The Atlas system of Morocco: studies on its geodynamic evolution

Lecture Notes in Earth Sciences Edited by Somdev Bhattacharji, Gerald M. Friedman, Horst J. Neugebauer and Adolf Seilacher

15 Volker H. Jacobshagen (Ed.)

The Atlas System of Morocco Studies on its Geodynamic Evolution

Springer-Verlag Berlin Heidelberg NewYork London Paris Tokyo

Editor Prof. Dr Volker H. Jacobshagen Frele Unlversltat Berhn FB Geowissenschaften, Instltut fur Geologle Altensteinstrasse 34a, D-1000 Berhn 33, FRG

ISBN 3-540-19086-4 Sprmger-Verlag Berhn Heidelberg N e w York ISBN 0-387-19086-4 Springer-Verlag New York Berlin Heidelberg

Ltbrary of Congress Catalogmg-tn-PubhcahonData Geodynamlc evoluhon of the Atlas system/ Morocco / Volker H Jacobshagen, ed p cm -(Lecture notes in earth scrences, 15) "One of the sources of this book was a conference on the geodynamJcaspects of the Atlas system, organtzed m Berltn in 1986" by the Forschungsgruppe "Mobrhtat akhver Kontlnentalrander"-Pref ISBN 0-387-19086-4 (U S ) 1 Geology-Morocco-Atlas Mountains 2 Geology-Morocco-Antt-Atlas Mountains 3 Geodynamics I. Jacobshagen, Volker II Senes QE339 MSG36 1988 556 4'4-dc 19 88-12329 Thrs work is subject to copyright All rights are reserved, whether the whole or part of the material ts concerned, specifically the rights of translation, reprmtrng, re-use of Jllustrattons, recttatlon, broadcasttng, reproduction on mrcrofilms or in other ways, and storage m data banks Duphcat~on of thrs pubhcahen or parts thereof rs only permitted under the provtsJonsof the German Copynght Law of September g, 1965, in its versron of June 24, 1985, and a copyright fee must always be paid Violations fall under the prosecution act of the German Copyrtght Law @ Spnnger-Veriag Berhn Hetdelberg 1988 Pnnted rn Germany Prmhng and binding. Druckhaus Beltz, Hemsbach/Bergstr 2132/3140-543210

PREFACE

The aim of this volume is to r e f l e c t the current state of geoscient i f i c a c t i v i t y focused on the geodynamic evolution of the Atlas system and to discuss new results and ideas. The volume provides a selection of papers on the geological history, structural development, and geophysical data of Morocco. I t was not possible to cover all areas of geoscientific i n t e r e s t , however, we hope to shed some l i g h t on the major geodynamic problems. One of the sources of this book was a conference on the geodynamic aspects of the Atlas system, organized in Berlin in 1986 by the Forschergruppe "Mobilit~t aktiver Kontinentalr~nder". The financial support for this project is being provided by the Deutsche Forscht:!~gsgemeinschaft and by the Freie Universit~t and the Technische Universi o t~t of Berlin (West). Furthermore we are very glad to mention that we have received a great number of publications by authors from d i f f e r e n t countries who did not take part in the Berlin conference. I t should be noted that, at present, a l o t of international geos c i e n t i f i c a c t i v i t y is concerned with Morocco. This is not only due to the fascinating complexity of Moroccan geology and the beautiful outcroups, that may provide the key to many general problems, but also t e s t i f i e s to the l i b e r a l and prescient science p o l i t i c s of the Moroccan a u t h o r i t i e s , most of all the Ministery of Energy and Mining in Rabat. Therefore, on behalf of all the authors from abroad, I take the opportunity to express our deepest gratitude to our host country, especially to the leaders of the Division de la G~ologie, the general director Dr. M. Bensaid and the directors M. Dahmani and M. Hammouda. Thanks are, furthermore, due to Springer Verlag (Berlin-HeidelbergNew York) for offering the opportunity to publish a comprehensive volume and especially to Dr. Wolfgang Engel for his patience and helpful advices. V. Jacobshagen

CONTENTS

INTRODUCTION JACOBSHAGEN, V . : Geodynamic M o r o c c o : an i n t r o d u c t i o n

evolution

of

the A t l a s

System, 3

ANTI-ATLAS WALLBRECHER, E.:

11 The A n t i - A t l a s

system t a n

overview

13

WALLBRECHER, E.: A d u c t i l e shear zone in t h e P a n a f r i c a n basement on t h e n o r t h w e s t e r n m a r g i n of t h e W e s t - A f r i can c r a t o n ( S i r w a dome, c e n t r a l A n t i - A t l a s )

19

BASSIAS, Y . , WALLBRECHER, E. and WILLGALLIS, A . : T e c t o n o t h e r m a l e v o l u t i o n o f t h e L a t e P a n a f r i c a n o r o g e n y in the Central Anti-Atlas ( S o u t h e r n Morocco)

43

BRABERS, P.M.: A p l a t e t e c t o n i c model f o r o r o g e n y in the A n t i - A t l a s , Morocco

61

BUGGISCH, W. and FLOGEL, E.: b o u n d a r y in t h e A n t i - A t l a s new r e s u l t s SDZUY, K. and GEYER, G.: Morocco

the

Panafrican

The P r e c a m b r i a n / C a m b r i a n ( M o r o c c o ) . D i s c u s s i o n ~nd 81

The base o f

t h e Cambrian

in 91

BUGGISCH, W. and SIEGERT, R,: P a l e o g e o g r a p h y and f a c i e s o f t h e d ~ r ~ s t e r m i n a u x ' ~ ( u p p e r m o s t Lower C a m b r i a n , Anti-Atl as/Morocco)

107

BUGGISCH, W.: D i a g e n e s i s and v e r y l o w - g r a d e metamorphism of the Lower Cambrian r o c k s in t h e A n t i - A t l a s (~4orocco)

123

HIGH AND MIDDLE ATLAS

129

SCHAER, J . - P . : Deformation of a r o u n d the T i c h k a g r a n i t e ,

i g n e o u s d i k e s i n and High A t l a s - Morocco

LORENZ, J . C . : S y n t h e s i s o f L a t e P a l e o z o i c s i c redbed s e d i m e n t a t i o n in Morocco WARME J . E . : J u r a s s i c c a r b o n a t e and E a s t e r n High A t l a s r i f t ,

f a c i e s of Morocco

131

and T r i a s 139 the Central 169

BRECHBQHLER, Y . A . , BERNASCONI, R. and S C H A E R , J . - P . : J u r a s s i c s e d i m e n t s o f t h e C e n t r a l High A t l a s o f Morocco: Deposition, b u r i a l and e r o s i o n h i s t o r y

201

FROITZHEIM, N., STETS, J. and WURSTER, P.: A s p e c t s of !qestern High A t l a s t e c t o n i c s

219

JACOBSHAGE,~I, V , BREDE, R , HAUPTMA~IH, M HEINITZ,W and 7YLKA, R : S t r u c t u r e and p o s t - P a l a e o z o i c evolution o f the c e n t r a l High A t l a s

245

Vl FRAISSINET, C., ZOUINE, E.M., MOREL,J.-L., POISSON, A . , ANDRIEUX, J . and FAURE-MURET, A.: S t r u c t u r a l e v o l u t i o n of the southern and n o r t h e r n Central High A t l a s in Paleogene and M i o - P l i o c e n e times

273

DRESNAY, R. du: Recent data on the geology of the M i d d l e A t l a s (Moro, cco)

293

HERBIG, H.-G.: Synsedimentary t e c t o n i c s in the Northern Middle A t l a s (Morocco) during the Late Cretaceous and Tertiary

321

SCH!,/ARZ, G. and WIGGER,P.J. Geophysical s t u d i e s of the e a r t h ' s c r u s t and upper mantle in the A t l a s system of Morocco

339

CENOZOIC BASINS

359

G~RLER, K., HELMDACH, F . - F . , GAEMERS, P., HEISSIG, K., HINSCH, W., M#DLER= K., SCHWARZHANS, ~I. and ZUCHT, M. The u p l i f t of the c e n t r a l High A t l a s as deduced from Neogene c o n t i n e n t a l sediments of the Ouarzazate prov i n c e , Morocco

361

HELMDACH, F . - F . : The ostracode fauna of the A i t region: systematic description

405

Kandoula

ST~BLEIN, G.: Geomorphological aspects of the Quaternary e v o l u t i o n of the Ouarzazate basin, Southern Morocco

433

SCHMIDT, K . - H . : Rates of s c a r p - r e t r e a t : neotectonic activity

445

a means of d a t i n g

SYNTHESES

463

DURAND-DELGA, M. and OLIVIER,Ph.: E v o l u t i o n of the Aiboran block margin from E a r l y Mesozoic to Early Miocene time

465

JACOBSHAGEN, V., G~RLER, K. and GIESE, P.: Geodynamic evol u t i o n of the A t l a s System (Morocco) in p o s t - P a l a e o z o i c times

481

Introduction

GEODYNAMIC EVOLUTION OF THE ATLAS SYSTEM, MOROCCO: AN INTRODUCTION Volker Jacobshagen I n s t i t u t fur Geologie, Freie Universit~t Berlin, A l t e n s t e i n s t r . 34 A, D-lOO0 Berlin

33

The Atlas System The northwestern margin of Africa, between the A t l a n t i c Ocean and the Lesser S y r t i s , is characterized by high mountain ranges, the elevation of which surmounts 3.000 m in the western part. Jbel Toubkal (4.165 m) in the High Atlas is the highest mountain of North Africa. These ranges have been synchronously u p l i f t e d since the Miocene. They d i f f e r , however, both in structure and geological

history. The Maghre-

bides consisting of the Rif and Tell Atlas along the Mediterranean coast are a typical Alpidic orogen with complex nappe structures. The ranges in t h e i r southern foreland, called the Middle and High Atlas, are anorogenic intracratonic mountain belts separated by elevated plateaus. The eastern prolongation of the High Atlas is formed by the Saharan and Tunisian Atlas. The Anti-Atlas, however, which l i e s south of the High Atlas in Morocco, originated by simple updoming of the Precambrian basement of the Sahara craton. As a l l these ranges are geographical

andgeological related to each other they have been named

the Atlas system by M. RICHTER (1970). Thus, we find a mobile zone in the southern foreland of the Maghrebide orogen, which in southern Morocco extends up to 500 km into the African plate. As the southern border of this zone diverges more and more from the Maghrebides to the southwest, we may assume that i t s geological evolution cannot be solely deduced from the Mediterranean orogeneses, but seems to be more complex.

Geotectonic subdivision of Morocco The Moroccan segment of the A t l a s For an o v e r v i e w , the p u b l i c a t i o n s FAURE-MURET (1962,

system is r e l a t i v e l y

of MICHARD (1976),

well

known.

CHOUBERT and

1971, 1973) and FAURE-MURET and CHOUBERT (1971) are

recommended. The g e o t e c t o n i c

s u b d i v i s i o n of the Moroccan A t l a s

system

is shown in f i g .

1 and is b r i e f l y described here:

SEA

0 0 \

0 o

x,,

£o$~bic((Ica

on

o

o

o

o

ii;iC~i~

~

o

re=l

o

~

I

~~

•

'~'

:~$~Z1

HAUTES PLATEAUX

"v

x,, "t"

ZOO*

~!iiiiiiii!!iiiiy

' ....

[3 ......

~

e=aeoz=c

[~

UpperProterozoic

Upp,r Ce,nz0ic vat0n,,0es

R.-uoont=°,

Post- Horcynioncover:

f--I

pi,te,u,

(~

Highend Middlektles

Precambrian [~

UpperCeonzoicbasins

Fig. I : Geotectonic sketch-map of the Atlas system of Morocco, based on MICHARD (1976) The narrow arc of the Rif orogen in the north is overthrust towards the S on an Upper Cenozoic marginal trough. To the SW, the orogenic

belt is bordered by the h i l l s and plains of the Moroccan Meseta. Its Variscan basement is exposed in large areas from under a cover of f l a t - l y i n g Mesozoic and Palaeogene sediments. To the SE, the Meseta and the Rif are bordered by the High and the Middle Atlas. Both ranges developed from early Mesozoic r i f t grabens which originated in correlation to the opening of the Western Tethys and of the North Atlant i c Ocean. Rifting was accompanied by mafic volcanism, which culminated in the Upper Triassic/Liassic and in the Dogger. Some intrusions, however, are of Cretaceous and even Palaeogene age. The structure of the High and Middle Atlas generally correspond to each other, both showing a basement of Palaeozoic or Precambrian rocks that is covered by thick Triassic and Jurassic sediments. Prevailing structures are steep f a u l t s , monoclines and s t r i k e - s l i p f a u l t s . In places, especially in the central and eastern High Atlas, overthrusts, box folds and even overturned folds e x i s t . However, essential c r i t e r i a of orogens such as: ophiolites, nappes, flysches, regional metamorphism and granitoids are not developed. Both ranges are accompanied by a few small Cenozoic basins. The Middle Atlas forks o f f from High Atlas in a northeastern d i r e c t i o n , enclosing the so-called High Plateaus of Oran. The Variscan basement of these plateaus is covered by Mesozoic sediments, which are in the southwest overlain by the Cenozoic of the Haute Moulouya. The High Atlas is separated from the Anti-Atlas by the Souss and the Ouarzazate basins ( " s i l l o n pr~africaine"). As suggested by RUSSO and RUSSO (1934), many geologists believe that the High Atlas is bordered to the S by an important f a u l t zone, known as the South Atlas Marginal Fault zone ("accident sud-atlasien"). Although several authors doubt that this f a u l t r e a l l y exists, others have already introduced i t to plate tectonic concepts. Some of them even believe i t to be the northwestern boundary of the African plate (e.g. DEWEY e t a l . , 1973). The Anti-Atlas was formed by wide-spanned updoming of marginal parts of the Sahara craton. Precambrian rocks form the core of major domes ("boutonnieres") in the axial zone of the range. Normally, this basement is covered by poorly deformed sediments of Uppermost Proterozoic or Lower Palaeozoic age. In the T a f i l a l t region (Southeast Morocco), the Anti-Atlas dips to the E beneath undeformed Cretaceous sediments of the Sahara platform.

Pliocene or even Quaternary volcanoes of alkaline character are widespread in Morocco, with the exception of the High Atlas. Their geodynamic meaning is not yet c l e a r l y understood. The structural development of Morocco south of the Rif and probably also the Mesozoic magmatism were preordained by a pattern of major f a u l t s in the pre-Mesozoic basement, the main directions of which are NE and ENE (DUBOURDIEU 1962, MATTAUER et a l . 1972) and, in addition, NNW and ESE. According to these authors, the f a u l t s are of late Palaeozoic o r i g i n . In the central Anti-Atlas, however, the same pattern o r i ginated as soon as the end of the Panafrican cycle (HEINITZ 1984).

Aspects of the geodynamic evolution Our present geoscientific knowledge of the four ranges of the Atlas system d i f f e r s widely. The Maghrebides have been intensely investigated for many decades. Usually they have been considered within the context of the alpine orogens of the Mediterranean region. To describe t h e i r structure and geologic history with some detail would f i l l

a separate

volume. The present book aims, however, mainly to shed some l i g h t on the development of the intracontinental A~las ranges, and to define the general relations between them. Most of the authors have interpreted t h e i r geodynamic evolution with respect to major plate tectonic events of the Mesozoic and Cenozoic periods. Under this aspect the Rif orogeny was only b r i e f l y reviewed, here, in connection with the alpine development of the Western Mediterranean region by DURAND-DELGA and OLIVIERo The geodynamic history of the Atlas system started very early. Within the Precambrian cores of the Anti-Atlas, traces of several orogenies are preserved (see WALLBRECHER, this v o l . ) . Recent studies have been mainly focussed on the Panafrican cycle for which plate tectonic models have been proposed by LEBLANC (1981), SCHERMERHORN et a l . (1986), and BRABERS ( t h i s v o l . ) . But also many new results concerning the Late Proterozoic and Lower Palaeozoic cover of the Anti-Atlas have been produced, which are referred to by BUGGISCH and FLOGEL, SDZUY and GEYER and by BUGGISCH and SIEGERT in the f i r s t part of this book. The very important Caledonian-Hercynian history, which is documented in the basement of the High and Middle Atlas and of the Moroccan Meseta, has recently been discussed during a symposium "Morocco and

Palaeozoic Orogenesis (I.B.C.P. Project No. 27, The Caledonide Orogen), held in Rabat in 1983. This epoch is, therefore, neglected in the present volume. A great many of recent investigations have been concentrated on post-Palaeozoic

stratigraphy, facies and structural evolution of the

Western and Central High Atlas mountains, but our knowledge of the Middle Atlas is at present r e l a t i v e l y poor. As both ranges play a key role in deciphering the Mesozoic and Cenozoic evolution of the Atlas system, the majority of contributions to this volume deal with these topics. Seismologic, geoelectric and magnetotelluric studies on a section across both ranges by SCHWARZ and WIGGER are of special

im-

portance for the understanding of geodynamic processes. The neotectonic history of the Atlas system is clearly reflected by the sedimentary f i l l i n g s of the Upper Cenozoic basins adjacent to the Atlas ranges as demonstrated mainly by the contributions of GUREER et al. and FRAISSINET et a l . , but also by geomorphologic features. Recent geotectonic hypotheses interpreted the evolution of the intracontinental Atlas ranges with respect to plate tectonic configurations and processes (MICHARD et al.

1975, MATTAUER et al. 1977,

COURBOULEIX e t a l . 1981, STETS and WURSTER 1981). All of them referred to the fundamental fractures of the pre-Mesozoic basement. I t is gene r a l l y agreed that these fractures have been repeatedly reactivated since the Triassic, while the orientation of the continental stressf i e l d s changed in time, due to the r e l a t i v e movements of the African and the European plates. MATTAUER et al. (1977) were of the opinion that the post-Palaeozoic development r e f l e c t s two major geotectonic processes of changing influence: the opening of the Atlantic and the collisions of the West Mediterranean regions. Deviating from this view, STETS and WURSTER (1981) proposed a geotectonic model for the High Atlas which is related only to the Atlantic opening and excluded Mediterranean influences. JACOBSHAGEN, GURLER and GIESE (this vol.) support MATTAUER's view in reference to new geological and geophysical data, they also suggest a mechanism to transmit compressional forces from the active northern margin over several hundred kilometers into the African plate. Acknowledgements: The manuscript would not have been realized without the help of W. Jung, G. Lindemann, D. Reich and C. Di Stefano which is g r a t e f u l l y acknowledged.

REFERENCES CHOUBERT, G. and FAURE-MURET, A. (1962): E v o l u t i o n du domaine a t l a s i aue marocain depuis le temps D a l 6 o z o i q u e . - Livre-m~m. P. F a l l o t , M~m. h. ser. Soc. g~ol. France, 1: 447-527, P a r i s . (1971): A f r i q u e o c c i d e n t a l e : socle or#cambrien et zones m o b i l e s . A n t i - A t l a s ( M a r o c ) . - in UNESCO, Tectonique d ' A f r i q u e , Sci. T e r r e , 6: 163-175, P a r i s . (1973): Moroccan R i f . CER ( e d , ) : Mesozoic - Cenozoic orogenic b e l t s : Soc. London.

in A.M. SPEN37-46, Geol.

COURBOULEIX, S., DELPONT, G. and DESTEUCQ, CH. (1981): Un grand d~crochement e s t - o u e s t au nord du Maroc ~ l ' o r i g i n e des s t r u c t u r e s p l i s s # e s a t l a s i q u e s . Arguments g#ologiques et e x p ~ r i m e n t a u x . B u l l . Soc. g#ol. France, ( V I I ) , 23: 33-43, P a r i s . DE~JEY, J . F , , PITMAN, ~J.C., RYAN, W.B. and BONNIN, J. (1973): Plate t e c t o n i c s and the e v o l u t i o n of the a l p i n e system. B u l l . g e o l . Soc. Amer., 84: 3137-3180. DUBOURDIEU, G. (1962): DynamiQue wegen#rienne de l ' A f r i q u e du Nord.L i v r e M~m. P. F a l l o t , M#m. h. ser. Soc. g~ol. France, I : 627644, P a r i s . FAURE-MURET, A. and CHOUBERT, G. (1971): Le Maroc. Domaine r i f a i n et a t l a s i q u e . - in UNESCO, Tectonique de l ' A f r i q u e , Sci. T e r r e , 6: 17-46, P a r i s . HEINITZ, W. (1984): Die Deformation des sediment~ren Deckgebirges im z e n t r a l e n A n t i - A t l a s (SUdmarokko).- B e r l i n e r geowiss. Abh., (A), 55: 1-84, B e r l i n . LEBLANC, M. (1981): O p h i o l i t e s pr6cambriennes et g?tes a r s e n i c s de c o b a l t (Bou Azzer - M a r o c ) . - Notes M#m. Serv. g~ol, Maroc, 280: 1-306, Rabat. MATTAUER, M., PROUST, F. and TAPPONIER, P. (1972): Major s t r i k e - s l i p f a u l t s of Late Hercynian age in Morocco.- Nature, 237: 160162. MATTAUER, M., TAPPONIER, P. and PROUST, F. (1977): Sur les m~canismes de f o r m a t i o n des cha?nes i n t r a c o n t i n e n t a l e s . L'exemple des cha?nes a t l a s i q u e s du Maroc.- B u l l . Soc. g#ol. France, ( V I I ) , 19: 521-526, P a r i s . MICHARD, A. (1976): Elements de g # o l o g i e m a r o c a i n e . - Notes M~m. Serv. g ~ o l . , 252: 1-408, Rabat. MICHARD, A., WESTPHAL, M., BOSSERT, A. and HAMZEH, R. (1975): T e c t o n i oue de blocs dans le socle a t l a s o - m ~ s # t i e n du Maroc: Une n o u v e l l e i n t e r p r e t a t i o n des donn~es g~ologiques et pal~omagn ~ t i a u e s . - Earth P l a n e t . Sci. L e t t e r s , 24: 363-368, Amsterdam. RICHTER, M. (1970): Das A t l a s - S y s t e m in N o r d a f r i k a . 120 (1968): 68-84, Hannover.

Z.dt.geol.

RUSSO, P. and RUSSO, L. (1934): Le grand a c c i d e n t s u d - a t l a s i e n . Soc. g#ol. France, (V), 4: 375-384, P a r i s .

Ges., Bull.

SCHERMERHORN, L . J . G . , WALLBRECHER, E. and HUCH, K.M. (1986): Der Subd u k t i o n s k o m p l e x , G r a n i t o p l u t o n i s m u s und S c h e r t e k t o n i k im Grundgebirge des Sirwa-Doms ( A n t i - A t l a s , Marokko).- Berliner qeowiss. Abh., ( A ) , 66: 301-332, B e r l i n . STETS,. J . , and WURSTER, P. (1981): Zur S t r u k t u r g e s c h i c h t e des Hohen A t l a s in Marokko.- Geol. Rdsch., 70: 801-841, S t u t t g a r t .

Anti-Atlas

[HE ANTI-ATLAS

SYSTEM:

AN OVERVIEW

Eckard Wallbrecher for Geologie und Paleontologic Universit~t Graz Heinrichstr. 26, A-8010 Graz

Institut

General Situation The Anti-Atlas System, (Fig.

which lies between the High Atlas and the Sahara

i) is a part of the perieratonic

(MICHARD 1976).

sis of the High Atlas, time,

zone of the West Sahara Craton

In the Alpine age, which was the time of the orogenethis system acted as a stable crust.

it was transgressed

an Upper Precambrian

only by the flat Cenoman-Turon

to Lower Paleozoic

sea.

At this It has

cover which was partly folded

during Varisean orogeny. The typical structures

of the Anti-Atlas are the numerous~

often deeply

eroded basement domes which break through the covering rocks as "boutonni@res"(button

holes).

These structures

are lined up in a chain

which runs from WSW towards ENE and thus follows the NW margin of the West Sahara Craton. The Anti-Atlas, Ougnate,

including the eastern branches Jbel Saghro and Jbel

forms the "l'@pine dorsale",

with peaks of 2500 m to 2700 m

height in Jbel Saghro and 3300 m in Jbel Sirw@. ever,

is formed by a Ponto-Pliocene

crystalline uplift. The Anti-Atlas is separated molasse basins:

The latter peak, how-

volcanic complex which lies on a

from the High Atlas by young intramontane

the Souss Basin in the west and the Warzazat and Er-

Rachidia Basins in the east. To the south, the Anti-Atlas borders on the Jbel Bani chain which is built up by the cuestas of a flat southerly dipping Cambro-S~lurian series.

Further to the south follows the Dra Plain and the late Paleo-

zoic layers of Jbel Ouarkziz,

which dip under the Meso- and Cenozoic

Hamadas of the West Sahara. The Basin of Tindouf forms a syneclise a sedimentary

on the West Sahara Craton with

filling of more than lO,O00 m depth.

The Boutonni@res The crystalline domes of the Anti-Atlas

form the northern border be-

tween the West Sahara Cr@ton and the Paleozoic

and Cenozoic mountains

14

ALGI[RS

SAF [ANGER

Anh-Atlas

Rif Atlas Touareg Shield

Westsahara Cralon

[ASABLAN[A

HARRAt(EEH

o..J

[talon

~Slo Luis £raton

~,,

•

Brazil

sOO,,,,k, Archean J.B..@el Bani T.6. Tindouf 6asia

Basin Tindouf f

~

/

,b~"~

S.A,F,SoulhAttasFauU CenozoicHotassetrouths ~'T] Upper Paleozoic ~ Lover Pateozoic

~ ~

•~ ~'

_ ~-

~

~0

,,~.~

~G~%,

Hamada

~

t~ '~E 200 km

Upper Infracambrium and Georgian BasementUplifts: B.O.i.BaSlfnOra i X, Kerdous Z. Zenaga Si. Sirra 8.k. Bou Azzer Sa. $argho Ou, Oognal

Figure i: Main structure of the northwestern margin of the West Sahara Craton of North Africa. CHOUBERT (1963) differentiated six orogenic cycles, among them

15

l)

Precambrium 0 This includes,

for example,

(Boutonniere de Kerdous) niere de E1 Graara).

the augengneisses

of Jbel Ouiharem

and the gneisses of Oued Assemlil

According

to CHOUBERT,

(Bouton-

these series belong

to the "Zagorides". 2)

Precambrium

I

This includes the micaschists tites of the Z~naga Series.

of the Kerdous Series and the migmaCHOUBERT calls these series the "Ber-

berides" 5)

Precambrium

II

To this belong epi- anA pyroclastic Azzer,

3bel Sirwa,

layers and greenstones

and Jbel Saghro.

of Bou

CHOUBERT calls these series

"Marrocanides". 4)

Precambrium

III

This includes the Warzazat Series, Infracambrian (Panafrican)

age.

which is of late Precambrian

It is considered

orogenesis.

to

to be a molasse of the last

It forms the base of the non-metamorphic

covering layers.

Radiometric Dating New rubidium/strontium in a separation

dating,

carried out by CHARLOT

between a metamorphic

volcano-sedimentary

and granitic

(1982),

foundation

resulted and a

cover.

Foundation The foundation

is formed by the chain of boutonni@res

and runs parallel

to the Dorsale de L~o in the south and to the Dorsale de Reguibat in the central part of the craton

(Fig. 1).

Only the western margin,

which begins at the western end of the Ivory Coast and leaves the African continent crystalline

somewhere near Rio de Oro, is of Archean age.

domes of the Anti-Atlas

All of the

are east of this line, and, there-

fore, outside of the Archean core of the craton. A second metamorphic granite),

event,

dated at 1.80 - 1.65 x i09 years

(Taznakht

took place as a thermic event which produced the metamorpho-

sis of the Azguemerzi

granite.

to the Dorsale de Requibat

The same age has also been attributed

(VACHETTE et al. 1973).

CHARLOT has pointed out that these events took place at about the same time as the Hudsonian orogeny in North America, Svekofennides Thus,

in Scandinavia,

the Anti-Atlas

the orogeny of the

and the Laxfordian orogeny in Scotland.

might be a link between the West African Craton and

North America or Northern Europe.

16

According to CHARLOT,

the Precambrium

II unit is probably Panafrican,

that is, about 700 - 600 x lO 6 years old. dating on zircons,

CHARLOT showed,

using U/Pb

thit no new thermic event has rejuvenated

these

rocks. The Cover The lowest of the covering layers is the Warzazat Series.

In Jbel

Sirwa and Jbel Saghro this series consists of ignimbrites, rhyolites, and conglomerates.

Granites and volcanites

have been dated at 604 and

560 x lO 6 years (CLAUER 1976, CLAUER & LEBLANC

1977, CHARLOT 1982).

Above the Warzazat Series follows the Adoudounien Series, an angular unconformity. to Georgian age. erates

It is considered

sometimes with

to be of late Infracambrian

The Adoudounien Series consists of a base of conglom-

followed by carbonates,

marls,

sandstones,

and,

finally,

carbon-

ates again. Volcanites

which are embedded in the lower carbonate

Azzer have been dated at 534 x lO 6 years. known. however,

In the Boutonni@re

layers at Bou

Later thermic events are not

de Kerdous and in the Boutonni@re

d'Ifni,

Variscan closing ages for biotite of 325 x lO 6 and 260 x lO 6

years, respectively,

have been measured.

Variscan elements of impor-

tance can only be found south of the Antiatlas chain in the monoclines of Jbel Bani and Jbel Ouarkziz.

They show NE-SW striking structures

the southern slope of the Anti-Atlas

on

in the SW and NW-SE striking struc-

tures on the slope of the Ougarta chain in the SE. Permian

to early Mesozoic extensional

opening of the Atlantic,

contributed

tectonic events,

caused by the

long doleritic dyke systems run-

ning SWINE. The longest of these systems shows its southernmost

outcrop

in the Jbel Ouarkziz and can be followed for more than 400 km as it cuts through the Bou Azzer structure and ends in the northeastern part of Jbel Saghro.

The youngest geodynamic

Tertiary-Quaternary

activity seems to have been a

elevation of the central part of the Anti-Atlas

an important Ponto-Pliocene

and

volcanism of Jbel Sirwa and eastern Jbel

Saghro. REFERENCES CHARLOT, R. (1982): Caract@risations des @v@nements @burn@ens et panafricains dans l'Anti-Atlas marocain. Apport de la m@thode g@ochronologique Rb-Sr. Notes et m@moires du Service G@ologJque du Maroc 313:106 p., Rabat. CHOUBERT, G. (1963): Histoire g@ologique du Pr@cambrien de l'AntiAtlas. (Tome 1). Notes et m@moires du Service G@ologique du Maroc 162:352 p., Rabat.

17

CLAUER, N. (1976): G@ochimie isotopique du strontium des milieux s@dimentaires. Application ~ la g@ochronologie de la couverture du craton ouest-africain. Th@se Doct. Etat, Univ. Strasbourg. CLAUER, N. & LEBLANC, U. (1977): Implications stratigraphiques d'une @tude g@ochronologique Rb-Sr sur m@tas@diments pr@cambriens de Bou Azzer (Anti-Atlas, Maroc). Notes et m@moires du Service G@ologique du Maroc 268: 7-22, Rabat. MICHARD, A. (1976): El@ments de G@ologie Marocaine. Notes et m@moires du Service G@ologique du Maroc 2 5 2 : 4 0 8 p., Rabat. VACHETTE, M; SOUGY, J.; CARON, J.P.; MARCHAND, J.; SIMON, B.; TEMPIER, C. (1973): Ages radiom~triques Rb/Sr de 2000 ~ 1700 M.A. de s@ries m@tamorphiques et granites intrusifs pr@cambriens dans la partie N e t NE de la dorsale Reguibat (Mauritanie septentrionale). C. R. 7e Collogue G@ologie Africaine, Florence.

A DUCTILE

SHEAR ZONE IN THE PANAFRICAN

MARGIN OF THE WEST AFRICAN CRATON

BASEMENT

ON THE NORTHWESTERN

(SIRWA DOME, CENTRAL ANTI-ATLAS

Eckard Wallbrecher fdr Geologie und Pal~ontologie Oniversit~t Graz Heinrichstr. 26, A-8010 Graz

Institut

ABSTRACT A wrench-fault Precambrian

type shear zone of at least 6 km width was found in the

basement

It is explained

of the Jbel Sirwa

by lateral

mass along the northern

slip of an (unknown)

folds with axes that steepen to a (dextral

in granitic

continuously

tachylitic

veins indicate

whole complex

high strain rates. ly Paleozoie).

Morocco).

continental

seismic

gliding

until they are The grain direction,

in superplastic

of the quartzes.

events after the plastic by either

the end of shearing,

The defor-

producing

in a northern

culminated

This must be explained towards

northwards

shear stress.

that increased

parent rocks,

mation with prism and grain boundary of the fabric.

northern

from south to north,

?) horizontal

fabric suffered mylonitization and that,

Anti-Atlas,

margin of the West African Craton.

mation in the shear zone increases perpendicular

(Central

deforPseudo-

softening

an elevation

or by occasionally

of the very

The age of the shear zone is late Panafrican

(Ear-

RESUME Dans le socle pr@cambrien une zone de cisaillement faille verticale mouvement

lateral

drant des plis, perpendiculaires

dana cette zone s'intensifie dont les axes se redressent @ une contrainte

prismatiques

litiques

indiquent

Marne),

de 6 km a produit

une

comme le r~sultat

do

(inconnue)

le long de la

vers le nord jusqu'& (?) dextrale.

du sud au nord.

pour atteindre

@tre

De m@me,

I1 s'intensifie

au

le degr@ de d@formations

sur lea joints des grains et sur les

des min@raux

l'existence

du sud au nord, engen-

horizontale

augmente

avee glissement

surfaces

central,

du craton ouest-africain.

des roches granitiques

superplastiques

On l'explique

d'une masse continentale

le degr@ de mylonitisation centre

(Anti-Atlas

d'une largeur minimale

de d@crochement.

bordure septentrionale La d@formation

du Jbel Sirwa

de quartz.

d'&v@nements

Des veines pseudotachy-

sismiques

apr@s la d@for-

20

mation plastique. Ceci est explicable soit par une @l@vation du complex entier vers la fin du cisaillement soit par des d@formations occasionelles tr@s rapides. L'@ge de la tectonique est tardi-panafricain (Pal@ozoique inf@rieur). INTRODUCTION The crystalline dome of the northeastern Jbel Sirwa belongs to a chain of Preeambrian basement uplifts, arranged in SW-NE direction,

parallel

to the northwestern margin of the West African Craton (Fig. i). First mapping of this area, the

f

geographic position of ~hich is indicated by the arrow in Fig. l, was carried out by CHOUBERT (1965).

Nonmetamorphic sediments

in the south of the area were investigated by JEANNETTE & TISSERANT (1977).

A comparison

of this uplift with the structure of Bou Azzer was carried

: ]:

~~00,,

out by LEBLANC (1981), who also described tectonic observations and distinguised two tectonic

Figure l: Precambrian uplifts of the Antiatlas and position of the research area in the Sirwa Dome

phases, From these works and from our own observations (SCHERMERHORN et al., 1986) the following sequences have been separated. l)

These are, from south to north (Fig. 2)

The Lmakhzan Formation,

a foreland sequence of epl- and pyroelas-

tic sediments, containing basaltic dykes and sills. 2) The Tachoukaoht Gneiss Formation, a sequence of intermediate gneisses, amphibolites and blastomylonites. 5) The Tourtit Ophiolite Complex, an imbricated and tectonically deformed metamorphic ophiolite sequence of ultrabasic,

basic and, to a

small extent, acid rocks, containing large lenses of the highly de~ formed Tourtit Granite-Mylonite. 4) The sedimentary and volcanic cover which forms the Uarzazat Grou~, a sequence which consists mostly of conglomerates,

ignimbrites,

and

rhyolites. 5) Postteetonic granitic intrusions and the N-S striking Daroufarnou k ~ D

of porphyric granite.

21

"~"

• • Anmid

-e

• • :K~

•

• .Ait N e b d a s

•

•

•

e

•

/ J

/

e

e

•

/, • /•

•

:_ , . [ .

!

.

. < '

.

.

,

•

II

[;i~;'--I Ooleritic dyke

I

o

ot

Oorogfornou dykes ,

---:~ A m O S S I R

~

Volconic rocks(Ptiocene)

m4-'x-~T] POSt t ect onic gronites

,

- ~

'

I~TI •

%

x

x

!:

~

!

!

!

t

!

Ouarzoz.te system

I

I

I For~lllnd series Gronite mylooites

-.. •

"

~ "

"

'

V_"=- "

~

'

x

x

x

x

."

x

I~'10phioLites

".

~---------'d Tochoukocht gneiss /Asdr emt foult

....

• ::•

. .

•

•

/ - / .

/(~) Locntion of equol oreo diogrom /~f~rood Anzel- Askaoun

5: ,

.

.

•

.

.

.

. "

I0

7o25'1

17o35 '

I

t

I

I

15kin

Figure 2: Tectonic and lithological units (SCHERMERHORN eL al., 1986, simplified). Numbers indicate the position of the fabric diagrams.

The petrography

and geochemistry

tonic implications (1986).

These authors

quences

in the following

l)

Opening

continent

Initiation Development

Continent

the fore-arc

of a volcanic

collision, ophiolites, Craton.

in the ophiolites Lateral

of the se-

and an unknown

zone.

Ophiolite

Gneiss

Complex)

Formation),

and a back-arc

basin. which thrusted

the northern

under the island Metamorphism

arc,

continent

and under

and imbrication

and in the Tachoukacht

Gneisses

under

the margin

of

are generated

(island

geosuture

shear zone which will be described

tectonical

horizontal

are separated

upthrust

et al.

arc).

which gives rise

in this paper.

SETTING

The general Generally

Craton

(the Tachoukacht

slip along the thus developed

to a 6 km wide

TECTONICAL

subduction

arc

(the Tourtit

Formation)

the West African

ments

evolution

the West African

of a south-dipping

by a fore-arc

(the Lamakhzan

5)

the geotectonic

and their geotec-

by SCHERMERHORN

steps:

of an ocean between

2)

4)

describe

in detail

north of it.

3)

flanked

of these sequences

have been described

style of the area

or only weakly by a steeply

from the albite-gneisses

(Fig. 2) is rather

inclined

northward

southern

dipping

simple.

foreland

sedi-

and E-W striking

of the Tachoukacht

Gneiss Formation

22

in the north. Tourtit

A similar

Ophiolite

strongly

Complex

imbricated,

Granite-Mylonite

further

contains

the ophiolites

the imbrication.

this formation

north.

This complex,

the E-W striking

part of the area

cover of late Precambrian farnou

separates

which has a thickness

In the northern salan Valley),

thrust

from the which is

layer of the Tourtit

of lO0 to 300 m.

(generally

are overlain

to Infracambrian

parallel

to the Assif Am-

by a younger age

which

horizontal

did not suffer

All these units are cut by the N-S striking

Darou-

Oikes.

Numerous

posttectonic

these units.

granite

A penetrative

intrusions

foliation

from south to north and culminates nounced

vortex

structure

are also

pattern

found in all of

develops

in the ophiolites

continuously

producing

and extreme shear deformation

a pro-

in the Tourtit

Granite-Mylonite.

Macroscopic l)

Tectonic

The Lmakhzan

The layers

Observations

Formation

of the foreland

sediments

generally

gles of about 30 to 60 degrees

(Diagr.

folding

a

unique

in the area of Tizwat, fold pattern

cernable

(degree

distribution

of great circle

= 21%) ++

Up to now,

a)

Here we f i n d siltites

++)

+++)

(sheet

of thin-bedded

w h i c h are deformed slip.

1 km

T a g h d o u t ) +++.

NNW-SSE s t r i k i n g

layers

bands ( F i g .

dextral

+)

at three places.

of Tizwat

vertical

in

from south to north

in the Assif Tamazirt,

north

kink

strike

could be observed

these series T h e s ~ are,

NE at an-

is not dis-

shear zones with brittle separation

dip towards

l) +. In spite of some local

3),

to

indicating

The o r i e n t a t i o n

of

Diagram

1

The position of places of fabric measurements for the diagrams are marked by the numbers in Fig. 2. All parameters describing quantitatively preferred orientation and fabric shape are derived from WALLBRECHER ~979). Numbers 1 to 3 are the eigenvectors of the orientation tensor; s is the center of gravity. All map names refer to the "Carte du Maroc"~ scale 1:50.000.

23

the bedding planes and of corrugation

linea-

tions in these beds are plotted

;~Ni

?:

in Dia-

gramm 2 a) and b) + b)

near Bou Wals

(trail from Souq at Tlata to Azib-n-lriri, sheet Tachoukacht). Here, there is a sequence of vertical SW-NE striking

shear

planes with distances in the dm range between which some

Figure 3: Kink bands in siltites of the Lmakhzan Formation, north of Tizwat.

rocks of 1 to 2 m width were dextrally c)

in the Assif Lmakhzan,

kacht).

(sheet Tachou-

layers of the Lmakhzan Formation

in

and shows a dextral

along kink bands.

Diagramm 2a

+)

300 m south of the car trail

Here we find a lens of serpentinite which is imbricated

the sedimentary movement

rotated.

Diagramm 2b

In all diagramms inner small circles are the cones of confidence for a level of 99% and the outer small circles are the spherical aperture, a deviation measure (WALLBRECHER, 1979).

24

2)

The Tachoukacht-Gneisses

Steep upthrusts served

of these gneisses

in the Assif Lmakhzan,

Izgaran

near Amassin

onto the southern

in the Assif Tamazirt,

foreland

~as ob-

and at the Tizi

(Fig. 4).

At their southern margin,

the Tach-

oukacht

Gneisses

"

L!]

r200m

S

begin with a fold pattern which sketched

is

in Fig.

T. Gneiss

5.

8osnlt

Conglomerotes

Lmakhzon Form.

The fold axes are Figure

4:

Profile

at Tizi Izgaran

flat and arranged in E-W direction

.

The folds S

are strongly

asymetric

long vertical

or steeply

ward dipping

limbs.

ing enveloping steeply

surface

plane surfaces indicate

south-

The result-

surface

northward.

veloping

and have

dips

L

~

I

This en-

and the axial

of the folds

a southward

tectonic

movement. In equal area diagrams,

Figure 5: Fold pattern of the Tachoukacht Gneisses, Assif Lmakhzan, south of the car trail

the

fold axes of cm to dm range form

a

flatly

dipping

eastward ...............

cluster

with

a center of gravity at 970/27 ° (direction of dip and angle of dip, Diagram The foliation

3a).

strikes

E-W and shows a center of gravity

at

174o/66 ° and a beginning transition the cluster

from

distribu-

tion to a girdle distribution

(Diagram 3b).

Since eigenvector which belongs

c

i,

to the

Figure 6: Fold with flat axis, Gneiss, south of the car trail

Tachoukacht

25

80 O A T E N

17g O A T E N

i

]

~

• .

'~

Diagram 3a smallest foliation pattern,

,IL-

o

.

Diagram 3b

eigenvalue

of the orientation

with the center is arrBnged

of gravity

tautozonally

which is established

about 200 m width, transition,

is followed

tensor

(SCHEIDEGGER,

tautozonality

to the minor

fold axes.

in the Lmakhzan

Valley

to the north,

probably

the same (Diagram

of the foliation

1965),

of the fold axes cluster,

by a zone in which the center of gravity

poles is approximately coincides

°

4"

I

coincides

"

.

4a).

with an also steeper cluster

This fold

in a zone of with a floating of foliation

However,

is much steeper

the

the axis of

(67o/430 ) and again

of the minor

fold axes

(71°/49 °,

Diagram 4b).

\.

oi

.....

."

.

.

2 "

"

"/

26

An even steeper structures

arrangement

with beginning

lel to the foliation circle distribution Diagrams

of the fold axes in the style of vortex

transition

88

o

poles is demonstrated

the contact

6a and b (Lmakhzan

ation shows almost

of the ophiolites,

5b

this pattern

of foliation

Valley north of the car trail).

the same picture

as before,

..

6a

"

continu-

and fold axes shown in The foli-

but the fold axes now

""

Diagram

by

DATEN

Diagram

ously turns into the arrangement Diagram

paral-

(great

•

5a

As we approach

pattern

near Tichkiwin.

2ZZ D A T E N ~

Diagram

cluster

= 40%) of the foliation

5a and b, measured

•

to a girdle distribution

plane and a stronger

.

o

Diagram 6b

.-

. .

.'.- .....

.

~'~

~2

"I,

.

"

o

27

are scattered

to a girdle distribution

gree of great circle distribution A similar

setting

drem Pass

(Diagram

of foliation

in the plane of foliation

= 66%, circular

7a and b).

The flatter

tion and fold axes fabrics must probably

3211 D A T E N

~ _

.'..

"

Diagram

2

"

aperture

and fold axes was measured

-" ""

arrangement

be explained

(de-

= 99%). at the As-

of both foliaby a tilting

of

-!

7 a

Diagram 7 b

the Asdrem bloc along the younger which raised the high plateau

Asdrem

fault

(CHOUBERT,

of Asdrem upwards

1965)

in relation

to the

plain of Tamazirt. A true vortex

style of fold axes was observed

the Assif Tamazirt, This extremely plained 5)

NNE of Tizwat,

southern

position

sheet Taghdout

of vortex

by a dextral bloc movement

The Tourtit

The ophiolite lization cumulates

Ophiolite

sequence

parallel resisted

formed by extreme

Complex

the schistosity. structures

Diagram

of Amassin.

The foliation

also strike E-W.

The schistosity with vertical

intrusion

fold measures

suffered

ophiolites,

and recrystal-

planes are de-

fold axes (Fig. 7).

of schistosity

and fold axes part of the

from the region west

planes of the Tourtit

This granite

fault.

Only the cores of basic

lOa and b in the northern

ll contains

as did the surrounding

schistosity

planes.

The vergency of most folds is dextral. Diagrams 9a and b show the arrangement in the southern, and Diagrams

might be ex-

and the Tourtit Granite-Mylonite

shows a very strong

ophiolites.

(Oiagram 8a and b).

structures

along the young Asdrem

to the schistosity vortex

E of the Asdrem Pass in

Granite-Mylonite

the same tectonical

treatment

which proves that the granitic

must be older than the shear tectonic

event.

28

10~ OATEN

Diagram 8a

i~

Diagram 8b

Figure 7: Vertical folds in serpentinites, Assif Tasrwin south of Tourtit

1 17 OATEN

S i':.-:.:' Diagram £ a

Diagram 9b

--~~'

x

29

258

DATEN ~.~_

•

Diagram 4)

Diagram

lOa

The Warzazat

The younger northern

191

OATEN

margin on top of the crysuplift shows the same tec-

tonical

deformation

the basement, (Diagram

10b

Group

cover which lies at the

talline

nounced

,%

patterns

and less penetrative 12).

schistosity

The distance

planes

tex structures

as

yet in a less proform between

is greater,

vor-

are not found so

D

often,

and are concentrated

certain

zones.

In the conglomerates, tion of the pebbles observed ure 8).

Diagram

12

in

a disseccould be

in some outcrops

(Fig-

30

Figure 8: Sheared pebbles in the conglomerates of the Warzazat Group south o¢ Ait Nebdas

Macroscopic

tectonic

observations:

The two most important ferred orientation, G%,were Diagrams

derived

statistical

other in Figure

9, as proposed the development

very narrow clusters

by BEER

in a zone of transition

an-

4 to 7), and

ending in vertical

This plot very clear-

with a tectonical (Diagram

style of 3), pass-

along a girdle

1--

®i,

I~

Southern morgm

folds

distributions

8 to ll).

(Diagrams

each

Cluster

/

cluster

To explain

this tectonic

we presume

a system of lateral

pattern

which moved a

bloc along the margin

of the West

African

thus causing

a (dextral?)

fault zone. The Tachoukacht Tourtit

(1981).

against

in which

which again show narrow

shear stresses

were plotted

beginning

gles of dip in the foliation

northern

of fold axes in each of the

of the fold axes distribution

the axes lie with various

finally

the degree of pre-

of flat fold axes in the south

an elongation

(Diagrams

parameters,

These two parameters

ly demonstrates

plane

and conclusions

R%, and the degree of great circle distribution,

from the distributions

1 to ll.

ing through

summary

Gneisses

Ophiolites,

ready suffered

Craton wrench and the

-6%-~ ]20

I.~0

160

tO0

which al-

upthrusting

onto

Figure 9: R% - G%, measures fold axes, Diagrams i to Ii

of

81

the southern

foreland

(SCHERMERHORN striking

et al.,

mobile

developed southern a certain

extension

planes

which

then served

continent

transformed

of folds,

deformation

the system

into an E-W

overprinted

(in connection of N-S striking

to

of south-

The steep thrust

in the gneisses for lateral

and thus,

pattern

and imbrication.

the stress

of this zone

while the more marginal

the older tectonic

generated

collision

The center

less strongly

as shear planes

in E-W direction

caused

porphyry

pattern

of upthrusting

the end of the shear

lantic)

vortex

could preserve

planes

and foliation

were hereby

of the zone were

degree,

pentinites

1986),

during

zone of at least 6 km width.

a distinct parts

ern directed

and imbrication

and in the ser-

displacement.

At

field was rebuilt, and an

with the opening dykes

of the At-

of the Daroufarnou

granites.

THE GRAIN FABRIC l)

The Tachoukacht

At the microscopic patterns. Rests

Gneiss scale,

It occurs

of the former

southern

margin

the foreland.

of the Tachoukacht

the garnets

turing.

The old grains

cated

reacted

to the foliation.

in a small

rim at the

close to the upthrust

quartz brittle

to this stress

to schistose

+ plagioclase deformation pattern

onto fab-

+ bio-

caused

by external

quartz and primarily

textures

(Fig-

with a diaph-

which was indi-

in the new grains

the parsgenesis albite

fabric

This deformation

went together toresis,

At first,

all older

i0) and frac-

feldspar

ure ll).

gneisses

had the paragenesis

+ amphibolite.

(Fig.

formed mortar

parallel

can only be observed

In this small zone we find a gneissic

tite + garnet cata61asis;

superimposes

as a grain reduction fabric

ric which originally

rotation

mylonitization

+ epidote

quartz

by +

+ chlor-

ite. Towards

the north,

deformation

rapidly

came more plastic, Tachoukacht into ductile

the beand the

Gneisses

turned

protomylites

Figure lO: Externally rotated garnet clast with asymmetric pressure shadows (Tachoukacht Gneiss, 60 x, Nic. X)

32

and blastomylonites (sensu HIGGINS,

1971).

At the beginning, tic deformation restricted

plas-

was

to single

shear bands with characteristically

elonga-

ted quartzes whlch show low angle grain boundaries.

Between

these

shear bands there is an old grain assemblage of quartz and feldspar

clasts.

Recovery

patterns

the Quartz

They form mosaics

triple junctions

the genetically stadium

preceding

strain heterogeneity spar paleoclasts

grains with charac-

structures

with lobate grain boundaries.

1978).

point 4, there is an intensive went through

an external

This

of rigid

In areas with unusually

The blasts are packed very densely,

about 5 mmo and obviously

with

and with the following

must be caused by the distribution

(PRICE,

near measuring

albite blastesis.

of isometric

and exist in the same sample together mortar

of recrystallization

ing, i.e.,

in Tach-

fabric can

also be observed. teristic

Figure ll: Mortar quartz structures oukacht Gneiss, 30 x, Nic. X)

of

strong

feldfold-

synkinematic

reach a size of

rotation

(Fig. 12).

Figure 12: Synkinematic albite blast (Tachoukacht Gneiss, 80 x, Nic X)

33

2)

The Tourtit Ophiolite

In the ultrabasic ed metamorphic

Complex and the Tourtit Granite. Mylonite

and basic rocks of this complex,

recrystallization

which follows the foliation planes

and which covers a probably preexisting In contrast,

the leucocratic migmatite

parallel to the foliation ness of some centimeters

of the serpentinites

1980).

ation culminate The genuine

or mylonitization. which are arranged

and which have a thick-

are characterized

and strain homogeneity.

by a

In these bands of

rocks most of the strike-slip movement must have taken

place under the conditions (WHITE et al.,

cataclasis schlieren,

to a few meters,

strong ductile deformation quartz-feldspar

there is an extend-

of strain softening

The fabric homogeneity

and superplasticity and superplastie

deform-

in the lens of Tourtit Granite.

granitoid grain fabric first developed

into cataclasites,

remnants of which survived in some nests within the Tourtit Granite. Plastic deformation

turned these cataclasites

mylonites with very fine subgrain aggregates of feldspar.

Only a few feldspar

marginal disintegration

into mylonites

clasts still exist.

into subgrains

and ultra-

of quartz and neoblasts These show a

and a schlieren-like

arrange-

ment of very small new grains in the pressure shadows of the clasts. The quartz grains are strongly elongated of >5:1.

Only low angle grain boundaries

An intercalation lonitic

with an average ratio of axes are visible.

of layers of elongated new quartz grains with a my-

fabric of very small feldspar and quartz aggregates

can be observed.

Mylonites

(Fig.

15)

of this type are generated at high temper-

atures and relatively low stresses and strain rates

(BOULLIER & GUE-

GUEN,

1975).

Accord-

ing to these authors the ratio T/Tm between the temperature

T at

the time of deformation and the melting temperature Tm of the mineral phase

(feldspar)

must be >0.5.

In the

entire area investigated~the geometric

pat-

tern of the grain fabric is characterized

Figure 13: Intercalation of elongated quartzes and feldspars with strong grain reduction (Tourtit Granite-Mylonite, 13 x,Nic~X)

34

by the perpendicular is manifested

relationship

by elongation

of the stretching

lineation,

of quartz and by feldspar

to the fold axes of the vortex structures

which

lineation

in the neighboring

rods,

serpen-

tinites. This means that the serpentinites folding and forming dough"

of B-tectonites

deformation

shear stress,

reacted

(SANDER,

(LISTER & PRICE,

whereas

1950) in a "rolling

1978) with b-axes vertical

most of the shear movement

dered as a simple shear deformation relatively

to the shear stress only by

small leucocratic

which can be consi-

must have been concentrated

migmatite

the

to the in the

bands and in the Tourtit

Granite. In the Tourtit which contain ed

contacts

optical

Granite-Mylonites, rounded

grains of parent rock.

to the host rock

microscopic

glassy material

(Fig.

14).

They show sharp,

sinuat-

Even though we have only

data up to now, and there is no indication

in these bands,

pseudotachylitic

1-2 mm wide dark veins were found

the observations

of

point strongly

to

bands

with their characteristic injection

veins

(of. SIBSON,

1975;

ALLEN,

PASSCHIER,

1979;

1982).

Pseudotachy-

lites are explained

as

former rock melts generated by rapid melting which is caused by friction

fusion along

seismically

active

fault

(ALLEN,

zones

19797 ERMANOVICS et a l . , 1972; FRANCIS; 1972;

GROCOTT, 1977;

SIBSON, 1 9 7 5 , 1 9 7 7 ; The f a c t less

that

the

vertically

pseudotachylites servation zone.

F i g u r e 14: Pseudotachylite vein Granite-Mylonite, 10 x, N i c . / / )

through

veins

the mylonite

were g e n e r a t e d

1979).

are undeformed and c u t more or

after

bands c l e a r l y

shows t h a t

the m y l o n i t i z a t i o n .

should begin with

brittle

behaviour

may d e v e l o p and s h o u l d be f o l l o w e d

w h i c h can be s u p p o r t e d caused by h y d r a t i o n

by a r e d u c t i o n

along a fault

during

zone.

a shear

w h i c h pseudo-

by a d u c t i l e

of the effective

the

T h i s ob-

does n o t c o n f o r m to t h e d e v e l o p m e n t e x p e c t e d a t

This

tachylites

WATTS & WILLIAMS, injection

(Tourtit

reaction

normal s t r e s s

85

These observations a)

The entire ductilely deformed

crustal TULLIS

level with transition and YUND

is mostly b)

may be explained

(1972)

temperature

An aseiamic

fabric

zone was elevated

to brittle

showed

rate was interrupted

deformation

Quartz-c-axes

by seismic

into a higher

and plastic

In experiments

of rock material

with a transition

which took place in the deeper

ways:

behaviour.

that this behaviour

controlled,

ductile

in two different

at about 400 ° C.

softening

of the

crust at a very low strain

events

in greater

depth.

patterns

Method The quartz-c-axes tively

distribution

in two separate

section

were measured

tions of defined BRECHER,

1986),

tation which

ways. with

On the one hand,

orientation.

Using

the thin sections were plotted parameters

to this method 1978)

was used

distributions caused

an integral

and to register

field of view,

which

ing the polarizer, the analyzer

the microscope. intensity

photometric

to obtain

analyses

the thin section,

is measured

The method

of SANDER

at each degree

system

fields

may be comIn the

of the microafter pass-

fixed on the phototube

is turned

through

of

360 ° , and

of Fotometer

In order to carry out these measurements the apparatus

of PRICE

fied as shown in Figure measurements

ties and storage by a computer K6nigswinter, were plotted

(1980) 15.

Movement

Gypsum Plate

of

Thin Section

of the intensi-

,J-

Microscope Polarizer

for this purpose by

BOro C. H. HILGERS,

Germany.

The intensities

as rose diagrams

field of view.

Analyzer

was modi-

of data are controlled

built

the Technisches

represent

of the

plate of 550 nm, and

a gypsum

stage

method

(1950).

turning.

the stage,

and

of the strain

light transmitted

by a photodiode

The microscope

measuring

a quick overview

of rigid clasts.

of monochromatic

is measured

into the orien-

area projection

is fixed by the optical

the intensity

(WALL-

In this orientation

the heterogeneity

pared to the axial distribution scope,

were then rotated

in sec-

program

were calculated.

in order

by the distribution

of a thin

stage method

a small computer

into an equal

quantita-

200 grains

they would have had in the field.

the quartz-c-axes

(PRICE,

were registered

the usual universal

their statistical Parallel

patterns

These diagrams

the distributions

of quarz-c-

(517 rim)

/

for each thus

Filter




ASRIRIEN

{

O - >

Z 0

Schistes

de i' l s s a f e n e

,,Calcoi res scoriac~s "

,,CompIexe [SSAFENtEN J

schist eux"

J

I

TI! m Z 0 N

,,S & r i e s c h i s t

o - c a I c a i r es"

AMOUSLEKIEN 1

Tab. 1. Stratigraphy of the uppermost Early Cambrian in the Anti-Atlas. trilobites and archaeocyathid bioherms in these Lower Cambrian sediments are evidence of an open marine environment in the western Anti-Atlas up to the central Anti-Atlas (particularly in the black limestones of the "calcaires de base'). The overlying series (schisto-calcaires, complexe schisteux and gr~s terminaux) mark the second regression in the Lower Cambrian prior to the Middle Cambrian transgression, which covered most parts of the Anti-Atlas. This paper describes the lithology and facies of the clastic sediments below the Middle/Upper Cambrian boundary and outlines paleogeographic maps and facies models of the western and central Anti-Atlas. This investigation is based on 40 sections (Fig. 1) and several hundred samples and thin-sections.

2, Previous work The first survey of the Cambrian in the Anti-Atlas by NELTNER (1938) established the stratigraphic outlines, which are still valid today. Based on trilobites, HUPE(1952) subdivided the Lower Cambrian into different zones and classified the clastic sediments below the Lower/Middle Cambrian boundary as Zone VI to Vllt. The term 'gres terminaux' was established by CHOUBERT (1952) for the sandstones of the trilobite Zone VII. In 1963, he named the sedimentary time point of the gr~s terminaux '6tage d'Asrir', referring to the type locality. The term 'niveau & Micmacca' or 'niveau d'Ourmast' was used for the Lower/Middle Cambrian transition beds. Volcanoclastic intercalations in these sequences were described in the western Anti-Atlas; in the High Atlas they were described by BOUDDA et al. (1974).The biostratigraphy of the Lower to Middle Cambrian boundary beds in the western AntiAtlas was studied in detail by GEYER (1984).

3. Llthostratigraphy The clastic sequences below the Lower/Middle Cambrian boundary are not at all uniform. The variety in lithology

109

~

Serie d'Akerouz

F~

Asririen

Niveau d'0urmast

~

'ssaferfien

0unein j

20,

i

400

S~&rie volcano- d~tritique

Astir

60 i

6,0~

Cornplexe schisteux

Khenig

km i

Aguerd

m

I~?

Tat(]

Air Sedrat

Akerouz

÷ 4- ..-.:-:~. -.-:7.-' Jr • ¢.-1 ~, 44-

-h/j/'~.' 4 f//.z

Fig. 2. Sections across the depositional area of the 'gr@sterminaux' in the Anti-Atlas.

documents a differentiated depositional area with an inferfingering of terrestrial and shallow marine sediments. In order to classify the different units, new terms will be introduced (underlined in Tab. 1) and existing terms will be defined more precisely in this paper. The term 'gr@s terminaux' is applied as a generic term for sandstones of the trilobite ZoneVIl. Five different paleogeographic units were recognized within the trilobite Zone VII (HUPE 1952) in the Anti-Atlas: (1) The eastern Anti-Atlas (between Foum Zguid/Tazenakht and Erfoud). This area is characterized by fluviatiie sandstones. (2) The central Anti-Atlas (between Tata and Foum Zguid) with the transition from continental to madne facies. (3) The northwestern Anti-Atlas (region of Tiznit-Taroudant) distinguished by marine sediments and thick volcanic sequences. (4) The southwestern Anti-Atlas (Tan-Tan, Goulimime, Bou Izakarn) characterized by the interfingedng of the 'Delta of Asrir' with shallow marine sediments. (5) The Ait Ounein section is situated north of the southern Atlas boundary fault and cannot be clearly correlated with the sections of the Anti-Atlas.

3.1. Eastern Anti-Atlas A 70 m thick sequence of white and grey sandstones alternating with red-violet mudstones is exposed in the

110

easternmost section of the Anti-Atlas about 60 km WNW of Erfoud. This S~rie d'Akerouz (Gdorgien continental) consists of laminated or sometimes cross-bedded sandstones. The bed thickness is about one meter. The grainsize parameters (mean = 0.06-1.0 mm; sorting and skewness) indicate transportation and sedimentation in slowly moving water• Pedogenic broken quartz-grains (PI. 1, Fig. 1) floating in a carbonate matrix indicate temporary sedimentation above the ground water table under arid conditions (caliche). The upper Lower Cambrian of the other sections of the eastern Anti-Atlas are characterized by the succession: gr&s d'Ourmast gr~s rouges complexe schisteux Red-violet mudstones, siltstones and thin-bedded fine-grained sandstones are typical of the comDlexe schisteux. Frequent ripple marks, small-scale cross-bedding, desiccation cracl~ and casts of halite argue for a depositional environment with occasional water currents, high salinity and abundant desiccation (lagoon to salt marsh, REINECK 1985).

3.1.1. Gr&s rouges The gr&s rouges are the typical 'gr&s terminaux' in the eastern Anti-Atlas. They consist of light-red, thin-bedded, 20 -

300 cm thick layers of fine-grained sandstones forming up to 15 m thick units. These units wedge out laterally

after short distances (maximum width about 300 m). Erosion channels with a few meters in width (point bars), layers

3z°7iVlar r akech 0 /~

. ..: .i'..'~ ~

' ...

'-'

)uorzozate

'/: i ' : . " . i . " . : . : - : : . . " : " . ' . ::. " A g a d i r / o Set ~u " d~t~ ~,t'~que "'7 7 '

~

\ g

o. ~Oei~.a

o ~o o:i o /xs o

o

o/ o

0

100 ~,m

0

o

}

I

I0 Q

90

......J_ 8~

7~

6 ~

Fig. 3. Paleocurrent pattern according to the interpretation of cross-bedding orientation.

5 °

111

Y °o o

I

/

/

|

gr~s d'Ourmost

~_000

/

Y

000 oo~

Y

C 0

a •

o

o

o o

oo o

o

o o

10

/

)

I00 o

/

schistes de l'lsscdene 100

1000~um

~tage d'Asrir M

Fig. 4. CM-patterns in the Passega-diagram.

with mud pebbles and asymmetric current ripples with a wave length of a few centimeters up to more than one meter are frequently observed. Arched trough-shaped cross-bedded sand bodies, several meters wide and about 50 cm thick, are typical of the gr~s rouges. Statistical measurements of cross-beddings (Fig. 3) indicate a W or WSW directed transport in outcrops at the southern margin of the Jebel Sarhro and W or WNW directed transport in the vicinity of the anticline (boutonniere) of Bou Azzer - El Graara. The sandstones are fine-grained (Md 0.06-0.07 mm), sorting is good to medium with a predominance of the fine-grained fraction; roundness ranges from rounded to subrounded. The pattern of the CM-diagram ranges from 'uniform' transport to 'graded suspension' (Fig. 4). According to the diagram of PETTIJOHN et al. (1973) the sandstones of the gr~s rouges are classified as 'arcosic arenites' or 'subarcoses' (Fig. 5, PI. 1, Figs. 2,3). The quartz content increases and the matrix content decreases from E to W. About 50 - 60% of the sandstones consist of quartz. Metamorphic quartz grains are unimportant in comparison with the non-metamorphic volcanic quartz with sharp extinction and resorption features which make up the bulk of the grains. At the southern margin of the anticline of the Jebel Sarhro, only 25 to 30% of

!12

I

quartz arenite

II

subarkose

]JI IV V VT VII

VII

(')uar tz

100%

A reni tes

Wackes

, sublith[c arenite / ~ arkosicarenJte lithic arenite quartzwacke fel dspafhic graywackeacke ~ ~ lithic gra ywacke / ~

..... /;.

50%

o ~eQY///

50%

100%

y

v

y

v

100%

Feldspar

Rock Fragments

"~ gr~'s rouges

v

•

~tc~ge d'Asrir

*

congtornerate d'Aguerd

•

gr~'s d' Ourrnast

Fig. 5. Classification of the upper Lower Cambrian sandstones.

the quartz grains exhibit undulatory extinction, whilst in the vicinity of the anticline of Bou Azzer - El Graara the amount of undulatory quartz grains is about 40%. Feldspars (plagioclase, orthoclase and microcline) make up about 20 - 30% of the sandstones. Rock fragments (up to 10%) consist of metamorphics and plutonics. The gr~s rouges are cemented by syntaxial overgrowth of detritic quartz and feldspar grains (PL 1, Figs. 2, 3). Two different source areas probably existed in the NE and SE according to the data from current directions and the modal composition of the gr~s rouges. The sandstones were deposited in braided river systems. The thickness of the sections Tazlaft and Talioulat (Fig. 2) indicates an increased rate of subsidence in this area. The base of these sections consists of bioturbated calcareous arenites; typical cross-bedding ismissing. The thick sequences of the eastern Anti-Atlas probably represent lake deposits. Recent braided rivers are usually characterized by the dominance of coarse-grained sand and gravel (REINECK & SINGH 1980), but fine-grained sediments are described as well, for instance from the Brahmaputra and the Mekong river (COLEMAN 1969; MORGAN 1970).

3.1.2. Gr~s d'Ourmast Dark or black, medium-grained to coarse-grained sandstones interbedded with thin green-black to dark-viobt mudstones occur at the base of the 'niveau d'Ourmast'. This formation is named 'gr~s d'Ourmast' in this paper. It is overlain by light-green mudstones and coquinas of fossils of the 'br~cche ~ Micmacca'. The thickness of the gr~s d'Ourmast varies considerably (Fig. 6). The orientation of cross-bedding suggests a NE - SW directed pattern of usually unimodal paleocurrents ; a NW - SE transport is measured in one section (IG = Ighels 21 Km NE Tazenakht). Asymmetric ripples and mud pebbles are common. Bioturbation and grazing trails can be observed in the upper part of the gr~s d'Ourmast. These horizons are partly cemented by carbonates and they

113

• MARRAKECH

A~(×I

Fig. 6. Thickness of the gr~s d'Ourmast.

~0 S QUARZAZATE HM

.o

TI

"// TZ.

1

,AZ

JB SD -

%~

TA

~ ~

~-

/ "

/

.I, .o.\ i B5

/

MR"

Ii TATA "T

may contain fossil fragments (trilobites, archaeocyathids and echinoderms). A different facies of the gr~s d'Ourmast is exposed in the Targant section (Tr). Lenticular and flaser bedding and small-scale cross-bedding of mudstones and fine-grained sandstones are attributes of a muddy to sandy tidal flat (REINECK 1972). The grain-size is not uniform: rolling and traction transport and transport in uniform to graded suspension is documented in CM-diagrams (Fig. 4). Whereas rolling transport dominated in the eastern Anti-Atlas, water energy decreased towards the central Anti-Atlas. In this area, fluvial transport was obviously unimportant in comparison with transport by waves and marine currents. The gr~s d'Ourmast are composed of dominating undulate quartz grains, not-undulate quartz, rock fragments (20 %)> feldspars (10-20%) and abundant matrix (up to 30%). According to the diagram (Fig. 5), the gr~s d'Ourmast consist of lithic and arcosic arenites and arcosicwackes. The depositional environment of the gr~s d'Ourmast was predominantly fluviatile in the E with increasing marine influence toward the W and toward the top of the sections (beach and delta sediments in the central Anti-Atlas and tidal flat deposits at Targant).

3.2. Central Anti-Atlas The facies of the gr~s rouges interfingers with the sediments of the 6tage d'Asrir in the central Anti-Atlas. A change in color from red to green is combined with the facies change. Cross-bedding measurements give evidence of a SW transport direction in the E, whereas the bimodal paleocurrent patterns in the sections IT and TT are indicative of an estuary or probably tidal-dominateddelta environment. 3.3. Southwestern Anti-Atlas The sedimentation of the southwestern Anti-Atlas is mainly influenced by the 'Delta of Asrir". The following facies units and formations can be distinguished: 1. Basin sediments 'schistes de I'lssaf~ne' 'S~rie de Jebel Tichinchine' 2. Bar and platform sediments 'Calcaires scoriac~s'

114

3. Delta sediments:

'etage d'Asrir' 'conglomerate d'Aguerd'

3.3.1. Schistes de rlssaf~ne The up to 350 m thick sequence consists of 25 m thick units of green mudstones and up to 3 m thick limestones. Crystal tufts occur in some layers. The fine-grained sediments are usually laminated, occasionally crossbedded and frequently bioturbated. According to the CM-diagram (Fig. 4), the sediment transport is to be classified as 'pelagic' to 'uniformly suspended' (PASSEGA 1964). The mudstones and siltstones are composed of quartz and feldspar grains floating in a matrix of carbonate, chlorite and sericite. The cystal tufts consist of plagioclase, sanidine, orthoclase and rock fragments. The feldspars are altered to sericite and carbonates. The reddish brown, massive, meter-thick sandy carbonate units consist of quartz and feldspar containing biosparites with trilobites, echinoderms and archaeocyathids (PI. 1, Fig. 4). Chlorite and oxychlorite are neomorphic in fossil shells. The schistes de I'lssaf~ne are open marine basin sediments deposited belowthe wave base.

3.3.2. Etage d'Asrir The ~tage d'Asrir can be correlated with the gr~s rouges. The up to 70 m thick basal sandstones are overlain by green mudstones in alternation with sandstones. The ~tage d'Asrir is overlain by the conglomerate of Aguerd or by the S~rie volcano-d6tdtique.The transition to the Middle Cambrian 'schistes & Paradoxides' is continuous in some sections of the western Anti-Atlas. While cross-bedding shows bimodal, E-W directed current patterns in the sections IT, T and TZ, no clear direction can be recognized in the western sections. The mean of the grain sizes varies from silt to coarse sand; the sorting is poor to good. All possibilities from rolling transport to uniform suspension are shown in the CM-diagram (Fig. 4). Accordingly, the modal composition is very inhomogeneous. Corresponding to the variability of the (~tage d'Asrir, the sediments were deposited in different environments as follows: ~38~

~

AGAO,R 8S~AL(~001/ / ~62/ ~

/

/

()./

~ 4 0 ~ elB

/

•

, O TATA

S Anti-Atlas:

predominantlagoon

SWAnti-Atlas:

prodeltato delta plat-

NWAnti-Atlas:

form shelftoshore, pyro-

The source area of the 'delta of Asrir' is situated south of the Anti-Atlas where mainly metamorphic rocks and inter-

/

mediate to basic volcanics were eroded. It is still an open question as to where

/ /

~e GOULrMIME eL

(3)

e" ~ - ~

I ~ '%0~/

the source area for the sediments of the 0 ~

50 ,

lOOkm

northwestern Anti-Atlas was situated.

,

Local emersion combined with erosion • K [2]

Fig. 7. Thickness of the Serie volcano-d~tritique.

evidence the redistribution of sediments in this area.

115

3.3.3. Conglomerate of Aguerd In the sections Aguerd (A), Asrir (AS), Khenig (K), and Tata (T) the etage d'Asrir is overlain by a 10 - 20 m thick sequence of massive quartzitic coarse sandstones and conglomerates. Planar and through-shaped cross-beds and scour fills indicate NE to NW directed paleocurrents. 65 - 95% of the components consist of quartz (recrystallized metaquartzites, volcanic quartz grains with embayments); feldspars and rock fragments are less abundant (Pi. 1, Fig. 5). The basal units of the conglomerate d'Aguerd were deposited in a littoral environment evidenced by the occurrence of Scolithos. Sedimentary structures and granulometric parameters substantiate a fluviatile environment for the sedimentation of the upper units. Therefore, the conglomerates of Aguerd probably represent the topsets of the Delta of Asrir.

3.4. Northwestern Anti-Atlas and Ait Ounein Only marine environment is known in the northwestern Anti-Atlas for the late Upper Cambrian and the Middle Cambrian period.

3.4.1. Calcaires scoriac#s The approximately 100 m thick reddish brown limy marls contain a rich fauna. A direct terrigenous influx cannot be observed; the few lithoclasts are of volcanic origin. Possibly, the calcaires scoriac~s were deposited on a submarine rise which separated the depositional area of the schistes de I'lssaf6ne from the open marine.

3.4.2. Series of Jebel Tichinchine The depositional area of the series of the Jebel Tichinchine is situated between the Delta of Asrir and the open marine northwestern Anti-Atlas. Green slaty mudstones with intercalated decimeter-thick limestones alternate with thin fine-grained sandstones. The increasing abundance of crystal tuffs in the upper part of the section is indicative of a correlation with the S~rie volcano-d~tritique. Biosparites and oolites (section BI) confirm a sedimentation in an open marine shallow water at the base of these sections. Other sedimentary structures indicate deposition below the wave base and a reduced environment within the sediment (pyrite). The area of deposition probably represents a partly restricted offshore basin.

3.4.3. S6rie volcano-d~tritique The Lower Cambrian section Amouslek (AL) is well known for its abundance of fossils. The section is situated about 30 km SW of Taroudant at the southern border of the Souss Plain. The S6rie volcano-d~tritique d'Jebel Issendalen is exposed at the top of the complete section. This series is known throughout the northwestern AntiAtlas. Some horizons of volcanics can even be traced south of Goulimime. The thickness amounts to approximately 100 m in the type section, decreasing towards the southwest and east (Fig. 7). Multiple eruption cycles characterize this series. Each cycle starts with splintery green tuffites, overlain by up to 10 m thick coarse-grained volcanoclastics. The cycle terminates with a I om thick horizon of precipitated iron oxides. The base o! the tuffitic layers consists of closely packed pyroclastic material (fragments of devitrified volcanic

116

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glasses). Dispersed chlorite, calcite and ore form the matrix. Non-vulcanogenetic compounds (for instance metaquartzites) become more frequent towards the top. The coarse-grained unit (PI. 1, Fig 6) is composed of trachytic rock fragments in a tuffaceous matrix. The S~rie volcano-detritique was deposited in a shallow marine basin by pyroclastic turbidites and ash-falls. The location of the eruption center cannot be definitely determined; CHOUBERT & FAURE-MURET (1956) supposed that this eruption center was situated about 1O0 km east of Agadir.

4. Conclusions 4.1. Serie d'lssaf~ne (Trilobite Zone IV-VI) (Fig. 8) Three lithologic units can be distinguished: the comptexe schisteux in the eastern Anti-Atlas, the schistes de I'lssaf~ne and the calcaires scodac~s in the western Anti-Atlas. The facies of the reddish mudstones of the compiexe schisteux

extends over several hundred kilometers

without a major change. The sediments were deposited in a flat lagoon or flute plain under aride climate. The unit complexe schisteux might have been deposited diachronously. The transition into lithology of the schistes de I'lssaf~ne is located east of Tata. Green mudstones, crystal tufts and intercalated carbonate horizons are predominant west of this boundary. Trilobites indicate an open marine environment for the schistes de I'lssaf~ne. The distinct increase in pyroclastics point to an intensified volcanic activity in the western Anti-Atlas. The carbonates of the calcaires scoriac~s were deposited on top of a wide submarine ridge in the westernmost part of the Anti-Atlas. This ridge probably separated the depostitional area of the schistes de I'lssaf~ne from the open ocean (Japetus ocean?).

4.2. Asririen (Trilobite Zone VII) (Fig. 9) The pattern of sedimentation is completely different in the Asririen. Whereas mudstones prevailed in the S6rie de I'lssat~ne, the Asririen is characterized by fine-grained detrital sediments which were distributed by several river systems into the Anti-Atlas basin. Two river systems can be traced in the eastern Anti-Atlas. The first exhibits a nearly E-W directed flow regime and the second a dominance of SE-NW sediment transport. Both river systems of the gr~s rouges represent the type of 'braided river' with slow water flow. The fluviatile facies of the gr~s rouges gradually passes into the marine sandstones of the ~tage d'Asrir in the central Anti-Atlas. Sediments of a tidal-dominated delta are found in the transitional area. During the same period, the Delta of Asrir developed in the southwestern Anti-Atlas prograding towards the marine basin in the north. The conglomerates of Aguerd represent the littoral to tluviatile topset bed of the Delta of Asrir. They are exposed throughout a width of appr. 300 km at the southern margin of the Anti-Atlas. The area of Issaf~ne is characterized by local erosion and redepostition of the sediments. The mudstones of the ~tage d'Asrir were deposited outside of the influx of sand. The carbonates of the S~rie d'Jebel Tichinchine were deposited in the former sedimentation area of the calaires scoriac~s. The upper Asririen exhibits an intensified volcanic activiy leading to an increased deposition of volcanoclastics.

119

Fig. 12. Model of the upper Lower Cambrian deposition area in the Anti-Atlas.

4.3. Transition Asririen - Ourmastien (Trilobite Zone VII - VIII) (Fig. 10) A new transgressive cycle of sedimentation started with the deposition of the gr~s d'Ourmast. The source area of these series was located in the north, in the region of the present-day High Atlas. The gr#s d'Ourmast wedge out towards the W. They are replaced by only few meters of mudstones and siltstones. The volcanic activity increased considerably in the western Anti-Atlas. The up to 100 m thick S6rie volcanod6tritique was formed by eruptions with submarine currents of pyroclastics and a considerable amount of ash-fall. The Delta of Asdr reached its maximum extent at the Asririen - Ourmastien boundary.

4.4. Ourmastien (Trilobite Zone Viii) (Fig. 11) Coquina horizons intercalated within the increasingly more fine-grained sediments of the gr~s d'Ourmast indicate the transgression of the sea which covered nearly the entire Anti-Atlas - with the exception of some areas in the SE and E. The prograding of the Delta of Asrir terminated. The volcanic activity decreased in the northwestern AntiAtlas. The schistes & Paradoxides are indicative of the Middle Cambrian transgression which considerably exceeded the former area of sedimentation in the Anti-Atlas.

120

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Plate 1: Uthology of the 'gr~s terminaux' (thin sections of Figs. 1 - 3, and 5 with crossed nicols;Figs. 1,4 - 6: scale = 1 mm, Figs. 2, 3: scale = 0.1 mm) Fig. 1.

Pedogenic broken quartz within a carbonate matrix. S(~rieof Akerouz (sample AK 11).

Fig. 2. Gres rouges. Detrital quartz cemented by secondary quartz overgrowth (sample AM 32). Fig. 3. Grl~,srouges. Detrital feldspar with secondary feldspar overgrowth (sample AZ). Fig. 4. Schistes de I'lssaf~ne. Sandy biosparite with echinoderms and trilobites (sample A 11). Fig.&

Congbmerate d'Aguerd. Component of partly recrystallized metaquartzite (sample T 16).

Fig. 6. S~rie volcano-d~tritique. Lithic arenite composed of reworked volcanic rock fragments (sample AL 18).

Acknowledgment The authors are indebted to the authorities of Morocco - particularly to Dr. Bensaid and M. Dahmani of the Minist~re de I'Energie et des Mines - for the permission to work in the Anti-Atlas and for support during fieldwork. We,gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (Bu 312-10/1-2). Furthermore, we wish to thank H. Bogle for the translation and proof reading.

References BOUDDA, A., CHOUBERT, G. & FAURE-MURET, A. (1974): Note au sujet des 'gres terminaux' du Cambrien inf~rieur de rAnti-Atlas et de la transgression du Cambrien moyen. - C. R. somm. Soc. g~ol. France, 5, 138140; Paris CHOUBERT, G. (1952): Histoire g~ologique du domaine de rAnti-Atlas. - Not. M6m. Serv. g~ol. Maroc, 1 0 0 , 7 7 194; Rabat (1963): Histoire geologique du Precambrien de I'Anti-Atlas. - Not. M~m. Sew. g6ol. Maroc, 162,352 pp.; Rabat CHOUBERT,

G. &

FAURE-MURET, A. (1956): L'activit6 volcanique de la fin du G(~orgien dans I'Anti-Atlas et le Haut

Atlas occidentaux. - C. R. Acad. Sci. Pads, 242, 2735-2738; Paris COLEMAN, J. M. (1969): Brahmaputra river, channel processes and sedimentation. - Sed. Geol., 3, 129-239; Amsterdam GEYER, G. (1984): Biostratigraphische und systematisch- pal~ontologische Untersuchungen im Grenzbereich Unter-/Mittelkambrium von Marokko. - Diss. Univ. W0rzburg, 229 pp.; W0rzburg HUPE, P. (1952): Contribution & I'~tage du Cambrien inf(~rieur et du Pr6carnbrien Ill de I'Anti-Atlas marocain. - Not. M~m. Sew. g{}ol Maroc, 103, 402 pp.; Rabat MORGAN, J. P. (1970): Depositional processes and products in the deltaic environment. - In: MORGAN, J. P. (ed): Deltaic sedimentation. - SEPM Spec. Paper, 15, 31-47; Tulsa NELTNER, L. (1938): Etudes g~ologiques dans le sud Marocain. - Not. M6m. Serv. g0ol. Maroc, 42, 298 pp.; Rabat PASSEGA, R. (1964): Grain size representation by CM-pattern as a geological tool. - J. Sed. Petrol., 34, 830-847; Menasha PETTIJOHN, F. J., POTTER, P. E. & SIEVER, R. (1973): Sand and Sandstones. - 618 pp.; Heidelberg (Springer) REINECK, H. E. (1972): Tidal flats. - SEPM Spec. Paper, 17,146-159; Tulsa --

(1985) Aktuogeologie klastischerSedimente. - 348 pp.; Frankfurt (Kramer)

REINECK, H. E. & SINGH, I. B. (1980): Depositional sedimentary environments. - 451 pp.; Heidelberg (Springer)

DIAGENESIS AND VERY LOW-GRADE METAMORPHISM OF THE LOWER CAMBRIAN ROCKS IN THE ANTI-ATLAS (MOROCCO)

Werner Buggisch Institut f0r Geologie, Universit&t Erlangen-N0rnberg, SchloBgarten 5, D-8520 Erlangen

Abstract The diagenesis and the very low-grade metamorphism of the 'gr~s terminaux' in the Anti-Atlas were studied on the basis of the 'crystallinity' of illite and the deformation of quartz. The illite crystallinity improves from E to W. The quartz cements of the quartzitic sandstones are not deformed in the eastern Anti-Atlas. Undulation of quartz cement is observed in the central Anti-Atlas. The deformation increases towards the WSW where subgrains of quartz are found in quartz veins.

Introduction The diagenesis and the very low-grade metamorphism (WINKLER 1967) were studied in the strata of the AntiAtlas on the basis of clay minerals, 'illite crystallinity', authigenic minerals and the grade of quartz deformation. The stratigraphy, paleogeography and facies of the Lower Cambrian strata are discussed by BUGGISCH & FLOGEL and BUGGISCH & SIEGERT (this volume). The illite crystallinity was measured in one section containing strata from the 'S@de des calcaires inf@rieurs' to the 'gr@s terminaux' of the central Anti-Atlas, in order to analyse the influence of the sedimentary environment. The quartz deformation and the illite crystallinity were studied in the gr~s terminaux between Erfoud in the E and Agadir and Tiglit in the W in order to establish the regional pattern of the diagenesis and very low-grade metamorphism.

Phyllosilicates The clay mineral association of the eastern Anti-Atlas is dominated by illite and mixed layer minerals. Furthermore montmorillonite and kaolinite were detected in the X-ray diffraction records. The stability of kaolinite and its replacement by illite and chlorite depends considerably on the K+/H + and (Mg++ + Fe++)/H+ concentration ratios (KISCH 1983). Therefore, the disappearance of kaolinite is documented in many deep wells at depths between 2000 m (90o C) and more than 5100 m (180o C), but not at as great a depth as the appearance of anchizonal illite "crystallinities" (references summarized in KISCH 1983). An overburden pressure has already resulted from more than 3000 m thick Middle Cambrian and Ordovician sediment in the region south of Ouarzazate. Therefore, the new formation of kaolinite during weathering (Cretaceous or later) must be taken into consideration as well. Chlorite is the main phyllosilicate in the upper Early Cambrian sediments of the western Anti-Atlas. The chlorites which were frequently altered to oxychlorites replaced detrital micas as well as fossil shells. The replacement of mica by chlorite is known from the diagenesis and the 'early metagenesis' (KARPOVA 1969 after KISCH 1983: Fig. 5-9). The matrix of the clastic sediments in the western Anti-Atlas is composed of authigenic chlorite and illite. Intergrowth of quartz, chlorite and illite/sericite forming 'beards' can be obsewed in thin sections of rock samples from the southwesternmost Anti-Atlas (PI. 1, Fig. 6, 7).

124

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