westward and
open
Open m a r i n e Middle
was
no
(Fig.9.3)
marine
sea-floor
so
that
region.
and Late Liassic
the
took
faunas
Dolerite The Atlas
spreading.
by
sedimentation
Mesozoic
Late
place
from
the rifts
in the rifts and red beds were deposited.
encroached
evaporitic
pull-apart
(1978),
the tectonic his-
(Fig.9.2A)
the relative e a s t w a r d m o v e m e n t of the A f r i c a n plate
in north-
Caire
Triassic
in
the
in the Rif basin
seaindi-
cate the c r e a t i o n of an A t l a n t i c seaway between Europe and the Portuguese and n o r t h w e s t A f r i c a n basins.
~ O c e a n l c Crusl
I
[;--:--I ContinentQI
~
£picontJnenlal Seo
I Conliner~ol Margin Oceonic
Crusl
Contlnentol 5ubduc'tion
Subduction
Volconlsm
c)
_~ -
0
~%
-- -
--
_
~__4.
Of Block
Sea
Pontides
BQsin
BETICS +
~.
+
=~-
÷
÷
L----k~
I-"'"
-/
÷
~ ~..' .~Z,J~-__'% iIJTyrrhenion~Bosln
+
+
+
~
+
+
÷
+
~
÷
.I,-
-
+
"
÷
÷
÷
+
UJ/
'
"'
" ".~'.'
÷
Syr'le
÷
+
+
A
÷
~x,
,
~
Basin÷
Figure 9.1: Tectonic map of the (Redrawn from Biju-Duval et al., 1977.)
÷
Toor~e,s +,
+
> ~ 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) ' " ~ ×'" × "~, , , , ,
<~//~
"vk/~F~'k/-/_~
i IGNEOUS CORE
x
DE POSITS
SHALLOW& D E E P - - MARINELIMESTONE& MARLS (THICK)'%t , , I. . . . , ,
,
SHALLOW MARINE,.., ~ ',,
i
i
X X
#~---
/ TRIASSIC
X
x
x
\
i
\
i
i
,
i
i
i
~
i
i
i
REOBEDE, EVAPORITES. BASALTE UNCONFORMITY
i
1Ookrn
,,~
Figure 9.6: Schematic section along the High Atlas fracture zone of M o r o c c o showing how Jurassic carbonate s e d i m e n t a t i o n was influenced by their geologic and paleogeographic settings. (Redrawn from Warme, 1988.)
9.2.4 The Saharan Atlas The M o r o c c a n M i d d l e and High Atlas
continue eastward into A l g e r i a as the
Saharan Atlas, where Mesozoic deposition took place in a more marine milieu
(Fig.9.3).
Here
the
Mesozoic-Cenozoic
sequence
is
probably
Up
to
541
o~
U O3 0
-o m
"E
s
o ~
u
°M
" cl
!
o
& E
L) U 0 .,J
u u o
.IJ
o
®
,.~ ~n 0 ,.~
0 oM -IJ U I 0 U
u o u
o O00o 0
o o o
oo
u
o
o e~
,,-I
542
10.7 km
thick
(Caire,
1978).
dolomite,
and d o l o m i t i c marl,
sists
of
fossiliferous
Lower
Cretaceous,
The
Triassic
is
represented
limestone, and evaporite;
limestone,
a sandstone
marl,
and
sandstone,
limestone
clays,
the Jurassic conand
sequence
by
dolomite.
with
The
fossiliferous
intercalations,
is very thick. The Upper Cretaceous consists of fossilif-
erous
and marl.
limestone
marine
limestone,
marl,
An upward and
shoaling
sandstone
and
sequence w h i c h
conglomerate
is
starts with followed
by
Lower Miocene clastics and marl. This is overlain by Late Miocene to Quaternary clastics
that accumulated along the northern border of the Saha-
ran Atlas. Late
Eocene
to
Oligocene
folding
and
faulting
affected
the
Saharan
Atlas and caused d ~ c o l l e m e n t at the top of the u n d e r l y i n g basement which produced broad
E-NNE-trending
folds
(Fig.9.7).
The tectonic evolution of
the Saharan Atlas will be m e n t i o n e d again in connection with the Tell Atlas, of which the former was its foreland.
9.2.5 T u n i s i a n A t l a s The
stratigraphy
termed
the
Bishop
(1976),
sent
and
paleogeography
intermediate
chain,
Burollet
outline
is
this
region
were described
(1967),
culled.
of
and by Salaj
Salaj's
which
Caire
in considerable (1978)
paleogeographic
from which
(1978)
detail
by
the pre-
reconstructions
were
based upon rich and w e l l - p r e s e r v e d foraminiferal microfaunas. A
zone of diapirs
separates
the northern
part of the Tunisian Atlas
from the northern Tunisian Alpine thrust zone (Fig.9.8).
The Tunisian At-
las
and
terminates
southward
against
the
Saharan
platform
to
the
east
against the T u n i s i a n part of the Pelagian block. As shown in the generalized
North
(Fig.9.3)
African
Early
Tunisia was
Mesozoic
paleogeographic
reconstruction
situated right in the centre of the Triassic evap-
orite basin. The
Triassic
is
more
trending Tunisian trough nental
detrital
stones,
red
evaporites,
Triassic evaporitic in central consists
complete
beds and
with
syn-sedimentary
of m a s s i v e dolomites (Fig.9.9A)
were
Tunisia
and
in
the
NNE-
fossiliferous
volcanics.
A
marine
Middle
to
limeUpper
sequence rests upon continental and lagoonal deposits In the latter region the Upper Triassic
and associated clays and anhydrite.
located
During
a shallow to brackish water environment occupied
the edge of the Saharan platform. mentation
southern
intercalated
and southern Tunisia.
the Jurassic
in
(Fig.9.9A) where the sequence consists of conti-
to
the
Zones of subsidence with pelagic sedi-
north
in
the
Tunisian
trough.
Carbonate
543
reefs d e v e l o p e d on the Pelagian p l a t f o r m and in the M i d d l e to Late Jurassic s h a l l o w - w a t e r algal mats became ubiquitous
in the central region and
on the w e s t e r n edge of the Pelagian platform.
C 'Z
/
~,,e\ :,'~'~o~N"~>,
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
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~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~ <
. ~ rj ,-..-I
.J
.¢
•
L
v
,--I O u ~
z r~ < -r
o
v~
IN
~ m
e~w
U3 < _J
z
' g"g
.J _J
mCu
.0N~
.
~'l:J I..l .~1 ~ ' ~ ~,-t M
E~ ~ U
550
radiolarian
shales
were
deposited
in the
Rif
over
the
Numidian
flysch,
followed by renewed w e s t w a r d thrusting of the internal zones. 9.2.7 The Tell
Atlas
Palinspastic Reconstruction Unlike role
the in
Rif
the
regional
transcurrent
paleogeographic
movement
evolution
of
did
the
not
Tell
play Atlas
a
decisive
in
Tunisia
(Fig.9.11), although some east-west movements did occur along a JurassicCretaceous dextral wrench tween
the
internal
zone known as the V i c a r i a n
thrust
zone
and
the
Saharan
Thus, except for southward nappe transport, of the
internal
zones of the Tell Atlas
line
(Fig.9.12)
foreland
(Caire,
the tectonic units
are believed
be-
1978).
(Fig.9.11)
to have originated
from the p a l e o g e o g r a p h i c realms of a M e s o z o i c - C e n o z o i c North African continental margin.
This margin existed parallel to the p r e s e n t - d a y Algerian
margin
(Fig.9.12),
and
basin,
continental
rise,
margin
was
differentiated
evolution
highly of
the
Tell
had
recognizable
(from
furrow and shelf. Atlas
into
shows
north
to
south)
oceanic
The North A f r i c a n continental
troughs
major
and
ridges.
departures
The
from
tectonic
that
of
the
Betic-Rifian domain.
i;
+
"=:++ A,,+,,,
A,<+,,
......
i=,
,,-:, , . C ~ : . : : ~ - ~
. .-"
I ~ " ~ . Z . ~ ; ~ Z ,
+, l~-,,.',.i-or-,+~++,r+-,+.+ ,+y ~ : ; + . / , . + _,__,__,_'__: ~++
+
+'+. +
.. +:++':f % + ~
1" ' + ~t + ""+ , '+ . '
_
; =~.,+u'to~
++": ,++ ~ ++.. :, ", ,++ +%.+.++ •
"-> ~ '
<..,,,,<<>'<,~
~
V
Zones
~ 4 9 _ . /
.-" s SHELF
,,o:o
-
..o.+.7@ '
~
;
A+,,,, it o .
,<
o , T o,
¥
,.+',
.... tCarj
~ , , , ~ ~ , , . +
++/
o
CONTINENTAL
<<:/~
..
+"
~"~'~"
Pl<,t~,, l
F
R
l
C
A
N
P
l
A
1"
Figure 9.12: Palinspastic reconstruction of the m a i n tural units in the Tell Atlas. (Redrawn from Cair~, 1978.)
F
0
R
H
struc-
S t r a t i g r a p h y and Tectonics of Structural Zones This will be examined d e s c r i b e d by Caire
from north to south along the t r a n s e c t
(1973,
(Fig.9.11A)
1978). The Kabyle massif is a c r y s t a l l i n e base-
551
ment
overlain
formed
atop
by
oceanic
oceanic
in the C r e t a c e o u s and
troughs
Miocene
ridges
because
in w h i c h
molasse
The
zone,
Kabyle
underlain
flysch ian
thrusts
also
the
shelf
furrow
like
a
which also The
ian
consist beds,
of
the
beds
Numidian
and
stones. in the
zone
basin
shows
the
from the
by detrital Numidian
in the Senonsome
northward
the
these
in w h i c h
Cretaceous
the
zone,
a
was
closed
In
After
shales,
de-
Neri-
the
area
north
was
de-
rocks as from the end
During
the
to the
the Senon-
re-deposition
furrow
Eocene
ensued
as
Tellian
sands. to
active
limestones,
Late
sedimentation and
with
Tellian
with
the
frequent
folded and o v e r t u r n e d
folded
Eocene
sandstones
region
in the north.
the
served
the Saharan At-
continental
Kabyle
tectonically
again
with
despite
zone,
the A l g e r i a n
furrows
furrow separated
oceanic
zone
furrows
intermediate
times
it re c e i v e d
external
furrows
01igocene-Miocene
the
the
continental
Cenozoic marls,
folding
and
including
discordantly
phosphate thrusting
deposition upon
of
strata
the
of Cre-
flysch.
sources
furrow there
which
received
Early M i o c e n e
nated
in
zone,
With
The Tellian
sandstones.
foredeep,
This
flysch
Early
Lower
of the Tellian
the Tellian
The
and intruded with u l t r a b a s i c
detrital
flysch
taceous-Eocene South
since
olistostromes.
Oligocene-Miocene
starts
borderland
the ocean basin
as
however,
zone
furrow.
up to the
conformable
shell
The
radiolarites,
Oligocene-Miocene
troughs.
the A l b i a n when it was
Intermediate beds
or
lay to the south;
until
at the time
with
and o v e r l a i n
are,
flysch
external
accumulated
metamorphosed,
the
massif.
over the adjoining
thrust d i r e c t i o n
There
furrows
Intermediate
of the J u r a s s i c
Trias s i c
ridges,
Oligocene-
Kabyle
flysch
the
The-dominant south.
its margins.
las p l a t f o r m
north
the
which
existed
into
accumulated.
in
geosynclinal
including
continental
tic
formed,
an
active
along
(Fig.9.12).
features
broken
and sliding
and limestones,
or Tellian
depocentres
deposits
sequence
Such
was
clastics
thrusting
internal
of
zone
remaining
formation
margin
chain
rim.
above
consist
terrigenous
the
continental
Bibanic
was
upper
strata
and shales.
the
contains
(Fig.9.11B) and
the
a Mesozoic
towards
of
plateau).
terrigenous
sandstones
in the Kabyle
South which
is
Miocene
sandstones
was
in
limestone
the Kabyle rim. rim
to
Mesozoic
(geanticlinal
by southward
by T r i a s s i c
Paleogene
and
the continental
thick
occurs
m a s s i f was d e f o r m e d structural
Paleozoic
during
and
Tellian
is filled
is an Early M i o c e n e with
sedimentary
fossiliferous
klippe
the main Alpine
directions furrow
of
slide
(A nappes),
nappes,
sand-
from
the
north
Figure
9.11B
phase.
most
those
basin,
and
and nappes
orogenic
while
molasse marls
of
that
which came
origi-
from
the
552
internal
zone
(C nappes)
contained Cretaceous
and
Numidian
flysch.
From
the M i d d l e M i o c e n e onwards the Tell Atlas has been experiencing uplift.
9.3 StraUgraphic Evolution of the Eastern Saharan Platform
9.3.1 S t r u c t u r a l F r a m e w o r k P o s t - H e r c y n i a n structural realignments and resultant east-west structures in the Saharan p l a t f o r m determined the major depocentres until the disintegration trends
of
Pangea
(Klitzsch,
structural
provinces,
transcurrent shelf
in
to
and
south.
Mesozoic
continental
low
which
seas
Dakhla basin,
Tunisia,
basins
from
the
sea level rise lie
in
Egypt's
classified
Libya,
and in
basins
are m a j o r
later m o d i f i e d
complex NE-SW
horsts, by
and
mostly
a
stable
Paleozoic-
north
(Fig.9.13A)
Egypt
the into
especially
(Kerdany and Cherif, unstable
shelf,
among
modified
which
are
the
(Fig.9.14A).
Mesozoic-Cenozoic wrench
during
1990). Major in-
sedimentary basins
and
basins
interior
(Fig.8.2). The structural inversion of the Early Mesozoic
platform
North A f r i c a n
structural
deposits and was i n t e r m i t t e n t l y overlapped by shal-
(1986)
fracture basins Pelagian
and
covered
the Misaha trough, and the Abyad basin
Clifford of
basins was
shelf
new
to the north with
stable
encroached
basins
shelf
created
Egypt consisted of two main
fault-bounded
The
periods of eustatic tracratonic
times
During the Mesozoic
an unstable
faults
the
Jurassic-Cretaceous
1986).
Tunisia
has
already
such as the Libya's
interior
during
mentioned.
Sirte and Egypt's
fracture basins,
by w r e n c h i n g
been
the Late
whereas
the
Cretaceous
Other Gulf
Dakhla
major
of Suez
basin was
dextral
movement
and d e f o r m a t i o n in the Atlas Mountains.
9.3.2 P a l e o g e o g r a p h i c Development
Triassic
From Sinai can
be
through the Gulf of Suez to Egypt thin Triassic
found
limited marine
with
Middle
Triassic
limestones
clastic beds
(Fig.9.15A),
e n c r o a c h m e n t over the A r a b i a n - N u b i a n
Shield
suggesting
(Kuss,
Triassic t r a n s g r e s s i o n was, however, more pronounced in Libya.
1989).
553
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MeSozoic and £enozoicmarineemb~,yme""'"'-=:" \.
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Mid-Mesozoic continental facies
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Upper
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• •
i
+..'/','-..~>~,'. '
".'
,
"\
Gu~.~ o~ ~ ? . . . : . ; : .
B
Figure 9.13: A, distribution of mid-Mesozoic environments the Saharan platform; B, paleogeography of the Late Cretaceous northern Africa. (Redrawn from Nairn, 1978; Clifford, 1986.)
of of
Jurassic
This
is the thickest
Sinai
(Fig.9.14B)
marine
algal
and most
with
carbonates.
which extended
mostly
complete
interval
lagoonal
facies
The Jurassic
in the northern and
accumulated
Middle-Late
mostly
part
of
Jurassic
in a depocentre
from Sinai through the present Nile delta to the Dakhla
554
A
- N-
S inal
fi a~ala
W adi
H ountaln
llena
!
-5-
I
Z
B
I
Figure 9.14: across Sinai.
Structural elements of NE Africa (A), and section (Redrawn from Klitzsch, 1986; Kuss, 1989.)
basin in the w e s t e r n desert. curred in the Jurassic of
the Middle
Jurassic
Several
of Egypt which
transgressive
(Fig.8.4),
reached
regressive
cycles oc-
the most extensive being that
Libya,
Tunisia,
and
Algeria
where
carbonates and fine clastics of similar age are known. The Late Jurassic-
555
Early Cretaceous was regressive with extensive alluvial sedimentation (Nubian sandstone) in Egypt (Fig.9.15B) and most of North Africa.
/
Hediterr c~nean Sea
~__5 -'_~
"_~
"
EXE3C:~_
~'
, i~ii~~
B
TRIASSIC iN EGYPT ~
Continental Tidalflats
~
ShaUow marine ntternating witfi •luvld deposits Aluvlal dep osits Marine shales JURA S "" ~ H a r i n e limestone SIC- EARLy
A
Mnrine
CRETACEOUS
%%_
"
:
•
II ~ ~ ' : . ' ~ _ . ~ l
II
I I ] Ir~ope, mari,, ,,, ,
I=,.,uo.o.I lll ~Fluvi.I
CENOMANIAN
Figure 9.15: Said, 1990a.)
-----
Mesozoic
= S,a,e with
C
paleogeography
bon~'~s
~_ j
CAMPANIAN
of
Egypt,
(Redrawn
D
from
556
Cretaceous
Periodic t r a n s g r e s s i o n s of the Neo-Tethys Ocean spread across the Saharan platform
and
became
more
widespread
climax in the C e n o m a n i a n - T u r o n i a n the Sahara
Continental ally
referred
mal
to
Jurassic-Cretaceous
as
the
all
Nubian
over most
strata
sandstone
lithostratigraphic were
(Fig.9.15B).
its
or
unit
deposited They
(Table 8.3), by
intertongue
northward
as
cycle,
fluvial and deltaic
with
Tunisia,
and
fluvial
nearshore
Libya,
in
southern Al-
rather than a for-
prograding
and phosphatic beds in Egypt
deposits
gener-
Intercalaire"
far as
and
marine
1987),
"Continental
these
northward
1990).
which contain carbonates
Peterson,
reaching
(Schrank,
of North Africa
Now regarded as the Nubian depositional
stones
progressed,
(Furon, 1963; Lefranc and Guiraud,
Late
accumulated
geria.
time
(Fig.9.13B) and is believed to have connected with the Gulf of
Guniea to the south
Group
as
(Fig.9.15C) when a seaway spread across
Algeria
sand-
systems
marine
facies
(Fig.9.15D;
9.16),
(Klitzsch,
1986;
1985).
N0ffhern Wod}~eno WodiQeno Qeno-Sofa~o Quse!r
Aswan
--bS to
Wadi Dakhel ~per [enomonion Cenomonian to ALbian Lower Carboniferous
Figure 9.16: Generalized section (Redrawn from Klitzsch, 1986.) Northwesterly
tilt
of
the
northern
across
Saharan
southern
platform
Tethys region c o n t i n u e d in the Middle and Late Cretaceous
Egypt.
towards
the
accompanied by
NNW-SSE rifting which formed the Sirte and Dakhla basins. The Sirte basin is a m a j o r C r e t a c e o u s - T e r t i a r y hydrocarbon
province c o n s i s t i n g of horsts
and
on
grabens
that
had
reefal
build-ups
Thick,
continental
quartzitic
sand.
which
a
major
influence
constitute
Lower
the
Cretaceous
sedimentation,
principal sands
Above the Lower Cretaceous
reservoirs
overlie
especially (Fig.8.29).
Cambrian-Ordovician
sands are thick Upper Creta-
557
ceous
shales with
taceous
thick micritic
(Clifford,
1986).
carbonates m a r k i n g
There are carbonate build-ups
within a c a r b o n a t e - s h a l e succession; Oil
is trapped
mainly
in
in the Paleocene
and Lower Eocene shales in the basin
centre pass upward into evaporites, shale.
the top of the Cre-
carbonates,
Paleocene
and finally into marl and
reefs
which
developed
on
the
Paleocene
and
crest of d e e p e r horst blocks.
Paleogene Maximum
Tertiary marine
extended
as
far
south
stable
shelf
tiary,
there was
stable
shelf
(Said,
1990b).
Dakhla
Shale,
folds
epeirogenic
The Paleocene the
Tarawan,
in
the
larger
conditions
during
in Egypt the
Late
the
In Egypt w h e r e
Cretaceous
cover in that
downwarps
and
(Upper Esna,
with
occurred
(Fig.9.17A).
a thinner sedimentary
where
The Oligocene in Egypt tinental
the Sudan
had d e v e l o p e d
Eocene strata carbonates
transgression
as
had
lower part
of
Early Ter-
region
than in the
deeper
depocentres
created
(Table 8.3)
and
is r e p r e s e n t e d the
Esna
such
Nummulites,
as
by the
Shale,
Thebes, M o k a t t a m and Maadi Groups)
foraminifera
the un-
while
are mostly
Aiveolina.
and
(Fig.9.17B) was deposited under p r e d o m i n a n t l y con-
with
fluviatite
facies
occuring
in
the
south;
a
n o r t h e r n shelf facies existed in which clays and m i n o r carbonates accumulated. Of c o n s i d e r a b l e are the p e t r i f i e d
paleoclimatic
forests
in which
cal climate and vegetation. pression
significance
Also,
a unique m a m m a l i a n
in the O l i g o c e n e
silicified tree trunks along the escarpments
fauna
Qatrani Formation
(Fig.9.17B),
suggest tropi-
in the Fayum de-
(Simons and Rasmussen,
ciated with s i l i c i f i e d logs in fluvial point bar and of the Gebel
of Egypt
1990)
is asso-
floodplain deposits
in a p a l e o e n v i r o n m e n t a l
set-
ting quite reminiscent of Karoo vertebrate localities in South Africa.
Neogene AS
from the Late
Egypt which
Cretaceous
culminated
pronounced
in the b r e a k - u p
structural
an event that
this
at
event work
Suffice
dramatically of
Egypt
to
affected
opened
state the
in the Neogene
the Red Sea grabens spread over
it
this
structural
(Said,
1990b).
and
that
place
this
later in tectonic
paleogeographical
After
the
Gulf
a marine
Egypt. M a r i n e deltaic
in
Shield and
is c o n s i d e r e d
juncture
in the Early Miocene,
large areas of northern
took
of the A r a b i a n - N u b i a n
the formation of the Gulf of Aden, chapter.
changes
of
frame-
Suez
and
transgression clays and flu-
558 vio-marine formed
deposits
Gulf
of
Suez
accumulated fluvial
in
northern
sedimentation
Egypt,
also
took
and
in
place
the
newly
(Fig.9.17C).
During a late Early Miocene regression the Gulf of Suez was isolated from the M e d i t e r r a n e a n
Sea,
and evaporites
formed
in the Gulf
of Suez,
later
extending into the Red Sea. Arid conditions began in the Late Miocene,
in
the course of which thick evaporite sequences accumulated in the Red SeaGulf
of
Suez
grabens,
and
the M e d i t e r r a n e a n
Sea
dried
up
(Hsu et
1973).
\ ?
(
?
.?,,~ I
u~mm ~.MA~z2°e , ~# ~
E3 Shelf deposits
~ Lacustrine . . . . . . .
~
%
(. o v';.. ~
I~lP°~t~ve ~
'~ r e Q $
~ L teas
PALEOCENE
Tayi \ / ,lakheil
~eir P~]She'funclerc°nfnenfa''rEIunci,,~", C
OLIGOCENE Figure 9.17: Cenozoic from Said, 1990b.)
7
MIOCENE paleogeographic
maps
of
Egypt.
(Redrawn
al.,
559 9.4 Evolution of the Atlantic Margin of Africa
9.4.1 Origin and Structure of the African Atlantic Margin Continental Moroccan duced
rifting in the Late Triassic
Atlas
some
(Fig.9.2A)
of
the
(Atlantic-type)
basins.
broad categories, torial
Atlantic
coastal basins
and
world's
the
in n o r t h w e s t Africa west of the
southward
classic
propagation
examples
As shown in Fig.9.18
namely:
of
of
marginal
rifting or
these basins
fall into four
the Northwest A f r i c a n coastal basins;
basins;
the
(Clifford,
Aptian
salt
basins;
the
pro-
divergent the Equa-
Southwest
African
1986). Each basin group has a common structural
style and s t r a t i g r a p h i c fill. Besides, there are m o d i f i c a t i o n s of the basic plan of the m a r g i n a l sag basins of the Equatorial A t l a n t i c by wrenching and by the construction of the Niger delta. Uchupi
(1989)
observed
that
the
African
Atlantic
marginal
basins
originated in Mesozoic rift systems that consisted of four main segments, each
separated
the rift
by oceanic
segments with
Northwest
coastal
fracture
lantic
segment
or t r a n s f o r m
the resultant basin types
basins
correspond
and Southern North A t l a n t i c modified
zones
(Fig.9.20)
contains
by transcurrent motion,
Northern
(Fig.9.19);
a different
as well
Combining
it is evident that the
to Uchupi's
rift segments
faults.
N o r t h Atlantic
the Equatorial At-
group
of basins
as the A p t i a n
that are
salt basins.
The
South A t l a n t i c rift segment includes the Southwest A f r i c a n basins and the basins along the margin of Southeast Africa Except
along
the
oceanic
fracture
(Fig.9.21).
shear
zones
which
the basic basin structure consists of half-grabens typical ture
pull-apart
zones.
zone has
structures.
Early Cretaceous
fragmented
into d e t a c h e d
blocks
Grabens,
shear motion
the basement some
1981)
further
to
in the
of which
steep scarps and basement ridges goke,
the
however,
have
(Fig.9.18). characterize
along
the
Equatorial
Ivory coast basin, subsided
as
in the Dahomey basin
east
separate
(Fig.9.20)
which
them,
These are the
frac-
fracture
for example,
grabens;
there
are
(Omatsola and Adealso
resulted
from
wrenching. In the South A t l a n t i c Mesozoic rift system w h i c h starts from the Torres-Walvis
ridge,
a
major
transform
margin
was
initiated
Jurassic along the roughly east-west F a l k l a n d - A g u l h a s tion
along
this
fracture
zone
fragmented
the
crust
in
fracture into
the
Late
zone. Mo-
north-south-
trending basement highs such as the Malvinas and Maurice Ewing banks, and
560
SALT DIAPIRS
--
--
- - L i
-qc
Benin Toga \ ~ t ' - ' - - ' J ~'~'~. r'--~./~ .) NIGERIA ,"',:~. GHANA "~ / / "7 ~ c ~ . .-- j
AMEROON
GABON NIGER DELTA MUD DIAPIRS
CONGO DELTA SALT DIAPIRS
F ~
salt
Diapirs
Mud Diapirs Over
ANCIENT ORANGE BELTA
&Km sediment
1-4Kin
I C. 6, NO.
M.
North i GUINEA •-
,.'..
'I /' I
t2-~ "
3
\
1 Under I Km ST. FRANCIS BASIN
Cuonzo Guinea Niger Delta Mocamedes
South
NIGER
","'.
,.;
(-
.
I~
GABON2CABINDA CUANZA ~OCAMEOES WALVIS -~.~, ,, s , I t - .,,. ;~1~.., . 3 ~-'
:.
-,.-. . . . ; ,': 3
/"~.::--."I~'--~I~-/: '- .';':::,,:".t:'
ROMANE$E E z/':. .,L~ - a,i :-; - - :11;.. ;. .~:! - " CHAIN F.Z.
/'..~!:"
~ :.. ;'".,..,'..?,
I
--
~
~ . ~
INEOGENE SEQUENCE L CRET.LMibEENE 2.PALEOGENE SEQUENCE 3.U, CRET, SEQUENCE 4. L, CRET, SEQUENCE ~CONTINENTAL U. JURASSIC SEQUENCE [~';?:c~BASEMENT
Figure 9.18: Broad subdivisions nental margin of Africa. (Redrawn Dingle, 1982).
ORANGE '
T ~:
~
CENOZOIC
~///////I VOLCANICS
of t h e e a s t e r n A t l a n t i c contip a r t l y f r o m E m e r y e t al., 1 9 7 5 ;
561
basins
including
the O u t e n i q u a ter
the Valdez
and Malvinas
sea-floor
spreading
and San Jorge
basins
in
South
plateau basin of South Africa
began
in
the
Early
Cretaceous
America,
and
(Fig.9.21).
Af-
these
structures
e x e r c i s e d m u c h influence over sedimentation.
rL-~)Diapirs; ~ Carnian- Norian evaporites /~ Diapirs; Rhaetian-Sinemurian / Hettangian evaporites k'L.~ /') Mesozoic marginal basins ~\ Mesozoic Rift basins , \~ s'~. ~. Basement highs \/~ Basement hinge CN Faults and fracture zones ~ .... Edge of coastal plain ,. Cross section [ .~, Lu-E/Hingezon'e 500 k m
I
//
,'
,"" '"
BRANCH
;
,"~egUibQ ~, Ossi f t
i*
PiSCF°~'JuYtru¢ Tacul?
9, ~'\C ~.,.~ t~
~5outhernNorlhAtlanti¢
Branch
Figure 9.19: Rifts at the end of Early Jurassic before seafloor spreading in the North Atlantic. AT, A a i u n - T a r f a y a basin; B, Blake plateau basin; BSFZ, Blake spur fracture zone; BT, Baltimore canyon; CN, Clinton - N e w b e r r r y fault system; CP, Carolina platform; CT, Carolina trough; EB, Essouira basin; F, Franklin basin; FP, Florida platform; GB, Georges bank basin; G-BL, G u i n e a - B i s s e a u - L i b e r i a plateau; LI, Long Island platform; MP, Mazagan plateau; N, Newark basin; SE, S a l i s b u r y embayment; SE, Southeast Georgia embayment. (Redrawn from Uchupi, 1989.) The p u l l - a p a r t margins between the t r a n s f o r m faults are c h a r a c t e r i z e d by a steep continental
basement hinge w h i c h
faces
seaward with
a relief
often e x c e e d i n g 8 km, and a marginal sag basin located at the base of the hinge,
and
(Fig.9.22A, the base
platforms B).
The
and
embayments
on
continental-oceanic
of the basement
scarp,
with
the
landward
crust
side
boundary
an attenuated
of
the
roughly
continental
the landward side and oceanic basement on the seaward side.
hinge
lies
at
crust on
562 Pre-existing rifted
zone
so
basement that
where
structures the
rifts
determined were
the
aligned
development
parallel
to
of the
the Pan-
A f r i c a n orogens,
e x t e n s i o n and crustal a t t e n u a t i o n was quite pronounced,
but
were
where
rifts
basin off Namibia and
not
doming
aligned
at
right
angles,
for
example
(Fig.9.21), extension has been limited. is
believed
to
have
preceded
rifting
the
Walvis
Crustal sagging in
the
Atlantic
(Uchupi, 1989)o
I
Oiapir, Aptian Evaporites Rift basins Marginal basins ~ P a n - A f r i ~ n fold belt [~Basement highs %
Hid-ocean
. , Basement hinge / Faults and fracture zones ~ ' Cross-section J , 500Km , l.l.. N Nigerdetta
[~
! ridge
Demerara
•
~
~mid-ocean ridge
~
L
NORTHWEST
.<x~-
~AFRICANPLATE
S
kx~XEquator,o.I fracture system .k~.~\ ~.~.~/" ~..'kk-. / tvorv coast basinOahomeybasih.~k-<~'~ 7
/"
NORTHWEST
SOUTH AMERI£ PLATE
. Calabar Fl~nk Cameroon Trend - Oouaia basin ~-Gabon basin
3urup~ arch Francisco "~A basin ~ Tucanusu! basin
A~... ~, % i~
SOUTHWEST AFRICAN % basin anza bssin
~Lundahig~~ZZ~ Mossamedes~~~- ~. basin ~~ - ~ ~ o%~
J_
o~nn ridge
Figure 9.20: Equatorial Atlantic branch of the A t l a n t i c Mesozoic rift system in Aptian times. (Redrawn from Uchupi, 1989.) In their structure,
lithic fill, and hydrocarbon potential the evolu-
tion of the Mesozoic Atlantic jugate basins (Figs.
9.19,
rift systems produced
symmetrical
and con-
along the African and the North and South A m e r i c a n margins 20, 21; 23).
563
The s t r a t i g r a p h i c stages
in
their
tectonic
syn-rift deposits evaporites,
fill in these basins evolution.
The
is a consistent sequence
record of the
generally
such as continental red beds, volcanics,
overlain
by d r i f t - p h a s e deposits
comprising
begins
with
carbonates and
open marine
ter-
rigenous sediments which accumulated after continental separation.
Torres Ridge
• i ~B~sin
~
~--'~ Pre-Riff ~.sin s
..~v\"
B~sms "~J'~s. I~s~zoic Harginal B~S ~)~""[,% \~~ - DA' - . . / ~ \ [ ~ l P o n - A f r i c a n fold belt 4-
4-
4-
444.
44-
"
,,
+
~-
+ + _..,.~,
Tondi|
Hi gh
~,
NORTH ~,3 PATAGONIA HASSIF I
~asin C>
500 Km
~
~n.alv,phs\:, ,,~\i-,-,-)'-q. ' /-
<
~0.~
~'~
,,
",-~,,~
S~AD,0~. ~ .
MASSIF-'-"
-
'/ ~ ' / ~
V-",,"
.
-
'
<,-
,.
"~ - ~' ".4,~ ~ ' %~,~,%'~ , , , -,, - ,) \
-'" /'"
~,0~
Cross section
-
;~ Agulhas
,
........ ram=e-Basement Hinge
~q'
"
,:,,',
/
v'/ ,
EAST ANTARCT]CA
i ' - ,' , '_'//ANTARCTI[A
',
-,
,,-;
"1
Figure 9.21: South Atlantic branch of the M e s o z o i c A t l a n t i c rift system in Late Jurassic before initiation of rifting in the Luderitz arch area. (Redrawn from Uchupi, 1989.)
9.4.2 N o r t h w e s t A f r i c a n Coastal Basins Several basins occur along the n o r t h w e s t e r n margin; saouira, Agadir, Aaiun-Tarfaya, Von
Rad
et ai.(1982)
and Senegal basins
and W i e d e m a n n
(1987)
these include the Es(Figs.
furnished
counts of the sequences in these basins w h i l e C l i f f o r d synthesis on their p e t r o l e u m geology.
9.19;
9.23A-D).
stratigraphical
ac-
(1986) p r e s e n t e d a
564
In the A a i u n - T a r f a y a rites,
and basalt
late rift
basin
extruded
Jurassic
(Fig.9.23C)
in the
clastics
typical
late tensional
and carbonates.
rift clastics, phase
These
clastic
sequence
in
which
succeeded by
are o v e r l a i n
drift Late Jurassic clastics and marls and carbonates; Cretaceous-Tertiary
are
the
evapo-
by early
and succeeded by a Paleocene-Eocene
is
g e n e r a l l y u n c o n f o r m a b l e upon Cretaceous deposits.
V,' OKmi~
E
~
I
200 i
~
'
600 RRA
'
600 ,
E 500Kin
W 0
VOLCANIC7-SY":rIN S I ?
L
0Kin
E 200Kin ,
~--
1° F OUTER ~
RIDGE
\
[
~"
k._
J:!'
A
l
/~y._
RIF~
,v ~BASIN~LKS, U,~ J OCEANIC 41-- CONTINENTAL
W 0
WALVI5
E
CENOZOIC \ (,00
200
600
I
BOO
1000
w 0
1200Km
BASIN Z l,OOKm
200
D SEDS. CONTINENTAL-+,O ,. CEANIC COLORADO
t
BASIN W - - - ~200 .----~
0}(111
'
........ / ~,~ISYN-RIFT
~-
_
1,00 ,
...... '"
B
SEDX"C"TGI
LK&U/~ OCEANIC ~H,,CONTINENTAL LUDERITZ ARCH l _ _ 600 l
800Km I
l
"
L
V ~.
.
j-.'f
DUTCR "")(-._~I[:~CCNTRALRIDGE
~
RIDGE M~N
OUTERBASIN
IISYN-RIFT SEDS.
BASIN LK~U~
OCEANJC"~CONtJNeNTAL ORANGE
BASIN
Figure 9.22: Cross-sections through the South A t l a n t i c branch of the M e s o z o i c A t l a n t i c rift system. (Redrawn from Uchupi, 1989.) The Senegal basin, contains starts
over
with
Cretaceous
6
km
Late
the o v e r l y i n g
the largest onshore embayment in northwest Africa, section
Jurassic
detrital
more calcareous
of
marine
sediments
(Fig.9.23D) dolomitic
overlie
in
the
Jurassic
onshore
part
Offshore,
Early
evaporites,
while
lithofacies developed on a western carbonate platform.
Late
Cretaceous-Tertiary
east i n t e r f i n g e r w e s t w a r d with marine regression
Early
which
limestones.
occurred
sequence
continental
terrigenous
deposits.
at the end of the M a a s t r i c h t i a n
Early T e r t i a r y transgression.
before
beds
In
in the
A pronounced a widespread
565
NNW
5SE
C ° n o , o i ~ ~ [~p~'" Permo~ Oceanic ~ s C o n t i n e n t a l
I M azagan F:: W "
5yn-rift seds Crustj
High Atlas Ranges
~E
Crust q l l q l P r o v ~ ~ ' " Syn~rlft..seds. ", ,nass~cj.- Conhnenta~ m a n t ~ ~ Paleozolc Crust Essauira" NW ~/E of Canary Is. Ridge Canary~~__---------~'~.,,
B
5E
+-T1-,+,]],] J
+ |
l
moo.° ? ~ Co+,+ne.,ot J Aaiun-Tarfaya'?'~rust
NW
C
SE
F'Car bonate J7.1km/s--/ Platform I .~_~t / cont.crust& .~ ~"--....syn-rift seds Oceanic " Cont~k : :.]
Is yet 3
Northern Senegal Sub- basin'~ W
9akar
Pateogene
t Jury / "'+Basaltic O~anic Extrusions I Crust
Figure 9.23: lantic branch.
Cross-sections through the (Redrawn from Uchupi, 1989.)
E
D
northern
North
At-
566
Seibold basin
of
(1982)
outlined the stratigraphical
Morocco
comprises
red
and
showed
detrital
that
the
Early
sediments
and
lagoonal
h i s t o r y of the Essaouira
Triassic
syn-rift
clays,
deposits
evaporites,
and
basalt flows and dolerite dykes, with thicker evaporites d e p o s i t e d in the early Jurassic.
After the evaporite phase a c i r c u m - A t l a n t i c transgression
occurred which is known also in eastern North America.
This was the main
phase when c a r b o n a t e platforms containing algal and coral reefs were constructed caused
all around
the margins
by the cooling
of the
of the North Atlantic.
lithosphere
Rapid
subsidence
(Onuoha and Ofoegbu,
1988)
al-
lowed the a c c u m u l a t i o n of over 5 km of shallow-water carbonate section in the
Essaouira
basin
the deep ocean deep-water monites,
(Fig.9.23B).
Starved
side of the carbonate
argillaceous
but
basin
conditions
platforms
pelagic
reddish
prevailed
on
and caused d e p o s i t i o n of limestones
containing
am-
and distal turbidites.
Uplift of the High Atlas,
starting in the Early Cretaceous,
supplied
o v e r w h e l m i n g amounts of clastic sediments with deltaic progradation which suppressed c a r b o n a t e deposition°
Mid-Cretaceous
(Barremian to Cenomanian)
p l a e o - a n o x i a w h i c h was w o r l d - w i d e due to r e s t r i c t e d oceanic caused the d e p o s i t i o n of carbonaceous excellent
hydrocarbon
source
rocks.
organic-rich black The
Upper
circulation,
shales that are
Cretaceous
is
an
anomal-
ously reduced interval in the North Atlantic marginal basins probably because
widespread
marine
Cenomanian-Turonian Thickness
and
reduction
the
could
d i s s o l u t i o n in the ocean, temporaneous quence
sediment
consists
of
level
with
Coniacian also
have
into
greatly been
the
Sahara
limited
due
to
during
sediment
enhanced
the
supply.
carbonate
or to the fact that there p r o b a b l y was penecon-
erosion silty
O l i g o c e n e turbidites; sea
transgressions
by
deep-sea
clays
and
currents.
claystones,
The with
Paleogene
se-
Paleocene
and
the latter was intensified by a p r o n o u n c e d drop of
concomittant
slumping,
and
cutting
of
submarine
canyons.
Because of arid climate Neogene deposits in the Essaouira basin form only a thin veneer. On the M o r o c c a n continental margin the M a z a g a n plateau a platform mantled Jurassic
on the landward
shallow w a t e r
carbonates
sive area with evaporite dipairs seaward
off
the basement
Paleo-oceanographic
hinge
side by Triassic suggesting m a r i n e
invasion.
overlain by continental along
interpretations
the w e s t e r n
(Uchupi,
1989)
(Fig.9.23A)
red beds with
edge
is
Early
An exten-
red beds occurs of
this
plateau.
of the Early Jurassic
d r i f t - p h a s e black to gray carbonates and argillaceous facies on the Mazagan plateau
reveal
that
the North Atlantic
Ocean was
up
to
500 km wide
and about 1,000 m deep with an estuarine-type v e r t i c a l l y stratified water mass w i t h slow b o t t o m and intermediate circulation.
567
Off
the
coast
islands b e g a n Cape
Verde
of
northwest
in the
and
later
Canary
caniclastics
to m a r i n e
of t u r b i d i t y
currents
ing submarine
Africa
the
Paleocene
Island
and
growth climaxed
archipelago.
sediments
these
uplift
from
chains
deep-sea
of
volcanic
in the M i o c e n e
Apart
island
and also confined
and
in the
contributing
diverted
currents
the
thus
volroutes
intensify-
erosion.
9.4.3 Equatorial Atlantic Basins The
Liberian
this
marginal
region
of
the Gulf
of the
Ivory Coast
eastern
basin. ward
of Guinea
ior
basin
coast
Although
extensions fracture
delta,
of
to
it
is
originated
it
the
basins,
westward
through
the
is
only
which
exception are
is
continuation
transcurrent
zones,
not
entrance
sometimes
trough
included
here.
into
Benue
underlies
the
movement
the Benue
the
of
in
extension
w h i l e the Keta basin w h i c h
the
fracture
hence
across
north
The Tano basin is an e a s t w a r d
(Fig.9.20),
of oceanic
sits
the
marginal
basins.
Ghana
basin,
which
basin
wrench-modified
termed
the
sag
Dahomey
along
land-
is an inter-
Rather,
the
trough,
is
Niger
consid-
ered. The s o u t h e r n m o s t flank This
is
located,
feature
separates
salt basins.
The
Niger
and
delta,
grabens
which
fossiliferous featheredge The ridge
the
Calabar is
of
Equatorial
flank
constitutes by
a
system
Albian-Maastrichtian flank
the Calabar
between
this
flank, basin
the eastern of
manner
trend
Aptian
into
and
the
are
sits
Douala
hinge
the Niger
beds
creating
from
NW-SE-trending
the Calabar (Fig.9.20). the
Aptian
line
of the
horsts
delta.
exposed
and
Highly
along
the
1987). on
evaporites
thus
trend
basins
(Nyong and Akpan,
volcanic
barred
is w h e r e
Cameroon
Atlantic
marine
Cameroon
of Guinea the
in a step-wise
effectively
the C a m e r o o n
north
descend
Cenozoic
from e n t e r i n g
of the Gulf
underlain
of the Calabar
which
continuity
embayment immediately
an
of
a major basin
oceanic
the
South
aseismic Atlantic
stratigraphic
immediately
dis-
south
of
trend.
Liberian Basin
This
extends
Leone
to
reveal taceous to the
the
from
Liberia
up
to
the
8 km
of
continental Farmington
Edina
southern
(Fig.8.2).
coast
Off
the
sediments,
conglomerates, Formation.
Sandstone
Formation
but
Guinea
Bissau
coast
onshore
only
2 km
with
plant
Tertiary
also
exposed
through
geophysical
sandstones
Thinner are
of
L i be r i a n
continental along
the
of
Sierra surveys
mostly
remains
Cre-
belong
sandstones
of
coast.
in
But
568
Sierra Leone the marine to estuarine Tertiary Bullom Group, m thick,
at least 100
consists of sands, kaolinitic clays, and lignite beds which also
bear plant and fish remains.
Ivory Coast Basin From available subsurface information 1987)
in
offshore
sediments nounced
Ghana
(Fig.9.24B)
of
unconformity
of A f r i c a
there Late
is
up
Jurassic
of Middle
(De Klasz,
to
2 km of to
Early
Cenomanian
age
1978; Mascle et al.,
basal
rift
Cretaceous
represents
continental age.
the
in a basin that was surrounded by Early Cretaceous America
from Africa
Cretaceous-Tertiary bidites
that were
and
occur
gas
pro-
from South America and the end of rapid syn-rift sedimentation landmasses
and proba-
bly c h a r a c t e r i z e d by fast sedimentation and rapid subsidence. South
A
separation
sequence
proximal
(Clifford,
shallow marine sands In the
Tano
caused
the
deposition
of the marginal and distal
1986)
of
the
sag basin
to prograding
in the
turbidites
Drifting of
overlying phase,
deltaic
(Belier
Upper
with
tur-
wedges.
field)
Oil
and
in
(Espoir field) as shown in Fig.9.24B).
basin
the basin
fill
is
referred
to as
the Appollonian
"system" in w h i c h there is a basal nonmarine Lower Cretaceous overlain by mostly marine
limestones,
sandstones
and shales
ranging
in age
from AI-
b i a n - C e n o m a n i a n to Eocene. A Santonian regression is m a r k e d by conglomerates
and
bituminous
Nauli L i m e s t o n e The
effects
Ivory Coast
sandstones,
(Wright et al., of
basin
locally
transcurrent (Mascle
overlain
by
the
Maastrichtian
1985). deformation
et al.,
1987).
are
very
Seismic
noticeable
lines
across
in
the
the off-
shore part of the basin show tilted sequences w h i c h
fill huge asymmetric
grabens
the
with
possible
Other structures formed
include
sedimentary
(Fig.9.24A).
local
This
and ridge
syntectonic
faults, basement extends
folding
in
exposed continental wedge
known
eastward
as
along
basal
sequence.
basement;
and a de-
the
Ivory
Coast
the continental
far as Ghana where continental
basement is exposed,
and southward
comes
basement.
Coast-Ghana
part
of
exposed
oceanic
The
Ivory
ridge
slope as it be-
ridge
is
regarded as the landward continuation of the Romanche fracture zone; this ridge
is
located
along
the
continent
oceanic
crust
boundary.
It caused
the landward ponding of sediments along the continental margin. According to Mascle et al.
(1987) the ridge shows evidence of both tensional
(asymmetric grabens) ing
folds
that
die
and compressional deformation w h i c h out near
the
continent.
been c o n t e m p o r a n e o u s w i t h progressive
Deformation
includes appears
faults shearto have
shearing when a deep basin was de-
v e l o p i n g b e t w e e n the A f r i c a n and South A m e r i c a n plates.
It ended abruptly
569 .<' ~
Abijo~ . ~
-L-/.
.'
:~.,'Y
~
;~'~;':::'-
~
.......
"C,6
..
,'.,.::~.r5
•
,~
,:S" ":"/.2;
->~. ~.
:"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<°QSth~ t ',11Mesozo ic ° ~ P a l e o c e n e ~'~JContinentQt la ~ " (Imoshole) ~ C e n o z o i ¢ Poleocene L~ PI ei sto c ene? co .: L:.,Ewekoro)
~ °
GULF OF" GUINEA
L.
| o~ ~Plioceoe-PleistoOe°e/~ ~ooosta,> ,_~
"
<
A
m a p of N i g e r i a (A); B, Recent (Redrawn from De Klasz, 1978;
573
.......,,,,,,,, ,,,
,J,,,
I AB
~
%/
,
M
\/ ~
(
\~-~~--~
.., \
I~ h / ~ / / ~
AS
AM A j o l i it. Mamu Fins /~ IN I m o & N k p o r o Sh I~e ~
~
/
Embayment "
~/
AB Abeokula Fro.
~
NE Nk~(agu Fm.
"~
KK Upper Cretaceous Un-
/
" " ~ / . tkpe-le ~ 1
~
A Mfamosing Limestone AS A s u R i v e r Group
,)
~"J'~, C / ~
" J ~
/'-J~
.n,o.,oo.
.,.
~
,E ' ~
~
:
~
,
~
'
'
-
,
1.'.".',:1 Rara~ic Sand,s,t ~
~
~
.
~
.
.
~
W._:_'_~-tT~-
Limestone
Fz
Oceanic
Shore
z
e.
Oligocene Miocene Shore Late
Shore
Early Middle
Eocene
Ish? re
,
Updip limitof Eocene 1 roliovers and growth f a u l t s { [ Anambra Basin ] Delta Complex i
I
~ t ~ enin a ./.~.i
~ " Continentalslope channelfills and
B
Eocenej Shore
Zone o f f a i r l y regular rottaver slructures
Timestr(Itlgrophlc unlt
turbiditic f a n s l n t h e A k o l c t
B
Fz
I "=- - l- -=
"
Charcot
Crust
j
~
~.~ D•--';'t
T2,,'~,
~ \ / ' /%
Zone of clay diapirism l Zone of collapsed I I crest structures Thesediapirs may have and back-tol very tittle reservoir back faulting J sand in structures I Coast Line I
/ \ ~
O -'I~5E
~ont,oeoto, crust
at FQ'--m--i-'~.-::'# "
Chain / ~ ~
........
~-'~'L-~'_ CFlank o l - - j,0Oban Hitls H
'""~"
~ . . ~ ' ~ , ~
,
Shale
~
~
~
Plio-Plelstocene
SSW
Ikang Trough
.iger 0.,,o C o - -
1 '
~
100 Km
A
OahomeyBasin "I"
A
A~/'~
/
Ituk High
1-
/ /:
ltuk-"2 ' ~ A IJa~aoar
/
~
,
"~
~
formation
-
1
C , ~ , / ~ .< ~- - F - "- - r ~ 7
NNE
-
C
M&Zn~>~
" Transitional oceoni'c*"
~continental
crust
100
Km
Figure 9.26: G e o l o g i c sketch map of southern N i g e r i a (A), and c r o s s - s e c t i o n s (B,C). (Redrawn from Reijers and Petters, 1987; Whiteman, 1982.)
574
Natural Levee ~
Fluviotile Sackiwamp Lagoonat Sediments
~Barrier
and
Sat Sated
~
Barrier Foot (inferbedded sand, silt and clay)
~
Marine cl(~y Transgresive Deposits
~
Environment
Lower floodplain
Mangrove swamp
Beach
River mouth bar Z 5 o~ to. < D e l t a - f r o n t piafform
Lcmgsh
Curre
/
/
/
/
....
....
,o,
Environmental Characterisf~cs
Channels and point bars, mottles, backswamps, Cut-off channels; similar" to baakswamps.
Strong reversing tidal currents, tidal flats
Channels and point-bars: Halnly layered cross-stratlfied f to v , c . sand and organic-rich silty clay, Abundant drifted plant debris. I n t e r - c r e e k flats and i n ~ r - s w a m p deltas
Strongwave attack on active beaches shore currents diverging tram delta tip, Soil formation and plant growth on beach ridges
Delta tip:Mainly e v e n l y laminated clean f, to m. sand.
Very strongwave action and r e v e r s ing tidal currents, Longshore current, Energy conditions decrease in~and and seaward from bar c r e s t with increase in depth,
Crests: Mainly clean, v.f, tom, sand with even Iamination cr oss-strati flcation, or c u t - a n d fill. Bar f l a n k s : Layered v . f , sand clean v.c. silt and clayey silt. Drifted plant remains s o m e times in thick layers
S t r o n g f o moderate wave and tidal current actton. Longshore currents. Guinea Currents. Rip c u r r e n t s . Energy c o n d i t arts decrease from shoreface tO outer edge.
Delta tip:On inner p l a t f o r m c o a r se v. f. sond, and v.c. sltt with e v en laminations= On outer platform layered v f, sand, v . c ~ s i t t , c l a y e y silt, and silty cloy with plant debris
Moderate toweak wave and tidal c u r r e nt action. Guinea Current.
v. f. sand, v. c. silt, c l a y e y sift, and silty clay, Coarser layers with ev. en lamination, cross-stratification Plant debris and mica flakes, Common to abundant mottles, D e l ta f l a n k s : L a y e r e d v.c, silt c l a yey silt and silty c l a y in shallower parts. Uniform fine claye~/ silty clays in deeper areas. Abanaant mottles.
Weakwave and tidal current action Deep ocean currents of unknown strength flowing northward over shelf edge,
Delta tip: Rq~e l a y e r e d v. c. sill, clayey silt and silly clay, Mainly uniform fine clayey silt and silty clay. Abundant mottles and pelagic foraminifera. Delta flanks: Mainly uniform fine silty clays. Abudant marries and pelagic Toraminifera.
Weak wave and tidal current action, Strong to moderate action inshore No or v e r y slow deposition of suspended fines, Abundant benthos Organic debris c o n c e n t r a t g d .
Mainly mottled v.f. tO v,c.quartz sands largely out of equdibr~m with prevailing c u r r e n t rand]lions Shell debris, Olauconite foraminfera, and clay-silt increase from shallow to deep water Partly Late Pleistocene in age. Dep0Sits inte(preted as of strondplain or~gm.
C3 <
Pro-delta slope
Open shelf
O- < Non -deposi tional
Lithofacies
Strong currents in channels, meander migration, periodic flooding of topstratum levees and backswamps, abundant plant growth
of the Recent Niger Figure 9.27: Sedimentary environments (Redrawn from Weber and delta and lithofacies characteristics. Daukoru, 1975; Whiteman, 1982.)
575
pressured shale towards the continental slope. Deltaic growth d e c l i n e d in the
Late
Pliocene-Pleistocene
sediment b y - p a s s i n g flooded
the
during
into deep-sea
Plio-Pleistocene
a
major
drop
in
sea-level,
fans. A Late P l e i s t o c e n e
offlap upper
and
with
transgression
lower deltaic
plains,
and
as sea-level stabilized, a new regressive offlap sequence developed. D e l t a i c - f r o n t sands account for the bulk of the N i g e r delta hydrocarbon production.
These
sands
significant
are
also
provide the traps.
interfinger
with
pro-delta
reservoirs.
Growth
cated
offshore
beds.
and
Turbidite
shale
diapirs
Growth faults are v e r y common in the Niger delta with
primary and s e c o n d a r y synthetic and antithetic distal
source
faults
part of the delta,
faults
(Fig.9.26C).
petroleum accumulations
In the
are also
lo-
includes
the
at the top and flanks of shale diapirs.
9.4.4 Aptian Salt Basins This
group
Douala,
of
basins
Gabon,
basins
which
are
(50,000 km 2) surfaces,
and
are
extends
Congo,
narrow
the
more
from
Cuanza,
Cameroon
and
embayments
Cuanza
basin
extensive
to
Mossamedes
Angola
(Fig.9.20).
These
with
the
Gabon
basin
having
the
largest
onshore,
(22,000 km 2)
offshore
and
basins
because
structurally,
the
consist of h a ! f - g r a b e n s which are m o s t l y d o w n - d r o p p e d seaward. basin
(Fig.9.28),
land
basins
The Cuanza
is however better exposed, with the Late J u r a s s i c - E a r l y
Cretaceous to Q u a t e r n a r y sedimentary fill o u t c r o p p i n g in a belt about 150 km wide.
Otherwise,
vegetated
coastal
the
plains
subaerial
parts
of these
in the northern
basins
part to
range
low-latitude
from
low
desert
in
Angola. C o n s i d e r a b l e stratigraphic u n i f o r m i t y exists in the A p t i a n Salt basin (Fig.9.29).
Above
the
faulted
crystalline
group, as the case m a y be in some basins
basement
Cenozoic succession comprises three distinct units: quence, Klasz, ceous
overlain 1978;
Cocobeach Uchupi
followed
1973).
the
by
Karoo
Super-
the Mesozoic-
a basal n o n m a r i n e se-
normal
marine
strata
(De
The basal J u r a s s i c to Early Creta-
and lacustrine rift sequence are v a r i o u s l y termed
"Gr~s
(Douala basin), M'Vone Sandstone and Marl, N ' D o m b o S a n d s t o n e and Series
(1989)
anoxic
filled w i t h
(Gabon),
deposition
lowed by rifting deep
evaporites,
Franks and Nairn,
fluviatile
de base"
by
or
(eg. Gabon basin),
lake
and of
clastic
and subsidence formed
organic-rich
the
which
Cuvo
Group
sediments
(Cuanza). in
the
Jurassic
in the Early Cretaceous, extended
dolomites,
green
from
Angola
shales
and
According
to
was
to fol-
d u r i n g which a Gabon,
and
carbonates.
the second rift pulse which began in the B a r r e m i a n the initial
was
During
rift,
now
broken into a series of secondary basins, was filled w i t h n o n - m a r i n e car-
576
bonates
and
the basin
subsided,
dolomites. tions)
shales.
The
region
was
and evaporites
A transgressive
accumulated
before
sheet
invaded
by the
were d e p o s i t e d
sand
(Gamba,
the formation
sea
from
which
the
grade
upward
Chela and Upper
of A p t i a n
south
as
into
Cuvo Forma-
salts.
z < l.u (o o
z
.< ..i
cc
<
Recent Pleistocel M i o c e n e {l M i o c e n e fi Eocene-
Paleocen( SenonianTuronian Cen oman~ Upper A[ (I) Dondo Lower All (I)Dondo Aptian re-Al~ti ecambr
Figure 9.28: from De Klasz,
The cycles cause was
Geologic 1978.)
evaporites, and
mostly
show great
of d e p o s i t i o n
restricted
by
sketch map of the Cuanza
halite
horizontal
in a large local
and
basin
barriers.
carnallite,
uniformity
were
over distances
in w h i c h
The
basin.
the
evaporites
inflow in
the
(Redrawn
deposited of
in
100 km be-
of marine water northern
basins
577
contain
a
higher
sulphates
than
much more
direct
the n o r t h e r n trated. parts
those of
is
potash
salts
difference
basin
between
salt basins
overlain
by
a
and
to the south.
to sea-water which
the evaporite
the A f r i c a n
evaporite
of
in Angola
access
part
Another
of
proportion
the
was
d o l o m i t e with anhydrite horizons
lower
from the
was more
(Angola)
in the
sequence
south,
confined
southern
in which
proportion
of
This was p r o b a b l y due to a
came
that
marine
a
whereas
and
concen-
and
northern
former area,
the main
containing
there was
limestone
and
renewed formation of
halite and anhydrite. Anoxic of
the
conditions
evaporites.
prevailed
This
was
in the Late Aptian,
betause
the
entire
after
basin
the
was
formation
closed
to the
north and isolated from the ocean to the south by the T o r r e s - W a l v i s ridge (Fig.9.20). carbonate Lower
This
Albian,
anoxic
to oxic ridge
unconformities basins
the deposition
the
marine
The
which
were
a result
of
by
outline
it
the
setting,
of
the
sequence
a few c a r b o n a t e and
apparent
reefal
transition
the b r e a c h i n g
subsidence is
Massive
in the upper Aptian-
during
and marl with
from one rift
clays.
highs
Cretaceous-Tertiary
caused
stratigraphic
originated
as
sediments,
Upper
clastics
of varved
atop basement
anoxic
conditions,
barrier.
fossiliferous this
to
were built
above
Walvis
From
led
platforms
Torres-
is
the western
the
parts
mostly
cycles,
sea-level
that
from
and
changes.
Aptian
Salt
of w h i c h
now
c o n s t i t u t e the Brazilian coastal basins. The
structural
and
stratigraphic
similarities
between
the
Mesozoic
sequence in the Aptian
salt basins and the Brazilian coastal basins have
attracted considerable
attention over the years.
ties exist among n o n m a r i n e and marine ostracodes 1966),
ammonites
Taquet,
basins
1972)
Reconcavo,
Camamu-Almada,
similari-
(Kromelbein and Wenger,
and vertebrates
1979). The Brazilian coastal basins
Alagoas, Santos)
(Reyment and Tait,
Strong faunal
(Buffetaut and
(Recife-Joao Pessoa,
Jequitinhonha-Espiritu
Sergipe-
Santo,
Campos,
share the same depositional history with the African A p t i a n (Ponte
and Asmus,
1973;
Uchupi,
1989).
The
rift sequence in Brazil consists of alluvial sands,
Late
Jurassic
salt
infra-
l a c u s t r i n e red shales
with sand interbeds and sandstones; o v e r l a i n by N e o c o m i a n r i f t - p h a s e fluvio-lacustrine Geral
Early
rift unit. carbonates, basins was
all
lesser
basins
sandstones,
Cretaceous These as
shales
volcanics
are overlain in
the
African
subsided when subsidence
and
by Aptian basins.
sea-floor
in
at the n o r t h e r n end.
the
some
(9.22E)
limestones,
constituting euxinic
shales,
Whereas
the
spreading
began
Sergipe-Alagoas
This p a l e o g e o g r a p h i c
and
with
the
Serra
the y o u n g e s t
syn-
evaporites,
southern
Brazilian
in the Albian, Recife
Joao
there Pessoa
situation h i n d e r e d
c o m m u n i c a t i o n between the North Atlantic and the South Atlantic.
and
free
The con-
578
nection was, however,
established in the Turonian when the Equatorial At-
lantic ocean opened. BAS CONGO
G AB 0 N
Maestrich.
~
............
b
.
>
~...'...
~
C UA N Z A
• MQnz~dT ~.~
~
-'--S .~
~5.'.'.'..':
,nhob,--<
- i~=~oo7~'-: :.
.
. c° Cenomomcm ~ % u g e
~
:....'....
" .
.
~
?,, :.;;:.!.:
"~'"'"
".:
j,.
S
I ;
.
--T~b°~.
,,onCe
~ .?. '<>,,;: , : . .... ~A=it~- , . . ._,~:.:,... ,~
T,~on~on
MOSSAMEDES
.
.
.
~.'..y
.
;.'? Z "." ".
,~
"~.
. . . . .
'~
•
,..,-."":"
:":
?
. . . .
,
^,b,oo
~Lo~"'I' -- • ~5°-i,. ~'~ Ro,,b~,! , " .........~
~':'''-':'
~', ".:.1. : .....
~C6co~.£c..h;
•
.
.-
¢
. . . .
~he,o~
~ J Co ~ ' "' . ' " '
• .'~Pt~. Indlenne., '' Bucomax'..:.-/..~ .~" ,.LOa~gO. ...... '. ".,
"/'//~C'uV'o.Y/.~
.'.
" . - ~ / ~ ' v o ~
.
•
~
"
~o+~'-,°~
. . . . . .
car[- Cf-t "*q"'~'t~, ~ • ,''r'~q~' ' -'? " }' = 'Cocob@(~ch'h'.' Lcde Jur ~
[
..
--°
. -
•
,
"/////////~
Evaporites
~
Continental Fclcies
Volcanic
~
Nerilic Facies
Strata Absent
~
Basina~ Facies
Figure 9.29: Stratigraphic correlations of m a r g i n a l basins. (Redrawn from Franks and Nairn, Clifford petroleum and
(1986)
producers
turbidite
post-salt
have
that as
(lacustrine)
marine
(Fig.9.30).
showed
The
carbonates
the
Aptian
hydrocarbon sands, and
transgressive
intertidal sand,
Cuvo also provide excellent reservoirs. role in p r o v i d i n g
structural
traps
basins
reservoirs
pre-salt
sheet
salt
South 1973.)
channel
which
pre-salt
freshwater
the
Atlantic
or
Gamba,
are
major
fluviatile
carbonate estuarine Chela,
and
reefs, sands Upper
Salt m o v e m e n t has played a major
(Fig.9.30),
e s p e c i a l l y where
the salt
579
W
E __
5 ~-
-=--_~-
2"
==
=~__~'__..____
:
.
.
.
.
.-~.......:.::.'..:...'.'..'.-:
.-...-.....
____. _"....
L
WEST
WEST
.
M A L O NGO
MALONGO
A
2Skin
EAST
8
W
i5
~
o
U - Upper, Eo-Eocene
Figure 9.30 : Cross-sections of A, offshore Congo basin; B, Cab/nda basin; C, Cuanza basin. (Redrawn from C l i f f o r d , 1986.)
580
unit
is
thick
overlain (Franks
by and
as
in
Gabon
(1,000-2,000 m)
sufficiently Nairn,
thick
1973).
and
overburden
Post-salt
in
to
offshore
trigger
hydrocarbon
Congo,
salt
migration
and
movement
from pre-salt
source beds followed the listric faults which a c c o m p a n i e d salt tectonics; localized
salt
drocarbon
plays
oil-field
in
Cabinda
over
basement
arches
separating
interior
grabens.
There
is
trapped the
solutions are
also
greater
cycles
the
(Fig.9.30B),
a c c u m u l a t i o n in this field. carbonate
offered m i g r a t o r y in
show
offshore pre-salt
also
pathways.
generated
the deeper
considerable
In the Cuanza basin
accumulations
in
Post-salt
direction.
In
the
hydrocarbons
offshore
are
margin
post-salt
hy-
Malongo from
hydrocarbon
(Fig.9.30C) the post-salt
platform
carbonates,
with
tur-
bidite sands d o m i n a t i n g in the offshore basinal sequences. 9.4.5 S o u t h w e s t A f r i c a n Marginal Basins Between
the
Torres-Walvis
ridge
complex
and
the
Faulkland-Agulhas
ture zone are three major rift basins--the Pelotas-Walvis ange
basin,
and
basin with (133 Ma)
at
northern
end
tremes bezi
such as
basin
(Uchupi,
were
1989).
lavas
and
in
this
lavas of and
Syn-rift
basalts rocks
deposits, The
Cape
the
such as the Stormberg dia-
Mozambique,
rifting
Karoo
at
Swaziland,
episode.
dykes
Younger
(146 Ma), and
and
Moveme
kimberlite
Zam-
volcanics basalt
plugs
and
in the offshore Orange basin are related to the tran(132-108 Ma)
in
the
(126 Ma)
Hauterivian
southern Lebombo
other
in
the
Karoo volcanics
with
ended
the Valanginian
sition from rifting to sea-floor spreading. Kaokovelt
Rifting
frac-
the Or-
spreading during
the Lebombo,
associated
(137 Ma),
(Fig.9.21).
sea-floor
end,
(210 Ma),
the rhyolitic
of M o z a m b i q u e
of
southern
lavas
amygdaloidal
Cape
initiation
the
and
lavas
the
the
basin,
the
of
Namibia
Pelotas-Walvis
and
The latter stage was when the were
extruded
Orange
(Uchupi,
basins
include
1989). clastic
but there is no evidence of evaporite and carbonate deposition. Walvis
basin
underwent
limited
crustal
extension
transverse alignment to the Pan-African Damara orogen. n a r r o w platform,
due
to
its
It therefore has a
and a n a r r o w marginal sub-basin which is situated at the
base of an 8-km high basement hinge zone (Fig.9.22A). The Orange basin which was aligned parallel to P r e c a m b r i a n structural trends
is, however,
more extensive with a larger marginal
is d i v i d e d into two segments by a central basement ridge stratigraphic al.
(1982)
richtian DSDP
site
succession
shows
times, 361
in
continuous resulting
(Fig.9.31)
the
Orange
deposition
basin
described
from Early Aptian
in a seaward
thickening
the
in
sequence
the
sub-basin that
(Fig.9.22C). by
et
through Maast-
clastic
distal
Tankard
The
part
wedge.
At
the
(continental
581
CO
,-4 .p I-i • ....
-~
o
"cl
%=
~
\
,,-,
v
0
= ',
/
"<< <"V (~-.":~--. ~-
o
o
o k % %
4-4 0
,,-I I/]
,,cl
o
b 'o
U .,-4 0 N 0
~...':~....
u
(D
,,-I
582
slope)
of
the
Orange
basin
fines
upward
bonaceous shales, m u d d y sandstones, water
pelagic
at the top. dients
clays
High
fossil
wood-bearing
car-
through gray and red shales, to deep-
in the upper part,
with
Upper
Cretaceous
sandstones
fluvial influx was m a i n t a i n e d by steep topographic gra-
caused by repeated down-faulting,
Onshore,
from
limestones
at Bogenfels
tilting and epeirogenic uplift.
in Namibia w h i c h contain Cenomanian am-
monites were d e p o s i t e d during a major transgression.
M i d d l e Santonian to
Late C a m p a n i a n i n o c e r a m u s - b e a r i n g deposits exposed near Bogenfels suggest other
extensive
marine
transgressions
along
the
continental
margin
of
southwest Africa. 9.4.6 South A f r i c a n T r a n s l a t i o n Margin The
eastern
cussed the
in this
Indian
evolved
as
fracture
an a
result
zone
the
part
of
(e.g.
it extends
southeasternmost of
the
The
0uteniqua bank;
basin
Algoa;
South
from the of
along
basins
or
the
is dis-
Atlantic
orginated
Ocean.
This
to and
margin
Falkland-Agulhas
the
created
during
continental
Zululand
grouped under the South African coastal basins
South
grabens
underlying
Oudtshoorn;
Africa
Africa
Atlantic
motion
various
of
margin
South
left-lateral
(Fig.9.31).
Agulhas
of the Republic
Although
the
integral
movement
termed
margin
section.
Ocean,
as
opened this
continental
basins)
shelf
have
been
(Fig.8.2).
Figure 9.21 shows the p a l e o g e o g r a p h y of the M i d d l e - L a t e Jurassic when rifting began in southeastern Africa and formed the grabens which underlie
the
coastal
basins.
nantly
continental
lated,
comprising
In
these
intertonguing (Dingle,
1978)
depocentres lithofacies the Enon
three
principal
(Uitenhage
Group)
conglomerates,
the
Kirkwood Formation,
and the estuarine or marginal marine
Infanta Formation.
In the Oudtshoorn basin where alluvial
the fossils
clastics of the fan conglomer-
of
deposits,
the
and playa
include dinosaur teeth and lignite which ac-
cumulated under semi-arid conditions. 4 km
accumu-
fluviatile
ates are t r a n s i t i o n a l d o w n s l o p e into alluvial plain sandstones lake mudstones,
predomi-
first
marine
In the Algoa basin where embayment
probably
there are
occurred
in
the
Late Jurassic when Africa separated from Antarctica. Continental separation created a coastal plain across the lower parts of braided graded
into
fluvial
systems
the newly
were highly indented.
which discharged
opened marine
into the rifts.
embayments,
the
Deltas
shorelines
of which
Marine circulation did not start until continental
b r e a k - u p began during the Late V a l a n g i n i a n as indicated in Fig.9.32. lier,
in the Late
pro-
Jurassic
the temporary marine
the O u t e n i q u a basin and in the Natal V a l l e y basin
incursion was (Fig.9.31).
Ear-
anoxic
in
Barremian-
583
Aptian
paleo-anoxia
southwest
Africa
the O u t e n i q u a
to
basin
entended the
all
Falkland
(Fig.9.32).
the
way
from
plateau,
It was
the
Maurice
offshore
Ewing
region
bank,
not until M a u r i c e
into
Ewing bank had
cleared South Africa at the end of the Albian that d e e p - w a t e r entered the South A t l a n t i c and the Indian Ocean,
and
of
conditions
thus ending anoxic sedi-
mentation.
&~0ZAMBIOUE
ZULULAND/ S ATA L
MAAS.
JC-1 (OFFSHORE NATAL)
OSDP 361 FALKLAND (OFFSHORE SWA PLATEAU S W CAPE) (BOGENFEL5) (DSDP 330)
TRANSKEI B~TE.~'N IQUA COAST NEEDS CAMP b|
i
...o,.o
CAMR
II
I
L_
SANT: CON" TURO~ CEN. ALB"
I
.......
ApT.
I ----4--
V-------
shares
shales
River
BARR. HAUT, VAL, BERR.
[ ~ Norine sediments [ ] Hiatus
[ %Continental "•o sediments
~ Samplqs from exposures on the AgulhasBank
Stratigraphic subjdivisions of Cretaceous Figure 9.32: ments in South Africa. (Redrawn from Dingle, 1978.)
sedi-
C r e t a c e o u s - T e r t i a r y deposition along the continental m a r g i n of southeastern Africa involved deltaic progradation conformities sions sions.
have
in
the
been
Late
(Fig.9.33),
Cenomanian-Coniacian
attributed
to
The terminal Cretaceous
regional
uplift
and regional un-
(Fig.9.32). due
These
regres-
to k i m b e r l i t e
intru-
regression was p r o b a b l y due to subsidence
of oceanic crust caused by the reduced rates of sea-floor spreading. Fragmentation shift
of
southern
Gondwana
induced
a
major
paleoclimatic
from the h y p e r - c o n t i n e n t a l i t y of Karoo times with a t t e n d a n t deser-
tification
(e.g.
(Fig.8.50,F-H)
to
Stormberg more
dune
humid
fields,
conditions,
playa with
lakes,
seasonal
moderately
high
streams) run-off
584 (Tankard et al.,
1982). Once the continental mass had reduced in size and
s h a l l o w seas expanded,
continental climate ameliorated.
NORTH
SOUTH
" "
t
~
Gamma-ray log 0 150 APtUnitm
"
Sonic log 40
140
rS/ff A
Margin[ fracture t
.....
200Kin
r,
~"%
j
LSO
t
>
C
Lignite
I
,~
Sandstone Siltstone
Dark-grayshale l]
Figure 9.33: North-south stratigraphic section and g a m m a - r a y log for the Agulhas Bank. (Redrawn from Tankard et ai.,1982.)
9.5 Evolution of the Eastern African Margin
9.5.1 Plate T e c t o n i c H i s t o r y Rifting and strike-slip tectonics were involved ern Gondwana, Middle
and in the opening of the
Jurassic
Both p r o c e s s e s Africa, wedges
stretching The
of deposits Based physical
the drifting
subsident
data
coastal
basins
from Kenya
and Somalia.
(Dualeh et al., of
(Fig.9.34)
with
to Somalia,
in East Africa
extensive
sedimentary
over a d i s t a n c e sometimes
contain
of nearly over
6 km
and
geo-
1978).
interpretations
1987; Tarling,
the e v o l u t i o n
margin
from M o z a m b i q u e
(Kamen-Kaye,
on
of M a d a g a s c a r
led to the creation of the continental m a r g i n of eastern
a highly
6,500 km.
et al.,
with
in the b r e a k - u p of east-
Indian Ocean w h i c h began in the
of
marine
1990;
and
Coffin
coastal
geological
and Rabinowitz,
1988;
Mascle
1988) the following phases have been recognized in
the eastern African margin.
First
there was
southeast-
585
erly
transform
the Davie ceous.
motion
fracture
This
of
zone
created
Madagascar (Fig.9.34)
rift
basins
and
India
(eastern
between m i d - J u r a s s i c in
the
Horn
of
Gondwana)
along
and Early Creta-
Africa
and
the
Somali
basin offshore. M a d a g a s c a r reached its present location off the coasts of Mozambique
and Tanzania during the Early Cretaceous.
Secondly,
there was
the N E - S W opening of the M a s c a r e n e basin in the Late Cretaceous, ing M a d a g a s c a r
from the
the
micro-continent
Seychelles
Eocene
Indian
subcontinent. from
India
Thirdly,
took
place
followed by the northward drift of India.
the in
Lastly,
separat-
separation the
of
Paleocene-
the East Africa
Rift System was initiated in the mid-Miocene.
, ' ,) '2,F-i:~~ ~
°OJ
r----~
I
. . , . , , , , .,.,-b,o,
,,-,,,--,, ,.,-,,-~,m1°~ ~.',
I-/%
ok9.,-=%--~ "
CK°~f:oibos~:ins
SB,o,o
I
H Ho.d.w~
I
\ f
1 ~ %\
I
llll
~ /
~ V~',
\ ~ \
j i.~
~
o~.~\/% ~"l)
9-
/ .'~_
~
e',#
",,,
s ~ \
; t---& ~.
~v-',<'--:~,q.... I
"\./
\
",'t\
"
A.tiir~ti~
00bbi~ S South S o m a l i a OU Outeniqua b a s i n
L lO00Kmj I s o b ~ t h s in metres
North S,SOMAL|A 1
I
T Tend.guru
~Basement
:° L
A
\\ _ ~ - - ~ ~ z ~
k
k
",~-
# I ..- x I '~i~'~l~-.'l~ ' ~Maj,ungs
. . . . . m
q
B
Z Zombezi
KENYA ...............
TANZANIA w ' '.
.
.
ZAMBEZI . . .
.
.
c,e~ac,ou, South MOZAMBIQUE-- / NATAL I& OUTENIQUAI~? ; ....
10 12
;'I 16 ~
,-,,.
'g~:{~ o~kl
~-'7::I ~4
-
,. NEOOENE SEQOE.CE
~OR.-C,ETACEOOS
2. PALEOGENE SEQUENCE
~.'>'..'.;-~
LAVAS
3, U. CRET. SEQUENCE ~CONTINENTAL 4, L. CRET. SEQUENCE 5 U. JURASSIC SEQUENCE I~:'.<-'4BASEMENT 6, K A R O 0 S E Q U E N C E
~
CENOZOIC
~///~/~ VOLCANICS
C
Figure 9.34: A, Jurassic coastal basins of eastern Africa; L, Lamu; M, Mombasa; D, D a r e s Salaam. B, tectonic features of the Indian Ocean. (Redrawn from Nairn, 1978; Tarling, 1988.). C, N o r t h - S o u t h c r o s s - s e c t i o n (Redrawn from Dingle, 1982).
I
586
Whereas oceanic the
virtually
the
entire
crust w h i c h extends
continental-oceanic
Mascarene
basin,
with
western
Somali
landward to about
crust the
boundary
Mozambique
basin
is
underlain
by
1,500 m b a t h y m e t r i c depth,
lies
east
channel
of
Madagascar
underlain
by
in
the
continental
crust. The Somali basin contains m o s t l y Cretaceous and y o u n g e r deposits.
9.5.2 P a l e o g e o g r a p h y Table 9.1 shows that the eastern African coastal basins are all underlain by Karoe deposits.
Over the Karoo basins the sea p r o g r e s s i v e l y encroached
from
times
Late
Permian
onwards,
approaching
southward
from
the
north
through the Somali basin into Kenya and Tanzania.
From the south another
seaway t r a n s g r e s s e d n o r t h w a r d towards Mozambique.
Kamen-Kaye
nished
a Permian-Tertiary
synthesis regional
for eastern Africa paleogeographic
transgressions however, gion
invertebrate
and vertebrate
that offers a c o m p r e h e n s i v e
overview,
are concerned.
as
far
as
the
(1978)
fur-
paleobiogeographic framework for a
extent
of
the
marine
The non-drift p h i l o s o p h y of Kamen-Kaye
is,
u n t e n a b l e in the light of the plate tectonic h i s t o r y of the re-
outlined
above.
The
Early
Triassic
was
marked
by
a
limited
trans-
g r e s s i o n since fossiliferous marine beds are found only in the Karoo beds of Madagascar.
In the Mandawa basin in Tanzania P e r m i a n - T r i a s s i c
evapor-
ites indicate m a r i n e influx from a marine embayment to the southeast. Along
the
coast of eastern Africa
the Jurassic
to T e r t i a r y deposits
represent an onlap sequence of a regressive continental m a r g i n Gierlowski-Kordesch, recorded This
by
the
1989).
occurrence
transgression
A of
created
widespread the
major
Jurassic
ammonite
Bouleiceras
embayments,
for
(Ernst and
transgression in m a n y
example
in
bique basin, the Selous basin and at Tendaguru in Tanzania,
is
basins.
the Mozam-
and all along
the coast, up to Somalia and Ethiopia. According
to
Kapilima
(1989)
widespread
marine
transgression
began
along the coast of Tanzania in the early Middle Jurassic with the deposition of detrital to oolitic and oncolitic reefal limestones which contain diagnostic Jurassic Inland
ammonite
was
assemblages.
followed
at Tendaguru,
Deepening
by a regression
with
the Late Jurassic
of
the
sea
Kimmeridgian
is represented
in
reef
the
Late
build-ups.
by an alternating
sequence of d i n o s a u r beds and marine beds w i t h ammonites and pelecypods. Among the
the d i n o s a u r s largest
dinosaurs
at
(Fig.8.57J).
land
was
the
animal
Tendaguru
sauropod
known
include
to
Brachiosaurus
have
Allosaurus
branchi
inhabited (Fig.8.57I),
the and
(Fig.8.57G), Earth.
Other
Kentrosaurus
587
<~J~
o~
L~_
~
: I~ ~
I01 IE ~
o
o
o
o
_90
o
m
o
ff
u~ o
C~ 04
o T~
o
"2
N
~
"~ E
0
rn
o
~ o E
.~
lJ -,..4
o~ o-c
o
o
~r-.,
y,-,
c_
N
o
.--
,~
E
o
_
c
.< o
-~
-i
,
a
.~
o
O N
•.-I "0
i
~P~
f, .
,-
chC OO ,-~4J
o
E o
o
O0 • , 4-I
m
o
m~I
u
z ¢j
w ~u
~z
lu
~z
o
o
o
~
o
-
o
o
o
o
A~YI1~31
g SNO33V13~3
31SSY~NI"
.
OlSSVI~I-'N6t3d
588
There was an extensive transgression in the Aptian-Albian, marked by the a m m o n i t e Tropaeum w h i c h occurs in South Africa, Mozambique, Madagascar, and Australia,
but is not found in Tanzania, Kenya and Somolia.
the M o z a m b i q u e basin argillaceous
facies
In
(Table 9.1) Early Cretaceous littoral to neritic (Maputo Formation) with sandy marls,
is laterally
equivalent to the deltaic sandstones of the Sena Formation.
The Cenoma-
n i a - T u r o n i a n in off-shore Mozambique is represented by the nonmarine Domo Formation which marked a pronounced regression like in South Africa. Open marine environments prevailed in the M o z a m b i q u e basin during the deposition of the Grudja Formation. In coastal Tanzania the J u r a s s i c - C r e t a c e o u s boundary is generally gradational and lies within a coral-bearing limestone; whereas at T e n d a g u r u the faunas suggest a hiatus beginning in the latest Jurassic and lasting through the first two stages of the Cretaceous
(Table 9.1).
Upper Cretaceous marine shelf deposits in Tanzania are rich in foraminifera, but are p e c u l i a r in containing g r a p h o g l y p t i d burrows which are u s u a l l y c h a r a c t e r i s t i c of deep-water flysch environments Gierlowski-Kordesch,
1989).
(Ernst and
In Kenya the outcropping Freretown Limestone
(Table 9.1) contains abundant Early Cretaceous bivalves, gastropods, and corals, while in the subsurface planktonic foraminifera reveal Late Cret a c e o u s - T e r t i a r y continuous marine sedimentation, as in Tanzania.
9.5.3 Selous and M a j u n g a Basins Located at the seaward end of the north-south Karoo Ruvu V a l l e y in northeastern Tanzania, basin motion
overlying along
the Selous
a pre-drift
the
Davie
basin Karoo
fracture
(Figs. 8.2; interior
zone
9.35A)
fracture
separated
the
is a M e s o z o i c basin.
eastern
sag
Transcurrent half
of the
Selous basin which now constitutes the Majunga basin in northern Madagascar
(Fig.8o2).
In both basins
c o n f o r m i t y separates
(Fig.9.35)
nonmarine Karoo clastics
overlying m a r i n e carbonates, marls, tics
(Clifford,
a Middle
1986).
Poorly
Jurassic
and lacustrine
break-up unshales
from
shales, w i t h m a r g i n a l - t o - m a r i n e clas-
developed
latest
Triassic
or
earliest
Jurassic e v a p o r i t e s mark the transition from n o n m a r i n e to m a r i n e sedimentation as the b r e a k - u p of Gondwana progressed.
589
2
~
SELOUS
0
BASIN
,1Rt~,S,51£
HAJUNGA
÷
BASIN
A
B
Figure 9.35: Cross-sections for the Selous and Majunga basins. (Redrawn from Clifford, 1986.)
9.5.4 Mesozoic Rift Basins in the Horn of Africa
This is a group of related basins in Kenya, Somalia, and Ethiopia which are separated by various basement uplifts
(Fig°9.36). They occupy the
passive Indian Ocean margin and comprise the Lamu embayment in southern Somalia and adjoining parts of Kenya; and in Somalia there are the LughMandera basin, the Somali coastal basin, the Somali embayment,
the AI-
Mado Darro basin, and the Barbera-Borama basin. The Lamu embayment is believed to be the southern extension of the Ogaden and the Lugh-Mandera basinslalthough in the subsurface an east-west basement ridge seems to separate the Lamu embayment
(Whiteman,
1981). Although the structural
pattern of the Lamu embayment and those along the Somali coast are generally obscured by flat-lying coastal plain deposits,
in the subsurface
geophysical surveys have delineated NE-SW-trending faults and pre-Tertiary highs in the Lamu embayment 1973; Whiteman,
(Peterson,
1985; Walters and Linton,
1981). Uplift of the Bur Aqabar and Nogal basement highs
were accompanied by downwarping in Ogaden, Somali coastal basin, Somali embayment,
and in the Lugh-Mandera basin. Although separated by a north-
590
m
J ,.~ -.~ 0
0
.~ g Z
xx ~x
o
"~
0
3 n,,
•~1 .--1
z~
,--I < ,,-t 14 0
O)
Ii)
°.-I kO tl-I
~tl-t
--I O ~
591
south trend of s e d i m e n t a r y eastward with
the Somali
Offshore, lies along
the
basin,
of Kenya
continental
the Ogaden basin
and
margin
the thickest
Somalia
fault
served
on
the
the Somali
sequence
ranging
downthrown
side
(Fig.9.36C)
to be the
shoreward
Dualeh
et of
al.
sic c o n t i n e n t a l shallow marine new phase
(1990)
the basins
widespread
Adigrat
chronous
regional
contains
rift
the Gulf
of Aden.
The the
Middle
Jurassic
These
to Late
Jurassic
in southern
are
Somalia
Hamanlei
embayment,
2,000 m
in
Somalia
into
the
Lugh-Mandera uplift.
After
throughout offshore.
the
extend
very
far
the
Late
basin
associated
in
eastern
continental
first
major
marine
of Ethiopia,
and
all
interbedded
less thins
beds
anhydrite occur in
embayment.
than and
the
of Middle
Shaly c a r b o n a t e s
from
of
in which
limestone
in the Lamu
and
coast
in
carbonates
coralline
with
Somalia.
basin,
deposited
of the
of which
the
transgression
shelf
and
on
A
is a dia-
the base
basin
were
and
peneplana-
Sandstone
unit,
Berbera
oolitic
beds
accumulated.
The A d i g r a t
Ethiopia,
depositional
by the d e p o s i t i o n
Jurassic
thickness
and
Carboniferous-Trias-
the end of regional
the
of Tethyan
Cretaceous
the
a
eastern was
Lugh-Mandera
complete
a renewed
inland.
with e v a p o r i t e in
which
Cretaceous-Tertiary.
there was
coastal
subsidence
where
equivalents,
comprise
(Fig.9.38A)
this
Following
Neocomian,
in
was regressive
Early
basin
Somali
is pre-
considered
300 m
to
becomes
The over
sandy
to
1985).
The Late J u r a s s i c tinued
its
Lugh-Mandera
and
(Peterson,
side
9 km of m a r i n e
deposits
evaporites
marked
(Fig.9.37).
in parts
ranges
Over
structural
troughs
shale in n o r t h e r n
Formation
the west
They
and
and dark gray m a r i n e the Somali
and
eastern
on the ocean
Peterson
the
shield was initiated
example
sequences age.
Karoo
recorded
Formation
basins.
the
Triassic-Middle for
and
Horn of Africa.
which
sandstone latest
slope).
regional
and P e r m o - T r i a s s i c
African
basalts,
Hamanlei
reconstructed
formation
tion of the eastern
of the
1985).
to T e r t i a r y
embayments,
created
clastics
of basin
on the
is located
Somali basin.
in the
rifting
(Peterson,
from Karoo
manifestations
part of w h i c h
(Fig.9.36B)
(continental
and Lamu
with the growth of the oceanic
histories
to m e r g e
system w h i c h d o w n d r o p p e d
side, w i t h a t h r o w of at least 4 km and c o n t i n e n t a l
seems
embayment.
the Somali
coastline
of a m a j o r
thinning,
But
retreat
partially
basin
transgression
Late Cretaceous
basin,
the
from
sediments
the
the
Bur
emergent
which
are
con-
in
prevailed
Somalia
in the A p t i a n
marine
by
was
sedimentation sea
which and
isolated
(Fig.9.36B)
marine of
deposition
Ogaden
in
the
did not
represented
592
3N39037Vcl
SN033'¢13~3
31SS~r
IN~13d - ' 0 8 ~IV 0
~,: • , ' ° . . " . . , °, . . • ° ~..'.'...--.. • . .:" "~" .... .'.-°''. o o.....°, • .° .'.. £ o ~.:..'..'." .;. :;..~ <. ~.. , • ° .. ° ..° ,. °°.. •
,."..'.'...
. •- o
~" . . ; . . . . - - o ...-~
~
....... ~:',...
o ~
:~..-.', z.. ..
o
0
0 .'~.~ . ~ 4J oE
1.4 0 ~J
°
~o Om
a)
•
0 04J 0
~44
14 m~
i :'.i'.'"':
O3 ~4U
V7 3N 3 9 0 3 7
Yd
J
~V3
S~030YJ-3~O
OISSV~Nr
"NLI3d-'08 ~ V 3
•,4 t,~
593
by the G u m b u r o G r o u p which contains rudistid limestones at the base, with locally marine
developed shales,
evaporites
sandstone,
and
in
Somalia;
minor
and
in
limestone.
the
Lamu
Cretaceous
embayment rocks
by
in
the
Horn of A f r i c a thin w e s t w a r d and northward to less than 500 m in much of eastern Ethiopia
(Fig.9o38B).
|-- . . . . . . . .
i
i --J I I f -;
Sudan
//
Sudan
/
,
il/ t,
°'.3, eO
/ ./
/ 0
Ethiopia
i
".-~
~" ,
/:I
"t. L EXPLANATION Main S e d i m e n t a r y Fach Oolitic and fossilifer carbonate
.
[]
Anhydritic - gypsifel ]
Narine
]
Mixed marine and clastic
shale
B
ca
'k.
CRETACEOUS
/-
£XPL
]
8osei M o i n Rc Sand
~
Anhy
[--I Mixed ClaSt
Volc(
Figure 9.38: M e s o z o i c - Cenozoic isopach maps for the Horn of Africa showing the main lithofacies. (Redrawn from Peterson, 1985.)
_
594
There during was
was
which
an the
deposited
stones.
The
extensive
marine
Paleocene-Eocene
in
northern
Aurado
transgression Aurado
Somalia
Formation
over
changes
in
marine
the
fossiliferous
end-Cretaceous
into
Early
Tertiary limestone
regressive
deep-water
marine
sand-
shale
and
shaly limestone to the east and southeast in Somalia.
It is succeeded by
a sequence of interbedded gypsum,
(Taleh Formation).
limestone and shale
The Taleh Formation changes eastward to m a i n l y dolomite, and to sandstone, red and green shale,
gypsum and limestone in the south and southeast.
n o r t h e r n Somalia the Taleh Formation comprising
limestone,
varicoloured
shales
Horn of Africa most
of
gypsum
toward
is over
Somalia,
and
marine sandstones,
and
the
shale
absent
that
southeast.
4 km thick in
In
is overlain by the Karkar Formation change
The
to
sandstones
Tertiary
sequence
in the Lamu embayment.
eastern
Ethiopia
and
in
the
It is thin
(Fig.9.38C).
in
Tertiary
shales, and limestones occur in the Lamu embayment and
in the Somali coastal basin, while clastics accumulated offshore. Total
marine
tonic phase
withdrawal
during
in eastern Africa.
the
Oligocene
Regional
ushered
doming preceded
in a
new
tec-
the break-up
of
the A r a b i a n - N u b i a n Shield, and the opening of the Red Sea and the Gulf of Aden.
These
were
followed
African Rift System. ment
in
Africa,
in
the
Miocene
by
the
formation
of
the
East
But before entering this last phase of rift develop-
let
us
conclude
the
Mesozoic
rift
story
by
examining
Early Cretaceous rifting across the heartland of west and central Africa.
9.6 West and Central African Cretaceous Rifts
9.6.1 O r i g i n It is now a well e s t a b l i s h e d fact that some of the m a j o r A t l a n t i c transform faults,
e s p e c i a l l y those of the Equatorial A t l a n t i c extend
(eg.,
Emery et al°,
ments
(Fig.8.2).
1975)
along major d e e p - s e a t e d
basement
landward
shear linea-
In a d d i t i o n to controlling the opening of the Equatorial
A t l a n t i c and the structures along the continental m a r g i n by shearing during Early Cretaceous, (Guiraud et al., genetic the
link
break-up
continental
Fairhead and Green,
is b e l i e v e d
strike-slip
the continental and
shear motion also p r o d u c e d i n t r a c o n t i n e n t a l rifting
1987;
to exist between
movements extensions
stages
margins.
during
that
resulted
of oceanic the
The Gulf
in
1989; M a s c l e et al., the
timing
transform
evolution
of Guinea
and
intracratonic
of
faults,
African
oceanic
landward as a complex system of shear lineaments,
1987). A
the nature rifting
and
of
along
the rifting
Mesozoic-Cenozoic
transform
faults extend
collectively
595
~
~ ~ Eor,yC~e,oceoos
Figure 9.39: Geodynamic model and rifts systems A f r i c a and the Sudan. (Redrawn from Fairhead and Green, termed
the
according
Central to
(approximating
African
Fairhead a
small
shear
and circle)
zone
Green to
(Fig.9.39).
(1989),
transmit
is
shear
oceanic ridge, deep into the A f r i c a n continent.
for West 1989.)
This
shear
ideally motion
from
zone,
orientated the
mid-
The Central A f r i c a n shear
596
zone
extends
in
Proterozoic Chad,
the
Foumban
the
into
direction
shear
Central
continues
ENE
the
zone
from
in
Gulf
Cameroon,
African
Republic,
Red
Hills
Sea
the
into
of
NE
of
and
Guinea
runs
western
Sudan
through
across
Sudan
southern
and
(Schandelmeier
the
probably
and
Pudlo,
1990). Strike-slip
motion
along
the
opening
of a complex
system
of
shear
Collectively
the
African those
zone.
Rift
System
of the Chad
central
Sudan,
(Fig.9.39).
propagated
northward
that
we
have
of crustal slip
along
According
the
and
opening
basin
to
the of
the
main
the
basin
axis
and
basins
of
South
the
the
m a jo r
Early
northward T e r m i n a t io n
deformation
in the
Benue
which
Early
Campanian, shear
change
from
vated
coincided
zone.
sinistral
Equatorial
change
in
and Green,
to
motion
Benue
trough of
the
folding
in the
reactivation
pro-
Benue during
the
Chad
South
At-
compressional
of
Santonian-
the
Central
of shearing was
caused
movement
the
This
Central
strike-
the
and
produced
zones.
the
in
and
strike-slip
fracture
between
58 km
zone
(130-119 Ma)
rifting
dextral
estimated
dextral
of
in the direction
dextral
oceanic
plate
with
A change
been basin
about
shear
began
propagation
(Fig.9.39A).
African
rifting
The
system
and
rifts.
Cretaceous
Ocean.
have Sirte
(1989)
African
into
to the shear to
caused
, while
Central
rift
trough
also
sides
southern
the
Green
trough
the Sudanese
Atlantic
constituted
and
in
the
Central
grouped
of
believed
both and
are
those
rifts
(Fig.9.39
the
Green
in
basins
are
Fairhead
in the Benue
West
perpendicular
rifts
caused
along
the
and
intracratonic
opened
Fairhead
rift
almost
zone
basins
as
basin,
basin
considered.
which
Chad
actually
lantic
Doseo
in the Chad
the
the e x t e n s i o n
trough
form
to
principal
Chad
slip m o v e m e n t
extension
motion
duced
to
already
sinistral
the
shear
extensional
are aligned
The
African
referred
the Doba
all of which
direction
which
of m a j o r
(WCARS),
basin,
Central
along
resulted
and
South
from
the
Atlantic
by a
reactisudden
(Fairhead
1989).
9.6.2 Benue T r o u g h This
is a Cretaceous
ria
(Fig.9.25),
the
Chad
Camero o n include ern
basin, through
the Mamfe
Cameroon,
(Ajakaiye, 1984)
and
1981;
reveal
folded rift basin
and extends through the
Yola
rift
Adighije,
crustal
the
Gongola
rift.
(Fig.9.39)
the Nupe
(Fig.9.39)
from the Niger delta
basin 1979;
thinning
Other
rift.
It
in central Ofoegbu, the
1985;
of
into
the
Nigeria
Nigeria.
Benue
lies across
branches
bifurcations
in southeastern
beneath
which
Nige-
and links n o r t h w a r d
Benue
and
Ofrey,
trough
southwest-
Geophysical
trough,
with
northern
1982; flanked
surveys Okereke, on both
597
sides
by
linear
sub-basins
thickest depocentres In its
lithic
(Agagu
(Fig.9.39),
fill,
and
Adighije,
1983)
that
were
magmatism
and deformation,
the Benue
trough has
evolved in direct response to Cretaceous plate tectonic processes Atlantic
Ocean,
1988).
At
its
sisted
of
a
especially inception
series
the South Atlantic
in the
of
isolated
(Bima Sandstone)
(Benkhelil,
Early Cretaceous depocentres
where there was mostly alluvial mentation
the
sometimes with over 6 km of strata.
fan,
the
or
braided
in the
1989;
Benue
Popoff,
trough
sub-basins
con-
(Fig.9.40A)
river and lacustrine
sedi-
in the north, with the Asu River Group in the
middle and southern parts of the Benue trough. The initial marine transgression entered the Benue trough in the Middle Albian
(Reyment,
1965)
from the Gulf of Guinea
(Fig.9.40)
and estab-
lished a stratified epeiric sea where anoxic bottom conditions from
Cenomanian
to
Coniacian
again in the Late Campanian. pelagic
organisms
tensively 1990)
were
(Fig,
Turonian
B)
which
(Petters
inoceramus,
to Coniacian in
the
had
and
Ekweozor,
deeper
earlier
foraminifera)
times,
been
carbonate
parts the
of site
are
pervasively
Nkalagu
intercalated
within
the
Late
Abakaliki clastic
(Petters
includes
Late
and
Ekweozor,
the Turonian
1982).
Eze-Aku
Cenomanian-Coniacian
transgression
Shoal water carbonCenomanian-Coniacian
Shale,
The
Nkalagu
and
the
witnessed
the
in the Benue trough which coming
1990; Reyment and Tait,
most
extensive
shows
the
in the northern ammonites, limestone
probable
from the Gulf of Guinea,
foraminifera)
Sukuliye,
extent
of
the
Saharan
a
seaway
Benue and Chad basin include highly
(Gongila
1986; Oti,
marine
Formation),
Numanha,
during
Figure
the Late
Deposits of the epeiric sea (Figs.9.40B; shales and
(Pindiga
marginal
Lamja Formations
and
marine
9.42A)
fossiliferous Fika
Formations),
lithologies
(Fig.9.41))
(eg.,
(Yolde,
at various
local-
(Odebode and Enu, 1986). Deltaic sandstones prograded into the mid-
dle and southern parts of the Benue trough as
Awgu
marine
1972), and into the Yola and Mamfe rifts.
Cretaceous transgression.
ities
Formation
Conaician
extended marine influence into the Chad basin (Avbovbo et al.,
Jessu,
trough
turbidites
(Fig.9.40B).
The
9.13B
ex-
(Oti,
the southern parts of which have been grouped into the
Formation
(Fig.9.41) Shales
and
accumulated turbidites
the of
1990) during the Middle Albian transgression.
dark gray shales,
prevailed 1982),
While thick organic-rich muds with abundant
(ammonites,
deposited
9.40
(Ojoh, ates
from
times
result
transgression.
of
regressive
pulses
during
(Amajor, the
Late
1990; Nwajide,
1990)
Cenomanian-Coniacian
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601
Compressional causing
severe
northeastern
deformation
folding
part
of
in
the
occurred
the
in
the
Abakaliki
Benue
trough
Santonian-Early
rift
where
(Fig.9.42B,
deformation
less, there was only m o d e r a t e folding and fracturing Alkaline the Benue
magmatism
trough
accompanied
(Okeke et al.,
intense in the A b a k a l i k i (Fig.9.42D),
Turonian
metamorphism.
sub-basin,
alkaline
Low-grade
Benue,
River Group After
due
nian-Maastrichtian basin,
while
posited Late
(Fig.9.40C) of
genesis
southern
deltaic
deltaic the
transgression
ironstones
(Adeleye,
The
In
the
northeastern
affected
the
Benue
deltaic
occurred
trough
Gombe
basin
displaced areas.
from
Late
Shale) the
but
sub-
were
de-
during
the
Nupe
without
basin
the
favoured
Ironstones) filled
the
Campa-
in the Anambra
in
was
and in the Asu
during
with
coal the this
predomi-
there
sandstone
was
deformation
(Fig.9.42D),
and
and
uplift
caused
the
The p o s t - d e f o r m a t i o n conti-
the Kerri-Kerri Formation thus occupies a structural po-
sition similar to the deltaic south
in
Lead-
(Fig.9.41).
n o r t h w e s t w a r d d i s p l a c e m e n t of the depocentre. nental sequence,
sub-basin
conditions
Batiti
contact
area.
brines
(Nkporo
Rather,
Nupe
by
this
paleo-anoxia
Westward
(Sakpe and
1976).
in
particularly
1979).
were
shales
also
basin.
n a n t l y fluviatile strata, the Nupe Group
which
in
of basinal
pronounced
transgression. Anambra
1989).
sedimentation
accompanied
accumulated
carbonaceous
sedimentation
adjoining
of oolitic
were
occurred
depocentres
under
the was
and was
and the A f i k p o - C a l a b a r
(Fig.9.41C) renewed
initial
1978)
in the A b a k a l i k i
coal measures
shallow marine
southward
Campanian
swamps
the
D).
(Benkhelil,
1989; Ukpong and Olade,
to the Anambra
In
intensity
in the W a n a k a n d e area of Ogoja
also
took place
(Akande and Mucke,
axis
where
to the m o b i l i z a t i o n
deformation
Abakaliki
and
Olade,
intrusives
metamorphism
zinc-copper m i n e r a l i z a t i o n the m i d d l e
rifting
1988;
Campanian
(Benkhelil,
coal measures
of the Anambra
basin
in the
1989).
9.6.3 C h a d Basin The
Chad
basin
(East
Niger
basin)
refers
to
a
group
of
buried rifts in w e s t - c e n t r a l Chad and southeastern Niger. (Louis,
1970)
terson,
1985; Petters,
mantle
of
and
stratigraphic
Quaternary
studies
(eg., A v b e v b o
NW-SE-trending G r a v i t y surveys
et al.,
desert
sands.
These
rifts
formed
on
side of the Central African shear zone, already mentioned. up to rine
4 km thick, strata
of
1986;
Pe-
1981) have confirmed these burried rifts beneath a
begins with
fluviatile
and
Permo-Jurassic lacustrine
to
the
northern
The sequence,
Early Cretaceous
origin,
which
belong
nonmato
the
602
"Continental
Intercalaire"
nigeriensis,
and
Spinosaurus
and invertebrates Niger
Republic
Group
(Fig.9.43).
(Fig.8.57H),
and Early Cretaceous (Lefranc
and
The
along
Ouranosaurus
dinosaurs with
other
vertebrates,
f l o r a n o c c u r in Cretaceous
Guiraud,
1990).
The
beds in
"Continental
Inter-
calaire" G r o u p is o v e r l a i n by C e n o m a n i a n - C o n i a c i a n m a r i n e shales and carbonates.
The
Upper
glauconitic,
and
Cretaceous
to the Santonian-Campanian. on
is
fossiliferous
a
clastic
shales
that
sequence
with
record marine
gypsiferous,
influence down
T e r t i a r y n o n m a r i n e beds resting u n c o n f o r m a b l y
Maastrichtian-Paleocene
continental
sandstones
stones, belong to the "Continental Terminal" Group
with
oolitic
(Lang et al.,
iron-
1990).
9.6.4 C a m e r o o n Cretaceous Rifts Poor exposures rift basins
greatly limit our knowledge of the successions
e s p e c i a l l y where drill
hole data
in African
are not available.
This
is
the case regarding the rift basins in Cameroon Republic, where extensions of the Benue trough are that
country.
Small
Koum, Babouri-Figuil, to
Albian
trough.
beds
found in the northern and
basins
similar
basins
southwestern parts of
(Fig.9.39B)
to
the
tains about 800 m of arkosic,
Bima
Sandstone
in
clays.
1990).
The
Cameroon,
and
fluviatile marine
rift is
as the
starts filled
sandstones
northern
Benue
also occur basin con-
conglomeratic beds, overlain by sandstones,
Dinosaur footprints,
1989; Jacobs et al.,
Mamfe
the
The Hama Koussou turtles,
codes, and silicified wood are common in these beds F!ynn et al.,
such
sills and flows, as well as dolerites,
(Maurin and Guiraud,
and fossiliferous
Cameroon
Mayo Rey, and Hama Koussou contain n o n m a r i n e Aptian
Basaltic dykes,
in these
in northern
from with
(Mamfe
1988,
ostra1988;
1989).
southeastern about
crocodiles,
(Brunet et al.,
4 km
Formation).
Nigeria
of
and
mostly
extends
Lower
into
Cretaceous
Cenomanian-Turonian
marginal
shales are p r e s e r v e d at the top of the sequence near the entrance
into the Benue trough
(Fig.9.41A).
9.6.5 S u d a n e s e Rift Basins Geophysical
investigations
fault-bounded (Fig.9.44A), ure 9.44A consist from
intracratonic
and
petroleum
rift
basins
exploration in
have
southern
and
at the eastern end of the Central A f r i c a n
shows
that
of v a r i a b l y
low-angle
these linked
listric
ated
sedimentary
rift
segment,
and
over
basins
normal
magmatic
7 km
in
are
half-grabens faulting fill
shear
generaly
orientated
and
grabens
full
up
to
basin,
15 km and
in
less
Fig-
NW-SE.
They
resulted
crust.
the
than
25
Sudan
zone.
which
in the continental
are
the Melut
uncovered central
Estim-
Muglad-Sudd 5 km
in the
603
4"~O7
0
r~
7= Z
v
<
O < m'U
U
"U
(D
H
Z~ O
0
I
Z < nm
Z W D
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I
rd
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2 0
604
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~ --
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3HRONC-
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150-C
-~ ~
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~
Sudanese
m~d-Oll~v(01fo~ floodp|oln
1150
d ~- ~;~/~/~(~ $ub-laeu=6 fo=
13~J' t ~°1~ O !Voto Bi rriosia r~fi'~iO nn
Figure 9.44 :
floodploln
800
~ : t ~.~ ~- • braided ttreom
='~ ~-~~ , : :
aHf Kebir
f ~lviol
t °==strtnl . . . . dpl~in [~11¢
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Cretaceous
rift
and p a l e o e n v i r o n m e n t s .
basins,
stratigraphic
(Redrawn from Wycisk e t
605
Bara,
Kosti,
cession
Khartoum,
(Wycisk et al.,
ated d e p o s i t i o n a l Late
Atbara,
Cretaceous
and
sauropod dinosaurs, the H u m a r basin Wycisk
et
1990)
cycles,
and Humar consists
Tertiary.
stratigraphic
of u n c o n f o r m i t y - b o u n d e d
Vertebrates,
al.
grouped
the
(Fig.9.44B).
A p r o b a b l e pre-drift
including
rift
basins
northwestern
parts
custrine
and
remains
of
beds
those
of
in
the
sequence of u n c e r t a i n age,
in
the
country
about
200 m
This unit overlies
in the M u g l a d - S u d d basin and is u n c o n f o r m a b l y over-
lain by 3-5 km of syn-rift N e o c o m i a n - B a r r e m i a n Sharaf
the
into
consists of strongly lithified quartz arenites.
P a n - A f r i c a n basement The
rift-rel-
1990).
Sudanese
and
suc-
the E a r l y Cretaceous,
occur in exposed continental Late C r e t a c e o u s
(Buffetaut et al.,
east-central,
tion.
The
which a c c u m u l a t e d during
southwest, thick,
basins.
Formation
comprises
fluvial-floodplain
strata,
cyclically
mudstones
and
the Sharaf
bedded
fine
Forma-
organic-rich
sandstones
la-
which
in-
terfinger with coarser alluvial fan and braided stream d e p o s i t s along the rift b o r d e r stones,
represents into
faults.
The A p t i a n - A l b i a n
shales with
intercalated
the deposits
which
lacustrine
sequence
(dark, o r g a n i c - r i c h
clay-
fine-grained
sandstones
and siltstones)
of a deep stratified,
sub-anoxic
to anoxic
lakes
turbidites
were
deltaic
sands
and
sub-lacustrine
d i s c h a r g e d in a setting quite similar to the m o d e r n East A f r i c a n lakes. A second rifting with an accompanying depositional phase began in the s o u t h w e s t e r n basins richtian.
The
coarsening
sequence
and a l l u v i a l the
late
in the Late Cenomanian,
Muglad-Sudd of
was
floodplain,
fan clastics.
Eocene
basin
during
shallow
The third and
which
the
and t e r m i n a t e d
filled
basin
with
a
lacustrine,
final
in the Maast-
generally braided
rifting e p i s o d e
filled
up
with
upwardstream, began
lacustrine
in and
floodplain clastics. In
the
east-central
Sudanese
rift
basins,
a
Late
Jurassic-earliest
Cretaceous m a r i n e t r a n s g r e s s i o n which came through the Horn of A f r i c a deposited a fossiliferous and glauconitic claystone and s i l t s t o n e sequence, which in the K h a r t o u m basin, ive basalts.
is underlain by coarse clastics
to Late Cretaceous with a sequence which changes floodplain
through
unconformably
and extrus-
The r a p i d l y subsiding K h a r t o u m basin was filled in the Early
lacustrine to alluvial
overlain
by M a a s t r i c h t i a n m a r g i n a l
which are alluvial fan deposits,
(from base upward)
fan deposits. playa
This
from
sequence
carbonates,
is
above
and the u n c o n f o r m a b l e Q u a t e r n a r y fluvia-
tile sediments of the Gezira Formation,
at the top.
606
9.7 Interior Sag Basins Mesozoic-Cenozoic
interior
sag basins
are m o s t l y
Africa. A m o n g these are the Iullemeden, hari basins was
under
(Fig.8.2).
Zaire, Okwango,
S e d i m e n t a t i o n in these basins,
continental
rifting either.
located
conditions,
and
was
not
in sub-Saharan
Etosha,
and Kala-
for the m o s t part,
profoundly
influenced
by
However the basins in central and southern parts of west-
ern Africa are u n d e r l a i n by Karoo rifts. 9.7.1 I u l l e m m e d e n Basin This basin has a s u b - r e c t a n g u l a r outline; separate basin
Paleozoic
extends
northwestern nonmarine
basin
from
Algeria,
Nigeria
where
strata
of Late
Kogbe and Lemoigne, are e x p o s e d
as we
its northern part is a c t u a l l y a
saw earlier
through it
is
Jurassic
(Ch. 8.5.3).
eastern
known
as
Mali,
the
The
Iullemmeden
western
Sokoto
to Early Cretaceous
Niger,
embayment. age
into Basal
(Kogbe,
1973;
1976) which belong to the Gundumi and Illo Formations
in the Sokoto
embayment.
These
are part of the
Intercalaire" Group in the rest of the Iullemmeden basin
"Continental
(Fig.9.43).
M e s o z o i c m a r i n e s e d i m e n t a t i o n in the Iullemmeden basin began with the Cenomanian
transgression
fossiliferous Cretaceous
shales
(Reyment
and
Due
to
richtian-Paleocene,
the
sea
produced
sequence
an o v e r l a p
(Dukamaje cycle. Dange
and
gypsiferous
Formation)
greater
therefore basin
encroached
1983).
into
and
the
faunas
(Halstead,
Cretaceous
nonmarine
during
Sokoto
the
and marginal by
the
marine
strata
of
marine
marls
in the
1974).
(Kalambaina
Sokoto
Eocene
and
marls
younger
This
(Taloka
and
shales
a Paleocene
marine
facies
Formation).
embayment
Lower Maast-
embayment.
in which M a a s t r i c h t i a n paralic beds
marine
succeeded
deposits
Upper
overlie
subsidence
Deposits of the Paleocene cycle include paralic Formations),
richtian-Paleocene brate
are
Sch6bel,
limestones
strata.
Formation)
and
contain
(Wurno and The
Maast-
rich verte-
ferruginous
clastics
b e l o n g i n g to the Gwandu Formation, are part of the regional T e r t i a r y continental deposits,
the "Continental Terminal"
9.7.2 Zaire Basin This is a broad d o w n w a r p centred on the Zaire craton, the Central
African
by an epeiric tolitic contains
and
basin
carbonates mostly
Congo
Republics
(Fig.8.2.).
in the Late Proterozoic when
and clastics
Karoo,
and lacustrine deposits
and Late (Cahen,
accumulated Jurassic
and extending into Previously
epiccntinental
(Fig.9.45A),
1989).
stroma-
the Zaire basin
to Early C r e t a c e o u s
1983,a,b; Mateer,
occupied
fluviatile
The Lualaba and
607
.:.:.:.:.:..:.:.~.....:':.:.......~....;.~.:...::.:.:.:...~.:.:.:::.:.~.:..:~:;::::::::.::.~.:::~::::.!::.{..~.....: " :':::':.'.:::: *" ~:
0
~i:.'.:'..: ;:!:" :i :w~:"'-:..':': :"-:i ::-.':":".::.: :." ~. ~: ~:: :::iY" ~:':-::2 :_-::./J:-':~'"':': """"-H_ES_07=9Lc'" """ "'-'L~" ":" " ' " ' - ' " "
---~: -_--_:-_:_-:z-z--_---z~ _ - - - - -
j-_- ~ ~ - - - - -
S" 1o ~ ~
~
,,
,~ ~ \ \
-,. ~
%%\ ~
", ~ ~ f : : :
%" %
~s I
I
1SKin
-.- \ ~ . . ' . ' . ' . ' . ' . ~ ' - ' - ' - ' - ' - "
~,i\i\
:::: : ~ " . . : ' . : - . . ' : i . - : ; : ! : : : ?
...........................................
J
A
Summary of the Mesozoic lffhologies in the Zaire basin Nsele Sfage,max. 110m
/
soft fine- grained sandstone, local tens of argiltite near the base.
B Kipola- Kimbau-Schwetz complex: reddish or greenish /fossi(iferous argillltes, intercalations of sandstones and / c o n g l o m e r a t e beds, this complex contains a marine /fauna. /whitish, reddish coherent sandstones with accessional /pebble horizons Inzia Stage, max. thickness Kinko-Luzubi complex of orgillites and soft sandstones, observed 290m ~ t = north includes greenish shales, and grey limestones ~.nd whitish fossillferous marls and sandsfones~ soft sandstones ranging from fine to coarse.
Kwango Series
\
o_
Kamina Series
<
Leia Stage (U.Jur.-hCr.)
J
lualaba Series =
o_ o_ m
\
Stanleyv~le Stage,mo~. thickness 8SOre,U. Jur.
iutshima argil[ite: an alternation of red, calcareous, sandy micaceous shales with fine- to medium-grained sandstone. Series of cross-bedded sands, Lignite traces, pre-end-Cret~ceous defined in Lower Lomami it consists of: cross-bedded sandstones, soft shaty sandstones, some argil[ites and bituminous shales; the Lifoko sandstones assigned to the Loia are now known to belong to the Sta nleyvlUe Stage (~) red and mottled marls and argiltites, some bituminous horizons (3) green shales and bituminous beds (2)argiUaceous and calcareous sandstones and bituminous beds (1) sandstones and conglomerates (Falls Conglomerate)
no undisputed Tries known, but red beds in mrious localities
<[ =¢
assigned to the Triassic, e.g., red beds of Lukuga, an alternation of red, green or gray shales with red, green, brown, or white sandstone with coarser sometimes conglomeratic base; red beds at Hakunga (north 5° ) contain a bivalve which resembles a Late Triassic form, fish remains and an Estheria sp. which elsewhere occur dated as Late TriassicRhaetic
F i g u r e 9.45: S c h e m a t i c s t r a t i g r a p h i c s e c t i o n a c r o s s the Zaire b a s i n (A) and M e s o z o i c l i t h o s t r a t i g r a p h y . (Redrawn from C l i f f o r d , 1986; Nairn, 1978.)
608
the Kamina
Series
(Fig.9.45B),
constitute
while
succession,
the
the Late J u r a s s i c - E a r l y Cretaceous
Kwango
Series
is
the
Upper
rich in freshwater fish, ostracodes,
Diamond-bearing
gravels
and conglomerates
sequence
Cretaceous
nonmarine
and palynomorphs.
at
the base
of
the Kwango
Series extend into Angola where they are known as the Calonda Formation; and in the Central African Republic similar deposits belong to the Carnot Sandstones these
(Censier,
formations,
1990).
Diamond
suggesting
the
is
produced
intrusion
of
from
paleo-placers
kimberlite
pipes
in
in the
Early Cretaceous. Clifford Zaire basin, in the
Upper
(1986)
in his appraisal
of the h y d r o c a r b o n potential
stressed the high total organic carbon content, Jurassic
Stanleyville
carbon source beds and reservoirs
shales
(Fig.9.45B).
of the
up to 18.5%,
Potential
hydro-
also exist in the u n d e r l y i n g Karoo se-
quence.
9.8. Tertiary Rifts and Ocean Basins
9.8.1 The Red Sea and the Gulf of Aden
Tectonic History The Red
Sea and
the Gulf
of Aden basins
(Fig.9.46)
are classic examples
of the t r a n s i t i o n from T e r t i a r y rifts to A t l a n t i c - t y p e m a r g i n a l sags that d e v e l o p e d along the trailing edges of young oceans. tectonic
evolution
of
the
Red
(1989) stressed the following.
Sea
and
Gulf
of
In his review of the
Aden
depression
Beydoun
Initial arching and uplift of the Arabian-
Nubian Shield in the Late Eocene was followed by crustal extension, ing and crustal attenuation in the Oligocene and Early Miocene.
rift-
There was
s e a - f l o o r spreading during which Arabia separated from A f r i c a and rotated counter-clockwise form fault collision
with
(Fig.9.46) and
horizontal motion
along
the Gulf
of Aqaba
trans-
leading to the opening of the Gulf of Suez, and the
suturing of Arabia with
Eurasia,
and the rise
of the Tau-
rus-Zagros fold and thrust belt. The initiation
of the Gulf of Aden has been a t t r i b u t e d to the propa-
gation w e s t w a r d of o c e a n - f l o o r spreading along the C a r l s b e r g ridge in the Indian Ocean Gulf
of
Aden
high heat
(Fig.9.34), (Fig.9.46)
during has
the Miocene.
a deep m e d i a n
flow in contrast with
the Red
The
valley
Sea that
spreading which
is
ridge the
has no central
in the site
of
ridge,
609
but
rather,
an
axial
The Red Sea axial
trough
trough
mostly
over
its
southern
and
is a l s o the s i t e of h i g h h e a t
~
central
parts.
flow.
Alluvial deposits(Quatenary)
[[~
Phanerozoic( Including VoleanicS ) Precambrian B asmenf"
-,,_,?//4/,:
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.
%VICTORIA
'
~.,
~
".AN G O Li_~_ ":' "
!
",
TANZ
I
~
-"<"°
:'-'~
f~
. /
_,- " ~-°.~119
Z A l B IA
M6~somede ,~,"
AN A
LusGkQe ZAMBEZI 126".. 136 ~ ......... "> ",,~ ..:,.,~,
~ :': ""
~--~---c=~,_... ", .-"
,ZIMBABWE ,>
168 "139" ,'~D.i:-i~z
Wlnldlloek ¢-
NAMIBIA
l t
.. ~
[
'.'. •
,4
! ~~o
'
KJmbirLY
s
/
173 .
/
AFRICA
SOUTH
,, 39 ,,
-
'~'q40-141-14Z
I~SWAZILAND
, %
77,Sutherlond
3.~ oRiverldale
CapeTov
~\AghuUaS •
Bank
Figure 10.4: Igneous ring complexes (Redrawn from Cahen et al., 1984.) Another atites,
kimberlite complexes ranging gabbros.
NE-trending
occurs
of arcuate
province
of
NE
Angola.
in the Angola
granites,
Carbonatites
quartz are
Thirty-six province.
syenites,
associated
southern
alkaline
in central Angola from Mossamedes
are known
from
belt
of
complexes
with
carbon-
on the coast towards alkaline
and
syenites,
several
of
and the
the
carbonatite
They include various
foid
with
Africa.
plutons
trachytes
to
complexes.
In
630
age
range
(159-89 Ma),
rock
type,
and
structural
alignment,
the Angola
alkaline province is similar to the Chilwa province. 10.2.6 Tectonic Controls of Ring Complex Emplacement The origin of ring complexes belongs to the broader theme of continental magmatism, mantle
a theme that has been dominated
plumes
or
hot
spot
activity
(e.g.
by the hypothesis Burke
et
al.,
1981).
(1990) in his discussion of the link between intracontinental anorogenic
magmatism,
and associated
lowing widely held view regarding
tin mineralization
of rising Sawkins
hot spots,
echoed
the
fol-
the likely role of hot spot activity.
"Where the relative motions of hot spots and overlying continental crust are negligible or very small, mantle hot spots impinge more substantially on overlying
continental
of generating
and appear
an array of igneous rocks.
alkaline mafic rocks, felsic
areas
suites".
For
where anorogenic
magmatism
been largely stationary,
which
in specific
instances
These can include basalts,
and carbonatites, Africa,
capable
per-
and peralkaline and peraluminous
is
characterized
is concentrated,
by
crustal
and as a continent
the hot spot mechanism
doming
that has
for anorogenic magmatism
is very plausible. Another Africa shear the
view
zones.
Control
occurrence
(e.g.
(Vail,
1989)
is
that
Phanerozoic
ring
complexes
in
show a distribution pattern which suggests control by deep-seated of
Fig.10.4).
country linear
rocks trends
trends,
by pre-existing
almost
all
crustal
complexes
These usually show parallel
and with of ring
associated complexes
dyke
line,
are continuous
credence to the hypothesis generated
along
the
complexes,
lieved to have
served
linear
groups
and
fractures.
(Fig.10.4).
Luderitz,
Angola,
fracture
Sometimes
Some
of these
and the Cameroon
zones,
thus lending
that ring complexes are the products of magma
continental
(Kinnaird and Bowden, of alkaline
swarms
with oceanic
is suggested by
definable
strike with the surrounding
are parallel
such as those of Damaraland,
volcanic
lineaments
within
extensions
of
oceanic
1987).
In addition
to controlling
these
reactivated
deep-seated
as channelways
fracture
the emplacement
lineaments
for hydrothermal
zones
are be-
mineralizing
flu-
ids. 10.2.7 Mineralization in Alkaline Complexes Of
all
the
Phanerozoic
Younger Granites
of the Jos plateau examples
of
alkaline
province
(Fig.10.3)
tin deposits
complexes
in Africa,
is the most mineralized. contain
which
are
the
West
African
The Younger Granites
some of the finest and best known
associated
with
anorogenic
granites
631
(Bowden and Kinnaird,
1978;
Sawkins,
following
1990).
The
Younger Granites Alluvial mineral tion
1982;
summary
is drawn from Sawkins
tin mining
export
Ekwere,
until
consequently
on the Jos from
of
cassiterite
70,000
tonnes
about
10,000
mineralized of
the
stockworks
biotite
(Fig.10.5),
and columbite
respectively.
the
which
Morginai z one quartz
~ '
tonnes
alluvial
anually
.
" ,
In the biotite tain and
grains,
<,~,;,,~-,,~
........
toc
cassiterite
greisen
wo,k,
and
zones
and
sulphides.
The
and
occasional
molybdenite.
menite,
zircon,
emeralds).
Other
thorite,
fluvial channels. The
minerals
in the roof
mineralization
,
tantalite
include
are
also con-
chalcopyrite,
also
occurs
fluorite,
gem-quality
as dissemi-
which
sphalerite,
Wolframite topaz,
and
type of granites.
occur
veins
occur in gravel pockets
styles
of mineralization
Hydrothermal
alterations,
four e c o n o m i c a l l y
important phases
and m i n e r a l o g i c a l
evidence.
An
cally
with
introduction
associated
portions
in
magnetite,
beryl
the il-
(aquamarines,
of ancient
and modern
Overburden depths of 30-40 m allow strip mining.
following
granites.
sulphides
monazite,
Placer deposits
and
depleted,
//
and quartz
pyrite
veins.
Total re-
for tin and niobium.
pyrite,
primary
tin
granites
and within
1,700
tonnes
become
primary
Produc-
about
140,000
reserves
the
to
the
earliest
1958.
since then. at
cassiterite
carry
in
Figure 10.5: Schematic section showing the main bedrock tin deposits associated with the Jos Plateau (Redrawn from Sawkins, 1990.)
nated
in
(1985).
Nigeria's
export
1985;
Roof Contact Ring zone zone dike pegmatit@c voiconics
~
'
supported
significantly
and disseminated
granites
Olade,
mineralization
are estimated
hold the greatest potential
Country rock
I
As
the
of p e t r o l e u m
tonnes in 1983, and has not recovered serves
of
1982;
(1990) and Wright et al.
plateau
the beginning
fell
Imeokparia,
the
followed
are which
found are
that are known
initial
columbite),
was
cassiterite
into the biotite granites.
by potassic
sodic of
among
from field
metasomatism niobium
metasomatism
the Nigerian
widespread,
(as
which
exhibit
, textural,
which
is
lo-
pyrochlore
or
introduced
Next came H + m e t a s o m a t i s m
some
result-
632
ing
in
greisenization,
menite,
with
the
formation
of monazite,
zircon,
and
followed by cassiterite and lesser amounts of w o l f r a m i t e
il-
and ru-
tile. L a r g e - s c a l e silica m e t a s o m a t i s m followed, w i t h the addition of cassiterite,
major
amounts
of
sphalerite,
and
lesser
chalcopyrite
and
galena. At this stage there was w a l l - r o c k a l t e r a t i o n involving the formation
of
chlorite
and
clay minerals
mineralizing
fluids
were
Kinnaird
Bowden
(1987)
and
other A f r i c a n
highly
adjacent
saline
attributed
ring complexes
to
with
the
the
veins.
temperatures
The
initial
above
300°C.
lack of m i n e r a l i z a t i o n
in the
to the lower temperatures
of their residual
fluids. Because
of
the
small
total
areas
occupied
by
biotite
granites
(the
m i n e r a l i z e d host rocks) among the Y o u n g e r Granites in Niger and Cameroon, there is lesser tin m i n e r a l i z a t i o n in those countries. tin p r o d u c t i o n
coming
from placers.
Cameroon has small
The Younger Granites
are also a po-
tential source of uranium m i n e r a l i z a t i o n in West Africa.
10.3 Basaltic Magrnatism
10.3.1 M e s o z o i c Basic Intrusives Cahen et al.
(1984) o b s e r v e d that a striking aspect of P h a n e r o z o i c anoro-
genic a c t i v i t y in Africa of M e s o z o i c
dolerite
is the existence in western and southern Africa
dykes
and
pear to be of the same age. the c o n t i n e n t
age,
include
which
are
These
intrusives
common
the
extensively
in a belt which
dolerite
around
extends
"Continental
to the
Bov~
in
basins.
sills
such
stone
of
Leone, 1983;
coast
swarms, as
The
consists Wells,
coastal A of
1962).
the
the
ap-
large areas on
basic
troctolitic An
body,
gabbro
interesting
1985).
in
the
intrudes
and
aspect
especially
sometimes
oc-
in the Taoudeni
and
subsurface, complex
anorthositic the
contain
the Paynesville
Freetown
of
They
Dyke swarms are
Sills,
sequence
the
are
and are not believed
and Liberia.
diabase which
and
sheets
(Fig.10.6).
from the Bov~ basin. Group
(Wright et al.,
especially
to Early
irregular
craton
basin,
northeastwards
basins,
and
African
Intercalaire"
the sedimentary
layered
sills,
West
Taoudeni
in Sierra Leone
intrude
the M o n r o v i a
Liberia.
dykes,
on
to be y o u n g e r than Early Jurassic curring
lavas which
basic intrusives of Late Triassic
exposed
outcrop
intrude up to the parallel
basic-to-acid
are exposed over
(Fig.10.6).
In West Africa tholeiitic Jurassic
associated
These dykes
in
rocks
tectonic
SandSierra
(Umeji,
setting
of
633
Permo-Triassic
dolerite
dykes
the W e s t A f r i c a n craton,
is that
except
and in the M o r o c c a n Atlas.
they are n e a r l y
all
restricted
to
in the southern part of the Mauritanides
This is in contrast to the P a l e o z o i c - T e r t i a r y
a n o r o g e n i c granites which are confined to P a n - A f r i c a n terranes.
r
%'Modeiro I. Cono,rLo
"1°~0
4~D80
8_5~/EI6Cenlrali IE
"k ~ .
Tripoli
4) 0 2' ..V 'Droo C X'
39
i89
A L G E R I A
"l
"*T,.
Haruj
{ ~" ..~ .¢_ jt
"" ~
,4 %"
Too udenni
~ " "~"
k
boPsin k
MAURiTANiA =' "
#
LIBYA
y
Ahog g or y /
~. ~" ~,-
MALl
~
f~ghei
~
,.
•":t~"
~,
2"
f
4
Alr
4
NI
"n~esti,
"R&
4
GER
yx
_Y
F~
CHAD
/'~ ~ ~ ~ ~'~" ~ ~ N
I
(
.5-Q.8 ../) 1 Jos 11-4 581,o i ~
q
NIGERIA
I~%. ! 284 COTE ~ o,
,
\k
A22
('~
o
I
~ X
%-------~JI5 f "
4,
) (I" Princip4 ~'25 / s~o
Tom%5
-- " f - " "
-
Figure 10.6: Distribution of m o s t l y basic Africa. (Redrawn from Cahen et al., 1984.) Southern
Africa
(Fig.10.7)
but m o s t l y in the Jurassic, throughout
the
South Africa,
Kalahari
Lesotho,
shows
between
craton,
the
They occur
as
volcanism
195 Ma and about
and
in
and Mozambique.
fault-bounded
~
emplacement
Karoo
basins
of
West
similar
155 Ma. in
in
dykes,
These occur
places
such
as
M a j o r d o l e r i t e dykes provinces oc-
cur along the Lebombo m o n o c l i n e and the Nuanetsi margin.
'
AMEROON\
£
isolated
syncline on the cratonic
outliers
in the
lower
Zam-
634
bezi valley.
Dolerite dykes also extend from the Lupata gorge to the Sabi
valley.
(o) Central parrot continent North Victoria Falls ( Zambesi Valley) Olivine.poor basalts (lO00m)
Tuli Syncline ( Limpopo Valley) Olivine-poor basalts Olivine.rich basolfs ( total z,O0m)
Transvaal (Springbok Flats) Olivine-poor basolts (400 m)
South Lesotho Ohvinepoor basalts 11SOOm)
(b) Eastern part of continent North Lupata (Zam besi Valley) Phonolites, rhyolites--~ Lupata trachytes j Series Rhyolite Basalts
-L .j-
Korea
Nuanetsi,Sobiand Northern Lebombo Rhyolite Gro u p Otivlne.poorbasolts Olivine-rich basalts Nephelinite (total 6.7kin)
South Southern Lebombo (Swazilandand lqozambique) Upper Basalts and alkaliclavas Rhyolite Group Olivine.poor basalts (estimated c.10Km)
B Figure 10.7: A, distribution of mostly basaltic volcanic rocks of southern Africa. B, generalized stratigraphy and thicknesses of Karoo lavas. (Redrawn from Cahen et al., 1984; Burke et al., 1981)
635
Mesozoic
basic
contemporaneous,
intrusives
were
in w e s t e r n
intruded during
and
southern Africa,
the b r e a k - u p
which
of Gondwana.
This
are is
supported by the fact that the sites of rifting and f l e x u r i n g during continental
separation
are
consistently
characterized
by
olivine-rich
basalts with rhyolites, whereas tholeiitic basalts are p r e d o m i n a n t on the cratons
(Cahen et al.,
In n o r t h e a s t e r n
1984).
Africa
dykes
are
unusually
the
Precam-
Sinai peninsular,
abundant
the Red
brian terranes
of the Eastern desert of Egypt,
Sea Hills,
in the Bayuda desert of the Sudan.
narrow, long,
and
in
T h e s e dykes which are
steeply dipping bodies a few meters thick and several
occur
in
(Cahen et al.,
swarms
which
1984).
may
extend
for
These are doleritic,
several
granitic,
kilometres
hundred and
kilometres
felsitic dykes,
the ages of w h i c h range from Precambrian to Cenozoic.
10.3.2 K a r o o V o l c a n i s m The
Karoo
plateau
Supergroup
basalts
age. Cox
in
km 2, a
lion km 2. The ness
Africa
of
is
overlain
of m o s t l y Late
South
fraction average
about
Africa, of
volcanics
an estimated
Early Jurassic
extensive
is about
Known as the Storm-
(Fig.10.7)
initial
extent
1,000 m,
10 km on the Lebombo monocline.
into the Cretaceous
very
to
(1982) gave d e t a i l e d accounts of Karoo
Karoo
thickness
by
Triassic
from w h i c h the following outline is drawn.
velcanics
140,000
southern
(1972) and Tankard et al.
volcanism, berg
in
and rhyolites
with
Karoo
of
cover
about
about
2 mil-
a maximum
thick-
volcanism
persisted
in the lower Zambezi v a l l e y w h e r e it is known as the
Lupata series. Karoo
basalts
composition.
occur
Vertical
mostly
as
horizontal
dolerite
and
Fig.10.7. olivine to
compositional
In
the
basalts
rhyolites.
northern at the
The
m i n e d near M e s s i n a
variations area
base
breccia
(Lebombo,
change
pipe
in Karoo
and
in South A f r i c a
replacement
preceded
the
of
Karoo
final
similar
separation
in the Early Cretaceous.
resulted of
lavas,
The stratipresented
in
for
example,
olivine-poor
basalts,
copper
deposit
that
is
formed h y d r o t h e r m a l l y by the leaching (Sawkins,
from
1990).
the Late J u r a s s i c - E a r l y Cre-
(Serra Geral Formation) basalts
are
Nuanetsi),
through
T o g e t h e r with their partial equivalents, outpourings
(Fig.10.7).
basalts
Tuli,
upward
of copper from c o p p e r - r i c h Karoo rift basalts
taceous Parana basalts
of
or feeder dykes are known for the S t o r m b e r g
the D r a k e n s b e r g basalts, and in the Tuli s y n c l i n e graphic
sills
of Brazil the
southwestern
(Fig.9.22E),
tensional
Africa
from
regime
the
which
South America
636
10.3.3 Kimberlites African
kimberlites
1984).
cluster
An older Jurassic
berlites
approximately
Karoo volcanics.
into
two main
overlapped
and
to Eocene
(93-53 Ma).
of
in Africa
Cahen
kimber!ites
t
/
followed
A second phase of kimberlite
Late Cretaceous kimberlites
age
to Early Cretaceous
intervals
(Cahen
(190-134 Ma) closely
the extrusion
emplacement
•
et al.
(1984)
North I'~roo
Asthen0
,,:::\I I(~
~".. " ..-....'....- ~ "...
Y_...
was during the
stressed
the
0
\
JI
points
CaJ~f~Fold 8e(t
that
(Fig.10.8).
o
R ecen t
coastline
of
In their review of the distribution
seem to be confined to Africa south of the Sahara
"
et al.,
group of kim-
South Falkland PLateau
I
Figure 10.8: A, distribution of kimberlites in Gondwana: solid dots, kimberlites; open dots, kimberlitic rocks; triangles, diamondiferous kimberlites; dashed lines, limits of Mesozoic kimberlites; dotted lines, limits of cratons. B, flat-plate subduction model for the origin of the Cape fold belt. (Redrawn from Helmstaedt and Gurney, 1984; Tankard et al., 1982.)
637
T h e y are best d e v e l o p e d
in South Africa,
Tanzania,
Zaire-Angola,
and be-
tween Sierra Leone and Ghana where m o s t l y the y o u n g e r g e n e r a t i o n of kimberlites are found
(Wright et al.,
1985).
A l t h o u g h k i m b e r l i t e s constitute a vast topic on which enormous ature
is available,
(Wright et al. sary
a brief m e n t i o n
1985)
because
the
is adequate
type
area
of
their
characteristics
for our present purpose.
for
kimberlites
is
in
This
South
liter-
and origin is neces-
Africa,
and
African kimberlites dominate the world's diamond production. Kimberlites rocks, and
carry
the
usually
porphyritic
comprise~mainly
carbonate,
often are
which
are
only
igneous
diatremes
rocks
depths
into the upper crust. depths
of
by
150 km
kimberlites
and Park,
of
pyroxene, They also
mantle
diamond. which
rocks.
They
rise
are
Kimberlites usually
rapidly
from
em-
great
forms at great pressures which obThey
after
rapidly,
green
olivine,
survive
their
diamond
is
to
reach
formation.
At
released
into
the
earth's
the
surface
placer
for 90% of the world's d i a m o n d p r o d u c t i o n
area
for kimberlites
the k i m b e r l i t e
Karoo d o l e r i t e xenoliths
below.
to
de-
(Guilbert
1986).
The type many
and
transport
weather
posits w h i c h account
Diamonds
gray
ilmenite megacrysts. and
contain magmas
fine-grained
phlogopite,
and
crustal
gas-rich
surface by rapid upward since
of
that
as
at
garnet
xenoliths
placed tain
serpentine,
with m a g n e s i a n
rounded
to
of
pipes
sills w h i c h
country
rock
is at K i m b e r l e y
intrude
are
Karoo
found in the
occur
in the
in South Africa.
strata
(Fig.10.9),
sedimentary
kimberlite
sequence.
pipes,
the
Here
including Since
pipes
are
therefore y o u n g e r than Karoo strata. Africa
is
the
world's
largest
supplier
of
gem
and
industrial
dia-
monds. A l l u v i a l d i a m o n d derived from M e s o z o i c k i m b e r l i t e s occur along the Vaal and Orange valleys in South Africa. vial
diamond
are
Zaire,
Central
Other
African
sources of M e s o z o i c allu-
Republic,
and
Sierra
Leone
in
West Africa. Helmstaedt Cretaceous ment
and
Gunery
interval
throughout
kimberlites
considered
the most
southern
important
Africa.
They
why
phase
supported
the
Jurassic
of k i m b e r l i t e the
view
that
to
Early
emplaceMesozoic
o r i g i n a t e d above a shallow subducted oceanic plate which un-
derthrust
the
duced
Cape
the
was
(1984)
region fold
during belt
in
the
Gondwanide
the
Late
orogeny
Permian
-
(Fig.10.8)
Early
and
Triassic.
pro-
Large-
scale s h a l l o w or flat-plate subduction under southern Gondwana,
prior to
continental
from
subducted
break-up, slabs
which
is
believed
could
have
to
have
caused
generated the
volatiles
voluminous
Karoo
the
flood
638
basalts.
The
volatiles
could,
additionally,
have
generated
upper mantle metasomatism and kimberlite magmatism crustal
lithosphere.
The
contribution
volcanism does not, however,
of
the
in the overlying sub-
above
fragmentation.
the
lithosphere
processes
processes
negate the view previously
roo volcanism was related to Gondwana fundamental
below
the
large-scale
stated,
Rather,
which
to
Karoo
that Ka-
it suggests
could
have
ac-
counted for the copious Karoo basaltic volcanism and attendant kimberlite intrusion which
had no counterparts
during
the break-up
of northwestern
Gondwana.
Crater facies kimberlite Wote r'~"
cove
Sandstone
:::.::If::::::
~"
=
Red beds
l~ :~ .~ "1o .o E
E
Beaufort,
=
~,o-~
~'" "
..............
--F " ~ ;..... ~' :-~,
;Ore
i°'~%~o'~o o=e, ~0~ I "
'
"
°"
, O~ ~
t~*
~ l
° ' o e 0
t ,
* ~ o t r e m e fa ~---'----'--~-kimberlite ~ o <= "tu ffisi t e "
Koroo d o l e r i t e sill
Karoo -200m Ecc° shales Dw y k° shale s O. o
~ n t e r SO
Om Present day E s u r f a c e Kimberley o p i p ° s a n d sills
o
"p
! i i ! i ! ~ i ! ! Venter sdorp quar tz-~----"--~ porphyry
o E
.-~
(2
~ L
ill
m
0 C Ib (U l:n.C .Q um~'N
L!!i!.:!i':i:"
,+
Kimbertite oE
; ~ ; %i ~; ill;I- o
N;I:F :-I .'
o. ~
~]
+
facies
\t'
* "tr'l Kor ~ ,Ventersdorp,Kimbertite
÷ "
~
1
* II c c +1 + ] * I
I
. I .41 ~
F.IjF
feeder dykes'
Tuff cone
I~J Finesed~me°,c os00d =so
%Hypobyssa[ I +1 kimberlite
m
t
ver Conglomerate Veal River L
tuff fntrusive bre¢'cios Sills
-1000 m
500 m !
Figure 10.9: Schematic cross-section of a hypothetical kimberlit° pipe shortly after emplacement; local names at the right denote outcrop erosion levels of specific pipes: (Redrawn from Guilbert and Park, 1986.)
639
10.3.4 C e n o z o i c
East A f r i c a n
Burke
et
ing place canism, doming. tle
in A f r i c a seven
outlined
the
tectonic
under diverse
setting
along
1989).
structural
in the East A f r i c a n Rift
hot
spots.
The hot
As p r e v i o u s l y noted
diapirism
Curtis,
(1981)
of
African
Cenozoic
Figure 10.10 shows that extensive intraplate v o l c a n i s m is tak-
is however,
identified
Hot Spots
Rift System
al.
volcanism.
Continental
the
However,
(Ch.
spots
9.8.2)
Eastern
branch
settings. System,
manifest
Most
where as
of the volBurke et al.
areas
of
crustal
doming has been a s c r i b e d to manof
the
Rift
Valley
the domes do not carry volcanoes,
(Karson
and rifts are
~ w aa'-"~J,"h Haruj Ih ID,
TAOUDENI E ,:~:~ Iforas' Cape Verde Ids.~'
'
Ain~ ~/:'~':'.:.~
..
'-.
~ED SEA SWE Danakil
La~'e Fc~uibine ~BA~N;:-~Jeb~-M.. °rra %.';...,':;, t "~, 1 ~ El Qbeid ~'outa Jos( .~ ~IlB,Blu --~ ," ~ .".-
Princi Sao To
"Naku,'~' ~u' - ~ SWELL
~K
Ngaounde're"
%Comores
/ C~- ~,~N.MADAGASCAR
Bid •
~
Plateau
St. Helena
CUBANGO~.:.'.:::l~ BASIN v&'EJ~/'~R /-Darnara
~
~,'21 /
~Gough~ Id.
MADAGASCAR • SWELL
/
I'" Discovery seamounts ~) BOUVet
Rodri guez ReuDion
S
Tristan da Cunha
"~
EAST AFRICAN SWELL ilimanjaro /'~'~
Mr. Cameroon
~
NEOGENE VOLCANICS NEOGENE UPLIFT
Figure 10.10: Distribution of hot spots and Neogene the A f r i c a n plate. (Redrawn from Burke et al., 1981.)
and
(.___~)
uplifts
on
640
mostly
located
(McConnell, the
East
Ababa
not
1972).
African
on
swells
There
Rift
in Ethiopia,
is
but
on
widespread
System,
volcanism
but
reactivated
occurrence
in some
areas,
is tholeiitic.
of
old
structures
alkaline
for
example
The d i s t r i b u t i o n
rocks near
in
Addis
of volcanism
along the rift is also very uneven. In the Tertiary.
Eastern The
branch
volcanics
with u n d e r s a t u r a t e d basalts, Karson
and
of the Rift V a l l e y
in Kenya
varieties,
nephelinites.
and Curtis
(1989)
trusive
rocks
of such large volumes
requires
the
chambers
and p l u t o n i c
In the W e s t e r n cally d i s t i n c t Toro-Ankole north
field.
volcanic
provinces
itic
and/or
basalts. layer
and
cumulates They
through
Eastern
of
trachytes, extrusives,
and other
concluded
that
intermediate
branch
of very
inthe
volcanics
large magma
are
three
These
et
petrographically
comprise,
volcanoes;
in southwest
al.
(1987)
attributed
to
interface,
a
rapid at
that
the
the
Western
basalts, rise
of
and the
the b e g i n n i n g
of
the
field in
and
south Kivu
(from the oldest)
alkaline
geochemi-
to south,
the Virunga
Uganda;
ob s e r v e d
show a sequence
basalts,
and
from n o r t h
branch
of tholei-
transitional
lower the
velocity
extensional
Late
many
Volcanic Centres
Cenozoic
other
(1984) an
is
belt,
from Ahaggar,
alignment,
volcanism
centres
these
There
of
northeastern Chad.
Mountain
through
in w e s t e r n
Sudan
in the A f r i c a n
doming
Air,
volcanic
Nigeria, An
Adamawa the
and
to the Jos line starts centres
continues
volcanic
Ngaoundere
(Fig.10.10).
east
centres
and shows volcanic
continent
(Fig.10.10)°
located
volcanic
southern
the C a m e r o o n
northern
of
are m o s t l y
alignment
chain of islands in
volume
alkaline
regime.
Extensive
et al.
the
Bufumbira
generally
they
of
branch.
in Uganda with extinct
transitional
Other Continental
with
there
to the m a n t l e - c r u s t
tectonic
the volume
Eastern
in the mid-
strongly
b e i n g phonolites,
enormous
of basaltic
fields.
Lualaba
This,
the
began
to
bodies.
branch
(Zaire)
volcanic
the
underneath
volcanic
fields
Kivu
on
estimated
underlying
volcanism
from m i l d l y
the commonest
Based
erupt i o n
existence
range
As
observed
of
the W e s t
in
the
Trans-Saharan
plateau
in Nigeria.
on the Cameroon
through
Lake
extends
volcanic
fields,
by
African
from the Gulf
line
is associated
ENE
Chad from
probably
Cahen
craton. mobile Another
of Guinea
as a
Mountain, to
Bui
Tibesti
the
in
Cameroon
to Jebel
Mara
641
In the Atlas belt of Morocco, plugs
of
Miocene
to
A l g e r i a and Tunisia there are volcanic
Quaternary
age.
Similar
volcanics
are
found
near
Dakar in Senegal. The above volcanics are p r e d o m i n a n t l y basaltic lavas b e l o n g i n g to the olivine-basalt-trachyte Mountains. calderas.
The
Cahen et al.
ing d e e p - s e a t e d on ring-dykes date
association.
volcanic
centres
faults. Volcanoes are sometimes
in the Younger Granite
Volcanism
has
occur
mountainous
in the Cameroon
relief
with
large
(1984) attributed their d i s t r i b u t i o n to pre-exist-
from the mid-Tertiary,
tivity.
Feldspathoids show
situated along the faults
complexes.
Most
with the M i o c e n e being
continued
down
to p r e s e n t - d a y
of
these volcanics
the time of peak acin one
form
or the
other in some places. An example was the Lake Nyos gas e r u p t i o n on August 2, 1986 on the n o r t h w e s t e r n part of the C a m e r o o n M o u n t a i n
(Freeth,
1987).
N o r t h e a s t Africa also w i t n e s s e d considerable Cenozoic v o l c a n i c activity.
Flood
basalts
and
shield volcanoes,
and t r a c h y t e
domes
are
exposed
in Libya, w h e r e trachytic rocks and basalts of M e s o z o i c age have been intersepted in oil wells.
In Libya basalts overlie Upper Cretaceous and Pa-
leocene sediments at Jebel as Sawda. A p p r o x i m a t e l y 40,000 km 2 of probable Oligocene
olivine
p o s i t s - a t Haruj
basalts
overlie
Eocene
and
Upper
Cretaceous
de-
(Fig.10.10).
In the Tibesti Mountains,
the highest peak in n o r t h e a s t Africa,
taceous and Paleocene sediments plosive
Lower
volcanic
centres.
Here
rocks are also predominant, Scattered b a s a l t i c
are o v e r l a i n by shield v o l c a n o e s basaltic
and
w i t h hot springs
acid
ignimbritic
indicating
Cre-
and exeruptive
recent activity.
and trachytic rocks occur at Jebel Uweinat;
along the
Red Sea coast of southern Egypt; and along the Nile V a l l e y in the Sudan.
10.3.5 O c e a n i c Hot Spots The A f r i c a n p l a t e contains several v o l c a n i c a r c h i p e l a g o e s and
Indian
Oceans
(Fig.10.10).
lands of the A t l a n t i c Ocean. to the m i d - o c e a n i c
ridges;
are
close
situated
quite
Here we
shall
be
in the Atlantic
concerned
with
the
is-
Some of these islands are located v e r y close
some lie along t r a n s f o r m faults; to the continental
margin.
w h i l e others
Although
the origin
of v o l c a n i c islands is still being debated, we shall for our present purpose and c o n v e n i e n c e include them among hot spot activities. A l o n g the continental margin of n o r t h w e s t A f r i c a groups
of
volcanic
lands.
Cahen
et al
islands, (1984)
the
Cape
summarized
Verde
there are two major
Archipelago,
the g e o l o g y
of
the
and Cape
lands. Two m a i n groups of volcanic rocks make up these islands.
Canary
Is-
Verde
Is-
A central
642
igneous
complex
of p r o b a b l y
lain by limestones by
Late
Miocene
in MaYo,
Late
Jurassic-Early
Cretaceous
one of the islands.
essexites,
carbonatites,
and
age
is over-
This complex is intruded nepheline
syenites.
These
a l k a l i n e intrusions were followed by the extrusion of stratovolcanoes and phonolites Islands which
in the latest Miocene and Pliocene.
were
constructed
formed
during
part of the ocean
an
early
floor.
In general
phase
This was
of
the Cape Verde
tholeiitic
volcanism
followed by s t r o n g l y un-
d e r s a t u r a t e d a l k a l i n e magmatism. The C a n a r y Islands consist of seven m a j o r v o l c a n i c islands. tend
for
nearly
northwest African Canaria, which
and
500 km coast.
Tenerife.
constitute
Eocene times,
east-west.
these
The
They
all
largest islands
Schmincke islands.
(1982)
lie
about
include
described
Although
volcanism
These ex-
I00 km
off
Fuerteventura, the
volcanic
began
the Gran
rocks
probably
in
the C a n a r y Islands were built m a i n l y d u r i n g the last 20 Ma
(Miocene). Saturated tholeiite
was
and T e n e r i f e
to
moderately
the
dominant
contain
undersaturated shield-building
alkali type
of
basalt magma.
with Gran
large caldera-forming ash flow eruptions,
local Canaria
the prod-
ucts of more d i f f e r e n t i a t e d magma. Minor amounts of trachyte are found in the w e s t e r n
islands;
and phonolitic plugs occur in the central and west-
ern islands. Schmincke mas
in
thick
the
(1982) linked the highly alkaline m a f i c u n d e r s a t u r a t e d mag-
Canaries
lithosphere
and
the
beneath
Cape
them.
Verde
This
Islands,
would
have
and only reduced partial m e l t i n g at greater depth. no
geological
or
geochemical
evidence
for
crust b e n e a t h any of the C a n a r y Islands. of the
islands
along
an oceanic
fracture
the
to
the
allowed
presence low heat
Schmincke
presence
of
He also d i s c o u n t e d zone.
Schmincke
of
a
flow,
(1982) found continental the location
suggested
that
islands are located over a zone of m a n t l e i n s t a b i l i t i e s along the boundary between oceanic and continental of the hot spot hypothesis.
lithosphere.
This
is another variant
Chapter 11 The Quaternary in Africa
11.1 Introduction The Q u a t e r n a r y
is the
1.8 m i l l i o n years referred as
the
which
to as the geological
nessed tively.
For
and climatic
during
alternations
reason
and
period to which
the Q u a t e r n a r y period
corresponded
this
in academic
Ice Ages, interval
spectacular in A f r i c a
latest geological
the
last
of Earth's history has often been assigned. the
is now characterized
climate
of cold dry phases
to
the
interpluvials
of
the
interests
because
of
Earth wit-
and w a r m wet
phases,
and
respec-
pluvials
Q u a t e r n a r y geology has w i t n e s s e d
public
changes.
which
2.5 to
Originally
on-going
Quite apart from the climatic
a recent
global
surge
ecological
impact of industrial
pollution there is a need to understand past climatic changes in order to predict what the future climate might be. Perhaps it is in the tropical region of the w o r l d such as A f r i c a that the
impact
of
widespread. posits,
were
laid
rose
and
valleys level;
fell
on
sediments,
in different
as to
the
ice-caps
waned
most
severe,
soils,
cave
parts
continental
resume
is
laterites,
shelf and shoreline
down
only
changes
loess,
lacustrine
accumulated
deltaic,
climatic
desert
colluvia,
deposits beds,
these
While
of
earth the
deep-ocean
margin
and
and waxed; at
next
de-
glacial
continent,
peat
fans and carbonates
deep-sea
rivers
the
and
terrace
and m o u n t a i n
African
sands,
down-cutting
varied,
alluvia,
floor.
rapidly abrupt
Sea-level
filled
change
their
of
lake levels rose and fell and some lakes never reappeared.
base
Forests
turned into deserts. All plant and animal groups r e s p o n d e d to these sharp climatic changes m o s t l y by m i g r a t i n g back and forth. African
vertebrates,
especially
the
crossed in the h i s t o r y of life,
primates,
as Man appeared
a
But s o m e w h e r e among major
threshold
was
at the dawn of the Qua-
ternary. For the frail new creature that had suddenly appeared with an unusually
prolonged
ardous
infancy
ecological
and
changes
childhood
and
what turned out to be, perhaps, human brain. tral
place
Since in
the
stress
dependence, contributed
the most
these to
of
this
period.
rapid
and
haz-
development
of
spectacular a d a p t i v e o r g a n - - t h e
"man is a child of the Quaternary" story
the
The
he occupies
evolution
of
his
a cen-
adaptive
strategies i n c l u d i n g his t e c h n o l o g y and culture not only e n s u r e d his survival, but using these tools, man g r a d u a l l y c o n q u e r e d and c h a n g e d his en-
644
vironment.
Man's m a s t e r y of nature in turn p r o d u c e d a n e g a t i v e feed-back
on climate, a new
vegetation,
geological
agent
soils, and water.
In this respect man appeared as
that
altered
has
greatly
the
geosphere
and
bio-
sphere. Africa occupies
an important place in the current w o r l d - w i d e
about global ecology. soil
erosion,
Recent ecological
desertification
and
concern
changes on the continent
the
destruction
of
the
such as
tropical
rain
forest e c o s y s t e m have attracted considerable attention. Since the Q u a t e r n a r y Period is replete with these p r o f o u n d and on-going changes therefore
on the surface of the Earth,
a multidisciplinary
geologist,
archeologist,
geographer,
scientist, oceanographer, Africa longest
is
the
and most
science
Tanzania,
record
oldest
and most
stable
continent.
It has
record
of man's
origin,
habitation
the
most
parts
fossils volcanic
and
in Kenya,
and
where
in the
created
an e x c e p t i o n a l l y
and artefacts,
Q u a t e r n a r y deposits
thus
long and unsur-
according
east
tuffs
which
are unfossiliferous,
(Fig.ll.l).
basins
in
Here
which
earth movements
excellent
beds
Interlayered within the sedimentary are
and
Unlike in-
the East African Q u a t e r n a r y sequences are
rift v a l l e y
sedimentary
and artefacts.
the
early
in the Hadar
the u n r i v a l l e d position as the cradle of mankind.
preserved
repeatedly
unfolded
fossils
complete and d i f f i c u l t to date, well
yielded
The discoveries of homonid sites in
of Lake Turkana
in Ethiopia
of A f r i c a
the soil
since the 1920s and later at Olduvai Gorge in
shores
of homonid
southern Africa
others,
biologist,
complete
and Awash valleys passed
among
and engineer.
(Figoll.l)
around
involves,
paleoanthropo!ogist,
technological and cultural evolution. southern A f r i c a
the study of the Q u a t e r n a r y is
that
excellent
geological
marker
have
preserve
sequences are
beds
that
have
yielded radiometric ages, by means of which homonid fossils and artefacts and
the
record
of
climatic
other parts of Africa, Like meaning. time
as
boundary has,
other
changes
have
been
dated
and
with
subdivisions
of geologic
time,
the Q u a t e r n a r y
has a dual
It can be used for the last 2.5 to 1.8 million years of geologic well of
as the
however,
for
the
at
deposits
(e.g.
rocks
Quaternary
remained
boundary
2.5 Ma
that
period
unresolved
(Fig.ll.2)
Boellstorff,
formed or
as
based
the some on
during
this
1988).
time.
The
Pliocene-Pleistocene authorities
studies
of
have
placed
continental
lower
boundary this
glacial
1978), while those working on oceanic oxygen
isotope records g e n e r a l l y favour an age of about 1.8 Ma al.,
correlated
and the world.
(e.g. Williams et
645
NORTHWEST DEEP
AFRICAN
- SEA
RECORD
Oxygen isotopic
record.
L
,~% A F R
Hormottom dust discharge.
Fluvial discharge / , ~ ( ~ % " ~ S A H A R A DE SER T sediments. ~ ~"Ancient rivercourses. Carbonate content Rock paintings. of deep sea Ancient dunes. sediments. Fossils in ancient takeand swamp deposits. Planktonic 2 ' ~ ~" " " "" '-.. ~ ... /&~ /~ foraminifera. . . ~ -- '.,- ~ /"~ Laterltes. mselbergs,r~ver ~ I terraces vegetational history/
~Hodar • "
•, o j
and
Awash
~
Vstteys
WEST AFRICA
P j
3
ZAIRE
D E E P - SEA
Carbonate
Faunal and floral
history
content of
[.'. 'L''
deep-sea sediments. Influx
of
A IR ' E * "BASIN "
FAN
V~.,
"~
sediments and plant
l
•
Homonid
~
of planktonic fossil
foraminifera.
Fast
and take basins,
h~.~
sites.
Line separating Low Africa to the Northwest from High Africa to the east and south,
|Fossil
I • Hakapansgat Sterkfontei. omonid and stone fools. ~ Sediments and periglacial features,
~unes /H
African Rift System
Limits
Homoo~ds aRd stone toots. Sediments and fossils in rift voUey
I:'tl ) Ancientlake levels, SOUTHERN!.~ ) Mouotaing~aciation. Tt~'/'~'ALAHARI FRICA~ ' / Vegetational.istor~
remains from the Zaire Basins. Migrations
[::".! ,~t..:..C/ ,~ '~'.'i .,~,~ ! ! / ~
of Deserts
I. Z. 3.
NW AFRICAN CONTINENTAL MARGIN ( CORE12392-I) OFFSHORE SENEGAL DELTA ( CORE 12 345 - 5) OFFSHORE NIGER DELTA (CORE W A L D A / KW-31) 4, ZAIRE DEEP - SEA FAN ( CORE l e O - 7)
Figure ii.i: African l e o c l i m a t i c indicators.
Bowen
(1978)
cept
in
18th
century
attempts sense, propo s e d
Europe
to the
traced where
to
the
term
and
and
International
t e r n a r y age on the basis
age
of
the
the
oldest
Tertiary
Quaternary human
Quaternary
the
Geological
Period.
artefacts
their
was
first
deposits.
use
of
Congress
the in
the Q u a t e r n a r y
This
later from
archeological
reinforced the
Olduvai
when
Quaternary
pa-
of the Q u a t e r n a r y applied In
one
in
Great
the
of
the
Quaternary
were the c h a r a c t e r i s t i c
of which
was
and
development
superficial
standardize
that human artefacts
the p r e c e d i n g
regions
the historical
alluvial
define
natural
in
Britain
element
implication
Gorge
was
a
time
in
1888
of the Quafrom
in the us-
discovered 2.5
midfirst
should be s e p a r a t e d
it was
con-
to
that
2.0 m.y.
646
old, and hence very close to the lower b o u n d a r y of the Q u a t e r n a r y Period. From the time it was first used,
the term Q u a t e r n a r y was also defined to
include deposits with fauna and flora that have living representatives.
u4 i-
CONTINENTAL ........... OPEN OCEAN GLACIATION K - A t ond SURFACE SHELF SLOPE {N. AMERICA ) MAGNETIC SCALE TEMPERATURE SEA LEVEL PRESENT PRESENT CONCEPTUAL I03 YEARS .~OLEER~,WARMER LOWER4--~HIGHER STAGES
EGYPT ' PLUVIALS
NILE STAGES ARCHEOLOGY
Neon~le
.J
<
~'~
Y Neonile
Q3 KubbQnivon ~: ,
~:z
.....
)
~_
-- ~ ~ "Sahamn |l ~: Altonion '! Soho.ran I Korosko
~
/--2
,}
MONIAN
~
/
•
Nobt;on
;jTwocree,on
~qAMIL[
~
Q~
~k~'k-~ ILLINOfAN
~
M~ddle PGleollthic
o,e;o,.o,,oo
~ Neonik~
Abbassian I concjtomerate Prenile ?
~
Late
PaleoUth~c
Armantic, n Cong~omerote
Pebble
Tools
~j ~ GJLSA -uJ
AFTONIAN j
Idfuon
Protonile
~LOUYAt
~
~E BRASKAN
~,N~'~COm~ENTAL ~'~%~GLACIATION
oz ~
Figure 11.2: C l a s s i f i c a t i o n of the Nile Q u a t e r n a r y deposits in Egypt. (Redrawn from Said, 1990c.) The d e v e l o p m e n t
of the glacial
theory in Europe
in the
19th Century
also brought about the fourth and most w i d e l y adopted concept of the Quaternary.
The
Q u a t e r n a r y was
accepted as the Glacial
Period during which
extensive and frequent continental glaciations occurred and molluscs from the cold
region m i g r a t e d
the Q u a t e r n a r y
as
tions t h r o u g h o u t broader period more
rocks.
concept,
the
universally
niques,
into the
lower
latitudes.
the time d i s t i n g u i s h e d
This
redefinition
by severe
climatic
the great part of the northern hemisphere"
climatic outside
"...
and
temperate
acceptable
and principles,
laid
the
region.
foundation
It
stratigraphic
called
for
for
methods,
introduced a
recognizing
the
this
application
refined
of
condi-
dating
of
tech-
for the subdivision and correlation of Quaternary
Equally important has been the need for reliable p a l e o c l i m a t i c in-
dicators w h i c h can be dated and correlated.
From the m i d - 1 9 t h century the
647
P l e i s t o c e n e has been accepted as the lower geologic epoch of the Quaternary, and the H o l o c e n e or Recent for the present epoch.
11.2 The Quaternary Physical Geography of Africa Before
examining
pertinent govern
the Q u a t e r n a r y
first
their
to
consider
distribution.
sedimentary
the The
successions
geomorphic
and
contemporary
climatic
physical
tures of Africa are not only significant because to
the
past",
but
because
the
Quaternary
in Africa factors
and
is
so
which
climatic
"the present
Period
it is fea-
is the key
short
and
the
A f r i c a n continent has remained so stable, that at no time during this period,
could
the geographical
cally d i f f e r e n t
framework of the continent
from what is today.
have been radi-
Even during the peak of the pluvials
ancient lakes Sudd, A r a o u a n e and Zaire existed w h e r e they are supposed to be, that is, in the great interior depressions or basins the
present
geography
and
climate
(Fig.ll.4A)
against w h i c h
to assess
paleoclimatic
(Fig.ll.4B) departures.
Throughout basement
the
continent
terrains
deposits
(Fig.ll.3)
which
Ahaggar, valley
especially
in West Africa
is receiving
Tibesti,
sharp contrast, rift
and
constitute
the
past p a l e o g e o g r a p h i c
in West
are
thickest
sediments
Cameroon
and
from
Ennedi
in
the
Africa the
from
Ethiopia
to
Lake
Mozambique
The
(Fig.ll.3)
Drakensberg north
Mountains
of w h i c h
form
the
lie the Kalahari
and
exposed
Chad
basin
uplifts
of
1976).
In
by the spectacular
and
within
great lakes and thick piles of Q u a t e r n a r y fluvio-lacustrine, debris.
the
(Burke,
the East African scenery is d o m i n a t e d
running
line
and alluvia.
surrounding
Mountains
base
(Fig.ll.3)
have thin Q u a t e r n a r y superficial deposits
Quaternary the
and explain
(Fig.ll.3). Also
backbone
of
desertlto
which
lie
and volcanic
southern
the w e s t
Africa
and vast
rolling plains to the east and north, where the b e s t - k n o w n Q u a t e r n a r y deposits draai
are
the cave earth of Taung,
Sterkfontein,
Makapansgat
and Krom-
(Fig.ll.l).
Climatically, and one third of arid and
three-quarters the continent
semi-arid
conditions
of Africa
is located w i t h i n
is affected by winds (Fig.ll.4).
Winds
which
the tropics
have
produced
from the A t l a n t i c
bring
m o i s t u r e to coastal West Africa in July when eastern and southern Africa are dry. While equatorial Africa remains wet in January and most of East Africa sweep
receives across
the
rainfall Sahara
later in April, and most
of West
dry and d u s t y N.E. Africa
down
to the
Trade winds coast.
Be-
cause it receives rainfall throughout the year dense rain forests used to
648
thrive
in the e q u a t o r i a l
maining or
part
montane
fuel wood forest
of
the
(Fig.ll.5A)
This means
continent
(Fig.ll.5A). for rural
region
Bush
Nouakchottlan TrQnsgresslon Inchlrian Transgression
is
the
mostly
burning,
to
savanna,
savanna
most to
~,,,~,,,~TIBEST1 ,
,," /z'"
t
desert, and
The
re-
mediterranean, the
cutting
of
of what used to be lowland
sahel,
and
sahel
upon forested
to
desert.
land.
ASWANDAI
~,
%,,,.,,t'~
• ~"
deforestation.
savanna,
are e n c r o a c h i n g
'%% t~
recent
overgrazing,
e n e r g y have converted
that the deserts
'-2m
before
/ " " ;" " -
%::
- L'C HAg ,~t"
of T Mangrove
Limit
4'~"~ Pa[eo~kes R Ruwenzori Hountalns +
~
-
I
/s&-e'~'J/~ /Io/ ,~i~ ~' S/ ~t-".)~ ~ ! Kcflombo Fcttts -..~tI ~
Raised" Beaches
t~ Sea Cliff .,,.-- Lagoon Coast Carat Coast
f"
Jids
Limit of Hangrove !D
It~
\ 4Victori= Falls !'~
1"Lake Nckuru 2. L~ke Naivasha 3,Lake Hagadl &-Lake Natron 5.Lake Manyara
O Coastal Dunes Sebkha or Satt Pans ~ - Wadi
• Periglacial Features A Hountain Glaciers
ETOSHA >PAN
Aughrabies
R.= Ruwenzor; Mountains E = Hr. Eigon K : Mr. Kenya J. = Mr. Kilimanjaro
Figure 11o3: Outline g e o m o r p h o l o g i c a l m a j o r Q u a t e r n a r y paleo-lakes.
map
of
Africa
showing
649
;
.~?od--/
JANUARY
"~,
\\ "
~~--------7
~'* " ~ ".9 / ' ~ H
. . . . . . . .
A "~,, 10 0 mm
RECENT
Rainfall
C LIH ATt'C PATTERN
/Dry
:.E. Tro,d e s ~ N.£. Monsoon
o,-y/--"r
%
Wester|i,s k
//
~ . / S . E . /
Dry..__~ k j Westerlies ~
//
JULY
~
Cold Fronts
£- Tredes JANUARY
;m100 mm Pluvial Rainfall
B
RECONSTRUCTED 6LACIAL PATTERN Figure 11.4:
Recent and "glacial" climate regimes for Africa.
11.3 Quaternary Deposits in Africa In this section attention will focus on representative Quaternary successions and faunas
in the rather contrasting
and southern Africa.
regions
of west,
east,
north
650
'~L
RECENT
VEO ATION
"INTER- GLACIA[~ V P E A G TT E E T R A N TO IN
[
PATTERN
MEAN ANNUAL RAINFALL
~
VEGETATION
<400ram I"
~ i IISAHEL
4004400 ~ 1400
DESERT
SAVANNA LOWLAND FOREST
MEDITERRANEAN MONTANE B.
A
Figure 11.5: Africa.
Recent
and
"glacial"
vegetational
patterns
for
11.3.1 West A f r i c a As a natural of
the
Chad
west
of
the
the
the m o v e m e n t
of
which
north
shifts
of the southern
Africa rior
is
savanna has
Africa,
namely:
quence,
(3)
and
produced
the
and
(i)
wet
and humid,
desert
coastal
savanna-sahel
types plain
sequence
This
Zone
rains
by
(ITCZ)
over most
is down near
sweep
across
the
part of west
while most
of the inte-
geomorphic
of Q u a t e r n a r y sequence,
and
the Chad
the coastal
bushes
(Fig.ll.5A).
four distinct the
with
and
is controlled
monsoon
winds
lies
Africa
includes
Taoudeni
Convergence
trade
thus low
In J a n u a r y when the ITCZ
north-east
south
separating
from
region the
south-west
lying
watershed
of West Africa
being v e r y close to the equator
is p e r p e t u a l l y
setting
dry
African
as
West A f r i c a
Africa
Inter-Tropical
part of the region. the
the
high
West
The climate
pressure
of
basins.
shields
in July and brings
(Fig.ll.4A)
However,
of
the
basement
Sahara. low
west
demarcation
Guinea
the
and
may be d e f i n e d
Zaire d r a i n a g e
Geomorphologically,
in the w e s t e r n
coast
and
topographic
plain,
region.
(Fig.ll.l)
mountains
from the Nile
(Fig.ll.l). coastal
the
West Africa
Ahaggar-Tibesti
Lake
basins
region
(4) the
(2)
and
sequences
climatic in west
the
basement
se-
Saharan
sequence.
The
651
geomorphic-climatic that different
control
of Quaternary
sequences
parent or bed rocks materials
ferent rates under different climates,
is due
to
the
undergo weathering
fact
at dif-
and different erosion and deposi-
tional processes operate under different climatic regimes. Coastal Plain S e q u e n c e s
Although known under different names the Bullom Group
in Sierra Leone,
in different countries
the Benin
Formation
West African
coastal plain Quaternary sequence consists
cross-bedded
poorly
sorted
fluvial
(for example
in Nigeria),
the
of predominantly
sands and kaolinitic
clays with
lig-
nite beds. The coastal plain sequence includes the deposits of the recent coastal
depositional
subsurface.
However,
environments,
but
its
base
from the coastal
Age cleavers,
ill-defined
in
the
the Benin Formation is about 2,000 m thick while the
Bullom Group is at least 100 m thick. Andah's facts
is
(1979a) review of the arte-
region of Ghana mentioned
Early and Middle
picks and flakes from beach deposits.
If explored,
Stone
the West
African coastal plains deposits could also yield the artefacts of prehistoric coastal fishing communities. palynology,
are
potentially
Thus, archeological methods,
useful
for
dating
the
coastal
including plains
se-
quences. S e q u e n c e s O v e r l y i n g B a s e m e n t in the Rain F o r e s t and Savanna Zones
The
lithological
characteristics
and origin
of the
superficial
deposits
in the rain forest and savanna parts of the Guinea basement arch are well known through the researches
of geomorphologists
1983; Thomas,
1974; Thomas and Thorpe,
toye,
soil
1972),
scientists
1968) and archaeologists basement (1979b), preting
Quaternary who
leached
are
summarized
residual complex.
Nigeria,
below
to
1979a;
Faniran and Jeje,
superficial
(Burke and Duro-
Montgomery,
Sowunmi,
of dating,
their
made
correlating deposits
near Accra, basic
1962;
Moss,
1987). A review of was
by and
Andah inter-
(laterites)
The Quaternary deposits
Asochrochona show
and
interpretations
the problems
the West African basement ment of southwestern
Smyth
(e.g. Andah,
stratigraphic
emphasized
the highly
(e.g.
(e.g.
1980), geologists
and Sierra
lithologic
on
on the baseLeone
successions
(Fig.ll.6). The which
sequence may
(Fig.ll,
be
usually
overlain
starts by
with
laterite
saprolite (Fig.ll,
or 6i)
weathered or
a
basement
stone
layer
6ii). This is overlain by sands or clays which have been washed
in by rain or moved upward by termites and worms. this sequence,
known as the Bodija Formation,
In southwestern Nigeria
is up to 8 m thick. The la-
652
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•• ~II " "I' l::1i ~1' 1 I;11111
; ~, i i ' ~, i ~:
~, x'r~
I
,~,
\
~
~
....
:"11.0.
liilll
::::
;
~
~
Sil$Od30
I
~
~
..~.
. . . .
~'
z
~ ~
~
If/~ .t,~,~i~,--.,li>,.I
.'.1>1~1.>1-
~
"" -.
:.'.-
..r--J
.:
I
~
. ~~
~.
,""
~ ~ I
.
,
~
Z
~
=
"-~
~.
~
'
. . . .
-':
I
~ ~ . . ~~ . - - - -
°~ ~
..
..........
~.~
-"I ..:
"l "-'.
= =
<
w
'
-
3~4VI
--
=
a ..,~
=
•
I"
:..
[',IL1]~I~L~!g:~"! -
i"l i
lilii;'li
~
-,~
z
i
.
\
-~
OVH3
::~i~lll
::,,
ill
-, ~
UO~
I
."
~
Jr
,
•
.,~
~ ' ~ © !
~,-/
~J •
653
terites
in the sequence point to cycles of savanna wet
the stone
layers
and older
laterites,
are considered while
there
is
the some
sequence
of
prospect
of mass w a s t i n g
the sandy and clayey topsoils
der p r e s e n t - d a y humid climate. materials,
as products
and dry seasons;
Because they contain little or no datable
events
of
can
relative
not
age
be
reconstructed,
interpretation
such as the Early Stone Age artefacts from A s o c h r o c h o n a basement the
alluvial
rain
bearing sandy
gravel
clay
fossil
sequence
forests
of
at
the
top
from
where
by a clayey
(Thomas
the
20,000 yr B.P.;
exposed
Leone
is overlain
wood
1,000 yr B.P.
is well
Sierra
and
a
artefacts
(Andah,
1979a). A
in the m i n i n g
excavations and
diamond sand
1980).
deposits
12,500-7,800 yr B.P.,
although
using
basal
alluvial
Thorpe,
alluvial
to Recent,
of basement
are forming un-
layer,
with
Radiocarbon
gave
ages
loamy
dates
of
sequences
on
36,000-
3,300-1,750 yr B.P.,
implying that the basement
in
corrundum-
and
are incom-
plete and may not represent more than the deposits of the Late Quaternary pluvials and interpluvials. S a v a n n a - S a h e l Sequences The
sedimentary
(Fig.ll,
sequences
6iii, iv)
sahel belt.
on
exemplify
the
the
Jos
Plateau
Quaternary
In Burkina Faso, a succession
terized by the repetition of lateritic
and
successions
(Gavaud,
Burkina of
the
Faso
savanna-
1972) w h i c h is charac-
layers and dune sands records the
alternation of laterite-forming savanna climate with the southward migrations
of Saharan dunes.
in mining ries
excavations.
comprising
custrine
sands
The sequence At
laterized, and
clays.
on the Jos plateau
the base
is the
deeply
weathered
These
so-called
is well
Plio-Pleistocene
are d i s c o n f o r m a b l y
followed by three fining-upward fluvial cyclothems.
exposed
Fluvio-volcanic overlain
Se-
fluvio-laby basalts,
The b e g i n n i n g of each
fluvial cycle is marked by laterite or tin-bearing gravels which
contain
Neolithic artefacts of the Nok culture. Western Saharan S u c c e s s i o n s Late P l e i s t o c e n e - H o l o c e n e lithologic sections are well exposed in pluvial lake
beds
in
the
western
d e s c r i b e d at Dibella northern Mali Petit-Maire,
Sahara
(Baumhaeur,
(Fig.ll.6vi, vii, viii) 1987), w e s t e r n Niger
(Fabre and Petit-Maire, 1987).
The Q u a t e r n a r y
and
(Gavaud,
have
1988; H i l l a i r e - M a r c e l et al.,
sedimentary
sequence
on
been
1972) and 1983;
the Nigerian
side of the Chad basin is up to 600 m thick and is known as the Chad Formation
comprising
(Fig.ll.6v).
The
fluvio-lacustrine
Pliocene-Pleistocene
clays
and
boundary
sand
in the
with Chad
diatomites Formation
is
654
not known because this unit has not been subjected to d e t a i l e d paleontological or p a l y n o l o g i c a l studies. In the northern
part of the Chad basin,
in the great
sandy plain of
Bilma, d e f l a t i o n hollows reveal ancient H o l o c e n e lake deposits at Dibella (Fig.ll.6vi),
comprising
a succession of diatomites
lateritic or calcrete paleosols. low
the
which
recognition
later
This
large
merged
of
small
into
a
paleogeographic
dia gibberula,
evidence
isolated
large
lake existed until
which alternate with
Radiocarbon dates and d i a t o m species allake
saline
about
5,370 yr B.P.
includes
lakes,
9,785 years
i0 km 2 wide
and
30 m
when it dried up.
old, deep.
The diatom
such as Rhopalo-
saline diatom species
R. musculus and Campylodiscus clypeus at the base of the
Dibella sequence followed by a transgressive freshwater diatom assemblage
Cyclotella stelligera, Synedra ulna and Melorisa granulata var. angustissima. S a l i n i t y fluctuations at the terminal phase of the lake are
with
suggested by the r e - a p p e a r a n c e of the basal saline d i a t o m assemblage. At the great (1983) ments
This
The rich mollusc
truncatus,
in northern Mali,
and dated highly fossiliferous
(Fig.ll.6viii).
lake.
nus
Erg Ine Sekane
uncovered
lake existed
assemblage
Biomphalaria
Hillaire-Marcel
at the
same time
includes Melanoides
pfeifferi,
et al.
paleo-lake terrace
Aspatharia
sedi-
as the Dibella
tuberculata,
sp.,
Buli-
Limicolaria
and
turriformis w h i c h is still living today in the swamps of southern Mali. V e r t e b r a t e fossils of the Nile perch, Lates niloticus, and bones of cows were
found
among
4,000 yr B.P.
the
remains
Artefacts
agricultural
life
such
of
Neolithic
as
mortars
for the Neolithic
settlements
and
people
which
grinders
were
suggest
whose diet
dated
sedentary
included
cereals.
This implies h y d r o l o g i c a l conditions which are d r a s t i c a l l y different from today's
desert
mentioning in the
conditions
in
northern
Mali.
At
this
that these pluvials did not only create
Sahara
desert,
but
these
were
also
periods
point
aquifer
systems
beneath
the
Sahara
were
is worth
lakes and settlements when
the
reserves that u n d e r l i e the Sahara today were replenished. the
it
recharged
groundwater
This means that during
the
plu-
vials. Two groups of paleo-lakes were recently investigated in Mall by Fabre and P e t i t - M a i r e to
as
the
logical,
(1988), one at Taoudeni and the other in what is referred
Taoudeni
depression
(Fig.ll.7).
C 14 dating a and paleontological
of humid and arid phases
At
both
criteria
locations
revealed an alternation
in the h y d r o l o g i c a l l y isolated Taoudeni depres-
sion which has always remained an area of inland drainage late g e o l o g i c a l history.
sedimento-
throughout its
Today this region receives only 5 mm mean annual
rainfall and has not been uninhabited
for over 4,000 years,
thus remain-
655
ing as one gott,
of
apart
the driest
from
its
places
economic
on earth.
value,
is
The Q u a t e r n a r y very
interesting
salt
at Agor-
paleoclimati-
cally.
Pl,,leou of ° L
Nite
Motmota
O
Ou~rzQzote
~9-"" Tooudeni Depression Argogott . , ~ o . /
Core
mQ,-,~
.
. t
+I~,,++ ,+.~ -.~,
~-.~e ~.,~O
LocotiOnof C gastern
j
J%Erg Ine
_ 'Khar~OU,-
Sohoron wadis
Sokone
IVORY
L. Turkana (I
2 3 4 5 6 7 B 9 11 12 13 15
BOOMPLAAS CANGO CAVE UITENHAGE ALIWAL NORTH ROSE COTTAGE ALEXANDERSFONTEIN KATHU WON DERWIERK CAVE HOMEB ETOSHA IMAKGA D1KGAO! KALOMBO NYIKA PLATEAU
17 '~9 20 22 23
]NYANGA MOUNTAIN WOLKBERG WONDERKRATER R]ETVLE BORDER CAVE
14
Figure 11.7: the text. Agorgott leo-lake saline salt
lake.
revealed
been
salt
layers dated
L, Rukwa ~/
y
17
PC 16X~ 11 K A I AH ARI 9
Luderitz ~
72
~e•
23"Y
St. Heten,',) 2o•3 ~ J Boy African
Quaternary
Taoudeni
7 million
Cores
Sodium
alternating have
in northern
in which
mine
quence.
Some
ohomo • • Eherangon Komironzovu=, ~ "2 A Mt K e n v ' a
/
was
tons
the
localities
salt
was
the
floor
to depths
just below
the
following
stratigraphic
muds
6,760 yr B.P.
In the
and
and m a g n e s i u m
of halite and vegetal
in
site of a late Quaternary
of mineable
taken
(glauberite)
mentioned
remains
alternating
deposited
in
pathe
of the Agorgott
paleoclimatic carbonate
se-
muds with
occur at the bottom and dark
layers
pollen
of
656
Sahelian and Sudanese elements mix with north S a h e l i a n or Saharan species.
The overlying
of w h i c h
are mined
stratigraphic
at present)
unit contains
separated
five halite
by g l a u b e r i t e
which have yielded radiocarbon dates of about
and
"drier"
layers
clay
(3
layers
6,700 to 4,000 yr B.P., for
the second p e r m a n e n t lake episode during which salt was formed.
The upper
s t r a t i g r a p h i c layers dated 3,840 yr B.P., show mud cracks suggesting that climatic
deteroriation
had
started.
The
top
horizon
comprises
a
red
clayey sandstone w i t h a local brown crust indicating a shift to the present arid phase. dry enough
the Holocene, permanent years, of
the
environment
lake could exist
was
much more
in the area,
humid
at least
than
today
marine
waters
Rather,
are
known
at
Agorgott
which
so that a
for a few thousands of
long enough for 7 million tons of salt to accumulate!
salts. zoic
It can therefore be surmized that while the climate was
for salts to precipitate at Agorgott d u r i n g the early part of
would
have
No evidence supplied
the
the source of the salts is b e l i e v e d to be the late Paleo-
(mid-Carboniferous)
marine and lagoonal transitional
beds which un-
derlie the Taoudeni basin. But
unlike
depression which, lakes
the
Taoudeni
paleo-salt
lying to the southeast,
in the Early Quaternary, and
swamps.
ter lakes
to
is dissected
the
north,
clayey
carbonate
Taoudeni
freshwater paleo-
beds
are
The first Holocene deposits
rest d i r e c t l y on Paleozoic
the
into smaller depressions
were occupied by small
Fossiliferous
remain of these paleo-lakes.
lake
formations.
now
all
that
of the freshwa-
The oldest Holocene de-
posits are paleosols and muds with freshwater molluscs which date younger than 9,000 yr B.P. dated
ern Mali
before
northern
part
clays dated tain
Further south,
9,320 yr B.P.,
conditions
of the
vegetal stems. outnumber
humid
enough
Continuous
sections
at the base and
ostracodes,
diatoms,
for
them of
in south-
to appear
in the
carbonate muds
4,440 yr B.P.
and
at the top con-
charophyte oogonia
and calcified
In these sediments epiphytic freshwater molluscs generally
the more
shoal water.
were
country.
8,300 yr B.P.
foraminifera,
at Erg Ine Sakane similar deposits are
implying that large lakes existed earlier
euryhaline
species,
thus
implying
low salinities
and
The climatic optimum implied by this lacustrine episode per-
sisted until 4,500 yr B.P. when the lakes dried up. Another is
their
interesting
aspect
cyclicity.
The
6,700 yr B.P., at A g o r g o t t sequences.
includes
contains
of
the
northern
lacustrine
four sequences;
Mali
optimum
paleo-lake between
sequence
8,300
and
one section of the saline member
8 or 9 glauberitic m u d / m a g n e s i t i c mud/salt
and clay
The salt layers also exhibit m i c r o s c a l e c y c l i c i t y which is re-
flected in colour v a r i a t i o n and clay content. A c c o r d i n g to Fabre and Petit-Maire
(1988)
these
depositional
cycles,
which
show
periodicities
657
ranging
from
400 years,
360-300 years,
250-200 years
down
to
50 years,
are indications of more frequent climatic oscillations within the familiar long periodicity climatic events which relate to changes ume, atmospheric cles
could
and oceanic circulation and global eustacy.
be due
to a
complex
interaction
of planetary,
spheric, hydrospheric and internal earth processes 1984; M6rner,
in ice volSmaller cysolar,
atmo-
(Kutzbach and Guetter,
1984).
11.3.2 North African Successions Gorler
et
al.
(1988)
Neogene
continental
east
Marrakesh.
of
documented
the
Ouarzazate Like
the
stratigraphy
basin East
African
1988), the Ahaggar and Tibesti Mountains ern Cape Province
(Hill,
gion of Quaternary of Quaternary basin
are
tently
tectonism
strata.
The
predominantly
shed during
bouring
1988),
Rift
structure
of
central
System
of
the
Morocco,
(Baker
et
al.,
(Burke, 1976) and the southeast-
the Ouarzazate basin of Morocco is a re-
involving
faulting,
Oligocene-Early fanglomerates
the paroxysmal
High Atlas Mountain.
and
(Figs. 11.7)
uplift
Pleistocene
(Fig.ll.8)
uplift
These beds
and deformation
sediments
which
were
and deformation are tabular,
of
of this intermit-
the neigh-
cross-bedded
sand-
stones and conglomerates which are exposed at the base of the paleo-valleys which
drained
nates
an alluvial
with
the central top unit
sandstones with numerous alluvial
from
layers
the
throughout
varying
(glacis)
(?) Late
levels
(Fig.ll.9),
termi-
Pleistocene
fills which resulted by renewed
uplift
from
phases
(?) and Early Pleistocene times.
the Pleistocene of
sequence
Pliocene/Early
channel
the
indurated
(St~blein, fluvial
unbalanced knickpoints
and the folded and tilted gravel layers. occur at 5 distinct
The Meogene
that were triggered
in Late Pliocene
uplift was effective dent
of
conglomeratic
fan sedimentation
of the High Atlas
High Atlas.
This
1988) as evi-
pediment
gravel
in the wadi gradient~
The terrace gravel layers which
levels in the Ouarzazate basin further attest to the
pronounced effects of the Quaternary climatic alternations
on the geomor-
phological development of the region. In neighbouring Tunisia interesting interpretations leosols and desert tional processes, southern bounded
Tunisia westward
Mediterranean
loess deposits were presented the
calcareous
by the
coastal
Great
Gulf
of
of Quaternary pa-
in the light of desert margin deposiby Coude-Gaussen Matmata
Eastern Gabes,
(Fig.ll.7)
Sahara is
and Rognon
Erg,
covered
plateau
and by
(1988).
In
which
is
eastward
deposits
of
by the peri-
desert loess. Although these loessic silts originated from the Sahara and accumulated
from time to
accumulation,
time by wind deposition
often mistakenly
included
on the plateau,
among aeolian
dunes,
their
took place
658
in
a
"pluvial"
type
of
paleoenvironment.
C 14
dates
Tunisian loess beds gave Late Pleistocene ages while is
suggested
loess
and
contents lating
in are
by
the
the
are therefore
and
calcareous
similar
today under
018
C 13
contents
concretions
of
the
in the
to that of M e d i t e r r a n e a n
dense
steppe v e g e t a t i o n
on
some
fraction
paleosols.
The
soils.
in
(Brunet
al.,
are accumu-
Steppe
conditions
1988)
Proposed strotlgraphic
5 ~ ~- ~
~D c (b ~U
m
correl~ion
E
ta~
Z
cn
C
A
-J
~
A
0_
E
7o E .o -
o
-
Strati@rop hic correlation oocordmg to the geobgicat mop Jbel Saghro-Dades
m c~ 0
I.L
1.~
O Z•
o_
..
-6
G,,
I
- - ( . )
£= -J ~
LU Z Z
C 0
m
_<w
<2_
o~o.J
~ 7
OCL: a l l IE "6
Continental red beds
GOCENE
o
?
u~ ?
Marine w, index fossils
the
isotopic
suggested for the Tunisian paleosols w h i c h are interbedded
w i t h i n the loesses
o
the
"pluvial" deposition fine
loesses which
and
of
~
~ OLIGO o. CENE
o 6
!
u2
~5 o
~
LATE EOCENE
EOCENE PALEOCENE
M.- EOCENE PALEOCENE
CRETACEOUS
C RETACEOUS
Figure 11.8: Schematic stratigraphic column for tal T e r t i a r y of the Ouarzazate basin in Morocco. G o r l e r et al., 1988.)
the continen(Redrawn from
659
SSE
NNW
HIGH A T L A S
BASIN OF OUARZAZATE
ANTI--ATLAS
mNN 000
- 4
hat 3825 D~sturbed M or ginal
Land of Khelas
OB~rn
Zone
Edge/Glac,s 3000
-
Glacis
~_ IC~i~
"~/Tel°uet "~"kk
~-.~
(:~ . . %v ~~; _- %
•
q5
~,
Dad es
--~-~ .,..
- -
--
........
r. .......
~
.
•
--
.
..,
i~
"----.-.. _ . . _ . .
--
l--_IV--//
/%00./'---
i
~ ' ~ ' ~
- - -
O b O 0 0 0 o
x
Y
1g30~.....,__,f~ - 2000
x-
X
-
×
X X
X
I
40
F~q
2
~
s
V ~
F~
3
p=~
6
~
km
X
I
g
FT~
11
~ 4
~ i
12
~
Is
Figure 11.9: Schematic cross-section through the Ouarzazate basin, i, wadis; 2, Dades and dra valley; 3, Pleistocene terraces of Draa valley; 4, Pleistocene glacis levels; 5, basin margin of upper Tertiary and intra-mountain basins; 6, Tertiary planation surfaces in mountains; 7, pushed and overthrust Cretaceous and Tertiary strata; 8, cuesta scarps; 9, Tertiary continental, lacustrine and marine beds; i0, locally overthrust Mesozoic; ii, basement; 12, uplifted mountain areas; 13, subsidence, compression and overthrusting; 15, sub-recent fault lines. (Redrawn from Gorler et al., 1988.)
660
11.3.3 The Nile V a l l e y Fill Since the thick and well d o c u m e n t e d Q u a t e r n a r y deposits ley
(Said,
1982,
cal
record
dating
model
the
runs
of the Nile val-
preserve a long stratigraphical
from
Quaternary
(Fig.ll.7) from
1990c)
the
Late
sequence
in
Miocene
(5-6 Ma),
northeastern
it
Africa.
and archaeologicould
serve
Although
as
the
a
Nile
today m o s t l y through the eastern Sahara desert being fed
highlands
of
Ethiopia
and Uganda,
sedimentation
was d e t e r m i n e d by alternating pluvials
and interpluvials.
only have
of
cut across
the great
wastes
the
Sahara
in
the valley
The Nile could
during
the pluvial
episodes when h e a v y rains fell on the Ethiopian highlands. During
the
Early
was a dry valley. cias
Pleistocene
most
Intense uplift
at the footslopes
of
Egypt was
desert
led to the a c c u m u l a t i o n
of valleys
and wadi
cliffs.
and
the Nile
of talus
But somewhere
brecin the
Early P l e i s t o c e n e a short pluvial seems to have set in leading to the accumulation highly
of
over
competent
40 m
of
river,
coarse
the
Protonile
Early P l e i s t o c e n e
(Fig.ll.2).
was
the
deposited
Pleistocene
in
deposits
Nile
detritus
The
occupied
up
to
Armant
the
Idfu Formation,
valley
in the Nile
(the
Nile
valley
in
A the
a coarse g r a v e l l y sand
Khartoum
(Fig.ll.10)
Formation).
in
Sudan.
The
Middle
valley are the cross-bedded
Qena Sands which are about 250 m thick in the subsurface and over 1,000 m thick
in
the
200,000 yr top.
The
Nile
B.P.
delta.
on
freshwater
the
Deposition
evidence
mollusc
of
of
the
late
assemblages
Qena
Sands
Acheulian of
the
ended
at
implements
Qena
Sands
about
near
the
comprise
a
n o r t h e r n assemblage with Corbicula artinii,
Unio gaillardoti and Muteline aegyptiaca w h i c h is distinct from the southern Ethiopian fauna (with Unio abyssinicus and Aspatharia calliaudi), on the basis of which Said (1982)
postulated
the
first
connection
between the northern
Nile
and Lake
Sudd
in Sudan and the Ethiopian rivers which today form the headwaters of the Nile.
The M i d d l e
Pleistocene
terminated with
a pluvial
during
which
the
A b b a s s i a gravel was d e p o s i t e d u n c o n f o r m a b l y on the Qena Sands. The was
overlying
comprise the
Late
greatly diminished
Pleistocene
four dry phase
Dandara,
thin,
Deir
el-Fakhuri
gravelly
gazelles
and
In general
(aggradational)
Masma-Ballana,
a l t e r n a t e w i t h wet phase the
Neonile deposits
in volume. Sahaba
deposits
Formations
sandy
and hippopotamus.
with
The
Makhadma
Ethiopian
known
and
(recessional) depoists,
Formations.
show that
the deposits
the river
of this phase
(from oldest)
Holocene
silts
as
which
the K o r o s k o - M a k h a d m a and sheet-wash
megafauna
deposits
such
as
are
horses,
In the Korosko Formation the p r e s e n c e of wadi
marls suggest a w a n i n g pluvial phase during which calcium carbonate marls were p r e c i p i t a t e d
from stagnant water. The m o l l u s c a n fauna of the Korosko
661
YOUNGERIHOLOCENE
"".---NILE DEL~TA
;
11 N E O N I L £ 10
NILE VALLEY
w
9 OLDER
L
z
8 NEONIL£
7
l
I
6 S
BASAL
T
&
NE ONILE
E
3 2
DEPOSITS
I
~ ~
o
ua
PRENL IEMIDDLE
D.
ETHIOP]ANRIFT
I,~
c~
DEPOS~ IA
R. Aw
\ m
PEDIMENTS X " X ~ / ~ ~ J ~
?
l
t
+
O
~S; . : +++++++÷++ ÷+÷++÷÷~ +÷
+%%%%% FLOODPLAIN / /
I.
WESTERN RIFT
~
L M~BUTU
~
.
~
--
_
/
/
B~d Iv
~
7.
//
,Be~ IZZ
~----:1-_
~ "
z/
~,th. 2nd. Foutt FauLt TANZANIA
~OOMOE
BEOS (L....P,.~., .....
/
c-~ . . . . . ~ . ~
GORGE ~ / ~ ~ VLAKE ~ r MANYARA / ~
BALBAL 1st. Fau{t RIFT
I' '__'_'l
~,~s~on~
~Phosphates
~C,oy~ ~ , / Y ~ Phosphotes LACUSTRINE PHOSPHATE DEPOSIT
AT LAKE MA NYARA
Figure
11.10:
LOWER
BEDS 80DO BEDS
I,~"~'T'~GALANA BOI BEDS (Holocene)
X/OLBUVA1
//n
~ L ~ 1 1 ~ ~ Sfh. Fault
BODO
Y I YC, /'~' t-n---I-~tJ~^-,;~;'~n.... i l l I-:--:zlKOOBI FORA /'-'~=""'~'1T'¢' • ~ FORMATION (PLiocene-LowerPLeistocene)
'~
~
~
<~°:
SEMLIKI SERIES I'."-'l (MiddLe Quaternary)" ~ ' ' ' l KA1SOSERIES I ~
~
BEDS
( Pteistocene) /L.TURKANA~ ,/KO/OB]FORA (Lower P~eistocene)(P{iocene)
UPPER QUATERNARY ,ED,MENTS
BODO
/ MIDDLE
UPPER/.
East African Quaternary successions.
662
include
Planorbis ehrenbergi,
Bulinus
bicula
fluminalis
suggest
which
all
truncatus, muddy
Lymnacea
waters.
Korosko were d e p o s i t e d during the Ikhtiariya pluvial. terian-Aterian
sp.,
The
and
Cor-
Mekhadma
and
They contain Mous-
cultures which suggest that at least 60,000 years ago man
made a grand a p p e a r a n c e both in the Nile valley and in the desert beyond (Said,
dry
1982).
River
channel
sands
and
phase
contain
molluscs.
flood
silts
Radiocarbon
the
dates
from
upper
silts
of the u n c o n f o r m a b l y overlying Deir el-Fakhuri layers,
each about
to
the
cessional deposits
two diatomite
18,000
Masma-Ballana
dunes
in ponds
gave
overlying
the i n t e r f i n g e r i n g tion contain
in Egypt
of
16,000 yr B.P.
70 cm thick,
that d e v e l o p e d on the Ballana dune fields.
ization
being
vegetation.
due
to
the
Radiocarbon
accumulation dates
from
of
cultural
the
Deir
debris
which
and
el-Fakhuri
17,000 to 14,000 yr B.P. for this minor wet phase.
Formaformed
The diatomites
lie paleosols which developed over the stabilized Ballana dunes,
and
The re-
over-
stabil-
increased
gave
ages~ of
Dates from the overly-
ing Sahaba F o r m a t i o n suggest the d e p o s i t i o n of these i n t e r f i n g e r i n g fluvial
and
dune
assemblage
sediments
is
like
that
between
13,700
and
12,060 yr B.P.
of the underlying
Ballana dry phase sediments.
lithologically
The
molluscan
similar Masma-
Southward in the Gezira plain near Khartoum,
carbon -14 dates also suggest that from about 18,000 to 12,000 yr B.P. the Blue
Nile
was
a
highly
seasonal
braided
river
transporting
gravel
and
sand during floods, while between about 12,000 yr B.P. and 500 yr B.P.
it
was a sinuous
it
river
d e p o s i t e d clays Egypt
began
at
flowing
(Hamilton, about
slowly through permanent 1982).
swamps
in which
The modern regime of the River Nile in
9,000 yr B.P.,
during
which
the
river
assumed
its
present gradient and deposited silts in its valley and delta.
11.3.4 East A f r i c a n Rift V a l l e y Successions Most
of
East
Africa.
Africa
is
underlain
by
basement
complex
just
like
West
The East A f r i c a n climate also shows an a l t e r n a t i o n of wet and dry
seasons w h i c h
is largely responsible
the region except
for the p r e p o n d e r a n c e of savanna in
on the high mountains.
As
in West Africa,
terferred e x t e n s i v e l y with the natural v e g e t a t i o n grazing,
through
man has in-
farming,
over-
bush burning and dependence on fuel wood for rural energy. His-
torically,
the concept of erosion or p l a n a t i o n surfaces in Africa was in-
spired
the
by
spectacular
East
African
summarized by Valeton and Mutakyahwa characterized rocks,
by
extensively
plateau
sceneries.
As
recently
(1987) many parts of East Africa are
elevated
plateaus
of
crystalline
basement
i n t e r r u p t e d by isolated highlands and broad linear valleys of tec-
tonic origin.
Uplift and erosion since Jurassic times have produced dif-
663
ferent wana,
planation
post-Gondwana,
Africa, are
surfaces
known
(from oldest
post-African
and Congo
to youngest) surfaces,
as
where,
the Gondas in West
thick lateritic and b a u x i t e - b e a r i n g soils v a r y i n g from 5 to 40 m,
preserved
on
the
relicts
of
the
planation
surfaces.
Beyond
these,
there are not m u c h similarities between the Q u a t e r n a r y successions of the East and West A f r i c a n overshadowed ially
of
Africa.
regions • The Q u a t e r n a r y of East A f r i c a
by the stratigraphy
the
great
rift
lakes,
of the which
East A f r i c a n
have
no
is largely
Rift V a l l e y
counterparts
espec-
elsewhere
in
The Q u a t e r n a r y of the East African Rift V a l l e y is considered be-
low under the four natural physiographic segments of the rift system.
Ethiopian Rift A
typical
eastern
Quaternary
fluvial
faulted m a r g i n
rift
part of the Ethiopian rift valley Beds
(Clark
clays Beds
et
al.,
1984),
(Lower Bodo Beds)
the
sequence
is
exposed
the
I n f o r m a l l y termed the Bodo
starts
fossiliferous
along
in the northern
with
Pliocene
in fault contact w i t h
silts
the Middle
and Bodo
sands and gravel and volcanic
The M i d d l e Bodo Beds of Early Pleistocene age are succeeded by the
faulted Upper Bodo Beds with similar age.
sequence
(Fig.ll.10).
which are
(brown clays with minor
ash).
valley
of the middle Awash River v a l l e y
The Bodo Beds
beds
contain
contain
Acheulian East
the
abundant
African
At
and
light duty artefacts canid,
crocodiles
artefacts
complex
sites.
cleavers
association
Australopithecus
homonid Oldowan
industrial
Acheulian
lithology,
are of great archeological
the hand
which
and
suggests
similar top axes
to
part
afarensis;
the
upper
the of
lower the
are m i x e d
with
(Clark
butchery
site
et of
the
beds
middle
carry
Acheulian
Upper
Bodo
Developed
are associated with bones
catfish
the
while
but of later Pleistocene
interest because the lower
al.,
of
early
from
other
Beds
upper
Oldowan-type
small
1984).
bovids,
Sometimes
a hippopotamus
beds
an
that
a
the
had
been
internal
lake
hunted and killed. In the
southern
d r a i n a g e basins lithologic,
part
of the Ethiopian
rift v a l l e y
such as Lake A b h 6 and the Ziway-Shala
radiocarbon,
lake level fluctuation
pollen
and
diatom
evidence
the
lakes have yielded of
Late
Quaternary
(Table ii.i).
Kenya Rift The prolific
paleoanthropological
stratigraphic
sequences
variously described Coppens,
1976;
and archeological
in the drainage
under different
Cohen,
1981).
The
basin
sites
of Lake
lithostratigraphic
Pliocene-Holocene
and Q u a t e r n a r y
Turkana names
have
been
(Howell and
succession
at
Koobi
664
>..
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d B 000'0 000'0~
000"0 E
-
d 9 000"09 -
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os 0 >.,r~ cr >
0 0 .,-I ,-I I~
.
0 ..~ -,.4
•
o,.
--
.~_~o~m
~
~o o
'~Se "~
.x~
cJ
o ._
o *a o~
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=o ~ .o
~c::
°~
o
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,--4 r/l dg
O00'~t -O00'OZ)
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NVI1VZVH9 "]LNV
665
Fora,
about
500 m
scriptions, studies
thick,
has
potassium-argon
and geochemical,
mental
interpretations
works of A b e l l McDougall
been
subjected
dates
and
are
Owen
detailed
lithologic
stratigraphy, in
paleoenviron-
Fig.ll.ll
based
on
(1979, 1981), Brock and Isaac
et
al.,
(1982),
Howell
Koobi
Fora
sequence
(1978),
de-
paleomagnetic
and c o m p r e h e n s i v e
summarized
(1982), B e h r e n s m e y e r
(1985),
tuff
paleontological which
to
and
the
(1974),
Williamson
(1982). Essentially,
the
(Fig.ll.ll)
comprises
fluvio-
lacustrine and deltaic silts, clays and sands which are a b u n d a n t l y interlayered by v o l c a n i c on
tuff
framework
for the
sions
Koobi
at
African
H.
between
Fora.
2.0
and
if
is
Turkana
inferred
provided
sequence
should
not
1.4 m . y
not
has
for
the
known
O18/O 16 during
of
thus
1984).
this
Africa.
Salinity
critical
(Williamson,
and
on
the
the
East
homonids
Homo
co-existed
hypothesis ancestor
1982)
stage
tocene
1982)
The
of
age
succes-
fluctuations
(Abell,
changes also had drastic effects
for
robustus
immediate
These ecological molluscs
standard
negating
variations
numerical
and a r t e f a c t
and A.
The
t i m e - s c a l e b~sed
excellent
the
boisei
habilis.
(Pilbeam,
from
alternations
H.
as
whole
(Fig.lloll), from
an
and homonid
serve
Australopithecus
descended
erectus
climatic
This
erectus,
erectus
The well documented numerical ages
sedimentary
Quaternary,
habilis,
Homo
tuff.
potassium-argon
of
suggest
homonid
that of
H.
Lake
marked
evolution.
on the Pliocene-Pleis-
diatom
assemblages
at
Lake
Turkana.
Tanzania Rift
The w o r l d - f a m o u s vai
Gorge
in
(Fig.ll.10), basin.
The
trine clays
as
is
gastropods
they
are
part
of
termed,
sequence,
the are
about
ages
of
Bed
I,
2.1 m.y
to
about
60 m
is exposed
country. well
100 m
15,000
Here
exposed
thick,
and sands interbedded with volcanic
Olduvai
clays w h i c h
northern
entire
potassium-argon base
Q u a t e r n a r y locality in Tanzania
the
yr
thick,
in
a
comprises
Olduvai
Beds
shallow
lake
fluvio-lacus-
lavas w h i c h have yielded
B.P.
(Hays,
comprising
i n t e r f i n g e r eastward with alluvial
in the Oldu-
the
1976).
marly
fan deposits.
At
the
lacustrine Terrestrial
such as slugs suggest damp conditions during the d e p o s i t i o n of
Bed I. A l t h o u g h it overlies Bed I conformably, the m i d d l e
that separates
vial-lacustrine
facies
Bed II has an u n c o n f o r m i t y in
a lower lacustrine
which
implies
sequence from an upper flu-
considerable
reduction
in
the
size
of the Q u a t e r n a r y lake. The v e r t e b r a t e fauna b e l o w the m i d d l e part of Bed II is rich with m a i n l y swamp-dwelling crocodiles and t u r t l e s
(Fig.ll.12),
666
.>_
o
oo.~o
"G ._ c~o
oo~
z
NvI]n3H3V
0"~ o
~"
",-~~o E
39V
~
]NOIS
(sloo I '
o ~
"~~ ' ~
A ' I ~1V ]
04J
alqqa d )
N v~oo',o
,~
rd 0
!as!oq ~n~aq&!dol~&snv
~ EooG o~:
•~
=
-
~.~
~o
sn~aq~d°l°Jlsnv
~"~'
7~ ....~
-.-t
sue,~Os o=o H ~!o4~jv
--s~qoq o=o H
4J~ m o
sua!dos sua!dos o=o H
o
iol~ V N V ] V O
NOIIVW~0:]
~
to],
V~O:I
~ m o o U,l:~ .,-i
,-'1
:I Tlt
°~
I~:
~°N
aNa~O10.
"
°
~
~
N
~
" °
3 0
~!I~
~1 ~ ' k
.L S I
-::iI~ " . ....
V':I I - ~
~
.
~
3N330
•
~
-.'. :~
I]
d
•,-.t ~ .,-4
667
whereas deer)
the
prOportiOn
abruptly
artefacts
in the
the upper
part.
lower Homo
are the h o m o n i d s
2"
of
increases
part
•
the and
habilis,
in Bed
f
savanna
at
and
top
plains
of
Bed
Developed
H.
dwellers
II.
Bed
Oldowan
erectus
II
(eg.
lions
contains
and A c h e u l i a n
and A u s t r a l o p i t h e c u s
and
Oldowan tools
cf.
in
boisei
II.
•
"
"::'@xi.wD~.f ~ , , ~
.
.
.
.
.
-
"~
Figure 11.12: Vertebrate faunas and r e c o n s t r u c t e d s c e n e r y of Olduvai Bed I and the lower part of Bed II. The faunas include several u n f a m i l i a r animals n o w extinct and some m o d e r n animals. (Redrawn from d i s p l a y in the British M u s e u m of Natural History, London.)
Higher nently which
in the Olduvai
fluviatile are
verteb r a t e s greater
still of
strata
sequence (Bed
interbedded Bed
reduction
III
and
of homonid
III,
with IV are
lake de p o s i t s IV,
Masek,
volcanic greatly
occupation
are r e p l a c e d Ndutu
tuffs. reduced
and
The
by predomi-
Naisiusiu
homonid
(Fig.ll.13).
and a c t i v i t i e s
Beds)
sites An
and even
at the time of the
668
Masek
Beds
(Middle Pleistocene)
reflects
the
onset
of climatic
desicca-
tion and large areas of dry savanna which have persisted till today.
--
",.,
,~{ ' ~:.
f ~
.
"
.
.
.
"
.
, ";
.
'-"~':'-"
q~'
~
~.-...._
.....
"3":- : - . ' ~ _ ...._.
".~'~l~lt"~..
Figure 11.13: Olduvai Bed III faunas showing an impoverished c o m m u n i t y in gravel beds. (Redrawn from display in the British M u s e u m of Natural History, London.) In the
southern part
Pliocene-Pleistocene
of Tanzania
different
the Lake Manyara basin and is mined at Minjingu, ent
small
alkaline
lake habitats
lacustrine phosphate deposit, Lake Manyara.
Lacustrine
and tuffs alternate with the phosphates.
I0 m thick,
existed.
A
occurs
in
5 km away from the pres-
clay beds,
algal
The rich bird,
limestones
fish and mollusc
faunas suggest a greatly expanded P l i o c e n e - P l e i s t o c e n e Lake Manyara which p r o b a b l y united with the nearby Lakes Natron and Magadi and formed a huge lake
(Schluter,
populations
of
1987) with abundant nutrient supply which attracted large birds
and
fish.
The
faeces
and
bones
of
these
organisms
w e r e the initial deposits that later underwent diagenesis into rock phosphate.
Western
Rift
The Q u a t e r n a r y strata that underlie the W e s t e r n Rift V a l l e y are not well exposed as in the Eastern Rift. Well dated lake levels occur around Lakes Malawi, posed
in
sequence
Tanganyika, Lakes
and Kivu
Amin
is e s t i m a t e d
and
(Table
Mobutu
to be
ii.i).
basins
Quaternary
(Fig.ll.10).
2,500 m thick
(Hamilton,
sequences
At
Lake
1982)
are ex-
Mobutu and
the
it com-
669
prises the Kaiso Beds overlain by upper Q u a t e r n a r y sediments. ies
of
Quaternary
Kaiso
Beds
oolitic
sediments
comprising
ferruginous
Kaiso
Beds
include
beds
also
contain
are
esposed
fossiliferous
layers,
sands
abundant
such
cleavers and A c h e u l i a n hand axes
Lake
to
tuff.
mammals, as
with
the
clay
beds
The
reptiles
hammer
(Bishop,
Amin,
greenish
and volcanic
molluscs,
artefacts
at
grey
faunas
and
stones,
Three serbasal
fish.
coves,
with
of
the
These
flakes,
1958). The Kaiso Beds are over-
lain by the c o a r s e - g r a i n e d unfossiliferous Semliki Series, w h i c h are succeeded by unnamed upper Quaternary sediments
(10,000-8,000 yr B.P.) with
fishes and human remains and stone tools. 11.3.5 Q u a t e r n a r y Deposits in Southern A f r i c a Marine
terrace
deposits
which
accumulated
Pleistocene sea level changes, along the South A t l a n t i c land,
Quaternary
drainage
basin,
several
stripping
during
as
occur the
1982).
the
and coastal plain eolian dune ridges occur
in
the
so-called
Widespread
Kalahari
of p o o r l y vegetated
Group,
the
Australopithecine
colluvial
deposits,
mantle east and central parts of southern Africa the
levels
and Indian ocean coasts of southern Africa.
deposits and
(Tankard et al.,
at
pediment
Cave
up to
(Fig.ll.14)
slopes during
In-
Vaal-Orange Breccias
10 m thick and suggest
the dry phases
between 30,000 B.P. and 12,000 yr B.P. Kalahari Basin
Here
the Q u a t e r n a r y
which
are
these
are
up
to
is represented
90 m
overlain
thick by
and
180 m
by
are
of
thick basal
fluvial
occasionally
red
shales,
conglomerates
cemented
marls
and
by
caliche;
duricrusts.
W i d e s p r e a d caliche crusts and dolomite in the sands of the Kalahari Group point to arid climate with saline conditions. p a l e o - l a k e depressions, porary
saline
though
not
lakes
suggest the p r e s e n c e of tem-
the
Kalahari
in
dated,
three
in
precisely
In the Etosha pan and other
algal stromatolites basin
the
morphologically
Late
Quaternary.
distinct
Al-
groups
of
fixed dunes in the Kalahari basin have p r e s e r v e d three periods of desertexpansion in the Late Pleistocene the
interdune
stones, moisture
areas
mudstones in
this
(Teller et al.,
are
and
limestones
normally
1990).
filled
(Lancaster, with
thin
suggesting
hyperarid
region
1981).
In the Namib desert
calcareous increased during
the
lacustrine
sand-
availability Late
of
Quaternary
670 Vaal-Orange Basin and Continental Shelf Q u a t e r n a r y deposits
including d i a m o n d - b e a r i n g gravels,
sequence of alluvial terraces and Brink, bones
1967). Near the confluence of both rivers
including
artefacts alluvial
the
suggest
Pliocene
a Late
deposits
which
occur in a complex
along the Orange and Vaal
Pliocene-Early
subplanifrons
Pleistocene
are c a l i c h e - c e m e n t e d
(Patridge
(Fig.ll.7) mammalian
Mammuthus
elephant
rivers
age
for
cobble-grade
the
and
oldest
conglomerates.
Rare fossils and artefacts suggest a Middle Pleistocene age for a younger l i t h o l o g i c a l l y similar set of braided floodplain gravel and sand beds.
-
L
~
O•
~ o
•
•
O0 •
j-~: \
J. Etosho •
% Peleowind
, •
Pan
Alignment ol •
"
/
~
/ ~
-
/ "--- l
%
/ O f
~
'
"~ ~
/.._,,Limit of cotiuvium
%
•
\
Ooom,<
•O
~
\
I
k
!i
Mok:,:n.g., / • /
/
Figure 11.14:
J
/ M
~_M~t__ ~
:
A
.
"~
inner
5tratigrophy of Sterkfonteincave deposits
Q u a t e r n a r y deposits of southern Africa. continental
shelf
off
SW
Africa
have
resulted
in a
marine
alluvial
diamond
fluctuations richest
between
highly
unique
fields
A recent
survey of the seafloor m o r p h o l o g y
thickness
cover,
and
reveals
stratigraphy
that during
the
of
in
a s s o c i a t i o n of
with
features which resulted
geophysical
Luderitz
Late T e r t i a r y and Quaternary
submarine geomorphic the
o°/~_.
~-
sea-level
sediment
~
~4"t.k~j ~ ~ . . ~ .~.~/~
Bay in South Africa,
cluding
~----/~
~
~ l~.~x~/~/~'~" ~z rk :/~//~ ~ / / i
-
Namibia and St. Helena world's
.j
I
-- / ~ ;
t41-0 Lilhologic "members"
the
~"
/
~6
Precclmbrian dolomite
the
/
i
/
q
, _ . - ,bo°e ilrnesi0neand fl0wslone,bone
On
#
T • ~ •clung ~
I~:A'I Muddy cal¢.reous sondslone, WL~ ~
~/~
I... i•
~__
II -,,-
/
...
0"0
~"
t
o" ° Limilof Kolohori - . : sand
•
~__
" " _ _ ~I ii Sterkfontein "• / //.~7:e " .:
\
,,,ed d0oe.
",..,
~_
:
t
i~'r direction~
•
_~
h
1
~
•
OO
\
/
"..
from sea-level
shallow
of the area,
the u n c o n s o l i d a t e d
lowstands
of
shelf
stillstands. in-
phosphatic
sea-level
diamonds
671
were
reworked
cut
terraces,
bedrock
from coastal cliffs,
depressions
(De Decker
and
concentrated
paleo-channels,
which
1989a).
deposits were
It was
eroded
also
reefs, during
during
the
in submarine wave-
gullies,
the
last
potholes
lowering major
of
and
sea-level
lowering
of
sea-
level at about 18,000 yr B.P., that the Orange R i v e r delta began to build seaward in this region. 120 m during
A c c o r d i n g to De Decker
the Last Glacial
Maximum,
(1989b)
and along
this
sea-level
strandline a b a r r i e r - b e a c h and lagoonal sediment complex ange
River
delta
subsequently
prograded
over
the
fell to
regressive formed.
shelf
with
paleoThe Or-
extensive
c o a s t - p a r a l l e l beach deposits mantling the shelf during the Recent transg r e s s i v e phase. A u s t r a l o p i t h e c i n e Cave B r e c c i a s In the Transvaal dolomites Swartkrans (King,
area
in South Africa
(Fig.ll.14) and
1951 a,b;
succession
are
known
Kromdraai,
which
Patridge,
1978).
in the Sterkfontein
such
as
brecciation
of
tion
of
insoluble
cave
at
contain
of
caves
in
Sterkfontein,
homonid-bearing
Precambrian Makapansgat,
cave
deposits
The l i t h o s t r a t i g r a p h y of a typical cave
cave reflects
cavern walls, earth
a system Taung,
and
several d i f f e r e n t processes
carbonate
the
flushing
precipitation, of
accumula-
colluvium
into
the
caves. Homonid remains and fossils occur m o s t l y in the c a r b o n a t e - c e m e n t e d colluvia and breccia. each separated stone
The Sterkfontein cave shows six s e d i m e n t a r y cycles
by a hiatus
accumulation
tains A u s t r a l o p i t h e c u s by the fifth cycle tocene). little
that may be a s s o c i a t e d with
(Fig.ll.14). africanus
The
fourth
sedimentary
(synonymous with A.
(M5) with H o m o cf. habilis
calcite cycle
or flow(M4)
afarensis)
con-
followed
(Late P l i o c e n e - E a r l y Pleis-
The cave sedimentary facies suggest r e l a t i v e l y dry climates with variationp while
the
vertebrate
faunas,
dominated
by
bovids,
is
consistent w i t h a savanna or wooded g r a s s l a n d w h e r e A u s t r a l o p i t h e c u s robustus and A. a f r i c a n u s roamed, being hunted by leopards.
11.4 Quaternary Paleoclimatic Reconstructions for Africa Although
the
cessions
were
paleoenvironmental mentioned
in
the
significance aforegoing
of A f r i c a n
section,
Quaternary
there
is
a
suc-
need
to
present a coherent Q u a t e r n a r y paleoclimatic scenario for Africa using the available e v i d e n c e from some of the regions where the record is extensive and well dated. No attempt is, however m a d e to correlate the A f r i c a n land record
of
Quaternary
paleoclimatic
changes
with
the oceanic
oxygen
iso-
672
tope stages
(eg. Williams
et al.,
1988).
Such a correlation
is, however,
greatly needed. W h e n pieced ous
sources
together,
yield
Pliocene-Early
a
the p a l e o c l i m a t i c
coherent
picture
Pleistocene phase.
This
information
of
a
nary sequence at Olduvai Gorge in Tanzania, Turkana
in
Kenya,
the
fairly
is evident
from these varidry
and
unstable
from the lower Quater-
the Koobi Fora region of Lake
Australopithecus-bearing
cave
deposits
in
South
Africa and the alluvial deposits in the Nile v a l l e y in Egypt. This unstable p a l e o c l i m a t e resulted in important speciation events amongst homonids (Fig.ll.ll) and amongst several groups of organisms. While
there is a Middle Pleistocene hiatus
with no p a l e o c l i m a t i c vide
the m i s s i n g
drier
but
there was tocene
was
evidence,
still
a wet
and
the
From
data
the Bed
moistier
phase,
transition.
paleoclimatic
record,
are
reveal than
that
this
point
In
the
during
onward
because
at Olduvai pro-
climate became
today.
Idfu pluvial,
available
in the Lake Turkana basin
IV and M a s e k Beds Nile
progressively
valley
in
the E a r l y - M i d d l e
both
oceanic
deep-sea
and
cores
Egypt Pleis-
continental
from
the
ocean
bottom of n o r t h w e s t Africa and from the Zaire deep-sea fan have retrieved sediments as old as the Middle Pleistocene. C a l c f u m c a r b o n a t e fluctuations in the Zaire d e e p - s e a conditions
d u r i n g most of the Middle
while
there
tions
in
is e v i d e n c e
East
Pleistocene
in
Africa. Egypt
of aridity The
major
(Fig.ll.2)
pluvial
and
point was solely an Egyptian river,
Pleistocene
in Egypt
(Jansen et
(Said, which
caused
the
fan reflect cold
1982)
al.,
terminated Nile,
the
which
up
cidence
of
lake
level
fluctuations
of A f r i c a
(eg.
Pachur
Middle to
this
to capture its Ethiopian and Sudanese
headwaters, was felt as a w a r m humid phase elsewhere in Africa. the faunal and p a l y n o l o g i c a l
1984),
and dry condi-
throughout
Africa
(e.g.
The coin-
Table
ii.I),
information from the Sahara and other parts
et al.,
1990;
Voight
et
al.,
!990)land
the
rich
a r c h e o l o g i c a l record from all parts of the continent d o c u m e n t Late Pleist o c e n e - H o l o c e n e climatic fluctuations.
11.4.1 The Land R e c o r d
Southern
Regional cently
and Eastern
overviews furnished
southern Africa South Africa.
Africa
of by
the
late Q u a t e r n a r y
Zinderen
Bakker
and
and by Deacon and Lancaster
paleoclimatic Coetzee (1988),
record were re-
(1988)
for
and Scott
East
and
(1989)
for
These workers reviewed the results of fossil pollen studies
673
over the last mals
35 years.
Avery's
(1988) record of South A f r i c a n micromam-
is also consistent with pollen p a l e o c l i m a t i c
scenarios.
Most of the
locations m e n t i o n e d are shown on Fig.ll.7. 130,000
-
80,000
yr
Deacon and L a n c a s t e r
B.P.
(1988) have shown that warm
to m i l d l y cool interglacial conditions p r e v a i l e d both on the coast and in the
hinterland
of
South
Africa
on
the
evidence
of
large
mammals
and
shelfish faunas. 80,000
-
(Fig.ll.7) cliff,
50,000
where
the
yr
the
At
B.P.
sequence
deposits
the
Boomplaas
is more
suggest
cooler
cave
complete
site
and
conditions
in
than
in South Africa Zimbabwe
in
the
at Red-
preceeding
phase. 50,000
grasses
-
at
32,000
the
yr
Oxygen isotopes C 13, and the p r e d o m i n a n c e of
B.P.
Wolkberg
and
Cango
cave
sites
in
South
Africa,
suggest
cooling trends. 32,000
-
28,000
yr
This
B.P.
was
a warm
and humid
phase
as
evident
from the upward m o v e m e n t of the tree line at Sacred Lake on Mount
Kenya
(Fig.ll.7);
a rapid increase in peat growth at K a m i r a n z o v u
in SW Uganda;
and
groundwater
deposition
higher
tufas in SW A f r i c a
levels,
stream
(Lancaster,
1989).
discharges However,
Africa a t e m p e r a t u r e drop was reported. the K a s h i r u location
peatbog
in
Burundi
30,000 yr B.P., upper now,
forest but
and v a l l e y swamps 28'S,
29 °
the
A site with w a r m p a l e o c l i m a t e from the central
34'E).
indicates
humid
than
in
climatic
At
this
conditions
late-glacial
of
at W o l k b e r g cave in South African
time
colder
is
highland
location,
the occurence of a montane conifer forest,
limit,
more
(3 °
and
prior
to
i n c l u d i n g the and drier
(Bonefille
and
than
Riollet,
1988). 28,000
-
20,000
yr
central A f r i c a was The
Kamiranzovou
B.P.
Swamp
shows
s l i g h t l y d r i e r conditions. ter 24,000 yr B.P., and
precipitation
deposits
at
the
m o i s t conditions deserts
received
32,000 years.
During this interval the climate in East and
fairly similar a
to that of the H o l o c e n e moist
slower
accumulation
A forest period
in
East
in East Africa
Africa
slightly
greater
savanna
than
today.
spring-site
of
and the existence of bushveld. higher
rainfall
than
during
Towards the end of this phase,
In
southern
show
22,000 yr B.P.
peat cool
The K a l a h a r i and Namibia any
period
in
the
last
the t e m p e r a t u r e d r o p p e d and
a r i d i t y spread in the Kalahari desert from south to north. at around
colder
Africa
Wonderkrater
The Lake Mak-
gadikgadi area continued to be humid and in the n o r t h e r n Kalahari, ity p e r s i s t e d
and
c u l m i n a t e d af-
during a wet phase in w h i c h temperatures w e r e
present
period.
In the Namib d e s e r t
humid-
the onset of
674
arid conditions
is evident
in the Homeb
K u i s e b R i v e r during a drier p e r i o d 20,000
-
mum during
16,000
which
which
there
is
Zaire
basin
was
yr
aridity
spread
East
African
mean
temperature
of
with
active
formation
and
its
ward
mountains
dune
surroundings,
expansion
belts
was
5-8
of
on M o u n t
Kamiranzovu
lowered
in
by
East
size
mountain
also
Uganda, Ishiba
suggest Lake
Kalombo
Falls,
logical
site in NE Angola,
Ngandu
and
900-1100
Africa.
a drop
indicating
humidity
was the
a
the
Zambia
and
on
the
at
in
was
arid
southern
part
high.
The down-
lower
Ruwenzori
colder drier climates
the
drop
desert
in the
and
for
of
line
Afroalpine
in p a l e o t e m p e r a t u r e ,
on
in northern
tree
Kalahari
except
and
rainforest
the
m,
The
Ericaceous
Mahoma
of Africa,
tropical
(Lancasterp 1988),
high
the last glacial maxi-
the whole
the
and the SW Cape where
the
Kenya
in
Even
reduced
°C in
represents
over nearly
evidence.
considerably
along the
from 23,000 to 19,000 yr B.P.
This period
B.P.
abundant
silts w h i c h a c c u m u l a t e d
while
Mountains, the
Mufo
are r e g i s t e r e d
at the
archeo-
on pollen
evidence. Van land
Zinderen
record
and
conti n e n t a l sea
information basin
on
on
land,
This
ter w i t h
spores,
able
change
and
coastal the
and
tropical
replaced
zone
was
northward
3,800
types
suggested
During
the
of
changes cold
the
were
waters
and
A considerwith
coastal
period Zaire
strongly
upwelling
pollen,
compared its
Zaire
- 15,400 yr
pollen.
basin with
in
the
organic mat-
grass
when
glacial
rainforest
the
with
excellent
in
18,600
terrigenous
and no m a n g r o v e
vegetation.
penetration
around
the
African
Zaire deep-
provides
prevailed
, together
Zaire
These
offshore
that
in the
desert.
the
depth
abundant
therefore
tropical
southwestern
from
maximum
contain
between
the
m water
rainforest
the a
is
comparisons
of
conditions
appearance
pollen
drew
record
last glacial
burnt
few forest
savanna
grassland
the
(1988)
obtained
than
in the cores
in v e g e t a t i o n
present-day grove
pollen
less
environmental
during
a black
many
at
the
Coetzee deep-sea
Fossil
from
interval
and
coeval
margin.
(Fig.ll.l)
B.P.
Bakker the
the man-
savanna
basin
and
while
correlated
the with
of
the
Benguela
in
South
Africa
suggesting
a mean
Current. At there
Wonderkrater was
temperature in
7.5 ° at
18,000
cured
the
between
Wonderwerk
and
than
southern yr B.P.,
of about
Boomplaas
migration
5-6 ° lower
occured
peratures
and
a downward
in
of
the
at present.
Cape
at
Wolkberg
and at U i t e n h a g e
5.5 ° and
26 ° and
29°S
Border
caves.
southern
highland
Large Cave
which
in
at Rose
landforms
temperature
recorded
cave
are known.
as evident
Periglacial
drops
and Cango
5°C r e s p e c t i v e l y latitude
Cape
vegetation
a drop
sites
where
Severe Cottage
are k n o w n
also of
tem-
frost occave
and
in the high
675
mountains dence
of Lesotho and at the southern Cape.
in
the
glacial
Kalahari
maximum.
pletely between ditions
suggests
Lake
less
humid
Paleo-Makgadikgadi
westward,
conditions periodically
g e o m o r p h i c eviduring
the
last
dried
out
com-
19,000 and 12,000 yr B.P., w h i l e s u b - h u m i d to humid con-
prevailed
in the northern Kalahari
at
16,000
- 13,00 yr B.P.
the M a l o p o v a l l e y in the southern Kalahari a perennial
In
river existed be-
tween about 17,000 and 15,000 yr B.P. 16,000
-
14,000
to
10,000
yr
This transitional
B.P.
period witnessed
considerable climatic and p h y t o g e o g r a p h i c changes in w h i c h v e g e t a t i o n responded
to
a
general
rise
in
temperature
and
an
important
increase
in
p r e c i p i t a t i o n between 12,600 and ii,000 yr B.P. On the East A f r i c a n mountains
vegetation
belts
migrated
to
higher
altitudes
between
12,500
yr
B.P. and 10,500 yr B.P. Trees replaced grassland at Sacred Lake, the E r i caceous
Zambia
grassbelt (Stager,
declined 1988)
at Cherangani
dry
conditions
in Kenya,
still
13,000 yr B.P., when the lake shrank and became The
vegetation
sites
cover
Swamps
(4,040 m).
influx
of
vegetation
mountains. yr B.P.
loam
on
the
occured
in
parts
and
(2,960 m)
rapid
Chesi
about
in
15,000-
c h e m i c a l l y concentrated.
higher
Lake Mahoma caused
at Lake at
erosion
of
colder
mountain
and at the Badda at Rutundu
Rwanda
where
the
and an ground
In the lowlands around the n o r t h e r n edge of Lake Victo-
changes
took
place
which
paralleled
those
on
the
high
The open v e g e t a t i o n which existed between ca 14,500 and 14,000
was p r o g r e s s i v e l y
that after
sparse
(3,140 m),
H e a v y rainfall
sand-rich
cover was sparse. ria
remained
of Lake Rutundu
but
prevailed
about
replaced
9,500 yr.
p a l y n o l o g i c a l ~indications
B.P., for
from about
lowland
higher
12,000 yr
forest became
rainfall
are
B.P.
onward,
established.
corroborated
so The
by
the
the
hy-
rise of the East A f r i c a n lake levels at about 12,000 yr B.P. In
southern
pothermal North, south
Africa
period
those
registered
which
humid
had
been
climate,
dry
for
during
example
at
Aliwal
situated at the b o u n d a r y between the dry semi-desert Karoo in the and
the
subhumid
grassland
g r a s s l a n d o c c u p i e d the area, This
parts more
vegetation
was
in
the
north.
At
12,000
yr
B.P.
pure
suggesting colder and more humid conditions.
replaced
twice
by
warm
dry
Karoo-type
vegetation
which was r e - e s t a b l i s h e d for the third time at about 9,600 yr B.P. Pollen spectra and charcoal analysis Boomplaas tween tures.
in the
14,200
and
show that the p r e v i o u s l y open v e g e t a t i o n at
south coastal 12,000yr
B.P.,
region was indicating
replaced higher
by
Olea
rainfall
w o o d l a n d beand
tempera-
This w o o d l a n d changed into thicket at about 10,000 yr B.P.
Kalahari humid phases
started between
13,000 and 12,000 yr B.P.,
In the so that
after a complete d e s i c c a t i o n of the Makgadikgadi pan d u r i n g the hypothermal period a n e w t r a n s g r e s s i o n occured here at 12,000 yr BoP.
The Etosha
676
Pan r e c e i v e d more
rainfall
v a l l e y in the south,
from 13,000
as did Malopo
from 13,000 to 10,000 yr B.P.
In general humid
Holocene.
to 12,000 yr B.P.,
conditions
persisted in East and southern
Africa during the Early Holocene until about 4,000 yr B.P. Pollen data in East Africa At
Sacred
point to d e f o r e s t a t i o n by man during the last two millennia. Lake
on
Mount
Kenya
and
on
Mount
Kilimanjaro
the
tree
line!
moved upward during the w a r m and moist Early Holocene but conditions became the
colder
and
glacier
Maximum Africa
drier
grew
lake down
levels to
towards
and
about
constructed occured
Zambia
4,000 yr
a moraine
between
and were
6,000
followed
arid conditions at about 3,500 yr B.P. In
southern
Africa
two
BOP., at
and
sets
on Mount Kenya
altitude
4,000
by a
yr
lowstand
(Stager,
different
when
an
of
B.P.
4,265 m. from
under
East
presumably
1988).
of
climatic
r e c o g n i z e d in spite of the lack of precise data.
change
have
been
In the northern more hu-
mid region of South Africa temperature and h u m i d i t y increase was punctuated
by a short
However,
the
conditions around
exhibit
4,000
alternating Kalahari
colder
southern yr.
interval
two
B.P.,
wet
and
between
boundary
semi-arid
the
about
in
separated
this
and
with
This
region
3,000
its by
at Wonderwerk.
conditions However,
4,000
Kalahari
periods
for example
dry
and the Namib.
of
yr.
overall
an
arid
was
episode
followed
extending
radiocarbon dates o b t a i n e d
BoP. drier
into
by the
from fluvial
deposits and calcretes in the Namib desert also suggest a humid phase between
about
4,000
1200 to A.D. Talbot parameters plants
1600
and
using of
tion.
and
1,200
yr
B.P.
(1989)
demonstrated
C3
lake
photosynthesis, eg.
terrestrial
level,
in addition
methods.
by a decrease
eg. to
can
a dry
that
found that Lake V i c t o r i a 15,120 yr B.P., circulation.
and be
the usual
Exposure
in total
Using these indicators
grasses
plants)
organic
phase
from A.D.
surfaces
organic
aquatic used
to
w h i l e L. Rukwa
plants, detect
and
in
lake
C4
fluctua-
geomorphological
on sediment cores retrieved
fell below
geochemical
and the proportion of
and mi-
sediments
carbon and hydrogen
of Lakes V i c t o r i a and Rukwa in East Africa,
B.P.
and
1989).
Livingstone
cropaleontological marked
B.P.
(total organic carbon; hydrogen content;
photosynthesis, tions
yr
(Vogel,
are
due to oxida-
from the bottom
Talbot and L i v i n g s t o n e
- 66 m some time between
(1989)
17,310 and
in the Early Holocene was deep with poor
On three occasions L. Rukwa dried up since 4,000 - 3,000 yr
677
The Sahara
Fabre
and
Petit-Maire
(1988)
summarized
Holocene
climatic
evolution
in
the western Sahara during the Q u a t e r n a r y based on p a l e o - l a k e evidence and g e n e r a l l y r e - a f f i r m e d the p a l e o c l i m a t i c pattern wet
phase
during
isotopic
stage
3
(ca.
e v i d e n c e of w h i c h Fontes and Gasse as yet inconclusive. 20,000
- 10,000 yr
during
isotope
(Fig.ll.15)
40,000
-
of a possible
20,000
(1989) and W h i t e m a n
yr
B.Po),
the
(1982) deem to be
There was an arid phase during isotope stage 2 (ca. B.P.).
stage
1
Extensive
(i0,000
freshwater
- 3,000
yr
lakes
B.P.)
and
with
swamps
occured
the onset
of cli-
matic d e t e r o r i a t i o n at about 7,000 yr B.P., and the b e g i n n i n g of the arid phase at about 3,000 yr B.P. In the S u d a n e s e part of the southeastern Sahara and
Gabriel
(1984)
found
Early
Holocene
Pachur et al.
fossiliferous
lake
and
(1990) ancient
wadi sediments
(Fig.ll.16), the ages of which range b e t w e e n 9,400 to less
than
B.P.
4,800 yr
The
lake
and
marsh
v e r t e b r a t e fauna w i t h fish, crocodile, m e s t i c a t e d cattle arid region,
(Fig.ll.16).
sediments
contain
hippopotamus,
a
diverse
land turtle,
and do-
At latitude 19 °N in what is now a hyper-
cattle rearing was still possible 3,500 years ago.
11.4.2 The O c e a n i c R e c o r d The ocean contains
both extrinsic and intrinsic Q u a t e r n a r y p a l e o c l i m a t i c
records. L a n d - d e r i v e d extraneous dust
record
fallouts,
intrinsic bonates, Nile
pollen
record chert
delta
to
contained
paleoclimatic
which
and
comprises
and
the
the
Africa
signals
constitute
fluvial
spores,
other
indigenous
glauconite.
western
in
comprises
plant
sediments
be
about
the
and
to
show
Quaternary
such
margin the
or
desert
diatoms.
sediments
continental
reviewed
extrinsic
sediments,
remains
marine
The A f r i c a n will
the
terrigenous
The
as carfrom
the
information
paleoenvironmental
changes as well as the imprints of these changes on the m o r p h o l o g y of the continental
shelf
and
shoreline.
The p a l e o c l i m a t i c
signals
in the
Zaire
deep-sea fan have a l r e a d y been mentioned. Foucault and Stanley poral
fluctuations
sediments
in
consistent
cores
with
in
(1989)
inferred climatic o s c i l l a t i o n s
amphibole-pyroxene
from
the
eastern
paleoclimatic
Nile
ratios delta.
interpretations
in
the
These
based
on
Late
oscillations changes
levels of Lake A b h 6 and the lakes
in the Ziway-Shala
basin
High
the
by
proportions
from Uganda
and
spond to periods yr B.P.
of
amphibole
the Sudan,
in
dated about
cores
supplied
40,000
to 20,000
of amphibole
and high p e r c e n t a g e
of
are the
in Ethiopia.
the
White
yr B.P.,
of high lake levels recorded before about
Low amounts
from temQuaternary
Nile
corre-
20,000-17,000
of p y r o x e n e
in the
678
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o
o
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g 7
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[
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if'
~~
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.,~ ::::::.1
-,-t
o c .iJ
o
-~i ~J
u
0
,...i c~1 4j m
c e ~
.~
~
~
--
,. ®_=
:~'~-, o
~
i~ ~-
o
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~
_
L__~
~o
I ! ~ ~!?l
~ ,'~
-::l
-a
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I
.
I.~. -: : ~. :: :1
o
m~F1
• N
0...~ w
,-.4
:.~ o0",~
>oo o
_
_
L
_
_
I
L
o. 14
679
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4
ot
-~
,-4
~.~
+,.o
_
,+,,
,,+ ~ ,.,
_ <,+,,>
~o
oo
'
,o;
(+ +.++
?
~
+ +
~
+
++ o
o m
-0 +
°
•
"
:
:'+
o+
ooo mZ o
+
++ -_+ ++-
.++++
+
-+ +
? ~
+.m m=
o+ ~-
~, o
u
~-
ooo
4"
+~
~+ ~+
+
.+:_
+
Zo
~
+
s
+
z
=
o,+ ¢=
++-+ += ~
o=
,~.
~
+
+.=
®
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p, E
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•
+,,
•
£:
)
+
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~-
0
"++I, ~J
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+~
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+ ~ + a u (.+ ' ( ~=
~o
~,I,",+++
r.
+~,'+.
- -
~ ~t ,":++ .., ~.. " ++
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(i)
®+,
~
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--+~:,.&,,o _ +
+ ~ ~ , + + , " , +
0 ° ~ 0
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.I .k +. ,~,, - . , . ,
-, ~ ~
(.'~-++I:~
m
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+
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..-,
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#'
+ . ... . .- _ . . ~
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-
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/ +','"-'-+ \,~'-'+.. ~+ "+= ," < +. -+,,, _-+++ +. ++++_I
\
"J
o
~=
'
. . .
~
, <
~ , ' + m°
T
/,, ~
.j
+
+a
/' El
.+++ +-
-''"+'~
(/) . . . . .
"
',++ - . .
" , ',-. . .
1
j
~m
680
cores
between
about
20,000
lake levels
from about
arid
An
phase.
4,000
yr
curred
increase
B.P.,
spans
around
8,6000
increase
in a m p h i b o l e
climatic
phase
that
to
12,000-10,000
20,000-17,000
yr B.P.,
in amphibole
between
a period
of high
lake
yr
(Gasse
near
the
induced
B.P.
levels
lake
found
low
7,000
in Djibouti
Fontes,
3.4 m may
levels
with
during the
dated
which
and
top at about
high
yr B.P.,
the interval
to
6,000
correspond
to 14,000-10,000
1989).
about
oc-
Another
be related
for
to
to the
1,500
yr
off Cap Blanc
in
B.P.
Hooghiemstra West
Africa
of
main
(Agwu
Air Layer w h i c h and
African
the
trade
hara
after was
around yr
zone
11,000 B.P.
forests
et al.
and
controlled
B.P.
by
to
the
presented
environments
took
the
south
fluvial
trade
yr B.P.,
of
runoff
runoff
savanna
species.
place
The
13,000
the
Sa-
in
the
increased
is recorded
its m a x i m u m
from about
belt
started
to
The core also reveals expansion
and
15 ° N, w h i c h
a description
that the
reflects
hu-
the
the
time,
and major
effects
of
gradients carbonate climatic
processes
Congo,
evolution
deposition
latitudinal
terrigenous
off
dominant
on
shelf
and an i n t e r p r e t a t i o n
and d e p o s i t i o n a l
(Fig.ll.7)
controls
present
runoff
off
extended
and the dominant
arid
of
as far north as about
Recent
of
reached
Sahara
around
Fluvial
core
yr B.P.
latitudinally.
B.P.
B.P.,
The
south of the Sahara.
shelves
climatic
distribution up
had
demonstrated
paleo-physiographic
yr
the
less
Late
properties
which
19,000-14,000 of
the
in the southern
belt
spike
yr
and m a x i m u m
12,500
the
inversion.
Savanna
became
percentage latitudinal
Jet or the Sahara
of trade winds
richer in w o o d y
(1988)
sedimentary continental
paleoenvironments.
and
Around
reveals
wind
shifted
first
the
during
Easterly
situation, but
13,000
yr B.P.
and became
a
zones
also
expansion
climate
and
around
conditions
Barusseau
Senegal,
trade
not
of
that reflects
and originates
the belt had
The
B.P.,
had m i g r a t e d
mid climatic
when
in the m o d e r n
yr
tropical
of Recent
the
southward
weakened.
14,000
shift n o r t h w a r d
African
core
a compressed
transport
as
registered
9,000-7,000
Guinea
and
Jet
strong winds
the
was
conditions
Easterly
stron g l y
This
and the A f r i c a n
above
there
northward
were
core
that
westwards
variation
vegetation
at higher altitudes
that
hyperarid
winds
~frican
1982).
winds
located
in a m a n n e r
12°N to 14-15°N during the period
Maximum under
occurs
shows
from about
Beug,
trade
blows
Blanc
is a d o w n co r e
northwest
and
of the p r e v a i l i n g
Sahara
there
and influx of pollen,
the
Quaternary
Cap
has shown that in a core
(Fig.ll.7),
concentration shifts
(1988)
C6te
of
of
acting
and
paleoclimatic
and
the
material
Late in
on
the
the
sediments. changes
on the West
d'Ivoire
Quaternary region
type,
From
combined
is
volume
18,000 with
yr eu-
681
static
sea-level
rise,
controlled
important
variations
in
sedimentary
conditions. During were
the
beginning
emplaced
nental dune
the
in paleo-valleys
shelf. sands
of
In the tropical
formed
during
emerged
shelf,
but
Giresse
et
(1988)
al.
Senegalo-Mauritanian T a f a r i t i a n with
which were arid
destroyed
up-dated
by
the
Quaternary
B.P.,
the
subsequent
sequences
sands conti-
aeolian
(Ogalian)
on
the
transgression.
chronostratigraphy
shoreline
followed
and Senegal,
period
the oldest transgressive deposits
yr
fluvial
incised on the exposed
"glacial"
A k c h a r i a n with the next regression, 40,000-30,000
regression
regions of M a u r i t a n i a
the
were
Wisconsinian
of
and
the
emerged
correlated
the
(mid-Pleistocene),
the
the A i o u j i a n with a t r a n s g r e s s i o n at
later by the Ogalian
and
the Nouakchot-
tian. Near the Equator, duced,
but
observed shelf
littoral
close
during
to
the
on the shelf of C6te d ' I v o i r e aeolian input was redunes
the low
present stand
fans on the continental ization d u r i n g
the
of that period shoreline.
of
sea
deposits
still visible
stillstand with
today.
larly the main ones
of the Holocene
mangrove
Offshore
and
Mauritania
structural
terrigenous recorded
during
the
B.P.).
This
all
along
first is
the
in
lows.
In the input
Shoreline
stabil-
allowed
the
dune
sands,
and
paleo-shorelines
which
are
sands,
river mouths,
sediments
tropical is
the
in deep-sea
regions
considerably
particu-
settled in morof
Senegal
reduced
but,
and in
aeolian contribution of silts and v e r y fine ma-
surficial
stillstand
represented outer
along
may be
by-passed
deposited
transgression
(fine shoreline peat)
remnants
sediments
from the equatorial
fluvial
their n o r t h e r n m o s t parts, is
Fine
and were
such as the Zaire, pelitic
phological
terial
level
rise and on the abyssal plain.
a c c u m u l a t i o n of littoral deposits lagoonal
occurred whose
shelf
of
sediments. the
Holocene
by a belt in West
Bioclastic
of
carbonates
sea-level
rise
formed
(12,000
yr
Amphistegina sands d i s t r i b u t e d
Africa.
This
relict
fauna
occurs
be-
tween the 80- and 120-m bathymetric depth. Off the coast of SW Africa, tern
of
the
abundance
Benguela
and
ocean
preservation
c l i m a t e - i n d u c e d changes
current of
(Fig.ll.4,A)
opaline
or
are
siliceous
nental
slope
stages
1 and 3 (last interglacials),
glacial
period
in
which
the opal
opal-rich
content
sediments
reflected
microfossils
the d i a t o m species composition of deep-sea sediments. v a r i a t i o n of opaline silica in an oceanic core
in the flow pat-
found
and
the in
There is a downcore
(Fig.ll.7)
are
in
in
from the contioxygen
isotope
whereas during the peak of the last
is v e r y much
opal is c o n t r o l l e d by the Benguela Current.
reduced.
The p r o d u c t i o n
of
At the point along the coast
682
where the B e n g u e l a Current changes its flow d i r e c t i o n and deviates westward, high
upwelling
of n u t r i e n t - r i c h
productivity
well-preserved
among
opal
coastal waters
siliceous
flora
plankton
(diatoms)
to
takes place and supports
which
the
contributes
sediments
on
rich
the
and
seafloor.
Thus, d u r i n g the last interglacial the Benguela Current d e v i a t e d westward at
24 ° S where the
studied core
and g e n e r a t e d o p a l - r i c h
is located
sediments.
(Diester-Haass
et al.,
But during the last glacial
1988)
stage the
current flowed for a longer distance northward before d e f l e c t i n g west,
in
w h i c h case n u t r i e n t - r i c h waters were not available at the location of the core
(lat. 24 ° S), but further to the north.
11.5 Aspects of Human Origin As r e c e n t l y o b s e r v e d by Simons
(1989) hominid evolution has remained one
of
Quaternary
the most
engaging
fields of
research,
not
only
because
of
its direct bearing on man's ancestry but because of the m a n y tantalizing aspects sites
of
have
its
continued
technological ever, of
of
human
fragmentary
record.
New discoveries
to sustain our knowledge
developments
considerable evolution,
and
of
man's
significance
are
the
to the
applications
from A f r i c a n
of h o m i n i d
earliest of
anatomical
cultural
interpretations improved
hominid
traits.
and How-
of
the course
dating
techniques
and genetics. Continuing tocene line,
Lake
research
Turkana
in deposits
at
basin
the
famous
in Kenya,
ranging between
hominid-bearing
this
1.0 and
time
Pliocene
along
3.5 Ma,
- Pleis-
the w e s t e r n
has
produced
shore-
the most
complete
specimen of Homo erectus known so far. This has established the
presence
in East Africa of a hyper-robust a u s t r a l o p i t h e c i n e at about 2.5
m i l l i o n years ago
(Harris et al.,
cently
discovered
in
the
Sillen
(1988).
Brain
and
warthogs
and
baboons
deposits
dated
in
between
1988). The earliest use of fire was re-
Swartkrans Sillen
found
association 1.0
cave
and
1.5
in
burnt
with Ma.
South bones
Africa of
it
Brain
antelopes,
Australopithecus However,
by
is
zebra,
robustus, not
and
clear
in who
"roasted" these animals, w h e t h e r Australopithecus or Homo. Simons'
(1989) review of the history of hominid discoveries in Africa
and their r a d i a t i o n into Europe and Asia, the-art account on paleoanthropology. est
discoveries
the
1920's,
rapidly
and
Simons
through
the
how
a m a s t e r l y state-of-
of Australopithecus
christening showed
furnishes
Starting from Raymond Dart's earli-
our
discoveries
of
knowledge
of
in
South
hominids
A. africanus,
A.
Africa
then
afarensis
in
unfolded and
A.
683
in Tanzania and Kenya, in sediments ranging in age from 3.6 to 1.6
boisei
m.y.
Although
their evolutionary relationships
(sensu lato)
bilis
ithecines ago that
and H o m o Homo
remain
illusive,
and H o m o
erectus
sapiens,
but it was not until about
made
erectus
the
Homo
ha-
are the links between the australop-
initial
hominid
1.0 m i l l i o n years
migration
out
of Africa
into Europe and Asia. How modern man s u b s e q u e n t l y o r i g i n a t e d - w h e t h e r by local d i f f e r e n t i a t i o n of H. parent
stock
Andrews (1988) DNA,
as
(1988), and Lewin summarized
and
Simons
(Fig.ll.ll)
argued
(1989)
advocated
genetic
favour of
insists
data,
moluminescence
sapiens
fact
that
hard
early
peared
around
of
Africa
(1988),
Stringer
and
S t r i n g e r and Andrews
nuclear
origin
and m i t o c h o n d r i a l
of m o d e r n
man,
while
the
Homo
However,
Simon
sapiens
92,000 yr B.P.,
60,000
yr
model"
Stringer and Andrews,
B.P.
of
(Fig.ll.ll) as the ancesor
neanderthalensis
from a cave
modern
East before
"out
for
age dates
Middle
mainly
a sub-Saharan
of m o d e r n man in Europe and Asia. and
Stringer
or from the African
that the evidence is not yet s u f f i c i e n t l y convinc-
ing to c h a l l e n g e H o m o recalcitrant
by
(1988) - is controversial.
modern
in
e r e c t u s w h e r e v e r he went,
the
This
site had
to
in
dismiss
still
modern
to
lend
humans
the
point
in A f r i c a
than N e a n d e r t a l
seems
of
are
Israel w h i c h
appeared
earlier
origin
1988; Lewin,
recent e v i d e n c e w h i c h remains
school
and
man,
therto the in
the
who ap-
credence
to
(Stringer,
the
1988;
1988).
11.6 Reflections on Contemporary Environmental Problems From the aforegoing, well-documented nar~ with
concomitant
now w i d e l y cyclical
it is evident that Africa is replete with a long and
record of changing climatic
believed
changes
that
the
affecting
landforms
iterative
mechanisms
changes w h i c h we have reviewed were
driven by cosmic or astronomical phenomena last 2,000 years, East
Africa
cal human
(Hamilton,
1982),
land degradation.
vironmental
also
degradation
deforestation,
pose
serious
the
impact
for i r r i g a t e d
that
It is
perpetuated
and might
the
have been
1984). But w i t h i n the
through
to
agriculture
d e r n o u r i s h e d populations.
global
exacerbated,
three
desertification,
threats
of
climatic
change
on
and p r o l o n g e d by lo-
Human action has a c c e l e r a t e d the rate of en-
African
c a p a c i t y to supply hydro-electricity, mand
and vegetation.
global
(M6rner,
d u r i n g the Quater-
ever since man began to d e f o r e s t the w o o d l a n d belts of
Africa has been s e r i o u s l y overprinted,
namely:
scenarios
interrelated
ecological
and soil erosion. man-made
lakes,
abuses,
These problems
especially
their
and also sustain the increasing de-
to feed Africa's
teeming
and at best un-
684
Among
the
human
mechanisms
for
global
climatic
change
are
the
in-
creasing g r e e n h o u s e effect of the abnormally high amounts of carbon dioxide,
the
nia,
the effects
impact of other gases
addition the
of
global
of sulfur,
such as nitrous
and the depletion
chlorofluorocarbon. environment,
their
Although
oxide,
these
immediate
methane
of the ozone
and
factors
are
potential
and ammo-
layer by the threatening
impact
are
very
severe on poor A f r i c a n nations whose marginal agricultural productivities are
further
worsened
by drought.
combat natural disasters, diate
and u n m i t i g a t e d
Since Africa
has
the
least
ability
to
any global ecological d i s a s t e r will bring imme-
catastrophe
to the
continent.
For example,
an un-
abated increase in global temperature would mean 6-8 ° rise in the temperature of the polar regions of the world polar
ice
and
sea-level
release
which
will
enough drown
water
all
(Woodwell,
into
the
the
1984). This would melt
oceans
coastlands
of
to
cause
the
a rise
world,
in
including
Africa. The annual d e s t r u c t i o n of about 7 million hectares of A f r i c a n forests by b u r n i n g
(Burngh,
1984)
does not only threaten
the release of copious amounts of carbon dioxide, mediate sion,
impact
which
of Africa
on Africa
have
already
of
accelerating
reached crises
and on African mountains
1988).
Ironically,
severe
in
the
global
climate
through
but it also has the im-
desertification
proportions
and
in the
soil
Sahel
ero-
region
and highlands
(Grosjean and Messerli,
as evident in eastern Nigeria,
soil erosion is equally
sandy
wet
coast
lands
in
the
equatorial
belts
of
West
Africa. The forest
solution
to African
conservation
development
following
and Swaziland national
(Hudson,
intervention.
natural disasters of A f r i c a ' s Glantz Saharan and,
in
problem which
1987),
After
affluent in Africa
the early days
crises
of
all,
societies,
which
have
in
lies
partly
through
already
planning
the historical
"...drought itself
Africa:
problems efforts,
in
must
succeeded
mitigate
and inter-
the
and s o c i o - p o l i t i c a l
be borne
in mind.
As
rural
in Kenya
impact
of
contexts
observed
in
is not the fundamental p r o b l e m in sub-
drought need
to
soil and
integrated
and through timely governmental
However,
in Africa,
prevails
be
in m a n y
no more
than
parts
of
a nuisance.
is poverty -- the lack of d e v e l o p m e n t
lie in Africa's
donors."
communal
the models
environmental
(1987):
ecological
through
the world The
real
-- the seeds of
colonial past and in unwise policy choices made in
independence by national
governments
and external
aid
References Abell,
P.I. 1982. Paleoclimate at Lake Turkana, Kenya, from oxygen isotope ratios of gatropod shells. Nature 297: 321-323.
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